Jonathan Hui’s SNAC Flag Marks the Infrastructure Link

Summary

RFC 10075 assigns bit 6 of the Router Advertisement Flags Extension Option to identify a sending router as a Stub Network Auto-Configuring (SNAC) router. The current SNAC architecture draft sets the bit on Adjacent Infrastructure Link (AIL) advertisements and shows it cleared on stub-network advertisements.
The draft records OpenThread support and automated tests for AIL advertisements. RFC 10075 adds the network boundary: where RA-Guard filters SNAC-router advertisements, transit devices never receive the flagged message.

The bit changes with the side of the router. The current SNAC architecture draft sets it in Router Advertisements on the Adjacent Infrastructure Link (AIL); its stub-network appendix shows the bit at 0 and says the router sets it only on AIL messages.

Jonathan Hui’s RFC 10075, published in October 2026, assigns bit 6 in the Router Advertisement (RA) Flags Extension Option as the SNAC Router flag. A set bit says that the router sending the RA is a SNAC router. The document is crisp about that meaning and equally crisp about its limit: the exact triggers for setting the bit to 1 are out of scope.

That side-specific signal matters because a SNAC router sits between networks with different jobs. A Stub Network Auto-Configuring router can connect a stub network to a transit network without an operator configuring every link by hand. RFC 10075 gives it a way to identify itself in an IPv6 Neighbor Discovery message. The architecture draft then places that signal on the infrastructure-facing link and keeps it clear in the stub-network appendix.

The document split now has a concrete interface rule. RFC 10075 is a Standards Track RFC: it establishes what a received flag says. The current SNAC architecture draft applies the flag to RAs sent over the AIL and records 0 for stub-network RAs. The appendix says the flag is only set on the AIL interface. The same draft reports that OpenThread added support in September 2024; its automated tests verify the flag and a Route Information Option on the AIL, and it says multiple-router scenarios are regularly tested in simulation.

The flag is bit 6 in the expanded Router Advertisement option defined by RFC 5175. That earlier RFC created room for more RA flags. The new work uses one of those positions for a specific distinction: a SNAC router’s advertisement versus an advertisement from a transit router. IANA’s IPv6 ND Router Advertisement Flags registry now records the entry as S - SNAC Router Flag and cites RFC 10075.

There is a second boundary: whether the message gets through. RFC 10075 says that on a network where RA-Guard filters RAs sent by SNAC routers, transit-network devices will never receive an RA carrying the flag. The standard can define the bit and the registry can name it; an operator’s filtering rule can still remove that signal from the transit side of the link.

Hui works on the kind of systems where this distinction has a practical home. The Connectivity Standards Alliance identifies him as a Principal Software Engineer at Google responsible for Thread implementation, maintainer of OpenThread, and Thread Group vice president of technology. That profile establishes his engineering roles; the RFC itself is the evidence for this particular contribution.

Implementers now have one documented implementation and test record to compare against the interface rule. The remaining evidence gap is a second independent implementation and packet captures from deployed networks; either could show whether the AIL boundary behaves consistently across products.