One document matched: draft-allman-tcp-early-rexmt-04.txt
Differences from draft-allman-tcp-early-rexmt-03.txt
Internet Engineering Task Force Mark Allman
INTERNET DRAFT ICSI
File: draft-allman-tcp-early-rexmt-04.txt Konstantin Avrachenkov
INRIA
Urtzi Ayesta
LAAS-CNRS
Josh Blanton
Ohio University
November 2006
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Early Retransmit for TCP and SCTP
Status of this Memo
By submitting this Internet-Draft, each author represents that any
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document proposes a new mechanism for TCP and SCTP that can be
used to recover lost segments when a connection's congestion window
is small. The "Early Retransmit" mechanism allows the transport to
reduce, in certain special circumstances, the number of duplicate
acknowledgments required to trigger a fast retransmission. This
allows the transport to use fast retransmit to recover packet losses
that would otherwise require a lengthy retransmission timeout.
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1 Introduction
Many researchers have studied problems with TCP [RFC793,RFC2581]
when the congestion window is small and have outlined possible
mechanisms to mitigate these problems
[Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]. SCTP's [RFC2960] loss
recovery and congestion control mechanisms are based on TCP and
therefore the same problems impact the performance of SCTP
connections. When the transport detects a missing segment, the
connection enters a loss recovery phase. There are several variants
of the loss recovery phase depending on a TCP's version. TCP can
use slow start based recovery or Fast Recovery [RFC2581], NewReno
[RFC2582], and loss recovery based on selective acknowledgments
(SACKs) [RFC2018,FF96,RFC3517]. SCTP's loss recovery is not as
varied due to the built-in selective acknowledgments.
All the above variants have two methods for loss recovery. First,
if an acknowledgment (ACK) for a given segment is not received in a
certain amount of time a retransmission timer fires and the segment
is resent [RFC2988,RFC2960]. Second, the ``Fast Retransmit''
algorithm resends a segment when three duplicate ACKs arrive at the
sender [Jac88,RFC2581]. Duplicate ACKs are triggered by
out-of-order arrivals at the receiver. However, because duplicate
ACKs from the receiver are triggered by both packet loss and packet
reordering in the network path, the sender waits for three duplicate
ACKs in an attempt to disambiguate packet loss from packet
reordering. When using small congestion windows it may not be
possible to generate the required number of duplicate ACKs to
trigger Fast Retransmit when a loss does happen.
Small windows can occur in a number of situations, such as:
(1) The connection is constrained by end-to-end congestion control
when the connection's share of the path is small, the path has a
small bandwidth-delay product or the transport is ascertaining
the available bandwidth in the first few round-trip times of
slow start.
(2) The connection is "application limited" and has only a limited
amount of data to send. This can happen any time the
application does not produce enough data to fill the congestion
window. A particular case when all connections become
application limited is as the connection ends.
(3) The connection is limited by the receiver's advertised window.
The transport's retransmission timeout (RTO) is based on measured
round-trip times (RTT) between the sender and receiver, as specified
in [RFC2988] (for TCP) and [RFC2960] (for SCTP). To prevent
spurious retransmissions of segments that are only delayed and not
lost, the minimum RTO is conservatively chosen to be 1 second.
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Therefore, it behooves TCP senders to detect and recover from as
many losses as possible without incurring a lengthy timeout during
which the connection remains idle. However, if not enough duplicate
ACKs arrive from the receiver, the Fast Retransmit algorithm is
never triggered---this situation occurs when the congestion window
is small, if a large number of segments in a window are lost or at
the end of a transfer as data drains from the network. For
instance, consider a congestion window (cwnd) of three segments. If
one segment is dropped by the network, then at most two duplicate
ACKs will arrive at the sender, assuming no ACK loss. Since three
duplicate ACKs are required to trigger Fast Retransmit, a timeout
will be required to resend the dropped packet.
[BPS+98] shows that roughly 56% of retransmissions sent by a busy
web server are sent after the RTO timer expires, while only 44% are
handled by Fast Retransmit. In addition, only 4% of the RTO
timer-based retransmissions could have been avoided with SACK, which
has to continue to disambiguate reordering from genuine loss.
Furthermore, [All00] shows that for one particular web server the
median transfer size is less than four segments, indicating that
more than half of the connections will be forced to rely on the RTO
timer to recover from any losses that occur. Thus, loss recovery
that does not rely on the conservative RTO is beneficial for short
TCP transfers.
The Limited Transmit mechanism introduced in [RFC3042] allows a TCP
sender to transmit previously unsent data upon the reception of each
of the two duplicate ACKs that precede a Fast Retransmit. SCTP
[RFC2960] uses SACK information to calculate the number of
outstanding segments in the network. Hence, when the first two
duplicate ACKs arrive at the sender they will indicate that data has
left the network and allow the sender to transmit new data (if
available) similar to TCP's Limited Transmit algorithm.
