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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5201

Network Working Group                                       R. Moskowitz
Internet-Draft                         ICSAlabs, a Division of TruSecure
Expires: August 25, 2005                                     Corporation
                                                             P. Nikander
                                                      P. Jokela (editor)
                                            Ericsson Research NomadicLab
                                                            T. Henderson
                                                      The Boeing Company
                                                       February 21, 2005

                         Host Identity Protocol
                         draft-ietf-hip-base-02

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 25, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo specifies the details of the Host Identity Protocol (HIP).


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   The overall description of protocol and the underlying architectural
   thinking is available in the separate HIP architecture specification.
   The Host Identity Protocol is used to establish a rapid
   authentication between two hosts and to provide continuity of
   communications between those hosts independent of the networking
   layer.

   The various forms of the Host Identity, Host Identity Tag (HIT) and
   Local Scope Identifier (LSI), are covered in detail.  It is described
   how they are used to support authentication and the establishment of
   keying material, which is then used for protecting subsequent HIP
   messages, and which can be used for generating session keys for other
   security protocols, such as IPsec Encapsulaed Security Payload (ESP).
   The basic state machine for HIP provides a HIP compliant host with
   the resiliency to avoid many denial-of-service (DoS) attacks.  The
   basic HIP exchange for two public hosts shows the actual packet flow.
   Other HIP exchanges, including those that work across NATs, are
   covered elsewhere.

















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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1  A new name space and identifiers . . . . . . . . . . . . .   6
     1.2  The HIP base exchange  . . . . . . . . . . . . . . . . . .   6
   2.   Conventions used in this document  . . . . . . . . . . . . .   8
   3.   Host Identifier (HI) and its representations . . . . . . . .   9
     3.1  Host Identity Tag (HIT)  . . . . . . . . . . . . . . . . .   9
       3.1.1  Restricting HIT values . . . . . . . . . . . . . . . .  10
       3.1.2  Generating a HIT from a HI . . . . . . . . . . . . . .  11
     3.2  Local Scope Identifier (LSI) . . . . . . . . . . . . . . .  12
   4.   Host Identity Protocol . . . . . . . . . . . . . . . . . . .  14
     4.1  HIP base exchange  . . . . . . . . . . . . . . . . . . . .  14
       4.1.1  HIP Cookie Mechanism . . . . . . . . . . . . . . . . .  15
       4.1.2  Authenticated Diffie-Hellman protocol  . . . . . . . .  18
       4.1.3  HIP replay protection  . . . . . . . . . . . . . . . .  19
     4.2  TCP and UDP pseudo-header computation for user data  . . .  20
     4.3  Updating a HIP association . . . . . . . . . . . . . . . .  20
     4.4  Error processing . . . . . . . . . . . . . . . . . . . . .  20
     4.5  Certificate distribution . . . . . . . . . . . . . . . . .  21
     4.6  Sending data on HIP packets  . . . . . . . . . . . . . . .  21
     4.7  Transport Formats  . . . . . . . . . . . . . . . . . . . .  21
   5.   HIP protocol overview  . . . . . . . . . . . . . . . . . . .  22
     5.1  HIP Scenarios  . . . . . . . . . . . . . . . . . . . . . .  22
     5.2  Refusing a HIP exchange  . . . . . . . . . . . . . . . . .  23
     5.3  Reboot and SA timeout restart of HIP . . . . . . . . . . .  23
     5.4  HIP State Machine  . . . . . . . . . . . . . . . . . . . .  23
       5.4.1  HIP States . . . . . . . . . . . . . . . . . . . . . .  24
       5.4.2  HIP State Processes  . . . . . . . . . . . . . . . . .  24
       5.4.3  Simplified HIP State Diagram . . . . . . . . . . . . .  28
   6.   Packet formats . . . . . . . . . . . . . . . . . . . . . . .  30
     6.1  Payload format . . . . . . . . . . . . . . . . . . . . . .  30
       6.1.1  HIP Controls . . . . . . . . . . . . . . . . . . . . .  31
       6.1.2  Checksum . . . . . . . . . . . . . . . . . . . . . . .  31
     6.2  HIP parameters . . . . . . . . . . . . . . . . . . . . . .  32
       6.2.1  TLV format . . . . . . . . . . . . . . . . . . . . . .  33
       6.2.2  Defining new parameters  . . . . . . . . . . . . . . .  35
       6.2.3  R1_COUNTER . . . . . . . . . . . . . . . . . . . . . .  36
       6.2.4  PUZZLE . . . . . . . . . . . . . . . . . . . . . . . .  37
       6.2.5  SOLUTION . . . . . . . . . . . . . . . . . . . . . . .  38
       6.2.6  DIFFIE_HELLMAN . . . . . . . . . . . . . . . . . . . .  39
       6.2.7  HIP_TRANSFORM  . . . . . . . . . . . . . . . . . . . .  40
       6.2.8  HOST_ID  . . . . . . . . . . . . . . . . . . . . . . .  41
       6.2.9  CERT . . . . . . . . . . . . . . . . . . . . . . . . .  42
       6.2.10   HMAC . . . . . . . . . . . . . . . . . . . . . . . .  43
       6.2.11   HMAC_2 . . . . . . . . . . . . . . . . . . . . . . .  43
       6.2.12   HIP_SIGNATURE  . . . . . . . . . . . . . . . . . . .  44
       6.2.13   HIP_SIGNATURE_2  . . . . . . . . . . . . . . . . . .  44


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       6.2.14   SEQ  . . . . . . . . . . . . . . . . . . . . . . . .  45
       6.2.15   ACK  . . . . . . . . . . . . . . . . . . . . . . . .  45
       6.2.16   ENCRYPTED  . . . . . . . . . . . . . . . . . . . . .  46
       6.2.17   NOTIFY . . . . . . . . . . . . . . . . . . . . . . .  47
       6.2.18   ECHO_REQUEST . . . . . . . . . . . . . . . . . . . .  50
       6.2.19   ECHO_RESPONSE  . . . . . . . . . . . . . . . . . . .  51
     6.3  ICMP messages  . . . . . . . . . . . . . . . . . . . . . .  51
       6.3.1  Invalid Version  . . . . . . . . . . . . . . . . . . .  51
       6.3.2  Other problems with the HIP header and packet
              structure  . . . . . . . . . . . . . . . . . . . . . .  51
       6.3.3  Invalid Cookie Solution  . . . . . . . . . . . . . . .  52
       6.3.4  Non-existing HIP association . . . . . . . . . . . . .  52
   7.   HIP Packets  . . . . . . . . . . . . . . . . . . . . . . . .  53
     7.1  I1 - the HIP initiator packet  . . . . . . . . . . . . . .  53
     7.2  R1 - the HIP responder packet  . . . . . . . . . . . . . .  54
     7.3  I2 - the second HIP initiator packet . . . . . . . . . . .  55
     7.4  R2 - the second HIP responder packet . . . . . . . . . . .  56
     7.5  CER - the HIP Certificate Packet . . . . . . . . . . . . .  57
     7.6  UPDATE - the HIP Update Packet . . . . . . . . . . . . . .  57
     7.7  NOTIFY - the HIP Notify Packet . . . . . . . . . . . . . .  58
     7.8  CLOSE - the HIP association closing packet . . . . . . . .  59
     7.9  CLOSE_ACK - the HIP closing acknowledgment packet  . . . .  59
   8.   Packet processing  . . . . . . . . . . . . . . . . . . . . .  61
     8.1  Processing outgoing application data . . . . . . . . . . .  61
     8.2  Processing incoming application data . . . . . . . . . . .  62
     8.3  HMAC and SIGNATURE calculation and verification  . . . . .  63
       8.3.1  HMAC calculation . . . . . . . . . . . . . . . . . . .  63
       8.3.2  Signature calculation  . . . . . . . . . . . . . . . .  63
     8.4  Initiation of a HIP exchange . . . . . . . . . . . . . . .  64
       8.4.1  Sending multiple I1s in parallel . . . . . . . . . . .  65
       8.4.2  Processing incoming ICMP Protocol Unreachable
              messages . . . . . . . . . . . . . . . . . . . . . . .  65
     8.5  Processing incoming I1 packets . . . . . . . . . . . . . .  66
       8.5.1  R1 Management  . . . . . . . . . . . . . . . . . . . .  66
       8.5.2  Handling malformed messages  . . . . . . . . . . . . .  67
     8.6  Processing incoming R1 packets . . . . . . . . . . . . . .  67
       8.6.1  Handling malformed messages  . . . . . . . . . . . . .  68
     8.7  Processing incoming I2 packets . . . . . . . . . . . . . .  69
       8.7.1  Handling malformed messages  . . . . . . . . . . . . .  70
     8.8  Processing incoming R2 packets . . . . . . . . . . . . . .  70
     8.9  Sending UPDATE packets . . . . . . . . . . . . . . . . . .  71
     8.10   Receiving UPDATE packets . . . . . . . . . . . . . . . .  71
       8.10.1   Handling a SEQ paramaeter in a received UPDATE
                message  . . . . . . . . . . . . . . . . . . . . . .  72
       8.10.2   Handling an ACK parameter in a received UPDATE
                packet . . . . . . . . . . . . . . . . . . . . . . .  72
     8.11   Processing CER packets . . . . . . . . . . . . . . . . .  73
     8.12   Processing NOTIFY packets  . . . . . . . . . . . . . . .  73


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     8.13   Processing CLOSE packets . . . . . . . . . . . . . . . .  73
     8.14   Processing CLOSE_ACK packets . . . . . . . . . . . . . .  73
     8.15   Dropping HIP associations  . . . . . . . . . . . . . . .  73
   9.   HIP KEYMAT . . . . . . . . . . . . . . . . . . . . . . . . .  75
   10.  HIP Fragmentation Support  . . . . . . . . . . . . . . . . .  77
   11.  HIP Policies . . . . . . . . . . . . . . . . . . . . . . . .  78
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  79
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  82
   14.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  83
   15.  References . . . . . . . . . . . . . . . . . . . . . . . . .  84
     15.1   Normative references . . . . . . . . . . . . . . . . . .  84
     15.2   Informative references . . . . . . . . . . . . . . . . .  85
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  86
   A.   Probabilities of HIT collisions  . . . . . . . . . . . . . .  87
   B.   Probabilities in the cookie calculation  . . . . . . . . . .  88
   C.   Using responder cookies  . . . . . . . . . . . . . . . . . .  89
   D.   Example checksums for HIP packets  . . . . . . . . . . . . .  92
     D.1  IPv6 HIP example (I1)  . . . . . . . . . . . . . . . . . .  92
     D.2  IPv4 HIP packet (I1) . . . . . . . . . . . . . . . . . . .  92
     D.3  TCP segment  . . . . . . . . . . . . . . . . . . . . . . .  92
   E.   384-bit group  . . . . . . . . . . . . . . . . . . . . . . .  94
        Intellectual Property and Copyright Statements . . . . . . .  95















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1.  Introduction

   The Host Identity Protocol (HIP) provides a rapid exchange of Host
   Identities between two hosts.  The protocol is designed to be
   resistant to Denial-of-Service (DoS) and Man-in-the-middle (MitM)
   attacks, and when used together with another suitable security
   protocol, such as Encapsulated Security Payload (ESP) [23], it
   provides DoS and MitM protection for upper layer protocols, such as
   TCP and UDP.

