Network Working Group R. Moskowitz Internet-Draft ICSAlabs, a Division of TruSecure Expires: December 13, 2007 Corporation P. Nikander P. Jokela (editor) Ericsson Research NomadicLab T. Henderson The Boeing Company June 11, 2007 Host Identity Protocol draft-ietf-hip-base-08 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 13, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Moskowitz, et al. Expires December 13, 2007 [Page 1] Internet-Draft Host Identity Protocol June 2007 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 name space for mutual peer authentication. The protocol is designed to be resistant to Denial-of-Service (DoS) and Man-in-the- middle (MitM) attacks, and when used together with another suitable security protocol, such as Encapsulated Security Payload (ESP), 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 Name Space and Identifiers . . . . . . . . . . . . 5 1.2. The HIP Base Exchange . . . . . . . . . . . . . . . . . . 6 1.3. Memo structure . . . . . . . . . . . . . . . . . . . . . 7 2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 8 2.1. Requirements Terminology . . . . . . . . . . . . . . . . 8 2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8 3. Host Identifier (HI) and its Representations . . . . . . . . 10 3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 10 3.2. Generating a HIT from a HI . . . . . . . . . . . . . . . 11 4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Creating a HIP Association . . . . . . . . . . . . . . . 12 4.1.1. HIP Puzzle Mechanism . . . . . . . . . . . . . . . . 13 4.1.2. Puzzle exchange . . . . . . . . . . . . . . . . . . . 14 4.1.3. Authenticated Diffie-Hellman Protocol . . . . . . . . 15 4.1.4. HIP Replay Protection . . . . . . . . . . . . . . . . 16 4.1.5. Refusing a HIP Exchange . . . . . . . . . . . . . . . 17 4.1.6. HIP Opportunistic Mode . . . . . . . . . . . . . . . 17 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 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 Moskowitz, et al. Expires December 13, 2007 [Page 2] Internet-Draft Host Identity Protocol June 2007 4.6. Certificate Distribution . . . . . . . . . . . . . . . . 31 5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 32 5.1. Payload Format . . . . . . . . . . . . . . . . . . . . . 32 5.1.1. Checksum . . . . . . . . . . . . . . . . . . . . . . 33 5.1.2. HIP Controls . . . . . . . . . . . . . . . . . . . . 33 5.1.3. HIP Fragmentation Support . . . . . . . . . . . . . . 34 5.2. HIP Parameters . . . . . . . . . . . . . . . . . . . . . 35 5.2.1. TLV Format . . . . . . . . . . . . . . . . . . . . . 37 5.2.2. Defining New Parameters . . . . . . . . . . . . . . . 39 5.2.3. R1_COUNTER . . . . . . . . . . . . . . . . . . . . . 40 5.2.4. PUZZLE . . . . . . . . . . . . . . . . . . . . . . . 41 5.2.5. SOLUTION . . . . . . . . . . . . . . . . . . . . . . 42 5.2.6. DIFFIE_HELLMAN . . . . . . . . . . . . . . . . . . . 43 5.2.7. HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . . 44 5.2.8. HOST_ID . . . . . . . . . . . . . . . . . . . . . . . 45 5.2.9. HMAC . . . . . . . . . . . . . . . . . . . . . . . . 46 5.2.10. HMAC_2 . . . . . . . . . . . . . . . . . . . . . . . 47 5.2.11. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . . 47 5.2.12. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . . 48 5.2.13. SEQ . . . . . . . . . . . . . . . . . . . . . . . . . 48 5.2.14. ACK . . . . . . . . . . . . . . . . . . . . . . . . . 49 5.2.15. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . . 50 5.2.16. NOTIFICATION . . . . . . . . . . . . . . . . . . . . 51 5.2.17. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . . 54 5.2.18. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . . 55 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 . . . . . . . . . . . . 57 5.3.2. R1 - the HIP Responder Packet . . . . . . . . . . . . 58 5.3.3. I2 - the Second HIP Initiator Packet . . . . . . . . 60 5.3.4. R2 - the Second HIP Responder Packet . . . . . . . . 61 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 . . . . . 63 5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet . . 64 5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 64 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 . . . . . . . . . . . . 65 6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 66 6.1. Processing Outgoing Application Data . . . . . . . . . . 66 6.2. Processing Incoming Application Data . . . . . . . . . . 67 6.3. Solving the Puzzle . . . . . . . . . . . . . . . . . . . 68 6.4. HMAC and SIGNATURE Calculation and Verification . . . . . 69 6.4.1. HMAC Calculation . . . . . . . . . . . . . . . . . . 69 Moskowitz, et al. Expires December 13, 2007 [Page 3] Internet-Draft Host Identity Protocol June 2007 6.4.2. Signature Calculation . . . . . . . . . . . . . . . . 71 6.5. HIP KEYMAT Generation . . . . . . . . . . . . . . . . . . 73 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 . . . . . . . . . . . . . . . . . . . . . . 76 6.7. Processing Incoming I1 Packets . . . . . . . . . . . . . 76 6.7.1. R1 Management . . . . . . . . . . . . . . . . . . . . 78 6.7.2. Handling Malformed Messages . . . . . . . . . . . . . 78 6.8. Processing Incoming R1 Packets . . . . . . . . . . . . . 78 6.8.1. Handling Malformed Messages . . . . . . . . . . . . . 80 6.9. Processing Incoming I2 Packets . . . . . . . . . . . . . 80 6.9.1. Handling Malformed Messages . . . . . . . . . . . . . 83 6.10. Processing Incoming R2 Packets . . . . . . . . . . . . . 83 6.11. Sending UPDATE Packets . . . . . . . . . . . . . . . . . 83 6.12. Receiving UPDATE Packets . . . . . . . . . . . . . . . . 84 6.12.1. Handling a SEQ parameter in a received UPDATE message . . . . . . . . . . . . . . . . . . . . . . . 85 6.12.2. Handling an ACK Parameter in a Received UPDATE Packet . . . . . . . . . . . . . . . . . . . . . . . 86 6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . . 86 6.14. Processing CLOSE Packets . . . . . . . . . . . . . . . . 86 6.15. Processing CLOSE_ACK Packets . . . . . . . . . . . . . . 87 6.16. Handling State Loss . . . . . . . . . . . . . . . . . . . 87 7. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 88 8. Security Considerations . . . . . . . . . . . . . . . . . . . 89 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 92 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 94 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 a HI . . . . . 100 Appendix C. Example Checksums for HIP Packets . . . . . . . . . 101 C.1. IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 101 C.2. IPv4 HIP Packet (I1) . . . . . . . . . . . . . . . . . . 101 C.3. TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 101 Appendix D. 384-bit Group . . . . . . . . . . . . . . . . . . . 103 Appendix E. OAKLEY Well-known group 1 . . . . . . . . . . . . . 104 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 105 Intellectual Property and Copyright Statements . . . . . . . . . 106 Moskowitz, et al. Expires December 13, 2007 [Page 4] Internet-Draft Host Identity Protocol June 2007 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 [I-D.ietf-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 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 ESP transport format with HIP" [I-D.ietf-hip-esp]: how to use Encapsulating Security Payload (ESP) for integrity protection and optional encryption o "End-Host Mobility and Multihoming with the Host Identity Protocol" [I-D.ietf-hip-mm]: how to support mobility and multihoming in HIP o "Host Identity Protocol (HIP) Domain Name System (DNS) Extensions" [I-D.ietf-hip-dns]: how to extend DNS to contain Host Identity information o "Host Identity Protocol (HIP) Rendezvous Extension" [I-D.ietf-hip-rvs]: using a rendezvous mechanism to contact mobile HIP hosts 1.1. A New Name Space and Identifiers The Host Identity Protocol introduces a new name space, the Host Identity name space. Some ramifications of this new namespace are explained in the HIP architecture description [I-D.ietf-hip-arch]. 