Request for APC document numbers

[Sakae OKUBO]

Archana,

The ACK packet certainly follows the line of thinking that I had.  However,
one issue with implementing this approach is that there are no fields in
H.323 to provide a "sequence number".  I suppose this could easily be
overcome.

I want to change direction just a bit.  I have been pushing for Annex E, but
I got resistance: people want to be able to use TCP or another transport.  I
understand that desire.  People have also expressed concerns that we're not
clearly separating the transport and application layers.  To some extent,
that is true.  However, I believe that with Annex E, we can draw that line a
little clear-- but it means modifications to Annex E.  Another issue is that
some people have disagreed with modifying Annex E :-)

Here's a thought: how about introducing a new session layer between the
transport and below the H.323 application?  This session layer would
actually encode messages in a special format for transmission on the wire.
For example, for TCP, we could modify the TPKT header and the payload to
carry not only the H.323 message, but also sequencing data and other
information.  We can allow fail-over addresses to be carried in this layer
and allow this layer to completely handle connection recovery.

When robustness is not available end-to-end, this layer in the middle would
send and receive messages as normal.  However, when robustness is available
end to end, it would "get in the middle" and transparently handle robustness
issues for the H.323 application.

The goal, of course, is to minimize the application changes and to minimize
changes to the transports below.

Certainly, this is a shift from my previous thinking, but it represents an
attempt to address all of the concerns people have raised.  What is your
opinion on this type of approach?

Paul

----- Original Message -----
From: "Archana Nehru" <archie at trillium.com>
To: <ITU-SG16 at mailbag.cps.intel.com>
Cc: "Mahesh Bhan" <mbhan at trillium.com>; "Chinmay Padhye"
<chinmay at trillium.com>; "Archana Nehru" <archie at trillium.com>; "Tim Chen"
<scc at trillium.com>; "Jill Caugherty" <jcaugher at cisco.com>; "Rich K. Bowen"
<rkbowen at cisco.com>; "'Paul E. Jones'" <paul.jones at ties.itu.ch>
Sent: Tuesday, April 25, 2000 9:19 PM
Subject: Issue in H.323 robustness not addressed by SCTP/DDP


> Hello,
>
> We think that SCTP/DDP by itself is not a complete solution
> for robustness (see PROBLEM B below) and certain changes
> need to be made in the H.323 layer to achieve robustness.
> For the sake of clarity, we restate the issues we need to address in
> order to achieve robustness:
>
> In the current H.323 specs, if the TCP connection for a H.323 call goes
> down, the call is  lost. To overcome this problem, we need:
>
> A. Fail over mechanism
>
>    Whenever an endpoint detects that the other side is  down (e.g.: TCP
>    connection failure/ no ACKs received in Annex E) the endpoint can
>    save an active H.323 call, if it knows about a "recovery H.323
>    address".
>
>    The "recovery address" is the back-up address that the endpoint can
>    use  to re-establish a TCP connection (for TCP) or to resend Annex E
>    data (UDP). From the  endpoint's point of view, the "recovery address"
>    represents a node that has enough  information about the H.323 call
>    to continue processing as if the failure had never occurred
>
>    The failure in the node could have been one of the following types:
>
>    1. Transport failure:  e.g. failed NIC, congested network.
>
>    2. Node failure: e.g. the entire gatekeeper fail. In this case, we
>    need a synchronization mechanism between the gatekeeper and its
>    backup so the active calls can be saved.
>
> B. Handle Call State Synchronization
>    We need to make sure that both legs of a H.323 call are in sync. When
>    an intermediate node  (e.g. Gk) fails, messages from an endpoint can
>   get lost. e.g.: Take the example of a lost RELEASE COMPLETE in the
>    following scenario:
>
>                              (CRASH)     RELCOMPLETE
>           EP2 <--------------  GK   <------------  EP1
>
>    EP1 sends a RELCOMPLETE to EP2 via the GK. The GK crashes, before
>    forwarding the RELCOMPLETE from EP1 to EP2. As a result EP1 thinks
>    the  call is released, while as the EP2 thinks the call is up.
>
>    As Paul has pointed out: several H.245 messages are problematic--
>    especially those related to conferencing, such as chair control,
>    terminal join/left, terminal you are seeing, etc.
>    UserInputIndication and any other "indication" message  that does not
>    require a response is an issue.
>
>
> POSSIBLE SOLUTION(s):
> ---------------------
>
> Solution to Problem A:
> ----------------------
> This problem can be solved using SCTP/DDP or modifying Annex E to
> include alternate addresses.
>
> Solution to PROBLEM B:
> ----------------------
>
> This problem cannot be solved using SCTP/DDP as it is inherent in the
> H.323 protocol. If we take the same example as above:
>
>                                 (CRASH)     RELCOMPLETE
>           EP2 <--------------  GK   <------------  EP1
>          (SCTP/DDP)           (SCTP/DDP)         (SCTP/DDP)
>
> what happens if the GK fails just after its SCTP layer finished sending
> an SCTP-ACK for the RELCOMPLETE message to EP1.  EP1 receives the
> SCTP-ACK and therefore considers the call released but EP2 never
> receives the RELCOMPLETE message. It is important to note here that
> "checkpointing" in the H.323 layer of the GK will not help since the ACK
> at the SCTP level is generated before RELCOMPLETE message is delivered
> to the H.323 layer of the GK.
>
> So we can solve the problem by having an "END-to-END acknowledgement
> mechanism"  to make sure that EP1 and EP2 are in sync even when the
> intermediate node fails.
>
> One approach as suggested by Paul is to modify Annex E to have
> end-to-end acknowledgement.  We want to point out that actually this is a
> H.323 layer problem.  By introducing end-to-end ack into Annex E, we
> will be trying to solve a protocol layer problem by making modifications
> in the transport layer mechanisms.  The problem of synchronization comes
> from the fact that the H.323 layer does not have an ACK for every
> message that is sent out.
>
> Alternatively, if we introduce an ACK packet for every H.323 message
> that currently has no ACK (e.g: H.245 commands/indications or H.225
> RELEASE COMPLETE), we can address the problem cleanly. This ACK message
> will be supported only by the nodes that support robustness. Unlike the
> Annex-E approach, this approach is independent of the transport layer
> protocol layer below H.323, and can also be applied to SCTP/DDP.
>
> Comments are welcome on this issue.
>
> Regards,
> Archana
>
>
>
>
>

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