





Network Working Group                                           S. Floyd
Request for Comments: 3649                                          ICSI
Category: Experimental                                     December 2003


               HighSpeed TCP for Large Congestion Windows

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 Internet Society (2003).  All Rights Reserved.

Abstract

   The proposals in this document are experimental.  While they may be
   deployed in the current Internet, they do not represent a consensus
   that this is the best method for high-speed congestion control.  In
   particular, we note that alternative experimental proposals are
   likely to be forthcoming, and it is not well understood how the
   proposals in this document will interact with such alternative
   proposals.

   This document proposes HighSpeed TCP, a modification to TCP's
   congestion control mechanism for use with TCP connections with large
   congestion windows.  The congestion control mechanisms of the current
   Standard TCP constrains the congestion windows that can be achieved
   by TCP in realistic environments.  For example, for a Standard TCP
   connection with 1500-byte packets and a 100 ms round-trip time,
   achieving a steady-state throughput of 10 Gbps would require an
   average congestion window of 83,333 segments, and a packet drop rate
   of at most one congestion event every 5,000,000,000 packets (or
   equivalently, at most one congestion event every 1 2/3 hours).  This
   is widely acknowledged as an unrealistic constraint.  To address this
   limitation of TCP, this document proposes HighSpeed TCP, and solicits
   experimentation and feedback from the wider community.










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Table of Contents

   1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2. The Problem Description.. . . . . . . . . . . . . . . . . . . .  3
   3. Design Guidelines.. . . . . . . . . . . . . . . . . . . . . . .  4
   4. Non-Goals.. . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5. Modifying the TCP Response Function.. . . . . . . . . . . . . .  6
   6. Fairness Implications of the HighSpeed Response
      Function. . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7. Translating the HighSpeed Response Function into
      Congestion Control Parameters . . . . . . . . . . . . . . . . . 12
   8. An alternate, linear response functions.. . . . . . . . . . . . 13
   9. Tradeoffs for Choosing Congestion Control Parameters. . . . . . 16
      9.1. The Number of Round-Trip Times between Loss Events . . . . 17
      9.2. The Number of Packet Drops per Loss Event, with Drop-Tail. 17
   10. Related Issues . . . . . . . . . . . . . . . . . . . . . . . . 18
      10.1. Slow-Start. . . . . . . . . . . . . . . . . . . . . . . . 18
      10.2. Limiting burstiness on short time scales. . . . . . . . . 19
      10.3. Other limitations on window size. . . . . . . . . . . . . 19
      10.4. Implementation issues.. . . . . . . . . . . . . . . . . . 19
   11. Deployment issues. . . . . . . . . . . . . . . . . . . . . . . 20
      11.1. Deployment issues of HighSpeed TCP. . . . . . . . . . . . 20
      11.2. Deployment issues of Scalable TCP . . . . . . . . . . . . 22
   12. Related Work in HighSpeed TCP. . . . . . . . . . . . . . . . . 23
   13. Relationship to other Work.. . . . . . . . . . . . . . . . . . 25
   14. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . 25
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   16. Normative References . . . . . . . . . . . . . . . . . . . . . 26
   17. Informative References . . . . . . . . . . . . . . . . . . . . 26
   18. Security Considerations. . . . . . . . . . . . . . . . . . . . 28
   19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 28
   A.  TCP's Loss Event Rate in Steady-State. . . . . . . . . . . . . 29
   B.  A table for a(w) and b(w). . . . . . . . . . . . . . . . . . . 30
   C.  Exploring the time to converge to fairness . . . . . . . . . . 32
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 33
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 34

1.  Introduction

   This document proposes HighSpeed TCP, a modification to TCP's
   congestion control mechanism for use with TCP connections with large
   congestion windows.  In a steady-state environment, with a packet
   loss rate p, the current Standard TCP's average congestion window is
   roughly 1.2/sqrt(p) segments.  This places a serious constraint on
   the congestion windows that can be achieved by TCP in realistic
   environments.  For example, for a Standard TCP connection with 1500-
   byte packets and a 100 ms round-trip time, achieving a steady-state
   throughput of 10 Gbps would require an average congestion window of



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   83,333 segments, and a packet drop rate of at most one congestion
   event every 5,000,000,000 packets (or equivalently, at most one
   congestion event every 1 2/3 hours).  The average packet drop rate of
   at most 2*10^(-10) needed for full link utilization in this
   environment corresponds to a bit error rate of at most 2*10^(-14),
   and this is an unrealistic requirement for current networks.

   To address this fundamental limitation of TCP and of the TCP response
   function (the function mapping the steady-state packet drop rate to
   TCP's average sending rate in packets per round-trip time), this
   document describes a modified TCP response function for regimes with
   higher congestion windows.  This document also solicits
   experimentation and feedback on HighSpeed TCP from the wider
   community.

   Because HighSpeed TCP's modified response function would only take
   effect with higher congestion windows, HighSpeed TCP does not modify
   TCP behavior in environments with heavy congestion, and therefore
   does not introduce any new dangers of congestion collapse.  However,
   if relative fairness between HighSpeed TCP connections is to be
   preserved, then in our view any modification to the TCP response
   function should be addressed in the IETF, rather than made as ad hoc
   decisions by individual implementors or TCP senders.  Modifications
   to the TCP response function would also have implications for
   transport protocols that use TFRC and other forms of equation-based
   congestion control, as these congestion control mechanisms directly
   use the TCP response function [RFC3448].

   This proposal for HighSpeed TCP focuses specifically on a proposed
   change to the TCP response function, and its implications for TCP.
   This document does not address what we view as a separate fundamental
   issue, of the mechanisms required to enable best-effort connections
   to *start* with large initial windows.  In our view, while HighSpeed
   TCP proposes a somewhat fundamental change to the TCP response
   function, at the same time it is a relatively simple change to
   implement in a single TCP sender, and presents no dangers in terms of
   congestion collapse.  In contrast, in our view, the problem of
   enabling connections to *start* with large initial windows is
   inherently more risky and structurally more difficult, requiring some
   form of explicit feedback from all of the routers along the path.
   This is another reason why we would propose addressing the problem of
   starting with large initial windows separately, and on a separate
   timetable, from the problem of modifying the TCP response function.








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2.  The Problem Description

   This section describes the number of round-trip times between
   congestion events required for a Standard TCP flow to achieve an
   average throughput of B bps, given packets of D bytes and a round-
   trip time of R seconds.  A congestion event refers to a window of
   data with one or more dropped or ECN-marked packets (where ECN stands
   for Explicit Congestion Notification).

   From Appendix A, achieving an average TCP throughput of B bps
   requires a loss event at most every BR/(12D) round-trip times.  This
   is illustrated in Table 1, for R = 0.1 seconds and D = 1500 bytes.
   The table also gives the average congestion window W of BR/(8D), and
   the steady-state packet drop rate P of 1.5/W^2.

    TCP Throughput (Mbps)   RTTs Between Losses     W       P
    ---------------------   -------------------   ----    -----
              1                    5.5             8.3    0.02
             10                   55.5            83.3    0.0002
            100                  555.5           833.3    0.000002
           1000                 5555.5          8333.3    0.00000002
          10000                55555.5         83333.3    0.0000000002

   Table 1: RTTs Between Congestion Events for Standard TCP, for
   1500-Byte Packets and a Round-Trip Time of 0.1 Seconds.

   This document proposes HighSpeed TCP, a minimal modification to TCP's
   increase and decrease parameters, for TCP connections with larger
   congestion windows, to allow TCP to achieve high throughput with more
   realistic requirements for the steady-state packet drop rate.
   Equivalently, HighSpeed TCP has more realistic requirements for the
   number of round-trip times between loss events.

3.  Design Guidelines

   Our proposal for HighSpeed TCP is motivated by the following
   requirements:

   *  Achieve high per-connection throughput without requiring
      unrealistically low packet loss rates.

   *  Reach high throughput reasonably quickly when in slow-start.

   *  Reach high throughput without overly long delays when recovering
      from multiple retransmit timeouts, or when ramping-up from a
      period with small congestion windows.





