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Host Identity Protocol Architecture

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9063.
Authors Robert Moskowitz , Miika Komu
Last updated 2021-07-15 (Latest revision 2019-02-14)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Gonzalo Camarillo
Shepherd write-up Show Last changed 2018-02-08
IESG IESG state Became RFC 9063 (Informational)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Éric Vyncke
Send notices to Gonzalo Camarillo <>
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
Network Working Group                                  R. Moskowitz, Ed.
Internet-Draft                                            HTT Consulting
Obsoletes: 4423 (if approved)                                    M. Komu
Intended status: Informational                                  Ericsson
Expires: August 18, 2019                               February 14, 2019

                  Host Identity Protocol Architecture


   This memo describes the Host Identity (HI) namespace, that provides a
   cryptographic namespace to applications, and the associated protocol
   layer, the Host Identity Protocol, located between the
   internetworking and transport layers, that supports end-host
   mobility, multihoming and NAT traversal.  Herein are presented the
   basics of the current namespaces, their strengths and weaknesses, and
   how a HI namespace will add completeness to them.  The roles of the
   HI namespace in the protocols are defined.

   This document obsoletes RFC 4423 and addresses the concerns raised by
   the IESG, particularly that of crypto agility.  The section on
   security considerations describe also measures against flooding
   attacks, usage of identities in access control lists, weaker types of
   identifiers and trust on first use.  This document incorporates
   lessons learned from the implementations of RFC 5201 and goes further
   to explain how HIP works as a secure signaling 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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

   This Internet-Draft will expire on August 18, 2019.

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Copyright Notice

   Copyright (c) 2019 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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.1.  Terms common to other documents . . . . . . . . . . . . . .   4
   2.2.  Terms specific to this and other HIP documents  . . . . . .   5
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.1.  A desire for a namespace for computing platforms  . . . . .   8
   4.  Host Identity namespace . . . . . . . . . . . . . . . . . . .   9
   4.1.  Host Identifiers  . . . . . . . . . . . . . . . . . . . . .  10
   4.2.  Host Identity Hash (HIH)  . . . . . . . . . . . . . . . . .  11
   4.3.  Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . .  11
   4.4.  Local Scope Identifier (LSI)  . . . . . . . . . . . . . . .  12
   4.5.  Storing Host Identifiers in directories . . . . . . . . . .  13
   5.  New stack architecture  . . . . . . . . . . . . . . . . . . .  14
   5.1.  On the multiplicity of identities . . . . . . . . . . . . .  15
   6.  Control plane . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.1.  Base exchange . . . . . . . . . . . . . . . . . . . . . . .  16
   6.2.  End-host mobility and multi-homing  . . . . . . . . . . . .  17
   6.3.  Rendezvous mechanism  . . . . . . . . . . . . . . . . . . .  18
   6.4.  Relay mechanism . . . . . . . . . . . . . . . . . . . . . .  18

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   6.5.  Termination of the control plane  . . . . . . . . . . . . .  18
   7.  Data plane  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  HIP and NATs  . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.1.  HIP and Upper-layer checksums . . . . . . . . . . . . . . .  20
   9.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. HIP policies  . . . . . . . . . . . . . . . . . . . . . . . .  21
   11. Security considerations . . . . . . . . . . . . . . . . . . .  21
   11.1.  MiTM Attacks . . . . . . . . . . . . . . . . . . . . . . .  22
   11.2.  Protection against flooding attacks  . . . . . . . . . . .  23
   11.3.  HITs used in ACLs  . . . . . . . . . . . . . . . . . . . .  24
   11.4.  Alternative HI considerations  . . . . . . . . . . . . . .  25
   11.5.  Trust On First Use . . . . . . . . . . . . . . . . . . . .  25
   12. IANA considerations . . . . . . . . . . . . . . . . . . . . .  28
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  28
   14. Changes from RFC 4423 . . . . . . . . . . . . . . . . . . . .  29
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
   15.1.  Normative References . . . . . . . . . . . . . . . . . . .  29
   15.2.  Informative references . . . . . . . . . . . . . . . . . .  31
   Appendix A.  Design considerations  . . . . . . . . . . . . . . .  38
   A.1.  Benefits of HIP . . . . . . . . . . . . . . . . . . . . . .  38
   A.2.  Drawbacks of HIP  . . . . . . . . . . . . . . . . . . . . .  41
   A.3.  Deployment and adoption considerations  . . . . . . . . . .  43
   A.3.1.  Deployment analysis . . . . . . . . . . . . . . . . . . .  43
   A.3.2.  HIP in 802.15.4 networks  . . . . . . . . . . . . . . . .  44
   A.3.3.  HIP and Internet of Things  . . . . . . . . . . . . . . .  44
   A.3.4.  Infrastructure Applications . . . . . . . . . . . . . . .  46
   A.3.5.  Management of Identities in a Commercial Product  . . . .  47
   A.4.  Answers to NSRG questions . . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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 to do too
   much with them.  Semantic overloading and functionality extensions
   have greatly complicated these namespaces.

   The proposed Host Identity namespace is also a global namespace, and
   it 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 (although there may be transient periods of time such as key

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   replacement when more than one identifier may be active).  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 6.  HIP provides for limited forms of trust between systems,
   enhances mobility, multi-homing and dynamic IP renumbering, aids in
   protocol translation / transition, and reduces 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 [RFC7402].  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.

   Much has been learned about HIP [RFC6538] 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

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   | 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-Adleman (RSA), Digital Signature Algorithm |
   |               | (DSA) and Elliptic Curve DSA (ECDSA) 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 the base
   specification [RFC7401] 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 6      |
   | 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          | A public key used as a name for a Host Identity.  |
   | Identifier    |                                                   |
   |               |                                                   |
   | 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 cryptographic hash used in creating the Host  |
   | Hash          | Identity Tag from the Host Identifier.            |
   |               |                                                   |
   | Host Identity | A 128-bit datum created by taking a cryptographic |
   | Tag           | hash over a Host Identifier plus bits to identify |
   |               | which hash used.                                  |
   |               |                                                   |
   | Local Scope   | A 32-bit datum denoting a Host Identity.          |
   | Identifier    |                                                   |
   |               |                                                   |
   | Public Host   | A published or publicly known Host Identifier     |
   | Identifier    | used as a public name for a Host Identity, and    |
   | and Identity  | the 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, establishing initial contact and sustaining of data flows
   between two hosts can be challenging due to private address realms
   and ephemeral nature of addresses.  Secondly, confidentiality 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

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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
   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 replacement can be handled transparently for
      legacy applications as the LSIs and HITs are compatible with IPv4
      and IPv6 addresses [RFC5338].  However, HIP-aware applications
      require some modifications from the developers, who may employ
      networking API extensions for HIP [RFC6317].

   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

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      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.  With the currently used hash size of 96
      bits [RFC7343], the figure is 2**48 (281 trillions).

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

   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.

   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, the 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.

   In theory, any name that can claim to be 'statistically globally
   unique' may serve as a Host Identifier.  In the HIP architecture, the
   public key of a private-public key pair has been chosen as the Host
   Identifier because it can be self-managed and it is computationally
   difficult to forge.  As specified in the Host Identity Protocol
   [RFC7401] specification, a public-key-based HI can authenticate the
   HIP packets and protect them from man-in-the-middle attacks.  Since
   authenticated datagrams are mandatory to provide much of HIP's
   denial-of-service protection, the Diffie-Hellman exchange in HIP base

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   exchange has to be authenticated.  Thus, only public-key HI and
   authenticated HIP messages are supported in practice.

   In this document, some non-cryptographic forms of HI and HIP are
   referenced, but cryptographic forms SHOULD be preferred because they
   are more secure than their non-cryptographic counterparts.  There has
   been past research in challenge puzzles to use non-cryptographic HI,
   for Radio Frequency IDentification (RFID), in an HIP exchange
   tailored to the workings of such challenges (as described further in
   [urien-rfid] and [urien-rfid-draft]).

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.

   An identity is based on public-private key cryptography in HIP.  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.  For instance, Back-to-My-Mac
   [RFC6281] from Apple comes pretty close to the functionality of HIP,
   but unlike HIP, it is based on non-cryptographic identifiers.

   The actual Host Identifiers are never directly used at the transport
   or network layers.  The corresponding Host Identifiers (public keys)
   may be stored in various DNS or other 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

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   Identities, the Local Scope Identifier (LSI), can also be used in
   protocols and APIs.

4.2.  Host Identity Hash (HIH)

   The Host Identity Hash (HIH) is the cryptographic hash algorithm used
   in producing the HIT from the HI.  It is also the hash used
   throughout the HIP protocol for consistency and simplicity.  It is
   possible for the two hosts in the HIP exchange to use different hash

   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 [RFC7401].

