Summary

  • IANA’s IPv6 ND Router Advertisement flags registry assigns bit 6 to S - SNAC Router Flag. The flag draft, revision 08, still prints TBD in its frozen text and leaves the circumstances for setting S to the companion SNAC work.
  • On the adjacent infrastructure link, the companion SNAC draft, revision 13, requires a SNAC router advertising its own suitable on-link prefix to set S. The receiver’s prefix comparison then determines whether its current prefix stays advertised or enters deprecation; RA-Guard can prevent the advertisement from arriving at all.

A number in a registry is a beginning

The IANA registry shows bit 6 as the S flag and points to the SNAC Router flag document. The 5 June revision of that document still calls the bit TBD and asks IANA for an allocation. Those are the two current documentary views. The registry supplies the number that an implementer needs; the draft states the proposed behavior and remains an Internet-Draft in the Datatracker’s RFC Editor queue.

The distinction the flag makes is narrow and valuable. A stub network auto-configuring router joins one or more stub networks to an adjacent infrastructure network. The flag draft says an RA with S set to one indicates that the sender is a SNAC router. It also says the precise trigger for setting the bit belongs elsewhere. Routers that do not operate as SNAC routers leave it clear; devices that do not understand it ignore it. That gives compatible routers a common label while allowing older receivers to continue ordinary RA processing. The label can distinguish an advertisement from a stub router from one sent by a non-SNAC router.

It cannot by itself tell a receiver which prefix to keep.

The companion SNAC simple draft, revision 13, supplies the missing operating rule. On the adjacent infrastructure link (AIL), a SNAC router advertising its own suitable on-link prefix must set S in that RA. Appendix C gives the opposite setting for RAs it sends on the stub-network link: S is zero there. The same physical router therefore sends different signals on different sides. Reading an RA without its sending interface is like reading a ballot without knowing which election it belongs to.

The losing prefix is chosen by the receiver

Suppose a SNAC router is already advertising a suitable on-link prefix on that adjacent link. It hears another RA with a different suitable prefix and S clear. Under revision 13 it moves its interface to STATE-DEPRECATING. This is the sharpest boundary in the decision table: an eligible non-SNAC advertisement takes precedence over the SNAC router’s advertised prefix. A network operator need not invent a score for the two prefixes. The flag and the companion rule already identify the branch.

Now set S on the received RA. The result is more selective. Equal prefixes leave the current advertising state alone. If the SNAC router is advertising a Unique Local Address prefix and receives a non-ULA suitable prefix, it deprecates its own. If both are ULAs, the numerically lower received prefix wins and the current one is deprecated. In the other listed cases the router keeps advertising its current prefix. That ordering gives cooperating stub routers a way to converge without pretending the first packet to arrive owns the link forever. It also means that S=1 is not a command to yield: sometimes it is exactly the reason to stay.

The comparison lives in a receiving implementation, not in IANA’s table. It depends on suitable on-link prefixes, the received S value, the existing advertised prefix and the link on which the RA arrived. A registry entry cannot perform that comparison. An operator trying to explain a prefix change should therefore inspect the received RA and the receiver’s state transition, rather than pointing at the registered bit as if the allocation itself changed the network.

A switch can make the bit disappear

The flag draft supplies an unusually blunt operational warning. An environment whose RA-Guard filters SNAC-router advertisements prevents devices on the transit network from receiving an RA with S set. A router may generate the right packet and still leave its neighbor with no S-bearing evidence. The registry entry remains correct. The implementation may also be correct. The missing delivery happens between them.

That makes a filter configuration part of the governance story. RA-Guard exists to constrain which Router Advertisements cross a link; the RFC describes the implementation and evasion issues around that enforcement. A network that depends on SNAC routers exchanging advertisements must decide which such advertisements its local policy permits. If it admits none, the receiving router cannot run the S-dependent branch on a packet it never sees. If it admits them, the operator still has to inspect origin and trust under the usual IPv6 Neighbor Discovery security model. The flag draft’s security section explicitly leaves the RFC 4861 security considerations in place.

There is no measured deployment outcome in these sources. A trace showing the AIL RA with S set, the local RA-Guard decision and the receiver’s resulting prefix state would establish what happened on a particular link. Until that trace exists, a bit assigned in a registry is evidence of a common vocabulary, not a packet-delivery receipt.

The hand-off worth watching

This is a small allocation with a large temptation attached. A registry number looks like completion: the standard has a name, the table has a row, the implementation can flip a bit. But the SNAC documents divide the work across institutions and machines. IANA records the number. The flag draft defines the signal and delegates its setting circumstances. The companion draft decides where a SNAC router sets it and how another SNAC router reacts. The link operator decides what its RA filter will pass. A receiver finally decides which prefix remains suitable to advertise.

That division is useful because it makes failure legible. If an RA leaves the wrong interface with S set, fix the sender. If the RA is filtered, inspect the link policy. If the receiver sees it but keeps the wrong prefix under the draft’s cases, inspect the implementation and its suitability inputs. If all those records agree and service still fails, the investigation moves beyond this flag. Calling every one of those outcomes “the SNAC bit did not work” would be an excellent way to lose the evidence.

The open decision is concrete: before treating bit six as proof of coordination, collect one received adjacent-link RA, its S value, its prefix, the filter outcome and the receiver’s state change. The number on IANA’s page starts that chain. The link finishes it.

Sources