Multicast Virtual Private Network (MVPN): Using Bidirectional P-Tunnels
updated by
- rfc-8534 — Explicit Tracking with Wildcard Routes in Multicast VPN
- rfc-9573 — MVPN/EVPN Tunnel Aggregation with Common Labels
updates
Extracted elements (27)
The Hierarchical Partitioned Method reduces core state compared to Flat Partitioning by multiplexing multiple inner MP2MP LSPs over a single outer MP2MP LSP using upstream-assigned PE Distinguisher Labels, but requires hardware support for upstream-assigned labels not universally available; hence both methods are specified.
The Unpartitioned Method cannot support the 'Partitioned Set of PEs' technique for C-BIDIR because receiving PEs cannot identify which PE transmitted a packet from the bidirectional tunnel; duplicate detection must instead use RFC 6513 Sections 9.1.2 or 9.1.3 techniques.
Using a bidirectional P-tunnel to instantiate an I-PMSI for an MVPN with n PE routers requires only a single multicast distribution tree rather than n separate P2MP trees (one per source PE), thereby saving significant state in the network's transit nodes.
This document updates RFCs 6513, 6514, and 6625. Where any conflict exists between the procedures specified here and those of the updated RFCs, the procedures of this document take precedence.
All VRFs of a single MVPN MUST be provisioned to use the same PMSI instantiation method (Flat Partitioned, Hierarchical Partitioned, or Unpartitioned) for that MVPN's I-PMSI. Similarly, all VRFs MUST use the same method for S-PMSIs instantiated by bidirectional P-tunnels.
An implementation SHOULD be able to accept and properly process a PTA that uses any legal type of mLDP tunnel identifier, even if it cannot advertise all tunnel identifier types.
If a PE Distinguisher Labels attribute is malformed (duplicate IP address or duplicate label), the 'treat-as-withdraw' error-handling approach described in [BGP-ERROR] MUST be used.
If Partitioned Methods are used, all VRFs of a single MVPN MUST all use the Flat Partitioned Method or all use the Hierarchical Partitioned Method; mixing the two variants within one MVPN is not allowed.
Implementation of bidirectional P-tunnels is OPTIONAL. If implemented, an implementation claiming compliance MUST support at least one of the two P-tunnel technologies (MP2MP LSPs or BIDIR-PIM). An implementation providing C-BIDIR support via partitioned sets of PEs MUST support the Partial Mesh of MP2MP P-tunnels variant using Flat Partitioned Method.
In Flat Partitioned I-PMSI reception, if a (C-*,C-G-BIDIR) packet is received on a P-tunnel other than the one expected from the distinguished PE's I-PMSI A-D route, the packet MUST be discarded.
In Flat Partitioned S-PMSI reception, if a (C-*,C-G-BIDIR) packet is received on a P-tunnel whose root node is not the distinguished PE for that C-flow, the packet MUST be discarded. This prevents data duplication.
In Hierarchical Partitioned I-PMSI reception, packets addressed to C-G that arrive with unexpected values of the top two labels (outer or inner P-tunnel) MUST be discarded.
In Hierarchical Partitioned S-PMSI reception, if a multicast data packet is received over a matching P-tunnel but does not carry the PE Distinguisher Label value assigned to the upstream PE for its C-flow, the packet MUST be discarded.
In the Flat Partitioned Method using BIDIR-PIM, each advertised P-tunnel MUST have a distinct P-group address; using MP2MP LSPs, each P-tunnel MUST have a distinct MP2MP FEC (distinct combination of root node and opaque value), and the advertising PE MUST be the PE identified in the root node field of the FEC.
In the Flat Partitioned Method, when a PE originates an I-PMSI or S-PMSI A-D route whose PTA specifies a bidirectional P-tunnel, the PE MUST be the root node of that P-tunnel.
In the Hierarchical Partitioned Method, PE Distinguisher Labels used in the context of a given MP2MP LSP MUST be allocated and advertised by the router that is the root node of that LSP, per RFC 5331 Section 7.
In the Hierarchical Partitioned Method, when a PE (PE1) transmits a C-BIDIR flow and the upstream PE for the C-RPA is PE2, the packet's MPLS label stack MUST carry the PE Distinguisher Label assigned to PE2's IP address, not PE1's label, by the P-tunnel's root node.
In the Unpartitioned Method, at least one VRF of the MVPN MUST originate an Intra-AS I-PMSI A-D route with a PTA specifying a bidirectional P-tunnel, and the disposition of a received packet MUST NOT depend upon the particular P-tunnel over which it was received.
In the Unpartitioned Method for S-PMSI reception, a PE MUST join the P-tunnel in the matching S-PMSI A-D route and MUST also join all P-tunnels advertised in other installed S-PMSI A-D routes containing the same (C-S,C-G) as the matching route.
When the PTA of an I-PMSI or S-PMSI A-D route identifies a BIDIR-PIM tunnel, the originator SHOULD NOT include a PE Distinguisher Labels attribute; if one is present, it MUST be ignored in the data plane and MUST be treated by BGP as an unsupported optional transitive attribute.
When using BIDIR-PIM P-tunnels to instantiate an I-PMSI or S-PMSI, it is RECOMMENDED that the path from each PE to the RPA consist entirely of point-to-point links to avoid complexity from BIDIR-PIM Designated Forwarder Election on multiaccess links.
When using Flat or Hierarchical Partitioned Methods, a C-BIDIR flow MUST be carried only on an I-PMSI or on a (C-*,C-G-BIDIR), (C-*,C-*-BIDIR), or (C-*,C-*) S-PMSI. A PE MUST NOT originate (C-S,C-G-BIDIR) S-PMSI A-D routes, and C-BIDIR packets MUST NOT be carried on (C-S,C-*) S-PMSIs.
Three PMSI instantiation methods are defined for bidirectional P-tunnels: Flat Partitioned (no upstream-assigned labels), Hierarchical Partitioned (uses PE Distinguisher Labels and upstream-assigned MPLS labels for LSP aggregation), and Unpartitioned (single or multiple bidir tunnels, no distinguished PE inference).
Two bidirectional P-tunnel technologies are supported: Multipoint-to-Multipoint (MP2MP) LSPs created via LDP extensions (RFC 6388), and BIDIR-PIM multicast distribution trees (RFC 5015). Other bidirectional tunnel technologies are outside the scope.
There are no security considerations beyond those of RFC 6513 (Multicast in MPLS/BGP IP VPNs), RFC 6514 (BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs), RFC 5015 (BIDIR-PIM), and RFC 6388 (LDP MP2MP extensions). The existing security frameworks from those documents fully apply.
The (C-*,C-*-BIDIR) wildcard for 'all BIDIR-PIM groups' is encoded with a multicast group length field containing 0x08 (8 bits) and a single-octet multicast group field containing 0x00. This encoding is distinct from the generic group wildcard defined in RFC 6625.
The PE Distinguisher Labels attribute specifies a set of <MPLS label, IP address> bindings. Within a single attribute, each IP address MUST appear at most once and each MPLS label MUST appear only once; otherwise the attribute is malformed.