4.3.  Host Identity Tag (HIT)

   A Host Identity Tag (HIT) is a 128-bit representation for a Host
   Identity.  Due to its size, it is suitable to be used in the existing
   sockets API in the place of IPv6 addresses (e.g., in sockaddr_in6
   structure, sin6_addr member) without modifying applications.  It is
   created from an HIH, an IPv6 prefix [RFC7343] 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

   In essence, the HIT is a hash over the public key.  As such, two
   algorithms affect the generation of a HIT: the public-key algorithm
   of the HI and the used HIH.  The two algorithms are encoded in the
   bit presentation of the HIT.  As the two communicating parties may
   support different algorithms, [RFC7401] defines the minimum set for
   interoperability.  For further interoperability, the responder may
   store its keys in DNS records, and thus the initiator may have to
   couple destination HITs with appropriate source HITs according to
   matching HIH.

   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.

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   Although it may be rare for an accidental collision to cause a single
   HIT mapping to more than one Host Identity, it may be the case that
   an attacker succeeds to find, by brute force or algorithmic weakness,
   a second Host Identity hashing to the same HIT.  This type of attack
   is known as a preimage attack, and the resistance to finding a second
   Host Identifier (public key) that hashes to the same HIT is called
   second preimage resistance.  Second preimage resistance in HIP is
   based on the hash algorithm strength and the length of the hash
   output used.  Through HIPv2 [RFC7401], this resistance is 96 bits
   (less than the 128 bit width of an IPv6 address field due to the
   presence of the ORCHID prefix [RFC7343]).  96 bits of resistance was
   considered acceptable strength during the design of HIP, but may
   eventually be considered insufficient for the threat model of an
   envisioned deployment.  One possible mitigation would be to augment
   the use of HITs in the deployment with the HIs themselves (and
   mechanisms to securely bind the HIs to the HITs), so that the HI
   becomes the final authority.  It also may be possible to increase the
   difficulty of brute force attack by making the generation of the HI
   more computationally difficult, such as the hash extension approach
   of SEND CGAs [RFC3972], although the HIP specifications through HIPv2
   do not provide such a mechanism.  Finally, deployments that do not
   use ORCHIDs (such as certain types of overlay networks) might also
   use the full 128-bit width of an IPv6 address field for the HIT.

4.4.  Local Scope Identifier (LSI)

   An LSI is a 32-bit localized representation for a Host Identity.  Due
   to its size, it is suitable to be used in the existing sockets API in
   the place of IPv4 addresses (e.g., in sockaddr_in structure, sin_addr
   member) without modifying applications.  The purpose of an LSI is to
   facilitate using Host Identities in existing APIs for IPv4-based
   applications.  LSIs are never transmitted on the wire; when an
   application sends data using a pair of LSIs, the HIP layer (or
   sockets handler) translates the LSIs to the corresponding HITs, and
   vice versa for receiving of data.  Besides facilitating HIP-based
   connectivity for legacy IPv4 applications, the LSIs are beneficial in
   two other scenarios [RFC6538].

   In the first scenario, two IPv4-only applications are residing on two
   separate hosts connected by IPv6-only network.  With HIP-based
   connectivity, the two applications are able to communicate despite of
   the mismatch in the protocol families of the applications and the
   underlying network.  The reason is that the HIP layer translates the
   LSIs originating from the upper layers into routable IPv6 locators
   before delivering the packets on the wire.

   The second scenario is the same as the first one, but with the
   difference that one of the applications supports only IPv6.  Now two

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   obstacles hinder the communication between the application: the
   addressing families of the two applications differ, and the
   application residing at the IPv4-only side is again unable to
   communicate because of the mismatch between addressing families of
   the application (IPv4) and network (IPv6).  With HIP-based
   connectivity for applications, this scenario works; the HIP layer can
   choose whether to translate the locator of an incoming packet into an
   LSI or HIT.

   Effectively, LSIs improve IPv6 interoperability at the network layer
   as described in the first scenario and at the application layer as
   depicted in the second example.  The interoperability mechanism
   should not be used to avoid transition to IPv6; the authors firmly
   believe in IPv6 adoption and encourage developers to port existing
   IPv4-only applications to use IPv6.  However, some proprietary,
   closed-source, IPv4-only applications may never see the daylight of
   IPv6, and the LSI mechanism is suitable for extending the lifetime of
   such applications even in IPv6-only networks.

   The main disadvantage of an LSI is its local scope.  Applications may
   violate layering principles and pass LSIs to each other in
   application-layer protocols.  As the LSIs are valid only in the
   context of the local host, they may represent an entirely different
   host when passed to another host.  However, it should be emphasized
   here that the LSI concept is effectively a host-based NAT and does
   not introduce any more issues than the prevalent middlebox based NATs
   for IPv4.  In other words, the applications violating layering
   principles are already broken by the NAT boxes that are ubiquitously

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 [RFC8005].

   Alternatively, or in addition to storing Host Identifiers in the DNS,
   they may be stored in various other directories.  For instance, a
   directory based on the Lightweight Directory Access Protocol (LDAP)
   or a Public Key Infrastructure (PKI) [RFC8002]  may be used.
   Alternatively, Distributed Hash Tables (DHTs) [RFC6537] have
   successfully been utilized [RFC6538].  Such a practice may allow them
   to be used for purposes other than pure host identification.

   Some types of applications may cache and use Host Identifiers
   directly, while others may indirectly discover them through symbolic

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   host name (such as FQDN) look up from a directory.  Even though Host
   Identities can have a substantially longer lifetime associated with
   them than routable IP addresses, directories may be a better approach
   to manage the lifespan of Host Identities.  For example, an LDAP-
   based directory or DHT can be used for locally published identities
   whereas DNS can be more suitable for public advertisement.

5.  New stack architecture

   One way to characterize Host Identity is to compare the proposed HI-
   based architecture with the current one.  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.  The left side illustrates the current TCP/
   IP architecture and the right side the HIP-based architecture.

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

                                 Figure 1

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   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 rather to Host Identities.  In practice, the Host
   Identities are exposed as LSIs and HITs for legacy applications and
   the transport layer to facilitate backward compatibility with
   existing networking APIs and stacks.

   The HIP layer is logically located at layer 3.5, between the
   transport and network layers, in the networking stack.  It acts as
   shim layer for transport data utilizing LSIs or HITs, but leaves
   other data intact.  The HIP layer translates between the two forms of
   HIP identifiers originating from the transport layer into routable
   IPv4/IPv6 addresses for the network layer, and vice versa for the
   reverse direction.

5.1.  On the multiplicity of identities

   A host may have multiple identities both at the client and server
   side.  This raises some additional concerns that are addressed in
   this section.

   For security reasons, it may be a bad idea to duplicate the same Host
   Identity on multiple hosts because the compromise of a single host
   taints the identities of the other hosts.  Management of machines
   with identical Host Identities may also present other challenges and,
   therefore, it is advisable to have a unique identity for each host.

   At the server side, utilizing DNS is a better alternative than a
   shared Host Identity to implement load balancing.  A single FQDN
   entry can be configured to refer to multiple Host Identities.  Each
   of the FQDN entries can be associated with the related locators, or a
   single shared locator in the case the servers are using the same HIP
   rendezvous server Section 6.3 or HIP relay server Section 6.4.

   Instead of duplicating identities, HIP opportunistic mode can be
   employed, where the initiator leaves out the identifier of the
   responder when initiating the key exchange and learns it upon the
   completion of the exchange.  The tradeoffs are related to lowered
   security guarantees, but a benefit of the approach is to avoid
   publishing of Host Identifiers in any directories [komu-leap].  Since
   many public servers already employ DNS as their directory,
   opportunistic mode may be more suitable for, e.g, peer-to-peer
   connectivity.  It is also worth noting that opportunistic mode is
   also required in practice when anycast IP addresses would be utilized
   as locators.

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   HIP opportunistic mode could be utilized in association with HIP
   rendezvous servers or HIP relay servers [komu-diss].  In such a
   scenario, the Initiator sends an I1 message with a wildcard
   destination HIT to the locator of a HIP rendezvous/relay server.
   When the receiving rendezvous/relay server is serving multiple
   registered Responders, the server can choose the ultimate destination
   HIT, thus acting as a HIP based load balancer.  However, this
   approach is still experimental and requires further investigation.

   At the client side, a host may have multiple Host Identities, for
   instance, for privacy purposes.  Another reason can be that the
   person utilizing the host employs different identities for different
   administrative domains as an extra security measure.  If a HIP-aware
   middlebox, such as a HIP-based firewall, is on the path between the
   client and server, the user or the underlying system should carefully
   choose the correct identity to avoid the firewall to unnecessarily
   drop HIP-based connectivity [komu-diss].

   Similarly, a server may have multiple Host Identities.  For instance,
   a single web server may serve multiple different administrative
   domains.  Typically, the distinction is accomplished based on the DNS
   name, but also the Host Identity could be used for this purpose.
   However, a more compelling reason to employ multiple identities are
   HIP-aware firewalls that are unable see the HTTP traffic inside the
   encrypted IPsec tunnel.  In such a case, each service could be
   configured with a separate identity, thus allowing the firewall to
   segregate the different services of the single web server from each
   other [lindqvist-enterprise].

