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Versions: 00 01 03 04 05 06 07 RFC 4828

Internet Engineering Task Force                              Sally Floyd
INTERNET-DRAFT                                                      ICIR
draft-ietf-dccp-tfrc-voip-03.txt                            Eddie Kohler
Expires: July 2006                                                  UCLA
                                                         13 January 2006


                   TCP Friendly Rate Control (TFRC):
                     the Small-Packet (SP) Variant



Status of this Memo

    By submitting this Internet-Draft, each author represents that any
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    This Internet-Draft will expire on July 2006.

Abstract

    TCP-Friendly Rate Control (TFRC) is a congestion control mechanism
    for unicast flows operating in a best-effort Internet environment
    [RFC 3448]. This document proposes TFRC-SP, a Small-Packet (SP)
    variant of TFRC.  TFRC was intended for applications that use a
    fixed packet size, and was designed to be reasonably fair when
    competing for bandwidth with TCP connections using the same packet



Floyd/Kohler                                                    [Page 1]

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    size.  The Small-Packet variant of TFRC is designed for applications
    that send small packets, where the design goal is to achieve the
    same bandwidth in bps as a TCP flow using packets of up to 1500
    bytes.  This variant is referred to in RFC 3448 as TFRC-PS, for
    applications that might vary their packet size in response to
    congestion.  TFRC-SP enforces a Min Interval of 10 ms between data
    packets, to prevent a single flow from sending small packets
    arbitrarily frequently.

    Flows using TFRC-SP compete reasonably fairly with large-packet TCP
    and TFRC flows in environments where large-packet flows and small-
    packet flows experience similar packet drop rates.  However, in
    environments where small-packet flows experience lower packet drop
    rates than large-packet flows (e.g., with Drop-Tail queues in units
    of bytes), TFRC-SP can receive considerably more than its share of
    the bandwidth.  We note however that in all scenarios TFRC-SP is
    better, in terms of congestion in the network, than the same
    application in the absence of congestion control.

































Floyd/Kohler                                                    [Page 2]

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    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:
     Changes from draft-ietf-dccp-tfrc-voip-02.txt:
      * Changed name from "VoIP variant of TFRC" to "TFRC-SP".
      * Added Section 4.5 on "The Nominal Packet Size", discussing
         possible differences in packet drop rates between small
         and large packets.
      * Added text to Section 5 on "A Comparison with RFC 3714".
      * Added text to Section 6 on "TFRC-SP with Applications that
          Modify the Packet Size"
      * Added simulations with small-packet TCP flows.
      * Added a Security Considerations section.
      * Minor editing.
     Changes from draft-ietf-dccp-tfrc-voip-01.txt:
      * Added modified algorithm for calculating the loss event rate,
          for short intervals with multiple packet drops.
      * Moved Faster Restart to a separate document.
      * Added simulations with a configured byte drop rate.
      * Added many more simulations, including Drop-Tail with a queue
        in bytes.
      * Added a discussion of unfairness for Drop-Tail with a queue
        in bytes.
     Changes from draft-ietf-dccp-tfrc-voip-00.txt:
      * Added more simulations.
      * Added a Related Work section.



























Floyd/Kohler                                                    [Page 3]

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                             Table of Contents

    1. Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   5
    2. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   5
    3. TFRC-SP Congestion Control. . . . . . . . . . . . . . . . . .   7
    4. TFRC-SP Discussion. . . . . . . . . . . . . . . . . . . . . .   8
       4.1. The TCP Throughput Equation. . . . . . . . . . . . . . .   8
       4.2. Accounting for Header Size . . . . . . . . . . . . . . .   9
       4.3. The TFRC-SP Min Interval . . . . . . . . . . . . . . . .   9
       4.4. Counting Packet Losses . . . . . . . . . . . . . . . . .  11
       4.5. The Nominal Packet Size. . . . . . . . . . . . . . . . .  11
    5. A Comparison with RFC 3714. . . . . . . . . . . . . . . . . .  13
    6. TFRC-SP with Applications that Modify the Packet
    Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
    7. Simulation Results. . . . . . . . . . . . . . . . . . . . . .  15
       7.1. Simulations with Configured Packet Drop Rates. . . . . .  16
       7.2. Simulations with Configured Byte Drop Rates. . . . . . .  19
       7.3. Packet Dropping Behavior at Routers with Drop-
       Tail Queues . . . . . . . . . . . . . . . . . . . . . . . . .  21
       7.4. Packet Dropping Behavior at Routers with AQM . . . . . .  24
    8. General Discussion. . . . . . . . . . . . . . . . . . . . . .  28
    9. Security Considerations . . . . . . . . . . . . . . . . . . .  29
    10. IANA Considerations. . . . . . . . . . . . . . . . . . . . .  29
    11. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
    A. Appendix: Related Work on Small-Packet Variants of
    TFRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
    B. Appendix: A Discussion of Packet Size and Packet
    Dropping . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
    Normative References . . . . . . . . . . . . . . . . . . . . . .  32
    Informative References . . . . . . . . . . . . . . . . . . . . .  32
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  33
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . .  33
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . .  33


















Floyd/Kohler                                                    [Page 4]

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1.  Conventions

    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 [RFC 2119].

2.  Introduction

    This document specifies TFRC-SP, a Small-Packet variant of TCP-
    friendly rate control (TFRC) [RFC 3448].

    TFRC was designed to be reasonably fair when competing for bandwidth
    with TCP flows, but to avoid the abrupt changes in the sending rate
    characteristic of TCP's congestion control mechanisms.  TFRC is
    intended for applications such as streaming media applications where
    a relatively smooth sending rate is of importance.  Conventional
    TFRC measures loss rates by estimating the loss event ratio as
    described in [RFC 3448], and uses this loss event rate to determine
    the sending rate in packets per round-trip time.  This has
    consequences for the rate a TFRC flow can achieve when sharing a
    bottleneck with large-packet TCP flows.  In particular, a low-
    bandwidth, small-packet TFRC flow sharing a bottleneck with high-
    bandwidth, large-packet TCP flows may be forced to slow down, even
    though the TFRC application's nominal rate in bytes per second is
    less than the rate achieved by the TCP flows.  Intuitively, this
    would be "fair" only if the network limitation was in packets per
    second (such as a routing lookup), rather than bytes per second
    (such as link bandwidth).  Conventional wisdom is that many of the
    network limitations in today's Internet are in bytes per second,
    even though the network limitations of the future might move back
    towards limitations in packets per second.

    TFRC-SP is intended for flows that need to send frequent small
    packets (limited by a minimum interval between packets of 10 ms).
    It will better support applications that do not want their sending
    rates in bytes per second to suffer from their use of small packets.
    This variant is restricted to applications that send packets no more
    than once every 10 ms (the Min Interval).  Given this restriction,
    TFRC-SP effectively calculates the TFRC fair rate as if the
    bottleneck restriction was in bytes per second.  Applications using
    TFRC-SP could have a fixed packet size, or could vary their packet
    size in response to congestion.

    TFRC-SP is motivated in part by the approach in RFC 3714, which
    argues that it is acceptable for VoIP flows to assume that the
    network limitation is in bytes per second (Bps) rather in packets
    per second (pps), and to have the same sending rate in bytes per
    second as a TCP flow with 1500-byte packets and the same packet drop



Floyd/Kohler                                        Section 2.  [Page 5]

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    rate.  RFC 3714 states the following:

        "While the ideal would be to have a transport protocol that is
        able to detect whether the bottleneck links along the path are
        limited in Bps or in pps, and to respond appropriately when the
        limitation is in pps, such an ideal is hard to achieve. We would
        not want to delay the deployment of congestion control for
        telephony traffic until such an ideal could be accomplished.  In
        addition, we note that the current TCP congestion control
        mechanisms are themselves not very effective in an environment
        where there is a limitation along the reverse path in pps.
        While the TCP mechanisms do provide an incentive to use large
        data packets, TCP does not include any effective congestion
        control mechanisms for the stream of small acknowledgement
        packets on the reverse path.  Given the arguments above, it
        seems acceptable to us to assume a network limitation in Bps
        rather than in pps in considering the minimum sending rate of
        telephony traffic."

    Translating the discussion in [RFC 3714] to the congestion control
    mechanisms of TFRC, it seems acceptable to standardize a variant of
    TFRC that allows low-bandwidth flows sending small packets to
    achieve a rough fairness with TCP flows in terms of the sending rate
    in Bps, rather than in terms of the sending rate in pps.  This is
    accomplished by TFRC-SP, a small modification to TFRC, as described
    below.

