Network Working Group                                       R. Moskowitz
Internet-Draft                         ICSAlabs, a Division of TruSecure
Expires: December 26, 2004                                   Corporation
                                                             P. Nikander
                                           Ericsson Research Nomadic Lab
                                                           June 27, 2004

                  Host Identity Protocol Architecture

Status of this Memo

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   This Internet-Draft will expire on December 26, 2004.

Copyright Notice

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


   This memo describes a snapshot of the reasoning behind a proposed new
   namespace, the Host Identity namespace, and a new protocol layer, the
   Host Identity Protocol, between the internetworking and transport
   layers.  Herein are presented the basics of the current namespaces,
   strengths and weaknesses, and how a new namespace will add
   completeness to them.  The roles of this new namespace in the
   protocols are defined.  The memo describes the thinking of the

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   authors as of Fall 2003.

Table of Contents

   1.   Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1  Terms common to other documents  . . . . . . . . . . . . .   5
     3.2  Terms specific to this and other HIP documents . . . . . .   5
   4.   Background . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1  A Desire for a Namespace for Computing Platforms . . . . .   8
   5.   Host Identity Namespace  . . . . . . . . . . . . . . . . . .  10
     5.1  Host Identifiers . . . . . . . . . . . . . . . . . . . . .  10
     5.2  Storing Host Identifiers in DNS  . . . . . . . . . . . . .  11
     5.3  Host Identity Tag (HIT)  . . . . . . . . . . . . . . . . .  11
     5.4  Local Scope Identifier (LSI) . . . . . . . . . . . . . . .  12
   6.   New Stack Architecture . . . . . . . . . . . . . . . . . . .  13
     6.1  Transport associations and end-points  . . . . . . . . . .  13
   7.   End-Host Mobility and Multi-Homing . . . . . . . . . . . . .  15
     7.1  Rendezvous mechanism . . . . . . . . . . . . . . . . . . .  15
     7.2  Protection against Flooding Attacks  . . . . . . . . . . .  16
   8.   HIP and IPsec  . . . . . . . . . . . . . . . . . . . . . . .  17
   9.   HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1  HIP and TCP Checksum . . . . . . . . . . . . . . . . . . .  18
   10.  HIP Policies . . . . . . . . . . . . . . . . . . . . . . . .  19
   11.  Benefits of HIP  . . . . . . . . . . . . . . . . . . . . . .  20
     11.1   HIP's Answers to NSRG questions  . . . . . . . . . . . .  21
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  23
     12.1   HITs used in ACLs  . . . . . . . . . . . . . . . . . . .  24
     12.2   Non-security Considerations  . . . . . . . . . . . . . .  25
   13.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  26
   14.  References (informative) . . . . . . . . . . . . . . . . . .  26
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  27
        Intellectual Property and Copyright Statements . . . . . . .  28

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1.  Disclaimer

   The purpose of this memo is to provide a stable reference point in
   the development of the Host Identity Protocol architecture.  This
   memo describes the thinking of the authors as of Fall 2003; their
   thinking may have evolved since then.  In occasions, this memo may be
   confusing or self-contradicting.  That is (partially) intentional,
   and reflects the snapshot nature of this memo.

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2.  Introduction

   The Internet has created two global namespaces: Internet Protocol
   (IP) addresses and Domain Name Service (DNS) names.  These two
   namespaces have a set of features and abstractions that have powered
   the Internet to what it is today.  They also have a number of
   weaknesses.  Basically, since they are all we have, we try and do too
   much with them.  Semantic overloading and functionality extensions
   have greatly complicated these namespaces.

   The Host Identity namespace fills an important gap between the IP and
   DNS namespaces.  The Host Identity namespace consists of Host
   Identifiers (HI).  A Host Identifier is cryptographic in its nature;
   it is the public key of an asymmetric key-pair.  A Host Identity is
   assigned to each host.  Each host will have at least one Host
   Identity and a corresponding Host Identifier, which can either be
   public (e.g.  published in DNS), or unpublished.  Client systems will
   tend to have both public and unpublished Identities.

   There is a subtle but important difference between Host Identities
   and Host Identifiers.  An Identity refers to the abstract entity that
   is identified.  An Identifier, on the other hand, refers to the
   concrete bit pattern that is used in the identification process.

   Although the Host Identities could be used in many authentication
   systems, the presented architecture introduces a new protocol, called
   the Host Identity Protocol (HIP), and a cryptographic exchange,
   called the HIP base exchange [4].  The new protocol provides for
   limited forms of trust between systems.  It enhances mobility,
   multi-homing and dynamic IP renumbering [7], aids in protocol
   translation / transition [4], and reduces certain types of
   denial-of-service (DoS) attacks [4].

