Computing TCP's Retransmission Timer
draft-paxson-tcp-rto-01

Versions: 00 01 rfc2988                                  Standards Track
Internet Engineering Task Force                              Vern Paxson
INTERNET DRAFT                                                     ACIRI
File: draft-paxson-tcp-rto-01.txt                            Mark Allman
                                                            NASA GRC/BBN
                                                             April, 2000
                                                  Expires: October, 2000


                  Computing TCP's Retransmission Timer


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
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Abstract

    This document defines the standard algorithm 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.

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 4.2.3.1
    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

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    is sent.  This document does not alter the behavior outlined in RFC
    2581 [APS99].

    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",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    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
    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 <- 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'

          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

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          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.

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).

    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.

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

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    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

    The following algorithm MUST be used 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).

    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).


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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
    packets.

    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
    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.

Acknowledgments

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

References

    [AP99] Allman, M. and V. Paxson, "On Estimating End-to-End Network
        Path Properties", SIGCOMM 99.

    [APS99] Allman, M., V. Paxson and W. R. 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
        Control", ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z.


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    [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.

Author's Addresses:

    Vern Paxson
    ACIRI / ICSI
    1947 Center Street
    Suite 600
    Berkeley, CA 94704-1198
    Phone: 510-642-4274 x302
    Fax: 510-643-7684
    vern@aciri.org
    http://www.aciri.org/vern/

    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
    mallman@grc.nasa.gov
    http://roland.grc.nasa.gov/~mallman




























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