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Versions: (draft-arkko-send-ndopt) 00 01 03 04 05 06 RFC 3971

Secure Neighbor Discovery Working                               J. Arkko
Group                                                           Ericsson
Internet-Draft                                                  J. Kempf
Expires: April 16, 2004                   DoCoMo Communications Labs USA
                                                           B. Sommerfeld
                                                        Sun Microsystems
                                                                 B. Zill
                                                               Microsoft
                                                             P. Nikander
                                                                Ericsson
                                                        October 17, 2003


                    SEcure Neighbor Discovery (SEND)
                        draft-ietf-send-ndopt-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on April 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   IPv6 nodes use the Neighbor Discovery Protocol (NDP) to discover
   other nodes on the link, to determine each the link-layer addresses
   of the nodes on the link, 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



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   specifies security mechanisms for NDP. Unlike to the original NDP
   specifications, these mechanisms do not make use of IPsec.

Table of Contents

   1.     Introduction . . . . . . . . . . . . . . . . . . . . . . .   4
   2.     Terms  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.     Neighbor and Router Discovery Overview . . . . . . . . . .   7
   4.     Secure Neighbor Discovery Overview . . . . . . . . . . . .  11
   5.     Neighbor Discovery Options . . . . . . . . . . . . . . . .  12
   5.1    Ordering of the new options  . . . . . . . . . . . . . . .  12
   5.2    CGA Option . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.2.1  Processing Rules for Senders . . . . . . . . . . . . . . .  14
   5.2.2  Processing Rules for Receivers . . . . . . . . . . . . . .  15
   5.2.3  Configuration  . . . . . . . . . . . . . . . . . . . . . .  15
   5.3    Signature Option . . . . . . . . . . . . . . . . . . . . .  15
   5.3.1  Processing Rules for Senders . . . . . . . . . . . . . . .  18
   5.3.2  Processing Rules for Receivers . . . . . . . . . . . . . .  18
   5.3.3  Configuration  . . . . . . . . . . . . . . . . . . . . . .  19
   5.4    Timestamp and Nonce options  . . . . . . . . . . . . . . .  20
   5.4.1  Timestamp Option . . . . . . . . . . . . . . . . . . . . .  20
   5.4.2  Nonce Option . . . . . . . . . . . . . . . . . . . . . . .  21
   5.4.3  Processing rules for senders . . . . . . . . . . . . . . .  22
   5.4.4  Processing rules for receivers . . . . . . . . . . . . . .  22
   5.5    Proxy Neighbor Discovery . . . . . . . . . . . . . . . . .  24
   6.     Authorization Delegation Discovery . . . . . . . . . . . .  25
   6.1    Delegation Chain Solicitation Message Format . . . . . . .  25
   6.2    Delegation Chain Advertisement Message Format  . . . . . .  27
   6.3    Trust Anchor Option  . . . . . . . . . . . . . . . . . . .  29
   6.4    Certificate Option . . . . . . . . . . . . . . . . . . . .  30
   6.5    Router Authorization Certificate Format  . . . . . . . . .  31
   6.5.1  Router Authorization Certificate Profile . . . . . . . . .  31
   6.6    Processing Rules for Routers . . . . . . . . . . . . . . .  32
   6.7    Processing Rules for Hosts . . . . . . . . . . . . . . . .  34
   7.     Securing Neighbor Discovery with SEND  . . . . . . . . . .  37
   7.1    Neighbor Solicitation Messages . . . . . . . . . . . . . .  37
   7.1.1  Sending Secure Neighbor Solicitations  . . . . . . . . . .  37
   7.1.2  Receiving Secure Neighbor Solicitations  . . . . . . . . .  37
   7.2    Neighbor Advertisement Messages  . . . . . . . . . . . . .  37
   7.2.1  Sending Secure Neighbor Advertisements . . . . . . . . . .  37
   7.2.2  Receiving Secure Neighbor Advertisements . . . . . . . . .  38
   7.3    Other Requirements . . . . . . . . . . . . . . . . . . . .  38
   8.     Securing Router Discovery with SEND  . . . . . . . . . . .  40
   8.1    Router Solicitation Messages . . . . . . . . . . . . . . .  40
   8.1.1  Sending Secure Router Solicitations  . . . . . . . . . . .  40
   8.1.2  Receiving Secure Router Solicitations  . . . . . . . . . .  40
   8.2    Router Advertisement Messages  . . . . . . . . . . . . . .  41
   8.2.1  Sending Secure Router Advertisements . . . . . . . . . . .  41



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   8.2.2  Receiving Secure Router Advertisements . . . . . . . . . .  41
   8.3    Redirect Messages  . . . . . . . . . . . . . . . . . . . .  41
   8.3.1  Sending Redirects  . . . . . . . . . . . . . . . . . . . .  41
   8.3.2  Receiving Redirects  . . . . . . . . . . . . . . . . . . .  42
   8.4    Other Requirements . . . . . . . . . . . . . . . . . . . .  42
   9.     Co-Existence of SEND and non-SEND nodes  . . . . . . . . .  43
   10.    Performance Considerations . . . . . . . . . . . . . . . .  45
   11.    Security Considerations  . . . . . . . . . . . . . . . . .  46
   11.1   Threats to the Local Link Not Covered by SEND  . . . . . .  46
   11.2   How SEND Counters Threats to Neighbor Discovery  . . . . .  47
   11.2.1 Neighbor Solicitation/Advertisement Spoofing . . . . . . .  47
   11.2.2 Neighbor Unreachability Detection Failure  . . . . . . . .  48
   11.2.3 Duplicate Address Detection DoS Attack . . . . . . . . . .  48
   11.2.4 Router Solicitation and Advertisement Attacks  . . . . . .  49
   11.2.5 Replay Attacks . . . . . . . . . . . . . . . . . . . . . .  49
   11.2.6 Neighbor Discovery DoS Attack  . . . . . . . . . . . . . .  49
   11.3   Attacks against SEND Itself  . . . . . . . . . . . . . . .  50
   12.    IANA Considerations  . . . . . . . . . . . . . . . . . . .  51
          Normative References . . . . . . . . . . . . . . . . . . .  52
          Informative References . . . . . . . . . . . . . . . . . .  54
          Authors' Addresses . . . . . . . . . . . . . . . . . . . .  55
   A.     Contributors . . . . . . . . . . . . . . . . . . . . . . .  57
   B.     IPR Considerations . . . . . . . . . . . . . . . . . . . .  58
   C.     Cache Management . . . . . . . . . . . . . . . . . . . . .  59
   D.     Comparison to AH-Based Approach  . . . . . . . . . . . . .  60
          Intellectual Property and Copyright Statements . . . . . .  63

























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1. Introduction

   IPv6 defines the Neighbor Discovery Protocol (NDP) in RFC 2461 [6].
   Nodes on the same link use NDP to discover each other's presence, to
   determine each other's link-layer addresses, to find routers, and to
   maintain reachability information about the paths to active
   neighbors. NDP is used both by 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.

   RFC 2461 called for the use of IPsec for protecting the NDP messages.
   However, it does not specify detailed instructions for using IPsec to
   secure NDP.  It turns out that in this particular application, IPsec
   can only be used with a manual configuration of security
   associations, due to chicken-and-egg problems in using IKE [22] [19].
   Furthermore, the number of such manually configured security
   associations needed for protecting NDP can be very large [23], making
   that approach impractical for most purposes.

   This document is organized as follows.  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 address ownership on
   individual nodes; routers are certified by a trust anchor [11].  The
   formats, procedures, and cryptographic mechanisms for the
   zero-configuration mechanism are described in a related specification
   [26].

   Section 6 describes the mechanism for distributing certificate chains
   to establish an authorization delegation chain to a common trust
   anchor.  The required new NDP options are discussed in Section 5.
   Section 7 and Section 8 show how to apply these components to
   securing Neighbor and Router Discovery.

   Finally, Section 9 discusses the co-existence of secure and
   non-secure Neighbor Discovery on the same link, Section 10 discusses
   performance considerations, and Section 11 discusses security
   considerations for Secure Neighbor Discovery.












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2. Terms

   Authorization Delegation Discovery (ADD)

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

   Cryptographically Generated Addresses (CGAs)

      A technique [26] [30] where the IPv6 address of a node is
      cryptographically generated using a one-way hash function from the
      node's public key and some other parameters.

   Duplicate Address Detection (DAD)

      A mechanism defined in RFC 2462 [7] that assures that two IPv6
      nodes on the same link are not using the same addresses.

