Path: utzoo!attcan!uunet!van-bc!cynic!curt From: curt@cynic.wimsey.bc.ca (Curt Sampson) Newsgroups: comp.mail.uucp Subject: Re: What should a new UUCP protocol do? Message-ID: Date: 11 Nov 90 11:01:19 GMT References: <+2m8g2.lb1@smurf.sub.org> Organization: Mad Artists' Technological Hangout, Vancouver, B.C. Lines: 48 urlichs@smurf.sub.org (Matthias Urlichs) writes: > Actually, it might be sufficient to NAK packets if there's an error but to > keep quiet otherwise. Using UUCP, this is OK because you're transmitting a > file and the sender can always back up as far as the receiver wants it to. > [...] > < scheme where packets are numbered modulo 'n', the maximum transmit window > < size for a retransmit-entire-window protocol like 'g' is n-1, but for a > < selective reject protocol it is (n/2)-1. > < > It might be a better idea to just use the offset into the file you're > transmitting. That way you wouldn't need to worry about windowing and the > machines involved could agree on these things on a dynamic basis. This is starting to look more and more like zmodem by the minute. Zmodem protocol returns NAKs only. One of the advantages of this scheme is that if you're using a half duplex link there will be no turnarounds until you actually have an error. Zmodem does use blocks of course, because you have to block it to be able to calculate the checksum and the like. It uses adaptive block sizing, though. The better implementations start with 256 byte blocks. They will move down as far as 32 byte blocks (on a very noisy line) or up as far as 8K blocks at 9600 or higher BPS. It can also send a short block of any length to finish off the file. Zmodem also has provisions for a restart on an interupted transfer (i.e., when you hang up or get hung up upon for whatever reason and then call back). It's just like a NAK in the transfer, actually. You just hand it a byte offset into the file and tell it to go. > One problem that you're not going to cure with this is that if you have a > full duplex link, about half the bandwidth is wasted. Fixing this would > probably require major mods to the uucico's involved. My copy of the uucp protocol description seemed to imply rather strongly that it would be possible to have a uucp session going in each direction. Obviously, the ones currently in use don't do this. It would be very nice to have, though. There is a protocol out there call Janus, I believe, that supports this. cjs curt@cynic.UUCP | "The unconscious self is the real genius. curt@cynic.wimsey.bc.ca | Your breathing goes wrong the minute your {uunet|ubc-cs}!van-bc!cynic!curt | conscious self meddles with it." --GBS