QUIC: A UDP-Based Multiplexed and Secure Transport

1.1. Document Structure

This document describes the core QUIC protocol and is structured as follows:¶

• Streams are the basic service abstraction that QUIC provides.¶

  • Section 2 describes core concepts related to streams,¶
  • Section 3 provides a reference model for stream states, and¶
  • Section 4 outlines the operation of flow control.¶

• Connections are the context in which QUIC endpoints communicate.¶

  • Section 5 describes core concepts related to connections,¶
  • Section 6 describes version negotiation,¶
  • Section 7 details the process for establishing connections,¶
  • Section 8 specifies critical denial of service mitigation mechanisms,¶
  • Section 9 describes how endpoints migrate a connection to a new network path,¶
  • Section 10 lists the options for terminating an open connection, and¶
  • Section 11 provides general guidance for error handling.¶

• Packets and frames are the basic unit used by QUIC to communicate.¶

  • Section 12 describes concepts related to packets and frames,¶
  • Section 13 defines models for the transmission, retransmission, and acknowledgement of data, and¶
  • Section 14 specifies rules for managing the size of packets.¶

• Finally, encoding details of QUIC protocol elements are described in:¶

  • Section 15 (Versions),¶
  • Section 16 (Integer Encoding),¶
  • Section 17 (Packet Headers),¶
  • Section 18 (Transport Parameters),¶
  • Section 19 (Frames), and¶
  • Section 20 (Errors).¶

Accompanying documents describe QUIC's loss detection and congestion control [QUIC-RECOVERY], and the use of TLS for key negotiation [QUIC-TLS].¶

This document defines QUIC version 1, which conforms to the protocol invariants in [QUIC-INVARIANTS].¶

1.2. Terms and Definitions

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Commonly used terms in the document are described below.¶

QUIC:

The transport protocol described by this document. QUIC is a name, not an acronym.¶

QUIC packet:

A complete processable unit of QUIC that can be encapsulated in a UDP datagram. Multiple QUIC packets can be encapsulated in a single UDP datagram.¶

Ack-eliciting Packet:

A QUIC packet that contains frames other than ACK, PADDING, and CONNECTION_CLOSE. These cause a recipient to send an acknowledgment (see Section 13.2.1).¶

Out-of-order packet:

A packet that does not increase the largest received packet number for its packet number space (Section 12.3) by exactly one. A packet can arrive out of order if it is delayed or if earlier packets are lost or delayed.¶

Endpoint:

An entity that can participate in a QUIC connection by generating, receiving, and processing QUIC packets. There are only two types of endpoint in QUIC: client and server.¶

Client:

The endpoint initiating a QUIC connection.¶

Server:

The endpoint accepting incoming QUIC connections.¶

Address:

When used without qualification, the tuple of IP version, IP address, UDP protocol, and UDP port number that represents one end of a network path.¶

Connection ID:

An opaque identifier that is used to identify a QUIC connection at an endpoint. Each endpoint sets a value for its peer to include in packets sent towards the endpoint.¶

Stream:

A unidirectional or bidirectional channel of ordered bytes within a QUIC connection. A QUIC connection can carry multiple simultaneous streams.¶

Application:

An entity that uses QUIC to send and receive data.¶

1.3. Notational Conventions

Packet and frame diagrams in this document use the format described in Section 3.1 of [RFC2360], with the following additional conventions:¶

[x]:

Indicates that x is optional¶

x (A):

Indicates that x is A bits long¶

x (A/B/C) ...:

Indicates that x is one of A, B, or C bits long¶

x (i) ...:

Indicates that x uses the variable-length encoding in Section 16¶

x (*) ...:

Indicates that x is variable-length¶

2.1. Stream Types and Identifiers

Streams can be unidirectional or bidirectional. Unidirectional streams carry data in one direction: from the initiator of the stream to its peer. Bidirectional streams allow for data to be sent in both directions.¶

Streams are identified within a connection by a numeric value, referred to as the stream ID. A stream ID is a 62-bit integer (0 to 2^62-1) that is unique for all streams on a connection. Stream IDs are encoded as variable-length integers (see Section 16). A QUIC endpoint MUST NOT reuse a stream ID within a connection.¶

The least significant bit (0x1) of the stream ID identifies the initiator of the stream. Client-initiated streams have even-numbered stream IDs (with the bit set to 0), and server-initiated streams have odd-numbered stream IDs (with the bit set to 1).¶

The second least significant bit (0x2) of the stream ID distinguishes between bidirectional streams (with the bit set to 0) and unidirectional streams (with the bit set to 1).¶

The least significant two bits from a stream ID therefore identify a stream as one of four types, as summarized in Table 1.¶

Within each type, streams are created with numerically increasing stream IDs. A stream ID that is used out of order results in all streams of that type with lower-numbered stream IDs also being opened.¶

The first bidirectional stream opened by the client has a stream ID of 0.¶

2.2. Sending and Receiving Data

STREAM frames (Section 19.8) encapsulate data sent by an application. An endpoint uses the Stream ID and Offset fields in STREAM frames to place data in order.¶

Endpoints MUST be able to deliver stream data to an application as an ordered byte-stream. Delivering an ordered byte-stream requires that an endpoint buffer any data that is received out of order, up to the advertised flow control limit.¶

QUIC makes no specific allowances for delivery of stream data out of order. However, implementations MAY choose to offer the ability to deliver data out of order to a receiving application.¶

An endpoint could receive data for a stream at the same stream offset multiple times. Data that has already been received can be discarded. The data at a given offset MUST NOT change if it is sent multiple times; an endpoint MAY treat receipt of different data at the same offset within a stream as a connection error of type PROTOCOL_VIOLATION.¶

Streams are an ordered byte-stream abstraction with no other structure visible to QUIC. STREAM frame boundaries are not expected to be preserved when data is transmitted, retransmitted after packet loss, or delivered to the application at a receiver.¶

An endpoint MUST NOT send data on any stream without ensuring that it is within the flow control limits set by its peer. Flow control is described in detail in Section 4.¶

2.3. Stream Prioritization

Stream multiplexing can have a significant effect on application performance if resources allocated to streams are correctly prioritized.¶

QUIC does not provide a mechanism for exchanging prioritization information. Instead, it relies on receiving priority information from the application that uses QUIC.¶

A QUIC implementation SHOULD provide ways in which an application can indicate the relative priority of streams. When deciding which streams to dedicate resources to, the implementation SHOULD use the information provided by the application.¶

2.4. Required Operations on Streams

There are certain operations which an application MUST be able to perform when interacting with QUIC streams. This document does not specify an API, but any implementation of this version of QUIC MUST expose the ability to perform the operations described in this section on a QUIC stream.¶

On the sending part of a stream, application protocols need to be able to:¶

• write data, understanding when stream flow control credit (Section 4.1) has successfully been reserved to send the written data;¶
• end the stream (clean termination), resulting in a STREAM frame (Section 19.8) with the FIN bit set; and¶
• reset the stream (abrupt termination), resulting in a RESET_STREAM frame (Section 19.4), if the stream was not already in a terminal state.¶

On the receiving part of a stream, application protocols need to be able to:¶

• read data; and¶
• abort reading of the stream and request closure, possibly resulting in a STOP_SENDING frame (Section 19.5).¶

Applications also need to be informed of state changes on streams, including when the peer has opened or reset a stream, when a peer aborts reading on a stream, when new data is available, and when data can or cannot be written to the stream due to flow control.¶

3.1. Sending Stream States

Figure 1 shows the states for the part of a stream that sends data to a peer.¶

       o
       | Create Stream (Sending)
       | Peer Creates Bidirectional Stream
       v
   +-------+
   | Ready | Send RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Send STREAM /             |
       |      STREAM_DATA_BLOCKED  |
       |                           |
       | Peer Creates              |
       |      Bidirectional Stream |
       v                           |
   +-------+                       |
   | Send  | Send RESET_STREAM     |
   |       |---------------------->|
   +-------+                       |
       |                           |
       | Send STREAM + FIN         |
       v                           v
   +-------+                   +-------+
   | Data  | Send RESET_STREAM | Reset |
   | Sent  |------------------>| Sent  |
   +-------+                   +-------+
       |                           |
       | Recv All ACKs             | Recv ACK
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Recvd |                   | Recvd |
   +-------+                   +-------+

The sending part of stream that the endpoint initiates (types 0 and 2 for clients, 1 and 3 for servers) is opened by the application. The "Ready" state represents a newly created stream that is able to accept data from the application. Stream data might be buffered in this state in preparation for sending.¶

Sending the first STREAM or STREAM_DATA_BLOCKED frame causes a sending part of a stream to enter the "Send" state. An implementation might choose to defer allocating a stream ID to a stream until it sends the first STREAM frame and enters this state, which can allow for better stream prioritization.¶

The sending part of a bidirectional stream initiated by a peer (type 0 for a server, type 1 for a client) starts in the "Ready" state when the receiving part is created.¶

In the "Send" state, an endpoint transmits - and retransmits as necessary - stream data in STREAM frames. The endpoint respects the flow control limits set by its peer, and continues to accept and process MAX_STREAM_DATA frames. An endpoint in the "Send" state generates STREAM_DATA_BLOCKED frames if it is blocked from sending by stream or connection flow control limits Section 4.1.¶

After the application indicates that all stream data has been sent and a STREAM frame containing the FIN bit is sent, the sending part of the stream enters the "Data Sent" state. From this state, the endpoint only retransmits stream data as necessary. The endpoint does not need to check flow control limits or send STREAM_DATA_BLOCKED frames for a stream in this state. MAX_STREAM_DATA frames might be received until the peer receives the final stream offset. The endpoint can safely ignore any MAX_STREAM_DATA frames it receives from its peer for a stream in this state.¶

Once all stream data has been successfully acknowledged, the sending part of the stream enters the "Data Recvd" state, which is a terminal state.¶

From any of the "Ready", "Send", or "Data Sent" states, an application can signal that it wishes to abandon transmission of stream data. Alternatively, an endpoint might receive a STOP_SENDING frame from its peer. In either case, the endpoint sends a RESET_STREAM frame, which causes the stream to enter the "Reset Sent" state.¶

An endpoint MAY send a RESET_STREAM as the first frame that mentions a stream; this causes the sending part of that stream to open and then immediately transition to the "Reset Sent" state.¶

Once a packet containing a RESET_STREAM has been acknowledged, the sending part of the stream enters the "Reset Recvd" state, which is a terminal state.¶

3.2. Receiving Stream States

Figure 2 shows the states for the part of a stream that receives data from a peer. The states for a receiving part of a stream mirror only some of the states of the sending part of the stream at the peer. The receiving part of a stream does not track states on the sending part that cannot be observed, such as the "Ready" state. Instead, the receiving part of a stream tracks the delivery of data to the application, some of which cannot be observed by the sender.¶

       o
       | Recv STREAM / STREAM_DATA_BLOCKED / RESET_STREAM
       | Create Bidirectional Stream (Sending)
       | Recv MAX_STREAM_DATA / STOP_SENDING (Bidirectional)
       | Create Higher-Numbered Stream
       v
   +-------+
   | Recv  | Recv RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Recv STREAM + FIN         |
       v                           |
   +-------+                       |
   | Size  | Recv RESET_STREAM     |
   | Known |---------------------->|
   +-------+                       |
       |                           |
       | Recv All Data             |
       v                           v
   +-------+ Recv RESET_STREAM +-------+
   | Data  |--- (optional) --->| Reset |
   | Recvd |  Recv All Data    | Recvd |
   +-------+<-- (optional) ----+-------+
       |                           |
       | App Read All Data         | App Read RST
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Read  |                   | Read  |
   +-------+                   +-------+

The receiving part of a stream initiated by a peer (types 1 and 3 for a client, or 0 and 2 for a server) is created when the first STREAM, STREAM_DATA_BLOCKED, or RESET_STREAM is received for that stream. For bidirectional streams initiated by a peer, receipt of a MAX_STREAM_DATA or STOP_SENDING frame for the sending part of the stream also creates the receiving part. The initial state for the receiving part of stream is "Recv".¶

The receiving part of a stream enters the "Recv" state when the sending part of a bidirectional stream initiated by the endpoint (type 0 for a client, type 1 for a server) enters the "Ready" state.¶

An endpoint opens a bidirectional stream when a MAX_STREAM_DATA or STOP_SENDING frame is received from the peer for that stream. Receiving a MAX_STREAM_DATA frame for an unopened stream indicates that the remote peer has opened the stream and is providing flow control credit. Receiving a STOP_SENDING frame for an unopened stream indicates that the remote peer no longer wishes to receive data on this stream. Either frame might arrive before a STREAM or STREAM_DATA_BLOCKED frame if packets are lost or reordered.¶

Before a stream is created, all streams of the same type with lower-numbered stream IDs MUST be created. This ensures that the creation order for streams is consistent on both endpoints.¶

In the "Recv" state, the endpoint receives STREAM and STREAM_DATA_BLOCKED frames. Incoming data is buffered and can be reassembled into the correct order for delivery to the application. As data is consumed by the application and buffer space becomes available, the endpoint sends MAX_STREAM_DATA frames to allow the peer to send more data.¶

When a STREAM frame with a FIN bit is received, the final size of the stream is known (see Section 4.4). The receiving part of the stream then enters the "Size Known" state. In this state, the endpoint no longer needs to send MAX_STREAM_DATA frames, it only receives any retransmissions of stream data.¶

Once all data for the stream has been received, the receiving part enters the "Data Recvd" state. This might happen as a result of receiving the same STREAM frame that causes the transition to "Size Known". After all data has been received, any STREAM or STREAM_DATA_BLOCKED frames for the stream can be discarded.¶

The "Data Recvd" state persists until stream data has been delivered to the application. Once stream data has been delivered, the stream enters the "Data Read" state, which is a terminal state.¶

Receiving a RESET_STREAM frame in the "Recv" or "Size Known" states causes the stream to enter the "Reset Recvd" state. This might cause the delivery of stream data to the application to be interrupted.¶

It is possible that all stream data is received when a RESET_STREAM is received (that is, from the "Data Recvd" state). Similarly, it is possible for remaining stream data to arrive after receiving a RESET_STREAM frame (the "Reset Recvd" state). An implementation is free to manage this situation as it chooses.¶

Sending RESET_STREAM means that an endpoint cannot guarantee delivery of stream data; however there is no requirement that stream data not be delivered if a RESET_STREAM is received. An implementation MAY interrupt delivery of stream data, discard any data that was not consumed, and signal the receipt of the RESET_STREAM. A RESET_STREAM signal might be suppressed or withheld if stream data is completely received and is buffered to be read by the application. If the RESET_STREAM is suppressed, the receiving part of the stream remains in "Data Recvd".¶

Once the application receives the signal indicating that the stream was reset, the receiving part of the stream transitions to the "Reset Read" state, which is a terminal state.¶

3.3. Permitted Frame Types

The sender of a stream sends just three frame types that affect the state of a stream at either sender or receiver: STREAM (Section 19.8), STREAM_DATA_BLOCKED (Section 19.13), and RESET_STREAM (Section 19.4).¶

A sender MUST NOT send any of these frames from a terminal state ("Data Recvd" or "Reset Recvd"). A sender MUST NOT send STREAM or STREAM_DATA_BLOCKED after sending a RESET_STREAM; that is, in the terminal states and in the "Reset Sent" state. A receiver could receive any of these three frames in any state, due to the possibility of delayed delivery of packets carrying them.¶

The receiver of a stream sends MAX_STREAM_DATA (Section 19.10) and STOP_SENDING frames (Section 19.5).¶

The receiver only sends MAX_STREAM_DATA in the "Recv" state. A receiver can send STOP_SENDING in any state where it has not received a RESET_STREAM frame; that is states other than "Reset Recvd" or "Reset Read". However there is little value in sending a STOP_SENDING frame in the "Data Recvd" state, since all stream data has been received. A sender could receive either of these two frames in any state as a result of delayed delivery of packets.¶

3.4. Bidirectional Stream States

A bidirectional stream is composed of sending and receiving parts. Implementations may represent states of the bidirectional stream as composites of sending and receiving stream states. The simplest model presents the stream as "open" when either sending or receiving parts are in a non-terminal state and "closed" when both sending and receiving streams are in terminal states.¶

Table 2 shows a more complex mapping of bidirectional stream states that loosely correspond to the stream states in HTTP/2 [HTTP2]. This shows that multiple states on sending or receiving parts of streams are mapped to the same composite state. Note that this is just one possibility for such a mapping; this mapping requires that data is acknowledged before the transition to a "closed" or "half-closed" state.¶

Note (*1):

A stream is considered "idle" if it has not yet been created, or if the receiving part of the stream is in the "Recv" state without yet having received any frames.¶

3.5. Solicited State Transitions

If an application is no longer interested in the data it is receiving on a stream, it can abort reading the stream and specify an application error code.¶

If the stream is in the "Recv" or "Size Known" states, the transport SHOULD signal this by sending a STOP_SENDING frame to prompt closure of the stream in the opposite direction. This typically indicates that the receiving application is no longer reading data it receives from the stream, but it is not a guarantee that incoming data will be ignored.¶

STREAM frames received after sending STOP_SENDING are still counted toward connection and stream flow control, even though these frames can be discarded upon receipt.¶

A STOP_SENDING frame requests that the receiving endpoint send a RESET_STREAM frame. An endpoint that receives a STOP_SENDING frame MUST send a RESET_STREAM frame if the stream is in the Ready or Send state. If the stream is in the Data Sent state and any outstanding data is declared lost, an endpoint SHOULD send a RESET_STREAM frame in lieu of a retransmission.¶

An endpoint SHOULD copy the error code from the STOP_SENDING frame to the RESET_STREAM frame it sends, but MAY use any application error code. The endpoint that sends a STOP_SENDING frame MAY ignore the error code carried in any RESET_STREAM frame it receives.¶

If the STOP_SENDING frame is received on a stream that is already in the "Data Sent" state, an endpoint that wishes to cease retransmission of previously-sent STREAM frames on that stream MUST first send a RESET_STREAM frame.¶

STOP_SENDING SHOULD only be sent for a stream that has not been reset by the peer. STOP_SENDING is most useful for streams in the "Recv" or "Size Known" states.¶

An endpoint is expected to send another STOP_SENDING frame if a packet containing a previous STOP_SENDING is lost. However, once either all stream data or a RESET_STREAM frame has been received for the stream - that is, the stream is in any state other than "Recv" or "Size Known" - sending a STOP_SENDING frame is unnecessary.¶

An endpoint that wishes to terminate both directions of a bidirectional stream can terminate one direction by sending a RESET_STREAM, and it can encourage prompt termination in the opposite direction by sending a STOP_SENDING frame.¶

4.1. Data Flow Control

QUIC employs a credit-based flow-control scheme similar to that in HTTP/2 [HTTP2], where a receiver advertises the number of bytes it is prepared to receive on a given stream and for the entire connection. This leads to two levels of data flow control in QUIC:¶

• Stream flow control, which prevents a single stream from consuming the entire receive buffer for a connection by limiting the amount of data that can be sent on any stream.¶
• Connection flow control, which prevents senders from exceeding a receiver's buffer capacity for the connection, by limiting the total bytes of stream data sent in STREAM frames on all streams.¶

A receiver sets initial credits for all streams by sending transport parameters during the handshake (Section 7.3). A receiver sends MAX_STREAM_DATA (Section 19.10) or MAX_DATA (Section 19.9) frames to the sender to advertise additional credit.¶

A receiver advertises credit for a stream by sending a MAX_STREAM_DATA frame with the Stream ID field set appropriately. A MAX_STREAM_DATA frame indicates the maximum absolute byte offset of a stream. A receiver could use the current offset of data consumed to determine the flow control offset to be advertised. A receiver MAY send MAX_STREAM_DATA frames in multiple packets in order to make sure that the sender receives an update before running out of flow control credit, even if one of the packets is lost.¶

