Multicast in L3VPNs

Bruce Davie1

bsd@cisco.com

draft-ietf-l3vpn-2547bis-mcast-03.txt

1. Not a draft co-author, or a multicast expert

Overview

Aiming to encourage more involvement in multicast L3VPN work by providing user-friendly overview of problem space

Focus more on problems that the current proposed solutions

Hard questions will be deflected to draft authors– Likewise questions such as “why did you choose to design it

that way?”

Attempts to make me look ignorant will be frowned upon

1draft-rosen-vpn-mcast-08.txt

Agenda

Recap of current (deployed) state (draft-rosen1)– See also draft-raggarwa-l3vpn-2547-mvpn-00.txt, draft-ycai-mboned-mvpn-deploy-00.txt

Enhancements/changes in L3VPN WG draft– Supporting multiple tree types– Aggregation– Carrying customer multicast routing in BGP– Inter-AS improvements

Note Well: WG draft is a superset of draft-rosen– i.e. deployed solutions will not be obsoleted by new draft

L3VPN Multicast—Motivation

Customers with IP multicast traffic would like to use MPLS VPN services

RFC 2547/4364 only addresses unicast As usual, multicast makes the problem harder

– Difficult to achieve same scalability as unicast

Multicast VPN—Current Deployments

Based on draft-rosen-vpn-mcast-08.txt

Many similarities to unicast, and some differences

CE-routers maintain PIM adjacency with PE-router only–Similar concept to 2547/4364 VPNs

P-routers do not hold (S, G) state for individual customers–Unlike unicast, there is some per-customer state in P-routers

PE-routers exchange customer routing information using PIM–Contrast to BGP for unicast

Customer multicast group addresses need not be unique –Same as 2547/4364

Multicast VPN—Current State (2)

Multicast domain is a set of multicast-enabled VRFs (mVRFs) that can send multicast traffic to each other

–e.g., VRFs associated with a single customer

Maps all (S, G) that can exist in a particular VPN to a single (S, G) group in the P-network

–This is the Multicast Distribution Tree (MDT)

–Amount of P-state is a function of # of VPNs rather than # of (S, G)s of all customers

–This is not as good as unicast, but better than the alternative

Mapping is achieved by encapsulating C-packet into P-packet using GRE

Customer BDefault MDT239.192.10.2

Customer BCE

Customer BCE

PE PE

Customer BCE

PE

Default Multicast Distribution Tree

PE routers build a default MDT in the global table for each mVRF using standard PIM procedures

All PEs participating in the same mVPN join the same Default MDT

Every mVRF must have a Default MDT MDT group addresses are defined by the provider

–Unrelated to the groups used by the customer Data MDTs may be created for high BW sources

Default Multicast Distribution Tree

Default MDT is used as a permanent channel for PIM control messages and low bandwidth streams

Access to the Default MDT is via a Multicast Tunnel Interface A PE is always a root (source) of the MDT A PE is also a leaf (receiver) to the MDT rooted on remote PEs

Customer BCustomer BDefault MDTDefault MDT239.192.10.2239.192.10.2

Customer BCE

Customer BCE

PE PE

Customer BCE

PE

Multicast Tunnel

Interface

Customer BCustomer BDefault MDTDefault MDT239.192.10.2239.192.10.2

RootLeaf

Limitations of draft-rosen

At least one multicast tree per customer in core– No option to aggregate multicast customers on one tree

Multicast traffic is GRE (not MPLS) encapsulated– Bandwidth and encaps/decaps cost– Operational cost—different mcast and unicast data planes

PIM the only fully described way to build core trees– Can’t leverage p2mp RSVP-TE, mLDP

PE-PE exchange of C-routes using per-customer PIM instances

Inter-AS challenges

PMSI: P-Multicast Service Interface

New terms introduced to decouple tree from service A PE delivers packet to PMSI, then all the required

PEs will get than packet and known which MVPN it belongs to

Three types of PMSI– MI-PMSI: Multipoint Inclusive, all → all

• All PEs of an MVPN can transmit to all PEs

– UI-PMSI: Unidirectional Inclusive, some → all• Unidirectional, selected PEs can transmit to all PEs