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   *  No additional feedback or support required from routers:

   For example, the goal is for acceptable performance in both ECN-
   capable and non-ECN-capable environments, and with Drop-Tail as well
   as with Active Queue Management such as RED in the routers.

   *  No additional feedback required from TCP receivers.

   *  TCP-compatible performance in environments with moderate or high
      congestion (e.g., packet drop rates of 1% or higher):

   Equivalently, the requirement is that there be no additional load on
   the network (in terms of increased packet drop rates) in environments
   with moderate or high congestion.

   *  Performance at least as good as Standard TCP in environments with
      moderate or high congestion.

   *  Acceptable transient performance, in terms of increases in the
      congestion window in one round-trip time, responses to severe
      congestion, and convergence times to fairness.

   Currently, users wishing to achieve throughputs of 1 Gbps or more
   typically open up multiple TCP connections in parallel, or use MulTCP
   [CO98,GRK99], which behaves roughly like the aggregate of N virtual
   TCP connections.  While this approach suffices for the occasional
   user on well-provisioned links, it leaves the parameter N to be
   determined by the user, and results in more aggressive performance
   and higher steady-state packet drop rates if used in environments
   with periods of moderate or high congestion.  We believe that a new
   approach is needed that offers more flexibility, more effectively
   scales to a wide range of available bandwidths, and competes more
   fairly with Standard TCP in congested environments.

4.  Non-Goals

   The following are explicitly *not* goals of our work:

   *  Non-goal: TCP-compatible performance in environments with very low
      packet drop rates.

   We note that our proposal does not require, or deliver, TCP-
   compatible performance in environments with very low packet drop
   rates, e.g., with packet loss rates of 10^-5 or 10^-6.  As we discuss
   later in this document, we assume that Standard TCP is unable to make
   effective use of the available bandwidth in environments with loss





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   rates of 10^-6 in any case, so that it is acceptable and appropriate
   for HighSpeed TCP to perform more aggressively than Standard TCP in
   such an environment.

   *  Non-goal: Ramping-up more quickly than allowed by slow-start.

   It is our belief that ramping-up more quickly than allowed by slow-
   start would necessitate more explicit feedback from routers along the
   path.  The proposal for HighSpeed TCP is focused on changes to TCP
   that could be effectively deployed in the current Internet
   environment.

   *  Non-goal: Avoiding oscillations in environments with only one-way,
      long-lived flows all with the same round-trip times.

   While we agree that attention to oscillatory behavior is useful,
   avoiding oscillations in aggregate throughput has not been our
   primary consideration, particularly for simplified environments
   limited to one-way, long-lived flows all with the same, large round-
   trip times.  Our assessment is that some oscillatory behavior in
   these extreme environments is an acceptable price to pay for the
   other benefits of HighSpeed TCP.

5.  Modifying the TCP Response Function

   The TCP response function, w = 1.2/sqrt(p), gives TCP's average
   congestion window w in MSS-sized segments, as a function of the
   steady-state packet drop rate p [FF98].  This TCP response function
   is a direct consequence of TCP's Additive Increase Multiplicative
   Decrease (AIMD) mechanisms of increasing the congestion window by
   roughly one segment per round-trip time in the absence of congestion,
   and halving the congestion window in response to a round-trip time
   with a congestion event.  This response function for Standard TCP is
   reflected in the table below.  In this proposal we restrict our
   attention to TCP performance in environments with packet loss rates
   of at most 10^-2, and so we can ignore the more complex response
   functions that are required to model TCP performance in more
   congested environments with retransmit timeouts.  From Appendix A, an
   average congestion window of W corresponds to an average of 2/3 W
   round-trip times between loss events for Standard TCP (with the
   congestion window varying from 2/3 W to 4/3 W).










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     Packet Drop Rate P   Congestion Window W    RTTs Between Losses
     ------------------   -------------------    -------------------
            10^-2                     12                8
            10^-3                     38               25
            10^-4                    120               80
            10^-5                    379              252
            10^-6                   1200              800
            10^-7                   3795             2530
            10^-8                  12000             8000
            10^-9                  37948            25298
            10^-10                120000            80000

   Table 2: TCP Response Function for Standard TCP.  The average
   congestion window W in MSS-sized segments is given as a function of
   the packet drop rate P.

   To specify a modified response function for HighSpeed TCP, we use
   three parameters, Low_Window, High_Window, and High_P.  To ensure TCP
   compatibility, the HighSpeed response function uses the same response
   function as Standard TCP when the current congestion window is at
   most Low_Window, and uses the HighSpeed response function when the
   current congestion window is greater than Low_Window.  In this
   document we set Low_Window to 38 MSS-sized segments, corresponding to
   a packet drop rate of 10^-3 for TCP.

   To specify the upper end of the HighSpeed response function, we
   specify the packet drop rate needed in the HighSpeed response
   function to achieve an average congestion window of 83000 segments.
   This is roughly the window needed to sustain 10 Gbps throughput, for
   a TCP connection with the default packet size and round-trip time
   used earlier in this document.  For High_Window set to 83000, we
   specify High_P of 10^-7; that is, with HighSpeed TCP a packet drop
   rate of 10^-7 allows the HighSpeed TCP connection to achieve an
   average congestion window of 83000 segments.  We believe that this
   loss rate sets an achievable target for high-speed environments,
   while still allowing acceptable fairness for the HighSpeed response
   function when competing with Standard TCP in environments with packet
   drop rates of 10^-4 or 10^5.

   For simplicity, for the HighSpeed response function we maintain the
   property that the response function gives a straight line on a log-
   log scale (as does the response function for Standard TCP, for low to
   moderate congestion).  This results in the following response
   function, for values of the average congestion window W greater than
   Low_Window:

     W = (p/Low_P)^S Low_Window,




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   for Low_P the packet drop rate corresponding to Low_Window, and for S
   as following constant [FRS02]:

     S = (log High_Window - log Low_Window)/(log High_P - log Low_P).

   (In this paper, "log x" refers to the log base 10.)  For example, for
   Low_Window set to 38, we have Low_P of 10^-3 (for compatibility with
   Standard TCP).  Thus, for High_Window set to 83000 and High_P set to
   10^-7, we get the following response function:

     W = 0.12/p^0.835.                                    (1)

   This HighSpeed response function is illustrated in Table 3 below.
   For HighSpeed TCP, the number of round-trip times between losses,
   1/(pW), equals 12.7 W^0.2, for W > 38 segments.

     Packet Drop Rate P   Congestion Window W    RTTs Between Losses
     ------------------   -------------------    -------------------
            10^-2                    12                   8
            10^-3                    38                  25
            10^-4                   263                  38
            10^-5                  1795                  57
            10^-6                 12279                  83
            10^-7                 83981                 123
            10^-8                574356                 180
            10^-9               3928088                 264
            10^-10             26864653                 388

   Table 3: TCP Response Function for HighSpeed TCP.  The average
   congestion window W in MSS-sized segments is given as a function of
   the packet drop rate P.

   We believe that the problem of backward compatibility with Standard
   TCP requires a response function that is quite close to that of
   Standard TCP for loss rates of 10^-1, 10^-2, or 10^-3.  We believe,
   however, that such stringent TCP-compatibility is not required for
   smaller loss rates, and that an appropriate response function is one
   that gives a plausible packet drop rate for a connection throughput
   of 10 Gbps.  This also gives a slowly increasing number of round-trip
   times between loss events as a function of a decreasing packet drop
   rate.

   Another way to look at the HighSpeed response function is to consider
   that HighSpeed TCP is roughly emulating the congestion control
   response of N parallel TCP connections, where N is initially one, and
   where N increases as a function of the HighSpeed TCP's congestion
   window.  Thus for the HighSpeed response function in Equation (1)
   above, the response function can be viewed as equivalent to that of



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   N(W) parallel TCP connections, where N(W) varies as a function of the
   congestion window W.  Recall that for a single standard TCP
   connection, the average congestion window equals 1.2/sqrt(p).  For N
   parallel TCP connections, the aggregate congestion window for the N
   connections equals N*1.2/sqrt(p).  From the HighSpeed response
   function in Equation (1) and the relationship above, we can derive
   the following:

    N(W) = 0.23*W^(0.4)

   for N(W) the number of parallel TCP connections emulated by the
   HighSpeed TCP response function, and for N(W) >= 1.  This is shown in
   Table 4 below.

