





Network Working Group                                      J. Arkko, Ed.
Request for Comments: 3971                                      Ericsson
Category: Standards Track                                       J. Kempf
                                          DoCoMo Communications Labs USA
                                                                 B. Zill
                                                               Microsoft
                                                             P. Nikander
                                                                Ericsson
                                                              March 2005


                    SEcure Neighbor Discovery (SEND)

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 (2005).

Abstract

   IPv6 nodes use the Neighbor Discovery Protocol (NDP) to discover
   other nodes on the link, to determine their link-layer addresses to
   find routers, and to maintain reachability information about the
   paths to active neighbors.  If not secured, NDP is vulnerable to
   various attacks.  This document specifies security mechanisms for
   NDP.  Unlike those in the original NDP specifications, these
   mechanisms do not use IPsec.

















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Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   3
       1.1.  Specification of Requirements . . . . . . . . . . . . .   4
   2.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Neighbor and Router Discovery Overview. . . . . . . . . . . .   6
   4.  Secure Neighbor Discovery Overview. . . . . . . . . . . . . .   8
   5.  Neighbor Discovery Protocol Options . . . . . . . . . . . . .   9
       5.1.  CGA Option. . . . . . . . . . . . . . . . . . . . . . .  10
             5.1.1.  Processing Rules for Senders. . . . . . . . . .  11
             5.1.2.  Processing Rules for Receivers. . . . . . . . .  12
             5.1.3.  Configuration . . . . . . . . . . . . . . . . .  13
       5.2.  RSA Signature Option. . . . . . . . . . . . . . . . . .  14
             5.2.1.  Processing Rules for Senders. . . . . . . . . .  16
             5.2.2.  Processing Rules for Receivers. . . . . . . . .  16
             5.2.3.  Configuration . . . . . . . . . . . . . . . . .  17
             5.2.4.  Performance Considerations. . . . . . . . . . .  18
       5.3.  Timestamp and Nonce Options . . . . . . . . . . . . . .  19
             5.3.1.  Timestamp Option. . . . . . . . . . . . . . . .  19
             5.3.2.  Nonce Option. . . . . . . . . . . . . . . . . .  20
             5.3.3.  Processing Rules for Senders. . . . . . . . . .  21
             5.3.4.  Processing Rules for Receivers. . . . . . . . .  21
   6.  Authorization Delegation Discovery. . . . . . . . . . . . . .  24
       6.1.  Authorization Model . . . . . . . . . . . . . . . . . .  24
       6.2.  Deployment Model. . . . . . . . . . . . . . . . . . . .  25
       6.3.  Certificate Format. . . . . . . . . . . . . . . . . . .  26
             6.3.1.  Router Authorization Certificate Profile. . . .  26
             6.3.2.  Suitability of Standard Identity Certificates .  29
       6.4.  Certificate Transport . . . . . . . . . . . . . . . . .  29
             6.4.1.  Certification Path Solicitation Message Format.  30
             6.4.2.  Certification Path Advertisement Message Format  32
             6.4.3.  Trust Anchor Option . . . . . . . . . . . . . .  34
             6.4.4.  Certificate Option. . . . . . . . . . . . . . .  36
             6.4.5.  Processing Rules for Routers. . . . . . . . . .  37
             6.4.6.  Processing Rules for Hosts. . . . . . . . . . .  38
       6.5.  Configuration . . . . . . . . . . . . . . . . . . . . .  39
   7.  Addressing. . . . . . . . . . . . . . . . . . . . . . . . . .  40
       7.1.  CGAs. . . . . . . . . . . . . . . . . . . . . . . . . .  40
       7.2.  Redirect Addresses. . . . . . . . . . . . . . . . . . .  40
       7.3.  Advertised Subnet Prefixes. . . . . . . . . . . . . . .  40
       7.4.  Limitations . . . . . . . . . . . . . . . . . . . . . .  41
   8.  Transition Issues . . . . . . . . . . . . . . . . . . . . . .  42
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
       9.1.  Threats to the Local Link Not Covered by SEND . . . . .  44
       9.2.  How SEND Counters Threats to NDP. . . . . . . . . . . .  45
             9.2.1.  Neighbor Solicitation/Advertisement Spoofing. .  45
             9.2.2.  Neighbor Unreachability Detection Failure . . .  46
             9.2.3.  Duplicate Address Detection DoS Attack. . . . .  46



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             9.2.4.  Router Solicitation and Advertisement Attacks .  46
             9.2.5.  Replay Attacks. . . . . . . . . . . . . . . . .  47
             9.2.6.  Neighbor Discovery DoS Attack . . . . . . . . .  48
       9.3.  Attacks against SEND Itself . . . . . . . . . . . . . .  48
   10. Protocol Values . . . . . . . . . . . . . . . . . . . . . . .  49
       10.1. Constants . . . . . . . . . . . . . . . . . . . . . . .  49
       10.2. Variables . . . . . . . . . . . . . . . . . . . . . . .  49
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   12. References. . . . . . . . . . . . . . . . . . . . . . . . . .  50
       12.1. Normative References. . . . . . . . . . . . . . . . . .  50
       12.2. Informative References. . . . . . . . . . . . . . . . .  51
   Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . .  53
       A.    Contributors and Acknowledgments. . . . . . . . . . . .  53
       B.    Cache Management. . . . . . . . . . . . . . . . . . . .  53
       C.    Message Size When Carrying Certificates . . . . . . . .  54
   Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . .  55
   Full Copyright Statements . . . . . . . . . . . . . . . . . . . .  56

1.  Introduction

   IPv6 defines the Neighbor Discovery Protocol (NDP) in RFCs 2461 [4]
   and 2462 [5].  Nodes on the same link use NDP to discover each
   other's presence and link-layer addresses, to find routers, and to
   maintain reachability information about the paths to active
   neighbors.  NDP is used by both hosts and routers.  Its functions
   include Neighbor Discovery (ND), Router Discovery (RD), Address
   Autoconfiguration, Address Resolution, Neighbor Unreachability
   Detection (NUD), Duplicate Address Detection (DAD), and Redirection.

   The original NDP specifications called for the use of IPsec to
   protect NDP messages.  However, the RFCs do not give detailed
   instructions for using IPsec to do this.  In this particular
   application, IPsec can only be used with a manual configuration of
   security associations, due to bootstrapping problems in using IKE
   [19, 15].  Furthermore, the number of manually configured security
   associations needed for protecting NDP can be very large [20], making
   that approach impractical for most purposes.

   The SEND protocol is designed to counter the threats to NDP.  These
   threats are described in detail in [22].  SEND is applicable in
   environments where physical security on the link is not assured (such
   as over wireless) and attacks on NDP are a concern.

   This document is organized as follows.  Sections 2 and 3 define some
   terminology and present a brief review of NDP, respectively.  Section
   4 describes the overall approach to securing NDP.  This approach
   involves the use of new NDP options to carry public key - based
   signatures.  A zero-configuration mechanism is used for showing



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   address ownership on individual nodes; routers are certified by a
   trust anchor [7].  The formats, procedures, and cryptographic
   mechanisms for the zero-configuration mechanism are described in a
   related specification [11].

   The required new NDP options are discussed in Section 5.  Section 6
   describes the mechanism for distributing certification paths to
   establish an authorization delegation chain to a trust anchor.

   Finally, Section 8 discusses the co-existence of secured and
   unsecured NDP on the same link, and Section 9 discusses security
   considerations for SEcure Neighbor Discovery (SEND).

   The use of identity certificates provisioned on end hosts for
   authorizing address use is out of the scope for this document, as is
   the security of NDP when the entity defending an address is not the
   same as the entity claiming that address (also known as "proxy ND").
   These are extensions of SEND that may be treated in separate
   documents, should the need arise.

1.1.  Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and
   "MAY" are to be interpreted as described in [2].

2.  Terms

   Authorization Delegation Discovery (ADD)

      A process through which SEND nodes can acquire a certification
      path from a peer node to a trust anchor.

   Certificate Revocation List (CRL)

      In one method of certificate revocation, an authority periodically
      issues a signed data structure called the Certificate Revocation
      List.  This is a time-stamped list identifying revoked
      certificates, signed by the issuer, and made freely available in a
      public repository.

   Certification Path Advertisement (CPA)

      The advertisement message used in the ADD process.






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   Certification Path Solicitation (CPS)

      The solicitation message used in the ADD process.

   Cryptographically Generated Address (CGA)

      A technique [11] whereby an IPv6 address of a node is
      cryptographically generated by using a one-way hash function from
      the node's public key and some other parameters.

   Distinguished Encoding Rules (DER)

      An encoding scheme for data values, defined in [12].

   Duplicate Address Detection (DAD)

      A mechanism assuring that two IPv6 nodes on the same link are not
      using the same address.

   Fully Qualified Domain Name (FQDN)

      A fully qualified domain name consists of a host and domain name,
      including the top-level domain.

   Internationalized Domain Name (IDN)

      Internationalized Domain Names can be used to represent domain
      names that contain characters outside the ASCII set.  See RFC 3490
      [9].

   Neighbor Discovery (ND)

      The Neighbor Discovery function of the Neighbor Discovery Protocol
      (NDP).  NDP contains functions besides ND.

   Neighbor Discovery Protocol (NDP)

      The IPv6 Neighbor Discovery Protocol [7, 8].

      The Neighbor Discovery Protocol is a part of ICMPv6 [6].

   Neighbor Unreachability Detection (NUD)

      A mechanism used for tracking the reachability of neighbors.







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   Non-SEND node

      An IPv6 node that does not implement this specification but uses
      only the Neighbor Discovery protocol defined in RFCs 2461 and
      2462, as updated, without security.

   Nonce

      An unpredictable random or pseudo-random number generated by a
      node and used exactly once.  In SEND, nonces are used to assure
      that a particular advertisement is linked to the solicitation that
      triggered it.

   Router Authorization Certificate

      An X.509v3 [7] public key certificate using the profile specified
      in Section 6.3.1.

   SEND node

      An IPv6 node that implements this specification.

   Router Discovery (RD)

      Router Discovery allows the hosts to discover what routers exist
      on the link, and what subnet prefixes are available.  Router
      Discovery is a part of the Neighbor Discovery Protocol.

   Trust Anchor

      Hosts are configured with a set of trust anchors to protect Router
      Discovery.  A trust anchor is an entity that the host trusts to
      authorize routers to act as routers.  A trust anchor configuration
      consists of a public key and some associated parameters (see
      Section 6.5 for a detailed explanation of these parameters).

