Pseudowire Status for Static Pseudowires
updated by
- rfc-7274 — Allocating and Retiring Special-Purpose MPLS Labels
updates
- rfc-5885 — Bidirectional Forwarding Detection (BFD) for the Pseudowire Virtual Circuit Connectivity Verification (VCCV)
Extracted elements (30)
Static PWs have no LDP control plane, so there is no native method to transmit PW or attachment circuit status between PE devices. This document defines an in-band method using the generic associated channel (ACh) to carry PW status codes without requiring a dynamic control plane.
All PW OAM status messages are by default sent to the adjacent PE to preserve backward compatibility with PEs that do not understand MS-PWs and cannot distinguish whether the adjacent PE is an S-PE or a T-PE. Direct T-PE-to-T-PE messaging (S-PE bypass) is an enhanced optional mode.
A PE forwarding a PW OAM status message to the adjacent PE across the PSN tunnel MUST set the PW label TTL field to 1. If the control word is not in use, the PE MUST place the GAL reserved label below the PW label with TTL set to 1.
A PE that establishes a PW using LDP MUST use the PW status TLV mechanism and MUST NOT use the static PW OAM status mechanisms for those PWs, except that S-PE bypass mode MAY be used when both T-PEs use LDP.
A PE that establishes an MPLS PW using means other than LDP (e.g., static configuration) MUST support some alternative method of status reporting.
A receiver starts a timer per the refresh rate in the packet. If a non-zero PW status message is not received within 3.5 times the specified timer value, the status condition times out and the default status of zero is assumed. Jitter in refresh transmissions is recommended but MUST remain within 3.5 times the timer value for 3 consecutive packets.
For S-PE bypass mode (message destined to the egress T-PE), a T-PE MUST set the PW label TTL to a value sufficient to reach the corresponding T-PE (greater than 1, per local policy). If the control word is not in use, the GAL label MUST also be placed below the PW label with TTL set to 1.
If a malformed TLV or an unknown TLV is received in a PW OAM status message, the TLV MUST be ignored, and the PE SHOULD report the event to the operator.
If a T-PE receives an LDP label mapping message with the B bit set in the Generic Protocol Flags TLV, it MAY send S-PE bypass status messages. If bit B is not set or the TLV is absent, the T-PE MUST NOT send S-PE bypass status messages.
If the S-PE bypass mode LDP flag bit in the Generic Protocol Flags interface parameter does not match in the FEC advertisement for both directions of a specific PW, that PW MUST NOT be enabled.
If the transmitting PE does not want to use a new timer value requested via acknowledgment, it MUST refresh the PW OAM message with its preferred timer value. The receiving PE MUST NOT request a new refresh timer value more than once per refresh interval.
PW OAM status messages MUST NOT be used as a connectivity verification method; their sole purpose is status signaling.
S-PE bypass mode MUST NOT be used to send messages that may originate at an S-PE (e.g., 'clear all failures' 0x00000001); doing so could cause race conditions between control-plane messages from S-PEs and data-plane messages from T-PEs. Only status codes 0x00000002, 0x00000004, 0x00000020, and 0x00000040 are permitted via bypass.
When S-PE bypass mode is enabled, PW Status TLVs received via this method have priority over those sent via LDP. However, the same PW Status TLVs MUST also be sent in LDP to keep S-PE state updated.
When the Pseudowire Status protocol for Static Pseudowires is used, the BFD VCCV status-signaling mechanisms (CV Types 0x08 and 0x20) from RFC 5885 MUST NOT be used to avoid duplicate or conflicting notifications. BFD VCCV for fault detection (CV types 0x04 and 0x10) MAY still be used.
When the PW status code is cleared to zero, the PW OAM message is sent normally but transmission ceases after 3 messages at one-second intervals. If acknowledged, a timer value of zero MUST be used, which SHOULD cause the sender to stop sending and resume normal operation.
A PE MAY acknowledge a PW status message by sending a reply with the same format and status code as the received message but with the A bit set. The receiver MAY use the refresh timer field in the acknowledgment to request a new refresh interval from the originator.
Because there is no TCP/IP session, PW OAM messages must be sent repeatedly for reliability. A new status message is sent immediately, retried twice more at one-second intervals if unacknowledged within one second, and then periodically at the refresh timer interval. The suggested default refresh timer value is 600 seconds.
For a static PW switched to another static PW at an S-PE, the LDP header in RFC 6073 procedures is replaced by the PW OAM header. The SP-PE TLV information MUST be configured in the S-PE or no SP-PE TLV will be sent.
S-PE bypass mode enables a T-PE using LDP to bypass all S-PEs in an MS-PW and send a status message directly to the remote T-PE in-band with PW PDUs. It is OPTIONAL and MUST be supported by both T-PEs; it MUST NOT be used if the first PW segment at either T-PE is not LDP-controlled.
The Refresh Timer in the PW OAM message is an unsigned integer specifying the retransmit interval in seconds (range 1–65535). A value of 0 means infinity: the message is never refreshed and never times out.
When a dynamic (LDP-controlled) PW segment adjacent to an S-PE has its label withdrawn, the S-PE sets the PW status code '0x00000001 - Pseudowire Not Forwarding' on the static PW segment. The S-PE withdraws its own LDP label only if it is not provisioned.
IANA assigned the Associated Channel Type value 0x0027 with description 'PW OAM Message' in the 'Pseudowire Associated Channel Types' registry.
IANA assigned the Pseudowire Interface Parameters Sub-TLV type value 0x18 with description 'PW Generic Protocol Flags' in the 'Pseudowire Interface Parameters Sub-TLV type Registry'.
IANA assigned the Pseudowire Switching Point PE sub-TLV Type value 0x07 with description 'Static PW/MPLS-TP PW segment ID of last PW segment traversed' in the 'Pseudowire Switching Point PE sub-TLV Type' registry.
IANA created the 'PW Generic Protocol Flags' registry for 16-bit flag strings. Bit 0 (mask 0x0001) is assigned as 'S-PE bypass mode'. Bits 1–15 are assigned via IETF Review. Allocation requests require a description of up to 65 characters.
The security measures described in RFC 4447, RFC 5085, and RFC 6073 are considered adequate for the PW OAM status mechanism defined in this document. No additional security mechanisms are introduced.
The PW Generic Protocol Flags sub-TLV (Type 0x18, Length always 4 octets) carries a 16-bit flags field. Bit B (position 31 of the 32-bit word, i.e., bit 0 of the flags) is set to request S-PE bypass mode. All other bits (R bits) are reserved, MUST be zero on transmission, and ignored on reception.
The PW OAM message carries a standard 32-bit ACH header (first nibble 0001b, version and reserved both zero), followed by a 16-bit Refresh Timer (unsigned integer, seconds, 0 = infinity), an 8-bit TLV Length, and an 8-bit Flags field containing the A (acknowledgment) flag bit. The channel type field is set to 0x0027.
The PW Status TLV (type 0x096A, 4-octet Status Code) is transported in the TLV field of the PW OAM message. The first 2 bits of the type field are reserved and MUST be zero on transmit and ignored on receipt (unlike RFC 4447 where those bits carry the 'E' and 'F' bits).