GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools

Problem, Formatting or Query -  Send Feedback

Was this page helpful?-10+1


rfc:std:std79

Internet Engineering Task Force (IETF) C. Kaufman Request for Comments: 7296 Microsoft STD: 79 P. Hoffman Obsoletes: 5996 VPN Consortium Category: Standards Track Y. Nir ISSN: 2070-1721 Check Point

                                                             P. Eronen
                                                           Independent
                                                            T. Kivinen
                                                         INSIDE Secure
                                                          October 2014
          Internet Key Exchange Protocol Version 2 (IKEv2)

Abstract

 This document describes version 2 of the Internet Key Exchange (IKE)
 protocol.  IKE is a component of IPsec used for performing mutual
 authentication and establishing and maintaining Security Associations
 (SAs).  This document obsoletes RFC 5996, and includes all of the
 errata for it.  It advances IKEv2 to be an Internet Standard.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7296.

Kaufman, et al. Standards Track [Page 1] RFC 7296 IKEv2bis October 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Kaufman, et al. Standards Track [Page 2] RFC 7296 IKEv2bis October 2014

Table of Contents

 1. Introduction ....................................................5
    1.1. Usage Scenarios ............................................7
         1.1.1. Security Gateway to Security Gateway in
                Tunnel Mode .........................................7
         1.1.2. Endpoint-to-Endpoint Transport Mode .................8
         1.1.3. Endpoint to Security Gateway in Tunnel Mode .........8
         1.1.4. Other Scenarios .....................................9
    1.2. The Initial Exchanges ......................................9
    1.3. The CREATE_CHILD_SA Exchange ..............................13
         1.3.1. Creating New Child SAs with the
                CREATE_CHILD_SA Exchange ...........................14
         1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA
                Exchange ...........................................16
         1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA
                Exchange ...........................................16
    1.4. The INFORMATIONAL Exchange ................................17
         1.4.1. Deleting an SA with INFORMATIONAL Exchanges ........18
    1.5. Informational Messages outside of an IKE SA ...............19
    1.6. Requirements Terminology ..................................20
    1.7. Significant Differences between RFC 4306 and RFC 5996 .....20
    1.8. Differences between RFC 5996 and This Document ............23
 2. IKE Protocol Details and Variations ............................23
    2.1. Use of Retransmission Timers ..............................24
    2.2. Use of Sequence Numbers for Message ID ....................25
    2.3. Window Size for Overlapping Requests ......................26
    2.4. State Synchronization and Connection Timeouts .............28
    2.5. Version Numbers and Forward Compatibility .................30
    2.6. IKE SA SPIs and Cookies ...................................32
         2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD .......35
    2.7. Cryptographic Algorithm Negotiation .......................35
    2.8. Rekeying ..................................................36
         2.8.1. Simultaneous Child SA Rekeying .....................38
         2.8.2. Simultaneous IKE SA Rekeying .......................40
         2.8.3. Rekeying the IKE SA versus Reauthentication ........42
    2.9. Traffic Selector Negotiation ..............................42
         2.9.1. Traffic Selectors Violating Own Policy .............45
         2.9.2. Traffic Selectors in Rekeying ......................46
    2.10. Nonces ...................................................46
    2.11. Address and Port Agility .................................47
    2.12. Reuse of Diffie-Hellman Exponentials .....................47
    2.13. Generating Keying Material ...............................48
    2.14. Generating Keying Material for the IKE SA ................49
    2.15. Authentication of the IKE SA .............................50
    2.16. Extensible Authentication Protocol Methods ...............52
    2.17. Generating Keying Material for Child SAs .................54
    2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange ........55

Kaufman, et al. Standards Track [Page 3] RFC 7296 IKEv2bis October 2014

    2.19. Requesting an Internal Address on a Remote Network .......56
    2.20. Requesting the Peer's Version ............................58
    2.21. Error Handling ...........................................58
         2.21.1. Error Handling in IKE_SA_INIT .....................59
         2.21.2. Error Handling in IKE_AUTH ........................59
         2.21.3. Error Handling after IKE SA is Authenticated ......60
         2.21.4. Error Handling Outside IKE SA .....................60
    2.22. IPComp ...................................................61
    2.23. NAT Traversal ............................................62
         2.23.1. Transport Mode NAT Traversal ......................66
    2.24. Explicit Congestion Notification (ECN) ...................70
    2.25. Exchange Collisions ......................................70
         2.25.1. Collisions while Rekeying or Closing Child SAs ....71
         2.25.2. Collisions while Rekeying or Closing IKE SAs ......71
 3. Header and Payload Formats .....................................72
    3.1. The IKE Header ............................................72
    3.2. Generic Payload Header ....................................75
    3.3. Security Association Payload ..............................77
         3.3.1. Proposal Substructure ..............................80
         3.3.2. Transform Substructure .............................81
         3.3.3. Valid Transform Types by Protocol ..................85
         3.3.4. Mandatory Transform IDs ............................85
         3.3.5. Transform Attributes ...............................86
         3.3.6. Attribute Negotiation ..............................88
    3.4. Key Exchange Payload ......................................89
    3.5. Identification Payloads ...................................90
    3.6. Certificate Payload .......................................92
    3.7. Certificate Request Payload ...............................95
    3.8. Authentication Payload ....................................97
    3.9. Nonce Payload .............................................98
    3.10. Notify Payload ...........................................99
         3.10.1. Notify Message Types .............................101
    3.11. Delete Payload ..........................................104
    3.12. Vendor ID Payload .......................................105
    3.13. Traffic Selector Payload ................................106
         3.13.1. Traffic Selector .................................108
    3.14. Encrypted Payload .......................................110
    3.15. Configuration Payload ...................................112
         3.15.1. Configuration Attributes .........................113
         3.15.2. Meaning of INTERNAL_IP4_SUBNET and
                 INTERNAL_IP6_SUBNET ..............................116
         3.15.3. Configuration Payloads for IPv6 ..................118
         3.15.4. Address Assignment Failures ......................119
    3.16. Extensible Authentication Protocol (EAP) Payload ........120
 4. Conformance Requirements ......................................122
 5. Security Considerations .......................................124
    5.1. Traffic Selector Authorization ...........................127

Kaufman, et al. Standards Track [Page 4] RFC 7296 IKEv2bis October 2014

 6. IANA Considerations ...........................................128
 7. References ....................................................128
    7.1. Normative References .....................................128
    7.2. Informative References ...................................130
 Appendix A. Summary of Changes from IKEv1 ........................136
 Appendix B. Diffie-Hellman Groups ................................137
   B.1. Group 1 - 768-bit MODP ....................................137
   B.2. Group 2 - 1024-bit MODP ...................................137
 Appendix C. Exchanges and Payloads ...............................138
   C.1. IKE_SA_INIT Exchange ......................................138
   C.2. IKE_AUTH Exchange without EAP .............................138
   C.3. IKE_AUTH Exchange with EAP ................................139
   C.4. CREATE_CHILD_SA Exchange for Creating or Rekeying
        Child SAs .................................................140
   C.5. CREATE_CHILD_SA Exchange for Rekeying the IKE SA ..........140
   C.6. INFORMATIONAL Exchange ....................................141
 Acknowledgements .................................................141
 Authors' Addresses ...............................................142

1. Introduction

 IP Security (IPsec) provides confidentiality, data integrity, access
 control, and data source authentication to IP datagrams.  These
 services are provided by maintaining shared state between the source
 and the sink of an IP datagram.  This state defines, among other
 things, the specific services provided to the datagram, which
 cryptographic algorithms will be used to provide the services, and
 the keys used as input to the cryptographic algorithms.
 Establishing this shared state in a manual fashion does not scale
 well.  Therefore, a protocol to establish this state dynamically is
 needed.  This document describes such a protocol -- the Internet Key
 Exchange (IKE).  Version 1 of IKE was defined in RFCs 2407 [DOI],
 2408 [ISAKMP], and 2409 [IKEV1].  IKEv2 replaced all of those RFCs.
 IKEv2 was defined in [IKEV2] (RFC 4306) and was clarified in [Clarif]
 (RFC 4718).  [RFC5996] replaced and updated RFCs 4306 and 4718.  This
 document replaces RFC 5996.  IKEv2 as stated in RFC 4306 was a change
 to the IKE protocol that was not backward compatible.  RFC 5996
 revised RFC 4306 to provide a clarification of IKEv2, making minimal
 changes to the IKEv2 protocol.  This document replaces RFC 5996,
 slightly revising it to make it suitable for progression to Internet
 Standard.  A list of the significant differences between RFCs 4306
 and 5996 is given in Section 1.7, and differences between RFC 5996
 and this document are given in Section 1.8.

Kaufman, et al. Standards Track [Page 5] RFC 7296 IKEv2bis October 2014

 IKE performs mutual authentication between two parties and
 establishes an IKE Security Association (SA) that includes shared
 secret information that can be used to efficiently establish SAs for
 Encapsulating Security Payload (ESP) [ESP] or Authentication Header
 (AH) [AH] and a set of cryptographic algorithms to be used by the SAs
 to protect the traffic that they carry.  In this document, the term
 "suite" or "cryptographic suite" refers to a complete set of
 algorithms used to protect an SA.  An initiator proposes one or more
 suites by listing supported algorithms that can be combined into
 suites in a mix-and-match fashion.  IKE can also negotiate use of IP
 Compression (IPComp) [IP-COMP] in connection with an ESP or AH SA.
 The SAs for ESP or AH that get set up through that IKE SA we call
 "Child SAs".
 All IKE communications consist of pairs of messages: a request and a
 response.  The pair is called an "exchange", and is sometimes called
 a "request/response pair".  The first two exchanges of messages
 establishing an IKE SA are called the IKE_SA_INIT exchange and the
 IKE_AUTH exchange; subsequent IKE exchanges are called either
 CREATE_CHILD_SA exchanges 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.
 An IKE message flow always consists of a request followed by a
 response.  It is the responsibility of the requester to ensure
 reliability.  If the response is not received within a timeout
 interval, the requester needs to retransmit the request (or abandon
 the connection).
 The first exchange of an IKE session, IKE_SA_INIT, negotiates
 security parameters for the IKE SA, sends nonces, and sends
 Diffie-Hellman values.
 The second exchange, IKE_AUTH, transmits identities, proves knowledge
 of the secrets corresponding to the two identities, and sets up an SA
 for the first (and often only) AH or ESP Child SA (unless there is
 failure setting up the AH or ESP Child SA, in which case the IKE SA
 is still established without the Child SA).

Kaufman, et al. Standards Track [Page 6] RFC 7296 IKEv2bis October 2014

 The types of subsequent exchanges are CREATE_CHILD_SA (which creates
 a Child SA) and INFORMATIONAL (which deletes an SA, reports error
 conditions, or does other housekeeping).  Every request requires a
 response.  An INFORMATIONAL request with no payloads (other than the
 empty Encrypted payload required by the syntax) is commonly used as a
 check for liveness.  These subsequent exchanges cannot be used until
 the initial exchanges have completed.
 In the description that follows, we assume that no errors occur.
 Modifications to the flow when errors occur are described in
 Section 2.21.

1.1. Usage Scenarios

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

1.1.1. Security Gateway to Security Gateway in Tunnel Mode

              +-+-+-+-+-+            +-+-+-+-+-+
              |         | IPsec      |         |
 Protected    |Tunnel   | tunnel     |Tunnel   |     Protected
 Subnet   <-->|Endpoint |<---------->|Endpoint |<--> Subnet
              |         |            |         |
              +-+-+-+-+-+            +-+-+-+-+-+
         Figure 1: Security Gateway to Security Gateway Tunnel
 In this scenario, neither endpoint of the IP connection implements
 IPsec, but network nodes between them protect traffic for part of the
 way.  Protection is transparent to the endpoints, and depends on
 ordinary routing to send packets through the tunnel endpoints for
 processing.  Each endpoint would announce the set of addresses
 "behind" it, and packets would be sent in tunnel mode where the inner
 IP header would contain the IP addresses of the actual endpoints.

Kaufman, et al. Standards Track [Page 7] RFC 7296 IKEv2bis October 2014

1.1.2. Endpoint-to-Endpoint Transport Mode

 +-+-+-+-+-+                                          +-+-+-+-+-+
 |         |                 IPsec transport          |         |
 |Protected|                or tunnel mode SA         |Protected|
 |Endpoint |<---------------------------------------->|Endpoint |
 |         |                                          |         |
 +-+-+-+-+-+                                          +-+-+-+-+-+
                    Figure 2: Endpoint to Endpoint
 In this scenario, both endpoints of the IP connection implement
 IPsec, as required of hosts in [IPSECARCH].  Transport mode will
 commonly be used with no inner IP header.  A single pair of addresses
 will be negotiated for packets to be protected by this SA.  These
 endpoints MAY implement application-layer access controls based on
 the IPsec authenticated identities of the participants.  This
 scenario enables the end-to-end security that has been a guiding
 principle for the Internet since [ARCHPRINC], [TRANSPARENCY], and a
 method of limiting the inherent problems with complexity in networks
 noted by [ARCHGUIDEPHIL].  Although this scenario may not be fully
 applicable to the IPv4 Internet, it has been deployed successfully in
 specific scenarios within intranets using IKEv1.  It should be more
 broadly enabled during the transition to IPv6 and with the adoption
 of IKEv2.
 It is possible in this scenario that one or both of the protected
 endpoints will be behind a network address translation (NAT) node, in
 which case the tunneled packets will have to be UDP encapsulated so
 that port numbers in the UDP headers can be used to identify
 individual endpoints "behind" the NAT (see Section 2.23).

1.1.3. Endpoint to Security Gateway in Tunnel Mode

 +-+-+-+-+-+                          +-+-+-+-+-+
 |         |         IPsec            |         |     Protected
 |Protected|         tunnel           |Tunnel   |     Subnet
 |Endpoint |<------------------------>|Endpoint |<--- and/or
 |         |                          |         |     Internet
 +-+-+-+-+-+                          +-+-+-+-+-+
             Figure 3: Endpoint to Security Gateway Tunnel
 In this scenario, a protected endpoint (typically a portable roaming
 computer) connects back to its corporate network through an IPsec-
 protected tunnel.  It might use this tunnel only to access
 information on the corporate network, or it might tunnel all of its
 traffic back through the corporate network in order to take advantage

Kaufman, et al. Standards Track [Page 8] RFC 7296 IKEv2bis October 2014

 of protection provided by a corporate firewall against Internet-based
 attacks.  In either case, the protected endpoint will want an IP
 address associated with the security gateway so that packets returned
 to it will go to the security gateway and be tunneled back.  This IP
 address may be static or may be dynamically allocated by the security
 gateway.  In support of the latter case, IKEv2 includes a mechanism
 (namely, configuration payloads) for the initiator to request an IP
 address owned by the security gateway for use for the duration of
 its SA.
 In this scenario, packets will use tunnel mode.  On each packet from
 the protected endpoint, the outer IP header will contain the source
 IP address associated with its current location (i.e., the address
 that will get traffic routed to the endpoint directly), while the
 inner IP header will contain the source IP address assigned by the
 security gateway (i.e., the address that will get traffic routed to
 the security gateway for forwarding to the endpoint).  The outer
 destination address will always be that of the security gateway,
 while the inner destination address will be the ultimate destination
 for the packet.
 In this scenario, it is possible that the protected endpoint will be
 behind a NAT.  In that case, the IP address as seen by the security
 gateway will not be the same as the IP address sent by the protected
 endpoint, and packets will have to be UDP encapsulated in order to be
 routed properly.  Interaction with NATs is covered in detail in
 Section 2.23.

1.1.4. Other Scenarios

 Other scenarios are possible, as are nested combinations of the
 above.  One notable example combines aspects of Sections 1.1.1 and
 1.1.3.  A subnet may make all external accesses through a remote
 security gateway using an IPsec tunnel, where the addresses on the
 subnet are routed to the security gateway by the rest of the
 Internet.  An example would be someone's home network being virtually
 on the Internet with static IP addresses even though connectivity is
 provided by an ISP that assigns a single dynamically assigned IP
 address to the user's security gateway (where the static IP addresses
 and an IPsec relay are provided by a third party located elsewhere).

1.2. The Initial Exchanges

 Communication using IKE always begins with IKE_SA_INIT and IKE_AUTH
 exchanges (known in IKEv1 as Phase 1).  These initial exchanges
 normally consist of four messages, though in some scenarios that
 number can grow.  All communications using IKE consist of request/
 response pairs.  We'll describe the base exchange first, followed by

Kaufman, et al. Standards Track [Page 9] RFC 7296 IKEv2bis October 2014

 variations.  The first pair of messages (IKE_SA_INIT) negotiate
 cryptographic algorithms, exchange nonces, and do a Diffie-Hellman
 exchange [DH].
 The second pair of messages (IKE_AUTH) authenticate the previous
 messages, exchange identities and certificates, and establish the
 first Child SA.  Parts of these messages are encrypted and integrity
 protected with keys established through the IKE_SA_INIT exchange, so
 the identities are hidden from eavesdroppers and all fields in all
 the messages are authenticated.  See Section 2.14 for information on
 how the encryption keys are generated.  (A man-in-the-middle attacker
 who cannot complete the IKE_AUTH exchange can nonetheless see the
 identity of the initiator.)
 All messages following the initial exchange are cryptographically
 protected using the cryptographic algorithms and keys negotiated in
 the IKE_SA_INIT exchange.  These subsequent messages use the syntax
 of the Encrypted payload described in Section 3.14, encrypted with
 keys that are derived as described in Section 2.14.  All subsequent
 messages include an Encrypted payload, even if they are referred to
 in the text as "empty".  For the CREATE_CHILD_SA, IKE_AUTH, or
 INFORMATIONAL exchanges, the message following the header is
 encrypted and the message including the header is integrity protected
 using the cryptographic algorithms negotiated for the IKE SA.
 Every IKE message contains a Message ID as part of its fixed header.
 This Message ID is used to match up requests and responses, and to
 identify retransmissions of messages.
 In the following descriptions, the payloads contained in the message
 are indicated by names as listed below.
 Notation    Payload
 -----------------------------------------
 AUTH        Authentication
 CERT        Certificate
 CERTREQ     Certificate Request
 CP          Configuration
 D           Delete
 EAP         Extensible Authentication
 HDR         IKE header (not a payload)
 IDi         Identification - Initiator
 IDr         Identification - Responder
 KE          Key Exchange
 Ni, Nr      Nonce
 N           Notify
 SA          Security Association
 SK          Encrypted and Authenticated

Kaufman, et al. Standards Track [Page 10] RFC 7296 IKEv2bis October 2014

 TSi         Traffic Selector - Initiator
 TSr         Traffic Selector - Responder
 V           Vendor ID
 The details of the contents of each payload are described in
 Section 3.  Payloads that may optionally appear will be shown in
 brackets, such as [CERTREQ]; this indicates that a Certificate
 Request payload can optionally be included.
 The initial exchanges are as follows:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SAi1, KEi, Ni  -->
 HDR contains the Security Parameter Indexes (SPIs), version numbers,
 Exchange Type, Message ID, 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 a quantity
 called SKEYSEED (see Section 2.14), from which all keys are derived
 for that IKE SA.  The messages that follow are encrypted and
 integrity protected in their entirety, with the exception of the
 message headers.  The keys used for the encryption and integrity
 protection are derived from SKEYSEED and are known as SK_e
 (encryption) and SK_a (authentication, a.k.a. integrity protection);
 see Sections 2.13 and 2.14 for details on the key derivation.  A
 separate SK_e and SK_a is computed for each direction.  In addition
 to the keys SK_e and SK_a derived from the Diffie-Hellman value for
 protection of the IKE SA, another quantity SK_d is derived and used
 for derivation of further keying material for Child SAs.  The
 notation SK { ... } indicates that these payloads are encrypted and
 integrity protected using that direction's SK_e and SK_a.
 HDR, SK {IDi, [CERT,] [CERTREQ,]
     [IDr,] AUTH, SAi2,
     TSi, TSr}  -->

Kaufman, et al. Standards Track [Page 11] RFC 7296 IKEv2bis October 2014

 The initiator asserts its identity with the IDi payload, proves
 knowledge of the secret corresponding to IDi and integrity protects
 the contents of the first message using the AUTH payload (see
 Section 2.15).  It might also send its certificate(s) in CERT
 payload(s) and a list of its trust anchors in CERTREQ payload(s).  If
 any CERT payloads are included, the first certificate provided MUST
 contain the public key used to verify the AUTH field.
 The optional payload IDr enables the initiator to specify to which of
 the responder's identities it wants to talk.  This is useful when the
 machine on which the responder is running is hosting multiple
 identities at the same IP address.  If the IDr proposed by the
 initiator is not acceptable to the responder, the responder might use
 some other IDr to finish the exchange.  If the initiator then does
 not accept the fact that responder used an IDr different than the one
 that was requested, the initiator can close the SA after noticing the
 fact.
 The Traffic Selectors (TSi and TSr) are discussed in Section 2.9.
 The initiator begins negotiation of a Child SA using the SAi2
 payload.  The final fields (starting with SAi2) are described in the
 description of the CREATE_CHILD_SA exchange.
                              <--  HDR, SK {IDr, [CERT,] AUTH,
                                       SAr2, TSi, TSr}
 The responder asserts its identity with the IDr payload, optionally
 sends one or more certificates (again with the certificate containing
 the public key used to verify AUTH listed first), authenticates its
 identity and protects the integrity of the second message with the
 AUTH payload, and completes negotiation of a Child SA with the
 additional fields described below in the CREATE_CHILD_SA exchange.
 Both parties in the IKE_AUTH exchange MUST verify that all signatures
 and Message Authentication Codes (MACs) are computed correctly.  If
 either side uses a shared secret for authentication, the names in the
 ID payload MUST correspond to the key used to generate the AUTH
 payload.
 Because the initiator sends its Diffie-Hellman value in the
 IKE_SA_INIT, it must guess the Diffie-Hellman group that the
 responder will select from its list of supported groups.  If the
 initiator guesses wrong, the responder will respond with a Notify
 payload of type INVALID_KE_PAYLOAD indicating the selected group.  In
 this case, the initiator MUST retry the IKE_SA_INIT with the
 corrected Diffie-Hellman group.  The initiator MUST again propose its
 full set of acceptable cryptographic suites because the rejection

Kaufman, et al. Standards Track [Page 12] RFC 7296 IKEv2bis October 2014

 message was unauthenticated and otherwise an active attacker could
 trick the endpoints into negotiating a weaker suite than a stronger
 one that they both prefer.
 If creating the Child SA during the IKE_AUTH exchange fails for some
 reason, the IKE SA is still created as usual.  The list of Notify
 message types in the IKE_AUTH exchange that do not prevent an IKE SA
 from being set up include at least the following: NO_PROPOSAL_CHOSEN,
 TS_UNACCEPTABLE, SINGLE_PAIR_REQUIRED, INTERNAL_ADDRESS_FAILURE, and
 FAILED_CP_REQUIRED.
 If the failure is related to creating the IKE SA (for example, an
 AUTHENTICATION_FAILED Notify error message is returned), the IKE SA
 is not created.  Note that although the IKE_AUTH messages are
 encrypted and integrity protected, if the peer receiving this Notify
 error message has not yet authenticated the other end (or if the peer
 fails to authenticate the other end for some reason), the information
 needs to be treated with caution.  More precisely, assuming that the
 MAC verifies correctly, the sender of the error Notify message is
 known to be the responder of the IKE_SA_INIT exchange, but the
 sender's identity cannot be assured.
 Note that IKE_AUTH messages do not contain KEi/KEr or Ni/Nr payloads.
 Thus, the SA payloads in the IKE_AUTH exchange cannot contain
 Transform Type 4 (Diffie-Hellman group) with any value other than
 NONE.  Implementations SHOULD omit the whole transform substructure
 instead of sending value NONE.

1.3. The CREATE_CHILD_SA Exchange

 The CREATE_CHILD_SA exchange is used to create new Child SAs and to
 rekey both IKE SAs and Child SAs.  This exchange consists of a single
 request/response pair, and some of its function was referred to as a
 Phase 2 exchange in IKEv1.  It MAY be initiated by either end of the
 IKE SA after the initial exchanges are completed.
 An SA is rekeyed by creating a new SA and then deleting the old one.
 This section describes the first part of rekeying, the creation of
 new SAs; Section 2.8 covers the mechanics of rekeying, including
 moving traffic from old to new SAs and the deletion of the old SAs.
 The two sections must be read together to understand the entire
 process of rekeying.
 Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
 section the term initiator refers to the endpoint initiating this
 exchange.  An implementation MAY refuse all CREATE_CHILD_SA requests
 within an IKE SA.

Kaufman, et al. Standards Track [Page 13] RFC 7296 IKEv2bis October 2014

 The CREATE_CHILD_SA request MAY optionally contain a KE payload for
 an additional Diffie-Hellman exchange to enable stronger guarantees
 of forward secrecy for the Child SA.  The keying material for the
 Child SA is a function of SK_d established during the establishment
 of the IKE SA, the nonces exchanged during the CREATE_CHILD_SA
 exchange, and the Diffie-Hellman value (if KE payloads are included
 in the CREATE_CHILD_SA exchange).
 If a CREATE_CHILD_SA exchange includes a KEi payload, at least one of
 the SA offers MUST include the Diffie-Hellman group of the KEi.  The
 Diffie-Hellman group of the KEi MUST be an element of the group the
 initiator expects the responder to accept (additional Diffie-Hellman
 groups can be proposed).  If the responder selects a proposal using a
 different Diffie-Hellman group (other than NONE), the responder MUST
 reject the request and indicate its preferred Diffie-Hellman group in
 the INVALID_KE_PAYLOAD Notify payload.  There are two octets of data
 associated with this notification: the accepted Diffie-Hellman group
 number in big endian order.  In the case of such a rejection, the
 CREATE_CHILD_SA exchange fails, and the initiator will probably retry
 the exchange with a Diffie-Hellman proposal and KEi in the group that
 the responder gave in the INVALID_KE_PAYLOAD Notify payload.
 The responder sends a NO_ADDITIONAL_SAS notification to indicate that
 a CREATE_CHILD_SA request is unacceptable because the responder is
 unwilling to accept any more Child SAs on this IKE SA.  This
 notification can also be used to reject IKE SA rekey.  Some minimal
 implementations may only accept a single Child SA setup in the
 context of an initial IKE exchange and reject any subsequent attempts
 to add more.