A receiver advertises credit for a connection by sending a MAX_DATA frame, which indicates the maximum of the sum of the absolute byte offsets of all streams. A receiver maintains a cumulative sum of bytes received on all streams, which is used to check for flow control violations. A receiver might use a sum of bytes consumed on all streams to determine the maximum data limit to be advertised.¶

A receiver can advertise a larger offset by sending MAX_STREAM_DATA or MAX_DATA frames. Once a receiver advertises an offset, it MAY advertise a smaller offset, but this has no effect.¶

A receiver MUST close the connection with a FLOW_CONTROL_ERROR error (Section 11) if the sender violates the advertised connection or stream data limits.¶

A sender MUST ignore any MAX_STREAM_DATA or MAX_DATA frames that do not increase flow control limits.¶

If a sender runs out of flow control credit, it will be unable to send new data and is considered blocked. A sender SHOULD send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame to indicate it has data to write but is blocked by flow control limits. If a sender is blocked for a period longer than the idle timeout (Section 10.2), the connection might be closed even when data is available for transmission. To keep the connection from closing, a sender that is flow control limited SHOULD periodically send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame when it has no ack-eliciting packets in flight.¶

4.2. Flow Credit Increments

Implementations decide when and how much credit to advertise in MAX_STREAM_DATA and MAX_DATA frames, but this section offers a few considerations.¶

To avoid blocking a sender, a receiver can send a MAX_STREAM_DATA or MAX_DATA frame multiple times within a round trip or send it early enough to allow for recovery from loss of the frame.¶

Control frames contribute to connection overhead. Therefore, frequently sending MAX_STREAM_DATA and MAX_DATA frames with small changes is undesirable. On the other hand, if updates are less frequent, larger increments to limits are necessary to avoid blocking a sender, requiring larger resource commitments at the receiver. There is a trade-off between resource commitment and overhead when determining how large a limit is advertised.¶

A receiver can use an autotuning mechanism to tune the frequency and amount of advertised additional credit based on a round-trip time estimate and the rate at which the receiving application consumes data, similar to common TCP implementations. As an optimization, an endpoint could send frames related to flow control only when there are other frames to send or when a peer is blocked, ensuring that flow control does not cause extra packets to be sent.¶

A blocked sender is not required to send STREAM_DATA_BLOCKED or DATA_BLOCKED frames. Therefore, a receiver MUST NOT wait for a STREAM_DATA_BLOCKED or DATA_BLOCKED frame before sending a MAX_STREAM_DATA or MAX_DATA frame; doing so could result in the sender being blocked for the rest of the connection. Even if the sender sends these frames, waiting for them will result in the sender being blocked for at least an entire round trip.¶

When a sender receives credit after being blocked, it might be able to send a large amount of data in response, resulting in short-term congestion; see Section 6.9 in [QUIC-RECOVERY] for a discussion of how a sender can avoid this congestion.¶

4.3. Handling Stream Cancellation

Endpoints need to eventually agree on the amount of flow control credit that has been consumed, to avoid either exceeding flow control limits or deadlocking.¶

On receipt of a RESET_STREAM frame, an endpoint will tear down state for the matching stream and ignore further data arriving on that stream. Without the offset included in RESET_STREAM, the two endpoints could disagree on the number of bytes that count towards connection flow control.¶

To remedy this issue, a RESET_STREAM frame (Section 19.4) includes the final size of data sent on the stream. On receiving a RESET_STREAM frame, a receiver definitively knows how many bytes were sent on that stream before the RESET_STREAM frame, and the receiver MUST use the final size of the stream to account for all bytes sent on the stream in its connection level flow controller.¶

RESET_STREAM terminates one direction of a stream abruptly. For a bidirectional stream, RESET_STREAM has no effect on data flow in the opposite direction. Both endpoints MUST maintain flow control state for the stream in the unterminated direction until that direction enters a terminal state, or until one of the endpoints sends CONNECTION_CLOSE.¶

4.4. Stream Final Size

The final size is the amount of flow control credit that is consumed by a stream. Assuming that every contiguous byte on the stream was sent once, the final size is the number of bytes sent. More generally, this is one higher than the offset of the byte with the largest offset sent on the stream, or zero if no bytes were sent.¶

For a stream that is reset, the final size is carried explicitly in a RESET_STREAM frame. Otherwise, the final size is the offset plus the length of a STREAM frame marked with a FIN flag, or 0 in the case of incoming unidirectional streams.¶

An endpoint will know the final size for a stream when the receiving part of the stream enters the "Size Known" or "Reset Recvd" state (Section 3).¶

An endpoint MUST NOT send data on a stream at or beyond the final size.¶

Once a final size for a stream is known, it cannot change. If a RESET_STREAM or STREAM frame is received indicating a change in the final size for the stream, an endpoint SHOULD respond with a FINAL_SIZE_ERROR error (see Section 11). A receiver SHOULD treat receipt of data at or beyond the final size as a FINAL_SIZE_ERROR error, even after a stream is closed. Generating these errors is not mandatory, but only because requiring that an endpoint generate these errors also means that the endpoint needs to maintain the final size state for closed streams, which could mean a significant state commitment.¶

4.5. Controlling Concurrency

An endpoint limits the cumulative number of incoming streams a peer can open. Only streams with a stream ID less than (max_stream * 4 + initial_stream_id_for_type) can be opened (see Table 5). Initial limits are set in the transport parameters (see Section 18.2) and subsequently limits are advertised using MAX_STREAMS frames (Section 19.11). Separate limits apply to unidirectional and bidirectional streams.¶

If a max_streams transport parameter or MAX_STREAMS frame is received with a value greater than 2^60, this would allow a maximum stream ID that cannot be expressed as a variable-length integer (see Section 16). If either is received, the connection MUST be closed immediately with a connection error of type STREAM_LIMIT_ERROR (see Section 10.3).¶

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a frame with a stream ID exceeding the limit it has sent MUST treat this as a connection error of type STREAM_LIMIT_ERROR (Section 11).¶

Once a receiver advertises a stream limit using the MAX_STREAMS frame, advertising a smaller limit has no effect. A receiver MUST ignore any MAX_STREAMS frame that does not increase the stream limit.¶

As with stream and connection flow control, this document leaves when and how many streams to advertise to a peer via MAX_STREAMS to implementations. Implementations might choose to increase limits as streams close to keep the number of streams available to peers roughly consistent.¶

An endpoint that is unable to open a new stream due to the peer's limits SHOULD send a STREAMS_BLOCKED frame (Section 19.14). This signal is considered useful for debugging. An endpoint MUST NOT wait to receive this signal before advertising additional credit, since doing so will mean that the peer will be blocked for at least an entire round trip, and potentially for longer if the peer chooses to not send STREAMS_BLOCKED frames.¶

5.1. Connection ID

Each connection possesses a set of connection identifiers, or connection IDs, each of which can identify the connection. Connection IDs are independently selected by endpoints; each endpoint selects the connection IDs that its peer uses.¶

The primary function of a connection ID is to ensure that changes in addressing at lower protocol layers (UDP, IP) don't cause packets for a QUIC connection to be delivered to the wrong endpoint. Each endpoint selects connection IDs using an implementation-specific (and perhaps deployment-specific) method which will allow packets with that connection ID to be routed back to the endpoint and identified by the endpoint upon receipt.¶

Connection IDs MUST NOT contain any information that can be used by an external observer (that is, one that does not cooperate with the issuer) to correlate them with other connection IDs for the same connection. As a trivial example, this means the same connection ID MUST NOT be issued more than once on the same connection.¶

Packets with long headers include Source Connection ID and Destination Connection ID fields. These fields are used to set the connection IDs for new connections; see Section 7.2 for details.¶

Packets with short headers (Section 17.3) only include the Destination Connection ID and omit the explicit length. The length of the Destination Connection ID field is expected to be known to endpoints. Endpoints using a load balancer that routes based on connection ID could agree with the load balancer on a fixed length for connection IDs, or agree on an encoding scheme. A fixed portion could encode an explicit length, which allows the entire connection ID to vary in length and still be used by the load balancer.¶

A Version Negotiation (Section 17.2.1) packet echoes the connection IDs selected by the client, both to ensure correct routing toward the client and to allow the client to validate that the packet is in response to an Initial packet.¶

A zero-length connection ID can be used when a connection ID is not needed to route to the correct endpoint. However, multiplexing connections on the same local IP address and port while using zero-length connection IDs will cause failures in the presence of peer connection migration, NAT rebinding, and client port reuse; and therefore MUST NOT be done unless an endpoint is certain that those protocol features are not in use.¶

When an endpoint uses a non-zero-length connection ID, it needs to ensure that the peer has a supply of connection IDs from which to choose for packets sent to the endpoint. These connection IDs are supplied by the endpoint using the NEW_CONNECTION_ID frame (Section 19.15).¶

5.2. Matching Packets to Connections

Incoming packets are classified on receipt. Packets can either be associated with an existing connection, or - for servers - potentially create a new connection.¶

Endpoints try to associate a packet with an existing connection. If the packet has a non-zero-length Destination Connection ID corresponding to an existing connection, QUIC processes that packet accordingly. Note that more than one connection ID can be associated with a connection; see Section 5.1.¶

If the Destination Connection ID is zero length and the addressing information in the packet matches the addressing information the endpoint uses to identify a connection with a zero-length connection ID, QUIC processes the packet as part of that connection. An endpoint can use just destination IP and port or both source and destination addresses for identification, though this makes connections fragile as described in Section 5.1.¶

Endpoints can send a Stateless Reset (Section 10.4) for any packets that cannot be attributed to an existing connection. A stateless reset allows a peer to more quickly identify when a connection becomes unusable.¶

Packets that are matched to an existing connection are discarded if the packets are inconsistent with the state of that connection. For example, packets are discarded if they indicate a different protocol version than that of the connection, or if the removal of packet protection is unsuccessful once the expected keys are available.¶

Invalid packets without packet protection, such as Initial, Retry, or Version Negotiation, MAY be discarded. An endpoint MUST generate a connection error if it commits changes to state before discovering an error.¶

5.3. Life of a QUIC Connection

A QUIC connection is a stateful interaction between a client and server, the primary purpose of which is to support the exchange of data by an application protocol. Streams (Section 2) are the primary means by which an application protocol exchanges information.¶

Each connection starts with a handshake phase, during which client and server establish a shared secret using the cryptographic handshake protocol [QUIC-TLS] and negotiate the application protocol. The handshake (Section 7) confirms that both endpoints are willing to communicate (Section 8.1) and establishes parameters for the connection (Section 7.3).¶

An application protocol can also operate in a limited fashion during the handshake phase. 0-RTT allows application messages to be sent by a client before receiving any messages from the server. However, 0-RTT lacks certain key security guarantees. In particular, there is no protection against replay attacks in 0-RTT; see [QUIC-TLS]. Separately, a server can also send application data to a client before it receives the final cryptographic handshake messages that allow it to confirm the identity and liveness of the client. These capabilities allow an application protocol to offer the option to trade some security guarantees for reduced latency.¶

The use of connection IDs (Section 5.1) allows connections to migrate to a new network path, both as a direct choice of an endpoint and when forced by a change in a middlebox. Section 9 describes mitigations for the security and privacy issues associated with migration.¶

For connections that are no longer needed or desired, there are several ways for a client and server to terminate a connection (Section 10).¶

5.4. Required Operations on Connections

There are certain operations which an application MUST be able to perform when interacting with the QUIC transport. This document does not specify an API, but any implementation of this version of QUIC MUST expose the ability to perform the operations described in this section on a QUIC connection.¶

When implementing the client role, applications need to be able to:¶

• open a connection, which begins the exchange described in Section 7;¶
• enable 0-RTT when available; and¶
• be informed when 0-RTT has been accepted or rejected by a server.¶

When implementing the server role, applications need to be able to:¶

• listen for incoming connections, which prepares for the exchange described in Section 7;¶
• if Early Data is supported, embed application-controlled data in the TLS resumption ticket sent to the client; and¶
• if Early Data is supported, retrieve application-controlled data from the client's resumption ticket and enable rejecting Early Data based on that information.¶

In either role, applications need to be able to:¶

• configure minimum values for the initial number of permitted streams of each type, as communicated in the transport parameters (Section 7.3);¶
• control resource allocation of various types, including flow control and the number of permitted streams of each type;¶
• identify whether the handshake has completed successfully or is still ongoing¶
• keep a connection from silently closing, either by generating PING frames (Section 19.2) or by requesting that the transport send additional frames before the idle timeout expires (Section 10.2); and¶
• immediately close (Section 10.3) the connection.¶

6.1. Sending Version Negotiation Packets

If the version selected by the client is not acceptable to the server, the server responds with a Version Negotiation packet (see Section 17.2.1). This includes a list of versions that the server will accept. An endpoint MUST NOT send a Version Negotiation packet in response to receiving a Version Negotiation packet.¶

This system allows a server to process packets with unsupported versions without retaining state. Though either the Initial packet or the Version Negotiation packet that is sent in response could be lost, the client will send new packets until it successfully receives a response or it abandons the connection attempt. As a result, the client discards all state for the connection and does not send any more packets on the connection.¶

A server MAY limit the number of Version Negotiation packets it sends. For instance, a server that is able to recognize packets as 0-RTT might choose not to send Version Negotiation packets in response to 0-RTT packets with the expectation that it will eventually receive an Initial packet.¶

6.2. Handling Version Negotiation Packets

When a client receives a Version Negotiation packet, it MUST abandon the current connection attempt. Version Negotiation packets are designed to allow future versions of QUIC to negotiate the version in use between endpoints. Future versions of QUIC might change how implementations that support multiple versions of QUIC react to Version Negotiation packets when attempting to establish a connection using this version. How to perform version negotiation is left as future work defined by future versions of QUIC. In particular, that future work will need to ensure robustness against version downgrade attacks; see Section 21.10.¶

6.3. Using Reserved Versions

For a server to use a new version in the future, clients need to correctly handle unsupported versions. To help ensure this, a server SHOULD include a version that is reserved for forcing version negotiation (0x?a?a?a?a as defined in Section 15) when generating a Version Negotiation packet.¶

The design of version negotiation permits a server to avoid maintaining state for packets that it rejects in this fashion.¶

A client MAY send a packet using a version that is reserved for forcing version negotiation. This can be used to solicit a list of supported versions from a server.¶

7.1. Example Handshake Flows

Details of how TLS is integrated with QUIC are provided in [QUIC-TLS], but some examples are provided here. An extension of this exchange to support client address validation is shown in Section 8.1.2.¶

Once any address validation exchanges are complete, the cryptographic handshake is used to agree on cryptographic keys. The cryptographic handshake is carried in Initial (Section 17.2.2) and Handshake (Section 17.2.4) packets.¶

Figure 3 provides an overview of the 1-RTT handshake. Each line shows a QUIC packet with the packet type and packet number shown first, followed by the frames that are typically contained in those packets. So, for instance the first packet is of type Initial, with packet number 0, and contains a CRYPTO frame carrying the ClientHello.¶

Note that multiple QUIC packets - even of different encryption levels - may be coalesced into a single UDP datagram (see Section 12.2), and so this handshake may consist of as few as 4 UDP datagrams, or any number more. For instance, the server's first flight contains packets from the Initial encryption level (obfuscation), the Handshake level, and "0.5-RTT data" from the server at the 1-RTT encryption level.¶

Client                                                  Server

Initial[0]: CRYPTO[CH] ->

                                 Initial[0]: CRYPTO[SH] ACK[0]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]

Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[0]: STREAM[0, "..."], ACK[0] ->

                            1-RTT[1]: STREAM[3, "..."], ACK[0]
                                       <- Handshake[1]: ACK[0]

Figure 4 shows an example of a connection with a 0-RTT handshake and a single packet of 0-RTT data. Note that as described in Section 12.3, the server acknowledges 0-RTT data at the 1-RTT encryption level, and the client sends 1-RTT packets in the same packet number space.¶

Client                                                  Server

Initial[0]: CRYPTO[CH]
0-RTT[0]: STREAM[0, "..."] ->

                                 Initial[0]: CRYPTO[SH] ACK[0]
                                  Handshake[0] CRYPTO[EE, FIN]
                          <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[1]: STREAM[0, "..."] ACK[0] ->

                            1-RTT[1]: STREAM[3, "..."], ACK[1]
                                       <- Handshake[1]: ACK[0]

7.2. Negotiating Connection IDs

A connection ID is used to ensure consistent routing of packets, as described in Section 5.1. The long header contains two connection IDs: the Destination Connection ID is chosen by the recipient of the packet and is used to provide consistent routing; the Source Connection ID is used to set the Destination Connection ID used by the peer.¶

During the handshake, packets with the long header (Section 17.2) are used to establish the connection ID that each endpoint uses. Each endpoint uses the Source Connection ID field to specify the connection ID that is used in the Destination Connection ID field of packets being sent to them. Upon receiving a packet, each endpoint sets the Destination Connection ID it sends to match the value of the Source Connection ID that they receive.¶

When an Initial packet is sent by a client that has not previously received an Initial or Retry packet from the server, it populates the Destination Connection ID field with an unpredictable value. This MUST be at least 8 bytes in length. Until a packet is received from the server, the client MUST use the same value unless it abandons the connection attempt and starts a new one. The initial Destination Connection ID is used to determine packet protection keys for Initial packets.¶

The client populates the Source Connection ID field with a value of its choosing and sets the SCID Len field to indicate the length.¶

The first flight of 0-RTT packets use the same Destination and Source Connection ID values as the client's first Initial.¶

Upon first receiving an Initial or Retry packet from the server, the client uses the Source Connection ID supplied by the server as the Destination Connection ID for subsequent packets, including any subsequent 0-RTT packets. That means that a client might change the Destination Connection ID twice during connection establishment, once in response to a Retry and once in response to the first Initial packet from the server. Once a client has received an Initial packet from the server, it MUST discard any packet it receives with a different Source Connection ID.¶

A client MUST only change the value it sends in the Destination Connection ID in response to the first packet of each type it receives from the server (Retry or Initial); a server MUST set its value based on the Initial packet. Any additional changes are not permitted; if subsequent packets of those types include a different Source Connection ID, they MUST be discarded. This avoids problems that might arise from stateless processing of multiple Initial packets producing different connection IDs.¶

The connection ID can change over the lifetime of a connection, especially in response to connection migration (Section 9); see Section 5.1.1 for details.¶

7.3. Transport Parameters

During connection establishment, both endpoints make authenticated declarations of their transport parameters. These declarations are made unilaterally by each endpoint. Endpoints are required to comply with the restrictions implied by these parameters; the description of each parameter includes rules for its handling.¶

The encoding of the transport parameters is detailed in Section 18.¶

QUIC includes the encoded transport parameters in the cryptographic handshake. Once the handshake completes, the transport parameters declared by the peer are available. Each endpoint validates the value provided by its peer.¶

Definitions for each of the defined transport parameters are included in Section 18.2.¶

An endpoint MUST treat receipt of a transport parameter with an invalid value as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

An endpoint MUST NOT send a parameter more than once in a given transport parameters extension. An endpoint SHOULD treat receipt of duplicate transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

A server MUST include the original_connection_id transport parameter (Section 18.2) if it sent a Retry packet to enable validation of the Retry, as described in Section 17.2.5.¶

7.4. Cryptographic Message Buffering

Implementations need to maintain a buffer of CRYPTO data received out of order. Because there is no flow control of CRYPTO frames, an endpoint could potentially force its peer to buffer an unbounded amount of data.¶

Implementations MUST support buffering at least 4096 bytes of data received in CRYPTO frames out of order. Endpoints MAY choose to allow more data to be buffered during the handshake. A larger limit during the handshake could allow for larger keys or credentials to be exchanged. An endpoint's buffer size does not need to remain constant during the life of the connection.¶

Being unable to buffer CRYPTO frames during the handshake can lead to a connection failure. If an endpoint's buffer is exceeded during the handshake, it can expand its buffer temporarily to complete the handshake. If an endpoint does not expand its buffer, it MUST close the connection with a CRYPTO_BUFFER_EXCEEDED error code.¶

