Re: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call - TEA attribute

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

Hi,

Please see zzh2> below.



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From: Bidgoli, Hooman (Nokia - CA/Ottawa) <[e-mail]>
Sent: Wednesday, April 6, 2022 11:41 PM
To: Jeffrey (Zhaohui) Zhang <[e-mail]>; Susan Hares <[e-mail]>; [e-mail]
Subject: RE: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call - TEA attribute

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Hi Sue/Jeffrey

As I am sure Jeffrey knows MP2MP is out of scope of SR-P2MP-Policy as the architectural draft name very clearly points this out.

Zzh2> Of course; but it would be nice to have a base mechanism that can be easily extended to cover MP2MP. Draft-bess does have that.

draft-ietf-pim-sr-p2mp-policy

This document describes an architecture to construct a Point-to-
   Multipoint (P2MP) tree to deliver Multi-point services in a Segment
   Routing domain.  A SR P2MP tree is constructed by stitching a set of
   Replication segments together.  A SR Point-to-Multipoint (SR P2MP)
   Policy is used to define and instantiate a P2MP tree which is
   computed by a PCE.

Again I want to point out the main vision behind SR-P2MP-POLICY. Simplicity and providing the providers a set of simple tools with low learning curve to deploy multicast services in their Segment Routing networks.
Even from standardization point of view that is the goal, hence specific, simplified methods are preferred over methods that are trying to address all multicast transport types.

Zzh2> draft-bess does have a lot of contents, but its method is *not* more complicated than draft-hb when it comes to a particular multicast tree like SR-P2MP. Section 1.7 and 3.4 in draft-bess cover SR-P2MP based on the generic mechanism (which is actually not much different from draft-hb's SR-specific approach besides how policy info is signaled, but as I said we can take draft-hb approach for signaling the policy info).

Zzh2> I can understand the desire to have a separate short spec that covers SR-P2MP only and I am fine with that, but it's still good to align/consolidate the approaches. In fact, some of my initial comments (and the thoughts that we might allow two ways of SR-P2MP signaling with BGP) were based on my understanding of draft-hb-02. Now I see the significant changes in -03 (of one route for each upstream/downstream) and I think draft-hb method is actually becoming similar to the draft-bess method (especially if we add back the per-downstream route that was in draft-bess-06 but removed in draft-bess-07) so we really need to think if we should have a single consolidate approach.

Zzh2> I will send out separate email to further comment on the Tunnel Encapsulation Attribute (TEA) encoding in draft-hb.

Zzh2> Thanks.
Zzh2> Jeffrey

Thanks
Hooman

From: Idr <[e-mail]<mailto:[e-mail]>> On Behalf Of Jeffrey (Zhaohui) Zhang
Sent: Wednesday, April 6, 2022 11:05 PM
To: Susan Hares <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Subject: Re: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call - TEA attribute

Hi Sue,

Please see zzh> below.



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From: Idr <[e-mail]<mailto:[e-mail]>> On Behalf Of Susan Hares
Sent: Tuesday, April 5, 2022 8:04 PM
To: [e-mail]<mailto:[e-mail]>
Subject: Re: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call - TEA attribute

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Jeffrey and Hooman:

This adoption call runs until 4/8/2022.  The reason I have extended the
adoption call is to determine what interaction there is between
draft-hb-idr-sr-p2mp-policy-04.txt (which has interest in IDR) and
draft-ietf-bess-bgp-multicast-controller-07.txt (adopted by BESS).

I need your input on these questions in order to advise BESS, PIM, and IDR
WG-chairs + WGS on the status of these drafts.   Please let me know if I am
Unclear on my questions.

This is the first of 3 lengthy technical messages.

