Network Working Group E. Blanton
Request for Comments: 3517 Purdue University
Category: Standards Track M. Allman
BBN/NASA GRC
K. Fall
Intel Research
L. Wang
University of Kentucky
April 2003
A Conservative Selective Acknowledgment (SACK)-based
Loss Recovery Algorithm for TCP
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document presents a conservative loss recovery algorithm for TCP
that is based on the use of the selective acknowledgment (SACK) TCP
option. The algorithm presented in this document conforms to the
spirit of the current congestion control specification (RFC 2581),
but allows TCP senders to recover more effectively when multiple
segments are lost from a single flight of data.
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 BCP 14, RFC 2119
[RFC2119].
Blanton, et al. Standards Track [Page 1]
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1 Introduction
This document presents a conservative loss recovery algorithm for TCP
that is based on the use of the selective acknowledgment (SACK) TCP
option. While the TCP SACK [RFC2018] is being steadily deployed in
the Internet [All00], there is evidence that hosts are not using the
SACK information when making retransmission and congestion control
decisions [PF01]. The goal of this document is to outline one
straightforward method for TCP implementations to use SACK
information to increase performance.
[RFC2581] allows advanced loss recovery algorithms to be used by TCP
[RFC793] provided that they follow the spirit of TCP's congestion
control algorithms [RFC2581, RFC2914]. [RFC2582] outlines one such
advanced recovery algorithm called NewReno. This document outlines a
loss recovery algorithm that uses the SACK [RFC2018] TCP option to
enhance TCP's loss recovery. The algorithm outlined in this
document, heavily based on the algorithm detailed in [FF96], is a
conservative replacement of the fast recovery algorithm [Jac90,
RFC2581]. The algorithm specified in this document is a
straightforward SACK-based loss recovery strategy that follows the
guidelines set in [RFC2581] and can safely be used in TCP
implementations. Alternate SACK-based loss recovery methods can be
used in TCP as implementers see fit (as long as the alternate
algorithms follow the guidelines provided in [RFC2581]). Please
note, however, that the SACK-based decisions in this document (such
as what segments are to be sent at what time) are largely decoupled
from the congestion control algorithms, and as such can be treated as
separate issues if so desired.
2 Definitions
The reader is expected to be familiar with the definitions given in
[RFC2581].
The reader is assumed to be familiar with selective acknowledgments
as specified in [RFC2018].
For the purposes of explaining the SACK-based loss recovery algorithm
we define four variables that a TCP sender stores:
"HighACK" is the sequence number of the highest byte of data that
has been cumulatively ACKed at a given point.
"HighData" is the highest sequence number transmitted at a given
point.
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"HighRxt" is the highest sequence number which has been
retransmitted during the current loss recovery phase.
"Pipe" is a sender's estimate of the number of bytes outstanding
in the network. This is used during recovery for limiting the
sender's sending rate. The pipe variable allows TCP to use a
fundamentally different congestion control than specified in
[RFC2581]. The algorithm is often referred to as the "pipe
algorithm".
For the purposes of this specification we define a "duplicate
acknowledgment" as a segment that arrives with no data and an
acknowledgment (ACK) number that is equal to the current value of
HighACK, as described in [RFC2581].
We define a variable "DupThresh" that holds the number of duplicate
acknowledgments required to trigger a retransmission. Per [RFC2581]
this threshold is defined to be 3 duplicate acknowledgments.
However, implementers should consult any updates to [RFC2581] to
determine the current value for DupThresh (or method for determining
its value).
Finally, a range of sequence numbers [A,B] is said to "cover"
sequence number S if A <= S <= B.
3 Keeping Track of SACK Information
For a TCP sender to implement the algorithm defined in the next
section it must keep a data structure to store incoming selective
acknowledgment information on a per connection basis. Such a data
structure is commonly called the "scoreboard". The specifics of the
scoreboard data structure are out of scope for this document (as long
as the implementation can perform all functions required by this
specification).
Note that this document refers to keeping account of (marking)
individual octets of data transferred across a TCP connection. A
real-world implementation of the scoreboard would likely prefer to
manage this data as sequence number ranges. The algorithms presented
here allow this, but require arbitrary sequence number ranges to be
marked as having been selectively acknowledged.
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4 Processing and Acting Upon SACK Information
For the purposes of the algorithm defined in this document the
scoreboard SHOULD implement the following functions:
Update ():
Given the information provided in an ACK, each octet that is
cumulatively ACKed or SACKed should be marked accordingly in the
scoreboard data structure, and the total number of octets SACKed
should be recorded.
