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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5201
Network Working Group R. Moskowitz
Internet-Draft ICSAlabs, a Division of TruSecure
Expires: September 3, 2006 Corporation
P. Nikander
P. Jokela (editor)
Ericsson Research NomadicLab
T. Henderson
The Boeing Company
March 2, 2006
Host Identity Protocol
draft-ietf-hip-base-05
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Copyright Notice
Copyright (C) The Internet Society (2006).
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
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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, suchs as TCP and UDP. Discussion related to this document
is going on at the IETF HIP Working Group mailing list.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. A New Name Space and Identifiers . . . . . . . . . . . . 5
1.2. The HIP Base Exchange . . . . . . . . . . . . . . . . . . 5
1.3. Memo structure . . . . . . . . . . . . . . . . . . . . . 6
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 7
2.1. Requirements Terminology . . . . . . . . . . . . . . . . 7
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
3. Host Identifier (HI) and its Representations . . . . . . . . 9
3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 9
3.2. Generating a HIT from a HI . . . . . . . . . . . . . . . 10
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
4.2. Updating a HIP Association . . . . . . . . . . . . . . . 17
4.3. Error Processing . . . . . . . . . . . . . . . . . . . . 18
4.4. HIP State Machine . . . . . . . . . . . . . . . . . . . . 19
4.4.1. HIP States . . . . . . . . . . . . . . . . . . . . . 20
4.4.2. HIP State Processes . . . . . . . . . . . . . . . . . 20
4.4.3. Simplified HIP State Diagram . . . . . . . . . . . . 27
4.5. User Data Considerations . . . . . . . . . . . . . . . . 29
4.5.1. TCP and UDP Pseudo-header Computation for User Data . 29
4.5.2. Sending Data on HIP Packets . . . . . . . . . . . . . 29
4.5.3. Transport Formats . . . . . . . . . . . . . . . . . . 29
4.5.4. Reboot and SA Timeout Restart of HIP . . . . . . . . 29
4.6. Certificate Distribution . . . . . . . . . . . . . . . . 30
5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 31
5.1. Payload Format . . . . . . . . . . . . . . . . . . . . . 31
5.1.1. Checksum . . . . . . . . . . . . . . . . . . . . . . 32
5.1.2. HIP Controls . . . . . . . . . . . . . . . . . . . . 32
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5.1.3. HIP Fragmentation Support . . . . . . . . . . . . . . 33
5.2. HIP Parameters . . . . . . . . . . . . . . . . . . . . . 33
5.2.1. TLV Format . . . . . . . . . . . . . . . . . . . . . 35
5.2.2. Defining New Parameters . . . . . . . . . . . . . . . 36
5.2.3. R1_COUNTER . . . . . . . . . . . . . . . . . . . . . 37
5.2.4. PUZZLE . . . . . . . . . . . . . . . . . . . . . . . 38
5.2.5. SOLUTION . . . . . . . . . . . . . . . . . . . . . . 39
5.2.6. DIFFIE_HELLMAN . . . . . . . . . . . . . . . . . . . 40
5.2.7. HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . . 41
5.2.8. HOST_ID . . . . . . . . . . . . . . . . . . . . . . . 42
5.2.9. HMAC . . . . . . . . . . . . . . . . . . . . . . . . 43
5.2.10. HMAC_2 . . . . . . . . . . . . . . . . . . . . . . . 43
5.2.11. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . . 44
5.2.12. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . . 45
5.2.13. SEQ . . . . . . . . . . . . . . . . . . . . . . . . . 45
5.2.14. ACK . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.2.15. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . . 47
5.2.16. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 48
5.2.17. ECHO_REQUEST . . . . . . . . . . . . . . . . . . . . 51
5.2.18. ECHO_RESPONSE . . . . . . . . . . . . . . . . . . . . 52
5.3. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . 52
5.3.1. I1 - the HIP Initiator Packet . . . . . . . . . . . . 53
5.3.2. R1 - the HIP Responder Packet . . . . . . . . . . . . 54
5.3.3. I2 - the Second HIP Initiator Packet . . . . . . . . 55
5.3.4. R2 - the Second HIP Responder Packet . . . . . . . . 57
5.3.5. UPDATE - the HIP Update Packet . . . . . . . . . . . 57
5.3.6. NOTIFY - the HIP Notify Packet . . . . . . . . . . . 58
5.3.7. CLOSE - the HIP Association Closing Packet . . . . . 59
5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet . . 59
5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 59
5.4.1. Invalid Version . . . . . . . . . . . . . . . . . . . 60
5.4.2. Other Problems with the HIP Header and Packet
Structure . . . . . . . . . . . . . . . . . . . . . . 60
5.4.3. Invalid Puzzle Solution . . . . . . . . . . . . . . . 60
5.4.4. Non-existing HIP Association . . . . . . . . . . . . 60
6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 62
6.1. Processing Outgoing Application Data . . . . . . . . . . 62
6.2. Processing Incoming Application Data . . . . . . . . . . 63
6.3. Solving the Puzzle . . . . . . . . . . . . . . . . . . . 64
6.4. HMAC and SIGNATURE Calculation and Verification . . . . . 65
6.4.1. HMAC Calculation . . . . . . . . . . . . . . . . . . 65
6.4.2. Signature Calculation . . . . . . . . . . . . . . . . 66
6.5. HIP KEYMAT Generation . . . . . . . . . . . . . . . . . . 67
6.6. Initiation of a HIP Exchange . . . . . . . . . . . . . . 68
6.6.1. Sending Multiple I1s in Parallel . . . . . . . . . . 69
6.6.2. Processing Incoming ICMP Protocol Unreachable
Messages . . . . . . . . . . . . . . . . . . . . . . 70
6.7. Processing Incoming I1 Packets . . . . . . . . . . . . . 70
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6.7.1. R1 Management . . . . . . . . . . . . . . . . . . . . 71
6.7.2. Handling Malformed Messages . . . . . . . . . . . . . 71
6.8. Processing Incoming R1 Packets . . . . . . . . . . . . . 71
6.8.1. Handling Malformed Messages . . . . . . . . . . . . . 73
6.9. Processing Incoming I2 Packets . . . . . . . . . . . . . 74
6.9.1. Handling Malformed Messages . . . . . . . . . . . . . 76
6.10. Processing Incoming R2 Packets . . . . . . . . . . . . . 76
6.11. Sending UPDATE Packets . . . . . . . . . . . . . . . . . 77
6.12. Receiving UPDATE Packets . . . . . . . . . . . . . . . . 78
6.12.1. Handling a SEQ parameter in a received UPDATE
message . . . . . . . . . . . . . . . . . . . . . . . 78
6.12.2. Handling an ACK Parameter in a Received UPDATE
Packet . . . . . . . . . . . . . . . . . . . . . . . 79
6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . . 80
6.14. Processing CLOSE Packets . . . . . . . . . . . . . . . . 80
6.15. Processing CLOSE_ACK Packets . . . . . . . . . . . . . . 80
6.16. Dropping HIP Associations . . . . . . . . . . . . . . . . 80
7. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 81
8. Security Considerations . . . . . . . . . . . . . . . . . . . 82
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 85
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 90
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 91
11.1. Normative References . . . . . . . . . . . . . . . . . . 91
11.2. Informative References . . . . . . . . . . . . . . . . . 92
Appendix A. Using Responder Puzzles . . . . . . . . . . . . . . 94
Appendix B. Generating a HIT from a HI . . . . . . . . . . . . . 95
Appendix C. Example Checksums for HIP Packets . . . . . . . . . 96
C.1. IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 96
C.2. IPv4 HIP Packet (I1) . . . . . . . . . . . . . . . . . . 96
C.3. TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 96
Appendix D. 384-bit Group . . . . . . . . . . . . . . . . . . . 98
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 99
Intellectual Property and Copyright Statements . . . . . . . . . 100
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1. Introduction
This memo specifies the details of the Host Identity Protocol (HIP).
A high-level description of the protocol and the underlying
architectural thinking is available in the separate HIP architecture
description [26]. 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 ayer 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,
including how HIP can be combined with a variant of the Encapsulating
Security Payload (ESP) for integrity protection and optional
encryption, mobility and multi-homing extensions to HIP, extensions
to the Domain Name System (DNS) for storing Host Identities there,
proposals on added HIP-related infrastructure into the networks, and
techniques for NAT traversal.
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 [26].
There are two main representations of the Host Identity, the full
Host Identifier (HI) and the Host Identity Tag (HIT The HI is a
public key and directly represents the Identity. Since there are
different public key algorithms that can be used with different key
lengths, the HI is not good for use as a packet identifier, or as an
index into the various operational tables needed to support HIP.
Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes
the operational representation. It is 128 bits long and is used in
the HIP payloads and to index the corresponding state in the end
hosts. 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
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establish communications context between hosts. The base exchange is
a Sigma-compliant [30] 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. [29]
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) [24] and other end-to-end security protocols. The base
protocol lacks the details for security association management and
much of the fine-grained policy control found in Internet Key
Exchange IKE RFC2409 [7] that allows IKE to support complex gateway
policies. Thus, HIP is not a replacement for IKE.
1.3. Memo structure
The rest of this memo is structured as follows. Section 2 defines
the central keywords, notation, and terms used throughout the rest of
the document. Section 3 defines the structure of the Host Identity
and its various representations. Section 4 gives an overview of the
HIP base exchange protocol. 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.
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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 [5].
2.2. Notation
[x] indicates that x is optional.
{x} indicates that x is encrypted.
X(y) indicates that y is a parameter of X.
<x>i indicates that x exists i times.
--> signifies "Initiator to Responder" communication (requests).
<-- signifies "Responder to Initiator" communication (replies).
| signifies concatenation of information-- e.g. X | Y is the
concatenation of X with Y.
Ltrunc (SHA-1(), K) denotes the lowest order K bits of the SHA-1
result.
2.3. Definitions
Unused Association Lifetime (UAL): Implementation-specific time for
which, if no packet is sent or received for this time interval, a
host MAY begin to tear down an active association.
Maximum Segment Lifetime (MSL): Maximum time that a TCP segment is
expected to spend in the network.
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 ~ 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 [25]
is used.
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Puzzle Hash Algorithm (PHASH): hash algorithm used to calculate the
puzzle hash. The algorithm is the same as is used to generate the
Responder's HIT.
Opportunistic mode: HIP base exchange where the Responder's HIT is
not a priori known to the Initiator.
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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 [26] for more details about the difference
between an identity and the corresponding identifier.
HIP implementations MUST support the Rivest Shamir Adelman (RSA) [15]
public key algorithm, and SHOULD support the Digital Signature
Algorithm (DSA) [13] 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.
"A Non-Routable IPv6 Prefix for Keyed Hash Identifiers" [22] has been
specified to store 128-bit hash based identifier called Keyed Hash
Identifier (KHI) under an 8-bit prefix, proposed to be allocated from
the IPv6 address block 1000::/4. The Host Identity Tag is a KHI
valid for the Context ID [22] value for HIP, 0xF0EF F02F BFF4 3D0F
E793 0C3C 6E61 74EA (The tag value has been generated randomly by the
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editor of this specification.) The following figure shows, for
informal purposes only, the format of a HIT specified by [22], and
used in this document:
1
0 2
0 1 2 3 4 5 6 7 8 ... 7
+-+-+-+-+-+-+-+-+-+-//-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix | Hash |
+-+-+-+-+-+-+-+-+-+-//-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix (8 bits) - Fixed prefix, TBD (0x11, TO BE DISCUSSED), as
defined per [22].
Encoding (120 bits) - Encoding of the public key and KHI context
identifier as defined per [22].
Additional values for the prefix (including different hash
algorithms, or other information) may be defined in the future. A
host may receive a HIT for which it does not understand the prefix.
In such a case, it will not be able to check the mapping between HI
and HIT.
3.2. Generating a HIT from a HI
The HIT MUST be generated according to the KHI generation method
described in [22] using a context ID value of 0xF0EF F02F BFF4 3D0F
E793 0C3C 6E61 74EA, and an input encoding the Host Identity field
(see Section 5.2.8) present in a HIP payload packet.
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 and DSA. Hence, either of the following applies:
The RSA public key is encoded as defined in RFC3110 [15] 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 [15].
