Internet Engineering Task Force Ethan Blanton INTERNET DRAFT Ohio University File: draft-blanton-dsack-use-00.txt Mark Allman BBN/NASA GRC July, 2001 Expires: January, 2002 Using TCP DSACKs and SCTP Duplicate TSNs to Detect Spurious Retransmissions 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 Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents 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 TCP and SCTP provide notification of duplicate segment receipt through DSACK and Duplicate TSN notification, respectively. This document presents a conservative method of using this information to identify unnecessary retransmissions. 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 TCP [RFC793] and SCTP [RFC2960] provide notification of duplicate segment receipt through 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. While DSACKs and duplicate TSN notifications can be caused by segments being duplicated by the network [Pax97] shows this is exceedingly rare. This document presents a conservative algorithm to disambiguate unnecessary retransmissions from loss events. Expries: January 2002 [Page 1]
draft-blanton-dsack-use-00.txt July 2001 This document is intended to outline a reasonable and safe algorithm for detecting spurious retransmissions and discuss some of the considerations involved. It is not intended to describe the only possible method for achieving this goal, although some of the guidelines in this document should be taken into consideration when designing an alternate algorithm. Additionally, this document does not outline what a TCP or SCTP sender may do after a spurious retransmission is detected. Some possibilities are mentioned in [RFC2883], such as reverting changes made to the congestion control state. Discussion of this topic is left to a companion document that makes no assumptions about the manner in which spurious retransmissions are detected [BA01b]. 3 The Algorithm The complexity of the algorithm used for detecting spurious retransmits depends on the purpose in determining this information. For instance, if a sender is only interested in keeping a count of the number of spurious retransmits the information can be derived directly from the returning DSACK or duplicate TSN notifications. However, if the purpose of detecting spurious retransmissions is to ``undo'' unnecessary changes made to the congestion control state, as suggested in [RFC2883], the data sender needs to ensure that all retransmissions in a particular window of data were spurious before reverting the congestion control state. In other words, we must ensure that DSACKs (or duplicate TSN notifications) do not mask real loss. For example, say segments N and N+1 are retransmitted. Assume that segment N was dropped by the network and 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 real congestion indication (i.e., segment N was lost). The following algorithm ensures that all retransmissions sent in a particular window were, in fact, needless. 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 if it has been retransmitted. (A.1) If it was retransmitted, mark it as a duplicate. (A.2) If not, processing of this DSACK or Duplicate TSN is complete and can be terminated. (This is a case when the network path duplicated a segment.) (B) Check all retransmitted segments in the previous window of data. Expries: January 2002 [Page 2]
draft-blanton-dsack-use-00.txt July 2001 (B.1) If all segments or chunks marked as retransmitted have also been marked as duplicate, 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 [BA01a] (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 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 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. As a more trivial example, a receiver could simply acknowledge every segment or chunk received as a duplicate as they arrive. This approach is more easily defeated by heuristics, but would nonetheless cause the algorithm in this document to come to an incorrect conclusion. The ECN nonce sum proposal [WES01] may help mitigate the ability of the receiver to hide real losses from the sender. References [BA01a] E. Blanton, M. Allman. A Conservative SACK-based Loss Recovery Algorithm for TCP. Internet-Draft draft-allman-tcp-sack-06.txt, July 2001. Work in progress. [BA01b] E. Blanton, M. Allman. Adjusting the Duplicate ACK fThreshold to Avoid Spurious Retransmits. Internet-Draft draft-blanton-tcp-dupack-thresh-adjust-00.txt, July 2001. Work in progress. [FF99] S. Floyd, K. Fall. Promoting the Use of End-to-End Congestion Control on the Internet. IEEE/ACM Transactions on Networking, August 1999. Expries: January 2002 [Page 3]
draft-blanton-dsack-use-00.txt July 2001 [Pax97] V. Paxson. End-to-End Internet Packet Dynamics. In ACM SIGCOMM, September 1997. [RFC793] Jon Postel. Transmission Control Protocol. Std 7, RFC 793. September 1981. [RFC2883] S. Floyd, J. Mahdavi, M. Mathis, M. 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. [WES00] D. Wetherall, D. Ely, N. Spring. Robust ECN Signaling with Nonces. Internet-Draft draft-ietf-tsvwg-tcp-nonce-00.txt, January 2001. Work in progress. Authors' Addresses: Ethan Blanton Ohio University Internetworking Research Lab Stocker Center Athens, OH 45701 eblanton@irg.cs.ohiou.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 mallman@bbn.com http://roland.grc.nasa.gov/~mallman Expries: January 2002 [Page 4]