     Congestion Window W         Number N(W) of Parallel TCPs
     -------------------         -------------------------
              1                            1
             10                            1
            100                            1.4
          1,000                            3.6
         10,000                            9.2
        100,000                           23.0

   Table 4: Number N(W) of parallel TCP connections roughly emulated by
   the HighSpeed TCP response function.

   In this document, we do not attempt to seriously evaluate the
   HighSpeed response function for congestion windows greater than
   100,000 packets.  We believe that we will learn more about the
   requirements for sustaining the throughput of best-effort connections
   in that range as we gain more experience with HighSpeed TCP with
   congestion windows of thousands and tens of thousands of packets.
   There also might be limitations to the per-connection throughput that
   can be realistically achieved for best-effort traffic, in terms of
   congestion window of hundreds of thousands of packets or more, in the
   absence of additional support or feedback from the routers along the
   path.

6.  Fairness Implications of the HighSpeed Response Function

   The Standard and Highspeed Response Functions can be used directly to
   infer the relative fairness between flows using the two response
   functions.  For example, given a packet drop rate P, assume that
   Standard TCP has an average congestion window of W_Standard, and
   HighSpeed TCP has a higher average congestion window of W_HighSpeed.






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   In this case, a single HighSpeed TCP connection is receiving
   W_HighSpeed/W_Standard times the throughput of a single Standard TCP
   connection competing in the same environment.

   This relative fairness is illustrated below in Table 5, for the
   parameters used for the Highspeed response function in the section
   above.  The second column gives the relative fairness, for the
   steady-state packet drop rate specified in the first column.  To help
   calibrate, the third column gives the aggregate average congestion
   window for the two TCP connections, and the fourth column gives the
   bandwidth that would be needed by the two connections to achieve that
   aggregate window and packet drop rate, given 100 ms round-trip times
   and 1500-byte packets.

     Packet Drop Rate P   Fairness  Aggregate Window  Bandwidth
     ------------------   --------  ----------------  ---------
            10^-2            1.0              24        2.8 Mbps
            10^-3            1.0              76        9.1 Mbps
            10^-4            2.2             383       45.9 Mbps
            10^-5            4.7            2174      260.8 Mbps
            10^-6           10.2           13479        1.6 Gbps
            10^-7           22.1           87776       10.5 Gbps

   Table 5: Relative Fairness between the HighSpeed and Standard
   Response Functions.

   Thus, for packet drop rates of 10^-4, a flow with the HighSpeed
   response function can expect to receive 2.2 times the throughput of a
   flow using the Standard response function, given the same round-trip
   times and packet sizes.  With packet drop rates of 10^-6 (or 10^-7),
   the unfairness is more severe, and we have entered the regime where a
   Standard TCP connection requires at most one congestion event every
   800 (or 2530) round-trip times in order to make use of the available
   bandwidth.  Our judgement would be that there are not a lot of TCP
   connections effectively operating in this regime today, with
   congestion windows of thousands of packets, and that therefore the
   benefits of the HighSpeed response function would outweigh the
   unfairness that would be experienced by Standard TCP in this regime.
   However, one purpose of this document is to solicit feedback on this
   issue.  The parameter Low_Window determines directly the point of
   divergence between the Standard and HighSpeed Response Functions.

   The third column of Table 5, the Aggregate Window, gives the
   aggregate congestion window of the two competing TCP connections,
   with HighSpeed and Standard TCP, given the packet drop rate specified
   in the first column.  From Table 5, a HighSpeed TCP connection would
   receive ten times the bandwidth of a Standard TCP in an environment
   with a packet drop rate of 10^-6.  This would occur when the two



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   flows sharing a single pipe achieved an aggregate window of 13479
   packets.  Given a round-trip time of 100 ms and a packet size of 1500
   bytes, this would occur with an available bandwidth for the two
   competing flows of 1.6 Gbps.

   Next we consider the time that it takes a standard or HighSpeed TCP
   flow to converge to fairness against a pre-existing HighSpeed TCP
   flow.  The worst case for convergence to fairness occurs when a new
   flow is starting up, competing against a high-bandwidth existing
   flow, and the new flow suffers a packet drop and exits slow-start
   while its window is still small.  In the worst case, consider that
   the new flow has entered the congestion avoidance phase while its
   window is only one packet.  A standard TCP flow in congestion
   avoidance increases its window by at most one packet per round-trip
   time, and after N round-trip times has only achieved a window of N
   packets (when starting with a window of 1 in the first round-trip
   time).  In contrast, a HighSpeed TCP flows increases much faster than
   a standard TCP flow while in the congestion avoidance phase, and we
   can expect its convergence to fairness to be much better.  This is
   shown in Table 6 below.  The script used to generate this table is
   given in Appendix C.

     RTT  HS_Window Standard_TCP_Window
     ---  --------- -------------------
     100       131        100
     200       475        200
     300      1131        300
     400      2160        400
     500      3601        500
     600      5477        600
     700      7799        700
     800     10567        800
     900     13774        900
    1000     17409       1000
    1100     21455       1100
    1200     25893       1200
    1300     30701       1300
    1400     35856       1400
    1500     41336       1500
    1600     47115       1600
    1700     53170       1700
    1800     59477       1800
    1900     66013       1900
    2000     72754       2000

   Table 6:  For a HighSpeed and a Standard TCP connection, the
   congestion window during congestion avoidance phase (starting with a
   congestion window of 1 packet during RTT 1).



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   The classic paper on relative fairness is from Chiu and Jain [CJ89].
   This paper shows that AIMD (Additive Increase Multiplicative
   Decrease) converges to fairness in an environment with synchronized
   congestion events.  From [CJ89], it is easy to see that MIMD and AIAD
   do not converge to fairness in this environment.  However, the
   results of [CJ89] do not apply to an asynchronous environment such as
   that of the current Internet, where the frequency of congestion
   feedback can be different for different flows.  For example, it has
   been shown that MIMD converges to fair states in a model with
   proportional instead of synchronous feedback in terms of packet drops
   [GV02].  Thus, we are not concerned about abandoning a strict model
   of AIMD for HighSpeed TCP.  However, we note that in an environment
   with Drop-Tail queue management, there is likely to be some
   synchronization of packet drops.  In this environment, the model of
   completely synchronous feedback does not hold, but the model of
   completely asynchronous feedback is not accurate either.  Fairness in
   Drop-Tail environments is discussed in more detail in Sections 9 and
   12.

7.  Translating the HighSpeed Response Function into Congestion Control
     Parameters

   For equation-based congestion control such as TFRC, the HighSpeed
   Response Function above could be used directly by the TFRC congestion
   control mechanism.  However, for TCP the HighSpeed response function
   has to be translated into additive increase and multiplicative
   decrease parameters.  The HighSpeed response function cannot be
   achieved by TCP with an additive increase of one segment per round-
   trip time and a multiplicative decrease of halving the current
   congestion window; HighSpeed TCP will have to modify either the
   increase or the decrease parameter, or both.  We have concluded that
   HighSpeed TCP is most likely to achieve an acceptable compromise
   between moderate increases and timely decreases by modifying both the
   increase and the decrease parameter.

   That is, for HighSpeed TCP let the congestion window increase by a(w)
   segments per round-trip time in the absence of congestion, and let
   the congestion window decrease to w(1-b(w)) segments in response to a
   round-trip time with one or more loss events.  Thus, in response to a
   single acknowledgement HighSpeed TCP increases its congestion window
   in segments as follows:

    w <- w + a(w)/w.

   In response to a congestion event, HighSpeed TCP decreases as
   follows:

    w <- (1-b(w))w.



