GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1


rfc:rfc5201

Network Working Group R. Moskowitz Request for Comments: 5201 ICSAlabs Category: Experimental P. Nikander

                                                        P. Jokela, Ed.
                                          Ericsson Research NomadicLab
                                                          T. Henderson
                                                    The Boeing Company
                                                            April 2008
                       Host Identity Protocol

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

IESG Note

 The following issues describe IESG concerns about this document.  The
 IESG expects that these issues will be addressed when future versions
 of HIP are designed.
 This document doesn't currently define support for parameterized
 (randomized) hashing in signatures, support for negotiation of a key
 derivation function, or support for combined encryption modes.
 HIP defines the usage of RSA in signing and encrypting data.  Current
 recommendations propose usage of, for example, RSA OAEP/PSS for these
 operations in new protocols.  Changing the algorithms to more current
 best practice should be considered.
 The current specification is currently using HMAC for message
 authentication.  This is considered to be acceptable for an
 experimental RFC, but future versions must define a more generic
 method for message authentication, including the ability for other
 MAC algorithms to be used.
 SHA-1 is no longer a preferred hashing algorithm.  This is noted also
 by the authors, and it is understood that future, non-experimental
 versions must consider more secure hashing algorithms.
 HIP requires that an incoming packet's IP address be ignored.  In
 simple cases this can be done, but when there are security policies
 based on incoming interface or IP address rules, the situation

Moskowitz, et al. Experimental [Page 1] RFC 5201 Host Identity Protocol April 2008

 changes.  The handling of data needs to be enhanced to cover
 different types of network and security configurations, as well as to
 meet local security policies.

Abstract

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

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   1.1.  A New Namespace and Identifiers . . . . . . . . . . . . .   5
   1.2.  The HIP Base Exchange . . . . . . . . . . . . . . . . . .   6
   1.3.  Memo Structure  . . . . . . . . . . . . . . . . . . . . .   7
 2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   7
   2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   7
   2.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   7
   2.3.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
 3.  Host Identifier (HI) and Its Representations  . . . . . . . .   8
   3.1.  Host Identity Tag (HIT) . . . . . . . . . . . . . . . . .   9
   3.2.  Generating a HIT from an HI . . . . . . . . . . . . . . .   9
 4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  10
   4.1.  Creating a HIP Association  . . . . . . . . . . . . . . .  10
     4.1.1.  HIP Puzzle Mechanism  . . . . . . . . . . . . . . . .  12
     4.1.2.  Puzzle Exchange . . . . . . . . . . . . . . . . . . .  13
     4.1.3.  Authenticated Diffie-Hellman Protocol . . . . . . . .  14
     4.1.4.  HIP Replay Protection . . . . . . . . . . . . . . . .  14
     4.1.5.  Refusing a HIP Exchange . . . . . . . . . . . . . . .  15
     4.1.6.  HIP Opportunistic Mode  . . . . . . . . . . . . . . .  16
   4.2.  Updating a HIP Association  . . . . . . . . . . . . . . .  18
   4.3.  Error Processing  . . . . . . . . . . . . . . . . . . . .  18
   4.4.  HIP State Machine . . . . . . . . . . . . . . . . . . . .  19
     4.4.1.  HIP States  . . . . . . . . . . . . . . . . . . . . .  20
     4.4.2.  HIP State Processes . . . . . . . . . . . . . . . . .  21
     4.4.3.  Simplified HIP State Diagram  . . . . . . . . . . . .  28
   4.5.  User Data Considerations  . . . . . . . . . . . . . . . .  30
     4.5.1.  TCP and UDP Pseudo-Header Computation for User Data .  30

Moskowitz, et al. Experimental [Page 2] RFC 5201 Host Identity Protocol April 2008

     4.5.2.  Sending Data on HIP Packets . . . . . . . . . . . . .  30
     4.5.3.  Transport Formats . . . . . . . . . . . . . . . . . .  30
     4.5.4.  Reboot and SA Timeout Restart of HIP  . . . . . . . .  30
   4.6.  Certificate Distribution  . . . . . . . . . . . . . . . .  31
 5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  31
   5.1.  Payload Format  . . . . . . . . . . . . . . . . . . . . .  31
     5.1.1.  Checksum  . . . . . . . . . . . . . . . . . . . . . .  33
     5.1.2.  HIP Controls  . . . . . . . . . . . . . . . . . . . .  33
     5.1.3.  HIP Fragmentation Support . . . . . . . . . . . . . .  33
   5.2.  HIP Parameters  . . . . . . . . . . . . . . . . . . . . .  34
     5.2.1.  TLV Format  . . . . . . . . . . . . . . . . . . . . .  37
     5.2.2.  Defining New Parameters . . . . . . . . . . . . . . .  38
     5.2.3.  R1_COUNTER  . . . . . . . . . . . . . . . . . . . . .  39
     5.2.4.  PUZZLE  . . . . . . . . . . . . . . . . . . . . . . .  40
     5.2.5.  SOLUTION  . . . . . . . . . . . . . . . . . . . . . .  41
     5.2.6.  DIFFIE_HELLMAN  . . . . . . . . . . . . . . . . . . .  42
     5.2.7.  HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . .  43
     5.2.8.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  44
     5.2.9.  HMAC  . . . . . . . . . . . . . . . . . . . . . . . .  45
     5.2.10. HMAC_2  . . . . . . . . . . . . . . . . . . . . . . .  46
     5.2.11. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . .  46
     5.2.12. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . .  47
     5.2.13. SEQ . . . . . . . . . . . . . . . . . . . . . . . . .  48
     5.2.14. ACK . . . . . . . . . . . . . . . . . . . . . . . . .  48
     5.2.15. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . .  49
     5.2.16. NOTIFICATION  . . . . . . . . . . . . . . . . . . . .  50
     5.2.17. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . .  54
     5.2.18. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . .  54
     5.2.19. ECHO_RESPONSE_SIGNED  . . . . . . . . . . . . . . . .  55
     5.2.20. ECHO_RESPONSE_UNSIGNED  . . . . . . . . . . . . . . .  56
   5.3.  HIP Packets . . . . . . . . . . . . . . . . . . . . . . .  56
     5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  58
     5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  58
     5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  61
     5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  62
     5.3.5.  UPDATE - the HIP Update Packet  . . . . . . . . . . .  62
     5.3.6.  NOTIFY - the HIP Notify Packet  . . . . . . . . . . .  63
     5.3.7.  CLOSE - the HIP Association Closing Packet  . . . . .  64
     5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet . .  64
   5.4.  ICMP Messages . . . . . . . . . . . . . . . . . . . . . .  65
     5.4.1.  Invalid Version . . . . . . . . . . . . . . . . . . .  65
     5.4.2.  Other Problems with the HIP Header and Packet
             Structure . . . . . . . . . . . . . . . . . . . . . .  65
     5.4.3.  Invalid Puzzle Solution . . . . . . . . . . . . . . .  65
     5.4.4.  Non-Existing HIP Association  . . . . . . . . . . . .  66
 6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  66
   6.1.  Processing Outgoing Application Data  . . . . . . . . . .  66
   6.2.  Processing Incoming Application Data  . . . . . . . . . .  67

Moskowitz, et al. Experimental [Page 3] RFC 5201 Host Identity Protocol April 2008

   6.3.  Solving the Puzzle  . . . . . . . . . . . . . . . . . . .  68
   6.4.  HMAC and SIGNATURE Calculation and Verification . . . . .  70
     6.4.1.  HMAC Calculation  . . . . . . . . . . . . . . . . . .  70
     6.4.2.  Signature Calculation . . . . . . . . . . . . . . . .  72
   6.5.  HIP KEYMAT Generation . . . . . . . . . . . . . . . . . .  74
   6.6.  Initiation of a HIP Exchange  . . . . . . . . . . . . . .  75
     6.6.1.  Sending Multiple I1s in Parallel  . . . . . . . . . .  76
     6.6.2.  Processing Incoming ICMP Protocol Unreachable
             Messages  . . . . . . . . . . . . . . . . . . . . . .  77
   6.7.  Processing Incoming I1 Packets  . . . . . . . . . . . . .  77
     6.7.1.  R1 Management . . . . . . . . . . . . . . . . . . . .  78
     6.7.2.  Handling Malformed Messages . . . . . . . . . . . . .  79
   6.8.  Processing Incoming R1 Packets  . . . . . . . . . . . . .  79
     6.8.1.  Handling Malformed Messages . . . . . . . . . . . . .  81
   6.9.  Processing Incoming I2 Packets  . . . . . . . . . . . . .  81
     6.9.1.  Handling Malformed Messages . . . . . . . . . . . . .  84
   6.10. Processing Incoming R2 Packets  . . . . . . . . . . . . .  84
   6.11. Sending UPDATE Packets  . . . . . . . . . . . . . . . . .  84
   6.12. Receiving UPDATE Packets  . . . . . . . . . . . . . . . .  85
     6.12.1. Handling a SEQ Parameter in a Received UPDATE
             Message . . . . . . . . . . . . . . . . . . . . . . .  86
     6.12.2. Handling an ACK Parameter in a Received UPDATE
             Packet  . . . . . . . . . . . . . . . . . . . . . . .  87
   6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . .  87
   6.14. Processing CLOSE Packets  . . . . . . . . . . . . . . . .  88
   6.15. Processing CLOSE_ACK Packets  . . . . . . . . . . . . . .  88
   6.16. Handling State Loss . . . . . . . . . . . . . . . . . . .  88
 7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . .  89
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  89
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  92
 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  93
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  95
   11.1. Normative References  . . . . . . . . . . . . . . . . . .  95
   11.2. Informative References  . . . . . . . . . . . . . . . . .  96
 Appendix A.  Using Responder Puzzles  . . . . . . . . . . . . . .  98
 Appendix B.  Generating a Public Key Encoding from an HI  . . . .  99
 Appendix C.  Example Checksums for HIP Packets  . . . . . . . . . 100
   C.1.  IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 100
   C.2.  IPv4 HIP Packet (I1)  . . . . . . . . . . . . . . . . . . 100
   C.3.  TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 101
 Appendix D.  384-Bit Group  . . . . . . . . . . . . . . . . . . . 101
 Appendix E.  OAKLEY Well-Known Group 1  . . . . . . . . . . . . . 102

Moskowitz, et al. Experimental [Page 4] RFC 5201 Host Identity Protocol April 2008

1. Introduction

 This memo specifies the details of the Host Identity Protocol (HIP).
 A high-level description of the protocol and the underlying
 architectural thinking is available in the separate HIP architecture
 description [RFC4423].  Briefly, the HIP architecture proposes an
 alternative to the dual use of IP addresses as "locators" (routing
 labels) and "identifiers" (endpoint, or host, identifiers).  In HIP,
 public cryptographic keys, of a public/private key pair, are used as
 Host Identifiers, to which higher layer protocols are bound instead
 of an IP address.  By using public keys (and their representations)
 as host identifiers, dynamic changes to IP address sets can be
 directly authenticated between hosts, and if desired, strong
 authentication between hosts at the TCP/IP stack level can be
 obtained.
 This memo specifies the base HIP protocol ("base exchange") used
 between hosts to establish an IP-layer communications context, called
 HIP association, prior to communications.  It also defines a packet
 format and procedures for updating an active HIP association.  Other
 elements of the HIP architecture are specified in other documents,
 such as.
 o  "Using the Encapsulating Security Payload (ESP) Transport Format
    with the Host Identity Protocol (HIP)" [RFC5202]: how to use the
    Encapsulating Security Payload (ESP) for integrity protection and
    optional encryption
 o  "End-Host Mobility and Multihoming with the Host Identity
    Protocol" [RFC5206]: how to support mobility and multihoming in
    HIP
 o  "Host Identity Protocol (HIP) Domain Name System (DNS) Extensions"
    [RFC5205]: how to extend DNS to contain Host Identity information
 o  "Host Identity Protocol (HIP) Rendezvous Extension" [RFC5204]:
    using a rendezvous mechanism to contact mobile HIP hosts

1.1. A New Namespace and Identifiers

 The Host Identity Protocol introduces a new namespace, the Host
 Identity namespace.  Some ramifications of this new namespace are
 explained in the HIP architecture description [RFC4423].
 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

Moskowitz, et al. Experimental [Page 5] RFC 5201 Host Identity Protocol April 2008

 lengths, the HI is not good for use as a packet identifier, or as an
 index into the various operational tables needed to support HIP.
 Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes
 the operational representation.  It is 128 bits long and is used in
 the HIP payloads and to index the corresponding state in the end
 hosts.  The HIT has an important security property in that it is
 self-certifying (see Section 3).

1.2. The HIP Base Exchange

 The HIP base exchange is a two-party cryptographic protocol used to
 establish communications context between hosts.  The base exchange is
 a Sigma-compliant [KRA03] four-packet exchange.  The first party is
 called the Initiator and the second party the Responder.  The four-
 packet design helps to make HIP DoS resilient.  The protocol
 exchanges Diffie-Hellman keys in the 2nd and 3rd packets, and
 authenticates the parties in the 3rd and 4th packets.  Additionally,
 the Responder starts a puzzle exchange in the 2nd packet, with the
 Initiator completing it in the 3rd packet before the Responder stores
 any state from the exchange.
 The exchange can use the Diffie-Hellman output to encrypt the Host
 Identity of the Initiator in the 3rd packet (although Aura, et al.,
 [AUR03] notes that such operation may interfere with packet-
 inspecting middleboxes), or the Host Identity may instead be sent
 unencrypted.  The Responder's Host Identity is not protected.  It
 should be noted, however, that both the Initiator's and the
 Responder's HITs are transported as such (in cleartext) in the
 packets, allowing an eavesdropper with a priori knowledge about the
 parties to verify their identities.
 Data packets start to flow after the 4th packet.  The 3rd and 4th HIP
 packets may carry a data payload in the future.  However, the details
 of this are to be defined later as more implementation experience is
 gained.
 An existing HIP association can be updated using the update mechanism
 defined in this document, and when the association is no longer
 needed, it can be closed using the defined closing mechanism.
 Finally, HIP is designed as an end-to-end authentication and key
 establishment protocol, to be used with Encapsulated Security Payload
 (ESP) [RFC5202] and other end-to-end security protocols.  The base
 protocol does not cover all the fine-grained policy control found in
 Internet Key Exchange (IKE) [RFC4306] that allows IKE to support
 complex gateway policies.  Thus, HIP is not a replacement for IKE.

Moskowitz, et al. Experimental [Page 6] RFC 5201 Host Identity Protocol April 2008

1.3. Memo Structure

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

2. Terms and Definitions

2.1. Requirements Terminology

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

2.2. Notation

 [x]   indicates that x is optional.
 {x}   indicates that x is encrypted.
 X(y)   indicates that y is a parameter of X.
 <x>i   indicates that x exists i times.
  1. → signifies "Initiator to Responder" communication (requests).
 <--   signifies "Responder to Initiator" communication (replies).
 |  signifies concatenation of information-- e.g., X | Y is the
    concatenation of X with Y.
 Ltrunc (SHA-1(), K)   denotes the lowest order K bits of the SHA-1
    result.

2.3. Definitions

 Unused Association Lifetime (UAL):   Implementation-specific time for
    which, if no packet is sent or received for this time interval, a
    host MAY begin to tear down an active association.
 Maximum Segment Lifetime (MSL):   Maximum time that a TCP segment is
    expected to spend in the network.

Moskowitz, et al. Experimental [Page 7] RFC 5201 Host Identity Protocol April 2008

 Exchange Complete (EC):   Time that the host spends at the R2-SENT
    before it moves to ESTABLISHED state.  The time is n * I2
    retransmission timeout, where n is about I2_RETRIES_MAX.
 HIT Hash Algorithm:   Hash algorithm used to generate a Host Identity
    Tag (HIT) from the Host Identity public key.  Currently SHA-1
    [FIPS95] is used.
 Responder's HIT Hash Algorithm (RHASH):   Hash algorithm used for
    various hash calculations in this document.  The algorithm is the
    same as is used to generate the Responder's HIT.  RHASH is defined
    by the Orchid Context ID.  For HIP, the present RHASH algorithm is
    defined in Section 3.2.  A future version of HIP may define a new
    RHASH algorithm by defining a new Context ID.
 Opportunistic mode:   HIP base exchange where the Responder's HIT is
    not known a priori to the Initiator.

3. Host Identifier (HI) and Its Representations

 In this section, the properties of the Host Identifier and Host
 Identifier Tag are discussed, and the exact format for them is
 defined.  In HIP, the public key of an asymmetric key pair is used as
 the Host Identifier (HI).  Correspondingly, the host itself is
 defined as the entity that holds the private key from the key pair.
 See the HIP architecture specification [RFC4423] for more details
 about the difference between an identity and the corresponding
 identifier.
 HIP implementations MUST support the Rivest Shamir Adelman (RSA/SHA1)
 [RFC3110] public key algorithm, and SHOULD support the Digital
 Signature Algorithm (DSA) [RFC2536] algorithm; other algorithms MAY
 be supported.
 A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
 protocols to represent the Host Identity.  The HIT is 128 bits long
 and has the following three key properties: i) it is the same length
 as an IPv6 address and can be used in address-sized fields in APIs
 and protocols, ii) it is self-certifying (i.e., given a HIT, it is
 computationally hard to find a Host Identity key that matches the
 HIT), and iii) the probability of HIT collision between two hosts is
 very low.
 Carrying HIs and HITs in the header of user data packets would
 increase the overhead of packets.  Thus, it is not expected that they
 are carried in every packet, but other methods are used to map the
 data packets to the corresponding HIs.  In some cases, this makes it
 possible to use HIP without any additional headers in the user data

Moskowitz, et al. Experimental [Page 8] RFC 5201 Host Identity Protocol April 2008

 packets.  For example, if ESP is used to protect data traffic, the
 Security Parameter Index (SPI) carried in the ESP header can be used
 to map the encrypted data packet to the correct HIP association.

3.1. Host Identity Tag (HIT)

 The Host Identity Tag is a 128-bit value -- a hashed encoding of the
 Host Identifier.  There are two advantages of using a hashed encoding
 over the actual Host Identity public key in protocols.  Firstly, its
 fixed length makes for easier protocol coding and also better manages
 the packet size cost of this technology.  Secondly, it presents a
 consistent format to the protocol whatever underlying identity
 technology is used.
 RFC 4843 [RFC4843] specifies 128-bit hash-based identifiers, called
 Overlay Routable Cryptographic Hash Identifiers (ORCHIDs).  Their
 prefix, allocated from the IPv6 address block, is defined in
 [RFC4843].  The Host Identity Tag is a type of ORCHID, based on a
 SHA-1 hash of the Host Identity, as defined in Section 2 of
 [RFC4843].

3.2. Generating a HIT from an HI

 The HIT MUST be generated according to the ORCHID generation method
 described in [RFC4843] using a context ID value of 0xF0EF F02F BFF4
 3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly
 by the editor of this specification), and an input that encodes the
 Host Identity field (see Section 5.2.8) present in a HIP payload
 packet.  The hash algorithm SHA-1 has to be used when generating HITs
 with this context ID.  If a new ORCHID hash algorithm is needed in
 the future for HIT generation, a new version of HIP has to be
 specified with a new ORCHID context ID associated with the new hash
 algorithm.
 For Identities that are either RSA or Digital Signature Algorithm
 (DSA) public keys, this input consists of the public key encoding as
 specified in the corresponding DNSSEC document, taking the algorithm-
 specific portion of the RDATA part of the KEY RR.  There are
 currently only two defined public key algorithms: RSA/SHA1 and DSA.
 Hence, either of the following applies:
    The RSA public key is encoded as defined in [RFC3110] Section 2,
    taking the exponent length (e_len), exponent (e), and modulus (n)
    fields concatenated.  The length (n_len) of the modulus (n) can be
    determined from the total HI Length and the preceding HI fields
    including the exponent (e).  Thus, the data to be hashed has the
    same length as the HI.  The fields MUST be encoded in network byte
    order, as defined in [RFC3110].

