Internet Engineering Task Force             R. Pereira, TimeStep Corp.
IP Security Working Group                    S. Anand, Microsoft Corp.
Internet Draft                                   B. Patel, Intel Corp.
Expires in six months
                                                       August 17, 1999

                   The ISAKMP Configuration Method

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   This document is a submission to the IETF Internet Protocol
   Security Working Group. Comments are solicited and should be
   addressed to the working group mailing list
   ( or to the editor(s).

   This document is an Internet-Draft.  Internet Drafts are working
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Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   This document describes a new ISAKMP method that allows
   configuration items to be exchanged securely by using both
   push/acknowledge or request/reply paradigms.

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Table of Contents

     1.   Introduction..............................................2
     1.1  Reader Prerequisites......................................2
     1.2  Specification of Requirements.............................3
     2.   Configuration Transaction.................................3
     3.   Configuration Method Exchange and Payload.................4
     3.1  Transaction Exchanges.....................................4
     3.2  Attribute Payload.........................................5
     3.3  Configuration Message Types...............................6
     3.4  Configuration Attributes..................................7
     3.5  Retransmission............................................9
     4.   Exchange Positioning......................................9
     5.   Specific Uses.............................................9
     5.1  Requesting an Internal Address...........................10
     5.2  Requesting the Peer's Version............................11
     6.   Enterprise Management Considerations.....................11
     7.   Security Considerations..................................11
     8.   References...............................................12
     9.   Acknowledgments..........................................12
     10.  Editors' Addresses.......................................13
     11.  Expiration...............................................13
     12.  Full Copyright Statement.................................14

1.  Introduction

   The ISAKMP protocol provides a framework to negotiate and generate
   Security Associations.  While negotiating SAs, it is sometimes
   quite useful to retrieve certain information from the other peer
   before the non-ISAKMP SA can be established.  Luckily, ISAKMP is
   also flexible enough to provide configuration information and do it
   securely.  This document will present a mechanism to extend ISAKMP
   to provide such functionality.

1.1 Reader Prerequisites

   It is assumed that the reader is familiar with the terms and
   concepts described in the "Security Architecture for the Internet
   Protocol" [ArchSec] and "IP Security Document Roadmap" [Thayer97]

   Readers are advised to be familiar with both [IKE] and [ISAKMP]
   because of the terminology used within this document and the fact
   that this document is an extension of both of those documents.

R. Pereira, S. Anand, B. Patel                                [Page 2]

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1.2 Specification of Requirements

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
   NOT", and "MAY" that appear in this document are to be interpreted
   as described in [Bradner97].

2.  Configuration Transaction

   A "Configuration Transaction" is defined as two configuration
   exchanges, the first being either a Set or a Request and the second
   being either an Acknowledge or a Reply, respectively.  A common
   identifier is used to identify the transaction between exchanges.

   There are two paradigms to follow for this method.

   o "Request/Reply" allows a host to request information from an
     informed hosts (a configuration manager).  If the attributes in
     the Request message are not empty, then these attributes are
     taken as suggestions for that attribute.  The Reply message MAY
     wish to choose those values, or return new values.  It MAY also
     add new attributes and not include some requested attributes.

     A Reply MUST always be sent when a Request is received, even if
     it is an empty Reply or if there are missing attributes in the
     Request.  This merely means that the requested attributes were
     not available or unknown.

       Initiator              Responder
       ---------------        --------------
       REQUEST          -->
                        <--   REPLY

   o "Set/Acknowledge" works on the push principle that allows a
     configuration manager (a host that wishes to send information to
     another host) to start the configuration transaction.  This code
     sends attributes that it wants the peer to alter.  The
     Acknowledge code MUST return the zero length attributes that it
     accepted.  Those attributes that it did not accept will NOT be
     sent back in the acknowledgement.

       Initiator              Responder
       ---------------        -------------------
       SET              -->
                        <--   ACKNOWLEDGE

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   Transactions are completed once the Reply or Acknowledge code is
   received.  If one is not received, the implementation MAY wish to
   retransmit the original exchange as detailed in a later section.

   The initiator and responder are not necessarily the same as the
   initiator and responder of the ISAKMP exchange.

