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RFC 4828 - TCP Friendly Rate Control (TFRC): The Small-Packet (S


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Network Working Group                                           S. Floyd
Request for Comments: 4828                                          ICIR
Category: Experimental                                         E. Kohler
                                                                    UCLA
                                                              April 2007

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

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document proposes a mechanism for further experimentation, but
   not for widespread deployment at this time in the global Internet.

   TCP-Friendly Rate Control (TFRC) is a congestion control mechanism
   for unicast flows operating in a best-effort Internet environment
   (RFC 3448).  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 size.  This
   document proposes TFRC-SP, a Small-Packet (SP) variant of TFRC, that
   is designed for applications that send small packets.  The design
   goal for TFRC-SP is to achieve the same bandwidth in bps (bits per
   second) as a TCP flow using packets of up to 1500 bytes.  TFRC-SP
   enforces a minimum 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.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions .....................................................5
   3. TFRC-SP Congestion Control ......................................5
   4. TFRC-SP Discussion ..............................................9
      4.1. Response Functions and Throughput Equations ................9
      4.2. Accounting for Header Size ................................12
      4.3. The TFRC-SP Min Interval ..................................13
      4.4. Counting Packet Losses ....................................14
      4.5. The Nominal Packet Size ...................................15
           4.5.1. Packet Size and Packet Drop Rates ..................15
           4.5.2. Fragmentation and the Path MTU .....................17
           4.5.3. The Nominal Segment Size and the Path MTU ..........17
      4.6. The Loss Interval Length for Short Loss Intervals .........18
   5. A Comparison with RFC 3714 .....................................19
   6. TFRC-SP with Applications that Modify the Packet Size ..........19
   7. Simulations ....................................................20
   8. General Discussion .............................................21
   9. Security Considerations ........................................22
   10. Conclusions ...................................................23
   11. Acknowledgements ..............................................24
   Appendix A. Related Work on Small-Packet Variants of TFRC .........25
   Appendix B. Simulation Results ....................................26
      B.1. Simulations with Configured Packet Drop Rates .............26
      B.2. Simulations with Configured Byte Drop Rates ...............30
      B.3. Packet Dropping Behavior at Routers with Drop-Tail
           Queues ....................................................32
      B.4. Packet Dropping Behavior at Routers with AQM ..............37
   Appendix C. Exploring Possible Oscillations in the Loss Event
      Rate ...........................................................42
   Appendix D. A Discussion of Packet Size and Packet Dropping .......43
   Normative References ..............................................44
   Informative References ............................................44

1.  Introduction

   This document specifies TFRC-SP, an experimental, Small-Packet
   variant of TCP-friendly Rate Control (TFRC) [RFC3448].

   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 [RFC3448], and uses this loss event rate to determine the sending
   rate in packets per round-trip time.  This has consequences for the
   rate that 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, with less than 1500 bytes per packet, 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 or minimum 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 or naturally-varying packet size, or
   could vary their packet size in response to congestion.  Applications
   that are not willing to be limited by a minimum interval of 10 ms
   between packets, or that want to send packets larger than 1500 bytes,
   should not use TFRC-SP.  However, for applications with a minimum
   interval of at least 10 ms between packets and with data packets of
   at most 1500 bytes, the performance of TFRC-SP should be at least as
   good as that from TFRC.

   RFC 3448, the protocol specification for TFRC, stated that TFRC-PS
   (for TFRC-PacketSize), a variant of TFRC for applications that have a
   fixed sending rate but vary their packet size in response to
   congestion, would be specified in a later document.  This document
   instead specifies TFRC-SP, a variant of TFRC designed for

   applications that send small packets, where applications could either
   have a fixed or varying packet size or could adapt their packet size
   in response to congestion.  However, as discussed in Section 6 of
   this document, there are many questions about how such an adaptive
   application would use TFRC-SP that are beyond the scope of this
   document, and that would need to be addressed in documents that are
   more application-specific.

   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 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 [RFC3714] 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.

   Initializing the Loss History after the First Loss Event: Section
   6.3.1 of RFC 3448 specifies that the TFRC receiver initializes the
   loss history after the first loss event by calculating the loss
   interval that would be required to produce the receive rate measured
   over the most recent round-trip time.  In calculating this loss
   interval, TFRC-SP uses the segment size of 1460 bytes, rather than
   the actual segment size used in the connection.

   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
   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.

2.  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 [RFC2119].

3.  TFRC-SP Congestion Control

   TFRC uses the TCP throughput equation given in Section 3.1 of
   [RFC3448], which gives the allowed sending rate X in bytes per second
   as a function of the loss event rate, segment size, and round-trip
   time.  [RFC3448] specifies that the segment size s used in the
   throughput equation should be the segment size used by the
   application, or the estimated mean segment size if there are
   variations in the segment size depending on the data.  This gives
   rough fairness with TCP flows using the same segment size.

