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rfc:rfc2408

Network Working Group D. Maughan Request for Comments: 2408 National Security Agency Category: Standards Track M. Schertler

                                                     Securify, Inc.
                                                       M. Schneider
                                           National Security Agency
                                                          J. Turner
                                            RABA Technologies, Inc.
                                                      November 1998
 Internet Security Association and Key Management Protocol (ISAKMP)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

 This memo describes a protocol utilizing security concepts necessary
 for establishing Security Associations (SA) and cryptographic keys in
 an Internet environment.  A Security Association protocol that
 negotiates, establishes, modifies and deletes Security Associations
 and their attributes is required for an evolving Internet, where
 there will be numerous security mechanisms and several options for
 each security mechanism.  The key management protocol must be robust
 in order to handle public key generation for the Internet community
 at large and private key requirements for those private networks with
 that requirement.  The Internet Security Association and Key
 Management Protocol (ISAKMP) defines the procedures for
 authenticating a communicating peer, creation and management of
 Security Associations, key generation techniques, and threat
 mitigation (e.g.  denial of service and replay attacks).  All of
 these are necessary to establish and maintain secure communications
 (via IP Security Service or any other security protocol) in an
 Internet environment.

Maughan, et. al. Standards Track [Page 1] RFC 2408 ISAKMP November 1998

Table of Contents

 1 Introduction                                                     4
   1.1 Requirements Terminology  . . . . . . . . . . . . . . . . .  5
   1.2 The Need for Negotiation  . . . . . . . . . . . . . . . . .  5
   1.3 What can be Negotiated?   . . . . . . . . . . . . . . . . .  6
   1.4 Security Associations and Management  . . . . . . . . . . .  7
     1.4.1 Security Associations and Registration  . . . . . . . .  7
     1.4.2 ISAKMP Requirements   . . . . . . . . . . . . . . . . .  8
   1.5 Authentication  . . . . . . . . . . . . . . . . . . . . . .  8
     1.5.1 Certificate Authorities   . . . . . . . . . . . . . . .  9
     1.5.2 Entity Naming   . . . . . . . . . . . . . . . . . . . .  9
     1.5.3 ISAKMP Requirements   . . . . . . . . . . . . . . . . . 10
   1.6 Public Key Cryptography . . . . . . . . . . . . . . . . . . 10
     1.6.1 Key Exchange Properties   . . . . . . . . . . . . . . . 11
     1.6.2 ISAKMP Requirements   . . . . . . . . . . . . . . . . . 12
   1.7 ISAKMP Protection . . . . . . . . . . . . . . . . . . . . . 12
     1.7.1 Anti-Clogging (Denial of Service)   . . . . . . . . . . 12
     1.7.2 Connection Hijacking  . . . . . . . . . . . . . . . . . 13
     1.7.3 Man-in-the-Middle Attacks   . . . . . . . . . . . . . . 13
   1.8 Multicast Communications  . . . . . . . . . . . . . . . . . 13
 2 Terminology and Concepts                                        14
   2.1 ISAKMP Terminology  . . . . . . . . . . . . . . . . . . . . 14
   2.2 ISAKMP Placement  . . . . . . . . . . . . . . . . . . . . . 16
   2.3 Negotiation Phases  . . . . . . . . . . . . . . . . . . . . 16
   2.4 Identifying Security Associations . . . . . . . . . . . . . 17
   2.5 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . 20
     2.5.1 Transport Protocol  . . . . . . . . . . . . . . . . . . 20
     2.5.2 RESERVED Fields   . . . . . . . . . . . . . . . . . . . 20
     2.5.3 Anti-Clogging Token ("Cookie") Creation   . . . . . . . 20
 3 ISAKMP Payloads                                                 21
   3.1 ISAKMP Header Format  . . . . . . . . . . . . . . . . . . . 21
   3.2 Generic Payload Header  . . . . . . . . . . . . . . . . . . 25
   3.3 Data Attributes . . . . . . . . . . . . . . . . . . . . . . 25
   3.4 Security Association Payload  . . . . . . . . . . . . . . . 27
   3.5 Proposal Payload  . . . . . . . . . . . . . . . . . . . . . 28
   3.6 Transform Payload . . . . . . . . . . . . . . . . . . . . . 29
   3.7 Key Exchange Payload  . . . . . . . . . . . . . . . . . . . 31
   3.8 Identification Payload  . . . . . . . . . . . . . . . . . . 32
   3.9 Certificate Payload . . . . . . . . . . . . . . . . . . . . 33
   3.10 Certificate Request Payload  . . . . . . . . . . . . . . . 34
   3.11 Hash Payload   . . . . . . . . . . . . . . . . . . . . . . 36
   3.12 Signature Payload  . . . . . . . . . . . . . . . . . . . . 37
   3.13 Nonce Payload  . . . . . . . . . . . . . . . . . . . . . . 37
   3.14 Notification Payload   . . . . . . . . . . . . . . . . . . 38
     3.14.1 Notify Message Types   . . . . . . . . . . . . . . . . 40
   3.15 Delete Payload   . . . . . . . . . . . . . . . . . . . . . 41
   3.16 Vendor ID Payload  . . . . . . . . . . . . . . . . . . . . 43

Maughan, et. al. Standards Track [Page 2] RFC 2408 ISAKMP November 1998

 4 ISAKMP Exchanges                                                44
   4.1 ISAKMP Exchange Types . . . . . . . . . . . . . . . . . . . 45
     4.1.1 Notation  . . . . . . . . . . . . . . . . . . . . . . . 46
   4.2 Security Association Establishment  . . . . . . . . . . . . 46
     4.2.1 Security Association Establishment Examples   . . . . . 48
   4.3 Security Association Modification . . . . . . . . . . . . . 50
   4.4 Base Exchange . . . . . . . . . . . . . . . . . . . . . . . 51
   4.5 Identity Protection Exchange  . . . . . . . . . . . . . . . 52
   4.6 Authentication Only Exchange  . . . . . . . . . . . . . . . 54
   4.7 Aggressive Exchange . . . . . . . . . . . . . . . . . . . . 55
   4.8 Informational Exchange  . . . . . . . . . . . . . . . . . . 57
 5 ISAKMP Payload Processing                                       58
   5.1 General Message Processing  . . . . . . . . . . . . . . . . 58
   5.2 ISAKMP Header Processing  . . . . . . . . . . . . . . . . . 59
   5.3 Generic Payload Header Processing . . . . . . . . . . . . . 61
   5.4 Security Association Payload Processing . . . . . . . . . . 62
   5.5 Proposal Payload Processing . . . . . . . . . . . . . . . . 63
   5.6 Transform Payload Processing  . . . . . . . . . . . . . . . 64
   5.7 Key Exchange Payload Processing . . . . . . . . . . . . . . 65
   5.8 Identification Payload Processing . . . . . . . . . . . . . 66
   5.9 Certificate Payload Processing  . . . . . . . . . . . . . . 66
   5.10 Certificate Request Payload Processing   . . . . . . . . . 67
   5.11 Hash Payload Processing  . . . . . . . . . . . . . . . . . 69
   5.12 Signature Payload Processing   . . . . . . . . . . . . . . 69
   5.13 Nonce Payload Processing   . . . . . . . . . . . . . . . . 70
   5.14 Notification Payload Processing  . . . . . . . . . . . . . 71
   5.15 Delete Payload Processing  . . . . . . . . . . . . . . . . 73
 6 Conclusions                                                     75
 A ISAKMP Security Association Attributes                          77
   A.1 Background/Rationale  . . . . . . . . . . . . . . . . . . . 77
   A.2 Internet IP Security DOI Assigned Value . . . . . . . . . . 77
   A.3 Supported Security Protocols  . . . . . . . . . . . . . . . 77
   A.4 ISAKMP Identification Type Values . . . . . . . . . . . . . 78
     A.4.1 ID_IPV4_ADDR  . . . . . . . . . . . . . . . . . . . . . 78
     A.4.2 ID_IPV4_ADDR_SUBNET . . . . . . . . . . . . . . . . . . 78
     A.4.3 ID_IPV6_ADDR  . . . . . . . . . . . . . . . . . . . . . 78
     A.4.4 ID_IPV6_ADDR_SUBNET   . . . . . . . . . . . . . . . . . 78
 B Defining a new Domain of Interpretation                         79
   B.1 Situation . . . . . . . . . . . . . . . . . . . . . . . . . 79
   B.2 Security Policies . . . . . . . . . . . . . . . . . . . . . 80
   B.3 Naming Schemes  . . . . . . . . . . . . . . . . . . . . . . 80
   B.4 Syntax for Specifying Security Services . . . . . . . . . . 80
   B.5 Payload Specification . . . . . . . . . . . . . . . . . . . 80
   B.6 Defining new Exchange Types . . . . . . . . . . . . . . . . 80
 Security Considerations                                           81
 IANA Considerations                                               81
 Domain of Interpretation                                          81
 Supported Security Protocols                                      82

Maughan, et. al. Standards Track [Page 3] RFC 2408 ISAKMP November 1998

 Acknowledgements                                                  82
 References                                                        82
 Authors' Addresses                                                85
 Full Copyright Statement                                          86

List of Figures

 1   ISAKMP Relationships  . . . . . . . . . . . . . . . . . . . 16
 2   ISAKMP Header Format  . . . . . . . . . . . . . . . . . . . 22
 3   Generic Payload Header  . . . . . . . . . . . . . . . . . . 25
 4   Data Attributes . . . . . . . . . . . . . . . . . . . . . . 26
 5   Security Association Payload  . . . . . . . . . . . . . . . 27
 6   Proposal Payload Format . . . . . . . . . . . . . . . . . . 28
 7   Transform Payload Format  . . . . . . . . . . . . . . . . . 30
 8   Key Exchange Payload Format . . . . . . . . . . . . . . . . 31
 9   Identification Payload Format . . . . . . . . . . . . . . . 32
 10  Certificate Payload Format  . . . . . . . . . . . . . . . . 33
 11  Certificate Request Payload Format  . . . . . . . . . . . . 34
 12  Hash Payload Format . . . . . . . . . . . . . . . . . . . . 36
 13  Signature Payload Format  . . . . . . . . . . . . . . . . . 37
 14  Nonce Payload Format  . . . . . . . . . . . . . . . . . . . 38
 15  Notification Payload Format . . . . . . . . . . . . . . . . 39
 16  Delete Payload Format . . . . . . . . . . . . . . . . . . . 42
 17  Vendor ID Payload Format  . . . . . . . . . . . . . . . . . 44

1 Introduction

 This document describes an Internet Security Association and Key
 Management Protocol (ISAKMP). ISAKMP combines the security concepts
 of authentication, key management, and security associations to
 establish the required security for government, commercial, and
 private communications on the Internet.
 The Internet Security Association and Key Management Protocol
 (ISAKMP) defines procedures and packet formats to establish,
 negotiate, modify and delete Security Associations (SA). SAs contain
 all the information required for execution of various network
 security services, such as the IP layer services (such as header
 authentication and payload encapsulation), transport or application
 layer services, or self-protection of negotiation traffic.  ISAKMP
 defines payloads for exchanging key generation and authentication
 data.  These formats provide a consistent framework for transferring
 key and authentication data which is independent of the key
 generation technique, encryption algorithm and authentication
 mechanism.

Maughan, et. al. Standards Track [Page 4] RFC 2408 ISAKMP November 1998

 ISAKMP is distinct from key exchange protocols in order to cleanly
 separate the details of security association management (and key
 management) from the details of key exchange.  There may be many
 different key exchange protocols, each with different security
 properties.  However, a common framework is required for agreeing to
 the format of SA attributes, and for negotiating, modifying, and
 deleting SAs.  ISAKMP serves as this common framework.
 Separating the functionality into three parts adds complexity to the
 security analysis of a complete ISAKMP implementation.  However, the
 separation is critical for interoperability between systems with
 differing security requirements, and should also simplify the
 analysis of further evolution of a ISAKMP server.
 ISAKMP is intended to support the negotiation of SAs for security
 protocols at all layers of the network stack (e.g., IPSEC, TLS, TLSP,
 OSPF, etc.).  By centralizing the management of the security
 associations, ISAKMP reduces the amount of duplicated functionality
 within each security protocol.  ISAKMP can also reduce connection
 setup time, by negotiating a whole stack of services at once.
 The remainder of section 1 establishes the motivation for security
 negotiation and outlines the major components of ISAKMP, i.e.
 Security Associations and Management, Authentication, Public Key
 Cryptography, and Miscellaneous items.  Section 2 presents the
 terminology and concepts associated with ISAKMP. Section 3 describes
 the different ISAKMP payload formats.  Section 4 describes how the
 payloads of ISAKMP are composed together as exchange types to
 establish security associations and perform key exchanges in an
 authenticated manner.  Additionally, security association
 modification, deletion, and error notification are discussed.
 Section 5 describes the processing of each payload within the context
 of ISAKMP exchanges, including error handling and associated actions.
 The appendices provide the attribute values necessary for ISAKMP and
 requirement for defining a new Domain of Interpretation (DOI) within
 ISAKMP.

1.1 Requirements Terminology

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
 document, are to be interpreted as described in [RFC-2119].

1.2 The Need for Negotiation

 ISAKMP extends the assertion in [DOW92] that authentication and key
 exchanges must be combined for better security to include security
 association exchanges.  The security services required for

Maughan, et. al. Standards Track [Page 5] RFC 2408 ISAKMP November 1998

 communications depends on the individual network configurations and
 environments.  Organizations are setting up Virtual Private Networks
 (VPN), also known as Intranets, that will require one set of security
 functions for communications within the VPN and possibly many
 different security functions for communications outside the VPN to
 support geographically separate organizational components, customers,
 suppliers, sub-contractors (with their own VPNs), government, and
 others.  Departments within large organizations may require a number
 of security associations to separate and protect data (e.g.
 personnel data, company proprietary data, medical) on internal
 networks and other security associations to communicate within the
 same department.  Nomadic users wanting to "phone home" represent
 another set of security requirements.  These requirements must be
 tempered with bandwidth challenges.  Smaller groups of people may
 meet their security requirements by setting up "Webs of Trust".
 ISAKMP exchanges provide these assorted networking communities the
 ability to present peers with the security functionality that the
 user supports in an authenticated and protected manner for agreement
 upon a common set of security attributes, i.e.  an interoperable
 security association.

1.3 What can be Negotiated?

 Security associations must support different encryption algorithms,
 authentication mechanisms, and key establishment algorithms for other
 security protocols, as well as IP Security.  Security associations
 must also support host-oriented certificates for lower layer
 protocols and user- oriented certificates for higher level protocols.
 Algorithm and mechanism independence is required in applications such
 as e-mail, remote login, and file transfer, as well as in session
 oriented protocols, routing protocols, and link layer protocols.
 ISAKMP provides a common security association and key establishment
 protocol for this wide range of security protocols, applications,
 security requirements, and network environments.
 ISAKMP is not bound to any specific cryptographic algorithm, key
 generation technique, or security mechanism.  This flexibility is
 beneficial for a number of reasons.  First, it supports the dynamic
 communications environment described above.  Second, the independence
 from specific security mechanisms and algorithms provides a forward
 migration path to better mechanisms and algorithms.  When improved
 security mechanisms are developed or new attacks against current
 encryption algorithms, authentication mechanisms and key exchanges
 are discovered, ISAKMP will allow the updating of the algorithms and
 mechanisms without having to develop a completely new KMP or patch
 the current one.

Maughan, et. al. Standards Track [Page 6] RFC 2408 ISAKMP November 1998

 ISAKMP has basic requirements for its authentication and key exchange
 components.  These requirements guard against denial of service,
 replay / reflection, man-in-the-middle, and connection hijacking
 attacks.  This is important because these are the types of attacks
 that are targeted against protocols.  Complete Security Association
 (SA) support, which provides mechanism and algorithm independence,
 and protection from protocol threats are the strengths of ISAKMP.

1.4 Security Associations and Management

 A Security Association (SA) is a relationship between two or more
 entities that describes how the entities will utilize security
 services to communicate securely.  This relationship is represented
 by a set of information that can be considered a contract between the
 entities.  The information must be agreed upon and shared between all
 the entities.  Sometimes the information alone is referred to as an
 SA, but this is just a physical instantiation of the existing
 relationship.  The existence of this relationship, represented by the
 information, is what provides the agreed upon security information
 needed by entities to securely interoperate.  All entities must
 adhere to the SA for secure communications to be possible.  When
 accessing SA attributes, entities use a pointer or identifier refered
 to as the Security Parameter Index (SPI). [SEC-ARCH] provides details
 on IP Security Associations (SA) and Security Parameter Index (SPI)
 definitions.

1.4.1 Security Associations and Registration

 The SA attributes required and recommended for the IP Security (AH,
 ESP) are defined in [SEC-ARCH].  The attributes specified for an IP
 Security SA include, but are not limited to, authentication
 mechanism, cryptographic algorithm, algorithm mode, key length, and
 Initialization Vector (IV).  Other protocols that provide algorithm
 and mechanism independent security MUST define their requirements for
 SA attributes.  The separation of ISAKMP from a specific SA
 definition is important to ensure ISAKMP can es tablish SAs for all
 possible security protocols and applications.
 NOTE: See [IPDOI] for a discussion of SA attributes that should be
 considered when defining a security protocol or application.
 In order to facilitate easy identification of specific attributes
 (e.g.  a specific encryption algorithm) among different network
 entites the attributes must be assigned identifiers and these
 identifiers must be registered by a central authority.  The Internet
 Assigned Numbers Authority (IANA) provides this function for the
 Internet.

