Network Working Group                                       R. Moskowitz
Internet-Draft                         ICSAlabs, a Division of TruSecure
Expires: September 30, 2003                                  Corporation
                                                             P. Nikander
                                           Ericsson Research Nomadic Lab
                                                              April 2003

                  Host Identity Protocol Architecture

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on September 30, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   This memo describes the reasoning behind proposing a new namespace,
   the Host Identity namespace, and a new layer, Host Identity Layer,
   between the internetworking and transport layers. Herein are
   presented the basics of the current namespaces, strengths and
   weaknesses, and how a new namespace will add completeness to them.
   This new namespace's roles in the protocols are defined.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1 A Desire for Namespace for Computing Platforms . . . . . . . .  5
   3.  Host Identity Namespace  . . . . . . . . . . . . . . . . . . .  7
   3.1 Host Identifiers . . . . . . . . . . . . . . . . . . . . . . .  7
   3.2 Storing Host Identifiers in DNS  . . . . . . . . . . . . . . .  8
   3.3 Host Identity Tag (HIT)  . . . . . . . . . . . . . . . . . . .  8
   3.4 Local Scope Identity (LSI) . . . . . . . . . . . . . . . . . .  8
   4.  The New Architecture . . . . . . . . . . . . . . . . . . . . . 10
   4.1 Transport associations and endpoints . . . . . . . . . . . . . 10
   5.  End-Host Mobility and Multi-Homing via HIP . . . . . . . . . . 12
   5.1 Rendezvous server  . . . . . . . . . . . . . . . . . . . . . . 12
   5.2 Protection against Flooding Attacks  . . . . . . . . . . . . . 13
   6.  HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.1 HIP and TCP Checksum . . . . . . . . . . . . . . . . . . . . . 14
   7.  HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Benefits of HIP  . . . . . . . . . . . . . . . . . . . . . . . 16
   8.1 HIP's Answers to NSRG questions  . . . . . . . . . . . . . . . 17
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.1 HITs used in ACLs  . . . . . . . . . . . . . . . . . . . . . . 20
   9.2 Non-security Considerations  . . . . . . . . . . . . . . . . . 21
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
       Intellectual Property and Copyright Statements . . . . . . . . 24

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1. Introduction

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

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

   Although the Host Identity can be used in many authentication
   systems, its design principle calls out for a new protocol and
   exchange [6] that will support limited forms of trust between
   systems, enhance mobility, multi-homing and dynamic IP renumbering,
   aid in protocol translation / transition, and greatly reduce denial
   of service (DoS) attacks.

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2. Background

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

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

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

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

   In the current Internet, the transport layers are coupled to the IP
   addresses.  Neither can evolve separately from the other.  IPng
   deliberations were framed by concerns of requiring a TCPng effort as

   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 addresses provide naming for both carbon and silicon based
   people.  Email addresses are extensions of Domain Names, only in so
   far as a named service is responsible for managing a person's mail.
   There is some anonymity in Email addresses.

   There are three critical deficiencies with the current namespaces.
   Dynamic readdressing cannot be directly managed.  Anonymity is not
   provided in a consistent, trustable manner.  And authentication for
   systems and datagrams is not provided.  All because computing
   platforms are not well named with the current namespaces.

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2.1 A Desire for 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
   either from mobility or renumbering.

   If the namespace for computing platforms is cryptographically based,
   it can also provide authentication services for IPsec.  If this
   namespace is locally created without requiring registration, it can
   provide anonymity.

   Such a namespace (for computing platforms) and the names in it should
   have the following characteristics:

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

      The namespace should fully decouple the internetworking layer from
      the higher layers.  The names should replace all occurrences of IP
      addresses within applications (like in the TCB).  This may require
      changes to the current APIs.  In the long run, it is probable that
      some new APIs are needed.

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

      The names should have a fixed length representation, for easy
      inclusion in datagrams and programming interfaces (e.g the TCB).

      Using the namespace should be affordable when used in protocols.
      This is primarily a packet size issue.  There is also a
      computational concern in affordability.

      The names must be statistically globally unique.  64 bits is
      inadequate (1% chance of collision in a population of 640M); thus
      approximately 100 or more bits should be used.

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

      It must be possible to create names locally. This can provide
      anonymity at the cost of making resolvability very difficult.

         Sometimes the names may contain a delegation component. This is

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         the cost of resolvability.

      The namespace should provide authentication services.  This is a
      preferred function.

      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, such a new namespace is called the Host Identity
   namespace.  Using Host Identities requires its own protocol layer
   (the Host Identity Protocol), between the internetworking and
   transport layers.  The names are based on Public Key Cryptography to
   supply authentication services. Properly designed, it can deliver all
   of the above stated requirements.

