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strongswan/doc/standards/rfc5996.txt

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Internet Engineering Task Force (IETF) C. Kaufman
Request for Comments: 5996 Microsoft
Obsoletes: 4306, 4718 P. Hoffman
Category: Standards Track VPN Consortium
ISSN: 2070-1721 Y. Nir
Check Point
P. Eronen
Independent
September 2010
Internet Key Exchange Protocol Version 2 (IKEv2)
Abstract
This document describes version 2 of the Internet Key Exchange (IKE)
protocol. IKE is a component of IPsec used for performing mutual
authentication and establishing and maintaining Security Associations
(SAs). This document replaces and updates RFC 4306, and includes all
of the clarifications from RFC 4718.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5996.
Kaufman, et al. Standards Track [Page 1]
RFC 5996 IKEv2bis September 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................5
1.1. Usage Scenarios ............................................6
1.1.1. Security Gateway to Security Gateway in
Tunnel Mode .........................................7
1.1.2. Endpoint-to-Endpoint Transport Mode .................7
1.1.3. Endpoint to Security Gateway in Tunnel Mode .........8
1.1.4. Other Scenarios .....................................9
1.2. The Initial Exchanges ......................................9
1.3. The CREATE_CHILD_SA Exchange ..............................13
1.3.1. Creating New Child SAs with the
CREATE_CHILD_SA Exchange ...........................14
1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA
Exchange ...........................................15
1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA
Exchange ...........................................16
1.4. The INFORMATIONAL Exchange ................................17
1.4.1. Deleting an SA with INFORMATIONAL Exchanges ........17
1.5. Informational Messages outside of an IKE SA ...............18
1.6. Requirements Terminology ..................................19
Kaufman, et al. Standards Track [Page 2]
RFC 5996 IKEv2bis September 2010
1.7. Significant Differences between RFC 4306 and This
Document ..................................................20
2. IKE Protocol Details and Variations ............................22
2.1. Use of Retransmission Timers ..............................23
2.2. Use of Sequence Numbers for Message ID ....................24
2.3. Window Size for Overlapping Requests ......................25
2.4. State Synchronization and Connection Timeouts .............26
2.5. Version Numbers and Forward Compatibility .................28
2.6. IKE SA SPIs and Cookies ...................................30
2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD .......33
2.7. Cryptographic Algorithm Negotiation .......................34
2.8. Rekeying ..................................................34
2.8.1. Simultaneous Child SA Rekeying .....................36
2.8.2. Simultaneous IKE SA Rekeying .......................39
2.8.3. Rekeying the IKE SA versus Reauthentication ........40
2.9. Traffic Selector Negotiation ..............................40
2.9.1. Traffic Selectors Violating Own Policy .............43
2.10. Nonces ...................................................44
2.11. Address and Port Agility .................................44
2.12. Reuse of Diffie-Hellman Exponentials .....................44
2.13. Generating Keying Material ...............................45
2.14. Generating Keying Material for the IKE SA ................46
2.15. Authentication of the IKE SA .............................47
2.16. Extensible Authentication Protocol Methods ...............50
2.17. Generating Keying Material for Child SAs .................52
2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange ........53
2.19. Requesting an Internal Address on a Remote Network .......53
2.20. Requesting the Peer's Version ............................55
2.21. Error Handling ...........................................56
2.21.1. Error Handling in IKE_SA_INIT .....................56
2.21.2. Error Handling in IKE_AUTH ........................57
2.21.3. Error Handling after IKE SA is Authenticated ......58
2.21.4. Error Handling Outside IKE SA .....................58
2.22. IPComp ...................................................59
2.23. NAT Traversal ............................................60
2.23.1. Transport Mode NAT Traversal ......................64
2.24. Explicit Congestion Notification (ECN) ...................68
2.25. Exchange Collisions ......................................68
2.25.1. Collisions while Rekeying or Closing Child SAs ....69
2.25.2. Collisions while Rekeying or Closing IKE SAs ......69
3. Header and Payload Formats .....................................69
3.1. The IKE Header ............................................70
3.2. Generic Payload Header ....................................73
3.3. Security Association Payload ..............................75
3.3.1. Proposal Substructure ..............................78
3.3.2. Transform Substructure .............................79
3.3.3. Valid Transform Types by Protocol ..................82
3.3.4. Mandatory Transform IDs ............................83
Kaufman, et al. Standards Track [Page 3]
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3.3.5. Transform Attributes ...............................84
3.3.6. Attribute Negotiation ..............................86
3.4. Key Exchange Payload ......................................87
3.5. Identification Payloads ...................................87
3.6. Certificate Payload .......................................90
3.7. Certificate Request Payload ...............................93
3.8. Authentication Payload ....................................95
3.9. Nonce Payload .............................................96
3.10. Notify Payload ...........................................97
3.10.1. Notify Message Types ..............................98
3.11. Delete Payload ..........................................101
3.12. Vendor ID Payload .......................................102
3.13. Traffic Selector Payload ................................103
3.13.1. Traffic Selector .................................105
3.14. Encrypted Payload .......................................107
3.15. Configuration Payload ...................................109
3.15.1. Configuration Attributes .........................110
3.15.2. Meaning of INTERNAL_IP4_SUBNET and
INTERNAL_IP6_SUBNET ..............................113
3.15.3. Configuration Payloads for IPv6 ..................115
3.15.4. Address Assignment Failures ......................116
3.16. Extensible Authentication Protocol (EAP) Payload ........117
4. Conformance Requirements ......................................118
5. Security Considerations .......................................120
5.1. Traffic Selector Authorization ...........................123
6. IANA Considerations ...........................................124
7. Acknowledgements ..............................................125
8. References ....................................................126
8.1. Normative References .....................................126
8.2. Informative References ...................................127
Appendix A. Summary of Changes from IKEv1 ........................132
Appendix B. Diffie-Hellman Groups ................................133
B.1. Group 1 - 768-bit MODP ....................................133
B.2. Group 2 - 1024-bit MODP ...................................133
Appendix C. Exchanges and Payloads ..............................134
C.1. IKE_SA_INIT Exchange .....................................134
C.2. IKE_AUTH Exchange without EAP .............................135
C.3. IKE_AUTH Exchange with EAP ...............................136
C.4. CREATE_CHILD_SA Exchange for Creating or Rekeying
Child SAs .................................................137
C.5. CREATE_CHILD_SA Exchange for Rekeying the IKE SA ..........137
C.6. INFORMATIONAL Exchange ....................................137
Kaufman, et al. Standards Track [Page 4]
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1. Introduction
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 document describes such a protocol -- the Internet Key
Exchange (IKE). Version 1 of IKE was defined in RFCs 2407 [DOI],
2408 [ISAKMP], and 2409 [IKEV1]. IKEv2 replaced all of those RFCs.
IKEv2 was defined in [IKEV2] (RFC 4306) and was clarified in [Clarif]
(RFC 4718). This document replaces and updates RFC 4306 and RFC
4718. IKEv2 was a change to the IKE protocol that was not backward
compatible. In contrast, the current document not only provides a
clarification of IKEv2, but makes minimum changes to the IKE
protocol. A list of the significant differences between RFC 4306 and
this document is given in Section 1.7.
IKE performs mutual authentication between two parties and
establishes an IKE security association (SA) that includes shared
secret information that can be used to efficiently establish SAs for
Encapsulating Security Payload (ESP) [ESP] or Authentication Header
(AH) [AH] and a set of cryptographic algorithms to be used by the SAs
to protect the traffic that they carry. In this document, the term
"suite" or "cryptographic suite" refers to a complete set of
algorithms used to protect an SA. An initiator proposes one or more
suites by listing supported algorithms that can be combined into
suites in a mix-and-match fashion. IKE can also negotiate use of IP
Compression (IPComp) [IP-COMP] in connection with an ESP or AH SA.
The SAs for ESP or AH that get set up through that IKE SA we call
"Child SAs".
All IKE communications consist of pairs of messages: a request and a
response. The pair is called an "exchange", and is sometimes called
a "request/response pair". The first exchange of messages
establishing an IKE SA are called the IKE_SA_INIT and IKE_AUTH
exchanges; subsequent IKE exchanges are called the CREATE_CHILD_SA or
INFORMATIONAL exchanges. In the common case, there is a single
IKE_SA_INIT exchange and a single IKE_AUTH exchange (a total of four
messages) to establish the IKE SA and the first Child SA. In
exceptional cases, there may be more than one of each of these
exchanges. In all cases, all IKE_SA_INIT exchanges MUST complete
before any other exchange type, then all IKE_AUTH exchanges MUST
Kaufman, et al. Standards Track [Page 5]
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complete, and following that, any number of CREATE_CHILD_SA and
INFORMATIONAL exchanges may occur in any order. In some scenarios,
only a single Child SA is needed between the IPsec endpoints, and
therefore there would be no additional exchanges. Subsequent
exchanges MAY be used to establish additional Child SAs between the
same authenticated pair of endpoints and to perform housekeeping
functions.
An 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 needs to retransmit the request (or abandon
the connection).
The first exchange of an IKE session, IKE_SA_INIT, negotiates
security parameters for the IKE SA, sends nonces, and sends Diffie-
Hellman values.
The second exchange, 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 or ESP Child SA (unless there is
failure setting up the AH or ESP Child SA, in which case the IKE SA
is still established without the Child SA).
The types of subsequent exchanges are CREATE_CHILD_SA (which creates
a Child SA) and INFORMATIONAL (which deletes an SA, reports error
conditions, or does other housekeeping). Every request requires a
response. An INFORMATIONAL request with no payloads (other than the
empty Encrypted payload required by the syntax) is commonly used as a
check for liveness. These subsequent exchanges cannot be used until
the initial exchanges have completed.
In the description that follows, we assume that no errors occur.
Modifications to the flow when errors occur are described in
Section 2.21.
1.1. Usage Scenarios
IKE is used to negotiate ESP or AH SAs in a number of different
scenarios, each with its own special requirements.
Kaufman, et al. Standards Track [Page 6]
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1.1.1. Security Gateway to Security Gateway in Tunnel Mode
+-+-+-+-+-+ +-+-+-+-+-+
| | IPsec | |
Protected |Tunnel | tunnel |Tunnel | Protected
Subnet <-->|Endpoint |<---------->|Endpoint |<--> Subnet
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 1: Security Gateway to Security Gateway 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 to send 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.