Maughan, et. al. Standards Track [Page 7] RFC 2408 ISAKMP November 1998

1.4.2 ISAKMP Requirements

 Security Association (SA) establishment MUST be part of the key
 management protocol defined for IP based networks.  The SA concept is
 required to support security protocols in a diverse and dynamic
 networking environment.  Just as authentication and key exchange must
 be linked to provide assurance that the key is established with the
 authenticated party [DOW92], SA establishment must be linked with the
 authentication and the key exchange protocol.
 ISAKMP provides the protocol exchanges to establish a security
 association between negotiating entities followed by the
 establishment of a security association by these negotiating entities
 in behalf of some protocol (e.g.  ESP/AH). First, an initial protocol
 exchange allows a basic set of security attributes to be agreed upon.
 This basic set provides protection for subsequent ISAKMP exchanges.
 It also indicates the authentication method and key exchange that
 will be performed as part of the ISAKMP protocol.  If a basic set of
 security attributes is already in place between the negotiating
 server entities, the initial ISAKMP exchange may be skipped and the
 establishment of a security association can be done directly.  After
 the basic set of security attributes has been agreed upon, initial
 identity authenticated, and required keys generated, the established
 SA can be used for subsequent communications by the entity that
 invoked ISAKMP.  The basic set of SA attributes that MUST be
 implemented to provide ISAKMP interoperability are defined in
 Appendix A.

1.5 Authentication

 A very important step in establishing secure network communications
 is authentication of the entity at the other end of the
 communication.  Many authentication mechanisms are available.
 Authentication mechanisms fall into two catagories of strength - weak
 and strong.  Sending cleartext keys or other unprotected
 authenticating information over a network is weak, due to the threat
 of reading them with a network sniffer.  Additionally, sending one-
 way hashed poorly-chosen keys with low entropy is also weak, due to
 the threat of brute-force guessing attacks on the sniffed messages.
 While passwords can be used for establishing identity, they are not
 considered in this context because of recent statements from the
 Internet Architecture Board [IAB].  Digital signatures, such as the
 Digital Signature Standard (DSS) and the Rivest-Shamir-Adleman (RSA)
 signature, are public key based strong authentication mechanisms.
 When using public key digital signatures each entity requires a
 public key and a private key.  Certificates are an essential part of
 a digital signature authentication mechanism.  Certificates bind a
 specific entity's identity (be it host, network, user, or

Maughan, et. al. Standards Track [Page 8] RFC 2408 ISAKMP November 1998

 application) to its public keys and possibly other security-related
 information such as privileges, clearances, and compartments.
 Authentication based on digital signatures requires a trusted third
 party or certificate authority to create, sign and properly
 distribute certificates.  For more detailed information on digital
 signatures, such as DSS and RSA, and certificates see [Schneier].

1.5.1 Certificate Authorities

 Certificates require an infrastructure for generation, verification,
 revocation, management and distribution.  The Internet Policy
 Registration Authority (IPRA) [RFC-1422] has been established to
 direct this infrastructure for the IETF. The IPRA certifies Policy
 Certification Authorities (PCA). PCAs control Certificate Authorities
 (CA) which certify users and subordinate entities.  Current
 certificate related work includes the Domain Name System (DNS)
 Security Extensions [DNSSEC] which will provide signed entity keys in
 the DNS. The Public Key Infrastucture (PKIX) working group is
 specifying an Internet profile for X.509 certificates.  There is also
 work going on in industry to develop X.500 Directory Services which
 would provide X.509 certificates to users.  The U.S. Post Office is
 developing a (CA) hierarchy.  The NIST Public Key Infrastructure
 Working Group has also been doing work in this area.  The DOD Multi
 Level Information System Security Initiative (MISSI) program has
 begun deploying a certificate infrastructure for the U.S. Government.
 Alternatively, if no infrastructure exists, the PGP Web of Trust
 certificates can be used to provide user authentication and privacy
 in a community of users who know and trust each other.

1.5.2 Entity Naming

 An entity's name is its identity and is bound to its public keys in
 certificates.  The CA MUST define the naming semantics for the
 certificates it issues.  See the UNINETT PCA Policy Statements
 [Berge] for an example of how a CA defines its naming policy.  When
 the certificate is verified, the name is verified and that name will
 have meaning within the realm of that CA. An example is the DNS
 security extensions which make DNS servers CAs for the zones and
 nodes they serve.  Resource records are provided for public keys and
 signatures on those keys.  The names associated with the keys are IP
 addresses and domain names which have meaning to entities accessing
 the DNS for this information.  A Web of Trust is another example.
 When webs of trust are set up, names are bound with the public keys.
 In PGP the name is usually the entity's e-mail address which has
 meaning to those, and only those, who understand e-mail.  Another web
 of trust could use an entirely different naming scheme.

Maughan, et. al. Standards Track [Page 9] RFC 2408 ISAKMP November 1998

1.5.3 ISAKMP Requirements

 Strong authentication MUST be provided on ISAKMP exchanges.  Without
 being able to authenticate the entity at the other end, the Security
 Association (SA) and session key established are suspect.  Without
 authentication you are unable to trust an entity's identification,
 which makes access control questionable.  While encryption (e.g.
 ESP) and integrity (e.g.  AH) will protect subsequent communications
 from passive eavesdroppers, without authentication it is possible
 that the SA and key may have been established with an adversary who
 performed an active man-in-the-middle attack and is now stealing all
 your personal data.
 A digital signature algorithm MUST be used within ISAKMP's
 authentication component.  However, ISAKMP does not mandate a
 specific signature algorithm or certificate authority (CA). ISAKMP
 allows an entity initiating communications to indicate which CAs it
 supports.  After selection of a CA, the protocol provides the
 messages required to support the actual authentication exchange.  The
 protocol provides a facility for identification of different
 certificate authorities, certificate types (e.g.  X.509, PKCS #7,
 PGP, DNS SIG and KEY records), and the exchange of the certificates
 identified.
 ISAKMP utilizes digital signatures, based on public key cryptography,
 for authentication.  There are other strong authentication systems
 available, which could be specified as additional optional
 authentication mechanisms for ISAKMP. Some of these authentication
 systems rely on a trusted third party called a key distribution
 center (KDC) to distribute secret session keys.  An example is
 Kerberos, where the trusted third party is the Kerberos server, which
 holds secret keys for all clients and servers within its network
 domain.  A client's proof that it holds its secret key provides
 authenticaton to a server.
 The ISAKMP specification does not specify the protocol for
 communicating with the trusted third parties (TTP) or certificate
 directory services.  These protocols are defined by the TTP and
 directory service themselves and are outside the scope of this
 specification.  The use of these additional services and protocols
 will be described in a Key Exchange specific document.

1.6 Public Key Cryptography

 Public key cryptography is the most flexible, scalable, and efficient
 way for users to obtain the shared secrets and session keys needed to
 support the large number of ways Internet users will interoperate.
 Many key generation algorithms, that have different properties, are

Maughan, et. al. Standards Track [Page 10] RFC 2408 ISAKMP November 1998

 available to users (see [DOW92], [ANSI], and [Oakley]).  Properties
 of key exchange protocols include the key establishment method,
 authentication, symmetry, perfect forward secrecy, and back traffic
 protection.
 NOTE: Cryptographic keys can protect information for a considerable
 length of time.  However, this is based on the assumption that keys
 used for protection of communications are destroyed after use and not
 kept for any reason.

1.6.1 Key Exchange Properties

 Key Establishment (Key Generation / Key Transport): The two common
 methods of using public key cryptography for key establishment are
 key transport and key generation.  An example of key transport is the
 use of the RSA algorithm to encrypt a randomly generated session key
 (for encrypting subsequent communications) with the recipient's
 public key.  The encrypted random key is then sent to the recipient,
 who decrypts it using his private key.  At this point both sides have
 the same session key, however it was created based on input from only
 one side of the communications.  The benefit of the key transport
 method is that it has less computational overhead than the following
 method.  The Diffie-Hellman (D-H) algorithm illustrates key
 generation using public key cryptography.  The D-H algorithm is begun
 by two users exchanging public information.  Each user then
 mathematically combines the other's public information along with
 their own secret information to compute a shared secret value.  This
 secret value can be used as a session key or as a key encryption key
 for encrypting a randomly generated session key.  This method
 generates a session key based on public and secret information held
 by both users.  The benefit of the D-H algorithm is that the key used
 for encrypting messages is based on information held by both users
 and the independence of keys from one key exchange to another
 provides perfect forward secrecy.  Detailed descriptions of these
 algorithms can be found in [Schneier].  There are a number of
 variations on these two key generation schemes and these variations
 do not necessarily interoperate.
 Key Exchange Authentication: Key exchanges may be authenticated
 during the protocol or after protocol completion.  Authentication of
 the key exchange during the protocol is provided when each party
 provides proof it has the secret session key before the end of the
 protocol.  Proof can be provided by encrypting known data in the
 secret session key during the protocol echange.  Authentication after
 the protocol must occur in subsequent commu nications.
 Authentication during the protocol is preferred so subsequent
 communications are not initiated if the secret session key is not
 established with the desired party.

Maughan, et. al. Standards Track [Page 11] RFC 2408 ISAKMP November 1998

 Key Exchange Symmetry: A key exchange provides symmetry if either
 party can initiate the exchange and exchanged messages can cross in
 transit without affecting the key that is generated.  This is
 desirable so that computation of the keys does not require either
 party to know who initated the exchange.  While key exchange symmetry
 is desirable, symmetry in the entire key management protocol may
 provide a vulnerablity to reflection attacks.
 Perfect Forward Secrecy: As described in [DOW92], an authenticated
 key exchange protocol provides perfect forward secrecy if disclosure
 of longterm secret keying material does not compromise the secrecy of
 the exchanged keys from previous communications.  The property of
 perfect forward secrecy does not apply to key exchange without
 authentication.

1.6.2 ISAKMP Requirements

 An authenticated key exchange MUST be supported by ISAKMP. Users
 SHOULD choose additional key establishment algorithms based on their
 requirements.  ISAKMP does not specify a specific key exchange.
 However, [IKE] describes a proposal for using the Oakley key exchange
 [Oakley] in conjunction with ISAKMP. Requirements that should be
 evaluated when choosing a key establishment algorithm include
 establishment method (generation vs.  transport), perfect forward
 secrecy, computational overhead, key escrow, and key strength.  Based
 on user requirements, ISAKMP allows an entity initiating
 communications to indicate which key exchanges it supports.  After
 selection of a key exchange, the protocol provides the messages
 required to support the actual key establishment.

1.7 ISAKMP Protection

1.7.1 Anti-Clogging (Denial of Service)

 Of the numerous security services available, protection against
 denial of service always seems to be one of the most difficult to
 address.  A "cookie" or anti-clogging token (ACT) is aimed at
 protecting the computing resources from attack without spending
 excessive CPU resources to determine its authenticity.  An exchange
 prior to CPU-intensive public key operations can thwart some denial
 of service attempts (e.g.  simple flooding with bogus IP source
 addresses).  Absolute protection against denial of service is
 impossible, but this anti-clogging token provides a technique for
 making it easier to handle.  The use of an anti-clogging token was
 introduced by Karn and Simpson in [Karn].

Maughan, et. al. Standards Track [Page 12] RFC 2408 ISAKMP November 1998

 It should be noted that in the exchanges shown in section 4, the
 anticlogging mechanism should be used in conjuction with a garbage-
 state collection mechanism; an attacker can still flood a server
 using packets with bogus IP addresses and cause state to be created.
 Such aggressive memory management techniques SHOULD be employed by
 protocols using ISAKMP that do not go through an initial, anti-
 clogging only phase, as was done in [Karn].

1.7.2 Connection Hijacking

 ISAKMP prevents connection hijacking by linking the authentication,
 key exchange and security association exchanges.  This linking
 prevents an attacker from allowing the authentication to complete and
 then jumping in and impersonating one entity to the other during the
 key and security association exchanges.

1.7.3 Man-in-the-Middle Attacks

 Man-in-the-Middle attacks include interception, insertion, deletion,
 and modification of messages, reflecting messages back at the sender,
 replaying old messages and redirecting messages.  ISAKMP features
 prevent these types of attacks from being successful.  The linking of
 the ISAKMP exchanges prevents the insertion of messages in the
 protocol exchange.  The ISAKMP protocol state machine is defined so
 deleted messages will not cause a partial SA to be created, the state
 machine will clear all state and return to idle.  The state machine
 also prevents reflection of a message from causing harm.  The
 requirement for a new cookie with time variant material for each new
 SA establishment prevents attacks that involve replaying old
 messages.  The ISAKMP strong authentication requirement prevents an
 SA from being established with anyone other than the intended party.
 Messages may be redirected to a different destination or modified but
 this will be detected and an SA will not be established.  The ISAKMP
 specification defines where abnormal processing has occurred and
 recommends notifying the appropriate party of this abnormality.

1.8 Multicast Communications

 It is expected that multicast communications will require the same
 security services as unicast communications and may introduce the
 need for additional security services.  The issues of distributing
 SPIs for multicast traffic are presented in [SEC-ARCH].  Multicast
 security issues are also discussed in [RFC-1949] and [BC].  A future
 extension to ISAKMP will support multicast key distribution.  For an
 introduction to the issues related to multicast security, consult the
 Internet Drafts, [RFC-2094] and [RFC-2093], describing Sparta's
 research in this area.

Maughan, et. al. Standards Track [Page 13] RFC 2408 ISAKMP November 1998

2 Terminology and Concepts

2.1 ISAKMP Terminology

 Security Protocol: A Security Protocol consists of an entity at a
 single point in the network stack, performing a security service for
 network communication.  For example, IPSEC ESP and IPSEC AH are two
 different security protocols.  TLS is another example.  Security
 Protocols may perform more than one service, for example providing
 integrity and confidentiality in one module.
 Protection Suite: A protection suite is a list of the security
 services that must be applied by various security protocols.  For
 example, a protection suite may consist of DES encryption in IP ESP,
 and keyed MD5 in IP AH. All of the protections in a suite must be
 treated as a single unit.  This is necessary because security
 services in different security protocols can have subtle
 interactions, and the effects of a suite must be analyzed and
 verified as a whole.
 Security Association (SA): A Security Association is a security-
 protocol- specific set of parameters that completely defines the
 services and mechanisms necessary to protect traffic at that security
 protocol location.  These parameters can include algorithm
 identifiers, modes, cryptographic keys, etc.  The SA is referred to
 by its associated security protocol (for example, "ISAKMP SA", "ESP
 SA", "TLS SA").
 ISAKMP SA: An SA used by the ISAKMP servers to protect their own
 traffic.  Sections 2.3 and 2.4 provide more details about ISAKMP SAs.
 Security Parameter Index (SPI): An identifier for a Security
 Assocation, relative to some security protocol.  Each security
 protocol has its own "SPI-space".  A (security protocol, SPI) pair
 may uniquely identify an SA. The uniqueness of the SPI is
 implementation dependent, but could be based per system, per
 protocol, or other options.  Depending on the DOI, additional
 information (e.g.  host address) may be necessary to identify an SA.
 The DOI will also determine which SPIs (i.e.  initiator's or
 responder's) are sent during communication.
 Domain of Interpretation: A Domain of Interpretation (DOI) defines
 payload formats, exchange types, and conventions for naming
 security-relevant information such as security policies or
 cryptographic algorithms and modes.  A Domain of Interpretation (DOI)
 identifier is used to interpret the payloads of ISAKMP payloads.  A
 system SHOULD support multiple Domains of Interpretation
 simultaneously.  The concept of a DOI is based on previous work by

Maughan, et. al. Standards Track [Page 14] RFC 2408 ISAKMP November 1998

 the TSIG CIPSO Working Group, but extends beyond security label
 interpretation to include naming and interpretation of security
 services.  A DOI defines:
  o  A "situation":  the set of information that will be used to
     determine the required security services.
  o  The set of security policies that must, and may, be supported.
  o  A syntax for the specification of proposed security services.
  o  A scheme for naming security-relevant information, including
     encryption algorithms, key exchange algorithms, security policy
     attributes, and certificate authorities.
  o  The specific formats of the various payload contents.
  o  Additional exchange types, if required.
 The rules for the IETF IP Security DOI are presented in [IPDOI].
 Specifications of the rules for customized DOIs will be presented in
 separate documents.
 Situation: A situation contains all of the security-relevant
 information that a system considers necessary to decide the security
 services required to protect the session being negotiated.  The
 situation may include addresses, security classifications, modes of
 operation (normal vs.  emergency), etc.
 Proposal: A proposal is a list, in decreasing order of preference, of
 the protection suites that a system considers acceptable to protect
 traffic under a given situation.
 Payload: ISAKMP defines several types of payloads, which are used to
 transfer information such as security association data, or key
 exchange data, in DOI-defined formats.  A payload consists of a
 generic payload header and a string of octects that is opaque to
 ISAKMP. ISAKMP uses DOI- specific functionality to synthesize and
 interpret these payloads.  Multiple payloads can be sent in a single
 ISAKMP message.  See section 3 for more details on the payload types,
 and [IPDOI] for the formats of the IETF IP Security DOI payloads.
 Exchange Type: An exchange type is a specification of the number of
 messages in an ISAKMP exchange, and the payload types that are
 contained in each of those messages.  Each exchange type is designed
 to provide a particular set of security services, such as anonymity
 of the participants, perfect forward secrecy of the keying material,
 authentication of the participants, etc.  Section 4.1 defines the

Maughan, et. al. Standards Track [Page 15] RFC 2408 ISAKMP November 1998

 default set of ISAKMP exchange types.  Other exchange types can be
 added to support additional key exchanges, if required.