Once the handshake completes, if an endpoint is unable to buffer all data in a CRYPTO frame, it MAY discard that CRYPTO frame and all CRYPTO frames received in the future, or it MAY close the connection with a CRYPTO_BUFFER_EXCEEDED error code. Packets containing discarded CRYPTO frames MUST be acknowledged because the packet has been received and processed by the transport even though the CRYPTO frame was discarded.¶

8.1. Address Validation During Connection Establishment

Connection establishment implicitly provides address validation for both endpoints. In particular, receipt of a packet protected with Handshake keys confirms that the client received the Initial packet from the server. Once the server has successfully processed a Handshake packet from the client, it can consider the client address to have been validated.¶

Prior to validating the client address, servers MUST NOT send more than three times as many bytes as the number of bytes they have received. This limits the magnitude of any amplification attack that can be mounted using spoofed source addresses. In determining this limit, servers only count the size of successfully processed packets.¶

Clients MUST ensure that UDP datagrams containing Initial packets have UDP payloads of at least 1200 bytes, adding padding to packets in the datagram as necessary. Sending padded datagrams ensures that the server is not overly constrained by the amplification restriction.¶

Packet loss, in particular loss of a Handshake packet from the server, can cause a situation in which the server cannot send when the client has no data to send and the anti-amplification limit is reached. In order to avoid this causing a handshake deadlock, clients MUST send a packet upon a probe timeout, as described in [QUIC-RECOVERY]. If the client has no data to retransmit and does not have Handshake keys, it MUST send an Initial packet in a UDP datagram of at least 1200 bytes. If the client has Handshake keys, it SHOULD send a Handshake packet.¶

A server might wish to validate the client address before starting the cryptographic handshake. QUIC uses a token in the Initial packet to provide address validation prior to completing the handshake. This token is delivered to the client during connection establishment with a Retry packet (see Section 8.1.2) or in a previous connection using the NEW_TOKEN frame (see Section 8.1.3).¶

In addition to sending limits imposed prior to address validation, servers are also constrained in what they can send by the limits set by the congestion controller. Clients are only constrained by the congestion controller.¶

Client                                                  Server

Initial[0]: CRYPTO[CH] ->

                                                <- Retry+Token

Initial+Token[1]: CRYPTO[CH] ->

                                 Initial[0]: CRYPTO[SH] ACK[1]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]

8.2. Path Validation

Path validation is used during connection migration (see Section 9 and Section 9.6) by the migrating endpoint to verify reachability of a peer from a new local address. In path validation, endpoints test reachability between a specific local address and a specific peer address, where an address is the two-tuple of IP address and port.¶

Path validation tests that packets (PATH_CHALLENGE) can be both sent to and received (PATH_RESPONSE) from a peer on the path. Importantly, it validates that the packets received from the migrating endpoint do not carry a spoofed source address.¶

Path validation can be used at any time by either endpoint. For instance, an endpoint might check that a peer is still in possession of its address after a period of quiescence.¶

Path validation is not designed as a NAT traversal mechanism. Though the mechanism described here might be effective for the creation of NAT bindings that support NAT traversal, the expectation is that one or other peer is able to receive packets without first having sent a packet on that path. Effective NAT traversal needs additional synchronization mechanisms that are not provided here.¶

An endpoint MAY bundle PATH_CHALLENGE and PATH_RESPONSE frames that are used for path validation with other frames. In particular, an endpoint may pad a packet carrying a PATH_CHALLENGE for PMTU discovery, or an endpoint may bundle a PATH_RESPONSE with its own PATH_CHALLENGE.¶

When probing a new path, an endpoint might want to ensure that its peer has an unused connection ID available for responses. The endpoint can send NEW_CONNECTION_ID and PATH_CHALLENGE frames in the same packet. This ensures that an unused connection ID will be available to the peer when sending a response.¶

8.3. Initiating Path Validation

To initiate path validation, an endpoint sends a PATH_CHALLENGE frame containing a random payload on the path to be validated.¶

An endpoint MAY send multiple PATH_CHALLENGE frames to guard against packet loss. However, an endpoint SHOULD NOT send multiple PATH_CHALLENGE frames in a single packet. An endpoint SHOULD NOT send a PATH_CHALLENGE more frequently than it would an Initial packet, ensuring that connection migration is no more load on a new path than establishing a new connection.¶

The endpoint MUST use unpredictable data in every PATH_CHALLENGE frame so that it can associate the peer's response with the corresponding PATH_CHALLENGE.¶

8.4. Path Validation Responses

On receiving a PATH_CHALLENGE frame, an endpoint MUST respond immediately by echoing the data contained in the PATH_CHALLENGE frame in a PATH_RESPONSE frame.¶

An endpoint MUST NOT send more than one PATH_RESPONSE frame in response to one PATH_CHALLENGE frame (see Section 13.3). The peer is expected to send more PATH_CHALLENGE frames as necessary to evoke additional PATH_RESPONSE frames.¶

8.5. Successful Path Validation

A new address is considered valid when a PATH_RESPONSE frame is received that contains the data that was sent in a previous PATH_CHALLENGE. Receipt of an acknowledgment for a packet containing a PATH_CHALLENGE frame is not adequate validation, since the acknowledgment can be spoofed by a malicious peer.¶

Note that receipt on a different local address does not result in path validation failure, as it might be a result of a forwarded packet (see Section 9.3.3) or misrouting. It is possible that a valid PATH_RESPONSE might be received in the future.¶

8.6. Failed Path Validation

Path validation only fails when the endpoint attempting to validate the path abandons its attempt to validate the path.¶

Endpoints SHOULD abandon path validation based on a timer. When setting this timer, implementations are cautioned that the new path could have a longer round-trip time than the original. A value of three times the larger of the current Probe Timeout (PTO) or the initial timeout (that is, 2*kInitialRtt) as defined in [QUIC-RECOVERY] is RECOMMENDED. That is:¶

   validation_timeout = max(3*PTO, 6*kInitialRtt)

Note that the endpoint might receive packets containing other frames on the new path, but a PATH_RESPONSE frame with appropriate data is required for path validation to succeed.¶

When an endpoint abandons path validation, it determines that the path is unusable. This does not necessarily imply a failure of the connection - endpoints can continue sending packets over other paths as appropriate. If no paths are available, an endpoint can wait for a new path to become available or close the connection.¶

A path validation might be abandoned for other reasons besides failure. Primarily, this happens if a connection migration to a new path is initiated while a path validation on the old path is in progress.¶

9.1. Probing a New Path

An endpoint MAY probe for peer reachability from a new local address using path validation Section 8.2 prior to migrating the connection to the new local address. Failure of path validation simply means that the new path is not usable for this connection. Failure to validate a path does not cause the connection to end unless there are no valid alternative paths available.¶

An endpoint uses a new connection ID for probes sent from a new local address; see Section 9.5 for further discussion. An endpoint that uses a new local address needs to ensure that at least one new connection ID is available at the peer. That can be achieved by including a NEW_CONNECTION_ID frame in the probe.¶

Receiving a PATH_CHALLENGE frame from a peer indicates that the peer is probing for reachability on a path. An endpoint sends a PATH_RESPONSE in response as per Section 8.2.¶

PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames are "probing frames", and all other frames are "non-probing frames". A packet containing only probing frames is a "probing packet", and a packet containing any other frame is a "non-probing packet".¶

9.2. Initiating Connection Migration

An endpoint can migrate a connection to a new local address by sending packets containing non-probing frames from that address.¶

Each endpoint validates its peer's address during connection establishment. Therefore, a migrating endpoint can send to its peer knowing that the peer is willing to receive at the peer's current address. Thus an endpoint can migrate to a new local address without first validating the peer's address.¶

When migrating, the new path might not support the endpoint's current sending rate. Therefore, the endpoint resets its congestion controller, as described in Section 9.4.¶

The new path might not have the same ECN capability. Therefore, the endpoint verifies ECN capability as described in Section 13.4.¶

Receiving acknowledgments for data sent on the new path serves as proof of the peer's reachability from the new address. Note that since acknowledgments may be received on any path, return reachability on the new path is not established. To establish return reachability on the new path, an endpoint MAY concurrently initiate path validation Section 8.2 on the new path.¶

9.3. Responding to Connection Migration

Receiving a packet from a new peer address containing a non-probing frame indicates that the peer has migrated to that address.¶

In response to such a packet, an endpoint MUST start sending subsequent packets to the new peer address and MUST initiate path validation (Section 8.2) to verify the peer's ownership of the unvalidated address.¶

An endpoint MAY send data to an unvalidated peer address, but it MUST protect against potential attacks as described in Section 9.3.1 and Section 9.3.2. An endpoint MAY skip validation of a peer address if that address has been seen recently. In particular, if an endpoint returns to a previously-validated path after detecting some form of spurious migration, skipping address validation and restoring loss detection and congestion state can reduce the performance impact of the attack.¶

An endpoint only changes the address that it sends packets to in response to the highest-numbered non-probing packet. This ensures that an endpoint does not send packets to an old peer address in the case that it receives reordered packets.¶

After changing the address to which it sends non-probing packets, an endpoint could abandon any path validation for other addresses.¶

Receiving a packet from a new peer address might be the result of a NAT rebinding at the peer.¶

After verifying a new client address, the server SHOULD send new address validation tokens (Section 8) to the client.¶

9.4. Loss Detection and Congestion Control

The capacity available on the new path might not be the same as the old path. Packets sent on the old path MUST NOT contribute to congestion control or RTT estimation for the new path.¶

On confirming a peer's ownership of its new address, an endpoint MUST immediately reset the congestion controller and round-trip time estimator for the new path to initial values (see Sections A.3 and B.3 in [QUIC-RECOVERY]) unless it has knowledge that a previous send rate or round-trip time estimate is valid for the new path. For instance, an endpoint might infer that a change in only the client's port number is indicative of a NAT rebinding, meaning that the new path is likely to have similar bandwidth and round-trip time. However, this determination will be imperfect. If the determination is incorrect, the congestion controller and the RTT estimator are expected to adapt to the new path. Generally, implementations are advised to be cautious when using previous values on a new path.¶

There may be apparent reordering at the receiver when an endpoint sends data and probes from/to multiple addresses during the migration period, since the two resulting paths may have different round-trip times. A receiver of packets on multiple paths will still send ACK frames covering all received packets.¶

While multiple paths might be used during connection migration, a single congestion control context and a single loss recovery context (as described in [QUIC-RECOVERY]) may be adequate. For instance, an endpoint might delay switching to a new congestion control context until it is confirmed that an old path is no longer needed (such as the case in Section 9.3.3).¶

A sender can make exceptions for probe packets so that their loss detection is independent and does not unduly cause the congestion controller to reduce its sending rate. An endpoint might set a separate timer when a PATH_CHALLENGE is sent, which is cancelled if the corresponding PATH_RESPONSE is received. If the timer fires before the PATH_RESPONSE is received, the endpoint might send a new PATH_CHALLENGE, and restart the timer for a longer period of time. This timer SHOULD be set as described in Section 5.3 of [QUIC-RECOVERY] and MUST NOT be more aggressive.¶

9.5. Privacy Implications of Connection Migration

Using a stable connection ID on multiple network paths allows a passive observer to correlate activity between those paths. An endpoint that moves between networks might not wish to have their activity correlated by any entity other than their peer, so different connection IDs are used when sending from different local addresses, as discussed in Section 5.1. For this to be effective endpoints need to ensure that connection IDs they provide cannot be linked by any other entity.¶

At any time, endpoints MAY change the Destination Connection ID they send to a value that has not been used on another path.¶

An endpoint MUST use a new connection ID if it initiates connection migration as described in Section 9.2 or probes a new network path as described in Section 9.1. An endpoint MUST use a new connection ID in response to a change in the address of a peer if the packet with the new peer address uses an active connection ID that has not been previously used by the peer.¶

Using different connection IDs for packets sent in both directions on each new network path eliminates the use of the connection ID for linking packets from the same connection across different network paths. Header protection ensures that packet numbers cannot be used to correlate activity. This does not prevent other properties of packets, such as timing and size, from being used to correlate activity.¶

Unintentional changes in path without a change in connection ID are possible. For example, after a period of network inactivity, NAT rebinding might cause packets to be sent on a new path when the client resumes sending.¶

A client might wish to reduce linkability by employing a new connection ID and source UDP port when sending traffic after a period of inactivity. Changing the UDP port from which it sends packets at the same time might cause the packet to appear as a connection migration. This ensures that the mechanisms that support migration are exercised even for clients that don't experience NAT rebindings or genuine migrations. Changing port number can cause a peer to reset its congestion state (see Section 9.4), so the port SHOULD only be changed infrequently.¶

An endpoint that exhausts available connection IDs cannot probe new paths or initiate migration, nor can it respond to probes or attempts by its peer to migrate. To ensure that migration is possible and packets sent on different paths cannot be correlated, endpoints SHOULD provide new connection IDs before peers migrate; see Section 5.1.1. If a peer might have exhausted available connection IDs, a migrating endpoint could include a NEW_CONNECTION_ID frame in all packets sent on a new network path.¶

9.6. Server's Preferred Address

QUIC allows servers to accept connections on one IP address and attempt to transfer these connections to a more preferred address shortly after the handshake. This is particularly useful when clients initially connect to an address shared by multiple servers but would prefer to use a unicast address to ensure connection stability. This section describes the protocol for migrating a connection to a preferred server address.¶

Migrating a connection to a new server address mid-connection is left for future work. If a client receives packets from a new server address not indicated by the preferred_address transport parameter, the client SHOULD discard these packets.¶

9.7. Use of IPv6 Flow-Label and Migration

Endpoints that send data using IPv6 SHOULD apply an IPv6 flow label in compliance with [RFC6437], unless the local API does not allow setting IPv6 flow labels.¶

The IPv6 flow label SHOULD be a pseudo-random function of the source and destination addresses, source and destination UDP ports, and the destination CID. The flow label generation MUST be designed to minimize the chances of linkability with a previously used flow label, as this would enable correlating activity on multiple paths (see Section 9.5).¶

A possible implementation is to compute the flow label as a cryptographic hash function of the source and destination addresses, source and destination UDP ports, destination CID, and a local secret.¶

10.1. Closing and Draining Connection States

The closing and draining connection states exist to ensure that connections close cleanly and that delayed or reordered packets are properly discarded. These states SHOULD persist for at least three times the current Probe Timeout (PTO) interval as defined in [QUIC-RECOVERY].¶

An endpoint enters a closing period after initiating an immediate close (Section 10.3). While closing, an endpoint MUST NOT send packets unless they contain a CONNECTION_CLOSE frame (see Section 10.3 for details). An endpoint retains only enough information to generate a packet containing a CONNECTION_CLOSE frame and to identify packets as belonging to the connection. The endpoint's selected connection ID and the QUIC version are sufficient information to identify packets for a closing connection; an endpoint can discard all other connection state. An endpoint MAY retain packet protection keys for incoming packets to allow it to read and process a CONNECTION_CLOSE frame.¶

The draining state is entered once an endpoint receives a signal that its peer is closing or draining. While otherwise identical to the closing state, an endpoint in the draining state MUST NOT send any packets. Retaining packet protection keys is unnecessary once a connection is in the draining state.¶

An endpoint MAY transition from the closing period to the draining period if it receives a CONNECTION_CLOSE frame or stateless reset, both of which indicate that the peer is also closing or draining. The draining period SHOULD end when the closing period would have ended. In other words, the endpoint can use the same end time, but cease retransmission of the closing packet.¶

Disposing of connection state prior to the end of the closing or draining period could cause delayed or reordered packets to generate an unnecessary stateless reset. Endpoints that have some alternative means to ensure that late-arriving packets on the connection do not induce a response, such as those that are able to close the UDP socket, MAY use an abbreviated draining period which can allow for faster resource recovery. Servers that retain an open socket for accepting new connections SHOULD NOT exit the closing or draining period early.¶

Once the closing or draining period has ended, an endpoint SHOULD discard all connection state. This results in new packets on the connection being handled generically. For instance, an endpoint MAY send a stateless reset in response to any further incoming packets.¶

The draining and closing periods do not apply when a stateless reset (Section 10.4) is sent.¶

An endpoint is not expected to handle key updates when it is closing or draining. A key update might prevent the endpoint from moving from the closing state to draining, but it otherwise has no impact.¶

While in the closing period, an endpoint could receive packets from a new source address, indicating a connection migration (Section 9). An endpoint in the closing state MUST strictly limit the number of packets it sends to this new address until the address is validated (see Section 8.2). A server in the closing state MAY instead choose to discard packets received from a new source address.¶

10.2. Idle Timeout

If the idle timeout is enabled by either peer, a connection is silently closed and its state is discarded when it remains idle for longer than the minimum of the max_idle_timeouts (see Section 18.2) and three times the current Probe Timeout (PTO).¶

Each endpoint advertises a max_idle_timeout, but the effective value at an endpoint is computed as the minimum of the two advertised values. By announcing a max_idle_timeout, an endpoint commits to initiating an immediate close (Section 10.3) if it abandons the connection prior to the effective value.¶

An endpoint restarts its idle timer when a packet from its peer is received and processed successfully. The idle timer is also restarted when sending an ack-eliciting packet (see [QUIC-RECOVERY]), but only if no other ack-eliciting packets have been sent since last receiving a packet. Restarting when sending packets ensures that connections do not prematurely time out when initiating new activity. An endpoint might need to send packets to avoid an idle timeout if it is unable to send application data due to being blocked on flow control limits; see Section 4.¶

An endpoint that sends packets near the end of the idle timeout period risks having those packets discarded if its peer enters the draining state before the packets arrive. If a peer could time out within a Probe Timeout (PTO; see Section 6.6 of [QUIC-RECOVERY]), it is advisable to test for liveness before sending any data that cannot be retried safely. Note that it is likely that only applications or application protocols will know what information can be retried.¶

10.3. Immediate Close

An endpoint sends a CONNECTION_CLOSE frame (Section 19.19) to terminate the connection immediately. A CONNECTION_CLOSE frame causes all streams to immediately become closed; open streams can be assumed to be implicitly reset.¶

After sending a CONNECTION_CLOSE frame, an endpoint immediately enters the closing state.¶

During the closing period, an endpoint that sends a CONNECTION_CLOSE frame SHOULD respond to any incoming packet that can be decrypted with another packet containing a CONNECTION_CLOSE frame. Such an endpoint SHOULD limit the number of packets it generates containing a CONNECTION_CLOSE frame. For instance, an endpoint could wait for a progressively increasing number of received packets or amount of time before responding to a received packet.¶

An endpoint is allowed to drop the packet protection keys when entering the closing period (Section 10.1) and send a packet containing a CONNECTION_CLOSE in response to any UDP datagram that is received. However, an endpoint without the packet protection keys cannot identify and discard invalid packets. To avoid creating an unwitting amplification attack, such endpoints MUST reduce the frequency with which it sends packets containing a CONNECTION_CLOSE frame. To minimize the state that an endpoint maintains for a closing connection, endpoints MAY send the exact same packet.¶

Note:

Allowing retransmission of a closing packet contradicts other advice in this document that recommends the creation of new packet numbers for every packet. Sending new packet numbers is primarily of advantage to loss recovery and congestion control, which are not expected to be relevant for a closed connection. Retransmitting the final packet requires less state.¶

New packets from unverified addresses could be used to create an amplification attack (see Section 8). To avoid this, endpoints MUST either limit transmission of CONNECTION_CLOSE frames to validated addresses or drop packets without response if the response would be more than three times larger than the received packet.¶

After receiving a CONNECTION_CLOSE frame, endpoints enter the draining state. An endpoint that receives a CONNECTION_CLOSE frame MAY send a single packet containing a CONNECTION_CLOSE frame before entering the draining state, using a CONNECTION_CLOSE frame and a NO_ERROR code if appropriate. An endpoint MUST NOT send further packets, which could result in a constant exchange of CONNECTION_CLOSE frames until the closing period on either peer ended.¶

