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Versions: 00 01                                                         
Network Working Group                                        P. Nikander
Internet-Draft                             Ericsson Research Nomadic Lab
Expires: January 13, 2005                                   T. Henderson
                                                      The Boeing Company
                                                           July 15, 2004

             Considerations on HIP based IPv6 multi-homing

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on January 13, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   The Host Identity Protocol implements the identifier locator
   separation by introducing a new name space and a new layer to the IP
   stack.  The new structure insulates the transport layer protocols
   from the internetworking layer, thereby allowing transport sessions
   to remain unaffected even if the underlying IP addresses change.
   That, in turn, seems to make it easier to solve the so called site
   multi-homing problem than without introducing such an indirection

   This document discusses how the proposed HIP mobility and
   multi-homing solution, described separately, would fit to the IETF

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   multi6 working group requirements.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Background . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Current venues for HIP work  . . . . . . . . . . . . . . . .  4
   1.3   Baseline HIP multi-homing mechanism  . . . . . . . . . . . .  4
   2.    HIP as a site-multi-homing solution  . . . . . . . . . . . .  6
   2.1   Hiding of underlying IP version  . . . . . . . . . . . . . .  6
   2.2   Integrated mobility  . . . . . . . . . . . . . . . . . . . .  6
   2.3   Architectural support for multi-realm connectivity . . . . .  6
   2.4   Integrated, mandatory end-to-end security  . . . . . . . . .  6
   2.5   High state setup cost  . . . . . . . . . . . . . . . . . . .  7
   2.6   Comparison with other Group-F multi6 proposals . . . . . . .  7
   3.    Using components of HIP or modified HIP instead of full
         HIP  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.1   Using HIP without IPsec ESP  . . . . . . . . . . . . . . . .  9
   3.2   Delaying HIP state setup . . . . . . . . . . . . . . . . . .  9
   3.2.1 Securing LHIP state setup  . . . . . . . . . . . . . . . . . 10
   3.3   Using HIP with routable AIDs . . . . . . . . . . . . . . . . 11
   4.    Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.1   Default router and source address selection  . . . . . . . . 12
   4.2   Selecting primary destination address  . . . . . . . . . . . 12
   4.3   Reacting to addresses becoming unreachable . . . . . . . . . 12
   5.    Evaluation against of RFC 3582 and MULTI6 Solution
         Questionaire . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.1   Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.2   Answers to MULTI6 Solution Questionaire  . . . . . . . . . . 13
   5.2.1 Routing  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.2.2 Identifiers and locators . . . . . . . . . . . . . . . . . . 14
   5.2.3 On the wire  . . . . . . . . . . . . . . . . . . . . . . . . 15
   5.2.4 Names, Hosts, Endpoints, or none of the above? . . . . . . . 17
   5.3   RFC 3582 Section 3 considerations  . . . . . . . . . . . . . 20
   5.3.1 Multi-Homing capabilities  . . . . . . . . . . . . . . . . . 20
   5.3.2 Additional requirements  . . . . . . . . . . . . . . . . . . 22
   5.4   Security considerations  . . . . . . . . . . . . . . . . . . 24
   6.    Security considerations  . . . . . . . . . . . . . . . . . . 25
   7.    Change log . . . . . . . . . . . . . . . . . . . . . . . . . 26
         Informative references . . . . . . . . . . . . . . . . . . . 27
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 28
         Intellectual Property and Copyright Statements . . . . . . . 29

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1. Introduction

   The IETF multi6 working group is currently calling various
   alternative solutions as components for an architectural analysis.
   The aim of that work is to try to understand the architectural design
   choices and their tradeoffs.

   This document discusses how a Host Identity Protocol (HIP) based
   approach could solve the multi6 site multi-homing problem. The draft
   also presents some ideas on how the HIP architecture could be split
   into components, some of which could be applied to the multi6 problem
   without adopting all of the current HIP proposal.

1.1 Background

   The Host Identity Protocol (HIP) is a proposal for changing the TCP/
   IP stack architecture by introducing a new name space and a new
   protocol layer to the stack.  The overall design is discussed in the
   HIP architecture document [3].  The actual protocol details are
   defined in the HIP protocol specification [2], and the mobility and
   multi-homing related extensions in the HIP mobility and multi-homing
   specification [4].  It is expected through this document that the
   reader is at least superficially familiar with the architecture
   document and the protocol specifications.

   The proposed HIP multi-homing mechanism [4] is primarily aimed to be
   a host multi-homing solution.  Basically, it allows two end hosts to
   inform each other of their IP connectivity.  That is, an end-host
   sends a set of IP addresses to its peer, and the peer makes sure that
   the end-host is reachable through (some of) these IP addresses.

   In the baseline HIP solution, a HIP base exchange protocol is run
   each time a new pair of hosts starts to communicate. This protocol is
   a four-way handshake, requiring public key cryptographic operations.
   While such a heavy exchange makes sense for applications where the
   hosts have a fairly long term relationship, e.g. for e-mail, disk
   access, etc., it may be too heavy for short term transactions, such
   as some forms of web browsing etc.  It is definitely unsuitable for
   DNS (see Section  Therefore, the architecture has been
   defined in such a way that each application can be configured either
   to use HIP or to use legacy IP, without the HIP overhead (and, of
   course, without any benefits from HIP).

