





Internet Engineering Task Force (IETF)                        T. Kivinen
Request for Comments: 7815                                 INSIDE Secure
Category: Informational                                       March 2016
ISSN: 2070-1721


Minimal Internet Key Exchange Version 2 (IKEv2) Initiator Implementation

Abstract

   This document describes a minimal initiator version of the Internet
   Key Exchange version 2 (IKEv2) protocol for constrained nodes.  IKEv2
   is a component of IPsec used for performing mutual authentication and
   establishing and maintaining Security Associations (SAs).  IKEv2
   includes several optional features, which are not needed in minimal
   implementations.  This document describes what is required from the
   minimal implementation and also describes various optimizations that
   can be done.  The protocol described here is interoperable with a
   full IKEv2 implementation using shared secret authentication (IKEv2
   does not require the use of certificate authentication).  This
   minimal initiator implementation can only talk to a full IKEv2
   implementation acting as the responder; thus, two minimal initiator
   implementations cannot talk to each other.

   This document does not update or modify RFC 7296 but provides a more
   compact description of the minimal version of the protocol.  If this
   document and RFC 7296 conflict, then RFC 7296 is the authoritative
   description.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7815.







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Copyright Notice

   Copyright (c) 2016 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
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   publication of this document.  Please review these documents
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   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.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Initial Exchange  . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Other Exchanges . . . . . . . . . . . . . . . . . . . . .  12
     2.3.  Generating Keying Material  . . . . . . . . . . . . . . .  12
   3.  Conformance Requirements  . . . . . . . . . . . . . . . . . .  13
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Header and Payload Formats . . . . . . . . . . . . .  17
     A.1.  The IKE Header  . . . . . . . . . . . . . . . . . . . . .  17
     A.2.  Generic Payload Header  . . . . . . . . . . . . . . . . .  19
     A.3.  Security Association Payload  . . . . . . . . . . . . . .  21
       A.3.1.  Proposal Substructure . . . . . . . . . . . . . . . .  23
       A.3.2.  Transform Substructure  . . . . . . . . . . . . . . .  24
       A.3.3.  Valid Transform Types by Protocol . . . . . . . . . .  26
       A.3.4.  Transform Attributes  . . . . . . . . . . . . . . . .  26
     A.4.  Key Exchange Payload  . . . . . . . . . . . . . . . . . .  27
     A.5.  Identification Payloads . . . . . . . . . . . . . . . . .  27
     A.6.  Certificate Payload . . . . . . . . . . . . . . . . . . .  29
     A.7.  Certificate Request Payload . . . . . . . . . . . . . . .  30
     A.8.  Authentication Payload  . . . . . . . . . . . . . . . . .  31
     A.9.  Nonce Payload . . . . . . . . . . . . . . . . . . . . . .  31
     A.10. Notify Payload  . . . . . . . . . . . . . . . . . . . . .  32
       A.10.1.  Notify Message Types . . . . . . . . . . . . . . . .  33
     A.11. Traffic Selector Payload  . . . . . . . . . . . . . . . .  34
       A.11.1.  Traffic Selector . . . . . . . . . . . . . . . . . .  36
     A.12. Encrypted Payload . . . . . . . . . . . . . . . . . . . .  37
   Appendix B.  Useful Optional Features . . . . . . . . . . . . . .  39
     B.1.  IKE SA Delete Notification  . . . . . . . . . . . . . . .  39
     B.2.  Raw Public Keys . . . . . . . . . . . . . . . . . . . . .  40
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  41
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  41













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1.  Introduction

   The Internet Protocol Suite is increasingly used on small devices
   with severe constraints on power, memory, and processing resources.
   This document describes a minimal IKEv2 implementation designed for
   use on such constrained nodes that is interoperable with "Internet
   Key Exchange Protocol Version 2 (IKEv2)" [RFC7296].

   A minimal IKEv2 implementation only supports the initiator end of the
   protocol.  It only supports the initial IKE_SA_INIT and IKE_AUTH
   exchanges and does not initiate any other exchanges.  It also replies
   with an empty (or error) message to all incoming requests.

   This means that most of the optional features of IKEv2 are left out:
   NAT traversal, IKE SA rekey, Child SA rekey, multiple Child SAs,
   deleting Child / IKE SAs, Configuration payloads, Extensible
   Authentication Protocol (EAP) authentication, COOKIEs, etc.

   Some optimizations can be done because of the limited set of
   supported features, and this text should not be considered for
   generic IKEv2 implementations (for example, Message IDs can be done
   as specified because minimal implementation is only sending out an
   IKE_SA_INIT and IKE_AUTH request and not any other request).

   This document is intended to be standalone, meaning everything needed
   to implement IKEv2 is copied here except the description of the
   cryptographic algorithms.  The IKEv2 specification has lots of
   background information and rationale that has been omitted from this
   document.

