RFC 2711
IPv6 Router Alert Option
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
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.
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.
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.
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
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.