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IPSEC Working Group                                   Charlie Kaufman
INTERNET-DRAFT                                                 editor
draft-ietf-ipsec-ikev2-08.txt                                May 2003


                 Internet Key Exchange (IKEv2) Protocol
                    <draft-ietf-ipsec-ikev2-08.txt>


                          Status of this Memo

   This document is a submission by the IPSEC Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the ipsec@lists.tislabs.com mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [Bra96]. Internet Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and working groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet Drafts Shadow
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   munnari.oz.au (Australia), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


Abstract

   This document describes version 2 of the IKE (Internet Key Exchange)
   protocol.  IKE is a component of IPsec used for performing mutual
   authentication and establishing and maintaining security
   associations.

   This version of IKE simplifies the design by removing options that
   were rarely used and simplifying the encoding. This version of the
   IKE specification combines the contents of what were previously
   separate documents, including ISAKMP (RFC 2408), IKE (RFC 2409), the
   Internet DOI (RFC 2407), NAT Traversal, Legacy authentication, and
   remote address acquisition.



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   Version 2 of IKE does not interoperate with version 1, but it has
   enough of the header format in common that both versions can
   unambiguously run over the same UDP port.

Table of Contents


   Abstract.....................................................1
   Requirements Terminology.....................................3
   1 IKE Protocol Overview......................................3
   1.1 Usage Scenarios..........................................5
   1.1.1 Security Gateway to Security Gateway Tunnel............5
   1.1.2 Endpoint to Endpoint Transport.........................6
   1.1.3 Endpoint to Security Gateway Transport.................6
   1.1.4 Other Scenarios........................................7
   1.2 The Initial Exchange.....................................7
   1.3 The CREATE_CHILD_SA Exchange.............................9
   1.4 The INFORMATIONAL Exchange..............................10
   1.5 Informational Messages outside of an IKE_SA.............12
   2 IKE Protocol Details and Variations.......................12
   2.1 Use of Retransmission Timers............................12
   2.2 Use of Sequence Numbers for Message ID..................13
   2.3 Window Size for overlapping requests....................13
   2.4 State Synchronization and Connection Timeouts...........14
   2.5 Version Numbers and Forward Compatibility...............16
   2.6 Cookies.................................................17
   2.7 Cryptographic Algorithm Negotiation.....................19
   2.8 Rekeying................................................20
   2.9 Traffic Selector Negotiation............................22
   2.10 Nonces.................................................24
   2.11 Address and Port Agility...............................24
   2.12 Reuse of Diffie-Hellman Exponentials...................24
   2.13 Generating Keying Material.............................25
   2.14 Generating Keying Material for the IKE_SA..............26
   2.15 Authentication of the IKE_SA...........................27
   2.16 Extended Authentication Protocol Methods...............28
   2.17 Generating Keying Material for CHILD_SAs...............29
   2.18 Rekeying IKE_SAs using a CREATE_CHILD_SA exchange......30
   2.19 Requesting an internal address on a remote network.....31
   2.20 Requesting a Peer's Version............................32
   2.21 Error Handling.........................................33
   2.22 IPcomp.................................................34
   2.23 NAT Traversal..........................................34
   2.24 ECN Notification.......................................36
   3 Header and Payload Formats................................37
   3.1 The IKE Header..........................................37
   3.2 Generic Payload Header..................................40
   3.3 Security Association Payload............................42



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   3.3.1 Proposal Substructure.................................44
   3.3.2 Transform Substructure................................45
   3.3.3 Mandatory Transform Types.............................48
   3.3.4 Mandatory Transform IDs...............................48
   3.3.5 Transform Attributes..................................49
   3.3.6 Attribute Negotiation.................................52
   3.4 Key Exchange Payload....................................53
   3.5 Identification Payloads.................................53
   3.6 Certificate Payload.....................................55
   3.7 Certificate Request Payload.............................57
   3.8 Authentication Payload..................................58
   3.9 Nonce Payload...........................................59
   3.10 Notify Payload.........................................60
   3.10.1 Notify Message Types.................................62
   3.11 Delete Payload.........................................67
   3.12 Vendor ID Payload......................................68
   3.13 Traffic Selector Payload...............................69
   3.13.1 Traffic Selector.....................................70
   3.14 Encrypted Payload......................................72
   3.15 Configuration Payload..................................73
   3.15.1 Configuration Attributes.............................76
   3.16 Extended Authentication Protocol (EAP) Payload.........78
   4 Conformance Requirements..................................80
   5 Security Considerations...................................82
   6 IANA Considerations.......................................83
   7 Intellectual property rights..............................84
   8 Acknowledgements..........................................84
   9 References................................................84
   9.1 Normative References....................................84
   9.2 Non-normative References................................85
   Appendix A: Summary of Changes from IKEv1...................88
   Appendix B: Diffie-Hellman Groups...........................90
   Change History..............................................93
   Editor's Address............................................97
   Full Copyright Statement....................................97

Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
   "MAY" that appear in this document are to be interpreted as described
   in [Bra97].

1 IKE Protocol Overview

   IP Security (IPsec) provides confidentiality, data integrity, access
   control, and data source authentication to IP datagrams. These
   services are provided by maintaining shared state between the source
   and the sink of an IP datagram. This state defines, among other



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   things, the specific services provided to the datagram, which
   cryptographic algorithms will be used to provide the services, and
   the keys used as input to the cryptographic algorithms.

   Establishing this shared state in a manual fashion does not scale
   well.  Therefore a protocol to establish this state dynamically is
   needed.  This memo describes such a protocol-- the Internet Key
   Exchange (IKE).  This is version 2 of IKE. Version 1 of IKE was
   defined in RFCs 2407, 2408, and 2409. This single document is
   intended to replace all three of those RFCs.

   IKE performs mutual authentication between two parties and
   establishes an IKE security association that includes shared secret
   information that can be used to efficiently establish SAs for ESP
   [RFC2406] and/or AH [RFC2402] and a set of cryptographic algorithms
   to be used to protect the SAs.  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 IPcomp [RFC2393] in
   connection with an ESP and/or AH SA.  We call the IKE SA an "IKE_SA".
   The SAs for ESP and/or AH that get set up through that IKE_SA we call
   "CHILD_SA"s.

   All IKE communications consist of pairs of messages: a request and a
   response. The pair is called an "exchange".  We call the first
   messages establishing an IKE_SA IKE_SA_INIT and IKE_AUTH exchanges
   and subsequent IKE exchanges CREATE_CHILD_SA or INFORMATIONAL
   exchanges. In the common case, there is a single IKE_SA_INIT exchange
   and a single IKE_AUTH exchange (a total of four messages) to
   establish the IKE_SA and the first CHILD_SA. In exceptional cases,
   there may be more than one of each of these exchanges. In all cases,
   all IKE_SA_INIT exchanges MUST complete before any other exchange
   type, then all IKE_AUTH exchanges MUST complete, and following that
   any number of CREATE_CHILD_SA and INFORMATIONAL exchanges may occur
   in any order.  In some scenarios, only a single CHILD_SA is needed
   between the IPsec endpoints and therefore there would be no
   additional exchanges. Subsequent exchanges MAY be used to establish
   additional CHILD_SAs between the same authenticated pair of endpoints
   and to perform housekeeping functions.

   IKE message flow always consists of a request followed by a response.
   It is the responsibility of the requester to ensure reliability.  If
   the response is not received within a timeout interval, the requester
   needs to retransmit the request (or abandon the connection).

   The first request/response of an IKE session negotiates security
   parameters for the IKE_SA, sends nonces, and sends Diffie-Hellman



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   values. We call the initial exchange IKE_SA_INIT (request and
   response).

   The second request/response, which we'll call IKE_AUTH transmits
   identities, proves knowledge of the secrets corresponding to the two
   identities, and sets up an SA for the first (and often only) AH
   and/or ESP CHILD_SA.

   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 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 should errors occur are described in
   section 2.21.

1.1 Usage Scenarios

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

1.1.1 Security Gateway to Security Gateway Tunnel

                    +-+-+-+-+-+            +-+-+-+-+-+
                    !         ! IPsec      !         !
       Protected    !Tunnel   ! Tunnel     !Tunnel   !     Protected
       Subnet   <-->!Endpoint !<---------->!Endpoint !<--> Subnet
                    !         !            !         !
                    +-+-+-+-+-+            +-+-+-+-+-+

             Figure 1:  Security Gateway to Security Gateway Tunnel

   In this scenario, neither endpoint of the IP connection implements
   IPsec, but network nodes between them protect traffic for part of the
   way. Protection is transparent to the endpoints, and depends on
   ordinary routing sending packets through the tunnel endpoints for
   processing. Each endpoint would announce the set of addresses
   "behind" it, and packets would be sent in Tunnel Mode where the inner
   IP header would contain the IP addresses of the actual endpoints.









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1.1.2 Endpoint to Endpoint Transport

       +-+-+-+-+-+                                          +-+-+-+-+-+
       !         !                 IPsec                    !         !
       !Protected!                 Tunnel                   !Protected!
       !Endpoint !<---------------------------------------->!Endpoint !
       !         !                                          !         !
       +-+-+-+-+-+                                          +-+-+-+-+-+

                       Figure 2:  Endpoint to Endpoint

   In this scenario, both endpoints of the IP connection implement
   IPsec. These endpoints may implement application layer access
   controls based on the authenticated identities of the participants.
   Transport mode will commonly be used with no inner IP header. If
   there is an inner IP header, the inner addresses will be the same as
   the outer addresses. A single pair of addresses will be negotiated
   for packets to be sent over this SA.

   It is possible in this scenario that one or both of the protected
   endpoints will be behind a network address translation (NAT) node, in
   which case the tunnelled packets will have to be UDP encapsulated so
   that port numbers in the UDP headers can be used to identify
   individual endpoints "behind" the NAT.

1.1.3 Endpoint to Security Gateway Transport

       +-+-+-+-+-+                          +-+-+-+-+-+
       !         !         IPsec            !         !     Protected
       !Protected!         Tunnel           !Tunnel   !     Subnet
       !Endpoint !<------------------------>!Endpoint !<--- and/or
       !         !                          !         !     Internet
       +-+-+-+-+-+                          +-+-+-+-+-+

                 Figure 3:  Endpoint to Security Gateway Tunnel

   In this scenario, a protected endpoint (typically a portable roaming
   computer) connects back to its corporate network through an IPsec
   protected tunnel. It might use this tunnel only to access information
   on the corporate network or it might tunnel all of its traffic back
   through the corporate network in order to take advantage of
   protection provided by a corporate firewall against Internet based
   attacks. In either case, the protected endpoint will want an IP
   address associated with the security gateway so that packets returned
   to it will go to the security gateway and be tunnelled back. This IP
   address may be static or may be dynamically allocated by the security
   gateway. In support of the latter case, IKEv2 includes a mechanism
   for the initiator to request an IP address owned by the security



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   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.

1.1.4 Other Scenarios

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

1.2 The Initial Exchanges

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

   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



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   the messages are authenticated.

   In the following description, the payloads contained in the message
   are indicated by names such as SA. The details of the contents of
   each payload are described later. Payloads which may optionally
   appear will be shown in brackets, such as [CERTREQ], would indicate
   that optionally a certificate request payload can be included.

   The initial exchanges are as follows:

       Initiator                          Responder
      -----------                        -----------
       HDR, SAi1, KEi, Ni   -->

   HDR contains the SPIs, version numbers, and flags of various sorts.
   The SAi1 payload states the cryptographic algorithms the Initiator
   supports for the IKE_SA.  The KE payload sends the Initiator's
   Diffie-Hellman value. Ni is the Initiator's nonce.

                            <--    HDR, SAr1, KEr, Nr, [CERTREQ]

   The Responder chooses a cryptographic suite from the Initiator's
   offered choices and expresses that choice in the SAr1 payload,
   completes the Diffie-Hellman exchange with the KEr payload, and sends
   its nonce in the Nr payload.

   At this point in the negotiation each party can generate SKEYSEED,
   from which all keys are derived for that IKE_SA.  All but the headers
   of all the messages that follow are encrypted and integrity
   protected.  The keys used for the encryption and integrity protection
   are derived from SKEYSEED and are known as SK_e (encryption) and SK_a
   (authentication, a.k.a.  integrity protection). A separate SK_e and
   SK_a is computed for each direction.  In addition to the keys SK_e
   and SK_a derived from the DH value for protection of the IKE_SA,
   another quantity SK_d is derived and used for derivation of further
   keying material for CHILD_SAs.  The notation SK { ... } indicates
   that these payloads are encrypted and integrity protected using that
   direction's SK_e and SK_a.

       HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
                  AUTH, SAi2, TSi, TSr}     -->

   The Initiator asserts her identity with the IDi payload, proves
   knowledge of the secret corresponding to IDi and integrity protects
   the contents of the first two messages using the AUTH payload (see
   section 2.15).  She might also send her certificate(s) in CERT
   payload(s) and a list of her trust anchors in CERTREQ payload(s). If
   any CERT payloads are included, the first certificate provided MUST



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   contain the public key used to verify the AUTH field.  The optional
   payload IDr enables Alice to specify which of Bob's identities she
   wants to talk to. This is useful when Bob is hosting multiple
   identities at the same IP address.  She begins negotiation of a
   CHILD_SA using the SAi2 payload. The final fields (starting with
   SAi2) are described in the description of the CREATE_CHILD_SA
   exchange.

                                   <--    HDR, SK {IDr, [CERT,] AUTH,
                                                SAr2, TSi, TSr}

   The Responder asserts his identity with the IDr payload, optionally
   sends one or more certificates (again with the certificate containing
   the public key used to verify AUTH listed first), authenticates his
   identity with the AUTH payload, and completes negotiation of a
   CHILD_SA with the additional fields described below in the
   CREATE_CHILD_SA exchange.

   The recipients of messages 3 and 4 MUST verify that all signatures
   and MACs are computed correctly and that the names in the ID payloads
   correspond to the keys used to generate the AUTH payload.

1.3 The CREATE_CHILD_SA Exchange

   This exchange consists of a single request/response pair, and was
   referred to as a phase 2 exchange in IKEv1. It MAY be initiated by
   either end of the IKE_SA after the initial exchanges are completed.

   All messages following the initial exchange are cryptographically
   protected using the cryptographic algorithms and keys negotiated in
   the first two messages of the IKE exchange using a syntax described
   in section 3.14.

   Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
   section the term Initiator refers to the endpoint initiating this
   exchange.

   A CHILD_SA is created by sending a CREATE_CHILD_SA request.  The
   CREATE_CHILD_SA request MAY optionally contain a KE payload for an
   additional Diffie-Hellman exchange to enable stronger guarantees of
   forward secrecy for the CHILD_SA. The keying material for the
   CHILD_SA is a function of SK_d established during the establishment
   of the IKE_SA, the nonces exchanged during the CREATE_CHILD_SA
   exchange, and the Diffie-Hellman value (if KE payloads are included
   in the CREATE_CHILD_SA exchange).

   In the CHILD_SA created as part of the initial exchange, a second KE
   payload and nonce MUST NOT be sent. The nonces from the initial



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   exchange are used in computing the keys for the CHILD_SA.

   The CREATE_CHILD_SA request contains:

       Initiator                                 Responder
      -----------                               -----------
       HDR, SK {SA, Ni, [KEi],
           [TSi, TSr]}             -->

   The Initiator sends SA offer(s) in the SA payload, a nonce in the Ni
   payload, optionally a Diffie-Hellman value in the KEi payload, and
   the proposed traffic selectors in the TSi and TSr payloads. If the SA
   offers include different Diffie-Hellman groups, KEi MUST be an
   element of the group the Initiator expects the responder to accept.
   If she guesses wrong, the CREATE_CHILD_SA exchange will fail and she
   will have to retry with a different KEi.

   The message following the header is encrypted and the message
   including the header is integrity protected using the cryptographic
   algorithms negotiated for the IKE_SA.

   The CREATE_CHILD_SA response contains:

                                  <--    HDR, SK {SA, Nr, [KEr],
                                               [TSi, TSr]}

   The Responder replies (using the same Message ID to respond) with the
   accepted offer in an SA payload, and a Diffie-Hellman value in the
   KEr payload if KEi was included in the request and the selected
   cryptographic suite includes that group.  If the responder chooses a
   cryptographic suite with a different group, it MUST reject the
   request. The initiator SHOULD repeat the request, but now with a KEi
   payload from the group the responder selected.

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

1.4 The INFORMATIONAL Exchange

   At various points during the operation of an IKE_SA, peers may desire
   to convey control messages to each other regarding errors or
   notifications of certain events. To accomplish this IKE defines an
   INFORMATIONAL exchange.  INFORMATIONAL exchanges MAY ONLY occur after
   the initial exchanges and are cryptographically protected with the
   negotiated keys.



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   Control messages that pertain to an IKE_SA MUST be sent under that
   IKE_SA. Control messages that pertain to CHILD_SAs MUST be sent under
   the protection of the IKE_SA which generated them (or its successor
   if the IKE_SA was replaced for the purpose of rekeying).

   Messages in an INFORMATIONAL Exchange contain zero or more
   Notification, Delete, and Configuration payloads. The Recipient of an
   INFORMATIONAL Exchange request MUST send some response (else the
   Sender will assume the message was lost in the network and will
   retransmit it). That response MAY be a message with no payloads. The
   request message in an INFORMATIONAL Exchange MAY also contain no
   payloads. This is the expected way an endpoint can ask the other
   endpoint to verify that it is alive.

   ESP and AH SAs always exist in pairs, with one SA in each direction.
   When an SA is closed, both members of the pair MUST be closed. When
   SAs are nested, as when data (and IP headers if in tunnel mode) are
   encapsulated first with IPcomp, then with ESP, and finally with AH
   between the same pair of endpoints, all of the SAs MUST be deleted
   together. Each endpoint MUST close its incoming SAs and allow the
   other endpoint to close the other SA in each pair. To delete an SA,
   an INFORMATIONAL Exchange with one or more delete payloads is sent
   listing the SPIs (as they would be expected in the headers of inbound
   packets) of the SAs to be deleted. The recipient MUST close the
   designated SAs. Normally, the reply in the INFORMATIONAL Exchange
   will contain delete payloads for the paired SAs going in the other
   direction. There is one exception.  If by chance both ends of a set
   of SAs independently decide to close them, each may send a delete
   payload and the two requests may cross in the network. If a node
   receives a delete request for SAs for which it has already issued a
   delete request, it MUST delete the outgoing SAs while processing the
   request and the incoming SAs while processing the response. In that
   case, the responses MUST NOT include delete payloads for the deleted
   SAs, since that would result in duplicate deletion and could in
   theory delete the wrong SA.

