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Network Working Group V. Paxson Request for Comments: 2988 ACIRI Category: Standards Track M. Allman

                                                          NASA GRC/BBN
                                                         November 2000
                Computing TCP's Retransmission Timer

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2000).  All Rights Reserved.


 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 of RFC 1122 and upgrades the requirement of
 supporting the algorithm from a SHOULD to a MUST.

1 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
 of RFC 1122 and upgrades the requirement of supporting the algorithm
 from a SHOULD to a MUST.  RFC 2581 [APS99] 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 2581

Paxson & Allman Standards Track [Page 1] RFC 2988 Computing TCP's Retransmission Timer November 2000

 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.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [Bra97].

2 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
 The rules governing the computation of SRTT, RTTVAR, and RTO are as
 (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 <- 3 seconds (per RFC 1122 [Bra89]), though the
       "backing off" on repeated retransmission discussed in (5.5)
       still applies.
          Note that some implementations may use a "heartbeat" timer
          that in fact yield a value between 2.5 seconds and 3
          seconds.  Accordingly, a lower bound of 2.5 seconds is also
          acceptable, providing that the timer will never expire
          faster than 2.5 seconds.  Implementations using a heartbeat
          timer with a granularity of G SHOULD not set the timer below
          2.5 + G seconds.
 (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'

Paxson & Allman Standards Track [Page 2] RFC 2988 Computing TCP's Retransmission Timer November 2000

       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
       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

3 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).

Paxson & Allman Standards Track [Page 3] RFC 2988 Computing TCP's Retransmission Timer November 2000

 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

4 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 more coarse 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.

5 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).

Paxson & Allman Standards Track [Page 4] RFC 2988 Computing TCP's Retransmission Timer November 2000

 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).
 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 backoff
 (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).

6 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
 The Internet to a considerable degree relies on the correct
 implementation of the RTO algorithm (as well as those described in
 RFC 2581) 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

Paxson & Allman Standards Track [Page 5] RFC 2988 Computing TCP's Retransmission Timer November 2000

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


 [AP99]  Allman, M. and V. Paxson, "On Estimating End-to-End Network
         Path Properties", SIGCOMM 99.
 [APS99] Allman, M., Paxson V. and W. Stevens, "TCP Congestion
         Control", RFC 2581, April 1999.
 [Bra89] Braden, R., "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.
 [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
 [KP87]  Karn, P. and C. Partridge, "Improving Round-Trip Time
         Estimates in Reliable Transport Protocols", SIGCOMM 87.
 [Pos81] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
         September 1981.

Paxson & Allman Standards Track [Page 6] RFC 2988 Computing TCP's Retransmission Timer November 2000

Author's Addresses

 Vern Paxson
 1947 Center Street
 Suite 600
 Berkeley, CA 94704-1198
 Phone: 510-666-2882
 Fax:   510-643-7684
 Mark Allman
 NASA Glenn Research Center/BBN Technologies
 Lewis Field
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 Phone: 216-433-6586
 Fax:   216-433-8705

Paxson & Allman Standards Track [Page 7] RFC 2988 Computing TCP's Retransmission Timer November 2000

Full Copyright Statement

 Copyright (C) The Internet Society (2000).  All Rights Reserved.
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 revoked by the Internet Society or its successors or assigns.
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 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Paxson & Allman Standards Track [Page 8]

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