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3. 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, but not
   limited to, an IP stack. A system can have multiple identities, some
   'well known', some anonymous.  A system may self assert its identity,
   or may use a third-party authenticator like DNSSEC, PGP, or X.509 to
   'notarize' the identity assertion.  DNSSEC is a "SHOULD" implement
   authenticator for the Host Identity namespace.

   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 a Host Identifier can be any name that can claim
   'statistically globally unique', a public key of a 'public key' pair
   makes the best Host Identifiers.  As documented in the Host Identity
   Protocol (HIP) specification [6], a public key based HI can
   authenticate the HIP packets and protect them for man-in-the-middle
   attacks.  And 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
   datagrams are supported in practice.  The non-cryptographic forms of
   HI and HIP are presented to complete the theory of HI, but should not
   be implemented as they could produce worse DoS attacks than the
   Internet has without HI.

3.1 Host Identifiers

   Host Identity adds two main features to Internet protocols.  The
   first is a decoupling of the internetworking and transport layers,
   see Section 4.  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.  Whenever the Host Identifier is a public key, this
   key can be used to authenticate security protocols like IPsec.

   The preferred structure of the Host Identity is that of a public key
   pair.  In that case the Host Identity is referred to by its public
   component, the public key.   Thus, the name representing the 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.  It the private key is possessed by more than
   one node, the Identity can be considered to be a distributed one.

   Any other Internet naming convention may be used for the Host

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   Identifiers.  However, these 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.

   The actual Host Identities are never directly used in any Internet
   protocols.  The corresponding Host Identifiers (public keys) may be
   stored in various DNS or LDAP directories as identified elsewhere in
   this document, and they are passed in the HIP protocol.  A Host
   Identity Tag (HIT) is used in other protocols to represent the Host
   Identities.  Another representation of the Host Identities, the Local
   Scope Identity (LSI), can also be used in protocols and APIs.  LSI's
   advantage over HIT is its size; its disadvantage is its local scope.

3.2 Storing Host Identifiers in DNS

   The Host Identifiers should be stored in DNS.  The exception to this
   is anonymous identities.  The HI is stored in a new RR type, to be
   defined.  This RR type is likely to be very similar to the IPSECKEY
   RR [7].

   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.

3.3 Host Identity Tag (HIT)

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

   When the Host Identity is a public key pair, HIT functions much like
   the SPI does in IPsec.  However, instead of being an arbitrary 32-bit
   value used to identify the Security Association for a datagram, a HIT
   identifies the public key pair that can validate the packet
   authentication.  HIT should be unique in the whole IP universe.  In
   the rare case that a single HIT happens to map to more than one Host
   Identities, 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.

3.4 Local Scope Identity (LSI)

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   An LSI is a 32 bit localized representation for a Host Identity. The
   purpose of an LSI is to facilitate using Host Identity in existing
   protocols and APIs.  The generation of LSI is to be determined; two
   candidate solutions are to let the peer pick its incoming LSI (like
   IPsec SPI) or to use a 32-bit subset of the HIT.

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

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4. The New 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 [8] and, e.g., the unpublished
   Internet-Draft Endpoints and Endpoint Names [10], currently the IP
   addresses embody the dual role of locators and endpoint 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
   endpoint name.

   In the HIP Architecture, the endpoint names and locators are
   separated from each other.  IP addresses continue to act as locators.
   The Host Identities take the role of endpoint identifiers.  It is
   important to understand that the endpoint 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.

   Process ------ Socket                  Process ------ Socket
                    |                                      |
                    |                                      |
                    |                                      |
                    |                                      |
   Endpoint         |                     Endpoint --- Host Identity
            \       |                                      |
              \     |                                      |
                \   |                                      |
                  \ |                                      |
   Location --- IP address                Location --- IP address

                                Figure 1

4.1 Transport associations and endpoints

   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 more bound to IP
   addresses but to Host Identities.