   While the redundancy (Section and especially transport layer
   survivability (Section make sense mostly for long term
   transport sessions, where the state setup may be amortized over a
   longer period of time, it would be benficial to avoid such a large
   state setup cost.  Therefore, in Section 3, we also describe how some

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   components of HIP could be applied to the multi6 site-multihoming
   problem without adopting all of HIP.

1.2 Current venues for HIP work

   HIP is being developed in an IETF Working Group within the Internet
   Area, and a parallel IRTF Research Group.  The charter of the IETF
   Working Group is to develop a minimal set of specifications that can
   enable HIP experimentation on a wide scale, including the base
   protocol, DNS resource record definition, and basic mobility and host
   multihoming. The charter of the IRTF Research Group is to study the
   potential effects on the Internet of wide scale HIP deployment, and
   more broadly, the consequences of wide scale adoption of any type of
   separation of the identifier and locator roles of IP addresses.

   It is currently expected that if the HIP mobility and multi-homing
   solution, or some aspects of it, are selected for further work at the
   multi6 working group, then the resulting work will be chartered at
   the multi6 WG.  In that case, some co-operation with the HIP WG and
   the IRTF HIP RG is needed.

1.3 Baseline HIP multi-homing mechanism

   The baseline HIP multi-homing mechanism is specified in [4].  Here we
   briefly summarize the mechanism, giving an outline to those impatient
   readers that don't have cycles to read the full HIP specifications.

   As mentioned above, when two HIP hosts start to communicate, they run
   the HIP base exchange and create an HIP association.  As a part of
   this association, a multi-homed host MAY send a list of IP addresses
   that it believes to belong to itself.  The recipient of these
   addresses stores them as potential addresses of the peer.  Before
   using any of new addresses, it SHOULD verify that the peer is indeed
   reachable through the address.  This verification MAY be skipped if
   the peer host is fully trusted; see [4] for details.  According to
   the current specification, an end-host MAY perform such reachability
   test at any time, subject to its local policy. Such a reachability
   test requires only the tested address to work, meaning that such a
   test can be delayed until the other address(es) become unreachable.

   The hosts are free to change the information about their addresses at
   any time.  However, note that especially in the case of site
   multi-homing, one of the addresses may become unreachable while the
   other one still works.  In the typical case, however, this does not
   require the host to inform its peers about the situation, since even
   the non-working address still logically exists.

   Establishing the initial multi-addressing situation, and all changes

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   to that, are protected with strong cryptography. There are no known
   vulnerabilities in the specified mechanisms.  (Note, however, that
   the fact that there are no _known_ vulnerabilities does not mean that
   there are no unknown ones.  There might be, and given the freshness
   of the specifications, there probably are.)

   The current specification outlines a method where one of the peer's
   addresses is considered as a primary address.  By default, all
   traffic is sent using this address.  This practise is similar to how
   SCTP multi-addressing works, and is designed to work well with
   current transport layer congestion control.  However, the HIP
   architecture itself would allow multiple addresses to be used in
   parallel, even for one transport session.  Experimentation of such
   practise is assumed to take place at the IRTF HIP RG.

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2. HIP as a site-multi-homing solution

   As mentioned above, HIP multi-homing is primarily designed as a
   solution for multi-homed end-hosts.  As such, it offers a
   multi-address based baseline solution, similar to other
   multi-addressing based multi6 proposals.  Efforts to adopt the
   approach to site multi-homing, especially in the case where some
   hosts within the site and outside of the site may be legacy non-HIP
   hosts, has been fairly minimal.

   It is expected that most of what applies to other multi-addressing
   based multi6 proposals apply also to HIP.

   Since the multi-homing aspects of HIP do not seem to considerably
   differ from other multi-address based proposals, the focus in this
   section is on the factors that differentiate HIP from the other
   solutions.  Multi-homing aspects are covered in Section 5.

2.1 Hiding of underlying IP version

   HIP hides the underlying IP version from applications. That is, an
   IPv4 legacy application can be run over IPv6, and vice versa, HIP
   acting as an insulation layer.  This also means that the HIP mobility
   and multi-homing solution allows existing transport sessions to
   change their underlying connectivity from IPv4 to IPv6 and vice
   versa, as long as both end-hosts remain reachable (either directly or
   through a gateway).

2.2 Integrated mobility

   The HIP multi-homing mechanism is fully integrated with mobility.  In
   fact, the two mechanism are so integrated that it would be very hard
   to make them separate.

2.3 Architectural support for multi-realm connectivity

   The HIP architecture allows HIP associations to be routed through
   layer 3.5 middle boxes, thereby extending the associations across
   multiple IP realms.  In other words, HIP would allow controlled
   NAT-traversal that does not have the ill effects of the current NAT
   practise.  However, fully realising such service requires more work,
   and is subject to study at the IRTF HIP RG.