   Internet Control Message Protocol version 6 (ICMPv6)

      The IPv6 control signaling protocol. Neighbor Discovery is a part
      of ICMPv6.

   Neighbor Discovery Protocol (NDP)

      The IPv6 Neighbor Discovery Protocol [6].

   Neighbor Discovery (ND)

      The Neighbor Discovery function of the Neighbor Discovery Protocol
      (NDP).  NDP contains also other functions but ND.

   Neighbor Unreachability Detection (NUD)

      This mechanism defined in RFC 2461 [6] is used for tracking the
      reachability of neighbors.

   Nonce

      A random number generated by a node and used exactly once, and
      never again. In SEND, nonces are used to ensure that a particular
      advertisement is linked to the solicitation that triggered it.

   Router Authorization Certificate

      An X.509v3 [11] PKC certificate using the profile specified in
      Section 6.5.1.




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   SEND node

      An IPv6 node that implements this specification.

   non-SEND node

      An IPv6 node that does not implement this specification but uses
      the legacy RFC 2461 and RFC 2462 mechanisms.

   Router Discovery (RD)

      The Router Discovery function of the Neighbor Discovery Protocol
      (NDP).






































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3. Neighbor and Router Discovery Overview

   IPv6 Neighbor and Router Discovery have several functions. Many of
   these functions are overloaded on a few central message types, such
   as the ICMPv6 Neighbor Discovery message.  In this section we review
   some of these tasks and their effects in order to understand better
   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 take precedence.

   In IPv6, many of the tasks traditionally preformed at lower the
   layers, such as ARP, have been moved to the IP layer.  As a
   consequence, a set of unified mechanisms can be applied across link
   layers, any introduced security mechanisms or other extensions can be
   adopted more easily, and a clear separation of the roles between the
   IP and link layer has been achieved.

   The main functions of IPv6 Neighbor Discovery are the following.

   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 [6].  NUD is
      security-sensitive, because an attacker could falsely claim that
      reachability exists when it in fact does not.

   o  Duplicate Address Detection (DAD) is used for preventing address
      collisions [7].  A node that intends to assign a new address to
      one of its interfaces first runs the DAD procedure to verify that
      there is no other node using the same address.  Since 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  Address Resolution is similar to IPv4 ARP [18].  The Address
      Resolution function resolves a node's IPv6 address to the
      corresponding link-layer address for nodes on the link.  Address
      Resolution is defined in Section 7.2 of RFC 2461 [6], and it is
      used for hosts and routers alike.  Again, no higher level traffic
      can proceed until the sender knows the hardware address of the
      destination node or the next hop router.  Note that like its
      predecessor in ARP, IPv6 Neighbor Discovery does not check the
      source link layer address 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 needs to be passed
      from layer to layer.




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   o  Address Autoconfiguration is used for automatically assigning
      addresses to a host [7]. This allows hosts to operate without
      explicit configuration related to IP connectivity.  The Address
      Autoconfiguration mechanism defined in [7] is stateless. To create
      IP addresses, the hosts use any prefix information delivered to
      them during Router Discovery, and then test the newly formed
      addresses for uniqueness using the DAD procedure.  A stateful
      mechanism, DHCPv6 [24], provides additional Autoconfiguration
      features.  Router and Prefix Discovery and Duplicate Address
      Detection have an effect on the Address Autoconfiguration tasks.

   o  The Redirect function is used for automatically redirecting hosts
      to an alternate router.  Redirect is specified in Section 8 of RFC
      2461 [6].  It is similar to the ICMPv4 Redirect function [17].

   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 [6].  The main purpose of Router
      Discovery is to find neighboring routers that are willing to
      forward packets on behalf of hosts.  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 to the destination node directly.
      Typically, address autoconfiguration and other tasks can not
      proceed until suitable routers and prefixes have been found.

   The Neighbor Discovery messages follow the ICMPv6 message format.
   They have ICMPv6 types from 133 to 137.  The IPv6 Next Header value
   for ICMPv6 is 58. The actual Neighbor Discovery message includes an
   NDP message header, consisting of an ICMPv6 header and ND
   message-specific data, and zero or more NDP options.

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

   The NDP message options are formatted in the Type-Length-Value
   format.

   All IPv6 NDP functions are realized using the following ICMPv6
   messages:

            ICMPv6 Type      Message
            ------------------------------------



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            133              Router Solicitation (RS)
            134              Router Advertisement (RA)
            135              Neighbor Solicitation (NS)
            136              Neighbor Advertisement (NA)
            137              Redirect

   The various functions are realized using these messages as follows:

   o  Router Discovery uses the RS and RA messages.

   o  Duplicate Address Detection uses the NS and NA messages.

   o  Address Autoconfiguration uses the NS, NA, RS, and RA messages.

   o  Address Resolution uses the NS and NA messages.

   o  Neighbor Unreachability Detection uses the NS and NA messages.

   o  Redirect uses the Redirect message.

   The NDP messages are always meant to be used within a link, and never
   intended to leak outside of it.  The destination and source addresses
   used in these messages are as follows:

   o  Neighbor Solicitation: The destination address is either the
      Solicited-Node multicast address, a unicast address, or an anycast
      address.  The source address is either the unspecified address (in
      DAD) or a unicast address assigned to the sending interface.  In a
      typical case, the source address is equal to the source address of
      the outgoing packet, locally triggering the need to send the
      solicitation.

   o  Neighbor Advertisement: The destination address is either a
      unicast address or the link-scoped All-Nodes multicast address
      [12].  The source address is a unicast address assigned to the
      sending interface.

   o  Router Solicitation: The destination address is typically the
      All-Routers multicast address [12].  The source address is either
      the unspecified address or a unicast address assigned to the
      sending interface.  An unspecified source address does not have
      any special semantics; it is just an optimization for startup.

   o  Router Advertisement: The destination address can be either a
      unicast or the link-scoped All-Nodes multicast address [12].  The
      source address is a link-local address assigned to the sending
      interface.




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   o  Redirect: This message is always sent to the source address of the
      packet that triggered the Redirect.  Hosts verify that the IP
      source address of the Redirect is the same as the current
      first-hop router for the specified ICMP Destination Address.
      Rules in [12] dictate that anycast, or multicast addresses may not
      be used as source addresses.  If the source address is an
      unspecified address, it is impossible to send a Redirect, since
      the unspecified address is forbidden as the destination address.
      Therefore, the destination address must always be a unicast
      address.

      The source address is a link-local address assigned to the sending
      interface.






































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4. Secure Neighbor Discovery Overview

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

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

   o  Certificate chains, anchored on trusted parties, are expected to
      certify the authority of routers.  A host and a router must have
      at least one common trust anchor before the host can adopt the
      router as its default router.  Delegation Chain Solicitation and
      Advertisement messages are used to discover a certificate chain to
      the trust anchor without requiring the actual Router Discovery
      messages to carry lengthy certificate chains.

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

      This specification also allows one to use non-CGA addresses and to
      use certificates to authorized their use.  However, the details of
      such use have been left for future work.

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

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

   o  In order to prevent replay attacks, two new Neighbor Discovery
      options, Timestamp and Nonce, are used.  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
      protected using the Nonce option.



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5. Neighbor Discovery Options

   The following new NDP options and mechanisms are REQUIRED to be
   implemented by all SEND nodes:

   o  The CGA option MAY be present in all Neighbor Discovery messages,
      and SHOULD be present in most cases.

   o  The Signature option is REQUIRED in all Neighbor Discovery
      messages.

   o  The Nonce option is REQUIRED in all Neighbor Discovery
      solicitations, and in all solicited advertisements.

   o  The Timestamp option is REQUIRED in all Neighbor Discovery
      advertisements and Redirects.

   o  Proxy Neighbor Discovery is not supported by this specification;
      it is planned to be specified in a future document.


5.1 Ordering of the new options

   The ordering of the new options MUST obey the following rules:

      The CGA option MUST appear before the Signature option.

      The Nonce option SHOULD appear before the Timestamp option.

      The Signature option MUST NOT be be followed CGA, Nonce, or
      Timestamp options.

      It is RECOMMENDED that the options appear in the following order:
      CGA, Nonce, Timestamp, Signature.


5.2 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     |            Modifier           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Collision Cnt |                  Reserved                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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     |                                                               |
     .                                                               .
     .                        Key Information                        .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                           Padding                             .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meaning of the fields is described as follows.

   Type

      TBD <To be assigned by IANA> for CGA.

