RFC 1072 (rfc1072) - Page 2 of 16

TCP extensions for long-delay paths

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