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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5006

Network Working Group                                      J. Jeong, Ed.
Internet-Draft                              ETRI/University of Minnesota
Expires: July 20, 2006                                           S. Park
                                                     SAMSUNG Electronics
                                                              L. Beloeil
                                                      France Telecom R&D
                                                          S. Madanapalli
                                                             SAMSUNG ISO
                                                        January 16, 2006


         IPv6 Router Advertisement Option for DNS Configuration
              draft-jeong-dnsop-ipv6-dns-discovery-07.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 20, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies a new IPv6 Router Advertisement option to
   allow IPv6 routers to advertise DNS recursive server addresses to
   IPv6 hosts.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Applicability Statements . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Neighbor Discovery Extension . . . . . . . . . . . . . . . . .  4
     5.1   Recursive DNS Server Option  . . . . . . . . . . . . . . .  5
     5.2   Procedure of DNS Configuration . . . . . . . . . . . . . .  7
       5.2.1   Procedure in IPv6 Router . . . . . . . . . . . . . . .  7
       5.2.2   Procedure in IPv6 Host . . . . . . . . . . . . . . . .  7
   6.  Implementation Considerations  . . . . . . . . . . . . . . . .  7
     6.1   DNS Server List Management . . . . . . . . . . . . . . . .  8
     6.2   Synchronization between DNS Server List and Resolver
           Repository . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2  Informative References . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 14



























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1.  Introduction

   Neighbor Discovery (ND) for IP Version 6 and IPv6 Stateless Address
   Autoconfiguration provide ways to configure either fixed or mobile
   nodes with one or more IPv6 addresses, default routes and some other
   parameters [4][5].  To support the access to additional services in
   the Internet that are identified by a DNS name, such as a web server,
   the configuration of at least one recursive DNS server is also needed
   for DNS name resolution.

   It is infeasible for nomadic hosts, such as laptops, to have to enter
   a DNS resolver each time they connect to a different wireless LAN
   (WLAN) such as IEEE 802.11 a/b/g [10]-[13].  This document provides a
   new way which uses a new IPv6 Router Advertisement option to allow
   IPv6 routers to advertise DNS recursive server addresses to IPv6
   hosts.

1.1  Applicability Statements

   RA-based DNS configuration is useful in the networks where an IPv6
   address is autoconfigured through IPv6 stateless address
   autoconfiguration, such as SOHO, home networks, cellular networks
   (e.g., 3GPP), MIPv6 (especially, HMIPv6), NEMO and MANET connected to
   the Internet.  Especially, the new RA option may be useful in some
   mobile environments where the addresses of the RDNSSes are added or
   deleted according to a mobile node's movement because the RA option
   includes a lifetime field that allows the mobile node to delete the
   expired entries for RDNSSes.  This lifetime field can be configured
   to a value that will require the mobile node to time out the RDNSS
   address entry in the previous network and to switch over to another
   RDNSS address in the same network.  Therefore, the lifetime field can
   allow the mobile node to use the RDNSSes announced in the network
   where it is placed.  As a result, the local RDNSS may provide the
   mobile node with quicker recursive DNS resolution service than the
   remote RDNSSes.  Using the lifetime field differentiate RA approach
   from DHCPv6 approach in that it allows mobile nodes to use local
   RDNSSes rather than remote RDNSSes in order to being able to reduce
   the DNS resolution delay [6]-[8].

2.  Definitions

   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 [3].

3.  Terminology

   This document uses the terminology described in [4][5].  In addition,



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   three new terms are defined below:

   o  Recursive DNS Server (RDNSS): Server which provides a recursive
      DNS resolution service.

   o  DNS Server List: Data structure for managing DNS Server
      Information existing in IPv6 protocol stack in addition to
      Neighbor Cache and Destination Cache for Neighbor Discovery [4].

   o  Resolver Repository: Configuration repository with RDNSS addresses
      which a DNS resolver on the host uses for DNS name resolution,
      such as Unix resolver file (i.e., /etc/resolv.conf) and Windows
      registry.

