





Network Working Group                                       R. Moskowitz
Request for Comments: 4423     ICSA Labs, a division of Cybertrust, Inc.
Category: Informational                                      P. Nikander
                                           Ericsson Research Nomadic Lab
                                                                May 2006


               Host Identity Protocol (HIP) Architecture


Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   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 (HIP), between the internetworking and
   transport layers.  Herein are presented the basics of the current
   namespaces, their 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
   authors as of Fall 2003.  The architecture may have evolved since.
   This document represents one stable point in that evolution of
   understanding.

Table of Contents

   1. Disclaimer ......................................................2
   2. Introduction ....................................................2
   3. Terminology .....................................................4
      3.1. Terms Common to Other Documents ............................4
      3.2. Terms Specific to This and Other HIP Documents .............4
   4. Background ......................................................6
      4.1. A Desire for a Namespace for Computing Platforms ...........6
   5. Host Identity Namespace .........................................8
      5.1. Host Identifiers ...........................................9
      5.2. Storing Host Identifiers in DNS ............................9
      5.3. Host Identity Tag (HIT) ...................................10
      5.4. Local Scope Identifier (LSI) ..............................10
   6. New Stack Architecture .........................................11



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      6.1. Transport Associations and End-points .....................11
   7. End-host Mobility and Multi-homing .............................12
      7.1. Rendezvous Mechanism ......................................13
      7.2. Protection against Flooding Attacks .......................13
   8. HIP and IPsec ..................................................14
   9. HIP and NATs ...................................................15
      9.1. HIP and TCP Checksums .....................................15
   10. Multicast .....................................................16
   11. HIP Policies ..................................................16
   12. Benefits of HIP ...............................................16
      12.1. HIP's Answers to NSRG Questions ..........................17
   13. Security Considerations .......................................19
      13.1. HITs Used in ACLs ........................................21
      13.2. Non-security considerations ..............................21
   14. Acknowledgements ..............................................22
   15. Informative References ........................................22

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.  Occasionally, this memo may be
   confusing or self-contradicting.  That is (partially) intentional,
   and it reflects the snapshot nature of this memo.

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and notes that the decision to publish is not based on IETF
   review.  However, the ideas put forth in this RFC have generated
   significant interest, including the formation of the IETF HIP Working
   Group and the IRTF HIP Research Group.  These groups are expected to
   generate further documents, sharing their findings with the whole
   Internet community.

2.  Introduction

   The Internet has two important 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 to do too
   much with them.  Semantic overloading and functionality extensions
   have greatly complicated these namespaces.

   The proposed Host Identity namespace fills an important gap between
   the IP and DNS namespaces.  The Host Identity namespace consists of
   Host Identifiers (HIs).  A Host Identifier is cryptographic in its



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   nature; it is the public key of an asymmetric key-pair.  Each host
   will have at least one Host Identity, but it will typically have more
   than one.  Each Host Identity uniquely identifies a single host;
   i.e., no two hosts have the same Host Identity.  The Host Identity,
   and the corresponding Host Identifier, can be either public (e.g.,
   published in the 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 Identifiers could be used in many authentication
   systems, such as the Internet Key Exchange (IKEv2) Protocol [9], the
   presented architecture introduces a new protocol, called the Host
   Identity Protocol (HIP), and a cryptographic exchange, called the HIP
   base exchange; see also Section 8.  The HIP protocols provide for
   limited forms of trust between systems, enhance mobility, multi-
   homing, and dynamic IP renumbering; aid in protocol
   translation/transition; and reduce certain types of denial-of-service
   (DoS) attacks.

   When HIP is used, the actual payload traffic between two HIP hosts is
   typically, but not necessarily, protected with IPsec.  The Host
   Identities are used to create the needed IPsec Security Associations
   (SAs) 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 packets.






















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3.  Terminology

3.1.  Terms Common to Other Documents

   +--------------+----------------------------------------------------+
   | Term         | Explanation                                        |
   +--------------+----------------------------------------------------+
   | public key   | The public key of an asymmetric cryptographic key  |
   |              | pair.  Used as a publicly known identifier for     |
   |              | cryptographic identity authentication.             |
   |              |                                                    |
   | Private key  | The private or secret key of 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   | An asymmetric cryptographic key pair consisting of |
   | pair         | public and private keys.  For example,             |
   |              | Rivest-Shamir-Adelman (RSA) and Digital Signature  |
   |              | Algorithm (DSA) key pairs are such key pairs.      |
   |              |                                                    |
   | 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.

















