[ RFC Index | RFC Search | Usenet FAQs | Web FAQs | Documents | Cities ]
Alternate Formats: rfc5201.txt | rfc5201.txt.pdf
RFC 5201 - Host Identity Protocol
|
 |
RFC5201 - Host Identity Protocol
Network Working Group R. Moskowitz
Request for Comments: 5201 ICSAlabs
Category: Experimental P. Nikander
P. Jokela, Ed.
Ericsson Research NomadicLab
T. Henderson
The Boeing Company
April 2008
Host Identity Protocol
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
IESG Note
The following issues describe IESG concerns about this document. The
IESG expects that these issues will be addressed when future versions
of HIP are designed.
This document doesn't currently define support for parameterized
(randomized) hashing in signatures, support for negotiation of a key
derivation function, or support for combined encryption modes.
HIP defines the usage of RSA in signing and encrypting data. Current
recommendations propose usage of, for example, RSA OAEP/PSS for these
operations in new protocols. Changing the algorithms to more current
best practice should be considered.
The current specification is currently using HMAC for message
authentication. This is considered to be acceptable for an
experimental RFC, but future versions must define a more generic
method for message authentication, including the ability for other
MAC algorithms to be used.
SHA-1 is no longer a preferred hashing algorithm. This is noted also
by the authors, and it is understood that future, non-experimental
versions must consider more secure hashing algorithms.
HIP requires that an incoming packet's IP address be ignored. In
simple cases this can be done, but when there are security policies
based on incoming interface or IP address rules, the situation
changes. The handling of data needs to be enhanced to cover
different types of network and security configurations, as well as to
meet local security policies.
Abstract
This memo specifies the details of the Host Identity Protocol (HIP).
HIP allows consenting hosts to securely establish and maintain shared
IP-layer state, allowing separation of the identifier and locator
roles of IP addresses, thereby enabling continuity of communications
across IP address changes. HIP is based on a Sigma-compliant Diffie-
Hellman key exchange, using public key identifiers from a new Host
Identity namespace for mutual peer authentication. The protocol is
designed to be resistant to denial-of-service (DoS) and man-in-the-
middle (MitM) attacks. When used together with another suitable
security protocol, such as the Encapsulated Security Payload (ESP),
it provides integrity protection and optional encryption for upper-
layer protocols, such as TCP and UDP.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. A New Namespace and Identifiers . . . . . . . . . . . . . 5
1.2. The HIP Base Exchange . . . . . . . . . . . . . . . . . . 6
1.3. Memo Structure . . . . . . . . . . . . . . . . . . . . . 7
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 7
2.1. Requirements Terminology . . . . . . . . . . . . . . . . 7
2.2. Notation . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7
3. Host Identifier (HI) and Its Representations . . . . . . . . 8
3.1. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . 9
3.2. Generating a HIT from an HI . . . . . . . . . . . . . . . 9
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Creating a HIP Association . . . . . . . . . . . . . . . 10
4.1.1. HIP Puzzle Mechanism . . . . . . . . . . . . . . . . 12
4.1.2. Puzzle Exchange . . . . . . . . . . . . . . . . . . . 13
4.1.3. Authenticated Diffie-Hellman Protocol . . . . . . . . 14
4.1.4. HIP Replay Protection . . . . . . . . . . . . . . . . 14
4.1.5. Refusing a HIP Exchange . . . . . . . . . . . . . . . 15
4.1.6. HIP Opportunistic Mode . . . . . . . . . . . . . . . 16
4.2. Updating a HIP Association . . . . . . . . . . . . . . . 18
4.3. Error Processing . . . . . . . . . . . . . . . . . . . . 18
4.4. HIP State Machine . . . . . . . . . . . . . . . . . . . . 19
4.4.1. HIP States . . . . . . . . . . . . . . . . . . . . . 20
4.4.2. HIP State Processes . . . . . . . . . . . . . . . . . 21
4.4.3. Simplified HIP State Diagram . . . . . . . . . . . . 28
4.5. User Data Considerations . . . . . . . . . . . . . . . . 30
4.5.1. TCP and UDP Pseudo-Header Computation for User Data . 30
4.5.2. Sending Data on HIP Packets . . . . . . . . . . . . . 30
4.5.3. Transport Formats . . . . . . . . . . . . . . . . . . 30
4.5.4. Reboot and SA Timeout Restart of HIP . . . . . . . . 30
4.6. Certificate Distribution . . . . . . . . . . . . . . . . 31
5. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . 31
5.1. Payload Format . . . . . . . . . . . . . . . . . . . . . 31
5.1.1. Checksum . . . . . . . . . . . . . . . . . . . . . . 33
5.1.2. HIP Controls . . . . . . . . . . . . . . . . . . . . 33
5.1.3. HIP Fragmentation Support . . . . . . . . . . . . . . 33
5.2. HIP Parameters . . . . . . . . . . . . . . . . . . . . . 34
5.2.1. TLV Format . . . . . . . . . . . . . . . . . . . . . 37
5.2.2. Defining New Parameters . . . . . . . . . . . . . . . 38
5.2.3. R1_COUNTER . . . . . . . . . . . . . . . . . . . . . 39
5.2.4. PUZZLE . . . . . . . . . . . . . . . . . . . . . . . 40
5.2.5. SOLUTION . . . . . . . . . . . . . . . . . . . . . . 41
5.2.6. DIFFIE_HELLMAN . . . . . . . . . . . . . . . . . . . 42
5.2.7. HIP_TRANSFORM . . . . . . . . . . . . . . . . . . . . 43
5.2.8. HOST_ID . . . . . . . . . . . . . . . . . . . . . . . 44
5.2.9. HMAC . . . . . . . . . . . . . . . . . . . . . . . . 45
5.2.10. HMAC_2 . . . . . . . . . . . . . . . . . . . . . . . 46
5.2.11. HIP_SIGNATURE . . . . . . . . . . . . . . . . . . . . 46
5.2.12. HIP_SIGNATURE_2 . . . . . . . . . . . . . . . . . . . 47
5.2.13. SEQ . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2.14. ACK . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2.15. ENCRYPTED . . . . . . . . . . . . . . . . . . . . . . 49
5.2.16. NOTIFICATION . . . . . . . . . . . . . . . . . . . . 50
5.2.17. ECHO_REQUEST_SIGNED . . . . . . . . . . . . . . . . . 54
5.2.18. ECHO_REQUEST_UNSIGNED . . . . . . . . . . . . . . . . 54
5.2.19. ECHO_RESPONSE_SIGNED . . . . . . . . . . . . . . . . 55
5.2.20. ECHO_RESPONSE_UNSIGNED . . . . . . . . . . . . . . . 56
5.3. HIP Packets . . . . . . . . . . . . . . . . . . . . . . . 56
5.3.1. I1 - the HIP Initiator Packet . . . . . . . . . . . . 58
5.3.2. R1 - the HIP Responder Packet . . . . . . . . . . . . 58
5.3.3. I2 - the Second HIP Initiator Packet . . . . . . . . 61
5.3.4. R2 - the Second HIP Responder Packet . . . . . . . . 62
5.3.5. UPDATE - the HIP Update Packet . . . . . . . . . . . 62
5.3.6. NOTIFY - the HIP Notify Packet . . . . . . . . . . . 63
5.3.7. CLOSE - the HIP Association Closing Packet . . . . . 64
5.3.8. CLOSE_ACK - the HIP Closing Acknowledgment Packet . . 64
5.4. ICMP Messages . . . . . . . . . . . . . . . . . . . . . . 65
5.4.1. Invalid Version . . . . . . . . . . . . . . . . . . . 65
5.4.2. Other Problems with the HIP Header and Packet
Structure . . . . . . . . . . . . . . . . . . . . . . 65
5.4.3. Invalid Puzzle Solution . . . . . . . . . . . . . . . 65
5.4.4. Non-Existing HIP Association . . . . . . . . . . . . 66
6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 66
6.1. Processing Outgoing Application Data . . . . . . . . . . 66
6.2. Processing Incoming Application Data . . . . . . . . . . 67
6.3. Solving the Puzzle . . . . . . . . . . . . . . . . . . . 68
6.4. HMAC and SIGNATURE Calculation and Verification . . . . . 70
6.4.1. HMAC Calculation . . . . . . . . . . . . . . . . . . 70
6.4.2. Signature Calculation . . . . . . . . . . . . . . . . 72
6.5. HIP KEYMAT Generation . . . . . . . . . . . . . . . . . . 74
6.6. Initiation of a HIP Exchange . . . . . . . . . . . . . . 75
6.6.1. Sending Multiple I1s in Parallel . . . . . . . . . . 76
6.6.2. Processing Incoming ICMP Protocol Unreachable
Messages . . . . . . . . . . . . . . . . . . . . . . 77
6.7. Processing Incoming I1 Packets . . . . . . . . . . . . . 77
6.7.1. R1 Management . . . . . . . . . . . . . . . . . . . . 78
6.7.2. Handling Malformed Messages . . . . . . . . . . . . . 79
6.8. Processing Incoming R1 Packets . . . . . . . . . . . . . 79
6.8.1. Handling Malformed Messages . . . . . . . . . . . . . 81
6.9. Processing Incoming I2 Packets . . . . . . . . . . . . . 81
6.9.1. Handling Malformed Messages . . . . . . . . . . . . . 84
6.10. Processing Incoming R2 Packets . . . . . . . . . . . . . 84
6.11. Sending UPDATE Packets . . . . . . . . . . . . . . . . . 84
6.12. Receiving UPDATE Packets . . . . . . . . . . . . . . . . 85
6.12.1. Handling a SEQ Parameter in a Received UPDATE
Message . . . . . . . . . . . . . . . . . . . . . . . 86
6.12.2. Handling an ACK Parameter in a Received UPDATE
Packet . . . . . . . . . . . . . . . . . . . . . . . 87
6.13. Processing NOTIFY Packets . . . . . . . . . . . . . . . . 87
6.14. Processing CLOSE Packets . . . . . . . . . . . . . . . . 88
6.15. Processing CLOSE_ACK Packets . . . . . . . . . . . . . . 88
6.16. Handling State Loss . . . . . . . . . . . . . . . . . . . 88
7. HIP Policies . . . . . . . . . . . . . . . . . . . . . . . . 89
8. Security Considerations . . . . . . . . . . . . . . . . . . . 89
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 92
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 93
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 95
11.1. Normative References . . . . . . . . . . . . . . . . . . 95
11.2. Informative References . . . . . . . . . . . . . . . . . 96
Appendix A. Using Responder Puzzles . . . . . . . . . . . . . . 98
Appendix B. Generating a Public Key Encoding from an HI . . . . 99
Appendix C. Example Checksums for HIP Packets . . . . . . . . . 100
C.1. IPv6 HIP Example (I1) . . . . . . . . . . . . . . . . . . 100
C.2. IPv4 HIP Packet (I1) . . . . . . . . . . . . . . . . . . 100
C.3. TCP Segment . . . . . . . . . . . . . . . . . . . . . . . 101
Appendix D. 384-Bit Group . . . . . . . . . . . . . . . . . . . 101
Appendix E. OAKLEY Well-Known Group 1 . . . . . . . . . . . . . 102
1. Introduction
This memo specifies the details of the Host Identity Protocol (HIP).
