Internet Engineering Task Force (IETF)
Request for Comments: 6317
Category: Experimental
ISSN: 2070-1721
M. Komu
Aalto University
T. Henderson
The Boeing Company
July 2011

Basic Socket Interface Extensions for

the Host Identity Protocol (HIP)


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

Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.

This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

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.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................5
   3. Name Resolution Process .........................................5
      3.1. Interaction with the Resolver ..............................5
      3.2. Interaction without a Resolver .............................6
   4. API Syntax and Semantics ........................................7
      4.1. Socket Family and Address Structure Extensions .............7
      4.2. Extensions to Resolver Data Structures .....................9
      4.3. The Use of getsockname() and getpeername() Functions ......12
      4.4. Selection of Source HIT Type ..............................12
      4.5. Verification of HIT Type ..................................13
      4.6. Explicit Handling of Locators .............................14
   5. Summary of New Definitions .....................................16
   6. Security Considerations ........................................16
   7. Contributors ...................................................17
   8. Acknowledgments ................................................17
   9. References .....................................................17
      9.1. Normative References ......................................17
      9.2. Informative References ....................................18

1. Introduction

This document defines the 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 extension for HIP is 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 endpoint identifiers and locators by introducing a new namespace to the TCP/IP stack. Shim6 [RFC5533] is another protocol based on an identity- locator split. The APIs specified in this document are specific to HIP, but have been designed as much as possible to not preclude its use with other protocols. The use of these APIs with other protocols is, nevertheless, for further study.

The APIs in this document are based on Host Identity Tags (HITs) that are defined as IPv6 addresses with the Overlay Routable Cryptographic Hash Identifiers (ORCHID) prefix [RFC4843]. ORCHIDs are derived from Host Identifiers using a hash and fitting the result into an IPv6 address. Such addresses are called HITs, and they can be distinguished from other IPv6 addresses via the ORCHID prefix. Note that ORCHIDs are presently an experimental allocation by IANA. If the ORCHID allocation were to expire and HIT generation were to use a different prefix in the future, most users of the API would not be impacted, unless they explicitly checked the ORCHID prefix on returned HITs. Users who check (for consistency) that HITs have a valid ORCHID prefix must monitor the IANA allocation for ORCHIDs and adapt their software in case the ORCHID allocation were to be removed at a future date.

Applications can observe the HIP layer and its identifiers in the networking stacks with varying degrees of visibility. [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 identifiers, such as Local Scope Identifiers (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. Similarly to other address families, AF_HIP can be used as an alias for PF_HIP. The extensions also describe a new socket address structure for sockets using 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 (Section 4.1.6 of [RFC5201]) by just relying on a peer locator. This document describes how to address both situations using "wildcards" as described in Section 4.1.1.

This document references one additional API document [RFC6316] that defines multihoming and explicit-locator handling. Most of the extensions defined in this document can be used independently of the above document.

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 an 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 illustrate how HIP-aware applications can use the Shim API [RFC6316] to manually map locators to HITs.

This document was published as experimental because a number of its normative references had experimental status. The success of this experiment can be evaluated by a thorough implementation of the APIs defined.

2. Terminology

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

   | Term    | Explanation                                            |
   | FQDN    | Fully Qualified Domain Name                            |
   | HIP     | Host Identity Protocol                                 |
   | HI      | Host Identifier                                        |
   | 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 |
   | RR      | Resource Record                                        |

Table 1

3. Name Resolution Process

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 hostname, 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 to one or more socket addresses within the PF_HIP family. The resolver, in turn, queries the DNS in step (b) to map the FQDN to one or more HIP RRs with the HIT and HI and possibly the rendezvous server of the Responder, and also (in parallel or sequentially) to resolve the FQDN into possibly one or more A and AAAA records. It should be noted that the FQDN may map to multiple Host Identifiers and locators, and this step may involve multiple DNS transactions, including queries for A, AAAA, HI, and possibly other resource records. The DNS server responds with a list of HIP resource records in step (c). Optionally, in step (d), the resolver caches the HIT-to-locator mapping with the HIP module. The resolver converts the HIP records to HITs and returns the HITs to the application contained in HIP socket address structures in step (e). Depending on the parameters for the resolver call, the resolver may also return other socket address structures to the application. Finally, the application receives the socket address structure(s) from the resolver and uses them in socket calls such as connect() in step (f).

