RFC 1072 (rfc1072) - Page 1 of 16

TCP extensions for long-delay paths

Network Working Group                                        V. Jacobson
Request for Comments: 1072                                           LBL
                                                               R. Braden
                                                                     ISI
                                                            October 1988


                  TCP Extensions for Long-Delay Paths


Status of This Memo

   This memo proposes a set of extensions to the TCP protocol to provide
   efficient operation over a path with a high bandwidth*delay product.
   These extensions are not proposed as an Internet standard at this
   time.  Instead, they are intended as a basis for further
   experimentation and research on transport protocol performance.
   Distribution of this memo is unlimited.

1. INTRODUCTION

   Recent work on TCP performance has shown that TCP can work well over
   a variety of Internet paths, ranging from 800 Mbit/sec I/O channels
   to 300 bit/sec dial-up modems [Jacobson88].  However, there is still
   a fundamental TCP performance bottleneck for one transmission regime:
   paths with high bandwidth and long round-trip delays.  The
   significant parameter is the product of bandwidth (bits per second)
   and round-trip delay (RTT in seconds); this product is the number of
   bits it takes to "fill the pipe", i.e., the amount of unacknowledged
   data that TCP must handle in order to keep the pipeline full.  TCP
   performance problems arise when this product is large, e.g.,
   significantly exceeds 10**5 bits.  We will refer to an Internet path
   operating in this region as a "long, fat pipe", and a network
   containing this path as an "LFN" (pronounced "elephan(t)").

   High-capacity packet satellite channels (e.g., DARPA's Wideband Net)
   are LFN's.  For example, a T1-speed satellite channel has a
   bandwidth*delay product of 10**6 bits or more; this corresponds to
   100 outstanding TCP segments of 1200 bytes each!  Proposed future
   terrestrial fiber-optical paths will also fall into the LFN class;
   for example, a cross-country delay of 30 ms at a DS3 bandwidth
   (45Mbps) also exceeds 10**6 bits.

   Clever algorithms alone will not give us good TCP performance over
   LFN's; it will be necessary to actually extend the protocol.  This
   RFC proposes a set of TCP extensions for this purpose.

   There are three fundamental problems with the current TCP over LFN



Jacobson & Braden