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rfc:rfc3971

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.

Arkko, et al. Standards Track [Page 1] RFC 3971 SEcure Neighbor Discovery March 2005

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

Arkko, et al. Standards Track [Page 2] RFC 3971 SEcure Neighbor Discovery March 2005

           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

Arkko, et al. Standards Track [Page 3] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 4] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 5] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 6] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 7] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 8] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 9] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 10] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 11] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 12] RFC 3971 SEcure Neighbor Discovery March 2005

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].

Arkko, et al. Standards Track [Page 13] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 14] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 15] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 16] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 17] RFC 3971 SEcure Neighbor Discovery March 2005

       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.

Arkko, et al. Standards Track [Page 18] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 19] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 20] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 21] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 22] RFC 3971 SEcure Neighbor Discovery March 2005

 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:
  1. 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.

Arkko, et al. Standards Track [Page 23] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 24] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 25] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 26] RFC 3971 SEcure Neighbor Discovery March 2005

 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 ...

Arkko, et al. Standards Track [Page 27] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 28] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 29] RFC 3971 SEcure Neighbor Discovery March 2005

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
    Checksum
       The ICMP checksum [6].

Arkko, et al. Standards Track [Page 30] RFC 3971 SEcure Neighbor Discovery March 2005

    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.

Arkko, et al. Standards Track [Page 31] RFC 3971 SEcure Neighbor Discovery March 2005

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
    Checksum
       The ICMP checksum [6].

Arkko, et al. Standards Track [Page 32] RFC 3971 SEcure Neighbor Discovery March 2005

    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.

Arkko, et al. Standards Track [Page 33] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 34] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 35] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 36] RFC 3971 SEcure Neighbor Discovery March 2005

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.

Arkko, et al. Standards Track [Page 37] RFC 3971 SEcure Neighbor Discovery March 2005

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

Arkko, et al. Standards Track [Page 38] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 39] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 40] RFC 3971 SEcure Neighbor Discovery March 2005

 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

Arkko, et al. Standards Track [Page 41] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 42] RFC 3971 SEcure Neighbor Discovery March 2005

 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

Arkko, et al. Standards Track [Page 43] RFC 3971 SEcure Neighbor Discovery March 2005

    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

Arkko, et al. Standards Track [Page 44] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 45] RFC 3971 SEcure Neighbor Discovery March 2005

    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

Arkko, et al. Standards Track [Page 46] RFC 3971 SEcure Neighbor Discovery March 2005

 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

Arkko, et al. Standards Track [Page 47] RFC 3971 SEcure Neighbor Discovery March 2005

    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

Arkko, et al. Standards Track [Page 48] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 49] RFC 3971 SEcure Neighbor Discovery March 2005

    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.

Arkko, et al. Standards Track [Page 50] RFC 3971 SEcure Neighbor Discovery March 2005

 [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.

Arkko, et al. Standards Track [Page 51] RFC 3971 SEcure Neighbor Discovery March 2005

 [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.

Arkko, et al. Standards Track [Page 52] RFC 3971 SEcure Neighbor Discovery March 2005

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

Arkko, et al. Standards Track [Page 53] RFC 3971 SEcure Neighbor Discovery March 2005

 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.

Arkko, et al. Standards Track [Page 54] RFC 3971 SEcure Neighbor Discovery March 2005

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

Arkko, et al. Standards Track [Page 55] RFC 3971 SEcure Neighbor Discovery March 2005

Full Copyright Statement

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 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.
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Acknowledgement

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Arkko, et al. Standards Track [Page 56]

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