4.  Overview

   This document defines a new ND option called RDNSS option that
   contains the addresses of recursive DNS servers.  Existing ND
   transport mechanisms (i.e., advertisements and solicitations) are
   used.  This works in the same way that hosts learn about routers and
   prefixes.  An IPv6 host can configure the IPv6 addresses of one or
   more RDNSSes via RA message periodically sent by router or solicited
   by a Router Solicitation (RS).

   Through the RDNSS option along with the prefix information option
   based on the ND protocol [4][5], an IPv6 host can perform its network
   configuration of its IPv6 address and RDNSS simultaneously without
   needing a separate message exchange for the RDNSS information.  The
   RA option for RDNSS can be used on any network that supports the use
   of ND.

   This approach requires RDNSS information to be configured in the
   routers sending the advertisements.  The configuration of RDNSS
   addresses in the routers can be done by manual configuration.  The
   automatic configuration or redistribution of RDNSS information is
   possible by running a DHCPv6 client running on the router [6]-[8].
   The automatic configuration of RDNSS addresses in the routers is out
   of scope in this document.

   The preference field of RDNSS option allows IPv6 hosts to select a
   primary RDNSS among several RDNSSes; this can be used for load
   balancing of RDNSSes.

5.  Neighbor Discovery Extension

   The IPv6 DNS configuration mechanism in this document needs a new ND
   option in Neighbor Discovery, Recursive DNS Server (RDNSS) option.




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5.1  Recursive DNS Server Option

   RDNSS option contains one or more IPv6 addresses of recursive DNS
   servers.  All of the addresses share the same preference and lifetime
   values.  If it is desirable to have different preference and lifetime
   values, multiple RDNSS options can be used.



      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Length    |  Pref |S|      Reserved       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Lifetime                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     :            Addresses of IPv6 Recursive DNS Servers            :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


           Figure 1: Recursive DNS Server (RDNSS) Option Format


   Figure 1 shows the format of RDNSS option.

   Fields:

        Type          8-bit identifier of the option type (TBD: IANA)
                            Option Name               Type
                            RDNSS option              (TBD)

        Length        8-bit unsigned integer.  The length of the
                      option (including the type and length fields)
                      in units of 8 octets.  The minimum value is 0x03
                      if one IPv6 address is contained in the option.
                      Every additional RDNSS address increases the
                      length by 0x02.  The length field is used by
                      the receiver to determine the number of IPv6
                      addresses in the option.

        Pref          The preference of an RDNSS.  A 4-bit unsigned
                      integer.  A decimal value of 15 indicates the
                      highest preference.  A value of zero means
                      unspecified.  The default value of preference
                      may be from 8 to 11 for manually configured or
                      RA-derived RDNSSes.  A preference less than 8



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                      means less preferred than manual configured
                      RDNSS and a preference greater than 11 means
                      more preferred.

        S             1-bit "Service open" flag.  When set, it indicates
                      that RDNSS(es) in the option can be available for
                      IPv6 hosts when they are detached from the origin
                      subnet where they obtained the RDNSSes.  This flag
                      SHOULD be set only if the routers or firewalls in
                      the network allow DNS Query messages to be routed
                      to the destination RDNSS without being filtered
                      out, and the RDNSS is configured to perform
                      recursive queries for all hosts regardless of
                      their addresses.

        Lifetime      32-bit unsigned integer.  The maximum time, in
                      seconds (relative to the time the packet is sent),
                      over which this RDNSS is used for name resolution.
                      Hosts MAY send a Router Solicitation to ensure the
                      RDNSS information is fresh before the interval
                      expires.  In order to provide fixed hosts with the
                      stable DNS service and allow mobile hosts to
                      prefer local RDNSSes to remote RDNSSes, the value
                      of lifetime should be at least as long as the
                      Maximum RA Interval (MaxRtrAdvInterval) in
                      RFC 2461, and be at most as long as two times
                      MaxRtrAdvInterval; Lifetime SHOULD be bounded as
                      follows:
                      MaxRtrAdvInterval<=Lifetime<=2*MaxRtrAdvInterval.
                      A value of all one bits (0xffffffff) represents
                      infinity.  A value of zero means that the RDNSS
                      MUST no longer be used.

