IPSEC Working Group Charlie Kaufman
INTERNET-DRAFT editor
draft-ietf-ipsec-ikev2-04.txt January 2003
Internet Key Exchange (IKEv2) Protocol
<draft-ietf-ipsec-ikev2-04.txt>
Status of this Memo
This document is a submission by the IPSEC Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipsec@lists.tislabs.com mailing list.
Distribution of this memo is unlimited.
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [Bra96]. Internet Drafts are
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Abstract
This document describes version 2 of the IKE (Internet Key Exchange)
protocol. IKE performs mutual authentication and establishes an IKE
security association that can be used to efficiently establish SAs
for ESP and/or AH. This version greatly simplifies IKE by replacing
the 8 possible phase 1 exchanges with a single exchange based on
either public signature keys or shared secret keys. The single
exchange provides identity hiding, yet works in 2 round trips (all
the identity hiding exchanges in IKE v1 required 3 round trips).
Latency of setup of an IPsec SA is further reduced from IKEv1 by
allowing setup of an SA for ESP and/or AH to be piggybacked on the
initial IKE exchange. It also improves security by allowing the
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Responder to be stateless until it can be assured that the Initiator
can receive at the claimed IP source address. This version also
presents the entire protocol in a single self-contained document, in
contrast to IKEv1, in which the protocol was described in ISAKMP (RFC
2408), IKE (RFC 2409), and the DOI (RFC 2407) documents.
Table of Contents
Abstract.....................................................1
1 Summary of Changes from IKEv1..............................3
2 Requirements Terminology...................................4
3 IKE Protocol Overview......................................5
3.1 Usage Scenarios..........................................6
3.1.1 Gateway to Gateway Tunnel..............................6
3.1.2 Endpoint to Endpoint Transport.........................6
3.1.3 Endpoint to Gateway Transport..........................7
3.1.4 Other Scenarios........................................8
3.2 The Initial Exchange.....................................8
3.3 The CREATE_CHILD_SA Exchange.............................9
3.4 The Informational Exchange..............................11
3.5 Informational Messages outside of an IKE-SA.............12
4 IKE Protocol Details and Variations.......................13
4.1 Use of Retransmission Timers............................13
4.2 Use of Sequence Numbers for Message ID..................13
4.3 Window Size for overlapping requests....................14
4.4 State Synchronization and Connection Timeouts...........15
4.5 Version Numbers and Forward Compatibility...............16
4.6 Cookies.................................................18
4.7 Cryptographic Algorithm Negotiation.....................20
4.8 Rekeying................................................20
4.9 Traffic Selector Negotiation............................21
4.10 Nonces.................................................23
4.11 Address and Port Agility...............................24
4.12 Reuse of Diffie-Hellman Exponentials...................24
4.13 Generating Keying Material.............................25
4.14 Generating Keying Material for the IKE-SA..............25
4.15 Authentication of the IKE-SA...........................26
4.16 Generating Keying Material for CHILD-SAs...............27
4.17 Rekaying IKE-SAs using a CREATE_CHILD_SA exchange......28
4.18 Requesting an internal address on a remote network.....28
4.19 Requesting a Peer's Version............................30
4.20 Error Handling.........................................30
4.21 IPcomp.................................................31
5 Header and Payload Formats................................32
5.1 The IKE Header..........................................32
5.2 Generic Payload Header..................................34
5.3 Security Association Payload............................35
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5.3.1 Proposal Substructure.................................36
5.4 Key Exchange Payload....................................38
5.5 Identification Payload..................................38
5.6 Certificate Payload.....................................40
5.7 Certificate Request Payload.............................41
5.8 Authentication Payload..................................42
5.9 Nonce Payload...........................................44
5.10 Notify Payload.........................................44
5.10.1 Notify Message Types.................................45
5.11 Delete Payload.........................................50
5.12 Vendor ID Payload......................................51
5.13 Traffic Selector Payload...............................53
5.13.1 Traffic Selector.....................................53
5.14 Encrypted Payload......................................55
5.15 Configuration Payload..................................57
5.15.1 Configuration Attributes.............................59
5.16 Other Payload types....................................61
6 Conformance Requirements..................................62
7 Security Considerations...................................62
8 IANA Considerations.......................................63
8.1 Transform Types and Attribute Values....................64
8.2 Exchange Types..........................................64
8.3 Payload Types...........................................64
9 Acknowledgements..........................................64
10 References...............................................64
10 Normative References.....................................64
10 Non-normative References.................................64
Appendix A: NAT Traversal...................................67
Appendix B: Diffie-Hellman Groups...........................69
Change History..............................................71
Author's Address............................................73
Full Copyright Statement....................................74
1 Summary of changes from IKEv1
The goals of this revision to IKE are:
1) To define the entire IKE protocol in a single document, rather
than three that cross reference one another;
2) To simplify IKE by replacing the eight different initial phase 1
exchanges with a single four message exchange (with changes in
authentication mechanisms affecting only a single AUTH payload rather
than restructuring the entire exchange);
3) To remove the Domain of Interpretation (DOI), Situation (SIT), and
Labeled Domain Identifier fields, and the Commit and Authentication
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only bits;
4) To decrease IKE's latency in the common case by making the initial
exchange be 2 round trips (4 messages), and allowing the ability to
piggyback setup of a CHILD-SA on that exchange;
5) To replace the cryptographic syntax for protecting the IKE
messages themselves with one based closely on ESP to simplify
implementation and security analysis;
6) To reduce the number of possible error states by making the
protocol reliable (all messages are acknowledged) and sequenced. This
allows shortening Phase 2 exchanges from 3 messages to 2;
7) To increase robustness by allowing the responder to not do
significant processing until it receives a message proving that the
initiator can receive messages at its claimed IP address, and not
commit any state to an exchange until the initiator can be
cryptographically authenticated;
8) To fix bugs such as the hash problem documented in [draft-ietf-
ipsec-ike-hash-revised-02.txt];
9) To specify Traffic Selectors in their own payloads type rather
than overloading ID payloads, and making more flexible the Traffic
Selectors that may be specified;
10) To replace the complex mix and match negotiation of cryptographic
algorithms with proposals based on suites of algorithms;
11) To specify required behavior under certain error conditions or
when data that is not understood is received in order to make it
easier to make future revisions in a way that does not break
backwards compatibility;
12) To incorporate ideas from draft-ietf-ipsec-nat-reqts-02.txt to
allow IKE to negotiate through NAT gateways;
12) To simplify and clarify how shared state is maintained in the
presence of network failures and Denial of Service attacks; and
13) To maintain existing syntax and magic numbers to the extent
possible to make it likely that implementations of IKEv1 can be
enhanced to support IKEv2 with minimum effort.
2 Requirements Terminology
Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
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"MAY" that appear in this document are to be interpreted as described
in [Bra97].
3 IKE Protocol Overview
IP Security (IPsec) provides confidentiality, data integrity, access
control, and data source authentication to IP datagrams. These
services are provided by maintaining shared state between the source
and the sink of an IP datagram. This state defines, among other
things, the specific services provided to the datagram, which
cryptographic algorithms will be used to provide the services, and
the keys used as input to the cryptographic algorithms.
Establishing this shared state in a manual fashion does not scale
well. Therefore a protocol to establish this state dynamically is
needed. This memo describes such a protocol-- the Internet Key
Exchange (IKE). This is version 2 of IKE. Version 1 of IKE was
defined in RFCs 2407, 2408, and 2409. This single document is
intended to replace all three of those RFCs.
IKE performs mutual authentication between two parties and
establishes an IKE security association that includes shared secret
information that can be used to efficiently establish SAs for ESP
(RFC 2406) and/or AH (RFC 2402). It also negotiates use of IPcomp
(RFC 2393) in connection with an ESP and/or AH SA. We call the IKE
SA an "IKE-SA". The SAs for ESP and/or AH that get set up through
that IKE-SA we call "CHILD-SA"s.
We call the first four messages establishing an IKE-SA a "phase 1"
exchange and subsequent IKE exchanges "phase 2", inheriting this
terminology from IKEv1. The phase 1 exchange establishes the IKE-SA
and the first CHILD-SA. In some scenarios, only a single CHILD-SA is
needed between the IPsec endpoints and therefore there would be no
phase 2 exchanges. Phase 2 exchanges MAY be used to establish
additional CHILD-SAs between the same authenticated pair of endpoints
and to perform housekeeping functions. The phase 1 exchange consists
of two request/response pairs. A phase 2 exchange is one
request/response pair, and can be used to create or delete a CHILD-
SA, rekey or delete the IKE-SA, or report information such as error
conditions.
IKE message flow always consists of a request followed by a response.
It is the responsibility of the requester to ensure reliability. If
the response is not received within a timeout interval, the requester
MUST retransmit the request (or abandon the connection).
The first request/response of a phase 1 exchange negotiates security
parameters for the IKE-SA, sends nonces, and sends Diffie-Hellman
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values. We call the initial exchange IKE_SA_INIT (request and
response).
The second request/response, which we'll call IKE_AUTH transmits
identities, proves knowledge of the secrets corresponding to the two
identities, and sets up an SA for the first (and often only) AH
and/or ESP CHILD-SA.
Phase 2 exchanges each consist of a single request/response pair. The
types of exchanges are CREATE_CHILD_SA (which creates a CHILD-SA), or
an Informational exchange which deletes an SA, reports error
conditions, or does other housekeeping. All these messages require a
response. An informational message with no payloads is commonly used
as a check for liveness.
In the description that follows, we assume that no errors occur.
Modifications to the flow should errors occur are described in
section 4.
3.1 Usage Scenarios
IKE is expected to be used to negotiate ESP and/or AH SAs in a number
of different scenarios, each with their own special requirements.
3.1.1 Gateway to Gateway Tunnel
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec ! !
Protected !Tunnel ! Tunnel !Tunnel ! Protected
Subnet <-->!Endpoint !<---------->!Endpoint !<--> Subnet
! ! ! !
+-+-+-+-+-+ +-+-+-+-+-+
Figure 1: Firewall to Firewall Tunnel
In this scenario, neither endpoint of the IP connection implements
IPsec, but network nodes between them protect traffic for part of the
way. Protection is transparent to the endpoints, and depends on
ordinary routing sending packets through the tunnel endpoints for
processing. Each endpoint would announce the set of addresses
"behind" it, and packets would be sent in Tunnel Mode where the inner
IP header would contain the IP addresses of the actual endpoints.
3.1.2 Endpoint to Endpoint Transport
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec ! !
!Protected! Tunnel !Protected!
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!Endpoint !<---------------------------------------->!Endpoint !
! ! ! !
+-+-+-+-+-+ +-+-+-+-+-+
Figure 2: Endpoint to Endpoint
In this scenario, both endpoints of the IP connection implement
IPsec. These endpoints may implement application layer access
controls based on the authenticated identities of the participants.
Transport mode will commonly be used with no inner IP header. If
there is an inner IP header, the inner addresses will be the same as
the outher addresses. A single pair of addresses will be negotiated
for packets to be sent over this SA.
It is possible in this scenario that one of the protected endpoints
will be behind a network address translation (NAT) node, in which
case the tunnelled packets will have to be UDP encapsulated so that
port numbers in the UDP headers can be used to identify individual
endpoints "behind" the NAT.
3.1.3 Endpoint to Gateway Transport
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec ! ! Protected
!Protected! Tunnel !Tunnel ! Subnet
!Endpoint !<------------------------>!Endpoint !<--- and/or
! ! ! ! Internet
+-+-+-+-+-+ +-+-+-+-+-+
Figure 3: Endpoint to Gateway
In this scenario, a protected endpoint (typically a portable roaming
computer) connects back to its corporate network through an IPsec
protected tunnel. It might use this tunnel only to access information
on the corporate network or it might tunnel all of its traffic back
through the corporate network in order to take advantage of
protection provided by a corporate firewall against Internet based
attacks. In either case, the protected endpoint will want an IP
address associated with the gateway so that packets returned to it
will go to the gateway and be tunnelled back. This IP address may be
static or may be dynamically allocated by the gateway. In support of
the latter case, IKEv2 includes a mechanism for the initiator to
request an IP address owned by the gateway for use for the duration
of its SA.
In this scenario, packets will use tunnel mode. On each packet from
the protected endpoint, the outer IP header will contain the source
IP address associated with its current location (i.e. the address
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that will get traffic routed to the endpoint directly) while the
inner IP header will contain the source IP address assigned by the
gateway (i.e. the address that will get traffic routed to the gateway
for forwarding to the endpoint). The outer destination address will
always be that of the gateway, while the inner destination address
will be the ultimate destination for the packet.
In this scenario, it is possible that the protected endpoint will be
behind a NAT. In that case, the IP address as seen by the gateway
will not be the same as the IP address sent by the protected
endpoint, and packets will have to be UDP encapsulated in order to be
routed properly.
3.1.4 Other Scenarios
Other scenarios are possible, as are nested combinations of the
above. One noteable example combines aspects of 3.1.1 and 3.1.3. A
subnet may make all external accesses through a remote gateway using
an IPsec tunnel, where the addresses on the subnet are routed to the
gateway by the rest of the Internet. An example would be someones
home network being virtually on the Internet with static IP addresses
even though connectivity is provided by an ISP that assigns a single
dynamically assigned IP address (where the static IP addresses and an
IPsec relay is provided by a third party located elsewhere).
3.2 The Initial Exchange
Communication using IKE always begins with an initial exchange (known
in IKEv1 as Phase 1). This initial exchange normally consists of four
messages, though in some scenarios that number can grow. All
communications using IKE consist of request/response pairs. We'll
describe the base exchange first, followed by variations. The first
pair of messages (IKE_SA_INIT) negotiate cryptographic algorithms,
exchange nonces, and do a Diffie-Hellman exchange.
The second pair of messages (IKE_AUTH) authenticate the previous
messages, exchange identities and certificates, and establish the
first CHILD-SA. Parts of these messages are encrypted and integrity
protected with keys established through the IKE_SA_INIT exchange, so
the identities are hidden from eavesdroppers and all fields in all
the messages are authenticated.
In the following description, the payloads contained in the message
are indicated by names such as SA. The details of the contents of
each payload are described later. Payloads which may optionally
appear will be shown in brackets, such as [CERTREQ], would indicate
that optionally a certificate request payload can be included.
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The initial exchange is as follows:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
HDR contains the SPIs, version numbers, and flags of various sorts.
The SAi1 payload states the cryptographic algorithms the Initiator
supports for the IKE-SA. The KE payload sends the Initiator's
Diffie-Hellman value. Ni is the Initiator's nonce.
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
The Responder chooses a cryptographic suite from the Initiator's
offered choices and expresses that choice in the SAr1 payload,
completes the Diffie-Hellman exchange with the KEr payload, and sends
its nonce in the Nr payload.
At this point in the negotiation each party can generate SKEYSEED,
from which all keys are derived for that IKE-SA. All but the headers
of all the messages that follow are encrypted and integrity
protected. The keys used for the encryption and integrity protection
are derived from SKEYSEED and are known as SK_e (encryption) and SK_a
(authentication, a.k.a. integrity protection). A separate SK_e and
SK_a is computed for each direction. The notation SK { ... }
indicates that these payloads are encrypted and integrity protected
using that direction's SK_e and SK_a.
HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, SAi2, TSi, TSr} -->
The Initiator asserts her identity with the IDi payload, proves
knowledge of the secret corresponding to IDi and integrity protects
the contents of the first two messages using the AUTH payload. She
might also send her certificate(s) in CERT payload(s) and a list of
her trust anchors in CERTREQ payload(s). If any CERT payloads are
included, the first certificate provided must contain the public key
used to verify the AUTH field. The optional payload IDr enables
Alice to specify which of Bob's identities she wants to talk to. This
is useful when Bob is hosting multiple identities at the same IP
address. She begins negotiation of a CHILD-SA using the SAi2
payload. The final fields (starting with SAi2) are described in the
description of the CREATE_CHILD_SA exchange.
<-- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
The Responder asserts his identity with the IDr payload, optionally
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sends one or more certificates (again with the certificate containing
the public key used to verify AUTH listed first), authenticates his
identity with the AUTH payload, and completes negotiation of a
CHILD-SA with the additional fields described below in the
CREATE_CHILD_SA exchange.
The recipients of messages 3 and 4 MUST verify that all signatures
and MACs are computed correctly and that the names in the ID payloads
correspond to the keys used to generate the AUTH payload.
3.3 The CREATE_CHILD_SA Exchange
This exchange consists of a single request/response pair, and was
referred to as a phase 2 exchange in IKEv1.
All messages following the initial exchange are cryptographically
protected using the cryptographic algorithms and keys negotiated in
the first two messages of the IKE exchange using a syntax described
in section 5.14.
Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
section the term Initiator refers to the endpoint initiating this
exchange.
A CHILD-SA is created by sending a CREATE_CHILD_SA request. The
CREATE_CHILD_SA request MAY optionally contain a KE payload for an
additional Diffie-Hellman exchange to enable stronger guarantees of
forward secrecy for the CHILD-SA. The keying material for the CHILD-
SA is a function of SK_d established during the establishment of the
IKE-SA, the nonces exchanged during the CREATE_CHILD_SA exchange, and
the Diffie-Hellman value (if KE payloads are included in the
CREATE_CHILD_SA exchange).
In the CHILD-SA created as part of the initial exchange, a second KE
payload and nonce MUST NOT be sent. The nonces from the initial
exchange are used in computing the keys for the CHILD-SA.
The CREATE_CHILD_SA request contains:
Initiator Responder
----------- -----------
HDR, SK {SA, Ni, [KEi],
[TSi, TSr]} -->
The Initiator sends SA offer(s) in the SA payload, a nonce in the Ni
payload, optionally a Diffie-Hellman value in the KEi payload, and
the proposed traffic selectors in the TSi and TSr payloads. If the SA
offers include different Diffie-Hellman groups, KEi must be an
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element of the group the Initiator expects the responder to accept.
If she guesses wrong, the CREATE_CHILD_SA exchange will fail and she
will have to retry with a different KEi.
The message past the header is encrypted and the message including
the header is integrity protected using the cryptographic algorithms
negotiated in Phase 1.
The CREATE_CHILD_SA response contains:
<-- HDR, SK {SA, Nr, [KEr],
[TSi, TSr]}
The Responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, a Diffie-Hellman value in the KEr
payload if KEi was included in the request and the selected
cryptographic suite includes that group. If the responder chooses a
cryptographic suite with a different group, it must reject the
request and have the initiator make another one.
The traffic selectors for traffic to be sent on that SA are specified
in the TS payloads, which may be a subset of what the Initiator of
the CHILD-SA proposed. Traffic selectors are omitted if this
CREATE_CHILD_SA request is being used to change the key of the IKE-
SA.
3.4 The Informational Exchange
At various points during the operation of an IKE-SA, peers may desire
to convey control messages to each other regarding errors or
notifications of certain events. To accomplish this IKE defines an
Informational exchange. Informational exchanges MUST occur after an
initial exchange and are cryptographically protected with the
negotiated keys.
Control messages that pertain to an IKE-SA MUST be sent under that
IKE-SA. Control messages that pertain to CHILD-SAs MUST be sent under
the protection of the IKE-SA which generated them (or its successor
if the IKE-SA was replaced for the purpose of rekeying).
Messages in an Informational Exchange contain zero or more
Notification or Delete payloads. The Recipient of an Informational
Exchange request MUST send some response (else the Sender will assume
the message was lost in the network and will retransmit it). That
response MAY be a message with no payloads. The request message in an
Informational Exchange MAY also contain no payloads. This is the
expected way an endpoint can ask the other endpoint to verify that it
is alive.
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ESP and AH SAs always exist in pairs, with one SA in each direction.
When an SA is closed, both members of the pair MUST be closed. When
SAs are nested, as when data (and IP headers if in tunnel mode) are
encapsulated first with IPcomp, then with ESP, and finally with AH
between the same pair of endpoints, all of the SAs MUST be deleted
together. Each endpoint MUST close the SAs it sends on and allow the
other endpoint to close the other SA in each pair. To delete an SA,
an Informational Exchange with one or more delete payloads is sent
listing the SPIs (as they would be placed in the headers of outbound
packets) of the SAs to be deleted. The recipient MUST close the
designated SAs. Normally, the reply in the Informational Exchange
will contain delete payloads for the paired SAs going in the other
direction. There is one exception. If by chance both ends of a set
of SAs independently decide to close them, each may send a delete
payload and the two requests may cross in the network. If a node
receives a delete request for SAs for which it has already issued a
delete request, it MUST delete the incoming SAs while processing the
request and the outgoing SAs while processing the response. In that
case, the responses MUST NOT include delete payloads for the deleted
SAs, since that would result in duplicate deletion and could in
theory delete the wrong SA.
A node SHOULD regard half open connections as anomalous and audit
their existence should they persist. Note that this specification
nowhere specifies time periods, so it is up to individual endpoints
to decide how long to wait. A node MAY refuse to accept incoming data
on half open connections but MUST NOT unilaterally close them and
reuse the SPIs. If connection state becomes sufficiently messed up, a
node MAY close the IKE-SA which will implicitly close all SAs
negotiated under it. It can then rebuild the SAs it needs on a clean
base under a new IKE-SA.
The Informational Exchange is defined as:
Initiator Responder
----------- -----------
HDR, SK {N, ..., D, ...} -->
<-- HDR, SK {N, ..., D, ...}
The processing of an Informational Exchange is determined by its
component payloads.
3.5 Informational Messages outside of an IKE-SA
If a packet arrives with an unrecognised SPI, it could be because the
receiving node has recently crashed and lost state or because of some
other system malfunction or attack. If the receiving node has an
active IKE-SA to the IP address from whence the packet came, it MAY
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send a notification of the wayward packet over that IKE-SA. If it
does not, it MAY send an Informational message without cryptographic
protection to the source IP address to alert it to a possible
problem.
4 IKE Protocol Details and Variations
IKE normally listens on UDP port 500, though IKE messages may also be
received on UDP port 4500 with a slightly different format. Since
UDP is a datagram (unreliable) protocol, IKE includes in its
definition recovery from transmission errors, including packet loss,
packet replay, and packet forgery. IKE is designed to function so
long as (1) at least one of a series of retransmitted packets reaches
its destination before timing out; and (2) the channel is not so full
of forged and replayed packets so as to exhaust the network or CPU
capacities of either endpoint. Even in the absence of those minimum
performance requirements, IKE is designed to fail cleanly (as though
the network were broken).
4.1 Use of Retransmission Timers
All messages in IKE exist in pairs: a request and a response. The
setup of an IKE-SA normally consists of two request/response pairs.
Once the IKE-SA is set up, either end of the security association may
initiate requests at any time, and there can be many requests and
responses "in flight" at any given moment. But each message is
labelled as either a request or a response and for each
request/response pair one end of the security association is the
Initiator and the other is the Responder.
For every pair of messages, the Initiator is responsible for
retransmission in the event of a timeout. The Responder MUST never
retransmit a response unless it receives a retransmission of the
request. In that event, the Responder MUST ignore the retransmitted
request except insofar as it triggers a retransmission of the
response. The Initiator MUST remember each request until it receives
the corresponding response. The Responder MUST remember each response
until it receives a request whose sequence number is larger than the
sequence number in the response plus his window size (see section
4.3).
IKE is a reliable protocol, in the sense that the Initiator MUST
retransmit a request until either it receives a corresponding reply
OR it deems the IKE security association to have failed and it
discards all state associated with the IKE-SA and any CHILD-SAs
negotiated using that IKE-SA.
4.2 Use of Sequence Numbers for Message ID
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Every IKE message contains a Message ID as part of its fixed header.
This Message ID is used to match up requests and responses, and to
identify retransmissions of messages.
The Message ID is a 32 bit quantity, which is zero for the first IKE
request in each direction. The IKE-SA initial setup messages will
always be numbered 0 and 1. Each endpoint in the IKE Security
Association maintains two "current" Message IDs: the next one to be
used for a request it initiates and the next one it expects to see
from the other end. These counters increment as requests are
generated and received. Responses always contain the same message ID
as the corresponding request. That means that after the initial
exchange, each integer n may appear as the message ID in four
distinct messages: The nth request from the original IKE Initiator,
the corresponding response, the nth request from the original IKE
Responder, and the corresponding response. If the two ends make very
different numbers of requests, the Message IDs in the two directions
can be very different. There is no ambiguity in the messages,
however, because each packet contains enough information to determine
which of the four messages a particular one is.
Note that Message IDs are cryptographically protected and provide
protection against message replays.
4.3 Window Size for overlapping requests
In order to maximize IKE throughput, an IKE endpoint MAY issue
multiple requests before getting a response to any of them. For
simplicity, an IKE implementation MAY choose to process requests
strictly in order and/or wait for a response to one request before
issuing another. Certain rules must be followed to assure
interoperability between implementations using different strategies.
After an IKE-SA is set up, either end can initiate one or more
requests. These requests may pass one another over the network. An
IKE endpoint MUST be prepared to accept and process a request while
it has a request outstanding in order to avoid a deadlock in this
situation. An IKE endpoint SHOULD be prepared to accept and process
multiple requests while it has a request outstanding.
An IKE endpoint MUST wait for a response to each of its messages
before sending a subsequent message unless it has received a Notify
message from its peer informing it that the peer is prepared to
maintain state for multiple outstanding messages in order to allow
greater throughput.
An IKE endpoint MUST NOT exceed the peer's stated window size for
transmitted IKE requests. In other words, if Bob stated his window
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size is N, then when Alice needs to make a request X, she MUST wait
until she has received responses to all requests up through request
X-N. An IKE endpoint MUST keep a copy of (or be able to regenerate
exactly) each request it has sent until it receives the corresponding
response. An IKE endpoint MUST keep a copy of (or be able to
regenerate exactly) the number of previous responses equal to its
declared window size in case its response was lost and the Initiator
requests its retransmission by retransmitting the request.
An IKE endpoint supporting a window size greater than one SHOULD be
capable of processing incoming requests out of order to maximize
performance in the event of network failures or packet reordering.
4.4 State Synchronization and Connection Timeouts
An IKE endpoint is allowed to forget all of its state associated with
an IKE-SA and the collection of corresponding CHILD-SAs at any time.
This is the anticipated behavior in the event of an endpoint crash
and restart. It is important when an endpoint either fails or
reinitializes its state that the other endpoint detect those
conditions and not continue to waste network bandwidth by sending
packets over those SAs and having them fall into a black hole.
Since IKE is designed to operate in spite of Denial of Service (DoS)
attacks from the network, an endpoint MUST NOT conclude that the
other endpoint has failed based on any routing information (e.g. ICMP
messages) or IKE messages that arrive without cryptographic
protection (e.g., notify messages complaining about unknown SPIs). An
endpoint MUST conclude that the other endpoint has failed only when
repeated attempts to contact it have gone unanswered for a timeout
period or when a cryptographically protected INITIAL-CONTACT
notification is received on a different IKE-SA to the same
authenticated identity. An endpoint SHOULD suspect that the other
endpoint has failed based on routing information and initiate a
request to see whether the other endpoint is alive. To check whether
the other side is alive, IKE specifies an empty Informational message
that (like all IKE requests) requires an acknowledgment. If a
cryptographically protected message has been received from the other
side recently, unprotected notifications MAY be ignored.
Implementations MUST limit the rate at which they take actions based
on unprotected messages.
Numbers of retries and lengths of timeouts are not covered in this
specification because they do not affect interoperability. It is
suggested that messages be retransmitted at least a dozen times over
a period of at least several minutes before giving up on an SA, but
different environments may require different rules. If there has only
been outgoing traffic on all of the SAs associated with an IKE-SA, it
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is essential to confirm liveness of the other endpoint to avoid black
holes. If no cryptographically protected messages have been received
on an IKE-SA or any of its CHILD-SAs recently, a liveness check MUST
be performed. Receipt of a fresh cryptographically protected message
on an IKE-SA or any of its CHILD-SAs assures liveness of the IKE-SA
and all of its CHILD-SAs. Note that this places requirements on the
failure modes of an IKE endpoint. An implementation MUST NOT continue
sending on any SA if some failure prevents it from receiving on all
of the associated SAs. If CHILD-SAs can fail independently from one
another without the associated IKE-SA being able to send a delete
message, then they MUST be negotiated by separate IKE-SAs.
There is a Denial of Service attack on the Initiator of an IKE-SA
that can be avoided if the Initiator takes the proper care. Since the
first two messages of an SA setup are not cryptographically
protected, an attacker could respond to the Initiator's message
before the genuine Responder and poison the connection setup attempt.
To prevent this, the Initiator MAY be willing to accept multiple
responses to its first message, treat each as potentially legitimate,
respond to it, and then discard all the invalid half open connections
when she receives a valid cryptographically protected response to any
one of her requests. Once a cryptographically valid response is
received, all subsequent responses should be ignored whether or not
they are cryptographically valid.
Note that with these rules, there is no reason to negotiate and agree
upon an SA lifetime. If IKE presumes the partner is dead, based on
repeated lack of acknowledgment to an IKE message, then the IKE SA
and all CHILD-SAs set up through that IKE-SA are deleted.
An IKE endpoint may at any time delete inactive CHILD-SAs to recover
resources used to hold their state. If an IKE endpoint chooses to do
so, it MUST send Delete payloads to the other end notifying it of the
deletion. It MAY similarly time out the IKE-SA. Closing the IKE-SA
implicitly closes all associated CHILD-SAs. In this case, an IKE
endpoint SHOULD send a Delete payload indicating that it has closed
the IKE-SA.
4.5 Version Numbers and Forward Compatibility
This document describes version 2.0 of IKE, meaning the major version
number is 2 and the minor version number is zero. It is likely that
some implementations will want to support both version 1.0 and
version 2.0, and in the future, other versions.
