RE: BFD Echo format

["Vengada Prasad Govindan (venggovi)" <[e-mail]>]

Hello Greg/ Nobo,
  Thanks for this thoughtful discussion, I would like to clarify the motivation of this draft:

1.       There are scenarios where fault detection be initiated and managed from one end since the logic to failover (to a backup PW) is present on only on that end. The other end may participate in the PW establishment (signaling) or the PW may be provisioned but does not need to be actively involved in fault detection. Also, the action  of failing over to a backup PW is made at one end. This motivated us to use SBFD instead of classical BFD. With classical BFD, though we could use echo in one direction, both ends need to actively participate in the BFD session (through Async and by allocating other BFD session resources).

2.       However the circuit (PW) in use is a bi-directional construct as you point out and hence the text in Sec2.2.2.
Thanks to Nobo for helping clarify this.
Prasad

From: Rtg-bfd [mailto:[e-mail]] On Behalf Of Nobo Akiya
Sent: Tuesday, March 24, 2015 7:36 AM
To: 'Gregory Mirsky'; [e-mail]
Subject: RE: BFD Echo format

Hi Greg,

I absolutely agree on the S-BFD reverse path needing to take the corresponding PW. I peeked into draft-gp-pals-seamless-vccv, and I did find a text for this.

2.2.2.  S-BFD Reflector Operation

      All point to point pseudowires are bidirectional, the S-BFD
      Reflector therefore reflects the S-BFD packet back to the
      Initiator using the VCCV channel of the reverse direction of the
      PW on which it was received.

So, even though this key aspect was not described in the presentation, it looks like the authors did consider this.

Thanks!

-Nobo

From: Gregory Mirsky [mailto:[e-mail]]
Sent: March-23-15 8:21 PM
To: Nobo Akiya; [e-mail]<mailto:[e-mail]>
Subject: RE: BFD Echo format

Hi Nobo,
thank you for your thoughtful and interesting, as always, comments.

I've realized, after the session, that encapsulation of BFD Echo over LSP or PW MUST be different from what defined in RFC 5880. I think it must follow the same principle as set in RFC 5884 for Async BFD mode.

And I agree that PW, unlike LSP (at least most LSP constructs), is a bi-directional construct. Thus, as I think of it, operators more interested in verifying data plane availability in both directions. If we propose two-way single-ended mechanism to check data path of the particular PW, then, I imagine, the return path MUST be in-band, not over IP path. I think that we need to note that in our discussion of S-BFD or BFD Echo over VCCV as a requirement. Then we can see whether extending BFD Echo to MPLS LSP and PW or using S-BFD bring better solution, particularly since we already have ICMP and LSP Ping over VCCV defined.

                Regards,
                                Greg

From: Nobo Akiya [mailto:[e-mail]]
Sent: Monday, March 23, 2015 6:19 PM
To: Gregory Mirsky; [e-mail]<mailto:[e-mail]>
Subject: RE: BFD Echo format

Hi Greg,

I believe this comment was in context of S-BFD for VCCV.

>From what I understand, the scenario which S-BFD for VCCV is looked into is the case where one end of the PW has very constrained resources. Using the classical BFD will require such devices to maintain a session object per PW, as opposed to S-BFD which only requires one reflector session.

Your comment about why not send BFD echo packets over the PW? Sending a probe (BFD echo) with IP destination having the sender address, across multiple physical hops, always has the danger of the probe packet still coming back to the sender despite issues. Yes this is more likely on LSPs like LDP, RSVP (and less likely on PW) but it is possible that PW label gets exposed "somewhere" and that label value happens to match a different LSP, picking up traversal to a different node.

Then there's the desire for return packets to come back on the reverse direction of the forward PW. Whether we go with S-BFD for VCCV or BFD echo, this (i.e., reflector bouncing back packets on the reverse PW) is a change that needs to be made/standardized.

So to me, S-BFD for VCCV makes sense.

Thanks!

-Nobo

From: Rtg-bfd [mailto:[e-mail]]<mailto:[mailto:[e-mail]]> On Behalf Of Gregory Mirsky
Sent: March-23-15 1:54 PM
To: [e-mail]<mailto:[e-mail]>
Subject: BFD Echo format

Dear All,
I stand corrected, BFD Echo format is not defined in the RFC 5880 but there requirement suggesting that BFD Discriminator MAY be used to demux Echo replies:
   The payload of a BFD Echo packet is a local matter, since only the
   sending system ever processes the content.  The only requirement is
   that sufficient information is included to demultiplex the received
   packet to the correct BFD session after it is looped back to the
   sender.  The contents are otherwise outside the scope of this
   specification.

                Regards,
                                Greg