Path: utzoo!utgpu!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!bloom-beacon!bu-cs!kwe
From: kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England))
Newsgroups: comp.sys.proteon
Subject: Re: TCP & UDP over PRONET80
Message-ID: <26687@bu-cs.BU.EDU>
Date: 15 Dec 88 16:35:43 GMT
References: <3756@jpl-devvax.JPL.NASA.GOV>
Reply-To: kwe@buit13.bu.edu (Kent England)
Followup-To: comp.sys.proteon
Organization: Boston Univ. Information Tech. Dept.
Lines: 42

In article <3756@jpl-devvax.JPL.NASA.GOV> 
mike@jpl-devvax.JPL.NASA.GOV (Mike Tankenson) writes:
>
>Question 1: How is UDP over Token Ring different than UDP over Ethernet?
>Here we're interested in collisions, checksum errors, slow receivers,
>retransmissions (built into the driver?).  I know collisions are eliminated
>with Token Ring -- I'm interested in basic UDP behavior.

	There are no collisions on Token Ring, but the datagram will
still have a variable waiting time to get on the channel.
	You could overrun a buffer on a Pronet-80 node, just like
could happen on Ethernet.  So you don't find much use for Pronet-80
interfaces in PC-XTs.  :-)
	Checksum errors and retransmissions.  Hmmm.  Pronet-80 doesn't
use CRC checking on the datagram like Ethernet does.  It encodes each
4-bit "character" into a 6-bit symbol.  Supposed to be better than CRC
in detecting single bit errors, but I wonder about the differences.
Can anyone explain the differences between Pronet-80 error detection
and Ethernet error detection?
	NFS runs UDP with no checksum.   Works fine on Ethernet.  Work
fine on Pronet-80?

>
>PS: I have spoken to Proteon on this, and they basically said that UDP is
>slightly more reliable on Token Ring (no collisions, etc), but that there
>is still NO GUARANTEE that UDP packets will ever get delivered.  Am I off
>base here?

	Pronet-80 is like Ethernet in that it *should* detect an error
in a frame (at least a single bit error) but it will drop the packet.
There is no buffering and retransmission in Ethernet.  I think there
is not any in Pronet-80, although there is link-level acknowledgement
in some token rings that would allow link-level retransmission.  I'm
not sure that that feature is used in any token ring implementations.
It's left for higher layers, which is reasonable given the low bit
error rates of most modern media.

	Collisions in CSMA/CD are not errors.  Frames that collide are
retransmitted by the MAC layer.  Bit errors are detected and frames
dropped.  That's why Ethernet is "unreliable".

	Kent England, Boston University