A high-level description of the protocol and the underlying
architectural thinking is available in the separate HIP architecture
description [RFC4423]. Briefly, the HIP architecture proposes an
alternative to the dual use of IP addresses as "locators" (routing
labels) and "identifiers" (endpoint, or host, identifiers). In HIP,
public cryptographic keys, of a public/private key pair, are used as
Host Identifiers, to which higher layer protocols are bound instead
of an IP address. By using public keys (and their representations)
as host identifiers, dynamic changes to IP address sets can be
directly authenticated between hosts, and if desired, strong
authentication between hosts at the TCP/IP stack level can be
obtained.
This memo specifies the base HIP protocol ("base exchange") used
between hosts to establish an IP-layer communications context, called
HIP association, prior to communications. It also defines a packet
format and procedures for updating an active HIP association. Other
elements of the HIP architecture are specified in other documents,
such as.
o "Using the Encapsulating Security Payload (ESP) Transport Format
with the Host Identity Protocol (HIP)" [RFC5202]: how to use the
Encapsulating Security Payload (ESP) for integrity protection and
optional encryption
o "End-Host Mobility and Multihoming with the Host Identity
Protocol" [RFC5206]: how to support mobility and multihoming in
HIP
o "Host Identity Protocol (HIP) Domain Name System (DNS) Extensions"
[RFC5205]: how to extend DNS to contain Host Identity information
o "Host Identity Protocol (HIP) Rendezvous Extension" [RFC5204]:
using a rendezvous mechanism to contact mobile HIP hosts
1.1. A New Namespace and Identifiers
The Host Identity Protocol introduces a new namespace, the Host
Identity namespace. Some ramifications of this new namespace are
explained in the HIP architecture description [RFC4423].
There are two main representations of the Host Identity, the full
Host Identifier (HI) and the Host Identity Tag (HIT). The HI is a
public key and directly represents the Identity. Since there are
different public key algorithms that can be used with different key
lengths, the HI is not good for use as a packet identifier, or as an
index into the various operational tables needed to support HIP.
Consequently, a hash of the HI, the Host Identity Tag (HIT), becomes
the operational representation. It is 128 bits long and is used in
the HIP payloads and to index the corresponding state in the end
hosts. The HIT has an important security property in that it is
self-certifying (see Section 3).
1.2. The HIP Base Exchange
The HIP base exchange is a two-party cryptographic protocol used to
establish communications context between hosts. The base exchange is
a Sigma-compliant [KRA03] four-packet exchange. The first party is
called the Initiator and the second party the Responder. The four-
packet design helps to make HIP DoS resilient. The protocol
exchanges Diffie-Hellman keys in the 2nd and 3rd packets, and
authenticates the parties in the 3rd and 4th packets. Additionally,
the Responder starts a puzzle exchange in the 2nd packet, with the
Initiator completing it in the 3rd packet before the Responder stores
any state from the exchange.
The exchange can use the Diffie-Hellman output to encrypt the Host
Identity of the Initiator in the 3rd packet (although Aura, et al.,
[AUR03] notes that such operation may interfere with packet-
inspecting middleboxes), or the Host Identity may instead be sent
unencrypted. The Responder's Host Identity is not protected. It
should be noted, however, that both the Initiator's and the
Responder's HITs are transported as such (in cleartext) in the
packets, allowing an eavesdropper with a priori knowledge about the
parties to verify their identities.
Data packets start to flow after the 4th packet. The 3rd and 4th HIP
packets may carry a data payload in the future. However, the details
of this are to be defined later as more implementation experience is
gained.
An existing HIP association can be updated using the update mechanism
defined in this document, and when the association is no longer
needed, it can be closed using the defined closing mechanism.
Finally, HIP is designed as an end-to-end authentication and key
establishment protocol, to be used with Encapsulated Security Payload
(ESP) [RFC5202] and other end-to-end security protocols. The base
protocol does not cover all the fine-grained policy control found in
Internet Key Exchange (IKE) [RFC4306] that allows IKE to support
complex gateway policies. Thus, HIP is not a replacement for IKE.
1.3. Memo Structure
The rest of this memo is structured as follows. Section 2 defines
the central keywords, notation, and terms used throughout the rest of
the document. Section 3 defines the structure of the Host Identity
and its various representations. Section 4 gives an overview of the
HIP base exchange protocol. Sections 5 and 6 define the detail
packet formats and rules for packet processing. Finally, Sections 7,
8, and 9 discuss policy, security, and IANA considerations,
respectively.
2. Terms and Definitions
2.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2.2. Notation
[x] indicates that x is optional.
{x} indicates that x is encrypted.
X(y) indicates that y is a parameter of X.
<x>i indicates that x exists i times.
--> signifies "Initiator to Responder" communication (requests).
<-- signifies "Responder to Initiator" communication (replies).
| signifies concatenation of information-- e.g., X | Y is the
concatenation of X with Y.
Ltrunc (SHA-1(), K) denotes the lowest order K bits of the SHA-1
result.
2.3. Definitions
Unused Association Lifetime (UAL): Implementation-specific time for
which, if no packet is sent or received for this time interval, a
host MAY begin to tear down an active association.
Maximum Segment Lifetime (MSL): Maximum time that a TCP segment is
expected to spend in the network.
Exchange Complete (EC): Time that the host spends at the R2-SENT
before it moves to ESTABLISHED state. The time is n * I2
retransmission timeout, where n is about I2_RETRIES_MAX.
HIT Hash Algorithm: Hash algorithm used to generate a Host Identity
Tag (HIT) from the Host Identity public key. Currently SHA-1
[FIPS95] is used.
Responder's HIT Hash Algorithm (RHASH): Hash algorithm used for
various hash calculations in this document. The algorithm is the
same as is used to generate the Responder's HIT. RHASH is defined
by the Orchid Context ID. For HIP, the present RHASH algorithm is
defined in Section 3.2. A future version of HIP may define a new
RHASH algorithm by defining a new Context ID.
Opportunistic mode: HIP base exchange where the Responder's HIT is
not known a priori to the Initiator.
3. Host Identifier (HI) and Its Representations
In this section, the properties of the Host Identifier and Host
Identifier Tag are discussed, and the exact format for them is
defined. In HIP, the public key of an asymmetric key pair is used as
the Host Identifier (HI). Correspondingly, the host itself is
defined as the entity that holds the private key from the key pair.
See the HIP architecture specification [RFC4423] for more details
about the difference between an identity and the corresponding
identifier.
HIP implementations MUST support the Rivest Shamir Adelman (RSA/SHA1)
[RFC3110] public key algorithm, and SHOULD support the Digital
Signature Algorithm (DSA) [RFC2536] algorithm; other algorithms MAY
be supported.
A hashed encoding of the HI, the Host Identity Tag (HIT), is used in
protocols to represent the Host Identity. The HIT is 128 bits long
and has the following three key properties: i) it is the same length
as an IPv6 address and can be used in address-sized fields in APIs
and protocols, ii) it is self-certifying (i.e., given a HIT, it is
computationally hard to find a Host Identity key that matches the
HIT), and iii) the probability of HIT collision between two hosts is
very low.
Carrying HIs and HITs in the header of user data packets would
increase the overhead of packets. Thus, it is not expected that they
are carried in every packet, but other methods are used to map the
data packets to the corresponding HIs. In some cases, this makes it
possible to use HIP without any additional headers in the user data
packets. For example, if ESP is used to protect data traffic, the
Security Parameter Index (SPI) carried in the ESP header can be used
to map the encrypted data packet to the correct HIP association.
3.1. Host Identity Tag (HIT)
The Host Identity Tag is a 128-bit value -- a hashed encoding of the
Host Identifier. There are two advantages of using a hashed encoding
over the actual Host Identity public key in protocols. Firstly, its
fixed length makes for easier protocol coding and also better manages
the packet size cost of this technology. Secondly, it presents a
consistent format to the protocol whatever underlying identity
technology is used.
RFC 4843 [RFC4843] specifies 128-bit hash-based identifiers, called
Overlay Routable Cryptographic Hash Identifiers (ORCHIDs). Their
prefix, allocated from the IPv6 address block, is defined in
[RFC4843]. The Host Identity Tag is a type of ORCHID, based on a
SHA-1 hash of the Host Identity, as defined in Section 2 of
[RFC4843].
3.2. Generating a HIT from an HI
The HIT MUST be generated according to the ORCHID generation method
described in [RFC4843] using a context ID value of 0xF0EF F02F BFF4
3D0F E793 0C3C 6E61 74EA (this tag value has been generated randomly
by the editor of this specification), and an input that encodes the
Host Identity field (see Section 5.2.8) present in a HIP payload
packet. The hash algorithm SHA-1 has to be used when generating HITs
with this context ID. If a new ORCHID hash algorithm is needed in
the future for HIT generation, a new version of HIP has to be
specified with a new ORCHID context ID associated with the new hash
algorithm.
For Identities that are either RSA or Digital Signature Algorithm
(DSA) public keys, this input consists of the public key encoding as
specified in the corresponding DNSSEC document, taking the algorithm-
specific portion of the RDATA part of the KEY RR. There are
currently only two defined public key algorithms: RSA/SHA1 and DSA.