An immediate close can be used after an application protocol has arranged to close a connection. This might be after the application protocols negotiates a graceful shutdown. The application protocol exchanges whatever messages that are needed to cause both endpoints to agree to close the connection, after which the application requests that the connection be closed. The application protocol can use a CONNECTION_CLOSE frame with an appropriate error code to signal closure.¶

10.4. Stateless Reset

A stateless reset is provided as an option of last resort for an endpoint that does not have access to the state of a connection. A crash or outage might result in peers continuing to send data to an endpoint that is unable to properly continue the connection. An endpoint MAY send a stateless reset in response to receiving a packet that it cannot associate with an active connection.¶

A stateless reset is not appropriate for signaling error conditions. An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it has sufficient state to do so.¶

To support this process, a token is sent by endpoints. The token is carried in the Stateless Reset Token field of a NEW_CONNECTION_ID frame. Servers can also specify a stateless_reset_token transport parameter during the handshake that applies to the connection ID that it selected during the handshake; clients cannot use this transport parameter because their transport parameters don't have confidentiality protection. These tokens are protected by encryption, so only client and server know their value. Tokens are invalidated when their associated connection ID is retired via a RETIRE_CONNECTION_ID frame (Section 19.16).¶

An endpoint that receives packets that it cannot process sends a packet in the following layout:¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|               Unpredictable Bits (38 ..)                ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                   Stateless Reset Token (128)                 +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This design ensures that a stateless reset packet is - to the extent possible - indistinguishable from a regular packet with a short header.¶

A stateless reset uses an entire UDP datagram, starting with the first two bits of the packet header. The remainder of the first byte and an arbitrary number of bytes following it that are set to unpredictable values. The last 16 bytes of the datagram contain a Stateless Reset Token.¶

To entities other than its intended recipient, a stateless reset will appear to be a packet with a short header. For the stateless reset to appear as a valid QUIC packet, the Unpredictable Bits field needs to include at least 38 bits of data (or 5 bytes, less the two fixed bits).¶

A minimum size of 21 bytes does not guarantee that a stateless reset is difficult to distinguish from other packets if the recipient requires the use of a connection ID. To prevent a resulting stateless reset from being trivially distinguishable from a valid packet, all packets sent by an endpoint SHOULD be padded to at least 22 bytes longer than the minimum connection ID that the endpoint might use. An endpoint that sends a stateless reset in response to packet that is 43 bytes or less in length SHOULD send a stateless reset that is one byte shorter than the packet it responds to.¶

These values assume that the Stateless Reset Token is the same as the minimum expansion of the packet protection AEAD. Additional unpredictable bytes are necessary if the endpoint could have negotiated a packet protection scheme with a larger minimum expansion.¶

An endpoint MUST NOT send a stateless reset that is three times or more larger than the packet it receives to avoid being used for amplification. Section 10.4.3 describes additional limits on stateless reset size.¶

Endpoints MUST discard packets that are too small to be valid QUIC packets. With the set of AEAD functions defined in [QUIC-TLS], packets that are smaller than 21 bytes are never valid.¶

Endpoints MUST send stateless reset packets formatted as a packet with a short header. However, endpoints MUST treat any packet ending in a valid stateless reset token as a stateless reset, as other QUIC versions might allow the use of a long header.¶

An endpoint MAY send a stateless reset in response to a packet with a long header. Sending a stateless reset is not effective prior to the stateless reset token being available to a peer. In this QUIC version, packets with a long header are only used during connection establishment. Because the stateless reset token is not available until connection establishment is complete or near completion, ignoring an unknown packet with a long header might be as effective as sending a stateless reset.¶

An endpoint cannot determine the Source Connection ID from a packet with a short header, therefore it cannot set the Destination Connection ID in the stateless reset packet. The Destination Connection ID will therefore differ from the value used in previous packets. A random Destination Connection ID makes the connection ID appear to be the result of moving to a new connection ID that was provided using a NEW_CONNECTION_ID frame (Section 19.15).¶

Using a randomized connection ID results in two problems:¶

• The packet might not reach the peer. If the Destination Connection ID is critical for routing toward the peer, then this packet could be incorrectly routed. This might also trigger another Stateless Reset in response; see Section 10.4.3. A Stateless Reset that is not correctly routed is an ineffective error detection and recovery mechanism. In this case, endpoints will need to rely on other methods - such as timers - to detect that the connection has failed.¶
• The randomly generated connection ID can be used by entities other than the peer to identify this as a potential stateless reset. An endpoint that occasionally uses different connection IDs might introduce some uncertainty about this.¶

This stateless reset design is specific to QUIC version 1. An endpoint that supports multiple versions of QUIC needs to generate a stateless reset that will be accepted by peers that support any version that the endpoint might support (or might have supported prior to losing state). Designers of new versions of QUIC need to be aware of this and either reuse this design, or use a portion of the packet other than the last 16 bytes for carrying data.¶

11.1. Connection Errors

Errors that result in the connection being unusable, such as an obvious violation of protocol semantics or corruption of state that affects an entire connection, MUST be signaled using a CONNECTION_CLOSE frame (Section 19.19). An endpoint MAY close the connection in this manner even if the error only affects a single stream.¶

Application protocols can signal application-specific protocol errors using the application-specific variant of the CONNECTION_CLOSE frame. Errors that are specific to the transport, including all those described in this document, are carried in the QUIC-specific variant of the CONNECTION_CLOSE frame.¶

A CONNECTION_CLOSE frame could be sent in a packet that is lost. An endpoint SHOULD be prepared to retransmit a packet containing a CONNECTION_CLOSE frame if it receives more packets on a terminated connection. Limiting the number of retransmissions and the time over which this final packet is sent limits the effort expended on terminated connections.¶

An endpoint that chooses not to retransmit packets containing a CONNECTION_CLOSE frame risks a peer missing the first such packet. The only mechanism available to an endpoint that continues to receive data for a terminated connection is to use the stateless reset process (Section 10.4).¶

11.2. Stream Errors

If an application-level error affects a single stream, but otherwise leaves the connection in a recoverable state, the endpoint can send a RESET_STREAM frame (Section 19.4) with an appropriate error code to terminate just the affected stream.¶

Resetting a stream without the involvement of the application protocol could cause the application protocol to enter an unrecoverable state. RESET_STREAM MUST only be instigated by the application protocol that uses QUIC.¶

The semantics of the application error code carried in RESET_STREAM are defined by the application protocol. Only the application protocol is able to cause a stream to be terminated. A local instance of the application protocol uses a direct API call and a remote instance uses the STOP_SENDING frame, which triggers an automatic RESET_STREAM.¶

Application protocols SHOULD define rules for handling streams that are prematurely cancelled by either endpoint.¶

12.1. Protected Packets

All QUIC packets except Version Negotiation packets use authenticated encryption with additional data (AEAD) [RFC5116] to provide confidentiality and integrity protection. Retry packets use AEAD to provide integrity protection. Details of packet protection are found in [QUIC-TLS]; this section includes an overview of the process.¶

Initial packets are protected using keys that are statically derived. This packet protection is not effective confidentiality protection. Initial protection only exists to ensure that the sender of the packet is on the network path. Any entity that receives the Initial packet from a client can recover the keys necessary to remove packet protection or to generate packets that will be successfully authenticated.¶

All other packets are protected with keys derived from the cryptographic handshake. The type of the packet from the long header or key phase from the short header are used to identify which encryption level - and therefore the keys - that are used. Packets protected with 0-RTT and 1-RTT keys are expected to have confidentiality and data origin authentication; the cryptographic handshake ensures that only the communicating endpoints receive the corresponding keys.¶

The packet number field contains a packet number, which has additional confidentiality protection that is applied after packet protection is applied (see [QUIC-TLS] for details). The underlying packet number increases with each packet sent in a given packet number space; see Section 12.3 for details.¶

12.2. Coalescing Packets

Initial (Section 17.2.2), 0-RTT (Section 17.2.3), and Handshake (Section 17.2.4) packets contain a Length field, which determines the end of the packet. The length includes both the Packet Number and Payload fields, both of which are confidentiality protected and initially of unknown length. The length of the Payload field is learned once header protection is removed.¶

Using the Length field, a sender can coalesce multiple QUIC packets into one UDP datagram. This can reduce the number of UDP datagrams needed to complete the cryptographic handshake and start sending data. This can also be used to construct PMTU probes (see Section 14.3.1). Receivers MUST be able to process coalesced packets.¶

Coalescing packets in order of increasing encryption levels (Initial, 0-RTT, Handshake, 1-RTT) makes it more likely the receiver will be able to process all the packets in a single pass. A packet with a short header does not include a length, so it can only be the last packet included in a UDP datagram. An endpoint SHOULD NOT coalesce multiple packets at the same encryption level.¶

Senders MUST NOT coalesce QUIC packets for different connections into a single UDP datagram. Receivers SHOULD ignore any subsequent packets with a different Destination Connection ID than the first packet in the datagram.¶

Every QUIC packet that is coalesced into a single UDP datagram is separate and complete. The receiver of coalesced QUIC packets MUST individually process each QUIC packet and separately acknowledge them, as if they were received as the payload of different UDP datagrams. For example, if decryption fails (because the keys are not available or any other reason), the receiver MAY either discard or buffer the packet for later processing and MUST attempt to process the remaining packets.¶

Retry packets (Section 17.2.5), Version Negotiation packets (Section 17.2.1), and packets with a short header (Section 17.3) do not contain a Length field and so cannot be followed by other packets in the same UDP datagram. Note also that there is no situation where a Retry or Version Negotiation packet is coalesced with another packet.¶

12.3. Packet Numbers

The packet number is an integer in the range 0 to 2^62-1. This number is used in determining the cryptographic nonce for packet protection. Each endpoint maintains a separate packet number for sending and receiving.¶

Packet numbers are limited to this range because they need to be representable in whole in the Largest Acknowledged field of an ACK frame (Section 19.3). When present in a long or short header however, packet numbers are reduced and encoded in 1 to 4 bytes (see Section 17.1).¶

Version Negotiation (Section 17.2.1) and Retry (Section 17.2.5) packets do not include a packet number.¶

Packet numbers are divided into 3 spaces in QUIC:¶

• Initial space: All Initial packets (Section 17.2.2) are in this space.¶
• Handshake space: All Handshake packets (Section 17.2.4) are in this space.¶
• Application data space: All 0-RTT and 1-RTT encrypted packets (Section 12.1) are in this space.¶

As described in [QUIC-TLS], each packet type uses different protection keys.¶

Conceptually, a packet number space is the context in which a packet can be processed and acknowledged. Initial packets can only be sent with Initial packet protection keys and acknowledged in packets which are also Initial packets. Similarly, Handshake packets are sent at the Handshake encryption level and can only be acknowledged in Handshake packets.¶

This enforces cryptographic separation between the data sent in the different packet sequence number spaces. Packet numbers in each space start at packet number 0. Subsequent packets sent in the same packet number space MUST increase the packet number by at least one.¶

0-RTT and 1-RTT data exist in the same packet number space to make loss recovery algorithms easier to implement between the two packet types.¶

A QUIC endpoint MUST NOT reuse a packet number within the same packet number space in one connection. If the packet number for sending reaches 2^62 - 1, the sender MUST close the connection without sending a CONNECTION_CLOSE frame or any further packets; an endpoint MAY send a Stateless Reset (Section 10.4) in response to further packets that it receives.¶

A receiver MUST discard a newly unprotected packet unless it is certain that it has not processed another packet with the same packet number from the same packet number space. Duplicate suppression MUST happen after removing packet protection for the reasons described in Section 9.3 of [QUIC-TLS]. An efficient algorithm for duplicate suppression can be found in Section 3.4.3 of [RFC4303].¶

Packet number encoding at a sender and decoding at a receiver are described in Section 17.1.¶

12.4. Frames and Frame Types

The payload of QUIC packets, after removing packet protection, consists of a sequence of complete frames, as shown in Figure 7. Version Negotiation, Stateless Reset, and Retry packets do not contain frames.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Frame 1 (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Frame 2 (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Frame N (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The payload of a packet that contains frames MUST contain at least one frame, and MAY contain multiple frames and multiple frame types. Frames always fit within a single QUIC packet and cannot span multiple packets.¶

Each frame begins with a Frame Type, indicating its type, followed by additional type-dependent fields:¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Frame Type (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Type-Dependent Fields (*)                 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The frame types defined in this specification are listed in Table 3. The Frame Type in ACK, STREAM, MAX_STREAMS, STREAMS_BLOCKED, and CONNECTION_CLOSE frames is used to carry other frame-specific flags. For all other frames, the Frame Type field simply identifies the frame. These frames are explained in more detail in Section 19.¶

The "Packets" column in Table 3 does not form part of the IANA registry (see Section 22.3). This column lists the types of packets that each frame type can appear in, indicated by the following characters:¶

I:

Initial (Section 17.2.2)¶

H:

Handshake (Section 17.2.4)¶

0:

0-RTT (Section 17.2.3)¶

1:

1-RTT (Section 17.3)¶

*:

A CONNECTION_CLOSE frame of type 0x1c can appear in Initial, Handshake, and 1-RTT packets, whereas a CONNECTION_CLOSE of type 0x1d can only appear in a 1-RTT packet.¶

Section 4 of [QUIC-TLS] provides more detail about these restrictions. Note that all frames can appear in 1-RTT packets.¶

An endpoint MUST treat the receipt of a frame of unknown type as a connection error of type FRAME_ENCODING_ERROR.¶

All QUIC frames are idempotent in this version of QUIC. That is, a valid frame does not cause undesirable side effects or errors when received more than once.¶

The Frame Type field uses a variable length integer encoding (see Section 16) with one exception. To ensure simple and efficient implementations of frame parsing, a frame type MUST use the shortest possible encoding. For frame types defined in this document, this means a single-byte encoding, even though it is possible to encode these values as a two-, four- or eight-byte variable length integer. For instance, though 0x4001 is a legitimate two-byte encoding for a variable-length integer with a value of 1, PING frames are always encoded as a single byte with the value 0x01. This rule applies to all current and future QUIC frame types. An endpoint MAY treat the receipt of a frame type that uses a longer encoding than necessary as a connection error of type PROTOCOL_VIOLATION.¶

13.1. Packet Processing

A packet MUST NOT be acknowledged until packet protection has been successfully removed and all frames contained in the packet have been processed. For STREAM frames, this means the data has been enqueued in preparation to be received by the application protocol, but it does not require that data is delivered and consumed.¶

Once the packet has been fully processed, a receiver acknowledges receipt by sending one or more ACK frames containing the packet number of the received packet.¶

13.2. Generating Acknowledgements

Endpoints acknowledge all packets they receive and process. However, only ack-eliciting packets cause an ACK frame to be sent within the maximum ack delay. Packets that are not ack-eliciting are only acknowledged when an ACK frame is sent for other reasons.¶

When sending a packet for any reason, an endpoint should attempt to bundle an ACK frame if one has not been sent recently. Doing so helps with timely loss detection at the peer.¶

In general, frequent feedback from a receiver improves loss and congestion response, but this has to be balanced against excessive load generated by a receiver that sends an ACK frame in response to every ack-eliciting packet. The guidance offered below seeks to strike this balance.¶

13.3. Retransmission of Information

QUIC packets that are determined to be lost are not retransmitted whole. The same applies to the frames that are contained within lost packets. Instead, the information that might be carried in frames is sent again in new frames as needed.¶

New frames and packets are used to carry information that is determined to have been lost. In general, information is sent again when a packet containing that information is determined to be lost and sending ceases when a packet containing that information is acknowledged.¶

• Data sent in CRYPTO frames is retransmitted according to the rules in [QUIC-RECOVERY], until all data has been acknowledged. Data in CRYPTO frames for Initial and Handshake packets is discarded when keys for the corresponding encryption level are discarded.¶
• Application data sent in STREAM frames is retransmitted in new STREAM frames unless the endpoint has sent a RESET_STREAM for that stream. Once an endpoint sends a RESET_STREAM frame, no further STREAM frames are needed.¶
• ACK frames carry the most recent set of acknowledgements and the Ack Delay from the largest acknowledged packet, as described in Section 13.2.1. Delaying the transmission of packets containing ACK frames or sending old ACK frames can cause the peer to generate an inflated RTT sample or unnecessarily disable ECN.¶
• Cancellation of stream transmission, as carried in a RESET_STREAM frame, is sent until acknowledged or until all stream data is acknowledged by the peer (that is, either the "Reset Recvd" or "Data Recvd" state is reached on the sending part of the stream). The content of a RESET_STREAM frame MUST NOT change when it is sent again.¶
• Similarly, a request to cancel stream transmission, as encoded in a STOP_SENDING frame, is sent until the receiving part of the stream enters either a "Data Recvd" or "Reset Recvd" state; see Section 3.5.¶
• Connection close signals, including packets that contain CONNECTION_CLOSE frames, are not sent again when packet loss is detected, but as described in Section 10.¶
• The current connection maximum data is sent in MAX_DATA frames. An updated value is sent in a MAX_DATA frame if the packet containing the most recently sent MAX_DATA frame is declared lost, or when the endpoint decides to update the limit. Care is necessary to avoid sending this frame too often as the limit can increase frequently and cause an unnecessarily large number of MAX_DATA frames to be sent.¶
• The current maximum stream data offset is sent in MAX_STREAM_DATA frames. Like MAX_DATA, an updated value is sent when the packet containing the most recent MAX_STREAM_DATA frame for a stream is lost or when the limit is updated, with care taken to prevent the frame from being sent too often. An endpoint SHOULD stop sending MAX_STREAM_DATA frames when the receiving part of the stream enters a "Size Known" state.¶
• The limit on streams of a given type is sent in MAX_STREAMS frames. Like MAX_DATA, an updated value is sent when a packet containing the most recent MAX_STREAMS for a stream type frame is declared lost or when the limit is updated, with care taken to prevent the frame from being sent too often.¶
• Blocked signals are carried in DATA_BLOCKED, STREAM_DATA_BLOCKED, and STREAMS_BLOCKED frames. DATA_BLOCKED frames have connection scope, STREAM_DATA_BLOCKED frames have stream scope, and STREAMS_BLOCKED frames are scoped to a specific stream type. New frames are sent if packets containing the most recent frame for a scope is lost, but only while the endpoint is blocked on the corresponding limit. These frames always include the limit that is causing blocking at the time that they are transmitted.¶
• A liveness or path validation check using PATH_CHALLENGE frames is sent periodically until a matching PATH_RESPONSE frame is received or until there is no remaining need for liveness or path validation checking. PATH_CHALLENGE frames include a different payload each time they are sent.¶
• Responses to path validation using PATH_RESPONSE frames are sent just once. The peer is expected to send more PATH_CHALLENGE frames as necessary to evoke additional PATH_RESPONSE frames.¶
• New connection IDs are sent in NEW_CONNECTION_ID frames and retransmitted if the packet containing them is lost. Retransmissions of this frame carry the same sequence number value. Likewise, retired connection IDs are sent in RETIRE_CONNECTION_ID frames and retransmitted if the packet containing them is lost.¶
• NEW_TOKEN frames are retransmitted if the packet containing them is lost. No special support is made for detecting reordered and duplicated NEW_TOKEN frames other than a direct comparison of the frame contents.¶
• PING and PADDING frames contain no information, so lost PING or PADDING frames do not require repair.¶
• The HANDSHAKE_DONE frame MUST be retransmitted until it is acknowledged.¶

Endpoints SHOULD prioritize retransmission of data over sending new data, unless priorities specified by the application indicate otherwise (see Section 2.3).¶

Even though a sender is encouraged to assemble frames containing up-to-date information every time it sends a packet, it is not forbidden to retransmit copies of frames from lost packets. A sender that retransmits copies of frames needs to handle decreases in available payload size due to change in packet number length, connection ID length, and path MTU. A receiver MUST accept packets containing an outdated frame, such as a MAX_DATA frame carrying a smaller maximum data than one found in an older packet.¶

