IPSEC Working Group                                   Charlie Kaufman
INTERNET-DRAFT                                                 editor
draft-ietf-ipsec-ikev2-04.txt                            January 2003

                 Internet Key Exchange (IKEv2) Protocol

                          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
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   To learn the current status of any Internet Draft, please check the
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   ftp.isi.edu (US West Coast).


   This document describes version 2 of the IKE (Internet Key Exchange)
   protocol.  IKE performs mutual authentication and establishes an IKE
   security association that can be used to efficiently establish SAs
   for ESP and/or AH. This version greatly simplifies IKE by replacing
   the 8 possible phase 1 exchanges with a single exchange based on
   either public signature keys or shared secret keys.  The single
   exchange provides identity hiding, yet works in 2 round trips (all
   the identity hiding exchanges in IKE v1 required 3 round trips).
   Latency of setup of an IPsec SA is further reduced from IKEv1 by
   allowing setup of an SA for ESP and/or AH to be piggybacked on the
   initial IKE exchange.  It also improves security by allowing the

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   Responder to be stateless until it can be assured that the Initiator
   can receive at the claimed IP source address.  This version also
   presents the entire protocol in a single self-contained document, in
   contrast to IKEv1, in which the protocol was described in ISAKMP (RFC
   2408), IKE (RFC 2409), and the DOI (RFC 2407) documents.

Table of Contents

   1 Summary of Changes from IKEv1..............................3
   2 Requirements Terminology...................................4
   3 IKE Protocol Overview......................................5
   3.1 Usage Scenarios..........................................6
   3.1.1 Gateway to Gateway Tunnel..............................6
   3.1.2 Endpoint to Endpoint Transport.........................6
   3.1.3 Endpoint to Gateway Transport..........................7
   3.1.4 Other Scenarios........................................8
   3.2 The Initial Exchange.....................................8
   3.3 The CREATE_CHILD_SA Exchange.............................9
   3.4 The Informational Exchange..............................11
   3.5 Informational Messages outside of an IKE-SA.............12
   4 IKE Protocol Details and Variations.......................13
   4.1 Use of Retransmission Timers............................13
   4.2 Use of Sequence Numbers for Message ID..................13
   4.3 Window Size for overlapping requests....................14
   4.4 State Synchronization and Connection Timeouts...........15
   4.5 Version Numbers and Forward Compatibility...............16
   4.6 Cookies.................................................18
   4.7 Cryptographic Algorithm Negotiation.....................20
   4.8 Rekeying................................................20
   4.9 Traffic Selector Negotiation............................21
   4.10 Nonces.................................................23
   4.11 Address and Port Agility...............................24
   4.12 Reuse of Diffie-Hellman Exponentials...................24
   4.13 Generating Keying Material.............................25
   4.14 Generating Keying Material for the IKE-SA..............25
   4.15 Authentication of the IKE-SA...........................26
   4.16 Generating Keying Material for CHILD-SAs...............27
   4.17 Rekaying IKE-SAs using a CREATE_CHILD_SA exchange......28
   4.18 Requesting an internal address on a remote network.....28
   4.19 Requesting a Peer's Version............................30
   4.20 Error Handling.........................................30
   4.21 IPcomp.................................................31
   5 Header and Payload Formats................................32
   5.1 The IKE Header..........................................32
   5.2 Generic Payload Header..................................34
   5.3 Security Association Payload............................35

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   5.3.1 Proposal Substructure.................................36
   5.4 Key Exchange Payload....................................38
   5.5 Identification Payload..................................38
   5.6 Certificate Payload.....................................40
   5.7 Certificate Request Payload.............................41
   5.8 Authentication Payload..................................42
   5.9 Nonce Payload...........................................44
   5.10 Notify Payload.........................................44
   5.10.1 Notify Message Types.................................45
   5.11 Delete Payload.........................................50
   5.12 Vendor ID Payload......................................51
   5.13 Traffic Selector Payload...............................53
   5.13.1 Traffic Selector.....................................53
   5.14 Encrypted Payload......................................55
   5.15 Configuration Payload..................................57
   5.15.1 Configuration Attributes.............................59
   5.16 Other Payload types....................................61
   6 Conformance Requirements..................................62
   7 Security Considerations...................................62
   8 IANA Considerations.......................................63
   8.1 Transform Types and Attribute Values....................64
   8.2 Exchange Types..........................................64
   8.3 Payload Types...........................................64
   9 Acknowledgements..........................................64
   10 References...............................................64
   10 Normative References.....................................64
   10 Non-normative References.................................64
   Appendix A: NAT Traversal...................................67
   Appendix B: Diffie-Hellman Groups...........................69
   Change History..............................................71
   Author's Address............................................73
   Full Copyright Statement....................................74

1 Summary of changes from IKEv1

   The goals of this revision to IKE are:

   1) To define the entire IKE protocol in a single document, rather
   than three that cross reference one another;

   2) To simplify IKE by replacing the eight different initial phase 1
   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

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   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 Phase 2 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-

   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 replace the complex mix and match negotiation of cryptographic
   algorithms with proposals based on suites of algorithms;

   11) 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;

   12) To incorporate ideas from draft-ietf-ipsec-nat-reqts-02.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.

2 Requirements Terminology

   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and

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   "MAY" that appear in this document are to be interpreted as described
   in [Bra97].

3 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
   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
   (RFC 2406) and/or AH (RFC 2402).  It also negotiates use of IPcomp
   (RFC 2393) 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.