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 Moskowitz, et al. Expires December 13, 2007 [Page 5] Internet-Draft Host Identity Protocol June 2007 different public key algorithms that can be used with different key lengths, the HI is not good for use as a packet identifier, or as an index into the various operational tables needed to support HIP. Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes the operational representation. It is 128 bits long and is used in the HIP payloads and to index the corresponding state in the end hosts. The HIT has an important security property in that it is self-certifying (see Section 3). 1.2. The HIP Base Exchange 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 packet 3 (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) [I-D.ietf-hip-esp] 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 RFC2409 [RFC2409] that allows IKE to support complex gateway policies. Thus, HIP is not a replacement for IKE. Moskowitz, et al. Expires December 13, 2007 [Page 6] Internet-Draft Host Identity Protocol June 2007 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. Section 5 and Section 6 define the detail packet formats and rules for packet processing. Finally, Section 7, Section 8, and Section 9 discuss policy, security, and IANA considerations, respectively. Moskowitz, et al. Expires December 13, 2007 [Page 7] Internet-Draft Host Identity Protocol June 2007 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 RFC2119 [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. 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. 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. Moskowitz, et al. Expires December 13, 2007 [Page 8] Internet-Draft Host Identity Protocol June 2007 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 can be determined by inspecting the Prefix of the ORCHID (HIT). The Prefix value has a one-to-one mapping to a hash function. Opportunistic mode: HIP base exchange where the Responder's HIT is not a priori known to the Initiator. Moskowitz, et al. Expires December 13, 2007 [Page 9] Internet-Draft Host Identity Protocol June 2007 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, 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 [I-D.ietf-hip-arch] 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 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 bits long 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. "An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers (ORCHID)" [RFC4843] has been specified to store 128-bit hash based identifier called Overlay Routable Cryptographic Hash Identifiers (ORCHID) under a prefix, proposed to be allocated from the IPv6 address block as defined in [RFC4843]. The Host Identity Tag is a Moskowitz, et al. Expires December 13, 2007 [Page 10] Internet-Draft Host Identity Protocol June 2007 type of ORCHID, based on a SHA-1 hash of the host identity (Section 2 of [RFC4843]). 3.2. Generating a HIT from a 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 encoding 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 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 is 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 [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 [RFC3110]. The DSA public key is encoded as defined in RFC2536 [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 [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 [RFC2536]. In Appendix B the public key encoding generation process is illustrated using pseudo-code. Moskowitz, et al. Expires December 13, 2007 [Page 11] Internet-Draft Host Identity Protocol June 2007 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. Section 5 and Section 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 for Host Identity Protocol will be assigned by IANA. For testing purposes, the protocol number 253 is currently used. This number has been reserved by IANA for experimental use (see [RFC3692]). 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 [I-D.ietf-hip-esp]. 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. 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 Moskowitz, et al. Expires December 13, 2007 [Page 12] Internet-Draft Host Identity Protocol June 2007 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 pre-computed R1 R1: puzzle, D-H, key, sig <------------------------- check sig remain stateless solve puzzle I2: solution, D-H, {key}, sig --------------------------> compute D-H check puzzle check sig R2: sig <-------------------------- check sig compute D-H 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. Moskowitz, et al. Expires December 13, 2007 [Page 13] Internet-Draft Host Identity Protocol June 2007 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. One possible way for a Responder to remain stateless but drop most spoofed I2s is to base the selection of the puzzle on some function over the Initiator's Host Identity. The idea is that the Responder has a (perhaps varying) number of pre-calculated R1 packets, and it selects one of these based on the information carried in I1. When the Responder then later receives I2, it checks that the puzzle in the I2 matches with the puzzle sent in the R1, thereby making it impractical for the attacker to first exchange one I1/R1, and then generate a large number of spoofed I2s that seemingly come from different IP addresses or use different HITs. The method does not protect from an attacker that uses fixed IP addresses and HITs, though. Against such an attacker 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. 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 RHASH algorithm. The lowest order K bits of the result MUST be zeros. The value K sets the difficulty of the puzzle. Moskowitz, et al. Expires December 13, 2007 [Page 14] Internet-Draft Host Identity Protocol June 2007 To generate a proper number J, the Initiator will have to generate a number of Js until one produces the hash target of zero. The Initiator SHOULD give up after exceeding the puzzle lifetime in the PUZZLE 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 pre-computation attacks, the Responder MUST select the number I in such a way that the Initiator cannot guess it. Furthermore, the construction MUST allow the Responder to verify that the value was indeed selected by it and not by the Initiator. See 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 parameters (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 the sent I, some secret, and possibly other related data. Using this same secret, received I in I2 packet and possible other data, the Receiver can verify that it has itself sent the I to the Initiator. The Responder MUST change such a secret periodically. 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 it 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 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 Moskowitz, et al. Expires December 13, 2007 [Page 15] Internet-Draft Host Identity Protocol June 2007 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. 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 Moskowitz, et al. Expires December 13, 2007 [Page 16] Internet-Draft Host Identity Protocol June 2007 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 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. 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 multiple security issues involved with opportunistic mode that must be carefully addressed in the implementation. Such a set up is vulnerable to, e.g., man-in-the-middle attacks, because the initializing node does not have any public key information about the peer. Moskowitz, et al. Expires December 13, 2007 [Page 17] Internet-Draft Host Identity Protocol June 2007 While this document defines the concept of the opportunistic mode, and outlines the basic signalling mechanism to trigger it; i.e., send an I1 with a NULL destination HIT, this document does not specify the details of the opportunistic mode. Especially, its security properties are not discussed beyond the warning above. However, the authors believe that using the opportunistic mode is no less secure than communicating, without any cryptographic protection, over the current Internet. It is expected that a separate document will describe the opportunistic mode in more detail, including its security properties. 