RFC3708

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.

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.

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].

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.

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.

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.

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.

References

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.

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.

Authors' Addresses

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

EMail: [email protected]

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

EMail: [email protected] http://www.icir.org/mallman/

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