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RFC 3457 - Requirements for IPsec Remote Access Scenarios


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Network Working Group                                           S. Kelly
Request for Comments: 3457                                     Airespace
Category: Informational                                   S. Ramamoorthi
                                                        Juniper Networks
                                                            January 2003

             Requirements for IPsec Remote Access Scenarios

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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

Abstract

   IPsec offers much promise as a secure remote access mechanism.
   However, there are a number of differing remote access scenarios,
   each having some shared and some unique requirements.  A thorough
   understanding of these requirements is necessary in order to
   effectively evaluate the suitability of a specific set of mechanisms
   for any particular remote access scenario.  This document enumerates
   the requirements for a number of common remote access scenarios.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . .   2
      1.1 Requirements Terminology . . . . . . . . . . . . . . . .  3
      1.2 Reader Prerequisites . . . . . . . . . . . . . . . . . .  3
      1.3 General Terminology  . . . . . . . . . . . . . . . . . .  4
      1.4 Document Content and Organization  . . . . . . . . . . .  4
   2. Overview  . . . . . . . . . . . . . . . . . . . . . . . . .   5
      2.1 Endpoint Authentication . . . . . . . . . . . . . . . .   6
         2.1.1 Machine-Level Authentication . . . . . . . . . . .   7
         2.1.2 User-Level Authentication  . . . . . . . . . . . .   7
         2.1.3 Combined User/Machine Authentication . . . . . . .   8
         2.1.4 Remote Access Authentication . . . . . . . . . . .   8
         2.1.5 Compatibility With Legacy Remote Access Mechanisms   9
      2.2 Remote Host Configuration  . . . . . . . . . . . . . . . 10
      2.3 Security Policy Configuration  . . . . . . . . . . . . . 11
      2.4 Auditing . . . . . . . . . . . . . . . . . . . . . . . . 12
      2.5 Intermediary Traversal . . . . . . . . . . . . . . . . . 13

   3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . .  13
      3.1 Telecommuters (Dialup/DSL/Cablemodem)  . . . . . . . . . 14
         3.1.1 Endpoint Authentication Requirements . . . . . . .  15
         3.1.2 Device Configuration Requirements  . . . . . . . .  16
         3.1.3 Policy Configuration Requirements  . . . . . . . .  17
         3.1.4 Auditing Requirements  . . . . . . . . . . . . . .  18
         3.1.5 Intermediary Traversal Requirements  . . . . . . .  18
      3.2 Corporate to Remote Extranet . . . . . . . . . . . . . . 19
         3.2.1 Authentication Requirements  . . . . . . . . . . .  19
         3.2.2 Device Configuration Requirements  . . . . . . . .  20
         3.2.3 Policy Configuration Requirements  . . . . . . . .  21
         3.2.4 Auditing Requirements  . . . . . . . . . . . . . .  21
         3.2.5 Intermediary Traversal Requirements  . . . . . . .  21
      3.3 Extranet Laptop to Home Corporate Net . . . . . . . . .  22
         3.3.1 Authentication Requirements  . . . . . . . . . . .  22
         3.3.2 Device Configuration Requirements  . . . . . . . .  23
         3.3.3 Policy Configuration Requirements  . . . . . . . .  23
         3.3.4 Auditing Requirements  . . . . . . . . . . . . . .  24
         3.3.5 Intermediary Traversal Requirements  . . . . . . .  24
      3.4 Extranet Desktop to Home Corporate Net . . . . . . . . . 25
         3.4.1 Authentication Requirements  . . . . . . . . . . .  25
         3.4.2 Device Configuration Requirements  . . . . . . . .  26
         3.4.3 Policy Configuration Requirements  . . . . . . . .  26
         3.4.4 Auditing Requirements  . . . . . . . . . . . . . .  26
         3.4.5 Intermediary Traversal Requirements  . . . . . . .  26
      3.5 Public System to Target Network . . . . . . . . . . . .  27
         3.5.1 Authentication Requirements  . . . . . . . . . . .  27
         3.5.2 Device Configuration Requirements  . . . . . . . .  28
         3.5.3 Policy  Configuration Requirements . . . . . . . .  28
         3.5.4 Auditing Requirements  . . . . . . . . . . . . . .  29
         3.5.5 Intermediary Traversal Requirements  . . . . . . .  29
   4. Scenario Commonalities  . . . . . . . . . . . . . . . . . .  29
   5. Security Considerations . . . . . . . . . . . . . . . . . .  30
   6. References  . . . . . . . . . . . . . . . . . . . . . . . .  30
   7. Acknowledgements  . . . . . . . . . . . . . . . . . . . . .  30
   8. Editors' Addresses. . . . . . . . . . . . . . . . . . . . .  30
   9. Full Copyright Statement  . . . . . . . . . . . . . . . . .  31

1. Introduction

   Until recently, remote access has typically been characterized by
   dial-up users accessing the target network via the Public Switched
   Telephone Network (PSTN), with the dial-up connection terminating at
   a Network Access Server (NAS) within the target domain.  The
   protocols facilitating this have usually been PPP-based, and access
   control, authorization, and accounting functions have typically been
   provided using one or more of a number of available mechanisms,
   including RADIUS [RADIUS].

   Note that for such access, it has often been assumed that the
   communications infrastructure supporting the ISP connection (the
   PSTN) is relatively secure, and poses no significant threats to
   communications integrity or confidentiality.  Based on this
   assumption, connection security has been limited to access control at
   the NAS based on username/passphrase pairs.  In reality, PSTN dialup
   connections have never been impervious to a determined adversary.

   The availability of widespread broadband access, in concert with the
   desire to reduce the cost of PSTN toll access, have driven the
   development of Internet-based remote access mechanisms.  In some
   cases, PPP-based tunneling mechanisms have been used to provide
   remote access by allowing the dial user to first access a local ISP
   account, and then tunnel an additional PPP connection over the
   Internet into the target network.  In the case of broadband users,
   such connections are tunneled directly over the Internet.  While
   these mechanisms have been lacking in terms of security features, the
   increasing availability of IPsec renders it possible to provide more
   secure remote access to the remote resources via the Internet.

   Remote access via the Internet has numerous benefits, financial and
   otherwise.  However, security is paramount, and this presents strong
   incentives for migration from the old dial-up model to a more secure
   IPsec-based remote access model.  Meeting the security requirements
   of various classes of remote access users presents a number of
   challenges.  It is the aim of this document to explore and enumerate
   the requirements of various IPsec remote access scenarios, without
   suggesting particular solutions for them.

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 [3].

1.2 Reader Prerequisites

   Reader familiarity with RFCs 2401-2412 is a minimum prerequisite to
   understanding the concepts discussed here.  Familiarity with concepts
   relating to Public Key Infrastructures (PKIs) is also necessary.
   Familiarity with RADIUS, PPP, PPTP, L2F, L2TP, and other remote
   access support protocols will also be helpful, though not strictly
   necessary.

