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Versions: (draft-moskowitz-hip-rfc5201-bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

Network Working Group                                  R. Moskowitz, Ed.
Internet-Draft                                                   Verizon
Obsoletes: 5201 (if approved)                                    T. Heer
Intended status: Standards Track                 RWTH Aachen University,
Expires: May 27, 2013                      Communication and Distributed
                                                           Systems Group
                                                               P. Jokela
                                            Ericsson Research NomadicLab
                                                            T. Henderson
                                                      The Boeing Company
                                                       November 23, 2012


                Host Identity Protocol Version 2 (HIPv2)
                     draft-ietf-hip-rfc5201-bis-10

Abstract

   This document specifies the details of the Host Identity Protocol
   (HIP).  HIP allows consenting hosts to securely establish and
   maintain shared IP-layer state, allowing separation of the identifier
   and locator roles of IP addresses, thereby enabling continuity of
   communications across IP address changes.  HIP is based on a SIGMA-
   compliant Diffie-Hellman key exchange, using public key identifiers
   from a new Host Identity namespace for mutual peer authentication.
   The protocol is designed to be resistant to denial-of-service (DoS)
   and man-in-the-middle (MitM) attacks.  When used together with
   another suitable security protocol, such as the Encapsulated Security
   Payload (ESP), it provides integrity protection and optional
   encryption for upper-layer protocols, such as TCP and UDP.

   This document obsoletes RFC 5201 and addresses the concerns raised by
   the IESG, particularly that of crypto agility.  It also incorporates
   lessons learned from the implementations of RFC 5201.

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 http://datatracker.ietf.org/drafts/current/.

   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



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 27, 2013.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.   A New Namespace and Identifiers  . . . . . . . . . . . .   7
     1.2.   The HIP Base Exchange (BEX)  . . . . . . . . . . . . . .   7
     1.3.   Memo Structure . . . . . . . . . . . . . . . . . . . . .   8
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   8
     2.1.   Requirements Terminology . . . . . . . . . . . . . . . .   8
     2.2.   Notation . . . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.   Definitions  . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Host Identity (HI) and its Structure  . . . . . . . . . . . .  10
     3.1.   Host Identity Tag (HIT)  . . . . . . . . . . . . . . . .  11
     3.2.   Generating a HIT from an HI  . . . . . . . . . . . . . .  11
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  12
     4.1.   Creating a HIP Association . . . . . . . . . . . . . . .  13
       4.1.1.  HIP Puzzle Mechanism  . . . . . . . . . . . . . . . .  14
       4.1.2.  Puzzle Exchange . . . . . . . . . . . . . . . . . . .  15



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       4.1.3.  Authenticated Diffie-Hellman Protocol with DH
               Group Negotiation . . . . . . . . . . . . . . . . . .  17
       4.1.4.  HIP Replay Protection . . . . . . . . . . . . . . . .  18
       4.1.5.  Refusing a HIP Base Exchange  . . . . . . . . . . . .  19
       4.1.6.  Aborting a HIP Base Exchange  . . . . . . . . . . . .  20
       4.1.7.  HIP Downgrade Protection  . . . . . . . . . . . . . .  20
       4.1.8.  HIP Opportunistic Mode  . . . . . . . . . . . . . . .  21
     4.2.   Updating a HIP Association . . . . . . . . . . . . . . .  24
     4.3.   Error Processing . . . . . . . . . . . . . . . . . . . .  24
     4.4.   HIP State Machine  . . . . . . . . . . . . . . . . . . .  25
       4.4.1.  State Machine Terminology . . . . . . . . . . . . . .  26
       4.4.2.  HIP States  . . . . . . . . . . . . . . . . . . . . .  27
       4.4.3.  HIP State Processes . . . . . . . . . . . . . . . . .  27
       4.4.4.  Simplified HIP State Diagram  . . . . . . . . . . . .  34
     4.5.   User Data Considerations . . . . . . . . . . . . . . . .  36
       4.5.1.  TCP and UDP Pseudo-Header Computation for User Data .  36
       4.5.2.  Sending Data on HIP Packets . . . . . . . . . . . . .  36
       4.5.3.  Transport Formats . . . . . . . . . . . . . . . . . .  36
       4.5.4.  Reboot, Timeout, and Restart of HIP . . . . . . . . .  36
     4.6.   Certificate Distribution . . . . . . . . . . . . . . . .  37
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  37
     5.1.   Payload Format . . . . . . . . . . . . . . . . . . . . .  37
       5.1.1.  Checksum  . . . . . . . . . . . . . . . . . . . . . .  38
       5.1.2.  HIP Controls  . . . . . . . . . . . . . . . . . . . .  39
       5.1.3.  HIP Fragmentation Support . . . . . . . . . . . . . .  39
     5.2.   HIP Parameters . . . . . . . . . . . . . . . . . . . . .  40
       5.2.1.  TLV Format  . . . . . . . . . . . . . . . . . . . . .  43
       5.2.2.  Defining New Parameters . . . . . . . . . . . . . . .  45
       5.2.3.  R1_COUNTER  . . . . . . . . . . . . . . . . . . . . .  46
       5.2.4.  PUZZLE  . . . . . . . . . . . . . . . . . . . . . . .  47
       5.2.5.  SOLUTION  . . . . . . . . . . . . . . . . . . . . . .  48
       5.2.6.  DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . .  49
       5.2.7.  DIFFIE_HELLMAN  . . . . . . . . . . . . . . . . . . .  50
       5.2.8.  HIP_CIPHER  . . . . . . . . . . . . . . . . . . . . .  51
       5.2.9.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  52
       5.2.10. HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  54
       5.2.11. TRANSPORT_FORMAT_LIST . . . . . . . . . . . . . . . .  55
       5.2.12. HIP_MAC . . . . . . . . . . . . . . . . . . . . . . .  56
       5.2.13. HIP_MAC_2 . . . . . . . . . . . . . . . . . . . . . .  56
       5.2.14. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . .  57
       5.2.15. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . .  58
       5.2.16. SEQ . . . . . . . . . . . . . . . . . . . . . . . . .  58
       5.2.17. ACK . . . . . . . . . . . . . . . . . . . . . . . . .  59
       5.2.18. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . .  60
       5.2.19. NOTIFICATION  . . . . . . . . . . . . . . . . . . . .  61
       5.2.20. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . .  65
       5.2.21. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . .  65
       5.2.22. ECHO_RESPONSE_SIGNED  . . . . . . . . . . . . . . . .  66



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       5.2.23. ECHO_RESPONSE_UNSIGNED  . . . . . . . . . . . . . . .  67
     5.3.   HIP Packets  . . . . . . . . . . . . . . . . . . . . . .  67
       5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  68
       5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  69
       5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  72
       5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  73
       5.3.5.  UPDATE - the HIP Update Packet  . . . . . . . . . . .  73
       5.3.6.  NOTIFY - the HIP Notify Packet  . . . . . . . . . . .  74
       5.3.7.  CLOSE - the HIP Association Closing Packet  . . . . .  75
       5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet . .  75
     5.4.   ICMP Messages  . . . . . . . . . . . . . . . . . . . . .  76
       5.4.1.  Invalid Version . . . . . . . . . . . . . . . . . . .  76
       5.4.2.  Other Problems with the HIP Header and Packet
               Structure . . . . . . . . . . . . . . . . . . . . . .  76
       5.4.3.  Invalid Puzzle Solution . . . . . . . . . . . . . . .  76
       5.4.4.  Non-Existing HIP Association  . . . . . . . . . . . .  77
   6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  77
     6.1.   Processing Outgoing Application Data . . . . . . . . . .  78
     6.2.   Processing Incoming Application Data . . . . . . . . . .  79
     6.3.   Solving the Puzzle . . . . . . . . . . . . . . . . . . .  79
     6.4.   HIP_MAC and SIGNATURE Calculation and Verification . . .  81
       6.4.1.  HMAC Calculation  . . . . . . . . . . . . . . . . . .  81
       6.4.2.  Signature Calculation . . . . . . . . . . . . . . . .  84
     6.5.   HIP KEYMAT Generation  . . . . . . . . . . . . . . . . .  86
     6.6.   Initiation of a HIP Base Exchange  . . . . . . . . . . .  87
       6.6.1.  Sending Multiple I1 Packets in Parallel . . . . . . .  88
       6.6.2.  Processing Incoming ICMP Protocol Unreachable
               Messages  . . . . . . . . . . . . . . . . . . . . . .  88
     6.7.   Processing Incoming I1 Packets . . . . . . . . . . . . .  89
       6.7.1.  R1 Management . . . . . . . . . . . . . . . . . . . .  90
       6.7.2.  Handling Malformed Messages . . . . . . . . . . . . .  91
     6.8.   Processing Incoming R1 Packets . . . . . . . . . . . . .  91
       6.8.1.  Handling of Malformed Messages  . . . . . . . . . . .  93
     6.9.   Processing Incoming I2 Packets . . . . . . . . . . . . .  94
       6.9.1.  Handling of Malformed Messages  . . . . . . . . . . .  96
     6.10.  Processing of Incoming R2 Packets  . . . . . . . . . . .  96
     6.11.  Sending UPDATE Packets . . . . . . . . . . . . . . . . .  97
     6.12.  Receiving UPDATE Packets . . . . . . . . . . . . . . . .  98
       6.12.1. Handling a SEQ Parameter in a Received UPDATE
               Message . . . . . . . . . . . . . . . . . . . . . . .  99
       6.12.2. Handling an ACK Parameter in a Received UPDATE
               Packet  . . . . . . . . . . . . . . . . . . . . . . . 100
     6.13.  Processing of NOTIFY Packets . . . . . . . . . . . . . . 100
     6.14.  Processing CLOSE Packets . . . . . . . . . . . . . . . . 100
     6.15.  Processing CLOSE_ACK Packets . . . . . . . . . . . . . . 101
     6.16.  Handling State Loss  . . . . . . . . . . . . . . . . . . 101
   7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . . 101
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . 102



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   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 105
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 107
   11. Changes from RFC 5201 . . . . . . . . . . . . . . . . . . . . 108
     11.1.  Changes from draft-ietf-hip-rfc5201-bis-09 . . . . . . . 108
     11.2.  Changes from draft-ietf-hip-rfc5201-bis-08 . . . . . . . 108
     11.3.  Changes from draft-ietf-hip-rfc5201-bis-07 . . . . . . . 108
     11.4.  Changes from draft-ietf-hip-rfc5201-bis-06 . . . . . . . 108
     11.5.  Changes from draft-ietf-hip-rfc5201-bis-05 . . . . . . . 109
     11.6.  Changes from draft-ietf-hip-rfc5201-bis-04 . . . . . . . 109
     11.7.  Changes from draft-ietf-hip-rfc5201-bis-03 . . . . . . . 111
     11.8.  Changes from draft-ietf-hip-rfc5201-bis-02 . . . . . . . 111
     11.9.  Changes from draft-ietf-hip-rfc5201-bis-01 . . . . . . . 112
     11.10. Changes from draft-ietf-hip-rfc5201-bis-00 . . . . . . . 114
     11.11. Contents of draft-ietf-hip-rfc5201-bis-00  . . . . . . . 114
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . 114
     12.1.  Normative References . . . . . . . . . . . . . . . . . . 114
     12.2.  Informative References . . . . . . . . . . . . . . . . . 117
   Appendix A.  Using Responder Puzzles  . . . . . . . . . . . . . . 119
   Appendix B.  Generating a Public Key Encoding from an HI  . . . . 120
   Appendix C.  Example Checksums for HIP Packets  . . . . . . . . . 120
     C.1.   IPv6 HIP Example (I1 packet) . . . . . . . . . . . . . . 121
     C.2.   IPv4 HIP Packet (I1 packet)  . . . . . . . . . . . . . . 121
     C.3.   TCP Segment  . . . . . . . . . . . . . . . . . . . . . . 122
   Appendix D.  ECDH and ECDSA 160 Bit Groups  . . . . . . . . . . . 122
   Appendix E.  HIT Suites and HIT Generation  . . . . . . . . . . . 122


























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1.  Introduction

   This document specifies the details of the Host Identity Protocol
   (HIP).  A high-level description of the protocol and the underlying
   architectural thinking is available in the separate HIP architecture
   description [I-D.ietf-hip-rfc4423-bis].  Briefly, the HIP
   architecture proposes an alternative to the dual use of IP addresses
   as "locators" (routing labels) and "identifiers" (endpoint, or host,
   identifiers).  In HIP, public cryptographic keys, of a public/private
   key pair, are used as Host Identifiers, to which higher layer
   protocols are bound instead of an IP address.  By using public keys
   (and their representations) as host identifiers, dynamic changes to
   IP address sets can be directly authenticated between hosts, and if
   desired, strong authentication between hosts at the TCP/IP stack
   level can be obtained.

   This memo specifies the base HIP protocol ("base exchange") used
   between hosts to establish an IP-layer communications context, called
   a HIP association, prior to communications.  It also defines a packet
   format and procedures for updating an active HIP association.  Other
   elements of the HIP architecture are specified in other documents,
   such as:

   o  "Using the Encapsulating Security Payload (ESP) transport format
      with the Host Identity Protocol (HIP)" [I-D.ietf-hip-rfc5202-bis]:
      how to use the Encapsulating Security Payload (ESP) for integrity
      protection and optional encryption

   o  "Host Mobility with the Host Identity Protocol"
      [I-D.ietf-hip-rfc5206-bis]: how to support host mobility in HIP

   o  "Host Identity Protocol (HIP) Domain Name System (DNS) Extensions"
      [I-D.ietf-hip-rfc5205-bis]: how to extend DNS to contain Host
      Identity information

   o  "Host Identity Protocol (HIP) Rendezvous Extension"
      [I-D.ietf-hip-rfc5204-bis]: using a rendezvous mechanism to
      contact mobile HIP hosts

   Since the HIP Base Exchange was first developed, there have been a
   few advances in cryptography and attacks against cryptographic
   systems.  As a result, all cryptographic protocols need to be agile.
   That is, it should be a part of the protocol to be able to switch
   from one cryptographic primitive to another.  It is important to
   support a reasonable set of mainstream algorithms to cater for
   different use cases and allow moving away from algorithms that are
   later discovered to be vulnerable This update to the Base Exchange
   includes this needed cryptographic agility while addressing the



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   downgrade attacks that such flexibility introduces.  In particular,
   Elliptic Curve support by Elliptic Curve DSA (ECDSA) and Elliptic
   Curve Diffie-Hellman (ECDH) and alternative hash functions have been
   added.

1.1.  A New Namespace and Identifiers

   The Host Identity Protocol introduces a new namespace, the Host
   Identity namespace.  Some ramifications of this new namespace are
   explained in the HIP architecture description
   [I-D.ietf-hip-rfc4423-bis].

   There are two main representations of the Host Identity, the full
   Host Identity (HI) and the Host Identity Tag (HIT).  The HI is a
   public key and directly represents the Identity of a host.  Since
   there are different public key algorithms that can be used with
   different key lengths, the HI, as such, is unsuitable for use as a
   packet identifier, or as an index into the various state-related
   implementation structures needed to support HIP.  Consequently, a
   hash of the HI, the Host Identity Tag (HIT), is used as the
   operational representation.  The HIT is 128 bits long and is used in
   the HIP headers and to index the corresponding state in the end
   hosts.  The HIT has an important security property in that it is
   self-certifying (see Section 3).

1.2.  The HIP Base Exchange (BEX)

   The HIP base exchange is a two-party cryptographic protocol used to
   establish communications context between hosts.  The base exchange is
   a SIGMA-compliant [KRA03] four-packet exchange.  The first party is
   called the Initiator and the second party the Responder.  The
   protocol exchanges Diffie-Hellman [DIF76] keys in the 2nd and 3rd
   packets, and authenticates the parties in the 3rd and 4th packets.
   The four-packet design helps to make HIP DoS resilient.  It allows
   the Responder to stay stateless until the IP address and the
   cryptographic puzzle is verified.  The Responder starts the puzzle
   exchange in the 2nd packet, with the Initiator completing it in the
   3rd packet before the Responder stores any state from the exchange.

   The exchange can use the Diffie-Hellman output to encrypt the Host
   Identity of the Initiator in the 3rd packet (although Aura, et al.,
   [AUR03] notes that such operation may interfere with packet-
   inspecting middleboxes), or the Host Identity may instead be sent
   unencrypted.  The Responder's Host Identity is not protected.  It
   should be noted, however, that both the Initiator's and the
   Responder's HITs are transported as such (in cleartext) in the
   packets, allowing an eavesdropper with a priori knowledge about the
   parties to identify them by their HITs.  Hence, encrypting the HI of



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   any party does not provide privacy against such attacker.

   Data packets start to flow after the 4th packet.  The 3rd and 4th HIP
   packets may carry a data payload in the future.  However, the details
   of this may be defined later.

   An existing HIP association can be updated using the update mechanism
   defined in this document, and when the association is no longer
   needed, it can be closed using the defined closing mechanism.

   Finally, HIP is designed as an end-to-end authentication and key
   establishment protocol, to be used with Encapsulated Security Payload
   (ESP) [I-D.ietf-hip-rfc5202-bis] and other end-to-end security
   protocols.  The base protocol does not cover all the fine-grained
   policy control found in Internet Key Exchange (IKE) [RFC4306] that
   allows IKE to support complex gateway policies.  Thus, HIP is not a
   complete replacement for IKE.

1.3.  Memo Structure

   The rest of this memo is structured as follows.  Section 2 defines
   the central keywords, notation, and terms used throughout the rest of
   the document.  Section 3 defines the structure of the Host Identity
   and its various representations.  Section 4 gives an overview of the
   HIP base exchange protocol.  Sections 5 and 6 define the detailed
   packet formats and rules for packet processing.  Finally, Sections 7,
   8, and 9 discuss policy, security, and IANA considerations,
   respectively.

2.  Terms and Definitions

2.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.2.  Notation

   [x]   indicates that x is optional.

   {x}   indicates that x is encrypted.

   X(y)   indicates that y is a parameter of X.







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   <x>i   indicates that x exists i times.

   -->   signifies "Initiator to Responder" communication (requests).

   <--   signifies "Responder to Initiator" communication (replies).

   |  signifies concatenation of information (e.g., X | Y is the
      concatenation of X with Y).

   Ltrunc (H(x), K)   denotes the lowest order #K bits of the result of
      the hash function H on the input x.

2.3.  Definitions

   HIP Base Exchange (BEX):  the handshake for establishing a new HIP
      association.

   Host Identity (HI):  The public key of the signature algorithm that
      represents the identity of the host.  In HIP, a host proves its
      identity by creating a signature with the private key belonging to
      its HI (c.f.  Section 3).

   Host Identity Tag (HIT):  A shorthand for the HI in IPv6 format.  It
      is generated by hashing the HI (c.f.  Section 3.1).

   HIT Suite:  A HIT Suite groups all cryptographic algorithms that are
      required to generate and use an HI and its HIT.  In particular,
      these algorithms are: 1) the public key signature algorithm and 2)
      the hash function, 3) the truncation (c.f.  Appendix E).

   HIP association:   The shared state between two peers after
      completion of the BEX.

   Initiator:   The host that initiates the BEX.  This role is typically
      forgotten once the BEX is completed.

   Responder:   The host that responds to the Initiator in the BEX.
      This role is typically forgotten once the BEX is completed.

   Responder's HIT Hash Algorithm (RHASH):   The Hash algorithm used for
      various hash calculations in this document.  The algorithm is the
      same as is used to generate the Responder's HIT.  The RHASH is the
      hash function defined by the HIT Suite of the Responder's HIT
      (c.f.  Appendix E).







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   Length of the Responder's HIT Hash Algorithm (RHASH_len):   The
      natural output length of RHASH in bits.

   Signed data:   Data that is signed is protected by a digital
      signature that was created by the sender of the data by using the
      private key of its HI.

   KDF:   The Key Derivation Function (KDF) is used for deriving the
      symmetric keys from the Diffie-Hellman key exchange.

   KEYMAT:   The keying material derived from the Diffie-Hellman key
      exchange by using the KDF.  Symmetric keys for encryption and
      integrity protection of HIP control and payload packets are drawn
      from this keying material.

3.  Host Identity (HI) and its Structure

   In this section, the properties of the Host Identity and Host
   Identity Tag are discussed, and the exact format for them is defined.
   In HIP, the public key of an asymmetric key pair is used as the Host
   Identity (HI).  Correspondingly, the host itself is defined as the
   entity that holds the private key of the key pair.  See the HIP
   architecture specification [I-D.ietf-hip-rfc4423-bis] for more
   details on the difference between an identity and the corresponding
   identifier.

   HIP implementations MUST support the Rivest Shamir Adelman (RSA)
   [RFC3110] public key algorithm and the Elliptic Curve Digital
   Signature Algorithm (ECDSA) for generating the HI as defined in
   Section 5.2.9.  Additional algorithms MAY be supported.

   A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
   protocols to represent the Host Identity.  The HIT is 128 bits long
   and has the following three key properties: i) it is the same length
   as an IPv6 address and can be used in fixed address-sized fields in
   APIs and protocols, ii) it is self-certifying (i.e., given a HIT, it
   is computationally hard to find a Host Identity key that matches the
   HIT), and iii) the probability of a HIT collision between two hosts
   is very low, hence, it is infeasible for an attacker to find a
   collision with a HIT that is in use.  For details on the security
   properties of the HIT see [I-D.ietf-hip-rfc4423-bis].

   The structure of the HIT is defined in [I-D.ietf-hip-rfc4843-bis].
   The HIT is an Overlay Routable Cryptographic Hash Identifier (ORCHID)
   and consists of three parts: first, an IANA assigned prefix to
   distinguish it from other IPv6 addresses.  Second, a four-bit
   encoding of the algorithms that were used for generating the HI and
   the hashed representation of HI.  Third, a 96-bit hashed



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   representation of the Host Identity.  The encoding of the ORCHID
   generation algorithm and the exact algorithm for generating the
   hashed representation is specified in Appendix E.

   Carrying HIs and HITs in the header of user data packets would
   increase the overhead of packets.  Thus, it is not expected that they
   are carried in every packet, but other methods are used to map the
   data packets to the corresponding HIs.  In some cases, this makes it
   possible to use HIP without any additional headers in the user data
   packets.  For example, if ESP is used to protect data traffic, the
   Security Parameter Index (SPI) carried in the ESP header can be used
   to map the encrypted data packet to the correct HIP association.

3.1.  Host Identity Tag (HIT)

   The Host Identity Tag is a 128-bit value -- a hashed encoding of the
   Host Identifier.  There are two advantages of using a hashed encoding
   over the actual variable-sized Host Identity public key in protocols.
   First, the fixed length of the HIT keeps packet sizes manageable and
   eases protocol coding.  Second, it presents a consistent format for
   the protocol, independent of the underlying identity technology in
   use.

   RFC 4843-bis [I-D.ietf-hip-rfc4843-bis] specifies 128-bit hash-based
   identifiers, called Overlay Routable Cryptographic Hash Identifiers,
   ORCHIDs.  Their prefix, allocated from the IPv6 address block, is
   defined in [I-D.ietf-hip-rfc4843-bis].  The Host Identity Tag is one
   type of an ORCHID.

   This document extends the original, experimental HIP specification
   [RFC5201] with measures to support crypto agility.  One of these
   measures is to allow different hash functions for creating a HIT.
   HIT Suites group the sets of algorithms that are required to generate
   and use a particular HIT.  The Suites are encoded in HIT Suite IDs.
   These HIT Suite IDs are transmitted in the ORCHID Generation
   Algorithm (OGA) field in the ORCHID.  With the HIT Suite ID in the
   OGA field, a hosts can tell from another host's HIT, whether it
   supports the necessary hash and signature algorithms to establish a
   HIP association with that host.

3.2.  Generating a HIT from an HI

   The HIT MUST be generated according to the ORCHID generation method
   described in [I-D.ietf-hip-rfc4843-bis] using a context ID value of
   0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA (this tag value has been
   generated randomly by the editor of this specification), and an input
   that encodes the Host Identity field (see Section 5.2.9) present in a
   HIP payload packet.  The set of hash function, signature algorithm,



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   and the algorithm used for generating the HIT from the HI depends on
   the HIT Suite (see Appendix E) and is indicated by the four bits of
   the ORCHID Generation Algorithm (OGA) field in the ORCHID.
   Currently, truncated SHA-1, truncated SHA-384, and truncated SHA-256
   [FIPS.180-2.2002] are defined as hashes for generating a HIT.

   For identities that are either RSA, Digital Signature Algorithm
   (DSA), or Elliptic Curve DSA (ECDSA) public keys, the ORCHID input
   consists of the public key encoding as specified for the Host
   Identity field of the HOST_ID parameter (see Section 5.2.9).  This
   document defines four algorithm profiles: RSA, DSA, ECDSA, and
   ECDSA_LOW.  The ECDSA_LOW profile is meant for devices with low
   computational capabilities.  Hence, one of the following applies:

      The RSA public key is encoded as defined in [RFC3110] Section 2,
      taking the exponent length (e_len), exponent (e), and modulus (n)
      fields concatenated.  The length (n_len) of the modulus (n) can be
      determined from the total HI Length and the preceding HI fields
      including the exponent (e).  Thus, the data that serves as input
      for the HIT generation has the same length as the HI.  The fields
      MUST be encoded in network byte order, as defined in [RFC3110].

      The DSA public key is encoded as defined in [RFC2536] Section 2,
      taking the fields T, Q, P, G, and Y, concatenated as input.  Thus,
      the data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
      where T is the size parameter as defined in [RFC2536].  The size
      parameter T, affecting the field lengths, MUST be selected as the
      minimum value that is long enough to accommodate P, G, and Y.  The
      fields MUST be encoded in network byte order, as defined in
      [RFC2536].

      The ECDSA public keys are encoded as defined in [RFC6090] Section
      4.2 and 6.

   In Appendix B, the public key encoding process is illustrated using
   pseudo-code.

4.  Protocol Overview

   This section is a simplified overview of the HIP protocol operation,
   and does not contain all the details of the packet formats or the
   packet processing steps.  Sections 5 and 6 describe in more detail
   the packet formats and packet processing steps, respectively, and are
   normative in case of any conflicts with this section.

   The protocol number 139 has been assigned by IANA to the Host
   Identity Protocol.




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   The HIP payload (Section 5.1) header could be carried in every IP
   datagram.  However, since HIP headers are relatively large (40
   bytes), it is desirable to 'compress' the HIP header so that the HIP
   header only occurs in control packets used to establish or change HIP
   association state.  The actual method for header 'compression' and
   for matching data packets with existing HIP associations (if any) is
   defined in separate documents, describing transport formats and
   methods.  All HIP implementations MUST implement, at minimum, the ESP
   transport format for HIP [I-D.ietf-hip-rfc5202-bis].

4.1.  Creating a HIP Association

   By definition, the system initiating a HIP base exchange is the
   Initiator, and the peer is the Responder.  This distinction is
   typically forgotten once the base exchange completes, and either
   party can become the Initiator in future communications.

   The HIP base exchange serves to manage the establishment of state
   between an Initiator and a Responder.  The first packet, I1,
   initiates the exchange, and the last three packets, R1, I2, and R2,
   constitute an authenticated Diffie-Hellman [DIF76] key exchange for
   session-key generation.  In the first two packets, the hosts agree on
   a set of cryptographic identifiers and algorithms that are then used
   in and after the exchange.  During the Diffie-Hellman key exchange, a
   piece of keying material is generated.  The HIP association keys are
   drawn from this keying material by using a Key Derivation Function
   (KDF).  If other cryptographic keys are needed, e.g., to be used with
   ESP, they are expected to be drawn from the same keying material by
   using the KDF.

