





Network Working Group                                         M. Bagnulo
Request for Comments: 5535                                          UC3M
Category: Standards Track                                      June 2009


                       Hash-Based Addresses (HBA)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Abstract

   This memo describes a mechanism to provide a secure binding between
   the multiple addresses with different prefixes available to a host
   within a multihomed site.  This mechanism employs either
   Cryptographically Generated Addresses (CGAs) or a new variant of the
   same theme that uses the same format in the addresses.  The main idea
   in the new variant is that information about the multiple prefixes is
   included within the addresses themselves.  This is achieved by
   generating the interface identifiers of the addresses of a host as



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   hashes of the available prefixes and a random number.  Then, the
   multiple addresses are generated by prepending the different prefixes
   to the generated interface identifiers.  The result is a set of
   addresses, called Hash-Based Addresses (HBAs), that are inherently
   bound to each other.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Overview ........................................................4
      3.1. Threat Model ...............................................4
      3.2. Overview ...................................................4
      3.3. Motivations for the HBA Design .............................5
   4. Cryptographic Generated Addresses (CGAs) Compatibility
      Considerations ..................................................6
   5. Multi-Prefix Extension for CGA ..................................8
   6. HBA-Set Generation ..............................................9
   7. HBA Verification ...............................................11
      7.1. Verification That a Particular HBA Address
           Corresponds to a Given CGA Parameter Data Structure .......11
      7.2. Verification That a Particular HBA Address Belongs to the
           HBA Set Associated with a Given CGA Parameter Data
           Structure .................................................11
   8. Example of HBA Application in a Multihoming Scenario ...........13
      8.1. Dynamic Address Set Support ...............................16
   9. DNS Considerations .............................................17
   10. IANA Considerations ...........................................18
   11. Security Considerations .......................................18
      11.1. Security Considerations When Using HBAs in the
            Shim6 Protocol ...........................................20
      11.2. Privacy Considerations ...................................22
      11.3. SHA-1 Dependency Considerations ..........................22
      11.4. DoS Attack Considerations ................................22
   12. Contributors ..................................................23
   13. Acknowledgments ...............................................23
   14. References ....................................................24
      14.1. Normative References .....................................24
      14.2. Informative References ...................................24












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1.  Introduction

   In order to preserve inter-domain routing system scalability, IPv6
   sites obtain addresses from their Internet Service Providers (ISPs).
   Such an addressing strategy significantly reduces the amount of
   routes in the global routing tables, since each ISP only announces
   routes to its own address blocks, rather than announcing one route
   per customer site.  However, this addressing scheme implies that
   multihomed sites will obtain multiple prefixes, one per ISP.
   Moreover, since each ISP only announces its own address block, a
   multihomed site will be reachable through a given ISP if the ISP
   prefix is contained in the destination address of the packets.  This
   means that, if an established communication needs to be routed
   through different ISPs during its lifetime, addresses with different
   prefixes will have to be used.  Changing the address used to carry
   packets of an established communication exposes the communication to
   numerous attacks, as described in [11], so security mechanisms are
   required to provide the required protection to the involved parties.
   This memo describes a tool that can be used to provide protection
   against some of the potential attacks, in particular against future/
   premeditated attacks (aka time shifting attacks in [12]).

   This memo describes a mechanism to provide a secure binding between
   the multiple addresses with different prefixes available to a host
   within a multihomed site.

   It should be noted that, as opposed to the mobility case where the
   addresses that will be used by the mobile node are not known a
   priori, the multiple addresses available to a host within the
   multihomed site are pre-defined and known in advance in most of the
   cases.  The mechanism proposed in this memo employs either
   Cryptographically Generated Addresses (CGAs) [2] or a new variant of
   the same theme that uses the same format in the addresses.  The new
   variant, Hash-Based Address (HBA), takes advantage of the address set
   stability.  In either case, a secure binding between the addresses of
   a node in a multihomed site can be provided.  CGAs employ public key
   cryptography and can deal with changing address sets.  HBAs employ
   only symmetric key cryptography, and have smaller computational
   requirements.

   For the purposes of the Shim6 protocol, the other characteristics of
   the CGAs and HBAs are similar.  Both can be generated by the host
   itself without any reliance on external infrastructure.  Both employ
   the same format of addresses and same format of data fed to generate
   the addresses.  It is not required that all interface identifiers of
   a node's addresses be equal, preserving some degree of privacy
   through changes in the addresses used during the communications.




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   The main idea in HBAs is that information about the multiple prefixes
   is included within the addresses themselves.  This is achieved by
   generating the interface identifiers of the addresses of a host as
   hashes of the available prefixes and a random number.  Then, the
   multiple addresses are obtained by prepending the different prefixes
   to the generated interface identifiers.  The result is a set of
   addresses that are inherently bound.  A cost-efficient mechanism is
   available to determine if two addresses belong to the same set, since
   given the prefix set and the additional parameters used to generate
   the HBA, a single hash operation is enough to verify if an HBA
   belongs to a given HBA set.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

3.  Overview

3.1.  Threat Model

   The threat analysis for the multihoming problem is described in [11].
   This analysis basically identifies attacks based on redirection of
   packets by a malicious attacker towards addresses that do not belong
   to the multihomed node.  There are essentially two types of
   redirection attacks: communication hijacking and flooding attacks.
   Communication hijacking attacks are about an attacker stealing on-
   going and/or future communications from a victim.  Flooding attacks
   are about redirecting the traffic generated by a legitimate source
   towards a third party, flooding it.  The HBA solution provides full
   protection against the communication hijacking attacks.  The Shim6
   protocol [9] protects against flooding attacks.  Residual threats are
   described in the "Security Considerations" section.

