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
