Media Access Control (MAC) Address Withdrawal over Static Pseudowire
Extracted elements (21)
A 32-bit sequence number is used instead of the 16-bit scheme in RFC 4385 to better handle scenarios where sequence numbers must be tracked across PW restarts and retransmissions over static pseudowires.
MAC withdraw signaling is explicitly a best-effort scheme designed to optimize convergence by reducing blackholes during PW failover, not to guarantee delivery. Fallback to MAC re-learning via bidirectional traffic or MAC aging is relied upon when signaling fails, causing only temporary blackouts.
The mechanism reuses in-band signaling from RFC 6478 (static PW status) combined with MAC withdrawal semantics from RFC 4762/RFC 7361, avoiding the need for a new transport infrastructure for static PWs that cannot use LDP.
The MAC withdraw OAM message MAY contain the MAC Flush Parameter TLV defined in RFC 7361 when static PWs are deployed in H-VPLS and PBB-VPLS scenarios, enabling compatibility with optimized MAC withdrawal in hierarchical deployments.
The retransmission interval and retry count are local matters that present no interworking issues; the default is a one-second interval with two retries, but operators MAY configure different values or use increasing backoff delay.
A MAC address withdraw OAM message MUST contain a Sequence Number TLV; otherwise, the entire message is dropped.
A MAC withdraw message with the R-bit set MUST be processed by resetting the send and receive sequence numbers first, before performing the rest of MAC withdraw message processing. The ACK is sent with the R-bit cleared.
During retransmission, if a need to send a new MAC withdraw message with an updated sequence number arises, retransmission of the older unacknowledged withdraw message MUST be suspended and the retransmit timer for the new sequence number MUST be initiated.
If the sequence number in a received MAC withdraw message is smaller than or equal to the value in the receiver's register, MAC TLVs are not processed, but an ACK with the received sequence number MUST be sent as a response.
Retransmission MUST cease when an ACK is received. A method of suppressing retransmissions MUST be implemented to avoid continuous retransmissions in the absence of acknowledgements.
The first 2 bits of the Sequence Number TLV are reserved and MUST be set to 0 on transmit and ignored on receipt.
The Sequence Number TLV MUST be the first TLV in the MAC withdraw OAM message.
The A-bit (Acknowledge bit) is set by a receiver to acknowledge receipt and processing of a MAC Address Withdraw OAM Message. In the ACK message, with A-bit set, MAC TLVs are excluded.
The MAC Address Withdraw OAM Message is a PW OAM message sent over the in-band PW Associated Channel (ACH) to signal MAC address withdrawal for statically provisioned pseudowires. It uses channel type 0x0028 and follows the message structure defined in RFC 6478.
The R-bit (Reset bit) is set by a sender to request reset of sequence numbers, used when a pseudowire is restarted and the sender has no local record of previous send and expected receive sequence numbers.
The Sequence Number field is 32 bits. Overflow detection is simplified: a sequence number exceeding 2,147,483,647 (0x7FFFFFFF) is considered an overflow and reset to 1, departing from RFC 4385's 16-bit scheme.
IANA assigned a new channel type 0x0028 ('MAC Withdraw OAM Message') from the 'MPLS Generalized Associated Channel (G-ACh) Types (including Pseudowire Associated Channel Types)' registry.
IANA assigned a new TLV Type 0x0001 ('Sequence Number TLV') from the existing LDP 'TLV Type Name Space' registry.
The security measures described in RFC 4447 (pseudowire setup and maintenance), RFC 5085 (VCCV), and RFC 6073 (segmented pseudowire) are considered adequate for the MAC address withdrawal over static pseudowire mechanism; no additional security measures are introduced.
Each PW sender maintains a transmitted sequence-number counter initialized to 1. The transmit counter is incremented on each new MAC withdrawal, reset to 1 after wrap or R-bit reset acknowledgement, and set to 2 in the first message after initialization. Each PW receiver maintains an expected sequence-number register initialized to 1, updated to the received sequence number when a higher-numbered message arrives.
The MAC withdraw PW OAM packet begins with a 4-byte OAM header (0001|Version|Reserved|0x0028), followed by Reserved (2 bytes), TLV Length (1 byte), and a Flags byte containing A-bit and R-bit. The Sequence Number TLV (type 0x0001) carries a 32-bit sequence number and MUST be the first TLV.