1.3.1. Creating New Child SAs with the CREATE_CHILD_SA Exchange

 A Child SA may be created by sending a CREATE_CHILD_SA request.  The
 CREATE_CHILD_SA request for creating a new Child SA is:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SK {SA, Ni, [KEi,]
            TSi, TSr}  -->
 The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
 payload, optionally a Diffie-Hellman value in the KEi payload, and
 the proposed Traffic Selectors for the proposed Child SA in the TSi
 and TSr payloads.
 The CREATE_CHILD_SA response for creating a new Child SA is:
                              <--  HDR, SK {SA, Nr, [KEr,]

Kaufman, et al. Standards Track [Page 14] RFC 7296 IKEv2bis October 2014

                                       TSi, TSr}
 The responder replies (using the same Message ID to respond) with the
 accepted offer in an SA payload, a nonce in the Nr payload, and a
 Diffie-Hellman value in the KEr payload if KEi was included in the
 request and the selected cryptographic suite includes that group.
 The Traffic Selectors for traffic to be sent on that SA are specified
 in the TS payloads in the response, which may be a subset of what the
 initiator of the Child SA proposed.
 The USE_TRANSPORT_MODE notification MAY be included in a request
 message that also includes an SA payload requesting a Child SA.  It
 requests that the Child SA use transport mode rather than tunnel mode
 for the SA created.  If the request is accepted, the response MUST
 also include a notification of type USE_TRANSPORT_MODE.  If the
 responder declines the request, the Child SA will be established in
 tunnel mode.  If this is unacceptable to the initiator, the initiator
 MUST delete the SA.  Note: Except when using this option to negotiate
 transport mode, all Child SAs will use tunnel mode.
 The ESP_TFC_PADDING_NOT_SUPPORTED notification asserts that the
 sending endpoint will not accept packets that contain Traffic Flow
 Confidentiality (TFC) padding over the Child SA being negotiated.  If
 neither endpoint accepts TFC padding, this notification is included
 in both the request and the response.  If this notification is
 included in only one of the messages, TFC padding can still be sent
 in the other direction.
 The NON_FIRST_FRAGMENTS_ALSO notification is used for fragmentation
 control.  See [IPSECARCH] for a fuller explanation.  Both parties
 need to agree to sending non-first fragments before either party does
 so.  It is enabled only if NON_FIRST_FRAGMENTS_ALSO notification is
 included in both the request proposing an SA and the response
 accepting it.  If the responder does not want to send or receive
 non-first fragments, it only omits NON_FIRST_FRAGMENTS_ALSO
 notification from its response, but does not reject the whole Child
 SA creation.
 An IPCOMP_SUPPORTED notification, covered in Section 2.22, can also
 be included in the exchange.
 A failed attempt to create a Child SA SHOULD NOT tear down the IKE
 SA: there is no reason to lose the work done to set up the IKE SA.
 See Section 2.21 for a list of error messages that might occur if
 creating a Child SA fails.

Kaufman, et al. Standards Track [Page 15] RFC 7296 IKEv2bis October 2014

1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA Exchange

 The CREATE_CHILD_SA request for rekeying an IKE SA is:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SK {SA, Ni, KEi} -->
 The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
 payload, and a Diffie-Hellman value in the KEi payload.  The KEi
 payload MUST be included.  A new initiator SPI is supplied in the SPI
 field of the SA payload.  Once a peer receives a request to rekey an
 IKE SA or sends a request to rekey an IKE SA, it SHOULD NOT start any
 new CREATE_CHILD_SA exchanges on the IKE SA that is being rekeyed.
 The CREATE_CHILD_SA response for rekeying an IKE SA is:
                              <--  HDR, SK {SA, Nr, KEr}
 The responder replies (using the same Message ID to respond) with the
 accepted offer in an SA payload, a nonce in the Nr payload, and a
 Diffie-Hellman value in the KEr payload if the selected cryptographic
 suite includes that group.  A new responder SPI is supplied in the
 SPI field of the SA payload.
 The new IKE SA has its message counters set to 0, regardless of what
 they were in the earlier IKE SA.  The first IKE requests from both
 sides on the new IKE SA will have Message ID 0.  The old IKE SA
 retains its numbering, so any further requests (for example, to
 delete the IKE SA) will have consecutive numbering.  The new IKE SA
 also has its window size reset to 1, and the initiator in this rekey
 exchange is the new "original initiator" of the new IKE SA.
 Section 2.18 also covers IKE SA rekeying in detail.

1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA Exchange

 The CREATE_CHILD_SA request for rekeying a Child SA is:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SK {N(REKEY_SA), SA, Ni, [KEi,]
     TSi, TSr}   -->
 The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
 payload, optionally a Diffie-Hellman value in the KEi payload, and
 the proposed Traffic Selectors for the proposed Child SA in the TSi
 and TSr payloads.

Kaufman, et al. Standards Track [Page 16] RFC 7296 IKEv2bis October 2014

 The notifications described in Section 1.3.1 may also be sent in a
 rekeying exchange.  Usually, these will be the same notifications
 that were used in the original exchange; for example, when rekeying a
 transport mode SA, the USE_TRANSPORT_MODE notification will be used.
 The REKEY_SA notification MUST be included in a CREATE_CHILD_SA
 exchange if the purpose of the exchange is to replace an existing ESP
 or AH SA.  The SA being rekeyed is identified by the SPI field in the
 Notify payload; this is the SPI the exchange initiator would expect
 in inbound ESP or AH packets.  There is no data associated with this
 Notify message type.  The Protocol ID field of the REKEY_SA
 notification is set to match the protocol of the SA we are rekeying,
 for example, 3 for ESP and 2 for AH.
 The CREATE_CHILD_SA response for rekeying a Child SA is:
                              <--  HDR, SK {SA, Nr, [KEr,]
                                       TSi, TSr}
 The responder replies (using the same Message ID to respond) with the
 accepted offer in an SA payload, a nonce in the Nr payload, and a
 Diffie-Hellman value in the KEr payload if KEi was included in the
 request and the selected cryptographic suite includes that group.
 The Traffic Selectors for traffic to be sent on that SA are specified
 in the TS payloads in the response, which may be a subset of what the
 initiator of the Child SA proposed.

1.4. The INFORMATIONAL Exchange

 At various points during the operation of an IKE SA, peers may desire
 to convey control messages to each other regarding errors or
 notifications of certain events.  To accomplish this, IKE defines an
 INFORMATIONAL exchange.  INFORMATIONAL exchanges MUST ONLY occur
 after the initial exchanges and are cryptographically protected with
 the negotiated keys.  Note that some informational messages, not
 exchanges, can be sent outside the context of an IKE SA.
 Section 2.21 also covers error messages in great detail.
 Control messages that pertain to an IKE SA MUST be sent under that
 IKE SA.  Control messages that pertain to Child SAs MUST be sent
 under the protection of the IKE SA that generated them (or its
 successor if the IKE SA was rekeyed).
 Messages in an INFORMATIONAL exchange contain zero or more
 Notification, Delete, and Configuration payloads.  The recipient of
 an INFORMATIONAL exchange request MUST send some response; otherwise,
 the sender will assume the message was lost in the network and will

Kaufman, et al. Standards Track [Page 17] RFC 7296 IKEv2bis October 2014

 retransmit it.  That response MAY be an empty message.  The request
 message in an INFORMATIONAL exchange MAY also contain no payloads.
 This is the expected way an endpoint can ask the other endpoint to
 verify that it is alive.
 The INFORMATIONAL exchange is defined as:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SK {[N,] [D,]
     [CP,] ...}  -->
                              <--  HDR, SK {[N,] [D,]
                                       [CP,] ...}
 The processing of an INFORMATIONAL exchange is determined by its
 component payloads.

1.4.1. Deleting an SA with INFORMATIONAL Exchanges

 ESP and AH SAs always exist in pairs, with one SA in each direction.
 When an SA is closed, both members of the pair MUST be closed (that
 is, deleted).  Each endpoint MUST close its incoming SAs and allow
 the other endpoint to close the other SA in each pair.  To delete an
 SA, an INFORMATIONAL exchange with one or more Delete payloads is
 sent listing the SPIs (as they would be expected in the headers of
 inbound packets) of the SAs to be deleted.  The recipient MUST close
 the designated SAs.  Note that one never sends Delete payloads for
 the two sides of an SA in a single message.  If there are many SAs to
 delete at the same time, one includes Delete payloads for the inbound
 half of each SA pair in the INFORMATIONAL exchange.
 Normally, the response in the INFORMATIONAL exchange will contain
 Delete payloads for the paired SAs going in the other direction.
 There is one exception.  If, by chance, both ends of a set of SAs
 independently decide to close them, each may send a Delete payload
 and the two requests may cross in the network.  If a node receives a
 delete request for SAs for which it has already issued a delete
 request, it MUST delete the outgoing SAs while processing the request
 and the incoming SAs while processing the response.  In that case,
 the responses MUST NOT include Delete payloads for the deleted SAs,
 since that would result in duplicate deletion and could in theory
 delete the wrong SA.
 Similar to ESP and AH SAs, IKE SAs are also deleted by sending an
 INFORMATIONAL exchange.  Deleting an IKE SA implicitly closes any
 remaining Child SAs negotiated under it.  The response to a request
 that deletes the IKE SA is an empty INFORMATIONAL response.

Kaufman, et al. Standards Track [Page 18] RFC 7296 IKEv2bis October 2014

 Half-closed ESP or AH connections are anomalous, and a node with
 auditing capability should probably audit their existence if they
 persist.  Note that this specification does not specify time periods,
 so it is up to individual endpoints to decide how long to wait.  A
 node MAY refuse to accept incoming data on half-closed connections
 but MUST NOT unilaterally close them and reuse the SPIs.  If
 connection state becomes sufficiently messed up, a node MAY close the
 IKE SA, as described above.  It can then rebuild the SAs it needs on
 a clean base under a new IKE SA.

1.5. Informational Messages outside of an IKE SA

 There are some cases in which a node receives a packet that it cannot
 process, but it may want to notify the sender about this situation.
 o  If an ESP or AH packet arrives with an unrecognized SPI.  This
    might be due to the receiving node having recently crashed and
    lost state, or because of some other system malfunction or attack.
 o  If an encrypted IKE request packet arrives on port 500 or 4500
    with an unrecognized IKE SPI.  This might be due to the receiving
    node having recently crashed and lost state, or because of some
    other system malfunction or attack.
 o  If an IKE request packet arrives with a higher major version
    number than the implementation supports.
 In the first case, if the receiving node has an active IKE SA to the
 IP address from whence the packet came, it MAY send an INVALID_SPI
 notification of the wayward packet over that IKE SA in an
 INFORMATIONAL exchange.  The Notification Data contains the SPI of
 the invalid packet.  The recipient of this notification cannot tell
 whether the SPI is for AH or ESP, but this is not important because
 in many cases the SPIs will be different for the two.  If no suitable
 IKE SA exists, the node MAY send an informational message without
 cryptographic protection to the source IP address, using the source
 UDP port as the destination port if the packet was UDP (UDP-
 encapsulated ESP or AH).  In this case, it should only be used by the
 recipient as a hint that something might be wrong (because it could
 easily be forged).  This message is not part of an INFORMATIONAL
 exchange, and the receiving node MUST NOT respond to it because doing
 so could cause a message loop.  The message is constructed as
 follows: there are no IKE SPI values that would be meaningful to the
 recipient of such a notification; using zero values or random values
 are both acceptable, this being the exception to the rule in
 Section 3.1 that prohibits zero IKE Initiator SPIs.  The Initiator

Kaufman, et al. Standards Track [Page 19] RFC 7296 IKEv2bis October 2014

 flag is set to 1, the Response flag is set to 0, and the version
 flags are set in the normal fashion; these flags are described in
 Section 3.1.
 In the second and third cases, the message is always sent without
 cryptographic protection (outside of an IKE SA), and includes either
 an INVALID_IKE_SPI or an INVALID_MAJOR_VERSION notification (with no
 notification data).  The message is a response message, and thus it
 is sent to the IP address and port from whence it came with the same
 IKE SPIs and the Message ID and Exchange Type are copied from the
 request.  The Response flag is set to 1, and the version flags are
 set in the normal fashion.

1.6. Requirements Terminology

 Definitions of the primitive terms in this document (such as Security
 Association or SA) can be found in [IPSECARCH].  It should be noted
 that parts of IKEv2 rely on some of the processing rules in
 [IPSECARCH], as described in various sections of this document.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [MUSTSHOULD].

1.7. Significant Differences between RFC 4306 and RFC 5996

 This document contains clarifications and amplifications to IKEv2
 [IKEV2].  Many of the clarifications are based on [Clarif].  The
 changes listed in that document were discussed in the IPsec Working
 Group and, after the Working Group was disbanded, on the IPsec
 mailing list.  That document contains detailed explanations of areas
 that were unclear in IKEv2, and is thus useful to implementers of
 IKEv2.
 The protocol described in this document retains the same major
 version number (2) and minor version number (0) as was used in
 RFC 4306.  That is, the version number is *not* changed from
 RFC 4306.  The small number of technical changes listed here are not
 expected to affect RFC 4306 implementations that have already been
 deployed at the time of publication of this document.
 This document makes the figures and references a bit more consistent
 than they were in [IKEV2].
 IKEv2 developers have noted that the SHOULD-level requirements in
 RFC 4306 are often unclear in that they don't say when it is OK to
 not obey the requirements.  They also have noted that there are MUST-
 level requirements that are not related to interoperability.  This

Kaufman, et al. Standards Track [Page 20] RFC 7296 IKEv2bis October 2014

 document has more explanation of some of these requirements.  All
 non-capitalized uses of the words SHOULD and MUST now mean their
 normal English sense, not the interoperability sense of [MUSTSHOULD].
 IKEv2 (and IKEv1) developers have noted that there is a great deal of
 material in the tables of codes in Section 3.10.1 in RFC 4306.  This
 leads to implementers not having all the needed information in the
 main body of the document.  Much of the material from those tables
 has been moved into the associated parts of the main body of the
 document.
 This document removes discussion of nesting AH and ESP.  This was a
 mistake in RFC 4306 caused by the lag between finishing RFC 4306 and
 RFC 4301.  Basically, IKEv2 is based on RFC 4301, which does not
 include "SA bundles" that were part of RFC 2401.  While a single
 packet can go through IPsec processing multiple times, each of these
 passes uses a separate SA, and the passes are coordinated by the
 forwarding tables.  In IKEv2, each of these SAs has to be created
 using a separate CREATE_CHILD_SA exchange.
 This document removes discussion of the INTERNAL_ADDRESS_EXPIRY
 configuration attribute because its implementation was very
 problematic.  Implementations that conform to this document MUST
 ignore proposals that have configuration attribute type 5, the old
 value for INTERNAL_ADDRESS_EXPIRY.  This document also removed
 INTERNAL_IP6_NBNS as a configuration attribute.
 This document removes the allowance for rejecting messages in which
 the payloads were not in the "right" order; now implementations
 MUST NOT reject them.  This is due to the lack of clarity where the
 orders for the payloads are described.
 The lists of items from RFC 4306 that ended up in the IANA registry
 were trimmed to only include items that were actually defined in
 RFC 4306.  Also, many of those lists are now preceded with the very
 important instruction to developers that they really should look at
 the IANA registry at the time of development because new items have
 been added since RFC 4306.
 This document adds clarification on when notifications are and are
 not sent encrypted, depending on the state of the negotiation at the
 time.
 This document discusses more about how to negotiate combined-mode
 ciphers.

Kaufman, et al. Standards Track [Page 21] RFC 7296 IKEv2bis October 2014

 In Section 1.3.2, "The KEi payload SHOULD be included" was changed to
 be "The KEi payload MUST be included".  This also led to changes in
 Section 2.18.
 In Section 2.1, there is new material covering how the initiator's
 SPI and/or IP is used to differentiate if this is a "half-open" IKE
 SA or a new request.
 This document clarifies the use of the critical flag in Section 2.5.
 In Section 2.8, "Note that, when rekeying, the new Child SA MAY have
 different Traffic Selectors and algorithms than the old one" was
 changed to "Note that, when rekeying, the new Child SA SHOULD NOT
 have different Traffic Selectors and algorithms than the old one".
 The new Section 2.8.2 covers simultaneous IKE SA rekeying.
 This document adds the restriction in Section 2.13 that all
 pseudorandom functions (PRFs) used with IKEv2 MUST take variable-
 sized keys.  This should not affect any implementations because there
 were no standardized PRFs that have fixed-size keys.
 Section 2.18 requires doing a Diffie-Hellman exchange when rekeying
 the IKE_SA.  In theory, RFC 4306 allowed a policy where the Diffie-
 Hellman exchange was optional, but this was not useful (or
 appropriate) when rekeying the IKE_SA.
 Section 2.21 has been greatly expanded to cover the different cases
 where error responses are needed and the appropriate responses to
 them.
 Section 2.23 clarified that, in NAT traversal, now both UDP-
 encapsulated IPsec packets and non-UDP-encapsulated IPsec packets
 need to be understood when receiving.
 Added Section 2.23.1 to describe NAT traversal when transport mode is
 requested.
 Added Section 2.25 to explain how to act when there are timing
 collisions when deleting and/or rekeying SAs, and two new error
 notifications (TEMPORARY_FAILURE and CHILD_SA_NOT_FOUND) were
 defined.
 In Section 3.6, "Implementations MUST support the "http:" scheme for
 hash-and-URL lookup.  The behavior of other URL schemes is not
 currently specified, and such schemes SHOULD NOT be used in the
 absence of a document specifying them" was added.

Kaufman, et al. Standards Track [Page 22] RFC 7296 IKEv2bis October 2014

 In Section 3.15.3, a pointer to a new document that is related to
 configuration of IPv6 addresses was added.
 Appendix C was expanded and clarified.

1.8. Differences between RFC 5996 and This Document

 Clarified in the Abstract and the Introduction section that the
 status of this document is Internet Standard.
 The new Section 2.9.2 covers Traffic Selectors in rekeying.
 Added reference to RFC 6989 when reusing Diffie-Hellman exponentials
 (Section 2.12).
 Added name "Last Substruc" for the Proposal Substructure and
 Transform Substructure header (Sections 3.3.1 and 3.3.2) for the 0
 (last) or 2/3 (more) field.
 Added reference to RFC 6989 when using groups that are not
 Sophie Germain Modular Exponentiation (MODP) groups (Section 3.3.2).
 Added reference to RFC 4945 in the Identification Payloads section
 (Section 3.5).
 Deprecated Raw RSA public keys in Section 3.6.  There is new work in
 progress adding a more generic format for raw public keys.
 Fixed Sections 3.6 and 3.10 as specified in the errata for RFC 5996
 (RFC Errata IDs 2707 and 3036).
 Added a note in the IANA Considerations section (Section 6) about
 deprecating the Raw RSA Key, and removed the old contents (which was
 already done during RFC 5996 processing).  Added a note that IANA
 should update all references to RFC 5996 to point to this document.

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, et al. Standards Track [Page 23] RFC 7296 IKEv2bis October 2014

 as to exhaust the network or CPU capacities of either endpoint.  Even
 in the absence of those minimum performance requirements, IKE is
 designed to fail cleanly (as though the network were broken).
 Although IKEv2 messages are intended to be short, they contain
 structures with no hard upper bound on size (in particular, digital
 certificates), and IKEv2 itself does not have a mechanism for
 fragmenting large messages.  IP defines a mechanism for fragmentation
 of oversized UDP messages, but implementations vary in the maximum
 message size supported.  Furthermore, use of IP fragmentation opens
 an implementation to denial-of-service (DoS) attacks [DOSUDPPROT].
 Finally, some NAT and/or firewall implementations may block IP
 fragments.
 All IKEv2 implementations MUST be able to send, receive, and process
 IKE messages that are up to 1280 octets long, and they SHOULD be able
 to send, receive, and process messages that are up to 3000 octets
 long.  IKEv2 implementations need to be aware of the maximum UDP
 message size supported and MAY shorten messages by leaving out some
 certificates or cryptographic suite proposals if that will keep
 messages below the maximum.  Use of the "Hash and URL" formats rather
 than including certificates in exchanges where possible can avoid
 most problems.  Implementations and configuration need to keep in
 mind, however, that if the URL lookups are possible only after the
 Child SA is established, recursion issues could prevent this
 technique from working.
 The UDP payload of all packets containing IKE messages sent on
 port 4500 MUST begin with the prefix of four zeros; otherwise, the
 receiver won't know how to handle them.

2.1. Use of Retransmission Timers

 All messages in IKE exist in pairs: a request and a response.  The
 setup of an IKE SA normally consists of two exchanges.  Once the IKE
 SA is set up, either end of the Security Association may initiate
 requests at any time, and there can be many requests and responses
 "in flight" at any given moment.  But each message is labeled as
 either a request or a response, and for each exchange, one end of the
 Security Association is the initiator and the other is the responder.
 For every pair of IKE messages, the initiator is responsible for
 retransmission in the event of a timeout.  The responder MUST never
 retransmit a response unless it receives a retransmission of the
 request.  In that event, the responder MUST ignore the retransmitted
 request except insofar as it causes a retransmission of the response.
 The initiator MUST remember each request until it receives the
 corresponding response.  The responder MUST remember each response

Kaufman, et al. Standards Track [Page 24] RFC 7296 IKEv2bis October 2014

 until it receives a request whose sequence number is larger than or
 equal to the sequence number in the response plus its window size
 (see Section 2.3).  In order to allow saving memory, responders are
 allowed to forget the response after a timeout of several minutes.
 If the responder receives a retransmitted request for which it has
 already forgotten the response, it MUST ignore the request (and not,
 for example, attempt constructing a new response).
 IKE is a reliable protocol: the initiator MUST retransmit a request
 until it either receives a corresponding response or deems the IKE SA
 to have failed.  In the latter case, the initiator discards all state
 associated with the IKE SA and any Child SAs that were negotiated
 using that IKE SA.  A retransmission from the initiator MUST be
 bitwise identical to the original request.  That is, everything
 starting from the IKE header (the IKE SA initiator's SPI onwards)
 must be bitwise identical; items before it (such as the IP and UDP
 headers) do not have to be identical.
 Retransmissions of the IKE_SA_INIT request require some special
 handling.  When a responder receives an IKE_SA_INIT request, it has
 to determine whether the packet is a retransmission belonging to an
 existing "half-open" IKE SA (in which case the responder retransmits
 the same response), or a new request (in which case the responder
 creates a new IKE SA and sends a fresh response), or it belongs to an
 existing IKE SA where the IKE_AUTH request has been already received
 (in which case the responder ignores it).
 It is not sufficient to use the initiator's SPI and/or IP address to
 differentiate between these three cases because two different peers
 behind a single NAT could choose the same initiator SPI.  Instead, a
 robust responder will do the IKE SA lookup using the whole packet,
 its hash, or the Ni payload.
 The retransmission policy for one-way messages is somewhat different
 from that for regular messages.  Because no acknowledgement is ever
 sent, there is no reason to gratuitously retransmit one-way messages.
 Given that all these messages are errors, it makes sense to send them
 only once per "offending" packet, and only retransmit if further
 offending packets are received.  Still, it also makes sense to limit
 retransmissions of such error messages.

2.2. Use of Sequence Numbers for Message ID

 Every IKE message contains a Message ID as part of its fixed header.
 This Message ID is used to match up requests and responses and to
 identify retransmissions of messages.  Retransmission of a message
 MUST use the same Message ID as the original message.

Kaufman, et al. Standards Track [Page 25] RFC 7296 IKEv2bis October 2014

 The Message ID is a 32-bit quantity, which is zero for the
 IKE_SA_INIT messages (including retries of the message due to
 responses such as COOKIE and INVALID_KE_PAYLOAD), and incremented for
 each subsequent exchange.  Thus, the first pair of IKE_AUTH messages
 will have an ID of 1, the second (when EAP is used) will be 2, and so
 on.  The Message ID is reset to zero in the new IKE SA after the IKE
 SA is rekeyed.
 Each endpoint in the IKE Security Association maintains two "current"
 Message IDs: the next one to be used for a request it initiates and
 the next one it expects to see in a request from the other end.
 These counters increment as requests are generated and received.
 Responses always contain the same Message ID as the corresponding
 request.  That means that after the initial exchange, each integer n
 may appear as the Message ID in four distinct messages: the nth
 request from the original IKE initiator, the corresponding response,
 the nth request from the original IKE responder, and the
 corresponding response.  If the two ends make a very different number
 of requests, the Message IDs in the two directions can be very
 different.  There is no ambiguity in the messages, however, because
 the Initiator and Response flags in the message header specify which
 of the four messages a particular one is.
 Throughout this document, "initiator" refers to the party who
 initiated the exchange being described.  The "original initiator"
 always refers to the party who initiated the exchange that resulted
 in the current IKE SA.  In other words, if the "original responder"
 starts rekeying the IKE SA, that party becomes the "original
 initiator" of the new IKE SA.
 Note that Message IDs are cryptographically protected and provide
 protection against message replays.  In the unlikely event that
 Message IDs grow too large to fit in 32 bits, the IKE SA MUST be
 closed or rekeyed.

2.3. Window Size for Overlapping Requests

 The SET_WINDOW_SIZE notification asserts that the sending endpoint is
 capable of keeping state for multiple outstanding exchanges,
 permitting the recipient to send multiple requests before getting a
 response to the first.  The data associated with a SET_WINDOW_SIZE
 notification MUST be 4 octets long and contain the big endian
 representation of the number of messages the sender promises to keep.
 The window size is always one until the initial exchanges complete.

Kaufman, et al. Standards Track [Page 26] RFC 7296 IKEv2bis October 2014

 An IKE endpoint MUST wait for a response to each of its messages
 before sending a subsequent message unless it has received a
 SET_WINDOW_SIZE Notify message from its peer informing it that the
 peer is prepared to maintain state for multiple outstanding messages
 in order to allow greater throughput.
 After an IKE SA is set up, in order to maximize IKE throughput, an
 IKE endpoint MAY issue multiple requests before getting a response to
 any of them, up to the limit set by its peer's SET_WINDOW_SIZE.
 These requests may pass one another over the network.  An IKE
 endpoint MUST be prepared to accept and process a request while it
 has a request outstanding in order to avoid a deadlock in this
 situation.  An IKE endpoint may also accept and process multiple
 requests while it has a request outstanding.
 An IKE endpoint MUST NOT exceed the peer's stated window size for
 transmitted IKE requests.  In other words, if the responder stated
 its window size is N, then when the initiator needs to make a request
 X, it MUST wait until it has received responses to all requests up
 through request X-N.  An IKE endpoint MUST keep a copy of (or be able
 to regenerate exactly) each request it has sent until it receives the
 corresponding response.  An IKE endpoint MUST keep a copy of (or be
 able to regenerate exactly) the number of previous responses equal to
 its declared window size in case its response was lost and the
 initiator requests its retransmission by retransmitting the request.
 An IKE endpoint supporting a window size greater than one ought to be
 capable of processing incoming requests out of order to maximize
 performance in the event of network failures or packet reordering.
 The window size is normally a (possibly configurable) property of a
 particular implementation, and is not related to congestion control
 (unlike the window size in TCP, for example).  In particular, what
 the responder should do when it receives a SET_WINDOW_SIZE
 notification containing a smaller value than is currently in effect
 is not defined.  Thus, there is currently no way to reduce the window
 size of an existing IKE SA; you can only increase it.  When rekeying
 an IKE SA, the new IKE SA starts with window size 1 until it is
 explicitly increased by sending a new SET_WINDOW_SIZE notification.
 The INVALID_MESSAGE_ID notification is sent when an IKE Message ID
 outside the supported window is received.  This Notify message
 MUST NOT be sent in a response; the invalid request MUST NOT be
 acknowledged.  Instead, inform the other side by initiating an
 INFORMATIONAL exchange with Notification Data containing the
 four-octet invalid Message ID.  Sending this notification is
 OPTIONAL, and notifications of this type MUST be rate limited.