   A node SHOULD regard half closed connections as anomalous and audit
   their existence should they persist. Note that this specification
   nowhere specifies time periods, so it is up to individual endpoints
   to decide how long to wait. A node MAY refuse to accept incoming data
   on half closed connections but MUST NOT unilaterally close them and
   reuse the SPIs. If connection state becomes sufficiently messed up, a
   node MAY close the IKE_SA which will implicitly close all SAs
   negotiated under it. It can then rebuild the SAs it needs on a clean
   base under a new IKE_SA.

   The INFORMATIONAL Exchange is defined as:




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       Initiator                        Responder
      -----------                      -----------
       HDR, SK {[N,] [D,] [CP,] ...} -->
                                   <-- HDR, SK {[N,] [D,] [CP], ...}

   The processing of an INFORMATIONAL Exchange is determined by its
   component payloads.

1.5 Informational Messages outside of an IKE_SA

   If a packet arrives with an unrecognized SPI, it could be because the
   receiving node has recently crashed and lost state or because of some
   other system malfunction or attack. If the receiving node has an
   active IKE_SA to the IP address from whence the packet came, it MAY
   send a notification of the wayward packet over that IKE_SA.  If it
   does not, it MAY send an Informational message without cryptographic
   protection to the source IP address and port to alert it to a
   possible problem.

2 IKE Protocol Details and Variations

   IKE normally listens and sends on UDP port 500, though IKE messages
   may also be received on UDP port 4500 with a slightly different
   format (see section 2.23).  Since UDP is a datagram (unreliable)
   protocol, IKE includes in its definition recovery from transmission
   errors, including packet loss, packet replay, and packet forgery. IKE
   is designed to function so long as (1) at least one of a series of
   retransmitted packets reaches its destination before timing out; and
   (2) the channel is not so full of forged and replayed packets so as
   to exhaust the network or CPU capacities of either endpoint. Even in
   the absence of those minimum performance requirements, IKE is
   designed to fail cleanly (as though the network were broken).

2.1 Use of Retransmission Timers

   All messages in IKE exist in pairs: a request and a response.  The
   setup of an IKE_SA normally consists of two request/response pairs.
   Once the IKE_SA is set up, either end of the security association may
   initiate requests at any time, and there can be many requests and
   responses "in flight" at any given moment. But each message is
   labelled as either a request or a response and for each
   request/response pair one end of the security association is the
   Initiator and the other is the Responder.

   For every pair of messages, the Initiator is responsible for
   retransmission in the event of a timeout. The Responder MUST never
   retransmit a response unless it receives a retransmission of the
   request. In that event, the Responder MUST ignore the retransmitted



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   request except insofar as it triggers a retransmission of the
   response. The Initiator MUST remember each request until it receives
   the corresponding response. The Responder MUST remember each response
   until it receives a request whose sequence number is larger than the
   sequence number in the response plus his window size (see section
   2.3).

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

2.2 Use of Sequence Numbers for Message ID

   Every IKE message contains a Message ID as part of its fixed header.
   This Message ID is used to match up requests and responses, and to
   identify retransmissions of messages.

   The Message ID is a 32 bit quantity, which is zero for the first IKE
   request in each direction. The IKE_SA initial setup messages will
   always be numbered 0 and 1.  Each endpoint in the IKE Security
   Association maintains two "current" Message IDs: the next one to be
   used for a request it initiates and the next one it expects to see in
   a request from the other end. These counters increment as requests
   are generated and received. Responses always contain the same message
   ID as the corresponding request. That means that after the initial
   exchange, each integer n may appear as the message ID in four
   distinct messages: The nth request from the original IKE Initiator,
   the corresponding response, the nth request from the original IKE
   Responder, and the corresponding response. If the two ends make very
   different numbers of requests, the Message IDs in the two directions
   can be very different. There is no ambiguity in the messages,
   however, because the (I)nitiator and (R)esponse bits in the message
   header specify which of the four messages a particular one is.

   Note that Message IDs are cryptographically protected and provide
   protection against message replays. In the unlikely event that
   Message IDs grow too large to fit in 32 bits, the IKE_SA MUST be
   closed. Rekeying an IKE_SA resets the sequence numbers.

2.3 Window Size for overlapping requests

   In order to maximize IKE throughput, an IKE endpoint MAY issue
   multiple requests before getting a response to any of them if the
   other endpoint has indicated its ability to handle such requests. For
   simplicity, an IKE implementation MAY choose to process requests
   strictly in order and/or wait for a response to one request before



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   issuing another. Certain rules must be followed to assure
   interoperability between implementations using different strategies.

   After an IKE_SA is set up, either end can initiate one or more
   requests. These requests may pass one another over the network. An
   IKE endpoint MUST be prepared to accept and process a request while
   it has a request outstanding in order to avoid a deadlock in this
   situation. An IKE endpoint SHOULD be prepared to accept and process
   multiple requests while it has a request outstanding.

   An IKE endpoint MUST wait for a response to each of its messages
   before sending a subsequent message unless it has received a
   SET_WINDOW_SIZE Notify message from its peer informing it that the
   peer is prepared to maintain state for multiple outstanding messages
   in order to allow greater throughput.

   An IKE endpoint MUST NOT exceed the peer's stated window size for
   transmitted IKE requests. In other words, if Bob stated his window
   size is N, then when Alice needs to make a request X, she MUST wait
   until she has received responses to all requests up through request
   X-N. An IKE endpoint MUST keep a copy of (or be able to regenerate
   exactly) each request it has sent until it receives the corresponding
   response. An IKE endpoint MUST keep a copy of (or be able to
   regenerate exactly) the number of previous responses equal to its
   declared window size in case its response was lost and the Initiator
   requests its retransmission by retransmitting the request.

   An IKE endpoint supporting a window size greater than one SHOULD be
   capable of processing incoming requests out of order to maximize
   performance in the event of network failures or packet reordering.

2.4 State Synchronization and Connection Timeouts

   An IKE endpoint is allowed to forget all of its state associated with
   an IKE_SA and the collection of corresponding CHILD_SAs at any time.
   This is the anticipated behavior in the event of an endpoint crash
   and restart. It is important when an endpoint either fails or
   reinitializes its state that the other endpoint detect those
   conditions and not continue to waste network bandwidth by sending
   packets over discarded SAs and having them fall into a black hole.

   Since IKE is designed to operate in spite of Denial of Service (DoS)
   attacks from the network, an endpoint MUST NOT conclude that the
   other endpoint has failed based on any routing information (e.g.,
   ICMP messages) or IKE messages that arrive without cryptographic
   protection (e.g., Notify messages complaining about unknown SPIs). An
   endpoint MUST conclude that the other endpoint has failed only when
   repeated attempts to contact it have gone unanswered for a timeout



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   period or when a cryptographically protected INITIAL_CONTACT
   notification is received on a different IKE_SA to the same
   authenticated identity. An endpoint SHOULD suspect that the other
   endpoint has failed based on routing information and initiate a
   request to see whether the other endpoint is alive. To check whether
   the other side is alive, IKE specifies an empty INFORMATIONAL message
   that (like all IKE requests) requires an acknowledgment. If a
   cryptographically protected message has been received from the other
   side recently, unprotected notifications MAY be ignored.
   Implementations MUST limit the rate at which they take actions based
   on unprotected messages.

   Numbers of retries and lengths of timeouts are not covered in this
   specification because they do not affect interoperability. It is
   suggested that messages be retransmitted at least a dozen times over
   a period of at least several minutes before giving up on an SA, but
   different environments may require different rules. If there has only
   been outgoing traffic on all of the SAs associated with an IKE_SA, it
   is essential to confirm liveness of the other endpoint to avoid black
   holes. If no cryptographically protected messages have been received
   on an IKE_SA or any of its CHILD_SAs recently, the system needs to
   perform a liveness check in order to prevent sending messages to a
   dead peer. Receipt of a fresh cryptographically protected message on
   an IKE_SA or any of its CHILD_SAs assures liveness of the IKE_SA and
   all of its CHILD_SAs. Note that this places requirements on the
   failure modes of an IKE endpoint. An implementation MUST NOT continue
   sending on any SA if some failure prevents it from receiving on all
   of the associated SAs. If CHILD_SAs can fail independently from one
   another without the associated IKE_SA being able to send a delete
   message, then they MUST be negotiated by separate IKE_SAs.

   There is a Denial of Service attack on the Initiator of an IKE_SA
   that can be avoided if the Initiator takes the proper care. Since the
   first two messages of an SA setup are not cryptographically
   protected, an attacker could respond to the Initiator's message
   before the genuine Responder and poison the connection setup attempt.
   To prevent this, the Initiator MAY be willing to accept multiple
   responses to its first message, treat each as potentially legitimate,
   respond to it, and then discard all the invalid half open connections
   when she receives a valid cryptographically protected response to any
   one of her requests.  Once a cryptographically valid response is
   received, all subsequent responses should be ignored whether or not
   they are cryptographically valid.

   Note that with these rules, there is no reason to negotiate and agree
   upon an SA lifetime. If IKE presumes the partner is dead, based on
   repeated lack of acknowledgment to an IKE message, then the IKE SA
   and all CHILD_SAs set up through that IKE_SA are deleted.



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   An IKE endpoint may at any time delete inactive CHILD_SAs to recover
   resources used to hold their state. If an IKE endpoint chooses to do
   so, it MUST send Delete payloads to the other end notifying it of the
   deletion. It MAY similarly time out the IKE_SA. Closing the IKE_SA
   implicitly closes all associated CHILD_SAs. In this case, an IKE
   endpoint SHOULD send a Delete payload indicating that it has closed
   the IKE_SA.

2.5 Version Numbers and Forward Compatibility

   This document describes version 2.0 of IKE, meaning the major version
   number is 2 and the minor version number is zero. It is likely that
   some implementations will want to support both version 1.0 and
   version 2.0, and in the future, other versions.

   The major version number should only be incremented if the packet
   formats or required actions have changed so dramatically that an
   older version node would not be able to interoperate with a newer
   version node if it simply ignored the fields it did not understand
   and took the actions specified in the older specification. The minor
   version number indicates new capabilities, and MUST be ignored by a
   node with a smaller minor version number, but used for informational
   purposes by the node with the larger minor version number. For
   example, it might indicate the ability to process a newly defined
   notification message. The node with the larger minor version number
   would simply note that its correspondent would not be able to
   understand that message and therefore would not send it.

   If an endpoint receives a message with a higher major version number,
   it MUST drop the message and SHOULD send an unauthenticated
   notification message containing the highest version number it
   supports.  If an endpoint supports major version n, and major version
   m, it MUST support all versions between n and m. If it receives a
   message with a major version that it supports, it MUST respond with
   that version number. In order to prevent two nodes from being tricked
   into corresponding with a lower major version number than the maximum
   that they both support, IKE has a flag that indicates that the node
   is capable of speaking a higher major version number.

   Thus the major version number in the IKE header indicates the version
   number of the message, not the highest version number that the
   transmitter supports. If A is capable of speaking versions n, n+1,
   and n+2, and B is capable of speaking versions n and n+1, then they
   will negotiate speaking n+1, where A will set the flag indicating
   ability to speak a higher version. If they mistakenly (perhaps
   through an active attacker sending error messages) negotiate to
   version n, then both will notice that the other side can support a
   higher version number, and they MUST break the connection and



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   reconnect using version n+1.

   Note that IKEv1 does not follow these rules, because there is no way
   in v1 of noting that you are capable of speaking a higher version
   number. So an active attacker can trick two v2-capable nodes into
   speaking v1.  When a v2-capable node negotiates down to v1, it SHOULD
   note that fact in its logs.

   Also for forward compatibility, all fields marked RESERVED MUST be
   set to zero by a version 2.0 implementation and their content MUST be
   ignored by a version 2.0 implementation ("Be conservative in what you
   send and liberal in what you receive"). In this way, future versions
   of the protocol can use those fields in a way that is guaranteed to
   be ignored by implementations that do not understand them.
   Similarly, payload types that are not defined are reserved for future
   use and implementations of version 2.0 MUST skip over those payloads
   and ignore their contents.

   IKEv2 adds a "critical" flag to each payload header for further
   flexibility for forward compatibility. If the critical flag is set
   and the payload type is unrecognized, the message MUST be rejected
   and the response to the IKE request containing that payload MUST
   include a Notify payload UNSUPPORTED_CRITICAL_PAYLOAD, indicating an
   unsupported critical payload was included. If the critical flag is
   not set and the payload type is unsupported, that payload MUST be
   ignored.

   While new payload types may be added in the future and may appear
   interleaved with the fields defined in this specification,
   implementations MUST send the payloads defined in this specification
   in the order shown in the figures in section 2 and implementations
   SHOULD reject as invalid a message with those payloads in any other
   order.

2.6 Cookies

   The term "cookies" originates with Karn and Simpson [RFC 2522] in
   Photuris, an early proposal for key management with IPsec.  It has
   persisted because the IETF has never rejected a proposal involving
   cookies. The ISAKMP fixed message header includes two eight octet
   fields titled "cookies", and that syntax is used by both IKEv1 and
   IKEv2 though in IKEv2 they are referred to as the IKE SPI and there
   is a new separate field in a Notify payload holding the cookie. The
   initial two eight octet fields in the header are used as a connection
   identifier at the beginning of IKE packets. Each endpoint chooses one
   of the two SPIs and SHOULD choose them so as to be unique identifiers
   of an IKE_SA. An SPI value of zero is special and indicates that the
   remote SPI value is not yet known by the sender.



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   Unlike ESP and AH where only the recipient's SPI appears in the
   header of a message, in IKE the sender's SPI is also sent in every
   message. Since the SPI chosen by the original initiator of the IKE_SA
   is always sent first, an endpoint with multiple IKE_SAs open that
   wants to find the appropriate IKE_SA using the SPI it assigned must
   look at the I(nitiator) Flag bit in the header to determine whether
   it assigned the first or the second eight octets.

   In the first message of an initial IKE exchange, the initiator will
   not know the responder's SPI value and will therefore set that field
   to zero.

   An expected attack against IKE is state and CPU exhaustion, where the
   target is flooded with session initiation requests from forged IP
   addresses. This attack can be made less effective if an
   implementation of a responder uses minimal CPU and commits no state
   to an SA until it knows the initiator can receive packets at the
   address from which he claims to be sending them. To accomplish this,
   a responder SHOULD - when it detects a large number of half-open
   IKE_SAs - reject initial IKE messages unless they contain a Notify
   payload of type COOKIE. It SHOULD instead send an unprotected IKE
   message as a response and include COOKIE Notify payload with the
   cookie data to be returned.  Initiators who receive such responses
   MUST retry the IKE_SA_INIT with a Notify payload of type COOKIE
   containing the responder supplied cookie data as the first payload.
   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



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   the SPI assigned by the initiator, while 'B' is the SPI assigned by
   the responder.

   An IKE implementation SHOULD implement its responder cookie
   generation in such a way as to not require any saved state to
   recognize its valid cookie when the second IKE_SA_INIT message
   arrives.  The exact algorithms and syntax they use to generate
   cookies does not affect interoperability and hence is not specified
   here. The following is an example of how an endpoint could use
   cookies to implement limited DOS protection.

   A good way to do this is to set the responder cookie to be:

      Cookie = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)

   where <secret> is a randomly generated secret known only to the
   responder and periodically changed. <VersionIDofSecret> should be
   changed whenever <secret> is regenerated.  The cookie can be
   recomputed when the IKE_SA_INIT arrives the second time and compared
   to the cookie in the received message. If it matches, the responder
   knows that SPIr was generated since the last change to <secret> and
   that IPi must be the same as the source address it saw the first
   time. Incorporating SPIi into the calculation assures that if
   multiple IKE_SAs are being set up in parallel they will all get
   different cookies (assuming the initiator chooses unique SPIi's).
   Incorporating Ni into the hash assures that an attacker who sees only
   message 2 can't successfully forge a message 3.

   If a new value for <secret> is chosen while there are connections in
   the process of being initialized, an IKE_SA_INIT might be returned
   with other than the current <VersionIDofSecret>.  The responder in
   that case MAY reject the message by sending another response with a
   new cookie or it MAY keep the old value of <secret> around for a
   short time and accept cookies computed from either one.  The
   responder SHOULD NOT accept cookies indefinitely after <secret> is
   changed, since that would defeat part of the denial of service
   protection. The responder SHOULD change the value of <secret>
   frequently, especially if under attack.

2.7 Cryptographic Algorithm Negotiation

   The payload type known as "SA" indicates a proposal for a set of
   choices of protocols (IKE, ESP, and/or AH) for the SA as well as
   cryptographic algorithms associated with each protocol.

   An SA consists of one or more proposals. Each proposal includes one
   or more protocols (usually one). Each protocol contains one or more
   transforms - each specifying a cryptographic algorithm. Each



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   transform contains zero or more attributes (attributes are only
   needed if the transform identifier does not completely specify the
   cryptographic algorithm).

   This hierarchical structure was designed to be able to efficiently
   encode proposals for cryptographic suites when the number of
   supported suites is large because multiple values are acceptable for
   multiple transforms. The responder MUST choose a single suite, which
   MAY be any subset of the SA proposal following the rules below:


      Each proposal contains one or more protocols. If a proposal is
      accepted, the SA response MUST contain the same protocols in the
      same order as the proposal. At most one proposal MAY be accepted.
      (Example: if a single proposal contains ESP and AH and that
      proposal is accepted, both ESP and AH MUST be accepted. If ESP and
      AH are included in separate proposals, only one of them MAY be
      accepted).

      Each protocol 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. (Example: if an ESP proposal includes transforms
      ENCR_3DES, ENCR_AES128, AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the
      accepted suite MUST contain one of the ENCR_ transforms and one of
      the AUTH_ transforms. Thus four combinations are acceptable).

   Since Alice sends her Diffie-Hellman value in the IKE_SA_INIT, she
   must guess at the Diffie-Hellman group that Bob will select from her
   list of supported groups.  If she guesses wrong, Bob will respond
   with a Notify payload of type INVALID_KE_PAYLOAD indicating the
   selected group.  In this case, Alice MUST retry the IKE_SA_INIT with
   the corrected Diffie-Hellman group. Alice MUST again propose her full
   set of acceptable cryptographic suites because the rejection message
   was unauthenticated and otherwise an active attacker could trick
   Alice and Bob into negotiating a weaker suite than a stronger one
   that they both prefer.

2.8 Rekeying

   IKE, ESP, and AH security associations use secret keys which SHOULD
   only be used for a limited amount of time and to protect a limited
   amount of data. This limits the lifetime of the entire security
   association. When the lifetime of a security association expires the
   security association must not be used.  If there is demand, new
   security associations MAY be established.  Reestablishment of
   security associations to take the place of ones which expire is
   referred to as "rekeying".



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   To 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 them MAY start new ones.
   Implementations SHOULD support in place rekeying of SAs, since doing
   so offers better performance and is likely to reduce the number
   packets lost during the transition.

