An ECN Extension to CONNECT-UDP

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Table of Contents

1. Introduction

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

2. Context Identifiers

The "Context Identifiers" section of [CONNECT-UDP] defines the concept of context IDs and how they can be used to extend CONNECT-UDP. When a client wishes to use ECN with CONNECT-UDP, it generates an unique client-allocated context ID. In this document, we'll refer to that context ID as the "chosen context ID". Note that, by definition of client-allocated context IDs, the chosen context ID will always be a non-zero even number. We also add the restriction that the chosen context ID MUST be strictly less than 10¹⁵. If the client has run out of available context ID values that match this requirement for this CONNECT-UDP request, it MUST NOT use the ECN extension with this CONNECT-UDP request.¶

3. ECN Header Definition

The "ECN" header field is an Item Structured Field, see Section 3.3 of [STRUCT-FIELD]; its value MUST be a Integer; any other value type MUST be handled as if the field were not present by recipients (for example, if this field is included multiple times, its type will become a List and the field will therefore be ignored). This document does not define any parameters for the "ECN" header field value, but future documents might define parameters. Receivers MUST ignore unknown parameters.¶

When present, the "ECN" header indicates that the sender supports this extension, and communicates the chosen context ID as the "ECN" field value.¶

For example, if the client chosen context ID is 42, it would send the following:¶

ECN: 42; foo=bar

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. When indicating support for this extension, the proxy send the client's chosen context ID as the "ECN" field value.¶

For example, the proxy could reply with:¶

ECN: 42

4. Encoding of ECN bits

When an HTTP Datagram [HTTP-DGRAM] associated with a CONNECT-UDP stream uses the chosen context ID as its context ID, its "Payload" field contains the following format (using the notation from the "Notational Conventions" section of [QUIC]):¶

CONNECT-UDP Payload with ECN {
  Must be Zero (6),
  ECN Bits (2),
  UDP Payload (..),
}

Must be Zero:

6 bits that MUST be sent as zero. Receivers MUST validate that these bits are zero and MUST silently drop the HTTP Datagram if they have any other value. Extensions to this mechanism MAY relax this requirement.¶

ECN Bits:

The ECN bits, sent in the same order as they appear in the IP header.¶

UDP Payload:

The UDP Payload, as defined in the "HTTP Datagram Payload Format" section of [CONNECT-UDP].¶

When the proxy receives a datagram with the chosen context ID, it sets the IP packet's ECN bits accordingly on the UDP packet it sends to the target. Similarly, in the other direction the ECN Bits field represents which ECN bits were seen on the UDP packets received from the target.¶

5. A Note about Future Extensions

This CONNECT-UDP extension uses an HTTP field to register its chosen context ID. Future extensions to CONNECT-UDP can use the same strategy to register their chosen context ID(s) via another HTTP field. This strategy is best for CONNECT-UDP extensions that only need to register context IDs during the HTTP request and response.¶

Some extensions may need to register context IDs after the request and response have been exchanged, for example an extension that wishes to compress QUIC connection IDs [QUIC] is not aware of all connection IDs at request time. In such cases, extensions can use new Capsule Types (see [HTTP-DGRAM]) to perform context ID registration.¶

6. Security Considerations

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

7. IANA Considerations

This document will request IANA to register the following entry in the "HTTP Field Name" registry:¶

Field Name:

ECN¶

Template:

None¶

Status:

provisional (permanent if this document is approved)¶

Reference:

This document¶

Comments:

None¶

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