6.  Control plane

   HIP decouples control and data plane from each other.  Two end-hosts
   initialize the control plane using a key exchange procedure called
   the base exchange.  The procedure can be assisted by HIP specific
   infrastructural intermediaries called rendezvous or relay servers.
   In the event of IP address changes, the end-hosts sustain control
   plane connectivity with mobility and multihoming extensions.
   Eventually, the end-hosts terminate the control plane and remove the
   associated state.

6.1.  Base exchange

   The base exchange is a key exchange procedure that authenticates the
   initiator and responder to each other using their public keys.
   Typically, the initiator is the client-side host and the responder is
   the server-side host.  The roles are used by the state machine of a
   HIP implementation, but discarded upon successful completion.

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   The exchange consists of four messages during which the hosts also
   create symmetric keys to protect the control plane with Hash-based
   message authentication codes (HMACs).  The keys can be also used to
   protect the data plane, and IPsec ESP [RFC7402] is typically used as
   the data-plane protocol, albeit HIP can also accommodate others.
   Both the control and data plane are terminated using a closing
   procedure consisting of two messages.

   In addition, the base exchange also includes a computational puzzle
   [RFC7401] that the initiator must solve.  The responder chooses the
   difficulty of the puzzle which permits the responder to delay new
   incoming initiators according to local policies, for instance, when
   the responder is under heavy load.  The puzzle can offer some
   resiliency against DoS attacks because the design of the puzzle
   mechanism allows the responder to remain stateless until the very end
   of the base exchange [aura-dos].  HIP puzzles have also been studied
   under steady-state DDoS attacks [beal-dos], on multiple adversary
   models with varying puzzle difficulties [tritilanunt-dos] and with
   ephemeral Host Identities [komu-mitigation].

6.2.  End-host mobility and multi-homing

   HIP decouples the transport from the internetworking layer, and binds
   the transport associations to the Host Identities (actually through
   either the HIT or LSI).  After the initial key exchange, the HIP
   layer maintains transport-layer connectivity and data flows using its
   mobility [RFC8046] and multihoming [RFC8047] extensions.
   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.  If one
   address becomes unusable, or a more preferred address becomes
   available, existing transport associations can easily be moved to
   another address.

   When a mobile node moves while communication is already on-going,
   address changes are rather straightforward.  The mobile node sends a
   HIP UPDATE packet to inform the peer of the new address(es), and the
   peer then verifies that the mobile node is reachable through these
   addresses.  This way, the peer can avoid flooding attacks as further
   discussed in Section 11.2.

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6.3.  Rendezvous mechanism

   Establishing a contact to a mobile, moving node is slightly more
   involved.  In order to start the HIP exchange, the initiator node has
   to know how to reach the mobile node.  For instance, the mobile node
   can employ Dynamic DNS [RFC2136] to update its reachability
   information in the DNS.  To avoid the dependency to DNS, HIP provides
   its own HIP-specific alternative: the HIP rendezvous mechanism as
   defined in HIP Rendezvous specifications [RFC8004].

   Using the HIP rendezvous extensions, the mobile node keeps the
   rendezvous infrastructure continuously updated with its current IP
   address(es).  The mobile nodes trusts the rendezvous mechanism in
   order to properly maintain their HIT and IP address mappings.

   The rendezvous mechanism is especially useful in scenarios where both
   of the nodes are expected to change their address at the same time.
   In such a case, the HIP UPDATE packets will cross each other in the
   network and never reach the peer node.

6.4.  Relay mechanism

   The HIP relay mechanism [I-D.ietf-hip-native-nat-traversal] is an
   alternative to the HIP rendezvous mechanism.  The HIP relay mechanism
   is more suitable for IPv4 networks with NATs because a HIP relay can
   forward all control and data plane communications in order to
   guarantee successful NAT traversal.

6.5.  Termination of the control plane

   The control plane between two hosts is terminated using a secure two-
   message exchange as specified in base exchange specification
   [RFC7401].  The related state (i.e. host associations) should be
   removed upon successful termination.

7.  Data plane

   The encapsulation format for the data plane used for carrying the
   application-layer traffic can be dynamically negotiated during the
   key exchange.  For instance, HICCUPS extensions [RFC6078] define one
   way to transport application-layer datagrams directly over the HIP
   control plane, protected by asymmetric key cryptography.  Also, SRTP
   has been considered as the data encapsulation protocol [hip-srtp].
   However, the most widely implemented method is the Encapsulated
   Security Payload (ESP) [RFC7402] that is protected by symmetric keys
   derived during the key exchange.  ESP Security Associations (SAs)
   offer both confidentiality and integrity protection, of which the

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   former can be disabled during the key exchange.  In the future, other
   ways of transporting application-layer data may be defined.

   The ESP SAs are established and terminated between the initiator and
   the responder hosts.  Usually, the hosts create at least two SAs, one
   in each direction (initiator-to-responder SA and responder-to-
   initiator SA).  If the IP addresses of either host changes, the HIP
   mobility extensions can be used to re-negotiate the corresponding

   On the wire, the difference in the use of identifiers between the HIP
   control and data plane is that the HITs are included in all control
   packets, but not in the data plane when ESP is employed.  Instead,
   the ESP employs SPI numbers that act as compressed HITs.  Any HIP-
   aware middlebox (for instance, a HIP-aware firewall) interested in
   the ESP based data plane should keep track between the control and
   data plane identifiers in order to associate them with each other.

   Since HIP does not negotiate any SA lifetimes, all lifetimes are
   subject to 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 and other data-plane protocols.

8.  HIP and NATs

   Passing packets between different IP addressing realms requires
   changing IP addresses in the packet header.  This may occur, 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 NATs
   are employed because private address spaces are overlapping.  In
   other words, two hosts cannot be distinguished from each other solely
   based on their IP address.  With HIP, the transport-layer end-points
   (i.e. applications) are bound to unique Host Identities rather than
   overlapping private addresses.  This allows two end-points to
   distinguish one other even when they are located in different private
   address realms.  Thus, the IP addresses are used only for routing
   purposes and can be changed freely by NATs when a packet between two
   HIP capable hosts traverses through multiple private address realms.

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   NAT traversal extensions for HIP [I-D.ietf-hip-native-nat-traversal]
   can be used to realize the actual end-to-end connectivity through NAT
   devices.  To support basic backward compatibility with legacy NATs,
   the extensions encapsulate both HIP control and data plane in UDP.
   The extensions define mechanisms for forwarding the two planes
   through an intermediary host called HIP relay and procedures to
   establish direct end-to-end connectivity by penetrating NATs.
   Besides this "native" NAT traversal mode for HIP, other NAT traversal
   mechanisms have been successfully utilized, such as Teredo [RFC4380]
   (as described in further detail in [varjonen-split]).

   Besides legacy NATs, a HIP-aware NAT has been designed and
   implemented [ylitalo-spinat].  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.

8.1.  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
   used.  This provides for IPv4 / IPv6 protocol translation.

9.  Multicast

   A number of studies investigating HIP-based multicast have been
   published (including [shields-hip], [xueyong-hip], [xueyong-hip],
   [amir-hip], [kovacshazi-host] and [xueyong-secure]).  In particular,
   so-called Bloom filters, that allow compressing of multiple labels
   into small data structures, may be a promising way forward
   [sarela-bloom].  However, the different schemes have not been adopted
   by the HIP working group (nor the HIP research group in IRTF), so the
   details are not further elaborated here.

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10.  HIP policies

   There are a number of variables that influence the HIP exchange 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 (that should expect to be
   rotated frequently in order to disrupt linkability/trackability).
   Although unpublished HIs will be rarely used as responder HIs, they
   are likely to be common for initiators.  As stated in [RFC7401], "all
   HIP implementations MUST support more than one simultaneous HI, at
   least one of which SHOULD be reserved for anonymous usage", and
   "support for more than two 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 initiator starts a HIP exchange without
   prior knowledge of the responder's HI) presents a security tradeoff.
   At the expense of being subject to MITM attacks, the opportunistic
   mode allows the initiator to learn the identity of the responder
   during communication rather than from an external directory.
   Opportunistic mode can be used for registration to HIP-based services
   [RFC8003] (i.e. utilized by HIP for its own internal purposes) or by
   the application layer [komu-leap].  For security reasons, especially
   the latter requires some involvement from the user to accept the
   identity of the responder similar to how SSH prompts the user when
   connecting to a server for the first time [pham-leap].  In practice,
   this can be realized in end-host based firewalls in the case of
   legacy applications [karvonen-usable] or with native APIs for HIP
   APIs [RFC6317] in the case of HIP-aware applications.

   As stated in [RFC7401], "Initiators MAY use a different HI for
   different Responders to provide basic privacy.  Whether such private
   HIs are used repeatedly with the same Responder, and how long these
   HIs are used, are decided by local policy and depend on the privacy
   requirements of the Initiator".

   According to [RFC7401], "Responders that only respond to selected
   Initiators require an Access Control List (ACL), representing for
   which hosts they accept HIP base exchanges, and the preferred
   transport format and local lifetimes.  Wildcarding SHOULD be
   supported for such ACLs, and also for Responders that offer public or
   anonymous services".

