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IPSEC Working Group                                          Baiju Patel
INTERNET-DRAFT                                                     Intel
Category: Standards Track                                  Bernard Aboba
<draft-ietf-ipsec-dhcp-12.txt>                                 Microsoft
                                                             Scott Kelly
                                                 RedCreek Communications
                                                             Vipul Gupta
                                                  Sun Microsystems, Inc.

               DHCPv4 Configuration of IPsec Tunnel Mode

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

This document is an Internet-Draft.  Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
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.

Copyright Notice

Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

In many remote access scenarios, a mechanism for making the remote host
appear to be present on the local corporate network is quite useful.
This may be accomplished by assigning the host a "virtual" address from
the corporate network, and then tunneling traffic via IPsec from the
host's ISP-assigned address to the corporate security gateway. In IPv4,
Dynamic Host Configuration Protocol (DHCP) provides for such remote host
configuration. This draft explores the requirements for host
configuration in IPsec tunnel mode, and describes how DHCPv4 may be
leveraged for configuration.






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Table of contents

1   Introduction...........................................  2
1.1 Terminology............................................  2
1.2 Requirements Language..................................  3
2.0 IPsec tunnel mode configuration requirements...........  3
2.1 DHCP configuration evaluation..........................  3
2.2 Summary................................................  4
3.0 Scenario overview......................................  4
3.1 Configuration walk-through.............................  5
4.0 Detailed description...................................  6
4.1 DHCPDISCOVER message processing........................  7
4.2 DHCP Relay behavior....................................  9
4.3 DHCPREQUEST message processing......................... 10
4.4 DHCPACK message processing............................. 10
4.5 Configuration policy................................... 10
5.0 Security Considerations................................ 11
6.0 IANA Considerations.................................... 11
7.0 Intellectual Property Statement........................ 12
8.0 References............................................. 12
9.0 Acknowledgments........................................ 14
10.0 Author's Address...................................... 14
11.0 Appendix - IKECFG evaluation.......................... 15
12.0 Full Copyright Statement ............................. 16

1.  Introduction

In many remote access scenarios, a mechanism for making the remote host
appear to be present on the local corporate network is quite useful.
This may be accomplished by assigning the host a "virtual" address from
the corporate network, and then tunneling traffic via IPsec from the
host's ISP-assigned address to the corporate security gateway. In IPv4,
Dynamic Host Configuration Protocol (DHCP) [3] provides for such remote
host configuration. This draft explores the requirements for host
configuration in IPsec tunnel mode, and describes how DHCPv4 may be
leveraged for configuration.

1.1.  Terminology

This document uses the following terms:

DHCP client
          A DHCP client or "client" is an Internet host using DHCP to
          obtain configuration parameters such as a network address.

DHCP server
          A DHCP server or "server" is an Internet host that returns
          configuration parameters to DHCP clients.



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1.2.  Requirements language

In this document, the key words "MAY", "MUST,  "MUST  NOT",  "optional",
"recommended",  "SHOULD",  and  "SHOULD  NOT",  are to be interpreted as
described in [1].

2.  IPsec tunnel mode configuration requirements

As described in [21], the configuration requirements of a host with an
IPsec tunnel mode interface include the need to obtain an IPv4 address
and other configuration parameters appropriate to the class of host. In
addition to meeting the basic requirements [21], the following
additional capabilities may be desirable:

   a. integration with existing IPv4 address management facilities
   b. support for address pool management
   c. reconfiguration when required
   d. support for fail-over
   e. maintaining security and simplicity in the IKE implementation.
   f. authentication where required

2.1.  DHCP configuration evaluation

Leveraging DHCP for configuration of IPsec tunnel mode meets the basic
requirements described in [21]. It also provides the additional
capabilities described above.

Basic configuration
          In IPv4, leveraging DHCPv4 [3] for the configuration of IPsec
          tunnel mode satisfies the basic requirements described in
          [21].  Since the required configuration parameters described
          in [21] are a subset of those already supported in DHCPv4
          options [5], no new DHCPv4 options are required, and no
          modifications to DHCPv4 [3] are required.