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 there exists an active HIP association or not. 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 Moskowitz, et al. Expires December 13, 2007 [Page 18] Internet-Draft Host Identity Protocol June 2007 other hand, if there are no existing HIP associations between the sender and receiver, or the receiver cannot reasonably determine the identity of the sender, the receiver MAY respond with a suitable ICMP message; see Section 5.4 for more details. The HIP protocol and state machine is designed to recover from one of the parties crashing and losing its state. The following scenarios describe the main use cases covered by the design. No prior state between the two systems. 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 Parameter Problem type to inform about non-existing HIP association (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 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 Moskowitz, et al. Expires December 13, 2007 [Page 19] Internet-Draft Host Identity Protocol June 2007 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 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 | +---------------------+---------------------------------------------+ Moskowitz, et al. Expires December 13, 2007 [Page 20] Internet-Draft Host Identity Protocol June 2007 4.4.2. HIP State Processes System behaviour in state UNASSOCIATED, Table 2. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | User data to send, | Send I1 and go to I1-SENT | | requiring a new HIP | | | association | | | | | | Receive I1 | Send R1 and stay at UNASSOCIATED | | | | | Receive I2, process | If successful, send R2 and go to R2-SENT | | | | | | If fail, stay at UNASSOCIATED | | | | | Receive user data | Optionally send ICMP as defined in | | for unknown HIP | Section 5.4 and stay at UNASSOCIATED | | association | | | | | | Receive CLOSE | Optionally send ICMP Parameter Problem and | | | stay at UNASSOCIATED | | | | | Receive ANYOTHER | Drop and stay at UNASSOCIATED | +---------------------+---------------------------------------------+ Table 2: UNASSOCIATED - Start state Moskowitz, et al. Expires December 13, 2007 [Page 21] Internet-Draft Host Identity Protocol June 2007 System behaviour in state I1-SENT, Table 3. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Receive I1 | If the local HIT is smaller than the peer | | | HIT, drop I1 and stay at I1-SENT | | | | | | If the local HIT is greater than the peer | | | HIT, send R1 and stay at I1_SENT | | | | | Receive I2, process | If successful, send R2 and go to R2-SENT | | | | | | If fail, stay at I1-SENT | | | | | Receive R1, process | If successful, send I2 and go to I2-SENT | | | | | | If fail, stay at I1-SENT | | | | | Receive ANYOTHER | Drop and stay at I1-SENT | | | | | Timeout, increment | If counter is less than I1_RETRIES_MAX, | | timeout counter | send I1 and stay at I1-SENT | | | | | | If counter is greater than I1_RETRIES_MAX, | | | go to E-FAILED | +---------------------+---------------------------------------------+ Table 3: I1-SENT - Initiating HIP Moskowitz, et al. Expires December 13, 2007 [Page 22] Internet-Draft Host Identity Protocol June 2007 System behaviour in state I2-SENT, Table 4. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Receive I1 | Send R1 and stay at I2-SENT | | | | | Receive R1, process | If successful, send I2 and cycle at I2-SENT | | | | | | If fail, stay at I2-SENT | | | | | Receive I2, process | If successful and local HIT is smaller than | | | the peer HIT, drop I2 and stay at I2-SENT | | | | | | If successful and local HIT is greater than | | | the peer HIT, send R2 and go to R2-SENT | | | | | | If fail, stay at I2-SENT | | | | | Receive R2, process | If successful, go to ESTABLISHED | | | | | | If fail, stay at I2-SENT | | | | | Receive ANYOTHER | Drop and stay at I2-SENT | | | | | Timeout, increment | If counter is less than I2_RETRIES_MAX, | | timeout counter | send I2 and stay at I2-SENT | | | | | | If counter is greater than I2_RETRIES_MAX, | | | go to E-FAILED | +---------------------+---------------------------------------------+ Table 4: I2-SENT - Waiting to finish HIP Moskowitz, et al. Expires December 13, 2007 [Page 23] Internet-Draft Host Identity Protocol June 2007 System behaviour in state R2-SENT, Table 5. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Receive I1 | Send R1 and stay at R2-SENT | | | | | Receive I2, process | If successful, send R2 and cycle at R2-SENT | | | | | | If fail, stay at R2-SENT | | | | | Receive R1 | Drop and stay at R2-SENT | | | | | Receive R2 | Drop and stay at R2-SENT | | | | | Receive data or | Move to ESTABLISHED | | UPDATE | | | | | | Exchange Complete | Move to ESTABLISHED | | Timeout | | +---------------------+---------------------------------------------+ Table 5: R2-SENT - Waiting to finish HIP Moskowitz, et al. Expires December 13, 2007 [Page 24] Internet-Draft Host Identity Protocol June 2007 System behaviour in state ESTABLISHED, Table 6. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Receive I1 | Send R1 and stay at ESTABLISHED | | | | | Receive I2, process | If successful, send R2, drop old HIP | | with puzzle and | association, establish a new HIP | | possible Opaque | association, go to R2-SENT | | data verification | | | | | | | If fail, stay at ESTABLISHED | | | | | Receive R1 | Drop and stay at ESTABLISHED | | | | | Receive R2 | Drop and stay at ESTABLISHED | | | | | Receive user data | Process and stay at ESTABLISHED | | for HIP association | | | | | | No packet | Send CLOSE and go to CLOSING | | sent/received | | | during UAL minutes | | | | | | Receive CLOSE, | If successful, send CLOSE_ACK and go to | | process | CLOSED | | | | | | If fail, stay at ESTABLISHED | +---------------------+---------------------------------------------+ Table 6: ESTABLISHED - HIP association established Moskowitz, et al. Expires December 13, 2007 [Page 25] Internet-Draft Host Identity Protocol June 2007 System behaviour in state CLOSING, Table 7. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | User data to send, | Send I1 and stay at CLOSING | | requires the | | | creation of another | | | incarnation of the | | | HIP association | | | | | | Receive I1 | Send R1 and stay at CLOSING | | | | | Receive I2, process | If successful, send R2 and go to R2-SENT | | | | | | If fail, stay at CLOSING | | | | | Receive R1, process | If successful, send I2 and go to I2-SENT | | | | | | If fail, stay at CLOSING | | | | | Receive CLOSE, | If successful, send CLOSE_ACK, discard | | process | state and go to CLOSED | | | | | | If fail, stay at CLOSING | | | | | Receive CLOSE_ACK, | If successful, discard state and go to | | process | UNASSOCIATED | | | | | | If fail, stay at CLOSING | | | | | Receive ANYOTHER | Drop and stay at CLOSING | | | | | Timeout, increment | If timeout sum is less than UAL+MSL | | timeout sum, reset | minutes, retransmit CLOSE and stay at | | timer | CLOSING | | | | | | If timeout sum is greater than UAL+MSL | | | minutes, go to UNASSOCIATED | +---------------------+---------------------------------------------+ Table 7: CLOSING - HIP association has not been used for UAL minutes Moskowitz, et al. Expires December 13, 2007 [Page 26] Internet-Draft Host Identity Protocol June 2007 System behaviour in state CLOSED, Table 8. