Multipoint LDP (mLDP) In-Band Signaling with Wildcards
updates
Extracted elements (16)
In IPTV deployments with a small number of content server sites, statically configured channels MAY be forwarded over MP-LSPs with wildcard encoding (source and/or group depending on provider address allocation policy), as an alternative to MSDP or BGP Auto-Discovery for Ingress LSR source discovery.
In mLDP in-band signaling deployments, RP-to-group mappings are typically statically configured rather than dynamically distributed over the MPLS core. Configuring an Anycast RP address at each multicast site is advisable to provide redundancy, avoiding a single point of failure at the RP.
In PIM-SM deployments with 'threshold infinity', last hop routers deliberately avoid joining source-specific trees and receive all traffic via the (*,G) shared tree. This is preferred when the RP lies on the shortest source-to-receiver path, making source trees add only state overhead. Wildcard source encoding efficiently maps this (*,G) tree to an MP-LSP in such deployments.
RFC 6826 and RFC 7246 allow only a single (S,G) tree per MP-LSP and do not support non-bidirectional ASM shared trees. Wildcard encodings were introduced to address both limitations: they allow aggregating multiple IP multicast trees onto one MP-LSP (saving routing state) and allow (*,G) shared trees to be attached to MP-LSPs.
When senders and receivers are directly connected to an MPLS domain, IGMP/MLD proxying (RFC 4605) can replace PIM entirely, treating the MPLS domain as a single hop. In this scenario the mLDP root is manually configured, and the MP-LSP must carry all sources for the ASM group, requiring a wildcard source in the Opaque Value TLV.
An Egress LSR supporting RFC 6826/7246 but not this document will never generate Opaque values containing wildcards. An Ingress LSR supporting only RFC 6826/7246 has no choice but to treat received FEC elements with all-zero source or group subfields as invalid, because the prior procedures do not function with all-zero values in those subfields.
Procedures for wildcard group usage in Bidir TLVs (Transit IPv4 Bidir, Transit IPv6 Bidir, Transit VPNv4 Bidir, Transit VPNv6 Bidir) are explicitly outside the scope of this document. Procedures for using both a wildcard source and a wildcard group in the same TLV are also outside scope. Bidir TLVs have no source address subfield, so wildcard source does not apply to them.
An Egress LSR MUST NOT use wildcard encodings when setting up a particular multipoint LSP unless it is known a priori that the Ingress LSR supports the procedures of this document. How this is determined is outside the scope of this document.
The combination of a wildcard source with an ASM group address MUST NOT be used unless it is known a priori that neither source discovery nor source pruning are needed. Use of mLDP in-band signaling is NOT RECOMMENDED for cases that require source discovery or source pruning, as no such procedures are specified in this document.
When the Ingress LSR receives a wildcard-source Opaque Value TLV and PIM is enabled for a non-bidirectional ASM group, it MUST act as if it received a (*,G) IGMP/MLD report from downstream and follow RFC 4601 procedures. For PIM-SSM groups, all known multicast sources MUST be forwarded down the MP-LSP. If PIM is not enabled, it MUST follow RFC 4605 (IGMP/MLD proxying) procedures.
When the Ingress LSR receives an in-band signaling Opaque Value TLV with a wildcard group address, it examines its IP multicast routing table for all streams whose source address matches the TLV's source subfield; all such streams SHOULD be forwarded down the identified MP-LSP.
RFC 7438 extends the mLDP in-band signaling Opaque Value TLVs defined in RFC 6826 and RFC 7246 (Transit IPv4 Source, Transit IPv6 Source, Transit VPNv4 Source, Transit VPNv6 Source) to allow wildcard values in the IP source address subfield, the IP group address subfield, or both. This enables a single MP-LSP to carry traffic from a set of IP multicast trees or from a PIM shared tree.
Three wildcard semantic cases are defined: (1) wildcard source with a non-SSM ASM group address identifies a PIM-SM shared tree (*,G); (2) wildcard source with an SSM group address identifies the collection of PIM trees for that group; (3) non-zero source with a wildcard group identifies the collection of PIM-SSM trees rooted at that source (S,*).
To determine the MP-LSP root (Ingress LSR) for a (*,G) set of multicast streams, the RP address or proxy device address is substituted for the source address 'S' in the root discovery procedures of RFC 6826 and RFC 7246. Policy-based alternatives for root node determination are also permitted.
There are no security considerations beyond those already documented in RFC 5036 (LDP Specification), RFC 6826 (mLDP In-Band Signaling for P2MP and MP2MP LSPs), and RFC 7246 (mLDP In-Band Signaling in a VRF Table Context). Implementers should consult those documents for the applicable security analysis.
A value of all zeroes in the IP source address subfield represents a wildcard source address; a value of all zeroes in the IP group address subfield represents a wildcard group address. The lengths of these subfields are unchanged from their definitions in RFC 6826 and RFC 7246.