3.2.  Overview

   The basic goal of the HBA mechanism is to securely bind together
   multiple IPv6 addresses that belong to the same multihomed host.
   This allows rerouting of traffic without worrying that the
   communication is being redirected to an attacker.  The technique that
   is used is to include a hash of the permitted prefixes in the
   low-order bits of the IPv6 address.

   So, eliding some details, say the available prefixes are A, B, C, and
   D, the host would generate a prefix list P consisting of (A,B,C,D)
   and a random number called Modifier M.  Then it would generate the
   new addresses:



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   A || H(M || A || P)

   B || H(M || B || P)

   C || H(M || C || P)

   D || H(M || D || P)

   Thus, given one valid address out of the group and the prefix list P
   and the random Modifier M it is possible to determine whether another
   address is part of the group by computing the hash and checking
   against the low-order bits.

3.3.  Motivations for the HBA Design

   The design of the HBA technique was driven by the following
   considerations:

   First of all, the goal of HBA is to provide a secure binding between
   the IPv6 address used as an identifier by the upper-layer protocols
   and the alternative locators available in the multihomed node so that
   redirection attacks are prevented.

   Second, in order to achieve such protection, the selected approach
   was to include security information in the identifier itself, instead
   of relying on third trusted parties to secure the binding, such as
   the ones based on repositories or Public Key Infrastructure.  This
   decision was driven by deployment considerations, i.e., the cost of
   deploying the trusted third-party infrastructure.

   Third, application support considerations described in [16] resulted
   in selecting routable IPv6 addresses to be used as identifiers.
   Hence, security information is stuffed within the interface
   identifier part of the IPv6 address.

   Fourth, performance considerations as described in [17] motivated the
   usage of a hash-based approach as opposed to a public-key-based
   approach based on pure Cryptographic Generated Addresses (CGA), in
   order to avoid imposing the performance of public key operations for
   every communication in multihomed environments.  The HBA approach
   presented in this document presents a cheaper alternative that is
   attractive to many common usage cases.  Note that the HBA approach
   and the CGA approaches are not mutually exclusive and that it is
   possible to generate addresses that are both valid CGA and HBA
   addresses providing the benefits of both approaches if needed.






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4.  Cryptographic Generated Addresses (CGAs) Compatibility
    Considerations

   As described in the previous section, the HBA technique uses the
   interface identifier part of the IPv6 address to encode information
   about the multiple prefixes available to a multihomed host.  However,
   the interface identifier is also used to carry cryptographic
   information when Cryptographic Generated Addresses (CGAs) [2] are
   used.  Therefore, conflicting usages of the interface identifier bits
   may result if this is not taken into account during the HBA design.
   There are at least two valid reasons to provide CGA-HBA
   compatibility:

   First, the current Secure Neighbor Discovery (SeND) specification [3]
   uses the CGAs defined in [2] to prove address ownership.  If HBAs are
   not compatible with CGAs, then nodes using HBAs for multihoming
   wouldn't be able to do Secure Neighbor Discovery using the same
   addresses (at least the parts of SeND that require CGAs).  This would
   imply that nodes would have to choose between security (from SeND)
   and fault tolerance (from IPv6 multihoming support provided by the
   Shim6 protocol [9]).  In addition to SeND, there are other protocols
   that are considered to benefit from the advantages offered by the CGA
   scheme, such as mobility support protocols [13].  Those protocols
   could not be used with HBAs if HBAs are not compatible with CGAs.

   Second, CGAs provide additional features that cannot be achieved
   using only HBAs.  In particular, because of its own nature, the HBA
   technique only supports a predetermined prefix set that is known at
   the time of the generation of the HBA set.  No additions of new
   prefixes to this original set are supported after the HBA set
   generation.  In most of the cases relevant for site multihoming, this
   is not a problem because the prefix set available to a multihomed set
   is not very dynamic.  New prefixes may be added in a multihomed site
   when a new ISP is available, but the timing of those events are
   rarely in the same time scale as the lifetime of established
   communications.  It is then enough for many situations that the new
   prefix is not available for established communications and that only
   new communications benefit from it.  However, in the case that such
   functionality is required, it is possible to use CGAs to provide it.
   This approach clearly requires that HBA and CGA approaches be
   compatible.  If this is the case, it then would be possible to create
   HBA/CGA addresses that support CGA and HBA functionality
   simultaneously.  The inputs to the HBA/CGA generation process will be
   both a prefix set and a public key.  In this way, a node that has
   established a communication using one address of the CGA/HBA set can
   tell its peer to use the HBA verification when one of the addresses





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   of its HBA/CGA set is used as locator in the communication or to use
   CGA (public-/private-key-based) verification when a new address that
   does not belong to the HBA/CGA set is used as locator in the
   communication.

   So, because of the aforementioned reasons, it is a goal of the HBA
   design to define HBAs in such a way that they are compatible with
   CGAs as defined in [2] and their usages described in [3]
   (consequently, to understand the rest of this note, the reader should
   be familiar with the CGA specification defined in [2]).  This means
   that it must be possible to generate addresses that are both an HBA
   and a CGA, i.e., that the interface identifier contains cryptographic
   information of CGA and the prefix-set information of an HBA.  The CGA
   specification already considers the possibility of including
   additional information into the CGA generation process through the
   usage of Extension Fields in the CGA Parameter Data Structure.  It is
   then possible to define a Multi-Prefix extension for CGA so that the
   prefix set information is included in the interface identifier
   generation process.

   Even though a CGA compatible approach is adopted, it should be noted
   that HBAs and CGAs are different concepts.  In particular, the CGA is
   inherently bound to a public key, while an HBA is inherently bound to
   a prefix set.  This means that a public key is not required to
   generate an HBA-only address.  Because of that, we define three
   different types of addresses:

   -  CGA-only addresses:  These are addresses generated as specified in
      [2] without including the Multi-Prefix extension.  They are bound
      to a public key and to a single prefix (contained in the basic CGA
      Parameter Data Structure).  These addresses can be used for SeND
      [3]; if used for multihoming, their application will have to be
      based on the public key usage.

