Host Identity Protocol                                           M. Komu
Internet-Draft                        Helsinki Institute for Information
Intended status: Experimental                                 Technology
Expires: January 14, November 23, 2009                                     Henderson
                                                      The Boeing Company
                                                           July 13, 2008
                                                            May 22, 2009

   Basic Socket Interface Extensions for Host Identity Protocol (HIP)
                      draft-ietf-hip-native-api-05
                      draft-ietf-hip-native-api-06

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Abstract

   This document defines extensions to the current sockets API for Host
   Identity Protocol (HIP).  The extensions focus on the use of public-
   key based identifiers discovered via DNS resolution, but define also
   interfaces for manual bindings between HITs and locators.  With the
   extensions, the application can also support more relaxed security
   models where the communication can be non-HIP based, according to
   local policies.  The extensions in document are experimental and
   provide basic tools for futher experimentation with policies.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4

   3.  API Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Interaction with the Resolver  . . . . . . . . . . . . . .  5
     3.2.  Interaction without a Resolver . . . . . . . . . . . . . .  5

   4.  API Syntax and Semantics . . . . . . . . . . . . . . . . . . .  6
     4.1.  Socket Family and Address Structure Extensions . . . . . .  6
     4.2.  Extensions to Resolver Data Structures . . . . . . . . . .  8
       4.2.1.  Resolver Usage . . . . . . . . . . . . . . . . . . . .  9
     4.3.  The Use of getsockname and getpeername Functions . . . . . 10
     4.4.  Validating HITs  . . . . . . . . . . . . . . . . . . . . . 10
     4.5.  Source HIT Selection by the System . . . . . . . . . . . . 11
     4.6.  Explicit Handling of Locators  . . . . . . . . . . . . . . 12

   5.  Summary of New Definitions . . . . . . . . . . . . . . . . . . 14

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15

   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15

   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15

   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15

   10. Normative References . . . . . . . . . . . . . . . . . . . . . 16

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18

1.  Introduction

   This document defines C-based sockets Application Programming
   Interface (API) extensions for handling HIP-based identifiers
   explicitly in HIP-aware applications.  It is up to the applications,
   or high-level programming languages or libraries, to manage the
   identifiers.  The extensions in this document are mainly related to
   the use case in which a DNS resolution step has occurred prior to the
   creation of a new socket, and assumes that the system has cached or
   is otherwise able to resolve identifiers to locators (IP addresses).
   The DNS extensions for HIP are described in [RFC5205].  The
   extensions also cover the case in which an application may want to
   explicitly provide suggested locators with the identifiers, including
   supporting the opportunistic case in which the system does not know
   the peer host identity.

   The Host Identity Protocol (HIP) [RFC4423] proposes a new
   cryptographic namespace by separating the roles of end-point
   identifiers and locators by introducing a new namespace to the TCP/IP
   stack.  SHIM6 [I-D.ietf-shim6-proto] is another protocol based on
   identity-locator split.  Note that the APIs specified in this
   document are specific to HIP.  However, the APIs here have been
   designed as much as possible so as not to preclude its use with other
   protocols.  The use of these APIs with other protocols is,
   nevertheless, for further study.

   Applications can observe the HIP layer and its identifiers in the
   networking stacks with varying degrees of visibility.
   [I-D.ietf-hip-applications]  [RFC5338]
   discusses the lowest levels of visibility in which applications are
   completely unaware of the underlying HIP layer.  Such HIP-unaware
   applications in some circumstances use HIP-
   based HIP-based identifiers, such as
   LSIs or HITs, instead of IPv4 or IPv6 addresses and cannot observe
   the identifier-locator bindings.

   This document specifies extensions to [RFC3493] to define a new
   socket address family, AF_HIP.  The macro AF_HIP is used as an alias
   for PF_HIP in this document because the distinction between AF and PF
   has been lost in practice.  The extensions also describe a new socket
   address structure for sockets using Host Identity Tags (HITs)
   explicitly and describe how the socket calls in [RFC3493] are adapted
   or extended as a result.

