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rfc:rfc4306

Network Working Group C. Kaufman, Ed. Request for Comments: 4306 Microsoft Obsoletes: 2407, 2408, 2409 December 2005 Category: Standards Track

               Internet Key Exchange (IKEv2) Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

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 version of the IKE specification combines the contents of what
 were previously separate documents, including Internet Security
 Association and Key Management Protocol (ISAKMP, RFC 2408), IKE (RFC
 2409), the Internet Domain of Interpretation (DOI, RFC 2407), Network
 Address Translation (NAT) Traversal, Legacy authentication, and
 remote address acquisition.
 Version 2 of IKE does not interoperate with version 1, but it has
 enough of the header format in common that both versions can
 unambiguously run over the same UDP port.

Kaufman Standards Track [Page 1] RFC 4306 IKEv2 December 2005

Table of Contents

 1. Introduction ....................................................3
    1.1. Usage Scenarios ............................................5
    1.2. The Initial Exchanges ......................................7
    1.3. The CREATE_CHILD_SA Exchange ...............................9
    1.4. The INFORMATIONAL Exchange ................................11
    1.5. Informational Messages outside of an IKE_SA ...............12
 2. IKE Protocol Details and Variations ............................12
    2.1. Use of Retransmission Timers ..............................13
    2.2. Use of Sequence Numbers for Message ID ....................14
    2.3. Window Size for Overlapping Requests ......................14
    2.4. State Synchronization and Connection Timeouts .............15
    2.5. Version Numbers and Forward Compatibility .................17
    2.6. Cookies ...................................................18
    2.7. Cryptographic Algorithm Negotiation .......................21
    2.8. Rekeying ..................................................22
    2.9. Traffic Selector Negotiation ..............................24
    2.10. Nonces ...................................................26
    2.11. Address and Port Agility .................................26
    2.12. Reuse of Diffie-Hellman Exponentials .....................27
    2.13. Generating Keying Material ...............................27
    2.14. Generating Keying Material for the IKE_SA ................28
    2.15. Authentication of the IKE_SA .............................29
    2.16. Extensible Authentication Protocol Methods ...............31
    2.17. Generating Keying Material for CHILD_SAs .................33
    2.18. Rekeying IKE_SAs Using a CREATE_CHILD_SA exchange ........34
    2.19. Requesting an Internal Address on a Remote Network .......34
    2.20. Requesting the Peer's Version ............................35
    2.21. Error Handling ...........................................36
    2.22. IPComp ...................................................37
    2.23. NAT Traversal ............................................38
    2.24. Explicit Congestion Notification (ECN) ...................40
 3. Header and Payload Formats .....................................41
    3.1. The IKE Header ............................................41
    3.2. Generic Payload Header ....................................44
    3.3. Security Association Payload ..............................46
    3.4. Key Exchange Payload ......................................56
    3.5. Identification Payloads ...................................56
    3.6. Certificate Payload .......................................59
    3.7. Certificate Request Payload ...............................61
    3.8. Authentication Payload ....................................63
    3.9. Nonce Payload .............................................64
    3.10. Notify Payload ...........................................64
    3.11. Delete Payload ...........................................72
    3.12. Vendor ID Payload ........................................73
    3.13. Traffic Selector Payload .................................74
    3.14. Encrypted Payload ........................................77

Kaufman Standards Track [Page 2] RFC 4306 IKEv2 December 2005

    3.15. Configuration Payload ....................................79
    3.16. Extensible Authentication Protocol (EAP) Payload .........84
 4. Conformance Requirements .......................................85
 5. Security Considerations ........................................88
 6. IANA Considerations ............................................90
 7. Acknowledgements ...............................................91
 8. References .....................................................91
    8.1. Normative References ......................................91
    8.2. Informative References ....................................92
 Appendix A: Summary of Changes from IKEv1 .........................96
 Appendix B: Diffie-Hellman Groups .................................97
    B.1. Group 1 - 768 Bit MODP ....................................97
    B.2. Group 2 - 1024 Bit MODP ...................................97

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 memo describes such a protocol -- the Internet Key
 Exchange (IKE).  This is version 2 of IKE.  Version 1 of IKE was
 defined in RFCs 2407, 2408, and 2409 [Pip98, MSST98, HC98].  This
 single document is intended to replace all three of those RFCs.
 Definitions of the primitive terms in this document (such as Security
 Association or SA) can be found in [RFC4301].
 Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
 "MAY" that appear in this document are to be interpreted as described
 in [Bra97].
 The term "Expert Review" is to be interpreted as defined in
 [RFC2434].
 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) [RFC4303] and/or Authentication
 Header (AH) [RFC4302] 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

Kaufman Standards Track [Page 3] RFC 4306 IKEv2 December 2005

 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) [IPCOMP] in connection with
 an ESP and/or AH SA.  We call the IKE SA an "IKE_SA".  The SAs for
 ESP and/or AH that get set up through that IKE_SA we call
 "CHILD_SAs".
 All IKE communications consist of pairs of messages: a request and a
 response.  The pair is called an "exchange".  We call the first
 messages establishing an IKE_SA IKE_SA_INIT and IKE_AUTH exchanges
 and subsequent IKE exchanges 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 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.
 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 request/response of an IKE session (IKE_SA_INIT) negotiates
 security parameters for the IKE_SA, sends nonces, and sends Diffie-
 Hellman values.
 The second request/response (IKE_AUTH) transmits identities, proves
 knowledge of the secrets corresponding to the two identities, and
 sets up an SA for the first (and often only) AH and/or ESP CHILD_SA.
 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.

Kaufman Standards Track [Page 4] RFC 4306 IKEv2 December 2005

 In the description that follows, we assume that no errors occur.
 Modifications to the flow should errors occur are described in
 section 2.21.

1.1. Usage Scenarios

 IKE is expected to be used to negotiate ESP and/or AH SAs in a number
 of different scenarios, each with its own special requirements.

1.1.1. Security Gateway to Security Gateway Tunnel

                  +-+-+-+-+-+            +-+-+-+-+-+
                  !         ! 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

     +-+-+-+-+-+                                          +-+-+-+-+-+
     !         !                 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 [RFC4301].  Transport mode will
 commonly be used with no inner IP header.  If there is an inner IP
 header, the inner addresses will be the same as the outer addresses.
 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 [RFC1958],

Kaufman Standards Track [Page 5] RFC 4306 IKEv2 December 2005

 [RFC2775], and a method of limiting the inherent problems with
 complexity in networks noted by [RFC3439].  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.
 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 Tunnel

     +-+-+-+-+-+                          +-+-+-+-+-+
     !         !         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
 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.

Kaufman Standards Track [Page 6] RFC 4306 IKEv2 December 2005

 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
 endpoint, and packets will have to be UDP encapsulated in order to be
 routed properly.

1.1.4. Other Scenarios

 Other scenarios are possible, as are nested combinations of the
 above.  One notable example combines aspects of 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.
 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
 E         Encrypted

Kaufman Standards Track [Page 7] RFC 4306 IKEv2 December 2005

 EAP       Extensible Authentication
 HDR       IKE Header
 IDi       Identification - Initiator
 IDr       Identification - Responder
 KE        Key Exchange
 Ni, Nr    Nonce
 N         Notify
 SA        Security Association
 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], indicate that optionally a certificate request
 payload can 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]
 The responder chooses a cryptographic suite from the initiator's
 offered choices and expresses that choice in the SAr1 payload,
 completes the Diffie-Hellman exchange with the KEr payload, and sends
 its nonce in the Nr payload.
 At this point in the negotiation, each party can generate SKEYSEED,
 from which all keys are derived for that IKE_SA.  All but the headers
 of all the messages that follow are encrypted and integrity
 protected.  The keys used for the encryption and integrity protection
 are derived from SKEYSEED and are known as SK_e (encryption) and SK_a
 (authentication, a.k.a.  integrity protection).  A separate SK_e and
 SK_a is computed for each direction.  In addition to the keys SK_e
 and SK_a derived from the DH 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.

Kaufman Standards Track [Page 8] RFC 4306 IKEv2 December 2005

     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 which of the responder's
 identities it wants to talk to.  This is useful when the machine on
 which the responder is running is hosting multiple identities at the
 same IP address.  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.
                                 <--    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.
 The recipients of messages 3 and 4 MUST verify that all signatures
 and MACs are computed correctly and that the names in the ID payloads
 correspond to the keys used to generate the AUTH payload.

1.3. The CREATE_CHILD_SA Exchange

 This exchange consists of a single request/response pair, and was
 referred to as a phase 2 exchange in IKEv1.  It MAY be initiated by
 either end of the IKE_SA after the initial exchanges are completed.
 All messages following the initial exchange are cryptographically
 protected using the cryptographic algorithms and keys negotiated in
 the first two messages of the IKE exchange.  These subsequent
 messages use the syntax of the Encrypted Payload described in section
 3.14.  All subsequent messages included an Encrypted Payload, even if
 they are referred to in the text as "empty".
 Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
 section the term "initiator" refers to the endpoint initiating this
 exchange.

Kaufman Standards Track [Page 9] RFC 4306 IKEv2 December 2005

 A CHILD_SA is created by sending a CREATE_CHILD_SA request.  The
 CREATE_CHILD_SA request MAY optionally contain a KE payload for an
 additional Diffie-Hellman exchange to enable stronger guarantees of
 forward secrecy for the CHILD_SA.  The keying material for the
 CHILD_SA is a function of SK_d established during the establishment
 of the IKE_SA, the nonces exchanged during the CREATE_CHILD_SA
 exchange, and the Diffie-Hellman value (if KE payloads are included
 in the CREATE_CHILD_SA exchange).
 In the CHILD_SA created as part of the initial exchange, a second KE
 payload and nonce MUST NOT be sent.  The nonces from the initial
 exchange are used in computing the keys for the CHILD_SA.
 The CREATE_CHILD_SA request contains:
     Initiator                                 Responder
    -----------                               -----------
     HDR, SK {[N], SA, Ni, [KEi],
         [TSi, TSr]}             -->
 The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
 payload, optionally a Diffie-Hellman value in the KEi payload, and
 the proposed traffic selectors in the TSi and TSr payloads.  If this
 CREATE_CHILD_SA exchange is rekeying an existing SA other than the
 IKE_SA, the leading N payload of type REKEY_SA MUST identify the SA
 being rekeyed.  If this CREATE_CHILD_SA exchange is not rekeying an
 existing SA, the N payload MUST be omitted.  If the SA offers include
 different Diffie-Hellman groups, KEi MUST be an element of the group
 the initiator expects the responder to accept.  If it guesses wrong,
 the CREATE_CHILD_SA exchange will fail, and it will have to retry
 with a different KEi.
 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.
 The CREATE_CHILD_SA response contains:
                                <--    HDR, SK {SA, Nr, [KEr],
                                             [TSi, TSr]}
 The responder replies (using the same Message ID to respond) with the
 accepted offer in an SA payload, and a Diffie-Hellman value in the
 KEr payload if KEi was included in the request and the selected
 cryptographic suite includes that group.  If the responder chooses a
 cryptographic suite with a different group, it MUST reject the
 request.  The initiator SHOULD repeat the request, but now with a KEi
 payload from the group the responder selected.

Kaufman Standards Track [Page 10] RFC 4306 IKEv2 December 2005

 The traffic selectors for traffic to be sent on that SA are specified
 in the TS payloads, which may be a subset of what the initiator of
 the CHILD_SA proposed.  Traffic selectors are omitted if this
 CREATE_CHILD_SA request is being used to change the key of the
 IKE_SA.

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.
 Control messages that pertain to an IKE_SA MUST be sent under that
 IKE_SA.  Control messages that pertain to CHILD_SAs MUST be sent
 under the protection of the IKE_SA which generated them (or its
 successor if the IKE_SA was replaced for the purpose of rekeying).
 Messages in an INFORMATIONAL exchange contain zero or more
 Notification, Delete, and Configuration payloads.  The Recipient of
 an INFORMATIONAL exchange request MUST send some response (else the
 Sender will assume the message was lost in the network and will
 retransmit it).  That response MAY be a message with no payloads.
 The request message in an INFORMATIONAL exchange MAY also contain no
 payloads.  This is the expected way an endpoint can ask the other
 endpoint to verify that it is alive.
 ESP and AH SAs always exist in pairs, with one SA in each direction.
 When an SA is closed, both members of the pair MUST be closed.  When
 SAs are nested, as when data (and IP headers if in tunnel mode) are
 encapsulated first with IPComp, then with ESP, and finally with AH
 between the same pair of endpoints, all of the SAs MUST be deleted
 together.  Each endpoint MUST close 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 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.  Normally, the reply in the INFORMATIONAL exchange
 will contain delete payloads for the paired SAs going in the other
 direction.  There is one exception.  If by chance both ends of a set
 of SAs independently decide to close them, each may send a delete
 payload and the two requests may cross in the network.  If a node
 receives a delete request for SAs for which it has already issued a
 delete request, it MUST delete the outgoing SAs while processing the
 request and the incoming SAs while processing the response.  In that

Kaufman Standards Track [Page 11] RFC 4306 IKEv2 December 2005

 case, the responses MUST NOT include delete payloads for the deleted
 SAs, since that would result in duplicate deletion and could in
 theory delete the wrong SA.
 A node SHOULD regard half-closed connections as anomalous and audit
 their existence should they persist.  Note that this specification
 nowhere specifies time periods, so it is up to individual endpoints
 to decide how long to wait.  A node MAY refuse to accept incoming
 data on half-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; doing so will implicitly close all
 SAs negotiated under it.  It can then rebuild the SAs it needs on a
 clean base under a new IKE_SA.
 The INFORMATIONAL exchange is defined as:
     Initiator                        Responder
    -----------                      -----------
     HDR, SK {[N,] [D,] [CP,] ...} -->
                                 <-- HDR, SK {[N,] [D,] [CP], ...}
 The processing of an INFORMATIONAL exchange is determined by its
 component payloads.

1.5. Informational Messages outside of an IKE_SA

 If an encrypted IKE packet arrives on port 500 or 4500 with an
 unrecognized SPI, it could be because the receiving node has recently
 crashed and lost state or because of some other system malfunction or
 attack.  If the receiving node has an active IKE_SA to the IP address
 from whence the packet came, it MAY send a notification of the
 wayward packet over that IKE_SA in an INFORMATIONAL exchange.  If it
 does not have such an IKE_SA, it MAY send an Informational message
 without cryptographic protection to the source IP address.  Such a
 message is not part of an informational exchange, and the receiving
 node MUST NOT respond to it.  Doing so could cause a message loop.

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

Kaufman Standards Track [Page 12] RFC 4306 IKEv2 December 2005

 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, X.509
 certificates), and IKEv2 itself does not have a mechanism for
 fragmenting large messages.  IP defines a mechanism for fragmentation
 of oversize UDP messages, but implementations vary in the maximum
 message size supported.  Furthermore, use of IP fragmentation opens
 an implementation to denial of service attacks [KPS03].  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 bytes long, and they SHOULD be able
 to send, receive, and process messages that are up to 3000 bytes
 long.  IKEv2 implementations SHOULD 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 should keep in
 mind, however, that if the URL lookups are possible only after the
 IPsec SA is established, recursion issues could prevent this
 technique from working.

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 request/response pairs.
 Once the IKE_SA is set up, either end of the security association may
 initiate requests at any time, and there can be many requests and
 responses "in flight" at any given moment.  But each message is
 labeled as either a request or a response, and for each
 request/response pair one end of the security association is the
 initiator and the other is the responder.
 For every pair of 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 triggers a retransmission of the
 response.  The initiator MUST remember each request until it receives
 the corresponding response.  The responder MUST remember each
 response until it receives a request whose sequence number is larger
 than the sequence number in the response plus its window size (see
 section 2.3).

Kaufman Standards Track [Page 13] RFC 4306 IKEv2 December 2005

 IKE is a reliable protocol, in the sense that the initiator MUST
 retransmit a request until either it receives a corresponding reply
 OR it deems the IKE security association to have failed and it
 discards all state associated with the IKE_SA and any CHILD_SAs
 negotiated using that IKE_SA.

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.
 The Message ID is a 32-bit quantity, which is zero for the first IKE
 request in each direction.  The IKE_SA initial setup messages will
 always be numbered 0 and 1.  Each endpoint in the IKE Security
 Association maintains two "current" Message IDs: the next one to be
 used for a request it initiates and the next one it expects to see 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 very
 different numbers of requests, the Message IDs in the two directions
 can be very different.  There is no ambiguity in the messages,
 however, because the (I)nitiator and (R)esponse bits in the message
 header specify which of the four messages a particular one is.
 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.  Rekeying an IKE_SA resets the sequence numbers.

2.3. Window Size for Overlapping Requests

 In order to maximize IKE throughput, an IKE endpoint MAY issue
 multiple requests before getting a response to any of them if the
 other endpoint has indicated its ability to handle such requests.
 For simplicity, an IKE implementation MAY choose to process requests
 strictly in order and/or wait for a response to one request before
 issuing another.  Certain rules must be followed to ensure
 interoperability between implementations using different strategies.
 After an IKE_SA is set up, either end can initiate one or more
 requests.  These requests may pass one another over the network.  An
 IKE endpoint MUST be prepared to accept and process a request while

Kaufman Standards Track [Page 14] RFC 4306 IKEv2 December 2005

 it has a request outstanding in order to avoid a deadlock in this
 situation.  An IKE endpoint SHOULD be prepared to accept and process
 multiple requests while it has a request outstanding.
 An IKE endpoint MUST wait for a response to each of its messages
 before sending a subsequent message unless it has received a
 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.
 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 SHOULD be
 capable of processing incoming requests out of order to maximize
 performance in the event of network failures or packet reordering.

