Network Working Group                                       R. Moskowitz
Internet-Draft                                                   Verizon
Obsoletes: 4423 (if approved)                         September 28, 2012
Intended status: Standards Track
Expires: April 1, 2013

                  Host Identity Protocol Architecture


   This memo describes a 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, 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.

   This document obsoletes RFC 4423 and addresses the concerns raised by
   the IESG, particularly that of crypto agility.  It incorporates
   lessons learned from the implementations of RFC 5201 and goes further
   to explain how HIP works as a secure signalling channel.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 1, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1.  Terms common to other documents  . . . . . . . . . . . . . .  5
   2.2.  Terms specific to this and other HIP documents . . . . . . .  5
   3.    Background . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.1.  A desire for a namespace for computing platforms . . . . . .  7
   4.    Host Identity namespace  . . . . . . . . . . . . . . . . . .  9
   4.1.  Host Identifiers . . . . . . . . . . . . . . . . . . . . . . 10
   4.2.  Host Identity Hash (HIH) . . . . . . . . . . . . . . . . . . 10
   4.3.  Host Identity Tag (HIT)  . . . . . . . . . . . . . . . . . . 11
   4.4.  Local Scope Identifier (LSI) . . . . . . . . . . . . . . . . 11
   4.5.  Storing Host Identifiers in Directories  . . . . . . . . . . 12
   5.    New stack architecture . . . . . . . . . . . . . . . . . . . 12
   5.1.  Transport associations and end-points  . . . . . . . . . . . 13
   6.    End-host mobility and multi-homing . . . . . . . . . . . . . 13
   6.1.  Rendezvous mechanism . . . . . . . . . . . . . . . . . . . . 14
   6.2.  Protection against flooding attacks  . . . . . . . . . . . . 14
   7.    HIP and ESP  . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.    HIP and MAC Security . . . . . . . . . . . . . . . . . . . . 16
   9.    HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . . 17
   9.1.  HIP and Upper-layer checksums  . . . . . . . . . . . . . . . 17
   10.   Multicast  . . . . . . . . . . . . . . . . . . . . . . . . . 18
   11.   HIP policies . . . . . . . . . . . . . . . . . . . . . . . . 18
   12.   Benefits of HIP  . . . . . . . . . . . . . . . . . . . . . . 18
   12.1. HIP's answers to NSRG questions  . . . . . . . . . . . . . . 19
   13.   Changes from RFC 4423  . . . . . . . . . . . . . . . . . . . 21
   14.   Security considerations  . . . . . . . . . . . . . . . . . . 21
   14.1. HITs used in ACLs  . . . . . . . . . . . . . . . . . . . . . 23
   14.2. Alternative HI considerations  . . . . . . . . . . . . . . . 24
   15.   IANA considerations  . . . . . . . . . . . . . . . . . . . . 24
   16.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 24
   17.   References . . . . . . . . . . . . . . . . . . . . . . . . . 25
   17.1. Normative References . . . . . . . . . . . . . . . . . . . . 25
   17.2. Informative references . . . . . . . . . . . . . . . . . . . 26
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 27

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1.  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 and 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.  A Host Identity conceptually refers to a
   computing platform, and there may be multiple such Host Identities
   per computing platform (because the platform may wish to present a
   different identity to different communicating peers).  The Host
   Identity namespace consists of Host Identifiers (HI).  There is
   exactly one Host Identifier for each Host Identity.  While this text
   later talks about non-cryptographic Host Identifiers, the
   architecture focuses on the case in which Host Identifiers are
   cryptographic in nature.  Specifically, the Host Identifier is the
   public key of an asymmetric key-pair.  Each Host Identity uniquely
   identifies a single host, i.e., no two hosts have the same Host
   Identity.  If two or more computing platforms have the same Host
   Identifier, then they are instantiating a distributed host.  The Host
   Identifier can either be public (e.g. published in the DNS), or
   unpublished.  Client systems will tend to have both public and
   unpublished Host Identifiers.

   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 IKEv2 [RFC4306], 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 7.  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 ESP.  The Host
   Identities are used to create the needed ESP Security Associations
   (SAs) and to authenticate the hosts.  When ESP is used, the actual
   payload IP packets do not differ in any way from standard ESP
   protected IP packets.

