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Network Working Group                                      Bernard Aboba
INTERNET-DRAFT                                                 Microsoft
Category: Informational
21 November 2000

            IPSEC Remote Access Protocol Evaluation Criteria

1.  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 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

To view the list Internet-Draft Shadow Directories, see

The distribution of this memo is unlimited.

2.  Copyright Notice

Copyright (C) The Internet Society (2000).  All Rights Reserved.

3.  Abstract

This document describes criteria for evaluation of IPSEC Remote Access
(IPSRA) protocols. In particular, this document focuses on criteria
relevant to voluntary tunneling.

4.  Introduction

This document describes criteria for evaluation of IPSEC Remote Access
(IPSRA) protocols. In particular, this document focuses on criteria
relevant to voluntary tunneling.

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5.  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].

Please note that the criteria specified in this document are to be used
in evaluating protocol submissions.  As such, the requirements language
refers to capabilities of these protocols; the protocol documents will
specify whether these features are required, recommended, or optional.
For example, requiring that a protocol support confidentiality is NOT
the same thing as requiring that all protocol traffic be encrypted.

A protocol submission is not compliant if it fails to satisfy one or
more of the must or must not requirements for the capabilities that it
implements.  A protocol submission that satisfies all the must, must
not, should and should not requirements for its capabilities is said to
be "unconditionally compliant"; one that satisfies all the must and must
not requirements but not all the should or should not requirements for
its protocols is said to be "conditionally compliant."

6.  Overview

Requirements may be divided into several categories:

   Multi-protocol requirements
   Configuration requirements
   Overhead requirements
   PKI transition requirements
   Authentication requirements
   Accounting and auditing requirements
   Security requirements

6.1.  Multiprotocol requirements

With the widespread acceptance of IP, the usage of alternative protocols
such as IPX, SNA, NetBEUI, and AppleTalk is declining rapidly. Thus
while multi-protocol networks are still common today, this is not
expected to be the case within five years.  For those networks requiring
multi-protocol support, alternatives are widely available, as described
in [19] and [26]. As a result, while an IPSEC remote access protocol MAY
provide multi-protocol support, this is at best a minor objective, and
protocol designers would be wise to optimize for IP.

Note that in order to provide for multi-protocol support, it is not
necessary to encapsulate GRE or PPP within IPSEC. Rather, IPSEC can
readily support multi-protocol tunneling via inclusion of the non-IP
protocol number in the "Next Header" field of the AH or ESP header.

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Since multi-protocol use is declining rapidly, support for multi-
protocol configuration such as what was provided in PPP, described in
[16]-[18], is not a requirement for an IPSEC remote access protocol.
Thus, it is feasible for an IPSEC remote access solution to focus solely
on IP configuration mechanisms.

6.2.  Configuration requirements

The configuration requirements of a host with an IPSEC remote access
interface are similar to those of a host needing to configure any other
kind of interface. These include the need:

   1. to obtain an IP address and other configuration parameters
      appropriate to the class of host
   2. to reconfigure when required
   3. to authenticate where required
   4. to support address pool management
   5. to support fail-over
   6. to integrate with existing IP address management
      facilities such as DHCP
   7. to maintain security and simplicity in the IKE implementation.

A configuration facility for IPSEC remote access MUST provide for both
IP address assignment as well as configuration for a wide variety of
parameters, such as those supported in DHCP [3]. Note that rich
configuration facilities have already proved necessary in wide variety
of cases outside of conventional LAN configuration.  For example, in the
case of PPP, IPCP, described in [4], was used to provide for IP address
assignment. However, it was found that additional configuration
parameters were necessary, so that non-standard extensions, described in
[7] were developed. Rather than continuing down the road towards
duplicating existing DHCP functionality, it was decided that it would be
preferable to support DHCPINFORM capabilities, described in [3].

A configuration facility for IPSEC remote access SHOULD support the
concept of a configuration lease, and MAY support the ability to force
reconfiguration of the client, in a manner such as that described in
[14]. Configuration leases permit recovery of unused IP address space,
and therefore result in more optimal use of addresses. The ability to
force reconfiguration of the client can be useful in a number of
circumstances, such as renumbering.

