An ECN Extension to CONNECT-UDP

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Table of Contents

1. Introduction

The CONNECT-UDP [CONNECT-UDP] method allows proxying UDP packets over HTTP. This document describes an extension to CONNECT-UDP that allows conveying ECN [ECN] information on proxied UDP packets.¶

Discussion of this work is encouraged to happen on the MASQUE IETF mailing list (masque@ietf.org) or on the GitHub repository which contains the draft: https://github.com/DavidSchinazi/draft-connect-udp-ecn.¶

2. ECN Header Definition

"ECN" is a Item Structured Header [STRUCT-HDR]. Its value MUST be a Boolean. Its ABNF is:¶

  ECN = sf-boolean

The "ECN" header indicates whether the sender supports this extension. A value of 1 indicates support whereas a value of 0 (or the absence of the header) indicates lack of support.¶

Clients MUST NOT indicate support for this extension unless they know that the protocol running over UDP that is being proxied supports ECN, and will react appropriately to Congestion Experienced (CE) markings.¶

Proxies MUST NOT indicate support for this extension unless they know they have the ability to read and write the IP ECN bits on its target-bound UDP sockets.¶

This extension is said to have been negotiated when both client and proxy indicated support for it in their CONNECT-UDP request and response.¶

3. Datagram Encoding of Proxied UDP Packets

If a client supports this extension and HTTP/3 datagrams [H3DGRAM], it can attempt to use datagrams for ECN information. This is done by allocating four datagram flow identifiers (as opposed to one in traditional CONNECT-UDP) and communicating them to the proxy using parameters on the "Datagram-Flow-Id" header. These parameters are "ect0", "ect1" and "ce". For example:¶

  Datagram-Flow-Id = 100; ect0=102; ect1=104; ce=106

If the proxy wishes to support datagram encoding of this extension, it echoes those parameters in its CONNECT-UDP response. The main flow identifier now represents Not-ECT, whereas the one in "ect0" represents ECT(0), "ect1" represents ECT(1) and "ce" represents CE; see Section 5 of [ECN] for the definition of these IP header fields.¶

When the proxy receives a datagram from the given flow identifier, it sets the IP packet's ECN bits accordingly on the UDP packet it sends to the target. Similarly, in the other direction the flow identifier represents which ECN bits were seen on the UDP packets received from the target.¶

4. Stream Encoding of Proxied UDP Packets

If HTTP/3 datagrams are not supported, the stream is used to convey UDP payloads, and the CONNECT-UDP Stream Chunk Type is used to indicate the values of the ECN bits, as defined below:¶

  +-------+-----------------+-----------+
  | Value |      Type       | ECN Field |
  +-------+-----------------+-----------+
  | 0x00  | UDP_PACKET      |  Not-ECT  |
  +-------+-----------------+-----------+
  | 0x31  | UDP_PACKET_ECT0 |  ECT(0)   |
  +-------+-----------------+-----------+
  | 0x32  | UDP_PACKET_ECT1 |  ECT(1)   |
  +-------+-----------------+-----------+
  | 0x33  | UDP_PACKET_CE   |  CE       |
  +-------+-----------------+-----------+

The proxy then uses the the CONNECT-UDP Stream Chunk Type on received UDP payloads to set the ECN bits on the IP packets it sends to the target, and in the reverse direction to indicate which ECN bits received from the target.¶

5. HTTP Intermediaries

HTTP/3 DATAGRAM flow identifiers are specific to a given HTTP/3 connection. However, in some cases, an HTTP request may travel across multiple HTTP connections if there are HTTP intermediaries involved; see Section 2.3 of [RFC7230].¶

Intermediaries that support this extension and HTTP/3 datagrams MUST negotiate flow identifiers separately on the client-facing and server-facing connections. This is accomplished by having the intermediary parse the "Datagram-Flow-Id" header on all CONNECT-UDP requests it receives, and sending the same value in the "Datagram-Flow-Id" header on the response. The intermediary will perform this individualy for all the parameters defined by this extension as well, in addition to the rules in the "HTTP Intermediaries" section of [CONNECT-UDP].¶

6. Security Considerations

This document does not have additional security considerations beyond those defined in [CONNECT-UDP].¶

7. IANA Considerations

  +-------------------+----------+--------+---------------+
  | Header Field Name | Protocol | Status |   Reference   |
  +-------------------+----------+--------+---------------+
  |        ECN        |   http   |  std   | This document |
  +-------------------+----------+--------+---------------+

  +-------+-----------------+-------------------------+---------------+
  | Value |      Type       |       Description       |   Reference   |
  +-------+-----------------+-------------------------+---------------+
  | 0x31  | UDP_PACKET_ECT0 | UDP payload with ECT(0) | This document |
  +-------+-----------------+-------------------------+---------------+
  | 0x32  | UDP_PACKET_ECT1 | UDP payload with ECT(1) | This document |
  +-------+-----------------+-------------------------+---------------+
  | 0x33  | UDP_PACKET_CE   | UDP payload with CE     | This document |
  +-------+-----------------+-------------------------+---------------+

Acknowledgments

This proposal was inspired directly or indirectly by prior work from many people. The author would like to thank contributors the MASQUE working group.¶

Author's Address