   To rekey a CHILD_SA within an existing IKE_SA, create a new,
   equivalent SA (see section 2.17 below), and when the new one is
   established, delete the old one.  To rekey an IKE_SA, establish a new
   equivalent IKE_SA (see section 2.18 below) with the peer to whom the
   old IKE_SA is shared using a CREATE_CHILD_SA within the existing
   IKE_SA. An IKE_SA so created inherits all of the original IKE_SA's
   CHILD_SAs.  Use the new IKE_SA for all control messages needed to
   maintain the CHILD_SAs created by the old IKE_SA, and delete the old
   IKE_SA. The Delete payload to delete itself MUST be the last request
   sent over an IKE_SA.

   SAs SHOULD be rekeyed proactively, i.e., the new SA should be
   established before the old one expires and becomes unusable. Enough
   time should elapse between the time the new SA is established and the
   old one becomes unusable so that traffic can be switched over to the
   new SA.

   A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
   were negotiated. In IKEv2, each end of the SA is responsible for
   enforcing its own lifetime policy on the SA and rekeying the SA when
   necessary.  If the two ends have different lifetime policies, the end
   with the shorter lifetime will end up always being the one to request
   the rekeying.  If an SA bundle has been inactive for a long time and
   if an endpoint would not initiate the SA in the absence of traffic,
   the endpoint MAY choose to close the SA instead of rekeying it when
   its lifetime expires. It SHOULD do so if there has been no traffic
   since the last time the SA was rekeyed.

   If the two ends have the same lifetime policies, it is possible that
   both will initiate a rekeying at the same time (which will result in
   redundant SAs). To reduce the probability of this happening, the
   timing of rekeying requests SHOULD be jittered (delayed by a random
   amount of time after the need for rekeying is noticed).

   This form of rekeying may temporarily result in multiple similar SAs
   between the same pairs of nodes. When there are two SAs eligible to
   receive packets, a node MUST accept incoming packets through either
   SA. If redundant SAs are created though such a collision, the SA



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   created with the lowest of the four nonces used in the two exchanges
   SHOULD be closed by the endpoint that created it.

   The node that initiated the surviving rekeyed SA SHOULD delete the
   replaced SA after the new one is established.

2.9 Traffic Selector Negotiation

   When an IP packet is received by an RFC2401 compliant IPsec subsystem
   and matches a "protect" selector in its SPD, the subsystem MUST
   protect that packet with IPsec. When no SA exists yet it is the task
   of IKE to create it. Maintenance of a system's SPD is outside the
   scope of IKE (see [PFKEY] for an example protocol), though some
   implementations might update their SPD in connection with the running
   of IKE (for an example scenario, see section 1.1.3).

   Traffic Selector (TS) payloads allow endpoints to communicate some of
   the information from their SPD to their peers. TS payloads specify
   the selection criteria for packets that will be forwarded over the
   newly set up SA.  This can serve as a consistency check in some
   scenarios to assure that the SPDs are consistent. In others, it
   guides the dynamic update of the SPD.

   Two TS payloads appear in each of the messages in the exchange that
   creates a CHILD_SA pair. Each TS payload contains one or more Traffic
   Selectors. Each Traffic Selector consists of an address range (IPv4
   or IPv6), a port range, and a protocol ID. In support of the scenario
   described in section 1.1.3, an initiator may request that the
   responder assign an IP address and tell the initiator what it is.

   IKEv2 allows the responder to choose a subset of the traffic proposed
   by the initiator.  This could happen when the configuration of the
   two endpoints are being updated but only one end has received the new
   information.  Since the two endpoints may be configured by different
   people, the incompatibility may persist for an extended period even
   in the absence of errors. It also allows for intentionally different
   configurations, as when one end is configured to tunnel all addresses
   and depends on the other end to have the up to date list.

   The first of the two TS payloads is known as TSi (Traffic Selector-
   initiator).  The second is known as TSr (Traffic Selector-responder).
   TSi specifies the source address of traffic forwarded from (or the
   destination address of traffic forwarded to) the initiator of the
   CHILD_SA pair. TSr specifies the destination address of the traffic
   forwarded from (or the source address of the traffic forwarded to)
   the responder of the CHILD_SA pair.  For example, if Alice initiates
   the creation of the CHILD_SA pair from Alice to Bob, and wishes to
   tunnel all traffic from subnet 10.2.16.* on Alice's side to subnet



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   18.16.*.* on Bob's side, Alice would include a single traffic
   selector in each TS payload. TSi would specify the address range
   (10.2.16.0 - 10.2.16.255) and TSr would specify the address range
   (18.16.0.0 - 18.16.255.255). Assuming that proposal was acceptable to
   Bob, he would send identical TS payloads back.

   The Responder is allowed to narrow the choices by selecting a subset
   of the traffic, for instance by eliminating or narrowing the range of
   one or more members of the set of traffic selectors, provided the set
   does not become the NULL set.

   It is possible for the Responder's policy to contain multiple smaller
   ranges, all encompassed by the Initiator's traffic selector, and with
   the Responder's policy being that each of those ranges should be sent
   over a different SA. Continuing the example above, Bob might have a
   policy of being willing to tunnel those addresses to and from Alice,
   but might require that each address pair be on a separately
   negotiated CHILD_SA. If Alice generated her request in response to an
   incoming packet from 10.2.16.43 to 18.16.2.123, there would be no way
   for Bob to determine which pair of addresses should be included in
   this tunnel, and he would have to make his best guess or reject the
   request with a status of SINGLE_PAIR_REQUIRED.

   To enable Bob to choose the appropriate range in this case, if Alice
   has initiated the SA due to a data packet, Alice 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, Alice would include in TSi two traffic
   selectors: the first containing the address range (10.2.16.43 -
   10.2.16.43) and the source port and protocol from the packet and the
   second containing (10.2.16.0 - 10.2.16.255) with all ports and
   protocols. She would similarly include two traffic selectors in TSr.

   If Bob's policy does not allow him to accept the entire set of
   traffic selectors in Alice's request, but does allow him to accept
   the first selector of TSi and TSr, then Bob MUST narrow the traffic
   selectors to a subset that includes Alice's first choices. In this
   example, Bob might respond with TSi being (10.2.16.43 - 10.2.16.43)
   with all ports and protocols.

   If Alice creates the CHILD_SA pair not in response to an arriving
   packet, but rather - say - upon startup, then there may be no
   specific addresses Alice prefers for the initial tunnel over any
   other.  In that case, the first values in TSi and TSr MAY be ranges
   rather than specific values, and Bob chooses a subset of Alice's TSi
   and TSr that are acceptable to him. If more than one subset is
   acceptable but their union is not, Bob MUST accept some subset and
   MAY include a Notify payload of type ADDITIONAL_TS_POSSIBLE to



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   indicate that Alice might want to try again. This case will only
   occur when Alice and Bob are configured differently from one another.
   If Alice and Bob agree on the granularity of tunnels, she will never
   request a tunnel wider than Bob will accept.

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 extract 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 prf. If the same random number source
   is used for both keys and nonces, care must be taken to ensure that
   the latter use does not compromise the former.

2.11 Address and Port Agility

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

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.

   Achieving perfect forward secrecy requires that when a connection is
   closed, each endpoint MUST forget not only the keys used by the
   connection but any information that could be used to recompute those
   keys. In particular, it MUST forget the secrets used in the Diffie-
   Hellman calculation and any state that may persist in the state of a
   pseudo-random number generator that could be used to recompute the
   Diffie-Hellman secrets.




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   Since the computing of Diffie-Hellman exponentials is computationally
   expensive, an endpoint may find it advantageous to reuse those
   exponentials for multiple connection setups. There are several
   reasonable strategies for doing this. An endpoint could choose a new
   exponential only periodically though this could result in less-than-
   perfect forward secrecy if some connection lasts for less than the
   lifetime of the exponential. Or it could keep track of which
   exponential was used for each connection and delete the information
   associated with the exponential only when some corresponding
   connection was closed. This would allow the exponential to be reused
   without losing perfect forward secrecy at the cost of maintaining
   more state.

   Decisions as to whether and when to reuse Diffie-Hellman exponentials
   is a private decision in the sense that it will not affect
   interoperability.  An implementation that reuses exponentials MAY
   choose to remember the exponential used by the other endpoint on past
   exchanges and if one is reused to avoid the second half of the
   calculation.

2.13 Generating Keying Material

   In the context of the IKE_SA, four cryptographic algorithms are
   negotiated: an encryption algorithm, an integrity protection
   algorithm, a Diffie-Hellman group, and a pseudo-random function
   (prf). The pseudo-random function is used for the construction of
   keying material for all of the cryptographic algorithms used in both
   the IKE_SA and the CHILD_SAs.

   We assume that each encryption algorithm and integrity protection
   algorithm uses a fixed size key, and that any randomly chosen value
   of that fixed size can serve as an appropriate key. For algorithms
   that accept a variable length key, a fixed key size MUST be specified
   as part of the cryptographic transform negotiated.  For integrity
   protection functions based on HMAC, the fixed key size is the size of
   the output of the underlying hash function. When the prf function
   takes a variable length key, variable length data, and produces a
   fixed length output (e.g., when using HMAC), the formulas in this
   document apply. When the key for the prf function has fixed length,
   the data provided as a key is truncated or padded with zeros as
   necessary unless exceptional processing is explained following the
   formula.

   Keying material will always be derived as the output of the
   negotiated prf algorithm. Since the amount of keying material needed
   may be greater than the size of the output of the prf algorithm, we
   will use the prf iteratively.  We will use the terminology prf+ to
   describe the function that outputs a pseudo-random stream based on



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   the inputs to a prf as follows: (where | indicates concatenation)

   prf+ (K,S) = T1 | T2 | T3 | T4 | ...

   where:
   T1 = prf (K, S | 0x01)
   T2 = prf (K, T1 | S | 0x02)
   T3 = prf (K, T2 | S | 0x03)
   T4 = prf (K, T3 | S | 0x04)

   continuing as needed to compute all required keys. The keys are taken
   from the output string without regard to boundaries (e.g., if the
   required keys are a 256 bit AES key and a 160 bit HMAC key, and the
   prf function generates 160 bits, the AES key will come from T1 and
   the beginning of T2, while the HMAC key will come from the rest of T2
   and the beginning of T3).

   The constant concatenated to the end of each string feeding the prf
   is a single octet. prf+ in this document is not defined beyond 255
   times the size of the prf output.

2.14 Generating Keying Material for the IKE_SA

   The shared keys are computed as follows.  A quantity called SKEYSEED
   is calculated from the nonces exchanged during the IKE_SA_INIT
   exchange and the Diffie-Hellman shared secret established during that
   exchange.  SKEYSEED is used to calculate five other secrets: SK_d
   used for deriving new keys for the CHILD_SAs established with this
   IKE_SA; SK_ai and SK_ar used as a key to the integrity protection
   algorithm for authenticating the component messages of subsequent
   exchanges; and SK_ei and SK_er used for encrypting (and of course
   decrypting) all subsequent exchanges.  SKEYSEED and its derivatives
   are computed as follows:

       SKEYSEED = prf(Ni | Nr, g^ir)

       {SK_d, SK_ai, SK_ar, SK_ei, SK_er}
                 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )

   (indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, and SK_er
   are taken in order from the generated bits of the prf+).  g^ir is the
   shared secret from the ephemeral Diffie-Hellman exchange.  g^ir is
   represented as a string of octets in big endian order padded with
   zeros if necessary to make it the length of the modulus. Ni and Nr
   are the nonces, stripped of any headers. If the negotiated prf takes
   a fixed length key and the lengths of Ni and Nr do not add up to that
   length, half the bits must come from Ni and half from Nr, taking the
   first bits of each.



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   The two directions of flow use different keys. The keys used to
   protect messages from the original initiator are SK_ai and SK_ei. The
   keys used to protect messages in the other direction are SK_ar and
   SK_er. Each algorithm takes a fixed number of bits of keying
   material, which is specified as part of the algorithm.  For integrity
   algorithms based on HMAC, the key size is always equal to the length
   of the output of the underlying hash function.

2.15 Authentication of the IKE_SA

   When not using extended authentication (see section 2.16), the peers
   are authenticated by having each sign (or MAC using a shared secret
   as the key) a block of data.  For the responder, the octets to be
   signed start with the first octet of the first SPI in the header of
   the second message and end with the last octet of the last payload in
   the second message.  Appended to this (for purposes of computing the
   signature) are the initiator's nonce Ni (just the value, not the
   payload containing it), and the value prf(SK_ar,IDr') where IDr' is
   the responder's ID payload excluding the fixed header. Note that
   neither the nonce Ni nor the value prf(SK_ar,IDr') are transmitted.
   Similarly, the initiator signs the first message, starting with the
   first octet of the first SPI in the header and ending with the last
   octet of the last payload.  Appended to this (for purposes of
   computing the signature) are the responder's nonce Nr, and the value
   prf(SK_ai,IDi'). In the above calculation, IDi' and IDr' are the
   entire ID payloads excluding the fixed header.  It is critical to the
   security of the exchange that each side sign the other side's nonce
   (see [SIGMA]).

   Note that all of the payloads are included under the signature,
   including any payload types not defined in this document. If the
   first message of the exchange is sent twice (the second time with a
   responder cookie and/or a different Diffie-Hellman group), it is the
   second version of the message that is signed.

   Optionally, messages 3 and 4 MAY include a certificate, or
   certificate chain providing evidence that the key used to compute a
   digital signature belongs to the name in the ID payload. The
   signature or MAC will be computed using algorithms dictated by the
   type of key used by the signer, and specified by the Auth Method
   field in the Authentication payload.  There is no requirement that
   the Initiator and Responder sign with the same cryptographic
   algorithms. The choice of cryptographic algorithms depends on the
   type of key each has.  In particular, the initiator may be using a
   shared key while the responder may have a public signature key and
   certificate.  It will commonly be the case (but it is not required)
   that if a shared secret is used for authentication that the same key
   is used in both directions.  Note that it is a common but typically



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   insecure practice to have a shared key derived solely from a user
   chosen password without incorporating another source of randomness.
   This is typically insecure because user chosen passwords are unlikely
   to have sufficient unpredictability to resist dictionary attacks and
   these attacks are not prevented in this authentication method.
   (Applications using password-based authentication for bootstrapping
   and IKE_SA should use the authentication method in section 2.16,
   which is designed to prevent off-line dictionary attacks).  The pre-
   shared key SHOULD contain as much unpredictability as the strongest
   key being negotiated.  In the case of a pre-shared key, the AUTH
   value is computed as:

      AUTH = prf(prf(Shared Secret,"Key Pad for IKEv2"), <message
      octets>)

   where the string "Key Pad for IKEv2" is ASCII encoded and not null
   terminated. The shared secret can be variable length. The pad string
   is added so that if the shared secret is derived from a password, 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. The management interface MAY accept other
   forms, like hex encoding. If the negotiated prf takes a fixed size
   key, the shared secret MUST be of that fixed size.

2.16 Extended Authentication Protocol Methods

   In addition to authentication using public key signatures and shared
   secrets, IKE supports authentication using methods defined in RFC
   2284 [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 are used in addition to a public key
   signature based authentication of the responder to the initiator.
   These methods are also referred to as "Legacy Authentication"
   mechanisms.

   While this memo references [EAP] with the intent that new methods can
   be added in the future without updating this specification, the
   protocols expected to be used most commonly are fully documented here
   and in section 3.16.  [EAP] defines an authentication protocol
   requiring a variable number of messages. Extended Authentication is
   implemented in IKE as additional IKE_AUTH exchanges that MUST be



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   completed in order to initialize the IKE_SA.

   An initiator indicates a desire to use extended authentication by
   leaving out the AUTH payload from message 3. By including an IDi
   payload but not an AUTH payload, the initiator has declared an
   identity but has not proven it. If the responder is willing to use an
   extended authentication method, it will place an EAP payload in
   message 4 and defer sending SAr2, TSi, and TSr until initiator
   authentication is complete in a subsequent IKE_AUTH exchange. In the
   case of a minimal extended authentication, 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, [AUTH] }     -->

                                  <--    HDR, SK {EAP, [AUTH],
                                                  SAr2, TSi, TSr }

   For EAP methods that create a shared key as a side effect of
   authentication, that shared key MUST be used by both the Initiator
   and Responder to generate an AUTH payload using the syntax for shared
   secrets specified in section 2.15. The shared key generated during an
   IKE exchange MUST NOT be used for any other purpose. For EAP methods
   that do not establish a shared key, there will be no AUTH payloads in
   the final messages.

   The Initiator of an IKE_SA using EAP SHOULD be capable of extending
   the initial protocol exchange to at least ten IKE_AUTH exchanges in
   the event the Responder sends notification messages and/or retries
   the authentication prompt. The protocol terminates when the Responder
   sends the Initiator an EAP payload containing either a success or
   failure type.

2.17 Generating Keying Material for CHILD_SAs

   CHILD_SAs are created either by being piggybacked on the IKE_AUTH
   exchange, or in a CREATE_CHILD_SA exchange. Keying material for them



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   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 with PFS 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 if necessary to
   make it the length of the modulus),

   A single CHILD_SA negotiation may result in multiple security
   associations. ESP and AH SAs exist in pairs (one in each direction),
   and four SAs could be created in a single CHILD_SA negotiation if a
   combination of ESP and AH is being negotiated.

   Keying material is taken from the expanded KEYMAT in the following
   order:

      All keys for SAs carrying data from the initiator to the responder
      are taken before SAs going in the reverse direction.

      If multiple protocols are negotiated, keying material is taken in
      the order in which the protocol headers will appear in the
      encapsulated packet.

      If a single protocol has both encryption and authentication keys,
      the encryption key is taken from the first octets of KEYMAT and
      the authentication key is taken from the next octets.

   Each cryptographic algorithm takes a fixed number of bits of keying
   material specified as part of the algorithm.

2.18 Rekeying IKE_SAs using a CREATE_CHILD_SA exchange

   The CREATE_CHILD_SA exchange can be used to re-key an existing IKE_SA
   (see section 2.8).  New Initiator and Responder SPIs are supplied in
   the SPI fields. The TS payloads are omitted when rekeying an IKE_SA.
   SKEYSEED for the new IKE_SA is computed using SK_d from the existing
   IKE_SA as follows:




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       SKEYSEED = prf(SK_d (old), [g^ir (new)] | Ni | Nr)

   where g^ir (new) is the shared secret from the ephemeral Diffie-
   Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
   octet string in big endian order padded with zeros if necessary to
   make it the length of the modulus) and Ni and Nr are the two nonces
   stripped of any headers.

   The new IKE_SA MUST reset its message counters to 0.

   SK_d, SK_ai, SK_ar, and SK_ei, and SK_er are computed from SKEYSEED
   as specified in section 2.14.

2.19 Requesting an internal address on a remote network

   Most commonly occurring in the endpoint to security gateway scenario,
   an endpoint may need an IP address in the network protected by the
   security gateway, and may need to have that address dynamically
   assigned. A request for such a temporary address can be included in
   any request to create a CHILD_SA (including the implicit request in
   message 3) by including a CP payload.

