Subject: Value of Cumulative TSN in SHUTDOWN without DATA transmitted

From: Dirk Tepelmann <[email-masked]>
Date: Mon, 12 Jan 2009 12:46:45 +0100

Hello,

in studying differences between RFC2960 and RFC4960 in using RFC4460 as 
well an issue raised on my side concerning the cumulative TSN ack value.
I could see that the handling in SACK(chapter 3.3.4) was changed:
old text: "This parameter contains the TSN of the last DATA chunk 
received in sequence before a gap.
new text: "This parameter contains the TSN of the last DATA chunk 
received in sequence before a gap. In case where no DATA has been 
received, this value is set to the peer's Initial TSN minus one."

I'm wondering how the correct behaviour should be in other cases.
E.g. SCTP association is established(remote side sent INIT) and no DATA 
were exchanged, the local side will send a SHUTDOWN. What is the correct 
value for the cumulative TSN?

In both RFC's it's stated: "This parameter contains the TSN of the last 
received chunk in sequence before any gap."

Shall the SHUTDOWN contain the Initial TSN, or minus one? And why there 
is the difference in handling of the cumulative TSN between SACK and 
SHUTDOWN?

Best regards,
Dirk

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Mon, 12 Jan 2009 05:15:31 -0700

Dirk,

There is no actual change in behaviour between 2960 and 4960.
RFC 4960 just clarifies the case you mention.  When an association
is started the cummulative ack value is set to the peer's initial
TSN minus one (modulo 2^32) and that is where it will stay until
a data chunk with the initial TSN value is acknowledged.  So the
SHUTDOWN (and any intervening SACK) in your case has the initial
TSN minus one as its value.  Did for RFC 2960, does for RFC 4960.

--brian

-- 
Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
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