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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 15 16 17 18 19 20 RFC 7401

Network Working Group                                  R. Moskowitz, Ed.
Internet-Draft                                                 ICSA labs
Obsoletes: 5201 (if approved)                                  P. Jokela
Intended status: Standards Track            Ericsson Research NomadicLab
Expires: April 26, 2011                                     T. Henderson
                                                      The Boeing Company
                                                                 T. Heer
                                                 RWTH Aachen University,
                                               Distributed Systems Group
                                                        October 23, 2010


                         Host Identity Protocol
                     draft-ietf-hip-rfc5201-bis-03

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 to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

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



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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on April 26, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   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 Identifier (HI) and Its Structure  . . . . . . . . . . .   9
     3.1.  Host Identity Tag (HIT) . . . . . . . . . . . . . . . . .  10
     3.2.  Generating a HIT from an HI . . . . . . . . . . . . . . .  11



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   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Creating a HIP Association  . . . . . . . . . . . . . . .  12
       4.1.1.  HIP Puzzle Mechanism  . . . . . . . . . . . . . . . .  14
       4.1.2.  Puzzle Exchange . . . . . . . . . . . . . . . . . . .  15
       4.1.3.  Authenticated Diffie-Hellman Protocol with DH
               Group Negotiation . . . . . . . . . . . . . . . . . .  16
       4.1.4.  HIP Replay Protection . . . . . . . . . . . . . . . .  17
       4.1.5.  Refusing a HIP Exchange . . . . . . . . . . . . . . .  18
       4.1.6.  Aborting a HIP Exchange . . . . . . . . . . . . . . .  19
       4.1.7.  HIP Downgrade Protection  . . . . . . . . . . . . . .  19
       4.1.8.  HIP Opportunistic Mode  . . . . . . . . . . . . . . .  20
     4.2.  Updating a HIP Association  . . . . . . . . . . . . . . .  23
     4.3.  Error Processing  . . . . . . . . . . . . . . . . . . . .  23
     4.4.  HIP State Machine . . . . . . . . . . . . . . . . . . . .  24
       4.4.1.  Timespan Definitions  . . . . . . . . . . . . . . . .  25
       4.4.2.  HIP States  . . . . . . . . . . . . . . . . . . . . .  25
       4.4.3.  HIP State Processes . . . . . . . . . . . . . . . . .  26
       4.4.4.  Simplified HIP State Diagram  . . . . . . . . . . . .  33
     4.5.  User Data Considerations  . . . . . . . . . . . . . . . .  35
       4.5.1.  TCP and UDP Pseudo-Header Computation for User Data .  35
       4.5.2.  Sending Data on HIP Packets . . . . . . . . . . . . .  35
       4.5.3.  Transport Formats . . . . . . . . . . . . . . . . . .  35
       4.5.4.  Reboot, Timeout, and Restart of HIP . . . . . . . . .  35
     4.6.  Certificate Distribution  . . . . . . . . . . . . . . . .  36
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  36
     5.1.  Payload Format  . . . . . . . . . . . . . . . . . . . . .  36
       5.1.1.  Checksum  . . . . . . . . . . . . . . . . . . . . . .  37
       5.1.2.  HIP Controls  . . . . . . . . . . . . . . . . . . . .  38
       5.1.3.  HIP Fragmentation Support . . . . . . . . . . . . . .  38
     5.2.  HIP Parameters  . . . . . . . . . . . . . . . . . . . . .  39
       5.2.1.  TLV Format  . . . . . . . . . . . . . . . . . . . . .  42
       5.2.2.  Defining New Parameters . . . . . . . . . . . . . . .  44
       5.2.3.  R1_COUNTER  . . . . . . . . . . . . . . . . . . . . .  45
       5.2.4.  PUZZLE  . . . . . . . . . . . . . . . . . . . . . . .  46
       5.2.5.  SOLUTION  . . . . . . . . . . . . . . . . . . . . . .  47
       5.2.6.  DIFFIE_HELLMAN  . . . . . . . . . . . . . . . . . . .  48
       5.2.7.  HIP_CIPHER  . . . . . . . . . . . . . . . . . . . . .  49
       5.2.8.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  50
       5.2.9.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  52
       5.2.10. DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . .  53
       5.2.11. HIP_MAC . . . . . . . . . . . . . . . . . . . . . . .  54
       5.2.12. HIP_MAC_2 . . . . . . . . . . . . . . . . . . . . . .  54
       5.2.13. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . .  55
       5.2.14. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . .  56
       5.2.15. SEQ . . . . . . . . . . . . . . . . . . . . . . . . .  56
       5.2.16. ACK . . . . . . . . . . . . . . . . . . . . . . . . .  57
       5.2.17. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . .  58
       5.2.18. NOTIFICATION  . . . . . . . . . . . . . . . . . . . .  59



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       5.2.19. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . .  63
       5.2.20. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . .  63
       5.2.21. ECHO_RESPONSE_SIGNED  . . . . . . . . . . . . . . . .  64
       5.2.22. ECHO_RESPONSE_UNSIGNED  . . . . . . . . . . . . . . .  65
     5.3.  HIP Packets . . . . . . . . . . . . . . . . . . . . . . .  65
       5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  66
       5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  67
       5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  70
       5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  71
       5.3.5.  UPDATE - the HIP Update Packet  . . . . . . . . . . .  71
       5.3.6.  NOTIFY - the HIP Notify Packet  . . . . . . . . . . .  72
       5.3.7.  CLOSE - the HIP Association Closing Packet  . . . . .  73
       5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet . .  73
     5.4.  ICMP Messages . . . . . . . . . . . . . . . . . . . . . .  74
       5.4.1.  Invalid Version . . . . . . . . . . . . . . . . . . .  74
       5.4.2.  Other Problems with the HIP Header and Packet
               Structure . . . . . . . . . . . . . . . . . . . . . .  74
       5.4.3.  Invalid Puzzle Solution . . . . . . . . . . . . . . .  74
       5.4.4.  Non-Existing HIP Association  . . . . . . . . . . . .  75
   6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  75
     6.1.  Processing Outgoing Application Data  . . . . . . . . . .  75
     6.2.  Processing Incoming Application Data  . . . . . . . . . .  76
     6.3.  Solving the Puzzle  . . . . . . . . . . . . . . . . . . .  77
     6.4.  HIP_MAC and SIGNATURE Calculation and Verification  . . .  79
       6.4.1.  HMAC Calculation  . . . . . . . . . . . . . . . . . .  79
       6.4.2.  Signature Calculation . . . . . . . . . . . . . . . .  81
     6.5.  HIP KEYMAT Generation . . . . . . . . . . . . . . . . . .  83
     6.6.  Initiation of a HIP Exchange  . . . . . . . . . . . . . .  84
       6.6.1.  Sending Multiple I1s in Parallel  . . . . . . . . . .  85
       6.6.2.  Processing Incoming ICMP Protocol Unreachable
               Messages  . . . . . . . . . . . . . . . . . . . . . .  86
     6.7.  Processing Incoming I1 Packets  . . . . . . . . . . . . .  86
       6.7.1.  R1 Management . . . . . . . . . . . . . . . . . . . .  87
       6.7.2.  Handling Malformed Messages . . . . . . . . . . . . .  88
     6.8.  Processing Incoming R1 Packets  . . . . . . . . . . . . .  88
       6.8.1.  Handling Malformed Messages . . . . . . . . . . . . .  90
     6.9.  Processing Incoming I2 Packets  . . . . . . . . . . . . .  90
       6.9.1.  Handling Malformed Messages . . . . . . . . . . . . .  93
     6.10. Processing Incoming R2 Packets  . . . . . . . . . . . . .  93
     6.11. Sending UPDATE Packets  . . . . . . . . . . . . . . . . .  94
     6.12. Receiving UPDATE Packets  . . . . . . . . . . . . . . . .  95
       6.12.1. Handling a SEQ Parameter in a Received UPDATE
               Message . . . . . . . . . . . . . . . . . . . . . . .  96
       6.12.2. Handling an ACK Parameter in a Received UPDATE
               Packet  . . . . . . . . . . . . . . . . . . . . . . .  96
     6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . .  97
     6.14. Processing CLOSE Packets  . . . . . . . . . . . . . . . .  97
     6.15. Processing CLOSE_ACK Packets  . . . . . . . . . . . . . .  98