Hence, either of the following applies:
The RSA public key is encoded as defined in [RFC3110] Section 2,
taking the exponent length (e_len), exponent (e), and modulus (n)
fields concatenated. The length (n_len) of the modulus (n) can be
determined from the total HI Length and the preceding HI fields
including the exponent (e). Thus, the data to be hashed has the
same length as the HI. The fields MUST be encoded in network byte
order, as defined in [RFC3110].
The DSA public key is encoded as defined in [RFC2536] Section 2,
taking the fields T, Q, P, G, and Y, concatenated. Thus, the data
to be hashed is 1 + 20 + 3 * 64 + 3 * 8 * T octets long, where T
is the size parameter as defined in [RFC2536]. The size parameter
T, affecting the field lengths, MUST be selected as the minimum
value that is long enough to accommodate P, G, and Y. The fields
MUST be encoded in network byte order, as defined in [RFC2536].
In Appendix B, the public key encoding process is illustrated using
pseudo-code.
4. Protocol Overview
The following material is an overview of the HIP protocol operation,
and does not contain all details of the packet formats or the packet
processing steps. Sections 5 and 6 describe in more detail the
packet formats and packet processing steps, respectively, and are
normative in case of any conflicts with this section.
The protocol number 139 has been assigned by IANA to the Host
Identity Protocol.
The HIP payload (Section 5.1) header could be carried in every IP
datagram. However, since HIP headers are relatively large (40
bytes), it is desirable to 'compress' the HIP header so that the HIP
header only occurs in control packets used to establish or change HIP
association state. The actual method for header 'compression' and
for matching data packets with existing HIP associations (if any) is
defined in separate documents, describing transport formats and
methods. All HIP implementations MUST implement, at minimum, the ESP
transport format for HIP [RFC5202].
4.1. Creating a HIP Association
By definition, the system initiating a HIP exchange is the Initiator,
and the peer is the Responder. This distinction is forgotten once
the base exchange completes, and either party can become the
Initiator in future communications.
The HIP base exchange serves to manage the establishment of state
between an Initiator and a Responder. The first packet, I1,
initiates the exchange, and the last three packets, R1, I2, and R2,
constitute an authenticated Diffie-Hellman [DIF76] key exchange for
session key generation. During the Diffie-Hellman key exchange, a
piece of keying material is generated. The HIP association keys are
drawn from this keying material. If other cryptographic keys are
needed, e.g., to be used with ESP, they are expected to be drawn from
the same keying material.
The Initiator first sends a trigger packet, I1, to the Responder.
The packet contains only the HIT of the Initiator and possibly the
HIT of the Responder, if it is known. Note that in some cases it may
be possible to replace this trigger packet by some other form of a
trigger, in which case the protocol starts with the Responder sending
the R1 packet.
The second packet, R1, starts the actual exchange. It contains a
puzzle -- a cryptographic challenge that the Initiator must solve
before continuing the exchange. The level of difficulty of the
puzzle can be adjusted based on level of trust with the Initiator,
current load, or other factors. In addition, the R1 contains the
initial Diffie-Hellman parameters and a signature, covering part of
the message. Some fields are left outside the signature to support
pre-created R1s.
In the I2 packet, the Initiator must display the solution to the
received puzzle. Without a correct solution, the I2 message is
discarded. The I2 also contains a Diffie-Hellman parameter that
carries needed information for the Responder. The packet is signed
by the sender.
The R2 packet finalizes the base exchange. The packet is signed.
The base exchange is illustrated below. The term "key" refers to the
Host Identity public key, and "sig" represents a signature using such
a key. The packets contain other parameters not shown in this
figure.
Initiator Responder
I1: trigger exchange
-------------------------->
select precomputed R1
R1: puzzle, D-H, key, sig
<-------------------------
check sig remain stateless
solve puzzle
I2: solution, D-H, {key}, sig
-------------------------->
compute D-H check puzzle
check sig
R2: sig
<--------------------------
check sig compute D-H
4.1.1. HIP Puzzle Mechanism
The purpose of the HIP puzzle mechanism is to protect the Responder
from a number of denial-of-service threats. It allows the Responder
to delay state creation until receiving I2. Furthermore, the puzzle
allows the Responder to use a fairly cheap calculation to check that
the Initiator is "sincere" in the sense that it has churned CPU
cycles in solving the puzzle.
The puzzle mechanism has been explicitly designed to give space for
various implementation options. It allows a Responder implementation
to completely delay session-specific state creation until a valid I2
is received. In such a case, a correctly formatted I2 can be
rejected only once the Responder has checked its validity by
computing one hash function. On the other hand, the design also
allows a Responder implementation to keep state about received I1s,
and match the received I2s against the state, thereby allowing the
implementation to avoid the computational cost of the hash function.
The drawback of this latter approach is the requirement of creating
state. Finally, it also allows an implementation to use other
combinations of the space-saving and computation-saving mechanisms.
The Responder can remain stateless and drop most spoofed I2s because
puzzle calculation is based on the Initiator's Host Identity Tag.
The idea is that the Responder has a (perhaps varying) number of pre-
calculated R1 packets, and it selects one of these based on the
information carried in I1. When the Responder then later receives
I2, it can verify that the puzzle has been solved using the
Initiator's HIT. This makes it impractical for the attacker to first
exchange one I1/R1, and then generate a large number of spoofed I2s
that seemingly come from different HITs. The method does not protect
from an attacker that uses fixed HITs, though. Against such an
attacker a viable approach may be to create a piece of local state,
and remember that the puzzle check has previously failed. See
Appendix A for one possible implementation. Implementations SHOULD
include sufficient randomness to the algorithm so that algorithmic
complexity attacks become impossible [CRO03].
The Responder can set the puzzle difficulty for Initiator, based on
its level of trust of the Initiator. Because the puzzle is not
included in the signature calculation, the Responder can use pre-
calculated R1 packets and include the puzzle just before sending the
R1 to the Initiator. The Responder SHOULD use heuristics to
determine when it is under a denial-of-service attack, and set the
puzzle difficulty value K appropriately; see below.
4.1.2. Puzzle Exchange
The Responder starts the puzzle exchange when it receives an I1. The
Responder supplies a random number I, and requires the Initiator to
find a number J. To select a proper J, the Initiator must create the
concatenation of I, the HITs of the parties, and J, and take a hash
over this concatenation using the RHASH algorithm. The lowest order
K bits of the result MUST be zeros. The value K sets the difficulty
of the puzzle.
To generate a proper number J, the Initiator will have to generate a
number of Js until one produces the hash target of zeros. The
Initiator SHOULD give up after exceeding the puzzle lifetime in the
PUZZLE parameter (Section 5.2.4). The Responder needs to re-create
the concatenation of I, the HITs, and the provided J, and compute the
hash once to prove that the Initiator did its assigned task.
To prevent precomputation attacks, the Responder MUST select the
number I in such a way that the Initiator cannot guess it.
Furthermore, the construction MUST allow the Responder to verify that
the value was indeed selected by it and not by the Initiator. See
Appendix A for an example on how to implement this.
Using the Opaque data field in an ECHO_REQUEST_SIGNED
(Section 5.2.17) or in an ECHO_REQUEST_UNSIGNED parameter
(Section 5.2.18), the Responder can include some data in R1 that the
Initiator must copy unmodified in the corresponding I2 packet. The
Responder can generate the Opaque data in various ways; e.g., using
some secret, the sent I, and possibly other related data. Using the
same secret, the received I (from the I2), and the other related data
(if any), the Receiver can verify that it has itself sent the I to
the Initiator. The Responder MUST periodically change such a used
secret.
It is RECOMMENDED that the Responder generates a new puzzle and a new
R1 once every few minutes. Furthermore, it is RECOMMENDED that the
Responder remembers an old puzzle at least 2*Lifetime seconds after
the puzzle has been deprecated. These time values allow a slower
Initiator to solve the puzzle while limiting the usability that an
old, solved puzzle has to an attacker.
NOTE: The protocol developers explicitly considered whether R1 should
include a timestamp in order to protect the Initiator from replay
attacks. The decision was to NOT include a timestamp.
NOTE: The protocol developers explicitly considered whether a memory
bound function should be used for the puzzle instead of a CPU-bound
function. The decision was not to use memory-bound functions. At
the time of the decision, the idea of memory-bound functions was
relatively new and their IPR status were unknown. Once there is more
experience about memory-bound functions and once their IPR status is
better known, it may be reasonable to reconsider this decision.
4.1.3. Authenticated Diffie-Hellman Protocol
The packets R1, I2, and R2 implement a standard authenticated Diffie-
Hellman exchange. The Responder sends one or two public Diffie-
Hellman keys and its public authentication key, i.e., its Host
Identity, in R1. The signature in R1 allows the Initiator to verify
that the R1 has been once generated by the Responder. However, since
it is precomputed and therefore does not cover all of the packet, it
does not protect from replay attacks.
When the Initiator receives an R1, it gets one or two public Diffie-
Hellman values from the Responder. If there are two values, it
selects the value corresponding to the strongest supported Group ID
and computes the Diffie-Hellman session key (Kij). It creates a HIP
association using keying material from the session key (see
Section 6.5), and may use the association to encrypt its public
authentication key, i.e., Host Identity. The resulting I2 contains
the Initiator's Diffie-Hellman key and its (optionally encrypted)
public authentication key. The signature in I2 covers all of the
packet.
The Responder extracts the Initiator Diffie-Hellman public key from
the I2, computes the Diffie-Hellman session key, creates a
corresponding HIP association, and decrypts the Initiator's public
authentication key. It can then verify the signature using the
authentication key.
The final message, R2, is needed to protect the Initiator from replay
attacks.
4.1.4. HIP Replay Protection
The HIP protocol includes the following mechanisms to protect against
malicious replays. Responders are protected against replays of I1
packets by virtue of the stateless response to I1s with presigned R1
messages. Initiators are protected against R1 replays by a
monotonically increasing "R1 generation counter" included in the R1.
Responders are protected against replays or false I2s by the puzzle
mechanism (Section 4.1.1 above), and optional use of opaque data.
Hosts are protected against replays to R2s and UPDATEs by use of a
less expensive HMAC verification preceding HIP signature
verification.
The R1 generation counter is a monotonically increasing 64-bit
counter that may be initialized to any value. The scope of the
counter MAY be system-wide but SHOULD be per Host Identity, if there
is more than one local host identity. The value of this counter
SHOULD be kept across system reboots and invocations of the HIP base
exchange. This counter indicates the current generation of puzzles.