A configuration facility for IPSEC remote access MUST support the
ability to authenticate the configuration conversation.  As noted in
[6], a number of security threats exist in IP address management, and so
authentication may be desirable in order to mitigate these threats.
Alternatively, it may be desirable to bind an IP address to a user or
machine ID for the purposes of supporting policy-based networking. Note

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that the need for authentication is particularly strong where forced
reconfiguration is supported.  Where DHCP authentication is implemented
for these purposes, IPSEC remote access addressing SHOULD permit
integration with DHCP authentication so as to permit universal coverage.

A configuration facility for IPSEC remote access SHOULD provide the
ability to be able to obtain an IP address within the appropriate
address pool. Today it is common to use distinct address pools for the
purposes of service differentiation, and therefore the ability to obtain
an address from the appropriate pool may be critical to the operation of
the network. Methods for pool assignment include the user identity
option as well as the user class option specified in [21].

A configuration facility for IPSEC remote access SHOULD NOT preclude the
ability to provide for fail-over capabilities in terms of IP address
management. With enterprise customers increasingly investing in IP
address management systems with fail-over capabilities, creating
additional pockets of addressing state creates the the need to provide
those additional pockets with fail-over capabilities equivalent to those
provided in DHCP fail-over, described in [8].

A configuration facility for IPSEC remote access MUST NOT compromise the
simplicity or security of IKE, described in [12].  Since IKE is a key
element of the Internet security architecture, it is critical to
maintain inter-operability as well as the ability to predict and analyze
the behavior of implementations.

6.3.  Overhead requirements

Given the increasing popularity of IPSEC remote access, it is inevitable
that this technology will be used in a wide variety of applications,
including transport of voice and video. These applications typically
involve transport of small payloads, and as a result the level of
overhead introduced by an IPSEC remote access protocol is of concern.

It is possible that existing header compression schemes may be operating
within an IPSEC remote access environment. These schemes are used to
reduce the size of IP headers encapsulated within the IPSEC remote
access headers.  For example, the IP header compression scheme described
in [23], or IPSEC minimal encapsulation, described in [22] may be in
use. Where voice or video payloads are being carried within RTP, the
IP/UDP/RTP compression scheme described in [24] may prove useful.

For the purpose of the comparisons that follow, the packet to be
encapsulated by the IPSEC remote access protocol may be assumed to have
been previously compressed by one of the above methods.  For comparison
purposes, overhead is calculated for a 700 octet packet as well as a 64
octet packet,  In this calculation we assume an IP header of 20 octets,

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a UDP header of 8 octets, an IPSEC header of 32 octets (corresponding to
MD5 IPSEC AH) and (where applicable) an L2TP header of 8 octets.
Overhead percentage is calculated as overhead/packet size.

As described below, for the case of the 64 octet packet, all known
methods result in excessive overhead. Thus an IPSEC remote access
protocol MAY provide additional mechanisms to reduce overhead in these

6.3.1.  IPSEC tunnel mode

In IPSEC tunnel mode, an IP header is used inside the IP and IPSEC
headers. Thus, an IP packet would require an additional 52 octets of
overhead for the addition of IP and IPSEC headers.

For a 64 octet packet, this results in overhead of 81.8%. For a 700
octet packet, overhead is 7.4%.

6.3.2.  IPSEC/L2TP

L2TP, defined in [19], involves encapsulation of PPP within UDP and
L2TP. Where IPSEC is used to provide security, 68 octets of overhead are

For a 64 octet packet, this results in overhead of 106.3%. For a 700
octet packet, overhead is 9.7%.

6.3.3.  IPSEC/L2TP with header compression

L2TP header compression, defined in [20], involves encapsulation of an
L2TPHC header, which can be as small as a single octet, within IP.  Thus
where IPSEC is used to provide security, 53 octets of overhead are

For a 64 octet packet, this results in overhead of 82.8%. For a 700
octet packet, overhead is 7.6%.

