Network Working Group                                 Steven M. Bellovin
Internet Draft                                        AT&T Labs Research

Expiration Date: April 2003                                 October 2002

               Guidelines for Mandating the Use of IPsec


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
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   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 should and
   should not be specified.

Bellovin                                                        [Page 1]

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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".

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

   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 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, use another
   standardized, well-understood security protocol.  Don't roll your

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 these can be configured manually, in general a key management
   component is used.  Additionally, the decision about whether and how
   to use IPsec is controlled by a policy database of some sort.

Bellovin                                                        [Page 2]

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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, when ESP is used in tunnel mode
   (see section 4.2), and if integrity/authentication is enabled, the IP
   header seen by the source and destination hosts is completely
   protected anyway.

   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.

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
   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

Bellovin                                                        [Page 3]

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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.

   One automated key exchange mechanism is available, Internet Key
   Exchange (IKE) [RFC2409].  A new, simpler version of IKE is currently
   being designed.  A second mechanism, Kerberized Internet Key Exchange
   (KINK) [KINK], is being defined.  It, of course, uses Kerberos, and
   is suitable if and only if a Kerberos infrastructure is available.

   IKE can use public key certificates or shared secrets.  Shared secret
   authentication is simpler, but doesn't scale as well, since each
   endpoint must share a unique secret with every peer with which it can
   communicate, in order to uniquely authenticate these peers.

   IKE also has public key variants.  In most real-world situations,
   these use locally-issued certificates.  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 from an existing PKI
   with IKE.  In practice, this is rare.  Furthermore, there not only is
   no global PKI for the Internet, 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.

   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.

Bellovin                                                        [Page 4]

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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.

   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.

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 do not always support it on a point-to-point basis,
   especially via a stable, well-defined API ur 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 [RFC2041].  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.

Bellovin                                                        [Page 5]

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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
   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 IPsecp 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 is written that defines new selectors for

   Implementation issues are also a concern here.  As before, too many
   vendors have not implemented the full specifications; 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

Bellovin                                                        [Page 6]

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7. Broadcast and Multicast

   Although the designers of IPsec tried to leave room for protection of
   multicast traffic, the design has not yet been completed.  There is,
   as yet, no key management for the general case.  Worse yet, an
   important component of over-the-wire IPsec -- replay protection -- is
   very difficult to implement in a multi-sender situation.  IPsec is
   thus inappropriate for such protocols unless and until suitable key
   management and replay protection mechanisms are defined and available
   in the target domain.  (Single-sender multicast can be supported, if
   suitable key management is available; the MSEC working group is
   developing such a protocol.)

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
   RFCs that rely on IPsec must specify the following:

     (a)  What selectors the initiator of the conversation (the client,
          in client-server architectures) should use?  In particular,
          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 security policy database entry types should be used by
          the responder (i.e., the server) when deciding whether or not
          to accept the IPsec connection request.

     (e)  What form of identification and authentication should be used.

     (f)  Which of the many variants of IKE must be supported.

     (g)  Is suitable IPsec support available in likely configurations
          of the products that would have to employ IPsec?

Bellovin                                                        [Page 7]

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9. Example

   Suppose that the designers of the Border Gateway Protocl (BGP)
   [RFC1771] wished to use IPsec for security, rather than the mechanism
   described in [2385].  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.)

          The issue of selectors is easy.  BGP already runs between
          manully-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.

          Clearly, transport mode is the proper choice.  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].)

     Key Management
          To permit replay detection, an automated key management system
          should be used, most likely IKE.

     Security Policy
          Connections should be accepted only from the designated peer.

          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 in some fashion, since other BGP policies
          are expressed in terms of the peer's IP address.

          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?

Bellovin                                                        [Page 8]

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          The working group and the IESG must make these determinations
          before deciding to use IPsec to protect BGP.

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, Barbara Fraser, Paul Hoffman, and Stephen Kent made
   many useful suggestions.

12. References

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

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

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

   [KINK]    "Kerberized Internet Negotiation of Keys (KINK)", M. Thomas
        and J. Vilhuber, work in progress, 2002.

   [RFC1771] "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter and T.
        Li.  RFC 1771, March 1995.

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

   [RFC2402] "IP Authentication Header", S. Kent and R. Atkinson, RFC
        2401, November 1998.

   [RFC2406] "IP Encapsulating Security Payload (ESP)", S. Kent and R.

Bellovin                                                        [Page 9]

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        Atkinson, RFC 2401, November 1998.

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

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

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

   [SECCONS] "Guidelines for Writing RFC Text on Security
        Considerations", E. Rescorla and B. Korver, work in progress,

13. Author Information

Steven M. Bellovin
AT&T Labs Research
Shannon Laboratory
180 Park Avenue
Florham Park, NJ 07932
Phone: +1 973-360-8656

Bellovin                                                       [Page 10]