| Internet-Draft | BDP_Frame Extension | October 2023 |
| Kuhn, et al. | Expires 24 April 2024 | [Page] |
This document describes the BDP_FRAME extension for QUIC. The frame enables the exchange of Congestion Control (CC) parameters related to the path characteristics between the receiver and the sender during a connection. These CC parameters allow a receiver to prepare to use any additional capacity that is made available when using Careful Resume. It also allows a receiver to request that previously computed CC parameters, are not used when the receiver has additional information about the current path or traffic that is to be sent.¶
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This document extends the Careful Resume method [I-D.ietf-tsvwg-careful-resume] to allow sender-generated CC parameters to be stored at the receiver. Transfering the CC parameters to a receiver, can release the sender from needing to retain CC parameter state for each receiver. This also allows a receiver to implement a policy that informs a sender whether the receiver desires the sender to reuse previously saved CC parameters.¶
It defines the method to exchange the CC parameters between a QUIC receiver and the sender in an interoperable manner. The process is outlined here:¶
For an established connection, the current RTT, measured capacity and receiver Endpoint Token are computed and are respectively stored as the saved_rtt, saved_capacity and saved_endpoint_token. These are placed within a BDP_FRAME. The sender also computes a secured hash over these CC parameters and sends this within the BDP_FRAME.¶
The receiver can read the non-encrypted portion of the BDP_FRAME parameters. It cannot verify the hash and is not permitted to modify any CC parameters.¶
At a later time, the receiver sends the BDP-FRAME to the sender to re-use previously computed CC parameters.¶
Upon reception, the sender MUST verify the hash, and only use the CC parameters when valid. This is to prevent a receiver from modifying the parameters. The sender is able to utilise the CC parameters in a received BDP_FRAME when it is connected to the same endpoint.¶
This section reviews three approaches to implement the storage of CC parameters:¶
(1) The saved CC parameters are stored at the sender ("Local storage") and are not sent to a receiver, this does not use a BDP_Frame;¶
(2) The saved CC parameters are transmitted to the receiver in a BDP_FRAME, these can be used when reconnecting, but a receiver does not read the CC parameters;¶
(3) The saved CC parameters are transmitted to a receiver in a BDP_Frame, which can use these when reconnecting. The receiver can also read, but not modify, the CC parameters to accept or not the use of these parameters.¶
The use of the BDP_Frame is negotiated with the receiver.¶
A sender can send a BDP_FRAME to the receiver with CC parameters that were collected during the Careful Resume Observe Phase.¶
The receiver can read the BDP_Frame content and later decide to send the BDP_FRAME back to the sender in the header of an Initial packet, or later in a connection. This frame needs to be received during the Reconaisance Phase of a Careful Resume sender.¶
Where the receiver is aware it is using a path with high Bandwidth-Delay Product (BDP), it can also adapt other protocol parameters to better utilize the available capacity, e.g., to estimate a larger size for the flow credit.¶
Some designs of application do not use long-lasting transport connections. Instead, they use a series of shorter connections, typically each using the same path. This style of application can benefit when the receiver provides an estimate of the expected characteristics (e.g., to adapt the content of quality for a video application; or anticipate the time taken to complete the transmission of an object). An example scenario considers a client using Dynamic Adaptive Streaming over HTTPS (DASH) that is unable to receive sufficient data to reach the desired video playback quality supported by the path, because for each video chunk, the transport CC method needs to independently determine the path capacity. In this example, a lower transfer rate is safe, but could lead to an overly conservative requested rate when the rate is based on the transport performance (i.e., the client could fail to increase the requested quality of video chunks, or to fill buffers to avoid stalling playback or to send high quality advertisements). These client requests could then be adapted based on the previously observed path characteristics, enabling a client to increase the requested quality of video chunks, to fill receiver buffers and avoid stalling playback.¶
BDP: Bandwdith Delay Product of the path (maximum path capacity);¶
saved_capacity: The capacity preserved from a previous connection;¶
saved_rtt: The preserved minimum RTT, corresponding to the minimum of a set RTT of measurements taken at the time when the saved_capacity was estimated;¶
endpoint_token: An Endpoint Token for a receiver;¶
secured hash: hash generated by the sender using a list of CC parameters that it selected. The sender uses a private key to protect the hash.¶
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.¶
Variable-length integer encoding is defined in section 16 of [RFC9000].¶
This section describes the use of a new QUIC Frame, the BDP_FRAME. This can be utilized by the congestion controller in the Careful Resume method.¶
Sending a BDP_FRAME is not be limited by flow control limits. The sender and the receiver MAY send multiple BDP_FRAMEs in both 1-RTT and 0-RTT connections. The rate of update SHOULD be limited (e.g., much less frequent than once for several RTTs).¶
The format of a BDP_FRAME is shown in Figure 1.¶
BDP_FRAME {
Type (i) = 0xXXX,
Hash (...),
Lifetime (i),
Saved Capacity (i),
Saved RTT (i),
Saved Endpoint Token (...)
