Internet Engineering Task Force M. Allman
INTERNET DRAFT ICSI
File: draft-allman-tcpm-bump-initcwnd-00.txt November 15, 2010
Initial Congestion Window Specification
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Abstract
This document specifies the progression of initial TCP congestion
window sizes over the next nine years.
1 Introduction
This document proposes a series of increases to TCP's [RFC790]
initial congestion window. For the first time in roughly nine years
the IETF is currently considering increasing the initial congestion
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window (IW). The current proposal is to increase IW from the
standard two to four segments [RFC5681] to ten segments [CDCM10].
While there is much evidence ([Chu09,CDCM10] and references therein)
that the increase has benefit, there is also some unease within the
community that stems from a lack of solid understanding of the
dynamics that the increase would cause system-wide. Such an
understanding is nearly impossible to apprehend. This document
presents an alternate approach that slowly increases the size of the
allowed IW over the next nine years---attaining a value of ten
segments over the course of the first three years and increasing to
15 segments by the end of the inflationary period. By making slow
and steady changes the community can continually assess the dynamics
and short-circuit the increases as necessary. We believe this
offers the best tradeoff between (1) assuring network safety and (2)
allowing for reasonable performance increases as network capacity
increases.
2 Initial Congestion Window Values
This document specifies experimental and standard values for the
initial window as a function of time in the following chart.
Year Exp. IW Std. IW
--------------------------------------------------------
pre-1998 (1,1*SMSS)
1998 (2--4,4380) [RFC2414] (1,1*SMSS)
1999 (2,2*SMSS) [RFC2581]
2002 (2--4,4380) [RFC3390]
2009 (2--4,4380) [RFC5681]
2011 (6,8760)
2012 (8,11680) (6,8760)
2013 (10,14600) (8,11680)
2014 (11,16060) (10,14600)
2015 (12,17520) (11,16060)
2016 (13,18980) (12,17520)
2017 (14,20440) (13,18980)
2018 (15,21900) (14,20440)
2019 (15,21900)
The (X,Y) tuples indicate the IW in terms of the maximum number of
segments, X, and the maximum number of octets, Y. The smaller of
these two values is used as the IW. In other words, (6,8760)
indicates that six segments of 1460 bytes each can be sent. If the
packet size is larger than 1460 bytes then the IW is bounded at 8760
bytes. If the segment size is smaller than 1460 bytes then the IW
is bounded by six segments.
Each IW value spends one year as experimental and assuming the
community does not find any broad problems because standard the
following year.
We take larger steps at the beginning of the time period than at the
end due to our confidence in each step. That is, [CDCM10] (and
references therein) shows that an IW of ten segments is reasonably
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safe today. Therefore, our first step to six segments seems
conservative, as does the overall progression to ten segments over
the next three years. After that the empirical basis for further
increases, which exists in some form today [CDCM10], is less
compelling and therefore further steps are increases of only a
single segment at a time.
We stress that the above is an upper bound on the allowable IW and
not a requirement to use the given value. Hosts may indeed have
reasons to be less aggressive in certain situations.
3 Short Circuiting
The table in the last section can be short-circuited if the IETF
finds particular issues with a given IW. While our expectation is
the IW values given in Section 2 are and will be safe for general
Internet use the progression can be canceled by making this document
obsolete.
4 Security Considerations
This document discusses the initial congestion window permitted for
TCP connections. Changing this value does not raise any known new
security issues with TCP.
5 IANA Considerations
None
Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window Size", RFC 2414, September 1998.
[RFC2581] Allman, M., Paxson V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC3390] Allman, M., Floyd, S., C. Partridge, "Increasing TCP's
Initial Window", RFC 3390, October 2002.
[RFC5681] Allman, M., Paxson V. and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
Non-Normative References
[Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century",
http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf, July 2009.
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[CDCM10] Chu, J., Dukkipati, N., Cheng, Y. and M. Mathis,
"Increasing TCP's Initial Window", Internet-Draft
draft-ietf-tcpm-initcwnd-00.txt (work in progress), October
2010.
Author's Addresses
Mark Allman
ICSI
1947 Center Street
Suite 600
Berkeley, CA 94704-1198
Phone: 440-235-1792
EMail: mallman@icir.org
http://www.icir.org/mallman/
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