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rfc:rfc5535

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
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 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

Bagnulo Standards Track [Page 1] RFC 5535 HBA June 2009

 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

Bagnulo Standards Track [Page 2] RFC 5535 HBA June 2009

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.

Bagnulo Standards Track [Page 3] RFC 5535 HBA June 2009

 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:

Bagnulo Standards Track [Page 4] RFC 5535 HBA June 2009

 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.

Bagnulo Standards Track [Page 5] RFC 5535 HBA June 2009

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

Bagnulo Standards Track [Page 6] RFC 5535 HBA June 2009

 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:
  1. 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.
  1. 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).
  1. 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.

Bagnulo Standards Track [Page 7] RFC 5535 HBA June 2009

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.

Bagnulo Standards Track [Page 8] RFC 5535 HBA June 2009

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

Bagnulo Standards Track [Page 9] RFC 5535 HBA June 2009

 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].

Bagnulo Standards Track [Page 10] RFC 5535 HBA June 2009

    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

Bagnulo Standards Track [Page 11] RFC 5535 HBA June 2009

 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.

Bagnulo Standards Track [Page 12] RFC 5535 HBA June 2009

    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.

Bagnulo Standards Track [Page 13] RFC 5535 HBA June 2009

                +-------+
                | 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.

Bagnulo Standards Track [Page 14] RFC 5535 HBA June 2009

 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

Bagnulo Standards Track [Page 15] RFC 5535 HBA June 2009

 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

Bagnulo Standards Track [Page 16] RFC 5535 HBA June 2009

 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

Bagnulo Standards Track [Page 17] RFC 5535 HBA June 2009

 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.

Bagnulo Standards Track [Page 18] RFC 5535 HBA June 2009

 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].

Bagnulo Standards Track [Page 19] RFC 5535 HBA June 2009

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:

Bagnulo Standards Track [Page 20] RFC 5535 HBA June 2009

 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,

Bagnulo Standards Track [Page 21] RFC 5535 HBA June 2009

 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

Bagnulo Standards Track [Page 22] RFC 5535 HBA June 2009

 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.

Bagnulo Standards Track [Page 23] RFC 5535 HBA June 2009

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.

Bagnulo Standards Track [Page 24] RFC 5535 HBA June 2009

 [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

Bagnulo Standards Track [Page 25]

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