    Maintaining incentives for large packets: Because the bottlenecks in
    the network in fact can include limitations in pps as well as in
    Bps, we pay special attention to the potential dangers of
    encouraging a large deployment of best-effort traffic in the
    Internet consisting entirely of small packets.  This is discussed in
    more detail in Section 4.3. In addition, as again discussed in
    Section 4.3, TFRC-SP includes the limitation of the Min Interval
    between packets of 10 ms.

    Packet drop rates as a function of packet size: TFRC-SP essentially
    assumes that the small-packet TFRC-SP flow receives roughly the same
    packet drop rate as a large-packet TFRC or TCP flow.  As we show,
    this assumption is not necessarily correct for all environments in
    the Internet.

    Calculating the loss event rate for TFRC-SP: TFRC-SP requires a
    modification in TFRC's calculation of the loss event rate, because a
    TFRC-SP connection can send many small packets when a standard TFRC
    or TCP connection would send a single large packet.  It is not
    possible for a standard TFRC or TCP connection to repeatedly send
    multiple packets per round-trip time in the face of a high packet



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    drop rate.  As a result, TCP and standard TFRC only respond to a
    single loss event per round-trip time, and are still able to detect
    and respond to increasingly heavy packet loss rates.  However, in a
    highly-congested environment, when a TCP connection might be
    sending, on average, one large packet per round-trip time, a
    corresponding TFRC-SP connection might be sending many small packets
    per round-trip time.  As a result, in order to maintain fairness
    with TCP, and to be able to detect changes in the degree of
    congestion, TFRC-SP needs to be sensitive to the actual packet drop
    rate during periods of high congestion.  This is discussed in more
    detail in the section below.

3.  TFRC-SP Congestion Control

    TFRC uses the TCP throughput equation given in Section 3.1 of [RFC
    3448], which gives the allowed sending rate X in bytes per second as
    a function of the loss event rate, packet size, and round-trip time.
    [RFC 3448] specifies that the packet size s used in the throughput
    equation should be the packet size used by the application, or the
    estimated mean packet size if there are variations in the packet
    size depending on the data.  This gives rough fairness with TCP
    flows using the same packet size.

    TFRC-SP changes this behavior in the following ways.

    o  The nominal packet size: The nominal packet size s is set to
       1460 bytes.  Following [RFC 3714], this provides a goal of
       fairness, in terms of the sending rate in bytes per second, with
       a TCP flow with 1460 bytes of application data per packet but
       with the same packet drop rate.

    o  Taking packet headers into account: The allowed transmit rate X
       in bytes per second is reduced by a factor that accounts for
       packet header size.  This gives the application some incentive,
       beyond the Min Interval, not to use unnecessarily small packets.
       In particular, we introduce a new parameter H, which represents
       the expected size in bytes of network and transport headers to be
       used on the TFRC connection's packets.  This is used to reduce
       the allowed transmit rate X as follows:

       X := X * s_true / (s_true + H),

       where s_true is the true average data packet size for the
       connection in bytes, excluding the transport and network headers.

       The H parameter is set to the constant 40 bytes.  Thus, if the
       TFRC-SP application used 40-byte data segments, the allowed
       transmit rate X would be halved to account for the fact that half



Floyd/Kohler                                        Section 3.  [Page 7]

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       of the sending rate would be used by the headers.  Section 4.2
       justifies this definition.  However, a connection using TFRC-SP
       MAY instead use a more precise estimate of H, based on the actual
       network and transport headers to be used on the connection's
       packets.  For example, a DCCP connection [DCCP] over IPv4, where
       data packets use the DCCP-Data packet type, and there are no IP
       or DCCP options, could set H to 20 + 12 = 32 bytes.

    o  Measuring the loss event rate in times of high loss: During short
       loss intervals (those at most two round-trip times), the loss
       rate is computed by counting the actual number of packets lost or
       marked, not by counting at most one loss event per loss interval.
       Without this change, TFRC-SP could send multiple packets per
       round-trip time even in the face of heavy congestion, for a
       steady-state behavior of multiple packets dropped each round-trip
       time.

       In standard TFRC, the TFRC receiver estimates the loss event rate
       by calculating the average loss interval in packets, and
       inverting to get the loss event rate.  Thus, for a short loss
       interval with N packets and K losses, standard TFRC calculates
       the size of that loss interval as N packets, contributing to a
       loss event rate of 1/N.  However, for TFRC-SP, for small loss
       intervals of at most two round-trip times, if the loss interval
       consists of N packets including K losses, the size of the loss
       interval is calculated as N/K, contributing to a loss event rate
       of K/N instead of 1/N.

    o  A minimum interval between packets: TFRC-SP enforces a Min
       Interval between packets of 10 ms.  A flow that wishes its
       transport protocol to exceed this Min Interval MUST use the
       conventional TFRC equations, rather than TFRC-SP.  The motivation
       for this is discussed below.

4.  TFRC-SP Discussion

4.1.  The TCP Throughput Equation

    TFRC-SP uses the TCP throughput equation given in [RFC 3448].  As
    shown in Table 1 of [RFC 3714], for high packet drop rates, this
    throughput equation gives rough fairness with the most aggressive
    possible current TCP: a SACK TCP flow using timestamps and ECN.
    Because it is not recommended for routers to use ECN-marking in
    highly-congested environments (e.g., with packet drop rates greater
    than 10%), we note that it would be useful to have a throughput
    equation with a somewhat more moderate sending rate for packet drop
    rates of 40% and above.




Floyd/Kohler                                      Section 4.1.  [Page 8]

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4.2.  Accounting for Header Size

    [RFC 3714] makes the optimistic assumption that the limitation of
    the network is in bandwidth in bytes per second (Bps), and not in
    CPU cycles or in packets per second (pps).  However, some attention
    must be paid to the load in pps as well as to the load in Bps.  Even
    aside from the Min Interval, TFRC-SP gives the application some
    incentive to use fewer but larger packets, when larger packets would
    suffice, by including the bandwidth used by the packet header in the
    allowed sending rate.

    As an example, a sender using 120-byte packets needs a TCP-friendly
    rate of 128 Kbps to send 96 Kbps of application data.  This is
    because the TCP-friendly rate is reduced by a factor of
    s_true/(s_true + H) = 120/160, to account for the effect of packet
    headers.  If the sender suddenly switched to 40-byte data segments,
    the allowed sending rate would reduce to 64 Kbps of application
    data; and the use of one-byte data segments would reduce the allowed
    sending rate to 3.12 Kbps of application data.  (In fact, the Min
    Interval would prevent senders from achieving these rates, since
    applications using TFRC-SP cannot send more than 100 packets per
    second.)

    Unless it has a more precise estimate of the header size, TFRC-SP
    assumes 40 bytes for the header size, although the header could be
    larger (due to IP options, IPv6, IP tunnels, and the like) or
    smaller (due to header compression, DCCP instead of UDP) on the
    wire.  Requiring the use of the exact header size in bytes would
    require significant additional complexity, and would have little
    additional benefit.  TFRC-SP's default assumption of a 40-byte
    header is sufficient to get a rough estimate of the throughput, and
    to give the application some incentive not to use unnecessarily-many
    small packets.  Because we are only aiming at rough fairness, and at
    a rough incentive for applications, the default use of a 40-byte
    header in the calculations of the header bandwidth seems sufficient.

4.3.  The TFRC-SP Min Interval

    The header size calculation provides an incentive for applications
    to use fewer, but larger, packets.  Another incentive is that when
    the path limitation is in pps, the application using more small
    packets is likely to cause higher packet drop rates, and to have to
    reduce its sending rate accordingly.  That is, if the congestion is
    in terms of pps, then the flow sending more pps will increase the
    packet drop rate, and have to adjust its sending rate accordingly.
    However, the increased congestion caused by the use of small packets
    in an environment limited by pps is experienced not only by the flow
    using the small packets, but by all of the competing traffic on that



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    congested link.  These incentives are therefore insufficient to
    provide sufficient protection for pps network limitations.

    TFRC-SP, then, includes a Min Interval of 10 ms.  This provides
    additional restrictions on the use of unnecessarily many small
    packets.

