Re: [bess] WG adoption and IPR poll for draft-zzhang-bess-bgp-multicast-03

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

Hi Gyan,

Please zzh4> below.

From: Gyan Mishra <[e-mail]>
Sent: Thursday, January 9, 2020 7:22 PM
To: Jeffrey (Zhaohui) Zhang <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: Re: [bess] WG adoption and IPR poll for draft-zzhang-bess-bgp-multicast-03


In-line comments

On Wed, Jan 8, 2020 at 1:48 PM Jeffrey (Zhaohui) Zhang <[e-mail]<mailto:[e-mail]>> wrote:
Hi Gyan,

Please see zzh3> below. I trimmed some text.

From: Gyan Mishra <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, January 8, 2020 2:50 AM
To: Jeffrey (Zhaohui) Zhang <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [bess] WG adoption and IPR poll for draft-zzhang-bess-bgp-multicast-03

Hi Jeffery

Pleas see in line Gyan>


Gyan> I actually read RFC 7938 when I was redesigning a data center architecture for stability using a L3 smaller fault domain design.  This BGP signaling of trees feature has to be used with eBGP and not iBGP as that requires IGP which would now be in the RIB for RPF path so would not work thus the "no IGP" requirement as per RFC 7938.  If you had directly connected iBGP peers and not loop-loop so that you don't need an IGP, could the BGP signaled tree feature still work. In theory your spine & leaf could all be directly connected iBGP peers and all now sit in one AS and not have an IGP.  This would eliminate the need to have ASNs deployed.

Zzh3> This draft does work for both eBGP and iBGP:

   How the BGP peer sessions are provisioned, whether EBGP or IBGP,
   whether statically, automatically (e.g., based on IGP neighbor
   discovery), or programmably via an external controller, is outside
   the scope of this document.

   In case of IBGP, it could be that every router peering with Route
   Reflectors, or hop by hop IBGP sessions could be used to exchange
   C-MCAST NLRIs for joins.  In the latter case, unless desired
   otherwise for reasons outside of the scope of this document, the hop
   by hop IBGP sessions SHOULD only be used to exchange C-MCAST NLRIs.

   Gyan> I am on the same page with you on the draft as it has a lot of merit.  I like the concept of leveraging BGP for multicast and using the proven MVPN procedures to instantiate PMSI trees now with hard stare end to end.

Few comments below:

The main objectives of this draft which should be incorporated in the Introduction is to provide an option for both service providers and enterprises that do not want to maintain soft state of multicast trees ; native PIM ASM or SSM based trees ; or MPLS based instantiated S-PMSI trees ; to use BGP multicast hard state to send joins/prune using proven service provider MVPN procedures.  Provide a means of network based source discovery for both ASM and SSM.

Zzh4> The introduction section has the following:

   1<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#section-1>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#page-3>
     1.1<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#section-1.1>.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#page-3>
      1.1.1<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#section-1.1.1>.  Native/unlabeled Multicast  . . . . . . . . . . . . .   3<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#page-3>
       1.1.2<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#section-1.1.2>.  Labeled Multicast . . . . . . . . . . . . . . . . . .   4<https://tools.ietf.org/html/draft-zzhang-bess-bgp-multicast-02#page-4>

Zzh4> It does cover the above points.

Would Default MDT UI/MI PMSI instantiated tree type 1 and 2 be considered hard state since the default MDT tree is always UP.  They could be an example to give to describe what is meant by hard state.

Zzh4>. Hard state vs. soft state means whether the state needs to be refreshed.

What is confusing is the mention in the introduction of the target deployment being for data centers BGP-only RFC 7938 deployments.  Since BGP multicast procedure can use both eBGP or IBGP with or w/o RR ; I would put the target deployments or use case in a separate section.  I think the reason why 7938 is mentioned is that in cases where BGP is solely used and protocol elimination is the goal, similar to P router “BGP free core” ; this allows for the elimination of PIM as well.  Does add confusion as part of intro since it makes the reader think that this is a eBGP only option for this feature  which it is not.