2.4 Integrated, mandatory end-to-end security

   HIP has its origin as a security solution, aiming to simplify IPsec
   administration and to address mobility at the same time.  Hence, HIP
   as defined today, requires that all payload traffic is protected with

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   IPsec ESP.  By default, this adds the overhead of carrying the ESP
   header and trailer in all packets.  Additionally, the current IPsec
   specifications mandate that either encryption or integrity protection
   of ESP MUST be used, i.e., it is not allowed to use IPsec without
   encryption and without integrity protection.  Hence, all HIP packets
   are subject to the cost of symmetric crypto processing on both
   sending and receiving ends.  While this cost is fairly minor in most
   modern architectures, it may have negative effects on small devices,
   such as PDAs, and large scale servers.

   In addition to the header overhead and computational cost, ESP breaks
   some middle box functionality by making it impossible to inspect and/
   or modify the packet contents.

   It should be noted that when end-to-end security is desirable, HIP
   adds no additional overhead compared to using standard IPsec
   mechanisms.  Hence, for applications were IPsec based security is
   adequate and desirable, HIP looks like an optimal or near-optimal
   multi-homing solution.

   In Section 3.1, below, we discuss how ESP could be replaced with
   other mechanisms for the case where end-to-end security is not needed
   for payload traffic.

2.5 High state setup cost

   The HIP base exchange is a four-way cryptographic authentication
   protocol, implementing a sigma [11] authenticated Diffie-Hellman
   exchange, with state-space and CPU-exhaustion denial-of-service
   protection. The initiator performs one public-key (DSA) signature and
   two signature verifications, while the responder performs one or two
   signatures and one verification. A single protocol run requires a few
   long integer exponentiations, taking a fraction of a second on modern
   CPU architectures.

2.6 Comparison with other Group-F multi6 proposals

   HIP has been classified into one of a set of multi6 proposals, known
   as "Group F", that propose a "Wedgelayer 3.5 / Fat IP" solution as
   shown in Figure 1.

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      |        Transport Protocols        |
      | AH | ESP | Frag/reass | Dest opts |
      |            Wedge layer            |
      |               IP                  |

                        Figure 1: Protocol stack

   Recently on the multi6 mailing list, a number of these Group F
   proposals have been contrasted with respect to how they make use of
   Application Identifiers (AIDs), Context Identifiers (CIDs), Context
   Identification Tags (CIDTs), and IP layer locators.  As presently
   specified in [2], HIP uses Host Identity Tags (HITs), which are
   hashes of the host identity public keys, as both AIDs and CIDs, and
   uses IPsec SPIs as CIDTs.

   Compared to other multi6 proposals, the state setup cost of HIP seems
   to be largest.  In Section 3.2 we discuss how this state setup cost
   might be delayed to a later moment, allowing two hosts to start
   communicating and creating the state only if they later determine
   that they want security or want to change active IP addresses. In
   Section 3.2.1 we discuss how the security properties of such delayed
   state setup might be improved with hash chains.

   In HIP, the AIDs have strong cryptographic properties but are not
   routable, which can cause problems for application level referrals.
   In Section 3.3, we discuss the potential for making HIP AIDs

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3. Using components of HIP or modified HIP instead of full HIP

   In this section, we first briefly describe how HIP could be modified
   so that it would impose less computational overhead, and then how
   some HIP-like ideas could relate to other multi6 proposals, and vice

3.1 Using HIP without IPsec ESP

   As mentioned above, some applications may have long lasting
   connections that would benefit from redundancy and transport layer
   survivability, but would not need end-to-end security. DRM protected
   video streams using application level encryption might be one such
   example.  In cases like that, it would be beneficial to use HIP
   without ESP.

   There seems at least to be two different ways how HIP could possibly
   be used without ESP:

   1.  One possibility is to replace the ESP header with a simple header
       that carries the SPI, similar to the M6 header proposed in the
       SIM proposal [9].  In the HIP case, the context tag (CIDT) would
       be the SPI.

   2.  Another possibility would be to allocate a single bit in the IP
       header, indicating whether in the receiving end the source and
       destination locators should be rewritten into identifiers or not.
       This would be somewhat similar to the NOID [7] and CB64 [8]
       proposals.  When ESP is not used, there is no replay protection,
       and therefore there is no need for multiple parallel SAs.  In
       this case, the CIDTs would be based on the IPv6 flow IDs and the

   Adding support for non-ESP communication would add a need for policy
   into HIP.  For each connection, the end-points would need to decide
   whether to use ESP or not.  Since one of the current HIP goals has
   been and still is simplicity, this feature has not been added to the
   current HIP specifications. From a functional point of view, the
   possibility of not using ESP is a mere performance optimization.

3.2 Delaying HIP state setup

   It might be possible to delay the actual HIP state setup. However, it
   would not be possible to use ESP before the state has been
   established.  The main benefit from this kind of optimization is to
   initially avoid the computational cost.

   This variant is tentatively called LHIP, for Lightweight HIP.

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   The idea goes as follows:

   1.  The Initiator sends an I1, just like in the current
       specification.  However, the I1 packet would contain an extension
       to indicate that it wants to delay state setup. It could also
       piggyback the initial payload, e.g., TCP SYN.

