Updates to the Fast Reroute Procedures for Co-routed Associated Bidirectional Label Switched Paths (LSPs)
draft-ietf-teas-assoc-corouted-bidir-frr-07

Thanks for the work everyone did on this document.

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Please expand "LSP" in the abstract.

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§3.3:

>  In this example, the mid-point node D may mistakenly associate LSP1
>  with the reverse LSP4 instead of the reverse LSP3 due to the matching
>  (Extended) ASSOCIATION Objects.

This doesn't seem right to me. LSP1 and LSP3 appear to be forward direction,
while LSP2 and LSP4 appear to be reverse direction. It's also not clear how LSP1
would be associated with LSP3 in this scenario, which is what this sentence
seems to be saying might happen.

Did this mean to say "...instead of the reverse LSP2..."?

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§4.2:

>  In order to associate the LSPs unambiguously at a mid-point node (see
>  Figure 3), the endpoint node MUST signal Extended ASSOCIATION Object
>  and add unique Extended Association ID for each associated forward
>  and reverse LSP pair forming the bidirectional LSP.  An endpoint node
>  MAY set the Extended Association ID to the value shown in Appendix A.

This phrasing is very confusing, as it implies that Appendix A is going to
contain a value that can be used as an ID (which would be at odds with
"unique"). Appendix A contains a format, not a value. I think what you mean is:

"...MAY set the Extended Association ID to a value formatted according to the
structure shown in Appendix A."

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Appendix A:

>  Extended Association ID in the Extended ASSOCIATION Object [RFC6780]
>  can be set to the value shown in the following example to uniquely

Same comment as above regarding the distinction between "value" and "format".

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Appendix A:

>     0                   1                   2                   3
>     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>    |                    IPv4 LSP Source Address                    |
>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
>    |           Reserved            |            LSP-ID             |
>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

What are implementors expected to set the "Reserved" bytes to?

Thanks for the work on this!

I just have one strictly editorial comment:

I found it odd that the introduction doesn't occur until section 2.  I think section 1 would make more sense if I had had the context from the introduction when reading it.

How does the unidirectional link failure logic and the revertive logic
interact?  That is, in the unidirectional failure case a node should be
detecting that there is a failure case and rerouting reverse traffic onto
the protection path to match the forward path.  But a node in the process
of reverting back on to the primary path (before its counterpart in the
other direction) would seem to observe the same packet/path behavior as in
the case of a unidirectional link failure.  Do we need to rely on the
flooding of link status information to differentiate between these cases?

Are the state-keeping and resource consumption burdens large for the
midpoint nodes that now must correlate whether they see traffic on
original/protection paths for associated flows?  (E.g., Section 4.1.3's
"when it receives the un-modified RSVP path messages and traffic".)
It seems like it should just be a linear scaling factor at worst, with no
real path to an attack, but perhaps there are security considerations
relating to router capacity.

Section 2

   In packet transport networks, there are requirements where the
   reverse LSP of a bidirectional LSP needs to follow the same path as
   its forward LSP [RFC6373].  [...]

Does this need a qualifier (e.g., "some packet transport networks" or
"there are sometimes requirements")?


Section 3.2

   tunnel S (on path B-F-G-D) to reach downstream MP node D whereas the
   upstream PLR node C reroute the protected reverse LSP2 traffic over
   the bypass tunnel N (on path C-I-H-A) to reach the upstream MP node
   A.  [...]

nit: "reroutes"

Section 4.1.1

   As shown in Figure 2, when using a node protection bypass tunnel with
   protected co-routed LSPs, asymmetry of paths can occur in the forward
   and reverse directions after a link failure [RFC8271].  In order to
   restore co-routing, the downstream MP node D (acting as an upstream
   PLR) SHOULD trigger the procedure to restore co-routing and reroute
   the protected reverse LSP2 RSVP Path messages and traffic over the
   bypass tunnel S (on path D-G-F-B) to the upstream MP node B upon

Why is this only a SHOULD?

Section 4.2

                                                        An endpoint node
   MAY set the Extended Association ID to the value shown in Appendix A.

The contents of Appendix A do not include a distinguished single value, but
rather a data structure, so I think that a phrase other than "to the
value" should be used.

   o  For double-sided provisioned bidirectional LSPs [RFC7551], both
      endpoints need to ensure that the bidirectional LSP has a unique
      Extended ASSOCIATION Object for each forward and reverse LSP pair
      by selecting appropriate unique Extended Association IDs signaled
      by them.

How does this signalling/selection process get the two endpoints to agree
on the same value?

Appendix A

(Again, "to the value" is not appropriate to describe the general format.
Perhaps, "to a value using the format".)

Please also explicitly describe the semantics of the "Reserved" field(s)
(i.e., set to zero on transmission and ignored on receipt).

I agree with Ben's comment about the unconventional document structure. I'm not an expert, but I think I understood the Introduction well enough to get through it before needing to look at the terminology for precise meanings, and it does seem odd compared to the vast majority of RFCs I've reviewed, so somewhat disorienting for readers.