Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions

Network Working Group                                         E. Blanton
Request for Comments: 3708                             Purdue University
Category: Experimental                                         M. Allman
                                                                    ICIR
                                                           February 2004


      Using TCP Duplicate Selective Acknowledgement (DSACKs) and
         Stream Control Transmission Protocol (SCTP) Duplicate
        Transmission Sequence Numbers (TSNs) to Detect Spurious
                            Retransmissions

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   TCP and Stream Control Transmission Protocol (SCTP) provide
   notification of duplicate segment receipt through Duplicate Selective
   Acknowledgement (DSACKs) and Duplicate Transmission Sequence Number
   (TSN) notification, respectively.  This document presents
   conservative methods of using this information to identify
   unnecessary retransmissions for various applications.

1.  Introduction

   TCP [RFC793] and SCTP [RFC2960] provide notification of duplicate
   segment receipt through duplicate selective acknowledgment (DSACK)
   [RFC2883] and Duplicate TSN notifications, respectively.  Using this
   information, a TCP or SCTP sender can generally determine when a
   retransmission was sent in error.  This document presents two methods
   for using duplicate notifications.  The first method is simple and
   can be used for accounting applications.  The second method is a
   conservative algorithm to disambiguate unnecessary retransmissions
   from loss events for the purpose of undoing unnecessary congestion
   control changes.

   This document is intended to outline reasonable and safe algorithms
   for detecting spurious retransmissions and discuss some of the
   considerations involved.  It is not intended to describe the only
   possible method for achieving the goal, although the guidelines in
   this document should be taken into consideration when designing
   alternate algorithms.  Additionally, this document does not outline
   what a TCP or SCTP sender may do after a spurious retransmission is
   detected.  A number of proposals have been developed (e.g.,
   [RFC3522], [SK03], [BDA03]), but it is not yet clear which of these
   proposals are appropriate.  In addition, they all rely on detecting
   spurious retransmits and so can share the algorithm specified in this
   document.

   Finally, we note that to simplify the text much of the following
   discussion is in terms of TCP DSACKs, while applying to both TCP and
   SCTP.

   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].

2.  Counting Duplicate Notifications

   For certain applications a straight count of duplicate notifications
   will suffice.  For instance, if a stack simply wants to know (for
   some reason) the number of spuriously retransmitted segments,
   counting all duplicate notifications for retransmitted segments
   should work well.  Another application of this strategy is to monitor
   and adapt transport algorithms so that the transport is not sending
   large amounts of spurious data into the network.  For instance,
   monitoring duplicate notifications could be used by the Early
   Retransmit [AAAB03] algorithm to determine whether fast
   retransmitting [RFC2581] segments with a lower than normal duplicate
   ACK threshold is working, or if segment reordering is causing
   spurious retransmits.

   More speculatively, duplicate notification has been proposed as an
   integral part of estimating TCP's total loss rate [AEO03] for the
   purposes of mitigating the impact of corruption-based losses on
   transport protocol performance.  [EOA03] proposes altering the
   transport's congestion response to the fraction of losses that are
   actually due to congestion by requiring the network to provide the
   corruption-based loss rate and making the transport sender estimate
   the total loss rate.  Duplicate notifications are a key part of
   estimating the total loss rate accurately [AEO03].

3.  Congestion/Duplicate Disambiguation Algorithm

   When the purpose of detecting spurious retransmissions is to "undo"
   unnecessary changes made to the congestion control state, as
   suggested in [RFC2883], the data sender ideally needs to determine:

   (a) That spurious retransmissions in a particular window of data do
       not mask real segment loss (congestion).

       For example, assume segments N and N+1 are retransmitted even
       though only segment N was dropped by the network (thus, segment
       N+1 was needlessly retransmitted).  When the sender receives the
       notification that segment N+1 arrived more than once it can
       conclude that segment N+1 was needlessly resent.  However, it
       cannot conclude that it is appropriate to revert the congestion
       control state because the window of data contained at least one
       valid congestion indication (i.e., segment N was lost).

   (b) That network duplication is not the cause of the duplicate
       notification.

       Determining whether a duplicate notification is caused by network
       duplication of a packet or a spurious retransmit is a nearly
       impossible task in theory.  Since [Pax97] shows that packet
       duplication by the network is rare, the algorithm in this section
       simply ceases to function when network duplication is detected
       (by receiving a duplication notification for a segment that was
       not retransmitted by the sender).

   The algorithm specified below gives reasonable, but not complete,
   protection against both of these cases.