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

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5. End-Host Mobility and Multi-Homing via HIP

   As HIP decouples the transport from the internetworking layer, and
   binds the transport associations to the Host Identifiers (through
   actually either the HIT or LSI), HIP can provide for a degree of
   internetworking mobility and multi-homing at a very low
   infrastructure cost.  HIP internetworking mobility includes IP
   address changes (via any method) to either the initiator or
   responder.  Thus, a system is considered mobile if its IP address can
   change dynamically for any reason like PPP, DHCP, IPv6 TLA
   reassignments, or a NAT 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 allows these IP addresses
   to be linked with each other, and if one address becomes unusable
   (e.g. due a network failure), existing transport associations can be
   easily 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 ESP packet from any address and totally
   ignore the source address for anything more than transmitting return
   packets.  However, as discussed in Section 5.2 below, the mobile node
   must send a HIP readdress packet to inform the peer of the new
   address(es) of the mobile node, and the peer must verify that the new
   addresses are reachable.  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

5.1 Rendezvous server

   Making a contact to a mobile node is slightly more involved.  The
   initiator node has to know where the mobile node is to start the HIP
   exchange.  HIP need not rely on Dynamic DNS for this function, but
   uses a rendezvous server.  Instead of registering its current dynamic
   address to the DNS server, the mobile node registers the address of
   the rendezvous server.  The mobile node keeps the rendezvous server
   continuously updated with its current IP address(es).  The rendezvous
   server simply forwards the initial HIP packet from an initiator to
   the mobile node at its current location. All further packets flow
   between the initiator and the mobile node. There is typically very
   little activity on the rendezvous server, address updates and initial
   HIP packet forwarding, thus one server can support a large number of
   potential mobile nodes.  The mobile nodes must trust the rendezvous
   server to properly maintain its HIT and IP address mapping.

   The rendezvous server is also needed if both of the nodes are mobile
   and happen to move at the same time.  In that case the HIP readdress

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   packets will cross each other in the network and never reach the peer
   node.  To solve this situation, the nodes should remember the
   rendezvous server address, and re-send the HIP readdress packet to
   the rendezvous server if no reply is received.

   The mobile node keeps its address current on the rendezvous server by
   setting up a HIP based SA with the rendezvous server and sending it
   HIP readdress packets.  A rendezvous server will permit two mobile
   systems to use HIP without any extraneous infrastructure, including
   DNSSEC if they have a method other than a DNS query to get each
   other's HI and HIT.

5.2 Protection against Flooding Attacks

   In an earlier version of this document the nodes were permitted to
   inform about address changes by simply sending packets with a new
   source address.  While receiving packets in HIP still does not rely
   on the source address for anything, it appears to be necessary to
   check the mobile node's reachability at the new address(es) before
   actually sending any larger amounts of traffic to the address.

   Blindly accepting new addresses would potentially lead to a flooding
   Denial-of-Service attack against third parties [9].  In a distributed
   flooding attack an attacker opens (anonymous) high volume HIP
   connections with a large number of hosts, 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 the
   node is separately checked at each address before actually using the
   address for larger amounts of traffic.

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

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6. 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) [3]  or NAT Protocol translation (NAT-PT) [2].

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

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

   The NAT systems cannot touch the datagrams within the 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 place holder for embedded IP addresses.

6.1 HIP and TCP Checksum

   There is no way for a host to know if any of the IP addresses in the
   IP header are the addresses used to calculate the TCP Checksum. That
   is, it is not feasible to calculate the TCP checksum using the IP
   addresses in the pseudo header; the addresses received in the
   incoming packet are not necessarily the same as they were on the
   sending host.  Furthermore, it is not possible to recompute the upper
   layer checksums in the NAT/NAT-PT system, since the traffic is IPsec
   protected.  Consequently, the TCP and UDP checksums are calculated
   using the HIT (or some other representation of the HI) in the place
   of the IP addresses in the pseudo header.

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7. 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 one for anonymous usage.
   Although anonymous HIs will be rarely used as responder HIs, they are
   likely be common for initiators.  Support for multiple HIs is

   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 transform and local lifetimes.

   Responders would need a similar policy, representing which hosts they
   accept HIP exchanges, and the preferred transform and local

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8. Benefits of HIP

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

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

      Non-mobile: The address doesn't change during the course of an

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

      Omniscient: Each host knows what address a partner host can use to
      send packets to it.

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

   In the current "post-classic" world, we are trying intentionally to
   get rid of the second invariant (both for mobility and for
   multi-homing), and we have been forced to give up the first and the
   fourth.  Realm Specific IP [4] 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, or more specifically,
   Fully Qualified Domain Names (FQDN).  FQDN names (and their
   extensions as email names) are Application Layer names; more
   frequently naming processes than a particular system.  This is why
   most systems on the internet are not registered in DNS; they do not
   have processes of interest to other Internet hosts.

   DNS names are indirect 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
   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

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   reversibility allows one to get a packet back to one's partner host.
   You don't care if the NLP changes in transit (mutable) and you don't
   care what NLP the partner is using (non-omniscient).

   Since all systems can have a Host Identity, every system can have an
   entry in the DNS.  The mobility features in HIP make it attractive to
   trusted 3rd parties to offer rendezvous servers.