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     6.16. Handling State Loss . . . . . . . . . . . . . . . . . . .  98
   7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . .  98
   8.  Changes from RFC 5201 . . . . . . . . . . . . . . . . . . . .  99
     8.1.  Changes from draft-ietf-hip-rfc5201-bis-02  . . . . . . .  99
     8.2.  Changes from draft-ietf-hip-rfc5201-bis-01  . . . . . . .  99
     8.3.  Changes from draft-ietf-hip-rfc5201-bis-00  . . . . . . . 101
     8.4.  Contents of draft-ietf-hip-rfc5201-bis-00 . . . . . . . . 101
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . 101
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 103
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 106
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . 107
     12.1. Normative References  . . . . . . . . . . . . . . . . . . 107
     12.2. Informative References  . . . . . . . . . . . . . . . . . 109
   Appendix A.  Using Responder Puzzles  . . . . . . . . . . . . . . 110
   Appendix B.  Generating a Public Key Encoding from an HI  . . . . 112
   Appendix C.  Example Checksums for HIP Packets  . . . . . . . . . 112
     C.1.  IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 113
     C.2.  IPv4 HIP Packet (I1)  . . . . . . . . . . . . . . . . . . 113
     C.3.  TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 113
   Appendix D.  ECDH-160 Group . . . . . . . . . . . . . . . . . . . 114
   Appendix E.  HIT Suites and HIT Generation  . . . . . . . . . . . 114






























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1.  Introduction

   This memo 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 [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
   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)" [RFC5202]: how to use the
      Encapsulating Security Payload (ESP) for integrity protection and
      optional encryption

   o  "End-Host Mobility and Multihoming with the Host Identity
      Protocol" [RFC5206]: how to support mobility and multihoming in
      HIP

   o  "Host Identity Protocol (HIP) Domain Name System (DNS) Extensions"
      [RFC5205]: how to extend DNS to contain Host Identity information

   o  "Host Identity Protocol (HIP) Rendezvous Extension" [RFC5204]:
      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 switch from one
   cryptographic primitive to another, and reasonable effort should be
   done to support a reasonable set of mainstream algorithms.  This
   update to the Base Exchange adds this needed cryptographic agility
   while addressing the downgrade attacks that such flexibility enables.
   In particular, Elliptic Curve support (ECDSA and ECDH) and
   alternative hash functions have been added.



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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 [rfc4423-bis].

   There are two main representations of the Host Identity, the full
   Host Identifier (HI) and the Host Identity Tag (HIT).  The HI is a
   public key and directly represents the Identity.  Since there are
   different public key algorithms that can be used with different key
   lengths, the HI is not good for use as a packet identifier, or as an
   index into the various operational tables needed to support HIP.
   Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes
   the operational representation.  It is 128 bits long and is used in
   the HIP payloads 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 four-
   packet design helps to make HIP DoS resilient.  The protocol
   exchanges Diffie-Hellman keys in the 2nd and 3rd packets, and
   authenticates the parties in the 3rd and 4th packets.  Additionally,
   the Responder starts a 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 verify their identities.

   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.



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   Finally, HIP is designed as an end-to-end authentication and key
   establishment protocol, to be used with Encapsulated Security Payload
   (ESP) [RFC5202] 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 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 detail
   packet formats and rules for packet processing.  Finally, Sections 7,
   9, and 10 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. `

   <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.






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2.3.  Definitions

   Host Identity (HI)  The Host Identity is the public key of a
      signature algorithm and 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)  The Host Identity Tag is 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).

   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).

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

3.  Host Identifier (HI) and Its Structure

   In this section, the properties of the Host Identifier and Host
   Identifier 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 Identifier (HI).  Correspondingly, the host itself is
   defined as the entity that holds the private key from the key pair.
   See the HIP architecture specification [rfc4423-bis] for more details
   about the difference between an identity and the corresponding
   identifier.

   HIP implementations MUST support the Rivest Shamir Adelman (RSA)
   [RFC3110] public key algorithm, and SHOULD support the Digital
   Signature Algorithm (DSA) [RFC2536] algorithms, and Elliptic Curve
   Digital Signature Algorithm (ECDSA) Section 5.2.8, ECDSA description;
   other 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 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



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   HIT), and iii) the probability of 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 [rfc4423-bis].

   The structure of the HIT is defined in [RFC4843-bis].  The HIT
   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
   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 Host Identity public key in protocols.  Firstly, its
   fixed length makes for easier protocol coding and also better manages
   the packet size cost of this technology.  Secondly, it presents a
   consistent format to the protocol whatever underlying identity
   technology is used.

   RFC 4843-bis [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
   [RFC4843-bis].  The Host Identity Tag is a type of ORCHID.

   This document extends [RFC5201] with measures to support crypto
   agility.  One of these measures is to allow for different hash
   functions for creating a HIT.  HIT Suites group 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 field in the ORCHID.  The HIT Suite
   ID in the OGA field enables a hosts tell from another host's HIT,
   whether it can successfully establish a HIP association with that
   host.




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3.2.  Generating a HIT from an HI

   The HIT MUST be generated according to the ORCHID generation method
   described in [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 5.2.8) present in a HIP
   payload packet.  The class of hash function, signature algorithm, 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 [FIPS.95-1.1993] 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 in the corresponding
   DNSSEC documents, taking the algorithm-specific portion of the RDATA
   part of the KEY RR.  There are currently only two defined public key
   algorithms: RSA/SHA-1 and DSA.  Hence, either 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.  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 key is encoded as defined in [fundamental-ecc]
      Section 4.2 and 6.

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

4.  Protocol Overview

   The following material is an overview of the HIP protocol operation,
   and does not contain all details of the packet formats or the packet



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   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.

   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 [RFC5202].

4.1.  Creating a HIP Association

   By definition, the system initiating a HIP exchange is the Initiator,
   and the peer is the Responder.  This distinction is 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 then are 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.  If other cryptographic keys are
   needed, e.g., to be used with ESP, they are expected to be drawn from
   the same keying material.

   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 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.




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   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
   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.  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 also contains a Diffie-Hellman parameter that
   carries needed information for the Responder.  The packet is signed
   by the sender.

   The R2 packet acknowledges the receipt of the I2 and finalizes the
   base exchange.  The packet is signed.

   The base exchange is illustrated below.  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








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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 I2.  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 churned CPU
   cycles in solving the puzzle.

   The puzzle mechanism has been explicitly designed to give space for
   various implementation options.  It allows a Responder implementation
   to completely delay session-specific state creation until a valid I2
   is received.  In such a case, a correctly formatted I2 can be
   rejected only once the Responder has checked its validity by
   computing one hash function.  On the other hand, the design also
   allows a Responder implementation to keep state about received I1s,
   and match the received I2s against the state, thereby allowing the
   implementation to avoid the computational cost of the hash function.
   The drawback of this latter approach is the requirement of creating
   state.  Finally, it also allows an implementation to use other
   combinations of the space-saving and computation-saving mechanisms.

   The Responder can remain stateless and drop most spoofed I2s 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 I1.  When the Responder then later receives
   I2, 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, and then generate a large number of spoofed I2s
   that seemingly come from different HITs.  The method does not protect
   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.  Implementations SHOULD
   include sufficient randomness to the algorithm so that algorithmic
   complexity attacks become impossible [CRO03].

   The Responder can set the puzzle difficulty for 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; see below.






