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RFC 5201

Host Identity Protocol

Pages: 104
Obsoleted by:  7401
Updated by:  6253
Part 1 of 3 – Pages 1 to 31
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Network Working Group                                       R. Moskowitz
Request for Comments: 5201                                      ICSAlabs
Category: Experimental                                       P. Nikander
                                                          P. Jokela, Ed.
                                            Ericsson Research NomadicLab
                                                            T. Henderson
                                                      The Boeing Company
                                                              April 2008


                         Host Identity Protocol

Status of This Memo

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

IESG Note

   The following issues describe IESG concerns about this document.  The
   IESG expects that these issues will be addressed when future versions
   of HIP are designed.

   This document doesn't currently define support for parameterized
   (randomized) hashing in signatures, support for negotiation of a key
   derivation function, or support for combined encryption modes.

   HIP defines the usage of RSA in signing and encrypting data.  Current
   recommendations propose usage of, for example, RSA OAEP/PSS for these
   operations in new protocols.  Changing the algorithms to more current
   best practice should be considered.

   The current specification is currently using HMAC for message
   authentication.  This is considered to be acceptable for an
   experimental RFC, but future versions must define a more generic
   method for message authentication, including the ability for other
   MAC algorithms to be used.

   SHA-1 is no longer a preferred hashing algorithm.  This is noted also
   by the authors, and it is understood that future, non-experimental
   versions must consider more secure hashing algorithms.

   HIP requires that an incoming packet's IP address be ignored.  In
   simple cases this can be done, but when there are security policies
   based on incoming interface or IP address rules, the situation
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   changes.  The handling of data needs to be enhanced to cover
   different types of network and security configurations, as well as to
   meet local security policies.