Note: SACK information is advisory and therefore SACKed data MUST
NOT be removed from TCP's retransmission buffer until the data is
cumulatively acknowledged [RFC2018].
IsLost (SeqNum):
This routine returns whether the given sequence number is
considered to be lost. The routine returns true when either
DupThresh discontiguous SACKed sequences have arrived above
'SeqNum' or (DupThresh * SMSS) bytes with sequence numbers greater
than 'SeqNum' have been SACKed. Otherwise, the routine returns
false.
SetPipe ():
This routine traverses the sequence space from HighACK to HighData
and MUST set the "pipe" variable to an estimate of the number of
octets that are currently in transit between the TCP sender and
the TCP receiver. After initializing pipe to zero the following
steps are taken for each octet 'S1' in the sequence space between
HighACK and HighData that has not been SACKed:
(a) If IsLost (S1) returns false:
Pipe is incremented by 1 octet.
The effect of this condition is that pipe is incremented for
packets that have not been SACKed and have not been determined
to have been lost (i.e., those segments that are still assumed
to be in the network).
(b) If S1 <= HighRxt:
Pipe is incremented by 1 octet.
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The effect of this condition is that pipe is incremented for
the retransmission of the octet.
Note that octets retransmitted without being considered lost are
counted twice by the above mechanism.
NextSeg ():
This routine uses the scoreboard data structure maintained by the
Update() function to determine what to transmit based on the SACK
information that has arrived from the data receiver (and hence
been marked in the scoreboard). NextSeg () MUST return the
sequence number range of the next segment that is to be
transmitted, per the following rules:
(1) If there exists a smallest unSACKed sequence number 'S2' that
meets the following three criteria for determining loss, the
sequence range of one segment of up to SMSS octets starting
with S2 MUST be returned.
(1.a) S2 is greater than HighRxt.
(1.b) S2 is less than the highest octet covered by any
received SACK.
(1.c) IsLost (S2) returns true.
(2) If no sequence number 'S2' per rule (1) exists but there
exists available unsent data and the receiver's advertised
window allows, the sequence range of one segment of up to SMSS
octets of previously unsent data starting with sequence number
HighData+1 MUST be returned.
(3) If the conditions for rules (1) and (2) fail, but there exists
an unSACKed sequence number 'S3' that meets the criteria for
detecting loss given in steps (1.a) and (1.b) above
(specifically excluding step (1.c)) then one segment of up to
SMSS octets starting with S3 MAY be returned.
Note that rule (3) is a sort of retransmission "last resort".
It allows for retransmission of sequence numbers even when the
sender has less certainty a segment has been lost than as with
rule (1). Retransmitting segments via rule (3) will help
sustain TCP's ACK clock and therefore can potentially help
avoid retransmission timeouts. However, in sending these
segments the sender has two copies of the same data considered
to be in the network (and also in the Pipe estimate). When an
ACK or SACK arrives covering this retransmitted segment, the
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sender cannot be sure exactly how much data left the network
(one of the two transmissions of the packet or both
transmissions of the packet). Therefore the sender may
underestimate Pipe by considering both segments to have left
the network when it is possible that only one of the two has.
We believe that the triggering of rule (3) will be rare and
that the implications are likely limited to corner cases
relative to the entire recovery algorithm. Therefore we leave
the decision of whether or not to use rule (3) to
implementors.
(4) If the conditions for each of (1), (2), and (3) are not met,
then NextSeg () MUST indicate failure, and no segment is
returned.
Note: The SACK-based loss recovery algorithm outlined in this
document requires more computational resources than previous TCP loss
recovery strategies. However, we believe the scoreboard data
structure can be implemented in a reasonably efficient manner (both
in terms of computation complexity and memory usage) in most TCP
implementations.
5 Algorithm Details
Upon the receipt of any ACK containing SACK information, the
scoreboard MUST be updated via the Update () routine.
Upon the receipt of the first (DupThresh - 1) duplicate ACKs, the
scoreboard is to be updated as normal. Note: The first and second
duplicate ACKs can also be used to trigger the transmission of
previously unsent segments using the Limited Transmit algorithm
[RFC3042].
When a TCP sender receives the duplicate ACK corresponding to
DupThresh ACKs, the scoreboard MUST be updated with the new SACK
information (via Update ()). If no previous loss event has occurred
on the connection or the cumulative acknowledgment point is beyond
the last value of RecoveryPoint, a loss recovery phase SHOULD be
initiated, per the fast retransmit algorithm outlined in [RFC2581].
