One document matched: draft-allman-tcp-early-rexmt-03.txt
Differences from draft-allman-tcp-early-rexmt-02.txt
Internet Engineering Task Force Mark Allman
INTERNET DRAFT ICIR
File: draft-allman-tcp-early-rexmt-03.txt Konstantin Avrachenkov
INRIA
Urtzi Ayesta
France Telecom R&D
Josh Blanton
Ohio University
December, 2003
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Early Retransmit for TCP and SCTP
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
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Internet-Drafts are draft documents valid for a maximum of six
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at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document proposes a new mechanism for TCP and SCTP that can be
used to more effectively 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1 Introduction
A number of researchers have pointed out that the loss recovery
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strategies employed by TCP [RFC793] and SCTP [RFC2960] do not work
well when the amount of outstanding data a TCP sender has injected
into the network is small. This can happen in a number of
situations, such as:
(1) 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.
(2) The connection is limited by the receiver-advertised window.
(3) 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.
Many researchers have studied problems with TCP when the congestion
window is small and have outlined possible mechanisms to mitigate
these problems [Mor97,BPS+98,Bal98,LK98,RFC3150,AA02]. SCTP's 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 using one of two methods.
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]. Second, the ``Fast
Retransmit'' algorithm resends a segment when three duplicate ACKs
arrive at the sender [Jac88,RFC2581]. However, because duplicate
ACKs from the receiver are also triggered by packet reordering in
the Internet, the sender waits for three duplicate ACKs in an
attempt to disambiguate segment loss from packet reordering. When
using small windows it may not be possible to generate the required
number of duplicate ACKs to trigger Fast Retransmit when a loss does
happen.
Once in a loss recovery phase, a number of techniques can be used to
retransmit lost segments. 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.
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.
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
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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
without relying 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.
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 Reduction of the Retransmission Threshold
The Early Retransmit algorithm calls for lowering the threshold for
triggering Fast Retransmit when the amount of outstanding data is
small and when no unsent data segments are enqueued. We define
variants of Early Retransmit for connections that do and do 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.)
If the following two conditions hold the sender can use Early
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Retransmit (regardless of SACK support).
(2.a) The amount of outstanding data (ownd) 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 and the connection does not
support SACK the duplicate ACK threshold used to trigger Fast
Retransmit MAY 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) hold and the connection does support
SACK Fast Retransmit MAY be used 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 has
arrived at the receiver. This mitigation is less robust in the face
of reordered segments than the standard Fast Retransmit threshold.
Research shows that a general reduction in the number of duplicate
ACKs required to trigger fast retransmission of a segment 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.
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
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 and the arrival of S3 triggers a second ACK
(because it is out-of-order). Both ACKs will cover the same
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segment (S1). Therefore, regardless of whether SACK is used
Early Retransmit can be performed by the sender (assuming no ACK
loss).
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 (and one-third of all segments)
when using the above algorithm. 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. Rather, the authors believe that the community's
consensus is that Early Retransmit is scoped enough that the worst
case problems are pathological and do not need mitigation at this
time. However, Appendix A offers a survey of possible mitigations.
3 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).
4 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.
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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.
[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:
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Analysis and Improvements. Proc. IEEE INFOCOM Conf., San
Francisco, CA, March 1998.
[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.
[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: 216-243-7361
mallman@icir.org
http://www.icir.org/mallman/
Konstantin Avrachenkov
INRIA
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2004 route des Lucioles, B.P.93
06902, Sophia Antipolis
France
Phone: 00 33 492 38 7751
Email: k.avrachenkov@sophia.inria.fr
http://www.inria.fr/mistral/personnel/K.Avrachenkov/moi.html
Urtzi Ayesta
France Telecom R&D
905 rue Albert Einstein
06921 Sophia Antipolis
France
Email: Urtzi.Ayesta@francetelecom.com
http://www.inria.fr/mistral/personnel/Urtzi.Ayesta/me.html
Josh Blanton
Ohio University
301 Stocker Center
Athens, OH 45701
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 2. 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 patalogical 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 a "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.
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
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[Pax97]. For instance, a sender may have to wait for 2 duplicate
ACKs and then T msec before Early Retransmitting a segment. 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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