Network Working Group                                        S. Bellovin
Internet-Draft                                       Columbia University
Intended status: Best Current                            October 7, 2007
Expires: April 9, 2008

          Guidelines for Mandating the Use of IPsec Version 2

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Security Considerations sections of many Internet Drafts say, in
   effect, "just use IPsec".  While this is sometimes correct, more
   often it will leave users without real, interoperable security
   mechanisms.  This memo offers some guidance on when IPsec Version 2
   should and should not be specified.

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1.  Introduction

   The Security Considerations sections of many Internet Drafts say, in
   effect, "just use IPsec".  While this is sometimes correct, more
   often it will leave users without real, interoperable security
   mechanisms.  IPsec is often unavailable in the likely endpoints.
   Even if it is available, it may not provide the proper granularity of
   protection.  Finally, if it is available and appropriate, the
   document mandating it needs to specify just how it is to be used.

   Recall that the goal is realistic, interoperable security.
   Specifying, as the only security mechanism, a configuration which is
   unavailable to -- and hence unusable by -- a majority of the user
   community is tantamount to saying "turn off security".

   This document describes how to specify the use of IPsec Version 2
   [RFC2401], including ESPv2 [RFC2406], AHv2 [RFC2402], and IKEv1
   [RFC2409].  A separate document will describe the IPsec Version3
   suite [RFC4301] [RFC4302] [RFC4303] [RFC4306].

   For further guidance on security considerations (including discussion
   of IPsec), see [RFC3552].

   NOTE: Many of the arguments below relate to the capabilities of
   current implementations of IPsec.  These may change over time; this
   advice based on the knowledge available to the IETF at publication


   The design of security protocols is a subtle and difficult art.  The
   cautions here about specifying the use of IPsec should NOT be taken
   to mean that that you should invent your own new security protocol
   for each new application.  If IPsec is a bad choice, using another
   standardized, well-understood security protocol will almost always
   give the best results for both implementation and deployment.
   Security protocols are very hard to design; rolling out a new one
   will require extensive theoretical and practical work to confirm its
   security properties and will incur both delay and uncertainty.

3.  The Pieces of IPsec

   IPsec is composed of a number of different pieces.  These can be used
   to provide confidentiality, integrity, and replay protection; though
   some of these can be configured manually, in general a key management
   component is used.  Additionally, the decision about whether and how

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   to use IPsec is controlled by a policy database of some sort.

3.1.  AH and ESP

   The Authentication Header (AH) [RFC2402] and the Encapsulating
   Security Protocol (ESP) [RFC2406] are the over-the-wire security
   protocols.  Both provide (optional) replay protection.  ESP typically
   is used to provide confidentiality (encryption), integrity and
   authentication for traffic.  ESP also can provide integrity and
   authentication without confidentiality, which makes it a good
   alternative to AH in most cases where confidentiality is not a
   required or desired service.  Finally, ESP can be used to provide
   confidentiality alone, although this is not recommended [Bell96].

   The difference in integrity protection offered by AH is that AH
   protects portions of the preceding IP header, including the source
   and destination address.  However, if ESP is used in tunnel mode (see
   Section 3.2) and integrity/authentication is enabled, the IP header
   seen by the source and destination hosts is completely protected

   AH can also protect those IP options that need to be seen by
   intermediate routers, but must be intact and authentic when delivered
   to the receiving system.  At this time, use (and existence) of such
   IP options is extremely rare.

   If an application requires such protection, and if the information to
   be protected cannot be inferred from the key management process, AH
   must be used.  (ESP is generally regarded as easier to implement;
   however, virtually all IPsec packages support both.)  If
   confidentiality is required, ESP must be used.  It is possible to use
   AH in conjunction with ESP, but this combination is rarely required.

   All variants of IPsec have problems with NAT boxes -- see [RFC3715]
   for details -- but AH is considerably more troublesome.  In
   environments where there is substantial likelihood that the two end-
   points will be separated by a NAT box -- this includes almost all
   services involving user-to-server traffic, as opposed to server-to-
   server traffic -- NAT traversal [RFC3948] should be mandated and AH
   should be avoided.  (Note that [RFC3948] is for ESP only, and cannot
   be used for AH.)

3.2.  Transport and Tunnel Mode

   AH and ESP can both be used in either transport mode or tunnel mode.
   In tunnel mode, the IPsec header is followed by an inner IP header;
   this is the normal usage for Virtual Private Networks (VPN), and it
   is generally required whenever either end of the IPsec-protected path

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   is not the ultimate IP destination, e.g., when IPsec is implemented
   in a firewall, router, etc.

   Transport mode is preferred for point-to-point communication, though
   tunnel mode can be used for this purpose.