   Length

      The length of the option, in units of 8 octets.

   Modifier

      A random number used in CGA generation.  Its semantics are defined
      in [26].

   Collision Cnt

      An 8-bit collision count, which can get values 0, 1 and 2. Its
      semantics are defined in [26].

   Reserved

      A 24-bit field reserved for future use.  The value MUST be
      initialized to zero by the sender, and MUST be ignored by the
      receiver.

   Key Information

      A variable length field containing the public key of the sender,
      represented as an ASN.1 type SubjectPublicKeyInfo [11], encoded as
      described in Section 4 of [26].

      This specification requires that if both the CGA option and the
      Signature option are present, then the publicKey field in the
      former option MUST be the public key referred by the Key Hash



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      field in the latter option.  Packets received with two different
      keys MUST be silently discarded.  Note that a future extension may
      provide a mechanism which allows the owner of an address and the
      signer to be different parties.

      The length of the Key Information field is determined by the ASN.1
      encoding.

   Padding

      A variable length field making the option length a multiple of 8.
      It begins after the ASN.1 encoding of the previous field has ends,
      and continues to the end of the option, as specified by the Length
      field.


5.2.1 Processing Rules for Senders

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

   The Modifier, Collision Cnt, and Key Information fields in the CGA
   option are filled in according to the rules presented above and in
   [26].  The used public key is taken from configuration; typically
   from a data structure associated with the source address.

   An address MUST be constructed as specified in Section 4 of [26].  In
   the typical case, the address is constructed long before it is used.

   Depending on the type of the message, this address appears in
   different places:

   Redirect

      The address MUST be the source address of the message.

   Neighbor Solicitation

      The address MUST be the Target Address for solicitations sent for
      the purpose of Duplicate Address Detection, and the source address
      of the message otherwise.

   Neighbor Advertisement

      The address MUST be the source address of the message.






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   Router Solicitation

      The address MUST be the source address of the message, unless the
      source address is the unspecified address.

   Router Advertisement

      The address MUST be the source address of the message.


5.2.2 Processing Rules for Receivers

   A message containing a CGA option MUST be checked as follows:

      If the interface has been configued to use CGA, it is REQUIRED
      that the receiving node verifies the source address of the packet
      using the algorithm described in Section 5 of [26].  The inputs
      for the algorithm are the contents of the Modifier, Collision Cnt,
      and the Key Information fields, the claimed address in the packet
      (as discussed in the previous section), and the minimum acceptable
      Sec value. If the CGA verification is successful, the recipient
      proceeds with the cryptographically more time consuming check of
      the signature.

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

5.2.3 Configuration

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

   minbits

      The minimum acceptable key length for the public keys used in the
      generation of the CGA address.  The default SHOULD be 1024 bits.
      Implementations MAY also set an upper limit in order to limit the
      amount of computation they need to perform when verifying packets
      that use these security associations.  Any implementation should
      follow prudent cryptographic practise in determining the
      appropriate key lengths.


5.3 Signature Option

   The Signature option allows public-key based signatures to be



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   attached to NDP messages. Both trust anchor authentication and CGAs
   can be used.  The format of the Signature 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     |  Pad Length   |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                          Key Hash                             |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                       Digital Signature                       .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                           Padding                             .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meaning of the fields is described below:

   Type

      TBD <To be assigned by IANA> for Signature.

   Length

      The length of the option, in units of 8 octets.

   Pad Length

      An 8-bit integer field, giving the length of the Pad field in
      units of an octet.

   Reserved

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




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   Key Hash

      A 128-bit field contains the most significant (leftmost) 128-bits
      of a SHA1 hash of the public key used for the constructing the
      signature.  The SHA1 is taken over the presentation used in the
      Key Information field in the CGA option.  Its purpose is to
      associate the signature to a particular key known by the receiver.
      Such a key can be either 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 contains the signature constructed using
      the sender's private key, over the the following sequence of
      octets:

      1.  The 128-bit CGA Type Tag [26] value for SEND, 0xXXXX XXXX XXXX
          XXXX XXXX XXXX XXXX XXXX (To be generated randomly).

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

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

      4.  The 32-bit ICMP header, i.e., the Type, Code, and Checksum
          fields.

      5.  The Neighbor Discovery message header, i.e., the Reserved
          field in the Router Solicitation message, the Cur Hop Limit,
          M, O, Reserved, Router Lifetime, Reachable Time, and Retrans
          Timer fields in the Router Advertisement message, Reserved and
          Target Address fields in the Neighbor Solicitation message, R,
          S, O, Reserved, and Target Address fields in the Neighbor
          Advertisement message, and Reserved, Target Address, and
          Destination Address fields in the Redirect message.

      6.  All NDP options preceding the Signature option.

      The signature is constructed using the RSA algorithm and MUST be
      encoded as private key encryption in PKCS#1 format [15].  The
      signature value is computed with the RSASSA-PKCS1-v2_1 algorithm
      and SHA-1 hash as defined in [15].

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





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      This variable length field contains padding, as many bytes as is
      given by the Pad Length Field.


5.3.1 Processing Rules for Senders

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

   o  The message is constructed in its entirety.

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

   o  For the purpose of constructing a signature, the following data
      items are concatenated:

      *  The 128-bit CGA Type Tag.

      *  The source address of the message.

      *  The destination address of the message.

      *  The contents of the message, starting from the ICMPv6 header,
         up to but excluding the Signature option.

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


5.3.2 Processing Rules for Receivers

   A message containing a Signature option MUST be checked as follows:

   o  The Signature option MUST appear as the last option.

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

   o  TheDigital Signature field MUST have correct encoding, and do not
      exceed the length of the Signature option.

   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
      authorization delegation chain MUST be known between the



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      receiver's trust anchor and the sender's public key.

      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.  The receiver MAY silently drop packets also otherwise,
   e.g., as a response to an apparent CPU exhausting DoS attack.

5.3.3 Configuration

   All nodes that support the reception of the Signature options MUST
   record the following configuration information for each separate
   Neighbor Discovery Protocol 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
         6.5.  The sender may claim additional authorization through the
         use of CGAs, but that is neither required nor verified.

      CGA

         The CGA property of the sender's address is verified as
         described in [26]. The sender may claim additional authority
         through a trust anchor, but that is neither required nor
         verified.

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





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   minSec

      The minimum acceptable Sec value, if CGA verification is required
      (see Section 2 in [26]). This parameter is intended to facilitate
      future extensions and experimental work.  Currently, the minSec
      value SHOULD always be set to zero.

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

   keypair

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

   CGA flag

      A flag that indicates whether CGA is used or is not used.  This
      flag may be per interface or per node.

   CGA parameters

      Optionally any information required to construct CGAs, including
      the used Sec and Modifier values, and the CGA address itself.


5.4 Timestamp and Nonce options

5.4.1 Timestamp Option

   The purpose of the Timestamp option is to ensure that unsolicited
   advertisements and redirects have not been replayed.  The format of
   the Timestamp 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                            +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where the fields are as follows:



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   Type

      TBD <To be assigned by IANA> for Timestamp.

   Length

      The length of the option, 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.

   Timestamp

      A 64-bit unsigned integer field containing a timestamp. The value
      indicates the number of seconds since January 1,, 1970 00:00 UTC,
      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.


5.4.2 Nonce Option

   The purpose of the Nonce option is to ensure that an advertisement is
   a fresh response to a solicitation sent earlier by the receiving same
   node. The format of the Nonce option is as 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 ...                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                                                               |
       .                                                               .
       .                                                               .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where the fields are as follows:

   Type

      TBD <To be assigned by IANA> for Nonce.




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   Length

      The length of the option, 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.


5.4.3 Processing rules for senders

   All solicitation messages MUST include a Nonce.  All solicited-for
   announcements MUST include a Nonce, copying the nonce value from the
   received solicitation.  When sending a solication, the sender MUST
   store the nonce internally so that it can recognize any replies
   containing that particular nonce.

   All NDP messages MUST include a Timestamp.  Senders SHOULD set the
   Timestamp field to the current time, according to their real time
   clock.

   If a message has both Nonce and Timestamp options, the Nonce option
   SHOULD precede the Timestamp option in order. The receiver MUST be
   prepared to receive them in any order, as per RFC 2461 [6] Section 9.

5.4.4 Processing rules for receivers

   The processing of the Nonce and Timestamp options depends on whether
   a packet is a solicited-for advertisement or not. A system may
   implement the distinction in various means. Section 5.4.4.1 defines
   the processing rules for solicited-for advertisements.  Section
   5.4.4.2 defines the processing rules for all other messages.