        Addresses of IPv6 Recursive DNS Servers
                      One or more 128-bit IPv6 addresses of the
                      recursive DNS servers.  The number of addresses
                      is determined by the Length field.  That is,
                      the number of addresses is equal to
                      (Length - 1) / 2.  For the maximum number of
                      RDNSS addresses in one RDNSS option, at most
                      three is recommended since three RDNSSes are
                      enough for DNS resolution service.  See Section
                      5.2 for how parts of RDNSSes in one RDNSS option
                      can be selected by a host.







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5.2  Procedure of DNS Configuration

   The procedure of DNS configuration through RDNSS option is the same
   as any other ND option [4].

5.2.1  Procedure in IPv6 Router

   An IPv6 router SHOULD include RDNSS option(s) in every solicited RA.
   When the router participates in routing and forwarding data packets,
   RDNSS option SHOULD be sent along with Prefix Information option
   included in unsolicited RAs.  Otherwise, the RDNSS option MUST NOT be
   advertised.

   Since one RDNSS option is recommended to have at most three RDNSSes,
   the router MAY send more than one RDNSS option if it would like to
   advertise more than three RDNSSes.

5.2.2  Procedure in IPv6 Host

   When an IPv6 host receives RDNSS option through RA, it checks whether
   the option is valid;

   o  If the RDNSS option is present, the host SHOULD copy the option's
      value into the DNS Server List and the Resolver Repository as long
      as the value of Length field is greater than or equal to the
      minimum value (0x03).  The host MAY ignore additional RDNSS
      addresses within an RDNSS option and/or additional RDNSS options
      within an RA when it has gathered a sufficient number of RDNSSes.
      It is expected that most implementations will use at least two or
      three RDNSSes, but few will use a fourth or subsequent RDNSS.

   o  If the RDNSS option is present but invalid (e.g., it has the
      length less than 0x03), the host SHOULD discard the option.

6.  Implementation Considerations

Note

   This non-normative section gives some hints for implementing the
   processing of RDNSS option in IPv6 host.


   For the configuration and management of RDNSS information, the
   advertised RDNSS addresses can be stored and managed in both the DNS
   Server List and the Resolver Repository.

   In environments where the RDNSS information is stored in user space
   and ND runs in the kernel, it is necessary to synchronize the DNS



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   Server List for RDNSSes in kernel space and the Resolver Repository
   in user space.  For the synchronization, the implementation where ND
   works in the kernel should provide the write operation for updating
   RDNSS information from the kernel to the Resolver Repository.  One
   simple approach to perform this is to have a daemon around (or a
   program that is called at the defined intervals) that keeps
   monitoring the lifetime of RDNSSes all the time.  Whenever there is
   an expired entry in the DNS Server List, the daemon can delete the
   corresponding entry from the Resolver Repository.

6.1  DNS Server List Management

   The kernel or user-space process (depending on where RAs are
   processed) should maintain a data structure called DNS Server List
   which keeps the list of RDNSSes.  Each entry of DNS Server List
   consists of RDNSS address, Preference, Expiration-time, and Service-
   open-flag as follows:

   o  RDNSS address: IPv6 address of Recursive DNS Server which is
      available for recursive DNS resolution service in the network
      advertising the RDNSS option; such a network is called site in
      this document.

   o  Preference: Preference field in RDNSS option allowing IPv6 hosts
      to select a primary RDNSS among several RDNSSes; the other RDNSSes
      except for the primary one can be used as backup.

   o  Expiration-time: Expiration time field giving the time when this
      entry becomes invalid.  Expiration-time is set to the value of
      Lifetime field of RDNSS option plus the current system time.
      Whenever a new RDNSS option with the same address is received,
      this field is updated to have a new expiration time.  When
      Expiration-time becomes less than the current system time, this
      entry is regared as expired.  The decision about whether to delete
      the expired entry depends on its Service-open-flag (See the
      explanation for Service-open-flag).

   o  Service-open-flag (S flag): Flag for deciding whether to delete
      the expired entry.  It is set to the value of "Service open" flag
      of RDNSS option.  When the entry has expired and Service-open-flag
      is 0, the expired entry is deleted from the DNS Server List.
      Otherwise, the entry is maintained.  That is, Service-open-flag
      set to 1 allows expired entry to be maintained.  It may be useful
      when an IPv6 host is nomadic or mobile node.  Service-open-flag
      allows an IPv6 host to continue to use expired RDNSSes located in
      other networks which it moved from.  When there is no available
      RDNSS in the new network (or subnet), the IPv6 host can still use
      the remote RDNSSes which it used for DNS name resolution before.