Address management integration
          Since DHCPv4 is widely deployed for address management today,
          reuse of DHCPv4 for IPsec tunnel mode address management
          enables compatibility and integration with existing addressing
          implementations and IPv4 address management software.

Address pool management
          As described in [18], DHCPv4 implementations support
          conditional behavior so that the address and configuration
          parameters assigned can be dependent on parameters included in
          the DHCPDISCOVER. This makes it possible for the security
          gateway to ensure that the remote host receives an IP address
          assignment from the appropriate address pool, such as via the



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          User Class option, described in [16].

Reconfiguration
          DHCP supports the concept of configuration leases, and there
          is a proposal for handling forced reconfiguration [14].

Fail-over support
          When leveraging DHCPv4, configuration and addressing state is
          kept on the DHCP server, not within the IKE implementation. As
          a result, the loss of a tunnel server does not result in the
          loss of configuration and addressing state, thus making it
          easier to support fail-over [8].

Security and simplicity
          Leveraging DHCPv4 also makes it easier to maintain security in
          the IKE implementation since no IKE modifications are required
          to support configuration.

Authentication
          Where DHCPv4 authentication [6] is required, this can be
          supported on an IPsec tunnel mode interface as it would be on
          any other interface.

2.2.  Summary

As described, DHCPv4 [3] meets the IPsec tunnel mode configuration
requirements [21], as well as providing additional capabilities. As
described in the Appendix, IKECFG [13] does not meet the basic
requirements, nor does it provide the additional capabilities. As a
result, DHCPv4 is the superior alternative for IPsec tunnel mode
configuration.

3.  Scenario overview

IPsec [2], [9]-[12] is a protocol suite defined to secure communication
at the network layer between communicating peers. Among many
applications enabled by IPsec, a useful application is to connect a
remote host to a corporate intranet via a security gateway, using IPsec
tunnel mode.  This host is then configured in such a manner so as to
provide it with a virtual presence on the internal network. This is
accomplished in the following manner:

A remote host on the Internet will connect to the security gateway and
then establish an IPsec tunnel to it.  The remote host then interacts
via the IPsec tunnel with a DHCPv4 server which provides the remote host
with an address from the corporate network address space. The remote
host subsequently uses this as the source address for all interactions
with corporate resources. Note that this implies that the corporate



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security gateway continues to recognize the host's original, routable IP
address as the tunnel endpoint. The virtual identity assumed by the
remote host when using the assigned address appears to the corporate
network as though it were situated behind a security gateway bearing the
original routable IP address. All the traffic between the remote host
and the intranet will be carried over the IPsec tunnel via the security
gateway as shown below:

                                       corporate net
 +------------------+                      |
 |    externally    |        +--------+    |   !~~~~~~~~~~!
 |+-------+ visible |        |        |    |   ! rmt host !
 ||virtual| host    |        |security|    |---! virtual  !
 || host  |         |--------|gateway/|    |   ! presence !
 ||       |<================>|  DHCP  |----|   !~~~~~~~~~~!
 |+-------+         |--------| Relay  |    |
 +------------------+   ^    +--------+    |   +--------+
                        |                  |---| DHCPv4 |
                      IPsec tunnel         |   | server |
                      with encapsulated    |   +--------+
                      traffic inside

This scenario assumes that the remote host already has Internet
connectivity and the host Internet interface is appropriately
configured. The mechanisms for configuration of the remote host's
address for the Internet interface are well defined; i.e., PPP IP
control protocol (IPCP), described in [4], DHCPv4, described in [3], and
static addressing. The mechanisms for auto-configuration of the intranet
are also standardized. It is also assumed that the remote host has
knowledge of the location of the security gateway. This can be
accomplished via DNS, using either A, KX [23], or SRV [24] records.

A typical configuration of the remote host in this application would use
two addresses: 1) an interface to connect to the Internet (Internet
interface), and 2) a virtual interface to connect to the intranet
(intranet interface). The IP address of the Internet and intranet
interfaces are used in the outer and inner headers of the IPsec tunnel
mode packet, respectively.