   An implementation may utilize 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 becomes 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
   the others is left as an implementation decision. However, typical
   policies may prefer existing entries over new ones, CGAs with a large
   Sec value over smaller Sec values, and so on.  The issue is briefly
   discussed in Appendix C.





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5.4.4.1 Processing solicited-for advertisements

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

   If the message does not contain a Nonce option, it MAY be considered
   as a non-solicited-for announcement, and processed according to
   Section 5.4.4.2.

   If the message does contain a Nonce option, but the Nonce value is
   not recognized, the message MUST be silently dropped.

   If the message is accepted, the receiver SHOULD store the receive
   time of the message and the time stamp time in the message, as
   specified in Section 5.4.4.2

5.4.4.2 Processing all other messages

   Receivers SHOULD be configured with an allowed timestamp Delta value
   and an allowed clock drift parameter.  The recommended default value
   for the allowed Delta is 3,600 seconds (1 hour) and for clock dritf
   1% (0.01).

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

      The receive time of the last received, acepted SEND message.  This
      is called RDlast.

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

   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 with
      respect to the receival time of the packet, RDnew:

        -Delta < (RDnew - TSnew) < +Delta

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

   o  If the timestamp is NOT within the boundaries but the message is a
      Neighbor Solicitation message that should be responded to by the
      receiver, the receiver MAY respond to the message.  However, if it



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      does respond to the message, it MUST NOT create a neighbor cache
      entry.  This allows nodes that have large difference in their
      clocks to still communicate 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 time stamp is checked
      against the previously received SEND message:

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

   o  If TSnew < TSlast, which is possible if packets arrive rapidly and
      out of order, TSlast MUST NOT be updated, i.e., the stored TSlast
      for a given node MUST NOT ever decrease.  Otherwise TSlast SHOULD
      be updated.  Independent on whether TSlast is updated or not,
      RDlast is updated in any case.


5.5 Proxy Neighbor Discovery

   The Target Address in Neighbor Advertisement is required to be equal
   to the source address of the packet, except in the case of proxy
   Neighbor Discovery.  Proxy Neighbor Discovery is not supported by
   this specification; it is planned to be specified in a future
   document.


























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6. Authorization Delegation Discovery

   Several protocols, including the IPv6 Neighbor Discovery Protocol,
   allow a node to automatically configure itself based on information
   it learns 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 information, when the node needs this information
   before being able to communicate with nodes outside of the link.

   Since the newly-connected node cannot communicate off-link, it can
   not be responsible for searching information to help validating the
   router(s); however, given a chain of appropriately signed
   certificates, it can check someone else's search results and conclude
   that a particular message comes from an authorized source.  In the
   typical case, a router, which is already connected to beyond the
   link, can (if necessary) communicate with off-link nodes and
   construct such a certificate chain.

   The Secure Neighbor Discovery Protocol introduces two new ICMPv6
   messages that are used between hosts and routers to allow the host to
   learn a certificate chain with the assistance of the router.  Where
   hosts themselves are certified by a trust anchor, these messages MAY
   also optionally be used between hosts to acquire the peer's
   certificate chain.  However, the details of such usage are left for
   future specification.

   The Delegation Chain Solicitation (DCS) message is sent by a host
   when it wishes to request a certificate chain between a router and
   the one of the host's trust anchors.  The Delegation Chain
   Advertisement (DCA) message is sent as an answer to the DCS message.
   It MAY be periodically sent to the link-scoped All-Nodes multicast
   address.  These messages are separate from the rest of Neighbor and
   Router Discovery, in order to reduce the effect of the potentially
   voluminous certificate chain information on other messages.

   The Authorization Delegation Discovery (ADD) process does not exclude
   other forms of discovering certificate chains. For instance, during
   fast movements mobile nodes may learn information - including the
   certificate chains - of the next router from a previous router.

6.1 Delegation Chain Solicitation Message Format

   Hosts send Delegation Chain Solicitations in order to prompt routers
   to generate Delegation Chain Advertisements quickly.

      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



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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Identifier           |          Reserved             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

      Source Address

         An IP address assigned to the sending interface, or 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

         TBD <To be assigned by IANA> for Delegation Chain Solicitation.

      Code

         0

      Checksum

         The ICMP checksum [8].

      Identifier

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




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      Reserved

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

   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 6.3.  If there are more than
         one Trust Anchor options, the options past the first one may
         contain any types of Trust Anchors.

      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.


6.2 Delegation Chain Advertisement Message Format

   Routers send out Delegation Chain Advertisement messages
   periodically, or in response to a Delegation Chain 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           |           Component           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

   IP Fields:

      Source Address

         MUST be a unicast address assigned to the interface from which
         this message is sent.

      Destination Address

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



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      Hop Limit

         255

   ICMP Fields:

      Type

         TBD <To be assigned by IANA> for Delegation Chain
         Advertisement.

      Code

         0

      Checksum

         The ICMP checksum [8].

      Identifier

         A 16-bit unsigned integer field, acting as an identifier to
         help matching advertisements to solicitations.  The Identifier
         field MUST be zero for unsolicited advertisements and MUST NOT
         be zero for solicited advertisements.

      Component

         A 16-bit unsigned integer field, used for informing the
         receiver which certificate is being sent, and how many are
         still left to be sent in the whole chain.

         A single advertisement MUST be broken into separately sent
         components if there is more than one Certificate option, in
         order to avoid excessive fragmentation at the IP layer.  Unlike
         the fragmentation at the IP layer, individual components of an
         advertisement may be stored and used before all the components
         have arrived; this makes them slightly more reliable and less
         prone to Denial-of-Service attacks.

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

         The components MUST be ordered so that the trust anchor end of
         the chain is the one sent first.  Each certificate sent after
         it can be verified with the previously sent certificates.  The



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         certificate of the sender comes last.

      Reserved

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

   Valid Options:

      Certificate

         One certificate is provided in each Certificate option, to
         establish a (part of a) certificate chain to a trust anchor.

      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.


6.3 Trust Anchor Option

   The format of the Trust Anchor option is as 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 ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where the fields are as follows:

   Type

      TBD <To be assigned by IANA> for Trust Anchor.

   Length

      The length of the option, (including the Type, Length, Name Type,



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      Name Length, and Name fields,) in units of 8 octets.

   Name Type

      The type of the name included in the Name field. This
      specification defines only one legal value 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 certificate Name, represented and encoded
      exactly as in the matching X.509v3 trust anchor certificate.

      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
      "preferred name syntax" DNS format, as specified in RFC 1034 [1]
      Section 3.5.  Additionally, the restrictions discussed in RFC 3280
      [11] Section 4.2.1.7 apply.

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


6.4 Certificate Option

   The format of the certificate option is as 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    |  Pad Length   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Certificate ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where the fields are as follows:




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   Type

      TBD <To be assigned by IANA> for Certificate.

   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

   Pad Length

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

   Certificate

      When the Cert Type field is set to 1, the Certificate field
      contains an X.509v3 certificate [11], as described in Section
      6.5.1.


6.5 Router Authorization Certificate Format

   The certificate chain of a router terminates in a Router
   Authorization Certificate that authorizes a specific IPv6 node to act
   as a router.  Because authorization chains are not a common practice
   in the Internet at the time this specification is being written, the
   chain MUST consist of standard Public Key Certificates (PKC, in the
   sense of [21]).  The certificates chain MUST start from the identity
   of a trust anchor that is 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.

6.5.1 Router Authorization Certificate Profile

   Router Authorization Certificates be X.509v3 certificates, as defined
   in RFC 3280 [11], and MUST contain at least one instance of the X.509
   extension for IP addresses, as defined in [13].  The parent
   certificates in the certificate chain MUST contain one or more X.509



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   IP address extensions, back up to the delegating authority (the
   Regional Address Registry or IANA) that delegated the original IP
   address space block.  The certificates for intermediate delegating
   authorities MUST contain X.509 IP address extension(s) for
   subdelegations. The router's certificate is signed by the delegating
   authority for the prefixes the router is authorized to to advertize.

