Subject: Re: draft-ietf-ipsra-reqmts-02.txt

From: "Scott G. Kelly" <[email-masked]>
Date: Fri, 24 Nov 2000 17:18:29 -0800

(posting from home...)

This was apparently too late (by an hour or so) for publication, so I
have to resubmit it December 10. In any event, the text is available in
my previous post.

Scott

---

From: "Scott G. Kelly" <[email-masked]>
Date: Fri, 24 Nov 2000 16:13:54 -0800

Dear ID Editor,

Please publish the attached draft as draft-ietf-ipsra-reqmts-02.txt.
This is a work item of the ipsec remote access working group (ipsra),
and updates an existing draft. Please announce the publication to the
ipsra mailing list at [email-masked].

Thanks!

Scott

IPsec Remote Access  Working Group                 Scott Kelly, RedCreek
INTERNET-DRAFT                                 Sankar Ramamoorthi, Nexsi
draft-ietf-ipsra-reqmts-02.txt                            November, 2000

             Requirements for IPsec Remote Access Scenarios

Status of This Memo

   This document is an Internet Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026]. Internet Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and working groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at

   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Comments on this document should be sent to the IETF IPsec remote
   access discussion list ([email-masked]).

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.

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                             Table of Contents

1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
1.4 Document Organization  . . . . . . . . . . . . . . . . . . . . .   5
2. Overview  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
2.1 Endpoint Authentication  . . . . . . . . . . . . . . . . . . . .   6
2.1.1 Machine-Level Authentication . . . . . . . . . . . . . . . . .   7
2.1.2 User-Level Authentication  . . . . . . . . . . . . . . . . . .   8
2.1.3 Combined User/Machine Authentication . . . . . . . . . . . . .   8
2.1.4 Remote Access Authentication . . . . . . . . . . . . . . . . .   9
2.1.5 Compatibility With Legacy Mechanisms . . . . . . . . . . . . .  10
2.2 Remote Host Configuration  . . . . . . . . . . . . . . . . . . .  10
2.3 Security Policy Configuration  . . . . . . . . . . . . . . . . .  12
2.4 Auditing . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
2.5 Intermediary Traversal . . . . . . . . . . . . . . . . . . . . .  13
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
3.1 Telecommuters (Dialup/DSL/Cablemodem)  . . . . . . . . . . . . .  14
3.1.1 Endpoint Authentication Requirements . . . . . . . . . . . . .  15
3.1.2 Device Configuration Requirements  . . . . . . . . . . . . . .  17
3.1.3 Policy Configuration Requirements  . . . . . . . . . . . . . .  18
3.1.4 Auditing Requirements  . . . . . . . . . . . . . . . . . . . .  19
3.1.5 Intermediary Traversal Requirements  . . . . . . . . . . . . .  20
3.2 Corporate to Remote Extranet . . . . . . . . . . . . . . . . . .  20
3.2.1 Authentication Requirements  . . . . . . . . . . . . . . . . .  21
3.2.2 Device Configuration Requirements  . . . . . . . . . . . . . .  21
3.2.3 Policy Configuration Requirements  . . . . . . . . . . . . . .  22
3.2.4 Auditing Requirements  . . . . . . . . . . . . . . . . . . . .  22
3.2.5 Intermediary Traversal Requirements  . . . . . . . . . . . . .  22
3.3 Extranet Laptop to Home Corporate Net  . . . . . . . . . . . . .  23
3.3.1 Authentication Requirements  . . . . . . . . . . . . . . . . .  23
3.3.2 Device Configuration Requirements  . . . . . . . . . . . . . .  24
3.3.3 Policy Configuration Requirements  . . . . . . . . . . . . . .  24
3.3.4 Auditing Requirements  . . . . . . . . . . . . . . . . . . . .  25
3.3.5 Intermediary Traversal Requirements  . . . . . . . . . . . . .  25
3.4 Extranet Desktop to Home Corporate Net . . . . . . . . . . . . .  26
3.4.1 Authentication Requirements  . . . . . . . . . . . . . . . . .  26
3.4.2 Device Configuration Requirements  . . . . . . . . . . . . . .  27
3.4.3 Policy Configuration Requirements  . . . . . . . . . . . . . .  27
3.4.4 Auditing Requirements  . . . . . . . . . . . . . . . . . . . .  27
3.4.5 Intermediary Traversal Requirements  . . . . . . . . . . . . .  27
3.5 Remote Dialup Laptop (Road Warrior) Access . . . . . . . . . . .  28
3.6 Public System to Corporate Network . . . . . . . . . . . . . . .  28
3.6.1 Authentication Requirements  . . . . . . . . . . . . . . . . .  28
3.6.2 Device Configuration Requirements  . . . . . . . . . . . . . .  30
3.6.3 Policy  Configuration Requirements . . . . . . . . . . . . . .  30
3.6.4 Auditing Requirements  . . . . . . . . . . . . . . . . . . . .  30