    One justification for the Min Interval is the practical one that the
    applications that currently want to send small packets are the VoIP
    applications that send at most one packet every 10 ms, so this
    restriction does not affect current traffic.  A second justification
    is that there is no pressing need for best-effort traffic in the
    current Internet to send small packets more frequently than once
    every 10 ms (rather than taking the 10 ms delay at the sender, and
    merging the two small packets into one larger one).  This 10 ms
    delay for merging small packets is likely to be dominated by the
    network propagation, transmission, and queueing delays of best-
    effort traffic in the current Internet.  As a result, our judgment
    would be that the benefit to the user of having less than 10 ms
    between packets is outweighed by the benefit to the network of
    avoiding unnecessarily many small packets.

    The Min Interval causes TFRC-SP not to support applications sending
    small packets very frequently.  Consider a TFRC flow with a fixed
    packet size of 100 bytes, but with a variable sending rate and a
    fairly uncongested path.  When this flow was sending at most 100
    pps, it would be able to use TFRC-SP.  If the flow wished to
    increase its sending rate to more than 100 pps, but to keep the same
    packet size, it would no longer be able to achieve this with TFRC-
    SP, and would have to switch to the default TFRC, receiving a
    dramatic, discontinuous decrease in its allowed sending rate.  This
    seems not only acceptable, but desirable for the global Internet.

    What is to prevent flows from opening multiple connections, each
    with a 10 ms Min Interval, and thereby getting around the limitation
    of the Min Interval?  Obviously, there is nothing to prevent flows
    from doing this, just as there is currently nothing to prevent flows
    from using UDP, or from opening multiple parallel TCP connections,
    or from using their own congestion control mechanism.  Of course,
    implementations or middleboxes are also free to limit the number of
    parallel TFRC connections opened to the same destination in times of
    congestion, if that seems desirable.  And flows that open multiple
    parallel connections are subject to the inconveniences of reordering
    and the like.







Floyd/Kohler                                     Section 4.3.  [Page 10]

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4.4.  Counting Packet Losses

    It is not possible for a TCP connection to persistently send
    multiple packets per round-trip time in the face of high congestion,
    with a steady-state with multiple packets dropped per round-trip
    time.  For TCP, when one or more packets are dropped each round-trip
    time, the sending rate is quickly dropped to less than one packet
    per round-trip time.  In addition, for TCP with Tahoe, NewReno, or
    SACK congestion control mechanisms, the response to congestion is
    largely independent of the number of packets dropped per round-trip
    time.

    As a result, standard TFRC can best achieve fairness with TCP, even
    in highly congested environments, by calculating the loss event rate
    rather than the packet drop rate, where a loss event is one or more
    packets dropped or marked from a window of data.

    However, with TFRC-SP, it is no longer possible to achieve fairness
    with TCP or with standard TFRC by counting only the loss event rate
    [WBL04].  Instead of sending one large packet per round-trip time,
    TFRC-SP could be sending N small packets (where N small packets
    equal one large 1500-byte packet).  The loss measurement used with
    TFRC-SP has to be able to detect a connection that is consistently
    receiving multiple packet losses or marks per round-trip time, to
    allow TFRC-SP to respond appropriately.

    In TFRC-SP, the loss event rate is calculated by counting at most
    one loss event in loss intervals longer than two round-trip times,
    and by counting each packet lost or marked in shorter loss
    intervals.  In particular, for a short loss interval with N packets,
    including K lost or marked packets, the loss interval length is
    calculated as N/K, instead as N.  The average loss interval I_mean
    is still averaged over the most recent eight loss intervals, as
    specified in Section 5.4 of RFC 3448.  Thus, if eight successive
    loss intervals are short loss intervals with N packets and K losses,
    the loss event rate is calculated as K/N, rather than as 1/N.

4.5.  The Nominal Packet Size

    The guidelines in Section 3 above say that the nominal packet size s
    is set to 1460 bytes, providing a goal of fairness, in terms of the
    sending rate in bytes per second, with a TCP flow with 1460 bytes of
    application data per packet but with the same packet drop rate.
    This follows the assumption that a TCP flow with 1460-byte packets
    will have a higher sending rate than a TCP flow with smaller
    packets.  While this assumption holds in an environment where the
    packet drop rate is independent of packet size, this assumption does
    not necessarily hold in an environment where larger packets are more



Floyd/Kohler                                     Section 4.5.  [Page 11]

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    likely to be dropped than are small packets.

    The table below shows the results of simulations with standard
    (SACK) TCP flows, where, for each *byte*, the packet containing that
    byte is dropped with probability p.  Consider the approximation for
    the TCP response function for packet drop rates up to 10% or so; for
    this environments, the sending rate in bytes per RTT is roughly 1.2
    s/sqrt(p), for a packet size of s bytes and packet drop rate p.
    Given a fixed *byte* drop rate p1, and a TCP packet size of s bytes,
    the packet drop rate is roughly s*p1, producing a sending rate in
    bytes per RTT of roughly 1.2 sqrt(s)/sqrt(p1).  Thus, for TCP in an
    environment with a fixed byte drop rate, the sending rate should
    grow roughly as sqrt(s), for packet drop rates up to 10% or so.

    Each row of Table 1 below shows a separate simulation with ten TCP
    connection and a fixed byte drop rate of 0.0001, with each
    simulation using a different segment size.  For each simulation, the
    TCP sending rate and goodput are averaged over the ten flows.  As
    one would expect from the paragraph above, the TCP sending rate
    grows somewhat less than linearly with an increase in packet size,
    up to a packet size of 1460 bytes, corresponding to a packet drop
    rate of 13%.  After that, further increases in the packet size
    result in a *decrease* in the TCP sending rate, as the TCP
    connection enters the regime of exponential backoff of the
    retransmit timer.

                  Segment   Packet      TCP Rates (Kbps)
                  Size (B)  DropRate   SendRate    Goodput
                  --------  --------   --------    -------
                      14      0.005       6.37       6.34
                     128      0.016      30.78      30.30
                     256      0.028      46.54      44.96
                     512      0.053      62.43      58.37
                    1460      0.134      94.15      80.02
                    4000      0.324      35.20      21.44
                    8000      0.531      15.36       5.76

               Table 1: TCP Median Send Rate vs. Packet Size I:
                            Byte Drop Rate 0.0001

    Table 2 below shows similar results for a byte drop rate of 0.001.
    In this case, the TCP sending rate grows with increasing packet size
    up to a packet size of 128 bytes, corresponding to a packet drop
    rate of 16%.  After than, the TCP sending rate decreases and then
    increases again, as the TCP connection enters the regime of
    exponential backoff of the retransmit timer.  Note that with this
    byte drop rate, with packet sizes of 4000 and 8000 bytes, the TCP
    sending rate increases but the TCP goodput rate remains essentially



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    zero.  This makes sense, as almost all packets that are sent are
    dropped.

                  Segment   Packet      TCP Rates (Kbps)
                  Size (B)  DropRate   SendRate    Goodput
                  --------  --------   --------    -------
                      14      0.053       1.68       1.56
                     128      0.159       7.66       6.13
                     256      0.248       6.21       4.32
                     512      0.402       1.84       1.11
                    1460      0.712       1.87       0.47
                    4000      0.870       3.20       0.00
                    8000      0.890       5.76       0.00

               Table 2: TCP Median Send Rate vs. Packet Size II:
                            Byte Drop Rate 0.001

    The TCP behavior in the presence of a fixed byte drop rate suggests
    that instead of the goal of a TFRC-SP flow achieving the same
    sending rate in bytes per second as a TCP flow using 1500-byte
    packets, it makes more sense to consider an ideal goal of a TFRC-SP
    flow achieving the same sending rate as a TCP flow with the optimal
    packet size, given that the packet size is at most 1500 bytes.  This
    does not mean that we need to change the TFRC-SP mechanisms for
    computing the allowed transmit rate;  this means simply that in
    evaluating the fairness of TFRC-SP, we should consider fairness
    relative to the TCP flow using the optimal packet size (though still
    at most 1500 bytes) for that environment.

5.  A Comparison with RFC 3714

    RFC 3714 considers the problems of fairness, potential congestion
    collapse, and poor user quality that could occur with the deployment
    of non-congestion-controlled IP telephony over congested best-effort
    networks.  The March 2004 document cites ongoing efforts in the
    IETF, including work on TFRC and DCCP.  RFC 3714 recommends that for
    best-effort traffic with applications that have a fixed or minimum
    sending rate, the application or transport protocol should monitor
    the packet drop rate, and discontinue sending for a period if the
    steady-state packet drop rate significantly exceeds the allowed
    threshold for that minimum sending rate.