   When HIP is used, the actual payload traffic between two HIP hosts is
   typically protected with IPsec.  The Host Identities are used to
   create the needed IPsec Security Associations (SA) and to
   authenticate the hosts.  When IPsec is used, the actual payload IP
   packets do not differ in any way from standard IPsec protected IP

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3.  Terminology

3.1  Terms common to other documents

   Public key The public key from an asymmetric cryptographic key pair.
      Used as a publicly known identifier for cryptographic identity
   Private key The private or secret key from an asymmetric
      cryptographic key pair.  Assumed to be known only to the party
      identified by the corresponding public key.  Used by the
      identified party to authenticate its identity to other parties.
   Public key pair An asymmetric cryptographic key pair consisting of a
      public and private keys.  For example, Rivest-Shamir-Adelman (RSA)
      and Digital Signature Algorithm (DSA) key pairs are such key
   End-point A communicating entity.  For historical reasons, the term
      'computing platform' is used in this document as a (rough) synonym
      for end-point.

3.2  Terms specific to this and other HIP documents

   It should be noted that many of the terms defined herein are
   tautologous, self-referential or defined through circular reference
   to other terms.  This is due to the succinct nature of the
   definitions.  See the text elsewhere in this document for more
   elaborate explanations.

   Computing platform An entity capable of communicating and computing,
      for example, a computer.  See the definition of 'End-point',
   HIP base exchange A cryptographic protocol defined in [4].  See also
      Section 8.
   HIP packet An IP packet that carries a 'Host Identity Protocol'
   Host Identity An abstract concept assigned to a 'computing platform'.
      See 'Host Identifier', below.
   Host Identity namespace A name space formed by all possible Host
   Host Identity Protocol A protocol used to carry and authenticate Host
      Identifiers and other information.
   Host Identity Tag A 128-bit datum created by taking a cryptographic
      hash over a Host Identifier.
   Host Identifier A public key used as a name for a Host Identity.
   Local Scope Identifier A 32-bit datum denoting a Host Identity.
   Public Host Identifier and Identity A published or publicly known
      Host Identfier used as a public name for a Host Identity, and the
      corresponding Identity.

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   Unpublished Host Identifier and Identity A Host Identifier that is
      not placed in any public directory, and the corresponding Host
      Identity.  Unpublished Host Identities are typically short living
      in nature, being often replaced and possibly used just once.
   Rendezvous Mechanism A mechanism used to locate mobile hosts based on
      their HIT.

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4.  Background

   The Internet is built from three principal components: computing
   platforms, packet transport (i.e.  internetworking) infrastructure,
   and services (applications).  The Internet exists to service two
   principal components: people and robotic services (silicon based
   people, if you will).  All these components need to be named in order
   to interact in a scalable manner.  Here we concentrate on naming
   computing platforms and packet transport elements.

   There are two principal namespaces in use in the Internet for these
   components: IP numbers, and Domain Names.  Email, HTTP and SIP
   addresses are really only extensions of Domain Names.

   IP numbers are a confounding of two namespaces, the names of a host's
   networking interfaces and the names of the locations ('confounding'
   is a term used in statistics to discuss metrics that are merged into
   one with a gain in indexing, but a loss in informational value).  The
   names of locations should be understood as denoting routing direction
   vectors, i.e., information that is used to deliver packets to their

   IP numbers name networking interfaces, and typically only when the
   interface is connected to the network.  Originally IP numbers had
   long-term significance.  Today, the vast number of interfaces use
   ephemeral and/or non-unique IP numbers.  That is, every time an
   interface is connected to the network, it is assigned an IP number.

   In the current Internet, the transport layers are coupled to the IP
   addresses.  Neither can evolve separately from the other.  IPng
   deliberations were strongly shaped by the decision that a
   corresponding TCPng would not be created.

   Domain Names provide hierarchically assigned names for some computing
   platforms and some services.  Each hierarchy is delegated from the
   level above; there is no anonymity in Domain Names.

   Email, SIP and WWW addresses provide naming for humans, autonomous
   applications, and documents.  Email, SIP and WWW addresses are
   extensions of Domain Names.

   There are three critical deficiencies with the current namespaces.
   Firstly, dynamic readdressing cannot be directly managed.  Secondly,
   anonymity is not provided in a consistent, trustable manner.
   Finally, authentication for systems and datagrams is not provided.
   All of these deficiencies arise because computing platforms are not
   well named with the current namespaces.

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4.1  A Desire for a Namespace for Computing Platforms

   An independent namespace for computing platforms could be used in
   end-to-end operations independent of the evolution of the
   internetworking layer and across the many internetworking layers.
   This could support rapid readdressing of the internetworking layer
   because of mobility, rehoming, or renumbering.

   If the namespace for computing platforms is based on public key
   cryptography, it can also provide authentication services.  If this
   namespace is locally created without requiring registration, it can
   provide anonymity.

