TCP Maintenance and Minor Extensions (tcpm) P. Hurtig
Internet-Draft A. Brunstrom
Intended status: Experimental Karlstad University
Expires: March 21, 2014 A. Petlund
Simula Research Laboratory AS
M. Welzl
University of Oslo
September 17, 2013
TCP and SCTP RTO Restart
draft-ietf-tcpm-rtorestart-01
Abstract
This document describes a modified algorithm for managing the TCP and
SCTP retransmission timers that provides faster loss recovery when
there is a small amount of outstanding data for a connection. The
modification allows the transport to restart its retransmission timer
more aggressively in situations where fast retransmit cannot be used.
This enables faster loss detection and recovery for connections that
are short-lived or application-limited.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 21, 2014.
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Copyright (c) 2013 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
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1. Introduction
TCP uses two mechanisms to detect segment loss. First, if a segment
is not acknowledged within a certain amount of time, a retransmission
timeout (RTO) occurs, and the segment is retransmitted [RFC6298].
While the RTO is based on measured round-trip times (RTTs) between
the sender and receiver, it also has a conservative lower bound of 1
second to ensure that delayed segments are not mistaken as lost.
Second, when a sender receives duplicate acknowledgments, the fast
retransmit algorithm infers segment loss and triggers a
retransmission [RFC5681]. Duplicate acknowledgments are generated by
a receiver when out-of-order segments arrive. As both segment loss
and segment reordering cause out-of-order arrival, fast retransmit
waits for three duplicate acknowledgments before considering the
segment as lost. In some situations, however, the number of
outstanding segments is not enough to trigger three duplicate
acknowledgments, and the sender must rely on lengthy RTOs for loss
recovery.
The number of outstanding segments can be small for several reasons:
(1) The connection is limited by the congestion control when the
path has a low total capacity (bandwidth-delay product) or the
connection's share of the capacity is small. It is also limited
by the congestion control in the first few RTTs of a connection or
after an RTO when the available capacity is probed using slow-
start.
(2) The connection is limited by the receiver's available buffer
space.
(3) The connection is limited by the application if the available
capacity of the path is not fully utilized (e.g. interactive
applications), or at the end of a transfer.
While the reasons listed above are valid for any flow, the third
reason is common for applications that transmit short flows, or use a
low transmission rate. Typical examples of applications that produce
short flows are web servers. [RJ10] shows that 70% of all web
objects, found at the top 500 sites, are too small for fast
retransmit to work. [BPS98] shows that about 56% of all
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retransmissions sent by a busy web server are sent after RTO expiry.
While the experiments were not conducted using SACK [RFC2018], only
4% of the RTO-based retransmissions could have been avoided.
Applications have a low transmission rate when data is sent in
response to actions, or as a reaction to real life events. Typical
examples of such applications are stock trading systems, remote
computer operations and online games. What is special about this
class of applications is that they are time-dependant, and extra
latency can reduce the application service level [P09]. Although
such applications may represent a small amount of data sent on the
network, a considerable number of flows have such properties and the
importance of low latency is high.
The RTO restart approach outlined in this document makes the RTO
slightly more aggressive when the number of outstanding segments is
small, in an attempt to enable faster loss recovery for all segments
while being robust to reordering. While it still conforms to the
requirement in [RFC6298] that segments must not be retransmitted
earlier than RTO seconds after their original transmission, it could
increase the risk of spurious timeout. Spurious timeouts typically
degrade the performance of flows with multiple bursts of data, as a
burst following a spurious timeout might not fit within the reduced
congestion window (cwnd).
While this document focuses on TCP, the described changes are also
valid for the Stream Control Transmission Protocol (SCTP) [RFC4960]
which has similar loss recovery and congestion control algorithms.
1.1. Requirements Language
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].
2. RTO Restart Overview
The RTO management algorithm described in [RFC6298] recommends that
the retransmission timer is restarted when an acknowledgment (ACK)
that acknowledges new data is received and there is still outstanding
data. The restart is conducted to guarantee that unacknowledged
segments will be retransmitted after approximately RTO seconds.
However, by restarting the timer on each incoming acknowledgment,
retransmissions are not typically triggered RTO seconds after their
previous transmission but rather RTO seconds after the last ACK
arrived. The duration of this extra delay depends on several factors
but is in most cases approximately one RTT. Hence, in most
situations the time before a retransmission is triggered is equal to
"RTO + RTT".