3.  Neighbor and Router Discovery Overview

   The Neighbor Discovery Protocol has several functions.  Many of these
   are overloaded on a few central message types, such as the ICMPv6
   Neighbor Advertisement message.  In this section, we review some of
   these tasks and their effects in order to better understand how the
   messages should be treated.  This section is not normative, and if
   this section and the original Neighbor Discovery RFCs are in
   conflict, the original RFCs, as updated, take precedence.






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   The main functions of NDP are as follows:

   o  The Router Discovery function allows IPv6 hosts to discover the
      local routers on an attached link.  Router Discovery is described
      in Section 6 of RFC 2461 [4].  The main purpose of Router
      Discovery is to find neighboring routers willing to forward
      packets on behalf of hosts.  Subnet prefix discovery involves
      determining which destinations are directly on a link; this
      information is necessary in order to know whether a packet should
      be sent to a router or directly to the destination node.

   o  The Redirect function is used for automatically redirecting a host
      to a better first-hop router, or to inform hosts that a
      destination is in fact a neighbor (i.e., on-link).  Redirect is
      specified in Section 8 of RFC 2461 [4].

   o  Address Autoconfiguration is used for automatically assigning
      addresses to a host [5].  This allows hosts to operate without
      explicit configuration related to IP connectivity.  The default
      autoconfiguration mechanism is stateless.  To create IP addresses,
      hosts use any prefix information delivered to them during Router
      Discovery and then test the newly formed addresses for uniqueness.
      A stateful mechanism, DHCPv6 [18], provides additional
      autoconfiguration features.

   o  Duplicate Address Detection (DAD) is used for preventing address
      collisions [5]: for instance, during Address Autoconfiguration.  A
      node that intends to assign a new address to one of its interfaces
      first runs the DAD procedure to verify that no other node is using
      the same address.  As the rules forbid the use of an address until
      it has been found unique, no higher layer traffic is possible
      until this procedure has been completed.  Thus, preventing attacks
      against DAD can help ensure the availability of communications for
      the node in question.

   o  The Address Resolution function allows a node on the link to
      resolve another node's IPv6 address to the corresponding link-
      layer address.  Address Resolution is defined in Section 7.2 of
      RFC 2461 [4], and it is used for hosts and routers alike.  Again,
      no higher level traffic can proceed until the sender knows the
      link layer address of the destination node or the next hop router.
      Note that the source link layer address on link layer frames is
      not checked against the information learned through Address
      Resolution.  This allows for an easier addition of network
      elements such as bridges and proxies and eases the stack
      implementation requirements, as less information has to be passed
      from layer to layer.




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   o  Neighbor Unreachability Detection (NUD) is used for tracking the
      reachability of neighboring nodes, both hosts and routers.  NUD is
      defined in Section 7.3 of RFC 2461 [4].  NUD is security
      sensitive, because an attacker could claim that reachability
      exists when in fact it does not.

   The NDP messages follow the ICMPv6 message format.  All NDP functions
   are realized by using the Router Solicitation (RS), Router
   Advertisement (RA), Neighbor Solicitation (NS), Neighbor
   Advertisement (NA), and Redirect messages.  An actual NDP message
   includes an NDP message header, consisting of an ICMPv6 header and ND
   message-specific data, and zero or more NDP options.  The NDP message
   options are formatted in the Type-Length-Value format.

                       <------------NDP Message---------------->
   *-------------------------------------------------------------*
   | IPv6 Header      | ICMPv6   | ND Message- | ND Message      |
   | Next Header = 58 | Header   | specific    | Options         |
   | (ICMPv6)         |          | data        |                 |
   *-------------------------------------------------------------*
                       <--NDP Message header-->

4.  Secure Neighbor Discovery Overview

   To secure the various functions in NDP, a set of new Neighbor
   Discovery options is introduced.  They are used to protect NDP
   messages.  This specification introduces these options, an
   authorization delegation discovery process, an address ownership
   proof mechanism, and requirements for the use of these components in
   NDP.

   The components of the solution specified in this document are as
   follows:

   o  Certification paths, anchored on trusted parties, are expected to
      certify the authority of routers.  A host must be configured with
      a trust anchor to which the router has a certification path before
      the host can adopt the router as its default router.
      Certification Path Solicitation and Advertisement messages are
      used to discover a certification path to the trust anchor without
      requiring the actual Router Discovery messages to carry lengthy
      certification paths.  The receipt of a protected Router
      Advertisement message for which no certification path is available
      triggers the authorization delegation discovery process.







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   o  Cryptographically Generated Addresses are used to make sure that
      the sender of a Neighbor Discovery message is the "owner" of the
      claimed address.  A public-private key pair is generated by all
      nodes before they can claim an address.  A new NDP option, the CGA
      option, is used to carry the public key and associated parameters.

      This specification also allows a node to use non-CGAs with
      certificates that authorize their use.  However, the details of
      such use are beyond the scope of this specification and are left
      for future work.

   o  A new NDP option, the RSA Signature option, is used to protect all
      messages relating to Neighbor and Router discovery.

      Public key signatures protect the integrity of the messages and
      authenticate the identity of their sender.  The authority of a
      public key is established either with the authorization delegation
      process, by using certificates, or through the address ownership
      proof mechanism, by using CGAs, or with both, depending on
      configuration and the type of the message protected.

      Note: RSA is mandated because having multiple signature algorithms
      would break compatibility between implementations or increase
      implementation complexity by forcing the implementation of
      multiple algorithms and the mechanism to select among them.  A
      second signature algorithm is only necessary as a recovery
      mechanism, in case a flaw is found in RSA.  If this happens, a
      stronger signature algorithm can be selected, and SEND can be
      revised.  The relationship between the new algorithm and the RSA-
      based SEND described in this document would be similar to that
      between the RSA-based SEND and Neighbor Discovery without SEND.
      Information signed with the stronger algorithm has precedence over
      that signed with RSA, in the same way that RSA-signed information
      now takes precedence over unsigned information.  Implementations
      of the current and revised specs would still be compatible.

   o  In order to prevent replay attacks, two new Neighbor Discovery
      options, Timestamp and Nonce, are introduced.  Given that Neighbor
      and Router Discovery messages are in some cases sent to multicast
      addresses, the Timestamp option offers replay protection without
      any previously established state or sequence numbers.  When the
      messages are used in solicitation-advertisement pairs, they are
      protected with the Nonce option.

5.  Neighbor Discovery Protocol Options

   The options described in this section MUST be supported.




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5.1.  CGA Option

   The CGA option allows the verification of the sender's CGA.  The
   format of the CGA option is described as follows:

     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     |   Pad Length  |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        CGA Parameters                         .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                           Padding                             .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      11

   Length

      The length of the option (including the Type, Length, Pad Length,
      Reserved, CGA Parameters, and Padding fields) in units of 8
      octets.

   Pad Length

      The number of padding octets beyond the end of the CGA Parameters
      field but within the length specified by the Length field.
      Padding octets MUST be set to zero by senders and ignored by
      receivers.

   Reserved

      An 8-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender and MUST be ignored by the
      receiver.






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   CGA Parameters

      A variable-length field containing the CGA Parameters data
      structure described in Section 4 of [11].

      This specification requires that if both the CGA option and the
      RSA Signature option are present, then the public key found from
      the CGA Parameters field in the CGA option MUST be that referred
      by the Key Hash field in the RSA Signature option.  Packets
      received with two different keys MUST be silently discarded.  Note
      that a future extension may provide a mechanism allowing the owner
      of an address and the signer to be different parties.

   Padding

      A variable-length field making the option length a multiple of 8,
      containing as many octets as specified in the Pad Length field.

5.1.1.  Processing Rules for Senders

   If the node has been configured to use SEND, the CGA option MUST be
   present in all Neighbor Solicitation and Advertisement messages and
   MUST be present in Router Solicitation messages unless they are sent
   with the unspecified source address.  The CGA option MAY be present
   in other messages.

   A node sending a message using the CGA option MUST construct the
   message as follows:

      The CGA Parameter field in the CGA option is filled according to
      the rules presented above and in [11].  The public key in the
      field is taken from the configuration used to generate the CGA,
      typically from a data structure associated with the source
      address.  The address MUST be constructed as specified in Section
      4 of [11].  Depending on the type of the message, this address
      appears in different places, as follows:

   Redirect

      The address MUST be the source address of the message.

   Neighbor Solicitation

      The address MUST be the Target Address for solicitations sent for
      Duplicate Address Detection; otherwise it MUST be the source
      address of the message.





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   Neighbor Advertisement

      The address MUST be the source address of the message.

   Router Solicitation

      The address MUST be the source address of the message.  Note that
      the CGA option is not used when the source address is the
      unspecified address.

   Router Advertisement

      The address MUST be the source address of the message.

5.1.2.  Processing Rules for Receivers

   Neighbor Solicitation and Advertisement messages without the CGA
   option MUST be treated as unsecured (i.e., processed in the same way
   as NDP messages sent by a non-SEND node).  The processing of
   unsecured messages is specified in Section 8.  Note that SEND nodes
   that do not attempt to interoperate with non-SEND nodes MAY simply
   discard the unsecured messages.

   Router Solicitation messages without the CGA option MUST also be
   treated as unsecured, unless the source address of the message is the
   unspecified address.

   Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
   Solicitation, and Router Advertisement messages containing a CGA
   option MUST be checked as follows:

      If the interface has been configured to use CGA, the receiving
      node MUST verify the source address of the packet by using the
      algorithm described in Section 5 of [11].  The inputs to the
      algorithm are the claimed address, as defined in the previous
      section, and the CGA Parameters field.

      If the CGA verification is successful, the recipient proceeds with
      a more time-consuming cryptographic check of the signature.  Note
      that even if the CGA verification succeeds, no claims about the
      validity of the use can be made until the signature has been
      checked.

   A receiver that does not support CGA or has not specified its use for
   a given interface can still verify packets by using trust anchors,
   even if a CGA is used on a packet.  In such a case, the CGA property
   of the address is simply left unverified.




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5.1.3.  Configuration

   All nodes that support the verification of the CGA option MUST record
   the following configuration information:

   minbits

      The minimum acceptable key length for public keys used in the
      generation of CGAs.  The default SHOULD be 1024 bits.
      Implementations MAY also set an upper limit for the amount of
      computation needed when verifying packets that use these security
      associations.  The upper limit SHOULD be at least 2048 bits.  Any
      implementation should follow prudent cryptographic practice in
      determining the appropriate key lengths.

   All nodes that support the sending of the CGA option MUST record the
   following configuration information:

   CGA parameters

      Any information required to construct CGAs, as described in [11].






