1.3 General Terminology

   o  Remote Access - this term is used to refer to the case in which
      the remote user does not necessarily reside at a fixed location,
      i.e., in which the user's IP address is not fixed, and therefore
      usually not known prior to connection establishment.

   o  Secure Remote Access - this term refers to remote access which is
      secured using elements of the IPsec protocol suite.

   o  IPsec Remote Access Client (IRAC)- this term is used to refer to
      the remote access user's system.

   o  IPsec Remote Access Server (IRAS) - this term refers to the device
      providing access to the target network.  An alternative term is
      "Security Gateway".

   o  Security GateWay (SGW) - this refers to the device providing
      access to the target network.  An alternative term is IRAS.

   o  Virtual IP address (VIP) - this term describes an address from a
      subnet local to the target network which is assigned to a remote
      client, giving the appearance that the remote client actually
      resides on the target network.

   o  Machine-Level Authentication - this term describes the case where
      the identity of a machine is verified by virtue of the machine's
      possession and application of some combination of authenticators.
      For a more complete definition, see section 2.

   o  User-Level Authentication - this term describes the case where the
      identity of a user (as opposed to that of a machine) is verified
      by virtue of the user's possession and application of some
      combination of authenticators.  For a more complete definition,
      see section 2.

   o  NAPT - Network Address/Port Translation

1.4 Document Content and Organization

   This document, while initially intended to simply outline
   requirements for various remote access scenarios, has come to include
   somewhat more than this.  This document has evolved from discussion
   within the IPsec Remote Access (IPSRA) working group.  As a result,
   it in some respects documents the evolution of this thought process.
   While this represents a departure from the typical form of a

   requirements document, the associated historical context should prove
   useful in interpreting the conclusions reached in the IPSRA working
   group.

   The balance of this document is organized as follows: First, there is
   a general overview of the basic requirements categories, including
   definitions relevant to these categories.  Following this is a
   section devoted to each remote access scenario.  Within each of these
   sections there are subsections detailing requirements specific to
   that scenario in each of the following areas: endpoint
   authentication, remote host configuration, policy configuration,
   auditing, and intermediary traversal.

2. Overview

   In a very general sense, all secure remote access scenarios have a
   similar high-level appearance:

                                         target network
                                              |
                                              |   +---+
   +-------------+             +-----------+  |---|   |
   |remote access|  Internet   | security  |  |   +---+
   |   client    |=============| gateway   |--|
   |   (IRAC)    |             |(SGW/IRAS) |  |   +---+
   +-------------+             +-----------+  |---|   |
                                              |   +---+

   In all cases, a remote client wishes to securely access resources
   either behind a SGW or on an IPsec-protected host, and/or wishes to
   provide other (specific) systems with secure access to the client's
   own resources.  There are numerous details which may differ,
   depending on the particular scenario.  For example, the IRAC may be
   within another corporate network, or connected to an ISP via dialup,
   DSL, or CATV media.  There may be additional intermediaries between
   the remote client and the security gateway, but ultimately, all of
   these configurations may be viewed somewhat equivalently from a high
   level.

   In general, there are several basic categories of requirements
   relevant to secure remote access scenarios, including endpoint
   authentication, remote host configuration, security policy
   configuration, auditing, and intermediary traversal.  Endpoint
   authentication refers to verification of the identities of the
   communication partners (e.g., the IRAC and the IRAS).  Remote host
   configuration refers to the device configuration parameters of the
   IRAC system.  Security policy configuration refers to IPsec policy
   configuration of both the security gateway and the remote host, and

   might also be termed "access control and authorization
   configuration".  Auditing refers to the generation and collection of
   connection status information which is required for the purpose of
   maintaining the overall security and integrity of the connected
   networks.  Intermediary traversal refers to the ability to pass
   secured traffic across intermediaries, some of which may modify the
   packets in some manner.  Such intermediaries include NAPT and
   firewall devices.  These various categories are treated in more
   detail below.

2.1 Endpoint Authentication

   Before discussing endpoint authentication with respect to remote
   access, it is important to distinguish between data source
   authentication and end user authentication.  Data source
   authentication in the IPsec context consists in providing assurance
   that a network packet originates from a specific endpoint, typically
   a user, host, or application.  IPsec offers mechanisms for this via
   AH or ESP.  End user authentication within the IPsec context consists
   in providing assurance that the endpoint is what or who it claims to
   be.  IPsec currently offers mechanisms for this as part of IKE
   [IKE].

   While the two types of authentication differ, they are not unrelated.
   In fact, data source authentication relies upon endpoint
   authentication, because it is possible to inject packets with a
   particular IP address into the Internet from many arbitrary
   locations.  In many instances, we cannot be certain that a packet
   actually originates from a particular host, or even from the network
   upon which that host resides.  To resolve this, one must first
   authenticate the particular endpoint somehow, and then bind the
   addressing information (e.g., IP address, protocol, port) of this
   endpoint into the trust relationship established by the
   authentication process.

   In the context of secure remote access, the authenticated entity may
   be a machine, a user (application), or both.  The authentication
   methods currently supported by IPsec range from preshared secrets to
   various signature and encryption schemes employing private keys and
   their corresponding public key certificates.  These mechanisms may be
   used to authenticate the end user alone, the device alone, or both
   the end user and the device.  These are each discussed in more detail
   below.

2.1.1 Machine-Level Authentication

   In the case where no user input is required in order for an
   authentication credential to be used, the entity authenticated will
   primarily be the device in which the credential is stored and the
   level of derived assurance regarding this authentication is directly
   related to how securely the machine's credential is maintained during
   both storage and use.  That is, a shared secret or a private key
   corresponding to a public key certificate may be either stored within
   the device or contained in another device which is securely
   accessible by the device (e.g., a smartcard).  If the knowledge
   required for the use of such authentication credentials is entirely
   contained within the subject device (i.e., no user input is
   required), then it is problematic to state that such credential usage
   authenticates anything other than the subject device.

   In some cases, a user may be required to satisfy certain criteria
   prior to being given access to stored credentials.  In such cases,
   the level of user authentication provided by the use of such
   credentials is somewhat difficult to derive.  If sufficiently strong
   access controls exist for the system housing the credential, then
   there may be a strong binding between the authorized system user and
   the credential.  However, at the time the credential is presented,
   the IRAS itself has no such assurance.  That is, the IRAS in
   isolation may have some level of assurance that a particular device
   (the one in which the credential resides) is the one from which
   access is being attempted, but there is no explicit assurance
   regarding the identity of the user of the system.  In order for the
   IRAS to derive additional assurance regarding the user identity, an
   additional user credential of some sort would be required.  This is
   discussed further below.