The major version number should only be incremented if the packet
formats or required actions have changed so dramatically that an
older version node would not be able to interoperate with a newer
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version node if it simply ignored the fields it did not understand
and took the actions specified in the older specification. The minor
version number indicates new capabilities, and MUST be ignored by a
node with a smaller minor version number, but used for informational
purposes by the node with the larger minor version number. For
example, it might indicate the ability to process a newly defined
notification message. The node with the larger minor version number
would simply note that its correspondent would not be able to
understand that message and therefore would not send it.
If an endpoint receives a message with a higher major version number,
it MUST drop the message and SHOULD send an unauthenticated
notification message containing the highest version number it
supports. If an endpoint supports major version n, and major version
m, it MUST support all versions between n and m. If it receives a
message with a major version that it supports, it MUST respond with
that version number. In order to prevent two nodes from being tricked
into corresponding with a lower major version number than the maximum
that they both support, IKE has a flag that indicates that the node
is capable of speaking a higher major version number.
Thus the major version number in the IKE header indicates the version
number of the message, not the highest version number that the
transmitter supports. If A is capable of speaking versions n, n+1,
and n+2, and B is capable of speaking versions n and n+1, then they
will negotiate speaking n+1, where A will set the flag indicating
ability to speak a higher version. If they mistakenly (perhaps
through an active attacker sending error messages) negotiate to
version n, then both will notice that the other side can support a
higher version number, and they MUST break the connection and
reconnect using version n+1.
Note that IKEv1 does not follow these rules, because there is no way
in v1 of noting that you are capable of speaking a higher version
number. So an active attacker can trick two v2-capable nodes into
speaking v1. When a v2-capable node negotiates down to v1, it SHOULD
note that fact in its logs.
Also for forward compatibility, all fields marked RESERVED MUST be
set to zero by a version 2.0 implementation and their content MUST be
ignored by a version 2.0 implementation ("Be conservative in what you
send and liberal in what you receive"). In this way, future versions
of the protocol can use those fields in a way that is guaranteed to
be ignored by implementations that do not understand them.
Similarly, payload types that are not defined are reserved for future
use and implementations of version 2.0 MUST skip over those payloads
and ignore their contents.
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IKEv2 adds a "critical" flag to each payload header for further
flexibility for forward compatibility. If the critical flag is set
and the payload type is unrecognised, the message MUST be rejected
and the response to the IKE request containing that payload MUST
include a notify payload UNSUPPORTED-CRITICAL-PAYLOAD, indicating an
unsupported critical payload was included. If the critical flag is
not set and the payload type is unsupported, that payload MUST be
ignored.
While new payload types may be added in the future and may appear
interleaved with the fields defined in this specification,
implementations MUST send the payloads defined in this specification
in order shown in section 3 and implementations SHOULD reject as
invalid a message with payloads in any other order.
4.6 Cookies
The term "cookies" originates with Karn and Simpson [RFC 2522] in
Photuris, an early proposal for key management with IPsec. It has
persisted because the IETF has never rejected a proposal involving
cookies. The ISAKMP fixed message header includes two eight octet
fields titled "cookies", and that syntax is used by both IKEv1 and
IKEv2 though in IKEv2 they are referred to as the IKE SPI and there
is a new separate field in a NOTIFY payload holding the cookie. The
initial two eight octet fields in the header are used as a connection
identifier at the beginning of IKE packets. Each endpoint chooses one
of the two SPIs and SHOULD choose them so as to be unique identifiers
of an IKE-SA. An SPI value of zero is special and indicates that the
remote SPI value is not yet known by the sender.
Unlike ESP and AH where only the recipient's SPI appears in the
header of a message, in IKE the sender's SPI is also sent in every
message. Since the SPI chosen by the original initiator of the IKE-SA
is always sent first, an endpoint with multiple IKE-SAs open that
wants to find the appropriate IKE-SA using the SPI it assigned must
look at the I(nitiator) Flag bit in the header to determine whether
it assigned the first or the second eight octets.
In the first message of an initial IKE exchange, the initiator will
not know the responder's SPI value and will therefore set that field
to zero.
An expected attack against IKE is state and CPU exhaustion, where the
target is flooded with session initiation requests from forged IP
addresses. This attack can be made less effective if an
implementation of a responder uses minimal CPU and commits no state
to an SA until it knows the initiator can receive packets at the
address from which he claims to be sending them. To accomplish this,
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a responder SHOULD - when it detects a large number of half-open
IKE-SAs - reject initial IKE messages unless they contain a notify
payload of type "cookie". It SHOULD instead send an unprotected IKE
message as a response and include its cookie in a notify payload.
Initiators who receive such responses MUST retry the IKE_SA_INIT with
the responder supplied cookie as the first payload. The initial
exchange will then be as follows:
Initiator Responder
----------- -----------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE-REQUIRED),
N(COOKIE)
HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->
<-- HDR(A,B), SAr1, KEr, Nr, [CERTREQ]
HDR(A,B), SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, SAi2, TSi, TSr} -->
<-- HDR(A,B), SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
The first two messages do not affect any initiator or responder state
except for communicating the cookie. In particular, the message
sequence numbers in the first four messages will all be zero and the
message sequence numbers in the last two messages will be one.
An IKE implementation SHOULD implement its responder cookie
generation is such a way as to not require any saved state to
recognise its valid cookie when the second IKE_SA_INIT message
arrives. The exact algorithms and syntax they use to generate
cookies does not affect interoperability and hence is not specified
here. The following is an example of how an endpoint could use
cookies to implement limited DOS protection.
A good way to do this is to set the responder cookie to be:
Cookie = <SecretVersionNumber> | Hash(IPi | SPIi | <secret>)
where <secret> is a randomly generated secret known only to the
responder and periodically changed. <SecretVersionNumber> should be
changed whenever <secret> is regenerated. This value can be
recomputed when the IKE_SA_INIT arrives the second time and compared
to the cookie in the received message. If it matches, the responder
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knows that SPIr was generated since the last change to <secret> and
that IPi must be the same as the source address it saw the first
time. Incorporating SPIi into the calculation assures that if
multiple IKE-SAs are being set up in parallel they will all get
different cookies (assuming the initiator chooses unique SPIi's).
If a new value for <secret> is chosen while there are connections in
the process of being initialized, an IKE_SA_INIT might be returned
with other than the current <SecretVersionNumber>. The responder in
that case MAY reject the message by sending another response with a
new cookie or it MAY keep the old value of <secret> around for a
short time and accept cookies computed from either one. The
responder SHOULD NOT accept cookies indefinitely after <secret> is
changed, since that would defeat part of the denial of service
protection.
4.7 Cryptographic Algorithm Negotiation
The payload type known as "SA" indicates a proposal for a set of
choices of protocols (IKE, ESP, and/or AH) for the SA as well as
cryptographic algorithms associated with each protocol.
An SA consists of one or more proposals. Each proposal includes a
Suite-ID, which implies one or more protocols and the associated
cryptographic algorithms.
Since Alice sends her Diffie-Hellman value in the IKE_SA_INIT, she
must guess at the Diffie-Hellman group that Bob will select from her
list of supported cryptographic suites. If she guesses wrong, Bob
will respond with a NOTIFY payload of type INVALID-KE-PAYLOAD
indicating the selected cryptographic suite. In this case, Alice
MUST retry the IKE_SA_INIT with the corrected Diffie-Hellman group.
Alice MUST again propose her full set of acceptable cryptographic
suites because the rejection message was unauthenticated and
otherwise an active attacker could trick Alice and Bob into
negotiating a weaker suite than a stronger one that they both prefer.
4.8 Rekeying
IKE, ESP, and AH security associations use secret keys which SHOULD
only be used for a limited amount of time and to protect a limited
amount of data. This limits the lifetime of the entire security
association. When the lifetime of a security association expires the
security association MUST NOT be used. If there is demand, new
security associations MAY be established. Reestablishment of
security associations to take the place of ones which expire is
referred to as "rekeying".
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To rekey a CHILD-SA, create a new, equivalent SA (see section 4.17
below), and when the new one is established, delete the old one. To
rekey an IKE-SA, establish a new equivalent IKE-SA (see section 4.20
below) with the peer to whom the old IKE-SA is shared using a Phase 2
negotiation within the existing IKE-SA. An IKE-SA so created inherits
all of the original IKE-SA's CHILD-SAs. Use the new IKE-SA for all
control messages needed to maintain the CHILD-SAs created by the old
IKE-SA, and delete the old IKE-SA. The Delete payload to delete
itself MUST be the last request sent over an IKE-SA.
SAs SHOULD be rekeyed proactively, i.e., the new SA should be
established before the old one expires and becomes unusable. Enough
time should elapse between the time the new SA is established and the
old one becomes unusable so that traffic can be switched over to the
new SA.
A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
were negotiated. In IKEv2, each end of the SA is responsible for
enforcing its own lifetime policy on the SA and rekeying the SA when
necessary. If the two ends have different lifetime policies, the end
with the shorter lifetime will end up always being the one to request
the rekeying. If an SA bundle has been inactive for a long time and
if an endpoint would not initiate the SA in the absense of traffic,
the endpoint MAY choose to close the SA instead of rekeying it when
its lifetime expires. It SHOULD do so if there has been no traffic
since the last time the SA was rekeyed.
If the two ends have the same lifetime policies, it is possible that
both will initiate a rekeying at the same time (which will result in
redundant SAs). To reduce the probability of this happening, the
timing of rekeying requests SHOULD be jittered (delayed by a random
amount of time after the need for rekeying is noticed).
This form of rekeying may temporarily result in multiple similar SAs
between the same pairs of nodes. When there are two SAs eligible to
receive packets, a node MUST accept incoming packets through either
SA. An endpoint SHOULD wait a random amount of time before closing a
redundant SA to prevent cycling.
The node that initiated the rekeyed SA SHOULD delete the replaced SA
after the new one is established.
4.9 Traffic Selector Negotiation
When an IP packet is received by an RFC2401 compliant IPsec subsystem
and matches a "protect" selector in its SPD, the subsystem MUST
protect that packet with IPsec. When no SA exists yet it is the task
of IKE to create it. Maintenance of of a system's SPD is outside the
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scope of IKE (see [PFKEY] for an example protocol), though some
implementations might update their SPD in connection with the running
of IKE (for an example scenario, see section 3.1.3).
Traffic Selector (TS) payloads allow endpoints to communicate some of
the information from their SPD to their peers. TS payloads specify
the selection criteria for packets that will be forwarded over the
newly set up SA. This can serve as a consistency check in some
scenarios to assure that the SPDs are consistent. In others, it
guides the dynamic update of the SPD.
Two TS payloads appear in each of the messages in the exchange that
creates a CHILD-SA pair. Each TS payload contains one or more Traffic
Selectors. Each Traffic Selector consists of an address range (IPv4
or IPv6), a port range, and a protocol ID. In support of the scenario
described in section 3.1.3, an initiator may request that the
responder assign an IP address and tell the initiator what it is.
IKEv2 allows the responder to choose a subset of the traffic proposed
by the initiator. This could happen when the configuration of the
two endpoints are being updated but only one end has received the new
information. Since the two endpoints may be configured by different
people, the incompatibility may persist for an extended period even
in the absense of errors. It also allows for intentionally different
configurations, as when one end is configured to tunnel all addresses
and depends on the other end to have the up to date list.
The first of the two TS payloads is known as TSi (Traffic Selector-
initiator). The second is known as TSr (Traffic Selector-responder).
TSi specifies the source address of traffic forwarded from (or the
destination address of traffic forwarded to) the initiator of the
CHILD-SA pair. TSr specifies the destination address of the traffic
forwarded from (or the source address of the traffic forwarded to)
the responder of the CHILD-SA pair. For example, if Alice initiates
the creation of the CHILD-SA pair from Alice to Bob, and wishes to
tunnel all traffic from subnet 10.2.16.* on Alice's side to subnet
18.16.*.* on Bob's side, Alice would include a single traffic
selector in each TS payload. TSi would specify the address range
(10.2.16.0 - 10.2.16.255) and TSr would specify the address range
(18.16.0.0 - 18.16.255.255). Assuming that proposal was acceptable to
Bob, he would send identical TS payloads back.
The Responder is allowed to narrow the choices by selecting a subset
of the traffic, for instance by eliminating or narrowing the range of
one or more members of the set of traffic selectors, provided the set
does not become the NULL set.
It is possible for the Responder's policy to contain multiple smaller
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ranges, all encompassed by the Initiator's traffic selector, and with
the Responder's policy being that each of those ranges should be sent
over a different SA. Continuing the example above, Bob might have a
policy of being willing to tunnel those addresses to and from Alice,
but might require that each address pair be on a separately
negotiated CHILD-SA. If Alice generated her request in response to an
incoming packet from 10.2.16.43 to 18.16.2.123, there would be no way
for Bob to determine which pair of addresses should be included in
this tunnel, and he would have to make his best guess or reject the
request with a status of SINGLE-PAIR-REQUIRED.
To enable Bob to choose the appropriate range in this case, if Alice
has initiated the SA due to a data packet, Alice MAY include as the
first traffic selector in each of TSi and TSr a very specific traffic
selector including the addresses in the packet triggering the
request. In the example, Alice would include in TSi two traffic
selectors: the first containing the address range (10.2.16.43 -
10.2.16.43) and the source port and protocol from the packet and the
second containing (10.2.16.0 - 10.2.16.255) with all ports and
protocols. She would similarly include two traffic selectors in TSr.
If Bob's policy does not allow him to accept the entire set of
traffic selectors in Alice's request, but does allow him to accept
the first selector of TSi and TSr, then Bob MUST narrow the traffic
selectors to a subset that includes Alice's first choices. In this
example, Bob might respond with TSi being (10.2.16.43 - 10.2.16.43)
with all ports and protocols.
If Alice creates the CHILD-SA pair not in response to an arriving
packet, but rather - say - upon startup, then there may be no
specific addresses Alice prefers for the initial tunnel over any
other. In that case, the first values in TSi and TSr MAY be ranges
rather than specific values, and Bob chooses a subset of Alice's TSi
and TSr that are acceptable to him. If more than one subset is
acceptable but their union is not, Bob MUST accept some subset and
MAY include a NOTIFY payload of type ADDITIONAL-TS-POSSIBLE to
indicate that Alice might want to try again. This case will only
occur when Alice and Bob are configured differently from one another.
If Alice and Bob agree on the granularity of tunnels, she will never
request a tunnel wider than Bob will accept.
4.10 Nonces
The IKE_SA_INIT messages each contain a nonce. These nonces are used
as inputs to cryptographic functions. The CREATE_CHILD_SA request
and the CREATE_CHILD_SA response also contain nonces. These nonces
are used to add freshness to the key derivation technique used to
obtain keys for CHILD-SAs. Nonces used in IKEv2 MUST therefore be
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randomly chosen and of size at least equal to the key size of the
strongest cryptographic algorithm used.
