Pseudowire (PW) Operations, Administration, and Maintenance (OAM) Message Mapping
Extracted elements (26)
TDM and CEP PWs use PWE3 control word bit flags (L bit, R bit) for OAM rather than PW status messages because TDM and SONET/SDH are inherently real-time and OAM indications must be generated or forwarded with minimal delay, which rules out messaging-protocol-based approaches.
Two OAM modes are defined: 'single emulated OAM loop' mode, where a single end-to-end NS OAM loop is emulated by transparently passing NS OAM messages over the PW, and 'coupled OAM loops' mode, where three OAM loops are joined at the AC/PW PE boundaries and NS OAM is never carried over the PW. The choice reflects whether a homogeneous end-to-end NS OAM loop or independent per-segment OAM is more appropriate for the service type.
In ATM port mode PW service (transparent cell transport), the PE does not track individual VPC/VCC status and cannot relay PW defect state over those connections. End-to-end ATM CC cells running CE-to-CE or across a segment spanning the PSN will time out and independently cause CEs or ATM switches to enter AIS state.
This document is scoped exclusively to single-segment PWs. While the mechanisms may also apply to terminating PEs (T-PEs) in multi-segment PW deployments, RFC 6073 Section 10 provides the authoritative procedures for generating or relaying PW status by a switching PE (S-PE). Ethernet PW OAM interworking is covered in a separate document.
A PE MUST use PW status notification messages to report all defects affecting the PW service, including defects detected by PSN fault detection mechanisms, VCCV-Ping or VCCV-BFD (CV types 0x04/0x10), PE internal forwarding failures between AC and PW, and AC or L2 network defects as governed by the applicable OAM mode.
A PE that establishes a PW using LDP and has negotiated use of the LDP status TLV per RFC 4447 Section 5.4.3 MUST use the PW status TLV mechanism for AC and PW status and defect notification. Such a PE MAY additionally use VCCV-BFD CV types 0x04 and 0x10 for fault detection only.
A PE using VCCV-BFD MUST use diagnostic code 0 to indicate correct BFD reception, MUST use code 1 when it stops receiving BFD control messages (PW down receive direction), MUST use code 3 to confirm session going down after receiving code 1 from peer (PW down transmit direction), and MUST use code 7 for administrative PW shutdown. All other defects MUST be communicated through LDP Status TLV or L2TP Circuit Status AVP.
For a L2TPv3/IP PSN, a PE SHOULD use the Circuit Status AVP as the mechanism for AC and PW status and defect notification. VCCV-BFD CV types 0x08 and 0x20 SHOULD NOT be used when the PW is established via the L2TP control plane.
For ATM PW in AC receive defect state with coupled OAM loops mode, PE1 MUST send FDI to PE2 and MUST commence ATM RDI cell insertion towards CE1. With single emulated OAM loop mode, PE1 MUST transparently relay or insert ATM AIS cells over the PW towards CE2; if the defect interferes with NS OAM, PE1 MUST send FDI to PE2.
For ATM PW on entry to the PW receive defect state, PE1 MUST commence AIS insertion into the corresponding AC and MUST cease CC cell generation. If the defect was detected by PE1 without receiving FDI from PE2, PE1 MUST notify PE2 by sending RDI. On exit, PE1 MUST cease AIS insertion, MUST resume CC generation, and MUST clear the RDI to PE2.
For CEP PW receive defect state entry, PE1 MUST commence AIS-P/V insertion into the AC and MUST set the R bit in all PW packets sent back to PE2. On exit, PE1 MUST cease AIS-P/V insertion and MUST clear the R bit. On AC receive defect state entry, PE1 MUST set the L bit; if DBA is not enabled, PE1 SHOULD insert AIS-V/P in the SDH/SONET client layer in outgoing PW packets.
For Frame Relay PW on entry to the PW receive defect state, PE1 MUST clear the Active bit for the FR AC in a full Q.933 status report. On exit (but not on transition to PW transmit defect state), PE1 MUST set the Active bit. On entry to PW transmit defect state, PE1 MUST also clear the Active bit and, if defect was detected without RDI from PE2, MUST notify PE2 by sending FDI.
For L2TPv3/IP, a PE MUST set the Active bit in the Circuit Status AVP to clear all faults and MUST clear the Active bit to convey any defect that cannot be explicitly represented with specific Circuit Status flags from RFC 3931 or RFC 5641.