1.1.2. Endpoint-to-Endpoint Transport Mode
+-+-+-+-+-+ +-+-+-+-+-+
| | IPsec transport | |
|Protected| or tunnel mode SA |Protected|
|Endpoint |<---------------------------------------->|Endpoint |
| | | |
+-+-+-+-+-+ +-+-+-+-+-+
Figure 2: Endpoint to Endpoint
In this scenario, both endpoints of the IP connection implement
IPsec, as required of hosts in [IPSECARCH]. Transport mode will
commonly be used with no inner IP header. A single pair of addresses
will be negotiated for packets to be protected by this SA. These
endpoints MAY implement application-layer access controls based on
the IPsec authenticated identities of the participants. This
scenario enables the end-to-end security that has been a guiding
principle for the Internet since [ARCHPRINC], [TRANSPARENCY], and a
method of limiting the inherent problems with complexity in networks
noted by [ARCHGUIDEPHIL]. Although this scenario may not be fully
applicable to the IPv4 Internet, it has been deployed successfully in
specific scenarios within intranets using IKEv1. It should be more
broadly enabled during the transition to IPv6 and with the adoption
of IKEv2.
Kaufman, et al. Standards Track [Page 7]
RFC 5996 IKEv2bis September 2010
It is possible in this scenario that one or both of the protected
endpoints will be behind a network address translation (NAT) node, in
which case the tunneled 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 (see Section 2.23).
1.1.3. Endpoint to Security Gateway in Tunnel Mode
+-+-+-+-+-+ +-+-+-+-+-+
| | IPsec | | Protected
|Protected| tunnel |Tunnel | Subnet
|Endpoint |<------------------------>|Endpoint |<--- and/or
| | | | Internet
+-+-+-+-+-+ +-+-+-+-+-+
Figure 3: Endpoint to Security Gateway Tunnel
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 security gateway so that packets returned
to it will go to the security gateway and be tunneled back. This IP
address may be static or may be dynamically allocated by the security
gateway. In support of the latter case, IKEv2 includes a mechanism
(namely, configuration payloads) for the initiator to request an IP
address owned by the security 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
that will get traffic routed to the endpoint directly), while the
inner IP header will contain the source IP address assigned by the
security gateway (i.e., the address that will get traffic routed to
the security gateway for forwarding to the endpoint). The outer
destination address will always be that of the security 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 security
gateway will not be the same as the IP address sent by the protected
Kaufman, et al. Standards Track [Page 8]
RFC 5996 IKEv2bis September 2010
endpoint, and packets will have to be UDP encapsulated in order to be
routed properly. Interaction with NATs is covered in detail in
Section 2.23.
1.1.4. Other Scenarios
Other scenarios are possible, as are nested combinations of the
above. One notable example combines aspects of Sections 1.1.1 and
1.1.3. A subnet may make all external accesses through a remote
security gateway using an IPsec tunnel, where the addresses on the
subnet are routed to the security gateway by the rest of the
Internet. An example would be someone's 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 to the user's security gateway (where the static IP addresses
and an IPsec relay are provided by a third party located elsewhere).
1.2. The Initial Exchanges
Communication using IKE always begins with IKE_SA_INIT and IKE_AUTH
exchanges (known in IKEv1 as Phase 1). These initial exchanges
normally consist 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 [DH].
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. See Section 2.14 for information on
how the encryption keys are generated. (A man-in-the-middle attacker
who cannot complete the IKE_AUTH exchange can nonetheless see the
identity of the initiator.)
All messages following the initial exchange are cryptographically
protected using the cryptographic algorithms and keys negotiated in
the IKE_SA_INIT exchange. These subsequent messages use the syntax
of the Encrypted payload described in Section 3.14, encrypted with
keys that are derived as described in Section 2.14. All subsequent
messages include an Encrypted payload, even if they are referred to
in the text as "empty". For the CREATE_CHILD_SA, IKE_AUTH, or
INFORMATIONAL exchanges, the message following the header is
encrypted and the message including the header is integrity protected
using the cryptographic algorithms negotiated for the IKE SA.
Kaufman, et al. Standards Track [Page 9]
RFC 5996 IKEv2bis September 2010
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.
In the following descriptions, the payloads contained in the message
are indicated by names as listed below.
Notation Payload
-----------------------------------------
AUTH Authentication
CERT Certificate
CERTREQ Certificate Request
CP Configuration
D Delete
EAP Extensible Authentication
HDR IKE header (not a payload)
IDi Identification - Initiator
IDr Identification - Responder
KE Key Exchange
Ni, Nr Nonce
N Notify
SA Security Association
SK Encrypted and Authenticated
TSi Traffic Selector - Initiator
TSr Traffic Selector - Responder
V Vendor ID
The details of the contents of each payload are described in section
3. Payloads that may optionally appear will be shown in brackets,
such as [CERTREQ]; this indicates that a Certificate Request payload
can optionally be included.
The initial exchanges are as follows:
Initiator Responder
-------------------------------------------------------------------
HDR, SAi1, KEi, Ni -->
HDR contains the Security Parameter Indexes (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]
Kaufman, et al. Standards Track [Page 10]
RFC 5996 IKEv2bis September 2010
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. The messages that
follow are encrypted and integrity protected in their entirety, with
the exception of the message headers. 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); see Sections 2.13 and 2.14 for details on the key
derivation. A separate SK_e and SK_a is computed for each direction.
In addition to the keys SK_e and SK_a derived from the Diffie-Hellman
value for protection of the IKE SA, another quantity SK_d is derived
and used for derivation of further keying material for Child SAs.
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 its identity with the IDi payload, proves
knowledge of the secret corresponding to IDi and integrity protects
the contents of the first message using the AUTH payload (see
Section 2.15). It might also send its certificate(s) in CERT
payload(s) and a list of its 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 the initiator to specify to which of
the responder's identities it wants to talk. This is useful when the
machine on which the responder is running is hosting multiple
identities at the same IP address. If the IDr proposed by the
initiator is not acceptable to the responder, the responder might use
some other IDr to finish the exchange. If the initiator then does
not accept the fact that responder used an IDr different than the one
that was requested, the initiator can close the SA after noticing the
fact.
The Traffic Selectors (TSi and TSr) are discussed in Section 2.9.
The initiator 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.
Kaufman, et al. Standards Track [Page 11]
RFC 5996 IKEv2bis September 2010
<-- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
The responder asserts its identity with the IDr payload, optionally
sends one or more certificates (again with the certificate containing
the public key used to verify AUTH listed first), authenticates its
identity and protects the integrity of the second message with the
AUTH payload, and completes negotiation of a Child SA with the
additional fields described below in the CREATE_CHILD_SA exchange.
Both parties in the IKE_AUTH exchange MUST verify that all signatures
and Message Authentication Codes (MACs) are computed correctly. If
either side uses a shared secret for authentication, the names in the
ID payload MUST correspond to the key used to generate the AUTH
payload.
Because the initiator sends its Diffie-Hellman value in the
IKE_SA_INIT, it must guess the Diffie-Hellman group that the
responder will select from its list of supported groups. If the
initiator guesses wrong, the responder will respond with a Notify
payload of type INVALID_KE_PAYLOAD indicating the selected group. In
this case, the initiator MUST retry the IKE_SA_INIT with the
corrected Diffie-Hellman group. The initiator MUST again propose its
full set of acceptable cryptographic suites because the rejection
message was unauthenticated and otherwise an active attacker could
trick the endpoints into negotiating a weaker suite than a stronger
one that they both prefer.
If creating the Child SA during the IKE_AUTH exchange fails for some
reason, the IKE SA is still created as usual. The list of Notify
message types in the IKE_AUTH exchange that do not prevent an IKE SA
from being set up include at least the following: NO_PROPOSAL_CHOSEN,
TS_UNACCEPTABLE, SINGLE_PAIR_REQUIRED, INTERNAL_ADDRESS_FAILURE, and
FAILED_CP_REQUIRED.
If the failure is related to creating the IKE SA (for example, an
AUTHENTICATION_FAILED Notify error message is returned), the IKE SA
is not created. Note that although the IKE_AUTH messages are
encrypted and integrity protected, if the peer receiving this Notify
error message has not yet authenticated the other end (or if the peer
fails to authenticate the other end for some reason), the information
needs to be treated with caution. More precisely, assuming that the
MAC verifies correctly, the sender of the error Notify message is
known to be the responder of the IKE_SA_INIT exchange, but the
sender's identity cannot be assured.
Kaufman, et al. Standards Track [Page 12]
RFC 5996 IKEv2bis September 2010
Note that IKE_AUTH messages do not contain KEi/KEr or Ni/Nr payloads.
Thus, the SA payloads in the IKE_AUTH exchange cannot contain
Transform Type 4 (Diffie-Hellman group) with any value other than
NONE. Implementations SHOULD omit the whole transform substructure
instead of sending value NONE.
1.3. The CREATE_CHILD_SA Exchange
The CREATE_CHILD_SA exchange is used to create new Child SAs and to
rekey both IKE SAs and Child SAs. This exchange consists of a single
request/response pair, and some of its function was referred to as a
Phase 2 exchange in IKEv1. It MAY be initiated by either end of the
IKE SA after the initial exchanges are completed.
An SA is rekeyed by creating a new SA and then deleting the old one.
This section describes the first part of rekeying, the creation of
new SAs; Section 2.8 covers the mechanics of rekeying, including
moving traffic from old to new SAs and the deletion of the old SAs.
The two sections must be read together to understand the entire
process of rekeying.
Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
section the term initiator refers to the endpoint initiating this
exchange. An implementation MAY refuse all CREATE_CHILD_SA requests
within an IKE SA.
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).
If a CREATE_CHILD_SA exchange includes a KEi payload, at least one of
the SA offers MUST include the Diffie-Hellman group of the KEi. The
Diffie-Hellman group of the KEi MUST be an element of the group the
initiator expects the responder to accept (additional Diffie-Hellman
groups can be proposed). If the responder selects a proposal using a
different Diffie-Hellman group (other than NONE), the responder MUST
reject the request and indicate its preferred Diffie-Hellman group in
the INVALID_KE_PAYLOAD Notify payload. There are two octets of data
associated with this notification: the accepted Diffie-Hellman group
number in big endian order. In the case of such a rejection, the
CREATE_CHILD_SA exchange fails, and the initiator will probably retry
the exchange with a Diffie-Hellman proposal and KEi in the group that
the responder gave in the INVALID_KE_PAYLOAD Notify payload.
Kaufman, et al. Standards Track [Page 13]
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The responder sends a NO_ADDITIONAL_SAS notification to indicate that
a CREATE_CHILD_SA request is unacceptable because the responder is
unwilling to accept any more Child SAs on this IKE SA. This
notification can also be used to reject IKE SA rekey. 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.
1.3.1. Creating New Child SAs with the CREATE_CHILD_SA Exchange
A Child SA may be created by sending a CREATE_CHILD_SA request. The
CREATE_CHILD_SA request for creating a new Child SA is:
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 for the proposed Child SA in the TSi
and TSr payloads.
The CREATE_CHILD_SA response for creating a new Child SA is:
<-- 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, and a Diffie-Hellman value in the
KEr payload if KEi was included in the request and the selected
cryptographic suite includes that group.