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   For Standard TCP, a(w) = 1 and b(w) = 1/2, regardless of the value of
   w.  HighSpeed TCP uses the same values of a(w) and b(w) for w <=
   Low_Window.  This section specifies a(w) and b(w) for HighSpeed TCP
   for larger values of w.

   For w = High_Window, we have specified a loss rate of High_P.  From
   [FRS02], or from elementary calculations, this requires the following
   relationship between a(w) and b(w) for w = High_Window:

    a(w) = High_Window^2 * High_P * 2 * b(w)/(2-b(w)).     (2)

   We use the parameter High_Decrease to specify the decrease parameter
   b(w) for w = High_Window, and use Equation (2) to derive the increase
   parameter a(w) for w = High_Window.  Along with High_P = 10^-7 and
   High_Window = 83000, for example, we specify High_Decrease = 0.1,
   specifying that b(83000) = 0.1, giving a decrease of 10% after a
   congestion event.  Equation (2) then gives a(83000) = 72, for an
   increase of 72 segments, or just under 0.1%, within a round-trip
   time, for w = 83000.

   This moderate decrease strikes us as acceptable, particularly when
   coupled with the role of TCP's ACK-clocking in limiting the sending
   rate in response to more severe congestion [BBFS01].  A more severe
   decrease would require a more aggressive increase in the congestion
   window for a round-trip time without congestion.  In particular, a
   decrease factor High_Decrease of 0.5, as in Standard TCP, would
   require an increase of 459 segments per round-trip time when w =
   83000.

   Given decrease parameters of b(w) = 1/2 for w = Low_Window, and b(w)
   = High_Decrease for w = High_Window, we are left to specify the value
   of b(w) for other values of w > Low_Window.  From [FRS02], we let
   b(w) vary linearly as the log of w, as follows:

    b(w) = (High_Decrease - 0.5) (log(w)-log(W)) / (log(W_1)-log(W)) +
   0.5,

   for W = Low_window and W_1 = High_window.  The increase parameter
   a(w) can then be computed as follows:

    a(w) = w^2 * p(w) * 2 * b(w)/(2-b(w)),

   for p(w) the packet drop rate for congestion window w.  From
   inverting Equation (1), we get p(w) as follows:

    p(w) = 0.078/w^1.2.





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   We assume that experimental implementations of HighSpeed TCP for
   further investigation will use a pre-computed look-up table for
   finding a(w) and b(w).  For example, the implementation from Tom
   Dunigan adjusts the a(w) and b(w) parameters every 0.1 seconds.  In
   the appendix we give such a table for our default values of
   Low_Window = 38, High_Window = 83,000, High_P = 10^-7, and
   High_Decrease = 0.1.  These are also the default values in the NS
   simulator; example simulations in NS can be run with the command
   "./test-all-tcpHighspeed" in the directory tcl/test.

8.  An alternate, linear response functions

   In this section we explore an alternate, linear response function for
   HighSpeed TCP that has been proposed by a number of other people, in
   particular by Glenn Vinnicombe and Tom Kelly.  Similarly, it has been
   suggested by others that a less "ad-hoc" guideline for a response
   function for HighSpeed TCP would be to specify a constant value for
   the number of round-trip times between congestion events.

   Assume that we keep the value of Low_Window as 38 MSS-sized segments,
   indicating when the HighSpeed response function diverges from the
   current TCP response function, but that we modify the High_Window and
   High_P parameters that specify the upper range of the HighSpeed
   response function.  In particular, consider the response function
   given by High_Window = 380,000 and High_P = 10^-7, with Low_Window =
   38 and Low_P = 10^-3 as before.

   Using the equations in Section 5, this would give the following
   Linear response function, for w > Low_Window:

     W = 0.038/p.

   This Linear HighSpeed response function is illustrated in Table 7
   below.  For HighSpeed TCP, the number of round-trip times between
   losses, 1/(pW), equals 1/0.38, or equivalently, 26, for W > 38
   segments.















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     Packet Drop Rate P   Congestion Window W    RTTs Between Losses
     ------------------   -------------------    -------------------
            10^-2                    12                   8
            10^-3                    38                  26
            10^-4                   380                  26
            10^-5                  3800                  26
            10^-6                 38000                  26
            10^-7                380000                  26
            10^-8               3800000                  26
            10^-9              38000000                  26
            10^-10            380000000                  26

   Table 7: An Alternate, Linear TCP Response Function for HighSpeed
   TCP.  The average congestion window W in MSS-sized segments is given
   as a function of the packet drop rate P.

   Given a constant decrease b(w) of 1/2, this would give an increase
   a(w) of w/Low_Window, or equivalently, a constant increase of
   1/Low_Window packets per acknowledgement, for w > Low_Window.
   Another possibility is Scalable TCP [K03], which uses a fixed
   decrease b(w) of 1/8 and a fixed increase per acknowledgement of
   0.01.  This gives an increase a(w) per window of 0.005 w, for a TCP
   with delayed acknowledgements, for pure MIMD.

   The relative fairness between the alternate Linear response function
   and the standard TCP response function is illustrated below in Table
   8.

     Packet Drop Rate P   Fairness  Aggregate Window  Bandwidth
     ------------------   --------  ----------------  ---------
            10^-2            1.0              24        2.8 Mbps
            10^-3            1.0              76        9.1 Mbps
            10^-4            3.2             500       60.0 Mbps
            10^-5           15.1            4179      501.4 Mbps
            10^-6           31.6           39200        4.7 Gbps
            10^-7          100.1          383795       46.0 Gbps

   Table 8: Relative Fairness between the Linear HighSpeed and Standard
   Response Functions.

   One attraction of the linear response function is that it is scale-
   invariant, with a fixed increase in the congestion window per
   acknowledgement, and a fixed number of round-trip times between loss
   events.  My own assumption would be that having a fixed length for
   the congestion epoch in round-trip times, regardless of the packet
   drop rate, would be a poor fit for an imprecise and imperfect world
   with routers with a range of queue management mechanisms, such as the
   Drop-Tail queue management that is common today.  For example, a



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   response function with a fixed length for the congestion epoch in
   round-trip times might give less clearly-differentiated feedback in
   an environment with steady-state background losses at fixed intervals
   for all flows (as might occur with a wireless link with occasional
   short error bursts, giving losses for all flows every N seconds
   regardless of their sending rate).

   While it is not a goal to have perfect fairness in an environment
   with synchronized losses, it would be good to have moderately
   acceptable performance in this regime.  This goal might argue against
   a response function with a constant number of round-trip times
   between congestion events.  However, this is a question that could
   clearly use additional research and investigation.  In addition,
   flows with different round-trip times would have different time
   durations for congestion epochs even in the model with a linear
   response function.

   The third column of Table 8, the Aggregate Window, gives the
   aggregate congestion window of two competing TCP connections, one
   with Linear HighSpeed TCP and one with Standard TCP, given the packet
   drop rate specified in the first column.  From Table 8, a Linear
   HighSpeed TCP connection would receive fifteen times the bandwidth of
   a Standard TCP in an environment with a packet drop rate of 10^-5.
   This would occur when the two flows sharing a single pipe achieved an
   aggregate window of 4179 packets.  Given a round-trip time of 100 ms
   and a packet size of 1500 bytes, this would occur with an available
   bandwidth for the two competing flows of 501 Mbps.  Thus, because the
   Linear HighSpeed TCP is more aggressive than the HighSpeed TCP
   proposed above, it also is less fair when competing with Standard TCP
   in a high-bandwidth environment.

9.  Tradeoffs for Choosing Congestion Control Parameters

   A range of metrics can be used for evaluating choices for congestion
   control parameters for HighSpeed TCP.  My assumption in this section
   is that for a response function of the form w = c/p^d, for constant c
   and exponent d, the only response functions that would be considered
   are response functions with 1/2 <= d <= 1.  The two ends of this
   spectrum are represented by current TCP, with d = 1/2, and by the
   linear response function described in Section 8 above, with d = 1.
   HighSpeed TCP lies somewhere in the middle of the spectrum, with d =
   0.835.