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3.6.5 Intermediary Traversal Requirements  . . . . . . . . . . . . .  30
4. Scenario Commonalities  . . . . . . . . . . . . . . . . . . . . .  30
5. Security Considerations . . . . . . . . . . . . . . . . . . . . .  31
6. Editors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31
7. References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
8. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  32
9. Full Copyright Statement  . . . . . . . . . . . . . . . . . . . .  32
Appendix A: Change Log . . . . . . . . . . . . . . . . . . . . . . .  32

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1. Introduction

   In the past, remote access has typically been characterized by dial-
   up users accessing the corporate network via the Public Switched
   Telephone Network (PSTN), with the dial-up connection terminating at
   a Network Access Server (NAS) within the corporate 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].

   In some cases, PPP-based tunneling mechanisms have been used to
   provide remote access by allowing the user to first dial into a local
   ISP account, and then tunnel an additional PPP connection over the
   Internet into the target network. 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
   corporate 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 to an 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

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     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 corporate network. An alternative term
     is "Security Gateway".

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

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

   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.

1.4 Document Organization

   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:

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                                         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

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   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, so that we cannot be certain in many instances 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

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   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 to some degree. 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,

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   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 from authenticating the user's machine.  It requires
   some form of user input, and often 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

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   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 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, but there are other mechanisms as well. An ideal
   IPsec remote access solution might utilize the components of the
   underlying user authentication 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
       systems such as RADIUS

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

     o if legacy 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

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   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 (local) corporate network:

                                          corporate 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

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   address. An additional internal corporate 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 corporate 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 corporate 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 corporate 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 corporate 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
   corporate network, while we wish to restrict Bob's access to specific
   segments. One way to accomplish this would be to statically assign
   internal corporate "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

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   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 network the IRAS
   protects. 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

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   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. Such modifications render the use
   of the AH protocol problematic. In the case of ESP, if encryption is
   employed the UDP/TCP port fields are unreadable (and 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 own home
       systems to access corporate resources

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

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

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

     o road warriors using their own laptop systems to access
       corporate resources via an arbitrary ISP dialup connection

     o remote users using a borrowed system (e.g. an airport
       kiosk) to access corporate 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

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   access the internet from the comfort of their homes or hotel rooms,
   and using this internet connection, access the resources of a
   corporate 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

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   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 3 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. Third, there is the
   possibility that the connection is being passively monitored prior to
   being secured, effectively eliminating the data confidentiality which
   the connection is supposed to provide.

   Mitigation of the first threat, the possiblity 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.

   Mitigation of the third threat, passive monitoring,  requires that it
   not be possible to monitor the unprotected data externally via system
   emissions, and that the system be running a sufficiently secure
   operating system to protect the unsecured portion of the data stream
   against malicious code.  This is perhaps the most difficult of the
   listed threats to deal with, and it requires that the IRAS make some
   assumptions about the IRAC system.