The Traffic Selectors for traffic to be sent on that SA are specified
in the TS payloads in the response, which may be a subset of what the
initiator of the Child SA proposed.
The USE_TRANSPORT_MODE notification MAY be included in a request
message that also includes an SA payload 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 will be established in
tunnel mode. If this is unacceptable to the initiator, the initiator
MUST delete the SA. Note: Except when using this option to negotiate
transport mode, all Child SAs will use tunnel mode.
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The ESP_TFC_PADDING_NOT_SUPPORTED notification asserts that the
sending endpoint will not accept packets that contain Traffic Flow
Confidentiality (TFC) padding over the Child SA being negotiated. If
neither endpoint accepts TFC padding, this notification is included
in both the request and the response. If this notification is
included in only one of the messages, TFC padding can still be sent
in the other direction.
The NON_FIRST_FRAGMENTS_ALSO notification is used for fragmentation
control. See [IPSECARCH] for a fuller explanation. Both parties
need to agree to sending non-first fragments before either party does
so. It is enabled only if NON_FIRST_FRAGMENTS_ALSO notification is
included in both the request proposing an SA and the response
accepting it. If the responder does not want to send or receive non-
first fragments, it only omits NON_FIRST_FRAGMENTS_ALSO notification
from its response, but does not reject the whole Child SA creation.
An IPCOMP_SUPPORTED notification, covered in Section 2.22, can also
be included in the exchange.
A failed attempt to create a Child SA SHOULD NOT tear down the IKE
SA: there is no reason to lose the work done to set up the IKE SA.
See Section 2.21 for a list of error messages that might occur if
creating a Child SA fails.
1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA Exchange
The CREATE_CHILD_SA request for rekeying an IKE SA is:
Initiator Responder
-------------------------------------------------------------------
HDR, SK {SA, Ni, KEi} -->
The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
payload, and a Diffie-Hellman value in the KEi payload. The KEi
payload MUST be included. A new initiator SPI is supplied in the SPI
field of the SA payload. Once a peer receives a request to rekey an
IKE SA or sends a request to rekey an IKE SA, it SHOULD NOT start any
new CREATE_CHILD_SA exchanges on the IKE SA that is being rekeyed.
The CREATE_CHILD_SA response for rekeying an IKE SA is:
<-- HDR, SK {SA, Nr, KEr}
The responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, and a Diffie-Hellman value in the
KEr payload if the selected cryptographic suite includes that group.
A new responder SPI is supplied in the SPI field of the SA payload.
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The new IKE SA has its message counters set to 0, regardless of what
they were in the earlier IKE SA. The first IKE requests from both
sides on the new IKE SA will have Message ID 0. The old IKE SA
retains its numbering, so any further requests (for example, to
delete the IKE SA) will have consecutive numbering. The new IKE SA
also has its window size reset to 1, and the initiator in this rekey
exchange is the new "original initiator" of the new IKE SA.
Section 2.18 also covers IKE SA rekeying in detail.
1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA Exchange
The CREATE_CHILD_SA request for rekeying a Child SA is:
Initiator Responder
-------------------------------------------------------------------
HDR, SK {N(REKEY_SA), 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 for the proposed Child SA in the TSi
and TSr payloads.
The notifications described in Section 1.3.1 may also be sent in a
rekeying exchange. Usually, these will be the same notifications
that were used in the original exchange; for example, when rekeying a
transport mode SA, the USE_TRANSPORT_MODE notification will be used.
The REKEY_SA notification MUST be included in a CREATE_CHILD_SA
exchange if the purpose of the exchange is to replace an existing ESP
or AH SA. The SA being rekeyed is identified by the SPI field in the
Notify payload; this is the SPI the exchange initiator would expect
in inbound ESP or AH packets. There is no data associated with this
Notify message type. The Protocol ID field of the REKEY_SA
notification is set to match the protocol of the SA we are rekeying,
for example, 3 for ESP and 2 for AH.
The CREATE_CHILD_SA response for rekeying a Child SA is:
<-- 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, and a Diffie-Hellman value in the
KEr payload if KEi was included in the request and the selected
cryptographic suite includes that group.
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The Traffic Selectors for traffic to be sent on that SA are specified
in the TS payloads in the response, which may be a subset of what the
initiator of the Child SA proposed.
1.4. 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 ONLY occur
after the initial exchanges and are cryptographically protected with
the negotiated keys. Note that some informational messages, not
exchanges, can be sent outside the context of an IKE SA. Section
2.21 also covers error messages in great detail.
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 that generated them (or its
successor if the IKE SA was rekeyed).
Messages in an INFORMATIONAL exchange contain zero or more
Notification, Delete, and Configuration payloads. The recipient of
an INFORMATIONAL exchange request MUST send some response; otherwise,
the sender will assume the message was lost in the network and will
retransmit it. That response MAY be an empty message. 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.
The INFORMATIONAL exchange is defined as:
Initiator Responder
-------------------------------------------------------------------
HDR, SK {[N,] [D,]
[CP,] ...} -->
<-- HDR, SK {[N,] [D,]
[CP], ...}
The processing of an INFORMATIONAL exchange is determined by its
component payloads.
1.4.1. Deleting an SA with INFORMATIONAL Exchanges
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 (that
is, deleted). Each endpoint MUST close its incoming SAs 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
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sent listing the SPIs (as they would be expected in the headers of
inbound packets) of the SAs to be deleted. The recipient MUST close
the designated SAs. Note that one never sends Delete payloads for
the two sides of an SA in a single message. If there are many SAs to
delete at the same time, one includes Delete payloads for the inbound
half of each SA pair in the INFORMATIONAL exchange.
Normally, the response 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 outgoing SAs while processing the request
and the incoming 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.
Similar to ESP and AH SAs, IKE SAs are also deleted by sending an
Informational exchange. Deleting an IKE SA implicitly closes any
remaining Child SAs negotiated under it. The response to a request
that deletes the IKE SA is an empty INFORMATIONAL response.
Half-closed ESP or AH connections are anomalous, and a node with
auditing capability should probably audit their existence if they
persist. Note that this specification does not specify 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-closed 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, as described above. It can then rebuild the SAs it needs on
a clean base under a new IKE SA.
1.5. Informational Messages outside of an IKE SA
There are some cases in which a node receives a packet that it cannot
process, but it may want to notify the sender about this situation.
o If an ESP or AH packet arrives with an unrecognized SPI. This
might be due to the receiving node having recently crashed and
lost state, or because of some other system malfunction or attack.
o If an encrypted IKE request packet arrives on port 500 or 4500
with an unrecognized IKE SPI. This might be due to the receiving
node having recently crashed and lost state, or because of some
other system malfunction or attack.
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o If an IKE request packet arrives with a higher major version
number than the implementation supports.
In the first case, if the receiving node has an active IKE SA to the
IP address from whence the packet came, it MAY send an INVALID_SPI
notification of the wayward packet over that IKE SA in an
INFORMATIONAL exchange. The Notification Data contains the SPI of
the invalid packet. The recipient of this notification cannot tell
whether the SPI is for AH or ESP, but this is not important because
the SPIs are supposed to be different for the two. If no suitable
IKE SA exists, the node MAY send an informational message without
cryptographic protection to the source IP address, using the source
UDP port as the destination port if the packet was UDP (UDP-
encapsulated ESP or AH). In this case, it should only be used by the
recipient as a hint that something might be wrong (because it could
easily be forged). This message is not part of an INFORMATIONAL
exchange, and the receiving node MUST NOT respond to it because doing
so could cause a message loop. The message is constructed as
follows: there are no IKE SPI values that would be meaningful to the
recipient of such a notification; using zero values or random values
are both acceptable, this being the exception to the rule in
Section 3.1 that prohibits zero IKE Initiator SPIs. The Initiator
flag is set to 1, the Response flag is set to 0, and the version
flags are set in the normal fashion; these flags are described in
Section 3.1.
In the second and third cases, the message is always sent without
cryptographic protection (outside of an IKE SA), and includes either
an INVALID_IKE_SPI or an INVALID_MAJOR_VERSION notification (with no
notification data). The message is a response message, and thus it
is sent to the IP address and port from whence it came with the same
IKE SPIs and the Message ID and Exchange Type are copied from the
request. The Response flag is set to 1, and the version flags are
set in the normal fashion.
1.6. Requirements Terminology
Definitions of the primitive terms in this document (such as Security
Association or SA) can be found in [IPSECARCH]. It should be noted
that parts of IKEv2 rely on some of the processing rules in
[IPSECARCH], as described in various sections of this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].
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1.7. Significant Differences between RFC 4306 and This Document
This document contains clarifications and amplifications to IKEv2
[IKEV2]. Many of the clarifications are based on [Clarif]. The
changes listed in that document were discussed in the IPsec Working
Group and, after the Working Group was disbanded, on the IPsec
mailing list. That document contains detailed explanations of areas
that were unclear in IKEv2, and is thus useful to implementers of
IKEv2.
The protocol described in this document retains the same major
version number (2) and minor version number (0) as was used in RFC
4306. That is, the version number is *not* changed from RFC 4306.
The small number of technical changes listed here are not expected to
affect RFC 4306 implementations that have already been deployed at
the time of publication of this document.
This document makes the figures and references a bit more consistent
than they were in [IKEV2].
IKEv2 developers have noted that the SHOULD-level requirements in RFC
4306 are often unclear in that they don't say when it is OK to not
obey the requirements. They also have noted that there are MUST-
level requirements that are not related to interoperability. This
document has more explanation of some of these requirements. All
non-capitalized uses of the words SHOULD and MUST now mean their
normal English sense, not the interoperability sense of [MUSTSHOULD].
IKEv2 (and IKEv1) developers have noted that there is a great deal of
material in the tables of codes in Section 3.10.1 in RFC 4306. This
leads to implementers not having all the needed information in the
main body of the document. Much of the material from those tables
has been moved into the associated parts of the main body of the
document.
This document removes discussion of nesting AH and ESP. This was a
mistake in RFC 4306 caused by the lag between finishing RFC 4306 and
RFC 4301. Basically, IKEv2 is based on RFC 4301, which does not
include "SA bundles" that were part of RFC 2401. While a single
packet can go through IPsec processing multiple times, each of these
passes uses a separate SA, and the passes are coordinated by the
forwarding tables. In IKEv2, each of these SAs has to be created
using a separate CREATE_CHILD_SA exchange.
This document removes discussion of the INTERNAL_ADDRESS_EXPIRY
configuration attribute because its implementation was very
problematic. Implementations that conform to this document MUST
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ignore proposals that have configuration attribute type 5, the old
value for INTERNAL_ADDRESS_EXPIRY. This document also removed
INTERNAL_IP6_NBNS as a configuration attribute.
This document removes the allowance for rejecting messages in which
the payloads were not in the "right" order; now implementations MUST
NOT reject them. This is due to the lack of clarity where the orders
for the payloads are described.