   This function provides address allocation to an IRAC (IPsec Remote
   Access Client) trying to tunnel into a network protected by an IRAS
   (IPsec Remote Access Server).  Since the IKE_AUTH exchange creates an
   IKE_SA and a CHILD_SA, the IRAC MUST request the IRAS controlled
   address (and optionally other information concerning the protected
   network) in the IKE_AUTH exchange.  The IRAS may procure an address
   for the IRAC from any number of sources such as a DHCP/BOOTP server
   or its own address pool.

       Initiator                           Responder
      -----------------------------       ---------------------------
       HDR, SK {IDi, [CERT,] [CERTREQ,]
        [IDr,] AUTH, CP(CFG_REQUEST),
        SAi2, TSi, TSr}              -->

                                     <--   HDR, SK {IDr, [CERT,] AUTH,
                                            CP(CFG_REPLY), SAr2,
                                            TSi, TSr}

   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



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   attributes the initiator wants returned in the response.

   For example, message from Initiator to Responder:
      CP(CFG_REQUEST)=
        INTERNAL_ADDRESS(0.0.0.0)
        INTERNAL_NETMASK(0.0.0.0)
        INTERNAL_DNS(0.0.0.0)
      TSi = (0, 0-65536,0.0.0.0-255.255.255.255)
      TSr = (0, 0-65536,0.0.0.0-255.255.255.255)

   NOTE: Traffic Selectors are a (protocol, port range, address range)

   Message from Responder to Initiator:

      CP(CFG_REPLY)=
        INTERNAL_ADDRESS(192.168.219.202)
        INTERNAL_NETMASK(255.255.255.0)
        INTERNAL_SUBNET(192.168.219.0/255.255.255.0)
      TSi = (0, 0-65536,192.168.219.202-192.168.219.202)
      TSr = (0, 0-65536,192.168.219.0-192.168.219.255)

   All returned values will be implementation dependent.  As can be seen
   in the above example, the IRAS MAY also send other attributes that
   were not included in CP(CFG_REQUEST) and MAY ignore the non-
   mandatory attributes that it does not support.

   The responder MUST not send a CFG_REPLY without having first received
   a CP(CFG_REQUEST) from the initiator, because we do not want the IRAS
   to perform an unnecessary configuration lookup if the IRAC cannot
   process the REPLY. In the case where the IRAS's configuration
   requires that CP be used for a given identity IDi, but IRAC has
   failed to send a CP(CFG_REQUEST), IRAS MUST fail the request, and
   terminate the IKE exchange with a FAILED_CP_REQUIRED error.

2.20 Requesting the Peer's Version

   An IKE peer wishing to inquire about the other peer's IKE software
   version information MAY use the method below.  This is an example of
   a configuration request within an INFORMATIONAL Exchange, after the
   IKE_SA and first CHILD_SA have been created.

   An IKE implementation MAY decline to give out version information
   prior to authentication or even after authentication to prevent
   trolling in case some implementation is known to have some security
   weakness. In that case, it MUST either return an empty string or no
   CP payload if CP is not supported.

       Initiator                           Responder



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      -----------------------------       --------------------------
      HDR, SK{CP(CFG_REQUEST)}      -->
                                    <--    HDR, SK{CP(CFG_REPLY)}

      CP(CFG_REQUEST)=
        APPLICATION_VERSION("")

      CP(CFG_REPLY)
        APPLICATION_VERSION("foobar v1.3beta, (c) Foo Bar Inc.")

2.21 Error Handling

   There are many kinds of errors that can occur during IKE processing.
   If a request is received that is badly formatted or unacceptable for
   reasons of policy (e.g., no matching cryptographic algorithms), the
   response MUST contain a Notify payload indicating the error. If an
   error occurs outside the context of an IKE request (e.g., the node is
   getting ESP messages on a nonexistent SPI), the node SHOULD initiate
   an INFORMATIONAL Exchange with a Notify payload describing the
   problem.

   Errors that occur before a cryptographically protected IKE_SA is
   established must be handled very carefully. There is a trade-off
   between wanting to be helpful in diagnosing a problem and responding
   to it and wanting to avoid being a dupe in a denial of service attack
   based on forged messages.

   If a node receives a message on UDP port 500 outside the context of
   an IKE_SA known to it (and not a request to start one), it may be the
   result of a recent crash of the node.  If the message is marked as a
   response, the node MAY audit the suspicious event but MUST NOT
   respond. If the message is marked as a request, the node MAY audit
   the suspicious event and MAY send a response. If a response is sent,
   the response MUST be sent to the IP address and port from whence it
   came with the same IKE SPIs and the Message ID copied. The response
   MUST NOT be cryptographically protected and MUST contain a Notify
   payload indicating INVALID_IKE_SPI.

   A node receiving such an unprotected Notify payload MUST NOT respond
   and MUST NOT change the state of any existing SAs. The message might
   be a forgery or might be a response the genuine correspondent was
   tricked into sending. A node SHOULD treat such a message (and also a
   network message like ICMP destination unreachable) as a hint that
   there might be problems with SAs to that IP address and SHOULD
   initiate a liveness test for any such IKE_SA. An implementation
   SHOULD limit the frequency of such tests to avoid being tricked into
   participating in a denial of service attack.




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   A node receiving a suspicious message from an IP address with which
   it has an IKE_SA MAY send an IKE Notify payload in an IKE
   INFORMATIONAL exchange over that SA. The recipient MUST NOT change
   the state of any SA's as a result but SHOULD audit the event to aid
   in diagnosing malfunctions. A node MUST limit the rate at which it
   will send messages in response to unprotected messages.

2.22 IPcomp

   Use of IP compression [IPCOMP] can be negotiated as part of the setup
   of a CHILD_SA. While IP compression involves an extra header in each
   packet and a CPI (compression parameter index), the virtual
   "compression association" has no life outside the ESP or AH SA that
   contains it. Compression associations disappear when the
   corresponding ESP or AH SA goes away, and is not explicitly mentioned
   in any DELETE payload.

   Negotiation of IP compression is separate from the negotiation of
   cryptographic parameters associated with a CHILD_SA. A node
   requesting a CHILD_SA MAY advertise its support for one or more
   compression algorithms though one or more Notify payloads of type
   IPCOMP_SUPPORTED. The response MAY indicate acceptance of a single
   compression algorithm with a Notify payload of type IPCOMP_SUPPORTED.
   These payloads MAY ONLY occur in the same messages that contain SA
   payloads.

   While there has been discussion of allowing multiple compression
   algorithms to be accepted and to have different compression
   algorithms available for the two directions of a CHILD_SA,
   implementations of this specification MUST NOT accept an IPcomp
   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.

2.23 NAT Traversal

   NAT (Network Address Translation) gateways are a controversial
   subject.  This section briefly describes what they are and how they
   are likely to act on IKE traffic. Many people believe that NATs are
   evil and that we should not design our protocols so as to make them
   work better. IKEv2 does specify some unintuitive processing rules in
   order that NATs are more likely to work.




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   NATs exist primarily because of the shortage of IPv4 addresses,
   though there are other rationales. IP nodes that are "behind" a NAT
   have IP addresses that are not globally unique, but rather are
   assigned from some space that is unique within the network behind the
   NAT but which are likely to be reused by nodes behind other NATs.
   Generally, nodes behind NATs can communicate with other nodes behind
   the same NAT and with nodes with globally unique addresses, but not
   with nodes behind other NATs.  There are exceptions to that rule.
   When those nodes make connections to nodes on the real Internet, the
   NAT gateway "translates" the IP source address to an address that
   will be routed back to the gateway. Messages to the gateway from the
   Internet have their destination addresses "translated" to the
   internal address that will route the packet to the correct endnode.

   NATs are designed to be "transparent" to endnodes. Neither software
   on the node behind the NAT nor the node on the Internet require
   modification to communicate through the NAT. Achieving this
   transparency is more difficult with some protocols than with others.
   Protocols that include IP addresses of the endpoints within the
   payloads of the packet will fail unless the NAT gateway understands
   the protocol and modifies the internal references as well as those in
   the headers. Such knowledge is inherently unreliable, is a network
   layer violation, and often results in subtle problems.

   Opening an IPsec connection through a NAT introduces special
   problems.  If the connection runs in transport mode, changing the IP
   addresses on packets will cause the checksums to fail and the NAT
   cannot correct the checksums because they are cryptographically
   protected. Even in tunnel mode, there are routing problems because
   transparently translating the addresses of AH and ESP packets
   requires special logic in the NAT and that logic is heuristic and
   unreliable in nature. For that reason, IKEv2 can negotiate UDP
   encapsulation of IKE, ESP, and AH packets.  This encoding is slightly
   less efficient but is easier for NATs to process. In addition,
   firewalls may be configured to pass IPsec traffic over UDP but not
   ESP/AH or vice versa.

   It is a common practice of NATs to translate TCP and UDP port numbers
   as well as addresses and use the port numbers of inbound packets to
   decide which internal node should get a given packet. For this
   reason, even though IKE packets MUST be sent from and to UDP port
   500, they MUST be accepted coming from any port and responses MUST be
   sent to the port from whence they came. This is because the ports may
   be modified as the packets pass through NATs. Similarly, IP addresses
   of the IKE endpoints are generally not included in the IKE payloads
   because the payloads are cryptographically protected and could not be
   transparently modified by NATs.




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   Port 4500 is reserved for UDP encapsulated ESP, AH, and IKE.  When
   working through a NAT, it is generally better to pass IKE packets
   over port 4500 because some older NATs modify IKE traffic on port 500
   in an attempt to transparently establish IPsec connections. Such NATs
   may interfere with the straightforward NAT traversal envisioned by
   this document, so an IPsec endpoint that discovers a NAT between it
   and its correspondent MUST send all subsequent traffic to and from
   port 4500, which NATs should not treat specially (as they might with
   port 500).

   The specific requirements for supporting NAT traversal are listed
   below.  Support for NAT traversal is optional. In this section only,
   requirements listed as MUST only apply to implementations supporting
   NAT traversal.

      IKE MUST listen on port 4500 as well as port 500. IKE MUST respond
      to the IP address and port from which packets arrived.

      The IKE responder MUST include in its IKE_SA_INIT response Notify
      payloads of type NAT_DETECTION_SOURCE_IP and
      NAT_DETECTION_DESTINATION_IP. The IKE initiator MUST check these
      payloads if present and if they do not match the addresses in the
      outer packet MUST tunnel all future IKE, ESP, and AH packets
      associated with this IKE_SA over UDP port 4500.

2.24 ECN Notification

   Sections 5.1.2.1 and 5.1.2.2 of [RFC 2401] specify that the IPv4 TOS
   octet and IPv6 traffic class octet are to be copied from the inner
   header to the outer header by the encapsulator and that the outer
   header is to be discarded (no change to inner header) by the
   decapsulator.  If ECN (see [RFC 3168]) is in use, ECT codepoints will
   be copied to the outer header, but if a router within the tunnel
   changes an ECT codepoint to a CE codepoint to indicate congestion,
   that indication will be discarded by the decapsulator.  This behavior
   is highly undesirable, and Section 9.2 of [RFC 3168] specifies
   changes to IPsec to avoid it.  These changes include two ECN
   operating modes and negotiation support to detect and cope with IPsec
   decapsulators that discard ECN congestion indications; use of ECN in
   the outer IP header of IPsec tunnels is not permitted when such
   discarding is a possibility.

   In order to avoid multiple ECN operating modes and negotiation,
   tunnel decapsulators for tunnel-mode Security Associations (SAs)
   created by IKEv2 MUST implement the following modifications to
   prevent discarding of ECN congestion indications.  IKEv2 tunnel-mode
   SA negotiation is handled by the USE_TRANSPORT_MODE Notify message
   type (see Section 3.10.1).  The following modifications *replace*



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   Section 9.2 of RFC 3168 and *update* Sections 5.1.2.1 and 5.1.2.2 of
   RFC 2401.

   Encapsulation and Decapsulation of packets for a tunnel-mode SA
   created by IKEv2 MUST NOT follow the modifications specified by
   Section 9.2 of RFC 3168 and its subsections.  Instead, the following
   modifications to encapsulation and decapsulation in Sections 5.1.2.1
   and 5.1.2.2 of RFC 2401 MUST be performed:

                         Outer Hdr at                 Inner Hdr at
    IPv4                 Encapsulator                 Decapsulator
      Header fields:     --------------------         ------------
        DS Field         copied from inner hdr (5)    no change
        ECN Field        copied from inner hdr        constructed (7)
    IPv6
      Header fields:
        DS Field         copied from inner hdr (6)    no change
        ECN Field        copied from inner hdr        constructed (7)

      (5)(6) If the packet will immediately enter a domain for which the
      DSCP value in the outer header is not appropriate, that value MUST
      be mapped to an appropriate value for the domain [RFC 2474].  Also
      see [RFC 2475] for further information.

      (7) If the ECN field in the inner header is set to ECT(0) or
      ECT(1) and the ECN field in the outer header is set to CE, then
      set the ECN field in the inner header to CE, otherwise make no
      change to the ECN field in the inner header.

      (5) and (6) are identical to match usage in [RFC2401], although
      they are different in [RFC2401].  These actions are not related to
      ECN, but are required for Differentiated Services support.  They
      are carried over to this document from RFC 3168 so that all of RFC
      3168's changes to IPsec can be made non-applicable to SAs created
      by IKEv2.


3 Header and Payload Formats

3.1 The IKE Header

   IKE messages use UDP ports 500 and/or 4500, with one IKE message per
   UDP datagram. Information from the UDP header is largely ignored
   except that the IP addresses and UDP ports from the headers are
   reversed and used for return packets. When sent on UDP port 500, IKE
   messages begin immediately following the UDP header. When sent on UDP
   port 4500, IKE messages have prepended four octets of zero.  These
   four octets of zero are not part of the IKE message and are not



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   included in any of the length fields or checksums defined by IKE.
   Each IKE message begins with the IKE header, denoted HDR in this
   memo. Following the header are one or more IKE payloads each
   identified by a "Next Payload" field in the preceding payload.
   Payloads are processed in the order in which they appear in an IKE
   message by invoking the appropriate processing routine according to
   the "Next Payload" field in the IKE header and subsequently according
   to the "Next Payload" field in the IKE payload itself until a "Next
   Payload" field of zero indicates that no payloads follow. If a
   payload of type "Encrypted" is found, that payload is decrypted and
   its contents parsed as additional payloads. An Encrypted payload MUST
   be the last payload in a packet and an encrypted payload MUST NOT
   contain another encrypted payload.

   The Recipient SPI in the header identifies an instance of an IKE
   security association. It is therefore possible for a single instance
   of IKE to multiplex distinct sessions with multiple peers.

   All multi-octet fields representing integers are laid out in big
   endian order (aka most significant byte first, or network byte
   order).

   The format of the IKE header is shown in Figure 4.
                           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



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         Exchange (including repeats of that message including a
         cookie) and MUST NOT be zero in any other message.

      o  Next Payload (1 octet) - Indicates the type of payload that
         immediately follows the header. The format and value of each
         payload is defined below.

      o  Major Version (4 bits) - indicates the major version of the IKE
         protocol in use.  Implementations based on this version of IKE
         MUST set the Major Version to 2. Implementations based on
         previous versions of IKE and ISAKMP MUST set the Major Version
         to 1. Implementations based on this version of IKE MUST reject
         or ignore messages containing a version number greater than
         2.

      o  Minor Version (4 bits) - indicates the minor version of the
         IKE protocol in use.  Implementations based on this version of
         IKE MUST set the Minor Version to 0. They MUST ignore the minor
         version number of received messages.

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

                       Exchange Type            Value

                       RESERVED                 0
                       Reserved for ISAKMP      1-31
                       Reserved for IKEv1       32-33
                       IKE_SA_INIT              34
                       IKE_AUTH                 35
                       CREATE_CHILD_SA          36
                       INFORMATIONAL            37
                       Reserved for IKEv2+      38-239
                       Reserved for private use 240-255

      o  Flags (1 octet) - indicates specific options that are set
         for the message. Presence of options are indicated by the
         appropriate bit in the flags field being set. The bits are
         defined LSB first, so bit 0 would be the least significant
         bit of the Flags octet. In the description below, a bit
         being 'set' means its value is '1', while 'cleared' means
         its value is '0'.

       --  X(reserved) (bits 0-2) - These bits MUST be cleared
           when sending and MUST be ignored on receipt.

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



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           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 was generated by the
           recipient.

       --  V(ersion) (bit 4 of Flags) - This bit indicates that
           the transmitter is capable of speaking a higher major
           version number of the protocol than the one indicated
           in the major version number field. Implementations of
           IKEv2 must clear this bit when sending and MUST ignore
           it in incoming messages.

       --  R(esponse) (bit 5 of Flags) - This bit indicates that
           this message is a response to a message containing
           the same message ID. This bit MUST be cleared in all
           request messages and MUST be set in all responses.
           An IKE endpoint MUST NOT generate a response to a
           message that is marked as being a response.

       --  X(reserved) (bits 6-7 of Flags) - These bits MUST be
           cleared when sending and MUST be ignored on receipt.

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

      o  Length (4 octets) - Length of total message (header + payloads)
         in octets.

3.2 Generic Payload Header

   Each IKE payload defined in sections 3.3 through 3.16 begins with a
   generic payload header, shown in Figure 5. Figures for each payload
   below will include the generic payload header but for brevity the
   description of each field will be omitted.

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

                         Figure 5:  Generic Payload Header

   The Generic Payload Header fields are defined as follows:



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   o  Next Payload (1 octet) - Identifier for the payload type of the
      next payload in the message.  If the current payload is the last
      in the message, then this field will be 0.  This field provides
      a "chaining" capability whereby additional payloads can be
      added to a message by appending it to the end of the message
      and setting the "Next Payload" field of the preceding payload
      to indicate the new payload's type. An Encrypted payload,
      which must always be the last payload of a message, is an
      exception. It contains data structures in the format of
      additional payloads. In the header of an Encrypted payload,
      the Next Payload field is set to the payload type of the first
      contained payload (instead of 0).

      Payload Type Values

          Next Payload Type               Notation  Value

          Security Association             SA         1
          Key Exchange                     KE         4
          Initiator Identification         IDi        5
          Certificate                      CERT       6
          Certificate Request              CERTREQ    7
          Authentication                   AUTH       9
          Nonce                            Ni, Nr    10
          Notification                     N         11
          Delete                           D         12
          Vendor ID                        V         13
          Initiator Traffic Selector       TSi       14
          Encrypted                        E         15
          Configuration                    CP        16
          Extended Authentication          EAP       17
          Responder Identication           IDr       18
          Responder Traffic Selector       TSr       19

      Payload type values 20-127 are reserved to IANA for future
      assignment in IKEv2 (see section 6). Payload type values 128-255
      are for private use among mutually consenting parties.

   o  Critical (1 bit) - MUST be set to zero if the sender wants
      the recipient to skip this payload if he does not
      understand the payload type code in the Next Payload field
      of the previous payload. MUST be set to one if the
      sender wants the recipient to reject this entire message
      if he does not understand the payload type. MUST be ignored
      by the recipient if the recipient understands the payload type
      code. 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



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      appears in the first octet. The reasoning behind not setting
      the critical bit for payloads defined in this document is
      that all implementations MUST understand all payload types
      defined in this document and therefore must ignore the
      Critical bit's value. Skipped payloads are expected to
      have valid Next Payload and Payload Length fields.

   o  RESERVED (7 bits) - MUST be sent as zero; MUST be ignored on
      receipt.