   Such a namespace (for computing platforms) and the names in it should
   have the following characteristics:
      The namespace should be applied to the IP 'kernel'.  The IP kernel
      is the 'component' between applications and the packet transport
      The namespace should fully decouple the internetworking layer from
      the higher layers.  The names should replace all occurrences of IP
      addresses within applications (like in the TCB).  This may require
      changes to the current APIs.  In the long run, it is probable that
      some new APIs are needed.
      The introduction of the namespace should not mandate any
      administrative infrastructure.  Deployment must come from the
      bottom up, in a pairwise deployment.
      The names should have a fixed length representation, for easy
      inclusion in datagram headers and existing programming interfaces
      (e.g the TCB).
      Using the namespace should be affordable when used in protocols.
      This is primarily a packet size issue.  There is also a
      computational concern in affordability.
      The names must be statistically globally unique.  64 bits is
      inadequate to make the probability of collisions sufficiently low
      (1% chance of collision in a population of 640M); thus,
      approximately 100 or more bits should be used.
      The names should have a localized abstraction so that it can be
      used in existing protocols and APIs.
      It must be possible to create names locally.  This can provide
      anonymity at the cost of making resolvability very difficult.
         Sometimes the names may contain a delegation component.  This
         is the cost of resolvability.
      The namespace should provide authentication services.
      The names should be long lived, but replaceable at any time.  This
      impacts access control lists; short lifetimes will tend to result
      in tedious list maintenance or require a namespace infrastructure
      for central control of access lists.

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   In this document, a new namespace approaching these ideas is called
   the Host Identity namespace.  Using Host Identities requires its own
   protocol layer, the Host Identity Protocol, between the
   internetworking and transport layers.  The names are based on public
   key cryptography to supply authentication services.  Properly
   designed, it can deliver all of the above stated requirements.

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5.  Host Identity Namespace

   A name in the Host Identity namespace, a Host Identifier (HI),
   represents a statistically globally unique name for naming any system
   with an IP stack.  This identity is normally associated with, but not
   limited to, an IP stack.  A system can have multiple identities, some
   'well known', some unpublished or 'anonymous'.  A system may self
   assert its own identity, or may use a third-party authenticator like
   DNSSEC, PGP, or X.509 to 'notarize' the identity assertion.  It is
   expected that the Host Identifiers will initially be authenticated
   with DNSSEC and that all implementations will support DNSSEC as a
   minimal baseline.

   In theory, any name that can claim to be 'statistically globally
   unique' may serve as a Host Identifier.  However, in the authors'
   opinion, a public key of a 'public key pair' makes the best Host
   Identifier.  As documented in the Host Identity Protocol
   specification [4], a public key based HI can authenticate the HIP
   packets and protect them for man-in-the-middle attacks.  Since
   authenticated datagrams are mandatory to provide much of HIP's
   denial-of-service protection, the Diffie-Hellman exchange in HIP has
   to be authenticated.  Thus, only public key HI and authenticated HIP
   messages are supported in practice.  In this document, the
   non-cryptographic forms of HI and HIP are presented to complete the
   theory of HI, but they should not be implemented as they could
   produce worse denial-of-service attacks than the Internet has without
   Host Identity.

5.1  Host Identifiers

   Host Identity adds two main features to Internet protocols.  The
   first is a decoupling of the internetworking and transport layers;
   see Section 6.  This decoupling will allow for independent evolution
   of the two layers.  Additionally, it can provide end-to-end services
   over multiple internetworking realms.  The second feature is host
   authentication.  Because the Host Identifier is a public key, this
   key can be used to authenticate security protocols like IPsec.

   The only completely defined structure of the Host Identity is that of
   a public/private key pair.  In this case, the Host Identity is
   referred to by its public component, the public key.  Thus, the name
   representing a Host Identity in the Host Identity namespace, i.e.
   the Host Identifier, is the public key.  In a way, the possession of
   the private key defines the Identity itself.  If the private key is
   possessed by more than one node, the Identity can be considered to be
   a distributed one.

   Architecturally, any other Internet naming convention might form a

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   usable base for Host Identifiers.  However, non-cryptographic names
   should only be used in situations of high trust - low risk.  That is
   any place where host authentication is not needed (no risk of host
   spoofing) and no use of IPsec.  However, at least for interconnected
   networks spanning several operational domains, the set of
   environments where the risk of host spoofing allowed by
   non-cryptographic Host Identifiers is acceptable is the null set.
   Hence, the current HIP documents do not specify how to use any other
   types of Host Identifiers but public keys.

   The actual Host Identities are never directly used in any Internet
   protocols.  The corresponding Host Identifiers (public keys) may be
   stored in various DNS or LDAP directories as identified elsewhere in
   this document, and they are passed in the HIP base exchange.  A Host
   Identity Tag (HIT) is used in other protocols to represent the Host
   Identity.  Another representation of the Host Identities, the Local
   Scope Identifier (LSI), can also be used in protocols and APIs.

5.2  Storing Host Identifiers in DNS

   The public Host Identifiers should be stored in DNS; the unpublished
   Host Identifiers should not be stored anywhere (besides the
   communicating hosts themselves).  The (public) HI is stored in a new
   RR type, to be defined.  This RR type is likely to be quite similar
   to the IPSECKEY RR [5].

   Alternatively, or in addition to storing Host Identifiers in the DNS,
   they may be stored in various kinds of Public Key Infrastructure
   (PKI).  Such a practice may allow them to be used for purposes other
   than pure host identification.