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   Much has been learned about HIP since [RFC4423] was published.  This
   document expands Host Identities beyond use to enable IP connectivity
   and security to general interhost secure signalling at any protocol
   layer.  The signal may establish a security association between the
   hosts, or simply pass information within the channel.

2.  Terminology

2.1.  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.  Public is |
   |               | a relative term here, ranging from known to peers |
   |               | only to known to the World.                       |
   |               |                                                   |
   | 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   |
   | pair          | of 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.      |

2.2.  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 and in RFC 5201
   [RFC5201-bis] 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 7.     |
   | exchange      |                                                   |
   |               |                                                   |
   | HIP packet    | An IP packet that carries a 'Host Identity        |
   |               | Protocol' message.                                |
   |               |                                                   |
   | Host Identity | An abstract concept assigned to a 'computing      |
   |               | platform'.  See 'Host Identifier', below.         |
   |               |                                                   |
   | Host Identity | A name space formed by all possible Host          |
   | namespace     | Identifiers.                                      |
   |               |                                                   |
   | Host Identity | A protocol used to carry and authenticate Host    |
   | Protocol      | Identifiers and other information.                |
   |               |                                                   |
   | Host Identity | The cryptograhic hash used in creating the Host   |
   | Hash          | Identity Tag from the Host Identity.              |
   |               |                                                   |
   | Host Identity | A 128-bit datum created by taking a cryptographic |
   | Tag           | hash over a Host Identifier plus bits to identify |
   |               | which hash used.                                  |
   |               |                                                   |
   | 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 Identfier 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       |
   | Host          | public directory, and the corresponding Host      |
   | Identifier    | Identity.  Unpublished Host Identities are        |
   | and Identity  | typically short lived in nature, being often      |
   |               | replaced and possibly used just once.             |
   |               |                                                   |
   | Rendezvous    | A mechanism used to locate mobile hosts based on  |
   | Mechanism     | their HIT.                                        |

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3.  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 addresses, 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.

   The IP addressing namespace has been overloaded to name both
   interfaces (at layer-3) and endpoints (for the endpoint-specific part
   of layer-3, and for layer-4).  In their role as interface names, IP
   addresses are sometimes called "locators" and serve as an endpoint
   within a routing topology.

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

   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.
   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.

3.1.  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

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   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' or stack.  The
      IP stack 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 so that it can be
      used in existing protocols and APIs.

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   o  It must be possible to create names locally.  When such names are
      not published, 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.

4.  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 [RFC2535], PGP, or X.509 to 'notarize' the identity assertion
   to another namespace.  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 specified in the Host Identity Protocol [RFC5201-bis]
   specification, 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 BEX 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

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   implemented as they could produce worse denial-of-service attacks
   than the Internet has without Host Identity.  There is on-going
   research in challenge puzzles to use non-cryptographic HI, like
   RFIDs, in an HIP exchange tailored to the workings of such

4.1.  Host Identifiers

   Host Identity adds two main features to Internet protocols.  The
   first is a decoupling of the internetworking and transport layers;
   see Section 5.  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 for authentication in security protocols like ESP.

   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 ESP.  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 Identifiers 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.

4.2.  Host Identity Hash (HIH)

   The Host Identity Hash is the cryptographic hash used in producing
   the HIT from the HI.  It is also the hash used through out the HIP

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   protocol for consistancy and simplicity.  It is possible to for the
   two hosts in the HIP exchange to use different hashes.

   Multiple HIHs within HIP are needed to address the moving target of
   creation and eventual compromise of cryptographic hashes.  This
   significantly complicates HIP and offers an attacker an additional
   downgrade attack that is mitigated in the HIP protocol.

4.3.  Host Identity Tag (HIT)

   A Host Identity Tag is a 128-bit representation for a Host Identity.
   It is created from an HIH and other information, like an IPv6 prefix
   and a hash identifier.  There are two advantages of using the HIT
   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.

   There can be multiple HITs per Host Identifier when multiple hashes
   are supported.  An Initator may have to initially guess which HIT to
   use for the Responder, typically based on what it prefers, until it
   learns the appropriate HIT through the HIP exchange.

   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.

4.4.  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.