The DSA public key is encoded as defined in RFC2536 [13] Section
2, taking the fields T, Q, P, G, and Y, concatenated. Thus, the
data to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long,
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where T is the size parameter as defined in RFC2536 [13]. The
size parameter T, affecting the field lengths, MUST be selected as
the minimum value that is long enough to accommodate P, G, and Y.
The fields MUST be encoded in network byte order, as defined in
RFC2536 [13].
In Appendix B the public key encoding generation process is
illustrated using pseudo-code.
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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 [19]).
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 [24].
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 a standard authenticated Diffie-Hellman key exchange for
session key generation. During the Diffie-Hellman key exchange, a
piece of keying material is generated. The HIP association keys are
drawn from this keying material. If other cryptographic keys are
needed, e.g., to be used with ESP, they are expected to be drawn from
the same keying material.
The Initiator first sends a trigger packet, I1, to the Responder.
The packet contains only the HIT of the Initiator and possibly the
HIT of the Responder, if it is known. 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
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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.
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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 algorithm complexity attacks become impossible [31].
The Responder can set the puzzle difficulty for Initiator, based on
its level of trust of the Initiator. The Responder SHOULD use
heuristics to determine when it is under a denial-of-service attack,
and set the puzzle difficulty value K appropriately; see below.
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 SHA-1
hash over this concatenation. The lowest order K bits of the result
MUST be zeros. The value K sets the difficulty of the puzzle.
To generate a proper number J, the Initiator will have to generate a
number of Js until one produces the hash target of zero. The
Initiator SHOULD give up after exceeding the puzzle lifetime in the
PUZZLE parameter (Section 5.2.4). The Responder needs to re-create
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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 parameter
(Section 5.2.17), 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 its public Diffie-Hellman key
and its public authentication key, i.e., its host identity, in R1.
The signature in R1 allows the Initiator to verify that the R1 has
been once generated by the Responder. However, since it is
precomputed and therefore does not cover all of the packet, it does
not protect from replay attacks.
When the Initiator receives an R1, it computes the Diffie-Hellman
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session key. 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
numbered R1s. Also, the R1 generation counter MUST NOT roll over; if
the counter is about to become exhausted, the corresponding HI must
be abandoned and replaced with a new one.
A host may receive more than one R1, either due to sending multiple
I1s (Section 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
after the first R1 reception to allow possibly multiple R1s to
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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.
4.2. 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.
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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 an HIP association
exists between the sender and receiver of a packet causing an error
condition, the receiver SHOULD respond with a NOTIFY packet. On the
other hand, if there are no existing HIP associations between the
sender and receiver, or the receiver cannot reasonably determine the
identity of the sender, the receiver MAY respond with a suitable ICMP
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 an HIP association, but the
receiver does not.
The system sends data on the outbound user data security
association. The receiver 'detects' the situation when it
receives a user data packet that it cannot match to any HIP
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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
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).
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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 |
+---------------------+---------------------------------------------+
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 |
| | |
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| Receive ANYOTHER | Drop and stay at UNASSOCIATED |
+---------------------+---------------------------------------------+
Table 2: UNASSOCIATED - Start state
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, go to E-FAILED |
| | |
| 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
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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 succesful 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, go to E-FAILED |
| | |
| Receive ANYOTHER | Drop and stay at I2-SENT |
| | |
| Timeout, increment | If counter is less than I2_RETRIES_MAX, |
| timeout counter | send I2 and stay at I2-SENT |
| | |
| | If counter is greater than I2_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 4: I2-SENT - Waiting to finish HIP
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System 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
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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
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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
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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
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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.
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+-+ +---------------------------+
I1 received, send R1 | | | |
| v v |
Datagram to send +--------------+ I2 received, send R2 |
+---------------| UNASSOCIATED |---------------+ |
| +--------------+ | |
v | |
+---------+ I2 received, send R2 | |
+---->| I1-SENT |---------------------------------------+ | |
| +---------+ | | |
| | +------------------------+ | | |
| | R1 received, | I2 received, send R2 | | | |
| v send I2 | v v v |
| +---------+ | +---------+ |
| +->| I2-SENT |------------+ | R2-SENT |<----+ |
| | +---------+ +---------+ | |
| | | | | |
| | | data| | |
| |receive | or| | |
| |R1, send | EC timeout| receive I2,| |
| |I2 |R2 received +--------------+ | send R2| |
| | +----------->| ESTABLISHED |<-------+| | |
| | +--------------+ | |
| | | | | | |
| | +------------+ | +------------------------+ |
| | recv| | | |
| | CLOSE,| No packet sent| | |
| | send| /received for | | |
| | CLOSE_ACK| UAL min, send | | |
| | | CLOSE | +---------+<-+ timeout | |
| | | +--->| CLOSING |--+ (UAL+MSL) | |
| | | +---------+ retransmit | |
+--|------------|----------------------+ | | | | CLOSE | |
| +------------|------------------------+ | | +----------------+ |
| | | +-----------+ +------------------|--+
| | +------------+ | receive CLOSE, CLOSE_ACK | |
| | | | send CLOSE_ACK received or | |
| | v v timeout | |
| | +--------+ (UAL+MSL) | |
| +------------------------| CLOSED |---------------------------+ |
+---------------------------+--------+------------------------------+
Datagram to send ^ | timeout (UAL+2MSL),
+-+ move to UNASSOCIATED
CLOSE received,
send CLOSE_ACK
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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 [11]
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 [24].
When new transport formats are defined, they get the type value from
the HIP Transform type value space 2048 - 4095. The order in which
the transport formats are presented in the R1 packet, is the
preferred order. The last of the transport formats MUST be ESP
transport format, represented by the ESP_TRANSFORM parameter.
4.5.4. Reboot and SA Timeout Restart of HIP
Simulating a loss of state is a potential DoS attack. The following
process has been crafted to manage state recovery without presenting
a DoS opportunity.
If a host reboots or the HIP association times out, it has lost its
HIP state. If the host that lost state has a datagram to send to the
peer, it simply restarts the HIP base exchange. After the base
exchange has completed, the Initiator can create a new SA and start
sending data. The peer does not reset its state until it receives a
valid I2 HIP packet.
If a system receives a user data packet that cannot be matched to any
existing HIP association, it is possible that it has lost the state
and its peer has not. It MAY send an ICMP packet with the Parameter
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Problem type, 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.
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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
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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 [27]. 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 devies.
If IPv6 is used to carry the HIP packet, the pseudo-header [11]
contains the source and destination IPv6 addresses, HIP packet length
in the pseudo-header length field, a zero field, and the HIP protocol
number (see Section 4) in the Next Header field. The length field is
in bytes and can be calculated from the HIP header length field: (HIP
Header Length + 1) * 8.
In case of using IPv4, the IPv4 UDP pseudo header format [1] is used.
In the pseudo header, the source and destination addresses are those
used in the IP header, the zero field is obviously zero, the protocol
is the HIP protocol number (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.
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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 MUST employ a reassembly algorithm that is
sufficiently resistant to DoS attacks.
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.
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+------------------+-------+----------+-----------------------------+
| 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. |
| | | | |
| NOTIFY | 832 | variable | Informational data |
| | | | |
| ECHO_REQUEST | 897 | variable | Opaque data to be echoed |
| | | | back; under signature |
| | | | |
| ECHO_RESPONSE | 961 | variable | Opaque data echoed back; |
| | | | under signature |
| | | | |
| HMAC | 61505 | 20 | HMAC based message |
| | | | authentication code, with |
| | | | key material from |
| | | | HIP_TRANSFORM |
| | | | |
| HMAC_2 | 61569 | 20 | HMAC based message |
| | | | authentication code, with |
| | | | key material from |
| | | | HIP_TRANSFORM |
| | | | |
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| HIP_SIGNATURE_2 | 61633 | variable | Signature of the R1 packet |
| | | | |
| HIP_SIGNATURE | 61697 | variable | Signature of the packet |
| | | | |
| ECHO_REQUEST | 63661 | variable | Opaque data to be echoed |
| | | | back; after signature |
| | | | |
| ECHO_RESPONSE | 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 order does not follow this rule, the packet is
considered to be malformed and it MUST be discarded.
Parameters using type values from 2048 up to 4095 are transport
formats. Currently, one transport format is defined: the ESP
transport format [24]. 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. If
padding is added, the Length field MUST NOT include the padding. Any
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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
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
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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. By default, sending of such a
new critical parameter SHOULD be off. In other words, the
management interface MUST allow vanilla standards-only mode as a
default configuration setting, and MAY allow new critical
payloads to be configured on (and off).
4. See section Section 9 for allocation rules regarding type codes.
5.2.3. R1_COUNTER
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R1 generation counter, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 128
Length 12
R1 generation
counter The current generation of valid puzzles
The R1_COUNTER parameter contains 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
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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.
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 parameter included in the R1; the contents of the
ECHO_REQUEST are then echoed back in the ECHO_RESPONSE, 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.
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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.
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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 /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 513
Length length in octets, excluding Type, Length, and
padding
Group ID defines values for p and g
Public Value the sender's public Diffie-Hellman key
The following Group IDs have been defined:
Group Value
Reserved 0
384-bit group 1
OAKLEY well known group 1 2
1536-bit MODP group 3
3072-bit MODP group 4
6144-bit MODP group 5
8192-bit MODP group 6
The MODP Diffie-Hellman groups are defined in [17]. The OAKLEY group
is defined in [8]. The OAKLEY well known group 5 is the same as the
1536-bit MODP group.
A HIP implementation MUST support Group IDs 1 and 3. The 384-bit
group can be used when lower security is enough (e.g. web surfing)
and when the equipment is not powerful enough (e.g. some PDAs).
Equipment powerful enough SHOULD implement also group ID 5. The 384-
bit group is defined in Appendix D.
To avoid unnecessary failures during the base exchange, the rest of
the groups SHOULD be implemented in hosts where resources are
adequate.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transform-ID #1 | Transform-ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transform-ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 577
Length length in octets, excluding Type, Length, and
padding
Transform-ID Defines the HIP Suite to be used
The following Suite-IDs are defined ([21],[10]):
Suite-ID Value
RESERVED 0
AES-CBC with HMAC-SHA1 1
3DES-CBC with HMAC-SHA1 2
3DES-CBC with HMAC-MD5 3
BLOWFISH-CBC with HMAC-SHA1 4
NULL-ENCRYPT with HMAC-SHA1 5
NULL-ENCRYPT with HMAC-MD5 6
There MUST NOT be more than six (6) HIP Suite-IDs in one HIP
transform parameter. The limited number of transforms sets the
maximum size of HIP_TRANSFORM parameter. The HIP_TRANSFORM parameter
MUST contain at least one of the mandatory Suite-IDs.
The Responder lists supported and desired Suite-IDs in order of
preference in the R1, up to the maximum of six Suite-IDs. In the I2,
the Initiator MUST choose and insert only one of the corresponding
Suite-IDs that will be used for generating the I2.
Mandatory implementations: AES-CBC with HMAC-SHA1 and NULL-ENCRYPTION
with HMAC-SHA1.
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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 [12] format. The
algorithms used in RDATA format are the following:
Algorithms Values
RESERVED 0
DSA 3 [RFC2536] (RECOMMENDED)
RSA 5 [RFC3110] (REQUIRED)
The following DI-types have been defined:
Type Value
none included 0
FQDN 1
NAI 2
FQDN Fully Qualified Domain Name, in binary format.
NAI Network Access Identifier
[23]
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The format for the FQDN is defined in RFC1035 [3] Section 3.1.
If there is no Domain Identifier, i.e. the DI-type field is zero,
also the DI Length field is set to zero.
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 |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 61505
Length 20
HMAC 160 low order bits of the HMAC computed over the
HIP packet, excluding the HMAC parameter and any
following parameters, such as HIP_SIGNATURE,
HIP_SIGNATURE_2, ECHO_REQUEST, or ECHO_RESPONSE.
The checksum field MUST be set to zero
and the HIP header length in the HIP common header
MUST be calculated not to cover any excluded
parameters when the HMAC is calculated.