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.
 Since IKE is designed to operate in spite of Denial of Service (DoS)
 attacks from the network, an endpoint MUST NOT conclude that the
 other endpoint has failed based on any routing information (e.g.,
 ICMP messages) or IKE messages that arrive without cryptographic
 protection (e.g., Notify messages complaining about unknown SPIs).
 An endpoint MUST conclude that the other endpoint has failed only
 when repeated attempts to contact it have gone unanswered for a
 timeout period or when a cryptographically protected INITIAL_CONTACT
 notification is received on a different IKE_SA to the same
 authenticated identity.  An endpoint SHOULD suspect that the other
 endpoint has failed based on routing information and initiate a
 request to see whether the other endpoint is alive.  To check whether
 the other side is alive, IKE specifies an empty INFORMATIONAL message

Kaufman Standards Track [Page 15] RFC 4306 IKEv2 December 2005

 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 message has been
 received from the other side recently, unprotected notifications MAY
 be ignored.  Implementations MUST limit the rate at which they take
 actions based on unprotected messages.
 Numbers of retries and lengths of timeouts are not covered in this
 specification because they do not affect interoperability.  It is
 suggested that messages be retransmitted at least a dozen times over
 a period of at least several minutes before giving up on an SA, but
 different environments may require different rules.  To be a good
 network citizen, retranmission 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.
 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 MUST NOT continue sending on
 any SA if some failure prevents it from receiving on all of the
 associated SAs.  If CHILD_SAs can fail independently from one another
 without the associated IKE_SA being able to send a delete message,
 then they MUST be negotiated by separate IKE_SAs.
 There is a Denial of Service attack on the initiator of an IKE_SA
 that can be avoided if the initiator takes the proper care.  Since
 the first two messages of an SA setup are not cryptographically
 protected, an attacker could respond to the initiator's message
 before the genuine responder and poison the connection setup attempt.
 To prevent this, the initiator MAY be willing to accept multiple
 responses to its first message, treat each as potentially legitimate,
 respond to it, and then discard all the invalid half-open connections
 when 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.

Kaufman Standards Track [Page 16] RFC 4306 IKEv2 December 2005

 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.

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 zero.  It is likely that
 some implementations will want to support both version 1.0 and
 version 2.0, and in the future, other versions.
 The major version number should 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
 notification message.  The node with the larger minor version number
 would simply note that its correspondent would not be able to
 understand that message and therefore would not send it.
 If an endpoint receives a message with a higher major version number,
 it MUST drop the message and SHOULD send an unauthenticated
 notification message containing the highest version number it
 supports.  If an endpoint supports major version n, and major version
 m, it MUST support all versions between n and m.  If it receives a
 message with a major version that it supports, it MUST respond with
 that version number.  In order to prevent two nodes from being
 tricked into corresponding with a lower major version number than the
 maximum that they both support, IKE has a flag that indicates that
 the node is capable of speaking a higher major version number.
 Thus, the major version number in the IKE header indicates the
 version number of the message, not the highest version number that
 the transmitter supports.  If 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 the flag indicating its ability to speak a higher
 version.  If they mistakenly (perhaps through an active attacker

Kaufman Standards Track [Page 17] RFC 4306 IKEv2 December 2005

 sending error messages) negotiate to version n, then both will notice
 that the other side can support a higher version number, and they
 MUST break the connection and reconnect using version n+1.
 Note that IKEv1 does not follow these rules, because there is no way
 in v1 of noting that you are capable of speaking a higher version
 number.  So an active attacker can trick two v2-capable nodes into
 speaking v1.  When a v2-capable node negotiates down to v1, it SHOULD
 note that fact in its logs.
 Also for forward compatibility, all fields marked RESERVED MUST be
 set to zero by a version 2.0 implementation and their content MUST be
 ignored by a version 2.0 implementation ("Be conservative in what you
 send and liberal in what you receive").  In this way, future versions
 of the protocol can use those fields in a way that is guaranteed to
 be ignored by implementations that do not understand them.
 Similarly, payload types that are not defined are reserved for future
 use; implementations of version 2.0 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.  If the critical flag is
 not set and the payload type is unsupported, that payload MUST be
 ignored.
 Although new payload types may be added in the future and may appear
 interleaved with the fields defined in this specification,
 implementations MUST send the payloads defined in this specification
 in the order shown in the figures in section 2 and implementations
 SHOULD reject as invalid a message with those payloads in any other
 order.

2.6. Cookies

 The term "cookies" originates with Karn and Simpson [RFC2522] in
 Photuris, an early proposal for key management with IPsec, and it has
 persisted.  The Internet Security Association and Key Management
 Protocol (ISAKMP) [MSST98] fixed message header includes two eight-
 octet fields titled "cookies", and that syntax is used by both IKEv1
 and IKEv2 though in IKEv2 they are referred to as the IKE SPI and
 there is a new separate field in a Notify payload holding the cookie.
 The initial two eight-octet fields in the header are used as a
 connection identifier at the beginning of IKE packets.  Each endpoint

Kaufman Standards Track [Page 18] RFC 4306 IKEv2 December 2005

 chooses one of the two SPIs and SHOULD choose them so as to be unique
 identifiers of an IKE_SA.  An SPI value of zero is special and
 indicates that the remote SPI value is not yet known by the sender.
 Unlike ESP and AH where only the recipient's SPI appears in the
 header of a message, in IKE the sender's SPI is also sent in every
 message.  Since the SPI chosen by the original initiator of the
 IKE_SA is always sent first, an endpoint with multiple IKE_SAs open
 that wants to find the appropriate IKE_SA using the SPI it assigned
 must look at the I(nitiator) Flag bit in the header to determine
 whether it assigned the first or the second eight octets.
 In the first message of an initial IKE exchange, the initiator will
 not know the responder's SPI value and will therefore set that field
 to zero.
 An expected attack against IKE is state and CPU exhaustion, where the
 target is flooded with session initiation requests from forged IP
 addresses.  This attack can be made less effective if an
 implementation of a responder uses minimal CPU and commits no state
 to an SA until it knows the initiator can receive packets at the
 address from which it claims to be sending them.  To accomplish this,
 a responder SHOULD -- when it detects a large number of half-open
 IKE_SAs -- reject initial IKE messages unless they contain a Notify
 payload of type COOKIE.  It SHOULD instead send an unprotected IKE
 message as a response and include COOKIE Notify payload with the
 cookie data to be returned.  Initiators who receive such responses
 MUST retry the IKE_SA_INIT with a Notify payload of type COOKIE
 containing the responder supplied cookie 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}

Kaufman Standards Track [Page 19] RFC 4306 IKEv2 December 2005

 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.
 An IKE implementation SHOULD 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 they use 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 into the hash
 ensures that an attacker who sees only message 2 can't successfully
 forge a message 3.
 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 denial of service
 protection.  The responder SHOULD change the value of <secret>
 frequently, especially if under attack.

Kaufman Standards Track [Page 20] RFC 4306 IKEv2 December 2005

2.7. Cryptographic Algorithm Negotiation

 The payload type known as "SA" indicates a proposal for a set of
 choices of IPsec protocols (IKE, ESP, and/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 or more protocols (usually one).  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 identifier 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 or more protocols.  If a proposal is
    accepted, the SA response MUST contain the same protocols in the
    same order as the proposal.  The responder MUST accept a single
    proposal or reject them all and return an error. (Example: if a
    single proposal contains ESP and AH and that proposal is accepted,
    both ESP and AH MUST be accepted.  If ESP and AH are included in
    separate proposals, the responder MUST accept only one of them).
    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.
 Since 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.

Kaufman Standards Track [Page 21] RFC 4306 IKEv2 December 2005

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
 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 SHOULD 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.  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.  Use the new IKE_SA for all control messages needed to
 maintain the CHILD_SAs created by the old IKE_SA, and delete the old
 IKE_SA.  The Delete payload to delete itself MUST be the last request
 sent over an IKE_SA.
 SAs SHOULD be rekeyed proactively, i.e., the new SA should be
 established before the old one expires and becomes unusable.  Enough
 time should elapse between the time the new SA is established and the
 old one becomes unusable so that traffic can be switched over to the
 new SA.
 A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
 were negotiated.  In IKEv2, each end of the SA is responsible for
 enforcing its own lifetime policy on the SA and rekeying the SA when
 necessary.  If the two ends have different lifetime policies, the end
 with the shorter lifetime will end up always being the one to request
 the rekeying.  If an SA bundle has been inactive for a long time and
 if an endpoint would not initiate the SA in the absence of traffic,
 the endpoint MAY choose to close the SA instead of rekeying it when
 its lifetime expires.  It SHOULD do so if there has been no traffic
 since the last time the SA was rekeyed.

Kaufman Standards Track [Page 22] RFC 4306 IKEv2 December 2005

 If the two ends have the same lifetime policies, it is possible that
 both will initiate a rekeying at the same time (which will result in
 redundant SAs).  To reduce the probability of this happening, the
 timing of rekeying requests SHOULD be jittered (delayed by a random
 amount of time after the need for rekeying is noticed).
 This form of rekeying may temporarily result in multiple similar SAs
 between the same pairs of nodes.  When there are two SAs eligible to
 receive packets, a node MUST accept incoming packets through either
 SA.  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.
 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 [RFC2474], [RFC2475], and section 4.1 of [RFC2983]).
 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.
 The node that initiated the surviving rekeyed SA SHOULD delete the
 replaced SA after the new one is established.
 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 want to 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 new SA, 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 SHOULD continue to
 send messages on the old SA until one of those events occurs.  When
 establishing a new SA, the responder MAY defer sending messages on a

Kaufman Standards Track [Page 23] RFC 4306 IKEv2 December 2005

 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 SHOULD interpret that as
 a likely packet loss and retransmit the CREATE_CHILD_SA request.  An
 initiator MAY send a dummy message on a newly created SA if it has no
 messages queued in order to assure the responder that the initiator
 is ready to receive messages.

2.9. Traffic Selector Negotiation

 When an IP packet is received by an RFC4301-compliant IPsec subsystem
 and matches a "protect" selector in its Security Policy Database
 (SPD), the subsystem MUST protect that packet with IPsec.  When no SA
 exists yet, it is the task of IKE to create it.  Maintenance of a
 system's SPD is outside the scope of IKE (see [PFKEY] for an example
 protocol), though some implementations might update their SPD in
 connection with the running of IKE (for an example scenario, see
 section 1.1.3).
 Traffic Selector (TS) payloads allow endpoints to communicate some of
 the information from their SPD to their peers.  TS payloads specify
 the selection criteria for packets that will be forwarded over the
 newly set up SA.  This can serve as a consistency check in some
 scenarios to assure that the SPDs are consistent.  In others, it
 guides the dynamic update of the SPD.
 Two TS payloads appear in each of the messages in the exchange that
 creates a CHILD_SA pair.  Each TS payload contains one or more
 Traffic Selectors.  Each Traffic Selector consists of an address
 range (IPv4 or IPv6), a port range, and an IP protocol ID.  In
 support of the scenario described in section 1.1.3, an initiator may
 request that the responder assign an IP address and tell the
 initiator what it is.
 IKEv2 allows the responder to choose a subset of the traffic proposed
 by the initiator.  This could happen when the 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.
 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

Kaufman Standards Track [Page 24] RFC 4306 IKEv2 December 2005

 forwarded to (or the source address of the traffic forwarded from)
 the responder of the CHILD_SA pair.  For example, if the original
 initiator request the creation of a CHILD_SA pair, and wishes to
 tunnel all traffic from subnet 192.0.1.* 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 (192.0.1.0 - 192.0.1.255) 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.  (Note: The IP address range 192.0.2.* has been reserved for
 use in examples in RFCs and similar documents.  This document needed
 two such ranges, and so also used 192.0.1.*. This should not be
 confused with any actual address.)
 The responder is allowed to narrow the choices by selecting a subset
 of the traffic, for instance by eliminating or narrowing the range of
 one or more members of the set of traffic selectors, provided the set
 does not become the NULL set.
 It is possible for the responder's policy to contain multiple smaller
 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 generated its
 request in response to an incoming packet from 192.0.1.43 to
 192.0.2.123, 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 status of
 SINGLE_PAIR_REQUIRED.
 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 (192.0.1.43 - 192.0.1.43) and the source port and IP
 protocol from the packet and the second containing (192.0.1.0 -
 192.0.1.255) with all ports and IP protocols.  The initiator would
 similarly include two traffic selectors in TSr.
 If the responder's policy does not allow it to accept the entire set
 of traffic selectors in the initiator's request, but does allow him
 to accept the first selector of TSi and TSr, then the responder MUST
 narrow the traffic selectors to a subset that includes the

Kaufman Standards Track [Page 25] RFC 4306 IKEv2 December 2005

 initiator's first choices.  In this example, the responder might
 respond with TSi being (192.0.1.43 - 192.0.1.43) with all ports and
 IP protocols.
 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 MAY be
 ranges rather than specific values, and the responder chooses a
 subset of the initiator's TSi and TSr that are acceptable.  If more
 than one subset is acceptable but their union is not, the responder
 MUST accept some subset and MAY include a Notify payload of type
 ADDITIONAL_TS_POSSIBLE to indicate that the initiator might want to
 try again.  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.  Such misconfigurations SHOULD be recorded in error logs.

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 pseudo-
 random 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 prf. ("prf" refers to
 "pseudo-random function", one of the cryptographic algorithms
 negotiated in the IKE exchange.)  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 it runs over.  The IP
 addresses and ports in the outer header are, however, not themselves
 cryptographically protected, and IKE is designed to work even through
 Network Address Translation (NAT) boxes.  An implementation MUST
 accept incoming 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.

Kaufman Standards Track [Page 26] RFC 4306 IKEv2 December 2005

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
 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.  In particular, it MUST forget the secrets used in the
 Diffie-Hellman calculation and any state that may persist in the
 state of a pseudo-random number generator that could be used to
 recompute the Diffie-Hellman secrets.
 Since the computing of Diffie-Hellman exponentials is computationally
 expensive, an endpoint may find it advantageous to reuse those
 exponentials for multiple connection setups.  There are several
 reasonable strategies for doing this.  An endpoint could choose a new
 exponential only periodically though this could result in less-than-
 perfect forward secrecy if some connection lasts for less than the
 lifetime of the exponential.  Or it could keep track of which
 exponential was used for each connection and delete the information
 associated with the exponential only when some corresponding
 connection was closed.  This would allow the exponential to be reused
 without losing perfect forward secrecy at the cost of maintaining
 more state.
 Decisions as to whether and when to reuse Diffie-Hellman exponentials
 is a private decision in the sense that it will not affect
 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.

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 pseudo-random function
 (prf).  The pseudo-random function is used for the construction of
 keying material for all of the cryptographic algorithms used in both
 the IKE_SA and the CHILD_SAs.

Kaufman Standards Track [Page 27] RFC 4306 IKEv2 December 2005

 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.  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.  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.  When the prf function takes a variable
 length key, variable length data, and produces a fixed-length output
 (e.g., when using HMAC), the formulas in this document apply.  When
 the key for the prf function has fixed length, the data provided as a
 key is truncated or padded with zeros as necessary unless exceptional
 processing is explained following the formula.
 Keying material will always be derived as the output of the
 negotiated prf algorithm.  Since the amount of keying material needed
 may be greater than the size of the output of the prf algorithm, we
 will use the prf iteratively.  We will use the terminology prf+ to
 describe the function that outputs a pseudo-random stream based on
 the inputs to a prf as follows: (where | indicates concatenation)
 prf+ (K,S) = T1 | T2 | T3 | T4 | ...
 where:
 T1 = prf (K, S | 0x01)
 T2 = prf (K, T1 | S | 0x02)
 T3 = prf (K, T2 | S | 0x03)
 T4 = prf (K, T3 | S | 0x04)
 continuing as needed to compute all required keys.  The keys are
 taken from the output string without regard to boundaries (e.g., if
 the required keys are a 256-bit 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 string feeding the prf
 is a single octet. prf+ in this document is not defined beyond 255
 times the size of the prf 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

Kaufman Standards Track [Page 28] RFC 4306 IKEv2 December 2005

 exchange.  SKEYSEED is used to calculate seven other secrets: SK_d
 used for deriving new keys for the CHILD_SAs established with this
 IKE_SA; SK_ai and SK_ar used as a key to the 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.
 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.  If the
 negotiated prf takes a fixed-length key and the lengths of Ni and Nr
 do not add up to that length, half the bits must come from Ni and
 half from Nr, taking the first bits of each.
 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.  Each algorithm takes a fixed number of bits of keying
 material, which is specified as part of the algorithm.  For integrity
 algorithms based on a keyed hash, the key size is always equal to the
 length of the output of the underlying hash function.

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 shared
 secret as the key) a block of data.  For the responder, the octets to
 be signed start with the first octet of the first SPI in the header
 of the second message and end with the last octet of the last payload
 in the second message.  Appended to this (for purposes of computing
 the signature) are the initiator's nonce Ni (just the value, not the
 payload containing it), and the value prf(SK_pr,IDr') where IDr' is
 the responder's ID payload excluding the fixed header.  Note that
 neither the nonce Ni nor the value prf(SK_pr,IDr') are transmitted.
 Similarly, the initiator signs the first message, starting with the
 first octet of the first SPI in the header and ending with the last
 octet of the last payload.  Appended to this (for purposes of

Kaufman Standards Track [Page 29] RFC 4306 IKEv2 December 2005

 computing the signature) are the responder's nonce Nr, and the value
 prf(SK_pi,IDi').  In the above calculation, IDi' and IDr' are the
 entire ID payloads excluding the fixed header.  It is critical to the
 security of the exchange that each side sign the other side's nonce.
 Note that all of the payloads are included under the signature,
 including any payload types not defined in this document.  If the
 first message of the exchange is sent twice (the second time with a
 responder cookie and/or a different Diffie-Hellman group), it is the
 second version of the message that is signed.
 Optionally, messages 3 and 4 MAY include a certificate, or
 certificate chain providing evidence that the key used to compute a
 digital signature belongs to the name in the ID payload.  The
 signature or MAC will be computed using algorithms dictated by the
 type of key used by the signer, 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 that the same key
 is used in both directions.  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 SHOULD contain as much unpredictability as the strongest
 key being negotiated.  In the case of a pre-shared key, the AUTH
 value is computed as:
    AUTH = prf(prf(Shared Secret,"Key Pad for IKEv2"), <msg octets>)
 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

Kaufman Standards Track [Page 30] RFC 4306 IKEv2 December 2005

 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.  If the negotiated prf takes a
 fixed-size key, the shared secret MUST be of that fixed size.

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 memo references [EAP] with the intent that new methods can
 be added in the future without updating this specification, some
 simpler variations are documented here and in section 3.16.  [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 extensible authentication by
 leaving out the AUTH payload from message 3.  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 extensible authentication method, it will place an Extensible
 Authentication Protocol (EAP) payload in message 4 and defer sending
 SAr2, TSi, and TSr until initiator authentication is complete in a
 subsequent IKE_AUTH exchange.  In the case of a minimal extensible
 authentication, the initial SA establishment will appear as follows:

Kaufman Standards Track [Page 31] RFC 4306 IKEv2 December 2005

     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 }
 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.  The
 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 SHOULD 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.