   The Initiator first sends a trigger packet, I1, to the Responder.
   The packet contains the HIT of the Initiator and possibly the HIT of
   the Responder, if it is known.  Moreover, the I1 packet initializes
   the negotiation of the Diffie-Hellman group that is used for
   generating the keying material.  Therefore, the I1 packet contains a
   list of Diffie Hellman Group IDs supported by the Initiator.  Note
   that in some cases it may be possible to replace this trigger packet
   by some other form of a trigger, in which case the protocol starts
   with the Responder sending the R1 packet.  In such cases, another
   mechanism to convey the Initiator's supported DH Groups (e.g., by
   using a default group) must be specified.

   The second packet, R1, starts the actual authenticated Diffie-Hellman
   exchange.  It contains a puzzle -- a cryptographic challenge that the
   Initiator must solve before continuing the exchange.  The level of
   difficulty of the puzzle can be adjusted based on level of trust with
   the Initiator, current load, or other factors.  In addition, the R1
   contains the Responder's Diffie-Hellman parameter and lists of



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   cryptographic algorithms supported by the Responder.  Based on these
   lists, the Initiator can continue, abort, or restart the base
   exchange with a different selection of cryptographic algorithms.
   Also, the R1 packet contains a signature that covers selected parts
   of the message.  Some fields are left outside the signature to
   support pre-created R1s.

   In the I2 packet, the Initiator MUST display the solution to the
   received puzzle.  Without a correct solution, the I2 message is
   discarded.  The I2 packet also contains a Diffie-Hellman parameter
   that carries needed information for the Responder.  The I2 packet is
   signed by the Initiator.

   The R2 packet acknowledges the receipt of the I2 packet and completes
   the base exchange.  The packet is signed by the Responder.

   The base exchange is illustrated below in Figure 1.  The term "key"
   refers to the Host Identity public key, and "sig" represents a
   signature using such a key.  The packets contain other parameters not
   shown in this figure.

      Initiator                              Responder

                   I1: DH list
                 -------------------------->
                                             select precomputed R1
                   R1: puzzle, DH, key, sig
                 <-------------------------
   check sig                                 remain stateless
   solve puzzle
                 I2: solution, DH, {key}, sig
                 -------------------------->
   compute DH                                check puzzle
                                             check sig
                           R2: sig
                 <--------------------------
   check sig                                 compute DH


                                 Figure 1

4.1.1.  HIP Puzzle Mechanism

   The purpose of the HIP puzzle mechanism is to protect the Responder
   from a number of denial-of-service threats.  It allows the Responder
   to delay state creation until receiving the I2 packet.  Furthermore,
   the puzzle allows the Responder to use a fairly cheap calculation to
   check that the Initiator is "sincere" in the sense that it has



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   churned enough CPU cycles in solving the puzzle.

   The puzzle allows a Responder implementation to completely delay
   session-specific state creation until a valid I2 packet is received.
   An I2 packet without valid puzzle solution can be rejected
   immediately once the Responder has checked the solution by computing
   only one hash function before state is created and CPU-intensive
   public-key signature verification and Diffie-Hellman key generation
   are performed.  By varying the difficulty of the puzzle, the
   Responder can frustrate CPU or memory targeted DoS attacks.

   The Responder can remain stateless and drop most spoofed I2 packets
   because puzzle calculation is based on the Initiator's Host Identity
   Tag. The idea is that the Responder has a (perhaps varying) number of
   pre-calculated R1 packets, and it selects one of these based on the
   information carried in the I1 packet.  When the Responder then later
   receives the I2 packet, it can verify that the puzzle has been solved
   using the Initiator's HIT.  This makes it impractical for the
   attacker to first exchange one I1/R1 packet, and then generate a
   large number of spoofed I2 packets that seemingly come from different
   HITs.  This method does not protect the Responder from an attacker
   that uses fixed HITs, though.  Against such an attacker, a viable
   approach may be to create a piece of local state, and remember that
   the puzzle check has previously failed.  See Appendix A for one
   possible implementation.  Responder implementations SHOULD include
   sufficient randomness in the puzzle values so that algorithmic
   complexity attacks become impossible [CRO03].

   The Responder can set the puzzle difficulty for the Initiator, based
   on its level of trust of the Initiator.  Because the puzzle is not
   included in the signature calculation, the Responder can use pre-
   calculated R1 packets and include the puzzle just before sending the
   R1 to the Initiator.  The Responder SHOULD use heuristics to
   determine when it is under a denial-of-service attack, and set the
   puzzle difficulty value #K appropriately as explained later.

4.1.2.  Puzzle Exchange

   The Responder starts the puzzle exchange when it receives an I1
   packet.  The Responder supplies a random number #I, and requires the
   Initiator to find a number J.  To select a proper #J, the Initiator
   must create the concatenation of #I, the HITs of the parties, and #J,
   and calculate a hash over this concatenation using the RHASH
   algorithm.  The lowest order #K bits of the result MUST be zeros.
   The value #K sets the difficulty of the puzzle.

   To generate a proper number #J, the Initiator will have to generate a
   number of Js until one produces the hash target of zeros.  The



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   Initiator SHOULD give up after exceeding the puzzle Lifetime in the
   PUZZLE parameter (as described in Section 5.2.4).  The Responder
   needs to re-create the concatenation of #I, the HITs, and the
   provided #J, and compute the hash once to prove that the Initiator
   completed its assigned task.

   To prevent precomputation attacks, the Responder MUST select the
   number #I in such a way that the Initiator cannot guess it.
   Furthermore, the construction MUST allow the Responder to verify that
   the value #I was indeed selected by it and not by the Initiator.  See
   Appendix A for an example on how to implement this.

   Using the Opaque data field in the PUZZLE (see Section 5.2.4), in an
   ECHO_REQUEST_SIGNED (see Section 5.2.20) or in an
   ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21), the Responder
   can include some data in R1 that the Initiator MUST copy unmodified
   in the corresponding I2 packet.  The Responder can use the opaque
   data to transfer a piece of local state information to the Initiator
   and back, for example to recognize that the I2 is a response to a
   previously sent R1.  The Responder can generate the Opaque data in
   various ways; e.g., using encryption or hashing with some secret, the
   sent #I, and possibly using other related data.  With the same
   secret, the received #I (from the I2 packet), and the other related
   data (if any), the Responder can verify that it has itself sent the
   #I to the Initiator.  The Responder MUST periodically change such a
   secret.

   It is RECOMMENDED that the Responder generates new secrets for the
   puzzle and new R1s once every few minutes.  Furthermore, it is
   RECOMMENDED that the Responder is able to verify valid puzzle
   solution at least Lifetime seconds after the puzzle secret has been
   deprecated.  This time value guarantees that the puzzle is valid for
   at least Lifetime and at most 2 * Lifetime seconds.  This limits the
   usability that an old, solved puzzle has to an attacker.  Moreover,
   it avoids problems with the validity of puzzles if the lifetime is
   relatively short compared to the network delay and the time for
   solving the puzzle.

   The puzzle value #I and the solution #J are inputs for deriving the
   keying material from the Diffie-Hellman key exchange (see
   Section 6.5).  Therefore, a Responder SHOULD NOT use the same puzzle
   #I with the same DH keys for the same Initiator twice to ensure that
   the derived keying material differs.  Such uniqueness can be
   achieved, for example, by using a counter as an additional input for
   generating #I. This counter can be increased for each processed I1
   packet.  The state of the counter can be transmitted in the Opaque
   data field in the PUZZLE (see Section 5.2.4), in an
   ECHO_REQUEST_SIGNED (see Section 5.2.20) or in an



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   ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21) without the need
   to establish state.

   NOTE: The protocol developers explicitly considered whether R1 should
   include a timestamp in order to protect the Initiator from replay
   attacks.  The decision was to NOT include a timestamp to avoid
   problems with global time synchronization.

   NOTE: The protocol developers explicitly considered whether a memory
   bound function should be used for the puzzle instead of a CPU-bound
   function.  The decision was not to use memory-bound functions.

4.1.3.  Authenticated Diffie-Hellman Protocol with DH Group Negotiation

   The packets R1, I2, and R2 implement a standard authenticated Diffie-
   Hellman exchange.  The Responder sends one of its public Diffie-
   Hellman keys and its public authentication key, i.e., its Host
   Identity, in R1.  The signature in the R1 packet allows the Initiator
   to verify that the R1 has been once generated by the Responder.
   However, since the R1 is precomputed and therefore does not cover
   association-specific information in the I1 packet, it does not
   protect from replay attacks.

   Before the actual authenticated Diffie-Hellman exchange, the
   Initiator expresses its preference regarding its choice of the DH
   groups in the I1 packet.  The preference is expressed as a sorted
   list of DH Group IDs.  The I1 packet is not protected by a signature.
   Therefore, this list is sent in an unauthenticated way to avoid
   costly computations for processing the I1 packet at the Responder
   side.  Based on the preferences of the Initiator, the Responder sends
   an R1 packet containing its most suitable public DH value.  The
   Responder also attaches a list of its own preferences to the R1 to
   convey the basis for the DH group selection to the Initiator.  This
   list is carried in the signed part of the R1 packet.  If the choice
   of the DH group value in the R1 does not match the preferences of the
   Initiator and the Responder, the Initiator can detect that the list
   of DH Group IDs in the I1 was manipulated (see below for details).

   If none of the DH Group IDs in the I1 packet is supported by the
   Responder, the Responder selects the DH Group most suitable for it
   regardless of the Initiator's preference.  It then sends the R1
   containing this DH Group and its list of supported DH Group IDs to
   the Initiator.

   When the Initiator receives an R1, it receives one of the Responder's
   public Diffie-Hellman values and the list of DH Group IDs supported
   by the Responder.  This list is covered by the signature in the R1
   packet to avoid forgery.  The Initiator compares the Group ID of the



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   public DH value in the R1 packet to the list of supported DH Group
   IDs in the R1 packets and to its own preferences expressed in the
   list of supported DH Group IDs.  The Initiator continues the BEX only
   if the Group ID of the public DH value of the Responder is the most
   preferred of the IDs supported by both the Initiator and Responder.
   Otherwise, the communication is subject of a downgrade attack and the
   Initiator MUST either restart the base exchange with a new I1 packet
   or abort the base exchange.  If the Responder's choice of the DH
   Group is not supported by the Initiator, the Initiator MAY abort the
   handshake or send a new I1 packet with a different list of supported
   DH Groups.  However, the Initiator MUST verify the signature of the
   R1 packet before restarting or aborting the handshake.  It MUST
   silently ignore the R1 packet if the signature is not valid.

   If the preferences regarding the DH Group ID match, the Initiator
   computes the Diffie-Hellman session key (Kij).  The Initiator creates
   a HIP association using keying material from the session key (see
   Section 6.5), and may use the HIP association to encrypt its public
   authentication key, i.e., the Host Identity.  The resulting I2 packet
   contains the Initiator's Diffie-Hellman key and its (optionally
   encrypted) public authentication key.  The signature of the I2
   message covers all parameters of the signed parameter ranges (see
   Section 5.2) in the packet without exceptions as in the R1.

   The Responder extracts the Initiator's Diffie-Hellman public key from
   the I2 packet, computes the Diffie-Hellman session key, creates a
   corresponding HIP association, and decrypts the Initiator's public
   authentication key.  It can then verify the signature using the
   authentication key.

   The final message, R2, completes the BEX and protects the Initiator
   against replay attacks because the Responder uses the shared key from
   the Diffie-Hellman exchange to create an HMAC as well as uses the
   private key of its Host Identity to sign the packet contents.

4.1.4.  HIP Replay Protection

   The HIP protocol includes the following mechanisms to protect against
   malicious packet replays.  Responders are protected against replays
   of I1 packets by virtue of the stateless response to I1 packets with
   pre-signed R1 messages.  Initiators are protected against R1 replays
   by a monotonically increasing "R1 generation counter" included in the
   R1.  Responders are protected against replays of forged I2 packets by
   the puzzle mechanism (see Section 4.1.1 above), and optional use of
   opaque data.  Hosts are protected against replays of R2 packets and
   UPDATEs by use of a less expensive HMAC verification preceding the
   HIP signature verification.




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   The R1 generation counter is a monotonically increasing 64-bit
   counter that may be initialized to any value.  The scope of the
   counter MAY be system-wide but there SHOULD be a separate counter for
   each Host Identity, if there is more than one local host identity.
   The value of this counter SHOULD be preserved across system reboots
   and invocations of the HIP base exchange.  This counter indicates the
   current generation of puzzles.  Implementations MUST accept puzzles
   from the current generation and MAY accept puzzles from earlier
   generations.  A system's local counter MUST be incremented at least
   as often as every time old R1s cease to be valid.  The local counter
   SHOULD never be decremented, otherwise the host exposes its peers to
   the replay of previously generated, higher numbered R1s.  The R1
   counter SHOULD NOT roll over.

   A host may receive more than one R1, either due to sending multiple
   I1 packets (see Section 6.6.1) or due to a replay of an old R1.  When
   sending multiple I1 packets to the same host, an Initiator SHOULD
   wait for a small amount of time (a reasonable time may be 2 *
   expected RTT) after the first R1 reception to allow possibly multiple
   R1s to arrive, and it SHOULD respond to an R1 among the set with the
   largest R1 generation counter.  If an Initiator is processing an R1
   or has already sent an I2 packet (still waiting for the R2 packet)
   and it receives another R1 with a larger R1 generation counter, it
   MAY elect to restart R1 processing with the fresher R1, as if it were
   the first R1 to arrive.

   Upon conclusion of an active HIP association with another host, the
   R1 generation counter associated with the peer host SHOULD be
   flushed.  A local policy MAY override the default flushing of R1
   counters on a per-HIT basis.  The reason for recommending the
   flushing of this counter is that there may be hosts where the R1
   generation counter (occasionally) decreases; e.g., due to hardware
   failure.

4.1.5.  Refusing a HIP Base Exchange

   A HIP-aware host may choose not to accept a HIP base exchange.  If
   the host's policy is to only be an Initiator, it should begin its own
   HIP base exchange.  A host MAY choose to have such a policy since
   only the privacy of the Initiator's HI is protected in the exchange.
   It should be noted that such behavior can introduce the risk of a
   race condition if each host's policy is to only be an Initiator, at
   which point the HIP base exchange will fail.

   If the host's policy does not permit it to enter into a HIP exchange
   with the Initiator, it should send an ICMP 'Destination Unreachable,
   Administratively Prohibited' message.  A more complex HIP packet is
   not used here as it actually opens up more potential DoS attacks than



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   a simple ICMP message.  A HIP NOTIFY message is not used because no
   HIP session exists between the two hosts at that time.

4.1.6.  Aborting a HIP Base Exchange

   Two HIP hosts may encounter situations in which they cannot complete
   a HIP base exchange because of insufficient support for cryptographic
   algorithms, in particular the HIT Suites and DH Groups.  After
   receiving the R1 packet, the Initiator can determine whether the
   Responder supports the required cryptographic operations to
   successfully establish a HIP association.  The Initiator can abort
   the BEX silently after receiving an R1 packet that indicates an
   unsupported set of algorithms.  The specific conditions are described
   below.

   The R1 packet contains a signed list of HIT Suite IDs as supported by
   the Responder.  Therefore, the Initiator can determine whether its
   source HIT is supported by the Responder.  If the HIT Suite ID of the
   Initiator's HIT is not contained in the list of HIT Suites in the R1,
   the Initiator MAY abort the handshake silently or MAY restart the
   handshake with a new I1 packet that contains a source HIT supported
   by the Responder.

   During the Handshake, the Initiator and the Responder agree on a
   single DH Group.  The Responder selects the DH Group and its DH
   public value in the R1 based on the list of DH Suite IDs in the I1
   packet.  If the responder supports none of the DH Groups requested by
   the Initiator, the Responder selects an arbitrary DH and replies with
   an R1 containing its list of supported DH Group IDs.  In such case,
   the Initiator receives an R1 packet containing the DH public value
   for an unrequested DH Group and also the Responder's DH Group list in
   the signed part of the R1 packet.  At this point, the Initiator MAY
   abort the handshake or MAY restart the handshake by sending a new I1
   packet containing a selection of DH Group IDs that is supported by
   the Responder.

4.1.7.  HIP Downgrade Protection

   In a downgrade attack, an attacker attempts to unnoticeably
   manipulate the packets of an Initiator and/or a Responder to
   influence the result of the cryptographic negotiations in the BEX to
   its favor.  As a result, the victims select weaker cryptographic
   algorithms than they would otherwise have selected without the
   attacker's interference.  Downgrade attacks can only be successful if
   they remain un-detected by the victims and the victims falsely assume
   a secure communication channel.

   In HIP, almost all packet parameters related to cryptographic



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   negotiations are covered by signatures.  These parameters cannot be
   directly manipulated in a downgrade attack without invalidating the
   signature.  However, signed packets can be subject to replay attacks.
   In such a replay attack, the attacker could use an old BEX packet
   with an outdated and weak selection of cryptographic algorithms and
   replay it instead of a more recent packet with a collection of
   stronger cryptographic algorithms.  Signed packets that could be
   subject to this replay attack are the R1 and I2 packet.  However,
   replayed R1 and I2 packets cannot be used to successfully establish a
   HIP BEX because these packets also contain the public DH values of
   the Initiator and the Responder.  Old DH values from replayed packets
   lead to invalid keying material and mismatching shared secrets
   because the attacker is unable to derive valid keying material from
   the DH public keys in the R1 and cannot generate a valid HMAC and
   signature for a replayed I2.

   In contrast to the first version of HIP [RFC5201],the version 2 of
   HIP defined in this document begins the negotiation of the DH Groups
   already in the first BEX packet, the I1.  The I1 packet is, by
   intention, not protected by a signature to avoid CPU-intensive
   cryptographic operations for processing floods of I1 packets targeted
   at the Responder.  Hence, the list of DH Group IDs in the I1 packet
   is vulnerable to forgery and manipulation.  To thwart an unnoticed
   manipulation of the I1 packet, the Responder chooses the DH Group
   deterministically and includes its own list of DH Group IDs in the
   signed part of the R1 packet.  The Initiator can detect an attempted
   downgrade attack by comparing the list of DH Group IDs in the R1
   packet to its own preferences in the I1 packet.  If the choice of the
   DH Group in the R1 packet does not equal to the best match of the two
   lists (the highest priority DH ID of the Responder that is present in
   the Initiator's DH list), the Initiator can conclude that its list in
   the I1 packet was altered by an attacker.  In this case, the
   Initiator can restart or abort the BEX.  As mentioned before, the
   detection of the downgrade attack is sufficient to prevent it.

4.1.8.  HIP Opportunistic Mode

   It is possible to initiate a HIP BEX even if the Responder's HI (and
   HIT) is unknown.  In this case, the initial I1 packet contains all
   zeros as the destination HIT.  This kind of connection setup is
   called opportunistic mode.

   The Responder may have multiple HITs due to multiple supported HIT
   Suites.  Since the Responder's HIT Suite in the opportunistic mode is
   not determined by the destination HIT of the I1 packet, the Responder
   can freely select a HIT of any HIT Suite.  The complete set of HIT
   Suites supported by the Initiator is not known to the Responder.
   Therefore, the Responder SHOULD should select its HIT from the same



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   HIT Suite as the Initiator's HIT (indicated by the HIT suite
   information in the OGA field of the Initiator's HIT) because this HIT
   Suite is obviously supported by the Initiator.  If the Responder
   selects a different HIT that is not supported by the Initiator, the
   Initiator MAY restart the BEX with an I1 packet with a source HIT
   that is contained in the list of the Responder's HIT Suites in the R1
   packet.

   Note that the Initiator cannot verify the signature of the R1 packet
   if the Responder's HIT Suite is not supported.  Therefore, the
   Initiator MUST treat R1 packets with unsupported Responder HITs as
   potentially forged and MUST NOT use any parameters from the
   unverified R1 besides the HIT Suite List.  Moreover, an Initiator
   that uses an unverified HIT Suite List from an R1 packet to determine
   a possible source HIT MUST verify that the HIT_SUITE_LIST in the
   first unverified R1 packet matches the HIT_SUITE_LIST in the second
   R1 packet for which the Initiator supports the signature algorithm.
   The Initiator MUST restart the BEX with a new I1 packet for which the
   algorithm was mentioned in the verifiable R1 if the two lists do not
   match.  This procedure is necessary to mitigate downgrade attacks.

   There are both security and API issues involved with the
   opportunistic mode.  These issues are described in the reminder of
   this section.

   Given that the Responder's HI is not known by the Initiator, there
   must be suitable API calls that allow the Initiator to request,
   directly or indirectly, that the underlying system initiates the HIP
   base exchange solely based on locators.  The Responder's HI will be
   tentatively available in the R1 packet, and in an authenticated form
   once the R2 packet has been received and verified.  Hence, the
   Responder's HIT could be communicated to the application via new API
   mechanisms.  However, with a backwards-compatible API the application
   sees only the locators used for the initial contact.  Depending on
   the desired semantics of the API, this can raise the following
   issues:

   o  The actual locators may later change if an UPDATE message is used,
      even if from the API perspective the session still appears to be
      between two specific locators.  However, the locator update is
      still secure and the session is still between the same nodes.

   o  Different sessions between the same two locators may result in
      connections to different nodes, if the implementation no longer
      remembers which identifier the peer had in an earlier session.
      This is possible when the peer's locator has changed for
      legitimate reasons or when an attacker pretends to be a node that
      has the peer's locator.  Therefore, when using opportunistic mode,



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      HIP implementations MUST NOT place any expectation that the peer's
      HI returned in the R1 message matches any HI previously seen from
      that address.

      If the HIP implementation and application do not have the same
      understanding of what constitutes a session, this may even happen
      within the same session.  For instance, an implementation may not
      know when HIP state can be purged for UDP-based applications.

   o  As with all HIP base exchanges, the handling of locator-based or
      interface-based policy is unclear for HIP in opportunistic mode.
      An application may create a connection to a specific locator
      because the application has knowledge of the security properties
      along the network to that locator.  If one of the nodes moves and
      the locators are updated, these security properties may not be
      maintained.  Depending on the security policy of the application,
      this may be a problem.  This is an area of ongoing study.  As an
      example, there is work to create an API that applications can use
      to specify their security requirements in a similar context
      [I-D.ietf-btns-c-api].

   In addition, the following security considerations apply.  The
   generation counter mechanism will be less efficient in protecting
   against replays of the R1 packet, given that the Responder can choose
   a replay that uses an arbitrary HI, not just the one given in the I1
   packet.

   More importantly, the opportunistic exchange is vulnerable to man-in-
   the-middle attacks, because the Initiator does not have any public
   key information about the peer.  To assess the impacts of this
   vulnerability, we compare it to vulnerabilities in current, non-HIP-
   capable communications.

   An attacker on the path between the two peers can insert itself as a
   man-in-the-middle by providing its own identifier to the Initiator
   and then initiating another HIP session towards the Responder.  For
   this to be possible, the Initiator must employ opportunistic mode,
   and the Responder must be configured to accept a connection from any
   HIP-enabled node.

   An attacker outside the path will be unable to do so, given that it
   cannot respond to the messages in the base exchange.

   These security properties are characteristic also of communications
   in the current Internet.  A client contacting a server without
   employing end-to-end security may find itself talking to the server
   via a man-in-the-middle, assuming again that the server is willing to
   talk to anyone.



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   If end-to-end security is in place, then the worst that can happen in
   both the opportunistic HIP and non-HIP (normal IP) cases is denial-
   of-service; an entity on the path can disrupt communications, but
   will be unable to successfully insert itself as a man-in-the-middle.

   However, once the opportunistic exchange has successfully completed,
   HIP provides confidentiality and integrity protection for the
   communications, and can securely change the locators of the
   endpoints.

   As a result, it is believed that the HIP opportunistic mode is at
   least as secure as current IP.

4.2.  Updating a HIP Association

   A HIP association between two hosts may need to be updated over time.
   Examples include the need to rekey expiring security associations,
   add new security associations, or change IP addresses associated with
   hosts.  The UPDATE packet is used for those and other similar
   purposes.  This document only specifies the UPDATE packet format and
   basic processing rules, with mandatory parameters.  The actual usage
   is defined in separate specifications.

   HIP provides a general purpose UPDATE packet, which can carry
   multiple HIP parameters, for updating the HIP state between two
   peers.  The UPDATE mechanism has the following properties:

      UPDATE messages carry a monotonically increasing sequence number
      and are explicitly acknowledged by the peer.  Lost UPDATEs or
      acknowledgments may be recovered via retransmission.  Multiple
      UPDATE messages may be outstanding under certain circumstances.

      UPDATE is protected by both HIP_MAC and HIP_SIGNATURE parameters,
      since processing UPDATE signatures alone is a potential DoS attack
      against intermediate systems.

      UPDATE packets are explicitly acknowledged by the use of an
      acknowledgment parameter that echoes an individual sequence number
      received from the peer.  A single UPDATE packet may contain both a
      sequence number and one or more acknowledgment numbers (i.e.,
      piggybacked acknowledgment(s) for the peer's UPDATE).

   The UPDATE packet is defined in Section 5.3.5.

4.3.  Error Processing

   HIP error processing behavior depends on whether or not there exists
   an active HIP association.  In general, if a HIP association exists



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   between the sender and receiver of a packet causing an error
   condition, the receiver SHOULD respond with a NOTIFY packet.  On the
   other hand, if there are no existing HIP associations between the
   sender and receiver, or the receiver cannot reasonably determine the
   identity of the sender, the receiver MAY respond with a suitable ICMP
   message; see Section 5.4 for more details.

   The HIP protocol and state machine are designed to recover from one
   of the parties crashing and losing its state.  The following
   scenarios describe the main use cases covered by the design.

      No prior state between the two systems.

         The system with data to send is the Initiator.  The process
         follows the standard four-packet base exchange, establishing
         the HIP association.

      The system with data to send has no state with the receiver, but
      the receiver has a residual HIP association.

         The system with data to send is the Initiator.  The Initiator
         acts as in no prior state, sending an I1 packet and receiving
         an R1 packet.  When the Responder receives a valid I2 packet,
         the old association is 'discovered' and deleted, and the new
         association is established.

      The system with data to send has a HIP association, but the
      receiver does not.

         The system sends data on the outbound user data security
         association.  The receiver 'detects' the situation when it
         receives a user data packet that it cannot match to any HIP
         association.  The receiving host MUST discard this packet.

         Optionally, the receiving host MAY send an ICMP packet, with
         the type Parameter Problem, to inform the sender that the HIP
         association does not exist (see Section 5.4), and it MAY
         initiate a new HIP BEX.  However, responding with these
         optional mechanisms is implementation or policy dependent.

4.4.  HIP State Machine

   The HIP protocol itself has little state.  In the HIP base exchange,
   there is an Initiator and a Responder.  Once the security
   associations (SAs) are established, this distinction is lost.  If the
   HIP state needs to be re-established, the controlling parameters are
   which peer still has state and which has a datagram to send to its
   peer.  The following state machine attempts to capture these



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   processes.

