Subject: [mpls] Question on draft-mirsky-mpls-bfd-directed

From: Loa Andersson <[email-masked]>
Date: Mon, 16 Feb 2015 16:55:23 +0800

Authors, Working Group,

I started to review draft-mirsky-mpls-bfd-directed, but got stuck on the
problem statement.

If I understand you correctly, but I admit that I may have
misunderstood, you are saying that the scope of the document are uni-
directional explicit routed LSPs. A typical example would be MPLS-TE
LSPs, right?

You also say that for such an LSPs the egress would use the shortest
path for its control message, while the ingress will send control 
messages in-band.

Now in Asynchronous mode the BFD session will form a loop ingress -
egress - ingress, where ingress to egress is on the the monitored LSP,
while egress to ingress is out-of-band, right?

You say that since the egress to ingress control message will follow the
shortest path, which might not be co-routed with the ingress to egress
LSP, missing responses can't be taken as an indication of that the uni-
directional LSP needs to be re-routed.

While I agree with that statement, I don't see that if you give the
egress an explicit return/reverse path that is co-routed with the
monitored LSP you are really in a better shape to draw any conclusions.
Failure to receive egress to ingress control messages does not
necessarily  tell you anything about the state of the monitored LSP.

/Loa
-- 

Loa Andersson                        email: [email-masked]
Senior MPLS Expert                          [email-masked]
Huawei Technologies (consultant)     phone: +46 739 81 21 64