11.  Security considerations

   This section includes discussion on some issues and solutions related
   to security in the HIP architecture.

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11.1.  MiTM Attacks

   HIP takes advantage of the 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
   of the initiator, making the HIP base exchange 4 packets long.  The
   first packet sent by the responder can be prebuilt to further
   mitigate the costs.  This packet also includes a computational puzzle
   that can optionally be used to further delay the initiator, for
   instance, when the responder is overloaded.  The details are
   explained in the base exchange specification [RFC7401].

   Man-in-the-middle (MitM) attacks are difficult to defend against,
   without third-party authentication.  A skillful MitM could easily
   handle all parts of the HIP base exchange, but HIP indirectly
   provides the following protection from a MitM attack.  If the
   responder's HI is retrieved from a signed DNS zone or securely
   obtained 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.

   Other types of MitM attacks against HIP can be mounted using ICMP
   messages that can be used to signal about problems.  As an overall
   guideline, the ICMP messages should be considered as unreliable
   "hints" and should be acted upon only after timeouts.  The exact
   attack scenarios and countermeasures are described in full detail the
   base exchange specification [RFC7401].

   A MitM attacker could try to replay older I1 or R1 messages using
   weaker cryptographic algorithms as described in section 4.1.4 in
   [RFC7401].  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

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   finish (or would be aborted after some retries).  As a drawback, this
   leads to a 6-way base exchange which may seem bad at first.  However,
   since this only occurs in an attack scenario and since the attack can
   be handled (so it is not interesting to mount anymore), we assume the
   subsequent messages do not represent a security threat.  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
   attacking easier, 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.

11.2.  Protection against flooding attacks

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

   Blindly accepting new addresses would potentially lead to flooding
   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 mobility extensions include a return
   routability check procedure 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 for host mobility with the Host
   Identity Protocol [RFC8046] 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

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11.3.  HITs used in ACLs

   At end-hosts, HITs can be used in IP-based access control lists at
   the application and network layers.  At middleboxes, HIP-aware
   firewalls [lindqvist-enterprise] can use HITs or public keys to
   control both ingress and egress access to networks or individual
   hosts, even in the presence of mobile devices because the HITs and
   public keys are topology independent.  As discussed earlier 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 occurring
   between two known hosts.  This may increase firewall security.

   A potential drawback of HITs in ACLs is their 'flatness' means they
   cannot be aggregated, and this could potentially result in larger
   table searches in HIP-aware firewalls.  A way to optimize this could
   be to utilize Bloom filters for grouping of HITs [sarela-bloom].
   However, it should be noted that it is also easier to exclude
   individual, misbehaving hosts out when the firewall rules concern
   individual HITs rather than groups.

   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
   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 have been attempts
   to develop a secure method to issue the HIT revocation notice

   Some of the HIP-aware middleboxes, such as firewalls
   [lindqvist-enterprise] or NATs [ylitalo-spinat], may observe the on-
   path traffic passively.  Such middleboxes are transparent by their
   nature and may not get a notification when a host moves to a
   different network.  Thus, such middleboxes should maintain soft state
   and timeout when the control and data plane between two HIP end-hosts
   has been idle too long.  Correspondingly, the two end-hosts may send
   periodically keepalives, such as UPDATE packets or ICMP messages
   inside the ESP tunnel, to sustain state at the on-path middleboxes.

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   One general limitation related to end-to-end encryption is that
   middleboxes may not be able to participate to the protection of data
   flows.  While the issue may affect also other protocols, Heer at al
   [heer-end-host] have analyzed the problem in the context of HIP.
   More specifically, when ESP is used as the data-plane protocol for
   HIP, the association between the control and data plane is weak and
   can be exploited under certain assumptions.  In the scenario, the
   attacker has already gained access to the target network protected by
   a HIP-aware firewall, but wants to circumvent the HIP-based firewall.
   To achieve this, the attacker passively observes a base exchange
   between two HIP hosts and later replays it.  This way, the attacker
   manages to penetrate the firewall and can use a fake ESP tunnel to
   transport its own data.  This is possible because the firewall cannot
   distinguish when the ESP tunnel is valid.  As a solution, HIP-aware
   middleboxes may participate to the control plane interaction by
   adding random nonce parameters to the control traffic, which the end-
   hosts have to sign to guarantee the freshness of the control traffic
   [heer-midauth].  As an alternative, extensions for transporting data
   plane directly over the control plane can be used [RFC6078].

11.4.  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, but examples of such protocol variants do exist
   ([urien-rfid], [urien-rfid-draft]).  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.  Such schemes may be employed for
   resource constrained devices, such as small sensors operating on
   battery power, but are not further analyzed here.

   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.

11.5.  Trust On First Use

   [RFC7435] highlights four design principles for Leap of Faith, or
   Trust On First Use (TOFU), protocols that apply also to opportunistic

   1.  Coexist with explicit policy

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   2.  Prioritize communication

   3.  Maximize security peer by peer

   4.  No misrepresentation of security

   According to the first TOFU design principle, "opportunistic security
   never displaces or preempts explicit policy".  Some application data
   may be too sensitive, so the related policy could require
   authentication (i.e, the public key or certificate) in such a case
   instead of the unauthenticated opportunistic mode.  In practice, this
   has been realized in HIP implementations as follows [RFC6538].

   The OpenHIP implementation allowed an Initiator to use opportunistic
   mode only with an explicitly configured Responder IP address, when
   the Responder's HIT is unknown.  At the Responder, OpenHIP had an
   option to allow opportunistic mode with any Initiator -- trust any

   HIP for Linux (HIPL) developers experimented with more fine-grained
   policies operating at the application level.  HIPL implementation
   utilized so called "LD_PRELOAD" hooking at the application layer that
   allowed a dynamically linked library to intercept socket-related
   calls without rebuilding the related application binaries.  The
   library acted as a shim layer between the application and transport
   layers.  The shim layer translated the non-HIP based socket calls
   from the application into HIP-based socket calls.  While the shim
   library involved some level of complexity as described in more detail
   in [komu-leap], it achieved the goal of applying opportunistic mode
   at the granularity of individual applications.

   The second TOFU principle essentially states that communication
   should be first class citizen instead of security.  So opportunistic
   mode should be, in general, allowed even if no authentication is
   present, and even possibly a fallback to non-encrypted communications
   could be allowed (if policy permits) instead of blocking
   communications.  In practice, this can be realized in three steps.
   In the first step, a HIP Initiator can look up the HI of a Responder
   from a directory such as DNS.  When the Initiator discovers a HI, it
   can use the HI for authentication and skip the rest of the following
   steps.  In the second step, the Initiator can, upon failing to find a
   HI, try opportunistic mode with the Responder.  In the third step,
   the Initiator can fall back to non-HIP based communications upon
   failing with opportunistic mode if the policy allows it.  This three
   step model has been implemented successfully and described in more
   detail in [komu-leap].

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   The third TOFU principle suggests that security should be maximized,
   so that at least opportunistic security would be employed.  The three
   step model described earlier prefers authentication when it is
   available, e.g., via DNS records (and possibly even via DNSSEC when
   available) and falls back to opportunistic mode when no out-of-band
   credentials are available.  As the last resort, fallback to non-HIP
   based communications can be used if the policy allows it.  Also,
   since perfect forward security (PFS) is explicitly mentioned in the
   third design principle, it is worth mentioning that HIP supports it.

   The fourth TOFU principle states that users and non-interactive
   applications should be properly informed about the level of security
   being applied.  In practice, non-HIP aware applications would assume
   no extra security being applied, so misleading at least a non-
   interactive application should not be possible.  In the case of
   interactive desktop applications, system-level prompts have been
   utilized in earlier HIP experiments [karvonen-usable], [RFC6538] to
   guide the user about the underlying HIP-based security.  In general,
   users in those experiments perceived when HIP-based security was
   being used versus not used.  However, the users failed to notice the
   difference between opportunistic and non-opportunistic HIP.  The
   reason for this was that the opportunistic HIP (i.e. lowered level of
   security) was not clearly indicated in the prompt.  This provided a
   valuable lesson to further improve the user interface.

   In the case of HIP-aware applications, native sockets APIs for HIP as
   specified in [RFC6317] can be used to develop application-specific
   logic instead of using generic system-level prompting.  In such case,
   the application itself can directly prompt the user or otherwise
   manage the situation in other ways.  In this case, also non-
   interactive applications can properly log the level of security being
   employed because the developer can now explicitly program the use of
   authenticated HIP, opportunistic HIP and plain-text communication.

   It is worth mentioning a few additional items discussed in [RFC7435].
   Related to active attacks, HIP has built-in protection against
   cipher-suite down-grade attacks as described in detail in [RFC7401].
   In addition, pre-deployed certificates could be used to mitigate
   against active attacks in the case of opportunistic mode as mentioned
   in [RFC6538].