4.11 Address and Port Agility
IKE runs over UDP ports 500 and 4500, and implicitly sets up ESP and
AH associations for the same IP addresses it runs over. The IP
addresses and ports in the outer header are, however, not themselves
cryptographically protected, and IKE is designed to work even through
Network Address Translation (NAT) boxes. An implementation MUST
accept incoming connection requests even if not received from UDP
port 500 or 4500, and MUST respond to the address and port from which
the request was received. IKE functions identically over IPv4 or
IPv6.
4.12 Reuse of Diffie-Hellman Exponentials
IKE generates keying material using an ephemeral Diffie-Hellman
exchange in order to gain the property of "perfect forward secrecy".
This means that once a connection is closed and its corresponding
keys are forgotten, even someone who has recorded all of the data
from the connection and gets access to all of the long term keys of
the two endpoints cannot reconstruct the keys used to protect the
conversation.
Achieving perfect forward secrecy requires that when a connection is
closed, each endpoint must forget not only the keys used by the
connection but any information that could be used to recompute those
keys. In particular, it must forget the secrets used in the Diffie-
Hellman calculation and any state that may persist in the state of a
pseudo-random number generater that could be used to recompute the
Diffie-Hellman secrets.
Since the computing of Diffie-Hellman exponentials is computationally
expensive, an endpoint may find it advantageous to reuse those
exponentials for multiple connection setups. There are several
reasonable strategies for doing this. An endpoint could choose a new
exponential only periodically though this could result in less-than-
perfect forward secrecy if some connection lasts for less than the
lifetime of the exponential. Or it could keep track of which
exponential was used for each connection and delete the information
associated with the exponential only when some corresponding
connection was closed. This would allow the exponential to be reused
without losing perfect forward secrecy at the cost of maintaining
more state.
Decisions as to whether and when to reuse Diffie-Hellman exponentials
is a private decision in the sense that it will not affect
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interoperability. An implementation that reuses exponentials may
choose to remember the exponential used by the other endpoint on past
exchanges and if one is reused to avoid the second half of the
calculation.
4.13 Generating Keying Material
In the context of the IKE-SA, three cryptographic algorithms are
negotiated: an encryption algorithm, a Diffie-Hellman group, and a
pseudo-random function (prf). The pseudo-random function is used both
for integrity protection of the IKE payloads and for the construction
of keying material for all of the cryptographic algorithms used in
both the IKE-SA and the CHILD-SAs.
We assume that each cryptographic algorithm accepts a fixed size key,
and that any randomly chosen value of that fixed size can serve as an
appropriate key. For functions that accept a variable length key, a
fixed key size MUST be specified as part of the cryptographic suite
negotiated. For prf functions based on HMAC, the fixed key size is
the size of the output of the HMAC.
Keying material will always be derived as the output of the
negotiated prf algorithm. Since the amount of keying material needed
may be greater than the size of the output of the prf algorithm, we
will use the prf iteratively. We will use the terminology prf+ to
describe the function that outputs a pseudo-random stream based on
the inputs to a prf as follows: (where | indicates concatenation)
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
as needed to compute all required keys. The keys are taken from the
output string without regard to boundaries (e.g. if the required keys
are a 256 bit AES key and a 160 bit HMAC key, and the prf function
generates 160 bits, the AES key will come from T1 and the beginning
of T2, while the HMAC key will come from the rest of T2 and the
beginning of T3).
The constant concatenated to the end of each string feeding the prf
is a single octet. prf+ in this document is not defined beyond 255
times the size of the prf output.
4.14 Generating Keying Material for the IKE-SA
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The shared keys are computed as follows. A quantity called SKEYSEED
is calculated from the nonces exchanged during the IKE_SA_INIT
exchange and the Diffie-Hellman shared secret established during that
exchange. SKEYSEED is used to calculate five other secrets: SK_d
used for deriving new keys for the CHILD-SAs established with this
IKE-SA; SK_ai and SK_ar used as a key to the prf algorithm for
authenticating the component messages of subsequent exchanges; and
SK_ei and SK_er used for encrypting (and of course decrypting) all
subsequent exchanges. SKEYSEED and its derivatives are computed as
follows:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d, SK_ai, SK_ar, SK_ei, SK_er}
= prf+ (SKEYSEED, g^ir | Ni | Nr | CKY-I | CKY-R)
(indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, and SK_er
are taken in order from the generated bits of the prf+). g^ir is the
shared secret from the ephemeral Diffie-Hellman exchange. Ni and Nr
are the nonces, stripped of any headers.
The two directions of flow use different keys. The keys used to
protect messages from the original initiator are SK_ai and SK_ei. The
keys used to protect messages in the other direction are SK_ar and
SK_er. Each algorithm takes a fixed number of bits of keying
material, which is specified as part of the algorithm. For integrity
algorithms based on HMAC, the key size is always equal to the length
of the underlying hash function.
4.15 Authentication of the IKE-SA
The peers are authenticated by having each sign (or MAC using a
shared secret as the key) a block of data. For the responder, the
octets to be signed start with the first octet of the first SPI in
the header of the second message and end with the last octet of the
last payload in the second message. Appended to this (for purposes
of computing the signature) is the initiator's nonce Ni (just the
value, not the payload containing it). Similarly, the initiator
signs the first message, starting with the first octet of the first
SPI in the header and ending with the last octet of the last payload.
Appended to this (for purposes of computing the signature) is the
responder's nonce Nr. It is critical to the security of the exchange
that each side sign the other side's nonce.
Note that all of the payloads are included under the signature,
including any payload types not defined in this document. If the
first message of the exchange is sent twice (the second time with a
responder cookie and/or a different Diffie-Hellman group), it is the
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second version of the message that is signed.
Optionally, messages 3 and 4 MAY include a certificate, or
certificate chain providing evidence that the key used to compute a
digital signature belongs to the name in the ID payload. The
signature or MAC will be computed using algorithms dictated by the
type of key used by the signer, an RSA-signed PKCS1-padded-hash for
an RSA digital signature, a DSS-signed SHA1-hash for a DSA digital
signature, or the negotiated PRF function for a pre-shared key.
There is no requirement that the Initiator and Responder sign with
the same cryptographic algorithms. The choice of cryptographic
algorithms depends on the type of key each has. This type is either
indicated in the certificate supplied or, if the keys were exchanged
out of band, the key types must have been similarly learned. In
particular, the initiator may be using a shared key while the
responder may have a public signature key and certificate. It will
commonly be the case (but it is not required) that if a shared secret
is used for authentication that the same key is used in both
directions. Note that it is a common but insecure practice to have a
shared key derived from a user chosen password. This is insecure
because user chosen passwords are unlikely to have sufficient
randomness to resist dictionary attacks. The pre-shared key SHOULD
contain as much randomness as the strongest key being negotiated. In
the case of a pre-shared key, the AUTH value is computed as:
AUTH = prf(Shared Secret | "Key Pad for IKEv2", <message bytes>)
where the string "Key Pad for IKEv2" is ASCII encoded and not null
terminated. The shared secret can be variable length. The pad string
is added so that if the shared secret is derived from a password,
this exchange will not compromise use of the same password in other
protocols. As noted above, deriving the shared secret from a
password is not secure. This construction is used because it is
anticipated that people will do it anyway.
4.16 Generating Keying Material for CHILD-SAs
CHILD-SAs are created either by being piggybacked on the phase 1
exchange, or in a phase 2 CREATE_CHILD_SA exchange. Keying material
for them is generated as follows:
KEYMAT = prf+(SK_d, Ni | Nr)
Where Ni and Nr are the Nonces from the IKE_SA_INIT exchange if this
request is the first CHILD-SA created or the fresh Ni and Nr from the
CREATE_CHILD_SA exchange if this is a subsequent creation.
For phase 2 exchanges with PFS the keying material is defined as:
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KEYMAT = prf+(SK_d, g^ir (ph2) | Ni | Nr )
where g^ir (ph2) is the shared secret from the ephemeral Diffie-
Hellman exchange of this phase 2 exchange,
A single CHILD-SA negotiation may result in multiple security
associations. ESP and AH SAs exist in pairs (one in each direction),
and four SAs could be created in a single CHILD-SA negotiation if a
combination of ESP and AH is being negotiated. KEYMAT is generated
as described in section 4.13.
Keying material is taken from the expanded KEYMAT in the following
order:
All keys for SAs carrying data from the initiator to the responder
are taken before SAs going in the reverse direction.
If multiple protocols are negotiated, keying material is taken in
the order in which the protocol headers will appear in the
encapsulated packet.
If a single protocol has both encryption and authentication keys,
the encryption key is taken from the first octets of KEYMAT and
the authentication key is taken from the next octets.
Each cryptographic algorithm takes a fixed number of bits of keying
material specified as part of the algorithm.
4.17 Rekeying IKE-SAs using a CREATE_CHILD_SA exchange
The CREATE_CHILD_SA exchange can be used to re-key an existing IKE-SA
(see section 4.8). New Initiator and Responder SPIs are supplied in
the SPI fields. The TS payloads are omitted when rekeying an IKE-SA.
SKEYSEED for the new IKE-SA is computed using SK_d from the existing
IKE-SA as follows:
SKEYSEED = prf(SK_d (old), [g^ir (ph2)] | Ni | Nr)
where g^ir (ph2) is the shared secret from the ephemeral Diffie-
Hellman exchange of this phase 2 exchange and Ni and Nr are the two
nonces stripped of any headers.
The new IKE-SA MUST reset its message counters to 0.
SK_d, SK_ai, SK_ar, and SK_ei, and SK_er are computed from SKEYSEED
as specified in section 4.14.
4.18 Requesting an internal address on a remote network
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Most commonly in the endpoint to gateway scenario, an endpoint may
need an IP address on the gateway's internal network, and may need to
have that address dynamically assigned. A request for such a
temporary address can be included in any request to create a CHILD-SA
(including the implicit request in message 3) by including a CP
payload.
This function provides address allocation to an IRAC trying to tunnel
into a network protected by an IRAS. Since the IKE_SA_AUTH exchange
creates an IKE-SA and a CHILD-SA the IRAC MUST request the internal
address, and optionally other information concerning the internal
network, in the IKE_SA_AUTH exchange. The may IRAS procure an
internal address for the IRAC from any number of sources such as a
DHCP/BOOTP server or its own address pool.
Initiator Responder
----------------------------- ---------------------------
HDR, SAi1, KEi, Ni, Nr,
SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, CP(CFG_REQUEST),
SAi2, TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2,
TSi, TSr}
CP(CFG_REQUEST) MUST contain at least an INTERNAL_ADDRESS attribute
(either IPv4 or IPv6) but MAY contain any number of additional
attributes the initiator wants returned in the response.
For example, message from Initiator to Responder:
CP(CFG_REQUEST)=
INTERNAL_ADDRESS(0.0.0.0)
INTERNAL_NETMASK(0.0.0.0)
INTERNAL_DNS(0.0.0.0)
TSi = (0, 0-65536,0.0.0.0-255.255.255.255)
TSr = (0, 0-65536,0.0.0.0-255.255.255.255)
NOTE: Traffic Selectors are a (protocol, port range, address range)
Message from Responder to Initiator:
CP(CFG_REPLY)=
INTERNAL_ADDRESS(192.168.219.202)
INTERNAL_NETMASK(255.255.255.0)
INTERNAL_SUBNET(192.168.219.0/255.255.255.0)
TSi = (0, 0-65536,192.168.219.202-192.168.219.202)
TSr = (0, 0-65536,192.168.219.0-192.168.219.255)
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All returned values will be implementation dependent. As can be seen
in the above example, the IRAS MAY also send other attributes that
were not included in CP(CFG_REQUEST) and MAY ignore the non-
mandatory attributes that it does not support.
4.19 Requesting the Peer's Version
An IKE peer wishing to inquire about the other peer's version
information MUST use the method below. This is an example of a
configuration request within an Informational Exchange, after the
IKE-SA and first CHILD-SA have been created.
An IKE implementation MAY decline to give out version information
prior to authentication or even after authentication to prevent
trolling in case some implementation is known to have some security
weakness. In that case, it MUST return an empty string.
Initiator Responder
----------------------------- --------------------------
HDR, SK{CP(CFG_REQUEST)} -->
<-- HDR, SK{CP(CFG_REPLY)}
CP(CFG_REQUEST)=
APPLICATION_VERSION("")
CP(CFG_REPLY)
APPLICATION_VERSION("foobar v1.3beta, (c) Foo Bar Inc.")
4.20 Error Handling
There are many kinds of errors that can occur during IKE processing.
If a request is received that is badly formatted or unacceptable for
reasons of policy (e.g. no matching cryptographic algorithms), the
response MUST contain a Notify payload indicating the error. If an
error occurs outside the context of an IKE request (e.g. the node is
getting ESP messages on a non-existent SPI), the node SHOULD initiate
an Informational Exchange with a Notify payload describing the
problem.
Errors that occur before a cryptographically protected IKE-SA is
established must be handled very carefully. There is a trade-off
between wanting to be helpful in diagnosing a problem and responding
to it and wanting to avoid being a dupe in a denial of service attack
based on forged messages.
If a node receives a message on UDP port 500 outside the context of
an IKE-SA known to it (and not a request to start one), it may be the
result of a recent crash of the node. If the message is marked as a
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response, the node MAY audit the suspicious event but MUST NOT
respond. If the message is marked as a request, the node MAY audit
the suspicious event and MAY send a response. If a response is sent,
the response MUST be sent to the IP address and port from whence it
came with the same IKE SPIs and the Message ID copied. The response
MUST NOT be cryptographically protected and MUST contain a notify
payload indicating INVALID-SPI.
A node receiving such an unprotected NOTIFY payload MUST NOT respond
and MUST NOT change the state of any existing SAs. The message might
be a forgery or might be a response the genuine correspondent was
tricked into sending. A node SHOULD treat such a message (and also a
network message like ICMP destination unreachable) as a hint that
there might be problems with SAs to that IP address and SHOULD
initiate a liveness test for any such IKE-SA. An implementation
SHOULD limit the frequency of such tests to avoid being tricked into
participating in a denial of service attack.
A node receiving a suspicious message from an IP address with which
it has an IKE-SA MAY send an IKE notify payload in an IKE
Informational exchange over that SA. The recipient MUST NOT change
the state of any SA's as a result but SHOULD audit the event to aid
in diagnosing malfunctions. A node MUST limit the rate at which it
will send messages in response to unprotected messages.
4.21 IPcomp
Use of IP compression [IPCOMP] can be negotiated as part of the setup
of a CHILD-SA. While IP compression involves an extra header in each
packet and a CPI (compression parameter index), the virtual
"compression association" has no life outside the ESP or AH SA that
contains it. Compression associations disappear when the
corresponding ESP or AH SA goes away, and is not explicitly mentioned
in any DELETE payload.
Negotiation of IP compression is separate from the negotiation of
cryptographic parameters associated with a CHILD-SA. A node
requesting a CHILD-SA MAY advertise its support for one or more
compression algorithms though one or more NOTIFY payloads of type
IPCOMP_SUPPORTED. The response MAY indicate acceptance of a single
compression algorithm with a NOTIFY payload of type IPCOMP_SUPPORTED.