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5.2.  RSA Signature Option

   The RSA Signature option allows public key-based signatures to be
   attached to NDP messages.  The format of the RSA Signature option is
   described in the following diagram:

     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     |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                          Key Hash                             |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                       Digital Signature                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                           Padding                             .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      12

   Length

      The length of the option (including the Type, Length, Reserved,
      Key Hash, Digital Signature, and Padding fields) in units of 8
      octets.

   Reserved

      A 16-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender, and MUST be ignored by the
      receiver.







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   Key Hash

      A 128-bit field containing the most significant (leftmost) 128
      bits of a SHA-1 [14] hash of the public key used for constructing
      the signature.  The SHA-1 hash is taken over the presentation used
      in the Public Key field of the CGA Parameters data structure
      carried in the CGA option.  Its purpose is to associate the
      signature to a particular key known by the receiver.  Such a key
      can either be stored in the certificate cache of the receiver or
      be received in the CGA option in the same message.

   Digital Signature

      A variable-length field containing a PKCS#1 v1.5 signature,
      constructed by using the sender's private key over the following
      sequence of octets:

      1. The 128-bit CGA Message Type tag [11] value for SEND, 0x086F
         CA5E 10B2 00C9 9C8C E001 6427 7C08.  (The tag value has been
         generated randomly by the editor of this specification.).

      2. The 128-bit Source Address field from the IP header.

      3. The 128-bit Destination Address field from the IP header.

      4. The 8-bit Type, 8-bit Code, and 16-bit Checksum fields from the
         ICMP header.

      5. The NDP message header, starting from the octet after the ICMP
         Checksum field and continuing up to but not including NDP
         options.

      6. All NDP options preceding the RSA Signature option.

      The signature value is computed with the RSASSA-PKCS1-v1_5
      algorithm and SHA-1 hash, as defined in [13].

      This field starts after the Key Hash field.  The length of the
      Digital Signature field is determined by the length of the RSA
      Signature option minus the length of the other fields (including
      the variable length Pad field).

   Padding

      This variable-length field contains padding, as many bytes long as
      remain after the end of the signature.





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5.2.1.  Processing Rules for Senders

   If the node has been configured to use SEND, Neighbor Solicitation,
   Neighbor Advertisement, Router Advertisement, and Redirect messages
   MUST contain the RSA Signature option.  Router Solicitation messages
   not sent with the unspecified source address MUST contain the RSA
   Signature option.

   A node sending a message with the RSA Signature option MUST construct
   the message as follows:

   o  The message is constructed in its entirety, without the RSA
      Signature option.

   o  The RSA Signature option is added as the last option in the
      message.

   o  The data to be signed is constructed as explained in Section 5.2,
      under the description of the Digital Signature field.

   o  The message, in the form defined above, is signed by using the
      configured private key, and the resulting PKCS#1 v1.5 signature is
      put in the Digital Signature field.

5.2.2.  Processing Rules for Receivers

   Neighbor Solicitation, Neighbor Advertisement, Router Advertisement,
   and Redirect messages without the RSA Signature option MUST be
   treated as unsecured (i.e., processed in the same way as NDP messages
   sent by a non-SEND node).  See Section 8.

   Router Solicitation messages without the RSA Signature option MUST
   also be treated as unsecured, unless the source address of the
   message is the unspecified address.

   Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
   Solicitation, and Router Advertisement messages containing an RSA
   Signature option MUST be checked as follows:

   o  The receiver MUST ignore any options that come after the first RSA
      Signature option.  (The options are ignored for both signature
      verification and NDP processing purposes.)

   o  The Key Hash field MUST indicate the use of a known public key,
      either one learned from a preceding CGA option in the same
      message, or one known by other means.





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   o  The Digital Signature field MUST have correct encoding and MUST
      not exceed the length of the RSA Signature option minus the
      Padding.

   o  The Digital Signature verification MUST show that the signature
      has been calculated as specified in the previous section.

   o  If the use of a trust anchor has been configured, a valid
      certification path (see Section 6.3) between the receiver's trust
      anchor and the sender's public key MUST be known.

      Note that the receiver may verify just the CGA property of a
      packet, even if, in addition to CGA, the sender has used a trust
      anchor.

   Messages that do not pass all the above tests MUST be silently
   discarded if the host has been configured to accept only secured ND
   messages.  The messages MAY be accepted if the host has been
   configured to accept both secured and unsecured messages but MUST be
   treated as an unsecured message.  The receiver MAY also otherwise
   silently discard packets (e.g., as a response to an apparent CPU
   exhausting DoS attack).

5.2.3.  Configuration

   All nodes that support the reception of the RSA Signature options
   MUST allow the following information to be configured for each
   separate NDP message type:

   authorization method

      This parameter determines the method through which the authority
      of the sender is determined.  It can have four values:

         trust anchor

            The authority of the sender is verified as described in
            Section 6.3.  The sender may claim additional authorization
            through the use of CGAs, but this is neither required nor
            verified.

         CGA

            The CGA property of the sender's address is verified as
            described in [11].  The sender may claim additional
            authority through a trust anchor, but this is neither
            required nor verified.




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         trust anchor and CGA

            Both the trust anchor and the CGA verification is required.

         trust anchor or CGA

            Either the trust anchor or the CGA verification is required.

   anchor

      The allowed trust anchor(s), if the authorization method is not
      set to CGA.

   All nodes that support sending RSA Signature options MUST record the
   following configuration information:

      keypair

         A public-private key pair.  If authorization delegation is in
         use, a certification path from a trust anchor to this key pair
         must exist.

      CGA flag

         A flag that indicates whether CGA is used or not.  This flag
         may be per interface or per node.  (Note that in future
         extensions of the SEND protocol, this flag may also be per
         subnet prefix.)

5.2.4.  Performance Considerations

   The construction and verification of the RSA Signature option is
   computationally expensive.  In the NDP context, however, hosts
   typically only have to perform a few signature operations as they
   enter a link, a few operations as they find a new on-link peer with
   which to communicate, or Neighbor Unreachability Detection with
   existing neighbors.

   Routers are required to perform a larger number of operations,
   particularly when the frequency of router advertisements is high due
   to mobility requirements.  Still, the number of required signature
   operations is on the order of a few dozen per second, some of which
   can be precomputed as explained below.  A large number of router
   solicitations may cause a higher demand for performing asymmetric
   operations, although the base NDP protocol limits the rate at which
   multicast responses to solicitations can be sent.





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   Signatures can be precomputed for unsolicited (multicast) Neighbor
   and Router Advertisements if the timing of the future advertisements
   is known.  Typically, solicited neighbor advertisements are sent to
   the unicast address from which the solicitation was sent.  Given that
   the IPv6 header is covered by the signature, it is not possible to
   precompute solicited advertisements.

5.3.  Timestamp and Nonce Options

5.3.1.  Timestamp Option

   The purpose of the Timestamp option is to make sure that unsolicited
   advertisements and redirects have not been replayed.  The format of
   this option is described in the following:

    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     |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                          Timestamp                            +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      13

   Length

      The length of the option (including the Type, Length, Reserved,
      and Timestamp fields) in units of 8 octets; i.e., 2.

   Reserved

      A 48-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender and MUST be ignored by the
      receiver.










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   Timestamp

      A 64-bit unsigned integer field containing a timestamp.  The value
      indicates the number of seconds since January 1, 1970, 00:00 UTC,
      by using a fixed point format.  In this format, the integer number
      of seconds is contained in the first 48 bits of the field, and the
      remaining 16 bits indicate the number of 1/64K fractions of a
      second.

      Implementation note: This format is compatible with the usual
      representation of time under UNIX, although the number of bits
      available for the integer and fraction parts may vary.

5.3.2.  Nonce Option

   The purpose of the Nonce option is to make sure that an advertisement
   is a fresh response to a solicitation sent earlier by the node.  The
   format of this option is described in the following:

     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     |  Nonce ...                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                                                               |
    .                                                               .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      14

   Length

      The length of the option (including the Type, Length, and Nonce
      fields) in units of 8 octets.

   Nonce

      A field containing a random number selected by the sender of the
      solicitation message.  The length of the random number MUST be at
      least 6 bytes.  The length of the random number MUST be selected
      so that the length of the nonce option is a multiple of 8 octets.






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5.3.3.  Processing Rules for Senders

   If the node has been configured to use SEND, all solicitation
   messages MUST include a Nonce.  When sending a solicitation, the
   sender MUST store the nonce internally so that it can recognize any
   replies containing that particular nonce.

   If the node has been configured to use SEND, all advertisements sent
   in reply to a solicitation MUST include a Nonce, copied from the
   received solicitation.  Note that routers may decide to send a
   multicast advertisement to all nodes instead of a response to a
   specific host.  In such a case, the router MAY still include the
   nonce value for the host that triggered the multicast advertisement.
   (Omitting the nonce value may cause the host to ignore the router's
   advertisement, unless the clocks in these nodes are sufficiently
   synchronized so that timestamps function properly.)

   If the node has been configured to use SEND, all solicitation,
   advertisement, and redirect messages MUST include a Timestamp.
   Senders SHOULD set the Timestamp field to the current time, according
   to their real time clocks.

5.3.4.  Processing Rules for Receivers

   The processing of the Nonce and Timestamp options depends on whether
   a packet is a solicited advertisement.  A system may implement the
   distinction in various ways.  Section 5.3.4.1 defines the processing
   rules for solicited advertisements.  Section 5.3.4.2 defines the
   processing rules for all other messages.

   In addition, the following rules apply in all cases:

   o  Messages received without at least one of the Timestamp and Nonce
      options MUST be treated as unsecured (i.e., processed in the same
      way as NDP messages sent by a non-SEND node).

   o  Messages received with the RSA Signature option but without the
      Timestamp option MUST be silently discarded.

   o  Solicitation messages received with the RSA Signature option but
      without the Nonce option MUST be silently discarded.

   o  Advertisements sent to a unicast destination address with the RSA
      Signature option but without a Nonce option SHOULD be processed as
      unsolicited advertisements.






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   o  An implementation MAY use some mechanism such as a timestamp cache
      to strengthen resistance to replay attacks.  When there is a very
      large number of nodes on the same link, or when a cache filling
      attack is in progress, it is possible that the cache holding the
      most recent timestamp per sender will become full.  In this case,
      the node MUST remove some entries from the cache or refuse some
      new requested entries.  The specific policy as to which entries
      are preferred over others is left as an implementation decision.
      However, typical policies may prefer existing entries to new ones,
      CGAs with a large Sec value to smaller Sec values, and so on.  The
      issue is briefly discussed in Appendix B.

   o  The receiver MUST be prepared to receive the Timestamp and Nonce
      options in any order, as per RFC 2461 [4], Section 9.

5.3.4.1.  Processing Solicited Advertisements

   The receiver MUST verify that it has recently sent a matching
   solicitation, and that the received advertisement contains a copy of
   the Nonce sent in the solicitation.