1.1  A new name space and identifiers

   The Host Identity Protocol introduces a new namespace, the Host
   Identity.  The effects of this change are explained in the companion
   document, the HIP architecture [21] specification.

   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.

1.2  The HIP base exchange

   The HIP base exchange is a two-party cryptographic protocol that
   consists of four packets.  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, it starts the cookie exchange in the
   2nd packet, completing it in the 3rd packet.

   The exchange uses the Diffie-Hellman exchange to hide the Host
   Identity of the Initiator in packet 3.  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 after the 4th packet.  The 3rd and 4th HIP packets
   may carry a data payload in the future.  However, the details of this
   are to be defined later as more implementation experience is gained.


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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) [23] and other end-to-end security protocols.  The base
   protocol lacks the details for security association management and
   much of the fine-grained policy control found in Internet Key
   Exchange IKE RFC2409 [8] that allows IKE to support complex gateway
   policies.  Thus, HIP is not a replacement for IKE.






















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2.  Conventions used in this document

   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 RFC2119 [5].























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3.  Host Identifier (HI) and its representations

   A 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 [21] for more details about the difference between an
   identity and the corresponding identifier.

   HIP implementations MUST support the Rivest Shamir Adelman (RSA) [14]
   public key algorithm, and SHOULD support the Digital Signature
   Algorithm (DSA) [13] algorithm; other algorithms MAY be supported.

   A hash 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
   HIT), and iii) the probability of HIT collision between two hosts is
   very low.

   In many environments, 128 bits is still considered large.  For
   example, currently used IPv4 based applications are constrained with
   32-bit address fields.  Another problem is that the cohabitation of
   IPv6 and HIP might require some applications to differentiate an IPv6
   address from a HIT.  Thus, a third representation, the Local Scope
   Identifier (LSI), may be needed.  There are two types of such LSIs:
   32-bit IPv4-compatible ones and 128-bit IPv6-compatible ones.  The
   LSI provides a compression of the HIT with only a local scope so that
   it can be carried efficiently in any application level packet and
   used in API calls.  The LSIs do not have the same properties as HITs
   (i.e., they are not self-certifying nor are they as unlikely to
   collide -- hence their local scope), and consequently they must be
   used more carefully.

   Finally, HIs, HITs, and LSIs are not expected to be carried
   explicitly in the headers of user data packets.  Depending on the
   form of further communication, other methods are used to map the data
   packet to the these representatives of host identities.  For example,
   if ESP is used to protect data traffic, the Security Parameter Index
   (SPI) can be used for this purpose.  In some cases, this makes it
   possible to use HIP without an additional explicit protocol header.

3.1  Host Identity Tag (HIT)

   The Host Identity Tag is a 128 bits long value -- a hash of the Host
   Identifier.  There are two advantages of using a hash over the actual
   Identity in protocols.  Firstly, its fixed length makes for easier


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   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.

   There are two types of HITs.  HITs of the first type, called _type 1
   HIT_, consist of 128 bits of the SHA-1 hash of the public key.  HITs
   of the second type consist of a Host Assigning Authority Field (HAA),
   and only the last 64 bits come from a SHA-1 hash of the Host
   Identity.  This latter format for HIT is recommended for 'well known'
   systems.  It is possible to support a resolution mechanism for these
   names in hierarchical directories, like the DNS.  Another use of HAA
   is in policy controls, see Section 11.

   As the type of a HIT cannot be determined by inspecting its contents,
   the HIT type must be communicated by some external means.

   When comparing HITs for equality, it is RECOMMENDED that conforming
   implementations ignore the TBD top most bits.  This is to allow
   better compatibility for legacy IPv6 applications; see [29].
   However, independent of how many bits are actually used for HIT
   comparison, it is also RECOMMENDED that the final equality decision
   is based on the public key and not the HIT, if possible.  See also
   Section 3.2 for related discussion.

   This document fully specifies only type 1 HITs.  HITs that consists
   of the HAA field and the hash are specified in [25].

   Any conforming implementation MUST be able to deal with Type 1 HITs.
   When handling other than type 1 HITs, the implementation is
   RECOMMENDED to explicitly learn and record the binding between the
   Host Identifier and the HIT, as it may not be able to generate such
   HITs from the Host Identifiers.

3.1.1  Restricting HIT values

   To facilitate experimentation and make certain kind of
   implementations easier, the following restrictions are temporarily
   placed on HITs.  These restriction are to be lifted at the end of
   2008.  That is, after January 1st 2009, any implementation claiming
   conformance to this specification MUST accept any HITs from peers and
   be able to process them normally.

   The restrictions: Before the end of 2008, all implementations SHOULD
   restrict the HITs they generate to ones whose upper-most (left-most)
   two bits are either binary01 or10.  That is, when generating new HIs,
   if the resulting HIT has as its first two bits as00 or11, the
   implementation SHOULD generate new HIs until it generates one that
   fulfills this restriction.  Additionally, a conforming implementation


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   MAY refuse to communicate with a peer that has a HIT with the
   upper-most bits either00 or11.  When refusing a HIP connection on
   this bases, the implementation MAY send an R2 with a NOTIFY payload,
   with the NOTIFY code being UNSUPPORTED_HIT_VALUE_RANGE.  Any such
   NOTIFYs may be rate-limited

   A rationale: One way to experimentally implement HIP is to use
   unmodified IPv6, TCP and UDP implementations in the stack, using HITs
   in the place of IPv6 addresses.  This modification makes it easier to
   use existing IPv6 data structures to hold HITs and to distinguish
   between the two data types.  If the IPv6 address space and the HIT
   value space overlap, it becomes hard to define secure IPsec policies
   without explicitly tagging the values either as HITs or IPv6
   addresses.

3.1.2  Generating a HIT from a HI

   The 128 or 64 hash bits in a HIT MUST be generated by taking the
   least significant 128 or 64 bits of the SHA-1 [22] hash of the Host
   Identifier as it is represented in the Host Identity field in a HIP
   payload packet.

   For Identities that are either RSA or DSA public keys, the HIT is
   formed as follows:
   1.  The public key is encoded as specified in the corresponding
       DNSSEC document, taking the algorithm specific portion of the
       RDATA part of the KEY RR.  There is currently only two defined
       public key algorithms: RSA and DSA.  Hence, either of the
       following applies:
          The RSA public key is encoded as defined in RFC3110 [14]
          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
          (hi_len) and the preceding HI fields including the exponent
          (e).  Thus, the data to be hashed has the same length as the
          HI (hi_len).  The fields MUST be encoded in network byte
          order, as defined in RFC3110 [14].
          The DSA public key is encoded as defined in RFC2536 [13]
          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 [13].  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 [13].
   2.  A SHA-1 hash [22] is calculated over the encoded key.
   3.  The least significant 128 or 64 bits of the hash result are used
       to create the HIT, as defined above.


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   The following pseudo-codes illustrates the process 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;

   }

   digest := SHA-1 ( buffer )

   hit_128 := low_order_bits ( digest, 128 )
   hit_haa := concatenate ( HAA, low_order_bits ( digest,  64 ) )

3.2  Local Scope Identifier (LSI)

   LSIs are 32 or 128 bits long localized representations of a Host
   Identity.  The purpose of an LSI is to facilitate using Host
   Identities in existing IPv4 or IPv6 based protocols and APIs.  The
   LSI can be used anywhere in system processes where IP addresses have
   traditionally been used, such as IPv4 and IPv6 API calls and FTP PORT
   commands.

   The IPv4-compatible LSIs MUST be allocated from the TBD subnet and
   the IPv6-compatible LSIs MUST be allocated from the TBD subnet.  That
   makes it easier to differentiate between LSIs and IP addresses at the
   API level.  By default, the low order 24 bits of an IPv4-compatible
   LSI are equal to the low order 24 bits of the corresponding HIT,
   while the low order TBD bits of an IPv6-compatible LSI are equal to
   the low order TBD bits of the corresponding HIT.


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   A host performing a HIP handshake may discover that the LSI formed
   from the peer's HIT collides with another LSI in use locally (i.e.,
   the lower 24 or TBD bits of two different HITs are the same).  In
   that case, the host MUST handle the LSI collision locally such that
   application calls can be disambiguated.  One possible means of doing
   so is to perform a Host NAT function to locally convert a peer's LSI
   into a different LSI value.  This would require the host to ensure
   that LSI bits on the wire (i.e., in the application data stream) are
   converted back to match that host's LSI.  Other alternatives for
   resolving LSI collisions may be added in the future.





















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4.  Host Identity Protocol

   The Host Identity Protocol is IP protocol TBD (number will be
   assigned by IANA).  The HIP payload (Section 6.1) header could be
   carried in every datagram.  However, since HIP datagrams are
   relatively large (at least 40 bytes), it is desirable to 'compress'
   the HIP header so that the HIP header only occur in datagrams to
   establish or change HIP state.  The actual method for header
   'compression' and matching data packets with existing HIP
   associations (if any) is defined in separate extension documents,
   describing transport formats and methods.  All HIP implementations
   MUST implement, at minimum, the ESP transport format for HIP [23].

   For testing purposes, the protocol number 99 is currently used.

4.1  HIP base exchange

   The HIP base exchange serves to manage the establishment of state
   between an Initiator and a Responder.  The last three packets of the
   exchange, R1, I2, and R2, constitute a standard authenticated
   Diffie-Hellman key exchange for session key generation.  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 only the HIT of the Initiator and possibly the
   HIT of the Responder, if it is known.

   The second packet, R1, starts the actual exchange.  It contains a
   puzzle, that is, a cryptographic challenge that the Initiator must
   solve before continuing the exchange.  In addition, it contains the
   initial Diffie-Hellman parameters and a signature, covering part 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 finalizes the base exchange.  The packet is signed.

   The base exchange is illustrated below.



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       Initiator                              Responder

                    I1: trigger exchange
                  -------------------------->
                                              select pre-computed R1
                    R1: puzzle, D-H, key, sig
                  <-------------------------
    check sig                                 remain stateless
    solve puzzle
                  I2: solution, D-H, {key}, sig
                  -------------------------->
    compute D-H                               check cookie
                                              check puzzle
                                              check sig
                            R2: sig
                  <--------------------------
    check sig                                 compute D-H

   In R1, the signature covers the packet, after setting the Initiator
   HIT, header checksum, and the PUZZLE parameter's Opaque and Random #I
   fields temporarily to zero, and excluding any TLVs that follow the
   signature.

   In I2, the signature covers the whole packet, excluding any TLVs that
   follow the signature.

   In R2, the signature and the HMAC cover the whole envelope.

   In this section we cover the overall design of the base exchange.
   The details are the subject of the rest of this memo.

4.1.1  HIP Cookie Mechanism

   The purpose of the HIP cookie 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
   included in the cookie 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 Cookie 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 validly formatted I2 can be rejected
   earliest 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,


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   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.

   One possible way for a Responder to remain stateless but drop most
   spoofed I2s is to base the selection of the cookie on some function
   over the Initiator's Host Identity.  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 checks that the cookie in
   the I2 matches with the cookie sent in the R1, thereby making 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 IP addresses or use different HITs.  The method does not
   protect from an attacker that uses fixed IP addresses and HITs,
   though.  Against such an attacker it is probably best to create a
   piece of local state, and remember that the puzzle check has
   previously failed.  See Appendix C for one possible implementation.
   Note, however, that the implementations MUST NOT use the exact
   implementation given in the appendix, and SHOULD include sufficient
   randomness to the algorithm so that algorithm complexity attacks
   become impossible [27].

