RFC6298

Internet Engineering Task Force (IETF) V. PaxsonRequest for Comments: 6298 ICSI/UC BerkeleyObsoletes: 2988 M. AllmanUpdates: 1122 ICSICategory: Standards Track J. ChuISSN: 2070-1721 Google M. Sargent CWRU June 2011

              Computing TCP's Retransmission Timer

Abstract

This document defines the standard algorithm that Transmission Control Protocol (TCP) senders are required to use to compute and manage their retransmission timer. It expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. This document obsoletes RFC 2988.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6298.

Copyright Notice

Copyright (c) 2011 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.

Introduction

The Transmission Control Protocol (TCP) [Pos81] uses a retransmission timer to ensure data delivery in the absence of any feedback from the remote data receiver. The duration of this timer is referred to as RTO (retransmission timeout). RFC 1122 [Bra89] specifies that the RTO should be calculated as outlined in [Jac88].

This document codifies the algorithm for setting the RTO. In addition, this document expands on the discussion in Section 4.2.3.1 of RFC 1122 and upgrades the requirement of supporting the algorithm from a SHOULD to a MUST. RFC 5681 [APB09] outlines the algorithm TCP uses to begin sending after the RTO expires and a retransmission is sent. This document does not alter the behavior outlined in RFC 5681 [APB09].

In some situations, it may be beneficial for a TCP sender to be more conservative than the algorithms detailed in this document allow. However, a TCP MUST NOT be more aggressive than the following algorithms allow. This document obsoletes RFC 2988 [PA00].

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 [Bra97].

The Basic Algorithm

To compute the current RTO, a TCP sender maintains two state variables, SRTT (smoothed round-trip time) and RTTVAR (round-trip time variation). In addition, we assume a clock granularity of G seconds.

The rules governing the computation of SRTT, RTTVAR, and RTO are as follows:

(2.1) Until a round-trip time (RTT) measurement has been made for a

     segment sent between the sender and receiver, the sender SHOULD
     set RTO <- 1 second, though the "backing off" on repeated
     retransmission discussed in (5.5) still applies.

     Note that the previous version of this document used an initial
     RTO of 3 seconds [PA00].  A TCP implementation MAY still use
     this value (or any other value > 1 second).  This change in the
     lower bound on the initial RTO is discussed in further detail
     in Appendix A.

(2.2) When the first RTT measurement R is made, the host MUST set

        SRTT <- R
        RTTVAR <- R/2
        RTO <- SRTT + max (G, K*RTTVAR)

     where K = 4.

(2.3) When a subsequent RTT measurement R' is made, a host MUST set

        RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'|
        SRTT <- (1 - alpha) * SRTT + alpha * R'

     The value of SRTT used in the update to RTTVAR is its value
     before updating SRTT itself using the second assignment.  That
     is, updating RTTVAR and SRTT MUST be computed in the above
     order.

     The above SHOULD be computed using alpha=1/8 and beta=1/4 (as
     suggested in [JK88]).

     After the computation, a host MUST update
     RTO <- SRTT + max (G, K*RTTVAR)

(2.4) Whenever RTO is computed, if it is less than 1 second, then the

     RTO SHOULD be rounded up to 1 second.

     Traditionally, TCP implementations use coarse grain clocks to
     measure the RTT and trigger the RTO, which imposes a large
     minimum value on the RTO.  Research suggests that a large
     minimum RTO is needed to keep TCP conservative and avoid
     spurious retransmissions [AP99].  Therefore, this specification
     requires a large minimum RTO as a conservative approach, while

     at the same time acknowledging that at some future point,
     research may show that a smaller minimum RTO is acceptable or
     superior.

(2.5) A maximum value MAY be placed on RTO provided it is at least 60

     seconds.

Taking RTT Samples

TCP MUST use Karn's algorithm [KP87] for taking RTT samples. That is, RTT samples MUST NOT be made using segments that were retransmitted (and thus for which it is ambiguous whether the reply was for the first instance of the packet or a later instance). The only case when TCP can safely take RTT samples from retransmitted segments is when the TCP timestamp option [JBB92] is employed, since the timestamp option removes the ambiguity regarding which instance of the data segment triggered the acknowledgment.