   Response functions with exponents less than 1/2 can be eliminated
   from consideration because they would be even worse than standard TCP
   in accommodating connections with high congestion windows.





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9.1.  The Number of Round-Trip Times between Loss Events

   Response functions with exponents greater than 1 can be eliminated
   from consideration because for these response functions, the number
   of round-trip times between loss events decreases as congestion
   decreases.  For a response function of w = c/p^d, with one loss event
   or congestion event every 1/p packets, the number of round-trip times
   between loss events is w^((1/d)-1)/c^(1/d).  Thus, for standard TCP
   the number of round-trip times between loss events is linear in w.
   In contrast, one attraction of the linear response function, as
   described in Section 8 above, is that it is scale-invariant, in terms
   of a fixed increase in the congestion window per acknowledgement, and
   a fixed number of round-trip times between loss events.

   However, for a response function with d > 1, the number of round-
   trip times between loss events would be proportional to w^((1/d)-1),
   for a negative exponent ((1/d)-1), setting smaller as w increases.
   This would seem undesirable.

9.2.  The Number of Packet Drops per Loss Event, with Drop-Tail

   A TCP connection increases its sending rate by a(w) packets per
   round-trip time, and in a Drop-Tail environment, this is likely to
   result in a(w) dropped packets during a single loss event.  One
   attraction of standard TCP is that it has a fixed increase per
   round-trip time of one packet, minimizing the number of packets that
   would be dropped in a Drop-Tail environment.  For an environment with
   some form of Active Queue Management, and in particular for an
   environment that uses ECN, the number of packets dropped in a single
   congestion event would not be a problem.  However, even in these
   environments, larger increases in the sending rate per round-trip
   time result in larger stresses on the ability of the queues in the
   router to absorb the fluctuations.

   HighSpeed TCP plays a middle ground between the metrics of a moderate
   number of round-trip times between loss events, and a moderate
   increase in the sending rate per round-trip time.  As shown in
   Appendix B, for a congestion window of 83,000 packets, HighSpeed TCP
   increases its sending rate by 70 packets per round-trip time,
   resulting in at most 70 packet drops when the buffer overflows in a
   Drop-Tail environment.  This increased aggressiveness is the price
   paid by HighSpeed TCP for its increased scalability.  A large number
   of packets dropped per congestion event could result in synchronized
   drops from multiple flows, with a possible loss of throughput as a
   result.






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   Scalable TCP has an increase a(w) of 0.005 w packets per round-trip
   time.  For a congestion window of 83,000 packets, this gives an
   increase of 415 packets per round-trip time, resulting in roughly 415
   packet drops per congestion event in a Drop-Tail environment.

   Thus, HighSpeed TCP and its variants place increased demands on queue
   management in routers, relative to Standard TCP.  (This is rather
   similar to the increased demands on queue management that would
   result from using N parallel TCP connections instead of a single
   Standard TCP connection.)

10.  Related Issues

10.1.  Slow-Start

   A companion internet-draft on "Limited Slow-Start for TCP with Large
   Congestion Windows" [F02b] proposes a modification to TCP's slow-
   start procedure that can significantly improve the performance of TCP
   connections slow-starting up to large congestion windows.  For TCP
   connections that are able to use congestion windows of thousands (or
   tens of thousands) of MSS-sized segments (for MSS the sender's
   MAXIMUM SEGMENT SIZE), the current slow-start procedure can result in
   increasing the congestion window by thousands of segments in a single
   round-trip time.  Such an increase can easily result in thousands of
   packets being dropped in one round-trip time.  This is often
   counter-productive for the TCP flow itself, and is also hard on the
   rest of the traffic sharing the congested link.

   [F02b] proposes Limited Slow-Start, limiting the number of segments
   by which the congestion window is increased for one window of data
   during slow-start, in order to improve performance for TCP
   connections with large congestion windows.  We have separated out
   Limited Slow-Start to a separate draft because it can be used both
   with Standard or with HighSpeed TCP.

   Limited Slow-Start is illustrated in the NS simulator, for snapshots
   after May 1, 2002, in the tests "./test-all-tcpHighspeed tcp1A" and
   "./test-all-tcpHighspeed tcpHighspeed1" in the subdirectory
   "tcl/lib".

   In order for best-effort flows to safely start-up faster than slow-
   start, e.g., in future high-bandwidth networks, we believe that it
   would be necessary for the flow to have explicit feedback from the
   routers along the path.  There are a number of proposals for this,
   ranging from a minimal proposal for an IP option that allows TCP SYN
   packets to collect information from routers along the path about the
   allowed initial sending rate [J02], to proposals with more power that
   require more fine-tuned and continuous feedback from routers.  These



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   proposals are all somewhat longer-term proposals than the HighSpeed
   TCP proposal in this document, requiring longer lead times and more
   coordination for deployment, and will be discussed in later
   documents.

10.2.  Limiting burstiness on short time scales

   Because the congestion window achieved by a HighSpeed TCP connection
   could be quite large, there is a possibility for the sender to send a
   large burst of packets in response to a single acknowledgement.  This
   could happen, for example, when there is congestion or reordering on
   the reverse path, and the sender receives an acknowledgement
   acknowledging hundreds or thousands of new packets.  Such a burst
   would also result if the application was idle for a short period of
   time less than a round-trip time, and then suddenly had lots of data
   available to send.  In this case, it would be useful for the
   HighSpeed TCP connection to have some method for limiting bursts.

   In this document, we do not specify TCP mechanisms for reducing the
   short-term burstiness.  One possible mechanism is to use some form of
   rate-based pacing, and another possibility is to use maxburst, which
   limits the number of packets that are sent in response to a single
   acknowledgement.  We would caution, however, against a permanent
   reduction in the congestion window as a mechanism for limiting
   short-term bursts.  Such a mechanism has been deployed in some TCP
   stacks, and our view would be that using permanent reductions of the
   congestion window to reduce transient bursts would be a bad idea
   [Fl03].

10.3.  Other limitations on window size

   The TCP header uses a 16-bit field to report the receive window size
   to the sender.  Unmodified, this allows a window size of at most
   2**16 = 65K bytes.  With window scaling, the maximum window size is
   2**30 = 1073M bytes [RFC 1323].  Given 1500-byte packets, this allows
   a window of up to 715,000 packets.

10.4.  Implementation issues

   One implementation issue that has been raised with HighSpeed TCP is
   that with congestion windows of 4MB or more, the handling of
   successive SACK packets after a packet is dropped becomes very time-
   consuming at the TCP sender [S03].  Tom Kelly's Scalable TCP includes
   a "SACK Fast Path" patch that addresses this problem.

   The issues addressed in the Web100 project, the Net100 project, and
   related projects about the tuning necessary to achieve high bandwidth
   data rates with TCP apply to HighSpeed TCP as well [Net100, Web100].



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11.  Deployment issues

11.1.  Deployment issues of HighSpeed TCP

   We do not claim that the HighSpeed TCP modification to TCP described
   in this paper is an optimal transport protocol for high-bandwidth
   environments.  Based on our experiences with HighSpeed TCP in the NS
   simulator [NS], on simulation studies [SA03], and on experimental
   reports [ABLLS03,D02,CC03,F03], we believe that HighSpeed TCP
   improves the performance of TCP in high-bandwidth environments, and
   we are documenting it for the benefit of the IETF community.  We
   encourage the use of HighSpeed TCP, and of its underlying response
   function, and we further encourage feedback about operational
   experiences with this or related modifications.

   We note that in environments typical of much of the current Internet,
   HighSpeed TCP behaves exactly as does Standard TCP today.  This is
   the case any time the congestion window is less than 38 segments.