   TFRC-SP changes this behavior in the following ways.

   o  The nominal segment size: The nominal segment size s defaults to
      1460 bytes.  Following [RFC3714], 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.  If the endpoint knows the MTU (Maximum
      Transmission Unit) of the path and the derived MSS (Maximum
      Segment Size) is less than 1460 bytes, then the endpoint SHOULD
      set the nominal segment size s to MSS bytes.  In addition, if the
      endpoint knows the MTU of the path and the resulting MSS is less
      than 536 bytes, then the endpoint MUST set the nominal segment
      size s to MSS bytes.

      However, this document does not require that TFRC-SP endpoints
      determine the path MTU.  While most paths allow an MSS of 1460
      bytes, some paths have a slightly smaller MSS due to tunnels
      (e.g., IPv6 over IPv4).  In some specific cases, IPv4 paths may
      experience a much smaller path MTU.  Due to the complications of
      estimating the path MTU, and to the fact that most paths support
      an MSS of at least 536 bytes, TFRC-SP as a default uses a nominal
      segment size of 1460 bytes.  The nominal segment size is discussed
      in more detail in Section 4.5.3.

   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 segment size for the
      connection in bytes, excluding the transport and network headers.
      Section 4.1 of RFC 3448 states that where the packet size varies
      naturally with the data, an estimate of the mean segment size can
      be used for s_true.  As suggested in Section 4.1 of [RFC3448bis],
      when an estimate of the mean segment size is used for s_true, the
      estimate SHOULD be calculated over at least the last four loss
      intervals.  However, this document does not specify a specific
      algorithm for estimating the mean segment size.

      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

      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 Datagram Congestion Control Protocol
      (DCCP) connection [RFC4340] 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.  However, if the TFRC implementation
      knows that the IP layer is using IPv6 instead of IPv4, then the
      connection using TFRC-SP MAY still use the default estimate of 40
      bytes for H instead of the actual size of 60 bytes, for simplicity
      of implementation.

   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.

      Section 5.4 of RFC 3448 specifies that the calculation of the
      average loss interval includes the most recent loss interval only
      if this increases the calculated average loss interval, as in the
      pseudocode below.  However, in TFRC-SP the calculated loss
      interval size for a short loss interval varies as a function of
      the number of packet losses that have been detected, allowing
      either increases or decreases in the calculated loss interval size
      for the current short loss interval as new packets are received.
      Therefore, TFRC-SP adds the restriction that the calculation of
      the average loss interval can include the most recent loss
      interval only if more than two round-trip times have passed since
      the beginning of that loss interval.

      Let the most recent loss intervals be I_0 to I_n, with I_0 being
      the interval including the most recent loss event, with the
      corresponding weights w_i as defined in RFC 3448.  In RFC 3448
      (Section 5.4), the average loss interval I_mean is calculated as
      follows:

                  I_tot0 = 0;
                  I_tot1 = 0;
                  W_tot = 0;
                  for (i = 0 to n-1) {
                    I_tot0 = I_tot0 + (I_i * w_i);
                    W_tot = W_tot + w_i;
                  }
                  for (i = 1 to n) {
                    I_tot1 = I_tot1 + (I_i * w_(i-1));
                  }
                  I_tot = max(I_tot0, I_tot1);
                  I_mean = I_tot/W_tot;

   In TFRC-SP, the average loss interval I_mean is instead calculated as
   follows:

                  I_tot0 = 0;
                  I_tot1 = 0;
                  W_tot = 0;
                  for (i = 0 to n-1) {
                    I_tot0 = I_tot0 + (I_i * w_i);
                    W_tot = W_tot + w_i;
                  }
                  for (i = 1 to n) {
                    I_tot1 = I_tot1 + (I_i * w_(i-1));
                  }
                  If the current loss interval I_0 is "short"
                    then I_tot = I_tot1;
                    else I_tot = max(I_tot0, I_tot1);
                  I_mean = I_tot/W_tot;

   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.  Response Functions and Throughput Equations

   TFRC uses the TCP throughput equation given in [RFC3448], with the
   sending rate X in bytes per second as follows:

                                   s
         X = ------------------------------------------------------- ,
             R*sqrt(2*p/3) + (4*R* (3*sqrt(3*p/8) * p * (1+32*p^2)))

   where:

      s is the packet size in bytes;

      R is the round-trip time in seconds;

      p is the loss event rate, between 0 and 1.0, of the number of loss
      events as a fraction of the number of packets transmitted.

   This equation uses a retransmission timeout (RTO) of 4*R, and assumes
   that the TCP connection sends an acknowledgement for every data
   packet.

   This equation essentially gives the response function for TCP as well
   as for standard TFRC (modulo TCP's range of sender algorithms for
   setting the RTO).  As shown in Table 1 of [RFC3714], 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 Explicit Congestion Notification (ECN).  Because it is
   not recommended for routers to use ECN-marking in highly-congested
   environments with high packet dropping/marking rates (Section 7 of
   [RFC3168]), 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.