Abstract

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. A New Namespace and Identifiers . . . . . . . . . . . . . 5 1.2. The HIP Base Exchange . . . . . . . . . . . . . . . . . . 6 1.3. Memo Structure . . . . . . . . . . . . . . . . . . . . . 7 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 7 2.1. Requirements Terminology . . . . . . . . . . . . . . . . 7 2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 3. Host Identifier (HI) and Its Representations . . . . . . . . 8 3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 9 3.2. Generating a HIT from an HI . . . . . . . . . . . . . . . 9 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 10 4.1. Creating a HIP Association . . . . . . . . . . . . . . . 10 4.1.1. HIP Puzzle Mechanism . . . . . . . . . . . . . . . . 12 4.1.2. Puzzle Exchange . . . . . . . . . . . . . . . . . . . 13 4.1.3. Authenticated Diffie-Hellman Protocol . . . . . . . . 14 4.1.4. HIP Replay Protection . . . . . . . . . . . . . . . . 14 4.1.5. Refusing a HIP Exchange . . . . . . . . . . . . . . . 15 4.1.6. HIP Opportunistic Mode . . . . . . . . . . . . . . . 16 4.2. Updating a HIP Association . . . . . . . . . . . . . . . 18 4.3. Error Processing . . . . . . . . . . . . . . . . . . . . 18 4.4. HIP State Machine . . . . . . . . . . . . . . . . . . . . 19 4.4.1. HIP States . . . . . . . . . . . . . . . . . . . . . 20 4.4.2. HIP State Processes . . . . . . . . . . . . . . . . . 21 4.4.3. Simplified HIP State Diagram . . . . . . . . . . . . 28 4.5. User Data Considerations . . . . . . . . . . . . . . . . 30 4.5.1. TCP and UDP Pseudo-Header Computation for User Data . 30
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       4.5.2.  Sending Data on HIP Packets . . . . . . . . . . . . .  30
       4.5.3.  Transport Formats . . . . . . . . . . . . . . . . . .  30
       4.5.4.  Reboot and SA Timeout Restart of HIP  . . . . . . . .  30
     4.6.  Certificate Distribution  . . . . . . . . . . . . . . . .  31
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  31
     5.1.  Payload Format  . . . . . . . . . . . . . . . . . . . . .  31
       5.1.1.  Checksum  . . . . . . . . . . . . . . . . . . . . . .  33
       5.1.2.  HIP Controls  . . . . . . . . . . . . . . . . . . . .  33
       5.1.3.  HIP Fragmentation Support . . . . . . . . . . . . . .  33
     5.2.  HIP Parameters  . . . . . . . . . . . . . . . . . . . . .  34
       5.2.1.  TLV Format  . . . . . . . . . . . . . . . . . . . . .  37
       5.2.2.  Defining New Parameters . . . . . . . . . . . . . . .  38
       5.2.3.  R1_COUNTER  . . . . . . . . . . . . . . . . . . . . .  39
       5.2.4.  PUZZLE  . . . . . . . . . . . . . . . . . . . . . . .  40
       5.2.5.  SOLUTION  . . . . . . . . . . . . . . . . . . . . . .  41
       5.2.6.  DIFFIE_HELLMAN  . . . . . . . . . . . . . . . . . . .  42
       5.2.7.  HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . .  43
       5.2.8.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  44
       5.2.9.  HMAC  . . . . . . . . . . . . . . . . . . . . . . . .  45
       5.2.10. HMAC_2  . . . . . . . . . . . . . . . . . . . . . . .  46
       5.2.11. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . .  46
       5.2.12. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . .  47
       5.2.13. SEQ . . . . . . . . . . . . . . . . . . . . . . . . .  48
       5.2.14. ACK . . . . . . . . . . . . . . . . . . . . . . . . .  48
       5.2.15. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . .  49
       5.2.16. NOTIFICATION  . . . . . . . . . . . . . . . . . . . .  50
       5.2.17. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . .  54
       5.2.18. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . .  54
       5.2.19. ECHO_RESPONSE_SIGNED  . . . . . . . . . . . . . . . .  55
       5.2.20. ECHO_RESPONSE_UNSIGNED  . . . . . . . . . . . . . . .  56
     5.3.  HIP Packets . . . . . . . . . . . . . . . . . . . . . . .  56
       5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  58
       5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  58
       5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  61
       5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  62
       5.3.5.  UPDATE - the HIP Update Packet  . . . . . . . . . . .  62
       5.3.6.  NOTIFY - the HIP Notify Packet  . . . . . . . . . . .  63
       5.3.7.  CLOSE - the HIP Association Closing Packet  . . . . .  64
       5.3.8.  CLOSE_ACK - the HIP Closing Acknowledgment Packet . .  64
     5.4.  ICMP Messages . . . . . . . . . . . . . . . . . . . . . .  65
       5.4.1.  Invalid Version . . . . . . . . . . . . . . . . . . .  65
       5.4.2.  Other Problems with the HIP Header and Packet
               Structure . . . . . . . . . . . . . . . . . . . . . .  65
       5.4.3.  Invalid Puzzle Solution . . . . . . . . . . . . . . .  65
       5.4.4.  Non-Existing HIP Association  . . . . . . . . . . . .  66
   6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  66
     6.1.  Processing Outgoing Application Data  . . . . . . . . . .  66
     6.2.  Processing Incoming Application Data  . . . . . . . . . .  67
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     6.3.  Solving the Puzzle  . . . . . . . . . . . . . . . . . . .  68
     6.4.  HMAC and SIGNATURE Calculation and Verification . . . . .  70
       6.4.1.  HMAC Calculation  . . . . . . . . . . . . . . . . . .  70
       6.4.2.  Signature Calculation . . . . . . . . . . . . . . . .  72
     6.5.  HIP KEYMAT Generation . . . . . . . . . . . . . . . . . .  74
     6.6.  Initiation of a HIP Exchange  . . . . . . . . . . . . . .  75
       6.6.1.  Sending Multiple I1s in Parallel  . . . . . . . . . .  76
       6.6.2.  Processing Incoming ICMP Protocol Unreachable
               Messages  . . . . . . . . . . . . . . . . . . . . . .  77
     6.7.  Processing Incoming I1 Packets  . . . . . . . . . . . . .  77
       6.7.1.  R1 Management . . . . . . . . . . . . . . . . . . . .  78
       6.7.2.  Handling Malformed Messages . . . . . . . . . . . . .  79
     6.8.  Processing Incoming R1 Packets  . . . . . . . . . . . . .  79
       6.8.1.  Handling Malformed Messages . . . . . . . . . . . . .  81
     6.9.  Processing Incoming I2 Packets  . . . . . . . . . . . . .  81
       6.9.1.  Handling Malformed Messages . . . . . . . . . . . . .  84
     6.10. Processing Incoming R2 Packets  . . . . . . . . . . . . .  84
     6.11. Sending UPDATE Packets  . . . . . . . . . . . . . . . . .  84
     6.12. Receiving UPDATE Packets  . . . . . . . . . . . . . . . .  85
       6.12.1. Handling a SEQ Parameter in a Received UPDATE
               Message . . . . . . . . . . . . . . . . . . . . . . .  86
       6.12.2. Handling an ACK Parameter in a Received UPDATE
               Packet  . . . . . . . . . . . . . . . . . . . . . . .  87
     6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . .  87
     6.14. Processing CLOSE Packets  . . . . . . . . . . . . . . . .  88
     6.15. Processing CLOSE_ACK Packets  . . . . . . . . . . . . . .  88
     6.16. Handling State Loss . . . . . . . . . . . . . . . . . . .  88
   7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . .  89
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  89
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  92
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  93
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  95
     11.1. Normative References  . . . . . . . . . . . . . . . . . .  95
     11.2. Informative References  . . . . . . . . . . . . . . . . .  96
   Appendix A.  Using Responder Puzzles  . . . . . . . . . . . . . .  98
   Appendix B.  Generating a Public Key Encoding from an HI  . . . .  99
   Appendix C.  Example Checksums for HIP Packets  . . . . . . . . . 100
     C.1.  IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 100
     C.2.  IPv4 HIP Packet (I1)  . . . . . . . . . . . . . . . . . . 100
     C.3.  TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 101
   Appendix D.  384-Bit Group  . . . . . . . . . . . . . . . . . . . 101
   Appendix E.  OAKLEY Well-Known Group 1  . . . . . . . . . . . . . 102
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1. Introduction