Moskowitz, et al. Experimental [Page 9] RFC 5201 Host Identity Protocol April 2008

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

4. Protocol Overview

 The following material is an overview of the HIP protocol operation,
 and does not contain all details of the packet formats or the packet
 processing steps.  Sections 5 and 6 describe in more detail the
 packet formats and packet processing steps, respectively, and are
 normative in case of any conflicts with this section.
 The protocol number 139 has been assigned by IANA to the Host
 Identity Protocol.
 The HIP payload (Section 5.1) header could be carried in every IP
 datagram.  However, since HIP headers are relatively large (40
 bytes), it is desirable to 'compress' the HIP header so that the HIP
 header only occurs in control packets used to establish or change HIP
 association state.  The actual method for header 'compression' and
 for matching data packets with existing HIP associations (if any) is
 defined in separate documents, describing transport formats and
 methods.  All HIP implementations MUST implement, at minimum, the ESP
 transport format for HIP [RFC5202].

4.1. Creating a HIP Association

 By definition, the system initiating a HIP exchange is the Initiator,
 and the peer is the Responder.  This distinction is forgotten once
 the base exchange completes, and either party can become the
 Initiator in future communications.
 The HIP base exchange serves to manage the establishment of state
 between an Initiator and a Responder.  The first packet, I1,
 initiates the exchange, and the last three packets, R1, I2, and R2,
 constitute an authenticated Diffie-Hellman [DIF76] key exchange for
 session key generation.  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.

Moskowitz, et al. Experimental [Page 10] RFC 5201 Host Identity Protocol April 2008

 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.  Note that in some cases it may
 be possible to replace this trigger packet by some other form of a
 trigger, in which case the protocol starts with the Responder sending
 the R1 packet.
 The second packet, R1, starts the actual exchange.  It contains a
 puzzle -- a cryptographic challenge that the Initiator must solve
 before continuing the exchange.  The level of difficulty of the
 puzzle can be adjusted based on level of trust with the Initiator,
 current load, or other factors.  In addition, the R1 contains the
 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.  The term "key" refers to the
 Host Identity public key, and "sig" represents a signature using such
 a key.  The packets contain other parameters not shown in this
 figure.
     Initiator                              Responder
                  I1: trigger exchange
                -------------------------->
                                            select precomputed R1
                  R1: puzzle, D-H, key, sig
                <-------------------------
  check sig                                 remain stateless
  solve puzzle
                I2: solution, D-H, {key}, sig
                -------------------------->
  compute D-H                               check puzzle
                                            check sig
                          R2: sig
                <--------------------------
  check sig                                 compute D-H

Moskowitz, et al. Experimental [Page 11] RFC 5201 Host Identity Protocol April 2008

4.1.1. HIP Puzzle Mechanism

 The purpose of the HIP puzzle mechanism is to protect the Responder
 from a number of denial-of-service threats.  It allows the Responder
 to delay state creation until receiving I2.  Furthermore, the puzzle
 allows the Responder to use a fairly cheap calculation to check that
 the Initiator is "sincere" in the sense that it has churned CPU
 cycles in solving the puzzle.
 The puzzle mechanism has been explicitly designed to give space for
 various implementation options.  It allows a Responder implementation
 to completely delay session-specific state creation until a valid I2
 is received.  In such a case, a correctly formatted I2 can be
 rejected only once the Responder has checked its validity by
 computing one hash function.  On the other hand, the design also
 allows a Responder implementation to keep state about received I1s,
 and match the received I2s against the state, thereby allowing the
 implementation to avoid the computational cost of the hash function.
 The drawback of this latter approach is the requirement of creating
 state.  Finally, it also allows an implementation to use other
 combinations of the space-saving and computation-saving mechanisms.
 The Responder can remain stateless and drop most spoofed I2s because
 puzzle calculation is based on the Initiator's Host Identity Tag.
 The idea is that the Responder has a (perhaps varying) number of pre-
 calculated R1 packets, and it selects one of these based on the
 information carried in I1.  When the Responder then later receives
 I2, it can verify that the puzzle has been solved using the
 Initiator's HIT.  This makes it impractical for the attacker to first
 exchange one I1/R1, and then generate a large number of spoofed I2s
 that seemingly come from different HITs.  The method does not protect
 from an attacker that uses fixed HITs, though.  Against such an
 attacker a viable approach may be to create a piece of local state,
 and remember that the puzzle check has previously failed.  See
 Appendix A for one possible implementation.  Implementations SHOULD
 include sufficient randomness to the algorithm so that algorithmic
 complexity attacks become impossible [CRO03].
 The Responder can set the puzzle difficulty for Initiator, based on
 its level of trust of the Initiator.  Because the puzzle is not
 included in the signature calculation, the Responder can use pre-
 calculated R1 packets and include the puzzle just before sending the
 R1 to the Initiator.  The Responder SHOULD use heuristics to
 determine when it is under a denial-of-service attack, and set the
 puzzle difficulty value K appropriately; see below.

Moskowitz, et al. Experimental [Page 12] RFC 5201 Host Identity Protocol April 2008

4.1.2. Puzzle Exchange

 The Responder starts the puzzle exchange when it receives an I1.  The
 Responder supplies a random number I, and requires the Initiator to
 find a number J.  To select a proper J, the Initiator must create the
 concatenation of I, the HITs of the parties, and J, and take a hash
 over this concatenation using the RHASH algorithm.  The lowest order
 K bits of the result MUST be zeros.  The value K sets the difficulty
 of the puzzle.
 To generate a proper number J, the Initiator will have to generate a
 number of Js until one produces the hash target of zeros.  The
 Initiator SHOULD give up after exceeding the puzzle lifetime in the
 PUZZLE parameter (Section 5.2.4).  The Responder needs to re-create
 the concatenation of I, the HITs, and the provided J, and compute the
 hash once to prove that the Initiator did its assigned task.
 To prevent precomputation attacks, the Responder MUST select the
 number I in such a way that the Initiator cannot guess it.
 Furthermore, the construction MUST allow the Responder to verify that
 the value was indeed selected by it and not by the Initiator.  See
 Appendix A for an example on how to implement this.
 Using the Opaque data field in an ECHO_REQUEST_SIGNED
 (Section 5.2.17) or in an ECHO_REQUEST_UNSIGNED parameter
 (Section 5.2.18), 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
 some secret, the sent I, and possibly other related data.  Using the
 same secret, the received I (from the I2), and the other related data
 (if any), the Receiver can verify that it has itself sent the I to
 the Initiator.  The Responder MUST periodically change such a used
 secret.
 It is RECOMMENDED that the Responder generates a new puzzle and a new
 R1 once every few minutes.  Furthermore, it is RECOMMENDED that the
 Responder remembers an old puzzle at least 2*Lifetime seconds after
 the puzzle has been deprecated.  These time values allow a slower
 Initiator to solve the puzzle while limiting the usability that an
 old, solved puzzle 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 to NOT include a timestamp.
 NOTE: The protocol developers explicitly considered whether a memory
 bound function should be used for the puzzle instead of a CPU-bound
 function.  The decision was not to use memory-bound functions.  At

Moskowitz, et al. Experimental [Page 13] RFC 5201 Host Identity Protocol April 2008

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

4.1.3. Authenticated Diffie-Hellman Protocol

 The packets R1, I2, and R2 implement a standard authenticated Diffie-
 Hellman exchange.  The Responder sends one or two public Diffie-
 Hellman keys and its public authentication key, i.e., its Host
 Identity, in R1.  The signature in R1 allows the Initiator to verify
 that the R1 has been once generated by the Responder.  However, since
 it is precomputed and therefore does not cover all of the packet, it
 does not protect from replay attacks.
 When the Initiator receives an R1, it gets one or two public Diffie-
 Hellman values from the Responder.  If there are two values, it
 selects the value corresponding to the strongest supported Group ID
 and computes the Diffie-Hellman session key (Kij).  It creates a HIP
 association using keying material from the session key (see
 Section 6.5), and may use the association to encrypt its public
 authentication key, i.e., Host Identity.  The resulting I2 contains
 the Initiator's Diffie-Hellman key and its (optionally encrypted)
 public authentication key.  The signature in I2 covers all of the
 packet.
 The Responder extracts the Initiator Diffie-Hellman public key from
 the I2, computes the Diffie-Hellman session key, creates a
 corresponding HIP association, and decrypts the Initiator's public
 authentication key.  It can then verify the signature using the
 authentication key.
 The final message, R2, is needed to protect the Initiator from replay
 attacks.

4.1.4. HIP Replay Protection

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

Moskowitz, et al. Experimental [Page 14] RFC 5201 Host Identity Protocol April 2008

 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 puzzles.
 Implementations MUST accept puzzles from the current generation and
 MAY accept puzzles from earlier generations.  A system's local
 counter MUST be incremented at least as often as every time old R1s
 cease to be valid, and SHOULD never be decremented, lest the host
 expose its peers to the replay of previously generated, higher
 numbered R1s.  The R1 counter SHOULD NOT roll over.
 A host may receive more than one R1, either due to sending multiple
 I1s (Section 6.6.1) or due to a replay of an old R1.  When sending
 multiple I1s, an Initiator SHOULD wait for a small amount of time (a
 reasonable time may be 2 * expected RTT) after the first R1 reception
 to allow possibly multiple R1s to arrive, and it SHOULD respond to an
 R1 among the set with the largest R1 generation counter.  If an
 Initiator is processing an R1 or has already sent an I2 (still
 waiting for R2) and it receives another R1 with a larger R1
 generation counter, it MAY elect to restart R1 processing with the
 fresher R1, as if it were the first R1 to arrive.
 Upon conclusion of an active HIP association with another host, the
 R1 generation counter associated with the peer host SHOULD be
 flushed.  A local policy MAY override the default flushing of R1
 counters on a per-HIT basis.  The reason for recommending the
 flushing of this counter is that there may be hosts where the R1
 generation counter (occasionally) decreases; e.g., due to hardware
 failure.

4.1.5. Refusing a HIP Exchange

 A HIP-aware host may choose not to accept a HIP exchange.  If the
 host's policy is to only be an Initiator, it should begin its own HIP
 exchange.  A host MAY choose to have such a policy since only the
 Initiator's HI is protected in the exchange.  There is a risk of a
 race condition if each host's policy is to only be an Initiator, at
 which point the HIP exchange will fail.
 If the host's policy does not permit it to enter into a HIP exchange
 with the Initiator, it should send an ICMP 'Destination Unreachable,
 Administratively Prohibited' message.  A more complex HIP packet is
 not used here as it actually opens up more potential DoS attacks than
 a simple ICMP message.

Moskowitz, et al. Experimental [Page 15] RFC 5201 Host Identity Protocol April 2008

4.1.6. HIP Opportunistic Mode

 It is possible to initiate a HIP negotiation even if the Responder's
 HI (and HIT) is unknown.  In this case, the connection initializing
 I1 packet contains NULL (all zeros) as the destination HIT.  This
 kind of connection setup is called opportunistic mode.
 There are both security and API issues involved with the
 opportunistic mode.
 Given that the Responder's HI is not known by the Initiator, there
 must be suitable API calls that allow the Initiator to request,
 directly or indirectly, that the underlying kernel initiate the HIP
 base exchange solely based on locators.  The Responder's HI will be
 tentatively available in the R1 packet, and in an authenticated form
 once the R2 packet has been received and verified.  Hence, it could
 be communicated to the application via new API mechanisms.  However,
 with a backwards-compatible API the application sees only the
 locators used for the initial contact.  Depending on the desired
 semantics of the API, this can raise the following issues:
 o  The actual locators may later change if an UPDATE message is used,
    even if from the API perspective the session still appears to be
    between specific locators.  The locator update is still secure,
    however, and the session is still between the same nodes.
 o  Different sessions between the same locators may result in
    connections to different nodes, if the implementation no longer
    remembers which identifier the peer had in another session.  This
    is possible when the peer's locator has changed for legitimate
    reasons or when an attacker pretends to be a node that has the
    peer's locator.  Therefore, when using opportunistic mode, HIP
    MUST NOT place any expectation that the peer's HI returned in the
    R1 message matches any HI previously seen from that address.
    If the HIP implementation and application do not have the same
    understanding of what constitutes a session, this may even happen
    within the same session.  For instance, an implementation may not
    know when HIP state can be purged for UDP-based applications.
 o  As with all HIP exchanges, the handling of locator-based or
    interface-based policy is unclear for opportunistic mode HIP.  An
    application may make a connection to a specific locator because
    the application has knowledge of the security properties along the
    network to that locator.  If one of the nodes moves and the
    locators are updated, these security properties may not be
    maintained.  Depending on the security policy of the application,
    this may be a problem.  This is an area of ongoing study.  As an

Moskowitz, et al. Experimental [Page 16] RFC 5201 Host Identity Protocol April 2008

    example, there is work to create an API that applications can use
    to specify their security requirements in a similar context
    [IPsec-APIs].
 In addition, the following security considerations apply.  The
 generation counter mechanism will be less efficient in protecting
 against replays of the R1 packet, given that the Responder can choose
 a replay that uses any HI, not just the one given in the I1 packet.
 More importantly, the opportunistic exchange is vulnerable to man-in-
 the-middle attacks, because the Initiator does not have any public
 key information about the peer.  To assess the impacts of this
 vulnerability, we compare it to vulnerabilities in current, non-HIP-
 capable communications.
 An attacker on the path between the two peers can insert itself as a
 man-in-the-middle by providing its own identifier to the Initiator
 and then initiating another HIP session towards the Responder.  For
 this to be possible, the Initiator must employ opportunistic mode,
 and the Responder must be configured to accept a connection from any
 HIP-enabled node.
 An attacker outside the path will be unable to do so, given that it
 cannot respond to the messages in the base exchange.
 These properties are characteristic also of communications in the
 current Internet.  A client contacting a server without employing
 end-to-end security may find itself talking to the server via a man-
 in-the-middle, assuming again that the server is willing to talk to
 anyone.
 If end-to-end security is in place, then the worst that can happen in
 both the opportunistic HIP and normal IP cases is denial-of-service;
 an entity on the path can disrupt communications, but will be unable
 to insert itself as a man-in-the-middle.
 However, once the opportunistic exchange has successfully completed,
 HIP provides integrity protection and confidentiality for the
 communications, and can securely change the locators of the
 endpoints.
 As a result, it is believed that the HIP opportunistic mode is at
 least as secure as current IP.

Moskowitz, et al. Experimental [Page 17] RFC 5201 Host Identity Protocol April 2008

4.2. Updating a HIP Association

 A HIP association between two hosts may need to be updated over time.
 Examples include the need to rekey expiring user data security
 associations, add new security associations, or change IP addresses
 associated with hosts.  The UPDATE packet is used for those and other
 similar purposes.  This document only specifies the UPDATE packet
 format and basic processing rules, with mandatory parameters.  The
 actual usage is defined in separate specifications.
 HIP provides a general purpose UPDATE packet, which can carry
 multiple HIP parameters, for updating the HIP state between two
 peers.  The UPDATE mechanism has the following properties:
    UPDATE messages carry a monotonically increasing sequence number
    and are explicitly acknowledged by the peer.  Lost UPDATEs or
    acknowledgments may be recovered via retransmission.  Multiple
    UPDATE messages may be outstanding under certain circumstances.
    UPDATE is protected by both HMAC and HIP_SIGNATURE parameters,
    since processing UPDATE signatures alone is a potential DoS attack
    against intermediate systems.
    UPDATE packets are explicitly acknowledged by the use of an
    acknowledgment parameter that echoes an individual sequence number
    received from the peer.  A single UPDATE packet may contain both a
    sequence number and one or more acknowledgment numbers (i.e.,
    piggybacked acknowledgment(s) for the peer's UPDATE).
 The UPDATE packet is defined in Section 5.3.5.

4.3. Error Processing

 HIP error processing behavior depends on whether or not there exists
 an active HIP association.  In general, if a HIP association exists
 between the sender and receiver of a packet causing an error
 condition, the receiver SHOULD respond with a NOTIFY packet.  On the
 other hand, if there are no existing HIP associations between the
 sender and receiver, or the receiver cannot reasonably determine the
 identity of the sender, the receiver MAY respond with a suitable ICMP
 message; see Section 5.4 for more details.
 The HIP protocol and state machine is designed to recover from one of
 the parties crashing and losing its state.  The following scenarios
 describe the main use cases covered by the design.

Moskowitz, et al. Experimental [Page 18] RFC 5201 Host Identity Protocol April 2008

    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 a HIP association, but the
    receiver does not.
       The system sends data on the outbound user data security
       association.  The receiver 'detects' the situation when it
       receives a user data packet that it cannot match to any HIP
       association.  The receiving host MUST discard this packet.
       Optionally, the receiving host MAY send an ICMP packet, with
       the type Parameter Problem, to inform the sender that the HIP
       association does not exist (see Section 5.4), and it MAY
       initiate a new HIP negotiation.  However, responding with these
       optional mechanisms is implementation or policy dependent.

4.4. HIP State Machine

 The HIP protocol itself has little state.  In the HIP base exchange,
 there is an Initiator and a Responder.  Once the security
 associations (SAs) are established, this distinction is lost.  If the
 HIP state needs to be re-established, the controlling parameters are
 which peer still has state and which has a datagram to send to its
 peer.  The following state machine attempts to capture these
 processes.
 The state machine is presented in a single system view, representing
 either an Initiator or a Responder.  There is not a complete overlap
 of processing logic here and in the packet definitions.  Both are
 needed to completely implement HIP.
 Implementors must understand that the state machine, as described
 here, is informational.  Specific implementations are free to
 implement the actual functions differently.  Section 6 describes the
 packet processing rules in more detail.  This state machine focuses

Moskowitz, et al. Experimental [Page 19] RFC 5201 Host Identity Protocol April 2008

 on the HIP I1, R1, I2, and R2 packets only.  Other states may be
 introduced by mechanisms in other specifications (such as mobility
 and multihoming).

