Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!rutgers!sri-spam!ames!sdcsvax!ucsdhub!hp-sdd!hplabs!ucbvax!G.BBN.COM!WESTCOTT From: WESTCOTT@G.BBN.COM Newsgroups: comp.protocols.tcp-ip Subject: Re: TCP and Loss (inherently lossy nets) Message-ID: <[G.BBN.COM].7-Oct-87.14:58:41.WESTCOTT> Date: Wed, 7-Oct-87 14:58:00 EDT Article-I.D.: <[G.BBN.COM].7-Oct-87.14:58:41.WESTCOTT> Posted: Wed Oct 7 14:58:00 1987 Date-Received: Sat, 10-Oct-87 11:08:15 EDT References: Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 34 Mike, Hop by hop retransmissions are necessary in packet radio networks because PRs must transmit over links with relatively low probability of success. Imagine a 5 hop path with a probability of 80% success per hop (~30% over net). When that is coupled with congestion and other problems along a longer Internet route, then it is extremely difficult and costly (in terms of packet transmissions) to attempt to maintain a reliable connection. Presently, PR nets also use the timing information from their hop acknowledgements to apply backpressure for congestion control; its known as the pacing algorithm. The reasoning is that rather than fill all the buffers in a data path, limit the packets forwarded to what the next PR can handle. If congestion appears on the far side of the net, traffic generation may be slowed by increasing the pacing delays all the way back to the source. Smart sources slow down, dumb sources get their packets dropped by the source's attached PR. Hop by hop retransmission is limited to several attempts along the routing path and a few attempts directed to "any PR who can route this packet toward the destination". The later is known as alternate routing and becomes most useful when you've lost connectivity with your neighbor (perhaps its driven under a bridge or behind a building). Because of the uncertainty of successful retransmission, PR nets count on a reliable end-to-end protocol as well. One way to look at your question is that PRnets, with per hop acknowledgements, approach the non-lossy nets in probability of successful transmissions. I agree with your thought that due to "congestion => dropped packets" behavior, simply using hop by hop acknowledgements seems fruitless. Jil