   The X.509 IP address extension MUST contain at least one
   addressesOrRanges element that contains an addressPrefix element with
   an IPv6 address prefix for a prefix the router or the intermediate
   entity is authorized to advertize.  If the entity is allowed to route
   any prefix, the used IPv6 address prefix is the null prefix, 0/0.
   The addressFamily element of the containing IPAddrBlocks sequence
   element MUST contain the IPv6 AFI (0002), as specified in [13] for
   IPv6 prefixes.  Instead of an addressPrefix element, the
   addressesOrRange element MAY contain an addressRange element for a
   range of prefixes, if more than one prefix is authorized.  The X.509
   IP address extension MAY contain additional IPv6 prefixes, expressed
   either as an addressPrefix or an addressRange.

   A SEND node receiving a Router Authorization Certificate MUST first
   check whether the certificate's signature was generated by the
   delegating authority.  Then the client MUST 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 prefixes or ranges, the client MAY attept to
   take an intersection of the ranges/prefixes, and use that
   intersection.  If the addressPrefix in the certificate is the null
   prefix, 0/0, such an intersection SHOULD be used.  (In that case the
   intersection is the parent prefix or range.)  If the resulting
   intersection is empty, the client MUST NOT accept the certificate.

   The above check SHOULD be done for all certificates in the chain
   received through DCA messages.  If any of the checks fail, the client
   MUST NOT accept the certificate.

   Since it is possible that some PKC 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 off by default.  That is, the system SHOULD have a default
   configuration that requires rigorious prefix and range checks.

6.6 Processing Rules for Routers




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   Routers SHOULD possess a key pair and a certificate from at least one
   certificate authority.

   A router MUST silently discard any received Delegation Chain
   Solicitation messages that do not satisfy all of the following
   validity checks:

   o  The IP Hop Limit field MUST have a value of 255, i.e., the packet
      could not possibly have been forwarded by a router.

   o  If the message includes an IP Authentication Header, the message
      authenticates correctly.

   o  ICMP Checksum is valid.

   o  ICMP Code is 0.

   o  ICMP length (derived from the IP length) is 8 or more octets.

   o  Identifier field is non-zero.

   o  All included options have a length that is greater than zero.

   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 MAY send unsolicited Delegation Chain Advertisements for
   their configured trust anchor(s).  When such advertisements are sent,
   their timing MUST follow the rules given for Router Advertisements in
   RFC 2461 [6].  The only defined options that may appear are the
   Certificate and Trust Anchor options. At least one Certificate option
   MUST be present.  Router SHOULD also include at least one Trust
   Anchor option to indicate the trust anchor on which the Certificate
   is based.

   In addition to sending periodic, unsolicited advertisements, a router
   sends 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



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   Solicited-Node multicast address corresponding to the source address.

   In a solicited-for advertisement, the router SHOULD include suitable
   Certificate options so that a delegation chain to the solicited trust
   anchor can be established.  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
   delegation chain was found.

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

   Rate limiting of Delegation Chain Advertisements is performed as
   specified for Router Advertisements in RFC 2461 [6].

6.7 Processing Rules for Hosts

   Hosts SHOULD possess the public key and trust anchor name of at least
   one certificate authority, they SHOULD possess their own key pair,
   and they MAY posses a certificate from the above mentioned
   certificate authority.

   A host MUST silently discard any received Delegation Chain
   Advertisement messages that do not satisfy all of the following
   validity checks:

   o  IP Source Address MUST be a unicast address.  Note that routers
      may use multiple addresses, and therefore this address not
      sufficient for the unique identification of routers.

   o  IP Destination Address MUST be either the link-scoped All-Nodes
      multicast address or the Solicited-Node multicast address
      corresponding to one of the unicast addresses assigned to the
      host.

   o  The IP Hop Limit field MUST have a value of 255, i.e., the packet
      could not possibly have been forwarded by a router.

   o  If the message includes an IP Authentication Header, the message
      authenticates correctly.

   o  ICMP Checksum is valid.



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   o  ICMP Code is 0.

   o  ICMP length (derived from the IP length) is 16 or more octets.

   o  All included options have a length that is greater than zero.

   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 Delegation Chain 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 certificate chains retrieved in Delegation Chain
   Discovery messages if they start from an anchor trusted by the host.
   The certificates chains SHOULD be verified, as defined in Section
   6.5, before storing them.  Routers are required to send the
   certificates one by one, starting from the trust anchor end of the
   chain. Except for temporary purposes to allow for message loss and
   reordering, hosts SHOULD NOT store certificates received in a
   Delegation Chain Advertisement unless they contain a certificate
   which can be immediately verified either to the trust anchor or to a
   certificate which has been verified earlier.

   Note that it may be useful to cache this information and implied
   verification results for use over multiple attachments to the
   network.

   When an interface becomes enabled, a host may be unwilling to wait
   for the next unsolicited Delegation Chain Advertisement.  To obtain
   such advertisements quickly, a host MAY transmit up to
   MAX_RTR_SOLICITATIONS Delegation Chain Solicitation messages, each
   separated by at least RTR_SOLICITATION_INTERVAL seconds.  Delegation
   Chain Solicitations MAY be sent after any of the following events:

   o  The interface is initialized at system startup time.

   o  The interface is reinitialized after a temporary interface failure
      or after being temporarily disabled by system management.

   o  The system changes from being a router to being a host, by having
      its IP forwarding capability turned off by system management.

   o  The host attaches to a link for the first time.



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   o  A movement has been indicated by lower layers or has been inferred
      from changed information in a Router Advertisement.

   o  The host re-attaches to a link after being detached for some time.

   o  A Router Advertisement has been received with a public key that is
      not stored in the hosts' cache of certificates, or there is no
      authorization delegation chain to the host's trust anchor.

   Delegation Chain Solicitations SHOULD NOT be sent if the host has a
   currently valid certificate chain from a reachable router to a trust
   anchor.

   When soliciting certificates for a router, a host MUST send
   Delegation Chain 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 it has already been selected.

   If two hosts want to establish trust with the DCS and DCA messages,
   the DCS 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 left for
   a future specification.

   Delegation Chain Solicitations SHOULD be rate limited and timed
   similarly with Router Solicitations, as specified in RFC 2461 [6].

   When processing possible advertisements sent as responses to a
   solicitation, the host MAY prefer to process first those
   advertisements with the same Identifier field value as in the
   solicitation.  This makes Denial-of-Service attacks against the
   mechanism harder (see Section 11.3).



















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7. Securing Neighbor Discovery with SEND

   This section describes how to use the mechanisms from Section 5,
   Section 6, and the reference [26] in order to provide security for
   Neighbor Discovery.

   There is no requirement that nodes use both Secure Neighbor Discovery
   (as described in this Section) and Secure Router Discovery (as
   described in Section 8.  They MAY be used indepedently.

7.1 Neighbor Solicitation Messages

   All Neighbor Solicitation messages are protected with SEND.

7.1.1 Sending Secure Neighbor Solicitations

   Secure Neighbor Solicitation messages are sent as described in RFC
   2461 and 2462, with the additional requirements as listed in the
   following:

      All Neighbor Solicitation messages sent MUST contain the Nonce,
      Timestamp, and Signature options, and MAY contain the CGA option.
      The Signature option MUST be constructed with the sender's key
      pair, using the configured authorization method(s), and if
      applicable, using the trust anchor and/or minSec value as
      configured.


7.1.2 Receiving Secure Neighbor Solicitations

   Received Neighbor Solicitation messages are processed as described in
   RFC 2461 and 2462, with the additional SEND-related requirements as
   listed in the following:

      Neighbor Solicitation messages received without the Nonce,
      Timestamp, or Signature option MUST be silently discarded.  The
      Signature option MUST be constructed with the expected
      authorization method(s), the used key being within the configured
      minimum (and maximum) allowable key size, and if applicable, using
      an acceptable trust anchor and/or minSec value.


7.2 Neighbor Advertisement Messages

   All Neighbor Advertisement messages are protected with SEND.

7.2.1 Sending Secure Neighbor Advertisements




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   Secure Neighbor Advertisement messages are sent as described in RFC
   2461 and 2462, with the additional requirements as listed in the
   following:

      All Neighbor Advertisement messages sent MUST be sent with the
      Timestamp and Signature options and MAY be sent with the CGA
      option. The Signature option MUST be constructed with the sender's
      key pair, setting the authorization method and additional
      information as configured.

      Neighbor Advertisements sent in response to a Neighbor
      Solicitation MUST additionally contain a copy of the Nonce option
      included in the solicitation.


7.2.2 Receiving Secure Neighbor Advertisements

   Received Neighbor Advertisement messages are processed as described
   in RFC 2461 and 2462, with the additional SEND-related requirements
   as listed in the following:

      Any eighbor Advertisement messages received without the Timestamp
      or Signature options MUST be silently discarded.  The Signature
      option MUST be constructed with the expected authorization
      method(s), the used key being within the configured minimum (and
      maximum) allowable key size, and if applicable, using an
      acceptable trust anchor and/or minSec value.