   The state machine is symmetric and is presented in a single system
   view, representing either an Initiator or a Responder.  The state
   machine is not a full representation of the processing logic.
   Additional processing rules are presented in the packet definitions.
   Hence, both are needed to completely implement HIP.

   This document extends the state machine as defined in [RFC5201] and
   introduces a restart option to allow for the negotiation of
   cryptographic algorithms.  The extension to the previous state
   machine in [RFC5201] is a transition from state I1-SENT to I1-SENT -
   the restart option.  An Initiator is required to restart the HIP base
   exchange if the Responder does not support the HIT Suite of the
   Initiator.  In this case, the Initiator restarts the HIP base
   exchange by sending a new I1 packet with a source HIT supported by
   the Responder.

   Implementors must understand that the state machine, as described
   here, is informational.  Specific implementations are free to
   implement the actual processing logic differently.  Section 6
   describes the packet processing rules in more detail.  This state
   machine focuses on the HIP I1, R1, I2, and R2 packets only.  New
   states and state transitions may be introduced by mechanisms in other
   specifications (such as mobility and multihoming).

4.4.1.  State Machine Terminology

   Unused Association Lifetime (UAL):   Implementation-specific time for
      which, if no packet is sent or received for this time interval, a
      host MAY begin to tear down an active HIP association.

   Maximum Segment Lifetime (MSL):   Maximum time that a TCP segment is
      expected to spend in the network.

   Exchange Complete (EC):   Time that the host spends at the R2-SENT
      state before it moves to the ESTABLISHED state.  The time is n *
      I2 retransmission timeout, where n is about I2_RETRIES_MAX.

   Receive ANYOTHER:   Any received packet for which no state
      transitions or processing rules are defined for a given state.










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4.4.2.  HIP States

   +---------------------+---------------------------------------------+
   | State               | Explanation                                 |
   +---------------------+---------------------------------------------+
   | UNASSOCIATED        | State machine start                         |
   |                     |                                             |
   | I1-SENT             | Initiating base exchange                    |
   |                     |                                             |
   | I2-SENT             | Waiting to complete base exchange           |
   |                     |                                             |
   | R2-SENT             | Waiting to complete base exchange           |
   |                     |                                             |
   | ESTABLISHED         | HIP association established                 |
   |                     |                                             |
   | CLOSING             | HIP association closing, no data can be     |
   |                     | sent                                        |
   |                     |                                             |
   | CLOSED              | HIP association closed, no data can be sent |
   |                     |                                             |
   | E-FAILED            | HIP base exchange failed                    |
   +---------------------+---------------------------------------------+

                            Table 1: HIP States

4.4.3.  HIP State Processes

   System behavior in state UNASSOCIATED, Table 2.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | User data to send,  | Send I1 and go to I1-SENT                   |
   | requiring a new HIP |                                             |
   | association         |                                             |
   |                     |                                             |
   | Receive I1          | Send R1 and stay at UNASSOCIATED            |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and go to R2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at UNASSOCIATED               |
   |                     |                                             |
   | Receive user data   | Optionally send ICMP as defined in          |
   | for an unknown HIP  | Section 5.4 and stay at UNASSOCIATED        |
   | association         |                                             |
   |                     |                                             |
   | Receive CLOSE       | Optionally send ICMP Parameter Problem and  |
   |                     | stay at UNASSOCIATED                        |



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   | Receive ANYOTHER    | Drop and stay at UNASSOCIATED               |
   +---------------------+---------------------------------------------+

                    Table 2: UNASSOCIATED - Start state

   System behavior in state I1-SENT, Table 3.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1 from     | If the local HIT is smaller than the peer   |
   | Responder           | HIT, drop I1 and stay at I1-SENT (see       |
   |                     | Section 6.5 for HIT comparison)             |
   |                     |                                             |
   |                     | If the local HIT is greater than the peer   |
   |                     | HIT, send R1 and stay at I1-SENT            |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and go to R2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at I1-SENT                    |
   |                     |                                             |
   | Receive R1, process | If the HIT Suite of the local HIT is not    |
   |                     | supported by the peer, select supported     |
   |                     | local HIT, send I1 and stay at I1-SENT      |
   |                     |                                             |
   |                     | If successful, send I2 and go to I2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at I1-SENT                    |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at I1-SENT                    |
   |                     |                                             |
   | Timeout             | Increment timeout counter                   |
   |                     |                                             |
   |                     | If counter is less than I1_RETRIES_MAX,     |
   |                     | send I1 and stay at I1-SENT                 |
   |                     |                                             |
   |                     | If counter is greater than I1_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

            Table 3: I1-SENT - Initiating the HIP Base Exchange










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   System behavior in state I2-SENT, Table 4.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay at I2-SENT                 |
   |                     |                                             |
   | Receive R1, process | If successful, send I2 and stay at I2-SENT  |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive I2, process | If successful and local HIT is smaller than |
   |                     | the peer HIT, drop I2 and stay at I2-SENT   |
   |                     |                                             |
   |                     | If successful and local HIT is greater than |
   |                     | the peer HIT, send R2 and go to R2-SENT     |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive R2, process | If successful, go to ESTABLISHED            |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at I2-SENT                    |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at I2-SENT                    |
   |                     |                                             |
   | Timeout             | Increment timeout counter                   |
   |                     |                                             |
   |                     | If counter is less than I2_RETRIES_MAX,     |
   |                     | send I2 and stay at I2-SENT                 |
   |                     |                                             |
   |                     | If counter is greater than I2_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

        Table 4: I2-SENT - Waiting to finish the HIP Base Exchange











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   System behavior in state R2-SENT, Table 5.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay at R2-SENT                 |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and stay at R2-SENT  |
   |                     |                                             |
   |                     | If fail, stay at R2-SENT                    |
   |                     |                                             |
   | Receive R1          | Drop and stay at R2-SENT                    |
   |                     |                                             |
   | Receive R2          | Drop and stay at R2-SENT                    |
   |                     |                                             |
   | Receive data or     | Move to ESTABLISHED                         |
   | UPDATE              |                                             |
   |                     |                                             |
   | Exchange Complete   | Move to ESTABLISHED                         |
   | Timeout             |                                             |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at ESTABLISHED                |
   |                     |                                             |
   | Receive CLOSE_ACK   | Drop and stay at R2-SENT                    |
   |                     |                                             |
   | Receive NOTIFY      | Process and stay at R2-SENT                 |
   +---------------------+---------------------------------------------+

                 Table 5: R2-SENT - Waiting to finish HIP



















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   System behavior in state ESTABLISHED, Table 6.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay at ESTABLISHED             |
   |                     |                                             |
   | Receive I2          | Process with puzzle and possible Opaque     |
   |                     | data verification                           |
   |                     |                                             |
   |                     | If successful, send R2, drop old HIP        |
   |                     | association, establish a new HIP            |
   |                     | association and go to R2-SENT               |
   |                     |                                             |
   |                     | If fail, stay at ESTABLISHED                |
   |                     |                                             |
   | Receive R1          | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive R2          | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive user data   | Process and stay at ESTABLISHED             |
   | for HIP association |                                             |
   |                     |                                             |
   | No packet           | Send CLOSE and go to CLOSING                |
   | sent/received       |                                             |
   | during UAL minutes  |                                             |
   |                     |                                             |
   | Receive UPDATE      | Process and stay at ESTABLISHED             |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at ESTABLISHED                |
   |                     |                                             |
   | Receive CLOSE_ACK   | Drop and stay at ESTABLISHED                |
   |                     |                                             |
   | Receive NOTIFY      | Process and stay at ESTABLISHED             |
   +---------------------+---------------------------------------------+

            Table 6: ESTABLISHED - HIP association established











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   System behavior in state CLOSING, Table 7.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | User data to send,  | Send I1 and stay at CLOSING                 |
   | requires the        |                                             |
   | creation of another |                                             |
   | incarnation of the  |                                             |
   | HIP association     |                                             |
   |                     |                                             |
   | Receive I1          | Send R1 and stay at CLOSING                 |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and go to R2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at CLOSING                    |
   |                     |                                             |
   | Receive R1, process | If successful, send I2 and go to I2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at CLOSING                    |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK, discard      |
   | process             | state and go to CLOSED                      |
   |                     |                                             |
   |                     | If fail, stay at CLOSING                    |
   |                     |                                             |
   | Receive CLOSE_ACK,  | If successful, discard state and go to      |
   | process             | UNASSOCIATED                                |
   |                     |                                             |
   |                     | If fail, stay at CLOSING                    |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at CLOSING                    |
   |                     |                                             |
   | Timeout             | Increment timeout sum and reset timer. If   |
   |                     | timeout sum is less than UAL+MSL minutes,   |
   |                     | retransmit CLOSE and stay at CLOSING        |
   |                     |                                             |
   |                     | If timeout sum is greater than UAL+MSL      |
   |                     | minutes, go to UNASSOCIATED                 |
   +---------------------+---------------------------------------------+

   Table 7: CLOSING - HIP association has not been used for UAL minutes









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   System behavior in state CLOSED, Table 8.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Datagram to send,   | Send I1, and stay at CLOSED                 |
   | requires the        |                                             |
   | creation of another |                                             |
   | incarnation of the  |                                             |
   | HIP association     |                                             |
   |                     |                                             |
   | Receive I1          | Send R1 and stay at CLOSED                  |
   |                     |                                             |
   | Receive I2, process | If successful, send R2 and go to R2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at CLOSED                     |
   |                     |                                             |
   | Receive R1, process | If successful, send I2 and go to I2-SENT    |
   |                     |                                             |
   |                     | If fail, stay at CLOSED                     |
   |                     |                                             |
   | Receive CLOSE,      | If successful, send CLOSE_ACK, stay at      |
   | process             | CLOSED                                      |
   |                     |                                             |
   |                     | If fail, stay at CLOSED                     |
   |                     |                                             |
   | Receive CLOSE_ACK,  | If successful, discard state and go to      |
   | process             | UNASSOCIATED                                |
   |                     |                                             |
   |                     | If fail, stay at CLOSED                     |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at CLOSED                     |
   |                     |                                             |
   | Timeout (UAL+2MSL)  | Discard state, and go to UNASSOCIATED       |
   +---------------------+---------------------------------------------+

    Table 8: CLOSED - CLOSE_ACK sent, resending CLOSE_ACK if necessary

   System behavior in state E-FAILED, Table 9.

   +-------------------------+-----------------------------------------+
   | Trigger                 | Action                                  |
   +-------------------------+-----------------------------------------+
   | Wait for                | Go to UNASSOCIATED. Re-negotiation is   |
   | implementation-specific | possible after moving to UNASSOCIATED   |
   | time                    | state.                                  |
   +-------------------------+-----------------------------------------+




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     Table 9: E-FAILED - HIP failed to establish association with peer

4.4.4.  Simplified HIP State Diagram

   The following diagram (Figure 2) shows the major state transitions.
   Transitions based on received packets implicitly assume that the
   packets are successfully authenticated or processed.












































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                               +--+       +----------------------------+
              recv I1, send R1 |  |       |                            |
                               |  v       v                            |
                             +--------------+  recv I2, send R2        |
            +----------------| UNASSOCIATED |----------------+         |
    datagram|  +--+          +--------------+                |         |
    to send,|  |  | Alg. not supported,                      |         |
     send I1|  |  | send I1                                  |         |
            v  |  v                                          |         |
         +---------+  recv I2, send R2                       |         |
   +---->| I1-SENT |--------------------------------------+  |         |
   |     +---------+            +----------------------+  |  |         |
   |          | recv R2,        | recv I2, send R2     |  |  |         |
   |          v send I2         |                      v  v  v         |
   |       +---------+          |                    +---------+       |
   |  +--->| I2-SENT |----------+     +--------------| R2-SENT |<---+  |
   |  |    +---------+                |              +---------+    |  |
   |  |          |  |recv R2          |        data or|             |  |
   |  |recv R1,  |  |                 |     EC timeout|             |  |
   |  |send I2   +--|-----------------+               |  receive I2,|  |
   |  |          |  |       +-------------+           |      send R2|  |
   |  |          |  +------>| ESTABLISHED |<----------+             |  |
   |  |          |          +-------------+                         |  |
   |  |          |            |  |  |      receive I2, send R2      |  |
   |  |          +------------+  |  +-------------------------------+  |
   |  |          |               +-----------+                      |  |
   |  |          |    no packet sent/received|    +---+             |  |
   |  |          |    for UAL min, send CLOSE|    |   |timeout      |  |
   |  |          |                           v    v   |(UAL+MSL)    |  |
   |  |          |                        +---------+ |retransmit   |  |
   +--|----------|------------------------| CLOSING |-+CLOSE        |  |
      |          |                        +---------+               |  |
      |          |                         | |   | |                |  |
      +----------|-------------------------+ |   | +----------------+  |
      |          |               +-----------+   +------------------|--+
      |          |               |recv CLOSE,      recv CLOSE_ACK   |  |
      |          +-------------+ |send CLOSE_ACK   or timeout       |  |
      |     recv CLOSE,        | |                 (UAL+MSL)        |  |
      |     send CLOSE_ACK     v v                                  |  |
      |                     +--------+  receive I2, send R2         |  |
      +---------------------| CLOSED |------------------------------+  |
                            +--------+                                 |
                             ^ |  |                                    |
   recv CLOSE, send CLOSE_ACK| |  |              timeout (UAL+2MSL)    |
                             +-+  +------------------------------------+


                                 Figure 2



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4.5.  User Data Considerations

4.5.1.  TCP and UDP Pseudo-Header Computation for User Data

   When computing TCP and UDP checksums on user data packets that flow
   through sockets bound to HITs, the IPv6 pseudo-header format
   [RFC2460] MUST be used, even if the actual addresses in the header of
   the packet are IPv4 addresses.  Additionally, the HITs MUST be used
   in place of the IPv6 addresses in the IPv6 pseudo-header.  Note that
   the pseudo-header for actual HIP payloads is computed differently;
   see Section 5.1.1.

4.5.2.  Sending Data on HIP Packets

   Other documents may define how to include user data in various HIP
   packets.  However, currently the HIP header is a terminal header, and
   not followed by any other headers.

4.5.3.  Transport Formats

   The actual data transmission format, used for user data after the HIP
   base exchange, is not defined in this document.  Such transport
   formats and methods are described in separate specifications.  All
   HIP implementations MUST implement, at minimum, the ESP transport
   format for HIP [I-D.ietf-hip-rfc5202-bis].  The transport format to
   be chosen is negotiated in the base exchange.  The Responder
   expresses its preference of the transport format in the
   TRANSPORT_FORMAT_LIST in the R1 packet and the Initiator selects one
   transform and adds the respective HIP parameter to the I2 packet.

4.5.4.  Reboot, Timeout, and Restart of HIP

   Simulating a loss of state is a potential DoS attack.  The following
   process has been crafted to manage state recovery without presenting
   a DoS opportunity.

   If a host reboots or the HIP association times out, it has lost its
   HIP state.  If the host that lost state has a datagram to send to the
   peer, it simply restarts the HIP base exchange.  After the base
   exchange has completed, the Initiator can create a new payload
   association and start sending data.  The peer does not reset its
   state until it receives a valid I2 packet.

   If a system receives a user data packet that cannot be matched to any
   existing HIP association, it is possible that it has lost the state
   and its peer has not.  It MAY send an ICMP packet with the Parameter
   Problem type, and with the pointer pointing to the referred HIP-
   related association information.  Reacting to such traffic depends on



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   the implementation and the environment where the implementation is
   used.

   If the host, that apparently has lost its state, decides to restart
   the HIP base exchange, it sends an I1 packet to the peer.  After the
   base exchange has been completed successfully, the Initiator can
   create a new HIP association and the peer drops its old payload
   associations and creates a new one.

4.6.  Certificate Distribution

   This document does not define how to use certificates or how to
   transfer them between hosts.  These functions are expected to be
   defined in a future specification as for HIP Version 1 [RFC6253].  A
   parameter type value, meant to be used for carrying certificates, is
   reserved, though: CERT, Type 768; see Section 5.2.

5.  Packet Formats

5.1.  Payload Format

   All HIP packets start with a fixed header.


   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Header   | Header Length |0| Packet Type |Version| RES.|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum             |           Controls            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Sender's Host Identity Tag (HIT)               |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Receiver's Host Identity Tag (HIT)              |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                        HIP Parameters                         /
   /                                                               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The HIP header is logically an IPv6 extension header.  However, this
   document does not describe processing for Next Header values other
   than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
   Future documents MAY define behavior for also other values.  However,
   current implementations MUST ignore trailing data if an unimplemented
   Next Header value is received.

   The Header Length field contains the combined length of the HIP
   Header and HIP parameters in 8-byte units, excluding the first 8
   bytes.  Since all HIP headers MUST contain the sender's and
   receiver's HIT fields, the minimum value for this field is 4, and
   conversely, the maximum length of the HIP Parameters field is
   (255*8)-32 = 2008 bytes.  Note: this sets an additional limit for
   sizes of parameters included in the Parameters field, independent of
   the individual parameter maximum lengths.

   The Packet Type indicates the HIP packet type.  The individual packet
   types are defined in the relevant sections.  If a HIP host receives a
   HIP packet that contains an unrecognized packet type, it MUST drop
   the packet.

   The HIP Version field is four bits.  The version defined in this
   document is 2.  The version number is expected to be incremented only
   if there are incompatible changes to the protocol.  Most extensions
   can be handled by defining new packet types, new parameter types, or
   new Controls (see Section 5.1.2) .

   The following three bits are reserved for future use.  They MUST be
   zero when sent, and they SHOULD be ignored when handling a received
   packet.

   The two fixed bits in the header are reserved for SHIM6 compatibility
   [RFC5533], Section 5.3.  For implementations adhering (only) to this
   specification, they MUST be set as shown when sending and MUST be
   ignored when receiving.  This is to ensure optimal forward
   compatibility.  Note that for implementations that implement other
   compatible specifications in addition to this specification, the
   corresponding rules may well be different.  For example, an
   implementation that implements both this specification and the SHIM6
   protocol may need to check these bits in order to determine how to
   handle the packet.

   The HIT fields are always 128 bits (16 bytes) long.

5.1.1.  Checksum

   Since the checksum covers the source and destination addresses in the
   IP header, it MUST be recomputed on HIP-aware NAT devices.



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   If IPv6 is used to carry the HIP packet, the pseudo-header [RFC2460]
   contains the source and destination IPv6 addresses, HIP packet length
   in the pseudo-header length field, a zero field, and the HIP protocol
   number (see Section 4) in the Next Header field.  The length field is
   in bytes and can be calculated from the HIP header length field: (HIP
   Header Length + 1) * 8.

   In case of using IPv4, the IPv4 UDP pseudo-header format [RFC0768] is
   used.  In the pseudo-header, the source and destination addresses are
   those used in the IP header, the zero field is obviously zero, the
   protocol is the HIP protocol number (see Section 4), and the length
   is calculated as in the IPv6 case.

5.1.2.  HIP Controls

   The HIP Controls field conveys information about the structure of the
   packet and capabilities of the host.

   The following fields have been defined:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | | | | | | | | | | | | | | | |A|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A - Anonymous:   If this is set, the sender's HI in this packet is
      anonymous, i.e., one not listed in a directory.  Anonymous HIs
      SHOULD NOT be stored.  This control is set in packets using
      anonymous sender HIs.  The peer receiving an anonymous HI in an R1
      or I2 may choose to refuse it.

   The rest of the fields are reserved for future use and MUST be set to
   zero in sent packets and ignored in received packets.

5.1.3.  HIP Fragmentation Support

   A HIP implementation MUST support IP fragmentation/reassembly.
   Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
   fragment generation is REQUIRED to be implemented in IPv4 (IPv4
   stacks and networks will usually do this by default) and RECOMMENDED
   to be implemented in IPv6.  In IPv6 networks, the minimum MTU is
   larger, 1280 bytes, than in IPv4 networks.  The larger MTU size is
   usually sufficient for most HIP packets, and therefore fragment
   generation may not be needed.  If a host expects to send HIP packets
   that are larger than the minimum IPv6 MTU, it MUST implement fragment
   generation even for IPv6.

   In IPv4 networks, HIP packets may encounter low MTUs along their
   routed path.  Since basic HIP, as defined in this document, does not



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   provide a mechanism to use multiple IP datagrams for a single HIP
   packet, support for path MTU discovery does not bring any value to
   HIP in IPv4 networks.  HIP-aware NAT devices SHOULD perform IPv4
   reassembly/fragmentation for HIP control packets.

   All HIP implementations have to be careful while employing a
   reassembly algorithm so that the algorithm is sufficiently resistant
   to DoS attacks.

   Certificate chains can cause the packet to be fragmented and
   fragmentation can open implementations to denial-of-service attacks
   [KAU03].  "Hash and URL" schemes as defined in [RFC6253] for HIP
   version 1 may be used to avoid fragmentation and mitigate resulting
   DoS attacks.

5.2.  HIP Parameters

   The HIP parameters carry information that is necessary for
   establishing and maintaining a HIP association.  For example, the
   peer's public keys as well as the signaling for negotiating ciphers
   and payload handling are encapsulated in HIP parameters.  Additional
   information, meaningful for end-hosts or middleboxes, may also be
   included in HIP parameters.  The specification of the HIP parameters
   and their mapping to HIP packets and packet types is flexible to
   allow HIP extensions to define new parameters and new protocol
   behavior.

   In HIP packets, HIP parameters are ordered according to their numeric
   type number and encoded in TLV format.

   The following parameter types are currently defined.




















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   +------------------------+-------+-----------+----------------------+
   | TLV                    | Type  | Length    | Data                 |
   +------------------------+-------+-----------+----------------------+
   | R1_COUNTER             | 129   | 12        | Puzzle generation    |
   |                        |       |           | counter              |
   |                        |       |           |                      |
   | PUZZLE                 | 257   | 12        | K and Random #I      |
   |                        |       |           |                      |
   | SOLUTION               | 321   | 20        | K, Random #I and     |
   |                        |       |           | puzzle solution J    |
   |                        |       |           |                      |
   | SEQ                    | 385   | 4         | UPDATE packet ID     |
   |                        |       |           | number               |
   |                        |       |           |                      |
   | ACK                    | 449   | variable  | UPDATE packet ID     |
   |                        |       |           | number               |
   |                        |       |           |                      |
   | DH_GROUP_LIST          | 511   | variable  | Ordered list of DH   |
   |                        |       |           | Group IDs supported  |
   |                        |       |           | by a host            |
   |                        |       |           |                      |
   | DIFFIE_HELLMAN         | 513   | variable  | public key           |
   |                        |       |           |                      |
   | HIP_CIPHER             | 579   | variable  | List of HIP          |
   |                        |       |           | encryption           |
   |                        |       |           | algorithms           |
   |                        |       |           |                      |
   | ENCRYPTED              | 641   | variable  | Encrypted part of a  |
   |                        |       |           | HIP packet           |
   |                        |       |           |                      |
   | HOST_ID                | 705   | variable  | Host Identity with   |
   |                        |       |           | Fully-Qualified      |
   |                        |       |           | Domain FQDN (Name)   |
   |                        |       |           | or Network Access    |
   |                        |       |           | Identifier (NAI)     |
   |                        |       |           |                      |
   | HIT_SUITE_LIST         | 715   | variable  | Ordered list of the  |
   |                        |       |           | HIT suites supported |
   |                        |       |           | by the Responder     |
   |                        |       |           |                      |
   | CERT                   | 768   | variable  | HI Certificate; used |
   |                        |       |           | to transfer          |
   |                        |       |           | certificates.        |
   |                        |       |           | Specified in a       |
   |                        |       |           | separate docment.    |
   |                        |       |           |                      |
   | NOTIFICATION           | 832   | variable  | Informational data   |
   |                        |       |           |                      |



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   | ECHO_REQUEST_SIGNED    | 897   | variable  | Opaque data to be    |
   |                        |       |           | echoed back; signed  |
   |                        |       |           |                      |
   | ECHO_RESPONSE_SIGNED   | 961   | variable  | Opaque data echoed   |
   |                        |       |           | back by request;     |
   |                        |       |           | signed               |
   |                        |       |           |                      |
   | TRANSPORT_FORMAT_LIST  | 2049  | Ordered   | variable             |
   |                        |       | list of   |                      |
   |                        |       | preferred |                      |
   |                        |       | HIP       |                      |
   |                        |       | transport |                      |
   |                        |       | type      |                      |
   |                        |       | numbers   |                      |
   |                        |       |           |                      |
   | HIP_MAC                | 61505 | variable  | HMAC-based message   |
   |                        |       |           | authentication code, |
   |                        |       |           | with key material    |
   |                        |       |           | from KEYMAT          |
   |                        |       |           |                      |
   | HIP_MAC_2              | 61569 | variable  | HMAC based message   |
   |                        |       |           | authentication code, |
   |                        |       |           | with key material    |
   |                        |       |           | from KEYMAT. Unlike  |
   |                        |       |           | HIP_MAC, the HOST_ID |
   |                        |       |           | parameter is         |
   |                        |       |           | included in          |
   |                        |       |           | HIP_MAC_2            |
   |                        |       |           | calculation.         |
   |                        |       |           |                      |
   | HIP_SIGNATURE_2        | 61633 | variable  | Signature used in R1 |
   |                        |       |           | packet               |
   |                        |       |           |                      |
   | HIP_SIGNATURE          | 61697 | variable  | Signature of the     |
   |                        |       |           | packet               |
   |                        |       |           |                      |
   | ECHO_REQUEST_UNSIGNED  | 63661 | variable  | Opaque data to be    |
   |                        |       |           | echoed back; after   |
   |                        |       |           | signature            |
   |                        |       |           |                      |
   | ECHO_RESPONSE_UNSIGNED | 63425 | variable  | Opaque data echoed   |
   |                        |       |           | back by request;     |
   |                        |       |           | after signature      |
   +------------------------+-------+-----------+----------------------+

   As the ordering (from lowest to highest) of HIP parameters is
   strictly enforced (see Section 5.2.1), the parameter type values for
   existing parameters have been spaced to allow for future protocol



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   extensions.

   The following parameter type number ranges are defined.

   +---------------+---------------------------------------------------+
   | Type Range    | Purpose                                           |
   +---------------+---------------------------------------------------+
   |     0 -  1023 | Handshake                                         |
   |               |                                                   |
   |  1024 -  2047 | Reserved                                          |
   |               |                                                   |
   |  2048 -  4095 | Parameters related to HIP transport formats       |
   |               |                                                   |
   |  4096 -  8191 | Signed parameters allocated through specification |
   |               | documents                                         |
   |               |                                                   |
   |  8192 - 32767 | Reserved                                          |
   |               |                                                   |
   | 32768 - 49151 | Free for experimentation. Signed parameters.      |
   |               |                                                   |
   | 41952 - 61439 | Reserved                                          |
   |               |                                                   |
   | 61440 - 64443 | Signatures and (signed) MACs                      |
   |               |                                                   |
   | 62464 - 63487 | Parameters that are neither signed nor MACed      |
   |               |                                                   |
   | 63488 - 64511 | Rendezvous and relaying                           |
   |               |                                                   |
   | 64512 - 65023 | Parameters that are neither signed nor MACed      |
   |               |                                                   |
   | 65024 - 65535 | Reserved                                          |
   +---------------+---------------------------------------------------+

   The process for defining new parameters is described in Section 5.2.2
   of this document.