    Bandwidth   Avg Cwnd w (pkts)    Increase a(w)   Decrease b(w)
    ---------   -----------------    -------------   -------------
      1.5 Mbps         12.5               1              0.50
     10 Mbps           83                 1              0.50
    100 Mbps          833                 6              0.35
      1 Gbps         8333                26              0.22
     10 Gbps        83333                70              0.10

   Table 9: Performance of a HighSpeed TCP connection

   To help calibrate, Table 9 considers a TCP connection with 1500-byte
   packets, an RTT of 100 ms (including average queueing delay), and no
   competing traffic, and shows the average congestion window if that
   TCP connection had a pipe all to itself and fully used the link
   bandwidth, for a range of bandwidths for the pipe.  This assumes that
   the TCP connection would use Table 12 in determining its increase and
   decrease parameters.  The first column of Table 9 gives the
   bandwidth, and the second column gives the average congestion window
   w needed to utilize that bandwidth.  The third column shows the
   increase a(w) in segments per RTT for window w.  The fourth column
   shows the decrease b(w) for that window w (where the TCP sender
   decreases the congestion window from w to w(1-b(w)) segments after a
   loss event).  When a loss occurs we note that the actual congestion
   window is likely to be greater than the average congestion window w
   in column 2, so the decrease parameter used could be slightly smaller
   than the one given in column 4 of Table 9.

   Table 9 shows that a HighSpeed TCP over a 10 Mbps link behaves
   exactly the same as a Standard TCP connection, even in the absence of



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   competing traffic.  One can think of the congestion window staying
   generally in the range of 55 to 110 segments, with the HighSpeed TCP
   behavior being exactly the same as the behavior of Standard TCP.  (If
   the congestion window is ever 128 segments or more, then the
   HighSpeed TCP increases by two segments per RTT instead of by one,
   and uses a decrease parameter of 0.44 instead of 0.50.)

   Table 9 shows that for a HighSpeed TCP connection over a 100 Mbps
   link, with no competing traffic, HighSpeed TCP behaves roughly as
   aggressively as six parallel TCP connections, increasing its
   congestion window by roughly six segments per round-trip time, and
   with a decrease parameter of roughly 1/3 (corresponding to decreasing
   down to 2/3-rds of its old congestion window, rather than to half, in
   response to a loss event).

   For a Standard TCP connection in this environment, the congestion
   window could be thought of as generally varying in the range of 550
   to 1100 segments, with an average packet drop rate of 2.2 * 10^-6
   (corresponding to a bit error rate of 1.8 * 10^-10), or equivalently,
   roughly 55 seconds between congestion events.  While a Standard TCP
   connection could sustain such a low packet drop rate in a carefully
   controlled environment with minimal competing traffic, we would
   contend that in an uncontrolled best-effort environment with even a
   small amount of competing traffic, the occasional congestion events
   from smaller competing flows could easily be sufficient to prevent a
   Standard TCP flow with no lower-speed bottlenecks from fully
   utilizing the available bandwidth of the underutilized 100 Mbps link.

   That is, we would contend that in the environment of 100 Mbps links
   with a significant amount of available bandwidth, Standard TCP would
   sometimes be unable to fully utilize the link bandwidth, and that
   HighSpeed TCP would be an improvement in this regard.  We would
   further contend that in this environment, the behavior of HighSpeed
   TCP is sufficiently close to that of Standard TCP that HighSpeed TCP
   would be safe to deploy in the current Internet.  We note that
   HighSpeed TCP can only use high congestion windows if allowed by the
   receiver's advertised window size.  As a result, even if HighSpeed
   TCP was ubiquitously deployed in the Internet, the impact would be
   limited to those TCP connections with an advertised window from the
   receiver of 118 MSS or larger.

   We do not believe that the deployment of HighSpeed TCP would serve as
   a block to the possible deployment of alternate experimental
   protocols for high-speed congestion control, such as Scalable TCP,
   XCP [KHR02], or FAST TCP [JWL03].  In particular, we don't expect
   HighSpeed TCP to interact any more poorly with alternative
   experimental proposals than would the N parallel TCP connections
   commonly used today in the absence of HighSpeed TCP.



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11.2.  Deployment issues of Scalable TCP

   We believe that Scalable TCP and HighSpeed TCP have sufficiently
   similar response functions that they could easily coexist in the
   Internet.  However, we have not investigated Scalable TCP
   sufficiently to be able to claim, in this document, that Scalable TCP
   is safe for a widespread deployment in the current Internet.

    Bandwidth   Avg Cwnd w (pkts)    Increase a(w)   Decrease b(w)
    ---------   -----------------    -------------   -------------
      1.5 Mbps         12.5               1              0.50
     10 Mbps           83                 0.4            0.125
    100 Mbps          833                 4.1            0.125
      1 Gbps         8333                41.6            0.125
     10 Gbps        83333               416.5            0.125

   Table 10: Performance of a Scalable TCP connection.

   Table 10 shows the performance of a Scalable TCP connection with
   1500-byte packets, an RTT of 100 ms (including average queueing
   delay), and no competing traffic.  The TCP connection is assumed to
   use delayed acknowledgements.  The first column of Table 10 gives the
   bandwidth, the second column gives the average congestion window
   needed to utilize that bandwidth, and the third and fourth columns
   give the increase and decrease parameters.

   Note that even in an environment with a 10 Mbps link, Scalable TCP's
   behavior is considerably different from that of Standard TCP.  The
   increase parameter is smaller than that of Standard TCP, and the
   decrease is smaller also, 1/8-th instead of 1/2.  That is, for 10
   Mbps links, Scalable TCP increases less aggressively than Standard
   TCP or HighSpeed TCP, but decreases less aggressively as well.

   In an environment with a 100 Mbps link, Scalable TCP has an increase
   parameter of roughly four segments per round-trip time, with the same
   decrease parameter of 1/8-th.  A comparison of Tables 9 and 10 shows
   that for this scenario of 100 Mbps links, HighSpeed TCP increases
   more aggressively than Scalable TCP.

   Next we consider the relative fairness between Standard TCP,
   HighSpeed TCP and Scalable TCP.  The relative fairness between
   HighSpeed TCP and Standard TCP was shown in Table 5 earlier in this
   document, and the relative fairness between Scalable TCP and Standard
   TCP was shown in Table 8.  Following the approach in Section 6, for a
   given packet drop rate p, for p < 10^-3, we can estimate the relative
   fairness between Scalable and HighSpeed TCP as
   W_Scalable/W_HighSpeed.  This relative fairness is shown in Table 11
   below.  The bandwidth in the last column of Table 11 is the aggregate



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   bandwidth of the two competing flows given 100 ms round-trip times
   and 1500-byte packets.

    Packet Drop Rate P   Fairness  Aggregate Window  Bandwidth
    ------------------   --------  ----------------  ---------
         10^-2            1.0              24        2.8 Mbps
         10^-3            1.0              76        9.1 Mbps
         10^-4            1.4             643       77.1 Mbps
         10^-5            2.1            5595      671.4 Mbps
         10^-6            3.1           50279        6.0 Gbps
         10^-7            4.5          463981       55.7 Gbps

   Table 11: Relative Fairness between the Scalable and HighSpeed
   Response Functions.

   The second row of Table 11 shows that for a Scalable TCP and a
   HighSpeed TCP flow competing in an environment with 100 ms RTTs and a
   10 Mbps pipe, the two flows would receive essentially the same
   bandwidth.  The next row shows that for a Scalable TCP and a
   HighSpeed TCP flow competing in an environment with 100 ms RTTs and a
   100 Mbps pipe, the Scalable TCP flow would receive roughly 50% more
   bandwidth than would HighSpeed TCP.  Table 11 shows the relative
   fairness in higher-bandwidth environments as well.  This relative
   fairness seems sufficient that there should be no problems with
   Scalable TCP and HighSpeed TCP coexisting in the same environment as
   Experimental variants of TCP.

   We note that one question that requires more investigation with
   Scalable TCP is that of convergence to fairness in environments with
   Drop-Tail queue management.

12.  Related Work in HighSpeed TCP

   HighSpeed TCP has been separately investigated in simulations by
   Sylvia Ratnasamy and by Evandro de Souza [SA03].  The simulations in
   [SA03] verify the fairness properties of HighSpeed TCP when sharing a
   link with Standard TCP.