Kaufman, et al. Standards Track [Page 27] RFC 7296 IKEv2bis October 2014

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.
 The INITIAL_CONTACT notification asserts that this IKE SA is the only
 IKE SA currently active between the authenticated identities.  It MAY
 be sent when an IKE SA is established after a crash, and the
 recipient MAY use this information to delete any other IKE SAs it has
 to the same authenticated identity without waiting for a timeout.
 This notification MUST NOT be sent by an entity that may be
 replicated (e.g., a roaming user's credentials where the user is
 allowed to connect to the corporate firewall from two remote systems
 at the same time).  The INITIAL_CONTACT notification, if sent, MUST
 be in the first IKE_AUTH request or response, not as a separate
 exchange afterwards; receiving parties MAY ignore it in other
 messages.
 Since IKE is designed to operate in spite of DoS attacks from the
 network, an endpoint MUST NOT conclude that the other endpoint has
 failed based on any routing information (e.g., ICMP messages) or IKE
 messages that arrive without cryptographic protection (e.g., Notify
 messages complaining about unknown SPIs).  An endpoint MUST conclude
 that the other endpoint has failed only when repeated attempts to
 contact it have gone unanswered for a timeout period or when a
 cryptographically protected INITIAL_CONTACT notification is received
 on a different IKE SA to the same authenticated identity.  An
 endpoint should suspect that the other endpoint has failed based on
 routing information and initiate a request to see whether the other
 endpoint is alive.  To check whether the other side is alive, IKE
 specifies an empty INFORMATIONAL request that (like all IKE requests)
 requires an acknowledgement (note that within the context of an IKE
 SA, an "empty" message consists of an IKE header followed by an
 Encrypted payload that contains no payloads).  If a cryptographically
 protected (fresh, i.e., not retransmitted) message has been received
 from the other side recently, unprotected Notify messages MAY be
 ignored.  Implementations MUST limit the rate at which they take
 actions based on unprotected messages.
 The number of retries and length of timeouts are not covered in this
 specification because they do not affect interoperability.  It is
 suggested that messages be retransmitted at least a dozen times over
 a period of at least several minutes before giving up on an SA, but

Kaufman, et al. Standards Track [Page 28] RFC 7296 IKEv2bis October 2014

 different environments may require different rules.  To be a good
 network citizen, retransmission times MUST increase exponentially to
 avoid flooding the network and making an existing congestion
 situation worse.  If there has only been outgoing traffic on all of
 the SAs associated with an IKE SA, it is essential to confirm
 liveness of the other endpoint to avoid black holes.  If no
 cryptographically protected messages have been received on an IKE SA
 or any of its Child SAs recently, the system needs to perform a
 liveness check in order to prevent sending messages to a dead peer.
 (This is sometimes called "dead peer detection" or "DPD", although it
 is really detecting live peers, not dead ones.)  Receipt of a fresh
 cryptographically protected message on an IKE SA or any of its Child
 SAs ensures liveness of the IKE SA and all of its Child SAs.  Note
 that this places requirements on the failure modes of an IKE
 endpoint.  An implementation needs to stop sending over any SA if
 some failure prevents it from receiving on all of the associated SAs.
 If a system creates Child SAs that can fail independently from one
 another without the associated IKE SA being able to send a delete
 message, then the system MUST negotiate such Child SAs using separate
 IKE SAs.
 One type of DoS attack on the initiator of an IKE SA can be avoided
 if the initiator takes 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 response as potentially legitimate, respond to each one,
 and then discard all the invalid half-open connections when it
 receives a valid cryptographically protected response to any one of
 its requests.  Once a cryptographically valid response is received,
 all subsequent responses should be ignored whether or not they are
 cryptographically valid.
 Note that with these rules, there is no reason to negotiate and agree
 upon an SA lifetime.  If IKE presumes the partner is dead, based on
 repeated lack of acknowledgement to an IKE message, then the IKE SA
 and all Child SAs set up through that IKE SA are deleted.
 An IKE endpoint may at any time delete inactive Child SAs to recover
 resources used to hold their state.  If an IKE endpoint chooses to
 delete Child SAs, it MUST send Delete payloads to the other end
 notifying it of the deletion.  It MAY similarly time out the IKE SA.
 Closing the IKE SA implicitly closes all associated Child SAs.  In
 this case, an IKE endpoint SHOULD send a Delete payload indicating
 that it has closed the IKE SA unless the other endpoint is no longer
 responding.

Kaufman, et al. Standards Track [Page 29] RFC 7296 IKEv2bis October 2014

2.5. Version Numbers and Forward Compatibility

 This document describes version 2.0 of IKE, meaning the major version
 number is 2 and the minor version number is 0.  This document is a
 replacement for [IKEV2].  It is likely that some implementations will
 want to support version 1.0 and version 2.0, and in the future, other
 versions.
 The major version number should be incremented only if the packet
 formats or required actions have changed so dramatically that an
 older version node would not be able to interoperate with a newer
 version node if it simply ignored the fields it did not understand
 and took the actions specified in the older specification.  The minor
 version number indicates new capabilities, and MUST be ignored by a
 node with a smaller minor version number, but used for informational
 purposes by the node with the larger minor version number.  For
 example, it might indicate the ability to process a newly defined
 Notify message type.  The node with the larger minor version number
 would simply note that its correspondent would not be able to
 understand that message and therefore would not send it.
 If an endpoint receives a message with a higher major version number,
 it MUST drop the message and SHOULD send an unauthenticated Notify
 message of type INVALID_MAJOR_VERSION containing the highest
 (closest) version number it supports.  If an endpoint supports major
 version n, and major version m, it MUST support all versions between
 n and m.  If it receives a message with a major version that it
 supports, it MUST respond with that version number.  In order to
 prevent two nodes from being tricked into corresponding with a lower
 major version number than the maximum that they both support, IKE has
 a flag that indicates that the node is capable of speaking a higher
 major version number.
 Thus, the major version number in the IKE header indicates the
 version number of the message, not the highest version number that
 the transmitter supports.  If the initiator is capable of speaking
 versions n, n+1, and n+2, and the responder is capable of speaking
 versions n and n+1, then they will negotiate speaking n+1, where the
 initiator will set a flag indicating its ability to speak a higher
 version.  If they mistakenly (perhaps through an active attacker
 sending error messages) negotiate to version n, then both will notice
 that the other side can support a higher version number, and they
 MUST break the connection and reconnect using version n+1.

Kaufman, et al. Standards Track [Page 30] RFC 7296 IKEv2bis October 2014

 Note that IKEv1 does not follow these rules, because there is no way
 in v1 of noting that you are capable of speaking a higher version
 number.  So an active attacker can trick two v2-capable nodes into
 speaking v1.  When a v2-capable node negotiates down to v1, it should
 note that fact in its logs.
 Also, for forward compatibility, all fields marked RESERVED MUST be
 set to zero by an implementation running version 2.0, and their
 content MUST be ignored by an implementation running version 2.0 ("Be
 conservative in what you send and liberal in what you receive" [IP]).
 In this way, future versions of the protocol can use those fields in
 a way that is guaranteed to be ignored by implementations that do not
 understand them.  Similarly, payload types that are not defined are
 reserved for future use; implementations of a version where they are
 undefined MUST skip over those payloads and ignore their contents.
 IKEv2 adds a "critical" flag to each payload header for further
 flexibility for forward compatibility.  If the critical flag is set
 and the payload type is unrecognized, the message MUST be rejected
 and the response to the IKE request containing that payload MUST
 include a Notify payload UNSUPPORTED_CRITICAL_PAYLOAD, indicating an
 unsupported critical payload was included.  In that Notify payload,
 the Notification Data contains the one-octet payload type.  If the
 critical flag is not set and the payload type is unsupported, that
 payload MUST be ignored.  Payloads sent in IKE response messages
 MUST NOT have the critical flag set.  Note that the critical flag
 applies only to the payload type, not the contents.  If the payload
 type is recognized, but the payload contains something that is not
 (such as an unknown transform inside an SA payload, or an unknown
 Notify Message Type inside a Notify payload), the critical flag is
 ignored.
 Although new payload types may be added in the future and may appear
 interleaved with the fields defined in this specification,
 implementations SHOULD send the payloads defined in this
 specification in the order shown in the figures in Sections 1 and 2;
 implementations MUST NOT reject as invalid a message with those
 payloads in any other order.

Kaufman, et al. Standards Track [Page 31] RFC 7296 IKEv2bis October 2014

2.6. IKE SA SPIs and Cookies

 The initial two eight-octet fields in the header, called the "IKE
 SPIs", are used as a connection identifier at the beginning of IKE
 packets.  Each endpoint chooses one of the two SPIs and MUST choose
 them so as to be unique identifiers of an IKE SA.  An SPI value of
 zero is special: it indicates that the remote SPI value is not yet
 known by the sender.
 Incoming IKE packets are mapped to an IKE SA only using the packet's
 SPI, not using (for example) the source IP address of the packet.
 Unlike ESP and AH where only the recipient's SPI appears in the
 header of a message, in IKE the sender's SPI is also sent in every
 message.  Since the SPI chosen by the original initiator of the IKE
 SA is always sent first, an endpoint with multiple IKE SAs open that
 wants to find the appropriate IKE SA using the SPI it assigned must
 look at the Initiator flag in the header to determine whether it
 assigned the first or the second eight octets.
 In the first message of an initial IKE exchange, the initiator will
 not know the responder's SPI value and will therefore set that field
 to zero.  When the IKE_SA_INIT exchange does not result in the
 creation of an IKE SA due to INVALID_KE_PAYLOAD, NO_PROPOSAL_CHOSEN,
 or COOKIE, the responder's SPI will be zero also in the response
 message.  However, if the responder sends a non-zero responder SPI,
 the initiator should not reject the response for only that reason.
 Two expected attacks against IKE are state and CPU exhaustion, where
 the target is flooded with session initiation requests from forged IP
 addresses.  These attacks can be made less effective if a responder
 uses minimal CPU and commits no state to an SA until it knows the
 initiator can receive packets at the address from which it claims to
 be sending them.

Kaufman, et al. Standards Track [Page 32] RFC 7296 IKEv2bis October 2014

 When a responder detects a large number of half-open IKE SAs, it
 SHOULD reply to IKE_SA_INIT requests with a response containing the
 COOKIE notification.  The data associated with this notification MUST
 be between 1 and 64 octets in length (inclusive), and its generation
 is described later in this section.  If the IKE_SA_INIT response
 includes the COOKIE notification, the initiator MUST then retry the
 IKE_SA_INIT request, and include the COOKIE notification containing
 the received data as the first payload, and all other payloads
 unchanged.  The initial exchange will then be as follows:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR(A,0), SAi1, KEi, Ni  -->
                              <--  HDR(A,0), N(COOKIE)
 HDR(A,0), N(COOKIE), SAi1,
     KEi, Ni  -->
                              <--  HDR(A,B), SAr1, KEr,
                                       Nr, [CERTREQ]
 HDR(A,B), SK {IDi, [CERT,]
     [CERTREQ,] [IDr,] AUTH,
     SAi2, TSi, TSr}  -->
                              <--  HDR(A,B), SK {IDr, [CERT,]
                                       AUTH, SAr2, TSi, TSr}
 The first two messages do not affect any initiator or responder state
 except for communicating the cookie.  In particular, the message
 sequence numbers in the first four messages will all be zero and the
 message sequence numbers in the last two messages will be one.  'A'
 is the SPI assigned by the initiator, while 'B' is the SPI assigned
 by the responder.
 An IKE implementation can implement its responder cookie generation
 in such a way as to not require any saved state to recognize its
 valid cookie when the second IKE_SA_INIT message arrives.  The exact
 algorithms and syntax used to generate cookies do not affect
 interoperability and hence are not specified here.  The following is
 an example of how an endpoint could use cookies to implement limited
 DoS protection.
 A good way to do this is to set the responder cookie to be:
 Cookie = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)
 where <secret> is a randomly generated secret known only to the
 responder and periodically changed and | indicates concatenation.
 <VersionIDofSecret> should be changed whenever <secret> is
 regenerated.  The cookie can be recomputed when the IKE_SA_INIT
 arrives the second time and compared to the cookie in the received

Kaufman, et al. Standards Track [Page 33] RFC 7296 IKEv2bis October 2014

 message.  If it matches, the responder knows that the cookie was
 generated since the last change to <secret> and that IPi must be the
 same as the source address it saw the first time.  Incorporating SPIi
 into the calculation ensures that if multiple IKE SAs are being set
 up in parallel they will all get different cookies (assuming the
 initiator chooses unique SPIi's).  Incorporating Ni in the hash
 ensures that an attacker who sees only message 2 can't successfully
 forge a message 3.  Also, incorporating SPIi in the hash prevents an
 attacker from fetching one cookie from the other end, and then
 initiating many IKE_SA_INIT exchanges all with different initiator
 SPIs (and perhaps port numbers) so that the responder thinks that
 there are a lot of machines behind one NAT box that are all trying to
 connect.
 If a new value for <secret> is chosen while there are connections in
 the process of being initialized, an IKE_SA_INIT might be returned
 with other than the current <VersionIDofSecret>.  The responder in
 that case MAY reject the message by sending another response with a
 new cookie or it MAY keep the old value of <secret> around for a
 short time and accept cookies computed from either one.  The
 responder should not accept cookies indefinitely after <secret> is
 changed, since that would defeat part of the DoS protection.  The
 responder should change the value of <secret> frequently, especially
 if under attack.
 When one party receives an IKE_SA_INIT request containing a cookie
 whose contents do not match the value expected, that party MUST
 ignore the cookie and process the message as if no cookie had been
 included; usually this means sending a response containing a new
 cookie.  The initiator should limit the number of cookie exchanges it
 tries before giving up, possibly using exponential back-off.  An
 attacker can forge multiple cookie responses to the initiator's
 IKE_SA_INIT message, and each of those forged cookie replies will
 cause two packets to be sent: one packet from the initiator to the
 responder (which will reject those cookies), and one response from
 responder to initiator that includes the correct cookie.
 A note on terminology: the term "cookies" originates with Karn and
 Simpson [PHOTURIS] in Photuris, an early proposal for key management
 with IPsec, and it has persisted.  The Internet Security Association
 and Key Management Protocol (ISAKMP) [ISAKMP] fixed message header
 includes two eight-octet fields called "cookies", and that syntax is
 used by both IKEv1 and IKEv2, although in IKEv2 they are referred to
 as the "IKE SPI" and there is a new separate field in a Notify
 payload holding the cookie.

Kaufman, et al. Standards Track [Page 34] RFC 7296 IKEv2bis October 2014

2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD

 There are two common reasons why the initiator may have to retry the
 IKE_SA_INIT exchange: the responder requests a cookie or wants a
 different Diffie-Hellman group than was included in the KEi payload.
 If the initiator receives a cookie from the responder, the initiator
 needs to decide whether or not to include the cookie in only the next
 retry of the IKE_SA_INIT request, or in all subsequent retries as
 well.
 If the initiator includes the cookie only in the next retry, one
 additional round trip may be needed in some cases.  An additional
 round trip is needed also if the initiator includes the cookie in all
 retries, but the responder does not support this.  For instance, if
 the responder includes the KEi payloads in cookie calculation, it
 will reject the request by sending a new cookie.
 If both peers support including the cookie in all retries, a slightly
 shorter exchange can happen.
 Initiator                   Responder
 -----------------------------------------------------------
 HDR(A,0), SAi1, KEi, Ni -->
                         <-- HDR(A,0), N(COOKIE)
 HDR(A,0), N(COOKIE), SAi1, KEi, Ni  -->
                         <-- HDR(A,0), N(INVALID_KE_PAYLOAD)
 HDR(A,0), N(COOKIE), SAi1, KEi', Ni -->
                         <-- HDR(A,B), SAr1, KEr, Nr
 Implementations SHOULD support this shorter exchange, but MUST NOT
 fail if other implementations do not support this shorter exchange.

2.7. Cryptographic Algorithm Negotiation

 The payload type known as "SA" indicates a proposal for a set of
 choices of IPsec protocols (IKE, ESP, or AH) for the SA as well as
 cryptographic algorithms associated with each protocol.
 An SA payload consists of one or more proposals.  Each proposal
 includes one protocol.  Each protocol contains one or more transforms
 -- each specifying a cryptographic algorithm.  Each transform
 contains zero or more attributes (attributes are needed only if the
 Transform ID does not completely specify the cryptographic
 algorithm).

Kaufman, et al. Standards Track [Page 35] RFC 7296 IKEv2bis October 2014

 This hierarchical structure was designed to efficiently encode
 proposals for cryptographic suites when the number of supported
 suites is large because multiple values are acceptable for multiple
 transforms.  The responder MUST choose a single suite, which may be
 any subset of the SA proposal following the rules below.
 Each proposal contains one protocol.  If a proposal is accepted, the
 SA response MUST contain the same protocol.  The responder MUST
 accept a single proposal or reject them all and return an error.  The
 error is given in a notification of type NO_PROPOSAL_CHOSEN.
 Each IPsec protocol proposal contains one or more transforms.  Each
 transform contains a Transform Type.  The accepted cryptographic
 suite MUST contain exactly one transform of each type included in the
 proposal.  For example: if an ESP proposal includes transforms
 ENCR_3DES, ENCR_AES w/keysize 128, ENCR_AES w/keysize 256,
 AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the accepted suite MUST contain one
 of the ENCR_ transforms and one of the AUTH_ transforms.  Thus, six
 combinations are acceptable.
 If an initiator proposes both normal ciphers with integrity
 protection as well as combined-mode ciphers, then two proposals are
 needed.  One of the proposals includes the normal ciphers with the
 integrity algorithms for them, and the other proposal includes all
 the combined-mode ciphers without the integrity algorithms (because
 combined-mode ciphers are not allowed to have any integrity algorithm
 other than "NONE").

2.8. Rekeying

 IKE, ESP, and AH Security Associations use secret keys that should be
 used only for a limited amount of time and to protect a limited
 amount of data.  This limits the lifetime of the entire Security
 Association.  When the lifetime of a Security Association expires,
 the Security Association MUST NOT be used.  If there is demand, new
 Security Associations MAY be established.  Reestablishment of
 Security Associations to take the place of ones that expire is
 referred to as "rekeying".
 To allow for minimal IPsec implementations, the ability to rekey SAs
 without restarting the entire IKE SA is optional.  An implementation
 MAY refuse all CREATE_CHILD_SA requests within an IKE SA.  If an SA
 has expired or is about to expire and rekeying attempts using the
 mechanisms described here fail, an implementation MUST close the IKE
 SA and any associated Child SAs and then MAY start new ones.
 Implementations may wish to support in-place rekeying of SAs, since
 doing so offers better performance and is likely to reduce the number
 of packets lost during the transition.

Kaufman, et al. Standards Track [Page 36] RFC 7296 IKEv2bis October 2014

 To rekey a Child SA within an existing IKE SA, create a new,
 equivalent SA (see Section 2.17 below), and when the new one is
 established, delete the old one.  Note that, when rekeying, the new
 Child SA SHOULD NOT have different Traffic Selectors and algorithms
 than the old one.
 To rekey an IKE SA, establish a new equivalent IKE SA (see
 Section 2.18 below) with the peer to whom the old IKE SA is shared
 using a CREATE_CHILD_SA within the existing IKE SA.  An IKE SA so
 created inherits all of the original IKE SA's Child SAs, and the new
 IKE SA is used for all control messages needed to maintain those
 Child SAs.  After the new equivalent IKE SA is created, the initiator
 deletes the old IKE SA, and the Delete payload to delete itself MUST
 be the last request sent over the old IKE SA.
 SAs should be rekeyed proactively, i.e., the new SA should be
 established before the old one expires and becomes unusable.  Enough
 time should elapse between the time the new SA is established and the
 old one becomes unusable so that traffic can be switched over to the
 new SA.
 A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
 were negotiated.  In IKEv2, each end of the SA is responsible for
 enforcing its own lifetime policy on the SA and rekeying the SA when
 necessary.  If the two ends have different lifetime policies, the end
 with the shorter lifetime will end up always being the one to request
 the rekeying.  If an SA has been inactive for a long time and if an
 endpoint would not initiate the SA in the absence of traffic, the
 endpoint MAY choose to close the SA instead of rekeying it when its
 lifetime expires.  It can also do so if there has been no traffic
 since the last time the SA was rekeyed.
 Note that IKEv2 deliberately allows parallel SAs with the same
 Traffic Selectors between common endpoints.  One of the purposes of
 this is to support traffic quality of service (QoS) differences among
 the SAs (see [DIFFSERVFIELD], [DIFFSERVARCH], and Section 4.1 of
 [DIFFTUNNEL]).  Hence unlike IKEv1, the combination of the endpoints
 and the Traffic Selectors may not uniquely identify an SA between
 those endpoints, so the IKEv1 rekeying heuristic of deleting SAs on
 the basis of duplicate Traffic Selectors SHOULD NOT be used.
 There are timing windows -- particularly in the presence of lost
 packets -- where endpoints may not agree on the state of an SA.  The
 responder to a CREATE_CHILD_SA MUST be prepared to accept messages on
 an SA before sending its response to the creation request, so there
 is no ambiguity for the initiator.  The initiator MAY begin sending
 on an SA as soon as it processes the response.  The initiator,

Kaufman, et al. Standards Track [Page 37] RFC 7296 IKEv2bis October 2014

 however, cannot receive on a newly created SA until it receives and
 processes the response to its CREATE_CHILD_SA request.  How, then, is
 the responder to know when it is OK to send on the newly created SA?
 From a technical correctness and interoperability perspective, the
 responder MAY begin sending on an SA as soon as it sends its response
 to the CREATE_CHILD_SA request.  In some situations, however, this
 could result in packets unnecessarily being dropped, so an
 implementation MAY defer such sending.
 The responder can be assured that the initiator is prepared to
 receive messages on an SA if either (1) it has received a
 cryptographically valid message on the other half of the SA pair, or
 (2) the new SA rekeys an existing SA and it receives an IKE request
 to close the replaced SA.  When rekeying an SA, the responder
 continues to send traffic on the old SA until one of those events
 occurs.  When establishing a new SA, the responder MAY defer sending
 messages on a new SA until either it receives one or a timeout has
 occurred.  If an initiator receives a message on an SA for which it
 has not received a response to its CREATE_CHILD_SA request, it
 interprets that as a likely packet loss and retransmits the
 CREATE_CHILD_SA request.  An initiator MAY send a dummy ESP message
 on a newly created ESP SA if it has no messages queued in order to
 assure the responder that the initiator is ready to receive messages.

2.8.1. Simultaneous Child SA Rekeying

 If the two ends have the same lifetime policies, it is possible that
 both will initiate a rekeying at the same time (which will result in
 redundant SAs).  To reduce the probability of this happening, the
 timing of rekeying requests SHOULD be jittered (delayed by a random
 amount of time after the need for rekeying is noticed).
 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 through such a collision, the SA
 created with the lowest of the four nonces used in the two exchanges
 SHOULD be closed by the endpoint that created it.  "Lowest" means an
 octet-by-octet comparison (instead of, for instance, comparing the
 nonces as large integers).  In other words, start by comparing the
 first octet; if they're equal, move to the next octet, and so on.  If
 you reach the end of one nonce, that nonce is the lower one.  The
 node that initiated the surviving rekeyed SA should delete the
 replaced SA after the new one is established.

Kaufman, et al. Standards Track [Page 38] RFC 7296 IKEv2bis October 2014

 The following is an explanation on the impact this has on
 implementations.  Assume that hosts A and B have an existing Child SA
 pair with SPIs (SPIa1,SPIb1), and both start rekeying it at the same
 time:
 Host A                            Host B
 -------------------------------------------------------------------
 send req1: N(REKEY_SA,SPIa1),
     SA(..,SPIa2,..),Ni1,..  -->
                              <--  send req2: N(REKEY_SA,SPIb1),
                                       SA(..,SPIb2,..),Ni2
 recv req2 <--
 At this point, A knows there is a simultaneous rekeying happening.
 However, it cannot yet know which of the exchanges will have the
 lowest nonce, so it will just note the situation and respond as
 usual.
 send resp2: SA(..,SPIa3,..),
      Nr1,..  -->
                              -->  recv req1
 Now B also knows that simultaneous rekeying is going on.  It responds
 as usual.
                             <--  send resp1: SA(..,SPIb3,..),
                                      Nr2,..
 recv resp1 <--
                             -->  recv resp2
 At this point, there are three Child SA pairs between A and B (the
 old one and two new ones).  A and B can now compare the nonces.
 Suppose that the lowest nonce was Nr1 in message resp2; in this case,
 B (the sender of req2) deletes the redundant new SA, and A (the node
 that initiated the surviving rekeyed SA), deletes the old one.
 send req3: D(SPIa1) -->
                              <--  send req4: D(SPIb2)
                              -->  recv req3
                              <--  send resp3: D(SPIb1)
 recv req4 <--
 send resp4: D(SPIa3) -->
 The rekeying is now finished.

Kaufman, et al. Standards Track [Page 39] RFC 7296 IKEv2bis October 2014

 However, there is a second possible sequence of events that can
 happen if some packets are lost in the network, resulting in
 retransmissions.  The rekeying begins as usual, but A's first packet
 (req1) is lost.
 Host A                            Host B
 -------------------------------------------------------------------
 send req1: N(REKEY_SA,SPIa1),
     SA(..,SPIa2,..),
     Ni1,..  -->  (lost)
                              <--  send req2: N(REKEY_SA,SPIb1),
                                       SA(..,SPIb2,..),Ni2
 recv req2 <--
 send resp2: SA(..,SPIa3,..),
     Nr1,.. -->
                              -->  recv resp2
                              <--  send req3: D(SPIb1)
 recv req3 <--
 send resp3: D(SPIa1) -->
                              -->  recv resp3
 From B's point of view, the rekeying is now completed, and since it
 has not yet received A's req1, it does not even know that there was
 simultaneous rekeying.  However, A will continue retransmitting the
 message, and eventually it will reach B.
 resend req1 -->
                              -->  recv req1
 To B, it looks like A is trying to rekey an SA that no longer exists;
 thus, B responds to the request with something non-fatal such as
 CHILD_SA_NOT_FOUND.
                              <--  send resp1: N(CHILD_SA_NOT_FOUND)
 recv resp1 <--
 When A receives this error, it already knows there was simultaneous
 rekeying, so it can ignore the error message.

2.8.2. Simultaneous IKE SA Rekeying

 Probably the most complex case occurs when both peers try to rekey
 the IKE_SA at the same time.  Basically, the text in Section 2.8
 applies to this case as well; however, it is important to ensure that
 the Child SAs are inherited by the correct IKE_SA.

Kaufman, et al. Standards Track [Page 40] RFC 7296 IKEv2bis October 2014

 The case where both endpoints notice the simultaneous rekeying works
 the same way as with Child SAs.  After the CREATE_CHILD_SA exchanges,
 three IKE SAs exist between A and B: the old IKE SA and two new IKE
 SAs.  The new IKE SA containing the lowest nonce SHOULD be deleted by
 the node that created it, and the other surviving new IKE SA MUST
 inherit all the Child SAs.
 In addition to normal simultaneous rekeying cases, there is a special
 case where one peer finishes its rekey before it even notices that
 other peer is doing a rekey.  If only one peer detects a simultaneous
 rekey, redundant SAs are not created.  In this case, when the peer
 that did not notice the simultaneous rekey gets the request to rekey
 the IKE SA that it has already successfully rekeyed, it SHOULD return
 TEMPORARY_FAILURE because it is an IKE SA that it is currently trying
 to close (whether or not it has already sent the delete notification
 for the SA).  If the peer that did notice the simultaneous rekey gets
 the delete request from the other peer for the old IKE SA, it knows
 that the other peer did not detect the simultaneous rekey, and the
 first peer can forget its own rekey attempt.
 Host A                      Host B
 -------------------------------------------------------------------
 send req1:
      SA(..,SPIa1,..),Ni1,.. -->
                           <-- send req2: SA(..,SPIb1,..),Ni2,..
                           --> recv req1
                           <-- send resp1: SA(..,SPIb2,..),Nr2,..
 recv resp1 <--
 send req3: D() -->
                           --> recv req3
 At this point, host B sees a request to close the IKE_SA.  There's
 not much more to do than to reply as usual.  However, at this point
 host B should stop retransmitting req2, since once host A receives
 resp3, it will delete all the state associated with the old IKE_SA
 and will not be able to reply to it.
                           <-- send resp3: ()
 The TEMPORARY_FAILURE notification was not included in RFC 4306, and
 support of the TEMPORARY_FAILURE notification is not negotiated.
 Thus, older peers that implement RFC 4306 but not this document may
 receive these notifications.  In that case, they will treat it the
 same as any other unknown error notification, and will stop the
 exchange.  Because the other peer has already rekeyed the exchange,
 doing so does not have any ill effects.