You indicated in
https://mailarchive.ietf.org/arch/msg/idr/8k4Pkcg0aqIwPFbSULXepsaHDSw/<https://urldefense.com/v3/__https:/mailarchive.ietf.org/arch/msg/idr/8k4Pkcg0aqIwPFbSULXepsaHDSw/__;!!NEt6yMaO-gk!WSzsQHy_SUQ2pW3C3graivvjrJLm2JLEVtUPyAYkHx-uuWaJLG17XPp60NV8pr-N$>

"When it comes to SR-P2MP state downloading, there are three aspects involved here:


  1.  NLRI to encode policy information
  2.  NLRI to encode <tree/path/instance, node> identification
  3.  Tunnel Encapsulation Attribute (TEA) that encodes actual replication branches

The draft-ietf-bess-(even when used for SR-P2MP) is aligned with other
non-SR multicast trees (IP/mLDP) for a unified approach,
while draft-hb is aligned to unicast BGP SR policy."

This post examines the TEA attribute discussion in section 3.1 of
draft-ietf-bess-bgp-multicast-controller-07.txt draft.
Section 3.1 specifies  two types of tunnels for TEA (RFC9012):
any encapsulation (3.1.1) and load balancing (3.1.2)

Zzh> These are two new tunnels (that are SR-agnostic); other existing tunnels can also be used.
Zzh> I now realize that I missed listing an SR-specific tunnel type "Segment List tunnel" in this section. It is discussed in section 3.4.3.1 and in the IANA section.

Zzh> Let me refer to draft-ietf-bess-bgp-multicast-controller as draft-bess, and refer to draft-hb-idr-sr-p2mp-policy as draft-hb.

plus interactions with PMSI (section 3.1.3), and

zzh> There is no interaction with PMSI. Section 3.1.3 mentioned S-PMSI route because of this:
zzh> the bgp-multicast draft is about hop-by-hop signaling and it uses S-PMSI route to signal state for MP2MP upstream traffic, but the bgp-multicast-controller draft does not use S-PMSI route as the Replication State route can be used to signal state for both downstream and upstream traffic.

4 Sub-TLVs.  In section 3.4.3 you provide signaling for the
Policy to enter this tree, but you do not indicate how the
two different tunnel types interact with
the tunnel types defined in draft-ietf-idr-segment-routing-te-policy-15.txt

zzh> The "anyencap" and "load-balancing" tunnel types are not related to draft-ietf-idr-segment-routing-te-policy at all.
Zzh> The "segment list" tunnel "has a Segment List sub-TLV as specified in Section 2.4.4 of [I-D.ietf-idr-segment-routing-te-policy]", as described in in 3.4.3.1.

Section 2.4.4 of draft-ietf-idr-segment-routing-te-policy-15.txt
contains a new tunnel type: SR Policy and two groups of
subTLVs: weight SubTLV and segment SubTLVS (A-K).

1) how do the 3 Tunnel types interact?
(all encapsulation, load balancing, and SR Policy type)

Zzh> draft-bess does not use SR Policy tunnel type even for SR-P2MP.

Please include in your discussion where this interaction is
Described in the text.

2)  How do these 3 tunnel types interact with the
PMSI tunnels or Source-Active AD routes (types 1-3),
or ASM SPT (type 5) or C-multicast overlays (Type 6-7)?
This is  (section 3.1.3)?

Zzh> They don't interact with those. 3.1.3 does not refer to those route types.

3) draft-bess-bgp-multicast-controller states in section 1.7
on sr-p2mp:
"An SR- Point-to-Multipoint (SR P2MP) policy is used to
Define and instantiate a P2MP tree which is computed by a
Controller."

Section 3.4.2 defines the SR P2MP policy as  BGP
Community container in the BGP Wide communities which
Contains candidate path preference + TLVS.
You state in section 3.4.2
"The replicate state route for replication segments signaled to the
root is also used to signal (parts of) the SR P2MP Policy -
the policy name, the set of leaves [..optional], the preference of CP
and other information".

3a) Are you proposing translating this from the SR Policy or what?

Zzh> In draft-bess, we did not define a separate route to encode the policy information. Rather, the policy information is encoded in a wide community that is attached to the replication state route for the tree root.
Zzh> In the offline discussion with the draft-hb co-authors long time ago, I did indicate that draft-bess can also take their approach to use a different route instead of using wide community. I alluded to that again in one of the emails for this adoption call.