By sending these two new segments the TCP sender is attempting to
induce additional duplicate ACKs (if appropriate) so that Fast
Retransmit will be triggered before the retransmission timeout
expires. The "Early Retransmit" mechanism outlined in this document
covers the case when previously unsent data is not available for
transmission or cannot be transmitted due to an advertised window
limitation.
Section 2 of this document outlines a small change to TCP and SCTP
senders that will decrease the reliance on the retransmission timer,
and thereby improve performance when Fast Retransmit cannot
otherwise be triggered. Section 3 discusses related work. Section
4 sketches security issues.
2 Early Retransmit Algorithm
The Early Retransmit algorithm calls for lowering the threshold for
triggering Fast Retransmit when the amount of outstanding data is
small and when no previously unsent data can be transmitted. We
define variants of Early Retransmit for connections that do and do
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not support selective acknowledgments (SACK) [RFC2018]. (Note: SCTP
includes SACK in the base protocol and so there is no need for the
non-SACK variant of Early Retransmit in SCTP.)
A non-SACK TCP sender MAY use Early Retransmit. Such a sender MUST
use the following two rules to determine when an Early Retransmit is
sent:
(2.a) The amount of outstanding data (ownd)---data sent but not yet
acknowledged---is less than 4*SMSS bytes.
(2.b) There is either no unsent data ready for transmission at the
sender or the advertised window does not permit new segments to
be transmitted.
When the above two conditions hold, the connection does not support
SACK, and the connection wishes to use Early Retransmit, the
duplicate ACK threshold used to trigger Fast Retransmit MUST be
reduced to:
ER_thresh = ceiling (ownd/SMSS) - 1 (1)
duplicate ACKs, where ownd is in terms of bytes.
When conditions (2.a) and (2.b) do not hold, the transport MUST NOT
use Early Retransmit, but rather prefer the standard mechanisms
(including Limited Transmit [RFC3042]).
When conditions (2.a) and (2.b) hold and the connection does support
SACK, Early Retransmit MUST be used only when "ownd - SMSS" bytes
have been SACKed.
In other words, when ownd is small enough that losing one segment
would not trigger Fast Retransmit, the trigger for Fast Retransmit
is reduced to receiving indications that all but one segment have
arrived at the receiver.
3 Discussion
The SACK variant of the Early Retransmit algorithm is preferred to
the non-SACK variant due to its robustness in the face of ACK loss
(since SACKs are sent redundantly) and due to interactions with the
delayed ACK timer. Consider a flight of three segments, S1...S3,
with S2 being dropped by the network. When S1 arrives it is
in-order and so the receiver may or may not delay the ACK, leading
to two scenarios:
(A) The ACK for S1 is delayed. In this case the arrival of S3 will
trigger an ACK to be transmitted covering segment S1 (which was
previously unacknowledged). In this case Early Retransmit
without SACK will not prevent an RTO because no duplicate ACKs
will arrive. However, with SACK the ACK for S1 will also
include SACK information indicating that S3 has arrived at the
receiver. The sender can then invoke Fast Retransmit on this
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ACK because ownd - SMSS bytes have been SACKed when the ACK
arrives.
(B) The ACK for S1 is not delayed. In this case the arrival of S1
triggers an ACK of previously unacknowledged data. The arrival
of S3 triggers a duplicate ACK (because it is out-of-order).
Both ACKs will cover the same segment (S1). Therefore,
regardless of whether SACK is used Early Retransmit can be
performed by the sender (assuming no ACK loss).
Early Retransmit is less robust in the face of reordered segments
than when using the standard Fast Retransmit threshold. Research
shows that a general reduction in the number of duplicate ACKs
required to trigger Fast Retransmit to two (rather than three) leads
to a reduction in the ratio of good to bad retransmits by a factor
of three [Pax97]. However, this analysis did not include the
additional conditioning on the event that the ownd was smaller than
4 segments and that no new data was available for transmission.
A number of studies have shown that network reordering is not a rare
event across some network paths. Various measurement studies have
shown that reordering along most paths is negligible, but along
certain paths can be quite prevalent [Pax97,BPS99,BS02,Pir05].
Evaluating Early Retransmit in the face of real packet reordering is
part of the experiment we hope to instigate with this document.