   -  CGA/HBA addresses:  These addresses are CGAs that include the
      Multi-Prefix extension in the CGA Parameter Data Structure used
      for their generation.  These addresses are bound to a public key
      and a prefix set and they provide both CGA and HBA
      functionalities.  They can be used for SeND as defined in [3] and
      for any usage defined for HBA (such as a Shim6 protocol).

   -  HBA-only addresses:  These addresses are bound to a prefix set but
      they are not bound to a public key.  Because HBAs are compatible
      with CGA, the CGA Parameter Data Structure will be used for their
      generation, but a random nonce will be included in the Public Key
      field instead of a public key.  These addresses can be used for
      HBA-based multihoming protocols, but they cannot be used for SeND.




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5.  Multi-Prefix Extension for CGA

   The Multi-Prefix extension has the following TLV format as defined in
   [8]:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Extension Type        |   Extension Data Length       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |P|                         Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                           Prefix[1]                           +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                           Prefix[2]                           +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                               .                               .
     .                               .                               .
     .                               .                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                           Prefix[n]                           +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Ext Type:  16-bit type identifier of the Multi-Prefix extension (see
      the "IANA Considerations" section).

   Ext Len:  16-bit unsigned integer.  Length of the Extension in
      octets, not including the first 4 octets.

   P flag:  Set if a public key is included in the Public Key field of
      the CGA Parameter Data Structure, reset otherwise.

   Reserved:  31-bit reserved field.  MUST be initialized to zero, and
      ignored upon receipt.

   Prefix[1...n]:  Vector of 64-bit prefixes, numbered 1 to n.









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6.  HBA-Set Generation

   The HBA generation process is based on the CGA generation process
   defined in Section 4 of [2].  The goal is to require the minimum
   amount of changes to the CGA generation process.  It should be noted
   that the following procedure is only valid for Sec values of 0, 1,
   and 2.  For other Sec values, RFC 4982 [10] has defined a CGA SEC
   registry that will contain the specifications used to generate CGAs.
   The generation procedures defined in such specifications must be used
   for Sec values other than 0, 1, or 2.

   The CGA generation process has three inputs: a 64-bit subnet prefix,
   a public key (encoded in DER as an ASN.1 structure of the type
   SubjectPublicKeyInfo), and the security parameter Sec.

   The main difference between the CGA generation and the HBA generation
   is that while a CGA can be generated independently, all the HBAs of a
   given HBA set have to be generated using the same parameters, which
   implies that the generation of the addresses of an HBA set will occur
   in a coordinated fashion.  In this memo, we will describe a mechanism
   to generate all the addresses of a given HBA set.  The generation
   process of each one of the HBA address of an HBA set will be heavily
   based in the CGA generation process defined in [2].  More precisely,
   the HBA set generation process will be defined as a sequence of
   lightly modified CGA generations.

   The changes required in the CGA generation process when generating a
   single HBA are the following: First, the Multi-Prefix extension has
   to be included in the CGA Parameter Data Structure.  Second, in the
   case that the address being generated is an HBA-only address, a
   random nonce will have to be used as input instead of a valid public
   key.  For backwards compatibility issues with pure CGAs, the random
   nonce MUST be encoded as a public key as defined in [2].  In
   particular, the random nonce MUST be formatted as a DER-encoded ASN.1
   structure of the type SubjectPublicKeyInfo, defined in the Internet
   X.509 certificate profile [5].  The algorithm identifier MUST be
   rsaEncryption, which is 1.2.840.113549.1.1.1, and the random nonce
   MUST be formatted by using the RSAPublicKey type as specified in
   Section 2.3.1 of RFC 3279 [4].  The random nonce length is 384 bits.

   The resulting HBA-set generation process is the following:

   The inputs to the HBA generation process are:

   o  A vector of n 64-bit prefixes,

   o  A Sec parameter, and




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   o  In the case of the generation of a set of HBA/CGA addresses, a
      public key is also provided as input (not required when generating
      HBA-only addresses).

   The output of the HBA generation process are:

   o  An HBA-set

   o  their respective CGA Parameter Data Structures

   The steps of the HBA-set generation process are:

   1. Multi-Prefix extension generation.  Generate the Multi-Prefix
      extension with the format defined in Section 5.  Include the
      vector of n 64-bit prefixes in the Prefix[1...n] fields.  The Ext
      Len field value is (n*8 + 4).  If a public key is provided, then
      the P flag is set to one.  Otherwise, the P flag is set to zero.

   2. Modifier generation.  Generate a Modifier as a random or
      pseudorandom 128-bit value.  If a public key has not been provided
      as an input, generate the Extended Modifier as a 384-bit random or
      pseudorandom value.  Encode the Extended Modifier value as an RSA
      key in a DER-encoded ASN.1 structure of the type
      SubjectPublicKeyInfo defined in the Internet X.509 certificate
      profile [5].

   3. Concatenate from left to right the Modifier, 9 zero octets, the
      encoded public key or the encoded Extended Modifier (if no public
      key was provided), and the Multi-Prefix extension.  Execute the
      SHA-1 algorithm on the concatenation.  Take the 112 leftmost bits
      of the SHA-1 hash value.  The result is Hash2.

   4. Compare the 16*Sec leftmost bits of Hash2 with zero.  If they are
      all zero (or if Sec=0), continue with step (5).  Otherwise,
      increment the Modifier by one and go back to step (3).

   5. Set the 8-bit collision count to zero.

   6. For i=1 to n (number of prefixes) do:

      6.1.  Concatenate from left to right the final Modifier value,
         Prefix[i], the collision count, the encoded public key or the
         encoded Extended Modifier (if no public key was provided), and
         the Multi-Prefix extension.  Execute the SHA-1 algorithm on the
         concatenation.  Take the 64 leftmost bits of the SHA-1 hash
         value.  The result is Hash1[i].





