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From: jqj@HOGG.CC.UOREGON.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Reliable Datagram ??? Protocols
Message-ID: <9010231706.AA25446@hogg.cc.uoregon.edu>
Date: 23 Oct 90 17:06:28 GMT
References: <9010231522.AA18327@ftp.com>
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I think we are being a bit ingenuous in assuming that the only reason one
might want "reliable datagrams" is to implement a sequenced packet protocol,
or that the only good way to get reliable packet delivery over IP is by using
TCP.  The current discussion does not, for example, address reliable broadcast
or any of a miriad of other real transaction-oriented applications for
reliable packet exchange where the cost of setting up a VC is prohibitive.
I would even go so far as to suggest that the lack of a standard RPX protocol
in the IP suite has inhibited development of reasonable applications that
would use it!

As an aside, 4.2bsd included Eric Cooper's courier compiler that implemented a
Xerox SPP-like protocol over TCP (message boundaries and message types are 
needed to implement the semantics of Courier).  As I recall, Eric just used
a simple counted string approach, where messages could span multiple strings
and end was delimited by an EOM bit in the message header, totally ignoring IP
packet boundaries, PUSHes, etc. (you have to guarantee a PUSH after the EOM,
I suppose).  Worked just fine if what you wanted was packet boundaries on TCP;
no changes to TCP needed.