   2.  The Responder checks it policy to see if it allows delayed state
       setup for the HITs and IP addresses in I1. If it doesn't, it
       sends an R1 as usual, and forgets about the packet.  On the other
       hand, if it does allow delayed state setup, it generates a new
       nonce for lightweight state set up.  The lightweight state will
       consist of the HITs, the IP addresses, and the nonce.  The
       recipient will remember these, either by storing them or
       algorithmically, for a limited delta period.  The Responder
       replies with a new packet (LR1), which contains the HITs and the
       nonce.  A TCP SYN ACK may be piggybacked on the LR1.

   3.  When the Initiator receives the LR1, it stores the nonce, and
       sends an LI2, containing just the HITs and the nonce.

   4.  When the Responder receives the LI2, it knows that some node is
       reachable at the given IP address.  However, it has no assurance
       that the host is actually the one identified by the HIT.  Hence,
       the HIT cannot be used for access control or any other security
       purposes -- any node might have claimed to be identified by the

   If the Initiator later wants to move or use ESP, it must update the
   lightweight state to a full HIP association. Similarily, if the
   Recipient later wants to move, use ESP, or open a new transport
   session in the reverse direction, it must update the state.  Note
   that the notion is asymmetric here: the Recipient must update the
   state also in the case of opening new transport sessions, since it
   has no assurance that the other host actually "owns" the given HIT.

   It is noteworthy that this lightweight setup is completely insecure,
   allowing the initator to use any HIT as an identifier for itself.  On
   the other hand, it adds very little overhead to the setup of an
   initial connection, allowing the TCP three-way handshake to be
   piggybacked on the protocol.

3.2.1 Securing LHIP state setup

   Based on some initial discussions, it may be feasible to bring some
   security to the LHIP state setup using hash chains.  However, futher
   analysis would be needed.  For an example of how hash chains could be
   used for securing multi6 related state setup, see [12].

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   If such hash chains (or something similar) was added to the creation
   of the lightweight state, the state could probably be used for
   securing changes in the mobility and multi-homing state of the hosts.
   However, it would not be sufficient for creating ESP SAs.

3.3 Using HIP with routable AIDs

   As mentioned above in Section 2.6, HIP completely decouples upper
   layer protocols (ULP) from IP layer locators.  Specifically, HITs are
   used in the transport layer pseudoheader for checksum computations,
   and HITs may be passed to the application, depending on the
   implementation.  Because locators are not used in the checksum, the
   transport layer entities cannot communicate until a protocol exchange
   establishes the AIDs to use for the session.  Because HITs may be
   passed to the application, application level referrals may cause a
   HIT, treated as a locator by the application, to be passed to another
   non-multi6-aware host.

   It seems possible to modify HIP to use routable AIDs rather than HITs
   as the AIDs.  One such possibility would be to use the lower-order
   bits of the HIT as the interface ID of the primary locator of the
   host, which is then combined with the subnet prefix to form a
   routable AID.  This type of AID has cryptographic authentication
   properties, although weaker than those of full HITs, and somewhat
   more susceptible to collisions. Such a change does not seem to
   detract from the cryptographic properties of a full HIP base
   exchange, which could be conducted later or upon detection of
   collision, or initially as presently defined in [2]. In combination
   with a lighter-weight initial exchange, this could make the protocol
   very similar to CB64 [8]. Note also that there is no requirement to
   use a crypto-based locator; if an initial exchange establishes
   context that prevents session theft, such as described in the WIMP
   [12] proposal, any type of AID may be used.  This type of operation
   may have certain privacy advantages.

   The change required to the HIP base specification would be to use the
   initial locators as the AIDs of the transport layer checksums.  Note
   that HITs may still be passed to applications for HIP-aware
   applications, but HITs would no longer be passed in place of locators
   to non-HIP-aware applications.

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4. Discussion

4.1 Default router and source address selection

   Source address selection must be based on first selecting the
   outgoing router, based on the current reachability state, and then
   source address to be used, not vice versa.

4.2 Selecting primary destination address

   Section 8.4 of [4] briefly discusses how the hosts should select the
   primary destination address to use for their peers.  It should be
   noted that the currently presented discussion is probably not the
   optimal way of solving the problem.  Further engineering and maybe
   some research is required on the topic.

4.3 Reacting to addresses becoming unreachable

   In the case of site multi-homing one of the addresses may become
   unreachable while the other one still works.  In the typical case,
   however, this does not require the host to inform its peers about the
   situation, since even the non-working address still logically exists.

   Brian Carpenter: It's just as well you don't require this
   notification. The last node to know that an address is unreachable is
   the node that address belongs to. Unreachability is discovered at the
   other end of the multihomed session.

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5. Evaluation against of RFC 3582 and MULTI6 Solution Questionaire

5.1 Approach

5.2 Answers to MULTI6 Solution Questionaire

5.2.1 Routing

   As HIP is implemented on top of IP, it does not directly affect basic
   IP routing.  Routing within any IP realm is performed just as today.
   The end-hosts maintain a binding table that maps Host Identifiers
   into a set of IP addresses, and the HIP mobility and multi-homing
   protocol [4] is used to update these bindings. Primary multi-homing solution idea

   The HIP mobility and multi-homing protocol [4] allows an end-host to
   send information about all of its IP addresses, both IPv4 and IPv6,
   to its peers.  The peers have to check reachability of these
   addresses prior to using them for sending large amount of traffic.
   This mechanism allows interacting HIP hosts to establish
   multi-addressing based multi-homing state.