   We call the first four messages establishing an IKE-SA a "phase 1"
   exchange and subsequent IKE exchanges "phase 2", inheriting this
   terminology from IKEv1. The phase 1 exchange establishes the IKE-SA
   and the first CHILD-SA. In some scenarios, only a single CHILD-SA is
   needed between the IPsec endpoints and therefore there would be no
   phase 2 exchanges. Phase 2 exchanges MAY be used to establish
   additional CHILD-SAs between the same authenticated pair of endpoints
   and to perform housekeeping functions.  The phase 1 exchange consists
   of two request/response pairs.  A phase 2 exchange is one
   request/response pair, and can be used to create or delete a CHILD-
   SA, rekey or delete the IKE-SA, or report information such as error

   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
   MUST retransmit the request (or abandon the connection).

   The first request/response of a phase 1 exchange 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

   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.

   Phase 2 exchanges each consist of a single request/response pair. The
   types of exchanges are CREATE_CHILD_SA (which creates a CHILD-SA), or
   an Informational exchange which deletes an SA, reports error
   conditions, or does other housekeeping.  All these messages require a
   response. An informational message with no payloads is commonly used
   as a check for liveness.

   In the description that follows, we assume that no errors occur.
   Modifications to the flow should errors occur are described in
   section 4.

3.1 Usage Scenarios

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

3.1.1 Gateway to Gateway Tunnel

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

                       Figure 1:  Firewall to Firewall 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.

3.1.2 Endpoint to Endpoint Transport

       +-+-+-+-+-+                                          +-+-+-+-+-+
       !         !                 IPsec                    !         !
       !Protected!                 Tunnel                   !Protected!

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       !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 outher 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 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.

3.1.3 Endpoint to Gateway Transport

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

                       Figure 3:  Endpoint to Gateway

   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 gateway so that packets returned to it
   will go to the gateway and be tunnelled back. This IP address may be
   static or may be dynamically allocated by the gateway. In support of
   the latter case, IKEv2 includes a mechanism for the initiator to
   request an IP address owned by the 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

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   that will get traffic routed to the endpoint directly) while the
   inner IP header will contain the source IP address assigned by the
   gateway (i.e. the address that will get traffic routed to the gateway
   for forwarding to the endpoint). The outer destination address will
   always be that of the 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 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.

3.1.4 Other Scenarios

   Other scenarios are possible, as are nested combinations of the
   above.  One noteable example combines aspects of 3.1.1 and 3.1.3. A
   subnet may make all external accesses through a remote gateway using
   an IPsec tunnel, where the addresses on the subnet are routed to the
   gateway by the rest of the Internet. An example would be someones
   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 (where the static IP addresses and an
   IPsec relay is provided by a third party located elsewhere).

3.2 The Initial Exchange

   Communication using IKE always begins with an initial exchange (known
   in IKEv1 as Phase 1). This initial exchange normally consists 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
   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.

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   The initial exchange is 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.  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. 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 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

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   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.

3.3 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.

   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 5.14.

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

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

   In the CHILD-SA created as part of the initial exchange, a second KE
   payload and nonce MUST NOT be sent. The nonces from the initial
   exchange are used in computing the keys for the CHILD-SA.

   The CREATE_CHILD_SA request contains:

       Initiator                                 Responder
      -----------                               -----------
       HDR, SK {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

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   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 past the header is encrypted and the message including
   the header is integrity protected using the cryptographic algorithms
   negotiated in Phase 1.

   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, 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 and have the initiator make another one.

   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-

3.4 The Informational Exchange

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

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

   Messages in an Informational Exchange contain zero or more
   Notification or Delete 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.

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   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 the SAs it sends on 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 placed in the headers of outbound
   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 incoming SAs while processing the
   request and the outgoing 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 open 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 open 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:

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

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

3.5 Informational Messages outside of an IKE-SA

   If a packet arrives with an unrecognised 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

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   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 to alert it to a possible

4 IKE Protocol Details and Variations

   IKE normally listens on UDP port 500, though IKE messages may also be
   received on UDP port 4500 with a slightly different format.  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).

4.1 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
   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

   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.

4.2 Use of Sequence Numbers for Message ID

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   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
   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 each packet contains enough information to determine
   which of the four messages a particular one is.

   Note that Message IDs are cryptographically protected and provide
   protection against message replays.

4.3 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. For
   simplicity, an IKE implementation MAY choose to process requests
   strictly in order and/or wait for a response to one request before
   issuing another. Certain rules must be followed to 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 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

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   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.

4.4 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 those SAs and having them fall into a black hole.

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

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   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, a liveness check MUST
   be performed. 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.

   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.

4.5 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

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   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
   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.

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   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 unrecognised, 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

   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 order shown in section 3 and implementations SHOULD reject as
   invalid a message with payloads in any other order.

4.6 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.

   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,

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   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 its cookie in a notify payload.
   Initiators who receive such responses MUST retry the IKE_SA_INIT with
   the responder supplied cookie 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-REQUIRED),

       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.

   An IKE implementation SHOULD implement its responder cookie
   generation is such a way as to not require any saved state to
   recognise 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 = <SecretVersionNumber> | Hash(IPi | SPIi | <secret>)

   where <secret> is a randomly generated secret known only to the
   responder and periodically changed. <SecretVersionNumber> should be
   changed whenever <secret> is regenerated.  This value 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

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   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).

   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 <SecretVersionNumber>.  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

4.7 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 a
   Suite-ID, which implies one or more protocols and the associated
   cryptographic algorithms.