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The extra delay can be significant, especially for applications that
use a lower RTOmin than the standard of 1 second and/or in
environments with high RTTs, e.g. mobile networks. The restart
approach is illustrated in Figure 1 where a TCP sender transmits
three segments to a receiver. The arrival of the first and second
segment triggers a delayed ACK [RFC1122], which restarts the RTO
timer at the sender. RTO restart is performed approximately one RTT
after the transmission of the third segment. Thus, if the third
segment is lost, as indicated in Figure 1, the effective loss
detection time is "RTO + RTT" seconds. In some situations, the
effective loss detection time becomes even longer. Consider a
scenario where only two segments are outstanding. If the second
segment is lost, the time to expire the delayed ACK timer will also
be included in the effective loss detection time.
Sender Receiver
...
DATA [SEG 1] ----------------------> (ack delayed)
DATA [SEG 2] ----------------------> (send ack)
DATA [SEG 3] ----X /-------- ACK
(restart RTO) <----------/
...
(RTO expiry)
DATA [SEG 3] ---------------------->
Figure 1: RTO restart example
During normal TCP bulk transfer the current RTO restart approach is
not a problem. Actually, as long as enough segments arrive at a
receiver to enable fast retransmit, RTO-based loss recovery should be
avoided. RTOs should only be used as a last resort, as they
drastically lower the congestion window compared to fast retransmit.
The current approach can therefore be beneficial -- it is described
in [EL04] to act as a "safety margin" that compensates for some of
the problems that the authors have identified with the standard RTO
calculation. Notably, the authors of [EL04] also state that "this
safety margin does not exist for highly interactive applications
where often only a single packet is in flight."
Although fast retransmit is preferrable there are situations where
timeouts are appropriate, or the only choice. For example, if the
network is severely congested and no segments arrive, RTO-based
recovery should be used. In this situation, the time to recover from
the loss(es) will not be the performance bottleneck. Furthermore,
for connections that do not utilize enough capacity to enable fast
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retransmit, RTO is the only choice. The time needed for loss
detection in such scenarios can become a serious performance
bottleneck.
3. RTO Restart Algorithm
To enable faster loss recovery for connections that are unable to use
fast retransmit, an alternative RTO restart can be used. By
resetting the timer to "RTO - T_earliest", where T_earliest is the
time elapsed since the earliest outstanding segment was transmitted,
retransmissions will always occur after exactly RTO seconds. This
approach makes the RTO more aggressive than the standardized approach
in [RFC6298] but still conforms to the requirement in [RFC6298] that
segments must not be retransmitted earlier than RTO seconds after
their original transmission.
This document specifies a sender-only modification to TCP and SCTP
which updates step 5.3 in Section 5 of [RFC6298] (and a similar
update in Section 6.3.2 of [RFC4960] for SCTP):
When an ACK is received that acknowledges new data:
(1) Set T_earliest = 0.
(2) If the following two conditions hold:
(a) The number of outstanding segments is less than four.
(b) There is no unsent data ready for transmission.
set T_earliest to the time elapsed since the earliest
outstanding segment was sent.
(3) Restart the retransmission timer so that it will expire after
"RTO - T_earliest" seconds (for the current value of RTO).
The update requires TCP implementations to track the time elapsed
since the transmission of the earliest outstanding segment
(T_earliest). As the alternative restart is used only when the
number of outstanding segments is less than four only four segments
need to be tracked. Furthermore, some implementations of TCP (e.g.
Linux TCP) already track the transmission times of all segments.
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4. Discussion
In this section, we discuss the applicability and a number of issues
surrounding the modified RTO restart.
4.1. Applicability
The currently standardized algorithm has been shown to add at least
one RTT to the loss recovery process in TCP [LS00] and SCTP
[HB08][PBP09]. For applications that have strict timing requirements
(e.g. interactive web and gaming) rather than throughput
requirements, the modified restart approach could be important
because the RTT and also the delayed ACK timer of receivers are often
large components of the effective loss recovery time. Measurements
in [HB08] have shown that the total transfer time of a lost segment
(including the original transmission time and the loss recovery time)
can be reduced by 35% using the suggested approach. These results
match those presented in [PGH06][PBP09], where the modified restart
approach is shown to significantly reduce retransmission latency.
