Subject: [rohc] TCP profile packet formats...

From: Mark West <[email-masked]>
Date: Tue, 30 Aug 2005 21:36:45 +0000 (GMT)

Hi all,

Hopefully it is only slightly unusual to have last-call comments from an
author of the draft.  However (confession time!), I haven't stepped back
and looked at the packet formats in detail for some time.  So having gone
through then, I thought that I'd try and pick up on a couple of
inconsistencies before anyone else does...

These comments, therefore, relate solely to the construction of the packet
formats.

* co_format_co_common

We had a cunning idea, based on TCP flag usage, that only 2 bits were
required to convey the RST, SYN and FIN flags, since certain combinations
were unlikely to the point of impossibility.  Does it actually make sense
to retain this encoding in the 'common' packet format?  Or is it better to
explicitly carry all three flags (which, I note, means finding another bit
from somewhere)?

[and I'm really not sure myself, hence the question!]

* co_format_rnd_7 / co_format_rnd_9

Both of these are distinguished by the combination of a sequence number
change (scaled and non-) and a window change.  Window changes are
advertised from the receiver.  Window updates can clearly occur without an
ack number change.  However, I can't think of any obvious cases where a
window change will occur *with* a SN change and *not* an ack change.
Format 8 neatly covers the (unscaled) ACK + window change.  I suggest
changing the scaled SN packet to cover the scaled ACK case, and losing the
unnecessary unscaled SN.

[this situation is also reflected in the non-random IP-ID formats]

My feeling is that, overall, we have a pragmatic set of formats that cover
TCP usage pretty well [many thanks to all my co-authors and especially
Kris for this!].  The above points are all that struck me on a more
thorough re-read and, of those, the latter one is the only substantive
comment.  (Obviously I await with interest the feelings of those more
experienced in the ways of TCP!)

Cheers,

Mark.

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