2.1.2 User-Level Authentication

   In some cases, the user may possess an authentication token
   (preshared key, private key, passphrase, etc.), and may provide this
   or some derivative of this whenever authentication is required.  If
   this token or derivative is delivered directly to the other endpoint
   without modification by the IRAC system, and if the IRAC system
   provides no further credentials of its own, then it is the user alone
   which has been authenticated.  That is, while there may be some
   assurance as to the network address from which the user is
   originating packets, there is no assurance as to the particular
   machine from which the user is attempting access.

2.1.3 Combined User/Machine Authentication

   To authenticate both the user and the system, user input of some sort
   is required in addition to a credential which is securely stored upon
   the device.  In some cases, such user input may be used in order to
   "complete" the credential stored on the device (e.g., a private key
   is password-encrypted), while in others the user's input is supplied
   independently of the stored credential.  In the case where the
   passphrase is applied to the credential prior to use, the level of
   assurance derived from successful application of the credential
   varies according to your viewpoint.

   From the perspective of a system consisting of user, IRAC, IRAS, and
   a collection of system protections and security procedures, it may be
   said that the user has been authenticated to an extent which depends
   upon the strength of the security procedures and system protections
   which are in place.  However, from the perspective of the IRAS alone,
   there is little assurance with respect to user identity.  That is,
   schemes requiring that stored credentials be modified by user input
   prior to use may only be said to provide user-level authentication
   within the context of the larger system, and then, the level of
   assurance derived is directly proportional to the weakest security
   attribute of the entire system.

   When considering remote access from a general perspective,
   assumptions regarding the overall system are liable to prove
   incorrect.  This is because the IRAS and the IRAC may not be within
   the same domain of control; extranet scenarios are a good example of
   this.  Hence, the most desirable joint user/machine authentication
   mechanisms in this context are those which provide a high level of
   assurance to both the IRAS and the IRAC, independently of the larger
   system of which the user, IRAS, and IRAC are a part.

2.1.4 Remote Access Authentication

   In the general case for remote access, authentication requirements
   are typically asymmetric.  From the IRAC's perspective, it is
   important to ensure that the IRAS at the other end of the connection
   is indeed what it seems to be, and not some rogue system masquerading
   as the SGW.  That is, the IRAC requires machine-level authentication
   for the IRAS.  This is fairly straightforward, given the
   authentication mechanisms supported by IKE and IPsec.  Further, this
   sort of authentication tends to persist through time, although the
   extent of this persistence depends upon the mechanism chosen.

   While machine-level authentication for the IRAS is sufficient, this
   is not the case for the IRAC.  Here, it is often important to know
   that the entity at the other end of the connection is one who is

   authorized to access local resources rather than someone who happened
   upon an unoccupied but otherwise authorized system, or a malicious
   Trojan horse application on that user's system, or some other
   unauthorized entity.  Authenticating the user presents different
   requirements than authenticating the user's machine; this requires
   some form of user input, and often the authentication must be
   periodically renewed.

   In situations where a high level of physical security does not exist,
   it is common to require a user-input secret as part of the
   authentication process, and then to periodically renew the
   authentication.  Furthermore, since such circumstances may include
   the possibility of the presence of a Trojan horse application on the
   IRAC system, one-time passphrase mechanisms are often advisable.
   Choosing passphrase mechanisms and renewal intervals which provide an
   acceptable level of risk, but which do not annoy the user too much,
   may be challenging.  It should be obvious that even this approach
   offers limited assurance in many cases.

   Clearly, there are variable assurance levels which are attainable
   with the various endpoint authentication techniques, and none of the
   techniques discussed offer absolute assurance.  Also, there are
   variations in the authentication requirements among different remote
   access scenarios.  This means there is no "cookie cutter" solution
   for this problem, and that individual scenarios must be carefully
   examined in order to derive specific requirements for each.  These
   are examined on a case by case basis below in the detailed scenario
   descriptions.

2.1.5 Compatibility With Legacy Remote Access Mechanisms

   There are a number of currently deployed remote access mechanisms
   which were installed prior to the deployment of IPsec.  Typically,
   these are dialup systems which rely upon RADIUS for user
   authentication and accounting, but there are other mechanisms as
   well.  An ideal IPsec remote access solution might utilize the
   components of the underlying framework without modification.
   Inasmuch as this is possible, this should be a goal.  However, there
   may be cases where this simply cannot be accomplished, due to
   security and/or other considerations.  In such cases, the IPsec
   remote access framework should be designed to accommodate migration
   from these mechanisms as painlessly as is possible.

   In general, proposed IPsec remote access mechanisms should meet the
   following goals:

      o  should provide direct support for legacy user authentication
         and accounting systems such as RADIUS

      o  should encourage migration from existing low-entropy
         password-based systems to more secure authentication systems

      o  if legacy user authentication support cannot be provided
         without some sort of migration, the impact of such migration
         should be minimized

      o  user authentication information must be protected against
         eavesdropping and replay (including the user identity)

      o  single sign-on capability should be provided in configurations
         employing load-balancing and/or redundancy

      o  n-factor authentication mechanisms should be supported

2.2 Remote Host Configuration

   Remote host configuration refers to the network-related device
   configuration of the client system.  This configuration may be fixed
   or dynamic.  It may be completely provided by the administrator of
   the network upon which the remote user currently resides (e.g., the
   ISP), or it may be partially provided by that administrator, with the
   balance provided by an entity on the remote corporate network which
   the client is accessing.  In general, this configuration may include
   the following:

      o IP address(es)
      o Subnet mask(s)
      o Broadcast address(es)
      o Host name
      o Domain name
      o Time offset
      o Servers (e.g., SMTP, POP, WWW, DNS/NIS, LPR,
        syslog, WINS, NTP, etc. )
      o Router(s)
      o Router discovery options
      o Static routes
      o MTU
      o Default TTL
      o Source routing options
      o IP Forwarding enable/disable
      o PMTU options
      o ARP cache timeout
      o X Windows options
      o NIS options
      o NetBIOS options
      o Vendor-specific options
      o (other options)

   Cases where such configuration is fixed are uninteresting; it is the
   cases where specific IRAC configuration occurs as a result of remote
   access with which we are concerned.  For example, in some cases the
   IRAC may be assigned a "virtual address", giving the appearance that
   it resides on the target network:

                                          target net
    +------------------+                      |
    |  Remote Access   |        +--------+    |   ( ~ ~ ~ ~ ~ )
    |+-------+ Client  |        |        |    |   (   IRAC    )
    ||virtual|         |        |security|    |~~~(  virtual  )
    || host  |         |--------|gateway |    |   (  presence )
    ||       |<================>|        |----|     ~ ~ ~ ~ ~
    |+-------+         |--------|        |    |
    +------------------+   ^    +--------+    |   +--------+
                           |                  |---|  local |
                         IPsec tunnel         |   |   host |
                         with encapsulated    |   +--------+
                         traffic inside

   In this case, the IRAC system begins with an externally routable
   address.  An additional target network address is assigned to the
   IRAC, and packets containing this assigned address are encapsulated,
   with the outer headers containing the IRAC's routable address, and
   forwarded to the IRAS through the tunnel.  This provides the IRAC
   with a virtual presence on the target network via an IPsec tunnel.
   Note that the IRAC now has two active addresses: the ISP-assigned
   address, and the VIP.