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

1.1. A New Namespace and Identifiers

The Host Identity Protocol introduces a new namespace, the Host Identity namespace. Some ramifications of this new namespace are explained in the HIP architecture description [RFC4423]. There are two main representations of the Host Identity, the full Host Identifier (HI) and the Host Identity Tag (HIT). The HI is a public key and directly represents the Identity. Since there are different public key algorithms that can be used with different key
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   lengths, the HI is not good for use as a packet identifier, or as an
   index into the various operational tables needed to support HIP.
   Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes
   the operational representation.  It is 128 bits long and is used in
   the HIP payloads and to index the corresponding state in the end
   hosts.  The HIT has an important security property in that it is
   self-certifying (see Section 3).

1.2. The HIP Base Exchange

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

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

2. Terms and Definitions

2.1. Requirements Terminology

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

2.2. Notation

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

2.3. Definitions

Unused Association Lifetime (UAL): Implementation-specific time for which, if no packet is sent or received for this time interval, a host MAY begin to tear down an active association. Maximum Segment Lifetime (MSL): Maximum time that a TCP segment is expected to spend in the network.
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   Exchange Complete (EC):   Time that the host spends at the R2-SENT
      before it moves to ESTABLISHED state.  The time is n * I2
      retransmission timeout, where n is about I2_RETRIES_MAX.

   HIT Hash Algorithm:   Hash algorithm used to generate a Host Identity
      Tag (HIT) from the Host Identity public key.  Currently SHA-1
      [FIPS95] is used.

   Responder's HIT Hash Algorithm (RHASH):   Hash algorithm used for
      various hash calculations in this document.  The algorithm is the
      same as is used to generate the Responder's HIT.  RHASH is defined
      by the Orchid Context ID.  For HIP, the present RHASH algorithm is
      defined in Section 3.2.  A future version of HIP may define a new
      RHASH algorithm by defining a new Context ID.

   Opportunistic mode:   HIP base exchange where the Responder's HIT is
      not known a priori to the Initiator.