Kaufman, et al. Standards Track [Page 41] RFC 7296 IKEv2bis October 2014

2.8.3. Rekeying the IKE SA versus Reauthentication

 Rekeying the IKE SA and reauthentication are different concepts in
 IKEv2.  Rekeying the IKE SA establishes new keys for the IKE SA and
 resets the Message ID counters, but it does not authenticate the
 parties again (no AUTH or EAP payloads are involved).
 Although rekeying the IKE SA may be important in some environments,
 reauthentication (the verification that the parties still have access
 to the long-term credentials) is often more important.
 IKEv2 does not have any special support for reauthentication.
 Reauthentication is done by creating a new IKE SA from scratch (using
 IKE_SA_INIT/IKE_AUTH exchanges, without any REKEY_SA Notify
 payloads), creating new Child SAs within the new IKE SA (without
 REKEY_SA Notify payloads), and finally deleting the old IKE SA (which
 deletes the old Child SAs as well).
 This means that reauthentication also establishes new keys for the
 IKE SA and Child SAs.  Therefore, while rekeying can be performed
 more often than reauthentication, the situation where "authentication
 lifetime" is shorter than "key lifetime" does not make sense.
 While creation of a new IKE SA can be initiated by either party
 (initiator or responder in the original IKE SA), the use of EAP and/
 or Configuration payloads means in practice that reauthentication has
 to be initiated by the same party as the original IKE SA.  IKEv2 does
 not currently allow the responder to request reauthentication in this
 case; however, there are extensions that add this functionality such
 as [REAUTH].

2.9. Traffic Selector Negotiation

 When an RFC4301-compliant IPsec subsystem receives an IP packet that
 matches a "protect" selector in its Security Policy Database (SPD),
 the subsystem protects that packet with IPsec.  When no SA exists
 yet, it is the task of IKE to create it.  Maintenance of a system's
 SPD is outside the scope of IKE, although some implementations might
 update their SPD in connection with the running of IKE (for an
 example scenario, see Section 1.1.3).
 Traffic Selector (TS) payloads allow endpoints to communicate some of
 the information from their SPD to their peers.  These must be
 communicated to IKE from the SPD (for example, the PF_KEY API [PFKEY]
 uses the SADB_ACQUIRE message).  TS payloads specify the selection
 criteria for packets that will be forwarded over the newly set up SA.

Kaufman, et al. Standards Track [Page 42] RFC 7296 IKEv2bis October 2014

 This can serve as a consistency check in some scenarios to assure
 that the SPDs are consistent.  In others, it guides the dynamic
 update of the SPD.
 Two TS payloads appear in each of the messages in the exchange that
 creates a Child SA pair.  Each TS payload contains one or more
 Traffic Selectors.  Each Traffic Selector consists of an address
 range (IPv4 or IPv6), a port range, and an IP protocol ID.
 The first of the two TS payloads is known as TSi (Traffic Selector-
 initiator).  The second is known as TSr (Traffic Selector-responder).
 TSi specifies the source address of traffic forwarded from (or the
 destination address of traffic forwarded to) the initiator of the
 Child SA pair.  TSr specifies the destination address of the traffic
 forwarded to (or the source address of the traffic forwarded from)
 the responder of the Child SA pair.  For example, if the original
 initiator requests the creation of a Child SA pair, and wishes to
 tunnel all traffic from subnet 198.51.100.* on the initiator's side
 to subnet 192.0.2.* on the responder's side, the initiator would
 include a single Traffic Selector in each TS payload.  TSi would
 specify the address range (198.51.100.0 - 198.51.100.255) and TSr
 would specify the address range (192.0.2.0 - 192.0.2.255).  Assuming
 that proposal was acceptable to the responder, it would send
 identical TS payloads back.
 IKEv2 allows the responder to choose a subset of the traffic proposed
 by the initiator.  This could happen when the configurations of the
 two endpoints are being updated but only one end has received the new
 information.  Since the two endpoints may be configured by different
 people, the incompatibility may persist for an extended period even
 in the absence of errors.  It also allows for intentionally different
 configurations, as when one end is configured to tunnel all addresses
 and depends on the other end to have the up-to-date list.
 When the responder chooses a subset of the traffic proposed by the
 initiator, it narrows the Traffic Selectors to some subset of the
 initiator's proposal (provided the set does not become the null set).
 If the type of Traffic Selector proposed is unknown, the responder
 ignores that Traffic Selector, so that the unknown type is not
 returned in the narrowed set.
 To enable the responder to choose the appropriate range in this case,
 if the initiator has requested the SA due to a data packet, the
 initiator SHOULD include as the first Traffic Selector in each of TSi
 and TSr a very specific Traffic Selector including the addresses in
 the packet triggering the request.  In the example, the initiator
 would include in TSi two Traffic Selectors: the first containing the
 address range (198.51.100.43 - 198.51.100.43) and the source port and

Kaufman, et al. Standards Track [Page 43] RFC 7296 IKEv2bis October 2014

 IP protocol from the packet and the second containing (198.51.100.0 -
 198.51.100.255) with all ports and IP protocols.  The initiator would
 similarly include two Traffic Selectors in TSr.  If the initiator
 creates the Child SA pair not in response to an arriving packet, but
 rather, say, upon startup, then there may be no specific addresses
 the initiator prefers for the initial tunnel over any other.  In that
 case, the first values in TSi and TSr can be ranges rather than
 specific values.
 The responder performs the narrowing as follows:
 o  If the responder's policy does not allow it to accept any part of
    the proposed Traffic Selectors, it responds with a TS_UNACCEPTABLE
    Notify message.
 o  If the responder's policy allows the entire set of traffic covered
    by TSi and TSr, no narrowing is necessary, and the responder can
    return the same TSi and TSr values.
 o  If the responder's policy allows it to accept the first selector
    of TSi and TSr, then the responder MUST narrow the Traffic
    Selectors to a subset that includes the initiator's first choices.
    In this example above, the responder might respond with TSi being
    (198.51.100.43 - 198.51.100.43) with all ports and IP protocols.
 o  If the responder's policy does not allow it to accept the first
    selector of TSi and TSr, the responder narrows to an acceptable
    subset of TSi and TSr.
 When narrowing is done, there may be several subsets that are
 acceptable but their union is not.  In this case, the responder
 arbitrarily chooses one of them, and MAY include an
 ADDITIONAL_TS_POSSIBLE notification in the response.  The
 ADDITIONAL_TS_POSSIBLE notification asserts that the responder
 narrowed the proposed Traffic Selectors but that other Traffic
 Selectors would also have been acceptable, though only in a separate
 SA.  There is no data associated with this Notify type.  This case
 will occur only when the initiator and responder are configured
 differently from one another.  If the initiator and responder agree
 on the granularity of tunnels, the initiator will never request a
 tunnel wider than the responder will accept.
 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

Kaufman, et al. Standards Track [Page 44] RFC 7296 IKEv2bis October 2014

 separately negotiated Child SA.  If the initiator didn't generate its
 request based on the packet, but (for example) upon startup, there
 would not be the very specific first Traffic Selectors helping the
 responder to select the correct range.  There would be no way for the
 responder to determine which pair of addresses should be included in
 this tunnel, and it would have to make a guess or reject the request
 with a SINGLE_PAIR_REQUIRED Notify message.
 The SINGLE_PAIR_REQUIRED error indicates that a CREATE_CHILD_SA
 request is unacceptable because its sender is only willing to accept
 Traffic Selectors specifying a single pair of addresses.  The
 requestor is expected to respond by requesting an SA for only the
 specific traffic it is trying to forward.
 Few implementations will have policies that require separate SAs for
 each address pair.  Because of this, if only some parts of the TSi
 and TSr proposed by the initiator are acceptable to the responder,
 responders SHOULD narrow the selectors to an acceptable subset rather
 than use SINGLE_PAIR_REQUIRED.

2.9.1. Traffic Selectors Violating Own Policy

 When creating a new SA, the initiator needs to avoid proposing
 Traffic Selectors that violate its own policy.  If this rule is not
 followed, valid traffic may be dropped.  If you use decorrelated
 policies from [IPSECARCH], this kind of policy violations cannot
 happen.
 This is best illustrated by an example.  Suppose that host A has a
 policy whose effect is that traffic to 198.51.100.66 is sent via
 host B encrypted using AES, and traffic to all other hosts in
 198.51.100.0/24 is also sent via B, but must use 3DES.  Suppose also
 that host B accepts any combination of AES and 3DES.
 If host A now proposes an SA that uses 3DES, and includes TSr
 containing (198.51.100.0 - 198.51.100.255), this will be accepted by
 host B.  Now, host B can also use this SA to send traffic from
 198.51.100.66, but those packets will be dropped by A since it
 requires the use of AES for this traffic.  Even if host A creates a
 new SA only for 198.51.100.66 that uses AES, host B may freely
 continue to use the first SA for the traffic.  In this situation,
 when proposing the SA, host A should have followed its own policy,
 and included a TSr containing ((198.51.100.0 - 198.51.100.65),
 (198.51.100.67 - 198.51.100.255)) instead.

Kaufman, et al. Standards Track [Page 45] RFC 7296 IKEv2bis October 2014

 In general, if (1) the initiator makes a proposal "for traffic X
 (TSi/TSr), do SA", and (2) for some subset X' of X, the initiator
 does not actually accept traffic X' with SA, and (3) the initiator
 would be willing to accept traffic X' with some SA' (!=SA), valid
 traffic can be unnecessarily dropped since the responder can apply
 either SA or SA' to traffic X'.

2.9.2. Traffic Selectors in Rekeying

 Rekeying is used to replace an existing Child SA with another.  If
 the new SA would be allowed to have a narrower set of selectors than
 the original, traffic that was allowed on the old SA would be dropped
 in the new SA, thus violating the idea of "replacing".  Thus, the new
 SA MUST NOT have narrower selectors than the original.  If the
 rekeyed SA would ever need to have a narrower scope than the
 currently used SA, that would mean that the policy was changed in a
 way such that the currently used SA is against the policy.  In that
 case, the SA should have been already deleted after the policy change
 took effect.
 When the initiator attempts to rekey the Child SA, the proposed
 Traffic Selectors SHOULD be either the same as, or a superset of, the
 Traffic Selectors used in the old Child SA.  That is, they would be
 the same as, or a superset of, the currently active (decorrelated)
 policy.  The responder MUST NOT narrow down the Traffic Selectors
 narrower than the scope currently in use.
 Because a rekeyed SA can never have a narrower scope than the one
 currently in use, there is no need for the selectors from the packet,
 so those selectors SHOULD NOT be sent.

2.10. Nonces

 The IKE_SA_INIT messages each contain a nonce.  These nonces are used
 as inputs to cryptographic functions.  The CREATE_CHILD_SA request
 and the CREATE_CHILD_SA response also contain nonces.  These nonces
 are used to add freshness to the key derivation technique used to
 obtain keys for Child SA, and to ensure creation of strong
 pseudorandom bits from the Diffie-Hellman key.  Nonces used in IKEv2
 MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
 be at least half the key size of the negotiated pseudorandom function
 (PRF).  However, the initiator chooses the nonce before the outcome
 of the negotiation is known.  Because of that, the nonce has to be
 long enough for all the PRFs being proposed.  If the same random
 number source is used for both keys and nonces, care must be taken to
 ensure that the latter use does not compromise the former.

Kaufman, et al. Standards Track [Page 46] RFC 7296 IKEv2bis October 2014

2.11. Address and Port Agility

 IKE runs over UDP ports 500 and 4500, and implicitly sets up ESP and
 AH associations for the same IP addresses over which it runs.  The IP
 addresses and ports in the outer header are, however, not themselves
 cryptographically protected, and IKE is designed to work even through
 Network Address Translation (NAT) boxes.  An implementation MUST
 accept incoming requests even if the source port is not 500 or 4500,
 and MUST respond to the address and port from which the request was
 received.  It MUST specify the address and port at which the request
 was received as the source address and port in the response.  IKE
 functions identically over IPv4 or IPv6.

2.12. Reuse of Diffie-Hellman Exponentials

 IKE generates keying material using an ephemeral Diffie-Hellman
 exchange in order to gain the property of "perfect forward secrecy".
 This means that once a connection is closed and its corresponding
 keys are forgotten, even someone who has recorded all of the data
 from the connection and gets access to all of the long-term keys of
 the two endpoints cannot reconstruct the keys used to protect the
 conversation without doing a brute force search of the session key
 space.
 Achieving perfect forward secrecy requires that when a connection is
 closed, each endpoint MUST forget not only the keys used by the
 connection but also any information that could be used to recompute
 those keys.
 Because computing Diffie-Hellman exponentials is computationally
 expensive, an endpoint may find it advantageous to reuse those
 exponentials for multiple connection setups.  There are several
 reasonable strategies for doing this.  An endpoint could choose a new
 exponential only periodically though this could result in less-than-
 perfect forward secrecy if some connection lasts for less than the
 lifetime of the exponential.  Or it could keep track of which
 exponential was used for each connection and delete the information
 associated with the exponential only when some corresponding
 connection was closed.  This would allow the exponential to be reused
 without losing perfect forward secrecy at the cost of maintaining
 more state.
 Whether and when to reuse Diffie-Hellman exponentials are private
 decisions in the sense that they will not affect interoperability.
 An implementation that reuses exponentials MAY choose to remember the
 exponential used by the other endpoint on past exchanges and if one
 is reused to avoid the second half of the calculation.  See [REUSE]

Kaufman, et al. Standards Track [Page 47] RFC 7296 IKEv2bis October 2014

 and [RFC6989] for a security analysis of this practice and for
 additional security considerations when reusing ephemeral
 Diffie-Hellman keys.

2.13. Generating Keying Material

 In the context of the IKE SA, four cryptographic algorithms are
 negotiated: an encryption algorithm, an integrity protection
 algorithm, a Diffie-Hellman group, and a pseudorandom function (PRF).
 The PRF is used for the construction of keying material for all of
 the cryptographic algorithms used in both the IKE SA and the
 Child SAs.
 We assume that each encryption algorithm and integrity protection
 algorithm uses a fixed-size key and that any randomly chosen value of
 that fixed size can serve as an appropriate key.  For algorithms that
 accept a variable-length key, a fixed key size MUST be specified as
 part of the cryptographic transform negotiated (see Section 3.3.5 for
 the definition of the Key Length transform attribute).  For
 algorithms for which not all values are valid keys (such as DES or
 3DES with key parity), the algorithm by which keys are derived from
 arbitrary values MUST be specified by the cryptographic transform.
 For integrity protection functions based on Hashed Message
 Authentication Code (HMAC), the fixed key size is the size of the
 output of the underlying hash function.
 It is assumed that PRFs accept keys of any length, but have a
 preferred key size.  The preferred key size MUST be used as the
 length of SK_d, SK_pi, and SK_pr (see Section 2.14).  For PRFs based
 on the HMAC construction, the preferred key size is equal to the
 length of the output of the underlying hash function.  Other types of
 PRFs MUST specify their preferred key size.
 Keying material will always be derived as the output of the
 negotiated PRF algorithm.  Since the amount of keying material needed
 may be greater than the size of the output of the PRF, the PRF is
 used iteratively.  The term "prf+" describes a function that outputs
 a pseudorandom stream based on the inputs to a pseudorandom function
 called "prf".

Kaufman, et al. Standards Track [Page 48] RFC 7296 IKEv2bis October 2014

 In the following, | indicates concatenation.  prf+ is defined as:
 prf+ (K,S) = T1 | T2 | T3 | T4 | ...
 where:
 T1 = prf (K, S | 0x01)
 T2 = prf (K, T1 | S | 0x02)
 T3 = prf (K, T2 | S | 0x03)
 T4 = prf (K, T3 | S | 0x04)
 ...
 This continues until all the material needed to compute all required
 keys has been output from prf+.  The keys are taken from the output
 string without regard to boundaries (e.g., if the required keys are a
 256-bit Advanced Encryption Standard (AES) key and a 160-bit HMAC
 key, and the prf function generates 160 bits, the AES key will come
 from T1 and the beginning of T2, while the HMAC key will come from
 the rest of T2 and the beginning of T3).
 The constant concatenated to the end of each prf function is a single
 octet.  The prf+ function is not defined beyond 255 times the size of
 the prf function output.

2.14. Generating Keying Material for the IKE SA

 The shared keys are computed as follows.  A quantity called SKEYSEED
 is calculated from the nonces exchanged during the IKE_SA_INIT
 exchange and the Diffie-Hellman shared secret established during that
 exchange.  SKEYSEED is used to calculate seven other secrets: SK_d
 used for deriving new keys for the Child SAs established with this
 IKE SA; SK_ai and SK_ar used as a key to the integrity protection
 algorithm for authenticating the component messages of subsequent
 exchanges; SK_ei and SK_er used for encrypting (and of course
 decrypting) all subsequent exchanges; and SK_pi and SK_pr, which are
 used when generating an AUTH payload.  The lengths of SK_d, SK_pi,
 and SK_pr MUST be the preferred key length of the PRF agreed upon.
 SKEYSEED and its derivatives are computed as follows:
 SKEYSEED = prf(Ni | Nr, g^ir)
 {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr}
                 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr)
 (indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
 SK_pi, and SK_pr are taken in order from the generated bits of the
 prf+).  g^ir is the shared secret from the ephemeral Diffie-Hellman
 exchange.  g^ir is represented as a string of octets in big endian

Kaufman, et al. Standards Track [Page 49] RFC 7296 IKEv2bis October 2014

 order padded with zeros if necessary to make it the length of the
 modulus.  Ni and Nr are the nonces, stripped of any headers.  For
 historical backward-compatibility reasons, there are two PRFs that
 are treated specially in this calculation.  If the negotiated PRF is
 AES-XCBC-PRF-128 [AESXCBCPRF128] or AES-CMAC-PRF-128 [AESCMACPRF128],
 only the first 64 bits of Ni and the first 64 bits of Nr are used in
 calculating SKEYSEED, but all the bits are used for input to the prf+
 function.
 The two directions of traffic flow use different keys.  The keys used
 to protect messages from the original initiator are SK_ai and SK_ei.
 The keys used to protect messages in the other direction are SK_ar
 and SK_er.

2.15. Authentication of the IKE SA

 When not using extensible authentication (see Section 2.16), the
 peers are authenticated by having each sign (or MAC using a padded
 shared secret as the key, as described later in this section) a block
 of data.  In these calculations, IDi' and IDr' are the entire ID
 payloads excluding the fixed header.  For the responder, the octets
 to be signed start with the first octet of the first SPI in the
 header of the second message (IKE_SA_INIT response) and end with the
 last octet of the last payload in the second message.  Appended to
 this (for the purposes of computing the signature) are the
 initiator's nonce Ni (just the value, not the payload containing it),
 and the value prf(SK_pr, IDr').  Note that neither the nonce Ni nor
 the value prf(SK_pr, IDr') are transmitted.  Similarly, the initiator
 signs the first message (IKE_SA_INIT request), starting with the
 first octet of the first SPI in the header and ending with the last
 octet of the last payload.  Appended to this (for purposes of
 computing the signature) are the responder's nonce Nr, and the value
 prf(SK_pi, IDi').  It is critical to the security of the exchange
 that each side sign the other side's nonce.
 The initiator's signed octets can be described as:
 InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
 GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
 RealIKEHDR =  SPIi | SPIr |  . . . | Length
 RealMessage1 = RealIKEHDR | RestOfMessage1
 NonceRPayload = PayloadHeader | NonceRData
 InitiatorIDPayload = PayloadHeader | RestOfInitIDPayload
 RestOfInitIDPayload = IDType | RESERVED | InitIDData
 MACedIDForI = prf(SK_pi, RestOfInitIDPayload)

Kaufman, et al. Standards Track [Page 50] RFC 7296 IKEv2bis October 2014

 The responder's signed octets can be described as:
 ResponderSignedOctets = RealMessage2 | NonceIData | MACedIDForR
 GenIKEHDR = [ four octets 0 if using port 4500 ] | RealIKEHDR
 RealIKEHDR =  SPIi | SPIr |  . . . | Length
 RealMessage2 = RealIKEHDR | RestOfMessage2
 NonceIPayload = PayloadHeader | NonceIData
 ResponderIDPayload = PayloadHeader | RestOfRespIDPayload
 RestOfRespIDPayload = IDType | RESERVED | RespIDData
 MACedIDForR = prf(SK_pr, RestOfRespIDPayload)
 Note that all of the payloads are included under the signature,
 including any payload types not defined in this document.  If the
 first message of the exchange is sent multiple times (such as with a
 responder cookie and/or a different Diffie-Hellman group), it is the
 latest version of the message that is signed.
 Optionally, messages 3 and 4 MAY include a certificate, or
 certificate chain providing evidence that the key used to compute a
 digital signature belongs to the name in the ID payload.  The
 signature or MAC will be computed using algorithms dictated by the
 type of key used by the signer, and specified by the Auth Method
 field in the Authentication payload.  There is no requirement that
 the initiator and responder sign with the same cryptographic
 algorithms.  The choice of cryptographic algorithms depends on the
 type of key each has.  In particular, the initiator may be using a
 shared key while the responder may have a public signature key and
 certificate.  It will commonly be the case (but it is not required)
 that, if a shared secret is used for authentication, the same key is
 used in both directions.
 Note that it is a common but typically insecure practice to have a
 shared key derived solely from a user-chosen password without
 incorporating another source of randomness.  This is typically
 insecure because user-chosen passwords are unlikely to have
 sufficient unpredictability to resist dictionary attacks and these
 attacks are not prevented in this authentication method.
 (Applications using password-based authentication for bootstrapping
 and IKE SA should use the authentication method in Section 2.16,
 which is designed to prevent off-line dictionary attacks.)  The
 pre-shared key needs to contain as much unpredictability as the
 strongest key being negotiated.  In the case of a pre-shared key, the
 AUTH value is computed as:
 For the initiator:
    AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                     <InitiatorSignedOctets>)

Kaufman, et al. Standards Track [Page 51] RFC 7296 IKEv2bis October 2014

 For the responder:
    AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                     <ResponderSignedOctets>)
 where the string "Key Pad for IKEv2" is 17 ASCII characters without
 null termination.  The shared secret can be variable length.  The pad
 string is added so that if the shared secret is derived from a
 password, the IKE implementation need not store the password in
 cleartext, but rather can store the value prf(Shared Secret,"Key Pad
 for IKEv2"), which could not be used as a password equivalent for
 protocols other than IKEv2.  As noted above, deriving the shared
 secret from a password is not secure.  This construction is used
 because it is anticipated that people will do it anyway.  The
 management interface by which the shared secret is provided MUST
 accept ASCII strings of at least 64 octets and MUST NOT add a null
 terminator before using them as shared secrets.  It MUST also accept
 a hex encoding of the shared secret.  The management interface MAY
 accept other encodings if the algorithm for translating the encoding
 to a binary string is specified.
 There are two types of EAP authentication (described in
 Section 2.16), and each type uses different values in the AUTH
 computations shown above.  If the EAP method is key-generating,
 substitute master session key (MSK) for the shared secret in the
 computation.  For non-key-generating methods, substitute SK_pi and
 SK_pr, respectively, for the shared secret in the two AUTH
 computations.

2.16. Extensible Authentication Protocol Methods

 In addition to authentication using public key signatures and shared
 secrets, IKE supports authentication using methods defined in
 RFC 3748 [EAP].  Typically, these methods are asymmetric (designed
 for a user authenticating to a server), and they may not be mutual.
 For this reason, these protocols are typically used to authenticate
 the initiator to the responder and MUST be used in conjunction with a
 public-key-signature-based authentication of the responder to the
 initiator.  These methods are often associated with mechanisms
 referred to as "Legacy Authentication" mechanisms.
 While this document references [EAP] with the intent that new methods
 can be added in the future without updating this specification, some
 simpler variations are documented here.  [EAP] defines an
 authentication protocol requiring a variable number of messages.
 Extensible authentication is implemented in IKE as additional
 IKE_AUTH exchanges that MUST be completed in order to initialize the
 IKE SA.

Kaufman, et al. Standards Track [Page 52] RFC 7296 IKEv2bis October 2014

 An initiator indicates a desire to use EAP by leaving out the AUTH
 payload from the first message in the IKE_AUTH exchange.  (Note that
 the AUTH payload is required for non-EAP authentication, and is thus
 not marked as optional in the rest of this document.)  By including
 an IDi payload but not an AUTH payload, the initiator has declared an
 identity but has not proven it.  If the responder is willing to use
 an EAP method, it will place an Extensible Authentication Protocol
 (EAP) payload in the response of the IKE_AUTH exchange and defer
 sending SAr2, TSi, and TSr until initiator authentication is complete
 in a subsequent IKE_AUTH exchange.  In the case of a minimal EAP
 method, the initial SA establishment will appear as follows:
 Initiator                         Responder
 -------------------------------------------------------------------
 HDR, SAi1, KEi, Ni  -->
                              <--  HDR, SAr1, KEr, Nr, [CERTREQ]
 HDR, SK {IDi, [CERTREQ,]
     [IDr,] SAi2,
     TSi, TSr}  -->
                              <--  HDR, SK {IDr, [CERT,] AUTH,
                                       EAP}
 HDR, SK {EAP}  -->
                              <--  HDR, SK {EAP (success)}
 HDR, SK {AUTH}  -->
                              <--  HDR, SK {AUTH, SAr2, TSi, TSr}
 As described in Section 2.2, when EAP is used, each pair of IKE SA
 initial setup messages will have their message numbers incremented;
 the first pair of IKE_AUTH messages will have an ID of 1, the second
 will be 2, and so on.
 For EAP methods that create a shared key as a side effect of
 authentication, that shared key MUST be used by both the initiator
 and responder to generate AUTH payloads in messages 7 and 8 using the
 syntax for shared secrets specified in Section 2.15.  The shared key
 from EAP is the field from the EAP specification named MSK.  This
 shared key generated during an IKE exchange MUST NOT be used for any
 other purpose.
 EAP methods that do not establish a shared key SHOULD NOT be used, as
 they are subject to a number of man-in-the-middle attacks [EAPMITM]
 if these EAP methods are used in other protocols that do not use a
 server-authenticated tunnel.  Please see the Security Considerations
 section for more details.  If EAP methods that do not generate a
 shared key are used, the AUTH payloads in messages 7 and 8 MUST be
 generated using SK_pi and SK_pr, respectively.

Kaufman, et al. Standards Track [Page 53] RFC 7296 IKEv2bis October 2014

 The initiator of an IKE SA using EAP needs to be capable of extending
 the initial protocol exchange to at least ten IKE_AUTH exchanges in
 the event the responder sends notification messages and/or retries
 the authentication prompt.  Once the protocol exchange defined by the
 chosen EAP authentication method has successfully terminated, the
 responder MUST send an EAP payload containing the Success message.
 Similarly, if the authentication method has failed, the responder
 MUST send an EAP payload containing the Failure message.  The
 responder MAY at any time terminate the IKE exchange by sending an
 EAP payload containing the Failure message.
 Following such an extended exchange, the EAP AUTH payloads MUST be
 included in the two messages following the one containing the EAP
 Success message.
 When the initiator authentication uses EAP, it is possible that the
 contents of the IDi payload is used only for Authentication,
 Authorization, and Accounting (AAA) routing purposes and selecting
 which EAP method to use.  This value may be different from the
 identity authenticated by the EAP method.  It is important that
 policy lookups and access control decisions use the actual
 authenticated identity.  Often the EAP server is implemented in a
 separate AAA server that communicates with the IKEv2 responder.  In
 this case, the authenticated identity, if different from that in the
 IDi payload, has to be sent from the AAA server to the IKEv2
 responder.

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).