3.3.  Key Management

   Any cryptographic system requires key management.  IPsec provides for
   both manual and automatic key management schemes.  Manual key
   management is easy; however, it doesn't scale very well.  Also,
   IPsec's replay protection mechanisms are not available if manual key
   management is used.  The need for automatic key exchange is discussed
   in more detail in [RFC4107].

   The primary automated key exchange mechanism for IPsec is the
   Internet Key Exchange (IKE) [RFC2409].  A new, simpler version of IKE
   has been approved, but there are few if any deployments [RFC4306].  A
   second mechanism, Kerberized Internet Negotiation of Keys (KINK)
   [RFC4430], has been defined.  It, of course, uses Kerberos, and is
   suitable if and only if a Kerberos infrastructure is available.

   If a decision to use IKE is made, the precise mode of operation must
   be specified as well.  IKE can be used in main mode or aggressive
   mode; both support digital signatures, two different ways of using
   public key encryption, and shared secrets for authentication.

   Shared secret authentication is simpler; however, it doesn't scale as
   well in many-to-many communication scenarios, since each endpoint
   must share a unique secret with every peer with which it can
   communicate.  Note, though, that using shared secrets in IKE is far
   preferable to manual keying.

   In most real-world situations where public key modes of IKE are used,
   locally-issued certificates are employed.  That is, the administrator
   of the system or network concerned will issue certificates to all
   authorized users.  These certificates are useful only for IPsec.

   It is sometimes possible to use certificates [RFC3280] from an
   existing public key infrastructure (PKI) with IKE.  In practice, this
   is rare.  Furthermore, there not only is no global PKI covering most
   Internet endpoints, there probably never will be one.  Designing a
   structure which assumes such a PKI is a mistake.  In particular,
   assuming that an arbitrary node will have an "authentic" certificate,
   issued by a mutually trusted third party and vouching for that node's
   identity, is wrong.  Again, such a PKI does not and probably will not
   exist.  Public key IKE is generally a good idea, but almost always
   with locally-issued certificates.

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   Note that public key schemes require a substantial amount of
   computation.  Protocol designers should consider whether or not such
   computations are feasible on devices of interest to their clientele.
   Using certificates roughly doubles the number of large
   exponentiations that must be performed, compared with shared secret
   versions of IKE.

   Today, even low-powered devices can generally perform enough
   computation to set up a limited number of security associations;
   concentration points, such as firewalls or VoIP servers, may require
   hardware assists, especially if many peers are expected to create
   security associations at about the same time.

   Using any automated key management mechanism can be difficult when
   trying to protect low-level protocols.  For example, even though
   [RFC2461] specified the use of IPsec to protect IPv6 Neighbor
   Discovery, it was impossible to do key management: nodes couldn't use
   IKE because it required IP-level communication, and that isn't
   possible before Neighbor Discovery associations are set up.

3.4.  Applications Program Interface (API)

   It is, in some sense, a misnomer to speak of the API as a part of
   IPsec, since that piece is missing on many systems.  To the extent
   that it does exist, it isn't standardized.  The problem is simple: it
   is difficult or impossible to request IPsec protection, or to tell if
   was used for given inbound packets or connections.

   There are additional problems:
   o  Applications have rarely access to such APIs.  Rather, IPsec is
      usually configured by a system or network administrator.
   o  Applications are unable to verify that IPsec services are being
      used underneath.
   o  Applications are unaware of the specific identities and properties
      of the protected channel provided by IPsec.  For instance, the
      IPsec key management mechanisms may be aware of the identity and
      authorization of the peer, but this information cannot be used by
      the application, nor linked to application-level decisions, such
      as access to resources reserved to the entity identified by this

   Router- or firewall-based IPsec implementations pose even greater
   problems, since there is no standardized over-the-wire protocol for
   communicating this information from outboard encryptors to hosts.

   By contrast, higher-layer security services such as TLS are able to
   provide the necessary control and assurance.

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4.  Availability of IPsec in Target Devices

   Although IPsec is now widely implemented, and is available for
   current releases of most host operating systems, it is less available
   for embedded systems.  Few hubs, network address translators, etc.,
   implement it, especially at the low end.  It is generally
   inappropriate to rely on IPsec when many of the endpoints are in this

   Even for host-to-host use, IPsec availability (and experience, and
   ease of use) has generally been for VPNs.  Hosts that support IPsec
   for VPN use frequently do not support it on a point-to-point basis,
   especially via a stable, well-defined API or user interface.

   Finally, few implementations support multiple layers of IPsec.  If a
   telecommuter is using IPsec in VPN mode to access an organizational
   network, he or she may not be able to employ a second level of IPsec
   to protect an application connection to a host within the
   organization.  (We note that such support is, in fact, mandated by
   Case 4 of Section 4.5 of [RFC2401].  Nevertheless, it is not widely
   available.)  The likelihood of such deployment scenarios should be
   taken into account when deciding whether or not to mandate IPsec.