   Detection of peer capabilities is also mentioned in the TOFU context.
   As discussed in this section, the three-step model can be used to
   detect peer capabilities.  A host can achieve the first step of
   authentication, i.e., discovery of a public key, via DNS, for
   instance.  If the host found no keys, the host can then try
   opportunistic mode as the second step.  Upon a timeout, the host can
   then proceed to the third step by falling back to non-HIP based

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   communications if the policy permits.  This last step is based on an
   implicit timeout rather an explicit (negative) acknowledgment like in
   the case of DNS, so the user may conclude prematurely that the
   connectivity has failed.  To speed up the detection phase by
   explicitly detecting if the peer supports opportunistic HIP,
   researchers have proposed TCP specific extensions [RFC6538],
   [komu-leap].  In a nutshell, an Initiator sends simultaneously both
   an opportunistic I1 packet and the related TCP SYN datagram equipped
   with a special TCP option to a peer.  If the peer supports HIP, it
   drops the SYN packet and responds with an R1.  If the peer is HIP
   incapable, it drops the HIP packet (and the unknown TCP option) and
   responds with a TCP SYN-ACK.  The benefit of the proposed scheme is
   faster, one round-trip fallback to non-HIP based communications.  The
   drawback is that the approach is tied to TCP (IP-options were also
   considered, but do not work well with firewalls and NATs).
   Naturally, the approach does not work against active attacker, but
   opportunistic mode is not anyway supposed to protect against such an

   It is worth noting that while the use of opportunistic mode has some
   benefits related to incremental deployment, it does not achieve all
   the benefits of authenticated HIP [komu-diss].  Namely, authenticated
   HIP supports persistent identifiers in the sense that hosts are
   identified with the same HI independently of their movement.
   Opportunistic HIP meets this goal only partially: after the first
   contact between two hosts, HIP can successfully sustain connectivity
   with its mobility management extensions, but problems emerge when the
   hosts close the HIP association and try to re-establish connectivity.
   As hosts can change their location, it is no longer guaranteed that
   the same IP address belongs to the same host.  The same address can
   be temporally assigned to different hosts, e.g., due to the reuse of
   IP addresses (e.g., by a DHCP service), overlapping private address
   realms (see also the discussion on Internet transparency in
   Appendix A.1) or due to an attempted attack.

12.  IANA considerations

   This document has no actions for IANA.

13.  Acknowledgments

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

   During the later stages of this document, when the editing baton was
   transferred to Pekka Nikander, the comments from the early
   implementers and others, including Jari Arkko, Jeff AhrenHolz, Tom

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   Henderson, Petri Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan
   Melen, Tim Shepard, Jukka Ylitalo, Sasu Tarkoma, and Jorma Wall, were
   invaluable.  Also, the comments from Lars Eggert, Spencer Dawkins,
   Dave Crocker and Erik Giesa were also useful.

   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 main effort to update and move HIP forward within the IETF
   process owes its impetuous to a number of HIP development teams.  The
   authors are grateful for Boeing, Helsinki Institute for Information
   Technology (HIIT), NomadicLab of Ericsson, and the three
   universities: RWTH Aachen, Aalto and University of Helsinki, for
   their efforts.  Without their collective efforts HIP would have
   withered as on the IETF vine as a nice concept.

   Thanks also for Suvi Koskinen for her help with proofreading and with
   the reference jungle.

14.  Changes from RFC 4423

   In a nutshell, the changes from RFC 4423 [RFC4423] are mostly
   editorial, including clarifications on topics described in a
   difficult way and omitting some of the non-architectural
   (implementation) details that are already described in other
   documents.  A number of missing references to the literature were
   also added.  New topics include the drawbacks of HIP, discussion on
   802.15.4 and MAC security, HIP for IoT scenarios, deployment
   considerations and description of the base exchange.

15.  References

15.1.  Normative References

              Moskowitz, R. and R. Hummen, "HIP Diet EXchange (DEX)",
              draft-ietf-hip-dex-06 (work in progress), December 2017.

              Keranen, A., Melen, J., and M. Komu, "Native NAT Traversal
              Mode for the Host Identity Protocol", draft-ietf-hip-
              native-nat-traversal-28 (work in progress), March 2018.

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   [RFC5482]  Eggert, L. and F. Gont, "TCP User Timeout Option",
              RFC 5482, DOI 10.17487/RFC5482, March 2009,

   [RFC6079]  Camarillo, G., Nikander, P., Hautakorpi, J., Keranen, A.,
              and A. Johnston, "HIP BONE: Host Identity Protocol (HIP)
              Based Overlay Networking Environment (BONE)", RFC 6079,
              DOI 10.17487/RFC6079, January 2011, <https://www.rfc-

   [RFC7086]  Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
              Protocol-Based Overlay Networking Environment (HIP BONE)
              Instance Specification for REsource LOcation And Discovery
              (RELOAD)", RFC 7086, DOI 10.17487/RFC7086, January 2014,

   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
              2014, <>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,

   [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 7402,
              DOI 10.17487/RFC7402, April 2015, <https://www.rfc-

   [RFC8002]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 8002, DOI 10.17487/RFC8002, October
              2016, <>.

   [RFC8003]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
              October 2016, <>.

   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
              October 2016, <>.

   [RFC8005]  Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
              October 2016, <>.

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   [RFC8046]  Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
              with the Host Identity Protocol", RFC 8046,
              DOI 10.17487/RFC8046, February 2017, <https://www.rfc-

   [RFC8047]  Henderson, T., Ed., Vogt, C., and J. Arkko, "Host
              Multihoming with the Host Identity Protocol", RFC 8047,
              DOI 10.17487/RFC8047, February 2017, <https://www.rfc-

15.2.  Informative references

              Amir, K., Forsgren, H., Grahn, K., Karvi, T., and G.
              Pulkkis, "Security and Trust of Public Key Cryptography
              for HIP and HIP Multicast", International Journal of
              Dependable and Trustworthy Information Systems (IJDTIS),
              2(3), 17-35, DOI: 10.4018/jdtis.2011070102, 2013.

              Aura, T., Nikander, P., and J. Leiwo, "DOS-resistant
              Authentication with Client Puzzles", 8th International
              Workshop on Security Protocols, pages 170-177. Springer, ,
              April 2001.

              Beal, J. and T. Shephard, "Deamplification of DoS Attacks
              via Puzzles",  , October 2004.

              Camarillo, G., Maeenpaeae, J., Keraenen, A., and V.
              Anderson, "Reducing delays related to NAT traversal in
              P2PSIP session establishments", IEEE Consumer
              Communications and Networking Conference (CCNC), pp.
              549-553 DOI: 10.1109/CCNC.2011.5766540, 2011.

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

              Heer, T., Hummen, R., Komu, M., Goetz, S., and K. Wehre,
              "End-host Authentication and Authorization for Middleboxes
              based on a Cryptographic Namespace", ICC2009 Communication
              and Information Systems Security Symposium, , 2009.

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              Heer, T. and M. Komu, "End-Host Authentication for HIP
              Middleboxes", Working draft draft-heer-hip-middle-auth-02,
              September 2009.

              Henderson, T. and D. Mattes, "HIP-based Virtual Private
              LAN Service (HIPLS)", Working draft draft-henderson-hip-
              vpls-07, Dec 2013.

   [hip-lte]  Liyanage, M., Kumar, P., Ylianttila, M., and A. Gurtov,
              "Novel secure VPN architectures for LTE backhaul
              networks", Security and Communication Networks DOI
              10.1002/sec.1411, November 2015.

              Tschofenig, H., Muenz, F., and M. Shanmugam, "Using SRTP
              transport format with HIP", Working draft draft-
              tschofenig-hiprg-hip-srtp-01, October 2005.

   [hummen]   Hummen, R., Hiller, J., Henze, M., and K. Wehrle, "Slimfit
              - A HIP DEX Compression Layer for the IP-based Internet of
              Things", Wireless and Mobile Computing, Networking and
              Communications (WiMob), 2013 IEEE 9th International
              Conference on , page 259-266. DOI:
              10.1109/WiMOB.2013.6673370, October 2013.

              "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, <

              "IEEE Draft Recommended Practice for Transort of Key
              Management Protocol (KMP) Datagrams", IEEE P802.15.9/D04,
              May 2015.

              Karvonen, K., Komu, M., and A. Gurtov, "Usable Security
              Management with Host Identity Protocol", 7th ACS/IEEE
              International Conference on Computer Systems and
              Applications, (AICCSA-2009), 2009.

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              Komu, M., Sethi, M., Mallavarapu, R., Oirola, H., Khan,
              R., and S. Tarkoma, "Secure Networking for Virtual
              Machines in the Cloud", International Workshop on Power
              and QoS Aware Computing (PQoSCom2012), IEEE, ISBN:
              978-1-4244-8567-3, September 2012.

              Komu, M., "A Consolidated Namespace for Network
              Applications, Developers, Administrators and Users",
              Dissertation, Aalto University, Espoo, Finland ISBN:
              978-952-60-4904-5 (printed), ISBN: 978-952-60-4905-2
              (electronic). , December 2012.

              Komu, M. and J. Lindqvist, "Leap-of-Faith Security is
              Enough for IP Mobility", 6th Annual IEEE Consumer
              Communications and Networking Conference IEEE CCNC 2009,
              Las Vegas, Nevada, , January 2009.