   The range between 32768 (2^15) and 49151 (2^15 + 2^14) are free for
   experimentation.  Types from this range SHOULD be selected in a
   random fashion to reduce the probability of collisions.

5.2.1.  TLV Format

   The TLV-encoded parameters are described in the following
   subsections.  The type-field value also describes the order of these
   fields in the packet.  The parameters MUST be included in the packet
   so that their types form an increasing order.  If multiple parameters
   with the same type number are in one packet, the parameters with the
   same type MUST be consecutive in the packet.  If the order does not



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   follow this rule, the packet is considered to be malformed and it
   MUST be discarded.

   Parameters using type values from 2048 up to 4095 are related to
   transport formats.  Currently, one transport format is defined: the
   ESP transport format [I-D.ietf-hip-rfc5202-bis].

   All of the encoded TLV parameters have a length (that includes the
   Type and Length fields), which is a multiple of 8 bytes.  When
   needed, padding MUST be added to the end of the parameter so that the
   total length is a multiple of 8 bytes.  This rule ensures proper
   alignment of data.  Any added padding bytes MUST be zeroed by the
   sender, and their values SHOULD NOT be checked by the receiver.

   The Length field indicates the length of the Contents field (in
   bytes).  Consequently, the total length of the TLV parameter
   (including Type, Length, Contents, and Padding) is related to the
   Length field according to the following formula:

   Total Length = 11 + Length - (Length + 3) % 8;

   where % is the modulo operator

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type            |C|             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     /                          Contents                             /
     /                                               +-+-+-+-+-+-+-+-+
     |                                               |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         Type code for the parameter. 16 bits long, C-bit
                  being part of the Type code.
       C          Critical.  One if this parameter is critical, and
                  MUST be recognized by the recipient, zero otherwise.
                  The C bit is considered to be a part of the Type
                  field.  Consequently, critical parameters are always
                  odd and non-critical ones have an even value.
     Length       Length of the Contents, in bytes excluding Type,
                  Length, and Padding.
     Contents     Parameter specific, defined by Type
     Padding      Padding, 0-7 bytes, added if needed

   Critical parameters (indicated by the odd type number) MUST be
   recognized by the recipient.  If a recipient encounters a critical



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   parameter that it does not recognize, it MUST NOT process the packet
   any further.  It MAY send an ICMP or NOTIFY, as defined in
   Section 4.3.

   Non-critical parameters MAY be safely ignored.  If a recipient
   encounters a non-critical parameter that it does not recognize, it
   SHOULD proceed as if the parameter was not present in the received
   packet.

5.2.2.  Defining New Parameters

   Future specifications may define new parameters as needed.  When
   defining new parameters, care must be taken to ensure that the
   parameter type values are appropriate and leave suitable space for
   other future extensions.  One must remember that the parameters MUST
   always be arranged in numerically increasing order by Type code,
   thereby limiting the order of parameters (see Section 5.2.1).

   The following rules MUST be followed when defining new parameters.

   1.  The low-order bit C of the Type code is used to distinguish
       between critical and non-critical parameters.  Hence, even
       parameter type numbers indicate non-critical parameters while odd
       parameter type numbers indicate critical parameters.

   2.  A new parameter MAY be critical only if an old implementation
       that ignored it would cause security problems.  In general, new
       parameters SHOULD be defined as non-critical, and expect a reply
       from the recipient.

   3.  If a system implements a new critical parameter, it MUST provide
       the ability to set the associated feature off, such that the
       critical parameter is not sent at all.  The configuration option
       MUST be well documented.  Implementations operating in a mode
       adhering to this specification MUST disable the sending of new
       critical parameters by default.  In other words, the management
       interface MUST allow vanilla standards-only mode as a default
       configuration setting, and MAY allow new critical payloads to be
       configured on (and off).

   4.  See Section 9 for allocation rules regarding Type codes.










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5.2.3.  R1_COUNTER

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Reserved, 4 bytes                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                R1 generation counter, 8 bytes                 |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           129
     Length         12
     R1 generation
       counter      The current generation of valid puzzles


   The R1_COUNTER parameter contains a 64-bit unsigned integer in
   network-byte order, indicating the current generation of valid
   puzzles.  The sender SHOULD increment this counter periodically.  It
   is RECOMMENDED that the counter value is incremented at least as
   often as old PUZZLE values are deprecated so that SOLUTIONs to them
   are no longer accepted.

   Support for the R1_COUNTER parameter is mandatory.  It SHOULD be
   included in the R1 (in which case, it is covered by the signature),
   and if present in the R1, it MUST be echoed (including the Reserved
   field verbatim) by the Initiator in the I2 packet.





















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5.2.4.  PUZZLE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  #K, 1 byte   |    Lifetime   |        Opaque, 2 bytes        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Random #I, RHASH_len/8 bytes             |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           257
     Length         4 + RHASH_len / 8
     #K             #K is the number of verified bits
     Lifetime       puzzle lifetime 2^(value-32) seconds
     Opaque         data set by the Responder, indexing the puzzle
     Random #I      random number of size RHASH_len bits


   Random #I is represented as a n-bit integer (where n is RHASH_len),
   #K and Lifetime as 8-bit integers, all in network byte order.

   The PUZZLE parameter contains the puzzle difficulty #K and a n-bit
   random integer #I. The Puzzle Lifetime indicates the time during
   which the puzzle solution is valid, and sets a time limit that should
   not be exceeded by the Initiator while it attempts to solve the
   puzzle.  The lifetime is indicated as a power of 2 using the formula
   2^(Lifetime-32) seconds.  A puzzle MAY be augmented with an
   ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in
   the R1; the contents of the echo request are then echoed back in the
   ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED parameter,
   allowing the Responder to use the included information as a part of
   its puzzle processing.

   The Opaque and Random #I field are not covered by the HIP_SIGNATURE_2
   parameter.













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5.2.5.  SOLUTION

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | #K, 1 byte   |   Reserved    |        Opaque, 2 bytes        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Random #I, n bytes                       |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Puzzle solution #J, RHASH_len/8 bytes              |
     /                                                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type               321
     Length             4 + RHASH_len /4
     #K                 #K is the number of verified bits
     Reserved           zero when sent, ignored when received
     Opaque             copied unmodified from the received PUZZLE
                        parameter
     Random #I          random number of size RHASH_len bits
     Puzzle solution #J random number of size RHASH_len bits

   Random #I and Random #J are represented as n-bit unsigned integers
   (where n is RHASH_len), #K as an 8-bit unsigned integer, all in
   network byte order.

   The SOLUTION parameter contains a solution to a puzzle.  It also
   echoes back the random difficulty #K, the Opaque field, and the
   puzzle integer #I.



















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5.2.6.  DH_GROUP_LIST

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | DH GROUP ID #1| DH GROUP ID #2| DH GROUP ID #3| DH GROUP ID #4|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | DH GROUP ID #n|                Padding                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           511
     Length         number of DH Group IDs
     DH GROUP ID    defines a DH GROUP ID supported by the host.
                    The list of IDs is ordered by preference of the
                    host. The list of define DH Group IDs in the
                    DIFFIE_HELLMAN parameter. Each DH Group ID is one
                    octet long.

   The DH_GROUP_LIST parameter contains the list of supported DH Group
   IDs of a host.  The Initiator sends the DH_GROUP_LIST in the I1
   packet, the Responder sends its own list in the signed part of the R1
   packet.  The DH Group IDs in the DH_GROUP_LIST are listed in the
   order of their preference of the host sending the list.  DH Group IDs
   that are listed first are preferred over the DH Group IDs listed
   later.  The information in the DH_GROUP_LIST allows the Responder to
   select the DH group preferred by itself and supported by the
   Initiator.  Based on the DH_GROUP_LIST in the R1 packet, the
   Initiator can determine if the Responder has selected the best
   possible choice based on the Initiator's and Responder's preferences.
   If the Responder's choice differs from the best choice, the Initiator
   can conclude that there was an attempted downgrade attack (see
   Section 4.1.7).

   When selecting the DH group for the DIFFIE_HELLMAN parameter in the
   R1 packet, the Responder MUST select the first DH Group ID in its
   DH_GROUP_LIST in the R1 packet that is compatible with one of the
   Suite IDs in the Initiator's DH_GROUP_LIST in the I1 packet.  The
   Responder MUST NOT select any other DH Group ID that is contained in
   both lists because a downgrade attack cannot be detected then.

   In general, hosts SHOULD prefer stronger groups over weaker ones if
   the computation overhead is not prohibitively high for the intended
   application.






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5.2.7.  DIFFIE_HELLMAN

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Group ID    |      Public Value Length      | Public Value  /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           513
     Length         length in octets, excluding Type, Length, and
                    Padding
     Group ID       defines values for p and g as well as the KDF
     Public Value   length of the following Public Value in octets
       Length
     Public Value   the sender's public Diffie-Hellman key

   The following Group IDs have been defined:

    Group                              KDF              Value
    Reserved                                            0
    DEPRECATED                                          1
    DEPRECATED                                          2
    1536-bit MODP group  [RFC3526]     HKDF [RFC5869]   3
    3072-bit MODP group  [RFC3526]     HKDF [RFC5869]   4
    DEPRECATED                                          5
    DEPRECATED                                          6
    NIST P-256 [RFC5903]               HKDF [RFC5869]   7
    NIST P-384 [RFC5903]               HKDF [RFC5869]   8
    NIST P-521 [RFC5903]               HKDF [RFC5869]   9
    SECP160R1  [SECG]                  HKDF [RFC5869]  10

   The MODP Diffie-Hellman groups are defined in [RFC3526].  The ECDH
   groups 7 - 9 are defined in [RFC5903] and [RFC6090].  ECDH group 10
   is covered in Appendix D.  Any ECDH used with HIP MUST have a co-
   factor of 1.

   The Group ID also defines the key derivation function that is to be
   used for deriving the symmstric keys for the HMAC and symmetric
   encryption from the keying material from the Diffie Hellman key
   exchange (see Section 6.5).

   A HIP implementation MUST implement Group ID 3.  The 160-bit



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   SECP160R1 group can be used when lower security is enough (e.g., web
   surfing) and when the equipment is not powerful enough (e.g., some
   PDAs).  Implementations SHOULD implement Group IDs 4 and 8.

   To avoid unnecessary failures during the base exchange, the rest of
   the groups SHOULD be implemented in hosts where resources are
   adequate.

5.2.8.  HIP_CIPHER

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Cipher ID #1         |          Cipher ID #2         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Cipher ID #n         |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           579
     Length         length in octets, excluding Type, Length, and
                    Padding
     Cipher ID      defines the cipher algorithm to be used for
                    encrypting the contents of the ENCRYPTED parameter

   The following Cipher IDs are defined:

        Suite ID           Value

        RESERVED           0
        NULL-ENCRYPT       1     ([RFC2410])
        AES-128-CBC        2     ([RFC3602])
        DEPRECATED         3
        AES-256-CBC        4     ([RFC3602])


   The sender of a HIP_CIPHER parameter MUST make sure that there are no
   more than six (6) Cipher IDs in one HIP_CIPHER parameter.
   Conversely, a recipient MUST be prepared to handle received transport
   parameters that contain more than six Cipher IDs by accepting the
   first six Cipher IDs and dropping the rest.  The limited number of
   transforms sets the maximum size of the HIP_CIPHER parameter.  As the
   default configuration, the HIP_CIPHER parameter MUST contain at least
   one of the mandatory Cipher IDs.  There MAY be a configuration option
   that allows the administrator to override this default.

   The Responder lists supported and desired Cipher IDs in order of



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   preference in the R1, up to the maximum of six Cipher IDs.  The
   Initiator MUST choose only one of the corresponding Cipher IDs.  This
   Cipher ID will be used for generating the ENCRYPTED parameter.

   Mandatory implementation: AES-128-CBC.  NULL-ENCRYPTION is included
   for testing purposes.

5.2.9.  HOST_ID

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          HI Length            |DI-type|      DI Length        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Algorithm            |         Host Identity         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |       Domain Identifier       /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                                               |    Padding    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type              705
     Length            length in octets, excluding Type, Length, and
                       Padding
     HI Length         length of the Host Identity in octets
     DI-type           type of the following Domain Identifier field
     DI Length         length of the Domain Identifier field in octets
     Algorithm         index to the employed algorithm
     Host Identity     actual Host Identity
     Domain Identifier the identifier of the sender

   The following DI-types have been defined:

         Type                    Value
         none included           0
         FQDN                    1
         NAI                     2


         FQDN            Fully Qualified Domain Name, in binary format.
         NAI             Network Access Identifier

   The format for the FQDN is defined in RFC 1035 [RFC1035] Section 3.1.
   The format for the NAI is defined in [RFC4282]

   A host MAY optionally associate the Host Identity with a single



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   Domain Identifier in the HOST_ID parameter.  If there is no Domain
   Identifier, i.e., the DI-type field is zero, the DI Length field is
   set to zero as well.

   The following HI Algorithms have been defined:

        Algorithm
        profiles         Values

        RESERVED         0
        DSA              3 [FIPS 186-3] (RECOMMENDED)
        RSA              5 [RFC3447]    (REQUIRED)
        ECDSA            7 [RFC4754]    (REQUIRED)
        ECDSA_LOW        9 [SECG]       (RECOMMENDED)

   For DSA, RSA, and ECDSA key types, profiles containing at least 112
   bits of security strength (as defined by [NIST.800-131A.2011]) should
   be used.  For RSA signature padding, the PSS method of padding
   [RFC3447] MUST be used.

   The Host Identity is derived from the DNSKEY format for RSA and DSA.
   For these, the Public Key field of the RDATA part from RFC 4034
   [RFC4034] is used.  For ECC we distinguish two different profiles:
   ECDSA and ECDSA_LOW.  ECC contains curves approved by NIST and
   defined in RFC 4754 [RFC4754].  ECDSA_LOW is defined for devices with
   low computational capabilities and uses shorter curves from SECG
   [SECG].  Any ECDSA used with HIP MUST have a co-factor of 1.

   For ECDSA and ECDSA_LOW Host Identities are represented by the
   following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          ECC Curve            |                               /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                         Public Key                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     ECC Curve     Curve label
     Public Key    Represented in Octet-string format
                   [RFC6090]

   For hosts that implement ECDSA as algorithm the following ECC curves
   are required:






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        Algorithm    Curve            Values

        ECDSA        RESERVED         0
        ECDSA        NIST P-256       1 [RFC4754]
        ECDSA        NIST P-384       2 [RFC4754]

   For hosts that implement the EDSA_LOW algorithm profile, the
   following curve is required:

        Algorithm    Curve            Values

        ECDSA_LOW    RESERVED         0
        ECDSA_LOW    SECP160R1        1 [SECG]

5.2.10.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   suite IDs of the Responder.  The Responder sends the HIT_SUITE_LIST
   in the signed part of the R1 packet.  Based on the HIT_SUITE_LIST,
   the Initiator can determine which source HITs are supported by the
   Responder.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ID #1     |     ID #2     |     ID #3     |     ID #4     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     ID #n     |                Padding                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           715
     Length         number of HIT Suite IDs
     ID             defines a HIT Suite ID supported by the host.
                    The list of IDs is ordered by preference of the
                    host. Each HIT Suite ID is one octet long. The four
                    higher-order bits of the ID field correspond to the
                    HIT Suite ID in the ORCHID OGA field. The four
                    lower-order bits are reserved and set to 0 and
                    ignored by the receiver.

   The HIT Suite ID indexes a HIT Suite.  HIT Suites are composed of
   signature algorithms as defined in Section 5.2.9 and hash functions.

   The ID field in the HIT_SUITE_LIST is defined as eight-bit field as
   opposed to the four-bit HIT Suite ID and OGA field in the ORCHID.
   This difference is a measure to accommodate larger HIT Suite IDs if



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   the 16 available values prove insufficient.  In that case, one of the
   16 values, zero, will be used to indicate that four additional bits
   of the ORCHID will be used to encode the HIT Suite ID.  Hence, the
   current four-bit HIT Suite-IDs only use the four higher order bits in
   the ID field.  Future documents may define the use of the four lower-
   order bits in the ID field.

   The following HIT Suites ID are defined:

        HIT Suite              ID
        RESERVED                0
        RSA,DSA/SHA-256         1    (REQUIRED)
        ECDSA/SHA-384           2    (RECOMMENDED)
        ECDSA_LOW/SHA-1         3    (RECOMMENDED)

5.2.11.  TRANSPORT_FORMAT_LIST

   The TRANSPORT_FORMAT_LIST parameter contains a list of the supported
   HIP transport formats (TFs) of the Responder.  The Responder sends
   the TRANSPORT_FORMAT_LIST in the signed part of the R1 packet.  Based
   on the TRANSPORT_FORMAT_LIST, the Initiator chooses one suitable
   transport format and includes the respective HIP transport format
   parameter in its response packet.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             Type              |             Length            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |          TF type #1           |           TF type #2          /
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  /          TF type #n           |             Padding           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Type           2049
  Length         2x number of TF types
  TF Type        defines a transport format (TF) type supported by the
                 host. The TF type numbers correspond to the HIP
                 parameter type numbers of the respective transform
                 parameters. The list of TF types is ordered by preference
                 of the sender

   The TF type numbers index the respective HIP parameters for the
   transport formats in the type number range between 2050 to 4095.  The
   parameters and their use is defined in separate documents.
   Currently, the only transport format defined is IPsec ESP
   [I-D.ietf-hip-rfc5202-bis].




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   For each listed TF type, the sender of the parameter MUST include the
   repective transport form parameter in the HIP packet.  The TF type in
   the TRANSPORT_FORM_LIST MUST be ignored if no matching transport form
   parameter is present in the packet.

5.2.12.  HIP_MAC

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                             HMAC                              |
     /                                                               /
     /                               +-------------------------------+
     |                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           61505
     Length         length in octets, excluding Type, Length, and
                    Padding
     HMAC           HMAC computed over the HIP packet, excluding the
                    HIP_MAC parameter and any following parameters, such
                    as HIP_SIGNATURE, HIP_SIGNATURE_2,
                    ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
                    The checksum field MUST be set to zero and the HIP
                    header length in the HIP common header MUST be
                    calculated not to cover any excluded parameters
                    when the HMAC is calculated.  The size of the
                    HMAC is the natural size of the hash computation
                    output depending on the used hash function.

   The HMAC uses RHASH as hash algorithm.  The calculation and
   verification process is presented in Section 6.4.1.

5.2.13.  HIP_MAC_2

   The HIP_MAC_2 is a MAC of the packet and the HOST_ID parameter of the
   sender while only the packet without HOST_ID of the sender is sent.
   The parameter structure is the same as in Section 5.2.12.  The fields
   are:









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     Type           61569
     Length         length in octets, excluding Type, Length, and
                    Padding
     HMAC           HMAC computed over the HIP packet, excluding the
                    HIP_MAC_2 parameter and any following parameters
                    such as HIP_SIGNATURE, HIP_SIGNATURE_2,
                    ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
                    and including an additional sender's HOST_ID
                    parameter during the HMAC calculation.  The checksum
                    field MUST be set to zero and the HIP header length
                    in the HIP common header MUST be calculated not to
                    cover any excluded parameters when the HMAC is
                    calculated.  The size of the HMAC is the natural
                    size of the hash computation output depending on the
                    used hash function.

   The HMAC uses RHASH as hash algorithm.  The calculation and
   verification process is presented in Section 6.4.1.

5.2.14.  HIP_SIGNATURE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SIG alg                    |            Signature          /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                               |             Padding           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           61697
     Length         length in octets, excluding Type, Length, and
                    Padding
     SIG alg        signature algorithm
     Signature      the signature is calculated over the HIP packet,
                    excluding the HIP_SIGNATURE parameter and any
                    parameters that follow the HIP_SIGNATURE parameter.
                    When the signature is calculated the checksum field
                    MUST be set to zero, and the HIP header length in
                    the HIP common header MUST be calculated only up to
                    the beginning of the HIP_SIGNATURE parameter.

   The signature algorithms are defined in Section 5.2.9.  The signature
   in the Signature field is encoded using the method depending on the
   signature algorithm (e.g., according to [RFC3110] in case of RSA/
   SHA-1, according to [RFC5702] in case of RSA/SHA-256, according to
   [RFC2536] in case of DSA, or according to [RFC6090] in case of



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   ECDSA).

   The HIP_SIGNATURE calculation and verification process are presented
   in Section 6.4.2.

5.2.15.  HIP_SIGNATURE_2

   The HIP_SIGNATURE_2 excludes the variable parameters in the R1 packet
   to allow R1 pre-creation.  The parameter structure is the same as in
   Section 5.2.14.  The fields are:

     Type           61633
     Length         length in octets, excluding Type, Length, and
                    Padding
     SIG alg        signature algorithm
     Signature      Within the R1 packet that contains the
                    HIP_SIGNATURE_2 parameter, the Initiator's HIT, the
                    checksum field, and the Opaque and Random #I fields
                    in the PUZZLE parameter MUST be set to zero while
                    computing the HIP_SIGNATURE_2 signature.  Further,
                    the HIP packet length in the HIP header MUST be
                    adjusted as if the HIP_SIGNATURE_2 was not in the
                    packet during the signature calculation, i.e., the
                    HIP packet length points to the beginning of
                    the HIP_SIGNATURE_2 parameter during signing and
                    verification.

   Zeroing the Initiator's HIT makes it possible to create R1 packets
   beforehand, to minimize the effects of possible DoS attacks.  Zeroing
   the Random #I and Opaque fields within the PUZZLE parameter allows
   these fields to be populated dynamically on precomputed R1s.

   Signature calculation and verification follows the process defined in
   Section 6.4.2.

5.2.16.  SEQ

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Update ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Type            385
     Length          8
     Update ID       32-bit sequence number




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   The Update ID is an unsigned number in network byte order,
   initialized by a host to zero upon moving to ESTABLISHED state.  The
   Update ID has scope within a single HIP association, and not across
   multiple associations or multiple hosts.  The Update ID is
   incremented by one before each new UPDATE that is sent by the host;
   the first UPDATE packet originated by a host has an Update ID of 0.

5.2.17.  ACK

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       peer Update ID 1                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                       peer Update ID n                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type            449
     Length          length in octets, excluding Type and Length
     peer Update ID  32-bit sequence number corresponding to the
                     Update ID being ACKed.


   The ACK parameter includes one or more Update IDs that have been
   received from the peer.  The number of peer Update IDs can be
   inferred from the length by dividing it by 4.























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5.2.18.  ENCRYPTED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              IV                               /
     /                                                               /
     /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
     /                        Encrypted data                         /
     /                                                               /
     /                               +-------------------------------+
     /                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           641
     Length         length in octets, excluding Type, Length, and
                    Padding
     Reserved       zero when sent, ignored when received
     IV             Initialization vector, if needed, otherwise
                    nonexistent.  The length of the IV is inferred from
                    the HIP_CIPHER.
     Encrypted      The data is encrypted using the encryption algorithm
       data         defined in the HIP_CIPHER parameter.

   The ENCRYPTED parameter encapsulates other parameters, the encrypted
   data, which holds one or more HIP parameters in block encrypted form.

   Consequently, the first fields in the encapsulated parameter(s) are
   Type and Length of the first such parameter, allowing the contents to
   be easily parsed after decryption.

   The field labeled "Encrypted data" consists of the output of one or
   more HIP parameters concatenated together that have been passed
   through an encryption algorithm.  Each of these inner parameters is
   padded according to the rules of Section 5.2.1 for padding individual
   parameters.  As a result, the concatenated parameters will be a block
   of data that is 8-byte aligned.

   Some encryption algorithms require that the data to be encrypted must
   be a multiple of the cipher algorithm block size.  In this case, the
   above block of data MUST include additional padding, as specified by
   the encryption algorithm.  The size of the extra padding is selected
   so that the length of the unencrypted data block is a multiple of the



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   cipher block size.  The encryption algorithm may specify padding
   bytes other than zero; for example, AES [FIPS.197.2001] uses the
   PKCS5 padding scheme (see section 6.1.1 of [RFC2898]) where the
   remaining n bytes to fill the block each have the value of n.  This
   yields an "unencrypted data" block that is transformed to an
   "encrypted data" block by the cipher suite.  This extra padding added
   to the set of parameters to satisfy the cipher block alignment rules
   is not counted in HIP TLV length fields, and this extra padding
   should be removed by the cipher suite upon decryption.

   Note that the length of the cipher suite output may be smaller or
   larger than the length of the set of parameters to be encrypted,
   since the encryption process may compress the data or add additional
   padding to the data.

   Once this encryption process is completed, the Encrypted data field
   is ready for inclusion in the parameter.  If necessary, additional
   Padding for 8-byte alignment is then added according to the rules of
   Section 5.2.1.

5.2.19.  NOTIFICATION

   The NOTIFICATION parameter is used to transmit informational data,
   such as error conditions and state transitions, to a HIP peer.  A
   NOTIFICATION parameter may appear in NOTIFY packets.  The use of the
   NOTIFICATION parameter in other packet types is for further study.

























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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Reserved             |      Notify Message Type      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               /
     /                   Notification Data                           /
     /                                               +---------------+
     /                                               |     Padding   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           832
     Length         length in octets, excluding Type, Length, and
                    Padding
     Reserved       zero when sent, ignored when received
     Notify Message specifies the type of notification
       Type
     Notification   informational or error data transmitted in addition
       Data         to the Notify Message Type.  Values for this field
                    are type specific (see below).
                    multiple of 8 bytes.

   Notification information can be error messages specifying why an HIP
   Security Association could not be established.  It can also be status
   data that a HIP implementation wishes to communicate with a peer
   process.  The table below lists the notification messages and their
   Notification Message Types.  HIP packets MAY contain multiple
   NOTIFICATION parameters if several problems exist or several
   independent pieces of information must be transmitted.

   To avoid certain types of attacks, a Responder SHOULD avoid sending a
   NOTIFICATION to any host with which it has not successfully verified
   a puzzle solution.

   Notify Message Types in the range 0-16383 are intended for reporting
   errors and in the range 16384-65535 for other status information.  An
   implementation that receives a NOTIFY packet with a Notify Message
   Type that indicates an error in response to a request packet (e.g.,
   I1, I2, UPDATE) SHOULD assume that the corresponding request has
   failed entirely.  Unrecognized error types MUST be ignored except
   that they SHOULD be logged.

   As currently defined, Notify Message Type values 1-10 are used for
   informing about errors in packet structures, values 11-20 for
   informing about problems in parameters.




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   Notification Data in NOTIFICATION parameters with status Notify
   Message Types MUST be ignored if not recognized.

     Notify Message Types - Errors             Value
     -----------------------------             -----

     UNSUPPORTED_CRITICAL_PARAMETER_TYPE        1

       Sent if the parameter type has the "critical" bit set and the
       parameter type is not recognized.  Notification Data contains the
       two-octet parameter type.

     INVALID_SYNTAX                             7

       Indicates that the HIP message received was invalid because some
       type, length, or value was out of range or because the request
       was otherwise malformed.  To avoid a denial- of-service
       attack using forged messages, this status may only be returned
       for packets whose HIP_MAC (if present) and SIGNATURE have been
       verified.  This status MUST be sent in response to any error not
       covered by one of the other status types, and SHOULD NOT contain
       details to avoid leaking information to someone probing a node.
       To aid debugging, more detailed error information SHOULD be
       written to a console or log.