   The effective response function of TFRC-SP can be derived from the
   TFRC response function by using a segment size s of 1460 bytes, and
   using the loss event rate actually experienced by the TFRC-SP flow.
   In addition, for loss intervals of at most two round-trip times, the
   loss event rate for TFRC-SP is estimated by counting the actual
   number of lost or marked packets, rather than by counting loss
   events.  In addition, the sending rate for TFRC-SP is constrained to
   be at most 100 packets per second.

   For an environment with a fixed packet drop rate p, regardless of
   packet size, the response functions of TCP, TFRC, and TFRC-SP are
   illustrated as follows, given in KBps (kilobytes per second), for a
   flow with a round-trip time of 100 ms:

                          <--  TCP and Standard TFRC  -->
               Packet     14-byte    536-byte   1460-byte
               DropRate   Segments   Segments   Segments
               --------   --------   --------   --------
               0.00001       209.25    2232.00    5812.49
               0.00003       120.79    1288.41    3355.24
               0.00010        66.12     705.25    1836.58
               0.00030        38.10     406.44    1058.45
               0.00100        20.74     221.23     576.12
               0.00300        11.76     125.49     326.79
               0.01000         6.07      64.75     168.61
               0.03000         2.99      31.90      83.07
               0.10000         0.96      10.21      26.58
               0.20000         0.29       3.09       8.06
               0.30000         0.11       1.12       2.93
               0.40000         0.05       0.48       1.26
               0.50000         0.02       0.24       0.63

              Table 1: Response Function for TCP and TFRC.
        Sending Rate in KBps, as a Function of Packet Drop Rate.

                          <----------  TFRC-SP  -------->
               Packet     14-byte    536-byte   1460-byte
               DropRate   Segments   Segments   Segments
               --------   --------   --------   --------
               0.00001       5.40      57.60     150.00
               0.00003       5.40      57.60     150.00
               0.00010       5.40      57.60     150.00
               0.00030       5.40      57.60     150.00
               0.00100       5.40      57.60     150.00
               0.00300       5.40      57.60     150.00
               0.01000       5.40      57.60     150.00
               0.03000       5.40      57.60      83.07
               0.10000       5.40      26.58      26.58
               0.20000       5.40       8.06       8.06
               0.30000       2.93       2.93       2.93
               0.40000       1.26       1.26       1.26
               0.50000       0.63       0.63       0.63

                Table 2: Response Function for TFRC-SP.
        Sending Rate in KBps, as a Function of Packet Drop Rate.
            Maximum Sending Rate of 100 Packets per Second.

   The calculations for Tables 1 and 2 use the packet loss rate for an
   approximation for the loss event rate p.  Scripts and graphs for the
   tables are available from [VOIPSIMS].  As the well-known TCP response
   function in Table 1 shows, the sending rate for TCP and standard TFRC
   varies linearly with segment size.  The TFRC-SP response function
   shown in Table 2 reflects the maximum sending rate of a hundred
   packets per second; when not limited by this maximum sending rate,
   the TFRC-SP flow receives the same sending rate in KBps as the TCP
   flow with 1460-byte segments, given the same packet drop rate.
   Simulations showing the TCP, standard TFRC, and TFRC-SP sending rates
   in response to a configured packet drop rate are given in Tables 7,
   8, and 9, and are consistent with the response functions shown here.

                            <--  TCP and Standard TFRC  -->
               Byte         14-byte    536-byte   1460-byte
               DropRate     Segments   Segments   Segments
               --------     --------   --------   --------
               0.0000001     284.76     929.61    1498.95
               0.0000003     164.39     536.17     863.16
               0.0000010      90.01     292.64     468.49
               0.0000030      51.92     167.28     263.68
               0.0000100      28.34      88.56     132.75
               0.0000300      16.21      46.67      61.70
               0.0001000       8.60      19.20      16.25
               0.0003000       4.56       4.95       1.70
               0.0010000       1.90       0.37       0.15
               0.0030000       0.52       0.05       0.06
               0.0100000       0.04       0.02       0.06
               0.0300000       0.00       0.02       0.06

               Table 3: Response Function for TCP and TFRC.
         Sending Rate in KBps, as a Function of Byte Drop Rate.

                            <----------  TFRC-SP  -------->
               Byte         14-byte    536-byte   1460-byte
               DropRate     Segments   Segments   Segments
               --------     --------   --------   --------
               0.0000001       5.40      57.60     150.00
               0.0000003       5.40      57.60     150.00
               0.0000010       5.40      57.60     150.00
               0.0000030       5.40      57.60     150.00
               0.0000100       5.40      57.60     132.75
               0.0000300       5.40      57.60      61.70
               0.0001000       5.40      50.00      16.25
               0.0003000       5.40      12.89       1.70
               0.0010000       5.40       0.95       0.15
               0.0030000       5.40       0.12       0.06
               0.0100000       1.10       0.06       0.06
               0.0300000       0.13       0.06       0.06

                 Table 4: Response Function for TFRC-SP.
         Sending Rate in KBps, as a Function of Byte Drop Rate.
            Maximum Sending Rate of 100 Packets per Second.