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

3.3 Security Association Payload

   The Security Association Payload, denoted SA in this memo, is used to
   negotiate attributes of a security association. Assembly of Security
   Association Payloads requires great peace of mind. An SA may contain
   multiple proposals, ordered from most preferred to least preferred.
   Each proposal may contain multiple protocols (where a protocol is
   IKE, ESP, or AH), each protocol may contain multiple transforms, and
   each transform may contain multiple attributes. When parsing an SA,
   an implementation MUST check that the total Payload Length is
   consistent with the payload's internal lengths and counts.
   Proposals, Transforms, and Attributes each have their own variable
   length encodings. They are nested such that the Payload Length of an
   SA includes the combined contents of the SA, Proposal, Transform, and
   Attribute information. The length of a Proposal includes the lengths
   of all Transforms and Attributes it contains. The length of a
   Transform includes the lengths of all Attributes it contains.

   The syntax of Security Associations, Proposals, Transforms, and
   Attributes is based on ISAKMP, however the semantics are somewhat
   different. The reason for the complexity and the hierarchy is to
   allow for multiple possible combinations of algorithms to be encoded
   in a single SA. Sometimes there is a choice of multiple algorithms,
   while other times there is a combination of algorithms.  For example,
   an Initiator might want to propose using (AH w/MD5 and ESP w/3DES) OR
   (ESP w/MD5 and 3DES).

   One of the reasons the semantics of the SA payload has changed from
   ISAKMP and IKEv1 is to make the encodings more compact in common
   cases.

   The Proposal structure contains within it a Proposal # and a
   SECURITY_PROTOCOL_ID.  Each structure MUST have the same Proposal #
   as the previous one or be one (1) greater. The first Proposal MUST
   have a Proposal # of one (1). If two successive structures have the



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   same Proposal number, it means that the proposal consists of the
   first structure AND the second. So a proposal of AH AND ESP would
   have two proposal structures, one for AH and one for ESP and both
   would have Proposal #1. A proposal of AH OR ESP would have two
   proposal structures, one for AH with proposal #1 and one for ESP with
   proposal #2.

   Each Proposal/Protocol structure is followed by one or more transform
   structures. The number of different transforms is generally
   determined by the Protocol. AH generally has a single transform: an
   integrity check algorithm. ESP generally has two: an encryption
   algorithm AND an integrity check algorithm. IKE generally has four
   transforms: a Diffie-Hellman group, an integrity check algorithm, a
   prf algorithm, and an encryption algorithm.  For each Protocol, the
   set of permissible transforms are assigned transform ID numbers,
   which appear in the header of each transform.

   If there are multiple transforms with the same Transform Type, the
   proposal is an OR of those transforms. If there are multiple
   Transforms with different Transform Types, the proposal is an AND of
   the different groups. For example, to propose ESP with (3DES or IDEA)
   and (HMAC_MD5 or HMAC_SHA), the ESP proposal would contain two
   Transform Type 1 candidates (one for 3DES and one for IDEA) and two
   Transform Type 2 candidates (one for HMAC_MD5 and one for HMAC_SHA).
   This effectively proposes four combinations of algorithms. If the
   Initiator wanted to propose only a subset of those - say (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.

   Note that the semantics of Transforms and Attributes are quite
   different than 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.










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                           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.

      The payload type for the Security Association Payload is one (1).

3.3.1 Proposal Substructure

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! 0 (last) or 2 !   RESERVED    !         Proposal Length       !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Proposal #    !  Protocol-Id  !    SPI Size   !# of Transforms!
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                        SPI (variable)                         ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                        <Transforms>                           ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 7:  Proposal Substructure

      o  0 (last) or 2 (more) (1 octet) - Specifies whether this is the
         last Proposal Substructure in the SA. This syntax is inherited
         from ISAKMP, but is unnecessary because the last Proposal
         could be identified from the length of the SA. The value (2)
         corresponds to a Payload Type of Proposal, 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) - Length of this proposal,
         including all transforms and attributes that follow.




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      o  Proposal # (1 octet) - When a proposal is made, the first
         proposal in an SA MUST be #1, and subsequent proposals
         MUST either be the same as the previous proposal (indicating
         an AND of the two proposals) or one more than the previous
         proposal (indicating an OR of the two proposals). When a
         proposal is accepted, all of the proposal numbers in the
         SA MUST be the same and MUST match the number on the
         proposal sent that was accepted.

      o  Protocol-Id (1 octet) - Specifies the protocol identifier
         for the current negotiation. Zero (0) indicates IKE,
         one (1) indicated ESP, and two (2) indicates AH.

      o  SPI Size (1 octet) - For an initial IKE_SA negotiation,
         this field MUST be zero; the SPI is obtained from the
         outer header. During subsequent negotiations,
         it is equal to the size, in octets, of the SPI of the
         corresponding protocol (8 for IKE, 4 for ESP and AH).

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

      o  SPI (variable) - The sending entity's SPI. Even if the SPI
         Size is not a multiple of 4 octets, there is no padding
         applied to the payload. When the SPI Size field is zero,
         this field is not present in the Security Association
         payload.

      o  Transforms (variable) - one or more transform substructures.


3.3.2 Transform Substructure

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

               Figure 8:  Transform Substructure

   o  0 (last) or 3 (more) (1 octet) - Specifies whether this is the



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      last Transform Substructure in the Proposal. This syntax is
      inherited from ISAKMP, but is unnecessary because the last
      Proposal could be identified from the length of the SA. The
      value (3) corresponds to a Payload Type of Transform, 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, she includes a transform substructure with
      transform ID = 0 as one of the options.

   o  Transform ID (1 octet) - The specific instance of the transform
      type being proposed.

   Transform Type Values

                               Transform    Used In
                                  Type
          Encryption Algorithm      1  (IKE and ESP)
          Pseudo-random Function    2  (IKE)
          Integrity Algorithm       3  (IKE, AH, and optional in ESP)
          Diffie-Hellman Group      4  (IKE and optional in AH and ESP)
          Extended Sequence Numbers 5  (Optional in AH and ESP)

          values 6-240 are reserved to IANA. Values 241-255 are for
          private use among mutually consenting parties.

   For Transform Type 1 (Encryption Algorithm), defined Transform IDs
   are:

          Name                     Number           Defined In
          RESERVED                    0
          ENCR_DES_IV64               1              (RFC1827)
          ENCR_DES                    2              (RFC2405)
          ENCR_3DES                   3              (RFC2451)
          ENCR_RC5                    4              (RFC2451)
          ENCR_IDEA                   5              (RFC2451)



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          ENCR_CAST                   6              (RFC2451)
          ENCR_BLOWFISH               7              (RFC2451)
          ENCR_3IDEA                  8              (RFC2451)
          ENCR_DES_IV32               9
          ENCR_RC4                   10
          ENCR_NULL                  11              (RFC2410)
          ENCR_AES_128_CBC           12
          ENCR_AES_128_CTR           13

          values 14-240 are reserved to IANA. Values 241-255 are for
          private use among mutually consenting parties.

   For Transform Type 2 (Pseudo-random Function), defined Transform IDs
   are:

          Name                     Number                 Defined In
          RESERVED                    0
          PRF_HMAC_MD5                1                   (RFC2104)
          PRF_HMAC_SHA1               2                   (RFC2104)
          PRF_HMAC_TIGER              3                   (RFC2104)
          PRF_AES128_CBC              4

          values 5-240 are reserved to IANA. Values 241-255 are for
          private use among mutually consenting parties.


   For Transform Type 3 (Integrity Algorithm), defined Transform IDs
   are:

          Name                     Number                 Defined In
          NONE                       0
          AUTH_HMAC_MD5_96           1                     (RFC2403)
          AUTH_HMAC_SHA1_96          2                     (RFC2404)
          AUTH_DES_MAC               3
          AUTH_KPDK_MD5              4                     (RFC1826)
          AUTH_AES_XCBC_96           5

          values 6-240 are reserved to IANA. Values 241-255 are for
          private use among mutually consenting parties.

   For Transform Type 4 (Diffie-Hellman Group), defined Transform IDs
   are:

          Name                                Number
          NONE                               0
          Defined in Appendix B              1 - 5
          Defined in [ADDGROUP]              14 - 18
          MODP (exponentiation)              201  (w/attributes)



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          ECP (elliptic curve over GF[P]     202  (w/attributes)
          EC2N (elliptic curve over GF[2^N]) 203  (w/attributes)

          values 6-13 and 19-200 are reserved to IANA for new MODP, ECP
          or EC2N groups. Values 204-255 are for private use among
          mutually consenting parties. Specification of values 201, 202
          or 203 allow peers to define a new Diffie-Hellman group in-
          line as part of the exchange. Private use of values 204-255
          may entail complete definition of a group or may require
          attributes to accompany them.


   For Transform Type 5 (Extended Sequence Numbers), defined Transform
   IDs are:

          Name                                Number
          No Extended Sequence Numbers       0
          Extended Sequence Numbers          1
          RESERVED                           2 - 255

          If Transform Type 5 is not included in a proposal, use of
          Extended Sequence Numbers is assumed.

3.3.3 Mandatory Transform Types

   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                   INTEG, D-H, ESN
            AH      INTEG                  D-H, ESN

3.3.4 Mandatory Transform IDs

   The specification of suites that MUST and SHOULD be supported for
   interoperability has been removed from this document because they are
   likely to change more rapidly than this document evolves.

   An important lesson learned from IKEv1 is that no system should only
   implement the mandatory algorithms and expect them to be the best
   choice for all customers. For example, at the time that this document



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   was being written, many IKEv1 implementers are starting to migrate to
   AES in CBC mode for VPN applications. Many IPsec systems based on
   IKEv2 will implement AES, longer Diffie-Hellman keys, and additional
   hash algorithms, and some IPsec customers already require these
   algorithms in addition to the ones listed above.

   It is likely that IANA will add additional transforms in the future,
   and some users may want to use private suites, especially for IKE
   where implementations should be capable of supporting different
   parameters, up to certain size limits. In support of this goal, all
   implementations of IKEv2 SHOULD include a management facility that
   allows specification (by a user or system administrator) of Diffie-
   Hellman parameters (the generator, modulus, and exponent lengths and
   values) for new DH groups. Implementations SHOULD provide a
   management interface via which these parameters and the associated
   transform IDs may be entered (by a user or system administrator), to
   enable negotiating such groups.

   All implementations of IKEv2 MUST include a management facility that
   enables a user or system administrator to specify the suites that are
   acceptable for use with IKE. Upon receipt of a payload with a set of
   transform IDs, the implementation MUST compare the transmitted
   transform IDs against those locally configured via the management
   controls, to verify that the proposed suite is acceptable based on
   local policy.  The implementation MUST reject SA proposals that are
   not authorized by these IKE suite controls.

3.3.5 Transform Attributes

   Each transform in a Security Association payload may include
   attributes that modify or complete the specification of the
   transform. These attributes are type/value pairs and are defined
   below. For example, if an encryption algorithm has a variable length
   key, the key length to be used may be specified as an attribute.
   Attributes can have a value with a fixed two octet length or a
   variable length value. For the latter, the attribute is encoded as
   type/length/value.














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                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !A!       Attribute Type        !    AF=0  Attribute Length     !
      !F!                             !    AF=1  Attribute Value      !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                   AF=0  Attribute Value                       !
      !                   AF=1  Not Transmitted                       !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 9:  Data Attributes

      o  Attribute Type (2 octets) - Unique identifier for each type of
         attribute (see below).

         The most significant bit of this field is the Attribute Format
         bit (AF). It indicates whether the data attributes follow the
         Type/Length/Value (TLV) format or a shortened Type/Value (TV)
         format.  If the AF bit is zero (0), then the Data Attributes
         are of the Type/Length/Value (TLV) form. If the AF bit is a
         one (1), then the Data Attributes are of the Type/Value form.

      o  Attribute Length (2 octets) - Length in octets of the Attribute
         Value.  When the AF bit is a one (1), the Attribute Value is
         only 2 octets and the Attribute Length field is not present.

      o  Attribute Value (variable length) - Value of the Attribute
         associated with the Attribute Type.  If the AF bit is a
         zero (0), this field has a variable length defined by the
         Attribute Length field.  If the AF bit is a one (1), the
         Attribute Value has a length of 2 octets.

   Note that while quite a few attribute types are defined, the only
   algorithms defined in this document that accept attributes are the
   defined on the fly Diffie-Hellman groups, whose use is optional and
   likely unusual. An IKEv2 implementation MAY ignore attributes if it
   does not support any algorithms that use them.

   Attributes described as basic MUST NOT be encoded using the variable
   length encoding.  Variable length attributes MUST NOT be encoded as
   basic even if their value can fit into two octets. NOTE: This is a
   change from IKEv1, where increased flexibility may have simplified
   the composer of messages but certainly complicated the parser.

         Attribute Type                 value        Attribute Format
      --------------------------------------------------------------
      RESERVED                           0-5
      Group Prime/Irreducible Polynomial  6                 TLV



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      Group Generator One                 7                 TLV
      Group Generator Two                 8                 TLV
      Group Curve A                       9                 TLV
      Group Curve B                      10                 TLV
      RESERVED                          11-13
      Key Length                         14                 TV
      Field Size                         15                 TV
      Group Order                        16                 TLV
      Block Size                         17                 TV

   values 0-5, 11-13, and 18-16383 are reserved to IANA. Values
   16384-32767 are for private use among mutually consenting parties.

   - Group Prime/Irreducible Polynomial

      The prime number of a MODP Diffie-Hellman group or the irreducible
      polynomial of an elliptic curve when specifying a private Diffie-
      Hellman group.

   - Generator One, Generator Two

      The X- and Y-coordinate of a point on an elliptic curve. When the
      Y-coordinate (generator two) is not given it can be computed with
      the X-coordinate and the definition of the curve.

   - Curve A, Curve B

      Coefficients from the definition of an elliptic curve:

          y^2 + xy = x^3 + (curve A)x^2 + (curve B)

   - Key Length

      When using an Encryption Algorithm that has a variable length key,
      this attribute specifies the key length in bits. (MUST use network
      byte order). This attribute MUST NOT be used when the specified
      Encryption Algorithm uses a fixed length key.

   - Field Size

      The field size, in bits, of a Diffie-Hellman group.

   - Group Order

      The group order of an elliptic curve group. Note the length of
      this attribute depends on the field size.

   - Block Size



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      The number of bits per block of a cipher with a variable block
      length.

3.3.6 Attribute Negotiation

   During security association negotiation Initiators present offers to
   Responders. Responders MUST select a single complete set of
   parameters from the offers (or reject all offers if none are
   acceptable).  If there are multiple proposals, the Responder MUST
   choose a single proposal number and return all of the Proposal
   substructures with that Proposal number.  If there are multiple
   Transforms with the same type the Responder MUST choose a single one.
   Any attributes of a selected transform MUST be returned unmodified.
   The Initiator of an exchange MUST check that the accepted offer is
   consistent with one of its proposals, and if not that response MUST
   be rejected.

   Negotiating Diffie-Hellman groups presents some special challenges.
   Diffie-Hellman groups are specified either using a defined group
   description (see Appendix B) or by defining all attributes of a group
   in an IKE proposal. Group attributes, such as group type or prime
   number MUST NOT be provided in conjunction with groups that have
   assigned codes. SA offers include proposed attributes and a Diffie-
   Hellman public number (KE) in the same message. If the Initiator
   offers to use one of several Diffie-Hellman groups, it SHOULD pick
   the one the Responder is most likely to accept and include a KE
   corresponding to that group. If the guess turns out to be wrong, the
   Responder will indicate the correct group in the response and the
   Initiator SHOULD pick an element of that group for its KE value in
   the third message. If the Initiator guesses wrong in a
   CREATE_CHILD_SA negotiation, no SA is created and the Initiator
   SHOULD retry with the correct group.

   Implementation Note:

      Certain negotiable attributes can have ranges or could have
      multiple acceptable values. These are the Diffie-Hellman group and
      the key length of a variable key length symmetric cipher. To
      further interoperability and to support upgrading endpoints
      independently, implementers of this protocol SHOULD accept values
      which they deem to supply greater security. For instance if a peer
      is configured to accept a variable lengthed cipher with a key
      length of X bits and is offered that cipher with a larger key
      length an implementation SHOULD accept the offer.

   Support of this capability allows an implementation to express a
   concept of "at least" a certain level of security-- "a key length of
   _at least_ X bits for cipher foo".



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3.4 Key Exchange Payload

   The Key Exchange Payload, denoted KE in this memo, is used to
   exchange Diffie-Hellman public numbers as part of a Diffie-Hellman
   key exchange.  The Key Exchange Payload consists of the IKE generic
   payload header followed by the Diffie-Hellman public value itself.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! DH Group #    !                RESERVED                       !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                       Key Exchange Data                       ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 10:  Key Exchange Payload Format

   A key exchange payload is constructed by copying one's Diffie-Hellman
   public value into the "Key Exchange Data" portion of the payload.
   The length of the Diffie-Hellman public value MUST be equal to the
   length of the prime modulus over which the exponentiation was
   performed, prepending zero bits to the value if necessary.

   The DH Group # identifies the Diffie-Hellman group in which the Key
   Exchange Data was computed (see Appendix B).  If the selected
   proposal uses a different Diffie-Hellman group, the message MUST be
   rejected with a Notify payload of type INVALID_KE_PAYLOAD.

   The payload type for the Key Exchange payload is four (4).

3.5 Identification Payloads

   The Identification Payloads, denoted IDi and IDr in this memo, allow
   peers to assert an identity to one another. This identity may be used
   for policy lookup, but does not necessarily have to match anything in
   the CERT payload; both fields may be used by an implementation to
   perform access control decisions.

   NOTE: In IKEv1, two ID payloads were used in each direction to hold
   Traffic Selector information for data passing over the SA. In IKEv2,
   this information is carried in Traffic Selector (TS) payloads (see
   section 3.13).

   The Identification Payload consists of the IKE generic payload header



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   followed by identification fields as follows:

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

               Figure 11:  Identification Payload Format

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

   o  RESERVED - MUST be sent as zero; MUST be ignored on receipt.

   o  Identification Data (variable length) - Value, as indicated by
      the Identification Type. The length of the Identification Data
      is computed from the size in the ID payload header.

