Multipoint LDP In-Band Signaling for Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
updated by
- rfc-7438 — Multipoint LDP (mLDP) In-Band Signaling with Wildcards
Extracted elements (17)
For PIM-SM in ASM mode to work with in-band signaling, active sources must be discovered by some means other than a shared tree across the MPLS domain (e.g., MSDP or BGP Source Active auto-discovery routes); this document does not specify source discovery and therefore does not fully specify ASM support.
For sender-only branches of a bidirectional tree, a MP2MP LSP is created; other branches receive the packets but drop them since they have no receivers. If this packet waste is undesirable, an alternative transport (e.g., a multipoint-to-point LSP or unicast to root) must be used.
In-band signaling carries the multicast tree identity ((S,G) or (*,G)) in the 'opaque value' field of the mLDP FEC Element, avoiding a separate out-of-band mapping protocol. PIM control messages are sent periodically outside the MPLS domain, but mLDP messages are not repeated periodically.
Transiting PIM shared trees (ASM *,G) across a transit LSP is out of scope because it is not possible to prune individual sources from the shared tree without an additional out-of-band protocol such as PIM or BGP.
If the root LSR does not recognize the opaque value type in a received FEC Element, the MP LSP will still be established, but the root node will not send any multicast data packets on it.
Source or RP addresses reachable only in a VPN context, and multicast groups operating in PIM Dense-Mode, are explicitly outside the scope of this document.
A bidirectional IP multicast tree (BIDIR-PIM) that must be transported over an MPLS network using in-band signaling MUST be transported using an MP2MP LSP.
An ingress LSR MUST NOT use a particular opaque value type in an mLDP FEC Element unless it knows (through provisioning or other out-of-band means) that the root LSR supports the root-node procedures for that opaque value type.
Bidirectional multicast produces two states: group-specific state (PIM Join received, subnet mask /32 for IPv4 or /128 for IPv6) and RP state (static RP mapping, variable subnet mask). Each state results in a separate MP2MP LSP; PIM on the root LSR merges the two LSPs by treating each LSP as a PIM-enabled interface.
Each IP multicast tree is mapped one-to-one to a P2MP or MP2MP LSP in the MPLS network. Operators should expect as many LSPs as there are IP multicast trees, and must monitor scalability of both PIM and mLDP state.
IANA allocated four values in the 'LDP MP Opaque Value Element basic type' registry: Transit IPv4 Source TLV type=3, Transit IPv6 Source TLV type=4, Transit IPv4 Bidir TLV type=5, Transit IPv6 Bidir TLV type=6.
The security considerations for this document are the same as those for the base LDP specification (RFC 5036); no additional security mechanisms are introduced or required by the in-band signaling procedures.
At the root LSR: on receiving a label mapping whose FEC contains a recognized transit opaque value, the multicast source and group are extracted and the downstream LDP neighbor is added to the olist of the (S,G) or (*,G) state (creating state if absent). On receiving a label withdraw, the downstream LDP neighbor is removed from the olist. Normal PIM processing follows in both cases.
Transit IPv4 Bidir TLV (Type=5, Length=9): encodes a 1-octet Mask Length (max 32), a 4-octet IPv4 RP address, and a 4-octet IPv4 multicast group address for bidirectional trees in the mLDP FEC opaque value field.
Transit IPv4 Source TLV (Type=3, Length=8): encodes a 4-octet IPv4 multicast source address followed by a 4-octet IPv4 multicast group address in the mLDP FEC opaque value field.
Transit IPv6 Bidir TLV (Type=6, Length=33): encodes a 1-octet Mask Length (max 128), a 16-octet IPv6 RP address, and a 16-octet IPv6 multicast group address for bidirectional trees in the mLDP FEC opaque value field.
Transit IPv6 Source TLV (Type=4, Length=32): encodes a 16-octet IPv6 multicast source address followed by a 16-octet IPv6 multicast group address in the mLDP FEC opaque value field.