5.3  Host Identity Tag (HIT)

   A Host Identity Tag is an 128-bit representation for a Host Identity.
   It is created by taking a cryptographic hash over the corresponding
   Host Identifier.  There are two advantages of using a hash over using
   the Host Identifier in protocols.  Firstly, its fixed length makes
   for easier protocol coding and also better manages the packet size
   cost of this technology.  Secondly, it presents the identity in a
   consistent format to the protocol independent of the cryptographic
   algorithms used.

   In the HIP packets, the HITs identify the sender and recipient of a
   packet.  Consequently, a HIT should be unique in the whole IP
   universe as long as it is being used.  In the extremely rare case
   that a single HIT happens to map to more than one Host Identity, the
   Host Identifiers (public keys) will make the final difference.  If
   there is more than one public key for a given node, the HIT acts as a

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   hint for the correct public key to use.

5.4  Local Scope Identifier (LSI)

   An LSI is a 32-bit localized representation for a Host Identity.  The
   purpose of an LSI is to facilitate using Host Identities in existing
   protocols and APIs.  LSI's advantage over HIT is its size; its
   disadvantage is its local scope.  The generation of LSIs is defined
   in the Host Identity Protocol specification [4].

   Examples of how LSIs can be used include: as the address in a FTP
   command and as the address in a socket call.  Thus, LSIs act as a
   bridge for Host Identities into IPv4-based protocols and APIs.

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6.  New Stack Architecture

   One way to characterize Host Identity is to compare the proposed new
   architecture with the current one.  As discussed above, the IP
   addresses can be seen to be a confounding of routing direction
   vectors and interface names.  Using the terminology from the IRTF
   Name Space Research Group Report [6] and, e.g., the unpublished
   Internet-Draft Endpoints and Endpoint Names [9] by Noel Chiappa, the
   IP addresses currently embody the dual role of locators and end-point
   identifiers.  That is, each IP address names a topological location
   in the Internet, thereby acting as a routing direction vector, or
   locator.  At the same time, the IP address names the physical network
   interface currently located at the point-of-attachment, thereby
   acting as a end-point name.

   In the HIP architecture, the end-point names and locators are
   separated from each other.  IP addresses continue to act as locators.
   The Host Identifiers take the role of end-point identifiers.  It is
   important to understand that the end-point names based on Host
   Identities are slightly different from interface names; a Host
   Identity can be simultaneously reachable through several interfaces.

   The difference between the bindings of the logical entities are
   illustrated in Figure 1.

   Service ------ Socket                  Service ------ Socket
                    |                                      |
                    |                                      |
                    |                                      |
                    |                                      |
   End-point        |                    End-point --- Host Identity
            \       |                                      |
              \     |                                      |
                \   |                                      |
                  \ |                                      |
   Location --- IP address                Location --- IP address

                                Figure 1

6.1  Transport associations and end-points

   Architecturally, HIP provides for a different binding of transport
   layer protocols.  That is, the transport layer associations, i.e.,
   TCP connections and UDP associations, are no longer bound to IP
   addresses but to Host Identities.

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   It is possible that a single physical computer hosts several logical
   end-points.  With HIP, each of these end-points would have a distinct
   Host Identity.  Furthermore, since the transport associations are
   bound to Host Identities, HIP provides for process migration and
   clustered servers.  That is, if a Host Identity is moved from one
   physical computer to another, it is also possible to simultaneously
   move all the transport associations without breaking them.
   Similarly, if it is possible to distribute the processing of a single
   Host Identity over several physical computers, HIP provides for
   cluster based services without any changes at the client end-point.

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7.  End-Host Mobility and Multi-Homing

   HIP decouples the transport from the internetworking layer, and binds
   the transport associations to the Host Identities (through actually
   either the HIT or LSI).  Consequently, HIP can provide for a degree
   of internetworking mobility and multi-homing at a low infrastructure
   cost.  HIP mobility includes IP address changes (via any method) to
   either party.  Thus, a system is considered mobile if its IP address
   can change dynamically for any reason like PPP, DHCP, IPv6 prefix
   reassignments, or a NAT device remapping its translation.  Likewise,
   a system is considered multi-homed if it has more than one globally
   routable IP address at the same time.  HIP links IP addresses
   together, when multiple IP addresses correspond to the same Host
   Identity, and if one address becomes unusable, or a more preferred
   address becomes available, existing transport associations can easily
   be moved to another address.

   When a node moves while communication is already on-going, address
   changes are rather straightforward.  The peer of the mobile node can
   just accept a HIP or an integrity protected IPsec packet from any
   address and totally ignore the source address.  However, as discussed
   in Section 7.2 below, a mobile node must send a HIP readdress packet
   to inform the peer of the new address(es), and the peer must verify
   that the mobile node is reachable through these addresses.  This is
   especially helpful for those situations where the peer node is
   sending data periodically to the mobile node (that is re-starting a
   connection after the initial connection).

7.1  Rendezvous mechanism

   Making a contact to a mobile node is slightly more involved.  In
   order to start the HIP exchange, the initiator node has to know how
   to reach the mobile node.  Although infrequently moving HIP nodes
   could use Dynamic DNS to update their reachability information in the
   DNS, an alternative to using DNS in this fashion is to use a piece of
   new static infrastructure to facilitate rendezvous between HIP nodes.