      Received Neighbor Advertisements sent to a unicast destination
      address without a Nonce option MUST be silently discarded.


7.3 Other Requirements

   Upon receiving a message for which the receiver has no certificate
   chain to a trust anchor, the receiver MAY use Authorization
   Delegation Discovery to learn the certificate chain of the peer.

   Nodes that use stateless address autoconfiguration, SHOULD generate a
   new CGA as specified in Section 4 of [26] for each new
   autoconfiguration run.  The nodes MAY continue to use the same public
   key and modifier, and start the process from Step 4.

   This specification does not address the protection of Neighbor
   Discovery packets for nodes that are configured with a static address
   (e.g., PREFIX::1).  Future certificate chain based authorization
   specifications are needed for such nodes.




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   It is outside the scope of this specification to describe the use of
   trust anchor authorization between nodes with dynamically changing
   addresses.  Such dynamically changing addresses may be the result of
   stateful or stateless address autoconfiguration, or through the use
   of RFC 3041 [9] addresses.  If the CGA method is not used, nodes
   would be required to exchange certificate chains 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 such certificates, since there are currently a few
   cases where such certificates are required 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.





































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8. Securing Router Discovery with SEND

   This section describes how to use the mechanisms from Section 5,
   Section 6, and the reference [26] in order to provide security for
   Router Discovery.

8.1 Router Solicitation Messages

   All Router Solicitation messages are protected with SEND.

8.1.1 Sending Secure Router Solicitations

   Secure Router Solicitation messages are sent as described in RFC
   2461, with the additional requirements as listed in the following:

      Router Solicitation messages sent with an unspecified source
      address MUST have the Nonce and Timestamp options.

      Other Router Solicitations MUST have the Nonce, Timestamp, and
      Signature options.  The Signature option MUST be configured with
      the sender's key pair, setting the authorization method and
      additional information as is configured.


8.1.2 Receiving Secure Router Solicitations

   Received Router Solicitation messages are processed as described in
   RFC 2461, with the additional SEND-related requirements as listed in
   the following:

      Router Solicitation message sent with an unspecified source
      address and without the Nonce or Timestamp options MUST be
      silently discarded.

      Router Solicitation messages received with another type of source
      address but without the Nonce, Timestamp, or Signature options
      MUST be silently discarded.

      The Signature option MUST be constructed with the configured
      authorization method(s), the used key being within the configured
      minimum (and maximum) allowable key size, and if applicable, using
      an acceptable trust anchor and/or minSec value.

      The configured authorization methods MUST include the trust anchor
      authorization method, and MAY be additionally configured to
      require CGA authorization.





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8.2 Router Advertisement Messages

   All Router Advertisement messages are protected with SEND.

8.2.1 Sending Secure Router Advertisements

   Secure Router Advertisement messages are sent as described in RFC
   2461, with the additional requirements as listed in the following:

      All Router Advertisement messages sent MUST contain a Timestamp
      and Signature options.  The Signature option MUST be configured to
      protect the advertisement with the trust anchor authorization
      method and MAY be configured to additionally protect it with the
      CGA authorization method.

      Router Advertisements sent in response to a Router Solicitation
      MUST contain a copy of the Nonce option included in the
      solicitation.


8.2.2 Receiving Secure Router Advertisements

   Received Router Advertisement messages are processed as described in
   RFC 2461, with the additional SEND-related requirements as listed in
   the following:

      Router Advertisement messages received without the Timestamp and
      Signature options MUST be silently discarded.

      Received Router Advertisements sent to a unicast destination
      address without a Nonce option MUST be silently discarded.

      The Signature option MUST be constructed with the configured
      authorization method(s), the used key being within the configured
      minimum (and maximum) allowable key size, and if applicable, using
      an acceptable trust anchor and/or minSec value.

      The configured authorization methods MUST include the trust anchor
      authorization method, and MAY be additionally configured to
      require CGA authorization.


8.3 Redirect Messages

   All Redirect messages are protected with SEND.

8.3.1 Sending Redirects




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   Secure Redirect messages are sent as described in RFC 2461, with the
   additional requirements as listed in the following:

      All Redirect messages sent MUST contain the Timestamp and
      Signature options.  The Signature option MUST be configured to use
      the trust anchor authorization method, and MAY be additionally
      configured to use the CGA method.


8.3.2 Receiving Redirects

   Received Redirect messages are processed as described in RFC 2461,
   with the additional SEND-related requirements as listed in the
   following:

      Redirect messages received without the Timestamp or Signature
      options MUST be silently discarded.

      The Signature option MUST be constructed with the configured
      authorization method(s), the used key being within the configured
      minimum (and maximum) allowable key size, and if applicable, using
      an acceptable trust anchor and/or minSec value.

      The configured authorization methods MUST include the trust anchor
      authorization method, and MAY be additionally configured to
      require CGA authorization.

      The receiver MUST verify that the Redirect message comes from an
      IP address to which the host may have earlier sent the packet that
      the Redirect message now partially returns.  That is, the source
      address of the Redirect message must be the default router or the
      on-link destination host for traffic sent to the destination of
      the returned packet.  If this is not the case, the message MUST be
      silently discarded.

      This step prevents a bogus router from sending a Redirect message
      when the host is not using the bogus router as a default router.


8.4 Other Requirements

   Hosts SHOULD use Authorization Delegation Discovery to learn the
   certificate chain of their default router (or peer host), as
   explained in Section 6.  The receipt of a protected Router
   Advertisement message for which no router Authorization Certificate
   and certificate chain is available triggers Authorization Delegation
   Discovery.




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9. Co-Existence of SEND and non-SEND nodes

   During the transition to secure links or as a policy consideration,
   network operators may want to run a particular link with a mixture of
   secure and insecure nodes.  Nodes that support SEND SHOULD support
   the use of SEND and the legacy Neighbor Discovery Protocol at the
   same time.

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

   SEND nodes send only secured messages.  Legacy Neighbor Discovery
   nodes will obviously send only insecure messages. Such nodes will (as
   per RFC 2461 [6]) ignore the unknown options and will treat secured
   messages in the same way as they treat insecure ones.  Secured and
   insecure nodes share the same network resources, such as prefixes and
   address spaces.

   In a mixed environment SEND routers and hosts follow the protocols
   defined in RFC 2461 and RFC 2462 with the following exceptions:

      All solicitations sent by SEND nodes MUST be secured.

      Unsolicited Neighbor and Router Advertisements sent by a SEND
      router MUST be secured.

      Secured solicitations MUST contain the Nonce option. Secured
      advertisements sent in response to a secured solicitation MUST
      contain a copy of the Nonce option from the solicitation.
      Unsolicited advertisements and ones sent in response to an
      insecure solicitation MUST NOT contain the Nonce option.

      A SEND node that uses the CGA authorization method for protecting
      Neighbor Solicitations SHOULD perform Duplicate Address Detection
      as follows.  If Duplicate Address Detection indicates the
      tentative address is already in use, generate a new tentative CGA
      address.  If after 3 consecutive attempts no non-unique address
      was generated, log a system error and give up attempting to
      generate an address for that interface.

      When performing Duplicate Address Detection for the first
      tentative address, accept both secured and insecure Neighbor
      Advertisements and Solicitations received as response to the
      Neighbor Solicitations.  When performing Duplicate Address
      Detection for the second or third tentative address, ignore



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      insecure Neighbor Advertisements and Solicitations.

      The node SHOULD have a configuration option that causes it to
      ignore insecure advertisements even when performing Duplicate
      Address Detection for the first tentative address. This
      configuration option SHOULD be disabled by default. (This is
      recovery mechanism for the unlikely case that attacks against the
      first address become common.)

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

      The conceptual sending algorithm is modified so that an insecure
      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.

      A SEND node SHOULD have a configuration option that causes it to
      ignore all insecure ND, RD and Redirect messages. (This can be
      used to enforce SEND-only networks.)


























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10. Performance Considerations

   The computations related to the Signature option are computationally
   relatively expensive.  In the application which Signature option has
   been designed for, however, the nodes typically have the need to
   perform only a few signature operations as they enter a link, and a
   few operations as they find a new on-link peer with which to
   communicate.

   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 ones per second, some of
   which can be precomputed as discussed below.  A large number of
   router solicitations may cause higher demand for performing
   asymmetric operations, although RFC 2461 limits the rate at which
   responses to solicitations can be sent.

