Layer 2 Tunneling Protocol Version 3 (L2TPv3) Extended Circuit Status Values
updates
- rfc-3931 — Layer Two Tunneling Protocol - Version 3 (L2TPv3)
- rfc-4349 — High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)
- rfc-4454 — Asynchronous Transfer Mode (ATM) over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
- rfc-4591 — Frame Relay over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
- rfc-4719 — Transport of Ethernet Frames over Layer 2 Tunneling Protocol Version 3 (L2TPv3)
Extracted elements (20)
The (A) bit was generalized from indicating only local AC state to indicating the status of the entire local pseudowire endpoint (AC plus PSN-facing termination) to align L2TPv3 with the finer-grained fault direction signaling already available in MPLS-based pseudowires via LDP (RFC 4446, RFC 4447).
The (N) New bit is deprecated because it is of little or no use: the information it conveys (new vs. existing circuit) can already be inferred from the L2TP control message type, and it is uncertain what actionable use the receiving PE can make of it beyond logging.
The Standby (S) bit was introduced to support session-level redundancy scenarios, enabling a pseudowire to be fully established by signaling and available for OAM and data-plane liveness checks (e.g., via VCCV/BFD) without forwarding user traffic—mirroring functionality already present in MPLS/LDP pseudowires.
In implementations prior to this specification, bits 0–13 MUST be set to zero as required by RFC 3931 Section 5.4.5. This constraint enables legacy implementations to interwork properly with new implementations that use the extended bits.
The requirement that (A) MUST NOT be set until all fault bits are cleared allows an endpoint using extended status bits to remain backward compatible with a remote endpoint that only understands the original A bit semantics.
This document updates RFC 3931, RFC 4349, RFC 4454, RFC 4591, and RFC 4719 to adopt the generalized (A) bit definition and to deprecate the (N) bit; it also removes two sentences from RFC 4719 Section 2.2(c) that mandated specific N bit values in ICRQ/ICRP and ICCN/SLI messages.
If any of the fault status bits (R, T, I, E) are set, then the (A) bit MUST be cleared. The fault bits are a more granular definition of (A), such that ORing them provides an inverted (A).
If multiple faults occur simultaneously, all fault status bits corresponding to each fault MUST be set; that is, they MUST be bitwise ORed together.
The (A) bit MUST NOT be set until all fault status bits (R, T, I, E) are cleared. This ensures backward compatibility with remote endpoints that do not understand the new status bits.
The (N) New bit is deprecated. It SHOULD be ignored on receipt and is OPTIONAL to set on sending. Prior implementations MUST set bits 0–13 to zero per RFC 3931 Section 5.4.5, enabling legacy interworking.
When a pseudowire is in Standby mode, any received user traffic SHOULD be dropped, and user traffic MUST NOT be transmitted from or into the L2TPv3 session.
The (A) Active bit is redefined to indicate whether the local pseudowire endpoint—including the local Attachment Circuit and local PSN-facing pseudowire termination—has no faults present (1) or has faults present (0). This generalizes the prior definition which only pertained to local access circuit state.
The (E) bit at position 10 indicates a Local PSN-facing PW egress Transmit Fault: a fault has occurred in the transmit direction between the local endpoint and the remote L2TP endpoint. Such a fault does not necessarily trigger an L2TP control connection timeout.
The (I) bit at position 11 indicates a Local PSN-facing PW ingress Receive Fault: a fault has occurred in the receive direction between the local endpoint and the remote L2TP endpoint. Detection means (e.g., BFD, PW Type-specific) are outside the scope of this document.
The (R) bit at position 13 indicates a Local Attachment Circuit ingress Receive Fault: an alarm or fault has occurred at the local AC such that it is unable to receive traffic, though it can still transmit.
The (S) bit at position 9 indicates that the pseudowire is in Standby mode: it has been signaled and is operationally established, but is NOT switching user traffic. The (S) bit may be set in conjunction with any other bit, including (A).
The (T) bit at position 12 indicates a Local Attachment Circuit egress Transmit Fault: a fault has occurred at the local AC such that it is unable to transmit traffic, though it can still receive.
IANA assigned five new Circuit Status Bits in the L2TP registry (managed per RFC 3931 Section 10.7): Bit 9 (S, Standby), Bit 10 (E, Local PSN-facing PW egress Tx Fault), Bit 11 (I, Local PSN-facing PW ingress Rx Fault), Bit 12 (T, Local AC egress Tx Fault), Bit 13 (R, Local AC ingress Rx Fault). Bit 14 (N, New) was updated to mark its use as deprecated.
Security considerations for the Circuit Status AVP are fully covered by the base L2TPv3 specification in RFC 3931 Section 8. This document introduces no additional security considerations beyond extending the attribute.
The Circuit Status AVP (Attribute Type 71) Attribute Value field is a 16-bit bitmap. The original format defined two bits: (A) Active at bit 15 (0x0001) and (N) New at bit 14 (0x0002). This document extends it to add five more bits: (R) at 13 (0x0004), (T) at 12 (0x0008), (I) at 11 (0x0010), (E) at 10 (0x0020), and (S) at 9 (0x0040), with remaining bits reserved.