Traditionally, TCP implementations have taken one RTT measurement at a time (typically, once per RTT). However, when using the timestamp option, each ACK can be used as an RTT sample. RFC 1323 [JBB92] suggests that TCP connections utilizing large congestion windows should take many RTT samples per window of data to avoid aliasing effects in the estimated RTT. A TCP implementation MUST take at least one RTT measurement per RTT (unless that is not possible per Karn's algorithm).

For fairly modest congestion window sizes, research suggests that timing each segment does not lead to a better RTT estimator [AP99]. Additionally, when multiple samples are taken per RTT, the alpha and beta defined in Section 2 may keep an inadequate RTT history. A method for changing these constants is currently an open research question.

Clock Granularity

There is no requirement for the clock granularity G used for computing RTT measurements and the different state variables. However, if the K*RTTVAR term in the RTO calculation equals zero, the variance term MUST be rounded to G seconds (i.e., use the equation given in step 2.3).

   RTO <- SRTT + max (G, K*RTTVAR)

Experience has shown that finer clock granularities (<= 100 msec) perform somewhat better than coarser granularities.

Note that [Jac88] outlines several clever tricks that can be used to obtain better precision from coarse granularity timers. These changes are widely implemented in current TCP implementations.

Managing the RTO Timer

An implementation MUST manage the retransmission timer(s) in such a way that a segment is never retransmitted too early, i.e., less than one RTO after the previous transmission of that segment.

The following is the RECOMMENDED algorithm for managing the retransmission timer:

(5.1) Every time a packet containing data is sent (including a

     retransmission), if the timer is not running, start it running
     so that it will expire after RTO seconds (for the current value
     of RTO).

(5.2) When all outstanding data has been acknowledged, turn off the

     retransmission timer.

(5.3) When an ACK is received that acknowledges new data, restart the

     retransmission timer so that it will expire after RTO seconds
     (for the current value of RTO).

When the retransmission timer expires, do the following:

(5.4) Retransmit the earliest segment that has not been acknowledged

     by the TCP receiver.

(5.5) The host MUST set RTO <- RTO * 2 ("back off the timer"). The

     maximum value discussed in (2.5) above may be used to provide
     an upper bound to this doubling operation.

(5.6) Start the retransmission timer, such that it expires after RTO

     seconds (for the value of RTO after the doubling operation
     outlined in 5.5).

(5.7) If the timer expires awaiting the ACK of a SYN segment and the

     TCP implementation is using an RTO less than 3 seconds, the RTO
     MUST be re-initialized to 3 seconds when data transmission
     begins (i.e., after the three-way handshake completes).

     This represents a change from the previous version of this
     document [PA00] and is discussed in Appendix A.

Note that after retransmitting, once a new RTT measurement is obtained (which can only happen when new data has been sent and acknowledged), the computations outlined in Section 2 are performed, including the computation of RTO, which may result in "collapsing" RTO back down after it has been subject to exponential back off (rule 5.5).

Note that a TCP implementation MAY clear SRTT and RTTVAR after backing off the timer multiple times as it is likely that the current SRTT and RTTVAR are bogus in this situation. Once SRTT and RTTVAR are cleared, they should be initialized with the next RTT sample taken per (2.2) rather than using (2.3).

Security Considerations

This document requires a TCP to wait for a given interval before retransmitting an unacknowledged segment. An attacker could cause a TCP sender to compute a large value of RTO by adding delay to a timed packet's latency, or that of its acknowledgment. However, the ability to add delay to a packet's latency often coincides with the ability to cause the packet to be lost, so it is difficult to see what an attacker might gain from such an attack that could cause more damage than simply discarding some of the TCP connection's packets.

