RFC 1323 (rfc1323) - Page 2 of 37

TCP Extensions for High Performance

RFC 1323          TCP Extensions for High Performance           May 1992


   Authors' Addresses ............................................... 37

1. INTRODUCTION

   The TCP protocol [Postel81] was designed to operate reliably over
   almost any transmission medium regardless of transmission rate,
   delay, corruption, duplication, or reordering of segments.
   Production TCP implementations currently adapt to transfer rates in
   the range of 100 bps to 10**7 bps and round-trip delays in the range
   1 ms to 100 seconds.  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 [Jacobson88a].

   The introduction of fiber optics is resulting in ever-higher
   transmission speeds, and the fastest paths are moving out of the
   domain for which TCP was originally engineered.  This memo defines a
   set of modest extensions to TCP to extend the domain of its
   application to match this increasing network capability.  It is based
   upon and obsoletes RFC-1072 [Jacobson88b] and RFC-1185 [Jacobson90b].

   There is no one-line answer to the question: "How fast can TCP go?".
   There are two separate kinds of issues, performance and reliability,
   and each depends upon different parameters.  We discuss each in turn.

   1.1  TCP Performance

      TCP performance depends not upon the transfer rate itself, but
      rather upon the product of the transfer rate and the round-trip
      delay.  This "bandwidth*delay product" measures the amount of data
      that would "fill the pipe"; it is the buffer space required at
      sender and receiver to obtain maximum throughput on the TCP
      connection over the path, i.e., the amount of unacknowledged data
      that TCP must handle in order to keep the pipeline full.  TCP
      performance problems arise when the bandwidth*delay product is
      large.  We 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 DS1-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.  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.

      There are three fundamental performance problems with the current
      TCP over LFN paths:



Jacobson, Braden, & Borman