These payloads MAY ONLY occur in the same messages that contain SA
payloads.
While there has been discussion of allowing multiple compression
algorithms to be accepted and to have different compression
algorithms available for the two directions of a CHILD-SA,
implementations of this specification MUST NOT accept an IPcomp
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algorithm that was not proposed, MUST NOT accept more than one, and
MUST NOT compress using an algorithm other than one proposed and
accepted in the setup of the CHILD-SA.
A side effect of separating the negotiation of IPcomp from
cryptographic parameters is that it is not possible to propose
multiple cryptographic suites and propose IP compression with some of
them but not others.
5 Header and Payload Formats
5.1 The IKE Header
IKE messages use UDP ports 500 and/or 4500, with one IKE message per
UDP datagram. Information from the UDP header is largely ignored
except that the IP addresses and UDP ports from the headers are
reversed and used for return packets. When sent of UDP port 500, IKE
messages begin immediately following the UDP header. When sent on UDP
port 4500, IKE messages have prepended for octets of zero. These
four octets of zero are not part of the IKE message and are not
included in any of the length fields or checksums defined by IKE.
Each IKE message begins with the IKE header, denoted HDR in this
memo. Following the header are one or more IKE payloads each
identified by a "Next Payload" field in the preceding payload.
Payloads are processed in the order in which they appear in an IKE
message by invoking the appropriate processing routine according to
the "Next Payload" field in the IKE header and subsequently according
to the "Next Payload" field in the IKE payload itself until a "Next
Payload" field of zero indicates that no payloads follow. If a
payload of type "Encrypted" is found, that payload is decrypted and
its contents parsed as additional payloads. An Encrypted payload MUST
be the last payload in a packet and an encrypted payload MUST NOT
contain another encrypted payload.
The Recipient SPI in the header identifies an instance of an IKE
security association. It is therefore possible for a single instance
of IKE to multiplex distinct sessions with multiple peers.
The format of the IKE header is shown in Figure 1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IKE-SA Initiator's SPI !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! IKE-SA Responder's SPI !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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! Next Payload ! MjVer ! MnVer ! Exchange Type ! Flags !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Message ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IKE Header Format
o Initiator's SPI (8 octets) - A value chosen by the
initiator to identify a unique IKE security association. This
value MUST NOT be zero.
o Responder's SPI (8 octets) - A value chosen by the
responder to identify a unique IKE security association. This
value MUST be zero in the first message of an IKE Initial
Exchange and MUST NOT be zero in any other message.
o Next Payload (1 octet) - Indicates the type of payload that
immediately follows the header. The format and value of each
payload is defined below.
o Major Version (4 bits) - indicates the major version of the IKE
protocol in use. Implementations based on this version of IKE
MUST set the Major Version to 2. Implementations based on
previous versions of IKE and ISAKMP MUST set the Major Version
to 1. Implementations based on this version of IKE MUST reject
(or ignore) messages containing a version number greater than
2.
o Minor Version (4 bits) - indicates the minor version of the
IKE protocol in use. Implementations based on this version of
IKE MUST set the Minor Version to 0. They MUST ignore the minor
version number of received messages.
o Exchange Type (1 octet) - indicates the type of exchange being
used. This dictates the payloads sent in each message and
message orderings in the exchanges.
Exchange Type Value
RESERVED 0
Reserved for ISAKMP 1-31
Reserved for IKEv1 32-33
IKE_SA_INIT 34
IKE_SA_AUTH 35
CREATE_CHILD_SA 36
Informational 37
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Reserved for IKEv2+ 38-239
Reserved for private use 240-255
o Flags (1 octet) - indicates specific options that are set
for the message. Presence of options are indicated by the
appropriate bit in the flags field being set. The bits are
defined LSB first, so bit 0 would be the least significant
bit of the Flags octet. In the description below, a bit
being 'set' means its value is '1', while 'cleared' means
its value is '0'.
-- R(eserved) (bits 0-2) - These bits MUST be cleared
when sending and MUST be ignored on receipt.
-- I(nitiator) (bit 3 of Flags) - This bit MUST be set in
messages sent by the original Initiator of the IKE-SA
and MUST be cleared in messages sent by the original
Responder. It is used by the recipient to determine
whether the message is a request or a response.
-- V(ersion) (bit 4 of Flags) - This bit indicates that
the transmitter is capable of speaking a higher major
version number of the protocol than the one indicated
in the major version number field. Implementations of
IKEv2 must clear this bit when sending and MUST ignore
it in incoming messages.
-- R(eserved) (bits 5-7 of Flags) - These bits MUST be
cleared when sending and MUST be ignored on receipt.
o Message ID (4 octets) - Message identifier used to control
retransmission of lost packets and matching of requests and
responses. See section 4.2. In the first message of a Phase 1
negotiation, the value MUST be set to 0. The response to that
message MUST also have a Message ID of 0.
o Length (4 octets) - Length of total message (header + payloads)
in octets. Session encryption can expand the size of an IKE
message and that is reflected in the total length of the
message.
5.2 Generic Payload Header
Each IKE payload defined in sections 5.3 through 5.14 begins with a
generic header, shown in Figure 2. Figures for each payload below
will include the generic payload header but for brevity the
description of each field will be omitted. The construction and
processing of the generic payload header is identical for each
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payload and will similarly be omitted.
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Generic Payload Header
The Generic Payload Header fields are defined as follows:
o Next Payload (1 octet) - Identifier for the payload type of the
next payload in the message. If the current payload is the last
in the message, then this field will be 0. This field provides
a "chaining" capability whereby additional payloads can be
added to a message by appending it to the end of the message
and setting the "Next Payload" field of the preceding payload
to indicate the new payload's type. For an Encrypted payload,
which must always be the last payload of a message, the Next
Payload field is set to the payload type of the first contained
payload.
o Critical (1 bit) - MUST be set to zero if the sender wants
the recipient to skip this payload if he does not
understand the payload type code in the Next Payload field
of the previous payload. MUST be set to one if the
sender wants the recipient to reject this entire message
if he does not understand the payload type. MUST be ignored
by the recipient if the recipient understands the payload type
code. SHOULD be set to zero for payload types defined in this
document. Note that the critical bit applies to the current
payload rather than the "next" payload whose type code
appears in the first octet. The reasoning behind not setting
the critical bit for payloads defined in this document is
that all implementations MUST understand all payload types
defined in this document and therefore must ignore the
Critical bit's value. Skipped payloads are expected to
have valid Next Payload and Payload Length fields.
o RESERVED (7 bits) - MUST be sent as zero; MUST be ignored.
o Payload Length (2 octets) - Length in octets of the current
payload, including the generic payload header.
5.3 Security Association Payload
The Security Association Payload, denoted SA in this memo, is used to
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negotiate attributes of a security association. An SA may contain
multiple proposals. Each proposal may propose multiple protocols
(where a protocol is IKE, ESP, or AH), along with a suite of
cryptographic algorithms to be used by the protocols. The
protocol(s), cryptographic algorithms, and any associated parameters
are determined by the suite number. An SA payload MAY contain
proposals for different protocols. For example, one suite might
contain AH and ESP, while another might contain only ESP and a third
only AH.
The Proposal structure contains within it a Proposal # and a Suite-
ID. The first proposal MUST have Proposal # = 1, the second MUST
have Proposal # = 2, etc. If the proposals are misnumbered, the
responder MUST reject all of them. Unrecognised Suite-IDs MUST be
ignored.
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ <Proposals> ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Security Association Payload
o Proposals (variable) - one or more proposal substructures.
The payload type for the Security Association Payload is one (1).
5.3.1 Proposal Substructure
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 (last) or 2 ! RESERVED ! Proposal Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Proposal # ! RESERVED-MBZ ! Suite-ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI(S) (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Proposal Substructure
o 0 (last) or 2 (more) (1 octet) - Specifies whether this is the
last Proposal Substructure in the SA. This syntax is inherited
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from ISAKMP, but is unnecessary because the last Proposal
could be identified from the length of the SA. The value (2)
corresponds to a Payload Type of Proposal, and the first
four octets of the Proposal structure are designed to look
somewhat like the header of a Payload.
o RESERVED (1 octet) - MUST be sent as zero; MUST be ignored.
o Proposal Length (2 octets) - Length of this proposal,
including the SPI
o Proposal # (1 octet) - In an SA payload requesting a new
SA is sent, it may contain multiple proposals. The first
proposal in an SA MUST be #1, and subsequent proposals
MUST be one greater than the previous proposal. When an
SA is accepted, the SA payload sent back MUST contain a
single proposal and the proposal number MUST match the
number in the accepted proposal.
o RESERVED-MBZ (1 octet) - This field is reserved for
possible use in specifying different kinds of proposals.
This field MUST be sent as zero and a proposal containing
a non-zero value MUST NOT be accepted. The negotiation
MAY still succeed if there is another acceptable
proposal in the SA payload.
o Suite-ID (2 octets) - This field specifies a suite of
protocols and cryptographic algorithms. See table below.
o SPI(S) (variable) - The sending entity's SPI(s). If the
suite proposed includes more than one protocol, the SPIs
are concatenated together in the order in which they would
appear in a packet sent using the suite (i.e. AH followed
by ESP). When an initial IKE-SA is being
proposed, SPIs are implicit from the IKE header and are not
repeated here. Note that no padding is applied.
For Suite-ID, the following values are defined:
Name Number Algorithms
IKE_CLASSIC 0 DH-Group #5 (1536 bits)
3DES encryption
HMAC-SHA1 integrity and prf
ESP_CLASSIC 1 3DES encryption
HMAC-SHA1 integrity
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<some AES variants, AH (?))
values 2-65000 are reserved to IANA. Values 65001-65533 are
for private use among mutually consenting parties.
5.4 Key Exchange Payload
The Key Exchange Payload, denoted KE in this memo, is used to
exchange Diffie-Hellman public numbers as part of a Diffie-Hellman
key exchange. The Key Exchange Payload consists of the IKE generic
header followed by the Diffie-Hellman public value itself.
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Suite-ID ! RESERVED (MBZ) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Key Exchange Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Key Exchange Payload Format
A key exchange payload is constructed by copying ones Diffie-Hellman
public value into the "Key Exchange Data" portion of the payload.
The length of the Diffie-Hellman public value MUST be equal to the
length of the prime modulus over which the exponentiation was
performed, prepending zero bits to the value if necessary.
A key exchange payload is processed by first checking whether the
length of the key exchange data (the "Payload Length" from the
generic header minus the size of the generic header) is equal to the
length of the prime modulus over which the exponentiation was
performed. The message should be treated as invalid if the payload is
not the expected size.
The Suite-ID is the identifier of the cryptographic suite from which
the Diffie-Hellman group was taken. If the selected proposal uses a
different Diffie-Hellman group, the message MUST be rejected with a
Notify payload of type INVALID-KE-PAYLOAD.
The payload type for the Key Exchange payload is four (4).
5.5 Identification Payload
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The Identification Payload, denoted ID in this memo, allows peers to
assert an identify to one another. The ID Payload names the identity
to be authenticated with the AUTH payload.
NOTE: In IKEv1, two ID payloads were used in each direction in Phase
2 to hold Traffic Selector information for data passing over the SA.
In IKEv2, this information is carried in Traffic Selector (TS)
payloads (see section 5.13).
The Identification Payload consists of the IKE generic header
followed by identification fields as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ID Type ! RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Identification Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Identification Payload Format
o ID Type (1 octet) - Specifies the type of Identification being
used.
o RESERVED - MUST be sent as zero; MUST be ignored.
o Identification Data (variable length) - Value, as indicated by
the Identification Type. The length of the Identification Data
is computed from the size in the ID payload header.
The payload type for the Identification Payload is five (5).
The following table lists the assigned values for the Identification
Type field, followed by a description of the Identification Data
which follows:
ID Type Value
------- -----
RESERVED 0
ID_IPV4_ADDR 1
A single four (4) octet IPv4 address.
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ID_FQDN 2
A fully-qualified domain name string. An example of a
ID_FQDN is, "lounge.org". The string MUST not contain any
terminators (e.g. NULL, CR, etc.).
ID_RFC822_ADDR 3
A fully-qualified RFC822 email address string, An example of
a ID_RFC822_ADDR is, "lizard@lounge.org". The string MUST
not contain any terminators.
ID_IPV6_ADDR 5
A single sixteen (16) octet IPv6 address.
ID_DER_ASN1_DN 9
The binary DER encoding of an ASN.1 X.500 Distinguished Name
[X.501].
ID_DER_ASN1_GN 10
The binary DER encoding of an ASN.1 X.500 GeneralName
[X.509].
ID_KEY_ID 11
An opaque octet stream which may be used to pass vendor-
specific information necessary to do certain proprietary
forms of identification.
5.6 Certificate Payload
The Certificate Payload, denoted CERT in this memo, provides a means
to transport certificates or other certificate-related information
via IKE. Certificate payloads SHOULD be included in an exchange if
certificates are available to the sender.
The Certificate Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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! Cert Encoding ! !
+-+-+-+-+-+-+-+-+ !
~ Certificate Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Certificate Payload Format
o Certificate Encoding (1 octet) - This field indicates the type
of certificate or certificate-related information contained
in the Certificate Data field.
Certificate Encoding Value
-------------------- -----
NONE 0
PKCS #7 wrapped X.509 certificate 1
PGP Certificate 2
DNS Signed Key 3
X.509 Certificate - Signature 4
Kerberos Token 6
Certificate Revocation List (CRL) 7
Authority Revocation List (ARL) 8
SPKI Certificate 9
X.509 Certificate - Attribute 10
RESERVED 11 - 255
o Certificate Data (variable length) - Actual encoding of
certificate data. The type of certificate is indicated
by the Certificate Encoding field.
The payload type for the Certificate Payload is six (6).
5.7 Certificate Request Payload
The Certificate Request Payload, denoted CERTREQ in this memo,
provides a means to request preferred certificates via IKE and can
appear in the first, second, or third message of Phase 1.
Certificate Request payloads SHOULD be included in an exchange
whenever the peer may have multiple certificates, some of which might
be trusted while others are not. If multiple root CA's are trusted,
then multiple Certificate Request payloads SHOULD be transmitted.
Empty (zero length) CA names MUST NOT be generated and SHOULD be
ignored.
The Certificate Request Payload is defined as follows:
1 2 3
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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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Cert Encoding ! !
+-+-+-+-+-+-+-+-+ !
~ Certification Authority ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Certificate Request Payload Format
o Certificate Encoding (1 octet) - Contains an encoding of the type
of certificate requested. Acceptable values are listed in
section 5.6.
o Certification Authority (variable length) - Contains an encoding
of an acceptable certification authority for the type of
certificate requested.
The payload type for the Certificate Request Payload is seven (7).
The Certificate Request Payload is constructed by setting the "Cert
Encoding" field to be the type of certificate being desired and the
"Certification Authority" field to a proper encoding of a
certification authority for the specified certificate. For example,
for an X.509 certificate this field would contain the Distinguished
Name encoding of the Issuer Name of an X.509 certification authority
acceptable to the sender of this payload.