   The Responder can set the puzzle difficulty for Initiator, based on
   its concern of trust of 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.

   The Responder starts the cookie 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 SHA-1
   hash over this concatenation.  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 zero.  The
   Initiator SHOULD give up after exceeding the puzzle lifetime in the
   PUZZLE TLV.  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 pre-computation 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 was indeed selected by it and not by the Initiator.  See


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   Appendix C for an example on how to implement this.

   Using the Opaque data field in an ECHO_REQUEST parameter, 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 the sent I,
   some secret, and possibly other related data.  Using this same
   secret, received I in I2 packet and possible other data, the Receiver
   can verify that it has itself sent the I to the Initiator.  The
   Responder MUST change the secret periodically.

   It is RECOMMENDED that the Responder generates a new cookie and a new
   R1 once every few minutes.  Furthermore, it is RECOMMENDED that the
   Responder remembers an old cookie at least 2*lifetime seconds after
   it has been deprecated.  These time values allow a slower Initiator
   to solve the cookie puzzle while limiting the usability that an old,
   solved cookie has to an attacker.

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

   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 SHA-1
   hash is created by concatenating, in network byte order, the
   following data, in the following order:
      64-bit random value I, in network byte order, as appearing in R1
      and I2.
      128-bit initiator HIT, in network byte order, as appearing in the
      HIP Payload in R1 and I2.
      128-bit responder HIT, in network byte order, as appearing in the
      HIP Payload in R1 and I2.
      64-bit random value J, in network byte order, as appearing in I2.
   In order to be a valid response cookie, the K low-order bits of the
   resulting SHA-1 digest must be zero.

   Notes:
      The length of the data to be hashed is 48 bytes.
      All the data in the hash input MUST be in network byte order.


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      The order of the initiator and responder HITs are different in the
      R1 and I2 packets, see Section 6.1.  Care must be taken to copy
      the values in right order to the hash input.

   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( SHA-1( 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( SHA-1( I | HIT-I | HIT-R | J ), K )
      Rejects if V != 0
      Accept if V == 0

   The Ltrunc (SHA-1(), K) denotes the lowest order K bits of the SHA-1
   result.

4.1.2  Authenticated Diffie-Hellman protocol

   The packets R1, I2, and R2 implement a standard authenticated
   Diffie-Hellman exchange.  The Responder sends its public
   Diffie-Hellman key 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 all of the packet, it
   does not protect from replay attacks.

   When the Initiator receives an R1, it computes the Diffie-Hellman
   session key.  It creates a HIP association using keying material from
   the session key (see Section 9), and uses the association to encrypt
   its public authentication key, i.e., host identity.  The resulting I2
   contains the Initiator's Diffie-Hellman key and its encrypted public
   authentication key.  The signature in I2 covers all of the packet.


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   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.

4.1.3  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 cookie
   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
   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 cookie
   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.  Also, the R1 generation counter MUST NOT roll
   over; if the counter is about to become exhausted, the corresponding
   HI must be abandoned and replaced with a new one.

   A host may receive more than one R1, either due to sending multiple
   I1s (Section 8.4.1) or due to a replay of an old 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.  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


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   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.

4.2  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 or LSIs, the IPv6 pseudo-header format
   [11] MUST be used.  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 6.1.2.

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

   The UPDATE packet is defined in Section 7.6.

4.4  Error processing

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


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   message; see Section 6.3 for more details.

4.5  Certificate distribution

   HIP does not define how to use certificates.  However, it does define
   a simple certificate transport mechanisms that MAY be used to
   implement certificate-based security policies.  The certificate
   payload is defined in Section 6.2.9, and the certificate packet in
   Section 7.5.

4.6  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.

4.7  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 [23].

   When new transport formats are defined, the corresponding parameters
   MUST have smaller type value than the ESP_TRANSFORM parameter.  The
   order in which the transport formats are presented in the R1 packet,
   is the preferred order.  The last of the transport formats MUST be
   ESP transport format, represented by the ESP_TRANSFORM parameter.











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5.  HIP protocol overview

   The following material is an overview of the HIP protocol operation.
   Section 8 describes the packet processing steps in more detail.

   A typical HIP packet flow is shown below, between an Initiator (I)
   and a Responder (R).  It illustrates the exchange of four HIP packets
   (I1, R1, I2, and R2).

    I --> Directory: lookup R
    I <-- Directory: return R's addresses, and HI and/or HIT
    I1      I --> R (Hi. Here is my I1, let's talk HIP)
    R1      I <-- R (OK. Here is my R1, handle this HIP cookie)
    I2      I --> R (Compute, compute, here is my counter I2)
    R2      I <-- R (OK.  Let's finish HIP with my R2)
    I --> R (data)
    I <-- R (data)

   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.

5.1  HIP Scenarios

   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.
         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 an 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
         Parameter Problem type to inform about non-existing HIP
         association (see Section 6.3), and it MAY initiate a new HIP


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         negotiation.  However, responding with these optional
         mechanisms is implementation or policy dependent.

5.2  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 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.

5.3  Reboot and SA timeout 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 times out, it has lost its HIP state.  If the
   system that lost state has a datagram to deliver to its peer, it
   simply restarts the HIP exchange.  The peer replies with an R1 HIP
   packet, but does not reset its state until it receives the I2 HIP
   packet.  The I2 packet MUST have a valid solution to the puzzle and,
   if inserted in R1, a valid Opaque data as well as a valid signature.
   Note that either the original Initiator or the Responder could end up
   restarting the exchange, becoming the new Initiator.

   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, 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.

   After sending the I1, the HIP negotiation proceeds as normally and,
   when successful, the SA is created at the initiating end.  The peer
   end removes the OLD SA and replaces it with the new one.

5.4  HIP State Machine

   The HIP protocol itself has little state.  In the HIP base exchange,
   there is an Initiator and a Responder.  Once the SAs are established,


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   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.

   Implementors must understand that the state machine, as described
   here, is informational.  Specific implementations are free to
   implement the actual functions differently.  Section 8 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 drafts (such as mobility and
   multihoming).

5.4.1  HIP States

   +---------------------+---------------------------------------------+
   | State               | Explanation                                 |
   +---------------------+---------------------------------------------+
   | UNASSOCIATED        | State machine start                         |
   |                     |                                             |
   | I1-SENT             | Initiating HIP                              |
   |                     |                                             |
   | I2-SENT             | Waiting to finish HIP                       |
   |                     |                                             |
   | R2-SENT             | Waiting to finish HIP                       |
   |                     |                                             |
   | 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                         |
   +---------------------+---------------------------------------------+

5.4.2  HIP State Processes

   +------------+
   |UNASSOCIATED| Start state
   +------------+


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   User data to send requiring a new HIP association, send I1 and go to I1-SENT
   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 for unknown HIP association, optionally send ICMP as
        defined in
   Section 6.3
    and stay at UNASSOCIATED
   Receive CLOSE, optionally send ICMP Parameter Problem and stay
        in UNASSOCIATED.

   Receive ANYOTHER, drop and stay at UNASSOCIATED

   +---------+
   | I1-SENT | Initiating HIP
   +---------+

   Receive I1, 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 successful, send I2 and go to I2-SENT
        if fail, go to E-FAILED

   Receive ANYOTHER, drop and stay at I1-SENT
   Timeout, increment timeout counter
        If counter is less than I1_RETRIES_MAX, send I1 and stay at I1-SENT
        If counter is greater than I1_RETRIES_MAX, go to E-FAILED

   +---------+
   | I2-SENT | Waiting to finish HIP
   +---------+

   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, send R2 and go to R2-SENT
        if fail, stay at I2-SENT
   Receive R2, process
        if successful, go to ESTABLISHED
        if fail, go to E-FAILED

   Receive ANYOTHER, drop and stay at I2-SENT


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   Timeout, increment timeout counter
        If counter is less than I2_RETRIES_MAX, send I2 and stay at I2-SENT
        If counter is greater than I2_RETRIES_MAX, go to E-FAILED

   +---------+
   | R2-SENT | Waiting to finish HIP
   +---------+

   Receive I1, send R1 and stay at R2-SENT
   Receive I2, process,
      if successful, send R2, and cycle at R2-SENT
      if failed, stay at R2-SENT
   Receive R1, drop and stay at R2-SENT
   Receive R2, drop and stay at R2-SENT

   Move to ESTABLISHED after an implementation specific time.

   +------------+
   |ESTABLISHED | HIP association established
   +------------+

   Receive I1, send R1 and stay at ESTABLISHED
   Receive I2, process with cookie and possible Opaque data verification
        if successful, send R2, drop old HIP association, establish a new
        HIP association, to to R2-SENT
        if fail, stay at ESTABLISHED
   Receive R1, drop and stay at ESTABLISHED
   Receive R2, drop and stay at ESTABLISHED

   Receive user data for HIP association, process and stay at ESTABLISHED
   No packet sent/received during UAL minutes, send CLOSE and go to CLOSING.
   Receive CLOSE, process
        if successful, send CLOSE_ACK and go to CLOSED
        if failed, stay at ESTABLISHED

   +---------+
   | CLOSING | HIP association has not been used for UAL (Unused
   +---------+ Association Lifetime) minutes.

   User data to send, requires the creation of another incarnation
       of the HIP association, started by sending an I1,
       and stay at CLOSING

   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


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   Receive R1, process
       if successful, send I2 and go to I2-SENT
       if fail, stay at CLOSING

   Receive CLOSE, process
       if successful, send CLOSE_ACK, discard state and go to CLOSED
       if failed, stay at CLOSING
   Receive CLOSE_ACK, process
       if successful, discard state and go to UNASSOCIATED
       if failed, stay at CLOSING

   Receive ANYOTHER, drop and stay at CLOSING

   Timeout, increment timeout sum, reset timer
       if timeout sum is less than UAL+MSL minutes, retransmit CLOSE
         and stay at CLOSING
       if timeout sum is greater than UAL+MSL minutes, go to
          UNASSOCIATED

   +--------+
   | CLOSED | CLOSE_ACK sent, resending CLOSE_ACK if necessary
   +--------+

   Datagram to send, requires the creation of another incarnation
       of the HIP association, started by sending an I1,
       and stay at CLOSED

   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, process
       if successful, send CLOSE_ACK, stay at CLOSED
       if failed, stay at CLOSED

   Receive CLOSE_ACK, process
       if successful, discard state and go to UNASSOCIATED
       if failed, stay at CLOSED

   Receive ANYOTHER, drop and stay at CLOSED

   Timeout (UAL + 2MSL), discard state and go to UNASSOCIATED


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   +----------+
   | E-FAILED | HIP failed to establish association with peer
   +----------+

   Move to UNASSOCIATED after an implementation specific time.  Re-negotiation
   is possible after moving to UNASSOCIATED state.


5.4.3  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.