     NO_DH_PROPOSAL_CHOSEN                     14

       None of the proposed group IDs was acceptable.

     INVALID_DH_CHOSEN                         15

       The DH Group ID field does not correspond to one offered
       by the Responder.

     NO_HIP_PROPOSAL_CHOSEN                    16

       None of the proposed HIT Suites or HIP Encryption Algorithms was
       acceptable.

     INVALID_HIP_CIPHER_CHOSEN                 17

       The HIP_CIPHER Crypto ID does not correspond to one offered by
       the Responder.

     UNSUPPORTED_HIT_SUITE                     20

       Sent in response to an I1 or R1 packet for which the HIT suite
       is not supported.



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     AUTHENTICATION_FAILED                     24

       Sent in response to a HIP signature failure, except when
       the signature verification fails in a NOTIFY message.

     CHECKSUM_FAILED                           26

       Sent in response to a HIP checksum failure.

     HIP_MAC_FAILED                            28

       Sent in response to a HIP HMAC failure.

     ENCRYPTION_FAILED                         32

       The Responder could not successfully decrypt the
       ENCRYPTED parameter.

     INVALID_HIT                               40

       Sent in response to a failure to validate the peer's
       HIT from the corresponding HI.

     BLOCKED_BY_POLICY                         42

       The Responder is unwilling to set up an association
       for some policy reason (e.g., received HIT is NULL
       and policy does not allow opportunistic mode).

     RESPONDER_BUSY_PLEASE_RETRY               44

       The Responder is unwilling to set up an association as it is
       suffering under some kind of overload and has chosen to shed load
       by rejecting the Initiator's request.  The Initiator may retry;
       however, the Initiator MUST find another (different) puzzle
       solution for any such retries.  Note that the Initiator may need
       to obtain a new puzzle with a new I1/R1 exchange.

     Notify Message Types - Status            Value
     -----------------------------            -----

     I2_ACKNOWLEDGEMENT                       16384

       The Responder has an I2 packet from the Initiator but had to
       queue the I2 packet for processing.  The puzzle was correctly
       solved and the Responder is willing to set up an association but
       currently has a number of I2 packets in the processing queue.
       The R2 packet is sent after the I2 packet was processed.



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5.2.20.  ECHO_REQUEST_SIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         897
     Length       length of the opaque data in octets
     Opaque data  opaque data, supposed to be meaningful only to the
                  node that sends ECHO_REQUEST_SIGNED and receives a
                  corresponding ECHO_RESPONSE_SIGNED or
                  ECHO_RESPONSE_UNSIGNED.

   The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
   that the sender wants to get echoed back in the corresponding reply
   packet.

   The ECHO_REQUEST_SIGNED and corresponding echo response parameters
   MAY be used for any purpose where a node wants to carry some state in
   a request packet and get it back in a response packet.  The
   ECHO_REQUEST_SIGNED is covered by the HIP_MAC and SIGNATURE.  A HIP
   packet can contain only one ECHO_REQUEST_SIGNED parameter and MAY
   contain multiple ECHO_REQUEST_UNSIGNED parameter.  The
   ECHO_REQUEST_SIGNED parameter MUST be responded to with an
   ECHO_RESPONSE_SIGNED.

5.2.21.  ECHO_REQUEST_UNSIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         63661
     Length       length of the opaque data in octets
     Opaque data  opaque data, supposed to be meaningful only to the
                  node that sends ECHO_REQUEST_UNSIGNED and receives a
                  corresponding ECHO_RESPONSE_UNSIGNED.

   The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
   that the sender wants to get echoed back in the corresponding reply



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   packet.

   The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
   MAY be used for any purpose where a node wants to carry some state in
   a request packet and get it back in a response packet.  The
   ECHO_REQUEST_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.  A
   HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
   It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
   in HIP packets passing by.  The creator of the ECHO_REQUEST_UNSIGNED
   (end-host or middlebox) has to create the Opaque field so that it can
   later identify and remove the corresponding ECHO_RESPONSE_UNSIGNED
   parameter.

   The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
   ECHO_RESPONSE_UNSIGNED parameter.

5.2.22.  ECHO_RESPONSE_SIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         961
     Length       length of the opaque data in octets
     Opaque data  opaque data, copied unmodified from the
                  ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                  parameter that triggered this response.

   The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data
   that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back.
   The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED
   parameter.

   The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be
   used for any purpose where a node wants to carry some state in a
   request packet and get it back in a response packet.  The
   ECHO_RESPONSE_SIGNED is covered by the HIP_MAC and SIGNATURE.










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5.2.23.  ECHO_RESPONSE_UNSIGNED

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opaque data (variable length)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type         63425
     Length       length of the opaque data in octets
     Opaque data  opaque data, copied unmodified from the
                  ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                  parameter that triggered this response.

   The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data
   that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
   wants to get echoed back.  The opaque data is copied unmodified from
   the corresponding echo request parameter.

   The echo request and ECHO_RESPONSE_UNSIGNED parameters MAY be used
   for any purpose where a node wants to carry some state in a request
   packet and get it back in a response packet.  The
   ECHO_RESPONSE_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.

5.3.  HIP Packets

   There are eight basic HIP packets (see Table 10).  Four are for the
   HIP base exchange, one is for updating, one is for sending
   notifications, and two are for closing a HIP association.




















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   +------------------+------------------------------------------------+
   |    Packet type   | Packet name                                    |
   +------------------+------------------------------------------------+
   |         1        | I1 - the HIP Initiator Packet                  |
   |                  |                                                |
   |         2        | R1 - the HIP Responder Packet                  |
   |                  |                                                |
   |         3        | I2 - the Second HIP Initiator Packet           |
   |                  |                                                |
   |         4        | R2 - the Second HIP Responder Packet           |
   |                  |                                                |
   |        16        | UPDATE - the HIP Update Packet                 |
   |                  |                                                |
   |        17        | NOTIFY - the HIP Notify Packet                 |
   |                  |                                                |
   |        18        | CLOSE - the HIP Association Closing Packet     |
   |                  |                                                |
   |        19        | CLOSE_ACK - the HIP Closing Acknowledgment     |
   |                  | Packet                                         |
   +------------------+------------------------------------------------+

               Table 10: HIP packets and packet type values

   Packets consist of the fixed header as described in Section 5.1,
   followed by the parameters.  The parameter part, in turn, consists of
   zero or more TLV-coded parameters.

   In addition to the base packets, other packet types may be defined
   later in separate specifications.  For example, support for mobility
   and multi-homing is not included in this specification.

   See Notation (Section 2.2) for the notation used in the operations.

   In the future, an optional upper-layer payload MAY follow the HIP
   header.  The Next Header field in the header indicates if there is
   additional data following the HIP header.  The HIP packet, however,
   MUST NOT be fragmented.  This limits the size of the possible
   additional data in the packet.

5.3.1.  I1 - the HIP Initiator Packet

   The HIP header values for the I1 packet:

     Header:
       Packet Type = 1
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT, or NULL




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     IP ( HIP ( DH_GROUP_LIST ) )

   The I1 packet contains the fixed HIP header and the Initiator's
   DH_GROUP_LIST.

   Valid control bits: none

   The Initiator receives the Responder's HIT either from a DNS lookup
   of the Responder's FQDN (see 5205-bis), from some other repository,
   or from a local table.  If the Initiator does not know the
   Responder's HIT, it may attempt to use opportunistic mode by using
   NULL (all zeros) as the Responder's HIT.  See also "HIP Opportunistic
   Mode" (Section 4.1.8).

   Since the I1 packet is so easy to spoof even if it were signed, no
   attempt is made to add to its generation or processing cost.

   The Initiator includes a DH_GROUP_LIST parameter in the I1 packet to
   inform the Responder of its preferred DH Group IDs.  Note that the
   DH_GROUP_LIST in the I1 packet is not protected by a signature.

   Implementations MUST be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.

5.3.2.  R1 - the HIP Responder Packet

   The HIP header values for the R1 packet:

     Header:
       Packet Type = 2
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( [ R1_COUNTER, ]
                PUZZLE,
                DIFFIE_HELLMAN,
                HIP_CIPHER,
                HOST_ID,
                HIT_SUITE_LIST,
                DH_GROUP_LIST,
                [ ECHO_REQUEST_SIGNED, ]
                HIP_SIGNATURE_2 )
                <, ECHO_REQUEST_UNSIGNED >i)

   Valid control bits: A

   If the Responder's HI is an anonymous one, the A control MUST be set.



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   The Initiator's HIT MUST match the one received in the I1 packet if
   the R1 is a response to an I1.  If the Responder has multiple HIs,
   the Responder's HIT used MUST match Initiator's request.  If the
   Initiator used opportunistic mode, the Responder may select freely
   among its HIs.  See also "HIP Opportunistic Mode" (Section 4.1.8).

   The R1 packet generation counter is used to determine the currently
   valid generation of puzzles.  The value is increased periodically,
   and it is RECOMMENDED that it is increased at least as often as
   solutions to old puzzles are no longer accepted.

   The Puzzle contains a Random #I and the difficulty #K.  The
   difficulty #K indicates the number of lower-order bits, in the puzzle
   hash result, that must be zeros; see Section 4.1.2.  The Random #I is
   not covered by the signature and must be zeroed during the signature
   calculation, allowing the sender to select and set the #I into a
   precomputed R1 packet just prior sending it to the peer.

   The Responder selects the Diffie-Hellman public value based on the
   Initiator's preference expressed in the DH_GROUP_LIST parameter in
   the I1 packet.  The Responder sends back its own preference based on
   which it chose the DH public value as DH_GROUP_LIST.  This allows the
   Initiator to determine whether its own DH_GROUP_LIST in the sent I1
   packet was manipulated by an attacker.

   The Diffie-Hellman public value is ephemeral, and values SHOULD NOT
   be reused across different HIP sessions.  Once the Responder has
   received a valid response to an R1 packet, that Diffie-Hellman value
   SHOULD be deprecated.  It it is possible that the Responder has sent
   the same Diffie-Hellman value to different hosts simultaneously in
   corresponding R1 packets and those responses should also be accepted.
   However, as a defense against I1 packet storms, an implementation MAY
   propose, and re-use unless avoidable, the same Diffie-Hellman value
   for a period of time, for example, 15 minutes.  By using a small
   number of different puzzles for a given Diffie-Hellman value, the R1
   packets can be precomputed and delivered as quickly as I1 packets
   arrive.  A scavenger process should clean up unused Diffie-Hellman
   values and puzzles.

   Re-using Diffie-Hellman public values opens up the potential security
   risk of more than one Initiator ending up with the same keying
   material (due to faulty random number generators).  Also, more than
   one Initiator using the same Responder public key half may lead to
   potentially easier cryptographic attacks and to imperfect forward
   security.

   However, these risks involved in re-using the same public value are
   statistical; that is, the authors are not aware of any mechanism that



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   would allow manipulation of the protocol so that the risk of the re-
   use of any given Responder Diffie-Hellman public key would differ
   from the base probability.  Consequently, it is RECOMMENDED that
   Responders avoid re-using the same DH key with multiple Initiators,
   but because the risk is considered statistical and not known to be
   manipulable, the implementations MAY re-use a key in order to ease
   resource-constrained implementations and to increase the probability
   of successful communication with legitimate clients even under an I1
   packet storm.  In particular, when it is too expensive to generate
   enough precomputed R1 packets to supply each potential Initiator with
   a different DH key, the Responder MAY send the same DH key to several
   Initiators, thereby creating the possibility of multiple legitimate
   Initiators ending up using the same Responder-side public key.
   However, as soon as the Responder knows that it will use a particular
   DH key, it SHOULD stop offering it.  This design is aimed to allow
   resource-constrained Responders to offer services under I1 packet
   storms and to simultaneously make the probability of DH key re-use
   both statistical and as low as possible.

   If the Responder uses the same DH keypair for multiple handshakes.
   It must take care to avoid small subgroup attacks [RFC2785].  To
   avoid these attacks, when receiving the I2 message, the Responder
   SHOULD validate the Initiators DH public key as described in
   [RFC2785] Section 3.1.  In case the validation fails, the Responder
   MUST NOT generate a DH shared key and MUST silently abort the HIP
   BEX.

   The HIP_CIPHER contains the encryption algorithms supported by the
   Responder to encrypt the contents of the ENCRYPTED parameter, in the
   order of preference.  All implementations MUST support AES [RFC3602].

   The HIT_SUITE_LIST parameter is an ordered list of the Responder's
   preferred and supported HIT Suites.  The list allows the Initiator to
   determine whether its own source HIT matches any suite supported by
   the Responder.

   The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED parameters contain
   data that the sender wants to receive unmodified in the corresponding
   response packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
   parameter.  The R1 packet may contain zero or more
   ECHO_REQUEST_UNSIGNED parameters as described in Section
   Section 5.2.21.

   The signature is calculated over the whole HIP packet as described in
   Section 5.2.15.  This allows the Responder to use precomputed R1s.
   The Initiator SHOULD validate this signature.  It MUST check that the
   Responder's HI matches with the one expected, if any.




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5.3.3.  I2 - the Second HIP Initiator Packet

   The HIP header values for the I2 packet:

     Header:
       Type = 3
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT

     IP ( HIP ( [R1_COUNTER,]
                SOLUTION,
                DIFFIE_HELLMAN,
                HIP_CIPHER,
                ENCRYPTED { HOST_ID } or HOST_ID,
                [ ECHO_RESPONSE_SIGNED ,]
                HIP_MAC,
                HIP_SIGNATURE
                <, ECHO_RESPONSE_UNSIGNED>i ) )

   Valid control bits: A

   The HITs used MUST match the ones used in the R1.

   If the Initiator's HI is an anonymous one, the A control MUST be set.

   If present in the I1 packet, the Initiator MUST include an unmodified
   copy of the R1_COUNTER parameter received in the corresponding R1
   packet into the I2 packet.

   The Solution contains the Random #I from R1 and the computed #J.  The
   low-order #K bits of the RHASH(I | ... | J) MUST be zero.

   The Diffie-Hellman value is ephemeral.  If precomputed, a scavenger
   process should clean up unused Diffie-Hellman values.  The Responder
   MAY re-use Diffie-Hellman values under some conditions as specified
   in Section 5.3.2.

   The HIP_CIPHER contains the single encryption transform selected by
   the Initiator, that it uses to encrypt the ENCRYPTED parameters.  The
   chosen cipher MUST correspond to one of the ciphers offered by the
   Responder in the R1.  All implementations MUST support AES [RFC3602].

   The Initiator's HI MAY be encrypted using the HIP_CIPHER encryption
   algorithm.  The keying material is derived from the Diffie-Hellman
   exchanged as defined in Section 6.5.

   The ECHO_RESPONSE_SIGNED and ECHO_RESPONSE_UNSIGNED contain the
   unmodified Opaque data copied from the corresponding echo request



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   parameter(s).

   The HMAC value in the HIP_MAC parameter is calculated over the whole
   HIP packet, excluding any parameters after the HIP_MAC, as described
   in Section 6.4.1.  The Responder MUST validate the HIP_MAC.

   The signature is calculated over the whole HIP packet, excluding any
   parameters after the HIP_SIGNATURE, as described in Section 5.2.14.
   The Responder MUST validate this signature.  The Responder uses the
   HI in the packet or a HI acquired by some other means for verifying
   the signature.

5.3.4.  R2 - the Second HIP Responder Packet

   The HIP header values for the R2 packet:

     Header:
       Packet Type = 4
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( HIP_MAC_2, HIP_SIGNATURE ) )


   Valid control bits: none

   The HIP_MAC_2 is calculated over the whole HIP packet, with
   Responder's HOST_ID parameter concatenated with the HIP packet.  The
   HOST_ID parameter is removed after the HMAC calculation.  The
   procedure is described in Section 6.4.1.

   The signature is calculated over the whole HIP packet.

   The Initiator MUST validate both the HIP_MAC and the signature.

5.3.5.  UPDATE - the HIP Update Packet

   Support for the UPDATE packet is MANDATORY.

   The HIP header values for the UPDATE packet:

     Header:
       Packet Type = 16
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( [SEQ, ACK, ] HIP_MAC, HIP_SIGNATURE ) )




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   Valid control bits: None

   The UPDATE packet contains mandatory HIP_MAC and HIP_SIGNATURE
   parameters, and other optional parameters.

   The UPDATE packet contains zero or one SEQ parameter.  The presence
   of a SEQ parameter indicates that the receiver MUST acknowledge the
   the UPDATE.  An UPDATE that does not contain a SEQ but only an ACK
   parameter is simply an acknowledgment of a previous UPDATE and itself
   MUST NOT be acknowledged by a separate ACK.  Such UPDATE packets
   containing only an ACK parameter do not require processing in
   relative order to other UPDATE packets.  An UPDATE packet without
   either a SEQ or an ACK parameter is invalid; such unacknowledged
   updates MUST instead use a NOTIFY packet.

   An UPDATE packet contains zero or one ACK parameters.  The ACK
   parameter echoes the SEQ sequence number of the UPDATE packet being
   ACKed.  A host MAY choose to acknowledge more than one UPDATE packet
   at a time; e.g., the ACK may contain the last two SEQ values
   received, for resilience against ACK loss.  ACK values are not
   cumulative; each received unique SEQ value requires at least one
   corresponding ACK value in reply.  Received ACKs that are redundant
   are ignored.  Hosts MUST implement the processing of ACKs with
   multiple SEQ numbers even if they do not implement sending ACKs with
   multiple SEQ numbers.

   The UPDATE packet may contain both a SEQ and an ACK parameter.  In
   this case, the ACK is being piggybacked on an outgoing UPDATE.  In
   general, UPDATEs carrying SEQ SHOULD be ACKed upon completion of the
   processing of the UPDATE.  A host MAY choose to hold the UPDATE
   carrying ACK for a short period of time to allow for the possibility
   of piggybacking the ACK parameter, in a manner similar to TCP delayed
   acknowledgments.

   A sender MAY choose to forgo reliable transmission of a particular
   UPDATE (e.g., it becomes overcome by events).  The semantics are such
   that the receiver MUST acknowledge the UPDATE, but the sender MAY
   choose to not care about receiving the ACK.

   UPDATEs MAY be retransmitted without incrementing SEQ.  If the same
   subset of parameters is included in multiple UPDATEs with different
   SEQs, the host MUST ensure that the receiver's processing of the
   parameters multiple times will not result in a protocol error.

5.3.6.  NOTIFY - the HIP Notify Packet

   Implementing the NOTIFY packet is optional.  The NOTIFY packet MAY be
   used to provide information to a peer.  Typically, NOTIFY is used to



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   indicate some type of protocol error or negotiation failure.  NOTIFY
   packets are unacknowledged.  The receiver can handle the packet only
   as informational, and SHOULD NOT change its HIP state (see
   Section 4.4.2) based purely on a received NOTIFY packet.

   The HIP header values for the NOTIFY packet:

     Header:
       Packet Type = 17
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT, or zero if unknown

     IP ( HIP (<NOTIFICATION>i, [HOST_ID, ] HIP_SIGNATURE) )

   Valid control bits: None

   The NOTIFY packet is used to carry one or more NOTIFICATION
   parameters.

5.3.7.  CLOSE - the HIP Association Closing Packet

   The HIP header values for the CLOSE packet:


     Header:
       Packet Type = 18
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( ECHO_REQUEST_SIGNED, HIP_MAC, HIP_SIGNATURE ) )

   Valid control bits: none

   The sender MUST include an ECHO_REQUEST_SIGNED used to validate
   CLOSE_ACK received in response, and both a HIP_MAC and a signature
   (calculated over the whole HIP packet).

   The receiver peer MUST reply with a CLOSE_ACK containing an
   ECHO_RESPONSE_SIGNED corresponding to the received
   ECHO_REQUEST_SIGNED.

5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet

   The HIP header values for the CLOSE_ACK packet:







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     Header:
       Packet Type = 19
       SRC HIT = Sender's HIT
       DST HIT = Recipient's HIT

     IP ( HIP ( ECHO_RESPONSE_SIGNED, HIP_MAC, HIP_SIGNATURE ) )

   Valid control bits: none

   The sender MUST include both an HMAC and signature (calculated over
   the whole HIP packet).

   The receiver peer MUST validate the ECHO_RESPONSE_SIGNED and validate
   both the HIP_MAC and the signature if the receiver has state for a
   HIP association.

5.4.  ICMP Messages

   When a HIP implementation detects a problem with an incoming packet,
   and it either cannot determine the identity of the sender of the
   packet or does not have any existing HIP association with the sender
   of the packet, it MAY respond with an ICMP packet.  Any such replies
   MUST be rate-limited as described in [RFC4443].  In most cases, the
   ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for
   ICMPv6), with the Pointer field pointing to the field that caused the
   ICMP message to be generated.

5.4.1.  Invalid Version

   If a HIP implementation receives a HIP packet that has an
   unrecognized HIP version number, it SHOULD respond, rate-limited,
   with an ICMP packet with type Parameter Problem, with the Pointer
   pointing to the Version/RES. byte in the HIP header.

5.4.2.  Other Problems with the HIP Header and Packet Structure

   If a HIP implementation receives a HIP packet that has other
   unrecoverable problems in the header or packet format, it MAY
   respond, rate-limited, with an ICMP packet with type Parameter
   Problem, the Pointer pointing to the field that failed to pass the
   format checks.  However, an implementation MUST NOT send an ICMP
   message if the checksum fails; instead, it MUST silently drop the
   packet.

5.4.3.  Invalid Puzzle Solution

   If a HIP implementation receives an I2 packet that has an invalid
   puzzle solution, the behavior depends on the underlying version of



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   IP.  If IPv6 is used, the implementation SHOULD respond with an ICMP
   packet with type Parameter Problem, the Pointer pointing to the
   beginning of the Puzzle solution #J field in the SOLUTION payload in
   the HIP message.

   If IPv4 is used, the implementation MAY respond with an ICMP packet
   with the type Parameter Problem, copying enough of bytes from the I2
   message so that the SOLUTION parameter fits into the ICMP message,
   the Pointer pointing to the beginning of the Puzzle solution #J
   field, as in the IPv6 case.  Note, however, that the resulting ICMPv4
   message exceeds the typical ICMPv4 message size as defined in
   [RFC0792].

5.4.4.  Non-Existing HIP Association

   If a HIP implementation receives a CLOSE or UPDATE packet, or any
   other packet whose handling requires an existing association, that
   has either a Receiver or Sender HIT that does not match with any
   existing HIP association, the implementation MAY respond, rate-
   limited, with an ICMP packet with the type Parameter Problem.  The
   Pointer of the ICMP Parameter Problem packet is set pointing to the
   beginning of the first HIT that does not match.

   A host MUST NOT reply with such an ICMP if it receives any of the
   following messages: I1, R2, I2, R2, and NOTIFY packet.  When
   introducing new packet types, a specification SHOULD define the
   appropriate rules for sending or not sending this kind of ICMP reply.

6.  Packet Processing

   Each host is assumed to have a single HIP protocol implementation
   that manages the host's HIP associations and handles requests for new
   ones.  Each HIP association is governed by a conceptual state
   machine, with states defined above in Section 4.4.  The HIP
   implementation can simultaneously maintain HIP associations with more
   than one host.  Furthermore, the HIP implementation may have more
   than one active HIP association with another host; in this case, HIP
   associations are distinguished by their respective HITs.  It is not
   possible to have more than one HIP association between any given pair
   of HITs.  Consequently, the only way for two hosts to have more than
   one parallel association is to use different HITs, at least at one
   end.

   The processing of packets depends on the state of the HIP
   association(s) with respect to the authenticated or apparent
   originator of the packet.  A HIP implementation determines whether it
   has an active association with the originator of the packet based on
   the HITs.  In the case of user data carried in a specific transport



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   format, the transport format document specifies how the incoming
   packets are matched with the active associations.

6.1.  Processing Outgoing Application Data

   In a HIP host, an application can send application-level data using
   an identifier specified via the underlying API.  The API can be a
   backwards-compatible API (see [RFC5338]), using identifiers that look
   similar to IP addresses, or a completely new API, providing enhanced
   services related to Host Identities.  Depending on the HIP
   implementation, the identifier provided to the application may be
   different; for example, it can be a HIT or an IP address.

   The exact format and method for transferring the user data from the
   source HIP host to the destination HIP host is defined in the
   corresponding transport format document.  The actual data is
   transferred in the network using the appropriate source and
   destination IP addresses.

   In this document, conceptual processing rules are defined only for
   the base case where both hosts have only single usable IP addresses;
   the multi-address multi-homing case is specified separately.

   The following conceptual algorithm describes the steps that are
   required for handling outgoing datagrams destined to a HIT.

   1.  If the datagram has a specified source address, it MUST be a HIT.
       If it is not, the implementation MAY replace the source address
       with a HIT.  Otherwise, it MUST drop the packet.

   2.  If the datagram has an unspecified source address, the
       implementation MUST choose a suitable source HIT for the
       datagram.  Selecting the source HIT is subject to local policy.

   3.  If there is no active HIP association with the given <source,
       destination> HIT pair, one MUST be created by running the base
       exchange.  While waiting for the base exchange to complete, the
       implementation SHOULD queue at least one packet per HIP
       association to be formed, and it MAY queue more than one.

   4.  Once there is an active HIP association for the given <source,
       destination> HIT pair, the outgoing datagram is passed to
       transport handling.  The possible transport formats are defined
       in separate documents, of which the ESP transport format for HIP
       is mandatory for all HIP implementations.

   5.  Before sending the packet, the HITs in the datagram are replaced
       with suitable IP addresses.  For IPv6, the rules defined in



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       [RFC3484] SHOULD be followed.  Note that this HIT-to-IP-address
       conversion step MAY also be performed at some other point in the
       stack, e.g., before wrapping the packet into the output format.

6.2.  Processing Incoming Application Data

   The following conceptual algorithm describes the incoming datagram
   handling when HITs are used at the receiving host as application-
   level identifiers.  More detailed steps for processing packets are
   defined in corresponding transport format documents.

   1.  The incoming datagram is mapped to an existing HIP association,
       typically using some information from the packet.  For example,
       such mapping may be based on the ESP Security Parameter Index
       (SPI).

   2.  The specific transport format is unwrapped, in a way depending on
       the transport format, yielding a packet that looks like a
       standard (unencrypted) IP packet.  If possible, this step SHOULD
       also verify that the packet was indeed (once) sent by the remote
       HIP host, as identified by the HIP association.

       Depending on the used transport mode, the verification method can
       vary.  While the HI (as well as HIT) is used as the higher-layer
       identifier, the verification method has to verify that the data
       packet was sent by the correct node identity and that the actual
       identity maps to this particular HIT.  When using ESP transport
       format [I-D.ietf-hip-rfc5202-bis], the verification is done using
       the SPI value in the data packet to find the corresponding SA
       with associated HIT and key, and decrypting the packet with that
       associated key.

   3.  The IP addresses in the datagram are replaced with the HITs
       associated with the HIP association.  Note that this IP-address-
       to-HIT conversion step MAY also be performed at some other point
       in the stack.

   4.  The datagram is delivered to the upper layer (e.g., UDP or TCP).
       When demultiplexing the datagram, the right upper-layer socket is
       selected based on the HITs.

6.3.  Solving the Puzzle

   This subsection describes the details for solving the puzzle.