4.4.1. HIP States

 +---------------------+---------------------------------------------+
 | State               | Explanation                                 |
 +---------------------+---------------------------------------------+
 | UNASSOCIATED        | State machine start                         |
 |                     |                                             |
 | I1-SENT             | Initiating base exchange                    |
 |                     |                                             |
 | I2-SENT             | Waiting to complete base exchange           |
 |                     |                                             |
 | R2-SENT             | Waiting to complete base exchange           |
 |                     |                                             |
 | ESTABLISHED         | HIP association established                 |
 |                     |                                             |
 | CLOSING             | HIP association closing, no data can be     |
 |                     | sent                                        |
 |                     |                                             |
 | CLOSED              | HIP association closed, no data can be sent |
 |                     |                                             |
 | E-FAILED            | HIP exchange failed                         |
 +---------------------+---------------------------------------------+
                          Table 1: HIP States

Moskowitz, et al. Experimental [Page 20] RFC 5201 Host Identity Protocol April 2008

4.4.2. HIP State Processes

 System behavior in state UNASSOCIATED, Table 2.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | User data to send,  | Send I1 and go to I1-SENT                   |
 | requiring a new HIP |                                             |
 | association         |                                             |
 |                     |                                             |
 | Receive I1          | Send R1 and stay at UNASSOCIATED            |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and go to R2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at UNASSOCIATED               |
 |                     |                                             |
 | Receive user data   | Optionally send ICMP as defined in          |
 | for unknown HIP     | Section 5.4 and stay at UNASSOCIATED        |
 | association         |                                             |
 |                     |                                             |
 | Receive CLOSE       | Optionally send ICMP Parameter Problem and  |
 |                     | stay at UNASSOCIATED                        |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at UNASSOCIATED               |
 +---------------------+---------------------------------------------+
                  Table 2: UNASSOCIATED - Start state

Moskowitz, et al. Experimental [Page 21] RFC 5201 Host Identity Protocol April 2008

 System behavior in state I1-SENT, Table 3.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1          | If the local HIT is smaller than the peer   |
 |                     | HIT, drop I1 and stay at I1-SENT            |
 |                     |                                             |
 |                     | If the local HIT is greater than the peer   |
 |                     | HIT, send R1 and stay at I1_SENT            |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and go to R2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at I1-SENT                    |
 |                     |                                             |
 | Receive R1, process | If successful, send I2 and go to I2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at I1-SENT                    |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at I1-SENT                    |
 |                     |                                             |
 | Timeout, increment  | If counter is less than I1_RETRIES_MAX,     |
 | timeout counter     | send I1 and stay at I1-SENT                 |
 |                     |                                             |
 |                     | If counter is greater than I1_RETRIES_MAX,  |
 |                     | go to E-FAILED                              |
 +---------------------+---------------------------------------------+
                   Table 3: I1-SENT - Initiating HIP

Moskowitz, et al. Experimental [Page 22] RFC 5201 Host Identity Protocol April 2008

 System behavior in state I2-SENT, Table 4.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1          | Send R1 and stay at I2-SENT                 |
 |                     |                                             |
 | Receive R1, process | If successful, send I2 and cycle at I2-SENT |
 |                     |                                             |
 |                     | If fail, stay at I2-SENT                    |
 |                     |                                             |
 | Receive I2, process | If successful and local HIT is smaller than |
 |                     | the peer HIT, drop I2 and stay at I2-SENT   |
 |                     |                                             |
 |                     | If successful and local HIT is greater than |
 |                     | the peer HIT, send R2 and go to R2-SENT     |
 |                     |                                             |
 |                     | If fail, stay at I2-SENT                    |
 |                     |                                             |
 | Receive R2, process | If successful, go to ESTABLISHED            |
 |                     |                                             |
 |                     | If fail, stay at I2-SENT                    |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at I2-SENT                    |
 |                     |                                             |
 | Timeout, increment  | If counter is less than I2_RETRIES_MAX,     |
 | timeout counter     | send I2 and stay at I2-SENT                 |
 |                     |                                             |
 |                     | If counter is greater than I2_RETRIES_MAX,  |
 |                     | go to E-FAILED                              |
 +---------------------+---------------------------------------------+
               Table 4: I2-SENT - Waiting to finish HIP

Moskowitz, et al. Experimental [Page 23] RFC 5201 Host Identity Protocol April 2008

 System behavior in state R2-SENT, Table 5.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1          | Send R1 and stay at R2-SENT                 |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and cycle at R2-SENT |
 |                     |                                             |
 |                     | If fail, stay at R2-SENT                    |
 |                     |                                             |
 | Receive R1          | Drop and stay at R2-SENT                    |
 |                     |                                             |
 | Receive R2          | Drop and stay at R2-SENT                    |
 |                     |                                             |
 | Receive data or     | Move to ESTABLISHED                         |
 | UPDATE              |                                             |
 |                     |                                             |
 | Exchange Complete   | Move to ESTABLISHED                         |
 | Timeout             |                                             |
 +---------------------+---------------------------------------------+
               Table 5: R2-SENT - Waiting to finish HIP

Moskowitz, et al. Experimental [Page 24] RFC 5201 Host Identity Protocol April 2008

 System behavior in state ESTABLISHED, Table 6.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1          | Send R1 and stay at ESTABLISHED             |
 |                     |                                             |
 | Receive I2, process | If successful, send R2, drop old HIP        |
 | with puzzle and     | association, establish a new HIP            |
 | possible Opaque     | association, go to R2-SENT                  |
 | data verification   |                                             |
 |                     |                                             |
 |                     | If fail, stay at ESTABLISHED                |
 |                     |                                             |
 | Receive R1          | Drop and stay at ESTABLISHED                |
 |                     |                                             |
 | Receive R2          | Drop and stay at ESTABLISHED                |
 |                     |                                             |
 | Receive user data   | Process and stay at ESTABLISHED             |
 | for HIP association |                                             |
 |                     |                                             |
 | No packet           | Send CLOSE and go to CLOSING                |
 | sent/received       |                                             |
 | during UAL minutes  |                                             |
 |                     |                                             |
 | Receive CLOSE,      | If successful, send CLOSE_ACK and go to     |
 | process             | CLOSED                                      |
 |                     |                                             |
 |                     | If fail, stay at ESTABLISHED                |
 +---------------------+---------------------------------------------+
          Table 6: ESTABLISHED - HIP association established

Moskowitz, et al. Experimental [Page 25] RFC 5201 Host Identity Protocol April 2008

 System behavior in state CLOSING, Table 7.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | User data to send,  | Send I1 and stay at CLOSING                 |
 | requires the        |                                             |
 | creation of another |                                             |
 | incarnation of the  |                                             |
 | HIP association     |                                             |
 |                     |                                             |
 | Receive I1          | Send R1 and stay at CLOSING                 |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and go to R2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at CLOSING                    |
 |                     |                                             |
 | Receive R1, process | If successful, send I2 and go to I2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at CLOSING                    |
 |                     |                                             |
 | Receive CLOSE,      | If successful, send CLOSE_ACK, discard      |
 | process             | state and go to CLOSED                      |
 |                     |                                             |
 |                     | If fail, stay at CLOSING                    |
 |                     |                                             |
 | Receive CLOSE_ACK,  | If successful, discard state and go to      |
 | process             | UNASSOCIATED                                |
 |                     |                                             |
 |                     | If fail, stay at CLOSING                    |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at CLOSING                    |
 |                     |                                             |
 | Timeout, increment  | If timeout sum is less than UAL+MSL         |
 | timeout sum, reset  | minutes, retransmit CLOSE and stay at       |
 | timer               | CLOSING                                     |
 |                     |                                             |
 |                     | If timeout sum is greater than UAL+MSL      |
 |                     | minutes, go to UNASSOCIATED                 |
 +---------------------+---------------------------------------------+
 Table 7: CLOSING - HIP association has not been used for UAL minutes

Moskowitz, et al. Experimental [Page 26] RFC 5201 Host Identity Protocol April 2008

 System behavior in state CLOSED, Table 8.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Datagram to send,   | Send I1, and stay at CLOSED                 |
 | requires the        |                                             |
 | creation of another |                                             |
 | incarnation of the  |                                             |
 | HIP association     |                                             |
 |                     |                                             |
 | Receive I1          | Send R1 and stay at CLOSED                  |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and go to R2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at CLOSED                     |
 |                     |                                             |
 | Receive R1, process | If successful, send I2 and go to I2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at CLOSED                     |
 |                     |                                             |
 | Receive CLOSE,      | If successful, send CLOSE_ACK, stay at      |
 | process             | CLOSED                                      |
 |                     |                                             |
 |                     | If fail, stay at CLOSED                     |
 |                     |                                             |
 | Receive CLOSE_ACK,  | If successful, discard state and go to      |
 | process             | UNASSOCIATED                                |
 |                     |                                             |
 |                     | If fail, stay at CLOSED                     |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at CLOSED                     |
 |                     |                                             |
 | Timeout (UAL+2MSL)  | Discard state, and go to UNASSOCIATED       |
 +---------------------+---------------------------------------------+
  Table 8: CLOSED - CLOSE_ACK sent, resending CLOSE_ACK if necessary

Moskowitz, et al. Experimental [Page 27] RFC 5201 Host Identity Protocol April 2008

 System behavior in state E-FAILED, Table 9.
 +-------------------------+-----------------------------------------+
 | Trigger                 | Action                                  |
 +-------------------------+-----------------------------------------+
 | Wait for                | Go to UNASSOCIATED.  Re-negotiation is  |
 | implementation-specific | possible after moving to UNASSOCIATED   |
 | time                    | state.                                  |
 +-------------------------+-----------------------------------------+
   Table 9: E-FAILED - HIP failed to establish association with peer

4.4.3. 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.

Moskowitz, et al. Experimental [Page 28] RFC 5201 Host Identity Protocol April 2008

                              +-+        +---------------------------+
         I1 received, send R1 | |        |                           |
                              | v        v                           |
          Datagram to send  +--------------+  I2 received, send R2   |
            +---------------| UNASSOCIATED |---------------+         |
    Send I1 |               +--------------+               |         |
            v                                              |         |
       +---------+  I2 received, send R2                   |         |
 +---->| I1-SENT |---------------------------------------+ |         |
 |     +---------+                                       | |         |
 |          |                 +------------------------+ | |         |
 |          | R1 received,    | I2 received, send R2   | | |         |
 |          v send I2         |                        v v v         |
 |     +---------+            |                   +---------+        |
 |  +->| I2-SENT |------------+                   | R2-SENT |<----+  |
 |  |  +---------+                                +---------+     |  |
 |  |          |                                     |            |  |
 |  |          |                                 data|            |  |
 |  |receive   |                                   or|            |  |
 |  |R1, send  |                           EC timeout| receive I2,|  |
 |  |I2        |R2 received +--------------+         |     send R2|  |
 |  |          +----------->| ESTABLISHED  |<-------+|            |  |
 |  |                       +--------------+                      |  |
 |  |                         |    |     |  receive I2, send R2   |  |
 |  |        recv+------------+    |     +------------------------+  |
 |  |      CLOSE,|                 |                              |  |
 |  |        send|   No packet sent|                              |  |
 |  |   CLOSE_ACK|   /received for |                   timeout    |  |
 |  |            |   UAL min, send |    +---------+<-+ (UAL+MSL)  |  |
 |  |            |           CLOSE +--->| CLOSING |--+ retransmit |  |
 |  |            |                      +---------+    CLOSE      |  |
 +--|------------|----------------------+ | |  | |                |  |
    +------------|------------------------+ |  | +----------------+  |
    |            |              +-----------+  +------------------|--+
    |            +------------+ | receive CLOSE,   CLOSE_ACK      |  |
    |                         | | send CLOSE_ACK   received or    |  |
    |                         | |                  timeout        |  |
    |                         | |                  (UAL+MSL)      |  |
    |                         v v                                 |  |
    |                        +--------+  receive I2, send R2      |  |
    +------------------------| CLOSED |---------------------------+  |
                             +--------+       /----------------------+
                               ^ |   \-------/  timeout (UAL+2MSL),
                               +-+              move to UNASSOCIATED
                CLOSE received, send CLOSE_ACK

Moskowitz, et al. Experimental [Page 29] RFC 5201 Host Identity Protocol April 2008

4.5. User Data Considerations

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

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

4.5.2. Sending Data on HIP Packets

 A future version of this document may define how to include user data
 on various HIP packets.  However, currently the HIP header is a
 terminal header, and not followed by any other headers.

4.5.3. Transport Formats

 The actual data transmission format, used for user data after the HIP
 base exchange, is not defined in this document.  Such transport
 formats and methods are described in separate specifications.  All
 HIP implementations MUST implement, at minimum, the ESP transport
 format for HIP [RFC5202].
 When new transport formats are defined, they get the type value from
 the HIP Transform type value space 2048-4095.  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.

4.5.4. 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 the HIP association times out, it has lost its
 HIP state.  If the host that lost state has a datagram to send to the
 peer, it simply restarts the HIP base exchange.  After the base
 exchange has completed, the Initiator can create a new SA and start
 sending data.  The peer does not reset its state until it receives a
 valid I2 HIP packet.
 If a system receives a user data packet that cannot be matched to any
 existing HIP association, it is possible that it has lost the state
 and its peer has not.  It MAY send an ICMP packet with the Parameter

Moskowitz, et al. Experimental [Page 30] RFC 5201 Host Identity Protocol April 2008

 Problem type, and with the pointer pointing to the referred HIP-
 related association information.  Reacting to such traffic depends on
 the implementation and the environment where the implementation is
 used.
 If the host, that apparently has lost its state, decides to restart
 the HIP base exchange, it sends an I1 packet to the peer.  After the
 base exchange has been completed successfully, the Initiator can
 create a new HIP association and the peer drops its old SA and
 creates a new one.

4.6. Certificate Distribution

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

5. Packet Formats

5.1. Payload Format

 All HIP packets start with a fixed header.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Next Header   | Header Length |0| Packet Type |  VER. | RES.|1|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Checksum             |           Controls            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Sender's Host Identity Tag (HIT)               |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |               Receiver's Host Identity Tag (HIT)              |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 /                        HIP Parameters                         /
 /                                                               /
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Moskowitz, et al. Experimental [Page 31] RFC 5201 Host Identity Protocol April 2008

 The HIP header is logically an IPv6 extension header.  However, this
 document does not describe processing for Next Header values other
 than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
 Future documents MAY do so.  However, current implementations MUST
 ignore trailing data if an unimplemented Next Header value is
 received.
 The Header Length field contains the length of the HIP Header and HIP
 parameters in 8-byte units, excluding the first 8 bytes.  Since all
 HIP headers MUST contain the sender's and receiver's HIT fields, the
 minimum value for this field is 4, and conversely, the maximum length
 of the HIP Parameters field is (255*8)-32 = 2008 bytes.  Note: this
 sets an additional limit for sizes of parameters included in the
 Parameters field, independent of the individual parameter maximum
 lengths.
 The Packet Type indicates the HIP packet type.  The individual packet
 types are defined in the relevant sections.  If a HIP host receives a
 HIP packet that contains an unknown packet type, it MUST drop the
 packet.
 The HIP Version is four bits.  The current version is 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 three bits are reserved for future use.  They MUST be
 zero when sent, and they SHOULD be ignored when handling a received
 packet.
 The two fixed bits in the header are reserved for potential SHIM6
 compatibility [SHIM6-PROTO].  For implementations adhering (only) to
 this specification, they MUST be set as shown when sending and MUST
 be ignored when receiving.  This is to ensure optimal forward
 compatibility.  Note that for implementations that implement other
 compatible specifications in addition to this specification, the
 corresponding rules may well be different.  For example, in the case
 that the forthcoming SHIM6 protocol happens to be compatible with
 this specification, an implementation that implements both this
 specification and the SHIM6 protocol may need to check these bits in
 order to determine how to handle the packet.
 The HIT fields are always 128 bits (16 bytes) long.

Moskowitz, et al. Experimental [Page 32] RFC 5201 Host Identity Protocol April 2008

5.1.1. Checksum

 Since the checksum covers the source and destination addresses in the
 IP header, it must be recomputed on HIP-aware NAT devices.
 If IPv6 is used to carry the HIP packet, the pseudo-header [RFC2460]
 contains the source and destination IPv6 addresses, HIP packet length
 in the pseudo-header length field, a zero field, and the HIP protocol
 number (see Section 4) in the Next Header field.  The length field is
 in bytes and can be calculated from the HIP header length field: (HIP
 Header Length + 1) * 8.
 In case of using IPv4, the IPv4 UDP pseudo-header format [RFC0768] is
 used.  In the pseudo-header, the source and destination addresses are
 those used in the IP header, the zero field is obviously zero, the
 protocol is the HIP protocol number (see Section 4), and the length
 is calculated as in the IPv6 case.

5.1.2. HIP Controls

 The HIP Controls section conveys information about the structure of
 the packet and capabilities of the host.
 The following fields have been defined:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | | | | | | | | | | | | | | | |A|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 A - Anonymous:   If this is set, the sender's HI in this packet is
    anonymous, i.e., one not listed in a directory.  Anonymous HIs
    SHOULD NOT be stored.  This control is set in packets R1 and/or
    I2.  The peer receiving an anonymous HI may choose to refuse it.
 The rest of the fields are reserved for future use and MUST be set to
 zero on sent packets and ignored on received packets.

5.1.3. HIP Fragmentation Support

 A HIP implementation must support IP fragmentation/reassembly.
 Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
 fragment generation is REQUIRED to be implemented in IPv4 (IPv4
 stacks and networks will usually do this by default) and RECOMMENDED
 to be implemented in IPv6.  In IPv6 networks, the minimum MTU is
 larger, 1280 bytes, than in IPv4 networks.  The larger MTU size is
 usually sufficient for most HIP packets, and therefore fragment

Moskowitz, et al. Experimental [Page 33] RFC 5201 Host Identity Protocol April 2008

 generation may not be needed.  If a host expects to send HIP packets
 that are larger than the minimum IPv6 MTU, it MUST implement fragment
 generation even for IPv6.
 In IPv4 networks, HIP packets may encounter low MTUs along their
 routed path.  Since HIP does not provide a mechanism to use multiple
 IP datagrams for a single HIP packet, support for path MTU discovery
 does not bring any value to HIP in IPv4 networks.  HIP-aware NAT
 devices MUST perform any IPv4 reassembly/fragmentation.
 All HIP implementations have to be careful while employing a
 reassembly algorithm so that the algorithm is sufficiently resistant
 to DoS attacks.
 Because certificate chains can cause the packet to be fragmented and
 fragmentation can open implementation to denial-of-service attacks
 [KAU03], it is strongly recommended that the separate document
 specifying the certificate usage in the HIP Base Exchange defines the
 usage of "Hash and URL" formats rather than including certificates in
 exchanges.  With this, most problems related to DoS attacks with
 fragmentation can be avoided.

5.2. HIP Parameters

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

Moskowitz, et al. Experimental [Page 34] RFC 5201 Host Identity Protocol April 2008

 +------------------------+-------+----------+-----------------------+
 | TLV                    | Type  | Length   | Data                  |
 +------------------------+-------+----------+-----------------------+
 | R1_COUNTER             | 128   | 12       | System Boot Counter   |
 |                        |       |          |                       |
 | PUZZLE                 | 257   | 12       | K and Random #I       |
 |                        |       |          |                       |
 | SOLUTION               | 321   | 20       | K, Random #I and      |
 |                        |       |          | puzzle solution J     |
 |                        |       |          |                       |
 | SEQ                    | 385   | 4        | Update packet ID      |
 |                        |       |          | number                |
 |                        |       |          |                       |
 | ACK                    | 449   | variable | Update packet ID      |
 |                        |       |          | number                |
 |                        |       |          |                       |
 | DIFFIE_HELLMAN         | 513   | variable | public key            |
 |                        |       |          |                       |
 | HIP_TRANSFORM          | 577   | variable | HIP Encryption and    |
 |                        |       |          | Integrity Transform   |
 |                        |       |          |                       |
 | ENCRYPTED              | 641   | variable | Encrypted part of I2  |
 |                        |       |          | packet                |
 |                        |       |          |                       |
 | HOST_ID                | 705   | variable | Host Identity with    |
 |                        |       |          | Fully-Qualified       |
 |                        |       |          | Domain FQDN (Name) or |
 |                        |       |          | Network Access        |
 |                        |       |          | Identifier (NAI)      |
 |                        |       |          |                       |
 | CERT                   | 768   | variable | HI Certificate; used  |
 |                        |       |          | to transfer           |
 |                        |       |          | certificates.  Usage  |
 |                        |       |          | is not currently      |
 |                        |       |          | defined, but it will  |
 |                        |       |          | be specified in a     |
 |                        |       |          | separate document     |
 |                        |       |          | once needed.          |
 |                        |       |          |                       |
 | NOTIFICATION           | 832   | variable | Informational data    |
 |                        |       |          |                       |
 | ECHO_REQUEST_SIGNED    | 897   | variable | Opaque data to be     |
 |                        |       |          | echoed back; under    |
 |                        |       |          | signature             |
 |                        |       |          |                       |
 | ECHO_RESPONSE_SIGNED   | 961   | variable | Opaque data echoed    |
 |                        |       |          | back; under signature |
 |                        |       |          |                       |

Moskowitz, et al. Experimental [Page 35] RFC 5201 Host Identity Protocol April 2008

 | HMAC                   | 61505 | variable | HMAC-based message    |
 |                        |       |          | authentication code,  |
 |                        |       |          | with key material     |
 |                        |       |          | from HIP_TRANSFORM    |
 |                        |       |          |                       |
 | HMAC_2                 | 61569 | variable | HMAC based message    |
 |                        |       |          | authentication code,  |
 |                        |       |          | with key material     |
 |                        |       |          | from HIP_TRANSFORM.   |
 |                        |       |          | Compared to HMAC, the |
 |                        |       |          | HOST_ID parameter is  |
 |                        |       |          | included in HMAC_2    |
 |                        |       |          | calculation.          |
 |                        |       |          |                       |
 | HIP_SIGNATURE_2        | 61633 | variable | Signature of the R1   |
 |                        |       |          | packet                |
 |                        |       |          |                       |
 | HIP_SIGNATURE          | 61697 | variable | Signature of the      |
 |                        |       |          | packet                |
 |                        |       |          |                       |
 | ECHO_REQUEST_UNSIGNED  | 63661 | variable | Opaque data to be     |
 |                        |       |          | echoed back; after    |
 |                        |       |          | signature             |
 |                        |       |          |                       |
 | ECHO_RESPONSE_UNSIGNED | 63425 | variable | Opaque data echoed    |
 |                        |       |          | back; after signature |
 +------------------------+-------+----------+-----------------------+
 Because the ordering (from lowest to highest) of HIP parameters is
 strictly enforced (see Section 5.2.1), the parameter type values for
 existing parameters have been spaced to allow for future protocol
 extensions.  Parameters numbered between 0-1023 are used in HIP
 handshake and update procedures and are covered by signatures.
 Parameters numbered between 1024-2047 are reserved.  Parameters
 numbered between 2048-4095 are used for parameters related to HIP
 transform types.  Parameters numbered between 4096 and (2^16 - 2^12)
 61439 are reserved.  Parameters numbered between 61440-62463 are used
 for signatures and signed MACs.  Parameters numbered between 62464-
 63487 are used for parameters that fall outside of the signed area of
 the packet.  Parameters numbered between 63488-64511 are used for
 rendezvous and other relaying services.  Parameters numbered between
 64512-65535 are reserved.