3.1.  Configuration walk-through

The configuration of the intranet interface of the IPsec tunnel mode
host is accomplished in the following steps:

a. The remote host establishes an IKE security association with
   the security gateway in a main mode or aggressive mode exchange.
   This IKE SA then serves to secure additional quick mode IPsec SAs.




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b. The remote host establishes a DHCP SA with the IPsec tunnel mode
   server in a quick mode exchange. The DHCP SA is an IPsec tunnel mode
   SA established to protect initial DHCPv4 traffic between the
   security gateway and the remote host. The DHCP SA MUST
   only be used for DHCP traffic. The details of how this
   SA is set up are described in Section 4.1.

c. DHCP messages are sent back and forth between the remote host and the
   DHCPv4 server. The traffic is protected between the remote host and the
   security gateway using the DHCP SA established in step b. After
   the DHCP conversation completes, the remote host's intranet interface
   obtains an IP address as well as other configuration parameters.

d. The remote host MAY request deletion of the DHCP SA since
   future DHCP messages will be carried over a new IPsec tunnel.
   Alternatively, the remote host and the security gateway
   MAY continue to use the same SA for all subsequent traffic
   by adding temporary SPD selectors in the same manner as is
   provided for name ID types in [2].

e. If a new IPsec tunnel is required, the remote host establishes
   a tunnel mode SA to the security gateway in a quick mode exchange.
   In this case, the new address assigned via DHCPv4 SHOULD be used
   in the quick mode ID.

At the end of the last step, the remote host is ready to communicate
with the intranet using an IPsec tunnel. All the IP traffic (including
future DHCPv4 messages) between the remote host and the intranet are now
tunneled over this IPsec tunnel mode SA.

Since the security parameters used for different SAs are based on the
unique requirements of the remote host and the security gateway, they
are not described in this document. The mechanisms described here work
best when the VPN is implemented using a virtual interface.

4.  Detailed description

This section provides details relating to the messages exchanged during
the setup and teardown of the DHCP SAs.












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4.1.  DHCPDISCOVER message processing

The events begin with the remote host intranet interface generating a
DHCPDISCOVER message. Details are described below:

   FIELD      OCTETS       DESCRIPTION
   -----      ------       -----------

   op            1  Message op code / message type.
                    1 = BOOTREQUEST, 2 = BOOTREPLY
   htype         1  Hardware address type. Set to value (TBD)
                    signifying an IPsec tunnel mode virtual interface.
   hlen          1  Hardware address length
   hops          1  Client sets to zero, optionally used by relay agents
                    when booting via a relay agent.
   xid           4  Transaction ID, a random number chosen by the
                    client, used by the client and server to associate
                    messages and responses between a client and a
                    server.
   secs          2  Filled in by client, seconds elapsed since client
                    began address acquisition or renewal process.
   flags         2  Flags. Broadcast bit MUST be set to zero.
   ciaddr        4  Client IP address; only filled in if client is in
                    BOUND, RENEW or REBINDING state.
   yiaddr        4  'your' (client) IP address.
   siaddr        4  IP address of next server to use in bootstrap;
                    returned in DHCPOFFER, DHCPACK by server.
   giaddr        4  Security gateway interface IPv4 address, used in booting
                    via a relay agent.
   chaddr       16  Client hardware address. Should be unique.
   sname        64  Optional server host name, null terminated string.
   file        128  Boot file name, null terminated string; "generic"
                    name or null in DHCPDISCOVER, fully qualified
                    directory-path name in DHCPOFFER.
   options     var  Optional parameters field.

           Table 1:  Description of fields in the DHCP message

The htype value is set to the value TBD, signifying a virtual IPsec
tunnel mode interface, in order to enable the DHCP server to
differentiate VPN from non-VPN requests.  The chaddr field of the
DHCPDISCOVER MUST include an identifier unique to the virtual subnet.
The client MUST use the same chaddr field in all subsequent messages
within the same DHCPv4 exchange.  In addition, the chaddr SHOULD be
persistent between reboots so that the DHCP server will be able to re-
assign the same address if desired.