The HMAC calculation and verification process is presented in
Section 6.4.1
5.2.10. HMAC_2
The parameter structure is the same as in Section 5.2.9. The fields
are:
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Type 61569
Length 20
HMAC 160 low order bits of the HMAC computed over the
HIP packet, excluding the HMAC parameter and any
following parameters such as HIP_SIGNATURE,
HIP_SIGNATURE_2, ECHO_REQUEST, or ECHO_RESPONSE,
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 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.
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 [15] in case of RSA, or
according to [13] in case of DSA).
The HIP_SIGNATURE calculation and verification process is presented
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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
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
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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.
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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
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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
[32] uses the PKCS5 padding scheme [14] (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. NOTIFY
The NOTIFY parameter is used to transmit informational data, such as
error conditions and state transitions, to a HIP peer. A NOTIFY
parameter may appear in the NOTIFY packet type. The use of the
NOTIFY parameter in other packet types is for further study.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ Notification data /
/ +---------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 832
Length length in octets, excluding Type, Length, and
Padding
Reserved zero when sent, ignored when received
Notify Message Specifies the type of notification
Type
Notification Informational or error data transmitted in addition
Data to the Notify Message Type. Values for this field
are type specific (see below).
Padding Any Padding, if necessary, to make the parameter a
multiple of 8 bytes.
Notification information can be error messages specifying why an SA
could not be established. It can also be status data that a process
managing an SA database wishes to communicate with a peer process.
The table below lists the Notification messages and their
corresponding values.
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To avoid certain types of attacks, a Responder SHOULD avoid sending a
NOTIFY to any host with which it has not successfully verified a
puzzle solution.
Types in the range 0 - 16383 are intended for reporting errors. An
implementation that receives a NOTIFY error parameter in response to
a request packet (e.g., I1, I2, UPDATE), SHOULD assume that the
corresponding request has failed entirely. Unrecognized error types
MUST be ignored except that they SHOULD be logged.
Notify payloads with status types MUST be ignored if not recognized.
NOTIFY PARAMETER - ERROR TYPES Value
------------------------------ -----
UNSUPPORTED_CRITICAL_PARAMETER_TYPE 1
Sent if the parameter type has the "critical" bit set and the
parameter type is not recognized. Notification Data contains
the two octet parameter type.
INVALID_SYNTAX 7
Indicates that the HIP message received was invalid because
some type, length, or value was out of range or because the
request was rejected for policy reasons. To avoid a denial of
service attack using forged messages, this status may only be
returned for packets whose HMAC (if present) and SIGNATURE have
been verified. This status MUST be sent in response to any
error not covered by one of the other status types, and should
not contain details to avoid leaking information to someone
probing a node. To aid debugging, more detailed error
information SHOULD be written to a console or log.
NO_DH_PROPOSAL_CHOSEN 14
None of the proposed group IDs was acceptable.
INVALID_DH_CHOSEN 15
The D-H Group ID field does not correspond to one offered
by the Responder.
NO_HIP_PROPOSAL_CHOSEN 16
None of the proposed HIP Transform crypto suites was
acceptable.
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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.
I2_ACKNOWLEDGEMENT 46
The Responder has received your I2 but had to queue
the I2 for processing. The puzzle was correctly solved
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and the Responder is willing to set up an association
but has currently a number of I2s in processing queue.
R2 will be sent after the I2 has been processed.
NOTIFY MESSAGES - STATUS TYPES Value
------------------------------ -----
(None defined at present)
5.2.17. ECHO_REQUEST
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63661 or 897
Length variable
Opaque data Opaque data, supposed to be meaningful only to the
node that sends ECHO_REQUEST and receives a
corresponding ECHO_RESPONSE.
The ECHO_REQUEST parameter contains an opaque blob of data that the
sender wants to get echoed back in the corresponding reply packet.
The ECHO_REQUEST and ECHO_RESPONSE parameters MAY be used for any
purpose where a node wants to carry some state in a request packet
and get it back in a response packet. The ECHO_REQUEST MAY be
covered by the HMAC and SIGNATURE. This is dictated by the Type
field selected for the parameter; Type 897 ECHO_REQUEST is covered
and Type 63661 is not covered. A HIP packet can contain only one
ECHO_REQUEST parameter.
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5.2.18. ECHO_RESPONSE
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63425 or 961
Length variable
Opaque data Opaque data, copied unmodified from the ECHO_REQUEST
parameter that triggered this response.
The ECHO_RESPONSE parameter contains an opaque blob of data that the
sender of the ECHO_REQUEST wants to get echoed back. The opaque data
is copied unmodified from the ECHO_REQUEST parameter.
The ECHO_REQUEST and ECHO_RESPONSE parameters MAY be used for any
purpose where a node wants to carry some state in a request packet
and get it back in a response packet. The ECHO_RESPONSE MAY be
covered by the HMAC and SIGNATURE. This is dictated by the Type
field selected for the parameter; Type 961 ECHO_RESPONSE is covered
and Type 63425 is not.
5.3. HIP Packets
There are eight basic HIP packets (see Table 11). Four are for the
HIP base exchange, one is for updating, one is for sending
notifications, and two for closing a HIP association.
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+-------------+---------------------------------------------------+
| Packet type | Packet name |
+-------------+---------------------------------------------------+
| 1 | I1 - the HIP Initiator Packet |
| | |
| 2 | R1 - the HIP Responder Packet |
| | |
| 3 | I2 - the Second HIP Initiator Packet |
| | |
| 4 | R2 - the Second HIP Responder Packet |
| | |
| 16 | UPDATE - the HIP Update Packet |
| | |
| 17 | NOTIFY - the HIP Notify Packet |
| | |
| 18 | CLOSE - the HIP Association Closing Packet |
| | |
| 19 | CLOSE_ACK - the HIP Closing Acknowledgment Packet |
+-------------+---------------------------------------------------+
Table 11: HIP packets and packet type numbers
Packets consist of the fixed header as described in Section 5.1,
followed by the parameters. The parameter part, in turn, consists of
zero or more parameter coded parameters.
In addition to the base packets, other packets types will be defined
later in separate specifications. For example, support for mobility
and multi-homing is not included in this specification.
See Notation (Section 2.2) for used operations.
In the future, an OPTIONAL upper layer payload MAY follow the HIP
header. The Next Header field in the header indicates if there is
additional data following the HIP header. The HIP packet, however,
MUST NOT be fragmented. This limits the size of the possible
additional data in the packet.
5.3.1. I1 - the HIP Initiator Packet
The HIP header values for the I1 packet:
Header:
Packet Type = 1
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT, or NULL
IP ( HIP () )
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The I1 packet contains only the fixed HIP header.
Valid control bits: none
The Initiator gets the Responder's HIT either from a DNS lookup of
the Responder's FQDN, from some other repository, or from a local
table. If the Initiator does not know the Responder's HIT, it may
attempt opportunistic mode by using NULL (all zeros) as the
Responder's HIT. If the Initiator sends a NULL as the Responder's
HIT, it MUST be able to handle all MUST and SHOULD algorithms from
Section 3, which are currently RSA and DSA.
Since this packet is so easy to spoof even if it were signed, no
attempt is made to add to its generation or processing cost.
Implementations MUST be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
5.3.2. R1 - the HIP Responder Packet
The HIP header values for the R1 packet:
Header:
Packet Type = 2
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( [ R1_COUNTER, ]
PUZZLE,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
HOST_ID,
[ ECHO_REQUEST, ]
HIP_SIGNATURE_2 )
[, ECHO_REQUEST ])
Valid control bits: A
If the Responder HI is an anonymous one, the A control MUST be set.
The Initiator HIT MUST match the one received in I1. If the
Responder has multiple HIs, the Responder HIT used MUST match
Initiator's request. If the Initiator used opportunistic mode, the
Responder may select freely among its HIs.
The R1 generation counter is used to determine the currently valid
generation of puzzles. The value is increased periodically, and it
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is RECOMMENDED that it is increased at least as often as solutions to
old puzzles are no longer accepted.
The Puzzle contains a random #I and the difficulty K. The difficulty
K is the number of bits that the Initiator must get zero in the
puzzle. The random #I is not covered by the signature and must be
zeroed during the signature calculation, allowing the sender to
select and set the #I into a pre-computed R1 just prior sending it to
the peer.
The Diffie-Hellman value is ephemeral, but can be reused over a
number of connections. In fact, as a defense against I1 storms, an
implementation MAY use the same Diffie-Hellman value for a period of
time, for example, 15 minutes. By using a small number of different
puzzles for a given Diffie-Hellman value, the R1 packets can be pre-
computed and delivered as quickly as I1 packets arrive. A scavenger
process should clean up unused DHs and puzzles.
The HIP_TRANSFORM contains the encryption and integrity algorithms
supported by the Responder to protect the HI exchange, in the order
of preference. All implementations MUST support the AES [18] with
HMAC-SHA-1-96 [6].
The ECHO_REQUEST contains data that the sender wants to receive
unmodified in the corresponding response packet in the ECHO_RESPONSE
parameter. The ECHO_REQUEST can be either covered by the signature,
or it can be left out from it. In the first case, the ECHO_REQUEST
gets Type number 897 and in the latter case 63661.
The signature is calculated over the whole HIP envelope, after
setting the Initiator HIT, header checksum as well as the Opaque
field and the Random #I in the PUZZLE parameter temporarily to zero,
and excluding any parameters that follow the signature, as described
in Section 5.2.12. This allows the Responder to use precomputed R1s.
The Initiator SHOULD validate this signature. It SHOULD check that
the Responder HI received matches with the one expected, if any.
5.3.3. I2 - the Second HIP Initiator Packet
The HIP header values for the I2 packet:
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Header:
Type = 3
SRC HIT = Initiator's HIT
DST HIT = Responder's HIT
IP ( HIP ( [R1_COUNTER,]
SOLUTION,
DIFFIE_HELLMAN,
HIP_TRANSFORM,
ENCRYPTED { HOST_ID } or HOST_ID,
[ ECHO_RESPONSE ,]
HMAC,
HIP_SIGNATURE
[, ECHO_RESPONSE] ) )
Valid control bits: A
The HITs used MUST match the ones used previously.
If the Initiator HI is an anonymous one, the A control MUST be set.
The Initiator MAY include an unmodified copy of the R1_COUNTER
parameter received in the corresponding R1 packet into the I2 packet.
The Solution contains the random # I from R1 and the computed # J.
The low order K bits of the PHASH(I | ... | J) MUST be zero.
The Diffie-Hellman value is ephemeral. If precomputed, a scavenger
process should clean up unused DHs.
The HIP_TRANSFORM contains the single encryption and integrity
transform selected by the Initiator, that will be used to protect the
HI exchange. The chosen transform MUST correspond to one offered by
the Responder in the R1. All implementations MUST support the AES
transform [18].
The Initiator's HI MAY be encrypted using the HIP_TRANSFORM
encryption algorithm. The keying material is derived from the
Diffie-Hellman exchanged as defined in Section 6.5.
The ECHO_RESPONSE contains the unmodified Opaque data copied from the
corresponding ECHO_REQUEST parameter. The ECHO_RESPONSE can be
either covered by the HMAC and SIGNATURE or not covered. In the
former case, the ECHO_RESPONSE gets Type number 961, in the latter it
is 63425.
The HMAC is calculated over whole HIP envelope, excluding any
parameters after the HMAC, as described in Section 6.4.1. The
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Responder MUST validate the HMAC.
The signature is calculated over whole HIP envelope, excluding any
parameters after the HIP_SIGNATURE, as described in Section 5.2.11.
The Responder MUST validate this signature. It MAY use either the HI
in the packet or the HI acquired by some other means.
5.3.4. R2 - the Second HIP Responder Packet
The HIP header values for the R2 packet:
Header:
Packet Type = 4
SRC HIT = Responder's HIT
DST HIT = Initiator's HIT
IP ( HIP ( HMAC_2, HIP_SIGNATURE ) )
Valid control bits: none
The HMAC_2 is calculated over whole HIP envelope, with Responder's
HOST_ID parameter concatenated with the HIP envelope. The HOST_ID
parameter is removed after the HMAC calculation. The procedure is
described in 8.3.1.
The signature is calculated over whole HIP envelope.
The Initiator MUST validate both the HMAC and the signature.
5.3.5. UPDATE - the HIP Update Packet
Support for the UPDATE packet is MANDATORY.
