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Versions: 00 01 02 03 04 05 RFC 5535

Network Working Group                                         M. Bagnulo
Internet-Draft                                                      UC3M
Expires: April 26, 2006                                 October 23, 2005


                       Hash Based Addresses (HBA)
                        draft-ietf-shim6-hba-01

Status of this Memo

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   This Internet-Draft will expire on April 26, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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.  The main idea 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 generated by prepending the
   different prefixes to the generated interface identifiers.  The
   result is a set of addresses, called Hash Based Addresses (HBAs),



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   that are inherently bound.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  CGA compatibility considerations . . . . . . . . . . . . . . .  4
   3.  Multi-Prefix Extension for CGA . . . . . . . . . . . . . . . .  6
   4.  HBA-Set Generation . . . . . . . . . . . . . . . . . . . . . .  7
   5.  HBA verification . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Example of HBA application to a multihoming scenario . . . . . 11
     6.1.  Dynamic Address Set Support  . . . . . . . . . . . . . . . 13
   7.  DNS considerations . . . . . . . . . . . . . . . . . . . . . . 14
   8.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Security considerations  . . . . . . . . . . . . . . . . . . . 14
     9.1.  Security considerations when using HBAs in the shim6
           protocol . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.2.  Privacy Considerations . . . . . . . . . . . . . . . . . . 18
     9.3.  Interaction with IPSec.  . . . . . . . . . . . . . . . . . 18
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     12.1. Changes from draft-ietf-shim6-hba-00 to
           draft-ietf-multi6-hba-01 . . . . . . . . . . . . . . . . . 19
     12.2. Changes from draft-ietf-multi6-hba-00 to
           draft-ietf-shim6-hba-00  . . . . . . . . . . . . . . . . . 19
     12.3. Changes from draft-bagnulo-multi6dt-hba-00 to
           draft-ietf-multi6-hba-00 . . . . . . . . . . . . . . . . . 19
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     13.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22


















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1.  Introduction

   In order to preserve inter-domain routing system scalability, IPv6
   sites obtain addresses from their Internet Service Providers.  Such
   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 [8], 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 (a.k.a. time shifting attacks in [9]).

   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 takes advantage of this
   address set stability, and provides a secure binding between all the
   addresses of a node in a multihomed site.  The mechanism does so
   without requiring the usage of public key cryptography, providing a
   cost efficient alternative to public key cryptography based schemes.

   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.  The main idea 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, called Hash Based Addresses (HBAs),
   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.  No public key operations are involved in the
   verification process.  In addition, it should also be noted that it
   is not required that all interface identifiers of the addresses of an



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   HBA set are equal, preserving some degree of privacy through changes
   in the addresses used during the communications.


2.  CGA compatibility considerations

   As described in 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 [1] 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 specification [2] uses
   the CGAs defined in [1] 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 shim6).  In addition to SeND, there are other
   protocols that are considering to benefit from the advantages offered
   by the CGA scheme, such as mobility support protocols [10].  Those
   protocols would also become incompatible 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 than 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 are
   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



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   tell its peer to use the HBA verification when one of the addresses
   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 a way that they are compatible with CGAs as
   defined in [1] and their usages described in [2] (Consequently, to
   understand the rest of this note, the reader should be familiar with
   the CGA specification defined in [1]).  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 a HBA is inherently bound to
   a prefix set.  This means that a public key is not strictly required
   to generate an HBA.  Because of that, we define three different types
   of addresses:

   - CGA-only addresses: These are addresses generated as specified in
      [1] 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
      [2] and 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 Parameters 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 [2] 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 CGA compatibility,
      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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3.  Multi-Prefix Extension for CGA

   The Multi-Prefix Extension has the following TLV format as defined in
   [6] :


      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 (TBD
      IANA) (Meanwhile, the 0x12 value is recommended for trails)

   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 if a additional Modifier
      bits are included in the CGA Parameter Data Structure.

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








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


4.  HBA-Set Generation

   The HBA generation process is based on the CGA generation process
   defined in section 4 of [1].  The goal is to require the minimum
   amount of changes to the CGA generation process.

   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 [1].  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 Parameters Data Structure.  Second, in the
   case that the address being generated is an HBA-only address, a
   random nonce (encoded in DER as an ASN.1 structure of the type
   SubjectPublicKeyInfo) will have to be used as input instead of a
   valid public key.

   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
   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 Parameters Data Structures

   The steps of the HBA-set generation process are:





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   1. Multi-Prefix Extension generation.  Generate the Multi-Prefix
      Extension with the format defined in section 3.  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.  Otherwise, the P flag is reset.

   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 a RSA
      key in a DER-encoded ASN.1 structure of the type
      SubjectPublicKeyInfo defined in the Internet X.509 certificate
      profile [3].

   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 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].

      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 [4].








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      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 Parameters 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 [1]]


5.  HBA verification

   HBAs are constructed as a CGA Extension, so a properly formated HBA
   and its correspondent CGA Parameter Data Structure will successfully
   finish the verification process described in section 5 of [1].  Such
   verification is useful when the goal is the verification of the
   binding between the public key and the HBA.

   However, for multihoming applications, it is also relevant to verify
   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, it is then needed to verify if an HBA belongs
   to the HBA set defined by a CGA Parameter Data Structure.  It should
   be noted that it may be needed 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, the CGA verification process as defined in
   [1] will fail, because the prefix included in the Subnet Prefix field
   of the CGA Parameter Data Structure will not match with the one of
   the HBA that is being verified.  However, this not means that this
   HBA does not belong to the HBA set.  In order to address this issue,
   it is only required to verify that the HBA prefix is included in
   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 [1].