3. Host Identifier (HI) and Its Representations

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

3.1. Host Identity Tag (HIT)

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

3.2. Generating a HIT from an HI

The HIT MUST be generated according to the ORCHID generation method described in [RFC4843] using a context ID value of 0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly by the editor of this specification), and an input that encodes the Host Identity field (see Section 5.2.8) present in a HIP payload packet. The hash algorithm SHA-1 has to be used when generating HITs with this context ID. If a new ORCHID hash algorithm is needed in the future for HIT generation, a new version of HIP has to be specified with a new ORCHID context ID associated with the new hash algorithm. For Identities that are either RSA or Digital Signature Algorithm (DSA) public keys, this input consists of the public key encoding as specified in the corresponding DNSSEC document, taking the algorithm- specific portion of the RDATA part of the KEY RR. There are currently only two defined public key algorithms: RSA/SHA1 and DSA. Hence, either of the following applies: The RSA public key is encoded as defined in [RFC3110] Section 2, taking the exponent length (e_len), exponent (e), and modulus (n) fields concatenated. The length (n_len) of the modulus (n) can be determined from the total HI Length and the preceding HI fields including the exponent (e). Thus, the data to be hashed has the same length as the HI. The fields MUST be encoded in network byte order, as defined in [RFC3110].
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      The DSA public key is encoded as defined in [RFC2536] Section 2,
      taking the fields T, Q, P, G, and Y, concatenated.  Thus, the data
      to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long, where T
      is the size parameter as defined in [RFC2536].  The size parameter
      T, affecting the field lengths, MUST be selected as the minimum
      value that is long enough to accommodate P, G, and Y.  The fields
      MUST be encoded in network byte order, as defined in [RFC2536].

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

4. Protocol Overview

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

4.1. Creating a HIP Association

By definition, the system initiating a HIP exchange is the Initiator, and the peer is the Responder. This distinction is forgotten once the base exchange completes, and either party can become the Initiator in future communications. The HIP base exchange serves to manage the establishment of state between an Initiator and a Responder. The first packet, I1, initiates the exchange, and the last three packets, R1, I2, and R2, constitute an authenticated Diffie-Hellman [DIF76] key exchange for session key generation. During the Diffie-Hellman key exchange, a piece of keying material is generated. The HIP association keys are drawn from this keying material. If other cryptographic keys are needed, e.g., to be used with ESP, they are expected to be drawn from the same keying material.
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   The Initiator first sends a trigger packet, I1, to the Responder.
   The packet contains only the HIT of the Initiator and possibly the
   HIT of the Responder, if it is known.  Note that in some cases it may
   be possible to replace this trigger packet by some other form of a
   trigger, in which case the protocol starts with the Responder sending
   the R1 packet.

   The second packet, R1, starts the actual exchange.  It contains a
   puzzle -- a cryptographic challenge that the Initiator must solve
   before continuing the exchange.  The level of difficulty of the
   puzzle can be adjusted based on level of trust with the Initiator,
   current load, or other factors.  In addition, the R1 contains the
   initial Diffie-Hellman parameters and a signature, covering part of
   the message.  Some fields are left outside the signature to support
   pre-created R1s.

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

   The R2 packet finalizes the base exchange.  The packet is signed.

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

       Initiator                              Responder

                    I1: trigger exchange
                  -------------------------->
                                              select precomputed R1
                    R1: puzzle, D-H, key, sig
                  <-------------------------
    check sig                                 remain stateless
    solve puzzle
                  I2: solution, D-H, {key}, sig
                  -------------------------->
    compute D-H                               check puzzle
                                              check sig
                            R2: sig
                  <--------------------------
    check sig                                 compute D-H
ToP   noToC   RFC5201 - Page 12