              Komu, M., Tarkoma, S., and A. Lukyanenko, "Mitigation of
              Unsolicited Traffic Across Domains with Host Identities
              and Puzzles", 15th Nordic Conference on Secure IT Systems
              (NordSec 2010), Springer Lecture Notes in Computer
              Science, Volume 7127, pp. 33-48, ISBN: 978-3-642-27936-2,
              October 2010.

              Kovacshazi, Z. and R. Vida, "Host Identity Specific
              Multicast", International conference on Networking and
              Services (ICNS'06), IEEE Computer Society, Los Alamitos,
              CA, USA,
              ICNS.2007.66, 2007.

              Levae, A., Komu, M., and S. Luukkainen, "Adoption Barriers
              of Network-layer Protocols: the Case of Host Identity
              Protocol", The International Journal of Computer and
              Telecommunications Networking, ISSN: 1389-1286, March

              Lindqvist, J., Vehmersalo, E., Manner, J., and M. Komu,
              "Enterprise Network Packet Filtering for Mobile
              Cryptographic Identities", International Journal of
              Handheld Computing Research, 1 (1), 79-94, , January-March

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   [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.

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

              Paine, R., "Beyond HIP: The End to Hacking As We Know It",
              BookSurge Publishing, ISBN: 1439256047, 9781439256046,

              Pham, V. and T. Aura, "Security Analysis of Leap-of-Faith
              Protocols", Seventh ICST International Conference on
              Security and Privacy for Communication Networks, ,
              September 2011.

              Ranjbar, A., Komu, M., Salmela, P., and T. Aura,
              "SynAPTIC: Secure and Persistent Connectivity for
              Containers", 2017 17th IEEE/ACM International Symposium on
              Cluster, Cloud and Grid Computing (CCGRID), Madrid, 2017,
              pp. 262-267 doi: 10.1109/CCGRID.2017.62, 2017.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC2535]  Eastlake 3rd, D., "Domain Name System Security
              Extensions", RFC 2535, DOI 10.17487/RFC2535, March 1999,

   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              DOI 10.17487/RFC2766, February 2000, <https://www.rfc-

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001, <https://www.rfc-

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   [RFC3102]  Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
              "Realm Specific IP: Framework", RFC 3102,
              DOI 10.17487/RFC3102, October 2001, <https://www.rfc-

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, 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, DOI 10.17487/RFC4225,
              December 2005, <>.

   [RFC4306]  Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006, <https://www.rfc-

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May
              2006, <>.

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

   [RFC5338]  Henderson, T., Nikander, P., and M. Komu, "Using the Host
              Identity Protocol with Legacy Applications", RFC 5338,
              DOI 10.17487/RFC5338, September 2008, <https://www.rfc-

   [RFC5887]  Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
              Still Needs Work", RFC 5887, DOI 10.17487/RFC5887, May
              2010, <>.

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   [RFC6078]  Camarillo, G. and J. Melen, "Host Identity Protocol (HIP)
              Immediate Carriage and Conveyance of Upper-Layer Protocol
              Signaling (HICCUPS)", RFC 6078, DOI 10.17487/RFC6078,
              January 2011, <>.

   [RFC6250]  Thaler, D., "Evolution of the IP Model", RFC 6250,
              DOI 10.17487/RFC6250, May 2011, <https://www.rfc-

   [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac (BTMM) Service",
              RFC 6281, DOI 10.17487/RFC6281, June 2011,

   [RFC6317]  Komu, M. and T. Henderson, "Basic Socket Interface
              Extensions for the Host Identity Protocol (HIP)",
              RFC 6317, DOI 10.17487/RFC6317, July 2011,

   [RFC6537]  Ahrenholz, J., "Host Identity Protocol Distributed Hash
              Table Interface", RFC 6537, DOI 10.17487/RFC6537, February
              2012, <>.

   [RFC6538]  Henderson, T. and A. Gurtov, "The Host Identity Protocol
              (HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538,
              March 2012, <>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

              Saerelae, M., Esteve Rothenberg, C., Zahemszky, A.,
              Nikander, P., and J. Ott, "BloomCasting: Security in Bloom
              filter based multicast",  , Lecture Notes in Computer
              Science 2012,  , pages 1-16,  Springer Berlin Heidelberg,

              Schuetz, S., Eggert, L., Schmid, S., and M. Brunner,
              "Protocol enhancements for intermittently connected
              hosts", SIGCOMM Comput. Commun. Rev., 35(3):5-18, , July

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              Shields, C. and J. Garcia-Luna-Aceves, "The HIP protocol
              for hierarchical multicast routing", Proceedings of the
              seventeenth annual ACM symposium on Principles of
              distributed computing, pages 257-266. ACM, New York, NY,
              USA, ISBN: 0-89791-977-7, DOI: 10.1145/277697.277744,

              "Identity-Defined Network (IDN) Architecture: Unified,
              Secure Networking Made Simple", White Paper , 2016.

              Tritilanunt, S., Boyd, C., Foo, E., and J. Nieto,
              "Examining the DoS Resistance of HIP", OTM Workshops (1),
              volume 4277 of Lecture Notes in Computer Science, pages
              616-625,Springer , 2006.

              Urien, P., Chabanne, H., Bouet, M., de Cunha, D., Guyot,
              V., Pujolle, G., Paradinas, P., Gressier, E., and J.
              Susini, "HIP-based RFID Networking Architecture", IFIP
              International Conference on Wireless and Optical
              Communications Networks, DOI: 10.1109/WOCN.2007.4284140,
              July 2007.

              Urien, P., Lee, G., and G. Pujolle, "HIP support for
              RFIDs", IRTF Working draft draft-irtf-hiprg-rfid-07, April

              Varjonen, S., Komu, M., and A. Gurtov, "Secure and
              Efficient IPv4/IPv6 Handovers Using Host-Based Identifier-
              Location Split", Journal of Communications Software and
              Systems, 6(1), 2010, ISSN: 18456421, 2010.

              Xin, G., "Host Identity Protocol Version 2.5", Master's
              Thesis, Aalto University, Espoo, Finland, , June 2012.

              Xueyong, Z., Zhiguo, D., and W. Xinling, "A Multicast
              Routing Algorithm Applied to HIP-Multicast Model",
              Proceedings of the 2011 International Conference on
              Network Computing and Information Security - Volume 01
              (NCIS '11), Vol. 1. IEEE Computer Society, Washington, DC,
              USA, pages 169-174, DOI: 10.1109/NCIS.2011.42, 2011.

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              Xueyong, Z. and J. Atwood, "A Secure Multicast Model for
              Peer-to-Peer and Access Networks Using the Host Identity
              Protocol", Consumer Communications and Networking
              Conference. CCNC 2007. 4th IEEE, pages 1098,1102, DOI:
              10.1109/CCNC.2007.221, January 2007.

              Ylitalo, J., "Secure Mobility at Multiple Granularity
              Levels over Heterogeneous Datacom Networks", Dissertation,
              Helsinki University of Technology, Espoo, Finland ISBN
              978-951-22-9531-9, 2008.

              Ylitalo, J., Salmela, P., and H. Tschofenig, "SPINAT:
              Integrating IPsec into overlay routing", Proceedings of
              the First International Conference on Security and Privacy
              for Emerging Areas in Communication Networks (SecureComm
              2005). Athens, Greece. IEEE Computer Society, pages
              315-326, ISBN: 0-7695-2369-2, September 2005.

              Zhang, D., Kuptsov, D., and S. Shen, "Host Identifier
              Revocation in HIP", IRTF Working draft draft-irtf-hiprg-
              revocation-05, Mar 2012.

Appendix A.  Design considerations

A.1.  Benefits of HIP

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

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

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

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

   4.  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 explicitly for reasons that will be obvious soon if not

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   In the current "post-classic" world, we are intentionally trying to
   get rid of the second invariant (both for mobility and for multi-
   homing), and we have been forced to give up the first and the fourth.
   Realm Specific IP [RFC3102] is an attempt to reinstate the fourth
   invariant without the first invariant.  IPv6 is attempts to reinstate
   the first invariant.

   Few client-side 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
   particular systems.  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 and 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 a host 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, HIP takes
   care of host identification, and reversibility allows a local host to
   receive a packet back from a remote host.  The address changes
   occurring during NAT transit (non-mutable) or host movement (non-
   omniscient or non-mobile) can be managed by the HIP layer.

   With the exception of High-Performance Computing applications, the
   Sockets API is the most common way to develop network applications.
   Applications use the Sockets API either directly or indirectly
   through some libraries or frameworks.  However, the Sockets API is
   based on the assumption of static IP addresses, and DNS with its
   lifetime values was invented at later stages during the evolution of
   the Internet.  Hence, the Sockets API does not deal with the lifetime
   of addresses [RFC6250].  As the majority of the end-user equipment is
   mobile today, their addresses are effectively ephemeral, but the
   Sockets API still gives a fallacious illusion of persistent IP
   addresses to the unwary developer.  HIP can be used to solidify this

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   illusion because HIP provides persistent surrogate addresses to the
   application layer in the form of LSIs and HITs.