   We assume the TCP sender has a data structure to hold selective
   acknowledgment information (e.g., as outlined in [RFC3517]).  The
   following steps require an extension of such a 'scoreboard' to
   incorporate a slightly longer history of retransmissions than called
   for in [RFC3517].  The following steps MUST be taken upon the receipt
   of each DSACK or duplicate TSN notification:

   (A) Check the corresponding sequence range or TSN to determine
       whether the segment has been retransmitted.

       (A.1) If the SACK scoreboard is empty (i.e., the TCP sender has
             received no SACK information from the receiver) and the
             left edge of the incoming DSACK is equal to SND.UNA,
             processing of this DSACK MUST be terminated and the
             congestion control state MUST NOT be reverted during the
             current window of data.  This clause intends to cover the
             case when an entire window of acknowledgments have been
             dropped by the network.  In such a case, the reverse path
             seems to be in a congested state and so reducing TCP's
             sending rate is the conservative approach.

       (A.2) If the segment was retransmitted exactly one time, mark it
             as a duplicate.

       (A.3) If the segment was retransmitted more than once processing
             of this DSACK MUST be terminated and the congestion control
             state MUST NOT be reverted to its previous state during the
             current window of data.

       (A.4) If the segment was not retransmitted the incoming DSACK
             indicates that the network duplicated the segment in
             question.  Processing of this DSACK MUST be terminated.  In
             addition, the algorithm specified in this document MUST NOT
             be used for the remainder of the connection, as future
             DSACK reports may be indicating network duplication rather
             than unnecessary retransmission.  Note that some techniques
             to further disambiguate network duplication from
             unnecessary retransmission (e.g., the TCP timestamp option
             [RFC1323]) may be used to refine the algorithm in this
             document further.  Using such a technique in conjunction
             with an algorithm similar to the one presented herein may
             allow for the continued use of the algorithm in the face of
             duplicated segments.  We do not delve into such an
             algorithm in this document due the current rarity of
             network duplication.  However, future work should include
             tackling this problem.

   (B) Assuming processing is allowed to continue (per the (A) rules),
       check all retransmitted segments in the previous window of data.

       (B.1) If all segments or chunks marked as retransmitted have also
             been marked as acknowledged and duplicated, we conclude
             that all retransmissions in the previous window of data
             were spurious and no loss occurred.

       (B.2) If any segment or chunk is still marked as retransmitted
             but not marked as duplicate, there are outstanding
             retransmissions that could indicate loss within this window
             of data.  We can make no conclusions based on this
             particular DSACK/duplicate TSN notification.

   In addition to keeping the state mentioned in [RFC3517] (for TCP) and
   [RFC2960] (for SCTP), an implementation of this algorithm must track
   all sequence numbers or TSNs that have been acknowledged as
   duplicates.

4.  Related Work

   In addition to the mechanism for detecting spurious retransmits
   outlined in this document, several other proposals for finding
   needless retransmits have been developed.

   [BA02] uses the algorithm outlined in this document as the basis for
   investigating several methods to make TCP more robust to reordered
   packets.

   The Eifel detection algorithm [RFC3522] uses the TCP timestamp option
   [RFC1323] to determine whether the ACK for a given retransmit is for
   the original transmission or a retransmission.  More generally,
   [LK00] outlines the benefits of detecting spurious retransmits and
   reverting from needless congestion control changes using the
   timestamp-based scheme or a mechanism that uses a "retransmit bit" to
   flag retransmits (and ACKs of retransmits).  The Eifel detection
   algorithm can detect spurious retransmits more rapidly than a DSACK-
   based scheme.  However, the tradeoff is that the overhead of the 12-
   byte timestamp option must be incurred in every packet transmitted
   for Eifel to function.

   The F-RTO scheme [SK03] slightly alters TCP's sending pattern
   immediately following a retransmission timeout and then observes the
   pattern of the returning ACKs.  This pattern can indicate whether the
   retransmitted segment was needed.  The advantage of F-RTO is that the
   algorithm only needs to be implemented on the sender side of the TCP
   connection and that nothing extra needs to cross the network (e.g.,
   DSACKs, timestamps, special flags, etc.).  The downside is that the
   algorithm is a heuristic that can be confused by network pathologies
   (e.g., duplication or reordering of key packets).  Finally, note that
   F-RTO only works for spurious retransmits triggered by the
   transport's retransmission timer.

   Finally, [AP99] briefly investigates using the time between
   retransmitting a segment via the retransmission timeout and the
   arrival of the next ACK as an indicator of whether the retransmit was
   needed.  The scheme compares this time delta with a fraction (f) of
   the minimum RTT observed thus far on the connection.  If the time
   delta is less than f*minRTT then the retransmit is labeled spurious.
   When f=1/2 the algorithm identifies roughly 59% of the needless
   retransmission timeouts and identifies needed retransmits only 2.5%
   of the time.  As with F-RTO, this scheme only detects spurious
   retransmits sent by the transport's retransmission timer.