Kaufman Standards Track [Page 32] RFC 4306 IKEv2 December 2005

 Following such an extended exchange, the EAP AUTH payloads MUST be
 included in the two messages following the one containing the EAP
 Success message.

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),
 and four SAs could be created in a single CHILD_SA negotiation if a
 combination of ESP and AH is being negotiated.
 Keying material MUST be taken from the expanded KEYMAT in the
 following order:
    All keys for SAs carrying data from the initiator to the responder
    are taken before SAs going in the reverse direction.
    If multiple IPsec protocols are negotiated, keying material is
    taken in the order in which the protocol headers will appear in
    the encapsulated packet.
    If a single protocol has both encryption and authentication keys,
    the encryption key is taken from the first octets of KEYMAT and
    the authentication key is taken from the next octets.
 Each cryptographic algorithm takes a fixed number of bits of keying
 material specified as part of the algorithm.

Kaufman Standards Track [Page 33] RFC 4306 IKEv2 December 2005

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 section 2.8).  New initiator and responder SPIs are supplied in
 the SPI fields.  The TS payloads are omitted when rekeying an IKE_SA.
 SKEYSEED for the new IKE_SA is computed using SK_d from the existing
 IKE_SA as follows:
     SKEYSEED = prf(SK_d (old), [g^ir (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 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.

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
 assigned.  A request for such a temporary address can be included in
 any request to create a CHILD_SA (including the implicit request in
 message 3) by including a CP payload.
 This function provides address allocation to an 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 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}

Kaufman Standards Track [Page 34] RFC 4306 IKEv2 December 2005

 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.
 For example, message from initiator to responder:
    CP(CFG_REQUEST)=
      INTERNAL_ADDRESS(0.0.0.0)
      INTERNAL_NETMASK(0.0.0.0)
      INTERNAL_DNS(0.0.0.0)
    TSi = (0, 0-65535,0.0.0.0-255.255.255.255)
    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 IKE exchange with a 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.

Kaufman Standards Track [Page 35] RFC 4306 IKEv2 December 2005

 An IKE implementation MAY decline to give out version information
 prior to authentication or even after authentication to prevent
 trolling in case some implementation is known to have some security
 weakness.  In that case, it MUST 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("")
    CP(CFG_REPLY) APPLICATION_VERSION("foobar v1.3beta, (c) Foo Bar
      Inc.")

2.21. Error Handling

 There are many kinds of errors that can occur during IKE processing.
 If a request is received that is badly formatted or unacceptable for
 reasons of policy (e.g., no matching cryptographic algorithms), the
 response MUST contain a Notify payload indicating the error.  If an
 error occurs outside the context of an IKE request (e.g., the node is
 getting ESP messages on a nonexistent SPI), the node SHOULD initiate
 an INFORMATIONAL exchange with a Notify payload describing the
 problem.
 Errors that occur before a cryptographically protected IKE_SA is
 established must be handled very carefully.  There is a trade-off
 between wanting to be helpful in diagnosing a problem and responding
 to it and wanting to avoid being a dupe in a denial of service attack
 based on forged messages.
 If a node receives a message on UDP port 500 or 4500 outside the
 context of an IKE_SA known to it (and not a request to start one), it
 may be the result of a recent crash of the node.  If the message is
 marked as a response, the node MAY audit the suspicious event but
 MUST NOT respond.  If the message is marked as a request, the node
 MAY audit the suspicious event and MAY send a response.  If a
 response is sent, the response MUST be sent to the IP address and
 port from whence it came with the same IKE SPIs and the Message ID
 copied.  The response MUST NOT be cryptographically protected and
 MUST contain a Notify payload indicating INVALID_IKE_SPI.
 A node receiving such an unprotected Notify payload MUST NOT respond
 and MUST NOT change the state of any existing SAs.  The message might
 be a forgery or might be a response the genuine correspondent was

Kaufman Standards Track [Page 36] RFC 4306 IKEv2 December 2005

 tricked into sending.  A node SHOULD treat such a message (and also a
 network message like ICMP destination unreachable) as a hint that
 there might be problems with SAs to that IP address and SHOULD
 initiate a liveness test for any such IKE_SA.  An implementation
 SHOULD limit the frequency of such tests to avoid being tricked into
 participating in a denial of service attack.
 A node receiving a suspicious message from an IP address with which
 it has an IKE_SA MAY send an IKE Notify payload in an IKE
 INFORMATIONAL exchange over that SA.  The recipient MUST NOT change
 the state of any SA's as a result but SHOULD audit the event to aid
 in diagnosing malfunctions.  A node MUST limit the rate at which it
 will send messages in response to unprotected messages.

2.22. IPComp

 Use of IP compression [IPCOMP] can be negotiated as part of the setup
 of a CHILD_SA.  While IP compression involves an extra header in each
 packet and a compression parameter index (CPI), the virtual
 "compression association" has no life outside the ESP or AH SA that
 contains it.  Compression associations disappear when the
 corresponding ESP or AH SA goes away.  It is not explicitly mentioned
 in any DELETE payload.
 Negotiation of IP compression is separate from the negotiation of
 cryptographic parameters associated with a CHILD_SA.  A node
 requesting a CHILD_SA MAY advertise its support for one or more
 compression algorithms through one or more Notify payloads of type
 IPCOMP_SUPPORTED.  The response MAY indicate acceptance of a single
 compression algorithm with a Notify payload of type IPCOMP_SUPPORTED.
 These payloads MUST NOT occur in messages that do not contain SA
 payloads.
 Although there has been discussion of allowing multiple compression
 algorithms to be accepted and to have different compression
 algorithms available for the two directions of a CHILD_SA,
 implementations of this specification MUST NOT accept an IPComp
 algorithm that was not proposed, MUST NOT accept more than one, and
 MUST NOT compress using an algorithm other than one proposed and
 accepted in the setup of the CHILD_SA.
 A side effect of separating the negotiation of IPComp from
 cryptographic parameters is that it is not possible to propose
 multiple cryptographic suites and propose IP compression with some of
 them but not others.

Kaufman Standards Track [Page 37] RFC 4306 IKEv2 December 2005

2.23. NAT Traversal

 Network Address Translation (NAT) gateways are a controversial
 subject.  This section briefly describes what they are and how they
 are likely to act on IKE traffic.  Many people believe that NATs are
 evil and that we should not design our protocols so as to make them
 work better.  IKEv2 does specify some unintuitive processing rules in
 order that NATs are more likely to work.
 NATs exist primarily because of the shortage of IPv4 addresses,
 though there are other rationales.  IP nodes that are "behind" a NAT
 have IP addresses that are not globally unique, but rather are
 assigned from some space that is unique within the network behind the
 NAT but that are likely to be reused by nodes behind other NATs.
 Generally, nodes behind NATs can communicate with other nodes behind
 the same NAT and with nodes with globally unique addresses, but not
 with nodes behind other NATs.  There are exceptions to that rule.
 When those nodes make connections to nodes on the real Internet, the
 NAT gateway "translates" the IP source address to an address that
 will be routed back to the gateway.  Messages to the gateway from the
 Internet have their destination addresses "translated" to the
 internal address that will route the packet to the correct endnode.
 NATs are designed to be "transparent" to endnodes.  Neither software
 on the node behind the NAT nor the node on the Internet requires
 modification to communicate through the NAT.  Achieving this
 transparency is more difficult with some protocols than with others.
 Protocols that include IP addresses of the endpoints within the
 payloads of the packet will fail unless the NAT gateway understands
 the protocol and modifies the internal references as well as those in
 the headers.  Such knowledge is inherently unreliable, is a network
 layer violation, and often results in subtle problems.
 Opening an IPsec connection through a NAT introduces special
 problems.  If the connection runs in transport mode, changing the IP
 addresses on packets will cause the checksums to fail and the NAT
 cannot correct the checksums because they are cryptographically
 protected.  Even in tunnel mode, there are routing problems because
 transparently translating the addresses of AH and ESP packets
 requires special logic in the NAT and that logic is heuristic and
 unreliable in nature.  For that reason, IKEv2 can negotiate UDP
 encapsulation of IKE and ESP packets.  This encoding is slightly less
 efficient but is easier for NATs to process.  In addition, firewalls
 may be configured to pass IPsec traffic over UDP but not ESP/AH or
 vice versa.

Kaufman Standards Track [Page 38] RFC 4306 IKEv2 December 2005

 It is a common practice of NATs to translate TCP and UDP port numbers
 as well as addresses and use the port numbers of inbound packets to
 decide which internal node should get a given packet.  For this
 reason, even though IKE packets MUST be sent from and to UDP port
 500, they MUST be accepted coming from any port and responses MUST be
 sent to the port from whence they came.  This is because the ports
 may be modified as the packets pass through NATs.  Similarly, IP
 addresses of the IKE endpoints are generally not included in the IKE
 payloads because the payloads are cryptographically protected and
 could not be transparently modified by NATs.
 Port 4500 is reserved for UDP-encapsulated ESP and IKE.  When working
 through a NAT, it is generally better to pass IKE packets over port
 4500 because some older NATs handle IKE traffic on port 500 cleverly
 in an attempt to transparently establish IPsec connections between
 endpoints that don't handle NAT traversal themselves.  Such NATs may
 interfere with the straightforward NAT traversal envisioned by this
 document, so an IPsec endpoint that discovers a NAT between it and
 its correspondent MUST send all subsequent traffic to and from port
 4500, which NATs should not treat specially (as they might with port
 500).
 The specific requirements for supporting NAT traversal [RFC3715] are
 listed below.  Support for NAT traversal is optional.  In this
 section only, requirements listed as MUST apply only to
 implementations supporting NAT traversal.
    IKE MUST listen on port 4500 as well as port 500.  IKE MUST
    respond to the IP address and port from which packets arrived.
    Both IKE initiator and responder MUST include in their IKE_SA_INIT
    packets Notify payloads of type NAT_DETECTION_SOURCE_IP and
    NAT_DETECTION_DESTINATION_IP.  Those payloads can be used to
    detect if there is NAT between the hosts, and which end is behind
    the NAT.  The location of the payloads in the IKE_SA_INIT packets
    are just after the Ni and Nr payloads (before the optional CERTREQ
    payload).
    If none of the NAT_DETECTION_SOURCE_IP payload(s) received matches
    the hash of the source IP and port found from the IP header of the
    packet containing the payload, it means that the other end is
    behind NAT (i.e., someone along the route changed the source
    address of the original packet to match the address of the NAT
    box).  In this case, this end should allow dynamic update of the
    other ends IP address, as described later.

Kaufman Standards Track [Page 39] RFC 4306 IKEv2 December 2005

    If the NAT_DETECTION_DESTINATION_IP payload received does not
    match the hash of the destination IP and port found from the IP
    header of the packet containing the payload, it means that this
    end is behind a NAT.  In this case, this end SHOULD start sending
    keepalive packets as explained in [Hutt05].
    The IKE initiator MUST check these payloads if present and if they
    do not match the addresses in the outer packet MUST tunnel all
    future IKE and ESP packets associated with this IKE_SA over UDP
    port 4500.
    To tunnel IKE packets over UDP port 4500, the IKE header has four
    octets of zero prepended and the result immediately follows the
    UDP header.  To tunnel ESP packets over UDP port 4500, the ESP
    header immediately follows the UDP header.  Since the first four
    bytes of the ESP header contain the SPI, and the SPI cannot
    validly be zero, it is always possible to distinguish ESP and IKE
    messages.
    The original source and destination IP address required for the
    transport mode TCP and UDP packet checksum fixup (see [Hutt05])
    are obtained from the Traffic Selectors associated with the
    exchange.  In the case of NAT traversal, the Traffic Selectors
    MUST contain exactly one IP address, which is then used as the
    original IP address.
    There are cases where a NAT box decides to remove mappings that
    are still alive (for example, the keepalive interval is too long,
    or the NAT box is rebooted).  To recover in these cases, hosts
    that are not behind a NAT SHOULD send all packets (including
    retransmission packets) to the IP address and port from the last
    valid authenticated packet from the other end (i.e., dynamically
    update the address).  A host behind a NAT SHOULD NOT do this
    because it opens a DoS attack possibility.  Any authenticated IKE
    packet or any authenticated UDP-encapsulated ESP packet can be
    used to detect that the IP address or the port has changed.
    Note that similar but probably not identical actions will likely
    be needed to make IKE work with Mobile IP, but such processing is
    not addressed by this document.

2.24. Explicit Congestion Notification (ECN)

 When IPsec tunnels behave as originally specified in [RFC2401], ECN
 usage is not appropriate for the outer IP headers because tunnel
 decapsulation processing discards ECN congestion indications to the
 detriment of the network.  ECN support for IPsec tunnels for IKEv1-
 based IPsec requires multiple operating modes and negotiation (see

Kaufman Standards Track [Page 40] RFC 4306 IKEv2 December 2005

 [RFC3168]).  IKEv2 simplifies this situation by requiring that ECN be
 usable in the outer IP headers of all tunnel-mode IPsec SAs created
 by IKEv2.  Specifically, tunnel encapsulators and decapsulators for
 all tunnel-mode SAs created by IKEv2 MUST support the ECN full-
 functionality option for tunnels specified in [RFC3168] and MUST
 implement the tunnel encapsulation and decapsulation processing
 specified in [RFC4301] to prevent discarding of ECN congestion
 indications.

3. Header and Payload Formats

3.1. The IKE Header

 IKE messages use UDP ports 500 and/or 4500, with one IKE message per
 UDP datagram.  Information from the beginning of the packet through
 the UDP header is largely ignored except that the IP addresses and
 UDP ports from the headers are reversed and used for return packets.
 When sent on UDP port 500, IKE messages begin immediately following
 the UDP header.  When sent on UDP port 4500, IKE messages have
 prepended four octets of zero.  These four octets of zero are not
 part of the IKE message and are not included in any of the length
 fields or checksums defined by IKE.  Each IKE message begins with the
 IKE header, denoted HDR in this memo.  Following the header are one
 or more IKE payloads each identified by a "Next Payload" field in the
 preceding payload.  Payloads are processed in the order in which they
 appear in an IKE message by invoking the appropriate processing
 routine according to the "Next Payload" field in the IKE header and
 subsequently according to the "Next Payload" field in the IKE payload
 itself until a "Next Payload" field of zero indicates that no
 payloads follow.  If a payload of type "Encrypted" is found, that
 payload is decrypted and its contents parsed as additional payloads.
 An Encrypted payload MUST be the last payload in a packet and an
 Encrypted payload MUST NOT contain another Encrypted payload.
 The Recipient SPI in the header identifies an instance of an IKE
 security association.  It is therefore possible for a single instance
 of IKE to multiplex distinct sessions with multiple peers.
 All multi-octet fields representing integers are laid out in big
 endian order (aka most significant byte first, or network byte
 order).
 The format of the IKE header is shown in Figure 4.

Kaufman Standards Track [Page 41] RFC 4306 IKEv2 December 2005

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                       IKE_SA Initiator's SPI                  !
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                       IKE_SA Responder's SPI                  !
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !  Next Payload ! MjVer ! MnVer ! Exchange Type !     Flags     !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                          Message ID                           !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                            Length                             !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 4:  IKE Header Format
    o  Initiator's SPI (8 octets) - A value chosen by the
       initiator to identify a unique IKE security association.  This
       value MUST NOT be zero.
    o  Responder's SPI (8 octets) - A value chosen by the
       responder to identify a unique IKE security association.  This
       value MUST be zero in the first message of an IKE Initial
       Exchange (including repeats of that message including a
       cookie) and MUST NOT be zero in any other message.
    o  Next Payload (1 octet) - Indicates the type of payload that
       immediately follows the header.  The format and value of each
       payload are defined below.
    o  Major Version (4 bits) - Indicates the major version of the IKE
       protocol in use.  Implementations based on this version of IKE
       MUST set the Major Version to 2.  Implementations based on
       previous versions of IKE and ISAKMP MUST set the Major Version
       to 1.  Implementations based on this version of IKE MUST reject
       or ignore messages containing a version number greater than
       2.
    o  Minor Version (4 bits) - Indicates the minor version of the
       IKE protocol in use.  Implementations based on this version of
       IKE MUST set the Minor Version to 0.  They MUST ignore the
       minor version number of received messages.
    o  Exchange Type (1 octet) - Indicates the type of exchange being
       used.  This constrains the payloads sent in each message and
       orderings of messages in an exchange.

Kaufman Standards Track [Page 42] RFC 4306 IKEv2 December 2005

                     Exchange Type            Value
                     RESERVED                 0-33
                     IKE_SA_INIT              34
                     IKE_AUTH                 35
                     CREATE_CHILD_SA          36
                     INFORMATIONAL            37
                     RESERVED TO IANA         38-239
                     Reserved for private use 240-255
    o  Flags (1 octet) - Indicates specific options that are set
       for the message.  Presence of options are indicated by the
       appropriate bit in the flags field being set.  The bits are
       defined LSB first, so bit 0 would be the least significant
       bit of the Flags octet.  In the description below, a bit
       being 'set' means its value is '1', while 'cleared' means
       its value is '0'.
  1. - X(reserved) (bits 0-2) - These bits MUST be cleared

when sending and MUST be ignored on receipt.

  1. - I(nitiator) (bit 3 of Flags) - This bit MUST be set in

messages sent by the original initiator of the IKE_SA

         and MUST be cleared in messages sent by the original
         responder.  It is used by the recipient to determine
         which eight octets of the SPI were generated by the
         recipient.
  1. - V(ersion) (bit 4 of Flags) - This bit indicates that

the transmitter is capable of speaking a higher major

         version number of the protocol than the one indicated
         in the major version number field.  Implementations of
         IKEv2 must clear this bit when sending and MUST ignore
         it in incoming messages.
  1. - R(esponse) (bit 5 of Flags) - This bit indicates that

this message is a response to a message containing

         the same message ID.  This bit MUST be cleared in all
         request messages and MUST be set in all responses.
         An IKE endpoint MUST NOT generate a response to a
         message that is marked as being a response.
  1. - X(reserved) (bits 6-7 of Flags) - These bits MUST be

cleared when sending and MUST be ignored on receipt.

Kaufman Standards Track [Page 43] RFC 4306 IKEv2 December 2005

    o  Message ID (4 octets) - Message identifier used to control
    retransmission of lost packets and matching of requests and
    responses.  It is essential to the security of the protocol
    because it is used to prevent message replay attacks.
    See sections 2.1 and 2.2.
    o  Length (4 octets) - Length of total message (header + payloads)
    in octets.

