The Path Computation Element Communication Protocol (PCEP) Extension for Wavelength Switched Optical Network (WSON) Routing and Wavelength Assignment (RWA)
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 (29)
Optical impairments are explicitly excluded from the RWA path computation scope of this document, as the physical-layer aspects are not currently standardized. The framework for optical impairments is deferred to RFC 6566.
The Combined Process (R&WA) architecture is chosen as the base, where a single PCE handles both routing and wavelength assignment. This simplifies the PCC interface and reflects the interdependency between routing and wavelength selection in transparent optical networks, where wavelength continuity constrains path choice.
Wavelength assignment is expressed in terms of labels and label sets (per RFC 7579/RFC 6205) rather than frequency-specific encoding, because a wavelength is a type of GMPLS label. This alignment with generic GMPLS label mechanisms allows reuse of existing label constraint encoding across the PCEP framework.
Signal processing capabilities (Optical Interface Class and Client Signal) are already addressed in GMPLS signaling (RFC 7581, RFC 7688). This document extends PCEP to carry the same constraints during path computation, ensuring the PCE-computed path is compatible with the signaling-layer signal compatibility requirements.
The Link Identifier Type field reuses the 'TE_LINK Object Class type name space (Value 11)' registry originally created for the Link Management Protocol (LMP, RFC 4204). An IANA note was added to that registry to reflect this additional usage, ensuring consistent type assignment across LMP and PCEP WSON contexts.
For any request containing a wavelength assignment, the PCC MUST specify a Wavelength Restriction TLV to constrain the wavelengths to be used, reflecting laser tunability or maintenance-related constraints.
If a PCE receives an RWA request and cannot process it due to insufficient memory, the PCE MUST send a PCErr with Error-Type=27 and Error-value=1, stop processing, and the PCC MUST cancel the corresponding RWA request.
If a PCE receives an RWA request but is not capable of RWA computation, the PCE MUST send a PCErr with Error-Type=27 and Error-value=2, stop processing, and the PCC MUST cancel the corresponding RWA computation.
If a PCE receives an unrecognized Action value in the Wavelength Restriction TLV, the PCE MUST send a PCErr message with Error-Type=27 and Error-value=3. Syntactical encoding errors (wrong link ID count, unknown identifier types, no matching link) also require a PCErr with Error-Type=27, Error-value=3.
If the M bit in the WA object is set to 0 (distributed wavelength assignment), the PCE MUST return a Label Set Field as described in Section 2.6 of RFC 7579 in the response. Unused flags in the WA object SHOULD be zeroed.
The PCE MUST have mechanisms to know signal processing capabilities at each interface (e.g., via TED through IGP or NMS) to perform signal compatibility checking during path computation.
The WA object MUST be encoded after the END-POINTS object in a PCReq message. At least one TLV (Wavelength Selection or Wavelength Restriction) MUST be present in the WA object.
The Wavelength Selection TLV MUST NOT be used when the M bit in the WA object is cleared (M=0); it is only applicable when explicit label control mode is requested (M=1).
A new NO-PATH-VECTOR TLV bit flag (Bit 23) is defined: when set by the PCE, it indicates no feasible route was found satisfying all RWA constraints (wavelength restriction, signal compatibility, etc.). This flag is carried in the NO-PATH object's NO-PATH-VECTOR TLV.
The Wavelength Assignment (WA) object (Object-Class 42, Object-Type 1) is a new PCEP object carrying wavelength-assignment-related constraints in a PCReq message. It contains a 16-bit Flags field (with M bit for allocation mode) followed by variable-length TLVs; at least one TLV MUST be present.
Two new endpoint restriction TLVs are defined for the END-POINTS Generalized Endpoint: Optical Interface Class List TLV (Type 11, encoding per RFC 7581 Section 4.1) and Client Signal Information TLV (Type 12, encoding per RFC 7581 Section 4.2). These carry signal compatibility constraints for RWA path computation.
Two new IRO subobjects are defined for signal processing inclusion (Types 8 and 9), mirroring the XRO subobjects except the X bit is replaced by the L bit per RFC 3209. These allow PCC/PCE to mandate specific signal processing types along the computed path.
Two new XRO subobjects are defined for signal processing exclusion: Type 8 (Optical Interface Class List) and Type 9 (Client Signal Information). Each has an X bit, 7-bit Type, Length, Reserved (8 bits), Attribute (8 bits, 0=Interface or 1=Node), and the respective information encoded per RFC 7581.
A new 'Wavelength Restriction TLV Action Values' subregistry was created, managed by Standards Action. Initial values: 0 = Inclusive List, 1 = Inclusive Range, 2–255 Unassigned. A new 'Wavelength Allocation TLV Flag Field' subregistry was also created with Bit 15 for Wavelength Allocation Mode.
IANA allocated four new PCEP TLV types: Type 8 (Wavelength Selection), Type 9 (Wavelength Restriction), Type 10 (Wavelength Allocation), Type 11 (Optical Interface Class List), and Type 12 (Client Signal Information) in the 'PCEP TLV Type Indicators' subregistry.
IANA allocated PCEP Object-Class 42 with Object-Type 1 for the new Wavelength Assignment (WA) object. A new 'WA Object Flag Field' subregistry was created under PCEP Numbers, managed by Standards Action; Bit 15 is defined as 'Wavelength Allocation Mode'.
New Error-Type 27 (WSON RWA error) was registered in 'PCEP-ERROR Object Error Types and Values' with Error-values: 1 (insufficient memory), 2 (RWA computation not supported), 3 (syntactical encoding error). XRO and IRO subobject Types 8 and 9 were also registered.
This document does not introduce new security issues beyond those in RFC 5440 (PCEP). To protect confidential WSON topology information, operators SHOULD use PCEPS (TLS-secured PCEP transport per RFC 8253).
The Link Identifier field supports three types: Type 1 (IPv4, 4-byte address), Type 2 (IPv6, 16-byte address), and Type 3 (Unnumbered IF ID: 4-byte TE Node ID + 4-byte Interface ID). Each type is preceded by an 8-bit Type field and 24 reserved bits. All link identifiers in the same list MUST be of the same type.
The WA object body encodes: Reserved (16 bits), Flags (16 bits, M bit at bit 15 for Wavelength Allocation Mode), followed by variable-length TLVs. The M bit distinguishes explicit label control (M=1) from Label Set-based distributed assignment (M=0).
The Wavelength Allocation TLV (Type 10) in PCRep messages encodes: Reserved (16 bits), Flags (16 bits with M bit at bit 15 for allocation mode), Link Identifier (variable, indicating the interface), and Allocated Wavelength(s) (encoded as per Section 4.3.2). This TLV is carried as an Attribute TLV in ERO Hop Attribute subobjects.
The Wavelength Constraint field is encoded as a Label Set Field per RFC 7579 Section 2.6, with the base label as a 32-bit LSC label per RFC 6205. It includes Action (4 bits), Num Labels (12 bits), Length (16 bits), Grid/C.S./Identifier/n fields, and optional additional fields. The Identifier field MUST always be 0.
The Wavelength Restriction TLV (Type 9) encodes one or more tuples of: Action (8 bits, 0=Inclusive List, 1=Inclusive Range), Count (8 bits, number of link identifiers), Reserved (16 bits), Link Identifiers (variable), and Wavelength Constraint (variable). Multiple action/restriction groups may be chained.
The Wavelength Selection TLV (Type 8) has a fixed length of 32 bits and encodes the wavelength selection constraint (order of wavelength assignment). Its encoding mirrors the WavelengthSelection sub-TLV from Section 4.2.2 of RFC 7689.