Summary

  • The IETF moved RFC 7506 to Historic and RFC 9570 changed LSP Ping from requiring Router Alert to requiring that senders not set it; IANA retains the two affected values as deprecated.
  • Historic, deprecated, configured, emitted, processed and observed are different states. An operator needs a retirement receipt before claiming that Router Alert 69 is gone from a network.

The change ticket has an authoritative link. The RFC Editor page labels RFC 7506 Historic. The IANA row for IPv6 Router Alert value 69 says MPLS OAM (DEPRECATED). The engineer closes the ticket: the old behavior has been retired.

But nobody has opened the device template that generates MPLS echo requests. No receiver version has been checked. No capture window has been declared. The standards conclusion is correct and the operational conclusion is unsupported.

That gap is easy to miss because protocol governance uses verbs that sound executable. A value is retired. A reply mode is removed. A document is made Historic. Yet the IETF does not have remote write access to deployed equipment. It changes the public rule and its traceable authority. Operators still have to change running systems and prove the result.

Historic is a decision about standing

RFC 7506 was published in April 2015 on the Standards Track. It allocated IPv6 Router Alert Option value 69 for MPLS Operations, Administration and Maintenance, including LSP Ping. Its current RFC Editor record identifies it as Historic.

The status did not arrive through a casual label edit. The status-change record is an archival IETF object. It says RFC 9570 retires Router Alert for MPLS OAM in IPv4 and IPv6, updates RFC 8029 and explains why RFC 7506 should be Historic. Its state is Approved - announcement sent.

The IESG statement on Historic designations supplies the important semantics. Obsolescence normally points to a newer version of the same technology. Historic status says the described technology is no longer current or recommended. It requires an IETF-wide Last Call and formal IESG action. The status-change document preserves the explanation and links it to the RFC.

That process establishes institutional standing. It tells an implementer which text no longer expresses current recommended practice. It does not attest that a configuration job ran successfully on a particular router.

The replacement rule is unusually explicit

RFC 9570 does more than offer a preference. The older RFC 8029 text required an MPLS echo request to carry Router Alert: value 0 for IPv4 or value 69 for IPv6. RFC 9570 replaces that sentence with a MUST NOT set Router Alert rule. It removes reply mode 3, the mode that requested a UDP reply with Router Alert. A conforming receiver should ignore Router Alert options that arrive on MPLS echo requests or replies.

The engineering case is also precise. LSP Ping already used concurrent mechanisms intended to prevent a request from travelling beyond the egress: special destination selection and a TTL or Hop Limit of one. The working group reported no implementation that relied on Router Alert for that protection and no implementation of reply mode 3. RFC 9570 recommends the IPv6 loopback address ::1/128 and other entropy sources where an operator needs to exercise ECMP paths.

Those statements justify the standards change. They do not support a universal inventory claim. “No implementation was reported to the working group” is bounded evidence about the record available to that group. It is not proof that no legacy image, lab tool, copied template or private implementation can still emit or interpret the option.

Deprecated values remain visible for a reason

IANA did not erase value 69. The current IPv6 Router Alert registry preserves it as MPLS OAM (DEPRECATED) and cites both RFC 7506 and RFC 9570. The LSP Ping parameters registry preserves reply mode 3 with the same deprecated state.

This is not ambiguity about whether new code should use them. RFC 9570 explains the compatibility meaning by reference to RFC 8126: deprecated values should not be used in new implementations, while deployed implementations already using them continue to work. Later, RFC 9805 closed the IPv6 Router Alert registry. Closure prevents the registry from serving as an open source of new assignments; it does not convert old packets into impossible packets or make deprecated value 69 reusable.

The retained row is operationally useful. A packet decoder can still name what it sees. A migration tool can identify a legacy value instead of calling it unknown. An auditor can connect an observation to the document that first assigned the number and the document that later deprecated it.

Deleting the history would make retirement harder to prove, not easier.

Receiving, sending and depending are separate questions

An inventory that asks only “Do we support RFC 7506?” is too blunt. A system can have at least four different relationships with value 69.

It may generate Router Alert in an echo request because a template or library still follows the older rule. It may accept a packet containing the value and ignore it, as RFC 9570 recommends. It may process the option on a slow or exceptional path even though LSP Ping itself no longer needs it. Or it may depend on the option for behavior that the new standards model says should be supplied by other mechanisms.

The last state carries the greatest migration risk, but all four matter. RFC 6398 discusses the processing and security cost of Router Alert and recommends against using it outside controlled environments. That makes continued exposure worth measuring. It does not license an allegation that a named device is vulnerable merely because its software recognizes the code point.

The negative test must also be honest. A capture with no value 69 packets during an hour proves absence in that observation window and on those capture points. It does not prove the option was removed from every dormant template or that an infrequent diagnostic path can never generate it.

Build an operational retirement receipt

The missing artifact is an operational retirement receipt. It begins with the documentary layer: the observed RFC 7506 status-change revision, RFC 9570 and the dated IANA rows. It then records the running layer separately.

For each sender, name the implementation and build, the active template and whether Router Alert can be emitted. For each receiver, record whether the option is ignored, specially processed or rejected. State the replacement controls: destination selection, TTL or Hop Limit, the chosen IPv6 address and the entropy source used for multipath testing. Attach a configuration result and a bounded capture or counter observation.

Exceptions need an owner, purpose, scope and expiry. A legacy peer that still sends value 69 may remain interoperable because RFC 9570 designed for that case. Compatibility is not permission to leave an exception ownerless. Nor should an ignored incoming option be reported as proof that every sender has migrated.

The receipt ends with four explicit conclusions rather than one green badge: documentary retirement, configured retirement, observed retirement and compatibility decommissioning. Each can be true, false, partial or stale.

This is where The Policy Mirror matters: the public rule and the device rule have to be compared without pretending they are the same object. Running-Code Primacy requires evidence that the changed rule is actually expressed and observed. Reality, Not Advocacy keeps the conclusion narrow. RFC 7506 is Historic. Router Alert 69 is deprecated. Whether either fact has reached a particular wire remains a question for evidence.

Sources