The lists of items from RFC 4306 that ended up in the IANA registry
were trimmed to only include items that were actually defined in RFC
4306. Also, many of those lists are now preceded with the very
important instruction to developers that they really should look at
the IANA registry at the time of development because new items have
been added since RFC 4306.
This document adds clarification on when notifications are and are
not sent encrypted, depending on the state of the negotiation at the
time.
This document discusses more about how to negotiate combined-mode
ciphers.
In Section 1.3.2, "The KEi payload SHOULD be included" was changed to
be "The KEi payload MUST be included". This also led to changes in
Section 2.18.
In Section 2.1, there is new material covering how the initiator's
SPI and/or IP is used to differentiate if this is a "half-open" IKE
SA or a new request.
This document clarifies the use of the critical flag in Section 2.5.
In Section 2.8, "Note that, when rekeying, the new Child SA MAY have
different Traffic Selectors and algorithms than the old one" was
changed to "Note that, when rekeying, the new Child SA SHOULD NOT
have different Traffic Selectors and algorithms than the old one".
The new Section 2.8.2 covers simultaneous IKE SA rekeying.
The new Section 2.9.2 covers Traffic Selectors in rekeying.
This document adds the restriction in Section 2.13 that all
pseudorandom functions (PRFs) used with IKEv2 MUST take variable-
sized keys. This should not affect any implementations because there
were no standardized PRFs that have fixed-size keys.
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Section 2.18 requires doing a Diffie-Hellman exchange when rekeying
the IKE_SA. In theory, RFC 4306 allowed a policy where the Diffie-
Hellman exchange was optional, but this was not useful (or
appropriate) when rekeying the IKE_SA.
Section 2.21 has been greatly expanded to cover the different cases
where error responses are needed and the appropriate responses to
them.
Section 2.23 clarified that, in NAT traversal, now both UDP-
encapsulated IPsec packets and non-UDP-encapsulated IPsec packets
need to be understood when receiving.
Added Section 2.23.1 to describe NAT traversal when transport mode is
requested.
Added Section 2.25 to explain how to act when there are timing
collisions when deleting and/or rekeying SAs, and two new error
notifications (TEMPORARY_FAILURE and CHILD_SA_NOT_FOUND) were
defined.
In Section 3.6, "Implementations MUST support the HTTP method for
hash-and-URL lookup. The behavior of other URL methods is not
currently specified, and such methods SHOULD NOT be used in the
absence of a document specifying them" was added.
In Section 3.15.3, a pointer to a new document that is related to
configuration of IPv6 addresses was added.
Appendix C was expanded and clarified.
2. IKE Protocol Details and Variations
IKE normally listens and sends on UDP port 500, though IKE messages
may also be received on UDP port 4500 with a slightly different
format (see Section 2.23). 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).
Although IKEv2 messages are intended to be short, they contain
structures with no hard upper bound on size (in particular, digital
certificates), and IKEv2 itself does not have a mechanism for
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fragmenting large messages. IP defines a mechanism for fragmentation
of oversized UDP messages, but implementations vary in the maximum
message size supported. Furthermore, use of IP fragmentation opens
an implementation to denial-of-service (DoS) attacks [DOSUDPPROT].
Finally, some NAT and/or firewall implementations may block IP
fragments.
All IKEv2 implementations MUST be able to send, receive, and process
IKE messages that are up to 1280 octets long, and they SHOULD be able
to send, receive, and process messages that are up to 3000 octets
long. IKEv2 implementations need to be aware of the maximum UDP
message size supported and MAY shorten messages by leaving out some
certificates or cryptographic suite proposals if that will keep
messages below the maximum. Use of the "Hash and URL" formats rather
than including certificates in exchanges where possible can avoid
most problems. Implementations and configuration need to keep in
mind, however, that if the URL lookups are possible only after the
Child SA is established, recursion issues could prevent this
technique from working.
The UDP payload of all packets containing IKE messages sent on port
4500 MUST begin with the prefix of four zeros; otherwise, the
receiver won't know how to handle them.
2.1. 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 exchanges. 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 labeled as
either a request or a response, and for each exchange, one end of the
Security Association is the initiator and the other is the responder.
For every pair of IKE 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 causes 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 or
equal to the sequence number in the response plus its window size
(see Section 2.3). In order to allow saving memory, responders are
allowed to forget the response after a timeout of several minutes.
If the responder receives a retransmitted request for which it has
already forgotten the response, it MUST ignore the request (and not,
for example, attempt constructing a new response).
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IKE is a reliable protocol: the initiator MUST retransmit a request
until it either receives a corresponding response or deems the IKE SA
to have failed. In the latter case, the initiator discards all state
associated with the IKE SA and any Child SAs that were negotiated
using that IKE SA. A retransmission from the initiator MUST be
bitwise identical to the original request. That is, everything
starting from the IKE header (the IKE SA initiator's SPI onwards)
must be bitwise identical; items before it (such as the IP and UDP
headers) do not have to be identical.
Retransmissions of the IKE_SA_INIT request require some special
handling. When a responder receives an IKE_SA_INIT request, it has
to determine whether the packet is a retransmission belonging to an
existing "half-open" IKE SA (in which case the responder retransmits
the same response), or a new request (in which case the responder
creates a new IKE SA and sends a fresh response), or it belongs to an
existing IKE SA where the IKE_AUTH request has been already received
(in which case the responder ignores it).
It is not sufficient to use the initiator's SPI and/or IP address to
differentiate between these three cases because two different peers
behind a single NAT could choose the same initiator SPI. Instead, a
robust responder will do the IKE SA lookup using the whole packet,
its hash, or the Ni payload.
The retransmission policy for one-way messages is somewhat different
from that for regular messages. Because no acknowledgement is ever
sent, there is no reason to gratuitously retransmit one-way messages.
Given that all these messages are errors, it makes sense to send them
only once per "offending" packet, and only retransmit if further
offending packets are received. Still, it also makes sense to limit
retransmissions of such error messages.
2.2. Use of Sequence Numbers for Message ID
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. Retransmission of a message
MUST use the same Message ID as the original message.
The Message ID is a 32-bit quantity, which is zero for the
IKE_SA_INIT messages (including retries of the message due to
responses such as COOKIE and INVALID_KE_PAYLOAD), and incremented for
each subsequent exchange. Thus, the first pair of IKE_AUTH messages
will have an ID of 1, the second (when EAP is used) will be 2, and so
on. The Message ID is reset to zero in the new IKE SA after the IKE
SA is rekeyed.
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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 in a request 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 a very different number
of requests, the Message IDs in the two directions can be very
different. There is no ambiguity in the messages, however, because
the Initiator and Response flags in the message header specify which
of the four messages a particular one is.
Throughout this document, "initiator" refers to the party who
initiated the exchange being described. The "original initiator"
always refers to the party who initiated the exchange that resulted
in the current IKE SA. In other words, if the "original responder"
starts rekeying the IKE SA, that party becomes the "original
initiator" of the new IKE SA.
Note that Message IDs are cryptographically protected and provide
protection against message replays. In the unlikely event that
Message IDs grow too large to fit in 32 bits, the IKE SA MUST be
closed or rekeyed.
2.3. Window Size for Overlapping Requests
The SET_WINDOW_SIZE notification asserts that the sending endpoint is
capable of keeping state for multiple outstanding exchanges,
permitting the recipient to send multiple requests before getting a
response to the first. The data associated with a SET_WINDOW_SIZE
notification MUST be 4 octets long and contain the big endian
representation of the number of messages the sender promises to keep.
The window size is always one until the initial exchanges complete.
An IKE endpoint MUST wait for a response to each of its messages
before sending a subsequent message unless it has received a
SET_WINDOW_SIZE 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.
After an IKE SA is set up, in order to maximize IKE throughput, an
IKE endpoint MAY issue multiple requests before getting a response to
any of them, up to the limit set by its peer's SET_WINDOW_SIZE.
These requests may pass one another over the network. An IKE
endpoint MUST be prepared to accept and process a request while it
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has a request outstanding in order to avoid a deadlock in this
situation. An IKE endpoint may also accept and process multiple
requests while it has a request outstanding.
An IKE endpoint MUST NOT exceed the peer's stated window size for
transmitted IKE requests. In other words, if the responder stated
its window size is N, then when the initiator needs to make a request
X, it MUST wait until it 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 ought to be
capable of processing incoming requests out of order to maximize
performance in the event of network failures or packet reordering.
The window size is normally a (possibly configurable) property of a
particular implementation, and is not related to congestion control
(unlike the window size in TCP, for example). In particular, what
the responder should do when it receives a SET_WINDOW_SIZE
notification containing a smaller value than is currently in effect
is not defined. Thus, there is currently no way to reduce the window
size of an existing IKE SA; you can only increase it. When rekeying
an IKE SA, the new IKE SA starts with window size 1 until it is
explicitly increased by sending a new SET_WINDOW_SIZE notification.
The INVALID_MESSAGE_ID notification is sent when an IKE Message ID
outside the supported window is received. This Notify message 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, and
notifications of this type MUST be rate limited.
2.4. 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 discarded SAs and having them fall into a black hole.
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The INITIAL_CONTACT 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.
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). The INITIAL_CONTACT notification, if sent, MUST
be in the first IKE_AUTH request or response, not as a separate
exchange afterwards; receiving parties MAY ignore it in other
messages.
Since IKE is designed to operate in spite of 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 acknowledgement (note that within the context of an IKE
SA, an "empty" message consists of an IKE header followed by an
Encrypted payload that contains no payloads). If a cryptographically
protected (fresh, i.e., not retransmitted) message has been received
from the other side recently, unprotected Notify messages MAY be
ignored. Implementations MUST limit the rate at which they take
actions based on unprotected messages.
The number of retries and length 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. To be a good
network citizen, retransmission times MUST increase exponentially to
avoid flooding the network and making an existing congestion
situation worse. If there has only been outgoing traffic on all of
the SAs associated with an IKE SA, it 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, the system needs to perform a
liveness check in order to prevent sending messages to a dead peer.
(This is sometimes called "dead peer detection" or "DPD", although it
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is really detecting live peers, not dead ones.) Receipt of a fresh
cryptographically protected message on an IKE SA or any of its Child
SAs ensures 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 needs to stop sending over any SA if
some failure prevents it from receiving on all of the associated SAs.
If a system creates Child SAs that can fail independently from one
another without the associated IKE SA being able to send a delete
message, then the system MUST negotiate such Child SAs using separate
IKE SAs.
There is a DoS 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 it
receives a valid cryptographically protected response to any one of
its 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 acknowledgement 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
delete Child SAs, 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 unless the other endpoint is no longer
responding.
2.5. 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 0. This document is a
replacement for [IKEV2]. It is likely that some implementations will
want to support version 1.0 and version 2.0, and in the future, other
versions.