   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.  LSIs
   also make it possible for some IPv4 applications to run over an IPv6

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4.5.  Storing Host Identifiers in Directories

   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 along with the
   supported HIHs are stored in a new RR type.  This RR type is defined
   in HIP DNS Extension [I-D.ietf-hip-rfc5205-bis].

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

5.  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 [nsrg-report] and, e.g., the
   unpublished Internet-Draft Endpoints and Endpoint Names
   [chiappa-endpoints], 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.

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   Transport ---- Socket                Transport ------ Socket
   association      |                   association        |
                    |                                      |
                    |                                      |
                    |                                      |
   End-point        |                    End-point --- Host Identity
            \       |                                      |
              \     |                                      |
                \   |                                      |
                  \ |                                      |
   Location --- IP address                Location --- IP address

                                 Figure 1

5.1.  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.

   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.

6.  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

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   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 ESP packet from any
   address and ignore the source address.  However, as discussed in
   Section 6.2 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).

6.1.  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 [RFC2136] 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 UPDATE packets will cross each other in the network and
   never reach the peer node.

   The HIP rendezvous mechanism is defined in HIP Rendezvous

6.2.  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.

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   Blindly accepting new addresses would potentially lead to flooding
   Denial-of-Service attacks against third parties [RFC4225].  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
   prevent this type of 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.

   A credit-based authorization approach Host Mobility with the Host
   Identity Protocol [I-D.ietf-hip-rfc5206-bis] can be used between
   hosts for sending data prior to completing the address tests.
   Otherwise, if 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.

7.  HIP and ESP

   The preferred way of implementing HIP is to use ESP to carry the
   actual data traffic.  As of today, the only completely defined method
   is to use ESP Encapsulated Security Payload (ESP) to carry the data
   packets [I-D.ietf-hip-rfc5202-bis].  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 provides mechanisms for middlebox

   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

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   granularity over other Key Management Protocols, such as IKE and
   IKEv2.  In other words, from an architectural point of view, HIP only
   supports host-to-host (or endpoint-to-endpoint) Security

   Originally, as 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 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 [Bel1998].

   It should be noted that there are already BITW implementations of HIP
   providing virtual private network (VPN) services.  This is still
   consistent to the SA bindings above.

   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.

8.  HIP and MAC Security

   The IEEE 802 standards have been defining MAC layered security.  Many
   of these standards use EAP [RFC3748] as a Key Management System (KMS)
   transport, but some like IEEE 802.15.4 [IEEE.802-15-4.2011] leave the
   KMS and its transport as "Out of Scope".

   HIP is well suited as a KMS in these environments.

   o  HIP is independent of IP addressing and can be directly
      transported over any network protocol.

   o  Master Keys in 802 protocols are strictly pair-based with group
      keys transported from the group controller using pair-wise keys.

   o  AdHoc 802 networks can be better served by a peer-to-peer KMS than
      the EAP client/server model.

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   o  Some devices are very memory constrained and a common KMS for both
      MAC and IP security represents a considerable code savings.

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) [RFC3022] or NAT Protocol translation (NAT-PT)

   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 ESP 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 ESP 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.

   An experimental HIP and NAT traversal is defined in [RFC5770].

9.1.  HIP and Upper-layer 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
   sending host.  Furthermore, it is not possible to recompute the
   upper-layer checksums in the NAT/NAT-PT system, since the traffic is
   ESP 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

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   used.  This provides for IPv4 / IPv6 protocol translation.

10.  Multicast

   Since its inception, a few studies have looked at 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 or similar directory service
   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.  This provides
   new challenges for systems or users to decide which type of HI to
   expose when they start a new session.

   Opportunistic mode (where the initator starts a HIP exchange without
   prior knowledge of the responder's HI) presents a policy tradeoff.
   It provides some security benefits but may be subject to MITM.

   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 doesn't change during the course of an

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

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   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.

   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 [RFC3102] 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 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 don't 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 their report [nsrg-report].  In this section, we provide
   answers to these questions.

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

          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
          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.  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

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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 [Nik2001] 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.  Changes from RFC 4423

   This section summarizes the changes made from [RFC4423].

14.  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
   ESP.  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

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   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
   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.