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      6.2.  Form an interface identifier from Hash1[i] by writing the
         value of Sec into the three leftmost bits and by setting bits 6
         and 7 (i.e., the "u" and "g" bits) both to zero.

      6.3.  Generate address HBA[i] by concatenating Prefix[i] and the
         64-bit interface identifier to form a 128-bit IPv6 address with
         the subnet prefix to the left and interface identifier to the
         right as in a standard IPv6 address [6].

      6.4.  Perform duplicate address detection if required.  If an
         address collision is detected, increment the collision count by
         one and go back to step (6).  However, after three collisions,
         stop and report the error.

      6.5.  Form the CGA Parameter Data Structure that corresponds to
         HBA[i] by concatenating from left to right the final Modifier
         value, Prefix[i], the final collision count value, the encoded
         public key or the encoded Extended Modifier, and the Multi-
         Prefix extension.

   Note: most of the steps of the process are taken from [2].

7.  HBA Verification

   The following procedure is only valid for Sec values of 0, 1, and 2.
   For other Sec values, RFC 4982 [10] has defined a CGA SEC registry
   that will contain the specifications used to verify CGAs.  The
   verification procedures defined in such specifications must be used
   for Sec values other than 0,1, or 2.

7.1.  Verification That a Particular HBA Address Corresponds to a Given
      CGA Parameter Data Structure

   HBAs are constructed as a CGA Extension, so a properly formatted HBA
   and its correspondent CGA Parameter Data Structure will successfully
   finish the verification process described in Section 5 of [2].  Such
   verification is useful when the goal is the verification of the
   binding between the public key and the HBA.

7.2.  Verification That a Particular HBA Address Belongs to the HBA Set
      Associated with a Given CGA Parameter Data Structure

   For multihoming applications, it is also relevant that the receiver
   of the HBA information verifies if a given HBA address belongs to a
   certain HBA set.  An HBA set is identified by a CGA Parameter Data
   structure that contains a Multi-Prefix extension.  So, the receiver
   needs to verify if a given HBA belongs to the HBA set defined by a
   CGA Parameter Data Structure.  It should be noted that the receiver



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   may need to verify if an HBA belongs to the HBA set defined by the
   CGA Parameter Data Structure of another HBA of the set.  If this is
   the case, HBAs will fail to pass the CGA verification process defined
   in [2], because the prefix included in the Subnet Prefix field of the
   CGA Parameter Data Structure will not match the prefix of the HBA
   that is being verified.  To verify if an HBA belongs to an HBA set
   associated with another HBA, verify that the HBA prefix is included
   in the prefix set defined in the Multi-Prefix extension, and if this
   is the case, then substitute the prefix included in the Subnet Prefix
   field by the prefix of the HBA, and then perform the CGA verification
   process defined in [2].

   So, the process to verify that an HBA belongs to an HBA set
   determined by a CGA Parameter Data Structure is called HBA
   verification and it is the following:

   The inputs to the HBA verification process are:

   o  An HBA

   o  A CGA Parameter Data Structure

   The steps of the HBA verification process are the following:

   1. Verify that the 64-bit HBA prefix is included in the prefix set of
      the Multi-Prefix extension.  If it is not included, the
      verification fails.  If it is included, replace the prefix
      contained in the Subnet Prefix field of the CGA Parameter Data
      Structure by the 64-bit HBA prefix.

   2. Run the verification process described in Section 5 of [2] with
      the HBA and the new CGA Parameters Data Structure (including the
      Multi-Prefix extension) as inputs.  The steps of the process are
      included below, extracted from [2]:

      2.1.  Check that the collision count in the CGA Parameter Data
         Structure is 0, 1, or 2.  The CGA verification fails if the
         collision count is out of the valid range.

      2.2.  Check that the subnet prefix in the CGA Parameter Data
         Structure is equal to the subnet prefix (i.e., the leftmost 64
         bits) of the address.  The CGA verification fails if the prefix
         values differ.  Note: This step always succeeds because of the
         action taken in step 1.







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      2.3.  Execute the SHA-1 algorithm on the CGA Parameter Data
         Structure.  Take the 64 leftmost bits of the SHA-1 hash value.
         The result is Hash1.

      2.4.  Compare Hash1 with the interface identifier (i.e., the
         rightmost 64 bits) of the address.  Differences in the three
         leftmost bits and in bits 6 and 7 (i.e., the "u" and "g" bits)
         are ignored.  If the 64-bit values differ (other than in the
         five ignored bits), the CGA verification fails.

      2.5.  Read the security parameter Sec from the three leftmost bits
         of the 64-bit interface identifier of the address.  (Sec is an
         unsigned 3-bit integer.)

      2.6.  Concatenate from left to right the Modifier, 9 zero octets,
         the public key, and any extension fields (in this case, the
         Multi-Prefix extension will be included, at least) that follow
         the public key in the CGA Parameter Data Structure.  Execute
         the SHA-1 algorithm on the concatenation.  Take the 112
         leftmost bits of the SHA-1 hash value.  The result is Hash2.

      2.7.  Compare the 16*Sec leftmost bits of Hash2 with zero.  If any
         one of them is non-zero, the CGA verification fails.
         Otherwise, the verification succeeds.  (If Sec=0, the CGA
         verification never fails at this step.)

8.  Example of HBA Application in a Multihoming Scenario

   In this section, we will describe a possible application of the HBA
   technique to IPv6 multihoming.

   We will consider the following scenario: a multihomed site obtains
   Internet connectivity through two providers: ISPA and ISPB.  Each
   provider has delegated a prefix to the multihomed site (PrefA::/nA
   and PrefB::/nb, respectively).  In order to benefit from multihoming,
   the hosts within the multihomed site will configure multiple IP
   addresses, one per available prefix.  The resulting configuration is
   depicted in the next figure.













