TCP Maintenance and Minor Extensions (tcpm) P. Hurtig
Internet-Draft A. Brunstrom
Intended status: Experimental Karlstad University
Expires: August 18, 2014 A. Petlund
Simula Research Laboratory AS
M. Welzl
University of Oslo
February 14, 2014
TCP and SCTP RTO Restart
draft-ietf-tcpm-rtorestart-02
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
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). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on August 18, 2014.
Copyright Notice
Copyright (c) 2014 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
Hurtig, et al. Expires August 18, 2014 [Page 1]
Internet-Draft TCP and SCTP RTO Restart February 2014
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.
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. [FDT13] shows that about 77% of all
Hurtig, et al. Expires August 18, 2014 [Page 2]
Internet-Draft TCP and SCTP RTO Restart February 2014
retransmissions sent by a major web service are sent after RTO
expiry. 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".
Hurtig, et al. Expires August 18, 2014 [Page 3]
Internet-Draft TCP and SCTP RTO Restart February 2014
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. However, for
connections that do not utilize enough capacity to enable fast
Hurtig, et al. Expires August 18, 2014 [Page 4]
Internet-Draft TCP and SCTP RTO Restart February 2014
retransmit, RTO-based loss detection is the only choice and the time
required for this 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 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 an OPTIONAL 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). A sender
that implements this method MUST follow the algorithm below:
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 a RTO
restart threshold (rrthresh). The rrthresh SHOULD be
set to 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).
This update needs TCP implementations to track the time elapsed since
the transmission of the earliest outstanding segment (T_earliest).
The modified restart is only necessary to conduct when fast
retransmit cannot be triggered, i.e., when there are less than four
segments outstanding. Therefore, only four segments need to be
tracked by the TCP implementation. Furthermore, some implementations
of TCP (e.g. Linux TCP) already track the transmission times of all
segments.
Hurtig, et al. Expires August 18, 2014 [Page 5]
Internet-Draft TCP and SCTP RTO Restart February 2014
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
[HB11][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 [HB11] 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.
There are also traffic types that do not benefit from a modified
restart behavior of the timer. One example of such traffic is bulk
transmission. The reason why bulk traffic does not benefit from RTO
restart is related to the number of outstanding segments that such
flows usually have. Fast retransmit [RFC5681], the preferred loss
recovery mechanism, is triggered whenever three duplicate
acknowledgments arrive at a TCP sender. 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
regarding the segment as lost. Considering this, bulk flows will
mostly use fast retransmit as they often have three or more
outstanding segments. Moreover, as the modified restart behavior is
not activated when there are four, or more, segments outstanding
there is no increased risk of recovering loss using timeouts instead
of fast retransmits.
Given RTO restart's ability to only work when it is beneficial for
the loss recovery process, it is suitable as a system-wide default
mechanism for TCP traffic.
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
Hurtig, et al. Expires August 18, 2014 [Page 6]
Internet-Draft TCP and SCTP RTO Restart February 2014
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.
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
Hurtig, et al. Expires August 18, 2014 [Page 7]
Internet-Draft TCP and SCTP RTO Restart February 2014
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, which is used as basis
for rrthresh. 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-
start). In contrast, RTO restart is trying to make the RTO more
appropriate in cases where there is no need to be overly cautious.
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 problem of restarting
the timer on an incoming ACK, 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, Richard Scheffenegger, and Nicolas Kuhn for
commenting the draft and the ideas behind it.
All the authors are supported by RITE (http://riteproject.eu/ ), a
research project (ICT-317700) funded by the European Community under
its Seventh Framework Program. The views expressed here are those of
the author(s) only. The European Commission is not liable for any
use that may be made of the information in this document.
7. IANA Considerations
This memo includes no request to IANA.
Hurtig, et al. Expires August 18, 2014 [Page 8]
Internet-Draft TCP and SCTP RTO Restart February 2014
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-...-01 to -02
o Changed the algorithm description in Section 3 to use formal RFC
2119 language.
o Changed last paragraph of Section 3 to clarify why the RTO restart
algorithm is active when less than four segments are outstanding.
o Added two paragraphs in Section 4.1 to clarify why the algorithm
can be turned on for all TCP traffic without having any negative
effects on traffic patterns that do not benefit from a modified
timer restart.
o Improved the wording throughout the document.
o Replaced and updated some references.
9.2. 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.
o Added acknowledgments.
10. References
10.1. Normative References
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
Hurtig, et al. Expires August 18, 2014 [Page 9]
Internet-Draft TCP and SCTP RTO Restart February 2014
[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.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, June
2011.
10.2. Informative References
[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.
[FDT13] Flach, T., Dukkipati, N., Terzis, A., Raghavan, B.,
Cardwell, N., Cheng, Y., Jain, A., Hao, S., Katz-Bassett,
E., and R. Govindan, "Reducing Web Latency: the Virtue of
Gentle Aggression", Proc. ACM SIGCOMM Conf., August 2013.
Hurtig, et al. Expires August 18, 2014 [Page 10]
Internet-Draft TCP and SCTP RTO Restart February 2014
[HB11] Hurtig, P. and A. Brunstrom, "SCTP: designed for timely
message delivery?", Springer Telecommunication Systems 47
(3-4), August 2011.
[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-01.txt, February 2013.
Authors' Addresses
Per Hurtig
Karlstad University
Universitetsgatan 2
Karlstad 651 88
Sweden
Phone: +46 54 700 23 35
Email: per.hurtig@kau.se
Hurtig, et al. Expires August 18, 2014 [Page 11]
Internet-Draft TCP and SCTP RTO Restart February 2014
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
Hurtig, et al. Expires August 18, 2014 [Page 12]