Implementations MUST accept puzzles from the current generation and
MAY accept puzzles from earlier generations. A system's local
counter MUST be incremented at least as often as every time old R1s
cease to be valid, and SHOULD never be decremented, lest the host
expose its peers to the replay of previously generated, higher
numbered R1s. The R1 counter SHOULD NOT roll over.
A host may receive more than one R1, either due to sending multiple
I1s (Section 6.6.1) or due to a replay of an old R1. When sending
multiple I1s, an Initiator SHOULD wait for a small amount of time (a
reasonable time may be 2 * expected RTT) after the first R1 reception
to allow possibly multiple R1s to arrive, and it SHOULD respond to an
R1 among the set with the largest R1 generation counter. If an
Initiator is processing an R1 or has already sent an I2 (still
waiting for R2) and it receives another R1 with a larger R1
generation counter, it MAY elect to restart R1 processing with the
fresher R1, as if it were the first R1 to arrive.
Upon conclusion of an active HIP association with another host, the
R1 generation counter associated with the peer host SHOULD be
flushed. A local policy MAY override the default flushing of R1
counters on a per-HIT basis. The reason for recommending the
flushing of this counter is that there may be hosts where the R1
generation counter (occasionally) decreases; e.g., due to hardware
failure.
4.1.5. Refusing a HIP Exchange
A HIP-aware host may choose not to accept a HIP exchange. If the
host's policy is to only be an Initiator, it should begin its own HIP
exchange. A host MAY choose to have such a policy since only the
Initiator's HI is protected in the exchange. There is a risk of a
race condition if each host's policy is to only be an Initiator, at
which point the HIP exchange will fail.
If the host's policy does not permit it to enter into a HIP exchange
with the Initiator, it should send an ICMP 'Destination Unreachable,
Administratively Prohibited' message. A more complex HIP packet is
not used here as it actually opens up more potential DoS attacks than
a simple ICMP message.
4.1.6. HIP Opportunistic Mode
It is possible to initiate a HIP negotiation even if the Responder's
HI (and HIT) is unknown. In this case, the connection initializing
I1 packet contains NULL (all zeros) as the destination HIT. This
kind of connection setup is called opportunistic mode.
There are both security and API issues involved with the
opportunistic mode.
Given that the Responder's HI is not known by the Initiator, there
must be suitable API calls that allow the Initiator to request,
directly or indirectly, that the underlying kernel initiate the HIP
base exchange solely based on locators. The Responder's HI will be
tentatively available in the R1 packet, and in an authenticated form
once the R2 packet has been received and verified. Hence, it could
be communicated to the application via new API mechanisms. However,
with a backwards-compatible API the application sees only the
locators used for the initial contact. Depending on the desired
semantics of the API, this can raise the following issues:
o The actual locators may later change if an UPDATE message is used,
even if from the API perspective the session still appears to be
between specific locators. The locator update is still secure,
however, and the session is still between the same nodes.
o Different sessions between the same locators may result in
connections to different nodes, if the implementation no longer
remembers which identifier the peer had in another session. This
is possible when the peer's locator has changed for legitimate
reasons or when an attacker pretends to be a node that has the
peer's locator. Therefore, when using opportunistic mode, HIP
MUST NOT place any expectation that the peer's HI returned in the
R1 message matches any HI previously seen from that address.
If the HIP implementation and application do not have the same
understanding of what constitutes a session, this may even happen
within the same session. For instance, an implementation may not
know when HIP state can be purged for UDP-based applications.
o As with all HIP exchanges, the handling of locator-based or
interface-based policy is unclear for opportunistic mode HIP. An
application may make a connection to a specific locator because
the application has knowledge of the security properties along the
network to that locator. If one of the nodes moves and the
locators are updated, these security properties may not be
maintained. Depending on the security policy of the application,
this may be a problem. This is an area of ongoing study. As an
example, there is work to create an API that applications can use
to specify their security requirements in a similar context
[IPsec-APIs].
In addition, the following security considerations apply. The
generation counter mechanism will be less efficient in protecting
against replays of the R1 packet, given that the Responder can choose
a replay that uses any HI, not just the one given in the I1 packet.
More importantly, the opportunistic exchange is vulnerable to man-in-
the-middle attacks, because the Initiator does not have any public
key information about the peer. To assess the impacts of this
vulnerability, we compare it to vulnerabilities in current, non-HIP-
capable communications.
An attacker on the path between the two peers can insert itself as a
man-in-the-middle by providing its own identifier to the Initiator
and then initiating another HIP session towards the Responder. For
this to be possible, the Initiator must employ opportunistic mode,
and the Responder must be configured to accept a connection from any
HIP-enabled node.
An attacker outside the path will be unable to do so, given that it
cannot respond to the messages in the base exchange.
These properties are characteristic also of communications in the
current Internet. A client contacting a server without employing
end-to-end security may find itself talking to the server via a man-
in-the-middle, assuming again that the server is willing to talk to
anyone.
If end-to-end security is in place, then the worst that can happen in
both the opportunistic HIP and normal IP cases is denial-of-service;
an entity on the path can disrupt communications, but will be unable
to insert itself as a man-in-the-middle.
However, once the opportunistic exchange has successfully completed,
HIP provides integrity protection and confidentiality for the
communications, and can securely change the locators of the
endpoints.
As a result, it is believed that the HIP opportunistic mode is at
least as secure as current IP.
4.2. Updating a HIP Association
A HIP association between two hosts may need to be updated over time.
Examples include the need to rekey expiring user data security
associations, add new security associations, or change IP addresses
associated with hosts. The UPDATE packet is used for those and other
similar purposes. This document only specifies the UPDATE packet
format and basic processing rules, with mandatory parameters. The
actual usage is defined in separate specifications.
HIP provides a general purpose UPDATE packet, which can carry
multiple HIP parameters, for updating the HIP state between two
peers. The UPDATE mechanism has the following properties:
UPDATE messages carry a monotonically increasing sequence number
and are explicitly acknowledged by the peer. Lost UPDATEs or
acknowledgments may be recovered via retransmission. Multiple
UPDATE messages may be outstanding under certain circumstances.
UPDATE is protected by both HMAC and HIP_SIGNATURE parameters,
since processing UPDATE signatures alone is a potential DoS attack
against intermediate systems.
UPDATE packets are explicitly acknowledged by the use of an
acknowledgment parameter that echoes an individual sequence number
received from the peer. A single UPDATE packet may contain both a
sequence number and one or more acknowledgment numbers (i.e.,
piggybacked acknowledgment(s) for the peer's UPDATE).
The UPDATE packet is defined in Section 5.3.5.
4.3. Error Processing
HIP error processing behavior depends on whether or not there exists
an active HIP association. In general, if a HIP association exists
between the sender and receiver of a packet causing an error
condition, the receiver SHOULD respond with a NOTIFY packet. On the
other hand, if there are no existing HIP associations between the
sender and receiver, or the receiver cannot reasonably determine the
identity of the sender, the receiver MAY respond with a suitable ICMP
message; see Section 5.4 for more details.
The HIP protocol and state machine is designed to recover from one of
the parties crashing and losing its state. The following scenarios
describe the main use cases covered by the design.
No prior state between the two systems.
The system with data to send is the Initiator. The process
follows the standard four-packet base exchange, establishing
the HIP association.
The system with data to send has no state with the receiver, but
the receiver has a residual HIP association.
The system with data to send is the Initiator. The Initiator
acts as in no prior state, sending I1 and getting R1. When the
Responder receives a valid I2, the old association is
'discovered' and deleted, and the new association is
established.
The system with data to send has a HIP association, but the
receiver does not.
The system sends data on the outbound user data security
association. The receiver 'detects' the situation when it
receives a user data packet that it cannot match to any HIP
association. The receiving host MUST discard this packet.
Optionally, the receiving host MAY send an ICMP packet, with
the type Parameter Problem, to inform the sender that the HIP
association does not exist (see Section 5.4), and it MAY
initiate a new HIP negotiation. However, responding with these
optional mechanisms is implementation or policy dependent.
4.4. HIP State Machine
The HIP protocol itself has little state. In the HIP base exchange,
there is an Initiator and a Responder. Once the security
associations (SAs) are established, this distinction is lost. If the
HIP state needs to be re-established, the controlling parameters are
which peer still has state and which has a datagram to send to its
peer. The following state machine attempts to capture these
processes.
The state machine is presented in a single system view, representing
either an Initiator or a Responder. There is not a complete overlap
of processing logic here and in the packet definitions. Both are
needed to completely implement HIP.
Implementors must understand that the state machine, as described
here, is informational. Specific implementations are free to
implement the actual functions differently. Section 6 describes the
packet processing rules in more detail. This state machine focuses
on the HIP I1, R1, I2, and R2 packets only. Other states may be
introduced by mechanisms in other specifications (such as mobility
and multihoming).