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                  +-------+
                  | Host2 |
                  |IPHost2|
                  +-------+
                      |
                      |
                  (Internet)
                   /      \
                  /        \
            +------+      +------+
            | ISPA |      | ISPB |
            |      |      |      |
            +------+      +------+
               |             |
                \            /
                 \          /
            +---------------------+
            | multihomed site     |
            | PA::/nA             |
            | PB::/nB    +------+ |
            |            |Host1 | |
            |            +------+ |
            +---------------------+

   We assume that both Host1 and Host2 support the Shim6 protocol.

   Host2 is not located in a multihomed site, so there is no need for it
   to create HBAs (it must be able to verify them though, in order to
   support the Shim6 protocol, as we will describe next).

   Host1 is located in the multihomed site, so it will generate its
   addresses as HBAs.  In order to do that, it needs to execute the
   HBA-set generation process as detailed in Section 6 of this memo.
   The inputs of the HBA-set generation process will be: a prefix vector
   containing the two prefixes available in its link, i.e., PA:LA::/64
   and PB:LB::/64, a Sec parameter value, and optionally a public key.
   In this case, we will assume that a public key is provided so that we
   can also illustrate how a renumbering event can be supported when
   HBA/CGA addresses are used (see the sub-section referring to dynamic
   address set support).  So, after executing the HBA-set generation
   process, Host1 will have: an HBA-set consisting in two addresses,
   i.e., PA:LA:iidA and PB:LB:iidB with their respective CGA Parameter
   Data Structures, i.e., CGA_PDS_A and CGA_PDS_B.  Note that iidA and
   iidB are different but both contain information about the prefix set
   available in the multihomed site.






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   We will next consider a communication between Host1 and Host2.
   Assume that both ISPs of the multihomed site are working properly, so
   any of the available addresses in Host1 can be used for the
   communication.  Suppose then that the communication is established
   using PA:LA:iidA and IPHost2 for Host1 and Host2, respectively.  So
   far, no special Shim6 support has been required, and PA:LA:iidA is
   used as any other global IP address.

   Suppose that at a certain moment, one of the hosts involved in the
   communication decides that multihoming support is required in this
   communication (this basically means that one of the hosts involved in
   the communication desires enhanced fault-tolerance capabilities for
   this communication, so that if an outage occurs, the communication
   can be re-homed to an alternative provider).

   At this moment, the Shim6 protocol Host-Pair Context establishment
   exchange will be performed between the two hosts (see [9]).  In this
   exchange, Host1 will send CGA_PDS_A to Host2.

   After the reception of CGA_PDS_A, Host2 will verify that the received
   CGA Parameter Data Structure corresponds to the address being used in
   the communication PA:LA:iidA.  This means that Host2 will execute the
   HBA verification process described in Section 7 of this memo with PA:
   LA:iidA and CGA_PDS_A as inputs.  In this case, the verification will
   succeed since the CGA Parameter Data Structure and the addresses used
   in the verification match.

   As long as there are no outages affecting the communication path
   through ISPA, packets will continue flowing.  If a failure affects
   the path through ISPA, Host1 will attempt to re-home the
   communication to an alternative address, i.e., PB:LB:iidB.  In order
   to accomplish this, after detecting the outage, Host1 will inform
   Host2 about the alternative address.  Host2 will verify that the new
   address belongs to the HBA set of the initial address.  In order to
   accomplish this, Host2 will execute the HBA verification process with
   the CGA Parameter Data Structure of the original address (i.e.,
   CGA_PDS_A) and the new address (i.e., PB:LB:iidB) as inputs.  The
   verification process will succeed because PB:LB::/64 has been
   included in the Multi-Prefix extension during the HBA-set generation
   process.  Additional verifications may be required to prevent
   flooding attacks (see the comments about flooding attacks prevention
   in the Security Considerations section of this memo).

   Once the new address is verified, it can be used as an alternative
   locator to re-home the communication, while preserving the original
   address (PA:LA:iidA) as an identifier for the upper layers.  This





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   means that following packets will be addressed to/from this new
   address.  Note that no additional HBA verification is required for
   the following packets, since the new valid address can be stored in
   Host2.

   In this example, only the HBA capabilities of the Host1 addresses
   were used.  In other words, neither the public key included in the
   CGA Parameter Data Structure nor its correspondent private key was
   used in the protocol.  In the following section, we will consider a
   case where its usage is required.

8.1.  Dynamic Address Set Support

   In the previous section, we have presented the mechanisms that allow
   a host to use different addresses of a predetermined set to exchange
   packets of a communication.  The set of addresses involved was
   predetermined and known when the communication was initiated.  To
   achieve such functionality, only HBA functionalities of the addresses
   were needed.  In this section, we will explore the case where the
   goal is to exchange packets using additional addresses that were not
   known when the communication was established.  An example of such a
   situation is when a new prefix is available in a site after a
   renumbering event.  In this case, the hosts that have the new address
   available may want to use it in communications that were established
   before the renumbering event.  In this case, HBA functionalities of
   the addresses are not enough and CGA capabilities are to be used.

   Consider then the previous case of the communication between Host1
   and Host2.  Suppose that the communication is up and running, as
   described earlier.  Host1 is using PA:LA:iidA and Host2 is using
   IPHost2 to exchange packets.  Now suppose that a new address, PC:LC:
   addC is available in Host1.  Note that this address is just a regular
   IPv6 address, and it is neither an HBA nor a CGA.  Host1 wants to use
   this new address in the existent communication with Host2.  It should
   be noted that the HBA mechanism described in the previous section
   cannot be used to verify this new address, since this address does
   not belong to the HBA set (since the prefix was not available at the
   moment of the generation of the HBA set).  This means that
   alternative verification mechanisms will be needed.