Zzh4> The text says “ONE target deployment”. There could be more. But I will change the wording to “one deployment scenario”. The following paragraph and sub-sections expand to other motivations.

Some operators don’t have an issue with the IP or labeled soft state with tree based models


Zzh4> Right – that’s why it says “some Data Center operators have been avoiding deploying multicast in their networks”.

Since the objective of this draft is to provide an alternative solution for operators  and not that this would provide a means to that end to eliminate tree based protocols such as pim or mldp but as an “option” if so desired.   I would stare as such.

Zzh4> This document has no mentioning of segment routing (where people talk about eliminating core state) and has the following in the abstract:

   This document specifies a BGP address family and related procedures
   that allow BGP to be used for setting up multicast distribution
   trees.


I don’t think tree based protocols are going away any time soon but I think it’s good to clarify that the direction from a standards perspective is not to eliminate but provide the optional flexibility for operators.

I think it’s a good idea to define what is meant by hard state and soft state.

Zzh4> Hard state and soft state are generally well-known, and I do have the following:

   o  Periodical protocol state refreshes due to soft state nature.


There maybe reason that operators may desire to keep the soft state if they don’t have much native or labeled state.  Or for telemetry or tracking purposes desire to maintain tree state to see how many trees are active within the network.  For scalability in cases of where soft state management is an issue now they have an option.

Zzh4> If they don’t have concerns that are addressed by this solution they certainly don’t have to use this. This is just an option.

ASM was built with LHR network based source discovery with MSDP and to that end it did add complexity but did allow a lot of flexibility that the MRIB contained all available groups so any receiver could join and group and any source could start streaming if they so desired.  Controls were built into ASM for that reason to limit what groups could be serviced  by an RP via ACL and what sources could stream with PIM accept register.  Controls were also put in place with MSDP SA propagation with SA filtering.  So the added flexibility of network based discovery was nice with ASM however with a major trade off of complexity as well as now added controls as to what valid sources can steam and what receivers can join what what steam.

Zzh4> ASM does not require MSDP. The real problem of ASM is with the complexity of source discovery that involves RP, register and RPT to SPT switchover). The solution in this draft addresses the complexity.

SSM was designed w/o network based discovery to eliminate the complexity of all the knobs to provide those controls that were built into ASM for the gain of simplicity : thus application based network discovery.  With SSM now all the controls are with the application /  server layer and removed from the network.  With application based source discovery at the application layer a channel list can now be provided which was done previously by legacy multicast apps such as SDR which detected the active groups on the network.

I think it maybe worthwhile mentioning reasons why SSM was built without network based discovery since now this draft will be adding the capability to SSM for network based source discovery.

Zzh4> That is outside the scope of this document. It’s better to be focused.

Regarding the multicast NLRI I was thinking how that would work and reason why the group is not needed is that the group is known with ASM model and if you have ASM and SSM overlay the  group is known by the app layer and so the network just needs the source to be learned for the LHR S,G join. Maybe worth mentioning in the draft.

Zzh4> The NLRIs do have (*,g) or (s,g) information. The LHR does need to know which group it needs to join, and that is a given.

In the introduction I would remove that soft state and tree building protocols as a reason why data centers avoid enabling multicast on the network.  There maybe some isolated corner cases however predominantly PIM soft state ASM or SSM has never been an issue.  To that end most data centers server deployments rely on multicast for clustering and data replication.  Also server based applications that can utilize multicast save on high bandwidth server to server east to west intra data center flows.  I do think BGP multicast would be an improvement and would add stability for data centers requiring multicast.

Zzh4> It is true some DC operators don’t like multicast trees. It’s true some others don’t mind having them. It’s ok to mention that this solution could suit for whoever needs multicast yet don’t want to run PIM.

