draft-paxson-tcpm-rfc2988bis-00.txt   draft-paxson-tcpm-rfc2988bis-01.txt 
Internet Engineering Task Force V. Paxson Internet Engineering Task Force V. Paxson
INTERNET DRAFT ICSI/UC Berkeley INTERNET DRAFT ICSI/UC Berkeley
File: draft-paxson-tcpm-rfc2988bis-00.txt M. Allman File: draft-paxson-tcpm-rfc2988bis-01.txt M. Allman
ICSI ICSI
J. Chu J. Chu
Google Google
February 2010 M. Sargent
CWRU
December 6, 2010
Computing TCP's Retransmission Timer Computing TCP's Retransmission Timer
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79. the provisions of BCP 78 and BCP 79.
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months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
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Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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publication of this document. Please review these documents publication of this document. Please review these documents
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Non-Normative References Non-Normative References
[AP99] Allman, M. and V. Paxson, "On Estimating End-to-End Network [AP99] Allman, M. and V. Paxson, "On Estimating End-to-End Network
Path Properties", SIGCOMM 99. Path Properties", SIGCOMM 99.
[Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century", [Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century",
http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf, July http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf, July
2009. 2009.
[SLS09] Schulman, A., Levin, D., and Spring, N., "CRAWDAD data set
umd/sigcomm2008 (v. 2009-03-02)",
http://crawdad.cs.dartmouth.edu/umd/sigcomm2008, March,
2009.
[HKA04] Henderson, T., Kotz, D., and Abyzov, I., "CRAWDAD trace
dartmouth/campus/tcpdump/fall03 (v. 2004-11-09)",
http://crawdad.cs.dartmouth.edu/dartmouth/campus/tcpdump/fall03,
November 2004.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer [Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer
Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988. Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988.
[JK88] Jacobson, V. and M. Karels, "Congestion Avoidance and [JK88] Jacobson, V. and M. Karels, "Congestion Avoidance and
Control", ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z. Control", ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.
[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time [KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", SIGCOMM 87. Estimates in Reliable Transport Protocols", SIGCOMM 87.
Author's Addresses Author's Addresses
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http://www.icir.org/mallman/ http://www.icir.org/mallman/
H.K. Jerry Chu H.K. Jerry Chu
Google, Inc. Google, Inc.
1600 Amphitheatre Parkway 1600 Amphitheatre Parkway
Mountain View, CA 94043 Mountain View, CA 94043
Phone: 650-253-3010 Phone: 650-253-3010
Email: hkchu@google.com Email: hkchu@google.com
Matt Sargent
Case Western Reserve University Olin Building
10900 Euclid Avenue
Room 505
Cleveland, OH 44106
Phone: 440-223-5932
Email: mts71@case.edu
Appendix A Appendix A
Choosing a reasonable initial RTO requires balancing two Choosing a reasonable initial RTO requires balancing two
competing considerations: competing considerations:
1. The initial RTO should be sufficiently large to cover most of the 1. The initial RTO should be sufficiently large to cover most of the
end-to-end paths to avoid spurious retransmissions and their end-to-end paths to avoid spurious retransmissions and their
associated negative performance impact. associated negative performance impact.
2. The initial RTO should be small enough to ensure a timely 2. The initial RTO should be small enough to ensure a timely
recovery from packet loss occurring before an RTT sample is recovery from packet loss occurring before an RTT sample is
taken. taken.
Traditionally, TCP has used 3 seconds as the initial RTO Traditionally, TCP has used 3 seconds as the initial RTO
[RFC1122,RFC2988]. This document calls for lowering this value to 1 [RFC1122,RFC2988]. This document calls for lowering this value to 1
second for the following reasons: second using the following rationale:
- Modern networks are simply faster than the state-of-the-art was - Modern networks are simply faster than the state-of-the-art was
at the time the initial RTO of 3 seconds was defined. at the time the initial RTO of 3 seconds was defined.
- Studies have found that the round-trip time of more than 97.5% of - Studies have found that the round-trip times of more than 97.5% of
the connections observed in a large scale analysis were less than the connections observed in a large scale analysis were less than
1 second [Chu09], suggesting that 1 second meets criteria 1 above. 1 second [Chu09], suggesting that 1 second meets criteria 1 above.
- In addition, the studies have observed retransmission rates within the - In addition, the studies observed retransmission rates within
three-way handshake of roughly 2%. This shows that reducing the the three-way handshake of roughly 2%. This shows that reducing
initial RTO has benefit to a non-negligible set of connections. the initial RTO has benefit to a non-negligible set of connections.