    In determining the allowed packet drop rate for a fixed sending
    rate, RFC 3714 assumes that an IP telephony flow should be allowed
    to use the same sending rate in bytes per second as a 1460-byte-
    packet TCP flow experiencing the same packet drop rate as that of
    the IP telephony flow.  As an example, following this guideline, a
    VoIP connection with a round-trip time of 0.1 sec and a minimum



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    sending rate of 64 kbps would be required to terminate or suspend
    when the persistent packet drop rate significantly exceeded 25%.

    One limitation of the lack of fine-grained control in the minimal
    mechanism described in RFC 3714 is that an IP telephony flow would
    not adapt its sending rate in response to congestion.  In contrast,
    with TFRC-SP, a small-packet flow would reduce its sending rate
    somewhat in response to moderate packet drop rates, possibly
    avoiding a period with unnecessarily-heavy packet drop rates in the
    network.

    Because RFC 3714 assumes that the allowed packet drop rate for an IP
    telephony flow is determined by the sending rate that a TCP would
    use *with the same packet drop rate*, the minimal mechanism in RFC
    3714 would not provide fairness between TCP and IP telephony traffic
    in an environment where small packets are less likely to be dropped
    than large packets.  In such an environment, the small-packet IP
    telephony flow would make the optimistic assumption that a large-
    packet TCP flow would receive the same packet drop rate as the IP
    telephony flow, and as a result the small-packet IP telephony flow
    would receive a larger fraction of the link bandwidth than a
    competing large-packet TCP flow.

6.  TFRC-SP with Applications that Modify the Packet Size

    One possible use for TFRC-SP would be with applications that
    maintain a fixed sending rate in packets per second, but modify
    their packet size in response to congestion.  TFRC-SP monitors the
    connection's packet drop rate, and determines the allowed sending
    rate in bytes per second.  Given an application with a fixed sending
    rate in packets per second, the TFRC-SP sender could determine the
    data packet size that would be needed for the sending rate in bytes
    per second not to exceed the allowed sending rate.  In environments
    where the packet drop rate is affected by the packet size,
    decreasing the packet size could also result in a decrease in the
    packet drop rate experienced by the flow.

    There are many questions about how an adaptive application would use
    TFRC-SP that are beyond the scope of this document.  In particular,
    an application might wish to avoid unnecessary reductions in the
    packet size.   In this case, an application might wait for some
    period of time before reducing the packet size, to avoid an
    unnecessary reduction in the packet size.  The details of how long
    an application might wait before reducing the packet size can be
    addressed in documents that are more application-specific.

    Similarly, an application using TFRC-SP might only have a few packet
    sizes that it is able to use.  In this case, the application might



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    not reduce the packet size until the current packet drop rate has
    significantly exceeded the packet drop rate threshold for the
    current sending rate, to avoid unnecessary oscillations between two
    packet sizes and two sending rates.  Again, the details will have to
    be addressed in documents that are more application-specific.

7.  Simulation Results

    This section explores the performance of TFRC-PS in simulation
    scenarios with configured packet or byte drop rates, and in
    scenarios with a range of queue management mechanisms at the
    congested link.  The simulations explore environments where standard
    TFRC significantly limits the throughput of small-packet flows, and
    TFRC-SP gives the desired throughput.  The simulations also explore
    environments where standard TFRC allows small-packet flows to
    receive good performance, while TFRC-SP is overly aggressive.

    The general lessons from the simulations are as follows.

    o  In scenarios where large and small packets receive similar packet
       drop rates, TFRC-SP gives roughly the desired sending rate
       (Sections 7.1, 7.3).

    o  In scenarios where each *byte* is equally likely to be dropped,
       standard TFRC gives reasonable fairness between small-packet TFRC
       flows and large-packet TCP flows (Section 7.2).

    o  In scenarios where small packets are less likely to be dropped
       than large packets, TFRC-SP does not give reasonable fairness
       between small-packet TFRC-SP flows and large-packet TCP flows;
       small-packet TFRC-SP flows can receive considerably more
       bandwidth than competing large-packet TCP flows (Sections 7.2,
       7.3, 7.4).

    o  Scenarios where small packets are less likely to be dropped than
       large packets include those with Drop-Tail queues in bytes, and
       with AQM mechanisms in byte mode (Sections 7.3, 7.4).

    Those who are not interested in the details of the simulations could
    proceed directly to Section 8 on General Discussion.

    TFRC-SP has been added to the NS simulator, and is illustrated in
    the validation test "./test-all-friendly" in the directory
    tcl/tests.  The simulation scripts for the simulations in this
    document are available at "http://www.icir.org/tfrc/voipsims.html".
    There is also a pointer to the document "Graphs for draft-ietf-dccp-
    tfrc-voip-03", which has graphs showing the information in tables in
    this document.



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7.1.  Simulations with Configured Packet Drop Rates

    In this section we describe simulation results from simulations
    comparing the throughput of standard (SACK) TCP flows, TCP flows
    with timestamps and ECN, TFRC-SP flows, and standard TFRC (Stnd
    TFRC) flows.  In these simulations we configure the router to
    randomly drop or mark packets at a specified rate, independently of
    the packet size.  For each specified packet drop rate, we give a
    flow's average sending rate in Kbps over the second half of a
    100-second simulation, averaged over ten flows.

                Packet       Send Rates (Kbps, 1460B seg)
                DropRate      TCP      ECN TCP      TFRC
                --------    --------   --------   --------
                   0.001    2020.85    1904.61     982.09
                   0.005     811.10     792.11     878.08
                   0.01      515.45     533.19     598.90
                   0.02      362.93     382.67     431.41
                   0.04      250.06     252.64     284.82
                   0.05      204.48     218.16     268.51
                   0.1       143.30     148.41     146.03
                   0.2        78.65      93.23*     55.14
                   0.3        26.26      59.65*     32.87
                   0.4         9.87      47.79*     25.45
                   0.5         3.53      32.01*     18.52

         * ECN scenarios marked with an asterisk are not realistic,
           as routers are not recommended to mark packets when packet
           drop/mark rates are 20% or higher.

                Table 3: Send Rate vs. Packet Drop Rate I:
                              1460B TFRC Segments
                  (1.184 Kbps Maximum TFRC Data Sending Rate)


    Table 3 shows the sending rate for a TCP and a standard TFRC flow
    for a range of configured packet drop rates, when both flows have
    1460-byte data packets, in order to illustrate the relative fairness
    of TCP and TFRC when both flows use the same packet size.  For
    example, a packet drop rate of 0.1 means that 10% of the TCP and
    TFRC packets are dropped.  The TFRC flow is configured to send at
    most 100 packets per second.  There is good relative fairness until
    the packet drop percentages reach 40 and 50%, when the TFRC flow
    receives three to five times more bandwidth than the standard TCP
    flow.  We note that an ECN TCP flow would receive a higher
    throughput than the TFRC flow.  However, we don't use the ECN TCP
    sending rate in these high-packet-drop scenarios as the target
    sending rate for TFRC, as routers are advised to drop rather than



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    mark packets during high levels of congestion.


                    < - - - - - - Send Rates (Kbps) - - - - - >
           Packet       TCP       ECN TCP     TFRC-SP   Stnd TFRC
          DropRate  (1460B seg) (1460B seg)  (14B seg)  (14B seg)
          --------  ----------- -----------  ---------  ---------
             0.001    1787.54     1993.03      17.71      17.69
             0.005     785.11      823.75      18.11      17.69
             0.01      533.38      529.01      17.69      17.80
             0.02      317.16      399.62      17.69      13.41
             0.04      245.42      260.57      17.69       8.84
             0.05      216.38      223.75      17.69       7.63
             0.1       142.75      138.36      17.69       4.29
             0.2        58.61       91.54*     17.80       1.94
             0.3        21.62       63.96*     10.26       1.00
             0.4        10.51       41.74*      4.78       0.77
             0.5         1.92       19.03*      2.41       0.56

         * ECN scenarios marked with an asterisk are not realistic,
           as routers are not recommended to mark packets when packet
           drop/mark rates are 20% or higher.