   In order to provide assurance regarding mitigation of this (third)

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   threat, a sufficiently secured operating system must have access to a
   machine credential, and this must be used to authenticate the system
   once system integrity has been verified. If further assurance
   regarding the user's identity is required in addition to this, it is
   possible that the IRAC system could provide such an assurance
   mechanism as part of its login procedure (prior to establishment of
   the remote access connection).  However, it is important to note that
   the IRAS derives no assurance from this, independently of the larger
   security system of which it is a part. If the IRAS requires
   independent assurance of the user identity, then some additional
   user-level authentication mechanism (e.g. a time-variant credential
   of some sort) must be combined with the IRAC machine authentication.

   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 machine authentication MUST be provided if
       protection from passive monitoring is desired. Further, such
       protection requires a that the IRAC be physically secured, and
       running a secure OS
     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 corporate 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 corporate address space. In the first case, there are
   no device configuration requirements which are not already satisfied

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

   In this case, the IRAC requires, at minimum, assignment of a
   corporate IP address. 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 SHOULD 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 corporate 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
   non-corporate packets leaving the IRAC to first traverse the tunnel
   to the corporate network, where they may be subjected to corporate
   policy.  A second approach which carries a bit less overhead entails
   modifying the IRAC's policy configuration to reflect that of the
   corporation during the time the IRAC is connected to the corporate
   network. In this case, traffic is not forced to loop through the
   corporate site prior to exiting or entering the IRAC. This requires

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   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 corporate
   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.

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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.

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   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

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   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 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 or
   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

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   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

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   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

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   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 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 or
     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.

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     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
   ----

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     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 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 or
     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

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     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 Remote Dialup Laptop (Road Warrior) Access

     This is a very common remote access scenario, and is virtually
     indistinguishable from the telecommuter scenario, except that the
     connections are typically dialup only, and hence, short-lived.

     Refer to section 3.1.1 for details.

3.6 Public System to Corporate Network

     This scenario entails a traveling user connecting to the corporate
     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 publically 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.

3.6.1 Authentication Requirements

     There are two variations to this scenario. In the first, no trust
     relationship exists between the corporate 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 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.

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     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 corporation. 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

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       MAY be used in conjunction with machine-level authentication
       of the IRAC system.
     o frequent renewal of user authentication MUST occur

3.6.2 Device Configuration Requirements

     None.

3.6.3 Policy  Configuration Requirements

     None.

3.6.4 Auditing Requirements

     The auditing requirements in this scenario are the same as for the
     telecommuter scenario. Session start/end times must 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 or
     time, a signaling mechanism is required by which the IRAC indicates
     that the connection remains in use.

3.6.5 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

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       be extensible.

     o Machine authentication for IRAC is generally only useful
       when combined with user authentication. Combined user and
       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. Editors' Addresses

   Scott Kelly
   RedCreek Communications
   3900 Newpark Mall Road
   Newark, CA 94560 USA
   email: [email-masked]
   Telephone: +1 (510) 745-3969

   Sankar Ramamoorthi
   Nexsi
   1959 Concourse Drive
   San Jose, CA 95131 USA
   E-mail: [email-masked]
   Telephone: +1 (408) 579-5718

7. 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]    C. Rigney, A. Rubens, W. Simpson, S. Willens,
               "Remote Authentication Dial In User Service

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               (RADIUS)", RFC2138

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

8. Acknowledgements

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

9. 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.

Appendix A: Change Log

   00 to 01:

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   o delete mobility requirements
   o add accounting requirements
   o extensively modify discussion of endpoint authentication, and
     machine vs. user authentication
   o delete roaming/wireless users and user-to-user connections from
     Scenarios bullet list
   o Add discussion of trojan horse applications to telecommuter scenario
   o add statement about encouraging migration to PKI-based systems to
     legacy compatibility section.
   o clarified language in section 2.3 (Security Policy Configuration)

   01 to 02:

   o add n-factor authentication to general requirements
   o change "accounting" to "auditing"
   o delete incoming/outgoing octet counts from auditing requirements
   o added intermediary traversal requirements
   o numerous general edits for clarity

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