   In order to verify this new address, CGA capabilities of PA:LA:iidA
   are used.  Note that the same address is used, only that the
   verification mechanism is different.  So, if Host1 wants to use PC:
   LC:addC to exchange packets in the established communication, it will
   use the UPDATE message defined in the Shim6 protocol [9], conveying
   the new address, PC:LC:addC, and this message will be signed using
   the private key corresponding to the public key contained in
   CGA_PDS_A.  When Host2 receives the message, it will verify the



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   signature using the public key contained in the CGA Parameter Data
   Structure associated with the address used for establishing the
   communication, i.e., CGA_PDS_A and PA:LA:iidA, respectively.  Once
   that the signature is verified, the new address (PC:LC:addC) can be
   used in the communication.

   In any case, a renumbering event has an impact on a site that is
   using the HBA technique.  In particular, the new prefix added will
   not be included in the existing HBA set, so it is only possible to
   use the new prefix with the existing HBA set if CGA capabilities are
   used.  While this is acceptable for the short term, in the long run,
   the site will need to renumber its HBA addresses.  In order to do
   that, it will need to re-generate the HBA sets assigned to hosts
   including the new prefix in the prefix set, which will result in
   different addresses, not only because we need to add a new address
   with the new prefix, but also because the addresses with the existing
   prefixes will also change because of the inclusion of a new prefix in
   the prefix set.  Moreover, since HBA addresses need to be generated
   locally, once these are generated after the renumbering event, the
   new address information needs to be conveyed to the DNS manager in
   case that such address information is to be published in the DNS (see
   DNS considerations section for more details).

9.  DNS Considerations

   HBA sets can be generated using any prefix set.  Actually, the only
   particularity of the HBA is that they contain information about the
   prefix set in the interface identifier part of the address in the
   form of a hash, but no assumption about the properties of prefixes
   used for the HBA generation is made.  This basically means that
   depending on the prefixes used for the HBA set generation, it may or
   may not be recommended to publish the resulting (HBA) addresses in
   the DNS.  For instance, when Unique Local Address (ULA) prefixes [18]
   are included in the HBA generation process, specific DNS
   considerations related to the local nature of the ULA should be taken
   into account and proper recommendations related to publishing such
   prefixes in the DNS should followed.  Moreover, among its addresses,
   a given host can have some HBAs and some other IPv6 addresses.  The
   consequence from this is that only HBA addresses will be bound
   together by the HBA technique, while other addresses would not be
   bound to the HBA set.  This would basically mean that if one of the
   other addresses is used for initiating a Shim6 communication, it
   won't be possible to use the HBA technique to bind the address used
   with the HBA set.  Furthermore, since HBA addresses are
   indistinguishable from other IPv6 addresses in their format, an
   initiator will not be able to distinguish, by merely looking at the





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   different addresses, which ones belong to the HBA set and which ones
   do not, so alternative means would be required the initiator is
   supposed to use only HBA for establishing communications in the
   presence of non-HBA addresses in the DNS.

   In addition, it should be noted that the actual HBA values are a
   result of the HBA generation procedure, meaning that they cannot be
   arbitrarily chosen.  This has an implication with respect to DNS
   management, because the party that generates the HBA address set
   needs to convey the address information to the DNS manager, so that
   the addresses are published and not the other way around.  The
   situation is similar to regular CGA addresses and even to the case
   where stateless address autoconfiguration is used.  In order to do
   that, it is possible to use Dynamic DNS updates [19] or other
   proprietary tools.  A similar consideration applies when the host
   wants to publish reverse-DNS entries.  Since the host needs to
   generate its HBA addresses, it will need to convey the address
   information to the DNS manager so the proper reverse-DNS entry is
   populated in case it is needed.  It should be noted that neither the
   Shim6 protocol nor the HBA technique rely on the reverse DNS for its
   proper functioning and the general reasons for requiring reverse-DNS
   population apply as for any other regular IPv6 address.

10.  IANA Considerations

   This document defines a new CGA Extension, the Multi-Prefix
   extension.  This extension has been assigned the CGA Extension Type
   value 0x0012.

11.  Security Considerations

   The goal of HBAs is to create a group of addresses that are securely
   bound, so that they can be used interchangeably when communicating
   with a node.  If there is no secure binding between the different
   addresses of a node, a number of attacks are enabled, as described in
   [11].  In particular, it would be possible for an attacker to
   redirect the communications of a victim to an address selected by the
   attacker, hijacking the communication.  When using HBAs, only the
   addresses belonging to an HBA set can be used interchangeably,
   limiting the addresses that can be used to redirect the communication
   to a predetermined set that belongs to the original node involved in
   the communication.  So, when using HBAs, a node that is communicating
   using address A can redirect the communication to a new address B if
   and only if B belongs to the same HBA set as A.







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   This means that if an attacker wants to redirect communications
   addressed to address HBA1 to an alternative address IPX, the attacker
   will need to create a CGA Parameter Data Structure that generates an
   HBA set that contains both HBA1 and IPX.

   In order to generate the required HBA set, the attacker needs to find
   a CGA Parameter Data Structure that fulfills the following
   conditions:

   o  the prefix of HBA1 and the prefix of IPX are included in the
      Multi-Prefix extension.

   o  HBA1 is included in the HBA set generated.

   Note: this assumes that it is acceptable for the attacker to redirect
   HBA1 to any address of the prefix of IPX.