For TDM PW AC receive defect state entry (single emulated OAM loop), PE1 SHOULD overwrite TDM data with AIS in outgoing PW packets, MUST set the L bit in these packets, MAY omit payload to conserve bandwidth, and a structure-aware PE1 SHOULD send RDI back towards CE1. On exit, PE1 MUST cease AIS overwriting, MUST clear the L bit, and a structure-aware PE1 MUST cease RDI.
For TDM PW receive defect state entry, PE1 MUST commence AIS insertion into the TDM AC and MUST set the R bit in all PW packets sent back to PE2. On exit, PE1 MUST cease AIS insertion and MUST clear the R bit. A structure-aware PE1 MUST commence RDI insertion into the AC on entry to PW transmit defect state.
In coupled OAM loops mode, the upstream PE MUST terminate and translate a received NS defect notification message into a PW defect notification message, and MUST signal local AC and PW failures using PW defect notification messages. The downstream PE MUST insert NS defect notification messages into the AC when it detects or is notified of PW or remote AC defects.
In single emulated OAM loop mode, the upstream PE MUST transparently relay NS OAM messages over the PW and MUST signal local AC defects using a NS defect notification sent over the PW. If the defect interferes with NS OAM message generation, the PE MUST instead signal AC defects using a PW defect notification message.
When a BFD session changes state, the PEs MUST use state change diagnostic codes (0, 1, 3, 7) per RFC 5880 and MUST override any previously signaled AC/PW status diagnostic codes (6 or 8). A defect generating code 6 or 8 does not necessarily bring the BFD session down; if the session does time out, code 1 MUST be used.
BFD CV types 0x04/0x10 restrict usage to four diagnostic codes: 0 (no diagnostic — correctly receiving), 1 (control detection time expired — stopped receiving), 3 (neighbor signaled session down), and 7 (administratively down). CV types 0x08/0x20 additionally use codes 6 (Concatenated Path Down) and 8 (Reverse Concatenated Path Down) to signal AC and forwarding defects without requiring the BFD session itself to go down.
The CEP PWE3 control word carries: L bit (AC forward defect indication), R bit (PW reverse defect indication for PSN failures), and N/P bit combination (signals loss of pointer to remote PE). Additionally, loss of packet synchronization itself triggers PW receive defect state entry for CEP PWs.
The LDP Status TLV defines six PW status code points: 0x00000000 (clear all failures), 0x00000001 (PW Not Forwarding), 0x00000002 (AC ingress Receive Fault), 0x00000004 (AC egress Transmit Fault), 0x00000008 (PSN-facing PW ingress Receive Fault), 0x00000010 (PSN-facing PW egress Transmit Fault). Forward defect indication maps to the logical OR of codes 0x00000002, 0x00000010, and 0x00000001; reverse defect indication maps to the logical OR of 0x00000004 and 0x00000008.
The TDM PWE3 control word carries OAM state via three fields: the L bit (AC forward defect indication — upstream PE signals NS network defects to downstream PE), the R bit (PW reverse defect indication — PE signals PSN failures to remote PE), and the M field (modifies the meaning of receive defects for structure-aware PEs). Reception of packets with R bit set causes PE entry into the PW transmit defect state.
Since the mechanisms of this document enable propagation of OAM messages and fault conditions between native service networks and PSNs, continuity of the end-to-end emulated service depends on a trust relationship between the operators of these networks. Security considerations for cross-operator scenarios are discussed in RFC 5254 Section 7; all generic PW security considerations from RFC 3985 Section 10 and VCCV security considerations from RFC 5085 apply.
A PE must track four abstract defect states: AC receive defect, AC transmit defect, PW receive defect, and PW transmit defect, reflecting observed conditions from both directions of the PW service on both AC and PW sides. When a PE enters both receive and transmit defect states for the same PW service simultaneously, the receive defect takes precedence in terms of consequent actions.
PE1 enters the PW receive defect state upon: receiving an FDI from PE2 indicating remote AC receive defect or downstream PW fault; detecting loss of PSN tunnel connectivity upstream; or detecting loss of PW connectivity via VCCV-BFD or VCCV-PING. PE1 exits when all detected defects have disappeared and PE2 has cleared any FDI. Loss of the PW control session (LDP, L2TP) results in the same consequent AC actions as entering the receive defect state.
PE1 enters the PW transmit defect state only when it receives an RDI from PE2 and is not already in the PW receive defect state. PE1 exits when PE2 clears the RDI or when PE1 transitions to the PW receive defect state. For L2TPv3/IP using only the basic Circuit Status AVP (RFC 3931), the transmit defect state is not valid.