Subject: Re: Window Scale option

From: Jason Thorpe <[email-masked]>
Date: Wed, 06 May 1998 00:21:36 -0700

On Tue, 5 May 1998 12:12:53 -0700 
 [email-masked] wrote:

 > I would not call it a "mindset", I would call it concern about
 > (possible lack of) interoperability.  However, the issue of breaking
 > some TCPs with options on non-SYN segments may well be less of a
 > concern than it was 8 years ago.

Unfortunately, there's still the concern of intermediate agents (e.g.
routers, terminal servers, etc.) completely losing if they encounter
non-SYN packets with options.

It's not too hard to find recent accounts of people having to disable
timestamps because their or their ISP's terminal server or ISDN gateway
can't cope with them.  (Presumably, they're looking that far into the
packet because they're trying to do header compression.)

Jason R. Thorpe                                       [email-masked]
NASA Ames Research Center                            Home: +1 408 866 1912
NAS: M/S 258-5                                       Work: +1 650 604 0935
Moffett Field, CA 94035                             Pager: +1 415 428 6939

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From: Matt Mathis <[email-masked]>
Date: 06 May 1998 13:05:58 -0400

>non-SYN packets with options.

We ran into this as well, and your diagnosis isn't quite correct.
There are a number of VJ header compression implementations that
completely and totally botch *all* TCP or IP options.  But under VJ
compression some packets such as SYNs and retransmissions (data which
is non-contigious with the prior segment) are not compressed, so they
make it through.  These implementations are just busted because they
are incorrectly compressing packets containing options.

This situation is precisely what started me thinking about initiating
timestamps later in the connection.  These paths are the very ones
that benefit the least and can least afford the overhead of
timestamps.  Under 1323, the TCP has to know in advance of the SYNACK
if timestamps are a gain or loss, and negotiate accordingly.  This is
way too early, long before TCP has had any opportunity to measure
anything useful about the path.

--MM--