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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