– Selective: S-PMSI, some → some• Unidirectional, selected PEs can transmit to

selected PEs

Types of Multicast Trees

Inclusive: contains all the PEs for a given MVPN

Selective: contains only a subset of PEs of a given MVPN

Aggregate– Carries traffic for more than one MVPN– May be either inclusive or selective

Supporting Multiple Tree Types

A PMSI is scoped to a single MVPN PMSI is instantiated using a tunnel

(or set of tunnels) Tunnels may be:

– PIM (any flavor)– MPLS (mLDP or p2mp RSVP-TE)– Unicast tunnels with ingress PE replication

Can map multiple PMSIs onto one tunnel (aggregation)

Encaps a function of tunnel, not service Single provider can mix and match tunnel types

Mappings to Old Terminology

Default MDT– MI-PMSI, instantiated by PIM Shared Tree or set

of PIM Source Trees

Data MDT– S-PMSI, instantiated by PIM Source Tree

New terminology helpful in:– Describing the complete set of options– Allowing multiple instantiations of same service,

without changing service spec

Autodiscovery

The process of discovering all the PEs with members in a given MVPN

Similar to RFC4364, but:– New address family MCAST-VPN– Contains address of the originating PE– Contains tunnel attribute to specify what sort of tunnel (e.g.

PIM-SSM, mLDP, etc.) can be supported by this PE, and whether aggregation is supported

• May contain a tunnel ID

Can also be used to discover set of PEs interested in a given group (to enable S-PMSI creation)

Aggregation Conflicting goals

– Scale: Minimize P-router state → Use as few trees as possible– Optimality: Send traffic at most once on each link, and only to PEs that need it → Use a tree for each customer multicast group

Solution: lots of options– Draft-rosen has one MDT per VPN, and optional data MDT for high BW or sparse customer groups– New draft also allows a tunnel to be shared among multiple mVPNs

• Better aggregation, less optimality• Requires a de-multiplexing field (e.g., MPLS label)

– Utility of aggregation depends on amount of “congruence” and traffic volume

PE-PE Routing Exchange In draft-rosen, C-PIM instances exchange PIM messages

over the MDT as if it were a LAN– Per-customer PIM peering among the PEs

– By contrast, one BGP instance carries all customer unicast routes among PEs

– PIM Hellos can be eliminated, but Join/Prunes remain

In new draft, BGP is proposed, as in unicast– New AFI/SAFI

– Advertisement contains essentially the same info as a PIM join or prune (source, group, PE sending the message)

– RDs are used to disambiguate customer multicast group and source addresses

– BGP route reflectors may be used

Inter-AS

Old (draft-rosen) approach: tunnel spans multiple ASes– Undesirable fate-sharing, must agree on tunnel type

New draft allows another approach: may “splice” tunnels from different ASes– Allows each AS to build its tunnels independently of

other ASes– Scaling now independent of number of PEs in other ASes

Inter-AS Overlay Tunnel

For a given MVPN, each AS independently builds an intra-AS tunnel

In addition, an “overlay tunnel” that spans multiple ASes is built

Each PE announces its MVPN membership info to the ASBRs with iBGP

An ASBR announces the AS MVPN membership to other ASBRs (in other ASes) using eBGP

This enables an AS-level spanning tree to be built among the set of ASes with members in this MVPN – Inter-AS tunnels spliced to intra-AS tunnels

Inter-AS Tunnels

ASBR1 ASBR2

Customer A Customer A

ASBR3

Customer A

Customer A

Intra-AS tunnels

Inter-AS tunnels

Other issues

RPF establishment– PEs need to know who their RPF PE is for a given route

Duplicate detection– Multihomed sites and switching from shared to source tree

can lead to a PE getting duplicate packets – draft proposes means to address this

C-RP Engineering– RP location in customer sites may cause “hairpin”– PEs may act as “outsourced” C-RPs– PEs may speak MSDP to C-RPs

Conclusions

WG draft builds on rosen draft without obsoleting it:– Support for more tree types, including PIM

variants, mLDP, RSVP-TE– Separation of service and mechanism– Aggregation support– More Inter-AS options with improved

independence– Possibility to use BGP for C-route exchange