   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 cryptographic suites.  If she guesses wrong, Bob
   will respond with a NOTIFY payload of type INVALID-KE-PAYLOAD
   indicating the selected cryptographic suite.  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.

4.8 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 rekey a CHILD-SA, create a new, equivalent SA (see section 4.17
   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 4.20
   below) with the peer to whom the old IKE-SA is shared using a Phase 2
   negotiation 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 absense 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. An endpoint SHOULD wait a random amount of time before closing a
   redundant SA to prevent cycling.

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

4.9 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 of a system's SPD is outside the

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   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 3.1.3).

   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 3.1.3, 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 absense 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
   18.16.*.* on Bob's side, Alice would include a single traffic
   selector in each TS payload. TSi would specify the address range
   ( - and TSr would specify the address range
   ( - 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

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   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 to, 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 MAY 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 ( - and the source port and protocol from the packet and the
   second containing ( - 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 ( -
   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
   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.

4.10 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-SAs. Nonces used in IKEv2 MUST therefore be

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   randomly chosen and of size at least equal to the key size of the
   strongest cryptographic algorithm used.

4.11 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

4.12 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

   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 generater that could be used to recompute the
   Diffie-Hellman secrets.

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

   Decisions as to whether and when to reuse Diffie-Hellman exponentials
   is a private decision in the sense that it will not affect

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   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

4.13 Generating Keying Material

   In the context of the IKE-SA, three cryptographic algorithms are
   negotiated:  an encryption algorithm, a Diffie-Hellman group, and a
   pseudo-random function (prf). The pseudo-random function is used both
   for integrity protection of the IKE payloads and 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 cryptographic algorithm accepts a fixed size key,
   and that any randomly chosen value of that fixed size can serve as an
   appropriate key. For functions that accept a variable length key, a
   fixed key size MUST be specified as part of the cryptographic suite
   negotiated.  For prf functions based on HMAC, the fixed key size is
   the size of the output of the HMAC.

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

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

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

   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.

4.14 Generating Keying Material for the IKE-SA

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   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 prf 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

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

       {SK_d, SK_ai, SK_ar, SK_ei, SK_er}
                 = prf+ (SKEYSEED, g^ir | Ni | Nr | CKY-I | CKY-R)

   (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.  Ni and Nr
   are the nonces, stripped of any headers.

   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 underlying hash function.

4.15 Authentication of the IKE-SA

   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) is the initiator's nonce Ni (just the
   value, not the payload containing it).  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) is the
   responder's nonce Nr.  It is critical to the security of the exchange
   that each side sign the other side's nonce.

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

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   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, an RSA-signed PKCS1-padded-hash for
   an RSA digital signature, a DSS-signed SHA1-hash for a DSA digital
   signature, or the negotiated PRF function for a pre-shared key.
   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. This type is either
   indicated in the certificate supplied or, if the keys were exchanged
   out of band, the key types must have been similarly learned.  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 insecure practice to have a
   shared key derived from a user chosen password. This is insecure
   because user chosen passwords are unlikely to have sufficient
   randomness to resist dictionary attacks.  The pre-shared key SHOULD
   contain as much randomness as the strongest key being negotiated.  In
   the case of a pre-shared key, the AUTH value is computed as:

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

   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,
   this exchange will not compromise use of the same password in other
   protocols.  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.

4.16 Generating Keying Material for CHILD-SAs

   CHILD-SAs are created either by being piggybacked on the phase 1
   exchange, or in a phase 2 CREATE_CHILD_SA exchange. Keying material
   for them is generated as follows:

      KEYMAT = prf+(SK_d, Ni | Nr)

   Where Ni and Nr are the Nonces from the IKE_SA_INIT exchange if this
   request is the first CHILD-SA created or the fresh Ni and Nr from the
   CREATE_CHILD_SA exchange if this is a subsequent creation.

   For phase 2 exchanges with PFS the keying material is defined as:

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      KEYMAT = prf+(SK_d, g^ir (ph2) | Ni | Nr )

   where g^ir (ph2) is the shared secret from the ephemeral Diffie-
   Hellman exchange of this phase 2 exchange,

   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.  KEYMAT is generated
   as described in section 4.13.

   Keying material is taken from the expanded KEYMAT in the following

      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.

4.17 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 4.8).  New Initiator and Responder SPIs are supplied in
   the SPI fields. The TS payloads are omitted when rekeying an IKE-SA.
   SKEYSEED for the new IKE-SA is computed using SK_d from the existing
   IKE-SA as follows:

       SKEYSEED = prf(SK_d (old), [g^ir (ph2)] | Ni | Nr)

   where g^ir (ph2) is the shared secret from the ephemeral Diffie-
   Hellman exchange of this phase 2 exchange 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 4.14.

4.18 Requesting an internal address on a remote network

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   Most commonly in the endpoint to gateway scenario, an endpoint may
   need an IP address on the gateway's internal network, 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

   This function provides address allocation to an IRAC trying to tunnel
   into a network protected by an IRAS.  Since the IKE_SA_AUTH exchange
   creates an IKE-SA and a CHILD-SA the IRAC MUST request the internal
   address, and optionally other information concerning the internal
   network, in the IKE_SA_AUTH exchange.  The may IRAS procure an
   internal address for the IRAC from any number of sources such as a
   DHCP/BOOTP server or its own address pool.

       Initiator                           Responder
      -----------------------------       ---------------------------
       HDR, SAi1, KEi, Ni, Nr,
        SK {IDi, [CERT,] [CERTREQ,]
        [IDr,] AUTH, CP(CFG_REQUEST),
        SAi2, TSi, TSr}              -->

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

   CP(CFG_REQUEST) MUST contain at least an INTERNAL_ADDRESS attribute
   (either IPv4 or IPv6) but MAY contain any number of additional
   attributes the initiator wants returned in the response.