   Some applications may accept incoming communications from any
   identifier.  Other applications may initiate outgoing communications
   without the knowledge of the peer identifier in Opportunistic Mode
   [RFC5201] by just relying on a peer locator.  This document describes
   how to address both situations using "wildcards" as described later
   in this document.

   There are two related API documents.  Multihoming and explicit
   locator-handling related APIs are defined in
   [I-D.ietf-shim6-multihome-shim-api].  IPsec related policy attributes
   and channel bindings APIs are defined in [I-D.ietf-btns-c-api].  Most
   of the extensions defined in this document can be used independently
   of the two mentioned related API documents.

   The identity-locator split introduced by HIP introduces some policy
   related challenges with datagram oriented sockets, opportunistic
   mode, and manual bindings between HITs and locators.  The extensions
   in this document are of experimental nature and provide basic tools
   for experimenting with policies.  Policy related issues are left for
   further experimentation.

   To recap, the extensions in this document have three goals.  The
   first goal is to allow HIP-aware applications to open sockets to
   other hosts based on the HITs alone, presuming that the underlying
   system can resolve the HITs to addresses used for initial contact.
   The second goal is that applications can explicitly initiate
   communications with unknown peer identifiers.  The third goal is to
   define how HIP-aware applications may provide suggested initial
   contact addresses along with the HITs.

2.  Terminology

   The terms used in this document are summarized in Table 1.

   +---------+---------------------------------------------------------+
   | Term    | Explanation                                             |
   +---------+---------------------------------------------------------+
   | HIP     | Host Identity Protocol                                  |
   | HIT     | Host Identity Tag, a 100-bit hash of a public key with  |
   |         | a 28 bit prefix                                         |
   | LSI     | Local Scope Identifier, a local, 32-bit descriptor for  |
   |         | a given public key.                                     |
   | Locator | Routable IPv4 or IPv6 address used at the lower layers  |
   +---------+---------------------------------------------------------+

                                  Table 1

3.  API Overview

   This section provides an overview of how the API can be used.  First,
   the case in which a resolver is involved in name resolution is
   described, and then the case in which no resolver is involved is
   described.

3.1.  Interaction with the Resolver

   Before an application can establish network communications with the
   entity named by a given FQDN or relative host name, the application
   must translate the name into the corresponding identifier(s).  DNS-
   based hostname-to-identifier translation is illustrated in Figure 1.
   The application calls the resolver in step a to resolve an FQDN step
   b.  The DNS server responds with a list of HITs and a set of locators
   step c.  Optionally in step d, the resolver caches the HIT to locator
   mapping to the HIP module.  The resolver returns the HITs to the
   application step e.  Finally, the application selects one HIT and
   uses it in a socket call such as connect() in step f.

                                              +----------+
                                              |          |
                                              |   DNS    |
                                              |          |
                                              +----------+
                                                  ^  |
                                        b. <FQDN> |  | c. <HITs+locators
                                                  |  v      = HITs+locs>
       +-------------+ a. getaddrinfo(<FQDN>)  +----------+
       |             |------------------------>|          |
       | Application |                         | Resolver |
       |             |<------------------------|          |
       +-------------+        e. <HITs>        +----------+
               |                                    |
               |                                    |
               | f. connect(<HIT>)                  | d. <HITs+locs>
               v                                    v
        +----------+                           +----------+
        |          |                           |          |
        |  TCP/IP  |                           |   HIP    |
        |  Stack   |                           |          |
        +----------+                           +----------+

                                 Figure 1

   In practice, the resolver functionality can be implemented in
   different ways.  For example, it may be implemented in existing
   resolver libraries or as a DNS proxy.

