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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.dcom,net.lan
Subject: Re: Re: Standards for commercial packet radio
Message-ID: <481@petrus.UUCP>
Date: Mon, 26-Aug-85 13:54:44 EDT
Article-I.D.: petrus.481
Posted: Mon Aug 26 13:54:44 1985
Date-Received: Tue, 27-Aug-85 06:41:16 EDT
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> As evidence to support this, they point to the fact that in 1980,
> X.25 had a datagram facility, but by 1984 nobody had implemented it,
> so it was deleted from the 1984 spec.

Some might consider this a cheap shot, but in my opinion the X.25 datagram
facility was such a brain-damaged afterthought that it never had a fair
chance in the first place. Worse, many of the PDNs out there assume at a
very low level that they're only providing virtual circuit service (e.g.,
Telenet, TYMNET) so implementing a datagram service would essentially
require the overhead of setting up and tearing down a virtual circuit on
each and every datagram.

> I have no idea how you are
> supposed to get through a virtual circuit oriented X.25 bottleneck to
> send IP packets out over the ARPANET, this must make an interesting story.

We are one of the relative handful of sites running the CSNET IP/X.25
interface, so I can describe it and our experiences in some detail. RFC-877
specifies a standard for the transmission of IP datagrams over X.25 virtual
circuits. Virtual circuits are established whenever there are datagrams to
send, and timed out after periods of inactivity.  "X.25 bottleneck" is
highly appropriate; because of the tiny packet size limits and flow control
windows, it is often necessary to open several parallel virtual circuits to
the same destination and multiplex datagrams among them, just so the
bandwidth of our 9600 baud "local loop" can be fully utilized.  Even then,
we're lucky to get 4800 baud left over for IP after all the redundant X.25
link level acks, packet level acks and so forth.

And the CCITT bigots claim that TCP/IP has too much "overhead".

Phil