Moskowitz, et al. Experimental [Page 36] RFC 5201 Host Identity Protocol April 2008

5.2.1. TLV Format

 The TLV-encoded parameters are described in the following
 subsections.  The type-field value also describes the order of these
 fields in the packet, except for type values from 2048 to 4095 which
 are reserved for new transport forms.  The parameters MUST be
 included in the packet such that their types form an increasing
 order.  If the parameter can exist multiple times in the packet, the
 type value may be the same in consecutive parameters.  If the order
 does not follow this rule, the packet is considered to be malformed
 and it MUST be discarded.
 Parameters using type values from 2048 up to 4095 are transport
 formats.  Currently, one transport format is defined: the ESP
 transport format [RFC5202].  The order of these parameters does not
 follow the order of their type value, but they are put in the packet
 in order of preference.  The first of the transport formats it the
 most preferred, and so on.
 All of the TLV parameters have a length (including Type and Length
 fields), which is a multiple of 8 bytes.  When needed, padding MUST
 be added to the end of the parameter so that the total length becomes
 a multiple of 8 bytes.  This rule ensures proper alignment of data.
 Any added padding bytes MUST be zeroed by the sender, and their
 values SHOULD NOT be checked by the receiver.
 Consequently, the Length field indicates the length of the Contents
 field (in bytes).  The total length of the TLV parameter (including
 Type, Length, Contents, and Padding) is related to the Length field
 according to the following formula:
 Total Length = 11 + Length - (Length + 3) % 8;
 where % is the modulo operator

Moskowitz, et al. Experimental [Page 37] RFC 5201 Host Identity Protocol April 2008

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type            |C|             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    /                          Contents                             /
    /                                               +-+-+-+-+-+-+-+-+
    |                                               |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type         Type code for the parameter.  16 bits long, C-bit
                 being part of the Type code.
      C          Critical.  One if this parameter is critical, and
                 MUST be recognized by the recipient, zero otherwise.
                 The C bit is considered to be a part of the Type
                 field.  Consequently, critical parameters are always
                 odd and non-critical ones have an even value.
    Length       Length of the Contents, in bytes.
    Contents     Parameter specific, defined by Type
    Padding      Padding, 0-7 bytes, added if needed
 Critical parameters MUST be recognized by the recipient.  If a
 recipient encounters a critical parameter that it does not recognize,
 it MUST NOT process the packet any further.  It MAY send an ICMP or
 NOTIFY, as defined in Section 4.3.
 Non-critical parameters MAY be safely ignored.  If a recipient
 encounters a non-critical parameter that it does not recognize, it
 SHOULD proceed as if the parameter was not present in the received
 packet.

5.2.2. Defining New Parameters

 Future specifications may define new parameters as needed.  When
 defining new parameters, care must be taken to ensure that the
 parameter type values are appropriate and leave suitable space for
 other future extensions.  One must remember that the parameters MUST
 always be arranged in increasing order by Type code, thereby limiting
 the order of parameters (see Section 5.2.1).
 The following rules must be followed when defining new parameters.
 1.  The low-order bit C of the Type code is used to distinguish
     between critical and non-critical parameters.

Moskowitz, et al. Experimental [Page 38] RFC 5201 Host Identity Protocol April 2008

 2.  A new parameter may be critical only if an old recipient ignoring
     it would cause security problems.  In general, new parameters
     SHOULD be defined as non-critical, and expect a reply from the
     recipient.
 3.  If a system implements a new critical parameter, it MUST provide
     the ability to set the associated feature off, such that the
     critical parameter is not sent at all.  The configuration option
     must be well documented.  Implementations operating in a mode
     adhering to this specification MUST disable the sending of new
     critical parameters.  In other words, the management interface
     MUST allow vanilla standards-only mode as a default configuration
     setting, and MAY allow new critical payloads to be configured on
     (and off).
 4.  See Section 9 for allocation rules regarding Type codes.

5.2.3. R1_COUNTER

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Reserved, 4 bytes                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                R1 generation counter, 8 bytes                 |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           128
    Length         12
    R1 generation
      counter      The current generation of valid puzzles
 The R1_COUNTER parameter contains a 64-bit unsigned integer in
 network-byte order, indicating the current generation of valid
 puzzles.  The sender is supposed to increment this counter
 periodically.  It is RECOMMENDED that the counter value is
 incremented at least as often as old PUZZLE values are deprecated so
 that SOLUTIONs to them are no longer accepted.
 The R1_COUNTER parameter is optional.  It SHOULD be included in the
 R1 (in which case, it is covered by the signature), and if present in
 the R1, it MAY be echoed (including the Reserved field verbatim) by
 the Initiator in the I2.

Moskowitz, et al. Experimental [Page 39] RFC 5201 Host Identity Protocol April 2008

5.2.4. PUZZLE

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  K, 1 byte    |    Lifetime   |        Opaque, 2 bytes        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Random #I, 8 bytes                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           257
    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 a 64-bit integer, K and Lifetime as 8-bit
 integers, all in network byte order.
 The PUZZLE parameter contains the puzzle difficulty K and a 64-bit
 puzzle random integer #I.  The Puzzle Lifetime indicates the time
 during which the puzzle solution is valid, and sets a time limit that
 should not be exceeded by the Initiator while it attempts to solve
 the puzzle.  The lifetime is indicated as a power of 2 using the
 formula 2^(Lifetime-32) seconds.  A puzzle MAY be augmented with an
 ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in
 the R1; the contents of the echo request are then echoed back in the
 ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED, allowing the
 Responder to use the included information as a part of its puzzle
 processing.
 The Opaque and Random #I field are not covered by the HIP_SIGNATURE_2
 parameter.

Moskowitz, et al. Experimental [Page 40] RFC 5201 Host Identity Protocol April 2008

5.2.5. SOLUTION

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | K, 1 byte     |   Reserved    |        Opaque, 2 bytes        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Random #I, 8 bytes                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Puzzle solution #J, 8 bytes                   |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type               321
    Length             20
    K                  K is the number of verified bits
    Reserved           zero when sent, ignored when received
    Opaque             copied unmodified from the received PUZZLE
                       parameter
    Random #I          random number
    Puzzle solution #J random number
 Random #I and Random #J are represented as 64-bit integers, K as an
 8-bit integer, all in network byte order.
 The SOLUTION parameter contains a solution to a puzzle.  It also
 echoes back the random difficulty K, the Opaque field, and the puzzle
 integer #I.

Moskowitz, et al. Experimental [Page 41] RFC 5201 Host Identity Protocol April 2008

5.2.6. DIFFIE_HELLMAN

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Group ID    |      Public Value Length      | Public Value  /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Group ID    |      Public Value Length      | Public Value  /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                               |            padding            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           513
    Length         length in octets, excluding Type, Length, and
                   padding
    Group ID       defines values for p and g
    Public Value   length of the following Public Value in octets
      Length
    Public Value   the sender's public Diffie-Hellman key
 The following Group IDs have been defined:
    Group                            Value
    Reserved                         0
    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 [RFC3526].  The OAKLEY
 well-known group 1 is defined in Appendix E.
 The sender can include at most two different Diffie-Hellman public
 values in the DIFFIE_HELLMAN parameter.  This gives the possibility,
 e.g., for a server to provide a weaker encryption possibility for a
 PDA host that is not powerful enough.  It is RECOMMENDED that the
 Initiator, receiving more than one public value, selects the stronger
 one, if it supports it.
 A HIP implementation MUST implement 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).  It

Moskowitz, et al. Experimental [Page 42] RFC 5201 Host Identity Protocol April 2008

 is REQUIRED that the default configuration allows Group ID 1 usage,
 but it is RECOMMENDED that applications that need stronger security
 turn Group ID 1 support off.  Equipment powerful enough SHOULD
 implement also Group ID 5.  The 384-bit group is defined in
 Appendix D.
 To avoid unnecessary failures during the base exchange, the rest of
 the groups SHOULD be implemented in hosts where resources are
 adequate.

5.2.7. 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            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Suite ID #1        |          Suite ID #2          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Suite ID #n        |             Padding           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           577
    Length         length in octets, excluding Type, Length, and
                   padding
    Suite ID       defines the HIP Suite to be used
 The following Suite IDs are defined ([RFC4307],[RFC2451]):
       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
 The sender of a HIP_TRANSFORM parameter MUST make sure that there are
 no more than six (6) HIP Suite IDs in one HIP_TRANSFORM parameter.
 Conversely, a recipient MUST be prepared to handle received transport
 parameters that contain more than six Suite IDs by accepting the
 first six Suite IDs and dropping the rest.  The limited number of
 transforms sets the maximum size of HIP_TRANSFORM parameter.  As the
 default configuration, the HIP_TRANSFORM parameter MUST contain at
 least one of the mandatory Suite IDs.  There MAY be a configuration
 option that allows the administrator to override this default.

Moskowitz, et al. Experimental [Page 43] RFC 5201 Host Identity Protocol April 2008

 The Responder lists supported and desired Suite IDs in order of
 preference in the R1, up to the maximum of six Suite IDs.  The
 Initiator MUST choose only one of the corresponding Suite IDs.  That
 Suite ID will be used for generating the I2.
 Mandatory implementations: AES-CBC with HMAC-SHA1 and NULL-ENCRYPTION
 with HMAC-SHA1.

5.2.8. HOST_ID

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          HI Length            |DI-type|      DI Length        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Host Identity                         /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                               |         Domain Identifier     /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                               |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type              705
    Length            length in octets, excluding Type, Length, and
                      Padding
    HI Length         length of the Host Identity in octets
    DI-type           type of the following Domain Identifier field
    DI Length         length of the FQDN or NAI in octets
    Host Identity     actual Host Identity
    Domain Identifier the identifier of the sender
 The Host Identity is represented in RFC 4034 [RFC4034] format.  The
 algorithms used in RDATA format are the following:
       Algorithms       Values
       RESERVED         0
       DSA              3 [RFC2536] (RECOMMENDED)
       RSA/SHA1         5 [RFC3110] (REQUIRED)
 The following DI-types have been defined:
        Type                    Value
        none included           0
        FQDN                    1
        NAI                     2

Moskowitz, et al. Experimental [Page 44] RFC 5201 Host Identity Protocol April 2008

        FQDN            Fully Qualified Domain Name, in binary format.
        NAI             Network Access Identifier
 The format for the FQDN is defined in RFC 1035 [RFC1035] Section 3.1.
 The format for NAI is defined in [RFC4282]
 If there is no Domain Identifier, i.e., the DI-type field is zero,
 the DI Length field is set to zero as well.

5.2.9. 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                              |
    /                                                               /
    /                               +-------------------------------+
    |                               |            Padding            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           61505
    Length         length in octets, excluding Type, Length, and
                   Padding
    HMAC           HMAC computed over the HIP packet, excluding the
                   HMAC parameter and any following parameters, such
                   as HIP_SIGNATURE, HIP_SIGNATURE_2,
                   ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
                   The checksum field MUST be set to zero and the HIP
                   header length in the HIP common header MUST be
                   calculated not to cover any excluded parameters
                   when the HMAC is calculated.  The size of the
                   HMAC is the natural size of the hash computation
                   output depending on the used hash function.
 The HMAC calculation and verification process is presented in
 Section 6.4.1.

Moskowitz, et al. Experimental [Page 45] RFC 5201 Host Identity Protocol April 2008

5.2.10. HMAC_2

 The parameter structure is the same as in Section 5.2.9.  The fields
 are:
    Type           61569
    Length         length in octets, excluding Type, Length, and
                   Padding
    HMAC           HMAC computed over the HIP packet, excluding the
                   HMAC parameter and any following parameters such
                   as HIP_SIGNATURE, HIP_SIGNATURE_2,
                   ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
                   and including an additional sender's HOST_ID
                   parameter during the HMAC calculation.  The
                   checksum field MUST be set to zero and the HIP
                   header length in the HIP common header MUST be
                   calculated not to cover any excluded parameters
                   when the HMAC is calculated.  The size of the
                   HMAC is the natural size of the hash computation
                   output depending on the used hash function.
 The HMAC calculation and verification process is presented in
 Section 6.4.1.

5.2.11. HIP_SIGNATURE

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SIG alg    |                  Signature                    /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                               |             Padding           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           61697
    Length         length in octets, excluding Type, Length, and
                   Padding
    SIG alg        signature algorithm
    Signature      the signature is calculated over the HIP packet,
                   excluding the HIP_SIGNATURE parameter and any
                   parameters that follow the HIP_SIGNATURE parameter.
                   The checksum field MUST be set to zero, and the HIP
                   header length in the HIP common header MUST be
                   calculated only to the beginning of the
                   HIP_SIGNATURE parameter when the signature is
                   calculated.

Moskowitz, et al. Experimental [Page 46] RFC 5201 Host Identity Protocol April 2008

 The signature algorithms are defined in Section 5.2.8.  The signature
 in the Signature field is encoded using the proper method depending
 on the signature algorithm (e.g., according to [RFC3110] in case of
 RSA/SHA1, or according to [RFC2536] in case of DSA).
 The HIP_SIGNATURE calculation and verification process is presented
 in Section 6.4.2.

5.2.12. HIP_SIGNATURE_2

 The parameter structure is the same as in Section 5.2.11.  The fields
 are:
 Type           61633
 Length         length in octets, excluding Type, Length, and
                Padding
 SIG alg        signature algorithm
 Signature      Within the R1 packet that contains the HIP_SIGNATURE_2
                parameter, the Initiator's HIT, the checksum
                field, and the Opaque and Random #I fields in the
                PUZZLE parameter MUST be set to zero while
                computing the HIP_SIGNATURE_2 signature.  Further,
                the HIP packet length in the HIP header MUST be
                adjusted as if the HIP_SIGNATURE_2 was not in the
                packet during the signature calculation, i.e., the
                HIP packet length points to the beginning of
                the HIP_SIGNATURE_2 parameter during signing and
                verification.
 Zeroing the Initiator's HIT makes it possible to create R1 packets
 beforehand, to minimize the effects of possible DoS attacks.  Zeroing
 the Random #I and Opaque fields within the PUZZLE parameter allows
 these fields to be populated dynamically on precomputed R1s.
 Signature calculation and verification follows the process in
 Section 6.4.2.

Moskowitz, et al. Experimental [Page 47] RFC 5201 Host Identity Protocol April 2008

5.2.13. SEQ

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Update ID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           385
    Length         4
    Update ID      32-bit sequence number
 The Update ID is an unsigned quantity, initialized by a host to zero
 upon moving to ESTABLISHED state.  The Update ID has scope within a
 single HIP association, and not across multiple associations or
 multiple hosts.  The Update ID is incremented by one before each new
 UPDATE that is sent by the host; the first UPDATE packet originated
 by a host has an Update ID of 0.

5.2.14. ACK

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       peer Update ID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type            449
    Length          variable (multiple of 4)
    peer Update ID  32-bit sequence number corresponding to the
                    Update ID being ACKed.
 The ACK parameter includes one or more Update IDs that have been
 received from the peer.  The Length field identifies the number of
 peer Update IDs that are present in the parameter.

Moskowitz, et al. Experimental [Page 48] RFC 5201 Host Identity Protocol April 2008

5.2.15. ENCRYPTED

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Reserved                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              IV                               /
    /                                                               /
    /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
    /                        Encrypted data                         /
    /                                                               /
    /                               +-------------------------------+
    /                               |            Padding            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           641
    Length         length in octets, excluding Type, Length, and
                   Padding
    Reserved       zero when sent, ignored when received
    IV             Initialization vector, if needed, otherwise
                   nonexistent.  The length of the IV is inferred from
                   the HIP transform.
    Encrypted      The data is encrypted using an encryption algorithm
      data         as defined in HIP transform.
 The ENCRYPTED parameter encapsulates another parameter, the encrypted
 data, which holds one or more HIP parameters in block encrypted form.
 Consequently, the first fields in the encapsulated parameter(s) are
 Type and Length of the first such parameter, allowing the contents to
 be easily parsed after decryption.
 The field labelled "Encrypted data" consists of the output of one or
 more HIP parameters concatenated together that have been passed
 through an encryption algorithm.  Each of these inner parameters is
 padded according to the rules of Section 5.2.1 for padding individual
 parameters.  As a result, the concatenated parameters will be a block
 of data that is 8-byte aligned.
 Some encryption algorithms require that the data to be encrypted must
 be a multiple of the cipher algorithm block size.  In this case, the
 above block of data MUST include additional padding, as specified by
 the encryption algorithm.  The size of the extra padding is selected
 so that the length of the unencrypted data block is a multiple of the

Moskowitz, et al. Experimental [Page 49] RFC 5201 Host Identity Protocol April 2008

 cipher block size.  The encryption algorithm may specify padding
 bytes other than zero; for example, AES [FIPS01] uses the PKCS5
 padding scheme (see section 6.1.1 of [RFC2898]) where the remaining n
 bytes to fill the block each have the value n.  This yields an
 "unencrypted data" block that is transformed to an "encrypted data"
 block by the cipher suite.  This extra padding added to the set of
 parameters to satisfy the cipher block alignment rules is not counted
 in HIP TLV length fields, and this extra padding should be removed by
 the cipher suite upon decryption.
 Note that the length of the cipher suite output may be smaller or
 larger than the length of the set of parameters to be encrypted,
 since the encryption process may compress the data or add additional
 padding to the data.
 Once this encryption process is completed, the Encrypted data field
 is ready for inclusion in the Parameter.  If necessary, additional
 Padding for 8-byte alignment is then added according to the rules of
 Section 5.2.1.

5.2.16. NOTIFICATION

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

Moskowitz, et al. Experimental [Page 50] RFC 5201 Host Identity Protocol April 2008

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Reserved             |      Notify Message Type      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               /
    /                   Notification Data                           /
    /                                               +---------------+
    /                                               |     Padding   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type           832
    Length         length in octets, excluding Type, Length, and
                   Padding
    Reserved       zero when sent, ignored when received
    Notify Message specifies the type of notification
      Type
    Notification   informational or error data transmitted in addition
      Data         to the Notify Message Type.  Values for this field
                   are type specific (see below).
    Padding        any Padding, if necessary, to make the parameter a
                   multiple of 8 bytes.
 Notification information can be error messages specifying why an SA
 could not be established.  It can also be status data that a process
 managing an SA database wishes to communicate with a peer process.
 The table below lists the Notification messages and their
 corresponding values.
 To avoid certain types of attacks, a Responder SHOULD avoid sending a
 NOTIFICATION to any host with which it has not successfully verified
 a puzzle solution.
 Types in the range 0-16383 are intended for reporting errors and in
 the range 16384-65535 for other status information.  An
 implementation that receives a NOTIFY packet with a NOTIFICATION
 error parameter in response to a request packet (e.g., I1, I2,
 UPDATE) SHOULD assume that the corresponding request has failed
 entirely.  Unrecognized error types MUST be ignored except that they
 SHOULD be logged.
 Notify payloads with status types MUST be ignored if not recognized.

Moskowitz, et al. Experimental [Page 51] RFC 5201 Host Identity Protocol April 2008

 NOTIFICATION 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 packets whose HMAC (if present) and SIGNATURE have
    been verified.  This status MUST be sent in response to any
    error not covered by one of the other status types, and should
    not contain details to avoid leaking information to someone
    probing a node.  To aid debugging, more detailed error
    information SHOULD be written to a console or log.
 NO_DH_PROPOSAL_CHOSEN                     14
    None of the proposed group IDs was acceptable.
 INVALID_DH_CHOSEN                         15
    The D-H Group ID field does not correspond to one offered
    by the Responder.
 NO_HIP_PROPOSAL_CHOSEN                    16
    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, except when
    the signature verification fails in a NOTIFY message.

