Hypertext Transfer Protocol Working Group C. Benfield
Internet-Draft October 09, 2015
Intended status: Standards Track
Expires: April 11, 2016

Peer-to-peer Extension to HTTP/2
draft-benfield-http2-p2p-02

Abstract

This document introduces a negotiated extension to HTTP/2 that turns a single HTTP/2 connection into a bi-directional communication channel.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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Table of Contents

1. Introduction

The HTTP/2 [RFC7540] specification provides an alternative framing layer for the semantics of HTTP/1.1 [RFC7231]. This framing layer in principle allows for both parties in a HTTP/2 session to send requests and responses. However, the HTTP/2 specification also requires that the semantics of HTTP/1.1 be preserved. This means that one party of the conversation is considered the client, and one the server. Only the client may send requests, and only the server may send responses.

This document introduces an extension that can be advertised by a HTTP/2 client. This extension allows both the client and the server to send requests and responses. Essentially, this extension changes the protocol such that the notion of ‘client’ and ‘server’ are defined on a per-stream basis, rather than a per-connection basis.

The principle of this extension is similar to the Reverse HTTP [I-D.lentczner-rhttp] proposal made in 2009. HTTP/2’s framing makes this a substantially more flexible extension than Reverse HTTP by allowing the client and server to vary on a per-stream basis, rather than affecting the whole connection.

1.1. Notational Conventions

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Terminology

The nature of this specification is that which peer is a ‘client’ and which is a ‘server’ changes from stream-to-stream. Therefore, the terms ‘listener’ and ‘dialer’ are introduced to unambiguously refer to peers.

The ‘dialer’, or dialing peer, is the peer that initiated the HTTP/2 connection. In a standard, non-peer-to-peer HTTP/2 transaction, the ‘dialer’ and the ‘client’ are the same.

The ‘listener’, or listening peer, is the peer that accepted the HTTP/2 connection. In a standard, non-peer-to-peer HTTP/2 transaction, the ‘listener’ and the ‘server’ are the same.

‘Client’ and ‘server’ are defined on a per-stream basis, following the rules in Section 2.3.1.

2. Additions to HTTP/2

This document introduces a new HTTP/2 setting ([RFC7540], Section 11.3) and a new HTTP/2 frame type ([RFC7540], Section 11.2), to allow for a HTTP/2 dialer to advertise its support for receiving server-initiated streams, and to allow a listener to advertise its support for receiving client-initiated pushed streams.

2.1. SETTINGS_PEER_TO_PEER Setting

The following new SETTINGS parameters ([RFC7540], Section 6.5.2) are defined:

2.2. CLIENT_AUTHORITY Frame

This document introduces the CLIENT_AUTHORITY frame. This frame MUST be emitted by a dialer after it sends a value of SETTINGS_PEER_TO_PEER of 1, and MUST NOT be emitted by a dialer any time after. The purpose of this frame is to allow a dialer to advertise the authority or authorities for which it is prepared to accept requests.

This frame always applies to a whole connection. Therefore, the stream identifier for CLIENT_AUTHORITY frames MUST be 0. If a listener receives a CLIENT_AUTHORITY frame whose stream identifier field is anything other than 0, it MUST respond with a connection error ([RFC7540] Section 5.4.1) of type PROTOCOL_ERROR.

2.2.1. Payload

Each CLIENT_AUTHORITY frame is made up of one or more of the following authority segments:

    +----------------------+
    | Authority Length (8) |
    +----------------------+----------------------------------+
    |                      Authority (*)                      |
    +---------------------------------------------------------+

Figure 1: Client Authority Frame Payload

Each segment begins with a one-byte field indicating the length of the authority string the client is asserting. That field is then followed by a single authority field. The authority MUST be sent in whatever character encoding is going to be expected by the dialer on receipt of the :authority pseudo-header field.

2.2.2. Semantics

Generally speaking, a listener or coalescing intermediary has no in-band method of validating that a dialer’s authority claims are valid. Therefore, a conforming listener MUST confirm a dialer’s authority claims using some out-of-band method: see Section 3 for more.

A dialer MUST NOT send a CLIENT_AUTHORITY frame after the first one. The CLIENT_AUTHORITY frame is considered to be a complete list of authorities: therefore, a dialer MUST start a new connection if it would like to change the list of authorities it claims.

2.3. HTTP Changes

From the perspective of other HTTP RFCs, such as RFC 7231 [RFC7231] and RFC 7540 [RFC7540], this extension changes whether a peer is considered a ‘client’ or a ‘server’ on a per-stream basis, instead of a per-connection basis, based on which peer opened the stream and how they did so.

The rest of the requirements of RFC 7231 [RFC7231] are preserved.

2.3.1. Client and Server

For the purpose of the rest of this document, ‘client’ and ‘server’ are defined on a per-stream basis. For a stream that is opened by means of a HEADERS frame, the peer that sent the initial headers frame is ‘client’ and the other peer is ‘server’. For a stream that is opened by means of a PUSH_PROMISE frame, the peer that sent the PUSH_PROMISE frame is ‘server’ and the other peer is ‘client’.

