GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc7401

Internet Engineering Task Force (IETF) R. Moskowitz, Ed. Request for Comments: 7401 HTT Consulting Obsoletes: 5201 T. Heer Category: Standards Track Hirschmann Automation and Control ISSN: 2070-1721 P. Jokela

                                          Ericsson Research NomadicLab
                                                          T. Henderson
                                              University of Washington
                                                            April 2015
              Host Identity Protocol Version 2 (HIPv2)

Abstract

 This document specifies the details of the Host Identity Protocol
 (HIP).  HIP allows consenting hosts to securely establish and
 maintain shared IP-layer state, allowing separation of the identifier
 and locator roles of IP addresses, thereby enabling continuity of
 communications across IP address changes.  HIP is based on a 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 Encapsulating Security Payload (ESP),
 it provides integrity protection and optional encryption for upper-
 layer protocols, such as TCP and UDP.
 This document obsoletes RFC 5201 and addresses the concerns raised by
 the IESG, particularly that of crypto agility.  It also incorporates
 lessons learned from the implementations of RFC 5201.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7401.

Moskowitz, et al. Standards Track [Page 1] RFC 7401 HIPv2 April 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................5
    1.1. A New Namespace and Identifiers ............................6
    1.2. The HIP Base Exchange (BEX) ................................6
    1.3. Memo Structure .............................................7
 2. Terms and Definitions ...........................................7
    2.1. Requirements Terminology ...................................7
    2.2. Notation ...................................................8
    2.3. Definitions ................................................8
 3. Host Identity (HI) and Its Structure ............................9
    3.1. Host Identity Tag (HIT) ...................................10
    3.2. Generating a HIT from an HI ...............................11
 4. Protocol Overview ..............................................12
    4.1. Creating a HIP Association ................................12
         4.1.1. HIP Puzzle Mechanism ...............................14
         4.1.2. Puzzle Exchange ....................................15
         4.1.3. Authenticated Diffie-Hellman Protocol with
                DH Group Negotiation ...............................17
         4.1.4. HIP Replay Protection ..............................18
         4.1.5. Refusing a HIP Base Exchange .......................19
         4.1.6. Aborting a HIP Base Exchange .......................20
         4.1.7. HIP Downgrade Protection ...........................20
         4.1.8. HIP Opportunistic Mode .............................21
    4.2. Updating a HIP Association ................................24
    4.3. Error Processing ..........................................24
    4.4. HIP State Machine .........................................25
         4.4.1. State Machine Terminology ..........................26
         4.4.2. HIP States .........................................27
         4.4.3. HIP State Processes ................................28
         4.4.4. Simplified HIP State Diagram .......................35

Moskowitz, et al. Standards Track [Page 2] RFC 7401 HIPv2 April 2015

    4.5. User Data Considerations ..................................37
         4.5.1. TCP and UDP Pseudo Header Computation for
                User Data ..........................................37
         4.5.2. Sending Data on HIP Packets ........................37
         4.5.3. Transport Formats ..................................37
         4.5.4. Reboot, Timeout, and Restart of HIP ................37
    4.6. Certificate Distribution ..................................38
 5. Packet Formats .................................................38
    5.1. Payload Format ............................................38
         5.1.1. Checksum ...........................................40
         5.1.2. HIP Controls .......................................40
         5.1.3. HIP Fragmentation Support ..........................40
    5.2. HIP Parameters ............................................41
         5.2.1. TLV Format .........................................44
         5.2.2. Defining New Parameters ............................46
         5.2.3. R1_COUNTER .........................................47
         5.2.4. PUZZLE .............................................48
         5.2.5. SOLUTION ...........................................49
         5.2.6. DH_GROUP_LIST ......................................50
         5.2.7. DIFFIE_HELLMAN .....................................51
         5.2.8. HIP_CIPHER .........................................52
         5.2.9. HOST_ID ............................................54
         5.2.10. HIT_SUITE_LIST ....................................56
         5.2.11. TRANSPORT_FORMAT_LIST .............................58
         5.2.12. HIP_MAC ...........................................59
         5.2.13. HIP_MAC_2 .........................................59
         5.2.14. HIP_SIGNATURE .....................................60
         5.2.15. HIP_SIGNATURE_2 ...................................61
         5.2.16. SEQ ...............................................61
         5.2.17. ACK ...............................................62
         5.2.18. ENCRYPTED .........................................62
         5.2.19. NOTIFICATION ......................................64
         5.2.20. ECHO_REQUEST_SIGNED ...............................67
         5.2.21. ECHO_REQUEST_UNSIGNED .............................68
         5.2.22. ECHO_RESPONSE_SIGNED ..............................69
         5.2.23. ECHO_RESPONSE_UNSIGNED ............................69
    5.3. HIP Packets ...............................................70
         5.3.1. I1 - the HIP Initiator Packet ......................71
         5.3.2. R1 - the HIP Responder Packet ......................72
         5.3.3. I2 - the Second HIP Initiator Packet ...............75
         5.3.4. R2 - the Second HIP Responder Packet ...............76
         5.3.5. UPDATE - the HIP Update Packet .....................77
         5.3.6. NOTIFY - the HIP Notify Packet .....................78
         5.3.7. CLOSE - the HIP Association Closing Packet .........78
         5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet ..79

Moskowitz, et al. Standards Track [Page 3] RFC 7401 HIPv2 April 2015

    5.4. ICMP Messages .............................................79
         5.4.1. Invalid Version ....................................79
         5.4.2. Other Problems with the HIP Header and
                Packet Structure ...................................80
         5.4.3. Invalid Puzzle Solution ............................80
         5.4.4. Non-existing HIP Association .......................80
 6. Packet Processing ..............................................80
    6.1. Processing Outgoing Application Data ......................81
    6.2. Processing Incoming Application Data ......................82
    6.3. Solving the Puzzle ........................................83
    6.4. HIP_MAC and SIGNATURE Calculation and Verification ........84
         6.4.1. HMAC Calculation ...................................84
         6.4.2. Signature Calculation ..............................87
    6.5. HIP KEYMAT Generation .....................................89
    6.6. Initiation of a HIP Base Exchange .........................90
         6.6.1. Sending Multiple I1 Packets in Parallel ............91
         6.6.2. Processing Incoming ICMP Protocol
                Unreachable Messages ...............................92
    6.7. Processing of Incoming I1 Packets .........................92
         6.7.1. R1 Management ......................................94
         6.7.2. Handling of Malformed Messages .....................94
    6.8. Processing of Incoming R1 Packets .........................94
         6.8.1. Handling of Malformed Messages .....................97
    6.9. Processing of Incoming I2 Packets .........................97
         6.9.1. Handling of Malformed Messages ....................100
    6.10. Processing of Incoming R2 Packets .......................101
    6.11. Sending UPDATE Packets ..................................101
    6.12. Receiving UPDATE Packets ................................102
         6.12.1. Handling a SEQ Parameter in a Received
                 UPDATE Message ...................................103
         6.12.2. Handling an ACK Parameter in a Received
                 UPDATE Packet ....................................104
    6.13. Processing of NOTIFY Packets ............................104
    6.14. Processing of CLOSE Packets .............................105
    6.15. Processing of CLOSE_ACK Packets .........................105
    6.16. Handling State Loss .....................................105
 7. HIP Policies ..................................................106
 8. Security Considerations .......................................106
 9. IANA Considerations ...........................................109
 10. Differences from RFC 5201 ....................................113
 11. References ...................................................117
    11.1. Normative References ....................................117
    11.2. Informative References ..................................119
 Appendix A. Using Responder Puzzles ..............................122
 Appendix B. Generating a Public Key Encoding from an HI ..........123

Moskowitz, et al. Standards Track [Page 4] RFC 7401 HIPv2 April 2015

 Appendix C. Example Checksums for HIP Packets ....................123
   C.1. IPv6 HIP Example (I1 Packet) ..............................124
   C.2. IPv4 HIP Packet (I1 Packet) ...............................124
   C.3. TCP Segment ...............................................125
 Appendix D. ECDH and ECDSA 160-Bit Groups ........................125
 Appendix E. HIT Suites and HIT Generation ........................125
 Acknowledgments ..................................................127
 Authors' Addresses ...............................................128

1. Introduction

 This document specifies the details of the Host Identity Protocol
 (HIP).  A high-level description of the protocol and the underlying
 architectural thinking is available in the separate HIP architecture
 description [HIP-ARCH].  Briefly, the HIP architecture proposes an
 alternative to the dual use of IP addresses as "locators" (routing
 labels) and "identifiers" (endpoint, or host, identifiers).  In HIP,
 public cryptographic keys, of a public/private key pair, are used as
 host identifiers, to which higher-layer protocols are bound instead
 of an IP address.  By using public keys (and their representations)
 as host identifiers, dynamic changes to IP address sets can be
 directly authenticated between hosts, and if desired, strong
 authentication between hosts at the TCP/IP stack level can be
 obtained.
 This memo specifies the base HIP protocol ("base exchange") used
 between hosts to establish an IP-layer communications context, called
 a HIP association, prior to communications.  It also defines a packet
 format and procedures for updating and terminating 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)" [RFC7402]: how to use the
    Encapsulating Security Payload (ESP) for integrity protection and
    optional encryption
 o  "Host Mobility with the Host Identity Protocol" [HIP-HOST-MOB]:
    how to support host mobility in HIP
 o  "Host Identity Protocol (HIP) Domain Name System (DNS) Extension"
    [HIP-DNS-EXT]: how to extend DNS to contain Host Identity
    information
 o  "Host Identity Protocol (HIP) Rendezvous Extension"
    [HIP-REND-EXT]: using a rendezvous mechanism to contact mobile HIP
    hosts

Moskowitz, et al. Standards Track [Page 5] RFC 7401 HIPv2 April 2015

 Since the HIP base exchange was first developed, there have been a
 few advances in cryptography and attacks against cryptographic
 systems.  As a result, all cryptographic protocols need to be agile.
 That is, the ability to switch from one cryptographic primitive to
 another should be a part of such protocols.  It is important to
 support a reasonable set of mainstream algorithms to cater to
 different use cases and allow moving away from algorithms that are
 later discovered to be vulnerable.  This update to the base exchange
 includes this needed cryptographic agility while addressing the
 downgrade attacks that such flexibility introduces.  In addition,
 Elliptic Curve support via Elliptic Curve DSA (ECDSA) and Elliptic
 Curve Diffie-Hellman (ECDH) has been added.

1.1. A New Namespace and Identifiers

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

1.2. The HIP Base Exchange (BEX)

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

Moskowitz, et al. Standards Track [Page 6] RFC 7401 HIPv2 April 2015

 The exchange can use the Diffie-Hellman output to encrypt the Host
 Identity of the Initiator in the 3rd packet (although Aura, et al.
 [AUR05] note that such operation may interfere with packet-inspecting
 middleboxes), or the Host Identity may instead be sent unencrypted.
 The Responder's Host Identity is not protected.  It should be noted,
 however, that both the Initiator's and the Responder's HITs are
 transported as such (in cleartext) in the packets, allowing an
 eavesdropper with a priori knowledge about the parties to identify
 them by their HITs.  Hence, encrypting the HI of any party does not
 provide privacy against such an attacker.
 Data packets start to flow after the 4th packet.  The 3rd and 4th HIP
 packets may carry a data payload in the future.  However, the details
 of this may be defined later.
 An existing HIP association can be updated using the update mechanism
 defined in this document, and when the association is no longer
 needed, it can be closed using the defined closing mechanism.
 Finally, HIP is designed as an end-to-end authentication and key
 establishment protocol, to be used with Encapsulating Security
 Payload (ESP) [RFC7402] 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) [RFC7296] that allows IKE to
 support complex gateway policies.  Thus, HIP is not a complete
 replacement for IKE.

1.3. Memo Structure

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

2. Terms and Definitions

2.1. Requirements Terminology

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

Moskowitz, et al. Standards Track [Page 7] RFC 7401 HIPv2 April 2015

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

2.3. Definitions

 HIP base exchange (BEX):  The handshake for establishing a new HIP
    association.
 Host Identity (HI):  The public key of the signature algorithm that
    represents the identity of the host.  In HIP, a host proves its
    identity by creating a signature with the private key belonging to
    its HI (cf. Section 3).
 Host Identity Tag (HIT):  A shorthand for the HI in IPv6 format.  It
    is generated by hashing the HI (cf. Section 3.1).
 HIT Suite:  A HIT Suite groups all cryptographic algorithms that are
    required to generate and use an HI and its HIT.  In particular,
    these algorithms are 1) the public key signature algorithm, 2) the
    hash function, and 3) the truncation (cf. Appendix E).
 HIP association:  The shared state between two peers after completion
    of the BEX.
 HIP packet:  A control packet carrying a HIP packet header relating
    to the establishment, maintenance, or termination of the HIP
    association.
 Initiator:  The host that initiates the BEX.  This role is typically
    forgotten once the BEX is completed.

Moskowitz, et al. Standards Track [Page 8] RFC 7401 HIPv2 April 2015

 Responder:  The host that responds to the Initiator in the BEX.  This
    role is typically forgotten once the BEX is completed.
 Responder's HIT hash algorithm (RHASH):  The hash algorithm used for
    various hash calculations in this document.  The algorithm is the
    same as is used to generate the Responder's HIT.  The RHASH is the
    hash function defined by the HIT Suite of the Responder's HIT
    (cf. Section 5.2.10).
 Length of the Responder's HIT hash algorithm (RHASH_len):  The
    natural output length of RHASH in bits.
 Signed data:  Data that is signed is protected by a digital signature
    that was created by the sender of the data by using the private
    key of its HI.
 KDF:  The Key Derivation Function (KDF) is used for deriving the
    symmetric keys from the Diffie-Hellman key exchange.
 KEYMAT:  The keying material derived from the Diffie-Hellman key
    exchange by using the KDF.  Symmetric keys for encryption and
    integrity protection of HIP packets and encrypted user data
    packets are drawn from this keying material.

3. Host Identity (HI) and Its Structure

 In this section, the properties of the Host Identity and Host
 Identity Tag are discussed, and the exact format for them is defined.
 In HIP, the public key of an asymmetric key pair is used as the Host
 Identity (HI).  Correspondingly, the host itself is defined as the
 entity that holds the private key of the key pair.  See the HIP
 architecture specification [HIP-ARCH] for more details on the
 difference between an identity and the corresponding identifier.
 HIP implementations MUST support the Rivest Shamir Adleman [RSA]
 public key algorithm and the Elliptic Curve Digital Signature
 Algorithm (ECDSA) for generating the HI as defined in Section 5.2.9.
 Additional algorithms MAY be supported.
 A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
 protocols to represent the Host Identity.  The HIT is 128 bits long
 and has the following three key properties: i) it is the same length
 as an IPv6 address and can be used in fixed address-sized fields in
 APIs and protocols; ii) it is self-certifying (i.e., given a HIT, it
 is computationally hard to find a Host Identity key that matches the
 HIT); and iii) the probability of a HIT collision between two hosts

Moskowitz, et al. Standards Track [Page 9] RFC 7401 HIPv2 April 2015

 is very low; hence, it is infeasible for an attacker to find a
 collision with a HIT that is in use.  For details on the security
 properties of the HIT, see [HIP-ARCH].
 The structure of the HIT is defined in [RFC7343].  The HIT is an
 Overlay Routable Cryptographic Hash Identifier (ORCHID) and consists
 of three parts: first, an IANA-assigned prefix to distinguish it from
 other IPv6 addresses; second, a four-bit encoding of the algorithms
 that were used for generating the HI and the hashed representation of
 HI; third, a 96-bit hashed representation of the Host Identity.  The
 encoding of the ORCHID generation algorithm and the exact algorithm
 for generating the hashed representation are specified in Appendix E
 and [RFC7343].
 Carrying HIs and HITs in the header of user data packets would
 increase the overhead of packets.  Thus, it is not expected that they
 are carried in every packet, but other methods are used to map the
 data packets to the corresponding HIs.  In some cases, this makes it
 possible to use HIP without any additional headers in the user data
 packets.  For example, if ESP is used to protect data traffic, the
 Security Parameter Index (SPI) carried in the ESP header can be used
 to map the encrypted data packet to the correct HIP association.

3.1. Host Identity Tag (HIT)

 The Host Identity Tag is a 128-bit value -- a hashed encoding of the
 Host Identifier.  There are two advantages of using a hashed encoding
 over the actual variable-sized Host Identity public key in protocols.
 First, the fixed length of the HIT keeps packet sizes manageable and
 eases protocol coding.  Second, it presents a consistent format for
 the protocol, independent of the underlying identity technology
 in use.
 RFC 7343 [RFC7343] specifies 128-bit hash-based identifiers, called
 ORCHIDs.  Their prefix, allocated from the IPv6 address block, is
 defined in [RFC7343].  The Host Identity Tag is one type of ORCHID.
 This document extends the original, experimental HIP specification
 [RFC5201] with measures to support crypto agility.  One of these
 measures allows different hash functions for creating a HIT.  HIT
 Suites group the sets of algorithms that are required to generate and
 use a particular HIT.  The Suites are encoded in HIT Suite IDs.
 These HIT Suite IDs are transmitted in the ORCHID Generation
 Algorithm (OGA) field in the ORCHID.  With the HIT Suite ID in the
 OGA ID field, a host can tell, from another host's HIT, whether it
 supports the necessary hash and signature algorithms to establish a
 HIP association with that host.

Moskowitz, et al. Standards Track [Page 10] RFC 7401 HIPv2 April 2015

3.2. Generating a HIT from an HI

 The HIT MUST be generated according to the ORCHID generation method
 described in [RFC7343] using a context ID value of 0xF0EF F02F BFF4
 3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly
 by the editor of this specification), and an input that encodes the
 Host Identity field (see Section 5.2.9) present in a HIP payload
 packet.  The set of hash function, signature algorithm, and the
 algorithm used for generating the HIT from the HI depends on the HIT
 Suite (see Section 5.2.10) and is indicated by the four bits of the
 OGA ID field in the ORCHID.  Currently, truncated SHA-1, truncated
 SHA-384, and truncated SHA-256 [FIPS.180-4.2012] are defined as
 hashes for generating a HIT.
 For identities that are either RSA, Digital Signature Algorithm (DSA)
 [FIPS.186-4.2013], or Elliptic Curve DSA (ECDSA) public keys, the
 ORCHID input consists of the public key encoding as specified for the
 Host Identity field of the HOST_ID parameter (see Section 5.2.9).
 This document defines four algorithm profiles: RSA, DSA, ECDSA, and
 ECDSA_LOW.  The ECDSA_LOW profile is meant for devices with low
 computational capabilities.  Hence, one of the following applies:
    The RSA public key is encoded as defined in [RFC3110], Section 2,
    taking the exponent length (e_len), exponent (e), and modulus (n)
    fields concatenated.  The length (n_len) of the modulus (n) can be
    determined from the total HI Length and the preceding HI fields
    including the exponent (e).  Thus, the data that serves as input
    for the HIT generation has the same length as the HI.  The fields
    MUST be encoded in network byte order, as defined in [RFC3110].
    The DSA public key is encoded as defined in [RFC2536], Section 2,
    taking the fields T, Q, P, G, and Y, concatenated as input.  Thus,
    the data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
    where T is the size parameter as defined in [RFC2536].  The size
    parameter T, affecting the field lengths, MUST be selected as the
    minimum value that is long enough to accommodate P, G, and Y.  The
    fields MUST be encoded in network byte order, as defined in
    [RFC2536].
    The ECDSA public keys are encoded as defined in Sections 4.2 and 6
    of [RFC6090].
 In Appendix B, the public key encoding process is illustrated using
 pseudo-code.

Moskowitz, et al. Standards Track [Page 11] RFC 7401 HIPv2 April 2015

4. Protocol Overview

 This section is a simplified overview of the HIP protocol operation,
 and does not contain all the details of the packet formats or the
 packet processing steps.  Sections 5 and 6 describe in more detail
 the packet formats and packet processing steps, respectively, and are
 normative in case of any conflicts with this section.
 The protocol number 139 has been assigned by IANA to the Host
 Identity Protocol.
 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 [RFC7402].

4.1. Creating a HIP Association

 By definition, the system initiating a HIP base exchange is the
 Initiator, and the peer is the Responder.  This distinction is
 typically forgotten once the base exchange completes, and either
 party can become the Initiator in future communications.
 The HIP base exchange serves to manage the establishment of state
 between an Initiator and a Responder.  The first packet, I1,
 initiates the exchange, and the last three packets, R1, I2, and R2,
 constitute an authenticated Diffie-Hellman [DIF76] key exchange for
 session-key generation.  In the first two packets, the hosts agree on
 a set of cryptographic identifiers and algorithms that are then used
 in and after the exchange.  During the Diffie-Hellman key exchange, a
 piece of keying material is generated.  The HIP association keys are
 drawn from this keying material by using a Key Derivation Function
 (KDF).  If other cryptographic keys are needed, e.g., to be used with
 ESP, they are expected to be drawn from the same keying material by
 using the KDF.
 The Initiator first sends a trigger packet, I1, to the Responder.
 The packet contains the HIT of the Initiator and possibly the HIT of
 the Responder, if it is known.  Moreover, the I1 packet initializes
 the negotiation of the Diffie-Hellman group that is used for
 generating the keying material.  Therefore, the I1 packet contains a
 list of Diffie-Hellman Group IDs supported by the Initiator.  Note
 that in some cases it may be possible to replace this trigger packet

Moskowitz, et al. Standards Track [Page 12] RFC 7401 HIPv2 April 2015

 with some other form of a trigger, in which case the protocol starts
 with the Responder sending the R1 packet.  In such cases, another
 mechanism to convey the Initiator's supported DH groups (e.g., by
 using a default group) must be specified.
 The second packet, R1, starts the actual authenticated Diffie-Hellman
 exchange.  It contains a puzzle -- a cryptographic challenge that the
 Initiator must solve before continuing the exchange.  The level of
 difficulty of the puzzle can be adjusted based on the level of trust
 with the Initiator, the current load, or other factors.  In addition,
 the R1 contains the Responder's Diffie-Hellman parameter and lists of
 cryptographic algorithms supported by the Responder.  Based on these
 lists, the Initiator can continue, abort, or restart the base
 exchange with a different selection of cryptographic algorithms.
 Also, the R1 packet contains a signature that covers selected parts
 of the message.  Some fields are left outside the signature to
 support pre-created R1s.
 In the I2 packet, the Initiator MUST display the solution to the
 received puzzle.  Without a correct solution, the I2 message is
 discarded.  The I2 packet also contains a Diffie-Hellman parameter
 that carries needed information for the Responder.  The I2 packet is
 signed by the Initiator.
 The R2 packet acknowledges the receipt of the I2 packet and completes
 the base exchange.  The packet is signed by the Responder.