   The exact mechanisms on how a host is supposed to perform address and
   path selection are not defined in the current HIP specifications.
   However, the required practise is assumed to be similar to any other
   multi-addressing based multi-homing solutions.  Hence, it is expected
   that the multi6 WG (instead of the HIP WG) will define the required

   Some of the above described variations of HIP allow delayed
   establishment of the full HIP association. However, the details such
   practise are currently undefined and there is no implementation
   experience on the aspect. Uniqueness

   [I don't understand what the title of this section refers to.] Mobility

   HIP addresses mobility.  A mobile host sends HIP readdressing
   information to all of its peer hosts, allowing them to update
   addressing information.

   Initial rendezvous is planned to be started with DNS. An initiating
   host that wants to contact a mobile host is supposed to look up the
   Host Identifier and a set of current IP addresses from the DNS.  The

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   set of current IP addresses may include real active addresses of the
   mobile host, addresses of a Rendezvous server, or both.

   Once the initiating host has a tentative set of addresses, it sends
   an HIP I1 packet to an address.  If the address is a real address of
   the mobile host, the mobile host will directly answer with an R1
   packet, and the rest of the HIP base exchange  is run between the
   used addresses.  At the end the hosts inform each other about their
   multi-addressing state.

   If the I1 destination address is an address of a Rendezvous server,
   the Rendezvous server will forward the packet to the currently
   registered address of the mobile host.  The mobile host will send an
   R1 directly back to the initiating host, and the rest of the HIP base
   exchange is run directly between the mobile host and the initiating

   If a mobile host changes its active address while the HIP base
   exchange is going on, there will be a timeout and the initating host
   needs to start again, either using another address from the set of
   addresses received from the DNS, or remaking the DNS query if

   All hosts that use rendezvous servers are assumed to include the
   rendezvous server address in their active address sets.  Hence, if
   two interacting mobile hosts move at the same time so that the
   readdressing indications cross each other in the network and get
   lost, the host will fall back to the rendezvous server address after
   a timeout.  (The length of the timeout is currently unspecified, and
   subject to local policy of the hosts.)  Hence, provided that the
   hosts have updated their current location to the rendezvous server,
   the hosts will be able to continue communications.

   The HIP mobility mechanism is expected to replace Mobile IP for all
   communication taking place between HIP enabled hosts.  When a HIP
   host is communicating with a legacy host, it may use Mobile IP,
   provided that the host stack includes both HIP and Mobile IP

5.2.2 Identifiers and locators Split identifiers and locators

   HIP is based on the idea of splitting identifiers and locators.
   Public cryptographic keys are used as identifiers.  IP addresses are
   used as locators.  From the routing point-of-view, IP addresses are
   used just like today.  From the applications and transport layer
   point of view, identifiers (in the form of HITs and LSIs [3]) replace

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   IP addresses, unless a change as described in Section 3.3 is made to
   HIP. Binding lifetime

   The lifetime of the binding from an identifier to a locator is
   defined in the protocol messages.  Typically it is equal to the
   lifetime of the locator.  The host creating the binding state simply
   accepts the lifetime from the sending host. Update of bindings

   The bindings are updated by a host sending HIP readdressing
   paramters, typically in a HIP UPDATE packet. A single packet may
   update several sets of bindings.

   Whenever a new address is associated with an identifier, the hosts
   must verify the reachability of the address before using the address
   for payload traffic. This procedure is required in order to block
   flooding attacks [6].

   Updating the bindings have no direct effect on transport connections,
   which will remain up.  Changes in the actual paths may have effects
   on transport connections, such as changes in QoS.

5.2.3 On the wire

   HIP, as currently defined, consists of two protocols. One is a new
   protocol, the HIP protocol, run directly on the the top of IP.  The
   other one is IPsec ESP.  Using telecom terminology, the HIP protocol
   forms a control plane, and all user plane traffic is encapsulated in

   As discussed above, it might be possible to use HIP without requiring
   all user plane traffic to be ESP encapsulated.  However, such
   practise has not been defined in detail and there are no
   implementation experience. Solution layer

   HIP can be consider to be a layer 3.5 solution.

   HIP is applied to every packet, in the form of encapsulating them
   into ESP envelopes.  The ESP SPI field is used to associate the
   packet with the right end-point identifiers in the receiving end.

   As described above in Section 3.1, it ESP was not used, a single bit
   in the IP header might suffice to allow the receiving host to

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   associate HIP and non-HIP traffic with the appropriate sockets.  In
   that case the source and destination IP addresses would be used to
   associate the packet with the right end-points. This practice has the
   drawback that does not allow multiple host identifiers to be hosted
   on a single node.

   If the single bit approach is deemed infeasible, it would be possible
   to create a new extension header that would contain a new
   demultiplexing field.  From the HIP demultiplexing point of view, the
   contents of the field would be similar to ESP SPI. Correctness of the selected layer

   Multi-homing is a phenomenon that clearly appears between hosts, not
   between applications or transport sessions.  Hence, a multi-homing
   solution should be located at a layer that has host granularity, and
   not any finer granularity.  This leaves out transport and higher
   layer solutions.

