Network Working Group                                       T. Henderson
Internet-Draft                                        The Boeing Company
Intended status: Informational                               P. Nikander
Expires: October 11, 2007                   Ericsson Research NomadicLab
                                                           April 9, 2007

       Using the Host Identity Protocol with Legacy Applications

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   The Host Identity Protocol (HIP) and architecture proposes to add a
   cryptographic name space for network stack names.  From an
   application viewpoint, HIP-enabled systems support a new address
   family of host identifiers, but it may be a long time until such HIP-
   aware applications are widely deployed even if host systems are
   upgraded.  This informational document discusses implementation and
   API issues relating to using HIP in situations in which the system is
   HIP-aware but the applications are not.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Approaches for supporting legacy applications  . . . . . . . .  5
     3.1.  Using IP addresses in applications . . . . . . . . . . . .  5
     3.2.  Using DNS to map domain names to HIs . . . . . . . . . . .  6
     3.3.  Connecting directly to a HIT . . . . . . . . . . . . . . .  8
     3.4.  Local address management . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

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1.  Introduction

   The Host Identity Protocol (HIP) [1] is an experimental effort in the
   IETF and IRTF to study a new public-key-based name space for use as
   host identifiers in Internet protocols.  Fully deployed, the HIP
   architecture will permit applications to explicitly request the
   system to send packets to another named host by expressing a
   location-independent name of the host when the system call to send
   packets is performed.  However, there will be a transition period
   during which systems become HIP-enabled but applications are not.

   When applications and systems are both HIP-aware, the coordination
   between the application and the system can be straightforward.  For
   example, using the terminology of the widely used sockets Application
   Programming Interface (API), the application can issue a system call
   to send packets to another host by naming it explicitly, and the
   system can perform the necessary name-to-address mapping to assign
   appropriate routable addresses to the packets.  To enable this, a new
   address family for hosts could be defined, and additional API
   extensions could be defined (such as allowing IP addresses to be
   passed in the system call, along with the host name, as hints of
   where to initially try to reach the host).

   This note does not define a native HIP API such as described above.
   Rather, this note is concerned with the scenario in which the
   application is not HIP-aware and a traditional IP-address-based API
   is used by the application.  To use HIP in such a situation, there
   are a few basic possibilities: i) allow applications to use IP
   addresses as before, and provide a mapping from IP address to host
   identifier (and back to IP address) within the system, ii) take
   advantage of domain name resolution to provide the application with
   either an alias for the host identifier or (in the case of IPv6) the
   host identity tag (HIT) itself, and iii) support the use of HITs
   directly (without prior DNS resolution) in place of IPv6 addresses.
   This note describes several variations of the above strategies and
   suggests some pros and cons to each approach.

   When HITs are used (rather than IP addresses) as peer names at the
   system API level, they can provide a type of "channel binding"
   (Section 1.1.6 of [2]) in that the ESP association formed by HIP is
   cryptographically bound to the name (HIT) invoked by the calling

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2.  Terminology

   Host Identity:  An abstract concept applied to a computing platform.

   Host Identifier (HI):  A public key of an asymmetric key pair used as
      a name for a Host Identity.  More details are available in [1].

   Host Identity Tag (HIT):  A 128-bit quantity composed with the hash
      of a Host Identity.  More details are available in [3] and [1].

   Local Scope Identifier (LSI):  A 32- or 128-bit quantity locally
      representing the Host Identity at the IPv4 or IPv6 API.

   Referral:   An event when the application passes what it believes to
      be an IP address to another application instance on another host,
      within its application data stream.  An example is the FTP PORT

   Resolver:  The system function used by applications to resolve domain
      names to IP addresses.

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3.  Approaches for supporting legacy applications

   This section provides examples of how legacy applications, using
   legacy APIs, can operate over a HIP-enabled system and use HIP.  The
   examples are organized by the name used by an application (or
   application user) to name the peer system: an IP address, a domain
   name, or a HIT.  Finally, some local address management issues are

   While the text below concentrates on the use of the sockets connect
   system call, the same argument is also valid for other system calls
   using socket addresses.

