RFC 1072 (rfc1072) - Page 2 of 16
TCP extensions for long-delay paths
Alternative Format: Original Text Document
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