   If the message contains a Nonce option but the Nonce value is not
   recognized, the message MUST be silently discarded.

   Otherwise, if the message does not contain a Nonce option, it MAY be
   considered an unsolicited advertisement and processed according to
   Section 5.3.4.2.

   If the message is accepted, the receiver SHOULD store the receive
   time of the message and the timestamp time in the message, as
   specified in Section 5.3.4.2.

5.3.4.2.  Processing All Other Messages

   Receivers SHOULD be configured with an allowed timestamp Delta value,
   a "fuzz factor" for comparisons, and an allowed clock drift
   parameter.  The recommended default value for the allowed Delta is
   TIMESTAMP_DELTA; for fuzz factor TIMESTAMP_FUZZ; and for clock drift,
   TIMESTAMP_DRIFT (see Section 10.2).

   To facilitate timestamp checking, each node SHOULD store the
   following information for each peer:

   o  The receive time of the last received and accepted SEND message.
      This is called RDlast.

   o  The time stamp in the last received and accepted SEND message.
      This is called TSlast.



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   An accepted SEND message is any successfully verified Neighbor
   Solicitation, Neighbor Advertisement, Router Solicitation, Router
   Advertisement, or Redirect message from the given peer.  The RSA
   Signature option MUST be used in such a message before it can update
   the above variables.

   Receivers SHOULD then check the Timestamp field as follows:

   o  When a message is received from a new peer (i.e., one that is not
      stored in the cache), the received timestamp, TSnew, is checked,
      and the packet is accepted if the timestamp is recent enough to
      the reception time of the packet, RDnew:

         -Delta < (RDnew - TSnew) < +Delta

      The RDnew and TSnew values SHOULD be stored in the cache as RDlast
      and TSlast.

   o  If the timestamp is NOT within the boundaries but the message is a
      Neighbor Solicitation message that the receiver should answer, the
      receiver SHOULD respond to the message.  However, even if it does
      respond to the message, it MUST NOT create a Neighbor Cache entry.
      This allows nodes that have large differences in their clocks to
      continue communicating with each other by exchanging NS/NA pairs.

   o  When a message is received from a known peer (i.e., one that
      already has an entry in the cache), the timestamp is checked
      against the previously received SEND message:

         TSnew + fuzz > TSlast + (RDnew - RDlast) x (1 - drift) - fuzz

      If this inequality does not hold, the receiver SHOULD silently
      discard the message.  If, on the other hand, the inequality holds,
      the receiver SHOULD process the message.

      Moreover, if the above inequality holds and TSnew > TSlast, the
      receiver SHOULD update RDlast and TSlast.  Otherwise, the receiver
      MUST NOT update RDlast or TSlast.

   As unsolicited messages may be used in a Denial-of-Service attack to
   make the receiver verify computationally expensive signatures, all
   nodes SHOULD apply a mechanism to prevent excessive use of resources
   for processing such messages.








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6.  Authorization Delegation Discovery

   NDP allows a node to configure itself automatically based on
   information learned shortly after connecting to a new link.  It is
   particularly easy to configure "rogue" routers on an unsecured link,
   and it is particularly difficult for a node to distinguish between
   valid and invalid sources of router information, because the node
   needs this information before communicating with nodes outside of the
   link.

   As the newly-connected node cannot communicate off-link, it cannot be
   responsible for searching information to help validate the router(s).
   However, given a certification path, the node can check someone
   else's search results and conclude that a particular message comes
   from an authorized source.  In the typical case, a router already
   connected beyond the link can communicate if necessary with off-link
   nodes and construct a certification path.

   The Secure Neighbor Discovery Protocol mandates a certificate format
   and introduces two new ICMPv6 messages used between hosts and routers
   to allow the host to learn a certification path with the assistance
   of the router.

6.1.  Authorization Model

   To protect Router Discovery, SEND requires that routers be authorized
   to act as routers.  This authorization is provisioned in both routers
   and hosts.  Routers are given certificates from a trust anchor, and
   the hosts are configured with the trust anchor(s) to authorize
   routers.  This provisioning is specific to SEND and does not assume
   that certificates already deployed for some other purpose can be
   used.

   The authorization for routers in SEND is twofold:

   o  Routers are authorized to act as routers.  The router belongs to
      the set of routers trusted by the trust anchor.  All routers in
      this set have the same authorization.

   o  Optionally, routers may also be authorized to advertise a certain
      set of subnet prefixes.  A specific router is given a specific set
      of subnet prefixes to advertise; other routers have an
      authorization to advertise other subnet prefixes.  Trust anchors
      may also delegate a certain set of subnet prefixes to someone
      (such as an ISP) who, in turn, delegates parts of this set to
      individual routers.





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   Note that while communicating with hosts, routers typically also
   present a number of other parameters beyond the above.  For instance,
   routers have their own IP addresses, subnet prefixes have lifetimes,
   and routers control the use of stateless and stateful address
   autoconfiguration.  However, the ability to be a router and the
   subnet prefixes are the most fundamental parameters to authorize.
   This is because the host needs to choose a router that it uses as its
   default router, and because the advertised subnet prefixes have an
   impact on the addresses the host uses.  The subnet prefixes also
   represent a claim about the topological location of the router in the
   network.

   Care should be taken if the certificates used in SEND are also used
   to provide authorization in other circumstances; for example, with
   routing protocols.  It is necessary to ensure that the authorization
   information is appropriate for all applications.  SEND certificates
   may authorize a larger set of subnet prefixes than the router is
   authorized to advertise on a given interface.  For instance, SEND
   allows the use of the null prefix, which might cause verification or
   routing problems in other applications.  It is RECOMMENDED that SEND
   certificates containing the null prefix are only used for SEND.

   Note that end hosts need not be provisioned with their own certified
   public keys, just as Web clients today do not require end host
   provisioning with certified keys.  Public keys for CGA generation do
   not need to be certified, as these keys derive their ability to
   authorize operations on the CGA by the tie to the address.

6.2.  Deployment Model

   The deployment model for trust anchors can be either a globally
   rooted public key infrastructure or a more local, decentralized
   deployment model similar to that currently used for TLS in Web
   servers.  The centralized model assumes a global root capable of
   authorizing routers and, optionally, the address space they
   advertise.  The end hosts are configured with the public keys of the
   global root.  The global root could operate, for instance, under the
   Internet Assigned Numbers Authority (IANA) or as a co-operative among
   Regional Internet Registries (RIRs).  However, no such global root
   currently exists.

   In the decentralized model, end hosts are configured with a
   collection of trusted public keys.  The public keys could be issued
   from various places; for example, a) a public key for the end host's
   own organization, b) a public key for the end host's home ISP and for
   ISPs with which the home ISP has a roaming agreement, or c) public
   keys for roaming brokers acting as intermediaries for ISPs that don't
   want to run their own certification authority.



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   This decentralized model works even when a SEND node is used both in
   networks that have certified routers and in networks that do not.  As
   discussed in Section 8, a SEND node can fall back to the use of a
   non-SEND router.  This makes it possible to start with a local trust
   anchor even if there is no trust anchor for all possible networks.

6.3.  Certificate Format

   The certification path of a router terminates in a Router
   Authorization Certificate that authorizes a specific IPv6 node to act
   as a router.  Because authorization paths are not a common practice
   in the Internet at the time of this writing, the path MUST consist of
   standard Public Key Certificates (PKC, in the sense of [8]).  The
   certification path MUST start from the identity of a trust anchor
   shared by the host and the router.  This allows the host to anchor
   trust for the router's public key in the trust anchor.  Note that
   there MAY be multiple certificates issued by a single trust anchor.

6.3.1.  Router Authorization Certificate Profile

   Router Authorization Certificates are X.509v3 certificates, as
   defined in RFC 3280 [7], and SHOULD contain at least one instance of
   the X.509 extension for IP addresses, as defined in [10].  The parent
   certificates in the certification path SHOULD contain one or more
   X.509 IP address extensions, back up to a trusted party (such as the
   user's ISP) that configured the original IP address block for the
   router in question, or that delegated the right to do so.  The
   certificates for the intermediate delegating authorities SHOULD
   contain X.509 IP address extension(s) for subdelegations.  The
   router's certificate is signed by the delegating authority for the
   subnet prefixes the router is authorized to advertise.

   The X.509 IP address extension MUST contain at least one
   addressesOrRanges element.  This element MUST contain an
   addressPrefix element containing an IPv6 address prefix for a prefix
   that the router or the intermediate entity is authorized to route.
   If the entity is allowed to route any prefix, the IPv6 address prefix
   used is the null prefix, ::/0.  The addressFamily element of the
   IPAddrBlocks sequence element MUST contain the IPv6 Address Family
   Identifier (0002), as specified in [10], for IPv6 subnet prefixes.
   Instead of an addressPrefix element, the addressesOrRange element MAY
   contain an addressRange element for a range of subnet prefixes, if
   more than one prefix is authorized.  The X.509 IP address extension
   MAY contain additional IPv6 subnet prefixes, expressed as either an
   addressPrefix or an addressRange.






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   A node receiving a Router Authorization Certificate MUST first check
   whether the certificate's signature was generated by the delegating
   authority.  Then the client SHOULD check whether all the
   addressPrefix or addressRange entries in the router's certificate are
   contained within the address ranges in the delegating authority's
   certificate, and whether the addressPrefix entries match any
   addressPrefix entries in the delegating authority's certificate.  If
   an addressPrefix or addressRange is not contained within the
   delegating authority's subnet prefixes or ranges, the client MAY
   attempt to take an intersection of the ranges/subnet prefixes and to
   use that intersection.  If the resulting intersection is empty, the
   client MUST NOT accept the certificate.  If the addressPrefix in the
   certificate is missing or is the null prefix, ::/0, the parent prefix
   or range SHOULD be used.  If there is no parent prefix or range, the
   subnet prefixes that the router advertises are said to be
   unconstrained (see Section 7.3).  That is, the router is allowed to
   advertise any prefix.

   The above checks SHOULD be done for all certificates in the path.  If
   any of the checks fail, the client MUST NOT accept the certificate.
   The client also has to perform validation of advertised subnet
   prefixes as discussed in Section 7.3.

   Hosts MUST check the subjectPublicKeyInfo field within the last
   certificate in the certificate path to ensure that only RSA public
   keys are used to attempt validation of router signatures.  Hosts MUST
   disregard the certificate for SEND if it does not contain an RSA key.

   As it is possible that some public key certificates used with SEND do
   not immediately contain the X.509 IP address extension element, an
   implementation MAY contain facilities that allow the prefix and range
   checks to be relaxed.  However, any such configuration options SHOULD
   be switched off by default.  The system SHOULD have a default
   configuration that requires rigorous prefix and range checks.