Kaufman, et al. Standards Track [Page 28]
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The major version number should be incremented only 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
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
Notify message type. 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 Notify
message of type INVALID_MAJOR_VERSION containing the highest
(closest) 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 the initiator is capable of speaking
versions n, n+1, and n+2, and the responder is capable of speaking
versions n and n+1, then they will negotiate speaking n+1, where the
initiator will set a flag indicating its 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 an implementation running version 2.0, and their
content MUST be ignored by an implementation running version 2.0 ("Be
conservative in what you send and liberal in what you receive" [IP]).
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
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understand them. Similarly, payload types that are not defined are
reserved for future use; implementations of a version where they are
undefined MUST skip over those payloads and ignore their contents.
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 unrecognized, 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. In that Notify payload,
the notification data contains the one-octet payload type. If the
critical flag is not set and the payload type is unsupported, that
payload MUST be ignored. Payloads sent in IKE response messages MUST
NOT have the critical flag set. Note that the critical flag applies
only to the payload type, not the contents. If the payload type is
recognized, but the payload contains something that is not (such as
an unknown transform inside an SA payload, or an unknown Notify
Message Type inside a Notify payload), the critical flag is ignored.
Although new payload types may be added in the future and may appear
interleaved with the fields defined in this specification,
implementations SHOULD send the payloads defined in this
specification in the order shown in the figures in Sections 1 and 2;
implementations MUST NOT reject as invalid a message with those
payloads in any other order.
2.6. IKE SA SPIs and Cookies
The initial two eight-octet fields in the header, called the "IKE
SPIs", are used as a connection identifier at the beginning of IKE
packets. Each endpoint chooses one of the two SPIs and MUST choose
them so as to be unique identifiers of an IKE SA. An SPI value of
zero is special: it indicates that the remote SPI value is not yet
known by the sender.
Incoming IKE packets are mapped to an IKE SA only using the packet's
SPI, not using (for example) the source IP address of the packet.
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 Initiator flag in the header to determine whether it
assigned the first or the second eight octets.
Kaufman, et al. Standards Track [Page 30]
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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. When the IKE_SA_INIT exchange does not result in the
creation of an IKE SA due to INVALID_KE_PAYLOAD, NO_PROPOSAL_CHOSEN,
or COOKIE (see Section 2.6), the responder's SPI will be zero also in
the response message. However, if the responder sends a non-zero
responder SPI, the initiator should not reject the response for only
that reason.
Two expected attacks against IKE are state and CPU exhaustion, where
the target is flooded with session initiation requests from forged IP
addresses. These attacks can be made less effective if 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 it claims to
be sending them.
When a responder detects a large number of half-open IKE SAs, it
SHOULD reply to IKE_SA_INIT requests with a response containing the
COOKIE notification. The data associated with this notification MUST
be between 1 and 64 octets in length (inclusive), and its generation
is described later in this section. If the IKE_SA_INIT response
includes the COOKIE notification, the initiator MUST then retry the
IKE_SA_INIT request, and include the COOKIE notification containing
the received data as the first payload, and all other payloads
unchanged. The initial exchange will then be as follows:
Initiator Responder
-------------------------------------------------------------------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), 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. 'A'
is the SPI assigned by the initiator, while 'B' is the SPI assigned
by the responder.
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An IKE implementation can implement its responder cookie generation
in such a way as to not require any saved state to recognize its
valid cookie when the second IKE_SA_INIT message arrives. The exact
algorithms and syntax used to generate cookies do not affect
interoperability and hence are 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 = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)
where <secret> is a randomly generated secret known only to the
responder and periodically changed and | indicates concatenation.
<VersionIDofSecret> should be changed whenever <secret> is
regenerated. The cookie 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 knows that the cookie 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 ensures that if multiple IKE SAs are being set
up in parallel they will all get different cookies (assuming the
initiator chooses unique SPIi's). Incorporating Ni in the hash
ensures that an attacker who sees only message 2 can't successfully
forge a message 3. Also, incorporating SPIi in the hash prevents an
attacker from fetching one cookie from the other end, and then
initiating many IKE_SA_INIT exchanges all with different initiator
SPIs (and perhaps port numbers) so that the responder thinks that
there are a lot of machines behind one NAT box that are all trying to
connect.
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 <VersionIDofSecret>. 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 DoS protection. The
responder should change the value of <secret> frequently, especially
if under attack.
When one party receives an IKE_SA_INIT request containing a cookie
whose contents do not match the value expected, that party MUST
ignore the cookie and process the message as if no cookie had been
included; usually this means sending a response containing a new
cookie. The initiator should limit the number of cookie exchanges it
tries before giving up, possibly using exponential back-off. An
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attacker can forge multiple cookie responses to the initiator's
IKE_SA_INIT message, and each of those forged cookie replies will
cause two packets to be sent: one packet from the initiator to the
responder (which will reject those cookies), and one response from
responder to initiator that includes the correct cookie.
A note on terminology: the term "cookies" originates with Karn and
Simpson [PHOTURIS] in Photuris, an early proposal for key management
with IPsec, and it has persisted. The Internet Security Association
and Key Management Protocol (ISAKMP) [ISAKMP] fixed message header
includes two eight-octet fields called "cookies", and that syntax is
used by both IKEv1 and IKEv2, although in IKEv2 they are referred to
as the "IKE SPI" and there is a new separate field in a Notify
payload holding the cookie.
2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD
There are two common reasons why the initiator may have to retry the
IKE_SA_INIT exchange: the responder requests a cookie or wants a
different Diffie-Hellman group than was included in the KEi payload.
If the initiator receives a cookie from the responder, the initiator
needs to decide whether or not to include the cookie in only the next
retry of the IKE_SA_INIT request, or in all subsequent retries as
well.
If the initiator includes the cookie only in the next retry, one
additional round trip may be needed in some cases. An additional
round trip is needed also if the initiator includes the cookie in all
retries, but the responder does not support this. For instance, if
the responder includes the KEi payloads in cookie calculation, it
will reject the request by sending a new cookie.
If both peers support including the cookie in all retries, a slightly
shorter exchange can happen.
Initiator Responder
-----------------------------------------------------------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE)
HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->
<-- HDR(A,0), N(INVALID_KE_PAYLOAD)
HDR(A,0), N(COOKIE), SAi1, KEi', Ni -->
<-- HDR(A,B), SAr1, KEr, Nr
Implementations SHOULD support this shorter exchange, but MUST NOT
fail if other implementations do not support this shorter exchange.
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2.7. Cryptographic Algorithm Negotiation
The payload type known as "SA" indicates a proposal for a set of
choices of IPsec protocols (IKE, ESP, or AH) for the SA as well as
cryptographic algorithms associated with each protocol.
An SA payload consists of one or more proposals. Each proposal
includes one protocol. Each protocol contains one or more transforms
-- each specifying a cryptographic algorithm. Each transform
contains zero or more attributes (attributes are needed only if the
Transform ID does not completely specify the cryptographic
algorithm).
This hierarchical structure was designed to efficiently encode
proposals for cryptographic suites when the number of supported
suites is large because multiple values are acceptable for multiple
transforms. The responder MUST choose a single suite, which may be
any subset of the SA proposal following the rules below.
Each proposal contains one protocol. If a proposal is accepted, the
SA response MUST contain the same protocol. The responder MUST
accept a single proposal or reject them all and return an error. The
error is given in a notification of type NO_PROPOSAL_CHOSEN.
Each IPsec protocol proposal contains one or more transforms. Each
transform contains a Transform Type. The accepted cryptographic
suite MUST contain exactly one transform of each type included in the
proposal. For example: if an ESP proposal includes transforms
ENCR_3DES, ENCR_AES w/keysize 128, ENCR_AES w/keysize 256,
AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the accepted suite MUST contain one
of the ENCR_ transforms and one of the AUTH_ transforms. Thus, six
combinations are acceptable.
If an initiator proposes both normal ciphers with integrity
protection as well as combined-mode ciphers, then two proposals are
needed. One of the proposals includes the normal ciphers with the
integrity algorithms for them, and the other proposal includes all
the combined-mode ciphers without the integrity algorithms (because
combined-mode ciphers are not allowed to have any integrity algorithm
other than "none").
2.8. Rekeying
IKE, ESP, and AH Security Associations use secret keys that should be
used only 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
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Security Associations MAY be established. Reestablishment of
Security Associations to take the place of ones that expire is
referred to as "rekeying".
To allow for minimal IPsec implementations, the ability to rekey SAs
without restarting the entire IKE SA is optional. An implementation
MAY refuse all CREATE_CHILD_SA requests within an IKE SA. If an SA
has expired or is about to expire and rekeying attempts using the
mechanisms described here fail, an implementation MUST close the IKE
SA and any associated Child SAs and then MAY start new ones.
Implementations may wish to support in-place rekeying of SAs, since
doing so offers better performance and is likely to reduce the number
of packets lost during the transition.
To rekey a Child SA within an existing IKE SA, create a new,
equivalent SA (see Section 2.17 below), and when the new one is
established, delete the old one. Note that, when rekeying, the new
Child SA SHOULD NOT have different Traffic Selectors and algorithms
than the old one.
To rekey an IKE SA, establish a new equivalent IKE SA (see
Section 2.18 below) with the peer to whom the old IKE SA is shared
using a CREATE_CHILD_SA within the existing IKE SA. An IKE SA so
created inherits all of the original IKE SA's Child SAs, and the new
IKE SA is used for all control messages needed to maintain those
Child SAs. After the new equivalent IKE SA is created, the initiator
deletes the old IKE SA, and the Delete payload to delete itself MUST
be the last request sent over the old 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 has been inactive for a long time and if an
endpoint would not initiate the SA in the absence of traffic, the
endpoint MAY choose to close the SA instead of rekeying it when its
lifetime expires. It can also do so if there has been no traffic
since the last time the SA was rekeyed.
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Note that IKEv2 deliberately allows parallel SAs with the same
Traffic Selectors between common endpoints. One of the purposes of
this is to support traffic quality of service (QoS) differences among
the SAs (see [DIFFSERVFIELD], [DIFFSERVARCH], and Section 4.1 of
[DIFFTUNNEL]). Hence unlike IKEv1, the combination of the endpoints
and the Traffic Selectors may not uniquely identify an SA between
those endpoints, so the IKEv1 rekeying heuristic of deleting SAs on
the basis of duplicate Traffic Selectors SHOULD NOT be used.
There are timing windows -- particularly in the presence of lost
packets -- where endpoints may not agree on the state of an SA. The
responder to a CREATE_CHILD_SA MUST be prepared to accept messages on
an SA before sending its response to the creation request, so there
is no ambiguity for the initiator. The initiator MAY begin sending
on an SA as soon as it processes the response. The initiator,
however, cannot receive on a newly created SA until it receives and
processes the response to its CREATE_CHILD_SA request. How, then, is
the responder to know when it is OK to send on the newly created SA?