   The payload types for the Identification Payload are five (5) for IDi
   and eighteen (18) for IDr.

   The following table lists the assigned values for the Identification
   Type field, followed by a description of the Identification Data
   which follows:

      ID Type                           Value
      -------                           -----
      RESERVED                            0

      ID_IPV4_ADDR                        1

            A single four (4) octet IPv4 address.

      ID_FQDN                             2

            A fully-qualified domain name string.  An example of a
            ID_FQDN is, "example.com".  The string MUST not contain any
            terminators (e.g., NULL, CR, etc.).

      ID_RFC822_ADDR                      3




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            A fully-qualified RFC822 email address string, An example of
            a ID_RFC822_ADDR is, "jsmith@example.com".  The string MUST
            not contain any terminators.

      ID_IPV6_ADDR                        5

            A single sixteen (16) octet IPv6 address.

      ID_DER_ASN1_DN                      9

            The binary DER encoding of an ASN.1 X.500 Distinguished Name
            [X.501].

      ID_DER_ASN1_GN                      10

            The binary DER encoding of an ASN.1 X.500 GeneralName
            [X.509].

      ID_KEY_ID                           11

            An opaque octet stream which may be used to pass an account
            name or 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 types. Implementations
   SHOULD be capable of generating and accepting all of these types.


3.6 Certificate Payload

   The Certificate Payload, denoted CERT in this memo, provides a means
   to transport certificates or other authentication related information
   via IKE. Certificate payloads SHOULD be included in an exchange if
   certificates are available to the sender unless the peer has
   indicated an ability to retrieve this information from elsewhere
   using an HTTP_CERT_LOOKUP_SUPPORTED Notify payload. Note that the
   term "Certificate Payload" is somewhat misleading, because not all
   authentication mechanisms use certificates and data other than
   certificates may be passed in this payload.








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   The Certificate Payload is defined as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Cert Encoding !                                               !
      +-+-+-+-+-+-+-+-+                                               !
      ~                       Certificate Data                        ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 12:  Certificate Payload Format

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

           Certificate Encoding               Value
           --------------------               -----
           RESERVED                             0
           PKCS #7 wrapped X.509 certificate    1
           PGP Certificate                      2
           DNS Signed Key                       3
           X.509 Certificate - Signature        4
           Kerberos Token                       6
           Certificate Revocation List (CRL)    7
           Authority Revocation List (ARL)      8
           SPKI Certificate                     9
           X.509 Certificate - Attribute       10
           Raw RSA Key                         11
           Hash and URL of PKIX certificate    12
           Hash and URL of PKIX bundle         13
           RESERVED                          14 - 200
           PRIVATE USE                      201 - 255

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

   The payload type for the Certificate Payload is six (6).

   Specific syntax is for some of the certificate type codes above is
   not defined in this document.  The types whose syntax is defined in
   this document are:

      X.509 Certificate - Signature (4) contains a BER encoded X.509



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      certificate.

      Certificate Revocation List (7) contains a BER encoded X.509
      certificate revocation list.

      Raw RSA Key (11) contains a PKCS #1 encoded RSA key.

      Hash and URL of PKIX certificate (12) contains a 20 octet SHA-1
      hash of a PKIX certificate followed by a variable length URL that
      resolves to the BER encoded certificate itself.

      Hash and URL of PKIX bundle (13) contains a 20 octet SHA-1 hash of
      a PKIX certificate bundle followed by a variable length URL the
      resolves to the BER encoded certificate bundle itself. The bundle
      is a BER encoded SEQUENCE of certificates and CRLs.

   Implementations MUST be capable of being configured to send and
   accept up to four X.509 certificates in support of authentication.
   Implementations SHOULD be capable of being configured to send and
   accept Raw RSA keys and the two Hash and URL formats.  If multiple
   certificates are sent, the first certificate MUST contain the public
   key used to sign the AUTH payload. The other certificates may be sent
   in any order.

3.7 Certificate Request Payload

   The Certificate Request Payload, denoted CERTREQ in this memo,
   provides a means to request preferred certificates via IKE and can
   appear in the IKE_INIT_SA response and/or the IKE_AUTH request.
   Certificate Request payloads MAY be included in an exchange when the
   sender needs to get the certificate of the receiver.  If multiple CAs
   are trusted and the cert encoding does not allow a list, then
   multiple Certificate Request payloads SHOULD be transmitted.

   The Certificate Request Payload is defined as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Cert Encoding !                                               !
      +-+-+-+-+-+-+-+-+                                               !
      ~                    Certification Authority                    ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 13:  Certificate Request Payload Format



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   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 seven (7).

   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 CAs.  Each is encoded as the SHA-1 hash
   of the Subject Public Key Info element (see section 4.1.2.7 of [RFC
   3280]) from each Trust Anchor certificate.  The twenty-octet hashes
   are concatenated and included with no other formatting.

   Note that the term "Certificate Request" is somewhat misleading, in
   that values other than certificates are defined in a "Certificate"
   payload and requests for those values can be present in a Certificate
   Request Payload. The syntax of the Certificate Request payload in
   such cases is not defined in this document.

   The Certificate Request Payload is processed by inspecting the "Cert
   Encoding" field to determine whether the processor has any
   certificates of this type. If so the "Certification Authority" field
   is inspected to determine if the processor has any certificates which
   can be validated up to one of the specified certification
   authorities. This can be a chain of certificates. If a certificate
   exists which satisfies the criteria specified in the Certificate
   Request Payload, the certificate MUST be sent back to the certificate
   requestor; if a certificate chain exists which goes back to the
   certification authority specified in the request the entire chain
   SHOULD be sent back to the certificate requestor. If multiple
   certificates or chains exist that satisfy the request, the sender
   MUST pick one. If no certificates exist then the Certificate Request
   Payload is ignored. This is not an error condition of the protocol.
   There may be cases where there is a preferred CA, but an alternate
   might be acceptable (perhaps after prompting a human operator).

3.8 Authentication Payload

   The Authentication Payload, denoted AUTH in this memo, contains data
   used for authentication purposes. The syntax of the Authentication
   data varies according to the Auth Method as specified below.




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   The Authentication Payload is defined as follows:

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

                 Figure 14:  Authentication Payload Format

   o  Auth Method (1 octet) - Specifies the method of authentication
      used. Values defined are:

        RSA Digital Signature (1) - Computed as specified in section
        2.15 using an RSA private key over a PKCS#1 padded hash.

        Shared Key Message Integrity Code (2) - Computed as specified in
        section 2.15 using the shared key associated with the identity
        in the ID payload and the negotiated prf function

        DSS Digital Signature (3) - Computed as specified in section
        2.15 using a DSS private key over a SHA-1 hash.

        The values 0 and 4-200 are reserved to IANA. The values 201-255
        are available for private use.

   o  Authentication Data (variable length) - see section 2.15.

      The payload type for the Authentication Payload is nine (9).

3.9 Nonce Payload

   The Nonce Payload, denoted Ni and Nr in this memo for the Initiator's
   and Responder's nonce respectively, contains random data used to
   guarantee liveness during an exchange and protect against replay
   attacks.









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   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 ten (10).

   The size of a Nonce MUST be between 8 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.




















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   The Notify Payload is defined as follows:

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

               Figure 16:  Notification Payload Format

   o  SECURITY_PROTOCOL_ID (1 octet) - If this notification concerns
      an existing SA, this field indicates the type of that SA.
      For IKE_SA notifications, this field MUST be one (1). For
      notifications concerning IPsec SAs this field MUST contain
      either (2) to indicate AH or (3) to indicate ESP. For
      notifications which do not relate to an existing SA, this
      field MUST be sent as zero and MUST be ignored on receipt.
      All other values for this field are reserved to IANA for future
      assignment.

   o  SPI Size (1 octet) - Length in octets of the SPI as defined by
      the SECURITY_PROTOCOL_ID or zero if no SPI is applicable.  For a
      notification concerning the IKE_SA, the SPI Size MUST be zero.

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

   o  SPI (variable length) - Security Parameter Index.

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

      The payload type for the Notification Payload is eleven (11).







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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 IKE V1 both for simplification and to avoid giving
   configuration information to probers.

   Types in the range 0 - 16383 are intended for reporting errors.  An
   implementation receiving a Notify payload with one of these types
   that it does not recognize in a response MUST assume that the
   corresponding request has failed entirely. Unrecognized error types
   in a request and status types in a request or response MUST be
   ignored except that they SHOULD be logged.

   Notify payloads with status types MAY be added to any message and
   MUST be ignored if not recognized. They are intended to indicate
   capabilities, and as part of SA negotiation are used to negotiate
   non-cryptographic parameters.

        NOTIFY MESSAGES - ERROR TYPES           Value
        -----------------------------           -----
        UNSUPPORTED_CRITICAL_PAYLOAD              1

            Sent if the payload has the "critical" bit set and the
            payload type is not recognized. Notification Data contains
            the one octet payload type.

        INVALID_IKE_SPI                           4

            Indicates an IKE message was received with an unrecognized
            destination SPI. This usually indicates that the recipient
            has rebooted and forgotten the existence of an IKE_SA.

        INVALID_MAJOR_VERSION                     5

            Indicates the recipient cannot handle the version of IKE
            specified in the header. The closest version number that the
            recipient can support will be in the reply header.

        INVALID_SYNTAX                            7

            Indicates the IKE message was received was invalid because
            some type, length, or value was out of range or because the
            request was rejected for policy reasons. To avoid a denial
            of service attack using forged messages, this status may



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            only be returned for and in an encrypted packet if the
            MESSAGE_ID and cryptographic checksum were valid. To avoid
            leaking information to someone probing a node, this status
            MUST be sent in response to any error not covered by one of
            the other status types. To aid debugging, more detailed
            error information SHOULD be written to a console or log.

        INVALID_MESSAGE_ID                        9

            Sent when an IKE MESSAGE_ID outside the supported window is
            received.  This Notify MUST NOT be sent in a response; the
            invalid request MUST NOT be acknowledged.  Instead, inform
            the other side by initiating an INFORMATIONAL exchange with
            Notification data containing the four octet invalid
            MESSAGE_ID. Sending this notification is optional and
            notifications of this type MUST be rate limited.

        INVALID_SPI                              11

            MAY be sent in an IKE INFORMATIONAL Exchange when a node
            receives an ESP or AH packet with an invalid SPI. The
            Notification Data contains the SPI of the invalid packet.
            This usually indicates a node has rebooted and forgotten an
            SA.  If this Informational Message is sent outside the
            context of an IKE_SA, it should only be used by the
            recipient as a "hint" that something might be wrong (because
            it could easily be forged).

        NO_PROPOSAL_CHOSEN                       14

            None of the proposed crypto suites was acceptable.

        INVALID_KE_PAYLOAD                       17

            The D-H Group # field in the KE payload is not the group #
            selected by the responder for this exchange. There is one
            octet of data associated with this notification: the
            accepted D-H Group #.

        AUTHENTICATION_FAILED                    24

            Sent in the response to an IKE_AUTH message when for some
            reason the authentication failed. There is no associated
            data.

        SINGLE_PAIR_REQUIRED                     34

            This error indicates that a CREATE_CHILD_SA request is



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            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 he is trying to forward.

        NO_ADDITIONAL_SAS                        35

            This error indicates that a CREATE_CHILD_SA request is
            unacceptable because the Responder is unwilling to accept
            any more CHILD_SAs on this IKE_SA. Some minimal
            implementations may only accept a single CHILD_SA setup in
            the context of an initial IKE exchange and reject any
            subsequent attempts to add more.

        INTERNAL_ADDRESS_FAILURE                 36
            Indicates an error assigning an internal address (i.e.,
            INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS) during the
            processing of a Configuration Payload by a Responder.  If
            this error is generated within an IKE_AUTH exchange no
            CHILD_SA will be created.

        FAILED_CP_REQUIRED                       37
            Sent by responder in the case where CP(CFG_REQUEST) was
            expected but not received, and so is a conflict with locally
            configured policy. There is no associated data.

        TS_UNACCEPTABLE                          38
            Indicates that none of the addresses/protocols/ports in the
            supplied traffic selectors is acceptable.

        RESERVED TO IANA - Errors             39 - 8191

        Private Use - Errors                8192 - 16383



        NOTIFY MESSAGES - STATUS TYPES           Value
        ------------------------------           -----

        RESERVED TO IANA - STATUS            16384 - 24577

        INITIAL_CONTACT                          24578

            This notification asserts that this IKE_SA is the only
            IKE_SA currently active between the authenticated
            identities. It MAY be sent when an IKE_SA is established
            after a crash, and the recipient MAY use this information to
            delete any other IKE_SAs it has to the same authenticated



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            identity without waiting for a timeout.  This notification
            MUST NOT be sent by an entity that may be replicated (e.g.,
            a roaming user's credentials where the user is allowed to
            connect to the corporate firewall from two remote systems at
            the same time).

        SET_WINDOW_SIZE                          24579

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

        ADDITIONAL_TS_POSSIBLE                   24580

            This notification asserts that the sending endpoint narrowed
            the proposed traffic selectors but that other traffic
            selectors would also have been acceptable, though only in a
            separate SA (see section 2.9). There is no data associated
            with this Notify type. It may only be sent as an additional
            payload in a message including accepted TSs.

        IPCOMP_SUPPORTED                         24581

            This notification may only be included in a message
            containing an SA payload negotiating a CHILD_SA and
            indicates a willingness by its sender to use IPcomp on this
            SA. The data associated with this notification includes a
            two octet IPcomp CPI followed by a one octet transform ID
            optionally followed by attributes whose length and format is
            defined by that transform ID. A message proposing an SA may
            contain multiple IPCOMP_SUPPORTED notifications to indicate
            multiple supported algorithms. A message accepting an SA may
            contain at most one.

            The transform IDs currently defined are:

                 NAME         NUMBER  DEFINED IN
                 -----------  ------  -----------
                 RESERVED       0
                 IPCOMP_OUI     1
                 IPCOMP_DEFLATE 2     RFC 2394
                 IPCOMP_LZS     3     RFC 2395




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                 values 4-240 are reserved to IANA. Values 241-255 are
                 for private use among mutually consenting parties.

        NAT_DETECTION_SOURCE_IP                  24582

            This notification is used to by its recipient to determine
            whether the source is behind a NAT box. The data associated
            with this notification is a SHA-1 digest of the SPIs (in the
            order they appear in the header), IP address and port on
            which this packet was sent.  There MAY be multiple Notify
            payloads of this type in a message if the sender does not
            know which of several network attachments will be used to
            send the packet. The recipient of this notification MAY
            compare the supplied value to a SHA-1 hash of the SPIs,
            source IP address and port and if they don't match it SHOULD
            enable NAT traversal (see section 2.23).  Alternately, it
            MAY reject the connection attempt if NAT traversal is not
            supported.

        NAT_DETECTION_DESTINATION_IP             24583

            This notification is used to by its recipient to determine
            whether it is behind a NAT box. The data associated with
            this notification is a SHA-1 digest of the SPIs (in the
            order they appear in the header), IP address and port to
            which this packet was sent.  The recipient of this
            notification MAY compare the supplied value to a hash of the
            SPIs, destination IP address and port and if they don't
            match it SHOULD invoke NAT traversal (see section 2.23). If
            they don't match, it means that this end is behind a NAT.
            Alternately, it MAY reject the connection attempt if NAT
            traversal is not supported.

        COOKIE                                   24584

            This notification MAY be included in an IKE_SA_INIT
            response. It indicates that the request should be retried
            with a copy of this notification as the first payload.  This
            notification MUST be included in an IKE_SA_INIT request
            retry if a COOKIE notification was included in the initial
            response.  The data associated with this notification MUST
            be between 1 and 64 octets in length (inclusive).

        USE_TRANSPORT_MODE                       24585

            This notification MAY be included in a request message that
            also includes an SA requesting a CHILD_SA. It requests that
            the CHILD_SA use transport mode rather than tunnel mode for



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            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.

        HTTP_CERT_LOOKUP_SUPPORTED               24586

            This notification MAY be included 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).

        RESERVED TO IANA - STATUS            24587 - 40959

        Private Use - STATUS                 40960 - 65535

3.11 Delete Payload

   The Delete Payload, denoted D in this memo, contains a protocol
   specific security association identifier that the sender has removed
   from its security association database and is, therefore, no longer
   valid.  Figure 17 shows the format of the Delete Payload. It is
   possible to send multiple SPIs in a Delete payload, however, each SPI
   MUST be for the same protocol. Mixing of Protocol Identifiers MUST
   NOT be performed in a 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 SECURITY_PROTOCOL_ID of 1
   (IKE) but no SPIs.  Deletion of a CHILD_SA, such as ESP or AH, will
   contain the SECURITY_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.













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   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        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! S_PROTOCOL_ID !   SPI Size    !           # of SPIs           !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~               Security Parameter Index(es) (SPI)              ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 17:  Delete Payload Format

   o  SECURITY_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 SECURITY_PROTOCOL_ID.  Zero for IKE (SPI is in message
      header) or four for AH and ESP.

   o  # of SPIs (2 octets) - The number of SPIs contained in the Delete
      payload.  The size of each SPI is defined by the SPI Size field.

   o  Security Parameter Index(es) (variable length) - Identifies the
      specific security association(s) to delete. The length of this
      field is determined by the SPI Size and # of SPIs fields.

      The payload type for the Delete Payload is twelve (12).

3.12 Vendor ID Payload

   The Vendor ID Payload contains a vendor defined constant.  The
   constant is used by vendors to identify and recognize remote
   instances of their implementations.  This mechanism allows a vendor
   to experiment with new features while maintaining backwards
   compatibility.

   A Vendor ID payload MAY announce that the sender is capable to
   accepting certain extensions to the protocol, or it MAY simply
   identify the implementation as an aid in debugging.  A Vendor ID
   payload MUST NOT change the interpretation of any information defined
   in this specification (i.e., it MUST be non-critical).  Multiple
   Vendor ID payloads MAY be sent. An implementation is NOT REQUIRED to
   send any Vendor ID payload at all.




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   A Vendor ID payload may be sent as part of any message.  Reception of
   a familiar Vendor ID payload allows an implementation to make use of
   Private USE numbers described throughout this memo-- private
   payloads, private exchanges, private notifications, etc. Unfamiliar
   Vendor IDs MUST be ignored.

   Writers of Internet-Drafts who wish to extend this protocol MUST
   define a Vendor ID payload to announce the ability to implement the
   extension in the Internet-Draft. It is expected that Internet-Drafts
   which gain acceptance and are standardized will be given "magic
   numbers" out of the Future Use range by IANA and the requirement to
   use a Vendor ID will go away.

   The Vendor ID Payload fields are defined as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                        Vendor ID (VID)                        ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 18:  Vendor ID Payload Format

   o  Vendor ID (variable length) - It is the responsibility of
      the person choosing the Vendor ID to assure its uniqueness
      in spite of the absence of any central registry for IDs.
      Good practice is to include a company name, a person name
      or some such. If you want to show off, you might include
      the latitude and longitude and time where you were when
      you chose the ID and some random input. A message digest
      of a long unique string is preferable to the long unique
      string itself.