   These simulations explore the relative fairness of HighSpeed TCP
   flows when competing with Standard TCP.  The simulation environment
   includes background forward and reverse-path TCP traffic limited by
   the TCP receive window, along with a small amount of forward and
   reverse-path traffic from the web traffic generator.  Most of the
   simulations so far explore performance on a simple dumbbell topology
   with a 1 Gbps link with a propagation delay of 50 ms.  Simulations
   have been run with Adaptive RED and with DropTail queue management.





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   The simulations in [SA03] explore performance with a varying number
   of competing flows, with the competing traffic being all standard
   TCP; all HighSpeed TCP; or a mix of standard and HighSpeed TCP.  For
   the simulations in [SA03] with RED queue management, the relative
   fairness between standard and HighSpeed TCP is consistent with the
   relative fairness predicted in Table 5.  For the simulations with
   Drop Tail queues, the relative fairness is more skewed, with the
   HighSpeed TCP flows receiving an even larger share of the link
   bandwidth.  This is not surprising; with Active Queue Management at
   the congested link, the fraction of packet drops received by each
   flow should be roughly proportional to that flow's share of the link
   bandwidth, while this property no longer holds with Drop Tail queue
   management.  We also note that relative fairness in simulations with
   Drop Tail queue management can sometimes depend on small details of
   the simulation scenario, and that Drop Tail simulations need special
   care to avoid phase effects [F92].

   [SA03] explores the bandwidth `stolen' by HighSpeed TCP from standard
   TCP by exploring the fraction of the link bandwidth N standard TCP
   flows receive when competing against N other standard TCP flows, and
   comparing this to the fraction of the link bandwidth the N standard
   TCP flows receive when competing against N HighSpeed TCP flows.  For
   the 1 Gbps simulation scenarios dominated by long-lived traffic, a
   small number of standard TCP flows are able to achieve high link
   utilization, and the HighSpeed TCP flows can be viewed as stealing
   bandwidth from the competing standard TCP flows, as predicted in
   Section 6 on the Fairness Implications of the HighSpeed Response
   Function.  However, [SA03] shows that when even a small fraction of
   the link bandwidth is used by more bursty, short TCP connections, the
   standard TCP flows are unable to achieve high link utilization, and
   the HighSpeed TCP flows in this case are not `stealing' bandwidth
   from the standard TCP flows, but instead are using bandwidth that
   otherwise would not be utilized.

   The conclusions of [SA03] are that "HighSpeed TCP behaved as forseen
   by its response function, and appears to be a real and viable option
   for use on high-speed wide area TCP connections."

   Future work that could be explored in more detail includes
   convergence times after new flows start-up; recovery time after a
   transient outage; the response to sudden severe congestion, and
   investigations of the potential for oscillations.  We invite
   contributions from others in this work.








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13.  Relationship to other Work

   Our assumption is that HighSpeed TCP will be used with the TCP SACK
   option, and also with the increased Initial Window of three or four
   segments, as allowed by [RFC3390].  For paths that have substantial
   reordering, TCP performance would be greatly improved by some of the
   mechanisms still in the research stages for robust performance in the
   presence of reordered packets.

   Our view is that HighSpeed TCP is largely orthogonal to proposals for
   higher PMTU (Path MTU) values [M02].  Unlike changes to the PMTU,
   HighSpeed TCP does not require any changes in the network or at the
   TCP receiver, and works well in the current Internet.  Our assumption
   is that HighSpeed TCP would be useful even with larger values for the
   PMTU.  Unlike the current congestion window, the PMTU gives no
   information about the bandwidth-delay product available to that
   particular flow.

   A related approach is that of a virtual MTU, where the actual MTU of
   the path might be limited [VMSS,S02].  The virtual MTU approach has
   not been fully investigated, and we do not explore the virtual MTU
   approach further in this document.

14.  Conclusions

   This document has proposed HighSpeed TCP, a modification to TCP's
   congestion control mechanism for use with TCP connections with large
   congestion windows.  We have explored this proposal in simulations,
   and others have explored HighSpeed TCP with experiments, and we
   believe HighSpeed TCP to be safe to deploy on the current Internet.
   We would welcome additional analysis, simulations, and particularly,
   experimentation.  More information on simulations and experiments is
   available from the HighSpeed TCP Web Page [HSTCP].  There are several
   independent implementations of HighSpeed TCP [D02,F03] and of
   Scalable TCP [K03] for further investigation.

15.  Acknowledgements

   The HighSpeed TCP proposal is from joint work with Sylvia Ratnasamy
   and Scott Shenker (and was initiated by Scott Shenker).  Additional
   investigations of HighSpeed TCP were joint work with Evandro de Souza
   and Deb Agarwal.  We thank Tom Dunigan for the implementation in the
   Linux 2.4.16 Web100 kernel, and for resulting experimentation with
   HighSpeed TCP.  We are grateful to the End-to-End Research Group, the
   members of the Transport Area Working Group, and to members of the
   IPAM program in Large Scale Communication Networks for feedback.  We
   thank Glenn Vinnicombe for framing the Linear response function in
   the parameters of HighSpeed TCP.  We are also grateful for



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RFC 3649                     HighSpeed TCP                 December 2003


   contributions and feedback from the following individuals: Les
   Cottrell, Mitchell Erblich, Jeffrey Hsu, Tom Kelly, Chuck Jackson,
   Matt Mathis, Jitendra Padhye, Andrew Reiter, Stanislav Shalunov, Alex
   Solan, Paul Sutter, Brian Tierney, Joe Touch.

16.  Normative References

   [RFC2581]  Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

17.  Informative References

   [ABLLS03]  A. Antony, J. Blom, C. de Laat, J. Lee, and W. Sjouw,
              "Microscopic Examination of TCP Flows over Transatlantic
              Links", iGrid2002 special issue, Future Generation
              Computer Systems, volume 19 issue 6 (2003), URL
              "http://www.science.uva.nl/~delaat/techrep-2003-2-
              tcp.pdf".

   [BBFS01]   Deepak Bansal, Hari Balakrishnan, Sally Floyd, and Scott
              Shenker, "Dynamic Behavior of Slowly-Responsive Congestion
              Control Algorithms", SIGCOMM 2001, August 2001.

   [CC03]     Fabrizio Coccetti and Les Cottrell, "TCP Stack
              Measurements on Lightly Loaded Testbeds", 2003.  URL
              "http://www-iepm.slac.stanford.edu/monitoring/bulk/fast/".

   [CJ89]     D. Chiu and R. Jain, "Analysis of the Increase and
              Decrease Algorithms for Congestion Avoidance in Computer
              Networks", Computer Networks and ISDN Systems, Vol. 17,
              pp. 1-14, 1989.

   [CO98]     J. Crowcroft and P. Oechslin, "Differentiated End-to-end
              Services using a Weighted Proportional Fair Share TCP",
              Computer Communication Review, 28(3):53--69, 1998.

   [D02]      Tom Dunigan, "Floyd's TCP slow-start and AIMD mods", URL
              "http://www.csm.ornl.gov/~dunigan/net100/floyd.html".

   [F03]      Gareth Fairey, "High-Speed TCP", 2003.  URL
              "http://www.hep.man.ac.uk/u/garethf/hstcp/".

   [F92]      S. Floyd and V. Jacobson, "On Traffic Phase Effects in
              Packet-Switched Gateways, Internetworking: Research and
              Experience", V.3 N.3, September 1992, p.115-156.  URL
              "http://www.icir.org/floyd/papers.html".





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   [Fl03]     Sally Floyd, "Re: [Tsvwg] taking NewReno (RFC 2582) to
              Proposed Standard", Email to the tsvwg mailing list, May
              14, 2003.

   URLs       "http://www1.ietf.org/mail-archive/working-
              groups/tsvwg/current/msg04086.html" and
              "http://www1.ietf.org/mail-archive/working-
              groups/tsvwg/current/msg04087.html".

   [FF98]     Floyd, S., and Fall, K., "Promoting the Use of End-to-End
              Congestion Control in the Internet", IEEE/ACM Transactions
              on Networking, August 1999.

