Multipoint LDP Extensions for Multi-Topology Routing
updates
- rfc-7307 — LDP Extensions for Multi-Topology
Extracted elements (23)
Since the {MT-ID, IPA} tuple is part of the mLDP FEC, there is no need to support multiple topology forwarding tables or per-topology label forwarding tables in mLDP; each MP LSP will be unique due to the tuple being part of the FEC and will have its own unique local label.
The {MT-ID, IPA} tuple is encoded as part of the mLDP FEC rather than as a separate signaling mechanism. This ensures that different MT-ID and IPA values result in unique MP-LSP FEC elements, avoiding conflicting MTR policies for the same mLDP FEC.
The term 'IGP Algorithm' (IPA) is used rather than 'Flexible Algorithm' (FA) throughout the document because, while FA is the current mechanism for creating sub-topologies, different algorithms might be defined in the future for this purpose, making the IPA field extensible to any non-default algorithm.
For LSP ping of MT MP LSPs, the rules and procedures for using the new sub-TLV in MPLS Echo Request messages are the same as defined for P2MP/MP2MP LDP FEC Stack sub-TLVs in RFC 6425; the only difference is that the Root LSR Address field is now sub-topology scoped.
The MT extensions defined in this document do not require any extension to procedures for support of the Typed Wildcard FEC Element (RFC 5918); these procedures apply as-is to Multipoint MT FEC wildcarding.
The MT extensions for MP FEC do not require any modifications to the End-of-LIB procedures from RFC 5919; those procedures apply as-is to MT MP FEC elements. An MT mLDP speaker MAY signal convergence per sub-topology using the MT Typed Wildcard MP FEC Element.
An mLDP speaker that has successfully advertised and negotiated the 'MT Multipoint' capability MUST support topology-scoped mLDP FECs in LDP messages and topology-scoped mLDP forwarding setup.
In the case of the Typed Wildcard FEC Element, the FEC element type MUST be one of the MP FECs (P2MP type 0x6, MP2MP-up type 0x7, or MP2MP-down type 0x8).
In the MT Multipoint Capability TLV, the U and F bits MUST be 1 and 0 respectively (per RFC 5561), and the Length field value MUST be 1 as there is no capability-specific data following the TLV.
The 8-bit Reserved field in the MT IP address family format MUST be zero on transmission and MUST be ignored on receipt.
This capability MAY be sent either in an Initialization message at session establishment or dynamically during the session via a Capability message, provided the Dynamic Announcement capability from RFC 5561 has been successfully negotiated.
When the MT extension defined in this specification is in use, and the Tunnel Type in the PMSI tunnel attribute is set to mLDP P2MP or MP2MP LSP, the IP MT-Scoped MP FEC element form of the respective FEC elements MUST be used.
Downstream forwarding interface selection for MT mLDP restricts candidate interfaces to those that are part of the same sub-topology signaled in the mLDP FEC element; all other RFC 6388 procedures apply unchanged.
The IPA (IGP Algorithm) field is an 8-bit identifier for the algorithm used to create the sub-topology, with permissible values tracked in the IANA 'IGP Algorithm Types' registry. It is encoded in the MT IP address family data and the MT MP FEC element.
The MT MP FEC element encodes the topology scope by using the 'MT IP' or 'MT IPv6' address family in the MP FEC element, with the root node address field extended to include an 8-bit Reserved field, an 8-bit IPA field, and a 16-bit MT-ID field following the IP address.
The 'MT Multipoint Capability' is a new LDP capability TLV (code point 0x0510) that an mLDP speaker advertises to peers to announce support for MTR and the procedures in this document. The U and F bits MUST be 1 and 0 respectively, the Length MUST be 1, and the S bit is set to 1 to announce or 0 to withdraw.
The Typed Wildcard MT MP FEC Element allows an LSR to perform wildcard MP FEC operations under the scope of a sub-topology. It encodes a 1-byte Typed Wildcard type (5), a 1-byte MP FEC type, a 1-byte length (6), the AF (MT IP or MT IPv6), an 8-bit Reserved field, an 8-bit IPA field, and a 16-bit MT-ID field.
Upstream LSR selection for MT mLDP uses the {MT-ID, IPA} tuple from the FEC to identify the sub-topology used to find the best path to the root address, then selects an LDP peer from the next-hop following RFC 6388 procedures.
IANA assigned the value 0x0510 from the 'TLV Type Name Space' registry in the 'Label Distribution Protocol (LDP) Parameters' group for the new 'MT Multipoint Capability' TLV defined by this document.
This extension to mLDP does not introduce any new security considerations beyond those already applied to the base LDP specification (RFC 5036), the LDP extensions for Multi-Topology (RFC 7307), the base mLDP specification (RFC 6388), and the MPLS security framework (RFC 5920).
For LSP ping of MT MP LSPs, the existing P2MP/MP2MP LDP FEC Stack sub-TLV sub-types are reused with 'MT IP' or 'MT IPv6' in the Address Family field, Address Length set to 8 (MT IP) or 20 (MT IPv6), and the Root LSR Address field extended with an 8-bit Reserved, 8-bit IPA, and 16-bit MT-ID suffix.
The MT IP address family data format consists of a 32-bit IPv4 address (or 128-bit IPv6 address) followed by an 8-bit Reserved field (MUST be zero on transmission, ignored on receipt), an 8-bit IPA field, and a 16-bit MT-ID field. This updates RFC 7307 by repurposing 8 bits from the previously 16-bit Reserved field.
The MT Multipoint Capability TLV format includes a U bit, F bit, a 14-bit TLV type field, a 16-bit Length field (value MUST be 1), and a 1-byte value field containing the S bit and 7 Reserved bits.