Moskowitz, et al. Standards Track [Page 13] RFC 7401 HIPv2 April 2015

 The base exchange is illustrated below in Figure 1.  The term "key"
 refers to the Host Identity public key, and "sig" represents a
 signature using such a key.  The packets contain other parameters not
 shown in this figure.
    Initiator                              Responder
                 I1: DH list
               -------------------------->
                                           select precomputed R1
                 R1: puzzle, DH, key, sig
               <-------------------------
 check sig                                 remain stateless
 solve puzzle
               I2: solution, DH, {key}, sig
               -------------------------->
 compute DH                                check puzzle
                                           check sig
                         R2: sig
               <--------------------------
 check sig                                 compute DH
                               Figure 1

4.1.1. HIP Puzzle Mechanism

 The purpose of the HIP puzzle mechanism is to protect the Responder
 from a number of denial-of-service threats.  It allows the Responder
 to delay state creation until receiving the I2 packet.  Furthermore,
 the puzzle allows the Responder to use a fairly cheap calculation to
 check that the Initiator is "sincere" in the sense that it has
 churned enough CPU cycles in solving the puzzle.
 The puzzle allows a Responder implementation to completely delay
 association-specific state creation until a valid I2 packet is
 received.  An I2 packet without a valid puzzle solution can be
 rejected immediately once the Responder has checked the solution.
 The solution can be checked by computing only one hash function, and
 invalid solutions can be rejected before state is created, and before
 CPU-intensive public-key signature verification and Diffie-Hellman
 key generation are performed.  By varying the difficulty of the
 puzzle, the Responder can frustrate CPU- or memory-targeted DoS
 attacks.
 The Responder can remain stateless and drop most spoofed I2 packets
 because puzzle calculation is based on the Initiator's Host Identity
 Tag.  The idea is that the Responder has a (perhaps varying) number
 of pre-calculated R1 packets, and it selects one of these based on

Moskowitz, et al. Standards Track [Page 14] RFC 7401 HIPv2 April 2015

 the information carried in the I1 packet.  When the Responder then
 later receives the I2 packet, it can verify that the puzzle has been
 solved using the Initiator's HIT.  This makes it impractical for the
 attacker to first exchange one I1/R1 packet, and then generate a
 large number of spoofed I2 packets that seemingly come from different
 HITs.  This method does not protect the Responder from an attacker
 that uses fixed HITs, though.  Against such an attacker, a viable
 approach may be to create a piece of local state, and remember that
 the puzzle check has previously failed.  See Appendix A for one
 possible implementation.  Responder implementations SHOULD include
 sufficient randomness in the puzzle values so that algorithmic
 complexity attacks become impossible [CRO03].
 The Responder can set the puzzle difficulty for the Initiator, based
 on its level of trust of the Initiator.  Because the puzzle is not
 included in the signature calculation, the Responder can use
 pre-calculated R1 packets and include the puzzle just before sending
 the R1 to the Initiator.  The Responder SHOULD use heuristics to
 determine when it is under a denial-of-service attack, and set the
 puzzle difficulty value #K appropriately, as explained later.

4.1.2. Puzzle Exchange

 The Responder starts the puzzle exchange when it receives an I1
 packet.  The Responder supplies a random number #I, and requires the
 Initiator to find a number #J.  To select a proper #J, the Initiator
 must create the concatenation of #I, the HITs of the parties, and #J,
 and calculate a hash over this concatenation using the RHASH
 algorithm.  The lowest-order #K bits of the result MUST be zeros.
 The value #K sets the difficulty of the puzzle.
 To generate a proper number #J, the Initiator will have to generate a
 number of #Js until one produces the hash target of zeros.  The
 Initiator SHOULD give up after exceeding the puzzle Lifetime in the
 PUZZLE parameter (as described in Section 5.2.4).  The Responder
 needs to re-create the concatenation of #I, the HITs, and the
 provided #J, and compute the hash once to prove that the Initiator
 completed its assigned task.
 To prevent precomputation attacks, the Responder MUST select the
 number #I in such a way that the Initiator cannot guess it.
 Furthermore, the construction MUST allow the Responder to verify that
 the value #I was indeed selected by it and not by the Initiator.  See
 Appendix A for an example on how to implement this.
 Using the Opaque data field in the PUZZLE (see Section 5.2.4) in an
 ECHO_REQUEST_SIGNED (see Section 5.2.20) or in an
 ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21), the Responder

Moskowitz, et al. Standards Track [Page 15] RFC 7401 HIPv2 April 2015

 can include some data in R1 that the Initiator MUST copy unmodified
 in the corresponding I2 packet.  The Responder can use the opaque
 data to transfer a piece of local state information to the Initiator
 and back -- for example, to recognize that the I2 is a response to a
 previously sent R1.  The Responder can generate the opaque data in
 various ways, e.g., using encryption or hashing with some secret, the
 sent #I, and possibly using other related data.  With the same
 secret, the received #I (from the I2 packet), and the other related
 data (if any), the Responder can verify that it has itself sent the
 #I to the Initiator.  The Responder MUST periodically change such a
 secret.
 It is RECOMMENDED that the Responder generates new secrets for the
 puzzle and new R1s once every few minutes.  Furthermore, it is
 RECOMMENDED that the Responder is able to verify a valid puzzle
 solution at least Lifetime seconds after the puzzle secret has been
 deprecated.  This time value guarantees that the puzzle is valid for
 at least Lifetime and at most 2 * Lifetime seconds.  This limits the
 usability that an old, solved puzzle has to an attacker.  Moreover,
 it avoids problems with the validity of puzzles if the lifetime is
 relatively short compared to the network delay and the time for
 solving the puzzle.
 The puzzle value #I and the solution #J are inputs for deriving the
 keying material from the Diffie-Hellman key exchange (see
 Section 6.5).  Therefore, to ensure that the derived keying material
 differs, a Responder SHOULD NOT use the same puzzle #I with the same
 DH keys for the same Initiator twice.  Such uniqueness can be
 achieved, for example, by using a counter as an additional input for
 generating #I.  This counter can be increased for each processed I1
 packet.  The state of the counter can be transmitted in the Opaque
 data field in the PUZZLE (see Section 5.2.4), in an
 ECHO_REQUEST_SIGNED parameter (see Section 5.2.20), or in an
 ECHO_REQUEST_UNSIGNED parameter (see Section 5.2.21) without the need
 to establish state.
 NOTE: The protocol developers explicitly considered whether R1 should
 include a timestamp in order to protect the Initiator from replay
 attacks.  The decision was to NOT include a timestamp, to avoid
 problems with global time synchronization.
 NOTE: The protocol developers explicitly considered whether a memory-
 bound function should be used for the puzzle instead of a CPU-bound
 function.  The decision was to not use memory-bound functions.

Moskowitz, et al. Standards Track [Page 16] RFC 7401 HIPv2 April 2015

4.1.3. Authenticated Diffie-Hellman Protocol with DH Group Negotiation

 The packets R1, I2, and R2 implement a standard authenticated
 Diffie-Hellman exchange.  The Responder sends one of its public
 Diffie-Hellman keys and its public authentication key, i.e., its Host
 Identity, in R1.  The signature in the R1 packet allows the Initiator
 to verify that the R1 has been once generated by the Responder.
 However, since the R1 is precomputed and therefore does not cover
 association-specific information in the I1 packet, it does not
 protect against replay attacks.
 Before the actual authenticated Diffie-Hellman exchange, the
 Initiator expresses its preference regarding its choice of the DH
 groups in the I1 packet.  The preference is expressed as a sorted
 list of DH Group IDs.  The I1 packet is not protected by a signature.
 Therefore, this list is sent in an unauthenticated way to avoid
 costly computations for processing the I1 packet at the Responder
 side.  Based on the preferences of the Initiator, the Responder sends
 an R1 packet containing its most suitable public DH value.  The
 Responder also attaches a list of its own preferences to the R1 to
 convey the basis for the DH group selection to the Initiator.  This
 list is carried in the signed part of the R1 packet.  If the choice
 of the DH group value in the R1 does not match the preferences of the
 Initiator and the Responder, the Initiator can detect that the list
 of DH Group IDs in the I1 was manipulated (see below for details).
 If none of the DH Group IDs in the I1 packet are supported by the
 Responder, the Responder selects the DH group most suitable for it,
 regardless of the Initiator's preference.  It then sends the R1
 containing this DH group and its list of supported DH Group IDs to
 the Initiator.
 When the Initiator receives an R1, it receives one of the Responder's
 public Diffie-Hellman values and the list of DH Group IDs supported
 by the Responder.  This list is covered by the signature in the R1
 packet to avoid forgery.  The Initiator compares the Group ID of the
 public DH value in the R1 packet to the list of supported DH Group
 IDs in the R1 packets and to its own preferences expressed in the
 list of supported DH Group IDs.  The Initiator continues the BEX only
 if the Group ID of the public DH value of the Responder is the most
 preferred of the IDs supported by both the Initiator and Responder.
 Otherwise, the communication is subject to a downgrade attack, and
 the Initiator MUST either restart the base exchange with a new I1
 packet or abort the base exchange.  If the Responder's choice of the
 DH group is not supported by the Initiator, the Initiator MAY abort
 the handshake or send a new I1 packet with a different list of
 supported DH groups.  However, the Initiator MUST verify the

Moskowitz, et al. Standards Track [Page 17] RFC 7401 HIPv2 April 2015

 signature of the R1 packet before restarting or aborting the
 handshake.  It MUST silently ignore the R1 packet if the signature is
 not valid.
 If the preferences regarding the DH Group ID match, the Initiator
 computes the Diffie-Hellman session key (Kij).  The Initiator creates
 a HIP association using keying material from the session key (see
 Section 6.5) and may use the HIP association to encrypt its public
 authentication key, i.e., the Host Identity.  The resulting I2 packet
 contains the Initiator's Diffie-Hellman key and its (optionally
 encrypted) public authentication key.  The signature of the I2
 message covers all parameters of the signed parameter ranges (see
 Section 5.2) in the packet without exceptions, as in the R1.
 The Responder extracts the Initiator's Diffie-Hellman public key from
 the I2 packet, computes the Diffie-Hellman session key, creates a
 corresponding HIP association, and decrypts the Initiator's public
 authentication key.  It can then verify the signature using the
 authentication key.
 The final message, R2, completes the BEX and protects the Initiator
 against replay attacks, because the Responder uses the shared key
 from the Diffie-Hellman exchange to create a Hashed Message
 Authentication Code (HMAC) and also uses the private key of its Host
 Identity to sign the packet contents.

4.1.4. HIP Replay Protection

 HIP includes the following mechanisms to protect against malicious
 packet replays.  Responders are protected against replays of I1
 packets by virtue of the stateless response to I1 packets with
 pre-signed R1 messages.  Initiators are protected against R1 replays
 by a monotonically increasing "R1 generation counter" included in
 the R1.  Responders are protected against replays of forged I2
 packets by the puzzle mechanism (see Section 4.1.1 above), and
 optional use of opaque data.  Hosts are protected against replays of
 R2 packets and UPDATEs by use of a less expensive HMAC verification
 preceding the HIP signature verification.
 The R1 generation counter is a monotonically increasing 64-bit
 counter that may be initialized to any value.  The scope of the
 counter MAY be system-wide, but there SHOULD be a separate counter
 for each Host Identity, if there is more than one local Host
 Identity.  The value of this counter SHOULD be preserved across
 system reboots and invocations of the HIP base exchange.  This
 counter indicates the current generation of puzzles.  Implementations
 MUST accept puzzles from the current generation and MAY accept
 puzzles from earlier generations.  A system's local counter MUST be

Moskowitz, et al. Standards Track [Page 18] RFC 7401 HIPv2 April 2015

 incremented at least as often as every time old R1s cease to be
 valid.  The local counter SHOULD never be decremented; otherwise, the
 host exposes its peers to the replay of previously generated, higher-
 numbered R1s.
 A host may receive more than one R1, either due to sending multiple
 I1 packets (see Section 6.6.1) or due to a replay of an old R1.  When
 sending multiple I1 packets to the same host, an Initiator SHOULD
 wait for a small amount of time (a reasonable time may be
 2 * expected RTT) after the first R1 reception to allow possibly
 multiple R1s to arrive, and it SHOULD respond to an R1 among the set
 with the largest R1 generation counter.  If an Initiator is
 processing an R1 or has already sent an I2 packet (still waiting for
 the R2 packet) and it receives another R1 with a larger R1 generation
 counter, it MAY elect to restart R1 processing with the fresher R1,
 as if it were the first R1 to arrive.
 The R1 generation counter may roll over or may become reset.  It is
 important for an Initiator to be robust to the loss of state about
 the R1 generation counter of a peer or to a reset of the peer's
 counter.  It is recommended that, when choosing between multiple R1s,
 the Initiator prefer to use the R1 that corresponds to the current R1
 generation counter, but that if it is unable to make progress with
 that R1, the Initiator may try the other R1s, beginning with the R1
 packet with the highest counter.

4.1.5. Refusing a HIP Base Exchange

 A HIP-aware host may choose not to accept a HIP base exchange.  If
 the host's policy is to only be an Initiator and policy allows the
 establishment of a HIP association with the original Initiator, it
 should begin its own HIP base exchange.  A host MAY choose to have
 such a policy since only the privacy of the Initiator's HI is
 protected in the exchange.  It should be noted that such behavior can
 introduce the risk of a race condition if each host's policy is to
 only be an Initiator, at which point the HIP base exchange will fail.
 If the host's policy does not permit it to enter into a HIP exchange
 with the Initiator, it should send an ICMP 'Destination Unreachable,
 Administratively Prohibited' message.  A more complex HIP packet is
 not used here as it actually opens up more potential DoS attacks than
 a simple ICMP message.  A HIP NOTIFY message is not used because no
 HIP association exists between the two hosts at that time.

Moskowitz, et al. Standards Track [Page 19] RFC 7401 HIPv2 April 2015

4.1.6. Aborting a HIP Base Exchange

 Two HIP hosts may encounter situations in which they cannot complete
 a HIP base exchange because of insufficient support for cryptographic
 algorithms, in particular the HIT Suites and DH groups.  After
 receiving the R1 packet, the Initiator can determine whether the
 Responder supports the required cryptographic operations to
 successfully establish a HIP association.  The Initiator can abort
 the BEX silently after receiving an R1 packet that indicates an
 unsupported set of algorithms.  The specific conditions are described
 below.
 The R1 packet contains a signed list of HIT Suite IDs as supported by
 the Responder.  Therefore, the Initiator can determine whether its
 source HIT is supported by the Responder.  If the HIT Suite ID of the
 Initiator's HIT is not contained in the list of HIT Suites in the R1,
 the Initiator MAY abort the handshake silently or MAY restart the
 handshake with a new I1 packet that contains a source HIT supported
 by the Responder.
 During the handshake, the Initiator and the Responder agree on a
 single DH group.  The Responder selects the DH group and its DH
 public value in the R1 based on the list of DH Group IDs in the I1
 packet.  If the Responder supports none of the DH groups requested by
 the Initiator, the Responder selects an arbitrary DH and replies with
 an R1 containing its list of supported DH Group IDs.  In such a case,
 the Initiator receives an R1 packet containing the DH public value
 for an unrequested DH group and also the Responder's DH group list in
 the signed part of the R1 packet.  At this point, the Initiator MAY
 abort the handshake or MAY restart the handshake by sending a new I1
 packet containing a selection of DH Group IDs that is supported by
 the Responder.

4.1.7. HIP Downgrade Protection

 In a downgrade attack, an attacker attempts to unnoticeably
 manipulate the packets of an Initiator and/or a Responder to
 influence the result of the cryptographic negotiations in the BEX in
 its favor.  As a result, the victims select weaker cryptographic
 algorithms than they would otherwise have selected without the
 attacker's interference.  Downgrade attacks can only be successful if
 they remain undetected by the victims and the victims falsely assume
 a secure communication channel.
 In HIP, almost all packet parameters related to cryptographic
 negotiations are covered by signatures.  These parameters cannot be
 directly manipulated in a downgrade attack without invalidating the
 signature.  However, signed packets can be subject to replay attacks.

Moskowitz, et al. Standards Track [Page 20] RFC 7401 HIPv2 April 2015

 In such a replay attack, the attacker could use an old BEX packet
 with an outdated and weak selection of cryptographic algorithms and
 replay it instead of a more recent packet with a collection of
 stronger cryptographic algorithms.  Signed packets that could be
 subject to this replay attack are the R1 and I2 packet.  However,
 replayed R1 and I2 packets cannot be used to successfully establish a
 HIP BEX because these packets also contain the public DH values of
 the Initiator and the Responder.  Old DH values from replayed packets
 lead to invalid keying material and mismatching shared secrets
 because the attacker is unable to derive valid keying material from
 the DH public keys in the R1 and cannot generate a valid HMAC and
 signature for a replayed I2.
 In contrast to the first version of HIP [RFC5201], version 2 of HIP
 as defined in this document begins the negotiation of the DH groups
 already in the first BEX packet, the I1.  The I1 packet is, by
 intention, not protected by a signature, to avoid CPU-intensive
 cryptographic operations processing floods of I1 packets targeted at
 the Responder.  Hence, the list of DH Group IDs in the I1 packet is
 vulnerable to forgery and manipulation.  To thwart an unnoticed
 manipulation of the I1 packet, the Responder chooses the DH group
 deterministically and includes its own list of DH Group IDs in the
 signed part of the R1 packet.  The Initiator can detect an attempted
 downgrade attack by comparing the list of DH Group IDs in the R1
 packet to its own preferences in the I1 packet.  If the choice of the
 DH group in the R1 packet does not equal the best match of the two
 lists (the highest-priority DH ID of the Responder that is present in
 the Initiator's DH list), the Initiator can conclude that its list in
 the I1 packet was altered by an attacker.  In this case, the
 Initiator can restart or abort the BEX.  As mentioned before, the
 detection of the downgrade attack is sufficient to prevent it.

4.1.8. HIP Opportunistic Mode

 It is possible to initiate a HIP BEX even if the Responder's HI (and
 HIT) is unknown.  In this case, the initial I1 packet contains all
 zeros as the destination HIT.  This kind of connection setup is
 called opportunistic mode.
 The Responder may have multiple HITs due to multiple supported HIT
 Suites.  Since the Responder's HIT Suite in the opportunistic mode is
 not determined by the destination HIT of the I1 packet, the Responder
 can freely select a HIT of any HIT Suite.  The complete set of HIT
 Suites supported by the Initiator is not known to the Responder.
 Therefore, the Responder SHOULD select its HIT from the same HIT
 Suite as the Initiator's HIT (indicated by the HIT Suite information
 in the OGA ID field of the Initiator's HIT) because this HIT Suite is
 obviously supported by the Initiator.  If the Responder selects a

Moskowitz, et al. Standards Track [Page 21] RFC 7401 HIPv2 April 2015

 different HIT that is not supported by the Initiator, the Initiator
 MAY restart the BEX with an I1 packet with a source HIT that is
 contained in the list of the Responder's HIT Suites in the R1 packet.
 Note that the Initiator cannot verify the signature of the R1 packet
 if the Responder's HIT Suite is not supported.  Therefore, the
 Initiator MUST treat R1 packets with unsupported Responder HITs as
 potentially forged and MUST NOT use any parameters from the
 unverified R1 besides the HIT_SUITE_LIST.  Moreover, an Initiator
 that uses an unverified HIT_SUITE_LIST from an R1 packet to determine
 a possible source HIT MUST verify that the HIT_SUITE_LIST in the
 first unverified R1 packet matches the HIT_SUITE_LIST in the second
 R1 packet for which the Initiator supports the signature algorithm.
 The Initiator MUST restart the BEX with a new I1 packet for which the
 algorithm was mentioned in the verifiable R1 if the two lists do not
 match.  This procedure is necessary to mitigate downgrade attacks.
 There are both security and API issues involved with the
 opportunistic mode.  These issues are described in the remainder of
 this section.
 Given that the Responder's HI is not known by the Initiator, there
 must be suitable API calls that allow the Initiator to request,
 directly or indirectly, that the underlying system initiates the HIP
 base exchange solely based on locators.  The Responder's HI will be
 tentatively available in the R1 packet, and in an authenticated form
 once the R2 packet has been received and verified.  Hence, the
 Responder's HIT could be communicated to the application via new API
 mechanisms.  However, with a backwards-compatible API the application
 sees only the locators used for the initial contact.  Depending on
 the desired semantics of the API, this can raise the following
 issues:
 o  The actual locators may later change if an UPDATE message is used,
    even if from the API perspective the association still appears to
    be between two specific locators.  However, the locator update is
    still secure, and the association is still between the same nodes.
 o  Different associations between the same two locators may result in
    connections to different nodes, if the implementation no longer
    remembers which identifier the peer had in an earlier association.
    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 implementations MUST NOT place any expectation that the peer's
    HI returned in the R1 message matches any HI previously seen from
    that address.

Moskowitz, et al. Standards Track [Page 22] RFC 7401 HIPv2 April 2015

    If the HIP implementation and application do not have the same
    understanding of what constitutes an association, this may even
    happen within the same association.  For instance, an
    implementation may not know when HIP state can be purged for
    UDP-based applications.
 In addition, the following security considerations apply.  The
 generation counter mechanism will be less efficient in protecting
 against replays of the R1 packet, given that the Responder can choose
 a replay that uses an arbitrary HI, not just the one given in the I1
 packet.
 More importantly, the opportunistic exchange is vulnerable to
 man-in-the-middle attacks, because the Initiator does not have any
 public key information about the peer.  To assess the impacts of this
 vulnerability, we compare it to vulnerabilities in current,
 non-HIP-capable communications.
 An attacker on the path between the two peers can insert itself as a
 man-in-the-middle by providing its own identifier to the Initiator
 and then initiating another HIP association towards the Responder.
 For this to be possible, the Initiator must employ opportunistic
 mode, and the Responder must be configured to accept a connection
 from any HIP-enabled node.
 An attacker outside the path will be unable to do so, given that it
 cannot respond to the messages in the base exchange.
 These security properties are characteristic also of communications
 in the current Internet.  A client contacting a server without
 employing end-to-end security may find itself talking to the server
 via a man-in-the-middle, assuming again that the server is willing to
 talk to anyone.
 If end-to-end security is in place, then the worst that can happen in
 both the opportunistic HIP and non-HIP (normal IP) cases is denial-
 of-service; an entity on the path can disrupt communications, but
 will be unable to successfully insert itself as a man-in-the-middle.
 However, once the opportunistic exchange has successfully completed,
 HIP provides confidentiality and integrity protection for the
 communications, and can securely change the locators of the
 endpoints.
 As a result, opportunistic mode in HIP offers a "better than nothing"
 security model.  Initially, a base exchange authenticated in the
 opportunistic mode involves a leap of faith subject to man-in-the-
 middle attacks, but subsequent datagrams related to the same HIP

Moskowitz, et al. Standards Track [Page 23] RFC 7401 HIPv2 April 2015

 association cannot be compromised by a new man-in-the-middle attack.
 Further, if the man-in-the-middle moves away from the path of the
 active association, the attack would be exposed after the fact.
 Thus, it can be stated that opportunistic mode in HIP is at least as
 secure as unprotected IP-based communications.

4.2. Updating a HIP Association

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

4.3. Error Processing

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

Moskowitz, et al. Standards Track [Page 24] RFC 7401 HIPv2 April 2015

 The HIP protocol and state machine are designed to recover from one
 of the parties crashing and losing its state.  The following
 scenarios describe the main use cases covered by the design.
    No prior state between the two systems.
       The system with data to send is the Initiator.  The process
       follows the standard four-packet base exchange, establishing
       the HIP association.
    The system with data to send has no state with the receiver, but
    the receiver has a residual HIP association.
       The system with data to send is the Initiator.  The Initiator
       acts as in no prior state, sending an I1 packet and receiving
       an R1 packet.  When the Responder receives a valid I2 packet,
       the old association is 'discovered' and deleted, and the new
       association is established.
    The system with data to send has a HIP association, but the
    receiver does not.
       The system sends data on the outbound user data security
       association.  The receiver 'detects' the situation when it
       receives a user data packet that it cannot match to any HIP
       association.  The receiving host MUST discard this packet.
       The receiving host SHOULD send an ICMP packet, with the type
       Parameter Problem, to inform the sender that the HIP
       association does not exist (see Section 5.4), and it MAY
       initiate a new HIP BEX.  However, responding with these
       optional mechanisms is implementation or policy dependent.  If
       the sending application doesn't expect a response, the system
       could possibly send a large number of packets in this state, so
       for this reason, the sending of one or more ICMP packets is
       RECOMMENDED.  However, any such responses MUST be rate-limited
       to prevent abuse (see Section 5.4).

4.4. HIP State Machine

 HIP 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.

