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rfc:rfc3649

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.

Floyd Experimental [Page 1] RFC 3649 HighSpeed TCP December 2003

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

Floyd Experimental [Page 2] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 3] RFC 3649 HighSpeed TCP December 2003

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.

Floyd Experimental [Page 4] RFC 3649 HighSpeed TCP December 2003

  • 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

Floyd Experimental [Page 5] RFC 3649 HighSpeed TCP December 2003

 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).

Floyd Experimental [Page 6] RFC 3649 HighSpeed TCP December 2003

   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,

Floyd Experimental [Page 7] RFC 3649 HighSpeed TCP December 2003

 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

Floyd Experimental [Page 8] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 9] RFC 3649 HighSpeed TCP December 2003

 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

Floyd Experimental [Page 10] RFC 3649 HighSpeed TCP December 2003

 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).

Floyd Experimental [Page 11] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 12] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 13] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 14] RFC 3649 HighSpeed TCP December 2003

   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

Floyd Experimental [Page 15] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 16] RFC 3649 HighSpeed TCP December 2003

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.

Floyd Experimental [Page 17] RFC 3649 HighSpeed TCP December 2003

 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

Floyd Experimental [Page 18] RFC 3649 HighSpeed TCP December 2003

 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].

Floyd Experimental [Page 19] RFC 3649 HighSpeed TCP December 2003

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

Floyd Experimental [Page 20] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 21] RFC 3649 HighSpeed TCP December 2003

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

Floyd Experimental [Page 22] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 23] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 24] RFC 3649 HighSpeed TCP December 2003

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

Floyd Experimental [Page 25] 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".

Floyd Experimental [Page 26] RFC 3649 HighSpeed TCP December 2003

 [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/".

Floyd Experimental [Page 27] 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.

Floyd Experimental [Page 28] RFC 3649 HighSpeed TCP December 2003

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.

Floyd Experimental [Page 29] RFC 3649 HighSpeed TCP December 2003

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

Floyd Experimental [Page 30] RFC 3649 HighSpeed TCP December 2003

  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.

Floyd Experimental [Page 31] RFC 3649 HighSpeed TCP December 2003

 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.

Floyd Experimental [Page 32] RFC 3649 HighSpeed TCP December 2003

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/

Floyd Experimental [Page 33] RFC 3649 HighSpeed TCP December 2003

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
 English.
 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.

Floyd Experimental [Page 34]

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