8.1 HIP's Answers to NSRG questions

   The IRTF Name Space Research Group has posed a number of evaluating
   questions in their report [8].  In this section, we provide answers
   to these questions.

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

          The HIP Host Identifiers make end-host mobility and
          multi-homing easier by separating the transport layer and
          internetworking layer from each other.  Among other things,
          this allows mobility and multi-homing accross the IPv4 and
          IPv6 internets.  They also make network re-numbering easier.
          At the conceptual level, they also make process migration and
          clustered servers easier to implement.  Furthermore, being
          cryptographic in nature, they also provide the basis for
          solving the security problems related to end-host mobility and

   2.  What does a stack name look like?

          A HIP Host Identifier 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 Host Identifiers are typically long lived, with a
          lifetime of years of more.  The lifetime of temporary Host
          Identifiers 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 HIP Host Identifiers live between the transport and
          internetworking layers.

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   5.  How is it used on the end points

          The HIP Host Identifiers, in the form of HITs or LSIs, are
          used by legacy applications as if they were IP addresses.
          Additionally, the Host Identifiers, as public keys, are used
          in a built in key agreement protocol to authenticate the
          Diffie-Hellman key exchange.

   6.  What administrative infrastructure is needed to support it?

          It is possible to use HIP opportunistically, without any
          infrastructure.  However, the gain full benefit from HIP, the
          Host Identifiers must be stored in the DNS, and a new
          infrastructure of Rendezvous servers is needed.

   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 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 Host Identifiers and IP addresses is
          sufficient.  However, if it becomes necessary to resolve Host
          Identifiers into IP addresses or back to DNS names, a flat,
          hash based resolution infrastructure is needed.  Such an
          infrastructure could be based on the ideas of Distributed Hash
          Tables, but would require significant new development and

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9. Security Considerations

   HIP takes advantage of the new Host Identity paradigm to provide
   secure authentication of hosts and provide a fast key exchange for
   IPsec 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.

   The Security Association for ESP is indexed by the SPI or HIT, not
   the SPI and IP address.  HIP enabled 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 ESP to DoS attacks.

   Denial-of-service attacks take advantage of the cost of start of
   state for a protocol on the responder compared to the 'cheapness' on
   the initiator.  HIP both allows 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 3-way
   cookie exchange instead of the initiator, making the HIP protocol 4
   packets long.  There are more details on this process in the HIP
   protocol document [6].

   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 spoofed, 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 HIP; but HIP indirectly provides the following protection
   from a MitM attack.  If the responder's HI is retrieved from a signed
   DNS zone by the initiator, the initiator can use this to validate 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 anonymous HI, it
   knowingly risks a MitM attack.  The responder may choose not to

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   accept a HIP exchange with an anonymous initiator.

   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' 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 delta time 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 rejection of a HIP
   initiation.  This is a simple ICMP Protocol 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.  The defense against this MitM
   attack is for the responder 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 exchange where this ICMP message is appropriate, it can be
   ignored at any other point in the exchange.

9.1 HITs used in ACLs

   It is expected that HITs will be used in ACLs.  Firewalls will use
   HITs to control egress and ingress to networks, with an assurance
   difficult to achieve today.

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

   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
   here to develop a secure method (like in CMP and CMC) to issue the
   HIT revocation notice.

   NATs, however, are transparent to the HIP aware systems by design.
   Thus, the host many 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

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   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.

9.2 Non-security Considerations

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

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   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

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

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

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

   [5]   Jokela, P., "Optimized Packet Structure for HIP",
         draft-jokela-hip-packets-01 (work in progress), November 2002.

   [6]   Moskowitz, R., "The Host Identity Payload",
         draft-moskowitz-hip-05 (work in progress), November 2001.

   [7]   Richardson, M., "A method for storing IPsec keying material in
         DNS", draft-ietf-ipseckey-rr-01 (work in progress), April 2003.

   [8]   Lear, E. and R. Droms, "What's In A Name:Thoughts from the
         NSRG", draft-irtf-nsrg-report-09 (work in progress), March

   [9]   Nikander, P., "Mobile IP version 6 Route Optimization Security
         Design Background", draft-nikander-mobileip-v6-ro-sec-00 (work
         in progress), April 2003.

   [10]  Chiappa, J., "Endpoints and Endpoint Names: A Proposed
         Enhancement  to the Internet Architecture", URL http://, 1999.

Authors' Addresses

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


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   Pekka Nikander
   Ericsson Research Nomadic Lab

   JORVAS  FIN-02420

   Phone: +358 9 299 1

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