3.2. Generic Payload Header

 Each IKE payload defined in sections 3.3 through 3.16 begins with a
 generic payload header, shown in Figure 5.  Figures for each payload
 below will include the generic payload header, but for brevity the
 description of each field will be omitted.
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 5:  Generic Payload Header
 The Generic Payload Header fields are defined as follows:
 o  Next Payload (1 octet) - Identifier for the payload type of the
    next payload in the message.  If the current payload is the last
    in the message, then this field will be 0.  This field provides a
    "chaining" capability whereby additional payloads can be added to
    a message by appending it to the end of the message and setting
    the "Next Payload" field of the preceding payload to indicate the
    new payload's type.  An Encrypted payload, which must always be
    the last payload of a message, is an exception.  It contains data
    structures in the format of additional payloads.  In the header of
    an Encrypted payload, the Next Payload field is set to the payload
    type of the first contained payload (instead of 0).
    Payload Type Values
        Next Payload Type               Notation  Value
        No Next Payload                              0
        RESERVED                                   1-32
        Security Association             SA         33
        Key Exchange                     KE         34
        Identification - Initiator       IDi        35

Kaufman Standards Track [Page 44] RFC 4306 IKEv2 December 2005

        Identification - Responder       IDr        36
        Certificate                      CERT       37
        Certificate Request              CERTREQ    38
        Authentication                   AUTH       39
        Nonce                            Ni, Nr     40
        Notify                           N          41
        Delete                           D          42
        Vendor ID                        V          43
        Traffic Selector - Initiator     TSi        44
        Traffic Selector - Responder     TSr        45
        Encrypted                        E          46
        Configuration                    CP         47
        Extensible Authentication        EAP        48
        RESERVED TO IANA                          49-127
        PRIVATE USE                              128-255
    Payload type values 1-32 should not be used so that there is no
    overlap with the code assignments for IKEv1.  Payload type values
    49-127 are reserved to IANA for future assignment in IKEv2 (see
    section 6).  Payload type values 128-255 are for private use among
    mutually consenting parties.
 o  Critical (1 bit) - MUST be set to zero if the sender wants the
    recipient to skip this payload if it does not understand the
    payload type code in the Next Payload field of the previous
    payload.  MUST be set to one if the sender wants the recipient to
    reject this entire message if it does not understand the payload
    type.  MUST be ignored by the recipient if the recipient
    understands the payload type code.  MUST be set to zero for
    payload types defined in this document.  Note that the critical
    bit applies to the current payload rather than the "next" payload
    whose type code appears in the first octet.  The reasoning behind
    not setting the critical bit for payloads defined in this document
    is that all implementations MUST understand all payload types
    defined in this document and therefore must ignore the Critical
    bit's value.  Skipped payloads are expected to have valid Next
    Payload and Payload Length fields.
 o  RESERVED (7 bits) - MUST be sent as zero; MUST be ignored on
    receipt.
 o  Payload Length (2 octets) - Length in octets of the current
    payload, including the generic payload header.

Kaufman Standards Track [Page 45] RFC 4306 IKEv2 December 2005

3.3. Security Association Payload

 The Security Association Payload, denoted SA in this memo, is used to
 negotiate attributes of a security association.  Assembly of Security
 Association Payloads requires great peace of mind.  An SA payload MAY
 contain multiple proposals.  If there is more than one, they MUST be
 ordered from most preferred to least preferred.  Each proposal may
 contain multiple IPsec protocols (where a protocol is IKE, ESP, or
 AH), each protocol MAY contain multiple transforms, and each
 transform MAY contain multiple attributes.  When parsing an SA, an
 implementation MUST check that the total Payload Length is consistent
 with the payload's internal lengths and counts.  Proposals,
 Transforms, and Attributes each have their own variable length
 encodings.  They are nested such that the Payload Length of an SA
 includes the combined contents of the SA, Proposal, Transform, and
 Attribute information.  The length of a Proposal includes the lengths
 of all Transforms and Attributes it contains.  The length of a
 Transform includes the lengths of all Attributes it contains.
 The syntax of Security Associations, Proposals, Transforms, and
 Attributes is based on ISAKMP; however, the semantics are somewhat
 different.  The reason for the complexity and the hierarchy is to
 allow for multiple possible combinations of algorithms to be encoded
 in a single SA.  Sometimes there is a choice of multiple algorithms,
 whereas other times there is a combination of algorithms.  For
 example, an initiator might want to propose using (AH w/MD5 and ESP
 w/3DES) OR (ESP w/MD5 and 3DES).
 One of the reasons the semantics of the SA payload has changed from
 ISAKMP and IKEv1 is to make the encodings more compact in common
 cases.
 The Proposal structure contains within it a Proposal # and an IPsec
 protocol ID.  Each structure MUST have the same Proposal # as the
 previous one or be one (1) greater.  The first Proposal MUST have a
 Proposal # of one (1).  If two successive structures have the same
 Proposal number, it means that the proposal consists of the first
 structure AND the second.  So a proposal of AH AND ESP would have two
 proposal structures, one for AH and one for ESP and both would have
 Proposal #1.  A proposal of AH OR ESP would have two proposal
 structures, one for AH with Proposal #1 and one for ESP with Proposal
 #2.
 Each Proposal/Protocol structure is followed by one or more transform
 structures.  The number of different transforms is generally
 determined by the Protocol.  AH generally has a single transform: an
 integrity check algorithm.  ESP generally has two: an encryption
 algorithm and an integrity check algorithm.  IKE generally has four

Kaufman Standards Track [Page 46] RFC 4306 IKEv2 December 2005

 transforms: a Diffie-Hellman group, an integrity check algorithm, a
 prf algorithm, and an encryption algorithm.  If an algorithm that
 combines encryption and integrity protection is proposed, it MUST be
 proposed as an encryption algorithm and an integrity protection
 algorithm MUST NOT be proposed.  For each Protocol, the set of
 permissible transforms is assigned transform ID numbers, which appear
 in the header of each transform.
 If there are multiple transforms with the same Transform Type, the
 proposal is an OR of those transforms.  If there are multiple
 Transforms with different Transform Types, the proposal is an AND of
 the different groups.  For example, to propose ESP with (3DES or
 IDEA) and (HMAC_MD5 or HMAC_SHA), the ESP proposal would contain two
 Transform Type 1 candidates (one for 3DES and one for IDEA) and two
 Transform Type 2 candidates (one for HMAC_MD5 and one for HMAC_SHA).
 This effectively proposes four combinations of algorithms.  If the
 initiator wanted to propose only a subset of those, for example (3DES
 and HMAC_MD5) or (IDEA and HMAC_SHA), there is no way to encode that
 as multiple transforms within a single Proposal.  Instead, the
 initiator would have to construct two different Proposals, each with
 two transforms.
 A given transform MAY have one or more Attributes.  Attributes are
 necessary when the transform can be used in more than one way, as
 when an encryption algorithm has a variable key size.  The transform
 would specify the algorithm and the attribute would specify the key
 size.  Most transforms do not have attributes.  A transform MUST NOT
 have multiple attributes of the same type.  To propose alternate
 values for an attribute (for example, multiple key sizes for the AES
 encryption algorithm), and implementation MUST include multiple
 Transforms with the same Transform Type each with a single Attribute.
 Note that the semantics of Transforms and Attributes are quite
 different from those in IKEv1.  In IKEv1, a single Transform carried
 multiple algorithms for a protocol with one carried in the Transform
 and the others carried in the Attributes.
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                          <Proposals>                          ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 6:  Security Association Payload

Kaufman Standards Track [Page 47] RFC 4306 IKEv2 December 2005

    o  Proposals (variable) - One or more proposal substructures.
    The payload type for the Security Association Payload is thirty
    three (33).

3.3.1. Proposal Substructure

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! 0 (last) or 2 !   RESERVED    !         Proposal Length       !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Proposal #    !  Protocol ID  !    SPI Size   !# of Transforms!
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                        SPI (variable)                         ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                        <Transforms>                           ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 7:  Proposal Substructure
    o  0 (last) or 2 (more) (1 octet) - Specifies whether this is the
       last Proposal Substructure in the SA.  This syntax is inherited
       from ISAKMP, but is unnecessary because the last Proposal could
       be identified from the length of the SA.  The value (2)
       corresponds to a Payload Type of Proposal in IKEv1, and the
       first 4 octets of the Proposal structure are designed to look
       somewhat like the header of a Payload.
    o  RESERVED (1 octet) - MUST be sent as zero; MUST be ignored on
       receipt.
    o  Proposal Length (2 octets) - Length of this proposal, including
       all transforms and attributes that follow.
    o  Proposal # (1 octet) - When a proposal is made, the first
       proposal in an SA payload MUST be #1, and subsequent proposals
       MUST either be the same as the previous proposal (indicating an
       AND of the two proposals) or one more than the previous
       proposal (indicating an OR of the two proposals).  When a
       proposal is accepted, all of the proposal numbers in the SA
       payload MUST be the same and MUST match the number on the
       proposal sent that was accepted.

Kaufman Standards Track [Page 48] RFC 4306 IKEv2 December 2005

    o  Protocol ID (1 octet) - Specifies the IPsec protocol identifier
       for the current negotiation.  The defined values are:
        Protocol               Protocol ID
        RESERVED                0
        IKE                     1
        AH                      2
        ESP                     3
        RESERVED TO IANA        4-200
        PRIVATE USE             201-255
    o  SPI Size (1 octet) - For an initial IKE_SA negotiation, this
       field MUST be zero; the SPI is obtained from the outer header.
       During subsequent negotiations, it is equal to the size, in
       octets, of the SPI of the corresponding protocol (8 for IKE, 4
       for ESP and AH).
    o  # of Transforms (1 octet) - Specifies the number of transforms
       in this proposal.
    o  SPI (variable) - The sending entity's SPI. Even if the SPI Size
       is not a multiple of 4 octets, there is no padding applied to
       the payload.  When the SPI Size field is zero, this field is
       not present in the Security Association payload.
    o  Transforms (variable) - One or more transform substructures.

3.3.2. Transform Substructure

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! 0 (last) or 3 !   RESERVED    !        Transform Length       !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !Transform Type !   RESERVED    !          Transform ID         !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                      Transform Attributes                     ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 8:  Transform Substructure
    o  0 (last) or 3 (more) (1 octet) - Specifies whether this is the
       last Transform Substructure in the Proposal.  This syntax is
       inherited from ISAKMP, but is unnecessary because the last
       Proposal could be identified from the length of the SA.  The

Kaufman Standards Track [Page 49] RFC 4306 IKEv2 December 2005

       value (3) corresponds to a Payload Type of Transform in IKEv1,
       and the first 4 octets of the Transform structure are designed
       to look somewhat like the header of a Payload.
    o  RESERVED - MUST be sent as zero; MUST be ignored on receipt.
    o  Transform Length - The length (in octets) of the Transform
       Substructure including Header and Attributes.
    o  Transform Type (1 octet) - The type of transform being
       specified in this transform.  Different protocols support
       different transform types.  For some protocols, some of the
       transforms may be optional.  If a transform is optional and the
       initiator wishes to propose that the transform be omitted, no
       transform of the given type is included in the proposal.  If
       the initiator wishes to make use of the transform optional to
       the responder, it includes a transform substructure with
       transform ID = 0 as one of the options.
    o  Transform ID (2 octets) - The specific instance of the
       transform type being proposed.
 Transform Type Values
                                   Transform    Used In
                                      Type
        RESERVED                        0
        Encryption Algorithm (ENCR)     1  (IKE and ESP)
        Pseudo-random Function (PRF)    2  (IKE)
        Integrity Algorithm (INTEG)     3  (IKE, AH, optional in ESP)
        Diffie-Hellman Group (D-H)      4  (IKE, optional in AH & ESP)
        Extended Sequence Numbers (ESN) 5  (AH and ESP)
        RESERVED TO IANA                6-240
        PRIVATE USE                     241-255
 For Transform Type 1 (Encryption Algorithm), defined Transform IDs
 are:
        Name                     Number           Defined In
        RESERVED                    0
        ENCR_DES_IV64               1              (RFC1827)
        ENCR_DES                    2              (RFC2405), [DES]
        ENCR_3DES                   3              (RFC2451)
        ENCR_RC5                    4              (RFC2451)
        ENCR_IDEA                   5              (RFC2451), [IDEA]
        ENCR_CAST                   6              (RFC2451)
        ENCR_BLOWFISH               7              (RFC2451)
        ENCR_3IDEA                  8              (RFC2451)

Kaufman Standards Track [Page 50] RFC 4306 IKEv2 December 2005

        ENCR_DES_IV32               9
        RESERVED                   10
        ENCR_NULL                  11              (RFC2410)
        ENCR_AES_CBC               12              (RFC3602)
        ENCR_AES_CTR               13              (RFC3664)
        values 14-1023 are reserved to IANA.  Values 1024-65535 are
        for private use among mutually consenting parties.
 For Transform Type 2 (Pseudo-random Function), defined Transform IDs
 are:
        Name                     Number               Defined In
        RESERVED                    0
        PRF_HMAC_MD5                1                 (RFC2104), [MD5]
        PRF_HMAC_SHA1               2                 (RFC2104), [SHA]
        PRF_HMAC_TIGER              3                 (RFC2104)
        PRF_AES128_XCBC             4                 (RFC3664)
        values 5-1023 are reserved to IANA.  Values 1024-65535 are for
        private use among mutually consenting parties.
 For Transform Type 3 (Integrity Algorithm), defined Transform IDs
 are:
        Name                     Number                 Defined In
        NONE                       0
        AUTH_HMAC_MD5_96           1                     (RFC2403)
        AUTH_HMAC_SHA1_96          2                     (RFC2404)
        AUTH_DES_MAC               3
        AUTH_KPDK_MD5              4                     (RFC1826)
        AUTH_AES_XCBC_96           5                     (RFC3566)
        values 6-1023 are reserved to IANA.  Values 1024-65535 are for
        private use among mutually consenting parties.
 For Transform Type 4 (Diffie-Hellman Group), defined Transform IDs
 are:
        Name                                Number
        NONE                               0
        Defined in Appendix B              1 - 2
        RESERVED                           3 - 4
        Defined in [ADDGROUP]              5
        RESERVED TO IANA                   6 - 13
        Defined in [ADDGROUP]              14 - 18
        RESERVED TO IANA                   19 - 1023
        PRIVATE USE                        1024-65535

Kaufman Standards Track [Page 51] RFC 4306 IKEv2 December 2005

 For Transform Type 5 (Extended Sequence Numbers), defined Transform
 IDs are:
        Name                                Number
        No Extended Sequence Numbers       0
        Extended Sequence Numbers          1
        RESERVED                           2 - 65535

3.3.3. Valid Transform Types by Protocol

 The number and type of transforms that accompany an SA payload are
 dependent on the protocol in the SA itself.  An SA payload proposing
 the establishment of an SA has the following mandatory and optional
 transform types.  A compliant implementation MUST understand all
 mandatory and optional types for each protocol it supports (though it
 need not accept proposals with unacceptable suites).  A proposal MAY
 omit the optional types if the only value for them it will accept is
 NONE.
        Protocol  Mandatory Types        Optional Types
          IKE     ENCR, PRF, INTEG, D-H
          ESP     ENCR, ESN              INTEG, D-H
          AH      INTEG, ESN             D-H

3.3.4. Mandatory Transform IDs

 The specification of suites that MUST and SHOULD be supported for
 interoperability has been removed from this document because they are
 likely to change more rapidly than this document evolves.
 An important lesson learned from IKEv1 is that no system should only
 implement the mandatory algorithms and expect them to be the best
 choice for all customers.  For example, at the time that this
 document was written, many IKEv1 implementers were starting to
 migrate to AES in Cipher Block Chaining (CBC) mode for Virtual
 Private Network (VPN) applications.  Many IPsec systems based on
 IKEv2 will implement AES, additional Diffie-Hellman groups, and
 additional hash algorithms, and some IPsec customers already require
 these algorithms in addition to the ones listed above.
 It is likely that IANA will add additional transforms in the future,
 and some users may want to use private suites, especially for IKE
 where implementations should be capable of supporting different
 parameters, up to certain size limits.  In support of this goal, all
 implementations of IKEv2 SHOULD include a management facility that
 allows specification (by a user or system administrator) of Diffie-
 Hellman (DH) parameters (the generator, modulus, and exponent lengths
 and values) for new DH groups.  Implementations SHOULD provide a

Kaufman Standards Track [Page 52] RFC 4306 IKEv2 December 2005

 management interface via which these parameters and the associated
 transform IDs may be entered (by a user or system administrator), to
 enable negotiating such groups.
 All implementations of IKEv2 MUST include a management facility that
 enables a user or system administrator to specify the suites that are
 acceptable for use with IKE.  Upon receipt of a payload with a set of
 transform IDs, the implementation MUST compare the transmitted
 transform IDs against those locally configured via the management
 controls, to verify that the proposed suite is acceptable based on
 local policy.  The implementation MUST reject SA proposals that are
 not authorized by these IKE suite controls.  Note that cryptographic
 suites that MUST be implemented need not be configured as acceptable
 to local policy.

3.3.5. Transform Attributes

 Each transform in a Security Association payload may include
 attributes that modify or complete the specification of the
 transform.  These attributes are type/value pairs and are defined
 below.  For example, if an encryption algorithm has a variable-length
 key, the key length to be used may be specified as an attribute.
 Attributes can have a value with a fixed two octet length or a
 variable-length value.  For the latter, the attribute is encoded as
 type/length/value.
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !A!       Attribute Type        !    AF=0  Attribute Length     !
    !F!                             !    AF=1  Attribute Value      !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                   AF=0  Attribute Value                       !
    !                   AF=1  Not Transmitted                       !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 9:  Data Attributes
    o  Attribute Type (2 octets) - Unique identifier for each type of
       attribute (see below).
       The most significant bit of this field is the Attribute Format
       bit (AF).  It indicates whether the data attributes follow the
       Type/Length/Value (TLV) format or a shortened Type/Value (TV)
       format.  If the AF bit is zero (0), then the Data Attributes
       are of the Type/Length/Value (TLV) form.  If the AF bit is a
       one (1), then the Data Attributes are of the Type/Value form.