3.  Configuration Method Exchange and Payload

3.1 Transaction Exchanges

   A new exchange mode is required for the configuration method.  This
   exchange is called the "Transaction Exchange" and has a value of 6.
   This exchange is quite similar to the Information exchange
   described in [ISAKMP] and [IKE], but allows for multi-exchange
   transactions instead of being a one-way transmittal of information.

   This specification protects ISAKMP Transaction Exchanges when

3.1.1 Protected Exchanges

   Once an ISAKMP security association has been established (and
   SKEYID_e and SKEYID_a have been generated), the ISAKMP Transaction
   Exchange is as follows:

           Initiator                        Responder
          -----------                      -----------
           HDR*, HASH, ATTR      -->
                                 <--        HDR*, HASH, ATTR

   Where the HASH payload contains the prf output, using SKEYID_a as
   the key, and the M-ID (ISAKMP header Message ID) unique to this
   exchange concatenated with all of the payloads after the HASH
   payload. In other words, the hash for the above exchange is:

           HASH = prf( SKEYID_a, M-ID | ATTR )

   Multiple ATTR payloads MAY NOT be present in the Transaction

   As noted, the message ID in the ISAKMP header-- as used in the prf
   computation-- is unique to this exchange and MUST NOT be the same
   as the message ID of another exchange.  The derivation of the
   initialization vector (IV) for the first message, used with
   SKEYID_e to encrypt the message, is described in Appendix B of
   [IKE].  Subsequent IVs are taken from the last ciphertext block of

R. Pereira, S. Anand, B. Patel                                [Page 4]

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   the previous message as described in [IKE].

3.1.2 Unprotected Exchanges

   If the ISAKMP security association has not yet been established at
   the time of the Transaction Exchange and the information being
   exchanged is not sensitive, the exchange MAY be done in the clear
   without an accompanying HASH payload.

           Initiator                        Responder
          -----------                      -----------
           HDR, ATTR           -->
                               <--          HDR, ATTR

   Multiple ATTR payloads MAY NOT be present in the Transaction

3.2 Attribute Payload

   A new payload is defined to carry attributes as well as the type of
   transaction message.

                           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  !   RESERVED    !         Payload Length        !
     !     Type      !   RESERVED    !           Identifier          !
     !                                                               !
     ~                           Attributes                          ~
     !                                                               !

   The Attributes Payload 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.

   o RESERVED (1 octet) - Unused, set to 0.

   o Payload Length (2 octets) - Length in octets of the current
     payload, including the generic payload header, the transaction-
     specific header and all attributes.  If the length does not
     match the length of the payload headers plus the attributes,
     (i.e. an attribute is half contained within this payload) then
     entire payload MUST be discarded.

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   o Attribute Message Type (1 octet) - Specifies the type of message
     represented by the attributes.  These are defined in the next

   o RESERVED (1 octet) - Unused, set to 0.

   o Identifier (2 octets) - An identifier used to reference a
     configuration transaction within the individual messages.

   o Attributes (variable length) - Zero or more ISAKMP Data
     Attributes as defined in [ISAKMP].  The attribute types are
     defined in a later section.

   The payload type for the Attributes Payload is 14.

3.3 Configuration Message Types

   These values are to be used within the Type field of an Attribute
   ISAKMP payload.

    Types                      Value
   ========================== ===========
    RESERVED                   0
    ISAKMP_CFG_REQUEST         1
    ISAKMP_CFG_REPLY           2
    ISAKMP_CFG_SET             3
    ISAKMP_CFG_ACK             4
    Reserved for Future Use    5-127
    Reserved for Private Use   128-255

   Messages with unknown types SHOULD be silently discarded.

R. Pereira, S. Anand, B. Patel                                [Page 6]

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3.4 Configuration Attributes

   Zero or more ISAKMP attributes [ISAKMP] are contained within an
   Attributes Payload. Zero length attribute values are usually sent
   in a Request and MUST NOT be sent in a Response.

   All IPv6 specific attributes are mandatory only if the
   implementation supports IPv6 and vice versa for IPv4.  Mandatory
   attributes are stated below.