   [FRS02]    Sally Floyd, Sylvia Ratnasamy, and Scott Shenker,
              "Modifying TCP's Congestion Control for High Speeds", May
              2002.  URL "http://www.icir.org/floyd/notes.html".

   [GRK99]    Panos Gevros, Fulvio Risso and Peter Kirstein, "Analysis
              of a Method for Differential TCP Service".  In Proceedings
              of the IEEE GLOBECOM'99, Symposium on Global Internet ,
              December 1999, Rio de Janeiro, Brazil.

   [GV02]     S. Gorinsky and H. Vin, "Extended Analysis of Binary
              Adjustment Algorithms", Technical Report TR2002-39,
              Department of Computer Sciences, The University of Texas
              at Austin, August 2002.  URL
              "http://www.cs.utexas.edu/users/gorinsky/pubs.html".

   [HSTCP]    HighSpeed TCP Web Page, URL
              "http://www.icir.org/floyd/hstcp.html".

   [J02]      Amit Jain and Sally Floyd, "Quick-Start for TCP and IP",
              Work in Progress, 2002.

   [JWL03]    Cheng Jin, David X. Wei and Steven H. Low, "FAST TCP for
              High-speed Long-distance Networks", Work in Progress, June
              2003.

   [K03]      Tom Kelly, "Scalable TCP: Improving Performance in
              HighSpeed Wide Area Networks", February 2003.  URL
              "http://www-lce.eng.cam.ac.uk/~ctk21/scalable/".

   [KHR02]    Dina Katabi, Mark Handley, and Charlie Rohrs, "Congestion
              Control for High Bandwidth-Delay Product Networks",
              SIGCOMM 2002.

   [M02]      Matt Mathis, "Raising the Internet MTU", Web Page, URL
              "http://www.psc.edu/~mathis/MTU/".



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RFC 3649                     HighSpeed TCP                 December 2003


   [Net100]   The DOE/MICS Net100 project.  URL
              "http://www.csm.ornl.gov/~dunigan/net100/".

   [NS]       The NS Simulator, "http://www.isi.edu/nsnam/ns/".

   [RFC 1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC3390]  Allman, M., Floyd, S. and C., Partridge, "Increasing TCP's
              Initial Window", RFC 3390, October 2002.

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

   [SA03]     Souza, E. and D.A., Agarwal, "A HighSpeed TCP Study:
              Characteristics and Deployment Issues", LBNL Technical
              Report LBNL-53215.  URL
              "http://www.icir.org/floyd/hstcp.html".

   [S02]      Stanislav Shalunov, "TCP Armonk", Work in Progress, 2002,
              URL "http://www.internet2.edu/~shalunov/tcpar/".

   [S03]      Alex Solan, private communication, 2003.

   [VMSS]     "Web100 at ORNL", Web Page,
              "http://www.csm.ornl.gov/~dunigan/netperf/web100.html".

   [Web100]   The Web100 project.  URL "http://www.web100.org/".

18.  Security Considerations

   This proposal makes no changes to the underlying security of TCP.

19.  IANA Considerations

   There are no IANA considerations regarding this document.














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A.  TCP's Loss Event Rate in Steady-State

   This section gives the number of round-trip times between congestion
   events for a TCP flow with D-byte packets, for D=1500, as a function
   of the connection's average throughput B in bps.  To achieve this
   average throughput B, a TCP connection with round-trip time R in
   seconds requires an average congestion window w of BR/(8D) segments.

   In steady-state, TCP's average congestion window w is roughly
   1.2/sqrt(p) segments.  This is equivalent to a lost event at most
   once every 1/p packets, or at most once every 1/(pw) = w/1.5 round-
   trip times.  Substituting for w, this is a loss event at most every
   (BR)/12D)round-trip times.

   An an example, for R = 0.1 seconds and D = 1500 bytes, this gives
   B/180000 round-trip times between loss events.



































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B.  A table for a(w) and b(w).

   This section gives a table for the increase and decrease parameters
   a(w) and b(w) for HighSpeed TCP, for the default values of Low_Window
   = 38, High_Window = 83000, High_P = 10^-7, and High_Decrease = 0.1.

        w  a(w)  b(w)
     ----  ----  ----
       38     1  0.50
      118     2  0.44
      221     3  0.41
      347     4  0.38
      495     5  0.37
      663     6  0.35
      851     7  0.34
     1058     8  0.33
     1284     9  0.32
     1529    10  0.31
     1793    11  0.30
     2076    12  0.29
     2378    13  0.28
     2699    14  0.28
     3039    15  0.27
     3399    16  0.27
     3778    17  0.26
     4177    18  0.26
     4596    19  0.25
     5036    20  0.25
     5497    21  0.24
     5979    22  0.24
     6483    23  0.23
     7009    24  0.23
     7558    25  0.22
     8130    26  0.22
     8726    27  0.22
     9346    28  0.21
     9991    29  0.21
    10661    30  0.21
    11358    31  0.20
    12082    32  0.20
    12834    33  0.20
    13614    34  0.19
    14424    35  0.19
    15265    36  0.19
    16137    37  0.19
    17042    38  0.18
    17981    39  0.18
    18955    40  0.18



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    19965    41  0.17
    21013    42  0.17
    22101    43  0.17
    23230    44  0.17
    24402    45  0.16
    25618    46  0.16
    26881    47  0.16
    28193    48  0.16
    29557    49  0.15
    30975    50  0.15
    32450    51  0.15
    33986    52  0.15
    35586    53  0.14
    37253    54  0.14
    38992    55  0.14
    40808    56  0.14
    42707    57  0.13
    44694    58  0.13
    46776    59  0.13
    48961    60  0.13
    51258    61  0.13
    53677    62  0.12
    56230    63  0.12
    58932    64  0.12
    61799    65  0.12
    64851    66  0.11
    68113    67  0.11
    71617    68  0.11
    75401    69  0.10
    79517    70  0.10
    84035    71  0.10
    89053    72  0.10
    94717    73  0.09

   Table 12: Parameters for HighSpeed TCP.
















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   This table was computed with the following Perl program:

    $top = 100000;
    $num = 38;
    if ($num == 38) {
      print "     w  a(w)  b(w)\n";
      print "  ----  ----  ----\n";
      print "    38     1  0.50\n";
      $oldb = 0.50;
      $olda = 1;
    }
    while ($num < $top) {
      $bw = (0.1 -0.5)*(log($num)-log(38))/(log(83000)-log(38))+0.5;
      $aw = ($num**2*2.0*$bw) / ((2.0-$bw)*$num**1.2*12.8);
      if ($aw > $olda + 1) {
         printf "%6d %5d  %3.2f0, $num, $aw, $bw;
         $olda = $aw;
      }
      $num ++;
    }

   Table 13: Perl Program for computing parameters for HighSpeed TCP.





























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C.  Exploring the time to converge to fairness.

   This section gives the Perl program used to compute the congestion
   window growth during congestion avoidance.

    $top = 2001;
    $hswin = 1;
    $regwin = 1;
    $rtt = 1;
    $lastrtt = 0;
    $rttstep = 100;
    if ($hswin == 1) {
      print "  RTT  HS_Window Standard_TCP_Window0;
      print "  ---  --------- -------------------0;
    }
    while ($rtt < $top) {
      $bw = (0.1 -0.5)*(log($hswin)-log(38))/(log(83000)-log(38))+0.5;
      $aw = ($hswin**2*2.0*$bw) / ((2.0-$bw)*$hswin**1.2*12.8);
      if ($aw < 1) {
          $aw = 1;
      }
      if ($rtt >= $lastrtt + $rttstep) {
        printf "%5d %9d %10d0, $rtt, $hswin, $regwin;
        $lastrtt = $rtt;
      }
      $hswin += $aw;
      $regwin += 1;
      $rtt ++;
    }

   Table 14: Perl Program for computing the window in congestion
   avoidance.

Author's Address

   Sally Floyd
   ICIR (ICSI Center for Internet Research)

   Phone: +1 (510) 666-2989
   EMail: floyd@acm.org
   URL: http://www.icir.org/floyd/










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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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