   Numerous additional numeric values from IANA registries have been
   omitted from this document; only those which are of interest for a
   minimal implementation are listed.

   The main body of this document describes how to use the shared secret
   authentication in IKEv2, as it is easiest to implement.  In some
   cases, that is not enough, and Appendix B.2 describes how to use raw
   public keys instead of shared secret authentication.

   For more information, check the full IKEv2 specification in [RFC7296]
   and [IKEV2IANA].

   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 [RFC2119].  The term
   "Constrained Node" is defined in "Terminology for Constrained-Node
   Networks" [RFC7228].




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1.1.  Use Cases

   One use case for this kind of minimal implementation is in small
   devices doing machine-to-machine communication.  In such
   environments, the node initiating connections can be very small, and
   the other end of the communication channel is some kind of larger
   device.

   An example of the small initiating node could be a remote garage door
   opener device, i.e., a device having buttons that open and close a
   garage door and that connects to the home area network server over a
   wireless link.

   Another example of such a device is some kind of sensor device, for
   example, a room temperature sensor, which sends periodic temperature
   data to some centralized node.

   Those devices usually sleep for a long time and only wake up
   periodically or because of user interaction.  The data transfer is
   always initiated from that sleeping node when they wake up; after
   they send packets, there might be ACKs or other packets coming back
   before they go back to sleep.  If some data needs to be transferred
   from a server node to the small device, it can be implemented by
   polling, i.e., the small node periodically polls for the server to
   see if it, for example, has some configuration changes or similar.
   While the device is sleeping, it will not maintain the IKEv2 SA.
   That is, it will always create the IKEv2 SA again when it wakes up.
   This means there is no need to do liveness checks for the server, as
   after the device wakes up again, the minimal implementation will
   start from the beginning again.

2.  Exchanges

2.1.  Initial Exchange

   All IKEv2 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".  Every request requires a response.

   For every pair of IKEv2 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.

   IKEv2 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.  A retransmission from the initiator MUST be bitwise




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   identical to the original request.  Retransmission times MUST
   increase exponentially.

   IKEv2 is run over UDP port 500.  All IKEv2 implementations MUST be
   able to send, receive, and process IKEv2 messages that are up to 1280
   octets long.  An implementation MUST accept incoming requests even if
   the source port is not 500 and MUST respond to the address and port
   from which the request was received.

   The minimal implementation of IKEv2 only uses the first two
   exchanges, called IKE_SA_INIT and IKE_AUTH.  These are used to create
   the IKE SA and the first Child SA.  In addition to those messages, a
   minimal IKEv2 implementation needs to understand the CREATE_CHILD_SA
   request enough to generate a CREATE_CHILD_SA response containing the
   NO_ADDITIONAL_SAS error notify.  It needs to understand the
   INFORMATIONAL request enough to generate an empty INFORMATIONAL
   response to it.  There is no requirement to be able to respond to any
   other requests.

   All messages following the IKE_SA_INIT exchange are cryptographically
   protected using the cryptographic algorithms and keys negotiated in
   the IKE_SA_INIT exchange.

   Every IKEv2 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.

   Minimal implementations only need to support the role of initiator,
   so it typically only sends an IKE_SA_INIT request that, when
   answered, is followed by an IKE_AUTH.  As those messages have fixed
   Message IDs (0 and 1), it does not need to keep track of its own
   Message IDs for outgoing requests after that.

   Minimal implementations can also optimize Message ID handling of the
   incoming requests, as they do not need to protect incoming requests
   against replays.  This is possible because minimal implementations
   will only return error or empty notification replies to incoming
   requests.  This means that any of those incoming requests do not have
   any effect on the minimal implementation, thus processing them again
   does not cause any harm.  Because of this, a minimal implementation
   can always answer a request coming in, with the same Message ID than
   what the request had, and then forget the request/response pair
   immediately.  This means there is no need to keep track of Message
   IDs of the incoming requests.







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   In the following descriptions, the payloads contained in the message
   are indicated by the names listed below.

   Notation    Payload
   -----------------------------------------
   AUTH        Authentication
   CERTREQ     Certificate Request
   D           Delete
   HDR         IKE header (not a payload)
   IDi         Identification - Initiator
   IDr         Identification - Responder
   KE          Key Exchange
   Ni, Nr      Nonce
   N           Notify
   SA          Security Association
   SK          Encrypted and Authenticated
   TSi         Traffic Selector - Initiator
   TSr         Traffic Selector - Responder

   The initial exchanges are as follows:

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=0, IKE_SA_INIT,
       Flags: Initiator, Message ID=0),
       SAi1, KEi, Ni  -->

                      <--  HDR(SPIi=xxx, SPIr=yyy, IKE_SA_INIT,
                               Flags: Response, Message ID=0),
                               SAr1, KEr, Nr, [CERTREQ]

   HDR contains the Security Parameter Indexes (SPIs), version numbers,
   and flags of various sorts.  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 IKEv2 packets are mapped to an IKE SA using only the
   packet's SPI, not using (for example) the source IP address of the
   packet.