Moskowitz, et al. Experimental [Page 52] RFC 5201 Host Identity Protocol April 2008

 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 parameter.
 INVALID_HIT                               40
    Sent in response to a failure to validate the peer's
    HIT from the corresponding HI.
 BLOCKED_BY_POLICY                         42
    The Responder is unwilling to set up an association
    for some policy reason (e.g., received HIT is NULL
    and policy does not allow opportunistic mode).
 SERVER_BUSY_PLEASE_RETRY                  44
    The Responder is unwilling to set up an association as it is
    suffering under some kind of overload and has chosen to shed load
    by rejecting the Initiator's request.  The Initiator may retry;
    however, the Initiator MUST find another (different) puzzle
    solution for any such retries.  Note that the Initiator may need
    to obtain a new puzzle with a new I1/R1 exchange.
 NOTIFY MESSAGES - STATUS TYPES           Value
 ------------------------------           -----
 I2_ACKNOWLEDGEMENT                        16384
    The Responder has an I2 from the Initiator but had to queue the I2
    for processing.  The puzzle was correctly solved and the Responder
    is willing to set up an association but currently has a number of
    I2s in the processing queue.  R2 will be sent after the I2 has
    been processed.

Moskowitz, et al. Experimental [Page 53] RFC 5201 Host Identity Protocol April 2008

5.2.17. ECHO_REQUEST_SIGNED

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Opaque data (variable length)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type         897
    Length       variable
    Opaque data  opaque data, supposed to be meaningful only to the
                 node that sends ECHO_REQUEST_SIGNED and receives a
                 corresponding ECHO_RESPONSE_SIGNED or
                 ECHO_RESPONSE_UNSIGNED.
 The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
 that the sender wants to get echoed back in the corresponding reply
 packet.
 The ECHO_REQUEST_SIGNED and corresponding echo response parameters
 MAY be used for any purpose where a node wants to carry some state in
 a request packet and get it back in a response packet.  The
 ECHO_REQUEST_SIGNED is covered by the HMAC and SIGNATURE.  A HIP
 packet can contain only one ECHO_REQUEST_SIGNED or
 ECHO_REQUEST_UNSIGNED parameter.  The ECHO_REQUEST_SIGNED parameter
 MUST be responded to with a corresponding echo response.
 ECHO_RESPONSE_SIGNED SHOULD be used, but if it is not possible, e.g.,
 due to a middlebox-provided response, it MAY be responded to with an
 ECHO_RESPONSE_UNSIGNED.

5.2.18. ECHO_REQUEST_UNSIGNED

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Opaque data (variable length)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type         63661
    Length       variable
    Opaque data  opaque data, supposed to be meaningful only to the
                 node that sends ECHO_REQUEST_UNSIGNED and receives a
                 corresponding ECHO_RESPONSE_UNSIGNED.

Moskowitz, et al. Experimental [Page 54] RFC 5201 Host Identity Protocol April 2008

 The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
 that the sender wants to get echoed back in the corresponding reply
 packet.
 The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
 MAY be used for any purpose where a node wants to carry some state in
 a request packet and get it back in a response packet.  The
 ECHO_REQUEST_UNSIGNED is not covered by the HMAC and SIGNATURE.  A
 HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
 It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
 in HIP packets passing by.  The sender has to create the Opaque field
 so that it can later identify and remove the corresponding
 ECHO_RESPONSE_UNSIGNED parameter.
 The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
 ECHO_RESPONSE_UNSIGNED parameter.

5.2.19. ECHO_RESPONSE_SIGNED

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Opaque data (variable length)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type         961
    Length       variable
    Opaque data  opaque data, copied unmodified from the
                 ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                 parameter that triggered this response.
 The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data
 that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back.
 The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED
 parameter.
 The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be
 used for any purpose where a node wants to carry some state in a
 request packet and get it back in a response packet.  The
 ECHO_RESPONSE_SIGNED is covered by the HMAC and SIGNATURE.

Moskowitz, et al. Experimental [Page 55] RFC 5201 Host Identity Protocol April 2008

5.2.20. ECHO_RESPONSE_UNSIGNED

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Type              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Opaque data (variable length)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Type         63425
    Length       variable
    Opaque data  opaque data, copied unmodified from the
                 ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                 parameter that triggered this response.
 The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data
 that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
 wants to get echoed back.  The opaque data is copied unmodified from
 the corresponding echo request parameter.
 The echo request and ECHO_RESPONSE_UNSIGNED parameters MAY be used
 for any purpose where a node wants to carry some state in a request
 packet and get it back in a response packet.  The
 ECHO_RESPONSE_UNSIGNED is not covered by the HMAC and SIGNATURE.

5.3. HIP Packets

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

Moskowitz, et al. Experimental [Page 56] RFC 5201 Host Identity Protocol April 2008

 +------------------+------------------------------------------------+
 |    Packet type   | Packet name                                    |
 +------------------+------------------------------------------------+
 |         1        | I1 - the HIP Initiator Packet                  |
 |                  |                                                |
 |         2        | R1 - the HIP Responder Packet                  |
 |                  |                                                |
 |         3        | I2 - the Second HIP Initiator Packet           |
 |                  |                                                |
 |         4        | R2 - the Second HIP Responder Packet           |
 |                  |                                                |
 |        16        | UPDATE - the HIP Update Packet                 |
 |                  |                                                |
 |        17        | NOTIFY - the HIP Notify Packet                 |
 |                  |                                                |
 |        18        | CLOSE - the HIP Association Closing Packet     |
 |                  |                                                |
 |        19        | CLOSE_ACK - the HIP Closing Acknowledgment     |
 |                  | Packet                                         |
 +------------------+------------------------------------------------+
             Table 10: HIP packets and packet type numbers
 Packets consist of the fixed header as described in Section 5.1,
 followed by the parameters.  The parameter part, in turn, consists of
 zero or more TLV-coded parameters.
 In addition to the base packets, other packet types will be defined
 later in separate specifications.  For example, support for mobility
 and multi-homing is not included in this specification.
 See Notation (Section 2.2) for used operations.
 In the future, an OPTIONAL upper-layer payload MAY follow the HIP
 header.  The Next Header field in the header indicates if there is
 additional data following the HIP header.  The HIP packet, however,
 MUST NOT be fragmented.  This limits the size of the possible
 additional data in the packet.

Moskowitz, et al. Experimental [Page 57] RFC 5201 Host Identity Protocol April 2008

5.3.1. I1 - the HIP Initiator Packet

 The HIP header values for the I1 packet:
    Header:
      Packet Type = 1
      SRC HIT = Initiator's HIT
      DST HIT = Responder's HIT, or NULL
    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 to use opportunistic mode by using NULL (all zeros) as the
 Responder's HIT.  See also "HIP Opportunistic Mode" (Section 4.1.6).
 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.
 Implementations MUST be able to handle a storm of received I1
 packets, discarding those with common content that arrive within a
 small time delta.

5.3.2. R1 - the HIP Responder Packet

 The HIP header values for the R1 packet:
    Header:
      Packet Type = 2
      SRC HIT = Responder's HIT
      DST HIT = Initiator's HIT
    IP ( HIP ( [ R1_COUNTER, ]
               PUZZLE,
               DIFFIE_HELLMAN,
               HIP_TRANSFORM,
               HOST_ID,
               [ ECHO_REQUEST_SIGNED, ]
               HIP_SIGNATURE_2 )
               <, ECHO_REQUEST_UNSIGNED >i)
 Valid control bits: A

Moskowitz, et al. Experimental [Page 58] RFC 5201 Host Identity Protocol April 2008

 If the Responder's HI is an anonymous one, the A control MUST be set.
 The Initiator's HIT MUST match the one received in I1.  If the
 Responder has multiple HIs, the Responder's HIT used MUST match
 Initiator's request.  If the Initiator used opportunistic mode, the
 Responder may select freely among its HIs.  See also "HIP
 Opportunistic Mode" (Section 4.1.6).
 The R1 generation counter is used to determine the currently valid
 generation of puzzles.  The value is increased periodically, and it
 is RECOMMENDED that it is increased at least as often as solutions to
 old puzzles are no longer accepted.
 The Puzzle contains a Random #I and the difficulty K.  The difficulty
 K indicates the number of lower-order bits, in the puzzle hash
 result, that must be zeros; see Section 4.1.2.  The Random #I is not
 covered by the signature and must be zeroed during the signature
 calculation, allowing the sender to select and set the #I into a
 precomputed R1 just prior sending it to the peer.
 The Diffie-Hellman value is ephemeral, and one value SHOULD be used
 only for one connection.  Once the Responder has received a valid
 response to an R1 packet, that Diffie-Hellman value SHOULD be
 deprecated.  Because it is possible that the Responder has sent the
 same Diffie-Hellman value to different hosts simultaneously in
 corresponding R1 packets, those responses should also be accepted.
 However, as a defense against I1 storms, an implementation MAY
 propose, and re-use if not avoidable, the same Diffie-Hellman value
 for a period of time, for example, 15 minutes.  By using a small
 number of different puzzles for a given Diffie-Hellman value, the R1
 packets can be precomputed and delivered as quickly as I1 packets
 arrive.  A scavenger process should clean up unused Diffie-Hellman
 values and puzzles.
 Re-using Diffie-Hellman public keys opens up the potential security
 risk of more than one Initiator ending up with the same keying
 material (due to faulty random number generators).  Also, more than
 one Initiator using the same Responder public key half may lead to
 potentially easier cryptographic attacks and to imperfect forward
 security.
 However, these risks involved in re-using the same key are
 statistical; that is, the authors are not aware of any mechanism that
 would allow manipulation of the protocol so that the risk of the re-
 use of any given Responder Diffie-Hellman public key would differ
 from the base probability.  Consequently, it is RECOMMENDED that
 implementations avoid re-using the same D-H key with multiple
 Initiators, but because the risk is considered statistical and not

Moskowitz, et al. Experimental [Page 59] RFC 5201 Host Identity Protocol April 2008

 known to be manipulable, the implementations MAY re-use a key in
 order to ease resource-constrained implementations and to increase
 the probability of successful communication with legitimate clients
 even under an I1 storm.  In particular, when it is too expensive to
 generate enough precomputed R1 packets to supply each potential
 Initiator with a different D-H key, the Responder MAY send the same
 D-H key to several Initiators, thereby creating the possibility of
 multiple legitimate Initiators ending up using the same Responder-
 side public key.  However, as soon as the Responder knows that it
 will use a particular D-H key, it SHOULD stop offering it.  This
 design is aimed to allow resource-constrained Responders to offer
 services under I1 storms and to simultaneously make the probability
 of D-H key re-use both statistical and as low as possible.
 If a future version of this protocol is considered, we strongly
 recommend that these issues be studied again.  Especially, the
 current design allows hosts to become potentially more vulnerable to
 a statistical, low-probability problem during I1 storm attacks than
 what they are if no attack is taking place; whether this is
 acceptable or not should be reconsidered in the light of any new
 experience gained.
 The HIP_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 [RFC3602]
 with HMAC-SHA-1-96 [RFC2404].
 The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED contains data that
 the sender wants to receive unmodified in the corresponding response
 packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
 parameter.
 The signature is calculated over the whole HIP envelope, after
 setting the Initiator's HIT, header checksum, as well as the Opaque
 field and the Random #I in the PUZZLE parameter temporarily to zero,
 and excluding any parameters that follow the signature, as described
 in Section 5.2.12.  This allows the Responder to use precomputed R1s.
 The Initiator SHOULD validate this signature.  It SHOULD check that
 the Responder's HI received matches with the one expected, if any.

Moskowitz, et al. Experimental [Page 60] RFC 5201 Host Identity Protocol April 2008

5.3.3. I2 - the Second HIP Initiator Packet

 The HIP header values for the I2 packet:
    Header:
      Type = 3
      SRC HIT = Initiator's HIT
      DST HIT = Responder's HIT
    IP ( HIP ( [R1_COUNTER,]
               SOLUTION,
               DIFFIE_HELLMAN,
               HIP_TRANSFORM,
               ENCRYPTED { HOST_ID } or HOST_ID,
               [ ECHO_RESPONSE_SIGNED ,]
               HMAC,
               HIP_SIGNATURE
               <, ECHO_RESPONSE_UNSIGNED>i ) )
 Valid control bits: A
 The HITs used MUST match the ones used previously.
 If the Initiator's HI is an anonymous one, the A control MUST be set.
 The Initiator MAY include an unmodified copy of the R1_COUNTER
 parameter received in the corresponding R1 packet into the I2 packet.
 The Solution contains the Random #I from R1 and the computed #J.  The
 low-order K bits of the RHASH(I | ... | J) MUST be zero.
 The Diffie-Hellman value is ephemeral.  If precomputed, a scavenger
 process should clean up unused Diffie-Hellman values.  The Responder
 may re-use Diffie-Hellman values under some conditions as specified
 in Section 5.3.2.
 The HIP_TRANSFORM contains the single encryption and integrity
 transform selected by the Initiator, that will be used to protect the
 HI exchange.  The chosen transform MUST correspond to one offered by
 the Responder in the R1.  All implementations MUST support the AES
 transform [RFC3602].
 The Initiator's HI MAY be encrypted using the HIP_TRANSFORM
 encryption algorithm.  The keying material is derived from the
 Diffie-Hellman exchanged as defined in Section 6.5.

Moskowitz, et al. Experimental [Page 61] RFC 5201 Host Identity Protocol April 2008

 The ECHO_RESPONSE_SIGNED and ECHO_RESPONSE_UNSIGNED contain the
 unmodified Opaque data copied from the corresponding echo request
 parameter.
 The HMAC is calculated over the whole HIP envelope, excluding any
 parameters after the HMAC, as described in Section 6.4.1.  The
 Responder MUST validate the HMAC.
 The signature is calculated over the whole HIP envelope, excluding
 any parameters after the HIP_SIGNATURE, as described in
 Section 5.2.11.  The Responder MUST validate this signature.  It MAY
 use either the HI in the packet or the HI acquired by some other
 means.

5.3.4. R2 - the Second HIP Responder Packet

 The HIP header values for the R2 packet:
    Header:
      Packet Type = 4
      SRC HIT = Responder's HIT
      DST HIT = Initiator's HIT
    IP ( HIP ( HMAC_2, HIP_SIGNATURE ) )
 Valid control bits: none
 The HMAC_2 is calculated over the whole HIP envelope, with
 Responder's HOST_ID parameter concatenated with the HIP envelope.
 The HOST_ID parameter is removed after the HMAC calculation.  The
 procedure is described in Section 6.4.1.
 The signature is calculated over the whole HIP envelope.
 The Initiator MUST validate both the HMAC and the signature.

5.3.5. UPDATE - the HIP Update Packet

 Support for the UPDATE packet is MANDATORY.
 The HIP header values for the UPDATE packet:
    Header:
      Packet Type = 16
      SRC HIT = Sender's HIT
      DST HIT = Recipient's HIT
    IP ( HIP ( [SEQ, ACK, ] HMAC, HIP_SIGNATURE ) )

Moskowitz, et al. Experimental [Page 62] RFC 5201 Host Identity Protocol April 2008

 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 forgo reliable transmission of a particular
 UPDATE (e.g., it becomes overcome by events).  The semantics are such
 that the receiver MUST acknowledge the UPDATE, but the sender MAY
 choose to not care about receiving the ACK.
 UPDATEs MAY be retransmitted without incrementing SEQ.  If the same
 subset of parameters is included in multiple UPDATEs with different
 SEQs, the host MUST ensure that the receiver's processing of the
 parameters multiple times will not result in a protocol error.

5.3.6. NOTIFY - the HIP Notify Packet

 The NOTIFY packet is OPTIONAL.  The NOTIFY packet MAY be used to
 provide information to a peer.  Typically, NOTIFY is used to indicate
 some type of protocol error or negotiation failure.  NOTIFY packets
 are unacknowledged.  The receiver can handle the packet only as
 informational, and SHOULD NOT change its HIP state (Section 4.4.1)
 based purely on a received NOTIFY packet.

Moskowitz, et al. Experimental [Page 63] RFC 5201 Host Identity Protocol April 2008

 The HIP header values for the NOTIFY packet:
    Header:
      Packet Type = 17
      SRC HIT = Sender's HIT
      DST HIT = Recipient's HIT, or zero if unknown
    IP ( HIP (<NOTIFICATION>i, [HOST_ID, ] HIP_SIGNATURE) )
 Valid control bits: None
 The NOTIFY packet is used to carry one or more NOTIFICATION
 parameters.

5.3.7. CLOSE - the HIP Association Closing Packet

 The HIP header values for the CLOSE packet:
    Header:
      Packet Type = 18
      SRC HIT = Sender's HIT
      DST HIT = Recipient's HIT
    IP ( HIP ( ECHO_REQUEST_SIGNED, HMAC, HIP_SIGNATURE ) )
 Valid control bits: none
 The sender MUST include an ECHO_REQUEST_SIGNED used to validate
 CLOSE_ACK received in response, and both an 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_RESPONSE_SIGNED corresponding to the received
 ECHO_REQUEST_SIGNED.

5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet

 The HIP header values for the CLOSE_ACK packet:
    Header:
      Packet Type = 19
      SRC HIT = Sender's HIT
      DST HIT = Recipient's HIT
    IP ( HIP ( ECHO_RESPONSE_SIGNED, HMAC, HIP_SIGNATURE ) )
 Valid control bits: none

Moskowitz, et al. Experimental [Page 64] RFC 5201 Host Identity Protocol April 2008

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

5.4. ICMP Messages

 When a HIP implementation detects a problem with an incoming packet,
 and it either cannot determine the identity of the sender of the
 packet or does not have any existing HIP association with the sender
 of the packet, it MAY respond with an ICMP packet.  Any such replies
 MUST be rate-limited as described in [RFC2463].  In most cases, the
 ICMP packet will have the Parameter Problem type (12 for ICMPv4, 4
 for ICMPv6), with the Pointer field pointing to the field that caused
 the ICMP message to be generated.

5.4.1. Invalid Version

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

5.4.2. Other Problems with the HIP Header and Packet Structure

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

5.4.3. Invalid Puzzle Solution

 If a HIP implementation receives an I2 packet that has an invalid
 puzzle solution, the behavior depends on the underlying version of
 IP.  If IPv6 is used, the implementation SHOULD respond with an ICMP
 packet with type Parameter Problem, the Pointer pointing to the
 beginning of the Puzzle solution #J field in the SOLUTION payload in
 the HIP message.
 If IPv4 is used, the implementation MAY respond with an ICMP packet
 with the type Parameter Problem, copying enough of bytes from the I2
 message so that the SOLUTION parameter fits into the ICMP message,
 the Pointer pointing to the beginning of the Puzzle solution #J

Moskowitz, et al. Experimental [Page 65] RFC 5201 Host Identity Protocol April 2008

 field, as in the IPv6 case.  Note, however, that the resulting ICMPv4
 message exceeds the typical ICMPv4 message size as defined in
 [RFC0792].

5.4.4. Non-Existing HIP Association

 If a HIP implementation receives a CLOSE or UPDATE packet, or any
 other packet whose handling requires an existing association, that
 has either a Receiver or Sender HIT that does not match with any
 existing HIP association, the implementation MAY respond, rate-
 limited, with an ICMP packet with the type Parameter Problem, and
 with the Pointer pointing to the beginning of the first HIT that does
 not match.
 A host MUST NOT reply with such an ICMP if it receives any of the
 following messages: I1, R2, I2, R2, and NOTIFY.  When introducing new
 packet types, a specification SHOULD define the appropriate rules for
 sending or not sending this kind of ICMP reply.

6. Packet Processing

 Each host is assumed to have a single HIP protocol implementation
 that manages the host's HIP associations and handles requests for new
 ones.  Each HIP association is governed by a conceptual state
 machine, with states defined above in Section 4.4.  The HIP
 implementation can simultaneously maintain HIP associations with more
 than one host.  Furthermore, the HIP implementation may have more
 than one active HIP association with another host; in this case, HIP
 associations are distinguished by their respective HITs.  It is not
 possible to have more than one HIP association between any given pair
 of HITs.  Consequently, the only way for two hosts to have more than
 one parallel association is to use different HITs, at least at one
 end.
 The processing of packets depends on the state of the HIP
 association(s) with respect to the authenticated or apparent
 originator of the packet.  A HIP implementation determines whether it
 has an active association with the originator of the packet based on
 the HITs.  In the case of user data carried in a specific transport
 format, the transport format document specifies how the incoming
 packets are matched with the active associations.