   Having obtained this virtual presence on the corporate network, the
   IRAC may now require other sorts of topology-related configuration,
   e.g., default routers, DNS server(s), etc., just as a dynamically
   configured host which physically resides upon the target network
   would.  It is this sort of configuration with which this requirements
   category is concerned.

2.3 Security Policy Configuration

   Security policy configuration refers to IPsec access policies for
   both the remote access client and the security gateway.  It may be
   desirable to configure access policies on connecting IRAC systems
   which will protect the target network.  For example, since a client
   has access to the Internet (via its routable address), other systems
   on the Internet also have some level of reciprocal access to the
   client.  In some cases, it may be desirable to block this Internet

   access (or force it to pass through the tunnel) while the client has
   a tunneled connection to the target network.  This is a matter of
   client security policy configuration.

   For the security gateway, it may also be desirable to dynamically
   adjust policies based upon the user with which a connection has been
   established.  For example, say there are two remote users, named
   Alice and Bob.  We wish to provide Alice with unrestricted access to
   the target network, while we wish to restrict Bob's access to
   specific segments.  One way to accomplish this would be to statically
   assign internal "virtual" addresses to each user in a one-to-one
   mapping, so that each user always has the same address.  Then, a
   particular user's access could be controlled via policies based upon
   the particular address.  However, this does not scale well.

   A more scalable solution for remote client access control would be to
   dynamically assign IP addresses from a specific pool based upon the
   authenticated endpoint identity, with access to specific resources
   controlled by address-based policies in the SGW.  This is very
   similar to the static mapping described above, except that a given
   group of users (those with identical access controls) would share a
   given pool of IP addresses (those which are granted the required
   access), rather than a given user always mapping to a given address.
   However, this also has scaling issues, though not as pronounced as
   for the static mapping.

   Alternatively, an arbitrary address could be assigned to a user, with
   the security gateway's policy being dynamically updated based upon
   the identity of the remote client (and its assigned virtual address)
   to permit access to particular resources.  In these cases, the
   relevant security policy configuration is specific to the IRAS,
   rather than to the IRAC.  Both IRAS and IRAC security policy
   configuration are encompassed by this requirements category.

2.4 Auditing

   Auditing is used here to refer to the collection and reporting of
   connection status information by the IRAS, for the purpose of
   maintaining the security and integrity of the IRAS protected network.
   For remote access, the following auditing information is useful from
   a security perspective:

      o connection start time
      o connection end time

   Note that the requirement for a connection-end-time attribute implies
   the need for a connection heartbeat mechanism of some sort so that
   the IRAS can accurately determine this quantity in cases where the

   IRAC does not explicitly terminate the connection.  Also note that
   the heartbeat mechanism in this case is always directed from the IRAC
   to the IRAS.

   In some cases, use of a heartbeat may negatively influence a
   connection.  For example, if the heartbeat interval is very short,
   and the connection is reset after loss of very few heartbeat packets,
   there is a possibility that network congestion could lead to
   unnecessary connection resets.  The heartbeat interval and reset
   threshold should be chosen with this in mind, and it should be
   possible to adjust these quantities either through configuration or
   negotiation.

2.5 Intermediary Traversal

   Intermediary traversal is used here to refer to passing a secured
   data stream through an intermediary such as a firewall or NAPT
   device.  In the case of firewalls, numerous deployed products do not
   recognize the IPsec protocol suite, making it difficult (sometimes
   impossible) to configure them to pass it through.  In such cases, a
   mechanism is required for making the data stream appear to be of a
   type which the firewall is capable of managing.

   In the case of NAPT devices, there are a number of issues with
   attempting to pass an encrypted or authenticated data stream.  For
   example, NAPT devices typically modify the source IP address and
   UDP/TCP port of outgoing packets, and the destination IP address and
   UDP/TCP port of incoming packets, and in some cases, they modify
   additional fields in the data portion of the packet.  Such
   modifications render the use of the AH protocol impossible.  In the
   case of ESP, the UDP/TCP port fields are sometimes unreadable and
   always unmodifiable, making meaningful translation by the NAPT device
   impossible.  There are numerous other protocol-field combinations
   which suffer similarly.  This requirements category is concerned with
   these issues.

3. Scenarios

   There are numerous remote access scenarios possible using IPsec.
   This section contains a brief summary enumeration of these, followed
   by a subsection devoted to each which explores the various
   requirements in terms of the categories defined above.

   The following scenarios are discussed:

   o  dialup/dsl/cablemodem telecommuters using their systems to access
      remote resources

   o  extranet users using local corporate systems to access the remote
      company network of a business partner

   o  extranet users using their own system within another company's
      network to access their home corporate network

   o  extranet users using a business partner's system (located on that
      partner's network) to access their home corporate network

   o  remote users using a borrowed system (e.g., an airport kiosk) to
      access target network resources

3.1 Telecommuters (Dialup/DSL/Cablemodem)

   The telecommuter scenario is one of the more common remote access
   scenarios.  The convenience and wide availability of Internet access
   makes this an attractive option under many circumstances.  Users may
   access the Internet from the comfort of their homes or hotel rooms,
   and using this Internet connection, access the resources of a target
   network.  In some cases, dialup accounts are used to provide the
   initial Internet access, while in others some type of "always-on"
   connection such as a DSL or CATV modem is used.

   The dialup and always-on cases are very similar, with two significant
   differences: address assignment mechanism and connection duration.
   In most dialup cases, the IRAC's IP address is dynamically assigned
   as part of connection setup, and with fairly high likelihood, it is
   different each time the IRAC connects.  DSL/CATV users, on the other
   hand, often have static IP addresses assigned to them, although
   dynamic assignment is on the increase.  As for connection duration,
   dialup remote access connections are typically short-lived, while
   always-on connections may maintain remote access connections for
   significantly longer periods of time.

   The general configuration in either case looks like this:

                                           corporate net
                                                  |  +----+
     +-----+   +-----+      /---/ Internet +---+  |--|    |
     |IRAC |---|modem|------|ISP|==========|SGW|--|  +----+
     +-----+   +-----+      /---/          +---+  |
                                                  |

   An alternative to this configuration entails placing a security
   gateway between the user's system and the modem, in which case this
   added SGW becomes the IRAC.  This is currently most common in cases
   where DSL/CATV connections are used.