   Signatures related to the use of the Signature option be precomputed
   for Multicast Neighbor and Router Advertisements. Typically,
   solicited 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 typically not possible to precompute
   solicited-for advertisements.



























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11. Security Considerations

11.1 Threats to the Local Link Not Covered by SEND

   SEND does not compensate for an insecure link layer.  In particular,
   there is no cryptographic binding in SEND between the link layer
   frame address and the IPv6 address.  On an insecure link layer that
   allows nodes to spoof the link layer address of other nodes, an
   attacker could disrupt IP service by sending out a Neighbor
   Advertisement having the source address on the link layer frame of a
   victim, a valid CGA address and a valid signature corresponding to
   itself, and a Target Link-layer Address extension corresponding to
   the victim.  The attacker could then proceed to cause a traffic
   stream to bombard the victim in a DoS attack.  To protect against
   such attacks, link layer security MUST be used.  An example of such
   for 802 type networks is port-based access control defined in the
   802.1X standard [34].

   Specifically, the 802.1X standard provides a mechanism by which a
   nodes can be authenticated to a particular point of attachment to a
   LAN (called a "port" in the standard). If the MAC on frames sent by a
   node does not correspond to the MAC of the node originally
   authenticated to this port, then the point of attachment drops the
   frames. Authorization to use the port is determined by the MAC
   address of the node that originally authenticated to the port. The
   way 802.1X protects against this attack is that, if a node
   authenticated to a particular port attempts to spoof the MAC address
   of another node, the port will drop the frames. Naturally, this
   requires that all ports by which nodes can attach to the LAN use
   802.1X authentication, and that all node physically attach through a
   port, as is the case with 802.3 switched LAN. For shared media, such
   as multiple nodes authenticated through the same 802.11 AP (which
   acts as a single port for all nodes), other measures are necessary,
   since an attacker on the wireless link can spoof the MAC address of a
   victim on the same wireless link.

   802.1X does not provide protection for the layer 2 frame - layer 3
   packet address binding in traffic (that is, real time filtering to
   check this binding), and neither does SEND.  802.1X provides
   authentication and filtering of MAC address to port; SEND provides
   protection for the layer 2 - layer 3 binding information in the
   Neighbor Discovery packet, via the CGA address (authorization to use
   the address via the public key) and the signature on the packet
   (authentication of contents as from authorized IP address possessor).

   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



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   address that they are claiming for their addresses; RFC 2461 [6].
   Subscribing to a multicast group requires that the nodes use MLD
   [20].  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 such an 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
   overwhelming) traffic.

11.2 How SEND Counters Threats to Neighbor Discovery

   The SEND protocol is designed to counter the threats to IPv6 Neighbor
   Discovery, as outlined in [27].  The following subsections contain a
   regression of the SEND protocol against the threats, to illustrate
   what aspects of the protocol counter each threat.

11.2.1 Neighbor Solicitation/Advertisement Spoofing

   This threat is defined in Section 4.1.1 of [27].  The threat is that
   a spoofed Neighbor Solicitation or Neighbor Advertisement causes 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.  There are two cases:

       1.  A router receiving a Router Solicitation with a firm IPv6
           source address and a Target Link-Layer Address extension
           inserts an entry for the IPv6 address into its Neighbor
           Cache.

       2.  A node doing 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.

   2.  Entries made as a result of a Neighbor Advertisement sent as a
       response to a Neighbor Solicitation for purposes of on-link
       address resolution.


11.2.1.1 Solicitations with Effect

   SEND counters the threat of solicitations with effect in the
   following ways:



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   1.  As discussed in Section 5, SEND nodes preferably send Router
       Solicitations with a CGA address and a Signature option, which
       the router can verify, so 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 RFC 2461.

   See Section 11.2.5, below, for discussion about replay protection and
   timestamps.

11.2.1.2 Address Resolution

   SEND counters attacks on address resolution by requiring that the
   responding node include a signature option on the packet, and that
   the node's interface identifier either be a CGA, or that the node be
   able to produce a certificate authorizing that node to use the public
   key.

   The Neighbor Solicitation and Advertisement pairs implement a
   challenge-response protocol, as explained in Section 7 and discussed
   in Section 11.2.5 below.

11.2.2 Neighbor Unreachability Detection Failure

   This attack is described in Section 4.1.2 of [27].  SEND counters
   this attack by requiring a node responding to Neighbor Solicitations
   sent as NUD probes to include a 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.

11.2.3 Duplicate Address Detection DoS Attack

   This attack is described in Section 4.1.3 of [27].  SEND counters
   this attack by requiring the Neighbor Advertisements sent as
   responses to DAD to include a 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 is used on a link that also connects to non-SEND
   nodes, the SEND node ignores any insecure Neighbor Solicitations or
   Advertisements that may be send by the non-SEND nodes.  This protects
   the SEND node from DAD DoS attacks by non-SEND nodes or attackers
   simulating to non-SEND nodes, at the cost of a potential address
   collision between a SEND node and non-SEND node.  The probability and
   effects of such an address collision are discussed in [26].




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11.2.4 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 [27].  SEND counters these attacks by requiring Router
   Advertisements to contain a Signature option, and that the signature
   is calculated using the public key of a node that can 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 the indication that
   the router is allowed to route any prefix. A Router Advertisement
   without these protections is dropped.

   SEND does not protect against brute force attacks on the router, such
   as DoS attacks, or compromise of the router, as described in Sections
   4.4.2 and 4.4.3 of [27].

11.2.5 Replay Attacks

   This attack is described in Section 4.3.1 of [27].  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 the
   advertisement to 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.  A window of vulnerability for replay attacks
   exists until the timestamp expires.

   When timestamps are used, SEND nodes are 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.
   Since most SEND nodes are likely to use fairly coarse grained
   timestamps, as explained in Section 5.4.1, this may affect some
   nodes.

11.2.6 Neighbor Discovery DoS Attack

   This attack is described in Section 4.3.2 of [27].  In this attack,
   the attacker bombards the router with packets for fictitious
   addresses on the link, causing the router to busy itself with
   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



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   cache management. These are all techniques involved in implementing
   Neighbor Discovery on the router.

11.3 Attacks against SEND Itself

   The CGAs have a 59-bit hash value. The security of the CGA mechanism
   has been discussed in [26].

   Some Denial-of-Service attacks against NDP and SEND itself remain.
   For instance, an attacker may try to produce a very high number of
   packets that a victim host or router has to verify using asymmetric
   methods.  While 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 using
   the verification process described in Section 5.3.2. In this process,
   a simple hash verification of the CGA property of the address is
   performed first before performing the more expensive signature
   verification.

   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 Signature option, and start selectively dropping
   packets if too many resources are spent. Implementations MAY also
   first drop 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 a large number of delegation chains to be discovered for
   different trust anchors.  Routers SHOULD defend against such attacks
   by caching discovered information (including negative responses) and
   by limiting the number of different discovery processes they engage
   in.

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









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12. 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 Delegation Chain Solicitation message, described in Section
      6.1.

   o  The Delegation Chain Advertisement message, described in Section
      6.2.

   This document defines six new Neighbor Discovery Protocol [6]
   options, which must be assigned Option Type values within the option
   numbering space for Neighbor Discovery Protocol messages:

   o  The Trust Anchor option, described in Section 6.3.

   o  The Certificate option, described in Section 6.4.

   o  The CGA option, described in Section 5.2.

   o  The Signature option, described in Section 5.3.

   o  The Timestamp option, described in Section 5.4.1.

   o  The Nonce option, described in Section 5.4.2.

   This document defines a new 128-bit CGA Message Type [26] value,
   0xXXXX XXXX XXXX XXXX XXXX XXXX XXXX XXXX (To be generated randomly).

   XXX: Use existing name spaces for these?

   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
   using standards action [5].

   Another new name space is allocated for the Cert Type field in the
   Certificate option. Future values of this field can be allocated
   using standards action [5].











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Normative References

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

   [2]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [3]   Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

   [4]   Piper, D., "The Internet IP Security Domain of Interpretation
         for ISAKMP", RFC 2407, November 1998.

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

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

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

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

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

   [10]  Bassham, L., Polk, W. and R. Housley, "Algorithms and
         Identifiers for the Internet X.509 Public Key Infrastructure
         Certificate and Certificate Revocation List (CRL) Profile", RFC
         3279, April 2002.

   [11]  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.