6.3.4.  IPSEC/L2TP with multiple PPP encapsulation

Using L2TP it is possible to include more than one PPP encapsulated
frame within a single L2TP packet. This results in a reduction in
overhead and attendant serialization time, at the expense of additional
delay required to accumulate additional packets. Where the serialization
time saved is greater than the additional delay, the tradeoff will be
worthwhile. This is typically the case at bandwidths of less than 100

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For the purposes of calculating overhead, let us assume that two 64
octet packets are bundled together. This results in overhead of 53.2%.
For a 700 octet packet, overhead is 4.9%.

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6.4.  Summary

A summary of the overhead computed for each method is given below:

|                       | Overhead        | Overhead        |
|   Protocol            | w/ 64 octet     | w/ 700 octet    |
|                       | packet          | packet          |
|                       |                 |                 |
|   IPSEC/L2TP          |    106.3%       |       9.7%      |
|                       |                 |                 |
|                       |                 |                 |
|   IPSEC/L2TP w/HC     |     82.8%       |       7.6%      |
|                       |                 |                 |
|                       |                 |                 |
|   IPSEC tunnel mode   |     81.8%       |       7.4%      |
|                       |                 |                 |
|                       |                 |                 |
|   IPSEC/L2TP w/       |     53.2%       |       4.9%      |
|   multiple PPP enc.   |                 |                 |
|                       |                 |                 |

6.5.  PKI transition requirements

An IPSEC remote access protocol MUST provide customers with the ability
to deploy the solution securely without requiring that clients implement
user certificates.  An IPSEC remote access solution MUST support user
authentication in the case where client and server machine certificates
are present and SHOULD support user authentication in the case where
only the server has a machine certificate, but the client does not have
any certificate.

The transition to a PKI may be divided into several steps:

a. Support for PKI on servers. This typically requires that
   machine certificates be deployed on the servers, along with
   appropriate certificate authorities and stores. It also requires
   that clients be capable of verifying the server's certificate
   against a current Certificate Revocation List (CRL). Since this
   will often require a client software upgrade, the work to
   transition to server certificates is comparable to the work
   required to deploy SSL/TLS-capable Web server and certificate-
   capable browsers.

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   Note that while some client software support for PKI must be
   assumed, in this step, it is not necessary for the clients to
   obtain their own machine or user certificates. Thus it is possible
   for the clients to continue to authenticate using only legacy
   methods during this phase of the transition.

b. Support for machine certificates on clients. This requires
   that machine certificates be deployed on clients. Completion
   of the previous step (a) often requires a client upgrade, which
   will typically also include support for client certificates. If
   the  infrastructure for machine auto-enrollment has also been put
   in place as part of the server PKI rollout, then there may not be
   much additional work required to complete this step, above what
   was already required for the previous step. Note that if the
   client only supports a machine certificate, then this may imply
   the use of a non-PKI method for user authentication in addition to
   the machine certificate.

c. Support for user certificates. This requires that user
   certificates be provided to users. Since storage of user
   certificates on the machine creates new vulnerabilities,
   smart-cards are typically be used to store the user certificates.
   Thus, a smart-card rollout may often be a prerequisite to
   deployment of user certificates. This in turn may require
   integration of smart-card provisioning with the existing
   identification system, such as the distribution of combined
   employee badge/smart-cards. Since this step may require
   considerable work above and beyond the tasks required to carry out
   transition steps a and b, support for legacy authentication
   methods will likely be required at least until this transition
   step is complete.

Thus, an IPSEC remote access solution MUST support transition step b,
and SHOULD support transition step a.

6.6.  Authentication requirements

An IPSEC remote access protocol MUST support user authentication and
MUST offer compatibility with legacy authentication methods commonly in
use today.  This includes PPP authentication methods, described in [40].
Note that since RADIUS, defined in [15], merely serves to encapsulate an
authentication method, it does not itself constitute an authentication

Since support for a variety of authentication methods is already
available within existing IETF standard frameworks such as such as
GSS_API [31], an IPSEC remote access protocol SHOULD support the GSS_API
authentication framework and MAY support other frameworks such as SASL

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[30] or EAP [29].