The Certificate Request Payload is processed by inspecting the "Cert
Encoding" field to determine whether the processor has any
certificates of this type. If so the "Certification Authority" field
is inspected to determine if the processor has any certificates which
can be validated up to the specified certification authority. This
can be a chain of certificates. If a certificate exists which
satisfies the criteria specified in the Certificate Request Payload
it MUST be sent back to the certificate requestor; if a certificate
chain exists which goes back to the certification authority specified
in the request the entire chain SHOULD be sent back to the
certificate requestor. If no certificates exist then no further
processing is performed-- this is not an error condition of the
protocol. There may be cases where there is a preferred CA, but an
alternate might be acceptable (perhaps after prompting a human
operator).
5.8 Authentication Payload
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The Authentication Payload, denoted AUTH in this memo, contains data
used for authentication purposes. The only authentication method
defined in this memo is digital signatures and therefore the contents
of this payload when used with this memo will be the output generated
by a digital signature function.
The Authentication Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Auth Method ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Authentication Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Authentication Payload Format
o Auth Method (1 octet) - Specifies the method of authentication
used. Values defined are:
Digital Signature (1) - Computed as specified in section 4.15
using the public key in the first CERT payload or known the the
recipient by some out of band means.
Shared Key Message Integrity Code (2) - Computed as specified in
section 4.15 using the shared key associated with the identity
in the ID payload.
o Authentication Data (variable length) - Data that results from
applying the digital signature function to the IKE state
(see section 3).
The payload type for the Authentication Payload is nine (9).
The Authentication Payload is constructed by computing a digital
signature (or secret key MAC) over part of one of the sender's
messages (see section 4.15). The result is placed in the
"Authentication Data" portion of the payload. The encoding depends
on the type of key being used to authenticate (see section 4.15).
The payload length is the size of the generic header plus the size of
the "Authentication Data" portion of the payload which depends on the
specific authentication method being used.
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The Authentication Payload is processed by extracting the
"Authentication Data" from the payload and verifying it according to
the specific authentication method being used. If the specified
authentication method is not supported or validation fails a NOTIFY
Error message of AUTHENTICATION-FAILED MUST be sent back to the peer
and the connection closed. (An exception to this case is that a peer
MAY treat unsupported, invalid, or missing authentication data as a
request to open an unauthenticated SA.
5.9 Nonce Payload
The Nonce Payload, denoted Ni and Nr in this memo for the Initiator's
and Responder's nonce respectively, contains random data used to
guarantee liveness during an exchange and protect against replay
attacks.
The Nonce Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Nonce Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Nonce Payload Format
o Nonce Data (variable length) - Contains the random data generated
by the transmitting entity.
The payload type for the Nonce Payload is ten (10).
The Nonce Payload is constructed by computing a pseudo-random value
and copying it into the "Nonce Data" field. The size of a Nonce MUST
be between 8 and 256 octets inclusive. Nonce values MUST NOT be
reused.
5.10 Notify Payload
The Notify Payload, denoted N in this document, is used to transmit
informational data, such as error conditions and state transitions to
an IKE peer. A Notify Payload may appear in a response message
(usually specifying why a request was rejected), or in an
Informational Exchange (to report an error not in an IKE request).
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The Notify Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol-ID ! SPI Size ! Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Security Parameter Index (SPI) ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Notification Data ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Notification Payload Format
o Protocol-Id (1 octet) - Specifies the protocol about which
this notification is being sent. For phase 1 notifications,
this field MUST be zero (0). For phase 2 notifications
concerning IPsec SAs this field will contain an IPsec
protocol (either ESP, or AH). For notifications
for which no protocol ID is relevant, this field MUST be
sent as zero and MUST be ignored.
o SPI Size (1 octet) - Length in octets of the SPI as defined by
the Protocol-Id or zero if no SPI is applicable. For phase 1
notification concerning the IKE-SA, the SPI Size MUST be zero.
o Notify Message Type (2 octets) - Specifies the type of
notification message.
o SPI (variable length) - Security Parameter Index.
o Notification Data (variable length) - Informational or error data
transmitted in addition to the Notify Message Type. Values for
this field are message specific, see below.
The payload type for the Notification Payload is eleven (11).
5.10.1 Notify Message Types
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.
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For example, a secure front end or security gateway may use the
Notify message to synchronize SA communication. The table below
lists the Notification messages and their corresponding values. The
number of different error statuses was greatly reduced from IKE V1
both for simplication and to avoid giving configuration information
to probers.
Types in the range 0 - 16383 are intended for reporting errors. An
implementation receiving a Notify payload with one of these types
that it does not recognise in a response MUST assume that the
corresponding request has failed entirely. Unrecognised error types
in a request and status types in a request or response MUST be
ignored except that they SHOULD be logged.
Notify payloads with status types MAY be added to any message and
MUST be ignored if not recognised. They are intended to indicate
capabilities, and as part of SA negotiation are used to negotiate
non-cryptographic parameters.
NOTIFY MESSAGES - ERROR TYPES Value
----------------------------- -----
UNSUPPORTED-CRITICAL-PAYLOAD 1
Sent if the payload has the "critical" bit set and the
payload type is not recognised. Notification Data contains
the one octet payload type.
INVALID-SPI 4
Indicates an IKE message was received with an unrecognized
destination SPI. This usually indicates that the recipient
has rebooted and forgotten the existence of an IKE-SA.
INVALID-MAJOR-VERSION 5
Indicates the recipient cannot handle the version of IKE
specified in the header. The closest version number that the
recipient can support will be in the reply header.
INVALID-SYNTAX 7
Indicates the IKE message was 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 and in an encrypted packet if the
MESSAGE-ID and cryptographic checksum were valid. To avoid
leaking information to someone probing a node, this status
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MUST be sent in response to any error not covered by one of
the other status codes. To aid debugging, more detailed
error information SHOULD be written to a console or log.
INVALID-MESSAGE-ID 9
Sent when an IKE MESSAGE-ID outside the supported window is
received. This Notify MUST NOT be sent in a response; the
invalid request MUST NOT be acknowledged. Instead, inform
the other side by initiating an Informational exchange with
Notification data containing the four octet invalid
MESSAGE-ID. Sending this notification is optional, MUST be
rate limited, and MUST NOT be sent unless an IKE-SA exists
to the sending address and port.
INVALID-SPI 11
MAY be sent in an IKE Informational Exchange when a node
receives an ESP or AH packet with an invalid SPI. The
Notification Data contains the SPI of the invalid packet.
This usually indicates a node has rebooted and forgotten an
SA. If this Informational Message is sent outside the
context of an IKE-SA, it should only be used by the
recipient as a "hint" that something might be wrong (because
it could easily be forged).
NO-PROPOSAL-CHOSEN 14
None of the proposed crypto suites was acceptable.
AUTHENTICATION-FAILED 24
Sent in the response to an IKE_AUTH message when for some
reason the authentication failed. There is no associated
data.
SINGLE-PAIR-REQUIRED 34
This error indicates that a Phase 2 SA request is
unacceptable because the Responder is willing to accept
traffic selectors specifying a single pair of addresses.
The Initiator is expected to respond by requesting an SA for
only the specific traffic he is trying to forward.
NO-ADDITIONAL-SAS 35
This error indicates that a Phase 2 SA request is
unacceptable because the Responder is unwilling to accept
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any more CHILD-SAs on this IKE-SA. Some minimal
implementations may only accept a single CHILD-SA setup in
the context of an initial IKE exchange and reject any
subsequent attempts to add more.
INTERNAL-ADDRESS-FAILURE 36
Indicates an error assigning an internal address (i.e.,
INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS) during the
processing of a Configuration Payload by a Responder. If
this error is generated within an IKE_AUTH exchange no
CHILD-SA will be created.
RESERVED TO IANA - Errors 37 - 8191
Private Use - Errors 8192 - 16383
NOTIFY MESSAGES - STATUS TYPES Value
------------------------------ -----
RESERVED TO IANA - STATUS 16384 - 24577
INITIAL-CONTACT 24578
This notification asserts that this IKE-SA is the only IKE-
SA currently active between the authenticated identities. It
MAY be sent when an IKE-SA is established after a crash, and
the recipient MAY use this information to delete any other
IKE-SAs it has to the same authenticated identity without
waiting for a timeout if those IKE-SAs reside at the IP
address from which this notification arrived. This
notification MUST NOT be sent by an entity that may be
replicated (e.g. a roaming user's credentials where the user
is allowed to connect to the corporate firewall from two
remote systems at the same time).
SET-WINDOW-SIZE 24579
This notification asserts that the sending endpoint is
capable of keeping state for multiple outstanding Phase 2
exchanges, permitting the recipient to send multiple Phase 2
requests before getting a response to the first. The data
associated with a SET-WINDOW-SIZE notification MUST be 4
octets long an contain the big endian represention of the
number of messages the sender promises to keep.
ADDITIONAL-TS-POSSIBLE 24580
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This notification asserts that the sending endpoint narrowed
the proposed traffic selectors but that other traffic
selectors would also have been acceptable, though only in a
separate SA. There is no data associated with this notify
type. It may only be sent as an additional payload in a
message including accepted TSs.
IPCOMP-SUPPORTED 24581
This notification may only be included in a message
containing an SA payload negotiating a CHILD-SA and
indicates a willingness by its sender to use IPcomp on this
SA. The data associated with this notification includes a
two byte IPcomp CPI followed by a one octet transform ID
optionally followed by attributes whose length and format is
defined by that transform ID. A message proposing an SA may
contain multiple IPCOMP-SUPPORTED notifications to indicate
multiple supported algorithms. A message accepting an SA may
contain at most one.
The transform IDs currently defined are:
NAME NUMBER DEFINED IN
----------- ------ -----------
RESERVED 0
IPCOMP_OUI 1
IPCOMP_DEFLATE 2 RFC 2394
IPCOMP_LZS 3 RFC 2395
values 4-240 are reserved to IANA. Values 241-255 are
for private use among mutually consenting parties.
NAT-DETECTION-SOURCE-IP 24582
This notification is used to by its recipient to determine
whether the source is behind a NAT box. The data associated
with this notification is a digest of the SPIs, IP address
and port on which this packet was sent. There MAY be
multiple notify payloads of this type in a message if the
sender does not know which of several network attachments
will be used to send the packet. The recipient of this
notification MAY compare the supplied value to a hash of the
source IP address and port and if they don't match it MAY
invoke NAT specific handling (like using UDP encapsulation
of ESP packets and subsequent IKE packets). The digest is
computed using the negotiated digest algorithm for the IKE-
SA.
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NAT-DETECTION-DESTINATION-IP 24583
This notification is used to by its recipient to determine
whether the it is behind a NAT box. The data associated with
this notification is a digest of the SPIs, IP address and
port to which this packet was sent. The recipient of this
notification MAY compare the supplied value to a hash of the
destination IP address and port and if they don't match it
MAY invoke NAT specific handling (like using UDP
encapsulation of ESP packets and subsequent IKE packets).
The digest is computed using the negotiated digest algorithm
for the IKE-SA.
COOKIE 24584
This notification MAY be included in an IKE_SA_INIT request
or response. In the response, it indicates that the request
should be retried with the COOKIE included in the request.
That data associated with this notification MUST be between
1 and 64 octets in length (inclusive).
USE-TRANSPORT-MODE 24585
This notification MAY be included in a request message that
also includes an SA requesting a CHILD-SA. It requests that
the CHILD-SA use transport mode rather than tunnel mode for
the SA created. If the request is accepted, the response
MUST also include a notification of type USE-TRANSPORT-MODE.
If the responder declines the request, the CHILD-SA can
still be established, but will use tunnel mode. If this is
unacceptable to the initiator, the initiator MUST delete the
SA.
RESERVED TO IANA - STATUS 24586 - 40959
Private Use - STATUS 40960 - 65535
5.11 Delete Payload
The Delete Payload, denoted D in this memo, contains a protocol-
specific security association identifier that the sender has removed
from its security association database and is, therefore, no longer
valid. Figure 14 shows the format of the Delete Payload. It is
possible to send multiple SPIs in a Delete payload, however, each SPI
MUST be for the same protocol. Mixing of Protocol Identifiers MUST
NOT be performed with the Delete payload. It is permitted, however,
to include multiple Delete payloads in a single Informational
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Exchange where each Delete payload lists SPIs for a different
protocol.
Deletion of the IKE-SA is indicated by a Protocol-Id of 0 (IKE) but
no SPIs. Deletion of a CHILD-SA, such as ESP or AH, will contain the
Protocol-Id of that protocol (e.g. ESP, AH) and the SPI is the
receiving entity's SPI(s).
The Delete Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol-Id ! SPI Size ! # of SPIs !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Security Parameter Index(es) (SPI) ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Delete Payload Format
o Protocol-Id (1 octet) - Must be zero for an IKE-SA, 50 for
ESP, or 51 for AH.
o SPI Size (1 octet) - Length in octets of the SPI as defined by
the Protocol-Id. Zero for IKE (SPI is in message header)
or four for AH and ESP.
o # of SPIs (2 octets) - The number of SPIs contained in the Delete
payload. The size of each SPI is defined by the SPI Size field.
o Security Parameter Index(es) (variable length) - Identifies the
specific security association(s) to delete. The length of this
field is determined by the SPI Size and # of SPIs fields.
The payload type for the Delete Payload is twelve (12).
5.12 Vendor ID Payload
The Vendor ID Payload contains a vendor defined constant. The
constant is used by vendors to identify and recognize remote
instances of their implementations. This mechanism allows a vendor
to experiment with new features while maintaining backwards
compatibility.
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A Vendor ID payload MAY announce that the sender is capable to
accepting certain extensions to the protocol, or it MAY simply
identify the implementation as an aid in debugging. If parameter
values "reserved for use by consenting parties" are used, they must
be preceded by a Vendor ID payload that disambiguates them. A Vendor
ID payload MUST NOT change the interpretation of any information
defined in this specification (i.e. it MUST be non-critical).
Multiple Vendor ID payloads MAY be sent. An implementation is NOT
REQUIRED to send any Vendor ID payload at all.
A Vendor ID payload may be sent as part of any message. Reception of
a familiar Vendor ID payload allows an implementation to make use of
Private USE numbers described throughout this memo-- private
payloads, private exchanges, private notifications, etc. Unfamiliar
Vendor IDs MUST be ignored.
Writers of Internet-Drafts who wish to extend this protocol MUST
define a Vendor ID payload to announce the ability to implement the
extension in the Internet-Draft. It is expected that Internet-Drafts
which gain acceptance and are standardized will be given "magic
numbers" out of the Future Use range by IANA and the requirement to
use a Vendor ID will go away.
The Vendor ID Payload fields are defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Vendor ID (VID) ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Vendor ID Payload Format
o Vendor ID (variable length) - It is the responsibility of
the person choosing the Vendor ID to assure its uniqueness
in spite of the absence of any central registry for IDs.
Good practice is to include a company name, a person name
or some such. If you want to show off, you might include
the latitude and longitude and time where you were when
you chose the ID and some random input. A message digest
of a long unique string is preferable to the long unique
string itself.