      The payload type for the Vendor ID Payload is thirteen (13).


3.13 Traffic Selector Payload

   The Traffic Selector Payload, denoted TS in this memo, allows peers
   to identify packet flows for processing by IPsec security services.
   The Traffic Selector Payload consists of the IKE generic payload
   header followed by individual traffic selectors as follows:





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                           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.

   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 fourteen (14)
   for TSi and nineteen (19) for TSr.

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     !  Protocol_ID  |       Selector Length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Start_Port          |           End_Port            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                         Starting Address                      ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                         Ending Address                        ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 20: Traffic Selector



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   o  TS Type (one octet) - Specifies the type of traffic selector.

   o  Protocol ID (1 octet) - Value specifying an associated IP
      protocol ID (e.g., UDP/TCP). A value of zero means that the
      Protocol ID is not relevant to this traffic selector--
      the SA can carry all protocols.

   o  Selector Length - Specifies the length of this Traffic
      Selector Substructure including the header.

   o  Start_Port (2 octets) - Value specifying the smallest port
      number allowed by this Traffic Selector. For protocols for
      which port is undefined, or if all ports are allowed by
      this Traffic Selector, this field MUST be zero.

   o  End_Port (2 octets) - Value specifying the largest port
      number allowed by this Traffic Selector. For protocols for
      which port is undefined, or it all ports are allowed by
      this Traffic Selector, this field MUST be 65535.

   o  Starting Address - The smallest address included in this
      Traffic Selector (length determined by TS type).

   o  Ending Address - The largest address included in this
      Traffic Selector (length determined by TS type).

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

      TS Type                           Value
      -------                           -----
      RESERVED                            0

      TS_IPV4_ADDR_RANGE                  7

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

      TS_IPV6_ADDR_RANGE                  8

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



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3.14 Encrypted Payload

   The Encrypted Payload, denoted SK{...} in this memo, contains other
   payloads in encrypted form. The Encrypted Payload, if present in a
   message, MUST be the last payload in the message. Often, it is the
   only payload in the message.

   The algorithms for encryption and integrity protection are negotiated
   during IKE_SA setup, and the keys are computed as specified in
   sections 2.14 and 2.18.

   The encryption and integrity protection algorithms are modelled after
   the ESP algorithms described in RFCs 2104, 2406, 2451. This document
   completely specifies the cryptographic processing of IKE data, but
   those documents should be consulted for design rationale. We assume a
   block cipher with a fixed block size and an integrity check algorithm
   that computes a fixed length checksum over a variable size message.

   The Payload Type for an Encrypted payload is fifteen (15).  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.



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   o  Payload Length - Includes the lengths of the header, IV,
      Encrypted IKE Payloads, Padding, Pad Length and Integrity
      Checksum Data.

   o  Initialization Vector - A randomly chosen value whose length
      is equal to the block length of the underlying encryption
      algorithm. Recipients MUST accept any value. Senders SHOULD
      either pick this value pseudo-randomly and independently for
      each message or use the final ciphertext block of the previous
      message sent. Senders MUST NOT use the same value for each
      message, use a sequence of values with low hamming distance
      (e.g., a sequence number), or use ciphertext from a received
      message.

   o  IKE Payloads are as specified earlier in this section. This
      field is encrypted with the negotiated cipher.

   o  Padding MAY contain any value chosen by the sender, and MUST
      have a length that makes the combination of the Payloads, the
      Padding, and the Pad Length to be a multiple of the encryption
      block size. This field is encrypted with the negotiated
      cipher.

   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.

3.15 Configuration Payload

   The Configuration payload, denoted CP in this document, is used to
   exchange configuration information between IKE peers.  Currently, the
   only defined uses for this exchange is for an IRAC to request an
   internal IP address from an IRAS or for either party to request
   version information from the other, but this payload is intended as a
   likely place for future extensions.

   Configuration payloads are of type CFG_REQUEST/CFG_REPLY or
   CFG_SET/CFG_ACK (see CFG Type in the payload description below).
   CFG_REQUEST and CFG_SET payloads may optionally be added to any IKE
   request. The IKE response MUST include either a corresponding



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   CFG_REPLY or CFG_ACK or a Notify payload with an error type
   indicating why the request could not be honored. An exception is that
   a minimal implementation MAY ignore all CFG_REQUEST and CFG_SET
   payloads, so a response message without a corresponding CFG_REPLY or
   CFG_ACK MUST be accepted as an indication that the request was not
   supported.

   "CFG_REQUEST/CFG_REPLY" allows an IKE endpoint to request information
   from its peer.  If an attribute in the CFG_REQUEST Configuration
   Payload is not zero length it is taken as a suggestion for that
   attribute.  The CFG_REPLY Configuration Payload MAY return that
   value, or a new one.  It MAY also add new attributes and not include
   some requested ones. Requestors MUST ignore returned attributes that
   they do not recognize.

   Some attributes MAY be multi-valued, in which case multiple attribute
   values of the same type are sent and/or returned. Generally, all
   values of an attribute are returned when the attribute is requested.
   For some attributes (in this version of the specification only
   internal addresses), multiple requests indicates a request that
   multiple values be assigned. For these attributes, the number of
   values returned SHOULD NOT exceed the number requested.

   If the data type requested in a CFG_REQUEST is not recognized or not
   supported, the responder MUST NOT return an error type but rather
   MUST either send a CFG_REPLY which MAY be empty or a reply not
   containing a CFG_REPLY payload at all. Error returns are reserved for
   cases where the request is recognized but cannot be performed as
   requested or the request is badly formatted.

   "CFG_SET/CFG_ACK" allows an IKE endpoint to push configuration data
   to its peer.  In this case the CFG_SET Configuration Payload contains
   attributes the initiator wants its peer to alter.  The responder MUST
   return a Configuration Payload if it accepted any of the
   configuration data and it MUST contain the attributes that the
   responder accepted with zero length data.  Those attributes that it
   did not accept MUST NOT be in the CFG_ACK Configuration Payload. If
   no attributes were accepted, the responder MUST return either an
   empty CFG_ACK payload or a response message without a CFG_ACK
   payload.  There are currently no defined uses for the CFG_SET/CFG_ACK
   exchange, though they may be used in connection with extensions based
   on Vendor IDs. An minimal implementation of this specification MAY
   ignore CFG_SET payloads.

   Extensions via the CP payload SHOULD NOT be used for general purpose
   management.  Its main intent is to provide a bootstrap mechanism to
   exchange information within IPsec from IRAS to IRAC.  While it MAY be
   useful to use such a method to exchange information between some



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   Security Gateways (SGW) or small networks, existing management
   protocols such as DHCP [DHCP], RADIUS [RADIUS], SNMP or LDAP [LDAP]
   should be preferred for enterprise management as well as subsequent
   information exchanges.

   The Configuration Payload is defined as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Next Payload  !C! RESERVED    !         Payload Length        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !   CFG Type    !                    RESERVED                   !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                   Configuration Attributes                    ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 22:  Configuration Payload Format

   The payload type for the Configuration Payload is 16.

   o  CFG Type (1 octet) - The type of exchange represented by the
      Configuration Attributes.

             CFG Type       Value
             ===========    =====
             RESERVED         0
             CFG_REQUEST      1
             CFG_REPLY        2
             CFG_SET          3
             CFG_ACK          4

      values 5-127 are reserved to IANA. Values 128-255 are for private
      use among mutually consenting parties.

   o  RESERVED (3 octets)  - MUST be sent as zero; MUST be ignored on
      receipt.

   o  Configuration Attributes (variable length) - These are type
      length values specific to the Configuration Payload and are
      defined below. There may be zero or more Configuration
      Attributes in this payload.







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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 (7 bits) - A unique identifier for each of the
      Configuration Attribute Types.

   o  Length (2 octets) - Length in octets of Value.

   o  Value (0 or more octets) - The variable length value of this
      Configuration Attribute.

   The following attribute types have been defined:

                                      Multi-
        Attribute Type          Value Valued Length
        ======================= ===== ====== ==================
         RESERVED                 0
         INTERNAL_IP4_ADDRESS     1    YES*  0 or 4 octets
         INTERNAL_IP4_NETMASK     2    NO    0 or 4 octets
         INTERNAL_IP4_DNS         3    YES   0 or 4 octets
         INTERNAL_IP4_NBNS        4    YES   0 or 4 octets
         INTERNAL_ADDRESS_EXPIRY  5    NO    0 or 4 octets
         INTERNAL_IP4_DHCP        6    YES   0 or 4 octets
         APPLICATION_VERSION      7    NO    0 or more
         INTERNAL_IP6_ADDRESS     8    YES*  0 or 16 octets
         INTERNAL_IP6_NETMASK     9    NO    0 or 16 octets
         INTERNAL_IP6_DNS        10    YES   0 or 16 octets
         INTERNAL_IP6_NBNS       11    YES   0 or 16 octets
         INTERNAL_IP6_DHCP       12    YES   0 or 16 octets
         INTERNAL_IP4_SUBNET     13    NO    0 or 8 octets
         SUPPORTED_ATTRIBUTES    14    NO    Multiple of 2
         INTERNAL_IP6_SUBNET     15    NO    17 octets

      * These attributes may be multi-valued on return only if



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        multiple values were requested.

        Types 16-16383 are reserved to IANA. Values 16384-32767 are for
        private use among mutually consenting parties.

      o  INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
         internal network, sometimes called a red node address or
         private address and MAY be a private address on the Internet.
         Multiple internal addresses MAY be requested by requesting
         multiple internal address attributes.  The responder MAY only
         send up to the number of addresses requested.

         The requested address is valid until the expiry time defined
         with the INTERNAL_ADDRESS EXPIRY attribute or there are no
         IKE_SAs between the peers.

      o  INTERNAL_IP4_NETMASK, INTERNAL_IP6_NETMASK - The internal
         network's netmask.  Only one netmask is allowed in the request
         and reply messages (e.g., 255.255.255.0) and it MUST be used
         only with an INTERNAL_ADDRESS attribute.

      o  INTERNAL_IP4_DNS, INTERNAL_IP6_DNS - Specifies an address of a
         DNS server within the network.  Multiple DNS servers MAY be
         requested.  The responder MAY respond with zero or more DNS
         server attributes.

      o  INTERNAL_IP4_NBNS, INTERNAL_IP6_NBNS - Specifies an address of
         a NetBios Name Server (WINS) within the network.  Multiple NBNS
         servers MAY be requested.  The responder MAY respond with zero
         or more NBNS server attributes.

      o  INTERNAL_ADDRESS_EXPIRY - Specifies the number of seconds that
         the host can use the internal IP address.  The host MUST renew
         the IP address before this expiry time.  Only one of these
         attributes MAY be present in the reply.

      o  INTERNAL_IP4_DHCP, INTERNAL_IP6_DHCP - Instructs the host to
         send any internal DHCP requests to the address contained within
         the attribute.  Multiple DHCP servers MAY be requested.  The
         responder MAY respond with zero or more DHCP server attributes.

      o  APPLICATION_VERSION - The version or application information of
         the IPsec host.  This is a string of printable ASCII characters
         that is NOT null terminated.

      o  INTERNAL_IP4_SUBNET - The protected sub-networks that this
         edge-device protects.  This attribute is made up of two fields;
         the first being an IP address and the second being a netmask.



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         Multiple sub-networks MAY be requested.  The responder MAY
         respond with zero or more sub-network attributes.

      o  SUPPORTED_ATTRIBUTES - When used within a Request, this
         attribute MUST be zero length and specifies a query to the
         responder to reply back with all of the attributes that it
         supports.  The response contains an attribute that contains a
         set of attribute identifiers each in 2 octets.  The length
         divided by 2 (octets) would state the number of supported
         attributes contained in the response.

      o  INTERNAL_IP6_SUBNET - The protected sub-networks that this
         edge-device protects.  This attribute is made up of two fields;
         the first being a 16 octet IPv6 address the second being a one
         octet prefix-length as defined in [ADDRIPV6].  Multiple
         sub-networks MAY be requested.  The responder MAY respond with
         zero or more sub-network attributes.

      Note that no recommendations are made in this document how an
      implementation actually figures out what information to send in a
      reply.  i.e., we do not recommend any specific method of an IRAS
      determining which DNS server should be returned to a requesting
      IRAC.

3.16 Extended Authentication Protocol (EAP) Payload

   The Extended Authentication Protocol Payload, denoted EAP in this
   memo, allows IKE_SAs to be authenticated using the protocol defined
   in RFC 2284 [EAP] and subsequent extensions to that protocol. The
   full set of acceptable values for the payload are defined elsewhere,
   but a short summary of RFC 2284 is included here to make this
   document stand alone in the common cases.

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ! Next Payload  !C!  RESERVED   !         Payload Length        !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       !                                                               !
       ~                       EAP Message                             ~
       !                                                               !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 24:  EAP Payload Format

      The payload type for an EAP Payload is 17.





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                            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 (one octet) indicates whether this message is a
      Request (1), Response (2), Success (3), or Failure (4).

   o  Identifier (one octet) is used in PPP to distinguish replayed
      messages from repeated ones. Since in IKE, EAP runs over a
      reliable protocol, it serves no function here. In a response
      message this octet MUST be set to match the identifier in the
      corresponding request. In other messages, this field MAY
      be set to any value.

   o  Length (two octets) is the length of the EAP message and MUST
      be four less than the Payload Length of the encapsulating
      payload.

   o  Type (one octet) is present only if the Code field is Request
      (1) or Response (2). For other types, 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 NAC or match the type of the data requested. The
      following types are defined in RFC 2284:

      1  Identity
      2  Notification
      3  NAK (Response Only)
      4  MD5-Challenge
      5  One-Time Password (OTP)
      6  Generic Token Card

   o  Type_Data (Variable Length) contains data depending on the Code
      and Type. In Requests other than MD5-Challenge, this field
      contains a prompt to be displayed to a human user. For NAK, it
      contains one octet suggesting the type of authentication the
      Initiator would prefer to use. For most other responses, it
      contains the authentication code typed by the human user.

   Note that since IKE passes an indication of initiator identity in
   message 3 of the protocol, EAP based prompts for Identity SHOULD NOT



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   be used.

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 preventing the bad guys from
   interoperating with one's systems. So a particular implementation may
   be configured with any of a number of restrictions concerning
   algorithms and trusted authorities that will prevent universal
   interoperability.

   IKEv2 is designed to permit minimal implementations that can
   interoperate with all compliant implementations. There are a series
   of optional features that can easily be ignored by a particular
   implementation if it does not support that feature. Those features
   include:

   Ability to negotiate SAs through a NAT and tunnel the resulting ESP
   SA over UDP.

   Ability to request (and respond to a request for) a temporary IP
   address on the remote end of a tunnel.

   Ability to support various types of legacy authentication.

   Ability to support window sizes greater than one.

   Ability to establish multiple ESP and/or AH SAs within a single
   IKE_SA.

   Ability to rekey SAs.

   To assure interoperability, all implementations MUST be capable of
   parsing all payload types (if only to skip over them) and to ignore
   payload types that it does not support unless the critical bit is set
   in the payload header. If the critical bit is set in an unsupported
   payload header, all implementations MUST reject the messages
   containing those payloads.

   Every implementation MUST be capable of doing four message
   IKE_SA_INIT and IKE_AUTH exchanges establishing two SAs (one for IKE,
   one for ESP and/or AH).  Implementations MAY be initiate-only or
   respond-only if appropriate for their platform. Every implementation
   MUST be capable of responding to an INFORMATIONAL exchange, but a
   minimal implementation MAY respond to any INFORMATIONAL message with
   an empty INFORMATIONAL reply. A minimal implementation MAY support



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   the CREATE_CHILD_SA exchange only in so far as to recognize requests
   and reject them with a Notify payload of type NO_ADDITIONAL_SAS. A
   minimal implementation need not be able to initiate CREATE_CHILD_SA
   or INFORMATIONAL exchanges. When an SA expires (based on locally
   configured values of either lifetime or octets passed), and
   implementation MAY either try to renew it with a CREATE_CHILD_SA
   exchange or it MAY delete (close) the old SA and create a new one. If
   the responder rejects the CREATE_CHILD_SA request with a
   NO_ADDITIONAL_SAS notification, the implementation MUST be capable of
   instead closing the old SA and creating a new one.

   Implementations are not required to support requesting temporary IP
   addresses or responding to such requests. If an implementation does
   support issuing such requests, it MUST include a CP payload in
   message 3 containing at least a field of type INTERNAL_IP4_ADDRESS or
   INTERNAL_IP6_ADDRESS. All other fields are optional. If an
   implementation supports responding to such requests, it MUST parse
   the CP payload of type CFG_REQUEST in message 3 and recognize a field
   of type INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS. If it supports
   leasing an address of the appropriate type, it MUST return a CP
   payload of type CFG_REPLY containing an address of the requested
   type. The responder SHOULD include all of the other related
   attributes if it has them.

   A minimal IPv4 responder implementation will ignore the contents of
   the CP payload except to determine that it includes an
   INTERNAL_IP4_ADDRESS attribute and will respond with the address and
   other related attributes regardless of whether the initiator
   requested them.

   A minimal IPv4 initiator will generate a CP payload containing only
   an INTERNAL_IP4_ADDRESS attribute and will parse the response
   ignoring attributes it does not know how to use. The only attribute
   it MUST be able to process is INTERNAL_ADDRESS_EXPIRY, which it must
   use to bound the lifetime of the SA unless it successfully renews the
   lease before it expires. Minimal initiators need not be able to
   request lease renewals and minimal responders need not respond to
   them.

   For an implementation to be called conforming to this specification,
   it MUST be possible to configure it to accept the following:

   PKIX Certificates containing and signed by RSA keys of size 1024 or
   2048 bits, where the ID passed is any of ID_KEY_ID, ID_FQDN,
   ID_RFC822_ADDR, or ID_DER_ASN1_DN.

   Shared key authentication where the ID passes is any of ID_KEY_ID,
   ID_FQDN, or ID_RFC822_ADDR.



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

5 Security Considerations

   Repeated re-keying using CREATE_CHILD_SA without PFS leaves all SAs
   vulnerable to cryptanalysis of a single key or overrun of either
   endpoint. Implementers should take note of this fact and set a limit
   on CREATE_CHILD_SA exchanges between exponentiations.  This memo does
   not prescribe such a limit.

   The strength of a key derived from a Diffie-Hellman exchange using
   any of the groups defined here depends on the inherent strength of
   the group, the size of the exponent used, and the entropy provided by
   the random number generator used. Due to these inputs it is difficult
   to determine the strength of a key for any of the defined groups.
   Diffie-Hellman group number two, when used with a strong random
   number generator and an exponent no less than 200 bits, is sufficient
   for use with 3DES.  Groups three through five provide greater
   security. 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
   conservative estimates when establishing policy and negotiating
   security parameters.