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4.1.2.  Puzzle Exchange

   The Responder starts the puzzle exchange when it receives an I1.  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 take 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
   Initiator SHOULD give up after exceeding the puzzle lifetime in the
   PUZZLE parameter (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 did 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 (Section 5.2.4), in an
   ECHO_REQUEST_SIGNED (Section 5.2.19) or in an ECHO_REQUEST_UNSIGNED
   parameter (Section 5.2.20), the Responder can include some data in R1
   that the Initiator must copy unmodified in the corresponding I2
   packet.  The Responder can generate the Opaque data in various ways;
   e.g., using encryption or hashing with some secret, the sent I, and
   possibly other related data.  Using the same secret, the received I
   (from the I2), and the other related data (if any), the Receiver can
   verify that it has itself sent the I to the Initiator.  The Responder
   MUST periodically change such a used 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.  These time values guarantee 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.

   The puzzle value I and the solution J are inputs for deriving the
   keying material from the Diffie Hellman key exchange (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 additional input for generating I.



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   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 (Section 5.2.4), in an ECHO_REQUEST_SIGNED
   (Section 5.2.19) or in an ECHO_REQUEST_UNSIGNED parameter
   (Section 5.2.20) 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.

   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.  At
   the time of the decision, the idea of memory-bound functions was
   relatively new and their IPR status were unknown.  Once there is more
   experience about memory-bound functions and once their IPR status is
   better known, it may be reasonable to reconsider this decision.

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 R1 allows the Initiator to verify
   that the R1 has been once generated by the Responder.  However, since
   it 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 on the Responder's
   side.  Based on the preferences of the Initiator, the Responder sends
   an R1 packet containing its most suitable public DH value.  It also
   attaches a list of its own preferences to the R1 to convey the basis
   for the DH group selection to the Initiator.

   If none of the DH Group IDs in the I1 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 gets 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



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   packet to avoid forgery.  The Initiator compares the Group ID of the
   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 matches the
   preferences of both Initiator and Responder.  Otherwise, the
   communication is subject of a downgrade attack and the Initiator must
   restart the key exchange with a new I1 packet or must abort the key
   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 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).  It creates a HIP
   association using keying material from the session key (see
   Section 6.5), and may use the association to encrypt its public
   authentication key, i.e., Host Identity.  The resulting I2 contains
   the Initiator's Diffie-Hellman key and its (optionally encrypted)
   public authentication key.  The signature in I2 covers all of the
   packet.

   The Responder extracts the Initiator Diffie-Hellman public key from
   the I2, 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, is needed to protect the Initiator from replay
   attacks.

4.1.4.  HIP Replay Protection

   The HIP protocol includes the following mechanisms to protect against
   malicious replays.  Responders are protected against replays of I1
   packets by virtue of the stateless response to I1s with presigned R1
   messages.  Initiators are protected against R1 replays by a
   monotonically increasing "R1 generation counter" included in the R1.
   Responders are protected against replays or false I2s by the puzzle
   mechanism (Section 4.1.1 above), and optional use of opaque data.
   Hosts are protected against replays to R2s and UPDATEs by use of a
   less expensive HMAC verification preceding HIP signature
   verification.

   The R1 generation counter is a monotonically increasing 64-bit
   counter that may be initialized to any value.  The scope of the



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   counter MAY be system-wide but SHOULD be per Host Identity, if there
   is more than one local host identity.  The value of this counter
   SHOULD be kept 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, and SHOULD never be decremented, lest the host
   expose 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
   I1s (Section 6.6.1) or due to a replay of an old R1.  When sending
   multiple I1s, 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 (still
   waiting for R2) 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 Exchange

   A HIP-aware host may choose not to accept a HIP exchange.  If the
   host's policy is to only be an Initiator, it should begin its own HIP
   exchange.  A host MAY choose to have such a policy since only the
   Initiator's HI is protected in the exchange.  There is a risk of a
   race condition if each host's policy is to only be an Initiator, at
   which point the HIP 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
   a simple ICMP message.







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4.1.6.  Aborting a HIP Exchange

   Two HIP hosts may encounter situations in which they cannot complete
   a HIP exchange because of insufficient suport 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 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, 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 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 selected by the Initiator,
   the Responder selects an arbitrary DH and replies an R1 containing
   its list of supported DH Group IDs.  In this case, the Initiator will
   receive an R1 packet containing the DH public value for an
   unsupported DH Group and 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 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 manipulates the packets of an
   Initiator and/or a Responder to unnoticeably 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
   have without the attacker's interference.  Downgrade attacks can only
   be successful if these are not detected by the victims and the
   victims assume a secure communication channel.

   In HIP, almost all packet parameters related to cryptographic
   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 selection of cryptographic algorithms and replay it



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   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 packet will
   lead to invalid keying material and mismatching shared secrets.

   In contrast to the first version of HIP [RFC5201], this version
   begins the negotiation of the DH Groups already in the first BEX
   packet, the I1.  The I1 is, by intention, not protected by a
   signature to avoid CPU-intensive cryptographic operations for
   processing floods of I1s.  Hence, the list of DH Group IDs in the I1
   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.  If the choice of the DH
   Group in the R1 packet does not equal the best match of the two
   lists, the Initiator can conclude that its list in the I1 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 negotiation even if the Responder's
   HI (and HIT) is unknown.  In this case, the connection initializing
   I1 packet contains NULL (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 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 use a Responder HIT of the same HIT Suite as 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



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   unverified R1 besides the HIT Suite List.  Moreover, an Initiator
   that uses a unverified HIT Suite List to determine a possible source
   HIT from an R1 packet 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 with a source
   HIT mentioned in the verifiable R1 if the two lists do not match to
   mitigate downgrade attacks.

   There are both security and API issues involved with the
   opportunistic mode.

   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 kernel initiate 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, it 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 specific locators.  The locator update is still secure,
      however, and the session is still between the same nodes.

   o  Different sessions between the same locators may result in
      connections to different nodes, if the implementation no longer
      remembers which identifier the peer had in another 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, HIP
      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 exchanges, the handling of locator-based or
      interface-based policy is unclear for opportunistic mode HIP.  An
      application may make 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



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      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
      [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 any 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 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.

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

   However, once the opportunistic exchange has successfully completed,
   HIP provides integrity protection and confidentiality 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.






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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 user data 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
   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 is 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.




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         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 I1 and getting R1.  When the
         Responder receives a valid I2, 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 negotiation.  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
   processes.

   The state machine is presented in a single system view, representing
   either an Initiator or a Responder.  There is not a complete overlap
   of processing logic here and in the packet definitions.  Both are
   needed to completely implement HIP.

   This document extends the state machine defined in [RFC5201] and
   introduces a restart option to allow for the negotiation of
   cryptographic algorithms.  The only change to the previous state
   machine is a transition from state I1-SENT to I1-SENT - the restart
   option.  An Initiator is required to restart the HIP exchange if the
   Responder does not support the HIT Suite of the Initiator.  In this



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   case, the Initiator restarts the HIP 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 functions 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.  Other states may be
   introduced by mechanisms in other specifications (such as mobility
   and multihoming).

4.4.1.  Timespan Definitions

   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 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
      before it moves to ESTABLISHED state.  The time is n * I2
      retransmission timeout, where n is about I2_RETRIES_MAX.