The Internet, to a considerable degree, relies on the correct implementation of the RTO algorithm (as well as those described in RFC 5681) in order to preserve network stability and avoid congestion collapse. An attacker could cause TCP endpoints to respond more aggressively in the face of congestion by forging acknowledgments for segments before the receiver has actually received the data, thus lowering RTO to an unsafe value. But to do so requires spoofing the acknowledgments correctly, which is difficult unless the attacker can monitor traffic along the path between the sender and the receiver. In addition, even if the attacker can cause the sender's RTO to reach too small a value, it appears the attacker cannot leverage this into much of an attack (compared to the other damage they can do if they can spoof packets belonging to the connection), since the sending TCP will still back off its timer in the face of an incorrectly transmitted packet's loss due to actual congestion.

The security considerations in RFC 5681 [APB09] are also applicable to this document.

Changes from RFC 2988

This document reduces the initial RTO from the previous 3 seconds [PA00] to 1 second, unless the SYN or the ACK of the SYN is lost, in which case the default RTO is reverted to 3 seconds before data transmission begins.

Acknowledgments

The RTO algorithm described in this memo was originated by Van Jacobson in [Jac88].

Much of the data that motivated changing the initial RTO from 3 seconds to 1 second came from Robert Love, Andre Broido, and Mike Belshe.

References

Normative References

[APB09] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, September 2009. [Bra89] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [Bra97] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [JBB92] Jacobson, V., Braden, R., and D. Borman, "TCP Extensions for High Performance", RFC 1323, May 1992. [Pos81] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.

Informative References

[AP99] Allman, M. and V. Paxson, "On Estimating End-to-End Network Path Properties", SIGCOMM 99. [Chu09] Chu, J., "Tuning TCP Parameters for the 21st Century", http://www.ietf.org/proceedings/75/slides/tcpm-1.pdf, July 2009. [SLS09] Schulman, A., Levin, D., and Spring, N., "CRAWDAD data set umd/sigcomm2008 (v. 2009-03-02)", http://crawdad.cs.dartmouth.edu/umd/sigcomm2008, March, 2009.

[HKA04] Henderson, T., Kotz, D., and Abyzov, I., "CRAWDAD trace dartmouth/campus/tcpdump/fall03 (v. 2004-11-09)", http://crawdad.cs.dartmouth.edu/dartmouth/campus/ tcpdump/fall03, November 2004. [Jac88] Jacobson, V., "Congestion Avoidance and Control", Computer Communication Review, vol. 18, no. 4, pp. 314-329, Aug. 1988. [JK88] Jacobson, V. and M. Karels, "Congestion Avoidance and Control", ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z. [KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time Estimates in Reliable Transport Protocols", SIGCOMM 87. [PA00] Paxson, V. and M. Allman, "Computing TCP's Retransmission Timer", RFC 2988, November 2000.

Appendix A. Rationale for Lowering the Initial RTO Choosing a reasonable initial RTO requires balancing two competingconsiderations: 1. The initial RTO should be sufficiently large to cover most of the end-to-end paths to avoid spurious retransmissions and their associated negative performance impact. 2. The initial RTO should be small enough to ensure a timely recovery from packet loss occurring before an RTT sample is taken. Traditionally, TCP has used 3 seconds as the initial RTO [Bra89][PA00]. This document calls for lowering this value to 1 secondusing the following rationale: - Modern networks are simply faster than the state-of-the-art was at the time the initial RTO of 3 seconds was defined. - Studies have found that the round-trip times of more than 97.5% of the connections observed in a large scale analysis were less than 1 second [Chu09], suggesting that 1 second meets criterion 1 above. - In addition, the studies observed retransmission rates within the three-way handshake of roughly 2%. This shows that reducing the initial RTO has benefit to a non-negligible set of connections. - However, roughly 2.5% of the connections studied in [Chu09] have an RTT longer than 1 second. For those connections, a 1 second initial RTO guarantees a retransmission during connection establishment (needed or not).