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The hlen and chaddr fields SHOULD be determined as follows:

a. If one or more LAN interfaces are available, the hlen and chaddr
   fields SHOULD be determined from the active LAN interface with
   the lowest interface number. If no active LAN interface is
   available, then the parameters SHOULD be determined from the LAN
   interface with the lowest interface number. This enables the
   chaddr to be persistent between reboots, as long as the LAN
   interface hardware is not removed.

b. If there is no LAN interface, the chaddr field SHOULD be
   determined by concatenating x'4000', the IPv4 address of the
   interface supplying network connectivity, and an additional
   octet. The x'4000' value indicates a locally administered
   unicast MAC address, thus guaranteeing that the constructed
   chaddr value will not conflict with a globally assigned value.

   The additional octet (which MAY represent an interface number)
   SHOULD be persistent between reboots, so that the chaddr value
   will be persistent across reboots if the assigned IPv4
   address remains consistent.

If the above prescription is followed, then the chaddr will always be
unique on the virtual subnet provided that the remote host only brings
up a single tunnel to the security gateway.  Where a LAN interface is
available, the chaddr will be globally unique.  When a non-LAN interface
is available and a unique Internet address is assigned to the remote
host, the chaddr will also be globally unique. Where a private IP
address [22] is assigned to a non-LAN interface, it will not be globally
unique.  However, in this case packets will not be routed back and forth
between the remote host and the security gateway unless the external
network and corporate network have a consistent addressing plan.  In
this case the private IP address assigned to the remote host will be
unique on the virtual subnet.

For use in DHCPv4 configuration of IPsec tunnel mode, the client-
identifier option MUST be unique within the virtual subnet and SHOULD be
persistent across reboots.  Possibilities include:

a. The htype/chaddr combination. If assigned as described above,
   this will be unique on the virtual subnet. It will be
   persistent across reboots for a LAN interface. If a
   non-LAN interface is used, it may not be persistent across
   reboots if the assigned IP address changes.

b. The machine FQDN concatenated with an interface number.
   Assuming that the machine FQDN does not conflict with that
   of another machine, this will be unique on the virtual



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   subnet as well as persistent across reboots.

c. The user NAI concatenated with an interface number. Assuming
   that the user is only connected to the VPN at one location,
   this will be unique on the subnet as well as persistent across
   reboots.

In order to deliver the DHCPDISCOVER packet from the intranet interface
to the security gateway, an IKE Phase 1 SA is established between the
Internet interface and the security gateway. A phase 2 (quick mode) DHCP
SA tunnel mode SA is then established. The key lifetime for the DHCP SA
SHOULD be on the order of minutes since it will only be temporary.  The
remote host SHOULD  use an IDci payload of 0.0.0.0/UDP/port 68 in the
quick mode exchange. The security gateway will use an IDcr payload of
its own Internet address/UDP/port 67  The DHCP SA is established as a
tunnel mode SA with filters set as follows:

 From remote host to security gateway: Any to Any, destination: UDP port 67
 From security gateway to remote host: Any to Any, destination: UDP port 68

Note that these filters will work not only for a client without
configuration, but also with a client that has previously obtained a
configuration lease, and is attempting to renew it. In the latter case,
the DHCP SA will initially be used to send a DHCPREQUEST rather than a
DHCPDISCOVER message.  The initial DHCPv4 message (DHCPDISCOVER or
DHCPREQUEST) is then tunneled to the security gateway using the tunnel
mode SA. Note that since the DHCPDISCOVER packet has a broadcast address
destination, the IPsec implementations on both the remote host and the
security gateway must be capable of handling this.

4.2.  DHCP Relay behavior

While other configurations are possible, typically the DHCPv4 server
will not reside on the same machine as the security gateway, which will
act as a DHCPv4 relay, inserting its address in the "giaddr" field. In
this case, the security gateway relays packets between the client and
the DHCPv4 server, but does not request or renew addresses on the
client's behalf.  While acting as a DHCP Relay, the security gateway MAY
implement DHCP Relay load balancing as described in [19].

