Network Working Group                                       T. Henderson
Internet-Draft                                        The Boeing Company
Intended status: Informational                               P. Nikander
Expires: October 11, 2007 May 21, 2008                       Ericsson Research NomadicLab
                                                           April 9,
                                                                 M. Komu
                                      Helsinki Institute for Information
                                                              Technology
                                                       November 18, 2007

       Using the Host Identity Protocol with Legacy Applications
                     draft-ietf-hip-applications-01
                     draft-ietf-hip-applications-02

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The

   This document is an informative overview of how legacy applications
   can be made to work with the Host Identity Protocol (HIP) and architecture (HIP).  HIP
   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 HIP-aware applications are widely deployed even if host systems
   are upgraded.  This informational document discusses implementation
   and
   API Application Programming Interface (API) issues relating to using
   HIP in situations in which the system is HIP-aware but the
   applications are not. not, and is intended to aid implementors and early
   adopters in thinking about and locally solving systems issues
   regarding the incremental deployment of HIP.

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  7
     3.3.  Connecting directly to a HIT . . . . . . . . . . . . . . .  8
     3.4.  Local address management . . . . . . . . . . . . . . . . .  8  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11 12
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12 13
   7.  Informative References . . . . . . . . . . . . . . . . . . . . 13 14
   Appendix A.  Changes from previous versions  . . . . . . . . . . . 15
     A.1.  From version-01 to version-02 (current)  . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 15 18

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 would permit applications to explicitly request the
   system to send packets to another named host by expressing a
   location-independent 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.  This
   informational document does not propose normative specification or
   even suggest that different HIP implementations use more uniform
   methods for legacy application support, but is intended instead to
   aid implementors and early adopters in thinking about and solving
   systems issues regarding the incremental deployment of HIP.

   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 document does not define a native HIP API such as described
   above.  Rather, this note document 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 document describes several variations of the above strategies
   and
   suggests some pros and cons to points out tradeoffs with 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 Encapsulating Security Payload (ESP) association formed by
   HIP is cryptographically bound to the name (HIT) invoked by the
   calling application.

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] [2] 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
      command.

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

3.  Approaches for supporting legacy applications

   This section provides examples of how legacy applications, using
   legacy APIs, can operate over on 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
   discussed.

   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

   Applications use IP addresses [4].  These uses include short-
   lived as short-lived local handles, long-lived 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 applications.

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 can not (can not) indicate a
   desire to use HIP through the standard sockets API, API when IP addresses
   are used, the decision to invoke HIP must be done on the basis of
   host policy.  For example, if when an IPsec-like IPsec-based 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 existing sockets API, such
   as in the TESLA approach [5]. [3].

   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
      action.
      action, or a binding between an IP address and a HIT could be
      stored in a configuration file or database.

   Opportunistically:

      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 Initiator
      could perform a reverse+forward lookup to learn the HIT associated
      with the address.  Alternatively,  Although the HIT could be
      stored in some type of approach should work under normal
      circumstances, it has not been tested to verify that there are no
      recursion or bootstrapping issues, particularly if HIP name service such as a distributed hash
      table (DHT), keyed by IP address. is used to
      secure the connection to the DNS servers.  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

   Using the opportunistic mode or using DNS in the above fashion can
   cause additional setup delays compared to using plain IP.  For
   opportunistic mode, a host must wait to learn whether the peer is
   HIP-capable, although the delays may be mitigated in some
   implementations by sending initial packets (e.g., TCP SYN) in
   parallel to the HIP I1 packet.  For DNS lookups, there are resolution
   latencies.

   Solutions preserving the use of solutions IP addresses in the applications have
   the benefit of better supporting support for applications that use IP addresses
   for long-lived application associations, callbacks, and referrals.  They have weaker security
   properties than the approaches outlined referrals,
   although it should be noted that applications are discouraged from
   using IP addresses in Section 3.2 this manner due to the frequent presence of
   NATs [4] 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 [5] could be able used to
   provide some additional initial authentication, but at a cost
   authenticate the bindings between IP address and host identifier, if
   the necessary DNSSEC records were available and trusted.

   The legacy application is unaware of
   initial resolution latency.  Note that this usage does not
   necessarily reveal to HIP and cannot therefore notify
   the user when the application uses HIP.  However, the operating
   system can notify the user of the legacy usage of HIP through a user agent.
   Further, it is possible for the user agent to name the network
   application that caused a HIP-related event.  This way, the user is
   aware when he or she is using HIP even though the legacy network
   application is
   being used. not.

   Using IP addresses at the application layer may not provide the full
   potential benefits of HIP mobility support.  It allows for mobility
   if one the system is able to readdress the existing long-lived, connected sockets upon
   a HIP readdress event.  However, as in current systems, mobility will
   break in the connectionless case case, when an application caches the IP
   address and repeatedly calls sendto(). sendto(), or in the case of TCP when the
   system later opens additional sockets to the same destination.