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      A host MAY delete expired entries in order to limit the storage
      needed for the DNS Server List.  Any least recently used (LRU)
      policy that reclaims entries that have expired with Service-open-
      flag set to 0 can be adopted for replacing the expired entries
      with the entries for newly announced RDNSSes [4].  For example,
      when the replacement is necessary, the IPv6 host can choose one
      whose Service-open-flag is turned off and whose Expiration-time is
      the least.

Note

   When the S flag is not set, an RDNSS may only be used as long as both
   the RA router lifetime and the RDNSS option lifetime have not
   expired.  When the S flag is set, the RDNSS may continue to be used
   after either or both of these lifetimes have expired and there are
   not any more eligible RDNSSes (with still valid lifetimes or learned
   through DHCP or manual configuration) are available [6]-[8].  RDNSSes
   with expired lifetimes should be used as a last resort only, as they
   may not be currently reachable or the RDNSS may not provide service
   to the host's current address.

6.2  Synchronization between DNS Server List and Resolver Repository

   When an IPv6 host receives the information of multiple RDNSSes within
   a site through an RA message with RDNSS option(s), it stores the
   RDNSS addresses in order into both the DNS Server List and the
   Resolver Repository.  The processing of the RDNSS option included in
   RA message is as follows:

      Step (a): Receive and parse RDNSS option(s).  Process only the
      first three RDNSS addresses in each RDNSS option if one RDNSS
      option has more than three RDNSS addresses.

      Step (b): Arrange the addresses of RDNSSes in a descending order,
      starting with the biggest value of "Pref" field of the RDNSS
      option and store them in both the DNS Server List and the Resolver
      Repository by sorting the entries, including newly added entries,
      in the descending order of preference value.  In the case where
      there are several routers advertising RDNSS option(s) in a subnet,
      "Pref" field is used to arrange the information.  When the
      preference is the same, the RDNSS announced earlier is more
      preferred.  Also, when one RDNSS option has multiple RDNSSes with
      the same preference, keep the order or the addresses in the
      option, so the first is preferred.

      Step (c): For each RDNSS option, check the following: If each
      value of "Lifetime" field is set to zero, regardless of the value
      of 'S' flag, delete the corresponding RDNSS entries from both the



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      DNS Server List and the Resolver Repository in order to let the
      RDNSSes be not used any more for certain reasons in network
      management, e.g., the breakdown of the RDNSS and a renumbering
      situation.

      Step (d): Delete each expired entry whose S flag is set off from
      DNS Server List and the RDNSS address corresponding to the entry
      from the Resolver Repository.  However, in mobile environment, in
      order that a mobile node can still use the RDNSS of the previous
      site when the host moves into another site and no RDNSS is
      available there, it MAY be allowed to maintain the entry whose S
      flag is on in both the DNS Server List and the Resolver
      Repository.  The expired RDNSS entry is regarded as having lower
      preference than the valid entries regardless of preference values.
      The sorting for expired RDNSS entries with the S flag set on is
      performed in both the DNS Server List and the Resolver Repository
      based on preference values.  In the case where the data structure
      for the DNS Server List is full of RDNSS entries, the expired
      entries MAY be deleted according to the LRU-based policy specified
      in Section 6.1.

7.  Security Considerations

   The security of RA option for RDNSS is the same as ND protocol
   security [4].  The RA option does not add any new vulnerability.

   It should be noted that the vulnerability of ND is not worse and is a
   subset of the attacks that any node attached to a LAN can do
   independently of ND.  A malicious node on a LAN can promiscuously
   receive packets for any router's MAC address and send packets with
   the router's MAC address as the source MAC address in the L2 header.
   As a result, the L2 switches send packets addressed to the router to
   the malicious node.  Also, this attack can send redirects that tell
   the hosts to send their traffic somewhere else.  The malicious node
   can send unsolicited RA or NA replies, answer RS or NS requests, etc.
   Also, an attacker could configure a host to send out RA with a
   fraudulent RDNSS address, which is presumably and easier avenue of
   attack than becoming a rogue router and having to process all traffic
   for the subnet.  It is necessary to disable the RA RDNSS option
   administatively to avoid this problem.  All of this can be done
   independently of implementing ND.  Therefore, the RA option for RDNSS
   does not add to the vulnerability.