4.4.1. HIP States
+---------------------+---------------------------------------------+
| State | Explanation |
+---------------------+---------------------------------------------+
| UNASSOCIATED | State machine start |
| | |
| I1-SENT | Initiating base exchange |
| | |
| I2-SENT | Waiting to complete base exchange |
| | |
| R2-SENT | Waiting to complete base exchange |
| | |
| ESTABLISHED | HIP association established |
| | |
| CLOSING | HIP association closing, no data can be |
| | sent |
| | |
| CLOSED | HIP association closed, no data can be sent |
| | |
| E-FAILED | HIP exchange failed |
+---------------------+---------------------------------------------+
Table 1: HIP States
4.4.2. HIP State Processes
System behavior in state UNASSOCIATED, Table 2.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| User data to send, | Send I1 and go to I1-SENT |
| requiring a new HIP | |
| association | |
| | |
| Receive I1 | Send R1 and stay at UNASSOCIATED |
| | |
| Receive I2, process | If successful, send R2 and go to R2-SENT |
| | |
| | If fail, stay at UNASSOCIATED |
| | |
| Receive user data | Optionally send ICMP as defined in |
| for unknown HIP | Section 5.4 and stay at UNASSOCIATED |
| association | |
| | |
| Receive CLOSE | Optionally send ICMP Parameter Problem and |
| | stay at UNASSOCIATED |
| | |
| Receive ANYOTHER | Drop and stay at UNASSOCIATED |
+---------------------+---------------------------------------------+
Table 2: UNASSOCIATED - Start state
System behavior in state I1-SENT, Table 3.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | If the local HIT is smaller than the peer |
| | HIT, drop I1 and stay at I1-SENT |
| | |
| | If the local HIT is greater than the peer |
| | HIT, send R1 and stay at I1_SENT |
| | |
| Receive I2, process | If successful, send R2 and go to R2-SENT |
| | |
| | If fail, stay at I1-SENT |
| | |
| Receive R1, process | If successful, send I2 and go to I2-SENT |
| | |
| | If fail, stay at I1-SENT |
| | |
| Receive ANYOTHER | Drop and stay at I1-SENT |
| | |
| Timeout, increment | If counter is less than I1_RETRIES_MAX, |
| timeout counter | send I1 and stay at I1-SENT |
| | |
| | If counter is greater than I1_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 3: I1-SENT - Initiating HIP
System behavior in state I2-SENT, Table 4.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay at I2-SENT |
| | |
| Receive R1, process | If successful, send I2 and cycle at I2-SENT |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive I2, process | If successful and local HIT is smaller than |
| | the peer HIT, drop I2 and stay at I2-SENT |
| | |
| | If successful and local HIT is greater than |
| | the peer HIT, send R2 and go to R2-SENT |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive R2, process | If successful, go to ESTABLISHED |
| | |
| | If fail, stay at I2-SENT |
| | |
| Receive ANYOTHER | Drop and stay at I2-SENT |
| | |
| Timeout, increment | If counter is less than I2_RETRIES_MAX, |
| timeout counter | send I2 and stay at I2-SENT |
| | |
| | If counter is greater than I2_RETRIES_MAX, |
| | go to E-FAILED |
+---------------------+---------------------------------------------+
Table 4: I2-SENT - Waiting to finish HIP
System behavior in state R2-SENT, Table 5.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay at R2-SENT |
| | |
| Receive I2, process | If successful, send R2 and cycle at R2-SENT |
| | |
| | If fail, stay at R2-SENT |
| | |
| Receive R1 | Drop and stay at R2-SENT |
| | |
| Receive R2 | Drop and stay at R2-SENT |
| | |
| Receive data or | Move to ESTABLISHED |
| UPDATE | |
| | |
| Exchange Complete | Move to ESTABLISHED |
| Timeout | |
+---------------------+---------------------------------------------+
Table 5: R2-SENT - Waiting to finish HIP
System behavior in state ESTABLISHED, Table 6.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay at ESTABLISHED |
| | |
| Receive I2, process | If successful, send R2, drop old HIP |
| with puzzle and | association, establish a new HIP |
| possible Opaque | association, go to R2-SENT |
| data verification | |
| | |
| | If fail, stay at ESTABLISHED |
| | |
| Receive R1 | Drop and stay at ESTABLISHED |
| | |
| Receive R2 | Drop and stay at ESTABLISHED |
| | |
| Receive user data | Process and stay at ESTABLISHED |
| for HIP association | |
| | |
| No packet | Send CLOSE and go to CLOSING |
| sent/received | |
| during UAL minutes | |
| | |
| Receive CLOSE, | If successful, send CLOSE_ACK and go to |
| process | CLOSED |
| | |
| | If fail, stay at ESTABLISHED |
+---------------------+---------------------------------------------+
Table 6: ESTABLISHED - HIP association established
System behavior in state CLOSING, Table 7.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| User data to send, | Send I1 and stay at CLOSING |
| requires the | |
| creation of another | |
| incarnation of the | |
| HIP association | |
| | |
| Receive I1 | Send R1 and stay at CLOSING |
| | |
| Receive I2, process | If successful, send R2 and go to R2-SENT |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive R1, process | If successful, send I2 and go to I2-SENT |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive CLOSE, | If successful, send CLOSE_ACK, discard |
| process | state and go to CLOSED |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive CLOSE_ACK, | If successful, discard state and go to |
| process | UNASSOCIATED |
| | |
| | If fail, stay at CLOSING |
| | |
| Receive ANYOTHER | Drop and stay at CLOSING |
| | |
| Timeout, increment | If timeout sum is less than UAL+MSL |
| timeout sum, reset | minutes, retransmit CLOSE and stay at |
| timer | CLOSING |
| | |
| | If timeout sum is greater than UAL+MSL |
| | minutes, go to UNASSOCIATED |
+---------------------+---------------------------------------------+
Table 7: CLOSING - HIP association has not been used for UAL minutes
System behavior in state CLOSED, Table 8.
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Datagram to send, | Send I1, and stay at CLOSED |
| requires the | |
| creation of another | |
| incarnation of the | |
| HIP association | |
| | |
| Receive I1 | Send R1 and stay at CLOSED |
| | |
| Receive I2, process | If successful, send R2 and go to R2-SENT |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive R1, process | If successful, send I2 and go to I2-SENT |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive CLOSE, | If successful, send CLOSE_ACK, stay at |
| process | CLOSED |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive CLOSE_ACK, | If successful, discard state and go to |
| process | UNASSOCIATED |
| | |
| | If fail, stay at CLOSED |
| | |
| Receive ANYOTHER | Drop and stay at CLOSED |
| | |
| Timeout (UAL+2MSL) | Discard state, and go to UNASSOCIATED |
+---------------------+---------------------------------------------+
Table 8: CLOSED - CLOSE_ACK sent, resending CLOSE_ACK if necessary
System behavior in state E-FAILED, Table 9.
+-------------------------+-----------------------------------------+
| Trigger | Action |
+-------------------------+-----------------------------------------+
| Wait for | Go to UNASSOCIATED. Re-negotiation is |
| implementation-specific | possible after moving to UNASSOCIATED |
| time | state. |
+-------------------------+-----------------------------------------+
Table 9: E-FAILED - HIP failed to establish association with peer
4.4.3. Simplified HIP State Diagram
The following diagram shows the major state transitions. Transitions
based on received packets implicitly assume that the packets are
successfully authenticated or processed.
+-+ +---------------------------+
I1 received, send R1 | | | |
| v v |
Datagram to send +--------------+ I2 received, send R2 |
+---------------| UNASSOCIATED |---------------+ |
Send I1 | +--------------+ | |
v | |
+---------+ I2 received, send R2 | |
+---->| I1-SENT |---------------------------------------+ | |
| +---------+ | | |
| | +------------------------+ | | |
| | R1 received, | I2 received, send R2 | | | |
| v send I2 | v v v |
| +---------+ | +---------+ |
| +->| I2-SENT |------------+ | R2-SENT |<----+ |
| | +---------+ +---------+ | |
| | | | | |
| | | data| | |
| |receive | or| | |
| |R1, send | EC timeout| receive I2,| |
| |I2 |R2 received +--------------+ | send R2| |
| | +----------->| ESTABLISHED |<-------+| | |
| | +--------------+ | |
| | | | | receive I2, send R2 | |
| | recv+------------+ | +------------------------+ |
| | CLOSE,| | | |
| | send| No packet sent| | |
| | CLOSE_ACK| /received for | timeout | |
| | | UAL min, send | +---------+<-+ (UAL+MSL) | |
| | | CLOSE +--->| CLOSING |--+ retransmit | |
| | | +---------+ CLOSE | |
+--|------------|----------------------+ | | | | | |
+------------|------------------------+ | | +----------------+ |
| | +-----------+ +------------------|--+
| +------------+ | receive CLOSE, CLOSE_ACK | |
| | | send CLOSE_ACK received or | |
| | | timeout | |
| | | (UAL+MSL) | |
| v v | |
| +--------+ receive I2, send R2 | |
+------------------------| CLOSED |---------------------------+ |
+--------+ /----------------------+
^ | \-------/ timeout (UAL+2MSL),
+-+ move to UNASSOCIATED
CLOSE received, send CLOSE_ACK
4.5. User Data Considerations
4.5.1. TCP and UDP Pseudo-Header Computation for User Data
When computing TCP and UDP checksums on user data packets that flow
through sockets bound to HITs, the IPv6 pseudo-header format
[RFC2460] MUST be used, even if the actual addresses on the packet
are IPv4 addresses. Additionally, the HITs MUST be used in the place
of the IPv6 addresses in the IPv6 pseudo-header. Note that the
pseudo-header for actual HIP payloads is computed differently; see
Section 5.1.1.
4.5.2. Sending Data on HIP Packets
A future version of this document may define how to include user data
on various HIP packets. However, currently the HIP header is a
terminal header, and not followed by any other headers.
4.5.3. Transport Formats
The actual data transmission format, used for user data after the HIP
base exchange, is not defined in this document. Such transport
formats and methods are described in separate specifications. All
HIP implementations MUST implement, at minimum, the ESP transport
format for HIP [RFC5202].
When new transport formats are defined, they get the type value from
the HIP Transform type value space 2048-4095. The order in which the
transport formats are presented in the R1 packet, is the preferred
order. The last of the transport formats MUST be ESP transport
format, represented by the ESP_TRANSFORM parameter.
4.5.4. Reboot and SA Timeout Restart of HIP
Simulating a loss of state is a potential DoS attack. The following
process has been crafted to manage state recovery without presenting
a DoS opportunity.
If a host reboots or the HIP association times out, it has lost its
HIP state. If the host that lost state has a datagram to send to the
peer, it simply restarts the HIP base exchange. After the base
exchange has completed, the Initiator can create a new SA and start
sending data. The peer does not reset its state until it receives a
valid I2 HIP packet.
If a system receives a user data packet that cannot be matched to any
existing HIP association, it is possible that it has lost the state
and its peer has not. It MAY send an ICMP packet with the Parameter
Problem type, and with the pointer pointing to the referred HIP-
related association information. Reacting to such traffic depends on
the implementation and the environment where the implementation is
used.
If the host, that apparently has lost its state, decides to restart
the HIP base exchange, it sends an I1 packet to the peer. After the
base exchange has been completed successfully, the Initiator can
create a new HIP association and the peer drops its old SA and
creates a new one.
4.6. Certificate Distribution
This document does not define how to use certificates or how to
transfer them between hosts. These functions are expected to be
defined in a future specification. A parameter type value, meant to
be used for carrying certificates, is reserved, though: CERT, Type
768; see Section 5.2.