4.1.1. HIP Puzzle Mechanism

The purpose of the HIP puzzle mechanism is to protect the Responder from a number of denial-of-service threats. It allows the Responder to delay state creation until receiving I2. Furthermore, the puzzle allows the Responder to use a fairly cheap calculation to check that the Initiator is "sincere" in the sense that it has churned CPU cycles in solving the puzzle. The puzzle mechanism has been explicitly designed to give space for various implementation options. It allows a Responder implementation to completely delay session-specific state creation until a valid I2 is received. In such a case, a correctly formatted I2 can be rejected only once the Responder has checked its validity by computing one hash function. On the other hand, the design also allows a Responder implementation to keep state about received I1s, and match the received I2s against the state, thereby allowing the implementation to avoid the computational cost of the hash function. The drawback of this latter approach is the requirement of creating state. Finally, it also allows an implementation to use other combinations of the space-saving and computation-saving mechanisms. The Responder can remain stateless and drop most spoofed I2s because puzzle calculation is based on the Initiator's Host Identity Tag. The idea is that the Responder has a (perhaps varying) number of pre- calculated R1 packets, and it selects one of these based on the information carried in I1. When the Responder then later receives I2, it can verify that the puzzle has been solved using the Initiator's HIT. This makes it impractical for the attacker to first exchange one I1/R1, and then generate a large number of spoofed I2s that seemingly come from different HITs. The method does not protect from an attacker that uses fixed HITs, though. Against such an attacker a viable approach may be to create a piece of local state, and remember that the puzzle check has previously failed. See Appendix A for one possible implementation. Implementations SHOULD include sufficient randomness to the algorithm so that algorithmic complexity attacks become impossible [CRO03]. The Responder can set the puzzle difficulty for Initiator, based on its level of trust of the Initiator. Because the puzzle is not included in the signature calculation, the Responder can use pre- calculated R1 packets and include the puzzle just before sending the R1 to the Initiator. The Responder SHOULD use heuristics to determine when it is under a denial-of-service attack, and set the puzzle difficulty value K appropriately; see below.
ToP   noToC   RFC5201 - Page 13

4.1.2. Puzzle Exchange

The Responder starts the puzzle exchange when it receives an I1. The Responder supplies a random number I, and requires the Initiator to find a number J. To select a proper J, the Initiator must create the concatenation of I, the HITs of the parties, and J, and take a hash over this concatenation using the RHASH algorithm. The lowest order K bits of the result MUST be zeros. The value K sets the difficulty of the puzzle. To generate a proper number J, the Initiator will have to generate a number of Js until one produces the hash target of zeros. The Initiator SHOULD give up after exceeding the puzzle lifetime in the PUZZLE parameter (Section 5.2.4). The Responder needs to re-create the concatenation of I, the HITs, and the provided J, and compute the hash once to prove that the Initiator did its assigned task. To prevent precomputation attacks, the Responder MUST select the number I in such a way that the Initiator cannot guess it. Furthermore, the construction MUST allow the Responder to verify that the value was indeed selected by it and not by the Initiator. See Appendix A for an example on how to implement this. Using the Opaque data field in an ECHO_REQUEST_SIGNED (Section 5.2.17) or in an ECHO_REQUEST_UNSIGNED parameter (Section 5.2.18), the Responder can include some data in R1 that the Initiator must copy unmodified in the corresponding I2 packet. The Responder can generate the Opaque data in various ways; e.g., using some secret, the sent I, and possibly other related data. Using the same secret, the received I (from the I2), and the other related data (if any), the Receiver can verify that it has itself sent the I to the Initiator. The Responder MUST periodically change such a used secret. It is RECOMMENDED that the Responder generates a new puzzle and a new R1 once every few minutes. Furthermore, it is RECOMMENDED that the Responder remembers an old puzzle at least 2*Lifetime seconds after the puzzle has been deprecated. These time values allow a slower Initiator to solve the puzzle while limiting the usability that an old, solved puzzle has to an attacker. NOTE: The protocol developers explicitly considered whether R1 should include a timestamp in order to protect the Initiator from replay attacks. The decision was to NOT include a timestamp. NOTE: The protocol developers explicitly considered whether a memory bound function should be used for the puzzle instead of a CPU-bound function. The decision was not to use memory-bound functions. At
ToP   noToC   RFC5201 - Page 14
   the time of the decision, the idea of memory-bound functions was
   relatively new and their IPR status were unknown.  Once there is more
   experience about memory-bound functions and once their IPR status is
   better known, it may be reasonable to reconsider this decision.

