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rfc:rfc2711

Network Working Group C. Partridge Request for Comments: 2711 BBN Category: Standards Track A. Jackson

                                                                  BBN
                                                         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.

Abstract

 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
 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 [Page 2] RFC 2711 IPv6 Router Alert Option October 1999

    Value:  A 2 octet code in network byte order with the following
    values:
       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
 interesting to the router.  The router's interest and the actions
 taken by employing Router Alert MUST be specified in the RFC of the
 protocol that mandates or allows the use of Router Alert.
 The final destination of the IPv6 datagram MUST ignore this option
 upon receipt to prevent multiple evaluations of the datagram.
 Unrecognized value fields MUST be silently ignored and the processing
 of the header continued.
 Routers that recognize the option will examine datagrams carrying it
 more closely to determine whether or not further processing is
 necessary.  The router only needs to parse the packet in sufficient
 detail to decide whether the packet contains something of interest.
 The value field can be used by an implementation to speed processing
 of the datagram within the transit router.
 Observe that further processing can involve protocol layers above
 IPv6.  E.g., for RSVP messages, the datagram will have to undergo UDP
 and RSVP protocol processing.  Once the datagram leaves the IPv6
 layer, there is considerable ambiguity about whether the router is
 acting as an IPv6 host or an IPv6 router.  Precisely how the router
 handles the contents is value-field specific.  However, if the
 processing required for the datagram involves examining the payload
 of the IPv6 datagram, then the interim router is performing a host
 function and SHOULD interpret the data as a host.

Partridge & Jackson Standards Track [Page 3] RFC 2711 IPv6 Router Alert Option October 1999

3.0 Impact on Other Protocols

 For this option to be effective, its use MUST be mandated in
 protocols that expect routers to perform significant processing on
 datagrams not directly addressed to them.  Routers are not required
 to examine the datagrams not addressed to them unless the datagrams
 include the router alert option.
 All IPv6 datagrams containing an RSVP message MUST contain this
 option within the IPv6 Hop-by-Hop Options Header of such datagrams.

4.0 Security Considerations

 Gratuitous use of this option can cause performance problems in
 routers.  A more severe attack is possible in which the router is
 flooded by bogus datagrams containing router alert options.
 The use of the option, if supported in a router, MAY therefore be
 limited by rate or other means by the transit router.

5.0 IANA Considerations

 The value field described in Section 2.1 is registered and maintained
 by IANA. New values are to be assigned via IETF Consensus as defined
 in RFC 2434 [RFC-2434].

6.0 Notice on Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

Partridge & Jackson Standards Track [Page 4] RFC 2711 IPv6 Router Alert Option October 1999

7.0 References

 [RFC-2119] Bradner, S., "Key words for use in RFC's to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1977.
 [RFC-2205] Braden, B. (ed.), Zhang, L., Berson, S., Herzog, S. and S.
            Jamin, "Resource ReSerVation Protocol (RSVP)", RFC 2205,
            September 1997.
 [RFC-2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 [RFC-2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.
 [RFC-2710] Deering, S., Fenner, W. and B. Haberman, "Multicast
            Listener Discovery (MLD) for IPv6", RFC 2710, October
            1999.

6.0 Authors' Addresses

 Craig Partridge
 BBN Technologies
 10 Moulton Street
 Cambridge, MA 02138
 USA
 Phone: +1 (617) 873-3000
 EMail: craig@bbn.com
 Alden Jackson
 BBN Technologies
 10 Moulton Street
 Cambridge, MA 02138
 USA
 Phone: +1 (617) 873-3000
 EMail: awjacks@bbn.com

Partridge & Jackson Standards Track [Page 5] RFC 2711 IPv6 Router Alert Option October 1999

7.0 Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Partridge & Jackson Standards Track [Page 6]

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