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Network Working Group S. Bellovin Request for Comments: 5406 Columbia University BCP: 146 February 2009 Category: Best Current Practice

        Guidelines for Specifying the Use of IPsec Version 2

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
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 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Abstract

 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.

Bellovin Best Current Practice [Page 1] RFC 5406 IPsec Usage February 2009

1. Introduction

 The Security Considerations sections of many Internet Drafts say, in
 effect, "just use IPsec".  While the use of IPsec is sometimes the
 correct security solution, more information is needed to provide
 interoperable security solutions.  In some cases, IPsec is
 unavailable in the likely endpoints.  If IPsec is unavailable to --
 and hence unusable by -- a majority of the users in a particular
 protocol environment, then the specification of IPsec is tantamount
 to saying "turn off security" within this community.  Further, when
 IPsec is available, the implementation may not provide the proper
 granularity of protection.  Finally, if IPsec is available and
 appropriate, the document mandating the use of IPsec needs to specify
 just how it is to be used.
 The goal of this document is to provide guidance to protocol
 designers on the specification of IPsec when it is the appropriate
 security mechanism.  The protocol specification is expected to
 provide realistic, interoperable security.  Therefore, guidance on
 the configuration of the various IPsec databases, such as the
 Security Policy Database (SPD), is often required.
 This document describes how to specify the use of IPsec Version 2
 [RFC2401] including the ESPv2 (Encapsulating Security Payload version
 2) [RFC2406], AHv2 (Authentication Header version 2) [RFC2402], and
 IKEv1 (Internet Key Exchange version 1) [RFC2409].  A separate
 document will describe the IPsec Version 3 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 is based on the knowledge available to the IETF at publication
 time.

2. WARNING

 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 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.

Bellovin Best Current Practice [Page 2] RFC 5406 IPsec Usage February 2009

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, generally 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.

3.1. AH and ESP

 The Authentication Header (AH) [RFC2402] and the Encapsulating
 Security Payload (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
 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.
 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
 endpoints 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.)

Bellovin Best Current Practice [Page 3] RFC 5406 IPsec Usage February 2009

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 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 also 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 [RFC4306], but many existing systems still use
 IKEv1.  This document does not discuss IKEv2 and IPsecv3.  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 [RFC5280] from an
 existing Public Key Infrastructure (PKI) with IKE.  In practice, this
 is rare.  Furthermore, not only is there no global PKI covering most

Bellovin Best Current Practice [Page 4] RFC 5406 IPsec Usage February 2009

 Internet endpoints, there probably never will be.  Designing a
 structure that 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 should almost
 always be used with locally issued certificates as opposed to
 certificates from an existing PKI.
 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. Application Programming Interface (API)

 It is, in some sense, a misnomer to speak of the API as a part of
 IPsec since this piece is missing on many systems.  To the extent
 that APIs exist, they aren't standardized.  The problem is simple:
 there is no portable way (and often no way at all) for an application
 to request IPsec protection, or to tell if it was used for given
 inbound packets or connections.
 There are additional problems:
 o  Applications rarely have 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.

Bellovin Best Current Practice [Page 5] RFC 5406 IPsec Usage February 2009

 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
    identity.
 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.

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
 category.
 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-user systems, where it may not be possible to use different
 certificates to distinguish between traffic from two different users.

Bellovin Best Current Practice [Page 6] RFC 5406 IPsec Usage February 2009

 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 IPsec; in
 particular, it becomes impossible to apply protection at any finer
 grain than "destination host".  Thus, traffic embedded in a Layer 2
 Tunneling Protocol (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, the Stream Control Transmission
 Protocol (SCTP) [RFC4960] 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
 employed.

Bellovin Best Current Practice [Page 7] RFC 5406 IPsec Usage February 2009

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.
 [RFC5374] 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
 interest.

8. Specifying IPsec

 Despite all of the caveats given above, it may still be appropriate
 to use IPsec in particular situations.  The range of choices makes 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 should the initiator of the conversation (the
     client, in client-server architectures) 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 user IDs (i.e., it 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 and certificates.
 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 IPsec or access any resource?  (This problem can
     appear with any security service, of course.)

Bellovin Best Current Practice [Page 8] RFC 5406 IPsec Usage February 2009

 h.  Which of the many variants of IKE must be supported?  Main mode?
     Aggressive mode?
     Note that there 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

 Let us now work through an example based on these guidelines.  We
 will use the Border Gateway Protocol (BGP) [RFC4271] to show how to
 evaluate and specify the use of IPsec for transmission security,
 rather than the mechanism described in [RFC2385].  Note carefully
 that we are not saying that IPsec is an appropriate choice here.
 Rather, we are demonstrating the necessary examination and
 specification process.  Also 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.
 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.

Bellovin Best Current Practice [Page 9] RFC 5406 IPsec Usage February 2009

 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 Autonomous System (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.

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, including most notably availability.  For
 example, using IPsec does little to defend against denial-of-service
 attacks; in some situations, i.e., on CPU-limited systems, it may
 contribute to the attacks.  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.

Bellovin Best Current Practice [Page 10] RFC 5406 IPsec Usage February 2009

12. References

12.1. Normative References

 [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.
 [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.
            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.
 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.
 [RFC5374]  Weis, B., Gross, G., and D. Ignjatic, "Multicast
            Extensions to the Security Architecture for the Internet
            Protocol", RFC 5374, November 2008.

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.

Bellovin Best Current Practice [Page 11] RFC 5406 IPsec Usage February 2009

 [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.
 [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.
 [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.

Bellovin Best Current Practice [Page 12] RFC 5406 IPsec Usage February 2009

 [RFC4535]  Harney, H., Meth, U., Colegrove, A., and G. Gross,
            "GSAKMP: Group Secure Association Key Management
            Protocol", RFC 4535, June 2006.
 [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
            RFC 4960, September 2007.

Author's Address

 Steven M. Bellovin
 Columbia University
 1214 Amsterdam Avenue
 MC 0401
 New York, NY  10027
 US
 Phone: +1 212 939 7149
 EMail: bellovin@acm.org

Bellovin Best Current Practice [Page 13]

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