   For Tables 3 and 4, the packet drop rate is calculated as 1-(1-b)^N,
   for a byte drop rate of b, and a packet size of N bytes.  These
   tables use the packet loss rate as an approximation for the loss
   event rate p.  The TCP response functions shown in Table 3 for fixed
   byte drop rates are rather different from the response functions
   shown in Table 1 for fixed packet drop rates; with higher byte drop
   rates, a TCP connection can have a higher sending rate using
   *smaller* packets.  Table 4 also shows that with fixed byte drop
   rates, the sending rate for TFRC-SP can be significantly higher than
   that for TCP or standard TFRC, regardless of the TCP segment size.
   This is because in this environment, with small packets, TFRC-SP
   receives a small packet drop rate, but is allowed to send at the
   sending rate of a TCP or standard TFRC flow using larger packets but
   receiving the same packet drop rate.

   Simulations showing TCP, standard TFRC, and TFRC-SP sending rates in
   response to a configured byte drop rate are given in Appendix B.2.

4.2.  Accounting for Header Size

   [RFC3714] 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) 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 an excessive amount of 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 is sufficient for both IPv4 and IPv6.

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
   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 amount of small packets used.

   One practical justification for the Min Interval is 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 an excessive
   amount of 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 is sending at most 100 pps,
   it would be able to use TFRC-SP.  If the flow wishes to increase its
   sending rate to more than 100 pps, but 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, 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.

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 of as N.  The average loss interval I_mean is still
   averaged over the eight most recent 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

4.5.1.  Packet Size and Packet Drop Rates

   The guidelines in Section 3 above say that the nominal segment 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 segments will
   have a higher sending rate than a TCP flow with smaller segments.
   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
   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 these
   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 5 below shows a separate simulation with ten TCP
   connections 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 5: TCP Median Send Rate vs. Packet Size I:
                            Byte Drop Rate 0.0001

   Table 6 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 that, 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
   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 6: 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.

4.5.2.  Fragmentation and the Path MTU

   This document doesn't specify TFRC-SP behavior in terms of packet
   fragmentation and Path MTU Discovery (PMTUD).  That is, should the
   transport protocol using TFRC-SP use PMTUD information to set an
   upper bound on the segment size?  Should the transport protocol allow
   packets to be fragmented in the network?  We leave these as questions
   for the transport protocol.  As an example, we note that DCCP
   requires that endpoints keep track of the current PMTU, and says that
   fragmentation should not be the default (Section 14 of [RFC4340]).

4.5.3.  The Nominal Segment Size and the Path MTU

   When TFRC-SP is used with a nominal segment size s of 1460 bytes on a
   path where the TCP MSS is in fact only 536 bytes, the TFRC-SP flow
   could receive almost three times the bandwidth, in bytes per second,
   as that of a TCP flow using an MSS of 536 bytes.  Similarly, in an
   environment with an MSS close to 4000 bytes, a TCP flow could receive
   almost three times the bandwidth of a TFRC-SP flow with its nominal
   segment size s of 1460 bytes.  In both cases, we feel that these
   levels of "unfairness" with factors of two or three are acceptable;
   in particular, they won't result in one flow grabbing all of the

   available bandwidth, to the exclusion of the competing TCP or TFRC-SP
   flow.

   All IPv4 *end hosts* are required to accept and reassemble IP packets
   of size 576 bytes [RFC791], but IPv4 *links* do not necessarily have
   to support this packet size.  In slow networks, the largest possible
   packet may take a considerable amount of time to send [RFC3819], and
   a smaller MTU may be desirable, e.g., hundreds of bytes.  If the
   first-hop link had a small MTU, then TCP would choose an
   appropriately small MSS [RFC879].  [RFC1144] quotes cases of very low
   link speeds where the MSS may be tens of bytes (and notes this is an
   extreme case).  We note that if TFRC-SP is used over a path with an
   MTU considerably smaller than 576 bytes, and the TFRC-SP flow uses a
   nominal segment size s of 1460 bytes, then the TFRC-SP flow could
   receive considerably more than three times the bandwidth of competing
   TCP flows.

   If TFRC-SP is used with a nominal segment size s of less than 536
   bytes (because the path MTU is known to be less than 576 bytes), then
   TFRC-SP is likely to be of minimal benefit to applications.  If
   TFRC-SP was to be used on paths that have a path MTU of considerably
   less than 576 bytes, and the transport protocol was not required to
   keep track of the path MTU, this could result in the TFRC-SP flow
   using the default nominal segment size of 1460 bytes, and as a result
   receiving considerably more bandwidth than competing TCP flows.  As a
   result, TFRC-SP is not recommended for use with transport protocols
   that don't maintain some knowledge of the path MTU.

