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

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

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

Status of This Memo

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

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

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

   TCP-Friendly Rate Control (TFRC) is a congestion control mechanism
   for unicast flows operating in a best-effort Internet environment
   (RFC 3448).  TFRC was intended for applications that use a fixed
   packet size, and was designed to be reasonably fair when competing
   for bandwidth with TCP connections using the same packet size.  This
   document proposes TFRC-SP, a Small-Packet (SP) variant of TFRC, that
   is designed for applications that send small packets.  The design
   goal for TFRC-SP is to achieve the same bandwidth in bps (bits per
   second) as a TCP flow using packets of up to 1500 bytes.  TFRC-SP
   enforces a minimum interval of 10 ms between data packets to prevent
   a single flow from sending small packets arbitrarily frequently.

   Flows using TFRC-SP compete reasonably fairly with large-packet TCP
   and TFRC flows in environments where large-packet flows and small-
   packet flows experience similar packet drop rates.  However, in
   environments where small-packet flows experience lower packet drop
   rates than large-packet flows (e.g., with Drop-Tail queues in units
   of bytes), TFRC-SP can receive considerably more than its share of
   the bandwidth.

Floyd & Kohler                Experimental                      [Page 1]
RFC 4828                  TFRC: The SP Variant                April 2007

Table of Contents

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

Floyd & Kohler                Experimental                      [Page 2]
RFC 4828                  TFRC: The SP Variant                April 2007

1.  Introduction

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

   TFRC was designed to be reasonably fair when competing for bandwidth