Support for existing frameworks is important since creating new
frameworks increases the complexity of developing new authentication
methods and while also complicating their deployment.  Rather than
fragmenting the market while increasing the cost of development, a more
useful approach is to unify the authentication frameworks, making it
possible for developers to create authentication modules that will be
usable in a variety of applications.

Work toward support of GSS_API within other frameworks is already
underway. For example, SASL, described in [30], already supports
negotiation of GSS_API as a method, as noted in [38]. Similarly, work on
permitting GSS_API to be used for initial authentication [41], has
enabled GSS_API support within EAP.

6.7.  Accounting and auditing requirements

An IPSEC remote access protocol SHOULD support accounting and auditing.
In order to support accounting, it is necessary to be able to accurately
determine the duration of the IPSEC remote access session. Without a
standardized IPSEC keep-alive this can prove difficult to achieve since
security associations may remain in place after a client disconnect or

6.8.  Security requirements

6.8.1.  Identity protection

An IPSEC remote access protocol MUST provide for identity protection.
Since IKE Aggressive Mode exposes the user identity, an IPSEC remote
access protocol MUST NOT rely on aggressive mode.  For example, an IPSEC
remote access solution MUST protect against man in the middle attacks
without requiring the use of Aggressive Mode.

Note that main mode with signature authentication is secure against
passive attacks but not active attacks.

6.8.2.  Denial of service attacks

Denial of service attacks can be characterized based on the capabilities
of the attacker:

   1. Attackers that can send and receive IP-address spoofed
      messages corresponding to one real party in the attack.
   2. Attackers that can only send IP-address spoofed messages
      corresponding to one real party in the attack (but not receive).

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   3. Attackers that can gain physical access to the device
      being attacked.

An IPSEC remote access protocol MUST provide protection against attacks
in categories 1 and 2. Attackers in category 3 are able to deny service
without having to attack on the wire protocols, so that there is little
that can be done to deter them within an IPSEC remote access protocol.

6.8.3.  Man-in-the-middle attacks

Use of pre-shared keys in main mode is vulnerable to man-in-the-middle
attacks when used for IPSEC remote access. This occurs since in main
mode it is necessary for SKEYID_e to be used prior to the receipt of the
identification payload. Therefore the selection of the pre-shared key
may only be based on information contained in the IP header. However, in
remote access situations, dynamic IP address assignment is the rule, so
that it is typically not possible to identify the required pre-shared
key based on the IP address.

Thus when pre-shared keys are used in IPSEC remote access, the same pre-
shared key is shared by a group of users and is no longer able to
function as an effective shared secret.  In this situation, neither the
client nor the server identifies itself during IKE phase 1; it is only
known that both parties are a member of the group with knowledge of the
pre-shared key. This permits anyone with access to the group pre-shared
key to act as a man-in-the-middle.

Note that this vulnerability does not occur in aggressive mode since the
identity payload is sent earlier in the exchange.  However, when
aggressive mode is used the user identity is exposed and this may be

7.  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

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

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[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-11.txt, June 1999.

[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-04.txt, June 1999.

[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] Pereira, R., Anand, S., Patel, B., "The ISAKMP Configuration
     Method", Internet draft (work in progress), draft-ietf-ipsec-
     isakmp-mode-cfg-05.txt, August 1999.

[14] De Schrijver, P., T'Joens, Y., "Dynamic host configuration : DHCP
     reconfigure extension", Internet draft (work in progress), draft-
     schrijvp-dhcpv4-reconfigure-00.txt, June 1999.

[15] Rigney, C., Rubens, A., Simpson, W., Willens, S., "Remote
     Authentication Dial In User Service (RADIUS)", RFC  2138, April,

[16] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
     RFC 1661, July 1994.

[17] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
     "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.

[18] Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, January 1994.