4.6.  The Loss Interval Length for Short Loss Intervals

   For a TFRC-SP receiver, the guidelines from Section 6 of RFC 3448
   govern when the receiver should send feedback messages.  In
   particular, from [RFC3448], "a feedback packet should ... be sent
   whenever a new loss event is detected without waiting for the end of
   an RTT".  In addition, feedback packets are sent at least once per
   RTT.

   For a TFRC-SP connection with a short current loss interval (less
   than two round-trip times), it is possible for the loss interval
   length calculated for that loss interval to change in odd ways as
   additional packet losses in that loss interval are detected.  To
   prevent unnecessary oscillations in the average loss interval,
   Section 3 specifies that the current loss interval can be included in
   the calculation of the average loss interval only if the current loss
   interval is longer than two round-trip times.

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 1500-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 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.

   Because RFC 3714 assumes that the allowed packet drop rate for an IP
   telephony flow is determined by the sending rate that a TCP flow
   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
   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.

   Because the allowed sending rate in TFRC-SP is in bytes per second
   rather than in packets per second, there is little opportunity for
   applications to manipulate the packet size in order to "game" the
   system.  This differs from TFRC in CCID 3 (Section 5.3 of [RFC4342]),
   where the allowed sending rate is in packets per second.  In
   particular, a TFRC-SP application that sends small packets for a
   while, building up a fast sending rate, and then switches to large
   packets, would not increase its overall sending rate by the change of
   packet size.

7.  Simulations

   This section describes the performance of TFRC-SP 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, described in detail in Appendix B, 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
      (Appendix B.1, B.2).

   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 (Appendix B.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, and in some cases large-
      packet TCP flows can be essentially starved by competing small-
      packet TFRC-SP flows (Appendix B.2, B.3, B.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 (Appendix B.3, B.4).  It has also
      been reported that wireless links are sometimes good enough to let
      small packets through, while larger packets have a much higher
      error rate, and hence a higher drop probability [S05].

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 segments sending 2.2 packets per round-trip time.  From Table 1
   of RFC 3714, this sending rate can be sustained with a 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 the regime of high packet drop rates, TCP performance is also
   affected by the TCP algorithm (e.g., SACK or not), the minimum RTO,
   the use of Limited Transmit (or lack thereof), the use of ECN, and
   the like.  It is good to ensure that simulations or experiments
   exploring fairness include the exploration of fairness with the most
   aggressive TCP mechanisms conformant with the current standards.  Our
   simulations use SACK TCP with Limited Transmit and with a minimum RTO
   of 200 ms.  The simulation results are largely the same with or
   without timestamps; timestamps were not used for simulations reported
   in this paper.  We didn't use TCP with ECN in setting the target
   sending rate for TFRC, because, as explained in Appendix B.1, our
   expectation is that in high packet drop regimes, routers will drop
   rather than mark packets, either from policy or from buffer overflow.

   Fairness with different packet header sizes: In an environment with
   IPv6 and/or several layers of network-layer tunnels (e.g., IPsec,
   Generic Routing Encapsulation (GRE)), the packet header could be 60,
   80, or 100 bytes instead of the default 40 bytes assumed in Section
   3.  For an application with small ten-byte data segments, this means
   that the actual packet size could be 70, 90, or 110 bytes, instead of
   the 50 bytes assumed by TFRC-SP in calculating the allowed sending
   rate.  Thus, a TFRC-SP application with large headers could receive
   more than twice the bandwidth (including the bandwidth used by
   headers) than a TFRC-SP application with small headers.  We do not
   expect this to be a problem; in particular, TFRC-SP applications with
   large headers will not significantly degrade the performance of
   competing TCP applications, or of competing TFRC-SP applications with
   small headers.

   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 new security considerations introduced in this document.

   The issues of the fairness of TFRC-SP with standard TFRC and TCP in a
   range of environments, including those with byte-based queue
   management at the congested routers, are discussed extensively in the
   main body of this document.

   General security considerations for TFRC are discussed in RFC 3448.
   As with TFRC in RFC 3448, TFRC-SP is not a transport protocol in its
   own right, but a congestion control mechanism that is intended to be
   used in conjunction with a transport protocol.  Therefore, security
   primarily needs to be considered in the context of a specific
   transport protocol and its authentication mechanisms.  As discussed
   for TFRC in RFC 3448, any transport protocol that uses TFRC-SP needs
   to protect against spoofed feedback, and to protect the congestion
   control mechanisms against incorrect information from the receiver.
   Again as discussed for TFRC in RFC 3448, we expect that protocols
   incorporating ECN with TFRC-SP will want to use the ECN nonce
   [RFC3540] to protect the sender from the accidental or malicious
   concealment of marked packets

   Security considerations for DCCP's Congestion Control ID 3, TFRC
   Congestion Control, the transport protocol that uses TFRC, are
   discussed in [RFC4342].  That document extensively discussed the
   mechanisms the sender can use to verify the information sent by the
   receiver, including the use of the ECN nonce.