   The following is an example of a certification path.  Suppose that
   isp_group_example.net is the trust anchor.  The host has this
   certificate:

      Certificate 1:
        Issuer: isp_group_example.net
        Validity: Jan 1, 2004 through Dec 31, 2004
        Subject: isp_group_example.net
        Extensions:
          IP address delegation extension:
             Prefixes: P1, ..., Pk
          ... possibly other extensions ...
        ... other certificate parameters ...



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   When the host attaches to a link served by
   router_x.isp_foo_example.net, it receives the following certification
   path:

      Certificate 2:
        Issuer: isp_group_example.net
        Validity: Jan 1, 2004 through Dec 31, 2004
        Subject: isp_foo_example.net
        Extensions:
          IP address delegation extension:
            Prefixes: Q1, ..., Qk
          ... possibly other extensions ...
        ... other certificate parameters ...

      Certificate 3:
        Issuer: isp_foo_example.net
        Validity: Jan 1, 2004 through Dec 31, 2004
        Subject: router_x.isp_foo_example.net
        Extensions:
          IP address delegation extension:
            Prefixes R1, ..., Rk
          ... possibly other extensions ...

        ... other certificate parameters ...

   When the three certificates are processed, the usual RFC 3280 [7]
   certificate path validation is performed.  Note, however, that when a
   node checks certificates received from a router, it typically does
   not have a connection to the Internet yet, and so it is not possible
   to perform an on-line Certificate Revocation List (CRL) check, if
   necessary.  Until this check is performed, acceptance of the
   certificate MUST be considered provisional, and the node MUST perform
   a check as soon as it has established a connection with the Internet
   through the router.  If the router has been compromised, it could
   interfere with the CRL check.  Should performance of the CRL check be
   disrupted or should the check fail, the node SHOULD immediately stop
   using the router as a default and use another router on the link
   instead.

   In addition, the IP addresses in the delegation extension MUST be a
   subset of the IP addresses in the delegation extension of the
   issuer's certificate.  So in this example, R1, ..., Rs must be a
   subset of Q1,...,Qr, and Q1,...,Qr must be a subset of P1,...,Pk.  If
   the certification path is valid, then router_foo.isp_foo_example.com
   is authorized to route the prefixes R1,...,Rs.






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6.3.2.  Suitability of Standard Identity Certificates

   As deployment of the IP address extension is, itself, not common, a
   network service provider MAY choose to deploy standard identity
   certificates on the router to supply the router's public key for
   signed Router Advertisements.

   If there is no prefix information further up in the certification
   path, a host interprets a standard identity certificate as allowing
   unconstrained prefix advertisements.

   If the other certificates contain prefix information, a standard
   identity certificate is interpreted as allowing those subnet
   prefixes.

6.4.  Certificate Transport

   The Certification Path Solicitation (CPS) message is sent by a host
   when it wishes to request a certification path between a router and
   one of the host's trust anchors.  The Certification Path
   Advertisement (CPA) message is sent in reply to the CPS message.
   These messages are kept separate from the rest of Neighbor and Router
   Discovery to reduce the effect of the potentially voluminous
   certification path information on other messages.

   The Authorization Delegation Discovery (ADD) process does not exclude
   other forms of discovering certification paths.  For instance, during
   fast movements, mobile nodes may learn information (including the
   certification paths) about the next router from a previous router, or
   nodes may be preconfigured with certification paths from roaming
   partners.

   Where hosts themselves are certified by a trust anchor, these
   messages MAY also optionally be used between hosts to acquire the
   peer's certification path.  However, the details of such usage are
   beyond the scope of this specification.















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6.4.1.  Certification Path Solicitation Message Format

   Hosts send Certification Path Solicitations in order to prompt
   routers to generate Certification Path Advertisements.

    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      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |          Component            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

      Source Address

         A link-local unicast address assigned to the sending interface,
         or to the unspecified address if no address is assigned to the
         sending interface.

      Destination Address

         Typically the All-Routers multicast address, the Solicited-Node
         multicast address, or the address of the host's default router.

      Hop Limit

         255

   ICMP Fields:

      Type

         148

      Code

         0

      Checksum

         The ICMP checksum [6].






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      Identifier

         A 16-bit unsigned integer field, acting as an identifier to
         help match advertisements to solicitations.  The Identifier
         field MUST NOT be zero, and its value SHOULD be randomly
         generated.  This randomness does not have to be
         cryptographically hard, as its purpose is only to avoid
         collisions.

      Component

         This 16-bit unsigned integer field is set to 65,535 if the
         sender seeks to retrieve all certificates.  Otherwise, it is
         set to the component identifier corresponding to the
         certificate that the receiver wants to retrieve (see Sections
         6.4.2 and 6.4.6).

   Valid Options:

      Trust Anchor

         One or more trust anchors that the client is willing to accept.
         The first (or only) Trust Anchor option MUST contain a DER
         Encoded X.501 Name; see Section 6.4.3.  If there is more than
         one Trust Anchor option, the options beyond the first may
         contain any type of trust anchor.

      Future versions of this protocol may define new option types.
      Receivers MUST silently ignore any options they do not recognize
      and continue processing the message.  All included options MUST
      have a length greater than zero.

      ICMP length (derived from the IP length) MUST be 8 or more octets.


















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6.4.2.  Certification Path Advertisement Message Format

   Routers send out Certification Path Advertisement messages in
   response to a Certification Path Solicitation.

    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      |     Code      |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Identifier           |        All Components         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Component            |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options ...
   +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

      Source Address

         A link-local unicast address assigned to the interface from
         which this message is sent.  Note that routers may use multiple
         addresses, and therefore this address is not sufficient for the
         unique identification of routers.

      Destination Address

         Either the Solicited-Node multicast address of the receiver or
         the link-scoped All-Nodes multicast address.

      Hop Limit

         255

   ICMP Fields:

      Type

         149

      Code

         0

      Checksum

         The ICMP checksum [6].



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      Identifier

         A 16-bit unsigned integer field, acting as an identifier to
         help match advertisements to solicitations.  The Identifier
         field MUST be zero for advertisements sent to the All-Nodes
         multicast address and MUST NOT be zero for others.

      All Components

         A 16-bit unsigned integer field, used to inform the receiver of
         the number of certificates in the entire path.

         A single advertisement SHOULD be broken into separately sent
         components if there is more than one certificate in the path,
         in order to avoid excessive fragmentation at the IP layer.

         Individual certificates in a path MAY be stored and used as
         received before all the certificates have arrived; this makes
         the protocol slightly more reliable and less prone to Denial-
         of-Service attacks.

         Examples of packet lengths of Certification Path Advertisement
         messages for typical certification paths are listed in Appendix
         C.

      Component

         A 16-bit unsigned integer field, used to inform the receiver
         which certificate is being sent.

         The first message in an N-component advertisement has the
         Component field set to N-1, the second set to N-2, and so on.
         A zero indicates that there are no more components coming in
         this advertisement.

         The sending of path components SHOULD be ordered so that the
         certificate after the trust anchor is sent first.  Each
         certificate sent after the first can be verified with the
         previously sent certificates.  The certificate of the sender
         comes last.  The trust anchor certificate SHOULD NOT be sent.

      Reserved

         An unused field.  It MUST be initialized to zero by the sender
         and MUST be ignored by the receiver.






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   Valid Options:

      Certificate

         One certificate is provided in each Certificate option to
         establish part of a certification path to a trust anchor.

         The certificate of the trust anchor itself SHOULD NOT be sent.

      Trust Anchor

         Zero or more Trust Anchor options may be included to help
         receivers decide which advertisements are useful for them.  If
         present, these options MUST appear in the first component of a
         multi-component advertisement.

      Future versions of this protocol may define new option types.
      Receivers MUST silently ignore any options they do not recognize
      and continue processing the message.  All included options MUST
      have a length that is greater than zero.

      The ICMP length (derived from the IP length) MUST be 8 or more
      octets.

6.4.3.  Trust Anchor Option

   The format of the Trust Anchor option is described in the following:

    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     |  Name Type    |  Pad  Length  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Name ...                                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          ... Padding                                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      15

   Length

      The length of the option (including the Type, Length, Name Type,
      Pad Length, and Name fields), in units of 8 octets.





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   Name Type

      The type of the name included in the Name field.  This
      specification defines two legal values for this field:

            1        DER Encoded X.501 Name
            2        FQDN

   Pad Length

      The number of padding octets beyond the end of the Name field but
      within the length specified by the Length field.  Padding octets
      MUST be set to zero by senders and ignored by receivers.

   Name

      When the Name Type field is set to 1, the Name field contains a
      DER encoded X.501 Name identifying the trust anchor.  The value is
      encoded as defined in [12] and [7].

      When the Name Type field is set to 2, the Name field contains a
      Fully Qualified Domain Name of the trust anchor; for example,
      "trustanchor.example.com".  The name is stored as a string, in the
      DNS wire format, as specified in RFC 1034 [1].  Additionally, the
      restrictions discussed in RFC 3280 [7], Section 4.2.1.7 apply.

      In the FQDN case, the Name field is an "IDN-unaware domain name
      slot", as defined in [9].  That is, it can contain only ASCII
      characters.  An implementation MAY support internationalized
      domain names (IDNs) using the ToASCII operation; see [9] for more
      information.

      All systems MUST support the DER Encoded X.501 Name.
      Implementations MAY support the FQDN name type.

   Padding

      A variable-length field making the option length a multiple of 8,
      beginning after the previous field ends and continuing to the end
      of the option, as specified by the Length field.











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6.4.4.  Certificate Option

   The format of the certificate option is described in the following:

    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     |  Cert Type    |    Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Certificate ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 ...       Padding                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      16

   Length

      The length of the option (including the Type, Length, Cert Type,
      Pad Length, and Certificate fields), in units of 8 octets.

   Cert Type

      The type of the certificate included in the Certificate field.
      This specification defines only one legal value for this field:

            1        X.509v3 Certificate, as specified below

   Reserved

      An 8-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender and MUST be ignored by the
      receiver.

   Certificate

      When the Cert Type field is set to 1, the Certificate field
      contains an X.509v3 certificate [7], as described in Section
      6.3.1.

   Padding

      A variable length field making the option length a multiple of 8,
      beginning after the ASN.1 encoding of the previous field [7, 15]
      ends and continuing to the end of the option, as specified by the
      Length field.