5. Packet Formats
5.1. Payload Format
All HIP packets start with a fixed header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Header Length |0| Packet Type | VER. | RES.|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Controls |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver's Host Identity Tag (HIT) |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ HIP Parameters /
/ /
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The HIP header is logically an IPv6 extension header. However, this
document does not describe processing for Next Header values other
than decimal 59, IPPROTO_NONE, the IPv6 'no next header' value.
Future documents MAY do so. However, current implementations MUST
ignore trailing data if an unimplemented Next Header value is
received.
The Header Length field contains the length of the HIP Header and HIP
parameters in 8-byte units, excluding the first 8 bytes. Since all
HIP headers MUST contain the sender's and receiver's HIT fields, the
minimum value for this field is 4, and conversely, the maximum length
of the HIP Parameters field is (255*8)-32 = 2008 bytes. Note: this
sets an additional limit for sizes of parameters included in the
Parameters field, independent of the individual parameter maximum
lengths.
The Packet Type indicates the HIP packet type. The individual packet
types are defined in the relevant sections. If a HIP host receives a
HIP packet that contains an unknown packet type, it MUST drop the
packet.
The HIP Version is four bits. The current version is 1. The version
number is expected to be incremented only if there are incompatible
changes to the protocol. Most extensions can be handled by defining
new packet types, new parameter types, or new controls.
The following three bits are reserved for future use. They MUST be
zero when sent, and they SHOULD be ignored when handling a received
packet.
The two fixed bits in the header are reserved for potential SHIM6
compatibility [SHIM6-PROTO]. For implementations adhering (only) to
this specification, they MUST be set as shown when sending and MUST
be ignored when receiving. This is to ensure optimal forward
compatibility. Note that for implementations that implement other
compatible specifications in addition to this specification, the
corresponding rules may well be different. For example, in the case
that the forthcoming SHIM6 protocol happens to be compatible with
this specification, an implementation that implements both this
specification and the SHIM6 protocol may need to check these bits in
order to determine how to handle the packet.
The HIT fields are always 128 bits (16 bytes) long.
5.1.1. Checksum
Since the checksum covers the source and destination addresses in the
IP header, it must be recomputed on HIP-aware NAT devices.
If IPv6 is used to carry the HIP packet, the pseudo-header [RFC2460]
contains the source and destination IPv6 addresses, HIP packet length
in the pseudo-header length field, a zero field, and the HIP protocol
number (see Section 4) in the Next Header field. The length field is
in bytes and can be calculated from the HIP header length field: (HIP
Header Length + 1) * 8.
In case of using IPv4, the IPv4 UDP pseudo-header format [RFC0768] is
used. In the pseudo-header, the source and destination addresses are
those used in the IP header, the zero field is obviously zero, the
protocol is the HIP protocol number (see Section 4), and the length
is calculated as in the IPv6 case.
5.1.2. HIP Controls
The HIP Controls section conveys information about the structure of
the packet and capabilities of the host.
The following fields have been defined:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | | | | | | | | | | | | | |A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A - Anonymous: If this is set, the sender's HI in this packet is
anonymous, i.e., one not listed in a directory. Anonymous HIs
SHOULD NOT be stored. This control is set in packets R1 and/or
I2. The peer receiving an anonymous HI may choose to refuse it.
The rest of the fields are reserved for future use and MUST be set to
zero on sent packets and ignored on received packets.
5.1.3. HIP Fragmentation Support
A HIP implementation must support IP fragmentation/reassembly.
Fragment reassembly MUST be implemented in both IPv4 and IPv6, but
fragment generation is REQUIRED to be implemented in IPv4 (IPv4
stacks and networks will usually do this by default) and RECOMMENDED
to be implemented in IPv6. In IPv6 networks, the minimum MTU is
larger, 1280 bytes, than in IPv4 networks. The larger MTU size is
usually sufficient for most HIP packets, and therefore fragment
generation may not be needed. If a host expects to send HIP packets
that are larger than the minimum IPv6 MTU, it MUST implement fragment
generation even for IPv6.
In IPv4 networks, HIP packets may encounter low MTUs along their
routed path. Since HIP does not provide a mechanism to use multiple
IP datagrams for a single HIP packet, support for path MTU discovery
does not bring any value to HIP in IPv4 networks. HIP-aware NAT
devices MUST perform any IPv4 reassembly/fragmentation.
All HIP implementations have to be careful while employing a
reassembly algorithm so that the algorithm is sufficiently resistant
to DoS attacks.
Because certificate chains can cause the packet to be fragmented and
fragmentation can open implementation to denial-of-service attacks
[KAU03], it is strongly recommended that the separate document
specifying the certificate usage in the HIP Base Exchange defines the
usage of "Hash and URL" formats rather than including certificates in
exchanges. With this, most problems related to DoS attacks with
fragmentation can be avoided.
5.2. HIP Parameters
The HIP Parameters are used to carry the public key associated with
the sender's HIT, together with related security and other
information. They consist of ordered parameters, encoded in TLV
format.
The following parameter types are currently defined.
+------------------------+-------+----------+-----------------------+
| TLV | Type | Length | Data |
+------------------------+-------+----------+-----------------------+
| R1_COUNTER | 128 | 12 | System Boot Counter |
| | | | |
| PUZZLE | 257 | 12 | K and Random #I |
| | | | |
| SOLUTION | 321 | 20 | K, Random #I and |
| | | | puzzle solution J |
| | | | |
| SEQ | 385 | 4 | Update packet ID |
| | | | number |
| | | | |
| ACK | 449 | variable | Update packet ID |
| | | | number |
| | | | |
| DIFFIE_HELLMAN | 513 | variable | public key |
| | | | |
| HIP_TRANSFORM | 577 | variable | HIP Encryption and |
| | | | Integrity Transform |
| | | | |
| ENCRYPTED | 641 | variable | Encrypted part of I2 |
| | | | packet |
| | | | |
| HOST_ID | 705 | variable | Host Identity with |
| | | | Fully-Qualified |
| | | | Domain FQDN (Name) or |
| | | | Network Access |
| | | | Identifier (NAI) |
| | | | |
| CERT | 768 | variable | HI Certificate; used |
| | | | to transfer |
| | | | certificates. Usage |
| | | | is not currently |
| | | | defined, but it will |
| | | | be specified in a |
| | | | separate document |
| | | | once needed. |
| | | | |
| NOTIFICATION | 832 | variable | Informational data |
| | | | |
| ECHO_REQUEST_SIGNED | 897 | variable | Opaque data to be |
| | | | echoed back; under |
| | | | signature |
| | | | |
| ECHO_RESPONSE_SIGNED | 961 | variable | Opaque data echoed |
| | | | back; under signature |
| | | | |
| HMAC | 61505 | variable | HMAC-based message |
| | | | authentication code, |
| | | | with key material |
| | | | from HIP_TRANSFORM |
| | | | |
| HMAC_2 | 61569 | variable | HMAC based message |
| | | | authentication code, |
| | | | with key material |
| | | | from HIP_TRANSFORM. |
| | | | Compared to HMAC, the |
| | | | HOST_ID parameter is |
| | | | included in HMAC_2 |
| | | | calculation. |
| | | | |
| HIP_SIGNATURE_2 | 61633 | variable | Signature of the R1 |
| | | | packet |
| | | | |
| HIP_SIGNATURE | 61697 | variable | Signature of the |
| | | | packet |
| | | | |
| ECHO_REQUEST_UNSIGNED | 63661 | variable | Opaque data to be |
| | | | echoed back; after |
| | | | signature |
| | | | |
| ECHO_RESPONSE_UNSIGNED | 63425 | variable | Opaque data echoed |
| | | | back; after signature |
+------------------------+-------+----------+-----------------------+
Because the ordering (from lowest to highest) of HIP parameters is
strictly enforced (see Section 5.2.1), the parameter type values for
existing parameters have been spaced to allow for future protocol
extensions. Parameters numbered between 0-1023 are used in HIP
handshake and update procedures and are covered by signatures.
Parameters numbered between 1024-2047 are reserved. Parameters
numbered between 2048-4095 are used for parameters related to HIP
transform types. Parameters numbered between 4096 and (2^16 - 2^12)
61439 are reserved. Parameters numbered between 61440-62463 are used
for signatures and signed MACs. Parameters numbered between 62464-
63487 are used for parameters that fall outside of the signed area of
the packet. Parameters numbered between 63488-64511 are used for
rendezvous and other relaying services. Parameters numbered between
64512-65535 are reserved.
5.2.1. TLV Format
The TLV-encoded parameters are described in the following
subsections. The type-field value also describes the order of these
fields in the packet, except for type values from 2048 to 4095 which
are reserved for new transport forms. The parameters MUST be
included in the packet such that their types form an increasing
order. If the parameter can exist multiple times in the packet, the
type value may be the same in consecutive parameters. If the order
does not follow this rule, the packet is considered to be malformed
and it MUST be discarded.
Parameters using type values from 2048 up to 4095 are transport
formats. Currently, one transport format is defined: the ESP
transport format [RFC5202]. The order of these parameters does not
follow the order of their type value, but they are put in the packet
in order of preference. The first of the transport formats it the
most preferred, and so on.
All of the TLV parameters have a length (including Type and Length
fields), which is a multiple of 8 bytes. When needed, padding MUST
be added to the end of the parameter so that the total length becomes
a multiple of 8 bytes. This rule ensures proper alignment of data.
Any added padding bytes MUST be zeroed by the sender, and their
values SHOULD NOT be checked by the receiver.
Consequently, the Length field indicates the length of the Contents
field (in bytes). The total length of the TLV parameter (including
Type, Length, Contents, and Padding) is related to the Length field
according to the following formula:
Total Length = 11 + Length - (Length + 3) % 8;
where % is the modulo operator
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |C| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Contents /
/ +-+-+-+-+-+-+-+-+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Type code for the parameter. 16 bits long, C-bit
being part of the Type code.
C Critical. One if this parameter is critical, and
MUST be recognized by the recipient, zero otherwise.
The C bit is considered to be a part of the Type
field. Consequently, critical parameters are always
odd and non-critical ones have an even value.
Length Length of the Contents, in bytes.
Contents Parameter specific, defined by Type
Padding Padding, 0-7 bytes, added if needed
Critical parameters MUST be recognized by the recipient. If a
recipient encounters a critical parameter that it does not recognize,
it MUST NOT process the packet any further. It MAY send an ICMP or
NOTIFY, as defined in Section 4.3.
