TCP fires off blocks to its partner at the far end of the connection. If it doesn't receive an acknowledge- ment in a reasonable amount of time it retransmits the blocks. The determination of what is reasonable is done by TCP's retransmission algorithm. There is no correct algorithm but some are better than others, where better is measured by the number of retransmis- sions done unnecessarily. BSD 4.2 had a retransmission algorithm which retransmitted quickly and often. This is exactly what you would want if you had a bunch of machines on an ethernet (a low delay network of large bandwidth). If you have a network of relatively longer delay and scarce bandwidth (e.g. 56kb lines), it tends to retransmit too aggressively. Therefore, it makes the networks and gateways pass more traffic than is really necessary for a given conversation. Retransmis- sion algorithms do adapt to the delay of the network
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after a few packets, but 4.2's adapts slowly in delay situations. BSD 4.3 does a lot better and tries to do the best for both worlds. It fires off a few retransmissions really quickly assuming it is on a low delay network, and then backs off very quickly. It also allows the delay to be about 4 minutes before it gives up and declares the connection broken.
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Appendix A
References to Remedial Information
Quaterman and Hoskins, "Notable Computer Networks",
Communications of the ACM, Vol 29, #10, pp. 932-971
(October, 1986).
Tannenbaum, Andrew S., Computer Networks, Prentice
Hall, 1981.
Hedrick, Chuck, Introduction to the Internet Protocols,
Anonymous FTP from topaz.rutgers.edu, directory
pub/tcp-ip-docs, file tcp-ip-intro.doc.
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Appendix B
List of Major RFCs
RFC-768 User Datagram Protocol (UDP)
RFC-791 Internet Protocol (IP)
RFC-792 Internet Control Message Protocol (ICMP)
RFC-793 Transmission Control Protocol (TCP)
RFC-821 Simple Mail Transfer Protocol (SMTP)
RFC-822 Standard for the Format of ARPA Internet Text Messages
RFC-854 Telnet Protocol
RFC-917 * Internet Subnets
RFC-919 * Broadcasting Internet Datagrams
RFC-922 * Broadcasting Internet Datagrams in the Presence of Subnets
RFC-940 * Toward an Internet Standard Scheme for Subnetting
RFC-947 * Multi-network Broadcasting within the Internet
RFC-950 * Internet Standard Subnetting Procedure
RFC-959 File Transfer Protocol (FTP)
RFC-966 * Host Groups: A Multicast Extension to the Internet Protocol
RFC-988 * Host Extensions for IP Multicasting
RFC-997 * Internet Numbers
RFC-1010 * Assigned Numbers
RFC-1011 * Official ARPA-Internet Protocols