2.2 ISAKMP Placement

 Figure 1 is a high level view of the placement of ISAKMP within a
 system context in a network architecture.  An important part of
 negotiating security services is to consider the entire "stack" of
 individual SAs as a unit.  This is referred to as a "protection
 suite".
   +------------+        +--------+                +--------------+
   !     DOI    !        !        !                !  Application !
   ! Definition ! <----> ! ISAKMP !                !    Process   !
   +------------+    --> !        !                !--------------!
  +--------------+   !   +--------+                ! Appl Protocol!
  ! Key Exchange !   !     ^  ^                    +--------------+
  !  Definition  !<--      !  !                           ^
  +--------------+         !  !                           !
                           !  !                           !
          !----------------!  !                           !
          v                   !                           !
      +-------+               v                           v
      !  API  !        +---------------------------------------------+
      +-------+        !                Socket Layer                 !
          !            !---------------------------------------------!
          v            !        Transport Protocol (TCP / UDP)       !
   +----------+        !---------------------------------------------!
   ! Security ! <----> !                     IP                      !
   ! Protocol !        !---------------------------------------------!
   +----------+        !             Link Layer Protocol             !
                       +---------------------------------------------+
                   Figure 1:  ISAKMP Relationships

2.3 Negotiation Phases

 ISAKMP offers two "phases" of negotiation.  In the first phase, two
 entities (e.g.  ISAKMP servers) agree on how to protect further
 negotiation traffic between themselves, establishing an ISAKMP SA.
 This ISAKMP SA is then used to protect the negotiations for the
 Protocol SA being requested.  Two entities (e.g.  ISAKMP servers) can
 negotiate (and have active) multiple ISAKMP SAs.

Maughan, et. al. Standards Track [Page 16] RFC 2408 ISAKMP November 1998

 The second phase of negotiation is used to establish security
 associations for other security protocols.  This second phase can be
 used to establish many security associations.  The security
 associations established by ISAKMP during this phase can be used by a
 security protocol to protect many message/data exchanges.
 While the two-phased approach has a higher start-up cost for most
 simple scenarios, there are several reasons that it is beneficial for
 most cases.
 First, entities (e.g.  ISAKMP servers) can amortize the cost of the
 first phase across several second phase negotiations.  This allows
 multiple SAs to be established between peers over time without having
 to start over for each communication.
 Second, security services negotiated during the first phase provide
 security properties for the second phase.  For example, after the
 first phase of negotiation, the encryption provided by the ISAKMP SA
 can provide identity protection, potentially allowing the use of
 simpler second-phase exchanges.  On the other hand, if the channel
 established during the first phase is not adequate to protect
 identities, then the second phase must negotiate adequate security
 mechanisms.
 Third, having an ISAKMP SA in place considerably reduces the cost of
 ISAKMP management activity - without the "trusted path" that an
 ISAKMP SA gives you, the entities (e.g.  ISAKMP servers) would have
 to go through a complete re-authentication for each error
 notification or deletion of an SA.
 Negotiation during each phase is accomplished using ISAKMP-defined
 exchanges (see section 4) or exchanges defined for a key exchange
 within a DOI.
 Note that security services may be applied differently in each
 negotiation phase.  For example, different parties are being
 authenticated during each of the phases of negotiation.  During the
 first phase, the parties being authenticated may be the ISAKMP
 servers/hosts, while during the second phase, users or application
 level programs are being authenticated.

2.4 Identifying Security Associations

 While bootstrapping secure channels between systems, ISAKMP cannot
 assume the existence of security services, and must provide some
 protections for itself.  Therefore, ISAKMP considers an ISAKMP
 Security Association to be different than other types, and manages
 ISAKMP SAs itself, in their own name space.  ISAKMP uses the two

Maughan, et. al. Standards Track [Page 17] RFC 2408 ISAKMP November 1998

 cookie fields in the ISAKMP header to identify ISAKMP SAs.  The
 Message ID in the ISAKMP Header and the SPI field in the Proposal
 payload are used during SA establishment to identify the SA for other
 security protocols.  The interpretation of these four fields is
 dependent on the operation taking place.
 The following table shows the presence or absence of several fields
 during SA establishment.  The following fields are necessary for
 various operations associated with SA establishment: cookies in the
 ISAKMP header, the ISAKMP Header Message ID field, and the SPI field
 in the Proposal payload.  An 'X' in the column means the value MUST
 be present.  An 'NA' in the column means a value in the column is Not
 Applicable to the operation.
#             Operation            I-Cookie  R-Cookie  Message ID  SPI

(1) Start ISAKMP SA negotiation X 0 0 0 (2) Respond ISAKMP SA negotiation X X 0 0 (3) Init other SA negotiation X X X X (4) Respond other SA negotiation X X X X (5) Other (KE, ID, etc.) X X X/0 NA (6) Security Protocol (ESP, AH) NA NA NA X

 In the first line (1) of the table, the initiator includes the
 Initiator Cookie field in the ISAKMP Header, using the procedures
 outlined in sections 2.5.3 and 3.1.
 In the second line (2) of the table, the responder includes the
 Initiator and Responder Cookie fields in the ISAKMP Header, using the
 procedures outlined in sections 2.5.3 and 3.1.  Additional messages
 may be exchanged between ISAKMP peers, depending on the ISAKMP
 exchange type used during the phase 1 negotiation.  Once the phase 1
 exchange is completed, the Initiator and Responder cookies are
 included in the ISAKMP Header of all subsequent communications
 between the ISAKMP peers.
 During phase 1 negotiations, the initiator and responder cookies
 determine the ISAKMP SA. Therefore, the SPI field in the Proposal
 payload is redundant and MAY be set to 0 or it MAY contain the
 transmitting entity's cookie.
 In the third line (3) of the table, the initiator associates a
 Message ID with the Protocols contained in the SA Proposal.  This
 Message ID and the initiator's SPI(s) to be associated with each
 protocol in the Proposal are sent to the responder.  The SPI(s) will
 be used by the security protocols once the phase 2 negotiation is
 completed.

Maughan, et. al. Standards Track [Page 18] RFC 2408 ISAKMP November 1998

 In the fourth line (4) of the table, the responder includes the same
 Message ID and the responder's SPI(s) to be associated with each
 protocol in the accepted Proposal.  This information is returned to
 the initiator.
 In the fifth line (5) of the table, the initiator and responder use
 the Message ID field in the ISAKMP Header to keep track of the in-
 progress protocol negotiation.  This is only applicable for a phase 2
 exchange and the value MUST be 0 for a phase 1 exchange because the
 combined cookies identify the ISAKMP SA. The SPI field in the
 Proposal payload is not applicable because the Proposal payload is
 only used during the SA negotiation message exchange (steps 3 and 4).
 In the sixth line (6) of the table, the phase 2 negotiation is
 complete.  The security protocols use the SPI(s) to determine which
 security services and mechanisms to apply to the communication
 between them.  The SPI value shown in the sixth line (6) is not the
 SPI field in the Proposal payload, but the SPI field contained within
 the security protocol header.
 During the SA establishment, a SPI MUST be generated.  ISAKMP is
 designed to handle variable sized SPIs.  This is accomplished by
 using the SPI Size field within the Proposal payload during SA
 establishment.  Handling of SPIs will be outlined by the DOI
 specification (e.g.  [IPDOI]).
 When a security association (SA) is initially established, one side
 assumes the role of initiator and the other the role of responder.
 Once the SA is established, both the original initiator and responder
 can initiate a phase 2 negotiation with the peer entity.  Thus,
 ISAKMP SAs are bidirectional in nature.
 Additionally, ISAKMP allows both initiator and responder to have some
 control during the negotiation process.  While ISAKMP is designed to
 allow an SA negotiation that includes multiple proposals, the
 initiator can maintain some control by only making one proposal in
 accordance with the initiator's local security policy.  Once the
 initiator sends a proposal containing more than one proposal (which
 are sent in decreasing preference order), the initiator relinquishes
 control to the responder.  Once the responder is controlling the SA
 establishment, the responder can make its policy take precedence over
 the initiator within the context of the multiple options offered by
 the initiator.  This is accomplished by selecting the proposal best
 suited for the responder's local security policy and returning this
 selection to the initiator.

Maughan, et. al. Standards Track [Page 19] RFC 2408 ISAKMP November 1998

2.5 Miscellaneous

2.5.1 Transport Protocol

 ISAKMP can be implemented over any transport protocol or over IP
 itself.  Implementations MUST include send and receive capability for
 ISAKMP using the User Datagram Protocol (UDP) on port 500.  UDP Port
 500 has been assigned to ISAKMP by the Internet Assigned Numbers
 Authority (IANA). Implementations MAY additionally support ISAKMP
 over other transport protocols or over IP itself.

2.5.2 RESERVED Fields

 The existence of RESERVED fields within ISAKMP payloads are used
 strictly to preserve byte alignment.  All RESERVED fields in the
 ISAKMP protocol MUST be set to zero (0) when a packet is issued.  The
 receiver SHOULD check the RESERVED fields for a zero (0) value and
 discard the packet if other values are found.

2.5.3 Anti-Clogging Token ("Cookie") Creation

 The details of cookie generation are implementation dependent, but
 MUST satisfy these basic requirements (originally stated by Phil Karn
 in [Karn]):
    1.    The cookie must depend on the specific parties.  This
          prevents an attacker from obtaining a cookie using a real IP
          address and UDP port, and then using it to swamp the victim
          with Diffie-Hellman requests from randomly chosen IP
          addresses or ports.
    2.    It must not be possible for anyone other than the issuing
          entity to generate cookies that will be accepted by that
          entity.  This implies that the issuing entity must use local
          secret information in the generation and subsequent
          verification of a cookie.  It must not be possible to deduce
          this secret information from any particular cookie.
    3.    The cookie generation function must be fast to thwart
          attacks intended to sabotage CPU resources.
 Karn's suggested method for creating the cookie is to perform a fast
 hash (e.g.  MD5) over the IP Source and Destination Address, the UDP
 Source and Destination Ports and a locally generated secret random
 value.  ISAKMP requires that the cookie be unique for each SA
 establishment to help prevent replay attacks, therefore, the date and
 time MUST be added to the information hashed.  The generated cookies
 are placed in the ISAKMP Header (described in section 3.1) Initiator

Maughan, et. al. Standards Track [Page 20] RFC 2408 ISAKMP November 1998

 and Responder cookie fields.  These fields are 8 octets in length,
 thus, requiring a generated cookie to be 8 octets.  Notify and Delete
 messages (see sections 3.14, 3.15, and 4.8) are uni-directional
 transmissions and are done under the protection of an existing ISAKMP
 SA, thus, not requiring the generation of a new cookie.  One
 exception to this is the transmission of a Notify message during a
 Phase 1 exchange, prior to completing the establishment of an SA.
 Sections 3.14 and 4.8 provide additional details.

3 ISAKMP Payloads

 ISAKMP payloads provide modular building blocks for constructing
 ISAKMP messages.  The presence and ordering of payloads in ISAKMP is
 defined by and dependent upon the Exchange Type Field located in the
 ISAKMP Header (see Figure 2).  The ISAKMP payload types are discussed
 in sections 3.4 through 3.15.  The descriptions of the ISAKMP
 payloads, messages, and exchanges (see Section 4) are shown using
 network octet ordering.

3.1 ISAKMP Header Format

 An ISAKMP message has a fixed header format, shown in Figure 2,
 followed by a variable number of payloads.  A fixed header simplifies
 parsing, providing the benefit of protocol parsing software that is
 less complex and easier to implement.  The fixed header contains the
 information required by the protocol to maintain state, process
 payloads and possibly prevent denial of service or replay attacks.
 The ISAKMP Header fields are defined as follows:
  o  Initiator Cookie (8 octets) - Cookie of entity that initiated SA
     establishment, SA notification, or SA deletion.
  o  Responder Cookie (8 octets) - Cookie of entity that is responding
     to an SA establishment request, SA notification, or SA deletion.

Maughan, et. al. Standards Track [Page 21] RFC 2408 ISAKMP November 1998

                       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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                          Initiator                            !
  !                            Cookie                             !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                          Responder                            !
  !                            Cookie                             !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !  Next Payload ! MjVer ! MnVer ! Exchange Type !     Flags     !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                          Message ID                           !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  !                            Length                             !
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 2:  ISAKMP Header Format
  o  Next Payload (1 octet) - Indicates the type of the first payload
     in the message.  The format for each payload is defined in
     sections 3.4 through 3.16.  The processing for the payloads is
     defined in section 5.
                      Next Payload Type       Value
                  NONE                           0
                  Security Association (SA)      1
                  Proposal (P)                   2
                  Transform (T)                  3
                  Key Exchange (KE)              4
                  Identification (ID)            5
                  Certificate (CERT)             6
                  Certificate Request (CR)       7
                  Hash (HASH)                    8
                  Signature (SIG)                9
                  Nonce (NONCE)                 10
                  Notification (N)              11
                  Delete (D)                    12
                  Vendor ID (VID)               13
                  RESERVED                   14 - 127
                  Private USE               128 - 255
  o  Major Version (4 bits) - indicates the major version of the ISAKMP
     protocol in use.  Implementations based on this version of the
     ISAKMP Internet-Draft MUST set the Major Version to 1.
     Implementations based on previous versions of ISAKMP Internet-
     Drafts MUST set the Major Version to 0.  Implementations SHOULD

Maughan, et. al. Standards Track [Page 22] RFC 2408 ISAKMP November 1998

     never accept packets with a major version number larger than its
     own.
  o  Minor Version (4 bits) - indicates the minor version of the
     ISAKMP protocol in use.  Implementations based on this version of
     the ISAKMP Internet-Draft MUST set the Minor Version to 0.
     Implementations based on previous versions of ISAKMP Internet-
     Drafts MUST set the Minor Version to 1.  Implementations SHOULD
     never accept packets with a minor version number larger than its
     own, given the major version numbers are identical.
  o  Exchange Type (1 octet) - indicates the type of exchange being
     used.  This dictates the message and payload orderings in the
     ISAKMP exchanges.
                          Exchange Type      Value
                       NONE                    0
                       Base                    1
                       Identity Protection     2
                       Authentication Only     3
                       Aggressive              4
                       Informational           5
                       ISAKMP Future Use     6 - 31
                       DOI Specific Use     32 - 239
                       Private Use         240 - 255
  o  Flags (1 octet) - indicates specific options that are set for the
     ISAKMP exchange.  The flags listed below are specified in the
     Flags field beginning with the least significant bit, i.e the
     Encryption bit is bit 0 of the Flags field, the Commit bit is bit
     1 of the Flags field, and the Authentication Only bit is bit 2 of
     the Flags field.  The remaining bits of the Flags field MUST be
     set to 0 prior to transmission.
  1. - E(ncryption Bit) (1 bit) - If set (1), all payloads following

the header are encrypted using the encryption algorithm

        identified in the ISAKMP SA. The ISAKMP SA Identifier is the
        combination of the initiator and responder cookie.  It is
        RECOMMENDED that encryption of communications be done as soon
        as possible between the peers.  For all ISAKMP exchanges
        described in section 4.1, the encryption SHOULD begin after
        both parties have exchanged Key Exchange payloads.  If the
        E(ncryption Bit) is not set (0), the payloads are not
        encrypted.

Maughan, et. al. Standards Track [Page 23] RFC 2408 ISAKMP November 1998

  1. - C(ommit Bit) (1 bit) - This bit is used to signal key exchange

synchronization. It is used to ensure that encrypted material

        is not received prior to completion of the SA establishment.
        The Commit Bit can be set (at anytime) by either party
        participating in the SA establishment, and can be used during
        both phases of an ISAKMP SA establishment.  However, the value
        MUST be reset after the Phase 1 negotiation.  If set(1), the
        entity which did not set the Commit Bit MUST wait for an
        Informational Exchange containing a Notify payload (with the
        CONNECTED Notify Message) from the entity which set the Commit
        Bit.  In this instance, the Message ID field of the
        Informational Exchange MUST contain the Message ID of the
        original ISAKMP Phase 2 SA negotiation.  This is done to
        ensure that the Informational Exchange with the CONNECTED
        Notify Message can be associated with the correct Phase 2 SA.
        The receipt and processing of the Informational Exchange
        indicates that the SA establishment was successful and either
        entity can now proceed with encrypted traffic communication.
        In addition to synchronizing key exchange, the Commit Bit can
        be used to protect against loss of transmissions over
        unreliable networks and guard against the need for multiple
        re-transmissions.
        NOTE: It is always possible that the final message of an
        exchange can be lost.  In this case, the entity expecting to
        receive the final message of an exchange would receive the
        Phase 2 SA negotiation message following a Phase 1 exchange or
        encrypted traffic following a Phase 2 exchange.  Handling of
        this situation is not standardized, but we propose the
        following possibilities.  If the entity awaiting the
        Informational Exchange can verify the received message (i.e.
        Phase 2 SA negotiation message or encrypted traffic), then
        they MAY consider the SA was established and continue
        processing.  The other option is to retransmit the last ISAKMP
        message to force the other entity to retransmit the final
        message.  This suggests that implementations may consider
        retaining the last message (locally) until they are sure the
        SA is established.
  1. - A(uthentication Only Bit) (1 bit) - This bit is intended for

use with the Informational Exchange with a Notify payload and

        will allow the transmission of information with integrity
        checking, but no encryption (e.g.  "emergency mode").  Section
        4.8 states that a Phase 2 Informational Exchange MUST be sent
        under the protection of an ISAKMP SA. This is the only
        exception to that policy.  If the Authentication Only bit is
        set (1), only authentication security services will be applied
        to the entire Notify payload of the Informational Exchange and

Maughan, et. al. Standards Track [Page 24] RFC 2408 ISAKMP November 1998

        the payload will not be encrypted.
  o  Message ID (4 octets) - Unique Message Identifier used to
     identify protocol state during Phase 2 negotiations.  This value
     is randomly generated by the initiator of the Phase 2
     negotiation.  In the event of simultaneous SA establishments
     (i.e.  collisions), the value of this field will likely be
     different because they are independently generated and, thus, two
     security associations will progress toward establishment.
     However, it is unlikely there will be absolute simultaneous
     establishments.  During Phase 1 negotiations, the value MUST be
     set to 0.
  o  Length (4 octets) - Length of total message (header + payloads)
     in octets.  Encryption can expand the size of an ISAKMP message.

3.2 Generic Payload Header

 Each ISAKMP payload defined in sections 3.4 through 3.16 begins with
 a generic header, shown in Figure 3, which provides a payload
 "chaining" capability and clearly defines the boundaries of a
 payload.
                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! Next Payload  !   RESERVED    !         Payload Length        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 3:  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
     the "chaining" capability.
  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.