   The mobile node keeps the rendezvous infrastructure continuously
   updated with its current IP address(es).  The mobile nodes must trust
   the rendezvous mechanism to properly maintain their HIT and IP
   address mappings.

   The rendezvous mechanism is also needed if both of the nodes happen
   to change their address at the same time, either because they are
   mobile and happen to move at the same time, because one of them is
   off-line for a while, or because of some other reason.  In such a
   case, the HIP readdress packets will cross each other in the network
   and never reach the peer node.

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   A separate document will specify the details of the HIP rendezvous

7.2  Protection against Flooding Attacks

   While the idea of informing about address changes by simply sending
   packets with a new source address appears appealing, it is not secure
   enough.  That is, even if HIP does not rely on the source address for
   anything (once the base exchange has been completed), it appears to
   be necessary to check a mobile node's reachability at the new address
   before actually sending any larger amounts of traffic to the new

   Blindly accepting new addresses would potentially lead to flooding
   Denial-of-Service attacks against third parties [8].  In a
   distributed flooding attack an attacker opens high volume HIP
   connections with a large number of hosts (using unpublished HIs), and
   then claims to all of these hosts that it has moved to a target
   node's IP address.  If the peer hosts were to simply accept the move,
   the result would be a packet flood to the target node's address.  To
   close this attack, HIP includes an address check mechanism where the
   reachability of a node is separately checked at each address before
   using the address for larger amounts of traffic.

   Whenever HIP is used between two hosts that fully trust each other,
   the hosts may optionally decide to skip the address tests.  However,
   such performance optimization must be restricted to peers that are
   known to be trustworthy and capable of protecting themselves from
   malicious software.

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8.  HIP and IPsec

   The preferred way of implementing HIP is to use IPsec to carry the
   actual data traffic.  As of today, the only completely defined method
   is to use IPsec Encapsulated Security Payload (ESP) to carry the data
   packets.  In the future, other ways of transporting payload data may
   be developed, including ones that do not use cryptographic

   In practise, the HIP base exchange uses the cryptographic Host
   Identifiers to set up a pair of ESP Security Associations (SAs) to
   enable ESP in an end-to-end manner.  This is implemented in a way
   that can span addressing realms.

   From a conceptual point of view, the IPsec Security Parameter Index
   (SPI) in ESP provides a simple compression of the HITs.  This does
   require per-HIT-pair SAs (and SPIs), and a decrease of policy
   granularity over other Key Management Protocols, such as IKE and
   IKEv2.  Future HIP extensions may provide for more granularity and
   creation of several ESP SAs between a pair of HITs.

   Since HIP is designed for host usage, not for gateways, only ESP
   transport mode is supported.  An ESP SA pair is indexed by the SPIs
   and the two HITs (both HITs since a system can have more than one
   HIT).  The SAs need not to be bound to IP addresses; all internal
   control of the SA is by the HITs.  Thus, a host can easily change its
   address using Mobile IP, DHCP, PPP, or IPv6 readdressing and still
   maintain the SAs.  Since the transports are bound to the SA (via an
   LSI or a HIT), any active transport is also maintained.  Thus, real
   world conditions like loss of a PPP connection and its
   re-establishment or a mobile handover will not require a HIP
   negotiation or disruption of transport services.  [11]

   Since HIP does not negotiate any SA lifetimes, all lifetimes are
   local policy.  The only lifetimes a HIP implementation MUST support
   are sequence number rollover (for replay protection), and SA
   timeout[4].  An SA times out if no packets are received using that
   SA.  Implementations MAY support lifetimes for the various ESP

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9.  HIP and NATs

   Passing packets between different IP addressing realms requires
   changing IP addresses in the packet header.  This may happen, for
   example, when a packet is passed between the public Internet and a
   private address space, or between IPv4 and IPv6 networks.  The
   address translation is usually implemented as Network Address
   Translation (NAT) [2] or NAT Protocol translation (NAT-PT) [1].

   In a network environment where the identification is based on the IP
   addresses, identifying the communicating nodes is difficult when NAT
   is used.  With HIP, the transport layer end-points are bound to the
   Host Identities.  Thus, a connection between two hosts can traverse
   many addressing realm boundaries.  The IP addresses are used only for
   routing purposes; the IP addresses may be changed freely during
   packet traversal.

   For a HIP based flow, a HIP-aware NAT or NAT-PT system tracks the
   mapping of HITs and the corresponding IPsec SPIs to an IP address.
   Many HITs can map to a single IP address on a NAT, simplifying
   connections on address poor NAT interfaces.  The NAT can gain much of
   its knowledge from the HIP packets themselves; however, some NAT
   configuration may be necessary.

   The NAT systems cannot touch the datagrams within the IPsec envelope,
   thus application specific address translation must be done in the end
   systems.  HIP provides for 'Distributed NAT', and uses the HIT or the
   LSI as a place holder for embedded IP addresses.