   Multi-homing can be considered to be either as an end-to-end or a
   routing level phenomenon.  In the case of end-host multi-homing,
   where a single host has multiple accesses to the Internet, the
   situation seems to be best modelled as an end-to-end one.
   Respectively, the case of intra-transit-provider connectivity, an
   extreme form of site multi-homing, is probably best modeled as a part
   of the overall routing topology.  Various types of end-site
   multi-homing (soho...multinational) fall on different locations on
   this axis.

   The IP layer contains both end-to-end and routing functions.  Hence,
   IP layer could implement both end-to-end and routing based
   multi-homing solutions.

   Since HIP introduces a new name space, Host Identifiers, it is best
   described as a shim or 3.5 layer solution [10].  In other words, it
   is end-to-end in nature, affecting some of the current IP layer
   end-to-end functionality, but relies clearly below the transport

   A layer 3.5 solution has a number of good properties:

      It is possible to continue using unmodified TCP and UDP.

      It would become possible to move much of the SCTP and DCCP
      multi-addressing functionality into the new layer.  Such
      functionality would then be shared between them and the legacy
      transport protocols, TCP and UDP.

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      The approach would make it easier to collect per-path MTU and RTT
      information, if seen appropriate from the transport point-of-view.

      The approach does not require any changes to the IP layer or the
      pseudo header.  (But see also Section 3.1.) Expansion of packet size

   The solution does not cause any expansion of packet size other than
   that caused by ESP.  If ESP is not used, the single-bit solution,
   outlined above, would allow HIP to be used without any expansion of
   packet size. Fragmentation

   It is expected that HIP solutions report a reduced MTU to upper
   layer, similar to current ESP practise.  Other than that, the
   standard ESP fragmentation practise is used.  The current
   implementations seem to work, but no-one has performed a detailed
   analysis. Changes to ICMP error semantics

   The current HIP specifications do not create any new ICMP error
   messages.  However, a detailed analysis is needed to see if there are
   any subtle changes to the current semantics.  Such an analysis has
   not been made.

5.2.4 Names, Hosts, Endpoints, or none of the above? Relationship with DNS

   It is expected that the HIT (or the HI) of each HIP host is stored
   into the DNS, in addition to the IP address(es).  When a HIP host
   starts to connect to another HIP host, it queries for both the HIT/HI
   and the addresses.  If a HIT/HI is received, the initiating host
   creates a piece of local state, attempts to create a HIP association
   with the peer upon first connection request. If the association is
   created, the hosts establish their multi-addressing state directly.
   The addresses stored in the DNS are not used beyond building the HIP
   association. If no HIT/HI are received, the initiating host falls
   back to using legacy IP.

   Defining the required new RR type is a working item for the HIP WG.

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Internet-Draft               HIP for MULTI6                    July 2004 Interactions with 2-faced DNS

   Interactions with 2-faced DNS have not been fully analyzed.  However,
   as HIP reduced the applications' dependency on IP addresses, it looks
   like that HIP would easily allow 2-faced DNS to be used.
   Furthermore, if there is a proper HIP aware security gateway between
   the two domains, it should be possible to fully control the creation
   of HIP associations between the domains. (Non)need for centralized registration

   HIP does not require centralized registration.  The identifiers are
   public keys, and typically self-generated.

   It is expected that the IRTF HIP RG will study how to provide a
   service similar to reverse mapping for the public keys. (No) Circular dependencies with DNS

   DNS is not expected to use HIP.  In a typical implementation, this is
   accomplised by configuring the DNS proxies and servers to bind/
   connect to IP addresses, not HITs. Multihomed DNS servers

   Multi-homed DNS servers are expected to continue direct utilization
   of multiple IP addresses. Application/API changes

   Most old code will just work.  Multi-party applications doing
   IP-address-based referrals will break, unless HIP uses routable AIDs
   as described in Section 3.3. The IRTF HIP RG will study how to
   support such multi-party applications.

   To gain full benefit from HIP, extensions to the current socket API
   are expected to be needed.  However, using such extensions is not
   required to benefit from the multi-addressing properties of HIP. Backward compatibility and incremental deployment with current

   The current implementations allow full compatibility with the current
   IPv6, with the exception of using a large but unused part of the IPv6
   address space to represent HITs internally.  No requirements are
   placed on non-multihomed, non-mobile legacy hosts.

   HIP is designed to be incrementally deployed.  It is expected that

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   HIP capable servers announce their capability to run HIP by listing
   the new resource record in the DNS.  Possibilities to run HIP
   opportunistically, without DNS, are to be studied at the IRTF HIP RG. Backward compatibility with IPv4

   HIP works with both IPv4 and IPv6, even allowing simultaneous use of
   both IPv4 and IPv6 connections.

   It has not been analyzed how HIP interacts with existing 6to4
   gateways.  Such work is not on the HIP WG charter, but may be pursued
   at the IRTF HIP RG. Interaction with middleboxes

   Firewalls. Since HIP introduces a new control protocol to be run
      directly over IP, and uses IPsec to secure payload traffic, HIP
      would break most current firewalls.  However, the HIP base
      exchange and the rest of the control protocol has been carefully
      designed to be friendly towards future firewalls, allowing HIP
      aware firewalls to control HIP traffic.