3.1.1 Endpoint Authentication Requirements

   The authentication requirements of this scenario depend in part upon
   the general security requirements of the network to which access is
   to be provided.  Assuming that the corporate SGW is physically
   secure, machine authentication for the SGW is sufficient.  If this
   assumption regarding physical security is incorrect, it is not clear
   that stronger authentication for the SGW could be guaranteed, and
   derivation of an effective mechanism for that case is beyond the
   scope of this document.

   For the IRAC, there are numerous threats to the integrity of the user
   authentication process.  Due to the open nature of common consumer
   operating systems, some of these threats are quite difficult to
   protect against.  For example, it is very difficult to assert, with
   any level of certainty, that a single user system which permits the
   downloading and running of arbitrary applications from the Internet
   has not been compromised, and that a covert application is not
   monitoring and interacting with the user's data at any point in time.

   However, there are 2 general threats we might realistically hope to
   somehow mitigate with appropriate authentication mechanisms if we can
   assume that the system has not been compromised in this manner.
   First, there is the possibility that a secure connection is
   established for a particular user, but that someone other than the
   intended user is currently using that connection.  Second, there is
   the possibility that the user's credential (password, hardware token,
   etc.)  has been somehow compromised, and is being used by someone
   other than the authorized user to gain access.

   Mitigation of the first threat, the possibility that someone other
   than the authorized user is currently using the connection,  requires
   periodic renewal of user authentication.  It should be clear that
   machine authentication will not suffice in this case, and that
   requiring periodic re-entry of an unchanging user password (which may
   be written on a post-it note which is stuck to the user's monitor)
   will have limited effectiveness.  Convincing verification of the
   continued presence of the authorized user will, in many cases,
   require periodic application of a time-variant credential.

   Mitigation of the second threat, credential compromise, is difficult,
   and depends upon a number of factors.  If the IRAC system is running
   a highly secure operating system, then a time-variant credential may
   again offer some value.  A static password is clearly deficient in
   this scenario, since it may be subject to either online or offline
   guessing, and eventually compromised - which is the threat we are
   attempting to mitigate.  However, if the IRAC operating system is not

   hardened,  the use of a time-variant credential is only effective if
   simultaneous access from more than one location is forbidden, and if
   the credential generation mechanism is not easily compromised.

   A second approach to the credential compromise problem entails using
   a PKI-based credential which is stored within a secure container of
   some sort, and which requires some user interaction prior to
   operation (e.g., a smartcard).  If such a credential requires
   periodic user interaction to continue operating (e.g., pin re-entry),
   this may help to limit the access of an unauthorized user who happens
   upon a connected but unattended systems.  However, choosing an
   acceptable refresh interval is a difficult problem, and if the pin is
   not
   time-variant, this provides limited additional assurance.

   To summarize, the following are the authentication requirements for
   the IRAS and IRAC:

   IRAS
   ----

   o  machine authentication MUST be provided.

   IRAC
   ----

   o  support for user authentication SHOULD be provided
   o  support for either user or machine authentication MUST be provided
   o  support for user authentication MUST be provided if protection
      from unauthorized connection use is desired.
   o  if user authentication is provided for short-lived dialup
      connections, periodic renewal MAY occur
   o  if user authentication is provided for always-on connections,
      periodic renewal SHOULD occur

3.1.2 Device Configuration Requirements

   There are 2 possibilities for device configuration in the
   telecommuter scenario: either access to the target network is
   permitted for the native ISP-assigned address of the telecommuter's
   system, or the telecommuter's system is assigned a virtual address
   from within the target address space.  In the first case, there are
   no device configuration requirements which are not already satisfied
   by the ISP.  However, this case is the exception, rather than the
   rule.

   The second case is far more common, due to the numerous benefits
   derived by providing the IRAC with a virtual presence on the target

   network.  For example, the virtual presence allows the client to
   receive subnet broadcasts, which permits it to use WINS on the target
   network.  In addition, if the IRAC tunnels all traffic to the target
   network, then the target policy can be applied to Internet traffic
   to/from the IRAC.

   In this case, the IRAC requires, at minimum, assignment of an IP
   address from the target network.  Typically, the IRAC requires
   anywhere from several more to many more elements of configuration
   information, depending upon the corporate network's level of
   topological complexity.  For a fairly complete list, see section 2.2.

   To summarize, the following are the device configuration requirements
   for the IRAC:

      o  support for a virtual IP (VIP) address MAY be provided
      o  if VIP support is provided, support for all device-related
         parameters listed in section 2.2 above SHOULD be provided
      o  support for address assignment based upon authenticated
         identity MAY be provided
      o  if authenticated address assignment is not supported, an
         identity-based dynamic policy update mechanism such as is
         described in [ARCH] MUST be supported.

3.1.3 Policy Configuration Requirements

   In terms of IRAC policy configuration, the most important issue
   pertains to whether the IRAC has direct Internet access enabled (for
   browsing, etc.) while a connection to the target network exists.
   This is important since the fact that the IRAC has access to sites on
   the Internet implies that those sites have some level of reciprocal
   access to the IRAC.  It may be desirable to completely eliminate this
   type of access while a tunnel is active.

   Alternatively, the risks may be mitigated somewhat by forcing all
   Internet-bound packets leaving the IRAC to first traverse the tunnel
   to the target network, where they may be subjected to target network
   policy.  A second approach which carries a bit less overhead entails
   modifying the IRAC's policy configuration to reflect that of the
   target network during the time the IRAC is connected.  In this case,
   traffic is not forced to loop through the target site prior to
   exiting or entering the IRAC.  This requires some sort of policy
   download (or modification) capability as part of the SA establishment
   process.  A third approach is to provide a configuration variable for
   the IRAC which permits specification of "tunnel-all", or "block all
   traffic not destined for the target network while the SA is up".

   In terms of IRAS configuration, it may be necessary to dynamically
   update the security policy database (SPD) when the remote user
   connects.  This is because transit selectors must be based upon
   network address parameters, but these cannot be known a priori in the
   remote access case.  As is noted above, this may be avoided by
   provision of a mechanism which permits address assignment based upon
   authenticated identity.

   To summarize, the following are the policy configuration requirements
   for the IRAS and IRAC:

   IRAS
   ----

      o  dynamic policy update mechanism based upon identity and
         assigned address MAY be supported.

      o  if address assignment-based policy update mechanism is not
         supported, address assignment based upon authenticated identity
         SHOULD be supported.

   IRAC
   ----

      o  IRAC SHOULD provide ability to configure for "tunnel-all"
         and/or "block-all" for traffic not destined for the remote
         network to which IPsec remote access is being provided.

      o  support for dynamic IRAS update of IRAC policy MAY be provided.