   In the R1 packet, the values #I and #K are sent in network byte
   order.  Similarly, in the I2 packet, the values #I and #J are sent in
   network byte order.  The hash is created by concatenating, in network



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   byte order, the following data, in the following order and using the
   RHASH algorithm:

      n-bit random value #I (where n is RHASH_len), in network byte
      order, as appearing in the R1 and I2 packets.

      128-bit Initiator's HIT, in network byte order, as appearing in
      the HIP Payload in the R1 and I2 packets.

      128-bit Responder's HIT, in network byte order, as appearing in
      the HIP Payload in the R1 and I2 packets.

      n-bit random value #J (where n is RHASH_len), in network byte
      order, as appearing in the I2 packet.

   In a valid response puzzle, the #K low-order bits of the resulting
   RHASH digest MUST be zero.

   Notes:

      i) The length of the data to be hashed is variable depending on
      the output length of the Responder's hash function RHASH.

      ii) All the data in the hash input MUST be in network byte order.

      iii) The order of the Initiator's and Responder's HITs are
      different in the R1 and I2 packets; see Section 5.1.  Care must be
      taken to copy the values in the right order to the hash input.

      iv) For a puzzle #I, there may exist multiple valid puzzle
      solutions #J.

   The following procedure describes the processing steps involved,
   assuming that the Responder chooses to precompute the R1 packets:

   Precomputation by the Responder:
      Sets up the puzzle difficulty #K.
      Creates a signed R1 and caches it.

   Responder:
      Selects a suitable cached R1.
      Generates a random number #I.
      Sends #I and #K in an R1.
      Saves #I and #K for a Delta time.







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   Initiator:
      Generates repeated attempts to solve the puzzle until a matching
      #J is found:
      Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K ) == 0
      Sends #I and #J in an I2.


   Responder:
      Verifies that the received #I is a saved one.
      Finds the right #K based on #I.
      Computes V := Ltrunc( RHASH( #I | HIT-I | HIT-R | #J ), #K )
      Rejects if V != 0
      Accept if V == 0

6.4.  HIP_MAC and SIGNATURE Calculation and Verification

   The following subsections define the actions for processing HIP_MAC,
   HIP_MAC_2, HIP_SIGNATURE and HIP_SIGNATURE_2 parameters.  The
   HIP_MAC_2 parameter is contained in the R2 packet.  The
   HIP_SIGNATURE_2 parameter is contained in the R1 packet.  The
   HIP_SIGNATURE and HIP_MAC parameter are contained in other HIP
   control packets.

6.4.1.  HMAC Calculation

   The HMAC uses RHASH as underlying hash function.  The type of RHASH
   depends on the HIT Suite of the Responder.  Hence, HMAC-SHA-256
   [RFC4868] is used for HIT Suite RSA/DSA/SHA-256, HMAC-SHA-1 [RFC2404]
   is used for HIT Suite ECDSA_LOW/SHA-1, and HMAC-SHA-384 [RFC4868] for
   HIT Suite ECDSA/SHA-384.

   The following process applies both to the HIP_MAC and HIP_MAC_2
   parameters.  When processing HIP_MAC_2, the difference is that the
   HIP_MAC calculation includes a pseudo HOST_ID field containing the
   Responder's information as sent in the R1 packet earlier.

   Both the Initiator and the Responder should take some care when
   verifying or calculating the HIP_MAC_2.  Specifically, the Initiator
   has to preserve the HOST_ID exactly as it was received in the R1
   packet until it receives the HIP_MAC_2 in the R2 packet.

   The scope of the calculation for HIP_MAC is:

   HMAC: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters of the packet that is
   being calculated with Type values ranging from 1 to (HIP_MAC's Type
   value - 1) and exclude parameters with Type values greater or equal



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   to HIP_MAC's Type value.

   During HIP_MAC calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC parameter.

   Parameter order is described in Section 5.2.1.

   The scope of the calculation for HIP_MAC_2 is:

   HIP_MAC_2: { HIP header | [ Parameters ] | HOST_ID }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values from 1 to (HIP_MAC_2's Type value -
   1) and exclude parameters with Type values greater or equal to
   HIP_MAC_2's Type value.

   During HIP_MAC_2 calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC_2 parameter and increased by the
      length of the concatenated HOST_ID parameter length (including
      type and length fields).

   o  HOST_ID parameter is exactly in the form it was received in the R1
      packet from the Responder.

   Parameter order is described in Section 5.2.1, except that the
   HOST_ID parameter in this calculation is added to the end.

   The HIP_MAC parameter is defined in Section 5.2.12 and the HIP_MAC_2
   parameter in Section 5.2.13.  The HMAC calculation and verification
   process (the process applies both to HIP_MAC and HIP_MAC_2 except
   where HIP_MAC_2 is mentioned separately) is as follows:

   Packet sender:

   1.  Create the HIP packet, without the HIP_MAC, HIP_SIGNATURE,
       HIP_SIGNATURE_2, or any other parameter with greater Type value
       than the HIP_MAC parameter has.

   2.  In case of HIP_MAC_2 calculation, add a HOST_ID (Responder)
       parameter to the end of the packet.



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   3.  Calculate the Header Length field in the HIP header including the
       added HOST_ID parameter in case of HIP_MAC_2.

   4.  Compute the HMAC using either HIP-gl or HIP-lg integrity key
       retrieved from KEYMAT as defined in Section 6.5.

   5.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
       packet.

   6.  Add the HIP_MAC parameter to the packet and any parameter with
       greater Type value than the HIP_MAC's (HIP_MAC_2's) that may
       follow, including possible HIP_SIGNATURE or HIP_SIGNATURE_2
       parameters

   7.  Recalculate the Length field in the HIP header.

   Packet receiver:

   1.  Verify the HIP header Length field.

   2.  Remove the HIP_MAC or HIP_MAC_2 parameter, as well as all other
       parameters that follow it with greater Type value including
       possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
       contents if they are needed later.

   3.  In case of HIP_MAC_2, build and add a HOST_ID parameter (with
       Responder information) to the packet.  The HOST_ID parameter
       should be identical to the one previously received from the
       Responder.

   4.  Recalculate the HIP packet length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of HIP_MAC_2, the
       length is calculated with the added HOST_ID parameter.

   5.  Compute the HMAC using either HIP-gl or HIP-lg integrity key as
       defined in Section 6.5 and verify it against the received HMAC.

   6.  Set Checksum and Header Length field in the HIP header to
       original values.  Note that the checksum and length fields
       contain incorrect values after this step.

   7.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
       packet before further processing.








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6.4.2.  Signature Calculation

   The following process applies both to the HIP_SIGNATURE and
   HIP_SIGNATURE_2 parameters.  When processing the HIP_SIGNATURE_2, the
   only difference is that instead of the HIP_SIGNATURE parameter, the
   HIP_SIGNATURE_2 parameter is used, and the Initiator's HIT and PUZZLE
   Opaque and Random #I fields are cleared (set to all zeros) before
   computing the signature.  The HIP_SIGNATURE parameter is defined in
   Section 5.2.14 and the HIP_SIGNATURE_2 parameter in Section 5.2.15.

   The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2
   is:

   HIP_SIGNATURE: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values from 1 to (HIP_SIGNATURE's Type
   value - 1).

   During signature calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE parameter.

   The parameter order is described in Section 5.2.1.

   HIP_SIGNATURE_2: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values ranging from 1 to
   (HIP_SIGNATURE_2's Type value - 1).

   During signature calculation, the following apply:

   o  In the HIP header, the Initiator's HIT field and Checksum fields
      are set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_SIGNATURE_2 parameter.

   o  PUZZLE parameter's Opaque and Random #I fields are set to zero.

   Parameter order is described in Section 5.2.1.

   The signature calculation and verification process (the process
   applies both to HIP_SIGNATURE and HIP_SIGNATURE_2 except in the case



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   where HIP_SIGNATURE_2 is separately mentioned) is as follows:

   Packet sender:

   1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
       other parameters that follow the HIP_SIGNATURE parameter.

   2.  Calculate the Length field and zero the Checksum field in the HIP
       header.  In case of HIP_SIGNATURE_2, set Initiator's HIT field in
       the HIP header as well as PUZZLE parameter's Opaque and Random #I
       fields to zero.

   3.  Compute the signature using the private key corresponding to the
       Host Identifier (public key).

   4.  Add the HIP_SIGNATURE parameter to the packet.

   5.  Add any parameters that follow the HIP_SIGNATURE parameter.

   6.  Recalculate the Length field in the HIP header, and calculate the
       Checksum field.

   Packet receiver:

   1.  Verify the HIP header Length field and checksum.

   2.  Save the contents of the HIP_SIGNATURE parameter and any other
       parameters following the HIP_SIGNATURE parameter and remove them
       from the packet.

   3.  Recalculate the HIP packet Length in the HIP header and clear the
       Checksum field (set it to all zeros).  In case of
       HIP_SIGNATURE_2, set Initiator's HIT field in the HIP header as
       well as PUZZLE parameter's Opaque and Random #I fields to zero.

   4.  Compute the signature and verify it against the received
       signature using the packet sender's Host Identity (public key).

   5.  Restore the original packet by adding removed parameters (in step
       2) and resetting the values that were set to zero (in step 3).

   The verification can use either the HI received from a HIP packet,
   the HI from a DNS query, if the FQDN has been received in the HOST_ID
   packet or one received by some other means.







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6.5.  HIP KEYMAT Generation

   HIP keying material is derived from the Diffie-Hellman session key,
   Kij, produced during the HIP base exchange (see Section 4.1.3).  The
   Initiator has Kij during the creation of the I2 packet, and the
   Responder has Kij once it receives the I2 packet.  This is why I2 can
   already contain encrypted information.

   The KEYMAT is derived by feeding Kij into the key derivation function
   defined by the DH Group ID.  Currently the only key derivation
   function defied in this document is the Hash-based Key Derivation
   Function (HKDF) [RFC5869] using the RHASH hash function.  Other
   documents may define new DH Group IDs and corresponding key
   distribution functions.

   In the following we provide the details for deriving the keying
   material using HKDF.

   where

   info    = sort(HIT-I | HIT-R)
   salt    =  #I | #J

   Sort(HIT-I | HIT-R) is defined as the network byte order
   concatenation of the two HITs, with the smaller HIT preceding the
   larger HIT, resulting from the numeric comparison of the two HITs
   interpreted as positive (unsigned) 128-bit integers in network byte
   order.  The #I and #J values are from the puzzle and its solution
   that were exchanged in R1 and I2 messages when this HIP association
   was set up.  Both hosts have to store #I and #J values for the HIP
   association for future use.

   The initial keys are drawn sequentially in the order that is
   determined by the numeric comparison of the two HITs, with comparison
   method described in the previous paragraph.  HOST_g denotes the host
   with the greater HIT value, and HOST_l the host with the lower HIT
   value.

   The drawing order for the four initial keys is as follows:

      HIP-gl encryption key for HOST_g's ENCRYPTED parameter

      HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets

      HIP-lg encryption key for HOST_l's ENCRYPTED parameter

      HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets




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   The number of bits drawn for a given algorithm is the "natural" size
   of the keys.  For the mandatory algorithms, the following sizes
   apply:

   AES  128 or 256 bits

   SHA-1  160 bits

   SHA-256  256 bits

   SHA-384  384 bits

   NULL  0 bits

   If other key sizes are used, they MUST be treated as different
   encryption algorithms and defined separately.

6.6.  Initiation of a HIP Base Exchange

   An implementation may originate a HIP base exchange to another host
   based on a local policy decision, usually triggered by an application
   datagram, in much the same way that an IPsec IKE key exchange can
   dynamically create a Security Association.  Alternatively, a system
   may initiate a HIP exchange if it has rebooted or timed out, or
   otherwise lost its HIP state, as described in Section 4.5.4.

   The implementation prepares an I1 packet and sends it to the IP
   address that corresponds to the peer host.  The IP address of the
   peer host may be obtained via conventional mechanisms, such as DNS
   lookup.  The I1 packet contents are specified in Section 5.3.1.  The
   selection of which source or destination Host Identity to use, if a
   Initiator or Responder has more than one to choose from, is typically
   a policy decision.

   The following steps define the conceptual processing rules for
   initiating a HIP base exchange:

   1.  The Initiator receives one or more of the Responder's HITs and
       one or more addresses either from a DNS lookup of the Responder's
       FQDN, from some other repository, or from a local database.  If
       the Initiator does not know the Responder's HIT, it may attempt
       opportunistic mode by using NULL (all zeros) as the Responder's
       HIT (see also "HIP Opportunistic Mode" (Section 4.1.8)).  If the
       Initiator can choose from multiple Responder HITs, it selects a
       HIT for which the Initiator supports the HIT Suite.

   2.  The Initiator sends an I1 packet to one of the Responder's
       addresses.  The selection of which address to use is a local



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       policy decision.

   3.  The Initiator includes the DH_GROUP_LIST in the I1 packet.  The
       selection and order of DH Group IDs in the DH_GROUP_LIST MUST be
       stored by the Initiator because this list is needed for later R1
       processing.  In most cases, the preferences regarding the DH
       Groups will be static, so no per-association storage is
       necessary.

   4.  Upon sending an I1 packet, the sender transitions to state I1-
       SENT, starts a timer for which the timeout value SHOULD be larger
       than the worst-case anticipated RTT.  The sender SHOULD also
       increment the timeout counter associated with the I1.

   5.  Upon timeout, the sender SHOULD retransmit the I1 packet and
       restart the timer, up to a maximum of I1_RETRIES_MAX tries.

6.6.1.  Sending Multiple I1 Packets in Parallel

   For the sake of minimizing the session establishment latency, an
   implementation MAY send the same I1 packet to more than one of the
   Responder's addresses.  However, it MUST NOT send to more than three
   (3) Responder addresses in parallel.  Furthermore, upon timeout, the
   implementation MUST refrain from sending the same I1 packet to
   multiple addresses.  That is, if it retries to initialize the
   connection after a timeout, it MUST NOT send the I1 packet to more
   than one destination address.  These limitations are placed in order
   to avoid congestion of the network, and potential DoS attacks that
   might occur, e.g., because someone's claim to have hundreds or
   thousands of addresses could generate a huge number of I1 packets
   from the Initiator.

   As the Responder is not guaranteed to distinguish the duplicate I1
   packets it receives at several of its addresses (because it avoids
   storing states when it answers back an R1 packet), the Initiator may
   receive several duplicate R1 packets.

   The Initiator SHOULD then select the initial preferred destination
   address using the source address of the selected received R1, and use
   the preferred address as a source address for the I2 packet.
   Processing rules for received R1s are discussed in Section 6.8.

6.6.2.  Processing Incoming ICMP Protocol Unreachable Messages

   A host may receive an ICMP 'Destination Protocol Unreachable' message
   as a response to sending a HIP I1 packet.  Such a packet may be an
   indication that the peer does not support HIP, or it may be an
   attempt to launch an attack by making the Initiator believe that the



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   Responder does not support HIP.

   When a system receives an ICMP 'Destination Protocol Unreachable'
   message while it is waiting for an R1 packet, it MUST NOT terminate
   waiting.  It MAY continue as if it had not received the ICMP message,
   and send a few more I1 packets.  Alternatively, it MAY take the ICMP
   message as a hint that the peer most probably does not support HIP,
   and return to state UNASSOCIATED earlier than otherwise.  However, at
   minimum, it MUST continue waiting for an R1 packet for a reasonable
   time before returning to UNASSOCIATED.

6.7.  Processing Incoming I1 Packets

   An implementation SHOULD reply to an I1 with an R1 packet, unless the
   implementation is unable or unwilling to set up a HIP association.
   If the implementation is unable to set up a HIP association, the host
   SHOULD send an ICMP Destination Protocol Unreachable,
   Administratively Prohibited, message to the I1 packet source IP
   address.  If the implementation is unwilling to set up a HIP
   association, the host MAY ignore the I1 packet.  This latter case may
   occur during a DoS attack such as an I1 packet flood.

   The implementation SHOULD be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.

   A spoofed I1 packet can result in an R1 attack on a system.  An R1
   packet sender MUST have a mechanism to rate-limit R1 packets sent to
   an address.

   It is RECOMMENDED that the HIP state machine does not transition upon
   sending an R1 packet.

   The following steps define the conceptual processing rules for
   responding to an I1 packet:

   1.  The Responder MUST check that the Responder's HIT in the received
       I1 packet is either one of its own HITs or NULL.  Otherwise it
       must drop the packet.

   2.  If the Responder is in ESTABLISHED state, the Responder MAY
       respond to this with an R1 packet, prepare to drop an existing
       HIP security association with the peer, and stay at ESTABLISHED
       state.

   3.  If the Responder is in I1-SENT state, it MUST make a comparison
       between the sender's HIT and its own (i.e., the receiver's) HIT.
       If the sender's HIT is greater than its own HIT, it should drop



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       the I1 packet and stay at I1-SENT.  If the sender's HIT is
       smaller than its own HIT, it SHOULD send the R1 packet and stay
       at I1-SENT.  The HIT comparison is performed as defined in
       Section 6.5.

   4.  If the implementation chooses to respond to the I1 packet with an
       R1 packet, it creates a new R1 or selects a precomputed R1
       according to the format described in Section 5.3.2.  It creates
       or chooses an R1 that contains its most preferred DH public value
       that is also contained in the DH_GROUP_LIST in the I1 packet.  If
       no suitable DH Group ID was contained in the DH_GROUP_LIST in the
       I1 packet, it sends an R1 with any suitable DH public key.

   5.  If the received Responder's HIT in the I1 is NULL, the Responder
       selects a HIT with a the same HIT Suite as the Initiator's HIT.
       If this HIT Suite is not supported by the Responder, it SHOULD
       select a REQUIRED HIT Suite from Section 5.2.10, which is
       currently RSA/DSA/SHA-256.  Other than that, selecting the HIT is
       a local policy matter.

   6.  The responder expresses its supported HIP transport formats in
       the TRANSPORT_FORMAT_LIST as described in Section 5.2.10.  The
       Responder MUST at least provide one payload transport format
       type.

   7.  The Responder sends the R1 packet to the source IP address of the
       I1 packet.

6.7.1.  R1 Management

   All compliant implementations MUST be able to produce R1 packets.  An
   R1 packet MAY be precomputed.  An R1 packet MAY be reused for time
   Delta T, which is implementation dependent, and SHOULD be deprecated
   and not used once a valid response I2 packet has been received from
   an Initiator.  During an I1 message storm, an R1 packet MAY be re-
   used beyond this limit.  R1 information MUST NOT be discarded until
   Delta S after T.  Time S is the delay needed for the last I2 packet
   to arrive back to the Responder.

   Implementations that support multiple DH groups MAY pre-compute R1
   packets for each supported group so that incoming I1 packets with
   different DH Group IDs in the DH_GROUP_LIST can be served quickly.

   An implementation MAY keep state about received I1 packets and match
   the received I2 packets against the state, as discussed in
   Section 4.1.1.





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6.7.2.  Handling Malformed Messages

   If an implementation receives a malformed I1 packet, it SHOULD NOT
   respond with a NOTIFY message, as such practice could open up a
   potential denial-of-service threat.  Instead, it MAY respond with an
   ICMP packet, as defined in Section 5.4.

6.8.  Processing Incoming R1 Packets

   A system receiving an R1 packet MUST first check to see if it has
   sent an I1 packet to the originator of the R1 packet (i.e., it is in
   state I1-SENT).  If so, it SHOULD process the R1 as described below,
   send an I2 packet, and transition to state I2-SENT, setting a timer
   to protect the I2 packet.  If the system is in state I2-SENT, it MAY
   respond to the R1 packet if the R1 packet has a larger R1 generation
   counter; if so, it should drop its state due to processing the
   previous R1 packet and start over from state I1-SENT.  If the system
   is in any other state with respect to that host, the system SHOULD
   silently drop the R1 packet.

   When sending multiple I1 packets, an Initiator SHOULD wait for a
   small amount of time after the first R1 reception to allow possibly
   multiple R1 packets to arrive, and it SHOULD respond to an R1 packet
   among the set with the largest R1 generation counter.

   The following steps define the conceptual processing rules for
   responding to an R1 packet:

   1.   A system receiving an R1 MUST first check to see if it has sent
        an I1 packet to the originator of the R1 packet (i.e., it has a
        HIP association that is in state I1-SENT and that is associated
        with the HITs in the R1).  Unless the I1 packet was sent in
        opportunistic mode (see Section 4.1.8), the IP addresses in the
        received R1 packet SHOULD be ignored by the R1 processing and,
        when looking up the right HIP association, the received R1
        packet SHOULD be matched against the associations using only the
        HITs.  If a match exists, the system should process the R1
        packet as described below.

   2.   Otherwise, if the system is in any other state than I1-SENT or
        I2-SENT with respect to the HITs included in the R1 packet, it
        SHOULD silently drop the R1 packet and remain in the current
        state.

   3.   If the HIP association state is I1-SENT or I2-SENT, the received
        Initiator's HIT MUST correspond to the HIT used in the original
        I1.  Also, the Responder's HIT MUST correspond to the one used
        in the I1, unless the I1 packet contained a NULL HIT.



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   4.   The system SHOULD validate the R1 signature before applying
        further packet processing, according to Section 5.2.15.

   5.   If the HIP association state is I1-SENT, and multiple valid R1
        packets are present, the system MUST select from among the R1
        packets with the largest R1 generation counter.

   6.   The system MUST check that the Initiator HIT Suite is contained
        in the HIT_SUITE_LIST parameter in the R1 packet (i.e., the
        Initiator's HIT Suite is supported by the Responder).  If the
        HIT Suite is supported by the Responder, the system proceeds
        normally.  Otherwise, the system MAY stay in state I1-sent and
        restart the BEX by sending a new I1 packet with an Initiator HIT
        that is supported by the Responder and hence is contained in the
        HIT_SUITE_LIST in the R1 packet.  The system MAY abort the BEX
        if no suitable source HIT is available.  The system SHOULD wait
        for an acceptable time span to allow further R1 packets with
        higher R1 generation counters or different HIT and HIT Suites to
        arrive before restarting or aborting the BEX.

   7.   The system MUST check that the DH Group ID in the DIFFIE_HELLMAN
        parameter in the R1 matches the first DH Suite ID in the
        Responder's DH_GROUP_LIST in the R1 packet that was also
        contained in the Initiator's DH_GROUP_LIST in the I1 packet.  If
        the DH Group ID of the DIFFIE_HELLMAN parameter does not express
        the Responder's best choice, the Initiator can conclude that the
        DH_GROUP_LIST in the I1 packet was adversely modified.  In such
        case, the Initiator MAY send a new I1 packet, however, it SHOULD
        NOT change its preference in the DH_GROUP_LIST in the new I1
        packet.  Alternatively, the Initiator MAY abort the HIP base
        exchange.

   8.   If the HIP association state is I2-SENT, the system MAY re-enter
        state I1-SENT and process the received R1 packet if it has a
        larger R1 generation counter than the R1 packet responded to
        previously.

   9.   The R1 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the R1 packet and returning
        to state UNASSOCIATED.  The system SHOULD consider dropping the
        R1 packet only if it used a NULL HIT in I1 packet.  If the A bit
        is set, the Responder's HIT is anonymous and SHOULD NOT be
        stored permanently.

   10.  The system SHOULD attempt to validate the HIT against the
        received Host Identity by using the received Host Identity to
        construct a HIT and verify that it matches the Sender's HIT.




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   11.  The system MUST store the received R1 generation counter for
        future reference.

   12.  The system attempts to solve the puzzle in the R1 packet.  The
        system MUST terminate the search after exceeding the remaining
        lifetime of the puzzle.  If the puzzle is not successfully
        solved, the implementation MAY either resend the I1 packet
        within the retry bounds or abandon the HIP base exchange.

   13.  The system computes standard Diffie-Hellman keying material
        according to the public value and Group ID provided in the
        DIFFIE_HELLMAN parameter.  The Diffie-Hellman keying material
        Kij is used for key extraction as specified in Section 6.5.

   14.  The system selects the HIP_CIPHER ID from the choices presented
        in the R1 packet and uses the selected values subsequently when
        generating and using encryption keys, and when sending the I2
        packet.  If the proposed alternatives are not acceptable to the
        system, it may either resend an I1 within the retry bounds or
        abandon the HIP base exchange.

   15.  The system chooses one suitable transport format from the
        TRANSPORT_FORMAT_LIST and includes the respective transport
        format parameter in the subsequent I2 packet.

   16.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   17.  The system prepares and sends an I2 packet, as described in
        Section 5.3.3.

   18.  The system SHOULD start a timer whose timeout value SHOULD be
        larger than the worst-case anticipated RTT, and MUST increment a
        timeout counter associated with the I2 packet.  The sender
        SHOULD retransmit the I2 packet upon a timeout and restart the
        timer, up to a maximum of I2_RETRIES_MAX tries.

   19.  If the system is in state I1-SENT, it SHALL transition to state
        I2-SENT.  If the system is in any other state, it remains in the
        current state.

6.8.1.  Handling of Malformed Messages

   If an implementation receives a malformed R1 message, it MUST
   silently drop the packet.  Sending a NOTIFY or ICMP would not help,
   as the sender of the R1 packet typically doesn't have any state.  An
   implementation SHOULD wait for some more time for a possibly well-
   formed R1, after which it MAY try again by sending a new I1 packet.



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6.9.  Processing Incoming I2 Packets

   Upon receipt of an I2 packet, the system MAY perform initial checks
   to determine whether the I2 packet corresponds to a recent R1 packet
   that has been sent out, if the Responder keeps such state.  For
   example, the sender could check whether the I2 packet is from an
   address or HIT for which the Responder has recently received an I1.
   The R1 packet may have had Opaque data included that was echoed back
   in the I2 packet.  If the I2 packet is considered to be suspect, it
   MAY be silently discarded by the system.

   Otherwise, the HIP implementation SHOULD process the I2 packet.  This
   includes validation of the puzzle solution, generating the Diffie-
   Hellman key, possibly decrypting the Initiator's Host Identity,
   verifying the signature, creating state, and finally sending an R2
   packet.

   The following steps define the conceptual processing rules for
   responding to an I2 packet:

   1.   The system MAY perform checks to verify that the I2 packet
        corresponds to a recently sent R1 packet.  Such checks are
        implementation dependent.  See Appendix A for a description of
        an example implementation.

   2.   The system MUST check that the Responder's HIT corresponds to
        one of its own HITs and MUST drop the packet otherwise.

   3.   The system MUST further check that the Initiator's HIT Suite is
        supported.  The Responder SHOULD silently drop I2 packets with
        unsupported Initiator HITs.

   4.   If the system's state machine is in the R2-SENT state, the
        system MAY check if the newly received I2 packet is similar to
        the one that triggered moving to R2-SENT.  If so, it MAY
        retransmit a previously sent R2 packet, reset the R2-SENT timer,
        and the state machine stays in R2-SENT.

   5.   If the system's state machine is in the I2-SENT state, the
        system makes a comparison between its local and sender's HITs
        (similarly as in Section 6.5).  If the local HIT is smaller than
        the sender's HIT, it should drop the I2 packet, use the peer
        Diffie-Hellman key and nonce #I from the R1 packet received
        earlier, and get the local Diffie-Hellman key and nonce #J from
        the I2 packet sent to the peer earlier.  Otherwise, the system
        should process the received I2 packet and drop any previously
        derived Diffie-Hellman keying material Kij it might have formed
        upon sending the I2 packet previously.  The peer Diffie-Hellman



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        key and the nonce #J are taken from the just arrived I2 packet.
        The local Diffie-Hellman key and the nonce I are the ones that
        were sent earlier in the R1 packet.

