Re: Issue in H.323 robustness not addressed by SCTP/DDP
Qiaobing,
Could you explain how SCTP/DDP layer below would handle the PROBLEM B. My understanding of the SCTP/DDP is based on the internet draft that exists right now. Just as to make clear that we are in sync about the problem, let me explain our view of SCTP/DDP functionality for this particular problem of H.323 robustness.
a) SCTP will help H.323 to route data to multiple IP addresses belonging to the SAME node(multi-homed host).
b) DDP extends this functionality by allowing an H.323 endpoint to send data not only to multiple IP addresses belonging to the SAME node but between physically DIFFERENT nodes .This is achieved by translating a "NAME" to a group of IP addresses.
If there is any other functionality in this context that is relevant for the discussion here, please do point it out.
Now coming to problem B that we had stated (I am modifying the diagram for clarity):
(CRASH) RELCOMPLETE EP2 <-------------- GK <------------ EP1 (SCTP/DDP) (SCTP/DDP) (SCTP/DDP)
EP1 sends a RELCOMPLETE to EP2 via the GK.
| H.323 layer of GK| | | ^ | | | | | ---|------------|--- Outgoing | | DDP/SCTP | | RELCOMP<--------- ------------ Incoming RELCOMP | | --------------------
The SCTP/DDP layer receives the RELCOMP and sends an ACK back to the EP1.so SCTP/DDP's job is over.Now the SCTP/DDP layer sends the RELCOMP message to the H.323 layer and the H.323 layer crashes.So there is no context of that RELCOMP message on the STANDBY. So our point is that this problem is outside the domain of the SCTP/DDP.
Btw, are you suggesting that we do a "checkpointing" in the H.323 layer for every message we received? I am sure you will agree that that will be very expensive.
-Regards Archana
-----Original Message----- From: Qiaobing Xie [mailto:xieqb@CIG.MOT.COM] Sent: Wednesday, April 26, 2000 1:22 PM To: ITU-SG16@mailbag.cps.intel.com Subject: Re: Issue in H.323 robustness not addressed by SCTP/DDP
Archana,
One thing you might have missed is that the DDP/SCTP fault-tolerance model is designed to provide robustness to the application in a *transparent* fashion. The state synchronization issue (your PROBLEM B) is a no-issue to DDP/SCTP model. In our model, a back-up GK will automatically kick in and continue forwarding the RELCOMPLETE to EP2, without either EP even noticing that the failure ever happened at all! There is NO application involvment required in this scenario.
-Qiaobing
Archana Nehru wrote:
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:
Transport failure: e.g. failed NIC, congested network.
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 <------------ EP1EP1 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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Archana Nehru