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 exchange failed                         |
   +---------------------+---------------------------------------------+

                            Table 1: HIP States



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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 unknown HIP     | Section 5.4 and stay at UNASSOCIATED        |
   | association         |                                             |
   |                     |                                             |
   | Receive CLOSE       | Optionally send ICMP Parameter Problem and  |
   |                     | stay at UNASSOCIATED                        |
   |                     |                                             |
   | Receive ANYOTHER    | Drop and stay at UNASSOCIATED               |
   +---------------------+---------------------------------------------+

                    Table 2: UNASSOCIATED - Start state























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   System behavior in state I1-SENT, Table 3.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | If the local HIT is smaller than the peer   |
   |                     | HIT, drop I1 and stay at I1-SENT            |
   |                     |                                             |
   |                     | 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 HIT Suite of own HIT is not supported by |
   |                     | the peer, select supported own 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  | If counter is less than I1_RETRIES_MAX,     |
   | timeout counter     | send I1 and stay at I1-SENT                 |
   |                     |                                             |
   |                     | If counter is greater than I1_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

                     Table 3: I1-SENT - Initiating HIP


















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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 cycle 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  | If counter is less than I2_RETRIES_MAX,     |
   | timeout counter     | 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 HIP













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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 cycle 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 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 | If successful, send R2, drop old HIP        |
   | with puzzle and     | association, establish a new HIP            |
   | possible Opaque     | association, go to R2-SENT                  |
   | data verification   |                                             |
   |                     |                                             |
   |                     | 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  | If timeout sum is less than UAL+MSL         |
   | timeout sum, reset  | minutes, retransmit CLOSE and stay at       |
   | timer               | 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 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       |  |
      |          +-------------+ |                 (UAL+MSL)        |  |
      |     recv CLOSE,        | |                                  |  |
      |     send CLOSE_ACK     v v                                  |  |
      |                     +--------+  receive I2, send R2         |  |
      +---------------------| CLOSED |------------------------------+  |
                            +--------+                                 |
                             ^ |  |                                    |
   recv CLOSE, send CLOSE_ACK| |  |              timeout (UAL+2MSL)    |
                             +-+  +------------------------------------+



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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 on the packet
   are IPv4 addresses.  Additionally, the HITs MUST be used in the 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

   A future version of this document may define how to include user data
   on 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 [RFC5202].

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 HIP 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
   the implementation and the environment where the implementation is
   used.

   If the host, that apparently has lost its state, decides to restart



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   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.  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 |  VER. | RES.|1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum             |           Controls            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Sender's Host Identity Tag (HIT)               |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Receiver's Host Identity Tag (HIT)              |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                        HIP Parameters                         /
   /                                                               /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   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 do so.  However, current implementations MUST



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   ignore trailing data if an unimplemented Next Header value is
   received.

   The Header Length field contains the 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 unknown packet type, it MUST drop the
   packet.

   The HIP Version is four bits.  The current version 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.

   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 potential SHIM6
   compatibility [RFC5533].  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, in the case
   that the forthcoming SHIM6 protocol happens to be compatible with
   this specification, 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.

   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



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   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 section 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 R1 and/or
      I2.  The peer receiving an anonymous HI may choose to refuse it.

   The rest of the fields are reserved for future use and MUST be set to
   zero on sent packets and ignored on 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 HIP does not 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 MUST perform any IPv4 reassembly/fragmentation.




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

   Because certificate chains can cause the packet to be fragmented and
   fragmentation can open implementation to denial-of-service attacks
   [KAU03], it is strongly recommended that the separate document
   specifying the certificate usage in the HIP Base Exchange defines the
   usage of "Hash and URL" formats rather than including certificates in
   exchanges.  With this, most problems related to DoS attacks with
   fragmentation can be avoided.

5.2.  HIP Parameters

   The HIP Parameters are used to carry the public key associated with
   the sender's HIT, together with related security and other
   information.  They consist of ordered parameters, encoded in TLV
   format.

   The following parameter types are currently defined.































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   +------------------------+-------+----------+-----------------------+
   | TLV                    | Type  | Length   | Data                  |
   +------------------------+-------+----------+-----------------------+
   | R1_COUNTER             | 128   | 12       | System Boot 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                |
   |                        |       |          |                       |
   | DIFFIE_HELLMAN         | 513   | variable | public key            |
   |                        |       |          |                       |
   | HIP_CIPHER             | 579   | variable | HIP encryption        |
   |                        |       |          | algorithm             |
   |                        |       |          |                       |
   | ENCRYPTED              | 641   | variable | Encrypted part of I2  |
   |                        |       |          | 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. Usage   |
   |                        |       |          | is currently not      |
   |                        |       |          | defined, but it will  |
   |                        |       |          | be specified in a     |
   |                        |       |          | separate document     |
   |                        |       |          | once needed.          |
   |                        |       |          |                       |
   | NOTIFICATION           | 832   | variable | Informational data    |
   |                        |       |          |                       |
   | ECHO_REQUEST_SIGNED    | 897   | variable | Opaque data to be     |
   |                        |       |          | echoed back; under    |
   |                        |       |          | signature             |



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   | ECHO_RESPONSE_SIGNED   | 961   | variable | Opaque data echoed    |
   |                        |       |          | back; under signature |
   |                        |       |          |                       |
   | DH_GROUP_LIST          | 2151  | variable | Ordered list of DH    |
   |                        |       |          | Group IDs supported   |
   |                        |       |          | by a host             |
   |                        |       |          |                       |
   | 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. Compared |
   |                        |       |          | to HIP_MAC, the       |
   |                        |       |          | HOST_ID parameter is  |
   |                        |       |          | included in HIP_MAC_2 |
   |                        |       |          | calculation.          |
   |                        |       |          |                       |
   | HIP_SIGNATURE_2        | 61633 | variable | Signature of the 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; after signature |
   +------------------------+-------+----------+-----------------------+

   Because 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
   extensions.

   The following parameter type number ranges are defined.










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   +---------------+---------------------------------------------------+
   | Type Range    | Purpose                                           |
   +---------------+---------------------------------------------------+
   |     0 -  1023 | Handshake                                         |
   |               |                                                   |
   |  1024 -  2047 | Reserved                                          |
   |               |                                                   |
   |  2048 -  8191 | Signed parameters allocated through specification |
   |               | documents                                         |
   |               |                                                   |
   |  8192 - 32767 | Reserved                                          |
   |               |                                                   |
   | 32768 - 49151 | Free for experimentation. Signed parameters.      |
   |               |                                                   |
   | 41952 - 61439 | Reserved                                          |
   |               |                                                   |
   | 61440 - 62463 | 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, except for type values from 2048 to 4095 which
   are reserved for new transport forms.  The parameters MUST be
   included in the packet such that their types form an increasing
   order.  If the parameter can exist multiple times in the packet, the
   type value may be the same in consecutive parameters.  If the order
   does not 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 transport
   formats.  Currently, one transport format is defined: the ESP



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   transport format [RFC5202].  The order of these parameters does not
   follow the order of their type value, but they are put in the packet
   in order of preference.  The first of the transport formats it the
   most preferred, and so on.

   All of the TLV parameters have a length (including 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 becomes
   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.

   Consequently, the Length field indicates the length of the Contents
   field (in bytes).  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 MUST be recognized by the recipient.  If a
   recipient encounters a critical 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.



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   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 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 recipient ignoring
       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.  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 10 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           128
     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 is supposed to 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.

   The R1_COUNTER parameter is optional.  It SHOULD be included in the
   R1 (in which case, it is covered by the signature), and if present in
   the R1, it MAY be echoed (including the Reserved field verbatim) by
   the Initiator in the I2.





















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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, n 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
   puzzle 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, 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, n 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 integers (where n is
   RHASH_len), K as an 8-bit 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.  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
     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                Value
    Reserved             0
    DEPRECATED           1
    DEPRECATED           2
    1536-bit MODP group  3 [RFC3526]
    3072-bit MODP group  4 [RFC3526]
    DEPRECATED           5
    DEPRECATED           6
    160-bit rnd. ECP grp 7 [App.  D,draft-mcgrew-fundamental-ecc-02.txt]
    256-bit rnd. ECP grp 8 [RFC4753,draft-mcgrew-fundamental-ecc-02.txt]
    384-bit rnd. ECP grp 9 [RFC4753,draft-mcgrew-fundamental-ecc-02.txt]
    521-bit rnd. ECP grp 10[RFC4753,draft-mcgrew-fundamental-ecc-02.txt]


   The MODP Diffie-Hellman groups are defined in [RFC3526].  The ECDH
   groups 8 - 10 are defined in [RFC4753] and [fundamental-ecc].  ECDH
   group 7 is covered in Appendix D.