3.1.4 Auditing Requirements

   For telecommuter sessions, session start/end times must be collected.
   Reliable derivation of session end time requires that the IRAC
   somehow periodically signify that the connection remains active.
   This is implied if the IRAS receives data from the IRAC over the
   connection, but in cases where no data is sent for some period of
   time, a signaling mechanism is required by which the IRAC indicates
   that the connection remains in use.

3.1.5 Intermediary Traversal Requirements

   If the address assigned by the ISP to the IRAC system is globally
   routable, and no intermediate devices between the IRAC and the IRAS
   perform NAPT operations on the data stream, then there are no
   additional requirements.  If NAPT operations are performed on the
   data stream, some mechanism must be provided in order to render these
   modifications transparent to the IPsec implementation.

3.2 Corporate to Remote Extranet

   Extranets are becoming increasingly common, especially as IPsec
   becomes more widely deployed.  In this scenario, a user from one
   corporation uses a local corporate system to access resources on
   another corporation's network.  Typically, these corporations are
   cooperating on some level, but not to the degree that unbridled
   access between the two networks would be acceptable.  Hence, this
   scenario is characterized by limited access.  The general topological
   appearance is similar to this:

          CORP A                                CORP B
             |                                      |
    +----+   |                                      |  +-----+
    |USER|---|                                      |--| S1  |
    +----+   |   +------++              ++------+   |  +-----+
             |---|SGW/FW||===Internet===||SGW/FW|---|
             |   +------++              ++------+   |  +-----+
             |     SGW-A                   SGW-B    |--| S2  |
             |                                      |  +-----+

   This is purposely simplified in order to illustrate some basic
   characteristics without getting bogged down in details.  At the edge
   of each network is a combination security gateway and firewall
   device.  These are labeled "SGW-A" and "SGW-B".  In this diagram,
   corporation B wishes to provide a user from corporation A with access
   to servers S1 and/or S2.  This may be accomplished in one of several
   different ways:

   1) an end-to-end SA is formed from USER to S1 or S2

   2) a tunnel-mode SA is formed between SGW-A and SGW-B which only
      permits traffic between S1/S2 and USER.

   3) a tunnel-mode SA is formed between USER and SGW-B which only
      permits traffic between S1/S2 and USER.

   These various cases are individually discussed with respect to each
   requirements category below.

3.2.1 Authentication Requirements

   For the corporate extranet scenario, the authentication requirements
   vary slightly depending upon the manner in which the connection is
   accomplished.  If only a particular user is permitted to access
   S1/S2, then user-level authentication is required.  If connection
   types (1) or (3) are used, this may be accomplished in the same
   manner as it would be for a telecommuter.  If connection type (2) is

   used, one of two things must occur: either SGW-A must provide some
   local mechanism for authenticating USER and SGW-B must trust this
   mechanism, or SGW-B must have some mechanism for authenticating USER
   independently of SGW-A.

   If access is permitted for anyone within corporation A, then machine
   authentication will suffice.  However, this is highly unlikely.  A
   slightly more likely situation might be one in which access is
   permitted to anyone within a particular organizational unit in
   corporation A.  This case is very similar the single user access case
   discussed above, and essentially has the same requirements in terms
   of the mechanism required for SGW-A, although machine authentication
   might suffice if the organizational unit which is permitted access
   has a sufficient level of physical security.  Again, this requires
   that corporation B trust corporation A in this regard.

   To summarize, the following are the authentication requirements for
   the IRAS and IRAC:

   IRAS
   ----

      o  machine authentication MUST be provided.

   IRAC
   ----

      o  support for either user or machine authentication MUST be
         provided
      o  support for a combination of user and machine authentication
         SHOULD be provided
      o  if user authentication is used, periodic renewal SHOULD occur

3.2.2 Device Configuration Requirements

   It is possible that corporation B would want to assign a virtual
   address to USER for the duration of the connection.  The only way
   this could be accomplished would be if USER were a tunnel endpoint
   (e.g., in cases (1) and (3)).  It is not clear what benefits, if any,
   this would offer.

   To summarize, the following are the device configuration requirements
   for the IRAC:

      o  support for a virtual address MAY be provided
      o  if VIP support is provided, support for all device-related
         parameters listed in section 2.2 above SHOULD be supported

      o  support for address assignment based upon authenticated
         identity SHOULD be supported
      o  if authenticated address assignment is not supported, an
         identity-based dynamic policy update mechanism such as is
         described in [ARCH] MUST be supported.

3.2.3 Policy Configuration Requirements

   Any of the cases discussed above would present some static policy
   configuration requirements.  Case (1) would require that SGW-A and
   SGW-B permit IPsec traffic to pass between USER and S1/S2.  Case (3)
   would have similar requirements, except that the IPsec traffic would
   be between USER and SGW-B.  Case (2) would require that the
   appropriate transit traffic be secured between USER and S1/S2.

   None of these cases require dynamic policy configuration.

3.2.4 Auditing Requirements

   For cases (1) and (3),  session start/end times must be collected.
   Reliable derivation of session end time requires that the IRAC
   somehow periodically signify that the connection remains active.
   This is implied if the IRAS receives data from the IRAC over the
   connection, but in cases where no data is sent for some period of
   time, a signaling mechanism is required by which the IRAC indicates
   that the connection remains in use.

   For case (2), the type(s) of required auditing data would depend upon
   whether traffic from multiple users were aggregated within a single
   tunnel or not.  If so, the notion of individual connection start/stop
   times would be lost.  If such measures are desired, this requires
   that per-user tunnels be set up between SGW-A and SGW-B, and that
   some sort of timeout interval be used to cause tunnel teardown when
   traffic does not flow for some interval of time.

3.2.5 Intermediary Traversal Requirements

   If the address assigned by the host network to the IRAC system is
   globally routable, and no intermediate devices between the IRAC and
   the IRAS perform NAPT operations on the data stream, then there are
   no additional requirements in this regard.  If NAPT operations are
   performed on the data stream, some mechanism must be provided in
   order to render these modifications transparent to the IPsec
   implementation.

   If a firewall situated at the edge of the host network cannot be
   configured to pass protocols in the IPsec suite, then some mechanism
   must be provided which converts the data stream to one which the

   firewall may be configured to pass.  If the firewall can be
   configured to pass IPsec protocols, then this must be accomplished
   prior to connection establishment.

