On Mar 11, 2011, at 7:44 AM, Will Yu wrote:
Hello,
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
There is no state for the path IP2-IP4, only for the remote address IP4.
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.
SCTP supervises remote addresses. So EP1 is supervising IP3 and IP4. So there
is one error counter for IP3
and one for IP4. There is also only a state per remote IP address. So after the
error counter for IP4
exceeds PMR, the IP address IP4 should be marked as unreachable.
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?
As said above, SCTP supervises remote addresses, not paths.
Best regards
Michael
PS: It might be better to send SCTP related questions to
tsvwg(_at_)ietf(_dot_)org
Brs,
Will Yu
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