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To: Archie Cobbs <archie@whistle.com>
cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: VJ compression and MAX_HDR 
In-reply-to: Your message of "Sun, 06 Jul 1997 16:56:47 PDT."
             <199707062356.QAA13745@bubba.whistle.com> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Sun, 06 Jul 1997 17:53:37 -0700
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>> This means that when Van Jacobson compression is active, any packets
>> sent to us with combined IP and TCP header length greater than MLEN
>> will be dropped, always.
>> 
>> This is an admittedly rare case, but my question is, why was this
>> done? What code would break if MAX_HDR is changed back to 128?
...
>After further review, it looks like the genesis of this change is
>the fact that sl_compress_tcp() requires the full TCP/IP headers
>to lie within the first mbuf, and sl_uncompress_tcp() requires the
>TCP/IP header (in the TYPE_UNCOMPRESSED_TCP case) to also be in a
>contiguous buffer...
>
>Plus the fact that m_pullup() won't work for values greater than MHLEN
>doesn't help.

   I think the case of IP+TCP header > MLEN is more than rare...I think it 
never happens. Even with all options, the IP header doesn't exceed 64 bytes.

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project