Kaufman, et al. Standards Track [Page 54] RFC 7296 IKEv2bis October 2014

 A single CREATE_CHILD_SA negotiation may result in multiple Security
 Associations.  ESP and AH SAs exist in pairs (one in each direction),
 so two SAs are created in a single Child SA negotiation for them.
 Furthermore, Child SA negotiation may include some future IPsec
 protocol(s) in addition to, or instead of, ESP or AH (for example,
 ROHC_INTEG as described in [ROHCV2]).  In any case, keying material
 for each Child SA MUST be taken from the expanded KEYMAT using the
 following rules:
 o  All keys for SAs carrying data from the initiator to the responder
    are taken before SAs going from the responder to the initiator.
 o  If multiple IPsec protocols are negotiated, keying material for
    each Child SA is taken in the order in which the protocol headers
    will appear in the encapsulated packet.
 o  If an IPsec protocol requires multiple keys, the order in which
    they are taken from the SA's keying material needs to be described
    in the protocol's specification.  For ESP and AH, [IPSECARCH]
    defines the order, namely: the encryption key (if any) MUST be
    taken from the first bits and the integrity key (if any) MUST be
    taken from the remaining bits.
 Each cryptographic algorithm takes a fixed number of bits of keying
 material specified as part of the algorithm, or negotiated in SA
 payloads (see Section 2.13 for description of key lengths, and
 Section 3.3.5 for the definition of the Key Length transform
 attribute).

2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange

 The CREATE_CHILD_SA exchange can be used to rekey an existing IKE SA
 (see Sections 1.3.2 and 2.8).  New initiator and responder SPIs are
 supplied in the SPI fields in the Proposal structures inside the
 Security Association (SA) payloads (not the SPI fields in the IKE
 header).  The TS payloads are omitted when rekeying an IKE SA.
 SKEYSEED for the new IKE SA is computed using SK_d from the existing
 IKE SA as follows:
 SKEYSEED = prf(SK_d (old), g^ir (new) | Ni | Nr)
 where g^ir (new) is the shared secret from the ephemeral Diffie-
 Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
 octet string in big endian order padded with zeros if necessary to
 make it the length of the modulus) and Ni and Nr are the two nonces
 stripped of any headers.

Kaufman, et al. Standards Track [Page 55] RFC 7296 IKEv2bis October 2014

 The old and new IKE SA may have selected a different PRF.  Because
 the rekeying exchange belongs to the old IKE SA, it is the old IKE
 SA's PRF that is used to generate SKEYSEED.
 The main reason for rekeying the IKE SA is to ensure that the
 compromise of old keying material does not provide information about
 the current keys, or vice versa.  Therefore, implementations MUST
 perform a new Diffie-Hellman exchange when rekeying the IKE SA.  In
 other words, an initiator MUST NOT propose the value "NONE" for the
 Diffie-Hellman transform, and a responder MUST NOT accept such a
 proposal.  This means that a successful exchange rekeying the IKE SA
 always includes the KEi/KEr payloads.
 The new IKE SA MUST reset its message counters to 0.
 SK_d, SK_ai, SK_ar, SK_ei, and SK_er are computed from SKEYSEED as
 specified in Section 2.14, using SPIi, SPIr, Ni, and Nr from the new
 exchange, and using the new IKE SA's PRF.

2.19. Requesting an Internal Address on a Remote Network

 Most commonly occurring in the endpoint-to-security-gateway scenario,
 an endpoint may need an IP address in the network protected by the
 security gateway and may need to have that address dynamically
 assigned.  A request for such a temporary address can be included in
 any request to create a Child SA (including the implicit request in
 message 3) by including a CP payload.  Note, however, it is usual to
 only assign one IP address during the IKE_AUTH exchange.  That
 address persists at least until the deletion of the IKE SA.
 This function provides address allocation to an IPsec Remote Access
 Client (IRAC) trying to tunnel into a network protected by an IPsec
 Remote Access Server (IRAS).  Since the IKE_AUTH exchange creates an
 IKE SA and a Child SA, the IRAC MUST request the IRAS-controlled
 address (and optionally other information concerning the protected
 network) in the IKE_AUTH exchange.  The IRAS may procure an address
 for the IRAC from any number of sources such as a DHCP/BOOTP
 (Bootstrap Protocol) server or its own address pool.
 Initiator                         Responder
 -------------------------------------------------------------------
  HDR, SK {IDi, [CERT,]
     [CERTREQ,] [IDr,] AUTH,
     CP(CFG_REQUEST), SAi2,
     TSi, TSr}  -->
                              <--  HDR, SK {IDr, [CERT,] AUTH,
                                       CP(CFG_REPLY), SAr2,
                                       TSi, TSr}

Kaufman, et al. Standards Track [Page 56] RFC 7296 IKEv2bis October 2014

 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()
 TSi = (0, 0-65535, 0.0.0.0-255.255.255.255)
 TSr = (0, 0-65535, 0.0.0.0-255.255.255.255)
 NOTE: Traffic Selectors contain (protocol, port range, address
 range).
 Message from responder to initiator:
 CP(CFG_REPLY)=
   INTERNAL_ADDRESS(192.0.2.202)
   INTERNAL_NETMASK(255.255.255.0)
   INTERNAL_SUBNET(192.0.2.0/255.255.255.0)
 TSi = (0, 0-65535, 192.0.2.202-192.0.2.202)
 TSr = (0, 0-65535, 192.0.2.0-192.0.2.255)
 All returned values will be implementation dependent.  As can be seen
 in the above example, the IRAS MAY also send other attributes that
 were not included in CP(CFG_REQUEST) and MAY ignore the non-mandatory
 attributes that it does not support.
 The responder MUST NOT send a CFG_REPLY without having first received
 a CP(CFG_REQUEST) from the initiator, because we do not want the IRAS
 to perform an unnecessary configuration lookup if the IRAC cannot
 process the REPLY.
 In the case where the IRAS's configuration requires that CP be used
 for a given identity IDi, but IRAC has failed to send a
 CP(CFG_REQUEST), IRAS MUST fail the request, and terminate the Child
 SA creation with a FAILED_CP_REQUIRED error.  The FAILED_CP_REQUIRED
 is not fatal to the IKE SA; it simply causes the Child SA creation to
 fail.  The initiator can fix this by later starting a new
 Configuration payload request.  There is no associated data in the
 FAILED_CP_REQUIRED error.

Kaufman, et al. Standards Track [Page 57] RFC 7296 IKEv2bis October 2014

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.
 An IKE implementation MAY decline to give out version information
 prior to authentication or even after authentication in case some
 implementation is known to have some security weakness.  In that
 case, it MUST either return an empty string or no CP payload if CP is
 not supported.
 Initiator                          Responder
 -------------------------------------------------------------------
 HDR, SK {CP(CFG_REQUEST)}  -->
                               <--  HDR, SK {CP(CFG_REPLY)}
 CP(CFG_REQUEST)=
   APPLICATION_VERSION("")
 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.
 The general rule is that if a request is received that is badly
 formatted, or unacceptable for reasons of policy (such as no matching
 cryptographic algorithms), the response contains a Notify payload
 indicating the error.  The decision whether or not to send such a
 response depends whether or not there is an authenticated IKE SA.
 If there is an error parsing or processing a response packet, the
 general rule is to not send back any error message because responses
 should not generate new requests (and a new request would be the only
 way to send back an error message).  Such errors in parsing or
 processing response packets should still cause the recipient to clean
 up the IKE state (for example, by sending a Delete for a bad SA).
 Only authentication failures (AUTHENTICATION_FAILED and EAP failure)
 and malformed messages (INVALID_SYNTAX) lead to a deletion of the IKE
 SA without requiring an explicit INFORMATIONAL exchange carrying a
 Delete payload.  Other error conditions MAY require such an exchange
 if policy dictates that this is needed.  If the exchange is
 terminated with EAP Failure, an AUTHENTICATION_FAILED notification is
 not sent.

Kaufman, et al. Standards Track [Page 58] RFC 7296 IKEv2bis October 2014

2.21.1. Error Handling in IKE_SA_INIT

 Errors that occur before a cryptographically protected IKE SA is
 established need to be handled very carefully.  There is a trade-off
 between wanting to help the peer to diagnose a problem and thus
 responding to the error and wanting to avoid being part of a DoS
 attack based on forged messages.
 In an IKE_SA_INIT exchange, any error notification causes the
 exchange to fail.  Note that some error notifications such as COOKIE,
 INVALID_KE_PAYLOAD or INVALID_MAJOR_VERSION may lead to a subsequent
 successful exchange.  Because all error notifications are completely
 unauthenticated, the recipient should continue trying for some time
 before giving up.  The recipient should not immediately act based on
 the error notification unless corrective actions are defined in this
 specification, such as for COOKIE, INVALID_KE_PAYLOAD, and
 INVALID_MAJOR_VERSION.

2.21.2. Error Handling in IKE_AUTH

 All errors that occur in an IKE_AUTH exchange, causing the
 authentication to fail for whatever reason (invalid shared secret,
 invalid ID, untrusted certificate issuer, revoked or expired
 certificate, etc.) SHOULD result in an AUTHENTICATION_FAILED
 notification.  If the error occurred on the responder, the
 notification is returned in the protected response, and is usually
 the only payload in that response.  Although the IKE_AUTH messages
 are encrypted and integrity protected, if the peer receiving this
 notification has not authenticated the other end yet, that peer needs
 to treat the information with caution.
 If the error occurs on the initiator, the notification MAY be
 returned in a separate INFORMATIONAL exchange, usually with no other
 payloads.  This is an exception for the general rule of not starting
 new exchanges based on errors in responses.
 Note, however, that request messages that contain an unsupported
 critical payload, or where the whole message is malformed (rather
 than just bad payload contents), MUST be rejected in their entirety,
 and MUST only lead to an UNSUPPORTED_CRITICAL_PAYLOAD or
 INVALID_SYNTAX Notification sent as a response.  The receiver should
 not verify the payloads related to authentication in this case.
 If authentication has succeeded in the IKE_AUTH exchange, the IKE SA
 is established; however, establishing the Child SA or requesting
 configuration information may still fail.  This failure does not
 automatically cause the IKE SA to be deleted.  Specifically, a
 responder may include all the payloads associated with authentication

Kaufman, et al. Standards Track [Page 59] RFC 7296 IKEv2bis October 2014

 (IDr, CERT, and AUTH) while sending error notifications for the
 piggybacked exchanges (FAILED_CP_REQUIRED, NO_PROPOSAL_CHOSEN, and so
 on), and the initiator MUST NOT fail the authentication because of
 this.  The initiator MAY, of course, for reasons of policy later
 delete such an IKE SA.
 In an IKE_AUTH exchange, or in the INFORMATIONAL exchange immediately
 following it (in case an error happened when processing a response to
 IKE_AUTH), the UNSUPPORTED_CRITICAL_PAYLOAD, INVALID_SYNTAX, and
 AUTHENTICATION_FAILED notifications are the only ones to cause the
 IKE SA to be deleted or not created, without a Delete payload.
 Extension documents may define new error notifications with these
 semantics, but MUST NOT use them unless the peer has been shown to
 understand them, such as by using the Vendor ID payload.

2.21.3. Error Handling after IKE SA is Authenticated

 After the IKE SA is authenticated, all requests having errors MUST
 result in a response notifying the other end of the error.
 In normal situations, there should not be cases where a valid
 response from one peer results in an error situation in the other
 peer, so there should not be any reason for a peer to send error
 messages to the other end except as a response.  Because sending such
 error messages as an INFORMATIONAL exchange might lead to further
 errors that could cause loops, such errors SHOULD NOT be sent.  If
 errors are seen that indicate that the peers do not have the same
 state, it might be good to delete the IKE SA to clean up state and
 start over.
 If a peer parsing a request notices that it is badly formatted (after
 it has passed the message authentication code checks and window
 checks) and it returns an INVALID_SYNTAX notification, then this
 error notification is considered fatal in both peers, meaning that
 the IKE SA is deleted without needing an explicit Delete payload.

2.21.4. Error Handling Outside IKE SA

 A node needs to limit the rate at which it will send messages in
 response to unprotected messages.
 If a node receives a message on UDP port 500 or 4500 outside the
 context of an IKE SA known to it (and the message is not a request to
 start an IKE SA), this may be the result of a recent crash of the
 node.  If the message is marked as a response, the node can audit the
 suspicious event but MUST NOT respond.  If the message is marked as a
 request, the node can audit the suspicious event and MAY send a
 response.  If a response is sent, the response MUST be sent to the IP

Kaufman, et al. Standards Track [Page 60] RFC 7296 IKEv2bis October 2014

 address and port from where it came with the same IKE SPIs and the
 Message ID copied.  The response MUST NOT be cryptographically
 protected and MUST contain an INVALID_IKE_SPI Notify payload.  The
 INVALID_IKE_SPI notification 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.
 A peer 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 that a genuine correspondent was
 tricked into sending.  A node should treat such a message (and also a
 network message like ICMP destination unreachable) as a hint that
 there might be problems with SAs to that IP address and should
 initiate a liveness check for any such IKE SA.  An implementation
 SHOULD limit the frequency of such tests to avoid being tricked into
 participating in a DoS attack.
 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.
 A node receiving a suspicious message from an IP address (and port,
 if NAT traversal is used) with which it has an IKE SA SHOULD send an
 IKE Notify payload in an IKE INFORMATIONAL exchange over that SA.
 The recipient MUST NOT change the state of any SAs as a result, but
 may wish to audit the event to aid in diagnosing malfunctions.

2.22. IPComp

 Use of IP Compression [IP-COMP] 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.  This Notify message 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 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.

Kaufman, et al. Standards Track [Page 61] RFC 7296 IKEv2bis October 2014

 The data associated with this Notify message 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.
 The Transform IDs are listed here.  The values in the following table
 are only current as of the publication date of RFC 4306.  Other
 values may have been added since then or will be added after the
 publication of this document.  Readers should refer to [IKEV2IANA]
 for the latest values.
 Name              Number   Defined In
 ----------------------------------------
 IPCOMP_OUI        1        (UNSPECIFIED)
 IPCOMP_DEFLATE    2        RFC 2394
 IPCOMP_LZS        3        RFC 2395
 IPCOMP_LZJH       4        RFC 3051
 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.
 In some cases, Robust Header Compression (ROHC) may be more
 appropriate than IP Compression.  [ROHCV2] defines the use of ROHC
 with IKEv2 and IPsec.

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 indeed specify some unintuitive processing
 rules so that NATs are more likely to work.

Kaufman, et al. Standards Track [Page 62] RFC 7296 IKEv2bis October 2014

 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 will use 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 UDP-encapsulated IPsec traffic but not
 plain, unencapsulated ESP/AH or vice versa.
 It is a common practice of NATs to translate TCP and UDP port numbers
 as well as addresses and use the port numbers of inbound packets to
 decide which internal node should get a given packet.  For this
 reason, even though IKE packets MUST be sent to and from UDP port 500
 or 4500, 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.

Kaufman, et al. Standards Track [Page 63] RFC 7296 IKEv2bis October 2014

 Port 4500 is reserved for UDP-encapsulated ESP and IKE.  An IPsec
 endpoint that discovers a NAT between it and its correspondent (as
 described below) MUST send all subsequent traffic from port 4500,
 which NATs should not treat specially (as they might with port 500).
 An initiator can use port 4500 for both IKE and ESP, regardless of
 whether or not there is a NAT, even at the beginning of IKE.  When
 either side is using port 4500, sending ESP with UDP encapsulation is
 not required, but understanding received UDP-encapsulated ESP packets
 is required.  UDP encapsulation MUST NOT be done on port 500.  If
 Network Address Translation Traversal (NAT-T) is supported (that is,
 if NAT_DETECTION_*_IP payloads were exchanged during IKE_SA_INIT),
 all devices MUST be able to receive and process both UDP-encapsulated
 ESP and non-UDP-encapsulated ESP packets at any time.  Either side
 can decide whether or not to use UDP encapsulation for ESP
 irrespective of the choice made by the other side.  However, if a NAT
 is detected, both devices MUST use UDP encapsulation for ESP.
 The specific requirements for supporting NAT traversal [NATREQ] are
 listed below.  Support for NAT traversal is optional.  In this
 section only, requirements listed as MUST apply only to
 implementations supporting NAT traversal.
 o  Both the 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 is just after the Ni and Nr payloads
    (before the optional CERTREQ payload).
 o  The data associated with the NAT_DETECTION_SOURCE_IP notification
    is a SHA-1 digest of the SPIs (in the order they appear in the
    header), IP address, and port from which this packet was sent.
    There MAY be multiple NAT_DETECTION_SOURCE_IP payloads in a
    message if the sender does not know which of several network
    attachments will be used to send the packet.
 o  The data associated with the NAT_DETECTION_DESTINATION_IP
    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.
 o  The recipient of either the NAT_DETECTION_SOURCE_IP or
    NAT_DETECTION_DESTINATION_IP notification MAY compare the supplied
    value to a SHA-1 hash of the SPIs, source or recipient IP address,
    and port (respectively), and if they don't match, it SHOULD enable

Kaufman, et al. Standards Track [Page 64] RFC 7296 IKEv2bis October 2014

    NAT traversal.  In the case there is a mismatch of the
    NAT_DETECTION_SOURCE_IP hash with all of the
    NAT_DETECTION_SOURCE_IP payloads received, the recipient MAY
    reject the connection attempt if NAT traversal is not supported.
    In the case of a mismatching NAT_DETECTION_DESTINATION_IP hash, it
    means that the system receiving the NAT_DETECTION_DESTINATION_IP
    payload is behind a NAT and that system SHOULD start sending
    keepalive packets as defined in [UDPENCAPS]; alternately, it MAY
    reject the connection attempt if NAT traversal is not supported.
 o  If none of the NAT_DETECTION_SOURCE_IP payload(s) received matches
    the expected value of the source IP and port found from the IP
    header of the packet containing the payload, it means that the
    system sending those payloads is behind a 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, the system
    receiving the payloads should allow dynamic updates of the other
    system's IP address, as described later.
 o  The IKE initiator MUST check the NAT_DETECTION_SOURCE_IP or
    NAT_DETECTION_DESTINATION_IP 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.
 o  To tunnel IKE packets over UDP port 4500, the IKE header has
    four octets of zeros 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 octets of the ESP header contain the SPI, and the SPI cannot
    validly be zero, it is always possible to distinguish ESP and IKE
    messages.
 o  Implementations MUST process received UDP-encapsulated ESP packets
    even when no NAT was detected.
 o  The original source and destination IP address required for the
    transport mode TCP and UDP packet checksum fixup (see [UDPENCAPS])
    are obtained from the Traffic Selectors associated with the
    exchange.  In the case of transport mode NAT traversal, the
    Traffic Selectors MUST contain exactly one IP address, which is
    then used as the original IP address.  This is covered in greater
    detail in Section 2.23.1.
 o  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).  This will be apparent to a host if
    it receives a packet whose integrity protection validates, but has

Kaufman, et al. Standards Track [Page 65] RFC 7296 IKEv2bis October 2014

    a different port, address, or both from the one that was
    associated with the SA in the validated packet.  When such a
    validated packet is found, a host that does not support other
    methods of recovery such as IKEv2 Mobility and Multihoming
    (MOBIKE) [MOBIKE], and that is not behind a NAT, SHOULD send all
    packets (including retransmission packets) to the IP address and
    port in the validated packet, and SHOULD store this as the new
    address and port combination for the SA (that is, they SHOULD
    dynamically update the address).  A host behind a NAT SHOULD NOT
    do this type of dynamic address update if a validated packet has
    different port and/or address values because it opens a possible
    DoS attack (such as allowing an attacker to break the connection
    with a single packet).  Also, dynamic address update should only
    be done in response to a new packet; otherwise, an attacker can
    revert the addresses with old replayed packets.  Because of this,
    dynamic updates can only be done safely if replay protection is
    enabled.  When IKEv2 is used with MOBIKE, dynamically updating the
    addresses described above interferes with MOBIKE's way of
    recovering from the same situation.  See Section 3.8 of [MOBIKE]
    for more information.

2.23.1. Transport Mode NAT Traversal

 Transport mode used with NAT traversal requires special handling of
 the Traffic Selectors used in the IKEv2.  The complete scenario looks
 like:
 +------+        +------+            +------+         +------+
 |Client| IP1    | NAT  | IPN1  IPN2 | NAT  |     IP2 |Server|
 |node  |<------>|  A   |<---------->|  B   |<------->|      |
 +------+        +------+            +------+         +------+
 (Other scenarios are simplifications of this complex case, so this
 discussion uses the complete scenario.)
 In this scenario, there are two address translating NATs: NAT A and
 NAT B.  NAT A is a dynamic NAT that maps the client's source address
 IP1 to IPN1.  NAT B is a static NAT configured so that connections
 coming to IPN2 address are mapped to the gateway's address IP2, that
 is, IPN2 destination address is mapped to IP2.  This allows the
 client to connect to a server by connecting to the IPN2.  NAT B does
 not necessarily need to be a static NAT, but the client needs to know
 how to connect to the server, and it can only do that if it somehow
 knows the outer address of the NAT B, that is, the IPN2 address.  If
 NAT B is a static NAT, then its address can be configured to the
 client's configuration.  Another option would be to find it using
 some other protocol (like DNS), but that is outside of scope of
 IKEv2.

Kaufman, et al. Standards Track [Page 66] RFC 7296 IKEv2bis October 2014

 In this scenario, both the client and server are configured to use
 transport mode for the traffic originating from the client node and
 destined to the server.
 When the client starts creating the IKEv2 SA and Child SA for sending
 traffic to the server, it may have a triggering packet with source IP
 address of IP1, and a destination IP address of IPN2.  Its Peer
 Authorization Database (PAD) and SPD needs to have a configuration
 matching those addresses (or wildcard entries covering them).
 Because this is transport mode, it uses exactly same addresses as the
 Traffic Selectors and outer IP address of the IKE packets.  For
 transport mode, it MUST use exactly one IP address in the TSi and TSr
 payloads.  It can have multiple Traffic Selectors if it has, for
 example, multiple port ranges that it wants to negotiate, but all TSi
 entries must use the IP1-IP1 range as the IP addresses, and all TSr
 entries must have the IPN2-IPN2 range as IP addresses.  The first
 Traffic Selector of TSi and TSr SHOULD have very specific Traffic
 Selectors including protocol and port numbers, such as from the
 packet triggering the request.
 NAT A will then replace the source address of the IKE packet from IP1
 to IPN1, and NAT B will replace the destination address of the IKE
 packet from IPN2 to IP2, so when the packet arrives to the server it
 will still have the exactly same Traffic Selectors that were sent by
 the client, but the IP address of the IKE packet has been replaced by
 IPN1 and IP2.
 When the server receives this packet, it normally looks in the Peer
 Authorization Database (PAD) described in RFC 4301 [IPSECARCH] based
 on the ID and then searches the SPD based on the Traffic Selectors.
 Because IP1 does not really mean anything to the server (it is the
 address client has behind the NAT), it is useless to do a lookup
 based on that if transport mode is used.  On the other hand, the
 server cannot know whether transport mode is allowed by its policy
 before it finds the matching SPD entry.
 In this case, the server should first check that the initiator
 requested transport mode, and then do address substitution on the
 Traffic Selectors.  It needs to first store the old Traffic Selector
 IP addresses to be used later for the incremental checksum fixup (the
 IP address in the TSi can be stored as the original source address
 and the IP address in the TSr can be stored as the original
 destination address).  After that, if the other end was detected as
 being behind a NAT, the server replaces the IP address in TSi
 payloads with the IP address obtained from the source address of the
 IKE packet received (that is, it replaces IP1 in TSi with IPN1).  If
 the server's end was detected to be behind NAT, it replaces the IP

Kaufman, et al. Standards Track [Page 67] RFC 7296 IKEv2bis October 2014

 address in the TSr payloads with the IP address obtained from the
 destination address of the IKE packet received (that is, it replaces
 IPN2 in TSr with IP2).
 After this address substitution, both the Traffic Selectors and the
 IKE UDP source/destination addresses look the same, and the server
 does SPD lookup based on those new Traffic Selectors.  If an entry is
 found and it allows transport mode, then that entry is used.  If an
 entry is found but it does not allow transport mode, then the server
 MAY undo the address substitution and redo the SPD lookup using the
 original Traffic Selectors.  If the second lookup succeeds, the
 server will create an SA in tunnel mode using real Traffic Selectors
 sent by the other end.
 This address substitution in transport mode is needed because the SPD
 is looked up using the addresses that will be seen by the local host.
 This will also ensure that the Security Association Database (SAD)
 entries for the tunnel exit checks and return packets are added using
 the addresses as seen by the local operating system stack.
 The most common case is that the server's SPD will contain wildcard
 entries matching any addresses, but this also allows making different
 SPD entries, for example, for different known NATs' outer addresses.
 After the SPD lookup, the server will do Traffic Selector narrowing
 based on the SPD entry it found.  It will again use the already
 substituted Traffic Selectors, and it will thus send back Traffic
 Selectors having IPN1 and IP2 as their IP addresses; it can still
 narrow down the protocol number or port ranges used by the Traffic
 Selectors.  The SAD entry created for the Child SA will have the
 addresses as seen by the server, namely IPN1 and IP2.
 When the client receives the server's response to the Child SA, it
 will do similar processing.  If the transport mode SA was created,
 the client can store the original returned Traffic Selectors as
 original source and destination addresses.  It will replace the IP
 addresses in the Traffic Selectors with the ones from the IP header
 of the IKE packet: it will replace IPN1 with IP1 and IP2 with IPN2.
 Then, it will use those Traffic Selectors when verifying the SA
 against sent Traffic Selectors, and when installing the SAD entry.

Kaufman, et al. Standards Track [Page 68] RFC 7296 IKEv2bis October 2014

 A summary of the rules for NAT traversal in transport mode is:
 For the client proposing transport mode:
  1. The TSi entries MUST have exactly one IP address, and that MUST

match the source address of the IKE SA.

  1. The TSr entries MUST have exactly one IP address, and that MUST

match the destination address of the IKE SA.

  1. The first TSi and TSr Traffic Selectors SHOULD have very specific

Traffic Selectors including protocol and port numbers, such as

   from the packet triggering the request.
  1. There MAY be multiple TSi and TSr entries.
  1. If transport mode for the SA was selected (that is, if the server

included USE_TRANSPORT_MODE notification in its response):

  1. Store the original Traffic Selectors as the received source and

destination address.

  1. If the server is behind a NAT, substitute the IP address in the

TSr entries with the remote address of the IKE SA.

  1. If the client is behind a NAT, substitute the IP address in the

TSi entries with the local address of the IKE SA.

  1. Do address substitution before using those Traffic Selectors

for anything other than storing original content of them.

     This includes verification that Traffic Selectors were narrowed
     correctly by the other end, creation of the SAD entry, and so on.
 For the responder, when transport mode is proposed by client:
  1. Store the original Traffic Selector IP addresses as received source

and destination address, in case undo address substitution is

   needed, to use as the "real source and destination address"
   specified by [UDPENCAPS], and for TCP/UDP checksum fixup.
  1. If the client is behind a NAT, substitute the IP address in the

TSi entries with the remote address of the IKE SA.

  1. If the server is behind a NAT, substitute the IP address in the

TSr entries with the local address of the IKE SA.

  1. Do PAD and SPD lookup using the ID and substituted Traffic

Selectors.

Kaufman, et al. Standards Track [Page 69] RFC 7296 IKEv2bis October 2014

  1. If no SPD entry was found, or (if found) the SPD entry does not

allow transport mode, undo the Traffic Selector substitutions.