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6.4.5.  Processing Rules for Routers

   A router MUST silently discard any received Certification Path
   Solicitation messages that do not conform to the message format
   defined in Section 6.4.1.  The contents of the Reserved field and of
   any unrecognized options MUST be ignored.  Future, backward-
   compatible changes to the protocol may specify the contents of the
   Reserved field or add new options; backward-incompatible changes may
   use different Code values.  The contents of any defined options that
   are not specified to be used with Router Solicitation messages MUST
   be ignored, and the packet processed in the normal manner.  The only
   defined option that may appear is the Trust Anchor option.  A
   solicitation that passes the validity checks is called a "valid
   solicitation".

   Routers SHOULD send advertisements in response to valid solicitations
   received on an advertising interface.  If the source address in the
   solicitation was the unspecified address, the router MUST send the
   response to the link-scoped All-Nodes multicast address.  If the
   source address was a unicast address, the router MUST send the
   response to the Solicited-Node multicast address corresponding to the
   source address, except when under load, as specified below.  Routers
   SHOULD NOT send Certification Path Advertisements more than
   MAX_CPA_RATE times within a second.  When there are more
   solicitations, the router SHOULD send the response to the All-Nodes
   multicast address regardless of the source address that appeared in
   the solicitation.

   In an advertisement, the router SHOULD include suitable Certificate
   options so that a certification path can be established to the
   solicited trust anchor (or a part of it, if the Component field in
   the solicitation is not equal to 65,535).  Note also that a single
   advertisement is broken into separately sent components and ordered
   in a particular way (see Section 6.4.2) when there is more than one
   certificate in the path.

   The anchor is identified by the Trust Anchor option.  If the Trust
   Anchor option is represented as a DER Encoded X.501 Name, then the
   Name must be equal to the Subject field in the anchor's certificate.
   If the Trust Anchor option is represented as an FQDN, the FQDN must
   be equal to an FQDN in the subjectAltName field of the anchor's
   certificate.  The router SHOULD include the Trust Anchor option(s) in
   the advertisement for which the certification path was found.

   If the router is unable to find a path to the requested anchor, it
   SHOULD send an advertisement without any certificates.  In this case,
   the router SHOULD include the Trust Anchor options that were
   solicited.



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6.4.6.  Processing Rules for Hosts

   A host MUST silently discard any received Certification Path
   Advertisement messages that do not conform to the message format
   defined in Section 6.4.2.  The contents of the Reserved field, and of
   any unrecognized options, MUST be ignored.  Future, backward-
   compatible changes to the protocol MAY specify the contents of the
   Reserved field or add new options; backward-incompatible changes MUST
   use different Code values.  The contents of any defined options not
   specified to be used with Certification Path Advertisement messages
   MUST be ignored, and the packet processed in the normal manner.  The
   only defined options that may appear are the Certificate and Trust
   Anchor options.  An advertisement that passes the validity checks is
   called a "valid advertisement".

   Hosts SHOULD store certification paths retrieved in Certification
   Path Discovery messages if they start from an anchor trusted by the
   host.  The certification paths MUST be verified, as defined in
   Section 6.3, before storing them.  Routers send the certificates one
   by one, starting from the trust anchor end of the path.

   Note: Except to allow for message loss and reordering for temporary
   purposes, hosts might not store certificates received in a
   Certification Path Advertisement unless they contain a certificate
   that can be immediately verified either to the trust anchor or to a
   certificate that has been verified earlier.  This measure is intended
   to prevent Denial-of-Service attacks, whereby an attacker floods a
   host with certificates that the host cannot validate and overwhelms
   memory for certificate storage.

   Note that caching this information, and the implied verification
   results between network attachments for use over multiple attachments
   to the network, can help improve performance.  But periodic
   certificate revocation checks are still needed, even with cached
   results, to make sure that the certificates are still valid.

   The host SHOULD retrieve a certification path when a Router
   Advertisement has been received with a public key that is not
   available from a certificate in the hosts' cache, or when there is no
   certification path to one of the host's trust anchors.  In these
   situations, the host MAY send a Certification Path Solicitation
   message to retrieve the path.  If there is no response within
   CPS_RETRY seconds, the message should be retried.  The wait interval
   for each subsequent retransmission MUST exponentially increase,
   doubling each time.  If there is no response after CPS_RETRY_MAX
   seconds, the host abandons the certification path retrieval process.
   If the host receives only a part of a certification path within
   CPS_RETRY_FRAGMENTS seconds of receiving the first part, it MAY in



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   addition transmit a Certification Path Solicitation message with the
   Component field set to a value not equal to 65,535.  This message can
   be retransmitted by using the same process as for the initial
   message.  If there are multiple missing certificates, additional CPS
   messages can be sent after getting a response to first one.  However,
   the complete retrieval process may last at most CPS_RETRY_MAX
   seconds.

   Certification Path Solicitations SHOULD NOT be sent if the host has a
   currently valid certification path from a reachable router to a trust
   anchor.

   When soliciting certificates for a router, a host MUST send
   Certification Path Solicitations either to the All-Routers multicast
   address, if it has not selected a default router yet, or to the
   default router's IP address, if a default router has already been
   selected.

   If two hosts want to establish trust with the CPS and CPA messages,
   the CPS message SHOULD be sent to the Solicited-Node multicast
   address of the receiver.  The advertisements SHOULD be sent as
   specified above for routers.  However, the exact details are outside
   the scope of this specification.

   When processing possible advertisements sent as responses to a
   solicitation, the host MAY prefer to process those advertisements
   with the same Identifier field value as that of the solicitation
   first.  This makes Denial-of-Service attacks against the mechanism
   harder (see Section 9.3).

6.5.  Configuration

   End hosts are configured with a set of trust anchors in order to
   protect Router Discovery.  A trust anchor configuration consists of
   the following items:

   o  A public key signature algorithm and associated public key, which
      may optionally include parameters.

   o  A name as described in Section 6.4.3.

   o  An optional public key identifier.

   o  An optional list of address ranges for which the trust anchor is
      authorized.

   If the host has been configured to use SEND, it SHOULD possess the
   above information for at least one trust anchor.



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   Routers are configured with a collection of certification paths and a
   collection of certificates containing certified keys, down to the key
   and certificate for the router itself.  Certified keys are required
   for routers so that a certification path can be established between
   the router's certificate and the public key of a trust anchor.

   If the router has been configured to use SEND, it should be
   configured with its own key pair and certificate, and with at least
   one certification path.

7.  Addressing

7.1.  CGAs

   By default, a SEND-enabled node SHOULD use only CGAs for its own
   addresses.  Other types of addresses MAY be used in testing, in
   diagnostics, or for other purposes.  However, this document does not
   describe how to choose between different types of addresses for
   different communications.  A dynamic selection can be provided by an
   API, such as the one defined in [21].

7.2.  Redirect Addresses

   If the Target Address and Destination Address fields in the ICMP
   Redirect message are equal, then this message is used to inform hosts
   that a destination is, in fact, a neighbor.  In this case, the
   receiver MUST verify that the given address falls within the range
   defined by the router's certificate.  Redirect messages failing this
   check MUST be treated as unsecured, as described in Section 7.3.

   Note that base NDP rules prevent a host from accepting a Redirect
   message from a router that the host is not using to reach the
   destination mentioned in the redirect.  This prevents an attacker
   from tricking a node into redirecting traffic when the attacker is
   not the default router.

7.3.  Advertised Subnet Prefixes

   The router's certificate defines the address range(s) that it is
   allowed to advertise securely.  A router MAY, however, advertise a
   combination of certified and uncertified subnet prefixes.
   Uncertified subnet prefixes are treated as unsecured (i.e., processed
   in the same way as unsecured router advertisements sent by non-SEND
   routers).  The processing of unsecured messages is specified in
   Section 8.  Note that SEND nodes that do not attempt to interoperate
   with non-SEND nodes MAY simply discard the unsecured information.





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   Certified subnet prefixes fall into the following two categories:

   Constrained

      If the network operator wants to constrain which routers are
      allowed to route particular subnet prefixes, routers should be
      configured with certificates having subnet prefixes listed in the
      prefix extension.  These routers SHOULD advertise the subnet
      prefixes that they are certified to route, or a subset thereof.

   Unconstrained

      Network operators that do not want to constrain routers this way
      should configure routers with certificates containing either the
      null prefix or no prefix extension at all.

   Upon processing a Prefix Information option within a Router
   Advertisement, nodes SHOULD verify that the prefix specified in this
   option falls within the range defined by the certificate, if the
   certificate contains a prefix extension.  Options failing this check
   are treated as containing uncertified subnet prefixes.

   Nodes SHOULD use one of the certified subnet prefixes for stateless
   autoconfiguration.  If none of the advertised subnet prefixes match,
   the host SHOULD use a different advertising router as its default
   router, if one is available.  If the node is performing stateful
   autoconfiguration, it SHOULD check the address provided by the DHCP
   server against the certified subnet prefixes and SHOULD NOT use the
   address if the prefix is not certified.

7.4.  Limitations

   This specification does not address the protection of NDP packets for
   nodes configured with a static address (e.g., PREFIX::1).  Future
   certification path-based authorization specifications are needed for
   these nodes.  This specification also does not apply to addresses
   generated by the IPv6 stateless address autoconfiguration from a
   fixed interface identifiers (such as EUI-64).

   It is outside the scope of this specification to describe the use of
   trust anchor authorization between nodes with dynamically changing
   addresses.  These addresses may be the result of stateful or
   stateless address autoconfiguration, or may have resulted from the
   use of RFC 3041 [17] addresses.  If the CGA method is not used, nodes
   are required to exchange certification paths that terminate in a
   certificate authorizing a node to use an IP address having a
   particular interface identifier.  This specification does not specify
   the format of these certificates, as there are currently only a few



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   cases where they are provided by the link layer, and it is up to the
   link layer to provide certification for the interface identifier.
   This may be the subject of a future specification.  It is also
   outside the scope of this specification to describe how stateful
   address autoconfiguration works with the CGA method.

   The Target Address in Neighbor Advertisement is required to be equal
   to the source address of the packet, except in proxy Neighbor
   Discovery, which is not supported by this specification.

8.  Transition Issues

   During the transition to secured links, or as a policy consideration,
   network operators may want to run a particular link with a mixture of
   nodes accepting secured and unsecured messages.  Nodes that support
   SEND SHOULD support the use of secured and unsecured NDP messages at
   the same time.

   In a mixed environment, SEND nodes receive both secured and unsecured
   messages but give priority to secured ones.  Here, the "secured"
   messages are those that contain a valid signature option, as
   specified above, and "unsecured" messages are those that contain no
   signature option.

   A SEND node SHOULD have a configuration option that causes it to
   ignore all unsecured Neighbor Solicitation and Advertisement, Router
   Solicitation and Advertisement, and Redirect messages.  This can be
   used to enforce SEND-only networks.  The default for this
   configuration option SHOULD be that both secured and unsecured
   messages are allowed.