Upon detecting losses, a sender MUST take appropriate congestion control action. The details of loss detection and congestion control are described in [QUIC-RECOVERY].¶

13.4. Explicit Congestion Notification

QUIC endpoints can use Explicit Congestion Notification (ECN) [RFC3168] to detect and respond to network congestion. ECN allows a network node to indicate congestion in the network by setting a codepoint in the IP header of a packet instead of dropping it. Endpoints react to congestion by reducing their sending rate in response, as described in [QUIC-RECOVERY].¶

To use ECN, QUIC endpoints first determine whether a path supports ECN marking and the peer is able to access the ECN codepoint in the IP header. A network path does not support ECN if ECN marked packets get dropped or ECN markings are rewritten on the path. An endpoint validates the use of ECN on the path, both during connection establishment and when migrating to a new path (Section 9).¶

14.1. Path Maximum Transmission Unit (PMTU)

The PMTU is the maximum size of the entire IP packet including the IP header, UDP header, and UDP payload. The UDP payload includes the QUIC packet header, protected payload, and any authentication fields. The PMTU can depend upon the current path characteristics. Therefore, the current largest UDP payload an implementation will send is referred to as the QUIC maximum packet size.¶

QUIC depends on a PMTU of at least 1280 bytes. This is the IPv6 minimum size [RFC8200] and is also supported by most modern IPv4 networks. All QUIC packets (except for PMTU probe packets) SHOULD be sized to fit within the maximum packet size to avoid the packet being fragmented or dropped [RFC8085].¶

An endpoint SHOULD use Datagram Packetization Layer PMTU Discovery ([DPLPMTUD]) or implement Path MTU Discovery (PMTUD) [RFC1191] [RFC8201] to determine whether the path to a destination will support a desired message size without fragmentation.¶

In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP packets larger than 1280 bytes. Assuming the minimum IP header size, this results in a QUIC maximum packet size of 1232 bytes for IPv6 and 1252 bytes for IPv4. A QUIC implementation MAY be more conservative in computing the QUIC maximum packet size to allow for unknown tunnel overheads or IP header options/extensions.¶

Each pair of local and remote addresses could have a different PMTU. QUIC implementations that implement any kind of PMTU discovery therefore SHOULD maintain a maximum packet size for each combination of local and remote IP addresses.¶

If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses has fallen below the size needed to support the smallest allowed maximum packet size, it MUST immediately cease sending QUIC packets, except for PMTU probe packets, on the affected path. An endpoint MAY terminate the connection if an alternative path cannot be found.¶

14.2. ICMP Packet Too Big Messages

PMTU discovery [RFC1191] [RFC8201] relies on reception of ICMP messages (e.g., IPv6 Packet Too Big messages) that indicate when a packet is dropped because it is larger than the local router MTU. DPLPMTUD can also optionally use these messages. This use of ICMP messages is potentially vulnerable to off-path attacks that successfully guess the addresses used on the path and reduce the PMTU to a bandwidth-inefficient value.¶

An endpoint MUST ignore an ICMP message that claims the PMTU has decreased below 1280 bytes.¶

The requirements for generating ICMP ([RFC1812], [RFC4443]) state that the quoted packet should contain as much of the original packet as possible without exceeding the minimum MTU for the IP version. The size of the quoted packet can actually be smaller, or the information unintelligible, as described in Section 1.1 of [DPLPMTUD].¶

QUIC endpoints SHOULD validate ICMP messages to protect from off-path injection as specified in [RFC8201] and Section 5.2 of [RFC8085]. This validation SHOULD use the quoted packet supplied in the payload of an ICMP message to associate the message with a corresponding transport connection [DPLPMTUD].¶

ICMP message validation MUST include matching IP addresses and UDP ports [RFC8085] and, when possible, connection IDs to an active QUIC session.¶

Further validation can also be provided:¶

• An IPv4 endpoint could set the Don't Fragment (DF) bit on a small proportion of packets, so that most invalid ICMP messages arrive when there are no DF packets outstanding, and can therefore be identified as spurious.¶
• An endpoint could store additional information from the IP or UDP headers to use for validation (for example, the IP ID or UDP checksum).¶

The endpoint SHOULD ignore all ICMP messages that fail validation.¶

An endpoint MUST NOT increase PMTU based on ICMP messages. Any reduction in the QUIC maximum packet size MAY be provisional until QUIC's loss detection algorithm determines that the quoted packet has actually been lost.¶

14.3. Datagram Packetization Layer PMTU Discovery

Section 6.3 of [DPLPMTUD] provides considerations for implementing Datagram Packetization Layer PMTUD (DPLPMTUD) with QUIC.¶

When implementing the algorithm in Section 5 of [DPLPMTUD], the initial value of BASE_PMTU SHOULD be consistent with the minimum QUIC packet size (1232 bytes for IPv6 and 1252 bytes for IPv4).¶

PING and PADDING frames can be used to generate PMTU probe packets. These frames might not be retransmitted if a probe packet containing them is lost. However, these frames do consume congestion window, which could delay the transmission of subsequent application data.¶

A PING frame can be included in a PMTU probe to ensure that a valid probe is acknowledged.¶

The considerations for processing ICMP messages in the previous section also apply if these messages are used by DPLPMTUD.¶

17.1. Packet Number Encoding and Decoding

Packet numbers are integers in the range 0 to 2^62-1 (Section 12.3). When present in long or short packet headers, they are encoded in 1 to 4 bytes. The number of bits required to represent the packet number is reduced by including the least significant bits of the packet number.¶

The encoded packet number is protected as described in Section 5.4 of [QUIC-TLS].¶

The sender MUST use a packet number size able to represent more than twice as large a range than the difference between the largest acknowledged packet and packet number being sent. A peer receiving the packet will then correctly decode the packet number, unless the packet is delayed in transit such that it arrives after many higher-numbered packets have been received. An endpoint SHOULD use a large enough packet number encoding to allow the packet number to be recovered even if the packet arrives after packets that are sent afterwards.¶

As a result, the size of the packet number encoding is at least one bit more than the base-2 logarithm of the number of contiguous unacknowledged packet numbers, including the new packet.¶

For example, if an endpoint has received an acknowledgment for packet 0xabe8bc, sending a packet with a number of 0xac5c02 requires a packet number encoding with 16 bits or more; whereas the 24-bit packet number encoding is needed to send a packet with a number of 0xace8fe.¶

At a receiver, protection of the packet number is removed prior to recovering the full packet number. The full packet number is then reconstructed based on the number of significant bits present, the value of those bits, and the largest packet number received on a successfully authenticated packet. Recovering the full packet number is necessary to successfully remove packet protection.¶

Once header protection is removed, the packet number is decoded by finding the packet number value that is closest to the next expected packet. The next expected packet is the highest received packet number plus one. For example, if the highest successfully authenticated packet had a packet number of 0xa82f30ea, then a packet containing a 16-bit value of 0x9b32 will be decoded as 0xa82f9b32. Example pseudo-code for packet number decoding can be found in Appendix A.¶

17.2. Long Header Packets

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|1|1|T T|X X X X|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (32)                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..160)            ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..160)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Long headers are used for packets that are sent prior to the establishment of 1-RTT keys. Once 1-RTT keys are available, a sender switches to sending packets using the short header (Section 17.3). The long form allows for special packets - such as the Version Negotiation packet - to be represented in this uniform fixed-length packet format. Packets that use the long header contain the following fields:¶

Header Form:

The most significant bit (0x80) of byte 0 (the first byte) is set to 1 for long headers.¶

Fixed Bit:

The next bit (0x40) of byte 0 is set to 1. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded.¶

Long Packet Type (T):

The next two bits (those with a mask of 0x30) of byte 0 contain a packet type. Packet types are listed in Table 5.¶

Type-Specific Bits (X):

The lower four bits (those with a mask of 0x0f) of byte 0 are type-specific.¶

Version:

The QUIC Version is a 32-bit field that follows the first byte. This field indicates which version of QUIC is in use and determines how the rest of the protocol fields are interpreted.¶

DCID Len:

The byte following the version contains the length in bytes of the Destination Connection ID field that follows it. This length is encoded as an 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. Servers SHOULD be able to read longer connection IDs from other QUIC versions in order to properly form a version negotiation packet.¶

Destination Connection ID:

The Destination Connection ID field follows the DCID Len and is between 0 and 20 bytes in length. Section 7.2 describes the use of this field in more detail.¶

SCID Len:

The byte following the Destination Connection ID contains the length in bytes of the Source Connection ID field that follows it. This length is encoded as a 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20 bytes. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. Servers SHOULD be able to read longer connection IDs from other QUIC versions in order to properly form a version negotiation packet.¶

Source Connection ID:

The Source Connection ID field follows the SCID Len and is between 0 and 20 bytes in length. Section 7.2 describes the use of this field in more detail.¶

In this version of QUIC, the following packet types with the long header are defined:¶

The header form bit, connection ID lengths byte, Destination and Source Connection ID fields, and Version fields of a long header packet are version-independent. The other fields in the first byte are version-specific. See [QUIC-INVARIANTS] for details on how packets from different versions of QUIC are interpreted.¶

The interpretation of the fields and the payload are specific to a version and packet type. While type-specific semantics for this version are described in the following sections, several long-header packets in this version of QUIC contain these additional fields:¶

Reserved Bits (R):

Two bits (those with a mask of 0x0c) of byte 0 are reserved across multiple packet types. These bits are protected using header protection (see Section 5.4 of [QUIC-TLS]). The value included prior to protection MUST be set to 0. An endpoint MUST treat receipt of a packet that has a non-zero value for these bits, after removing both packet and header protection, as a connection error of type PROTOCOL_VIOLATION. Discarding such a packet after only removing header protection can expose the endpoint to attacks (see Section 9.3 of [QUIC-TLS]).¶

Packet Number Length (P):

In packet types which contain a Packet Number field, the least significant two bits (those with a mask of 0x03) of byte 0 contain the length of the packet number, encoded as an unsigned, two-bit integer that is one less than the length of the packet number field in bytes. That is, the length of the packet number field is the value of this field, plus one. These bits are protected using header protection (see Section 5.4 of [QUIC-TLS]).¶

Length:

The length of the remainder of the packet (that is, the Packet Number and Payload fields) in bytes, encoded as a variable-length integer (Section 16).¶

Packet Number:

The packet number field is 1 to 4 bytes long. The packet number has confidentiality protection separate from packet protection, as described in Section 5.4 of [QUIC-TLS]. The length of the packet number field is encoded in the Packet Number Length bits of byte 0 (see above).¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|1|  Unused (7) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Version (32)                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..2040)           ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..2040)              ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Supported Version 1 (32)                 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   [Supported Version 2 (32)]                ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   [Supported Version N (32)]                ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+
|1|1| 0 |R R|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (32)                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..160)            ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..160)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Token Length (i)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Token (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Length (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Packet Number (8/16/24/32)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Payload (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+
|1|1| 1 |R R|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (32)                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..160)            ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..160)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Length (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Packet Number (8/16/24/32)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Payload (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+
|1|1| 2 |R R|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (32)                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..160)            ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..160)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Length (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Packet Number (8/16/24/32)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Payload (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|1|1| 3 | Unused|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (32)                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Destination Connection ID (0..160)            ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SCID Len (8)  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Source Connection ID (0..160)               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Retry Token (*)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                   Retry Integrity Tag (128)                   +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3. Short Header Packets

This version of QUIC defines a single packet type which uses the short packet header.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|0|1|S|R|R|K|P P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                Destination Connection ID (0..160)           ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Packet Number (8/16/24/32)              ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Protected Payload (*)                   ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The short header can be used after the version and 1-RTT keys are negotiated. Packets that use the short header contain the following fields:¶

Header Form:

The most significant bit (0x80) of byte 0 is set to 0 for the short header.¶

Fixed Bit:

The next bit (0x40) of byte 0 is set to 1. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded.¶

Spin Bit (S):

The third most significant bit (0x20) of byte 0 is the latency spin bit, set as described in Section 17.3.1.¶

Reserved Bits (R):

The next two bits (those with a mask of 0x18) of byte 0 are reserved. These bits are protected using header protection (see Section 5.4 of [QUIC-TLS]). The value included prior to protection MUST be set to 0. An endpoint MUST treat receipt of a packet that has a non-zero value for these bits, after removing both packet and header protection, as a connection error of type PROTOCOL_VIOLATION. Discarding such a packet after only removing header protection can expose the endpoint to attacks (see Section 9.3 of [QUIC-TLS]).¶

Key Phase (K):

The next bit (0x04) of byte 0 indicates the key phase, which allows a recipient of a packet to identify the packet protection keys that are used to protect the packet. See [QUIC-TLS] for details. This bit is protected using header protection (see Section 5.4 of [QUIC-TLS]).¶

Packet Number Length (P):

The least significant two bits (those with a mask of 0x03) of byte 0 contain the length of the packet number, encoded as an unsigned, two-bit integer that is one less than the length of the packet number field in bytes. That is, the length of the packet number field is the value of this field, plus one. These bits are protected using header protection (see Section 5.4 of [QUIC-TLS]).¶

Destination Connection ID:

The Destination Connection ID is a connection ID that is chosen by the intended recipient of the packet. See Section 5.1 for more details.¶

Packet Number:

The packet number field is 1 to 4 bytes long. The packet number has confidentiality protection separate from packet protection, as described in Section 5.4 of [QUIC-TLS]. The length of the packet number field is encoded in Packet Number Length field. See Section 17.1 for details.¶

Protected Payload:

Packets with a short header always include a 1-RTT protected payload.¶

The header form bit and the connection ID field of a short header packet are version-independent. The remaining fields are specific to the selected QUIC version. See [QUIC-INVARIANTS] for details on how packets from different versions of QUIC are interpreted.¶

18.1. Reserved Transport Parameters

Transport parameters with an identifier of the form 31 * N + 27 for integer values of N are reserved to exercise the requirement that unknown transport parameters be ignored. These transport parameters have no semantics, and may carry arbitrary values.¶

18.2. Transport Parameter Definitions

This section details the transport parameters defined in this document.¶

Many transport parameters listed here have integer values. Those transport parameters that are identified as integers use a variable-length integer encoding (see Section 16) and have a default value of 0 if the transport parameter is absent, unless otherwise stated.¶

The following transport parameters are defined:¶

original_connection_id (0x0000):

The value of the Destination Connection ID field from the first Initial packet sent by the client. This transport parameter is only sent by a server. This is the same value sent in the "Original Destination Connection ID" field of a Retry packet (see Section 17.2.5). A server MUST include the original_connection_id transport parameter if it sent a Retry packet.¶

max_idle_timeout (0x0001):

The max idle timeout is a value in milliseconds that is encoded as an integer; see (Section 10.2). Idle timeout is disabled when both endpoints omit this transport parameter or specify a value of 0.¶

stateless_reset_token (0x0002):

A stateless reset token is used in verifying a stateless reset; see Section 10.4. This parameter is a sequence of 16 bytes. This transport parameter MUST NOT be sent by a client, but MAY be sent by a server. A server that does not send this transport parameter cannot use stateless reset (Section 10.4) for the connection ID negotiated during the handshake.¶

max_packet_size (0x0003):

The maximum packet size parameter is an integer value that limits the size of packets that the endpoint is willing to receive. This indicates that packets larger than this limit will be dropped. The default for this parameter is the maximum permitted UDP payload of 65527. Values below 1200 are invalid. This limit only applies to protected packets (Section 12.1).¶

initial_max_data (0x0004):

The initial maximum data parameter is an integer value that contains the initial value for the maximum amount of data that can be sent on the connection. This is equivalent to sending a MAX_DATA (Section 19.9) for the connection immediately after completing the handshake.¶

initial_max_stream_data_bidi_local (0x0005):

This parameter is an integer value specifying the initial flow control limit for locally-initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that sends the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x0; in server transport parameters, this applies to streams with the least significant two bits set to 0x1.¶

initial_max_stream_data_bidi_remote (0x0006):

This parameter is an integer value specifying the initial flow control limit for peer-initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x1; in server transport parameters, this applies to streams with the least significant two bits set to 0x0.¶

initial_max_stream_data_uni (0x0007):

This parameter is an integer value specifying the initial flow control limit for unidirectional streams. This limit applies to newly created unidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x3; in server transport parameters, this applies to streams with the least significant two bits set to 0x2.¶

initial_max_streams_bidi (0x0008):

The initial maximum bidirectional streams parameter is an integer value that contains the initial maximum number of bidirectional streams the peer may initiate. If this parameter is absent or zero, the peer cannot open bidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.¶

initial_max_streams_uni (0x0009):

The initial maximum unidirectional streams parameter is an integer value that contains the initial maximum number of unidirectional streams the peer may initiate. If this parameter is absent or zero, the peer cannot open unidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.¶

ack_delay_exponent (0x000a):

The ACK delay exponent is an integer value indicating an exponent used to decode the ACK Delay field in the ACK frame (Section 19.3). If this value is absent, a default value of 3 is assumed (indicating a multiplier of 8). Values above 20 are invalid.¶

max_ack_delay (0x000b):

The maximum ACK delay is an integer value indicating the maximum amount of time in milliseconds by which the endpoint will delay sending acknowledgments. This value SHOULD include the receiver's expected delays in alarms firing. For example, if a receiver sets a timer for 5ms and alarms commonly fire up to 1ms late, then it should send a max_ack_delay of 6ms. If this value is absent, a default of 25 milliseconds is assumed. Values of 2^14 or greater are invalid.¶

disable_active_migration (0x000c):

The disable active migration transport parameter is included if the endpoint does not support active connection migration (Section 9). Peers of an endpoint that sets this transport parameter MUST NOT send any packets, including probing packets (Section 9.1), from a local address or port other than that used to perform the handshake. This parameter is a zero-length value.¶

preferred_address (0x000d):

The server's preferred address is used to effect a change in server address at the end of the handshake, as described in Section 9.6. The format of this transport parameter is shown in Figure 18. This transport parameter is only sent by a server. Servers MAY choose to only send a preferred address of one address family by sending an all-zero address and port (0.0.0.0:0 or ::.0) for the other family. IP addresses are encoded in network byte order. The CID Length field contains the length of the Connection ID field.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       IPv4 Address (32)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         IPv4 Port (16)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                      IPv6 Address (128)                       +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         IPv6 Port (16)        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CID Length (8)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Connection ID (*)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                   Stateless Reset Token (128)                 +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

active_connection_id_limit (0x000e):

The active connection ID limit is an integer value specifying the maximum number of connection IDs from the peer that an endpoint is willing to store. This value includes the connection ID received during the handshake, that received in the preferred_address transport parameter, and those received in NEW_CONNECTION_ID frames. Unless a zero-length connection ID is being used, the value of the active_connection_id_limit parameter MUST be no less than 2. If this transport parameter is absent, a default of 2 is assumed. When a zero-length connection ID is being used, the active_connection_id_limit parameter MUST NOT be sent.¶

If present, transport parameters that set initial flow control limits (initial_max_stream_data_bidi_local, initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni) are equivalent to sending a MAX_STREAM_DATA frame (Section 19.10) on every stream of the corresponding type immediately after opening. If the transport parameter is absent, streams of that type start with a flow control limit of 0.¶

A client MUST NOT include server-only transport parameters (original_connection_id, stateless_reset_token, or preferred_address). A server MUST treat receipt of any of these transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

19.1. PADDING Frame

The PADDING frame (type=0x00) has no semantic value. PADDING frames can be used to increase the size of a packet. Padding can be used to increase an initial client packet to the minimum required size, or to provide protection against traffic analysis for protected packets.¶

A PADDING frame has no content. That is, a PADDING frame consists of the single byte that identifies the frame as a PADDING frame.¶

19.2. PING Frame

Endpoints can use PING frames (type=0x01) to verify that their peers are still alive or to check reachability to the peer. The PING frame contains no additional fields.¶

The receiver of a PING frame simply needs to acknowledge the packet containing this frame.¶

