Increasing TCP's Initial Window
RFC 3390
| Document | Type |
RFC - Proposed Standard
(November 2002; Errata)
Obsoletes RFC 2414
Updates RFC 2581
|
|
|---|---|---|---|
| Last updated | 2015-12-23 | ||
| Stream | IETF | ||
| Formats | plain text pdf html bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | No shepherd assigned | ||
| IESG | IESG state | RFC 3390 (Proposed Standard) | |
| Consensus Boilerplate | Unknown | ||
| Telechat date | |||
| Responsible AD | Allison Mankin | ||
| IESG note | Published as RFC 3390 [Note from Allison] | ||
| Send notices to | <mankin@psg.com>, <jon.peterson@neustar.biz> | ||
Network Working Group M. Allman
Request for Comments: 3390 BBN/NASA GRC
Obsoletes: 2414 S. Floyd
Updates: 2581 ICIR
Category: Standards Track C. Partridge
BBN Technologies
October 2002
Increasing TCP's Initial Window
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies an optional standard for TCP to increase the
permitted initial window from one or two segment(s) to roughly 4K
bytes, replacing RFC 2414. It discusses the advantages and
disadvantages of the higher initial window, and includes discussion
of experiments and simulations showing that the higher initial window
does not lead to congestion collapse. Finally, this document
provides guidance on implementation issues.
Terminology
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. TCP Modification
This document obsoletes [RFC2414] and updates [RFC2581] and specifies
an increase in the permitted upper bound for TCP's initial window
from one or two segment(s) to between two and four segments. In most
cases, this change results in an upper bound on the initial window of
roughly 4K bytes (although given a large segment size, the permitted
initial window of two segments may be significantly larger than 4K
bytes).
Allman, et. al. Standards Track [Page 1]
RFC 3390 Increasing TCP's Initial Window October 2002
The upper bound for the initial window is given more precisely in
(1):
min (4*MSS, max (2*MSS, 4380 bytes)) (1)
Note: Sending a 1500 byte packet indicates a maximum segment size
(MSS) of 1460 bytes (assuming no IP or TCP options). Therefore,
limiting the initial window's MSS to 4380 bytes allows the sender to
transmit three segments initially in the common case when using 1500
byte packets.
Equivalently, the upper bound for the initial window size is based on
the MSS, as follows:
If (MSS <= 1095 bytes)
then win <= 4 * MSS;
If (1095 bytes < MSS < 2190 bytes)
then win <= 4380;
If (2190 bytes <= MSS)
then win <= 2 * MSS;
This increased initial window is optional: a TCP MAY start with a
larger initial window. However, we expect that most general-purpose
TCP implementations would choose to use the larger initial congestion
window given in equation (1) above.
This upper bound for the initial window size represents a change from
RFC 2581 [RFC2581], which specified that the congestion window be
initialized to one or two segments.
This change applies to the initial window of the connection in the
first round trip time (RTT) of data transmission following the TCP
three-way handshake. Neither the SYN/ACK nor its acknowledgment
(ACK) in the three-way handshake should increase the initial window
size above that outlined in equation (1). If the SYN or SYN/ACK is
lost, the initial window used by a sender after a correctly
transmitted SYN MUST be one segment consisting of MSS bytes.
TCP implementations use slow start in as many as three different
ways: (1) to start a new connection (the initial window); (2) to
restart transmission after a long idle period (the restart window);
and (3) to restart transmission after a retransmit timeout (the loss
window). The change specified in this document affects the value of
the initial window. Optionally, a TCP MAY set the restart window to
the minimum of the value used for the initial window and the current
value of cwnd (in other words, using a larger value for the restart
window should never increase the size of cwnd). These changes do NOT
change the loss window, which must remain 1 segment of MSS bytes (to
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