PCE Communication Protocol (PCEP) Extensions for Label Switched Path (LSP) Scheduling with Stateful PCE
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 (31)
Grace periods (Grace-Before, Grace-After) are provided to extend the LSP lifetime slightly before and after the scheduled interval without reserving network resources (like bandwidth) during those periods, allowing the LSP to carry best-effort traffic while the reserved interval is cleanly bounded.
LSP scheduling in PCEP is preferred over existing NMS-based path pre-establishment because traditionally established paths reserve resources indefinitely, preventing other services from using bandwidth when the service is idle. Centralized scheduling in PCEP enables resources to be used by other services when not needed, improving network resource utilization efficiency.
The C flag in the scheduled TLV separates responsibility for LSP setup/teardown between PCC and PCE, allowing flexibility for deployments where either the PCC (C=1) or the delegated PCE (C=0) is better positioned to trigger signaling at the scheduled start time.
The elastic time interval feature (within -P and Q) is provided so that users who need a scheduled LSP for interval [Ta, Tb] but have some flexibility on exact timing can allow the PCE to optimize placement within the elastic range, enabling better resource utilization across the network.
A legacy PCEP implementation that does not understand this specification will treat the scheduled TLV as an unknown TLV and ignore it, as per Section 7.1 of RFC 5440, allowing backward compatibility with non-scheduling-aware peers.
A Duration value of 0 MUST NOT be used. The Duration SHOULD be greater than MINIMUM-DURATION (5 seconds). At expiry of Duration, the LSP MUST be torn down and deleted.
A SCHED-LSP-ATTRIBUTE TLV MUST NOT provide both grace period and elastic range for an LSP; only one may be configured.
For activation, the PCC MUST initiate setup of the scheduled LSP at the start time. On scheduled usage expiry, the PCC MUST initiate LSP removal based on the C flag in the SCHED-LSP-ATTRIBUTE TLV.
For PCC-initiated scheduled LSPs, a PCC MUST delegate the scheduled LSP via PCRpt message including scheduling information (start time and duration). Upon receiving delegation, the stateful PCE MUST check whether the parameters are valid.
For periodical LSP scheduling, if there is no path satisfying constraints for some intervals, the LSP MUST NOT be set up at all, and a PCErr MUST be generated with Error-Type=29 and Error-value=5.
The B bit MUST be set by both PCEP peers in order to support LSP scheduling for path computation. Both the PD bit and the B bit MUST be set to 1 by both PCEP peers in order to support periodical LSP scheduling.
The LSP object for a scheduled LSP MUST NOT include both SCHED-LSP-ATTRIBUTE and SCHED-PD-LSP-ATTRIBUTE TLVs simultaneously; only one scheduling type (normal or periodical) is allowed.
The PCEP extensions for scheduling MUST NOT be used if one or both PCEP speakers have not set the corresponding capability bits. If a speaker supports the extensions but did not advertise, upon receipt of LSP object with scheduled TLV it MUST generate PCErr with Error-Type=19, Error-value=15.
The SCHED-LSP-ATTRIBUTE TLV MUST NOT be included unless both PCEP peers have set the B (LSP-SCHEDULING-CAPABILITY) bit to 1. If received when the B bit was not set, the peer MUST generate a PCErr with Error-Type=19 and Error-value=15.
The SCHED-PD-LSP-ATTRIBUTE TLV MUST NOT be included unless both PCEP peers have set the B and PD bits to 1. If received when either bit is zero, the peer MUST generate a PCErr with Error-Type=19 and Error-value=15.
The scheduled TED MUST be maintained by all PCEs within the network environment.
The scheduled TLV MUST be included in the LSP object for scheduled LSPs in PCRpt, PCUpd, PCReq, and PCRep messages. If missing, the receiving PCEP speaker MUST send a PCErr with Error-Type=6 and Error-value=16 (Scheduled TLV missing).
The stateful PCE MUST update its local scheduled LSP-DB and scheduled TED with the scheduled LSP and MUST update scheduled LSP parameters on any network events using the PCUpd message to the PCC.