The payload type for the Vendor ID Payload is thirteen (13).
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5.13 Traffic Selector Payload
The Traffic Selector Payload, denoted TS in this memo, allows peers
to identify packet flows for processing by IPsec security services.
The Traffic Selector Payload consists of the IKE generic header
followed by individual traffic selectors as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Number of TSs ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ <Traffic Selectors> ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: Traffic Selectors Payload Format
o Number of TSs (1 octet) - Number of traffic selectors
being provided.
o RESERVED - This field MUST be sent as zero and MUST be ignored.
o Traffic Selectors (variable length) - one or more individual
traffic selectors.
The length of the Traffic Selector payload includes the TS header and
all the traffic selectors.
The payload type for the Traffic Selector payload is fourteen (14).
5.13.1 Traffic Selector
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! TS Type ! Protocol ID | Selector Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start-Port | End-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Starting Address ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
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~ Ending Address ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: Traffic Selector
o TS Type (one octet) - Specifies the type of traffic selector.
o Protocol ID (1 octet) - Value specifying an associated IP
protocol ID (e.g. UDP/TCP). A value of zero means that the
Protocol ID is not relevant to this traffic selector--
the SA can carry all protocols.
o Selector Length - Specifies the length of this Traffic
Selector Substructure including the header.
o Start-Port (2 octets) - Value specifying the smallest port
number allowed by this Traffic Selector. For protocols for
which port is undefined, or if all ports are allowed by
this Traffic Selector, this field MUST be zero.
o End-Port (2 octets) - Value specifying the largest port
number allowed by this Traffic Selector. For protocols for
which port is undefined, or it all ports are allowed by
this Traffic Selector, this field MUST be 65535.
o Starting Address - The smallest address included in this
Traffic Selector (length determined by TS type).
o Ending Address - The largest address included in this
Traffic Selector (length determined by TS type).
The following table lists the assigned values for the Traffic
Selector Type field and the corresponding Address Selector Data.
TS Type Value
------- -----
RESERVED 0
TS_IPV4_ADDR_RANGE 7
A range of IPv4 addresses, represented by two four (4) octet
values. The first value is the beginning IPv4 address
(inclusive) and the second value is the ending IPv4 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.
TS_IPV6_ADDR_RANGE 8
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A range of IPv6 addresses, represented by two sixteen (16)
octet values. The first value is the beginning IPv6 address
(inclusive) and the second value is the ending IPv6 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.
TS_IPV4_ADDR_REQUEST 9
This TS type requests that the responder assign an IPv4
address for use with this SA. The length of the addresses
field is zero.
TS_IPV6_ADDR_REQUEST 10
This TS type requests that the responder assign an IPv6
address for use with this SA. The length of the addresses
field is zero.
5.14 Encrypted Payload
The Encrypted Payload, denoted SK{...} in this memo, contains other
payloads in encrypted form. The Encrpted Payload, if present in a
message, must be the last payload in the message. Often, it is the
only payload in the message.
The algorithms for encryption and integrity protection are negotiated
during IKE-SA setup, and the keys are computed as specified in
sections 4.14 and 4.17.
The encryption and integrity protection algorithms are modelled after
the ESP algorithms described in RFCs 2104, 2406, 2451. This document
completely specifies the cryptographic processing of IKE data, but
those documents should be consulted for design rationale. We assume a
block cipher with a fixed block size and an integrity check algorithm
that computes a fixed length checksum over a variable size message.
The mandatory to implement algorithms are AES-128-CBC and HMAC-SHA1.
The Payload Type for an Encrypted payload is fifteen (15). The
Encrypted Payload consists of the IKE generic header followed by
individual fields as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Initialization Vector !
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! (length is block size for encryption algorithm) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Encrypted IKE Payloads !
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ! Padding (0-255 octets) !
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
! ! Pad Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: Encrypted Payload Format
o Next Payload - The payload type of the first embedded payload.
Since the Encrypted payload must be last in a message, there
is no need to specify a payload type for a payload beyond it.
o Payload Length - Includes the lengths of the IV, Padding, and
Authentication data.
o Initialization Vector - A randomly chosen value whose length
is equal to the block length of the underlying encryption
algorithm. Recipients MUST accept any value. Senders SHOULD
either pick this value pseudo-randomly and independently for
each message or use the final ciphertext block of the previous
message sent. Senders MUST NOT use the same value for each
message, use a sequence of values with low hamming distance
(e.g. a sequence number), or use ciphertext from a received
message.
o IKE Payloads are as specified earlier in this section. This
field is encrypted with the negotiated cipher.
o Padding may contain any value chosen by the sender, and must
have a length that makes the combination of the Payloads, the
Padding, and the Pad Length to be a multiple of the encryption
block size. This field is encrypted with the negotiated
cipher.
o Pad Length is the length of the Padding field. The sender
SHOULD set the Pad Length to the minimum value that makes
the combination of the Payloads, the Padding, and the Pad
Length a multiple of the block size, but the recipient MUST
accept any length that results in proper alignment. This
field is encrypted with the negotiated cipher.
o Integrity Checksum Data is the cryptographic checksum of
the entire message starting with the Fixed IKE Header
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through the Pad Length. The checksum MUST be computed over
the encrypted message.
5.15 Configuration Payload
The Configuration payload, denoted CP in this document, is used to
exchange configuration information between IKE peers. Currently, the
only defined uses for this exchange is for an IRAC to request an
internal IP address from an IRAS or for either party to request
version information from the other, but this payload is intended as a
likely place for future extensions.
Configuration payloads are of type CFG_REQUEST/CFG_REPLY or
CFG_SET/CFG_ACK (see CFG Type in the payload description below).
CFG_REQUEST and CFG_SET payloads may optionally be added to any IKE
request. The IKE response MUST include either a corresponding
CFG_REPLY or CFG_ACK or a Notify payload with an error code
indicating why the request could not be honored.
"CFG_REQUEST/CFG_REPLY" allows an IKE endpoint to request information
from its peer. If an attribute in the CFG_REQUEST Configuration
Payload is not zero length it is taken as a suggestion for that
attribute. The CFG_REPLY Configuration Payload MAY return that
value, or a new one. It MAY also add new attributes and not include
some requested ones. Requestors MUST ignore returned attributes that
they do not recognise.
Some attributes MAY be multi-valued, in which case multiple attribute
values of the same type are sent and/or returned. Generally, all
values of an attribute are returned when the attribute is requested.
For some attributes (in this version of the specification only
internal addresses), multiple requests indicates a request that
multiple values be assigned. For these attributes, the number of
values returned SHOULD NOT exceed the number requested.
If the data type requested in a CFG_REQUEST is not recognised or not
supported, the responder MUST NOT return an error code but rather
MUST send a CFG_REPLY which MAY be empty. Error returns are reserved
for cases where the request is recognised but cannot be performed as
requested or the request is badly formatted.
"CFG_SET/CFG_ACK" allows an IKE endpoint to push configuration data
to its peer. In this case the CFG_SET Configuration Payload contains
attributes the initiator wants its peer to alter. The responder MUST
return a Configuration Payload and it MUST contain the zero length
attributes that the responder accepted. Those attributes that it did
not accept MUST NOT be in the CFG_ACK Configuration Payload. There
are currently no defined uses for the CFG_SET/CFG_ACK exchange,
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though they may be used in connection with extensions based on Vendor
IDs. An implementation of this specification without extensions MUST
recognise the CFG_SET payload but MUST always respond with an empty
CFG_ACK.
Extensions via the CP payload SHOULD NOT be used for general purpose
management. Its main intent is to provide a bootstrap mechanism to
exchange information within IPSec from IRAS to IRAC. While it MAY be
useful to use such a method to exchange information between some
Security Gateways (SGW) or small networks, existing management
protocols such as DHCP [DHCP], RADIUS [RADIUS], SNMP or LDAP [LDAP]
should be preferred for enterprise management as well as subsequent
information exchanges.
The Configuration Payload is defined as follows:
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 Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! CFG Type ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Configuration Attributes ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 19: Configuration Payload Format
The payload type for the Configuration Payload is 16.
o CFG Type (1 octet) - The type of exchange represented by the
Configuration Attributes.
CFG Type Value
=========== =====
RESERVED 0
CFG_REQUEST 1
CFG_REPLY 2
CFG_SET 3
CFG_ACK 4
values 5-127 are reserved to IANA. Values 128-255 are for private
use among mutually consenting parties.
o RESERVED (3 octets) - MUST be sent as zero; MUST be ignored.
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o Configuration Attribute (variable length) - These are type length
values specific to the Configuration Payload and are defined
below. There may be zero or more Configuration Attributes in this
payload.
5.15.1 Configuration Attributes
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!R| Attribute Type ! Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: Configuration Attribute Format
o Reserved (1 bit) - This bit MUST be set to zero and MUST be
ignored.
o Attribute Type (7 bits) - A unique identifier for each of the
Configuration Attribute Types.
o Length (2 octets) - Length in octets of Value.
o Value (0 or more octets) - The variable length value of this
Configuration Attribute.
The following attribute types have been defined:
MUST Multi-
Attribute Type Value Support Valued Length
======================= ===== ======= ====== ==================
RESERVED 0
INTERNAL_IP4_ADDRESS 1 YES YES* 0 or 4 octets
INTERNAL_IP4_NETMASK 2 NO NO 0 or 4 octets
INTERNAL_IP4_DNS 3 NO YES 0 or 4 octets
INTERNAL_IP4_NBNS 4 NO YES 0 or 4 octets
INTERNAL_ADDRESS_EXPIRY 5 YES NO 0 or 4 octets
INTERNAL_IP4_DHCP 6 NO YES 0 or 4 octets
APPLICATION_VERSION 7 YES NO 0 or more
INTERNAL_IP6_ADDRESS 8 YES YES* 0 or 16 octets
INTERNAL_IP6_NETMASK 9 NO NO 0 or 16 octets
INTERNAL_IP6_DNS 10 NO YES 0 or 16 octets
INTERNAL_IP6_NBNS 11 NO YES 0 or 16 octets
INTERNAL_IP6_DHCP 12 NO YES 0 or 16 octets
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INTERNAL_IP4_SUBNET 13 YES NO 0 or 8 octets
SUPPORTED_ATTRIBUTES 14 YES NO Multiple of 2
INTERNAL_IP6_SUBNET 15 YES NO 17 octets
* These attributes may be multi-valued on return only if
multiple values were requested.
Types 16-16383 are reserved to IANA. Values 16384-32767 are for
private use among mutually consenting parties.
o INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
internal network, sometimes called a red node address or
private
address and MAY be a private address on the Internet. Multiple
internal addresses MAY be requested by requesting multiple
internal address attributes. The responder MAY only send up to
the number of addresses requested.
The requested address is valid until the expiry time defined
with
the INTERNAL_ADDRESS EXPIRY attribute or there are no IKE-SAs
between the peers.
o INTERNAL_IP4_NETMASK, INTERNAL_IP6_NETMASK - The internal
network's netmask. Only one netmask is allowed in the request
and
reply messages (e.g. 255.255.255.0) and it MUST be used only
with
an INTERNAL_ADDRESS attribute.
o INTERNAL_IP4_DNS, INTERNAL_IP6_DNS - Specifies an address of a
DNS server within the network. Multiple DNS servers MAY be
requested. The responder MAY respond with zero or more DNS
server
attributes.
o INTERNAL_IP4_NBNS, INTERNAL_IP6_NBNS - Specifies an address of
a
NetBios Name Server (WINS) within the network. Multiple NBNS
servers MAY be requested. The responder MAY respond with zero
or
more NBNS server attributes.
o INTERNAL_ADDRESS_EXPIRY - Specifies the number of seconds that
the host can use the internal IP address. The host MUST renew
the
IP address before this expiry time. Only one of these
attributes
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MAY be present in the reply.
o INTERNAL_IP4_DHCP, INTERNAL_IP6_DHCP - Instructs the host to
send any internal DHCP requests to the address contained within
the attribute. Multiple DHCP servers MAY be requested. The
responder MAY respond with zero or more DHCP server attributes.
o APPLICATION_VERSION - The version or application information of
the IPSec host. This is a string of printable ASCII characters
that is NOT null terminated.
o INTERNAL_IP4_SUBNET - The protected sub-networks that this
edge-
device protects. This attribute is made up of two fields; the
first being an IP address and the second being a netmask.
Multiple sub-networks MAY be requested. The responder MAY
respond
with zero or more sub-network attributes.
o SUPPORTED_ATTRIBUTES - When used within a Request, this
attribute must be zero length and specifies a query to the
responder to reply back with all of the attributes that it
supports. The response contains an attribute that contains a
set
of attribute identifiers each in 2 octets. The length divided
by
2 (bytes) would state the number of supported attributes
contained
in the response.
o INTERNAL_IP6_SUBNET - The protected sub-networks that this
edge-
device protects. This attribute is made up of two fields; the
first being a 16 octet IPv6 address the second being a one
octet
prefix-mask as defined in [ADDRIPV6]. Multiple sub-networks
MAY
be requested. The responder MAY respond with zero or more sub-
network attributes.
Note that no recommendations are made in this document how an
implementation actually figures out what information to send in a
reply. i.e. we do not recommend any specific method of an IRAS
determining which DNS server should be returned to a requesting
IRAC.
5.16 Other Payload Types
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Payload type values 17-127 are reserved to IANA for future assignment
in IKEv2 (see section 10). Payload type values 128-255 are for
private use among mutually consenting parties.
6 Conformance Requirements
In order to assure that all implementations of IKEv2 can
interoperate, there are MUST support requirements in addition to
those listed elsewhere. Of course, IKEv2 is a security protocol, and
one of its major functions is preventing the bad guys from
interoperating with one's systems. So a particular implementation may
be configured with any of a number of restrictions concerning
algorithms and trusted authorities that will prevent universal
interoperability. For an implementation to be called conforming to
this specification, it MUST be possible to configure it to accept the
following:
RSA keys: 1024 and 2048 bits
Cert types/lengths/algs
Symmetric key (pwd authentication)
setup 1 CHILD-SA in first 4 messages; may reject subsequent. Must
respond to "pings". Must accept "deletes". Must respond to all
messages; may ignore all but delete (what if ignores delete?).
....
7 Security Considerations
Repeated re-keying using Phase 2 without PFS can consume the entropy
of the Diffie-Hellman shared secret. Implementers should take note of
this fact and set a limit on Phase 2 Exchanges between
exponentiations. This memo does not prescribe such a limit.
The strength of a key derived from a Diffie-Hellman exchange using
any of the groups defined here depends on the inherent strength of
the group, the size of the exponent used, and the entropy provided by
the random number generator used. Due to these inputs it is difficult
to determine the strength of a key for any of the defined groups.
Diffie-Hellman group number two when used with a strong random number
generator and an exponent no less than 160 bits is sufficient to use
for 3DES. Groups three through five provide greater security. Group
one is for historic purposes only and does not provide sufficient
strength to the required cipher (although it is sufficient for use
with DES, which is also for historic use only). Implementations
should make note of these conservative estimates when establishing
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policy and negotiating security parameters.