                                              |          |
                                              |   DNS    |
                                              |          |
                                                  ^  |
                                   b. QNAME=FQDN  |  | c. HIP and
                                                  |  |    A/AAAA
                                                  |  v    RR(s)
       +-------------+ a. getaddrinfo(<FQDN>)  +----------+
       |             |------------------------>|          |
       | Application |                         | Resolver |
       |             |<------------------------|          |
       +-------------+        e. <HITs>        +----------+
               |                                    |
               |                                    | d. HIP and
               | f. connect(<HIT>)                  |    A/AAAA
               |    or any other socket call        |    RR(s)
               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 HIP-aware interposing agent.

3.2. Interaction without a Resolver

The extensions in this document focus on the use of the resolver to map hostnames to HITs and locators in HIP-aware applications. The resolver may implicitly associate a HIT with the corresponding locator(s) by 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 Section 4.6. Full support for all of the extensions defined in this document requires a number of shim socket options [RFC6316] 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 constants are aliases to each other. These definitions shall be defined as a result of including <sys/socket.h>.

When the socket() function is called with PF_HIP as the first argument (domain), it attempts to create a socket for HIP communication. If HIP is not supported, socket() follows its default behavior and returns -1, and sets errno to EAFNOSUPPORT.

Figure 2 shows the recommended implementation of the socket address structure for HIP in Portable Operating System Interface (POSIX) format.

            #include <netinet/hip.h>

typedef struct in6_addr hip_hit_t;

            struct sockaddr_hip {
                      uint8_t        ship_len;
                      sa_family_t    ship_family;
                      in_port_t      ship_port;
                      uint32_t       ship_flags;
                      hip_hit_t      ship_hit;

Figure 2

uint8_t ship_len: This field defines the length of the structure. Implementations that do not define this field typically embed the information in the following ship_family field.

sa_family_t ship_family: This mandatory field identifies the structure as a sockaddr_hip structure. It overlays the sa_family field of the sockaddr structure. Its value must be AF_HIP.

in_port_t ship_port: This mandatory field contains the transport protocol port number. It is handled in the same way as the sin_port field of the sockaddr_in structure. The port number is stored in network byte order.

uint32_t ship_flags: This mandatory bit field contains auxiliary flags. This document does not define any flags. This field is included for future extensions.

hip_hit_t ship_hit: This mandatory field contains the endpoint identifier. When the system passes a sockaddr_hip structure to the application, the value of this field is set to a valid HIT, IPv4, or IPv6 address, as discussed in Section 4.5. When the application passes a sockaddr_hip structure to the system, this field must be set to a HIT or a wildcard address as discussed in Section 4.1.1.

Some applications rely on system-level access control, either implicit or explicit (such as the 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. Applications operating on sockaddr_hip structures can use memcmp() or a similar function to compare the ship_hit fields. It should also be noted that different connection attempts between the same two hosts can result in different HITs, because a host is allowed to have multiple HITs.

4.1.1. HIP Wildcard Addresses

HIP wildcard addresses are similar to IPv4 and IPv6 wildcard addresses. They can be used instead of specific HITs in the ship_hit field for local and remote endpoints in sockets API calls such as bind(), connect(), sendto(), or sendmsg().

In order to bind to all local IPv4 and IPv6 addresses and HIP HITs, the ship_hit field must be set to HIP_ENDPOINT_ANY. In order to bind to all local HITs, ship_hit must contain HIP_HIT_ANY. To only bind to all local public HITs, the ship_hit field must be HIP_HIT_ANY_PUB. The value HIP_HIT_ANY_TMP binds a socket to all local anonymous identifiers only as specified in [RFC4423]. The system may label anonymous identifiers as such depending on whether they have been published or not. After binding a socket via one of the HIP_HIT_ANY_* wildcard addresses, the application is guaranteed to receive only HIP-based data flows. With the HIP_ENDPOINT_ANY wildcard address, the socket accepts HIP, IPv6, and IPv4-based data flows.

When a socket is bound or connected via a sockaddr_hip structure, i.e., the PF_HIP protocol family, the system returns only addresses of the AF_HIP family, i.e., sockaddr_hip structures, for this socket. This applies to all functions that provide addresses to the application, such as accept() or recvfrom(). If the data flow is based on HIP, the ship_hit field contains the peer's HIT. For a non-HIP IPv6 data flow, the field contains the peer's IPv6 address. For a non-HIP IPv4 data flow, the field contains the peer's IPv4 address in IPv4-mapped IPv6 address format as described in Section 3.7 of [RFC3493]. Section 4.5 describes how the application can verify the type of address returned by the sockets API calls.