3.2.  Interaction without a Resolver

   The extensions in this document focus on the use of the resolver to
   map host names to HITs and locators in HIP-aware applications.  The
   resolver associates implicitly the HIT with the locator(s) by e.g.
   communicating the HIT-to-IP mapping to the HIP daemon.  However, it
   is possible that an application operates directly on a peer HIT
   without interacting with the resolver.  In such a case, the
   application may resort to the system to map the peer HIT to an IP
   address.  Alternatively, the application can explicitly map the HIT
   to an IP address using socket options as specified in
   [I-D.ietf-shim6-multihome-shim-api].  Full support for all of the
   extensions defined in this draft requires shim socket options to be
   implemented by the system.

4.  API Syntax and Semantics

   In this section, we describe the native HIP APIs using the syntax of
   the C programming language.  We limit the description to the
   interfaces and data structures that are either modified or completely
   new, because the native HIP APIs are otherwise identical to the
   sockets API [POSIX].

4.1.  Socket Family and Address Structure Extensions

   The sockets API extensions define a new protocol family, PF_HIP, and
   a new address family, AF_HIP.  The AF_HIP and PF_HIP are aliases to
   each other.  These definition shall be defined as a result of
   including <sys/socket.h>.

   The use of the PF_HIP constant is mandatory with the socket()
   function when an application uses the native HIP APIs.  The
   application gives the PF_HIP constant as the first argument (domain)
   to the socket() function.  The system returns a positive integer
   representing a socket descriptor when the system supports HIP.
   Otherwise, the system returns -1 and sets errno to EAFNOSUPPORT.

   Figure 2 shows socket address structure for HIP.

           #include <netinet/in.h>

           typedef struct in6_addr hip_hit_t;

           struct sockaddr_hip {
                     sa_family_t    ship_family;
                     in_port_t      ship_port;
                     uint32_t       ship_pad;
                     uint64_t       ship_flags;
                     hip_hit_t      ship_hit;
                     uint8_t        ship_reserved[16];
           };

                                 Figure 2
   Figure 2 is in in 4.3BSD format.  The family of the socket,
   ship_family, is set to AF_HIP.  The port number ship_port is two
   octets in network byte order. and the ship_hit is 16 octets in
   network byte order.  An implementation may have extra member(s) in
   this structure.

   The application usually sets the ship_hit field using the resolver.
   However, the application can use three special wildcard macros to set
   a value directly into the ship_hit field.  The macros are
   HIP_HIT_ANY, HIP_HIT_ANY_PUB, HIP_HIT_ANY_TMP and HIP_ADDR_ANY.  The
   first three equal to a HIT value associated with a wildcard HIT of
   any, public, or anonymous type.  The fourth macro, HIP_ADDR_ANY,
   denotes both HIP_HIT_ANY or any IPv4 or IPv6 address.  The
   HIP_HIT_ANY equals to HIP_HIT_ANY_PUB or HIP_HIT_ANY_TMP.  The
   anonymous identifiers refer to the use anonymous identifiers as
   specified in [RFC4423].  The system may designate anonymous
   identifiers as meta data associated with a HIT depending on whether
   it has been published or not.  However, there is no difference in the
   classes of HITs from the HIP protocol perspective,

   The application can use the HIP_HIT_ANY_* and HIP_ADDR_ANY macros to
   accept incoming communications to all of the HITs of the local host.
   Incoming communications refers here to the functions such as bind(),
   recvfrom() and recvmsg().  The HIP_HIT_* macros are similar to the
   sockets API macros INADDR_ANY and IN6ADDR_ANY_INIT, but they are
   applicable to HITs only.  After initial contact with the peer, the
   application can discover the local and peer HITs using getsockname()
   and getpeername() calls in the context of connection oriented
   sockets.  The difference between the use of the HIP_HIT_* and
   HIP_ADDR_ANY macros here is that the former allows only HIP-based
   communications but the latter also allows communications without HIP.

