Internet DRAFT - draft-allman-tcpm-no-initwin
Internet Engineering Task Force M. Allman
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
The list of Internet-Draft Shadow Directories can be accessed at
This Internet-Draft will expire on May 24, 2016.
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."
This specification removes the specification of TCP's initial
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
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
(4) The initial cwnd MUST be bounded by the receiver's advertised
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
(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
(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.
[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.
[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.
International Computer Science Institute
1947 Center St. Suite 600
Berkeley, CA 94704
Expires: May 23, 2016 [Page 3]