                                +-+        +------------------------------+
           I1 received, send R1 | |        |                              |
                                | v        v                              |
            Datagram to send  +--------------+  I2 received, send R2      |
              +---------------| UNASSOCIATED |---------------+            |
              |               +--------------+               |            |
              v                                              |            |
         +---------+  I2 received, send R2                   |            |
   +---->| I1-SENT |---------------------------------------+ |            |
   |     +---------+                                       | |            |
   |          |                 +------------------------+ | |            |
   |          | R1 received,    | I2 received, send R2   | | |            |
   |          v send I2         |                        v v v            |
   |     +---------+            |                     +---------+         |
   |  +->| I2-SENT |------------+                     | R2-SENT |<-----+  |
   |  |  +---------+                                  +---------+      |  |
   |  |          |                                      |              |  |
   |  |          |                                      |              |  |
   |  |receive   |                                      |              |  |
   |  |R1, send  |                             timeout, |   receive I2,|  |
   |  |I2        |R2 received +--------------+ data     |       send R2|  |
   |  |          +----------->| ESTABLISHED  |<---------+              |  |
   |  |                       +--------------+                         |  |
   |  |                            |    |  |                           |  |
   |  |                            |    |  +---------------------------+  |
   |  |                            |    |                              |  |
   |  |                            |    | No packet sent/received      |  |
   |  |                            |    | for UAL min, send CLOSE      |  |
   |  |                            |    |                              |  |
   |  |                            |    |    +---------+<-+ timeout    |  |
   |  |                            |    +--->| CLOSING |--+ (UAL+MSL)  |  |
   |  |                            |         +---------+    retransmit |  |
   +--+----------------------------+---------+ | |  | |     CLOSE      |  |


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   |  +----------------------------+-----------+ |  | +----------------+  |
   |  |                            | +-----------+  +------------------+--+
   |  |                            | | receive CLOSE,   CLOSE_ACK      |  |
   |  |                            | | send CLOSE_ACK   received or    |  |
   |  |                            v v                  timeout        |  |
   |  |                           +--------+            (UAL+MSL)      |  |
   |  +---------------------------| CLOSED |---------------------------+  |
   +------------------------------+--------+------------------------------+
   Datagram to send                    ^ |            timeout (UAL+2MSL),
                                       +-+            move to UNASSOCIATED
                                    CLOSE received,
                                    send CLOSE_ACK




















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6.  Packet formats

6.1  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   |  Payload Len  |     Type      |  VER. |  RES. |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Controls             |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                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, implementations MUST 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 bytes 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 TLVs included in the Parameters
   field, independent of the individual TLV 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


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   packet.

   The HIP Version is four bits.  The current version is 1.  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 four bits are reserved for future use.  They MUST be
   zero when sent, and they SHOULD be ignored when handling a received
   packet.

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

6.1.1  HIP Controls

   The HIP control section transfers information about the structure of
   the packet and capabilities of the host.

   The following fields have been defined:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SHT | DHT | | | | | | | | |C|A|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   C - Certificate One or more certificate packets (CER) follows this
      HIP packet (see Section 7.5).
   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.
   SHT - Sender's HIT Type Currently the following values are specified:
         0 RESERVED
         1 Type 1 HIT
         2 Type 2 HIT
         3-6 UNASSIGNED
         7 RESERVED
   DHT - Destination's HIT Type Using the same values as SHT.
   The rest of the fields are reserved for future use and MUST be set to
   zero on sent packets and ignored on received packets.

6.1.2  Checksum

   The checksum field is located at the same location within the header
   as the checksum field in UDP packets, enabling hardware assisted
   checksum generation and verification.  Note that since the checksum
   covers the source and destination addresses in the IP header, it must
   be recomputed on HIP-aware NAT boxes.


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   If IPv6 is used to carry the HIP packet, the pseudo-header [11]
   contains the source and destination IPv6 addresses, HIP packet length
   in the pseudo-header length field, a zero field, and the HIP protocol
   number (TBD, 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 [1] 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 (TBD, see Section 4), and the length is
   calculated as in the IPv6 case.

6.2  HIP parameters

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

   The following parameter types are currently defined.

   +-----------------+-------+----------+------------------------------+
   | TLV             | Type  | Length   | Data                         |
   +-----------------+-------+----------+------------------------------+
   | R1_COUNTER      | 2     | 12       | System Boot Counter          |
   |                 |       |          |                              |
   | PUZZLE          | 5     | 12       | K and Random #I              |
   |                 |       |          |                              |
   | SOLUTION        | 7     | 20       | K, Random #I and puzzle      |
   |                 |       |          | solution J                   |
   |                 |       |          |                              |
   | SEQ             | 11    | 4        | Update packet ID number      |
   |                 |       |          |                              |
   | ACK             | 13    | variable | Update packet ID number      |
   |                 |       |          |                              |
   | DIFFIE_HELLMAN  | 15    | variable | public key                   |
   |                 |       |          |                              |
   | HIP_TRANSFORM   | 17    | variable | HIP Encryption and Integrity |
   |                 |       |          | Transform                    |
   |                 |       |          |                              |
   | ENCRYPTED       | 21    | variable | Encrypted part of I2 or CER  |
   |                 |       |          | packets                      |
   |                 |       |          |                              |
   | HOST_ID         | 35    | variable | Host Identity with Fully     |
   |                 |       |          | Qualified Domain Name or NAI |
   |                 |       |          |                              |
   | CERT            | 64    | variable | HI Certificate               |


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   |                 |       |          |                              |
   | NOTIFY          | 256   | variable | Informational data           |
   |                 |       |          |                              |
   | ECHO_REQUEST    | 1022  | variable | Opaque data to be echoed     |
   |                 |       |          | back; under signature        |
   |                 |       |          |                              |
   | ECHO_RESPONSE   | 1024  | variable | Opaque data echoed back;     |
   |                 |       |          | under signature              |
   |                 |       |          |                              |
   | HMAC            | 65245 | 20       | HMAC based message           |
   |                 |       |          | authentication code, with    |
   |                 |       |          | key material from            |
   |                 |       |          | HIP_TRANSFORM                |
   |                 |       |          |                              |
   | HMAC_2          | 65247 | 20       | HMAC based message           |
   |                 |       |          | authentication code, with    |
   |                 |       |          | key material from            |
   |                 |       |          | HIP_TRANSFORM                |
   |                 |       |          |                              |
   | HIP_SIGNATURE_2 | 65277 | variable | Signature of the R1 packet   |
   |                 |       |          |                              |
   | HIP_SIGNATURE   | 65279 | variable | Signature of the packet      |
   |                 |       |          |                              |
   | ECHO_REQUEST    | 65281 | variable | Opaque data to be echoed     |
   |                 |       |          | back                         |
   |                 |       |          |                              |
   | ECHO_RESPONSE   | 65283 | variable | Opaque data echoed back;     |
   |                 |       |          | after signature              |
   +-----------------+-------+----------+------------------------------+

6.2.1  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 into the packet so that the types form an increasing order.
   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
   transport format [23].  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.  First one of the transport formats it the most
   preferred, and so on.


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   All 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.  If
   padding is added, the Length field MUST NOT include the padding.  Any
   added padding bytes MUST be set zero by the sender, but their content
   SHOULD NOT be checked on the receiving end.

   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;

       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
        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.
      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 4.4.

   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.



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6.2.2  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 the increasing order by the type code, thereby
   limiting the order of parameters.

   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.
   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 configure the associated feature off, such that
       the critical parameter is not sent at all.  The configuration
       option must be well documented.  By default, sending of such a
       new critical parameter SHOULD be off.  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.  The following type codes are reserved for future base protocol
       extensions, and may be assigned only through an appropriate WG or
       RFC action.
          0 - 511
          65024 - 65535
   5.  The following type codes are reserved for experimentation and
       private use.  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.
          32768 - 49141
   6.  All other parameter type codes MUST be registered by the IANA.
       See Section 13.







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6.2.3  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           2
      Length         12
      R1 generation
      counter   The current generation of valid puzzles

   The R1_COUNTER parameter contains an 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 in verbatim)
   by the Initiator in the I2.











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6.2.4  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, 8 bytes                                           |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           5
      Length         12
      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

   Random #I is represented as 64-bit integer, K and Lifetime as 8-bit
   integer, all in network byte order.

   The PUZZLE parameter contains the puzzle difficulty K and an 64-bit
   puzzle random integer #I.  Puzzle Lifetime indicates the time during
   which the puzzle solution is valid and sets a time limit for
   initiator which it should not exceed while trying to solve the
   puzzle.  The lifetime is indicated as power of 2 using formula
   2^(Lifetime-32) seconds.  A puzzle MAY be augmented by including an
   ECHO_REQUEST parameter to an R1.  The contents of the ECHO_REQUEST
   are then echoed back in ECHO_RESPONSE, allowing the Responder to use
   the included information as a part of puzzle processing.

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








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6.2.5  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, 8 bytes                                            |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Puzzle solution #J, 8 bytes                                   |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           7
      Length         20
      K              K is the number of verified bits
      Reserved       zero when sent, ignored when received
      Opaque         Copied unmodified from the received PUZZLE TLV
      Random #I      random number
      Puzzle solution
      #J             random number

   Random #I, and Random #J are represented as 64-bit integers, K as
   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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6.2.6  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                    /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                               |            padding            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           15
      Length         length in octets, excluding Type, Length, and padding
      Group ID       defines values for p and g
      Public Value   the sender's public Diffie-Hellman key

   The following Group IDs have been defined:

      Group                            Value
      Reserved                         0
      384-bit group                    1
      OAKLEY well known group 1        2
      1536-bit MODP group              3
      3072-bit MODP group              4
      6144-bit MODP group              5
      8192-bit MODP group              6

   The MODP Diffie-Hellman groups are defined in [18].  The OAKLEY group
   is defined in [9].  The OAKLEY well known group 5 is the same as the
   1536-bit MODP group.

   A HIP implementation MUST support Group IDs 1 and 3.  The 384-bit
   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).
   Equipment powerful enough SHOULD implement also group ID 5.  The
   384-bit group is defined in Appendix E.

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





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6.2.7  HIP_TRANSFORM

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Transform-ID #1      |       Transform-ID #2         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Transform-ID #n      |             Padding           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           17
      Length         length in octets, excluding Type, Length, and padding
      Transform-ID   Defines the HIP Suite to be used

   The following Suite-IDs are defined ([20],[24]):

   XXX: Deprecate MD5 in the light of recent development?

         Suite-ID                          Value

         RESERVED                          0
         AES-CBC with HMAC-SHA1            1
         3DES-CBC with HMAC-SHA1           2
         3DES-CBC with HMAC-MD5            3
         BLOWFISH-CBC with HMAC-SHA1       4
         NULL-ENCRYPT with HMAC-SHA1       5
         NULL-ENCRYPT with HMAC-MD5        6

   There MUST NOT be more than six (6) HIP Suite-IDs in one HIP
   transform TLV.  The limited number of transforms sets the maximum
   size of HIP_TRANSFORM TLV.  The HIP_TRANSFORM TLV MUST contain at
   least one of the mandatory Suite-IDs.

   Mandatory implementations: AES-CBC with HMAC-SHA1 and NULL-ENCRYPTION
   with HMAC-SHA1.







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6.2.8  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              35
      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 RFC2535 [12] format.  The
   algorithms used in RDATA format are the following:

         Algorithms       Values

         RESERVED         0
         DSA              3 [RFC2536] (RECOMMENDED)
         RSA              5 [RFC3110] (REQUIRED)

   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, in binary format.  The
                          format of the NAI is login@FQDN.