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



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   the groups SHOULD be implemented in hosts where resources are
   adequate.

5.2.7.  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 parts of the HIP packet

   The following Cipher IDs are defined:

        Suite ID                          Value

        RESERVED                          0
        NULL-ENCRYPT                      1     ([RFC2410])
        AES-128-CBC                       2     ([RFC3602])
        3DES-CBC                          3     ([RFC2451])
        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
   preference in the R1, up to the maximum of six Cipher IDs.  The
   Initiator MUST choose only one of the corresponding Cipher IDs.  That
   Cipher ID will be used for generating the ENCRYPTED parameter.

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



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   for testing purposes.  NULL-ENCRYPTION SHOULD NOT be configurable via
   the UI.

5.2.8.  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        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         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 FQDN or NAI in octets
     Host Identity     actual Host Identity
     Domain Identifier the identifier of the sender

   The Host Identity is represented in the DNSKEY format for RSA and
   DSA.  For these, the Public Key field from RFC 4034 [RFC4034] is
   used.  For ECC Host Identities this field is defined here directly.

        Algorithms       Values

        RESERVED         0
        DSA              3 [RFC2536] (RECOMMENDED)
        RSA              5 [RFC3110] (REQUIRED)
        ECDSA            7 [fundamental-ecc] (RECOMMENDED)

   For ECDSA the Host Identity is represented by the following fields:











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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          ECC Curve            |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Public Key                            /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Required ECC Curve values are:

        Curve            Values

        RESERVED         0
        NIST-ECDSA-256   1 [RFC4754]
        NIST-ECDSA-384   2 [RFC4754]
        brainpoolP160r1  3 [RFC5639]

   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 NAI is defined in [RFC4282]

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















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5.2.9.  HIT_SUITE_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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     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 correspond to the HIT Suite ID in
                    the ORCHID OGA field. The four lower-order bits are
                    set to 0.

   The ID field in the HIT_SUITE_LIST is defined as eight-bit field
   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
   the 16 available values prove insufficient.  In that case, one of the
   16 values (0) 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-1           1    (REQUIRED)
        ECDSA/SHA-256           2    (RECOMMENDED)
        ECDSA/SHA-384           3    (RECOMMENDED)

   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.







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5.2.10.  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           2151
     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 it 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.  DH Group IDs that are listed first are
   preferred compared to 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 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.

   When selecting the DH group for the DIFFIE_HELLMAN parameter in the
   R1 packet, the Responder MUST select the first DH Group ID in the
   Responder's DH_GROUP_LIST that is contained in the Initiator's
   DH_GROUP_LIST.













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5.2.11.  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.

5.2.12.  HIP_MAC_2

   The parameter structure is the same as in Section 5.2.11.  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 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.

5.2.13.  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.
                    The checksum field MUST be set to zero, and the HIP
                    header length in the HIP common header MUST be
                    calculated only to the beginning of the
                    HIP_SIGNATURE parameter when the signature is
                    calculated.

   The signature algorithms are defined in Section 5.2.8.  The signature
   in the Signature field is encoded using the proper 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



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   to [RFC2536] in case of DSA, or according to [fundamental-ecc] in
   case of ECDSA).

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

5.2.14.  HIP_SIGNATURE_2

   The parameter structure is the same as in Section 5.2.13.  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 in
   Section 6.4.2.

5.2.15.  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         4
     Update ID      32-bit sequence number



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   The Update ID is an unsigned quantity, 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.

5.2.16.  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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type            449
     Length          variable (multiple of 4)
     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 Length field identifies the number of
   peer Update IDs that are present in the parameter.

























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5.2.17.  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 an encryption algorithm
       data         as defined in the HIP_CIPHER parameter.

   The ENCRYPTED parameter encapsulates another parameter, 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 labelled "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 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.

5.2.18.  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 the NOTIFY packet type.  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).
     Padding        any Padding, if necessary, to make the parameter a
                    multiple of 8 bytes.

   Notification information can be error messages specifying why an SA
   could not be established.  It can also be status data that a process
   managing an SA database wishes to communicate with a peer process.
   The table below lists the Notification messages and their
   corresponding values.

   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.

   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 NOTIFICATION
   error parameter 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.

   Notify payloads with status types MUST be ignored if not recognized.

     NOTIFICATION PARAMETER - ERROR TYPES     Value
     ------------------------------------     -----




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     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 rejected for policy reasons.  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.

     AUTHENTICATION_FAILED                     24

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




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     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).

     SERVER_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 MESSAGES - STATUS TYPES           Value
     ------------------------------           -----

     I2_ACKNOWLEDGEMENT                       16384

       The Responder has an I2 from the Initiator but had to queue the
       I2 for processing.  The puzzle was correctly solved and the
       Responder is willing to set up an association but currently has a
       number of I2s in the processing queue.  R2 will be sent after the
       I2 has been processed.








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5.2.19.  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       variable
     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 or
   ECHO_REQUEST_UNSIGNED parameter.  The ECHO_REQUEST_SIGNED parameter
   MUST be responded to with a corresponding echo response.
   ECHO_RESPONSE_SIGNED SHOULD be used, but if it is not possible, e.g.,
   due to a middlebox-provided response, it MAY be responded to with an
   ECHO_RESPONSE_UNSIGNED.

5.2.20.  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       variable
     Opaque data  opaque data, supposed to be meaningful only to the
                  node that sends ECHO_REQUEST_UNSIGNED and receives a
                  corresponding ECHO_RESPONSE_UNSIGNED.




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

5.2.21.  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       variable
     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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5.2.22.  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       variable
     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 numbers

   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 will 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 used 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 gets the Responder's HIT either from a DNS lookup of
   the Responder's FQDN, 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 this 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 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 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 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 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.  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 I1 was
   manipulated by an attacker.

   The Diffie-Hellman public value is ephemeral, and one value SHOULD be
   used only for one connection.  Once the Responder has received a
   valid response to an R1 packet, that Diffie-Hellman value SHOULD be
   deprecated.  Because it is possible that the Responder has sent the
   same Diffie-Hellman value to different hosts simultaneously in
   corresponding R1 packets, those responses should also be accepted.
   However, as a defense against I1 storms, an implementation MAY
   propose, and re-use if not 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 keys 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 key 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
   implementations 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 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 storms and to simultaneously make the probability
   of DH key re-use both statistical and as low as possible.

   If a future version of this protocol is considered, we strongly
   recommend that these issues be studied again.  Especially, the
   current design allows hosts to become potentially more vulnerable to
   a statistical, low-probability problem during I1 storm attacks than
   what they are if no attack is taking place; whether this is
   acceptable or not should be reconsidered in the light of any new
   experience gained.

   The HIP_CIPHER contains the encryption algorithms supported by the
   Responder to encrypt 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 is suitable.

   The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED contains data that
   the sender wants to receive unmodified in the corresponding response
   packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
   parameter.

   The signature is calculated over the whole HIP envelope, after
   setting the Initiator's HIT, header checksum, as well as the Opaque
   field and the Random #I in the PUZZLE parameter temporarily to zero,
   and excluding any parameters that follow the signature, as described
   in Section 5.2.14.  This allows the Responder to use precomputed R1s.
   The Initiator SHOULD validate this signature.  It SHOULD check that
   the Responder's HI received 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 previously.

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

   The Initiator MAY 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 will be used to encrypt the ENCRYPTED parameter.
   The chosen cipher MUST correspond to one offered by the Responder in
   the R1.  All implementations MUST support AES m [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
   parameter.