Since DHCP Relays are stateless, the security gateway SHOULD insert
appropriate information in the DHCP message prior to forwarding to one
or more DHCP servers. This enables the security gateway to route the
corresponding  DHCPOFFER message(s) back to the remote host on the
correct IPsec tunnel, without having to keep state gleaned from the
DISCOVER, such as a table of the xid, chaddr and tunnel.





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If the security gateway maintains a separate subnet for each IPsec
tunnel, then this can be accomplished by inserting the appropriate
interface address in the giaddr field. Alternatively, the security
gateway can utilize the DHCP Relay Agent Information Option [17]. In
this case, the virtual port number of the tunnel is inserted in the
Agent Circuit ID Sub-option (sub-option code 1).

To learn the internal IP address of the client in order to route packets
to it, the security gateway will typically snoop the yiaddr field within
the DHCPACK and plumb a corresponding route as part of DHCP Relay
processing.

Where allocating a separate subnet for each tunnel is not feasible, and
the DHCP server does not support the Relay Agent Information Option,
stateless Relay Agent behavior will not be possible. In such cases,
implementations MAY devise a mapping between the xid, chaddr, and tunnel
in order to route the DHCP server response to the appropriate tunnel
endpoint. Note that this is particularly undesirable in large VPN
servers where the resulting state will be substantial.

4.3.  DHCPREQUEST message processing

After the Internet interface has received the DHCPOFFER message, it
forwards this to the intranet interface after IPsec processing. The
intranet interface then responds by creating a DHCPREQUEST message,
which is tunneled to security gateway using the DHCP SA.

4.4.  DHCPACK message processing

The DHCPv4 server then replies with a DHCPACK or DHCPNAK message, which
is forwarded down the DHCP SA by the security gateway. The remote host
Internet interface then forwards the DHCPACK or DHCPNAK message to the
intranet interface after IPsec processing.

After processing of the DHCPACK, the intranet interface is configured
and the Internet interface can establish a new IPsec tunnel mode SA to
the security gateway.  The remote host may now delete the DHCP tunnel
mode SA. All future DHCP messages sent by the client, including
DHCPREQUEST, DHCPINFORM, DHCPDECLINE, and DHCPRELEASE messages will use
the newly established VPN SA. Similarly, all DHCP messages subsequently
sent by the DHCPv4 server will be forwarded by the security gateway
(acting as a DHCP Relay) using the IPsec tunnel mode SA, including
DHCPOFFER, DHCPACK, and DHCPNAK messages.

It SHOULD be possible to configure the remote host to forward all
Internet-bound traffic through the tunnel. While this adds overhead to
round-trips between the remote host and the Internet, it provides some
added security in return for this, in that the corporate security



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gateway may now filter traffic as it would if the remote host were
physically located on the corporate network.

4.5.  Configuration policy

Several mechanisms can be used to enable remote hosts to be assigned
different configurations. For example, clients may use the User Class
Option [16] to request various configuration profiles.  The DHCPv4
server may also take a number of other variables into account, including
the htype/chaddr;  the host name option; the client-identifier option;
the DHCP Relay Agent Information option [17]; the vendor-class-
identifier option; the vendor-specific information option; or the subnet
selection option [15].

Conditional configuration of clients, described in [18], can be used to
solve a number of problems, including assignment of options based on the
client operating system; assignment of groups of clients to address
ranges subsequently used to determine quality of service; allocation of
special address ranges for remote hosts; assignment of static routes to
clients [20], etc.  As noted in the security considerations, these
mechanisms, while useful, do not enhance security since they can be
evaded by a remote host choosing its own IP address.

5.  Security Considerations

This protocol is secured using IPsec, and as a result the DHCP packets
flowing between the remote host and the security gateway are
authenticated and integrity protected.

However, since the security gateway acts as a DHCP Relay, no protection
is afforded the DHCP packets in the portion of the path between the
security gateway and the DHCP server, unless DHCP authentication is
used.

Note that authenticated DHCP cannot be used as an access control
mechanism. This is because a remote host can always set its own IP
address and thus evade any  security measures based on DHCP
authentication.

As a result, the assigned address MUST NOT be depended upon for
security. Instead, the security gateway can use other techniques such as
instantiating packet filters or quick mode selectors on a per-tunnel
basis.

