Summary

  • RFC 6426 extends LSP ping for MPLS-TP on-demand connectivity verification and route tracing. The Requester chooses the probe, encapsulation, TTL, reply mode, and optional reverse-path request; the Responder supplies only evidence it can validate at its position.
  • A reply can identify a TTL-selected intermediate point, not the egress. Receipt alone is therefore not authority to declare a bidirectional service.
  • The Requester must validate the response interface, label stack, applicable FEC, identity and reply mode; if a Reverse-path Target FEC Stack TLV is present, it must validate that too. A failed validation requires dropping the response and should be reported.

What the packet means

IP-encapsulated operation uses an address from 127/8 and UDP inside the MPLS stack. Non-IP operation uses the Associated Channel Header (ACH), without dependence on IP routing. In non-IP ACH operation with reply mode 4, the reply must use the reverse LSP through ACH, without IP/UDP. A node without that return path should drop the request; silence is not positive evidence of a healthy reverse path.

The Requester and Responder have separate authority. The Requester selects a static LSP or pseudowire (PW) FEC, the TTL sequence, reply mode, and whether the R flag requests reverse-path FEC information. The Responder can report only what is valid at the point that receives the request. It must not set R in an echo response. When R was set in the request, the Responder should attach a Reverse-path Target FEC Stack TLV. Co-routed and associated bidirectional LSPs may nevertheless use a different reverse FEC. A response without R/TLV evidence does not prove a reverse FEC.

For route tracing, TTL-driven expiry directs requests to particular LSP points. DSMAP and DDMAP support the mapping of destinations and replies; they do not turn an intermediate response into egress evidence. Source Identifier and Destination Identifier TLVs make the claimed source, target, and administrative context explicit. Across administrative boundaries, Source Identifier TLVs are recommended so unexpected or unknown sources can be filtered.

GAL is carriage for OAM and is not the target FEC label being verified. It must not receive a Nil FEC TLV and must not appear in DSMAP or DDMAP, although interface and label-stack TLVs include it. Confusing GAL with the target FEC creates a false validation result.

Concrete verification fixture

An operator tests a statically provisioned LSP FEC 10.0.0.1/32 or a PW FEC, sends an MPLS-encapsulated probe with TTL 2, records reply mode 4, and requests reverse information with R set. The capture must correlate Source Identifier, Destination Identifier, responder identity, ingress interface, complete label stack, target FEC, Reverse-path Target FEC Stack TLV when present, ACH or 127/8-plus-UDP encapsulation, and the observed TTL-expiry point. The response is accepted only if the local interface and label stack match the expected FEC and any reverse stack validates against the requester’s inventory. R must be absent in the response. An unexpected source, a label mismatch, an invalid reverse FEC, or an invalid interface means: drop, log, and report the failed validation.

Repeat TTL values to distinguish the intended egress from intermediate responders. For P2MP with IP addressing, RFC 6425 procedures must be supported; without IP addressing, RFC 6426 applies its non-IP procedures. Identify the branch and do not assume one reply represents every leaf. A node that does not support, or cannot use, the requested reply mode must drop the request. Do not use ACH on-demand CV with ECMP: the ACH header may change hashing, so the probe can follow a path different from data traffic.

RFC 4379 supplies the LSP ping base procedures; RFC 5586 defines G-ACh carriage; RFC 5860 states MPLS-TP OAM requirements; RFC 5884 provides BFD/MPLS context. RFC 5920 concerns the MPLS security framework, RFC 5921 the transport-profile architecture, RFC 6370 identifiers, and RFC 6371 the OAM framework and authority boundary.

The method helps operators interrogate static and non-IP paths, locate transit points, and separate forward evidence from reverse evidence. It is on-demand, not continuous assurance. The fact packet supplies no evidence about vendor adoption, deployment prevalence, probe latency or loss, operational incident history, commercial value, or customer outcomes; those remain unknown. No allegation is established here.

Sources