   The application also uses the HIP_HIT_ANY macro in ship_hit field to
   establish outgoing communications in Opportunistic mode [RFC5201],
   i.e., when the application knows the remote peer locator but not the
   HIT.  Outgoing communications refers here to the use of functions
   such as connect(), sendto() and sendmsg().  However, the application
   must
   should first associate the socket with at least one IP address of the
   peer using SHIM_LOCLIST_PEER_PREF socket option.  The use of the
   HIP_HIT_ANY macro guarantees that the communications will be based on
   HIP or none at all.

   The use of HIP_ADDR_ANY macro in the context of outgoing
   communications is left for further experimentation.  It could be used
   for establishing a non-HIP based connectivity when HIP-based
   connectivity was unsuccessful.

   Some applications rely on system level access control, either
   implicit or explicit (such as accept_filter() function found on BSD-
   based systems), but such discussion is out of scope.  Other
   applications implement access control themselves by using the HITs.
   In such a case, the application can compare two HITs using memcmp()
   or similar function.  It should be noticed that different connection
   attempts between the same two hosts can result in different HITs
   because a host is allowed to have multiple HITs.

4.2.  Extensions to Resolver Data Structures

   The HIP APIs introduce a new addrinfo flag, HIP_PREFER_ORCHID, to be
   used by application to query for both HIT and locator information via
   the getaddrinfo() resolver function [RFC3493].  The getaddrinfo()
   function uses a data structure used for both input to and output from
   the resolver.  The data structure is illustrated in Figure 3.

          #include <netdb.h>

          struct addrinfo {
              int       ai_flags;          /* e.g. AI_EXTFLAGS */
              int       ai_family;         /* e.g. AF_HIP */
              int       ai_socktype;       /* e.g. SOCK_STREAM */
              int       ai_protocol;       /* 0 or IPPROTO_HIP */
              socklen_t ai_addrlen;        /* size of *ai_addr  */
              struct    sockaddr *ai_addr; /* sockaddr_hip */
              char     *ai_canonname;     /* canon. name of the host */
              struct    addrinfo *ai_next; /* next endpoint */
              int       ai_eflags;         /* RFC5014 extension */
          };

                                 Figure 3

   Application must set both the flag AI_EXTFLAGS [RFC5014] in ai_flags
   and HIP_PREFER_ORCHID in the ai_eflags, or otherwise the resolver
   does not return sockaddr_hip data structures.  The resolver returns
   EAI_BADFLAGS when it does not support HIP_PREFER_ORCHID or
   AI_EXTFLAGS flags.

   The system may have a HIP-aware interposing DNS agent as described in
   section 3.2 in [RFC5014].  In such a case, the DNS agent returns
   transparently LSIs or HITs in sockaddr_in and sockaddr_in6 structures
   when available.  To disable this behaviour, the application sets
   AI_EXTFLAGS and AI_NO_ORCHID flags.

   Application denotes its preference for public and anonymous types of
   HITs using HIP_PREFER_SRC_PUBLIC and HIP_PREFER_SRC_TMP flags in the
   ai_eflags field.  If the application sets neither of the flags, the
   resolver returns both public and anonymous HITs.

   The simultaneous use of both HIP_PREFER_ORCHID and
   HIP_PREFER_PASSIVE_* flags produces a single sockaddr_hip structure
   containing a wildcard address that the application can use either for
   incoming (node argument is NULL in getaddrinfo) or outgoing
   communications (node argument is non-NULL).  For example,
   HIP_PREFER_PASSIVE_HIT_TMP flag produces one sockaddr_hip structure
   that contains a HIP_HIT_ANY_TMP in the ship_hit field.

   The resolver sets the ai_family field to AF_HIP in the addrinfo
   structure when ai_addr points to a sockaddr_hip structure.

   When ai_protocol field is set to zero, the resolver also returns
   locators in sockaddr_in and sockaddr_in6 structures in addition to
   sockaddr_hip structures.  The resolver returns only sockaddr_hip
   structures when the application has set the ai_protocol field to
   IPPROTO_HIP or a sockaddr_hip structure is given as the hint argument
   to the resolver.