   Do PAD and SPD lookup again using the ID and original Traffic
   Selectors, but also searching for tunnel mode SPD entry (that
   is, fall back to tunnel mode).
  1. However, if a transport mode SPD entry was found, do normal

traffic selection narrowing based on the substituted Traffic

   Selectors and SPD entry.  Use the resulting Traffic Selectors when
   creating SAD entries, and when sending Traffic Selectors back to
   the client.

2.24. Explicit Congestion Notification (ECN)

 When IPsec tunnels behave as originally specified in [IPSECARCH-OLD],
 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 [ECN]).  IKEv2 simplifies this situation by requiring that ECN
 be usable in the outer IP headers of all tunnel mode Child 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 [ECN] and
 MUST implement the tunnel encapsulation and decapsulation processing
 specified in [IPSECARCH] to prevent discarding of ECN congestion
 indications.

2.25. Exchange Collisions

 Because IKEv2 exchanges can be initiated by either peer, it is
 possible that two exchanges affecting the same SA partly overlap.
 This can lead to a situation where the SA state information is
 temporarily not synchronized, and a peer can receive a request that
 it cannot process in a normal fashion.
 Obviously, using a window size greater than 1 leads to more complex
 situations, especially if requests are processed out of order.  This
 section concentrates on problems that can arise even with a window
 size of 1, and recommends solutions.
 A TEMPORARY_FAILURE notification SHOULD be sent when a peer receives
 a request that cannot be completed due to a temporary condition such
 as a rekeying operation.  When a peer receives a TEMPORARY_FAILURE
 notification, it MUST NOT immediately retry the operation; it MUST
 wait so that the sender may complete whatever operation caused the
 temporary condition.  The recipient MAY retry the request one or more
 times over a period of several minutes.  If a peer continues to

Kaufman, et al. Standards Track [Page 70] RFC 7296 IKEv2bis October 2014

 receive TEMPORARY_FAILURE on the same IKE SA after several minutes,
 it SHOULD conclude that the state information is out of sync and
 close the IKE SA.
 A CHILD_SA_NOT_FOUND notification SHOULD be sent when a peer receives
 a request to rekey a Child SA that does not exist.  The SA that the
 initiator attempted to rekey is indicated by the SPI field in the
 Notify payload, which is copied from the SPI field in the REKEY_SA
 notification.  A peer that receives a CHILD_SA_NOT_FOUND notification
 SHOULD silently delete the Child SA (if it still exists) and send a
 request to create a new Child SA from scratch (if the Child SA does
 not yet exist).

2.25.1. Collisions while Rekeying or Closing Child SAs

 If a peer receives a request to rekey a Child SA that it is currently
 trying to close, it SHOULD reply with TEMPORARY_FAILURE.  If a peer
 receives a request to rekey a Child SA that it is currently rekeying,
 it SHOULD reply as usual, and SHOULD prepare to close redundant SAs
 later based on the nonces (see Section 2.8.1).  If a peer receives a
 request to rekey a Child SA that does not exist, it SHOULD reply with
 CHILD_SA_NOT_FOUND.
 If a peer receives a request to close a Child SA that it is currently
 trying to close, it SHOULD reply without a Delete payload (see
 Section 1.4.1).  If a peer receives a request to close a Child SA
 that it is currently rekeying, it SHOULD reply as usual, with a
 Delete payload.  If a peer receives a request to close a Child SA
 that does not exist, it SHOULD reply without a Delete payload.
 If a peer receives a request to rekey the IKE SA, and it is currently
 creating, rekeying, or closing a Child SA of that IKE SA, it SHOULD
 reply with TEMPORARY_FAILURE.

2.25.2. Collisions while Rekeying or Closing IKE SAs

 If a peer receives a request to rekey an IKE SA that it is currently
 rekeying, it SHOULD reply as usual, and SHOULD prepare to close
 redundant SAs and move inherited Child SAs later based on the nonces
 (see Section 2.8.2).  If a peer receives a request to rekey an IKE SA
 that it is currently trying to close, it SHOULD reply with
 TEMPORARY_FAILURE.
 If a peer receives a request to close an IKE SA that it is currently
 rekeying, it SHOULD reply as usual, and forget about its own rekeying
 request.  If a peer receives a request to close an IKE SA that it is
 currently trying to close, it SHOULD reply as usual, and forget about
 its own close request.

Kaufman, et al. Standards Track [Page 71] RFC 7296 IKEv2bis October 2014

 If a peer receives a request to create or rekey a Child SA when it is
 currently rekeying the IKE SA, it SHOULD reply with
 TEMPORARY_FAILURE.  If a peer receives a request to delete a Child SA
 when it is currently rekeying the IKE SA, it SHOULD reply as usual,
 with a Delete payload.

3. Header and Payload Formats

 In the tables in this section, some cryptographic primitives and
 configuration attributes are marked as "UNSPECIFIED".  These are
 items for which there are no known specifications and therefore
 interoperability is currently impossible.  A future specification may
 describe their use, but until such specification is made,
 implementations SHOULD NOT attempt to use items marked as
 "UNSPECIFIED" in implementations that are meant to be interoperable.

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 zeros.  These four octets of zeros 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 document.  Following the header are
 one or more IKE payloads each identified by a Next Payload field in
 the preceding payload.  Payloads are identified in the order in which
 they appear in an IKE message by looking in 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 responder's 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, including
 multiple sessions per peer.
 All multi-octet fields representing integers are laid out in big
 endian order (also known as "most significant byte first", or
 "network byte order").

Kaufman, et al. Standards Track [Page 72] RFC 7296 IKEv2bis October 2014

 The format of the IKE header is shown in Figure 4.
                      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).
 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 document's version
    (version 2) of IKE MUST reject or ignore messages containing a
    version number greater than 2 with an INVALID_MAJOR_VERSION
    notification message as described in Section 2.5.
 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.

Kaufman, et al. Standards Track [Page 73] RFC 7296 IKEv2bis October 2014

 o  Exchange Type (1 octet) - Indicates the type of exchange being
    used.  This constrains the payloads sent in each message in an
    exchange.  The values in the following table are only current as
    of the publication date of RFC 4306.  Other values may have been
    added since then or will be added after the publication of this
    document.  Readers should refer to [IKEV2IANA] for the latest
    values.
    Exchange Type             Value
    ----------------------------------
    IKE_SA_INIT               34
    IKE_AUTH                  35
    CREATE_CHILD_SA           36
    INFORMATIONAL             37
 o  Flags (1 octet) - Indicates specific options that are set for the
    message.  Presence of options is indicated by the appropriate bit
    in the flags field being set.  The bits are as follows:
      +-+-+-+-+-+-+-+-+
      |X|X|R|V|I|X|X|X|
      +-+-+-+-+-+-+-+-+
    In the description below, a bit being 'set' means its value is
    '1', while 'cleared' means its value is '0'.  'X' bits MUST be
    cleared when sending and MUST be ignored on receipt.
  • R (Response) - 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 (with one exception;
       see Section 2.21.2).
  • V (Version) - 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.
  • I (Initiator) - 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.  This bit changes to reflect who
       initiated the last rekey of the IKE SA.

Kaufman, et al. Standards Track [Page 74] RFC 7296 IKEv2bis October 2014

 o  Message ID (4 octets, unsigned integer) - 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, unsigned integer) - Length of the 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 each one 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);
    conversely, the Next Payload field of the last contained payload
    is set to zero.  The payload type values are listed here.  The
    values in the following table are only current as of the
    publication date of RFC 4306.  Other values may have been added
    since then or will be added after the publication of this
    document.  Readers should refer to [IKEV2IANA] for the latest
    values.

Kaufman, et al. Standards Track [Page 75] RFC 7296 IKEv2bis October 2014

    Next Payload Type                Notation  Value
    --------------------------------------------------
    No Next Payload                             0
    Security Association             SA         33
    Key Exchange                     KE         34
    Identification - Initiator       IDi        35
    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 and Authenticated      SK         46
    Configuration                    CP         47
    Extensible Authentication        EAP        48
    (Payload type values 1-32 should not be assigned in the
    future so that there is no overlap with the code assignments
    for IKEv1.)
 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.  See Section 2.5 for more
    information on this bit.
 o  RESERVED (7 bits) - MUST be sent as zero; MUST be ignored on
    receipt.
 o  Payload Length (2 octets, unsigned integer) - Length in octets of
    the current payload, including the generic payload header.

Kaufman, et al. Standards Track [Page 76] RFC 7296 IKEv2bis October 2014

 Many payloads contain fields marked as "RESERVED".  Some payloads in
 IKEv2 (and historically in IKEv1) are not aligned to 4-octet
 boundaries.

3.3. Security Association Payload

 The Security Association payload, denoted SA in this document, 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 contains a single IPsec protocol (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 ESP with either
 (3DES and HMAC_MD5) or (AES and HMAC_SHA1).
 One of the reasons the semantics of the SA payload have changed from
 ISAKMP and IKEv1 is to make the encodings more compact in common
 cases.
 The Proposal structure contains within it a Proposal Num and an IPsec
 protocol ID.  Each structure MUST have a proposal number one (1)
 greater than the previous structure.  The first Proposal in the
 initiator's SA payload MUST have a Proposal Num of one (1).  One
 reason to use multiple proposals is to propose both standard crypto
 ciphers and combined-mode ciphers.  Combined-mode ciphers include
 both integrity and encryption in a single encryption algorithm, and
 MUST either offer no integrity algorithm or a single integrity
 algorithm of "NONE", with no integrity algorithm being the
 RECOMMENDED method.  If an initiator wants to propose both combined-
 mode ciphers and normal ciphers, it must include two proposals: one
 will have all the combined-mode ciphers, and the other will have all

Kaufman, et al. Standards Track [Page 77] RFC 7296 IKEv2bis October 2014

 the normal ciphers with the integrity algorithms.  For example, one
 such proposal would have two proposal structures.  Proposal 1 is ESP
 with AES-128, AES-192, and AES-256 bits in Cipher Block Chaining
 (CBC) mode, with either HMAC-SHA1-96 or XCBC-96 as the integrity
 algorithm; Proposal 2 is AES-128 or AES-256 in GCM mode with an
 8-octet Integrity Check Value (ICV).  Both proposals allow but do not
 require the use of ESNs (Extended Sequence Numbers).  This can be
 illustrated as:
 SA Payload
    |
    +--- Proposal #1 ( Proto ID = ESP(3), SPI size = 4,
    |     |            7 transforms,      SPI = 0x052357bb )
    |     |
    |     +-- Transform ENCR ( Name = ENCR_AES_CBC )
    |     |     +-- Attribute ( Key Length = 128 )
    |     |
    |     +-- Transform ENCR ( Name = ENCR_AES_CBC )
    |     |     +-- Attribute ( Key Length = 192 )
    |     |
    |     +-- Transform ENCR ( Name = ENCR_AES_CBC )
    |     |     +-- Attribute ( Key Length = 256 )
    |     |
    |     +-- Transform INTEG ( Name = AUTH_HMAC_SHA1_96 )
    |     +-- Transform INTEG ( Name = AUTH_AES_XCBC_96 )
    |     +-- Transform ESN ( Name = ESNs )
    |     +-- Transform ESN ( Name = No ESNs )
    |
    +--- Proposal #2 ( Proto ID = ESP(3), SPI size = 4,
          |            4 transforms,      SPI = 0x35a1d6f2 )
          |
          +-- Transform ENCR ( Name = AES-GCM with a 8 octet ICV )
          |     +-- Attribute ( Key Length = 128 )
          |
          +-- Transform ENCR ( Name = AES-GCM with a 8 octet ICV )
          |     +-- Attribute ( Key Length = 256 )
          |
          +-- Transform ESN ( Name = ESNs )
          +-- Transform ESN ( Name = No ESNs )
 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 two transforms:
 Extended Sequence Numbers (ESNs) and an integrity check algorithm.
 ESP generally has three: ESN, an encryption algorithm, and an
 integrity check algorithm.  IKE generally has four transforms: a
 Diffie-Hellman group, an integrity check algorithm, a PRF algorithm,

Kaufman, et al. Standards Track [Page 78] RFC 7296 IKEv2bis October 2014

 and an encryption algorithm.  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
 AES-CBC) and (HMAC_MD5 or HMAC_SHA), the ESP proposal would contain
 two Transform Type 1 candidates (one for 3DES and one for AEC-CBC)
 and two Transform Type 3 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), an 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
 o  Proposals (variable) - One or more proposal substructures.

Kaufman, et al. Standards Track [Page 79] RFC 7296 IKEv2bis October 2014

 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Last Substruc |   RESERVED    |         Proposal Length       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Proposal Num  |  Protocol ID  |    SPI Size   |Num  Transforms|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ~                        SPI (variable)                         ~
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                        <Transforms>                           ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 7: Proposal Substructure
 o  Last Substruc (1 octet) - Specifies whether or not this is the
    last Proposal Substructure in the SA.  This field has a value of 0
    if this was the last Proposal Substructure, and a value of 2 if
    there are more Proposal Substructures.  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 four octets
    of the Proposal structure are designed to look somewhat like the
    header of a payload.
 o  RESERVED (1 octet) - MUST be sent as zero; MUST be ignored on
    receipt.
 o  Proposal Length (2 octets, unsigned integer) - Length of this
    proposal, including all transforms and attributes that follow.
 o  Proposal Num (1 octet) - When a proposal is made, the first
    proposal in an SA payload MUST be 1, and subsequent proposals MUST
    be one more than the previous proposal (indicating an OR of the
    two proposals).  When a proposal is accepted, the proposal number
    in the SA payload MUST match the number on the proposal sent that
    was accepted.

Kaufman, et al. Standards Track [Page 80] RFC 7296 IKEv2bis October 2014

 o  Protocol ID (1 octet) - Specifies the IPsec protocol identifier
    for the current negotiation.  The values in the following table
    are only current as of the publication date of RFC 4306.  Other
    values may have been added since then or will be added after the
    publication of this document.  Readers should refer to [IKEV2IANA]
    for the latest values.
    Protocol                Protocol ID
    -----------------------------------
    IKE                     1
    AH                      2
    ESP                     3
 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  Num 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Last Substruc |   RESERVED    |        Transform Length       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Transform Type |   RESERVED    |          Transform ID         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 ~                      Transform Attributes                     ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 8: Transform Substructure
 o  Last Substruc (1 octet) - Specifies whether or not this is the
    last Transform Substructure in the Proposal.  This field has a
    value of 0 if this was the last Transform Substructure, and a

Kaufman, et al. Standards Track [Page 81] RFC 7296 IKEv2bis October 2014

    value of 3 if there are more Transform Substructures.  This syntax
    is inherited from ISAKMP, but is unnecessary because the last
    transform could be identified from the length of the proposal.
    The value (3) corresponds to a payload type of Transform in IKEv1,
    and the first four 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.
 The Transform Type values are listed below.  The values in the
 following table are only current as of the publication date of
 RFC 4306.  Other values may have been added since then or will be
 added after the publication of this document.  Readers should refer
 to [IKEV2IANA] for the latest values.
 Description                     Trans.  Used In
                                 Type
 ------------------------------------------------------------------
 Encryption Algorithm (ENCR)     1       IKE and ESP
 Pseudorandom 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
 (*) Negotiating an integrity algorithm is mandatory for the
 Encrypted payload format specified in this document.  For example,
 [AEAD] specifies additional formats based on authenticated
 encryption, in which a separate integrity algorithm is not
 negotiated.

Kaufman, et al. Standards Track [Page 82] RFC 7296 IKEv2bis October 2014

 For Transform Type 1 (Encryption Algorithm), the Transform IDs are
 listed below.  The values in the following table are only current as
 of the publication date of RFC 4306.  Other values may have been
 added since then or will be added after the publication of this
 document.  Readers should refer to [IKEV2IANA] for the latest values.
 Name                 Number      Defined In
 ---------------------------------------------------
 ENCR_DES_IV64        1           (UNSPECIFIED)
 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           (UNSPECIFIED)
 ENCR_DES_IV32        9           (UNSPECIFIED)
 ENCR_NULL            11          [RFC2410]
 ENCR_AES_CBC         12          [RFC3602]
 ENCR_AES_CTR         13          [RFC3686]
 For Transform Type 2 (Pseudorandom Function), the Transform IDs are
 listed below.  The values in the following table are only current as
 of the publication date of RFC 4306.  Other values may have been
 added since then or will be added after the publication of this
 document.  Readers should refer to [IKEV2IANA] for the latest values.
 Name                        Number    Defined In
 ------------------------------------------------------------------
 PRF_HMAC_MD5                1         [RFC2104], [MD5]
 PRF_HMAC_SHA1               2         [RFC2104], [FIPS.180-4.2012]
 PRF_HMAC_TIGER              3         (UNSPECIFIED)
 For Transform Type 3 (Integrity Algorithm), defined Transform IDs are
 listed below.  The values in the following table are only current as
 of the publication date of RFC 4306.  Other values may have been
 added since then or will be added after the publication of this
 document.  Readers should refer to [IKEV2IANA] for the latest values.
 Name                 Number   Defined In
 ----------------------------------------
 NONE                 0
 AUTH_HMAC_MD5_96     1        [RFC2403]
 AUTH_HMAC_SHA1_96    2        [RFC2404]
 AUTH_DES_MAC         3        (UNSPECIFIED)
 AUTH_KPDK_MD5        4        (UNSPECIFIED)
 AUTH_AES_XCBC_96     5        [RFC3566]

Kaufman, et al. Standards Track [Page 83] RFC 7296 IKEv2bis October 2014

 For Transform Type 4 (Diffie-Hellman group), defined Transform IDs
 are listed below.  The values in the following table are only current
 as of the publication date of RFC 4306.  Other values may have been
 added since then or will be added after the publication of this
 document.  Readers should refer to [IKEV2IANA] for the latest values.
 Name                Number      Defined In
 ------------------------------------------
 NONE                    0
 768-bit MODP Group      1       Appendix B
 1024-bit MODP Group     2       Appendix B
 1536-bit MODP Group     5       [ADDGROUP]
 2048-bit MODP Group     14      [ADDGROUP]
 3072-bit MODP Group     15      [ADDGROUP]
 4096-bit MODP Group     16      [ADDGROUP]
 6144-bit MODP Group     17      [ADDGROUP]
 8192-bit MODP Group     18      [ADDGROUP]
 Although ESP and AH do not directly include a Diffie-Hellman
 exchange, a Diffie-Hellman group MAY be negotiated for the Child SA.
 This allows the peers to employ Diffie-Hellman in the CREATE_CHILD_SA
 exchange, providing perfect forward secrecy for the generated Child
 SA keys.
 Note that the MODP Diffie-Hellman groups listed above do not need any
 special validity tests to be performed, but other types of groups
 (elliptic curve groups, and MODP groups with small subgroups) need to
 have some additional tests performed on them to use them securely.
 See "Additional Diffie-Hellman Tests for IKEv2" ([RFC6989]) for more
 information.
 For Transform Type 5 (Extended Sequence Numbers), defined Transform
 IDs are listed below.  The values in the following table are only
 current as of the publication date of RFC 4306.  Other values may
 have been added since then or will be added after the publication of
 this document.  Readers should refer to [IKEV2IANA] for the latest
 values.
 Name                               Number
 --------------------------------------------
 No Extended Sequence Numbers       0
 Extended Sequence Numbers          1
 Note that an initiator who supports ESNs will usually include two ESN
 transforms, with values "0" and "1", in its proposals.  A proposal
 containing a single ESN transform with value "1" means that using
 normal (non-extended) sequence numbers is not acceptable.

Kaufman, et al. Standards Track [Page 84] RFC 7296 IKEv2bis October 2014

 Numerous additional Transform Types have been defined since the
 publication of RFC 4306.  Please refer to the IANA "Internet Key
 Exchange Version 2 (IKEv2) Parameters" registry for details.

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
 (*) Negotiating an integrity algorithm is mandatory for the
 Encrypted payload format specified in this document.  For example,
 [AEAD] specifies additional formats based on authenticated
 encryption, in which a separate integrity algorithm is not
 negotiated.

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.  At the
 time of publication of this document, [RFC4307] specifies these
 suites, but note that it might be updated in the future, and other
 RFCs might specify different sets of suites.
 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.
 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 parameters (the generator, modulus, and exponent lengths and
 values) for new Diffie-Hellman groups.  Implementations SHOULD

Kaufman, et al. Standards Track [Page 85] RFC 7296 IKEv2bis October 2014

 provide a management interface through 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.  The set of valid attributes depends on the transform.
 Currently, only a single attribute type is defined: the Key Length
 attribute is used by certain encryption transforms with variable-
 length keys (see below for details).
 The attributes are type/value pairs and are defined below.
 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 Format (AF) (1 bit) - Indicates whether the data
    attribute follows the Type/Length/Value (TLV) format or a
    shortened Type/Value (TV) format.  If the AF bit is zero (0), then
    the attribute uses TLV format; if the AF bit is one (1), the TV
    format (with two-byte value) is used.

Kaufman, et al. Standards Track [Page 86] RFC 7296 IKEv2bis October 2014

 o  Attribute Type (15 bits) - Unique identifier for each type of
    attribute (see below).
 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.
 The only currently defined attribute type (Key Length) is fixed
 length; the variable-length encoding specification is included only
 for future extensions.  Attributes described as fixed length MUST NOT
 be encoded using the variable-length encoding unless that length
 exceeds two bytes.  Variable-length attributes MUST NOT be encoded as
 fixed-length 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.
 The values in the following table are only current as of the
 publication date of RFC 4306.  Other values may have been added since
 then or will be added after the publication of this document.
 Readers should refer to [IKEV2IANA] for the latest values.
 Attribute Type         Value         Attribute Format
 ------------------------------------------------------------
 Key Length (in bits)   14            TV
 Values 0-13 and 15-17 were used in a similar context in IKEv1, and
 should not be assigned except to matching values.
 The Key Length attribute specifies the key length in bits (MUST use
 network byte order) for certain transforms as follows:
 o  The Key Length attribute MUST NOT be used with transforms that use
    a fixed-length key.  For example, this includes ENCR_DES,
    ENCR_IDEA, and all the Type 2 (Pseudorandom Function) and Type 3
    (Integrity Algorithm) transforms specified in this document.  It
    is recommended that future Type 2 or 3 transforms do not use this
    attribute.
 o  Some transforms specify that the Key Length attribute MUST be
    always included (omitting the attribute is not allowed, and
    proposals not containing it MUST be rejected).  For example, this
    includes ENCR_AES_CBC and ENCR_AES_CTR.

Kaufman, et al. Standards Track [Page 87] RFC 7296 IKEv2bis October 2014

 o  Some transforms allow variable-length keys, but also specify a
    default key length if the attribute is not included.  For example,
    these transforms include ENCR_RC5 and ENCR_BLOWFISH.
 Implementation note: 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 for this capability allows a responder to express a concept
 of "at least" a certain level of security -- "a key length of _at
 least_ X bits for cipher Y".  However, as the attribute is always
 returned unchanged (see the next section), an initiator willing to
 accept multiple key lengths has to include multiple transforms with
 the same Transform Type, each with a different Key Length attribute.

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.  If the selected proposal has 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
 MUST terminate the exchange.
 If the responder receives a proposal that contains a Transform Type
 it does not understand, or a proposal that is missing a mandatory
 Transform Type, it MUST consider this proposal unacceptable; however,
 other proposals in the same SA payload are processed as usual.
 Similarly, if the responder receives a transform that it does not
 understand, or one that contains a Transform Attribute it does not
 understand, it MUST consider this transform unacceptable; other
 transforms with the same Transform Type are processed as usual.  This
 allows new Transform Types and Transform Attributes to be defined in
 the future.
 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

Kaufman, et al. Standards Track [Page 88] RFC 7296 IKEv2bis October 2014

 include a KE corresponding to that group.  If the responder selects a
 proposal using a different Diffie-Hellman group (other than NONE),
 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.  If one of the proposals offered
 is for the Diffie-Hellman group of NONE, and the responder selects
 that Diffie-Hellman group, then it MUST ignore the initiator's KE
 payload and omit the KE payload from the response.

3.4. Key Exchange Payload

 The Key Exchange payload, denoted KE in this document, 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        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Diffie-Hellman Group Num    |           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 for MODP groups 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 Diffie-Hellman Group Num identifies the Diffie-Hellman group in
 which the Key Exchange Data was computed (see Section 3.3.2).  This
 Diffie-Hellman Group Num MUST match a Diffie-Hellman group specified
 in a proposal in the SA payload that is sent in the same message, and
 SHOULD match the Diffie-Hellman group in the first group in the first
 proposal, if such exists.  If none of the proposals in that SA
 payload specifies a Diffie-Hellman group, the KE payload MUST NOT be

Kaufman, et al. Standards Track [Page 89] RFC 7296 IKEv2bis October 2014

 present.  If the selected proposal uses a different Diffie-Hellman
 group (other than NONE), the message MUST be rejected with a Notify
 payload of type INVALID_KE_PAYLOAD.  See also Sections 1.2 and 2.7.
 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 document,
 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.  When using the
 ID_IPV4_ADDR/ID_IPV6_ADDR identity types in IDi/IDr payloads, IKEv2
 does not require this address to match the address in the IP header
 of IKEv2 packets, or anything in the TSi/TSr payloads.  The contents
 of IDi/IDr are used purely to fetch the policy and authentication
 data related to the other party.
 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).
 The Peer Authorization Database (PAD) as described in RFC 4301
 [IPSECARCH] describes the use of the ID payload in IKEv2 and provides
 a formal model for the binding of identity to policy in addition to
 providing services that deal more specifically with the details of
 policy enforcement.  The PAD is intended to provide a link between
 the SPD and the IKE Security Association management.  See
 Section 4.4.3 of RFC 4301 for more details.
 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

Kaufman, et al. Standards Track [Page 90] RFC 7296 IKEv2bis October 2014

 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 semantics for the
 Identification Type field.  The values in the following table are
 only current as of the publication date of RFC 4306.  Other values
 may have been added since then or will be added after the publication
 of this document.  Readers should refer to [IKEV2IANA] for the latest
 values.
 ID Type                           Value
 -------------------------------------------------------------------
 ID_IPV4_ADDR                        1
    A single four (4) octet IPv4 address.
 ID_FQDN                             2
    A fully-qualified domain name string.  An example of an ID_FQDN
    is "example.com".  The string MUST NOT contain any terminators
    (e.g., NULL, CR, etc.).  All characters in the ID_FQDN are ASCII;
    for an "internationalized domain name", the syntax is as defined
    in [IDNA], for example "xn--tmonesimerkki-bfbb.example.net".
 ID_RFC822_ADDR                      3
    A fully-qualified RFC 822 email address string.  An example of a
    ID_RFC822_ADDR is "jsmith@example.com".  The string MUST NOT
    contain any terminators.  Because of [EAI], implementations would
    be wise to treat this field as UTF-8 encoded text, not as
    pure ASCII.
 ID_IPV6_ADDR                        5
    A single sixteen (16) octet IPv6 address.
 ID_DER_ASN1_DN                      9
    The binary Distinguished Encoding Rules (DER) encoding of an
    ASN.1 X.500 Distinguished Name [PKIX].
 ID_DER_ASN1_GN                      10
    The binary DER encoding of an ASN.1 X.509 GeneralName [PKIX].

Kaufman, et al. Standards Track [Page 91] RFC 7296 IKEv2bis October 2014

 ID_KEY_ID                           11
    An opaque octet stream that may be used to pass vendor-
    specific information necessary to do certain proprietary
    types of identification.
 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 four 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 instead of ID_IPV4_ADDR for
 IP addresses.
 EAP [EAP] does not mandate the use of any particular type of
 identifier, but often EAP is used with Network Access Identifiers
 (NAIs) defined in [NAI].  Although NAIs look a bit like email
 addresses (e.g., "joe@example.com"), the syntax is not exactly the
 same as the syntax of email address in [MAILFORMAT].  For those NAIs
 that include the realm component, the ID_RFC822_ADDR identification
 type SHOULD be used.  Responder implementations should not attempt to
 verify that the contents actually conform to the exact syntax given
 in [MAILFORMAT], but instead should accept any reasonable-looking
 NAI.  For NAIs that do not include the realm component, the ID_KEY_ID
 identification type SHOULD be used.
 See "The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and
 PKIX" ([RFC4945]) for more information about matching Identification
 payloads and the contents of the PKIX Certificates.