   Recent work in the shim6 group has categorized the ways in which
   current applications use IP addresses [4].  These uses include short-
   lived local handles, long-lived application associations, callbacks,
   referrals, and identity comparisons.  Each of the below mechanisms
   has implications on these different uses of IP addresses by legacy

3.1.  Using IP addresses in applications

   Consider the case in which an application issues a "connect(ip)"
   system call to set the default destination to a system named by
   address "ip", but for which we would like to enable HIP to protect
   the communications.  Since the application or user does not (can not)
   indicate a desire to use HIP through the standard sockets API, the
   decision to invoke HIP must be done on the basis of host policy.  For
   example, if an IPsec-like implementation of HIP is being used, a
   policy may be entered into the security policy database that mandates
   to use or try HIP based on a match on the source or destination IP
   address, or other factors.  The mapping of IP address to host
   identifier may be implemented by modifying the host operating system
   or by wrapping the existings sockets API, such as in the TESLA
   approach [5].

   There are a number of ways that HIP could be used in such a scenario.

   Manual configuration:

      Pre-existing SAs may be available due to previous administrative

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      The system could send an I1 to the Responder with an empty value
      for Responder HIT.

   Using DNS to map IP addresses to HIs:

      If the responder has host identifiers registered in the forward
      DNS zone and has a PTR record in the reverse zone, the initiating
      system could perform a reverse+forward lookup to learn the HIT
      associated with the address.  Alternatively, the HIT could be
      stored in some type of HIP name service such as a distributed hash
      table (DHT), keyed by IP address.  Unless secured with DNS
      security extensions, the use of the reverse DNS map is subject to
      well-known security limitations (an attacker may cause an
      incorrect IP address to domain name binding to occur).

   These types of solutions have the benefit of better supporting
   applications that use IP addresses for long-lived application
   associations, callbacks, and referrals.  They have weaker security
   properties than the approaches outlined in Section 3.2 and
   Section 3.3, however, because the binding between host identifier and
   address is weak and not visible to the application or user.  In fact,
   the semantics of the application's "connect(ip)" call may be
   interpreted as "connect me to the system reachable at IP address ip"
   but perhaps no stronger semantics than that.  HIP can be used in this
   case to provide perfect forward secrecy and authentication, but not
   to strongly authenticate the peer at the onset of communications.
   DNS with security extensions (DNSSEC) [6], if trusted, may be able to
   provide some additional initial authentication, but at a cost of
   initial resolution latency.  Note that this usage does not
   necessarily reveal to the user of the legacy application that HIP is
   being used.

   Using IP addresses at the application layer may not provide the full
   potential benefits of HIP mobility support.  It allows for mobility
   if one is able to readdress the existing sockets upon a HIP readdress
   event.  However, mobility will break in the connectionless case when
   an application caches the IP address and repeatedly calls sendto().

3.2.  Using DNS to map domain names to HIs

   In the previous section, it was pointed out that a HIP-enabled system
   might make use of DNS to transparently fetch host identifiers prior
   to the onset of communication.  For applications that make use of
   DNS, the name resolution process is another opportunity to use HIP.
   If host identifiers are bound to domain names (with a trusted DNS)

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   the following are possible:

   Return HIP LSIs and HITs instead of IP addresses:

      The system resolver could be configured to return a Local Scope
      Identifier (LSI) or HIT rather than an IP address, if HIP
      information is available in the DNS that binds a particular domain
      name to a host identifier, and otherwise to return an IP address
      as usual.  The system can then maintain a mapping between LSI and
      host identifier and perform the appropriate conversion at the
      system call interface or below.  The application uses the LSI or
      HIT as it would an IP address.

