APC number allocation
Reddy, Paul K
paul.k.reddy at INTEL.COM
Thu May 4 01:09:42 EDT 2000
Archana, et al,
In addition to these messages, there are other messages where, if one was
lost, there would be issues, including:
communication mode command
terminal capability set (including TCS=0)
end session command
Certainly, there are others and if those commands are lost a queue, as you
suggest, the call may become very unstable.
In addition to the H.245 commands above, any H.225.0 signaling may be an
issue, too. I believe people have stated that the H.450.x series protocols
have robustness designed within the protocol, but I have not looked for
issues. For example, if a particular ReturnResult message is lost, what
state would things be in?
Other services are built upon H.225.0, as well, including Annex K and Annex
L. In addition, we are now relying on the H.225.0 layer to tunnel other
protocols (ISUP and QSIG) as described in Annexes M.1 and M.2. There is
really no way to define what the state of the call might be in if any of
those messages are lost.
----- Original Message -----
From: "Archana Nehru" <archie at TRILLIUM.COM>
To: <ITU-SG16 at mailbag.cps.intel.com>
Sent: Wednesday, May 03, 2000 8:41 PM
Subject: H.323 Robustness
> hi all,
> This is following our discussion in the last teleconference. In the last
> discussion we wanted answers to the following questions:
> a) do we need "call-state synchronisation" between the two legs
> of a call when an intermediate GK fails? Is it ok to assume
> that things will sort themselves out without any significant
> b) Are there any cases where absence of "call-synchronisation"
> procedures can lead to hung resources or call-release
> if any) of a stable call?
> c) Are there any other issues(e.g: degraded service) that we
> need to take care of in the absence of "call-synchronisation"?
> d) if the answer to b)is yes, then do we need an ACK at the H.323
> layer if H.323 layer runs over SCTP/DDP?
> After the teleconference, we went back to do some study and discussions
> Randy and Qiaobing and here is a summary of that:
> a) While SCTP/DDP provides fault tolerance at the transport layer,
> it cannot handle the case where a GK fails after the message
> is ACKed at the SCTP layer of the GK. So in
> a case like:
> (CRASH) RELCOMPLETE
> EP2 <-------------- GK <------------ EP1
> (SCTP/DDP) (SCTP/DDP) (SCTP/DDP)
> NODE FAILS)
> when EP1 sends a RELCOMPLETE message to the GK, the SCTP/DDP sends an
> SCTP-ACK to the EP1 and if the GK node fails after this step, then the
> RELCOMPLETE message is lost. SCTP/DDP layer cannot detect
> such failures and therefore it is upto the H.323 protocol layer
> to recover from it (if required).
> So, we agreed that messages at the GK can get lost even with SCTP/DDP.
> Please note that it implies that in a normal GK implementation, the H.323
> layer will probably use a "queue" to exchange messages with the SCTP/DDP
> layer. The SCTP/DDP will put all those messages in this "queue" for which
> has sent an SCTP-ACK. Therefore when the H.323 layer fails, we lose all
> messages that were present in the "queue" and these messages may belong to
> multiple calls.
> In other words, failure of the H.323 layer is not trivial, since it
> mean loss of just one message belonging to that "one particular call" that
> was being processed at the time of the failure. It means the loss of all
> those messages that were present in the "queue" at the time of the failure
> which may belong to multiple calls.
> Having said this, we wanted to identify the impact of the lost messages at
> the GK and at the endpoints. I am enclosing a table of some of the
> messages that can be lost and what they might potentially translate to.
> please note that this tabel is not exhaustive . As of now, the current
> specs does not talk
> about the action that should be taken if for a particular
> command/indication, the terminal doesnot respond as desired. I guess the
> assumption is that the message is delivered reliably.
> We would like to discuss the issues listed in the tables with the group to
> get an idea of how current implementations behave if these messages are
> lost. Depending on the general consensus, we can conclude whether or not
> ACK should be introduced.
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