Next, we note two "worst case" scenarios for Early Retransmit:
(1) Persistent reordering of segments, coupled with an application
that does not constantly send data, can result in large numbers
of needless retransmissions when using Early Retransmit. For
instance, consider an application that sends data two segments
at a time, followed by an idle period when no data is queued for
delivery by TCP. If the network consistently reorders the two
segments, the sender will needlessly retransmit one out of every
two unique segments transmitted when using the above algorithm
(meaning that one-third of all segments sent are needless
retransmissions). However, this would only be a problem for
long-lived connections from applications that transmit in
spurts.
(2) Similar to the above, consider the case of 2 segment transfers
that always experience reordering. Just as in (1) above, one
out of every two unique data segments will be retransmitted
needlessly, therefore one-third of the traffic will be spurious.
Currently this document offers no suggestion on how to mitigate the
above problems. However, the worst cases are likely pathological
and part of the experiments that this document hopes to trigger
would involve better understanding of whether such theoretical worst
case scenarios are prevalent in the network and in general to
explore the tradeoff between spurious fast retransmits and the delay
imposed by the RTO. Appendix A does offer a survey of possible
mitigations.
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4 Related Work
Deployment of Explicit Congestion Notification (ECN) [Flo94,RFC3168]
may benefit connections with small congestion window sizes
[RFC2884]. ECN provides a method for indicating congestion to the
end-host without dropping segments. While some segment drops may
still occur, ECN may allow TCP to perform better with small cwnd
sizes because the sender will be required to detect less segment
loss [RFC2884].
[Bal98] outlines another solution to the problem of having no new
segments to transmit into the network when the first two duplicate
ACKs arrive. In response to these duplicate ACKs, a TCP sender
transmits zero-byte segments to induce additional duplicate ACKs.
This method preserves the robustness of the standard Fast Retransmit
algorithm at the cost of injecting segments into the network that do
not deliver any data (and, therefore are potentially wasting network
resources).
5 Security Considerations
The security considerations found in [RFC2581] apply to this
document. No additional security problems have been identified with
Early Retransmit at this time.
Acknowledgments
We thank Sally Floyd for her feedback in discussions about Early
Retransmit. We also thank Sally Floyd and Hari Balakrishnan who
helped with a large portion of the text of this document when it was
part of a separate document. Armando Caro and many members of the
tsvwg mailing list provided good discussions that helped shape this
document.
Normative References
[RFC793] Jon Postel. Transmission Control Protocol. Std 7, RFC
793. September 1981.
[RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow.
TCP Selective Acknowledgement Options. RFC 2018, October 1996.
[RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens. TCP
Congestion Control. RFC 2581, April 1999.
[RFC2883] Sally Floyd, Jamshid Mahdavi, Matt Mathis, Matt Podolsky.
An Extension to the Selective Acknowledgement (SACK) Option for
TCP. RFC 2883, July 2000.
[RFC2960] R. Stewart, Q. Xie, K. Morneault, C. Sharp, H.
Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang, V.
Paxson. Stream Control Transmission Protocol. October 2000.
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[RFC2988] Vern Paxson, Mark Allman. Computing TCP's Retransmission
Timer. RFC 2988, April 2000.
[RFC3042] Mark Allman, Hari Balakrishnan, Sally Floyd. Enhancing
TCP's Loss Recovery Using Limited Transmit. RFC 3042, January
2001.
[RFC3522] Reiner Ludwig, Michael Meyer. The Eifel Detection
Algorithm for TCP. RFC 3522, April 2003.
Informative References
[AA02] Urtzi Ayesta, Konstantin Avrachenkov, "The Effect of the
Initial Window Size and Limited Transmit Algorithm on the
Transient Behavior of TCP Transfers", In Proc. of the 15th ITC
Internet Specialist Seminar, Wurzburg, July 2002.
[All00] Mark Allman. A Server-Side View of WWW Characteristics.
ACM Computer Communications Review, October 2000.
[Bal98] Hari Balakrishnan. Challenges to Reliable Data Transport
over Heterogeneous Wireless Networks. Ph.D. Thesis, University
of California at Berkeley, August 1998.
[BPS+98] Hari Balakrishnan, Venkata Padmanabhan, Srinivasan Seshan,
Mark Stemm, and Randy Katz. TCP Behavior of a Busy Web Server:
Analysis and Improvements. Proc. IEEE INFOCOM Conf., San
Francisco, CA, March 1998.
[BS02] John Bellardo, Stefan Savage. Measuring Packet Reordering,
ACM/USENIX Internet Measurement Workshop, November 2002.
[FF96] Kevin Fall, Sally Floyd. Simulation-based Comparisons of
Tahoe, Reno, and SACK TCP. ACM Computer Communication Review,
July 1996.
[Flo94] Sally Floyd. TCP and Explicit Congestion Notification. ACM
Computer Communication Review, October 1994.