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   The format for the FQDN is defined in RFC1035 [3] Section 3.1.

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

6.2.9  CERT

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Cert count   |   Cert ID     |   Cert type   |               /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                          Certificate                          /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                               |            Padding            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           64
      Length         length in octets, excluding Type, Length, and padding
      Cert count     total count of certificates that are sent, possibly
                     in several consecutive CER packets
      Cert ID        the order number for this certificate
      Cert Type      describes the type of the certificate

   The receiver must know the total number (Cert count) of certificates
   that it will receive from the sender, related to the R1 or I2.  The
   Cert ID identifies the particular certificate and its order in the
   certificate chain.  The numbering in Cert ID MUST go from 1 to Cert
   count.

   The following certificate types are defined:

      Cert format    Type number
      X.509 v3       1

   The encoding format for X.509v3 certificate is defined in [15].







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6.2.10  HMAC

       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                              |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           65245
      Length         20
      HMAC           160 low order bits of the HMAC computed over the HIP
                     packet, excluding the HMAC parameter and any
                     following HIP_SIGNATURE or HIP_SIGNATURE_2
                     parameters.  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 HMAC calculation and verification process is presented in
   Section 8.3.1

6.2.11  HMAC_2

   The TLV structure is the same as in Section 6.2.10.  The fields are:

      Type           65247
      Length         20
      HMAC           160 low order bits of the HMAC computed over the HIP
                     packet, excluding the HMAC parameter and any
                     following HIP_SIGNATURE or HIP_SIGNATURE_2
                     parameters and including an additional sender's
                     HOST_ID TLV 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 HMAC calculation and verification process is presented in
   Section 8.3.1


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6.2.12  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           65279 (2^16-2^8-1)
      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 TLV field and any TLVs
                     that follow the HIP_SIGNATURE TLV.  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 TLV when the signature
                     is calculated.

   The signature algorithms are defined in Section 6.2.8.  The signature
   in the Signature field is encoded using the proper method depending
   on the signature algorithm (e.g.  according to [14] in case of RSA,
   or according to [13] in case of DSA).

   The HIP_SIGNATURE calculation and verification process is presented
   in Section 8.3.2

6.2.13  HIP_SIGNATURE_2

   The TLV structure is the same as in Section 6.2.12.  The fields are:

      Type           65277 (2^16-2^8-3)
      Length         length in octets, excluding Type, Length, and Padding
      SIG alg        Signature algorithm
      Signature      the signature is calculated over the HIP R1 packet,
                     excluding the HIP_SIGNATURE_2 TLV field and any
                     TLVs that follow the HIP_SIGNATURE_2 TLV.  Initiator's
                     HIT, checksum field, and the Opaque and Random #I
                     fields in the PUZZLE TLV MUST be set to zero while
                     computing the HIP_SIGNATURE_2 signature.  Further, the
                     HIP packet length in the HIP header MUST be
                     calculated to the beginning of the HIP_SIGNATURE_2
                     TLV when the signature is calculated.


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

   Signature calculation and verification follows the process in
   Section 8.3.2.

6.2.14  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           11
      Length         4
      Update ID      32-bit sequence number

   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 (i.e., the first UPDATE packet
   originated by a host has an Update ID of 1).

6.2.15  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            13
      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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6.2.16  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           21
      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 transform.
      Encrypted      The data is encrypted using an encryption algorithm as
      data           defined in HIP transform.
      Padding        Any Padding, if necessary, to make the TLV a multiple
                     of 8 bytes.

   The encrypted data is in TLV format itself.  Consequently, the first
   fields in the contents are Type and Length, allowing the contents to
   be easily parsed after decryption.  Each of the TLVs to be encrypted,
   must be padded according to rules in Section 6.2.1 before encryption.

   If the encryption algorithm requires the length of the data to be
   encrypted to be a multiple of the cipher algorithm block size,
   thereby necessitating padding, and if the encryption algorithm does
   not specify the padding contents, then an implementation MUST append
   the TLV parameter that is to be encrypted with an additional padding,
   so that the length of the resulting cleartext is a multiple of the
   cipher block size length.  Such a padding MUST be constructed as
   specified in [19] Section 2.4.  On the other hand, if the data to be
   encrypted is already a multiple of the block size, or if the
   encryption algorithm does specify padding as per [19] Section 2.4,
   then such additional padding SHOULD NOT be added.

   The Length field in the inside, to be encrypted TLV does not include
   the padding.  The Length field in the outside ENCRYPTED TLV is the


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   length of the data after encryption (including the Reserved field,
   the IV field, and the output from the encryption process specified
   for that suite, but not any additional external padding).  Note that
   the length of the cipher suite output may be smaller or larger than
   the length of the data to be encrypted, since the encryption process
   may compress the data or add additional padding to the data.

   The ENCRYPTED payload may contain additional external padding, if the
   result of encryption, the TLV header and the IV is not a multiple of
   8 bytes.  The contents of this external padding MUST follow the rules
   given in Section 6.2.1.

6.2.17  NOTIFY

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

       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           256
      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 TLV 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.


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

   Types in the range 0 - 16383 are intended for reporting errors.  An
   implementation that receives a NOTIFY 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.

      NOTIFY PARAMETER - ERROR TYPES           Value
      ------------------------------           -----

      UNSUPPORTED_CRITICAL_PARAMETER_TYPE        1

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

      INVALID_SYNTAX                             7

         Indicates that the HIP message received was invalid because
         some type, length, or value was out of range or because the
         request was rejected for policy reasons.  To avoid a denial
         of service attack using forged messages, this status may
         only be returned for and in an encrypted packet if the
         message ID and cryptographic checksum were valid.  To avoid
         leaking information to someone probing a node, this status
         MUST be sent in response to any error not covered by one of
         the other status types.  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 D-H Group ID field does not correspond to one offered
         by the responder.

      NO_HIP_PROPOSAL_CHOSEN                    16


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         None of the proposed HIP Transform crypto suites was
         acceptable.

      INVALID_HIP_TRANSFORM_CHOSEN              17

         The HIP Transform crypto suite does not correspond to
         one offered by the responder.

      AUTHENTICATION_FAILED                     24

         Sent in response to a HIP signature failure.

      CHECKSUM_FAILED                           26

         Sent in response to a HIP checksum failure.

      HMAC_FAILED                               28

         Sent in response to a HIP HMAC failure.

      ENCRYPTION_FAILED                         32

         The responder could not successfully decrypt the
         ENCRYPTED TLV.

      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 your request.
         You may retry if you wish, however you MUST find
         another (different) puzzle solution for any such
         retries.  Note that you may need to obtain a new
         puzzle with a new I1/R1 exchange.

      I2_ACKNOWLEDGEMENT                        46


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

      NOTIFY MESSAGES - STATUS TYPES           Value
      ------------------------------           -----

      (None defined at present)

6.2.18  ECHO_REQUEST

       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         65281 or 1022
      Length       variable
      Opaque data  Opaque data, supposed to be meaningful only to the
                   node that sends ECHO_REQUEST and receives a corresponding
                   ECHO_RESPONSE.

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

   The ECHO_REQUEST and 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 MAY be
   covered by the HMAC and SIGNATURE.  This is dictated by the Type
   field selected for the parameter; Type 1022 ECHO_REQUEST is covered
   and Type 65281 is not.







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6.2.19  ECHO_RESPONSE

       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         65283 or 1024
      Length       variable
      Opaque data  Opaque data, copied unmodified from the ECHO_REQUEST
                   parameter that triggered this response.

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

   The ECHO_REQUEST and 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_RESPONSE MAY be
   covered by the HMAC and SIGNATURE.  This is dictated by the Type
   field selected for the parameter; Type 1024 ECHO_RESPONSE is covered
   and Type 65283 is not.

6.3  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 [4].  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.

6.3.1  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.

6.3.2  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


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   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.

6.3.3  Invalid Cookie Solution

   If a HIP implementation receives an I2 packet that has an invalid
   cookie solution, the behaviour 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 form the I2
   message so that the SOLUTION parameter fits in to 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 [2].

6.3.4  Non-existing HIP association

   If a HIP implementation receives a CLOSE, or UPDATE packet, or any
   other packet whose handling requires an existing association, that
   has either a Receiver or Sender HIT that does not match with any
   existing HIP association, the implementation MAY respond, rate
   limited, with an ICMP packet with the type Parameter Problem, the
   Pointer pointing to the 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, CER, and NOTIFY.  When
   introducing new packet types, a specification SHOULD define the
   appropriate rules for sending or not sending this kind of ICMP
   replies.







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7.  HIP Packets

   There are nine basic HIP packets.  Four are for the HIP base
   exchange, one is for updating, one is for sending certificates, one
   for sending notifications, and two for closing a HIP association.

   Packets consist of the fixed header as described in Section 6.1,
   followed by the parameters.  The parameter part, in turn, consists of
   zero or more TLV coded parameters.

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

   Packet representation uses the following operations:

      ()      parameter
      x{y}    operation x on content y
      <x>i    x exists i times
      []      optional parameter
      x | y   x or y

   In the future, an OPTIONAL upper layer payload MAY follow the HIP
   header.  The payload proto 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.

7.1  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

      IP ( HIP () )

   The I1 packet contains only the fixed HIP header.

   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 opportunistic mode by using NULL (all zeros) as the
   Responder's HIT.  If the Initiator send a NULL as the Responder's


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   HIT, it MUST be able to handle all MUST and SHOULD algorithms from
   Section 3, which are currently RSA and DSA.

   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.

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

7.2  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_TRANSFORM,
                 HOST_ID,
                 [ ECHO_REQUEST, ]
                 HIP_SIGNATURE_2 )
                 [, ECHO_REQUEST ])

   Valid control bits: C, A

   The R1 packet may be followed by one or more CER packets.  In this
   case, the C-bit in the control field MUST be set.

   If the responder HI is an anonymous one, the A control MUST be set.

   The initiator HIT MUST match the one received in I1.  If the
   Responder has multiple HIs, the responder HIT used MUST match
   Initiator's request.  If the Initiator used opportunistic mode, the
   Responder may select freely among its HIs.

   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 not accepted any longer.

   The Puzzle contains a random #I and the difficulty K.  The difficulty
   K is the number of bits that the Initiator must get zero in the
   puzzle.  The random #I is not covered by the signature and must be


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   zeroed during the signature calculation, allowing the sender to
   select and set the #I into a pre-computed R1 just prior sending it to
   the peer.

   The Diffie-Hellman value is ephemeral, but can be reused over a
   number of connections.  In fact, as a defense against I1 storms, an
   implementation MAY use the same Diffie-Hellman value for a period of
   time, for example, 15 minutes.  By using a small number of different
   Cookies for a given Diffie-Hellman value, the R1 packets can be
   pre-computed and delivered as quickly as I1 packets arrive.  A
   scavenger process should clean up unused DHs and Cookies.

   The HIP_TRANSFORM contains the encryption and integrity algorithms
   supported by the Responder to protect the HI exchange, in the order
   of preference.  All implementations MUST support the AES [10] with
   HMAC-SHA-1-96 [6].

   The ECHO_REQUEST contains data that the sender wants to receive
   unmodified in the corresponding response packet in the ECHO_RESPONSE
   parameter.  The ECHO_REQUEST can be either covered by the signature,
   or it can be left out from it.  In the first case, the ECHO_REQUEST
   gets Type number 1022 and in the latter case 65281.