From a technical correctness and interoperability perspective, the
responder MAY begin sending on an SA as soon as it sends its response
to the CREATE_CHILD_SA request. In some situations, however, this
could result in packets unnecessarily being dropped, so an
implementation MAY defer such sending.
The responder can be assured that the initiator is prepared to
receive messages on an SA if either (1) it has received a
cryptographically valid message on the other half of the SA pair, or
(2) the new SA rekeys an existing SA and it receives an IKE request
to close the replaced SA. When rekeying an SA, the responder
continues to send traffic on the old SA until one of those events
occurs. When establishing a new SA, the responder MAY defer sending
messages on a new SA until either it receives one or a timeout has
occurred. If an initiator receives a message on an SA for which it
has not received a response to its CREATE_CHILD_SA request, it
interprets that as a likely packet loss and retransmits the
CREATE_CHILD_SA request. An initiator MAY send a dummy ESP message
on a newly created ESP SA if it has no messages queued in order to
assure the responder that the initiator is ready to receive messages.
2.8.1. Simultaneous Child SA Rekeying
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).
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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. If redundant SAs are created though such a collision, the SA
created with the lowest of the four nonces used in the two exchanges
SHOULD be closed by the endpoint that created it. "Lowest" means an
octet-by-octet comparison (instead of, for instance, comparing the
nonces as large integers). In other words, start by comparing the
first octet; if they're equal, move to the next octet, and so on. If
you reach the end of one nonce, that nonce is the lower one. The
node that initiated the surviving rekeyed SA should delete the
replaced SA after the new one is established.
The following is an explanation on the impact this has on
implementations. Assume that hosts A and B have an existing Child SA
pair with SPIs (SPIa1,SPIb1), and both start rekeying it at the same
time:
Host A Host B
-------------------------------------------------------------------
send req1: N(REKEY_SA,SPIa1),
SA(..,SPIa2,..),Ni1,.. -->
<-- send req2: N(REKEY_SA,SPIb1),
SA(..,SPIb2,..),Ni2
recv req2 <--
At this point, A knows there is a simultaneous rekeying happening.
However, it cannot yet know which of the exchanges will have the
lowest nonce, so it will just note the situation and respond as
usual.
send resp2: SA(..,SPIa3,..),
Nr1,.. -->
--> recv req1
Now B also knows that simultaneous rekeying is going on. It responds
as usual.
<-- send resp1: SA(..,SPIb3,..),
Nr2,..
recv resp1 <--
--> recv resp2
At this point, there are three Child SA pairs between A and B (the
old one and two new ones). A and B can now compare the nonces.
Suppose that the lowest nonce was Nr1 in message resp2; in this case,
B (the sender of req2) deletes the redundant new SA, and A (the node
that initiated the surviving rekeyed SA), deletes the old one.
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send req3: D(SPIa1) -->
<-- send req4: D(SPIb2)
--> recv req3
<-- send resp3: D(SPIb1)
recv req4 <--
send resp4: D(SPIa3) -->
The rekeying is now finished.
However, there is a second possible sequence of events that can
happen if some packets are lost in the network, resulting in
retransmissions. The rekeying begins as usual, but A's first packet
(req1) is lost.
Host A Host B
-------------------------------------------------------------------
send req1: N(REKEY_SA,SPIa1),
SA(..,SPIa2,..),
Ni1,.. --> (lost)
<-- send req2: N(REKEY_SA,SPIb1),
SA(..,SPIb2,..),Ni2
recv req2 <--
send resp2: SA(..,SPIa3,..),
Nr1,.. -->
--> recv resp2
<-- send req3: D(SPIb1)
recv req3 <--
send resp3: D(SPIa1) -->
--> recv resp3
From B's point of view, the rekeying is now completed, and since it
has not yet received A's req1, it does not even know that there was
simultaneous rekeying. However, A will continue retransmitting the
message, and eventually it will reach B.
resend req1 -->
--> recv req1
To B, it looks like A is trying to rekey an SA that no longer exists;
thus, B responds to the request with something non-fatal such as
CHILD_SA_NOT_FOUND.
<-- send resp1: N(CHILD_SA_NOT_FOUND)
recv resp1 <--
When A receives this error, it already knows there was simultaneous
rekeying, so it can ignore the error message.
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2.8.2. Simultaneous IKE SA Rekeying
Probably the most complex case occurs when both peers try to rekey
the IKE_SA at the same time. Basically, the text in Section 2.8
applies to this case as well; however, it is important to ensure that
the Child SAs are inherited by the correct IKE_SA.
The case where both endpoints notice the simultaneous rekeying works
the same way as with Child SAs. After the CREATE_CHILD_SA exchanges,
three IKE SAs exist between A and B: the old IKE SA and two new IKE
SAs. The new IKE SA containing the lowest nonce SHOULD be deleted by
the node that created it, and the other surviving new IKE SA MUST
inherit all the Child SAs.
In addition to normal simultaneous rekeying cases, there is a special
case where one peer finishes its rekey before it even notices that
other peer is doing a rekey. If only one peer detects a simultaneous
rekey, redundant SAs are not created. In this case, when the peer
that did not notice the simultaneous rekey gets the request to rekey
the IKE SA that it has already successfully rekeyed, it SHOULD return
TEMPORARY_FAILURE because it is an IKE SA that it is currently trying
to close (whether or not it has already sent the delete notification
for the SA). If the peer that did notice the simultaneous rekey gets
the delete request from the other peer for the old IKE SA, it knows
that the other peer did not detect the simultaneous rekey, and the
first peer can forget its own rekey attempt.
Host A Host B
-------------------------------------------------------------------
send req1:
SA(..,SPIa1,..),Ni1,.. -->
<-- send req2: SA(..,SPIb1,..),Ni2,..
--> recv req1
<-- send resp1: SA(..,SPIb2,..),Nr2,..
recv resp1 <--
send req3: D() -->
--> recv req3
At this point, host B sees a request to close the IKE_SA. There's
not much more to do than to reply as usual. However, at this point
host B should stop retransmitting req2, since once host A receives
resp3, it will delete all the state associated with the old IKE_SA
and will not be able to reply to it.
<-- send resp3: ()
The TEMPORARY_FAILURE notification was not included in RFC 4306, and
support of the TEMPORARY_FAILURE notification is not negotiated.
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Thus, older peers that implement RFC 4306 but not this document may
receive these notifications. In that case, they will treat it the
same as any other unknown error notification, and will stop the
exchange. Because the other peer has already rekeyed the exchange,
doing so does not have any ill effects.
2.8.3. Rekeying the IKE SA versus Reauthentication
Rekeying the IKE SA and reauthentication are different concepts in
IKEv2. Rekeying the IKE SA establishes new keys for the IKE SA and
resets the Message ID counters, but it does not authenticate the
parties again (no AUTH or EAP payloads are involved).
Although rekeying the IKE SA may be important in some environments,
reauthentication (the verification that the parties still have access
to the long-term credentials) is often more important.
IKEv2 does not have any special support for reauthentication.
Reauthentication is done by creating a new IKE SA from scratch (using
IKE_SA_INIT/IKE_AUTH exchanges, without any REKEY_SA Notify
payloads), creating new Child SAs within the new IKE SA (without
REKEY_SA Notify payloads), and finally deleting the old IKE SA (which
deletes the old Child SAs as well).
This means that reauthentication also establishes new keys for the
IKE SA and Child SAs. Therefore, while rekeying can be performed
more often than reauthentication, the situation where "authentication
lifetime" is shorter than "key lifetime" does not make sense.
While creation of a new IKE SA can be initiated by either party
(initiator or responder in the original IKE SA), the use of EAP
and/or Configuration payloads means in practice that reauthentication
has to be initiated by the same party as the original IKE SA. IKEv2
does not currently allow the responder to request reauthentication in
this case; however, there are extensions that add this functionality
such as [REAUTH].
2.9. Traffic Selector Negotiation
When an RFC4301-compliant IPsec subsystem receives an IP packet that
matches a "protect" selector in its Security Policy Database (SPD),
the subsystem protects that packet with IPsec. When no SA exists
yet, it is the task of IKE to create it. Maintenance of a system's
SPD is outside the scope of IKE, although some implementations might
update their SPD in connection with the running of IKE (for an
example scenario, see Section 1.1.3).
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Traffic Selector (TS) payloads allow endpoints to communicate some of
the information from their SPD to their peers. These must be
communicated to IKE from the SPD (for example, the PF_KEY API [PFKEY]
uses the SADB_ACQUIRE message). 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 an IP protocol ID.
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 to (or the source address of the traffic forwarded from)
the responder of the Child SA pair. For example, if the original
initiator requests the creation of a Child SA pair, and wishes to
tunnel all traffic from subnet 198.51.100.* on the initiator's side
to subnet 192.0.2.* on the responder's side, the initiator would
include a single Traffic Selector in each TS payload. TSi would
specify the address range (198.51.100.0 - 198.51.100.255) and TSr
would specify the address range (192.0.2.0 - 192.0.2.255). Assuming
that proposal was acceptable to the responder, it would send
identical TS payloads back.
IKEv2 allows the responder to choose a subset of the traffic proposed
by the initiator. This could happen when the configurations 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 absence 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.
When the responder chooses a subset of the traffic proposed by the
initiator, it narrows the Traffic Selectors to some subset of the
initiator's proposal (provided the set does not become the null set).
If the type of Traffic Selector proposed is unknown, the responder
ignores that Traffic Selector, so that the unknown type is not
returned in the narrowed set.
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To enable the responder to choose the appropriate range in this case,
if the initiator has requested the SA due to a data packet, the
initiator SHOULD 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, the initiator
would include in TSi two Traffic Selectors: the first containing the
address range (198.51.100.43 - 198.51.100.43) and the source port and
IP protocol from the packet and the second containing (198.51.100.0 -
198.51.100.255) with all ports and IP protocols. The initiator would
similarly include two Traffic Selectors in TSr. If the initiator
creates the Child SA pair not in response to an arriving packet, but
rather, say, upon startup, then there may be no specific addresses
the initiator prefers for the initial tunnel over any other. In that
case, the first values in TSi and TSr can be ranges rather than
specific values.
The responder performs the narrowing as follows:
o If the responder's policy does not allow it to accept any part of
the proposed Traffic Selectors, it responds with a TS_UNACCEPTABLE
Notify message.
o If the responder's policy allows the entire set of traffic covered
by TSi and TSr, no narrowing is necessary, and the responder can
return the same TSi and TSr values.
o If the responder's policy allows it to accept the first selector
of TSi and TSr, then the responder MUST narrow the Traffic
Selectors to a subset that includes the initiator's first choices.