3.3 Data Attributes

 There are several instances within ISAKMP where it is necessary to
 represent Data Attributes.  An example of this is the Security
 Association (SA) Attributes contained in the Transform payload

Maughan, et. al. Standards Track [Page 25] RFC 2408 ISAKMP November 1998

 (described in section 3.6).  These Data Attributes are not an ISAKMP
 payload, but are contained within ISAKMP payloads.  The format of the
 Data Attributes provides the flexibility for representation of many
 different types of information.  There can be multiple Data
 Attributes within a payload.  The length of the Data Attributes will
 either be 4 octets or defined by the Attribute Length field.  This is
 done using the Attribute Format bit described below.  Specific
 information about the attributes for each domain will be described in
 a DOI document, e.g.  IPSEC DOI [IPDOI].
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !A!       Attribute Type        !    AF=0  Attribute Length     !
   !F!                             !    AF=1  Attribute Value      !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                   AF=0  Attribute Value                       .
   .                   AF=1  Not Transmitted                       .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 4:  Data Attributes
 The Data Attributes fields are defined as follows:
  o  Attribute Type (2 octets) - Unique identifier for each type of
     attribute.  These attributes are defined as part of the DOI-
     specific information.
     The most significant bit, or Attribute Format (AF), indicates
     whether the data attributes follow the Type/Length/Value (TLV)
     format or a shortened Type/Value (TV) format.  If the AF bit is a
     zero (0), then the Data Attributes are of the Type/Length/Value
     (TLV) form.  If the AF bit is a one (1), then the Data Attributes
     are of the Type/Value form.
  o  Attribute Length (2 octets) - Length in octets of the Attribute
     Value.  When the AF bit is a one (1), the Attribute Value is only
     2 octets and the Attribute Length field is not present.
  o  Attribute Value (variable length) - Value of the attribute
     associated with the DOI-specific Attribute Type.  If the AF bit
     is a zero (0), this field has a variable length defined by the
     Attribute Length field.  If the AF bit is a one (1), the
     Attribute Value has a length of 2 octets.

Maughan, et. al. Standards Track [Page 26] RFC 2408 ISAKMP November 1998

3.4 Security Association Payload

 The Security Association Payload is used to negotiate security
 attributes and to indicate the Domain of Interpretation (DOI) and
 Situation under which the negotiation is taking place.  Figure 5
 shows the format of the Security Association payload.
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   RESERVED    !         Payload Length        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !              Domain of Interpretation  (DOI)                  !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                           Situation                           ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 5:  Security Association Payload
  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 MUST NOT
     contain the values for the Proposal or Transform payloads as they
     are considered part of the security association negotiation.  For
     example, this field would contain the value "10" (Nonce payload)
     in the first message of a Base Exchange (see Section 4.4) and the
     value "0" in the first message of an Identity Protect Exchange
     (see Section 4.5).
  o  RESERVED (1 octet) - Unused, set to 0.
  o  Payload Length (2 octets) - Length in octets of the entire
     Security Association payload, including the SA payload, all
     Proposal payloads, and all Transform payloads associated with the
     proposed Security Association.
  o  Domain of Interpretation (4 octets) - Identifies the DOI (as
     described in Section 2.1) under which this negotiation is taking
     place.  The DOI is a 32-bit unsigned integer.  A DOI value of 0
     during a Phase 1 exchange specifies a Generic ISAKMP SA which can
     be used for any protocol during the Phase 2 exchange.  The
     necessary SA Attributes are defined in A.4.  A DOI value of 1 is
     assigned to the IPsec DOI [IPDOI].  All other DOI values are
     reserved to IANA for future use.  IANA will not normally assign a
     DOI value without referencing some public specification, such as

Maughan, et. al. Standards Track [Page 27] RFC 2408 ISAKMP November 1998

     an Internet RFC. Other DOI's can be defined using the description
     in appendix B.  This field MUST be present within the Security
     Association payload.
  o  Situation (variable length) - A DOI-specific field that
     identifies the situation under which this negotiation is taking
     place.  The Situation is used to make policy decisions regarding
     the security attributes being negotiated.  Specifics for the IETF
     IP Security DOI Situation are detailed in [IPDOI].  This field
     MUST be present within the Security Association payload.

3.5 Proposal Payload

 The Proposal Payload contains information used during Security
 Association negotiation.  The proposal consists of security
 mechanisms, or transforms, to be used to secure the communications
 channel.  Figure 6 shows the format of the Proposal Payload.  A
 description of its use can be found in section 4.2.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Proposal #   !  Protocol-Id  !    SPI Size   !# of Transforms!
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                        SPI (variable)                         !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 6:  Proposal Payload Format
 The Proposal Payload fields are defined as follows:
  o  Next Payload (1 octet) - Identifier for the payload type of the
     next payload in the message.  This field MUST only contain the
     value "2" or "0".  If there are additional Proposal payloads in
     the message, then this field will be 2.  If the current Proposal
     payload is the last within the security association proposal,
     then this field will be 0.
  o  RESERVED (1 octet) - Unused, set to 0.
  o  Payload Length (2 octets) - Length in octets of the entire
     Proposal payload, including generic payload header, the Proposal
     payload, and all Transform payloads associated with this
     proposal.  In the event there are multiple proposals with the
     same proposal number (see section 4.2), the Payload Length field

Maughan, et. al. Standards Track [Page 28] RFC 2408 ISAKMP November 1998

     only applies to the current Proposal payload and not to all
     Proposal payloads.
  o  Proposal # (1 octet) - Identifies the Proposal number for the
     current payload.  A description of the use of this field is found
     in section 4.2.
  o  Protocol-Id (1 octet) - Specifies the protocol identifier for the
     current negotiation.  Examples might include IPSEC ESP, IPSEC AH,
     OSPF, TLS, etc.
  o  SPI Size (1 octet) - Length in octets of the SPI as defined by
     the Protocol-Id.  In the case of ISAKMP, the Initiator and
     Responder cookie pair from the ISAKMP Header is the ISAKMP SPI,
     therefore, the SPI Size is irrelevant and MAY be from zero (0) to
     sixteen (16).  If the SPI Size is non-zero, the content of the
     SPI field MUST be ignored.  If the SPI Size is not a multiple of
     4 octets it will have some impact on the SPI field and the
     alignment of all payloads in the message.  The Domain of
     Interpretation (DOI) will dictate the SPI Size for other
     protocols.
  o  # of Transforms (1 octet) - Specifies the number of transforms
     for the Proposal.  Each of these is contained in a Transform
     payload.
  o  SPI (variable) - The sending entity's SPI. In the event the SPI
     Size is not a multiple of 4 octets, there is no padding applied
     to the payload, however, it can be applied at the end of the
     message.
 The payload type for the Proposal Payload is two (2).

3.6 Transform Payload

 The Transform Payload contains information used during Security
 Association negotiation.  The Transform payload consists of a
 specific security mechanism, or transforms, to be used to secure the
 communications channel.  The Transform payload also contains the
 security association attributes associated with the specific
 transform.  These SA attributes are DOI-specific.  Figure 7 shows the
 format of the Transform Payload.  A description of its use can be
 found in section 4.2.

Maughan, et. al. Standards Track [Page 29] RFC 2408 ISAKMP November 1998

                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Transform #  !  Transform-Id !           RESERVED2           !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                        SA Attributes                          ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 7:  Transform Payload Format
 The Transform Payload fields are defined as follows:
  o  Next Payload (1 octet) - Identifier for the payload type of the
     next payload in the message.  This field MUST only contain the
     value "3" or "0".  If there are additional Transform payloads in
     the proposal, then this field will be 3.  If the current
     Transform payload is the last within the proposal, 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, Transform values,
     and all SA Attributes.
  o  Transform # (1 octet) - Identifies the Transform number for the
     current payload.  If there is more than one transform proposed
     for a specific protocol within the Proposal payload, then each
     Transform payload has a unique Transform number.  A description
     of the use of this field is found in section 4.2.
  o  Transform-Id (1 octet) - Specifies the Transform identifier for
     the protocol within the current proposal.  These transforms are
     defined by the DOI and are dependent on the protocol being
     negotiated.
  o  RESERVED2 (2 octets) - Unused, set to 0.
  o  SA Attributes (variable length) - This field contains the
     security association attributes as defined for the transform
     given in the Transform-Id field.  The SA Attributes SHOULD be
     represented using the Data Attributes format described in section
     3.3.  If the SA Attributes are not aligned on 4-byte boundaries,

Maughan, et. al. Standards Track [Page 30] RFC 2408 ISAKMP November 1998

     then subsequent payloads will not be aligned and any padding will
     be added at the end of the message to make the message 4-octet
     aligned.
 The payload type for the Transform Payload is three (3).

3.7 Key Exchange Payload

 The Key Exchange Payload supports a variety of key exchange
 techniques.  Example key exchanges are Oakley [Oakley], Diffie-
 Hellman, the enhanced Diffie-Hellman key exchange described in X9.42
 [ANSI], and the RSA-based key exchange used by PGP. Figure 8 shows
 the format of the Key Exchange payload.
 The Key Exchange Payload fields are defined as follows:
  o  Next Payload (1 octet) - Identifier for the payload type of the
     nextpayload in the message.  If the current payload is the last
     in the message, then this field will be 0.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                       Key Exchange Data                       ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 8:  Key Exchange Payload Format
  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.
  o  Key Exchange Data (variable length) - Data required to generate a
     session key.  The interpretation of this data is specified by the
     DOI and the associated Key Exchange algorithm.  This field may
     also contain pre-placed key indicators.
 The payload type for the Key Exchange Payload is four (4).

Maughan, et. al. Standards Track [Page 31] RFC 2408 ISAKMP November 1998

3.8 Identification Payload

 The Identification Payload contains DOI-specific data used to
 exchange identification information.  This information is used for
 determining the identities of communicating peers and may be used for
 determining authenticity of information.  Figure 9 shows the format
 of the Identification Payload.
 The Identification 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.
  o  ID Type (1 octet) - Specifies the type of Identification being
     used.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !   ID Type     !             DOI Specific ID Data              !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                   Identification Data                         ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 9:  Identification Payload Format
     This field is DOI-dependent.
  o  DOI Specific ID Data (3 octets) - Contains DOI specific
     Identification data.  If unused, then this field MUST be set to
     0.
  o  Identification Data (variable length) - Contains identity
     information.  The values for this field are DOI-specific and the
     format is specified by the ID Type field.  Specific details for
     the IETF IP Security DOI Identification Data are detailed in
     [IPDOI].

Maughan, et. al. Standards Track [Page 32] RFC 2408 ISAKMP November 1998

 The payload type for the Identification Payload is five (5).

3.9 Certificate Payload

 The Certificate Payload provides a means to transport certificates or
 other certificate-related information via ISAKMP and can appear in
 any ISAKMP message.  Certificate payloads SHOULD be included in an
 exchange whenever an appropriate directory service (e.g.  Secure DNS
 [DNSSEC]) is not available to distribute certificates.  The
 Certificate payload MUST be accepted at any point during an exchange.
 Figure 10 shows the format of the Certificate Payload.
 NOTE: Certificate types and formats are not generally bound to a DOI
 - it is expected that there will only be a few certificate types, and
 that most DOIs will accept all of these types.
 The Certificate 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.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Cert Encoding !                                               !
   +-+-+-+-+-+-+-+-+                                               !
   ~                       Certificate Data                        ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 10:  Certificate Payload Format
  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.
  o  Certificate Encoding (1 octet) - This field indicates the type of
     certificate or certificate-related information contained in the
     Certificate Data field.

Maughan, et. al. Standards Track [Page 33] RFC 2408 ISAKMP November 1998

                        Certificate Type            Value
                NONE                                   0
                PKCS #7 wrapped X.509 certificate      1
                PGP Certificate                        2
                DNS Signed Key                         3
                X.509 Certificate - Signature          4
                X.509 Certificate - Key Exchange       5
                Kerberos Tokens                        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).

3.10 Certificate Request Payload

 The Certificate Request Payload provides a means to request
 certificates via ISAKMP and can appear in any message.  Certificate
 Request payloads SHOULD be included in an exchange whenever an
 appropriate directory service (e.g.  Secure DNS [DNSSEC]) is not
 available to distribute certificates.  The Certificate Request
 payload MUST be accepted at any point during the exchange.  The
 responder to the Certificate Request payload MUST send its
 certificate, if certificates are supported, based on the values
 contained in the payload.  If multiple certificates are required,
 then multiple Certificate Request payloads SHOULD be transmitted.
 Figure 11 shows the format of the Certificate Request Payload.
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   RESERVED    !         Payload Length        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Cert. Type   !                                               !
   +-+-+-+-+-+-+-+-+                                               !
   ~                    Certificate Authority                      ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Figure 11:  Certificate Request Payload Format

Maughan, et. al. Standards Track [Page 34] RFC 2408 ISAKMP November 1998

 The Certificate 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.
  o  Certificate Type (1 octet) - Contains an encoding of the type of
     certificate requested.  Acceptable values are listed in section
     3.9.
  o  Certificate Authority (variable length) - Contains an encoding of
     an acceptable certificate authority for the type of certificate
     requested.  As an example, for an X.509 certificate this field
     would contain the Distinguished Name encoding of the Issuer Name
     of an X.509 certificate authority acceptable to the sender of
     this payload.  This would be included to assist the responder in
     determining how much of the certificate chain would need to be
     sent in response to this request.  If there is no specific
     certificate authority requested, this field SHOULD not be
     included.
 The payload type for the Certificate Request Payload is seven (7).

Maughan, et. al. Standards Track [Page 35] RFC 2408 ISAKMP November 1998

3.11 Hash Payload

 The Hash Payload contains data generated by the hash function
 (selected during the SA establishment exchange), over some part of
 the message and/or ISAKMP state.  This payload may be used to verify
 the integrity of the data in an ISAKMP message or for authentication
 of the negotiating entities.  Figure 12 shows the format of the Hash
 Payload.
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   RESERVED    !         Payload Length        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                           Hash Data                           ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 12:  Hash Payload Format
 The Hash 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.
  o  Hash Data (variable length) - Data that results from applying the
     hash routine to the ISAKMP message and/or state.

Maughan, et. al. Standards Track [Page 36] RFC 2408 ISAKMP November 1998

3.12 Signature Payload

 The Signature Payload contains data generated by the digital
 signature function (selected during the SA establishment exchange),
 over some part of the message and/or ISAKMP state.  This payload is
 used to verify the integrity of the data in the ISAKMP message, and
 may be of use for non-repudiation services.  Figure 13 shows the
 format of the Signature Payload.
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   RESERVED    !         Payload Length        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                         Signature Data                        ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 13:  Signature Payload Format
 The Signature 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.
  o  Signature Data (variable length) - Data that results from
     applying the digital signature function to the ISAKMP message
     and/or state.
 The payload type for the Signature Payload is nine (9).

3.13 Nonce Payload

 The Nonce Payload contains random data used to guarantee liveness
 during an exchange and protect against replay attacks.  Figure 14
 shows the format of the Nonce Payload.  If nonces are used by a
 particular key exchange, the use of the Nonce payload will be
 dictated by the key exchange.  The nonces may be transmitted as part
 of the key exchange data, or as a separate payload.  However, this is
 defined by the key exchange, not by ISAKMP.

Maughan, et. al. Standards Track [Page 37] RFC 2408 ISAKMP November 1998

                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                            Nonce Data                         ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 14:  Nonce Payload Format
 The Nonce 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.
  o  Nonce Data (variable length) - Contains the random data generated
     by the transmitting entity.
 The payload type for the Nonce Payload is ten (10).

3.14 Notification Payload

 The Notification Payload can contain both ISAKMP and DOI-specific
 data and is used to transmit informational data, such as error
 conditions, to an ISAKMP peer.  It is possible to send multiple
 Notification payloads in a single ISAKMP message.  Figure 15 shows
 the format of the Notification Payload.
 Notification which occurs during, or is concerned with, a Phase 1
 negotiation is identified by the Initiator and Responder cookie pair
 in the ISAKMP Header.  The Protocol Identifier, in this case, is
 ISAKMP and the SPI value is 0 because the cookie pair in the ISAKMP
 Header identifies the ISAKMP SA. If the notification takes place
 prior to the completed exchange of keying information, then the
 notification will be unprotected.

Maughan, et. al. Standards Track [Page 38] RFC 2408 ISAKMP November 1998

 Notification which occurs during, or is concerned with, a Phase 2
 negotiation is identified by the Initiator and Responder cookie pair
 in the ISAKMP Header and the Message ID and SPI associated with the
 current negotiation.  One example for this type of notification is to
 indicate why a proposal was rejected.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !              Domain of Interpretation  (DOI)                  !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Protocol-ID  !   SPI Size    !      Notify Message Type      !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                Security Parameter Index (SPI)                 ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                       Notification Data                       ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 15:  Notification Payload Format
 The Notification 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.
  o  Domain of Interpretation (4 octets) - Identifies the DOI (as
     described in Section 2.1) under which this notification is taking
     place.  For ISAKMP this value is zero (0) and for the IPSEC DOI
     it is one (1).  Other DOI's can be defined using the description
     in appendix B.
  o  Protocol-Id (1 octet) - Specifies the protocol identifier for the
     current notification.  Examples might include ISAKMP, IPSEC ESP,
     IPSEC AH, OSPF, TLS, etc.

Maughan, et. al. Standards Track [Page 39] RFC 2408 ISAKMP November 1998

  o  SPI Size (1 octet) - Length in octets of the SPI as defined by
     the Protocol-Id.  In the case of ISAKMP, the Initiator and
     Responder cookie pair from the ISAKMP Header is the ISAKMP SPI,
     therefore, the SPI Size is irrelevant and MAY be from zero (0) to
     sixteen (16).  If the SPI Size is non-zero, the content of the
     SPI field MUST be ignored.  The Domain of Interpretation (DOI)
     will dictate the SPI Size for other protocols.
  o  Notify Message Type (2 octets) - Specifies the type of
     notification message (see section 3.14.1).  Additional text, if
     specified by the DOI, is placed in the Notification Data field.
  o  SPI (variable length) - Security Parameter Index.  The receiving
     entity's SPI. The use of the SPI field is described in section
     2.4.  The length of this field is determined by the SPI Size
     field and is not necessarily aligned to a 4 octet boundary.
  o  Notification Data (variable length) - Informational or error data
     transmitted in addition to the Notify Message Type.  Values for
     this field are DOI-specific.
 The payload type for the Notification Payload is eleven (11).