6.1. Processing Outgoing Application Data

 In a HIP host, an application can send application-level data using
 an identifier specified via the underlying API.  The API can be a
 backwards-compatible API (see [HIP-APP]), using identifiers that look
 similar to IP addresses, or a completely new API, providing enhanced

Moskowitz, et al. Experimental [Page 66] RFC 5201 Host Identity Protocol April 2008

 services related to Host Identities.  Depending on the HIP
 implementation, the identifier provided to the application may be
 different; for example, it can be a HIT or an IP address.
 The exact format and method for transferring the data from the source
 HIP host to the destination HIP host is defined in the corresponding
 transport format document.  The actual data is transferred in the
 network using the appropriate source and destination IP addresses.
 In this document, conceptual processing rules are defined only for
 the base case where both hosts have only single usable IP addresses;
 the multi-address multi-homing case will be specified separately.
 The following conceptual algorithm describes the steps that are
 required for handling outgoing datagrams destined to a HIT.
 1.  If the datagram has a specified source address, it MUST be a HIT.
     If it is not, the implementation MAY replace the source address
     with a HIT.  Otherwise, it MUST drop the packet.
 2.  If the datagram has an unspecified source address, the
     implementation must choose a suitable source HIT for the
     datagram.
 3.  If there is no active HIP association with the given <source,
     destination> HIT pair, one must be created by running the base
     exchange.  While waiting for the base exchange to complete, the
     implementation SHOULD queue at least one packet per HIP
     association to be formed, and it MAY queue more than one.
 4.  Once there is an active HIP association for the given <source,
     destination> HIT pair, the outgoing datagram is passed to
     transport handling.  The possible transport formats are defined
     in separate documents, of which the ESP transport format for HIP
     is mandatory for all HIP implementations.
 5.  Before sending the packet, the HITs in the datagram are replaced
     with suitable IP addresses.  For IPv6, the rules defined in
     [RFC3484] SHOULD be followed.  Note that this HIT-to-IP-address
     conversion step MAY also be performed at some other point in the
     stack, e.g., before wrapping the packet into the output format.

6.2. Processing Incoming Application Data

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

Moskowitz, et al. Experimental [Page 67] RFC 5201 Host Identity Protocol April 2008

 1.  The incoming datagram is mapped to an existing HIP association,
     typically using some information from the packet.  For example,
     such mapping may be based on the ESP Security Parameter Index
     (SPI).
 2.  The specific transport format is unwrapped, in a way depending on
     the transport format, yielding a packet that looks like a
     standard (unencrypted) IP packet.  If possible, this step SHOULD
     also verify that the packet was indeed (once) sent by the remote
     HIP host, as identified by the HIP association.
     Depending on the used transport mode, the verification method can
     vary.  While the HI (as well as HIT) is used as the higher-layer
     identifier, the verification method has to verify that the data
     packet was sent by a node identity and that the actual identity
     maps to this particular HIT.  When using ESP transport format
     [RFC5202], the verification is done using the SPI value in the
     data packet to find the corresponding SA with associated HIT and
     key, and decrypting the packet with that associated key.
 3.  The IP addresses in the datagram are replaced with the HITs
     associated with the HIP association.  Note that this IP-address-
     to-HIT conversion step MAY also be performed at some other point
     in the stack.
 4.  The datagram is delivered to the upper layer.  When
     demultiplexing the datagram, the right upper-layer socket is
     based on the HITs.

6.3. Solving the Puzzle

 This subsection describes the puzzle-solving details.
 In R1, the values I and K are sent in network byte order.  Similarly,
 in I2, the values I and J are sent in network byte order.  The hash
 is created by concatenating, in network byte order, the following
 data, in the following order and using the RHASH algorithm:
    64-bit random value I, in network byte order, as appearing in R1
    and I2.
    128-bit Initiator's HIT, in network byte order, as appearing in
    the HIP Payload in R1 and I2.
    128-bit Responder's HIT, in network byte order, as appearing in
    the HIP Payload in R1 and I2.
    64-bit random value J, in network byte order, as appearing in I2.

Moskowitz, et al. Experimental [Page 68] RFC 5201 Host Identity Protocol April 2008

 In order to be a valid response puzzle, the K low-order bits of the
 resulting RHASH digest must be zero.
 Notes:
    i) The length of the data to be hashed is 48 bytes.
    ii) All the data in the hash input MUST be in network byte order.
    iii) The order of the Initiator's and Responder's HITs are
    different in the R1 and I2 packets; see Section 5.1.  Care must be
    taken to copy the values in the right order to the hash input.
 The following procedure describes the processing steps involved,
 assuming that the Responder chooses to precompute the R1 packets:
 Precomputation by the Responder:
    Sets up the puzzle difficulty K.
    Creates a signed R1 and caches it.
 Responder:
    Selects a suitable cached R1.
    Generates a random number I.
    Sends I and K in an R1.
    Saves I and K for a Delta time.
 Initiator:
    Generates repeated attempts to solve the puzzle until a matching J
    is found:
    Ltrunc( RHASH( I | HIT-I | HIT-R | J ), K ) == 0
    Sends I and J in an I2.
 Responder:
    Verifies that the received I is a saved one.
    Finds the right K based on I.
    Computes V := Ltrunc( RHASH( I | HIT-I | HIT-R | J ), K )
    Rejects if V != 0
    Accept if V == 0

Moskowitz, et al. Experimental [Page 69] RFC 5201 Host Identity Protocol April 2008

6.4. HMAC and SIGNATURE Calculation and Verification

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

6.4.1. HMAC Calculation

 The following process applies both to the HMAC and HMAC_2 parameters.
 When processing HMAC_2, the difference is that the HMAC calculation
 includes a pseudo HOST_ID field containing the Responder's
 information as sent in the R1 packet earlier.
 Both the Initiator and the Responder should take some care when
 verifying or calculating the HMAC_2.  Specifically, the Responder
 should preserve other parameters than the HOST_ID when sending the
 R2.  Also, the Initiator has to preserve the HOST_ID exactly as it
 was received in the R1 packet.
 The scope of the calculation for HMAC and HMAC_2 is:
 HMAC: { HIP header | [ Parameters ] }
 where Parameters include all HIP parameters of the packet that is
 being calculated with Type values from 1 to (HMAC's Type value - 1)
 and exclude parameters with Type values greater or equal to HMAC's
 Type value.
 During HMAC calculation, the following applies:
 o  In the HIP header, the Checksum field is set to zero.
 o  In the HIP header, the Header Length field value is calculated to
    the beginning of the HMAC parameter.
 Parameter order is described in Section 5.2.1.
 HMAC_2: { HIP header | [ Parameters ] | HOST_ID }
 where Parameters include all HIP parameters for the packet that is
 being calculated with Type values from 1 to (HMAC_2's Type value - 1)
 and exclude parameters with Type values greater or equal to HMAC_2's
 Type value.
 During HMAC_2 calculation, the following applies:
 o  In the HIP header, the Checksum field is set to zero.

Moskowitz, et al. Experimental [Page 70] RFC 5201 Host Identity Protocol April 2008

 o  In the HIP header, the Header Length field value is calculated to
    the beginning of the HMAC_2 parameter and added to the length of
    the concatenated HOST_ID parameter length.
 o  HOST_ID parameter is exactly in the form it was received in the R1
    packet from the Responder.
 Parameter order is described in Section 5.2.1, except that the
 HOST_ID parameter in this calculation is added to the end.
 The HMAC parameter is defined in Section 5.2.9 and the HMAC_2
 parameter in Section 5.2.10.  The HMAC calculation and verification
 process (the process applies both to HMAC and HMAC_2 except where
 HMAC_2 is mentioned separately) is as follows:
 Packet sender:
 1.  Create the HIP packet, without the HMAC, HIP_SIGNATURE,
     HIP_SIGNATURE_2, or any other parameter with greater Type value
     than the HMAC parameter has.
 2.  In case of HMAC_2 calculation, add a HOST_ID (Responder)
     parameter to the end of the packet.
 3.  Calculate the Header Length field in the HIP header including the
     added HOST_ID parameter in case of HMAC_2.
 4.  Compute the HMAC using either HIP-gl or HIP-lg integrity key
     retrieved from KEYMAT as defined in Section 6.5.
 5.  In case of HMAC_2, remove the HOST_ID parameter from the packet.
 6.  Add the HMAC parameter to the packet and any parameter with
     greater Type value than the HMAC's (HMAC_2's) that may follow,
     including possible HIP_SIGNATURE or HIP_SIGNATURE_2 parameters
 7.  Recalculate the Length field in the HIP header.
 Packet receiver:
 1.  Verify the HIP header Length field.
 2.  Remove the HMAC or HMAC_2 parameter, as well as all other
     parameters that follow it with greater Type value including
     possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
     contents if they will be needed later.

Moskowitz, et al. Experimental [Page 71] RFC 5201 Host Identity Protocol April 2008

 3.  In case of HMAC_2, build and add a HOST_ID parameter (with
     Responder information) to the packet.  The HOST_ID parameter
     should be identical to the one previously received from the
     Responder.
 4.  Recalculate the HIP packet length in the HIP header and clear the
     Checksum field (set it to all zeros).  In case of HMAC_2, the
     length is calculated with the added HOST_ID parameter.
 5.  Compute the HMAC using either HIP-gl or HIP-lg integrity key as
     defined in Section 6.5 and verify it against the received HMAC.
 6.  Set Checksum and Header Length field in the HIP header to
     original values.
 7.  In case of HMAC_2, remove the HOST_ID parameter from the packet
     before further processing.

6.4.2. Signature Calculation

 The following process applies both to the HIP_SIGNATURE and
 HIP_SIGNATURE_2 parameters.  When processing HIP_SIGNATURE_2, the
 only difference is that instead of HIP_SIGNATURE parameter, the
 HIP_SIGNATURE_2 parameter is used, and the Initiator's HIT and PUZZLE
 Opaque and Random #I fields are cleared (set to all zeros) before
 computing the signature.  The HIP_SIGNATURE parameter is defined in
 Section 5.2.11 and the HIP_SIGNATURE_2 parameter in Section 5.2.12.
 The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2
 is:
 HIP_SIGNATURE: { HIP header | [ Parameters ] }
 where Parameters include all HIP parameters for the packet that is
 being calculated with Type values from 1 to (HIP_SIGNATURE's Type
 value - 1).
 During signature calculation, the following apply:
 o  In the HIP header, the Checksum field is set to zero.
 o  In the HIP header, the Header Length field value is calculated to
    the beginning of the HIP_SIGNATURE parameter.
 Parameter order is described in Section 5.2.1.

Moskowitz, et al. Experimental [Page 72] RFC 5201 Host Identity Protocol April 2008

 HIP_SIGNATURE_2: { HIP header | [ Parameters ] }
 where Parameters include all HIP parameters for the packet that is
 being calculated with Type values from 1 to (HIP_SIGNATURE_2's Type
 value - 1).
 During signature calculation, the following apply:
 o  In the HIP header, the Initiator's HIT field and Checksum fields
    are set to zero.
 o  In the HIP header, the Header Length field value is calculated to
    the beginning of the HIP_SIGNATURE_2 parameter.
 o  PUZZLE parameter's Opaque and Random #I fields are set to zero.
 Parameter order is described in Section 5.2.1.
 Signature calculation and verification process (the process applies
 both to HIP_SIGNATURE and HIP_SIGNATURE_2 except in the case where
 HIP_SIGNATURE_2 is separately mentioned):
 Packet sender:
 1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
     parameters that follow the HIP_SIGNATURE parameter.
 2.  Calculate the Length field and zero the Checksum field in the HIP
     header.  In case of HIP_SIGNATURE_2, set Initiator's HIT field in
     the HIP header as well as PUZZLE parameter's Opaque and Random #I
     fields to zero.
 3.  Compute the signature using the private key corresponding to the
     Host Identifier (public key).
 4.  Add the HIP_SIGNATURE parameter to the packet.
 5.  Add any parameters that follow the HIP_SIGNATURE parameter.
 6.  Recalculate the Length field in the HIP header, and calculate the
     Checksum field.

Moskowitz, et al. Experimental [Page 73] RFC 5201 Host Identity Protocol April 2008

 Packet receiver:
 1.  Verify the HIP header Length field.
 2.  Save the contents of the HIP_SIGNATURE parameter and any
     parameters following the HIP_SIGNATURE parameter and remove them
     from the packet.
 3.  Recalculate the HIP packet Length in the HIP header and clear the
     Checksum field (set it to all zeros).  In case of
     HIP_SIGNATURE_2, set Initiator's HIT field in HIP header as well
     as PUZZLE parameter's Opaque and Random #I fields to zero.
 4.  Compute the signature and verify it against the received
     signature using the packet sender's Host Identifier (public key).
 5.  Restore the original packet by adding removed parameters (in step
     2) and resetting the values that were set to zero (in step 3).
 The verification can use either the HI received from a HIP packet,
 the HI from a DNS query, if the FQDN has been received in the HOST_ID
 packet, or one received by some other means.

6.5. HIP KEYMAT Generation

 HIP keying material is derived from the Diffie-Hellman session key,
 Kij, produced during the HIP base exchange (Section 4.1.3).  The
 Initiator has Kij during the creation of the I2 packet, and the
 Responder has Kij once it receives the I2 packet.  This is why I2 can
 already contain encrypted information.
 The KEYMAT is derived by feeding Kij and the HITs into the following
 operation; the | operation denotes concatenation.
  KEYMAT = K1 | K2 | K3 | ...
        where
  K1   = RHASH( Kij | sort(HIT-I | HIT-R) | I | J | 0x01 )
  K2   = RHASH( Kij | K1 | 0x02 )
  K3   = RHASH( Kij | K2 | 0x03 )
  ...
  K255 = RHASH( Kij | K254 | 0xff )
  K256 = RHASH( Kij | K255 | 0x00 )
  etc.

Moskowitz, et al. Experimental [Page 74] RFC 5201 Host Identity Protocol April 2008

 Sort(HIT-I | HIT-R) is defined as the network byte order
 concatenation of the two HITs, with the smaller HIT preceding the
 larger HIT, resulting from the numeric comparison of the two HITs
 interpreted as positive (unsigned) 128-bit integers in network byte
 order.
 I and J values are from the puzzle and its solution that were
 exchanged in R1 and I2 messages when this HIP association was set up.
 Both hosts have to store I and J values for the HIP association for
 future use.
 The initial keys are drawn sequentially in the order that is
 determined by the numeric comparison of the two HITs, with comparison
 method described in the previous paragraph.  HOST_g denotes the host
 with the greater HIT value, and HOST_l the host with the lower HIT
 value.
 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
 SHA-1  160 bits
 NULL  0 bits
 If other key sizes are used, they must be treated as different
 encryption algorithms and defined separately.

6.6. Initiation of a HIP Exchange

 An implementation may originate a HIP exchange to another host based
 on a local policy decision, usually triggered by an application
 datagram, in much the same way that an IPsec IKE key exchange can

Moskowitz, et al. Experimental [Page 75] RFC 5201 Host Identity Protocol April 2008

 dynamically create a Security Association.  Alternatively, a system
 may initiate a HIP exchange if it has rebooted or timed out, or
 otherwise lost its HIP state, as described in Section 4.5.4.
 The implementation prepares an I1 packet and sends it to the IP
 address that corresponds to the peer host.  The IP address of the
 peer host may be obtained via conventional mechanisms, such as DNS
 lookup.  The I1 contents are specified in Section 5.3.1.  The
 selection of which Host Identity to use, if a host has more than one
 to choose from, is typically a policy decision.
 The following steps define the conceptual processing rules for
 initiating a HIP exchange:
 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
     using NULL (all zeros) as the Responder's HIT.  See also "HIP
     Opportunistic Mode" (Section 4.1.6).
 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.

6.6.1. Sending Multiple I1s in Parallel

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

Moskowitz, et al. Experimental [Page 76] RFC 5201 Host Identity Protocol April 2008

 As the Responder is not guaranteed to distinguish the duplicate I1s
 it receives at several of its addresses (because it avoids storing
 states when it answers back an R1), the Initiator may receive several
 duplicate R1s.
 The Initiator SHOULD then select the initial preferred destination
 address using the source address of the selected received R1, and use
 the preferred address as a source address for the I2.  Processing
 rules for received R1s are discussed in Section 6.8.

6.6.2. Processing Incoming ICMP Protocol Unreachable Messages

 A host may receive an ICMP 'Destination Protocol Unreachable' message
 as a response to sending a HIP I1 packet.  Such a packet may be an
 indication that the peer does not support HIP, or it may be an
 attempt to launch an attack by making the Initiator believe that the
 Responder does not support HIP.
 When a system receives an ICMP 'Destination Protocol Unreachable'
 message while it is waiting for an R1, it MUST NOT terminate the
 wait.  It MAY continue as if it had not received the ICMP message,
 and send a few more I1s.  Alternatively, it MAY take the ICMP message
 as a hint that the peer most probably does not support HIP, and
 return to state UNASSOCIATED earlier than otherwise.  However, at
 minimum, it MUST continue waiting for an R1 for a reasonable time
 before returning to UNASSOCIATED.

6.7. Processing Incoming I1 Packets

 An implementation SHOULD reply to an I1 with an R1 packet, unless the
 implementation is unable or unwilling to set up a HIP association.
 If the implementation is unable to set up a HIP association, the host
 SHOULD send an ICMP Destination Protocol Unreachable,
 Administratively Prohibited, message to the I1 source address.  If
 the implementation is unwilling to set up a HIP association, the host
 MAY ignore the I1.  This latter case may occur during a DoS attack
 such as an I1 flood.
 The implementation MUST be able to handle a storm of received I1
 packets, discarding those with common content that arrive within a
 small time delta.
 A spoofed I1 can result in an R1 attack on a system.  An R1 sender
 MUST have a mechanism to rate-limit R1s to an address.
 It is RECOMMENDED that the HIP state machine does not transition upon
 sending an R1.

Moskowitz, et al. Experimental [Page 77] RFC 5201 Host Identity Protocol April 2008

 The following steps define the conceptual processing rules for
 responding to an I1 packet:
 1.  The Responder MUST check that the Responder's HIT in the received
     I1 is either one of its own HITs or NULL.
 2.  If the Responder is in ESTABLISHED state, the Responder MAY
     respond to this with an R1 packet, prepare to drop existing SAs,
     and stay at ESTABLISHED state.
 3.  If the Responder is in I1-SENT state, it must make a comparison
     between the sender's HIT and its own (i.e., the receiver's) HIT.
     If the sender's HIT is greater than its own HIT, it should drop
     the I1 and stay at I1-SENT.  If the sender's HIT is smaller than
     its own HIT, it should send R1 and stay at I1-SENT.  The HIT
     comparison goes similarly as in Section 6.5.
 4.  If the implementation chooses to respond to the I1 with an R1
     packet, it creates a new R1 or selects a precomputed R1 according
     to the format described in Section 5.3.2.
 5.  The R1 MUST contain the received Responder's HIT, unless the
     received HIT is NULL, in which case the Responder SHOULD select a
     HIT that is constructed with the MUST algorithm in Section 3,
     which is currently RSA.  Other than that, selecting the HIT is a
     local policy matter.
 6.  The Responder sends the R1 to the source IP address of the I1
     packet.

6.7.1. R1 Management

 All compliant implementations MUST produce R1 packets.  An R1 packet
 MAY be precomputed.  An R1 packet MAY be reused for time Delta T,
 which is implementation dependent, and SHOULD be deprecated and not
 used once a valid response I2 packet has been received from an
 Initiator.  During an I1 message storm, an R1 packet may be re-used
 beyond this limit.  R1 information MUST NOT be discarded until Delta
 S after T.  Time S is the delay needed for the last I2 to arrive back
 to the Responder.
 An implementation MAY keep state about received I1s and match the
 received I2s against the state, as discussed in Section 4.1.1.

