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IETF RFC 2711

IPv6 Router Alert Option

Last modified on Friday, October 1st, 1999

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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 © 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 top


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 top

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 top

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 top

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 top

RFC 2711 IPv6 Router Alert Option October 1999 7.0 Full Copyright Statement Copyright © 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 top

IPv6 Router Alert Option RFC TOTAL SIZE: 11973 bytes PUBLICATION DATE: Friday, October 1st, 1999 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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