In this example above, the responder might respond with TSi being
(198.51.100.43 - 198.51.100.43) with all ports and IP protocols.
o If the responder's policy does not allow it to accept the first
selector of TSi and TSr, the responder narrows to an acceptable
subset of TSi and TSr.
When narrowing is done, there may be several subsets that are
acceptable but their union is not. In this case, the responder
arbitrarily chooses one of them, and MAY include an
ADDITIONAL_TS_POSSIBLE notification in the response. The
ADDITIONAL_TS_POSSIBLE notification asserts that the responder
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. This case
will occur only when the initiator and responder are configured
differently from one another. If the initiator and responder agree
on the granularity of tunnels, the initiator will never request a
tunnel wider than the responder will accept.
Kaufman, et al. Standards Track [Page 42]
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It is possible for the responder's policy to contain multiple smaller
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, the responder
might have a policy of being willing to tunnel those addresses to and
from the initiator, but might require that each address pair be on a
separately negotiated Child SA. If the initiator didn't generate its
request based on the packet, but (for example) upon startup, there
would not be the very specific first Traffic Selectors helping the
responder to select the correct range. There would be no way for the
responder to determine which pair of addresses should be included in
this tunnel, and it would have to make a guess or reject the request
with a SINGLE_PAIR_REQUIRED Notify message.
The SINGLE_PAIR_REQUIRED error indicates that a CREATE_CHILD_SA
request is unacceptable because its sender is only willing to accept
Traffic Selectors specifying a single pair of addresses. The
requestor is expected to respond by requesting an SA for only the
specific traffic it is trying to forward.
Few implementations will have policies that require separate SAs for
each address pair. Because of this, if only some parts of the TSi
and TSr proposed by the initiator are acceptable to the responder,
responders SHOULD narrow the selectors to an acceptable subset rather
than use SINGLE_PAIR_REQUIRED.
2.9.1. Traffic Selectors Violating Own Policy
When creating a new SA, the initiator needs to avoid proposing
Traffic Selectors that violate its own policy. If this rule is not
followed, valid traffic may be dropped. If you use decorrelated
policies from [IPSECARCH], this kind of policy violations cannot
happen.
This is best illustrated by an example. Suppose that host A has a
policy whose effect is that traffic to 198.51.100.66 is sent via host
B encrypted using AES, and traffic to all other hosts in
198.51.100.0/24 is also sent via B, but must use 3DES. Suppose also
that host B accepts any combination of AES and 3DES.
If host A now proposes an SA that uses 3DES, and includes TSr
containing (198.51.100.0-198.51.100.255), this will be accepted by
host B. Now, host B can also use this SA to send traffic from
198.51.100.66, but those packets will be dropped by A since it
requires the use of AES for this traffic. Even if host A creates a
new SA only for 198.51.100.66 that uses AES, host B may freely
continue to use the first SA for the traffic. In this situation,
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when proposing the SA, host A should have followed its own policy,
and included a TSr containing ((198.51.100.0-
198.51.100.65),(198.51.100.67-198.51.100.255)) instead.
In general, if (1) the initiator makes a proposal "for traffic X
(TSi/TSr), do SA", and (2) for some subset X' of X, the initiator
does not actually accept traffic X' with SA, and (3) the initiator
would be willing to accept traffic X' with some SA' (!=SA), valid
traffic can be unnecessarily dropped since the responder can apply
either SA or SA' to traffic X'.
2.10. 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 SA, and to ensure creation of strong
pseudorandom bits from the Diffie-Hellman key. Nonces used in IKEv2
MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
be at least half the key size of the negotiated pseudorandom function
(PRF). However, the initiator chooses the nonce before the outcome
of the negotiation is known. Because of that, the nonce has to be
long enough for all the PRFs being proposed. If the same random
number source is used for both keys and nonces, care must be taken to
ensure that the latter use does not compromise the former.
2.11. 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 over which it runs. 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 requests even if the source port is not 500 or 4500,
and MUST respond to the address and port from which the request was
received. It MUST specify the address and port at which the request
was received as the source address and port in the response. IKE
functions identically over IPv4 or IPv6.
2.12. 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
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the two endpoints cannot reconstruct the keys used to protect the
conversation without doing a brute force search of the session key
space.
Achieving perfect forward secrecy requires that when a connection is
closed, each endpoint MUST forget not only the keys used by the
connection but also any information that could be used to recompute
those keys.
Because computing 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.
Whether and when to reuse Diffie-Hellman exponentials are private
decisions in the sense that they will not affect 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. See [REUSE]
for a security analysis of this practice and for additional security
considerations when reusing ephemeral Diffie-Hellman keys.
2.13. Generating Keying Material
In the context of the IKE SA, four cryptographic algorithms are
negotiated: an encryption algorithm, an integrity protection
algorithm, a Diffie-Hellman group, and a pseudorandom function (PRF).
The PRF is used 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 encryption algorithm and integrity protection
algorithm uses a fixed-size key and that any randomly chosen value of
that fixed size can serve as an appropriate key. For algorithms that
accept a variable-length key, a fixed key size MUST be specified as
part of the cryptographic transform negotiated (see Section 3.3.5 for
the definition of the Key Length transform attribute). For
algorithms for which not all values are valid keys (such as DES or
3DES with key parity), the algorithm by which keys are derived from
arbitrary values MUST be specified by the cryptographic transform.
Kaufman, et al. Standards Track [Page 45]
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For integrity protection functions based on Hashed Message
Authentication Code (HMAC), the fixed key size is the size of the
output of the underlying hash function.
It is assumed that PRFs accept keys of any length, but have a
preferred key size. The preferred key size MUST be used as the
length of SK_d, SK_pi, and SK_pr (see Section 2.14). For PRFs based
on the HMAC construction, the preferred key size is equal to the
length of the output of the underlying hash function. Other types of
PRFs MUST specify their preferred key size.
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, the PRF is
used iteratively. The term "prf+" describes a function that outputs
a pseudorandom stream based on the inputs to a pseudorandom function
called "prf".
In the following, | indicates concatenation. prf+ is defined as:
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)
...
This continues until all the material needed to compute all required
keys has been output from prf+. The keys are taken from the output
string without regard to boundaries (e.g., if the required keys are a
256-bit Advanced Encryption Standard (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 prf function is a single
octet. The prf+ function is not defined beyond 255 times the size of
the prf function output.
2.14. Generating Keying Material for the IKE SA
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 seven other secrets: SK_d
used for deriving new keys for the Child SAs established with this
Kaufman, et al. Standards Track [Page 46]
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IKE SA; SK_ai and SK_ar used as a key to the integrity protection
algorithm for authenticating the component messages of subsequent
exchanges; SK_ei and SK_er used for encrypting (and of course
decrypting) all subsequent exchanges; and SK_pi and SK_pr, which are
used when generating an AUTH payload. The lengths of SK_d, SK_pi,
and SK_pr MUST be the preferred key length of the PRF agreed upon.
SKEYSEED and its derivatives are computed as follows:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )
(indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
SK_pi, and SK_pr are taken in order from the generated bits of the
prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
exchange. g^ir is represented as a string of octets in big endian
order padded with zeros if necessary to make it the length of the
modulus. Ni and Nr are the nonces, stripped of any headers. For
historical backward-compatibility reasons, there are two PRFs that
are treated specially in this calculation. If the negotiated PRF is
AES-XCBC-PRF-128 [AESXCBCPRF128] or AES-CMAC-PRF-128 [AESCMACPRF128],
only the first 64 bits of Ni and the first 64 bits of Nr are used in
calculating SKEYSEED, but all the bits are used for input to the prf+
function.
The two directions of traffic 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.
2.15. Authentication of the IKE SA
When not using extensible authentication (see Section 2.16), the
peers are authenticated by having each sign (or MAC using a padded
shared secret as the key, as described later in this section) a block
of data. In these calculations, IDi' and IDr' are the entire ID
payloads excluding the fixed header. For the responder, the octets
to be signed start with the first octet of the first SPI in the
header of the second message (IKE_SA_INIT response) and end with the
last octet of the last payload in the second message. Appended to
this (for the purposes of computing the signature) are the
initiator's nonce Ni (just the value, not the payload containing it),
and the value prf(SK_pr, IDr'). Note that neither the nonce Ni nor
the value prf(SK_pr, IDr') are transmitted. Similarly, the initiator
signs the first message (IKE_SA_INIT request), starting with the
first octet of the first SPI in the header and ending with the last
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octet of the last payload. Appended to this (for purposes of
computing the signature) are the responder's nonce Nr, and the value
prf(SK_pi, IDi'). It is critical to the security of the exchange
that each side sign the other side's nonce.
The initiator's signed octets can be described as:
InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
RealIKEHDR = SPIi | SPIr | . . . | Length
RealMessage1 = RealIKEHDR | RestOfMessage1
NonceRPayload = PayloadHeader | NonceRData
InitiatorIDPayload = PayloadHeader | RestOfInitIDPayload
RestOfInitIDPayload = IDType | RESERVED | InitIDData
MACedIDForI = prf(SK_pi, RestOfInitIDPayload)
The responder's signed octets can be described as:
ResponderSignedOctets = RealMessage2 | NonceIData | MACedIDForR
GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
RealIKEHDR = SPIi | SPIr | . . . | Length
RealMessage2 = RealIKEHDR | RestOfMessage2
NonceIPayload = PayloadHeader | NonceIData
ResponderIDPayload = PayloadHeader | RestOfRespIDPayload
RestOfRespIDPayload = IDType | RESERVED | RespIDData
MACedIDForR = prf(SK_pr, RestOfRespIDPayload)
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 multiple times (such as with a
responder cookie and/or a different Diffie-Hellman group), it is the
latest 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, and specified by the Auth Method
field in the Authentication payload. 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. 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, the same key is
used in both directions.
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Note that it is a common but typically insecure practice to have a
shared key derived solely from a user-chosen password without
incorporating another source of randomness. This is typically
insecure because user-chosen passwords are unlikely to have
sufficient unpredictability to resist dictionary attacks and these
attacks are not prevented in this authentication method.
(Applications using password-based authentication for bootstrapping
and IKE SA should use the authentication method in Section 2.16,
which is designed to prevent off-line dictionary attacks.) The pre-
shared key needs to contain as much unpredictability as the strongest
key being negotiated. In the case of a pre-shared key, the AUTH
value is computed as:
For the initiator:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<InitiatorSignedOctets>)
For the responder:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<ResponderSignedOctets>)
where the string "Key Pad for IKEv2" is 17 ASCII characters without
null termination. The shared secret can be variable length. The pad
string is added so that if the shared secret is derived from a
password, the IKE implementation need not store the password in
cleartext, but rather can store the value prf(Shared Secret,"Key Pad
for IKEv2"), which could not be used as a password equivalent for
protocols other than IKEv2. 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. The
management interface by which the shared secret is provided MUST
accept ASCII strings of at least 64 octets and MUST NOT add a null
terminator before using them as shared secrets. It MUST also accept
a hex encoding of the shared secret. The management interface MAY
accept other encodings if the algorithm for translating the encoding
to a binary string is specified.
