Network Working Group C. Partridge
Request for Comments: 2711 BBN
Category: Standards Track A. Jackson
IPv6 Router Alert Option
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (1999). All Rights Reserved.
This memo describes a new IPv6 Hop-by-Hop Option type that alerts
transit routers to more closely examine the contents of an IP
datagram. This option is useful for situations where a datagram
addressed to a particular destination contains information that may
require special processing by routers along the path.
New protocols, such as RSVP, use control datagrams which, while
addressed to a particular destination, contain information that needs
to be examined, and in some case updated, by routers along the path
between the source and destination. It is desirable to forward
regular datagrams as rapidly as possible, while ensuring that the
router processes these special control datagrams appropriately.
Currently, however, the only way for a router to determine if it
needs to examine a datagram is to at least partially parse upper
layer data in all datagrams. This parsing is expensive and slow.
This situation is undesirable.
This document defines a new option within the IPv6 Hop-by-Hop Header.
The presence of this option in an IPv6 datagram informs the router
that the contents of this datagram is of interest to the router and
to handle any control data accordingly. The absence of this option
in an IPv6 datagram informs the router that the datagram does not
contain information needed by the router and hence can be safely
Partridge & Jackson Standards Track [Page 1]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
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].
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.
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
There MUST only be one option of this type, regardless of value,
per Hop-by-Hop header.
Partridge & Jackson Standards Track [Page 2]RFC 2711 IPv6 Router Alert Option October 1999
Value: A 2 octet code in network byte order with the following
0 Datagram contains a Multicast Listener Discovery