                Table 4: Send Rate vs. Packet Drop Rate II:
                               14B TFRC Segments
                   (5.6 Kbps Maximum TFRC Data Sending Rate)


    Table 4 shows the results of simulations where each TFRC-SP flow has
    a maximum data sending rate of 5.6 Kbps, with 14-byte data packets
    and a 32-byte packet header for DCCP and IP.  Each TCP flow has a
    receive window of 100 packets and a data packet size of 1460 bytes,
    with a 40-byte packet header for TCP and IP.  The TCP flow uses SACK
    TCP with Limited Transmit, but without timestamps or ECN.  Each flow
    has a round-trip time of 240 ms in the absence of queueing delay.

    The TFRC sending rate in Table 4 is the sending rate for the 14-byte
    data packet with the 32-byte packet header.  Thus, only 30% of the
    TFRC sending rate is for data, and with a packet drop rate of p,
    only a fraction 1-p of that data makes it to the receiver.  Thus,
    the TFRC data receive rate can be considerably less than the TFRC
    sending rate in the table.  Because TCP uses large packets, 97% of
    the TCP sending rate is for data, and the same fraction 1-p of that
    data makes it to the receiver.

    Table 4 shows that for the 5.6 Kbps data stream with TFRC, Standard
    TFRC (Stnd TFRC) gives a very poor sending rate in bps, relative to
    the sending rate for the large-packet TCP connection.  In contrast,



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    the sending rate for the TFRC-SP flow is relatively close to the
    desired goal of fairness in bps with TCP.

    Table 4 shows that with TFRC-SP, the 5.6 Kbps data stream doesn't
    reduce its sending rate until packet drop rates greater than 20%, as
    desired.  With packet drop rates of 30-40%, the sending rate for the
    TFRC-SP flow is somewhat less than that of the average large-packet
    TCP flow, while for packet drop rates of 50% the sending rate for
    the TFRC-SP flow is somewhat greater than that of the average large-
    packet TCP flow.


                    < - - - - - - Send Rates (Kbps) - - - - - >
           Packet       TCP       ECN TCP    TFRC-SP   Stnd TFRC
          DropRate  (1460B seg) (1460B seg) (200B seg) (200B seg)
          --------  ----------- ----------- ---------- ----------
             0.001    1908.98     1869.24     183.45     178.35
             0.005     854.69      835.10     185.06     138.06
             0.01      564.10      531.10     185.33      92.43
             0.02      365.38      369.10     185.57      62.18
             0.04      220.80      257.81     185.14      45.43
             0.05      208.97      219.41     180.08      39.44
             0.1       141.67      143.88     127.33      21.96
             0.2        62.66       91.87*     54.66       9.40
             0.3        16.63       65.52*     24.50       4.73
             0.4         6.62       42.00*     13.47       3.35
             0.5         1.01       21.34*     10.51       2.92

         * ECN scenarios marked with an asterisk are not realistic,
           as routers are not recommended to mark packets when packet
           drop/mark rates are 20% or higher.

                Table 5: Sending Rate vs. Packet Drop Rate III:
                             200B TFRC Segments
                 (160 Kbps Maximum TFRC Data Sending Rate)

    Table 5 shows results with configured packet drop rates when the
    TFRC flow uses 200-byte data packets, with a maximum data sending
    rate of 160 Kbps.  As in Table 4, the performance of Standard TFRC
    is quite poor, while the performance of TFRC-SP is essentially as
    desired for packet drop rates up to 30%.  Again as expected, with
    packet drop rates of 40-50% the TFRC-SP sending rate is somewhat
    higher than desired.

    For these simulations, the sending rate of a TCP connection using
    timestamps is similar to the sending rate shown for a standard TCP
    connection without timestamps.




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7.2.  Simulations with Configured Byte Drop Rates

    In this section we explore simulations where the router is
    configured to drop or mark each *byte* at a specified rate.  When a
    byte is chosen to be dropped (or marked), the entire packet
    containing that byte is dropped (or marked).

                            < - - - - - Send Rates (Kbps) - - - - - >
         Byte       TCP                           TFRC-SP    Stnd TFRC
       DropRate   SegSize     TCP      ECN TCP    (14B seg)  (14B seg)
       --------   -------   --------   --------   ---------  ---------
        0.00001     1460     423.02     431.26      17.69      17.69
        0.0001      1460     117.41     114.34      17.69      17.69
        0.001        128      10.78      11.50      17.69       8.37
        0.005         14       1.75       2.89      18.39       1.91
        0.010       1460       0.31       0.26       7.07       0.84
        0.020       1460       0.29       0.26       1.61       0.43
        0.040       1460       0.12       0.26*      0.17       0.12
        0.050       1460       0.15       0.26*      0.08       0.06

         * ECN scenarios marked with an asterisk are not realistic,
           as routers are not recommended to mark packets when packet
           drop/mark rates are 20% or higher.

           TFRC's maximum data sending rate is 5.6 Kbps.

            Table 6: Sending Rate vs. Byte Drop Rate


    Table 6 shows the TCP and TFRC send rates for various byte drop
    rates.  For each byte drop rate, Table 6 shows the TCP sending rate,
    with and without ECN, for the TCP segment size that gives the
    highest TCP sending rate.  Simulations were run with TCP segments of
    14, 128, 256, 512, and 1460 bytes.  Thus, for a byte drop rate of
    0.00001, the table shows the TCP sending rate with 1460-byte data
    segments, but with a byte drop rate of 0.001, the table shows the
    TCP sending rate with 128-byte data segments.  For each byte drop
    rate, Table 6 also shows the TFRC-SP and that Standard TFRC sending
    rates.  With configured byte drop rates, TFRC-SP gives an unfair
    advantage to the TFRC-SP flow, while Standard TFRC gives essentially
    the desired performance.










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                        TCP Pkt     TFRC Pkt
               Byte     DropRate    DropRate       TCP/TFRC
             DropRate  (1460B seg)  (14B seg)   Pkt Drop Ratio
             --------  -----------  ---------   --------------
              0.00001     0.015       0.0006        26.59
              0.0001      0.13        0.0056        24.94
              0.001       0.77        0.054         14.26
              0.005       0.99        0.24           4.08
              0.01        1.00        0.43           2.32
              0.05        1.00        0.94           1.05

            Table 7: Packet Drop Rate Ratio vs. Byte Drop Rate


    Table 7 converts the byte drop rate p to packet drop rates for the
    TCP and TFRC packets, where the packet drop rate for an N-byte
    packet is 1-(1-p)^N.  Thus, a byte drop rate of 0.001, with each
    byte being dropped with probability 0.001, converts to a packet drop
    rate of 0.77, or 77%, for the 1500-byte TCP packets, and a packet
    drop rate of 0.054, or 5.4%, for the 56-byte TFRC packets.

    The right column of Table 7 shows the ratio between the TCP packet
    drop rate and the TFRC packet drop rate.  For low byte drop rates,
    this ratio is close to 26.8, the ratio between the TCP and TFRC
    packet sizes.  For high byte drop rates, where even most small TFRC
    packets are likely to be dropped, this drop ratio approaches 1.  As
    Table 6 shows, with byte drop rates, the Standard TFRC sending rate
    is close to optimal, competing fairly with a TCP connection using
    the optimal packet size within the allowed range.  In contrast, the
    TFRC-SP connection gets more than its share of the bandwidth, though
    it does reduce its sending rate for a byte drop rate of 0.01 or more
    (corresponding to a TFRC-SP *packet* drop rate of 0.43.

    Table 6 essentially shows three separate regions.  In the low-
    congestion region, with byte drop rates less than 0.0001, the TFRC-
    SP connection is sending at its maximum sending rate.  In this
    region the optimal TCP connection is the one with 1460-byte packets,
    and the TCP sending rate goes as 1/sqrt(p), for packet drop rate p.
    This low-congestion region holds for packet drop rates up to 10-15%.

    In the middle region of Table 6, with byte drop rates from 0.0001 to
    0.005, the optimal TCP segment size is a function of the byte drop
    rate.  In particular, the optimal TCP segment size seems to be the
    one that keeps the packet drop rate at most 15%, keeping the TCP
    connection in the regime controlled by a 1/sqrt(p) sending rate, for
    packet drop rate p.  For a TCP packet size of S bytes (including
    headers), and a *byte* drop rate of B, the packet drop rate is
    roughly B*S.  For a given byte drop rate in this regime, if the



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    optimal TCP performance occurs with a packet size chosen to give a
    packet drop rate of at most 15%, keeping the TCP connection out of
    the regime of exponential backoffs of the retransmit timer, then
    this requires B*S = 0.15, or S = 0.15/B.