   For example, message from Initiator to Responder:
      TSi = (0, 0-65536,
      TSr = (0, 0-65536,

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

   Message from Responder to Initiator:

      TSi = (0, 0-65536,
      TSr = (0, 0-65536,

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   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.

4.19 Requesting the Peer's Version

   An IKE peer wishing to inquire about the other peer's version
   information MUST 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 return an empty string.

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


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

4.20 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 non-existent SPI), the node SHOULD initiate
   an Informational Exchange with a Notify payload describing the

   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

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   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-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.

   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.

4.21 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

   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

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   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.

5 Header and Payload Formats

5.1 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 of UDP port 500, IKE
   messages begin immediately following the UDP header. When sent on UDP
   port 4500, IKE messages have prepended for octets of zero.  These
   four octets of zero are not part of the IKE message and are not
   included in any of the length fields or checksums defined by IKE.
   Each IKE message begins with the IKE header, denoted HDR in this
   memo. Following the header are one or more IKE payloads each
   identified by a "Next Payload" field in the preceding payload.
   Payloads are processed in the order in which they appear in an IKE
   message by invoking the appropriate processing routine according to
   the "Next Payload" field in the IKE header and subsequently according
   to the "Next Payload" field in the IKE payload itself until a "Next
   Payload" field of zero indicates that no payloads follow. If a
   payload of type "Encrypted" is found, that payload is decrypted and
   its contents parsed as additional payloads. An Encrypted payload MUST
   be the last payload in a packet and an encrypted payload MUST NOT
   contain another encrypted payload.

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

   The format of the IKE header is shown in Figure 1.
                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      !                       IKE-SA Initiator's SPI                  !
      !                                                               !
      !                       IKE-SA Responder's SPI                  !
      !                                                               !

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      !  Next Payload ! MjVer ! MnVer ! Exchange Type !     Flags     !
      !                          Message ID                           !
      !                            Length                             !

                       Figure 1:  IKE Header Format

      o  Initiator's SPI (8 octets) - A value chosen by the
         initiator to identify a unique IKE security association. This
         value MUST NOT be zero.

      o  Responder's SPI (8 octets) - A value chosen by the
         responder to identify a unique IKE security association. This
         value MUST be zero in the first message of an IKE Initial
         Exchange 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

      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 dictates the payloads sent in each message and
         message orderings in the exchanges.

                       Exchange Type            Value

                       RESERVED                 0
                       Reserved for ISAKMP      1-31
                       Reserved for IKEv1       32-33
                       IKE_SA_INIT              34
                       IKE_SA_AUTH              35
                       CREATE_CHILD_SA          36
                       Informational            37

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                       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'.

       --  R(eserved) (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
           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
           whether the message is a request or a response.

       --  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(eserved) (bits 5-7 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. See section 4.2. In the first message of a Phase 1
         negotiation, the value MUST be set to 0. The response to that
         message MUST also have a Message ID of 0.

      o  Length (4 octets) - Length of total message (header + payloads)
         in octets. Session encryption can expand the size of an IKE
         message and that is reflected in the total length of the

5.2 Generic Payload Header

   Each IKE payload defined in sections 5.3 through 5.14 begins with a
   generic header, shown in Figure 2. Figures for each payload below
   will include the generic payload header but for brevity the
   description of each field will be omitted. The construction and
   processing of the generic payload header is identical for each

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   payload and will similarly be omitted.

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

                         Figure 2:  Generic Payload Header

   The Generic Payload Header fields are defined as follows:

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

   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. SHOULD be set to zero for payload types defined in this
      document. Note that the critical bit applies to the current
      payload rather than the "next" payload whose type code
      appears in the first octet. The reasoning behind not setting
      the critical bit for payloads defined in this document is
      that all implementations MUST understand all payload types
      defined in this document and therefore must ignore the
      Critical bit's value. Skipped payloads are expected to
      have valid Next Payload and Payload Length fields.

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

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

5.3 Security Association Payload

   The Security Association Payload, denoted SA in this memo, is used to

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   negotiate attributes of a security association.  An SA may contain
   multiple proposals. Each proposal may propose multiple protocols
   (where a protocol is IKE, ESP, or AH), along with a suite of
   cryptographic algorithms to be used by the protocols. The
   protocol(s), cryptographic algorithms, and any associated parameters
   are determined by the suite number. An SA payload MAY contain
   proposals for different protocols. For example, one suite might
   contain AH and ESP, while another might contain only ESP and a third
   only AH.

   The Proposal structure contains within it a Proposal # and a Suite-
   ID.  The first proposal MUST have Proposal # = 1, the second MUST
   have Proposal # = 2, etc. If the proposals are misnumbered, the
   responder MUST reject all of them. Unrecognised Suite-IDs MUST be

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

               Figure 3:  Security Association Payload

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

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

5.3.1 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 #    ! RESERVED-MBZ  !           Suite-ID            !
      ~                        SPI(S)  (variable)                     ~

               Figure 4:  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

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         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.

      o  Proposal Length (2 octets) - Length of this proposal,
         including the SPI

      o  Proposal # (1 octet) - In an SA payload requesting a new
         SA is sent, it may contain multiple proposals. The first
         proposal in an SA MUST be #1, and subsequent proposals
         MUST be one greater than the previous proposal. When an
         SA is accepted, the SA payload sent back MUST contain a
         single proposal and the proposal number MUST match the
         number in the accepted proposal.