3.6. Certificate Payload

 The Certificate payload, denoted CERT in this document, 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.  The Hash and
 URL formats of the Certificate payloads should be used in case 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.

Kaufman, et al. Standards Track [Page 92] RFC 7296 IKEv2bis October 2014

 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.  The values in the following table are
    only current as of the publication date of RFC 4306.  Other values
    may have been added since then or will be added after the
    publication of this document.  Readers should refer to [IKEV2IANA]
    for the latest values.
    Certificate Encoding                 Value
    ----------------------------------------------------
    PKCS #7 wrapped X.509 certificate    1   UNSPECIFIED
    PGP Certificate                      2   UNSPECIFIED
    DNS Signed Key                       3   UNSPECIFIED
    X.509 Certificate - Signature        4
    Kerberos Token                       6   UNSPECIFIED
    Certificate Revocation List (CRL)    7
    Authority Revocation List (ARL)      8   UNSPECIFIED
    SPKI Certificate                     9   UNSPECIFIED
    X.509 Certificate - Attribute        10  UNSPECIFIED
    Deprecated (was Raw RSA Key)         11  DEPRECATED
    Hash and URL of X.509 certificate    12
    Hash and URL of X.509 bundle         13
 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).

Kaufman, et al. Standards Track [Page 93] RFC 7296 IKEv2bis October 2014

 Specific syntax for some of the certificate type codes above is not
 defined in this document.  The types whose syntax is defined in this
 document are:
 o  "X.509 Certificate - Signature" contains a DER-encoded X.509
    certificate whose public key is used to validate the sender's AUTH
    payload.  Note that with this encoding, if a chain of certificates
    needs to be sent, multiple CERT payloads are used, only the first
    of which holds the public key used to validate the sender's AUTH
    payload.
 o  "Certificate Revocation List" contains a DER-encoded X.509
    certificate revocation list.
 o  Hash and URL encodings allow IKE messages to remain short by
    replacing long data structures with a 20-octet SHA-1 hash (see
    [FIPS.180-4.2012]) 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 DoS attacks that become
    easier to mount when IKE messages are large enough to require IP
    fragmentation [DOSUDPPROT].
 The "Hash and URL of a bundle" type uses the following ASN.1
 definition for the 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) } ;
 CertificateOrCRL ::= CHOICE {
   cert [0] Certificate,
   crl  [1] CertificateList }
 CertificateBundle ::= SEQUENCE OF CertificateOrCRL
 END

Kaufman, et al. Standards Track [Page 94] RFC 7296 IKEv2bis October 2014

 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
 two Hash and URL formats (with HTTP URLs).  If multiple certificates
 are sent, the first certificate MUST contain the public key
 associated with the private key used to sign the AUTH payload.  The
 other certificates may be sent in any order.
 Implementations MUST support the "http:" scheme for hash-and-URL
 lookup.  The behavior of other URL schemes [URLS] is not currently
 specified, and such schemes SHOULD NOT be used in the absence of a
 document specifying them.

3.7. Certificate Request Payload

 The Certificate Request payload, denoted CERTREQ in this document,
 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.
 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.
 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).

Kaufman, et al. Standards Track [Page 95] RFC 7296 IKEv2bis October 2014

 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 [PKIX]) from each Trust Anchor certificate.
 The 20-octet hashes are concatenated and included with no other
 formatting.
 The contents of the Certification Authority field are defined only
 for X.509 certificates, which are types 4, 12, and 13.  Other values
 SHOULD NOT be used until Standards-Track specifications that specify
 their use are published.
 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:
 o  is configured to use certificate authentication,
 o  is allowed to send a CERT payload,
 o  has matching CA trust policy governing the current negotiation,
    and
 o  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, et al. Standards Track [Page 96] RFC 7296 IKEv2bis October 2014

 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).
 The HTTP_CERT_LOOKUP_SUPPORTED 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).

3.8. Authentication Payload

 The Authentication payload, denoted AUTH in this document, 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.  The types of signatures are listed here.  The values in the
    following table are only current as of the publication date of
    RFC 4306.  Other values may have been added since then or will be
    added after the publication of this document.  Readers should
    refer to [IKEV2IANA] for the latest values.

Kaufman, et al. Standards Track [Page 97] RFC 7296 IKEv2bis October 2014

 Mechanism                              Value
 -----------------------------------------------------------------
 RSA Digital Signature                  1
    Computed as specified in Section 2.15 using an RSA private key
    with RSASSA-PKCS1-v1_5 signature scheme specified in [PKCS1]
    (implementers should note that IKEv1 used a different method for
    RSA signatures).  To promote interoperability, implementations
    that support this type SHOULD support signatures that use SHA-1
    as the hash function and SHOULD use SHA-1 as the default hash
    function when generating signatures.  Implementations can use the
    certificates received from a given peer as a hint for selecting a
    mutually understood hash function for the AUTH payload signature.
    Note, however, that the hash algorithm used in the AUTH payload
    signature doesn't have to be the same as any hash algorithm(s)
    used in the certificate(s).
 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.
 DSS Digital Signature                  3
    Computed as specified in Section 2.15 using a DSS private key
    (see [DSS]) over a SHA-1 hash.
 o  RESERVED - MUST be sent as zero; MUST be ignored on receipt.
 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 as Ni and Nr in this document for the
 initiator's and responder's nonce, respectively, contains random data
 used to guarantee liveness during an exchange and protect against
 replay attacks.

Kaufman, et al. Standards Track [Page 98] RFC 7296 IKEv2bis October 2014

 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 the Nonce Data 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, et al. Standards Track [Page 99] RFC 7296 IKEv2bis October 2014

 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 whose SPI is given in the SPI field, this field indicates the
    type of that SA.  For notifications concerning Child SAs, this
    field MUST contain either (2) to indicate AH or (3) to indicate
    ESP.  Of the notifications defined in this document, the SPI is
    included only with INVALID_SELECTORS, REKEY_SA, and
    CHILD_SA_NOT_FOUND.  If the SPI field is empty, this field MUST be
    sent as zero and MUST be ignored on receipt.
 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 and
    the field must be empty.
 o  Notify Message Type (2 octets) - Specifies the type of
    notification message.
 o  SPI (variable length) - Security Parameter Index.
 o  Notification Data (variable length) - Status 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, et al. Standards Track [Page 100] RFC 7296 IKEv2bis October 2014

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, and 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.
 More information on error handling can be found in Section 2.21.
 The values in the following table are only current as of the
 publication date of RFC 4306, plus two error types added in this
 document.  Other values may have been added since then or will be
 added after the publication of this document.  Readers should refer
 to [IKEV2IANA] for the latest values.
 NOTIFY messages: error types              Value
 -------------------------------------------------------------------
 UNSUPPORTED_CRITICAL_PAYLOAD              1
     See Section 2.5.
 INVALID_IKE_SPI                           4
     See Section 2.21.
 INVALID_MAJOR_VERSION                     5
     See Section 2.5.
 INVALID_SYNTAX                            7
     Indicates the IKE message that was received was invalid because
     some type, length, or value was out of range or because the
     request was rejected for policy reasons.  To avoid a DoS
     attack using forged messages, this status may only be
     returned for and in an encrypted packet if the Message ID and

Kaufman, et al. Standards Track [Page 101] RFC 7296 IKEv2bis October 2014

     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
     See Section 2.3.
 INVALID_SPI                              11
     See Section 1.5.
 NO_PROPOSAL_CHOSEN                       14
     None of the proposed crypto suites was acceptable.  This can be
     sent in any case where the offered proposals (including but not
     limited to SA payload values, USE_TRANSPORT_MODE notify,
     IPCOMP_SUPPORTED notify) are not acceptable for the responder.
     This can also be used as "generic" Child SA error when Child SA
     cannot be created for some other reason.  See also Section 2.7.
 INVALID_KE_PAYLOAD                       17
     See Sections 1.2 and 1.3.
 AUTHENTICATION_FAILED                    24
     Sent in the response to an IKE_AUTH message when, for some
     reason, the authentication failed.  There is no associated
     data.  See also Section 2.21.2.
 SINGLE_PAIR_REQUIRED                     34
     See Section 2.9.
 NO_ADDITIONAL_SAS                        35
     See Section 1.3.
 INTERNAL_ADDRESS_FAILURE                 36
     See Section 3.15.4.
 FAILED_CP_REQUIRED                       37
     See Section 2.19.
 TS_UNACCEPTABLE                          38
     See Section 2.9.

Kaufman, et al. Standards Track [Page 102] RFC 7296 IKEv2bis October 2014

 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 Child SA.
 TEMPORARY_FAILURE                        43
     See Section 2.25.
 CHILD_SA_NOT_FOUND                       44
     See Section 2.25.
 NOTIFY messages: status types            Value
 -------------------------------------------------------------------
 INITIAL_CONTACT                          16384
     See Section 2.4.
 SET_WINDOW_SIZE                          16385
     See Section 2.3.
 ADDITIONAL_TS_POSSIBLE                   16386
     See Section 2.9.
 IPCOMP_SUPPORTED                         16387
     See Section 2.22.
 NAT_DETECTION_SOURCE_IP                  16388
     See Section 2.23.
 NAT_DETECTION_DESTINATION_IP             16389
     See Section 2.23.
 COOKIE                                   16390
     See Section 2.6.
 USE_TRANSPORT_MODE                       16391
     See Section 1.3.1.
 HTTP_CERT_LOOKUP_SUPPORTED               16392
     See Section 3.6.
 REKEY_SA                                 16393
     See Section 1.3.3.

Kaufman, et al. Standards Track [Page 103] RFC 7296 IKEv2bis October 2014

 ESP_TFC_PADDING_NOT_SUPPORTED            16394
     See Section 1.3.1.
 NON_FIRST_FRAGMENTS_ALSO                 16395
     See Section 1.3.1.

3.11. Delete Payload

 The Delete payload, denoted D in this document, 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 the 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    |          Num 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.
 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.

Kaufman, et al. Standards Track [Page 104] RFC 7296 IKEv2bis October 2014

 o  Num of SPIs (2 octets, unsigned integer) - 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 Num 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 document, 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 of
 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 document, such as
 private payloads, private exchanges, private notifications, etc.
 Unfamiliar Vendor IDs MUST be ignored.
 Writers of documents who wish to extend this protocol MUST define a
 Vendor ID payload to announce the ability to implement the extension
 in the document.  It is expected that documents 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, et al. Standards Track [Page 105] RFC 7296 IKEv2bis October 2014

 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 information.
    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 document, 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, et al. Standards Track [Page 106] RFC 7296 IKEv2bis October 2014

 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.
 There is no requirement that TSi and TSr contain the same number of
 individual Traffic Selectors.  Thus, they are interpreted as follows:
 a packet matches a given TSi/TSr if it matches at least one of the
 individual selectors in TSi, and at least one of the individual
 selectors in TSr.
 For instance, the following Traffic Selectors:
 TSi = ((17, 100, 198.51.100.66-198.51.100.66),
        (17, 200, 198.51.100.66-198.51.100.66))
 TSr = ((17, 300, 0.0.0.0-255.255.255.255),
        (17, 400, 0.0.0.0-255.255.255.255))
 would match UDP packets from 198.51.100.66 to anywhere, with any of
 the four combinations of source/destination ports (100,300),
 (100,400), (200,300), and (200, 400).
 Thus, some types of policies may require several Child SA pairs.  For
 instance, a policy matching only source/destination ports (100,300)
 and (200,400), but not the other two combinations, cannot be
 negotiated as a single Child SA pair.

Kaufman, et al. Standards Track [Page 107] RFC 7296 IKEv2bis October 2014

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 (such as UDP, TCP, and 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 (2 octets, unsigned integer) - Specifies the
    length of this Traffic Selector substructure including the header.
 o  Start Port (2 octets, unsigned integer) - Value specifying the
    smallest port number allowed by this Traffic Selector.  For
    protocols for which port is undefined (including protocol 0), or
    if all ports are allowed, this field MUST be zero.  ICMP and
    ICMPv6 Type and Code values, as well as Mobile IP version 6
    (MIPv6) mobility header (MH) Type values, are represented in this
    field as specified in Section 4.4.1.1 of [IPSECARCH].  ICMP Type
    and Code values are treated as a single 16-bit integer port
    number, with Type in the most significant eight bits and Code in
    the least significant eight bits.  MIPv6 MH Type values are
    treated as a single 16-bit integer port number, with Type in the
    most significant eight bits and the least significant eight bits
    set to zero.

Kaufman, et al. Standards Track [Page 108] RFC 7296 IKEv2bis October 2014

 o  End Port (2 octets, unsigned integer) - Value specifying the
    largest port number allowed by this Traffic Selector.  For
    protocols for which port is undefined (including protocol 0), or
    if all ports are allowed, this field MUST be 65535.  ICMP and
    ICMPv6 Type and Code values, as well as MIPv6 MH Type values, are
    represented in this field as specified in Section 4.4.1.1 of
    [IPSECARCH].  ICMP Type and Code values are treated as a single
    16-bit integer port number, with Type in the most significant
    eight bits and Code in the least significant eight bits.  MIPv6 MH
    Type values are treated as a single 16-bit integer port number,
    with Type in the most significant eight bits and the least
    significant eight bits set to zero.
 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 [IPSECARCH] that wish to indicate
 "ANY" ports MUST set the start port to 0 and the end port to 65535;
 note that according to [IPSECARCH], "ANY" includes "OPAQUE".  Systems
 working with [IPSECARCH] that wish to indicate "OPAQUE" ports, but
 not "ANY" ports, MUST set the start port to 65535 and the end port
 to 0.
 The Traffic Selector types 7 and 8 can also refer to ICMP or ICMPv6
 type and code fields, as well as MH Type fields for the IPv6 mobility
 header [MIPV6].  Note, however, that neither ICMP nor MIPv6 packets
 have separate source and destination fields.  The method for
 specifying the Traffic Selectors for ICMP and MIPv6 is shown by
 example in Section 4.4.1.3 of [IPSECARCH].

Kaufman, et al. Standards Track [Page 109] RFC 7296 IKEv2bis October 2014

 The following table lists values for the Traffic Selector Type field
 and the corresponding Address Selector Data.  The values in the
 following table are only current as of the publication date of
 RFC 4306.  Other values may have been added since then or will be
 added after the publication of this document.  Readers should refer
 to [IKEV2IANA] for the latest values.
 TS Type                            Value
 -------------------------------------------------------------------
 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.
 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.

3.14. Encrypted Payload

 The Encrypted payload, denoted SK {...} in this document, 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.  This payload is also called the
 "Encrypted and Authenticated" payload.
 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.
 This document specifies the cryptographic processing of Encrypted
 payloads using a block cipher in CBC mode and an integrity check
 algorithm that computes a fixed-length checksum over a variable size
 message.  The design is modeled after the ESP algorithms described in
 RFCs 2104 [HMAC], 4303 [ESP], and 2451 [ESPCBC].  This document
 completely specifies the cryptographic processing of IKE data, but
 those documents should be consulted for design rationale.  Future
 documents may specify the processing of Encrypted payloads for other
 types of transforms, such as counter mode encryption and
 authenticated encryption algorithms.  Peers MUST NOT negotiate
 transforms for which no such specification exists.

Kaufman, et al. Standards Track [Page 110] RFC 7296 IKEv2bis October 2014

 When an authenticated encryption algorithm is used to protect the IKE
 SA, the construction of the Encrypted payload is different than what
 is described here.  See [AEAD] for more information on authenticated
 encryption algorithms and their use in IKEv2.
 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:
                      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,
    initialization vector (IV), Encrypted IKE payloads, Padding, Pad
    Length, and Integrity Checksum Data.
 o  Initialization Vector - For CBC mode ciphers, the length of the
    initialization vector (IV) is equal to the block length of the
    underlying encryption algorithm.  Senders MUST select a new
    unpredictable IV for every message; recipients MUST accept any
    value.  The reader is encouraged to consult [MODES] for advice on
    IV generation.  In particular, using the final ciphertext block of

Kaufman, et al. Standards Track [Page 111] RFC 7296 IKEv2bis October 2014

    the previous message is not considered unpredictable.  For modes
    other than CBC, the IV format and processing is specified in the
    document specifying the encryption algorithm and mode.
 o  IKE payloads are as specified earlier in this section.  This field
    is encrypted with the negotiated cipher.
 o  Padding MAY contain any value chosen by the sender, and MUST have
    a length that makes the combination of the payloads, the Padding,
    and the Pad Length to be a multiple of the encryption block size.
    This field is encrypted with the negotiated cipher.
 o  Pad Length is the length of the Padding field.  The sender SHOULD
    set the Pad Length to the minimum value that makes the combination
    of the payloads, the Padding, and the Pad Length a multiple of the
    block size, but the recipient MUST accept any length that results
    in proper alignment.  This field is encrypted with the negotiated
    cipher.
 o  Integrity Checksum Data is the cryptographic checksum of the
    entire message starting with the Fixed IKE header 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.
 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

Kaufman, et al. Standards Track [Page 112] RFC 7296 IKEv2bis October 2014

 The payload type for the Configuration payload is forty-seven (47).
 o  CFG Type (1 octet) - The type of exchange represented by the
    Configuration Attributes.  The values in the following table are
    only current as of the publication date of RFC 4306.  Other values
    may have been added since then or will be added after the
    publication of this document.  Readers should refer to [IKEV2IANA]
    for the latest values.
    CFG Type           Value
    --------------------------
    CFG_REQUEST        1
    CFG_REPLY          2
    CFG_SET            3
    CFG_ACK            4
 o  RESERVED (3 octets) - MUST be sent as zero; MUST be ignored on
    receipt.
 o  Configuration Attributes (variable length) - These are type length
    value (TLV) structures 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, unsigned integer) - Length in octets of value.
 o  Value (0 or more octets) - The variable-length value of this
    Configuration Attribute.  The following lists the attribute types.

Kaufman, et al. Standards Track [Page 113] RFC 7296 IKEv2bis October 2014

 The values in the following table are only current as of the
 publication date of RFC 4306 (except INTERNAL_ADDRESS_EXPIRY and
 INTERNAL_IP6_NBNS, which were removed by RFC 5996).  Other values may
 have been added since then or will be added after the publication of
 this document.  Readers should refer to [IKEV2IANA] for the latest
 values.
    Attribute Type           Value  Multi-Valued  Length
    ------------------------------------------------------------
    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_IP4_DHCP        6      YES           0 or 4 octets
    APPLICATION_VERSION      7      NO            0 or more
    INTERNAL_IP6_ADDRESS     8      YES*          0 or 17 octets
    INTERNAL_IP6_DNS         10     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.

 o  INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
    internal network, sometimes called a red node address or private
    address, and it MAY be a private address on the Internet.  In a
    request message, the address specified is a requested address (or
    a zero-length address 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 octets 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 16-octet IPv6 address, and the second is a
    one-octet prefix-length as defined in [ADDRIPV6].  The requested
    address is valid as long as this IKE SA (or its rekeyed
    successors) requesting the address is valid.  This is described in
    more detail in Section 3.15.3.
 o  INTERNAL_IP4_NETMASK - The internal network's netmask.  Only one
    netmask is allowed in the request and response messages (e.g.,
    255.255.255.0), and it MUST be used only with an
    INTERNAL_IP4_ADDRESS attribute.  INTERNAL_IP4_NETMASK in a
    CFG_REPLY means roughly the same thing as INTERNAL_IP4_SUBNET

Kaufman, et al. Standards Track [Page 114] RFC 7296 IKEv2bis October 2014

    containing the same information ("send traffic to these addresses
    through me"), but also implies a link boundary.  For instance, the
    client could use its own address and the netmask to calculate the
    broadcast address of the link.  An empty INTERNAL_IP4_NETMASK
    attribute can be included in a CFG_REQUEST to request this
    information (although the gateway can send the information even
    when not requested).  Non-empty values for this attribute in a
    CFG_REQUEST do not make sense and thus MUST NOT be included.
 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 - 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_IP4_DHCP, INTERNAL_IP6_DHCP - Instructs the host to send
    any internal DHCP requests to the address contained within the
    attribute.  Multiple DHCP servers MAY be requested.  The responder
    MAY respond with zero or more DHCP server attributes.
 o  APPLICATION_VERSION - The version or application information of
    the IPsec host.  This is a string of printable ASCII characters
    that is NOT null terminated.
 o  INTERNAL_IP4_SUBNET - The protected sub-networks that this edge-
    device protects.  This attribute is made up of two fields: the
    first being an IP address and the second being a netmask.
    Multiple sub-networks MAY be requested.  The responder MAY respond
    with zero or more sub-network attributes.  This is discussed in
    more detail in Section 3.15.2.
 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.
 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 16-octet IPv6 address, and the second is a
    one-octet prefix-length as defined in [ADDRIPV6].  Multiple

Kaufman, et al. Standards Track [Page 115] RFC 7296 IKEv2bis October 2014

    sub-networks MAY be requested.  The responder MAY respond with
    zero or more sub-network attributes.  This is discussed in more
    detail in Section 3.15.2.
 Note that no recommendations are made in this document as to how an
 implementation actually figures out what information to send in a
 response.  That is, we do not recommend any specific method of an
 IRAS determining which DNS server should be returned to a requesting
 IRAC.
 The CFG_REQUEST and CFG_REPLY pair 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.  Unrecognized or unsupported attributes
 MUST be ignored in both requests and responses.
 The CFG_SET and CFG_ACK pair 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 the Configuration
 payload 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
 implementation of this specification MAY ignore CFG_SET payloads.

3.15.2. Meaning of INTERNAL_IP4_SUBNET and INTERNAL_IP6_SUBNET

 INTERNAL_IP4/6_SUBNET attributes can indicate additional subnets,
 ones that need one or more separate SAs, that can be reached through
 the gateway that announces the attributes.  INTERNAL_IP4/6_SUBNET
 attributes may also express the gateway's policy about what traffic
 should be sent through the gateway; the client can choose whether
 other traffic (covered by TSr, but not in INTERNAL_IP4/6_SUBNET) is
 sent through the gateway or directly to the destination.  Thus,
 traffic to the addresses listed in the INTERNAL_IP4/6_SUBNET
 attributes should be sent through the gateway that announces the
 attributes.  If there are no existing Child SAs whose Traffic
 Selectors cover the address in question, new SAs need to be created.

Kaufman, et al. Standards Track [Page 116] RFC 7296 IKEv2bis October 2014

 For instance, if there are two subnets, 198.51.100.0/26 and
 192.0.2.0/24, and the client's request contains the following:
 CP(CFG_REQUEST) =
   INTERNAL_IP4_ADDRESS()
 TSi = (0, 0-65535, 0.0.0.0-255.255.255.255)
 TSr = (0, 0-65535, 0.0.0.0-255.255.255.255)
 then a valid response could be the following (in which TSr and
 INTERNAL_IP4_SUBNET contain the same information):
 CP(CFG_REPLY) =
   INTERNAL_IP4_ADDRESS(198.51.100.234)
   INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
   INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
 TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
 TSr = ((0, 0-65535, 198.51.100.0-198.51.100.63),
        (0, 0-65535, 192.0.2.0-192.0.2.255))
 In these cases, the INTERNAL_IP4_SUBNET does not really carry any
 useful information.
 A different possible response would have been this:
 CP(CFG_REPLY) =
   INTERNAL_IP4_ADDRESS(198.51.100.234)
   INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
   INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
 TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
 TSr = (0, 0-65535, 0.0.0.0-255.255.255.255)
 That response would mean that the client can send all its traffic
 through the gateway, but the gateway does not mind if the client
 sends traffic not included by INTERNAL_IP4_SUBNET directly to the
 destination (without going through the gateway).
 A different situation arises if the gateway has a policy that
 requires the traffic for the two subnets to be carried in separate
 SAs.  Then a response like this would indicate to the client that
 if it wants access to the second subnet, it needs to create a
 separate SA:
 CP(CFG_REPLY) =
   INTERNAL_IP4_ADDRESS(198.51.100.234)
   INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
   INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
 TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
 TSr = (0, 0-65535, 198.51.100.0-198.51.100.63)

Kaufman, et al. Standards Track [Page 117] RFC 7296 IKEv2bis October 2014

 INTERNAL_IP4_SUBNET can also be useful if the client's TSr included
 only part of the address space.  For instance, if the client requests
 the following:
 CP(CFG_REQUEST) =
   INTERNAL_IP4_ADDRESS()
 TSi = (0, 0-65535, 0.0.0.0-255.255.255.255)
 TSr = (0, 0-65535, 192.0.2.155-192.0.2.155)
 then the gateway's response might be:
 CP(CFG_REPLY) =
   INTERNAL_IP4_ADDRESS(198.51.100.234)
   INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
   INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
 TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
 TSr = (0, 0-65535, 192.0.2.155-192.0.2.155)
 Because the meaning of INTERNAL_IP4_SUBNET/INTERNAL_IP6_SUBNET in
 CFG_REQUESTs is unclear, they cannot be used reliably in
 CFG_REQUESTs.

3.15.3. Configuration Payloads for IPv6

 The Configuration payloads for IPv6 are based on the corresponding
 IPv4 payloads, and do not fully follow the "normal IPv6 way of doing
 things".  In particular, IPv6 stateless autoconfiguration or router
 advertisement messages are not used, neither is neighbor discovery.
 Note that there is an additional document that discusses IPv6
 configuration in IKEv2, [IPV6CONFIG].  At the present time, it is an
 experimental document, but there is a hope that with more
 implementation experience, it will gain the same standards treatment
 as this document.

Kaufman, et al. Standards Track [Page 118] RFC 7296 IKEv2bis October 2014

 A client can be assigned an IPv6 address using the
 INTERNAL_IP6_ADDRESS Configuration payload.  A minimal exchange might
 look like this:
 CP(CFG_REQUEST) =
   INTERNAL_IP6_ADDRESS()
   INTERNAL_IP6_DNS()
 TSi = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
 TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
 CP(CFG_REPLY) =
   INTERNAL_IP6_ADDRESS(2001:DB8:0:1:2:3:4:5/64)
   INTERNAL_IP6_DNS(2001:DB8:99:88:77:66:55:44)
 TSi = (0, 0-65535, 2001:DB8:0:1:2:3:4:5 - 2001:DB8:0:1:2:3:4:5)
 TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
 The client MAY send a non-empty INTERNAL_IP6_ADDRESS attribute in the
 CFG_REQUEST to request a specific address or interface identifier.
 The gateway first checks if the specified address is acceptable, and
 if it is, returns that one.  If the address was not acceptable, the
 gateway attempts to use the interface identifier with some other
 prefix; if even that fails, the gateway selects another interface
 identifier.
 The INTERNAL_IP6_ADDRESS attribute also contains a prefix length
 field.  When used in a CFG_REPLY, this corresponds to the
 INTERNAL_IP4_NETMASK attribute in the IPv4 case.
 Although this approach to configuring IPv6 addresses is reasonably
 simple, it has some limitations.  IPsec tunnels configured using
 IKEv2 are not fully featured "interfaces" in the IPv6 addressing
 architecture sense [ADDRIPV6].  In particular, they do not
 necessarily have link-local addresses, and this may complicate the
 use of protocols that assume them, such as [MLDV2].

