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From: tasman@CS.WISC.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP Urgent Pointer *Standard*
Message-ID: <9102130158.AA04006@hal.cs.wisc.edu>
Date: 13 Feb 91 01:58:26 GMT
References: <1991Feb11.205402.12401@b11.ingr.com>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 34

Ed Sale writes:

> My question boils down to this:
>
>	If a TCP segment is carrying just one byte of data, and that
>	one byte is urgent data, what value should the TCP header's
>	Urgent Pointer field contain, 0 or 1? (In a *STANDARD* TCP.)
> 
> Also, in my experience with many TCP implementations, there is
> apparently confusion about the value that the Urgent Pointer should
> have, as they do not appear to all be setting this field the same way.
> I guess even if I choose to implement *STANDARD* Urgent Pointer
> processing I'm still not going to be able to converse effectively
> with every TCP implementation out there :-(.


     A brief answer to Ed's question is that the Urgent Pointer field should
contain 0.

     For a more detailed treatment of the philosophy and implementation of TCP
Urgent, you may wish to read an article that I wrote last summer:

	Tasman, M., "Telnet Output Discard Processing,"
	ConneXions: The Interoperability Report, Volume 4, No. 6, June 1990.

     A footnote to the article addresses the compatibility issue.  Due to
the specification confusion concerning TCP Urgent, some early implementations
contained an "off by one" calculation error.  Rather than alter the behavior
of their TCP implementation, at least one supplier chose instead to compensate
in the application programs.  The preceding will make more sense if you
read the article.

					Mitchell Tasman
					UW-Madison Computer Sciences Dept.