   6.   If the system's state machine is in the I1-SENT state, and the
        HITs in the I2 packet match those used in the previously sent I1
        packet, the system uses this received I2 packet as the basis for
        the HIP association it was trying to form, and stops
        retransmitting I1 packets (provided that the I2 packet passes
        the additional checks below).

   7.   If the system's state machine is in any other state than R2-
        SENT, the system SHOULD check that the echoed R1 generation
        counter in the I2 packet is within the acceptable range if the
        counter is included.  Implementations MUST accept puzzles from
        the current generation and MAY accept puzzles from earlier
        generations.  If the generation counter in the newly received I2
        packet is outside the accepted range, the I2 packet is stale
        (and perhaps replayed) and SHOULD be dropped.

   8.   The system MUST validate the solution to the puzzle by computing
        the hash described in Section 5.3.3 using the same RHASH
        algorithm.

   9.   The I2 packet MUST have a single value in the HIP_CIPHER
        parameter, which MUST match one of the values offered to the
        Initiator in the R1 packet.

   10.  The system must derive Diffie-Hellman keying material Kij based
        on the public value and Group ID in the DIFFIE_HELLMAN
        parameter.  This key is used to derive the HIP association keys,
        as described in Section 6.5.  If the Diffie-Hellman Group ID is
        unsupported, the I2 packet is silently dropped.

   11.  The encrypted HOST_ID is decrypted by the Initiator's encryption
        key defined in Section 6.5.  If the decrypted data is not a
        HOST_ID parameter, the I2 packet is silently dropped.

   12.  The implementation SHOULD also verify that the Initiator's HIT
        in the I2 packet corresponds to the Host Identity sent in the I2
        packet.  (Note: some middleboxes may not able to make this
        verification.)

   13.  The system MUST verify the HIP_MAC according to the procedures
        in Section 5.2.12.

   14.  The system MUST verify the HIP_SIGNATURE according to
        Section 5.2.14 and Section 5.3.3.



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   15.  If the checks above are valid, then the system proceeds with
        further I2 processing; otherwise, it discards the I2 and its
        state machine remains in the same state.

   16.  The I2 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the I2 and the state machine
        returns to state UNASSOCIATED.  If the A bit is set, the
        Initiator's HIT is anonymous and should not be stored
        permanently.

   17.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   18.  Upon successful processing of an I2 message when the system's
        state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-
        SENT, an R2 packet is sent and the system's state machine
        transitions to state R2-SENT.

   19.  Upon successful processing of an I2 packet when the system's
        state machine is in state ESTABLISHED, the old HIP association
        is dropped and a new one is installed, an R2 packet is sent, and
        the system's state machine transitions to R2-SENT.

   20.  Upon the system's state machine transitioning to R2-SENT, the
        system starts a timer.  The state machine transitions to
        ESTABLISHED if some data has been received on the incoming HIP
        association, or an UPDATE packet has been received (or some
        other packet that indicates that the peer system's state machine
        has moved to ESTABLISHED).  If the timer expires (allowing for
        maximal amount of retransmissions of I2 packets), the state
        machine transitions to ESTABLISHED.

6.9.1.  Handling of Malformed Messages

   If an implementation receives a malformed I2 message, the behavior
   SHOULD depend on how many checks the message has already passed.  If
   the puzzle solution in the message has already been checked, the
   implementation SHOULD report the error by responding with a NOTIFY
   packet.  Otherwise, the implementation MAY respond with an ICMP
   message as defined in Section 5.4.

6.10.  Processing of Incoming R2 Packets

   An R2 packet received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
   results in the R2 packet being dropped and the state machine staying
   in the same state.  If an R2 packet is received in state I2-SENT, it
   MUST be processed.




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   The following steps define the conceptual processing rules for an
   incoming R2 packet:

   1.  If the system is in any other state than I2-SENT, the R2 packet
       is silently dropped.

   2.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in the R1 packet that caused the
       transition to the I1-SENT state.

   3.  The system MUST verify the HIP_MAC_2 according to the procedures
       in Section 5.2.13.

   4.  The system MUST verify the HIP signature according to the
       procedures in Section 5.2.14.

   5.  If any of the checks above fail, there is a high probability of
       an ongoing man-in-the-middle or other security attack.  The
       system SHOULD act accordingly, based on its local policy.

   6.  Upon successful processing of the R2 packet, the state machine
       transitions to state ESTABLISHED.

6.11.  Sending UPDATE Packets

   A host sends an UPDATE packet when it intends to update some
   information related to a HIP association.  There are a number of
   possible scenarios when this can occur, e.g., mobility management and
   rekeying of an existing ESP Security Association.  The following
   paragraphs define the conceptual rules for sending an UPDATE packet
   to the peer.  Additional steps can be defined in other documents
   where the UPDATE packet is used.

   The sequence of UPDATE messages is indicated by their SEQ parameter.
   Before sending an UPDATE message, the system first determines whether
   there are any outstanding UPDATE messages that may conflict with the
   new UPDATE message under consideration.  When multiple UPDATEs are
   outstanding (not yet acknowledged), the sender must assume that such
   UPDATEs may be processed in an arbitrary order by the receiver.
   Therefore, any new UPDATEs that depend on a previous outstanding
   UPDATE being successfully received and acknowledged MUST be postponed
   until reception of the necessary ACK(s) occurs.  One way to prevent
   any conflicts is to only allow one outstanding UPDATE at a time.
   However, allowing multiple UPDATEs may improve the performance of
   mobility and multihoming protocols.

   The following steps define the conceptual processing rules for
   sending UPDATE packets.



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   1.  The first UPDATE packet is sent with Update ID of zero.
       Otherwise, the system increments its own Update ID value by one
       before continuing the steps below.

   2.  The system creates an UPDATE packet that contains a SEQ parameter
       with the current value of Update ID.  The UPDATE packet MAY also
       include zero or more ACKs of the peer's Update ID(s) from
       previously received UPDATE SEQ parameter(s)

   3.  The system sends the created UPDATE packet and starts an UPDATE
       timer.  The default value for the timer is 2 * RTT estimate.  If
       multiple UPDATEs are outstanding, multiple timers are in effect.

   4.  If the UPDATE timer expires, the UPDATE is resent.  The UPDATE
       can be resent UPDATE_RETRY_MAX times.  The UPDATE timer SHOULD be
       exponentially backed off for subsequent retransmissions.  If no
       acknowledgment is received from the peer after UPDATE_RETRY_MAX
       times, the HIP association is considered to be broken and the
       state machine SHOULD move from state ESTABLISHED to state CLOSING
       as depicted in Section 4.4.4.  The UPDATE timer is cancelled upon
       receiving an ACK from the peer that acknowledges receipt of the
       UPDATE.

6.12.  Receiving UPDATE Packets

   When a system receives an UPDATE packet, its processing depends on
   the state of the HIP association and the presence and values of the
   SEQ and ACK parameters.  Typically, an UPDATE message also carries
   optional parameters whose handling is defined in separate documents.

   For each association, a host stores the peer's next expected in-
   sequence Update ID ("peer Update ID").  Initially, this value is
   zero.  Update ID comparisons of "less than" and "greater than" are
   performed with respect to a circular sequence number space.  Hence, a
   wrap around after 2^32 updates has to be expected and MUST be handled
   accordingly.

   The sender MAY send multiple outstanding UPDATE messages.  These
   messages are processed in the order in which they are received at the
   receiver (i.e., no re-sequencing is performed).  When processing
   UPDATEs out-of-order, the receiver MUST keep track of which UPDATEs
   were previously processed, so that duplicates or retransmissions are
   ACKed and not reprocessed.  A receiver MAY choose to define a receive
   window of Update IDs that it is willing to process at any given time,
   and discard received UPDATEs falling outside of that window.

   The following steps define the conceptual processing rules for
   receiving UPDATE packets.



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   1.  If there is no corresponding HIP association, the implementation
       MAY reply with an ICMP Parameter Problem, as specified in
       Section 5.4.4.

   2.  If the association is in the ESTABLISHED state and the SEQ (but
       not ACK) parameter is present, the UPDATE is processed and
       replied to as described in Section 6.12.1.

   3.  If the association is in the ESTABLISHED state and the ACK (but
       not SEQ) parameter is present, the UPDATE is processed as
       described in Section 6.12.2.

   4.  If the association is in the ESTABLISHED state and there is both
       an ACK and SEQ in the UPDATE, the ACK is first processed as
       described in Section 6.12.2, and then the rest of the UPDATE is
       processed as described in Section 6.12.1.

6.12.1.  Handling a SEQ Parameter in a Received UPDATE Message

   The following steps define the conceptual processing rules for
   handling a SEQ parameter in a received UPDATE packet.

   1.  If the Update ID in the received SEQ is not the next in the
       sequence of Update IDs and is greater than the receiver's window
       for new UPDATEs, the packet MUST be dropped.

   2.  If the Update ID in the received SEQ corresponds to an UPDATE
       that has recently been processed, the packet is treated as a
       retransmission.  The HIP_MAC verification (next step) MUST NOT be
       skipped.  (A byte-by-byte comparison of the received and a stored
       packet would be acceptable, though.)  It is recommended that a
       host caches UPDATE packets sent with ACKs to avoid the cost of
       generating a new ACK packet to respond to a replayed UPDATE.  The
       system MUST acknowledge, again, such (apparent) UPDATE message
       retransmissions but SHOULD also consider rate-limiting such
       retransmission responses to guard against replay attacks.

   3.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   4.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   5.  If a new SEQ parameter is being processed, the parameters in the
       UPDATE are then processed.  The system MUST record the Update ID
       in the received SEQ parameter, for replay protection.




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   6.  An UPDATE acknowledgment packet with ACK parameter is prepared
       and sent to the peer.  This ACK parameter MAY be included in a
       separate UPDATE or piggybacked in an UPDATE with SEQ parameter,
       as described in Section 5.3.5.  The ACK parameter MAY acknowledge
       more than one of the peer's Update IDs.

6.12.2.  Handling an ACK Parameter in a Received UPDATE Packet

   The following steps define the conceptual processing rules for
   handling an ACK parameter in a received UPDATE packet.

   1.  The sequence number reported in the ACK must match with an UPDATE
       packet sent earlier that has not already been acknowledged.  If
       no match is found or if the ACK does not acknowledge a new
       UPDATE, the packet MUST either be dropped if no SEQ parameter is
       present, or the processing steps in Section 6.12.1 are followed.

   2.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
       verification fails, the packet MUST be dropped.

   3.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
       verification fails, the packet SHOULD be dropped and an error
       message logged.

   4.  The corresponding UPDATE timer is stopped (see Section 6.11) so
       that the now acknowledged UPDATE is no longer retransmitted.  If
       multiple UPDATEs are acknowledged, multiple timers are stopped.

6.13.  Processing of NOTIFY Packets

   Processing of NOTIFY packets is OPTIONAL.  If processed, any errors
   in a received NOTIFICATION parameter SHOULD be logged.  Received
   errors MUST be considered only as informational, and the receiver
   SHOULD NOT change its HIP state (see Section 4.4.2) purely based on
   the received NOTIFY message.

6.14.  Processing CLOSE Packets

   When the host receives a CLOSE message, it responds with a CLOSE_ACK
   message and moves to CLOSED state.  (The authenticity of the CLOSE
   message is verified using both HIP_MAC and SIGNATURE).  This
   processing applies whether or not the HIP association state is
   CLOSING in order to handle simultaneous CLOSE messages from both ends
   that cross in flight.

   The HIP association is not discarded before the host moves to the
   UNASSOCIATED state.




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   Once the closing process has started, any new need to send data
   packets triggers creating and establishing of a new HIP association,
   starting with sending of an I1 packet.

   If there is no corresponding HIP association, the CLOSE packet is
   dropped.

6.15.  Processing CLOSE_ACK Packets

   When a host receives a CLOSE_ACK message, it verifies that it is in
   CLOSING or CLOSED state and that the CLOSE_ACK was in response to the
   CLOSE.  A host can map CLOSE_ACK messages to CLOSE messages by
   comparing the value of ECHO_REQUEST_SIGNED (in the CLOSE packet) to
   the value of ECHO_RESPONSE_SIGNED (in the CLOSE_ACK packet).

   The CLOSE_ACK contains the HIP_MAC and the SIGNATURE parameters for
   verification.  The state is discarded when the state changes to
   UNASSOCIATED and, after that, the host MAY respond with an ICMP
   Parameter Problem to an incoming CLOSE message (see Section 5.4.4).

6.16.  Handling State Loss

   In the case of a system crash and unanticipated state loss, the
   system SHOULD delete the corresponding HIP state, including the
   keying material.  That is, the state SHOULD NOT be stored in long-
   term storage.  If the implementation does drop the state (as
   RECOMMENDED), it MUST also drop the peer's R1 generation counter
   value, unless a local policy explicitly defines that the value of
   that particular host is stored.  An implementation MUST NOT store a
   peer's R1 generation counters by default, but storing R1 generation
   counter values, if done, MUST be configured by explicit HITs.

7.  HIP Policies

   There are a number of variables that will influence the HIP base
   exchanges that each host must support.  All HIP implementations MUST
   support more than one simultaneous HI, at least one of which SHOULD
   be reserved for anonymous usage.  Although anonymous HIs will be
   rarely used as Responders' HIs, they will be common for Initiators.
   Support for more than two HIs is RECOMMENDED.

   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 is decided by local
   policy and depends on the privacy requirements of the Initiator.

   The value of #K used in the HIP R1 must be chosen with care.  Too
   high numbers of #K will exclude clients with weak CPUs because these



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   devices cannot solve the puzzle within reasonable time. #K should
   only be raised if a Responder is under high load, i.e., it cannot
   process all incoming HIP handshakes any more.  If a responder is not
   under high load, K SHOULD be 0.

   Responders that only respond to selected Initiators require an ACL,
   representing for which hosts they accept HIP base exchanges, and the
   preferred transform and local lifetimes.  Wildcarding SHOULD be
   supported for such ACLs, and also for Responders that offer public or
   anonymous services.

8.  Security Considerations

   HIP is designed to provide secure authentication of hosts.  HIP also
   attempts to limit the exposure of the host to various denial-of-
   service and man-in-the-middle (MitM) attacks.  In doing so, 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.

   Denial-of-service attacks often take advantage of asymmetries in the
   cost of an starting an association.  One example of such asymmetry is
   the need of a Responder to store local state while a malicious
   Initiator can stay stateless.  HIP makes no attempt to increase the
   cost of the start of state at the Initiator, but makes an effort to
   reduce the cost for the Responder.  This is accomplished by having
   the Responder start the 3-way exchange instead of the Initiator,
   making the HIP protocol 4 packets long.  In doing this, the first
   packet from the Responder, R1, becomes a 'stock' packet that the
   Responder MAY use many times, until some Initiator has provided a
   valid response to such an R1 packet.  During an I1 packet storm, the
   host may reuse the same DH value also even if some Initiator has
   provided a valid response using that particular DH value.  However,
   such behavior is discouraged and should be avoided.  Using the same
   Diffie-Hellman values and random puzzle #I value has some risks.
   This risk needs to be balanced against a potential storm of HIP I1
   packets.

   This shifting of the start of state cost to the Initiator in creating
   the I2 HIP packet presents another DoS attack.  The attacker can
   spoof the I1 packet and the Responder sends out the R1 HIP packet.
   This could conceivably tie up the 'Initiator' with evaluating the R1
   HIP packet, and creating the I2 packet.  The defense against this
   attack is to simply ignore any R1 packet where a corresponding I1
   packet was not sent (as defined in Section 6.8 step 1).

   The R1 packet is considerably larger than the I1 packet.  This
   asymmetry can be exploited in an reflection attack.  A malicious



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   attacker could spoof the IP address of a victim and send a flood of
   I1 messages to a powerful Responder.  For each small I1 packet, the
   Responder would send a larger R1 packet to the victim.  The
   difference in packet sizes can further amplify a flooding attack
   against the victim.  To avoid such reflection attacks, the Responder
   SHOULD rate limit the sending of R1 packets in general or SHOULD rate
   limit the sending of R1 packets to a specific IP address.

   Floods of forged I2 packets form a second kind of DoS attack.  Once
   the attacking Initiator has solved the puzzle, it can send packets
   with spoofed IP source addresses with either an invalid HIP signature
   or invalid encrypted HIP payload (in the ENCRYPTED parameter).  This
   would take resources in the Responder's part to reach the point to
   discover that the I2 packet cannot be completely processed.  The
   defense against this attack is after N bad I2 packets with the same
   puzzle solution, the Responder would discard any I2 packets that
   contain the given solution.  This will shut down the attack.  The
   attacker would have to request another R1 packet and use that to
   launch a new attack.  The Responder could increase the value of #K
   while under attack.  Keeping a list of solutions from malformed
   packets requires that the Responder keeps state for these malformed
   I2 packets.  This state has to be kept until the R1 counter is
   increased.  As malformed packets are generally filtered by their
   checksum before signature verification, only solutions in packets
   that are forged to pass the checksum and puzzle are put to the
   blacklist.  In addition, a valid puzzle is required before a new list
   entry is created.  Hence, attackers that intend to flood the
   blacklist must solve puzzles first.

   A third form of DoS attack is emulating the restart of state after a
   reboot of one of the peers.  A restarting host would send an I1
   packet to the peers, which would respond with an R1 packet even if it
   were in the ESTABLISHED state.  If the I1 packet were spoofed, the
   resulting R1 packet would be received unexpectedly by the spoofed
   host and would be dropped, as in the first case above.

   A fourth form of DoS attack is emulating closing of the HIP
   association.  HIP relies on timers and a CLOSE/CLOSE_ACK handshake to
   explicitly signal the end of a HIP association.  Because both CLOSE
   and CLOSE_ACK messages contain a HIP_MAC, an outsider cannot close a
   connection.  The presence of an additional SIGNATURE allows
   middleboxes to inspect these messages and discard the associated
   state (for e.g., firewalling, SPI-based NATing, etc.).  However, the
   optional behavior of replying to CLOSE with an ICMP Parameter Problem
   packet (as described in Section 5.4.4) might allow an attacker
   spoofing the source IP address to send CLOSE messages to launch
   reflection attacks.




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   A fifth form of DoS attack is replaying R1s to cause the Initiator to
   solve stale puzzles and become out of synchronization with the
   Responder.  The R1 generation counter is a monotonically increasing
   counter designed to protect against this attack, as described in
   Section 4.1.4.

   Man-in-the-middle attacks are difficult to defend against, without
   third-party authentication.  A skillful MitM could easily handle all
   parts of HIP, but HIP indirectly provides the following protection
   from a MitM attack.  If the Responder's HI is retrieved from a signed
   DNS zone, a certificate, or through some other secure means, the
   Initiator can use this to validate the R1 HIP packet.

   Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
   certificate, or otherwise securely available, the Responder can
   retrieve the HI (after having got the I2 HIP packet) and verify that
   the HI indeed can be trusted.

   The HIP Opportunistic Mode concept has been introduced in this
   document, but this document does not specify what the semantics of
   such a connection setup are for applications.  There are certain
   concerns with opportunistic mode, as discussed in Section 4.1.8.

   NOTIFY messages are used only for informational purposes and they are
   unacknowledged.  A HIP implementation cannot rely solely on the
   information received in a NOTIFY message because the packet may have
   been replayed.  An implementation SHOULD NOT change any state
   information purely based on a received NOTIFY message.

   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' messages are to be expected and may be used for a DoS
   attack.  Against an Initiator, the attack would look like the
   Responder does not support HIP, but shortly after receiving the ICMP
   message, the Initiator would receive a valid R1 HIP packet.  Thus, to
   protect from this attack, an Initiator SHOULD NOT react to an ICMP
   message until a reasonable delta time to get the real Responder's R1
   HIP packet.  A similar attack against the Responder is more involved.
   Normally, if an I1 message received by a Responder was a bogus one
   sent by an attacker, the Responder may receive an ICMP message from
   the IP address the R1 message was sent to.  However, a sophisticated
   attacker can try to take advantage of such a behavior and try to
   break up the HIP base exchange by sending such an ICMP message to the
   Responder before the Initiator has a chance to send a valid I2
   message.  Hence, the Responder SHOULD NOT act on such an ICMP
   message.  Especially, it SHOULD NOT remove any minimal state created
   when it sent the R1 HIP packet (if it did create one), but wait for
   either a valid I2 HIP packet or the natural timeout (that is, if R1
   packets are tracked at all).  Likewise, the Initiator SHOULD ignore



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   any ICMP message while waiting for an R2 HIP packet, and SHOULD
   delete any pending state only after a natural timeout.

9.  IANA Considerations

   IANA has reserved protocol number 139 for the Host Identity Protocol.

   This document defines a new 128-bit value under the CGA Message Type
   namespace [RFC3972], 0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA, to be
   used for HIT generation as specified in ORCHID
   [I-D.ietf-hip-rfc4843-bis].

   This document uses HIP version number 2 for the four-bit Version
   field in a HIP protocol packet defined in [RFC5201].

   This document also creates a set of new namespaces.  These are
   described below.

   Packet Type

      The 7-bit Packet Type field in a HIP protocol packet describes the
      type of a HIP protocol message.  It is defined in Section 5.1.
      The current values are defined in Sections 5.3.1 through 5.3.8.

      New values are assigned through IETF Review or IESG Approval
      [RFC5226].

   HIT Suite

      The four-bit HIT Suite ID uses the OGA field in the ORCHID to
      express the type of the HIT.  This document defines two HIT Suites
      (see Appendix E).

      The HIT Suite ID is also carried in the four higher-order bits of
      the ID field in the HIT_SUITE_LIST parameter.  The four lower-
      order bits are reserved for future extensions of the HIT Suite ID
      space beyond 16 values.

      At the time being, the HIT Suite uses only four bits because these
      bits have to be carried in the HIT.  Using more bits for the HIT
      Suite ID reduces the cryptographic strength of the HIT.  HIT Suite
      IDs must be allocated carefully to avoid namespace exhaustion.
      Moreover, deprecated IDs should be reused after an appropriate
      time span.  If 16 Suite IDs prove insufficient and more HIT Suite
      IDs are needed concurrently, more bits can be used for the HIT
      Suite ID by using one HIT Suite ID (0) to indicate that more bits
      should be used.  The HIT_SUITE_LIST parameter already supports
      8-bit HIT Suite IDs, should longer IDs be needed.  Possible



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      extensions of the HIT Suite ID space to accommodate eight bits and
      new HIT Suite IDs are defined through IETF Review or IESG
      Approval.

   Parameter Type

      The 16-bit Type field in a HIP parameter describes the type of the
      parameter.  It is defined in Section 5.2.1.  The current values
      are defined in Sections 5.2.3 through 5.2.23.

      With the exception of the assigned Type codes, the Type codes 0
      through 1023 and 61440 through 65535 are reserved for future base
      protocol extensions, and are assigned through IETF Review or IESG
      Approval.

      The Type codes 32768 through 49151 are reserved for
      experimentation.  Types SHOULD be selected in a random fashion
      from this range, thereby reducing the probability of collisions.
      A method employing genuine randomness (such as flipping a coin)
      SHOULD be used.

      All other Type codes are assigned through First Come First Served,
      with Specification Required [RFC5226].

   Group ID

      The eight-bit Group ID values appear in the DIFFIE_HELLMAN
      parameter and the DH_GROUP_LIST parameter and are defined in
      Section 5.2.7.  New values are assigned through IETF Review or
      IESG Approval.

   HIP Cipher ID

      The 16-bit Cipher ID values in a HIP_CIPHER parameter are defined
      in Section 5.2.8.  New values either from the reserved or
      unassigned space are assigned through IETF Review or IESG
      Approval.

   DI-Type

      The four-bit DI-Type values in a HOST_ID parameter are defined in
      Section 5.2.9.  New values are assigned through IETF Review or
      IESG Approval.

   Notify Message Type

      The 16-bit Notify Message Type values in a NOTIFICATION parameter
      are defined in Section 5.2.19.



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      Notify Message Type values 1-10 are used for informing about
      errors in packet structures, values 11-20 for informing about
      problems in parameters containing cryptographic related material,
      values 21-30 for informing about problems in authentication or
      packet integrity verification.  Parameter numbers above 30 can be
      used for informing about other types of errors or events.  Values
      51-8191 are error types reserved to be allocated by IANA.  Values
      8192-16383 are error types for experimentation.  Values 16385-
      40959 are status types to be allocated by IANA, and values 40960-
      65535 are status types for experimentation.  New values in ranges
      51-8191 and 16385-40959 are assigned through First Come First
      Served, with Specification Required.

10.  Acknowledgments

   The drive to create HIP came to being after attending the MALLOC
   meeting at the 43rd IETF meeting.  Baiju Patel and Hilarie Orman
   really gave the original author, Bob Moskowitz, the assist to get HIP
   beyond 5 paragraphs of ideas.  It has matured considerably since the
   early versions thanks to extensive input from IETFers.  Most
   importantly, its design goals are articulated and are different from
   other efforts in this direction.  Particular mention goes to the
   members of the NameSpace Research Group of the IRTF.  Noel Chiappa
   provided valuable input at early stages of discussions about
   identifier handling and Keith Moore the impetus to provide
   resolvability.  Steve Deering provided encouragement to keep working,
   as a solid proposal can act as a proof of ideas for a research group.

   Many others contributed; extensive security tips were provided by
   Steve Bellovin.  Rob Austein kept the DNS parts on track.  Paul
   Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
   for the Initiator to respond, but easy for the Responder to validate.
   Bill Sommerfeld supplied the Birthday concept, which later evolved
   into the R1 generation counter, to simplify reboot management.  Erik
   Nordmark supplied the CLOSE-mechanism for closing connections.
   Rodney Thayer and Hugh Daniels provided extensive feedback.  In the
   early times of this document, John Gilmore kept Bob Moskowitz
   challenged to provide something of value.

   During the later stages of this document, when the editing baton was
   transferred to Pekka Nikander, the input from the early implementors
   was invaluable.  Without having actual implementations, this document
   would not be on the level it is now.

   In the usual IETF fashion, a large number of people have contributed
   to the actual text or ideas.  The list of these people include Jeff
   Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Andrew
   McGregor, Julien Laganier, Miika Komu, Mika Kousa, Jan Melen, Henrik



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   Petander, Michael Richardson, Rene Hummen, Tim Shepard, Jorma Wall,
   Xin Gu, and Jukka Ylitalo.  Our apologies to anyone whose name is
   missing.

   Once the HIP Working Group was founded in early 2004, a number of
   changes were introduced through the working group process.  Most
   notably, the original document was split in two, one containing the
   base exchange and the other one defining how to use ESP.  Some
   modifications to the protocol proposed by Aura, et al., [AUR03] were
   added at a later stage.

11.  Changes from RFC 5201

   This section summarizes the changes made from [RFC5201].

11.1.  Changes from draft-ietf-hip-rfc5201-bis-09

   o  Editorial changes based on working group last call.

