IANA Considerations for the IPv4 and IPv6 Router Alert Options
updates
Extracted elements (10)
IPv4 and IPv6 Router Alert Option Value registries are kept as separate registries rather than being aligned. The initial allocations in RFC 2711 and the aggregation-level allocations in RFC 3175 made a unified numbering scheme impossible after the fact.
IPv6 RAO value 3 is removed because it is a duplicate: aggregation level 0 means end-to-end signaling, which already has IPv6 RAO value 1 assigned. RFC 3175 defined values 3-35 (33 values) for nesting levels 0-31 (32 levels), creating an off-by-one inconsistency. The fix redefines IPv6 so values 4-35 represent levels 1-32.
Aggregated Reservation nesting levels are numbered differently in IPv4 and IPv6: nesting levels begin at 1 for IPv4 (RAO values 1-32) and at 1 for IPv6 after this document's correction (RAO values 4-35). Implementors must not assume the same RAO value encodes the same nesting level across both protocol versions.
For IPv4, value 0 means 'Router shall examine packet' (used by RSVP and IGMP), whereas for IPv6, value 0 means the datagram contains a Multicast Listener Discovery message. The semantics of value 0 differ between the two protocol versions.
New IPv4 Router Alert Option Values in the range 33-65502 are to be assigned via IETF Review as defined in RFC 5226.
The Router Alert Option (RAO) Value field is two octets in length (up to 65536 distinct values) and carries extra information used by routers to decide whether to more closely examine a datagram. It is present in both IPv4 (RFC 2113) and IPv6 (RFC 2711) variants of the option.
The existing IANA registry for IPv6 Router Alert Option Values (originally specified in RFC 2711) is updated: value 3 (RSVP Aggregation level 0) is removed and reserved for possible future use; values 65503-65534 are designated for experimental use.
This document creates a new IANA registry for IPv4 Router Alert Option Values. The two-octet Value field is managed by IANA with initial allocations: 0 (Router shall examine packet), 1-32 (Aggregated Reservation Nesting Levels), 33-65502 (available for IANA assignment), 65503-65534 (experimental use), 65535 (reserved).
Production networks do not necessarily support experimental code points in IP option headers. The network scope of support for experimental RAO values must be evaluated carefully before deploying them across extended domains such as the public Internet, as they risk disrupting stable network operation.
When experimental RAO values cross multiple administrative domains, experimenters must assume that the same values may be used simultaneously by other experiments, creating a risk of interference. Network administrators within a single domain should ensure consistent use of each experimental value to avoid intra-domain interference.