   The SAi1 payload states the cryptographic algorithms the initiator
   supports for the IKE SA.  The KEi and KEr payloads contain Diffie-
   Hellman values, and Ni and Nr are the nonces.  The SAr1 contains the
   chosen cryptographic suite from the initiator's offered choices.  A
   minimal implementation using shared secrets will ignore the CERTREQ
   payload.




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   Minimal implementation will most likely support exactly one set of
   cryptographic algorithms, meaning the SAi1 payload will be static.
   It needs to check that the SAr1 received matches the proposal it
   sent.

   At this point in the negotiation, each party can generate SKEYSEED,
   from which all keys are derived for that IKE SA.

   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 )

   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)
   ...

   (indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
   SK_pi, and SK_pr are taken in order from the generated bits of the
   prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
   exchange. g^ir is represented as a string of octets in big endian
   order padded with zeros if necessary to make it the length of the
   modulus.  Ni and Nr are the nonces, stripped of any headers.

   The SK_d is used for deriving new keys for the Child SAs.  The SK_ai
   and SK_ar are used as a key to the integrity protection algorithm for
   authenticating the component messages of subsequent exchanges.  The
   SK_ei and SK_er are used for encrypting (and of course decrypting)
   all subsequent exchanges.  The SK_pi and SK_pr 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 Pseudorandom Function (PRF)
   agreed upon.

   A separate SK_e and SK_a is computed for each direction.  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.  The notation SK { ... } indicates that these
   payloads are encrypted and integrity protected using that direction's
   SK_e and SK_a.







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   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
       Flags: Initiator, Message ID=1),
       SK {IDi, AUTH, SAi2, TSi, TSr,
           N(INITIAL_CONTACT)}  -->

                     <--  HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
                                 Response, Message ID=1),
                                 SK {IDr, AUTH, SAr2, TSi, TSr}

   The initiator asserts its identity with the IDi payload, proves
   knowledge of the secret corresponding to IDi, and integrity protects
   the contents of the first message using the AUTH payload.  The
   responder asserts its identity with the IDr payload, authenticates
   its identity, and protects the integrity of the second message with
   the AUTH payload.

   As minimal implementation usually has only one host where it
   connects, that means it has only one shared secret.  This means it
   does not need to care about the IDr payload that much.  If the other
   end sends an AUTH payload that the initiator can verify using the
   shared secret it has, then it knows the other end is the peer it was
   configured to talk to.

   In the IKE_AUTH request, the initiator sends the SA offer(s) in the
   SAi2 payload and the proposed Traffic Selectors (TSs) for the Child
   SA in the TSi and TSr payloads.  The responder replies with the
   accepted offer in an SAr2 payload and with the selected Traffic
   Selectors.  The selected Traffic Selectors may be a subset of what
   the initiator proposed.

   In the minimal implementation, both SA payloads and TS payloads are
   going to be mostly static.  The SA payload will have the SPI value
   used in the Encapsulating Security Payload (ESP), but the algorithms
   are most likely going to be the one and only supported set.  The TS
   payloads on the initiator end will most likely say from any to any,
   i.e., full wildcard ranges, or from the local IP to the remote IP.
   In the wildcard case, the responder quite often narrows the range
   down to the one IP address pair.  Using a single IP address pair as
   the Traffic Selectors when sending the IKE_AUTH request will simplify
   processing as the responder will either accept the IP address pair or
   return an error.  If wildcard ranges are used, there is a possibility
   that the responder will narrow the Traffic Selector range to range
   that is not acceptable by the initiator.

   The IKE_AUTH (and IKE_SA_INIT) response may contain multiple status
   notification payloads that can be ignored by minimal implementations.



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   There can also be Vendor ID, Certificate, Certificate Request, or
   Configuration payloads, but any payload unknown to minimal
   implementations can simply be skipped over (response messages cannot
   have critical unsupported payloads).

   The exchange above includes N(INITIAL_CONTACT) notification in the
   request as that is quite commonly sent by a minimal implementation.
   It indicates to the other end that the initiator does not have any
   other IKE SAs between it and the responder, and if there is any left
   from previous runs, those can be deleted by the responder.  As
   minimal implementations delete IKE SAs without sending IKE SA delete
   requests, this will help the responder to clean up leftover state.

   When using shared secret authentication, the peers are authenticated
   by having each calculating a Message Authentication Code (MAC) over a
   block of data:

   For the initiator:
      AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                       <InitiatorSignedOctets>)
   For the responder:
      AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
                       <ResponderSignedOctets>)

   The string "Key Pad for IKEv2" is 17 ASCII characters without null
   termination.  The implementation can precalculate the inner prf and
   only store the output of it.  This is possible because a minimal
   IKEv2 implementation usually only supports one PRF.