3.3 Extranet Laptop to Home Corporate Net

   The use of a laptop while visiting another corporation presents
   another increasingly common extranet scenario.  In this case, a user
   works temporarily within another corporation, perhaps as part of a
   service agreement of some sort.  The user brings along a CORP-A
   laptop which is assigned a CORP-B address either statically or
   dynamically, and the user wishes to securely access resources on
   CORP-A's network using this laptop.  This scenario has the following
   appearance:

          CORP A                                CORP B
             |                                      |
    +----+   |                                      |  +--------+
    |POP |---|                                      |--| CORP-A |
    +----+   |   +------++              ++------+   |  | laptop |
             |---|SGW/FW||===Internet===||SGW/FW|---|  +--------+
             |   +------++              ++------+   |
    +----+   |     SGW-A                   SGW-B    |
    |FTP |---|                                      |
    +----+   |                                      |

   This is very similar to the telecommuter scenario, but it differs in
   several important ways.  First, in this case there is often a SGW
   and/or firewall at the edge of CORP-B's site.  Second, there may be a
   significantly increased risk that a long-lived connection could
   become accessible to someone other than the intended user.

3.3.1 Authentication Requirements

   In most cases, the only acceptable connections from CORP-A's
   perspective are between the laptop and either SGW-A or the CORP-A
   servers the laptop wishes to access.  Most of the considerations
   applied to the telecommuter also apply here, and user-level
   authentication is required to provide assurance that the user who
   initiated the connection is still the active user.  As an added
   precaution, a combination of user-level and machine-level
   authentication may be warranted in some cases.  Further, in either
   case this authentication should be renewed frequently.

   To summarize, the following are the authentication requirements for
   the IRAS and IRAC:

   IRAS
   ----

      o  machine authentication MUST be provided.

   IRAC
   ----

      o  support for machine authentication SHOULD be provided
      o  support for user authentication MUST be provided
      o  support for a combination of user and machine authentication
         SHOULD be provided
      o  periodic renewal of user authentication MUST occur

3.3.2 Device Configuration Requirements

   The device configuration requirements in this scenario are the same
   as for the telecommuter, i.e., the laptop may be assigned a virtual
   presence on the corporate network, and if so, will require full
   infrastructure configuration.

   To summarize, the following are the device configuration requirements
   for the IRAC:

      o  support for a virtual address MAY be provided
      o  if VIP support is provided, support for all device-related
         parameters listed in section 2.2 above SHOULD be supported
      o  support for address assignment based upon authenticated
         identity SHOULD be supported
      o  if authenticated address assignment is not supported, an
         identity-based dynamic policy update mechanism such as is
         described in [ARCH] MUST be supported.

3.3.3 Policy Configuration Requirements

   The policy configuration requirements in this scenario differ from
   those of the telecommuter, in that the laptop cannot be assigned a
   policy which requires all traffic to be forwarded to CORP-A via the
   tunnel.  This is due to the fact that the laptop has a CORP-B
   address, and as such, may have traffic destined to CORP-B.  If this
   traffic were tunneled to CORP-A, there might be no return path to
   CORP-B except via the laptop.  On the other hand, Internet-bound
   traffic could be subjected to this restriction if desired, and/or all
   traffic other than that between CORP-A and the laptop could be
   blocked for the duration of the connection.

   IRAC
   ----

      o  support for IRAS update of IRAC policy MAY be provided.

      o  if IRAS update of IRAC policy is not supported, IRAC MAY
         support IRAS directives to "block-all" for non-tunneled
         traffic.

      o  IRAC SHOULD provide ability to configure for "tunnel-all"
         and/or "block-all" for traffic not destined for the remote
         network to which IPsec remote access is being provided.

3.3.4 Auditing Requirements

   The auditing requirements in this scenario are the same as for the
   telecommuter scenario.  Session start/end times must be collected.
   Reliable derivation of session end time requires that the IRAC
   somehow periodically signify that the connection remains active.
   This is implied if the IRAS receives data from the IRAC over the
   connection, but in cases where no data is sent for some period of
   time, a signaling mechanism is required by which the IRAC indicates
   that the connection remains in use.

3.3.5 Intermediary Traversal Requirements

   If the address assigned by the host network to the IRAC system is
   globally routable, and no intermediate devices between the IRAC and
   the IRAS perform NAPT operations on the data stream, then there are
   no additional requirements in this regard.  If NAPT operations are
   performed on the data stream, some mechanism must be provided in
   order to render these modifications transparent to the IPsec
   implementation.

   If a firewall situated at the edge of the host network cannot be
   configured to pass protocols in the IPsec suite, then some mechanism
   must be provided which converts the data stream to one which the
   firewall may be configured to pass.  If the firewall can be
   configured to pass IPsec protocols, then this must be accomplished
   prior to connection establishment.

3.4 Extranet Desktop to Home Corporate Net

   This is very similar to the extranet laptop scenario discussed above,
   except that a higher degree of trust for CORP-B is required by
   CORP-A.  This scenario has the following appearance:

           CORP A                                CORP B
             |                                      |
    +----+   |                                      |  +--------+
    |POP |---|                                      |--| CORP-B |
    +----+   |   +------++              ++------+   |  |desktop |
             |---|SGW/FW||===Internet===||SGW/FW|---|  +--------+
             |   +------++              ++------+   |
    +----+   |     SGW-A                   SGW-B    |
    |FTP |---|                                      |
    +----+   |                                      |

3.4.1 Authentication Requirements

   The authentication requirements for the desktop extranet scenario are
   very similar to those of the extranet laptop scenario discussed
   above.  The primary difference lies in the authentication type which
   may be used, i.e., in the laptop case, CORP-A can derive some
   assurance that the connection is coming from one of CORP-A's systems
   if a securely stored machine credential is stored on and used by on
   the laptop.  In the desktop case this is not possible, since CORP-A
   does not own the IRAC system.

   To summarize, the following are the authentication requirements for
   the IRAS and IRAC:

   IRAS
   ----

     o machine authentication MUST be provided.

   IRAC
   ----

      o  support for machine authentication MAY be provided
      o  support for user authentication MUST be provided
      o  support for a combination of user and machine authentication
         MAY be provided
      o  periodic renewal of user authentication MUST occur

3.4.2 Device Configuration Requirements

   The device configuration requirements in this scenario are the same
   as for the laptop extranet scenario, i.e., the desktop system may be
   assigned a virtual presence on the corporate network, and if so, will
   require full infrastructure configuration.  However, this seems less
   likely than in the laptop scenario, given CORP-A's lack of control
   over the software configuration of CORP-B's desktop system.

3.4.3 Policy Configuration Requirements

   The policy configuration requirements are quite similar to those of
   the extranet laptop, except that in this scenario there is even less
   control over CORP-B's desktop than there would be over the laptop.
   This means it may not be possible to restrict traffic in any way at
   the desktop system.

3.4.4 Auditing Requirements

   The auditing requirements in this scenario are the same as for the
   telecommuter scenario.  Session start/end times must be collected.
   Reliable derivation of session end time requires that the IRAC
   somehow periodically signify that the connection remains active.
   This is implied if the IRAS receives data from the IRAC over the
   connection, but in cases where no data is sent for some period of
   time, a signaling mechanism is required by which the IRAC indicates
   that the connection remains in use.

