Re: [mpls] Review of draft-mirsky-mpls-bfd-directed-03

[Gregory Mirsky <[e-mail]>]

Hi Nobo,
many thanks for your thoughtful comments and help to improve the document.
Would the following change address your concern:
OLD:
But the reverse
   direction of the BFD session would still follow the shortest path
   route and that might lead to the following problems detecting
   failures on the unidirectional explicit path:

   o  detection of a failure on the reverse path cannot reliably be
      interpreted as bi-directional defect and thus trigger, for
      example, protection switchover of the forward direction;

   o  if a failure of the reverse path had been ignored, the ingress
      node would not receive indication of forward direction failure
      from its egress peer.
NEW:
But the reverse
   direction of the BFD session would still follow the shortest path
   route and that might lead to the following problem in detecting
   failures on the unidirectional explicit path:

   o  a failure detection by ingress node on the reverse path cannot be
      interpreted as bi-directional failure with all the certainty and
      thus trigger, for example, protection switchover of the forward
      direction without possibility of being false positive or false
      negative.

                Regards,
                                Greg

From: Nobo Akiya [mailto:[e-mail]]
Sent: Saturday, August 15, 2015 8:27 PM
To: Gregory Mirsky
Cc: Mach Chen; mpls; [e-mail]; Ross Callon
Subject: Re: Review of draft-mirsky-mpls-bfd-directed-03

Hi March, Greg,

My apologies for very delayed response.

Please see in-line with [NOBO].

Note I've snipped out sections that I accept your response.


> Section 2
>
> o if reverse direction is in Down state, the head-end node would not
>    receive indication of forward direction failure from its egress
>    peer.
>
> [NOBO] I found above to be slightly unclear in 2 aspects.
>
> Since this section is describing the limitations of using BFD on an
> unidirectional explicitly routed path, what does "if reverse direction is in Down state" mean?
>
> Additionally, "would not receive indication of forward direction
> failure from its egress peer" is a bit cryptic. If the egress peer
> detected that forward direction failed (e.g., BFD session timed out),
> then the egress peer will send few BFD control packets with DOWN
> state, IP routed. Thus the ingress peer will notice the problem of the forward direction.
>
> Readers can likely benefit from having this bullet point better clarified.
>

GIM≫ Would the following wording be more deterministic:
   o  failure detection on the reverse path cannot reliably be
      interpreted as bi-directional failure and thus trigger, for
      example, protection switchover of the forward direction;

   o  if a failure of the reverse path being ignored, the ingress node
      would not receive indication of forward direction failure from its
      egress peer.


[NOBO] The second bullet may create more confusion. Can we keep just the first bullet and add a bit of clarifications as below?

* a failure detection on the reverse path cannot reliably be
  interpreted as bi-directional failure and thus trigger, for
  example, protection switchover of the forward direction.
  When the BFD session on the egress node declares a failure
  (i.e., forward direction failure), the failure may or may not
  reach the ingress node. Therefore, the BFD session on the
  ingress node declaring failure may be a result of just the
  reverse path failure or both forward and reverse paths
  failures.

Regards,
Nobo