}
This contains the following fields:¶
Hash (secured_hash): The secured_hash is generated by the sender using CC parameters from the BDP_FRAME. The sender encrypts the hash to prevent the receiver from reading it, and allow the sender bto verify that the contents were not changed.¶
Lifetime (extension_lifetime): The extension_lifetime is a value in milliseconds, encoded as a variable-length integer. This follows the design of a NewSessionTicket of TLS [RFC8446]. This represents the validity in time of this extension.¶
Saved capacity estimate (saved_capacity): The saved_capacity is a value in bytes, encoded as a variable-length integer. The available capacity can be estimated for the previous connection by the sender. Using the previous values of bytes_in_flight defined in [RFC9002] can result in overshoot of the bottleneck capacity, and ought to be used carefully. It is advised to not only use the amount of bytes in flight, but also the goodput.¶
Saved RTT (saved_rtt): The saved_rtt is a value in milliseconds, encoded as a variable-length integer. The saved_rtt is set to the min_rtt. NOTE: The min_rtt defined in [RFC9002], does not track a decreasing RTT: therefore the min_rtt reported might be larger than the actual minimum RTT measured during the 1-RTT connection.¶
Saved Endpoint Token (saved_endpoint_token) : Note: The Endpoint Token is defined in [I-D.ietf-tsvwg-careful-resume], and is discussed in the context of this protocol exchange in a later section.¶
The receiver can accept the transmission of BDP_FRAMEs from the sender by using the enable_bdp transport extension.¶
enable_bdp (0xTBD): in the 1-RTT connection, the receiver indicates to the sender that it wishes to receive BDP extension Frames. The default value is 0. In this specification, enable_bdp values larger than 3 are reserved for future, and the receipt of these values MUST be treated as a connection error of type TRANSPORT_PARAMETER_ERROR [RFC9000].¶
0: Default value. If the receiver does not send an enable_bdp parameter, the sender considers that the receiver does not support, or does not wish to activate, the BDP extension.¶
1: The receiver indicates to the sender that it wishes to receive BDP_FRAMEs and activates the optimization.¶
2: The receiver indicates that it does not wish to receive BDP Frames, but activates the optimization.¶
3: The receiver indicates that it wishes to receive BDP_FRAMEs, but does not activate the optimization.¶
This Transport Parameter is encoded as described in Section 18 of [RFC9000].¶
If the receiver activates the extension, it agrees to receive and read BDP_FRAMEs. If the receiver activates the optimization, it allows the sender to utilise the previously computed CC parameters. A receiver could then perform session resumption optimization without actually reading the previous CC parameters.¶
Care is needed in the use of any temporal information to assure safe use of the Internet and to be robust to changes in traffic patterns, network routing and link/node failures. There are also cases where using the CC parameters of a previous connection are not appropriate, and a need to evaluate the potential for malicious use of the method. The specification for the QUIC transport protocol [RFC9000] therefore notes "Generally, implementations are advised to be cautious when using previous values on a new path."¶
Careful exploitation of the CC parameters is discussed in [I-D.ietf-tsvwg-careful-resume].¶
A receiver using the BDP_FRAME extension has the choice of allowing the reuse of the previous CC parameters, or requesting the sender to not reuse these parameters.¶
This extension MUST NOT provide an opportunity for the current connection to be a vector for an amplification attack. The QUIC address validation process, used to prevent amplification attacks, SHOULD be performed [RFC9000].¶
The CC parameters are measured by the sender by observing a previous connection to the same receiver. During transmission, the BDP_Frame is protected by the mechanism that protects the exchange of the 0-RTT transport parameters. For version 1 of QUIC, the this is protected using the mechanism that already protects the "initial_max_data" parameter. This is defined in sections 4.5 to 4.7 of [RFC9001]. It provides a way for the sender to verify that the CC parameters proposed by the receiver are the same as those that the sender sent to the receiver during a previous connection.¶
Although a BDP_Frame is encrypted on transmission, a receiver could modify the value of the received CC parameters. The sender detects this by adding a secured hash over the CC parameters. If the receiver modifies any CC parameter, the result of the hash would be different. If a sender fails to veridy the secured hash, it MUST not use the CC parameters.¶
An example of an implementation where the sender computes an Endpoint Token that seeks to uniquely identify the receiver is provided in Section 4.3. The implementation details are independent from the specification of BDP_Frame.¶
Four cases are identified where Careful Resume is not appropriate and using the saved CC parameters could increase congestion:¶
The network path has changed and the new path is different. This might be evident from a change of local interface, a change in the client or sender IP address, or similar indication from the network. The saved CC parameters are not appropriate.¶
The network path has changed, but the new path is not known to be different. This case might be accompanied by a change in the RTT, or evident by loss observed at the start of the new connection. This case could be detected in the Careful Resume Reconaissance Phase. The saved CC parameters are not appropriate to the current path.¶
The network conditions have changed and it is discovered that the current capacity is less than the previously estimated bottleneck capacity. Using the saved CC parameters would then increase congestion, and the flow needs to adjust to a lower safe rate.¶