An application uses the sockets API as follows to set up a connection or to send messages in HIP opportunistic mode (cf. [RFC5201]). First, the application associates a socket with at least one IP address of the destination peer via setting the SHIM_LOCLIST_PEER_PREF socket option. It then uses outgoing socket functions such as connect(), sendto(), or sendmsg() with the HIP_ENDPOINT_ANY or HIP_HIT_ANY wildcard address in the ship_hit field of the sockaddr_hip structure. With the HIP_HIT_ANY address, the underlying system allows only HIP-based data flows with the corresponding socket. For incoming packets, the system discards all non-HIP-related traffic arriving at the socket. For outgoing packets, the system returns -1 in the socket call and sets errno to an appropriate error type when the system failed to deliver the packet over a HIP-based data channel. The semantics of using HIP_ENDPOINT_ANY are the subject of further experimentation in the context of opportunistic mode. Such use may result in a data flow either with or without HIP.

4.2. Extensions to Resolver Data Structures

The HIP APIs introduce a new address family, AF_HIP, that HIP-aware applications can use to control the address type returned from the getaddrinfo() function [RFC3493] [POSIX]. The getaddrinfo() function uses a data structure called addrinfo in its "hints" and "res" arguments, which are described in more detail in the next section. The addrinfo data structure is illustrated in Figure 3.

        #include <netdb.h>
        struct addrinfo {
            int       ai_flags;          /* e.g., AI_CANONNAME */
            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;         /* RFC 5014 extension */

Figure 3

An application resolving with the ai_family field set to AF_UNSPEC in the hints argument may receive any kind of socket address structures, including sockaddr_hip. When the application wants to receive only HITs contained in sockaddr_hip structures, it should set the ai_family field to AF_HIP. Otherwise, the resolver does not return any sockaddr_hip structures. The resolver returns EAI_FAMILY when AF_HIP is requested but not supported.

The resolver ignores the AI_PASSIVE flag when the application sets the family in hints to AF_HIP.

The system may have a HIP-aware interposing DNS agent as described in Section 3.2 of [RFC5338]. In such a case, the DNS agent may, according to local policy, transparently return LSIs or HITs in sockaddr_in and sockaddr_in6 structures when available. A HIP-aware application can override this local policy in two ways. First, the application can set the family to AF_HIP in the hints argument of getaddrinfo() when it requests only sockaddr_hip structures. Second, the application can set the AI_NO_HIT flag to prevent the resolver from returning HITs in any kind of data structures.

When getaddrinfo() returns resolved outputs in the output "res" argument, it sets the family to AF_HIP when the related structure is sockaddr_hip.

4.2.1. Resolver Usage

A HIP-aware application creates the sockaddr_hip structures manually or obtains them from the resolver. The explicit configuration of locators is described in [RFC6316]. This document defines

"automated" resolver extensions for the getaddrinfo() resolver [RFC3493]. Other resolver calls, such as gethostbyname() and getservbyname(), are not defined in this document. The getaddrinfo() resolver interface is shown in Figure 4.

            #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 nodename, servname, and hints as its input arguments. It places the result of the query into the res output argument. The return value is zero on success, or a non-zero error value on error. The nodename argument specifies the hostname to be resolved; a NULL argument denotes the HITs of the local host. The servname parameter declares the port number to be set in the socket addresses in the res output argument. The nodename and servname arguments cannot both be NULL at the same time.

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 endpoint is acceptable.

The output argument "res" is dynamically allocated by the resolver. The application frees the res argument with the free_addrinfo function. The res argument contains a linked list of the resolved endpoints. The linked list contains only sockaddr_hip structures when the input argument has the family set to AF_HIP. When the family is zero, the list contains sockaddr_hip structures before sockaddr_in and sockaddr_in6 structures.

The resolver can return a HIT that maps to multiple locators. The resolver may cache the locator mappings with the HIP module. The HIP module manages the multiple locators according to system policies of the host. The multihoming document [RFC6316] describes how an application can override system default policies.

It should be noted 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 sockaddr_hip structure does not contain a HIT when the application uses the HIP_HIT_ANY_* or HIP_ENDPOINT_ANY constants. In such a case, the application can discover the local and peer HITs using the getsockname() and getpeername() functions after the socket is connected. The functions getsockname() and getpeername() always output a sockaddr_hip structure when the family of the socket is AF_HIP. The application should be prepared to also handle IPv4 and IPv6 addresses in the ship_hit field, as described in Section 4.1, in the context of the HIP_ENDPOINT_ANY constant.