4.2.1.  Resolver Usage

   A HIP-aware application creates the sockaddr_hip structures
   explicitly or obtains them from the resolver.  The explicit
   configuration of locators is described in
   [I-D.ietf-shim6-multihome-shim-api].  This document defines
   "automated" resolver extensions for getaddrinfo() resolver [RFC3493].

           #include <netdb.h>

           int getaddrinfo(const char *nodename,
                           const char *servname,
                           const struct addrinfo *hints,
                           struct addrinfo **res)
           void free_addrinfo(struct addrinfo *res)

                                 Figure 4

   As described in [RFC3493], the getaddrinfo function takes the
   nodename, servname, and hints as its input arguments.  It places the
   result of the query into the res argument.  The return value is zero
   on success, or a non-zero error value on error.  The nodename
   argument specifies the host name to be resolved; a NULL argument
   denotes the local host.  The servname parameter declares the port
   number to be set in the socket addresses in the res output argument.
   Both the nodename and servname cannot be NULL.

   The input argument "hints" acts like a filter that defines the
   attributes required from the resolved endpoints.  A NULL hints
   argument indicates that any kind of endpoints are acceptable.

   The output argument "res" is dynamically allocated by the resolver.
   The application frees res argument with the free_addrinfo function.
   The res argument contains a linked list of the resolved endpoints.
   The linked list contains sockaddr_hip structures only when the input
   argument has the HIP_PREFER_ORCHID flag set in ai_eflags.  The
   resolver inserts HITs before any locators.  When the
   HIP_PREFER_ORCHID flag is set, the resolver does not return LSIs or
   HITs encapsulated into sockaddr_in or sockaddr_in6 data structures as
   described in [I-D.ietf-hip-applications]. [RFC5338].

   Resolver can return a HIT which maps to multiple locators.  The
   resolver may cache the locator mappings to the HIP module.  The HIP
   module manages the multiple locators according to system policies of
   the host.  The multihoming document
   [I-D.ietf-shim6-multihome-shim-api] describes how an application can
   override system default policies.

   It should be noticed that the application can configure the HIT
   explicitly without setting the locator or the resolver can fail to
   resolve any locator.  In this scenario, the application relies on the
   system to map the HIT to an IP address.  When the system fails to
   provide the mapping, it returns -1 in the called sockets API function
   to the application and sets errno to EADDRNOTAVAIL.

4.3.  The Use of getsockname and getpeername Functions

   The application usually discovers the local or peer HITs from the
   sockaddr_hip structures returned by getaddrinfo().  However, the
   sockaddr_hip structure does not contain a HIT when the application
   uses the HIP_HIT_ANY_* macros.  In such a case, the application
   discovers the local and peer HITs using the getsockname() and
   getpeername() functions.  The functions return sockaddr_hip
   structures when the family of the socket is AF_HIP.

4.4.  Validating HITs

   An application that uses the HIP_ADDR_ANY macro may want to check if
   the local or peer address is an orchid-based HIT [RFC4843].  Also,
   the application may want to verify whether a HIT is public or
   anonymous.  The application accomplishes these using a new function
   called sockaddr_is_srcaddr() which is illustrated in Figure 5.

         #include <netinet/in.h>

         short sockaddr_is_srcaddr(struct sockaddr *srcaddr
                                   uint64_t flags);

                                 Figure 5
   The sockaddr_is_srcaddr() function operates in the same way as
   inet6_is_srcaddr() function [RFC5014] which can be used to verify the
   type of an address belonging to the localhost.  The difference is
   that sockaddr_is_srcaddr() function handles sockaddr_hip structures
   in addition to sockaddr_in6, and possibly some other socket
   structures in further extensions.  The function has also 64 bit flags
   instead of 32 bits.  This new function handles the same flags as
   defined in [RFC5014] in addition to some HIP-specific flags listed in
   Table 2.