      o  RESERVED-MBZ (1 octet) - This field is reserved for
         possible use in specifying different kinds of proposals.
         This field MUST be sent as zero and a proposal containing
         a non-zero value MUST NOT be accepted. The negotiation
         MAY still succeed if there is another acceptable
         proposal in the SA payload.

      o  Suite-ID (2 octets) - This field specifies a suite of
         protocols and cryptographic algorithms. See table below.

      o  SPI(S) (variable) - The sending entity's SPI(s). If the
         suite proposed includes more than one protocol, the SPIs
         are concatenated together in the order in which they would
         appear in a packet sent using the suite (i.e. AH followed
         by ESP). When an initial IKE-SA is being
         proposed, SPIs are implicit from the IKE header and are not
         repeated here. Note that no padding is applied.

   For Suite-ID, the following values are defined:

          Name            Number   Algorithms
          IKE_CLASSIC       0       DH-Group #5 (1536 bits)
                                    3DES encryption
                                    HMAC-SHA1 integrity and prf

          ESP_CLASSIC       1       3DES encryption
                                    HMAC-SHA1 integrity

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             <some AES variants, AH (?))

          values 2-65000 are reserved to IANA. Values 65001-65533 are
          for private use among mutually consenting parties.

5.4 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
   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        !
      !            Suite-ID           !       RESERVED (MBZ)          !
      !                                                               !
      ~                       Key Exchange Data                       ~
      !                                                               !

                Figure 7:  Key Exchange Payload Format

   A key exchange payload is constructed by copying ones 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.

   A key exchange payload is processed by first checking whether the
   length of the key exchange data (the "Payload Length" from the
   generic header minus the size of the generic header) is equal to the
   length of the prime modulus over which the exponentiation was
   performed. The message should be treated as invalid if the payload is
   not the expected size.

   The Suite-ID is the identifier of the cryptographic suite from which
   the Diffie-Hellman group was taken. 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).

5.5 Identification Payload

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   The Identification Payload, denoted ID in this memo, allows peers to
   assert an identify to one another. The ID Payload names the identity
   to be authenticated with the AUTH payload.

   NOTE: In IKEv1, two ID payloads were used in each direction in Phase
   2 to hold Traffic Selector information for data passing over the SA.
   In IKEv2, this information is carried in Traffic Selector (TS)
   payloads (see section 5.13).

   The Identification Payload consists of the IKE generic header
   followed by identification fields as follows:

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

               Figure 8:  Identification Payload Format

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

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

   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 type for the Identification Payload is five (5).

   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.

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      ID_FQDN                             2

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

      ID_RFC822_ADDR                      3

            A fully-qualified RFC822 email address string, An example of
            a ID_RFC822_ADDR is, "lizard@lounge.org".  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

      ID_DER_ASN1_GN                      10

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

      ID_KEY_ID                           11

            An opaque octet stream which may be used to pass vendor-
            specific information necessary to do certain proprietary
            forms of identification.

5.6 Certificate Payload

   The Certificate Payload, denoted CERT in this memo, provides a means
   to transport certificates or other certificate-related information
   via IKE. Certificate payloads SHOULD be included in an exchange if
   certificates are available to the sender.

   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        !

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      ! Cert Encoding !                                               !
      +-+-+-+-+-+-+-+-+                                               !
      ~                       Certificate Data                        ~
      !                                                               !

                Figure 9:  Certificate Payload Format

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

                 Certificate Encoding               Value
                 --------------------               -----
                 NONE                                 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
                 RESERVED                          11 - 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).

5.7 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 first, second, or third message of Phase 1.
   Certificate Request payloads SHOULD be included in an exchange
   whenever the peer may have multiple certificates, some of which might
   be trusted while others are not.  If multiple root CA's are trusted,
   then multiple Certificate Request payloads SHOULD be transmitted.

   Empty (zero length) CA names MUST NOT be generated and SHOULD be

   The Certificate Request Payload is defined as follows:

                           1                   2                   3

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       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 10:  Certificate Request Payload Format

   o  Certificate Encoding (1 octet) - Contains an encoding of the type
      of certificate requested.  Acceptable values are listed in
      section 5.6.

   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 Request Payload is constructed by setting the "Cert
   Encoding" field to be the type of certificate being desired and the
   "Certification Authority" field to a proper encoding of a
   certification authority for the specified certificate. For example,
   for an X.509 certificate this field would contain the Distinguished
   Name encoding of the Issuer Name of an X.509 certification authority
   acceptable to the sender of this payload.

   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 the specified certification authority. This
   can be a chain of certificates. If a certificate exists which
   satisfies the criteria specified in the Certificate Request Payload
   it 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 no certificates exist then no further
   processing is performed-- 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

5.8 Authentication Payload

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   The Authentication Payload, denoted AUTH in this memo, contains data
   used for authentication purposes. The only authentication method
   defined in this memo is digital signatures and therefore the contents
   of this payload when used with this memo will be the output generated
   by a digital signature function.

   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 11:  Authentication Payload Format

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

        Digital Signature (1) - Computed as specified in section 4.15
        using the public key in the first CERT payload or known the the
        recipient by some out of band means.

        Shared Key Message Integrity Code (2) - Computed as specified in
        section 4.15 using the shared key associated with the identity
        in the ID payload.

   o  Authentication Data (variable length) - Data that results from
      applying the digital signature function to the IKE state
      (see section 3).