   Section 4.1.6 of the base HIP protocol specification [1] states that
   implementations that learn of HIT-to-IP address bindings through the
   use of HIP opportunistic mode must not enforce those bindings on
   later communications sessions.  This implies that when IP addresses
   are used by the applications, systems that attempt to
   opportunistically set up HIP must not assume that later sessions to
   the same address will communicate with the same host.

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) DNS),
   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  This technique has been used in
      overlay networking experiments such as the Internet Indirection
      Infrastructure (i3).

   Locally use a HIP-specific domain name suffix: prefix:

      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). ping, or processes that generate
      system log files).  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. prefix.
      Specifically, if the application requests to resolve
      "www.example.com.hip" "HIP-
      www.example.com" (or some similar suffix), prefix string), 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 a prefix rather than suffix is recommended, and the use of domain name suffixes a
      string delimiter that is discussed not a dot (".") is also recommended, to
      reduce the likelihood that such modified DNS names are mistakenly
      treated as names rooted at a new top-level domain.

   Fetch HIP records transparently:

      A third option would be for the system to opportunistically query
      for HIP records in parallel to other DNS resource records, and to
      temporarily cache the HITs returned with a DNS lookup, indexed by
      the IP addresses returned in [7]. the same entry, and pass the IP
      addresses up to the application as usual.  If an application
      connects to one of those IP addresses within a short time after
      the lookup, initiate a base exchange using the cached HITs.  The
      benefit is that this removes the uncertainty/delay associated with
      opportunistic HIP, because the DNS record suggests that the peer
      is HIP-capable.

   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 through an overlay.  For example, a special IP address
   that has some location invariance is the identifier-address discussed overlay, in [8].  A term other than LSI may be needed for these routable
   identifiers, since they which case it would no longer be locally scoped.  When
   using DNS, returning a routable identifier would avoid the
   aforementioned problems with referrals.
   preferable for applications to handle such names instead of IP
   addresses.  However, the cost such networks are out of
   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. scope of this document.

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. identifiers.  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  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]. [2].

   This approach also has a few challenges.  Using HITs can be more
   cumbersome for human users (due to the flat HIT name space) than
   using either IPv6 addresses or domain names, 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 HIP-
   aware hosts (else the initial HIP associations would fail to
   establish), the resulting referral 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 binds 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] [6] 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.

   In the case when resolvers can return multiple destination
   identifiers for an application, it may be configured that some of the
   identifiers can be HIP-based identifiers, and the rest can be IPv4 or
   IPv6 addresses.  The system resolver may return HIP-based identifiers
   in front of the list of identifiers when the underlying system and
   policies support HIP.  An application processing the identifiers
   sequentially will then first try a HIP-based connection and only then
   other non-HIP based connections.  However, certain applications may
   launch the connections in parallel.  In such a case, the non-HIP
   connections may succeed before HIP connections.  Based on local
   system policies, a system may disallow such behaviour and return only
   HIP-based identifiers when they are found from DNS.

   Some applications may try to bind a socket to a specific local
   address.
   address, or may implement server-side access control lists based on
   socket calls such as getsockname() and getpeername() in the C-based
   socket APIs.  If the local address specified is an IP address, again,
   the 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 to the local application can only come arrive 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 local identifier, it is a
   matter of local policy as to how to select a HI to serve as a
   source local
   identifier.

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 systems that bind to a wildcard may face
   problems when multiple HITs or LSIs are defined.  These problems are
   not intended specific to provide HIP per se, but are also encountered in non-HIP
   multihoming scenarios with applications not designed for multihoming.

   As an example, consider a complete risk analysis.  Such client application that sends an
   analysis would, in any case, be dependent on the actual UDP
   datagram to a server that is bound to a wildcard.  The server
   application receives the packet using HIP, recvfrom() and is therefore considered out of scope. sends a response
   using sendto().  The three outlined scenarios differ considerably in their security
   properties. problem here is that sendto() may actually use a
   different server HIT than the client assumes.  The client will drop
   the response packet when the client implements access control on the
   UDP socket (e.g. using connect()).

   Reimplementing the server application using the sendmsg() and
   recvmsg() to support multihoming (particularly considering the
   anchillary data) would be the ultimate solution to this problem, but
   with legacy applications is not an option.  As a workaround, we make
   suggestion for servers providing UDP-based services with non-
   multihoming capable services.  Such servers should announce only the
   HIT that matches to the default outgoing HIT of the host to avoid
   such problems.

   Finally, some applications may create a connection to a local HIT.
   In such a case, the local system may use NULL encryption to avoid
   unnecessary encryption overhead, and may be otherwise more permissive
   than usual such as excluding authentication, Diffie-Hellman exchange,
   and puzzle.