   The need to support multiple hashes for generating the HIT from the
   HI affords the MitM a potentially powerful downgrade attack due to
   the a-priori need of the HIT in the HIP base exchange.  The base
   exchange has been augmented to deal with such an attack by restarting
   on detecting the attack.  At worst this would only lead to a
   situation in which the base exchange would never finish (or would be
   aborted after some retries).  As a drawback, this leads to an 6-way
   base exchange which may seem bad at first.  However, since this only
   happens in an attack scenario and since the attack can be handled (so
   it is not interesting to mount anymore), we assume the additional
   messages are not a problem at all.  Since the MitM cannot be
   successful with a downgrade attack, these sorts of attacks will only
   occur as 'nuisance' attacks.  So, the base exchange would still be
   usually just four packets even though implementations must be
   prepared to protect themselves against the downgrade attack.

   In HIP, the Security Association for ESP is indexed by the SPI; the
   source address is always ignored, and the destination address may be
   ignored as well.  Therefore, HIP-enabled 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

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   check should not increase the exposure of 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.

14.1.  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 7,
   once a HIP session has been established, the SPI value in an ESP
   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
   ESP usage, dynamically opening 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.

   A potential of HITs in ACLs is their 'flatness' means they cannot be
   aggregated and this could result in large table searches

   There has been considerable bad experience with distributed ACLs that
   contain public key related material, for example, with 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

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   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.

14.2.  Alternative HI 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
   a 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
   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.

15.  IANA considerations

   This document has no actions for IANA.

16.  Acknowledgments

   For the people historically involved in the early stages of HIP, see

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   the Acknowledgements section in the Host Identity Protocol

   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.

   The authors want to express their special thanks to Tom Henderson,
   who took the burden of editing the document in response to IESG
   comments at the time when both of the authors were busy doing other
   things.  Without his perseverance original document might have never
   made it as RFC4423.

   This latest effort to update and move HIP forward within the IETF
   process owes its impetuous to the three HIP development teams:
   Boeing, HIIT (Helsinki Institute for Information Technology), and
   NomadicLab of Ericsson.  Without their collective efforts HIP would
   have withered as on the IETF vine as a nice concept.

17.  References

17.1.  Normative References

              Moskowitz, R., Heer, T., Jokela, P., and T. Henderson,
              "Host Identity Protocol Version 2 (HIPv2)",
              draft-ietf-hip-rfc5201-bis-09 (work in progress),
              July 2012.

              Jokela, P., Moskowitz, R., and J. Melen, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)",
              draft-ietf-hip-rfc5202-bis-01 (work in progress),
              September 2012.

              Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", draft-ietf-hip-rfc5204-bis-02 (work
              in progress), September 2012.

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              Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", draft-ietf-hip-rfc5205-bis-02
              (work in progress), September 2012.

              Henderson, T., Vogt, C., and J. Arkko, "Host Mobility with
              the Host Identity Protocol", draft-ietf-hip-rfc5206-bis-04
              (work in progress), July 2012.

17.2.  Informative references

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              February 2000.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3102]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
              "Realm Specific IP: Framework", RFC 3102, October 2001.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
              Material in DNS", RFC 4025, March 2005.

   [RFC4225]  Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
              Nordmark, "Mobile IP Version 6 Route Optimization Security
              Design Background", RFC 4225, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC5770]  Komu, M., Henderson, T., Tschofenig, H., Melen, J., and A.

Moskowitz                 Expires April 1, 2013                [Page 26]

Internet-Draft     Host Identity Protocol Architecture    September 2012

              Keranen, "Basic Host Identity Protocol (HIP) Extensions
              for Traversal of Network Address Translators", RFC 5770,
              April 2010.

              Lear, E. and R. Droms, "What's In A Name:Thoughts from the
              NSRG", draft-irtf-nsrg-report-10 (work in progress),
              September 2003.

              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              15.4: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Low-Rate Wireless Personal
              Area Networks (WPANs)", IEEE Standard 802.15.4,
              September 2011, <

              Chiappa, J., "Endpoints and Endpoint Names: A Proposed
              Enhancement  to the Internet Architecture",
              URL, 1999.

   [Nik2001]  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.

   [Bel1998]  Bellovin, S., "EIDs, IPsec, and HostNAT", in Proceedings
              of 41th IETF, Los Angeles, CA,
              March 1998.

Author's Address

   Robert Moskowitz
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA


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