    In the high-congestion regime of Table 6, with high congestion and
    with byte drop rates of 0.01 and higher, the TCP performance is
    dominated by the exponential backoff of the retransmit timer
    regardless of the segment size.  Even a 40-byte packet with a zero-
    byte data segment would have a packet drop rate of at least 33%.  In
    this regime, the optimal TCP *sending* rate comes with a large,
    1460-byte data segment, but the TCP sending rate is low with any
    segment size, considerably less than one packet per round-trip time.
    We note that in this regime, while a larger packet gives a higher
    TCP *sending* rate, a smaller packet gives a better *goodput* rate.

    In general, Tables 4 and 5 show good performance for TFRC-SP in
    environments with stable packet drop rates, where the decision to
    drop a packet is independent of the packet size.  However, in some
    environments the packet size might affect the likelihood that a
    packet is dropped.  For example, with heavy congestion and a Drop
    Tail queue with a fixed number of bytes rather than a fixed number
    of packet-sized buffers, small packets might be more likely than
    large packets to find room at the end of an almost-full queue.   As
    a further complication, in a scenario with Active Queue Management,
    the AQM mechanism could either be in packet mode, dropping each
    packet with equal probability, or in byte mode, dropping each byte
    with equal probability.  Sections 7.3 and 7.4 show simulations with
    packets dropped at Drop Tail or AQM queues, rather that from a
    probabilistic process.

7.3.  Packet Dropping Behavior at Routers with Drop-Tail Queues

    One of the problems with comparing the throughput of two flows using
    different packet sizes is that the packet size itself can influence
    the packet drop rate [V00, WBL04].

    The default TFRC was designed for rough fairness with TCP, for TFRC
    and TCP flows with the same packet size and experiencing the same
    packet drop rate.  When the issue of fairness between flows with
    different packets sizes is addressed, it matters whether the packet
    drop rates experienced by the flows is related to the packet size.
    That is, are small packets just as likely to be dropped as large TCP
    packets, or are the smaller packets less likely to be dropped
    [WBL04]? And what is the relationship between the packet-dropping
    behavior of the path, and the loss event measurements of TFRC?





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                     < - - - - - Send Rates in Kbps - - - - >
            Web        TCP (1460B seg)     TFRC-SP (200B seg)
          Sessions   DropRate  SendRate    DropRate  SendRate
          --------   --------  --------    --------  --------
              10       0.04     316.18       0.05     183.05
              25       0.07     227.47       0.07     181.23
              50       0.08     181.10       0.08     178.32
             100       0.14      85.97       0.12     151.42
             200       0.17      61.20       0.14      73.88
             400       0.20      27.79       0.18      36.81
             800       0.29       3.50       0.27      16.33
            1600       0.37       0.63       0.33       6.29

        Table 8: Drop and Send Rates for Drop-Tail Queues in Packets


    Table 8 shows the results of the second half of 100-second
    simulations, with five TCP connections and five TFRC-SP connections
    competing with web traffic in a topology with a 3 Mbps shared link.
    The TFRC-SP application generates 200-byte data packets every 10 ms,
    for a maximum data rate of 160 Kbps.  The five long-lived TCP
    connections use a data packet size of 1460 bytes, and the web
    traffic uses a data packet size of 512 bytes.  The five TCP
    connections have roundtrip times from 40 to 240 ms, and the five
    TFRC connections have the same set of round-trip times.  The SACK
    TCP connections in these simulations use the default parameters in
    the NS simulator, with Limited Transmit, and a minimum RTO of 200
    ms.  Adding timestamps to the TCP connection didn't change the
    results appreciably.  The simulations include reverse-path traffic,
    to add some small control packets to the forward path, and some
    queueing delay to the reverse path.  The number of web sessions is
    varied to create different levels of congestion.  The Drop-Tail
    queue is in units of packets, which each packet arriving to the
    queue requires a single buffer, regardless of the packet size.

    Table 8 shows the average TCP and TFRC sending rates, each averaged
    over the five flows.  As expected, the TFRC-SP flows see similar
    packet drop rates as the TCP flows, though the TFRC-SP flows
    receives higher throughput than the TCP flows with packet drop rates
    of 25% or higher.











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                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.06     239.81       0.00     185.19
                25       0.09     189.02       0.01     184.95
                50       0.14      99.46       0.01     185.07
               100       0.20      16.42       0.02     183.77
               200       0.26       4.46       0.03     181.98
               400       0.29       4.61       0.05     151.88
               800       0.49       1.01       0.08     113.10
              1600       0.65       0.67       0.12      65.17

     Table 9: Drop and Send Rates for Drop-Tail Queues in Bytes I:
                              1460B TCP Segments


    However, the fairness results can change significantly if the Drop-
    Tail queue at the congested output link is in units of bytes rather
    than packets.  For a queue in packets, the queue has a fixed number
    of buffers, and each buffer can hold exactly one packet, regardless
    of its size in bytes.  For a queue in bytes, the queue has a fixed
    number of *bytes*, and an almost-full queue might have room for a
    small packet but not for a large one.  This, for a simulation with a
    Drop-Tail queue in bytes, large packets are more likely to be
    dropped than are small ones.  The NS simulator doesn't yet have a
    more realistic intermediate model, where the queue has a fixed
    number of buffers, each buffer has a fixed number of bytes, and each
    packet would require one or more free buffers.  In this model, a
    small packet would use one buffer, while a larger packet would
    require several buffers.

    As Table 9 shows, with a Drop-Tail queue in bytes, the TFRC-SP flow
    sees a much smaller packet drop rate than the TCP flow, and as a
    consequence receives a much larger sending rate.  For the
    simulations in Table 9, the TFRC-SP flows use 200-byte data
    segments, while the long-lived TCP flows use 1460-byte data
    segments.  For example, when the five TCP flows and five TFRC-SP
    flows share the link with 800 web sessions, the five TCP flows see
    an average drop rate of 49% in the second half of the simulation,
    while the five TFRC-SP flows receive an average drop rate of 8%, and
    as a consequence receive more than 100 times the throughput of the
    TCP flows.  This raises serious questions about making the
    assumption that flows with small packets see the same packet drop
    rate as flows with larger packets.  Further work will have to
    include an investigation into the range of realistic Internet
    scenarios, in terms of whether large packets are considerably more
    likely to be dropped than are small ones.



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                       < - - - - - Send Rates in Kbps - - - - >
              Web        TCP (512B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.02     335.05       0.00     185.16
                25       0.02     289.94       0.00     185.36
                50       0.04     139.99       0.01     184.98
               100       0.06      53.50       0.01     184.66
               200       0.10      16.14       0.04     167.87
               400       0.16       6.36       0.07     114.85
               800       0.24       0.90       0.11      67.23
              1600       0.42       0.35       0.18      39.32

     Table 10: Drop and Send Rates for Drop-Tail Queues in Bytes II:
                               512B TCP Segments


    Table 10 shows that in the scenario the long-lived TCP flows receive
    a higher packet drop rate than the TFRC-SP flows, and receive
    considerably less throughput, even when the long-lived TCP flows use
    512-byte segments.

7.4.  Packet Dropping Behavior at Routers with AQM

    As expected, the packet dropping behavior also can be varied by
    varying the Active Queue Management (AQM) mechanism in the router.
    When the routers use RED (Random Early Detection), there are several
    parameters than can affect the packet dropping or marking behavior
    as a function of the packet size.

    First, as with Drop-Tail, the RED queue can be either in units of
    packets or of bytes.  This can affect the packet dropping behavior
    when RED is unable to control the average queue size, and the queue
    overflows.

    Second, and orthogonally, RED can be configured to be either in
    packet mode or in byte mode.  In packet mode, each *packet* has the
    same probability of being dropped by RED, while in byte mode, each
    *byte* has the same probability of being dropped.  In packet mode,
    large-packet and small-packet flows receive roughly the same packet
    drop rate, while in byte mode, large-packet and small-packet flows
    with the same throughput in bps receive roughly the same *number* of
    packet drops.  The simulations reported in the appendix show that
    for RED in packet mode, the packet drop rates for the TFRC-SP flows
    are similar to those for the TCP flows, with a resulting acceptable
    throughput for the TFRC-SP flows.   This is true with the queue in
    packets or in bytes, and with or without Adaptive RED (discussed
    below).  As we show below, this fairness between TCP and TFRC-SP



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    flows does not hold for RED in byte mode.

