Subject: [Idr] Stephen Farrell's No Objection on draft-ietf-idr-add-paths-14: (with COMMENT)

From: "Stephen Farrell" <[email-masked]>
Date: Tue, 03 May 2016 03:36:11 -0700

Stephen Farrell has entered the following ballot position for
draft-ietf-idr-add-paths-14: No Objection

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COMMENT:
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- section 2: This bit wasn't entirely clear to me: "A
BGP speaker that re-advertises a route MUST generate its
own Path Identifier to be associated with the
re-advertised route." It wasn't clear to me if that
means it's disallowed to use the same path identifier or
not when re-advertising. If it's not allowed that'd seem
to warrant a "MUST NOT" statement, or to say that the
path identifier re-advertised MUST be different from
that in the received advertisement - "it's own" doesn't
quite say the same to me.

- section 5: I wasn't clear what happens in the
following case. Alice advertises prefix-A with no path
identifier, then prefix-A with some path identifier.
Next Alice withdraws prefix-A with no associated path
identifier.  What happens when I get that sequence? (It
may well be clear for those readers who know more about
BGP.)

- What is "bestpath"? If that's defined elsewhere a
reference would be good. (A quick duckduckgo only showed
me Cisco specific answers for bestpath, but others for
"best path.")

- Section 5 (last para): what would "special care" here
mean? If that'd be clear to relevant readers, that's
fine.

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From: "Alvaro Retana (aretana)" <[email-masked]>
Date: Tue, 3 May 2016 15:25:07 +0000

On 5/3/16, 6:36 AM, "Stephen Farrell" <[email-masked]> wrote:

[Author hat on.]

Stephen:

Hi!

Please see my comments below.

Thanks!

Alvaro.

How the Path ID is selected is a local matter -- the important piece is
that the local router can use it to uniquely identify a specific path.
The intent here is to have every BGP speaker create the Path ID and not
just take the easy path of simply readvertising because there may be
collisions between the Path ID assigned by neighbors and the local ones --
that's the reason for the MUST.

The local assignment may result in the same Path ID as the one which was
received.  That is fine -- as long as the local router did the assignment
itself.

>may well be clear for those readers who know more about

That should never happen.  Section 5 says that if the capability has been
negotiated (Alice is able to send multiple paths and Bob is able to
receive multiple paths), then Alice "MUST generate a route update for the
<AFI, SAFI> based on the combination of the address prefix and the Path
Identifier, and use the extended NLRI encodings...".

If Alice does advertise prefix-A with no Path ID then Bob will detect an
"invalid network field" and reset the session, which is what is specified
in the base BGP spec [rfc4271] -- this document doesn't change the
processing of UPDATES, just the interpretation of the NLRI.

>me Cisco specific answers for bestpath, but others for

Good point!  I guess we're all just used to the colloquial term where
everyone knows it is the best route [rfc4271].   I'll update in the next
revision.