3.14.1 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.
 For example, a secure front end or security gateway may use the
 Notify message to synchronize SA communication.  The table below
 lists the Nofitication messages and their corresponding values.
 Values in the Private Use range are expected to be DOI-specific
 values.
                    NOTIFY MESSAGES - ERROR TYPES
                         Errors               Value
               INVALID-PAYLOAD-TYPE             1
               DOI-NOT-SUPPORTED                2
               SITUATION-NOT-SUPPORTED          3
               INVALID-COOKIE                   4
               INVALID-MAJOR-VERSION            5
               INVALID-MINOR-VERSION            6
               INVALID-EXCHANGE-TYPE            7
               INVALID-FLAGS                    8
               INVALID-MESSAGE-ID               9
               INVALID-PROTOCOL-ID             10
               INVALID-SPI                     11

Maughan, et. al. Standards Track [Page 40] RFC 2408 ISAKMP November 1998

               INVALID-TRANSFORM-ID            12
               ATTRIBUTES-NOT-SUPPORTED        13
               NO-PROPOSAL-CHOSEN              14
               BAD-PROPOSAL-SYNTAX             15
               PAYLOAD-MALFORMED               16
               INVALID-KEY-INFORMATION         17
               INVALID-ID-INFORMATION          18
               INVALID-CERT-ENCODING           19
               INVALID-CERTIFICATE             20
               CERT-TYPE-UNSUPPORTED           21
               INVALID-CERT-AUTHORITY          22
               INVALID-HASH-INFORMATION        23
               AUTHENTICATION-FAILED           24
               INVALID-SIGNATURE               25
               ADDRESS-NOTIFICATION            26
               NOTIFY-SA-LIFETIME              27
               CERTIFICATE-UNAVAILABLE         28
               UNSUPPORTED-EXCHANGE-TYPE       29
               UNEQUAL-PAYLOAD-LENGTHS         30
               RESERVED (Future Use)        31 - 8191
               Private Use                8192 - 16383
                    NOTIFY MESSAGES - STATUS TYPES
                        Status              Value
                CONNECTED                   16384
                RESERVED (Future Use)   16385 - 24575
                DOI-specific codes     24576 - 32767
                Private Use            32768 - 40959
                RESERVED (Future Use)  40960 - 65535

3.15 Delete Payload

 The Delete Payload contains a protocol-specific security association
 identifier that the sender has removed from its security association
 database and is, therefore, no longer valid.  Figure 16 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.
 Deletion which is concerned with an ISAKMP SA will contain a
 Protocol-Id of ISAKMP and the SPIs are the initiator and responder
 cookies from the ISAKMP Header.  Deletion which is concerned with a
 Protocol SA, such as ESP or AH, will contain the Protocol-Id of that
 protocol (e.g.  ESP, AH) and the SPI is the sending entity's SPI(s).

Maughan, et. al. Standards Track [Page 41] RFC 2408 ISAKMP November 1998

 NOTE: The Delete Payload is not a request for the responder to delete
 an SA, but an advisory from the initiator to the responder.  If the
 responder chooses to ignore the message, the next communication from
 the responder to the initiator, using that security association, will
 fail.  A responder is not expected to acknowledge receipt of a Delete
 payload.
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ! Next Payload  !   RESERVED    !         Payload Length        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !              Domain of Interpretation  (DOI)                  !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !  Protocol-Id  !   SPI Size    !           # of SPIs           !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~               Security Parameter Index(es) (SPI)              ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 16:  Delete Payload Format
 The Delete 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.
  o  Domain of Interpretation (4 octets) - Identifies the DOI (as
     described in Section 2.1) under which this deletion is taking
     place.  For ISAKMP this value is zero (0) and for the IPSEC DOI
     it is one (1).  Other DOI's can be defined using the description
     in appendix B.
  o  Protocol-Id (1 octet) - ISAKMP can establish security
     associations for various protocols, including ISAKMP and IPSEC.
     This field identifies which security association database to
     apply the delete request.

Maughan, et. al. Standards Track [Page 42] RFC 2408 ISAKMP November 1998

  o  SPI Size (1 octet) - Length in octets of the SPI as defined by
     the Protocol-Id.  In the case of ISAKMP, the Initiator and
     Responder cookie pair is the ISAKMP SPI. In this case, the SPI
     Size would be 16 octets for each SPI being deleted.
  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.  Values for this
     field are DOI and protocol specific.  The length of this field is
     determined by the SPI Size and # of SPIs fields.
 The payload type for the Delete Payload is twelve (12).

3.16 Vendor ID Payload

 The Vendor ID Payload contains a vendor defined constant.  The
 constant is used by vendors to identify and recognize remote
 instances of their implementations.  This mechanism allows a vendor
 to experiment with new features while maintaining backwards
 compatibility.  This is not a general extension facility of ISAKMP.
 Figure 17 shows the format of the Vendor ID Payload.
 The Vendor ID payload is not an announcement from the sender that it
 will send private payload types.  A vendor sending the Vendor ID MUST
 not make any assumptions about private payloads that it may send
 unless a Vendor ID is received as well.  Multiple Vendor ID payloads
 MAY be sent.  An implementation is NOT REQUIRED to understand any
 Vendor ID payloads.  An implementation is NOT REQUIRED to send any
 Vendor ID payload at all.  If a private payload was sent without
 prior agreement to send it, a compliant implementation may reject a
 proposal with a notify message of type INVALID-PAYLOAD-TYPE.
 If a Vendor ID payload is sent, it MUST be sent during the Phase 1
 negotiation.  Reception of a familiar Vendor ID payload in the Phase
 1 negotiation allows an implementation to make use of Private USE
 payload numbers (128-255), described in section 3.1 for vendor
 specific extensions during Phase 2 negotiations.  The definition of
 "familiar" is left to implementations to determine.  Some vendors may
 wish to implement another vendor's extension prior to
 standardization.  However, this practice SHOULD not be widespread and
 vendors should work towards standardization instead.
 The vendor defined constant MUST be unique.  The choice of hash and
 text to hash is left to the vendor to decide.  As an example, vendors
 could generate their vendor id by taking a plain (non-keyed) hash of
 a string containing the product name, and the version of the product.

Maughan, et. al. Standards Track [Page 43] RFC 2408 ISAKMP November 1998

 A hash is used instead of a vendor registry to avoid local
 cryptographic policy problems with having a list of "approved"
 products, to keep away from maintaining a list of vendors, and to
 allow classified products to avoid having to appear on any list.  For
 instance:
 "Example Company IPsec.  Version 97.1"
 (not including the quotes) has MD5 hash:
 48544f9b1fe662af98b9b39e50c01a5a, when using MD5file.  Vendors may
 include all of the hash, or just a portion of it, as the payload
 length will bound the data.  There are no security implications of
 this hash, so its choice is arbitrary.
                        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        !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   !                                                               !
   ~                        Vendor ID (VID)                        ~
   !                                                               !
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 17:  Vendor ID Payload Format
 The Vendor ID 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.
  o  Vendor ID (variable length) - Hash of the vendor string plus
     version (as described above).
 The payload type for the Vendor ID Payload is thirteen (13).

4 ISAKMP Exchanges

 ISAKMP supplies the basic syntax of a message exchange.  The basic
 building blocks for ISAKMP messages are the payload types described
 in section 3.  This section describes the procedures for SA

Maughan, et. al. Standards Track [Page 44] RFC 2408 ISAKMP November 1998

 establishment and SA modification, followed by a default set of
 exchanges that MAY be used for initial interoperability.  Other
 exchanges will be defined depending on the DOI and key exchange.
 [IPDOI] and [IKE] are examples of how this is achieved.  Appendix B
 explains the procedures for accomplishing these additions.

4.1 ISAKMP Exchange Types

 ISAKMP allows the creation of exchanges for the establishment of
 Security Associations and keying material.  There are currently five
 default Exchange Types defined for ISAKMP. Sections 4.4 through 4.8
 describe these exchanges.  Exchanges define the content and ordering
 of ISAKMP messages during communications between peers.  Most
 exchanges will include all the basic payload types - SA, KE, ID, SIG
 - and may include others.  The primary difference between exchange
 types is the ordering of the messages and the payload ordering within
 each message.  While the ordering of payloads within messages is not
 mandated, for processing efficiency it is RECOMMENDED that the
 Security Association payload be the first payload within an exchange.
 Processing of each payload within an exchange is described in section
 5.
 Sections 4.4 through 4.8 provide a default set of ISAKMP exchanges.
 These exchanges provide different security protection for the
 exchange itself and information exchanged.  The diagrams in each of
 the following sections show the message ordering for each exchange
 type as well as the payloads included in each message, and provide
 basic notes describing what has happened after each message exchange.
 None of the examples include any "optional payloads", like
 certificate and certificate request.  Additionally, none of the
 examples include an initial exchange of ISAKMP Headers (containing
 initiator and responder cookies) which would provide protection
 against clogging (see section 2.5.3).
 The defined exchanges are not meant to satisfy all DOI and key
 exchange protocol requirements.  If the defined exchanges meet the
 DOI requirements, then they can be used as outlined.  If the defined
 exchanges do not meet the security requirements defined by the DOI,
 then the DOI MUST specify new exchange type(s) and the valid
 sequences of payloads that make up a successful exchange, and how to
 build and interpret those payloads.  All ISAKMP implementations MUST
 implement the Informational Exchange and SHOULD implement the other
 four exchanges.  However, this is dependent on the definition of the
 DOI and associated key exchange protocols.

Maughan, et. al. Standards Track [Page 45] RFC 2408 ISAKMP November 1998

 As discussed above, these exchange types can be used in either phase
 of negotiation.  However, they may provide different security
 properties in each of the phases.  With each of these exchanges, the
 combination of cookies and SPI fields identifies whether this
 exchange is being used in the first or second phase of a negotiation.

4.1.1 Notation

 The following notation is used to describe the ISAKMP exchange types,
 shown in the next section, with the message formats and associated
 payloads:
   HDR is an ISAKMP header whose exchange type defines the payload
        orderings
   SA is an SA negotiation payload with one or more Proposal and
        Transform payloads. An initiator MAY provide multiple proposals
        for negotiation; a responder MUST reply with only one.
   KE is the key exchange payload.
   IDx is the identity payload for "x". x can be: "ii" or "ir"
        for the ISAKMP initiator and responder, respectively, or x can
        be: "ui", "ur" (when the ISAKMP daemon is a proxy negotiator),
        for the user initiator and responder, respectively.
   HASH is the hash payload.
   SIG is the signature payload. The data to sign is exchange-specific.
   AUTH is a generic authentication mechanism, such as HASH or SIG.
   NONCE is the nonce payload.
   '*' signifies payload encryption after the ISAKMP header. This
        encryption MUST begin immediately after the ISAKMP header and
        all payloads following the ISAKMP header MUST be encrypted.
   => signifies "initiator to responder" communication
   <= signifies "responder to initiator" communication

4.2 Security Association Establishment

 The Security Association, Proposal, and Transform payloads are used
 to build ISAKMP messages for the negotiation and establishment of
 SAs.  An SA establishment message consists of a single SA payload
 followed by at least one, and possibly many, Proposal payloads and at
 least one, and possibly many, Transform payloads associated with each
 Proposal payload.  Because these payloads are considered together,
 the SA payload will point to any following payloads and not to the
 Proposal payload included with the SA payload.  The SA Payload
 contains the DOI and Situation for the proposed SA. Each Proposal
 payload contains a Security Parameter Index (SPI) and ensures that
 the SPI is associated with the Protocol-Id in accordance with the
 Internet Security Architecture [SEC-ARCH].  Proposal payloads may or
 may not have the same SPI, as this is implementation dependent.  Each

Maughan, et. al. Standards Track [Page 46] RFC 2408 ISAKMP November 1998

 Transform Payload contains the specific security mechanisms to be
 used for the designated protocol.  It is expected that the Proposal
 and Transform payloads will be used only during SA establishment
 negotiation.  The creation of payloads for security association
 negotiation and establishment described here in this section are
 applicable for all ISAKMP exchanges described later in sections 4.4
 through 4.8.  The examples shown in 4.2.1 contain only the SA,
 Proposal, and Transform payloads and do not contain other payloads
 that might exist for a given ISAKMP exchange.
 The Proposal payload provides the initiating entity with the
 capability to present to the responding entity the security protocols
 and associated security mechanisms for use with the security
 association being negotiated.  If the SA establishment negotiation is
 for a combined protection suite consisting of multiple protocols,
 then there MUST be multiple Proposal payloads each with the same
 Proposal number.  These proposals MUST be considered as a unit and
 MUST NOT be separated by a proposal with a different proposal number.
 The use of the same Proposal number in multiple Proposal payloads
 provides a logical AND operation, i.e.  Protocol 1 AND Protocol 2.
 The first example below shows an ESP AND AH protection suite.  If the
 SA establishment negotiation is for different protection suites, then
 there MUST be multiple Proposal payloads each with a monotonically
 increasing Proposal number.  The different proposals MUST be
 presented in the initiator's preference order.  The use of different
 Proposal numbers in multiple Proposal payloads provides a logical OR
 operation, i.e.  Proposal 1 OR Proposal 2, where each proposal may
 have more than one protocol.  The second example below shows either
 an AH AND ESP protection suite OR just an ESP protection suite.  Note
 that the Next Payload field of the Proposal payload points to another
 Proposal payload (if it exists).  The existence of a Proposal payload
 implies the existence of one or more Transform payloads.
 The Transform payload provides the initiating entity with the
 capability to present to the responding entity multiple mechanisms,
 or transforms, for a given protocol.  The Proposal payload identifies
 a Protocol for which services and mechanisms are being negotiated.
 The Transform payload allows the initiating entity to present several
 possible supported transforms for that proposed protocol.  There may
 be several transforms associated with a specific Proposal payload
 each identified in a separate Transform payload.  The multiple
 transforms MUST be presented with monotonically increasing numbers in
 the initiator's preference order.  The receiving entity MUST select a
 single transform for each protocol in a proposal or reject the entire
 proposal.  The use of the Transform number in multiple Transform
 payloads provides a second level OR operation, i.e.  Transform 1 OR
 Transform 2 OR Transform 3.  Example 1 below shows two possible
 transforms for ESP and a single transform for AH. Example 2 below

Maughan, et. al. Standards Track [Page 47] RFC 2408 ISAKMP November 1998

 shows one transform for AH AND one transform for ESP OR two
 transforms for ESP alone.  Note that the Next Payload field of the
 Transform payload points to another Transform payload or 0.  The
 Proposal payload delineates the different proposals.
 When responding to a Security Association payload, the responder MUST
 send a Security Association payload with the selected proposal, which
 may consist of multiple Proposal payloads and their associated
 Transform payloads.  Each of the Proposal payloads MUST contain a
 single Transform payload associated with the Protocol.  The responder
 SHOULD retain the Proposal # field in the Proposal payload and the
 Transform # field in each Transform payload of the selected Proposal.
 Retention of Proposal and Transform numbers should speed the
 initiator's protocol processing by negating the need to compare the
 respondor's selection with every offered option.  These values enable
 the initiator to perform the comparison directly and quickly.  The
 initiator MUST verify that the Security Association payload received
 from the responder matches one of the proposals sent initially.

4.2.1 Security Association Establishment Examples

 This example shows a Proposal for a combined protection suite with
 two different protocols.  The first protocol is presented with two
 transforms supported by the proposer.  The second protocol is
 presented with a single transform.  An example for this proposal
 might be: Protocol 1 is ESP with Transform 1 as 3DES and Transform 2
 as DES AND Protocol 2 is AH with Transform 1 as SHA. The responder
 MUST select from the two transforms proposed for ESP. The resulting
 protection suite will be either (1) 3DES AND SHA OR (2) DES AND SHA,
 depending on which ESP transform was selected by the responder.  Note
 this example is shown using the Base Exchange.
                          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
    /+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Nonce    !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SA Pay ! Domain of Interpretation (DOI) !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                           Situation                           !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Proposal !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Prop 1 ! Proposal # = 1! Protocol-Id ! SPI Size !# of Trans. = 2! Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   \ !                         SPI (variable)                        !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Transform!   RESERVED    !         Payload Length        !

Maughan, et. al. Standards Track [Page 48] RFC 2408 ISAKMP November 1998

  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 1 ! Transform # 1 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 2 ! Transform # 2 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Prop 1 ! Proposal # = 1! Protocol ID ! SPI Size !# of Trans. = 1! Prot 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   \ !                         SPI (variable)                        !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 1 ! Transform # 1 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    \+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 This second example shows a Proposal for two different protection
 suites.  The SA Payload was omitted for space reasons.  The first
 protection suite is presented with one transform for the first
 protocol and one transform for the second protocol.  The second
 protection suite is presented with two transforms for a single
 protocol.  An example for this proposal might be:  Proposal 1 with
 Protocol 1 as AH with Transform 1 as MD5 AND Protocol 2 as ESP with
 Transform 1 as 3DES. This is followed by Proposal 2 with Protocol 1
 as ESP with Transform 1 as DES and Transform 2 as 3DES. The responder
 MUST select from the two different proposals.  If the second Proposal
 is selected, the responder MUST select from the two transforms for
 ESP. The resulting protection suite will be either (1) MD5 AND 3DES
 OR the selection between (2) DES OR (3) 3DES.
                          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
    /+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Proposal !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Prop 1 ! Proposal # = 1! Protocol ID ! SPI Size !# of Trans. = 1! Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   \ !                         SPI (variable)                        !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !

Maughan, et. al. Standards Track [Page 49] RFC 2408 ISAKMP November 1998

  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 1 ! Transform # 1 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Proposal !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Prop 1 ! Proposal # = 1! Protocol ID ! SPI Size !# of Trans. = 1! Prot 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   \ !                         SPI (variable)                        !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 1 ! Transform # 1 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Prop 2 ! Proposal # = 2! Protocol ID ! SPI Size !# of Trans. = 2! Prot 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   \ !                         SPI (variable)                        !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = Transform!   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 1 ! Transform # 1 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    >+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   / ! NP = 0        !   RESERVED    !         Payload Length        !
  /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Tran 2 ! Transform # 2 ! Transform ID ! RESERVED2 !

  \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \ !                         SA Attributes                         !
    \+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.3 Security Association Modification

 Security Association modification within ISAKMP is accomplished by
 creating a new SA and initiating communications using that new SA.
 Deletion of the old SA can be done anytime after the new SA is
 established.  Deletion of the old SA is dependent on local security
 policy.  Modification of SAs by using a "Create New SA followed by
 Delete Old SA" method is done to avoid potential vulnerabilities in
 synchronizing modification of existing SA attributes.  The procedure
 for creating new SAs is outlined in section 4.2.  The procedure for
 deleting SAs is outlined in section 5.15.

Maughan, et. al. Standards Track [Page 50] RFC 2408 ISAKMP November 1998

 Modification of an ISAKMP SA (phase 1 negotiation) follows the same
 procedure as creation of an ISAKMP SA. There is no relationship
 between the two SAs and the initiator and responder cookie pairs
 SHOULD be different, as outlined in section 2.5.3.
 Modification of a Protocol SA (phase 2 negotiation) follows the same
 procedure as creation of a Protocol SA. The creation of a new SA is
 protected by the existing ISAKMP SA. There is no relationship between
 the two Protocol SAs.  A protocol implementation SHOULD begin using
 the newly created SA for outbound traffic and SHOULD continue to
 support incoming traffic on the old SA until it is deleted or until
 traffic is received under the protection of the newly created SA. As
 stated previously in this section, deletion of an old SA is then
 dependent on local security policy.

4.4 Base Exchange

 The Base Exchange is designed to allow the Key Exchange and
 Authentication related information to be transmitted together.
 Combining the Key Exchange and Authentication-related information
 into one message reduces the number of round-trips at the expense of
 not providing identity protection.  Identity protection is not
 provided because identities are exchanged before a common shared
 secret has been established and, therefore, encryption of the
 identities is not possible.  The following diagram shows the messages
 with the possible payloads sent in each message and notes for an
 example of the Base Exchange.
                       BASE EXCHANGE

# Initiator Direction Responder NOTE (1) HDR; SA; NONCE ⇒ Begin ISAKMP-SA or Proxy negotiation

(2) ⇐ HDR; SA; NONCE

                                Basic SA agreed upon

(3) HDR; KE; ⇒

   IDii; AUTH                   Key Generated (by responder)
                                Initiator Identity Verified by
                                Responder

(4) ⇐ HDR; KE;

                       IDir; AUTH
                                Responder Identity Verified by
                                Initiator Key Generated (by
                                initiator) SA established

Maughan, et. al. Standards Track [Page 51] RFC 2408 ISAKMP November 1998

 In the first message (1), the initiator generates a proposal it
 considers adequate to protect traffic for the given situation.  The
 Security Association, Proposal, and Transform payloads are included
 in the Security Association payload (for notation purposes).  Random
 information which is used to guarantee liveness and protect against
 replay attacks is also transmitted.  Random information provided by
 both parties SHOULD be used by the authentication mechanism to
 provide shared proof of participation in the exchange.
 In the second message (2), the responder indicates the protection
 suite it has accepted with the Security Association, Proposal, and
 Transform payloads.  Again, random information which is used to
 guarantee liveness and protect against replay attacks is also
 transmitted.  Random information provided by both parties SHOULD be
 used by the authentication mechanism to provide shared proof of
 participation in the exchange.  Local security policy dictates the
 action of the responder if no proposed protection suite is accepted.
 One possible action is the transmission of a Notify payload as part
 of an Informational Exchange.
 In the third (3) and fourth (4) messages, the initiator and
 responder, respectively, exchange keying material used to arrive at a
 common shared secret and identification information.  This
 information is transmitted under the protection of the agreed upon
 authentication function.  Local security policy dictates the action
 if an error occurs during these messages.  One possible action is the
 transmission of a Notify payload as part of an Informational
 Exchange.

4.5 Identity Protection Exchange

 The Identity Protection Exchange is designed to separate the Key
 Exchange information from the Identity and Authentication related
 information.  Separating the Key Exchange from the Identity and
 Authentication related information provides protection of the
 communicating identities at the expense of two additional messages.
 Identities are exchanged under the protection of a previously
 established common shared secret.  The following diagram shows the
 messages with the possible payloads sent in each message and notes
 for an example of the Identity Protection Exchange.

Maughan, et. al. Standards Track [Page 52] RFC 2408 ISAKMP November 1998

                  IDENTITY PROTECTION EXCHANGE

# Initiator Direction Responder NOTE (1) HDR; SA ⇒ Begin ISAKMP-SA or

                                                  Proxy negotiation

(2) ⇐ HDR; SA

                                                  Basic SA agreed upon

(3) HDR; KE; NONCE ⇒ (4) ⇐ HDR; KE; NONCE

                                                  Key Generated (by
                                                  Initiator and
                                                  Responder)

(5) HDR*; IDii; AUTH ⇒

                                                  Initiator Identity
                                                  Verified by
                                                  Responder

(6) ⇐ HDR*; IDir; AUTH

                                                  Responder Identity
                                                  Verified by
                                                  Initiator
                                                  SA established
 In the first message (1), the initiator generates a proposal it
 considers adequate to protect traffic for the given situation.  The
 Security Association, Proposal, and Transform payloads are included
 in the Security Association payload (for notation purposes).
 In the second message (2), the responder indicates the protection
 suite it has accepted with the Security Association, Proposal, and
 Transform payloads.  Local security policy dictates the action of the
 responder if no proposed protection suite is accepted.  One possible
 action is the transmission of a Notify payload as part of an
 Informational Exchange.
 In the third (3) and fourth (4) messages, the initiator and
 responder, respectively, exchange keying material used to arrive at a
 common shared secret and random information which is used to
 guarantee liveness and protect against replay attacks.  Random
 information provided by both parties SHOULD be used by the
 authentication mechanism to provide shared proof of participation in
 the exchange.  Local security policy dictates the action if an error
 occurs during these messages.  One possible action is the
 transmission of a Notify payload as part of an Informational
 Exchange.
 In the fifth (5) and sixth (6) messages, the initiator and responder,
 respectively, exchange identification information and the results of
 the agreed upon authentication function.  This information is

Maughan, et. al. Standards Track [Page 53] RFC 2408 ISAKMP November 1998

 transmitted under the protection of the common shared secret.  Local
 security policy dictates the action if an error occurs during these
 messages.  One possible action is the transmission of a Notify
 payload as part of an Informational Exchange.

4.6 Authentication Only Exchange

 The Authentication Only Exchange is designed to allow only
 Authentication related information to be transmitted.  The benefit of
 this exchange is the ability to perform only authentication without
 the computational expense of computing keys.  Using this exchange
 during negotiation, none of the transmitted information will be
 encrypted.  However, the information may be encrypted in other
 places.  For example, if encryption is negotiated during the first
 phase of a negotiation and the authentication only exchange is used
 in the second phase of a negotiation, then the authentication only
 exchange will be encrypted by the ISAKMP SAs negotiated in the first
 phase.  The following diagram shows the messages with possible
 payloads sent in each message and notes for an example of the
 Authentication Only Exchange.
                   AUTHENTICATION ONLY EXCHANGE

# Initiator Direction Responder NOTE (1) HDR; SA; NONCE ⇒ Begin ISAKMP-SA or

                                                Proxy negotiation

(2) ⇐ HDR; SA; NONCE;

                               IDir; AUTH
                                                Basic SA agreed upon
                                                Responder Identity
                                                Verified by Initiator

(3) HDR; IDii; AUTH ⇒

                                                Initiator Identity
                                                Verified by Responder
                                                SA established
 In the first message (1), the initiator generates a proposal it
 considers adequate to protect traffic for the given situation.  The
 Security Association, Proposal, and Transform payloads are included
 in the Security Association payload (for notation purposes).  Random
 information which is used to guarantee liveness and protect against
 replay attacks is also transmitted.  Random information provided by
 both parties SHOULD be used by the authentication mechanism to
 provide shared proof of participation in the exchange.
 In the second message (2), the responder indicates the protection
 suite it has accepted with the Security Association, Proposal, and
 Transform payloads.  Again, random information which is used to

Maughan, et. al. Standards Track [Page 54] RFC 2408 ISAKMP November 1998

 guarantee liveness and protect against replay attacks is also
 transmitted.  Random information provided by both parties SHOULD be
 used by the authentication mechanism to provide shared proof of
 participation in the exchange.  Additionally, the responder transmits
 identification information.  All of this information is transmitted
 under the protection of the agreed upon authentication function.
 Local security policy dictates the action of the responder if no
 proposed protection suite is accepted.  One possible action is the
 transmission of a Notify payload as part of an Informational
 Exchange.
 In the third message (3), the initiator transmits identification
 information.  This information is transmitted under the protection of
 the agreed upon authentication function.  Local security policy
 dictates the action if an error occurs during these messages.  One
 possible action is the transmission of a Notify payload as part of an
 Informational Exchange.

4.7 Aggressive Exchange

 The Aggressive Exchange is designed to allow the Security
 Association, Key Exchange and Authentication related payloads to be
 transmitted together.  Combining the Security Association, Key
 Exchange, and Authentication-related information into one message
 reduces the number of round-trips at the expense of not providing
 identity protection.  Identity protection is not provided because
 identities are exchanged before a common shared secret has been
 established and, therefore, encryption of the identities is not
 possible.  Additionally, the Aggressive Exchange is attempting to
 establish all security relevant information in a single exchange.
 The following diagram shows the messages with possible payloads sent
 in each message and notes for an example of the Aggressive Exchange.

Maughan, et. al. Standards Track [Page 55] RFC 2408 ISAKMP November 1998

                      AGGRESSIVE EXCHANGE

# Initiator Direction Responder NOTE (1) HDR; SA; KE; ⇒ Begin ISAKMP-SA or

                                               Proxy negotiation
   NONCE; IDii                                 and Key Exchange

(2) ⇐ HDR; SA; KE;

                            NONCE; IDir; AUTH
                                               Initiator Identity
                                               Verified by Responder
                                               Key Generated
                                               Basic SA agreed upon

(3) HDR*; AUTH ⇒

                                               Responder Identity
                                               Verified by Initiator
                                               SA established
 In the first message (1), the initiator generates a proposal it
 considers adequate to protect traffic for the given situation.  The
 Security Association, Proposal, and Transform payloads are included
 in the Security Association payload (for notation purposes).  There
 can be only one Proposal and one Transform offered (i.e.  no choices)
 in order for the aggressive exchange to work.  Keying material used
 to arrive at a common shared secret and random information which is
 used to guarantee liveness and protect against replay attacks are
 also transmitted.  Random information provided by both parties SHOULD
 be used by the authentication mechanism to provide shared proof of
 participation in the exchange.  Additionally, the initiator transmits
 identification information.
 In the second message (2), the responder indicates the protection
 suite it has accepted with the Security Association, Proposal, and
 Transform payloads.  Keying material used to arrive at a common
 shared secret and random information which is used to guarantee
 liveness and protect against replay attacks is also transmitted.
 Random information provided by both parties SHOULD be used by the
 authentication mechanism to provide shared proof of participation in
 the exchange.  Additionally, the responder transmits identification
 information.  All of this information is transmitted under the
 protection of the agreed upon authentication function.  Local
 security policy dictates the action of the responder if no proposed
 protection suite is accepted.  One possible action is the
 transmission of a Notify payload as part of an Informational
 Exchange.

Maughan, et. al. Standards Track [Page 56] RFC 2408 ISAKMP November 1998

 In the third (3) message, the initiator transmits the results of the
 agreed upon authentication function.  This information is transmitted
 under the protection of the common shared secret.  Local security
 policy dictates the action if an error occurs during these messages.
 One possible action is the transmission of a Notify payload as part
 of an Informational Exchange.

4.8 Informational Exchange

 The Informational Exchange is designed as a one-way transmittal of
 information that can be used for security association management.
 The following diagram shows the messages with possible payloads sent
 in each message and notes for an example of the Informational
 Exchange.
                    INFORMATIONAL EXCHANGE
  #   Initiator  Direction Responder  NOTE
 (1)  HDR*; N/D     =>                Error Notification or Deletion
 In the first message (1), the initiator or responder transmits an
 ISAKMP Notify or Delete payload.
 If the Informational Exchange occurs prior to the exchange of keying
 meterial during an ISAKMP Phase 1 negotiation, there will be no
 protection provided for the Informational Exchange.  Once keying
 material has been exchanged or an ISAKMP SA has been established, the
 Informational Exchange MUST be transmitted under the protection
 provided by the keying material or the ISAKMP SA.
 All exchanges are similar in that with the beginning of any exchange,
 cryptographic synchronization MUST occur.  The Informational Exchange
 is an exchange and not an ISAKMP message.  Thus, the generation of an
 Message ID (MID) for an Informational Exchange SHOULD be independent
 of IVs of other on-going communication.  This will ensure
 cryptographic synchronization is maintained for existing
 communications and the Informational Exchange will be processed
 correctly.  The only exception to this is when the Commit Bit of the
 ISAKMP Header is set.  When the Commit Bit is set, the Message ID
 field of the Informational Exchange MUST contain the Message ID of
 the original ISAKMP Phase 2 SA negotiation, rather than a new Message
 ID (MID). This is done to ensure that the Informational Exchange with
 the CONNECTED Notify Message can be associated with the correct Phase
 2 SA. For a description of the Commit Bit, see section 3.1.

Maughan, et. al. Standards Track [Page 57] RFC 2408 ISAKMP November 1998

5 ISAKMP Payload Processing

 Section 3 describes the ISAKMP payloads.  These payloads are used in
 the exchanges described in section 4 and can be used in exchanges
 defined for a specific DOI. This section describes the processing for
 each of the payloads.  This section suggests the logging of events to
 a system audit file.  This action is controlled by a system security
 policy and is, therefore, only a suggested action.

5.1 General Message Processing

 Every ISAKMP message has basic processing applied to insure protocol
 reliability, and to minimize threats, such as denial of service and
 replay attacks.  All processing SHOULD include packet length checks
 to insure the packet received is at least as long as the length given
 in the ISAKMP Header.  If the ISAKMP message length and the value in
 the Payload Length field of the ISAKMP Header are not the same, then
 the ISAKMP message MUST be rejected.  The receiving entity (initiator
 or responder) MUST do the following:
 1.  The event, UNEQUAL PAYLOAD LENGTHS, MAY be logged in the
     appropriate system audit file.
 2.  An Informational Exchange with a Notification payload containing
     the UNEQUAL-PAYLOAD-LENGTHS message type MAY be sent to the
     transmitting entity.  This action is dictated by a system
     security policy.
 When transmitting an ISAKMP message, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Set a timer and initialize a retry counter.
     NOTE: Implementations MUST NOT use a fixed timer.  Instead,
     transmission timer values should be adjusted dynamically based on
     measured round trip times.  In addition, successive
     retransmissions of the same packet should be separated by
     increasingly longer time intervals (e.g., exponential backoff).
 2.  If the timer expires, the ISAKMP message is resent and the retry
     counter is decremented.
 3.  If the retry counter reaches zero (0), the event, RETRY LIMIT
     REACHED, MAY be logged in the appropriate system audit file.
 4.  The ISAKMP protocol machine clears all states and returns to
     IDLE.

Maughan, et. al. Standards Track [Page 58] RFC 2408 ISAKMP November 1998

5.2 ISAKMP Header Processing

 When creating an ISAKMP message, the transmitting entity (initiator
 or responder) MUST do the following:
 1.  Create the respective cookie.  See section 2.5.3 for details.
 2.  Determine the relevant security characteristics of the session
     (i.e. DOI and situation).
 3.  Construct an ISAKMP Header with fields as described in section
     3.1.
 4.  Construct other ISAKMP payloads, depending on the exchange type.
 5.  Transmit the message to the destination host as described in
     section5.1.
 When an ISAKMP message is received, the receiving entity (initiator
 or responder) MUST do the following:
 1.  Verify the Initiator and Responder "cookies".  If the cookie
     validation fails, the message is discarded and the following
     actions are taken:
     (a)  The event, INVALID COOKIE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-COOKIE message type MAY be sent to
          the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Check the Next Payload field to confirm it is valid.  If the Next
     Payload field validation fails, the message is discarded and the
     following actions are taken:
     (a)  The event, INVALID NEXT PAYLOAD, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-PAYLOAD-TYPE message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.
 3.  Check the Major and Minor Version fields to confirm they are
     correct (see section 3.1).  If the Version field validation
     fails, the message is discarded and the following actions are

Maughan, et. al. Standards Track [Page 59] RFC 2408 ISAKMP November 1998

     taken:
     (a)  The event, INVALID ISAKMP VERSION, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-MAJOR-VERSION or INVALID-MINOR-
          VERSION message type MAY be sent to the transmitting entity.
          This action is dictated by a system security policy.
 4.  Check the Exchange Type field to confirm it is valid.  If the
     Exchange Type field validation fails, the message is discarded
     and the following actions are taken:
     (a)  The event, INVALID EXCHANGE TYPE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-EXCHANGE-TYPE message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 5.  Check the Flags field to ensure it contains correct values.  If
     the Flags field validation fails, the message is discarded and
     the following actions are taken:
     (a)  The event, INVALID FLAGS, MAY be logged in the appropriate
          systemaudit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-FLAGS message type MAY be sent to the
          transmitting entity.  This action is dictated by a system
          security policy.
 6.  Check the Message ID field to ensure it contains correct values.
     If the Message ID validation fails, the message is discarded and
     the following actions are taken:
     (a)  The event, INVALID MESSAGE ID, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-MESSAGE-ID message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.
 7.  Processing of the ISAKMP message continues using the value in the
     Next Payload field.

