Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!udel!gatech!hubcap!ncrcae!ncr-sd!crash!simpact!jeh
From: jeh@simpact.com
Newsgroups: comp.mail.uucp
Subject: Re: New UUCP Protocol (was: Re: Zmodem added to UUCP)
Message-ID: <690.25315ee6@simpact.com>
Date: 10 Oct 89 10:15:50 GMT
References: <672.2530C0D4@mudos.ann-arbor.mi.us>
Organization: Simpact Associates, San Diego CA
Lines: 37

In article <672.2530C0D4@mudos.ann-arbor.mi.us>, mju@mudos.ann-arbor.mi.us
 (Marc Unangst) writes:
> In article <1989Oct9.003603.3693@esegue.segue.boston.ma.us>,
 johnl@esegue.segue.boston.ma.us (John R. Levine) writes:
>  >PEP, though, is really a
>  >half-duplex protocol, and fakes full-duplex by turning the line around.  That
>  >means that the zmodem ACK packets cause a performance hit and spoofing would
>  >still be a win.
> 
> Sigh.  This is why we want Zmodem -- Zmodem is a streaming ACKless protocol.
> The only time a receiving Zmodem sends anything back to the sender is if
> it gets a bad block, or at the end of each file.
> 

This may be a concern for other types of modems, but not for Telebits in 
PEP/uucp spoofing mode.  The uucp acks and naks do NOT get sent end-to-end;
the modems do their own ACKs, NAKs, and retransmission; a few of the PEP 
channels are reserved for "supervisory data" in both directions, and so line
turnaround is not required. 

ZMODEM fans take note:  I never said that it wouldn't be worthwhile to do 
ZMODEM under uucp.  I expect that most uucp sites wouldn't find it all that
beneficial (perhaps for reasons having nothing to do with technical merit), 
but if it'll solve some problems for some, go with it!  That's why uucp 
supports multiple protocol options!  

Only, please make sure that the problems and solutions are real... preferably
by experiment and measurement rather than theorizing.  (Note that ZMODEM
performance with dedicated systems, ie when at least one end is a PC, is not
necessarily indicative of what will happen when both ends are multiuser
timesharing systems.)  As I've said, I have my doubts that ZMODEM will be
all that big an improvement, but I'm just theorizing myself... 

	--- Jamie Hanrahan, Simpact Associates, San Diego CA
Chair, VMSnet [DECUS uucp] and Internals Working Groups, DECUS VAX Systems SIG 
Internet:  jeh@simpact.com, or if that fails, jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!jeh