The HIP header values for the UPDATE packet:
Header:
Packet Type = 16
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( [SEQ, ACK, ] HMAC, HIP_SIGNATURE ) )
Valid control bits: None
The UPDATE packet contains mandatory HMAC and HIP_SIGNATURE
parameters, and other optional parameters.
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The UPDATE packet contains zero or one SEQ parameter. The presence
of a SEQ parameter indicates that the receiver MUST ack the UPDATE.
An UPDATE that does not contain a SEQ parameter is simply an ACK of a
previous UPDATE and itself MUST not be acked.
An UPDATE packet contains zero or one ACK parameters. The ACK
parameter echoes the SEQ sequence number of the UPDATE packet being
acked. A host MAY choose to ack more than one UPDATE packet at a
time; e.g., the ACK may contain the last two SEQ values received, for
robustness to ack loss. ACK values are not cumulative; each received
unique SEQ value requires at least one corresponding ACK value in
reply. Received ACKs that are redundant are ignored.
The UPDATE packet may contain both a SEQ and an ACK parameter. In
this case, the ACK is being piggybacked on an outgoing UPDATE. In
general, UPDATEs carrying SEQ SHOULD be acked upon completion of the
processing of the UPDATE. A host MAY choose to hold the UPDATE
carrying ACK for a short period of time to allow for the possibility
of piggybacking the ACK parameter, in a manner similar to TCP delayed
acknowledgments.
A sender MAY choose to forego reliable transmission of a particular
UPDATE (e.g., it becomes overcome by events). The semantics are such
that the receiver MUST acknowledge the UPDATE but the sender MAY
choose to not care about receiving the ACK.
UPDATEs MAY be retransmitted without incrementing SEQ. If the same
subset of parameters is included in multiple UPDATEs with different
SEQs, the host MUST ensure that receiver processing of the parameters
multiple times will not result in a protocol error.
5.3.6. NOTIFY - the HIP Notify Packet
The NOTIFY packet is OPTIONAL. The NOTIFY packet MAY be used to
provide information to a peer. Typically, NOTIFY is used to indicate
some type of protocol error or negotiation failure. NOTIFY packets
are unacknowledged.
The HIP header values for the NOTIFY packet:
Header:
Packet Type = 17
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT, or zero if unknown
IP ( HIP (<NOTIFY>i, [HOST_ID, ] HIP_SIGNATURE) )
Valid control bits: None
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The NOTIFY packet is used to carry one or more NOTIFY parameters.
5.3.7. CLOSE - the HIP Association Closing Packet
The HIP header values for the CLOSE packet:
Header:
Packet Type = 18
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( ECHO_REQUEST, HMAC, HIP_SIGNATURE ) )
Valid control bits: none
The sender MUST include an ECHO_REQUEST used to validate CLOSE_ACK
received in response, and both an HMAC and a signature (calculated
over the whole HIP envelope).
The receiver peer MUST validate both the HMAC and the signature if it
has a HIP association state, and MUST reply with a CLOSE_ACK
containing an ECHO_REPLY corresponding to the received ECHO_REQUEST.
5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet
The HIP header values for the CLOSE_ACK packet:
Header:
Packet Type = 19
SRC HIT = Sender's HIT
DST HIT = Recipient's HIT
IP ( HIP ( ECHO_REPLY, HMAC, HIP_SIGNATURE ) )
Valid control bits: none
The sender MUST include both an HMAC and signature (calculated over
the whole HIP envelope).
The receiver peer MUST validate both the HMAC and the signature.
5.4. ICMP Messages
When a HIP implementation detects a problem with an incoming packet,
and it either cannot determine the identity of the sender of the
packet or does not have any existing HIP association with the sender
of the packet, it MAY respond with an ICMP packet. Any such replies
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MUST be rate limited as described in [4]. In most cases, the ICMP
packet will have the Parameter Problem type (12 for ICMPv4, 4 for
ICMPv6), with the Pointer field pointing to the field that caused the
ICMP message to be generated.
5.4.1. Invalid Version
If a HIP implementation receives a HIP packet that has an
unrecognized HIP version number, it SHOULD respond, rate limited,
with an ICMP packet with type Parameter Problem, the Pointer pointing
to the VER./RES. byte in the HIP header.
5.4.2. Other Problems with the HIP Header and Packet Structure
If a HIP implementation receives a HIP packet that has other
unrecoverable problems in the header or packet format, it MAY
respond, rate limited, with an ICMP packet with type Parameter
Problem, the Pointer pointing to the field that failed to pass the
format checks. However, an implementation MUST NOT send an ICMP
message if the Checksum fails; instead, it MUST silently drop the
packet.
5.4.3. Invalid Puzzle Solution
If a HIP implementation receives an I2 packet that has an invalid
puzzle solution, the behavior depends on the underlying version of
IP. If IPv6 is used, the implementation SHOULD respond with an ICMP
packet with type Parameter Problem, the Pointer pointing to the
beginning of the Puzzle solution #J field in the SOLUTION payload in
the HIP message.
If IPv4 is used, the implementation MAY respond with an ICMP packet
with the type Parameter Problem, copying enough of bytes from the I2
message so that the SOLUTION parameter fits into the ICMP message,
the Pointer pointing to the beginning of the Puzzle solution #J
field, as in the IPv6 case. Note, however, that the resulting ICMPv4
message exceeds the typical ICMPv4 message size as defined in [2].
5.4.4. Non-existing HIP Association
If a HIP implementation receives a CLOSE, or UPDATE packet, or any
other packet whose handling requires an existing association, that
has either a Receiver or Sender HIT that does not match with any
existing HIP association, the implementation MAY respond, rate
limited, with an ICMP packet with the type Parameter Problem, the
Pointer pointing to the beginning of the first HIT that does not
match.
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A host MUST NOT reply with such an ICMP if it receives any of the
following messages: I1, R2, I2, R2, and NOTIFY. When introducing new
packet types, a specification SHOULD define the appropriate rules for
sending or not sending this kind of ICMP replies.
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6. Packet Processing
Each host is assumed to have a single HIP protocol implementation
that manages the host's HIP associations and handles requests for new
ones. Each HIP association is governed by a conceptual state
machine, with states defined above in Section 4.4. The HIP
implementation can simultaneously maintain HIP associations with more
than one host. Furthermore, the HIP implementation may have more
than one active HIP association with another host; in this case, HIP
associations are distinguished by their respective HITs. It is not
possible to have more than one HIP association between any given pair
of HITs. Consequently, the only way for two hosts to have more than
one parallel association is to use different HITs, at least at one
end.
The processing of packets depends on the state of the HIP
association(s) with respect to the authenticated or apparent
originator of the packet. A HIP implementation determines whether it
has an active association with the originator of the packet based on
the HITs. In the case of user data carried in a specific transport
format, the transport format document specifies how the incoming
packets are matched with the active associations.
6.1. Processing Outgoing Application Data
In a HIP host, an application can send application level data using
an identifier specified via the underlying API. The API can be a
backwards compatible API (see [28]), using identifiers that look
similar to IP addresses, or a completely new API, providing enhanced
services related to Host Identities. Depending on the HIP
implementation, the identifier provided to the application may be
different; it can be e.g. a HIT or an IP address.
The exact format and method for transferring the data from the source
HIP host to the destination HIP host is defined in the corresponding
transport format document. The actual data is transferred in the
network using the appropriate source and destination IP addresses.
In this document, conceptual processing rules are defined only for
the base case where both hosts have only single usable IP addresses;
the multi-address multi-homing case will be specified separately.
The following conceptual algorithm describes the steps that are
required for handling outgoing datagrams destined to a HIT.
1. If the datagram has a specified source address, it MUST be a HIT.
If it is not, the implementation MAY replace the source address
with a HIT. Otherwise it MUST drop the packet.
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2. If the datagram has an unspecified source address, the
implementation must choose a suitable source HIT for the
datagram.
3. If there is no active HIP association with the given < source,
destination > HIT pair, one must be created by running the base
exchange. While waiting for the base exchange to complete, the
implementation SHOULD queue at least one packet per HIP
association to be formed, and it MAY queue more than one.
4. Once there is an active HIP association for the given < source,
destination > HIT pair, the outgoing datagram is passed to
transport handling. The possible transport formats are defined
in separate documents, of which the ESP transport format for HIP
is mandatory for all HIP implementations.
5. Before sending the packet, the HITs in the datagram are replaced
with suitable IP addresses. For IPv6, the rules defined in [16]
SHOULD be followed. Note that this HIT-to-IP-address conversion
step MAY also be performed at some other point in the stack,
e.g., before wrapping the packet into the output format.
6.2. Processing Incoming Application Data
The following conceptual algorithm describes the incoming datagram
handling when HITs are used at the receiving host as application
level identifiers. More detailed steps for processing packets are
defined in corresponding transport format documents.
1. The incoming datagram is mapped to an existing HIP association,
typically using some information from the packet. For example,
such mapping may be based on ESP Security Parameter Index (SPI).
2. The specific transport format is unwrapped, in a way depending on
the transport format, yielding a packet that looks like a
standard (unencrypted) IP packet. If possible, this step SHOULD
also verify that the packet was indeed (once) sent by the remote
HIP host, as identified by the HIP association.
3. The IP addresses in the datagram are replaced with the HITs
associated with the HIP association. Note that this IP-address-
to-HIT conversion step MAY also be performed at some other point
in the stack.
4. The datagram is delivered to the upper layer. Demultiplexing the
datagram the right upper layer socket is based on the HITs.
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6.3. Solving the Puzzle
This subsection describes the puzzle solving details.
In R1, the values I and K are sent in network byte order. Similarly,
in I2 the values I and J are sent in network byte order. The SHA-1
hash is created by concatenating, in network byte order, the
following data, in the following order:
64-bit random value I, in network byte order, as appearing in R1
and I2.
128-bit Initiator HIT, in network byte order, as appearing in the
HIP Payload in R1 and I2.
128-bit Responder HIT, in network byte order, as appearing in the
HIP Payload in R1 and I2.
64-bit random value J, in network byte order, as appearing in I2.
In order to be a valid response puzzle, the K low-order bits of the
resulting PHASH digest must be zero.
Notes:
i) The length of the data to be hashed is 48 bytes.
ii) All the data in the hash input MUST be in network byte order.
iii) The order of the Initiator and Responder HITs are different
in the R1 and I2 packets, see Section 5.1. Care must be taken to
copy the values in right order to the hash input.
The following procedure describes the processing steps involved,
assuming that the Responder chooses to precompute the R1 packets:
Precomputation by the Responder:
Sets up the puzzle difficulty K.
Creates a signed R1 and caches it.
Responder:
Selects a suitable cached R1.
Generates a random number I.
Sends I and K in an R1.
Saves I and K for a Delta time.
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Initiator:
Generates repeated attempts to solve the puzzle until a matching J
is found:
Ltrunc( PHASH( I | HIT-I | HIT-R | J ), K ) == 0
Sends I and J in an I2.
Responder:
Verifies that the received I is a saved one.
Finds the right K based on I.
Computes V := Ltrunc( PHASH( I | HIT-I | HIT-R | J ), K )
Rejects if V != 0
Accept if V == 0
6.4. HMAC and SIGNATURE Calculation and Verification
The following subsections define the actions for processing HMAC,
HIP_SIGNATURE and HIP_SIGNATURE_2 parameters.
6.4.1. HMAC Calculation
The following process applies both to the HMAC and HMAC_2 parameters.
When processing HMAC_2, the difference is that the HMAC calculation
includes a pseudo HOST_ID field containing the Responder's
information as sent in the R1 packet earlier.
Both the Initiator and the Responder should take some care when
verifying or calculating the HMAC_2. Specifically, the Responder
should preserve other parameters than the HOST_ID when sending the
R2. Also, the Initiator has to preserve the HOST_ID exactly as it
was received in the R1 packet.
The HMAC parameter is defined in Section 5.2.9 and HMAC_2 parameter
in Section 5.2.10. HMAC calculation and verification process:
Packet sender:
1. Create the HIP packet, without the HMAC or any possible
HIP_SIGNATURE or HIP_SIGNATURE_2 parameters.
2. In case of HMAC_2 calculation, add a HOST_ID (Responder)
parameter to the packet.
