Network Working Group O. Mazahir
Internet-Draft J. Padhye
Expires: June 11, 2013 Microsoft
W. Chan
R. Peon
Google
R. Trace
Microsoft
S. Loreto
Ericsson
G. Montenegro
Microsoft
December 8, 2012
HTTP 2.0 Principles for Flow Control
draft-montenegro-httpbis-http2-fc-principles-01
Abstract
This document states the principles for flow control in HTTP 2.0.
Status of this Memo
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This Internet-Draft will expire on June 11, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012
carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Principles for Flow Control in HTTP 2.0 Multiplexing . . . . . 4
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Normative References . . . . . . . . . . . . . . . . . . . 6
4.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
HTTP/2.0 introduces multiplexed streams over a given TCP connection.
In HTTP 1.X, there is no interleaving of Request/Response pairs.
Thus, any flow control issues are mostly left to the underlying TCP
implementation. In HTTP 2.0, each Request/Response pair uses a
separate stream, sharing the same TCP connection with other such
pairs over different streams. All such streams will be vying for a
common underlying resource of a single TCP connection. Given that
this interaction among all the streams is not visible to the TCP
implementation, handling the interaction among them has to be solved
at the HTTP 2.0 multiplexing layer. There are issues of
prioritization, head-of-line blocking and flow control. Perhaps the
most complex aspect is that of flow control. It may be that flow
control for HTTP 2.0 multiplexing will follow a path similar to what
TCP's complex dynamics have followed throughout the years. In
particular, TCP congestion control has seen a constant progress of
improved specifications based on measurements and research of the
networking community. What the TCP community recognized early on was
that this was a hard problem. Thus, the best course of action was to
agree on a minimal set of rules or principles (e.g., TCP
"friendliness"). Many TCP congestion control algorithms are then
possible as a (mostly) local implementation issue giving rise to TCP
Reno, Tahoe, Vegas, CTCP, and many more.
Flow control for HTTP 2.0 multiplexing over TCP is also a complex
issue. This document proposes (1) a set of principles aimed at
preventing egregious behavior, while allowing for future and ongoing
improvement of flow control algorithms, and (2) a simple flow control
algorithm that could be implemented in the absence of better schemes
(TBD). Other flow control algorithms with subsequent improvements
should be specified in separate documents without encumbering nor
delaying the base HTTP 2.0 specification. This is similar to how the
myriad TCP congestion algorithms published so far have been specified
separately from the base TCP documents.
The goal of this document is to propose additional text to the
HTTP/2.0 specification. The starting point for HTTP/2.0, the SPDY
[I-D.mbelshe-httpbis-spdy] protocol, does not have much language with
respect to flow control. Hence, the text below is offered as a new
section or sections within the HTTP/2.0 document.
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2. Principles for Flow Control in HTTP 2.0 Multiplexing
Flow control for Multiplexing in HTTP 2.0 must follow these
principles:
1. Flow control is hop by hop (where "hop" means an HTTP 2.0 hop),
and not end-to-end.
2. Flow control is based on window update messages. It is
essentially a credit-based scheme.
3. Flow control is directional and is determined by the receiver.
Flow control MAY be declared by the receiver and MUST be heeded
by the sender. For example, a client, a server or a proxy (in
their role as a "receiver") independently advertise their flow
control preference. The other side when operating as a "sender"
must heed that preference.
4. Flow control in the direction towards the receiver can be OFF or
ON as determined by the receiver. It is OFF if no flow control
is advertised by the receiver, or if the receiver declares
"infinite" credit to the sender.
5. HTTP 2.0 should only standardize the format of the window update
message and its semantics. In particular, the algorithms used by
the receiver to decide when to send window update messages, and
how much to update the window by, are not mandated in the spec.
The draft should, however, provide some illustrative examples.
NOTE: Whether flow control operates on a per-stream basis, on a per-
session (per-TCP connection) basis or on both a per-stream and a per-
session basis is TBD.
The spec will not define the algorithms the sender will use to manage
priorities among streams and to minimize head of the line blocking.
This is included for completeness, but is essentially independent of
flow-control.
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3. Acknowledgements
This document was produced using the xml2rfc tool [RFC2629].
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4. References
4.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-21 (work in progress),
October 2012.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-21 (work in progress),
October 2012.
4.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
[I-D.mbelshe-httpbis-spdy]
Belshe, M. and R. Peon, "SPDY Protocol",
draft-mbelshe-httpbis-spdy-00 (work in progress),
February 2012.
[I-D.montenegro-httpbis-speed-mobility]
Trace, R., Foresti, A., Singhal, S., Mazahir, O., Nielsen,
H., Raymor, B., Rao, R., and G. Montenegro, "HTTP Speed+
Mobility", draft-montenegro-httpbis-speed-mobility-02
(work in progress), June 2012.
[I-D.tarreau-httpbis-network-friendly]
Tarreau, W., Jeffries, A., and A. Croy, "Proposal for a
Network-Friendly HTTP Upgrade",
draft-tarreau-httpbis-network-friendly-00 (work in
progress), March 2012.
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Authors' Addresses
Osama Mazahir
Microsoft
Email: OsamaM@microsoft.com
Jitu Padhye
Microsoft
Email: padhye@microsoft.com
William Chan
Google
Email: willchan@chromium.org
Roberto Peon
Google
Email: fenix@google.com
Rob Trace
Microsoft
Email: Rob.Trace@microsoft.com
Salvatore Loreto
Ericsson
Email: salvatore.loreto@ericsson.com
Gabriel Montenegro
Microsoft
Email: Gabriel.Montenegro@microsoft.com
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