[Idr] Fwd: I-D Action: draft-kaliraj-idr-multinexthop-attribute-01.txt

Robert Raszuk <robert@raszuk.net> Fri, 11 June 2021 10:10 UTC

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From: Robert Raszuk <robert@raszuk.net>
Date: Fri, 11 Jun 2021 12:09:47 +0200
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To: Kaliraj Vairavakkalai <kaliraj@juniper.net>, minto@juniper.net
Cc: "idr@ietf. org" <idr@ietf.org>
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Subject: [Idr] Fwd: I-D Action: draft-kaliraj-idr-multinexthop-attribute-01.txt
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Dear authors,

I have read yr draft with interest as some perhaps recall we have discussed
this topic in the past number of times.


1. First nit - why do you say label must be 3107bis label ? (3.4.2.
Labeled IP nexthop) MPLS label is a label and I am not sure how one method
of label distribution matter here.

2a. What is PNH ? It first occurs in section 3 as "PNH-Len" or
"PNH-address", but it is never explained in the draft. Is this Path Next
Hop ?

2b. Would it be better to call this new attribute MNH MultiNextHop ?


3. In section 3.1.1 you describe how to assure that NH in MP_REACH is also
part of MultiNext Hop Attribute. But I do not see any discussion on how to
treat or ignore next hops in MP_REACH when a new attribute is present and
is valid.

4. In section 3.1.2 you define behaviour of RR advertising paths from non
MultiNexthop paths and those which carry new attributes ... But you should
make it clear that this is only about step in best path selection
(or candidate selection). There can be other criteria before we even get to
that step.

5. Now the most important question - how do you plan to handle atomic
withdraws ? I assume the plan is to readvertise the path with MNH - the
removed next hop. So by implicit withdraw this next hop will be removed.
The draft is silent on this. Now if the removed next hop was selected by
some receivers as best (due to and it was not arriving as part of
MP_REACH this proposal require significant implementation changes on how
BGP best path selection is triggered, how it runs, how it populates results
to the RIB/FIB. I think a new section is needed in detailed discussing the

6. How do you envision max-prefix safety knobs to work here ? On the
surface it may seem orthogonal - but it is not. Today folks use this to
protect infrastructure from for example operator's mistakes. Here one
received path may fill the MNH attribute with 100s of next hops and as
being optional and transitive will be distributed to all routers all over
the world.

7. Observe that metric to next hops is dynamic. So some implementations
capable of next hop tracking register with RIB all next hops and each time
metric changes they get a call back. Here we are effectively talking about
exploding this 10x or 100x or more ...

Many thx,

---------- Forwarded message ---------
From: <internet-drafts@ietf.org>
Date: Fri, Jun 11, 2021 at 10:56 AM
Subject: I-D Action: draft-kaliraj-idr-multinexthop-attribute-01.txt
To: <i-d-announce@ietf.org>

A New Internet-Draft is available from the on-line Internet-Drafts

        Title           : BGP MultiNexthop attribute
        Authors         : Kaliraj Vairavakkalai
                          Minto Jeyananth
        Filename        : draft-kaliraj-idr-multinexthop-attribute-01.txt
        Pages           : 12
        Date            : 2021-06-11

   Today, a BGP speaker can advertise one nexthop for a set of NLRIs in
   an Update.  This nexthop can be encoded in either the BGP-Nexthop
   attribute (code 3), or inside the MP_REACH attribute (code 14).

   For cases where multiple nexthops need to be advertised, BGP-Addpath
   is used.  Though Addpath allows basic ability to advertise multiple-
   nexthops, it does not allow the sender to specify desired
   relationship between the multiple nexthops being advertised e.g.,
   relative-preference, type of load-balancing.  These are local
   decisions at the receiving speaker based on path-selection between
   the various additional-paths, which may tie-break on some arbitrary
   step like Router-Id.

   Some scenarios with a BGP-free core may benefit from having a
   mechanism, where egress-node can signal multiple-nexthops along with
   their relationship to ingress nodes.  This document defines a new BGP
   attribute "MultiNexthop" that can be used for this purpose.

   This attribute can be used for both labeled and unlabled BGP
   families.  For labeled-families, it is used for a different purpose
   in "downstream allocation" case than "upstream allocation" scenarios.

The IETF datatracker status page for this draft is:

There is also an htmlized version available at:

A diff from the previous version is available at:

Internet-Drafts are also available by anonymous FTP at:

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