4.4. Selection of Source HIT Type

A client-side application can choose its source HIT by, for example, querying all of the local HITs with getaddrinfo() and associating one of them with the socket using bind(). This section describes another method for a client-side application to affect the selection of the source HIT type where the application does not call bind() explicitly. Instead, the application just specifies the preferred requirements for the source HIT type.

The sockets API for source address selection [RFC5014] defines socket options to allow applications to influence source address selection mechanisms. In some cases, HIP-aware applications may want to influence source HIT selection, in particular whether an outbound connection should use a published or anonymous HIT. Similar to IPV6_ADDR_PREFERENCES defined in [RFC5014], the socket option HIT_PREFERENCES is defined for HIP-based sockets. This socket option can be used with setsockopt() and getsockopt() calls to set and get the HIT selection preferences affecting a HIP-enabled socket. The socket option value (optval) is a 32-bit unsigned integer argument. The argument consists of a number of flags where each flag indicates an address selection preference that modifies one of the rules in the default HIT selection; these flags are shown in Table 2.

          | Socket Option             | Purpose                 |
          | HIP_PREFER_SRC_HIT_TMP    | Prefer an anonymous HIT |
          | HIP_PREFER_SRC_HIT_PUBLIC | Prefer a public HIT     |

Table 2

If the system is unable to assign the type of HIT that is requested, at HIT selection time, the socket call (connect(), sendto(), or sendmsg()) will fail, and errno will be set to EINVAL. If the application tries to set both of the above flags for the same socket, this also results in the error EINVAL.

4.5. Verification of HIT Type

An application that uses the HIP_ENDPOINT_ANY constant may want to check whether the actual communication was based on HIP or not. Also, the application may want to verify whether a HIT belonging to the local host is public or anonymous. The application accomplishes this using a new function called sockaddr_is_srcaddr(), which is illustrated in Figure 5.

         #include <netinet/hip.h>

short sockaddr_is_srcaddr(struct sockaddr *srcaddr,

                                   uint64_t flags);

Figure 5

The sockaddr_is_srcaddr() function operates in the same way as the inet6_is_srcaddr() function [RFC5014], which can be used to verify the type of an address belonging to the local host. The difference is that the sockaddr_is_srcaddr() function handles sockaddr_hip structures in addition to sockaddr_in6, and possibly other socket structures in further extensions. Also, the length of the flags argument is 64 bits instead of 32 bits, because the new function handles the same flags as defined in [RFC5014], in addition to two HIP-specific flags, HIP_PREFER_SRC_HIT_TMP and HIP_PREFER_SRC_HIT_PUBLIC. With these two flags, the application can distinguish anonymous HITs from public HITs.

When given an AF_INET6 socket, sockaddr_is_srcaddr() behaves the same way as the inet6_is_srcaddr() function as described in [RFC5014]. With an AF_HIP socket, the function returns 1 when the HIT contained in the socket address structure corresponds to a valid HIT of the local host and the HIT satisfies the given flags. The function returns -1 when the HIT does not belong to the local host or the flags are not valid. The function returns 0 when the preference flags are valid but the HIT does not match the given flags. The function also returns 0 on a sockaddr_hip structure containing a HIP_ENDPOINT_ANY or HIP_HIT_ANY_* wildcard.

The sockaddr_is_srcaddr() interface applies only to local HITs. Applications can call the function hip_is_hit() to verify that the given hit_hit_t pointer has the HIT prefix. The function is illustrated in Figure 6.

         #include <netinet/hip.h>
         short hip_is_hit(hip_hit_t *hit);

Figure 6

The hip_is_hit() function returns 1 when the given argument contains the HIT prefix. The function returns -1 on error and sets errno appropriately. The function returns 0 when the argument does not have the HIT prefix. The function also returns 0 when the argument is a HIP_ENDPOINT_ANY or HIP_HIT_ANY_* wildcard.