Kaufman Standards Track [Page 53] RFC 4306 IKEv2 December 2005

    o  Attribute Length (2 octets) - Length in octets of the Attribute
       Value.  When the AF bit is a one (1), the Attribute Value is
       only 2 octets and the Attribute Length field is not present.
    o  Attribute Value (variable length) - Value of the Attribute
       associated with the Attribute Type.  If the AF bit is a zero
       (0), this field has a variable length defined by the Attribute
       Length field.  If the AF bit is a one (1), the Attribute Value
       has a length of 2 octets.
 Note that only a single attribute type (Key Length) is defined, and
 it is fixed length.  The variable-length encoding specification is
 included only for future extensions.  The only algorithms defined in
 this document that accept attributes are the AES-based encryption,
 integrity, and pseudo-random functions, which require a single
 attribute specifying key width.
 Attributes described as basic MUST NOT be encoded using the
 variable-length encoding.  Variable-length attributes MUST NOT be
 encoded as basic even if their value can fit into two octets.  NOTE:
 This is a change from IKEv1, where increased flexibility may have
 simplified the composer of messages but certainly complicated the
 parser.
       Attribute Type                 Value        Attribute Format
    --------------------------------------------------------------
    RESERVED                           0-13 Key Length (in bits)
    14                 TV RESERVED                           15-17
    RESERVED TO IANA                   18-16383 PRIVATE USE
    16384-32767
 Values 0-13 and 15-17 were used in a similar context in IKEv1 and
 should not be assigned except to matching values.  Values 18-16383
 are reserved to IANA.  Values 16384-32767 are for private use among
 mutually consenting parties.
  1. Key Length
    When using an Encryption Algorithm that has a variable-length key,
    this attribute specifies the key length in bits (MUST use network
    byte order).  This attribute MUST NOT be used when the specified
    Encryption Algorithm uses a fixed-length key.

Kaufman Standards Track [Page 54] RFC 4306 IKEv2 December 2005

3.3.6. Attribute Negotiation

 During security association negotiation, initiators present offers to
 responders.  Responders MUST select a single complete set of
 parameters from the offers (or reject all offers if none are
 acceptable).  If there are multiple proposals, the responder MUST
 choose a single proposal number and return all of the Proposal
 substructures with that Proposal number.  If there are multiple
 Transforms with the same type, the responder MUST choose a single
 one.  Any attributes of a selected transform MUST be returned
 unmodified.  The initiator of an exchange MUST check that the
 accepted offer is consistent with one of its proposals, and if not
 that response MUST be rejected.
 Negotiating Diffie-Hellman groups presents some special challenges.
 SA offers include proposed attributes and a Diffie-Hellman public
 number (KE) in the same message.  If in the initial exchange the
 initiator offers to use one of several Diffie-Hellman groups, it
 SHOULD pick the one the responder is most likely to accept and
 include a KE corresponding to that group.  If the guess turns out to
 be wrong, the responder will indicate the correct group in the
 response and the initiator SHOULD pick an element of that group for
 its KE value when retrying the first message.  It SHOULD, however,
 continue to propose its full supported set of groups in order to
 prevent a man-in-the-middle downgrade attack.
 Implementation Note:
    Certain negotiable attributes can have ranges or could have
    multiple acceptable values.  These include the key length of a
    variable key length symmetric cipher.  To further interoperability
    and to support upgrading endpoints independently, implementers of
    this protocol SHOULD accept values that they deem to supply
    greater security.  For instance, if a peer is configured to accept
    a variable-length cipher with a key length of X bits and is
    offered that cipher with a larger key length, the implementation
    SHOULD accept the offer if it supports use of the longer key.
 Support of this capability allows an implementation to express a
 concept of "at least" a certain level of security -- "a key length of
 _at least_ X bits for cipher Y".

Kaufman Standards Track [Page 55] RFC 4306 IKEv2 December 2005

3.4. Key Exchange Payload

 The Key Exchange Payload, denoted KE in this memo, is used to
 exchange Diffie-Hellman public numbers as part of a Diffie-Hellman
 key exchange.  The Key Exchange Payload consists of the IKE generic
 payload header followed by the Diffie-Hellman public value itself.
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !          DH Group #           !           RESERVED            !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                       Key Exchange Data                       ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 10:  Key Exchange Payload Format
 A key exchange payload is constructed by copying one's Diffie-Hellman
 public value into the "Key Exchange Data" portion of the payload.
 The length of the Diffie-Hellman public value MUST be equal to the
 length of the prime modulus over which the exponentiation was
 performed, prepending zero bits to the value if necessary.
 The DH Group # identifies the Diffie-Hellman group in which the Key
 Exchange Data was computed (see section 3.3.2).  If the selected
 proposal uses a different Diffie-Hellman group, the message MUST be
 rejected with a Notify payload of type INVALID_KE_PAYLOAD.
 The payload type for the Key Exchange payload is thirty four (34).

3.5. Identification Payloads

 The Identification Payloads, denoted IDi and IDr in this memo, allow
 peers to assert an identity to one another.  This identity may be
 used for policy lookup, but does not necessarily have to match
 anything in the CERT payload; both fields may be used by an
 implementation to perform access control decisions.
 NOTE: In IKEv1, two ID payloads were used in each direction to hold
 Traffic Selector (TS) information for data passing over the SA.  In
 IKEv2, this information is carried in TS payloads (see section 3.13).

Kaufman Standards Track [Page 56] RFC 4306 IKEv2 December 2005

 The Identification Payload consists of the IKE generic payload header
 followed by identification fields as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !   ID Type     !                 RESERVED                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                   Identification Data                         ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 11:  Identification Payload Format
 o  ID Type (1 octet) - Specifies the type of Identification being
    used.
 o  RESERVED - MUST be sent as zero; MUST be ignored on receipt.
 o  Identification Data (variable length) - Value, as indicated by the
    Identification Type.  The length of the Identification Data is
    computed from the size in the ID payload header.
 The payload types for the Identification Payload are thirty five (35)
 for IDi and thirty six (36) for IDr.
 The following table lists the assigned values for the Identification
 Type field, followed by a description of the Identification Data
 which follows:
    ID Type                           Value
    -------                           -----
    RESERVED                            0
    ID_IPV4_ADDR                        1
          A single four (4) octet IPv4 address.
    ID_FQDN                             2
          A fully-qualified domain name string.  An example of a
          ID_FQDN is, "example.com".  The string MUST not contain any
          terminators (e.g., NULL, CR, etc.).

Kaufman Standards Track [Page 57] RFC 4306 IKEv2 December 2005

    ID_RFC822_ADDR                      3
          A fully-qualified RFC822 email address string, An example of
          a ID_RFC822_ADDR is, "jsmith@example.com".  The string MUST
          not contain any terminators.
    Reserved to IANA                    4
    ID_IPV6_ADDR                        5
          A single sixteen (16) octet IPv6 address.
    Reserved to IANA                    6 - 8
    ID_DER_ASN1_DN                      9
          The binary Distinguished Encoding Rules (DER) encoding of an
          ASN.1 X.500 Distinguished Name [X.501].
    ID_DER_ASN1_GN                      10
          The binary DER encoding of an ASN.1 X.500 GeneralName
          [X.509].
    ID_KEY_ID                           11
          An opaque octet stream which may be used to pass vendor-
          specific information necessary to do certain proprietary
          types of identification.
    Reserved to IANA                    12-200
    Reserved for private use            201-255
 Two implementations will interoperate only if each can generate a
 type of ID acceptable to the other.  To assure maximum
 interoperability, implementations MUST be configurable to send at
 least one of ID_IPV4_ADDR, ID_FQDN, ID_RFC822_ADDR, or ID_KEY_ID, and
 MUST be configurable to accept all of these types.  Implementations
 SHOULD be capable of generating and accepting all of these types.
 IPv6-capable implementations MUST additionally be configurable to
 accept ID_IPV6_ADDR.  IPv6-only implementations MAY be configurable
 to send only ID_IPV6_ADDR.

Kaufman Standards Track [Page 58] RFC 4306 IKEv2 December 2005

3.6. Certificate Payload

 The Certificate Payload, denoted CERT in this memo, provides a means
 to transport certificates or other authentication-related information
 via IKE.  Certificate payloads SHOULD be included in an exchange if
 certificates are available to the sender unless the peer has
 indicated an ability to retrieve this information from elsewhere
 using an HTTP_CERT_LOOKUP_SUPPORTED Notify payload.  Note that the
 term "Certificate Payload" is somewhat misleading, because not all
 authentication mechanisms use certificates and data other than
 certificates may be passed in this payload.
 The Certificate Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Cert Encoding !                                               !
    +-+-+-+-+-+-+-+-+                                               !
    ~                       Certificate Data                        ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 12:  Certificate Payload Format
    o  Certificate Encoding (1 octet) - This field indicates the type
       of certificate or certificate-related information contained in
       the Certificate Data field.
         Certificate Encoding               Value
         --------------------               -----
         RESERVED                             0
         PKCS #7 wrapped X.509 certificate    1
         PGP Certificate                      2
         DNS Signed Key                       3
         X.509 Certificate - Signature        4
         Kerberos Token                       6
         Certificate Revocation List (CRL)    7
         Authority Revocation List (ARL)      8
         SPKI Certificate                     9
         X.509 Certificate - Attribute       10
         Raw RSA Key                         11
         Hash and URL of X.509 certificate   12
         Hash and URL of X.509 bundle        13
         RESERVED to IANA                  14 - 200
         PRIVATE USE                      201 - 255

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    o  Certificate Data (variable length) - Actual encoding of
       certificate data.  The type of certificate is indicated by the
       Certificate Encoding field.
 The payload type for the Certificate Payload is thirty seven (37).
 Specific syntax is for some of the certificate type codes above is
 not defined in this document.  The types whose syntax is defined in
 this document are:
    X.509 Certificate - Signature (4) contains a DER encoded X.509
    certificate whose public key is used to validate the sender's AUTH
    payload.
    Certificate Revocation List (7) contains a DER encoded X.509
    certificate revocation list.
    Raw RSA Key (11) contains a PKCS #1 encoded RSA key (see [RSA] and
    [PKCS1]).
    Hash and URL encodings (12-13) allow IKE messages to remain short
    by replacing long data structures with a 20 octet SHA-1 hash (see
    [SHA]) of the replaced value followed by a variable-length URL
    that resolves to the DER encoded data structure itself.  This
    improves efficiency when the endpoints have certificate data
    cached and makes IKE less subject to denial of service attacks
    that become easier to mount when IKE messages are large enough to
    require IP fragmentation [KPS03].
    Use the following ASN.1 definition for an X.509 bundle:
          CertBundle
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) pkix(7) id-mod(0)
              id-mod-cert-bundle(34) }
          DEFINITIONS EXPLICIT TAGS ::=
          BEGIN
          IMPORTS
            Certificate, CertificateList
            FROM PKIX1Explicit88
               { iso(1) identified-organization(3) dod(6)
                 internet(1) security(5) mechanisms(5) pkix(7)
                 id-mod(0) id-pkix1-explicit(18) } ;

Kaufman Standards Track [Page 60] RFC 4306 IKEv2 December 2005

         CertificateOrCRL ::= CHOICE {
           cert [0] Certificate,
           crl  [1] CertificateList }
         CertificateBundle ::= SEQUENCE OF CertificateOrCRL
         END
 Implementations MUST be capable of being configured to send and
 accept up to four X.509 certificates in support of authentication,
 and also MUST be capable of being configured to send and accept the
 first two Hash and URL formats (with HTTP URLs).  Implementations
 SHOULD be capable of being configured to send and accept Raw RSA
 keys.  If multiple certificates are sent, the first certificate MUST
 contain the public key used to sign the AUTH payload.  The other
 certificates may be sent in any order.

3.7. Certificate Request Payload

 The Certificate Request Payload, denoted CERTREQ in this memo,
 provides a means to request preferred certificates via IKE and can
 appear in the IKE_INIT_SA response and/or the IKE_AUTH request.
 Certificate Request payloads MAY be included in an exchange when the
 sender needs to get the certificate of the receiver.  If multiple CAs
 are trusted and the cert encoding does not allow a list, then
 multiple Certificate Request payloads SHOULD be transmitted.
 The Certificate Request Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Cert Encoding !                                               !
    +-+-+-+-+-+-+-+-+                                               !
    ~                    Certification Authority                    ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 13:  Certificate Request Payload Format
 o  Certificate Encoding (1 octet) - Contains an encoding of the type
    or format of certificate requested.  Values are listed in section
    3.6.

Kaufman Standards Track [Page 61] RFC 4306 IKEv2 December 2005

 o  Certification Authority (variable length) - Contains an encoding
    of an acceptable certification authority for the type of
    certificate requested.
 The payload type for the Certificate Request Payload is thirty eight
 (38).
 The Certificate Encoding field has the same values as those defined
 in section 3.6. The Certification Authority field contains an
 indicator of trusted authorities for this certificate type.  The
 Certification Authority value is a concatenated list of SHA-1 hashes
 of the public keys of trusted Certification Authorities (CAs).  Each
 is encoded as the SHA-1 hash of the Subject Public Key Info element
 (see section 4.1.2.7 of [RFC3280]) from each Trust Anchor
 certificate.  The twenty-octet hashes are concatenated and included
 with no other formatting.
 Note that the term "Certificate Request" is somewhat misleading, in
 that values other than certificates are defined in a "Certificate"
 payload and requests for those values can be present in a Certificate
 Request Payload.  The syntax of the Certificate Request payload in
 such cases is not defined in this document.
 The Certificate Request Payload is processed by inspecting the "Cert
 Encoding" field to determine whether the processor has any
 certificates of this type.  If so, the "Certification Authority"
 field is inspected to determine if the processor has any certificates
 that can be validated up to one of the specified certification
 authorities.  This can be a chain of certificates.
 If an end-entity certificate exists that satisfies the criteria
 specified in the CERTREQ, a certificate or certificate chain SHOULD
 be sent back to the certificate requestor if the recipient of the
 CERTREQ:
  1. is configured to use certificate authentication,
  1. is allowed to send a CERT payload,
  1. has matching CA trust policy governing the current negotiation, and
  1. has at least one time-wise and usage appropriate end-entity

certificate chaining to a CA provided in the CERTREQ.

 Certificate revocation checking must be considered during the
 chaining process used to select a certificate.  Note that even if two
 peers are configured to use two different CAs, cross-certification
 relationships should be supported by appropriate selection logic.

Kaufman Standards Track [Page 62] RFC 4306 IKEv2 December 2005

 The intent is not to prevent communication through the strict
 adherence of selection of a certificate based on CERTREQ, when an
 alternate certificate could be selected by the sender that would
 still enable the recipient to successfully validate and trust it
 through trust conveyed by cross-certification, CRLs, or other out-
 of-band configured means.  Thus, the processing of a CERTREQ should
 be seen as a suggestion for a certificate to select, not a mandated
 one.  If no certificates exist, then the CERTREQ is ignored.  This is
 not an error condition of the protocol.  There may be cases where
 there is a preferred CA sent in the CERTREQ, but an alternate might
 be acceptable (perhaps after prompting a human operator).

3.8. Authentication Payload

 The Authentication Payload, denoted AUTH in this memo, contains data
 used for authentication purposes.  The syntax of the Authentication
 data varies according to the Auth Method as specified below.
 The Authentication Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Auth Method   !                RESERVED                       !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                      Authentication Data                      ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 14:  Authentication Payload Format
 o  Auth Method (1 octet) - Specifies the method of authentication
    used.  Values defined are:
      RSA Digital Signature (1) - Computed as specified in section
      2.15 using an RSA private key over a PKCS#1 padded hash (see
      [RSA] and [PKCS1]).
      Shared Key Message Integrity Code (2) - Computed as specified in
      section 2.15 using the shared key associated with the identity
      in the ID payload and the negotiated prf function
      DSS Digital Signature (3) - Computed as specified in section
      2.15 using a DSS private key (see [DSS]) over a SHA-1 hash.

Kaufman Standards Track [Page 63] RFC 4306 IKEv2 December 2005

      The values 0 and 4-200 are reserved to IANA.  The values 201-255
      are available for private use.
 o  Authentication Data (variable length) - see section 2.15.
 The payload type for the Authentication Payload is thirty nine (39).

3.9. Nonce Payload

 The Nonce Payload, denoted Ni and Nr in this memo for the initiator's
 and responder's nonce respectively, contains random data used to
 guarantee liveness during an exchange and protect against replay
 attacks.
 The Nonce Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                            Nonce Data                         ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 15:  Nonce Payload Format
 o  Nonce Data (variable length) - Contains the random data generated
    by the transmitting entity.
 The payload type for the Nonce Payload is forty (40).
 The size of a Nonce MUST be between 16 and 256 octets inclusive.
 Nonce values MUST NOT be reused.

3.10. Notify Payload

 The Notify Payload, denoted N in this document, is used to transmit
 informational data, such as error conditions and state transitions,
 to an IKE peer.  A Notify Payload may appear in a response message
 (usually specifying why a request was rejected), in an INFORMATIONAL
 Exchange (to report an error not in an IKE request), or in any other
 message to indicate sender capabilities or to modify the meaning of
 the request.

Kaufman Standards Track [Page 64] RFC 4306 IKEv2 December 2005

 The Notify Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !  Protocol ID  !   SPI Size    !      Notify Message Type      !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                Security Parameter Index (SPI)                 ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                       Notification Data                       ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 16:  Notify Payload Format
 o  Protocol ID (1 octet) - If this notification concerns an existing
    SA, this field indicates the type of that SA.  For IKE_SA
    notifications, this field MUST be one (1).  For notifications
    concerning IPsec SAs this field MUST contain either (2) to
    indicate AH or (3) to indicate ESP.  For notifications that do not
    relate to an existing SA, this field MUST be sent as zero and MUST
    be ignored on receipt.  All other values for this field are
    reserved to IANA for future assignment.
 o  SPI Size (1 octet) - Length in octets of the SPI as defined by the
    IPsec protocol ID or zero if no SPI is applicable.  For a
    notification concerning the IKE_SA, the SPI Size MUST be zero.
 o  Notify Message Type (2 octets) - Specifies the type of
    notification message.
 o  SPI (variable length) - Security Parameter Index.
 o  Notification Data (variable length) - Informational or error data
    transmitted in addition to the Notify Message Type.  Values for
    this field are type specific (see below).
 The payload type for the Notify Payload is forty one (41).