5.  Security Considerations

   It is possible for the receiver to falsely indicate spurious
   retransmissions in the case of actual loss, potentially causing a TCP
   or SCTP sender to inaccurately conclude that no loss took place (and
   possibly cause inappropriate changes to the senders congestion
   control state).

   Consider the following scenario: A receiver watches every segment or
   chunk that arrives and acknowledges any segment that arrives out of
   order by more than some threshold amount as a duplicate, assuming
   that it is a retransmission.  A sender using the above algorithm will
   assume that the retransmission was spurious.

   The ECN nonce sum proposal [RFC3540] could possibly help mitigate the
   ability of the receiver to hide real losses from the sender with
   modest extension.  In the common case of receiving an original
   transmission and a spurious retransmit a receiver will have received
   the nonce from the original transmission and therefore can "prove" to
   the sender that the duplication notification is valid.  In the case
   when the receiver did not receive the original and is trying to
   improperly induce the sender into transmitting at an inappropriately
   high rate, the receiver will not know the ECN nonce from the original
   segment and therefore will probabilistically not be able to fool the
   sender for long.  [RFC3540] calls for disabling nonce sums on
   duplicate ACKs, which means that the nonce sum is not directly
   suitable for use as a mitigation to the problem of receivers lying
   about DSACK information.  However, future efforts may be able to use
   [RFC3540] as a starting point for building protection should it be
   needed.

6.  Acknowledgments

   Sourabh Ladha and Reiner Ludwig made several useful comments on an
   earlier version of this document.  The second author thanks BBN
   Technologies and NASA's Glenn Research Center for supporting this
   work.

7. References

7.1.  Normative References

   [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
             793, September 1981.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2883] Floyd, S., Mahdavi, J., Mathis, M. and M. Podolsky, "An
             Extension to the Selective Acknowledgement (SACK) Option
             for TCP", RFC 2883, July 2000.

   [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
             Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
             L. and V. Paxson, "Stream Control Transmission Protocol",
             RFC 2960, October 2000.

7.2.  Informative References

   [AAAB03]  Allman, M., Avrachenkov, K., Ayesta, U. and J. Blanton,
             "Early Retransmit for TCP", Work in Progress, June 2003.

   [AEO03]   Allman, M., Eddy, E. and S. Ostermann, "Estimating Loss
             Rates With TCP", Work in Progress, August 2003.

   [AP99]    Allman, M. and V. Paxson, "On Estimating End-to-End Network
             Path Properties", SIGCOMM 99.

   [BA02]    Blanton, E. and M. Allman.  On Making TCP More Robust to
             Packet Reordering.  ACM Computer Communication Review,
             32(1), January 2002.

   [BDA03]   Blanton, E., Dimond, R. and M. Allman, "Practices for TCP
             Senders in the Face of Segment Reordering", Work in
             Progress, February 2003.

   [EOA03]   Eddy, W., Ostermann, S. and M. Allman, "New Techniques for
             Making Transport Protocols Robust to Corruption-Based
             Loss", Work in Progress, July 2003.

   [LK00]    R. Ludwig, R. H. Katz.  The Eifel Algorithm: Making TCP
             Robust Against Spurious Retransmissions.  ACM Computer
             Communication Review, 30(1), January 2000.

   [Pax97]   V. Paxson.  End-to-End Internet Packet Dynamics.  In ACM
             SIGCOMM, September 1997.

   [RFC1323] Jacobson, V., Braden, R. and D. Borman,  "TCP Extensions
             for High Performance", RFC 1323, May 1992.

   [RFC3517] Blanton, E., Allman, M., Fall, K. and L. Wang, "A
             Conservative Selective Acknowledgment (SACK)-based Loss
             Recovery Algorithm for TCP", RFC 3517, April 2003.

   [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for
             TCP," RFC 3522, April 2003.

   [RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit
             Congestion Notification (ECN) Signaling with Nonces", RFC
             3540, June 2003.

   [SK03]    Sarolahti, P. and M. Kojo, "F-RTO: An Algorithm for
             Detecting Spurious Retransmission Timeouts with TCP and
             SCTP", Work in Progress, June 2003.

8.  Authors' Addresses

   Ethan Blanton
   Purdue University Computer Sciences
   1398 Computer Science Building
   West Lafayette, IN  47907

   EMail: eblanton@cs.purdue.edu


   Mark Allman
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704-1198
   Phone: 216-243-7361

   EMail: mallman@icir.org
   http://www.icir.org/mallman/

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