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   The HMAC is calculated over the whole HIP envelope, 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 envelope, excluding
   any parameters after the HIP_SIGNATURE, as described in
   Section 5.2.13.  The Responder MUST validate this signature.  It MAY
   use either the HI in the packet or the HI acquired by some other
   means.

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 envelope, with
   Responder's HOST_ID parameter concatenated with the HIP envelope.
   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 envelope.

   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 ) )

   Valid control bits: None




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   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 ACK the UPDATE.
   An UPDATE that does not contain a SEQ parameter is simply an ACK of a
   previous UPDATE and itself MUST NOT be ACKed.

   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 ACK more than one UPDATE packet at a
   time; e.g., the ACK may contain the last two SEQ values received, for
   robustness to 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.

   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

   The NOTIFY packet is OPTIONAL.  The NOTIFY packet MAY be used to
   provide information to a peer.  Typically, NOTIFY is used to 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 (Section 4.4.2)
   based purely on a received NOTIFY packet.

   The HIP header values for the NOTIFY packet:







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     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 an HIP_MAC and a signature
   (calculated over the whole HIP envelope).

   The receiver peer MUST validate both the HIP_MAC and the signature if
   it has a HIP association state, and 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:

     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




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

   The receiver peer MUST validate both the HMAC and the signature.

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 [RFC2463].  In most cases, the
   ICMP packet will have 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, the Pointer pointing
   to the VER./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
   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



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   [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, and
   with the Pointer 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.  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
   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



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   different; for example, it can be a HIT or an IP address.

   The exact format and method for transferring the 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 will be 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.

   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
       [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.





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   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 a node identity and that the actual identity
       maps to this particular HIT.  When using ESP transport format
       [RFC5202], 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.  When
       demultiplexing the datagram, the right upper-layer socket is
       based on the HITs.

6.3.  Solving the Puzzle

   This subsection describes the puzzle-solving details.

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

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

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





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

   In order to be 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.

   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.

   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








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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.

6.4.1.  HMAC Calculation

   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 Responder
   should preserve other parameters than the HOST_ID when sending the
   R2.  Also, the Initiator has to preserve the HOST_ID exactly as it
   was received in the R1 packet.

   The scope of the calculation for HIP_MAC and HIP_MAC_2 is:

   HMAC: { HIP header | [ Parameters ] }

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

   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.





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   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC_2 parameter and added to the length
      of the concatenated HOST_ID parameter length.

   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 5.2.11 and the HIP_MAC_2
   parameter in Section 5.2.12.  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.

   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 will be needed later.



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   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.

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

6.4.2.  Signature Calculation

   The following process applies both to the HIP_SIGNATURE and
   HIP_SIGNATURE_2 parameters.  When processing HIP_SIGNATURE_2, the
   only difference is that instead of 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 5.2.13 and the HIP_SIGNATURE_2 parameter in Section 5.2.14.

   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 apply:

   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.

   Parameter order is described in Section 5.2.1.

   HIP_SIGNATURE_2: { HIP header | [ Parameters ] }




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   where Parameters include all HIP parameters for the packet that is
   being calculated with Type values 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.

   Signature calculation and verification process (the process applies
   both to HIP_SIGNATURE and HIP_SIGNATURE_2 except in the case where
   HIP_SIGNATURE_2 is separately mentioned):

   Packet sender:

   1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
       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.

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




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   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 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 Identifier (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.

6.5.  HIP KEYMAT Generation

   HIP keying material is derived from the Diffie-Hellman session key,
   Kij, produced during the HIP base exchange (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 HKDF [RFC5869] using the
   RHASH hash function.

   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.

   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.




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   The drawing order for initial keys:

      HIP-gl encryption key for HOST_g's outgoing HIP packets

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

      HIP-lg encryption key (currently unused) for HOST_l's outgoing HIP
      packets

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

   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 Exchange

   An implementation may originate a HIP 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 contents are specified in Section 5.3.1.  The
   selection of which Host Identity to use, if a host has more than one
   to choose from, is typically a policy decision.

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





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   1.  The Initiator gets 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 table.  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 to one of the Responder's addresses.
       The selection of which address to use is a local 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, the sender transitions to state I1-SENT,
       starts a timer whose timeout value SHOULD be larger than the
       worst-case anticipated RTT, and SHOULD increment a timeout
       counter associated with the I1.

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

6.6.1.  Sending Multiple I1s in Parallel

   For the sake of minimizing the session establishment latency, an
   implementation MAY send the same I1 to more than one of the
   Responder's addresses.  However, it MUST NOT send to more than three
   (3) 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 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
   happen, e.g., because someone's claim to have hundreds or thousands
   of addresses could generate a huge number of I1 messages from the
   Initiator.

   As the Responder is not guaranteed to distinguish the duplicate I1s
   it receives at several of its addresses (because it avoids storing
   states when it answers back an R1), the Initiator may receive several
   duplicate R1s.

   The Initiator SHOULD then select the initial preferred destination



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   address using the source address of the selected received R1, and use
   the preferred address as a source address for the I2.  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
   Responder does not support HIP.

   When a system receives an ICMP 'Destination Protocol Unreachable'
   message while it is waiting for an R1, it MUST NOT terminate the
   wait.  It MAY continue as if it had not received the ICMP message,
   and send a few more I1s.  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 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 source address.  If
   the implementation is unwilling to set up a HIP association, the host
   MAY ignore the I1.  This latter case may occur during a DoS attack
   such as an I1 flood.

   The implementation MUST 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 can result in an R1 attack on a system.  An R1 sender
   MUST have a mechanism to rate-limit R1s to an address.

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

   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 is either one of its own HITs or NULL.




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   2.  If the Responder is in ESTABLISHED state, the Responder MAY
       respond to this with an R1 packet, prepare to drop existing SAs,
       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
       the I1 and stay at I1-SENT.  If the sender's HIT is smaller than
       its own HIT, it should send R1 and stay at I1-SENT.  The HIT
       comparison goes similarly as in Section 6.5.

   4.  If the implementation chooses to respond to the I1 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 an arbitrary DH public key.

   5.  The R1 MUST contain the received Responder's HIT, unless the
       received HIT is NULL, in which case the Responder SHOULD select a
       HIT that is constructed with the MUST algorithm in Section 3,
       which is currently RSA.  Other than that, selecting the HIT is a
       local policy matter.

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

6.7.1.  R1 Management

   All compliant implementations MUST 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 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 I1s and match the
   received I2s 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 message, it SHOULD NOT
   respond with a NOTIFY message, as such practice could open up a
   potential denial-of-service danger.  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 MUST first check to see if it has sent an I1
   to the originator of the R1 (i.e., it is in state I1-SENT).  If so,
   it SHOULD process the R1 as described below, send an I2, and go to
   state I2-SENT, setting a timer to protect the I2.  If the system is
   in state I2-SENT, it MAY respond to an R1 if the R1 has a larger R1
   generation counter; if so, it should drop its state due to processing
   the previous R1 and start over from state I1-SENT.  If the system is
   in any other state with respect to that host, it SHOULD silently drop
   the R1.

   When sending multiple I1s, an Initiator SHOULD wait for a small
   amount of time 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.

   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 to the originator of the R1 (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 was sent in opportunistic
        mode (see Section 4.1.8), the IP addresses in the received R1
        packet SHOULD be ignored and, when looking up the right HIP
        association, the received R1 SHOULD be matched against the
        associations using only the HITs.  If a match exists, the system
        should process the R1 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, it SHOULD
        silently drop the R1 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,
        and the I1 and the Responder's HIT MUST correspond to the one
        used, unless the I1 contained a NULL HIT.

   4.   The system SHOULD validate the R1 signature before applying
        further packet processing, according to Section 5.2.14.



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   5.   If the HIP association state is I1-SENT, and multiple valid R1s
        are present, the system MUST select from among the R1s 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 a 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 acceptable time span to allow further R1 packets with higher
        R1 generation counters to arrive before restarting or aborting
        the BEX.

