RFC 2711 (rfc2711) - Page 2 of 6
IPv6 Router Alert Option
Alternative Format: Original Text Document
RFC 2711 IPv6 Router Alert Option October 1999
routed without further datagram parsing. Hosts originating IPv6
datagrams are required to include this option in certain
circumstances.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-2119].
2.0 Approach
The goal is to provide an efficient mechanism whereby routers can
know when to intercept datagrams not addressed to them without having
to extensively examine every datagram. The described solution is to
define a new IPv6 Hop-by-Hop Header option having the semantic
"routers should examine this datagram more closely" and require
protocols such as RSVP to use this option. This approach incurs
little or no performance penalty on the forwarding of normal
datagrams. Not including this option tells the router that there is
no need to closely examine the contents of the datagram.
2.1 Syntax
The router alert option has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0|0 0 1 0 1|0 0 0 0 0 0 1 0| Value (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
length = 2
The first three bits of the first byte are zero and the value 5 in
the remaining five bits is the Hop-by-Hop Option Type number.
[RFC-2460] specifies the meaning of the first three bits. By
zeroing all three, this specification requires that nodes not
recognizing this option type should skip over this option and
continue processing the header and that the option must not change
en route.
There MUST only be one option of this type, regardless of value,
per Hop-by-Hop header.
Partridge & Jackson Standards Track