Moskowitz, et al. Standards Track [Page 25] RFC 7401 HIPv2 April 2015

 The state machine is symmetric and is presented in a single system
 view, representing either an Initiator or a Responder.  The state
 machine is not a full representation of the processing logic.
 Additional processing rules are presented in the packet definitions.
 Hence, both are needed to completely implement HIP.
 This document extends the state machine as defined in [RFC5201] and
 introduces a restart option to allow for the negotiation of
 cryptographic algorithms.  The extension to the previous state
 machine in [RFC5201] is a transition from state I1-SENT back again to
 I1-SENT; namely, the restart option.  An Initiator is required to
 restart the HIP base exchange if the Responder does not support the
 HIT Suite of the Initiator.  In this case, the Initiator restarts the
 HIP base exchange by sending a new I1 packet with a source HIT
 supported by the Responder.
 Implementors must understand that the state machine, as described
 here, is informational.  Specific implementations are free to
 implement the actual processing logic differently.  Section 6
 describes the packet processing rules in more detail.  This state
 machine focuses on the HIP I1, R1, I2, and R2 packets only.  New
 states and state transitions may be introduced by mechanisms in other
 specifications (such as mobility and multihoming).

4.4.1. State Machine Terminology

 Unused Association Lifetime (UAL):  Implementation-specific time for
    which, if no packet is sent or received for this time interval, a
    host MAY begin to tear down an active HIP association.
 Maximum Segment Lifetime (MSL):  Maximum time that a HIP packet is
    expected to spend in the network.  A default value of 2 minutes
    has been borrowed from [RFC0793] because it is a prevailing
    assumption for packet lifetimes.
 Exchange Complete (EC):  Time that the host spends at the R2-SENT
    state before it moves to the ESTABLISHED state.  The time is n *
    I2 retransmission timeout, where n is about I2_RETRIES_MAX.
 Receive ANYOTHER:  Any received packet for which no state transitions
    or processing rules are defined for a given state.

Moskowitz, et al. Standards Track [Page 26] RFC 7401 HIPv2 April 2015

4.4.2. HIP States

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

Moskowitz, et al. Standards Track [Page 27] RFC 7401 HIPv2 April 2015

4.4.3. HIP State Processes

 System behavior in state UNASSOCIATED, Table 2.
 +----------------------------+--------------------------------------+
 | Trigger                    | Action                               |
 +----------------------------+--------------------------------------+
 | User data to send,         | Send I1 and go to I1-SENT            |
 | requiring a new HIP        |                                      |
 | association                |                                      |
 |                            |                                      |
 | Receive I1                 | Send R1 and stay at UNASSOCIATED     |
 |                            |                                      |
 | Receive I2, process        | If successful, send R2 and go to     |
 |                            | R2-SENT                              |
 |                            |                                      |
 |                            | If fail, stay at UNASSOCIATED        |
 |                            |                                      |
 | Receive user data for an   | Optionally send ICMP as defined in   |
 | unknown HIP association    | Section 5.4 and stay at UNASSOCIATED |
 |                            |                                      |
 | 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. Standards Track [Page 28] RFC 7401 HIPv2 April 2015

 System behavior in state I1-SENT, Table 3.
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1 from     | If the local HIT is smaller than the peer   |
 | Responder           | HIT, drop I1 and stay at I1-SENT (see       |
 |                     | Section 6.5 for HIT comparison)             |
 |                     |                                             |
 |                     | If the local HIT is greater than the peer   |
 |                     | HIT, send R1 and stay at I1-SENT            |
 |                     |                                             |
 | Receive I2, process | If successful, send R2 and go to R2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at I1-SENT                    |
 |                     |                                             |
 | Receive R1, process | If the HIT Suite of the local HIT is not    |
 |                     | supported by the peer, select supported     |
 |                     | local HIT, send I1, and stay at I1-SENT     |
 |                     |                                             |
 |                     | If successful, send I2 and go to I2-SENT    |
 |                     |                                             |
 |                     | If fail, stay at I1-SENT                    |
 |                     |                                             |
 | Receive ANYOTHER    | Drop and stay at I1-SENT                    |
 |                     |                                             |
 | Timeout             | Increment trial counter                     |
 |                     |                                             |
 |                     | If counter is less than I1_RETRIES_MAX,     |
 |                     | send I1 and stay at I1-SENT                 |
 |                     |                                             |
 |                     | If counter is greater than I1_RETRIES_MAX,  |
 |                     | go to E-FAILED                              |
 +---------------------+---------------------------------------------+
          Table 3: I1-SENT - Initiating the HIP Base Exchange

Moskowitz, et al. Standards Track [Page 29] RFC 7401 HIPv2 April 2015

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

Moskowitz, et al. Standards Track [Page 30] RFC 7401 HIPv2 April 2015

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

Moskowitz, et al. Standards Track [Page 31] RFC 7401 HIPv2 April 2015

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

Moskowitz, et al. Standards Track [Page 32] RFC 7401 HIPv2 April 2015

 System behavior in state CLOSING, Table 7.
 +----------------------------+--------------------------------------+
 | Trigger                    | Action                               |
 +----------------------------+--------------------------------------+
 | User data to send,         | Send I1 and go to I1-SENT            |
 | 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, process     | If successful, send CLOSE_ACK,       |
 |                            | discard state, and go to CLOSED      |
 |                            |                                      |
 |                            | If fail, stay at CLOSING             |
 |                            |                                      |
 | Receive CLOSE_ACK, process | If successful, discard state and go  |
 |                            | to UNASSOCIATED                      |
 |                            |                                      |
 |                            | If fail, stay at CLOSING             |
 |                            |                                      |
 | Receive ANYOTHER           | Drop and stay at CLOSING             |
 |                            |                                      |
 | Timeout                    | Increment timeout sum and reset      |
 |                            | timer.  If timeout sum is less than  |
 |                            | UAL+MSL minutes, retransmit CLOSE    |
 |                            | and stay at CLOSING.                 |
 |                            |                                      |
 |                            | If timeout sum is greater than       |
 |                            | UAL+MSL minutes, go to UNASSOCIATED  |
 +----------------------------+--------------------------------------+
 Table 7: CLOSING - HIP Association Has Not Been Used for UAL Minutes

Moskowitz, et al. Standards Track [Page 33] RFC 7401 HIPv2 April 2015

 System behavior in state CLOSED, Table 8.
 +----------------------------------------+--------------------------+
 | Trigger                                | Action                   |
 +----------------------------------------+--------------------------+
 | Datagram to send, requires the         | Send I1 and stay at      |
 | creation of another incarnation of the | CLOSED                   |
 | 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, process                 | If successful, send      |
 |                                        | CLOSE_ACK and stay at    |
 |                                        | CLOSED                   |
 |                                        |                          |
 |                                        | If fail, stay at CLOSED  |
 |                                        |                          |
 | Receive CLOSE_ACK, process             | If successful, discard   |
 |                                        | state and go to          |
 |                                        | 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. Standards Track [Page 34] RFC 7401 HIPv2 April 2015

 System behavior in state E-FAILED, Table 9.
 +-------------------------+-----------------------------------------+
 | Trigger                 | Action                                  |
 +-------------------------+-----------------------------------------+
 | Wait for                | Go to UNASSOCIATED.  Renegotiation is   |
 | implementation-specific | possible after moving to UNASSOCIATED   |
 | time                    | state.                                  |
 +-------------------------+-----------------------------------------+
   Table 9: E-FAILED - HIP Failed to Establish Association with Peer

4.4.4. Simplified HIP State Diagram

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

Moskowitz, et al. Standards Track [Page 35] RFC 7401 HIPv2 April 2015

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

Moskowitz, et al. Standards Track [Page 36] RFC 7401 HIPv2 April 2015

4.5. User Data Considerations

4.5.1. TCP and UDP Pseudo Header Computation for User Data

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

4.5.2. Sending Data on HIP Packets

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

4.5.3. Transport Formats

 The actual data transmission format, used for user data after the HIP
 base exchange, is not defined in this document.  Such transport
 formats and methods are described in separate specifications.  All
 HIP implementations MUST implement, at minimum, the ESP transport
 format for HIP [RFC7402].  The transport format to be chosen is
 negotiated in the base exchange.  The Responder expresses its
 preference regarding the transport format in the
 TRANSPORT_FORMAT_LIST in the R1 packet, and the Initiator selects one
 transport format and adds the respective HIP parameter to the I2
 packet.

4.5.4. Reboot, Timeout, and Restart of HIP

 Simulating a loss of state is a potential DoS attack.  The following
 process has been crafted to manage state recovery without presenting
 a DoS opportunity.
 If a host reboots or the HIP association times out, it has lost its
 HIP state.  If the host that lost state has a datagram to send to the
 peer, it simply restarts the HIP base exchange.  After the base
 exchange has completed, the Initiator can create a new payload
 association and start sending data.  The peer does not reset its
 state until it receives a valid I2 packet.
 If a system receives a user data packet that cannot be matched to any
 existing HIP association, it is possible that it has lost the state
 and its peer has not.  It MAY send an ICMP packet with the Parameter
 Problem type, and with the Pointer pointing to the referred

Moskowitz, et al. Standards Track [Page 37] RFC 7401 HIPv2 April 2015

 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 payload associations
 and creates a new one.

4.6. Certificate Distribution

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

5. Packet Formats

5.1. Payload Format

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

Moskowitz, et al. Standards Track [Page 38] RFC 7401 HIPv2 April 2015

 The HIP header is logically an IPv6 extension header.  However, this
 document does not describe processing for Next Header values other
 than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
 Future documents MAY define behavior for other values.  However,
 current implementations MUST ignore trailing data if an unimplemented
 Next Header value is received.
 The Header Length field contains the combined length of the HIP
 Header and HIP parameters in 8-byte units, excluding the first
 8 bytes.  Since all HIP headers MUST contain the sender's and
 receiver's HIT fields, the minimum value for this field is 4, and
 conversely, the maximum length of the HIP Parameters field is
 (255 * 8) - 32 = 2008 bytes (see Section 5.1.3 regarding HIP
 fragmentation).  Note: this sets an additional limit for sizes of
 parameters included in the Parameters field, independent of the
 individual parameter maximum lengths.
 The Packet Type indicates the HIP packet type.  The individual packet
 types are defined in the relevant sections.  If a HIP host receives a
 HIP packet that contains an unrecognized packet type, it MUST drop
 the packet.
 The HIP Version field is four bits.  The version defined in this
 document is 2.  The version number is expected to be incremented only
 if there are incompatible changes to the protocol.  Most extensions
 can be handled by defining new packet types, new parameter types, or
 new Controls (see Section 5.1.2).
 The following three bits are reserved for future use.  They MUST be
 zero when sent, and they MUST be ignored when handling a received
 packet.
 The two fixed bits in the header are reserved for SHIM6 compatibility
 [RFC5533], Section 5.3.  For implementations adhering (only) to this
 specification, they MUST be set as shown when sending and MUST be
 ignored when receiving.  This is to ensure optimal forward
 compatibility.  Note that for implementations that implement other
 compatible specifications in addition to this specification, the
 corresponding rules may well be different.  For example, an
 implementation that implements both this specification and the SHIM6
 protocol may need to check these bits in order to determine how to
 handle the packet.
 The HIT fields are always 128 bits (16 bytes) long.

Moskowitz, et al. Standards Track [Page 39] RFC 7401 HIPv2 April 2015

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 5.1) in the Next Header field.  The Length field
 is in bytes and can be calculated from the HIP Header Length field:
 (HIP Header Length + 1) * 8.
 In case of using IPv4, the IPv4 UDP pseudo header format [RFC0768] is
 used.  In the pseudo header, the source and destination addresses are
 those used in the IP header, the zero field is obviously zero, the
 protocol is the HIP protocol number (see Section 4), and the length
 is calculated as in the IPv6 case.

5.1.2. HIP Controls

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

5.1.3. HIP Fragmentation Support

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

Moskowitz, et al. Standards Track [Page 40] RFC 7401 HIPv2 April 2015

 generation may not be needed.  If it is expected that a host will
 send HIP packets that are larger than the minimum IPv6 MTU, the
 implementation MUST implement fragment generation even for IPv6.
 In IPv4 networks, HIP packets may encounter low MTUs along their
 routed path.  Since basic HIP, as defined in this document, does not
 provide a mechanism to use multiple IP datagrams for a single HIP
 packet, support for path MTU discovery does not bring any value to
 HIP in IPv4 networks.  HIP-aware NAT devices SHOULD perform IPv4
 reassembly/fragmentation for HIP packets.
 All HIP implementations have to be careful while employing a
 reassembly algorithm so that the algorithm is sufficiently resistant
 to DoS attacks.
 Certificate chains can cause the packet to be fragmented, and
 fragmentation can open implementations to denial-of-service attacks
 [KAU03].  "Hash and URL" schemes as defined in [RFC6253] for HIP
 version 1 may be used to avoid fragmentation and mitigate resulting
 DoS attacks.

5.2. HIP Parameters

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

Moskowitz, et al. Standards Track [Page 41] RFC 7401 HIPv2 April 2015

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

Moskowitz, et al. Standards Track [Page 42] RFC 7401 HIPv2 April 2015

 | NOTIFICATION           | 832   | variable  | Informational data   |
 |                        |       |           |                      |
 | ECHO_REQUEST_SIGNED    | 897   | variable  | Opaque data to be    |
 |                        |       |           | echoed back; signed  |
 |                        |       |           |                      |
 | ECHO_RESPONSE_SIGNED   | 961   | variable  | Opaque data echoed   |
 |                        |       |           | back by request;     |
 |                        |       |           | signed               |
 |                        |       |           |                      |
 | TRANSPORT_FORMAT_LIST  | 2049  | Ordered   | variable             |
 |                        |       | list of   |                      |
 |                        |       | preferred |                      |
 |                        |       | HIP       |                      |
 |                        |       | transport |                      |
 |                        |       | type      |                      |
 |                        |       | numbers   |                      |
 |                        |       |           |                      |
 | HIP_MAC                | 61505 | variable  | HMAC-based message   |
 |                        |       |           | authentication code, |
 |                        |       |           | with key material    |
 |                        |       |           | from KEYMAT          |
 |                        |       |           |                      |
 | HIP_MAC_2              | 61569 | variable  | HMAC-based message   |
 |                        |       |           | authentication code, |
 |                        |       |           | with key material    |
 |                        |       |           | from KEYMAT.  Unlike |
 |                        |       |           | HIP_MAC, the HOST_ID |
 |                        |       |           | parameter is         |
 |                        |       |           | included in          |
 |                        |       |           | HIP_MAC_2            |
 |                        |       |           | calculation.         |
 |                        |       |           |                      |
 | HIP_SIGNATURE_2        | 61633 | variable  | Signature used in R1 |
 |                        |       |           | packet               |
 |                        |       |           |                      |
 | HIP_SIGNATURE          | 61697 | variable  | Signature of the     |
 |                        |       |           | packet               |
 |                        |       |           |                      |
 | ECHO_REQUEST_UNSIGNED  | 63661 | variable  | Opaque data to be    |
 |                        |       |           | echoed back; after   |
 |                        |       |           | signature            |
 |                        |       |           |                      |
 | ECHO_RESPONSE_UNSIGNED | 63425 | variable  | Opaque data echoed   |
 |                        |       |           | back by request;     |
 |                        |       |           | after signature      |
 +------------------------+-------+-----------+----------------------+

Moskowitz, et al. Standards Track [Page 43] RFC 7401 HIPv2 April 2015

 As the ordering (from lowest to highest) of HIP parameters is
 strictly enforced (see Section 5.2.1), the parameter type values for
 existing parameters have been spaced to allow for future protocol
 extensions.
 The following parameter type number ranges are defined.
 +---------------+---------------------------------------------------+
 | Type Range    | Purpose                                           |
 +---------------+---------------------------------------------------+
 | 0 -  1023     | Handshake                                         |
 |               |                                                   |
 | 1024 -   2047 | Reserved                                          |
 |               |                                                   |
 | 2048 -   4095 | Parameters related to HIP transport formats       |
 |               |                                                   |
 | 4096 -   8191 | Signed parameters allocated through specification |
 |               | documents                                         |
 |               |                                                   |
 | 8192 -  32767 | Reserved                                          |
 |               |                                                   |
 | 32768 - 49151 | Reserved for Private Use.  Signed parameters.     |
 |               |                                                   |
 | 49152 - 61439 | Reserved                                          |
 |               |                                                   |
 | 61440 - 62463 | Signatures and (signed) MACs                      |
 |               |                                                   |
 | 62464 - 63487 | Parameters that are neither signed nor MACed      |
 |               |                                                   |
 | 63488 - 64511 | Rendezvous and relaying                           |
 |               |                                                   |
 | 64512 - 65023 | Parameters that are neither signed nor MACed      |
 |               |                                                   |
 | 65024 - 65535 | Reserved                                          |
 +---------------+---------------------------------------------------+
 The process for defining new parameters is described in Section 5.2.2
 of this document.
 The range between 32768 (2^15) and 49151 (2^15 + 2^14) is Reserved
 for Private Use.  Types from this range SHOULD be selected in a
 random fashion to reduce the probability of collisions.

5.2.1. TLV Format

 The TLV-encoded parameters are described in the following
 subsections.  The Type field value also describes the order of these
 fields in the packet.  The parameters MUST be included in the packet

Moskowitz, et al. Standards Track [Page 44] RFC 7401 HIPv2 April 2015

 so that their types form an increasing order.  If multiple parameters
 with the same type number are in one packet, the parameters with the
 same type MUST be consecutive in the packet.  If the order does not
 follow this rule, the packet is considered to be malformed and it
 MUST be discarded.
 Parameters using type values from 2048 up to 4095 are related to
 transport formats.  Currently, one transport format is defined: the
 ESP transport format [RFC7402].
 All of the encoded TLV parameters have a length (that includes the
 Type and Length fields), which is a multiple of 8 bytes.  When
 needed, padding MUST be added to the end of the parameter so that the
 total length is a multiple of 8 bytes.  This rule ensures proper
 alignment of data.  Any added padding bytes MUST be zeroed by the
 sender, and their values SHOULD NOT be checked by the receiver.
 The Length field indicates the length of the Contents field (in
 bytes).  Consequently, the total length of the TLV parameter
 (including Type, Length, Contents, and Padding) is related to the
 Length field according to the following formula:
 Total Length = 11 + Length - (Length + 3) % 8;
 where % is the modulo operator.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type            |C|             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                          Contents                             /
   /                                               +-+-+-+-+-+-+-+-+
   |                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type         Type code for the parameter.  16 bits long, C-bit
                being part of the Type code.
   C            Critical.  One if this parameter is critical and
                MUST be recognized by the recipient, zero otherwise.
                The C-bit is considered to be a part of the Type
                field.  Consequently, critical parameters are always
                odd, and non-critical ones have an even value.
   Length       Length of the Contents, in bytes, excluding Type,
                Length, and Padding
   Contents     Parameter specific, defined by Type
   Padding      Padding, 0-7 bytes, added if needed

Moskowitz, et al. Standards Track [Page 45] RFC 7401 HIPv2 April 2015

 Critical parameters (indicated by the odd type number value) 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 numerically increasing order by Type code,
 thereby limiting the order of parameters (see Section 5.2.1).
 The following rules MUST be followed when defining new parameters.
 1.  The low-order bit C of the Type code is used to distinguish
     between critical and non-critical parameters.  Hence, even
     parameter type numbers indicate non-critical parameters while odd
     parameter type numbers indicate critical parameters.
 2.  A new parameter MAY be critical only if an old implementation
     that ignored it would cause security problems.  In general, new
     parameters SHOULD be defined as non-critical, and expect a reply
     from the recipient.
 3.  If a system implements a new critical parameter, it MUST provide
     the ability to set the associated feature off, such that the
     critical parameter is not sent at all.  The configuration option
     MUST be well documented.  Implementations operating in a mode
     adhering to this specification MUST disable the sending of new
     critical parameters by default.  In other words, the management
     interface MUST allow vanilla standards-only mode as a default
     configuration setting, and MAY allow new critical payloads to be
     configured on (and off).
 4.  See Section 9 for allocation rules regarding Type codes.

Moskowitz, et al. Standards Track [Page 46] RFC 7401 HIPv2 April 2015

5.2.3. R1_COUNTER

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Reserved, 4 bytes                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                R1 generation counter, 8 bytes                 |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           129
   Length         12
   R1 generation
     counter      The current generation of valid puzzles
 The R1_COUNTER parameter contains a 64-bit unsigned integer in
 network byte order, indicating the current generation of valid
 puzzles.  The sender SHOULD increment this counter periodically.  It
 is RECOMMENDED that the counter value is incremented at least as
 often as old PUZZLE values are deprecated so that SOLUTIONs to them
 are no longer accepted.
 Support for the R1_COUNTER parameter is mandatory, although its
 inclusion in the R1 packet 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 MUST be echoed (including the Reserved field verbatim) by
 the Initiator in the I2 packet.

Moskowitz, et al. Standards Track [Page 47] RFC 7401 HIPv2 April 2015

5.2.4. PUZZLE

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

Moskowitz, et al. Standards Track [Page 48] RFC 7401 HIPv2 April 2015

5.2.5. SOLUTION

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

Moskowitz, et al. Standards Track [Page 49] RFC 7401 HIPv2 April 2015

5.2.6. DH_GROUP_LIST

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | DH GROUP ID #1| DH GROUP ID #2| DH GROUP ID #3| DH GROUP ID #4|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | DH GROUP ID #n|                Padding                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           511
   Length         number of DH Group IDs
   DH GROUP ID    identifies a DH GROUP ID supported by the host.
                  The list of IDs is ordered by preference of the
                  host.  The possible DH Group IDs are defined
                  in the DIFFIE_HELLMAN parameter.  Each DH
                  Group ID is one octet long.
 The DH_GROUP_LIST parameter contains the list of supported DH Group
 IDs of a host.  The Initiator sends the DH_GROUP_LIST in the I1
 packet, and the Responder sends its own list in the signed part of
 the R1 packet.  The DH Group IDs in the DH_GROUP_LIST are listed in
 the order of their preference of the host sending the list.  DH Group
 IDs that are listed first are preferred over the DH Group IDs listed
 later.  The information in the DH_GROUP_LIST allows the Responder to
 select the DH group preferred by itself and supported by the
 Initiator.  Based on the DH_GROUP_LIST in the R1 packet, the
 Initiator can determine if the Responder has selected the best
 possible choice based on the Initiator's and Responder's preferences.
 If the Responder's choice differs from the best choice, the Initiator
 can conclude that there was an attempted downgrade attack (see
 Section 4.1.7).
 When selecting the DH group for the DIFFIE_HELLMAN parameter in the
 R1 packet, the Responder MUST select the first DH Group ID in its
 DH_GROUP_LIST in the R1 packet that is compatible with one of the
 Suite IDs in the Initiator's DH_GROUP_LIST in the I1 packet.  The
 Responder MUST NOT select any other DH Group ID that is contained in
 both lists, because then a downgrade attack cannot be detected.
 In general, hosts SHOULD prefer stronger groups over weaker ones if
 the computation overhead is not prohibitively high for the intended
 application.

Moskowitz, et al. Standards Track [Page 50] RFC 7401 HIPv2 April 2015

5.2.7. DIFFIE_HELLMAN

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Group ID    |      Public Value Length      | Public Value  /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           513
   Length         length in octets, excluding Type, Length, and
                  Padding
   Group ID       identifies values for p and g as well as the KDF
   Public Value   length of the following Public Value in octets
     Length
   Public Value   the sender's public Diffie-Hellman key
 A single DIFFIE_HELLMAN parameter may be included in selected HIP
 packets based on the DH Group ID selected (Section 5.2.6).  The
 following Group IDs have been defined; values are assigned by this
 document:
  Group                              KDF              Value
  Reserved                                            0
  DEPRECATED                                          1
  DEPRECATED                                          2
  1536-bit MODP group  [RFC3526]     HKDF [RFC5869]   3
  3072-bit MODP group  [RFC3526]     HKDF [RFC5869]   4
  DEPRECATED                                          5
  DEPRECATED                                          6
  NIST P-256 [RFC5903]               HKDF [RFC5869]   7
  NIST P-384 [RFC5903]               HKDF [RFC5869]   8
  NIST P-521 [RFC5903]               HKDF [RFC5869]   9
  SECP160R1  [SECG]                  HKDF [RFC5869]  10
  2048-bit MODP group  [RFC3526]     HKDF [RFC5869]  11
 The MODP Diffie-Hellman groups are defined in [RFC3526].  ECDH
 groups 7-9 are defined in [RFC5903] and [RFC6090].  ECDH group 10
 is covered in Appendix D.  Any ECDH used with HIP MUST have a
 co-factor of 1.

