HighSpeed TCP for Large Congestion Windows
RFC 3649

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