Re: [mpls] MPLS-RT review of draft-zzhang-mpls-rmr-multicast

["Jeffrey (Zhaohui) Zhang" <[e-mail]>]

Hi Huub,

Thank you for your review and comments. Please see zzh> below.



Non-Juniper
From: Huub van Helvoort <[e-mail]>
Sent: Monday, April 8, 2019 5:58 AM
To: [e-mail]
Cc: [e-mail]; [e-mail]
Subject: MPLS-RT review of draft-zzhang-mpls-rmr-multicast

All,

I've been selected as an MPLS-RT reviewer for draft-zzhang-mpls-rmr-multicast,
which is currently a candidate for MPLS WG adoption.

In the abstract is stated:

   With Resilient MPLS Rings (RMR), although all existing multicast

   procedures and solutions can work as is, there are optimizations that

   could be done for RSVP-TE P2MP tunnel signaling and Fast-ReRouting

   for both mLDP and RSVP-TE P2MP tunnels.
I have carefully read the draft but I could not find any justification
for the optimisation that could be done.

Zzh> By "justification", do you mean "need"? I thought the following text provides that:

   For a conventionally signaled RSVP-TE P2MP tunnel, an ingress LSR
   discovers leaves and signals one sub-LSP for each leaf.  Even though
   the forwarding state is merged at each hop (i.e, one incoming label
   mapping to multiple outgoing entries), the control plane maintains
   individual sub-LSP state.  This leads to lots of redundant state on
   routers close to the ingress.

Zzh> Or do you mean "why" the optimization COULD be done? The key is that this is a ring, and:

   ... As the PATH message passes along the ring, the leaves
   send RESV messages, but only one RESV message reaches the tunnel
   ingress.

Zzh> If you mean "how" it is done, I thought the following describes it:

   ... With RMR, this can be optimized such
   that only a single LSP is signaled, with all the leaves listed in the
   PATH message.  As the PATH message passes along the ring, the leaves
   send RESV messages, but only one RESV message reaches the tunnel
   ingress.

   The ingress LSR may also send PATH messages in both directions, so
   that the tunnel is set up in such a way that minimum delay is
   incurred for traffic to reach all leaves.  Alternatively, the ingress
   may send PATH message only in one direction for best bandwidth
   utilization.  For example, a leaf D is three hops away from the
   ingress A in clockwise direction (A,B,C,D) and four hops away in the
   other direction (A,E,F,G,D), but G is also a leaf so it may be better
   to just send the PATH message in the anticlockwise direction.

   Each router establishes forwarding state accordingly.  Transit
   routers switches traffic towards downstream.  A transit router could
   also be a leaf router and in that case it does "drop and continue" -
   sends traffic off the ring and switches traffic downstream.


2.1.  Tunnel Protection and FRR

   Each node on a ring signals two counter-rotating MP2P RSVP-TE LSPs to
   itself.  As these LSPs are self-signaled after the discovery of the
   ring, they can be used to protect P2MP LSPs on ring.  So neither mLDP
   nor RSVP-TE has to setup a separate P2P bypass LSPs for link and node
   protection.


I also did not find any indication how this optimisation has to be
deployed. Do all the nodes in the ring, or in the network have to
support this optimisation before it becomes effective.

Zzh> The context is RMR - Resilient MPLS Ring. All routers on the ring need to support RMR and the multicast optimization for this to be effective. I can make that clear.

I think these issues have to be documented before this draft can
be adopted as WG draft.

Zzh> Besides explicitly stating that all routers on the ring need to support the RSVP-TE P2MP optimization for it to be effective, please advise further how the first issue can be better documented.

Thanks.
Jeffrey

Best regards, Huub.


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