    The third RED parameter that affects the packet dropping or marking
    behavior as a function of packet size is whether the RED mechanism
    is using Standard RED or Adaptive RED;  Adaptive RED tries to
    maintain the same average queue size, regardless of the packet drop
    rate.  The use of Adaptive RED allows the RED mechanism to function
    more effectively in the presence of high packet drop rates (e.g.,
    greater than 10%).  Without Adaptive RED, there is a fixed dropping
    threshold, and all arriving packets are dropped when the dropping or
    marking rate exceeds this threshold.  In contrast, with Adaptive
    RED, the dropping function is adapted to accommodate high-drop-rate
    regimes.  One consequence is that when byte mode is used with
    Adaptive RED, the byte mode extends even to high-drop-rate regimes.
    When byte mode is used with standard RED, however, the byte mode is
    no longer in use when the drop rate exceeds the fixed dropping
    threshold (set by default to 10% in the NS simulator).

    In the simulations in this section, we explore the TFRC-SP behavior
    over some of this range of scenarios.  In this simulations, as in
    Section 7.3 above, the application for the TFRC-SP flow uses
    200-byte data packets, generating 100 packets per second.


                       < - - - - - Send Rates in Kbps - - - - >
              Web        TCP (1460B seg)     TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.05     305.76       0.04     182.82
                25       0.06     224.16       0.06     175.91
                50       0.09     159.12       0.08     152.51
               100       0.13      90.77       0.11     106.13
               200       0.14      48.53       0.14      70.25
               400       0.20      22.08       0.19      41.50
               800       0.27       3.55       0.25      17.50
              1600       0.42       1.87       0.34       8.81

      Table 11: Drop and Send Rates for RED Queues in Packet Mode


    For the simulations in Table 11, with a congested router with a RED
    queue in packet mode, the results are similar to those with a Drop-
    Tail queue in packets, as in Table 8 above.  The TFRC-SP flow
    receives similar packet drop rates as the TCP flow, though it
    receives higher throughput in the more congested environments.  The
    simulations are similar with a RED queue in packet mode with the
    queue in bytes, and with or without Adaptive RED.  In these
    simulations, TFRC-SP gives roughly the desired performance.



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                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.06     272.16       0.02     184.37
                25       0.07     175.82       0.02     184.06
                50       0.10      75.65       0.04     180.56
               100       0.14      38.98       0.07     151.65
               200       0.19      16.66       0.11     106.80
               400       0.26       4.85       0.15      69.41
               800       0.35       3.12       0.20      27.07
              1600       0.42       0.67       0.29      10.68

        Table 12: Drop and Send Rates for RED Queues in Byte Mode


    Table 12 shows that with a standard RED queue in byte mode instead
    of packet mode, there is a somewhat greater different between the
    packet drop rates between the TCP and TFRC-SP flows, particularly
    for lower packet drop rates.  For the simulation in Table 12, the
    packet drop rates for the TCP flows can range from 1 1/2 to four
    times greater than the packet drop rates for the TFRC-SP flows.
    However, because the TFRC-SP flow has an upper bound on the sending
    rate, its sending rate is not affected in the lower packet-drop-rate
    regimes; its sending rate is only affected in the regimes with
    packet drop rates of 10% or more.  The sending rate for TFRC-SP in
    the scenarios in Table 12 with higher packet drop rates are greater
    than desired, e.g., for the scenarios with 400 web sessions or
    greater.  However, the results with TFRC-SP are at least better than
    that of small-packet flows with no congestion control at all.


                       < - - - - - Send Rates in Kbps - - - - >
              Web        TCP (512B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.01     337.86       0.01     184.06
                25       0.02     258.71       0.01     184.03
                50       0.02     184.71       0.01     183.99
               100       0.04      63.63       0.03     184.43
               200       0.08      28.95       0.06     149.80
               400       0.12      17.03       0.10      88.21
               800       0.24       5.94       0.15      36.80
              1600       0.32       3.37       0.21      19.45

        Table 13: Drop and Send Rates for RED Queues in Byte Mode





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    Table 13 shows that with a standard RED queue in byte mode and with
    long-lived TCP flows with 512-byte data segments, there is only a
    moderate difference between the packet drop rate for the 552-byte
    TCP packets and the 240-byte TFRC-SP packets.  However, the sending
    rate for TFRC-SP in the scenarios in Table 13 with higher packet
    drop rates are still greater than desired, even given the goal of
    fairness with TCP flows with 1500-byte data segments instead of
    512-byte data segments.


                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.04     318.10       0.02     185.34
                25       0.08     175.34       0.03     184.38
                50       0.10      81.60       0.04     181.95
               100       0.12      28.51       0.05     178.79
               200       0.20       3.65       0.06     173.78
               400       0.27       1.44       0.08     161.41
               800       0.40       0.58       0.06     159.62
              1600       0.55       0.29       0.02     180.92

    Table 14: Drop and Send Rates with Adaptive RED Queues in Byte Mode


    For the simulations in Table 14, the congested router uses an
    Adaptive RED queue in byte mode.

    For this router, the output queue is in units of bytes rather than
    of packets, each *byte* is dropped with the same probability, and
    because of the use of Adaptive RED, this byte-dropping mode extends
    even for the high-packet-drop-rate regime.

    As Table 14 shows, for a scenario with Adaptive RED in byte mode,
    the packet drop rate for the TFRC-SP traffic is *much* lower than
    that for the TCP traffic, and as a consequence, the sending rate for
    the TFRC-SP traffic in a highly congested environment is *much*
    higher than that of the TCP traffic.  In fact, in these scenarios
    the TFRC-SP congestion control mechanisms are largely ineffective
    for the small-packet traffic.

    We note that the unfairness in these simulations, in favor of TFRC-
    SP, is even greater than the unfairness shown in Table 9 for a Drop-
    Tail queue in bytes.  At the same time, it is not known if there is
    any deployment in the Internet of any routers with Adaptive RED in
    byte mode, or of any AQM mechanisms with similar behavior;  we don't
    know the extent of the deployment of standard RED, or or any of the



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    proposed AQM mechanisms.

                       < - - - - - Send Rates in Kbps - - - - >
              Web        TCP (512B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10       0.01     306.56       0.01     185.11
                25       0.02     261.41       0.01     184.41
                50       0.02     185.07       0.01     184.54
               100       0.04      59.25       0.03     181.58
               200       0.08      16.32       0.06     150.87
               400       0.12      11.53       0.10      98.10
               800       0.25       5.85       0.15      46.59
              1600       0.32       1.43       0.22      19.40

    Table 15: Drop and Send Rates for Adaptive RED Queues in Byte Mode


    Table 15 shows that when TFRC-SP with 240-byte packets competes with
    TCP with 552-byte packets in a scenario with Adaptive RED in byte
    mode, the TFRC-SP flows still receive more bandwidth that the TCP
    flows, but the level of unfairness is less severe, and the packet
    drop rates of the TCP flows is at most twice that of the TFRC-SP
    flows.  That is, the TFRC-SP flows still receive more than their
    share of the bandwidth, but the TFRC-SP congestion control is more
    effective that than in Table 14 above.

8.  General Discussion

    Dropping rates for small packets: The goal of TFRC-SP is for small-
    packet TFRC-SP flows to have rough fairness with large-packet TCP
    flows in the sending rate in bps, in a scenario where each packet
    receives roughly the same probability of being dropped.  In a
    scenario where large packets are more likely to be dropped than
    small packets, or where flows with a bursty sending rate are more
    likely to have packets dropped than are flows with a smooth sending
    rate, small-packet TFRC-SP flows can receive significantly more
    bandwidth than competing large-packet TCP flows.

    The accuracy of the TCP response function used in TFRC: For
    applications with a maximum sending rate of 96 Kbps or less (i.e.,
    packets of at most 120 bytes) TFRC-SP only restricts the sending
    rate when the packet drop rate is fairly high, e.g., greater than
    10%.  [Derivation: A TFRC-SP flow with a 200 ms round-trip time and
    a maximum sending rate with packet headers of 128 Kbps would have a
    sending rate in bytes per second equivalent to a TCP flow with
    1460-byte packets sending 2.2 packets per round-trip time.  From
    Table 1 of RFC 3714, this sending rate can be sustained with a



Floyd/Kohler                                       Section 8.  [Page 28]

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    packet drop rate slightly greater than 10%.]  In this high-packet-
    drop regime, the performance of TFRC-SP is determined in part by the
    accuracy of the TCP response function in representing the actual
    sending rate of a TCP connection.