   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:





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   o  An HBA
   o  An 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 [1] 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 [1]

      2.1. Check that the collision count in the CGA Parameters 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 Parameters 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 is trivially successful
         because step 1]

      2.3. Execute the SHA-1 algorithm on the CGA Parameters 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,
         and 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 Parameters 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.





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      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.)



6.  Example of HBA application to a multihoming scenario

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

   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 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.



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





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   We assume that both Host1 and Host2 support the shim6 protocol.

   Host2 in not located in a multihomed site, so there is no need for
   him 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 4 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.

   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 rehomed to an alternative provider).

   At this moment, the shim6 protocol Host-Pair Context establishment
   exchange will be perfomed between the two hosts (see [7].).  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 5 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.



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   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.  For that,
   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.  For that, 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) ad identifier for the upper layers.  This 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.

   Eventually, the communication will end and the associated shim6
   context information will be discarded.

   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.

6.1.  Dynamic Address Set Support

   In the previous section we have presented the mechanisms that allow a
   host to use different addresses of a pre-determined set to exchange
   packets of a communication.  The set of addresses involved was pre-
   determined 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 situation
   is for instance 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.




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   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 message of the shim6 protocol, 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 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.


7.  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.


8.  IANA considerations

   This document defines a new CGA Extension, the Multi-Prefix
   Extension.  This extension has been assigned the CGA Extension Type
   value TBD (IANA).


9.  Security considerations




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   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
   [8].  It particular, it would 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 well, pre-determined 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 than A.

   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 Parameters 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.

   (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).

   The protection against brute force attacks can be improved 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



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   estimate the number of times that the generation is to be repeated in
   O(2^(59+16*Sec)).

9.1.  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 6 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 6, 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
   don'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 [8], [9].  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 Strcutre.  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,
   However, HBAs make very difficult to launch a flooding attack towards
   a specific address.  It is possible though, to launch a flooding
   attack against a prefix.

   Suppose that an attacker has easy access to a prefix PX::/nX and that
   he wants to launch a flooding attack to a host located in the address
   P:iid.  The attack would consist in establishing a communication with
   a server S and requesting a heavy flow from it.  Then simply redirect
   the flow to P:iid, flooding the target.  In order to perform this
   attack the attacker need to generate an HBA set including P and PX in
   the prefix set and that the resulting HBA set contains P:iid.  In
   order to do this, the attacker need 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 element 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,
   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.






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9.2.  Privacy Considerations

   HBAs can be used as RFC 3041 [5]. 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 3041 address may be more expensive.

9.3.  Interaction with IPSec.

   In the case that both IPSec and CGA/HBA address are used
   simultaneously, it is possible that two public keys are available in
   a node, one for IPSec and another one for the CGA/HBA operation.  In
   this case, an improved security can be achieved by verifying that the
   keys are related somehow, (in particular if the same key is used for
   both purposes).


10.  Contributors

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


11.  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 [1].

   Jari Arkko, Matthew Ford, Francis Dupont, Mohan Parthasarathy, Pekka
   Savola and Brian Carpenter have reviewed this document and provided
   valuable comments.

   We would also like to thanks Francis Dupont (ENST) for providing the
   first implementation of HBA






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12.  Change Log

   To be removed prior publication

12.1.  Changes from draft-ietf-shim6-hba-00 to draft-ietf-multi6-hba-01

   Changed the format of the Multi-Prefix extension to make it compliant
   with the generic TLV format proposed for CGA extensions

   Added IANA considerations section

   Added DNS considerations section

12.2.  Changes from draft-ietf-multi6-hba-00 to draft-ietf-shim6-hba-00

   Editorial changes

12.3.  Changes from draft-bagnulo-multi6dt-hba-00 to
       draft-ietf-multi6-hba-00

   Added "Example of HBA application to a multihoming scenario" section

   Added Privacy Considerations section

   Added flooding attacks comments in the Security Considerations
   section

   Added the Multi-Prefix extension in step 6.1 of the HBA-set
   generation process

   Added the Security considerations when using HBAs in a multi6
   protocol sub-section in the Security Considerations section

   Added Ext type value recommended for trials

   Changed the name of the draft

   Some rewording


13.  References

13.1.  Normative References

   [1]  Aura, T., "Cryptographically Generated Addresses (CGA)",
        RFC 3972, April 2004.

   [2]  Arkko, J., Kempf, J., Sommerfeld, B., Zill, B., and P. Nikander,



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        "SEcure Neighbor Discovery (SEND)", RFC 3971, July 2004.

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

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

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

   [6]  Bagnulo, M. and J. Arkko, "Cryptographically Generated Addresses
        (CGA) Extension Field Format", draft-bagnulo-shim6-cga-ext-02
        (work in progress), October 2005.

   [7]  Nordmark, E., "Level 3 multihoming shim protocol",
        draft-ietf-shim6-proto-01 (work in progress), October 2005.

13.2.  Informative References

   [8]   Nordmark, E., "Threats relating to IPv6 multihoming solutions",
         draft-ietf-multi6-multihoming-threats-01 (work in progress),
         September 2004.

   [9]   Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
         Nordmark, "Mobile IP version 6 Route Optimization Security
         Design Background", draft-ietf-mip6-ro-sec-01 (work in
         progress), July 2004.

   [10]  Haddad, W., Madour, L., Arkko, J., and F. Dupont, "Applying
         Cryptographically Generated Addresses to Optimize MIPv6  (CGA-
         OMIPv6)", draft-haddad-mip6-cga-omipv6-02 (work in progress),
         June 2004.

















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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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