   A SEND node MAY also have a configuration option whereby it disables
   the use of SEND completely, even for the messages it sends itself.
   This configuration option SHOULD be switched off by default; that is,
   SEND is used.  Plain (non-SEND) NDP nodes will obviously send only
   unsecured messages.  Per RFC 2461 [4], such nodes will ignore the
   unknown options and will treat secured messages in the same way that
   they treat unsecured ones.  Secured and unsecured nodes share the
   same network resources, such as subnet prefixes and address spaces.

   SEND nodes configured to use SEND at least in their own messages
   behave in a mixed environment as explained below.

   SEND adheres to the rules defined for the base NDP protocol, with the
   following exceptions:

   o  All solicitations sent by a SEND node MUST be secured.




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   o  Unsolicited advertisements sent by a SEND node MUST be secured.

   o  A SEND node MUST send a secured advertisement in response to a
      secured solicitation.  Advertisements sent in response to an
      unsecured solicitation MUST be secured as well, but MUST NOT
      contain the Nonce option.

   o  A SEND node that uses the CGA authorization method to protect
      Neighbor Solicitations SHOULD perform Duplicate Address Detection
      as follows.  If Duplicate Address Detection indicates that the
      tentative address is already in use, the node generates a new
      tentative CGA.  If after three consecutive attempts no non-unique
      address is generated, it logs a system error and gives up
      attempting to generate an address for that interface.

      When performing Duplicate Address Detection for the first
      tentative address, the node accepts both secured and unsecured
      Neighbor Advertisements and Solicitations received in response to
      the Neighbor Solicitations.  When performing Duplicate Address
      Detection for the second or third tentative address, it ignores
      unsecured Neighbor Advertisements and Solicitations.  (The
      security implications of this are discussed in Section 9.2.3 and
      in [11].)

   o  The node MAY have a configuration option whereby it ignores
      unsecured advertisements, even when performing Duplicate Address
      Detection for the first tentative address.  This configuration
      option SHOULD be disabled by default.  This is a recovery
      mechanism for cases in which attacks against the first address
      become common.

   o  The Neighbor Cache, Prefix List, and Default Router list entries
      MUST have a secured/unsecured flag that indicates whether the
      message that caused the creation or last update of the entry was
      secured or unsecured.  Received unsecured messages MUST NOT cause
      changes to existing secured entries in the Neighbor Cache, Prefix
      List, or Default Router List.  Received secured messages MUST
      cause an update of the matching entries, which MUST be flagged as
      secured.

   o  Neighbor Solicitations for the purpose of Neighbor Unreachability
      Detection (NUD) MUST be sent to that neighbor's solicited-nodes
      multicast address if the entry is not secured with SEND.

      Upper layer confirmations on unsecured neighbor cache entries
      SHOULD NOT update neighbor cache state from STALE to REACHABLE on
      a SEND node if the neighbor cache entry has never previously been
      REACHABLE.  This ensures that if an entry spoofing a valid SEND



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      host is created by a non-SEND attacker without being solicited,
      NUD will be done with the entry for data transmission within five
      seconds of use.

      As a result, in mixed mode, attackers can take over a Neighbor
      Cache entry of a SEND node for a longer time only if (a) the SEND
      node was not communicating with the victim node, so that there is
      no secure entry for it, and (b) the SEND node is not currently on
      the link (or is unable to respond).

   o  The conceptual sending algorithm is modified so that an unsecured
      router is selected only if there is no reachable SEND router for
      the prefix.  That is, the algorithm for selecting a default router
      favors reachable SEND routers over reachable non-SEND ones.

   o  A node MAY adopt a router sending unsecured messages, or a router
      for which secured messages have been received but for which full
      security checks have not yet been completed, while security
      checking is underway.  Security checks in this case include
      certification path solicitation, certificate verification, CRL
      checks, and RA signature checks.  A node MAY also adopt a router
      sending unsecured messages if a router known to be secured becomes
      unreachable, but because the unreachability may be the result of
      an attack it SHOULD attempt to find a router known to be secured
      as soon as possible.  Note that although this can speed up
      attachment to a new network, accepting a router that is sending
      unsecured messages or for which security checks are not complete
      opens the node to possible attacks.  Nodes that choose to accept
      such routers do so at their own risk.  The node SHOULD, in any
      case, prefer a router known to be secure as soon as one is made
      available with completed security checks.

9.  Security Considerations

9.1.  Threats to the Local Link Not Covered by SEND

   SEND does not provide confidentiality for NDP communications.

   SEND does not compensate for an unsecured link layer.  For instance,
   there is no assurance that payload packets actually come from the
   same peer against which the NDP was run.

   There may not be cryptographic binding in SEND between the link layer
   frame address and the IPv6 address.  An unsecured link layer could
   allow nodes to spoof the link layer address of other nodes.  An
   attacker could disrupt IP service by sending out a Neighbor
   Advertisement on an unsecured link layer, with the link layer source
   address on the frame set as the source address of a victim, a valid



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   CGA address and a valid signature corresponding to itself, and a
   Target Link-layer Address extension corresponding to the victim.  The
   attacker could then make a traffic stream bombard the victim in a DoS
   attack.  This cannot be prevented just by securing the link layer.

   Even on a secured link layer, SEND does not require that the
   addresses on the link layer and Neighbor Advertisements correspond.
   However, performing these checks is RECOMMENDED if the link layer
   technology permits.

   Prior to participating in Neighbor Discovery and Duplicate Address
   Detection, nodes must subscribe to the link-scoped All-Nodes
   Multicast Group and the Solicited-Node Multicast Group for the
   address that they are claiming as their addresses; RFC 2461 [4].
   Subscribing to a multicast group requires that the nodes use MLD
   [16].  MLD contains no provision for security.  An attacker could
   send an MLD Done message to unsubscribe a victim from the Solicited-
   Node Multicast address.  However, the victim should be able to detect
   this attack because the router sends a Multicast-Address-Specific
   Query to determine whether any listeners are still on the address, at
   which point the victim can respond to avoid being dropped from the
   group.  This technique will work if the router on the link has not
   been compromised.  Other attacks using MLD are possible, but they
   primarily lead to extraneous (but not necessarily overwhelming)
   traffic.

9.2.  How SEND Counters Threats to NDP

   The SEND protocol is designed to counter the threats to NDP, as
   outlined in [22].  The following subsections contain a regression of
   the SEND protocol against the threats, to illustrate which aspects of
   the protocol counter each threat.

9.2.1.  Neighbor Solicitation/Advertisement Spoofing

   This threat is defined in Section 4.1.1 of [22].  The threat is that
   a spoofed message may cause a false entry in a node's Neighbor Cache.
   There are two cases:

   1. Entries made as a side effect of a Neighbor Solicitation or Router
      Solicitation.  A router receiving a Router Solicitation with a
      Target Link-Layer Address extension and the IPv6 source address
      unequal to the unspecified address inserts an entry for the IPv6
      address into its Neighbor Cache.  Also, a node performing
      Duplicate Address Detection (DAD) that receives a Neighbor
      Solicitation for the same address regards the situation as a
      collision and ceases to solicit for the address.




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      In either case, SEND counters these threats by requiring that the
      RSA Signature and CGA options be present in these solicitations.

      SEND nodes can send Router Solicitation messages with a CGA source
      address and a CGA option, which the router can verify, so that the
      Neighbor Cache binding is correct.  If a SEND node must send a
      Router Solicitation with the unspecified address, the router will
      not update its Neighbor Cache, as per base NDP.

   2. Entries made as a result of a Neighbor Advertisement message.
      SEND counters this threat by requiring that the RSA Signature and
      CGA options be present in these advertisements.

   Also see Section 9.2.5, below, for discussion about replay protection
   and timestamps.

9.2.2.  Neighbor Unreachability Detection Failure

   This attack is described in Section 4.1.2 of [22].  SEND counters it
   by requiring that a node responding to Neighbor Solicitations sent as
   NUD probes include an RSA Signature option and proof of authorization
   to use the interface identifier in the address being probed.  If
   these prerequisites are not met, the node performing NUD discards the
   responses.

9.2.3.  Duplicate Address Detection DoS Attack

   This attack is described in Section 4.1.3 of [22].  SEND counters
   this attack by requiring that the Neighbor Advertisements sent as
   responses to DAD include an RSA Signature option and proof of
   authorization to use the interface identifier in the address being
   tested.  If these prerequisites are not met, the node performing DAD
   discards the responses.

   When a SEND node performs DAD, it may listen for address collisions
   from non-SEND nodes for the first address it generates, but not for
   new attempts.  This protects the SEND node from DAD DoS attacks by
   non-SEND nodes or attackers simulating non-SEND nodes, at the cost of
   a potential address collision between a SEND node and a non-SEND
   node.  The probability and effects of such an address collision are
   discussed in [11].

9.2.4.  Router Solicitation and Advertisement Attacks

   These attacks are described in Sections 4.2.1, 4.2.4, 4.2.5, 4.2.6,
   and 4.2.7 of [22].  SEND counters them by requiring that Router
   Advertisements contain an RSA Signature option, and that the
   signature is calculated by using the public key of a node that can



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   prove its authorization to route the subnet prefixes contained in any
   Prefix Information Options.  The router proves its authorization by
   showing a certificate containing the specific prefix or an indication
   that the router is allowed to route any prefix.  A Router
   Advertisement without these protections is discarded.

   SEND does not protect against brute force attacks on the router, such
   as DoS attacks, or against compromise of the router, as described in
   Sections 4.4.2 and 4.4.3 of [22].

9.2.5.  Replay Attacks

   This attack is described in Section 4.3.1 of [22].  SEND protects
   against attacks in Router Solicitation/Router Advertisement and
   Neighbor Solicitation/Neighbor Advertisement transactions by
   including a Nonce option in the solicitation and requiring that the
   advertisement include a matching option.  Together with the
   signatures, this forms a challenge-response protocol.

   SEND protects against attacks from unsolicited messages such as
   Neighbor Advertisements, Router Advertisements, and Redirects by
   including a Timestamp option.  The following security issues are
   relevant only for unsolicited messages:

   o  A window of vulnerability for replay attacks exists until the
      timestamp expires.

      However, such vulnerabilities are only useful for attackers if the
      advertised parameters change during the window.  Although some
      parameters (such as the remaining lifetime of a prefix) change
      often, radical changes typically happen only in the context of
      some special case, such as switching to a new link layer address
      due to a broken interface adapter.