3.15.4. Address Assignment Failures

 If the responder encounters an error while attempting to assign an IP
 address to the initiator during the processing of a Configuration
 payload, it responds with an INTERNAL_ADDRESS_FAILURE notification.
 The IKE SA is still created even if the initial Child SA cannot be
 created because of this failure.  If this error is generated within
 an IKE_AUTH exchange, no Child SA will be created.  However, there
 are some more complex error cases.
 If the responder does not support Configuration payloads at all, it
 can simply ignore all Configuration payloads.  This type of
 implementation never sends INTERNAL_ADDRESS_FAILURE notifications.

Kaufman, et al. Standards Track [Page 119] RFC 7296 IKEv2bis October 2014

 If the initiator requires the assignment of an IP address, it will
 treat a response without CFG_REPLY as an error.
 The initiator may request a particular type of address (IPv4 or IPv6)
 that the responder does not support, even though the responder
 supports Configuration payloads.  In this case, the responder simply
 ignores the type of address it does not support and processes the
 rest of the request as usual.
 If the initiator requests multiple addresses of a type that the
 responder supports, and some (but not all) of the requests fail, the
 responder replies with the successful addresses only.  The responder
 sends INTERNAL_ADDRESS_FAILURE only if no addresses can be assigned.
 If the initiator does not receive the IP address(es) required by its
 policy, it MAY keep the IKE SA up and retry the Configuration payload
 as separate INFORMATIONAL exchange after suitable timeout, or it MAY
 tear down the IKE SA by sending a Delete payload inside a separate
 INFORMATIONAL exchange and later retry IKE SA from the beginning
 after some timeout.  Such a timeout should not be too short
 (especially if the IKE SA is started from the beginning) because
 these error situations may not be able to be fixed quickly; the
 timeout should likely be several minutes.  For example, an address
 shortage problem on the responder will probably only be fixed when
 more entries are returned to the address pool when other clients
 disconnect or when responder is reconfigured with larger address
 pool.

3.16. Extensible Authentication Protocol (EAP) Payload

 The Extensible Authentication Protocol payload, denoted EAP in this
 document, allows IKE SAs to be authenticated using the protocol
 defined in RFC 3748 [EAP] and subsequent extensions to that protocol.
 When using EAP, an appropriate EAP method needs to be selected.  Many
 of these methods have been defined, specifying the protocol's use
 with various authentication mechanisms.  EAP method types are listed
 in [EAP-IANA].  A short summary of the EAP format is included here
 for clarity.

Kaufman, et al. Standards Track [Page 120] RFC 7296 IKEv2bis October 2014

                      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).
 o  Identifier (1 octet) - Used in PPP to distinguish replayed
    messages from repeated ones.  Since in IKE, EAP runs over a
    reliable protocol, the Identifier serves no function here.  In a
    response message, this octet MUST be set to match the identifier
    in the corresponding request.
 o  Length (2 octets, unsigned integer) - The length of the EAP
    message.  MUST be four less than the Payload Length of the
    encapsulating payload.
 o  Type (1 octet) - 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.  Note that since IKE passes an
    indication of initiator identity in the first message in the
    IKE_AUTH exchange, the responder SHOULD NOT send EAP Identity
    requests (type 1).  The initiator MAY, however, respond to such
    requests if it receives them.

Kaufman, et al. Standards Track [Page 121] RFC 7296 IKEv2bis October 2014

 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 the
 first message in the IKE_AUTH exchange, the responder SHOULD NOT send
 EAP Identity requests.  The initiator MAY, 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.
 IKEv2 is designed to permit minimal implementations that can
 interoperate with all compliant implementations.  The following are
 features that can be omitted in a minimal implementation:
 o  Ability to negotiate SAs through a NAT and tunnel the resulting
    ESP SA over UDP.
 o  Ability to request (and respond to a request for) a temporary IP
    address on the remote end of a tunnel.
 o  Ability to support EAP-based authentication.
 o  Ability to support window sizes greater than one.
 o  Ability to establish multiple ESP or AH SAs within a single
    IKE SA.
 o  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.

Kaufman, et al. Standards Track [Page 122] RFC 7296 IKEv2bis October 2014

 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 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 request in the INFORMATIONAL
 exchange with an empty response (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), an 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 deleting the old SA and creating a
 new one.
 Implementations are not required to support requesting temporary IP
 addresses or responding to such requests.  If an implementation does
 support issuing such requests and its policy requires using temporary
 IP addresses, it MUST include a CP payload in the first message in
 the IKE_AUTH exchange 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 the first message
 in the IKE_AUTH exchange 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 may include any other related attributes.
 For an implementation to be called conforming to this specification,
 it MUST be possible to configure it to accept the following:
 o  Public Key Infrastructure using X.509 (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.
 o  Shared key authentication where the ID passed is any of ID_KEY_ID,
    ID_FQDN, or ID_RFC822_ADDR.

Kaufman, et al. Standards Track [Page 123] RFC 7296 IKEv2bis October 2014

 o  Authentication where the responder is authenticated using PKIX
    Certificates and the initiator is authenticated using shared key
    authentication.

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) not only can probe the initiator for its identity but may,
 by using CERTREQ payloads, 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.  Implementers should take note of this fact and set a
 limit on CREATE_CHILD_SA exchanges between exponentiations.  This
 document 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

Kaufman, et al. Standards Track [Page 124] RFC 7296 IKEv2bis October 2014

 negotiated including the PRF).  In fact, the extensible framework of
 IKE encourages the definition of more groups; use of elliptic curve
 groups may greatly increase strength using much smaller numbers.
 It is assumed that all Diffie-Hellman exponents are erased from
 memory after use.
 The IKE_SA_INIT and IKE_AUTH exchanges happen before the initiator
 has been authenticated.  As a result, an implementation of this
 protocol needs to be completely robust when deployed on any insecure
 network.  Implementation vulnerabilities, particularly DoS attacks,
 can be exploited by unauthenticated peers.  This issue is
 particularly worrisome because of the unlimited number of messages in
 EAP-based authentication.
 The strength of all keys is limited by the size of the output of the
 negotiated PRF.  For this reason, a PRF 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 pseudorandom source
 (see [RANDOMNESS]).  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

Kaufman, et al. Standards Track [Page 125] RFC 7296 IKEv2bis October 2014

 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-
 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 authentication.  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
 authentication 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 [DOSUDPPROT].
 Admission control is critical to the security of the protocol.  For
 example, trust anchors used for identifying IKE peers should probably
 be different than those used for other forms of trust, such as those
 used to identify public web servers.  Moreover, although IKE provides
 a great deal of leeway in defining the security policy for a trusted

Kaufman, et al. Standards Track [Page 126] RFC 7296 IKEv2bis October 2014

 peer's identity, credentials, and the correlation between them,
 having such security policy defined explicitly is essential to a
 secure implementation.

5.1. Traffic Selector Authorization

 IKEv2 relies on information in the Peer Authorization Database (PAD)
 when determining what kind of Child SAs a peer is allowed to create.
 This process is described in Section 4.4.3 of [IPSECARCH].  When a
 peer requests the creation of a Child SA with some Traffic Selectors,
 the PAD must contain "Child SA Authorization Data" linking the
 identity authenticated by IKEv2 and the addresses permitted for
 Traffic Selectors.
 For example, the PAD might be configured so that authenticated
 identity "sgw23.example.com" is allowed to create Child SAs for
 192.0.2.0/24, meaning this security gateway is a valid
 "representative" for these addresses.  Host-to-host IPsec requires
 similar entries, linking, for example, "fooserver4.example.com" with
 198.51.100.66/32, meaning this identity is a valid "owner" or
 "representative" of the address in question.
 As noted in [IPSECARCH], "It is necessary to impose these constraints
 on creation of child SAs to prevent an authenticated peer from
 spoofing IDs associated with other, legitimate peers".  In the
 example given above, a correct configuration of the PAD prevents
 sgw23 from creating Child SAs with address 198.51.100.66, and
 prevents fooserver4 from creating Child SAs with addresses from
 192.0.2.0/24.
 It is important to note that simply sending IKEv2 packets using some
 particular address does not imply a permission to create Child SAs
 with that address in the Traffic Selectors.  For example, even if
 sgw23 would be able to spoof its IP address as 198.51.100.66, it
 could not create Child SAs matching fooserver4's traffic.
 The IKEv2 specification does not specify how exactly IP address
 assignment using Configuration payloads interacts with the PAD.  Our
 interpretation is that when a security gateway assigns an address
 using Configuration payloads, it also creates a temporary PAD entry
 linking the authenticated peer identity and the newly allocated inner
 address.
 It has been recognized that configuring the PAD correctly may be
 difficult in some environments.  For instance, if IPsec is used
 between a pair of hosts whose addresses are allocated dynamically
 using DHCP, it is extremely difficult to ensure that the PAD

Kaufman, et al. Standards Track [Page 127] RFC 7296 IKEv2bis October 2014

 specifies the correct "owner" for each IP address.  This would
 require a mechanism to securely convey address assignments from the
 DHCP server, and link them to identities authenticated using IKEv2.
 Due to this limitation, some vendors have been known to configure
 their PADs to allow an authenticated peer to create Child SAs with
 Traffic Selectors containing the same address that was used for the
 IKEv2 packets.  In environments where IP spoofing is possible (i.e.,
 almost everywhere) this essentially allows any peer to create Child
 SAs with any Traffic Selectors.  This is not an appropriate or secure
 configuration in most circumstances.  See [H2HIPSEC] for an extensive
 discussion about this issue, and the limitations of host-to-host
 IPsec in general.

6. IANA Considerations

 [IKEV2] defined many field types and values.  IANA has already
 registered those types and values in [IKEV2IANA], so they are not
 listed here again.
 One item has been deprecated from the "IKEv2 Certificate Encodings"
 registry: "Raw RSA Key".
 IANA has updated all references to RFC 5996 to point to this
 document.

7. References

7.1. Normative References

 [ADDGROUP] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
            Diffie-Hellman groups for Internet Key Exchange (IKE)",
            RFC 3526, May 2003,
            <http://www.rfc-editor.org/info/rfc3526>.
 [ADDRIPV6] Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, February 2006,
            <http://www.rfc-editor.org/info/rfc4291>.
 [AEAD]     Black, D. and D. McGrew, "Using Authenticated Encryption
            Algorithms with the Encrypted Payload of the Internet Key
            Exchange version 2 (IKEv2) Protocol", RFC 5282, August
            2008, <http://www.rfc-editor.org/info/rfc5282>.

Kaufman, et al. Standards Track [Page 128] RFC 7296 IKEv2bis October 2014

 [AESCMACPRF128]
            Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
            Advanced Encryption Standard-Cipher-based Message
            Authentication Code-Pseudo-Random Function-128 (AES-CMAC-
            PRF-128) Algorithm for the Internet Key Exchange Protocol
            (IKE)", RFC 4615, August 2006,
            <http://www.rfc-editor.org/info/rfc4615>.
 [AESXCBCPRF128]
            Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
            Internet Key Exchange Protocol (IKE)", RFC 4434, February
            2006, <http://www.rfc-editor.org/info/rfc4434>.
 [EAP]      Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
            Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
            3748, June 2004, <http://www.rfc-editor.org/info/rfc3748>.
 [ECN]      Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP", RFC
            3168, September 2001,
            <http://www.rfc-editor.org/info/rfc3168>.
 [ESPCBC]   Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
            Algorithms", RFC 2451, November 1998,
            <http://www.rfc-editor.org/info/rfc2451>.
 [IKEV2IANA]
            IANA, "Internet Key Exchange Version 2 (IKEv2)
            Parameters",
            <http://www.iana.org/assignments/ikev2-parameters/>.
 [IPSECARCH]
            Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, December 2005,
            <http://www.rfc-editor.org/info/rfc4301>.
 [MUSTSHOULD]
            Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [PKCS1]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, February 2003,
            <http://www.rfc-editor.org/info/rfc3447>.

Kaufman, et al. Standards Track [Page 129] RFC 7296 IKEv2bis October 2014

 [PKIX]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008,
            <http://www.rfc-editor.org/info/rfc5280>.
 [RFC4307]  Schiller, J., "Cryptographic Algorithms for Use in the
            Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
            December 2005, <http://www.rfc-editor.org/info/rfc4307>.
 [UDPENCAPS]
            Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
            Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
            3948, January 2005,
            <http://www.rfc-editor.org/info/rfc3948>.
 [URLS]     Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.

7.2. Informative References

 [AH]       Kent, S., "IP Authentication Header", RFC 4302, December
            2005, <http://www.rfc-editor.org/info/rfc4302>.
 [ARCHGUIDEPHIL]
            Bush, R. and D. Meyer, "Some Internet Architectural
            Guidelines and Philosophy", RFC 3439, December 2002,
            <http://www.rfc-editor.org/info/rfc3439>.
 [ARCHPRINC]
            Carpenter, B., "Architectural Principles of the Internet",
            RFC 1958, June 1996,
            <http://www.rfc-editor.org/info/rfc1958>.
 [Clarif]   Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
            Implementation Guidelines", RFC 4718, October 2006,
            <http://www.rfc-editor.org/info/rfc4718>.
 [DES]      American National Standards Institute, "American National
            Standard for Information Systems-Data Link Encryption",
            ANSI X3.106, 1983.
 [DH]       Diffie, W. and M. Hellman, "New Directions in
            Cryptography", IEEE Transactions on Information Theory,
            V.IT-22 n. 6, June 1977.

Kaufman, et al. Standards Track [Page 130] RFC 7296 IKEv2bis October 2014

 [DIFFSERVARCH]
            Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
            and W. Weiss, "An Architecture for Differentiated
            Services", RFC 2475, December 1998,
            <http://www.rfc-editor.org/info/rfc2475>.
 [DIFFSERVFIELD]
            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, <http://www.rfc-editor.org/info/rfc2474>.
 [DIFFTUNNEL]
            Black, D., "Differentiated Services and Tunnels", RFC
            2983, October 2000,
            <http://www.rfc-editor.org/info/rfc2983>.
 [DOI]      Piper, D., "The Internet IP Security Domain of
            Interpretation for ISAKMP", RFC 2407, November 1998,
            <http://www.rfc-editor.org/info/rfc2407>.
 [DOSUDPPROT]
            Kaufman, C., Perlman, R., and B. Sommerfeld, "DoS
            protection for UDP-based protocols", ACM Conference on
            Computer and Communications Security, October 2003.
 [DSS]      National Institute of Standards and Technology, U.S.
            Department of Commerce, "Digital Signature Standard
            (DSS)", FIPS 186-4, July 2013,
            <http://nvlpubs.nist.gov/nistpubs/FIPS/
            NIST.FIPS.186-4.pdf>.
 [EAI]      Yang, A., Steele, S., and N. Freed, "Internationalized
            Email Headers", RFC 6532, February 2012,
            <http://www.rfc-editor.org/info/rfc6532>.
 [EAP-IANA] IANA, "Extensible Authentication Protocol (EAP) Registry:
            Method Types",
            <http://http://www.iana.org/assignments/eap-eke/>.
 [EAPMITM]  Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
            in Tunneled Authentication Protocols", November 2002,
            <http://eprint.iacr.org/2002/163>.
 [ESP]      Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
            4303, December 2005,
            <http://www.rfc-editor.org/info/rfc4303>.

Kaufman, et al. Standards Track [Page 131] RFC 7296 IKEv2bis October 2014

 [EXCHANGEANALYSIS]
            Perlman, R. and C. Kaufman, "Analysis of the IPsec key
            exchange Standard", WET-ICE Security Conference, MIT,
            2001, <http://www.computer.org/csdl/proceedings/
            wetice/2001/1269/00/12690150.pdf>.
 [FIPS.180-4.2012]
            National Institute of Standards and Technology, U.S.
            Department of Commerce, "Secure Hash Standard (SHS)", FIPS
            180-4, March 2012,
            <http://csrc.nist.gov/publications/fips/fips180-4/
            fips-180-4.pdf>.
 [H2HIPSEC] Aura, T., Roe, M., and A. Mohammed, "Experiences with
            Host-to-Host IPsec", 13th International Workshop on
            Security Protocols, Cambridge, UK, April 2005.
 [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
            Hashing for Message Authentication", RFC 2104, February
            1997, <http://www.rfc-editor.org/info/rfc2104>.
 [IDEA]     Lai, X., "On the Design and Security of Block Ciphers",
            ETH Series in Information Processing, v. 1, Konstanz:
            Hartung-Gorre Verlag, 1992.
 [IDNA]     Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, August 2010,
            <http://www.rfc-editor.org/info/rfc5890>.
 [IKEV1]    Harkins, D. and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998,
            <http://www.rfc-editor.org/info/rfc2409>.
 [IKEV2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
            4306, December 2005,
            <http://www.rfc-editor.org/info/rfc4306>.
 [IP]       Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981, <http://www.rfc-editor.org/info/rfc791>.
 [IP-COMP]  Shacham, A., Monsour, B., Pereira, R., and M. Thomas, "IP
            Payload Compression Protocol (IPComp)", RFC 3173,
            September 2001, <http://www.rfc-editor.org/info/rfc3173>.

Kaufman, et al. Standards Track [Page 132] RFC 7296 IKEv2bis October 2014

 [IPSECARCH-OLD]
            Kent, S. and R. Atkinson, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998,
            <http://www.rfc-editor.org/info/rfc2401>.
 [IPV6CONFIG]
            Eronen, P., Laganier, J., and C. Madson, "IPv6
            Configuration in Internet Key Exchange Protocol Version 2
            (IKEv2)", RFC 5739, February 2010,
            <http://www.rfc-editor.org/info/rfc5739>.
 [ISAKMP]   Maughan, D., Schneider, M., and M. Schertler, "Internet
            Security Association and Key Management Protocol
            (ISAKMP)", RFC 2408, November 1998,
            <http://www.rfc-editor.org/info/rfc2408>.
 [MAILFORMAT]
            Resnick, P., Ed., "Internet Message Format", RFC 5322,
            October 2008, <http://www.rfc-editor.org/info/rfc5322>.
 [MD5]      Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
            April 1992, <http://www.rfc-editor.org/info/rfc1321>.
 [MIPV6]    Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
            in IPv6", RFC 6275, July 2011,
            <http://www.rfc-editor.org/info/rfc6275>.
 [MLDV2]    Vida, R. and L. Costa, "Multicast Listener Discovery
            Version 2 (MLDv2) for IPv6", RFC 3810, June 2004,
            <http://www.rfc-editor.org/info/rfc3810>.
 [MOBIKE]   Eronen, P., "IKEv2 Mobility and Multihoming Protocol
            (MOBIKE)", RFC 4555, June 2006,
            <http://www.rfc-editor.org/info/rfc4555>.
 [MODES]    Dworkin, M., "Recommendation for Block Cipher Modes of
            Operation", National Institute of Standards and
            Technology, NIST Special Publication 800-38A 2001 Edition,
            December 2001.
 [NAI]      Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
            Network Access Identifier", RFC 4282, December 2005,
            <http://www.rfc-editor.org/info/rfc4282>.
 [NATREQ]   Aboba, B. and W. Dixon, "IPsec-Network Address Translation
            (NAT) Compatibility Requirements", RFC 3715, March 2004,
            <http://www.rfc-editor.org/info/rfc3715>.

Kaufman, et al. Standards Track [Page 133] RFC 7296 IKEv2bis October 2014

 [OAKLEY]   Orman, H., "The OAKLEY Key Determination Protocol", RFC
            2412, November 1998,
            <http://www.rfc-editor.org/info/rfc2412>.
 [PFKEY]    McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
            Management API, Version 2", RFC 2367, July 1998,
            <http://www.rfc-editor.org/info/rfc2367>.
 [PHOTURIS] Karn, P. and W. Simpson, "Photuris: Session-Key Management
            Protocol", RFC 2522, March 1999,
            <http://www.rfc-editor.org/info/rfc2522>.
 [RANDOMNESS]
            Eastlake 3rd, D., Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005, <http://www.rfc-editor.org/info/rfc4086>.
 [REAUTH]   Nir, Y., "Repeated Authentication in Internet Key Exchange
            (IKEv2) Protocol", RFC 4478, April 2006,
            <http://www.rfc-editor.org/info/rfc4478>.
 [REUSE]    Menezes, A. and B. Ustaoglu, "On Reusing Ephemeral Keys In
            Diffie-Hellman Key Agreement Protocols", December 2008,
            <http://www.cacr.math.uwaterloo.ca/techreports/2008/
            cacr2008-24.pdf>.
 [RFC4945]  Korver, B., "The Internet IP Security PKI Profile of
            IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007,
            <http://www.rfc-editor.org/info/rfc4945>.
 [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
            "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
            5996, September 2010,
            <http://www.rfc-editor.org/info/rfc5996>.
 [RFC6989]  Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
            Tests for the Internet Key Exchange Protocol Version 2
            (IKEv2)", RFC 6989, July 2013,
            <http://www.rfc-editor.org/info/rfc6989>.
 [ROHCV2]   Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and C.
            Bormann, "IKEv2 Extensions to Support Robust Header
            Compression over IPsec", RFC 5857, May 2010,
            <http://www.rfc-editor.org/info/rfc5857>.

Kaufman, et al. Standards Track [Page 134] RFC 7296 IKEv2bis October 2014

 [SIGMA]    Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' Approach to
            Authenticated Diffie-Hellman and its Use in the IKE
            Protocols", Advances in Cryptography - CRYPTO 2003
            Proceedings LNCS 2729, 2003,
            <http://www.informatik.uni-trier.de/~ley/db/conf/crypto/
            crypto2003.html>.
 [SKEME]    Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
            Mechanism for Internet", IEEE Proceedings of the 1996
            Symposium on Network and Distributed Systems Security,
            1996.
 [TRANSPARENCY]
            Carpenter, B., "Internet Transparency", RFC 2775, February
            2000, <http://www.rfc-editor.org/info/rfc2775>.

Kaufman, et al. Standards Track [Page 135] RFC 7296 IKEv2bis October 2014

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
      [EXCHANGEANALYSIS];
 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;
 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 in order to make it
      easier to make future revisions in a way that does not break
      backward compatibility;

Kaufman, et al. Standards Track [Page 136] RFC 7296 IKEv2bis October 2014

 11.  To simplify and clarify how shared state is maintained in the
      presence of network failures and DoS 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 [OAKLEY].
 The strength supplied by group 1 may not be sufficient for typical
 uses 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, et al. Standards Track [Page 137] RFC 7296 IKEv2bis October 2014

Appendix C. Exchanges and Payloads

 This appendix contains a short summary of the IKEv2 exchanges, and
 what payloads can appear in which message.  This appendix is purely
 informative; if it disagrees with the body of this document, the
 other text is considered correct.
 Vendor ID (V) payloads may be included in any place in any message.
 This sequence here shows what are the most logical places for them.

C.1. IKE_SA_INIT Exchange

 request             --> [N(COOKIE),]
                         SA, KE, Ni,
                         [N(NAT_DETECTION_SOURCE_IP)+,
                          N(NAT_DETECTION_DESTINATION_IP),]
                         [V+][N+]
 normal response     <-- SA, KE, Nr,
 (no cookie)             [N(NAT_DETECTION_SOURCE_IP),
                          N(NAT_DETECTION_DESTINATION_IP),]
                         [[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
                         [V+][N+]
 cookie response     <-- N(COOKIE),
                         [V+][N+]
 different Diffie-   <-- N(INVALID_KE_PAYLOAD),
 Hellman group           [V+][N+]
 wanted

C.2. IKE_AUTH Exchange without EAP

 request             --> IDi, [CERT+,]
                         [N(INITIAL_CONTACT),]
                         [[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
                         [IDr,]
                         AUTH,
                         [CP(CFG_REQUEST),]
                         [N(IPCOMP_SUPPORTED)+,]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, TSi, TSr,
                         [V+][N+]

Kaufman, et al. Standards Track [Page 138] RFC 7296 IKEv2bis October 2014

 response            <-- IDr, [CERT+,]
                         AUTH,
                         [CP(CFG_REPLY),]
                         [N(IPCOMP_SUPPORTED),]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, TSi, TSr,
                         [N(ADDITIONAL_TS_POSSIBLE),]
                         [V+][N+]
 error in Child SA  <--  IDr, [CERT+,]
 creation                AUTH,
                         N(error),
                         [V+][N+]

C.3. IKE_AUTH Exchange with EAP

 first request       --> IDi,
                         [N(INITIAL_CONTACT),]
                         [[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
                         [IDr,]
                         [CP(CFG_REQUEST),]
                         [N(IPCOMP_SUPPORTED)+,]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, TSi, TSr,
                         [V+][N+]
 first response      <-- IDr, [CERT+,] AUTH,
                         EAP,
                         [V+][N+]
                   / --> EAP
 repeat 1..N times |
                   \ <-- EAP

Kaufman, et al. Standards Track [Page 139] RFC 7296 IKEv2bis October 2014

 last request        --> AUTH
 last response       <-- AUTH,
                         [CP(CFG_REPLY),]
                         [N(IPCOMP_SUPPORTED),]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, TSi, TSr,
                         [N(ADDITIONAL_TS_POSSIBLE),]
                         [V+][N+]

C.4. CREATE_CHILD_SA Exchange for Creating or Rekeying Child SAs

 request             --> [N(REKEY_SA),]
                         [CP(CFG_REQUEST),]
                         [N(IPCOMP_SUPPORTED)+,]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, Ni, [KEi,] TSi, TSr,
                         [V+][N+]
 normal              <-- [CP(CFG_REPLY),]
 response                [N(IPCOMP_SUPPORTED),]
                         [N(USE_TRANSPORT_MODE),]
                         [N(ESP_TFC_PADDING_NOT_SUPPORTED),]
                         [N(NON_FIRST_FRAGMENTS_ALSO),]
                         SA, Nr, [KEr,] TSi, TSr,
                         [N(ADDITIONAL_TS_POSSIBLE),]
                         [V+][N+]
 error case          <-- N(error)
 different Diffie-   <-- N(INVALID_KE_PAYLOAD),
 Hellman group           [V+][N+]
 wanted

C.5. CREATE_CHILD_SA Exchange for Rekeying the IKE SA

 request             --> SA, Ni, KEi,
                         [V+][N+]
 response            <-- SA, Nr, KEr,
                         [V+][N+]

Kaufman, et al. Standards Track [Page 140] RFC 7296 IKEv2bis October 2014

C.6. INFORMATIONAL Exchange

 request             --> [N+,]
                         [D+,]
                         [CP(CFG_REQUEST)]
 response            <-- [N+,]
                         [D+,]
                         [CP(CFG_REPLY)]

Acknowledgements

 Many individuals in the IPsecME Working Group were very helpful in
 contributing ideas and text for this document, as well as in
 reviewing the clarifications suggested by others.
 The acknowledgements from the IKEv2 document were:
 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 Smyslov helped refine the design of the
 Traffic Selector negotiation.

Kaufman, et al. Standards Track [Page 141] RFC 7296 IKEv2bis October 2014

Authors' Addresses

 Charlie Kaufman
 Microsoft
 1 Microsoft Way
 Redmond, WA  98052
 United States
 EMail: charliekaufman@outlook.com
 Paul Hoffman
 VPN Consortium
 127 Segre Place
 Santa Cruz, CA  95060
 United States
 Phone: 1-831-426-9827
 EMail: paul.hoffman@vpnc.org
 Yoav Nir
 Check Point Software Technologies Ltd.
 5 Hasolelim St.
 Tel Aviv 6789735
 Israel
 EMail: ynir.ietf@gmail.com
 Pasi Eronen
 Independent
 EMail: pe@iki.fi
 Tero Kivinen
 INSIDE Secure
 Eerikinkatu 28
 HELSINKI  FI-00180
 Finland
 EMail: kivinen@iki.fi

Kaufman, et al. Standards Track [Page 142]

/data/webs/external/dokuwiki/data/pages/rfc/std/std79.txt · Last modified: 2014/10/24 23:16 (external edit)