I have a query on SCTP standard (RFC 4960). In section 8.2 Path Failure Detection, it describes the action which end points should take in detecting path unavailable.
When its peer endpoint is multi-homed, an endpoint should keep an
error counter for each of the destination transport addresses of the
peer endpoint.
Each time the T3-rtx timer expires on any address, or when a
HEARTBEAT sent to an idle address is not acknowledged within an RTO,
the error counter of that destination address will be incremented.
When the value in the error counter exceeds the protocol parameter
’Path.Max.Retrans’ of that destination address, the endpoint should
mark the destination transport address as inactive, and a
notification SHOULD be sent to the upper layer.
In my case, I connected two end points through four IP address. EP1 owns IP1 and IP2, EP2 owns IP3 and IP4. And IP1 is the primary address for EP1, while IP3 is the primary address for EP2. There is no other packet in this association except with Heartbeat and Heartbeat_ACK.
IP1--------------------IP3
End \ / End
Point1 * Point2
/ \
IP2--------------------IP4
When IP4 is unavailable, the heartbeat messages from IP1 and IP2 with the destination IP4 cannot received acknowledgement. In this case, PMR in each EP is set to 4. I observed that EP1 used IP1 and IP2 as source address to send the heartbeat messages to IP4 respectively. When the heartbeat message from source address IP2 exceed 4 times, it marked the path(IP2-IP4) is unreachable. And then IP1 exceed, it would mark the destination IP4 is unavailable. In my opinion, this result is reasonable with the recognition of IP2-IP4 is a path and IP1-IP4 is another path for the association. Consequently, the PMR parameter is for determining which PATH could regard as unavailable.
But in the section 8.2 Path Failure Detection, the increasing counter action is based on different destination transport address, not on the different source-destination address pair. I think it make me confused that counter will impact the result in my case with the assumption RFC regard the destination address failure as the critical element for Path Failure Detection.
Would you kindly help me clarify the behaviors in Path Failure Detection, or figure out if I have any misunderstanding in this case?
Brs,
Will Yu
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