   In the following calculations, IDi' and IDr' are the entire ID
   payloads excluding the fixed header, and the Ni and Nr are only the
   values, not the payloads containing it.  Note that neither the nonce
   Ni/Nr nor the value prf(SK_pr, IDr')/prf(SK_pi, IDi') are
   transmitted.

   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 in that first message.  Appended
   to this (for purposes of computing the signature) are the responder's
   nonce Nr and the value prf(SK_pi, IDi').

   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 that
   second message.  Appended to this are the initiator's nonce Ni and
   the value prf(SK_pr, IDr').





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   The initiator's signed octets can be described as:

   InitiatorSignedOctets = RealMessage1 | NonceRData | MACedIDForI
   RealIKEHDR =  SPIi | SPIr |  . . . | Length
   RealMessage1 = RealIKEHDR | RestOfMessage1
   NonceRPayload = PayloadHeader | NonceRData
   InitiatorIDPayload = PayloadHeader | RestOfInitIDPayload
   RestOfInitIDPayload = IDType | RESERVED | InitIDData
   MACedIDForI = prf(SK_pi, RestOfInitIDPayload)

   The responder's signed octets can be described as:

   ResponderSignedOctets = RealMessage2 | NonceIData | MACedIDForR
   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 inside the RestOfMessageX are included
   under the signature, including any payload types not listed in this
   document.

   The initiator might also get an unauthenticated response back that
   has a notification payload with an error code inside.  As that error
   code will be unauthenticated and may be faked, there is no need to do
   anything for those.  A minimal implementation can simply ignore those
   errors and retransmit its request until it times out, and if that
   happens, then the IKE SA (and Child SA) creation failed.

   The responder might also reply with an IKE_AUTH response packet that
   does not contain the payloads needed to set up a Child SA (SAr2, TSi,
   and TSr) but instead contain AUTH payload and an error.  Minimal
   implementation that does not support the CREATE_CHILD_SA exchange
   cannot recover from this scenario.  It can delete the IKE SA and
   start over from the beginning (which might fail again if this is a
   configuration error, or it might succeed if this was temporal
   failure).












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2.2.  Other Exchanges

   Minimal implementations MUST be able to reply to INFORMATIONAL
   requests by sending back an empty INFORMATIONAL response:

   Minimal implementation            Other end
   -------------------------------------------------------------------
                      <--  HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
                                  Flags: none,  Message ID=m),
                                  SK {...}

   HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
       Flags: Initiator | Response,
       Message ID=m),
       SK {}  -->

   Minimal implementations MUST be able to reply to incoming
   CREATE_CHILD_SA requests.  A typical implementation will reject the
   CREATE_CHILD_SA exchanges by sending a NO_ADDITIONAL_SAS error notify
   back:

   Minimal implementation            Other end
   -------------------------------------------------------------------
                      <--  HDR(SPIi=xxx, SPIy=yyy, CREATE_CHILD_SA,
                                  Flags: none, Message ID=m),
                                  SK {...}

   HDR(SPIi=xxx, SPIr=yyy, CREATE_CHILD_SA,
       Flags: Initiator | Response, Message ID=m),
       SK {N(NO_ADDITIONAL_SAS)}  -->

   Note that INFORMATIONAL and CREATE_CHILD_SA requests might contain
   unsupported critical payloads, in which case a compliant
   implementation MUST ignore the request and send a response message
   back that has the UNSUPPORTED_CRITICAL_PAYLOAD notification.  That
   notification payload data contains a 1-octet payload type of the
   unsupported critical payload.

2.3.  Generating Keying Material

   The keying material for the Child SA created by the IKE_AUTH exchange
   is generated as follows:

   KEYMAT = prf+(SK_d, Ni | Nr)

   Where Ni and Nr are the nonces from the IKE_SA_INIT exchange.





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   A single CHILD_SA negotiation may result in multiple Security
   Associations.  ESP and Authentication Header (AH) SAs exist in pairs
   (one in each direction), so two SAs are created in a single Child SA
   negotiation for them.  The 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 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.

3.  Conformance Requirements

   For an implementation to be called conforming to the RFC 7296
   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.

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

   This document only supports the second bullet; it does not support
   PKIX Certificates at all.  As full RFC 7296 responders must also
   support that shared key authentication, this allows a minimal
   implementation to be able to interoperate with all implementations
   that are compliant with RFC 7296.

   PKIX Certificates are left out from the minimal implementation as
   those would add quite a lot of complexity to the implementation.  The
   actual code changes needed in the IKEv2 protocol are small, but the
   certificate validation code would be more complex than the whole



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   minimal IKEv2 implementation itself.  If public-key-based
   authentication is needed for scalability reasons, then raw public
   keys would probably be the best compromise (see Appendix B.2).