The stored CC parameters are too old. It is no longer reasonable to expect the path to have same characteristics, and the the saved CC parameters are not appropriate.¶
In all these cases, a sender needs to return to the normal CC behavior. The method can still be used to characterize the new path, enabling later flows to use the Careful Resume method.¶
In a simple network scenario, the sending endpoint could use the IP source address to identify a path. This could work when one globally-allocated IP address is set per interface. There are many cases where the IP address would not an acceptable to identify a path. Section 8 of [RFC9040] describes cases where the IP address is not a suitable value when performing TCP control block sharing. In general the IP address of the sender is made public in the network-layer header of IP packets. When sharing internal state, [RFC6973] identifies relevant privacy considerations.¶
Examples of network uses where a source address is not a suitable endpoint token include:¶
The sending endpoint might not be identifiable remotely from its IP address because a device on the network path translates the address using a form of NAT/NAPT. In this case, a private IP address might be used, which does not identify a specific endpoint.¶
In some cases, a sender can choose to vary the source address over time to avoid linkability in the observable IP header, e.g., when the source address embeds private information, such as an endpoint's MAC address/EIDID.¶
Note: There are use-cases where for the purpose of identifying a path, the token does not need to be globally unique, but needs to be sufficiently unique to prevent attempts to misrepresent the path being used such as an attack on the congestion controller. Using a smaller size of token can add to the ambiguity set, reducing this likability.¶
NOTE: A different Endpoint Token is used for each direction of transmission. A receiver might decide not to provide an Endpoint Token to a sender, to avoid exposing additional linkable information (but also preventing use of any mechanism that relies on the token).¶
The sender computes an Endpoint Token that seeks to uniquely identify the path that it uses to communicate with the receiver (1) this is associated with the path information it sends. The Endpoint Token ought to be encrypted to avoid sending linkable information observable to eavesdroppers on the path. The receiver stores the path information together with the Endpoint Token, together with the sender's address/name (2). When the receiver later wishes the sender to use the stored path information it returns the information to the sender (3) together with the Endpoint Token. The sender recomputes the Endpoint Token and compares this with the received Endpoint Token before using the CC parameters.¶
A number of security-related topics have been identified concerning the potential exposure of the identity on the path. This information can also be visible in the IP source address or higher-layer data, but can be hidden from a remote endpoint using methods such as MASQUE proxy. When used to inform the transport system using a layered proxy, the transport endpoint token refers to the endpoints of the outer QUIC header, and hence the proxy itself, not the end-to-end communication relayed by the proxy.¶
A sender might decide to not use this method if it has a strong requirement to prevent flows being linkable with previous flows to the same endpoint. A decision not to provide an Endpoint Token necessarily prevents the sender from requesting the receiver to return path information to allow the same CC parameters to be re-used, potentially strengthening privacy but consequently eliminating any performance benefits.¶
The authors thank Gabriel Montenegro, Patrick McManus, Ian Swett, Igor Lubashev, Robin Marx, Roland Bless and Franklin Simo for their fruitful comments on earlier versions of this document.¶
The authors particularly thank Tom Jones for co-authoring previous versions of this document.¶
{XXX-Editor note: Text is required to register the BDP_Frame and the enable_bdp transport parameter. Parameters are registered using the procedure defined in [RFC9000].}¶
TBD: Text is required to register the BDP_FRAME and the enable_bdp transport parameter. Parameters are registered using the procedure defined in [RFC9000].¶
Security considerations for the CC method are discussed in the Security Considerations section of Careful Resume.¶
The sender MUST check the integrity of the saved_rtt and saved_capacity parameters received from a receiver.¶
There are several solutions to avoid attacks by malicious receivers:¶
Solution #1 : The sender stores a local estimate of the capacity and RTT parameters as the saved_capacity and saved_rtt.¶
Solution #2 : The sender sends the estimate of the capacity and RTT parameters to the receiver as the saved_capacity and saved_rtt in a block of CC parameters that is authenticated by the secure hash including local key information. The parameters may be encryptedd and are not read by the rceeiver. The receiver resends the same CC parameters for a new connection. The sender can use its local key information to authenticate the CC parameters, without needing to keep a local copy of the parameters.¶
Solution #3 : This approach is the same as above, except the receiver can read the saved_rtt and saved_capacity parameters and could enable a receiver to decide whether the new CC parameters are thought appropriate, based on receiver-side information about the network conditions, connectivity, or needs of the new connection. The sender will verify that the receiver did not change the CC parameters inside the frame.¶
This section to be rmeoved prior to publication.¶
-00 Introduced session tickets and BDP_Frames¶
-01 Reviewed receiver actions when a receiver holds CC parameters¶
-02 Interim version to align with terminology in Careful Resume¶
-03 Rewritten to align with Careful Resume and use the BDP_Frame method. Removed annexe 1, and discussion of session tickets, prefering BDP Frames.¶