   Unknown private attributes SHOULD be silently discarded.

   The following attributes are currently defined:

    Attribute                 Value   Type       Length
   ========================= ======= ========== =====================
    RESERVED                    0
    INTERNAL_IP4_ADDRESS        1     Variable   0 or 4 octets
    INTERNAL_IP4_NETMASK        2     Variable   0 or 4 octets
    INTERNAL_IP4_DNS            3     Variable   0 or 4 octets
    INTERNAL_IP4_NBNS           4     Variable   0 or 4 octets
    INTERNAL_ADDRESS_EXPIRY     5     Variable   0 or 4 octets
    INTERNAL_IP4_DHCP           6     Variable   0 or 4 octets
    APPLICATION_VERSION         7     Variable   0 or more
    INTERNAL_IP6_ADDRESS        8     Variable   0 or 16 octets
    INTERNAL_IP6_NETMASK        9     Variable   0 or 16 octets
    INTERNAL_IP6_DNS           10     Variable   0 or 16 octets
    INTERNAL_IP6_NBNS          11     Variable   0 or 16 octets
    INTERNAL_IP6_DHCP          12     Variable   0 or 16 octets
    INTERNAL_IP4_SUBNET        13     Variable   0 or 4 octets
    SUPPORTED_ATTRIBUTES       14     Variable   0 or multiples of 2
    Reserved for future use    15-16383
    Reserved for private use   16384-32767

     address within the internal network.  This address is sometimes
     called a red node address or a private address and MAY be a
     private address on the Internet.  Multiple internal addresses
     MAY be requested by requesting multiple internal address
     attributes.  The responder MAY only send up to the number of
     addresses requested.

     The requested address is valid until the expiry time defined
     with the INTERNAL_ADDRESS EXPIRY attribute or until the ISAKMP
     SA that was used to secure the request expires.  The address MAY
     also expire when the IPSec (phase 2) SA expires, if the request
     is associated with a phase 2 negotiation.  If no ISAKMP SA was
     used to secure the request, then the response MUST include an

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     expiry or the host MUST expire the SA after an implementation-
     defined time.

     An implementation MUST support this attribute.

     network's netmask.  Only one netmask is allowed in the request
     and reply messages. (e.g.

     An implementation MUST support this attribute.

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

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

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

     An implementation MUST support this attribute.

   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.

     This attribute does not need to be secured.

     An implementation MUST support this attribute.

   o INTERNAL_IP4_SUBNET û The protected sub-networks that this edge-
     device protects.  Multiple sub-networks MAY be requested.  The
     responder MAY respond with zero or more sub-networks attributes.

     An implementation MUST support this attribute.

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

     An implementation MUST support this attribute.

   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 edge
   device) determining which DNS server should be returned to a
   requesting host.

3.5 Retransmission

   Retransmission SHOULD follow the same retransmission rules used
   with standard ISAKMP messages.

4.  Exchange Positioning

   The exchange and messages defined within this document MAY appear
   at any time.  Because of security considerations with most
   attributes, the exchange SHOULD be secured with an ISAKMP phase 1

   Depending on the type of transaction and the information being
   exchanged, the exchange MAY be dependant on an ISAKMP phase 1 SA
   negotiation, a phase 2 SA negotiation, or none of the above.

   The next section details specific functions and their position
   within an ISAKMP negotiation.

5.  Specific Uses

   The following descriptions detail how to perform specific functions
   using this protocol.  Other functions are possible and thus this
   list is not a complete list of all of the possibilities.  While
   other functions are possible, the functions listed below MUST be
   performed as detailed in this document to preserve interoperability
   among different vendor's implementations.

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5.1 Requesting an Internal Address

   This function provides address allocation to a remote host trying
   to tunnel into a network protected by an edge device.   The remote
   host requests an address and optionally other information
   concerning the internal network from the edge device.  The edge
   device procures an internal address for the remote host from any
   number of sources such as a DHCP/BOOTP server or an its own address

    Initiator                           Responder
   -----------------------------       -------------------------------
    HDR*, HASH, ATTR1(REQUEST)    -->
                                  <--   HDR*, HASH, ATTR2(REPLY)

   ATTR1(REQUEST) MUST contain at least an INTERNAL_ADDRESS attribute
   (either IPv4 or IPv6) but MAY also contain any number of additional
   attributes that it wants returned in the response.