- However, roughly 2.5% of the connections studied in [Chu09] have - However, roughly 2.5% of the connections studied in [Chu09] have
an RTT longer than 1 second. For those connections, a 1 second an RTT longer than 1 second. For those connections, a 1 second
initial RTO guarantees a retransmission during connection establishment initial RTO guarantees a retransmission during connection
(needed or not). establishment (needed or not).
When this happens, this document calls for reverting to an initial When this happens, this document calls for reverting to an initial
RTO of 3 seconds for the data transmission phase. Therefore, the RTO of 3 seconds for the data transmission phase. Therefore, the
implications of the spurious retransmission are modest: (1) an implications of the spurious retransmission are modest: (1) an
extra SYN is transmitted into the network, and (2) according to extra SYN is transmitted into the network, and (2) according to
[RFC5681] the initial congestion window will be limited to 1 [RFC5681] the initial congestion window will be limited to 1
segment. While (2) clearly puts such connections at a segment. While (2) clearly puts such connections at a
disadvantage, this document at least resets the RTO such that the disadvantage, this document at least resets the RTO such that the
connection will not continually run into problems with a short connection will not continually run into problems with a short
timeout. (Of course, if the RTT is more than three seconds, the timeout. (Of course, if the RTT is more than three seconds, the
connection will still encounter difficulties. But that is not a new connection will still encounter difficulties. But that is not a
issue for TCP.) new issue for TCP.)
In addition, we note that when using timestamps the TCP will be In addition, we note that when using timestamps, TCP will be able
able to take an RTT sample even in the presence of a spurious to take an RTT sample even in the presence of a spurious
retransmission, hence avoiding concern (2) above. retransmission, facilitating convergence to a correct RTT estimate
when the RTT exceeds 1 second.
As an additional check on the results presented in [Chu09], we
analyzed packet traces of client behavior collected at four
different vantage points at different times, as follows:
Name Dates Pkts. Cnns. Clnts. Servs.
--------------------------------------------------------
LBL-1 Oct/05--Mar/06 292M 242K 228 74K
LBL-2 Nov/09--Feb/10 1.1B 1.2M 1047 38K
ICSI-1 Sep/11--18/07 137M 2.1M 193 486K
ICSI-2 Sep/11--18/08 163M 1.9M 177 277K
ICSI-3 Sep/14--21/09 334M 3.1M 170 253K
ICSI-4 Sep/11--18/10 298M 5M 183 189K
Dartmouth Jan/4--21/04 1B 4M 3782 132K
SIGCOMM Aug/17--21/08 11.6M 133K 152 29K
The "LBL" data was taken at the Lawrence Berkeley National
Laboratory, the "ICSI" data from the International Computer Science
Institute, the "SIGCOMM" data from the wireless network that served
the attendees of SIGCOMM 2008, and the "Dartmouth" data was
collected from Dartmouth College's wireless network. The latter two
datasets are available from the CRAWDAD data repository
[HKA04,SLS09]. The table lists the dates of the data collections,
the number of packets collected, the number of TCP connections
observed, the number of local clients monitored, and the number of
remote servers contacted. We consider only connections initiated
near the tracing vantage point.
Analysis of these datasets finds the prevalence of retransmitted
SYNs to be between 0.03% (ICSI-4) to roughly 2% (LBL-1 and
Dartmouth).
We then analyzed the data to determine the number of
additional---and spurious---retransmissions that would have been
incurred if the initial RTO was assumed to be 1 second. In most of
the datasets, the proportion of connections with spurious
retransmits was less than 0.1%. However, in the Dartmouth dataset
approximately 1.1% of the connections would have sent a spurious
retransmit with a lower initial RTO. We attribute this to the fact
that the monitored network is wireless and therefore susceptible to
additional delays from RF effects.
Finally, there are obviously performance benefits from
retransmitting lost SYNs with a reduced initial RTO. Across our
datasets, the percentage of connections that retransmitted a SYN and
would realize at least a 10% performance improvement by using the
smaller initial RTO specified in this document ranges from 43%
(LBL-1) to 87% (ICSI-4). The percentage of connections that would
realize at least a 50% performance improvement ranges from 17%
(ICSI-1 and SIGCOMM) to 73% (ICSI-4).
From the data to which we have access, we conclude that the lower
initial RTO is likely to be beneficial to many connections, and
harmful to relatively few.
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