TCP extensions for long-delay paths
RFC 1072

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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                                               [Page 1]
RFC 1072          TCP Extensions for Long-Delay Paths       October 1988

   paths:

   (1)  Window Size Limitation

        The TCP header uses a 16 bit field to report the receive window
        size to the sender.  Therefore, the largest window that can be
        used is 2**16 = 65K bytes.  (In practice, some TCP
        implementations will "break" for windows exceeding 2**15,
        because of their failure to do unsigned arithmetic).

        To circumvent this problem, we propose a new TCP option to allow
        windows larger than 2**16. This option will define an implicit
        scale factor, to be used to multiply the window size value found
        in a TCP header to obtain the true window size.

   (2)  Cumulative Acknowledgments

        Any packet losses in an LFN can have a catastrophic effect on
        throughput.  This effect is exaggerated by the simple cumulative
        acknowledgment of TCP.  Whenever a segment is lost, the
        transmitting TCP will (eventually) time out and retransmit the
        missing segment. However, the sending TCP has no information
        about segments that may have reached the receiver and been
        queued because they were not at the left window edge, so it may
        be forced to retransmit these segments unnecessarily.

        We propose a TCP extension to implement selective
        acknowledgements.  By sending selective acknowledgments, the
        receiver of data can inform the sender about all segments that
        have arrived successfully, so the sender need retransmit only
        the segments that have actually been lost.

        Selective acknowledgments have been included in a number of
        experimental Internet protocols -- VMTP [Cheriton88], NETBLT
        [Clark87], and RDP [Velten84].  There is some empirical evidence
        in favor of selective acknowledgments -- simple experiments with
        RDP have shown that disabling the selective acknowlegment
        facility greatly increases the number of retransmitted segments
        over a lossy, high-delay Internet path [Partridge87].  A
        simulation study of a simple form of selective acknowledgments
        added to the ISO transport protocol TP4 also showed promise of
        performance improvement [NBS85].

Jacobson & Braden                                               [Page 2]
RFC 1072          TCP Extensions for Long-Delay Paths       October 1988

   (3)  Round Trip Timing