Dual-Homing Coordination for MPLS Transport Profile (MPLS-TP) Pseudowires Protection
Extracted elements (25)
Dual-homing PW local protection is preferred over end-to-end PW switchover for AC failures in mobile backhaul networks because massive PW switchover in response to AC failures at the mobile core site can cause congestion by diverting traffic away from network planners' preferred paths. Local switchover keeps the working PW unchanged while only switching at the dual-homing PE side.
The coordination information is sent on the DNI-PW over G-ACh rather than a separate out-of-band channel because the DNI-PW is already required between dual-homing PEs for traffic forwarding during protection switching, reusing the same path for control keeps the mechanism self-contained.
Three consecutive DHC messages are sent for status changes and switchover requests rather than a single message to ensure fast protection switching even if one or two packets are lost or corrupted, providing reliability without requiring reliable transport.
On the single-homing PE side (e.g., PE3), the working PW and protection PW are treated as if they terminate on the same remote PE node, so normal MPLS-TP linear protection coordination procedures (RFC 6378, RFC 7271, RFC 7324) apply unchanged on the single-homing PE without modification.
If it is determined on one dual-homing PE that switchover is needed, that PE MUST set the S bit in the Dual-Node Switching TLV and send it to the other dual-homing PE immediately using three consecutive DHC messages.
If PE1 detects failure of the working PW, it MUST inform PE2 of the working PW state using the PW Status TLV in DHC messages and change the forwarding status of PW1 to standby. On receipt, PE2 SHOULD change PW2 to active and set up connections per the forwarding state machine.
Reserved bits in the Flags fields of both PW Status TLV and Dual-Node Switching TLV MUST be set to 0 on transmission and MUST be ignored upon receipt.
The default interval of the first three rapid DHC messages is RECOMMENDED to be 3.3 ms (similar to RFC 6378), and the default interval of subsequent periodic messages is 1 second. Both intervals SHALL be configurable by the operator.
The Dual-Homing Group ID MUST be the same at both PEs in the same dual-homing group.
To avoid DNI-PW failure due to a single link failure, it is RECOMMENDED that multiple diverse links be deployed between the dual-homing PEs, and the underlying LSP protection mechanism SHOULD be enabled.
When a change of service PW status is detected, the dual-homing PE MUST reflect it in the PW Status TLV and send it to the other dual-homing PE as quickly as possible using three consecutive DHC messages. After the three rapid messages, it MUST send the most recently transmitted status periodically on a continual basis.
When a dual-homing PE determines the active service PW needs to switch from working to protection PW, it MUST send the Dual-Node Switching TLV to the other dual-homing PE as quickly as possible using three consecutive DHC messages, after which the protection PW becomes active and the TLV MUST be sent periodically.
The Dual-Node Interconnection (DNI-PW) is a dedicated pseudowire provisioned between the two dual-homing PE nodes. It carries both DHC coordination messages and service traffic during protection switching, making its robustness critical to the protection mechanism.
The Dual-Node Switching TLV (Type=2) is used by one dual-homing PE to send protection state coordination to its peer. The S bit indicates which service PW carries traffic (0=working, 1=protection), and the P bit identifies the sender's role.
The MPLS-TP Dual-Homing Coordination (DHC) message is sent on the DNI-PW between dual-homing PEs over the Generic Associated Channel (G-ACh). It consists of a 4-octet G-ACh common header, a 4-octet Dual-Homing Group ID, a 2-octet TLV Length, a 2-octet Reserved field, and a variable-length TLV section.
The PW Status TLV (Type=1) is used by a dual-homing PE to report its service PW status to the peer dual-homing PE. The F bit signals Signal Fail (SF) and the D bit signals Signal Degrade (SD), which may originate locally or from the remote PE.
IANA assigned G-ACh channel type 0x0009 for the 'MPLS-TP Dual-Homing Coordination message' in the 'MPLS Generalized Associated Channel (G-ACh) Types' subregistry within the 'Generic Associated Channel (G-ACh) Parameters' registry.
IANA created the 'MPLS-TP DHC TLVs' subregistry within the 'Generic Associated Channel (G-ACh) Parameters' registry with initial allocations: 0x0000 Reserved, 0x0001 PW Status (20 bytes), 0x0002 Dual-Node Switching (16 bytes). Allocation policy is IETF Review.
The DHC message carries control information; if injected or modified by an attacker, the dual-homing PEs might disagree on which PE delivers CE traffic, enabling a denial-of-service attack against the CE. The DHC message MUST be used within a trusted MPLS-TP network domain as described in RFC 6941.
The DHC message is carried in the G-ACh, which is essentially connection-oriented, so injection or modification of control messages requires subversion of a transit node. Such subversion is generally hard in connection-oriented MPLS networks and impossible to protect against at the protocol level; management-level techniques are more appropriate.
Uniqueness of identifiers (Dual-Homing Group ID, Node IDs) is guaranteed by the assigning operator. Failure to use unique values within the specified scope could result in operational problems; see RFC 6370 for identifier uniqueness details.
The dual-homing PE forwarding state machine determines traffic forwarding based on three inputs: Service PW state (Active/Standby), AC state (Active/Standby), and DNI-PW state (Up/Down). Key transitions: Service PW Active + AC Active + DNI-PW Up → forward Service PW to AC; Service PW Active + AC Standby + DNI-PW Up → forward Service PW to DNI-PW; Service PW Standby + AC Active + DNI-PW Up → forward DNI-PW to AC; all Standby or DNI-PW Down with any Standby → drop all packets.
The DHC message header includes: the G-ACh common header (4 octets, first nibble 0001), DHC Channel Type (16-bit, value 0x0009), Dual-Homing Group ID (32-bit unsigned integer), TLV Length (16-bit), and Reserved (16-bit).
The Dual-Node Switching TLV (Type=2, Length=16 bytes) contains: Destination Dual-Homing PE Node_ID (32-bit), Source Dual-Homing PE Node_ID (32-bit), DNI-PW ID (32-bit), and Flags (32-bit, P bit for working/protection role, S bit indicating active service PW — 0=working PW, 1=protection PW).
The PW Status TLV (Type=1, Length=20 bytes) contains: Destination Dual-Homing PE Node_ID (32-bit), Source Dual-Homing PE Node_ID (32-bit), DNI-PW ID (32-bit), Flags (32-bit, P bit indicates working/protection role), and Service PW Status (32-bit, F bit for Signal Fail, D bit for Signal Degrade).