   The persistent identifiers as provided by HIP are useful in multiple
   scenarios (see, e.g., [ylitalo-diss] or [komu-diss], for a more
   elaborate discussion):

   o  When a mobile host moves physically between two different WLAN
      networks and obtains a new address, an application using the
      identifiers remains isolated regardless of the topology changes
      while the underlying HIP layer re-establishes connectivity (i.e. a
      horizontal handoff).

   o  Similarly, the application utilizing the identifiers remains again
      unaware of the topological changes when the underlying host
      equipped with WLAN and cellular network interfaces switches
      between the two different access technologies (i.e. a vertical

   o  Even when hosts are located in private address realms,
      applications can uniquely distinguish different hosts from each
      other based on their identifiers.  In other words, it can be
      stated that HIP improves Internet transparency for the application
      layer [komu-diss].

   o  Site renumbering events for services can occur due to corporate
      mergers or acquisitions, or by changes in Internet Service
      Provider.  They can involve changing the entire network prefix of
      an organization, which is problematic due to hard-coded addresses
      in service configuration files or cached IP addresses at the
      client side [RFC5887].  Considering such human errors, a site
      employing location-independent identifiers as promoted by HIP may
      experience fewer problems while renumbering their network.

   o  More agile IPv6 interoperability can be achieved, as discussed in
      Section 4.4.  IPv6-based applications can communicate using HITs
      with IPv4-based applications that are using LSIs.  Additionally,
      the underlying network type (IPv4 or IPv6) becomes independent of
      the addressing family of the application.

   o  HITs (or LSIs) can be used in IP-based access control lists as a
      more secure replacement for IPv6 addresses.  Besides security, HIT
      based access control has two other benefits.  First, the use of
      HITs can potentially halve the size of access control lists
      because separate rules for IPv4 are not needed [komu-diss].
      Second, HIT-based configuration rules in HIP-aware middleboxes
      remain static and independent of topology changes, thus
      simplifying administrative efforts particularly for mobile

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      environments.  For instance, the benefits of HIT-based access
      control have been harnessed in the case of HIP-aware firewalls,
      but can be utilized directly at the end-hosts as well [RFC6538].

   While some of these benefits could be and have been redundantly
   implemented by individual applications, providing such generic
   functionality at the lower layers is useful because it reduces
   software development effort and networking software bugs (as the
   layer is tested with multiple applications).  It also allows the
   developer to focus on building the application itself rather than
   delving into the intricacies of mobile networking, thus facilitating
   separation of concerns.

   HIP could also be realized by combining a number of different
   protocols, but the complexity of the resulting software may become
   substantially larger, and the interaction between multiple possibly
   layered protocols may have adverse effects on latency and throughput.
   It is also worth noting that virtually nothing prevents realizing the
   HIP architecture, for instance, as an application-layer library,
   which has been actually implemented in the past [xin-hip-lib].
   However, the tradeoff in moving the HIP layer to the application
   layer is that legacy applications may not be supported.

A.2.  Drawbacks of HIP

   In computer science, many problems can be solved with an extra layer
   of indirection.  However, the indirection always involves some costs
   as there is no such a thing as "free lunch".  In the case of HIP, the
   main costs could be stated as follows:

   o  In general, an additional layer and a namespace always involve
      some initial effort in terms of implementation, deployment and
      maintenance.  Some education of developers and administrators may
      also be needed.  However, the HIP community at the IETF has spent
      years in experimenting, exploring, testing, documenting and
      implementing HIP to ease the adoption costs.

   o  HIP introduces a need to manage HIs and requires a centralized
      approach to manage HIP-aware endpoints at scale.  What were
      formerly IP address-based ACLs are now trusted HITs, and the HIT
      to IP address mappings as well as access policies must be managed.
      HIP-aware endpoints must also be able to operate autonomously to
      ensure mobility and availability (an endpoint must be able to run
      without having to have a persistent management connection).  The
      users who want this better security and mobility of HIs instead of
      IP address based ACLs have to then manage this additional
      'identity layer' in a non-persistent fashion.  As exemplified in
      Appendix A.3.5, these challenges have been already solved in an

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      infrastructure setting to distribute policy and manage the
      mappings and trust relationships between HIP-aware endpoints.

   o  HIP decouples identifier and locator roles of IP addresses.
      Consequently, a mapping mechanism is needed to associate them
      together.  A failure to map a HIT to its corresponding locator may
      result in failed connectivity because a HIT is "flat" by its
      nature and cannot be looked up from the hierarchically organized
      DNS.  HITs are flat by design due to a security tradeoff.  The
      more bits are allocated for the hash in the HIT, the less likely
      there will be (malicious) collisions.

   o  From performance viewpoint, HIP control and data plane processing
      introduces some overhead in terms of throughput and latency as
      elaborated below.

   Related to deployment drawbacks, firewalls are commonly used to
   control access to various services and devices in the current
   Internet.  Since HIP introduces an additional namespace, it is
   expected that also the HIP namespace would be filtered for unwanted
   connectivity.  While this can be achieved with existing tools
   directly in the end-hosts, filtering at the middleboxes requires
   modifications to existing firewall software or additional middleboxes

   The key exchange introduces some extra latency (two round trips) in
   the initial transport-layer connection establishment between two
   hosts.  With TCP, additional delay occurs if the underlying network
   stack implementation drops the triggering SYN packet during the key
   exchange.  The same cost may also occur during HIP handoff
   procedures.  However, subsequent TCP sessions using the same HIP
   association will not bear this cost (within the key lifetime).  Both
   the key exchange and handoff penalties can be minimized by caching
   TCP packets.  The latter case can further be optimized with TCP user
   timeout extensions [RFC5482] as described in further detail by
   Schuetz et al [schuetz-intermittent].

   The most CPU-intensive operations involve the use of the asymmetric
   keys and Diffie-Hellman key derivation at the control plane, but this
   occurs only during the key exchange, its maintenance (handoffs,
   refreshing of key material) and tear-down procedures of HIP
   associations.  The data plane is typically implemented with ESP
   because it has a smaller overhead due to symmetric key encryption.
   Naturally, even ESP involves some overhead in terms of latency
   (processing costs) and throughput (tunneling) (see, e.g.,
   [ylitalo-diss] for a performance evaluation).

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A.3.  Deployment and adoption considerations

   This section describes some deployment and adoption considerations
   related to HIP from a technical perspective.

A.3.1.  Deployment analysis

   HIP has been adapted and deployed in an industrial control network in
   a production factory, in which HIP's strong network layer identity
   supports the secure coexistence of the control network with many
   untrusted network devices operated by third-party vendors
   [paine-hip].  Similarly, HIP has also been included in a security
   product to support layer-two Virtual Private Networks
   [henderson-vpls] to enable security zones in a supervisory control
   and data acquisition (SCADA) network.  However, HIP has not been a
   "wild success" [RFC5218] in the Internet as argued by Levae et al
   [leva-barriers].  Here, we briefly highlight some of their findings
   based on interviews with 19 experts from the industry and academia.

   From a marketing perspective, the demand for HIP has been low and
   substitute technologies have been favored.  Another identified reason
   has been that some technical misconceptions related to the early
   stages of HIP specifications still persist.  Two identified
   misconceptions are that HIP does not support NAT traversal, and that
   HIP must be implemented in the OS kernel.  Both of these claims are
   untrue; HIP does have NAT traversal extensions
   [I-D.ietf-hip-native-nat-traversal], and kernel modifications can be
   avoided with modern operating systems by diverting packets for
   userspace processing.

   The analysis by Levae et al clarifies infrastructural requirements
   for HIP.  In a minimal set up, a client and server machine have to
   run HIP software.  However, to avoid manual configurations, usually
   DNS records for HIP are set up.  For instance, the popular DNS server
   software Bind9 does not require any changes to accommodate DNS
   records for HIP because they can be supported in binary format in its
   configuration files [RFC6538].  HIP rendezvous servers and firewalls
   are optional.  No changes are required to network address points,
   NATs, edge routers or core networks.  HIP may require holes in legacy

   The analysis also clarifies the requirements for the host components
   that consist of three parts.  First, a HIP control plane component is
   required, typically implemented as a userspace daemon.  Second, a
   data plane component is needed.  Most HIP implementations utilize the
   so called BEET mode of ESP that has been available since Linux kernel
   2.6.27, but the BEET mode is also included as a userspace component
   in a few of the implementations.  Third, HIP systems usually provide

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   a DNS proxy for the local host that translates HIP DNS records to
   LSIs and HITs, and communicates the corresponding locators to HIP
   userspace daemon.  While the third component is not mandatory, it is
   very useful for avoiding manual configurations.  The three components
   are further described in the HIP experiment report [RFC6538].

   Based on the interviews, Levae et al suggest further directions to
   facilitate HIP deployment.  Transitioning a number of HIP
   specifications to the standards track in IETF has already taken
   place, but the authors suggest other additional measures based on the
   interviews.  As a more radical measure, the authors suggest to
   implement HIP as a purely application-layer library [xin-hip-lib] or
   other kind of middleware.  On the other hand, more conservative
   measures include focusing on private deployments controlled by a
   single stakeholder.  As a more concrete example of such a scenario,
   HIP could be used by a single service provider to facilitate secure
   connectivity between its servers [komu-cloud].