The following steps MUST be taken:
(1) RecoveryPoint = HighData
When the TCP sender receives a cumulative ACK for this data octet
the loss recovery phase is terminated.
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(2) ssthresh = cwnd = (FlightSize / 2)
The congestion window (cwnd) and slow start threshold (ssthresh)
are reduced to half of FlightSize per [RFC2581].
(3) Retransmit the first data segment presumed dropped -- the segment
starting with sequence number HighACK + 1. To prevent repeated
retransmission of the same data, set HighRxt to the highest
sequence number in the retransmitted segment.
(4) Run SetPipe ()
Set a "pipe" variable to the number of outstanding octets
currently "in the pipe"; this is the data which has been sent by
the TCP sender but for which no cumulative or selective
acknowledgment has been received and the data has not been
determined to have been dropped in the network. It is assumed
that the data is still traversing the network path.
(5) In order to take advantage of potential additional available
cwnd, proceed to step (C) below.
Once a TCP is in the loss recovery phase the following procedure MUST
be used for each arriving ACK:
(A) An incoming cumulative ACK for a sequence number greater than
RecoveryPoint signals the end of loss recovery and the loss
recovery phase MUST be terminated. Any information contained in
the scoreboard for sequence numbers greater than the new value of
HighACK SHOULD NOT be cleared when leaving the loss recovery
phase.
(B) Upon receipt of an ACK that does not cover RecoveryPoint the
following actions MUST be taken:
(B.1) Use Update () to record the new SACK information conveyed
by the incoming ACK.
(B.2) Use SetPipe () to re-calculate the number of octets still
in the network.
(C) If cwnd - pipe >= 1 SMSS the sender SHOULD transmit one or more
segments as follows:
(C.1) The scoreboard MUST be queried via NextSeg () for the
sequence number range of the next segment to transmit (if any),
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and the given segment sent. If NextSeg () returns failure (no
data to send) return without sending anything (i.e., terminate
steps C.1 -- C.5).
(C.2) If any of the data octets sent in (C.1) are below HighData,
HighRxt MUST be set to the highest sequence number of the
retransmitted segment.
(C.3) If any of the data octets sent in (C.1) are above HighData,
HighData must be updated to reflect the transmission of
previously unsent data.
(C.4) The estimate of the amount of data outstanding in the
network must be updated by incrementing pipe by the number of
octets transmitted in (C.1).
(C.5) If cwnd - pipe >= 1 SMSS, return to (C.1)
5.1 Retransmission Timeouts
In order to avoid memory deadlocks, the TCP receiver is allowed to
discard data that has already been selectively acknowledged. As a
result, [RFC2018] suggests that a TCP sender SHOULD expunge the SACK
information gathered from a receiver upon a retransmission timeout
"since the timeout might indicate that the data receiver has
reneged." Additionally, a TCP sender MUST "ignore prior SACK
information in determining which data to retransmit." However, a
SACK TCP sender SHOULD still use all SACK information made available
during the slow start phase of loss recovery following an RTO.
If an RTO occurs during loss recovery as specified in this document,
RecoveryPoint MUST be set to HighData. Further, the new value of
RecoveryPoint MUST be preserved and the loss recovery algorithm
outlined in this document MUST be terminated. In addition, a new
recovery phase (as described in section 5) MUST NOT be initiated
until HighACK is greater than or equal to the new value of
RecoveryPoint.
As described in Sections 4 and 5, Update () SHOULD continue to be
used appropriately upon receipt of ACKs. This will allow the slow
start recovery period to benefit from all available information
provided by the receiver, despite the fact that SACK information was
expunged due to the RTO.
If there are segments missing from the receiver's buffer following
processing of the retransmitted segment, the corresponding ACK will
contain SACK information. In this case, a TCP sender SHOULD use this
SACK information when determining what data should be sent in each
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segment of the slow start. The exact algorithm for this selection is
not specified in this document (specifically NextSeg () is
inappropriate during slow start after an RTO). A relatively
straightforward approach to "filling in" the sequence space reported
as missing should be a reasonable approach.