[19] Townsley, W.M., Valencia, A., Rubens, A., Pall, G.S., Zorn, G., and
     Palter, B., "Layer Two Tunneling Protocol L2TP", RFC 2661, August

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[20] Valencia, A. J., "L2TP Header Compression (``L2TPHC'')", Internet
     draft (work in progress), draft-ietf-l2tpext-l2tphc-03.txt, October

[21] Stump, G., Droms, R., Gu, Y., Vyaghrapuri, R., Demirtjis, A.,
     Beser, B., Privat, J., "The User Class Option for DHCP", Internet
     draft (work in progress), draft-ietf-dhc-userclass-04.txt, October

[22] Perkins, C., "Minimal Encapsulation Within IP", RFC 2004, October

[23] Degermark, M., Nordgren, B., Pink, S., "IP Header Compression", RFC
     2507, February 1999.

[24] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
     Low-Speed Serial Links", RFC 2508, February 1999.

[25] Engan, M., Casner, S. and C. Bormann, "IP Header Compression for
     PPP", RFC 2509, February 1999.

[26] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W., Zorn,
     G., "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, July

[27] Rigney, C., "RADIUS  Accounting", RFC 2139, April 1997.

[28] Rigney, C., Willens, S., Calhoun, P., "RADIUS Extensions", draft-
     ietf-radius-ext-04.txt, Internet Draft (work in progress), May

[29] Blunk, L., Vollbrecht, J., "PPP Extensible Authentication Protocol
     (EAP)", RFC 2284, March 1998.

[30] Myers, J., "Simple Authentication and Security Layer (SASL)", FC
     2222, October 1997.

[31] Linn, J., "Generic Security Service Application Program Interface,
     Version 2", RFC 2078, January 1997.

[33] Kohl, J., Neuman, C., "The Kerberos Network Authentication Service
     (V5)", RFC 1510, September 1993.

[34] Neuman, B. C., Ts'o, T., "Kerberos: An Authentication Service for
     Computer Networks", IEEE Communications, 32(9):33-38, September

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[35] Tung, B., Neuman, B. C., Hur, M., Medvinsky, A., Medvinsky, S.,
     Wray, J., Trostle, J., "Public Key Cryptography for Initial
     Authentication in Kerberos", Internet draft (work in progress),
     draft-ietf-cat-kerberos-pk-init-08.txt, May 1999.

[36] Baize, E., Pinkas., D., "The Simple and Protected GSS-API
     Negotiation Mechanism", RFC 2478, December 1998.

[37] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964,
     June 1996.

[38] Myers, J., "SASL GSSAPI mechanisms", Internet draft (work in
     progress), draft-ietf-cat-sasl-gssapi-00.txt, March 1999.

[39] Piper, D., "A GSS-API Authentication Mode for IKE", Internet draft
     (work in progress), draft-ietf-ipsec-isakmp-gss-auth-02.txt,
     December 1998.

[40] Lloyd, B., Simpson, W., "PPP Authentication Protocols", RFC 1334,
     October 1992.

[41] Swift, M., Trostle, J., "Initial Authentication and Pass Through
     Authentication Using Kerberos V5 and the GSS-API (IAKERB)",
     Internet draft (work in progress), draft-ietf-cat-iakerb-04.txt,
     October 1999.

[42] Pereira, R., Beaulieu, S., "Extended Authentication within
     ISAKMP/Oakley", Internet-draft (work in progress), draft-ietf-
     ipsec-isakmp-xauth-05.txt, September, 1999.

8.  Security Considerations

This document, being a requirements document, does not have any security
concerns. The security requirements on protocols to be evaluated using
this document are described throughout the document.

9.  IANA Considerations

This draft does not create any new number spaces for IANA

10.  Acknowledgments

Thanks to Scott Kelly of Redcreek Communications and Ari Huttunen of
Data Fellows for useful discussions of this problem space.

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11.  Authors' Addresses

Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

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

12.  Full Copyright Statement

Copyright (C) The Internet Society (2000).  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

13.  Expiration Date

This memo is filed as <draft-aboba-ipsra-req-02.txt>, and  expires
August 1, 2001.

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