      SEND nodes are also protected against replay attacks as long as
      they cache the state created by the message containing the
      timestamp.  The cached state allows the node to protect itself
      against replayed messages.  However, once the node flushes the
      state for whatever reason, an attacker can re-create the state by
      replaying an old message while the timestamp is still valid.
      Because most SEND nodes are likely to use fairly coarse-grained
      timestamps, as explained in Section 5.3.1, this may affect some
      nodes.

   o  Attacks against time synchronization protocols such as NTP [23]
      may cause SEND nodes to have an incorrect timestamp value.  This
      can be used to launch replay attacks, even outside the normal
      window of vulnerability.  To protect against these attacks, it is



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      recommended that SEND nodes keep independently maintained clocks
      or apply suitable security measures for the time synchronization
      protocols.

9.2.6.  Neighbor Discovery DoS Attack

   This attack is described in Section 4.3.2 of [22].  In it, the
   attacker bombards the router with packets for fictitious addresses on
   the link, causing the router to busy itself by performing Neighbor
   Solicitations for addresses that do not exist.  SEND does not address
   this threat because it can be addressed by techniques such as rate
   limiting Neighbor Solicitations, restricting the amount of state
   reserved for unresolved solicitations, and clever cache management.
   These are all techniques involved in implementing Neighbor Discovery
   on the router.

9.3.  Attacks against SEND Itself

   The CGAs have a 59-bit hash value.  The security of the CGA mechanism
   has been discussed in [11].

   Some Denial-of-Service attacks remain against NDP and SEND itself.
   For instance, an attacker may try to produce a very high number of
   packets that a victim host or router has to verify by using
   asymmetric methods.  Although safeguards are required to prevent an
   excessive use of resources, this can still render SEND non-
   operational.

   When CGA protection is used, SEND deals with the DoS attacks by using
   the verification process described in Section 5.2.2.  In this
   process, a simple hash verification of the CGA property of the
   address is performed before the more expensive signature
   verification.  However, even if the CGA verification succeeds, no
   claims about the validity of the message can be made until the
   signature has been checked.

   When trust anchors and certificates are used for address validation
   in SEND, the defenses are not quite as effective.  Implementations
   SHOULD track the resources devoted to the processing of packets
   received with the RSA Signature option and start selectively
   discarding packets if too many resources are spent.  Implementations
   MAY also first discard packets that are not protected with CGA.

   The Authorization Delegation Discovery process may also be vulnerable
   to Denial-of-Service attacks.  An attack may target a router by
   requesting that a large number of certification paths be discovered
   for different trust anchors.  Routers SHOULD defend against such
   attacks by caching discovered information (including negative



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   responses) and by limiting the number of different discovery
   processes in which they engage.

   Attackers may also target hosts by sending a large number of
   unnecessary certification paths, forcing hosts to spend useless
   memory and verification resources on them.  Hosts can defend against
   such attacks by limiting the amount of resources devoted to the
   certification paths and their verification.  Hosts SHOULD also
   prioritize advertisements sent as a response to solicitations the
   hosts have sent about unsolicited advertisements.

10.  Protocol Values

10.1.  Constants

   Host constants:

         CPS_RETRY                      1 second
         CPS_RETRY_FRAGMENTS            2 seconds
         CPS_RETRY_MAX                 15 seconds

   Router constants:

         MAX_CPA_RATE                  10 times per second

10.2.  Variables

         TIMESTAMP_DELTA               300 seconds (5 minutes)
         TIMESTAMP_FUZZ                  1 second
         TIMESTAMP_DRIFT                 1 % (0.01)

11.  IANA Considerations

   This document defines two new ICMP message types, used in
   Authorization Delegation Discovery.  These messages must be assigned
   ICMPv6 type numbers from the informational message range:

   o  The Certification Path Solicitation message (148), described in
      Section 6.4.1.

   o  The Certification Path Advertisement message (149), described in
      Section 6.4.2.

   This document defines six new Neighbor Discovery Protocol [4]
   options, which must be assigned Option Type values within the option
   numbering space for Neighbor Discovery Protocol messages:

      o  The CGA option (11), described in Section 5.1.



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      o  The RSA Signature option (12), described in Section 5.2.

      o  The Timestamp option (13), described in Section 5.3.1.

      o  The Nonce option (14), described in Section 5.3.2.

      o  The Trust Anchor option (15), described in Section 6.4.3.

      o  The Certificate option (16), described in Section 6.4.4.

   This document defines a new 128-bit value under the CGA Message Type
   [11] namespace, 0x086F CA5E 10B2 00C9 9C8C E001 6427 7C08.

   This document defines a new name space for the Name Type field in the
   Trust Anchor option.  Future values of this field can be allocated by
   using Standards Action [3].  The current values for this field are

      1  DER Encoded X.501 Name

      2  FQDN

   Another new name space is allocated for the Cert Type field in the
   Certificate option.  Future values of this field can be allocated by
   using Standards Action [3].  The current values for this field are

      1  X.509v3 Certificate

12.  References

12.1.  Normative References

   [1]   Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.

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

   [3]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October
         1998.

   [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.





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   [6]   Conta, A. and S. Deering, "Internet Control Message Protocol
         (ICMPv6) for the Internet Protocol Version 6 (IPv6)
         Specification", RFC 2463, December 1998.

   [7]  Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [8]  Farrell, S. and R. Housley, "An Internet Attribute Certificate
         Profile for Authorization", RFC 3281, April 2002.

   [9]  Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
         Domain Names in Applications (IDNA)", RFC 3490, March 2003.

   [10]  Lynn, C., Kent, S. and K. Seo, "X.509 Extensions for IP
         Addresses and AS Identifiers", RFC 3779, June 2004.

   [11]  Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
         3972, March 2005.

   [12]  International Telecommunications Union, "Information Technology
         - ASN.1 encoding rules: Specification of Basic Encoding Rules
         (BER), Canonical Encoding Rules (CER) and Distinguished
         Encoding Rules (DER)", ITU-T Recommendation X.690, July 2002.

   [13]  RSA Laboratories, "RSA Encryption Standard, Version 2.1", PKCS
         1, November 2002.

   [14]  National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS PUB 180-1, April 1995,
         <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.

12.2.  Informative References

   [15]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [16]  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
         Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [17]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [18]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.





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   [19]  Arkko, J., "Effects of ICMPv6 on IKE and IPsec Policies", Work
         in Progress, March 2003.

   [20]  Arkko, J., "Manual SA Configuration for IPv6 Link Local
         Messages", Work in Progress, June 2002.

   [21]  Nordmark, E., Chakrabarti, S. and J. Laganier, "IPv6 Socket API
         for Address Selection", Work in Progress, October 2003.

   [22]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
         Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

   [23]  Bishop, M., "A Security Analysis of the NTP Protocol", Sixth
         Annual Computer Security Conference Proceedings, December 1990.





































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Appendix A.  Contributors and Acknowledgments

   Tuomas Aura contributed the transition mechanism specification in
   Section 8.  Jonathan Trostle contributed the certification path
   example in Section 6.3.1.  Bill Sommerfeld was involved with much of
   the early design work.

   The authors would also like to thank Tuomas Aura, Bill Sommerfeld,
   Erik Nordmark, Gabriel Montenegro, Pasi Eronen, Greg Daley, Jon Wood,
   Julien Laganier, Francis Dupont, Pekka Savola, Wenxiao He, Valtteri
   Niemi, Mike Roe, Russ Housley, Thomas Narten, and Steven Bellovin for
   interesting discussions in this problem space and for feedback
   regarding the SEND protocol.

Appendix B.  Cache Management

   In this section, we outline a cache management algorithm that allows
   a node to remain partially functional even under a cache-filling DoS
   attack.  This appendix is informational, and real implementations
   SHOULD use different algorithms in order to avoid the dangers of a
   mono-cultural code.

   There are at least two distinct cache-related attack scenarios:

   1. There are a number of nodes on a link, and someone launches a
      cache filling attack.  The goal here is to make sure that the
      nodes can continue to communicate even if the attack is going on.

   2. There is already a cache-filling attack going on, and a new node
      arrives to the link.  The goal here is to make it possible for the
      new node to become attached to the network, in spite of the
      attack.

   As the intent is to limit the damage to existing, valid cache
   entries, it is clearly better to be very selective in throwing out
   entries.  Reducing the timestamp Delta value is very discriminatory
   against nodes with a large clock difference, as an attacker can
   reduce its clock difference arbitrarily.  Throwing out old entries
   just because their clock difference is large therefore seems like a
   bad approach.

   It is reasonable to have separate cache spaces for new and old
   entries, where when under attack, the newly cached entries would be
   more readily dropped.  One could track traffic and only allow
   reasonable new entries that receive genuine traffic to be converted
   into old cache entries.  Although such a scheme can make attacks
   harder, it will not fully prevent them.  For example, an attacker
   could send a little traffic (i.e., a ping or TCP syn) after each NS



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   to trick the victim into promoting its cache entry to the old cache.
   To counter this, the node can be more intelligent in keeping its
   cache entries than it would be just by having a black/white old/new
   boundary.

   Distinction of the Sec parameter from the CGA Parameters when forcing
   cache entries out -- by keeping entries with larger Sec parameters
   preferentially -- also appears to be a possible approach, as CGAs
   with higher Sec parameters are harder to spoof.

Appendix C.  Message Size When Carrying Certificates

   In one example scenario using SEND, an Authorization Delegation
   Discovery test run was made with a certification path length of 4.
   Three certificates are sent by using Certification Path Advertisement
   messages, as the trust anchor's certificate is already known by both
   parties.  With a key length of 1024 bits, the certificate lengths in
   the test run ranged from 864 to 888 bytes; the variation is due to
   the differences in the certificate issuer names and address prefix
   extensions.  The different certificates had between 1 and 4 address
   prefix extensions.

   The three Certification Path Advertisement messages ranged from 1050
   to 1,066 bytes on an Ethernet link layer.  The certificate itself
   accounts for the bulk of the packet.  The rest is the trust anchor
   option, ICMP header, IPv6 header, and link layer header.

























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Authors' Addresses

   Jari Arkko
   Ericsson
   Jorvas  02420
   Finland

   EMail: jari.arkko@ericsson.com


   James Kempf
   DoCoMo Communications Labs USA
   181 Metro Drive
   San Jose, CA  94043
   USA

   EMail: kempf@docomolabs-usa.com


   Brian Zill
   Microsoft Research
   One Microsoft Way
   Redmond, WA 98052
   USA

   EMail: bzill@microsoft.com


   Pekka Nikander
   Ericsson
   Jorvas  02420
   Finland

   EMail: Pekka.Nikander@nomadiclab.com

















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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM 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.

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   The IETF takes no position regarding the validity or scope of any
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

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







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