3.4.5 Intermediary Traversal Requirements

   If the address assigned by the host network to the IRAC system is
   globally routable, and no intermediate devices between the IRAC and
   the IRAS perform NAPT operations on the data stream, then there are
   no additional requirements in this regard.  If NAPT operations are
   performed on the data stream, some mechanism must be provided in
   order to render these modifications transparent to the IPsec
   implementation.

   If a firewall situated at the edge of the host network cannot be
   configured to pass protocols in the IPsec suite, then some mechanism
   must be provided which converts the data stream to one which the
   firewall may be configured to pass.  If the firewall can be
   configured to pass IPsec protocols, then this must be accomplished
   prior to connection establishment.

3.5 Public System to Target Network

   This scenario entails a traveling user connecting to the target
   network using a public system owned by someone else.  A commonly
   cited example is an airport kiosk.  This looks very similar to the
   extranet desktop scenario, except that in the extranet scenario,
   CORP-A might have a trust relationship with CORP-B, whereas in this
   scenario, CORP-A may not trust a publicly accessible system.  Note
   that a trust relationship between CORP-A and the owner of the public
   system may exist, but in many cases will not.

3.5.1 Authentication Requirements

   There are two variations to this scenario.  In the first, no trust
   relationship exists between the target network and the borrowed
   system.  In the second, some trust relationship does exist.  In the
   case where no trust relationship exists, machine authentication is
   out of the question, as it is meaningless in this context.  Further,
   since such a system could easily capture a passphrase, use of a
   static passphrase from such a system would seem to be ill-advised.

   If a one-time passphrase were used, this would mitigate the risk of
   passphrase capture by the public system.  On the other hand, if it is
   acknowledged that such capture is a real threat (i.e., the system
   itself is malicious), then it must also be recognized that any data
   transmitted and received via the resulting session would not be
   confidential or reliable with respect to this malicious system, and
   that the system could not be trusted to have actually disconnected
   when the user walks away.  This suggests that accessing non-trivial
   information from such a system would be imprudent.

   Another possible user authentication option would be a smartcard.
   However, many smartcards require a pin or passphrase to "unlock"
   them, which requires some level of trust in the kiosk to not record
   the pin.  Hence, this approach suffers from drawbacks similar to
   those of the static passphrase in this regard.  The primary
   difference would be that the pin/passphrase could not be used alone
   for access in the smartcard case.

   In cases where a trust relationship with the owner of the public
   system exists, the trust level would modulate the risk levels
   discussed above.  For example, if a sufficient level of trust for the
   system owner exists, use of a static passphrase might present no more
   risk than if this were permitted from a system owned by the accessed
   target.  However, the primary benefit of such a trust relationship
   would be derived from the ability to authenticate the machine from

   which the user is attempting access.  For example, a security policy
   requiring that remote access only be permitted with combined
   user/machine authentication might be effected, with further control
   regarding which machines were allowed.

   An additional issue to be dealt with in either case pertains to
   verification of the identity of the IRAS.  If the IRAC were to be
   misdirected somehow, a man in the middle attack could be effected,
   with the obtained password being then used for malicious access to
   the true IRAS.  Note that even a one-time password mechanism offers
   little protection in this case.  In order to avert such an attack,
   the IRAC must possess some certifiable or secret knowledge of the
   IRAS prior to attempting to connect.  Note that in the case where no
   trust relationship exists, this is not possible.

   To summarize, the following are the authentication requirements for
   the IRAS and IRAC:

   IRAS
   ----

      o  machine authentication MUST be provided.

   IRAC
   ----

      o  in cases where no trust relationship exists between the
         accessed network and the system owner, sensitive data SHOULD
         NOT be transmitted in either direction.
      o  in cases where a trust relationship exists between the accessed
         network and the system owner, machine authentication SHOULD be
         supported.
      o  in cases where a trust relationship exists between the accessed
         network and the system owner, a static passphrase MAY be used
         in conjunction with machine-level authentication of the IRAC
         system.
      o  frequent renewal of user authentication MUST occur

3.5.2 Device Configuration Requirements

   None.

3.5.3 Policy  Configuration Requirements

   None.

3.5.4 Auditing Requirements

   The auditing requirements in this scenario are the same as for the
   telecommuter scenario.  Session start/end times must be collected.
   Reliable derivation of session end time requires that the IRAC
   somehow periodically signify that the connection remains active.
   This is implied if the IRAS receives data from the IRAC over the
   connection, but in cases where no data is sent for some period of
   time, a signaling mechanism is required by which the IRAC indicates
   that the connection remains in use.

3.5.5 Intermediary Traversal Requirements

   If the address of the IRAC system is globally routable, and no
   intermediate devices between the IRAC and the IRAS perform NAPT
   operations on the data stream, then there are no additional
   requirements in this regard.  If NAPT operations are performed on the
   data stream, some mechanism must be provided in order to render these
   modifications transparent to the IPsec implementation.

4. Scenario Commonalities

   As we examine the various remote access scenarios, a general set of
   common requirements emerge.  Following is a summary:

   o  Support for user authentication is required in almost all
      scenarios

   o  Machine authentication for the IRAS is required in all scenarios

   o  A mechanism for providing device configuration for the IRAC is
      required in most scenarios.  Such a mechanism must be extensible.

   o  Machine authentication for IRAC is generally only useful when
      combined with user authentication.  Combined user and machine
      authentication is useful in some scenarios.

   o  Dynamic IRAC policy configuration is useful in several scenarios.

   o  Most scenarios require auditing for session start/stop times.

   o  An intermediary traversal mechanism may be required in any of the
      scenarios.

5. Security Considerations

   The topic of this document is secure remote access.  Security
   considerations are discussed throughout the document.

6. References

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

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

   [RADIUS]    Rigney, C., Rubens, A., Simpson, W. and S. Willens,
               "Remote Authentication Dial In User Service (RADIUS)",
               RFC 2865, June 2000.

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

7. Acknowledgements

   The editors would like to acknowledge the many helpful comments of
   Sara Bitan, Steve Kent, Mark Townsley, Bernard Aboba, Mike Horn, and
   other members of the ipsra working group who have made helpful
   comments on this work.

8. Editors' Addresses

   Scott Kelly
   Airespace
   110 Nortech Pkwy
   San Jose CA 95134 USA

   Phone: +1 (408) 941-0500
   EMail: scott@hyperthought.com

   Sankar Ramamoorthi
   Juniper Networks
   1194 North Mathilda Ave
   Sunnyvale CA 94089-1206 USA

   Phone: +1 (408) 936-2630
   EMail: sankarr@juniper.net

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