9.1  HIP and TCP Checksum

   There is no way for a host to know if any of the IP addresses in the
   IP header are the addresses used to calculate the TCP checksum.  That
   is, it is not feasible to calculate the TCP checksum using the actual
   IP addresses in the pseudo header; the addresses received in the
   incoming packet are not necessarily the same as they were on the
   sending host.  Furthermore, it is not possible to recompute the upper
   layer checksums in the NAT/NAT-PT system, since the traffic is IPsec
   protected.  Consequently, the TCP and UDP checksums are calculated
   using the HITs in the place of the IP addresses in the pseudo header.
   Furthermore, only the IPv6 pseudo header format is used.  This
   provides for IPv4 / IPv6 protocol translation.

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10.  HIP Policies

   There are a number of variables that will influence the HIP exchanges
   that each host must support.  All HIP implementations should support
   at least 2 HIs, one to publish in DNS and an unpublished one for
   anonymous usage.  Although unpublished HIs will be rarely used as
   responder HIs, they are likely be common for initiators.  Support for
   multiple HIs is recommended.

   Many initiators would want to use a different HI for different
   responders.  The implementations should provide for a policy of
   initiator HIT to responder HIT.  This policy should also include
   preferred transforms and local lifetimes.

   Responders would need a similar policy, describing the hosts allowed
   to participate in HIP exchanges, and the preferred transforms and
   local lifetimes.

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11.  Benefits of HIP

   In the beginning, the network layer protocol (i.e.  IP) had the
   following four "classic" invariants:
      Non-mutable: The address sent is the address received.
      Non-mobile: The address doesn't change during the course of an
      Reversible: A return header can always be formed by reversing the
      source and destination addresses.
      Omniscient: Each host knows what address a partner host can use to
      send packets to it.

   Actually, the fourth can be inferred from 1 and 3, but it is worth
   mentioning for reasons that will be obvious soon if not already.

   In the current "post-classic" world, we are trying intentionally to
   get rid of the second invariant (both for mobility and for
   multi-homing), and we have been forced to give up the first and the
   fourth.  Realm Specific IP [3] is an attempt to reinstate the fourth
   invariant without the first invariant.  IPv6 is an attempt to
   reinstate the first invariant.

   Few systems on the Internet have DNS names that are meaningful to
   them.  That is, if they have a Fully Qualified Domain Name (FQDN),
   that typically belongs to a NAT device or a dial-up server, and does
   not really identify the system itself but its current connectivity.
   FQDN names (and their extensions as email names) are Application
   Layer names; more frequently naming services than a particular
   system.  This is why many systems on the internet are not registered
   in DNS; they do not have services of interest to other Internet

   DNS names are references to IP addresses.  This only demonstrates the
   interrelationship of the networking and application layers.  DNS, as
   the Internet's only deployed, distributed, database is also the
   repository of other namespaces, due in part to DNSSEC and application
   specific key records.  Although each namespace can be stretched (IP
   with v6, DNS with KEY records), neither can adequately provide for
   host authentication or act as a separation between internetworking
   and transport layers.

   The Host Identity (HI) namespace fills an important gap between the
   IP and DNS namespaces.  An interesting thing about the HI is that it
   actually allows one to give-up all but the 3rd Network Layer
   invariant.  That is to say, as long as the source and destination
   addresses in the network layer protocol are reversible, then things
   work ok because HIP takes care of host identification, and
   reversibility allows one to get a packet back to one's partner host.

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   You don't care if the network layer address changes in transit
   (mutable) and you don't care what network layer address the partner
   is using (non-omniscient).

11.1  HIP's Answers to NSRG questions

   The IRTF Name Space Research Group has posed a number of evaluating
   questions in their report [6].  In this section, we provide answers
   to these questions.
   1.  How would a stack name improve the overall functionality of the
          At the fundamental level, HI decouples the internetworking
          layer from the transport layer, allowing each to evolve
          separately.  At the same time, the decoupling makes end-host
          mobility and multi-homing easier.  It also allows mobility and
          multi-homing across the IPv4 and IPv6 networks.  HIs make
          network renumbering easier.  At the conceptual level, they
          also make process migration and clustered servers easier to
          implement.  Furthermore, being cryptographic in nature, they
          provide the basis for solving the security problems related to
          end-host mobility and multi-homing.
   2.  What does a stack name look like?
          A HI is a cryptographic public key.  However, instead of using
          the keys directly, most protocols use a fixed size hash of the
          public key.
   3.  What is its lifetime?
          HIP provides both stable and temporary Host Identifiers.
          Stable HIs are typically long lived, with a lifetime of years
          or more.  The lifetime of temporary HIs depends on how long
          the upper layer connections and applications need them, and
          can range from a few seconds to years.
   4.  Where does it live in the stack?
          The HIs live between the transport and internetworking layers.
   5.  How is it used on the end points
          The Host Identifiers may be used directly or indirectly (in
          the form of HITs or LSIs) by applications when they access
          network services.  Additionally, the Host Identifiers, as
          public keys, are used in the built in key agreement protocol,
          called the HIP base exchange, to authenticate the hosts to
          each other.
   6.  What administrative infrastructure is needed to support it?
          In some environments, it is possible to use HIP
          opportunistically, without any infrastructure.  However, to
          gain full benefit from HIP, the HIs must be stored in the DNS
          or a PKI, and a new rendezvous mechanism is needed.  Such a
          new rendezvous mechanism may need new infrastructure to be