Moskowitz, et al. Standards Track [Page 51] RFC 7401 HIPv2 April 2015

 The Group ID also defines the key derivation function that is to be
 used for deriving the symmetric keys for the HMAC and symmetric
 encryption from the keying material from the Diffie-Hellman key
 exchange (see Section 6.5).
 A HIP implementation MUST implement Group ID 3.  The 160-bit
 SECP160R1 group can be used when lower security is enough (e.g., web
 surfing) and when the equipment is not powerful enough (e.g., some
 PDAs).  Implementations SHOULD implement Group IDs 4 and 8.
 To avoid unnecessary failures during the base exchange, the rest of
 the groups SHOULD be implemented in hosts where resources are
 adequate.

5.2.8. HIP_CIPHER

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Cipher ID #1         |          Cipher ID #2         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Cipher ID #n         |             Padding           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           579
   Length         length in octets, excluding Type, Length, and
                  Padding
   Cipher ID      identifies the cipher algorithm to be used for
                  encrypting the contents of the ENCRYPTED parameter

Moskowitz, et al. Standards Track [Page 52] RFC 7401 HIPv2 April 2015

 The following Cipher IDs are defined:
      Suite ID           Value
      RESERVED           0
      NULL-ENCRYPT       1     ([RFC2410])
      AES-128-CBC        2     ([RFC3602])
      RESERVED           3     (unused value)
      AES-256-CBC        4     ([RFC3602])
 The sender of a HIP_CIPHER parameter MUST make sure that there are no
 more than six (6) Cipher IDs in one HIP_CIPHER parameter.
 Conversely, a recipient MUST be prepared to handle received transport
 parameters that contain more than six Cipher IDs by accepting the
 first six Cipher IDs and dropping the rest.  The limited number of
 Cipher IDs sets the maximum size of the HIP_CIPHER parameter.  As the
 default configuration, the HIP_CIPHER parameter MUST contain at least
 one of the mandatory Cipher IDs.  There MAY be a configuration option
 that allows the administrator to override this default.
 The Responder lists supported and desired Cipher IDs in order of
 preference in the R1, up to the maximum of six Cipher IDs.  The
 Initiator MUST choose only one of the corresponding Cipher IDs.  This
 Cipher ID will be used for generating the ENCRYPTED parameter.
 Mandatory implementation: AES-128-CBC.  Implementors SHOULD support
 NULL-ENCRYPT for testing/debugging purposes but MUST NOT offer or
 accept this value unless explicitly configured for testing/debugging
 of HIP.

Moskowitz, et al. Standards Track [Page 53] RFC 7401 HIPv2 April 2015

5.2.9. HOST_ID

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          HI Length            |DI-Type|      DI Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Algorithm            |         Host Identity         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |       Domain Identifier       /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type               705
   Length             length in octets, excluding Type, Length, and
                      Padding
   HI Length          length of the Host Identity in octets
   DI-Type            type of the following Domain Identifier field
   DI Length          length of the Domain Identifier field in octets
   Algorithm          index to the employed algorithm
   Host Identity      actual Host Identity
   Domain Identifier  the identifier of the sender
 The following DI-Types have been defined:
       Type                    Value
       none included           0
       FQDN                    1
       NAI                     2
       FQDN            Fully Qualified Domain Name, in binary format
       NAI             Network Access Identifier
 The format for the FQDN is defined in RFC 1035 [RFC1035],
 Section 3.1.  The format for the NAI is defined in [RFC4282].
 A host MAY optionally associate the Host Identity with a single
 Domain Identifier in the HOST_ID parameter.  If there is no Domain
 Identifier, i.e., the DI-Type field is zero, the DI Length field is
 set to zero as well.

Moskowitz, et al. Standards Track [Page 54] RFC 7401 HIPv2 April 2015

 The following HI Algorithms have been defined:
      Algorithm profiles   Values
      RESERVED             0
      DSA                  3 [FIPS.186-4.2013]  (RECOMMENDED)
      RSA                  5 [RFC3447]          (REQUIRED)
      ECDSA                7 [RFC4754]          (REQUIRED)
      ECDSA_LOW            9 [SECG]             (RECOMMENDED)
 For DSA, RSA, and ECDSA key types, profiles containing at least
 112 bits of security strength (as defined by [NIST.800-131A.2011])
 should be used.  For RSA signature padding, the Probabilistic
 Signature Scheme (PSS) method of padding [RFC3447] MUST be used.
 The Host Identity is derived from the DNSKEY format for RSA and DSA.
 For these, the Public Key field of the RDATA part from RFC 4034
 [RFC4034] is used.  For Elliptic Curve Cryptography (ECC), we
 distinguish two different profiles: ECDSA and ECDSA_LOW.  ECC
 contains curves approved by NIST and defined in RFC 4754 [RFC4754].
 ECDSA_LOW is defined for devices with low computational capabilities
 and uses shorter curves from the Standards for Efficient Cryptography
 Group [SECG].  Any ECDSA used with HIP MUST have a co-factor of 1.
 For ECDSA and ECDSA_LOW, Host Identities are represented by the
 following fields:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          ECC Curve            |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                         Public Key                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ECC Curve     Curve label
   Public Key    Represented in octet-string format [RFC6090]
 For hosts that implement ECDSA as the algorithm, the following ECC
 curves are required:
      Algorithm    Curve            Values
      ECDSA        RESERVED         0
      ECDSA        NIST P-256       1 [RFC4754]
      ECDSA        NIST P-384       2 [RFC4754]

Moskowitz, et al. Standards Track [Page 55] RFC 7401 HIPv2 April 2015

 For hosts that implement the ECDSA_LOW algorithm profile, the
 following curve is required:
      Algorithm    Curve            Values
      ECDSA_LOW    RESERVED         0
      ECDSA_LOW    SECP160R1        1 [SECG]

5.2.10. HIT_SUITE_LIST

 The HIT_SUITE_LIST parameter contains a list of the supported HIT
 Suite IDs of the Responder.  The Responder sends the HIT_SUITE_LIST
 in the signed part of the R1 packet.  Based on the HIT_SUITE_LIST,
 the Initiator can determine which source HIT Suite IDs are supported
 by the Responder.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ID #1     |     ID #2     |     ID #3     |     ID #4     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ID #n     |                Padding                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           715
   Length         number of HIT Suite IDs
   ID             identifies a HIT Suite ID supported by the host.
                  The list of IDs is ordered by preference of the
                  host.  Each HIT Suite ID is one octet long.  The
                  four higher-order bits of the ID field correspond
                  to the HIT Suite ID in the ORCHID OGA ID field.  The
                  four lower-order bits are reserved and set to 0
                  by the sender.  The reception of an ID with the
                  four lower-order bits not set to 0 SHOULD be
                  considered as an error that MAY result in a
                  NOTIFICATION of type UNSUPPORTED_HIT_SUITE.
 The HIT Suite ID indexes a HIT Suite.  HIT Suites are composed of
 signature algorithms as defined in Section 5.2.9, and hash functions.
 The ID field in the HIT_SUITE_LIST is defined as an eight-bit field,
 as opposed to the four-bit HIT Suite ID and OGA ID field in the
 ORCHID.  This difference is a measure to accommodate larger HIT Suite
 IDs if the 16 available values prove insufficient.  In that case, one
 of the 16 values, zero, will be used to indicate that four additional
 bits of the ORCHID will be used to encode the HIT Suite ID.  Hence,

Moskowitz, et al. Standards Track [Page 56] RFC 7401 HIPv2 April 2015

 the current four-bit HIT Suite IDs only use the four higher-order
 bits in the ID field.  Future documents may define the use of the
 four lower-order bits in the ID field.
 The following HIT Suite IDs are defined, and the relationship between
 the four-bit ID value used in the OGA ID field and the eight-bit
 encoding within the HIT_SUITE_LIST ID field is clarified:
      HIT Suite       Four-bit ID    Eight-bit encoding
      RESERVED            0             0x00
      RSA,DSA/SHA-256     1             0x10           (REQUIRED)
      ECDSA/SHA-384       2             0x20           (RECOMMENDED)
      ECDSA_LOW/SHA-1     3             0x30           (RECOMMENDED)
 The following table provides more detail on the above HIT Suite
 combinations.  The input for each generation algorithm is the
 encoding of the HI as defined in Section 3.2.  The output is 96 bits
 long and is directly used in the ORCHID.
 +-------+----------+--------------+------------+--------------------+
 | Index | Hash     | HMAC         | Signature  | Description        |
 |       | function |              | algorithm  |                    |
 |       |          |              | family     |                    |
 +-------+----------+--------------+------------+--------------------+
 |     0 |          |              |            | Reserved           |
 |       |          |              |            |                    |
 |     1 | SHA-256  | HMAC-SHA-256 | RSA, DSA   | RSA or DSA HI      |
 |       |          |              |            | hashed with        |
 |       |          |              |            | SHA-256, truncated |
 |       |          |              |            | to 96 bits         |
 |       |          |              |            |                    |
 |     2 | SHA-384  | HMAC-SHA-384 | ECDSA      | ECDSA HI hashed    |
 |       |          |              |            | with SHA-384,      |
 |       |          |              |            | truncated to 96    |
 |       |          |              |            | bits               |
 |       |          |              |            |                    |
 |     3 | SHA-1    | HMAC-SHA-1   | ECDSA_LOW  | ECDSA_LOW HI       |
 |       |          |              |            | hashed with SHA-1, |
 |       |          |              |            | truncated to 96    |
 |       |          |              |            | bits               |
 +-------+----------+--------------+------------+--------------------+
                         Table 10: HIT Suites

Moskowitz, et al. Standards Track [Page 57] RFC 7401 HIPv2 April 2015

 The hash of the Responder as defined in the HIT Suite determines the
 HMAC to be used for the RHASH function.  The HMACs currently defined
 here are HMAC-SHA-256 [RFC4868], HMAC-SHA-384 [RFC4868], and
 HMAC-SHA-1 [RFC2404].

5.2.11. TRANSPORT_FORMAT_LIST

 The TRANSPORT_FORMAT_LIST parameter contains a list of the supported
 HIP transport formats (TFs) of the Responder.  The Responder sends
 the TRANSPORT_FORMAT_LIST in the signed part of the R1 packet.  Based
 on the TRANSPORT_FORMAT_LIST, the Initiator chooses one suitable
 transport format and includes the respective HIP transport format
 parameter in its response packet.
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          TF type #1           |           TF type #2          /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /          TF type #n           |             Padding           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           2049
   Length         2x number of TF types
   TF Type        identifies a transport format (TF) type supported
                  by the host.  The TF type numbers correspond to
                  the HIP parameter type numbers of the respective
                  transport format parameters.  The list of TF types
                  is ordered by preference of the sender.
 The TF type numbers index the respective HIP parameters for the
 transport formats in the type number range between 2050 and 4095.
 The parameters and their use are defined in separate documents.
 Currently, the only transport format defined is IPsec ESP [RFC7402].
 For each listed TF type, the sender of the TRANSPORT_FORMAT_LIST
 parameter MUST include the respective transport format parameter in
 the HIP packet.  The receiver MUST ignore the TF type in the
 TRANSPORT_FORMAT_LIST if no matching transport format parameter is
 present in the packet.

Moskowitz, et al. Standards Track [Page 58] RFC 7401 HIPv2 April 2015

5.2.12. HIP_MAC

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             HMAC                              |
   /                                                               /
   /                               +-------------------------------+
   |                               |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           61505
   Length         length in octets, excluding Type, Length, and
                  Padding
   HMAC           HMAC computed over the HIP packet, excluding the
                  HIP_MAC parameter and any following parameters,
                  such as HIP_SIGNATURE, HIP_SIGNATURE_2,
                  ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
                  The Checksum field MUST be set to zero, and the
                  HIP header length in the HIP common header MUST be
                  calculated not to cover any excluded parameters
                  when the HMAC is calculated.  The size of the
                  HMAC is the natural size of the hash computation
                  output depending on the used hash function.
 The HMAC uses RHASH as the hash algorithm.  The calculation and
 verification process is presented in Section 6.4.1.

5.2.13. HIP_MAC_2

 HIP_MAC_2 is a MAC of the packet and the HI of the sender in the form
 of a HOST_ID parameter when that parameter is not actually included
 in the packet.  The parameter structure is the same as the structure
 shown in Section 5.2.12.  The fields are as follows:
   Type           61569
   Length         length in octets, excluding Type, Length, and
                  Padding
   HMAC           HMAC computed over the HIP packet, excluding the
                  HIP_MAC_2 parameter and any following parameters
                  such as HIP_SIGNATURE, HIP_SIGNATURE_2,
                  ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
                  and including an additional sender's HOST_ID
                  parameter during the HMAC calculation.  The
                  Checksum field MUST be set to zero, and the HIP

Moskowitz, et al. Standards Track [Page 59] RFC 7401 HIPv2 April 2015

                  header length in the HIP common header MUST be
                  calculated not to cover any excluded parameters
                  when the HMAC is calculated.  The size of the
                  HMAC is the natural size of the hash computation
                  output depending on the used hash function.
 The HMAC uses RHASH as the hash algorithm.  The calculation and
 verification process is presented in Section 6.4.1.

5.2.14. HIP_SIGNATURE

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    SIG alg                    |            Signature          /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |             Padding           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           61697
   Length         length in octets, excluding Type, Length, and
                  Padding
   SIG alg        signature algorithm
   Signature      the signature is calculated over the HIP packet,
                  excluding the HIP_SIGNATURE parameter and any
                  parameters that follow the HIP_SIGNATURE
                  parameter.  When the signature is calculated, the
                  Checksum field MUST be set to zero, and the HIP
                  header length in the HIP common header MUST be
                  calculated only up to the beginning of the
                  HIP_SIGNATURE parameter.
 The signature algorithms are defined in Section 5.2.9.  The signature
 in the Signature field is encoded using the method depending on the
 signature algorithm (e.g., according to [RFC3110] in the case of RSA/
 SHA-1, [RFC5702] in the case of RSA/SHA-256, [RFC2536] in the case of
 DSA, or [RFC6090] in the case of ECDSA).
 HIP_SIGNATURE calculation and verification follow the process defined
 in Section 6.4.2.

Moskowitz, et al. Standards Track [Page 60] RFC 7401 HIPv2 April 2015

5.2.15. HIP_SIGNATURE_2

 HIP_SIGNATURE_2 excludes the variable parameters in the R1 packet to
 allow R1 pre-creation.  The parameter structure is the same as the
 structure shown in Section 5.2.14.  The fields are as follows:
   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 follow the process defined in
 Section 6.4.2.

5.2.16. SEQ

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Update ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type            385
   Length          4
   Update ID       32-bit sequence number
 The Update ID is an unsigned number in network byte order,
 initialized by a host to zero upon moving to ESTABLISHED state.  The
 Update ID has scope within a single HIP association, and not across

Moskowitz, et al. Standards Track [Page 61] RFC 7401 HIPv2 April 2015

 multiple associations or multiple hosts.  The Update ID is
 incremented by one before each new UPDATE that is sent by the host;
 the first UPDATE packet originated by a host has an Update ID of 0.

5.2.17. ACK

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       peer Update ID 1                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                       peer Update ID n                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type             449
   Length           length in octets, excluding Type and Length
   peer Update ID   32-bit sequence number corresponding to the
                    Update ID being ACKed
 The ACK parameter includes one or more Update IDs that have been
 received from the peer.  The number of peer Update IDs can be
 inferred from the length by dividing it by 4.

5.2.18. ENCRYPTED

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Reserved                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              IV                               /
   /                                                               /
   /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                        Encrypted data                         /
   /                                                               /
   /                               +-------------------------------+
   /                               |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type           641
   Length         length in octets, excluding Type, Length, and
                  Padding
   Reserved       zero when sent, ignored when received

Moskowitz, et al. Standards Track [Page 62] RFC 7401 HIPv2 April 2015

   IV             Initialization vector, if needed, otherwise
                  nonexistent.  The length of the IV is inferred from
                  the HIP_CIPHER.
   Encrypted      The data is encrypted using the encryption algorithm
     data         defined in the HIP_CIPHER parameter.
 The ENCRYPTED parameter encapsulates other parameters, the encrypted
 data, which holds one or more HIP parameters in block encrypted form.
 Consequently, the first fields in the encapsulated parameter(s) are
 Type and Length of the first such parameter, allowing the contents to
 be easily parsed after decryption.
 The field labeled "Encrypted data" consists of the output of one or
 more HIP parameters concatenated together that have been passed
 through an encryption algorithm.  Each of these inner parameters is
 padded according to the rules of Section 5.2.1 for padding individual
 parameters.  As a result, the concatenated parameters will be a block
 of data that is 8-byte aligned.
 Some encryption algorithms require that the data to be encrypted must
 be a multiple of the cipher algorithm block size.  In this case, the
 above block of data MUST include additional padding, as specified by
 the encryption algorithm.  The size of the extra padding is selected
 so that the length of the unencrypted data block is a multiple of the
 cipher block size.  The encryption algorithm may specify padding
 bytes other than zero; for example, AES [FIPS.197.2001] uses the
 PKCS5 padding scheme (see Section 6.1.1 of [RFC2898]) where the
 remaining n bytes to fill the block each have the value of n.  This
 yields an "unencrypted data" block that is transformed to an
 "encrypted data" block by the cipher suite.  This extra padding added
 to the set of parameters to satisfy the cipher block alignment rules
 is not counted in HIP TLV Length fields, and this extra padding
 should be removed by the cipher suite upon decryption.
 Note that the length of the cipher suite output may be smaller or
 larger than the length of the set of parameters to be encrypted,
 since the encryption process may compress the data or add additional
 padding to the data.
 Once this encryption process is completed, the Encrypted data field
 is ready for inclusion in the parameter.  If necessary, additional
 Padding for 8-byte alignment is then added according to the rules of
 Section 5.2.1.

Moskowitz, et al. Standards Track [Page 63] RFC 7401 HIPv2 April 2015

5.2.19. NOTIFICATION

 The NOTIFICATION parameter is used to transmit informational data,
 such as error conditions and state transitions, to a HIP peer.  A
 NOTIFICATION parameter may appear in NOTIFY packets.  The use of the
 NOTIFICATION parameter in other packet types is for further study.
    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
     Data           addition to the Notify Message Type.  Values
                    for this field are type specific (see below).
 Notification information can be error messages specifying why a HIP
 Security Association could not be established.  It can also be status
 data that a HIP implementation wishes to communicate with a peer
 process.  The table below lists the notification messages and their
 Notify Message Types.  HIP packets MAY contain multiple NOTIFICATION
 parameters if several problems exist or several independent pieces of
 information must be transmitted.
 To avoid certain types of attacks, a Responder SHOULD avoid sending a
 NOTIFICATION to any host with which it has not successfully verified
 a puzzle solution.
 Notify Message Types in the range 0-16383 are intended for reporting
 errors, and those in the range 16384-65535 are for other status
 information.  An implementation that receives a NOTIFY packet with a
 Notify Message Type that indicates an error in response to a request

Moskowitz, et al. Standards Track [Page 64] RFC 7401 HIPv2 April 2015

 packet (e.g., I1, I2, UPDATE) SHOULD assume that the corresponding
 request has failed entirely.  Unrecognized error types MUST be
 ignored, except that they SHOULD be logged.
 As currently defined, Notify Message Type values 1-10 are used for
 informing about errors in packet structures, and values 11-20 for
 informing about problems in parameters.
 Notification Data in NOTIFICATION parameters where the Notify Message
 Type is in the status range MUST be ignored if not recognized.
   Notify Message Types - Errors             Value
   -----------------------------             -----
   UNSUPPORTED_CRITICAL_PARAMETER_TYPE        1
     Sent if the parameter type has the "critical" bit set and the
     parameter type is not recognized.  Notification Data contains the
     two-octet parameter type.
   INVALID_SYNTAX                             7
     Indicates that the HIP message received was invalid because some
     type, length, or value was out of range or because the request
     was otherwise malformed.  To avoid a denial-of-service
     attack using forged messages, this status may only be returned
     for packets whose HIP_MAC (if present) and SIGNATURE have been
     verified.  This status MUST be sent in response to any error not
     covered by one of the other status types and SHOULD NOT contain
     details, to avoid leaking information to someone probing a node.
     To aid debugging, more detailed error information SHOULD be
     written to a console or log.
   NO_DH_PROPOSAL_CHOSEN                     14
     None of the proposed Group IDs were acceptable.
   INVALID_DH_CHOSEN                         15
     The DH Group ID field does not correspond to one offered
     by the Responder.
   NO_HIP_PROPOSAL_CHOSEN                    16
     None of the proposed HIT Suites or HIP Encryption Algorithms were
     acceptable.

Moskowitz, et al. Standards Track [Page 65] RFC 7401 HIPv2 April 2015

   INVALID_HIP_CIPHER_CHOSEN                 17
     The HIP_CIPHER Crypto ID does not correspond to one offered by
     the Responder.
   UNSUPPORTED_HIT_SUITE                     20
     Sent in response to an I1 or R1 packet for which the HIT Suite
     is not supported.
   AUTHENTICATION_FAILED                     24
     Sent in response to a HIP signature failure, except when
     the signature verification fails in a NOTIFY message.
   CHECKSUM_FAILED                           26
     Sent in response to a HIP checksum failure.
   HIP_MAC_FAILED                            28
     Sent in response to a HIP HMAC failure.
   ENCRYPTION_FAILED                         32
     The Responder could not successfully decrypt the
     ENCRYPTED parameter.
   INVALID_HIT                               40
     Sent in response to a failure to validate the peer's
     HIT from the corresponding HI.
   BLOCKED_BY_POLICY                         42
     The Responder is unwilling to set up an association
     for some policy reason (e.g., the received HIT is NULL
     and the policy does not allow opportunistic mode).
   RESPONDER_BUSY_PLEASE_RETRY               44
     The Responder is unwilling to set up an association, as it is
     suffering under some kind of overload and has chosen to shed load
     by rejecting the Initiator's request.  The Initiator may retry;
     however, the Initiator MUST find another (different) puzzle
     solution for any such retries.  Note that the Initiator may need
     to obtain a new puzzle with a new I1/R1 exchange.

Moskowitz, et al. Standards Track [Page 66] RFC 7401 HIPv2 April 2015

   Notify Message Types - Status            Value
   -----------------------------            -----
   I2_ACKNOWLEDGEMENT                       16384
     The Responder has an I2 packet from the Initiator but had to
     queue the I2 packet for processing.  The puzzle was correctly
     solved, and the Responder is willing to set up an association but
     currently has a number of I2 packets in the processing queue.
     The R2 packet is sent after the I2 packet was processed.

5.2.20. ECHO_REQUEST_SIGNED

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Opaque data (variable length)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type          897
   Length        length of the opaque data in octets
   Opaque data   opaque data, supposed to be meaningful only to
                 the node that sends ECHO_REQUEST_SIGNED and
                 receives a corresponding ECHO_RESPONSE_SIGNED or
                 ECHO_RESPONSE_UNSIGNED
 The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
 that the sender wants to get echoed back in the corresponding reply
 packet.
 The ECHO_REQUEST_SIGNED and corresponding echo response parameters
 MAY be used for any purpose where a node wants to carry some state in
 a request packet and get it back in a response packet.  The
 ECHO_REQUEST_SIGNED is covered by the HIP_MAC and SIGNATURE.  A HIP
 packet can contain only one ECHO_REQUEST_SIGNED parameter and MAY
 contain multiple ECHO_REQUEST_UNSIGNED parameters.  The
 ECHO_REQUEST_SIGNED parameter MUST be responded to with an
 ECHO_RESPONSE_SIGNED.