Kaufman Standards Track [Page 65] RFC 4306 IKEv2 December 2005

3.10.1. Notify Message Types

 Notification information can be error messages specifying why an SA
 could not be established.  It can also be status data that a process
 managing an SA database wishes to communicate with a peer process.
 The table below lists the Notification messages and their
 corresponding values.  The number of different error statuses was
 greatly reduced from IKEv1 both for simplification and to avoid
 giving configuration information to probers.
 Types in the range 0 - 16383 are intended for reporting errors.  An
 implementation receiving a Notify payload with one of these types
 that it does not recognize in a response MUST assume that the
 corresponding request has failed entirely.  Unrecognized error types
 in a request and status types in a request or response MUST be
 ignored except that they SHOULD be logged.
 Notify payloads with status types MAY be added to any message and
 MUST be ignored if not recognized.  They are intended to indicate
 capabilities, and as part of SA negotiation are used to negotiate
 non-cryptographic parameters.
      NOTIFY MESSAGES - ERROR TYPES           Value
      -----------------------------           -----
      RESERVED                                  0
      UNSUPPORTED_CRITICAL_PAYLOAD              1
          Sent if the payload has the "critical" bit set and the
          payload type is not recognized.  Notification Data contains
          the one-octet payload type.
      INVALID_IKE_SPI                           4
          Indicates an IKE message was received with an unrecognized
          destination SPI.  This usually indicates that the recipient
          has rebooted and forgotten the existence of an IKE_SA.
      INVALID_MAJOR_VERSION                     5
          Indicates the recipient cannot handle the version of IKE
          specified in the header.  The closest version number that
          the recipient can support will be in the reply header.
      INVALID_SYNTAX                            7
          Indicates the IKE message that was received was invalid
          because some type, length, or value was out of range or

Kaufman Standards Track [Page 66] RFC 4306 IKEv2 December 2005

          because the request was rejected for policy reasons.  To
          avoid a denial of service attack using forged messages, this
          status may only be returned for and in an encrypted packet
          if the message ID and cryptographic checksum were valid.  To
          avoid leaking information to someone probing a node, this
          status MUST be sent in response to any error not covered by
          one of the other status types.  To aid debugging, more
          detailed error information SHOULD be written to a console or
          log.
      INVALID_MESSAGE_ID                        9
          Sent when an IKE message ID outside the supported window is
          received.  This Notify MUST NOT be sent in a response; the
          invalid request MUST NOT be acknowledged.  Instead, inform
          the other side by initiating an INFORMATIONAL exchange with
          Notification data containing the four octet invalid message
          ID.  Sending this notification is optional, and
          notifications of this type MUST be rate limited.
      INVALID_SPI                              11
          MAY be sent in an IKE INFORMATIONAL exchange when a node
          receives an ESP or AH packet with an invalid SPI.  The
          Notification Data contains the SPI of the invalid packet.
          This usually indicates a node has rebooted and forgotten an
          SA.  If this Informational Message is sent outside the
          context of an IKE_SA, it should be used by the recipient
          only as a "hint" that something might be wrong (because it
          could easily be forged).
      NO_PROPOSAL_CHOSEN                       14
          None of the proposed crypto suites was acceptable.
      INVALID_KE_PAYLOAD                       17
          The D-H Group # field in the KE payload is not the group #
          selected by the responder for this exchange.  There are two
          octets of data associated with this notification: the
          accepted D-H Group # in big endian order.
      AUTHENTICATION_FAILED                    24
          Sent in the response to an IKE_AUTH message when for some
          reason the authentication failed.  There is no associated
          data.

Kaufman Standards Track [Page 67] RFC 4306 IKEv2 December 2005

      SINGLE_PAIR_REQUIRED                     34
      This 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.
      NO_ADDITIONAL_SAS                        35
      This error indicates that a CREATE_CHILD_SA request is
      unacceptable because the responder is unwilling to accept any
      more CHILD_SAs on this IKE_SA.  Some minimal implementations may
      only accept a single CHILD_SA setup in the context of an initial
      IKE exchange and reject any subsequent attempts to add more.
      INTERNAL_ADDRESS_FAILURE                 36
      Indicates an error assigning an internal address (i.e.,
      INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS) during the
      processing of a Configuration Payload by a responder.  If this
      error is generated within an IKE_AUTH exchange, no CHILD_SA will
      be created.
      FAILED_CP_REQUIRED                       37
      Sent by responder in the case where CP(CFG_REQUEST) was expected
      but not received, and so is a conflict with locally configured
      policy.  There is no associated data.
      TS_UNACCEPTABLE                          38
      Indicates that none of the addresses/protocols/ports in the
      supplied traffic selectors is acceptable.
      INVALID_SELECTORS                        39
          MAY be sent in an IKE INFORMATIONAL exchange when a node
          receives an ESP or AH packet whose selectors do not match
          those of the SA on which it was delivered (and that caused
          the packet to be dropped).  The Notification Data contains
          the start of the offending packet (as in ICMP messages) and
          the SPI field of the notification is set to match the SPI of
          the IPsec SA.
      RESERVED TO IANA - Error types         40 - 8191
      Private Use - Errors                8192 - 16383

Kaufman Standards Track [Page 68] RFC 4306 IKEv2 December 2005

      NOTIFY MESSAGES - STATUS TYPES           Value
      ------------------------------           -----
      INITIAL_CONTACT                          16384
          This notification asserts that this IKE_SA is the only
          IKE_SA currently active between the authenticated
          identities.  It MAY be sent when an IKE_SA is established
          after a crash, and the recipient MAY use this information to
          delete any other IKE_SAs it has to the same authenticated
          identity without waiting for a timeout.  This notification
          MUST NOT be sent by an entity that may be replicated (e.g.,
          a roaming user's credentials where the user is allowed to
          connect to the corporate firewall from two remote systems at
          the same time).
      SET_WINDOW_SIZE                          16385
          This 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.  Window size is always
          one until the initial exchanges complete.
      ADDITIONAL_TS_POSSIBLE                   16386
          This notification asserts that the sending endpoint narrowed
          the proposed traffic selectors but that other traffic
          selectors would also have been acceptable, though only in a
          separate SA (see section 2.9).  There is no data associated
          with this Notify type.  It may be sent only as an additional
          payload in a message including accepted TSs.
      IPCOMP_SUPPORTED                         16387
          This notification may be included only in a message
          containing an SA payload negotiating a CHILD_SA and
          indicates a willingness by its sender to use IPComp on this
          SA.  The data associated with this notification includes a
          two-octet IPComp CPI followed by a one-octet transform ID
          optionally followed by attributes whose length and format
          are defined by that transform ID.  A message proposing an SA
          may contain multiple IPCOMP_SUPPORTED notifications to
          indicate multiple supported algorithms.  A message accepting
          an SA may contain at most one.

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          The transform IDs currently defined are:
               NAME         NUMBER  DEFINED IN
               -----------  ------  -----------
               RESERVED       0
               IPCOMP_OUI     1
               IPCOMP_DEFLATE 2     RFC 2394
               IPCOMP_LZS     3     RFC 2395
               IPCOMP_LZJH    4     RFC 3051
               values 5-240 are reserved to IANA.  Values 241-255 are
               for private use among mutually consenting parties.
      NAT_DETECTION_SOURCE_IP                  16388
          This notification is used by its recipient to determine
          whether the source is behind a NAT box.  The data associated
          with this notification is a SHA-1 digest of the SPIs (in the
          order they appear in the header), IP address, and port on
          which this packet was sent.  There MAY be multiple Notify
          payloads of this type in a message if the sender does not
          know which of several network attachments will be used to
          send the packet.  The recipient of this notification MAY
          compare the supplied value to a SHA-1 hash of the SPIs,
          source IP address, and port, and if they don't match it
          SHOULD enable NAT traversal (see section 2.23).
          Alternately, it MAY reject the connection attempt if NAT
          traversal is not supported.
      NAT_DETECTION_DESTINATION_IP             16389
          This notification is used by its recipient to determine
          whether it is behind a NAT box.  The data associated with
          this notification is a SHA-1 digest of the SPIs (in the
          order they appear in the header), IP address, and port to
          which this packet was sent.  The recipient of this
          notification MAY compare the supplied value to a hash of the
          SPIs, destination IP address, and port, and if they don't
          match it SHOULD invoke NAT traversal (see section 2.23).  If
          they don't match, it means that this end is behind a NAT and
          this end SHOULD start sending keepalive packets as defined
          in [Hutt05].  Alternately, it MAY reject the connection
          attempt if NAT traversal is not supported.

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      COOKIE                                   16390
          This notification MAY be included in an IKE_SA_INIT
          response.  It indicates that the request should be retried
          with a copy of this notification as the first payload.  This
          notification MUST be included in an IKE_SA_INIT request
          retry if a COOKIE notification was included in the initial
          response.  The data associated with this notification MUST
          be between 1 and 64 octets in length (inclusive).
      USE_TRANSPORT_MODE                       16391
          This 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.
          Note: The ECN decapsulation modifications specified in
          [RFC4301] MUST be performed for every tunnel mode SA created
          by IKEv2.
      HTTP_CERT_LOOKUP_SUPPORTED               16392
          This notification MAY be included in any message that can
          include a CERTREQ payload and indicates that the sender is
          capable of looking up certificates based on an HTTP-based
          URL (and hence presumably would prefer to receive
          certificate specifications in that format).
      REKEY_SA                                 16393
          This 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 SPI field identifies the SA
          being rekeyed.  There is no data.
      ESP_TFC_PADDING_NOT_SUPPORTED            16394
          This notification asserts that the sending endpoint will NOT
          accept packets that contain Flow Confidentiality (TFC)
          padding.

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      NON_FIRST_FRAGMENTS_ALSO                 16395
          Used for fragmentation control.  See [RFC4301] for
          explanation.
      RESERVED TO IANA - STATUS TYPES      16396 - 40959
      Private Use - STATUS TYPES           40960 - 65535

3.11. Delete Payload

 The Delete Payload, denoted D in this memo, contains a protocol-
 specific security association identifier that the sender has removed
 from its security association database and is, therefore, no longer
 valid.  Figure 17 shows the format of the Delete Payload.  It is
 possible to send multiple SPIs in a Delete payload; however, each SPI
 MUST be for the same protocol.  Mixing of protocol identifiers MUST
 NOT be performed in a Delete payload.  It is permitted, however, to
 include multiple Delete payloads in a single INFORMATIONAL exchange
 where each Delete payload lists SPIs for a different protocol.
 Deletion of the IKE_SA is indicated by a protocol ID of 1 (IKE) but
 no SPIs.  Deletion of a CHILD_SA, such as ESP or AH, will contain the
 IPsec protocol ID of that protocol (2 for AH, 3 for ESP), and the SPI
 is the SPI the sending endpoint would expect in inbound ESP or AH
 packets.
 The Delete Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Protocol ID   !   SPI Size    !           # of SPIs           !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~               Security Parameter Index(es) (SPI)              ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 17:  Delete Payload Format
 o  Protocol ID (1 octet) - Must be 1 for an IKE_SA, 2 for AH, or 3
    for ESP.

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 o  SPI Size (1 octet) - Length in octets of the SPI as defined by the
    protocol ID.  It MUST be zero for IKE (SPI is in message header)
    or four for AH and ESP.
 o  # of SPIs (2 octets) - The number of SPIs contained in the Delete
    payload.  The size of each SPI is defined by the SPI Size field.
 o  Security Parameter Index(es) (variable length) - Identifies the
    specific security association(s) to delete.  The length of this
    field is determined by the SPI Size and # of SPIs fields.
 The payload type for the Delete Payload is forty two (42).

3.12. Vendor ID Payload

 The Vendor ID Payload, denoted V in this memo, contains a vendor
 defined constant.  The constant is used by vendors to identify and
 recognize remote instances of their implementations.  This mechanism
 allows a vendor to experiment with new features while maintaining
 backward compatibility.
 A Vendor ID payload MAY announce that the sender is capable to
 accepting certain extensions to the protocol, or it MAY simply
 identify the implementation as an aid in debugging.  A Vendor ID
 payload MUST NOT change the interpretation of any information defined
 in this specification (i.e., the critical bit MUST be set to 0).
 Multiple Vendor ID payloads MAY be sent.  An implementation is NOT
 REQUIRED to send any Vendor ID payload at all.
 A Vendor ID payload may be sent as part of any message.  Reception of
 a familiar Vendor ID payload allows an implementation to make use of
 Private USE numbers described throughout this memo -- private
 payloads, private exchanges, private notifications, etc.  Unfamiliar
 Vendor IDs MUST be ignored.
 Writers of Internet-Drafts who wish to extend this protocol MUST
 define a Vendor ID payload to announce the ability to implement the
 extension in the Internet-Draft.  It is expected that Internet-Drafts
 that gain acceptance and are standardized will be given "magic
 numbers" out of the Future Use range by IANA, and the requirement to
 use a Vendor ID will go away.

Kaufman Standards Track [Page 73] RFC 4306 IKEv2 December 2005

 The Vendor ID Payload fields are defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                        Vendor ID (VID)                        ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 18:  Vendor ID Payload Format
 o  Vendor ID (variable length) - It is the responsibility of the
    person choosing the Vendor ID to assure its uniqueness in spite of
    the absence of any central registry for IDs.  Good practice is to
    include a company name, a person name, or some such.  If you want
    to show off, you might include the latitude and longitude and time
    where you were when you chose the ID and some random input.  A
    message digest of a long unique string is preferable to the long
    unique string itself.
 The payload type for the Vendor ID Payload is forty three (43).

3.13. Traffic Selector Payload

 The Traffic Selector Payload, denoted TS in this memo, allows peers
 to identify packet flows for processing by IPsec security services.
 The Traffic Selector Payload consists of the IKE generic payload
 header followed by individual traffic selectors as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Number of TSs !                 RESERVED                      !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                       <Traffic Selectors>                     ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 19:  Traffic Selectors Payload Format
 o  Number of TSs (1 octet) - Number of traffic selectors being
    provided.

Kaufman Standards Track [Page 74] RFC 4306 IKEv2 December 2005

 o  RESERVED - This field MUST be sent as zero and MUST be ignored on
    receipt.
 o  Traffic Selectors (variable length) - One or more individual
    traffic selectors.
 The length of the Traffic Selector payload includes the TS header and
 all the traffic selectors.
 The payload type for the Traffic Selector payload is forty four (44)
 for addresses at the initiator's end of the SA and forty five (45)
 for addresses at the responder's end.

3.13.1. Traffic Selector

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !   TS Type     !IP Protocol ID*|       Selector Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Start Port*         |           End Port*           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                         Starting Address*                     ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                         Ending Address*                       ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 20: Traffic Selector
  • Note: All fields other than TS Type and Selector Length depend on

the TS Type. The fields shown are for TS Types 7 and 8, the only two

 values currently defined.
 o  TS Type (one octet) - Specifies the type of traffic selector.
 o  IP protocol ID (1 octet) - Value specifying an associated IP
    protocol ID (e.g., UDP/TCP/ICMP).  A value of zero means that the
    protocol ID is not relevant to this traffic selector -- the SA can
    carry all protocols.
 o  Selector Length - Specifies the length of this Traffic Selector
    Substructure including the header.

Kaufman Standards Track [Page 75] RFC 4306 IKEv2 December 2005

 o  Start Port (2 octets) - Value specifying the smallest port number
    allowed by this Traffic Selector.  For protocols for which port is
    undefined, or if all ports are allowed, this field MUST be zero.
    For the ICMP protocol, the two one-octet fields Type and Code are
    treated as a single 16-bit integer (with Type in the most
    significant eight bits and Code in the least significant eight
    bits) port number for the purposes of filtering based on this
    field.
 o  End Port (2 octets) - Value specifying the largest port number
    allowed by this Traffic Selector.  For protocols for which port is
    undefined, or if all ports are allowed, this field MUST be 65535.
    For the ICMP protocol, the two one-octet fields Type and Code are
    treated as a single 16-bit integer (with Type in the most
    significant eight bits and Code in the least significant eight
    bits) port number for the purposed of filtering based on this
    field.
 o  Starting Address - The smallest address included in this Traffic
    Selector (length determined by TS type).
 o  Ending Address - The largest address included in this Traffic
    Selector (length determined by TS type).
 Systems that are complying with [RFC4301] that wish to indicate "ANY"
 ports MUST set the start port to 0 and the end port to 65535; note
 that according to [RFC4301], "ANY" includes "OPAQUE".  Systems
 working with [RFC4301] that wish to indicate "OPAQUE" ports, but not
 "ANY" ports, MUST set the start port to 65535 and the end port to 0.
 The following table lists the assigned values for the Traffic
 Selector Type field and the corresponding Address Selector Data.
    TS Type                           Value
    -------                           -----
    RESERVED                           0-6
    TS_IPV4_ADDR_RANGE                  7
          A range of IPv4 addresses, represented by two four-octet
          values.  The first value is the beginning IPv4 address
          (inclusive) and the second value is the ending IPv4 address
          (inclusive).  All addresses falling between the two
          specified addresses are considered to be within the list.

Kaufman Standards Track [Page 76] RFC 4306 IKEv2 December 2005

    TS_IPV6_ADDR_RANGE                  8
          A range of IPv6 addresses, represented by two sixteen-octet
          values.  The first value is the beginning IPv6 address
          (inclusive) and the second value is the ending IPv6 address
          (inclusive).  All addresses falling between the two
          specified addresses are considered to be within the list.
    RESERVED TO IANA                    9-240
    PRIVATE USE                         241-255

3.14. Encrypted Payload

 The Encrypted Payload, denoted SK{...} or E in this memo, contains
 other payloads in encrypted form.  The Encrypted Payload, if present
 in a message, MUST be the last payload in the message.  Often, it is
 the only payload in the message.
 The algorithms for encryption and integrity protection are negotiated
 during IKE_SA setup, and the keys are computed as specified in
 sections 2.14 and 2.18.
 The encryption and integrity protection algorithms are modeled after
 the ESP algorithms described in RFCs 2104 [KBC96], 4303 [RFC4303],
 and 2451 [ESPCBC].  This document completely specifies the
 cryptographic processing of IKE data, but those documents should be
 consulted for design rationale.  We require a block cipher with a
 fixed block size and an integrity check algorithm that computes a
 fixed-length checksum over a variable size message.
 The payload type for an Encrypted payload is forty six (46).  The
 Encrypted Payload consists of the IKE generic payload header followed
 by individual fields as follows:

Kaufman Standards Track [Page 77] RFC 4306 IKEv2 December 2005

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C!  RESERVED   !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                     Initialization Vector                     !
    !         (length is block size for encryption algorithm)       !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    Encrypted IKE Payloads                     ~
    +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !               !             Padding (0-255 octets)            !
    +-+-+-+-+-+-+-+-+                               +-+-+-+-+-+-+-+-+
    !                                               !  Pad Length   !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    Integrity Checksum Data                    ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 21:  Encrypted Payload Format
 o  Next Payload - The payload type of the first embedded payload.
    Note that this is an exception in the standard header format,
    since the Encrypted payload is the last payload in the message and
    therefore the Next Payload field would normally be zero.  But
    because the content of this payload is embedded payloads and there
    was no natural place to put the type of the first one, that type
    is placed here.
 o  Payload Length - Includes the lengths of the header, IV, Encrypted
    IKE Payloads, Padding, Pad Length, and Integrity Checksum Data.
 o  Initialization Vector - A randomly chosen value whose length is
    equal to the block length of the underlying encryption algorithm.
    Recipients MUST accept any value.  Senders SHOULD either pick this
    value pseudo-randomly and independently for each message or use
    the final ciphertext block of the previous message sent.  Senders
    MUST NOT use the same value for each message, use a sequence of
    values with low hamming distance (e.g., a sequence number), or use
    ciphertext from a received message.
 o  IKE Payloads are as specified earlier in this section. This field
    is encrypted with the negotiated cipher.
 o  Padding MAY contain any value chosen by the sender, and MUST have
    a length that makes the combination of the Payloads, the Padding,
    and the Pad Length to be a multiple of the encryption block size.
    This field is encrypted with the negotiated cipher.