4.1.3. Authenticated Diffie-Hellman Protocol

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

4.1.4. HIP Replay Protection

The HIP protocol includes the following mechanisms to protect against malicious replays. Responders are protected against replays of I1 packets by virtue of the stateless response to I1s with presigned R1 messages. Initiators are protected against R1 replays by a monotonically increasing "R1 generation counter" included in the R1. Responders are protected against replays or false I2s by the puzzle mechanism (Section 4.1.1 above), and optional use of opaque data. Hosts are protected against replays to R2s and UPDATEs by use of a less expensive HMAC verification preceding HIP signature verification.
ToP   noToC   RFC5201 - Page 15
   The R1 generation counter is a monotonically increasing 64-bit
   counter that may be initialized to any value.  The scope of the
   counter MAY be system-wide but SHOULD be per Host Identity, if there
   is more than one local host identity.  The value of this counter
   SHOULD be kept across system reboots and invocations of the HIP base
   exchange.  This counter indicates the current generation of puzzles.
   Implementations MUST accept puzzles from the current generation and
   MAY accept puzzles from earlier generations.  A system's local
   counter MUST be incremented at least as often as every time old R1s
   cease to be valid, and SHOULD never be decremented, lest the host
   expose its peers to the replay of previously generated, higher
   numbered R1s.  The R1 counter SHOULD NOT roll over.

   A host may receive more than one R1, either due to sending multiple
   I1s (Section 6.6.1) or due to a replay of an old R1.  When sending
   multiple I1s, an Initiator SHOULD wait for a small amount of time (a
   reasonable time may be 2 * expected RTT) after the first R1 reception
   to allow possibly multiple R1s to arrive, and it SHOULD respond to an
   R1 among the set with the largest R1 generation counter.  If an
   Initiator is processing an R1 or has already sent an I2 (still
   waiting for R2) and it receives another R1 with a larger R1
   generation counter, it MAY elect to restart R1 processing with the
   fresher R1, as if it were the first R1 to arrive.

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

4.1.5. Refusing a HIP Exchange

A HIP-aware host may choose not to accept a HIP exchange. If the host's policy is to only be an Initiator, it should begin its own HIP exchange. A host MAY choose to have such a policy since only the Initiator's HI is protected in the exchange. There is a risk of a race condition if each host's policy is to only be an Initiator, at which point the HIP exchange will fail. If the host's policy does not permit it to enter into a HIP exchange with the Initiator, it should send an ICMP 'Destination Unreachable, Administratively Prohibited' message. A more complex HIP packet is not used here as it actually opens up more potential DoS attacks than a simple ICMP message.
ToP   noToC   RFC5201 - Page 16

4.1.6. HIP Opportunistic Mode

It is possible to initiate a HIP negotiation even if the Responder's HI (and HIT) is unknown. In this case, the connection initializing I1 packet contains NULL (all zeros) as the destination HIT. This kind of connection setup is called opportunistic mode. There are both security and API issues involved with the opportunistic mode. Given that the Responder's HI is not known by the Initiator, there must be suitable API calls that allow the Initiator to request, directly or indirectly, that the underlying kernel initiate the HIP base exchange solely based on locators. The Responder's HI will be tentatively available in the R1 packet, and in an authenticated form once the R2 packet has been received and verified. Hence, it could be communicated to the application via new API mechanisms. However, with a backwards-compatible API the application sees only the locators used for the initial contact. Depending on the desired semantics of the API, this can raise the following issues: o The actual locators may later change if an UPDATE message is used, even if from the API perspective the session still appears to be between specific locators. The locator update is still secure, however, and the session is still between the same nodes. o Different sessions between the same locators may result in connections to different nodes, if the implementation no longer remembers which identifier the peer had in another session. This is possible when the peer's locator has changed for legitimate reasons or when an attacker pretends to be a node that has the peer's locator. Therefore, when using opportunistic mode, HIP MUST NOT place any expectation that the peer's HI returned in the R1 message matches any HI previously seen from that address. If the HIP implementation and application do not have the same understanding of what constitutes a session, this may even happen within the same session. For instance, an implementation may not know when HIP state can be purged for UDP-based applications. o As with all HIP exchanges, the handling of locator-based or interface-based policy is unclear for opportunistic mode HIP. An application may make a connection to a specific locator because the application has knowledge of the security properties along the network to that locator. If one of the nodes moves and the locators are updated, these security properties may not be maintained. Depending on the security policy of the application, this may be a problem. This is an area of ongoing study. As an
ToP   noToC   RFC5201 - Page 17
      example, there is work to create an API that applications can use
      to specify their security requirements in a similar context
      [IPsec-APIs].