   The remaining fields that can be changed at will by the attacker in
   order to meet the above conditions are: the Modifier, other prefixes
   in the Multi-Prefix extension, and other extensions.  In any case, in
   order to obtain the desired HBA set, the attacker will have to use a
   brute-force attack, which implies the generation of multiple HBA sets
   with different parameters (for instance with a different Modifier)
   until the desired conditions are meet.  The expected number of times
   that the generation process will have to be repeated until the
   desired HBA set is found is exponentially related with the number of
   bits containing hash information included in the interface identifier
   of the HBA.  Since 59 of the 64 bits of the interface identifier
   contain hash bits, then the expected number of generations that will
   have to be performed by the attacker are O(2^59).  Note: We assume
   brute force is the best attack against HBA/CGAs.  Also, note that the
   assumption that the Sec tool defined in [2] multiplies the attack
   factor holds for brute-force attacks but may not hold for other
   attack classes.

   The protection against brute-force attacks can be improved by
   increasing the Sec parameter.  A non-zero Sec parameter implies that
   steps 3-4 of the generation process will be repeated O(2^(16*Sec))
   times (expected number of times).  If we assimilate the cost of
   repeating the steps 3-4 to the cost of generating the HBA address, we
   can estimate the number of times that the generation is to be
   repeated in O(2^(59+16*Sec)), in the case of Sec values of 1 and 2.
   For other Sec values, Sec protection mechanisms will be defined by
   the specifications pointed by the CGA SEC registry defined in RFC
   4982 [10].






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11.1.  Security Considerations When Using HBAs in the Shim6 Protocol

   In this section, we will analyze the security provided by HBAs in the
   context of a Shim6 protocol as described in Section 8 of this memo.

   First of all, it must be noted that HBAs cannot prevent
   man-in-the-middle (hereafter MITM) attacks.  This means that in the
   scenario described in Section 8, if an attacker is located along the
   path between Host1 and Host2 during the lifetime of the
   communication, the attacker will be able to change the addresses used
   for the communication.  This means that he will be able to change the
   addresses used in the communication, adding or removing prefixes at
   his will.  However, the attacker must make sure that the CGA
   Parameter Data Structure and the HBA set is changed accordingly.
   This essentially means that the attacker will have to change the
   interface identifier part of the addresses involved, since a change
   in the prefix set will result in different interface identifiers of
   the addresses of the HBA set, unless the appropriate Modifier value
   is used (which would require O(2(59+16*Sec)) attempts).  So, HBA
   doesn't provide MITM attacks protection, but a MITM attacker will
   have to change the address used in the communication in order to
   change the prefix set valid for the communication.

   HBAs provide protection against time shifting attacks [11], [12].  In
   the multihoming context, an attacker would perform a time shifted
   attack in the following way: an attacker placed along the path of the
   communication will modify the packets to include an additional
   address as a valid address for the communication.  Then the attacker
   would leave the on-path location, but the effects of the attack would
   remain (i.e., the address would still be considered as a valid
   address for that communication).  Next we will present how HBAs can
   be used to prevent such attacks.

   If the attacker is not on-path when the initial CGA Parameter Data
   Structure is exchanged, his only possibility to launch a redirection
   attack is to fake the signature of the message for adding new
   addresses using CGA capabilities of the addresses.  This implies
   discovering the public key used in the CGA Parameter Data Structure
   and then cracking the key pair, which doesn't seem feasible.  So in
   order to launch a redirection attack, the attacker needs to be
   on-path when the CGA Parameter Data Structure is exchanged, so he can
   modify it.  Now, in order to launch the redirection attack, the
   attacker needs to add his own prefix in the prefix set of the CGA
   Parameter Data Structure.  We have seen in the previous section that
   there are two possible approaches for this:






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   1. Find the right Modifier value, so that the address initially used
      in the communication is contained in the new HBA set.  The cost of
      this attack is O(2(59+16*Sec)) iterations of the generation
      process, so it is deemed unfeasible.

   2. Use any Modifier value, so that the address initially used in the
      communication is probably not included in the HBA set.  In this
      case, the attacker must remain on-path, since he needs to rewrite
      the address carried in the packets (if not, the endpoints will
      notice a change in the address used in the communication).  This
      essentially means that the attacker cannot launch a time shifted
      attack, but he must be a full-time man-in-the-middle.

   So, the conclusion is that HBAs provide protection against time
   shifted attacks

   HBAs do not provide complete protection against flooding attacks,
   and, as a result, the SHIM6 protocol has other means to deal with
   them.  However, HBAs make it very difficult to launch a flooding
   attack towards a specific address.  It is possible though, to launch
   a flooding attack against a prefix.  And of course, the protection
   that HBA offers applies only to nodes that employ it; HBA provides no
   solution for general-purpose flooding-attack protection for other
   nodes.

   Suppose that an attacker has easy access to a prefix PX::/nX and that
   he wants to launch a flooding attack on a host located in the address
   P:iid.  The attack would consist of establishing communication with a
   server S and requesting a heavy flow from it.  Then simply
   redirecting the flow to P:iid, flooding the target.  In order to
   perform this attack, the attacker needs to generate an HBA set
   including P and PX in the prefix set, and be sure that the resulting
   HBA set contains P:iid.  In order to do this, the attacker needs to
   find the appropriate Modifier value.  The expected number of attempts
   required to find such Modifier value is O(2(59+16*Sec)), as presented
   earlier.  So, we can conclude that such attack is not feasible.

   However, the target of a flooding attack is not limited to specific
   hosts, but it can also be launched against other elements of the
   infrastructure, such as router or access links.  In order to do that,
   the attacker can establish a communication with a server S and
   request a download of a heavy flow.  Then, the attacker redirects the
   communication to any address of the target network.  Even if the
   target address is not assigned to any host, the flow will flood the
   access link of the target site, and the site access router will also
   suffer the overload.  Such attack cannot be prevented using HBAs,





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   since the attacker can easily generate an HBA set using his own
   prefix and the target network prefix.  In order to prevent such
   attacks, additional mechanisms are required, such as reachability
   tests.