   The signature is calculated over the whole HIP envelope, after
   setting the initiator HIT, header checksum as well as the Opaque
   field and the Random #I in the PUZZLE parameter temporarily to zero,
   and excluding any TLVs that follow the signature, as described in
   Section 6.2.13.  This allows the Responder to use precomputed R1s.
   The Initiator SHOULD validate this signature.  It SHOULD check that
   the responder HI received matches with the one expected, if any.

7.3  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_TRANSFORM,
                 ENCRYPTED { HOST_ID },
                 [ ECHO_RESPONSE ,]
                 HMAC,
                 HIP_SIGNATURE


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                 [, ECHO_RESPONSE] ) )

   Valid control bits: C, A

   The HITs used MUST match the ones used previously.

   If the initiator 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 SHA-1(I | ...  | J) MUST be zero.

   The Diffie-Hellman value is ephemeral.  If precomputed, a scavenger
   process should clean up unused DHs.

   The HIP_TRANSFORM contains the encryption and integrity used to
   protect the HI exchange selected by the Initiator.  All
   implementations MUST support the AES transform [10].

   The Initiator's HI is encrypted using the HIP_TRANSFORM encryption
   algorithm.  The keying material is derived from the Diffie-Hellman
   exchanged as defined in Section 9.

   The ECHO_RESPONSE contains the the unmodified Opaque data copied from
   the corresponding ECHO_REQUEST TLV.  The ECHO_RESPONSE can be either
   covered by the signature, or it can be left out from it.  In the
   first case, the ECHO_RESPONSE gets Type number 1024 and in the latter
   case 65283.

   The HMAC is calculated over whole HIP envelope, excluding any TLVs
   after the HMAC, as described in Section 8.3.1.  The Responder MUST
   validate the HMAC.

   The signature is calculated over whole HIP envelope, excluding any
   TLVs after the HIP_SIGNATURE, as described in Section 6.2.12.  The
   Responder MUST validate this signature.  It MAY use either the HI in
   the packet or the HI acquired by some other means.

7.4  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


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      IP ( HIP ( HMAC_2, HIP_SIGNATURE ) )

   Valid control bits: none

   The HMAC_2 is calculated over whole HIP envelope, with Responder's
   HOST_ID TLV concatenated with the HIP envelope.  The HOST_ID TLV is
   removed after the HMAC calculation.  The procedure is described in
   8.3.1.

   The signature is calculated over whole HIP envelope.

   The Initiator MUST validate both the HMAC and the signature.

7.5  CER - the HIP Certificate Packet

   The CER packet is OPTIONAL.

   The Optional CER packets over the Announcer's HI by a higher level
   authority known to the Recipient is an alternative method for the
   Recipient to trust the Announcer's HI (over DNSSEC or PKI).

   The HIP header values for CER packet:

      Header:
        Packet Type = 5
        SRC HIT = Announcer's HIT
        DST HIT = Recipient's HIT

      IP ( HIP ( <CERT>i , HIP_SIGNATURE ) )  or

      IP ( HIP ( ENCRYPTED { <CERT>i }, HIP_SIGNATURE ) )

   Valid control bits: None

   Certificates in the CER packet MAY be encrypted.  The encryption
   algorithm is provided in the HIP transform of the previous (R1 or I2)
   packet.

7.6  UPDATE - the HIP Update Packet

   Support for the UPDATE packet is MANDATORY.

   The HIP header values for the UPDATE packet:




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      Header:
        Packet Type = 6
        SRC HIT = Sender's HIT
        DST HIT = Recipient's HIT

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

   Valid control bits: None

   The UPDATE packet contains mandatory HMAC 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 forego 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 retransmitting without incrementing SEQ.  If the same
   subset of parameters is included in multiple UPDATEs with different
   SEQs, the host MUST ensure that receiver processing of the parameters
   multiple times will not result in a protocol error.

7.7  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.


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   The HIP header values for the NOTIFY packet:

      Header:
        Packet Type = 7
        SRC HIT = Sender's HIT
        DST HIT = Recipient's HIT, or zero if unknown

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

   Valid control bits: None

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

7.8  CLOSE - the HIP association closing packet

   The HIP header values for the CLOSE packet:

      Header:
        Packet Type = 8
        SRC HIT = Sender's HIT
        DST HIT = Recipient's HIT

      IP ( HIP ( ECHO_REQUEST, HMAC, HIP_SIGNATURE ) )

   Valid control bits: none

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

   The receiver peer MUST validate both the HMAC and the signature if it
   has a HIP association state, and MUST reply with a CLOSE_ACK
   containing an ECHO_REPLY corresponding to the received ECHO_REQUEST.

7.9  CLOSE_ACK - the HIP closing acknowledgment packet

   The HIP header values for the CLOSE_ACK packet:

      Header:
        Packet Type = 9
        SRC HIT = Sender's HIT
        DST HIT = Recipient's HIT

      IP ( HIP ( ECHO_REPLY, HMAC, 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.
























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8.  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 5.4.  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 associations 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.

8.1  Processing outgoing application data

   In a HIP host, an application can send application level data using
   HITs or LSIs as source and destination identifiers.  The HITs and
   LSIs may be specified via a backwards compatible API (see [29]) or a
   completely new API.  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
   transmitted in the network using the appropriate source and
   destination IP addresses.  Here, we specify the processing rules only
   for the base case where both hosts have only single usable IP
   addresses; the multi-address multi-homing case will be specified
   separately.

   If the IPv4 or IPv6 backward compatible APIs and therefore LSIs are
   supported, it is assumed that the LSIs will be converted into proper
   HITs somewhere in the stack.  The exact location of the conversion is
   an implementation specific issue and not discussed here.  The
   following conceptual algorithm discusses only HITs, with the
   assumption that the LSI-to-HIT conversion takes place somewhere.

   The following steps define the conceptual processing rules for
   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


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       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.  In selecting a proper local HIT, the implementation
       SHOULD consult the table of currently active HIP sessions, and
       preferably select a HIT that already has an active session with
       the target HIT.
   3.  If there is no active HIP session with the given < source,
       destination > HIT pair, one must be created by running the base
       exchange.  The implementation SHOULD queue at least one packet
       per HIP session to be formed, and it MAY queue more than one.
   4.  Once there is an active HIP session 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.  The HITs in the datagram are replaced with suitable IP addresses.
       For IPv6, the rules defined in [16] 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.

8.2  Processing incoming application data

   The transport format and method (defined in separate specifications)
   determines the format in which incoming HIP packets arrive to the
   host.  The following steps define the conceptual processing rules for
   incoming datagrams.  The specific transport format and method
   specifications define in more detail the packet processing, related
   to the method.

   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 IPsec Security Parameter Index (SPI)
       or a protocol port number.
   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.
   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.  Demultiplexing the
       datagram the right upper layer socket is based on the HITs (or
       LSIs).


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8.3  HMAC and SIGNATURE calculation and verification

   The following subsections define the actions for processing HMAC,
   HIP_SIGNATURE and HIP_SIGNATURE_2 TLVs.

8.3.1  HMAC calculation

   The following process applies both to the HMAC and HMAC_2 TLVs.  When
   processing HMAC_2, the difference is that the HMAC calculation
   includes pseudo HOST_ID field containing the Responder's information
   as sent in the R1 packet earlier.

   The HMAC TLV is defined in Section 6.2.10 and HMAC_2 TLV in
   Section 6.2.11. HMAC calculation and verification process:

   Packet sender:
   1.  Create the HIP packet, without the HMAC or any possible
       HIP_SIGNATURE or HIP_SIGNATURE_2 TLVs.
   2.  In case of HMAC_2 calculation, add a HOST_ID (Responder) TLV to
       the packet.
   3.  Calculate the Length field in the HIP header.
   4.  Compute the HMAC.
   5.  In case of HMAC_2, remove the HOST_ID TLV from the packet.
   6.  Add the HMAC TLV to the packet and any HIP_SIGNATURE or
       HIP_SIGNATURE_2 TLVs that may follow.
   7.  Recalculate the Length field in the HIP header.

   Packet receiver:
   1.  Verify the HIP header Length field.
   2.  Remove the HMAC or HMAC_2 TLV, and if the packet contains any
       HIP_SIGNATURE or HIP_SIGNATURE_2 fields, remove them too, saving
       the contents if they will be needed later.
   3.  In case of HMAC_2, build and add a HOST_ID TLV (with Responder
       information) to the packet.
   4.  Recalculate the HIP packet length in the HIP header and clear the
       Checksum field (set it to all zeros).
   5.  Compute the HMAC and verify it against the received HMAC.
   6.  In case of HMAC_2, remove the HOST_ID TLV from the packet before
       further processing.

8.3.2  Signature calculation

   The following process applies both to the HIP_SIGNATURE and
   HIP_SIGNATURE_2 TLVs.  When processing HIP_SIGNATURE_2, the only
   difference is that instead of HIP_SIGNATURE TLV, the HIP_SIGNATURE_2
   TLV 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 TLV is defined in Section 6.2.12 and the


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   HIP_SIGNATURE_2 TLV in Section 6.2.13.

   Signature calculation and verification process:

   Packet sender:
   1.  Create the HIP packet without the HIP_SIGNATURE TLV or any TLVs
       that follow the HIP_SIGNATURE TLV.
   2.  Calculate the Length field in the HIP header.
   3.  Compute the signature.
   4.  Add the HIP_SIGNATURE TLV to the packet.
   5.  Add any TLVs that follow the HIP_SIGNATURE TLV.
   6.  Recalculate the Length field in the HIP header.

   Packet receiver:
   1.  Verify the HIP header Length field.
   2.  Save the contents of the HIP_SIGNATURE TLV and any TLVs following
       the HIP_SIGNATURE TLV and remove them from the packet.
   3.  Recalculate the HIP packet Length in the HIP header and clear the
       Checksum field (set it to all zeros).
   4.  Compute the signature and verify it against the received
       signature.

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

8.4  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 5.3.

   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 7.1.  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:
   1.  The Initiator gets the Responder's HIT 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


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       using NULL (all zeros) as the responder's HIT.
   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.  Upon sending an I1, the sender shall transition to state I1-SENT,
       start a timer whose timeout value should be larger than the
       worst-case anticipated RTT, and shall increment a timeout counter
       associated with the I1.
   4.  Upon timeout, the sender SHOULD retransmit the I1 and restart the
       timer, up to a maximum of I1_RETRIES_MAX tries.

8.4.1  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.  These limitations are placed order to avoid
   congestion of the network, and potential DoS attacks that might
   happen, e.g., because someone claims to have hundreds or thousands of
   addresses.

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

   The Initiator SHOULD then select the initial preferred destination
   address using the source address of the selected received R1, and use
   the preferred address as a source address for the I2.  Processing
   rules for received R1s are discussed in Section 8.6.

8.4.2  Processing incoming ICMP Protocol Unreachable messages

   A host may receive an ICMP Destination Protocol Unreachable message
   as a response to sending an 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.


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8.5  Processing incoming I1 packets

   An implementation SHOULD reply to an I1 with an R1 packet, unless the
   implementation is unable or unwilling to setup a HIP association.  If
   the implementation is unable to setup 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 setup 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.