   7.   The system MUST check that the DH Group ID in the DH parameter
        in the R1 matches the first DH Suite ID in the Responder's
        DH_GROUP_LIST in the R1 that was also contained in the
        Initiator's DH_GROUP_LIST in the I1.  If the two DH Group ID of
        the DH parameter does not express the Responder's best choice,
        the Initiator can conclude that the DH_GROUP_LIST in the I1 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.  Alternatively, the Initiator MAY
        abort the HIP exchange.

   8.   If the HIP association state is I2-SENT, the system MAY reenter
        state I1-SENT and process the received R1 if it has a larger R1
        generation counter than the R1 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 and returning to
        state UNASSOCIATED.  The system SHOULD consider dropping the R1
        only if it used a NULL HIT in I1.  If the A bit is set, the
        Responder's HIT is anonymous and should not be stored.

   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.

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

   12.  The system attempts to solve the puzzle in R1.  The system MUST
        terminate the search after exceeding the remaining lifetime of
        the puzzle.  If the puzzle is not successfully solved, the



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        implementation may either resend I1 within the retry bounds or
        abandon the HIP 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.  If
        the received Diffie-Hellman Group ID is not supported, the
        implementation may either resend I1 within the retry bounds or
        abandon the HIP exchange.

   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.
        If the proposed alternatives are not acceptable to the system,
        it may either resend I1 within the retry bounds or abandon the
        HIP exchange.

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

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

   17.  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.  The sender SHOULD
        retransmit the I2 upon a timeout and restart the timer, up to a
        maximum of I2_RETRIES_MAX tries.

   18.  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 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 typically doesn't have any state.  An
   implementation SHOULD wait for some more time for a possibly good R1,
   after which it MAY try again by sending a new I1 packet.

6.9.  Processing Incoming I2 Packets

   Upon receipt of an I2, the system MAY perform initial checks to
   determine whether the I2 corresponds to a recent R1 that has been
   sent out, if the Responder keeps such state.  For example, the sender
   could check whether the I2 is from an address or HIT that has



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   recently received an R1 from it.  The R1 may have had Opaque data
   included that was echoed back in the I2.  If the I2 is considered to
   be suspect, it MAY be silently discarded by the system.

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

   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 corresponds
        to a recently sent R1.  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.

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

   4.   If the system's state machine is in the R2-SENT state, the
        system MAY check if the newly received I2 is similar to the one
        that triggered moving to R2-SENT.  If so, it MAY retransmit a
        previously sent R2, 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 previously.  The peer Diffie-Hellman 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
        earlier sent in the R1 packet.

   6.   If the system's state machine is in the I1-SENT state, and the
        HITs in the I2 match those used in the previously sent I1, the
        system uses this received I2 as the basis for the HIP
        association it was trying to form, and stops retransmitting I1



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        (provided that the I2 passes the below additional checks).

   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 I2 is within the acceptable range.  Implementations
        MUST accept puzzles from the current generation and MAY accept
        puzzles from earlier generations.  If the newly received I2 is
        outside the accepted range, the I2 is stale (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 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 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 corresponds to the Host Identity sent in the I2.
        (Note: some middleboxes may not able to make this verification.)

   13.  The system MUST verify the HMAC according to the procedures in
        Section 5.2.11.

   14.  The system MUST verify the HIP_SIGNATURE according to
        Section 5.2.13 and Section 5.3.3.

   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.





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

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

   19.  Upon successful processing of an I2 when the system's state
        machine is in state ESTABLISHED, the old HIP association is
        dropped and a new one is installed, an R2 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 retransmissions of I2s), the state machine transitions
        to ESTABLISHED.

6.9.1.  Handling 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 Incoming R2 Packets

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

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

   1.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in the R1.

   2.  The system MUST verify the HIP_MAC_2 according to the procedures
       in Section 5.2.12.





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

   4.  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.

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

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

6.11.  Sending UPDATE Packets

   A host sends an UPDATE packet when it wants to update some
   information related to a HIP association.  There are a number of
   likely situations, 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 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.  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.

   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 below steps.

   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 an ACK of the peer's Update ID found in a received UPDATE
       SEQ parameter, if any.

   3.  The system sends the created UPDATE packet and starts an UPDATE
       timer.  The default value for the timer is 2 * RTT estimate.  If



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       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, the peer's next expected in-sequence Update ID
   ("peer Update ID") is stored.  Initially, this value is zero.  Update
   ID comparisons of "less than" and "greater than" are performed with
   respect to a circular sequence number space.

   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.

   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.




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   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 OK, though.)  It is recommended that a host cache
       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.

   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.





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   1.  The sequence number reported in the ACK must match with an
       earlier sent UPDATE packet 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 newly acknowledged, multiple timers are
       stopped.

6.13.  Processing NOTIFY Packets

   Processing 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 (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 CLOSE messages from both ends that cross
   in flight.

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

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

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






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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 (using the included ECHO_RESPONSE_SIGNED in response to the
   sent ECHO_REQUEST_SIGNED).

   The CLOSE_ACK uses HIP_MAC and SIGNATURE 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 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 on stable 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 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 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.

   Many Initiators would want to use a different HI for different
   Responders.  The implementations SHOULD provide for an ACL of
   Initiator's HIT to Responder's HIT.  This ACL SHOULD also include
   preferred transform and local lifetimes.

   The value of K used in the HIP R1 packet can also vary by policy.  K
   should never be greater than 20, but for trusted partners it could be
   as low as 0.

   Responders would need a similar ACL, representing which hosts they
   accept HIP exchanges, and the preferred transform and local
   lifetimes.  Wildcarding SHOULD be supported for this ACL also.






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8.  Changes from RFC 5201

   This section summarizes the changes made from [RFC5201].

8.1.  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.

   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.

8.2.  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.





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   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.

   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.




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   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.

   o  Added text about RHASH depending on OGA.

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

   o  Added reasoning why BIS document is needed.

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

   o  RFC5201 was submitted as draft-RFC.

9.  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 so doing, HIP
   itself is subject to its own DoS and MitM attacks that potentially
   could be more damaging to a host's ability to conduct business as
   usual.

   Denial-of-service attacks often take advantage of the cost of start
   of state for a protocol on the Responder compared to the 'cheapness'
   on the Initiator.  HIP makes no attempt to increase the cost of the
   start of state on the Initiator, but makes an effort to reduce the
   cost to the Responder.  This is done by having the Responder start
   the 3-way exchange instead of the Initiator, making the HIP protocol
   4 packets long.  In doing this, packet 2 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 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.



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   This shifting of the start of state cost to the Initiator in creating
   the I2 HIP packet, presents another DoS attack.  The attacker spoofs
   the I1 HIP 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 HIP packet.  The defense against this
   attack is to simply ignore any R1 packet where a corresponding I1 was
   not sent.

   A second form of DoS attack arrives in the I2 HIP packet.  Once the
   attacking Initiator has solved the puzzle, it can send packets with
   spoofed IP source addresses with either an invalid encrypted HIP
   payload component or a bad HIP signature.  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, the Responder would discard any I2s
   that contain the given Initiator HIT.  This will shut down the
   attack.  The attacker would have to request another R1 and use that
   to launch a new attack.  The Responder could up the value of K while
   under attack.  On the downside, valid I2s might get dropped too.

   A third form of DoS attack is emulating the restart of state after a
   reboot of one of the partners.  A restarting host would send an I1 to
   a peer, which would respond with an R1 even if it were in the
   ESTABLISHED state.  If the I1 were spoofed, the resulting R1 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 the end of state.  HIP
   relies on timers plus a CLOSE/CLOSE_ACK handshake to explicitly
   signal the end of a HIP association.  Because both CLOSE and
   CLOSE_ACK messages contain an 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 IP spoofer
   sending CLOSE messages to launch reflection attacks.