10.  Conclusions

   This document has specified TFRC-SP, a Small-Packet (SP) variant of
   TFRC, designed for applications that send small packets, with at most
   a hundred packets per second, but that desire the same sending rate
   in bps as a TCP connection experiencing the same packet drop rate but
   sending packets of 1500 bytes.  TFRC-SP competes reasonably well with
   large-packet TCP and TFRC flows in environments where large-packet
   flows and small-packet flows experience similar packet drop rates,
   but receives more than its share of the bandwidth in bps in
   environments where small packets are less likely to be dropped or
   marked than are large packets.  As a result, TFRC-SP is experimental,
   and is not intended for widespread deployment at this time in the
   global Internet.

   In order to allow experimentation with TFRC-SP in the Datagram
   Congestion Control Protocol (DCCP), an experimental Congestion
   Control IDentifier (CCID) will be used, based on TFRC-SP but
   specified in a separate document.

11.  Acknowledgements

   We thank Tom Phelan for discussions of TFRC-SP and for his paper
   exploring the fairness between TCP and TFRC-SP flows.  We thank Lars
   Eggert, Gorry Fairhurst, Mark Handley, Miguel Garcia, Ladan Gharai,
   Richard Nelson, Colin Perkins, Juergen Quittek, Pete Sholander,
   Magnus Westerlund, and Joerg Widmer for feedback on earlier versions
   of this document.  We also thank the DCCP Working Group for feedback
   and discussions.

Appendix A.  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 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).

   An important contribution would be to investigate the range of
   behaviors actually present in today's networks, in terms of packet
   dropping as a function of packet size.

Appendix B.  Simulation Results

   This appendix reports on the simulation results outlined in
   Section 7.  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 and graphs for the
   simulations in this document are available at [VOIPSIMS].

B.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 7: Send Rate vs. Packet Drop Rate I:
                              1460B TFRC Segments
                  (1.184 Kbps Maximum TFRC Data Sending Rate)

   Table 7 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 segments, 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
   at these high packet drop rates, if ECN-marking was still being used
   instead of packet dropping.  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
   mark packets during high levels of congestion (Section 7 of
   [RFC3168]).  In addition, there is likely to be buffer overflow in
   scenarios with such high packet dropping/marking rates, forcing
   routers to drop packets instead of ECN-marking them.

                    < - - - - - - 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 8: Send Rate vs. Packet Drop Rate II:
                               14B TFRC Segments
                   (5.6 Kbps Maximum TFRC Data Sending Rate)

   Table 8 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 8 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 8 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,
   the sending rate for the TFRC-SP flow is relatively close to the
   desired goal of fairness in bps with TCP.

   Table 8 shows that with TFRC-SP, the 5.6 Kbps data stream doesn't
   reduce its sending rate until packet drop rates are 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 9: Sending Rate vs. Packet Drop Rate III:
                             200B TFRC Segments
                 (160 Kbps Maximum TFRC Data Sending Rate)

   Table 9 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 8, 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.

B.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 10: Sending Rate vs. Byte Drop Rate

   Table 10 shows the TCP and TFRC send rates for various byte drop
   rates.  For each byte drop rate, Table 10 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 10 also
   shows the TFRC-SP and 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.

                        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 11: Packet Drop Rate Ratio vs. Byte Drop Rate

   Table 11 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 11 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 10 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 10 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 segments, 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 10, 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
   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 10, 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 8 and 9 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 B.3 and B.4 show simulations with
   packets dropped at Drop-Tail or AQM queues, rather that from a
   probabilistic process.

B.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?

                     < - - - - - 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 12: Drop and Send Rates for Drop-Tail Queues in Packets

   Table 12 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
   round-trip 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 12 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.

                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10      0.061     239.81      0.004     185.19
                25      0.089     189.02      0.006     184.95
                50      0.141      99.46      0.013     185.07
               100      0.196      16.42      0.022     183.77
               200      0.256       4.46      0.032     181.98
               400      0.291       4.61      0.051     151.88
               800      0.487       1.01      0.078     113.10
              1600      0.648       0.67      0.121      65.17

     Table 13: 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.  Thus, 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.

   The scenarios in Table 13 are identical to those in Table 12, except
   that the Drop-Tail queue is in units of bytes instead of packets.
   Thus, five TCP connections and five TFRC-SP connections compete with
   web traffic in a topology with a 3 Mbps shared link, with each TFRC-
   SP application generating 200-byte data packets every 10 ms, for a
   maximum data rate of 160 Kbps.  The number of web sessions is varied
   to create different levels of congestion.  However, instead of Drop-
   Tail queues able to accommodate at most a hundred packets, the
   routers' Drop-Tail queues are each able to accommodate at most 50,000
   bytes (computed in NS as a hundred packets times the mean packet size
   of 500 bytes).

   As Table 13 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 13, 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.

                       < - - - - - 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 14: Drop and Send Rates for Drop-Tail Queues in Bytes II:
                               512B TCP Segments

   Table 14 shows that, in these scenarios, 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.