Moskowitz, et al. Standards Track [Page 67] RFC 7401 HIPv2 April 2015

5.2.21. ECHO_REQUEST_UNSIGNED

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Opaque data (variable length)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type          63661
   Length        length of the opaque data in octets
   Opaque data   opaque data, supposed to be meaningful only to
                 the node that sends ECHO_REQUEST_UNSIGNED and
                 receives a corresponding ECHO_RESPONSE_UNSIGNED
 The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
 that the sender wants to get echoed back in the corresponding reply
 packet.
 The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
 MAY be used for any purpose where a node wants to carry some state in
 a request packet and get it back in a response packet.  The
 ECHO_REQUEST_UNSIGNED is not covered by the HIP_MAC and SIGNATURE.  A
 HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
 It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
 in HIP packets passing by.  The creator of the ECHO_REQUEST_UNSIGNED
 (end host or middlebox) has to create the Opaque field so that it can
 later identify and remove the corresponding ECHO_RESPONSE_UNSIGNED
 parameter.
 The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
 ECHO_RESPONSE_UNSIGNED parameter.

Moskowitz, et al. Standards Track [Page 68] RFC 7401 HIPv2 April 2015

5.2.22. ECHO_RESPONSE_SIGNED

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

5.2.23. ECHO_RESPONSE_UNSIGNED

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type              |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Opaque data (variable length)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Type          63425
   Length        length of the opaque data in octets
   Opaque data   opaque data, copied unmodified from the
                 ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
                 parameter that triggered this response
 The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data
 that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
 wants to get echoed back.  The opaque data is copied unmodified from
 the corresponding echo request parameter.

Moskowitz, et al. Standards Track [Page 69] RFC 7401 HIPv2 April 2015

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

5.3. HIP Packets

 There are eight basic HIP packets (see Table 11).  Four are for the
 HIP base exchange, one is for updating, one is for sending
 notifications, and two are for closing a HIP association.  Support
 for the NOTIFY packet type is optional, but support for all other HIP
 packet types listed below is mandatory.
 +------------------+------------------------------------------------+
 |   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 11: HIP Packets and Packet Type Values
 Packets consist of the fixed header as described in Section 5.1,
 followed by the parameters.  The parameter part, in turn, consists of
 zero or more TLV-coded parameters.
 In addition to the base packets, other packet types may be defined
 later in separate specifications.  For example, support for mobility
 and multihoming is not included in this specification.
 See "Notation" (Section 2.2) for the notation used in the operations.

Moskowitz, et al. Standards Track [Page 70] RFC 7401 HIPv2 April 2015

 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 into multiple extension headers by setting the
 Next Header field in a HIP header to the HIP protocol number.  This
 limits the size of the possible additional data in the packet.

5.3.1. I1 - the HIP Initiator Packet

 The HIP header values for the I1 packet:
   Header:
     Packet Type = 1
     SRC HIT = Initiator's HIT
     DST HIT = Responder's HIT, or NULL
   IP ( HIP ( DH_GROUP_LIST ) )
 The I1 packet contains the fixed HIP header and the Initiator's
 DH_GROUP_LIST.
 Valid control bits: None
 The Initiator receives the Responder's HIT from either a DNS lookup
 of the Responder's FQDN (see [HIP-DNS-EXT]), some other repository,
 or a local table.  If the Initiator does not know the Responder's
 HIT, it may attempt to use opportunistic mode by using NULL (all
 zeros) as the Responder's HIT.  See also "HIP Opportunistic Mode"
 (Section 4.1.8).
 Since the I1 packet is so easy to spoof even if it were signed, no
 attempt is made to add to its generation or processing cost.
 The Initiator includes a DH_GROUP_LIST parameter in the I1 packet to
 inform the Responder of its preferred DH Group IDs.  Note that the
 DH_GROUP_LIST in the I1 packet is not protected by a signature.
 Implementations MUST be able to handle a storm of received I1
 packets, discarding those with common content that arrive within a
 small time delta.

Moskowitz, et al. Standards Track [Page 71] RFC 7401 HIPv2 April 2015

5.3.2. R1 - the HIP Responder Packet

 The HIP header values for the R1 packet:
   Header:
     Packet Type = 2
     SRC HIT = Responder's HIT
     DST HIT = Initiator's HIT
   IP ( HIP ( [ R1_COUNTER, ]
              PUZZLE,
              DIFFIE_HELLMAN,
              HIP_CIPHER,
              HOST_ID,
              HIT_SUITE_LIST,
              DH_GROUP_LIST,
              [ ECHO_REQUEST_SIGNED, ]
              TRANSPORT_FORMAT_LIST,
              HIP_SIGNATURE_2 )
              <, ECHO_REQUEST_UNSIGNED >i)
 Valid control bits: A
 If the Responder's HI is an anonymous one, the A control MUST be set.
 The Initiator's HIT MUST match the one received in the I1 packet if
 the R1 is a response to an I1.  If the Responder has multiple HIs,
 the Responder's HIT used MUST match the Initiator's request.  If the
 Initiator used opportunistic mode, the Responder may select freely
 among its HIs.  See also "HIP Opportunistic Mode" (Section 4.1.8).
 The R1 packet generation counter is used to determine the currently
 valid generation of puzzles.  The value is increased periodically,
 and it is RECOMMENDED that it is increased at least as often as
 solutions to old puzzles are no longer accepted.
 The puzzle contains a Random #I and the difficulty #K.  The
 difficulty #K indicates the number of lower-order bits, in the puzzle
 hash result, that must be zeros; see Section 4.1.2.  The Random #I is
 not covered by the signature and must be zeroed during the signature
 calculation, allowing the sender to select and set the #I into a
 precomputed R1 packet just prior to sending it to the peer.
 The Responder selects the DIFFIE_HELLMAN Group ID and Public Value
 based on the Initiator's preference expressed in the DH_GROUP_LIST
 parameter in the I1 packet.  The Responder sends back its own
 preference based on which it chose the DH public value as

Moskowitz, et al. Standards Track [Page 72] RFC 7401 HIPv2 April 2015

 DH_GROUP_LIST.  This allows the Initiator to determine whether its
 own DH_GROUP_LIST in the sent I1 packet was manipulated by an
 attacker.
 The Diffie-Hellman public value is ephemeral, and values SHOULD NOT
 be reused across different HIP associations.  Once the Responder has
 received a valid response to an R1 packet, that Diffie-Hellman value
 SHOULD be deprecated.  It is possible that the Responder has sent the
 same Diffie-Hellman value to different hosts simultaneously in
 corresponding R1 packets, and those responses should also be
 accepted.  However, as a defense against I1 packet storms, an
 implementation MAY propose, and reuse unless avoidable, the same
 Diffie-Hellman value for a period of time -- for example, 15 minutes.
 By using a small number of different puzzles for a given
 Diffie-Hellman value, the R1 packets can be precomputed and delivered
 as quickly as I1 packets arrive.  A scavenger process should clean up
 unused Diffie-Hellman values and puzzles.
 Reusing Diffie-Hellman public values opens up the potential security
 risk of more than one Initiator ending up with the same keying
 material (due to faulty random number generators).  Also, more than
 one Initiator using the same Responder public key half may lead to
 potentially easier cryptographic attacks and to imperfect forward
 security.
 However, these risks involved in reusing the same public value 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
 reuse of any given Responder Diffie-Hellman public key would differ
 from the base probability.  Consequently, it is RECOMMENDED that
 Responders avoid reusing the same DH key with multiple Initiators,
 but because the risk is considered statistical and not known to be
 manipulable, the implementations MAY reuse a key in order to ease
 resource-constrained implementations and to increase the probability
 of successful communication with legitimate clients even under an I1
 packet storm.  In particular, when it is too expensive to generate
 enough precomputed R1 packets to supply each potential Initiator with
 a different DH key, the Responder MAY send the same DH key to several
 Initiators, thereby creating the possibility of multiple legitimate
 Initiators ending up using the same Responder-side public key.
 However, as soon as the Responder knows that it will use a particular
 DH key, it SHOULD stop offering it.  This design is aimed to allow
 resource-constrained Responders to offer services under I1 packet
 storms and to simultaneously make the probability of DH key reuse
 both statistical and as low as possible.

Moskowitz, et al. Standards Track [Page 73] RFC 7401 HIPv2 April 2015

 If the Responder uses the same DH key pair for multiple handshakes,
 it must take care to avoid small subgroup attacks [RFC2785].  To
 avoid these attacks, when receiving the I2 message, the Responder
 SHOULD validate the Initiator's DH public key as described in
 [RFC2785], Section 3.1.  If the validation fails, the Responder MUST
 NOT generate a DH shared key and MUST silently abort the HIP BEX.
 The HIP_CIPHER parameter contains the encryption algorithms supported
 by the Responder to encrypt the contents of the ENCRYPTED parameter,
 in the order of preference.  All implementations MUST support AES
 [RFC3602].
 The HIT_SUITE_LIST parameter is an ordered list of the Responder's
 preferred and supported HIT Suites.  The list allows the Initiator to
 determine whether its own source HIT matches any suite supported by
 the Responder.
 The ECHO_REQUEST_SIGNED and ECHO_REQUEST_UNSIGNED parameters contain
 data that the sender wants to receive unmodified in the corresponding
 response packet in the ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED
 parameter.  The R1 packet may contain zero or more
 ECHO_REQUEST_UNSIGNED parameters as described in Section 5.2.21.
 The TRANSPORT_FORMAT_LIST parameter is an ordered list of the
 Responder's preferred and supported transport format types.  The list
 allows the Initiator and the Responder to agree on a common type for
 payload protection.  This parameter is described in Section 5.2.11.
 The signature is calculated over the whole HIP packet as described in
 Section 5.2.15.  This allows the Responder to use precomputed R1s.
 The Initiator SHOULD validate this signature.  It MUST check that the
 Responder's HI matches with the one expected, if any.

Moskowitz, et al. Standards Track [Page 74] RFC 7401 HIPv2 April 2015

5.3.3. I2 - the Second HIP Initiator Packet

 The HIP header values for the I2 packet:
   Header:
     Packet Type = 3
     SRC HIT = Initiator's HIT
     DST HIT = Responder's HIT
   IP ( HIP ( [R1_COUNTER,]
              SOLUTION,
              DIFFIE_HELLMAN,
              HIP_CIPHER,
              ENCRYPTED { HOST_ID } or HOST_ID,
              [ ECHO_RESPONSE_SIGNED, ]
              TRANSPORT_FORMAT_LIST,
              HIP_MAC,
              HIP_SIGNATURE
              <, ECHO_RESPONSE_UNSIGNED>i ) )
 Valid control bits: A
 The HITs used MUST match the ones used in the R1.
 If the Initiator's HI is an anonymous one, the A control bit MUST
 be set.
 If present in the I1 packet, the Initiator MUST include an unmodified
 copy of the R1_COUNTER parameter received in the corresponding R1
 packet into the I2 packet.
 The Solution contains the Random #I from R1 and the computed #J.  The
 low-order #K bits of the RHASH( #I | ... | #J ) MUST be zero.
 The Diffie-Hellman value is ephemeral.  If precomputed, a scavenger
 process should clean up unused Diffie-Hellman values.  The Responder
 MAY reuse Diffie-Hellman values under some conditions as specified in
 Section 5.3.2.
 The HIP_CIPHER contains the single encryption suite selected by the
 Initiator, that it uses to encrypt the ENCRYPTED parameters.  The
 chosen cipher MUST correspond to one of the ciphers offered by the
 Responder in the R1.  All implementations MUST support AES [RFC3602].
 The Initiator's HI MAY be encrypted using the HIP_CIPHER encryption
 algorithm.  The keying material is derived from the Diffie-Hellman
 exchange as defined in Section 6.5.

Moskowitz, et al. Standards Track [Page 75] RFC 7401 HIPv2 April 2015

 The ECHO_RESPONSE_SIGNED and ECHO_RESPONSE_UNSIGNED contain the
 unmodified opaque data copied from the corresponding echo request
 parameter(s).
 The TRANSPORT_FORMAT_LIST contains the single transport format type
 selected by the Initiator.  The chosen type MUST correspond to one of
 the types offered by the Responder in the R1.  Currently, the only
 transport format defined is the ESP transport format ([RFC7402]).
 The HMAC value in the HIP_MAC parameter is calculated over the whole
 HIP packet, excluding any parameters after the HIP_MAC, as described
 in Section 6.4.1.  The Responder MUST validate the HIP_MAC.
 The signature is calculated over the whole HIP packet, excluding any
 parameters after the HIP_SIGNATURE, as described in Section 5.2.14.
 The Responder MUST validate this signature.  The Responder uses the
 HI in the packet or an HI acquired by some other means for verifying
 the signature.

5.3.4. R2 - the Second HIP Responder Packet

 The HIP header values for the R2 packet:
   Header:
     Packet Type = 4
     SRC HIT = Responder's HIT
     DST HIT = Initiator's HIT
   IP ( HIP ( HIP_MAC_2, HIP_SIGNATURE ) )
 Valid control bits: None
 The HIP_MAC_2 is calculated over the whole HIP packet, with the
 Responder's HOST_ID parameter concatenated with the HIP packet.  The
 HOST_ID parameter is removed after the HMAC calculation.  The
 procedure is described in Section 6.4.1.
 The signature is calculated over the whole HIP packet.
 The Initiator MUST validate both the HIP_MAC and the signature.

Moskowitz, et al. Standards Track [Page 76] RFC 7401 HIPv2 April 2015

5.3.5. UPDATE - the HIP Update Packet

 The HIP header values for the UPDATE packet:
   Header:
     Packet Type = 16
     SRC HIT = Sender's HIT
     DST HIT = Recipient's HIT
   IP ( HIP ( [SEQ, ACK, ] HIP_MAC, HIP_SIGNATURE ) )
 Valid control bits: None
 The UPDATE packet contains mandatory HIP_MAC and HIP_SIGNATURE
 parameters, and other optional parameters.
 The UPDATE packet contains zero or one SEQ parameter.  The presence
 of a SEQ parameter indicates that the receiver MUST acknowledge the
 UPDATE.  An UPDATE that does not contain a SEQ but only an ACK
 parameter is simply an acknowledgment of a previous UPDATE and itself
 MUST NOT be acknowledged by a separate ACK parameter.  Such UPDATE
 packets containing only an ACK parameter do not require processing in
 relative order to other UPDATE packets.  An UPDATE packet without
 either a SEQ or an ACK parameter is invalid; such unacknowledged
 updates MUST instead use a NOTIFY packet.
 An UPDATE packet contains zero or one ACK parameter.  The ACK
 parameter echoes the SEQ sequence number of the UPDATE packet being
 ACKed.  A host MAY choose to acknowledge more than one UPDATE packet
 at a time; e.g., the ACK parameter may contain the last two SEQ
 values received, for resilience against packet loss.  ACK values are
 not cumulative; each received unique SEQ value requires at least one
 corresponding ACK value in reply.  Received ACK parameters that are
 redundant are ignored.  Hosts MUST implement the processing of ACK
 parameters with multiple SEQ sequence numbers even if they do not
 implement sending ACK parameters with multiple SEQ sequence numbers.
 The UPDATE packet may contain both a SEQ and an ACK parameter.  In
 this case, the ACK parameter 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 an ACK parameter for a short period of time
 to allow for the possibility of piggybacking the ACK parameter, in a
 manner similar to TCP delayed acknowledgments.

Moskowitz, et al. Standards Track [Page 77] RFC 7401 HIPv2 April 2015

 A sender MAY choose to forego reliable transmission of a particular
 UPDATE (e.g., it becomes overcome by events).  The semantics are such
 that the receiver MUST acknowledge the UPDATE, but the sender MAY
 choose to not care about receiving the ACK parameter.
 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 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 (see Section 4.4.2) based purely on a received
 NOTIFY packet.
 The HIP header values for the NOTIFY packet:
   Header:
     Packet Type = 17
     SRC HIT = Sender's HIT
     DST HIT = Recipient's HIT, or zero if unknown
   IP ( HIP (<NOTIFICATION>i, [HOST_ID, ] HIP_SIGNATURE) )
 Valid control bits: None
 The NOTIFY packet is used to carry one or more NOTIFICATION
 parameters.

5.3.7. CLOSE - the HIP Association Closing Packet

 The HIP header values for the CLOSE packet:
   Header:
     Packet Type = 18
     SRC HIT = Sender's HIT
     DST HIT = Recipient's HIT
   IP ( HIP ( ECHO_REQUEST_SIGNED, HIP_MAC, HIP_SIGNATURE ) )
 Valid control bits: None

Moskowitz, et al. Standards Track [Page 78] RFC 7401 HIPv2 April 2015

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

5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet

 The HIP header values for the CLOSE_ACK packet:
   Header:
     Packet Type = 19
     SRC HIT = Sender's HIT
     DST HIT = Recipient's HIT
   IP ( HIP ( ECHO_RESPONSE_SIGNED, HIP_MAC, HIP_SIGNATURE ) )
 Valid control bits: None
 The sender MUST include both an HMAC and signature (calculated over
 the whole HIP packet).
 The receiver peer MUST validate the ECHO_RESPONSE_SIGNED and validate
 both the HIP_MAC and the signature if the receiver has state for a
 HIP association.

5.4. ICMP Messages

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

5.4.1. Invalid Version

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

Moskowitz, et al. Standards Track [Page 79] RFC 7401 HIPv2 April 2015

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, with 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, with 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 bytes from the I2
 message so that the SOLUTION parameter fits into the ICMP message,
 with the Pointer pointing to the beginning of the Puzzle solution #J
 field, as in the IPv6 case.  Note, however, that the resulting ICMPv4
 message exceeds the typical ICMPv4 message size as defined in
 [RFC0792].

5.4.4. Non-existing HIP Association

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

6. Packet Processing

 Each host is assumed to have a single HIP 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

Moskowitz, et al. Standards Track [Page 80] RFC 7401 HIPv2 April 2015

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

6.1. Processing Outgoing Application Data

 In a HIP host, an application can send application-level data using
 an identifier specified via the underlying API.  The API can be a
 backwards-compatible API (see [RFC5338]), using identifiers that look
 similar to IP addresses, or a completely new API, providing enhanced
 services related to Host Identities.  Depending on the HIP
 implementation, the identifier provided to the application may be
 different; for example, it can be a HIT or an IP address.
 The exact format and method for transferring the user data from the
 source HIP host to the destination HIP host are 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 multihoming case is specified separately.
 The following conceptual algorithm describes the steps that are
 required for handling outgoing datagrams destined to a HIT.
 1.  If the datagram has a specified source address, it MUST be a HIT.
     If it is not, the implementation MAY replace the source address
     with a HIT.  Otherwise, it MUST drop the packet.
 2.  If the datagram has an unspecified source address, the
     implementation MUST choose a suitable source HIT for the
     datagram.  Selecting the source HIT is subject to local policy.

Moskowitz, et al. Standards Track [Page 81] RFC 7401 HIPv2 April 2015

 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 user data 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
     [RFC6724] SHOULD be followed.  Note that this HIT-to-IP-address
     conversion step MAY also be performed at some other point in the
     stack, e.g., before wrapping the packet into the output format.

6.2. Processing Incoming Application Data

 The following conceptual algorithm describes the incoming datagram
 handling when HITs are used at the receiving host as application-
 level identifiers.  More detailed steps for processing packets are
 defined in corresponding transport format documents.
 1.  The incoming datagram is mapped to an existing HIP association,
     typically using some information from the packet.  For example,
     such mapping may be based on the ESP Security Parameter Index
     (SPI).
 2.  The specific transport format is unwrapped, in a way depending on
     the transport format, yielding a packet that looks like a
     standard (unencrypted) IP packet.  If possible, this step SHOULD
     also verify that the packet was indeed (once) sent by the remote
     HIP host, as identified by the HIP association.
     Depending on the used transport mode, the verification method can
     vary.  While the HI (as well as the HIT) is used as the higher-
     layer identifier, the verification method has to verify that the
     data packet was sent by the correct node identity and that the
     actual identity maps to this particular HIT.  When using the ESP
     transport format [RFC7402], 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.

Moskowitz, et al. Standards Track [Page 82] RFC 7401 HIPv2 April 2015

 3.  The IP addresses in the datagram are replaced with the HITs
     associated with the HIP association.  Note that this IP-address-
     to-HIT conversion step MAY also be performed at some other point
     in the stack.
 4.  The datagram is delivered to the upper layer (e.g., UDP or TCP).
     When demultiplexing the datagram, the right upper-layer socket is
     selected based on the HITs.

6.3. Solving the Puzzle

 This subsection describes the details for solving the puzzle.
 In the R1 packet, the values #I and #K are sent in network byte
 order.  Similarly, in the I2 packet, the values #I and #J are sent in
 network byte order.  The hash is created by concatenating, in network
 byte order, the following data, in the following order and using the
 RHASH algorithm:
    n-bit random value #I (where n is RHASH_len), in network byte
    order, as appearing in the R1 and I2 packets.
    128-bit Initiator's HIT, in network byte order, as appearing in
    the HIP Payload in the R1 and I2 packets.
    128-bit Responder's HIT, in network byte order, as appearing in
    the HIP Payload in the R1 and I2 packets.
    n-bit random value #J (where n is RHASH_len), in network byte
    order, as appearing in the I2 packet.
 In a valid response puzzle, the #K low-order bits of the resulting
 RHASH digest MUST be zero.
 Notes:
      i) The length of the data to be hashed is variable, depending on
         the output length of the Responder's hash function RHASH.
     ii) All the data in the hash input MUST be in network byte order.
    iii) The orderings of the Initiator's and Responder's HITs are
         different in the R1 and I2 packets; see Section 5.1.  Care
         must be taken to copy the values in the right order to the
         hash input.
     iv) For a puzzle #I, there may exist multiple valid puzzle
         solutions #J.

Moskowitz, et al. Standards Track [Page 83] RFC 7401 HIPv2 April 2015

 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
    Accepts if V == 0

6.4. HIP_MAC and SIGNATURE Calculation and Verification

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

6.4.1. HMAC Calculation

 The HMAC uses RHASH as the underlying hash function.  The type of
 RHASH depends on the HIT Suite of the Responder.  Hence, HMAC-SHA-256
 [RFC4868] is used for HIT Suite RSA/DSA/SHA-256, HMAC-SHA-1 [RFC2404]
 is used for HIT Suite ECDSA_LOW/SHA-1, and HMAC-SHA-384 [RFC4868] is
 used for HIT Suite ECDSA/SHA-384.
 The following process applies both to the HIP_MAC and HIP_MAC_2
 parameters.  When processing HIP_MAC_2, the difference is that the
 HIP_MAC calculation includes a pseudo HOST_ID field containing the
 Responder's information as sent in the R1 packet earlier.

Moskowitz, et al. Standards Track [Page 84] RFC 7401 HIPv2 April 2015

 Both the Initiator and the Responder should take some care when
 verifying or calculating the HIP_MAC_2.  Specifically, the Initiator
 has to preserve the HOST_ID exactly as it was received in the R1
 packet until it receives the HIP_MAC_2 in the R2 packet.
 The scope of the calculation for HIP_MAC is as follows:
 HMAC: { HIP header | [ Parameters ] }
 where Parameters include all of the packet's HIP parameters with type
 values ranging from 1 to (HIP_MAC's type value - 1), and excluding
 those parameters with type values greater than or equal to HIP_MAC's
 type value.
 During HIP_MAC 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_MAC parameter.
 Parameter order is described in Section 5.2.1.
 The scope of the calculation for HIP_MAC_2 is as follows:
 HIP_MAC_2: { HIP header | [ Parameters ] | HOST_ID }
 where Parameters include all of the packet's HIP parameters with type
 values from 1 to (HIP_MAC_2's type value - 1), and excluding those
 parameters with type values greater than or equal to HIP_MAC_2's type
 value.
 During HIP_MAC_2 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_MAC_2 parameter and increased by the
    length of the concatenated HOST_ID parameter length (including the
    Type and Length fields).
 o  The 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.

