Internet Engineering Task Force                              Mark Allman
INTERNET DRAFT                                              BBN/NASA GRC
File: draft-ietf-ippm-btc-cap-00.txt                      February, 2001
                                                   Expires: August, 2001

                A Bulk Transfer Capacity Methodology for
                           Cooperating Hosts

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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    This document specifies a specific Bulk Transfer Capacity (BTC)
    metric based on the BTC framework outlined in [MA00].

1   Introduction

    This document specifies a methodology that performs Bulk Transfer
    Capacity (BTC) measurements based on the BTC framework outlined in
    [MA00].  This particular methodology assumes cooperating processes
    on the sender and receiver.  As outlined in [MA00], there are a
    number of considerations that need to be made when writing a
    particular BTC metric.  This document specifies these items for a
    BTC methodology that uses cooperating processes on the sender and

    Readers are assumed to be familiar with [MA00] and [RFC2581].  The
    terminology used to describe the congestion control algorithms in
    this document is taken from [RFC2581].

    We implemented this methodology in two programs, cap and capd.  The
    discussion in this document is conducted in terms of these two
    programs.  However, alternate programs can be written that conform
    to this BTC methodology.

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2   Congestion Control Algorithm Specifications

    As specified in section 2 of [MA00], each BTC document must tightly
    specify several details of the congestion control algorithms that
    are not tightly specified in [RFC2581].  The following is the
    specification of those details for the sender's behavior in the
    defined methodology:

      * Window Increase During Congestion Avoidance: During congestion
        avoidance, cap counts the number of packets that are
        acknowledged (ACKed) by the cumulative acknowledgment, denoted
        SA.  When SA becomes greater than or equal to the current value
        of the congestion window (cwnd), SA is decreased by the current
        value of cwnd and cwnd is increased by 1 segment unless the
        increase is not possible due to the configured advertised window

      * When To Enter Congestion Avoidance.  [RFC2581] allows TCP to
        use either slow start or congestion avoidance when cwnd equals
        ssthresh.  Cap uses congestion avoidance.

      * Cap uses a segment size of 1500 bytes by default.  The segment
        size can be changed using a command-line option.  Cap does not
        use Path MTU Discovery [RFC1191].

      * By default, cap assumes 40 bytes of header are prepended to
        each segment (default TCP and IP headers).  When using
        timestamps [RFC1323] the header size is increased by 12 bytes.
        Additionally, when using selective acknowledgments (SACKs)
        [RFC2018] the header size on returning ACKs depends on the
        number of SACK blocks being returned (per [RFC2018]).

      * The algorithm for calculating the retransmission timeout (RTO)
        is similar to the algorithm outlined in [RFC2988].  The
        algorithm is fully specified in section 3.

    [MA00] recommends each BTC take a number of ancillary metrics, in
    addition to a simple BTC measurement.  Cap does not perform any of
    these ancillary metrics, but can produce a segment trace which may
    be used to derive these metrics via post-processing.

3   Calculating the Retransmission Timeout

    The RTO used in this BTC methodology is generally outlined in
    [RFC2988].  The following is a sketch of the initial conditions, as
    well as a discussion of how our estimator differs from the one
    outlined in [RFC2988].  The reader is assumed familiar with

    Cap takes high-precision round-trip time (RTT) measurements and
    converts these into a retransmission timeout (RTO) based on a clock
    with a given granularity.  The RTO is initialized as follows:

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      * The default clock granularity, G, is 500 ms.  However, the clock
        granularity may be changed via a command-line option.

      * The initial RTO in clock ticks is: (int)(3 seconds / G).

      * When cap is started, the first heartbeat is determined by
        generating a uniform random number between 0-G and subtracting
        the obtained value from the current time.  The time of the first
        heartbeat is denoted HB_FIRST.

      * We define bounds on the RTO, as follows:

            MIN_TICKS = ceil (1.0 / G)
            MAX_TICKS = ceil (64.0 / G)

    The RTO is calculated based on RTT measurements.  We derive RTT
    measurements in one of two ways.  When the timestamp option is
    enabled by the user, we use the timestamps in incoming ACKs to take
    RTT measurements.  Otherwise, we time one segment and its
    corresponding ACK per RTT, as outlined in [RFC2988].  We update the
    SRTT and RTTVAR upon taking each sample as defined in [RFC2988].

    The timer is armed in the situations outlined in [RFC2988].  Each
    time we are the timer the following algorithm is used to convert the
    fine-grained SRTT and RTTVAR values to a course-grained RTO

        now = get_current_time;
        if (!SRTT)
            ticks = 3.0 / G
            rto = SRTT + (4 * RTTVAR)
            ticks = ceil (rto / G)
        ticks *= BACKOFF
        if (ticks < MIN_TICKS)
            ticks = MIN_TICKS
        else if (ticks > MAX_TICKS)
            ticks = MAX_TICKS
        so_far = now - HB_FIRST;
        so_far_ticks = (int)(so_far / G)
        gone = so_far - (so_far_ticks * G)
        partial = G - gone;
        full = (ticks - 1) * G
        real_rto = full + partial
        arm_timer (real_rto)

4   Receiver Specification

    The receiving process, capd, sends ``ACKs'', UDP datagrams, to
    the sender with the following properties.

      * Each ACK contains a cumulative sequence number, as done in TCP.

      * The default size of an ACK is 40 bytes.

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      * In the case when capd echoes the timestamp sent by cap the ACK
        consists of 52 bytes.

      * By default ACKs are sent in response to every incoming data

      * The user may enable the use of delayed ACKs [RFC1122,RFC2581]
        via a command-line option.

5   Conclusion

    This document specifies a BTC methodology involving two processes
    based on the framework outlined in [MA00].  This methodology has
    been shown to accurately gauge the BTC of a given network path over
    various network conditions [All01].

6   Security Considerations

    The BTC methodology outlined in this document does not pose security
    problems beyond those expressed in the BTC framework document [MA00].


    Thanks to Vern Paxson for encouraging the development of cap.


    [All01] Mark Allman Measuring End-to-End Bulk Transfer Capacity,
        February 2001. Under review.

    [MA00] Matt Mathis, Mark Allman. A Framework for Defining Empirical
        Bulk Transfer Capacity Metrics, February 2001. Internet-Draft
        draft-ietf-ippm-btc-framework-05.txt (work in progress).

    [RFC1191] Jeff Mogul, Steve Deering, "Path MTU Discovery", RFC 1191,
        November 1990.

    [RFC1323] Van Jacobson, Robert Braden, David Borman, "TCP Extensions
        for High Performance", RFC 1323, May 1992.

    [RFC2018] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow,
        "TCP Selective Acknowledgment Options", RFC 2018, 1996.

    [RFC2581] Mark Allman, Vern Paxson, W. Richard Stevens, "TCP
        Congestion Control", RFC 2581, April 1999.

    [RFC2988] Vern Paxson, Mark Allman, "Computing TCP's Retransmission
        Timer", RFC 2988, November 2000.

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Author's Address:

    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

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