Non-critical parameters MAY be safely ignored. If a recipient
encounters a non-critical parameter that it does not recognize, it
SHOULD proceed as if the parameter was not present in the received
packet.
5.2.2. Defining New Parameters
Future specifications may define new parameters as needed. When
defining new parameters, care must be taken to ensure that the
parameter type values are appropriate and leave suitable space for
other future extensions. One must remember that the parameters MUST
always be arranged in increasing order by Type code, thereby limiting
the order of parameters (see Section 5.2.1).
The following rules must be followed when defining new parameters.
1. The low-order bit C of the Type code is used to distinguish
between critical and non-critical parameters.
2. A new parameter may be critical only if an old recipient ignoring
it would cause security problems. In general, new parameters
SHOULD be defined as non-critical, and expect a reply from the
recipient.
3. If a system implements a new critical parameter, it MUST provide
the ability to set the associated feature off, such that the
critical parameter is not sent at all. The configuration option
must be well documented. Implementations operating in a mode
adhering to this specification MUST disable the sending of new
critical parameters. In other words, the management interface
MUST allow vanilla standards-only mode as a default configuration
setting, and MAY allow new critical payloads to be configured on
(and off).
4. See Section 9 for allocation rules regarding Type codes.
5.2.3. R1_COUNTER
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, 4 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R1 generation counter, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 128
Length 12
R1 generation
counter The current generation of valid puzzles
The R1_COUNTER parameter contains a 64-bit unsigned integer in
network-byte order, indicating the current generation of valid
puzzles. The sender is supposed to increment this counter
periodically. It is RECOMMENDED that the counter value is
incremented at least as often as old PUZZLE values are deprecated so
that SOLUTIONs to them are no longer accepted.
The R1_COUNTER parameter is optional. It SHOULD be included in the
R1 (in which case, it is covered by the signature), and if present in
the R1, it MAY be echoed (including the Reserved field verbatim) by
the Initiator in the I2.
5.2.4. PUZZLE
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| K, 1 byte | Lifetime | Opaque, 2 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random #I, 8 bytes |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 257
Length 12
K K is the number of verified bits
Lifetime puzzle lifetime 2^(value-32) seconds
Opaque data set by the Responder, indexing the puzzle
Random #I random number
Random #I is represented as a 64-bit integer, K and Lifetime as 8-bit
integers, all in network byte order.
The PUZZLE parameter contains the puzzle difficulty K and a 64-bit
puzzle random integer #I. The Puzzle Lifetime indicates the time
during which the puzzle solution is valid, and sets a time limit that
should not be exceeded by the Initiator while it attempts to solve
the puzzle. The lifetime is indicated as a power of 2 using the
formula 2^(Lifetime-32) seconds. A puzzle MAY be augmented with an
ECHO_REQUEST_SIGNED or an ECHO_REQUEST_UNSIGNED parameter included in
the R1; the contents of the echo request are then echoed back in the
ECHO_RESPONSE_SIGNED or in the ECHO_RESPONSE_UNSIGNED, allowing the
Responder to use the included information as a part of its puzzle
processing.
The Opaque and Random #I field are not covered by the HIP_SIGNATURE_2
parameter.
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.
5.2.6. DIFFIE_HELLMAN
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID | Public Value Length | Public Value /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group ID | Public Value Length | Public Value /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 513
Length length in octets, excluding Type, Length, and
padding
Group ID defines values for p and g
Public Value length of the following Public Value in octets
Length
Public Value the sender's public Diffie-Hellman key
The following Group IDs have been defined:
Group Value
Reserved 0
384-bit group 1
OAKLEY well-known group 1 2
1536-bit MODP group 3
3072-bit MODP group 4
6144-bit MODP group 5
8192-bit MODP group 6
The MODP Diffie-Hellman groups are defined in [RFC3526]. The OAKLEY
well-known group 1 is defined in Appendix E.
The sender can include at most two different Diffie-Hellman public
values in the DIFFIE_HELLMAN parameter. This gives the possibility,
e.g., for a server to provide a weaker encryption possibility for a
PDA host that is not powerful enough. It is RECOMMENDED that the
Initiator, receiving more than one public value, selects the stronger
one, if it supports it.
A HIP implementation MUST implement Group IDs 1 and 3. The 384-bit
group can be used when lower security is enough (e.g., web surfing)
and when the equipment is not powerful enough (e.g., some PDAs). It
is REQUIRED that the default configuration allows Group ID 1 usage,
but it is RECOMMENDED that applications that need stronger security
turn Group ID 1 support off. Equipment powerful enough SHOULD
implement also Group ID 5. The 384-bit group is defined in
Appendix D.
To avoid unnecessary failures during the base exchange, the rest of
the groups SHOULD be implemented in hosts where resources are
adequate.
5.2.7. HIP_TRANSFORM
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite ID #1 | Suite ID #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suite ID #n | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 577
Length length in octets, excluding Type, Length, and
padding
Suite ID defines the HIP Suite to be used
The following Suite IDs are defined ([RFC4307],[RFC2451]):
Suite ID Value
RESERVED 0
AES-CBC with HMAC-SHA1 1
3DES-CBC with HMAC-SHA1 2
3DES-CBC with HMAC-MD5 3
BLOWFISH-CBC with HMAC-SHA1 4
NULL-ENCRYPT with HMAC-SHA1 5
NULL-ENCRYPT with HMAC-MD5 6
The sender of a HIP_TRANSFORM parameter MUST make sure that there are
no more than six (6) HIP Suite IDs in one HIP_TRANSFORM parameter.
Conversely, a recipient MUST be prepared to handle received transport
parameters that contain more than six Suite IDs by accepting the
first six Suite IDs and dropping the rest. The limited number of
transforms sets the maximum size of HIP_TRANSFORM parameter. As the
default configuration, the HIP_TRANSFORM parameter MUST contain at
least one of the mandatory Suite IDs. There MAY be a configuration
option that allows the administrator to override this default.
The Responder lists supported and desired Suite IDs in order of
preference in the R1, up to the maximum of six Suite IDs. The
Initiator MUST choose only one of the corresponding Suite IDs. That
Suite ID will be used for generating the I2.
Mandatory implementations: AES-CBC with HMAC-SHA1 and NULL-ENCRYPTION
with HMAC-SHA1.
5.2.8. HOST_ID
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HI Length |DI-type| DI Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Identity /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Domain Identifier /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 705
Length length in octets, excluding Type, Length, and
Padding
HI Length length of the Host Identity in octets
DI-type type of the following Domain Identifier field
DI Length length of the FQDN or NAI in octets
Host Identity actual Host Identity
Domain Identifier the identifier of the sender
The Host Identity is represented in RFC 4034 [RFC4034] format. The
algorithms used in RDATA format are the following:
Algorithms Values
RESERVED 0
DSA 3 [RFC2536] (RECOMMENDED)
RSA/SHA1 5 [RFC3110] (REQUIRED)
The following DI-types have been defined:
Type Value
none included 0
FQDN 1
NAI 2
FQDN Fully Qualified Domain Name, in binary format.
NAI Network Access Identifier
The format for the FQDN is defined in RFC 1035 [RFC1035] Section 3.1.
The format for NAI is defined in [RFC4282]
If there is no Domain Identifier, i.e., the DI-type field is zero,
the DI Length field is set to zero as well.
5.2.9. HMAC
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC |
/ /
/ +-------------------------------+
| | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 61505
Length length in octets, excluding Type, Length, and
Padding
HMAC HMAC computed over the HIP packet, excluding the
HMAC parameter and any following parameters, such
as HIP_SIGNATURE, HIP_SIGNATURE_2,
ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED.
The checksum field MUST be set to zero and the HIP
header length in the HIP common header MUST be
calculated not to cover any excluded parameters
when the HMAC is calculated. The size of the
HMAC is the natural size of the hash computation
output depending on the used hash function.
The HMAC calculation and verification process is presented in
Section 6.4.1.
5.2.10. HMAC_2
The parameter structure is the same as in Section 5.2.9. The fields
are:
Type 61569
Length length in octets, excluding Type, Length, and
Padding
HMAC HMAC computed over the HIP packet, excluding the
HMAC parameter and any following parameters such
as HIP_SIGNATURE, HIP_SIGNATURE_2,
ECHO_REQUEST_UNSIGNED, or ECHO_RESPONSE_UNSIGNED,
and including an additional sender's HOST_ID
parameter during the HMAC calculation. The
checksum field MUST be set to zero and the HIP
header length in the HIP common header MUST be
calculated not to cover any excluded parameters
when the HMAC is calculated. The size of the
HMAC is the natural size of the hash computation
output depending on the used hash function.
The HMAC calculation and verification process is presented in
Section 6.4.1.
5.2.11. HIP_SIGNATURE
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIG alg | Signature /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 61697
Length length in octets, excluding Type, Length, and
Padding
SIG alg signature algorithm
Signature the signature is calculated over the HIP packet,
excluding the HIP_SIGNATURE parameter and any
parameters that follow the HIP_SIGNATURE parameter.
The checksum field MUST be set to zero, and the HIP
header length in the HIP common header MUST be
calculated only to the beginning of the
HIP_SIGNATURE parameter when the signature is
calculated.
The signature algorithms are defined in Section 5.2.8. The signature
in the Signature field is encoded using the proper method depending
on the signature algorithm (e.g., according to [RFC3110] in case of
RSA/SHA1, or according to [RFC2536] in case of DSA).
The HIP_SIGNATURE calculation and verification process is presented
in Section 6.4.2.
5.2.12. HIP_SIGNATURE_2
The parameter structure is the same as in Section 5.2.11. The fields
are:
Type 61633
Length length in octets, excluding Type, Length, and
Padding
SIG alg signature algorithm
Signature Within the R1 packet that contains the HIP_SIGNATURE_2
parameter, the Initiator's HIT, the checksum
field, and the Opaque and Random #I fields in the
PUZZLE parameter MUST be set to zero while
computing the HIP_SIGNATURE_2 signature. Further,
the HIP packet length in the HIP header MUST be
adjusted as if the HIP_SIGNATURE_2 was not in the
packet during the signature calculation, i.e., the
HIP packet length points to the beginning of
the HIP_SIGNATURE_2 parameter during signing and
verification.