4.  Implementation Status

   This document describes a minimal implementation written by the
   author of this document.  The minimal implementation supported the
   base IKE_SA_INIT and IKE_AUTH exchanges and successfully
   interoperated with a full IKEv2 server.  This minimal implementation
   was presented in the Interconnecting Smart Objects with Internet
   Workshop in Prague in March 2011 [Kiv11].  This implementation was
   written as proof of concept in perl.

   There was another proof-of-concept implementation written in python,
   which also interoperated with a full IKEv2 server.

   Both implementations were written just for demonstration purposes and
   included fixed configuration built into the code, and both also
   implemented ESP, ICMP, and IP layers to the level that was needed to
   send and receive one ICMP echo packet.  Both implementations were
   about 1000 lines of code excluding cryptographic libraries but
   including ESP, ICMP, and IP layers.

5.  Security Considerations

   As this implements the same protocol as RFC 7296, this means all
   security considerations from it also apply to this document.























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6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

6.2.  Informative References

   [EAI]      Yang, A., Steele, S., and N. Freed, "Internationalized
              Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
              2012, <http://www.rfc-editor.org/info/rfc6532>.

   [IDNA]     Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [IKEV2IANA]
              IANA, "Internet Key Exchange Version 2 (IKEv2)
              Parameters",
              <http://www.iana.org/assignments/ikev2-parameters>.

   [IPSEARCH] Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <http://www.rfc-editor.org/info/rfc4301>.

   [Kiv11]    Kivinen, T., "Interconnecting Smart Objects with Internet
              Workshop 2011-03025; IKEv2 and Smart Objects", March 2011,
              <https://www.iab.org/wp-content/IAB-uploads/2011/04/
              Kivinen.pdf>.

   [MODES]    National Institute of Standards and Technology, U.S.
              Department of Commerce, "Recommendation for Block Cipher
              Modes of Operation", SP 800-38A, 2001.

   [PKCS1]    Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.




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   [RFC5280]  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, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

   [RFC7619]  Smyslov, V. and P. Wouters, "The NULL Authentication
              Method in the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,
              <http://www.rfc-editor.org/info/rfc7619>.

   [RFC7670]  Kivinen, T., Wouters, P., and H. Tschofenig, "Generic Raw
              Public-Key Support for IKEv2", RFC 7670,
              DOI 10.17487/RFC7670, January 2016,
              <http://www.rfc-editor.org/info/rfc7670>.



























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Appendix A.  Header and Payload Formats

   This appendix describes actual packet payload formats.  This is
   required to make the document self-contained.  The descriptions are
   mostly copied from RFC 7296, and more information can be found from
   there.

   Various payloads contain RESERVED fields, and those MUST be sent as
   zero and MUST be ignored on receipt.

   All multi-octet fields representing integers are laid out in big
   endian order (also known as "most significant byte first" or "network
   byte order").

A.1.  The IKE Header

   Each IKEv2 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 format of the IKE header is shown in Figure 1.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IKE SA Initiator's SPI                  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IKE SA Responder's SPI                  |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Payload | MjVer | MnVer | Exchange Type |     Flags     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Message ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Length                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1:  IKE Header Format




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   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.

   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 and MUST drop the messages with a
      higher major version number.

   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 zero.  They MUST ignore the minor
      version number of received messages.

   o  Exchange Type (1 octet) - Indicates the type of exchange being
      used.  This constrains the payloads sent in each message in an
      exchange.

      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|
        +-+-+-+-+-+-+-+-+











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      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 IKEv2 endpoint MUST NOT generate a response to a
         message that is marked as being a response.

      *  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 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 8 octets of the SPI were generated
         by the recipient.  This bit changes to reflect who initiated
         the last rekey of the IKE SA.

   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.

   o  Length (4 octets, unsigned integer) - Length of the total message
      (header + payloads) in octets.

A.2.  Generic Payload Header

   Each IKE payload begins with a generic payload header, as shown in
   Figure 2.  Figures for each payload below will include the generic
   payload header, but for brevity, the description of each field will
   be omitted.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2:  Generic Payload Header






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   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 zero.  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
      zero); conversely, the Next Payload field of the last contained
      payload is set to zero).  The payload type values needed for
      minimal implementations are listed here.

      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
      Traffic Selector - Initiator     TSi        44
      Traffic Selector - Responder     TSr        45
      Encrypted and Authenticated      SK         46

   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 1 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.

   o  Payload Length (2 octets, unsigned integer) - Length in octets of
      the current payload, including the generic payload header.




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A.3.  Security Association Payload

   The Security Association payload, denoted SA in this document, is
   used to negotiate attributes of a Security Association.

   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;
   currently, the only attribute is the Key Length attribute for
   variable-length ciphers, meaning there is exactly zero or one
   attribute).

   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.  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.

   Minimal implementation can create very simple SA proposal, i.e.,
   include one proposal, which contains exactly one transform for each
   Transform Type.  It is important to only include one Diffie-Hellman
   group in the proposal, so there is no need to do INVALID_KE_PAYLOAD
   processing in responses.