[Jac88] Van Jacobson. Congestion Avoidance and Control. ACM
SIGCOMM 1988.
[LK98] Dong Lin, H.T. Kung. TCP Fast Recovery Strategies: Analysis
and Improvements. Proceedings of InfoCom, March 1998.
[Mor97] Robert Morris. TCP Behavior with Many Flows. Proceedings
of the Fifth IEEE International Conference on Network Protocols.
October 1997.
[Pax97] Vern Paxson. End-to-End Internet Packet Dynamics. ACM
SIGCOMM, September 1997.
[Pir05] N. M. Piratla, "A Theoretical Foundation, Metrics and
Modeling of Packet Reordering and Methodology of Delay Modeling
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using Inter-packet Gaps," Ph.D. Dissertation, Department of
Electrical and Computer Engineering, Colorado State University,
Fort Collins, CO, Fall 2005.
[RFC2582] Sally Floyd, Tom Henderson. The NewReno Modification to
TCP's Fast Recovery Algorithm. RFC 2582, April 1999.
[RFC2884] Jamal Hadi Salim and Uvaiz Ahmed. Performance Evaluation
of Explicit Congestion Notification (ECN) in IP Networks. RFC
2884, July 2000.
[RFC3150] Spencer Dawkins, Gabriel Montenegro, Markku Kojo, Vincent
Magret. End-to-end Performance Implications of Slow Links. RFC
3150, July 2001.
[RFC3168] K. K. Ramakrishnan, Sally Floyd, David Black. The
Addition of Explicit Congestion Notification (ECN) to IP. RFC
3168, September 2001.
[RFC3517] Ethan Blanton, Mark Allman, Kevin Fall, Lili Wang. A
Conservative Selective Acknowledgment (SACK)-based Loss Recovery
Algorithm for TCP. RFC 3517, April 2003.
Author's Addresses:
Mark Allman
ICSI Center for Internet Research (ICIR)
1947 Center Street, Suite 600
Berkeley, CA 94704-1198
Phone: 440-235-1792
mallman@icir.org
http://www.icir.org/mallman/
Konstantin Avrachenkov
INRIA
2004 route des Lucioles, B.P.93
06902, Sophia Antipolis
France
Phone: 00 33 492 38 7751
k.avrachenkov@sophia.inria.fr
http://www.inria.fr/mistral/personnel/K.Avrachenkov/moi.html
Urtzi Ayesta
LAAS-CNRS
7 Avenue Colonel Roche
31077 Toulouse
France
urtzi@laas.fr
http://www.laas.fr/~urtzi
Josh Blanton
Ohio University
301 Stocker Center
Athens, OH 45701
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jblanton@irg.cs.ohiou.edu
Appendix A: Research Issues in Adjusting the Duplicate ACK Threshold
Decreasing the number of duplicate ACKs required to trigger Fast
Retransmit, as suggested in section 2, has the drawback of making
Fast Retransmit less robust in the face of minor network reordering.
Two egregious examples of problems caused by reordering are given in
section 3. This appendix outlines several schemes that have been
suggested to mitigate the problems caused to Early Retransmit by
reordering. These methods need further research before they are
suggested for general use (and, current consensus is that the cases
that make Early Retransmit unnecessarily retransmit a large amount
of data are pathological and therefore these mitigations are not
generally required).
MITIGATION A.1: Allow a connection to use Early Retransmit as long
as the algorithm is not injecting "too much" spurious data into the
network. For instance, using the information provided by TCP's
DSACK option [RFC2883] or SCTP's Duplicate-TSN notification, a
sender can determine when segments sent via Early Retransmit are
needless. Likewise, using Eifel [RFC3522] the sender can detect
spurious Early Retransmits. Once spurious Early Retransmits are
detected the sender can either eliminate the use of Early Retransmit
or limit the use of the algorithm to ensure that an acceptably small
fraction of the connection's transmissions are not spurious. For
example, a connection could stop using Early Retransmit after the
first spurious retransmit is detected.
Alternatively, if a sender cannot reliably determine if an Early
Retransmitted segment is spurious or not the sender could simply
limit Early Retransmits either to some fixed number per connection
(e.g., Early Retransmit is allowed only once per connection) or to
some small percentage of the total traffic being transmitted.
MITIGATION A.2: Allow a connection to trigger Early Retransmit using
the criteria given in section 2, in addition to a "small" timeout
[Pax97]. For instance, a sender may have to wait for 2 duplicate
ACKs and then T msec before Early Retransmit is invoked. The added
time gives reordered acknowledgments time to arrive at the sender
and avoid a needless retransmit. Designing a method for choosing an
appropriate timeout is part of the research that would need to be
involved in this scheme.
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