Zeroing the Initiator's HIT makes it possible to create R1 packets
beforehand, to minimize the effects of possible DoS attacks. Zeroing
the Random #I and Opaque fields within the PUZZLE parameter allows
these fields to be populated dynamically on precomputed R1s.
Signature calculation and verification follows the process in
Section 6.4.2.
5.2.13. SEQ
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Update ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 385
Length 4
Update ID 32-bit sequence number
The Update ID is an unsigned quantity, initialized by a host to zero
upon moving to ESTABLISHED state. The Update ID has scope within a
single HIP association, and not across multiple associations or
multiple hosts. The Update ID is incremented by one before each new
UPDATE that is sent by the host; the first UPDATE packet originated
by a host has an Update ID of 0.
5.2.14. ACK
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| peer Update ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 449
Length variable (multiple of 4)
peer Update ID 32-bit sequence number corresponding to the
Update ID being ACKed.
The ACK parameter includes one or more Update IDs that have been
received from the peer. The Length field identifies the number of
peer Update IDs that are present in the parameter.
5.2.15. ENCRYPTED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IV /
/ /
/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
/ Encrypted data /
/ /
/ +-------------------------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 641
Length length in octets, excluding Type, Length, and
Padding
Reserved zero when sent, ignored when received
IV Initialization vector, if needed, otherwise
nonexistent. The length of the IV is inferred from
the HIP transform.
Encrypted The data is encrypted using an encryption algorithm
data as defined in HIP transform.
The ENCRYPTED parameter encapsulates another parameter, the encrypted
data, which holds one or more HIP parameters in block encrypted form.
Consequently, the first fields in the encapsulated parameter(s) are
Type and Length of the first such parameter, allowing the contents to
be easily parsed after decryption.
The field labelled "Encrypted data" consists of the output of one or
more HIP parameters concatenated together that have been passed
through an encryption algorithm. Each of these inner parameters is
padded according to the rules of Section 5.2.1 for padding individual
parameters. As a result, the concatenated parameters will be a block
of data that is 8-byte aligned.
Some encryption algorithms require that the data to be encrypted must
be a multiple of the cipher algorithm block size. In this case, the
above block of data MUST include additional padding, as specified by
the encryption algorithm. The size of the extra padding is selected
so that the length of the unencrypted data block is a multiple of the
cipher block size. The encryption algorithm may specify padding
bytes other than zero; for example, AES [FIPS01] uses the PKCS5
padding scheme (see section 6.1.1 of [RFC2898]) where the remaining n
bytes to fill the block each have the value n. This yields an
"unencrypted data" block that is transformed to an "encrypted data"
block by the cipher suite. This extra padding added to the set of
parameters to satisfy the cipher block alignment rules is not counted
in HIP TLV length fields, and this extra padding should be removed by
the cipher suite upon decryption.
Note that the length of the cipher suite output may be smaller or
larger than the length of the set of parameters to be encrypted,
since the encryption process may compress the data or add additional
padding to the data.
Once this encryption process is completed, the Encrypted data field
is ready for inclusion in the Parameter. If necessary, additional
Padding for 8-byte alignment is then added according to the rules of
Section 5.2.1.
5.2.16. NOTIFICATION
The NOTIFICATION parameter is used to transmit informational data,
such as error conditions and state transitions, to a HIP peer. A
NOTIFICATION parameter may appear in the NOTIFY packet type. The use
of the NOTIFICATION parameter in other packet types is for further
study.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ Notification Data /
/ +---------------+
/ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 832
Length length in octets, excluding Type, Length, and
Padding
Reserved zero when sent, ignored when received
Notify Message specifies the type of notification
Type
Notification informational or error data transmitted in addition
Data to the Notify Message Type. Values for this field
are type specific (see below).
Padding any Padding, if necessary, to make the parameter a
multiple of 8 bytes.
Notification information can be error messages specifying why an SA
could not be established. It can also be status data that a process
managing an SA database wishes to communicate with a peer process.
The table below lists the Notification messages and their
corresponding values.
To avoid certain types of attacks, a Responder SHOULD avoid sending a
NOTIFICATION to any host with which it has not successfully verified
a puzzle solution.
Types in the range 0-16383 are intended for reporting errors and in
the range 16384-65535 for other status information. An
implementation that receives a NOTIFY packet with a NOTIFICATION
error parameter in response to a request packet (e.g., I1, I2,
UPDATE) SHOULD assume that the corresponding request has failed
entirely. Unrecognized error types MUST be ignored except that they
SHOULD be logged.
Notify payloads with status types MUST be ignored if not recognized.
NOTIFICATION PARAMETER - ERROR TYPES Value
------------------------------------ -----
UNSUPPORTED_CRITICAL_PARAMETER_TYPE 1
Sent if the parameter type has the "critical" bit set and the
parameter type is not recognized. Notification Data contains
the two-octet parameter type.
INVALID_SYNTAX 7
Indicates that the HIP message received was invalid because
some type, length, or value was out of range or because the
request was rejected for policy reasons. To avoid a denial-
of-service attack using forged messages, this status may only be
returned for packets whose HMAC (if present) and SIGNATURE have
been verified. This status MUST be sent in response to any
error not covered by one of the other status types, and should
not contain details to avoid leaking information to someone
probing a node. To aid debugging, more detailed error
information SHOULD be written to a console or log.
NO_DH_PROPOSAL_CHOSEN 14
None of the proposed group IDs was acceptable.
INVALID_DH_CHOSEN 15
The D-H Group ID field does not correspond to one offered
by the Responder.
NO_HIP_PROPOSAL_CHOSEN 16
None of the proposed HIP Transform crypto suites was
acceptable.
INVALID_HIP_TRANSFORM_CHOSEN 17
The HIP Transform crypto suite does not correspond to
one offered by the Responder.
AUTHENTICATION_FAILED 24
Sent in response to a HIP signature failure, except when
the signature verification fails in a NOTIFY message.
CHECKSUM_FAILED 26
Sent in response to a HIP checksum failure.
HMAC_FAILED 28
Sent in response to a HIP HMAC failure.
ENCRYPTION_FAILED 32
The Responder could not successfully decrypt the
ENCRYPTED parameter.
INVALID_HIT 40
Sent in response to a failure to validate the peer's
HIT from the corresponding HI.
BLOCKED_BY_POLICY 42
The Responder is unwilling to set up an association
for some policy reason (e.g., received HIT is NULL
and policy does not allow opportunistic mode).
SERVER_BUSY_PLEASE_RETRY 44
The Responder is unwilling to set up an association as it is
suffering under some kind of overload and has chosen to shed load
by rejecting the Initiator's request. The Initiator may retry;
however, the Initiator MUST find another (different) puzzle
solution for any such retries. Note that the Initiator may need
to obtain a new puzzle with a new I1/R1 exchange.
NOTIFY MESSAGES - STATUS TYPES Value
------------------------------ -----
I2_ACKNOWLEDGEMENT 16384
The Responder has an I2 from the Initiator but had to queue the I2
for processing. The puzzle was correctly solved and the Responder
is willing to set up an association but currently has a number of
I2s in the processing queue. R2 will be sent after the I2 has
been processed.
5.2.17. ECHO_REQUEST_SIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 897
Length variable
Opaque data opaque data, supposed to be meaningful only to the
node that sends ECHO_REQUEST_SIGNED and receives a
corresponding ECHO_RESPONSE_SIGNED or
ECHO_RESPONSE_UNSIGNED.
The ECHO_REQUEST_SIGNED parameter contains an opaque blob of data
that the sender wants to get echoed back in the corresponding reply
packet.
The ECHO_REQUEST_SIGNED and corresponding echo response parameters
MAY be used for any purpose where a node wants to carry some state in
a request packet and get it back in a response packet. The
ECHO_REQUEST_SIGNED is covered by the HMAC and SIGNATURE. A HIP
packet can contain only one ECHO_REQUEST_SIGNED or
ECHO_REQUEST_UNSIGNED parameter. The ECHO_REQUEST_SIGNED parameter
MUST be responded to with a corresponding echo response.
ECHO_RESPONSE_SIGNED SHOULD be used, but if it is not possible, e.g.,
due to a middlebox-provided response, it MAY be responded to with an
ECHO_RESPONSE_UNSIGNED.
5.2.18. ECHO_REQUEST_UNSIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63661
Length variable
Opaque data opaque data, supposed to be meaningful only to the
node that sends ECHO_REQUEST_UNSIGNED and receives a
corresponding ECHO_RESPONSE_UNSIGNED.
The ECHO_REQUEST_UNSIGNED parameter contains an opaque blob of data
that the sender wants to get echoed back in the corresponding reply
packet.
The ECHO_REQUEST_UNSIGNED and corresponding echo response parameters
MAY be used for any purpose where a node wants to carry some state in
a request packet and get it back in a response packet. The
ECHO_REQUEST_UNSIGNED is not covered by the HMAC and SIGNATURE. A
HIP packet can contain one or more ECHO_REQUEST_UNSIGNED parameters.
It is possible that middleboxes add ECHO_REQUEST_UNSIGNED parameters
in HIP packets passing by. The sender has to create the Opaque field
so that it can later identify and remove the corresponding
ECHO_RESPONSE_UNSIGNED parameter.
The ECHO_REQUEST_UNSIGNED parameter MUST be responded to with an
ECHO_RESPONSE_UNSIGNED parameter.
5.2.19. ECHO_RESPONSE_SIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 961
Length variable
Opaque data opaque data, copied unmodified from the
ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
parameter that triggered this response.
The ECHO_RESPONSE_SIGNED parameter contains an opaque blob of data
that the sender of the ECHO_REQUEST_SIGNED wants to get echoed back.
The opaque data is copied unmodified from the ECHO_REQUEST_SIGNED
parameter.
The ECHO_REQUEST_SIGNED and ECHO_RESPONSE_SIGNED parameters MAY be
used for any purpose where a node wants to carry some state in a
request packet and get it back in a response packet. The
ECHO_RESPONSE_SIGNED is covered by the HMAC and SIGNATURE.
5.2.20. ECHO_RESPONSE_UNSIGNED
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque data (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 63425
Length variable
Opaque data opaque data, copied unmodified from the
ECHO_REQUEST_SIGNED or ECHO_REQUEST_UNSIGNED
parameter that triggered this response.
The ECHO_RESPONSE_UNSIGNED