   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.

   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.



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   ESP generally has three: ESN, an encryption algorithm, and an
   integrity check algorithm.  IKEv2 generally has four transforms: a
   Diffie-Hellman group, an integrity check algorithm, a PRF algorithm,
   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.

   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.  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.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                          <Proposals>                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3:  Security Association Payload

   o  Proposals (variable) - One or more proposal substructures.

















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A.3.1.  Proposal Substructure

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 0 (last) or 2 |   RESERVED    |         Proposal Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Proposal Num  |  Protocol ID  |    SPI Size   |Num  Transforms|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                        SPI (variable)                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                        <Transforms>                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4:  Proposal Substructure

   o  0 (last) or 2 (more) (1 octet) - Specifies whether this is the
      last Proposal Substructure in the SA.

   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.  When a proposal is
      accepted, the proposal number in the SA payload MUST match the
      number on the proposal sent that was accepted.

   o  Protocol ID (1 octet) - Specifies the IPsec protocol identifier
      for the current negotiation.

      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 and 4 for ESP and
      AH).

   o  Num Transforms (1 octet) - Specifies the number of transforms in
      this proposal.




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   o  SPI (variable) - The sending entity's SPI.  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.

A.3.2.  Transform Substructure

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 0 (last) or 3 |   RESERVED    |        Transform Length       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Transform Type |   RESERVED    |          Transform ID         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                      Transform Attributes                     ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5:  Transform Substructure

   o  0 (last) or 3 (more) (1 octet) - Specifies whether this is the
      last Transform Substructure in the Proposal.

   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.











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   The relevant Transform Type values are listed below.  For more
   information see [RFC7296].

   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

   For Transform Type 1 (Encryption Algorithm), the relevant Transform
   IDs are listed below.

   Name                 Number
   ---------------------------
   ENCR_AES_CBC         12
   ENCR_AES-CCM_8       14

   For Transform Type 2 (Pseudorandom Function), the relevant Transform
   IDs are listed below.

   Name                        Number
   ----------------------------------
   PRF_HMAC_SHA1               2

   For Transform Type 3 (Integrity Algorithm), the relevant Transform
   IDs are listed below.

   Name                 Number
   ---------------------------
   AUTH_HMAC_SHA1_96    2
   AUTH_AES_XCBC_96     5

   For Transform Type 4 (Diffie-Hellman group), the relevant Transform
   IDs are listed below.

   Name               Number
   -------------------------
   1536-bit MODP      5
   2048-bit MODP      14









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   For Transform Type 5 (Extended Sequence Numbers), the relevant
   Transform IDs are listed below.

   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.

A.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

A.3.4.  Transform Attributes

   Transform Type 1 (Encryption Algorithm) transforms might include one
   transform attribute: Key Length.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|       Attribute Type        |        Attribute Value        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 6:  Data Attributes

   o  Attribute Type (15 bits) - Unique identifier for each type of
      attribute (see below).

   o  Attribute Value - Value of the attribute associated with the
      attribute type.



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   Attribute Type         Value
   ----------------------------
   Key Length (in bits)   14

   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.

   o  Some transforms specify that the Key Length attribute MUST be
      always included.  For example, ENCR_AES_CBC.

A.4.  Key Exchange Payload

                        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 7:  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 modular
   exponentiation groups (MODPs) 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.  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.

A.5.  Identification Payloads

   The Identification payloads, denoted IDi and IDr in this document,
   allow peers to assert an identity to one another.  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



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   of IDi/IDr are used purely to fetch the policy and authentication
   data related to the other party.  In minimal implementation, it might
   be easiest to always use KEY_ID type.  This allows the ID payload to
   be static.  Using an IP address has problems in environments where IP
   addresses are dynamically allocated.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   ID Type     |                 RESERVED                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                   Identification Data                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 8:  Identification Payload Format

   o  ID Type (1 octet) - Specifies the type of Identification being
      used.

   o  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 following table lists the assigned semantics for the
   Identification Type field.

   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 based [RFC5322].
      An example of an 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.



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   ID_IPV6_ADDR                        5
      A single sixteen (16) octet IPv6 address.

   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.  Minimal implementation might use
      this type to send out a serial number or similar device-specific
      unique static Identification Data for the device.

A.6.  Certificate Payload

                        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 9:  Certificate Payload Format

   o  Certificate Encoding (1 octet) - This field indicates the type of
      certificate or certificate-related information contained in the
      Certificate Data field.

      Certificate Encoding                 Value
      ----------------------------------------------------
      X.509 Certificate - Signature        4
      Raw Public Key                       15

   o  Certificate Data (variable length) - Actual encoding of
      certificate data.  The type of certificate is indicated by the
      Certificate Encoding field.

   The syntax of the types above 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.





