Path Computation Element Communication Protocol (PCEP) Extensions for Associated Bidirectional Label Switched Paths (LSPs)
updated by
- rfc-9756 — Update to the IANA Path Communication Element Protocol (PCEP) Numbers Registration Procedures and the Allowance of Experimental Error Codes
Extracted elements (27)
Double-sided initiation has the advantage that two existing unidirectional LSPs in opposite directions can be associated into a bidirectional LSP without significantly increasing operational complexity, making it suitable for associating independently provisioned LSPs.
If the Bidirectional LSP Association Group TLV is absent, it implies the LSP is a forward LSP and is not co-routed. This default avoids mandatory inclusion of the TLV for the common forward non-co-routed case, reducing message overhead.
Single-sided initiation is preferred for OAM use cases where an endpoint needs to know both forward and reverse paths. It also allows the originating PCC to dynamically compute co-routed forward and reverse paths and can be deployed where PCEP is only enabled on originating endpoint nodes, since remote nodes create reverse tunnels via RSVP-TE.
A Bidirectional LSP Association MUST NOT have both LSPs identified as the same direction (both forward or both reverse). Violation MUST result in PCErr Error-Type=26, Error-value=17 (Bidirectional LSP direction mismatch).
A Bidirectional LSP Association MUST NOT have one LSP identified as co-routed and the other as non-co-routed. Violation MUST result in PCErr Error-Type=26, Error-value=18 (Bidirectional LSP co-routed mismatch).
An LSP (forward or reverse) MUST NOT be part of more than one Bidirectional LSP Association. Violation results in a PCErr with Error-Type=26 (Association Error), Error-value=14 (Association group mismatch).
Both PCCs MUST report the forward and reverse LSPs in the Bidirectional LSP Association to the PCE via a PCRpt message.
Capability exchange for Bidirectional LSP Association Types MUST be done before use: the PCEP speaker MUST include the Association Types in the ASSOC-Type-List TLV and MUST receive the same from the peer before using these Association Types in PCEP messages.
During state synchronization, a PCC MUST report all existing Bidirectional LSP Associations to the stateful PCE. After synchronization, the PCE MUST remove all previous Bidirectional LSP Associations absent in the report.
For co-routed LSPs, the Bidirectional LSP Association Group TLV MUST be present and the C flag set for both forward and reverse LSPs. For reverse LSPs, the TLV MUST be present and the R flag set. For forward LSPs within the TLV, the R flag MUST be reset.
For PCC-initiated Single-Sided Bidirectional LSP Association, the PCC MUST send a PCRpt message to the PCE with both forward and reverse LSPs. For Double-Sided, both PCCs MUST send PCRpt with their forward LSPs.
For PCE-initiated Single-Sided Bidirectional LSP Association, the PCE MUST send a PCInitiate message to the originating endpoint node with both forward and reverse LSPs. For Double-Sided, the PCE MUST send PCInitiate to both endpoint nodes with their respective forward LSPs.
For Single-Sided Bidirectional LSP Association, the same RSVP-TE tunnel MUST contain both the forward and reverse LSPs on the originating node (albeit with reversed endpoint nodes). Tunnel mismatch results in PCErr Error-Type=26, Error-value=15.
If a PCE speaker receives a Bidirectional LSP Association Type it does not support, it MUST send PCErr with Error-Type=26, Error-value=1 (Association Type not supported). If PST is not RSVP-TE (value 0), the speaker MUST return PCErr Error-Type=26, Error-value=16.
In the single-sided PCC-initiated case, the originating endpoint node SHOULD delegate both forward and reverse LSPs of a tunnel together to a stateful PCE in order to avoid any race condition.
The Bidirectional LSP Association Group TLV MUST NOT be present more than once; if it appears more than once, only the first occurrence is processed and others MUST be ignored.
The LSP-IDENTIFIERS TLV (per RFC 8231) MUST be included in all forward and reverse LSPs of an associated bidirectional LSP.
The LSPs in a Bidirectional LSP Association MUST have matching endpoint nodes in the reverse directions. If a PCE speaker receives a non-compliant LSP, it MUST send PCErr Error-Type=26, Error-value=19 (Endpoint mismatch in the association group).
The operator-configured Association Range MUST be set for Bidirectional LSP Association Types to delineate a range of Association Identifiers for operator-configured associations and avoid identifier clashes within the scope of the Association Source.
In the Single-Sided Bidirectional LSP Association, the reverse LSP of a bidirectional LSP is identified by the PCE using two different PLSP-IDs: one (e.g., P2) allocated at the ingress node PCC and another (e.g., P3) at the egress node PCC. The forward LSP and Double-Sided LSPs follow standard RFC 8231 PLSP-ID allocation.
Two new Association Types are defined: 'Single-Sided Bidirectional LSP Association' (Type 4) and 'Double-Sided Bidirectional LSP Association' (Type 5), using the generic ASSOCIATION object (Object-Class value 40) defined in RFC 8697. In single-sided, both forward and reverse LSPs are initiated on one endpoint; in double-sided, forward LSPs are initiated on both endpoints independently.
IANA assigned six new Error-values for Error-Type 26 (Association Error) in the 'PCEP-ERROR Object Error Types and Values' subregistry: 14 (Association group mismatch), 15 (Tunnel mismatch in the association group), 16 (Path Setup Type not supported), 17 (Bidirectional LSP direction mismatch), 18 (Bidirectional LSP co-routed mismatch), 19 (Endpoint mismatch in the association group).
IANA assigned TLV Type 54 ('Bidirectional LSP Association Group TLV') in the 'PCEP TLV Type Indicators' subregistry within the 'PCEP Numbers' registry.
IANA assigned two new values in the 'ASSOCIATION Type Field' subregistry within the 'PCEP Numbers' registry: Type 4 (Single-Sided Bidirectional LSP Association) and Type 5 (Double-Sided Bidirectional LSP Association).
IANA created a new 'Bidirectional LSP Association Group TLV Flag Field' subregistry within the 'PCEP Numbers' registry, managed by Standards Action. Initial entries: bits 0–29 unassigned, bit 30 = C (Co-routed Path), bit 31 = R (Reverse LSP).
Security considerations from RFC 5440, RFC 8231, and RFC 8281 apply. Malicious PCEP speakers could exploit the two new Association Types (Single-Sided and Double-Sided Bidirectional LSP Association) as described in RFC 8697. Securing the PCEP session using TLS (RFC 8253) is RECOMMENDED.
The Bidirectional LSP Association Group TLV has Type=54, Length=4 bytes, and a 32-bit Flags field. Bit 31 (R) indicates a reverse LSP; bit 30 (C) indicates a co-routed bidirectional LSP. Bits 0–29 are unassigned and MUST be set to 0 on send and ignored on receipt.