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Datagram to send, | Send I1, and stay at CLOSED | | requires the | | | creation of another | | | incarnation of the | | | HIP association | | | | | | Receive I1 | Send R1 and stay at CLOSED | | | | | Receive I2, process | If successful, send R2 and go to R2-SENT | | | | | | If fail, stay at CLOSED | | | | | Receive R1, process | If successful, send I2 and go to I2-SENT | | | | | | If fail, stay at CLOSED | | | | | Receive CLOSE, | If successful, send CLOSE_ACK, stay at | | process | CLOSED | | | | | | If fail, stay at CLOSED | | | | | Receive CLOSE_ACK, | If successful, discard state and go to | | process | UNASSOCIATED | | | | | | If fail, stay at CLOSED | | | | | Receive ANYOTHER | Drop and stay at CLOSED | | | | | Timeout (UAL+2MSL) | Discard state and go to UNASSOCIATED | +---------------------+---------------------------------------------+ Table 8: CLOSED - CLOSE_ACK sent, resending CLOSE_ACK if necessary Moskowitz, et al. Expires December 13, 2007 [Page 27] Internet-Draft Host Identity Protocol June 2007 System behaviour in state E-FAILED, Table 9. +---------------------+---------------------------------------------+ | Trigger | Action | +---------------------+---------------------------------------------+ | Wait for | Go to UNASSOCIATED. Re-negotiation is | | implementation | possible after moving to UNASSOCIATED | | specific time | state. | +---------------------+---------------------------------------------+ Table 9: E-FAILED - HIP failed to establish association with peer 4.4.3. Simplified HIP State Diagram The following diagram shows the major state transitions. Transitions based on received packets implicitly assume that the packets are successfully authenticated or processed. Moskowitz, et al. Expires December 13, 2007 [Page 28] Internet-Draft Host Identity Protocol June 2007 +-+ +---------------------------+ 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 |---------------------------+ | +---------------------------+--------+ /----------------------+ Datagram to send, send I1 ^ | \-------/ timeout (UAL+2MSL), +-+ move to UNASSOCIATED CLOSE received, send CLOSE_ACK Moskowitz, et al. Expires December 13, 2007 [Page 29] Internet-Draft Host Identity Protocol June 2007 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 [I-D.ietf-hip-esp]. When new transport formats are defined, they get the type value from the HIP Transform type value space 2048 - 4095. The order in which the transport formats are presented in the R1 packet, is the preferred order. The last of the transport formats MUST be ESP transport format, represented by the ESP_TRANSFORM parameter. 4.5.4. Reboot and SA Timeout Restart of HIP Simulating a loss of state is a potential DoS attack. The following process has been crafted to manage state recovery without presenting a DoS opportunity. If a host reboots or the HIP association times out, it has lost its HIP state. If the host that lost state has a datagram to send to the peer, it simply restarts the HIP base exchange. After the base exchange has completed, the Initiator can create a new SA and start sending data. The peer does not reset its state until it receives a valid I2 HIP packet. If a system receives a user data packet that cannot be matched to any existing HIP association, it is possible that it has lost the state and its peer has not. It MAY send an ICMP packet with the Parameter Moskowitz, et al. Expires December 13, 2007 [Page 30] Internet-Draft Host Identity Protocol June 2007 Problem type, the Pointer pointing to the referred HIP-related association information. Reacting to such traffic depends on the implementation and the environment where the implementation is used. 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 HIP base specification does not define how to use certificates or how to transfer them between hosts. These functions are defined in a separate specification. A parameter type value, meant to be used for carrying certificates, is reserved, though: CERT, Type 768; see Section 5.2. Moskowitz, et al. Expires December 13, 2007 [Page 31] Internet-Draft Host Identity Protocol June 2007 5. Packet Formats 5.1. Payload Format All HIP packets start with a fixed header. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Next Header | Header Length |0| Packet Type | VER. | RES.|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | Controls | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sender's Host Identity Tag (HIT) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Receiver's Host Identity Tag (HIT) | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / HIP Parameters / / / | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The HIP header is logically an IPv6 extension header. However, this document does not describe processing for Next Header values other than decimal 59, IPPROTO_NONE, the IPv6 no next header value. Future documents MAY do so. However, current implementations MUST ignore trailing data if an unimplemented Next Header value is received. The Header Length field contains the length of the HIP Header and HIP parameters in 8 bytes units, excluding the first 8 bytes. Since all HIP headers MUST contain the sender's and receiver's HIT fields, the minimum value for this field is 4, and conversely, the maximum length of the HIP Parameters field is (255*8)-32 = 2008 bytes. Note: this sets an additional limit for sizes of 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 Moskowitz, et al. Expires December 13, 2007 [Page 32] Internet-Draft Host Identity Protocol June 2007 HIP packet that contains an unknown packet type, it MUST drop the packet. The HIP Version is four bits. The current version is 1. The version number is expected to be incremented only if there are incompatible changes to the protocol. Most extensions can be handled by defining new packet types, new parameter types, or new controls. The following three bits are reserved for future use. They MUST be zero when sent, and they SHOULD be ignored when handling a received packet. The two fixed bits in the header are reserved for potential SHIM6 compatibility [I-D.ietf-shim6-proto]. For implementations adhering (only) to this specification, they MUST be set as shown when sending and MUST be ignored when receiving. This is to ensure optimal forward compatibility. Note that implementations that implement other compatible specifications in addition to this specification, the corresponding rules may well be different. For example, in the case that the forthcoming SHIM6 protocol happens to be compatible with this specification, an implementation that implements both this specification and the SHIM6 protocol may need to check these bits in order to determine how to handle the packet. The HIT fields are always 128 bits (16 bytes) long. 5.1.1. Checksum Since the checksum covers the source and destination addresses in the IP header, it must be recomputed on HIP-aware NAT devices. If IPv6 is used to carry the HIP packet, the pseudo-header [RFC2460] contains the source and destination IPv6 addresses, HIP packet length in the pseudo-header length field, a zero field, and the HIP protocol number (see Section 4) in the Next Header field. The length field is in bytes and can be calculated from the HIP header length field: (HIP Header Length + 1) * 8. In case of using IPv4, the IPv4 UDP pseudo header format [RFC0768] is used. In the pseudo header, the source and destination addresses are those used in the IP header, the zero field is obviously zero, the protocol is the HIP protocol number (see Section 4), and the length is calculated as in the IPv6 case. 5.1.2. HIP Controls The HIP Controls section conveys information about the structure of the packet and capabilities of the host. Moskowitz, et al. Expires December 13, 2007 [Page 33] Internet-Draft Host Identity Protocol June 2007 The following fields have been defined: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | | | | | | | | | |A| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A - Anonymous: If this is set, the sender's HI in this packet is anonymous, i.e., one not listed in a directory. Anonymous HIs SHOULD NOT be stored. This control is set in packets R1 and/or I2. The peer receiving an anonymous HI may choose to refuse it. The rest of the fields are reserved for future use and MUST be set to zero on sent packets and ignored on received packets. 