Internet Engineering Task Force M. Allman INTERNET-DRAFT ICSI draft-allman-tcpm-rto-consider-01.txt May 24, 2012 Retransmission Timeout Considerations Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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. This Internet-Draft will expire on November 24, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Abstract This document provides for high-level guidance for retransmission timeout schemes appropriate for general use in the Internet. 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 Expires: November 24, 2012 [Page 1]
draft-allman-tcpm-rto-consider-01.txt May 2012 [RFC2119]. 1 Introduction Despite our best intentions and most robust mechanisms, reliability in networking ultimately requires a timeout and re-try mechanism. Often there are more timely and precise mechanisms for repairing loss (e.g., TCP's fast retransmit [RFC5681], NewReno [RFC6582] or selective acknowledgment scheme [RFC2018,RFC3517]) which require information exchange between components in the system. Such communication cannot be guaranteed. To the contrary, we can always depend on the passage of time and therefore our ultimate backstop to ensuring reliability is a timeout. (Note: There is a case when we cannot count on the passage of time, but in this case we believe repairing loss will be a moot point and hence we do not further consider this case in this document.) Various protocols have defined their own timeout mechanisms (e.g., TCP [RFC6298], SCTP [RFC4960]). The specifics of retransmission timeouts often represent a particular tradeoff between correctness and responsiveness. Therefore, we have found that even though the procedures are standardized, implementations also often add their own subtle imprint on the specifics of the process to tilt the tradeoff between correctness and responsiveness in some way. At this point we recognize that often the specifics are not crucial for network safety. Hence, in this document we outline the high-level principles that are crucial for any retransmission timeout scheme to leverage. The intent is to then allow implementations of protocols and applications to instantiate mechanisms that best realize their specific goals within this framework. These specific mechanisms could be standardized or ad-hoc, but as long as they adhere to the guidelines given in this document they would be considered consistent with the standards. 2 Guidelines We now list the four guidelines that apply when utilizing a retransmission timeout (RTO). (1) In the absence of any knowledge about the round-trip time (RTT) of a path the RTO MUST be conservatively set to no less than 1 second, per TCP's current default RTO [RFC6298]. This guideline ensures two important aspects of the RTO. First, when transmitting into an unknown network, retransmissions will not be sent before an ACK would reasonably be expected to arrive and hence possibly waste scarce network resources. Second, as noted below, sometimes retransmissions can lead to ambiguities in assessing the RTT of a network path. Therefore, it is especially important for the first RTT sample to be free of ambiguities such that there is a baseline for the remainder of the communication. (2) We specify three guidelines that pertain to the sampling of the Expires: November 24, 2012 [Page 2]
draft-allman-tcpm-rto-consider-01.txt May 2012 RTT. (a) In steady state the RTO MUST be set based on recent observations of both the RTT and the variance of the RTT. In other words, the RTO should be based on a reasonable amount of time the sender should wait for an acknowledgment of the data before retransmitting the given data. (b) RTT observations MUST be taken regularly. The exact definition of "regularly" is deliberately left vague. TCP takes an RTT sample once per RTT, or if using the timestamp option [RFC1323] on each acknowledgment arrival. [AP99] shows that both these approaches result in roughly equivalent performance for the RTO estimator. Additionally, [AP99] shows that taking only a single RTT sample per TCP connection is also suboptimal. Therefore, for the purpose of this guideline we state that RTT samples SHOULD be taken at least every RTT or as frequently as data is exchanged and ACKed if that happens less frequently than every RTT. However, we also recognize that it may not always be practical to take an RTT sample this often in all cases and hence this requirement is explicitly a "SHOULD" and not a "MUST". (c) RTT samples used in the computation of the RTO MUST NOT be ambiguous. Assume two copies of some segment X are transmitted at times t0 and t1 and then segment X is acknowledged at time t2. It is not clear which copy of X triggered the ACK and hence the actual RTT is either t2-t1 or t2-t0, but which could be a mystery. Therefore, in this situation we use Karn's algorithm [KP87,RFC6298] to use neither version of the RTT sample and hence not update the RTO. There are cases where two copies of some data are transmitted in a way whereby the sender can tell which is being acknowledged by an incoming ACK. E.g., TCP's timestamp option [RFC1323] allows for segments to be uniquely identified and hence avoid the ambiguity. In such cases there is no ambiguity and the resulting samples can update the RTO. (3) Each time the RTO fires and causes a retransmission the value of the RTO MUST be exponentially backed off such that the next firing requires a longer interval. The backoff may be removed after the successful transmission of non-retransmitted data. This ensures network safety. (4) Retransmission timeouts MUST be taken as indications of congestion in the network and the sending rate adapted using a Expires: November 24, 2012 [Page 3]