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

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

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

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

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

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

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

   As a result, it is believed that the HIP opportunistic mode is at
   least as secure as current IP.
ToP   noToC   RFC5201 - Page 18

4.2. Updating a HIP Association

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

4.3. Error Processing

HIP error processing behavior depends on whether or not there exists an active HIP association. In general, if a HIP association exists between the sender and receiver of a packet causing an error condition, the receiver SHOULD respond with a NOTIFY packet. On the other hand, if there are no existing HIP associations between the sender and receiver, or the receiver cannot reasonably determine the identity of the sender, the receiver MAY respond with a suitable ICMP message; see Section 5.4 for more details. The HIP protocol and state machine is designed to recover from one of the parties crashing and losing its state. The following scenarios describe the main use cases covered by the design.
ToP   noToC   RFC5201 - Page 19
      No prior state between the two systems.

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

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

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

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

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

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

4.4. HIP State Machine

The HIP protocol itself has little state. In the HIP base exchange, there is an Initiator and a Responder. Once the security associations (SAs) are established, this distinction is lost. If the HIP state needs to be re-established, the controlling parameters are which peer still has state and which has a datagram to send to its peer. The following state machine attempts to capture these processes. The state machine is presented in a single system view, representing either an Initiator or a Responder. There is not a complete overlap of processing logic here and in the packet definitions. Both are needed to completely implement HIP. Implementors must understand that the state machine, as described here, is informational. Specific implementations are free to implement the actual functions differently. Section 6 describes the packet processing rules in more detail. This state machine focuses
ToP   noToC   RFC5201 - Page 20
   on the HIP I1, R1, I2, and R2 packets only.  Other states may be
   introduced by mechanisms in other specifications (such as mobility
   and multihoming).

4.4.1. HIP States

+---------------------+---------------------------------------------+ | State | Explanation | +---------------------+---------------------------------------------+ | UNASSOCIATED | State machine start | | | | | I1-SENT | Initiating base exchange | | | | | I2-SENT | Waiting to complete base exchange | | | | | R2-SENT | Waiting to complete base exchange | | | | | ESTABLISHED | HIP association established | | | | | CLOSING | HIP association closing, no data can be | | | sent | | | | | CLOSED | HIP association closed, no data can be sent | | | | | E-FAILED | HIP exchange failed | +---------------------+---------------------------------------------+ Table 1: HIP States
ToP   noToC   RFC5201 - Page 21

4.4.2. HIP State Processes

System behavior in state UNASSOCIATED, Table 2. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | User data to send, | Send I1 and go to I1-SENT | | requiring a new HIP | | | association | | | | | | Receive I1 | Send R1 and stay at UNASSOCIATED | | | | | Receive I2, process | If successful, send R2 and go to R2-SENT | | | | | | If fail, stay at UNASSOCIATED | | | | | Receive user data | Optionally send ICMP as defined in | | for unknown HIP | Section 5.4 and stay at UNASSOCIATED | | association | | | | | | Receive CLOSE | Optionally send ICMP Parameter Problem and | | | stay at UNASSOCIATED | | | | | Receive ANYOTHER | Drop and stay at UNASSOCIATED | +---------------------+---------------------------------------------+ Table 2: UNASSOCIATED - Start state
ToP   noToC   RFC5201 - Page 22
   System behavior in state I1-SENT, Table 3.

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

                     Table 3: I1-SENT - Initiating HIP
ToP   noToC   RFC5201 - Page 23
   System behavior in state I2-SENT, Table 4.

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

                 Table 4: I2-SENT - Waiting to finish HIP
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   System behavior in state R2-SENT, Table 5.