Moskowitz, et al. Standards Track [Page 85] RFC 7401 HIPv2 April 2015

 The HIP_MAC parameter is defined in Section 5.2.12 and the HIP_MAC_2
 parameter in Section 5.2.13.  The HMAC calculation and verification
 process (the process applies both to HIP_MAC and HIP_MAC_2, except
 where HIP_MAC_2 is mentioned separately) is as follows:
 Packet sender:
 1.  Create the HIP packet, without the HIP_MAC, HIP_SIGNATURE,
     HIP_SIGNATURE_2, or any other parameter with greater type value
     than the HIP_MAC parameter has.
 2.  In case of HIP_MAC_2 calculation, add a HOST_ID (Responder)
     parameter to the end of the packet.
 3.  Calculate the Header Length field in the HIP header, including
     the added HOST_ID parameter in case of HIP_MAC_2.
 4.  Compute the HMAC using either the HIP-gl or HIP-lg integrity key
     retrieved from KEYMAT as defined in Section 6.5.
 5.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
     packet.
 6.  Add the HIP_MAC parameter to the packet and any parameter with
     greater type value than the HIP_MAC's (HIP_MAC_2's) that may
     follow, including possible HIP_SIGNATURE or HIP_SIGNATURE_2
     parameters.
 7.  Recalculate the Length field in the HIP header.
 Packet receiver:
 1.  Verify the HIP Header Length field.
 2.  Remove the HIP_MAC or HIP_MAC_2 parameter, as well as all other
     parameters that follow it with greater type value including
     possible HIP_SIGNATURE or HIP_SIGNATURE_2 fields, saving the
     contents if they are needed later.
 3.  In case of HIP_MAC_2, build and add a HOST_ID parameter (with
     Responder information) to the packet.  The HOST_ID parameter
     should be identical to the one previously received from the
     Responder.
 4.  Recalculate the HIP packet length in the HIP header and clear the
     Checksum field (set it to all zeros).  In case of HIP_MAC_2, the
     length is calculated with the added HOST_ID parameter.

Moskowitz, et al. Standards Track [Page 86] RFC 7401 HIPv2 April 2015

 5.  Compute the HMAC using either the HIP-gl or HIP-lg integrity key
     as defined in Section 6.5 and verify it against the received
     HMAC.
 6.  Set the Checksum and Header Length fields in the HIP header to
     original values.  Note that the Checksum and Length fields
     contain incorrect values after this step.
 7.  In case of HIP_MAC_2, remove the HOST_ID parameter from the
     packet before further processing.

6.4.2. Signature Calculation

 The following process applies both to the HIP_SIGNATURE and
 HIP_SIGNATURE_2 parameters.  When processing the HIP_SIGNATURE_2
 parameter, the only difference is that instead of the HIP_SIGNATURE
 parameter, the HIP_SIGNATURE_2 parameter is used, and the Initiator's
 HIT and PUZZLE Opaque and Random #I fields are cleared (set to all
 zeros) before computing the signature.  The HIP_SIGNATURE parameter
 is defined in Section 5.2.14 and the HIP_SIGNATURE_2 parameter in
 Section 5.2.15.
 The scope of the calculation for HIP_SIGNATURE and HIP_SIGNATURE_2 is
 as follows:
 HIP_SIGNATURE: { HIP header | [ Parameters ] }
 where Parameters include all of the packet's HIP parameters with type
 values from 1 to (HIP_SIGNATURE's type value - 1).
 During signature calculation, the following apply:
 o  In the HIP header, the Checksum field is set to zero.
 o  In the HIP header, the Header Length field value is calculated to
    the beginning of the HIP_SIGNATURE parameter.
 Parameter order is described in Section 5.2.1.
 HIP_SIGNATURE_2: { HIP header | [ Parameters ] }
 where Parameters include all of the packet's HIP parameters with type
 values ranging from 1 to (HIP_SIGNATURE_2's type value - 1).

Moskowitz, et al. Standards Track [Page 87] RFC 7401 HIPv2 April 2015

 During signature calculation, the following apply:
 o  In the HIP header, both the Checksum and the Receiver's HIT 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  The PUZZLE parameter's Opaque and Random #I fields are set to
    zero.
 Parameter order is described in Section 5.2.1.
 The signature calculation and verification process (the process
 applies both to HIP_SIGNATURE and HIP_SIGNATURE_2, except in the case
 where HIP_SIGNATURE_2 is separately mentioned) is as follows:
 Packet sender:
 1.  Create the HIP packet without the HIP_SIGNATURE parameter or any
     other parameters that follow the HIP_SIGNATURE parameter.
 2.  Calculate the Length field and zero the Checksum field in the HIP
     header.  In case of HIP_SIGNATURE_2, set the Initiator's HIT
     field in the HIP header as well as the PUZZLE parameter's Opaque
     and Random #I fields to zero.
 3.  Compute the signature using the private key corresponding to the
     Host Identifier (public key).
 4.  Add the HIP_SIGNATURE parameter to the packet.
 5.  Add any parameters that follow the HIP_SIGNATURE parameter.
 6.  Recalculate the Length field in the HIP header, and calculate the
     Checksum field.
 Packet receiver:
 1.  Verify the HIP Header Length field and checksum.
 2.  Save the contents of the HIP_SIGNATURE parameter and any other
     parameters following the HIP_SIGNATURE parameter, and remove them
     from the packet.

Moskowitz, et al. Standards Track [Page 88] RFC 7401 HIPv2 April 2015

 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 the Initiator's HIT field in the HIP header
     as well as the PUZZLE parameter's Opaque and Random #I fields
     to zero.
 4.  Compute the signature and verify it against the received
     signature using the packet sender's Host Identity (public key).
 5.  Restore the original packet by adding removed parameters (in
     step 2) and resetting the values that were set to zero (in
     step 3).
 The verification can use either the HI received from a HIP packet;
 the HI retrieved from a DNS query, if the FQDN has been received in
 the HOST_ID parameter; or an HI 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 (see Section 4.1.3).  The
 Initiator has Kij during the creation of the I2 packet, and the
 Responder has Kij once it receives the I2 packet.  This is why I2 can
 already contain encrypted information.
 The KEYMAT is derived by feeding Kij into the key derivation function
 defined by the DH Group ID.  Currently, the only key derivation
 function defined in this document is the Hash-based Key Derivation
 Function (HKDF) [RFC5869] using the RHASH hash function.  Other
 documents may define new DH Group IDs and corresponding key
 distribution functions.
 In the following, we provide the details for deriving the keying
 material using HKDF.
 where
 info    = sort(HIT-I | HIT-R)
 salt    =  #I | #J
 Sort(HIT-I | HIT-R) is defined as the network byte order
 concatenation of the two HITs, with the smaller HIT preceding the
 larger HIT, resulting from the numeric comparison of the two HITs
 interpreted as positive (unsigned) 128-bit integers in network byte
 order.  The #I and #J values are from the puzzle and its solution
 that were exchanged in R1 and I2 messages when this HIP association
 was set up.  Both hosts have to store #I and #J values for the HIP
 association for future use.

Moskowitz, et al. Standards Track [Page 89] RFC 7401 HIPv2 April 2015

 The initial keys are drawn sequentially in the order that is
 determined by the numeric comparison of the two HITs, with the
 comparison method described in the previous paragraph.  HOST_g
 denotes the host with the greater HIT value, and HOST_l the host with
 the lower HIT value.
 The drawing order for the four initial keys is as follows:
    HIP-gl encryption key for HOST_g's ENCRYPTED parameter
    HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets
    HIP-lg encryption key for HOST_l's ENCRYPTED parameter
    HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets
 The number of bits drawn for a given algorithm is the "natural" size
 of the keys.  For the mandatory algorithms, the following sizes
 apply:
    AES       128 or 256 bits
    SHA-1     160 bits
    SHA-256   256 bits
    SHA-384   384 bits
    NULL      0 bits
 If other key sizes are used, they MUST be treated as different
 encryption algorithms and defined separately.

6.6. Initiation of a HIP Base Exchange

 An implementation may originate a HIP base exchange to another host
 based on a local policy decision, usually triggered by an application
 datagram, in much the same way that an IPsec IKE key exchange can
 dynamically create a Security Association.  Alternatively, a system
 may initiate a HIP exchange if it has rebooted or timed out, or
 otherwise lost its HIP state, as described in Section 4.5.4.
 The implementation prepares an I1 packet and sends it to the IP
 address that corresponds to the peer host.  The IP address of the
 peer host may be obtained via conventional mechanisms, such as DNS
 lookup.  The I1 packet contents are specified in Section 5.3.1.  The

Moskowitz, et al. Standards Track [Page 90] RFC 7401 HIPv2 April 2015

 selection of which source or destination Host Identity to use, if an
 Initiator or Responder has more than one to choose from, is typically
 a policy decision.
 The following steps define the conceptual processing rules for
 initiating a HIP base exchange:
 1.  The Initiator receives one or more of the Responder's HITs and
     one or more addresses from either a DNS lookup of the Responder's
     FQDN, some other repository, or a local database.  If the
     Initiator does not know the Responder's HIT, it may attempt
     opportunistic mode by using NULL (all zeros) as the Responder's
     HIT (see also "HIP Opportunistic Mode" (Section 4.1.8)).  If the
     Initiator can choose from multiple Responder HITs, it selects a
     HIT for which the Initiator supports the HIT Suite.
 2.  The Initiator sends an I1 packet to one of the Responder's
     addresses.  The selection of which address to use is a local
     policy decision.
 3.  The Initiator includes the DH_GROUP_LIST in the I1 packet.  The
     selection and order of DH Group IDs in the DH_GROUP_LIST MUST be
     stored by the Initiator, because this list is needed for later R1
     processing.  In most cases, the preferences regarding the DH
     groups will be static, so no per-association storage is
     necessary.
 4.  Upon sending an I1 packet, the sender transitions to state
     I1-SENT and starts a timer for which the timeout value SHOULD be
     larger than the worst-case anticipated RTT.  The sender SHOULD
     also increment the trial counter associated with the I1.
 5.  Upon timeout, the sender SHOULD retransmit the I1 packet and
     restart the timer, up to a maximum of I1_RETRIES_MAX tries.

6.6.1. Sending Multiple I1 Packets in Parallel

 For the sake of minimizing the association establishment latency, an
 implementation MAY send the same I1 packet to more than one of the
 Responder's addresses.  However, it MUST NOT send to more than three
 (3) Responder addresses in parallel.  Furthermore, upon timeout, the
 implementation MUST refrain from sending the same I1 packet to
 multiple addresses.  That is, if it retries to initialize the
 connection after a timeout, it MUST NOT send the I1 packet to more
 than one destination address.  These limitations are placed in order
 to avoid congestion of the network, and potential DoS attacks that

Moskowitz, et al. Standards Track [Page 91] RFC 7401 HIPv2 April 2015

 might occur, e.g., because someone's claim to have hundreds or
 thousands of addresses could generate a huge number of I1 packets
 from the Initiator.
 As the Responder is not guaranteed to distinguish the duplicate I1
 packets it receives at several of its addresses (because it avoids
 storing states when it answers back an R1 packet), the Initiator may
 receive several duplicate R1 packets.
 The Initiator SHOULD then select the initial preferred destination
 address using the source address of the selected received R1, and use
 the preferred address as a source address for the I2 packet.
 Processing rules for received R1s are discussed in Section 6.8.

6.6.2. Processing Incoming ICMP Protocol Unreachable Messages

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

6.7. Processing of Incoming I1 Packets

 An implementation SHOULD reply to an I1 with an R1 packet, unless the
 implementation is unable or unwilling to set up a HIP association.
 If the implementation is unable to set up a HIP association, the host
 SHOULD send an 'ICMP Destination Protocol Unreachable,
 Administratively Prohibited' message to the I1 packet source IP
 address.  If the implementation is unwilling to set up a HIP
 association, the host MAY ignore the I1 packet.  This latter case may
 occur during a DoS attack such as an I1 packet flood.
 The implementation SHOULD be able to handle a storm of received I1
 packets, discarding those with common content that arrive within a
 small time delta.

Moskowitz, et al. Standards Track [Page 92] RFC 7401 HIPv2 April 2015

 A spoofed I1 packet can result in an R1 attack on a system.  An R1
 packet sender MUST have a mechanism to rate-limit R1 packets sent to
 an address.
 It is RECOMMENDED that the HIP state machine does not transition upon
 sending an R1 packet.
 The following steps define the conceptual processing rules for
 responding to an I1 packet:
 1.  The Responder MUST check that the Responder's HIT in the received
     I1 packet is either one of its own HITs or NULL.  Otherwise, it
     must drop the packet.
 2.  If the Responder is in ESTABLISHED state, the Responder MAY
     respond to this with an R1 packet, prepare to drop an existing
     HIP security association with the peer, and stay at ESTABLISHED
     state.
 3.  If the Responder is in I1-SENT state, it MUST make a comparison
     between the sender's HIT and its own (i.e., the receiver's) HIT.
     If the sender's HIT is greater than its own HIT, it should drop
     the I1 packet and stay at I1-SENT.  If the sender's HIT is
     smaller than its own HIT, it SHOULD send the R1 packet and stay
     at I1-SENT.  The HIT comparison is performed as defined in
     Section 6.5.
 4.  If the implementation chooses to respond to the I1 packet with an
     R1 packet, it creates a new R1 or selects a precomputed R1
     according to the format described in Section 5.3.2.  It creates
     or chooses an R1 that contains its most preferred DH public value
     that is also contained in the DH_GROUP_LIST in the I1 packet.  If
     no suitable DH Group ID was contained in the DH_GROUP_LIST in the
     I1 packet, it sends an R1 with any suitable DH public key.
 5.  If the received Responder's HIT in the I1 is NULL, the Responder
     selects a HIT with the same HIT Suite as the Initiator's HIT.  If
     this HIT Suite is not supported by the Responder, it SHOULD
     select a REQUIRED HIT Suite from Section 5.2.10, which is
     currently RSA/DSA/SHA-256.  Other than that, selecting the HIT is
     a local policy matter.

Moskowitz, et al. Standards Track [Page 93] RFC 7401 HIPv2 April 2015

 6.  The Responder expresses its supported HIP transport formats in
     the TRANSPORT_FORMAT_LIST as described in Section 5.2.11.  The
     Responder MUST provide at least one payload transport format
     type.
 7.  The Responder sends the R1 packet to the source IP address of the
     I1 packet.

6.7.1. R1 Management

 All compliant implementations MUST be able to produce R1 packets;
 even if a device is configured by policy to only initiate
 associations, it must be able to process I1s in cases of recovery
 from loss of state or key exhaustion.  An R1 packet MAY be
 precomputed.  An R1 packet MAY be reused for a short time period,
 denoted here as "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 reused beyond the normal Delta T.  R1 information MUST
 NOT be discarded until a time period "Delta S" (again, implementation
 dependent) after the R1 packet is no longer being offered.  Delta S
 is the assumed maximum time needed for the last I2 packet in response
 to the R1 packet to arrive back at the Responder.
 Implementations that support multiple DH groups MAY precompute R1
 packets for each supported group so that incoming I1 packets with
 different DH Group IDs in the DH_GROUP_LIST can be served quickly.
 An implementation MAY keep state about received I1 packets and match
 the received I2 packets against the state, as discussed in
 Section 4.1.1.

6.7.2. Handling of Malformed Messages

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

6.8. Processing of Incoming R1 Packets

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

Moskowitz, et al. Standards Track [Page 94] RFC 7401 HIPv2 April 2015

 previous R1 packet and start over from state I1-SENT.  If the system
 is in any other state with respect to that host, the system SHOULD
 silently drop the R1 packet.
 When sending multiple I1 packets, an Initiator SHOULD wait for a
 small amount of time after the first R1 reception to allow possibly
 multiple R1 packets to arrive, and it SHOULD respond to an R1 packet
 among the set with the largest R1 generation counter.
 The following steps define the conceptual processing rules for
 responding to an R1 packet:
 1.   A system receiving an R1 MUST first check to see if it has sent
      an I1 packet to the originator of the R1 packet (i.e., it has a
      HIP association that is in state I1-SENT and that is associated
      with the HITs in the R1).  Unless the I1 packet was sent in
      opportunistic mode (see Section 4.1.8), the IP addresses in the
      received R1 packet SHOULD be ignored by the R1 processing and,
      when looking up the right HIP association, the received R1
      packet SHOULD be matched against the associations using only the
      HITs.  If a match exists, the system should process the R1
      packet as described below.
 2.   Otherwise, if the system is in any state other than I1-SENT or
      I2-SENT with respect to the HITs included in the R1 packet, it
      SHOULD silently drop the R1 packet and remain in the current
      state.
 3.   If the HIP association state is I1-SENT or I2-SENT, the received
      Initiator's HIT MUST correspond to the HIT used in the original
      I1.  Also, the Responder's HIT MUST correspond to the one used
      in the I1, unless the I1 packet contained a NULL HIT.
 4.   The system SHOULD validate the R1 signature before applying
      further packet processing, according to Section 5.2.15.
 5.   If the HIP association state is I1-SENT, and multiple valid R1
      packets are present, the system MUST select from among the R1
      packets with the largest R1 generation counter.
 6.   The system MUST check that the Initiator's HIT Suite is
      contained in the HIT_SUITE_LIST parameter in the R1 packet
      (i.e., the Initiator's HIT Suite is supported by the Responder).
      If the HIT Suite is supported by the Responder, the system
      proceeds normally.  Otherwise, the system MAY stay in state
      I1-SENT and restart the BEX by sending a new I1 packet with an
      Initiator HIT that is supported by the Responder and hence is
      contained in the HIT_SUITE_LIST in the R1 packet.  The system

Moskowitz, et al. Standards Track [Page 95] RFC 7401 HIPv2 April 2015

      MAY abort the BEX if no suitable source HIT is available.  The
      system SHOULD wait for an acceptable time span to allow further
      R1 packets with higher R1 generation counters or different HIT
      and HIT Suites to arrive before restarting or aborting the BEX.
 7.   The system MUST check that the DH Group ID in the DIFFIE_HELLMAN
      parameter in the R1 matches the first DH Group ID in the
      Responder's DH_GROUP_LIST in the R1 packet, and also that this
      Group ID corresponds to a value that was included in the
      Initiator's DH_GROUP_LIST in the I1 packet.  If the DH Group ID
      of the DIFFIE_HELLMAN parameter does not express the Responder's
      best choice, the Initiator can conclude that the DH_GROUP_LIST
      in the I1 packet was adversely modified.  In such a case, the
      Initiator MAY send a new I1 packet; however, it SHOULD NOT
      change its preference in the DH_GROUP_LIST in the new I1 packet.
      Alternatively, the Initiator MAY abort the HIP base exchange.
 8.   If the HIP association state is I2-SENT, the system MAY re-enter
      state I1-SENT and process the received R1 packet if it has a
      larger R1 generation counter than the R1 packet responded to
      previously.
 9.   The R1 packet may have the A-bit set -- in this case, the system
      MAY choose to refuse it by dropping the R1 packet and returning
      to state UNASSOCIATED.  The system SHOULD consider dropping the
      R1 packet only if it used a NULL HIT in the I1 packet.  If the
      A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be
      stored permanently.
 10.  The system SHOULD attempt to validate the HIT against the
      received Host Identity by using the received Host Identity to
      construct a HIT and verify that it matches the Sender's HIT.
 11.  The system MUST store the received R1 generation counter for
      future reference.
 12.  The system attempts to solve the puzzle in the R1 packet.  The
      system MUST terminate the search after exceeding the remaining
      lifetime of the puzzle.  If the puzzle is not successfully
      solved, the implementation MAY either resend the I1 packet
      within the retry bounds or abandon the HIP base exchange.
 13.  The system computes standard Diffie-Hellman keying material
      according to the public value and Group ID provided in the
      DIFFIE_HELLMAN parameter.  The Diffie-Hellman keying material
      Kij is used for key extraction as specified in Section 6.5.

Moskowitz, et al. Standards Track [Page 96] RFC 7401 HIPv2 April 2015

 14.  The system selects the HIP_CIPHER ID from the choices presented
      in the R1 packet and uses the selected values subsequently when
      generating and using encryption keys, and when sending the I2
      packet.  If the proposed alternatives are not acceptable to the
      system, it may either resend an I1 within the retry bounds or
      abandon the HIP base exchange.
 15.  The system chooses one suitable transport format from the
      TRANSPORT_FORMAT_LIST and includes the respective transport
      format parameter in the subsequent I2 packet.
 16.  The system initializes the remaining variables in the associated
      state, including Update ID counters.
 17.  The system prepares and sends an I2 packet, as described in
      Section 5.3.3.
 18.  The system SHOULD start a timer whose timeout value SHOULD be
      larger than the worst-case anticipated RTT, and MUST increment a
      trial counter associated with the I2 packet.  The sender SHOULD
      retransmit the I2 packet upon a timeout and restart the timer,
      up to a maximum of I2_RETRIES_MAX tries.
 19.  If the system is in state I1-SENT, it SHALL transition to state
      I2-SENT.  If the system is in any other state, it remains in the
      current state.

6.8.1. Handling of Malformed Messages

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

6.9. Processing of Incoming I2 Packets

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

Moskowitz, et al. Standards Track [Page 97] RFC 7401 HIPv2 April 2015

 Otherwise, the HIP implementation SHOULD process the I2 packet.  This
 includes validation of the puzzle solution, generating the
 Diffie-Hellman key, possibly decrypting the Initiator's Host
 Identity, verifying the signature, creating state, and finally
 sending an R2 packet.
 The following steps define the conceptual processing rules for
 responding to an I2 packet:
 1.   The system MAY perform checks to verify that the I2 packet
      corresponds to a recently sent R1 packet.  Such checks are
      implementation dependent.  See Appendix A for a description of
      an example implementation.
 2.   The system MUST check that the Responder's HIT corresponds to
      one of its own HITs and MUST drop the packet otherwise.
 3.   The system MUST further check that the Initiator's HIT Suite is
      supported.  The Responder SHOULD silently drop I2 packets with
      unsupported Initiator HITs.
 4.   If the system's state machine is in the R2-SENT state, the
      system MAY check to see if the newly received I2 packet is
      similar to the one that triggered moving to R2-SENT.  If so, it
      MAY retransmit a previously sent R2 packet and reset the R2-SENT
      timer, and the state machine stays in R2-SENT.
 5.   If the system's state machine is in the I2-SENT state, the
      system MUST make a comparison between its local and sender's
      HITs (similar to the comparison method described in
      Section 6.5).  If the local HIT is smaller than the sender's
      HIT, it should drop the I2 packet, use the peer Diffie-Hellman
      key and nonce #I from the R1 packet received earlier, and get
      the local Diffie-Hellman key and nonce #J from the I2 packet
      sent to the peer earlier.  Otherwise, the system should process
      the received I2 packet and drop any previously derived
      Diffie-Hellman keying material Kij it might have formed upon
      sending the I2 packet previously.  The peer Diffie-Hellman key
      and the nonce #J are taken from the I2 packet that just arrived.
      The local Diffie-Hellman key and the nonce #I are the ones that
      were sent earlier in the R1 packet.
 6.   If the system's state machine is in the I1-SENT state, and the
      HITs in the I2 packet match those used in the previously sent I1
      packet, the system uses this received I2 packet as the basis for
      the HIP association it was trying to form, and stops
      retransmitting I1 packets (provided that the I2 packet passes
      the additional checks below).

