Updates to MPLS Transport Profile (MPLS-TP) Linear Protection in Automatic Protection Switching (APS) Mode
updates
- rfc-7271 — MPLS Transport Profile (MPLS-TP) Linear Protection to Match the Operational Expectations of Synchronous Digital Hierarchy, Optical Transport Network, and Ethernet Transport Network Operators
Extracted elements (12)
In cold reboot scenarios, no valid SF/SD indications may be present; the PSC Control Logic restarts as if no local requests are present and responds to SF/SD only when detected later. This is a non-normative clarification of expected restart behavior.
Remembering the active path at initialization (when no local request exists) minimizes traffic switchovers when the remote node is still in operation, and does not cause problems even if the remembered path is no longer valid due to local inputs that occurred while the node was out of operation.
This document does not introduce backward compatibility issues with RFC 7271 implementations. If a node implementing RFC 8234 restarts, the new state changes will not cause problems at a remote node implementing RFC 7271, and both ends will converge to the same local and remote states.
If an SF-W or SF-P exists as the highest local request at initialization, the node MUST start at the PF:W:L or UA:P:L state, respectively.
If the local node receives an EXER message as the first protocol message after initialization and the remote EXER becomes the top-priority global request, the local node MUST set the bridge/selector position per the Path value in the EXER message and transit to the E::R state.
In case any local SD exists at initialization, the local SD MUST be considered as an input to the Local Request Logic only after the local node has received the first protocol message from the remote node and completed processing.
When the local SF-P is cleared and local/remote request priorities are re-evaluated, the last received remote message MUST be treated as NR (since it may be invalid due to the previous protection path failure) and only the local request shall be evaluated.
When the PSC Control Logic is initialized, it MUST stop the WTR timer if running and clear any operator command in the Local Request Logic.
No new security issues are raised beyond those documented in RFC 7271. Tightening the description of initialization behavior may help protect networks from restart attacks.
At initialization with no local request: if the node remembers no active path or remembers the working path as active, it starts in Normal state; if it remembers the protection path as active, it starts in WTR state sending NR(0,1) or DNR state sending DNR(0,1) depending on reversion configuration.
The 'ignore' flag ('i' in state transition tables) is clarified: if re-evaluation is triggered and the flag is set, the state machine transits to the supposed state (Normal or DNR per footnotes); if re-evaluation is not triggered and the flag is set, the state machine remains in the current state and continues transmitting the current PSC message.
The remote state transition table is modified in four cells: for states N, PF:W:R, and PF:DW:R upon receiving a DNR message, the transition is now explicitly DNR (replacing note (10)); and for state N upon receiving WTR, cell (13) is retained. This ensures the two end nodes converge after initialization.