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
                                                          May 13, 1998

                   The ISAKMP Configuration Method

Status of this Memo

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

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   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  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
       3.1.1. Protected Exchanges...................................4
       3.1.2. Unprotected Exchanges.................................5
     3.2. Attribute Payload..........................................5
     3.3. Configuration Message Types................................6
     3.4. Configuration Attributes...................................6
     3.5. Retransmission.............................................8
   4. Exchange Positioning...........................................8
   5. Examples.......................................................8
     5.1. Example 1: Requesting an Internal Address..................8
     5.2. Example 2: Central Configuration Manager...................9
     5.3. Example 3: Requesting Peer's Version.......................9
   6. Enterprise Management Considerations..........................10
   7. Security Considerations.......................................10
   8. References....................................................10
   9. Acknowledgments...............................................11
   10. Editors' Addresses...........................................11
   11. Full Copyright Statement.....................................12

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.

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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 "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

   o "Request/Reply" allows a host to request information from an
     informed host (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.

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

   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.

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   If a badly formatted exchange is received, the message SHOULD be
   silently discarded and MAY be logged locally, as per local policy.
   Badly formatted exchanges MIGHT include those with unknown codes or
   unknown attributes.

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 the 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 the 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 )

   Note that more than one ATTR payload MAY be after the HASH payload
   and each ATTR payload represents an individual transaction:

           Initiator                        Responder
          -----------                      -----------
           HDR*, HASH, ATTR1, ATTR2   -->
                                      <--   HDR*, HASH, ATTR1, ATTR2

           HASH = prf( SKEYID_a, M-ID | ATTR1 | ATTR2 )

   As noted, the message ID in the ISAKMP header-- as used in the prf

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   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
   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, the exchange is done in the
   clear without an accompanying HASH payload.

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

   Multiple ATTR payloads MAY be present in the Transaction Exchange
   with each representing an individual 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-

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     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.

   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 fourteen (14).

3.3. Configuration Message Types

    Types                      Value
   ========================== ===========
    RESERVED                   0
    ISKAMP_CFG_REQUEST         1
    ISKAMP_CFG_REPLY           2
    ISKAMP_CFG_SET             3
    ISKAMP_CFG_ACK             4
    Reserved for Future Use    5-127
    Reserved for Private Use   128-255

   Messages with unknown types SHOULD be silently discarded.

3.4. Configuration Attributes

   Zero or more ISAKMP attributes [ISAKMP] are contained within an
   Attributes Payload. 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     Basic/Variable   0 or 2 or 4 octets
    INTERNAL_IP4_DHCP        6     Variable         0 or 4 octets

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    APPLICATION_VERSION      7     Variable         variable
    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
    Reserved for future use 13-16383
    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.  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
     expiry or the host MUST expire the SA after an implementation-
     defined time.

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

   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.

   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.

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   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

   It is hoped that more attribute types will be defined in future
   documents.  Some suggestions would be to distribute local policy,
   or even authenticate certificates.

   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 consideration with certain
   attributes, the exchange SHOULD be secured with a 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.

5.   Examples

   Some examples of this configuration method follow.  These are meant
   only as examples and should not be thought of as the only
   possibilities for this protocol.

5.1. Example 1: Requesting an Internal Address

   Initiator requesting their internal IP address from an edge device
   by a mobile host.

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

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   This Transaction Exchange would occur after an ISAKMP phase 1 SA
   had already occurred and before an ISAKMP phase 2 negotiation
   started, since that negotiation requires the internal address.

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

5.2. Example 2: Central Configuration Manager

   A central configuration manager application sends out policy
   information to an IPSec host.

   Initiator                           Responder
   -----------------------------      --------------------------
   HDR*, HASH, ATTR1(SET)         -->
                                  <--  HDR*, HASH, ATTR2(ACK)

   ATTR1(SET) =

   ATTR2(ACK) =

5.3. Example 3: Requesting Peer's Version

   An IPSec host inquires about the peer's version information (with
   or without security).

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


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

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
  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", draft-ietf-ipsec-arch-sec-04

   [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", draft-ietf-ipsec-isakmp-09

   [IKE]       D. Harkins, D. Carrel, "The Internet Key Exchange
               (IKE)", draft-ietf-ipsec-isakmp-oakley-07

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

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   [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

9.   Acknowledgments

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

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

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11.  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

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