3b) how are the atoms used in the policy?

Zzh> A policy has a name and a list of tree leaves among other things. As described in 3.4.2, a string atom can encode the policy name, and a prefix list atom can encode the set of leaves.
Zzh> Again, the policy can be encoded in a new route instead of a wide community (to align with draft-hb).

4) Give an example of how a simple scenario might be implemented
With section 3.4.3.1 and 3.4.3.2?

Zzh> Consider a tree node that has two replication branches. Branch 1 is to a downstream node1 with a node sid (label) 101; branch 2 is to another downstream node2 with a node sid (label) 102. Suppose that the path to node1 does not matter, but to node2 we want to use a particular path represented by a segment list <151, 152, 102>. Suppose traffic sent to this node on the tree has incoming tree sid/label 1000, and traffic replicated to node1 and node2 has tree sid/label 1001 and 1002 respectively (the same tree sid/label 1000 could also be used for all incoming/outgoing traffic, depending on the operator's choice - this is discussed in section 1.4).

Zzh> With the 3.4.3.1 approach, the tunnel encapsulation attribute (TEA) that is attached to the Replication State route will include three tunnels:
Zzh> 1) tunnel 1 for upstream information: anyencap tunnel; rpf sub-tlv (to indicate this is for upstream - section 3.1.4); tree-label-stack sub-tlv with label 1000 (section 3.1.5)
Zzh> 2) tunnel 2 for replication branch 1 to node1: segment list tunnel; segment-list sub-tlv (3.4.3.1) with segment list <101, 1001>
Zzh> 3) tunnel 3 for replication branch 2 to node2: segment list tunnel; segment-list sub-tlv (3.4.3.1) with segment list <151, 152, 102, 1002>

Zzh> With this TEA encoding, the receiving node programs its forwarding state so that incoming traffic with sid/label 1000 will be replicated to two downstream nodes using segment list <101, 1001> and <151, 152, 102, 1002> respectively.
Zzh> Encoding the upstream information as a tunnel has the advantage that in case of MP2MP, that upstream tunnel will also encode outgoing information for upstream traffic (from tunnel leaves towards root and other leaves), and a downstream tunnel will also have tree-label-stack sub-tlv to encode incoming information for upstream traffic.

Zzh> Section 3.4.3.2 of draft-bess basically says there could be another way to encode the replication information as specified in draft-hb. That section was added as an (unfinished/unsuccessful) attempt to consolidate both draft-bess and draft-hb approaches.
Zzh> In this example, my understanding of draft-hb approach is that three routes are needed (in addition to the route for the policy information):
Zzh> 1) a "Replication Segment Binding Sid" route; its NLRI will encode the incoming SID (1000 in this example). This is specified in 3.1.2 of draft-hb.
Zzh> 2) a "Replication segment Route OIF" route (3.1.3) , with a TEA that has a single tunnel of type Replication-Segment-oif (3.2.3) for branch 1
Zzh> 3) a "Replication segment Route OIF" route (3.1.3) , with a TEA that has a single tunnel of type Replication-Segment-oif (3.2.3) for branch 2

Zzh> The main difference is that one route is used for each upstream/downstream in draft-hb while in draft-bess, a single route is used with a TEA encoding all upstream/downstream information. I suppose there are pros and cons for each way, and I'll not get into which is better in this email, but one thing to notice is that the draft-hb way is not MP2MP friendly (obviously it is "out of scope" now, but it's good to be extensible).
Zzh> Thanks.
Zzh> Jeffrey

Thank you, Sue


From: Jeffrey (Zhaohui) Zhang [mailto:[e-mail]]
Sent: Friday, March 25, 2022 10:47 AM
To: Susan Hares; [e-mail]<mailto:[e-mail]>
Cc: 'BESS'; '[e-mail]'
Subject: RE: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call

[+ BESS, PIM]

Hi,

I realized that in a hurry I did not respond to the specific questions below. Please see zzh> next to the questions.