As described in [17], a number of issues arise when forwarding DHCP
client requests from untrusted sources. These include DHCP exhaustion
attacks, and spoofing of the client identifier option or client MAC
address. These issues can be partially addressed through use of the DHCP



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Relay Information Option [17].

6.  IANA Considerations

This draft requires that an htype value be allocated for use with IPsec
tunnel mode, as described in section 4.1. Note that DHCP relies on the
arp-parameters registry for definition of both the hrd parameter in ARP
and the htype parameter in BOOTP/DHCP. As a result, an assignment in the
arp-parameters registry is required, even though IPsec-DHCP will never
use that parameter for ARP purposes, since conceptually BOOTP/DHCP and
ARP share the arp-parameters registry.

This draft does not create any new number spaces for IANA
administration.

7.  Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to  pertain
to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; neither does it represent that it has made any
effort to identify any such rights.  Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11.  Copies of claims of
rights made available for publication and any assurances of licenses to
be made available, or the result of an attempt made to obtain a general
license or permission for the use of such proprietary rights by
implementors or users of this specification can be obtained from the
IETF Secretariat.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
which may cover technology that may be required to practice this
standard.  Please address the information to the IETF Executive
Director.

8.  References


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

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

[3]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
     1997.



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[4]  McGregor, G., "The PPP Internet Protocol Control Protocol (IPCP)",
     RFC 1332, May 1992.

[5]  Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
     Extensions", RFC 2132, March 1997.

[6]  Droms, R., Arbaugh, W., "Authentication for DHCP Messages",
     Internet draft (work in progress), draft-ietf-dhc-
     authentication-16.txt, January 2001.

[7]  Cobb, S., "PPP Internet Protocol Control Protocol Extensions for
     Name Server Addresses", RFC 1877, December 1995.

[8]  Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S., Rabil, G.,
     Dooley, M., Kapur, A., "DHCP Failover Protocol", Internet draft
     (work in progress), draft-ietf-dhc-failover-08.txt, July 2000.

[9]  Kent,S., Atkinson, R., "IP Authentication Header", RFC 2402,
     November 1998.

[10] Kent,S., Atkinson, R., "IP Encapsulating Security Payload (ESP)",
     RFC 2406, November 1998.

[11] Piper, D., "The Internet IP Security Domain of Interpretation of
     ISAKMP", RFC 2407, November 1998.

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

[13] Dukes, D., Pereira, R., "The ISAKMP Configuration Method", Internet
     draft (work in progress), draft-dukes-ike-mode-cfg-00.txt, October
     2000.

[14] De Schrijver, P., T'Joens, Y., Hublet, C., "Dynamic host
     configuration : DHCP reconfigure extension", Internet draft (work
     in progress), draft-ietf-dhc-pv4-reconfigure-04.txt, April 2001.

[15] Waters, G., "The IPv4 Subnet Selection Option for DHCP", RFC 3011,
     November 2000.

[16] Stump, G., Droms, R., Gu, Y., Vyaghrapuri, R., Demirtjis, A.,
     Beser, B., Privat, J., "The User Class Option for DHCP", RFC 3004,
     November 2000.

[17] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
     January 2001.





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[18] Droms, R., and Lemon, T., The DHCP Handbook, Macmillan,
     Indianapolis, Indiana, 1999.

[19] Volz, B., Gonczi, S., Lemon, T., Stevens, R., "DHC Load Balancing
     Algorithm", RFC 3074, February 2001.

[20] Lemon, T., "The Classless Static Route Option for DHCP", Internet
     draft (work in progress), draft-ietf-dhc-csr-04.txt, February 2001.

[21] Kelly, S., Ramamoorthi, S., "Requirements for IPsec Remote Access
     Scenarios", Internet draft (work in progress), draft-ietf-ipsra-
     reqmts-03.txt, January 2001.

[22] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and E.
     Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918,
     February 1996.

[23] Atkinson, R., "Key Exchange Delegation Record for the DNS", RFC
     2230, November 1997.