3. Calculate the Length field in the HIP header.
4. Compute the HMAC.
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5. In case of HMAC_2, remove the HOST_ID parameter from the packet.
6. Add the HMAC parameter to the packet and any HIP_SIGNATURE or
HIP_SIGNATURE_2 parameters that may follow.
7. Recalculate the Length field in the HIP header.
Packet receiver:
1. Verify the HIP header Length field.
2. Remove the HMAC or HMAC_2 parameter, and if the packet contains
any HIP_SIGNATURE or HIP_SIGNATURE_2 fields, remove them too,
saving the contents if they will be needed later.
3. In case of HMAC_2, build and add a HOST_ID parameter (with
Responder information) to the packet. The HOST_ID parameter
should be identical to the one previously received from the
Responder.
4. Recalculate the HIP packet length in the HIP header and clear the
Checksum field (set it to all zeros).
5. Compute the HMAC and verify it against the received HMAC.
6. In case of HMAC_2, remove the HOST_ID parameter from the packet
before further processing.
6.4.2. Signature Calculation
The following process applies both to the HIP_SIGNATURE and
HIP_SIGNATURE_2 parameters. When processing HIP_SIGNATURE_2, the
only difference is that instead of HIP_SIGNATURE parameter, the
HIP_SIGNATURE_2 parameter is used, and the Initiator's HIT and PUZZLE
Opaque and Random #I fields are cleared (set to all zeros) before
computing the signature. The HIP_SIGNATURE parameter is defined in
Section 5.2.11 and the HIP_SIGNATURE_2 parameter in Section 5.2.12.
Signature calculation and verification process:
Packet sender:
1. Create the HIP packet without the HIP_SIGNATURE parameter or any
parameters that follow the HIP_SIGNATURE parameter.
2. Calculate the Length field and zero the Checksum field in the HIP
header.
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3. Compute the signature.
4. Add the HIP_SIGNATURE parameter to the packet.
5. Add any parameters that follow the HIP_SIGNATURE parameter.
6. Recalculate the Length field in the HIP header, and calculate the
Checksum field.
Packet receiver:
1. Verify the HIP header Length field.
2. Save the contents of the HIP_SIGNATURE parameter and any
parameters following the HIP_SIGNATURE parameter and remove them
from the packet.
3. Recalculate the HIP packet Length in the HIP header and clear the
Checksum field (set it to all zeros).
4. Compute the signature and verify it against the received
signature.
The verification can use either the HI received from a HIP packet,
the HI from a DNS query, if the FQDN has been received in the HOST_ID
packet, or one received by some other means.
6.5. HIP KEYMAT Generation
HIP keying material is derived from the Diffie-Hellman Kij produced
during the HIP base exchange. The Initiator has Kij during the
creation of the I2 packet, and the Responder has Kij once it receives
the I2 packet. This is why I2 can already contain encrypted
information.
The KEYMAT is derived by feeding Kij and the HITs into the following
operation; the | operation denotes concatenation.
KEYMAT = K1 | K2 | K3 | ...
where
K1 = SHA-1( Kij | sort(HIT-I | HIT-R) | I | J | 0x01 )
K2 = SHA-1( Kij | K1 | 0x02 )
K3 = SHA-1( Kij | K2 | 0x03 )
...
K255 = SHA-1( Kij | K254 | 0xff )
K256 = SHA-1( Kij | K255 | 0x00 )
etc.
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Sort(HIT-I | HIT-R) is defined as the network byte order
concatenation of the two HITs, with the smaller HIT preceding the
larger HIT, resulting from the numeric comparison of the two HITs
interpreted as positive (unsigned) 128-bit integers in network byte
order.
I and J values are from the puzzle and its solution that were
exchanged in R1 and I2 messages when this HIP association was set up.
Both hosts have to store I and J values for the HIP association for
future use.
The initial keys are drawn sequentially in the order that is
determined by the numeric comparison of the two HITs, with comparison
method described in the previous paragraph. HOST_g denotes the host
with the greater HIT value, and HOST_l the host with the lower HIT
value.
The drawing order for initial keys:
HIP-gl encryption key for HOST_g's outgoing HIP packets
HIP-gl integrity (HMAC) key for HOST_g's outgoing HIP packets
HIP-lg encryption key (currently unused) for HOST_l's outgoing HIP
packets
HIP-lg integrity (HMAC) key for HOST_l's outgoing HIP packets
The number of bits drawn for a given algorithm is the "natural" size
of the keys. For the mandatory algorithms, the following sizes
apply:
AES 128 bits
SHA-1 160 bits
NULL 0 bits
6.6. Initiation of a HIP Exchange
An implementation may originate a HIP exchange to another host based
on a local policy decision, usually triggered by an application
datagram, in much the same way that an IPsec IKE key exchange can
dynamically create a Security Association. Alternatively, a system
may initiate a HIP exchange if it has rebooted or timed out, or
otherwise lost its HIP state, as described in Section 4.5.4.
The implementation prepares an I1 packet and sends it to the IP
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address that corresponds to the peer host. The IP address of the
peer host may be obtained via conventional mechanisms, such as DNS
lookup. The I1 contents are specified in Section 5.3.1. The
selection of which host identity to use, if a host has more than one
to choose from, is typically a policy decision.
The following steps define the conceptual processing rules for
initiating a HIP exchange:
1. The Initiator gets the Responder's HIT and one or more addresses
either from a DNS lookup of the Responder's FQDN, from some other
repository, or from a local table. If the Initiator does not
know the Responder's HIT, it may attempt opportunistic mode by
using NULL (all zeros) as the Responder's HIT.
2. The Initiator sends an I1 to one of the Responder's addresses.
The selection of which address to use is a local policy decision.
3. Upon sending an I1, the sender shall transition to state I1-SENT,
start a timer whose timeout value should be larger than the
worst-case anticipated RTT, and shall increment a timeout counter
associated with the I1.
4. Upon timeout, the sender SHOULD retransmit the I1 and restart the
timer, up to a maximum of I1_RETRIES_MAX tries.
6.6.1. Sending Multiple I1s in Parallel
For the sake of minimizing the session establishment latency, an
implementation MAY send the same I1 to more than one of the
Responder's addresses. However, it MUST NOT send to more than three
(3) addresses in parallel. Furthermore, upon timeout, the
implementation MUST refrain from sending the same I1 packet to
multiple addresses. These limitations are placed order to avoid
congestion of the network, and potential DoS attacks that might
happen, e.g., because someone claims to have hundreds or thousands of
addresses.
As the Responder is not guaranteed to distinguish the duplicate I1's
it receives at several of its addresses (because it avoids to store
states when it answers back an R1), the Initiator may receive several
duplicate R1's.
The Initiator SHOULD then select the initial preferred destination
address using the source address of the selected received R1, and use
the preferred address as a source address for the I2. Processing
rules for received R1s are discussed in Section 6.8.
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6.6.2. Processing Incoming ICMP Protocol Unreachable Messages
A host may receive an ICMP Destination Protocol Unreachable message
as a response to sending an HIP I1 packet. Such a packet may be an
indication that the peer does not support HIP, or it may be an
attempt to launch an attack by making the Initiator believe that the
Responder does not support HIP.
When a system receives an ICMP Destination Protocol Unreachable
message while it is waiting for an R1, it MUST NOT terminate the
wait. It MAY continue as if it had not received the ICMP message,
and send a few more I1s. Alternatively, it MAY take the ICMP message
as a hint that the peer most probably does not support HIP, and
return to state UNASSOCIATED earlier than otherwise. However, at
minimum, it MUST continue waiting for an R1 for a reasonable time
before returning to UNASSOCIATED.
6.7. Processing Incoming I1 Packets
An implementation SHOULD reply to an I1 with an R1 packet, unless the
implementation is unable or unwilling to setup a HIP association. If
the implementation is unable to setup a HIP association, the host
SHOULD send an ICMP Destination Protocol Unreachable,
Administratively Prohibited, message to the I1 source address. If
the implementation is unwilling to setup a HIP association, the host
MAY ignore the I1. This latter case may occur during a DoS attack
such as an I1 flood.
The implementation MUST be able to handle a storm of received I1
packets, discarding those with common content that arrive within a
small time delta.
A spoofed I1 can result in an R1 attack on a system. An R1 sender
MUST have a mechanism to rate limit R1s to an address.
It is RECOMMENDED that the HIP state machine does not transition upon
sending an R1.
The following steps define the conceptual processing rules for
responding to an I1 packet:
1. The Responder MUST check that the Responder HIT in the received
I1 is either one of its own HITs, or NULL.
2. If the Responder is in ESTABLISHED state, the Responder MAY
respond to this with an R1 packet, prepare to drop existing SAs
and stay at ESTABLISHED state.
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3. If the Responder is in I1-SENT state, it must make a comparison
between the sender's HIT and its own HIT. If the sender's HIT is
greater than its own HIT, it should drop the I1 and stay at I1-
SENT. If the sender's HIT is smaller than its own HIT, it should
send R1 and stay at I1-SENT. The HIT comparison goes similarly
as in Section 6.5.
4. If the implementation chooses to respond to the I1 with an R1
packet, it creates a new R1 or selects a precomputed R1 according
to the format described in Section 5.3.2.
5. The R1 MUST contain the received Responder HIT, unless the
received HIT is NULL, in which case the Responder SHOULD select a
HIT that is constructed with the MUST algorithm in Section 3,
which is currently RSA. Other than that, selecting the HIT is a
local policy matter.
6. The Responder sends the R1 to the source IP address of the I1
packet.
6.7.1. R1 Management
All compliant implementations MUST produce R1 packets. An R1 packet
MAY be precomputed. An R1 packet MAY be reused for time Delta T,
which is implementation dependent. R1 information MUST not be
discarded until Delta S after T. Time S is the delay needed for the
last I2 to arrive back to the Responder.
An implementation MAY keep state about received I1s and match the
received I2s against the state, as discussed in Section 4.1.1.
6.7.2. Handling Malformed Messages
If an implementation receives a malformed I1 message, it SHOULD NOT
respond with a NOTIFY message, as such practice could open up a
potential denial-of-service danger. Instead, it MAY respond with an
ICMP packet, as defined in Section 5.4.
6.8. Processing Incoming R1 Packets
A system receiving an R1 MUST first check to see if it has sent an I1
to the originator of the R1 (i.e., it is in state I1-SENT). If so,
it SHOULD process the R1 as described below, send an I2, and go to
state I2-SENT, setting a timer to protect the I2. If the system is
in state I2-SENT, it MAY respond to an R1 if the R1 has a larger R1
generation counter; if so, it should drop its state due to processing
the previous R1 and start over from state I1-SENT. If the system is
in any other state with respect to that host, it SHOULD silently drop
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the R1.
When sending multiple I1s, an Initiator SHOULD wait for a small
amount of time after the first R1 reception to allow possibly
multiple R1s to arrive, and it SHOULD respond to an R1 among the set
with the largest R1 generation counter.
The following steps define the conceptual processing rules for
responding to an R1 packet:
1. A system receiving an R1 MUST first check to see if it has sent
an I1 to the originator of the R1 (i.e., it has a HIP
association that is in state I1-SENT and that is associated with
the HITs in the R1. IP addresses in the received R1 packet
SHOULD be ignored and the match SHOULD be based on HITs only).
If so, it should process the R1 as described below. Note that
when the connection was initialized in opportunistic mode, HITs
cannot be used, but the Initiator must rely on the Responder's
IP address in the received R1 packet.
2. Otherwise, if the system is in any other state than I1-SENT or
I2-SENT with respect to the HITs included in the R1, it SHOULD
silently drop the R1 and remain in the current state.
3. If the HIP association state is I1-SENT or I2-SENT, the received
Initiator's HIT MUST correspond to the HIT used in the original,
I1 and the Responder's HIT MUST correspond to the one used,
unless the I1 contained a NULL HIT.
4. The system SHOULD validate the R1 signature before applying
further packet processing, according to Section 5.2.12.
5. If the HIP association state is I1-SENT, and multiple valid R1s
are present, the system SHOULD select from among the R1s with
the largest R1 generation counter.
6. If the HIP association state is I2-SENT, the system MAY reenter
state I1-SENT and process the received R1 if it has a larger R1
generation counter than the R1 responded to previously.