   To show the potential negative effect of TFRC-SP in such an
   environment, we consider a simulation with N TCP flows, N TFRC-SP
   flows, and 10*N web sessions, for N ranging from 1 to 50, so that the
   demand increases from all types of traffic, with routers with Drop-
   Tail queues in bytes.

                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)      TFRC-SP (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10      0.014    2085.36      0.001     180.29
                20      0.040     788.88      0.004     183.87
                30      0.074     248.80      0.006     182.94
                40      0.113     163.20      0.008     185.33
                50      0.127      94.70      0.011     185.14
                60      0.177      53.24      0.015     185.30
                70      0.174      35.31      0.016     185.07
                80      0.221      19.38      0.019     183.91
                90      0.188      15.63      0.019     180.42
               100      0.265       7.08      0.023     176.71
               200      0.324       0.38      0.042     139.48
               300      0.397       0.32      0.076      93.53
               400      0.529       0.40      0.100      70.40
               500      0.610       0.37      0.121      57.59

     Table 15: Drop and Send Rates for Drop-Tail Queues in Bytes III:
                      TFRC-SP, 1460B TCP Segments

                       < - - - - - Send Rates in Kbps - - - - >
              Web       TCP (1460B seg)       TFRC (200B seg)
            Sessions   DropRate  SendRate    DropRate  SendRate
            --------   --------  --------    --------  --------
                10      0.016    1926.00      0.002     178.47
                20      0.030     805.20      0.003     178.23
                30      0.062     346.24      0.005     161.19
                40      0.093     219.18      0.007     136.28
                50      0.110     132.77      0.010      83.02
                60      0.170      88.88      0.014      69.75
                70      0.149      70.73      0.015      55.59
                80      0.213      43.17      0.020      42.82
                90      0.188      36.19      0.017      43.61
               100      0.233      24.77      0.026      35.17
               200      0.311       5.46      0.034      24.85
               300      0.367       3.74      0.049      20.19
               400      0.421       2.59      0.055      17.71
               500      0.459       1.10      0.069      15.95
     Table 16: Drop and Send Rates for Drop-Tail Queues in Bytes IV:
                   Standard TFRC, 1460B TCP Segments

   Table 15 shows simulations using TFRC-SP, while Table 16 shows
   simulations using TFRC instead of TFRC-SP.  This is the only
   difference between the simulations in the two tables.  Note that when
   TFRC-SP is used, the TCP flows and web traffic are essentially

   starved, while the TFRC-SP flows each continue to send 57 Kbps.  In
   contrast, when standard TFRC is used instead of TFRC-SP for the flows
   sending 200-byte segments, the TCP flows are not starved (although
   they still don't receive their "share" of the link bandwidth when
   their packet drop rates are 30% or higher).  These two sets of
   simulations illustrate the dangers of a widespread deployment of
   TFRC-SP in an environment where small packets are less likely to be
   dropped than large ones.

B.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 in units of either
   packets or 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.  [EA03] assessed the impact of byte vs. packet modes on
   RED performance.

   The simulations reported below 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 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 these simulations, as in
   Section B.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 17: Drop and Send Rates for RED Queues in Packet Mode

   For the simulations in Table 17, 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 12 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.

                       < - - - - - 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 18: Drop and Send Rates for RED Queues in Byte Mode

   Table 18 shows that with a standard RED queue in byte mode instead of
   packet mode, there is a somewhat greater difference between the
   packet drop rates of the TCP and TFRC-SP flows, particularly for
   lower packet drop rates.  For the simulation in Table 18, 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 18 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 19: Drop and Send Rates for RED Queues in Byte Mode

   Table 19 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 rates
   for TFRC-SP in the scenarios in Table 19 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 20: Drop and Send Rates with Adaptive RED Queues in Byte Mode

   For the simulations in Table 20, 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, and each *byte* is dropped with the same probability.  Also,
   because of the use of Adaptive RED, this byte-dropping mode extends
   even for the high-packet-drop-rate regime.

   As Table 20 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.

   The simulation with 1600 web servers is of particular concern,
   because the TCP packet drop rate increases, while the TFRC-SP packet
   drop rate decreases significantly.  This is due to a detail of the
   Adaptive RED implementation in the NS simulator, and not about the
   dynamics of TFRC-SP.  In particular, Adaptive RED is configured not
   to "adapt" to packet drop rates over 50%.  When the packet drop rate
   is at most 50%, Adaptive RED is moderately successful in keeping the
   packet drop rate proportional to the packet size.  TCP packets are
   six times larger than the TFRC-SP packets (including headers), so the
   TCP packets should see a packet drop rate roughly six times larger.

   But for packet drop rates over 50%, Adaptive RED is no longer in this
   regime, so it is no longer successful in keeping the drop rate for
   TCP packets at most six times the drop rate of the TFRC-SP packets.

