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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Additions to HTTP/2 . . . . . . . . . . . . . . . . . . . . . 3
2.1. SETTINGS_PEER_TO_PEER Setting . . . . . . . . . . . . . . 3
2.2. CLIENT_AUTHORITY Frame . . . . . . . . . . . . . . . . . 4
2.2.1. Payload . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.2. Semantics . . . . . . . . . . . . . . . . . . . . . . 4
2.3. HTTP Changes . . . . . . . . . . . . . . . . . . . . . . 5
2.3.1. Client and Server . . . . . . . . . . . . . . . . . . 5
2.3.2. Stream IDs . . . . . . . . . . . . . . . . . . . . . 5
2.4. Dialer Behavioral Changes . . . . . . . . . . . . . . . . 5
2.5. Listener Behavioral Changes . . . . . . . . . . . . . . . 6
2.6. PUSH_PROMISE . . . . . . . . . . . . . . . . . . . . . . 6
2.7. Other Extensions . . . . . . . . . . . . . . . . . . . . 6
3. Authority Validation . . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. HTTP/2 Frame Type Registry Update . . . . . . . . . . . . 7
4.2. HTTP/2 Settings Registry Update . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Changelog . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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
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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:
o SETTINGS_PEER_TO_PEER (0xTBA): Informs the remote endpoint of
whether the sender supports the peer-to-peer extension to HTTP/2.
A value of 1 indicates that the peer-to-peer extension is
supported. Any other value, or the absence of this setting,
indicates that the peer-to-peer extension is not supported.
This setting MUST NOT be emitted by the listener on the HTTP/2
connection. If the dialer receives this setting from the listener
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it MUST respond with a connection error ([RFC7540] Section 5.4.1)
of type PROTOCOL_ERROR.
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.
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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.
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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.
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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:
o validating authorities against a TLS certificate presented by the
dialer during TLS handshake.
o confirming that a reverse DNS lookup for the dialer IP returns the
authority asserted by the dialer.
o a static list of IP addresses trusted for a given authority.
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.
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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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI
10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
6.2. Informative References
[I-D.lentczner-rhttp]
Lentczner, M. and D. Preston, "Reverse HTTP", draft-
lentczner-rhttp-00 (work in progress), March 2009.
Appendix A. Changelog
(This appendix to be deleted by the RFC Editor.)
Since -01:
o Introduce the terms 'dialer' and 'listener'.
o Clarify the terms 'client' and 'server'.
o Clarify what stream IDs are used by which peer.
o Remove the ability to send multiple CLIENT_AUTHORITY frames.
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o Correctly credit David Dias and Juan Benet for their role.
Since -00:
o Clarified the semantics behind multiple CLIENT_AUTHORITY frames.
o Removed the requirement for servers to issue
SETTINGS_PEER_TO_PEER, instead allowing the extension to be purely
client-advertised.
Author's Address
Cory Benfield
Email: cory@lukasa.co.uk
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