Subject: Query regarding number of paths for Multi-homing scenario

From: Padmalochan Moharana <[email-masked]>
Date: Thu, 5 Aug 2010 17:45:38 +0530

Hi All,

I have some doubt regarding the number path use by an sctp association
during multi-homing scenario.

Suppose end point “A” is single homed (having IP 192.168.1.1) and
endpoint “B” is multi-home (having IP 192.168.1.2 and 192.168.1.3). As
per definition of path in the rfc-4960 (section 1.3.)

Path: The route taken by the SCTP packets sent by one SCTP endpoint to
a specific destination transport address of its peer SCTP endpoint.
Sending to different destination transport addresses does not
necessarily guarantee getting separate paths.

As for endpoint “A” two destination address is available then two path
is available to send packets to endpoint B from A.

As for endpoint “B” only one address is available then only one path
is available to send data to endpoint A.

Again as per the section
6.4.1. Failover from an Inactive Destination Address

When retransmitting data that timed out, if the endpoint is
multi-homed, it should consider each source-destination address pair
in its retransmission selection policy.  When retransmitting timed-out
data, the endpoint should attempt to pick the most divergent source-
destination pair from the original source-destination pair to which
the packet was transmitted.

My doubts:
1-	How many paths are available for endpoint B to reach endpoint A?
2-	Should endpoint B send HEARTBEAT to endpoint A from 192.168.1.3
(secondary IP of B) in regular interval to test the reachablity of A
using that IP?
3-	Can endpoint B send DATA to A (192.168.1.1) from IP 192.168.1.3
(secondary IP of B)?
4-	Should endpoint A discard the DATA that received from
192.168.1.3(secondary IP of B)?

Regards,
Padmalochan

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From: Michael Tuexen <[email-masked]>
Date: Thu, 5 Aug 2010 14:29:02 +0200

> 1-	How many paths are available for endpoint B to reach endpoint A?
There is only one destination address available for endpoint B.
> (secondary IP of B) in regular interval to test the reachablity of A
HEARTBEATs are used to monitor idle destination addresses. So endpoint B
supervises only the one IP address of endpoint A.
> 3-	Can endpoint B send DATA to A (192.168.1.1) from IP 192.168.1.3
Sure. Endpoint A will accept all SCTP packets, no matter if they come
from 192.168.1.2 or 192.168.1.3. However, the ports must be OK and the
v-tag must be OK.
> 192.168.1.3(secondary IP of B)?
No.

Best regards
Michael

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From: Padmalochan Moharana <[email-masked]>
Date: Thu, 5 Aug 2010 18:13:28 +0530

Hi Michael,

Thanks for the quick answer. I have one more doubt.

In below scenario, suppose the network cable connected  to NIC having
primary IP 192.168.1.2  is unplugged. Should endpoint B send data to
endpoint A using secondary IP i.e. 192.168.1.3?

Br,
Padmalochan

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From: Michael Tuexen <[email-masked]>
Date: Thu, 5 Aug 2010 15:14:25 +0200

> endpoint A using secondary IP i.e. 192.168.1.3?
This is implementation dependent. You can not rely on it.
Most likely your setup is not substantially better than a
single homed one.
Also having multiple IP addresses from the same subnet
on one or more interface of a host results in platform
specific behaviour on the IP layer.

Best regards
Michael