   NAT. HIP, as currently defined, does not work with IP-multiplexing
      NAT boxes.  On the other hand, it would be fairly trivial to build
      HIP aware NAT devices that would allow multiple Host Identities to
      be NATed behind one IP address.

   Web caches. Since HIP encrypts by default all traffic, HIP does not
      work with existing web caches or other application level middle
      boxes.  If HIP was to be used without IPsec (see Section 3.1), Web
      proxies, and transparent application layer middle boxes might
      work.  However, that hasn't been analyzed. Implications on scoped addressing

   It has not been analyzed how HIP would affect scoped addressing.

   Multicast. Not analyzed.

   Link local. HIP should not have any effects on link local addresses
      or using them.

   Son-of-Sitelocal. It looks like HIP might reduce need for site local
      kind of addresses.

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Internet-Draft               HIP for MULTI6                    July 2004 Layer 2 implications

   HIP, as such, does not seem to have any direct implications on layer
   2 or neighbor discovery.  However, given that HIP introduces a public
   key per host, it might be possible to further simplify ND and layer 2
   security mechanisms. Referrals

   As HIP replaces IP addresses with HITs in application data
   structures, and since HITs cannot be currently resolved into IP
   addresses, multi-party applications doing IP-address-based referrals
   will not work.  The IRTF HIP RG will study the support of such
   multi-party applications. Legal stuff / trade marks and name space management

   Public keys or their hashes are not mnemonic.  The name space does
   not need to be managed.

5.3 RFC 3582 Section 3 considerations

5.3.1 Multi-Homing capabilities Redundancy

   Path redundancy is fully supported, similar to other solutions based
   on multi-addressing.  More specifically, as soon as the hosts have
   established multi-addressing state by exchanging REA payloads, the
   hosts may use the different transit providers interchangeably.  The
   current HIP specifications do not specify how a host detects a path
   failure; such a mechanism is expected to be specified in the multi6

   If a failure occurs before the multi-addressing state has been
   established, e.g., before the HIP base exchange has been completed,
   the hosts may try to re-create the HIP state using different IP
   addresses, if available, e.g., from the DNS.  However, the HIP
   specifications do not currently discuss such a situation, and the
   actual behaviour depends on local implementation. Load sharing

   Load sharing is supported, in the sense that the hosts may use
   different transit providers interchangeably, similar to other
   solutions based on multi-addressing.

   The current specification does include a feature that allows a host

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   to control the primary address that it wants its peer to use.  If
   more fine grained control is required, suitable policy mechanisms
   could be developed on the top of HIP. Performance

   By default, HIP based multi-homing does not require
   intra-transit-provider links to be used.  This is similar to other
   multi-addressing based solutions.

   As HIP insulates the transport sessions from the IP addresses, HIP
   allows more freedom in source or destination address based policy

   The baseline HIP solution adds a small delay before the first
   transport session between a pair of hosts is established.  The
   duration of the delay depends on latency and available CPU resources,
   consisting of two round trips of latency and requiring hosts to
   compute Diffie-Hellman shared key, one or two DSA signatures, and
   verify one or two DSA signatures.  Using short keys is allowed by the
   protocol, subject to local policy considerations. Policy

   As of today, HIP does not contain any policy control mechanisms.
   However, adding such mechanisms seems to be fairly straightforward,
   not differing from other multi-addressing based solutions. Simplicity

   HIP requires both end hosts to be changed.  Most applications do not
   require any changes; applications that use explicit referral may need
   to be made HIP aware. Incremental deployment is fully supported.

   Legacy IPv6 (and IPv4) hosts can be used at a multi-homed site either
   as such, in which case they do not benefit from multi-homing, or
   through a HIP proxy, located at the site.  If a multi-homed site
   wants to benefit from multi-homing when communicating with legacy
   hosts outside of the site, a HIP proxy must be deployed somewhere
   close to the core network.

   The exact details of HIP proxies have not been defined yet.

   The current HIP implementations, with limited multi-homing support,
   have around 10 000 lines of C code. Adding full multi-homing support,
   as defined in [4] is expected to add less than 3 000 lines of code.

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Internet-Draft               HIP for MULTI6                    July 2004 Transport layer survivability

   The solution provides full re-homing transparency for all transport
   layer sessions, similar to other multi-addressing based solutions.

   Most of the known current implementations do not support transparency
   for raw IP, as raw IP is considered to be located the host identity
   layer. Impact on DNS

   HIP adds a new DNS resource record for each HIP capable host.  The
   details are to be defined in the HIP WG.  This new resource record
   will be queried along with the IP addresses, thereby adding little

   For redundancy in initial connections, a HIP capable host should list
   multiple IP addresses in the DNS. However, these addresses are used
   only for the initial connection.  Once the multi-addressing state has
   been established, the hosts are independent of DNS. Packet filtering

   HIP is similar to other multi-addressing based solutions.

5.3.2 Additional requirements Scalability

   The solution does not impose new requirements on the routing system.