A.3.2.  HIP in 802.15.4 networks

   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 commonly 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.

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

A.3.3.  HIP and Internet of Things

   HIP requires certain amount computational resources from a device due
   to cryptographic processing.  HIP scales down to phones and small
   system-on-chip devices (such as Raspberry Pis, Intel Edison), but
   small sensors operating with small batteries have remained
   problematic.  Different extensions to the HIP have been developed to
   scale HIP down to smaller devices, typically with different security
   tradeoffs.  For example, the non-cryptographic identifiers have been

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   proposed in RFID scenarios.  The slimfit approach [hummen] proposes a
   compression layer for HIP to make it more suitable for constrained
   networks.  The approach is applied to a light-weight version of HIP
   (i.e.  "Diet HIP") in order to scale down to small sensors.

   The HIP Diet Exchange [I-D.ietf-hip-dex] design aims at reducing the
   overhead of the employed cryptographic primitives by omitting public-
   key signatures and hash functions.  In doing so, the main goal is to
   still deliver similar security properties to the Base Exchange (BEX).

   DEX is primarily designed for computation or memory- constrained
   sensor/actuator devices.  Like BEX, it is expected to be used
   together with a suitable security protocol such as the Encapsulated
   Security Payload (ESP) for the protection of upper layer protocol
   data.  In addition, DEX can also be used as a keying mechanism for
   security primitives at the MAC layer, e.g., for IEEE 802.15.9
   networks ([IEEE.802-15-9].

   The main differences between HIP BEX and DEX are:

   1.  Minimum collection of cryptographic primitives to reduce the
       protocol overhead.

       *  Static Elliptic Curve Diffie-Hellman key pairs for peer
          authentication and encryption of the session key.

       *  AES-CTR for symmetric encryption and AES-CMAC for MACing

       *  A simple fold function for HIT generation.

   2.  Forfeit of Perfect Forward Secrecy with the dropping of an
       ephemeral Diffie-Hellman key agreement.

   3.  Forfeit of digital signatures with the removal of a hash
       function.  Reliance on ECDH derived key used in HIP_MAC to prove
       ownership of the private key.

   4.  Diffie-Hellman derived key ONLY used to protect the HIP packets.
       A separate secret exchange within the HIP packets creates the
       session key(s).

   5.  Optional retransmission strategy tailored to handle the
       potentially extensive processing time of the employed
       cryptographic operations on computationally constrained devices.

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A.3.4.  Infrastructure Applications

   HIP experimentation report [RFC6538] enumerates a number of client
   and server applications that have been trialed with HIP.  Based on
   the report, this section highlights and complements some potential
   ways how HIP could be exploited in existing infrastructure such as
   routers, gateways and proxies.

   HIP has been successfully used with forward web proxies (i.e.,
   client-side proxies).  HIP was used between a client host (web
   browser) and a forward proxy (Apache server) that terminated the HIP/
   ESP-tunnel.  The forward web proxy translated HIP-based traffic
   originating from the client into non-HIP traffic towards any web
   server in the Internet.  Consequently, the HIP-capable client could
   communicate with HIP-incapable web servers.  This way, the client
   could utilize mobility support as provided by HIP while using the
   fixed IP address of the web proxy, for instance, to access services
   that were allowed only from the IP address range of the proxy.

   HIP has also been experimented with reverse web proxies (i.e. server-
   side proxies) as described in more detail in [komu-cloud].  In this
   scenario, a HIP-incapable client accessed a HIP-capable web service
   via an intermediary load balancer (that was a web based load balancer
   implementation called HAProxy).  The load balancer translated non-HIP
   traffic originating from the client into HIP-based traffic for the
   web service (consisting of front-end and back-end servers).  Both the
   load balancer and the web service were located in a datacenter.  One
   of the key benefits for encrypting the web traffic with HIP in this
   scenario was to support a private-public cloud scenario (i.e. hybrid
   cloud) where the load balancer, front-end servers and back-end
   servers can be located in different datacenters and, thus, the
   traffic needs to protected when it passes through potentially
   insecure networks between the borders of the private and public

   While HIP could be used to secure access to intermediary devices
   (e.g., access to switches with legacy telnet), it has also been used
   to secure intermittent connectivity between middlebox infrastructure.
   For instance, earlier research [komu-mitigation] utilized HIP between
   Secure Mail Transport Protocol (SMTP) servers in order to exploit the
   computational puzzles of HIP as a spam mitigation mechanism.  A
   rather obvious practical challenge in this approach was the lack of
   HIP adoption on existing SMTP servers.

   To avoid deployment hurdles with existing infrastructure, HIP could
   be applied in the context of new protocols with little deployment.
   Namely, HIP has been experimented in the context of a new protocol,
   peer-to-peer SIP [camarillo-p2psip].  The work has resulted in a

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   number of related RFCs [RFC6078], [RFC6079], [RFC7086].  The key idea
   in the research work was to avoid redundant, time consuming ICE
   procedures by grouping different connections (i.e.  SIP and media
   streams) together using the low-layer HIP which executes NAT
   traversal procedures only once per host.  An interesting aspect in
   the approach was the use of P2P-SIP infrastructure as rendezvous
   servers for HIP control plane instead of utilizing the traditional
   HIP rendezvous services [RFC8004].

   Researchers have proposed to use HIP in cellular networks as a
   mobility, multihoming and security solution. [hip-lte] provides a
   security analysis and simulation measurements of using HIP in Long
   Term Evolution (LTE) backhaul networks.

   HIP has been experimented with securing cloud internal connectivity.
   First with virtual machines [komu-cloud] and then later also between
   Linux containers [ranjbar-synaptic].  In both cases, HIP was
   suggested as a solution NAT traversal that could be utilized both
   internally by a cloud network and between multi-cloud deployments.
   Specifically in the former case, HIP was beneficial sustaining
   connectivity with a virtual machine while it migrates to a new
   location.  In the latter case, Software-Defined Networking (SDN)
   controller acted as rendezvous server for HIP-capable containers.
   The controller enforced strong replay protection by adding middlebox
   nonces [heer-end-host] to the passing HIP base exchange and UPDATE

A.3.5.  Management of Identities in a Commercial Product

   Tempered Networks provides HIP-based products.  They refer to their
   platform as Identity-Defined Networking (IDN) [tempered-networks]
   because of HIP's identity-first networking architecture.  Their
   objective has been to make it simple and non-disruptive to deploy HIP
   enabled services widely in production environments with the purpose
   of enabling transparent device authentication and authorization,
   cloaking, segmentation, and end-to-end networking.  The goal is to
   eliminate much of the circular dependencies, exploits, and layered
   complexity of traditional "address-defined networking" that prevents
   mobility and verifiable device access control.  The products in the
   portfolio of Tempered Networks utilize HIP as follows:

   o  HIP Switches / Gateways - these are physical or virtual appliances
      that serve as the HIP gateway and policy enforcement point for non
      HIP-aware applications and devices located behind it.  No IP or
      infrastructure changes are required in order to connect, cloak,
      and protect the non-HIP aware devices.  Currently known supported
      platforms for HIP gateways are: x86 and ARM chipsets, ESXi, Hyper-
      V, KVM, AWS, Azure, and Google clouds.

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   o  HIP Relays / Rendezvous - are physical or virtual appliances that
      serve as identity based routers authorizing and bridging HIP
      endpoints without decrypting the HIP session.  A HIP Relay can be
      deployed as a standalone appliance or in a cluster for horizontal
      scaling.  All HIP aware endpoints and the devices they're
      connecting and protecting can remain privately addressed, The
      appliances eliminate IP conflicts, tunnel through NAT and CGNAT,
      and require no changes to the underlay infrastructure.  The only
      requirement is that a HIP endpoint should have outbound access to
      the Internet and that a HIP Relay should have a public address.

   o  HIP-Aware Clients and Servers - software that installs in the
      host's network stack and enforces policy for that host.  HIP
      clients support split tunneling.  Both HIP client and HIP server
      can interface with the local host firewall and HIP Server can be
      locked down to listen only on the port used for HIP, making the
      server invisible from unauthorized devices.  Currently known
      supported platforms are Windows, OSX, iOS, Android, Ubuntu, CentOS
      and other Linux derivatives.

   o  Policy Orchestration Managers - a physical or virtual appliance
      that serves as the engine to define and distribute network and
      security policy (HI and IP mappings, overlay networks and
      whitelist policies etc.) to HIP-aware endpoints.  Orchestration
      does not need to persist to the HIP endpoints and vice versa
      allowing for autonomous host networking and security.

A.4.  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.

   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?

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          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 the rendezvous mechanism is needed [RFC8005].

   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.

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   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.

Authors' Addresses

   Robert Moskowitz (editor)
   HTT Consulting
   Oak Park


   Miika Komu
   Hirsalantie 11
   02420 Jorvas


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