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   o  "Raw Public Key" contains a raw public key.  In essence, the
      Certificate Payload contains the SubjectPublicKeyInfo part of the
      PKIX Certificate (see Section 4.1.2.7 of [RFC5280]).  This is a
      quite simple ASN.1 object that contains mostly static parts before
      the actual public key values.  See [RFC7670] for more information.

A.7.  Certificate Request Payload

                        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 (CA)               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 10:  Certificate Request Payload Format

   o  Certificate Encoding (1 octet) - Contains an encoding of the type
      or format of certificate requested.

   o  Certification Authority (variable length) - Contains an encoding
      of an acceptable certification authority for the type of
      certificate requested.

   The Certificate Encoding field has the same values as those defined
   by the certificate payload.  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.
   Each is encoded as the SHA-1 hash of the Subject Public Key Info
   element (see Section 4.1.2.7 of [RFC5280]) from each Trust Anchor
   certificate.  The 20-octet hashes are concatenated and included with
   no other formatting.














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A.8.  Authentication Payload

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Auth Method   |                RESERVED                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                      Authentication Data                      ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 11:  Authentication Payload Format

   o  Auth Method (1 octet) - Specifies the method of authentication
      used.

   Mechanism                              Value
   -----------------------------------------------------------------
   RSA Digital Signature                  1
      Using an RSA private key with an RSASSA-PKCS1-v1_5 signature
      scheme specified in [PKCS1]; see Section 2.15 of [RFC7296] for
      details.

   Shared Key Message Integrity Code      2
      Computed as specified earlier using the shared key associated
      with the identity in the ID payload and the negotiated PRF.

   o  Authentication Data (variable length) - see Section 2.1.

A.9.  Nonce Payload

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                            Nonce Data                         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 12:  Nonce Payload Format

   o  Nonce Data (variable length) - Contains the random data generated
      by the transmitting entity.



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   The size of the Nonce Data MUST be between 16 and 256 octets,
   inclusive.  Nonce values MUST NOT be reused.

A.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.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Protocol ID  |   SPI Size    |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                Security Parameter Index (SPI)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Notification Data                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 13:  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.  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 SPI field must be empty.

   o  Notify Message Type (2 octets) - Specifies the type of
      notification message.

   o  SPI (variable length) - Security Parameter Index.






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   o  Notification Data (variable length) - Status or error data
      transmitted in addition to the Notify Message Type.  Values for
      this field are type specific.

A.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.

   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.

   NOTIFY messages: error types              Value
   -------------------------------------------------------------------
   UNSUPPORTED_CRITICAL_PAYLOAD              1
       Indicates that the 1-octet payload type included in the
       Notification Data field is unknown.

   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
       Denial-of-Service (DoS) attack using forged messages, this
       status may only be returned for and in an encrypted packet if
       the Message ID and cryptographic checksum were valid.  To avoid
       leaking information to someone probing a node, this status MUST
       be sent in response to any error not covered by one of the other
       status types.  To aid debugging, more detailed error information
       should be written to a console or log.

   NO_PROPOSAL_CHOSEN                       14
       None of the proposed crypto suites was acceptable.  This can be
       sent in any case where the offered proposals are not acceptable
       for the responder.

   NO_ADDITIONAL_SAS                        35
       Specifies that the node is unwilling to accept any more Child
       SAs.



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   NOTIFY messages: status types            Value
   -------------------------------------------------------------------
   INITIAL_CONTACT                          16384
       Asserts that this IKE SA is the only IKE SA currently active
       between the authenticated identities.

A.11.  Traffic Selector Payload

   Traffic Selector (TS) payloads allow endpoints to communicate some of
   the information from their Security Policy Database (SPD) to their
   peers.  TS payloads specify the selection criteria for packets that
   will be forwarded over the newly set up SA.

                        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 14:  Traffic Selectors Payload Format

   o  Number of TSs (1 octet) - Number of Traffic Selectors being
      provided.

   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.

   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.

   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.





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   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.

   IKEv2 allows the responder to choose a subset of the traffic proposed
   by the initiator.

   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, if the
   initiator has requested the SA due to a data packet, the initiator
   SHOULD include as the first Traffic Selector in each TSi and TSr a
   very specific Traffic Selector including the addresses in the packet
   triggering the request.  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.

   As minimal implementations might only support one SA, the Traffic
   Selectors will usually be from the initiator's IP address to the
   responder's IP address (i.e., no port or protocol selectors and only
   one range).



















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A.11.1.  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 15: Traffic Selector

   o  TS Type (1 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 - 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.

   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.

   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).






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   The following table lists values for the Traffic Selector Type field
   and the corresponding Address Selector Data.

   TS Type                            Value
   -------------------------------------------------------------------
   TS_IPV4_ADDR_RANGE                  7
       A range of IPv4 addresses, represented by two 4-octet
       values.  The first value is the beginning IPv4 address
       (inclusive), and the second value is the ending IPv4 address
       (inclusive).  All addresses falling between the two specified
       addresses are considered to be within the list.