5.1.3. HIP Fragmentation Support A HIP implementation must support IP fragmentation / reassembly. Fragment reassembly MUST be implemented in both IPv4 and IPv6, but fragment generation is REQUIRED to be implemented in IPv4 (IPv4 stacks and networks will usually do this by default) and RECOMMENDED to be implemented in IPv6. In IPv6 networks, the minimum MTU is larger, 1280 bytes, than in IPv4 networks. The larger MTU size is usually sufficient for most HIP packets, and therefore fragment generation may not be needed. If a host expects to send HIP packets that are larger than the minimum IPv6 MTU, it MUST implement fragment generation even for IPv6. In IPv4 networks, HIP packets may encounter low MTUs along their routed path. Since HIP does not provide a mechanism to use multiple IP datagrams for a single HIP packet, support for path MTU discovery does not bring any value to HIP in IPv4 networks. HIP-aware NAT devices MUST perform any IPv4 reassembly/fragmentation. All HIP implementations have to be careful while employing a reassembly algorithm so that the algorithm is sufficiently resistant to DoS attacks. Because certificate chains can cause the packet to be fragmented and fragmentation can open implementation to denial of service attacks [KAU03], it is strongly recommended that the separate document specifying the certificate usage in HIP Base Exchange defines the usage of "Hash and URL" formats rather than including certificates in exchanges. With this, most problems related to DoS attacks with fragmentation can be avoided. Moskowitz, et al. Expires December 13, 2007 [Page 34] Internet-Draft Host Identity Protocol June 2007 5.2. HIP Parameters The HIP Parameters are used to carry the public key associated with the sender's HIT, together with related security and other information. They consist of ordered parameters, encoded in TLV format. The following parameter types are currently defined. Moskowitz, et al. Expires December 13, 2007 [Page 35] Internet-Draft Host Identity Protocol June 2007 +------------------------+-------+----------+-----------------------+ | TLV | Type | Length | Data | +------------------------+-------+----------+-----------------------+ | R1_COUNTER | 128 | 12 | System Boot Counter | | | | | | | PUZZLE | 257 | 12 | K and Random #I | | | | | | | SOLUTION | 321 | 20 | K, Random #I and | | | | | puzzle solution J | | | | | | | SEQ | 385 | 4 | Update packet ID | | | | | number | | | | | | | ACK | 449 | variable | Update packet ID | | | | | number | | | | | | | DIFFIE_HELLMAN | 513 | variable | public key | | | | | | | HIP_TRANSFORM | 577 | variable | HIP Encryption and | | | | | Integrity Transform | | | | | | | ENCRYPTED | 641 | variable | Encrypted part of I2 | | | | | packet | | | | | | | HOST_ID | 705 | variable | Host Identity with | | | | | Fully Qualified | | | | | Domain Name or NAI | | | | | | | CERT | 768 | variable | HI Certificate; used | | | | | to transfer | | | | | certificates. Usage | | | | | defined in a separate | | | | | document. | | | | | | | NOTIFICATION | 832 | variable | Informational data | | | | | | | ECHO_REQUEST_SIGNED | 897 | variable | Opaque data to be | | | | | echoed back; under | | | | | signature | | | | | | | ECHO_RESPONSE_SIGNED | 961 | variable | Opaque data echoed | | | | | back; under signature | | | | | | | HMAC | 61505 | variable | HMAC based message | | | | | authentication code, | | | | | with key material | | | | | from HIP_TRANSFORM | | | | | | Moskowitz, et al. Expires December 13, 2007 [Page 36] Internet-Draft Host Identity Protocol June 2007 | HMAC_2 | 61569 | variable | HMAC based message | | | | | authentication code, | | | | | with key material | | | | | from HIP_TRANSFORM. | | | | | Compared to HMAC, the | | | | | HOST_ID parameter is | | | | | included in HMAC_2 | | | | | calculation. | | | | | | | HIP_SIGNATURE_2 | 61633 | variable | Signature of the R1 | | | | | packet | | | | | | | HIP_SIGNATURE | 61697 | variable | Signature of the | | | | | packet | | | | | | | ECHO_REQUEST_UNSIGNED | 63661 | variable | Opaque data to be | | | | | echoed back; after | | | | | signature | | | | | | | ECHO_RESPONSE_UNSIGNED | 63425 | variable | Opaque data echoed | | | | | back; after signature | +------------------------+-------+----------+-----------------------+ Because the ordering (from lowest to highest) of HIP parameters is strictly enforced (see Section 5.2.1), the parameter type values for existing parameters have been spaced to allow for future protocol extensions. Parameters numbered between 0-1023 are used in HIP handshake and update procedures and are covered by signatures. Parameters numbered between 1024-2047 are reserved. Parameters numbered between 2048-4095 are used for parameters related to HIP transform types. Parameters numbered between 4096 and (2^16 - 2^12) 61439 are reserved. Parameters numbered between 61440-62463 are used for signatures and signed MACs. Parameters numbered between 62464- 63487 are used for parameters that fall outside of the signed area of the packet. Parameters numbered between 63488-64511 are used for rendezvous and other relaying services. Parameters numbered between 64512-65535 are reserved. 5.2.1. TLV Format The TLV-encoded parameters are described in the following subsections. The type-field value also describes the order of these fields in the packet, except for type values from 2048 to 4095 which are reserved for new transport forms. The parameters MUST be included in the packet such that their types form an increasing order. If the parameter can exist multiple times in the packet, the type value may be the same in consecutive parameters. If the order does not follow this rule, the packet is considered to be malformed Moskowitz, et al. Expires December 13, 2007 [Page 37] Internet-Draft Host Identity Protocol June 2007 and it MUST be discarded. Parameters using type values from 2048 up to 4095 are transport formats. Currently, one transport format is defined: the ESP transport format [I-D.ietf-hip-esp]. The order of these parameters does not follow the order of their type value, but they are put in the packet in order of preference. The first of the transport formats it the most preferred, and so on. All of the TLV parameters have a length (including Type and Length fields) which is a multiple of 8 bytes. When needed, padding MUST be added to the end of the parameter so that the total length becomes a multiple of 8 bytes. This rule ensures proper alignment of data. Any added padding bytes MUST be zeroed by the sender, and their values SHOULD NOT be checked by the receiver. Consequently, the Length field indicates the length of the Contents field (in bytes). The total length of the TLV parameter (including Type, Length, Contents, and Padding) is related to the Length field according to the following formula: Total Length = 11 + Length - (Length + 3) % 8; 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type |C| Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Contents / / +-+-+-+-+-+-+-+-+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type Type code for the parameter. 16 bits long, C-bit being part of the Type code. C Critical. One if this parameter is critical, and MUST be recognized by the recipient, zero otherwise. The C bit is considered to be a part of the Type field. Consequently, critical parameters are always odd and non-critical ones have an even value. Length Length of the Contents, in bytes. Contents Parameter specific, defined by Type Padding Padding, 0-7 bytes, added if needed Critical parameters MUST be recognized by the recipient. If a recipient encounters a critical parameter that it does not recognize, it MUST NOT process the packet any further. It MAY send an ICMP or Moskowitz, et al. Expires December 13, 2007 [Page 38] Internet-Draft Host Identity Protocol June 2007 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 the 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. 2. A new parameter may be critical only if an old recipient ignoring it would cause security problems. In general, new parameters SHOULD be defined as non-critical, and expect a reply from the recipient. 