Moskowitz, et al. Experimental [Page 78] RFC 5201 Host Identity Protocol April 2008

6.7.2. Handling Malformed Messages

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

6.8. Processing Incoming R1 Packets

 A system receiving an R1 MUST first check to see if it has sent an I1
 to the originator of the R1 (i.e., it is in state I1-SENT).  If so,
 it SHOULD process the R1 as described below, send an I2, and go to
 state I2-SENT, setting a timer to protect the I2.  If the system is
 in state I2-SENT, it MAY respond to an R1 if the R1 has a larger R1
 generation counter; if so, it should drop its state due to processing
 the previous R1 and start over from state I1-SENT.  If the system is
 in any other state with respect to that host, it SHOULD silently drop
 the R1.
 When sending multiple I1s, an Initiator SHOULD wait for a small
 amount of time after the first R1 reception to allow possibly
 multiple R1s to arrive, and it SHOULD respond to an R1 among the set
 with the largest R1 generation counter.
 The following steps define the conceptual processing rules for
 responding to an R1 packet:
 1.   A system receiving an R1 MUST first check to see if it has sent
      an I1 to the originator of the R1 (i.e., it has a HIP
      association that is in state I1-SENT and that is associated with
      the HITs in the R1).  Unless the I1 was sent in opportunistic
      mode (see Section 4.1.6), the IP addresses in the received R1
      packet SHOULD be ignored and, when looking up the right HIP
      association, the received R1 SHOULD be matched against the
      associations using only the HITs.  If a match exists, the system
      should process the R1 as described below.
 2.   Otherwise, if the system is in any other state than I1-SENT or
      I2-SENT with respect to the HITs included in the R1, it SHOULD
      silently drop the R1 and remain in the current state.
 3.   If the HIP association state is I1-SENT or I2-SENT, the received
      Initiator's HIT MUST correspond to the HIT used in the original,
      and the I1 and the Responder's HIT MUST correspond to the one
      used, unless the I1 contained a NULL HIT.
 4.   The system SHOULD validate the R1 signature before applying
      further packet processing, according to Section 5.2.12.

Moskowitz, et al. Experimental [Page 79] RFC 5201 Host Identity Protocol April 2008

 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.
 7.   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.
 8.   The system SHOULD attempt to validate the HIT against the
      received Host Identity by using the received Host Identity to
      construct a HIT and verify that it matches the Sender's HIT.
 9.   The system MUST store the received R1 generation counter for
      future reference.
 10.  The system attempts to solve the puzzle in R1.  The system MUST
      terminate the search after exceeding the remaining lifetime of
      the puzzle.  If the puzzle is not successfully solved, the
      implementation may either resend I1 within the retry bounds or
      abandon the HIP exchange.
 11.  The system computes standard Diffie-Hellman keying material
      according to the public value and Group ID provided in the
      DIFFIE_HELLMAN parameter.  The Diffie-Hellman keying material
      Kij is used for key extraction as specified in Section 6.5.  If
      the received Diffie-Hellman Group ID is not supported, the
      implementation may either resend I1 within the retry bounds or
      abandon the HIP exchange.
 12.  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.
 13.  The system initializes the remaining variables in the associated
      state, including Update ID counters.
 14.  The system prepares and sends an I2, as described in
      Section 5.3.3.

Moskowitz, et al. Experimental [Page 80] RFC 5201 Host Identity Protocol April 2008

 15.  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.
 16.  If the system is in state I1-SENT, it shall transition to state
      I2-SENT.  If the system is in any other state, it remains in the
      current state.

6.8.1. Handling Malformed Messages

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

6.9. Processing Incoming I2 Packets

 Upon receipt of an I2, the system MAY perform initial checks to
 determine whether the I2 corresponds to a recent R1 that has been
 sent out, if the Responder keeps such state.  For example, the sender
 could check whether the I2 is from an address or HIT that has
 recently received an R1 from it.  The R1 may have had Opaque data
 included that was echoed back in the I2.  If the I2 is considered to
 be suspect, it MAY be silently discarded by the system.
 Otherwise, the HIP implementation SHOULD process the I2.  This
 includes validation of the puzzle solution, generating the Diffie-
 Hellman key, decrypting the Initiator's Host Identity, verifying the
 signature, creating state, and finally sending an R2.
 The following steps define the conceptual processing rules for
 responding to an I2 packet:
 1.   The system MAY perform checks to verify that the I2 corresponds
      to a recently sent R1.  Such checks are implementation
      dependent.  See Appendix A for a description of an example
      implementation.
 2.   The system MUST check that the Responder's HIT corresponds to
      one of its own HITs.

Moskowitz, et al. Experimental [Page 81] RFC 5201 Host Identity Protocol April 2008

 3.   If the system's state machine is in the R2-SENT state, the
      system MAY check if the newly received I2 is similar to the one
      that triggered moving to R2-SENT.  If so, it MAY retransmit a
      previously sent R2, reset the R2-SENT timer, and the state
      machine stays in R2-SENT.
 4.   If the system's state machine is in the I2-SENT state, the
      system makes a comparison between its local and sender's HITs
      (similarly as in Section 6.5).  If the local HIT is smaller than
      the sender's HIT, it should drop the I2 packet, use the peer
      Diffie-Hellman key and nonce I from the R1 packet received
      earlier, and get the local Diffie-Hellman key and nonce J from
      the I2 packet sent to the peer earlier.  Otherwise, the system
      should process the received I2 packet and drop any previously
      derived Diffie-Hellman keying material Kij it might have formed
      upon sending the I2 previously.  The peer Diffie-Hellman key and
      the nonce J are taken from the just arrived I2 packet.  The
      local Diffie-Hellman key and the nonce I are the ones that were
      earlier sent in the R1 packet.
 5.   If the system's state machine is in the I1-SENT state, and the
      HITs in the I2 match those used in the previously sent I1, the
      system uses this received I2 as the basis for the HIP
      association it was trying to form, and stops retransmitting I1
      (provided that the I2 passes the below additional checks).
 6.   If the system's state machine is in any other state than R2-
      SENT, the system SHOULD check that the echoed R1 generation
      counter in I2 is within the acceptable range.  Implementations
      MUST accept puzzles from the current generation and MAY accept
      puzzles from earlier generations.  If the newly received I2 is
      outside the accepted range, the I2 is stale (perhaps replayed)
      and SHOULD be dropped.
 7.   The system MUST validate the solution to the puzzle by computing
      the hash described in Section 5.3.3 using the same RHASH
      algorithm.
 8.   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.
 9.   The system must derive Diffie-Hellman keying material Kij based
      on the public value and Group ID in the DIFFIE_HELLMAN
      parameter.  This key is used to derive the HIP association keys,
      as described in Section 6.5.  If the Diffie-Hellman Group ID is
      unsupported, the I2 packet is silently dropped.

Moskowitz, et al. Experimental [Page 82] RFC 5201 Host Identity Protocol April 2008

 10.  The encrypted HOST_ID is decrypted by the Initiator encryption
      key defined in Section 6.5.  If the decrypted data is not a
      HOST_ID parameter, the I2 packet is silently dropped.
 11.  The implementation SHOULD also verify that the Initiator's HIT
      in the I2 corresponds to the Host Identity sent in the I2.
      (Note: some middleboxes may not able to make this verification.)
 12.  The system MUST verify the HMAC according to the procedures in
      Section 5.2.9.
 13.  The system MUST verify the HIP_SIGNATURE according to
      Section 5.2.11 and Section 5.3.3.
 14.  If the checks above are valid, then the system proceeds with
      further I2 processing; otherwise, it discards the I2 and its
      state machine remains in the same state.
 15.  The I2 packet may have the A bit set -- in this case, the system
      MAY choose to refuse it by dropping the I2 and the state machine
      returns to state UNASSOCIATED.  If the A bit is set, the
      Initiator's HIT is anonymous and should not be stored.
 16.  The system initializes the remaining variables in the associated
      state, including Update ID counters.
 17.  Upon successful processing of an I2 when the system's state
      machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-SENT,
      an R2 is sent and the system's state machine transitions to
      state R2-SENT.
 18.  Upon successful processing of an I2 when the system's state
      machine is in state ESTABLISHED, the old HIP association is
      dropped and a new one is installed, an R2 is sent, and the
      system's state machine transitions to R2-SENT.
 19.  Upon the system's state machine transitioning to R2-SENT, the
      system starts a timer.  The state machine transitions to
      ESTABLISHED if some data has been received on the incoming HIP
      association, or an UPDATE packet has been received (or some
      other packet that indicates that the peer system's state machine
      has moved to ESTABLISHED).  If the timer expires (allowing for
      maximal retransmissions of I2s), the state machine transitions
      to ESTABLISHED.

Moskowitz, et al. Experimental [Page 83] RFC 5201 Host Identity Protocol April 2008

6.9.1. Handling Malformed Messages

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

6.10. Processing Incoming R2 Packets

 An R2 received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
 results in the R2 being dropped and the state machine staying in the
 same state.  If an R2 is received in state I2-SENT, it SHOULD be
 processed.
 The following steps define the conceptual processing rules for an
 incoming R2 packet:
 1.  The system MUST verify that the HITs in use correspond to the
     HITs that were received in the R1.
 2.  The system MUST verify the HMAC_2 according to the procedures in
     Section 5.2.10.
 3.  The system MUST verify the HIP signature according to the
     procedures in Section 5.2.11.
 4.  If any of the checks above fail, there is a high probability of
     an ongoing man-in-the-middle or other security attack.  The
     system SHOULD act accordingly, based on its local policy.
 5.  If the system is in any other state than I2-SENT, the R2 is
     silently dropped.
 6.  Upon successful processing of the R2, the state machine moves to
     state ESTABLISHED.

6.11. Sending UPDATE Packets

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

Moskowitz, et al. Experimental [Page 84] RFC 5201 Host Identity Protocol April 2008

 The system first determines whether there are any outstanding UPDATE
 messages that may conflict with the new UPDATE message under
 consideration.  When multiple UPDATEs are outstanding (not yet
 acknowledged), the sender must assume that such UPDATEs may be
 processed in an arbitrary order.  Therefore, any new UPDATEs that
 depend on a previous outstanding UPDATE being successfully received
 and acknowledged MUST be postponed until reception of the necessary
 ACK(s) occurs.  One way to prevent any conflicts is to only allow one
 outstanding UPDATE at a time.  However, allowing multiple UPDATEs may
 improve the performance of mobility and multihoming protocols.
 The following steps define the conceptual processing rules for
 sending UPDATE packets.
 1.  The first UPDATE packet is sent with Update ID of zero.
     Otherwise, the system increments its own Update ID value by one
     before continuing the below steps.
 2.  The system creates an UPDATE packet that contains a SEQ parameter
     with the current value of Update ID.  The UPDATE packet may also
     include an ACK of the peer's Update ID found in a received UPDATE
     SEQ parameter, if any.
 3.  The system sends the created UPDATE packet and starts an UPDATE
     timer.  The default value for the timer is 2 * RTT estimate.  If
     multiple UPDATEs are outstanding, multiple timers are in effect.
 4.  If the UPDATE timer expires, the UPDATE is resent.  The UPDATE
     can be resent UPDATE_RETRY_MAX times.  The UPDATE timer SHOULD be
     exponentially backed off for subsequent retransmissions.  If no
     acknowledgment is received from the peer after UPDATE_RETRY_MAX
     times, the HIP association is considered to be broken and the
     state machine should move from state ESTABLISHED to state CLOSING
     as depicted in Section 4.4.3.  The UPDATE timer is cancelled upon
     receiving an ACK from the peer that acknowledges receipt of the
     UPDATE.

6.12. Receiving UPDATE Packets

 When a system receives an UPDATE packet, its processing depends on
 the state of the HIP association and the presence and values of the
 SEQ and ACK parameters.  Typically, an UPDATE message also carries
 optional parameters whose handling is defined in separate documents.
 For each association, the peer's next expected in-sequence Update ID
 ("peer Update ID") is stored.  Initially, this value is zero.  Update
 ID comparisons of "less than" and "greater than" are performed with
 respect to a circular sequence number space.

Moskowitz, et al. Experimental [Page 85] RFC 5201 Host Identity Protocol April 2008

 The sender may send multiple outstanding UPDATE messages.  These
 messages are processed in the order in which they are received at the
 receiver (i.e., no resequencing is performed).  When processing
 UPDATEs out-of-order, the receiver MUST keep track of which UPDATEs
 were previously processed, so that duplicates or retransmissions are
 ACKed and not reprocessed.  A receiver MAY choose to define a receive
 window of Update IDs that it is willing to process at any given time,
 and discard received UPDATEs falling outside of that window.
 The following steps define the conceptual processing rules for
 receiving UPDATE packets.
 1.  If there is no corresponding HIP association, the implementation
     MAY reply with an ICMP Parameter Problem, as specified in
     Section 5.4.4.
 2.  If the association is in the ESTABLISHED state and the SEQ (but
     not ACK) parameter is present, the UPDATE is processed and
     replied to as described in Section 6.12.1.
 3.  If the association is in the ESTABLISHED state and the ACK (but
     not SEQ) parameter is present, the UPDATE is processed as
     described in Section 6.12.2.
 4.  If the association is in the ESTABLISHED state and there is both
     an ACK and SEQ in the UPDATE, the ACK is first processed as
     described in Section 6.12.2, and then the rest of the UPDATE is
     processed as described in Section 6.12.1.

6.12.1. Handling a SEQ Parameter in a Received UPDATE Message

 The following steps define the conceptual processing rules for
 handling a SEQ parameter in a received UPDATE packet.
 1.  If the Update ID in the received SEQ is not the next in the
     sequence of Update IDs and is greater than the receiver's window
     for new UPDATEs, the packet MUST be dropped.
 2.  If the Update ID in the received SEQ corresponds to an UPDATE
     that has recently been processed, the packet is treated as a
     retransmission.  The HMAC verification (next step) MUST NOT be
     skipped.  (A byte-by-byte comparison of the received and a stored
     packet would be OK, though.)  It is recommended that a host cache
     UPDATE packets sent with ACKs to avoid the cost of generating a
     new ACK packet to respond to a replayed UPDATE.  The system MUST
     acknowledge, again, such (apparent) UPDATE message
     retransmissions but SHOULD also consider rate-limiting such
     retransmission responses to guard against replay attacks.

Moskowitz, et al. Experimental [Page 86] RFC 5201 Host Identity Protocol April 2008

 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 parameters in the
     UPDATE are then processed.  The system MUST record the Update ID
     in the received SEQ parameter, for replay protection.
 6.  An UPDATE acknowledgment packet with ACK parameter is prepared
     and sent to the peer.  This ACK parameter may be included in a
     separate UPDATE or piggybacked in an UPDATE with SEQ parameter,
     as described in Section 5.3.5.  The ACK parameter MAY acknowledge
     more than one of the peer's Update IDs.

6.12.2. Handling an ACK Parameter in a Received UPDATE Packet

 The following steps define the conceptual processing rules for
 handling an ACK parameter in a received UPDATE packet.
 1.  The sequence number reported in the ACK must match with an
     earlier sent UPDATE packet that has not already been
     acknowledged.  If no match is found or if the ACK does not
     acknowledge a new UPDATE, the packet MUST either be dropped if no
     SEQ parameter is present, or the processing steps in
     Section 6.12.1 are followed.
 2.  The system MUST verify the 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 6.11) so
     that the now acknowledged UPDATE is no longer retransmitted.  If
     multiple UPDATEs are newly acknowledged, multiple timers are
     stopped.

6.13. Processing NOTIFY Packets

 Processing NOTIFY packets is OPTIONAL.  If processed, any errors in a
 received NOTIFICATION parameter SHOULD be logged.  Received errors
 MUST be considered only as informational, and the receiver SHOULD NOT
 change its HIP state (Section 4.4.1) purely based on the received
 NOTIFY message.

Moskowitz, et al. Experimental [Page 87] RFC 5201 Host Identity Protocol April 2008

6.14. Processing CLOSE Packets

 When the host receives a CLOSE message, it responds with a CLOSE_ACK
 message and moves to CLOSED state.  (The authenticity of the CLOSE
 message is verified using both HMAC and SIGNATURE).  This processing
 applies whether or not the HIP association state is CLOSING in order
 to handle CLOSE messages from both ends that cross in flight.
 The HIP association is not discarded before the host moves from the
 UNASSOCIATED state.
 Once the closing process has started, any need to send data packets
 will trigger creating and establishing of a new HIP association,
 starting with sending an I1.
 If there is no corresponding HIP association, the CLOSE packet is
 dropped.

6.15. Processing CLOSE_ACK Packets

 When a host receives a CLOSE_ACK message, it verifies that it is in
 CLOSING or CLOSED state and that the CLOSE_ACK was in response to the
 CLOSE (using the included ECHO_RESPONSE_SIGNED in response to the
 sent ECHO_REQUEST_SIGNED).
 The CLOSE_ACK uses HMAC and SIGNATURE for verification.  The state is
 discarded when the state changes to UNASSOCIATED and, after that, the
 host MAY respond with an ICMP Parameter Problem to an incoming CLOSE
 message (see Section 5.4.4).

6.16. Handling State Loss

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

Moskowitz, et al. Experimental [Page 88] RFC 5201 Host Identity Protocol April 2008

7. HIP Policies

 There are a number of variables that will influence the HIP exchanges
 that each host must support.  All HIP implementations MUST support
 more than one simultaneous HI, at least one of which SHOULD be
 reserved for anonymous usage.  Although anonymous HIs will be rarely
 used as Responders' HIs, they will be common for Initiators.  Support
 for more than two HIs is RECOMMENDED.
 Many Initiators would want to use a different HI for different
 Responders.  The implementations SHOULD provide for an ACL of
 Initiator's HIT to Responder's HIT.  This ACL SHOULD also include
 preferred transform and local lifetimes.
 The value of K used in the HIP R1 packet can also vary by policy.  K
 should never be greater than 20, but for trusted partners it could be
 as low as 0.
 Responders would need a similar ACL, representing which hosts they
 accept HIP exchanges, and the preferred transform and local
 lifetimes.  Wildcarding SHOULD be supported for this ACL also.

8. Security Considerations

 HIP is designed to provide secure authentication of hosts.  HIP also
 attempts to limit the exposure of the host to various denial-of-
 service and man-in-the-middle (MitM) attacks.  In so doing, HIP
 itself is subject to its own DoS and MitM attacks that potentially
 could be more damaging to a host's ability to conduct business as
 usual.
 The 384-bit Diffie-Hellman Group is targeted to be used in hosts that
 either do not require or are not powerful enough for handling strong
 cryptography.  Although there is a risk that with suitable equipment
 the encryption can be broken in real time, the 384-bit group can
 provide some protection for end-hosts that are not able to handle any
 stronger cryptography.  When the security provided by the 384-bit
 group is not enough for applications on a host, the support for this
 group should be turned off in the configuration.
 Denial-of-service attacks often take advantage of the cost of start
 of state for a protocol on the Responder compared to the 'cheapness'
 on the Initiator.  HIP makes no attempt to increase the cost of the
 start of state on the Initiator, but makes an effort to reduce the
 cost to the Responder.  This is done by having the Responder start
 the 3-way exchange instead of the Initiator, making the HIP protocol
 4 packets long.  In doing this, packet 2 becomes a 'stock' packet
 that the Responder MAY use many times, until some Initiator has

Moskowitz, et al. Experimental [Page 89] RFC 5201 Host Identity Protocol April 2008

 provided a valid response to such an R1 packet.  During an I1 storm,
 the host may reuse the same D-H value also even if some Initiator has
 provided a valid response using that particular D-H value.  However,
 such behavior is discouraged and should be avoided.  Using the same
 Diffie-Hellman values and random puzzle #I value has some risks.
 This risk needs to be balanced against a potential storm of HIP I1
 packets.
 This shifting of the start of state cost to the Initiator in creating
 the I2 HIP packet, presents another DoS attack.  The attacker spoofs
 the I1 HIP packet and the Responder sends out the R1 HIP packet.
 This could conceivably tie up the 'Initiator' with evaluating the R1
 HIP packet, and creating the I2 HIP packet.  The defense against this
 attack is to simply ignore any R1 packet where a corresponding I1 was
 not sent.
 A second form of DoS attack arrives in the I2 HIP packet.  Once the
 attacking Initiator has solved the puzzle, it can send packets with
 spoofed IP source addresses with either an invalid encrypted HIP
 payload component or a bad HIP signature.  This would take resources
 in the Responder's part to reach the point to discover that the I2
 packet cannot be completely processed.  The defense against this
 attack is after N bad I2 packets, the Responder would discard any I2s
 that contain the given Initiator HIT.  This will shut down the
 attack.  The attacker would have to request another R1 and use that
 to launch a new attack.  The Responder could up the value of K while
 under attack.  On the downside, valid I2s might get dropped too.
 A third form of DoS attack is emulating the restart of state after a
 reboot of one of the partners.  A restarting host would send an I1 to
 a peer, which would respond with an R1 even if it were in the
 ESTABLISHED state.  If the I1 were spoofed, the resulting R1 would be
 received unexpectedly by the spoofed host and would be dropped, as in
 the first case above.
 A fourth form of DoS attack is emulating the end of state.  HIP
 relies on timers plus a CLOSE/CLOSE_ACK handshake to explicitly
 signal the end of a HIP association.  Because both CLOSE and
 CLOSE_ACK messages contain an HMAC, an outsider cannot close a
 connection.  The presence of an additional SIGNATURE allows
 middleboxes to inspect these messages and discard the associated
 state (for e.g., firewalling, SPI-based NATing, etc.).  However, the
 optional behavior of replying to CLOSE with an ICMP Parameter Problem
 packet (as described in Section 5.4.4) might allow an IP spoofer
 sending CLOSE messages to launch reflection attacks.