   For example:


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

   This Transaction Exchange MUST occur after an ISAKMP phase 1 SA is
   already established and before an ISAKMP phase 2 negotiation has
   started, since that negotiation requires the internal address.

   Initial Negotiation:
     MainMode or AggressiveMode
     TransactionMode (IP Address request)

   Subsequent address requests would be done without the phase 1
   negotiation when there already exists a phase 1 SA.

R. Pereira, S. Anand, B. Patel                               [Page 10]

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   Subsequent Negotiations:
     TransactionMode (IP Address request)

5.2 Requesting the Peer's Version

   An IPSec host wishing to inquire about the other peer's version
   information (with or without security) MUST use this method.

    Initiator                           Responder
   -----------------------------       --------------------------
    HDR, ATTR1(REQUEST)           -->
                                  <--   HDR, ATTR2(REPLY)


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

   The return text string will be implementation dependent.  This
   transaction MAY be done at any time and with or without any other
   ISAKMP exchange and because the version information MAY be deemed
   not sensitive, security is optional.

6.  Enterprise Management Considerations

   The method defined in this document SHOULD NOT be used for wide
   scale management.  Its main intent is to provide a bootstrap
   mechanism to exchange information within IPSec.  While it MAY be
   useful to use such a method of exchange information to some
   outlying IPSec hosts or small networks, existing management
   protocols such as DHCP [DHCP], RADIUS [RADIUS], SNMP or LDAP [LDAP]
   should be considered for enterprise management as well as
   subsequent information exchanges.

7.  Security Considerations

   This entire draft discusses a new ISAKMP configuration method to
   allow IPSec-enabled entities to acquire and share configuration

   The draft mandates that this exchange should normally occur after
   the Phase I Security Association has been set up and that the
   entire exchange be protected by that Phase I SA.  Thus the exchange
   is as secure as any Phase II SA negotiation.

   This exchange MAY be secured (encrypted and authenticated) by other

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   means as well, such as pre-configured ESP or data-link security.

8.  References

   [ArchSec]   S. Kent, R. Atkinson, "Security Architecture for the
               Internet Protocol", RFC2401

   [Bradner97] S. Bradner, "Key words for use in RFCs to indicate
               Requirement Levels", RFC2119

   [ISAKMP]    D. Maughan, M. Schertler, M. Schneider, J. Turner,
               "Internet Security Association and Key Management
               Protocol", RFC2408

   [IKE]       D. Harkins, D. Carrel, "The Internet Key Exchange
               (IKE)", RFC2409

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

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

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

   [ESP]       S. Kent, "IP Encapsulating Security Payload (ESP)",

9.  Acknowledgments

   The editors would like to thank Stephane Beaulieu, Tim Jenkins,
   Peter Ford, Bob Moskowitz and Shawn Mamros.

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10. Editors' Addresses

     Roy Pereira
     TimeStep Corporation
     +1 (613) 599-3610 x 4808

     Sanjay Anand
     Microsoft Corporation
     +1 (206) 936-6367

     Baiju V. Patel
     Intel Corporation
     +1 (503) 264 2422

   The IPSec working group can be contacted via the IPSec working
   group's mailing list ( or through its chairs:

     Robert Moskowitz
     International Computer Security Association

     Theodore Y. Ts'o
     Massachusetts Institute of Technology

11. Expiration

   This draft expires January 17, 2000.

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12. Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain
   it or assist in its implementation may be prepared, copied,
   published and distributed, in whole or in part, without restriction
   of any kind, provided that the above copyright notice and this
   paragraph are included on all such copies and derivative works.
   However, this document itself may not be modified in any way, such
   as by removing the copyright notice or references to the Internet
   Society or other Internet organizations, except as needed for the
   purpose of developing Internet standards in which case the
   procedures for copyrights defined in the Internet Standards process
   must be followed, or as required to translate it into languages
   other than English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on

R. Pereira, S. Anand, B. Patel                               [Page 14]