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

                 Table 5: R2-SENT - Waiting to finish HIP
ToP   noToC   RFC5201 - Page 25
   System behavior in state ESTABLISHED, Table 6.

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

            Table 6: ESTABLISHED - HIP association established
ToP   noToC   RFC5201 - Page 26
   System behavior in state CLOSING, Table 7.

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

   Table 7: CLOSING - HIP association has not been used for UAL minutes
ToP   noToC   RFC5201 - Page 27
   System behavior in state CLOSED, Table 8.

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

    Table 8: CLOSED - CLOSE_ACK sent, resending CLOSE_ACK if necessary
ToP   noToC   RFC5201 - Page 28
   System behavior in state E-FAILED, Table 9.

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

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

4.4.3. Simplified HIP State Diagram

The following diagram shows the major state transitions. Transitions based on received packets implicitly assume that the packets are successfully authenticated or processed.
ToP   noToC   RFC5201 - Page 29
                                +-+        +---------------------------+
           I1 received, send R1 | |        |                           |
                                | v        v                           |
            Datagram to send  +--------------+  I2 received, send R2   |
              +---------------| UNASSOCIATED |---------------+         |
      Send I1 |               +--------------+               |         |
              v                                              |         |
         +---------+  I2 received, send R2                   |         |
   +---->| I1-SENT |---------------------------------------+ |         |
   |     +---------+                                       | |         |
   |          |                 +------------------------+ | |         |
   |          | R1 received,    | I2 received, send R2   | | |         |
   |          v send I2         |                        v v v         |
   |     +---------+            |                   +---------+        |
   |  +->| I2-SENT |------------+                   | R2-SENT |<----+  |
   |  |  +---------+                                +---------+     |  |
   |  |          |                                     |            |  |
   |  |          |                                 data|            |  |
   |  |receive   |                                   or|            |  |
   |  |R1, send  |                           EC timeout| receive I2,|  |
   |  |I2        |R2 received +--------------+         |     send R2|  |
   |  |          +----------->| ESTABLISHED  |<-------+|            |  |
   |  |                       +--------------+                      |  |
   |  |                         |    |     |  receive I2, send R2   |  |
   |  |        recv+------------+    |     +------------------------+  |
   |  |      CLOSE,|                 |                              |  |
   |  |        send|   No packet sent|                              |  |
   |  |   CLOSE_ACK|   /received for |                   timeout    |  |
   |  |            |   UAL min, send |    +---------+<-+ (UAL+MSL)  |  |
   |  |            |           CLOSE +--->| CLOSING |--+ retransmit |  |
   |  |            |                      +---------+    CLOSE      |  |
   +--|------------|----------------------+ | |  | |                |  |
      +------------|------------------------+ |  | +----------------+  |
      |            |              +-----------+  +------------------|--+
      |            +------------+ | receive CLOSE,   CLOSE_ACK      |  |
      |                         | | send CLOSE_ACK   received or    |  |
      |                         | |                  timeout        |  |
      |                         | |                  (UAL+MSL)      |  |
      |                         v v                                 |  |
      |                        +--------+  receive I2, send R2      |  |
      +------------------------| CLOSED |---------------------------+  |
                               +--------+       /----------------------+
                                 ^ |   \-------/  timeout (UAL+2MSL),
                                 +-+              move to UNASSOCIATED
                  CLOSE received, send CLOSE_ACK
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4.5. User Data Considerations

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

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

4.5.2. Sending Data on HIP Packets

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

4.5.3. Transport Formats

The actual data transmission format, used for user data after the HIP base exchange, is not defined in this document. Such transport formats and methods are described in separate specifications. All HIP implementations MUST implement, at minimum, the ESP transport format for HIP [RFC5202]. When new transport formats are defined, they get the type value from the HIP Transform type value space 2048-4095. The order in which the transport formats are presented in the R1 packet, is the preferred order. The last of the transport formats MUST be ESP transport format, represented by the ESP_TRANSFORM parameter.

4.5.4. Reboot and SA Timeout Restart of HIP

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

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

4.6. Certificate Distribution

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


(page 31 continued on part 2)

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