11.2.  Changes from draft-ietf-hip-rfc5201-bis-08

   o  Issue 29: Use different RSA mode OEAP/PSS, elevate ECDSA to
      REQUIRED status

   o  Issue 35: limiting ECC cofactor to 1

   o  Changed text regarding issue 33 reusing DH values

   o  Fix tracker issue 32 on Domain Identifier normative text

11.3.  Changes from draft-ietf-hip-rfc5201-bis-07

   o  Removed lingering references to SHA-1 as the mandatory hash
      algorithm (which was changed to SHA-256 in the -02 draft version).

   o  For parameter type number changes, changed "IETF Review" to "IETF
      Review or IESG Approval".

   o  Updated Appendix C checksum examples to conform to HIPv2 packets.

11.4.  Changes from draft-ietf-hip-rfc5201-bis-06

   o  Made echoing the R1_COUNTER in the I2 mandatory if the R1 contains
      an R1_COUNTER.  This required to make the R1 counter a critical
      parameter.  Hence, the parameter type number of the R1_COUNTER
      changed from 128 to 129.





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   o  Made KDF dependent on DH Group to enable negotiation of the KDF.

11.5.  Changes from draft-ietf-hip-rfc5201-bis-05

   o  Changed type number of DH_GROUP_LIST from 2151 to 511 because it
      was in the number space that is reserved for the HIP transport
      mode negotiations.

   o  Added transport form type list parameter.  Transport forms are now
      negotiated with this list instead of by their order in the HIP
      packet.  This allows to remove the exception of the transport
      format parameters that were ordered by their preference instead of
      by their type number.  This should remove complexity from
      implementations.

   o  Clarify that in HIP signature processing, the restored checksum
      and length fields have been rendered invalid by the previous
      steps.

   o  Clarify behavior for when UPDATE does not contain SEQ or ACQ
      (disallow this).

   o  For namespace changes, changed "IETF Review" to "IETF Review or
      IESG Approval".

   o  Addressed IESG comment about ignoring packet IP addresses.

   o  Permit using Anonymous HI control in packets other than R1/I2.

   o  Fixed minor reference error (RFC2418, RFC2410).

   o  Deleted comment that NULL-ENCRYPTION SHOULD NOT be configurable
      via the UI.

   o  Editorial changes.

11.6.  Changes from draft-ietf-hip-rfc5201-bis-04

   o  Clarifications of the Security Considerations section.  One DoS
      defense mechanism was changed to be more effective and less prone
      to misuse.

   o  Minor clarifications of the state machine.

   o  Clarified text on HIP puzzle.

   o  Added names and references for figures.




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   o  Extended the definitions section.

   o  Added a reference to the HIP Version 1 certificate document.

   o  Added Initiator, Responder, HIP association, and signed data to
      the definitions section.

   o  Changed parameter figure for PUZZLE and SOLUTION to use
      RHASH_len/8 instead of n-byte.

   o  Replaced occurrences of SHOULD not with SHOULD NOT.

   o  Changed text to reflect the fact that several
      ECHO_REQUEST_UNSIGNED parameters may be present in an R1 and
      several ECHO_RESPONSE parameters may be present in an I2.

   o  Added text on verifying the ECHO_RESPONSE_SIGNED parameter in
      CLOSE_ACK.

   o  Changed wording from HMAC to HIP_MAC in Section 5.3.8.

   o  Reflected fact that the UPDATE packet MAY include zero or more
      ACKs.

   o  Added BEX to Definitions section.

   o  Changed HIP_SIGNATURE algorithm field from 8 bit to 16 bit to
      achieve alignment with the HOST_ID parameters.

   o  Fixed the wrong figures of the SEQ and ACK parameters.  SEQ always
      contains ONE update ID.  ACK may acknowledge SEVERAL update IDs.

   o  Added wording that several NOTIFY parameters may be present in a
      HIP packet.

   o  Changed wording for the ECHO_RESPONSE_SIGNED parameter.  Also
      lifted the restriction that only one ECHO_RESPONSE_UNSIGNED
      parameter MUST be present in each HIP control packet.  This did
      contradict the definition of the ECHO_RESPONSE_UNSIGNED parameter.

   o  Changed IETF Consensus to IETF Review or IESG Approval in IANA
      section.

   o  Aligned use of I, J, and K. Now I is #I, J is #J and K is #K
      throughout the document.

   o  Updated references.




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   o  Editorial changes.

11.7.  Changes from draft-ietf-hip-rfc5201-bis-03

   o  Editorial changes to improve clarity and readability.

   o  Removed obsoleted (not applicable) attack from security
      consideration section.

   o  Added a requirement that hosts MUST support processing of ACK
      parameters with several SEQ numbers even when they do not support
      sending such parameters.

   o  Removed note on memory bound puzzles.  The use of memory bound
      puzzles was reconsidered but no convincing arguments for inclusion
      in this document have been made on the list.

   o  Changed references to reference the new bis documents.

   o  Specified the ECC curves and the hashes used for these.

   o  Specified representation of ECC curves in the HI.

   o  Added text on the dependency between RHASH and HMAC.

   o  Rephrased part of the security considerations to make them
      clearer.

   o  Clarified the use of HITs in opportunistic mode.

   o  Clarified the difference between HIP_MAC and HIP_MAC_2 as well as
      between SIGNATURE and SIGNATURE_2.

   o  Changed NOTIFY name for value 44 from SERVER_BUSY_PLEASE_RETRY to
      RESPONDER_BUSY_PLEASE_RETRY.

   o  Mentioned that there are multiple valid puzzle solutions.

11.8.  Changes from draft-ietf-hip-rfc5201-bis-02

   o  Added recommendation to not use puzzle #I twice for the same host
      to avoid identical key material.

   o  Revised state machine and added missing event handling.

   o  Added UNSUPPORTED_HIT_SUITE to NOTIFY to indicate unsupported HIT
      suites.




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   o  Revised parameter type numbers (corresponding to IANA allocations)
      and added missing "free for experimentation" range to the
      description.

   o  Clarifying note on the use of the C bit in the parameter type
      numbers.

11.9.  Changes from draft-ietf-hip-rfc5201-bis-01

   o  Changed RHASH-len to RHASH_len to avoid confusion in calculations
      (- could be minus)

   o  Added RHASH_len to list of abbreviations

   o  Fixed length of puzzle #I and #J to be 1*RHASH_len

   o  Changed RHASH-len to RHASH_len to avoid confusion in calculations
      (- could be minus)

   o  Added RHASH_len to list of abbreviations

   o  Fixed length of puzzle #I and #J to be 1*RHASH_len

   o  Included HIT_SUITEs.

   o  Added DH negotiation to I1 and R1.

   o  Added DH_LIST parameter.

   o  Added text for DH Group negotiation.

   o  Removed second DH public value from DH parameter.

   o  Added ECC to HI generation.

   o  Added Responder HIT selection to opportunistic mode.

   o  Added ECDSA HI text and references (not complete yet).

   o  Added separate section on aborting BEX.

   o  Added separate section on downgrade attack prevention.

   o  Added text about DH Group selection for use cases without I1.

   o  Removed type range allocation for parameters related to HIP
      transform types.




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   o  New type range allocation for parameters that are only covered by
      a signature if a signature is present (Applies to DH_GROUP_LIST).

   o  Renamed HIP_TRANSFORM to HIP_CIPHER and removed hashes from it -
      hashes are determined by RHASH.

   o  The length of #I and #J for the puzzle now depends on RHASH.

   o  New keymat generation.

   o  Puzzle seed and solution now use RHASH and have variable length.

   o  Moved timing definitions closer to state machine.

   o  Simplified text regarding puzzle lifetime.

   o  Clarified the description of the use of #I in the puzzle

   o  Removed "Opportunistic mode" description from general definitions.

   o  More consistency across the old RFC5201 text.  Aligned
      capitalization and abbreviations.

   o  Extended protocol overview to include restart option.

   o  Extended state machine to include restart option because of
      unsupported Algorithms.

   o  Replaced SHA-1 with SHA-256 for required implementation.

   o  Added OGA list parameter (715) for detecting the Responder's set
      of OGAs.

   o  Added Appendix on ORCHID use in HITs.

   o  Added truncated SHA-256 option for HITs.

   o  Added truncated SHA-1 option for HITs.

   o  Added text about new ORCHID structure to HIT overview.

   o  Moved Editor role to Robert Moskowitz.

   o  Added SHA-256 to puzzle parameter.

   o  Generalized LTRUNC to be hash-function agnostic.





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   o  Added text about RHASH depending on OGA.

11.10.  Changes from draft-ietf-hip-rfc5201-bis-00

   o  Added reasoning why BIS document is needed.

11.11.  Contents of draft-ietf-hip-rfc5201-bis-00

   o  RFC5201 was submitted as draft-RFC.

12.  References

12.1.  Normative References

   [FIPS.180-2.2002]           National Institute of Standards and
                               Technology, "Secure Hash Standard",
                               FIPS PUB 180-2, August 2002, <http://
                               csrc.nist.gov/publications/fips/
                               fips180-2/fips180-2.pdf>.

   [I-D.ietf-hip-rfc4843-bis]  Laganier, J. and F. Dupont, "An IPv6
                               Prefix for Overlay Routable Cryptographic
                               Hash Identifiers Version 2 (ORCHIDv2)",
                               draft-ietf-hip-rfc4843-bis-02 (work in
                               progress), September 2012.

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

   [NIST.800-131A.2011]        National Institute of Standards and
                               Technology, "Transitions:  Recommendation
                               for Transitioning the Use of
                               Cryptographic Algorithms and Key
                               Lengths", NIST 800-131A, January 2011.

   [RFC0768]                   Postel, J., "User Datagram Protocol",
                               STD 6, RFC 768, August 1980.

   [RFC1035]                   Mockapetris, P., "Domain names -
                               implementation and specification",
                               STD 13, RFC 1035, November 1987.

   [RFC2119]                   Bradner, S., "Key words for use in RFCs
                               to Indicate Requirement Levels", BCP 14,



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                               RFC 2119, March 1997.

   [RFC2404]                   Madson, C. and R. Glenn, "The Use of
                               HMAC-SHA-1-96 within ESP and AH",
                               RFC 2404, November 1998.

   [RFC2410]                   Glenn, R. and S. Kent, "The NULL
                               Encryption Algorithm and Its Use With
                               IPsec", RFC 2410, November 1998.

   [RFC2460]                   Deering, S. and R. Hinden, "Internet
                               Protocol, Version 6 (IPv6)
                               Specification", RFC 2460, December 1998.

   [RFC2536]                   Eastlake, D., "DSA KEYs and SIGs in the
                               Domain Name System (DNS)", RFC 2536,
                               March 1999.

   [RFC2785]                   Zuccherato, R., "Methods for Avoiding the
                               "Small-Subgroup" Attacks on the Diffie-
                               Hellman Key Agreement Method for S/MIME",
                               RFC 2785, March 2000.

   [RFC2898]                   Kaliski, B., "PKCS #5: Password-Based
                               Cryptography Specification Version 2.0",
                               RFC 2898, September 2000.

   [RFC3110]                   Eastlake, D., "RSA/SHA-1 SIGs and RSA
                               KEYs in the Domain Name System (DNS)",
                               RFC 3110, May 2001.

   [RFC3447]                   Jonsson, J. and B. Kaliski, "Public-Key
                               Cryptography Standards (PKCS) #1: RSA
                               Cryptography Specifications Version 2.1",
                               RFC 3447, February 2003.

   [RFC3484]                   Draves, R., "Default Address Selection
                               for Internet Protocol version 6 (IPv6)",
                               RFC 3484, February 2003.

   [RFC3526]                   Kivinen, T. and M. Kojo, "More Modular
                               Exponential (MODP) Diffie-Hellman groups
                               for Internet Key Exchange (IKE)",
                               RFC 3526, May 2003.

   [RFC3602]                   Frankel, S., Glenn, R., and S. Kelly,
                               "The AES-CBC Cipher Algorithm and Its Use
                               with IPsec", RFC 3602, September 2003.



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   [RFC3972]                   Aura, T., "Cryptographically Generated
                               Addresses (CGA)", RFC 3972, March 2005.

   [RFC4034]                   Arends, R., Austein, R., Larson, M.,
                               Massey, D., and S. Rose, "Resource
                               Records for the DNS Security Extensions",
                               RFC 4034, March 2005.

   [RFC4282]                   Aboba, B., Beadles, M., Arkko, J., and P.
                               Eronen, "The Network Access Identifier",
                               RFC 4282, December 2005.

   [RFC4443]                   Conta, A., Deering, S., and M. Gupta,
                               "Internet Control Message Protocol
                               (ICMPv6) for the Internet Protocol
                               Version 6 (IPv6) Specification",
                               RFC 4443, March 2006.

   [RFC4754]                   Fu, D. and J. Solinas, "IKE and IKEv2
                               Authentication Using the Elliptic Curve
                               Digital Signature Algorithm (ECDSA)",
                               RFC 4754, January 2007.

   [RFC4868]                   Kelly, S. and S. Frankel, "Using HMAC-
                               SHA-256, HMAC-SHA-384, and HMAC-SHA-512
                               with IPsec", RFC 4868, May 2007.

   [RFC5201]                   Moskowitz, R., Nikander, P., Jokela, P.,
                               and T. Henderson, "Host Identity
                               Protocol", RFC 5201, April 2008.

   [RFC5702]                   Jansen, J., "Use of SHA-2 Algorithms with
                               RSA in DNSKEY and RRSIG Resource Records
                               for DNSSEC", RFC 5702, October 2009.

   [RFC5869]                   Krawczyk, H. and P. Eronen, "HMAC-based
                               Extract-and-Expand Key Derivation
                               Function (HKDF)", RFC 5869, May 2010.

   [RFC5903]                   Fu, D. and J. Solinas, "Elliptic Curve
                               Groups modulo a Prime (ECP Groups) for
                               IKE and IKEv2", RFC 5903, June 2010.

   [RFC6090]                   McGrew, D., Igoe, K., and M. Salter,
                               "Fundamental Elliptic Curve Cryptography
                               Algorithms", RFC 6090, February 2011.





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12.2.  Informative References

   [AUR03]                     Aura, T., Nagarajan, A., and A. Gurtov,
                               "Analysis of the HIP Base Exchange
                               Protocol", in Proceedings of 10th
                               Australasian Conference on Information
                               Security and Privacy, July 2003.

   [CRO03]                     Crosby, SA. and DS. Wallach, "Denial of
                               Service via Algorithmic Complexity
                               Attacks", in Proceedings of Usenix
                               Security Symposium 2003,  Washington,
                               DC., August 2003.

   [DIF76]                     Diffie, W. and M. Hellman, "New
                               Directions in Cryptography", IEEE
                               Transactions on Information Theory vol.
                               IT-22, number 6, pages 644-654, Nov 1976.

   [FIPS.197.2001]             National Institute of Standards and
                               Technology, "Advanced Encryption Standard
                               (AES)", FIPS PUB 197, November 2001, <htt
                               p://csrc.nist.gov/publications/fips/
                               fips197/fips-197.pdf>.

   [I-D.ietf-btns-c-api]       Richardson, M., Williams, N., Komu, M.,
                               and S. Tarkoma, "C-Bindings for IPsec
                               Application Programming Interfaces",
                               draft-ietf-btns-c-api-04 (work in
                               progress), March 2009.

   [I-D.ietf-hip-rfc4423-bis]  Moskowitz, R., "Host Identity Protocol
                               Architecture",
                               draft-ietf-hip-rfc4423-bis-05 (work in
                               progress), September 2012.

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

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

   [I-D.ietf-hip-rfc5206-bis]  Henderson, T., Vogt, C., and J. Arkko,
                               "Host Mobility with the Host Identity



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                               Protocol", draft-ietf-hip-rfc5206-bis-04
                               (work in progress), July 2012.

   [KAU03]                     Kaufman, C., Perlman, R., and B.
                               Sommerfeld, "DoS protection for UDP-based
                               protocols", ACM Conference on Computer
                               and Communications Security , Oct 2003.

   [KRA03]                     Krawczyk, H., "SIGMA: The 'SIGn-and-MAc'
                               Approach to Authenticated Diffie-Hellman
                               and Its Use in the IKE-Protocols", in
                               Proceedings of CRYPTO 2003, pages 400-
                               425, August 2003.

   [RFC0792]                   Postel, J., "Internet Control Message
                               Protocol", STD 5, RFC 792,
                               September 1981.

   [RFC3849]                   Huston, G., Lord, A., and P. Smith, "IPv6
                               Address Prefix Reserved for
                               Documentation", RFC 3849, July 2004.

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

   [RFC5226]                   Narten, T. and H. Alvestrand, "Guidelines
                               for Writing an IANA Considerations
                               Section in RFCs", BCP 26, RFC 5226,
                               May 2008.

   [RFC5338]                   Henderson, T., Nikander, P., and M. Komu,
                               "Using the Host Identity Protocol with
                               Legacy Applications", RFC 5338,
                               September 2008.

   [RFC5533]                   Nordmark, E. and M. Bagnulo, "Shim6:
                               Level 3 Multihoming Shim Protocol for
                               IPv6", RFC 5533, June 2009.

   [RFC5747]                   Wu, J., Cui, Y., Li, X., Xu, M., and C.
                               Metz, "4over6 Transit Solution Using IP
                               Encapsulation and MP-BGP Extensions",
                               RFC 5747, March 2010.

   [RFC6253]                   Heer, T. and S. Varjonen, "Host Identity
                               Protocol Certificates", RFC 6253,
                               May 2011.



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   [SECG]                      SECG, "Recommended Elliptic Curve Domain
                               Parameters", SEC 2 , 2000,
                               <http://www.secg.org/>.

Appendix A.  Using Responder Puzzles

   As mentioned in Section 4.1.1, the Responder may delay state creation
   and still reject most spoofed I2 packets by using a number of pre-
   calculated R1 packets and a local selection function.  This appendix
   defines one possible implementation in detail.  The purpose of this
   appendix is to give the implementors an idea on how to implement the
   mechanism.  If the implementation is based on this appendix, it MAY
   contain some local modification that makes an attacker's task harder.

   The Responder creates a secret value S, that it regenerates
   periodically.  The Responder needs to remember the two latest values
   of S.  Each time the S is regenerated, the R1 generation counter
   value is incremented by one.

   The Responder generates a pre-signed R1 packet.  The signature for
   pre-generated R1s must be recalculated when the Diffie-Hellman key is
   recomputed or when the R1_COUNTER value changes due to S value
   regeneration.

   When the Initiator sends the I1 packet for initializing a connection,
   the Responder receives the HIT and IP address from the packet, and
   generates an #I value for the puzzle.  The #I value is set to the
   pre-signed R1 packet.

       #I value calculation:
       #I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), n)
       where n = RHASH_len

   The RHASH algorithm is the same that is used to generate the
   Responder's HIT value.

   From an incoming I2 packet, the Responder receives the required
   information to validate the puzzle: HITs, IP addresses, and the
   information of the used S value from the R1_COUNTER.  Using these
   values, the Responder can regenerate the #I, and verify it against
   the #I received in the I2 packet.  If the #I values match, it can
   verify the solution using #I, #J, and difficulty #K. If the #I values
   do not match, the I2 is dropped.

       puzzle_check:
       V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), #K )
       if V != 0, drop the packet




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   If the puzzle solution is correct, the #I and #J values are stored
   for later use.  They are used as input material when keying material
   is generated.

   Keeping state about failed puzzle solutions depends on the
   implementation.  Although it is possible for the Responder not to
   keep any state information, it still may do so to protect itself
   against certain attacks (see Section 4.1.1).

Appendix B.  Generating a Public Key Encoding from an HI

   The following pseudo-code illustrates the process to generate a
   public key encoding from an HI for both RSA and DSA.

   The symbol := denotes assignment; the symbol += denotes appending.
   The pseudo-function encode_in_network_byte_order takes two
   parameters, an integer (bignum) and a length in bytes, and returns
   the integer encoded into a byte string of the given length.

   switch ( HI.algorithm )
   {

   case RSA:
   buffer := encode_in_network_byte_order ( HI.RSA.e_len,
             ( HI.RSA.e_len > 255 ) ? 3 : 1 )
   buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
   buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )
   break;

   case DSA:
   buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
   buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
   buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
                                            8 * HI.DSA.T )
   buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
                                            8 * HI.DSA.T )
   buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
                                            8 * HI.DSA.T )
   break;

   }

Appendix C.  Example Checksums for HIP Packets

   The HIP checksum for HIP packets is specified in Section 5.1.1.
   Checksums for TCP and UDP packets running over HIP-enabled security
   associations are specified in Section 3.5.  The examples below use
   [RFC3849] and [RFC5747] addresses, and HITs with the prefix of



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   2001:10 followed by zeros, followed by a decimal 1 or 2,
   respectively.

   The following example is defined only for testing the checksum
   calculation.

C.1.  IPv6 HIP Example (I1 packet)

     Source Address:                 2001:d88::1
     Destination Address:            2001:d88::2
     Upper-Layer Packet Length:      48              0x30
     Next Header:                    139             0x8b
     Payload Protocol:               59              0x3b
     Header Length:                  4               0x4
     Packet Type:                    1               0x1
     Version:                        2               0x2
     Reserved:                       1               0x1
     Control:                        0               0x0
     Checksum:                       6878            0x1ade
     Sender's HIT  :                 2001:10::1
     Receiver's HIT:                 2001:10::2
     DH_GROUP_LIST type:             511             0x1ff
     DH_GROUP_LIST length:           3               0x3
     DH_GROUP_LIST group IDs:        3,4,8

C.2.  IPv4 HIP Packet (I1 packet)

   The IPv4 checksum value for the example I1 packet is shown below.

     Source Address:                 192.0.2.1
     Destination Address:            192.0.2.2
     Upper-Layer Packet Length:      48              0x30
     Next Header:                    139             0x8b
     Payload Protocol:               59              0x3b
     Header Length:                  4               0x4
     Packet Type:                    1               0x1
     Version:                        2               0x2
     Reserved:                       1               0x1
     Control:                        0               0x0
     Checksum:                       61934           0xf1ee
     Sender's HIT  :                 2001:10::1
     Receiver's HIT:                 2001:10::2
     DH_GROUP_LIST type:             511             0x1ff
     DH_GROUP_LIST length:           3               0x3
     DH_GROUP_LIST group IDs:        3,4,8






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C.3.  TCP Segment

   Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
   use the IPv6 pseudo-header format [RFC2460], with the HITs used in
   place of the IPv6 addresses.

     Sender's HIT:                   2001:10::1
     Receiver's HIT:                 2001:10::2
     Upper-Layer Packet Length:      20              0x14
     Next Header:                    6               0x06
     Source port:                    65500           0xffdc
     Destination port:               22              0x0016
     Sequence number:                1               0x00000001
     Acknowledgment number:          0               0x00000000
     Header length:                  20              0x14
     Flags:                          SYN             0x02
     Window size:                    65535           0xffff
     Checksum:                       28618           0x6fca
     Urgent pointer:                 0               0x0000


       0x0000:  6000 0000 0014 0640 2001 0010 0000 0000
       0x0010:  0000 0000 0000 0001 2001 0010 0000 0000
       0x0020:  0000 0000 0000 0002 ffdc 0016 0000 0001
       0x0030:  0000 0000 5002 ffff 6fca 0000

Appendix D.  ECDH and ECDSA 160 Bit Groups

   The ECDH and ECDSA 160-bit group SECP160R1 is rated at 80 bits
   symmetric strength.  Once this was considered appropriate for one
   year of security.  Today these groups should be used only when the
   host is not powerful enough (e.g., some embedded devices) and when
   security requirements are low (e.g., long-term confidentiality is not
   required).

Appendix E.  HIT Suites and HIT Generation

   The HIT as an ORCHID [I-D.ietf-hip-rfc4843-bis] consists of three
   parts: A 28-bit prefix, a 4-bit encoding of the ORCHID generation
   algorithm (OGA) and the representation of the public key.  The OGA is
   an index pointing to the specific algorithm by which the public key
   and the 96-bit hashed encoding is generated.  The OGA is protocol
   specific and is to be interpreted as defined below for all protocols
   that use the same context ID as HIP.  HIP groups sets of valid
   combinations of signature and hash algorithms into HIT Suites.  These
   HIT suites are addressed by an index, which is transmitted in the OGA
   field of the ORCHID.




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   The set of used HIT Suites will be extended to counter the progress
   in computation capabilities and vulnerabilities in the employed
   algorithms.  The intended use of the HIT Suites is to introduce a new
   HIT Suite and phase out an old one before it becomes insecure.  Since
   the 4-bit OGA field only permits 15 HIT Suites (the HIT Suite with ID
   0 is reserved) to be used in parallel, phased-out HIT Suites must be
   reused at some point.  In such a case, there will be a rollover of
   the HIT Suite ID and the next newly introduced HIT Suite will start
   with a lower HIT Suite index than the previously introduced one.  The
   rollover effectively deprecates the reused HIT Suite.  For a smooth
   transition, the HIT Suite should be deprecated a considerable time
   before the HIT Suite index is reused.

   Since the number of HIT Suites is tightly limited to 16, the HIT
   Suites must be assigned carefully.  Hence, sets of suitable
   algorithms are grouped in a HIT Suite.

   The HIT Suite of the Responder's HIT determines the RHASH and the
   hash function to be used for the HMAC in HIP control packets as well
   as the signature algorithm family used for generating the HI.  The
   list of HIT Suites is defined in Table 11.

   The following HIT Suites are defined for HIT generation.  The input
   for each generation algorithm is the encoding of the HI as defined in
   Section 3.2.  The output is 96 bits long and is directly used in the
   ORCHID.

   +-------+----------+--------------+------------+--------------------+
   | Index | Hash     | HMAC         | Signature  | Description        |
   |       | function |              | algorithm  |                    |
   |       |          |              | family     |                    |
   +-------+----------+--------------+------------+--------------------+
   |     0 |          |              |            | Reserved           |
   |     1 | SHA-256  | HMAC-SHA-256 | RSA, DSA   | RSA or DSA HI      |
   |       |          |              |            | hashed with        |
   |       |          |              |            | SHA-256, truncated |
   |       |          |              |            | to 96 bits         |
   |     2 | SHA-384  | HMAC-SHA-384 | ECDSA      | ECDSA HI hashed    |
   |       |          |              |            | with SHA-384,      |
   |       |          |              |            | truncated to 96    |
   |       |          |              |            | bits               |
   |     3 | SHA-1    | HMAC-SHA-1   | ECDSA_LOW  | ECDSA_LOW HI       |
   |       |          |              |            | hashed with SHA-1, |
   |       |          |              |            | truncated to 96    |
   |       |          |              |            | bits               |
   +-------+----------+--------------+------------+--------------------+

                           Table 11: HIT Suites



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   The hash of the responder as defined in the HIT Suite determines the
   HMAC to be used for the HMAC parameter.  The HMACs currently defined
   here are HMAC-SHA-256 [RFC4868], HMAC-SHA-384 [RFC4868], and HMAC-
   SHA-1 [RFC2404].

Authors' Addresses

   Robert Moskowitz (editor)
   Verizon Telcom and Business
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA
   USA

   EMail: robert.moskowitz@verizonbusiness.com


   Tobias Heer
   RWTH Aachen University, Communication and Distributed Systems Group
   Ahornstrasse 55
   Aachen  52062
   Germany

   EMail: heer@cs.rwth-aachen.de
   URI:   http://www.comsys.rwth-aachen.de/team/tobias-heer/


   Petri Jokela
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   EMail: petri.jokela@nomadiclab.com


   Thomas R. Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   EMail: thomas.r.henderson@boeing.com









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