7. The R1 packet may have the A bit set -- in this case, the system
MAY choose to refuse it by dropping the R1 and returning to
state UNASSOCIATED. The system SHOULD consider dropping the R1
only if it used a NULL HIT in I1. If the A bit is set, the
Responder's HIT is anonymous and should not be stored.
8. The system SHOULD attempt to validate the HIT against the
received Host Identity.
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9. The system MUST store the received R1 generation counter for
future reference.
10. The system attempts to solve the puzzle in R1. The system MUST
terminate the search after exceeding the remaining lifetime of
the puzzle. If the puzzle is not successfully solved, the
implementation may either resend I1 within the retry bounds or
abandon the HIP exchange.
11. The system computes standard Diffie-Hellman keying material
according to the public value and Group ID provided in the
DIFFIE_HELLMAN parameter. The Diffie-Hellman keying material
Kij is used for key extraction as specified in Section 6.5. If
the received Diffie-Hellman Group ID is not supported, the
implementation may either resend I1 within the retry bounds or
abandon the HIP exchange.
12. The system selects the HIP transform from the choices presented
in the R1 packet and uses the selected values subsequently when
generating and using encryption keys, and when sending the I2.
If the proposed alternatives are not acceptable to the system,
it may either resend I1 within the retry bounds or abandon the
HIP exchange.
13. The system initializes the remaining variables in the associated
state, including Update ID counters.
14. The system prepares and sends an I2, as described in
Section 5.3.3.
15. The system SHOULD start a timer whose timeout value should be
larger than the worst-case anticipated RTT, and MUST increment a
timeout counter associated with the I2. The sender SHOULD
retransmit the I2 upon a timeout and restart the timer, up to a
maximum of I2_RETRIES_MAX tries.
16. If the system is in state I1-SENT, it shall transition to state
I2-SENT. If the system is in any other state, it remains in the
current state.
6.8.1. Handling Malformed Messages
If an implementation receives a malformed R1 message, it MUST
silently drop the packet. Sending a NOTIFY or ICMP would not help,
as the sender of the R1 typically doesn't have any state. An
implementation SHOULD wait for some more time for a possible good R1,
after which it MAY try again by sending a new I1 packet.
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6.9. Processing Incoming I2 Packets
Upon receipt of an I2, the system MAY perform initial checks to
determine whether the I2 corresponds to a recent R1 that has been
sent out, if the Responder keeps such state. For example, the sender
could check whether the I2 is from an address or HIT that has
recently received an R1 from it. The R1 may have had Opaque data
included that was echoed back in the I2. If the I2 is considered to
be suspect, it MAY be silently discarded by the system.
Otherwise, the HIP implementation SHOULD process the I2. This
includes validation of the puzzle solution, generating the Diffie-
Hellman key, decrypting the Initiator's Host Identity, verifying the
signature, creating state, and finally sending an R2.
The following steps define the conceptual processing rules for
responding to an I2 packet:
1. The system MAY perform checks to verify that the I2 corresponds
to a recently sent R1. Such checks are implementation
dependent. See Appendix A for a description of an example
implementation.
2. The system MUST check that the Responder's HIT corresponds to
one of its own HITs.
3. If the system is in the R2-SENT state, it MAY check if the newly
received I2 is similar to the one that triggered moving to R2-
SENT. If so, it MAY retransmit a previously sent R2, reset the
R2-SENT timer, and stay in R2-SENT.
4. If the system is in the I2-SENT state, it makes a comparison
between its local and sender's HITs (similarly as in
Section 6.5). If the local HIT is smaller than the sender's
HIT, it should drop the I2 packet. Otherwise, the system should
process the received I2 packet.
5. To avoid the possibility to end up with different session keys
due to symmetric operation of the peer nodes, the Diffie-Hellman
key, I, and J selection is also based on the HIT comparison. If
the local HIT is smaller than the peer HIT, the system uses peer
Diffie-Hellman key and nonce I from the R1 packet received
earlier. The local Diffie-Hellman key and nonce J are taken
from the I2 packet sent to the peer earlier. Otherwise, it uses
peer Diffie-Hellman key and nonce J from the just arrived I2.
The local Diffie-Hellman key and nonce I are the ones that it
sent ealier in the R1 packet.
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6. If the system is in any other state than R2-SENT, it SHOULD
check that the echoed R1 generation counter in I2 is within the
acceptable range. Implementations MUST accept puzzles from the
current generation and MAY accept puzzles from earlier
generations. If the newly received I2 is outside the accepted
range, the I2 is stale (perhaps replayed) and SHOULD be dropped.
7. The system MUST validate the solution to the puzzle by computing
the hash described in Section 5.3.3 using the same hash
algorithm used to generate the Responder's HIT.
8. The I2 MUST have a single value in the HIP_TRANSFORM parameter,
which MUST match one of the values offered to the Initiator in
the R1 packet.
9. The system must derive Diffie-Hellman keying material Kij based
on the public value and Group ID in the DIFFIE_HELLMAN
parameter. This key is used to derive the HIP association keys,
as described in Section 6.5. If the Diffie-Hellman Group ID is
unsupported, the I2 packet is silently dropped.
10. The encrypted HOST_ID decrypted by the Initiator encryption key
defined in Section 6.5. If the decrypted data is not a HOST_ID
parameter, the I2 packet is silently dropped.
11. The implementation SHOULD also verify that the Initiator's HIT
in the I2 corresponds to the Host Identity sent in the I2.
12. The system MUST verify the HMAC according to the procedures in
Section 5.2.9.
13. The system MUST verify the HIP_SIGNATURE according to
Section 5.2.11 and Section 5.3.3.
14. If the checks above are valid, then the system proceeds with
further I2 processing; otherwise, it discards the I2 and remains
in the same state.
15. The I2 packet may have the A bit set -- in this case, the system
MAY choose to refuse it by dropping the I2 and returning to
state UNASSOCIATED. If the A bit is set, the Initiator's HIT is
anonymous and should not be stored.
16. The system initializes the remaining variables in the associated
state, including Update ID counters.
17. Upon successful processing of an I2 in states UNASSOCIATED, I1-
SENT, I2-SENT, and R2-SENT, an R2 is sent and the state machine
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transitions to state R2-SENT.
18. Upon successful processing of an I2 in state ESTABLISHED, the
old HIP association is dropped and a new one is installed, an R2
is sent, and the state machine transitions to R2-SENT.
19. Upon transitioning to R2-SENT, start a timer. Move to
ESTABLISHED if some data has been received on the incoming HIP
association, or an UPDATE packet has been received (or some
other packet that indicates that the peer has moved to
ESTABLISHED). If the timer expires (allowing for maximal
retransmissions of I2s), move to UNASSOCIATED.
6.9.1. Handling Malformed Messages
If an implementation receives a malformed I2 message, the behavior
SHOULD depend on how much checks the message has already passed. If
the puzzle solution in the message has already been checked, the
implementation SHOULD report the error by responding with a NOTIFY
packet. Otherwise the implementation MAY respond with an ICMP
message as defined in Section 5.4.
6.10. Processing Incoming R2 Packets
An R2 received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
results in the R2 being dropped and the state machine staying in the
same state. If an R2 is received in state I2-SENT, it SHOULD be
processed.
The following steps define the conceptual processing rules for
incoming R2 packet:
1. The system MUST verify that the HITs in use correspond to the
HITs that were received in R1.
2. The system MUST verify the HMAC_2 according to the procedures in
Section 5.2.10.
3. The system MUST verify the HIP signature according to the
procedures in Section 5.2.11.
4. If any of the checks above fail, there is a high probability of
an ongoing man-in-the-middle or other security attack. The
system SHOULD act accordingly, based on its local policy.
5. If the system is in any other state than I2-SENT, the R2 is
silently dropped.
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6. Upon successful processing of the R2, the state machine moves to
state ESTABLISHED.
6.11. Sending UPDATE Packets
A host sends an UPDATE packet when it wants to update some
information related to a HIP association. There are a number of
likely situations, e.g. mobility management and rekeying of an
existing ESP Security Association. The following paragraphs define
the conceptual rules for sending an UPDATE packet to the peer.
Additional steps can be defined in other documents where the UPDATE
packet is used.
The system first determines whether there are any outstanding UPDATE
messages that may conflict with the new UPDATE message under
consideration. When multiple UPDATEs are outstanding (not yet
acknowledged), the sender must assume that such UPDATEs may be
processed in an arbitrary order. Therefore, any new UPDATEs that
depend on a previous outstanding UPDATE being successfully received
and acknowledged MUST be postponed until reception of the necessary
ACK(s) occurs. One way to prevent any conflicts is to only allow one
outstanding UPDATE at a time, but allowing multiple UPDATEs may
improve the performance of mobility and multihoming protocols.
1. The first UPDATE packet is sent with Update ID of zero.
Otherwise, the system increments its own Update ID value by one
before continuing the below steps.
2. The system creates an UPDATE packet that contains a SEQ parameter
with the current value of Update ID. The UPDATE packet may also
include an ACK of the peer's Update ID found in a received UPDATE
SEQ parameter, if any.
3. The system sends the created UPDATE packet and starts an UPDATE
timer. The default value for the timer is 2 * RTT estimate. If
multiple UPDATEs are outstanding, multiple timers are in effect.
4. If the UPDATE timer expires, the UPDATE is resent. The UPDATE
can be resent UPDATE_RETRY_MAX times. The UPDATE timer SHOULD be
exponentially backed off for subsequent retransmissions. If no
acknowledgment is received from the peer after UPDATE_RETRY_MAX
times, the HIP association is considered to be broken and the
state machine should move from state ESTABLISHED to state CLOSING
as depicted in Section 4.4.3. The UPDATE timer is cancelled upon
receiving an ACK from the peer that acknowledges receipt of the
UPDATE.
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6.12. Receiving UPDATE Packets
When a system receives an UPDATE packet, its processing depends on
the state of the HIP association and the presence of and values of
the SEQ and ACK parameters. Typically, an UPDATE message also
carries optional parameters whose handling is defined in separate
documents.
For each association, the peer's next expected in-sequence Update ID
("peer Update ID") is stored. Initially, this value is zero. Update
ID comparisons of "less than" and "greater than" are performed with
respect to a circular sequence number space.
The sender may send multiple outstanding UPDATE messages. These
messages are processed in the order in which they are received at the
receiver (i.e., no resequencing is performed). When processing
UPDATEs out-of-order, the receiver MUST keep track of which UPDATEs
were previously processed, so that duplicates or retransmissions are
ACKed and not reprocessed. A receiver MAY choose to define a receive
window of Update IDs that it is willing to process at any given time,
and discard received UPDATEs falling outside of that window.
1. If there is no corresponding HIP association, the implementation
MAY reply with an ICMP Parameter Problem, as specified in
Section 5.4.4.
2. If the association is in the ESTABLISHED state and the SEQ (but
not ACK) parameter is present, the UPDATE is processed and
replied as described in Section 6.12.1.
3. If the association is in the ESTABLISHED state and the ACK (but
not SEQ) parameter is present, the UPDATE is processed as
described in Section 6.12.2.
4. If the association is in the ESTABLISHED state and there is both
an ACK and SEQ in the UPDATE, the ACK is first processed as
described in Section 6.12.2 and then the rest of the UPDATE is
processed as described in Section 6.12.1.
6.12.1. Handling a SEQ parameter in a received UPDATE message
1. If the Update ID in the received SEQ is not the next in sequence
Update ID and is greater than the receiver's window for new
UPDATEs, the packet MUST be dropped.
2. If the Update ID in the received SEQ corresponds to an UPDATE
that has recently been processed, the packet is treated as a
retransmission. The HMAC verification (next step) MUST NOT be
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skipped. (A byte-by-byte comparison of the received and a stored
packet would be OK, though.) It is recommended that a host cache
UPDATE packets sent with ACKs to avoid the cost of generating a
new ACK packet to respond to a replayed UPDATE. The system MUST
acknowledge, again, such (apparent) UPDATE message
retransmissions but SHOULD also consider rate-limiting such
retransmission responses to guard against replay attacks.
3. The system MUST verify the HMAC in the UPDATE packet. If the
verification fails, the packet MUST be dropped.