[24] Gulbrandsen , A., . Vixie, P., Esibov, L. "A DNS RR for specifying
     the location of services (DNS SRV)", RFC 2782, February 2000.

9.  Acknowledgments

This draft has been enriched by comments from John Richardson and
Prakash Iyer of Intel, Gurdeep Pall and Peter Ford of Microsoft.

10.  Authors' Addresses

Baiju V. Patel
Intel Corp, JF3-206
2511 NE 25th Ave
Hillsboro, OR 97124

Phone: +1 503 264-2422
EMail: baiju.v.patel@intel.com

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

Phone: +1 425 936-6605
EMail: bernarda@microsoft.com

Scott Kelly
RedCreek Communications



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3900 Newpark Mall Road
Newark, CA 94560

Phone: +1 510 745-3969
Email: skelly@redcreek.com

Vipul Gupta
Sun Microsystems, Inc.
901 San Antonio Rd.
Palo Alto, CA 94303

Phone: +1 650 786 3614
Fax: +1 650 786 6445
EMail: vipul.gupta@eng.sun.com

11.  Appendix - IKECFG evaluation

Alternatives to DHCPv4, such as ISAKMP CFG, described in [13], do not
meet the basic requirements described in [21], nor do they provide the
additional capabilities of DHCPv4.

Basic configuration
          While ISAKMP CFG can provide for IP address assignment as well
          as configuration of a few additional parameters such as the
          DNS server and WINS server addresses, the rich configuration
          facilities of DHCPv4 are not supported. Past experience with
          similar configuration mechanisms within PPP IPCP [7] has
          taught us that it is not viable merely to support minimal
          configuration.  Eventually, either much of the functionality
          embodied in the DHCPv4 options [5] is duplicated or support
          for DHCPINFORM [3] will be required.

Address management integration
          Since IKECFG is not integrated with existing IP address
          management facilities, it is difficult to integrate it with
          policy management services that may be dependent on the user
          to IP address binding.

Address pool management
          IKECFG does not provide a mechanism for the remote host to
          indicate a preference for a particular address pool. This
          makes it difficult to support address pool management.

Reconfiguration
          IKECFG does not support the concept of configuration leases or
          reconfiguration.





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Fail-over support
          Since IKECFG creates a separate pool of address state, it
          complicates the provisioning of network utility-class
          reliability, both in the IP address management system and in
          the security gateways themselves.

Security and simplicity
          As past history with PPP IPCP demonstrates, once it is decided
          to provide non-integrated address management and configuration
          facilities within IKE, it will be difficult to limit the
          duplication of effort to address assignment. Instead, it will
          be tempting to also duplicate the configuration,
          authentication and fail-over facilities of DHCPv4. This
          duplication will greatly increase the scope of work,
          eventually compromising the security of IKE.

Authentication
          While IKECFG can support mutual authentication of the IPsec
          tunnel endpoints, it is difficult to integrate IKECFG with
          DHCPv4 authentication [6].  This is because the security
          gateway will not typically have access to the client
          credentials necessary to issue an DHCPv4 authentication option
          on the client's behalf.

As a result, security gateways implementing IKECFG typically request
allocation of an IP address on their own behalf, and then assign this to
the client via IKECFG. Since IKECFG does not support the concept of an
address lease, the security gateway will need to do the renewal itself.
This complicates the renewal process.

Since RFC 2131 [3] assumes that a DHCPREQUEST will not contain a filled
in giaddr field when generated during RENEWING state, the DHCPACK will
be sent directly to the client, which will not be expecting it. As a
result, it is either necessary for the security gateway to add special
code to avoid forwarding such packets, or to wait until REBINDING state.
Since [3] does not specify that the giaddr field cannot be filled in
when in the REBINDING state, the security gateway may put its own
address in the giaddr field when in REBINDING state, thereby ensuring
that it can receive the renewal response without treating it as a
special case.

12.  Full Copyright Statement

Copyright (C) The Internet Society (2001).  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,



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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."

13.  Expiration Date

This memo is filed as <draft-ietf-ipsec-dhcp-12.txt>,  and  expires
November 1, 2001.































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