   If Secure Neighbor Discovery (SEND) protocol is used as the security
   mechanism for ND, all the ND options including RDNSS option are also
   automatically included in the signatures [9], so the RDNSS transport
   is integrity-protected.  However, since any valid SEND node can still
   insert RDNSS options, SEND cannot verify who is or is not authorized



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   to send the options.

8.  IANA Considerations

Note

   This section will be removed after the assignment of RDNSS option
   type.


   The IANA should assign a new IPv6 Neighbor Discovery Option type for
   the RDNSS option defined in this document.  The IANA registry for
   these options is:

   http://www.iana.org/assignments/icmpv6-parameters

9.  Acknowledgements

   This draft has greatly benefited from inputs by Robert Hinden, Pekka
   Savola, Iljitsch van Beijnum, Brian Haberman and Tim Chown.  The
   authors appreciate their contribution.

10.  References

10.1  Normative References

   [1]  Bradner, S., "IETF Rights in Contributions", RFC 3978,
        March 2005.

   [2]  Bradner, S., "Intellectual Property Rights in IETF Technology",
        RFC 3668, February 2004.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [4]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [5]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

10.2  Informative References

   [6]   Droms, R., Ed., "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [7]   Droms, R., "Stateless Dynamic Host Configuration Protocol
         (DHCP) Service for IPv6", RFC 3736, April 2004.



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   [8]   Droms, R., Ed., "DNS Configuration options for Dynamic Host
         Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
         December 2003.

   [9]   Arkko, J., Ed., "SEcure Neighbor Discovery (SEND)", RFC 3971,
         March 2005.

   [10]  ANSI/IEEE Std 802.11, "Part 11: Wireless LAN Medium Access
         Control (MAC) and Physical Layer (PHY) Specifications",
         March 1999.

   [11]  IEEE Std 802.11a, "Part 11: Wireless LAN Medium Access Control
         (MAC) and Physical Layer (PHY) specifications: High-speed
         Physical Layer in the 5 GHZ Band", September 1999.

   [12]  IEEE Std 802.11b, "Part 11: Wireless LAN Medium Access Control
         (MAC) and Physical Layer (PHY) specifications: Higher-Speed
         Physical Layer Extension in the 2.4 GHz Band", September 1999.

   [13]  IEEE P802.11g/D8.2, "Part 11: Wireless LAN Medium Access
         Control (MAC) and Physical Layer (PHY) specifications: Further
         Higher Data Rate Extension in the 2.4 GHz Band", April 2003.


Authors' Addresses

   Jaehoon Paul Jeong (editor)
   ETRI/Department of Computer Science and Engineering
   University of Minnesota
   117 Pleasant Street SE
   Minneapolis, MN  55455
   US

   Phone: +1 651 587 7774
   Fax:   +1 612 625 2002
   Email: jjeong@cs.umn.edu
   URI:   http://www.cs.umn.edu/~jjeong/














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   Soohong Daniel Park
   Mobile Platform Laboratory
   SAMSUNG Electronics
   416 Maetan-3dong, Yeongtong-Gu
   Suwon, Gyeonggi-Do  443-742
   Korea

   Phone: +82 31 200 4508
   Email: soohong.park@samsung.com


   Luc Beloeil
   France Telecom R&D
   42, rue des coutures
   BP 6243
   14066 CAEN Cedex 4
   France

   Phone: +33 02 3175 9391
   Email: luc.beloeil@francetelecom.com


   Syam Madanapalli
   AMSUNG India Software Operations
   J. P. Techno Park, 3/1
   Millers Road
   Bangalore  560052
   India

   Phone: +91 80 51197777
   Email: syam@samsung.com




















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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights 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; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
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Jeong, et al.             Expires July 20, 2006                [Page 14]


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