4. The system MAY verify the SIGNATURE in the UPDATE packet. If the
verification fails, the packet SHOULD be dropped and an error
message logged.
5. If a new SEQ parameter is being processed, the parameters in the
UPDATE are then processed. The system MUST record the Update ID
in the received SEQ parameter, for replay protection.
6. An UPDATE acknowledgement packet with ACK parameter is prepared
and sent to the peer. This ACK parameter may be included in a
separate UPDATE or piggybacked in an UPDATE with SEQ parameter,
as described in Section Section 5.3.5. The ACK parameter MAY
acknowledge more than one of the peer's Update IDs.
6.12.2. Handling an ACK Parameter in a Received UPDATE Packet
1. The sequence number reported in the ACK must match with an
earlier sent UPDATE packet that has not already been
acknowledged. If no match is found or if the ACK does not
acknowledge a new UPDATE, the packet MUST either be dropped if no
SEQ parameter is present, or the processing steps in
Section 6.12.1 are followed.
2. The system MUST verify the HMAC in the UPDATE packet. If the
verification fails, the packet MUST be dropped.
3. The system MAY verify the SIGNATURE in the UPDATE packet. If the
verification fails, the packet SHOULD be dropped and an error
message logged.
4. The corresponding UPDATE timer is stopped (see Section 6.11) so
that the now acknowledged UPDATE is no longer retransmitted. If
multiple UPDATEs are newly acknowledged, multiple timers are
stopped.
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6.13. Processing NOTIFY Packets
Processing NOTIFY packets is OPTIONAL. If processed, any errors
noted by the NOTIFY parameter SHOULD be taken into account by the HIP
state machine (e.g., by terminating a HIP handshake), and the error
SHOULD be logged.
6.14. Processing CLOSE Packets
When the host receives a CLOSE message it responds with a CLOSE_ACK
message and moves to CLOSED state. (The authenticity of the CLOSE
message is verified using both HMAC and SIGNATURE). This processing
applies whether or not the HIP association state is CLOSING in order
to handle CLOSE messages from both ends crossing in flight.
The HIP association is not discarded before the host moves from the
UNASSOCIATED state.
Once the closing process has started, any need to send data packets
will trigger creating and establishing of a new HIP association,
starting with sending an I1.
If there is no corresponding HIP association, the CLOSE packet is
dropped.
6.15. Processing CLOSE_ACK Packets
When a host receives a CLOSE_ACK message it verifies that it is in
CLOSING or CLOSED state and that the CLOSE_ACK was in response to the
CLOSE (using the included ECHO_REPLY in response to the sent
ECHO_REQUEST).
The CLOSE_ACK uses HMAC and SIGNATURE for verification. The state is
discarded when the state changes to UNASSOCIATED and, after that, the
host MAY respond with an ICMP Parameter Problem to an incoming CLOSE
message (See Section 5.4.4).
6.16. Dropping HIP Associations
A HIP implementation is free to drop a HIP association at any time,
based on its own policy. If a HIP host decides to drop a HIP
association, it deletes the corresponding HIP state, including the
keying material. The implementation MUST also drop the peer's R1
generation counter value, unless a local policy explicitly defines
that the value of that particular host is stored. An implementation
MUST NOT store R1 generation counters by default, but storing R1
generation counter values, if done, MUST be configured by explicit
HITs.
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7. HIP Policies
There are a number of variables that will influence the HIP exchanges
that each host must support. All HIP implementations MUST support
more than one simultaneous HIs, at least one of which SHOULD be
reserved for anonymous usage. Although anonymous HIs will be rarely
used as Responder HIs, they will be common for Initiators. Support
for more than two HIs is RECOMMENDED.
Many Initiators would want to use a different HI for different
Responders. The implementations SHOULD provide for an ACL of
Initiator HIT to Responder HIT. This ACL SHOULD also include
preferred transform and local lifetimes.
The value of K used in the HIP R1 packet can also vary by policy. K
should never be greater than 20, but for trusted partners it could be
as low as 0.
Responders would need a similar ACL, representing which hosts they
accept HIP exchanges, and the preferred transform and local
lifetimes. Wildcarding SHOULD be supported for this ACL also.
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8. Security Considerations
HIP is designed to provide secure authentication of hosts. HIP also
attempts to limit the exposure of the host to various denial-of-
service and man-in-the-middle (MitM) attacks. In so doing, HIP
itself is subject to its own DoS and MitM attacks that potentially
could be more damaging to a host's ability to conduct business as
usual.
Denial-of-service attacks take advantage of the cost of start of
state for a protocol on the Responder compared to the 'cheapness' on
the Initiator. HIP makes no attempt to increase the cost of the
start of state on the Initiator, but makes an effort to reduce the
cost to the Responder. This is done by having the Responder start
the 3-way exchange instead of the Initiator, making the HIP protocol
4 packets long. In doing this, packet 2 becomes a 'stock' packet
that the Responder MAY use many times. The duration of use is a
paranoia versus throughput concern. Using the same Diffie-Hellman
values and random puzzle #I has some risk. This risk needs to be
balanced against a potential storm of HIP I1 packets.
This shifting of the start of state cost to the Initiator in creating
the I2 HIP packet, presents another DoS attack. The attacker spoofs
the I1 HIP packet and the Responder sends out the R1 HIP packet.
This could conceivably tie up the 'Initiator' with evaluating the R1
HIP packet, and creating the I2 HIP packet. The defense against this
attack is to simply ignore any R1 packet where a corresponding I1 was
not sent.
A second form of DoS attack arrives in the I2 HIP packet. Once the
attacking Initiator has solved the puzzle, it can send packets with
spoofed IP source addresses with either invalid encrypted HIP payload
component or a bad HIP signature. This would take resources in the
Responder's part to reach the point to discover that the I2 packet
cannot be completely processed. The defense against this attack is
after N bad I2 packets, the Responder would discard any I2s that
contain the given Initiator HIT. Thus will shut down the attack.
The attacker would have to request another R1 and use that to launch
a new attack. The Responder could up the value of K while under
attack. On the downside, valid I2s might get dropped too.
A third form of DoS attack is emulating the restart of state after a
reboot of one of the partners. A host restarting would send an I1 to
a peer, which would respond with an R1 even if it were in the
ESTABLISHED state. If the I1 were spoofed, the resulting R1 would be
received unexpectedly by the spoofed host and would be dropped, as in
the first case above.
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A fourth form of DoS attack is emulating the end of state. HIP
relies on timers plus a CLOSE/CLOSE_ACK handshake to explicitly
signals the end of a state. Because both CLOSE and CLOSE_ACK
messages contain an HMAC, an outsider cannot close a connection. The
presence of an additional SIGNATURE allows middle-boxes to inspect
these messages and discard the associated state (for e.g.,
firewalling, SPI-based NATing, etc.). However, the optional behavior
of replying to CLOSE with an ICMP Parameter Problem packet (as
described in Section 5.4.4) might allow an IP spoofer sending CLOSE
messages to launch reflection attacks.
A fifth form of DoS attack is replaying R1s to cause the Initiator to
solve stale puzzles and become out of synchronization with the
Responder. The R1 generation counter is a monotonically increasing
counter designed to protect against this attack, as described in
section Section 4.1.4.
Man-in-the-middle attacks are difficult to defend against, without
third-party authentication. A skillful MitM could easily handle all
parts of HIP; but HIP indirectly provides the following protection
from a MitM attack. If the Responder's HI is retrieved from a signed
DNS zone, a certificate, or through some other secure means, the
Initiator can use this to validate the R1 HIP packet.
Likewise, if the Initiator's HI is in a secure DNS zone, a trusted
certificate, or otherwise securely available, the Responder can
retrieve it after it gets the I2 HIP packet and validate that.
However, since an Initiator may choose to use an anonymous HI, it
knowingly risks a MitM attack. The Responder may choose not to
accept a HIP exchange with an anonymous Initiator.
If an Initiator wants to use opportunistic mode, it is vulnerable to
man-in-the-middle attacks. Furthermore, the available HI types are
limited to the MUST implement algorithms, as per Section 3. Hence,
if a future specification deprecates the current MUST implement
algorithm(s) and replaces it (them) with some new one(s), backward
compatibility cannot be preserved.
Since not all hosts will ever support HIP, ICMP 'Destination Protocol
Unreachable' are to be expected and present a DoS attack. Against an
Initiator, the attack would look like the Responder does not support
HIP, but shortly after receiving the ICMP message, the Initiator
would receive a valid R1 HIP packet. Thus to protect from this
attack, an Initiator should not react to an ICMP message until a
reasonable delta time to get the real Responder's R1 HIP packet. A
similar attack against the Responder is more involved. First an ICMP
message is expected if the I1 was a DoS attack and the real owner of
the spoofed IP address does not support HIP. The Responder SHOULD
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NOT act on this ICMP message to remove the minimal state from the R1
HIP packet (if it has one), but wait for either a valid I2 HIP packet
or the natural timeout of the R1 HIP packet. This is to allow for a
sophisticated attacker that is trying to break up the HIP exchange.
Likewise, the Initiator should ignore any ICMP message while waiting
for an R2 HIP packet, deleting state only after a natural timeout.
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9. IANA Considerations
This document specifies the IP protocol number 253 to be used with
Host Identity Protocol during the experimental phase. This number
has been reserved by IANA for experimental use (see [19].
This document defines a new 128-bit value under the CGA Message Type
namespace [20], 0xF0EF F02F BFF4 3D0F E793 0C3C 6E61 74EA.
This document also creates a set of new name spaces. These are
described below.
Packet Type
The 7-bit Packet Type field in a HIP protocol packet describes the
type of a HIP protocol message. It is defined in Section 5.1.
The current values are defined in Section 5.3.1 through
Section 5.3.8 and are listed below:
* I1 is 1.
* R1 is 2.
* I2 is 3.
* R2 is 4.
* UPDATE is 16.
* NOTIFY is 17.
* CLOSE is 18.
* CLOSE_ACK is 19.
New values are assigned through IETF Consensus [9].
HIP Version
The four bit Version field in a HIP protocol packet describes the
version of the HIP protocol. It is defined in Section 5.1. The
only currently defined value is 1. New values are assigned
through IETF Consensus.
Parameter Type
The 16 bit Type field in a HIP parameters describes the type of
the parameter. It is defined in Section 5.2.1. The current
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values are defined in Section 5.2.3 through Section 5.2.18 and are
listed below:
* R1_COUNTER is 128.
* PUZZLE is 257.
* SOLUTION is 321.
* SEQ is 385.
* ACK is 449.
* DIFFIE_HELLMAN is 513.
* HIP_TRANSFORM is 577.
* ENCRYPTED is 641.
* HOST_ID is 705.
* CERT is 768.
* NOTIFY is 832.
* ECHO_REQUEST is 897.
* ECHO_RESPONSE is 961.
* HMAC is 61505.
* HMAC_2 is 61569.
* HIP_SIGNATURE_2 is 61633.
* HIP_SIGNATURE is 61697.
* ECHO_REQUEST is 63661.
* ECHO_RESPONSE is 63425.
The type codes 0 through 1023 and 61440 through 65535 are reserved
for future base protocol extensions, and are assigned through IETF
Consensus.
The type codes 32768 through 49141 are reserved for
experimentation and private use. Types SHOULD be selected in a
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random fashion from this range, thereby reducing the probability
of collisions. A method employing genuine randomness (such as
flipping a coin) SHOULD be used.
All other type codes are assigned through First Come First Served,
with Specification Required [9].
Group ID
The eight bit Group ID values appear in the DIFFIE_HELLMAN
parameter, defined in Section 5.2.6. The currently defined values
are listed below:
* 384-bit group is 1.
* OAKLEY well known group 1 is 2.
* 1536-bit MODP group is 3.
* 3072-bit MODP group is 4.
* 6144-bit MODP group is 5.
* 8192-bit MODP group is 6.
* Value 0 is reserved.
New values either from the reserved or unassigned space are
assigned through IETF Consensus.
Suite ID
The 16 bit Suite ID values in a HIP_TRANSFORM parameter are
defined in Section 5.2.7. The currently defined values are listed
below:
* AES-CBC with HMAC-SHA1 is 1.
* 3DES-CBC with HMAC-SHA1 is 2.