    In this regime of high packet drop rates, TCP performance is also
    affected by the TCP algorithm (e.g., SACK or not), by the minimum
    RTO, by the use or not of Limited Transmit, by the use of timestamps
    and/or of ECN, and the like.  It is good to insure that simulations
    or experiments exploring fairness include the exploration of
    fairness with the most aggressive TCP mechanisms conformance with
    the current standards.  Our simulations use SACK TCP with Limited
    Transmit and with a minimum RTO of 200 ms.  Adding the use of
    timestamps has not made a big difference.  We haven't used TCP with
    ECN, because our judgment is that in high packet drop regimes, it is
    preferable for AQM mechanisms to drop rather than mark packets.

    General issues for TFRC: The congestion control mechanisms in TFRC
    and TFRC-SP limit a flow's sending rate in packets per second.
    Simulations by Tom Phelan [P04] explore how such a limitation in
    sending rate can lead to unfairness for the TFRC flow in some
    scenarios, e.g., when competing with bursty TCP web traffic, in
    scenarios with low levels of statistical multiplexing at the
    congested link.

9.  Security Considerations

    There are no security considerations introduced in this document.

    General security considerations for TFRC are discussed in RFC 3448.
    The security considerations for TFRC include the need to protect
    against spoofed feedback, and the need for protection mechanisms to
    protect the congestion control mechanisms against incorrect
    information from the receiver.

    Security considerations for DCCP's Congestion Control ID 3, TFRC
    Congestion Control, are discussed in [CCID 3 PROFILE].  That
    document extensively discussed the mechanisms the sender can use to
    verify the information sent by the receiver.

10.  IANA Considerations

    There are no IANA considerations in this document.

11.  Thanks

    We thank Tom Phelan for discussions of TFRC-SP and for his paper
    exploring the fairness between TCP and TFRC-SP flows.  We thank



Floyd/Kohler                                      Section 11.  [Page 29]

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    Joerg Widmer for feedback on earlier versions of this draft.  We
    also thank the DCCP Working Group for feedback and discussions.

A.  Appendix: Related Work on Small-Packet Variants of TFRC

    Other proposals for variants of TFRC for applications with variable
    packet sizes include [WBL04] and [V00]. [V00] proposed that TFRC
    should be modified so that flows are not penalized by sending
    smaller packets.  In particular, [V00] proposes using the path MTU
    in the TCP-friendly equation, instead of the actual packet size used
    by TFRC, and counting the packet drop rate by using an estimation
    algorithm that aggregates both packet drops and received packets
    into MTU-sized units.

    [WBL04] also argues that adapting TFRC for variable packet sizes by
    just using the packet size of a reference TCP flow in the TFRC
    equation would not suffice in the high-packet-loss regime; such a
    modified TFRC would have a strong bias in favor of smaller packets,
    because multiple lost packets in a single round-trip time would be
    aggregated into a single packet loss.  [WBL04] proposes modifying
    the loss measurement process to account for the bias in favor of
    smaller packets.

    The TFRC-SP variant of TFRC proposed in our document differs from
    [WBL04] in restricting its attention to flows that send at most 100
    packets per second.  The TFRC-SP variant proposed in our document
    also differs from the straw proposal discussed in [WBL04] in that
    the allowed bandwidth includes the bandwidth used by packet headers.

    [WBL04] discusses four methods for modifying the loss measurement
    process, "unbiasing", "virtual packets", "random sampling", and
    "Loss Insensitive Period (LIP) scaling".  [WBL04] finds only the
    second and third methods sufficiently robust when the network drops
    packets independently of packet size.  They find only the second
    method sufficiently robust when the network is more likely to drop
    large packets than small packets.  Our method for calculating the
    loss event rate is somewhat similar to the random sampling method
    proposed in [WBL04], except that randomization is not used; instead
    of randomization, the exact packet loss rate is computed for short
    loss intervals, and the standard loss event rate calculation is used
    for longer loss intervals.  [WBL04] includes simulations with a
    Bernoulli loss model, a Bernoulli loss model with a drop rate
    varying over time, and a Gilbert loss model, as well as more
    realistic simulations with a range of TCP and TFRC flows.

    [WBL04] produces both a byte-mode and a packet-mode variant of the
    TFRC transport protocol, for connections over paths with per-byte
    and per-packet dropping respectively.  We would argue that in the



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    absence of transport-level mechanisms for determining whether the
    packet dropping in the network is per-packet, per-byte, or somewhere
    in between, a single TFRC implementation is needed, independently of
    the packet-dropping behaviors of the routers along the path.  It
    would of course be preferable to have a mechanism that gives roughly
    acceptable behavior, to the connection and to the network as a
    whole, on paths with both per-byte and per-packet dropping (and on
    paths with multiple congested routers, some with per-byte dropping
    mechanisms, some with per-packet dropping mechanisms, and some with
    dropping mechanisms that lie somewhere between per-byte and per-
    packet).

    A first step will be to investigate the range of behaviors actually
    present in today's networks, in terms of packet-dropping as a
    function of packet size.  We will report on these investigations in
    a separate document.

B.  Appendix: A Discussion of Packet Size and Packet Dropping

    The table below gives a general summary of the relative advantages
    of packet-dropping behavior at routers independent of packet size,
    versus packet dropping behavior where large packets are more likely
    to be dropped than small ones.




























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    Advantages of Packet Dropping Independent of Packet Size:
    ---------------------------------------------------------
    1.  Adds another incentive for end nodes to use large packets.

    2.  Matches an environment with a limitation in pps rather than
        bps.
    ---------------------------------------------------------

    Advantages of Packet Dropping as a Function of Packet Size:
    ---------------------------------------------------------
    1.  Small control packets are less likely to be dropped than are
        large data packets, improving TCP performance.

    2.  Matches an environment with a limitation in bps rather than
        pps.

    3.  Reduces the penalty of TCP and other transport protocols
        against flows with small packets (where the allowed sending
        rate is roughly a linear function of packet size).

    4.  A queue limited in bytes is better than a queue limited in
        packets for matching the worst-case queue size to the
        worst-case queueing delay in seconds.
    ---------------------------------------------------------


Normative References

    [RFC 2119] S. Bradner. Key Words For Use in RFCs to Indicate
        Requirement Levels. RFC 2119.

    [RFC 2434] T. Narten and H. Alvestrand.  Guidelines for Writing an
        IANA Considerations Section in RFCs.  RFC 2434.

    [RFC 3448] M. Handley, S. Floyd, J. Padhye, and J. Widmer, TCP
        Friendly Rate Control (TFRC): Protocol Specification, RFC 3448,
        Proposed Standard, January 2003.

Informative References

    [CCID 3 PROFILE] S. Floyd, E. Kohler, and J. Padhye.  Profile for
        DCCP Congestion Control ID 3: TFRC Congestion Control.  draft-
        ietf-dccp-ccid3-11.txt, work in progress, March 2005.

    [DCCP] E. Kohler, M. Handley, and S. Floyd.  Datagram Congestion
        Control Protocol, draft-ietf-dccp-spec-13.txt, work in progress,
        December 2005.




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    [P04] T. Phelan, TFRC with Self-Limiting Sources, October 2004.  URL
        "http://www.phelan-4.com/dccp/".

    [RFC 3714] S. Floyd and J. Kempf, Editors.  IAB Concerns Regarding
        Congestion Control for Voice Traffic in the Internet.  RFC 3714.

    [V00] P. Vasallo.  Variable Packet Size Equation-Based Congestion
        Control.  ICSI Technical Report TR-00-008, April 2000.  URL
        "http://www.icsi.berkeley.edu/techreports/2000.abstracts/
        tr-00-008.html".

    [WBL04] J. Widmer, C. Boutremans, and Jean-Yves Le Boudec.
        Congestion Control for Flows with Variable Packet Size, ACM CCR,
        34(2), 2004.

Authors' Addresses

    Sally Floyd <floyd@icir.org>
    ICSI Center for Internet Research
    1947 Center Street, Suite 600
    Berkeley, CA 94704
    USA

    Eddie Kohler <kohler@cs.ucla.edu>
    4531C Boelter Hall
    UCLA Computer Science Department
    Los Angeles, CA 90095
    USA

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Floyd/Kohler                                                   [Page 34]


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