Moskowitz, et al. Standards Track [Page 98] RFC 7401 HIPv2 April 2015

 7.   If the system's state machine is in any state other than
      R2-SENT, the system SHOULD check that the echoed R1 generation
      counter in the I2 packet is within the acceptable range if the
      counter is included.  Implementations MUST accept puzzles from
      the current generation and MAY accept puzzles from earlier
      generations.  If the generation counter in the newly received I2
      packet is outside the accepted range, the I2 packet is stale
      (and perhaps replayed) and SHOULD be dropped.
 8.   The system MUST validate the solution to the puzzle by computing
      the hash described in Section 5.3.3 using the same RHASH
      algorithm.
 9.   The I2 packet MUST have a single value in the HIP_CIPHER
      parameter, which MUST match one of the values offered to the
      Initiator in the R1 packet.
 10.  The system must derive Diffie-Hellman keying material Kij based
      on the public value and Group ID in the DIFFIE_HELLMAN
      parameter.  This key is used to derive the HIP association keys,
      as described in Section 6.5.  If the Diffie-Hellman Group ID is
      unsupported, the I2 packet is silently dropped.
 11.  The encrypted HOST_ID is decrypted by the Initiator's encryption
      key defined in Section 6.5.  If the decrypted data is not a
      HOST_ID parameter, the I2 packet is silently dropped.
 12.  The implementation SHOULD also verify that the Initiator's HIT
      in the I2 packet corresponds to the Host Identity sent in the I2
      packet.  (Note: some middleboxes may not be able to make this
      verification.)
 13.  The system MUST process the TRANSPORT_FORMAT_LIST parameter.
      Other documents specifying transport formats (e.g., [RFC7402])
      contain specifications for handling any specific transport
      selected.
 14.  The system MUST verify the HIP_MAC according to the procedures
      in Section 5.2.12.
 15.  The system MUST verify the HIP_SIGNATURE according to
      Sections 5.2.14 and 5.3.3.
 16.  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.

Moskowitz, et al. Standards Track [Page 99] RFC 7401 HIPv2 April 2015

 17.  The I2 packet may have the A-bit set -- in this case, the system
      MAY choose to refuse it by dropping the I2 and the state machine
      returns to state UNASSOCIATED.  If the A-bit is set, the
      Initiator's HIT is anonymous and should not be stored
      permanently.
 18.  The system initializes the remaining variables in the associated
      state, including Update ID counters.
 19.  Upon successful processing of an I2 message when the system's
      state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or
      R2-SENT, an R2 packet is sent and the system's state machine
      transitions to state R2-SENT.
 20.  Upon successful processing of an I2 packet when the system's
      state machine is in state ESTABLISHED, the old HIP association
      is dropped and a new one is installed, an R2 packet is sent, and
      the system's state machine transitions to R2-SENT.
 21.  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 a
      maximal amount of retransmissions of I2 packets), the state
      machine transitions to ESTABLISHED.

6.9.1. Handling of Malformed Messages

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

Moskowitz, et al. Standards Track [Page 100] RFC 7401 HIPv2 April 2015

6.10. Processing of Incoming R2 Packets

 An R2 packet received in state UNASSOCIATED, I1-SENT, or ESTABLISHED
 results in the R2 packet being dropped and the state machine staying
 in the same state.  If an R2 packet is received in state I2-SENT, it
 MUST be processed.
 The following steps define the conceptual processing rules for an
 incoming R2 packet:
 1.  If the system is in any state other than I2-SENT, the R2 packet
     is silently dropped.
 2.  The system MUST verify that the HITs in use correspond to the
     HITs that were received in the R1 packet that caused the
     transition to the I1-SENT state.
 3.  The system MUST verify the HIP_MAC_2 according to the procedures
     in Section 5.2.13.
 4.  The system MUST verify the HIP signature according to the
     procedures in Section 5.2.14.
 5.  If any of the checks above fail, there is a high probability of
     an ongoing man-in-the-middle or other security attack.  The
     system SHOULD act accordingly, based on its local policy.
 6.  Upon successful processing of the R2 packet, the state machine
     transitions to state ESTABLISHED.

6.11. Sending UPDATE Packets

 A host sends an UPDATE packet when it intends to update some
 information related to a HIP association.  There are a number of
 possible scenarios when this can occur, e.g., mobility management and
 rekeying of an existing ESP Security Association.  The following
 paragraphs define the conceptual rules for sending an UPDATE packet
 to the peer.  Additional steps can be defined in other documents
 where the UPDATE packet is used.
 The sequence of UPDATE messages is indicated by their SEQ parameter.
 Before sending an UPDATE message, the system first determines whether
 there are any outstanding UPDATE messages that may conflict with the
 new UPDATE message under consideration.  When multiple UPDATEs are
 outstanding (not yet acknowledged), the sender must assume that such
 UPDATEs may be processed in an arbitrary order by the receiver.
 Therefore, any new UPDATEs that depend on a previous outstanding
 UPDATE being successfully received and acknowledged MUST be postponed

Moskowitz, et al. Standards Track [Page 101] RFC 7401 HIPv2 April 2015

 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 an Update ID of zero.
     Otherwise, the system increments its own Update ID value by one
     before continuing the steps below.
 2.  The system creates an UPDATE packet that contains a SEQ parameter
     with the current value of the Update ID.  The UPDATE packet MAY
     also include zero or more ACKs of the peer's Update ID(s) from
     previously received UPDATE SEQ parameter(s).
 3.  The system sends the created UPDATE packet and starts an UPDATE
     timer.  The default value for the timer is 2 * RTT estimate.  If
     multiple UPDATEs are outstanding, multiple timers are in effect.
 4.  If the UPDATE timer expires, the UPDATE is resent.  The UPDATE
     can be resent UPDATE_RETRY_MAX times.  The UPDATE timer SHOULD be
     exponentially backed off for subsequent retransmissions.  If no
     acknowledgment is received from the peer after UPDATE_RETRY_MAX
     times, the HIP association is considered to be broken and the
     state machine SHOULD move from state ESTABLISHED to state CLOSING
     as depicted in Section 4.4.4.  The UPDATE timer is cancelled upon
     receiving an ACK from the peer that acknowledges receipt of the
     UPDATE.

6.12. Receiving UPDATE Packets

 When a system receives an UPDATE packet, its processing depends on
 the state of the HIP association and the presence and values of the
 SEQ and ACK parameters.  Typically, an UPDATE message also carries
 optional parameters whose handling is defined in separate documents.
 For each association, a host stores the peer's next expected
 in-sequence Update ID ("peer Update ID").  Initially, this value is
 zero.  Update ID comparisons of "less than" and "greater than" are
 performed with respect to a circular sequence number space.  Hence, a
 wraparound after 2^32 updates has to be expected and MUST be handled
 accordingly.
 The sender MAY send multiple outstanding UPDATE messages.  These
 messages are processed in the order in which they are received at the
 receiver (i.e., no resequencing is performed).  When processing

Moskowitz, et al. Standards Track [Page 102] RFC 7401 HIPv2 April 2015

 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 are both
     an ACK and SEQ in the UPDATE, the ACK is first processed as
     described in Section 6.12.2, and then the rest of the UPDATE is
     processed as described in Section 6.12.1.

6.12.1. Handling a SEQ Parameter in a Received UPDATE Message

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

Moskowitz, et al. Standards Track [Page 103] RFC 7401 HIPv2 April 2015

 3.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
     verification fails, the packet MUST be dropped.
 4.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
     verification fails, the packet SHOULD be dropped and an error
     message logged.
 5.  If a new SEQ parameter is being processed, the parameters in the
     UPDATE are then processed.  The system MUST record the Update ID
     in the received SEQ parameter, for replay protection.
 6.  An UPDATE acknowledgment packet with the 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
     the SEQ parameter, as described in Section 5.3.5.  The ACK
     parameter MAY acknowledge more than one of the peer's Update IDs.

6.12.2. Handling an ACK Parameter in a Received UPDATE Packet

 The following steps define the conceptual processing rules for
 handling an ACK parameter in a received UPDATE packet:
 1.  The sequence number reported in the ACK must match with an UPDATE
     packet sent earlier that has not already been acknowledged.  If
     no match is found or if the ACK does not acknowledge a new
     UPDATE, then either the packet MUST be dropped if no SEQ
     parameter is present, or the processing steps in Section 6.12.1
     are followed.
 2.  The system MUST verify the HIP_MAC in the UPDATE packet.  If the
     verification fails, the packet MUST be dropped.
 3.  The system MAY verify the SIGNATURE in the UPDATE packet.  If the
     verification fails, the packet SHOULD be dropped and an error
     message logged.
 4.  The corresponding UPDATE timer is stopped (see Section 6.11) so
     that the now-acknowledged UPDATE is no longer retransmitted.  If
     multiple UPDATEs are acknowledged, multiple timers are stopped.

6.13. Processing of NOTIFY Packets

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

Moskowitz, et al. Standards Track [Page 104] RFC 7401 HIPv2 April 2015

6.14. Processing of CLOSE Packets

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

6.15. Processing of CLOSE_ACK Packets

 When a host receives a CLOSE_ACK message, it verifies that it is in
 the CLOSING or CLOSED state and that the CLOSE_ACK was in response to
 the CLOSE.  A host can map CLOSE_ACK messages to CLOSE messages by
 comparing the value of ECHO_REQUEST_SIGNED (in the CLOSE packet) to
 the value of ECHO_RESPONSE_SIGNED (in the CLOSE_ACK packet).
 The CLOSE_ACK contains the HIP_MAC and the SIGNATURE parameters for
 verification.  The state is discarded when the state changes to
 UNASSOCIATED and, after that, the host MAY respond with an ICMP
 Parameter Problem to an incoming CLOSE message (see Section 5.4.4).

6.16. Handling State Loss

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

Moskowitz, et al. Standards Track [Page 105] RFC 7401 HIPv2 April 2015

7. HIP Policies

 There are a number of variables that will influence the HIP base
 exchanges that each host must support.  All HIP implementations MUST
 support more than one simultaneous HI, at least one of which SHOULD
 be reserved for anonymous usage.  Although anonymous HIs will be
 rarely used as Responders' HIs, they will be common for Initiators.
 Support for more than two HIs is RECOMMENDED.
 Initiators MAY use a different HI for different Responders to provide
 basic privacy.  Whether such private HIs are used repeatedly with the
 same Responder, and how long these HIs are used, are decided by local
 policy and depend on the privacy requirements of the Initiator.
 The value of #K used in the HIP R1 must be chosen with care.  Values
 of #K that are too high will exclude clients with weak CPUs because
 these devices cannot solve the puzzle within a reasonable amount of
 time.  #K should only be raised if a Responder is under high load,
 i.e., it cannot process all incoming HIP handshakes any more.  If a
 Responder is not under high load, #K SHOULD be 0.
 Responders that only respond to selected Initiators require an Access
 Control List (ACL), representing for which hosts they accept HIP base
 exchanges, and the preferred transport format and local lifetimes.
 Wildcarding SHOULD be supported for such ACLs, and also for
 Responders that offer public or anonymous services.

8. Security Considerations

 HIP is designed to provide secure authentication of hosts.  HIP also
 attempts to limit the exposure of the host to various denial-of-
 service and man-in-the-middle (MitM) attacks.  In doing so, HIP
 itself is subject to its own DoS and MitM attacks that potentially
 could be more damaging to a host's ability to conduct business as
 usual.
 Denial-of-service attacks often take advantage of asymmetries in the
 cost of starting an association.  One example of such asymmetry is
 the need of a Responder to store local state while a malicious
 Initiator can stay stateless.  HIP makes no attempt to increase the
 cost of the start of state at the Initiator, but makes an effort to
 reduce the cost for the Responder.  This is accomplished by having
 the Responder start the 3-way exchange instead of the Initiator,
 making the HIP exchange 4 packets long.  In doing this, the first
 packet from the Responder, R1, becomes a 'stock' packet that the
 Responder MAY use many times, until some Initiator has provided a
 valid response to such an R1 packet.  During an I1 packet storm, the
 host may reuse the same DH value also, even if some Initiator has

Moskowitz, et al. Standards Track [Page 106] RFC 7401 HIPv2 April 2015

 provided a valid response using that particular DH value.  However,
 such behavior is discouraged and should be avoided.  Using the same
 Diffie-Hellman values and random puzzle #I value has some risks.
 This risk needs to be balanced against a potential storm of HIP I1
 packets.
 This shifting of the start of state cost to the Initiator in creating
 the I2 HIP packet presents another DoS attack.  The attacker can
 spoof the I1 packet, and the Responder sends out the R1 HIP packet.
 This could conceivably tie up the 'Initiator' with evaluating the R1
 HIP packet, and creating the I2 packet.  The defense against this
 attack is to simply ignore any R1 packet where a corresponding I1
 packet was not sent (as defined in Section 6.8, step 1).
 The R1 packet is considerably larger than the I1 packet.  This
 asymmetry can be exploited in a reflection attack.  A malicious
 attacker could spoof the IP address of a victim and send a flood of
 I1 messages to a powerful Responder.  For each small I1 packet, the
 Responder would send a larger R1 packet to the victim.  The
 difference in packet sizes can further amplify a flooding attack
 against the victim.  To avoid such reflection attacks, the Responder
 SHOULD rate-limit the sending of R1 packets in general or SHOULD
 rate-limit the sending of R1 packets to a specific IP address.
 Floods of forged I2 packets form a second kind of DoS attack.  Once
 the attacking Initiator has solved the puzzle, it can send packets
 with spoofed IP source addresses with either an invalid HIP signature
 or invalid encrypted HIP payload (in the ENCRYPTED parameter).  This
 would take resources in the Responder's part to reach the point to
 discover that the I2 packet cannot be completely processed.  The
 defense against this attack is that after N bad I2 packets with the
 same puzzle solution, the Responder would discard any I2 packets that
 contain the given solution.  This will shut down the attack.  The
 attacker would have to request another R1 packet and use that to
 launch a new attack.  The Responder could increase the value of #K
 while under attack.  Keeping a list of solutions from malformed
 packets requires that the Responder keeps state for these malformed
 I2 packets.  This state has to be kept until the R1 counter is
 increased.  As malformed packets are generally filtered by their
 checksum before signature verification, only solutions in packets
 that are forged to pass the checksum and puzzle are put into the
 blacklist.  In addition, a valid puzzle is required before a new list
 entry is created.  Hence, attackers that intend to flood the
 blacklist must solve puzzles first.

Moskowitz, et al. Standards Track [Page 107] RFC 7401 HIPv2 April 2015

 A third form of DoS attack is emulating the restart of state after a
 reboot of one of the peers.  A restarting host would send an I1
 packet to the peers, which would respond with an R1 packet even if it
 were in the ESTABLISHED state.  If the I1 packet were spoofed, the
 resulting R1 packet would be received unexpectedly by the spoofed
 host and would be dropped, as in the first case above.
 A fourth form of DoS attack is emulating the closing of the HIP
 association.  HIP relies on timers and a CLOSE/CLOSE_ACK handshake to
 explicitly signal the end of a HIP association.  Because both CLOSE
 and CLOSE_ACK messages contain a HIP_MAC, an outsider cannot close a
 connection.  The presence of an additional SIGNATURE allows
 middleboxes to inspect these messages and discard the associated
 state (e.g., for firewalling, SPI-based NATing, etc.).  However, the
 optional behavior of replying to CLOSE with an ICMP Parameter Problem
 packet (as described in Section 5.4.4) might allow an attacker
 spoofing the source IP address to send CLOSE messages to launch
 reflection attacks.
 A fifth form of DoS attack is replaying R1s to cause the Initiator to
 solve stale puzzles and become out of synchronization with the
 Responder.  The R1 generation counter is a monotonically increasing
 counter designed to protect against this attack, as described in
 Section 4.1.4.
 Man-in-the-middle attacks are difficult to defend against, without
 third-party authentication.  A skillful MitM could easily handle all
 parts of HIP, but HIP indirectly provides the following protection
 from a MitM attack.  If the Responder's HI is retrieved from a signed
 DNS zone, a certificate, or through some other secure means, the
 Initiator can use this to validate the R1 HIP packet.
 Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
 certificate, or otherwise securely available, the Responder can
 retrieve the HI (after having got the I2 HIP packet) and verify that
 the HI indeed can be trusted.
 The HIP "opportunistic mode" concept has been introduced in this
 document, but this document does not specify what the semantics of
 such a connection setup are for applications.  There are certain
 concerns with opportunistic mode, as discussed in Section 4.1.8.
 NOTIFY messages are used only for informational purposes, and they
 are unacknowledged.  A HIP implementation cannot rely solely on the
 information received in a NOTIFY message because the packet may have
 been replayed.  An implementation SHOULD NOT change any state
 information purely based on a received NOTIFY message.

Moskowitz, et al. Standards Track [Page 108] RFC 7401 HIPv2 April 2015

 Since not all hosts will ever support HIP, ICMP 'Destination Protocol
 Unreachable' messages are to be expected and may be used for a DoS
 attack.  Against an Initiator, the attack would look like the
 Responder does not support HIP, but shortly after receiving the ICMP
 message, the Initiator would receive a valid R1 HIP packet.  Thus, to
 protect against 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 behavior and try to break
 up the HIP base exchange by sending such an ICMP message to the
 Responder before the Initiator has a chance to send a valid I2
 message.  Hence, the Responder SHOULD NOT act on such an ICMP
 message.  Especially, it SHOULD NOT remove any minimal state created
 when it sent the R1 HIP packet (if it did create one), but wait for
 either a valid I2 HIP packet or the natural timeout (that is, if R1
 packets are tracked at all).  Likewise, the Initiator SHOULD ignore
 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
 and included it in the "IPv6 Extension Header Types" registry
 [RFC7045] and the "Assigned Internet Protocol Numbers" registry.  The
 reference in both of these registries has been updated from [RFC5201]
 to this specification.
 The reference to the 128-bit value under the CGA Message Type
 namespace [RFC3972] of "0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA"
 has been changed from [RFC5201] to this specification.
 The following changes to the "Host Identity Protocol (HIP)
 Parameters" have been made.  In many cases, the changes involved
 updating the reference from [RFC5201] to this specification, but
 there are some differences as outlined below.  Allocation terminology
 is defined in [RFC5226]; any existing references to "IETF Consensus"
 can be replaced with "IETF Review" as per [RFC5226].

Moskowitz, et al. Standards Track [Page 109] RFC 7401 HIPv2 April 2015

 HIP Version
    This document adds the value "2" to the existing registry.  The
    value of "1" has been left with a reference to [RFC5201].
 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.
    All existing values referring to [RFC5201] have been updated to
    refer to this specification.  Other values have been left
    unchanged.
 HIT Suite ID
    This specification creates a new registry for "HIT Suite ID".
    This is different than the existing registry for "Suite ID", which
    can be left unmodified for version 1 of the protocol ([RFC5201]).
    The registry has been closed to new registrations.
    The four-bit HIT Suite ID uses the OGA ID field in the ORCHID to
    express the type of the HIT.  This document defines three HIT
    Suites (see Section 5.2.10).
    The HIT Suite ID is also carried in the four higher-order bits of
    the ID field in the HIT_SUITE_LIST parameter.  The four
    lower-order bits are reserved for future extensions of the HIT
    Suite ID space beyond 16 values.
    For the time being, the HIT Suite uses only four bits because
    these bits have to be carried in the HIT.  Using more bits for the
    HIT Suite ID reduces the cryptographic strength of the HIT.  HIT
    Suite IDs must be allocated carefully to avoid namespace
    exhaustion.  Moreover, deprecated IDs should be reused after an
    appropriate time span.  If 15 Suite IDs (the zero value is
    initially reserved) prove to be insufficient and more HIT Suite
    IDs are needed concurrently, more bits can be used for the HIT
    Suite ID by using one HIT Suite ID (0) to indicate that more bits
    should be used.  The HIT_SUITE_LIST parameter already supports
    8-bit HIT Suite IDs, should longer IDs be needed.  However,
    RFC 7343 [RFC7343] does not presently support such an extension.
    We suggest trying the rollover approach described in Appendix E
    first.  Possible extensions of the HIT Suite ID space to
    accommodate eight bits and new HIT Suite IDs are defined through
    IETF Review.

Moskowitz, et al. Standards Track [Page 110] RFC 7401 HIPv2 April 2015

    Requests to register reused values should include a note that the
    value is being reused after a deprecation period, to ensure
    appropriate IETF review and approval.
 Parameter Type
    The 16-bit Type field in a HIP parameter describes the type of the
    parameter.  It is defined in Section 5.2.1.  The current values
    are defined in Sections 5.2.3 through 5.2.23.  The existing
    "Parameter Types" registry has been updated as follows.
    A new value (129) for R1_COUNTER has been introduced, with a
    reference to this specification, and the existing value (128) for
    R1_COUNTER has been left in place with a reference to [RFC5201].
    This documents the change in value that has occurred in version 2
    of this protocol.  For clarity, the name for the value 128 has
    been changed from "R1_COUNTER" to "R1_Counter (v1 only)".
    A new value (579) for a new Parameter Type HIP_CIPHER has been
    added, with reference to this specification.  This Parameter Type
    functionally replaces the HIP_TRANSFORM Parameter Type
    (value 577), which has been left in the table with the existing
    reference to [RFC5201].  For clarity, the name for the
    value 577 has been changed from "HIP_TRANSFORM" to
    "HIP_TRANSFORM (v1 only)".
    A new value (715) for a new Parameter Type HIT_SUITE_LIST has been
    added, with reference to this specification.
    A new value (2049) for a new Parameter Type TRANSPORT_FORMAT_LIST
    has been added, with reference to this specification.
    The name of the HMAC Parameter Type (value 61505) has been changed
    to HIP_MAC.  The name of the HMAC_2 Parameter Type (value 61569)
    has been changed to HIP_MAC_2.  The reference has been changed to
    this specification.
    All other Parameter Types that reference [RFC5201] have been
    updated to refer to this specification, and Parameter Types that
    reference other RFCs are unchanged.
    The Type codes 32768 through 49151 (not 49141: a value corrected
    from a previous version of this table) have been Reserved for
    Private Use.  Implementors SHOULD select types 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. Standards Track [Page 111] RFC 7401 HIPv2 April 2015

    Where the existing ranges once stated "First Come First Served
    with Specification Required", this has been changed to
    "Specification Required".
 Group ID
    The eight-bit Group ID values appear in the DIFFIE_HELLMAN
    parameter and the DH_GROUP_LIST parameter and are defined in
    Section 5.2.7.  This registry has been updated based on the new
    values specified in Section 5.2.7; values noted as being
    DEPRECATED can be left in the table with reference to [RFC5201].
    New values are assigned through IETF Review.
 HIP Cipher ID
    The 16-bit Cipher ID values in a HIP_CIPHER parameter are defined
    in Section 5.2.8.  This is a new registry.  New values from either
    the reserved or unassigned space are assigned through IETF Review.
 DI-Type
    The four-bit DI-Type values in a HOST_ID parameter are defined in
    Section 5.2.9.  New values are assigned through IETF Review.  All
    existing values referring to [RFC5201] have been updated to refer
    to this specification.
 HI Algorithm
    The 16-bit Algorithm values in a HOST_ID parameter are defined in
    Section 5.2.9.  This is a new registry.  New values from either
    the reserved or unassigned space are assigned through IETF Review.
 ECC Curve Label
    When the HI Algorithm values in a HOST_ID parameter are defined to
    the values of either "ECDSA" or "ECDSA_LOW", a new registry is
    needed to maintain the values for the ECC Curve Label as defined
    in Section 5.2.9.  This might be handled by specifying two
    algorithm-specific subregistries named "ECDSA Curve Label" and
    "ECDSA_LOW Curve Label".  New values are to be assigned through
    IETF Review.

Moskowitz, et al. Standards Track [Page 112] RFC 7401 HIPv2 April 2015

 Notify Message Type
    The 16-bit Notify Message Type values in a NOTIFICATION parameter
    are defined in Section 5.2.19.
    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,
    and 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.
    The existing registration procedures have been updated as follows.
    The range from 1-50 can remain as "IETF Review".  The range from
    51-8191 has been marked as "Specification Required".  Values
    8192-16383 remain as "Reserved for Private Use".  Values
    16384-40959 have been marked as "Specification Required".  Values
    40960-65535 remain as "Reserved for Private Use".
    The following updates to the values have been made to the existing
    registry.  All existing values referring to [RFC5201] have been
    updated to refer to this specification.
    INVALID_HIP_TRANSFORM_CHOSEN has been renamed to
    INVALID_HIP_CIPHER_CHOSEN with the same value (17).
    A new value of 20 for the type UNSUPPORTED_HIT_SUITE has been
    added.
    HMAC_FAILED has been renamed to HIP_MAC_FAILED with the same
    value (28).
    SERVER_BUSY_PLEASE_RETRY has been renamed to
    RESPONDER_BUSY_PLEASE_RETRY with the same value (44).