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   +--------------+----------------------------------------------------+
   | Term         | Explanation                                        |
   +--------------+----------------------------------------------------+
   | computing    | An entity capable of communicating and computing,  |
   | platform     | for example, a computer.  See the definition of    |
   |              | 'end-point', above.                                |
   |              |                                                    |
   | HIP base     | A cryptographic protocol; see also Section 8.      |
   | exchange     |                                                    |
   |              |                                                    |
   | HIP packet   | An IP packet that carries a 'Host Identity         |
   |              | Protocol' message.                                 |
   |              |                                                    |
   | Host         | An abstract concept assigned to a 'computing       |
   | Identity     | platform'.  See 'Host Identifier', below.          |
   |              |                                                    |
   | Host         | A namespace formed by all possible Host            |
   | Identity     | Identifiers.                                       |
   | namespace    |                                                    |
   |              |                                                    |
   | Host         | A protocol used to carry and authenticate Host     |
   | Identity     | Identifiers and other information.                 |
   | Protocol     |                                                    |
   |              |                                                    |
   | Host         | A 128-bit datum created by taking a cryptographic  |
   | Identity Tag | hash over a Host Identifier.                       |
   |              |                                                    |
   | Host         | A public key used as a name for a Host Identity.   |
   | Identifier   |                                                    |
   |              |                                                    |
   | Local Scope  | A 32-bit datum denoting a Host Identity.           |
   | Identifier   |                                                    |
   |              |                                                    |
   | Public Host  | A published or publicly known Host Identifier used |
   | Identifier   | as a public name for a Host Identity, and the      |
   | and Identity | corresponding Identity.                            |
   |              |                                                    |
   | Unpublished  | A Host Identifier that is not placed in any public |
   | Host         | directory, and the corresponding Host Identity.    |
   | Identifier   | Unpublished Host Identities are typically          |
   | and Identity | shortlived in nature, being often replaced and     |
   |              | possibly used just once.                           |
   |              |                                                    |
   | Rendezvous   | A mechanism used to locate mobile hosts based on   |
   | Mechanism    | their Host Identity Tag (HIT).                    |
   +--------------+----------------------------------------------------+





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4.  Background

   The Internet is built from three principal components: computing
   platforms (end-points), 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.  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, HTTP, and SIP addresses all
   reference 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
   destinations.

   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.

   There are three critical deficiencies with the current namespaces.
   First, dynamic readdressing cannot be directly managed.  Second,
   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.

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.



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   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:

   o  The namespace should be applied to the IP 'kernel'.  The IP kernel
      is the 'component' between applications and the packet transport
      infrastructure.

   o  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 Transport Control
      Block, TCB).  This may require changes to the current APIs.  In
      the long run, it is probable that some new APIs are needed.

   o  The introduction of the namespace should not mandate any
      administrative infrastructure.  Deployment must come from the
      bottom up, in a pairwise deployment.

   o  The names should have a fixed-length representation, for easy
      inclusion in datagram headers and existing programming interfaces
      (e.g., the TCB).

   o  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.

   o  Name collisions should be avoided as much as possible.  The
      mathematics of the birthday paradox can be used to estimate the
      chance of a collision in a given population and hash space.  In
      general, for a random hash space of size n bits, we would expect
      to obtain a collision after approximately 1.2*sqrt(2**n) hashes
      were obtained.  For 64 bits, this number is roughly 4 billion.  A
      hash size of 64 bits may be too small to avoid collisions in a
      large population; for example, there is a 1% chance of collision
      in a population of 640M. For 100 bits (or more), we would not
      expect a collision until approximately 2**50 (1 quadrillion)
      hashes were generated.

   o  The names should have a localized abstraction that can be used in
      existing protocols and APIs.




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   o  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.

   o  The namespace should provide authentication services.

   o  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.

   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.

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
   DNS Security (DNSSEC) [2], Pretty Good Privacy (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 will be specified in the Host Identity Protocol
   specification, a public-key-based HI can authenticate the HIP packets
   and protect them from man-in-the-middle attacks.  Since authenticated
   datagrams are mandatory to provide much of HIP's DoS 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 DoS attacks than the
   Internet has without Host Identity.





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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.  In addition, 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 for authentication in 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
   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 Lightweight Directory Access Protocol (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
   Resource Record (RR) type, to be defined.  This RR type is likely to
   be quite similar to the IPSECKEY RR [6].





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   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 a 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.  First, its fixed length makes for
   easier protocol coding and also better manages the packet size cost
   of this technology.  Second, 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 of
   a single HIT mapping 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 hint
   for the correct public key to use.

5.4.  Local Scope Identifier (LSI)

   A Local Scope Identifier (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.

   Examples of how LSIs can be used include: as the address in an 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 [7] and, e.g., the unpublished
   Internet Draft "Endpoints and Endpoint Names" [10] 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 an 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 is
   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.

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, Dynamic Host
   Configuration Protocol (DHCP), IPv6 prefix reassignments, or a
   Network Address Translation (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 ongoing, 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 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 restarting a
   connection after the initial connection).












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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 [1] 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.

   A separate document will specify the details of the HIP rendezvous
   mechanism.

7.2.  Protection against Flooding Attacks

   Although 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 address.

   Blindly accepting new addresses would potentially lead to flooding
   DoS 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,



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   such performance optimization must be restricted to peers that are
   known to be trustworthy and capable of protecting themselves from
   malicious software.

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 Encapsulating 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
   protection.

   In practice, 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.