4.2. Spurious Timeouts
This document describes a modified RTO restart behavior that, in some
situations, reduces the loss detection time and thereby increases the
risk of spurious timeouts. In theory, the retransmission timer has a
lower bound of 1 second [RFC6298], which limits the risk of having
spurious timeouts. However, in practice most implementations use a
significantly lower value. Initial measurements, conducted by the
authors, show slight increases in the number of spurious timeouts
when such lower values are used. However, further experiments, in
different environments and with different types of traffic, are
encouraged to quantify such increases more reliably.
Does a slightly increased risk matter? Generally, spurious timeouts
have a negative effect on TCP/SCTP performance as the congestion
window is reduced to one segment [RFC5681], limiting an application's
ability to transmit large amounts of data instantaneously. However,
with respect to RTO restart spurious timeouts are only a problem for
applications transmitting multiple bursts of data within a single
flow. Other types of flows, e.g. long-lived bulk flows, are not
affected as the algorithm is only applied when the amount of
outstanding segments is less than four and no previously unsent data
is available. Furthermore, short-lived and application-limited flows
are typically not affected as they are too short to experience the
effect of congestion control or have a transmission rate that is
quickly attainable.
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While a slight increase in spurious timeouts has been observed using
the modified RTO restart approach, it is not clear whether the
effects of this increase mandate any future algorithmic changes or
not -- especially since most modern operating systems already include
mechanisms to detect [RFC3522][RFC3708][RFC5682] and resolve
[RFC4015] possible problems with spurious retransmissions. Further
experimentation is needed to determine this and thereby move this
specification from experimental to proposed standard.
5. Related Work
There are several proposals that address the problem of not having
enough ACKs for loss recovery. In what follows, we explain why the
mechanism described here is complementary to these approaches:
The limited transmit mechanism [RFC3042] allows a TCP sender to
transmit a previously unsent segment for each of the first two
duplicate acknowledgments. By transmitting new segments, the sender
attempts to generate additional duplicate acknowledgments to enable
fast retransmit. However, limited transmit does not help if no
previously unsent data is ready for transmission or if the receiver
has no buffer space. [RFC5827] specifies an early retransmit
algorithm to enable fast loss recovery in such situations. By
dynamically lowering the number of duplicate acknowledgments needed
for fast retransmit (dupthresh), based on the number of outstanding
segments, a smaller number of duplicate acknowledgments are needed to
trigger a retransmission. In some situations, however, the algorithm
is of no use or might not work properly. First, if a single segment
is outstanding, and lost, it is impossible to use early retransmit.
Second, if ACKs are lost, the early retransmit cannot help. Third,
if the network path reorders segments, the algorithm might cause more
unnecessary retransmissions than fast retransmit.
Following the fast retransmit mechanism standardized in [RFC5681]
this draft assumes a value of 3 for dupthresh. However, by
considering a dynamic value for dupthresh a tighter integration with
early retransmit (or other experimental algorithms) could also be
possible.
Tail Loss Probe [TLP] is a proposal to send up to two "probe
segments" when a timer fires which is set to a value smaller than the
RTO. A "probe segment" is a new segment if new data is available,
else a retransmission. The intention is to compensate for sluggish
RTO behavior in situations where the RTO greatly exceeds the RTT,
which, according to measurements reported in [TLP], is not uncommon.
The Probe timeout (PTO) is normally two RTTs, and a spurious PTO is
less risky than a spurious RTO because it would not have the same
negative effects (clearing the scoreboard and restarting with slow-
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start). In contrast, RTO restart is a small sender-only modification
of the RTO management algorithm and does not require an additional
timer or the use of SACK.
TLP is applicable in situations where RTO restart does not apply, and
it could overrule (yielding a similar general behavior, but with a
lower timeout) RTO restart in cases where the number of outstanding
segments is smaller than four and no new segments are available for
transmission. The PTO has the same inherent restart problems as the
RTO timer and could be combined with the modified restart approach to
offer more consistent timeouts.
6. Acknowledgements
The authors wish to thank Godred Fairhurst, Yuchung Cheng, Mark
Allman, Anantha Ramaiah and Richard Scheffenegger for commenting the
draft and the ideas behind it.