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

   The Authentication Payload is constructed by computing a digital
   signature (or secret key MAC) over part of one of the sender's
   messages (see section 4.15).  The result is placed in the
   "Authentication Data" portion of the payload.  The encoding depends
   on the type of key being used to authenticate (see section 4.15).
   The payload length is the size of the generic header plus the size of
   the "Authentication Data" portion of the payload which depends on the
   specific authentication method being used.

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   The Authentication Payload is processed by extracting the
   "Authentication Data" from the payload and verifying it according to
   the specific authentication method being used. If the specified
   authentication method is not supported or validation fails a NOTIFY
   Error message of AUTHENTICATION-FAILED MUST be sent back to the peer
   and the connection closed. (An exception to this case is that a peer
   MAY treat unsupported, invalid, or missing authentication data as a
   request to open an unauthenticated SA.

5.9 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

   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 12:  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 Nonce Payload is constructed by computing a pseudo-random value
   and copying it into the "Nonce Data" field. The size of a Nonce MUST
   be between 8 and 256 octets inclusive. Nonce values MUST NOT be

5.10 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), or in an
   Informational Exchange (to report an error not in an IKE 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        !
      !  Protocol-ID  !   SPI Size    !      Notify Message Type      !
      !                                                               !
      ~                Security Parameter Index (SPI)                 ~
      !                                                               !
      !                                                               !
      ~                       Notification Data                       ~
      !                                                               !

               Figure 13:  Notification Payload Format

   o  Protocol-Id (1 octet) - Specifies the protocol about which
      this notification is being sent. For phase 1 notifications,
      this field MUST be zero (0). For phase 2 notifications
      concerning IPsec SAs this field will contain an IPsec
      protocol (either ESP, or AH). For notifications
      for which no protocol ID is relevant, this field MUST be
      sent as zero and MUST be ignored.

   o  SPI Size (1 octet) - Length in octets of the SPI as defined by
      the Protocol-Id or zero if no SPI is applicable.  For phase 1
      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 message specific, see below.

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

5.10.1 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.

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   For example, a secure front end or security gateway may use the
   Notify message to synchronize SA communication.  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 simplication 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 recognise in a response MUST assume that the
   corresponding request has failed entirely. Unrecognised 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 recognised. They are intended to indicate
   capabilities, and as part of SA negotiation are used to negotiate
   non-cryptographic parameters.

        NOTIFY MESSAGES - ERROR TYPES           Value
        -----------------------------           -----

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

        INVALID-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
            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

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            MUST be sent in response to any error not covered by one of
            the other status codes. 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, MUST be
            rate limited, and MUST NOT be sent unless an IKE-SA exists
            to the sending address and port.

        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.

        AUTHENTICATION-FAILED                    24

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

        SINGLE-PAIR-REQUIRED                     34

            This error indicates that a Phase 2 SA request is
            unacceptable because the Responder is willing to accept
            traffic selectors specifying a single pair of addresses.
            The Initiator 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 Phase 2 SA request is
            unacceptable because the Responder is unwilling to accept

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            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.

        RESERVED TO IANA - Errors             37 - 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 identity without
            waiting for a timeout if those IKE-SAs reside at the IP
            address from which this notification arrived.  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 Phase 2
            exchanges, permitting the recipient to send multiple Phase 2
            requests before getting a response to the first. The data
            associated with a SET-WINDOW-SIZE notification MUST be 4
            octets long an contain the big endian represention of the
            number of messages the sender promises to keep.

        ADDITIONAL-TS-POSSIBLE                   24580

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            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. 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 byte 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

                 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 digest of the SPIs, 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 hash of the
            source IP address and port and if they don't match it MAY
            invoke NAT specific handling (like using UDP encapsulation
            of ESP packets and subsequent IKE packets). The digest is
            computed using the negotiated digest algorithm for the IKE-

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        NAT-DETECTION-DESTINATION-IP             24583

            This notification is used to by its recipient to determine
            whether the it is behind a NAT box. The data associated with
            this notification is a digest of the SPIs, 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
            destination IP address and port and if they don't match it
            MAY invoke NAT specific handling (like using UDP
            encapsulation of ESP packets and subsequent IKE packets).
            The digest is computed using the negotiated digest algorithm
            for the IKE-SA.

        COOKIE                                   24584

            This notification MAY be included in an IKE_SA_INIT request
            or response. In the response, it indicates that the request
            should be retried with the COOKIE included in the request.
            That 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
            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 can
            still be established, but will use tunnel mode. If this is
            unacceptable to the initiator, the initiator MUST delete the

        RESERVED TO IANA - STATUS            24586 - 40959

        Private Use - STATUS                 40960 - 65535

5.11 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 14 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 with the Delete payload. It is permitted, however,
   to include multiple Delete payloads in a single Informational

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   Exchange where each Delete payload lists SPIs for a different

   Deletion of the IKE-SA is indicated by a Protocol-Id of 0 (IKE) but
   no SPIs.  Deletion of a CHILD-SA, such as ESP or AH, will contain the
   Protocol-Id of that protocol (e.g.  ESP, AH) and the SPI is the
   receiving entity's SPI(s).

   The Delete Payload is defined as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      !  Protocol-Id  !   SPI Size    !           # of SPIs           !
      !                                                               !
      ~               Security Parameter Index(es) (SPI)              ~
      !                                                               !

                  Figure 14:  Delete Payload Format

   o  Protocol-Id (1 octet) - Must be zero for an IKE-SA, 50 for
      ESP, or 51 for AH.

   o  SPI Size (1 octet) - Length in octets of the SPI as defined by
      the 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).