Looks like that there are some comments on BESS/PIM list and I will go through those to see if I have any addition/follow-up on those.



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From: Idr <[e-mail]<mailto:[e-mail]>> On Behalf Of Jeffrey (Zhaohui) Zhang
Sent: Friday, March 25, 2022 6:30 AM
To: Susan Hares <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Subject: Re: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call

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I am sorry for responding late. I somehow missed this.

I think we should discuss the relationship with daft-ietf-bess-bgp-multicast-controller further before adopting this.

Thanks.
Jeffrey



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From: Idr <[e-mail]<mailto:[e-mail]>> On Behalf Of Susan Hares
Sent: Thursday, March 10, 2022 10:28 AM
To: [e-mail]<mailto:[e-mail]>
Subject: Re: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022) - Adoption call

[External Email. Be cautious of content]

IDR WG:

If you just wish to respond to the IDR list,
you may respond to this email on the adoption call.

Cheers, Sue

From: Idr [mailto:[e-mail]] On Behalf Of Susan Hares
Sent: Thursday, March 10, 2022 9:55 AM
To: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: [Idr] draft-hb-idr-sr-p2mp-policy (3/10 to 3/24/2022)

This begins a 2 week WG adoption call for:
draft-hb-idr-sr-p2mp-policy from (3/10 to 3/24/2022)

You can obtain the draft at:
https://datatracker.ietf.org/doc/draft-hb-idr-sr-p2mp-policy/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-hb-idr-sr-p2mp-policy/__;!!NEt6yMaO-gk!TfiPI1NfecN3db3pj6WZ8paxUr4s6OvmVZ91mapddPFeCkFZJodxFk8aTGCpYg34$>

In your comments for this call please consider:

Zzh> I want to point out that https://datatracker.ietf.org/doc/draft-ietf-bess-bgp-multicast-controller/<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/draft-ietf-bess-bgp-multicast-controller/__;!!NEt6yMaO-gk!WSzsQHy_SUQ2pW3C3graivvjrJLm2JLEVtUPyAYkHx-uuWaJLG17XPp60HRG_456$> is another way to do the same. I also explained in https://mailarchive.ietf.org/arch/msg/idr/KObeSgKPu3HRbd0ZN7L7fWq_Eto/<https://urldefense.com/v3/__https:/mailarchive.ietf.org/arch/msg/idr/KObeSgKPu3HRbd0ZN7L7fWq_Eto/__;!!NEt6yMaO-gk!WSzsQHy_SUQ2pW3C3graivvjrJLm2JLEVtUPyAYkHx-uuWaJLG17XPp60IkA8lid$> why it was in the bess WG.
Zzh> In addition, the bess draft supports *other* multicast trees (IP, mLDP besides SR-P2MP) using a consistent way.

1)  Does this technology support the SR P2MP features
that distributes candidate paths which connect
a multicast distribution tree (tree to leaves).

Zzh> It is one way to use BGP to support that. The bess draft specifies another way.

2) Is the technology correctly specified for the
NLRI (AFI/SAFI) and the tunnel encapsulation attribute
additions (sections 2 and 3)?

Zzh> The specified SAFI and tunnel encapsulation attribute additions are one way for the BGP signaling for SR-P2MP. The bess draft specifies another way.

3) Does the P2MP policy operation (section 4)
provide enough information for those implementing this
technology and those deploying the technology?

4) Do you think this multicast technology is a good
Place to start for P2MP policy advertisement via BGP?

Zzh> Both ways are good place to start. We just need to figure out how to proceed with the two proposals.

5) Do you think this SR P2MP policies should not be advertised
via BGP?

Zzh> I do think BGP signaling for SR P2MP is appropriate. We just need to discuss the two ways and figure out how to proceed. The authors have discussed before though we have not reached consensus.
Zzh> Thanks!
Zzh> Jeffrey

Cheers, Susan Hares