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 get or set locator information with one of the following shim socket options as defined in the multihoming extensions in [RFC6316]. The related socket options are summarized briefly in Table 3.

   | optname             | description                                 |
   | SHIM_LOC_LOCAL_PREF | Get or set the preferred locator on the     |
   |                     | local side for the context associated with  |
   |                     | the socket.                                 |
   | SHIM_LOC_PEER_PREF  | Get or set the preferred locator on the     |
   |                     | remote side for the context associated with |
   |                     | the socket.                                 |
   | SHIM_LOCLIST_LOCAL  | Get or set a list of locators associated    |
   |                     | with the local Endpoint Identifier (EID).   |
   | SHIM_LOCLIST_PEER   | Get or set a list of locators associated    |
   |                     | with the peer's EID.                        |
   | SHIM_LOC_LOCAL_SEND | Set or get the default source locator of    |
   |                     | outgoing IP packets.                        |
   | SHIM_LOC_PEER_SEND  | Set or get the default destination locator  |
   |                     | of outgoing IP packets.                     |

Table 3

As an example of locator mappings, a connection-oriented application creates a HIP-based socket and sets the SHIM_LOCLIST_PEER socket option on the socket. The HIP module uses the first address contained in the option if multiple addresses are provided. If the application provides one or more addresses in the 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. The application can override the default peer locator by setting the SHIM_LOC_PEER_PREF socket option if necessary. Finally, the application provides a specific HIT in the ship_hit field of the sockaddr_hip in the connect() system call. If the system cannot reach the HIT at one of the addresses provided, the outbound sockets API functions (connect(), sendmsg(), etc.) return -1 and set errno to EINVALIDLOCATOR.

Applications may also choose to associate local addresses with sockets. The procedures specified in [RFC6316] are followed in this case.

Another use case is to use the opportunistic mode when the destination HIT is specified as a wildcard. The application sets one or more destination addresses using the SHIM_LOCLIST_PEER socket option as described earlier in this section, and then calls 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 using datagram-oriented sockets can use ancillary data to control the locators, as described in detail in [RFC6316].

5. Summary of New Definitions

Table 4 summarizes the new constants 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_ENDPOINT_ANY      |
                | <netinet/hip.h> | HIP_HIT_PREFERENCES   |
                | <netinet/hip.h> | hip_hit_t             |
                | <netdb.h>       | AI_NO_HIT             |
                | <netinet/hip.h> | sockaddr_hip          |
                | <netinet/hip.h> | sockaddr_is_srcaddr() |
                | <netinet/hip.h> | hip_is_hit()          |

Table 4

6. Security Considerations

This document describes an API for HIP and therefore depends on the mechanisms defined in the HIP protocol suite. Security concerns associated with HIP itself are specified in [RFC4423], [RFC4843], [RFC5201], [RFC5205], and [RFC5338].

The HIP_ENDPOINT_ANY constant can be used to accept incoming data flows or create outgoing data flows without HIP. The application should use the sockaddr_is_srcaddr() function to validate the type of connection in order to, for example, inform the user of the lack of HIP-based security. The use of the HIP_HIT_ANY_* constants is recommended in security-critical applications and systems.

It should be noted that the wildcards described in this document are not suitable for identifying end hosts. Instead, applications should use getsockname() and getpeername() as described in Section 4.3 to identify an end host.

Future proofing of HITs was discussed during the design of this API. If HITs longer than 128 bits are required at the application layer, this will require explicit support from the applications, because they can store or cache HITs with their explicit sizes. To support longer HITs, further extensions of this API may define an additional flag for getaddrinfo() to generate different kinds of socket address structures for HIP.

7. Contributors

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

8. Acknowledgments

Kristian Slavov, Julien Laganier, Jaakko Kangasharju, Mika Kousa, Jan Melen, Andrew McGregor, Sasu Tarkoma, Lars Eggert, Joe Touch, Antti Jarvinen, Anthony Joseph, Teemu Koponen, Jari Arkko, Ari Keranen, Juha-Matti Tapio, Shinta Sugimoto, Philip Matthews, Joakim Koskela, Jeff Ahrenholz, Tobias Heer, and Gonzalo Camarillo have provided valuable ideas and feedback. Thanks to Nick Stoughton from the Austin group for POSIX-related comments. Thanks also to the APPS area folks, including Stephane Bortzmeyer, Chris Newman, Tony Finch, "der Mouse", and Keith Moore.

9. References

9.1. Normative References

   [POSIX]     "IEEE Std. 1003.1-2008 Standard for Information
               Technology -- Portable Operating System Interface
               (POSIX).  Open group Technical Standard: Base
               Specifications, Issue 7", September 2008,
   [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., Ed., and T.
               Henderson, "Host Identity Protocol", RFC 5201, April
   [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.
   [RFC6316]   Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, Ed.,
               "Sockets Application Program Interface (API) for
               Multihoming Shim", RFC 6316, July 2011.

9.2. Informative References

   [RFC5533]   Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
               Shim Protocol for IPv6", RFC 5533, June 2009.

Authors' Addresses

Miika Komu
Aalto University

   Phone: +358505734395
   Fax:   +358947025014

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