11.2.  Privacy Considerations

   HBAs can be used as RFC 4941 [7] addresses.  If a node wants to use
   temporary addresses, it will need to periodically generate new HBA
   sets.  The effort required for this operation depends on the Sec
   parameter value.  If Sec=0, then the cost of generating a new HBA set
   is similar to the cost of generating a random number, i.e., one
   iteration of the HBA set generation procedure.  However, if Sec>0,
   then the cost of generating an HBA set is significantly increased,
   since it required O(2(16*Sec)) iterations of the generation process.
   In this case, depending on the frequency of address change required,
   the support for RFC 4941 address may be more expensive.

11.3.  SHA-1 Dependency Considerations

   Recent attacks on currently used hash functions have motivated a
   considerable amount of concern in the Internet community.  The
   recommended approach [14] [15] to deal with this issue is first to
   analyze the impact of these attacks on the different Internet
   protocols that use hash functions, and second to make sure that the
   different Internet protocols that use hash functions are capable of
   migrating to an alternative (more secure) hash function without a
   major disruption in the Internet operation.

   The aforementioned analysis for CGAs and their extensions (including
   HBAs) is performed in RFC 4982 [10].  The conclusion of the analysis
   is that the security of the protocols using CGAs and their extensions
   are not affected by the recently available attacks against hash
   functions.  In spite of that, the CGA specification [2] was updated
   by RFC 4982 [10] to enable the support of alternative hash functions.

11.4.  DoS Attack Considerations

   In order to use the HBA technique, the owner of the HBA set must
   inform its peer about the CGA Parameter Data Structure in order to
   allow the peer to verify that the different HBAs belong to the same
   HBA set.  Such information must then be stored by the peer to verify
   alternative addresses in the future.  This can be a vector for DoS
   attacks, since the peer must commit resources (in this particular
   case memory) to be able to use the HBA technique for address
   verification.  It is then possible for an attacker to launch a DoS
   attack by conveying HBA information to a victim, imposing on the
   victim to use memory for storing HBA related state, and eventually



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   running out of memory for other genuine operations.  In order to
   prevent such an attack, protocols that use the HBA technique should
   implement proper DoS prevention techniques.

   For instance, the Shim6 protocol [9] includes a 4-way handshake to
   establish the Shim6 context and, in particular, to establish the HBA-
   related state.  In this 4-way handshake, the receiver remains
   stateless during the first 2 messages, while the initiator must keep
   state throughout the exchange of the 4 messages so that the cost of
   the context establishment is higher in memory terms for the initiator
   (i.e., the potential attacker) than for the receiver (i.e., the
   potential victim).  In addition to that, the 4-way handshake prevents
   the usage of spoofed addresses from off-path attacker, since the
   initiator must be able to receive information through the address it
   has used as source address, enabling the tracking of the location
   from which the attack was launched.

12.  Contributors

   This document was originally produced by a MULTI6 design team
   consisting of (in alphabetical order): Jari Arkko, Marcelo Bagnulo,
   Iljitsch van Beijnum, Geoff Huston, Erik Nordmark, Margaret
   Wasserman, and Jukka Ylitalo.

13.  Acknowledgments

   The initial discussion about HBA benefited from contributions from
   Alberto Garcia-Martinez, Tuomas Aura, and Arturo Azcorra.

   The HBA-set generation and HBA verification processes described in
   this document contain several steps extracted from [2].

   Jari Arkko, Matthew Ford, Francis Dupont, Mohan Parthasarathy, Pekka
   Savola, Brian Carpenter, Eric Rescorla, Robin Whittle, Matthijs
   Mekking, Hannes Tschofenig, Spencer Dawkins, Lars Eggert, Tim Polk,
   Peter Koch, Niclas Comstedt, David Ward, and Sam Hartman have
   reviewed this document and provided valuable comments.

   The text included in Section 3.2 was provided by Eric Rescorla.

   The author would also like to thank Francis Dupont for providing the
   first implementation of HBA.









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14.  References

14.1.  Normative References

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

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

   [3]   Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
         Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [4]   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.

   [5]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley,
         R., and W. Polk, "Internet X.509 Public Key Infrastructure
         Certificate and Certificate Revocation List (CRL) Profile",
         RFC 5280, May 2008.

   [6]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 4291, February 2006.

   [7]   Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions
         for Stateless Address Autoconfiguration in IPv6", RFC 4941,
         September 2007.

   [8]   Bagnulo, M. and J. Arkko, "Cryptographically Generated
         Addresses (CGA) Extension Field Format", RFC 4581,
         October 2006.

   [9]   Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming Shim
         Protocol for IPv6", RFC 5533, June 2009.

   [10]  Bagnulo, M. and J. Arkko, "Support for Multiple Hash Algorithms
         in Cryptographically Generated Addresses (CGAs)", RFC 4982,
         July 2007.

14.2.  Informative References

   [11]  Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming
         Solutions", RFC 4218, October 2005.






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   [12]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
         Nordmark, "Mobile IP Version 6 Route Optimization Security
         Design Background", RFC 4225, December 2005.

   [13]  Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
         Optimization for Mobile IPv6", RFC 4866, May 2007.

   [14]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes
         in Internet Protocols", RFC 4270, November 2005.

   [15]  Bellovin, S. and E. Rescorla, "Deploying a New Hash Algorithm",
         2005 September.

   [16]  Nordmark, E., "Multi6 Application Referral Issues", Work
         in Progress, October 2004.

   [17]  Bagnulo, M., Garcia-Martinez, A., and A. Azcorra, "Efficient
         Security for IPv6 Multihoming", ACM Computer Communications
         Review Vol 35 n 2, April 2005.

   [18]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
         Addresses", RFC 4193, October 2005.

   [19]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
         Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
         April 1997.

Author's Address

   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911
   SPAIN

   Phone: 34 91 6249500
   EMail: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es













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