* 3DES-CBC with HMAC-MD5 is 3.
* BLOWFISH-CBC with HMAC-SHA1 is 4.
* NULL-ENCRYPT with HMAC-SHA1 is 5.
* NULL-ENCRYPT with HMAC-MD5 is 6.
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* Value 0 is reserved.
New values either from the reserved or unassigned space are
assigned through IETF Consensus.
DI-Type
The four bit DI-Type values in a HOST_ID parameter are defined in
Section 5.2.8. The currently defined values are listed below:
* None included is 0.
* FQDN is 1.
* NAI is 2.
New values are assigned through IETF Consensus.
Notify Message Type
The 16 bit Notify Message Type field in a NOTIFY parameter is
defined in Section 5.2.16. The currently defined values are
listed below:
* UNSUPPORTED_CRITICAL_PARAMETER_TYPE is 1.
* INVALID_SYNTAX is 7.
* NO_DH_PROPOSAL_CHOSEN is 14.
* INVALID_DH_CHOSEN is 15.
* NO_HIP_PROPOSAL_CHOSEN is 16.
* INVALID_HIP_TRANSFORM_CHOSEN is 17.
* AUTHENTICATION_FAILED is 24.
* CHECKSUM_FAILED is 26.
* HMAC_FAILED is 28.
* ENCRYPTION_FAILED is 32.
* INVALID_HIT is 40.
* BLOCKED_BY_POLICY is 42.
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* SERVER_BUSY_PLEASE_RETRY is 44.
New values are assigned through First Come First Served, with
Specification Required.
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10. Acknowledgments
The drive to create HIP came to being after attending the MALLOC
meeting at the 43rd IETF meeting. Baiju Patel and Hilarie Orman
really gave the original author, Bob Moskowitz, the assist to get HIP
beyond 5 paragraphs of ideas. It has matured considerably since the
early drafts thanks to extensive input from IETFers. Most
importantly, its design goals are articulated and are different from
other efforts in this direction. Particular mention goes to the
members of the NameSpace Research Group of the IRTF. Noel Chiappa
provided the framework for LSIs and Keith Moore the impetus to
provide resolvability. Steve Deering provided encouragement to keep
working, as a solid proposal can act as a proof of ideas for a
research group.
Many others contributed; extensive security tips were provided by
Steve Bellovin. Rob Austein kept the DNS parts on track. Paul
Kocher taught Bob Moskowitz how to make the puzzle exchange expensive
for the Initiator to respond, but easy for the Responder to validate.
Bill Sommerfeld supplied the Birthday concept, which later evolved
into the R1 generation counter, to simplify reboot management. Erik
Nordmark supplied CLOSE-mechanism for closing connections. Rodney
Thayer and Hugh Daniels provide extensive feedback. In the early
times of this draft, John Gilmore kept Bob Moskowitz challenged to
provide something of value.
During the later stages of this document, when the editing baton was
transfered to Pekka Nikander, the input from the early implementors
were invaluable. Without having actual implementations, this
document would not be on the level it is now.
In the usual IETF fashion, a large number of people have contributed
to the actual text or ideas. The list of these people include Jeff
Ahrenholz, Francis Dupont, Derek Fawcus, George Gross, Andrew
McGregor, Julien Laganier, Miika Komu, Mika Kousa, Jan Melen, Henrik
Petander, Michael Richardson, Tim Shepard, Jorma Wall, and Jukka
Ylitalo. Our apologies to anyone whose name is missing.
Once the HIP Working Group was founded in early 2004, a number of
changes were introduced through the working group process. Most
notably, the original draft was split in two, one containing the base
exchange and the other one defining how to use ESP. Some
modifications to the protocol proposed by Aura et al. [29] were added
at a later stage.
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11. References
11.1. Normative References
[1] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[2] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC 1885,
December 1995.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
and AH", RFC 2404, November 1998.
[7] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[8] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412,
November 1998.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[10] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher Algorithms",
RFC 2451, November 1998.
[11] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[12] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[13] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
(DNS)", RFC 2536, March 1999.
[14] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898, September 2000.
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[15] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
System (DNS)", RFC 3110, May 2001.
[16] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[17] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003.
[18] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September 2003.
[19] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[20] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[21] Schiller, J., "Cryptographic Algorithms for use in the Internet
Key Exchange Version 2", draft-ietf-ipsec-ikev2-algorithms-05
(work in progress), April 2004.
[22] Nikander, P., "A Non-Routable IPv6 Prefix for Keyed Hash
Identifiers (KHI)", draft-laganier-ipv6-khi-00 (work in
progress), September 2005.
[23] Aboba, B., "The Network Access Identifier",
draft-ietf-radext-rfc2486bis-06 (work in progress), July 2005.
[24] Jokela, P., "Using ESP transport format with HIP",
draft-ietf-hip-esp-01 (work in progress), October 2005.
[25] NIST, "FIPS PUB 180-1: Secure Hash Standard", April 1995.
11.2. Informative References
[26] Moskowitz, R. and P. Nikander, "Host Identity Protocol
Architecture", draft-ietf-hip-arch-03 (work in progress),
August 2005.
[27] Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
protocol", draft-ietf-shim6-proto-03 (work in progress),
December 2005.
[28] Henderson, T. and P. Nikander, "Using HIP with Legacy
Applications", draft-henderson-hip-applications-01 (work in
progress), July 2005.
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[29] Aura, T., Nagarajan, A., and A. Gurtov, "Analysis of the HIP
Base Exchange Protocol", in Proceedings of 10th Australasian
Conference on Information Security and Privacy, July 2003.
[30] Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and Its Use in the IKE-Protocols",
in Proceedings of CRYPTO 2003, pages 400-425, August 2003.
[31] Crosby, SA. and DS. Wallach, "Denial of Service via Algorithmic
Complexity Attacks", in Proceedings of Usenix Security
Symposium 2003, Washington, DC., August 2003.
[32] NIST, "FIPS PUB 197: Advanced Encryption Standard", Nov 2001.
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Appendix A. Using Responder Puzzles
As mentioned in Section 4.1.1, the Responder may delay state creation
and still reject most spoofed I2s by using a number of pre-calculated
R1s and a local selection function. This appendix defines one
possible implementation in detail. The purpose of this appendix is
to give the implementors an idea on how to implement the mechanism.
If the implementation is based on this appendix, it MAY contain some
local modification that makes an attacker's task harder.
The Responder creates a secret value S, that it regenerates
periodically. The Responder needs to remember two latest values of
S. Each time the S is regenerated, R1 generation counter value is
incremented by one.
The Responder generates a pre-signed R1 packet. The signature for
pre-generated R1s must be recalculated when the Diffie-Hellman key is
recomputed or when the R1_COUNTER value changes due to S value
regeneration.
When the Initiator sends the I1 packet for initializing a connection,
the Responder gets the HIT and IP address from the packet, and
generates an I-value for the puzzle. The I value is set to the pre-
signed R1 packet.
I value calculation:
I = Ltrunc( PHASH ( S | HIT-I | HIT-R | IP-I | IP-R ), 64)
The PHASH algorithm is the same that is used to generate the
Responder's HIT value.
From an incoming I2 packet, the Responder gets the required
information to validate the puzzle: HITs, IP addresses, and the
information of the used S value from the R1_COUNTER. Using these
values, the Responder can regenerate the I, and verify it against the
I received in the I2 packet. If the I values match, it can verify
the solution using I, J, and difficulty K. If the I values do not
match, the I2 is dropped.
puzzle_check:
V := Ltrunc( PHASH( I2.I | I2.hit_i | I2.hit_r | I2.J ), K )
if V != 0, drop the packet
If the puzzle solution is correct, the I and J values are stored for
later use. They are used as input material when keying material is
generated.
The Responder SHOULD NOT keep state about failed puzzle solutions.
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Appendix B. Generating a HIT from a HI
The following pseudo-codes illustrate the process to generate a
public key encoding from a HI for both RSA and DSA.
The symbol := denotes assignment; the symbol += denotes appending.
The pseudo-function encode_in_network_byte_order takes two
parameters, an integer (bignum) and a length in bytes, and returns
the integer encoded into a byte string of the given length.
switch ( HI.algorithm )
{
case RSA:
buffer := encode_in_network_byte_order ( HI.RSA.e_len,
( HI.RSA.e_len > 255 ) ? 3 : 1 )
buffer += encode_in_network_byte_order ( HI.RSA.e, HI.RSA.e_len )
buffer += encode_in_network_byte_order ( HI.RSA.n, HI.RSA.n_len )
break;
case DSA:
buffer := encode_in_network_byte_order ( HI.DSA.T , 1 )
buffer += encode_in_network_byte_order ( HI.DSA.Q , 20 )
buffer += encode_in_network_byte_order ( HI.DSA.P , 64 +
8 * HI.DSA.T )
buffer += encode_in_network_byte_order ( HI.DSA.G , 64 +
8 * HI.DSA.T )
buffer += encode_in_network_byte_order ( HI.DSA.Y , 64 +
8 * HI.DSA.T )
break;
}
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Appendix C. Example Checksums for HIP Packets
The HIP checksum for HIP packets is specified in Section 6.1.2.
Checksums for TCP and UDP packets running over HIP-enabled security
associations are specified in Section 3.5. The examples below use IP
addresses of 192.168.0.1 and 192.168.0.2 (and their respective IPv4-
compatible IPv6 formats), and HITs with the first two bits "01"
followed by 124 zeroes followed by a decimal 1 or 2, respectively.
C.1. IPv6 HIP Example (I1)
Source Address: ::192.168.0.1
Destination Address: ::192.168.0.2
Upper-Layer Packet Length: 40 0x28
Next Header: 253 0xfd
Payload Protocol: 59 0x3b
Header Length: 4 0x4
Packet Type: 1 0x1
Version: 1 0x1
Reserved: 1 0x1
Control: 0 0x0
Checksum: 8046 0x1f6e
Sender's HIT : 1100::1
Receiver's HIT: 1100::2
C.2. IPv4 HIP Packet (I1)
The IPv4 checksum value for the same example I1 packet is the same as
the IPv6 checksum (since the checksums due to the IPv4 and IPv6
pseudo-header components are the same).
C.3. TCP Segment
Regardless of whether IPv6 or IPv4 is used, the TCP and UDP sockets
use the IPv6 pseudo-header format [11], with the HITs used in place
of the IPv6 addresses.
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Sender's HIT: 1100::0001
Receiver's HIT: 1100::0002
Upper-Layer Packet Length: 20 0x14
Next Header: 6 0x06
Source port: 65500 0xffdc
Destination port: 22 0x0016
Sequence number: 1 0x00000001
Acknowledgment number: 0 0x00000000
Header length: 20 0x14
Flags: SYN 0x02
Window size: 65535 0xffff
Checksum: 60301 0xeb8d
Urgent pointer: 0 0x0000
0x0000: 6000 0000 0014 0640 1100 0000 0000 0000
0x0010: 0000 0000 0000 0002 1100 0000 0000 0000
0x0020: 0000 0000 0000 0002 ffdc 0016 0000 0001
0x0030: 0000 0000 5002 ffff 8deb 0000
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Appendix D. 384-bit Group
This 384-bit group is defined only to be used with HIP. NOTE: The
security level of this group is very low! The encryption may be
broken in a very short time, even real-time. It should be used only
when the host is not powerful enough (e.g. some PDAs) and when
security requirements are low (e.g. during normal web surfing).
This prime is: 2^384 - 2^320 - 1 + 2^64 * { [ 2^254 pi] + 5857 }
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B13B202 FFFFFFFF FFFFFFFF
The generator is: 2.
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Authors' Addresses
Robert Moskowitz
ICSAlabs, a Division of TruSecure Corporation
1000 Bent Creek Blvd, Suite 200
Mechanicsburg, PA
USA
Email: rgm@icsalabs.com
Pekka Nikander
Ericsson Research NomadicLab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
Email: pekka.nikander@nomadiclab.com
Petri Jokela
Ericsson Research NomadicLab
JORVAS FIN-02420
FINLAND
Phone: +358 9 299 1
Email: petri.jokela@nomadiclab.com
Thomas R. Henderson
The Boeing Company
P.O. Box 3707
Seattle, WA
USA
Email: thomas.r.henderson@boeing.com
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