Label Advertisement Discipline for LDP Forwarding Equivalence Classes (FECs)
updates
- rfc-3212 — Constraint-Based LSP Setup using LDP
- rfc-4447 — Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)
- rfc-5036 — LDP Specification
- rfc-5918 — Label Distribution Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class (FEC)
- rfc-6388 — Label Distribution Protocol Extensions for Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
- rfc-7140 — LDP Extensions for Hub and Spoke Multipoint Label Switched Path
Extracted elements (15)
The negotiated label advertisement mode in LDP (carried in the Initialization message) is only indicative, not a strict rule. For some FEC types, the advertising behavior is governed by the FEC type itself rather than the negotiated session-level mode. This document clarifies that fact to remove ambiguity.
This document updates RFCs 3212, 4447, 5036, 5918, 6388, and 7140 to specify the label advertisement discipline for all currently defined LDP FEC types, ensuring consistent behavior across implementations regardless of the session-negotiated mode.
Each document defining an LDP FEC MUST state the applicability of the negotiated label advertisement discipline for label binding advertisements for that FEC. If the negotiated discipline does not apply, the document MUST also explicitly state the discipline to be used.
Label advertisement discipline types defined for LDP FECs are: DU (Downstream Unsolicited), DoD (Downstream on Demand), As negotiated (DU or DoD), Upstream (per RFC 6389), Not applicable, and Unknown.
The CR-LSP FEC (0x04, RFC 3212) uses Downstream on Demand (DoD) label advertisement discipline, independent of the session-negotiated mode.
The HSMP-upstream FEC (0x09) uses DU discipline; the HSMP-downstream FEC (0x0A) uses DU and Upstream discipline (both defined in RFC 7140).
The P2MP FEC (0x06), MP2MP-up FEC (0x07), and MP2MP-down FEC (0x08), all defined in RFC 6388, use Downstream Unsolicited (DU) label advertisement discipline.
The P2MP PW Upstream FEC Element (0x82) uses Upstream label advertisement discipline; the P2MP PW Downstream FEC Element (0x84) uses DU discipline.
The Prefix FEC (0x02) has a label advertisement discipline of 'As negotiated (DU or DoD)', meaning label bindings follow the session-level negotiated advertisement mode.
The Protection FEC Element (0x83) uses Downstream Unsolicited (DU) label advertisement discipline.
The PWid FEC Element (0x80) and Generalized PWid FEC Element (0x81), both defined in RFC 4447, use Downstream Unsolicited (DU) label advertisement discipline.
The Typed Wildcard FEC Element (0x05, RFC 5918) has a label advertisement discipline of 'Not applicable'.
The Wildcard FEC (0x01) has a label advertisement discipline of 'Not applicable', meaning the session-negotiated mode does not govern its label binding advertisements.
IANA's 'Forwarding Equivalence Class (FEC) Type Name Space' registry under 'Label Distribution Protocol (LDP) Parameters' is extended with a new 'Label Advertisement Discipline' column. Possible values are DU, DoD, As negotiated (DU or DoD), Upstream, Not applicable, and Unknown.
This document only clarifies the applicability of an LDP session's label advertisement mode and does not add any new LDP security mechanisms or considerations beyond those already defined in RFC 5036.