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 enough.  Here we mean that the delegation chain from an application point the
   reverse IP root should be trusted (typical trust anchor issues), and
   the DNS zone administrators in charge of view. the netblock should be
   trusted to put in the right information.

   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
   initial 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 zones are secured (or the HITs
   are 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 practices 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
   established.

   When the system provides (spoofs) LSIs or HITs instead of IP
   addresses as the result of name resolution, the resultant fields may
   inadvertently show up in user interfaces and system logs, which may
   cause operational concerns for some network administrators.

5.  IANA Considerations

   This document has no actions for IANA.

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.  Erik Nordmark provided the
   taxonomy of how applications use IP addresses in a previously expired
   Internet Draft.  The document received substantial and useful
   comments during the review phase from David Black, Pekka Savola, Lars
   Eggert, and the DNS directorate.

7.  Informative References

   [1]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson, "Host
        Identity Protocol", draft-ietf-hip-base-07 draft-ietf-hip-base-10 (work in progress), February
        October 2007.

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

   [3]  Nikander, P., and F. Dupont, "An IPv6 Prefix for
        Overlay Routable Cryptographic Hash Identifiers (ORCHID)", draft-laganier-ipv6-khi-07 (work in
        progress), February
        RFC 4843, April 2007.

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

   [5]

   [3]  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]

   [4]  Carpenter, B., "Architectural Principles of the Internet",
        RFC 1958, June 1996.

   [5]  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]

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

Appendix A.  Changes from previous versions

   This section is to be removed by the RFC Editor before publication.
   It summarizes resolution of issues raised in the following reviews:
   (1) IESG last call, (2) Gen-ART review, and (3) DNS directorate
   review.  Mobility and secdir reviews did not result in actionable
   comments.

A.1.  From version-01 to version-02 (current)

   Better clarity in the abstract and introduction about the goal of the
   draft; namely, that it is informational to help implementors and
   early adopters think about and solve deployment issues (comment from
   Pekka Savola).

   Delete the second paragraph of 3 about the general applicability of
   replacing IP addresses with LSIs and HITs at the socket layer.
   (comment from Pekka Savola).

   Delete comments in Section 3.2 on routable LSIs, as this is seen to
   be out of scope and potentially controversial or incomplete (comment
   from David Black).

   Delete reference to Erik Nordmark's shim6 application referral draft,
   since it is a dead draft (comment from David Black).  Instead, Erik
   is cited in the acknowledgments section for providing the taxonomy of
   IP address usage scenarios.

   Clarify (and reference the base spec) in Sec. 3.1 that use of the
   opportunistic mode requires that systems not enforce that the
   HIT-to-IP address bindings learned will pertain to subsequent
   sessions to that IP address.

   Section 3.2 drew comments from several reviewers.  First, David Black
   raised the issue that spoofing IP addresses with HITs or LSIs raises
   risks that it may turn up in log records; this has been noted in the
   text.  The section on using a DNS suffix to signal the preferred use
   of HIP was objected to by members of the DNS directorate and others
   (including the co-author Pekka Nikander), due to concern that queries
   to a new TLD might leak out.  The current draft instead recommends a
   DNS prefix instead of suffix, due to a suggestion by Thomas Narten.

   In section 3.1, clarify recursion issues that may arise when doing
   reverse+forward lookup of HIP records from DNS (comment from Pekka
   Savola).

   Clarify more specifically in security considerations section the
   DNSSEC trust assumptions or security considerations (outline of text
   provided by Pekka Savola, and similar comment raised by Peter Koch).

   Clarified in security considerations section that IP address spoofing
   could cause some operational difficulties if they unexpectedly show
   up in log files or UIs (comment from David Black).

   Clarified in Sec. 3.1 that opportunistic and DNS techniques can incur
   additional latency when compared to plain IP (comment from Lars
   Eggert)

   Added third option to Section 3.2 for using DNS (transparently
   fetching HIP resource records when doing other RR queries), suggested
   by Lars Eggert and also by Olaf Kolkman.

   Incorporated last-call comments from Miika Komu, which were all
   handled in Section 3.4: i) clarify multihoming issue for servers with
   multiple HITs, when receiving UDP, ii) clarify a problem that might
   arise for applications that do parallel connect, and iii) suggest
   that loopback HIP connections could use a NULL encryption.

   Removed expired references and updated active references.

   Incorporated additional review comments from Miika Komu, and some
   suggested replacement text, and added him as a co-author.

Authors' Addresses

   Tom

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

   Email: thomas.r.henderson@boeing.com

   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

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

   Miika Komu
   Helsinki Institute for Information Technology
   Metsaenneidonkuja 4
   Helsinki  FIN-02420
   FINLAND

   Phone: +358503841531
   Email: miika@iki.fi

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