Maughan, et. al. Standards Track [Page 60] RFC 2408 ISAKMP November 1998

5.3 Generic Payload Header Processing

 When creating any of the ISAKMP Payloads described in sections 3.4
 through 3.15 a Generic Payload Header is placed at the beginning of
 these payloads.  When creating the Generic Payload Header, the
 transmitting entity (initiator or responder) MUST do the following:
 1.  Place the value of the Next Payload in the Next Payload field.
     These values are described in section 3.1.
 2.  Place the value zero (0) in the RESERVED field.
 3.  Place the length (in octets) of the payload in the Payload Length
     field.
 4.  Construct the payloads as defined in the remainder of this
     section.
 When any of the ISAKMP Payloads are received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Check the Next Payload field to confirm it is valid.  If the Next
     Payload field validation fails, the message is discarded and the
     following actions are taken:
     (a)  The event, INVALID NEXT PAYLOAD, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-PAYLOAD-TYPE message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Verify the RESERVED field contains the value zero.  If the value
     in the RESERVED field is not zero, the message is discarded and
     the following actions are taken:
     (a)  The event, INVALID RESERVED FIELD, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the BAD-PROPOSAL-SYNTAX or PAYLOAD-MALFORMED
          message type MAY be sent to the transmitting entity.  This
          action is dictated by a system security policy.
 3.  Process the remaining payloads as defined by the Next Payload
     field.

Maughan, et. al. Standards Track [Page 61] RFC 2408 ISAKMP November 1998

5.4 Security Association Payload Processing

 When creating a Security Association Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the Domain of Interpretation for which this negotiation
     is being performed.
 2.  Determine the situation within the determined DOI for which this
     negotiation is being performed.
 3.  Determine the proposal(s) and transform(s) within the situation.
     These are described, respectively, in sections 3.5 and 3.6.
 4.  Construct a Security Association payload.
 5.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Security Association payload is received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Determine if the Domain of Interpretation (DOI) is supported.  If
     the DOI determination fails, the message is discarded and the
     following actions are taken:
     (a)  The event, INVALID DOI, MAY be logged in the appropriate
          system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the DOI-NOT-SUPPORTED message type MAY be sent to
          the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Determine if the given situation can be protected.  If the
     Situation determination fails, the message is discarded and the
     following actions are taken:
     (a)  The event, INVALID SITUATION, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the SITUATION-NOT-SUPPORTED message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 3.  Process the remaining payloads (i.e.  Proposal, Transform) of the
     Security Association Payload.  If the Security Association

Maughan, et. al. Standards Track [Page 62] RFC 2408 ISAKMP November 1998

     Proposal (as described in sections 5.5 and 5.6) is not accepted,
     then the following actions are taken:
     (a)  The event, INVALID PROPOSAL, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the NO-PROPOSAL-CHOSEN message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.

5.5 Proposal Payload Processing

 When creating a Proposal Payload, the transmitting entity (initiator
 or responder) MUST do the following:
 1.  Determine the Protocol for this proposal.
 2.  Determine the number of proposals to be offered for this protocol
     and the number of transforms for each proposal.  Transforms are
     described in section 3.6.
 3.  Generate a unique pseudo-random SPI.
 4.  Construct a Proposal payload.
 When a Proposal payload is received, the receiving entity (initiator
 or responder) MUST do the following:
 1.  Determine if the Protocol is supported.  If the Protocol-ID field
     is invalid, the payload is discarded and the following actions
     are taken:
     (a)  The event, INVALID PROTOCOL, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-PROTOCOL-ID message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Determine if the SPI is valid.  If the SPI is invalid, the
     payload is discarded and the following actions are taken:
     (a)  The event, INVALID SPI, MAY be logged in the appropriate
          system audit file.

Maughan, et. al. Standards Track [Page 63] RFC 2408 ISAKMP November 1998

     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-SPI message type MAY be sent to the
          transmitting entity.  This action is dictated by a system
          security policy.
 3.  Ensure the Proposals are presented according to the details given
     in section 3.5 and 4.2.  If the proposals are not formed
     correctly, the following actions are taken:
     (a)  Possible events, BAD PROPOSAL SYNTAX, INVALID PROPOSAL, are
          logged in the appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the BAD-PROPOSAL-SYNTAX or PAYLOAD-MALFORMED
          message type MAY be sent to the transmitting entity.  This
          action is dictated by a system security policy.
 4.  Process the Proposal and Transform payloads as defined by the
     Next Payload field.  Examples of processing these payloads are
     given in section 4.2.1.

5.6 Transform Payload Processing

 When creating a Transform Payload, the transmitting entity (initiator
 or responder) MUST do the following:
 1.  Determine the Transform # for this transform.
 2.  Determine the number of transforms to be offered for this
     proposal.  Transforms are described in sections 3.6.
 3.  Construct a Transform payload.
 When a Transform payload is received, the receiving entity (initiator
 or responder) MUST do the following:
 1.  Determine if the Transform is supported.  If the Transform-ID
     field contains an unknown or unsupported value, then that
     Transform payload MUST be ignored and MUST NOT cause the
     generation of an INVALID TRANSFORM event.  If the Transform-ID
     field is invalid, the payload is discarded and the following
     actions are taken:
     (a)  The event, INVALID TRANSFORM, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-TRANSFORM-ID message type MAY be sent

Maughan, et. al. Standards Track [Page 64] RFC 2408 ISAKMP November 1998

          to the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Ensure the Transforms are presented according to the details
     given in section 3.6 and 4.2.  If the transforms are not formed
     correctly, the following actions are taken:
     (a)  Possible events, BAD PROPOSAL SYNTAX, INVALID TRANSFORM,
          INVALID ATTRIBUTES, are logged in the appropriate system
          audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the BAD-PROPOSAL-SYNTAX, PAYLOAD-MALFORMED or
          ATTRIBUTES-NOT-SUPPORTED message type MAY be sent to the
          transmitting entity.  This action is dictated by a system
          security policy.
 3.  Process the subsequent Transform and Proposal payloads as defined
     by the Next Payload field.  Examples of processing these payloads
     are given in section 4.2.1.

5.7 Key Exchange Payload Processing

 When creating a Key Exchange Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the Key Exchange to be used as defined by the DOI.
 2.  Determine the usage of the Key Exchange Data field as defined by
     the DOI.
 3.  Construct a Key Exchange payload.
 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Key Exchange payload is received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Determine if the Key Exchange is supported.  If the Key Exchange
     determination fails, the message is discarded and the following
     actions are taken:
     (a)  The event, INVALID KEY INFORMATION, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-KEY-INFORMATION message type MAY be

Maughan, et. al. Standards Track [Page 65] RFC 2408 ISAKMP November 1998

          sent to the transmitting entity.  This action is dictated by
          a system security policy.

5.8 Identification Payload Processing

 When creating an Identification Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the Identification information to be used as defined by
     the DOI (and possibly the situation).
 2.  Determine the usage of the Identification Data field as defined
     by the DOI.
 3.  Construct an Identification payload.
 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When an Identification payload is received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Determine if the Identification Type is supported.  This may be
     based on the DOI and Situation.  If the Identification
     determination fails, the message is discarded and the following
     actions are taken:
     (a)  The event, INVALID ID INFORMATION, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-ID-INFORMATION message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.

5.9 Certificate Payload Processing

 When creating a Certificate Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the Certificate Encoding to be used.  This may be
     specified by the DOI.
 2.  Ensure the existence of a certificate formatted as defined by the
     Certificate Encoding.
 3.  Construct a Certificate payload.

Maughan, et. al. Standards Track [Page 66] RFC 2408 ISAKMP November 1998

 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Certificate payload is received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Determine if the Certificate Encoding is supported.  If the
     Certificate Encoding is not supported, the payload is discarded
     and the following actions are taken:
     (a)  The event, INVALID CERTIFICATE TYPE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-CERT-ENCODING message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 2.  Process the Certificate Data field.  If the Certificate Data is
     invalid or improperly formatted, the payload is discarded and the
     following actions are taken:
     (a)  The event, INVALID CERTIFICATE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-CERTIFICATE message type MAY be sent
          to the transmitting entity.  This action is dictated by a
          system security policy.

5.10 Certificate Request Payload Processing

 When creating a Certificate Request Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the type of Certificate Encoding to be requested.  This
     may be specified by the DOI.
 2.  Determine the name of an acceptable Certificate Authority which
     is to be requested (if applicable).
 3.  Construct a Certificate Request payload.
 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Certificate Request payload is received, the receiving entity
 (initiator or responder) MUST do the following:

Maughan, et. al. Standards Track [Page 67] RFC 2408 ISAKMP November 1998

 1.  Determine if the Certificate Encoding is supported.  If the
     Certificate Encoding is invalid, the payload is discarded and the
     following actions are taken:
     (a)  The event, INVALID CERTIFICATE TYPE, MAY be logged in
          the appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-CERT-ENCODING message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
     If the Certificate Encoding is not supported, the payload is
     discarded and the following actions are taken:
     (a)  The event, CERTIFICATE TYPE UNSUPPORTED, MAY be logged in
          the appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the CERT-TYPE-UNSUPPORTED message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 2.  Determine if the Certificate Authority is supported for the
     specified Certificate Encoding.  If the Certificate Authority is
     invalid or improperly formatted, the payload is discarded and the
     following actions are taken:
     (a)  The event, INVALID CERTIFICATE AUTHORITY, MAY be logged in
          the appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-CERT-AUTHORITY message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 3.  Process the Certificate Request.  If a requested Certificate Type
     with the specified Certificate Authority is not available, then
     the payload is discarded and the following actions are taken:
     (a)  The event, CERTIFICATE-UNAVAILABLE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the CERTIFICATE-UNAVAILABLE message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.

Maughan, et. al. Standards Track [Page 68] RFC 2408 ISAKMP November 1998

5.11 Hash Payload Processing

 When creating a Hash Payload, the transmitting entity (initiator or
 responder) MUST do the following:
 1.  Determine the Hash function to be used as defined by the SA
     negotiation.
 2.  Determine the usage of the Hash Data field as defined by the DOI.
 3.  Construct a Hash payload.
 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Hash payload is received, the receiving entity (initiator or
 responder) MUST do the following:
 1.  Determine if the Hash is supported.  If the Hash determination
     fails, the message is discarded and the following actions are
     taken:
     (a)  The event, INVALID HASH INFORMATION, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-HASH-INFORMATION message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.
 2.  Perform the Hash function as outlined in the DOI and/or Key
     Exchange protocol documents.  If the Hash function fails, the
     message is discarded and the following actions are taken:
     (a)  The event, INVALID HASH VALUE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the AUTHENTICATION-FAILED message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.

5.12 Signature Payload Processing

 When creating a Signature Payload, the transmitting entity (initiator
 or responder) MUST do the following:

Maughan, et. al. Standards Track [Page 69] RFC 2408 ISAKMP November 1998

 1.  Determine the Signature function to be used as defined by the SA
     negotiation.
 2.  Determine the usage of the Signature Data field as defined by the
     DOI.
 3.  Construct a Signature payload.
 4.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Signature payload is received, the receiving entity (initiator
 or responder) MUST do the following:
 1.  Determine if the Signature is supported.  If the Signature
     determination fails, the message is discarded and the following
     actions are taken:
     (a)  The event, INVALID SIGNATURE INFORMATION, MAY be logged in
          the appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the INVALID-SIGNATURE message type MAY be sent to
          the transmitting entity.  This action is dictated by a
          system security policy.
 2.  Perform the Signature function as outlined in the DOI and/or Key
     Exchange protocol documents.  If the Signature function fails,
     the message is discarded and the following actions are taken:
     (a)  The event, INVALID SIGNATURE VALUE, MAY be logged in the
          appropriate system audit file.
     (b)  An Informational Exchange with a Notification payload
          containing the AUTHENTICATION-FAILED message type MAY be
          sent to the transmitting entity.  This action is dictated by
          a system security policy.

5.13 Nonce Payload Processing

 When creating a Nonce Payload, the transmitting entity (initiator or
 responder) MUST do the following:
 1.  Create a unique random value to be used as a nonce.
 2.  Construct a Nonce payload.

Maughan, et. al. Standards Track [Page 70] RFC 2408 ISAKMP November 1998

 3.  Transmit the message to the receiving entity as described in
     section 5.1.
 When a Nonce payload is received, the receiving entity (initiator or
 responder) MUST do the following:
 1.  There are no specific procedures for handling Nonce payloads.
     The procedures are defined by the exchange types (and possibly
     the DOI and Key Exchange descriptions).

5.14 Notification Payload Processing

 During communications it is possible that errors may occur.  The
 Informational Exchange with a Notify Payload provides a controlled
 method of informing a peer entity that errors have occurred during
 protocol processing.  It is RECOMMENDED that Notify Payloads be sent
 in a separate Informational Exchange rather than appending a Notify
 Payload to an existing exchange.
 When creating a Notification Payload, the transmitting entity
 (initiator or responder) MUST do the following:
 1.  Determine the DOI for this Notification.
 2.  Determine the Protocol-ID for this Notification.
 3.  Determine the SPI size based on the Protocol-ID field.  This
     field is necessary because different security protocols have
     different SPI sizes.  For example, ISAKMP combines the Initiator
     and Responder cookie pair (16 octets) as a SPI, while ESP and AH
     have 4 octet SPIs.
 4.  Determine the Notify Message Type based on the error or status
     message desired.
 5.  Determine the SPI which is associated with this notification.
 6.  Determine if additional Notification Data is to be included.
     This is additional information specified by the DOI.
 7.  Construct a Notification payload.
 8.  Transmit the message to the receiving entity as described in
     section 5.1.
 Because the Informational Exchange with a Notification payload is a
 unidirectional message a retransmission will not be performed.  The
 local security policy will dictate the procedures for continuing.

Maughan, et. al. Standards Track [Page 71] RFC 2408 ISAKMP November 1998

 However, we RECOMMEND that a NOTIFICATION PAYLOAD ERROR event be
 logged in the appropriate system audit file by the receiving entity.
 If the Informational Exchange occurs prior to the exchange of keying
 material during an ISAKMP Phase 1 negotiation there will be no
 protection provided for the Informational Exchange.  Once the keying
 material has been exchanged or the ISAKMP SA has been established,
 the Informational Exchange MUST be transmitted under the protection
 provided by the keying material or the ISAKMP SA.
 When a Notification payload is received, the receiving entity
 (initiator or responder) MUST do the following:
 1.  Determine if the Informational Exchange has any protection
     applied to it by checking the Encryption Bit and the
     Authentication Only Bit in the ISAKMP Header.  If the Encryption
     Bit is set, i.e.  the Informational Exchange is encrypted, then
     the message MUST be decrypted using the (in-progress or
     completed) ISAKMP SA. Once the decryption is complete the
     processing can continue as described below.  If the
     Authentication Only Bit is set, then the message MUST be
     authenticated using the (in-progress or completed) ISAKMP SA.
     Once the authentication is completed, the processing can continue
     as described below.  If the Informational Exchange is not
     encrypted or authentication, the payload processing can continue
     as described below.
 2.  Determine if the Domain of Interpretation (DOI) is supported.  If
     the DOI determination fails, the payload is discarded and the
     following action is taken:
     (a)  The event, INVALID DOI, MAY be logged in the appropriate
          system audit file.
 3.  Determine if the Protocol-Id is supported.  If the Protocol-Id
     determination fails, the payload is discarded and the following
     action is taken:
     (a)  The event, INVALID PROTOCOL-ID, MAY be logged in the
          appropriate system audit file.
 4.  Determine if the SPI is valid.  If the SPI is invalid, the
     payload is discarded and the following action is taken:
     (a)  The event, INVALID SPI, MAY be logged in the appropriate
          system audit file.

Maughan, et. al. Standards Track [Page 72] RFC 2408 ISAKMP November 1998

 5.  Determine if the Notify Message Type is valid.  If the Notify
     Message Type is invalid, the payload is discarded and the
     following action is taken:
     (a)  The event, INVALID MESSAGE TYPE, MAY be logged in the
          appropriate system audit file.
 6.  Process the Notification payload, including additional
     Notification Data, and take appropriate action, according to
     local security policy.

5.15 Delete Payload Processing

 During communications it is possible that hosts may be compromised or
 that information may be intercepted during transmission.  Determining
 whether this has occurred is not an easy task and is outside the
 scope of this memo.  However, if it is discovered that transmissions
 are being compromised, then it is necessary to establish a new SA and
 delete the current SA.
 The Informational Exchange with a Delete Payload provides a
 controlled method of informing a peer entity that the transmitting
 entity has deleted the SA(s).  Deletion of Security Associations MUST
 always be performed under the protection of an ISAKMP SA. The
 receiving entity SHOULD clean up its local SA database.  However,
 upon receipt of a Delete message the SAs listed in the Security
 Parameter Index (SPI) field of the Delete payload cannot be used with
 the transmitting entity.  The SA Establishment procedure must be
 invoked to re-establish secure communications.
 When creating a Delete Payload, the transmitting entity (initiator or
 responder) MUST do the following:
 1.  Determine the DOI for this Deletion.
 2.  Determine the Protocol-ID for this Deletion.
 3.  Determine the SPI size based on the Protocol-ID field.  This
     field is necessary because different security protocols have
     different SPI sizes.  For example, ISAKMP combines the Initiator
     and Responder cookie pair (16 octets) as a SPI, while ESP and AH
     have 4 octet SPIs.
 4.  Determine the # of SPIs to be deleted for this protocol.
 5.  Determine the SPI(s) which is (are) associated with this
     deletion.