In the draft it mentions that earlier versions of this draft user C-Multicast for signaling which was change to S-PMSI leaf Type 4 routes so we can set up the tree from FEC root instead of leaves.  This is similar to P2MP TE P-Tunnel where the root advertises BGP route type 3 with “leaf info required” bit set ; when the receiver PE gets the route it responds with type 4 leaf-ad.  Is that correct?

Zzh4> S-PMSI routes are used for two purposes in this document – see 2.2.5 and 2.2.6.

For labeled use case can this be used for all P-tunnel types MP2MP P2MP mLDP, P2MP TE.  Would PIM Rosen GRE still be valid P tunnel supported.  Maybe good to mention all the P tunnels supported for labeled BGP multicast.

Zzh4> This is about establishing native/labeled multicast trees. The resulting trees can be used for MVPN (BGP-MVPN or Rosen MVPN) as P-tunnels, but in the context of this document,  you ca just forget about P tunnels.

As far as the MVPN procedures instantiation of S-PMSI trees is that being completed reused from RFC 6513 6514  - all the same route types supporting ASM and SSM types 3-7.  Only type 1 and 2 for default MDT instantiation of I-PMSI trees is not supported.

Zzh4> Other than that some NLRI types are similar to MVPN, you can ignore its relevance with MVPN.
Zzh4> Thanks!
Zzh4> Jeffrey

Correct?

Kind regards,

Gyan

Zzh3> RFC 7938 uses eBGP which does not require IGP, so the following text is perfect (after removing P2MP tunnel wording):

   This section provides some motivation for BGP signaling for native
   and labeld multicast.  One target deployment would be a Data Center
   that requires multicast but uses BGP as its only routing protocol
   [RFC7938<https://urldefense.com/v3/__https:/tools.ietf.org/html/rfc7938__;!!NEt6yMaO-gk!X9DfJ6RoZtXubIffaIRRYd0TgIz3lgYayTfkKv2a7LIImWQqa9eDPnSJyZ1k8qhB$>].  In such a deployment, it would be desirable to support
   multicast by extending the deployed routing protocol, without
   requiring the deployment of tree building protocols such as PIM,
   mLDP, RSVP-TE P2MP, and without requiring an IGP.

Zzh3> Then the following talks about other scenarios beyond DC:

   Additionally, compared to PIM, BGP based signaling has several
   advantage as described in the following section, and may be desired
   in non-DC deployment scenarios as well.

Zzh3> I will change “compared to PIM” to “compared to PIM/mLDP”.

Gyan> So with this feature the last hop router signals join similar to mLDP inband via BGP and the join is sent via BGP signalled tree.

Zzh3> It’s the PIM joins from LHRs or mLDP label mappings from leaves replaced with BGP messages, not that “the join is sent via BGP signalled tree”.

 Gyan>  With the BGP trees using the same MVPN mLDP procedures is their a concept of PMSI-I inclusive trees c-tree p-tree so a single tree  P2MP or MP2MP can be shared by all groups or is it 1-1 mapping group to tree.

Zzh3> MVPN/EVPN is for overlay – multiple C-flows can be transported over a single I/S-PMSI which are instantiated by underlay tunnels. This draft is about establish trees/tunnels, which can instantiate MVPN/EVPN I/S-PMSI (among other things).

Gyan>  Is their any way to minimize per GDA state with BGP trees.

Zzh3> What does GDA mean? Group Destination Address?
Zzh3> Anyway, so far the only efficient replication solution w/o incurring per-tree/tunnel state is BIER. BGP-signaled multicast does still have per-tree/tunnel state just like with PIM/mLDP. The only difference is how the state is signaled.

 Gyan> For the BGP trees is it possible use the same MVPN BGP A-D and c-tree p-tree Type 6 & 7 routes BGP multicast c-signalling.