Kaufman Standards Track [Page 78] RFC 4306 IKEv2 December 2005

 o  Pad Length is the length of the Padding field. The sender SHOULD
    set the Pad Length to the minimum value that makes the combination
    of the Payloads, the Padding, and the Pad Length a multiple of the
    block size, but the recipient MUST accept any length that results
    in proper alignment.  This field is encrypted with the negotiated
    cipher.
 o  Integrity Checksum Data is the cryptographic checksum of the
    entire message starting with the Fixed IKE Header through the Pad
    Length.  The checksum MUST be computed over the encrypted message.
    Its length is determined by the integrity algorithm negotiated.

3.15. Configuration Payload

 The Configuration payload, denoted CP in this document, is used to
 exchange configuration information between IKE peers.  The exchange
 is for an IRAC to request an internal IP address from an IRAS and to
 exchange other information of the sort that one would acquire with
 Dynamic Host Configuration Protocol (DHCP) if the IRAC were directly
 connected to a LAN.
 Configuration payloads are of type CFG_REQUEST/CFG_REPLY or
 CFG_SET/CFG_ACK (see CFG Type in the payload description below).
 CFG_REQUEST and CFG_SET payloads may optionally be added to any IKE
 request.  The IKE response MUST include either a corresponding
 CFG_REPLY or CFG_ACK or a Notify payload with an error type
 indicating why the request could not be honored.  An exception is
 that a minimal implementation MAY ignore all CFG_REQUEST and CFG_SET
 payloads, so a response message without a corresponding CFG_REPLY or
 CFG_ACK MUST be accepted as an indication that the request was not
 supported.
 "CFG_REQUEST/CFG_REPLY" allows an IKE endpoint to request information
 from its peer.  If an attribute in the CFG_REQUEST Configuration
 Payload is not zero-length, it is taken as a suggestion for that
 attribute.  The CFG_REPLY Configuration Payload MAY return that
 value, or a new one.  It MAY also add new attributes and not include
 some requested ones.  Requestors MUST ignore returned attributes that
 they do not recognize.
 Some attributes MAY be multi-valued, in which case multiple attribute
 values of the same type are sent and/or returned.  Generally, all
 values of an attribute are returned when the attribute is requested.
 For some attributes (in this version of the specification only
 internal addresses), multiple requests indicates a request that
 multiple values be assigned.  For these attributes, the number of
 values returned SHOULD NOT exceed the number requested.

Kaufman Standards Track [Page 79] RFC 4306 IKEv2 December 2005

 If the data type requested in a CFG_REQUEST is not recognized or not
 supported, the responder MUST NOT return an error type but rather
 MUST either send a CFG_REPLY that MAY be empty or a reply not
 containing a CFG_REPLY payload at all.  Error returns are reserved
 for cases where the request is recognized but cannot be performed as
 requested or the request is badly formatted.
 "CFG_SET/CFG_ACK" allows an IKE endpoint to push configuration data
 to its peer.  In this case, the CFG_SET Configuration Payload
 contains attributes the initiator wants its peer to alter.  The
 responder MUST return a Configuration Payload if it accepted any of
 the configuration data and it MUST contain the attributes that the
 responder accepted with zero-length data.  Those attributes that it
 did not accept MUST NOT be in the CFG_ACK Configuration Payload.  If
 no attributes were accepted, the responder MUST return either an
 empty CFG_ACK payload or a response message without a CFG_ACK
 payload.  There are currently no defined uses for the CFG_SET/CFG_ACK
 exchange, though they may be used in connection with extensions based
 on Vendor IDs.  An minimal implementation of this specification MAY
 ignore CFG_SET payloads.
 Extensions via the CP payload SHOULD NOT be used for general purpose
 management.  Its main intent is to provide a bootstrap mechanism to
 exchange information within IPsec from IRAS to IRAC.  While it MAY be
 useful to use such a method to exchange information between some
 Security Gateways (SGW) or small networks, existing management
 protocols such as DHCP [DHCP], RADIUS [RADIUS], SNMP, or LDAP [LDAP]
 should be preferred for enterprise management as well as subsequent
 information exchanges.
 The Configuration Payload is defined as follows:
                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ! Next Payload  !C! RESERVED    !         Payload Length        !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !   CFG Type    !                    RESERVED                   !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !                                                               !
    ~                   Configuration Attributes                    ~
    !                                                               !
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 22:  Configuration Payload Format
 The payload type for the Configuration Payload is forty seven (47).

Kaufman Standards Track [Page 80] RFC 4306 IKEv2 December 2005

 o  CFG Type (1 octet) - The type of exchange represented by the
    Configuration Attributes.
           CFG Type       Value
           ===========    =====
           RESERVED         0
           CFG_REQUEST      1
           CFG_REPLY        2
           CFG_SET          3
           CFG_ACK          4
    values 5-127 are reserved to IANA.  Values 128-255 are for private
    use among mutually consenting parties.
 o  RESERVED (3 octets)  - MUST be sent as zero; MUST be ignored on
    receipt.
 o  Configuration Attributes (variable length) - These are type length
    values specific to the Configuration Payload and are defined
    below.  There may be zero or more Configuration Attributes in this
    payload.

3.15.1. Configuration Attributes

                         1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    !R|         Attribute Type      !            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    ~                             Value                             ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 23:  Configuration Attribute Format
 o  Reserved (1 bit) - This bit MUST be set to zero and MUST be
    ignored on receipt.
 o  Attribute Type (15 bits) - A unique identifier for each of the
    Configuration Attribute Types.
 o  Length (2 octets) - Length in octets of Value.
 o  Value (0 or more octets) - The variable-length value of this
    Configuration Attribute.

Kaufman Standards Track [Page 81] RFC 4306 IKEv2 December 2005

 The following attribute types have been defined:
                                    Multi-
      Attribute Type          Value Valued Length
      ======================= ===== ====== ==================
       RESERVED                 0
       INTERNAL_IP4_ADDRESS     1    YES*  0 or 4 octets
       INTERNAL_IP4_NETMASK     2    NO    0 or 4 octets
       INTERNAL_IP4_DNS         3    YES   0 or 4 octets
       INTERNAL_IP4_NBNS        4    YES   0 or 4 octets
       INTERNAL_ADDRESS_EXPIRY  5    NO    0 or 4 octets
       INTERNAL_IP4_DHCP        6    YES   0 or 4 octets
       APPLICATION_VERSION      7    NO    0 or more
       INTERNAL_IP6_ADDRESS     8    YES*  0 or 17 octets
       RESERVED                 9
       INTERNAL_IP6_DNS        10    YES   0 or 16 octets
       INTERNAL_IP6_NBNS       11    YES   0 or 16 octets
       INTERNAL_IP6_DHCP       12    YES   0 or 16 octets
       INTERNAL_IP4_SUBNET     13    YES   0 or 8 octets
       SUPPORTED_ATTRIBUTES    14    NO    Multiple of 2
       INTERNAL_IP6_SUBNET     15    YES   17 octets
  • These attributes may be multi-valued on return only if multiple

values were requested.

    Types 16-16383 are reserved to IANA.  Values 16384-32767 are for
    private use among mutually consenting parties.
    o  INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
       internal network, sometimes called a red node address or
       private address and MAY be a private address on the Internet.
       In a request message, the address specified is a requested
       address (or zero if no specific address is requested).  If a
       specific address is requested, it likely indicates that a
       previous connection existed with this address and the requestor
       would like to reuse that address.  With IPv6, a requestor MAY
       supply the low-order address bytes it wants to use.  Multiple
       internal addresses MAY be requested by requesting multiple
       internal address attributes.  The responder MAY only send up to
       the number of addresses requested.  The INTERNAL_IP6_ADDRESS is
       made up of two fields: the first is a sixteen-octet IPv6
       address and the second is a one-octet prefix-length as defined
       in [ADDRIPV6].
       The requested address is valid until the expiry time defined
       with the INTERNAL_ADDRESS EXPIRY attribute or there are no
       IKE_SAs between the peers.

Kaufman Standards Track [Page 82] RFC 4306 IKEv2 December 2005

    o  INTERNAL_IP4_NETMASK - The internal network's netmask.  Only
       one netmask is allowed in the request and reply messages (e.g.,
       255.255.255.0), and it MUST be used only with an
       INTERNAL_IP4_ADDRESS attribute.
    o  INTERNAL_IP4_DNS, INTERNAL_IP6_DNS - Specifies an address of a
       DNS server within the network.  Multiple DNS servers MAY be
       requested.  The responder MAY respond with zero or more DNS
       server attributes.
    o  INTERNAL_IP4_NBNS, INTERNAL_IP6_NBNS - Specifies an address of
       a NetBios Name Server (WINS) within the network.  Multiple NBNS
       servers MAY be requested.  The responder MAY respond with zero
       or more NBNS server attributes.
    o  INTERNAL_ADDRESS_EXPIRY - Specifies the number of seconds that
       the host can use the internal IP address.  The host MUST renew
       the IP address before this expiry time.  Only one of these
       attributes MAY be present in the reply.
    o  INTERNAL_IP4_DHCP, INTERNAL_IP6_DHCP - Instructs the host to
       send any internal DHCP requests to the address contained within
       the attribute.  Multiple DHCP servers MAY be requested.  The
       responder MAY respond with zero or more DHCP server attributes.
    o  APPLICATION_VERSION - The version or application information of
       the IPsec host.  This is a string of printable ASCII characters
       that is NOT null terminated.
    o  INTERNAL_IP4_SUBNET - The protected sub-networks that this
       edge-device protects.  This attribute is made up of two fields:
       the first is an IP address and the second is a netmask.
       Multiple sub-networks MAY be requested.  The responder MAY
       respond with zero or more sub-network attributes.
    o  SUPPORTED_ATTRIBUTES - When used within a Request, this
       attribute MUST be zero-length and specifies a query to the
       responder to reply back with all of the attributes that it
       supports.  The response contains an attribute that contains a
       set of attribute identifiers each in 2 octets.  The length
       divided by 2 (octets) would state the number of supported
       attributes contained in the response.

Kaufman Standards Track [Page 83] RFC 4306 IKEv2 December 2005

    o  INTERNAL_IP6_SUBNET - The protected sub-networks that this
       edge-device protects.  This attribute is made up of two fields:
       the first is a sixteen-octet IPv6 address and the second is a
       one-octet prefix-length as defined in [ADDRIPV6].  Multiple
       sub-networks MAY be requested.  The responder MAY respond with
       zero or more sub-network attributes.
    Note that no recommendations are made in this document as to how
    an implementation actually figures out what information to send in
    a reply.  That is, we do not recommend any specific method of an
    IRAS determining which DNS server should be returned to a
    requesting IRAC.

3.16. Extensible Authentication Protocol (EAP) Payload

 The Extensible Authentication Protocol Payload, denoted EAP in this
 memo, allows IKE_SAs to be authenticated using the protocol defined
 in RFC 3748 [EAP] and subsequent extensions to that protocol.  The
 full set of acceptable values for the payload is defined elsewhere,
 but a short summary of RFC 3748 is included here to make this
 document stand alone in the common cases.
                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! Next Payload  !C!  RESERVED   !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !                                                               !
     ~                       EAP Message                             ~
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 24:  EAP Payload Format
    The payload type for an EAP Payload is forty eight (48).
                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !     Code      ! Identifier    !           Length              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !     Type      ! Type_Data...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
                    Figure 25:  EAP Message Format
 o  Code (1 octet) indicates whether this message is a Request (1),
    Response (2), Success (3), or Failure (4).

Kaufman Standards Track [Page 84] RFC 4306 IKEv2 December 2005

 o  Identifier (1 octet) is used in PPP to distinguish replayed
    messages from repeated ones.  Since in IKE, EAP runs over a
    reliable protocol, it serves no function here.  In a response
    message, this octet MUST be set to match the identifier in the
    corresponding request.  In other messages, this field MAY be set
    to any value.
 o  Length (2 octets) is the length of the EAP message and MUST be
    four less than the Payload Length of the encapsulating payload.
 o  Type (1 octet) is present only if the Code field is Request (1) or
    Response (2).  For other codes, the EAP message length MUST be
    four octets and the Type and Type_Data fields MUST NOT be present.
    In a Request (1) message, Type indicates the data being requested.
    In a Response (2) message, Type MUST either be Nak or match the
    type of the data requested.  The following types are defined in
    RFC 3748:
    1  Identity
    2  Notification
    3  Nak (Response Only)
    4  MD5-Challenge
    5  One-Time Password (OTP)
    6  Generic Token Card
 o  Type_Data (Variable Length) varies with the Type of Request and
    the associated Response.  For the documentation of the EAP
    methods, see [EAP].
 Note that since IKE passes an indication of initiator identity in
 message 3 of the protocol, the responder SHOULD NOT send EAP Identity
 requests.  The initiator SHOULD, however, respond to such requests if
 it receives them.

4. Conformance Requirements

 In order to assure that all implementations of IKEv2 can
 interoperate, there are "MUST support" requirements in addition to
 those listed elsewhere.  Of course, IKEv2 is a security protocol, and
 one of its major functions is to allow only authorized parties to
 successfully complete establishment of SAs.  So a particular
 implementation may be configured with any of a number of restrictions
 concerning algorithms and trusted authorities that will prevent
 universal interoperability.

Kaufman Standards Track [Page 85] RFC 4306 IKEv2 December 2005

 IKEv2 is designed to permit minimal implementations that can
 interoperate with all compliant implementations.  There are a series
 of optional features that can easily be ignored by a particular
 implementation if it does not support that feature.  Those features
 include:
    Ability to negotiate SAs through a NAT and tunnel the resulting
    ESP SA over UDP.
    Ability to request (and respond to a request for) a temporary IP
    address on the remote end of a tunnel.
    Ability to support various types of legacy authentication.
    Ability to support window sizes greater than one.
    Ability to establish multiple ESP and/or AH SAs within a single
    IKE_SA.
    Ability to rekey SAs.
 To assure interoperability, all implementations MUST be capable of
 parsing all payload types (if only to skip over them) and to ignore
 payload types that it does not support unless the critical bit is set
 in the payload header.  If the critical bit is set in an unsupported
 payload header, all implementations MUST reject the messages
 containing those payloads.
 Every implementation MUST be capable of doing four-message
 IKE_SA_INIT and IKE_AUTH exchanges establishing two SAs (one for IKE,
 one for ESP and/or AH).  Implementations MAY be initiate-only or
 respond-only if appropriate for their platform.  Every implementation
 MUST be capable of responding to an INFORMATIONAL exchange, but a
 minimal implementation MAY respond to any INFORMATIONAL message with
 an empty INFORMATIONAL reply (note that within the context of an
 IKE_SA, an "empty" message consists of an IKE header followed by an
 Encrypted payload with no payloads contained in it).  A minimal
 implementation MAY support the CREATE_CHILD_SA exchange only in so
 far as to recognize requests and reject them with a Notify payload of
 type NO_ADDITIONAL_SAS.  A minimal implementation need not be able to
 initiate CREATE_CHILD_SA or INFORMATIONAL exchanges.  When an SA
 expires (based on locally configured values of either lifetime or
 octets passed), and implementation MAY either try to renew it with a
 CREATE_CHILD_SA exchange or it MAY delete (close) the old SA and
 create a new one.  If the responder rejects the CREATE_CHILD_SA
 request with a NO_ADDITIONAL_SAS notification, the implementation
 MUST be capable of instead closing the old SA and creating a new one.

Kaufman Standards Track [Page 86] RFC 4306 IKEv2 December 2005

 Implementations are not required to support requesting temporary IP
 addresses or responding to such requests.  If an implementation does
 support issuing such requests, it MUST include a CP payload in
 message 3 containing at least a field of type INTERNAL_IP4_ADDRESS or
 INTERNAL_IP6_ADDRESS.  All other fields are optional.  If an
 implementation supports responding to such requests, it MUST parse
 the CP payload of type CFG_REQUEST in message 3 and recognize a field
 of type INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS.  If it supports
 leasing an address of the appropriate type, it MUST return a CP
 payload of type CFG_REPLY containing an address of the requested
 type.  The responder SHOULD include all of the other related
 attributes if it has them.
 A minimal IPv4 responder implementation will ignore the contents of
 the CP payload except to determine that it includes an
 INTERNAL_IP4_ADDRESS attribute and will respond with the address and
 other related attributes regardless of whether the initiator
 requested them.
 A minimal IPv4 initiator will generate a CP payload containing only
 an INTERNAL_IP4_ADDRESS attribute and will parse the response
 ignoring attributes it does not know how to use.  The only attribute
 it MUST be able to process is INTERNAL_ADDRESS_EXPIRY, which it must
 use to bound the lifetime of the SA unless it successfully renews the
 lease before it expires.  Minimal initiators need not be able to
 request lease renewals and minimal responders need not respond to
 them.
 For an implementation to be called conforming to this specification,
 it MUST be possible to configure it to accept the following:
 PKIX Certificates containing and signed by RSA keys of size 1024 or
 2048 bits, where the ID passed is any of ID_KEY_ID, ID_FQDN,
 ID_RFC822_ADDR, or ID_DER_ASN1_DN.
 Shared key authentication where the ID passes is any of ID_KEY_ID,
 ID_FQDN, or ID_RFC822_ADDR.
 Authentication where the responder is authenticated using PKIX
 Certificates and the initiator is authenticated using shared key
 authentication.