Maughan, et. al. Standards Track [Page 73] RFC 2408 ISAKMP November 1998

 6.  Construct a Delete payload.
 7.  Transmit the message to the receiving entity as described in
     section 5.1.
 Because the Informational Exchange with a Delete payload is a
 unidirectional message a retransmission will not be performed.  The
 local security policy will dictate the procedures for continuing.
 However, we RECOMMEND that a DELETE PAYLOAD ERROR event be logged in
 the appropriate system audit file by the receiving entity.
 As described above, the Informational Exchange with a Delete payload
 MUST be transmitted under the protection provided by an ISAKMP SA.
 When a Delete payload is received, the receiving entity (initiator or
 responder) MUST do the following:
 1.  Because the Informational Exchange is protected by some security
     service (e.g.  authentication for an Auth-Only SA, encryption for
     other exchanges), the message MUST have these security services
     applied using the ISAKMP SA. Once the security service processing
     is complete the processing can continue as described below.  Any
     errors that occur during the security service processing will be
     evident when checking information in the Delete payload.  The
     local security policy SHOULD dictate any action to be taken as a
     result of security service processing errors.
 2.  Determine if the Domain of Interpretation (DOI) is supported.  If
     the DOI determination fails, the payload is discarded and the
     following action is taken:
     (a)  The event, INVALID DOI, MAY be logged in the appropriate
          system audit file.
 3.  Determine if the Protocol-Id is supported.  If the Protocol-Id
     determination fails, the payload is discarded and the following
     action is taken:
     (a)  The event, INVALID PROTOCOL-ID, MAY be logged in the
          appropriate system audit file.
 4.  Determine if the SPI is valid for each SPI included in the Delete
     payload.  For each SPI that is invalid, the following action is
     taken:
     (a)  The event, INVALID SPI, MAY be logged in the appropriate
          system audit file.

Maughan, et. al. Standards Track [Page 74] RFC 2408 ISAKMP November 1998

 5.  Process the Delete payload and take appropriate action, according
     to local security policy.  As described above, one appropriate
     action SHOULD include cleaning up the local SA database.

6 Conclusions

 The Internet Security Association and Key Management Protocol
 (ISAKMP) is a well designed protocol aimed at the Internet of the
 future.  The massive growth of the Internet will lead to great
 diversity in network utilization, communications, security
 requirements, and security mechanisms.  ISAKMP contains all the
 features that will be needed for this dynamic and expanding
 communications environment.
 ISAKMP's Security Association (SA) feature coupled with
 authentication and key establishment provides the security and
 flexibility that will be needed for future growth and diversity.
 This security diversity of multiple key exchange techniques,
 encryption algorithms, authentication mechanisms, security services,
 and security attributes will allow users to select the appropriate
 security for their network, communications, and security needs.  The
 SA feature allows users to specify and negotiate security
 requirements with other users.  An additional benefit of supporting
 multiple techniques in a single protocol is that as new techniques
 are developed they can easily be added to the protocol.  This
 provides a path for the growth of Internet security services.  ISAKMP
 supports both publicly or privately defined SAs, making it ideal for
 government, commercial, and private communications.
 ISAKMP provides the ability to establish SAs for multiple security
 protocols and applications.  These protocols and applications may be
 session-oriented or sessionless.  Having one SA establishment
 protocol that supports multiple security protocols eliminates the
 need for multiple, nearly identical authentication, key exchange and
 SA establishment protocols when more than one security protocol is in
 use or desired.  Just as IP has provided the common networking layer
 for the Internet, a common security establishment protocol is needed
 if security is to become a reality on the Internet.  ISAKMP provides
 the common base that allows all other security protocols to
 interoperate.
 ISAKMP follows good security design principles.  It is not coupled to
 other insecure transport protocols, therefore it is not vulnerable or
 weakened by attacks on other protocols.  Also, when more secure
 transport protocols are developed, ISAKMP can be easily migrated to
 them.  ISAKMP also provides protection against protocol related
 attacks.  This protection provides the assurance that the SAs and
 keys established are with the desired party and not with an attacker.

Maughan, et. al. Standards Track [Page 75] RFC 2408 ISAKMP November 1998

 ISAKMP also follows good protocol design principles.  Protocol
 specific information only is in the protocol header, following the
 design principles of IPv6.  The data transported by the protocol is
 separated into functional payloads.  As the Internet grows and
 evolves, new payloads to support new security functionality can be
 added without modifying the entire protocol.

Maughan, et. al. Standards Track [Page 76] RFC 2408 ISAKMP November 1998

A ISAKMP Security Association Attributes

A.1 Background/Rationale

 As detailed in previous sections, ISAKMP is designed to provide a
 flexible and extensible framework for establishing and managing
 Security Associations and cryptographic keys.  The framework provided
 by ISAKMP consists of header and payload definitions, exchange types
 for guiding message and payload exchanges, and general processing
 guidelines.  ISAKMP does not define the mechanisms that will be used
 to establish and manage Security Associations and cryptographic keys
 in an authenticated and confidential manner.  The definition of
 mechanisms and their application is the purview of individual Domains
 of Interpretation (DOIs).
 This section describes the ISAKMP values for the Internet IP Security
 DOI, supported security protocols, and identification values for
 ISAKMP Phase 1 negotiations.  The Internet IP Security DOI is
 MANDATORY to implement for IP Security.  [Oakley] and [IKE] describe,
 in detail, the mechanisms and their application for establishing and
 managing Security Associations and cryptographic keys for IP
 Security.

A.2 Internet IP Security DOI Assigned Value

 As described in [IPDOI], the Internet IP Security DOI Assigned Number
 is one (1).

A.3 Supported Security Protocols

 Values for supported security protocols are specified in the most
 recent "Assigned Numbers" RFC [STD-2].  Presented in the following
 table are the values for the security protocols supported by ISAKMP
 for the Internet IP Security DOI.
                     Protocol Assigned Value
                     RESERVED        0
                     ISAKMP          1
 All DOIs MUST reserve ISAKMP with a Protocol-ID of 1.  All other
 security protocols within that DOI will be numbered accordingly.
 Security protocol values 2-15359 are reserved to IANA for future use.
 Values 15360-16383 are permanently reserved for private use amongst
 mutually consenting implementations.  Such private use values are
 unlikely to be interoperable across different implementations.

Maughan, et. al. Standards Track [Page 77] RFC 2408 ISAKMP November 1998

A.4 ISAKMP Identification Type Values

 The following table lists the assigned values for the Identification
 Type field found in the Identification payload during a generic Phase
 1 exchange, which is not for a specific protocol.
                            ID Type       Value
                      ID_IPV4_ADDR          0
                      ID_IPV4_ADDR_SUBNET   1
                      ID_IPV6_ADDR          2
                      ID_IPV6_ADDR_SUBNET   3

A.4.1 ID_IPV4_ADDR

 The ID_IPV4_ADDR type specifies a single four (4) octet IPv4 address.

A.4.2 ID_IPV4_ADDR_SUBNET

 The ID_IPV4_ADDR_SUBNET type specifies a range of IPv4 addresses,
 represented by two four (4) octet values.  The first value is an IPv4
 address.  The second is an IPv4 network mask.  Note that ones (1s) in
 the network mask indicate that the corresponding bit in the address
 is fixed, while zeros (0s) indicate a "wildcard" bit.

A.4.3 ID_IPV6_ADDR

 The ID_IPV6_ADDR type specifies a single sixteen (16) octet IPv6
 address.

A.4.4 ID_IPV6_ADDR_SUBNET

 The ID_IPV6_ADDR_SUBNET type specifies a range of IPv6 addresses,
 represented by two sixteen (16) octet values.  The first value is an
 IPv6 address.  The second is an IPv6 network mask.  Note that ones
 (1s) in the network mask indicate that the corresponding bit in the
 address is fixed, while zeros (0s) indicate a "wildcard" bit.

Maughan, et. al. Standards Track [Page 78] RFC 2408 ISAKMP November 1998

B Defining a new Domain of Interpretation

 The Internet DOI may be sufficient to meet the security requirements
 of a large portion of the internet community.  However, some groups
 may have a need to customize some aspect of a DOI, perhaps to add a
 different set of cryptographic algorithms, or perhaps because they
 want to make their security-relevant decisions based on something
 other than a host id or user id.  Also, a particular group may have a
 need for a new exchange type, for example to support key management
 for multicast groups.
 This section discusses guidelines for defining a new DOI. The full
 specification for the Internet DOI can be found in [IPDOI].
 Defining a new DOI is likely to be a time-consuming process.  If at
 all possible, it is recommended that the designer begin with an
 existing DOI and customize only the parts that are unacceptable.
 If a designer chooses to start from scratch, the following MUST be
 defined:
  o  A "situation":  the set of information that will be used to
     determine the required security services.
  o  The set of security policies that must be supported.
  o  A scheme for naming security-relevant information, including
     encryption algorithms, key exchange algorithms, etc.
  o  A syntax for the specification of proposed security services,
     attributes, and certificate authorities.
  o  The specific formats of the various payload contents.
  o  Additional exchange types, if required.

B.1 Situation

 The situation is the basis for deciding how to protect a
 communications channel.  It must contain all of the data that will be
 used to determine the types and strengths of protections applied in
 an SA. For example, a US Department of Defense DOI would probably use
 unpublished algorithms and have additional special attributes to
 negotiate.  These additional security attributes would be included in
 the situation.

Maughan, et. al. Standards Track [Page 79] RFC 2408 ISAKMP November 1998

B.2 Security Policies

 Security policies define how various types of information must be
 categorized and protected.  The DOI must define the set of security
 policies supported, because both parties in a negotiation must trust
 that the other party understands a situation, and will protect
 information appropriately, both in transit and in storage.  In a
 corporate setting, for example, both parties in a negotiation must
 agree to the meaning of the term "proprietary information" before
 they can negotiate how to protect it.
 Note that including the required security policies in the DOI only
 specifies that the participating hosts understand and implement those
 policies in a full system context.

B.3 Naming Schemes

 Any DOI must define a consistent way to name cryptographic
 algorithms, certificate authorities, etc.  This can usually be done
 by using IANA naming conventions, perhaps with some private
 extensions.

B.4 Syntax for Specifying Security Services

 In addition to simply specifying how to name entities, the DOI must
 also specify the format for complete proposals of how to protect
 traffic under a given situation.

B.5 Payload Specification

 The DOI must specify the format of each of the payload types.  For
 several of the payload types, ISAKMP has included fields that would
 have to be present across all DOI (such as a certificate authority in
 the certificate payload, or a key exchange identifier in the key
 exchange payload).

B.6 Defining new Exchange Types

 If the basic exchange types are inadequate to meet the requirements
 within a DOI, a designer can define up to thirteen extra exchange
 types per DOI.  The designer creates a new exchange type by choosing
 an unused exchange type value, and defining a sequence of messages
 composed of strings of the ISAKMP payload types.
 Note that any new exchange types must be rigorously analyzed for
 vulnerabilities.  Since this is an expensive and imprecise
 undertaking, a new exchange type should only be created when
 absolutely necessary.

Maughan, et. al. Standards Track [Page 80] RFC 2408 ISAKMP November 1998

Security Considerations

 Cryptographic analysis techniques are improving at a steady pace.
 The continuing improvement in processing power makes once
 computationally prohibitive cryptographic attacks more realistic.
 New cryptographic algorithms and public key generation techniques are
 also being developed at a steady pace.  New security services and
 mechanisms are being developed at an accelerated pace.  A consistent
 method of choosing from a variety of security services and mechanisms
 and to exchange attributes required by the mechanisms is important to
 security in the complex structure of the Internet.  However, a system
 that locks itself into a single cryptographic algorithm, key exchange
 technique, or security mechanism will become increasingly vulnerable
 as time passes.
 UDP is an unreliable datagram protocol and therefore its use in
 ISAKMP introduces a number of security considerations.  Since UDP is
 unreliable, but a key management protocol must be reliable, the
 reliability is built into ISAKMP. While ISAKMP utilizes UDP as its
 transport mechanism, it doesn't rely on any UDP information (e.g.
 checksum, length) for its processing.
 Another issue that must be considered in the development of ISAKMP is
 the effect of firewalls on the protocol.  Many firewalls filter out
 all UDP packets, making reliance on UDP questionable in certain
 environments.
 A number of very important security considerations are presented in
 [SEC-ARCH].  One bears repeating.  Once a private session key is
 created, it must be safely stored.  Failure to properly protect the
 private key from access both internal and external to the system
 completely nullifies any protection provided by the IP Security
 services.

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.

Domain of Interpretation

 The Domain of Interpretation (DOI) is a 32-bit field which identifies
 the domain under which the security association negotiation is taking
 place.  Requests for assignments of new DOIs must be accompanied by a
 standards-track RFC which describes the specific domain.

Maughan, et. al. Standards Track [Page 81] RFC 2408 ISAKMP November 1998

Supported Security Protocols

 ISAKMP is designed to provide security association negotiation and
 key management for many security protocols.  Requests for identifiers
 for additional security protocols must be accompanied by a
 standards-track RFC which describes the security protocol and its
 relationship to ISAKMP.

Acknowledgements

 Dan Harkins, Dave Carrel, and Derrell Piper of Cisco Systems provided
 design assistance with the protocol and coordination for the [IKE]
 and [IPDOI] documents.
 Hilarie Orman, via the Oakley key exchange protocol, has
 significantly influenced the design of ISAKMP.
 Marsha Gross, Bill Kutz, Mike Oehler, Pete Sell, and Ruth Taylor
 provided significant input and review to this document.
 Scott Carlson ported the TIS DNSSEC prototype to FreeBSD for use with
 the ISAKMP prototype.
 Jeff Turner and Steve Smalley contributed to the prototype
 development and integration with ESP and AH.
 Mike Oehler and Pete Sell performed interoperability testing with
 other ISAKMP implementors.
 Thanks to Carl Muckenhirn of SPARTA, Inc.  for his assistance with
 LaTeX.

References

 [ANSI]     ANSI, X9.42:  Public Key Cryptography for the Financial
            Services Industry -- Establishment of Symmetric Algorithm
            Keys Using Diffie-Hellman, Working Draft, April 19, 1996.
 [BC]       Ballardie, A., and J. Crowcroft, Multicast-specific
            Security Threats and Countermeasures, Proceedings of 1995
            ISOC Symposium on Networks & Distributed Systems Security,
            pp. 17-30, Internet Society, San Diego, CA, February 1995.
 [Berge]    Berge, N., "UNINETT PCA Policy Statements", RFC 1875,
            December 1995.

Maughan, et. al. Standards Track [Page 82] RFC 2408 ISAKMP November 1998

 [CW87]     Clark, D.D. and D.R. Wilson, A Comparison of Commercial
            and Military Computer Security Policies, Proceedings of
            the IEEE Symposium on Security & Privacy, Oakland, CA,
            1987, pp. 184-193.
 [DNSSEC]   D. Eastlake III, Domain Name System Protocol Security
            Extensions, Work in Progress.
 [DOW92]    Diffie, W., M.Wiener, P. Van Oorschot, Authentication and
            Authenticated Key Exchanges, Designs, Codes, and
            Cryptography, 2, 107-125, Kluwer Academic Publishers,
            1992.
 [IAB]      Bellovin, S., "Report of the IAB Security Architecture
            Workshop", RFC 2316, April 1998.
 [IKE]      Harkins, D., and D. Carrel, "The Internet Key Exchange
            (IKE)", RFC 2409, November 1998.
 [IPDOI]    Piper, D., "The Internet IP Security Domain of
            Interpretation for ISAKMP", RFC 2407, November 1998.
 [Karn]     Karn, P., and B. Simpson, Photuris:  Session Key
            Management Protocol, Work in Progress.
 [Kent94]   Steve Kent, IPSEC SMIB, e-mail to ipsec@ans.net, August
            10, 1994.
 [Oakley]   Orman, H., "The Oakley Key Determination Protocol",  RFC
            2412, November 1998.
 [RFC-1422] Kent, S., "Privacy Enhancement for Internet Electronic
            Mail:  Part II: Certificate-Based Key Management", RFC
            1422, February 1993.
 [RFC-1949] Ballardie, A., "Scalable Multicast Key Distribution", RFC
            1949, May 1996.
 [RFC-2093] Harney, H., and C. Muckenhirn, "Group Key Management
            Protocol (GKMP) Specification", RFC 2093, July 1997.
 [RFC-2094] Harney, H., and C. Muckenhirn, "Group Key Management
            Protocol (GKMP) Architecture", RFC 2094, July 1997.
 [RFC-2119] Bradner, S., "Key Words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Maughan, et. al. Standards Track [Page 83] RFC 2408 ISAKMP November 1998

 [Schneier] Bruce Schneier, Applied Cryptography - Protocols,
            Algorithms, and Source Code in C (Second Edition), John
            Wiley & Sons, Inc., 1996.
 [SEC-ARCH] Atkinson, R., and S. Kent, "Security Architecture for the
            Internet Protocol", RFC 2401, November 1998.
 [STD-2]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
            1700, October 1994.  See also:
            http://www.iana.org/numbers.html

Maughan, et. al. Standards Track [Page 84] RFC 2408 ISAKMP November 1998

Authors' Addresses

 Douglas Maughan
 National Security Agency
 ATTN: R23
 9800 Savage Road
 Ft.  Meade, MD. 20755-6000
 Phone:  301-688-0847
 EMail:wdm@tycho.ncsc.mil
 Mark Schneider
 National Security Agency
 ATTN: R23
 9800 Savage Road
 Ft.  Meade, MD. 20755-6000
 Phone:  301-688-0851
 EMail:mss@tycho.ncsc.mil
 Mark Schertler
 Securify, Inc.
 2415-B Charleston Road
 Mountain View, CA 94043
 Phone:  650-934-9303
 EMail:mjs@securify.com
 Jeff Turner
 RABA Technologies, Inc.
 10500 Little Patuxent Parkway
 Columbia, MD. 21044
 Phone:  410-715-9399
 EMail:jeff.turner@raba.com

Maughan, et. al. Standards Track [Page 85] RFC 2408 ISAKMP November 1998

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 an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Maughan, et. al. Standards Track [Page 86]

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