Zzh3> This draft uses a different address family and new route types similar to MVPN type-3/4 routes instead of type-6/7 routes:

   The joins are carried in BGP Updates with MCAST-TREE SAFI and S-PMSI/
   Leaf A-D routes defined in this document.  The updates are targeted
   at the upstream neighbor by use of Route Targets.  [Note - earlier
   version of this draft uses C-multicast route to send joins.  We're
   now switching to S-PMSI/Leaf routes for three reasons. a) when the
   routes go through RRs, we have to distinguish different routes based
   on upstream router and downstream router.  This leads to Leaf routes.
   b) for labeled bidirectional trees, we need to signal "upstream fec".
   S-PMSI suits this very well. c) we may want to allow the option of
   setting up trees from the roots instead of from the leaves.  S-PMSI
   suits that very well.]

   Gyan> Doing so could you leverage the PMSI-I inclusive tree MVPN feature so you don't have per GDA state

Zzh3> As explained earlier, I-PMSI is irrelevant.

    Gyan> Source discovery is only necessary with ASM not SSM. With SSM the receiver is "source" aware so does not require any discovery mechanism.
So with SSM which requires IGMPv3 enabled on the receiver last hop router subnets and on the source first hop router subnet for the both to be "source aware" ; for the receiver now to send the (S,G) join for the channel since it is now source aware. How the receiver gets that source awareness is from the server URI that the user connects to which has the S,G information ; server has to be also  source aware and has S,G channel available that can be joined. With IGMPv3 the packet  accommodate the Source information in the S,G join sent along the RPF path to the source. You mention that SSM deployment has been limited but in fact the opposite and reason why ASM is being officially deprecated by the IETF for inter domain multicast routing. IPv6 does not even have MSDP support since with ASM MSDP source discovery and propagation is not necessary since no RPs exist all disparate ASM multicast domains can now be collapsed into a single SSM domain. ASM MSDP/Anycast has its complexities which is why IPv6 nixed the idea of integrating MSDP into the architecture. Thus IPv6 only supports SSM for inter-domain multicast routing. I would keep the comment about ASM complexity which is true but remove mention of SSM.  I would not mention any gains with less state as you would still have to maintain IGMP join state with BGP with 1-1 mappings of GDA to tree so the tree state is not being eliminated.

Zzh3> To do SSM you need to know sources ahead of time.
Zzh3> In a true ASM scenario, there are multiple sources sending to the same group and receivers don’t necessarily know which sources will be sending. Even though for some applications the receivers can get that source information from some servers/URIs (which is what I refer to as “application based source discovery”), there are still many situations where the receivers just want to do (*,g) IGMP join and leave the source discover to the network.
Zzh3> As for deprecating inter-domain ASM, please note the following:

   This document does not make any statement on the use of ASM within a
   single domain or organisation, and therefore does not preclude its
   use.  Indeed, there are application contexts for which ASM is
   currently still widely considered well-suited within a single domain.

Zzh3> More importantly, to use SSM you need to know sources first – either the receivers somehow learns/knows the sources or the network will figure it out. This draft provides a way for the LHRs to figure out where the sources are and then apply SSM procedures.
Zzh> Notice that we’re not saying SSM is bad. Rather, SSM is what we want to do, but the draft is about BGP-SSM (a step up from PIM-SSM) with BGP-based source discovery.

   Gyan> "While PIM-SSM removes the complexity of PIM-ASM, it requires that

   multicast sources are known apriori.  There have not been a good way

   of discovering sources, so its deployment has been limited."

Zzh3> To clarify, the above text is not implying to move away from SSM. Rather, it is to explain why we introduce network-based source discovery via BGP in this draft so that SSM can be used w/o requiring application-based source discovery.

Zzh3> Thanks!
Zzh3> Jeffrey
--
Gyan  Mishra
Network Engineering & Technology
Verizon
Silver Spring, MD 20904
Phone: [phone]
Email: [e-mail]<mailto:[e-mail]>