Kaufman Standards Track [Page 87] RFC 4306 IKEv2 December 2005

5. Security Considerations

 While this protocol is designed to minimize disclosure of
 configuration information to unauthenticated peers, some such
 disclosure is unavoidable.  One peer or the other must identify
 itself first and prove its identity first.  To avoid probing, the
 initiator of an exchange is required to identify itself first, and
 usually is required to authenticate itself first.  The initiator can,
 however, learn that the responder supports IKE and what cryptographic
 protocols it supports.  The responder (or someone impersonating the
 responder) can probe the initiator not only for its identity, but
 using CERTREQ payloads may be able to determine what certificates the
 initiator is willing to use.
 Use of EAP authentication changes the probing possibilities somewhat.
 When EAP authentication is used, the responder proves its identity
 before the initiator does, so an initiator that knew the name of a
 valid initiator could probe the responder for both its name and
 certificates.
 Repeated rekeying using CREATE_CHILD_SA without additional Diffie-
 Hellman exchanges leaves all SAs vulnerable to cryptanalysis of a
 single key or overrun of either endpoint.  Implementers should take
 note of this fact and set a limit on CREATE_CHILD_SA exchanges
 between exponentiations.  This memo does not prescribe such a limit.
 The strength of a key derived from a Diffie-Hellman exchange using
 any of the groups defined here depends on the inherent strength of
 the group, the size of the exponent used, and the entropy provided by
 the random number generator used.  Due to these inputs, it is
 difficult to determine the strength of a key for any of the defined
 groups.  Diffie-Hellman group number two, when used with a strong
 random number generator and an exponent no less than 200 bits, is
 common for use with 3DES.  Group five provides greater security than
 group two.  Group one is for historic purposes only and does not
 provide sufficient strength except for use with DES, which is also
 for historic use only.  Implementations should make note of these
 estimates when establishing policy and negotiating security
 parameters.
 Note that these limitations are on the Diffie-Hellman groups
 themselves.  There is nothing in IKE that prohibits using stronger
 groups nor is there anything that will dilute the strength obtained
 from stronger groups (limited by the strength of the other algorithms
 negotiated including the prf function).  In fact, the extensible
 framework of IKE encourages the definition of more groups; use of
 elliptical curve groups may greatly increase strength using much
 smaller numbers.

Kaufman Standards Track [Page 88] RFC 4306 IKEv2 December 2005

 It is assumed that all Diffie-Hellman exponents are erased from
 memory after use.  In particular, these exponents MUST NOT be derived
 from long-lived secrets like the seed to a pseudo-random generator
 that is not erased after use.
 The strength of all keys is limited by the size of the output of the
 negotiated prf function.  For this reason, a prf function whose
 output is less than 128 bits (e.g., 3DES-CBC) MUST NOT be used with
 this protocol.
 The security of this protocol is critically dependent on the
 randomness of the randomly chosen parameters.  These should be
 generated by a strong random or properly seeded pseudo-random source
 (see [RFC4086]).  Implementers should take care to ensure that use of
 random numbers for both keys and nonces is engineered in a fashion
 that does not undermine the security of the keys.
 For information on the rationale of many of the cryptographic design
 choices in this protocol, see [SIGMA] and [SKEME].  Though the
 security of negotiated CHILD_SAs does not depend on the strength of
 the encryption and integrity protection negotiated in the IKE_SA,
 implementations MUST NOT negotiate NONE as the IKE integrity
 protection algorithm or ENCR_NULL as the IKE encryption algorithm.
 When using pre-shared keys, a critical consideration is how to assure
 the randomness of these secrets.  The strongest practice is to ensure
 that any pre-shared key contain as much randomness as the strongest
 key being negotiated.  Deriving a shared secret from a password,
 name, or other low-entropy source is not secure.  These sources are
 subject to dictionary and social engineering attacks, among others.
 The NAT_DETECTION_*_IP notifications contain a hash of the addresses
 and ports in an attempt to hide internal IP addresses behind a NAT.
 Since the IPv4 address space is only 32 bits, and it is usually very
 sparse, it would be possible for an attacker to find out the internal
 address used behind the NAT box by trying all possible IP addresses
 and trying to find the matching hash.  The port numbers are normally
 fixed to 500, and the SPIs can be extracted from the packet.  This
 reduces the number of hash calculations to 2^32.  With an educated
 guess of the use of private address space, the number of hash
 calculations is much smaller.  Designers should therefore not assume
 that use of IKE will not leak internal address information.
 When using an EAP authentication method that does not generate a
 shared key for protecting a subsequent AUTH payload, certain man-in-
 the-middle and server impersonation attacks are possible [EAPMITM].
 These vulnerabilities occur when EAP is also used in protocols that
 are not protected with a secure tunnel.  Since EAP is a general-

Kaufman Standards Track [Page 89] RFC 4306 IKEv2 December 2005

 purpose authentication protocol, which is often used to provide
 single-signon facilities, a deployed IPsec solution that relies on an
 EAP authentication method that does not generate a shared key (also
 known as a non-key-generating EAP method) can become compromised due
 to the deployment of an entirely unrelated application that also
 happens to use the same non-key-generating EAP method, but in an
 unprotected fashion.  Note that this vulnerability is not limited to
 just EAP, but can occur in other scenarios where an authentication
 infrastructure is reused.  For example, if the EAP mechanism used by
 IKEv2 utilizes a token authenticator, a man-in-the-middle attacker
 could impersonate the web server, intercept the token authentication
 exchange, and use it to initiate an IKEv2 connection.  For this
 reason, use of non-key-generating EAP methods SHOULD be avoided where
 possible.  Where they are used, it is extremely important that all
 usages of these EAP methods SHOULD utilize a protected tunnel, where
 the initiator validates the responder's certificate before initiating
 the EAP exchange.  Implementers SHOULD describe the vulnerabilities
 of using non-key-generating EAP methods in the documentation of their
 implementations so that the administrators deploying IPsec solutions
 are aware of these dangers.
 An implementation using EAP MUST also use a public-key-based
 authentication of the server to the client before the EAP exchange
 begins, even if the EAP method offers mutual authentication.  This
 avoids having additional IKEv2 protocol variations and protects the
 EAP data from active attackers.
 If the messages of IKEv2 are long enough that IP-level fragmentation
 is necessary, it is possible that attackers could prevent the
 exchange from completing by exhausting the reassembly buffers.  The
 chances of this can be minimized by using the Hash and URL encodings
 instead of sending certificates (see section 3.6).  Additional
 mitigations are discussed in [KPS03].

6. IANA Considerations

 This document defines a number of new field types and values where
 future assignments will be managed by the IANA.
 The following registries have been created by the IANA:
    IKEv2 Exchange Types (section 3.1)
    IKEv2 Payload Types (section 3.2)
    IKEv2 Transform Types (section 3.3.2)
        IKEv2 Transform Attribute Types (section 3.3.2)
        IKEv2 Encryption Transform IDs (section 3.3.2)
        IKEv2 Pseudo-random Function Transform IDs (section 3.3.2)
        IKEv2 Integrity Algorithm Transform IDs (section 3.3.2)

Kaufman Standards Track [Page 90] RFC 4306 IKEv2 December 2005

        IKEv2 Diffie-Hellman Transform IDs (section 3.3.2)
    IKEv2 Identification Payload ID Types (section 3.5)
    IKEv2 Certificate Encodings (section 3.6)
    IKEv2 Authentication Method (section 3.8)
    IKEv2 Notify Message Types (section 3.10.1)
        IKEv2 Notification IPCOMP Transform IDs (section 3.10.1)
    IKEv2 Security Protocol Identifiers (section 3.3.1)
    IKEv2 Traffic Selector Types (section 3.13.1)
    IKEv2 Configuration Payload CFG Types (section 3.15)
    IKEv2 Configuration Payload Attribute Types (section 3.15.1)
 Note: When creating a new Transform Type, a new registry for it must
 be created.
 Changes and additions to any of those registries are by expert
 review.

7. Acknowledgements

 This document is a collaborative effort of the entire IPsec WG.  If
 there were no limit to the number of authors that could appear on an
 RFC, the following, in alphabetical order, would have been listed:
 Bill Aiello, Stephane Beaulieu, Steve Bellovin, Sara Bitan, Matt
 Blaze, Ran Canetti, Darren Dukes, Dan Harkins, Paul Hoffman, John
 Ioannidis, Charlie Kaufman, Steve Kent, Angelos Keromytis, Tero
 Kivinen, Hugo Krawczyk, Andrew Krywaniuk, Radia Perlman, Omer
 Reingold, and Michael Richardson.  Many other people contributed to
 the design.  It is an evolution of IKEv1, ISAKMP, and the IPsec DOI,
 each of which has its own list of authors.  Hugh Daniel suggested the
 feature of having the initiator, in message 3, specify a name for the
 responder, and gave the feature the cute name "You Tarzan, Me Jane".
 David Faucher and Valery Smyzlov helped refine the design of the
 traffic selector negotiation.

8. References

8.1. Normative References

 [ADDGROUP] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003.
 [ADDRIPV6] Hinden, R. and S. Deering, "Internet Protocol Version 6
            (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [Bra97]    Bradner, S., "Key Words for use in RFCs to indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Kaufman Standards Track [Page 91] RFC 4306 IKEv2 December 2005

 [EAP]      Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
            Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
            3748, June 2004.
 [ESPCBC]   Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
            Algorithms", RFC 2451, November 1998.
 [Hutt05]   Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
            Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
            3948, January 2005.
 [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP", RFC
            3168, September 2001.
 [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
            X.509 Public Key Infrastructure Certificate and
            Certificate Revocation List (CRL) Profile", RFC 3280,
            April 2002.
 [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, December 2005.

8.2. Informative References

 [DES]      ANSI X3.106, "American National Standard for Information
            Systems-Data Link Encryption", American National Standards
            Institute, 1983.
 [DH]       Diffie, W., and Hellman M., "New Directions in
            Cryptography", IEEE Transactions on Information Theory, V.
            IT-22, n. 6, June 1977.
 [DHCP]     Droms, R., "Dynamic Host Configuration Protocol", RFC
            2131, March 1997.
 [DSS]      NIST, "Digital Signature Standard", FIPS 186, National
            Institute of Standards and Technology, U.S. Department of
            Commerce, May, 1994.
 [EAPMITM]  Asokan, N., Nierni, V., and Nyberg, K., "Man-in-the-Middle
            in Tunneled Authentication Protocols",
            http://eprint.iacr.org/2002/163, November 2002.

Kaufman Standards Track [Page 92] RFC 4306 IKEv2 December 2005

 [HC98]     Harkins, D. and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998.
 [IDEA]     Lai, X., "On the Design and Security of Block Ciphers,"
            ETH Series in Information Processing, v. 1, Konstanz:
            Hartung-Gorre Verlag, 1992.
 [IPCOMP]   Shacham, A., Monsour, B., Pereira, R., and M.  Thomas, "IP
            Payload Compression Protocol (IPComp)", RFC 3173,
            September 2001.
 [KPS03]    Kaufman, C., Perlman, R., and Sommerfeld, B., "DoS
            protection for UDP-based protocols", ACM Conference on
            Computer and Communications Security, October 2003.
 [KBC96]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104, February
            1997.
 [LDAP]     Wahl, M., Howes, T., and S  Kille, "Lightweight Directory
            Access Protocol (v3)", RFC 2251, December 1997.
 [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992.
 [MSST98]   Maughan, D., Schertler, M., Schneider, M., and J. Turner,
            "Internet Security Association and Key Management Protocol
            (ISAKMP)", RFC 2408, November 1998.
 [Orm96]    Orman, H., "The OAKLEY Key Determination Protocol", RFC
            2412, November 1998.
 [PFKEY]    McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
            Management API, Version 2", RFC 2367, July 1998.
 [PKCS1]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, February 2003.
 [PK01]     Perlman, R., and Kaufman, C., "Analysis of the IPsec key
            exchange Standard", WET-ICE Security Conference, MIT,2001,
            http://sec.femto.org/wetice-2001/papers/radia-paper.pdf.
 [Pip98]    Piper, D., "The Internet IP Security Domain Of
            Interpretation for ISAKMP", RFC 2407, November 1998.

Kaufman Standards Track [Page 93] RFC 4306 IKEv2 December 2005

 [RADIUS]   Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)", RFC
            2865, June 2000.
 [RFC4086]  Eastlake, D., 3rd, Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005.
 [RFC1958]  Carpenter, B., "Architectural Principles of the Internet",
            RFC 1958, June 1996.
 [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474, December
            1998.
 [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
            and W. Weiss, "An Architecture for Differentiated
            Service", RFC 2475, December 1998.
 [RFC2522]  Karn, P. and W. Simpson, "Photuris: Session-Key Management
            Protocol", RFC 2522, March 1999.
 [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775, February
            2000.
 [RFC2983]  Black, D., "Differentiated Services and Tunnels", RFC
            2983, October 2000.
 [RFC3439]  Bush, R. and D. Meyer, "Some Internet Architectural
            Guidelines and Philosophy", RFC 3439, December 2002.
 [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
            (NAT) Compatibility Requirements", RFC 3715, March 2004.
 [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December
            2005.
 [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
            4303, December 2005.
 [RSA]      Rivest, R., Shamir, A., and Adleman, L., "A Method for
            Obtaining Digital Signatures and Public-Key
            Cryptosystems", Communications of the ACM, v. 21, n. 2,
            February 1978.

Kaufman Standards Track [Page 94] RFC 4306 IKEv2 December 2005

 [SHA]      NIST, "Secure Hash Standard", FIPS 180-1, National
            Institute of Standards and Technology, U.S. Department of
            Commerce, May 1994.
 [SIGMA]    Krawczyk, H., "SIGMA: the `SIGn-and-MAc' Approach to
            Authenticated Diffie-Hellman and its Use in the IKE
            Protocols", in Advances in Cryptography - CRYPTO 2003
            Proceedings, LNCS 2729, Springer, 2003.  Available at:
            http://www.informatik.uni-trier.de/~ley/db/conf/
            crypto/crypto2003.html.
 [SKEME]    Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
            Mechanism for Internet", from IEEE Proceedings of the 1996
            Symposium on Network and Distributed Systems Security.
 [X.501]    ITU-T Recommendation X.501: Information Technology - Open
            Systems Interconnection - The Directory: Models, 1993.
 [X.509]    ITU-T Recommendation X.509 (1997 E): Information
            Technology - Open Systems Interconnection - The Directory:
            Authentication Framework, June 1997.

Kaufman Standards Track [Page 95] RFC 4306 IKEv2 December 2005

Appendix A: Summary of changes from IKEv1

 The goals of this revision to IKE are:
 1) To define the entire IKE protocol in a single document, replacing
 RFCs 2407, 2408, and 2409 and incorporating subsequent changes to
 support NAT Traversal, Extensible Authentication, and Remote Address
 acquisition;
 2) To simplify IKE by replacing the eight different initial exchanges
 with a single four-message exchange (with changes in authentication
 mechanisms affecting only a single AUTH payload rather than
 restructuring the entire exchange) see [PK01];
 3) To remove the Domain of Interpretation (DOI), Situation (SIT), and
 Labeled Domain Identifier fields, and the Commit and Authentication
 only bits;
 4) To decrease IKE's latency in the common case by making the initial
 exchange be 2 round trips (4 messages), and allowing the ability to
 piggyback setup of a CHILD_SA on that exchange;
 5) To replace the cryptographic syntax for protecting the IKE
 messages themselves with one based closely on ESP to simplify
 implementation and security analysis;
 6) To reduce the number of possible error states by making the
 protocol reliable (all messages are acknowledged) and sequenced.
 This allows shortening CREATE_CHILD_SA exchanges from 3 messages to
 2;
 7) To increase robustness by allowing the responder to not do
 significant processing until it receives a message proving that the
 initiator can receive messages at its claimed IP address, and not
 commit any state to an exchange until the initiator can be
 cryptographically authenticated;
 8) To fix cryptographic weaknesses such as the problem with
 symmetries in hashes used for authentication documented by Tero
 Kivinen;
 9) To specify Traffic Selectors in their own payloads type rather
 than overloading ID payloads, and making more flexible the Traffic
 Selectors that may be specified;
 10) To specify required behavior under certain error conditions or
 when data that is not understood is received, to make it easier to
 make future revisions that do not break backward compatibility;

Kaufman Standards Track [Page 96] RFC 4306 IKEv2 December 2005

 11) To simplify and clarify how shared state is maintained in the
 presence of network failures and Denial of Service attacks; and
 12) To maintain existing syntax and magic numbers to the extent
 possible to make it likely that implementations of IKEv1 can be
 enhanced to support IKEv2 with minimum effort.

Appendix B: Diffie-Hellman Groups

 There are two Diffie-Hellman groups defined here for use in IKE.
 These groups were generated by Richard Schroeppel at the University
 of Arizona.  Properties of these primes are described in [Orm96].
 The strength supplied by group one may not be sufficient for the
 mandatory-to-implement encryption algorithm and is here for historic
 reasons.
 Additional Diffie-Hellman groups have been defined in [ADDGROUP].

B.1. Group 1 - 768 Bit MODP

 This group is assigned id 1 (one).
 The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 } Its
 hexadecimal value is:
      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
      8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
      302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
      A63A3620 FFFFFFFF FFFFFFFF
 The generator is 2.

B.2. Group 2 - 1024 Bit MODP

 This group is assigned id 2 (two).
 The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
 Its hexadecimal value is:
      FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
      8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
      302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
      A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
      49286651 ECE65381 FFFFFFFF FFFFFFFF
 The generator is 2.

Kaufman Standards Track [Page 97] RFC 4306 IKEv2 December 2005

Editor's Address

 Charlie Kaufman
 Microsoft Corporation
 1 Microsoft Way
 Redmond, WA 98052
 Phone: 1-425-707-3335
 EMail: charliek@microsoft.com

Kaufman Standards Track [Page 98] RFC 4306 IKEv2 December 2005

Full Copyright Statement

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Kaufman Standards Track [Page 99]

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