Signaling Root-Initiated Point-to-Multipoint Pseudowire Using LDP
updates
- rfc-7385 — IANA Registry for P-Multicast Service Interface (PMSI) Tunnel Type Code Points
Extracted elements (29)
Interface parameters cannot be negotiated in P2MP signaling because the LDP Label Mapping Message is only sent by the R-PE to all L-PEs unidirectionally; instead, L-PEs check their values against R-PE thresholds rather than performing a bidirectional negotiation as in P2P PW.
The R-PE sends Label Mapping Messages as soon as the transport LSP ID is known regardless of the LSP's operational state, and does not withdraw labels when the transport LSP goes down. L-PEs also retain P2MP PW labels regardless of transport LSP status, decoupling PW signaling from transport LSP lifecycle to simplify operations.
This document updates RFC 7385 by reassigning PMSI Tunnel Type value 0xFF from 'Reserved' to 'wildcard transport tunnel type', enabling Typed Wildcard FEC matching across all transport tunnel types for P2MP PW signaling.
A P2MP PW MUST be associated with a transport LSP established using either RSVP-TE or mLDP. When using mLDP, the new L2VPN-MCAST application TLV Opaque Value Element type MUST be used in the P2MP FEC Element.
An LSR MUST be capable of handling PW status to support P2MP PWs, and MUST NOT claim P2MP PW capability by sending the Capability TLV if it is not also capable of handling PW status.
An R-PE and each L-PE MUST be configured with the same FEC as defined in Section 3.2 prior to P2MP PW establishment.
For interface parameters signaled by an R-PE, each L-PE must check that its configured threshold values (e.g., MTU size) are less than or equal to the R-PE's value; for other parameters like CEP/TDM Payload Bytes, the value MUST exactly match the values signaled by the R-PE.
If an L-PE fails to join the P2MP PSN LSP, it MUST NOT enable the PW, MUST notify the user, and MUST send a PW status message with status code 0x00000008 (Local PSN-facing PW Receive Fault) to the R-PE.
If an R-PE signals a PW with a PW Type, Control Word mode, or interface parameters that an L-PE cannot accept, the L-PE MUST NOT enable the PW and MUST send a PW status message with status code 0x00000001 (Pseudowire Not Forwarding) to the R-PE.
Once an L-PE successfully processes a Label Mapping Message for a P2MP PW, it MUST send appropriate PW status per RFC 8077; a PW status message from an L-PE to an R-PE MUST contain the P2P PW Downstream FEC Element to identify the PW.
The capability of supporting P2MP PWs MUST be advertised to all LDP peers using the P2MP PW Capability TLV; if the peer does not support dynamic capability advertisement, the TLV MUST be included in the LDP Initialization message.
The LDP liberal label retention mode MUST be used, and per RFC 5036, the Label Request message MUST also be supported for P2MP PW signaling.
The PMSI Tunnel Type field in the Typed Wildcard FEC MUST be set to 'wildcard' (0xFF) for the P2P PW Downstream FEC Element typed wildcard; this field only applies to the Typed Wildcard P2MP PW Upstream FEC Element.
When an L-PE receives a PW Label Mapping Message, it MUST verify the associated P2MP PSN LSP is in place. If it is not in place and the type is LDP P2MP LSP, the L-PE MUST attempt to join that LSP; if RSVP-TE, the L-PE SHOULD wait for the transport LSP to be signaled.
The optional P2P PW Downstream FEC Element (FEC type 0x84) is sent by the R-PE to individual L-PEs to signal an optional unidirectional P2P PW allowing L-PEs to send traffic back to the R-PE; it uses a downstream-assigned label.
The P2MP PW Upstream FEC Element (FEC type 0x82) is the primary FEC used by the R-PE to signal P2MP PW to all L-PEs; it carries the AGI, SAII identifying the P2MP PW root, and the PMSI Tunnel Info identifying the underlying transport LSP.
The PMSI Tunnel Info field within the P2MP PW Upstream FEC Element encodes the PMSI Tunnel Type (from RFC 6514), PMSI Tunnel Type Length, and Transport LSP ID. For mLDP, the Tunnel Identifier is a P2MP FEC Element; for RSVP-TE, it is the RSVP-TE tunnel identifier.
The Selective Tree Interface Parameter sub-TLV (type 0x1B) in the optional parameters of the P2MP PW Upstream FEC Element allows selective multicast tree association. Absence of this sub-TLV means the P2MP PW is associated with all multicast traffic originating from the root.
IANA has assigned FEC type values 0x82 (P2MP PW Upstream FEC Element) and 0x84 (P2P PW Downstream FEC Element) in the LDP 'Forwarding Equivalence Class (FEC) Type Name Space' registry.
IANA has assigned LDP TLV Type 0x0703 for the 'P2MP PW Capability TLV' in the LDP 'TLV Type Name Space' registry per RFC 5036.
IANA has assigned mLDP Opaque Value Element Type 13 ('L2VPN-MCAST application TLV') in the 'LDP MP Opaque Value Element basic type' registry (RFC 6388); the value is a 32-bit integer unique in the context of the root's address, with Length 4.
IANA has assigned sub-TLV type 0x1B ('Selective Tree Interface Parameter') in the 'Pseudowire Interface Parameters Sub-TLV type Registry' (RFC 4446).
IANA has updated value 0xFF in the 'P-Multicast Service Interface Tunnel (PMSI Tunnel) Tunnel Types' registry (RFC 7385) from 'Reserved' to 'wildcard transport tunnel type', as specified by this document.
The security measures from RFC 8077 are generally adequate, but the use of MD5 to secure the LDP control plane is no longer considered sufficiently secure. Implementations should be designed to migrate to a stronger cryptographic hash function for LDP authentication.
The LDP P2MP PW Capability TLV (type 0x0703) is a 6-octet structure containing U bit, F bit, the TLV Code Point (16 bits), a 16-bit Length field (MUST be set to 2), and an S bit plus reserved fields; the S bit indicates advertisement (1) or withdrawal (0) of the capability.
The P2MP PW Upstream FEC Element (type 0x82) contains an 8-bit FEC type, C bit, 15-bit PW Type, PW Info Length, variable-length AGI (type/length/value), SAII (AII Type 2 format), PMSI Tunnel Type (1 octet), PMSI TT Length (1 octet), Transport LSP ID, and optional parameters.
The P2P PW Downstream FEC Element (type 0x84) contains the same field structure as the P2MP PW Upstream FEC Element but without the PMSI Tunnel Info fields; it is used to signal optional unidirectional P2P traffic from L-PEs back to the R-PE.
The Selective Tree Interface Parameter sub-TLV (type 0x1B) carries a 1-octet sub-TLV type, 1-octet length, 1-octet Multicast Source Length, variable-length Multicast Source, 1-octet Multicast Group Length, and variable-length Multicast Group, identifying a specific (S,G) multicast tree.
The Typed Wildcard FEC format for P2MP PW FEC Elements reuses the RFC 6667 format (Typed Wcard=0x5, Type, Len=3, R bit, PW Type) and adds a 1-octet PMSI Tunnel Type field; this extends the Type field to include 0x82 (P2MP PW Upstream) and 0x84 (P2P PW Downstream).