   From a multi-homing point of view, the only required piece of new
   infrastructure is the new DNS resource record.  Adding such records
   scales approximately as well as the DNS does today.

   HIP uses approximately 120 bit pseudo-random identifiers for
   identifying hosts.  According to the birthday paradigm, as long as
   the total number of hosts remains considerably lower than sqrt(2^120)
   = 2^60, the probability of identifier collisions remains low.  If the
   number of hosts is expected to grow larger, the length of the
   identifier can be doubled with minor modifications to the solution. Impact on routers

   The solution does not require changes to IPv6 routers, other than
   what the multi6 wg has already determined useful for all
   multi-addressing based solutions.

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   HIP provides full backwards compatibility with legacy hosts.
   Whenever one of the two communicating hosts is not HIP aware, the
   applications fall back to legacy IP.

   HIP does require changes to the host stack.  These changes can be
   classified into two classes:

   1.  Basic packet processing must be changed to recognize HITs on
       outgoing packets, and incoming ESP protected HIP packets must
       replace the IP addresses with HITs.  This change is minor, and
       can be implemented either integral to IPsec, or separate.  In a
       typical implementation, the number of changed and/or added lines
       of code is a few hundred.

   2.  A HIP protocol implementation must be added to the stack.  This
       change is a logically separate function. In a typical
       implementation, the number of lines of code required is in order
       of 10000-20000.

   The solution does not _require_ changes to the socket API or
   transport layer.  However, it is _expected_ that the socket API and
   the transport layer will be changed in order to gain full benefit
   from HIP.

   As it is currently defined, HIP breaks some multi-party applications
   that use IP addresses for referral. Solutions to this problem is a
   research topic, being studied at the IRTF HIP RG. Interactions between hosts and the routing system

   HIP does not require interaction between hosts and the routing
   system, other than what the multi6 wg has already determined for
   other multi-addressing based solutions. Operations and management

   HIP implementations are expected to include a facility that allows an
   administrator to view HIT to address mapping.  There is no HIP MIB or
   PIB, but it can be expected to be added as a working item for the HIP
   working group in the future. Co-operationg between transit providers

   HIP does not require any co-operation between transit providers.  If
   such co-operation is available, HIP would benefit from it similar to
   other multi-addressing based solutions.

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Internet-Draft               HIP for MULTI6                    July 2004 Multiple solutions

   HIP should work well with multi-homing solutions that are located
   solely at the IP layer, i.e., below HIP.

   Interoperability with other multi-addressing based solutions depend
   on many details, and need to be analyzed case-by-case.

5.4 Security considerations

   HIP attempts to raise the security baseline in the Internet by
   employing IPsec ESP protection by default.

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6. Security considerations

   HIP security has been extensively discussed in [3] and [2].  Mobility
   and multicast related security issues have been briefly discussed in
   [4].  As this draft is more a discussion draft and not a protocol
   specification, security considerations related to using HIP
   components instead of full HIP are currently not discussed anywhere.
   Such a discussion is planned to be added at a later stage, if this
   draft goes forward.

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7. Change log

   Changes between this version (-01) and -00 draft

      - added Section 2.6 comparing HIP with other group F multi6

      - added Section 3.3 describing how HIP could be possibly changed
      to include routable AIDs

      - updated references to HIP WG and HIP RG (Section 1.2)

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Informative references

   [1]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

   [2]   Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-00
         (work in progress), June 2004.

   [3]   Moskowitz, R., "Host Identity Protocol Architecture",
         draft-moskowitz-hip-arch-06 (work in progress), June 2004.

   [4]   Nikander, P., "End-Host Mobility and Multi-Homing with Host
         Identity Protocol", draft-nikander-hip-mm-01 (work in
         progress), January 2004.

   [5]   Nikander, P., "Mobile IP version 6 Route Optimization Security
         Design Background", draft-nikander-mobileip-v6-ro-sec-02 (work
         in progress), December 2003.

   [6]   Nordmark, E. and T. Li, "Threats relating to IPv6 multihoming
         solutions", draft-nordmark-multi6-threats-02 (work in
         progress), June 2004.

   [7]   Nordmark, E., "Multihoming without IP Identifiers",
         draft-nordmark-multi6-noid-01 (work in progress), October 2003.

   [8]   Nordmark, E., "Multihoming using 64-bit Crypto-based IDs",
         draft-nordmark-multi6-cb64-00 (work in progress), November

   [9]   Nordmark, E., "Strong Identity Multihoming using 128 bit
         Identifiers (SIM/CBID128)", draft-nordmark-multi6-sim-01 (work
         in progress), October 2003.

         draft-crocker-mast-analysis-01 (work in progress), October

   [11]  Krawczyk, H., "The SIGMA family of key-exchange protocols",

   [12]  Ylitalo, J., "Weak Identifier Multihoming Protocol Framework
         (WIMP-F)", draft-ylitalo-multi6-wimp-01 (work in progress),
         June 2004.

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Authors' Addresses

   Pekka Nikander
   Ericsson Research Nomadic Lab

   JORVAS  FIN-02420

   Phone: +358 9 299 1
   EMail: pekka.nikander@nomadiclab.com

   Tom Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA

   EMail: thomas.r.henderson@boeing.com

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