5.  Endpoints

   [RFC2401] describes many different forms of endpoint identifier.
   These include source addresses (both IPv4 and IPv6), host names
   (possibly as embedded in X.500 certificates), and user IDs (again,
   possibly as embedded in a certificate).  Not all forms of identifier
   are available on all implementations; in particular, user-granularity
   identification is not common.  This is especially a concern for
   multi-users systems, where it may not be possible to use different
   certificates to distinguish between traffic from two different users.

   Again, we note that the ability to provide fine-grained protection,
   such as keying each connection separately, and with per-user
   credentials, was one of the original design goals of IPsec.
   Nevertheless, only a few platforms support it.  Indeed, some
   implementations do not even support using port numbers when deciding
   whether or not to apply IPsec protection.

6.  Selectors and the SPD

   Section 4.4 of [RFC2401] describes the Security Policy Database (SPD)
   and "selectors" used to decide what traffic should be protected by
   IPsec.  Choices include source and destination addresses (or address

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   ranges), protocol numbers (i.e., 6 for TCP and 17 for UDP), and port
   numbers for TCP and UDP.  Protocols whose protection requirements
   cannot be described in such terms are poorer candidates for IPsec; in
   particular, it becomes impossible to apply protection at any finer
   grain than "destination host".  Thus, traffic embedded in an L2TP
   [RFC2661] session cannot be protected selectively by IPsec above the
   L2TP layer, because IPsec has no selectors defined that let it peer
   into the L2TP packet to find the TCP port numbers.  Similarly, SCTP
   [RFC2960] did not exist when [RFC2401] was written; thus, protecting
   individual SCTP applications on the basis of port number could not be
   done until a new document was written [RFC3554] that defined new
   selectors for IPsec, and implementations appeared.

   Furthermore, in a world that runs to a large extent on dynamically
   assigned addresses and often uses dynamically assigned port numbers
   as well, an all-or-nothing policy for VPNs can work well; other
   policies, however, can be difficult to create in any usable form.

   The granularity of protection available may have side-effects.  If
   certain traffic between a pair of machines is protected by IPsec,
   does the implementation permit other traffic to be unprotected, or
   protected by different policies?  Alternatively, if the
   implementation is such that it is only capable of protecting all
   traffic or none, does the device have sufficient CPU capacity to
   encrypt everything?  Note that some low-end devices may have limited
   secure storage capacity for keys, etc.

   Implementation issues are also a concern here.  As before, too many
   vendors have not implemented the full specification; too many IPsec
   implementations are not capable of using port numbers in their
   selectors.  Protection of traffic between two hosts is thus on an all
   or nothing basis when these non-compliant implementations are

7.  Broadcast and Multicast

   Although the designers of IPsec tried to leave room for protection of
   multicast traffic, a complete design wasn't finished until much
   later.  As such, many IPsec implementations do not support multicast.
   [I-D.ietf-msec-ipsec-extensions] describes extensions to IPsec to
   support it.  Other relevant documents include [RFC3830], [RFC3547],
   and [RFC4535],

   Because of the delay, protocol designers who use multicast should
   consider the availability of these extensions in target platforms of

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8.  Mandating IPsec

   Despite all of the caveats given above, it may still be appropriate
   to use IPsec in particular situations.  The range of choices make it
   mandatory to define precisely how IPsec is to be used.  Authors of
   standards documents that rely on IPsec must specify the following:

   a.  What selectors the initiator of the conversation (the client, in
       client-server architectures) should use?  What addresses, port
       numbers, etc., are to be used?
   b.  What IPsec protocol is to be used: AH or ESP?  What mode is to be
       employed: transport mode or tunnel mode?
   c.  What form of key management is appropriate?
   d.  What form of identification should be used?  Choices include IP
       address, DNS name with or without a user name, and X.500
       distinguished name.
   e.  If the application server will switch userids (i.e., is a login
       service of some sort) and user name identification is used, is a
       new security association negotiated that utilizes a user-
       granularity certificate?  If so, when?
   f.  What form of authentication should be used?  Choices include pre-
       shared secrets, certificates, and (for IKEv2) an EAP exchange
   g.  How are the participants authorized to perform the operations
       that they request?  For instance, are all devices with a
       certificate from a particular source allowed to use any
       application with with IPsec or access any resource?  (This
       problem can appear with any security service, of course.)
   h.  Which of the many variants of IKE must be supported?  Main mode?
       Aggressive mode?

       Note that the are two different versions of IKE, IKE and IKEv2.
       IKEv2 is simpler and cleaner, but is not yet widely available.
       You must specify which version of IKE you require.
   i.  Is suitable IPsec support available in likely configurations of
       the products that would have to employ IPsec?