   While it would be possible, at least in theory, to use some existing
   cryptographic protocol, such as IKEv2 together with Host Identifiers,
   to establish the needed SAs, HIP defines a new protocol.  There are a
   number of historical reasons for this, and there are also a few
   architectural reasons.  First, IKE and IKEv2 were not designed with
   middle boxes in mind.  As adding a new naming layer allows one to
   potentially add a new forwarding layer (see Section 9, below), it is
   very important that the HIP protocols are friendly toward any middle
   boxes.

   Second, 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.  In particular, the current thinking is limited to a situation
   where, conceptually, there is only one pair of SAs between any given
   pair of HITs.  In other words, from an architectural point of view,
   HIP only supports host-to-host (or endpoint-to-endpoint) Security
   Associations.  If two hosts need more pairs of parallel SAs, they
   should use separate HITs for that.  However, 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 or so-called
   Bump-in-the-Wire (BITW) implementations, 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 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.



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   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 [12].

   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.
   An SA times out if no packets are received using that SA.
   Implementations may support lifetimes for the various ESP transforms.

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) [4] or NAT Protocol Translation (NAT-PT) [3].

   In a network environment where 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; they 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.
   The NAT system has to learn mappings both from HITs and from SPIs to
   IP addresses.  Many HITs (and SPIs) 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.

   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 placeholder for embedded IP addresses.

9.1.  HIP and TCP Checksums

   There is no way for a host to know if any of the IP addresses in an
   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



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   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.

10.  Multicast

   Back in the Fall of 2003, there were little if any concrete thoughts
   about how HIP might affect IP-layer or application-layer multicast.

11.  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.

12.  Benefits of HIP

   In the beginning, the network layer protocol (i.e., IP) had the
   following four "classic" invariants:

   o  Non-mutable: The address sent is the address received.

   o  Non-mobile: The address does not change during the course of an
      "association".

   o  Reversible: A return header can always be formed by reversing the
      source and destination addresses.

   o  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.



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   In the current "post-classic" world, we are intentionally trying 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 [5] 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.  That
   is, if they have a Fully Qualified Domain Name (FQDN), that name
   typically belongs to a NAT device or a dial-up server, and does not
   really identify the system itself but its current connectivity.
   FQDNs (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 the
   DNS; they do not have services of interest to other Internet hosts.

   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-specific 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.
   You do not care if the network-layer address changes in transit
   (mutable), and you do not care what network-layer address the partner
   is using (non-omniscient).

12.1.  HIP's Answers to NSRG Questions

   The IRTF Name Space Research Group has posed a number of evaluating
   questions in its report [7].  In this section, we provide answers to
   these questions.

   1.  How would a stack name improve the overall functionality of the
       Internet?

          HIP decouples the internetworking layer from the transport
          layer, allowing each to evolve separately.  The decoupling
          makes end-host mobility and multi-homing easier, also across



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          IPv4 and IPv6 networks.  HIs make network renumbering easier,
          and 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.  In addition, 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
          deployed.

   7.  If we add an additional layer, would it make the address list in
       Stream Control Transmission Protocol (SCTP) unnecessary?

          Yes.





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   8.  What additional security benefits would a new naming scheme
       offer?

          HIP reduces dependency on IP addresses, making the so-called
          address ownership [11] 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.

13.  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 DoS 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.

   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



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   packet is fixed and easily replayed, the initiator reacts to it only
   if it has just started a connection to the responder.

   MitM 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 an 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 an
   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
   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 have to either have unique content, and
   thus difficult to generate, resulting in yet another DoS attack, or
   be 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.





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13.1.  HITs Used in ACLs

   It is expected that HITs will be used in Access Control Lists (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 practice, firewalls can inspect 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 HIP packets allow a capable firewall to ensure that the
   HIP exchange is indeed happening between two known hosts.  This may
   increase firewall security.

   There has been considerable bad experience with distributed ACLs that
   contain public-key-related material, for example, with Secure SHell
   Protocol (SSH).  If the owner of a 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.

   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.

13.2.  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 to carry 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



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   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.

14.  Acknowledgements

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

   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,
   Lars Eggert, 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.

15.  Informative References

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

   [2]   Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033, March
         2005.

         Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

         Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Protocol Modifications for the DNS Security Extensions", RFC
         4035, March 2005

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




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

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

   [6]   Richardson, M., "A Method for Storing IPsec Keying Material in
         DNS", RFC 4025, March 2005.

   [7]   Lear, E. and R. Droms, "What's In A Name: Thoughts from the
         NSRG", Work in Progress, September 2003.

   [8]   Nikander, P., et al, "Mobile IP Version 6 Route Optimization
         Security Design Background", RFC 4225, December 2005.

   [9]   Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
         4306, December 2005.

   [10]  Chiappa, J., "Endpoints and Endpoint Names: A Proposed
         Enhancement to the Internet Architecture", URL
         http://users.exis.net/~jnc/tech/endpoints.txt, 1999.

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

   [12]  Bellovin, S., "EIDs, IPsec, and HostNAT", in Proceedings of the
         41st IETF, Los Angeles, CA, March 1998.

Authors' Addresses

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

   EMail: rgm@icsalabs.com


   Pekka Nikander
   Ericsson Research Nomadic Lab
   JORVAS  FIN-02420
   FINLAND

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



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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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