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   7.  If we add an additional layer would it make the address list in
       SCTP unnecessary?
   8.  What additional security benefits would a new naming scheme
          HIP reduces dependency on IP addresses, making the so called
          address ownership [10] problems easier to solve.  In practice,
          HIP provides security for end-host mobility and multi-homing.
          Furthermore, since HIP Host Identifiers are public keys,
          standard public key certificate infrastructures can be applied
          on the top of HIP.
   9.  What would the resolution mechanisms be, or what characteristics
       of a resolution mechanisms would be required?
          For most purposes, an approach where DNS names are resolved
          simultaneously to HIs and IP addresses is sufficient.
          However, if it becomes necessary to resolve HIs into IP
          addresses or back to DNS names, a flat resolution
          infrastructure is needed.  Such an infrastructure could be
          based on the ideas of Distributed Hash Tables, but would
          require significant new development and deployment.

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12.  Security Considerations

   HIP takes advantage of the new Host Identity paradigm to provide
   secure authentication of hosts and to provide a fast key exchange for
   IPsec.  HIP also attempts to limit the exposure of the host to
   various denial-of-service (DoS) and man-in-the-middle (MitM) attacks.
   In so doing, HIP itself is subject to its own DoS and MitM attacks
   that potentially could be more damaging to a host's ability to
   conduct business as usual.

   Resource exhausting Denial-of-service attacks take advantage of the
   cost of setting up a state for a protocol on the responder compared
   to the 'cheapness' on the initiator.  HIP allows a responder to
   increase the cost of the start of state on the initiator and makes an
   effort to reduce the cost to the responder.  This is done by having
   the responder start the authenticated Diffie-Hellman exchange instead
   of the initiator, making the HIP base exchange 4 packets long.  There
   are more details on this process in the Host Identity Protocol
   specification [4].

   HIP optionally supports opportunistic negotiation.  That is, if a
   host receives a start of transport without a HIP negotiation, it can
   attempt to force a HIP exchange before accepting the connection.
   This has the potential for DoS attacks against both hosts.  If the
   method to force the start of HIP is expensive on either host, the
   attacker need only spoof a TCP SYN.  This would put both systems into
   the expensive operations.  HIP avoids this attack by having the
   responder send a simple HIP packet that it can pre-build.  Since this
   packet is fixed and easily replayed, the initiator only reacts to it
   if it has just started a connection to the responder.

   Man-in-the-middle attacks are difficult to defend against, without
   third-party authentication.  A skillful MitM could easily handle all
   parts of the HIP base exchange, but HIP indirectly provides the
   following protection from a MitM attack.  If the responder's HI is
   retrieved from a signed DNS zone or secured by some other means, the
   initiator can use this to authenticate the signed HIP packets.
   Likewise, if the initiator's HI is in a secure DNS zone, the
   responder can retrieve it and validate the signed HIP packets.
   However, since an initiator may choose to use an unpublished HI, it
   knowingly risks a MitM attack.  The responder may choose not to
   accept a HIP exchange with an initiator using an unknown HI.

   In HIP, the Security Association for IPsec is indexed by the SPI; the
   source address is always ignored, and the destination address may be
   ignored as well.  Therefore, HIP enabled IPsec Encapsulated Security
   Payload (ESP) is IP address independent.  This might seem to make it
   easier for an attacker, but ESP with replay protection is already as

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   well protected as possible, and the removal of the IP address as a
   check should not increase the exposure of IPsec ESP to DoS attacks.

   Since not all hosts will ever support HIP, ICMPv4 'Destination
   Unreachable, Protocol Unreachable' and ICMPv6 'Parameter Problem,
   Unrecognized Next Header' messages are to be expected and present a
   DoS attack.  Against an initiator, the attack would look like the
   responder does not support HIP, but shortly after receiving the ICMP
   message, the initiator would receive a valid HIP packet.  Thus, to
   protect against this attack, an initiator should not react to an ICMP
   message until a reasonable time has passed, allowing it to get the
   real responder's HIP packet.  A similar attack against the responder
   is more involved.

   Another MitM attack is simulating a responder's administrative
   rejection of a HIP initiation.  This is a simple ICMP 'Destination
   Unreachable, Administratively Prohibited' message.  A HIP packet is
   not used because it would either have to have unique content, and
   thus difficult to generate, resulting in yet another DoS attack, or
   just as spoofable as the ICMP message.  Like in the previous case,
   the defense against this attack is for the initiator to wait a
   reasonable time period to get a valid HIP packet.  If one does not
   come, then the initiator has to assume that the ICMP message is
   valid.  Since this is the only point in the HIP base exchange where
   this ICMP message is appropriate, it can be ignored at any other
   point in the exchange.