 When this happens, this document calls for reverting to an initial  RTO of 3 seconds for the data transmission phase.  Therefore, the  implications of the spurious retransmission are modest: (1) an  extra SYN is transmitted into the network, and (2) according to RFC  5681 [APB09] the initial congestion window will be limited to 1  segment.  While (2) clearly puts such connections at a  disadvantage, this document at least resets the RTO such that the  connection will not continually run into problems with a short  timeout.  (Of course, if the RTT is more than 3 seconds, the  connection will still encounter difficulties.  But that is not a  new issue for TCP.)
 In addition, we note that when using timestamps, TCP will be able  to take an RTT sample even in the presence of a spurious  retransmission, facilitating convergence to a correct RTT estimate  when the RTT exceeds 1 second.

As an additional check on the results presented in [Chu09], weanalyzed packet traces of client behavior collected at four differentvantage points at different times, as follows: Name Dates Pkts. Cnns. Clnts. Servs.--------------------------------------------------------LBL-1 Oct/05--Mar/06 292M 242K 228 74KLBL-2 Nov/09--Feb/10 1.1B 1.2M 1047 38KICSI-1 Sep/11--18/07 137M 2.1M 193 486KICSI-2 Sep/11--18/08 163M 1.9M 177 277KICSI-3 Sep/14--21/09 334M 3.1M 170 253KICSI-4 Sep/11--18/10 298M 5M 183 189KDartmouth Jan/4--21/04 1B 4M 3782 132KSIGCOMM Aug/17--21/08 11.6M 133K 152 29K The "LBL" data was taken at the Lawrence Berkeley NationalLaboratory, the "ICSI" data from the International Computer ScienceInstitute, the "SIGCOMM" data from the wireless network that servedthe attendees of SIGCOMM 2008, and the "Dartmouth" data was collectedfrom Dartmouth College's wireless network. The latter two datasetsare available from the CRAWDAD data repository [HKA04] [SLS09]. Thetable lists the dates of the data collections, the number of packetscollected, the number of TCP connections observed, the number oflocal clients monitored, and the number of remote servers contacted.We consider only connections initiated near the tracing vantagepoint. Analysis of these datasets finds the prevalence of retransmitted SYNsto be between 0.03% (ICSI-4) to roughly 2% (LBL-1 and Dartmouth). We then analyzed the data to determine the number of additional andspurious retransmissions that would have been incurred if the initialRTO was assumed to be 1 second. In most of the datasets, theproportion of connections with spurious retransmits was less than0.1%. However, in the Dartmouth dataset, approximately 1.1% of theconnections would have sent a spurious retransmit with a lowerinitial RTO. We attribute this to the fact that the monitorednetwork is wireless and therefore susceptible to additional delaysfrom RF effects. Finally, there are obviously performance benefits from retransmittinglost SYNs with a reduced initial RTO. Across our datasets, thepercentage of connections that retransmitted a SYN and would realizeat least a 10% performance improvement by using the smaller initialRTO specified in this document ranges from 43% (LBL-1) to 87%(ICSI-4). The percentage of connections that would realize at leasta 50% performance improvement ranges from 17% (ICSI-1 and SIGCOMM) to73% (ICSI-4).

From the data to which we have access, we conclude that the lowerinitial RTO is likely to be beneficial to many connections, andharmful to relatively few. Authors' Addresses Vern PaxsonICSI/UC Berkeley1947 Center StreetSuite 600Berkeley, CA 94704-1198 Phone: 510-666-2882EMail: [email protected]://www.icir.org/vern/

Mark AllmanICSI1947 Center StreetSuite 600Berkeley, CA 94704-1198 Phone: 440-235-1792EMail: [email protected]://www.icir.org/mallman/

H.K. Jerry ChuGoogle, Inc.1600 Amphitheatre ParkwayMountain View, CA 94043 Phone: 650-253-3010EMail: [email protected]

Matt SargentCase Western Reserve UniversityOlin Building10900 Euclid AvenueRoom 505Cleveland, OH 44106 Phone: 440-223-5932EMail: [email protected]