Internet Engineering Task Force M. Allman INTERNET-DRAFT ICSI File: draft-allman-tcpm-no-initwin-00.txt November 24, 2015 Intended Status: Best Current Practice Expires: May 24, 2016 Removing TCP's Initial Congestion Window Status of this Memo This document may not be modified, and derivative works of it may not be created, except to format it for publication as an RFC or to translate it into languages other than English. 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), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This Internet-Draft will expire on May 24, 2016. Copyright Notice Copyright (c) 2015 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 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." Abstract This specification removes the specification of TCP's initial congestion window. Terminology Expires: May 23, 2016 [Page 1] draft-allman-tcpm-no-initwin-00.txt November 2015 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 BCP 14, RFC 2119 [RFC2119]. 1 Introduction TCP connections may choose the initial value of the congestion window (cwnd) and are not beholden to [RFC6928] or previous specifications of the initial cwnd, provided that: (1) The 3WHS MUST complete without a retransmission. (2) For initial windows of more than 10 segments, the initial window of segments MUST be paced evenly across the first round-trip time (as measured during the 3WHS). (3) Since the initial cwnd has no relationship to the available capacity of the network path, in the case of loss within the initial window of segments sent, the cwnd MUST be set to SMSS * ((IW - R) / 2) instead of simply halving the cwnd. Here, the IW is the size of the initial cwnd (in segments) and R is the number of retransmitted segments within the initial transmission window. (4) The initial cwnd MUST be bounded by the receiver's advertised window. 2 Reasoning The reasoning behind this proposal is mostly taken from [LAJW07]. (a) The author thinks that talking about the initial window for the better part of two decades is probably enough. And, definitely boring. (b) Traffic is heavy tailed and most TCP connections cannot use an overly large IW as they are short. (c) An overly aggressive IW is likely to congestion local networks before burdening remote portions of the path. (d) Routers should be using Active Queue Management [RFC2309] to protect from overly aggressive flows. (e) Receivers cannot be overrun as they can exercise control via the advertised window. (f) TCP's congestion control algorithms remain in force and therefore even if a sender transmits too aggressively, this aggression will not be a prolonged event. (g) Ultimately, being egregiously overly aggressive will not be in Expires: May 23, 2016 [Page 2] draft-allman-tcpm-no-initwin-00.txt November 2015 the sender's best interest---e.g., there will be a fight for local resources among the sender's own connections---and therefore there is an incentive to be reasonable. 3 Security Considerations A large IW allows TCP to send a large burst of traffic, but an attacker that can tune a TCP to do this can also simply send a large amount of traffic. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC6928] J. Chu, N. Dukkipati, Y. Cheng, M. Mathis. Increasing TCP's Initial Window, RFC 6928, April 2013. Informative References [LAJW07] Dan Liu, Mark Allman, Shudong Jin, Limin Wang. Congestion Control Without a Startup Phase. Workshop on Protocols for Fast Long-Distance Networks (PFLDnet), February 2007. [RFC2309] B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering, D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L. Peterson, K. Ramakrishnan, S. Shenker, J. Wroclawski, L. Zhang. Recommendations on Queue Management and Congestion Avoidance in the Internet, RFC 2309, April 1998. Authors' Addresses Mark Allman International Computer Science Institute 1947 Center St. Suite 600 Berkeley, CA 94704 EMail: mallman@icir.org http://www.icir.org/mallman Expires: May 23, 2016 [Page 3]