   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



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   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.  However, since an Initiator may choose
   to use an anonymous HI, it knowingly risks a MitM attack.  The
   Responder may choose not to accept a HIP exchange with an anonymous
   Initiator.

   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.  It SHOULD NOT change any state information based
   purely on a received NOTIFY message.

   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' messages are to be expected and present a DoS attack.
   Against an Initiator, the attack would look like the Responder does
   not support HIP, but shortly after receiving the ICMP message, the
   Initiator would receive a valid 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 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
   any ICMP message while waiting for an R2 HIP packet, and should
   delete any pending state only after a natural timeout.

10.  IANA Considerations

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




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   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 [RFC4843-bis].

   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 Consensus [RFC2434].

   HIP Version

      The four-bit Version field in a HIP protocol packet describes the
      version of the HIP protocol.  It is defined in Section 5.1.  The
      currently defined values are 1 and 2.  The version of this
      document is 2.  New values are assigned through IETF Consensus.

   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.

      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
      extensions of the HIT Suite ID space to eight-bit and new HIT
      Suite IDs are defined through IETF consensus.






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   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 5.2.22.

      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 Consensus.

      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 [RFC2434].

   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 5.2.6.  New values either from the reserved or unassigned
      space are assigned through IETF Consensus.

   HIP Cipher ID

      The 16-bit Cipher ID values in a HIP_CIPHER parameter are defined
      in Section 5.2.7.  New values either from the reserved or
      unassigned space are assigned through IETF Consensus.

   DI-Type

      The four-bit DI-Type values in a HOST_ID parameter are defined in
      Section 5.2.8.  New values are assigned through IETF Consensus.

   Notify Message Type

      The 16-bit Notify Message Type values in a NOTIFICATION parameter
      are defined in Section 5.2.18.

      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



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      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.

11.  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
   Petander, Michael Richardson, Rene Hummen, Tim Shepard, Jorma Wall,
   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



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   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.

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>.

   [FIPS.95-1.1993]   National Institute of Standards and Technology,
                      "Codes for the Identification of Federal and
                      Federally Assisted Organizations", FIPS PUB 95-1,
                      January 1993.

   [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, 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.

   [RFC2451]          Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
                      Algorithms", RFC 2451, November 1998.

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

   [RFC2463]          Conta, A. and S. Deering, "Internet Control
                      Message Protocol (ICMPv6) for the Internet
                      Protocol Version 6 (IPv6) Specification",
                      RFC 2463, December 1998.

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

   [RFC2898]          Kaliski, B., "PKCS #5: Password-Based Cryptography



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                      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.

   [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.

   [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.

   [RFC4307]          Schiller, J., "Cryptographic Algorithms for Use in
                      the Internet Key Exchange Version 2 (IKEv2)",
                      RFC 4307, December 2005.

   [RFC4753]          Fu, D. and J. Solinas, "ECP Groups For IKE and
                      IKEv2", RFC 4753, January 2007.

   [RFC4843-bis]      Nikander, P., Laganier, J., and F. Dupont, "STUB:
                      An IPv6 Prefix for Overlay Routable Cryptographic
                      Hash Identifiers (ORCHID)",
                      draft-laganier-rfc4843-bis-00 (work in progress),
                      February 2010.

   [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,



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                      April 2008.

   [RFC5202]          Jokela, P., Moskowitz, R., and P. Nikander, "Using
                      the Encapsulating Security Payload (ESP) Transport
                      Format with the Host Identity Protocol (HIP)",
                      RFC 5202, 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.

   [fundamental-ecc]  McGrew, D. and K. Igoe, "Fundamental Elliptic
                      Curve Cryptography Algorithms",
                      draft-mcgrew-fundamental-ecc-03 (work in
                      progress), May 2010.

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, <http://csrc.nist.gov/
                      publications/fips/fips197/fips-197.pdf>.

   [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



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                      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.

   [RFC2434]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 2434, October 1998.

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

   [RFC5204]          Laganier, J. and L. Eggert, "Host Identity
                      Protocol (HIP) Rendezvous Extension", RFC 5204,
                      April 2008.

   [RFC5205]          Nikander, P. and J. Laganier, "Host Identity
                      Protocol (HIP) Domain Name System (DNS)
                      Extensions", RFC 5205, April 2008.

   [RFC5206]          Nikander, P., Henderson, T., Vogt, C., and J.
                      Arkko, "End-Host Mobility and Multihoming with the
                      Host Identity Protocol", RFC 5206, April 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.

   [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.

   [rfc4423-bis]      Moskowitz, R., "Host Identity Protocol
                      Architecture", draft-moskowitz-hip-rfc4423-bis-01
                      (work in progress), June 2010.

Appendix A.  Using Responder Puzzles

   As mentioned in Section 4.1.1, the Responder may delay state creation
   and still reject most spoofed I2s by using a number of pre-calculated
   R1s and a local selection function.  This appendix defines one



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   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 gets 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 gets 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

   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).




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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 IP
   addresses of 192.168.0.1 and 192.168.0.2 (and their respective IPv4-
   compatible IPv6 formats), and HITs with the prefix of 2001:10
   followed by zeros, followed by a decimal 1 or 2, respectively.

   The following example is defined only for testing a checksum
   calculation.  The address format for the IPv4-compatible IPv6 address
   is not a valid one, but using these IPv6 addresses when testing an
   IPv6 implementation gives the same checksum output as an IPv4
   implementation with the corresponding IPv4 addresses.




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C.1.  IPv6 HIP Example (I1)

     Source Address:                 ::192.168.0.1
     Destination Address:            ::192.168.0.2
     Upper-Layer Packet Length:      40              0x28
     Next Header:                    139             0x8b
     Payload Protocol:               59              0x3b
     Header Length:                  4               0x4
     Packet Type:                    1               0x1
     Version:                        1               0x1
     Reserved:                       1               0x1
     Control:                        0               0x0
     Checksum:                       446             0x1be
     Sender's HIT  :                 2001:10::1
     Receiver's HIT:                 2001:10::2

C.2.  IPv4 HIP Packet (I1)

   The IPv4 checksum value for the same example I1 packet is the same as
   the IPv6 checksum (since the checksums due to the IPv4 and IPv6
   pseudo-header components are the same).

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




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Appendix D.  ECDH-160 Group

   The ECDH-160 group is rated at 80 bits strength.  Once this was
   considered appropriate for one year of security.  Today should be
   used only when the host is not powerful enough (e.g., some PDAs) and
   when security requirements are low (e.g., during normal web surfing).

Appendix E.  HIT Suites and HIT Generation

   The HIT as an ORCHID [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.

   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.





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   +-------+----------+-----------------+------------------------------+
   | Index | Hash     | Signature       | Description                  |
   |       | function | algorithm       |                              |
   |       |          | family          |                              |
   +-------+----------+-----------------+------------------------------+
   |     0 |          |                 | Reserved                     |
   |     1 | SHA-1    | RSA, DSA        | RSA or DSA HI hashed with    |
   |       |          |                 | SHA-1, truncated to 96 bits  |
   |     2 | SHA-256  | ECDSA           | ECDSA HI hashed with         |
   |       |          |                 | SHA-256, truncated to 96     |
   |       |          |                 | bits                         |
   |     3 | SHA-384  | ECDSA           | ECDSA HI hashed with         |
   |       |          |                 | SHA-384, truncated to 96     |
   |       |          |                 | bits                         |
   +-------+----------+-----------------+------------------------------+

                           Table 11: HIT Suites

Authors' Addresses

   Robert Moskowitz (editor)
   ICSA labs, An Independent Division of Verizon Business
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA
   USA

   EMail: robert.moskowitz@icsalabs.com


   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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   Tobias Heer
   RWTH Aachen University, Distributed Systems Group
   Ahornstrasse 55
   Aachen  52062
   Germany

   EMail: heer@cs.rwth-aachen.de
   URI:   http://ds.cs.rwth-aachen.de/members/heer











































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