12.1  HITs used in ACLs

   It is expected that HITs will be used in ACLs.  Future firewalls can
   use HITs to control egress and ingress to networks, with an assurance
   level difficult to achieve today.  As discussed above in Section 8,
   once a HIP session has been established, the SPI value in an IPsec
   packet may be used as an index, indicating the HITs.  In practise,
   the firewalls can inspect the HIP packets to learn of the bindings
   between HITs, SPI values, and IP addresses.  They can even explicitly
   control IPsec usage, dynamically opening IPsec ESP only for specific
   SPI values and IP addresses.  The signatures in the HIP packets allow
   a capable firewall to make sure that the HIP exchange is indeed
   happening between two known hosts.  This may increase firewall

   There has been considerable bad experience with distributed ACLs that
   contain public key related material, for example, with SSH.  If the
   owner of the key needs to revoke it for any reason, the task of
   finding all locations where the key is held in an ACL may be
   impossible.  If the reason for the revocation is due to private key
   theft, this could be a serious issue.

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   A host can keep track of all of its partners that might use its HIT
   in an ACL by logging all remote HITs.  It should only be necessary to
   log responder hosts.  With this information, the host can notify the
   various hosts about the change to the HIT.  There has been no attempt
   to develop a secure method to issue the HIT revocation notice.

   HIP-aware NATs, however, are transparent to the HIP aware systems by
   design.  Thus, the host may find it difficult to notify any NAT that
   is using a HIT in an ACL.  Since most systems will know of the NATs
   for their network, there should be a process by which they can notify
   these NATs of the change of the HIT.  This is mandatory for systems
   that function as responders behind a NAT.  In a similar vein, if a
   host is notified of a change in a HIT of an initiator, it should
   notify its NAT of the change.  In this manner, NATs will get updated
   with the HIT change.

12.2  Non-security Considerations

   The definition of the Host Identifier states that the HI need not be
   a public key.  It implies that the HI could be any value; for example
   an FQDN.  This document does not describe how to support such a
   non-cryptographic HI.  A non-cryptographic HI would still offer the
   services of the HIT or LSI for NAT traversal.  It would be possible
   carry the HITs in HIP packets that had neither privacy nor
   authentication.  Since such a mode would offer so little additional
   functionality for so much addition to the IP kernel, it has not been
   defined.  Given how little public key cryptography HIP requires, HIP
   should only be implemented using public key Host Identities.

   If it is desirable to use HIP in a low security situation where
   public key computations are considered expensive, HIP can be used
   with very short Diffie-Hellman and Host Identity keys.  Such use
   makes the participating hosts vulnerable to MitM and connection
   hijacking attacks.  However, it does not cause flooding dangers,
   since the address check mechanism relies on the routing system and
   not on cryptographic strength.

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13.  Acknowledgments

   For the people historically involved in the early stages of HIP, see
   the Acknowledgements section in the Host Identity Protocol
   specification [4].

   During the later stages of this document, when the editing baton was
   transfered to Pekka Nikander, the comments from the early
   implementors and others, including Jari Arkko, Tom Henderson, Petri
   Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan Melen, Tim
   Shepard, Jukka Ylitalo, and Jorma Wall, were invaluable.  Finally,
   Spencer Dawkins and Dave Crocker provided valuable input during the
   final stages of publication, most of which was incorporated but some
   of which the authors decided to ignore in order to get this document
   published in the first place.

14  References (informative)

   [1]   Tsirtsis, G. and P. Srisuresh, "Network Address Translation -
         Protocol Translation (NAT-PT)", RFC 2766, February 2000.

   [2]   Srisuresh, P. and K. Egevang, "Traditional IP Network Address
         Translator (Traditional NAT)", RFC 3022, January 2001.

   [3]   Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
         Specific IP: Framework", RFC 3102, October 2001.

   [4]   Moskowitz, R., Nikander, P. and P. Jokela, "Host Identity
         Protocol", draft-moskowitz-hip-09 (work in progress), February

   [5]   Richardson, M., "A method for storing IPsec keying material in
         DNS", draft-ietf-ipseckey-rr-10 (work in progress), April 2004.

   [6]   Lear, E. and R. Droms, "What's In A Name:Thoughts from the
         NSRG", draft-irtf-nsrg-report-10 (work in progress), September

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

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

   [9]   Chiappa, J., "Endpoints and Endpoint Names: A Proposed
         Enhancement  to the Internet Architecture", URL

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, 1999.

   [10]  Nikander, P., "Denial-of-Service, Address Ownership, and Early
         Authentication in the IPv6 World", in Proceesings of Security
         Protocols, 9th International Workshop,  Cambridge, UK, April
         25-27 2001, LNCS 2467, pp. 12-26,  Springer, 2002.

   [11]  Bellovin, S., "EIDs, IPsec, and HostNAT", in Proceesings of
         41th IETF, Los Angeles, CA, March 1998.

Authors' Addresses

   Robert Moskowitz
   ICSAlabs, a Division of TruSecure Corporation
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA


   Pekka Nikander
   Ericsson Research Nomadic Lab

   JORVAS  FIN-02420

   Phone: +358 9 299 1

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