9.  Example

   Suppose that the designers of the Border Gateway Protocol (BGP)
   [RFC4271] wished to use IPsec for security, rather than the mechanism
   described in [RFC2385].  Does it meet these criteria?  (Note that the
   deeper security issues raised by BGP are not addressed by IPsec or
   any other transmission security mechanism.  See [Kent00a] and
   [Kent00b] for more details.)

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   Selectors        BGP runs between manually-configured pairs of hosts
                    on TCP port 179.  The appropriate selector would be
                    the pair of BGP speakers, for that port only.  Note
                    that the router's "loopback address" is almost
                    certainly the address to use.
   Mode             Transport mode would be the proper choice if IPsec
                    were used.  The information being communicated is
                    generally not confidential, so encryption need not
                    be used.  Either AH or ESP can be used; if ESP is
                    used, the sender's IP address would need to be
                    checked against the IP address asserted in the key
                    management exchange.  (This check is mandated by
                    [RFC2401].)  For the sake of interoperability,
                    either AH or ESP would need to be specified as
                    mandatory to implement.
   Key Management   To permit replay detection, an automated key
                    management system should be used, most likely IKE.
                    Again, the RFC author should pick one.
   Security Policy  Connections should be accepted only from the
                    designated peer.  (Note that this restriction
                    applies only to BGP.  If the router -- or any IPsec
                    host -- runs multiple services with different
                    security needs, each such service requires its own
                    security policy.)
   Authentication   Given the number of BGP-speaking routers used
                    internally by large ISPs, it is likely that shared
                    key mechanisms are inadequate.  Consequently,
                    certificate-based IKE must be supported.  However,
                    shared secret mode is reasonable on peering links,
                    or (perhaps) on links between ISPs and customers.
                    Whatever scheme is used, it must tie back to a
                    source IP address or AS number in some fashion,
                    since other BGP policies are expressed in these
                    terms.  If certificates are used, would they use IP
                    addresses or AS numbers?  Which?
   Availability     For this scenario, availability is the crucial
                    question.  Do likely BGP speakers -- both backbone
                    routers and access routers -- support the profile of
                    IPsec described above?  Will use of IPsec, with its
                    attendant expensive cryptographic operations, raise
                    the issue of new denial of service attacks?  The
                    working group and the IESG must make these
                    determinations before deciding to use IPsec to
                    protect BGP.

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10.  Security Considerations

   IPsec provides transmission security and simple access control only.
   There are many other dimensions to protocol security that are beyond
   the scope of this memo.  Within its scope, the security of any
   resulting protocol depends heavily on the accuracy of the analysis
   that resulted in a decision to use IPsec.

11.  Acknowledgments

   Ran Atkinson, Lakshminath Dondeti, Barbara Fraser, Paul Hoffman, Russ
   Housley, Stephen Kent, Eric Fleischman, assorted members of the IESG,
   and a plethora of others have made many useful suggestions.

12.  References

12.1.  Normative References

              Weis, B., "Multicast Extensions to the Security
              Architecture for the Internet  Protocol",
              draft-ietf-msec-ipsec-extensions-06 (work in progress),
              July 2007.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [RFC3554]  Bellovin, S., Ioannidis, J., Keromytis, A., and R.
              Stewart, "On the Use of Stream Control Transmission
              Protocol (SCTP) with IPsec", RFC 3554, July 2003.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.

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              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

12.2.  Informative References

   [Bell96]   Bellovin, S., "Problem Areas for the IP Security
              Protocols", Proc. Sixth Usenix Security Symposium pp. 205-
              214, 1996.

   [Kent00a]  Kent, S., Lynn, C., and K. Seo, "Secure Border Gateway
              Protocol (Secure-BGP)", IEEE Journal on Selected Areas in
              Communications 18:4 pp. 582-592, 2000.

   [Kent00b]  Kent, S., Lynn, C., Mikkelson, J., and K. Seo, "Secure
              Border Gateway Protocol (Secure-BGP) -- Real World
              Performance and Deployment Issues", Proc. Network and
              Distributed System Security Symposium (NDSS), 2000.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

   [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
              (NAT) Compatibility Requirements", RFC 3715, March 2004.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.

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              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4430]  Sakane, S., Kamada, K., Thomas, M., and J. Vilhuber,
              "Kerberized Internet Negotiation of Keys (KINK)",
              RFC 4430, March 2006.

   [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
              "GSAKMP: Group Secure Association Key Management
              Protocol", RFC 4535, June 2006.

Author's Address

   Steven M. Bellovin
   Columbia University
   1214 Amsterdam Avenue
   MC 0401
   New York, NY  10027

   Phone: +1 212 939 7149

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