The PING frame can be used to keep a connection alive when an application or application protocol wishes to prevent the connection from timing out. An application protocol SHOULD provide guidance about the conditions under which generating a PING is recommended. This guidance SHOULD indicate whether it is the client or the server that is expected to send the PING. Having both endpoints send PING frames without coordination can produce an excessive number of packets and poor performance.¶

A connection will time out if no packets are sent or received for a period longer than the time negotiated using the max_idle_timeout transport parameter (see Section 10). However, state in middleboxes might time out earlier than that. Though REQ-5 in [RFC4787] recommends a 2 minute timeout interval, experience shows that sending packets every 15 to 30 seconds is necessary to prevent the majority of middleboxes from losing state for UDP flows.¶

19.3. ACK Frames

Receivers send ACK frames (types 0x02 and 0x03) to inform senders of packets they have received and processed. The ACK frame contains one or more ACK Ranges. ACK Ranges identify acknowledged packets. If the frame type is 0x03, ACK frames also contain the sum of QUIC packets with associated ECN marks received on the connection up until this point. QUIC implementations MUST properly handle both types and, if they have enabled ECN for packets they send, they SHOULD use the information in the ECN section to manage their congestion state.¶

QUIC acknowledgements are irrevocable. Once acknowledged, a packet remains acknowledged, even if it does not appear in a future ACK frame. This is unlike TCP SACKs ([RFC2018]).¶

Packets from different packet number spaces can be identified using the same numeric value. An acknowledgment for a packet needs to indicate both a packet number and a packet number space. This is accomplished by having each ACK frame only acknowledge packet numbers in the same space as the packet in which the ACK frame is contained.¶

Version Negotiation and Retry packets cannot be acknowledged because they do not contain a packet number. Rather than relying on ACK frames, these packets are implicitly acknowledged by the next Initial packet sent by the client.¶

An ACK frame is shown in Figure 19.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Largest Acknowledged (i)                ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          ACK Delay (i)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       ACK Range Count (i)                   ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       First ACK Range (i)                   ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          ACK Ranges (*)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          [ECN Counts]                       ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ACK frames contain the following fields:¶

Largest Acknowledged:

A variable-length integer representing the largest packet number the peer is acknowledging; this is usually the largest packet number that the peer has received prior to generating the ACK frame. Unlike the packet number in the QUIC long or short header, the value in an ACK frame is not truncated.¶

ACK Delay:

A variable-length integer representing the time delta in microseconds between when this ACK was sent and when the largest acknowledged packet, as indicated in the Largest Acknowledged field, was received by this peer. The value of the ACK Delay field is scaled by multiplying the encoded value by 2 to the power of the value of the ack_delay_exponent transport parameter set by the sender of the ACK frame (see Section 18.2). Scaling in this fashion allows for a larger range of values with a shorter encoding at the cost of lower resolution. Because the receiver doesn't use the ACK Delay for Initial and Handshake packets, a sender SHOULD send a value of 0.¶

ACK Range Count:

A variable-length integer specifying the number of Gap and ACK Range fields in the frame.¶

First ACK Range:

A variable-length integer indicating the number of contiguous packets preceding the Largest Acknowledged that are being acknowledged. The First ACK Range is encoded as an ACK Range (see Section 19.3.1) starting from the Largest Acknowledged. That is, the smallest packet acknowledged in the range is determined by subtracting the First ACK Range value from the Largest Acknowledged.¶

ACK Ranges:

Contains additional ranges of packets which are alternately not acknowledged (Gap) and acknowledged (ACK Range); see Section 19.3.1.¶

ECN Counts:

The three ECN Counts; see Section 19.3.2.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           [Gap (i)]                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          [ACK Range (i)]                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           [Gap (i)]                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          [ACK Range (i)]                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                               ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           [Gap (i)]                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          [ACK Range (i)]                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   smallest = largest - ack_range

   largest = previous_smallest - gap - 2

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        ECT(0) Count (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        ECT(1) Count (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        ECN-CE Count (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

19.4. RESET_STREAM Frame

An endpoint uses a RESET_STREAM frame (type=0x04) to abruptly terminate the sending part of a stream.¶

After sending a RESET_STREAM, an endpoint ceases transmission and retransmission of STREAM frames on the identified stream. A receiver of RESET_STREAM can discard any data that it already received on that stream.¶

An endpoint that receives a RESET_STREAM frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

The RESET_STREAM frame is shown in Figure 22.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream ID (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Application Error Code (i)                 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Final Size (i)                       ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

RESET_STREAM frames contain the following fields:¶

Stream ID:

A variable-length integer encoding of the Stream ID of the stream being terminated.¶

Application Protocol Error Code:

A variable-length integer containing the application protocol error code (see Section 20.1) which indicates why the stream is being closed.¶

Final Size:

A variable-length integer indicating the final size of the stream by the RESET_STREAM sender, in unit of bytes.¶

19.5. STOP_SENDING Frame

An endpoint uses a STOP_SENDING frame (type=0x05) to communicate that incoming data is being discarded on receipt at application request. STOP_SENDING requests that a peer cease transmission on a stream.¶

A STOP_SENDING frame can be sent for streams in the Recv or Size Known states (see Section 3.1). Receiving a STOP_SENDING frame for a locally-initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a STOP_SENDING frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

The STOP_SENDING frame is shown in Figure 23.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream ID (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Application Error Code (i)                 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

STOP_SENDING frames contain the following fields:¶

Stream ID:

A variable-length integer carrying the Stream ID of the stream being ignored.¶

Application Error Code:

A variable-length integer containing the application-specified reason the sender is ignoring the stream (see Section 20.1).¶

19.6. CRYPTO Frame

The CRYPTO frame (type=0x06) is used to transmit cryptographic handshake messages. It can be sent in all packet types except 0-RTT. The CRYPTO frame offers the cryptographic protocol an in-order stream of bytes. CRYPTO frames are functionally identical to STREAM frames, except that they do not bear a stream identifier; they are not flow controlled; and they do not carry markers for optional offset, optional length, and the end of the stream.¶

The CRYPTO frame is shown in Figure 24.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Offset (i)                         ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Length (i)                         ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Crypto Data (*)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

CRYPTO frames contain the following fields:¶

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this CRYPTO frame.¶

Length:

A variable-length integer specifying the length of the Crypto Data field in this CRYPTO frame.¶

Crypto Data:

The cryptographic message data.¶

There is a separate flow of cryptographic handshake data in each encryption level, each of which starts at an offset of 0. This implies that each encryption level is treated as a separate CRYPTO stream of data.¶

The largest offset delivered on a stream - the sum of the offset and data length - cannot exceed 2^62-1. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or CRYPTO_BUFFER_EXCEEDED.¶

Unlike STREAM frames, which include a Stream ID indicating to which stream the data belongs, the CRYPTO frame carries data for a single stream per encryption level. The stream does not have an explicit end, so CRYPTO frames do not have a FIN bit.¶

19.7. NEW_TOKEN Frame

A server sends a NEW_TOKEN frame (type=0x07) to provide the client with a token to send in the header of an Initial packet for a future connection.¶

The NEW_TOKEN frame is shown in Figure 25.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Token Length (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            Token (*)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NEW_TOKEN frames contain the following fields:¶

Token Length:

A variable-length integer specifying the length of the token in bytes.¶

Token:

An opaque blob that the client may use with a future Initial packet. The token MUST NOT be empty. An endpoint MUST treat receipt of a NEW_TOKEN frame with an empty Token field as a connection error of type FRAME_ENCODING_ERROR.¶

An endpoint might receive multiple NEW_TOKEN frames that contain the same token value if packets containing the frame are incorrectly determined to be lost. Endpoints are responsible for discarding duplicate values, which might be used to link connection attempts; see Section 8.1.3.¶

Clients MUST NOT send NEW_TOKEN frames. Servers MUST treat receipt of a NEW_TOKEN frame as a connection error of type PROTOCOL_VIOLATION.¶

19.8. STREAM Frames

STREAM frames implicitly create a stream and carry stream data. The STREAM frame takes the form 0b00001XXX (or the set of values from 0x08 to 0x0f). The value of the three low-order bits of the frame type determines the fields that are present in the frame.¶

• The OFF bit (0x04) in the frame type is set to indicate that there is an Offset field present. When set to 1, the Offset field is present. When set to 0, the Offset field is absent and the Stream Data starts at an offset of 0 (that is, the frame contains the first bytes of the stream, or the end of a stream that includes no data).¶
• The LEN bit (0x02) in the frame type is set to indicate that there is a Length field present. If this bit is set to 0, the Length field is absent and the Stream Data field extends to the end of the packet. If this bit is set to 1, the Length field is present.¶
• The FIN bit (0x01) of the frame type is set only on frames that contain the final size of the stream. Setting this bit indicates that the frame marks the end of the stream.¶

An endpoint that receives a STREAM frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

The STREAM frames are shown in Figure 26.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Stream ID (i)                       ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         [Offset (i)]                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         [Length (i)]                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream Data (*)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

STREAM frames contain the following fields:¶

Stream ID:

A variable-length integer indicating the stream ID of the stream (see Section 2.1).¶

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this STREAM frame. This field is present when the OFF bit is set to 1. When the Offset field is absent, the offset is 0.¶

Length:

A variable-length integer specifying the length of the Stream Data field in this STREAM frame. This field is present when the LEN bit is set to 1. When the LEN bit is set to 0, the Stream Data field consumes all the remaining bytes in the packet.¶

Stream Data:

The bytes from the designated stream to be delivered.¶

When a Stream Data field has a length of 0, the offset in the STREAM frame is the offset of the next byte that would be sent.¶

The first byte in the stream has an offset of 0. The largest offset delivered on a stream - the sum of the offset and data length - cannot exceed 2^62-1, as it is not possible to provide flow control credit for that data. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or FLOW_CONTROL_ERROR.¶

19.9. MAX_DATA Frame

The MAX_DATA frame (type=0x10) is used in flow control to inform the peer of the maximum amount of data that can be sent on the connection as a whole.¶

The MAX_DATA frame is shown in Figure 27.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Maximum Data (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MAX_DATA frames contain the following fields:¶

Maximum Data:

A variable-length integer indicating the maximum amount of data that can be sent on the entire connection, in units of bytes.¶

All data sent in STREAM frames counts toward this limit. The sum of the largest received offsets on all streams - including streams in terminal states - MUST NOT exceed the value advertised by a receiver. An endpoint MUST terminate a connection with a FLOW_CONTROL_ERROR error if it receives more data than the maximum data value that it has sent, unless this is a result of a change in the initial limits (see Section 7.3.1).¶

19.10. MAX_STREAM_DATA Frame

The MAX_STREAM_DATA frame (type=0x11) is used in flow control to inform a peer of the maximum amount of data that can be sent on a stream.¶

A MAX_STREAM_DATA frame can be sent for streams in the Recv state (see Section 3.1). Receiving a MAX_STREAM_DATA frame for a locally-initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a MAX_STREAM_DATA frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

The MAX_STREAM_DATA frame is shown in Figure 28.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream ID (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Maximum Stream Data (i)                  ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MAX_STREAM_DATA frames contain the following fields:¶

Stream ID:

The stream ID of the stream that is affected encoded as a variable-length integer.¶

Maximum Stream Data:

A variable-length integer indicating the maximum amount of data that can be sent on the identified stream, in units of bytes.¶

When counting data toward this limit, an endpoint accounts for the largest received offset of data that is sent or received on the stream. Loss or reordering can mean that the largest received offset on a stream can be greater than the total size of data received on that stream. Receiving STREAM frames might not increase the largest received offset.¶

The data sent on a stream MUST NOT exceed the largest maximum stream data value advertised by the receiver. An endpoint MUST terminate a connection with a FLOW_CONTROL_ERROR error if it receives more data than the largest maximum stream data that it has sent for the affected stream, unless this is a result of a change in the initial limits (see Section 7.3.1).¶

19.11. MAX_STREAMS Frames

The MAX_STREAMS frames (type=0x12 and 0x13) inform the peer of the cumulative number of streams of a given type it is permitted to open. A MAX_STREAMS frame with a type of 0x12 applies to bidirectional streams, and a MAX_STREAMS frame with a type of 0x13 applies to unidirectional streams.¶

The MAX_STREAMS frames are shown in Figure 29;¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Maximum Streams (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MAX_STREAMS frames contain the following fields:¶

Maximum Streams:

A count of the cumulative number of streams of the corresponding type that can be opened over the lifetime of the connection. Stream IDs cannot exceed 2^62-1, as it is not possible to encode stream IDs larger than this value. Receipt of a frame that permits opening of a stream larger than this limit MUST be treated as a FRAME_ENCODING_ERROR.¶

Loss or reordering can cause a MAX_STREAMS frame to be received which states a lower stream limit than an endpoint has previously received. MAX_STREAMS frames which do not increase the stream limit MUST be ignored.¶

An endpoint MUST NOT open more streams than permitted by the current stream limit set by its peer. For instance, a server that receives a unidirectional stream limit of 3 is permitted to open stream 3, 7, and 11, but not stream 15. An endpoint MUST terminate a connection with a STREAM_LIMIT_ERROR error if a peer opens more streams than was permitted.¶

Note that these frames (and the corresponding transport parameters) do not describe the number of streams that can be opened concurrently. The limit includes streams that have been closed as well as those that are open.¶

19.12. DATA_BLOCKED Frame

A sender SHOULD send a DATA_BLOCKED frame (type=0x14) when it wishes to send data, but is unable to due to connection-level flow control (see Section 4). DATA_BLOCKED frames can be used as input to tuning of flow control algorithms (see Section 4.2).¶

The DATA_BLOCKED frame is shown in Figure 30.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Data Limit (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DATA_BLOCKED frames contain the following fields:¶

Data Limit:

A variable-length integer indicating the connection-level limit at which blocking occurred.¶

19.13. STREAM_DATA_BLOCKED Frame

A sender SHOULD send a STREAM_DATA_BLOCKED frame (type=0x15) when it wishes to send data, but is unable to due to stream-level flow control. This frame is analogous to DATA_BLOCKED (Section 19.12).¶

An endpoint that receives a STREAM_DATA_BLOCKED frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

The STREAM_DATA_BLOCKED frame is shown in Figure 31.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream ID (i)                        ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Stream Data Limit (i)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

STREAM_DATA_BLOCKED frames contain the following fields:¶

Stream ID:

A variable-length integer indicating the stream which is flow control blocked.¶

Stream Data Limit:

A variable-length integer indicating the offset of the stream at which the blocking occurred.¶

19.14. STREAMS_BLOCKED Frames

A sender SHOULD send a STREAMS_BLOCKED frame (type=0x16 or 0x17) when it wishes to open a stream, but is unable to due to the maximum stream limit set by its peer (see Section 19.11). A STREAMS_BLOCKED frame of type 0x16 is used to indicate reaching the bidirectional stream limit, and a STREAMS_BLOCKED frame of type 0x17 indicates reaching the unidirectional stream limit.¶

A STREAMS_BLOCKED frame does not open the stream, but informs the peer that a new stream was needed and the stream limit prevented the creation of the stream.¶

The STREAMS_BLOCKED frames are shown in Figure 32.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Stream Limit (i)                     ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

STREAMS_BLOCKED frames contain the following fields:¶

Stream Limit:

A variable-length integer indicating the stream limit at the time the frame was sent. Stream IDs cannot exceed 2^62-1, as it is not possible to encode stream IDs larger than this value. Receipt of a frame that encodes a larger stream ID MUST be treated as a STREAM_LIMIT_ERROR or a FRAME_ENCODING_ERROR.¶

19.15. NEW_CONNECTION_ID Frame

An endpoint sends a NEW_CONNECTION_ID frame (type=0x18) to provide its peer with alternative connection IDs that can be used to break linkability when migrating connections (see Section 9.5).¶

The NEW_CONNECTION_ID frame is shown in Figure 33.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Sequence Number (i)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Retire Prior To (i)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Length (8)  |                                               |
+-+-+-+-+-+-+-+-+       Connection ID (8..160)                  +
|                                                             ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                                                               +
|                                                               |
+                   Stateless Reset Token (128)                 +
|                                                               |
+                                                               +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

NEW_CONNECTION_ID frames contain the following fields:¶

Sequence Number:

The sequence number assigned to the connection ID by the sender. See Section 5.1.1.¶

Retire Prior To:

A variable-length integer indicating which connection IDs should be retired. See Section 5.1.2.¶

Length:

An 8-bit unsigned integer containing the length of the connection ID. Values less than 1 and greater than 20 are invalid and MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Connection ID:

A connection ID of the specified length.¶

Stateless Reset Token:

A 128-bit value that will be used for a stateless reset when the associated connection ID is used (see Section 10.4).¶

An endpoint MUST NOT send this frame if it currently requires that its peer send packets with a zero-length Destination Connection ID. Changing the length of a connection ID to or from zero-length makes it difficult to identify when the value of the connection ID changed. An endpoint that is sending packets with a zero-length Destination Connection ID MUST treat receipt of a NEW_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

Transmission errors, timeouts and retransmissions might cause the same NEW_CONNECTION_ID frame to be received multiple times. Receipt of the same frame multiple times MUST NOT be treated as a connection error. A receiver can use the sequence number supplied in the NEW_CONNECTION_ID frame to identify new connection IDs from old ones.¶

If an endpoint receives a NEW_CONNECTION_ID frame that repeats a previously issued connection ID with a different Stateless Reset Token or a different sequence number, or if a sequence number is used for different connection IDs, the endpoint MAY treat that receipt as a connection error of type PROTOCOL_VIOLATION.¶

The Retire Prior To field counts connection IDs established during connection setup and the preferred_address transport parameter (see Section 5.1.2). The Retire Prior To field MUST be less than or equal to the Sequence Number field. Receiving a value greater than the Sequence Number MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Once a sender indicates a Retire Prior To value, smaller values sent in subsequent NEW_CONNECTION_ID frames have no effect. A receiver MUST ignore any Retire Prior To fields that do not increase the largest received Retire Prior To value.¶

An endpoint that receives a NEW_CONNECTION_ID frame with a sequence number smaller than the Retire Prior To field of a previously received NEW_CONNECTION_ID frame MUST immediately send a corresponding RETIRE_CONNECTION_ID frame that retires the newly received connection ID.¶

19.16. RETIRE_CONNECTION_ID Frame

An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x19) to indicate that it will no longer use a connection ID that was issued by its peer. This may include the connection ID provided during the handshake. Sending a RETIRE_CONNECTION_ID frame also serves as a request to the peer to send additional connection IDs for future use (see Section 5.1). New connection IDs can be delivered to a peer using the NEW_CONNECTION_ID frame (Section 19.15).¶

Retiring a connection ID invalidates the stateless reset token associated with that connection ID.¶

The RETIRE_CONNECTION_ID frame is shown in Figure 34.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Sequence Number (i)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

RETIRE_CONNECTION_ID frames contain the following fields:¶

Sequence Number:

The sequence number of the connection ID being retired. See Section 5.1.2.¶

Receipt of a RETIRE_CONNECTION_ID frame containing a sequence number greater than any previously sent to the peer MUST be treated as a connection error of type PROTOCOL_VIOLATION.¶

The sequence number specified in a RETIRE_CONNECTION_ID frame MUST NOT refer to the Destination Connection ID field of the packet in which the frame is contained. The peer MAY treat this as a connection error of type FRAME_ENCODING_ERROR.¶

An endpoint cannot send this frame if it was provided with a zero-length connection ID by its peer. An endpoint that provides a zero-length connection ID MUST treat receipt of a RETIRE_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

19.17. PATH_CHALLENGE Frame

Endpoints can use PATH_CHALLENGE frames (type=0x1a) to check reachability to the peer and for path validation during connection migration.¶

The PATH_CHALLENGE frame is shown in Figure 35.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                           Data (64)                           +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PATH_CHALLENGE frames contain the following fields:¶

Data:

This 8-byte field contains arbitrary data.¶

A PATH_CHALLENGE frame containing 8 bytes that are hard to guess is sufficient to ensure that it is easier to receive the packet than it is to guess the value correctly.¶

The recipient of this frame MUST generate a PATH_RESPONSE frame (Section 19.18) containing the same Data.¶

19.18. PATH_RESPONSE Frame

The PATH_RESPONSE frame (type=0x1b) is sent in response to a PATH_CHALLENGE frame. Its format is identical to the PATH_CHALLENGE frame (Section 19.17).¶

If the content of a PATH_RESPONSE frame does not match the content of a PATH_CHALLENGE frame previously sent by the endpoint, the endpoint MAY generate a connection error of type PROTOCOL_VIOLATION.¶

19.19. CONNECTION_CLOSE Frames

An endpoint sends a CONNECTION_CLOSE frame (type=0x1c or 0x1d) to notify its peer that the connection is being closed. The CONNECTION_CLOSE with a frame type of 0x1c is used to signal errors at only the QUIC layer, or the absence of errors (with the NO_ERROR code). The CONNECTION_CLOSE frame with a type of 0x1d is used to signal an error with the application that uses QUIC.¶

If there are open streams that haven't been explicitly closed, they are implicitly closed when the connection is closed.¶

The CONNECTION_CLOSE frames are shown in Figure 36.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Error Code (i)                      ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       [ Frame Type (i) ]                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Reason Phrase Length (i)                 ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Reason Phrase (*)                    ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

CONNECTION_CLOSE frames contain the following fields:¶

Error Code:

A variable length integer error code which indicates the reason for closing this connection. A CONNECTION_CLOSE frame of type 0x1c uses codes from the space defined in Section 20. A CONNECTION_CLOSE frame of type 0x1d uses codes from the application protocol error code space; see Section 20.1¶

Frame Type:

A variable-length integer encoding the type of frame that triggered the error. A value of 0 (equivalent to the mention of the PADDING frame) is used when the frame type is unknown. The application-specific variant of CONNECTION_CLOSE (type 0x1d) does not include this field.¶

Reason Phrase Length:

A variable-length integer specifying the length of the reason phrase in bytes. Because a CONNECTION_CLOSE frame cannot be split between packets, any limits on packet size will also limit the space available for a reason phrase.¶

Reason Phrase:

A human-readable explanation for why the connection was closed. This can be zero length if the sender chooses to not give details beyond the Error Code. This SHOULD be a UTF-8 encoded string [RFC3629].¶

The application-specific variant of CONNECTION_CLOSE (type 0x1d) can only be sent using an 1-RTT packet ([QUIC-TLS], Section 4). When an application wishes to abandon a connection during the handshake, an endpoint can send a CONNECTION_CLOSE frame (type 0x1c) with an error code of 0x15a ("user_canceled" alert; see [TLS13]) in an Initial or a Handshake packet.¶

19.20. HANDSHAKE_DONE frame

The server uses the HANDSHAKE_DONE frame (type=0x1e) to signal confirmation of the handshake to the client. The HANDSHAKE_DONE frame contains no additional fields.¶

This frame can only be sent by the server. Servers MUST NOT send a HANDSHAKE_DONE frame before completing the handshake. A server MUST treat receipt of a HANDSHAKE_DONE frame as a connection error of type PROTOCOL_VIOLATION.¶

19.21. Extension Frames

QUIC frames do not use a self-describing encoding. An endpoint therefore needs to understand the syntax of all frames before it can successfully process a packet. This allows for efficient encoding of frames, but it means that an endpoint cannot send a frame of a type that is unknown to its peer.¶

An extension to QUIC that wishes to use a new type of frame MUST first ensure that a peer is able to understand the frame. An endpoint can use a transport parameter to signal its willingness to receive one or more extension frame types with the one transport parameter.¶

Extensions that modify or replace core protocol functionality (including frame types) will be difficult to combine with other extensions which modify or replace the same functionality unless the behavior of the combination is explicitly defined. Such extensions SHOULD define their interaction with previously-defined extensions modifying the same protocol components.¶

Extension frames MUST be congestion controlled and MUST cause an ACK frame to be sent. The exception is extension frames that replace or supplement the ACK frame. Extension frames are not included in flow control unless specified in the extension.¶

An IANA registry is used to manage the assignment of frame types; see Section 22.3.¶

20.1. Application Protocol Error Codes

Application protocol error codes are 62-bit unsigned integers, but the management of application error codes is left to application protocols. Application protocol error codes are used for the RESET_STREAM frame (Section 19.4), the STOP_SENDING frame (Section 19.5), and the CONNECTION_CLOSE frame with a type of 0x1d (Section 19.19).¶

21.1. Handshake Denial of Service

As an encrypted and authenticated transport QUIC provides a range of protections against denial of service. Once the cryptographic handshake is complete, QUIC endpoints discard most packets that are not authenticated, greatly limiting the ability of an attacker to interfere with existing connections.¶

Once a connection is established QUIC endpoints might accept some unauthenticated ICMP packets (see Section 14.2), but the use of these packets is extremely limited. The only other type of packet that an endpoint might accept is a stateless reset (Section 10.4) which relies on the token being kept secret until it is used.¶

During the creation of a connection, QUIC only provides protection against attack from off the network path. All QUIC packets contain proof that the recipient saw a preceding packet from its peer.¶

Addresses cannot change during the handshake, so endpoints can discard packets that are received on a different network path.¶

The Source and Destination Connection ID fields are the primary means of protection against off-path attack during the handshake. These are required to match those set by a peer. Except for an Initial and stateless reset packets, an endpoint only accepts packets that include a Destination Connection ID field that matches a value the endpoint previously chose. This is the only protection offered for Version Negotiation packets.¶

The Destination Connection ID field in an Initial packet is selected by a client to be unpredictable, which serves an additional purpose. The packets that carry the cryptographic handshake are protected with a key that is derived from this connection ID and salt specific to the QUIC version. This allows endpoints to use the same process for authenticating packets that they receive as they use after the cryptographic handshake completes. Packets that cannot be authenticated are discarded. Protecting packets in this fashion provides a strong assurance that the sender of the packet saw the Initial packet and understood it.¶

These protections are not intended to be effective against an attacker that is able to receive QUIC packets prior to the connection being established. Such an attacker can potentially send packets that will be accepted by QUIC endpoints. This version of QUIC attempts to detect this sort of attack, but it expects that endpoints will fail to establish a connection rather than recovering. For the most part, the cryptographic handshake protocol [QUIC-TLS] is responsible for detecting tampering during the handshake.¶

Endpoints are permitted to use other methods to detect and attempt to recover from interference with the handshake. Invalid packets may be identified and discarded using other methods, but no specific method is mandated in this document.¶

21.2. Amplification Attack

An attacker might be able to receive an address validation token (Section 8) from a server and then release the IP address it used to acquire that token. At a later time, the attacker may initiate a 0-RTT connection with a server by spoofing this same address, which might now address a different (victim) endpoint. The attacker can thus potentially cause the server to send an initial congestion window's worth of data towards the victim.¶

Servers SHOULD provide mitigations for this attack by limiting the usage and lifetime of address validation tokens (see Section 8.1.3).¶

21.3. Optimistic ACK Attack

An endpoint that acknowledges packets it has not received might cause a congestion controller to permit sending at rates beyond what the network supports. An endpoint MAY skip packet numbers when sending packets to detect this behavior. An endpoint can then immediately close the connection with a connection error of type PROTOCOL_VIOLATION (see Section 10.3).¶

21.4. Slowloris Attacks

The attacks commonly known as Slowloris [SLOWLORIS] try to keep many connections to the target endpoint open and hold them open as long as possible. These attacks can be executed against a QUIC endpoint by generating the minimum amount of activity necessary to avoid being closed for inactivity. This might involve sending small amounts of data, gradually opening flow control windows in order to control the sender rate, or manufacturing ACK frames that simulate a high loss rate.¶

QUIC deployments SHOULD provide mitigations for the Slowloris attacks, such as increasing the maximum number of clients the server will allow, limiting the number of connections a single IP address is allowed to make, imposing restrictions on the minimum transfer speed a connection is allowed to have, and restricting the length of time an endpoint is allowed to stay connected.¶

21.5. Stream Fragmentation and Reassembly Attacks

An adversarial sender might intentionally send fragments of stream data in order to cause disproportionate receive buffer memory commitment and/or creation of a large and inefficient data structure.¶

An adversarial receiver might intentionally not acknowledge packets containing stream data in order to force the sender to store the unacknowledged stream data for retransmission.¶

The attack on receivers is mitigated if flow control windows correspond to available memory. However, some receivers will over-commit memory and advertise flow control offsets in the aggregate that exceed actual available memory. The over-commitment strategy can lead to better performance when endpoints are well behaved, but renders endpoints vulnerable to the stream fragmentation attack.¶

QUIC deployments SHOULD provide mitigations against stream fragmentation attacks. Mitigations could consist of avoiding over-committing memory, limiting the size of tracking data structures, delaying reassembly of STREAM frames, implementing heuristics based on the age and duration of reassembly holes, or some combination.¶

21.6. Stream Commitment Attack

An adversarial endpoint can open lots of streams, exhausting state on an endpoint. The adversarial endpoint could repeat the process on a large number of connections, in a manner similar to SYN flooding attacks in TCP.¶

Normally, clients will open streams sequentially, as explained in Section 2.1. However, when several streams are initiated at short intervals, loss or reordering may cause STREAM frames that open streams to be received out of sequence. On receiving a higher-numbered stream ID, a receiver is required to open all intervening streams of the same type (see Section 3.2). Thus, on a new connection, opening stream 4000000 opens 1 million and 1 client-initiated bidirectional streams.¶

The number of active streams is limited by the initial_max_streams_bidi and initial_max_streams_uni transport parameters, as explained in Section 4.5. If chosen judiciously, these limits mitigate the effect of the stream commitment attack. However, setting the limit too low could affect performance when applications expect to open large number of streams.¶

21.7. Peer Denial of Service

QUIC and TLS both contain messages that have legitimate uses in some contexts, but that can be abused to cause a peer to expend processing resources without having any observable impact on the state of the connection.¶

Messages can also be used to change and revert state in small or inconsequential ways, such as by sending small increments to flow control limits.¶

If processing costs are disproportionately large in comparison to bandwidth consumption or effect on state, then this could allow a malicious peer to exhaust processing capacity.¶

While there are legitimate uses for all messages, implementations SHOULD track cost of processing relative to progress and treat excessive quantities of any non-productive packets as indicative of an attack. Endpoints MAY respond to this condition with a connection error, or by dropping packets.¶

21.8. Explicit Congestion Notification Attacks

An on-path attacker could manipulate the value of ECN codepoints in the IP header to influence the sender's rate. [RFC3168] discusses manipulations and their effects in more detail.¶

An on-the-side attacker can duplicate and send packets with modified ECN codepoints to affect the sender's rate. If duplicate packets are discarded by a receiver, an off-path attacker will need to race the duplicate packet against the original to be successful in this attack. Therefore, QUIC endpoints ignore the ECN codepoint field on an IP packet unless at least one QUIC packet in that IP packet is successfully processed; see Section 13.4.¶

21.9. Stateless Reset Oracle

Stateless resets create a possible denial of service attack analogous to a TCP reset injection. This attack is possible if an attacker is able to cause a stateless reset token to be generated for a connection with a selected connection ID. An attacker that can cause this token to be generated can reset an active connection with the same connection ID.¶

If a packet can be routed to different instances that share a static key, for example by changing an IP address or port, then an attacker can cause the server to send a stateless reset. To defend against this style of denial service, endpoints that share a static key for stateless reset (see Section 10.4.2) MUST be arranged so that packets with a given connection ID always arrive at an instance that has connection state, unless that connection is no longer active.¶

In the case of a cluster that uses dynamic load balancing, it's possible that a change in load balancer configuration could happen while an active instance retains connection state; even if an instance retains connection state, the change in routing and resulting stateless reset will result in the connection being terminated. If there is no chance in the packet being routed to the correct instance, it is better to send a stateless reset than wait for connections to time out. However, this is acceptable only if the routing cannot be influenced by an attacker.¶

21.10. Version Downgrade

This document defines QUIC Version Negotiation packets in Section 6, which can be used to negotiate the QUIC version used between two endpoints. However, this document does not specify how this negotiation will be performed between this version and subsequent future versions. In particular, Version Negotiation packets do not contain any mechanism to prevent version downgrade attacks. Future versions of QUIC that use Version Negotiation packets MUST define a mechanism that is robust against version downgrade attacks.¶

21.11. Targeted Attacks by Routing

Deployments should limit the ability of an attacker to target a new connection to a particular server instance. This means that client-controlled fields, such as the initial Destination Connection ID used on Initial and 0-RTT packets SHOULD NOT be used by themselves to make routing decisions. Ideally, routing decisions are made independently of client-selected values; a Source Connection ID can be selected to route later packets to the same server.¶

21.12. Overview of Security Properties

A complete security analysis of QUIC is outside the scope of this document. This section provides an informal description of the desired security properties as an aid to implementors and to help guide protocol analysis.¶

QUIC assumes the threat model described in [SEC-CONS] and provides protections against many of the attacks that arise from that model.¶

For this purpose, attacks are divided into passive and active attacks. Passive attackers have the capability to read packets from the network, while active attackers also have the capability to write packets into the network. However, a passive attack may involve an attacker with the ability to cause a routing change or other modification in the path taken by packets that comprise a connection.¶

Attackers are additionally categorized as either on-path attackers or off-path attackers; see Section 3.5 of [SEC-CONS]. An on-path attacker can read, modify, or remove any packet it observes such that it no longer reaches its destination, while an off-path attacker observes the packets, but cannot prevent the original packet from reaching its intended destination. An off-path attacker can also transmit arbitrary packets.¶

Properties of the handshake, protected packets, and connection migration are considered separately.¶

22.1. Registration Policies for QUIC Registries

All QUIC registries allow for both provisional and permanent registration of codepoints. This section documents policies that are common to these registries.¶

22.2. QUIC Transport Parameter Registry

IANA [SHALL add/has added] a registry for "QUIC Transport Parameters" under a "QUIC" heading.¶

The "QUIC Transport Parameters" registry governs a 16-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy [RFC8126].¶

In addition to the fields in Section 22.1.1, permanent registrations in this registry MUST include the following fields:¶

Parameter Name:

A short mnemonic for the parameter.¶

The initial contents of this registry are shown in Table 6.¶

Additionally, each value of the format 31 * N + 27 for integer values of N (that is, 27, 58, 89, ...) are reserved and MUST NOT be assigned by IANA.¶

22.3. QUIC Frame Type Registry

IANA [SHALL add/has added] a registry for "QUIC Frame Types" under a "QUIC" heading.¶

The "QUIC Frame Types" registry governs a 62-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy [RFC8126], except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of [RFC8126].¶

In addition to the fields in Section 22.1.1, permanent registrations in this registry MUST include the following fields:¶

Frame Name:

A short mnemonic for the frame type.¶

In addition to the advice in Section 22.1, specifications for new permanent registrations SHOULD describe the means by which an endpoint might determine that it can send the identified type of frame. An accompanying transport parameter registration (see Section 22.2) is expected for most registrations. Specifications for permanent registrations also needs to describe the format and assigned semantics of any fields in the frame.¶

The initial contents of this registry are tabulated in Table 3.¶

22.4. QUIC Transport Error Codes Registry

IANA [SHALL add/has added] a registry for "QUIC Transport Error Codes" under a "QUIC" heading.¶

The "QUIC Transport Error Codes" registry governs a 62-bit space. This space is split into three spaces that are governed by different policies. Permanent registrations in this registry are assigned using the Specification Required policy [RFC8126], except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of [RFC8126].¶

In addition to the fields in Section 22.1.1, permanent registrations in this registry MUST include the following fields:¶

Code:

A short mnemonic for the parameter.¶

Description:

A brief description of the error code semantics, which MAY be a summary if a specification reference is provided.¶

The initial contents of this registry are shown in Table 7.¶

C.1. Since draft-ietf-quic-transport-24

• Added HANDSHAKE_DONE to signal handshake confirmation (#2863, #3142, #3145)¶
• Add integrity check to Retry packets (#3014, #3274, #3120)¶
• Specify handling of reordered NEW_CONNECTION_ID frames (#3194, #3202)¶
• Require checking of sequence numbers in RETIRE_CONNECTION_ID (#3037, #3036)¶
• active_connection_id_limit is enforced (#3193, #3197, #3200, #3201)¶
• Correct overflow in packet number decode algorithm (#3187, #3188)¶
• Allow use of CRYPTO_BUFFER_EXCEEDED for CRYPTO frame errors (#3258, #3186)¶
• Define applicability and scope of NEW_TOKEN (#3150, #3152, #3155, #3156)¶
• Tokens from Retry and NEW_TOKEN must be differentiated (#3127, #3128)¶
• Allow CONNECTION_CLOSE in response to invalid token (#3168, #3107)¶
• Treat an invalid CONNECTION_CLOSE as an invalid frame (#2475, #3230, #3231)¶
• Throttle when sending CONNECTION_CLOSE after discarding state (#3095, #3157)¶
• Application-variant of CONNECTION_CLOSE can only be sent in 0-RTT or 1-RTT packets (#3158, #3164)¶
• Advise sending while blocked to avoid idle timeout (#2744, #3266)¶
• Define error codes for invalid frames (#3027, #3042)¶
• Idle timeout is symmetric (#2602, #3099)¶
• Prohibit IP fragmentation (#3243, #3280)¶
• Define the use of provisional registration for all registries (#3109, #3020, #3102, #3170)¶
• Packets on one path must not adjust values for a different path (#2909, #3139)¶

C.2. Since draft-ietf-quic-transport-23

• Allow ClientHello to span multiple packets (#2928, #3045)¶
• Client Initial size constraints apply to UDP datagram payload (#3053, #3051)¶

• Stateless reset changes (#2152, #2993)¶

  • tokens need to be compared in constant time¶
  • detection uses UDP datagrams, not packets¶
  • tokens cannot be reused (#2785, #2968)¶
• Clearer rules for sharing of UDP ports and use of connection IDs when doing so (#2844, #2851)¶
• A new connection ID is necessary when responding to migration (#2778, #2969)¶
• Stronger requirements for connection ID retirement (#3046, #3096)¶
• NEW_TOKEN cannot be empty (#2978, #2977)¶
• PING can be sent at any encryption level (#3034, #3035)¶
• CONNECTION_CLOSE is not ack-eliciting (#3097, #3098)¶
• Frame encoding error conditions updated (#3027, #3042)¶
• Non-ack-eliciting packets cannot be sent in response to non-ack-eliciting packets (#3100, #3104)¶
• Servers have to change connection IDs in Retry (#2837, #3147)¶

C.3. Since draft-ietf-quic-transport-22

• Rules for preventing correlation by connection ID tightened (#2084, #2929)¶
• Clarified use of CONNECTION_CLOSE in Handshake packets (#2151, #2541, #2688)¶
• Discourage regressions of largest acknowledged in ACK (#2205, #2752)¶
• Improved robustness of validation process for ECN counts (#2534, #2752)¶
• Require endpoints to ignore spurious migration attempts (#2342, #2893)¶
• Transport parameter for disabling migration clarified to allow NAT rebinding (#2389, #2893)¶
• Document principles for defining new error codes (#2388, #2880)¶
• Reserve transport parameters for greasing (#2550, #2873)¶
• A maximum ACK delay of 0 is used for handshake packet number spaces (#2646, #2638)¶
• Improved rules for use of congestion control state on new paths (#2685, #2918)¶
• Removed recommendation to coordinate spin for multiple connections that share a path (#2763, #2882)¶
• Allow smaller stateless resets and recommend a smaller minimum on packets that might trigger a stateless reset (#2770, #2869, #2927, #3007).¶
• Provide guidance around the interface to QUIC as used by application protocols (#2805, #2857)¶
• Frames other than STREAM can cause STREAM_LIMIT_ERROR (#2825, #2826)¶
• Tighter rules about processing of rejected 0-RTT packets (#2829, #2840, #2841)¶
• Explanation of the effect of Retry on 0-RTT packets (#2842, #2852)¶
• Cryptographic handshake needs to provide server transport parameter encryption (#2920, #2921)¶
• Moved ACK generation guidance from recovery draft to transport draft (#1860, #2916).¶