Note that these limitations are on the Diffie-Hellman groups
themselves. There is nothing in IKE which prohibits using stronger
groups nor is there anything which will dilute the strength obtained
from stronger groups. In fact, the extensible framework of IKE
encourages the definition of more groups; use of elliptical curve
groups may greatly increase strength using much smaller numbers.
It is assumed that the Diffie-Hellman exponents in this exchange are
erased from memory after use. In particular, these exponents MUST NOT
be derived from long-lived secrets like the seed to a pseudo-random
generator that is not erased after use.
The security of this protocol is critically dependent on the
randomness of the Diffie-Hellman exponents, which should be generated
by a strong random or properly seeded pseudo-random source (see
RFC1715). While the protocol was designed to be secure even if the
Nonces and other values specified as random are not strongly random,
they should similarly be generated from a strong random source as
part of a conservative design.
8 IANA Considerations
This document contains many "magic numbers" to be maintained by the
IANA. This section explains the criteria to be used by the IANA to
assign additional numbers in each of these lists.
Cryptographic Suite-IDs
Error Codes
Status Codes
IPcomp Transform IDs
Configuration request types
Configuration attribute types
Payload Types
IKE Exchange Types
Values of the Cryptographic Suite-ID define a set of cryptographic
algorithms to be used in an IKE, ESP, or AH SA. Requests for
assignment of new values must be accompanied by a reference to an RFC
that describes how to use these algorithms.
This memo defines three exchange types for use with IKEv2. Requests
for assignment of new exchange types MUST be accompanied by an RFC
which defines the following:
- the purpose of and need for the new exchange.
- the payloads (mandatory and optional) that accompany
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messages in the exchange.
- the phase of the exchange.
- requirements the new exchange has on existing
exchanges which have assigned numbers.
Payloads are defined in this memo to convey information between
peers. New payloads may be required when defining a new
authentication method or exchange. Requests for new payload types
MUST be accompanied by an RFC which defines the physical layout of
the payload and the fields it contains. All payloads MUST use the
same generic header defined in Figure 2.
9 Acknowledgements
This document is a collaborative effort of the entire IPsec WG. If
there were no limit to the number of authors that could appear on an
RFC, the following, in alphabetical order, would have been listed:
Bill Aiello, Stephane Beaulieu, Steve Bellovin, Sara Bitan, Matt
Blaze, Ran Canetti, Darren Dukes, Dan Harkins, Paul Hoffman, J.
Ioannidis, Steve Kent, Angelos Keromytis, Tero Kivinen, Hugo
Krawczyk, Andrew Krywaniuk, Radia Perlman, O. Reingold. Many other
people contributed to the design. It is an evolution of IKEv1,
ISAKMP, and the IPSec DOI, each of which has its own list of authors.
Hugh Daniel suggested the feature of having the initiator, in message
3, specify a name for the responder, and gave the feature the cute
name "You Tarzan, Me Jane". David Faucher and Valery Smyzlov helped
refine the design of the traffic selector negotiation.
10 References
10.1 Normative References
[Bra96] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[Bra97] Bradner, S., "Key Words for use in RFCs to indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2 Non-normative References
[Ble98] Bleichenbacher, D., "Chosen Ciphertext Attacks against
Protocols Based on RSA Encryption Standard PKCS#1", Advances
in Cryptology Eurocrypt '98, Springer-Verlag, 1998.
[BR94] Bellare, M., and Rogaway P., "Optimal Asymmetric
Encryption", Advances in Cryptology Eurocrypt '94,
Springer-Verlag, 1994.
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[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption", American National Standards
Institute, 1983.
[DH] Diffie, W., and Hellman M., "New Directions in
Cryptography", IEEE Transactions on Information Theory, V.
IT-22, n. 6, June 1977.
[DHCP] R. Droms, "Dynamic Host Configuration Protocol",
RFC2131
[DSS] NIST, "Digital Signature Standard", FIPS 186, National
Institute of Standards and Technology, U.S. Department of
Commerce, May, 1994.
[HC98] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[IDEA] Lai, X., "On the Design and Security of Block Ciphers," ETH
Series in Information Processing, v. 1, Konstanz: Hartung-
Gorre Verlag, 1992
[Ker01] Keronytis, A., Sommerfeld, B., "The 'Suggested ID' Extension
for IKE", draft-keronytis-ike-id-00.txt, 2001
[KBC96] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[LDAP] M. Wahl, T. Howes, S. Kille., "Lightweight Directory
Access Protocol (v3)", RFC2251
[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
April 1992.
[MSST98] Maughhan, D., Schertler, M., Schneider, M., and Turner, J.
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[Orm96] Orman, H., "The Oakley Key Determination Protocol", RFC
2412, November 1998.
[PFKEY] McDonald, D., Metz, C., and Phan, B., "PFKEY Key Management
API, Version 2", RFC2367, July 1998.
[PKCS1] Kaliski, B., and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2", September 1998.
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[PK01] Perlman, R., and Kaufman, C., "Analysis of the IPsec key
exchange Standard", WET-ICE Security Conference, MIT, 2001,
http://sec.femto.org/wetice-2001/papers/radia-paper.pdf.
[Pip98] Piper, D., "The Internet IP Security Domain Of
Interpretation for ISAKMP", RFC 2407, November 1998.
[RADIUS] C. Rigney, A. Rubens, W. Simpson, S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC2138
[RSA] Rivest, R., Shamir, A., and Adleman, L., "A Method for
Obtaining Digital Signatures and Public-Key Cryptosystems",
Communications of the ACM, v. 21, n. 2, February 1978.
[SHA] NIST, "Secure Hash Standard", FIPS 180-1, National Institute
of Standards and Technology, U.S. Department of Commerce,
May 1994.
[SKEME] Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
Mechanism for Internet", from IEEE Proceedings of the 1996
Symposium on Network and Distributed Systems Security.
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Appendix A: NAT Traversal
NAT (Network Address Translation) gateways are a controversial
subject. This appendix briefly describes what they are and how they
are likely to act on IKE traffic. Many people believe that NATs are
evil and that we should not design our protocols so as to make them
work better. IKEv2 does specify some unintuitive processing rules in
order that NATs are more likely to work.
NATs exist primarily because of the shortage of IPv4 addresses,
though there are other rationales. IP nodes that are "behind" a NAT
have IP addresses that are not globally unique, but rather are
assigned from some space that is unique within the network behind the
NAT but which are likely to be reused by nodes behind other NATs.
Generally, nodes behind NATs can communicate with other nodes behind
the same NAT and with nodes with globally unique addresses, but not
with nodes behind other NATs. There are exceptions to that rule.
When those nodes make connections to nodes on the real Internet, the
NAT gateway "translates" the IP source address to an address that
will be routed back to the gateway. Messages to the gateway from the
Internet have their destination addresses "translated" to the
internal address that will route the packet to the correct endnode.
NATs are designed to be "transparent" to endnodes. Neither software
on the node behind the NAT nor the node on the Internet require
modification to communicate through the NAT. Achieving this
transparency is more difficult with some protocols than with others.
Protocols that include IP addresses of the endpoints within the
payloads of the packet will fail unless the NAT gateway understands
the protocol and modifies the internal references as well as those in
the headers. Such knowledge is inherently unreliable, is a network
layer violation, and often results in subtle problems.
Opening an IPsec connection through a NAT introduces special
problems. If the connection runs in transport mode, changing the IP
addresses on packets will cause the checksums to fail and the NAT
cannot correct the checksums because they are cryptographically
protected. Even in tunnel mode, there are routing problems because
transparently translating the addresses of AH and ESP packets
requires special logic in the NAT and that logic is heuristic and
unreliable in nature. For that reason, IKEv2 can negotiate UDP
encapsulation of ESP and AH packets. This encoding is slightly less
efficient but is easier for NATs to process. In addition, firewalls
may be configured to pass IPsec traffic over UDP but not ESP/AH or
vice versa.
It is a common practice of NATs to translate TCP and UDP port numbers
as well as addresses and use the port numbers of inbound packets to
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decide which internal node should get a given packet. For this
reason, even though IKE packets MUST be sent from and to UDP port
500, they SHOULD be accepted coming from any port and responses
SHOULD be sent to the port from whence they came. This is because the
ports may be modified as the packets pass through NATs. Similarly, IP
addresses of the IKE endpoints are generally not included in the IKE
payloads because the payloads are cryptographically protected and
could not be transparently modified by NATs.
Port 4500 is reserved for UDP encapsulated ESP, AH, and IKE. When
working through a NAT, it is generally better to pass IKE packets
over port 4500 because some older NATs modify IKE traffic on port 500
in an attempt to transparently establish IPsec connections. Such NATs
may interfere with the straightforward NAT traversal envisioned by
this document, so an IPsec endpoint that discovers a NAT between it
and its correspondent SHOULD send all subsequent traffic to and from
port 4500, which all NATs should know run the NAT-friendly protocol.
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Appendix B: Diffie-Hellman Groups
There are 5 groups different Diffie-Hellman groups defined for use in
IKE. These groups were generated by Richard Schroeppel at the
University of Arizona. Properties of these primes are described in
[Orm96].
The strength supplied by group one may not be sufficient for the
mandatory-to-implement encryption algorithm and is here for historic
reasons.
B.1 Group 1 - 768 Bit MODP
IKE implementations MAY support a MODP group with the following prime
and generator. This group is assigned id 1 (one).
The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 }
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A63A3620 FFFFFFFF FFFFFFFF
The generator is 2.
B.2 Group 2 - 1024 Bit MODP
IKE implementations SHOULD support a MODP group with the following
prime and generator. This group is assigned id 2 (two).
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The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
Its hexadecimal value is:
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
49286651 ECE65381 FFFFFFFF FFFFFFFF
The generator is 2.
B.3 Group 3 - 155 Bit EC2N
IKE implementations MAY support a EC2N group with the following
characteristics. This group is assigned id 3 (three). The curve is
based on the Galois Field GF[2^155]. The field size is 155. The
irreducible polynomial for the field is:
u^155 + u^62 + 1.
The equation for the elliptic curve is:
y^2 + xy = x^3 + ax^2 + b.
Field Size: 155
Group Prime/Irreducible Polynomial:
0x0800000000000000000000004000000000000001
Group Generator One: 0x7b
Group Curve A: 0x0
Group Curve B: 0x07338f
Group Order: 0x0800000000000000000057db5698537193aef944
The data in the KE payload when using this group is the value x from
the solution (x,y), the point on the curve chosen by taking the
randomly chosen secret Ka and computing Ka*P, where * is the
repetition of the group addition and double operations, P is the
curve point with x coordinate equal to generator 1 and the y
coordinate determined from the defining equation. The equation of
curve is implicitly known by the Group Type and the A and B
coefficients. There are two possible values for the y coordinate;
either one can be used successfully (the two parties need not agree
on the selection).
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B.4 Group 4 - 185 Bit EC2N
IKE implementations MAY support a EC2N group with the following
characteristics. This group is assigned id 4 (four). The curve is
based on the Galois Field GF[2^185]. The field size is 185. The
irreducible polynomial for the field is:
u^185 + u^69 + 1.
The equation for the elliptic curve is:
y^2 + xy = x^3 + ax^2 + b.
Field Size: 185
Group Prime/Irreducible Polynomial:
0x020000000000000000000000000000200000000000000001
Group Generator One: 0x18
Group Curve A: 0x0
Group Curve B: 0x1ee9
Group Order: 0x01ffffffffffffffffffffffdbf2f889b73e484175f94ebc
The data in the KE payload when using this group will be identical to
that as when using Oakley Group 3 (three).
B.5 Group 5 - 1536 Bit MODP
IKE implementations MUST support a MODP group with the following
prime and generator. This group is assigned id 5 (five).
The prime is 2^1536 - 2^1472 - 1 + 2^64 * {[2^1406 pi] + 741804}.
Its hexadecimal value is
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8
FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
The generator is 2.
Change History
H.1 Changes from IKEv2-00 to IKEv2-01 February 2002
1) Changed Appendix B to specify the encryption and authentication
processing for IKE rather than referencing ESP. Simplified the format
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by removing idiosyncracies not needed for IKE.
2) Added option for authentication via a shared secret key.
3) Specified different keys in the two directions of IKE messages.
Removed requirement of different cookies in the two directions since
now no longer required.
4) Change the quantities signed by the two ends in AUTH fields to
assure the two parties sign different quantities.
5) Changed reference to AES to AES_128.
6) Removed requirement that Diffie-Hellman be repeated when rekeying
IKE-SA.
7) Fixed typos.
8) Clarified requirements around use of port 500 at the remote end in
support of NAT.
9) Clarified required ordering for payloads.
10) Suggested mechanisms for avoiding DoS attacks.
11) Removed claims in some places that the first phase 2 piggybacked
on phase 1 was optional.
H.2 Changes from IKEv2-01 to IKEv2-02 April 2002
1) Moved the Initiator CERTREQ payload from message 1 to message 3.
2) Added a second optional ID payload in message 3 for the Initiator
to name a desired Responder to support the case where multiple named
identities are served by a single IP address.
3) Deleted the optimization whereby the Diffie-Hellman group did not
need to be specified in phase 2 if it was the same as in phase 1 (it
complicated the design with no meaningful benefit).
4) Added a section on the implications of reusing Diffie-Hellman
expontentials
5) Changed the specification of sequence numbers to being at 0 in
both directions.
6) Many editorial changes and corrections, the most significant being
a global replace of "byte" with "octet".
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H.3 Changes from IKEv2-02 to IKEv2-03 October 2002
1) Reorganized the document moving introductory material to the
front.
2) Simplified the specification of Traffic Selectors to allow only
IPv4 and IPv6 address ranges, as was done in the JFK spec.
3) Fixed the problem brought up by David Faucher with the fix
suggested by Valery Smyslov. If Bob needs to narrow the selector
range, but has more than one matching narrower range, then if Alice's
first selector is a single address pair, Bob chooses the range that
encompasses that.
4) To harmonize with the JFK spec, changed the exchange so that the
initial exchange can be completed in four messages even if the
responder must invoke an anti-clogging defense and the initiator
incorrectly anticipates the responder's choice of Diffie-Hellman
group.
5) Replaced the hierarchical SA payload with a simplified version
that only negotiates suites of cryptographic algorithms.
H.4 Changes from IKEv2-03 to IKEv2-04 January 2003
1) Integrated NAT traversal changes (including Appendix A).
2) Moved the anti-clogging token (cookie) from the SPI to a NOTIFY
payload; changed negotation back to 6 messages when a cookie is
needed.
3) Made capitalization of IKE-SA and CHILD-SA consistent.
4) Changed how IPcomp was negotiated.
5) Added usage scenarios.
6) Added configuration payload for acquiring internal addresses on
remote networks.
7) Added negotiation of tunnel vs transport mode.
Author's Address
Charlie Kaufman charlie_kaufman@notesdev.ibm.com IBM
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