draft-allman-tcpm-rto-consider-01.txt May 2012 standard mechanism (e.g., TCP collapses the congestion window to one segment [RFC5681]). This ensures network safety. An exception is made to this rule if a standard mechanism is used to determine that a particular loss is due to a non-congestion event (e.g., bit errors). In such a case a congestion control action is not required. 3 Discussion We note that research has shown the tension between responsiveness and correctness of TCP's RTO seems to be a fundamental tradeoff [AP99]. That is, making TCP's RTO more aggressive (via the EWMA gains, lowering the minimum RTO, etc.) can reduce the time spent waiting on needed RTOs. However, at the same time such aggressiveness leads to more needless RTOs, as well. Therefore, being as aggressive as the guidelines sketched in the last section allow in any particular situation may not be the best course of action (e.g., because an RTO expiration carries a requirement to slow down). While the tradeoff between responsiveness and correctness seems fundamental, the tradeoff can be made less relevant if the sender can detect and recover from spurious RTOs. Several mechanisms have been proposed for this purpose, such as Eifel [RFC3522], F-RTO [RFC5682] and DSACK [RFC2883,RFC3708]. Using such mechanisms may allow a data originator to tip towards being more responsive without incurring (as much of) the attendant costs of needless retransmits. Also, note, that in addition to the experiments discussed in [AP99], the Linux TCP implementation has been using various non-standard RTO mechanisms for many years seemingly without large scale problems (e.g., using different EWMA gains). Also, a number of implementations use minimum RTOs that are less than the 1 second specified in [RFC6298]. While the precise implications of this may show more spurious retransmits (per [AP99]) we are aware of no large scale problems caused by this change to the minimum RTO. Finally, we note that while allowing implementations to be more aggressive may in fact increase the number of needless retransmissions the above guidelines fail safe in that they insist on exponential backoff of the RTO and a transmission rate reduction. Therefore, allowing implementers latitude in their instantiations of an RTO mechanism does not somehow open the flood gates to aggressive behavior. Since there is a downside to being aggressive the incentives for proper behavior are retained in the mechanism. 4 Security Considerations This document does not alter the security properties of retransmission timeout mechanisms. See [RFC6298] for a discussion of these within the context of TCP. Expires: November 24, 2012 [Page 4]
draft-allman-tcpm-rto-consider-01.txt May 2012 Acknowledgments This document benefits from years of discussions with Sally Floyd, Shawn Ostermann, Vern Paxson and the members of the TCPM and TCP-IMPL working groups. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Informative References [AP99] Allman, M., V. Paxson, "On Estimating End-to-End Network Path Properties", Proceedings of the ACM SIGCOMM Technical Symposium, September 1999. [KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time Estimates in Reliable Transport Protocols", SIGCOMM 87. [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP Selective Acknowledgment Options", RFC 2018, October 1996. [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An Extension to the Selective Acknowledgement (SACK) Option for TCP", RFC 2883, July 2000. [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., M. Meyer, "The Eifel Detection Algorithm for TCP", RFC 3522, april 2003. [RFC3708] Blanton, E., M. Allman, "Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions", RFC 3708, February 2004. [RFC4960] Stweart, R., "Stream Control Transmission Protocol", RFC 4960, September 2007. [RFC5682] Sarolahti, P., M. Kojo, K. Yamamoto, M. Hata, "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP", RFC 5682, September 2009. [RFC6298] Paxson, V., M. Allman, H.K. Chu, M. Sargent, "Computing TCP's Retransmission Timer", June 2011, RFC 6298. [RFC6582] Henderson, T.,, S. Floyd, A. Gurtov, Y. Nishida, "The NewReno Modification to TCP's Fast Recovery Algorithm", April 2012, RFC 6582. Expires: November 24, 2012 [Page 5]
draft-allman-tcpm-rto-consider-01.txt May 2012 Authors' Addresses Mark Allman International Computer Science Institute 1947 Center St. Suite 600 Berkeley, CA 94704 Phone: 440-235-1792 EMail: mallman@icir.org http://www.icir.org/mallman Expires: November 24, 2012 [Page 6]