LDP Extensions for Multi-Topology
updated by
- rfc-9658 — Multipoint LDP Extensions for Multi-Topology Routing
Extracted elements (30)
Non-reserved, non-special IGP MT-ID values can be carried in LDP without translation, but reserved or special IGP MT-ID values require translation to corresponding LDP MT-IDs. The allocation policy for new MT-ID values is deferred to a separate future document.
The use of multiple topologies for LDP does not require different label spaces for each topology; an LSR can use the same label space for all MT FECs as for the default topology, and signaling for different topologies can be done within a single LDP session. This avoids data-plane changes.
Existing MIB modules for MPLS LSR (RFC 3813) and MPLS LDP managed objects (RFC 3815) do not provide the necessary information to support MT extensions, as the absence of MT-ID as an index means there is no way to disambiguate different topology instances.
For LSP ping with MT LDP LSPs, the reply packet for a non-return-path-specified Echo Request must go through the default topology rather than the topology where the Echo Request traveled.
The MPLS-MT solution is backwards compatible with existing LDP enhancements defined in RFC 5036, including message authenticity, integrity, and topology loop detection. Legacy nodes that do not support MT should not receive any MT-related LDP messages.
An LDP speaker MUST NOT send messages containing MT FEC elements unless the peer has advertised the MT capability.
An LSR MUST discard the entire LDP message before sending a Notification message with 'Invalid Topology ID' status when it receives a message with an FEC element specifying an MT-ID that is not locally known or not supported.
If an LSR does not support Dynamic Capability Announcement, it MUST reset the session with its peer whenever the LSR changes its local capability with regards to supporting LDP MT.
If an LSR has not advertised MT capability, its peer MUST NOT send to this LSR any LDP messages with FEC elements that include an MT identifier.
If an LSR receives a FEC element with an MT-ID value that is 'Unassigned' for future use and not IANA allocated yet, the LSR MUST abort the processing of the FEC element and SHOULD send a notification message with status code 'Invalid Topology ID' to the sender.
MT-ID value 0 MUST be ignored on receipt in the MT IP/MT IPv6 Address Family format so as not to cause conflict or confusion with existing non-MT procedures.
The MT-ID field of the MT Typed Wildcard FEC element MUST be set to 'Wildcard Topology' (65535) when specified in the MT Capability TLV.
The S-bit in the MT Capability TLV MUST be 1 if used in an LDP Initialization message. MAY be set to 0 or 1 in a dynamic Capability message to advertise or withdraw the capability, respectively.
The U-bit in the Multi-Topology Capability TLV MUST be 1 so that the TLV will be silently ignored by a recipient if it is unknown. The F-bit MUST be 0 per LDP Capabilities (RFC 5561).
MT-ID is a 16-bit value used to represent the Multi-Topology ID. MT-ID 0 corresponds to the default topology. Two new Address Families, 'MT IP' (AF 29) and 'MT IPv6' (AF 30), extend IP address families to include topology scope by appending a 16-bit MT-ID field after the IP address.
The LDP MT Capability advertisement allows an LDP speaker to announce MT support during session initialization (via Initialization message) or after session establishment (via Capability message, if Dynamic Capability Announcement has been negotiated per RFC 5561).
The MT Typed Wildcard FEC element allows an LSR to perform wildcard FEC operations under the scope of a specific topology (using a particular MT-ID) or across all topologies (using MT-ID set to 'Wildcard Topology' = 65535).
Topology-Scoped FEC: To set up MT LSPs for IP prefixes under a given topology scope, the LDP prefix-related FEC element is extended to include topology information; the MT-ID becomes an attribute of the prefix-related FEC element and all FEC-Label binding operations are performed under the context of a given topology.
Two new LSP ping FEC sub-types are defined: MT LDP IPv4 FEC (sub-type 31, length 8) and MT LDP IPv6 FEC (sub-type 32, length 20), both used in the Target FEC Stack TLV of RFC 4379 to support data-plane failure detection for MT LDP LSPs.
New LDP Capability TLV 'Multi-Topology Capability' (0x050C) registered in the LDP Parameters 'TLV Type Name Space' registry, and new Status Code 'Invalid Topology ID' (0x00000031) registered in the LDP Parameters 'Status Code Name Space' registry.
New 'MPLS Multi-Topology Identifiers' registry created under the MPLS category. Reserved values: 0 (default topology), 1 (IPv4 in-band management), 2 (IPv6 routing topology), 3 (IPv4 multicast topology), 4 (IPv6 multicast topology), 5 (IPv6 in-band management), 6-3995 (unassigned IGP topologies, Standards Action), 3996-4095 (Experimental), 4096-65534 (unassigned MPLS topologies, Standards Action), 65535 (Wildcard Topology).
Two new address family numbers registered under IANA 'Address Family Numbers': 29 for 'MT IP: Multi-Topology IP version 4' and 30 for 'MT IPv6: Multi-Topology IP version 6'.
Two new sub-TLV types registered under TLV type 1 (Target FEC Stack) in the MPLS LSP Ping Parameters registry: sub-type 31 for MT LDP IPv4 prefix and sub-type 32 for MT LDP IPv6 prefix.
The use of MT over existing MPLS solutions does not offer any specific security benefit. General LDP communication security threats (spoofing, privacy violations, denial of service) apply; mitigations are described in RFC 5036 and RFC 5920.
MT IP Address Family format: 32-bit IPv4 address followed by a 16-bit Reserved field and a 16-bit MT-ID field. MT IPv6 Address Family format: 128-bit IPv6 address followed by a 16-bit Reserved field and a 16-bit MT-ID field.
MT LDP IPv4 FEC Sub-TLV (Type=31, Length=8): 32-bit IPv4 prefix, 8-bit Prefix Length, 8-bit MBZ (Must Be Zero), and 16-bit MT-ID.
MT LDP IPv6 FEC Sub-TLV (Type=32, Length=20): 128-bit IPv6 prefix, 8-bit Prefix Length, 8-bit MBZ, and 16-bit MT-ID.
MT Prefix FEC Element: 1-byte FEC type (2), 2-byte Address Family (MT IP or MT IPv6), 1-byte PreLen, variable-length Prefix, followed by 16-bit Reserved and 16-bit MT-ID fields.
MT Typed Wildcard FEC Element: 1-byte Typed Wildcard type (5), 1-byte FEC Type, 1-byte Len=6, 2-byte AF (MT IP or MT IPv6), and 2-byte MT-ID field.
Multi-Topology Capability TLV: U-bit (MUST be 1), F-bit (MUST be 0), 2-byte capability type (IANA assigned 0x050C), 2-byte Length, S-bit, Reserved, and one or more Typed Wildcard FEC elements as capability data.