   Under no circumstances does the HIP state machine 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 HIT in the received
       I1 is either one of its own HITs, or NULL.
   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 implementation chooses to respond to the I1 with and R1
       packet, it creates a new R1 or selects a precomputed R1 according
       to the format described in Section 7.2.
   4.  The R1 MUST contain the received responder 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.
   5.  The responder sends the R1 to the source IP address of the I1
       packet.

8.5.1  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.  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.

   An implementation MAY keep state about received I1s and match the


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   received I2s against the state, as discussed in Section 4.1.1.

8.5.2  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 6.3.

8.6  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).  If so, it 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,
        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 6.2.13.
   5.  If the HIP association state is I1-SENT, and multiple valid R1s
        are present, the system SHOULD select from among the R1s with
        the largest R1 generation counter.
   6.  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.


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   7.  The R1 packet may have the C bit set -- in this case, the system
        should anticipate the receipt of HIP CER packets that contain
        the host identity corresponding to the responder's HIT.
   8.  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.
   9.  The system SHOULD attempt to validate the HIT against the
        received Host Identity.
   10.  The system MUST store the received R1 generation counter for
        future reference.
   11.  The system attempts to solve the cookie puzzle in R1.  The
        system MUST terminate the search after exceeding the remaining
        lifetime of the puzzle.  If the cookie puzzle is not
        successfully solved, the implementation may either resend I1
        within the retry bounds or abandon the HIP exchange.
   12.  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 9.  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.
   13.  The system selects the HIP transform 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.
   14.  The system initialized the remaining variables in the associated
        state, including Update ID counters.
   15.  The system prepares and sends an I2, as described in
        Section 7.3.
   16.  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.
   17.  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.

8.6.1  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


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   implementation SHOULD wait for some more time for a possible good R1,
   after which it MAY try again by sending a new I1 packet.

8.7  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
   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 cookie 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 C 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.  If the system is in the R2-SENT state, it 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 stay in R2-SENT.
   4.  If the system is in any other state, it 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.
   5.  The system MUST validate the solution to the cookie puzzle by
        computing the SHA-1 hash described in Section 7.3.
   6.  The I2 MUST have a single value in the HIP_TRANSFORM parameter,
        which MUST match one of the values offered to the Initiator in
        the R1 packet.
   7.  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 9.  If the Diffie-Hellman Group ID is
        unsupported, the I2 packet is silently dropped.
   8.  The encrypted HOST_ID decrypted by the Initiator encryption key
        defined in Section 9.  If the decrypted data is not an HOST_ID


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        parameter, the I2 packet is silently dropped.
   9.  The implementation SHOULD also verify that the Initiator's HIT in
        the I2 corresponds to the Host Identity sent in the I2.
   10.  The system MUST verify the HMAC according to the procedures in
        Section 6.2.10.
   11.  The system MUST verify the HIP_SIGNATURE according to
        Section 6.2.12 and Section 7.3.
   12.  If the checks above are valid, then the system proceeds with
        further I2 processing; otherwise, it discards the I2 and remains
        in the same state.
   13.  The I2 packet may have the C bit set -- in this case, the system
        should anticipate the receipt of HIP CER packets that contain
        the host identity corresponding to the responder's HIT.
   14.  The I2 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the I2 and returning to
        state UNASSOCIATED.  If the A bit is set, the Initiator's HIT is
        anonymous and should not be stored.
   15.  The system initialized the remaining variables in the associated
        state, including Update ID counters.
   16.  Upon successful processing of an I2 in states UNASSOCIATED,
        I1-SENT, I2-SENT, and R2-SENT, an R2 is sent and the state
        machine transitions to state ESTABLISHED.
   17.  Upon successful processing of an I2 in state ESTABLISHED, the
        old HIP association is dropped and a new one is installed, an R2
        is sent, and the state machine transitions to R2-SENT.
   18.  Upon transitioning to R2-SENT, start a timer.  Leave R2-SENT if
        either the timer expires (allowing for maximal retransmission of
        I2s), 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 has moved to
        ESTABLISHED).

8.7.1  Handling malformed messages

   If an implementation receives a malformed I2 message, the behaviour
   SHOULD depend on how much 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 6.3.

8.8  Processing incoming R2 packets

   An R2 received in states UNASSOCIATED, I1-SENT, ESTABLISHED, or
   REKEYING 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.


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   The following steps define the conceptual processing rules for
   incoming R2 packet:
   1.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in R1.
   2.  The system MUST verify the HMAC_2 according to the procedures in
       Section 6.2.11.
   3.  The system MUST verify the HIP signature according to the
       procedures in Section 6.2.12.
   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.

8.9  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.
   1.  The system increments its own Update ID value by one.
   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 Update ID found in the 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.
   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.

8.10  Receiving UPDATE packets

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

   1.  If there is no corresponding HIP association, the implementation
       MAY reply with an ICMP Parameter Problem, as specified in
       Section 6.3.4.


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   2.  If the association is in the ESTABLISHED state and the SEQ
       parameter is present, the UPDATE is processed and replied as
       described in Section 8.10.1.
   3.  Additionally (or alternatively), if the association is in the
       ESTABLISHED state and there is an ACK (of outstanding Update ID)
       in the UPDATE, the UPDATE is processed as described in
       Section 8.10.2.

8.10.1  Handling a SEQ paramaeter in a received UPDATE message

   1.  If the Update ID in the received SEQ is smaller than the stored
       Update ID for the peer host, the packet MUST BE dropped as a
       duplicate.
   2.  If the Update ID in the received SEQ is equal to the stored
       Update ID for the host, the packet is treated as a
       retransmission.  The HMAC verification (next step) MUST NOT be
       skipped.  (A byte-by-byte comparison of the received and a store
       packet would be OK, though.)  It is recommended that a host cache
       the last packet that was acked 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 HMAC 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 system MUST record
       the Update ID in the received SEQ parameter, for replay
       protection.
   6.  An UPDATE acknowledgement packet with ACK parameter is prepared
       and send to the peer.

8.10.2  Handling an ACK parameter in a received UPDATE packet

   1.  The UPDATE packet with ACK must match to an earlier sent UPDATE
       packet.  If no match is found, the packet MUST be dropped.
   2.  The system MUST verify the HMAC 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 8.9) so
       that the now acknowledged UPDATE is no longer retransmitted.



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8.11  Processing CER packets

   Processing CER packets is OPTIONAL, and currently undefined.

8.12  Processing NOTIFY packets

   Processing NOTIFY packets is OPTIONAL.  If processed, any errors
   noted by the NOTIFY parameter SHOULD be taken into account by the HIP
   state machine (e.g., by terminating a HIP handshake), and the error
   SHOULD be logged.

8.13  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 HMAC and SIGNATURE).  This processing
   applies whether or not the HIP association state is CLOSING in order
   to handle CLOSE messages from both ends crossing 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 implementation MAY
   reply to a CLOSE with an ICMP Parameter Problem, as specified in
   Section 6.3.4.

8.14  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_REPLY in response to the sent
   ECHO_REQUEST).

   The CLOSE_ACK uses HMAC and SIGNATURE for verification.  The state is
   discarded when the state changes to UNASSOCIATED and, after that,
   NOTIFY is sent as a response to a CLOSE message.

8.15  Dropping HIP associations

   A HIP implementation is free to drop a HIP association at any time,
   based on its own policy.  If a HIP host decides to drop a HIP
   association, it deletes the corresponding HIP state, including the
   keying material.  The implementation MUST also drop the peer's R1
   generation counter value, unless a local policy explicitly defines


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   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.
























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9.  HIP KEYMAT

   HIP keying material is derived from the Diffie-Hellman Kij produced
   during the HIP base exchange.  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 and the HITs into the following
   operation; the | operation denotes concatenation.

    KEYMAT = K1 | K2 | K3 | ...
          where

    K1   = SHA-1( Kij | sort(HIT-I | HIT-R) | 0x01 )
    K2   = SHA-1( Kij | K1 | 0x02 )
    K3   = SHA-1( Kij | K2 | 0x03 )
    ...
    K255 = SHA-1( Kij | K254 | 0xff )
    K256 = SHA-1( Kij | K255 | 0x00 )
    etc.

   Sort(HIT-I | HIT-R) is defined as the network byte order
   concatenation of the two HITs, with the smaller HIT preceding the
   larger HIT, resulting from the numeric comparison of the two HITs
   interpreted as positive (unsigned) 128-bit integers in network byte
   order.

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

   The drawing order for 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 bits



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   SHA-1 160 bits
   NULL 0 bits

























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10.  HIP Fragmentation Support

   A HIP implementation must support IP fragmentation / reassembly.
   Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
   fragment generation MUST be implemented only in IPv4 (IPv4 stacks and
   networks will usually do this by default) and SHOULD be implemented
   in IPv6.  In the IPv6 world, the minimum MTU is larger, 1280 bytes,
   than in the IPv4 world.  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 the IPv4 world, 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 of path MTU discovery
   does not bring any value to HIP in the IPv4 world.  HIP-aware NAT
   systems MUST perform any IPv4 reassembly/fragmentation.

   All HIP implementations MUST employ a reassembly algorithm that is
   sufficiently resistant against DoS attacks.















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11.  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 HIs, at least one of which SHOULD be
   reserved for anonymous usage.  Although anonymous HIs will be rarely
   used as responder 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 HIT to responder HIT.  This ACL SHOULD also include
   preferred transform and local lifetimes.  For HITs with HAAs,
   wildcarding SHOULD be supported.  Thus if a Community of Interest,
   like Banking, gets an RAA, a single ACL could be used.  A global
   wildcard would represent the general policy to be used.  Policy
   selection would be from most specific to most general.

   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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12.  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 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.  The duration of use is a
   paranoia versus throughput concern.  Using the same Diffie-Hellman
   values and random puzzle #I has some risk.  This risk needs to be
   balanced against a potential storm of HIP I1 packets.

   This shifting of the start of state cost to the Initiator in creating
   the I2 HIP packet, presents another DoS attack.  The attacker 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 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.  Thus 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 host restarting 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.


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   A fourth form of DoS attack is emulating the end of state.  HIP
   relies on timers plus a CLOSE/CLOSE_ACK handshake to explicitly
   signals the end of a state.  Because both CLOSE and CLOSE_ACK
   messages contain an HMAC, an outsider cannot close a connection.  The
   presence of an additional SIGNATURE allows middle-boxes 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 6.3.4) 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 Section 4.1.3.

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

   Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
   certificate, or otherwise securely available, the Responder can
   retrieve it after it gets the I2 HIP packet and validate that.
   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.

   If an initiator wants to use opportunistic mode, it is vulnerable to
   man-in-the-middle attacks.  Furthermore, the available HI types are
   limited to the MUST implement algorithms, as per Section 3.  Hence,
   if a future specification deprecates the current MUST implement
   algorithm(s) and replaces it (them) with some new one(s), backward
   compatibility cannot be preserved.

   Since not all hosts will ever support HIP, ICMP 'Destination Protocol
   Unreachable' are to be expected and present a DoS attack.  Against an
   Initiator, the attack would look like the Responder does not support
   HIP, but shortly after receiving the ICMP message, the Initiator
   would receive a valid 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.  First an ICMP
   message is expected if the I1 was a DoS attack and the real owner of
   the spoofed IP address does not support HIP.  The Responder SHOULD


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   NOT act on this ICMP message to remove the minimal state from the R1
   HIP packet (if it has one), but wait for either a valid I2 HIP packet
   or the natural timeout of the R1 HIP packet.  This is to allow for a
   sophisticated attacker that is trying to break up the HIP exchange.
   Likewise, the Initiator should ignore any ICMP message while waiting
   for an R2 HIP packet, deleting state only after a natural timeout.























