Subject: Questions on BFD for MPLS LSP

From: "Sunil C Gutlapalli" <[email-masked]>
Date: Mon, 16 Nov 2009 14:59:09 -0800

Hi

I have some question on BFD for MPLS LSP's

1.	MPLS LSP's are uni-directional and BFD sessions are for
Bi-directional failure detection. If we have an LSP from Ingress to Egress,
BFD control packets are sent to UDP port 3784 (BFD single hop session port)
from ingress to egress. Control packets from Egress to ingress are sent to
UDP port 4784 (multi hop session port). So does the session is single hop
session or multi hop session? What is the session behavior in ingress and
egress? 
2.	If LSP takes path1 from ingress to egress and the shortest path from
egress to ingress is path2, control packets takes different paths, if path2
fails then BFD session goes down. But LSP path is still up. How this case
will be handled? 
3.	If we have LSP from Ingress to egress and another LSP from egress to
ingress, do we need to create two BFD sessions or one session? 

If both LSP's take same path, then link failure causes both LSP's down, but
if they take different paths, one link failure causes both LSP's down.

Sunil

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From: Vishwas Manral <[email-masked]>
Date: Mon, 16 Nov 2009 15:38:22 -0800

Hi Sunil,

Thanks for the detailed mail.

> MPLS LSP’s are uni-directional and BFD sessions are for Bi-directional
> failure detection. If we have an LSP from Ingress to Egress, BFD control
> packets are sent to UDP port 3784 (BFD single hop session port) from ingress
> to egress. Control packets from Egress to ingress are sent to UDP port 4784
> (multi hop session port). So does the session is single hop session or multi
> hop session? What is the session behavior in ingress and egress?
Sunil with MPLS LSP's they have made special kinds of sessions which
can be multihop in one direction and single hop in the other. There
however was no requirement to model it that way, we could as well have
used multhop in both directions.

> If LSP takes path1 from ingress to egress and the shortest path from egress
> to ingress is path2, control packets takes different paths, if path2 fails
> then BFD session goes down. But LSP path is still up. How this case will be
> handled?
BFD is better suited for bidirectional LSP's than unidirectional ones.
In case BFD session goes down the LSP is assumed to be down, even
though the backward path may have gone down.

> If we have LSP from Ingress to egress and another LSP from egress to
We could as well use one session if the two sessions are bound togather in BFD.

> If both LSP’s take same path, then link failure causes both LSP’s down, but
> if they take different paths, one link failure causes both LSP’s down.
That is correct.

> Sunil
Thanks,
Vishwas

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From: Mach Chen <[email-masked]>
Date: Tue, 17 Nov 2009 17:06:26 +0800

Hi Sunil,

Regarding to your questions, especially for the question 2 and 3,  we also 
had the same doubts couple months ago.

Since the reverse direction of a "BFD for MPLS LSPs" sesssion follows 
routing and the return path is "randomly" detetermined by the egress LSR, 
the failures in return path may lead to false positives.

In addition to that, for bidirectional LSP or a pair of unidirectional LSP, 
there will be two separate BFD sessions( one for each direction) 
established, and these two BFD sessions have no any inherent relationship, 
it will result in not double the BFD sessions over the LSP, but failing to 
perform protection and switching when the bidirectional LSP or the pair of 
undirectional LSPs are desired to operated as a single entity.

We submitted a draft that is intended to reslove these issues/limitation. 
This draft was just presented at MPLS WG, Hiroshima.

Any comments and sugguestions are appreciated!

PS:

The links:

Slides:
http://tools.ietf.org/agenda/76/slides/mpls-2.ppt

Draft:
http://tools.ietf.org/html?draft=draft-chen-mpls-bfd-enhancement-00

Best regards,
Mach

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From: "Sunil C Gutlapalli" <[email-masked]>
Sent: Tuesday, November 17, 2009 6:59 AM
To: <[email-masked]>
Subject: Questions on BFD for MPLS LSP

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From: Vishwas Manral <[email-masked]>
Date: Tue, 17 Nov 2009 09:59:39 -0800

Hi Mach,

Though I have not yet read through the draft, it seems that the draft
could be of interest. LEt me read the draft and get back to you with
comments.

Thanks,
Vishwas

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From: Mach Chen <[email-masked]>
Date: Thu, 19 Nov 2009 08:51:27 +0800

Hi Vishwas,

Looking forward to receive you comments!

Thanks,
Mach

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From: "Vishwas Manral" <[email-masked]>
Sent: Wednesday, November 18, 2009 1:59 AM
To: "Mach Chen" <[email-masked]>
Cc: "Sunil C Gutlapalli" <[email-masked]>; <[email-masked]>; "So 
Ning" <[email-masked]>
Subject: Re: Questions on BFD for MPLS LSP