            +-----------------------+-------------------------+
            | Flag                  | Purpose                 |
            +-----------------------+-------------------------+
            | HIP_PREFER_ORCHID     | The identifier is a HIT |
            | HIP_PREFER_SRC_TMP    | Anonymous HIT           |
            | HIP_PREFER_SRC_PUBLIC | Public HIT              |
            +-----------------------+-------------------------+

                                  Table 2

4.5.  Source HIT Selection by the System

   Some applications initiate communications by specifying only the
   destination identifier and let the underlying system specify the
   source.  When the system selects the source HIT, the system should
   apply the rules specified in [RFC3484] according to the default
   policy table for HITs shown in Table 3.

                 +-----------------+------------+-------+
                 | HIT Type        | Precedence | Label |
                 +-----------------+------------+-------+
                 | Anonymous DSA   | 110        | 5     |
                 | Anonymous RSA   | 120        | 6     |
                 | Public DSA      | 130        | 7     |
                 | Public RSA      | 140        | 8     |
                 | [RFC3484] rules | 50-100     | 7     |
                 +-----------------+------------+-------+

                                  Table 3

   When application using a AF_HIP-based socket does not specify the
   source identifier, the system selects the source identifier on the
   behalf of the application according to the precedence in the above
   table.  For example, the system prefers public (published) keys
   before anonymous keys because they work better for referral purposes.
   RSA-based keys are preferred over DSA based because RSA is the
   default algorithm in HIP.

   When system provides multiple keys of same type, but with different
   key lengths, the longer keys should have a higher preference.  As
   example, system providing two public RSA keys of different size would
   give the smaller key preference value 140 and 145 for the larger.
   The preference value should not exceed 150.  Systems supporting more
   than 10 keys of same key size may use digits to further fragment the
   precedence namespace.  IPv6 addresses have the lowest precedence
   value to denote that HITs have a higher precedence when operating on
   AF_HIP-based sockets.

   [RFC5014] specifies flags for the getaddrinfo resolver and socket
   options for Mobile IPv6.  The resolver, operating under
   HIP_PREFER_ORCHID flag, or the socket handler, operating on a AF_HIP-
   based socket, may encounter such flags or options.  In such a case
   the resolver or socket handler should silenty ignore the flags or
   options without returning an error.  However, a HIP-aware application
   may use the HIP-specific flags HIP_PREFER_ORCHID, HIP_PREFER_SRC_TMP
   or HIP_PREFER_SRC_PUBLIC in getsockopt(), setsockopt(), getaddrinfo()
   calls and in the anchillary data of datagram packets as specified in
   [RFC5014].  The level of the socket options should be set to SOL_SHIM
   [I-D.ietf-shim6-multihome-shim-api] and the option name should be
   HIP_HIT_PREFERENCES.

4.6.  Explicit Handling of Locators

   The system resolver, or the HIP module, maps HITs to locators
   implicitly.  However, some applications may want to specify initial
   locator mappings explicitly.  In such a case, the application first
   creates a socket with AF_HIP as the domain argument.  Second, the
   application may set locator information with one of the following
   shim socket options as defined in the multihoming extensions in
   [I-D.ietf-shim6-multihome-shim-api]:

   +-----------------------------+-----+-----+-----------------+-------+
   | optname                     | get | set | description     | dtype |
   +-----------------------------+-----+-----+-----------------+-------+
   | SHIM_LOC_LOCAL_PREF         | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | local side for  |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOC_PEER_PREF          | o   | o   | Get or set the  | *1    |
   |                             |     |     | preferred       |       |
   |                             |     |     | locator on the  |       |
   |                             |     |     | remote side for |       |
   |                             |     |     | the context     |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the socket.     |       |
   | SHIM_LOCLIST_LOCAL          | o   | o   | Get or set a    | *2    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the local EID.  |       |
   | SHIM_LOCLIST_PEER           | o   | o   | Get or set a    | *2    |
   |                             |     |     | list of         |       |
   |                             |     |     | locators        |       |
   |                             |     |     | associated with |       |
   |                             |     |     | the peer's EID. |       |
   | SHIM_LOC_LOCAL_SEND         | o   | o   | Request use of  | *2    |
   |                             |     |     | specific        |       |
   |                             |     |     | locator as      |       |
   |                             |     |     | source locator  |       |
   |                             |     |     | of outgoing IP  |       |
   |                             |     |     | packets.        |       |
   | SHIM_LOC_PEER_SEND          | o   | o   | Request use of  | *2    |
   |                             |     |     | specific        |       |
   |                             |     |     | locator as      |       |
   |                             |     |     | destination     |       |
   |                             |     |     | locator of      |       |
   |                             |     |     | outgoing IP     |       |
   |                             |     |     | packets.        |       |
   +-----------------------------+-----+-----+-----------------+-------+
   *1: Pointer to a shim_locator which is defined in Section 7 of
   draft-ietf-shim6-multihome-shim-api.
   *2: Pointer to an array of shim_locator.

                                 Figure 6

   Finally, the application creates a valid sockaddr_hip structure and
   associates the socket also with the sockaddr_hip structure by calling
   some socket-related function, such as connect() or bind().

   The usage and semantics for typical use cases are as follows:

   An application that initiates a connection using a connection
   oriented socket to a particular host at a known address or set of
   addresses can invoke SHIM_LOCLIST_PEER socket option.  The HIP module
   uses the first address (if multiple are provided, or else the
   application can override this by setting SHIM_LOC_PEER_PREF to one of
   the addresses in SHIM_LOCLIST_PEER.  The application later provides a
   specific HIT in the ship_hit field of the sockaddr_hip in the
   connect() system call.  If the application provides one or more
   addresses in SHIM_LOCLIST_PEER setsockopt call, the system should not
   connect to the host via another destination address, in case the
   application intends to restrict the range of addresses permissible as
   a policy choice.  If the system cannot reach the provided HIT at one
   of the addresses provided, the outbound socket API functions
   (connect, sendmsg, etc.) return -1 and set errno to EINVALIDLOCATOR.

   Another common use case is to set up an association in opportunistic
   mode, when the destination HIT is specified as a wildcard.  This can
   be accomplished by setting one or more destination addresses using
   the SHIM_LOCLIST_PEER socket option as described above and then
   calling connect() with the wildcard HIT.  The connect() call returns
   -1 and sets errno to EADDRNOTAVAIL when the application connects to a
   wildcard without specifying any destination address.

   Applications may also choose to associate local addresses with
   sockets.  The procedures specified in
   [I-D.ietf-shim6-multihome-shim-api] are followed in this case.

5.  Summary of New Definitions

   Table 4 summarizes the new macro and structures defined in this
   document.

             +-----------------+-----------------------------+
             | Header          | Definition                  |
             +-----------------+-----------------------------+
             | <sys/socket.h>  | AF_HIP                      |
             | <sys/socket.h>  | PF_HIP                      |
             | <netinet/in.h>  | IPPROTO_HIP                 |
             | <netinet/hip.h> | HIP_HIT_ANY                 |
             | <netinet/hip.h> | HIP_HIT_ANY_PUB             |
             | <netinet/hip.h> | HIP_HIT_ANY_TMP             |
             | <netinet/hip.h> | HIP_ADDR_ANY                |
             | <netinet/hip.h> | HIP_HIT_PREFERENCES         |
             | <netinet/hip.h> | hip_hit_t                   |
             | <netdb.h>       | HIP_PREFER_ORCHID           |
             | <netdb.h>       | HIP_PREFER_SRC_TMP          |
             | <netdb.h>       | HIP_PREFER_SRC_PUBLIC       |
             | <netdb.h>       | HIP_PREFER_PASSIVE_HIT_TMP  |
             | <netdb.h>       | HIP_PREFER_PASSIVE_HIT_PUB  |
             | <netdb.h>       | HIP_PREFER_PASSIVE_HIT_ANY  |
             | <netdb.h>       | HIP_PREFER_PASSIVE_ADDR_ANY |
             | <netinet/hip.h> | sockaddr_hip                |
             | <netinet/hip.h> | sockaddr_is_srcaddr         |
             +-----------------+-----------------------------+

