Subject: RE: [Tsvwg] Re: U-SCTP comments

From: [email-masked]
Date: Wed, 14 Mar 2001 14:33:09 +0200

> I agree.  If we are going to go to the trouble of parsing the whole
> packet before checksumming, then there is no percentage in including
> the padding bytes.
> 
> I'm a little concerned about parsing the packet based on data that has
> not (yet) been confirmed by checksum.  At the very least it will
> require some very careful thought.  Invalid chunk types and invalid
> lengths are two problems that leap to mind.
> 
> Is there are fast way to detect packets with P-DATA chunks in them?
> I'd really rather not pollute my checksum fast-path with them.

	What about using with them the same restriction as with COOKIE ECHO
and COOKIE ACK? I will propose that in case a datagram carries any P-DATA,
the first chunk in the datagram should be a P-DATA chunk... So we should
just look for the value 0xc3 in the first two bytes after the header (bytes
13th and 14th)... It would be a quickly check, and in case that number is
there, we should make the parsing before verifying if the checksum is
correct, otherwise not...

	This can bring up some problems, since the DATA chunks (and I guess
that the P-DATA chunks as well, that's what logically I would do, but it is
not explicitly stated... Maybe the end of section 5 would be the best place
to state it...) must be inside the datagram in ascending order of TSNs (RFC
2960, section 6.10, third paragraph), so, in case there are DATA and P-DATA
chunks mixed in the same datagram, the first P-DATA should have a TSN
previous to any other TSN in that datagram... This would affect the way we
bundle the chunks inside a datagram... nothing that couldn't be done...
Also, if an application makes use of these P-DATA chunks, maybe it won't
even use the DATA chunks, so this restriction won't even been noticed...

	Any comments about this? Thanks!

	BR Iván Arias Rodríguez

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From: "La Monte Henry Piggy Yarroll" <[email-masked]>
Date: Thu, 15 Mar 2001 07:57:24 -0600

[email-masked] writes:
> > ["La Monte Henry Piggy Yarroll" <[email-masked]> wrote:]

I like this solution.  I was going to propose a P-DATA-PRESENT control
chunk that would have to be the first chunk in the packet, but just
saying that you can't bundle P-DATA with control chunks (AND the first
P-DATA must be the first chunk) accomplishes much the same thing
without a new control chunk.  Hmm... that DOES mean most of your SACKs
in an unreliable association would have to travel stand-alone.