3. If a system implements a new critical parameter, it MUST provide the ability to configure the associated feature off, such that the critical parameter is not sent at all. The configuration option must be well documented. Implementations operating in a mode adhering to this specification MUST disable the sending of new critical parameters. In other words, the management interface MUST allow vanilla standards-only mode as a default configuration setting, and MAY allow new critical payloads to be configured on (and off). 4. See section Section 9 for allocation rules regarding type codes. Moskowitz, et al. Expires December 13, 2007 [Page 39] Internet-Draft Host Identity Protocol June 2007 5.2.3. R1_COUNTER 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved, 4 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | R1 generation counter, 8 bytes | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 128 Length 12 R1 generation counter The current generation of valid puzzles The R1_COUNTER parameter contains an 64-bit unsigned integer in network byte order, indicating the current generation of valid puzzles. The sender is supposed to increment this counter periodically. It is RECOMMENDED that the counter value is incremented at least as often as old PUZZLE values are deprecated so that SOLUTIONs to them are no longer accepted. The R1_COUNTER parameter is optional. It SHOULD be included in the R1 (in which case it is covered by the signature), and if present in the R1, it MAY be echoed (including the Reserved field verbatim) by the Initiator in the I2. Moskowitz, et al. Expires December 13, 2007 [Page 40] Internet-Draft Host Identity Protocol June 2007 5.2.4. PUZZLE 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | K, 1 byte | Lifetime | Opaque, 2 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Random # I, 8 bytes | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 257 Length 12 K K is the number of verified bits Lifetime Puzzle lifetime 2^(value-32) seconds Opaque Data set by the Responder, indexing the puzzle Random #I random number Random #I is represented as 64-bit integer, K and Lifetime as 8-bit integer, all in network byte order. The PUZZLE parameter contains the puzzle difficulty K and a 64-bit puzzle random integer #I. The Puzzle Lifetime indicates the time during which the puzzle solution is valid, and sets a time limit which should not be exceeded by the Initiator while it attempts to solve the puzzle. The lifetime is indicated as a power of 2 using the formula 2^(Lifetime-32) seconds. A puzzle MAY be augmented with an ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in the R1; the contents of the echo request are then echoed back in the ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED, allowing the Responder to use the included information as a part of its puzzle processing. The Opaque and Random #I field are not covered by the HIP_SIGNATURE_2 parameter. Moskowitz, et al. Expires December 13, 2007 [Page 41] Internet-Draft Host Identity Protocol June 2007 5.2.5. SOLUTION 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | K, 1 byte | Reserved | Opaque, 2 bytes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Random #I, 8 bytes | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Puzzle solution #J, 8 bytes | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 321 Length 20 K K is the number of verified bits Reserved zero when sent, ignored when received Opaque copied unmodified from the received PUZZLE parameter Random #I random number Puzzle solution #J random number Random #I, and Random #J are represented as 64-bit integers, K as an 8-bit integer, all in network byte order. The SOLUTION parameter contains a solution to a puzzle. It also echoes back the random difficulty K, the Opaque field, and the puzzle integer #I. Moskowitz, et al. Expires December 13, 2007 [Page 42] Internet-Draft Host Identity Protocol June 2007 5.2.6. DIFFIE_HELLMAN 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group ID | Public Value Length | Public Value / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Group ID | Public Value Length | Public Value / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 513 Length length in octets, excluding Type, Length, and padding Group ID defines values for p and g Public Value length of the following Public Value in octets Length Public Value the sender's public Diffie-Hellman key The following Group IDs have been defined: Group Value Reserved 0 384-bit group 1 OAKLEY well known group 1 2 1536-bit MODP group 3 3072-bit MODP group 4 6144-bit MODP group 5 8192-bit MODP group 6 The MODP Diffie-Hellman groups are defined in [RFC3526]. The OAKLEY well known group 1 is defined in Appendix E. The sender can include at most two different Diffie-Hellman public values in the DIFFIE_HELLMAN parameter. This gives the possibility e.g. for a server to provide a weaker encryption possibility for a PDA host that is not powerful enough. It is RECOMMENDED that the Initiator, receiving more than one public values selects the stronger one, if it supports it. A HIP implementation MUST implement Group IDs 1 and 3. The 384-bit group can be used when lower security is enough (e.g. web surfing) and when the equipment is not powerful enough (e.g. some PDAs). It Moskowitz, et al. Expires December 13, 2007 [Page 43] Internet-Draft Host Identity Protocol June 2007 is REQUIRED that the default configuration allows Group ID 1 usage, but it is RECOMMENDED that applications that need stronger security turn Group ID 1 support off. Equipment powerful enough SHOULD implement also group ID 5. The 384-bit group is defined in Appendix D. To avoid unnecessary failures during the base exchange, the rest of the groups SHOULD be implemented in hosts where resources are adequate. 5.2.7. HIP_TRANSFORM 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Suite-ID #1 | Suite-ID #2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Suite-ID #n | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 577 Length length in octets, excluding Type, Length, and padding Suite-ID Defines the HIP Suite to be used The following Suite-IDs are defined ([RFC4307],[RFC2451]): Suite-ID Value RESERVED 0 AES-CBC with HMAC-SHA1 1 3DES-CBC with HMAC-SHA1 2 3DES-CBC with HMAC-MD5 3 BLOWFISH-CBC with HMAC-SHA1 4 NULL-ENCRYPT with HMAC-SHA1 5 NULL-ENCRYPT with HMAC-MD5 6 The sender of a HIP transform parameter MUST make sure that there are no more than six (6) HIP Suite-IDs in one HIP transform parameter. Conversely, a recipient MUST be prepared to handle received transport parameters that contain more than six Suite-IDs by accepting the first six Suite-IDs and dropping the rest. The limited number of transforms sets the maximum size of HIP_TRANSFORM parameter. As the default configuration, the HIP_TRANSFORM parameter MUST contain at least one of the mandatory Suite-IDs. There MAY be a configuration option that allows the administrator to override this default. Moskowitz, et al. Expires December 13, 2007 [Page 44] Internet-Draft Host Identity Protocol June 2007 The Responder lists supported and desired Suite-IDs in order of preference in the R1, up to the maximum of six Suite-IDs. The Initiator MUST choose only one of the corresponding Suite-IDs. That Suite-ID will be used for generating the I2. Mandatory implementations: AES-CBC with HMAC-SHA1 and NULL-ENCRYPTION with HMAC-SHA1. 