                                  Table 4

6.  IANA Considerations

   No IANA considerations.

7.  Security Considerations

   No security considerations currently.

8.  Contributors

   Thanks for Jukka Ylitalo and Pekka Nikander for their original
   contribution, time and effort to the native HIP APIs.  Thanks for
   Yoshifuji Hideaki for his contributions to this document.

9.  Acknowledgements

   Kristian Slavov, Julien Laganier, Jaakko Kangasharju, Mika Kousa, Jan
   Melen, Andrew McGregor, Sasu Tarkoma, Lars Eggert, Joe Touch, Antti
   Jaervinen, Anthony Joseph, Teemu Koponen, Jari Arkko, Ari Keraenen,
   Juha-Matti Tapio, Shinta Sugimoto, Philip Matthews, Jan Melen and
   Gonzalo Camarillo have also provided valuable ideas or feedback.
   Thanks also for the APPS area folks, including Stephane Bortzmeyer,
   Chris Newman, Tony Finch, "der Mouse" and Keith Moore.

10.  Normative References

   [I-D.ietf-btns-c-api]
              Richardson, M., Williams, N., Komu, M., and S. Tarkoma,
              "IPsec
              "C-Bindings for IPsec Application Programming Interfaces",
              draft-ietf-btns-c-api-03
              draft-ietf-btns-c-api-04 (work in progress),
              February 2008.

   [I-D.ietf-hip-applications]
              Henderson, T., Nikander, P., and M. Komu, "Using the Host
              Identity Protocol with Legacy Applications",
              draft-ietf-hip-applications-04 (work in progress),
              July 2008. March 2009.

   [I-D.ietf-shim6-multihome-shim-api]
              Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto,
              "Socket Application Program Interface (API) for
              Multihoming Shim", draft-ietf-shim6-multihome-shim-api-05 draft-ietf-shim6-multihome-shim-api-08
              (work in progress), February 2008. May 2009.

   [I-D.ietf-shim6-proto]
              Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", draft-ietf-shim6-proto-10 draft-ietf-shim6-proto-12 (work
              in progress), February 2008. 2009.

   [POSIX]    Institute of Electrical and Electronics Engineers, "IEEE
              Std. 1003.1-2001 Standard for Information Technology -
              Portable Operating System Interface (POSIX)", Dec 2001.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC4843]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix
              for Overlay Routable Cryptographic Hash Identifiers
              (ORCHID)", RFC 4843, April 2007.

   [RFC5014]  Nordmark, E., Chakrabarti, S., and J. Laganier, "IPv6
              Socket API for Source Address Selection", RFC 5014,
              September 2007.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5205]  Nikander, P. and J. Laganier, "Host Identity Protocol
              (HIP) Domain Name System (DNS) Extensions", RFC 5205,
              April 2008.

   [RFC5338]  Henderson, T., Nikander, P., and M. Komu, "Using the Host
              Identity Protocol with Legacy Applications", RFC 5338,
              September 2008.

Authors' Addresses

   Miika Komu
   Helsinki Institute for Information Technology
   Metsaenneidonkuja 4
   Helsinki
   Finland

   Phone: +358503841531
   Fax:   +35896949768
   Email: miika@iki.fi
   URI:   http://www.iki.fi/miika/

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

   Email: thomas.r.henderson@boeing.com

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