5.12 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

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   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. If parameter
   values "reserved for use by consenting parties" are used, they must
   be preceded by a Vendor ID payload that disambiguates them. 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.

   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 15:  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).

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5.13 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 header
   followed by individual traffic selectors as follows:

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      ! Next Payload  !C!  RESERVED   !         Payload Length        !
      ! Number of TSs !                 RESERVED                      !
      !                                                               !
      ~                       <Traffic Selectors>                     ~
      !                                                               !

               Figure 16:  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.

   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).

5.13.1 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                      ~
      !                                                               !
      !                                                               !

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      ~                         Ending Address                        ~
      !                                                               !

                  Figure 17: Traffic Selector

   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

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            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.

      TS_IPV4_ADDR_REQUEST                9

            This TS type requests that the responder assign an IPv4
            address for use with this SA. The length of the addresses
            field is zero.

      TS_IPV6_ADDR_REQUEST                10

            This TS type requests that the responder assign an IPv6
            address for use with this SA. The length of the addresses
            field is zero.

5.14 Encrypted Payload

   The Encrypted Payload, denoted SK{...} in this memo, contains other
   payloads in encrypted form. The Encrpted 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 4.14 and 4.17.

   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 mandatory to implement algorithms are AES-128-CBC and HMAC-SHA1.

   The Payload Type for an Encrypted payload is fifteen (15).  The
   Encrypted Payload consists of the IKE generic 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                     !

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      !         (length is block size for encryption algorithm)       !
      !                    Encrypted IKE Payloads                     !
      +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !               !             Padding (0-255 octets)            !
      +-+-+-+-+-+-+-+-+                               +-+-+-+-+-+-+-+-+
      !                                               !  Pad Length   !
      ~                    Integrity Checksum Data                    ~

               Figure 18:  Encrypted Payload Format

   o  Next Payload - The payload type of the first embedded payload.
      Since the Encrypted payload must be last in a message, there
      is no need to specify a payload type for a payload beyond it.

   o  Payload Length - Includes the lengths of the IV, Padding, and
      Authentication 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

   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

   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

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      through the Pad Length. The checksum MUST be computed over
      the encrypted message.

5.15 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
   CFG_REPLY or CFG_ACK or a Notify payload with an error code
   indicating why the request could not be honored.

   "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 recognise.

   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 recognised or not
   supported, the responder MUST NOT return an error code but rather
   MUST send a CFG_REPLY which MAY be empty. Error returns are reserved
   for cases where the request is recognised 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 and it MUST contain the zero length
   attributes that the responder accepted.  Those attributes that it did
   not accept MUST NOT be in the CFG_ACK Configuration Payload. There
   are currently no defined uses for the CFG_SET/CFG_ACK exchange,

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   though they may be used in connection with extensions based on Vendor
   IDs. An implementation of this specification without extensions MUST
   recognise the CFG_SET payload but MUST always respond with an empty

   Extensions via the CP payload SHOULD NOT be used for general purpose
   management.  Its main intent is to provide a bootstrap mechanism to
   exchange information within IPSec from IRAS to IRAC.  While it MAY be
   useful to use such a method to exchange information between some
   Security Gateways (SGW) or small networks, existing management
   protocols such as DHCP [DHCP], RADIUS [RADIUS], SNMP or LDAP [LDAP]
   should be preferred for enterprise management as well as subsequent
   information exchanges.

   The Configuration Payload is defined as follows:

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

               Figure 19:  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.

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   o  Configuration Attribute (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

5.15.1 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 20:  Configuration Attribute Format

   o  Reserved (1 bit) - This bit MUST be set to zero and MUST be

   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:

                                      MUST    Multi-
        Attribute Type          Value Support Valued Length
        ======================= ===== ======= ====== ==================
         RESERVED                 0
         INTERNAL_IP4_ADDRESS     1     YES    YES*  0 or 4 octets
         INTERNAL_IP4_NETMASK     2     NO     NO    0 or 4 octets
         INTERNAL_IP4_DNS         3     NO     YES   0 or 4 octets
         INTERNAL_IP4_NBNS        4     NO     YES   0 or 4 octets
         INTERNAL_ADDRESS_EXPIRY  5     YES    NO    0 or 4 octets
         INTERNAL_IP4_DHCP        6     NO     YES   0 or 4 octets
         APPLICATION_VERSION      7     YES    NO    0 or more
         INTERNAL_IP6_ADDRESS     8     YES    YES*  0 or 16 octets
         INTERNAL_IP6_NETMASK     9     NO     NO    0 or 16 octets
         INTERNAL_IP6_DNS        10     NO     YES   0 or 16 octets
         INTERNAL_IP6_NBNS       11     NO     YES   0 or 16 octets
         INTERNAL_IP6_DHCP       12     NO     YES   0 or 16 octets

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         INTERNAL_IP4_SUBNET     13     YES    NO    0 or 8 octets
         SUPPORTED_ATTRIBUTES    14     YES    NO    Multiple of 2
         INTERNAL_IP6_SUBNET     15     YES    NO    17 octets

      * These attributes may be multi-valued on return only if
        multiple values were requested.