All the authors are funded by the European Community under its
Seventh Framework Programme through the Reducing Internet Transport
Latency (RITE) project (ICT-317700). The views expressed are solely
those of the author(s).
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This document discusses a change in how to set the retransmission
timer's value when restarted. This change does not raise any new
security issues with TCP or SCTP.
9. Changes from Previous Versions
9.1. Changes from draft-ietf-...-00 to -01
o Improved the wording throughout the document.
o Removed the possibility for a connection limited by the receiver's
advertised window to use RTO restart, decreasing the risk of
spurious retransmission timeouts.
o Added a section that discusses the applicability of and problems
related to the RTO restart mechanism.
o Updated the text describing the relationship to TLP to reflect
updates made in this draft.
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o Added acknowledgments.
10. References
10.1. Normative References
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3042] Allman, M., Balakrishnan, H., and S. Floyd, "Enhancing
TCP's Loss Recovery Using Limited Transmit", RFC 3042,
January 2001.
[RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm
for TCP", RFC 3522, April 2003.
[RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective
Acknowledgement (DSACKs) and Stream Control Transmission
Protocol (SCTP) Duplicate Transmission Sequence Numbers
(TSNs) to Detect Spurious Retransmissions", RFC 3708,
February 2004.
[RFC4015] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm
for TCP", RFC 4015, February 2005.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata,
"Forward RTO-Recovery (F-RTO): An Algorithm for Detecting
Spurious Retransmission Timeouts with TCP", RFC 5682,
September 2009.
[RFC5827] Allman, M., Avrachenkov, K., Ayesta, U., Blanton, J., and
P. Hurtig, "Early Retransmit for TCP and Stream Control
Transmission Protocol (SCTP)", RFC 5827, May 2010.
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[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
10.2. Informative References
[BPS98] Balakrishnan, H., Padmanabhan, V., Seshan, S., Stemm, M.,
and R. Katz, "TCP Behavior of a Busy Web Server: Analysis
and Improvements", Proc. IEEE INFOCOM Conf., March 1998.
[EL04] Ekstroem, H. and R. Ludwig, "The Peak-Hopper: A New End-
to-End Retransmission Timer for Reliable Unicast
Transport", IEEE INFOCOM 2004, March 2004.
[HB08] Hurtig, P. and A. Brunstrom, "SCTP: designed for timely
message delivery?", Springer Telecommunication Systems,
May 2010.
[LS00] Ludwig, R. and K. Sklower, "The Eifel retransmission
timer", ACM SIGCOMM Comput. Commun. Rev., 30(3), July
2000.
[P09] Petlund, A., "Improving latency for interactive, thin-
stream applications over reliable transport", Unipub PhD
Thesis, Oct 2009.
[PBP09] Petlund, A., Beskow, P., Pedersen, J., Paaby, E., Griwodz,
C., and P. Halvorsen, "Improving SCTP Retransmission
Delays for Time-Dependent Thin Streams", Springer
Multimedia Tools and Applications, 45(1-3), 2009.
[PGH06] Pedersen, J., Griwodz, C., and P. Halvorsen,
"Considerations of SCTP Retransmission Delays for Thin
Streams", IEEE LCN 2006, November 2006.
[RJ10] Ramachandran, S., "Web metrics: Size and number of
resources", Google
http://code.google.com/speed/articles/web-metrics.html,
May 2010.
[TLP] Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
"TCP Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", Internet-draft draft-dukkipati-tcpm-tcp-
loss-probe-00.txt, July 2012.
Authors' Addresses
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Per Hurtig
Karlstad University
Universitetsgatan 2
Karlstad 651 88
Sweden
Phone: +46 54 700 23 35
Email: per.hurtig@kau.se
Anna Brunstrom
Karlstad University
Universitetsgatan 2
Karlstad 651 88
Sweden
Phone: +46 54 700 17 95
Email: anna.brunstrom@kau.se
Andreas Petlund
Simula Research Laboratory AS
P.O. Box 134
Lysaker 1325
Norway
Phone: +47 67 82 82 00
Email: apetlund@simula.no
Michael Welzl
University of Oslo
PO Box 1080 Blindern
Oslo N-0316
Norway
Phone: +47 22 85 24 20
Email: michawe@ifi.uio.no
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