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Network Working Group                                        S. BellovinRequest for Comments: 5406                           Columbia UniversityBCP: 146                                                   February 2009Category: Best Current PracticeGuidelines for Specifying the Use of IPsec Version 2Status 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 toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   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 20091.  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 20093.  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 (seeSection 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 20093.2.  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 mostBellovin                 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 ofSection 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 SPDSection 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 20097.  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 200912.  References12.1.  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.orgBellovin                 Best Current Practice                 [Page 13]