Path: utzoo!attcan!uunet!know!sdd.hp.com!ucsd!ucbvax!MATHOM.CISCO.COM!BILLW
From: BILLW@MATHOM.CISCO.COM (William "Chops" Westfield)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Reliable Datagram ??? Protocols
Message-ID: <12632005207.2.BILLW@mathom.cisco.com>
Date: 23 Oct 90 06:37:02 GMT
References: <9010230140.AA07826@WLV.IMSD.CONTEL.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 29


    I may have missed the point but doesn't a PUSH accomplish the same thing?
    [make TCP a datagram-oriented protocol]

Well, no.  A major part that is missing is a specification for the
interface between TCP and the next layer up (in ISO, this would likely
be a whole separtate document.)  In particular, if a receiver gets
two packets with PUSH set, the interface may put both packets in a
single buffer.  To quote RFC793:

    The exact push point might not be visible to the receiving user and
    the push function does not supply a record boundary marker.

Also, on the sender side, push does not preclude use of algorithms such
as slow start, Nagle, or re-packetization on retransmit.  (Hopefully,
a system using a datagram oriented protocol does not involve situations
where these are important (well, slow start would still be useful - you
just have to do it with datagrams rather than stream data))

Finally, TCP as is will send many datagrams if you present more than a
packet-sizes worth of data.  For a datagram oriented system, you would
force it to send a fragmented IP packets instead (and the maximum
segment size would have a slightly different meaning.)

The changes to any particular TCP to achieve a reliable datagram model
would not be significant, but it would take a little work.

BillW
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