   [12]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
         Addressing Architecture", RFC 3513, April 2003.

   [13]  Lynn, C., "X.509 Extensions for IP Addresses and AS
         Identifiers", draft-ietf-pkix-x509-ipaddr-as-extn-02 (work in
         progress), September 2003.

   [14]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in



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         IPv6", draft-ietf-mobileip-ipv6-24 (work in progress), July
         2003.

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

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










































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Informative References

   [17]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
         792, September 1981.

   [18]  Plummer, D., "Ethernet Address Resolution Protocol: Or
         converting network protocol addresses to 48.bit Ethernet
         address for transmission on Ethernet hardware", STD 37, RFC
         826, November 1982.

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

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

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

   [22]  Arkko, J., "Effects of ICMPv6 on IKE",
         draft-arkko-icmpv6-ike-effects-02 (work in progress), March
         2003.

   [23]  Arkko, J., "Manual Configuration of Security Associations for
         IPv6 Neighbor  Discovery", draft-arkko-manual-icmpv6-sas-02
         (work in progress), March 2003.

   [24]  Droms, R., "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
         November 2002.

   [25]  Kent, S., "IP Encapsulating Security Payload (ESP)",
         draft-ietf-ipsec-esp-v3-06 (work in progress), July 2003.

   [26]  Aura, T., "Cryptographically Generated Addresses (CGA)",
         draft-ietf-send-cga-01 (work in progress), August 2003.

   [27]  Nikander, P., "IPv6 Neighbor Discovery trust models and
         threats", draft-ietf-send-psreq-03 (work in progress), April
         2003.

   [28]  Montenegro, G. and C. Castelluccia, "SUCV Identifiers and
         Addresses", draft-montenegro-sucv-03 (work in progress), July
         2002.

   [29]  International Organization for Standardization, "The Directory
         - Authentication Framework", ISO Standard X.509, 2000.




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   [30]  O'Shea, G. and M. Roe, "Child-proof Authentication for MIPv6",
         Computer Communications Review, April 2001.

   [31]  Nikander, P., "Denial-of-Service, Address Ownership, and Early
         Authentication in the IPv6 World", Proceedings of the Cambridge
         Security Protocols Workshop, April 2001.

   [32]  Arkko, J., Aura, T., Kempf, J., Mantyla, V., Nikander, P. and
         M. Roe, "Securing IPv6 Neighbor Discovery", Wireless Security
         Workshop, September 2002.

   [33]  Montenegro, G. and C. Castelluccia, "Statistically Unique and
         Cryptographically Verifiable (SUCV) Identifiers and Addresses",
         NDSS, February 2002.

   [34]  Institute of Electrical and Electronics Engineers, "Local and
         Metropolitan Area Networks: Port-Based Network Access Control",
         IEEE Standard 802.1X, September 2001.


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


   Bill Sommerfeld
   Sun Microsystems
   1 Network Drive UBUR02-212
   Burlington, MA  01803
   USA

   EMail: sommerfeld@east.sun.com




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   Brian Zill
   Microsoft

   USA

   EMail: bzill@microsoft.com


   Pekka Nikander
   Ericsson

   Jorvas  02420
   Finland

   EMail: Pekka.Nikander@nomadiclab.com




































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Appendix A. Contributors

   Tuomas Aura contributed the transition mechanism specification in
   Section 9.















































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Appendix B. IPR Considerations

   The optional CGA part of SEND uses public keys and hashes to prove
   address ownership. Several IPR claims have been made about such
   methods.














































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Appendix C. 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 he dangers of
   monocultural 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 clearly 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, inspite of the
       attack.

   From this point of view, it is clearly better to be very selective in
   how to throw out entries.  Reducing the timestamp Delta value is very
   discriminative against those nodess that have a large clock
   difference, while an attacker can reduce its clock difference into
   arbitrarily small.  Throwing out old entries just because their clock
   difference is large seems like a bad approach.

   A reasonable idea seems to be to have a separate cache space for new
   entries and old entries, and under an attack more eagerly drop new
   cache entries than old ones.  One could track traffic, and only allow
   those new entries that receive genuine traffic to be converted into
   old cache entries.  While such a scheme will 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 to trick the
   victim into promoting its cache entry to the old cache.  Hence, the
   node may be more intelligent in keeping its cache entries, and not
   just have a black/white old/new boundary.

   It also looks like a good idea to consider the sec parameter when
   forcing cache entries out, and let those entries with a larger sec a
   higher chance of staying in.










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Appendix D. Comparison to AH-Based Approach

   This approach has the following benefits compared to the previous
   Working Group document approach:

   o  The full implementation of the security mechanism, including
      Nonces and CGAs, exists within one module.  There is no need to
      analyze the security of the mechanism across NDP, IPsec, and CGA
      layers.

   o  The CGA part of the solution has been separated into its own
      specification.  This is possible because the CGA handling is done
      in its own option.  (The authorization method configuration flag
      is the only thing common to the CGA and Signature options.)

   o  No extensions or modifications of IPsec processing are required:
      SPD entries are not required to distinguish ICMP types, AH does
      not need to support public keys or CGAs, and destination address
      acgnostic security associations are not needed.

   o  It is not necessary to allocate a new multicast address to
      represent the Solicited-Node multicast address for SEND nodes.

   o  It is not necessary to change the Neighbor Discovery behavior with
      regards to the use of the unspecified address.  Since all
      information is available within the Neighbor Discovery messages,
      unspecified source addresses can be used, still being able to
      correlate the CGA property with the Target Address in a Neighbor
      Solicitation during Duplicate Address Detection.

   o  The transition mechanisms for links with both SEND and non-SEND
      nodes are significantly simpler.  In particular, non-SEND nodes
      will be able to receive DAD probes and other messages sent by the
      SEND nodes.

   o  Only a single set of Neighbor Discovery messages from the router
      needs to be transmitted on a link.  This helps avoid extra
      overhead for mobility beacons and other frequently occurring
      messaging.

   o  Given that the asymmetric computations required in SEND are
      computationally expensive, it is necessary to control the number
      of these operations in order to avoid Denial-of-Service attacks.
      This control is easier to arrange with "application layer"
      information.  For instance, a router need not verify more Router
      Solicitations with an unspecified source address than it can
      respond to according to the RFC 2461 rules.




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   o  There is no need for an API to communicate certificate chains
      requests and certificate chains between the IPsec and Neighbor
      Discovery modules.

      Also, a good implementation of SEND would not require the user to
      configure it (beyond perhaps enabling it).  In order to achieve
      this with IPsec, a set of policy entries needs to be automatically
      created upon system start.

   o  There is no need for the CGA parameters to be stored both in the
      IPsec and Neighbor Discovery modules, where they are needed for
      the construction of Authentication Headers and addresses,
      respectively.

   o  It is not necessary to change existing BITS or BITW IPsec
      implementations to support SEND and AH_RSA_Sig.  There would have
      been two problems associated with such changes:

      *  A SEND implementation in such environment could not proceed
         until this modification were completed.

      *  Typical hardware that processes IPsec packets may not be easily
         changed to process asymmetric transforms.  (Of course, such
         packets can be passed to the main CPU at the node, assuming
         this can easily be done in the given implementation.)

   o  In addition, many IPsec implementations are highly optimized
      because they are on the fast path for packet processing.  For
      example, the Linux implementation runs in the kernel interrupt
      thread. Some of the SEND modifications might have required IPsec
      processing to wait on a semaphore while, for example, a
      certificate chain is fetched, an operation that takes place out of
      band in regular IPsec processing because it is done using IKE.
      While it might have been possible that the implemenation could
      have been arranged so that general IPsec processing wasqn't
      impacted, the resulting code would have been more complex.

   The use of IPsec to protect NDP would have been possible, but the
   limits and capabilities of IPsec would have to be stretched. Small
   changes in the NDP protocol (or our understanding of the issues)
   might have caused a situation which had no longer been easily handled
   when the "application" and the security existed at different layers.
   Although IPsec as defined in RFC 2402 just defines a header format,
   RFC 2401 and the ensuing years of implementation have evolved a
   complex interconnected set of components for IPsec which would have
   required some modification to accommodate SEND.

   On the other hand, IPsec is the current solution for securing NDP in



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   the original NDP RFCs.  Even if the current IPsec can be used only in
   very limited networks to secure NDP, it could have been argued that
   it would have been logical to continue its use. Also, the existence
   of an asymmetric transform in IPsec would have been potentially
   useful in other contexts as well.














































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

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











































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