   Note that these limitations are on the Diffie-Hellman groups
   themselves.  There is nothing in IKE which prohibits using stronger
   groups nor is there anything which will dilute the strength obtained
   from stronger groups (limited by the strength of the other algorithms
   negotiated including the prf function).  In fact, the extensible
   framework of IKE encourages the definition of more groups; use of
   elliptical curve groups may greatly increase strength using much
   smaller numbers.

   It is assumed that all Diffie-Hellman exponents are erased from
   memory after use. In particular, these exponents MUST NOT be derived
   from long-lived secrets like the seed to a pseudo-random generator
   that is not erased after use.

   The strength of all keys are limited by the size of the output of the
   negotiated prf function. For this reason, a prf function whose output
   is less than 128 bits (e.g., 3DES-CBC) MUST never be used with this
   protocol.

   The security of this protocol is critically dependent on the
   randomness of the randomly chosen parameters. These should be
   generated by a strong random or properly seeded pseudo-random source



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   (see [RFC1750]).  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].

   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
   from the IKE peer.  As the IPv4 address space is only 32 bits, and it
   is usually very sparse, it might be possible for the 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 limits the hash calculations down to 2^32. If
   educated guess of use of private address space is done, then the
   number of hash calculations needed to find out the internal IP
   address goes down to the 2^24 + 2 * (2^16).

6 IANA Considerations

   This document contains many "magic numbers" to be maintained by the
   Internet Assigned Numbers Authority (IANA). While in many cases the
   values were chosen so as to avoid collisions with other
   specifications, these should be considered a new IANA registry for
   IKEv2. The tables to be maintained are:

   Payload Types
   Transform Types
   For each Transform Type defined, the assigned Transform values
   Authentication Method
   Security Protocol ID
   Error codes
   Status codes
   IPcomp Transform IDs
   Configuration request types
   Configuration attribute types







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7 Intellectual Property Rights

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.

8 Acknowledgements

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

9 References

9.1 Normative References

   [ADDGROUP] Kivinen, T., and Kojo, M., "More Modular Exponential
              (MODP) Diffie-Hellman groups for Internet Key
              Exchange (IKE)", RFC 3526, May 2003.

   [ADDRIPV6] Hinden, R., and Deering, S.,
              "Internet Protocol Version 6 (IPv6) Addressing
              Architecture", RFC 3513, April 2003.

   [Bra96]    Bradner, S., "The Internet Standards Process --
              Revision 3", BCP 9, RFC 2026, October 1996.

   [Bra97]    Bradner, S., "Key Words for use in RFCs to indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [EAP]      Blunk, L. and Vollbrecht, J., "PPP Extensible
              Authentication Protocol (EAP), RFC 2284, March 1998.

   [ESPCBC]   Pereira, R., Adams, R., "The ESP CBC-Mode Cipher
              Algorithms", RFC 2451, November 1998.

   [RFC 3280] Housley, R., Polk, W., Ford, W., Solo, D., "Internet



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              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

9.2 Non-normative References

   [Ble98]    Bleichenbacher, D., "Chosen Ciphertext Attacks against
              Protocols Based on RSA Encryption Standard PKCS#1",
              Advances in Cryptology Eurocrypt '98, Springer-Verlag,
              1998.

   [BR94]     Bellare, M., and Rogaway P., "Optimal Asymmetric
              Encryption", Advances in Cryptology Eurocrypt '94,
              Springer-Verlag, 1994.

   [DES]      ANSI X3.106, "American National Standard for Information
              Systems-Data Link Encryption", American National Standards
              Institute, 1983.

   [DH]       Diffie, W., and Hellman M., "New Directions in
              Cryptography", IEEE Transactions on Information Theory, V.
              IT-22, n. 6, June 1977.

   [DHCP]     R. Droms, "Dynamic Host Configuration Protocol",
              RFC2131

   [DSS]      NIST, "Digital Signature Standard", FIPS 186, National
              Institute of Standards and Technology, U.S. Department of
              Commerce, May, 1994.

   [HC98]     Harkins, D., Carrel, D., "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [Hutt02]   Huttunen, A. et. al., "UDP Encapsulation of IPsec
              Packets", draft-ietf-ipsec-udp-encaps-05.txt, December
              2002.

   [IDEA]     Lai, X., "On the Design and Security of Block Ciphers,"
              ETH Series in Information Processing, v. 1, Konstanz:
              Hartung-Gorre Verlag, 1992

   [IPCOMP]   Shacham, A., Monsour, R., Pereira, R., and Thomas, M., "IP
              Payload Compression Protocol (IPComp)", RFC 2393,
              August 1998.

   [Ker01]    Keromytis, A., Sommerfeld, B., "The 'Suggested ID'
              Extension for IKE", draft-keromytis-ike-id-00.txt, 2001




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   [KBC96]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [LDAP]     M. Wahl, T. Howes, S. Kille., "Lightweight Directory
              Access Protocol (v3)", RFC2251

   [MD5]      Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
              April 1992.

   [MSST98]   Maughhan, D., Schertler, M., Schneider, M., and Turner, J.
              "Internet Security Association and Key Management Protocol
              (ISAKMP)", RFC 2408, November 1998.

   [Orm96]    Orman, H., "The Oakley Key Determination Protocol", RFC
              2412, November 1998.

   [PFKEY]    McDonald, D., Metz, C., and Phan, B., "PFKEY Key
              Management API, Version 2", RFC2367, July 1998.

   [PKCS1]    Kaliski, B., and J. Staddon, "PKCS #1: RSA Cryptography
              Specifications Version 2", September 1998.

   [PK01]     Perlman, R., and Kaufman, C., "Analysis of the IPsec key
              exchange Standard", WET-ICE Security Conference, MIT,2001,
              http://sec.femto.org/wetice-2001/papers/radia-paper.pdf.

   [Pip98]    Piper, D., "The Internet IP Security Domain Of
              Interpretation for ISAKMP", RFC 2407, November 1998.

   [RADIUS]   C. Rigney, A. Rubens, W. Simpson, S. Willens, "Remote
              Authentication Dial In User Service (RADIUS)", RFC2138

   [RFC1750]  Eastlake, D., Crocker, S., and Schiller, J., "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [RFC2401]  Kent, S., and Atkinson, R., "Security Architecture for
              the Internet Protocol", RFC 2401, November 1998.

   [RFC2474]  Nichols, K., Blake, S., Baker, F. and Black, D.,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
              and Weiss, W., "An Architecture for Differentiated
              Services", RFC 2475, December 1998.




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   [RFC2522]  Karn, P., and Simpson, W., "Photuris: Session-Key
              Management Protocol", RFC 2522, March 1999.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and Black, D.,
              "The Addition of Explicit Congestion Notification (ECN)
              to IP", RFC 3168, September 2001.

   [RSA]      Rivest, R., Shamir, A., and Adleman, L., "A Method for
              Obtaining Digital Signatures and Public-Key
              Cryptosystems", Communications of the ACM, v. 21, n. 2,
              February 1978.

   [SHA]      NIST, "Secure Hash Standard", FIPS 180-1, National
              Institute of Standards and Technology, U.S. Department
              of Commerce, May 1994.

   [SIGMA]    Krawczyk, H., "SIGMA: the `SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and its Use in the IKE
              Protocols", in Advances in Cryptography - CRYPTO 2003
              Proceedings, LNCS 2729, Springer, 2003. Available at:
              http://www.ee.technion.ac.il/~hugo/sigma.html

   [SKEME]    Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
              Mechanism for Internet", from IEEE Proceedings of the
              1996 Symposium on Network and Distributed Systems
              Security.

   [X.501]    ITU-T Recommendation X.501: Information Technology -
              Open Systems Interconnection - The Directory: Models,
              1993.

   [X.509]    ITU-T Recommendation X.509 (1997 E): Information
              Technology - Open Systems Interconnection - The
              Directory: Authentication Framework, June 1997.

















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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, Extended 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);

   3) To remove the Domain of Interpretation (DOI), Situation (SIT), and
   Labeled Domain Identifier fields, and the Commit and Authentication
   only bits;

   4) To decrease IKE's latency in the common case by making the initial
   exchange be 2 round trips (4 messages), and allowing the ability to
   piggyback setup of a CHILD-SA on that exchange;

   5) To replace the cryptographic syntax for protecting the IKE
   messages themselves with one based closely on ESP to simplify
   implementation and security analysis;

   6) To reduce the number of possible error states by making the
   protocol reliable (all messages are acknowledged) and sequenced. This
   allows shortening CREATE_CHILD_SA exchanges from 3 messages to 2;

   7) To increase robustness by allowing the responder to not do
   significant processing until it receives a message proving that the
   initiator can receive messages at its claimed IP address, and not
   commit any state to an exchange until the initiator can be
   cryptographically authenticated;

   8) To fix bugs such as the hash problem documented in [draft-ietf-
   ipsec-ike-hash-revised-02.txt];

   9) To specify Traffic Selectors in their own payloads type rather
   than overloading ID payloads, and making more flexible the Traffic
   Selectors that may be specified;

   10) To specify required behavior under certain error conditions or
   when data that is not understood is received in order to make it
   easier to make future revisions in a way that does not break
   backwards compatibility;



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   11) To incorporate ideas from draft-ietf-ipsec-nat-reqts-04.txt to
   allow IKE to negotiate through NAT gateways;

   12) To simplify and clarify how shared state is maintained in the
   presence of network failures and Denial of Service attacks; and

   13) To maintain existing syntax and magic numbers to the extent
   possible to make it likely that implementations of IKEv1 can be
   enhanced to support IKEv2 with minimum effort.










































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Appendix B: Diffie-Hellman Groups

   There are 5 different Diffie-Hellman groups defined for use in IKE.
   These groups were generated by Richard Schroeppel at the University
   of Arizona. Properties of these primes are described in [Orm96].

   The strength supplied by group one may not be sufficient for the
   mandatory-to-implement encryption algorithm and is here for historic
   reasons.

   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).






















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   The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
   Its hexadecimal value is:

        FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
        8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
        302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
        A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
        49286651 ECE65381 FFFFFFFF FFFFFFFF

   The generator is 2.

B.3 Group 3 - 155 Bit EC2N

   This group is assigned id 3 (three). The curve is based on the Galois
   Field GF[2^155]. The field size is 155. The irreducible polynomial
   for the field is:
      u^155 + u^62 + 1.
   The equation for the elliptic curve is:
      y^2 + xy = x^3 + ax^2 + b.

   Field Size:                         155
   Group Prime/Irreducible Polynomial:
                0x0800000000000000000000004000000000000001
   Group Generator One:                0x7b
   Group Curve A:                      0x0
   Group Curve B:                      0x07338f
   Group Order: 0x0800000000000000000057db5698537193aef944

   The data in the KE payload when using this group is the value x from
   the solution (x,y), the point on the curve chosen by taking the
   randomly chosen secret Ka and computing Ka*P, where * is the
   repetition of the group addition and double operations, P is the
   curve point with x coordinate equal to generator 1 and the y
   coordinate determined from the defining equation. The equation of
   curve is implicitly known by the Group Type and the A and B
   coefficients. There are two possible values for the y coordinate;
   either one can be used successfully (the two parties need not agree
   on the selection).

B.4 Group 4 - 185 Bit EC2N

   This group is assigned id 4 (four). The curve is based on the Galois
   Field GF[2^185]. The field size is 185. The irreducible polynomial
   for the field is:
      u^185 + u^69 + 1.

   The  equation for the elliptic curve is:
      y^2 + xy = x^3 + ax^2 + b.



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   Field Size:                         185
   Group Prime/Irreducible Polynomial:
                0x020000000000000000000000000000200000000000000001
   Group Generator One:                0x18
   Group Curve A:                      0x0
   Group Curve B:                      0x1ee9
   Group Order: 0x01ffffffffffffffffffffffdbf2f889b73e484175f94ebc

   The data in the KE payload when using this group will be identical to
   that as when using Oakley Group 3 (three).

B.5 Group 5 - 1536 Bit MODP

   This group is assigned id 5 (five).

   The prime is 2^1536 - 2^1472 - 1 + 2^64 * {[2^1406 pi] + 741804}.
   Its hexadecimal value is

        FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08
        8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B
        302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9
        A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6
        49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8
        FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
        670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF

   The generator is 2.
























IKEv2                                                          [Page 92]


INTERNET DRAFT                                                  May 2003


Change History

H.1 Changes from IKEv2-00 to IKEv2-01 February 2002

   1) Changed Appendix B to specify the encryption and authentication
   processing for IKE rather than referencing ESP. Simplified the format
   by removing idiosyncrasies not needed for IKE.

   2) Added option for authentication via a shared secret key.

   3) Specified different keys in the two directions of IKE messages.
   Removed requirement of different cookies in the two directions since
   now no longer required.

   4) Change the quantities signed by the two ends in AUTH fields to
   assure the two parties sign different quantities.

   5) Changed reference to AES to AES_128.

   6) Removed requirement that Diffie-Hellman be repeated when rekeying
   IKE_SA.

   7) Fixed typos.

   8) Clarified requirements around use of port 500 at the remote end in
   support of NAT.

   9) Clarified required ordering for payloads.

   10) Suggested mechanisms for avoiding DoS attacks.

   11) Removed claims in some places that the first phase 2 piggybacked
   on phase 1 was optional.

H.2 Changes from IKEv2-01 to IKEv2-02 April 2002

   1) Moved the Initiator CERTREQ payload from message 1 to message 3.

   2) Added a second optional ID payload in message 3 for the Initiator
   to name a desired Responder to support the case where multiple named
   identities are served by a single IP address.

   3) Deleted the optimization whereby the Diffie-Hellman group did not
   need to be specified in phase 2 if it was the same as in phase 1 (it
   complicated the design with no meaningful benefit).

   4) Added a section on the implications of reusing Diffie-Hellman
   expontentials



IKEv2                                                          [Page 93]


INTERNET DRAFT                                                  May 2003


   5) Changed the specification of sequence numbers to being at 0 in
   both directions.

   6) Many editorial changes and corrections, the most significant being
   a global replace of "byte" with "octet".

H.3 Changes from IKEv2-02 to IKEv2-03 October 2002

   1) Reorganized the document moving introductory material to the
   front.

   2) Simplified the specification of Traffic Selectors to allow only
   IPv4 and IPv6 address ranges, as was done in the JFK spec.

   3) Fixed the problem brought up by David Faucher with the fix
   suggested by Valery Smyslov. If Bob needs to narrow the selector
   range, but has more than one matching narrower range, then if Alice's
   first selector is a single address pair, Bob chooses the range that
   encompasses that.

   4) To harmonize with the JFK spec, changed the exchange so that the
   initial exchange can be completed in four messages even if the
   responder must invoke an anti-clogging defense and the initiator
   incorrectly anticipates the responder's choice of Diffie-Hellman
   group.

   5) Replaced the hierarchical SA payload with a simplified version
   that only negotiates suites of cryptographic algorithms.

H.4 Changes from IKEv2-03 to IKEv2-04 January 2003

   1) Integrated NAT traversal changes (including Appendix A).

   2) Moved the anti-clogging token (cookie) from the SPI to a NOTIFY
   payload; changed negotiation back to 6 messages when a cookie is
   needed.

   3) Made capitalization of IKE_SA and CHILD_SA consistent.

   4) Changed how IPcomp was negotiated.

   5) Added usage scenarios.

   6) Added configuration payload for acquiring internal addresses on
   remote networks.

   7) Added negotiation of tunnel vs transport mode.




IKEv2                                                          [Page 94]


INTERNET DRAFT                                                  May 2003


H.5 Changes from IKEv2-04 to IKEv2-05 February 2003

   1) Shortened Abstract

   2) Moved NAT Traversal from Appendix to section 2. Moved changes from
   IKEv2 to Appendix A. Renumbered sections.

   3) Made language more consistent. Removed most references to Phase 1
   and Phase 2.

   4) Made explicit the requirements for support of NAT Traversal.

   5) Added support for Extended Authentication Protocol methods.

   6) Added Response bit to message header.

   7) Made more explicit the encoding of Diffie-Hellman numbers in key
   expansion algorithms.

   8) Added ID payloads to AUTH payload computation.

   9) Expanded set of defined cryptographic suites.

   10) Added text for MUST/SHOULD support for ID payloads.

   11) Added new certificate formats and added MUST/SHOULD text.

   12) Clarified use of CERTREQ.

   13) Deleted "MUST SUPPORT" column in CP payload specification (it was
   inconsistent with surrounding text).

   14) Extended and clarified Conformance Requirements section,
   including specification of a minimal implementation.

   15) Added text to specify ECN handling.

H.6 Changes from IKEv2-05 to IKEv2-06 March 2003

   1) Changed the suite based crypto negotiation back to ala carte.

   2) Eliminated some awkward page breaks, typographical errors, and
   other formatting issues.

   3) Tightened language describing cryptographic strength.

   4) Added references.




IKEv2                                                          [Page 95]


INTERNET DRAFT                                                  May 2003


   5) Added more specific error codes.

   6) Added rationale for unintuitive key generation hash with shared
   secret based authentication.

   7) Changed the computation of the authenticating AUTH payload as
   proposed by Hugo Krawczyk.

   8) Changed the dashes (-) to underscores (_) in the names of fields
   and constants.

H.7 Changes from IKEv2-06 to IKEv2-07 April 2003

   1) Added a list of payload types to section 3.2.

   2) Clarified use of SET_WINDOW_SIZE Notify payload.

   3) Removed references to COOKIE_REQUIRED Notify payload.

   4) Specified how to use a prf with a fixed key size.

   5) Removed g^ir from data processed by prf+.

   6) Strengthened cautions against using passwords as shared keys.

   7) Renamed Protocol_id field SECURITY_PROTOCOL_ID when it is not the
   Protocol ID from IP, and changed its values for consistency with
   IKEv1.

   8) Clarified use of ID payload in access control decisions.

   9) Gave IDr and TSr their own payload type numbers.

   10) Added Intellectual Property rights section.

   11) Clarified some issues in NAT Traversal.

H.8 Changes from IKEv2-07 to IKEv2-08 May 2003

   1) Numerous editorial corrections and clarifications.

   2) Renamed Gateway to Security Gateway.

   3) Made explicit that the ability to rekey SAs without restarting IKE
   was optional.

   4) Removed last references to MUST and SHOULD ciphersuites.




IKEv2                                                          [Page 96]


INTERNET DRAFT                                                  May 2003


   5) Changed examples to "example.com".

   6) Changed references to status codes to status types.

   7) Simplified IANA Considerations section

   8) Updated References

Editor's Address

   Charlie Kaufman
   charlie_kaufman@notesdev.ibm.com
   IBM

Full Copyright Statement

   "Copyright (C) The Internet Society (2003). All Rights Reserved.

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IKEv2                                                          [Page 97]


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