   Locally use a HIP-specific domain name suffix:

      One drawback to spoofing the DNS resolution is that some
      applications actually may want to fetch IP addresses (e.g.,
      diagnostic applications such as ping).  One way to provide finer
      granularity on whether the resolver returns an IP address or an
      LSI is to distinguish by the presence of a domain name suffix.
      Specifically, if the application requests to resolve
      "www.example.com.hip" (or some similar suffix), then the system
      returns an LSI, while if the application requests to resolve
      "www.example.com", IP address(es) are returned as usual.  Caution
      against the use of domain name suffixes is discussed in [7].

   Since the LSI or HIT is non-routable, a couple of potential hazards
   arise, in the case of referrals, callbacks, and long-lived
   application associations.  First, applications that perform referrals
   may pass the LSI to another system that has no system context to
   resolve the LSI back to a host identifier or an IP address.  Note
   that these are the same type of applications that will likely break
   if used over certain types of network address translators (NATs).
   Second, applications may cache the results of DNS queries for a long
   time, and it may be hard for a HIP system to determine when to
   perform garbage collection on the LSI bindings.  However, when using
   HITs, the security of using the HITs for identity comparison may be
   stronger than in the case of using IP addresses.

   It may be possible for an LSI or HIT to be routable or resolvable,
   either directly or on an overlay.  For example, a special IP address
   that has some location invariance is the identifier-address discussed
   in [8].  A term other than LSI may be needed for these routable
   identifiers, since they would no longer be locally scoped.  When
   using DNS, returning a routable identifier would avoid the
   aforementioned problems with referrals.  However, the cost of

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   routability may be that the hash binding between the routable
   identifier and the host identifier would be weakened, since more bits
   may be allocated to the hierarchical part.

3.3.  Connecting directly to a HIT

   The previous two sections describe the use of IP addresses and LSIs
   as local handles to a host identifier.  A third approach, for IPv6
   applications, is to configure the application to connect directly to
   a HIT (e.g., "connect(HIT)" as a socket call).  Although more
   cumbersome for human users (due to the flat HIT name space) than
   using either IPv6 addresses or domain names, this scenario has
   stronger security semantics, because the application is asking the
   system to send packets specifically to the named peer system.  HITs
   have been defined as Overlay Routable Cryptographic Hash Identifiers
   (ORCHIDs) such that they cannot be confused with routable IP
   addresses; see [3].

   Another challenge with this approach is in actually finding the IP
   addresses to use, based on the HIT.  Some type of HIT resolution
   service would be needed in this case.

   A third challenge of this approach is in supporting callbacks and
   referrals to possibly non-HIP-aware hosts.  However, since most
   communications in this case would likely be to other HIP-aware hosts
   (else the initial HIP associations would fail to establish), the
   problem may otherwise be that the peer host supports HIP but is not
   able to perform HIT resolution for some reason.

3.4.  Local address management

   The previous sections focused mainly on client behavior (HIP
   initiator).  We must also consider the behavior for servers.
   Typically, a server may bind to a wildcard IP address and well-known
   port.  In the case of HIP use with legacy server implementations,
   there are again a few options.  As in Section 3.1 above, the system
   may be configured manually to always, optionally (depending on the
   client behavior), or never use HIP with a particular service, as a
   matter of policy, when the server specifies a wildcard (IP) address.

   When a system API call such as getaddrinfo [9] is used for resolving
   local addresses, it may also return HITs or LSIs, if the system has
   assigned HITs or LSIs to internal virtual interfaces (common in many
   HIP implementations).  The application may use such identifiers as
   addresses in subsequent socket calls.

   Some applications may try to bind a socket to a specific local
   address.  If the local address specified is an IP address, again, the

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   underlying system may be configured to still use HIP.  If the local
   address specified is a HIT (Section 3.3), the system should enforce
   that connections can only come to the specified HIT.  If a system has
   many HITs, an application that binds to a single HIT cannot accept
   connections to the other HITs in the system.  It may be possible for
   a system to specify a special ORCHID value as a local HIT wildcard
   value, if such wildcarding among local HIs is desired.