5.2.8. HOST_ID 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HI Length |DI-type| DI Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Host Identity / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | Domain Identifier / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 705 Length length in octets, excluding Type, Length, and Padding HI Length Length of the Host Identity in octets DI-type type of the following Domain Identifier field DI Length length of the FQDN or NAI in octets Host Identity actual host identity Domain Identifier the identifier of the sender The Host Identity is represented in RFC2535 [RFC2535] format. The algorithms used in RDATA format are the following: Algorithms Values RESERVED 0 DSA 3 [RFC2536] (RECOMMENDED) RSA/SHA1 5 [RFC3110] (REQUIRED) The following DI-types have been defined: Moskowitz, et al. Expires December 13, 2007 [Page 45] Internet-Draft Host Identity Protocol June 2007 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 RFC1035 [RFC1035] Section 3.1. The format for Network Access Identifier is defined in [I-D.ietf-radext-rfc2486bis] If there is no Domain Identifier, i.e. the DI-type field is zero, also the DI Length field is set to zero. 5.2.9. HMAC 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | HMAC | / / / +-------------------------------+ | | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 61505 Length length in octets, excluding Type, Length, and Padding HMAC HMAC computed over the HIP packet, excluding the HMAC parameter and any following parameters, such as HIP_SIGNATURE, HIP_SIGNATURE_2, ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED. The checksum field MUST be set to zero and the HIP header length in the HIP common header MUST be calculated not to cover any excluded parameters when the HMAC is calculated. The size of the HMAC is the natural size of the hash computation output depending on the used hash function. The HMAC calculation and verification process is presented in Section 6.4.1 Moskowitz, et al. Expires December 13, 2007 [Page 46] Internet-Draft Host Identity Protocol June 2007 5.2.10. HMAC_2 The parameter structure is the same as in Section 5.2.9. The fields are: Type 61569 Length length in octets, excluding Type, Length, and Padding HMAC HMAC computed over the HIP packet, excluding the HMAC parameter and any following parameters such as HIP_SIGNATURE, HIP_SIGNATURE_2, ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED, and including an additional sender's HOST_ID parameter during the HMAC calculation. The checksum field MUST be set to zero and the HIP header length in the HIP common header MUST be calculated not to cover any excluded parameters when the HMAC is calculated. The size of the HMAC is the natural size of the hash computation output depending on the used hash function. The HMAC calculation and verification process is presented in Section 6.4.1 5.2.11. HIP_SIGNATURE 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SIG alg | Signature / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 61697 Length length in octets, excluding Type, Length, and Padding SIG alg Signature algorithm Signature the signature is calculated over the HIP packet, excluding the HIP_SIGNATURE parameter and any parameters that follow the HIP_SIGNATURE parameter. The checksum field MUST be set to zero, and the HIP header length in the HIP common header MUST be calculated only to the beginning of the HIP_SIGNATURE parameter when the signature is calculated. Moskowitz, et al. Expires December 13, 2007 [Page 47] Internet-Draft Host Identity Protocol June 2007 The signature algorithms are defined in Section 5.2.8. The signature in the Signature field is encoded using the proper method depending on the signature algorithm (e.g. according to [RFC3110] in case of RSA/SHA1, or according to [RFC2536] in case of DSA). The HIP_SIGNATURE calculation and verification process is presented in Section 6.4.2 5.2.12. HIP_SIGNATURE_2 The parameter structure is the same as in Section 5.2.11. The fields are: Type 61633 Length length in octets, excluding Type, Length, and Padding SIG alg Signature algorithm Signature the signature is calculated over the HIP R1 packet, excluding the HIP_SIGNATURE_2 parameter and any parameters that follow. Initiator's HIT, 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 calculated to the beginning of the HIP_SIGNATURE_2 parameter when the signature is calculated. Zeroing the Initiator's HIT makes it possible to create R1 packets beforehand to minimize the effects of possible DoS attacks. Zeroing the I and Opaque fields allows these fields to be populated dynamically on precomputed R1s. Signature calculation and verification follows the process in Section 6.4.2. 5.2.13. SEQ 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Update ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 385 Length 4 Update ID 32-bit sequence number Moskowitz, et al. Expires December 13, 2007 [Page 48] Internet-Draft Host Identity Protocol June 2007 The Update ID is an unsigned quantity, initialized by a host to zero upon moving to ESTABLISHED state. The Update ID has scope within a single HIP association, and not across multiple associations or multiple hosts. The Update ID is incremented by one before each new UPDATE that is sent by the host; the first UPDATE packet originated by a host has an Update ID of 0. 5.2.14. ACK 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | peer Update ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 449 Length variable (multiple of 4) peer Update ID 32-bit sequence number corresponding to the Update ID being ACKed. The ACK parameter includes one or more Update IDs that have been received from the peer. The Length field identifies the number of peer Update IDs that are present in the parameter. Moskowitz, et al. Expires December 13, 2007 [Page 49] Internet-Draft Host Identity Protocol June 2007 5.2.15. ENCRYPTED 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IV / / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / / Encrypted data / / / / +-------------------------------+ / | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 641 Length length in octets, excluding Type, Length, and Padding Reserved zero when sent, ignored when received IV Initialization vector, if needed, otherwise nonexistent. The length of the IV is inferred from the HIP transform. Encrypted The data is encrypted using an encryption algorithm data as defined in HIP transform. Padding Any Padding, if necessary, to make the parameter a multiple of 8 bytes. The ENCRYPTED parameter encapsulates another parameter, the encrypted data, which is also in TLV format. Consequently, the first fields in the encapsulated parameter(s) are Type and Length, allowing the contents to be easily parsed after decryption. Both the ENCRYPTED parameter and the encapsulated parameter(s) MUST be padded. The padding needed for the ENCRYPTED parameter is referred as the "outer" padding. Correspondingly, the padding for the parameter(s) encapsulated within the ENCRYPTED parameter is referred as the "inner" padding. The inner padding follows exactly the rules of Section 5.2.1. The outer padding also follows the same rules but with an exception. Namely, some algorithms require that the data to be encrypted must be a multiple of the cipher algorithm block size. In this case, the outer padding MUST include extra padding, as specified by the encryption algorithm. The size of the extra padding is selected so Moskowitz, et al. Expires December 13, 2007 [Page 50] Internet-Draft Host Identity Protocol June 2007 that the length of the ENCRYPTED is the minimum value that is both multiple of eight and the cipher block size. The encryption algorithm may specify padding bytes other than zero; for example, AES [FIPS01] uses the PKCS5 padding scheme [RFC2898] (see section 6.1.1) where the remaining n bytes to fill the block each have the value n. Note that the length of the cipher suite output may be smaller or larger than the length of the data to be encrypted, since the encryption process may compress the data or add additional padding to the data. 5.2.16. NOTIFICATION The NOTIFICATION parameter is used to transmit informational data, such as error conditions and state transitions, to a HIP peer. A NOTIFICATION parameter may appear in the NOTIFY packet type. The use of the NOTIFICATION parameter in other packet types is for further study. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Notify Message Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | / / Notification data / / +---------------+ / | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 832 Length length in octets, excluding Type, Length, and Padding Reserved zero when sent, ignored when received Notify Message Specifies the type of notification Type Notification Informational or error data transmitted in addition Data to the Notify Message Type. Values for this field are type specific (see below). Padding Any Padding, if necessary, to make the parameter a multiple of 8 bytes. Notification information can be error messages specifying why an SA could not be established. It can also be status data that a process managing an SA database wishes to communicate with a peer process. The table below lists the Notification messages and their Moskowitz, et al. Expires December 13, 2007 [Page 51] Internet-Draft Host Identity Protocol June 2007 corresponding values. To avoid certain types of attacks, a Responder SHOULD avoid sending a NOTIFICATION to any host with which it has not successfully verified a puzzle solution. Types in the range 0 - 16383 are intended for reporting errors and in the range 16384 - 65535 for other status information. An implementation that receives a NOTIFY packet with an NOTIFICATION error parameter in response to a request packet (e.g., I1, I2, UPDATE), SHOULD assume that the corresponding request has failed entirely. Unrecognized error types MUST be ignored except that they SHOULD be logged. Notify payloads with status types MUST be ignored if not recognized. NOTIFICATION PARAMETER - ERROR TYPES Value ------------------------------------ ----- UNSUPPORTED_CRITICAL_PARAMETER_TYPE 1 Sent if the parameter type has the "critical" bit set and the parameter type is not recognized. Notification Data contains the two octet parameter type. INVALID_SYNTAX 7 Indicates that the HIP message received was invalid because some type, length, or value was out of range or because the request was rejected for policy reasons. To avoid a denial of service attack using forged messages, this status may only be returned for packets whose HMAC (if present) and SIGNATURE have been verified. This status MUST be sent in response to any error not covered by one of the other status types, and should not contain details to avoid leaking information to someone probing a node. To aid debugging, more detailed error information SHOULD be written to a console or log. NO_DH_PROPOSAL_CHOSEN 14 None of the proposed group IDs was acceptable. INVALID_DH_CHOSEN 15 The D-H Group ID field does not correspond to one offered by the Responder. NO_HIP_PROPOSAL_CHOSEN 16 Moskowitz, et al. Expires December 13, 2007 [Page 52] Internet-Draft Host Identity Protocol June 2007 None of the proposed HIP Transform crypto suites was acceptable. INVALID_HIP_TRANSFORM_CHOSEN 17 The HIP Transform crypto suite does not correspond to one offered by the Responder. AUTHENTICATION_FAILED 24 Sent in response to a HIP signature failure, except when the signature verification fails in a NOTIFY message. CHECKSUM_FAILED 26 Sent in response to a HIP checksum failure. HMAC_FAILED 28 Sent in response to a HIP HMAC failure. ENCRYPTION_FAILED 32 The Responder could not successfully decrypt the ENCRYPTED parameter. INVALID_HIT 40 Sent in response to a failure to validate the peer's HIT from the corresponding HI. BLOCKED_BY_POLICY 42 The Responder is unwilling to set up an association for some policy reason (e.g. received HIT is NULL and policy does not allow opportunistic mode). SERVER_BUSY_PLEASE_RETRY 44 The Responder is unwilling to set up an association as it is suffering under some kind of overload and has chosen to shed load by rejecting your request. You may retry if you wish, however you MUST find another (different) puzzle solution for any such retries. Note that you may need to obtain a new puzzle with a new I1/R1 exchange. NOTIFY MESSAGES - STATUS TYPES Value Moskowitz, et al. Expires December 13, 2007 [Page 53] Internet-Draft Host Identity Protocol June 2007 ------------------------------ ----- I2_ACKNOWLEDGEMENT 16384 The Responder has received your I2 but had to queue the I2 for processing. The puzzle was correctly solved and the Responder is willing to set up an association but has currently a number of I2s in processing queue. R2 will be sent after the I2 has been processed. 5.2.17. ECHO_REQUEST_SIGNED 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque data (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 897 Length variable Opaque data Opaque data, supposed to be meaningful only to the node that sends ECHO_REQUEST_SIGNED and receives a corresponding ECHO_RESPONSE_SIGNED or ECHO_RESPONSE_UNSIGNED. The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data that the sender wants to get echoed back in the corresponding reply packet. The ECHO_REQUEST_SIGNED and corresponding echo response parameters MAY be used for any purpose where a node wants to carry some state in a request packet and get it back in a response packet. The ECHO_REQUEST_SIGNED is covered by the HMAC and SIGNATURE. A HIP packet can contain only one ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED parameter. The ECHO_REQUEST_SIGNED parameter MUST be responded with a corresponding echo response. ECHO_RESPONSE_SIGNED SHOULD be used, but if it is not possible, e.g. due to a middle-box provided response, it MAY be responded with an ECHO_RESPONSE_UNSIGNED. Moskowitz, et al. Expires December 13, 2007 [Page 54] Internet-Draft Host Identity Protocol June 2007 5.2.18. ECHO_REQUEST_UNSIGNED 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque data (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 63661 Length variable Opaque data Opaque data, supposed to be meaningful only to the node that sends ECHO_REQUEST_UNSIGNED and receives a corresponding ECHO_RESPONSE_UNSIGNED. The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data that the sender wants to get echoed back in the corresponding reply packet. The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters MAY be used for any purpose where a node wants to carry some state in a request packet and get it back in a response packet. The ECHO_REQUEST_UNSIGNED is not covered by the HMAC and SIGNATURE. A HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters. It is possible that middle-boxes add ECHO_REQUEST_UNSIGNED parameters in HIP packets passing by. The sender has to create the Opaque field so that it can later identify the corresponding ECHO_RESPONSE_UNSIGNED parameter. The ECHO_REQUEST_UNSIGNED parameter MUST be responded with an ECHO_RESPONSE_UNSIGNED parameter. 5.2.19. ECHO_RESPONSE_SIGNED 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque data (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 961 Length variable Opaque data Opaque data, copied unmodified from the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED parameter that triggered this response. Moskowitz, et al. Expires December 13, 2007 [Page 55] Internet-Draft Host Identity Protocol June 2007 The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back. The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED parameter. The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be used for any purpose where a node wants to carry some state in a request packet and get it back in a response packet. The ECHO_RESPONSE_SIGNED is covered by the HMAC and SIGNATURE. 5.2.20. ECHO_RESPONSE_UNSIGNED 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opaque data (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 63425 Length variable Opaque data Opaque data, copied unmodified from the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED parameter that triggered this response. The ECHO_RESPONSE_UNSIGNED parameter contains an opaque blob of data that the sender of the ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED wants to get echoed back. The opaque data is copied unmodified from the corresponding echo request parameter. The echo request and ECHO_RESPONSE_UNSIGNED parameters MAY be used for any purpose where a node wants to carry some state in a request packet and g