   We note that the unfairness in these simulations, in favor of TFRC-
   SP, is even more severe than the unfairness shown in Table 13 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 of any of the
   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 21: Drop and Send Rates for Adaptive RED Queues in Byte Mode

   Table 21 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 than the TCP
   flows, but the level of unfairness is less severe, and the packet
   drop rates of the TCP flows are 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 than that in Table 20 above.

Appendix C.  Exploring Possible Oscillations in the Loss Event Rate

   TFRC-SP estimates the loss interval size differently for short and
   long loss intervals, counting only one loss event for long loss
   intervals, but counting all packet losses as loss events for the
   short loss intervals.  One question that has been raised is whether
   this can lead to oscillations in the average loss event rate in
   environments where there are many packet drops in a single loss
   event, and loss events switch from short to long and vice versa.  As
   an example, consider a loss interval with N packets, including N/4
   losses.  If this loss interval is short (at most two round-trip
   times), the loss interval length is measured as 4 packets.  If this
   loss interval is long, then the loss interval length is measured as N
   packets.

   If the loss interval switching from short to long and back leads to
   severe oscillations in the average packet drop rate, and therefore in
   the allowed sending rate, one solution would be to have a more
   gradual transition between the calculation of the loss interval
   length for short and long loss intervals.  For example, one
   possibility would be to use all of the packet losses for a loss
   interval of one round-trip time in calculating the loss interval
   length, to use 2/3 of the packet losses from a loss interval of two
   round-trip times, to use 1/3 of the packet losses from a loss
   interval of three round-trip time, and to use only one packet loss
   from a loss interval of four or more round-trip times.  This more
   gradual mechanism would give a transition to counting all losses for
   a loss interval of only one round-trip time, and counting only one
   loss event for a loss interval of four or more round-trip times.

   However, our simulations so far have not shown a great difference in
   oscillations in the estimate loss event rate between the default
   mechanism for estimating the loss interval length for short loss
   interval and the gradual mechanism described above.  Simulation
   scripts are available from [VOIPSIMS].  Therefore, for now we are
   staying with the simple default mechanism for estimating the loss
   event rate for short loss intervals described in this document.

Appendix D.  A Discussion of Packet Size and Packet Dropping

   The lists below give 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.

   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

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3448]      Handley, M., Floyd, S., Padhye, J., and J. Widmer,
                  "TCP Friendly Rate Control (TFRC): Protocol
                  Specification", RFC 3448, January 2003.

Informative References

   [EA03]         W. Eddy and M. Allman.  A Comparison of RED's Byte and
                  Packet Modes, Computer Networks, 42(2), June 2003.

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

   [RFC791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                  September 1981.

   [RFC879]       Postel, J., "TCP maximum segment size and related
                  topics", RFC 879, November 1983.

   [RFC1144]      Jacobson, V., "Compressing TCP/IP headers for low-
                  speed serial links", RFC 1144, February 1990.

   [RFC3168]      Ramakrishnan, K., Floyd, S., and D. Black, "The
                  Addition of Explicit Congestion Notification (ECN) to
                  IP", RFC 3168, September 2001.

   [RFC3540]      Spring, N., Wetherall, D., and D. Ely, "Robust
                  Explicit Congestion Notification (ECN) Signaling with
                  Nonces", RFC 3540, June 2003.

   [RFC3714]      Floyd, S. and J. Kempf, "IAB Concerns Regarding
                  Congestion Control for Voice Traffic in the Internet",
                  RFC 3714, March 2004.

   [RFC3819]      Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
                  Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
                  and L. Wood, "Advice for Internet Subnetwork
                  Designers", BCP 89, RFC 3819, July 2004.

   [RFC4340]      Kohler, E., Handley, M., and S. Floyd, "Datagram
                  Congestion Control Protocol (DCCP)", RFC 4340, March
                  2006.

   [RFC4342]      Floyd, S., Kohler, E., and J. Padhye, "Profile for
                  Datagram Congestion Control Protocol (DCCP) Congestion
                  Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
                  4342, March 2006.

   [RFC3448bis]   Handley, M., Floyd, S., Padhye, J., and J. Widmer,
                  "TCP Friendly Rate Control (TFRC): Protocol
                  Specification", Work in Progress, March 2007.

   [S05]          Peter Sholander, private communications, August 2005.
                  Citation for acknowledgement purposes only.

   [V00]          P. Vasallo.  Variable Packet Size Equation-Based
                  Congestion Control.  ICSI Technical Report TR-00-008,
                  April 2000, <http://www.icsi.berkeley.edu/cgi-bin/
                  pubs/publication.pl?ID=001183>

   [VOIPSIMS]     Web page <http://www.icir.org/tfrc/voipsims.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
   ICSI Center for Internet Research
   1947 Center Street, Suite 600
   Berkeley, CA 94704
   USA

   EMail: floyd@icir.org

   Eddie Kohler
   4531C Boelter Hall
   UCLA Computer Science Department
   Los Angeles, CA 90095
   USA

   EMail: kohler@cs.ucla.edu

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