C.4. Since draft-ietf-quic-transport-21

• Connection ID lengths are now one octet, but limited in version 1 to 20 octets of length (#2736, #2749)¶

C.5. Since draft-ietf-quic-transport-20

• Error codes are encoded as variable-length integers (#2672, #2680)¶
• NEW_CONNECTION_ID includes a request to retire old connection IDs (#2645, #2769)¶
• Tighter rules for generating and explicitly eliciting ACK frames (#2546, #2794)¶
• Recommend having only one packet per encryption level in a datagram (#2308, #2747)¶
• More normative language about use of stateless reset (#2471, #2574)¶
• Allow reuse of stateless reset tokens (#2732, #2733)¶
• Allow, but not require, enforcing non-duplicate transport parameters (#2689, #2691)¶
• Added an active_connection_id_limit transport parameter (#1994, #1998)¶
• max_ack_delay transport parameter defaults to 0 (#2638, #2646)¶
• When sending 0-RTT, only remembered transport parameters apply (#2458, #2360, #2466, #2461)¶
• Define handshake completion and confirmation; define clearer rules when it encryption keys should be discarded (#2214, #2267, #2673)¶
• Prohibit path migration prior to handshake confirmation (#2309, #2370)¶
• PATH_RESPONSE no longer needs to be received on the validated path (#2582, #2580, #2579, #2637)¶
• PATH_RESPONSE frames are not stored and retransmitted (#2724, #2729)¶
• Document hack for enabling routing of ICMP when doing PMTU probing (#1243, #2402)¶

C.6. Since draft-ietf-quic-transport-19

• Refine discussion of 0-RTT transport parameters (#2467, #2464)¶
• Fewer transport parameters need to be remembered for 0-RTT (#2624, #2467)¶
• Spin bit text incorporated (#2564)¶
• Close the connection when maximum stream ID in MAX_STREAMS exceeds 2^62 - 1 (#2499, #2487)¶
• New connection ID required for intentional migration (#2414, #2413)¶
• Connection ID issuance can be rate-limited (#2436, #2428)¶
• The "QUIC bit" is ignored in Version Negotiation (#2400, #2561)¶
• Initial packets from clients need to be padded to 1200 unless a Handshake packet is sent as well (#2522, #2523)¶
• CRYPTO frames can be discarded if too much data is buffered (#1834, #2524)¶
• Stateless reset uses a short header packet (#2599, #2600)¶

C.7. Since draft-ietf-quic-transport-18

• Removed version negotiation; version negotiation, including authentication of the result, will be addressed in the next version of QUIC (#1773, #2313)¶
• Added discussion of the use of IPv6 flow labels (#2348, #2399)¶
• A connection ID can't be retired in a packet that uses that connection ID (#2101, #2420)¶
• Idle timeout transport parameter is in milliseconds (from seconds) (#2453, #2454)¶
• Endpoints are required to use new connection IDs when they use new network paths (#2413, #2414)¶
• Increased the set of permissible frames in 0-RTT (#2344, #2355)¶

C.8. Since draft-ietf-quic-transport-17

• Stream-related errors now use STREAM_STATE_ERROR (#2305)¶
• Endpoints discard initial keys as soon as handshake keys are available (#1951, #2045)¶
• Expanded conditions for ignoring ICMP packet too big messages (#2108, #2161)¶
• Remove rate control from PATH_CHALLENGE/PATH_RESPONSE (#2129, #2241)¶
• Endpoints are permitted to discard malformed initial packets (#2141)¶
• Clarified ECN implementation and usage requirements (#2156, #2201)¶
• Disable ECN count verification for packets that arrive out of order (#2198, #2215)¶
• Use Probe Timeout (PTO) instead of RTO (#2206, #2238)¶
• Loosen constraints on retransmission of ACK ranges (#2199, #2245)¶
• Limit Retry and Version Negotiation to once per datagram (#2259, #2303)¶
• Set a maximum value for max_ack_delay transport parameter (#2282, #2301)¶
• Allow server preferred address for both IPv4 and IPv6 (#2122, #2296)¶
• Corrected requirements for migration to a preferred address (#2146, #2349)¶
• ACK of non-existent packet is illegal (#2298, #2302)¶

C.9. Since draft-ietf-quic-transport-16

• Stream limits are defined as counts, not maximums (#1850, #1906)¶
• Require amplification attack defense after closing (#1905, #1911)¶
• Remove reservation of application error code 0 for STOPPING (#1804, #1922)¶
• Renumbered frames (#1945)¶
• Renumbered transport parameters (#1946)¶
• Numeric transport parameters are expressed as varints (#1608, #1947, #1955)¶
• Reorder the NEW_CONNECTION_ID frame (#1952, #1963)¶

• Rework the first byte (#2006)¶

  • Fix the 0x40 bit¶
  • Change type values for long header¶
  • Add spin bit to short header (#631, #1988)¶
  • Encrypt the remainder of the first byte (#1322)¶
  • Move packet number length to first byte¶
  • Move ODCIL to first byte of retry packets¶
  • Simplify packet number protection (#1575)¶
• Allow STOP_SENDING to open a remote bidirectional stream (#1797, #2013)¶
• Added mitigation for off-path migration attacks (#1278, #1749, #2033)¶
• Don't let the PMTU to drop below 1280 (#2063, #2069)¶
• Require peers to replace retired connection IDs (#2085)¶
• Servers are required to ignore Version Negotiation packets (#2088)¶
• Tokens are repeated in all Initial packets (#2089)¶
• Clarified how PING frames are sent after loss (#2094)¶
• Initial keys are discarded once Handshake are available (#1951, #2045)¶
• ICMP PTB validation clarifications (#2161, #2109, #2108)¶

C.10. Since draft-ietf-quic-transport-15

Substantial editorial reorganization; no technical changes.¶

C.11. Since draft-ietf-quic-transport-14

• Merge ACK and ACK_ECN (#1778, #1801)¶
• Explicitly communicate max_ack_delay (#981, #1781)¶
• Validate original connection ID after Retry packets (#1710, #1486, #1793)¶
• Idle timeout is optional and has no specified maximum (#1765)¶
• Update connection ID handling; add RETIRE_CONNECTION_ID type (#1464, #1468, #1483, #1484, #1486, #1495, #1729, #1742, #1799, #1821)¶
• Include a Token in all Initial packets (#1649, #1794)¶
• Prevent handshake deadlock (#1764, #1824)¶

C.12. Since draft-ietf-quic-transport-13

• Streams open when higher-numbered streams of the same type open (#1342, #1549)¶
• Split initial stream flow control limit into 3 transport parameters (#1016, #1542)¶
• All flow control transport parameters are optional (#1610)¶
• Removed UNSOLICITED_PATH_RESPONSE error code (#1265, #1539)¶
• Permit stateless reset in response to any packet (#1348, #1553)¶
• Recommended defense against stateless reset spoofing (#1386, #1554)¶
• Prevent infinite stateless reset exchanges (#1443, #1627)¶
• Forbid processing of the same packet number twice (#1405, #1624)¶
• Added a packet number decoding example (#1493)¶
• More precisely define idle timeout (#1429, #1614, #1652)¶
• Corrected format of Retry packet and prevented looping (#1492, #1451, #1448, #1498)¶
• Permit 0-RTT after receiving Version Negotiation or Retry (#1507, #1514, #1621)¶
• Permit Retry in response to 0-RTT (#1547, #1552)¶
• Looser verification of ECN counters to account for ACK loss (#1555, #1481, #1565)¶
• Remove frame type field from APPLICATION_CLOSE (#1508, #1528)¶

C.13. Since draft-ietf-quic-transport-12

• Changes to integration of the TLS handshake (#829, #1018, #1094, #1165, #1190, #1233, #1242, #1252, #1450, #1458)¶

  • The cryptographic handshake uses CRYPTO frames, not stream 0¶
  • QUIC packet protection is used in place of TLS record protection¶
  • Separate QUIC packet number spaces are used for the handshake¶
  • Changed Retry to be independent of the cryptographic handshake¶
  • Added NEW_TOKEN frame and Token fields to Initial packet¶
  • Limit the use of HelloRetryRequest to address TLS needs (like key shares)¶
• Enable server to transition connections to a preferred address (#560, #1251, #1373)¶
• Added ECN feedback mechanisms and handling; new ACK_ECN frame (#804, #805, #1372)¶
• Changed rules and recommendations for use of new connection IDs (#1258, #1264, #1276, #1280, #1419, #1452, #1453, #1465)¶
• Added a transport parameter to disable intentional connection migration (#1271, #1447)¶
• Packets from different connection ID can't be coalesced (#1287, #1423)¶
• Fixed sampling method for packet number encryption; the length field in long headers includes the packet number field in addition to the packet payload (#1387, #1389)¶
• Stateless Reset is now symmetric and subject to size constraints (#466, #1346)¶
• Added frame type extension mechanism (#58, #1473)¶

C.14. Since draft-ietf-quic-transport-11

• Enable server to transition connections to a preferred address (#560, #1251)¶
• Packet numbers are encrypted (#1174, #1043, #1048, #1034, #850, #990, #734, #1317, #1267, #1079)¶
• Packet numbers use a variable-length encoding (#989, #1334)¶
• STREAM frames can now be empty (#1350)¶

C.15. Since draft-ietf-quic-transport-10

• Swap payload length and packed number fields in long header (#1294)¶
• Clarified that CONNECTION_CLOSE is allowed in Handshake packet (#1274)¶
• Spin bit reserved (#1283)¶
• Coalescing multiple QUIC packets in a UDP datagram (#1262, #1285)¶
• A more complete connection migration (#1249)¶
• Refine opportunistic ACK defense text (#305, #1030, #1185)¶
• A Stateless Reset Token isn't mandatory (#818, #1191)¶
• Removed implicit stream opening (#896, #1193)¶
• An empty STREAM frame can be used to open a stream without sending data (#901, #1194)¶
• Define stream counts in transport parameters rather than a maximum stream ID (#1023, #1065)¶
• STOP_SENDING is now prohibited before streams are used (#1050)¶
• Recommend including ACK in Retry packets and allow PADDING (#1067, #882)¶
• Endpoints now become closing after an idle timeout (#1178, #1179)¶
• Remove implication that Version Negotiation is sent when a packet of the wrong version is received (#1197)¶

C.16. Since draft-ietf-quic-transport-09

• Added PATH_CHALLENGE and PATH_RESPONSE frames to replace PING with Data and PONG frame. Changed ACK frame type from 0x0e to 0x0d. (#1091, #725, #1086)¶
• A server can now only send 3 packets without validating the client address (#38, #1090)¶
• Delivery order of stream data is no longer strongly specified (#252, #1070)¶
• Rework of packet handling and version negotiation (#1038)¶
• Stream 0 is now exempt from flow control until the handshake completes (#1074, #725, #825, #1082)¶
• Improved retransmission rules for all frame types: information is retransmitted, not packets or frames (#463, #765, #1095, #1053)¶
• Added an error code for server busy signals (#1137)¶
• Endpoints now set the connection ID that their peer uses. Connection IDs are variable length. Removed the omit_connection_id transport parameter and the corresponding short header flag. (#1089, #1052, #1146, #821, #745, #821, #1166, #1151)¶

C.17. Since draft-ietf-quic-transport-08

• Clarified requirements for BLOCKED usage (#65, #924)¶
• BLOCKED frame now includes reason for blocking (#452, #924, #927, #928)¶
• GAP limitation in ACK Frame (#613)¶
• Improved PMTUD description (#614, #1036)¶
• Clarified stream state machine (#634, #662, #743, #894)¶
• Reserved versions don't need to be generated deterministically (#831, #931)¶
• You don't always need the draining period (#871)¶
• Stateless reset clarified as version-specific (#930, #986)¶
• initial_max_stream_id_x transport parameters are optional (#970, #971)¶
• Ack Delay assumes a default value during the handshake (#1007, #1009)¶
• Removed transport parameters from NewSessionTicket (#1015)¶

C.18. Since draft-ietf-quic-transport-07

• The long header now has version before packet number (#926, #939)¶
• Rename and consolidate packet types (#846, #822, #847)¶
• Packet types are assigned new codepoints and the Connection ID Flag is inverted (#426, #956)¶
• Removed type for Version Negotiation and use Version 0 (#963, #968)¶

• Streams are split into unidirectional and bidirectional (#643, #656, #720, #872, #175, #885)¶

  • Stream limits now have separate uni- and bi-directional transport parameters (#909, #958)¶
  • Stream limit transport parameters are now optional and default to 0 (#970, #971)¶
• The stream state machine has been split into read and write (#634, #894)¶
• Employ variable-length integer encodings throughout (#595)¶

• Improvements to connection close¶

  • Added distinct closing and draining states (#899, #871)¶
  • Draining period can terminate early (#869, #870)¶
  • Clarifications about stateless reset (#889, #890)¶
• Address validation for connection migration (#161, #732, #878)¶
• Clearly defined retransmission rules for BLOCKED (#452, #65, #924)¶
• negotiated_version is sent in server transport parameters (#710, #959)¶
• Increased the range over which packet numbers are randomized (#864, #850, #964)¶

C.19. Since draft-ietf-quic-transport-06

• Replaced FNV-1a with AES-GCM for all "Cleartext" packets (#554)¶
• Split error code space between application and transport (#485)¶
• Stateless reset token moved to end (#820)¶
• 1-RTT-protected long header types removed (#848)¶
• No acknowledgments during draining period (#852)¶
• Remove "application close" as a separate close type (#854)¶
• Remove timestamps from the ACK frame (#841)¶
• Require transport parameters to only appear once (#792)¶

C.20. Since draft-ietf-quic-transport-05

• Stateless token is server-only (#726)¶
• Refactor section on connection termination (#733, #748, #328, #177)¶
• Limit size of Version Negotiation packet (#585)¶
• Clarify when and what to ack (#736)¶
• Renamed STREAM_ID_NEEDED to STREAM_ID_BLOCKED¶
• Clarify Keep-alive requirements (#729)¶

C.21. Since draft-ietf-quic-transport-04

• Introduce STOP_SENDING frame, RESET_STREAM only resets in one direction (#165)¶
• Removed GOAWAY; application protocols are responsible for graceful shutdown (#696)¶
• Reduced the number of error codes (#96, #177, #184, #211)¶
• Version validation fields can't move or change (#121)¶
• Removed versions from the transport parameters in a NewSessionTicket message (#547)¶
• Clarify the meaning of "bytes in flight" (#550)¶
• Public reset is now stateless reset and not visible to the path (#215)¶
• Reordered bits and fields in STREAM frame (#620)¶
• Clarifications to the stream state machine (#572, #571)¶
• Increased the maximum length of the Largest Acknowledged field in ACK frames to 64 bits (#629)¶
• truncate_connection_id is renamed to omit_connection_id (#659)¶
• CONNECTION_CLOSE terminates the connection like TCP RST (#330, #328)¶
• Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)¶

C.22. Since draft-ietf-quic-transport-03

• Change STREAM and RESET_STREAM layout¶
• Add MAX_STREAM_ID settings¶

C.23. Since draft-ietf-quic-transport-02

• The size of the initial packet payload has a fixed minimum (#267, #472)¶
• Define when Version Negotiation packets are ignored (#284, #294, #241, #143, #474)¶
• The 64-bit FNV-1a algorithm is used for integrity protection of unprotected packets (#167, #480, #481, #517)¶
• Rework initial packet types to change how the connection ID is chosen (#482, #442, #493)¶
• No timestamps are forbidden in unprotected packets (#542, #429)¶
• Cryptographic handshake is now on stream 0 (#456)¶
• Remove congestion control exemption for cryptographic handshake (#248, #476)¶
• Version 1 of QUIC uses TLS; a new version is needed to use a different handshake protocol (#516)¶
• STREAM frames have a reduced number of offset lengths (#543, #430)¶

• Split some frames into separate connection- and stream- level frames (#443)¶

  • WINDOW_UPDATE split into MAX_DATA and MAX_STREAM_DATA (#450)¶
  • BLOCKED split to match WINDOW_UPDATE split (#454)¶
  • Define STREAM_ID_NEEDED frame (#455)¶
• A NEW_CONNECTION_ID frame supports connection migration without linkability (#232, #491, #496)¶

• Transport parameters for 0-RTT are retained from a previous connection (#405, #513, #512)¶

  • A client in 0-RTT no longer required to reset excess streams (#425, #479)¶
• Expanded security considerations (#440, #444, #445, #448)¶

C.24. Since draft-ietf-quic-transport-01

• Defined short and long packet headers (#40, #148, #361)¶
• Defined a versioning scheme and stable fields (#51, #361)¶
• Define reserved version values for "greasing" negotiation (#112, #278)¶
• The initial packet number is randomized (#35, #283)¶
• Narrow the packet number encoding range requirement (#67, #286, #299, #323, #356)¶
• Defined client address validation (#52, #118, #120, #275)¶
• Define transport parameters as a TLS extension (#49, #122)¶
• SCUP and COPT parameters are no longer valid (#116, #117)¶
• Transport parameters for 0-RTT are either remembered from before, or assume default values (#126)¶
• The server chooses connection IDs in its final flight (#119, #349, #361)¶
• The server echoes the Connection ID and packet number fields when sending a Version Negotiation packet (#133, #295, #244)¶
• Defined a minimum packet size for the initial handshake packet from the client (#69, #136, #139, #164)¶
• Path MTU Discovery (#64, #106)¶
• The initial handshake packet from the client needs to fit in a single packet (#338)¶
• Forbid acknowledgment of packets containing only ACK and PADDING (#291)¶
• Require that frames are processed when packets are acknowledged (#381, #341)¶
• Removed the STOP_WAITING frame (#66)¶
• Don't require retransmission of old timestamps for lost ACK frames (#308)¶
• Clarified that frames are not retransmitted, but the information in them can be (#157, #298)¶
• Error handling definitions (#335)¶
• Split error codes into four sections (#74)¶
• Forbid the use of Public Reset where CONNECTION_CLOSE is possible (#289)¶
• Define packet protection rules (#336)¶
• Require that stream be entirely delivered or reset, including acknowledgment of all STREAM frames or the RESET_STREAM, before it closes (#381)¶
• Remove stream reservation from state machine (#174, #280)¶
• Only stream 1 does not contribute to connection-level flow control (#204)¶
• Stream 1 counts towards the maximum concurrent stream limit (#201, #282)¶
• Remove connection-level flow control exclusion for some streams (except 1) (#246)¶
• RESET_STREAM affects connection-level flow control (#162, #163)¶
• Flow control accounting uses the maximum data offset on each stream, rather than bytes received (#378)¶
• Moved length-determining fields to the start of STREAM and ACK (#168, #277)¶
• Added the ability to pad between frames (#158, #276)¶
• Remove error code and reason phrase from GOAWAY (#352, #355)¶
• GOAWAY includes a final stream number for both directions (#347)¶
• Error codes for RESET_STREAM and CONNECTION_CLOSE are now at a consistent offset (#249)¶
• Defined priority as the responsibility of the application protocol (#104, #303)¶

C.25. Since draft-ietf-quic-transport-00

• Replaced DIVERSIFICATION_NONCE flag with KEY_PHASE flag¶
• Defined versioning¶
• Reworked description of packet and frame layout¶
• Error code space is divided into regions for each component¶
• Use big endian for all numeric values¶

C.26. Since draft-hamilton-quic-transport-protocol-01

• Adopted as base for draft-ietf-quic-tls¶
• Updated authors/editors list¶
• Added IANA Considerations section¶
• Moved Contributors and Acknowledgments to appendices¶