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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Mazahir, et al. Expires June 11, 2013 [Page 1] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 Mazahir, et al. Expires June 11, 2013 [Page 2] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 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. Mazahir, et al. Expires June 11, 2013 [Page 3] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 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. Mazahir, et al. Expires June 11, 2013 [Page 4] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 3. Acknowledgements This document was produced using the xml2rfc tool [RFC2629]. Mazahir, et al. Expires June 11, 2013 [Page 5] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 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. Mazahir, et al. Expires June 11, 2013 [Page 6] Internet-Draft Principles for Flow Control in HTTP 2.0 December 2012 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 Mazahir, et al. Expires June 11, 2013 [Page 7]