10. Differences from RFC 5201

 This section summarizes the technical changes made from [RFC5201].
 This section is informational, intended to help implementors of the
 previous protocol version.  If any text in this section contradicts
 text in other portions of this specification, the text found outside
 of this section should be considered normative.
 This document specifies the HIP Version 2 protocol, which is not
 interoperable with the HIP Version 1 protocol specified in [RFC5201].
 The main technical changes are the inclusion of additional
 cryptographic agility features, and an update of the mandatory and
 optional algorithms, including Elliptic Curve support via the

Moskowitz, et al. Standards Track [Page 113] RFC 7401 HIPv2 April 2015

 Elliptic Curve DSA (ECDSA) and Elliptic Curve Diffie-Hellman (ECDH)
 algorithms.  The mandatory cryptographic algorithm implementations
 have been updated, such as replacing HMAC-SHA-1 with HMAC-SHA-256 and
 the RSA/SHA-1 signature algorithm with RSASSA-PSS, and adding ECDSA
 to RSA as mandatory public key types.  This version of HIP is also
 aligned with the ORCHID revision [RFC7343].
 The following changes have been made to the protocol operation.
 o  Section 4.1.3 describes the new process for Diffie-Hellman group
    negotiation, an aspect of cryptographic agility.  The Initiator
    may express a preference for the choice of a DH group in the I1
    packet and may suggest multiple possible choices.  The Responder
    replies with a preference based on local policy and the options
    provided by the Initiator.  The Initiator may restart the base
    exchange if the option chosen by the Responder is unsuitable
    (unsupported algorithms).
 o  Another aspect of cryptographic agility that has been added is the
    ability to use different cryptographic hash functions to generate
    the HIT.  The Responder's HIT hash algorithm (RHASH) terminology
    was introduced to support this.  In addition, HIT Suites have been
    introduced to group the set of cryptographic algorithms used
    together for public key signature, hash function, and hash
    truncation.  The use of HIT Suites constrains the combinatorial
    possibilities of algorithm selection for different functions.  HIT
    Suite IDs are related to the ORCHID OGA ID field ([RFC7343]).
 o  The puzzle mechanism has been slightly changed, in that the #I
    parameter depends on the HIT hash function (RHASH) selected, and
    the specification now advises against reusing the same #I value to
    the same Initiator; more details are provided in Sections 4.1.2
    and 5.2.4).
 o  Section 4.1.4 was extended to cover details about R1 generation
    counter rollover or reset.
 o  Section 4.1.6 was added to describe procedures for aborting a HIP
    base exchange.
 o  Section 4.1.7 provides guidance on avoiding downgrade attacks on
    the cryptographic algorithms.
 o  Section 4.1.8 on opportunistic mode has been updated to account
    for cryptographic agility by adding HIT selection procedures.

Moskowitz, et al. Standards Track [Page 114] RFC 7401 HIPv2 April 2015

 o  The HIP KEYMAT generation has been updated as described in
    Section 6.5 to make the key derivation function a negotiable
    aspect of the protocol.
 o  Packet processing for the I1, R1, and I2 packets has been updated
    to account for new parameter processing.
 o  This specification adds a requirement that hosts MUST support
    processing of ACK parameters with several SEQ sequence numbers
    even when they do not support sending such parameters.
 o  This document now clarifies that several ECHO_REQUEST_UNSIGNED
    parameters may be present in an R1 and that several ECHO_RESPONSE
    parameters may be present in an I2.
 o  Procedures for responding to version mismatches with an ICMP
    Parameter Problem have been added.
 o  The security considerations section (Section 8) has been updated
    to remove possible attacks no longer considered applicable.
 o  The use of the Anonymous bit for making the sender's Host Identity
    anonymous is now supported in packets other than the R1 and I2.
 o  Support for the use of a NULL HIP CIPHER is explicitly limited to
    debugging and testing HIP and is no longer a mandatory algorithm
    to support.
 The following changes have been made to the parameter types and
 encodings (Section 5.2).
 o  Four new parameter types have been added: DH_GROUP_LIST,
    HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST.
 o  Two parameter types have been renamed: HMAC has been renamed to
    HIP_MAC, and HMAC2 has been renamed to HIP_MAC_2.
 o  One parameter type is deprecated: HIP_TRANSFORM.  Functionally, it
    has been replaced by the HIP_CIPHER but with slightly different
    semantics (hashes have been removed and are now determined by
    RHASH).
 o  The TRANSPORT_FORMAT_LIST parameter allows transports to be
    negotiated with the list instead of by their order in the
    HIP packet.

Moskowitz, et al. Standards Track [Page 115] RFC 7401 HIPv2 April 2015

 o  The type code for the R1_COUNTER has been changed from 128 to 129
    to reflect that it is now considered a Critical parameter and must
    be echoed when present in R1.
 o  The PUZZLE and SOLUTION parameter lengths are now variable and
    dependent on the RHASH length.
 o  The Diffie-Hellman Group IDs supported have been updated.
 o  The HOST_ID parameter now requires specification of an Algorithm.
 o  The NOTIFICATION parameter supports new Notify Message Type
    values.
 o  The HIP_SIGNATURE algorithm field has been changed from 8 bits to
    16 bits to achieve alignment with the HOST_ID parameters.
 o  The specification clarifies that the SEQ parameter always contains
    one update ID but that the ACK parameter may acknowledge several
    update IDs.
 o  The restriction that only one ECHO_RESPONSE_UNSIGNED parameter
    must be present in each HIP packet has been removed.
 o  The document creates a new type range allocation for parameters
    that are only covered by a signature if a signature is present and
    applies it to the newly created DH_GROUP_LIST parameter.
 o  The document clarifies that several NOTIFY parameters may be
    present in a packet.
 The following changes have been made to the packet contents
 (Section 5.3).
 o  The I1 packet now carries the Initiator's DH_GROUP_LIST.
 o  The R1 packet now carries the HIP_CIPHER, HIT_SUITE_LIST,
    DH_GROUP_LIST, and TRANSPORT_FORMAT_LIST parameters.
 o  The I2 packet now carries the HIP_CIPHER and TRANSPORT_FORMAT_LIST
    parameters.
 o  This document clarifies that UPDATE packets that do not contain
    either a SEQ or ACK parameter are invalid.

Moskowitz, et al. Standards Track [Page 116] RFC 7401 HIPv2 April 2015

11. References

11.1. Normative References

 [FIPS.180-4.2012]
            National Institute of Standards and Technology, "Secure
            Hash Standard (SHS)", FIPS PUB 180-4, March 2012,
            <http://csrc.nist.gov/publications/fips/fips180-4/
            fips-180-4.pdf>.
 [NIST.800-131A.2011]
            National Institute of Standards and Technology,
            "Transitions: Recommendation for Transitioning the Use of
            Cryptographic Algorithms and Key Lengths", NIST
            SP 800-131A, January 2011, <http://csrc.nist.gov/
            publications/nistpubs/800-131A/sp800-131A.pdf>.
 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980, <http://www.rfc-editor.org/info/rfc768>.
 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, September 1981, <http://www.rfc-editor.org/
            info/rfc793>.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987,
            <http://www.rfc-editor.org/info/rfc1035>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
            ESP and AH", RFC 2404, November 1998,
            <http://www.rfc-editor.org/info/rfc2404>.
 [RFC2410]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
            Its Use With IPsec", RFC 2410, November 1998,
            <http://www.rfc-editor.org/info/rfc2410>.
 [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998,
            <http://www.rfc-editor.org/info/rfc2460>.
 [RFC2536]  Eastlake 3rd, D., "DSA KEYs and SIGs in the Domain Name
            System (DNS)", RFC 2536, March 1999,
            <http://www.rfc-editor.org/info/rfc2536>.

Moskowitz, et al. Standards Track [Page 117] RFC 7401 HIPv2 April 2015

 [RFC3110]  Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
            Domain Name System (DNS)", RFC 3110, May 2001,
            <http://www.rfc-editor.org/info/rfc3110>.
 [RFC3526]  Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003, <http://www.rfc-editor.org/
            info/rfc3526>.
 [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
            Algorithm and Its Use with IPsec", RFC 3602,
            September 2003, <http://www.rfc-editor.org/info/rfc3602>.
 [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
            RFC 3972, March 2005, <http://www.rfc-editor.org/
            info/rfc3972>.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, March 2005, <http://www.rfc-editor.org/
            info/rfc4034>.
 [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
            Network Access Identifier", RFC 4282, December 2005,
            <http://www.rfc-editor.org/info/rfc4282>.
 [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
            Message Protocol (ICMPv6) for the Internet Protocol
            Version 6 (IPv6) Specification", RFC 4443, March 2006,
            <http://www.rfc-editor.org/info/rfc4443>.
 [RFC4754]  Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using
            the Elliptic Curve Digital Signature Algorithm (ECDSA)",
            RFC 4754, January 2007, <http://www.rfc-editor.org/
            info/rfc4754>.
 [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
            HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
            May 2007, <http://www.rfc-editor.org/info/rfc4868>.
 [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
            and RRSIG Resource Records for DNSSEC", RFC 5702,
            October 2009, <http://www.rfc-editor.org/info/rfc5702>.
 [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
            "Default Address Selection for Internet Protocol Version 6
            (IPv6)", RFC 6724, September 2012,
            <http://www.rfc-editor.org/info/rfc6724>.

Moskowitz, et al. Standards Track [Page 118] RFC 7401 HIPv2 April 2015

 [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
            Routable Cryptographic Hash Identifiers Version 2
            (ORCHIDv2)", RFC 7343, September 2014,
            <http://www.rfc-editor.org/info/rfc7343>.
 [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the
            Encapsulating Security Payload (ESP) Transport Format with
            the Host Identity Protocol (HIP)", RFC 7402, April 2015,
            <http://www.rfc-editor.org/info/rfc7402>.

11.2. Informative References

 [AUR05]    Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of the
            HIP Base Exchange Protocol", in Proceedings of the 10th
            Australasian Conference on Information Security and
            Privacy, July 2005.
 [CRO03]    Crosby, S. and D. Wallach, "Denial of Service via
            Algorithmic Complexity Attacks", in Proceedings of the
            12th USENIX Security Symposium, Washington, D.C.,
            August 2003.
 [DIF76]    Diffie, W. and M. Hellman, "New Directions in
            Cryptography", IEEE Transactions on Information Theory
            Volume IT-22, Number 6, pages 644-654, November 1976.
 [FIPS.186-4.2013]
            National Institute of Standards and Technology, "Digital
            Signature Standard (DSS)", FIPS PUB 186-4, July 2013,
            <http://nvlpubs.nist.gov/nistpubs/FIPS/
            NIST.FIPS.186-4.pdf>.
 [FIPS.197.2001]
            National Institute of Standards and Technology, "Advanced
            Encryption Standard (AES)", FIPS PUB 197, November 2001,
            <http://csrc.nist.gov/publications/fips/fips197/
            fips-197.pdf>.
 [HIP-ARCH] Moskowitz, R., Ed., and M. Komu, "Host Identity Protocol
            Architecture", Work in Progress,
            draft-ietf-hip-rfc4423-bis-09, October 2014.
 [HIP-DNS-EXT]
            Laganier, J., "Host Identity Protocol (HIP) Domain Name
            System (DNS) Extension", Work in Progress,
            draft-ietf-hip-rfc5205-bis-06, January 2015.

Moskowitz, et al. Standards Track [Page 119] RFC 7401 HIPv2 April 2015

 [HIP-HOST-MOB]
            Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
            with the Host Identity Protocol", Work in Progress,
            draft-ietf-hip-rfc5206-bis-08, January 2015.
 [HIP-REND-EXT]
            Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
            Rendezvous Extension", Work in Progress,
            draft-ietf-hip-rfc5204-bis-05, December 2014.
 [KAU03]    Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
            protection for UDP-based protocols", in Proceedings of the
            10th ACM Conference on Computer and Communications
            Security, October 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, <http://www.rfc-editor.org/
            info/rfc792>.
 [RFC2785]  Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
            Attacks on the Diffie-Hellman Key Agreement Method for
            S/MIME", RFC 2785, March 2000,
            <http://www.rfc-editor.org/info/rfc2785>.
 [RFC2898]  Kaliski, B., "PKCS #5: Password-Based Cryptography
            Specification Version 2.0", RFC 2898, September 2000,
            <http://www.rfc-editor.org/info/rfc2898>.
 [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, February 2003,
            <http://www.rfc-editor.org/info/rfc3447>.
 [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
            Reserved for Documentation", RFC 3849, July 2004,
            <http://www.rfc-editor.org/info/rfc3849>.
 [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
            "Host Identity Protocol", RFC 5201, April 2008,
            <http://www.rfc-editor.org/info/rfc5201>.

Moskowitz, et al. Standards Track [Page 120] RFC 7401 HIPv2 April 2015

 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008, <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5338]  Henderson, T., Nikander, P., and M. Komu, "Using the Host
            Identity Protocol with Legacy Applications", RFC 5338,
            September 2008, <http://www.rfc-editor.org/info/rfc5338>.
 [RFC5533]  Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
            Shim Protocol for IPv6", RFC 5533, June 2009,
            <http://www.rfc-editor.org/info/rfc5533>.
 [RFC5737]  Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
            Reserved for Documentation", RFC 5737, January 2010,
            <http://www.rfc-editor.org/info/rfc5737>.
 [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
            Key Derivation Function (HKDF)", RFC 5869, May 2010,
            <http://www.rfc-editor.org/info/rfc5869>.
 [RFC5903]  Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
            Prime (ECP Groups) for IKE and IKEv2", RFC 5903,
            June 2010, <http://www.rfc-editor.org/info/rfc5903>.
 [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
            Curve Cryptography Algorithms", RFC 6090, February 2011,
            <http://www.rfc-editor.org/info/rfc6090>.
 [RFC6253]  Heer, T. and S. Varjonen, "Host Identity Protocol
            Certificates", RFC 6253, May 2011,
            <http://www.rfc-editor.org/info/rfc6253>.
 [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
            of IPv6 Extension Headers", RFC 7045, December 2013,
            <http://www.rfc-editor.org/info/rfc7045>.
 [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
            Kivinen, "Internet Key Exchange Protocol Version 2
            (IKEv2)", STD 79, RFC 7296, October 2014,
            <http://www.rfc-editor.org/info/rfc7296>.
 [RSA]      Rivest, R., Shamir, A., and L. Adleman, "A Method for
            Obtaining Digital Signatures and Public-Key
            Cryptosystems", Communications of the ACM 21 (2),
            pp. 120-126, February 1978.
 [SECG]     SECG, "Recommended Elliptic Curve Domain Parameters",
            SEC 2 Version 2.0, January 2010, <http://www.secg.org/>.

Moskowitz, et al. Standards Track [Page 121] RFC 7401 HIPv2 April 2015

Appendix A. Using Responder Puzzles

 As mentioned in Section 4.1.1, the Responder may delay state creation
 and still reject most spoofed I2 packets by using a number of
 pre-calculated R1 packets and a local selection function.  This
 appendix defines one possible implementation in detail.  The purpose
 of this appendix is to give the implementors an idea of how to
 implement the mechanism.  If the implementation is based on this
 appendix, it MAY contain some local modification that makes an
 attacker's task harder.
 The Responder creates a secret value S, that it regenerates
 periodically.  The Responder needs to remember the two latest values
 of S.  Each time the S is regenerated, the R1 generation counter
 value is incremented by one.
 The Responder generates a pre-signed R1 packet.  The signature for
 pre-generated R1s must be recalculated when the Diffie-Hellman key is
 recomputed or when the R1_COUNTER value changes due to S value
 regeneration.
 When the Initiator sends the I1 packet for initializing a connection,
 the Responder receives the HIT and IP address from the packet, and
 generates an #I value for the puzzle.  The #I value is set to the
 pre-signed R1 packet.
     #I value calculation:
     #I = Ltrunc( RHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), n)
     where n = RHASH_len
 The RHASH algorithm is the same as is used to generate the
 Responder's HIT value.
 From an incoming I2 packet, the Responder receives the required
 information to validate the puzzle: HITs, IP addresses, and the
 information of the used S value from the R1_COUNTER.  Using these
 values, the Responder can regenerate the #I, and verify it against
 the #I received in the I2 packet.  If the #I values match, it can
 verify the solution using #I, #J, and difficulty #K.  If the #I
 values do not match, the I2 is dropped.
     puzzle_check:
     V := Ltrunc( RHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), #K )
     if V != 0, drop the packet
 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. Standards Track [Page 122] RFC 7401 HIPv2 April 2015

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

Appendix B. Generating a Public Key Encoding from an HI

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

Appendix C. Example Checksums for HIP Packets

 The HIP checksum for HIP packets is specified in Section 5.1.1.
 Checksums for TCP and UDP packets running over HIP-enabled security
 associations are specified in Section 4.5.1.  The examples below use
 [RFC3849] and [RFC5737] addresses, and HITs with the prefix of
 2001:20 followed by zeros, followed by a decimal 1 or 2,
 respectively.

Moskowitz, et al. Standards Track [Page 123] RFC 7401 HIPv2 April 2015

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

C.1. IPv6 HIP Example (I1 Packet)

   Source Address:                 2001:db8::1
   Destination Address:            2001:db8::2
   Upper-Layer Packet Length:      48              0x30
   Next Header:                    139             0x8b
   Payload Protocol:               59              0x3b
   Header Length:                  5               0x5
   Packet Type:                    1               0x1
   Version:                        2               0x2
   Reserved:                       1               0x1
   Control:                        0               0x0
   Checksum:                       6750            0x1a5e
   Sender's HIT:                   2001:20::1
   Receiver's HIT:                 2001:20::2
   DH_GROUP_LIST type:             511             0x1ff
   DH_GROUP_LIST length:           3               0x3
   DH_GROUP_LIST Group IDs:        3,4,8

C.2. IPv4 HIP Packet (I1 Packet)

 The IPv4 checksum value for the example I1 packet is shown below.
   Source Address:                 192.0.2.1
   Destination Address:            192.0.2.2
   Upper-Layer Packet Length:      48              0x30
   Next Header:                    139             0x8b
   Payload Protocol:               59              0x3b
   Header Length:                  5               0x5
   Packet Type:                    1               0x1
   Version:                        2               0x2
   Reserved:                       1               0x1
   Control:                        0               0x0
   Checksum:                       61902           0xf1ce
   Sender's HIT:                   2001:20::1
   Receiver's HIT:                 2001:20::2
   DH_GROUP_LIST type:             511             0x1ff
   DH_GROUP_LIST length:           3               0x3
   DH_GROUP_LIST Group IDs:        3,4,8

Moskowitz, et al. Standards Track [Page 124] RFC 7401 HIPv2 April 2015

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:                   2001:20::1
   Receiver's HIT:                 2001:20::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
   Data offset:                    5               0x5
   Flags:                          SYN             0x02
   Window size:                    65535           0xffff
   Checksum:                       28586           0x6faa
   Urgent pointer:                 0               0x0000

Appendix D. ECDH and ECDSA 160-Bit Groups

 The ECDH and ECDSA 160-bit group SECP160R1 is rated at 80 bits
 symmetric strength.  This was once considered appropriate for one
 year of security.  Today, these groups should be used only when the
 host is not powerful enough (e.g., some embedded devices) and when
 security requirements are low (e.g., long-term confidentiality is not
 required).

Appendix E. HIT Suites and HIT Generation

 The HIT as an ORCHID [RFC7343] consists of three parts: A 28-bit
 prefix, a 4-bit encoding of the ORCHID generation algorithm (OGA),
 and a hash that includes the Host Identity and a context ID.  The OGA
 is an index pointing to the specific algorithm by which the public
 key and the 96-bit hashed encoding are generated.  The OGA is
 protocol specific and is to be interpreted as defined below for all
 protocols that use the same context ID as HIP.  HIP groups sets of
 valid combinations of signature and hash algorithms into HIT Suites.
 These HIT Suites are addressed by an index, which is transmitted in
 the OGA ID field of the ORCHID.
 The set of used HIT Suites will be extended to counter the progress
 in computation capabilities and vulnerabilities in the employed
 algorithms.  The intended use of the HIT Suites is to introduce a new
 HIT Suite and phase out an old one before it becomes insecure.  Since
 the 4-bit OGA ID field only permits 15 HIT Suites to be used at the
 same time (the HIT Suite with ID 0 is reserved), phased-out HIT

Moskowitz, et al. Standards Track [Page 125] RFC 7401 HIPv2 April 2015

 Suites must be reused at some point.  In such a case, there will be a
 rollover of the HIT Suite ID and the next newly introduced HIT Suite
 will start with a lower HIT Suite index than the previously
 introduced one.  The rollover effectively deprecates the reused HIT
 Suite.  For a smooth transition, the HIT Suite should be deprecated a
 considerable time before the HIT Suite index is reused.
 Since the number of HIT Suites is tightly limited to 16, the HIT
 Suites must be assigned carefully.  Hence, sets of suitable
 algorithms are grouped in a HIT Suite.
 The HIT Suite of the Responder's HIT determines the RHASH and the
 hash function to be used for the HMAC in HIP packets as well as the
 signature algorithm family used for generating the HI.  The list of
 HIT Suites is defined in Table 10.

Moskowitz, et al. Standards Track [Page 126] RFC 7401 HIPv2 April 2015

Acknowledgments

 The drive to create HIP came to being after attending the MALLOC
 meeting at the 43rd IETF meeting.  Baiju Patel and Hilarie Orman
 really gave the original author, Bob Moskowitz, the assist to get HIP
 beyond 5 paragraphs of ideas.  It has matured considerably since the
 early versions thanks to extensive input from IETFers.  Most
 importantly, its design goals are articulated and are different from
 other efforts in this direction.  Particular mention goes to the
 members of the NameSpace Research Group of the IRTF.  Noel Chiappa
 provided valuable input at early stages of discussions about
 identifier handling and Keith Moore the impetus to provide
 resolvability.  Steve Deering provided encouragement to keep working,
 as a solid proposal can act as a proof of ideas for a research group.
 Many others contributed; extensive security tips were provided by
 Steve Bellovin.  Rob Austein kept the DNS parts on track.  Paul
 Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
 for the Initiator to respond, but easy for the Responder to validate.
 Bill Sommerfeld supplied the Birthday concept, which later evolved
 into the R1 generation counter, to simplify reboot management.  Erik
 Nordmark supplied the CLOSE-mechanism for closing connections.
 Rodney Thayer and Hugh Daniels provided extensive feedback.  In the
 early times of this document, John Gilmore kept Bob Moskowitz
 challenged to provide something of value.
 During the later stages of this document, when the editing baton was
 transferred to Pekka Nikander, the input from the early implementors
 was invaluable.  Without having actual implementations, this document
 would not be on the level it is now.
 In the usual IETF fashion, a large number of people have contributed
 to the actual text or ideas.  The list of these people includes Jeff
 Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Xin Gu, Rene
 Hummen, Miika Komu, Mika Kousa, Julien Laganier, Andrew McGregor, 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. [AUR05] were
 added at a later stage.

Moskowitz, et al. Standards Track [Page 127] RFC 7401 HIPv2 April 2015

Authors' Addresses

 Robert Moskowitz (editor)
 HTT Consulting
 Oak Park, MI
 United States
 EMail: rgm@labs.htt-consult.com
 Tobias Heer
 Hirschmann Automation and Control
 Stuttgarter Strasse 45-51
 Neckartenzlingen  72654
 Germany
 EMail: tobias.heer@belden.com
 Petri Jokela
 Ericsson Research NomadicLab
 Jorvas  FIN-02420
 Finland
 Phone: +358 9 299 1
 EMail: petri.jokela@nomadiclab.com
 Thomas R. Henderson
 University of Washington
 Campus Box 352500
 Seattle, WA
 United States
 EMail: tomhend@u.washington.edu

Moskowitz, et al. Standards Track [Page 128]

/data/webs/external/dokuwiki/data/pages/rfc/rfc7401.txt · Last modified: 2015/04/09 20:38 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki