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From: barns@GATEWAY.MITRE.ORG
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Question on TCP use of IP TOS
Message-ID: <9101222113.AA17980@gateway.mitre.org>
Date: 22 Jan 91 21:13:18 GMT
References: <1991Jan21.200559.1426@ultra.com>
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Perhaps I can call myself at least a quasi-expert?  Well, nothing
says TOS is negotiated, except for the precedence, for which a
negotiation is clearly described in the protocol spec.  I think
one can reasonably infer that each end does what it wants with
the delay/throughput/reliability bits without regard to the other end.

You probably already know that Host Requirements (RFCs 1122/1123)
says to use TOS in accordance with recommendations in Assigned
Numbers.  If both ends did this, there should at least not be
randomness, and under current recommendations, I believe there
is no asymmetry.  However, setting low delay on naked ACKs was
discussed, and John Lekashman said it was a good thing and made
performance better for his (NASA?) situation.  The case in question
was a high-bandwidth satellite path and a low-bandwidth terrestrial
path.  RTT was a bottleneck and finagling the TOS in this way
allowed more data into the satellite pipe.  Or something like that.
I'd call it legal, possibly a desirable thing, but not common
practice in vanilla software as far as I know.

Bill Barns / MITRE-Washington / barns@gateway.mitre.org