When an LSP is configured with relative time (R bit=1), PCEs and PCCs MUST synchronize their clocks; it is RECOMMENDED that NTP (RFC 5905) be used for clock synchronization.
The SCHED-LSP-ATTRIBUTE TLV (Type 49) is an optional TLV within the LSP object for normal LSP scheduling. It has a fixed length of 16 octets and carries flags (R, C, A, G), Start-Time (32-bit, seconds), Duration (32-bit, seconds), and GrB/Elastic-Lower-Bound and GrA/Elastic-Upper-Bound (each 16 bits).
The SCHED-PD-LSP-ATTRIBUTE TLV (Type 50) is an optional TLV within the LSP object for periodical LSP scheduling. It has a fixed length of 20 octets and extends SCHED-LSP-ATTRIBUTE with Opt (4-bit repeat option), NR (12-bit number of repeats), Reserved (8 bits), and Repeat-time-length (32-bit, seconds).
Two new flag bits are defined in the STATEFUL-PCE-CAPABILITY TLV: B (LSP-SCHEDULING-CAPABILITY) at bit position 22, indicating support for LSP scheduling, and PD (PD-LSP-CAPABILITY) at bit position 21, indicating support for periodical LSP scheduling.
IANA allocated three new PCEP-ERROR values: Error-Type 6, Error-value 16 (Scheduled TLV missing); Error-Type 19, Error-value 15 (Attempted LSP scheduling while capability not advertised); Error-Type 29, Error-value 5 (Constraints could not be met for some intervals).
IANA allocated two new bits in the STATEFUL-PCE-CAPABILITY TLV Flag Field: Bit 22=LSP-SCHEDULING-CAPABILITY (B-bit) and Bit 21=PD-LSP-CAPABILITY (PD-bit).
IANA allocated two new PCEP TLV Type Indicators: value 49 for SCHED-LSP-ATTRIBUTE and value 50 for SCHED-PD-LSP-ATTRIBUTE, in the 'Path Computation Element Protocol (PCEP) Numbers' registry.
IANA created a new 'SCHED-PD-LSP-ATTRIBUTE TLV Opt Field' subregistry with values: 0=Reserved, 1=REPEAT-EVERY-MONTH, 2=REPEAT-EVERY-YEAR, 3=REPEAT-EVERY-REPEAT-TIME-LENGTH, 4-14=Unassigned, 15=Reserved. New values assigned by Standards Action.
IANA created a new 'Schedule TLVs Flag Field' subregistry defining bits 4-7: Bit 4=Relative Time (R-bit), Bit 5=PCC Responsible (C-bit), Bit 6=LSP Activated (A-bit), Bit 7=Grace Period Included (G-bit). New values assigned by Standards Action.
Scheduling information in PCEP messages can reveal sensitive details about network operations, and a single PCEP message can instruct a PCC to set up and tear down an LSP repeatedly (e.g., every second), potentially causing significant network impact. Deployments SHOULD use PCEP security mechanisms like TCP-AO (RFC 5925) or PCEPS (RFC 8253), and PCCs may need to apply rate limiting to scheduled LSP processing.
A scheduled LSP transitions through: (1) inactive/pre-signaled state before start time — path computed but not signaled on LSRs; (2) active state at start time — LSP signaled and carrying traffic; (3) deleted state at expiry of duration — LSP torn down. The A flag in the scheduled TLV signals activation; the C flag determines whether the PCC or PCE is responsible for triggering transitions.
The SCHED-LSP-ATTRIBUTE TLV wire format is 16 octets: Type=49 (16 bits), Length (16 bits), Flags (8 bits including R/C/A/G bits), Reserved (24 bits), Start-Time (32 bits), Duration (32 bits), GrB/Elastic-Lower-Bound (16 bits), GrA/Elastic-Upper-Bound (16 bits).
The SCHED-PD-LSP-ATTRIBUTE TLV wire format is 20 octets: Type=50 (16 bits), Length (16 bits), Flags+Opt+NR+Reserved (32 bits), Start-Time (32 bits), Duration (32 bits), Repeat-time-length (32 bits), GrB/Elastic-Lower-Bound (16 bits), GrA/Elastic-Upper-Bound (16 bits).