IPv6 Router Alert Option
RFC 2711

Document Type RFC - Proposed Standard (October 1999; No errata)
Updated by RFC 6398
Authors Alden Jackson  , Craig Partridge 
Last updated 2013-03-02
Stream IETF
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IESG IESG state RFC 2711 (Proposed Standard)
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Network Working Group                                      C. Partridge
Request for Comments: 2711                                          BBN
Category: Standards Track                                    A. Jackson
                                                           October 1999

                        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 Notice

   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.

1.0  Introduction

   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",
   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                     [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
                  message [RFC-2710].
         1        Datagram contains RSVP message.
         2        Datagram contains an Active Networks message.
         3-65535  Reserved to IANA for future use.

      Alignment requirement: 2n+0

      Values are registered and maintained by the IANA.  See section 5.0
      for more details.

2.2  Semantics

   The option indicates that the contents of the datagram may be
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