   TS_IPV6_ADDR_RANGE                  8
       A range of IPv6 addresses, represented by two 16-octet
       values.  The first value is the beginning IPv6 address
       (inclusive), and the second value is the ending IPv6 address
       (inclusive).  All addresses falling between the two specified
       addresses are considered to be within the list.

A.12.  Encrypted Payload

   The Encrypted payload, denoted as SK{...} in this document, contains
   other payloads in encrypted form.

                        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 the encryption algorithm)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    Encrypted IKE Payloads                     ~
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |             Padding (0-255 octets)            |
   +-+-+-+-+-+-+-+-+                               +-+-+-+-+-+-+-+-+
   |                                               |  Pad Length   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                    Integrity Checksum Data                    ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 16:  Encrypted Payload Format









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   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;
      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 Cipher Block Chaining (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 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.









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Appendix B.  Useful Optional Features

   There are some optional features of IKEv2, which might be useful for
   minimal implementations in some scenarios.  Such features include raw
   public keys authentication and sending an IKE SA delete notification.

B.1.  IKE SA Delete Notification

   In some scenarios, a minimal implementation device creates an IKE SA,
   sends one or few packets, perhaps gets some packets back, and then
   the device goes back to sleep, forgetting the IKE SA.  In such
   scenarios, it would be nice for the minimal implementation to send
   the IKE SA delete notification to tell the other end that the IKE SA
   is going away, so it can free the resources.

   Deleting the IKE SA can be done by sending one packet with a fixed
   Message ID and with only one payload inside the Encrypted payload.
   The other end will send back an empty response:

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
       Flags: Initiator, Message ID=2),
       SK {D}  -->

                      <--  HDR(SPIi=xxx, SPIr=yyy, INFORMATIONAL,
                               Flags: Response, Message ID=2),
                               SK {}

   The Delete payload format is:

                        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.





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   o  SPI Size (1 octet) - Length in octets of the SPI as defined by the
      protocol ID.  It MUST be zero for IKE (SPI is in the message
      header).

   o  Num of SPIs (2 octets, unsigned integer) - The number of SPIs
      contained in the Delete payload.  This MUST be zero for IKE.

   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.  This
      field is empty for the IKE SA delete.

B.2.  Raw Public Keys

   In some scenarios, the shared secret authentication is not safe
   enough, as anybody who knows the secret can impersonate the server.
   If the shared secret is printed on the side of the device, then
   anybody who gets physical access to the device can read it.  In such
   environments, public key authentication allows stronger
   authentication with minimal operational overhead.  Certificate
   support is quite complex, and minimal implementations do not usually
   have need for them.  Using Raw Public Keys is much simpler, and it
   scales similar to certificates.  The fingerprint of the raw public
   key can still be distributed by, for example, printing it on the side
   of the device allowing setup similar to using a shared secret.

   Raw public keys can also be used in a "leap of faith" or baby duck
   style initial setup, where the device imprints itself to the first
   device it sees when it boots up the first time.  After that initial
   connection, it stores the fingerprint of the Raw Public Key of the
   server in its own configuration and verifies that it never changes
   (unless a "reset to factory settings" or similar command is issued).

   This changes the initial IKE_AUTH payloads as follows:

   Initiator                         Responder
   -------------------------------------------------------------------
   HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH,
       Flags: Initiator, Message ID=1),
       SK {IDi, CERT, AUTH, SAi2, TSi, TSr,
           N(INITIAL_CONTACT)}  -->

                     <--  HDR(SPIi=xxx, SPIr=yyy, IKE_AUTH, Flags:
                                 Response, Message ID=1),
                                 SK {IDr, CERT, AUTH, SAr2, TSi, TSr}






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   The CERT payloads contain the raw public keys used to sign the hash
   of the InitiatorSignedOctects/ResponderSignedOctects when generating
   an AUTH payload.  Minimal implementations should use SHA-1 as the
   hash function as that is the "SHOULD" support algorithm specified in
   RFC 7296, so it is the most likely one that is supported by all
   devices.

   Note that RFC 7296 already obsoleted the old Raw RSA Key method, and
   "Generic Raw Public-Key Support for IKEv2" [RFC7670] adds a new
   format to allow using any types of raw public keys with IKEv2.  This
   document only specifies how to use the new format.

   In these setups, it might be possible that authenticating the server
   is not needed at all.  If a minimal device is sending, for example,
   sensor information to the server, the server wants to verify that the
   sensor is who it claims to be using raw public keys, but the sensor
   does not really care who the server is.  In such cases, the NULL
   authentication method [RFC7619] would be useful, as it allows devices
   to do one-way authentication.

Acknowledgements

   Most of the content of this document is copied from RFC 7296.

Author's Address

   Tero Kivinen
   INSIDE Secure
   Eerikinkatu 28
   HELSINKI  FI-00180
   FINLAND

   Email: kivinen@iki.fi


















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