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

      o  INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
         internal network, sometimes called a red node address or
         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
         the INTERNAL_ADDRESS EXPIRY attribute or there are no IKE-SAs
         between the peers.

         network's netmask.  Only one netmask is allowed in the request
         reply messages (e.g. and it MUST be used only
         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

      o  INTERNAL_IP4_NBNS, INTERNAL_IP6_NBNS - Specifies an address of
         NetBios Name Server (WINS) within the network.  Multiple NBNS
         servers MAY be requested.  The responder MAY respond with zero
         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
         IP address before this expiry time.  Only one of these

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         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
         device protects.  This attribute is made up of two fields; the
         first being an IP address and the second being a netmask.
         Multiple sub-networks MAY be requested.  The responder MAY
         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
         of attribute identifiers each in 2 octets.  The length divided
         2 (bytes) would state the number of supported attributes
         in the response.

      o  INTERNAL_IP6_SUBNET - The protected sub-networks that this
         device protects.  This attribute is made up of two fields; the
         first being a 16 octet IPv6 address the second being a one
         prefix-mask as defined in [ADDRIPV6].  Multiple sub-networks
         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

5.16 Other Payload Types

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   Payload type values 17-127 are reserved to IANA for future assignment
   in IKEv2 (see section 10). Payload type values 128-255 are for
   private use among mutually consenting parties.

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

   RSA keys: 1024 and 2048 bits

   Cert types/lengths/algs

   Symmetric key (pwd authentication)

   setup 1 CHILD-SA in first 4 messages; may reject subsequent. Must
   respond to "pings". Must accept "deletes". Must respond to all
   messages; may ignore all but delete (what if ignores delete?).


7 Security Considerations

   Repeated re-keying using Phase 2 without PFS can consume the entropy
   of the Diffie-Hellman shared secret. Implementers should take note of
   this fact and set a limit on Phase 2 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 160 bits is sufficient to use
   for 3DES.  Groups three through five provide greater security. Group
   one is for historic purposes only and does not provide sufficient
   strength to the required cipher (although it is sufficient for use
   with DES, which is also for historic use only). Implementations
   should make note of these conservative estimates when establishing

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   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. 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 the Diffie-Hellman exponents in this exchange 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 security of this protocol is critically dependent on the
   randomness of the Diffie-Hellman exponents, which should be generated
   by a strong random or properly seeded pseudo-random source (see
   RFC1715). While the protocol was designed to be secure even if the
   Nonces and other values specified as random are not strongly random,
   they should similarly be generated from a strong random source as
   part of a conservative design.

8 IANA Considerations

   This document contains many "magic numbers" to be maintained by the
   IANA.  This section explains the criteria to be used by the IANA to
   assign additional numbers in each of these lists.

   Cryptographic Suite-IDs
   Error Codes
   Status Codes
   IPcomp Transform IDs
   Configuration request types
   Configuration attribute types
   Payload Types
   IKE Exchange Types

   Values of the Cryptographic Suite-ID define a set of cryptographic
   algorithms to be used in an IKE, ESP, or AH SA.  Requests for
   assignment of new values must be accompanied by a reference to an RFC
   that describes how to use these algorithms.

   This memo defines three exchange types for use with IKEv2. Requests
   for assignment of new exchange types MUST be accompanied by an RFC
   which defines the following:

          - the purpose of and need for the new exchange.
          - the payloads (mandatory and optional) that accompany

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          messages in the exchange.
          - the phase of the exchange.
          - requirements the new exchange has on existing
          exchanges which have assigned numbers.

   Payloads are defined in this memo to convey information between
   peers. New payloads may be required when defining a new
   authentication method or exchange. Requests for new payload types
   MUST be accompanied by an RFC which defines the physical layout of
   the payload and the fields it contains. All payloads MUST use the
   same generic header defined in Figure 2.

9 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.

10 References

10.1 Normative References

   [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.

10.2 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.

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   [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",

   [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.

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

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

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

   [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.

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   [PK01]   Perlman, R., and Kaufman, C., "Analysis of the IPsec key
            exchange Standard", WET-ICE Security Conference, MIT, 2001,

   [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

   [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.

   [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.

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Appendix A: NAT Traversal

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

   NATs exist primarily because of the shortage of IPv4 addresses,
   though there are other rationales. IP nodes that are "behind" a NAT
   have IP addresses that are not globally unique, but rather are
   assigned from some space that is unique within the network behind the
   NAT but 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 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

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   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 SHOULD be accepted coming from any port and responses
   SHOULD be sent to the port from whence they came. This is because the
   ports may be modified as the packets pass through NATs. Similarly, IP
   addresses of the IKE endpoints are generally not included in the IKE
   payloads because the payloads are cryptographically protected and
   could not be transparently modified by NATs.

   Port 4500 is reserved for UDP encapsulated ESP, 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 SHOULD send all subsequent traffic to and from
   port 4500, which all NATs should know run the NAT-friendly protocol.

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Appendix B: Diffie-Hellman Groups

   There are 5 groups 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

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

B.1 Group 1 - 768 Bit MODP

   IKE implementations MAY support a MODP group with the following prime
   and generator. 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

   The generator is 2.

B.2 Group 2 - 1024 Bit MODP

   IKE implementations SHOULD support a MODP group with the following
   prime and generator. 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

   IKE implementations MAY support a EC2N group with the following
   characteristics. 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:
   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).

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B.4 Group 4 - 185 Bit EC2N

   IKE implementations MAY support a EC2N group with the following
   characteristics. 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.

   Field Size:                         185
   Group Prime/Irreducible Polynomial:
   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

   IKE implementations MUST support a MODP group with the following
   prime and generator. 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.

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

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   by removing idiosyncracies 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

   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

   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".

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

   1) Reorganized the document moving introductory material to the

   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

   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 negotation back to 6 messages when a cookie is

   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.

Author's Address

   Charlie Kaufman charlie_kaufman@notesdev.ibm.com IBM

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