6 Managing the RTO Timer
The standard TCP RTO estimator is defined in [RFC2988]. Due to the
fact that the SACK algorithm in this document can have an impact on
the behavior of the estimator, implementers may wish to consider how
the timer is managed. [RFC2988] calls for the RTO timer to be
re-armed each time an ACK arrives that advances the cumulative ACK
point. Because the algorithm presented in this document can keep the
ACK clock going through a fairly significant loss event,
(comparatively longer than the algorithm described in [RFC2581]), on
some networks the loss event could last longer than the RTO. In this
case the RTO timer would expire prematurely and a segment that need
not be retransmitted would be resent.
Therefore we give implementers the latitude to use the standard
[RFC2988] style RTO management or, optionally, a more careful variant
that re-arms the RTO timer on each retransmission that is sent during
recovery MAY be used. This provides a more conservative timer than
specified in [RFC2988], and so may not always be an attractive
alternative. However, in some cases it may prevent needless
retransmissions, go-back-N transmission and further reduction of the
congestion window.
7 Research
The algorithm specified in this document is analyzed in [FF96], which
shows that the above algorithm is effective in reducing transfer time
over standard TCP Reno [RFC2581] when multiple segments are dropped
from a window of data (especially as the number of drops increases).
[AHKO97] shows that the algorithm defined in this document can
greatly improve throughput in connections traversing satellite
channels.
8 Security Considerations
The algorithm presented in this paper shares security considerations
with [RFC2581]. A key difference is that an algorithm based on SACKs
is more robust against attackers forging duplicate ACKs to force the
TCP sender to reduce cwnd. With SACKs, TCP senders have an
additional check on whether or not a particular ACK is legitimate.
While not fool-proof, SACK does provide some amount of protection in
this area.
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Acknowledgments
The authors wish to thank Sally Floyd for encouraging this document
and commenting on early drafts. The algorithm described in this
document is loosely based on an algorithm outlined by Kevin Fall and
Sally Floyd in [FF96], although the authors of this document assume
responsibility for any mistakes in the above text. Murali Bashyam,
Ken Calvert, Tom Henderson, Reiner Ludwig, Jamshid Mahdavi, Matt
Mathis, Shawn Ostermann, Vern Paxson and Venkat Venkatsubra provided
valuable feedback on earlier versions of this document. We thank
Matt Mathis and Jamshid Mahdavi for implementing the scoreboard in ns
and hence guiding our thinking in keeping track of SACK state.
The first author would like to thank Ohio University and the Ohio
University Internetworking Research Group for supporting the bulk of
his work on this project.
Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2581] Allman, M., Paxson, V. and R. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
Informative References
[AHKO97] Mark Allman, Chris Hayes, Hans Kruse, Shawn Ostermann. TCP
Performance Over Satellite Links. Proceedings of the Fifth
International Conference on Telecommunications Systems,
Nashville, TN, March, 1997.
[All00] Mark Allman. A Web Server's View of the Transport Layer.
ACM Computer Communication Review, 30(5), October 2000.
[FF96] Kevin Fall and Sally Floyd. Simulation-based Comparisons
of Tahoe, Reno and SACK TCP. Computer Communication
Review, July 1996.
Blanton, et al. Standards Track [Page 10]
RFC 3517 SACK-based Loss Recovery for TCP April 2003
[Jac90] Van Jacobson. Modified TCP Congestion Avoidance Algorithm.
Technical Report, LBL, April 1990.
[PF01] Jitendra Padhye, Sally Floyd. Identifying the TCP Behavior
of Web Servers, ACM SIGCOMM, August 2001.
[RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
TCP's Fast Recovery Algorithm", RFC 2582, April 1999.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3042] Allman, M., Balakrishnan, H, and S. Floyd, "Enhancing TCP's
Loss Recovery Using Limited Transmit", RFC 3042, January
2001.
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Blanton, et al. Standards Track [Page 11]
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Authors' Addresses
Ethan Blanton
Purdue University Computer Sciences
1398 Computer Science Building
West Lafayette, IN 47907
EMail: eblanton@cs.purdue.edu
Mark Allman
BBN Technologies/NASA Glenn Research Center
Lewis Field
21000 Brookpark Rd. MS 54-5
Cleveland, OH 44135
Phone: 216-433-6586
Fax: 216-433-8705
EMail: mallman@bbn.com
http://roland.grc.nasa.gov/~mallman
Kevin Fall
Intel Research
2150 Shattuck Ave., PH Suite
Berkeley, CA 94704
EMail: kfall@intel-research.net
Lili Wang
Laboratory for Advanced Networking
210 Hardymon Building
University of Kentucky
Lexington, KY 40506-0495
EMail: lwang0@uky.edu
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RFC, FYI, BCP