Transcript Multicast in L3VPNs Bruce Davie 1

Multicast in L3VPNs
draft-ietf-l3vpn-2547bis-mcast-03.txt
Bruce Davie1
[email protected]
1. Not a draft co-author, or a multicast expert
Overview
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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?”
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Attempts to make me look ignorant will be frowned
upon
Agenda
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Recap of current (deployed) state (draft-rosen1)
– See also draft-raggarwa-l3vpn-2547-mvpn-00.txt,
draft-ycai-mboned-mvpn-deploy-00.txt
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Enhancements/changes in L3VPN WG draft
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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
1draft-rosen-vpn-mcast-08.txt
L3VPN Multicast—Motivation
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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
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Based on draft-rosen-vpn-mcast-08.txt
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Many similarities to unicast, and some differences
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CE-routers maintain PIM adjacency with PE-router only
–Similar concept to 2547/4364 VPNs
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P-routers do not hold (S, G) state for individual customers
–Unlike unicast, there is some per-customer state in P-routers
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PE-routers exchange customer routing information using PIM
–Contrast to BGP for unicast
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Customer multicast group addresses need not be unique
–Same as 2547/4364
Multicast VPN—Current State (2)
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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
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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
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Mapping is achieved by encapsulating C-packet into
P-packet using GRE
Default Multicast Distribution Tree
Customer B
Default MDT
239.192.10.2
PE
PE
PE
Customer B
CE
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Customer B
CE
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
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Customer B
CE
Data MDTs may be created for high BW sources
Default Multicast Distribution Tree
Customer B
Default MDT
239.192.10.2
Root
Leaf
PE
PE
PE
Customer B
CE
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Multicast
Tunnel
Interface
Customer B
CE
Customer B
CE
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
Limitations of draft-rosen
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At least one multicast tree per customer in core
– No option to aggregate multicast customers on one tree
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Multicast traffic is GRE (not MPLS) encapsulated
– Bandwidth and encaps/decaps cost
– Operational cost—different mcast and unicast data planes
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PIM the only fully described way to build core trees
– Can’t leverage p2mp RSVP-TE, mLDP
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PE-PE exchange of C-routes using per-customer PIM
instances
Inter-AS challenges
PMSI: P-Multicast Service Interface
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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
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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
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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
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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
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Default MDT
– MI-PMSI, instantiated by PIM Shared Tree or set
of PIM Source Trees
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Data MDT
– S-PMSI, instantiated by PIM Source Tree
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New terminology helpful in:
– Describing the complete set of options
– Allowing multiple instantiations of same service,
without changing service spec
Autodiscovery
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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
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Can also be used to discover set of PEs interested in
a given group (to enable S-PMSI creation)
Aggregation
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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
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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
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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
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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
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Old (draft-rosen) approach: tunnel spans multiple
ASes
– Undesirable fate-sharing, must agree on tunnel type
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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
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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
Customer A
Customer A
ASBR1
ASBR2
Customer A
ASBR3
Intra-AS tunnels
Inter-AS tunnels
Customer A
Other issues
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RPF establishment
– PEs need to know who their RPF PE is for a given route
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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
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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
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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