There are two types of EAP authentication (described in
Section 2.16), and each type uses different values in the AUTH
computations shown above. If the EAP method is key-generating,
substitute master session key (MSK) for the shared secret in the
computation. For non-key-generating methods, substitute SK_pi and
SK_pr, respectively, for the shared secret in the two AUTH
computations.
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2.16. Extensible Authentication Protocol Methods
In addition to authentication using public key signatures and shared
secrets, IKE supports authentication using methods defined in RFC
3748 [EAP]. Typically, these methods are asymmetric (designed for a
user authenticating to a server), and they may not be mutual. For
this reason, these protocols are typically used to authenticate the
initiator to the responder and MUST be used in conjunction with a
public-key-signature-based authentication of the responder to the
initiator. These methods are often associated with mechanisms
referred to as "Legacy Authentication" mechanisms.
While this document references [EAP] with the intent that new methods
can be added in the future without updating this specification, some
simpler variations are documented here. [EAP] defines an
authentication protocol requiring a variable number of messages.
Extensible Authentication is implemented in IKE as additional
IKE_AUTH exchanges that MUST be completed in order to initialize the
IKE SA.
An initiator indicates a desire to use EAP by leaving out the AUTH
payload from the first message in the IKE_AUTH exchange. (Note that
the AUTH payload is required for non-EAP authentication, and is thus
not marked as optional in the rest of this document.) By including
an IDi payload but not an AUTH payload, the initiator has declared an
identity but has not proven it. If the responder is willing to use
an EAP method, it will place an Extensible Authentication Protocol
(EAP) payload in the response of the IKE_AUTH exchange and defer
sending SAr2, TSi, and TSr until initiator authentication is complete
in a subsequent IKE_AUTH exchange. In the case of a minimal EAP
method, the initial SA establishment will appear as follows:
Initiator Responder
-------------------------------------------------------------------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERTREQ,]
[IDr,] SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
EAP }
HDR, SK {EAP} -->
<-- HDR, SK {EAP (success)}
HDR, SK {AUTH} -->
<-- HDR, SK {AUTH, SAr2, TSi, TSr }
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As described in Section 2.2, when EAP is used, each pair of IKE SA
initial setup messages will have their message numbers incremented;
the first pair of AUTH messages will have an ID of 1, the second will
be 2, and so on.
For EAP methods that create a shared key as a side effect of
authentication, that shared key MUST be used by both the initiator
and responder to generate AUTH payloads in messages 7 and 8 using the
syntax for shared secrets specified in Section 2.15. The shared key
from EAP is the field from the EAP specification named MSK. This
shared key generated during an IKE exchange MUST NOT be used for any
other purpose.
EAP methods that do not establish a shared key SHOULD NOT be used, as
they are subject to a number of man-in-the-middle attacks [EAPMITM]
if these EAP methods are used in other protocols that do not use a
server-authenticated tunnel. Please see the Security Considerations
section for more details. If EAP methods that do not generate a
shared key are used, the AUTH payloads in messages 7 and 8 MUST be
generated using SK_pi and SK_pr, respectively.
The initiator of an IKE SA using EAP needs to be capable of extending
the initial protocol exchange to at least ten IKE_AUTH exchanges in
the event the responder sends notification messages and/or retries
the authentication prompt. Once the protocol exchange defined by the
chosen EAP authentication method has successfully terminated, the
responder MUST send an EAP payload containing the Success message.
Similarly, if the authentication method has failed, the responder
MUST send an EAP payload containing the Failure message. The
responder MAY at any time terminate the IKE exchange by sending an
EAP payload containing the Failure message.
Following such an extended exchange, the EAP AUTH payloads MUST be
included in the two messages following the one containing the EAP
Success message.
When the initiator authentication uses EAP, it is possible that the
contents of the IDi payload is used only for Authentication,
Authorization, and Accounting (AAA) routing purposes and selecting
which EAP method to use. This value may be different from the
identity authenticated by the EAP method. It is important that
policy lookups and access control decisions use the actual
authenticated identity. Often the EAP server is implemented in a
separate AAA server that communicates with the IKEv2 responder. In
this case, the authenticated identity, if different from that in the
IDi payload, has to be sent from the AAA server to the IKEv2
responder.
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2.17. Generating Keying Material for Child SAs
A single Child SA is created by the IKE_AUTH exchange, and additional
Child SAs can optionally be created in CREATE_CHILD_SA exchanges.
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 CREATE_CHILD_SA exchanges including an optional Diffie-Hellman
exchange, the keying material is defined as:
KEYMAT = prf+(SK_d, g^ir (new) | Ni | Nr )
where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros in the high-order
bits if necessary to make it the length of the modulus).
A single CHILD_SA negotiation may result in multiple Security
Associations. ESP and AH SAs exist in pairs (one in each direction),
so two SAs are created in a single Child SA negotiation for them.
Furthermore, Child SA negotiation may include some future IPsec
protocol(s) in addition to, or instead of, ESP or AH (for example,
ROHC_INTEG as described in [ROHCV2]). In any case, keying material
for each Child SA MUST be taken from the expanded KEYMAT using the
following rules:
o All keys for SAs carrying data from the initiator to the responder
are taken before SAs going from the responder to the initiator.
o If multiple IPsec protocols are negotiated, keying material for
each Child SA is taken in the order in which the protocol headers
will appear in the encapsulated packet.
o If an IPsec protocol requires multiple keys, the order in which
they are taken from the SA's keying material needs to be described
in the protocol's specification. For ESP and AH, [IPSECARCH]
defines the order, namely: the encryption key (if any) MUST be
taken from the first bits and the integrity key (if any) MUST be
taken from the remaining bits.
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Each cryptographic algorithm takes a fixed number of bits of keying
material specified as part of the algorithm, or negotiated in SA
payloads (see Section 2.13 for description of key lengths, and
Section 3.3.5 for the definition of the Key Length transform
attribute).
2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange
The CREATE_CHILD_SA exchange can be used to rekey an existing IKE SA
(see Sections 1.3.2 and 2.8). New initiator and responder SPIs are
supplied in the SPI fields in the Proposal structures inside the
Security Association (SA) payloads (not the SPI fields in the IKE
header). 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 (new) | Ni | Nr)
where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros if necessary to
make it the length of the modulus) and Ni and Nr are the two nonces
stripped of any headers.
The old and new IKE SA may have selected a different PRF. Because
the rekeying exchange belongs to the old IKE SA, it is the old IKE
SA's PRF that is used to generate SKEYSEED.
The main reason for rekeying the IKE SA is to ensure that the
compromise of old keying material does not provide information about
the current keys, or vice versa. Therefore, implementations MUST
perform a new Diffie-Hellman exchange when rekeying the IKE SA. In
other words, an initiator MUST NOT propose the value "NONE" for the
Diffie-Hellman transform, and a responder MUST NOT accept such a
proposal. This means that a successful exchange rekeying the IKE SA
always includes the KEi/KEr payloads.
The new IKE SA MUST reset its message counters to 0.
SK_d, SK_ai, SK_ar, SK_ei, and SK_er are computed from SKEYSEED as
specified in Section 2.14, using SPIi, SPIr, Ni, and Nr from the new
exchange, and using the new IKE SA's PRF.
2.19. Requesting an Internal Address on a Remote Network
Most commonly occurring in the endpoint-to-security-gateway scenario,
an endpoint may need an IP address in the network protected by the
security gateway and may need to have that address dynamically
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RFC 5996 IKEv2bis September 2010
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. Note, however, it is usual to
only assign one IP address during the IKE_AUTH exchange. That
address persists at least until the deletion of the IKE SA.
This function provides address allocation to an IPsec Remote Access
Client (IRAC) trying to tunnel into a network protected by an IPsec
Remote Access Server (IRAS). Since the IKE_AUTH exchange creates an
IKE SA and a Child SA, the IRAC MUST request the IRAS-controlled
address (and optionally other information concerning the protected
network) in the IKE_AUTH exchange. The IRAS may procure an address
for the IRAC from any number of sources such as a DHCP/BOOTP
(Bootstrap Protocol) server or its own address pool.
Initiator Responder
-------------------------------------------------------------------
HDR, SK {IDi, [CERT,]
[CERTREQ,] [IDr,] AUTH,
CP(CFG_REQUEST), SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2,
TSi, TSr}
In all cases, the CP payload MUST be inserted before the SA payload.
In variations of the protocol where there are multiple IKE_AUTH
exchanges, the CP payloads MUST be inserted in the messages
containing the SA payloads.
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.
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RFC 5996 IKEv2bis September 2010
For example, message from initiator to responder:
CP(CFG_REQUEST)=
INTERNAL_ADDRESS()
TSi = (0, 0-65535,0.0.0.0-255.255.255.255)
TSr = (0, 0-65535,0.0.0.0-255.255.255.255)
NOTE: Traffic Selectors contain (protocol, port range, address
range).
Message from responder to initiator:
CP(CFG_REPLY)=
INTERNAL_ADDRESS(192.0.2.202)
INTERNAL_NETMASK(255.255.255.0)
INTERNAL_SUBNET(192.0.2.0/255.255.255.0)
TSi = (0, 0-65535,192.0.2.202-192.0.2.202)
TSr = (0, 0-65535,192.0.2.0-192.0.2.255)
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.
The responder MUST NOT send a CFG_REPLY without having first received
a CP(CFG_REQUEST) from the initiator, because we do not want the IRAS
to perform an unnecessary configuration lookup if the IRAC cannot
process the REPLY.
In the case where the IRAS's configuration requires that CP be used
for a given identity IDi, but IRAC has failed to send a
CP(CFG_REQUEST), IRAS MUST fail the request, and terminate the Child
SA creation with a FAILED_CP_REQUIRED error. The FAILED_CP_REQUIRED
is not fatal to the IKE SA; it simply causes the Child SA creation to
fail. The initiator can fix this by later starting a new
Configuration payload request. There is no associated data in the
FAILED_CP_REQUIRED error.
2.20. Requesting the Peer's Version
An IKE peer wishing to inquire about the other peer's IKE software
version information MAY 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.
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An IKE implementation MAY decline to give out version information
prior to authentication or even after authentication in case some
implementation is known to have some security weakness. In that
case, it MUST either return an empty string or no CP payload if CP is
not supported.
Initiator Responder
-------------------------------------------------------------------
HDR, SK{CP(CFG_REQUEST)} -->
<-- HDR, SK{CP(CFG_REPLY)}
CP(CFG_REQUEST)=
APPLICATION_VERSION("")