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13.  IANA Considerations

   IANA has assigned IP Protocol number TBD to HIP.

   IANA needs to create registries for:
   1.  HIP packet types
   2.  HIP parameter types






















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14.  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 drafts 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 the framework for LSIs 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 cookie 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.
   Rodney Thayer and Hugh Daniels provide extensive feedback.  In the
   early times of this draft, John Gilmore kept Bob Moskowitz challenged
   to provide something of value.

   During the later stages of this document, when the editing baton was
   transfered to Pekka Nikander, the input from the early implementors
   were 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, 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 draft was split in two, one containing the base
   exchange and the other one defining how to use ESP.





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15.  References

15.1  Normative references

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

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

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

   [4]   Conta, A. and S. Deering, "Internet Control Message Protocol
         (ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC 1885,
         December 1995.

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

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

   [7]   Maughan, D., Schneider, M. and M. Schertler, "Internet Security
         Association and Key Management Protocol (ISAKMP)", RFC 2408,
         November 1998.

   [8]   Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [9]   Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412,
         November 1998.

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

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

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

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

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


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   [15]  Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

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

   [17]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
         Addressing Architecture", RFC 3513, April 2003.

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

   [19]  Kent, S., "IP Encapsulating Security Payload (ESP)",
         Internet-Draft draft-ietf-ipsec-esp-v3-05, April 2003.

   [20]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         Internet-Draft draft-ietf-ipsec-ikev2-07, April 2003.

   [21]  Moskowitz, R., "Host Identity Protocol Architecture",
         Internet-Draft draft-moskowitz-hip-arch-03, May 2003.

   [22]  NIST, "FIPS PUB 180-1: Secure Hash Standard", April 1995.

   [23]  Jokela, P., Moskowitz, R. and P. Nikander, "Using ESP transport
         format with HIP", Internet-Draft draft-jokela-hip-esp-00,
         January 2005.

15.2  Informative references

   [24]  Bellovin, S. and W. Aiello, "Just Fast Keying (JFK)",
         Internet-Draft draft-ietf-ipsec-jfk-04, July 2002.

   [25]  Moskowitz, R. and P. Nikander, "Using Domain Name System (DNS)
         with Host Identity Protocol (HIP)",
         Internet-Draft draft-nikander-hip-dns-00 (to be issued), June
         2003.

   [26]  Nikander, P., "SPI assisted NAT traversal (SPINAT) with Host
         Identity Protocol (HIP)",
         Internet-Draft draft-nikander-hip-nat-00 (to be issued), June
         2003.

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


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   [28]  Nikander, P., "A Bound End-to-End Tunnel (BEET) mode for ESP",
         Internet-Draft draft-nikander-esp-beet-mode-00 (expired), Oct
         2003.

   [29]  Henderson, T., "Using HIP with Legacy Applications",
         Internet-Draft draft-henderson-hip-applications-00.txt, Feb
         2005.

Authors' Addresses

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

   Email: rgm@icsalabs.com

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: pekka.nikander@nomadiclab.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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Appendix A.  Probabilities of HIT collisions

   The birthday paradox sets a bound for the expectation of collisions.
   It is based on the square root of the number of values.  A 64-bit
   hash, then, would put the chances of a collision at 50-50 with 2^32
   hosts (4 billion).  A 1% chance of collision would occur in a
   population of 640M and a .001% collision chance in a 20M population.
   A 128 bit hash will have the same .001% collision chance in a 9x10^16
   population.





















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Appendix B.  Probabilities in the cookie calculation

   A question: Is it guaranteed that the Initiator is able to solve the
   puzzle in this way when the K value is large?

   Answer: No, it is not guaranteed.  But it is not guaranteed even in
   the old mechanism, since the Initiator may start far away from J and
   arrive to J after far too many steps.  If we wanted to make sure that
   the Initiator finds a value, we would need to give some hint of a
   suitable J, and I don't think we want to do that.

   In general, if we model the hash function with a random function, the
   probability that one iteration gives are result with K zero bits is
   2^-K.  Thus, the probability that one iteration does _not_ give K
   zero bits is (1 - 2^-K).  Consequently, the probability that 2^K
   iterations does not give K zero bits is (1 - 2^-K)^(2^K).

   Since my calculus starts to be rusty, I made a small experiment and
   found out that

     lim     (1 - 2^-k)^(2^k)     = 0.36788
     k->inf

     lim     (1 - 2^-k)^(2^(k+1)) = 0.13534
     k->inf

     lim     (1 - 2^-k)^(2^(k+2)) = 0.01832
     k->inf

     lim     (1 - 2^-k)^(2^(k+3)) = 0.000335
     k->inf

   Thus, if hash functions were random functions, we would need about
   2^(K+3) iterations to make sure that the probability of a failure is
   less than 1% (actually less than 0.04%).  Now, since my perhaps
   flawed understanding of hash functions is that they are "flatter"
   than random functions, 2^(K+3) is probably an overkill.  OTOH, the
   currently suggested 2^K is clearly too little.






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Appendix C.  Using responder cookies

   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
   possible implementation in detail.  The purpose of this appendix is
   to give the implementors an idea on how to implement the mechanism.
   The method described in this appendix SHOULD NOT be used in any real
   implementation.  If the implementation is based on this appendix, it
   SHOULD contain some local modification that makes an attacker's task
   harder.

   The basic idea is to create a cheap, varying local mapping function
   f:

      f( IP-I, IP-R, HIT-I, HIT-R ) -> cookie-index

    That is, given the Initiator's and Responder's IP addresses and
   HITs, the function returns an index to a cookie.  When processing an
   I1, the cookie is embedded in an pre-computed R1, and the Responder
   simply sends that particular R1 to the Initiator.  When processing an
   I2, the cookie may still be embedded in the R1, or the R1 may be
   deprecated (and replaced with a new one), but the cookie is still
   there.  If the received cookie does not match with the R1 or saved
   cookie, the I2 is simply dropped.  That prevents the Initiator from
   generating spoofed I2s with a probability that depends on the number
   of pre-computed R1s.

   As a concrete example, let us assume that the Responder has an array
   of R1s.  Each slot in the array contains a timestamp, an R1, and an
   old cookie that was sent in the previous R1 that occupied that
   particular slot.  The Responder replaces one R1 in the array every
   few minutes, thereby replacing all the R1s gradually.

   To create a varying mapping function, the Responder generates a
   random number every few minutes.  The octets in the IP addresses and
   HITs are XORed together, and finally the result is XORed with the
   random number.  Using pseudo-code, the function looks like the
   following.

   Pre-computation:
       r1 := random number

   Index computation:
       index := r1    XOR hit_r[0] XOR hit_r[1] XOR ... XOR hit_r[15]
       index := index XOR hit_i[0] XOR hit_i[1] XOR ... XOR hit_i[15]
       index := index XOR  ip_r[0] XOR  ip_r[1] XOR ... XOR  ip_r[15]
       index := index XOR  ip_i[0] XOR  ip_i[1] XOR ... XOR  ip_i[15]


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    The index gives the slot used in the array.

   It is possible that an Initiator receives an I1, and while it is
   computing I2, the Responder deprecates an R1 and/or chooses a new
   random number for the mapping function.  Therefore the Responder must
   remember the cookies used in deprecated R1s and the previous random
   number.

   To check an received I2, the Responder can use a simple algorithm,
   expressed in pseudo-code as follows.

     If I2.hit_r does not match my_hits, drop the packet.

     index := compute_index(current_random_number, I2)
     If current_cookie[index]  == I2.cookie, go to cookie check.
     If previous_cookie[index] == I2.cookie, go to cookie check.

     index := compute_index(previous_random_number, I2)
     If current_cookie[index]  == I2.cookie, go to cookie check.
     If previous_cookie[index] == I2.cookie, go to cookie check.

     Drop packet.

   cookie_check:
     V := Ltrunc( SHA-1( I2.I, I2.hit_i, I2.hit_r, I2.J ), K )
     if V != 0, drop the packet.

   Whenever the Responder receives an I2 that fails on the index check,
   it can simply drop the packet on the floor and forget about it.  New
   I2s with the same or other spoofed parameters will get dropped with a
   reasonable probability and minimal effort.

   If a Responder receives an I2 that passes the index check but fails
   on the puzzle check, it should create a state indicating this.  After
   two or three failures the Responder should cease checking the puzzle
   but drop the packets directly.  This saves the Responder from the
   SHA-1 calculations.  Such block should not last long, however, or
   there would be a danger that a legitimate Initiator could be blocked
   from getting connections.

   A key for the success of the defined scheme is that the mapping
   function must be considerably cheaper than computing SHA-1.  It also
   must detect any changes in the IP addresses, and preferably most
   changes in the HITs.  Checking the HITs is not that essential,
   though, since HITs are included in the cookie computation, too.

   The effectivity of the method can be varied by varying the size of
   the array containing pre-computed R1s.  If the array is large, the


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   probability that an I2 with a spoofed IP address or HIT happens to
   map to the same slot is fairly slow.  However, a large array means
   that each R1 has a fairly long life time, thereby allowing an
   attacker to utilize one solved puzzle for a longer time.
























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Appendix D.  Example checksums for HIP packets

   The HIP checksum for HIP packets is specified in Section 6.1.2.
   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 type 1 HITs with the first two
   bits "01" followed by 124 zeroes followed by a decimal 1 or 2,
   respectively.

D.1  IPv6 HIP example (I1)

      Source Address:                 ::c0a8:0001
      Destination Address:            ::c0a8:0002
      Upper-Layer Packet Length:      40              0x28
      Next Header:                    99              0x63
      Payload Protocol:               59              0x3b
      Header Length:                  4               0x04
      Packet Type:                    1               0x01
      Version:                        1               0x1
      Reserved:                       0               0x0
      Control:                        0               0x0000
      Checksum:                       49672           0xc208
      Sender's HIT:                   4000::0001
      Receiver's HIT:                 4000::0002

D.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).

D.3  TCP segment

   Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
   use the IPv6 pseudo-header format [8], with the HITs used in place of
   the IPv6 addresses.







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      Sender's HIT:                   4000::0001
      Receiver's HIT:                 4000::0002
      Upper-Layer Packet Length:      20              0x14
      Next Header:                    6               0x06
      Source port:                    32769           0x8001
      Destination port:               22              0x0016
      Sequence number:                1               0x00000001
      Acknowledgment number:          0               0x00000000
      Header length:                  20              0x14
      Flags:                          SYN             0x02
      Window size:                    5840            0x16d0
      Checksum:                       54519           0xd4f7
      Urgent pointer:                 0               0x0000



















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Appendix E.  384-bit group

   This 384-bit group is defined only to be used with HIP.  NOTE: The
   security level of this group is very low!  The encryption may be
   broken in a very short time, even real-time.  It 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).

   This prime is: 2^384 - 2^320 - 1 + 2^64 * { [ 2^254 pi] + 5857 }

   Its hexadecimal value is:

        FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
        29024E08 8A67CC74 020BBEA6 3B13B202 FFFFFFFF FFFFFFFF

   The generator is: 2.


















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