   When a host has multiple HIs and the socket behavior does not
   prescribe the use of any particular HI as a source identifier, it is
   a matter of local policy as to how to select a HI to serve as a
   source identifier.

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4.  Security Considerations

   In this section we discuss the security of the system in general
   terms, outlining some of the security properties.  However, this
   section is not intended to provide a complete risk analysis.  Such an
   analysis would, in any case, be dependent on the actual application
   using HIP, and is therefore considered out of scope.

   The three outlined scenarios differ considerably in their security
   properties.  There are further differences related to whether DNSSEC
   is used or not, and whether the DNSSEC zones are considered
   trustworthy enough from an application point of view.

   When IP addresses are used to represent the peer system, the security
   properties depend on the the configuration method.  With manual
   configuration, the security of the system is comparable to a non-HIP
   system with similar IPsec policies.  The security semantics of an
   opportunistic key exchange are roughly equal to current non-secured
   IP; the exchange is vulnerable to man-in-the-middle attacks.
   However, the system is less vulnerable to connection hijacking
   attacks.  If the DNS is used, if both maps are secured (or the HITs
   stored in the reverse DNS record) and the client trusts the DNSSEC
   signatures, the system may provide a fairly high security level.
   However, much depends on the details of the implementation, the
   security and administrative practises used when signing the DNS
   zones, and other factors.

   Using the forward DNS to map a domain name into an LSI is a case that
   is closest to the most typical use scenarios today.  If DNSSEC is
   used, the result is fairly similar to the current use of certificates
   with TLS.  If DNSSEC is not used, the result is fairly similar to the
   current use of plain IP, with the exception that HIP provides
   protection against connection hijacking attacks.

   If the application is basing its operations on HITs, the connections
   become automatically secured due to the implicit channel bindings in
   HIP.  That is, when the application makes a connect(HIT) system call,
   the resulting packets will either be sent to a node possessing the
   corresponding private key or the security association will fail to be

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5.  IANA Considerations

   This document has no actions for IANA.

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6.  Acknowledgments

   Jeff Ahrenholz, Gonzalo Camarillo, Alberto Garcia, Miika Komu, Teemu
   Koponen, Julien Laganier, and Jukka Ylitalo have provided comments on
   different versions of this draft.

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7.  Informative References

   [1]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-07
        (work in progress), February 2007.

   [2]  Linn, J., "Generic Security Service Application Program
        Interface Version 2, Update 1", RFC 2743, January 2000.

   [3]  Nikander, P., "An IPv6 Prefix for Overlay Routable Cryptographic
        Hash Identifiers  (ORCHID)", draft-laganier-ipv6-khi-07 (work in
        progress), February 2007.

   [4]  Nordmark, E., "Shim6 Application Referral Issues",
        draft-ietf-shim6-app-refer-00 (work in progress), July 2005.

   [5]  Salz, J., Balakrishnan, H., and A. Snoeren, "TESLA:  A
        Transparent, Extensible Session-Layer Architecture for End-to-
        end Network Services",  Proceedings of USENIX Symposium on
        Internet Technologies and Systems (USITS), pages 211-224,
        March 2003.

   [6]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033,
        March 2005.

   [7]  Faltstrom, P., "Design Choices When Expanding DNS",
        draft-iab-dns-choices-04 (work in progress), October 2006.

   [8]  Bagnulo, M. and E. Nordmark, "Level 3 multihoming shim
        protocol", draft-ietf-shim6-proto-07 (work in progress),
        December 2006.

   [9]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
        Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493,
        February 2003.

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Authors' Addresses

   Tom Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA

   Email: thomas.r.henderson@boeing.com

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420

   Phone: +358 9 299 1
   Email: pekka.nikander@nomadiclab.com

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