2.3.2. Stream IDs

RFC 7540 [RFC7540] Section 5.1.1 applies restrictions on what stream IDs MUST be used by a given peer.

This document amends that section to state that streams initiated by a dialer MUST use odd-numbered stream identifiers, and streams initiated by a listener MUST use even-numbered stream identifiers. This ensures that there will be no conflict when both peers are actively creating streams.

The other limitations of RFC 7540 [RFC7540] Section 5.1.1 continue to apply.

2.4. Dialer Behavioral Changes

When a dialer emits the SETTINGS_PEER_TO_PEER setting with a value of 1, it is informing the listener that it is willing to accept HTTP requests from the server, allowing the listener to open streams with HEADERS frames. This lifts some of the restrictions of RFC 7540 [RFC7540] Section 8.

If a dialer has sent the SETTINGS_PEER_TO_PEER setting with a value of 1, the dialer MUST NOT reject an attempt by the listener to change the value of SETTINGS_ENABLE_PUSH to 1.

If the dialer, subsequent to sending SETTINGS_PEER_TO_PEER with value 1, receives from the listener a value of SETTINGS_ENABLE_PUSH of 1, it MAY open streams by sending PUSH_PROMISE frames.

2.5. Listener Behavioral Changes

When a listener receives the SETTINGS_PEER_TO_PEER setting from the dialer with a value of 1, it MAY at any point afterwards issue a non-zero value for SETTINGS_ENABLE_PUSH. This allows dialers to open streams with PUSH_PROMISE, subject to some limitations (see Section 2.6), and also lifts some of the restrictions of RFC 7540 [RFC7540] Section 8: specifically those sections that only allow listeners to send PUSH_PROMISE frames, and only allow dialers to receive them.

A HTTP/2 listener, before receiving SETTINGS_PEER_TO_PEER, must have SETTINGS_ENABLE_PUSH equal to 0, as per [RFC7540] Section 8.2. However, once a listener has received SETTINGS_PEER_TO_PEER, it MAY set SETTINGS_ENABLE_PUSH equal to 1. If it does not, it is assumed that SETTINGS_ENABLE_PUSH remains at 0, and the listener is unwilling to accept pushed streams.

2.6. PUSH_PROMISE

Whichever peer is client on a given stream MUST NOT send PUSH_PROMISE frames on that stream. All other limitations about PUSH_PROMISE frames in RFC 7540 [RFC7540] continue to apply.

If a peer attempts to send a PUSH_PROMISE frame on a stream in which it is the client, the peer that is server for that stream MUST treat this event as a connection error ([RFC7540] Section 5.4.1) of type PROTOCOL_ERROR.

2.7. Other Extensions

When this extension is deployed with other extensions to HTTP/2, the behaviour of this extension does not change. All other extensions that refer to ‘client’ or ‘server’ SHOULD be treated as though those terms apply on a per-stream basis.

If other extensions apply ‘server’ or ‘client’ to the whole connection (e.g. for settings in SETTINGS frames, which are sent on stream 0), then both peers SHOULD be considered clients and both peers should be considered servers.

3. Authority Validation

Generally speaking, a listener or coalescing intermediary has no in-band method of validating that a dialer’s authority claims are valid. Therefore, a conforming listener MUST confirm a dialer’s authority claims using some out-of-band method.

This specification does not lay out in detail any proposed mechanism for doing this validation, as the best approach may vary from deployment to deployment. However, some options include:

The only requirement is that a listener MUST implement some form of validation, and then MUST treat any attempt by a dialer to assert an authority that it cannot validate as a connection error ([RFC7540] Section 5.4.1) of type PROTOCOL_ERROR.

4. IANA Considerations

4.1. HTTP/2 Frame Type Registry Update

This document updates the HTTP/2 Frame Type registry ([RFC7540], Section 11.2). The entries in the following table are registered by this document.

Name Code Section
CLIENT_AUTHORITY TBD Section 2.2

4.2. HTTP/2 Settings Registry Update

This document updates the registry for HTTP/2 Settings ([RFC7540], Section 11.4). The entries in the following table are registered by this document.

Name Code Initial Value Section
PEER_TO_PEER TBD 0 Section 2.1

5. Acknowledgements

Thanks to David Dias, Juan Benet, and Fedor Indutny for the original idea, and Amos Jeffries, Mike Bishop, and Ilari Liusvaara for their follow-up.

Thanks also to Tyrel Souza, Donald Stufft, and Paul Kehrer for proofreading.

Thanks to David Reid for pointing out the Reverse HTTP proposal [I-D.lentczner-rhttp].

Thanks to Amos Jeffries for proposing an advertised extension, rather than a negotiated one.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[RFC7540] Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015.

6.2. Informative References

[I-D.lentczner-rhttp] Lentczner, M. and D. Preston, "Reverse HTTP", Internet-Draft draft-lentczner-rhttp-00, March 2009.

Appendix A. Changelog

(This appendix to be deleted by the RFC Editor.)

Since -01:

Since -00:

Author's Address

Cory Benfield EMail: cory@lukasa.co.uk