IPv6 Router Alert Option
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Extracted elements (15)
The option was introduced because the only prior way for a router to determine whether it needed to examine a datagram was to partially parse upper-layer data in every datagram, which was expensive and slow. The Router Alert option avoids this overhead by explicitly signaling router interest.
The top 3 bits of the option type byte are zeroed, following RFC 2460 semantics: nodes not recognizing this option type will skip over it and continue processing, and the option must not change en route.
Routers are not required to examine datagrams not addressed to them unless those datagrams include the Router Alert option. Protocols relying on router inspection of transit datagrams must mandate use of this option.
All IPv6 datagrams containing an RSVP message MUST include the Router Alert option within the IPv6 Hop-by-Hop Options Header.
For Router Alert to be effective, its use MUST be mandated in protocols that expect routers to perform significant processing on datagrams not directly addressed to them.
If processing a datagram with the Router Alert option requires examining the IPv6 payload, the transit router is performing a host function and SHOULD interpret the data as a host.
The final destination of an IPv6 datagram carrying the Router Alert option MUST ignore this option upon receipt to prevent multiple evaluations of the datagram.
The router's interest and actions taken by employing Router Alert MUST be specified in the RFC of the protocol that mandates or allows its use.
There MUST only be one Router Alert option, regardless of value, per Hop-by-Hop header.
Unrecognized Router Alert value fields MUST be silently ignored and processing of the header continued.
The IPv6 Router Alert Option is a new Hop-by-Hop Option type that signals transit routers to more closely examine the contents of a datagram. Its presence indicates the datagram may require special processing by routers along the path; its absence signals safe forwarding without further parsing.
The Value field is a 2-octet code identifying the type of content requiring router attention: 0 = Multicast Listener Discovery (MLD), 1 = RSVP message, 2 = Active Networks message, 3-65535 reserved for IANA future use.
The Router Alert Option Value field is registered and maintained by IANA. New values are assigned via IETF Consensus as defined in RFC 2434. Initial values: 0 (MLD), 1 (RSVP), 2 (Active Networks), 3-65535 reserved.
Gratuitous or malicious use of the Router Alert option can cause performance problems in routers. A severe attack involves flooding a router with bogus datagrams containing Router Alert options. Transit routers MAY rate-limit or otherwise restrict processing of this option to mitigate such attacks.
The Router Alert option is 4 bytes total: a 1-byte type field (0x05 with top 3 bits zero), a 1-byte length field (value=2), and a 2-octet Value field in network byte order. Alignment requirement is 2n+0.