Moskowitz, et al. Experimental [Page 90] RFC 5201 Host Identity Protocol April 2008

 A fifth form of DoS attack is replaying R1s to cause the Initiator to
 solve stale puzzles and become out of synchronization with the
 Responder.  The R1 generation counter is a monotonically increasing
 counter designed to protect against this attack, as described in
 Section 4.1.4.
 Man-in-the-middle attacks are difficult to defend against, without
 third-party authentication.  A skillful MitM could easily handle all
 parts of HIP, but HIP indirectly provides the following protection
 from a MitM attack.  If the Responder's HI is retrieved from a signed
 DNS zone, a certificate, or through some other secure means, the
 Initiator can use this to validate the R1 HIP packet.
 Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
 certificate, or otherwise securely available, the Responder can
 retrieve the HI (after having got the I2 HIP packet) and verify that
 the HI indeed can be trusted.  However, since an Initiator may choose
 to use an anonymous HI, it knowingly risks a MitM attack.  The
 Responder may choose not to accept a HIP exchange with an anonymous
 Initiator.
 The HIP Opportunistic Mode concept has been introduced in this
 document, but this document does not specify what the semantics of
 such a connection setup are for applications.  There are certain
 concerns with opportunistic mode, as discussed in Section 4.1.6.
 NOTIFY messages are used only for informational purposes and they are
 unacknowledged.  A HIP implementation cannot rely solely on the
 information received in a NOTIFY message because the packet may have
 been replayed.  It SHOULD NOT change any state information based
 purely on a received NOTIFY message.
 Since not all hosts will ever support HIP, ICMP 'Destination Protocol
 Unreachable' messages are to be expected and present a DoS attack.
 Against an Initiator, the attack would look like the Responder does
 not support HIP, but shortly after receiving the ICMP message, the
 Initiator would receive a valid R1 HIP packet.  Thus, to protect from
 this attack, an Initiator should not react to an ICMP message until a
 reasonable delta time to get the real Responder's R1 HIP packet.  A
 similar attack against the Responder is more involved.  Normally, if
 an I1 message received by a Responder was a bogus one sent by an
 attacker, the Responder may receive an ICMP message from the IP
 address the R1 message was sent to.  However, a sophisticated
 attacker can try to take advantage of such a behavior and try to
 break up the HIP exchange by sending such an ICMP message to the
 Responder before the Initiator has a chance to send a valid I2
 message.  Hence, the Responder SHOULD NOT act on such an ICMP
 message.  Especially, it SHOULD NOT remove any minimal state created

Moskowitz, et al. Experimental [Page 91] RFC 5201 Host Identity Protocol April 2008

 when it sent the R1 HIP packet (if it did create one), but wait for
 either a valid I2 HIP packet or the natural timeout (that is, if R1
 packets are tracked at all).  Likewise, the Initiator should ignore
 any ICMP message while waiting for an R2 HIP packet, and should
 delete any pending state only after a natural timeout.

9. IANA Considerations

 IANA has reserved protocol number 139 for the Host Identity Protocol.
 This document defines a new 128-bit value under the CGA Message Type
 namespace [RFC3972], 0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA, to be
 used for HIT generation as specified in ORCHID [RFC4843].
 This document also creates a set of new namespaces.  These are
 described below.
 Packet Type
    The 7-bit Packet Type field in a HIP protocol packet describes the
    type of a HIP protocol message.  It is defined in Section 5.1.
    The current values are defined in Sections 5.3.1 through 5.3.8.
    New values are assigned through IETF Consensus [RFC2434].
 HIP Version
    The four-bit Version field in a HIP protocol packet describes the
    version of the HIP protocol.  It is defined in Section 5.1.  The
    only currently defined value is 1.  New values are assigned
    through IETF Consensus.
 Parameter Type
    The 16-bit Type field in a HIP parameter describes the type of the
    parameter.  It is defined in Section 5.2.1.  The current values
    are defined in Sections 5.2.3 through 5.2.20.
    With the exception of the assigned Type codes, the Type codes 0
    through 1023 and 61440 through 65535 are reserved for future base
    protocol extensions, and are assigned through IETF Consensus.
    The Type codes 32768 through 49141 are reserved for
    experimentation.  Types SHOULD be selected in a random fashion
    from this range, thereby reducing the probability of collisions.
    A method employing genuine randomness (such as flipping a coin)
    SHOULD be used.

Moskowitz, et al. Experimental [Page 92] RFC 5201 Host Identity Protocol April 2008

    All other Type codes are assigned through First Come First Served,
    with Specification Required [RFC2434].
 Group ID
    The eight-bit Group ID values appear in the DIFFIE_HELLMAN
    parameter and are defined in Section 5.2.6.  New values either
    from the reserved or unassigned space are assigned through IETF
    Consensus.
 Suite ID
    The 16-bit Suite ID values in a HIP_TRANSFORM parameter are
    defined in Section 5.2.7.  New values either from the reserved or
    unassigned space are assigned through IETF Consensus.
 DI-Type
    The four-bit DI-Type values in a HOST_ID parameter are defined in
    Section 5.2.8.  New values are assigned through IETF Consensus.
 Notify Message Type
    The 16-bit Notify Message Type values in a NOTIFICATION parameter
    are defined in Section 5.2.16.
    Notify Message Type values 1-10 are used for informing about
    errors in packet structures, values 11-20 for informing about
    problems in parameters containing cryptographic related material,
    values 21-30 for informing about problems in authentication or
    packet integrity verification.  Parameter numbers above 30 can be
    used for informing about other types of errors or events.  Values
    51-8191 are error types reserved to be allocated by IANA.  Values
    8192-16383 are error types for experimentation.  Values 16385-
    40959 are status types to be allocated by IANA, and values 40960-
    65535 are status types for experimentation.  New values in ranges
    51-8191 and 16385-40959 are assigned through First Come First
    Served, with Specification Required.

10. Acknowledgments

 The drive to create HIP came to being after attending the MALLOC
 meeting at the 43rd IETF meeting.  Baiju Patel and Hilarie Orman
 really gave the original author, Bob Moskowitz, the assist to get HIP
 beyond 5 paragraphs of ideas.  It has matured considerably since the
 early versions thanks to extensive input from IETFers.  Most
 importantly, its design goals are articulated and are different from
 other efforts in this direction.  Particular mention goes to the

Moskowitz, et al. Experimental [Page 93] RFC 5201 Host Identity Protocol April 2008

 members of the NameSpace Research Group of the IRTF.  Noel Chiappa
 provided valuable input at early stages of discussions about
 identifier handling and Keith Moore the impetus to provide
 resolvability.  Steve Deering provided encouragement to keep working,
 as a solid proposal can act as a proof of ideas for a research group.
 Many others contributed; extensive security tips were provided by
 Steve Bellovin.  Rob Austein kept the DNS parts on track.  Paul
 Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
 for the Initiator to respond, but easy for the Responder to validate.
 Bill Sommerfeld supplied the Birthday concept, which later evolved
 into the R1 generation counter, to simplify reboot management.  Erik
 Nordmark supplied the CLOSE-mechanism for closing connections.
 Rodney Thayer and Hugh Daniels provided extensive feedback.  In the
 early times of this document, John Gilmore kept Bob Moskowitz
 challenged to provide something of value.
 During the later stages of this document, when the editing baton was
 transferred to Pekka Nikander, the input from the early implementors
 was invaluable.  Without having actual implementations, this document
 would not be on the level it is now.
 In the usual IETF fashion, a large number of people have contributed
 to the actual text or ideas.  The list of these people include Jeff
 Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Andrew
 McGregor, Julien Laganier, Miika Komu, Mika Kousa, Jan Melen, Henrik
 Petander, Michael Richardson, Tim Shepard, Jorma Wall, and Jukka
 Ylitalo.  Our apologies to anyone whose name is missing.
 Once the HIP Working Group was founded in early 2004, a number of
 changes were introduced through the working group process.  Most
 notably, the original document was split in two, one containing the
 base exchange and the other one defining how to use ESP.  Some
 modifications to the protocol proposed by Aura, et al., [AUR03] were
 added at a later stage.

Moskowitz, et al. Experimental [Page 94] RFC 5201 Host Identity Protocol April 2008

11. References

11.1. Normative References

 [FIPS95]       NIST, "FIPS PUB 180-1: Secure Hash Standard",
                April 1995.
 [RFC0768]      Postel, J., "User Datagram Protocol", STD 6, RFC 768,
                August 1980.
 [RFC1035]      Mockapetris, P., "Domain names - implementation and
                specification", STD 13, RFC 1035, November 1987.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2404]      Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
                within ESP and AH", RFC 2404, November 1998.
 [RFC2451]      Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
                Algorithms", RFC 2451, November 1998.
 [RFC2460]      Deering, S. and R. Hinden, "Internet Protocol, Version
                6 (IPv6) Specification", RFC 2460, December 1998.
 [RFC2463]      Conta, A. and S. Deering, "Internet Control Message
                Protocol (ICMPv6) for the Internet Protocol Version 6
                (IPv6) Specification", RFC 2463, December 1998.
 [RFC2536]      Eastlake, D., "DSA KEYs and SIGs in the Domain Name
                System (DNS)", RFC 2536, March 1999.
 [RFC2898]      Kaliski, B., "PKCS #5: Password-Based Cryptography
                Specification Version 2.0", RFC 2898, September 2000.
 [RFC3110]      Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the
                Domain Name System (DNS)", RFC 3110, May 2001.
 [RFC3484]      Draves, R., "Default Address Selection for Internet
                Protocol version 6 (IPv6)", RFC 3484, February 2003.
 [RFC3526]      Kivinen, T. and M. Kojo, "More Modular Exponential
                (MODP) Diffie-Hellman groups for Internet Key Exchange
                (IKE)", RFC 3526, May 2003.
 [RFC3602]      Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC
                Cipher Algorithm and Its Use with IPsec", RFC 3602,
                September 2003.

Moskowitz, et al. Experimental [Page 95] RFC 5201 Host Identity Protocol April 2008

 [RFC3972]      Aura, T., "Cryptographically Generated Addresses
                (CGA)", RFC 3972, March 2005.
 [RFC4034]      Arends, R., Austein, R., Larson, M., Massey, D., and
                S. Rose, "Resource Records for the DNS Security
                Extensions", RFC 4034, March 2005.
 [RFC4282]      Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                Network Access Identifier", RFC 4282, December 2005.
 [RFC4307]      Schiller, J., "Cryptographic Algorithms for Use in the
                Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
                December 2005.
 [RFC4843]      Nikander, P., Laganier, J., and F. Dupont, "An IPv6
                Prefix for Overlay Routable Cryptographic Hash
                Identifiers (ORCHID)", RFC 4843, April 2007.
 [RFC5202]      Jokela, P., Moskowitz, R., and P. Nikander, "Using the
                Encapsulating Security Payload (ESP) Transport Format
                with the Host Identity Protocol (HIP)", RFC 5202,
                April 2008.

11.2. Informative References

 [AUR03]        Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of
                the HIP Base Exchange Protocol", in Proceedings
                of 10th Australasian Conference on Information
                Security and  Privacy, July 2003.
 [CRO03]        Crosby, SA. and DS. Wallach, "Denial of Service via
                Algorithmic Complexity Attacks", in Proceedings
                of Usenix Security Symposium 2003,  Washington, DC.,
                August 2003.
 [DIF76]        Diffie, W. and M. Hellman, "New Directions in
                Cryptography", IEEE Transactions on Information
                Theory vol. IT-22, number 6, pages 644-654, Nov 1976.
 [FIPS01]       NIST, "FIPS PUB 197: Advanced Encryption Standard",
                Nov 2001.
 [HIP-APP]      Henderson, T., Nikander, P., and M. Komu, "Using the
                Host Identity Protocol with Legacy Applications", Work
                in Progress, November 2007.

Moskowitz, et al. Experimental [Page 96] RFC 5201 Host Identity Protocol April 2008

 [IPsec-APIs]   Richardson, M., Williams, N., Komu, M., and S.
                Tarkoma, "IPsec Application Programming Interfaces",
                Work in Progress, February 2008.
 [KAU03]        Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
                protection for UDP-based protocols", ACM Conference on
                Computer and Communications Security , Oct 2003.
 [KRA03]        Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
                Authenticated Diffie-Hellman and Its Use in the IKE-
                Protocols", in Proceedings of CRYPTO 2003, pages 400-
                425, August 2003.
 [RFC0792]      Postel, J., "Internet Control Message Protocol",
                STD 5, RFC 792, September 1981.
 [RFC2412]      Orman, H., "The OAKLEY Key Determination Protocol",
                RFC 2412, November 1998.
 [RFC2434]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26,
                RFC 2434, October 1998.
 [RFC4306]      Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
                RFC 4306, December 2005.
 [RFC4423]      Moskowitz, R. and P. Nikander, "Host Identity Protocol
                (HIP) Architecture", RFC 4423, May 2006.
 [RFC5204]      Laganier, J. and L. Eggert, "Host Identity Protocol
                (HIP) Rendezvous Extension", RFC 5204, April 2008.
 [RFC5205]      Nikander, P. and J. Laganier, "Host Identity Protocol
                (HIP) Domain Name System (DNS) Extensions", RFC 5205,
                April 2008.
 [RFC5206]      Henderson, T., Ed., "End-Host Mobility and Multihoming
                with the Host Identity Protocol", RFC 5206,
                April 2008.
 [SHIM6-PROTO]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3
                Multihoming Shim Protocol for IPv6", Work in Progress,
                February 2008.

Moskowitz, et al. Experimental [Page 97] RFC 5201 Host Identity Protocol April 2008

Appendix A. Using Responder Puzzles

 As mentioned in Section 4.1.1, the Responder may delay state creation
 and still reject most spoofed I2s by using a number of pre-calculated
 R1s and a local selection function.  This appendix defines one
 possible implementation in detail.  The purpose of this appendix is
 to give the implementors an idea on how to implement the mechanism.
 If the implementation is based on this appendix, it MAY contain some
 local modification that makes an attacker's task harder.
 The Responder creates a secret value S, that it regenerates
 periodically.  The Responder needs to remember the two latest values
 of S.  Each time the S is regenerated, the R1 generation counter
 value is incremented by one.
 The Responder generates a pre-signed R1 packet.  The signature for
 pre-generated R1s must be recalculated when the Diffie-Hellman key is
 recomputed or when the R1_COUNTER value changes due to S value
 regeneration.
 When the Initiator sends the I1 packet for initializing a connection,
 the Responder gets the HIT and IP address from the packet, and
 generates an I value for the puzzle.  The I value is set to the pre-
 signed R1 packet.
      I value calculation:
      I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), 64)
 The RHASH algorithm is the same that is used to generate the
 Responder's HIT value.
 From an incoming I2 packet, the Responder gets the required
 information to validate the puzzle: HITs, IP addresses, and the
 information of the used S value from the R1_COUNTER.  Using these
 values, the Responder can regenerate the I, and verify it against the
 I received in the I2 packet.  If the I values match, it can verify
 the solution using I, J, and difficulty K.  If the I values do not
 match, the I2 is dropped.
      puzzle_check:
      V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), K )
      if V != 0, drop the packet
 If the puzzle solution is correct, the I and J values are stored for
 later use.  They are used as input material when keying material is
 generated.

Moskowitz, et al. Experimental [Page 98] RFC 5201 Host Identity Protocol April 2008

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

Appendix B. Generating a Public Key Encoding from an HI

 The following pseudo-code illustrates the process to generate a
 public key encoding from an HI for both RSA and DSA.
 The symbol := denotes assignment; the symbol += denotes appending.
 The pseudo-function encode_in_network_byte_order takes two
 parameters, an integer (bignum) and a length in bytes, and returns
 the integer encoded into a byte string of the given length.
 switch ( HI.algorithm )
 {
 case RSA:
  buffer := encode_in_network_byte_order ( HI.RSA.e_len,
            ( HI.RSA.e_len > 255 ) ? 3 : 1 )
  buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
  buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )
  break;
 case DSA:
  buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
  buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
  buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
                                           8 * HI.DSA.T )
  buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
                                           8 * HI.DSA.T )
  buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
                                           8 * HI.DSA.T )
  break;
 }

Moskowitz, et al. Experimental [Page 99] RFC 5201 Host Identity Protocol April 2008

Appendix C. Example Checksums for HIP Packets

 The HIP checksum for HIP packets is specified in Section 5.1.1.
 Checksums for TCP and UDP packets running over HIP-enabled security
 associations are specified in Section 3.5.  The examples below use IP
 addresses of 192.168.0.1 and 192.168.0.2 (and their respective IPv4-
 compatible IPv6 formats), and HITs with the prefix of 2001:10
 followed by zeros, followed by a decimal 1 or 2, respectively.
 The following example is defined only for testing a checksum
 calculation.  The address format for the IPv4-compatible IPv6 address
 is not a valid one, but using these IPv6 addresses when testing an
 IPv6 implementation gives the same checksum output as an IPv4
 implementation with the corresponding IPv4 addresses.

C.1. IPv6 HIP Example (I1)

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

C.2. IPv4 HIP Packet (I1)

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

Moskowitz, et al. Experimental [Page 100] RFC 5201 Host Identity Protocol April 2008

C.3. TCP Segment

 Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
 use the IPv6 pseudo-header format [RFC2460], with the HITs used in
 place of the IPv6 addresses.
    Sender's HIT:                   2001:10::1
    Receiver's HIT:                 2001:10::2
    Upper-Layer Packet Length:      20              0x14
    Next Header:                    6               0x06
    Source port:                    65500           0xffdc
    Destination port:               22              0x0016
    Sequence number:                1               0x00000001
    Acknowledgment number:          0               0x00000000
    Header length:                  20              0x14
    Flags:                          SYN             0x02
    Window size:                    65535           0xffff
    Checksum:                       28618           0x6fca
    Urgent pointer:                 0               0x0000
      0x0000:  6000 0000 0014 0640 2001 0010 0000 0000
      0x0010:  0000 0000 0000 0001 2001 0010 0000 0000
      0x0020:  0000 0000 0000 0002 ffdc 0016 0000 0001
      0x0030:  0000 0000 5002 ffff 6fca 0000

Appendix D. 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.

Moskowitz, et al. Experimental [Page 101] RFC 5201 Host Identity Protocol April 2008

Appendix E. OAKLEY Well-Known Group 1

 See also [RFC2412] for definition of OAKLEY well-known group 1.
 OAKLEY Well-Known Group 1: A 768-bit prime
 The prime is 2^768 - 2^704 - 1 + 2^64 * { [2^638 pi] + 149686 }.
 The hexadecimal value is:
     FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
     29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
     EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
     E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF
 This has been rigorously verified as a prime.
 The generator is: 22 (decimal)

Moskowitz, et al. Experimental [Page 102] RFC 5201 Host Identity Protocol April 2008

Authors' Addresses

 Robert Moskowitz
 ICSAlabs, An Independent Division of Verizon Business Systems
 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 (editor)
 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

Moskowitz, et al. Experimental [Page 103] RFC 5201 Host Identity Protocol April 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Moskowitz, et al. Experimental [Page 104]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5201.txt · Last modified: 2008/04/17 23:19 (external edit)