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RFC 5014

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IPv6 Socket API for Source Address Selection


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Network Working Group                                        E. Nordmark
Request for Comments: 5014                        Sun Microsystems, Inc.
Category: Informational                                   S. Chakrabarti
                                                         Azaire Networks
                                                             J. Laganier
                                                        DoCoMo Euro-Labs
                                                          September 2007

              IPv6 Socket API for Source Address Selection

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.


   The IPv6 default address selection document (RFC 3484) describes the
   rules for selecting source and destination IPv6 addresses, and
   indicates that applications should be able to reverse the sense of
   some of the address selection rules through some unspecified API.
   However, no such socket API exists in the basic (RFC 3493) or
   advanced (RFC 3542) IPv6 socket API documents.  This document fills
   that gap partially by specifying new socket-level options for source
   address selection and flags for the getaddrinfo() API to specify
   address selection based on the source address preference in
   accordance with the socket-level options that modify the default
   source address selection algorithm.  The socket API described in this
   document will be particularly useful for IPv6 applications that want
   to choose between temporary and public addresses, and for Mobile IPv6
   aware applications that want to use the care-of address for
   communication.  It also specifies socket options and flags for
   selecting Cryptographically Generated Address (CGA) or non-CGA source

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Definition Of Terms  . . . . . . . . . . . . . . . . . . . . .  5
   3.  Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Design Alternatives  . . . . . . . . . . . . . . . . . . . . .  6
   5.  Address Preference Flags . . . . . . . . . . . . . . . . . . .  7
   6.  Additions to the Socket Interface  . . . . . . . . . . . . . .  9
   7.  Additions to the Protocol-Independent Nodename Translation . . 10
   8.  Application Requirements . . . . . . . . . . . . . . . . . . . 11
   9.  Usage Example  . . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Implementation Notes . . . . . . . . . . . . . . . . . . . . . 13
   11. Mapping to Default Address Selection Rules . . . . . . . . . . 14
   12. IPv4-Mapped IPv6 Addresses . . . . . . . . . . . . . . . . . . 16
   13. Validating Source Address Preferences  . . . . . . . . . . . . 16
   14. Summary of New Definitions . . . . . . . . . . . . . . . . . . 19
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   16. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     17.1.  Normative References  . . . . . . . . . . . . . . . . . . 20
     17.2.  Informative References  . . . . . . . . . . . . . . . . . 20
   Appendix A.  Per-Packet Address Selection Preference . . . . . . . 21
   Appendix B.  Intellectual Property Statement . . . . . . . . . . . 22

1.  Introduction

   [RFC3484] specifies the default address selection rules for IPv6
   [RFC2460].  This document defines socket API extensions that allow
   applications to override the default choice of source address
   selection.  It therefore indirectly affects the destination address
   selection through getaddrinfo().  Privacy considerations [RFC3041]
   have introduced "public" and "temporary" addresses.  IPv6 Mobility
   [RFC3775] introduces "home address" and "care-of address" definitions
   in the mobile systems.

   The default address selection rules in [RFC3484], in summary, are
   that a public address is preferred over a temporary address, that a
   mobile IPv6 home address is preferred over a care-of address, and
   that a larger scope address is preferred over a smaller scope
   address.  Although it is desirable to have default rules for address
   selection, an application may want to reverse certain address
   selection rules for efficiency and other application-specific

   Currently, IPv6 socket API extensions provide mechanisms to choose a
   specific source address through simple bind() operation or
   IPV6_PKTINFO socket option [RFC3542].  However, in order to use
   bind() or IPV6_PKTINFO socket option, the application itself must

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   make sure that the source address is appropriate for the destination
   address (e.g., with respect to the interface used to send packets to
   the destination).  The application also needs to verify the
   appropriateness of the source address scope with respect to the
   destination address and so on.  This can be quite complex for the
   application, since in effect, it needs to implement all the default
   address selection rules in order to change its preference with
   respect to one of the rules.

   The mechanism presented in this document allows the application to
   specify attributes of the source addresses it prefers while still
   having the system perform the rest of the address selection rules.
   For instance, if an application specifies that it prefers to use a
   care-of address over a home address as the source address and if the
   host has two care-of addresses, one public and one temporary, then
   the host would select the public care-of address by following the
   default address selection rule for preferring a public over a
   temporary address.

   A socket option has been deemed useful for this purpose, as it
   enables an application to specify address selection preferences on a
   per-socket basis.  It can also provide the flexibility of enabling
   and disabling address selection preferences in non-connected (UDP)
   sockets.  The socket option uses a set of flags for specifying
   address selection preferences.  Since the API should not assume a
   particular implementation method of the address selection [RFC3484]
   in the network layer or in getaddrinfo(), the corresponding set of
   flags are also defined for getaddrinfo(), as it depends on the source
   address selection.

   As a result, this document introduces several flags for address
   selection preferences that alter the default address selection
   [RFC3484] for a number of rules.  It analyzes the usefulness of
   providing API functionality for different default address selection
   rules; it provides API to alter only those rules that are possibly
   used by certain classes of applications.  In addition, it also
   considers CGA [RFC3972] and non-CGA source addresses when CGA
   addresses are available in the system.  In the future, more source
   flags may be added to expand the API as the needs may arise.

   The approach in this document is to allow the application to specify
   preferences for address selection and not to be able to specify hard
   requirements.  For instance, an application can set a flag to prefer
   a temporary source address, but if no temporary source addresses are
   available at the node, a public address would be chosen instead.

   Specifying hard requirements for address selection would be
   problematic for several reasons.  The major one is that, in the vast

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   majority of cases, the application would like to be able to
   communicate even if an address with the 'optimal' attributes is not
   available.  For instance, an application that performs very short,
   e.g., UDP, transactional exchanges (e.g., DNS queries), might prefer
   to use a care-of address when running on a mobile host that is away
   from home since this provides a short roundtrip time in many cases.
   But if the application is running on a mobile host that is at home,
   or running on a host that isn't providing Mobile IPv6, then it
   doesn't make sense for the application to fail due to no care-of
   address being available.  Also, in particular, when using UDP sockets
   and the sendto() or sendmsg() primitives, the use of hard
   requirements would have been problematic, since the set of available
   IP addresses might very well have changed from when the application
   called getaddrinfo() until it called sendto() or sendmsg(), which
   would introduce new failure modes.

   For the few applications that have hard requirements on the
   attributes of the IP addresses they use, this document defines a
   verification function that allows such applications to properly fail
   to communicate when their address selection requirements are not met.

   Furthermore, the approach is to define two flags for each rule that
   can be modified so that an application can specify its preference for
   addresses selected as per the rule, the opposite preference (i.e., an
   address selected as per the rule reverted), or choose not to set
   either of the flags relating to that rule and leave it up to the
   system default (Section 4).  This approach allows different
   implementations to have different system defaults, and works with
   getaddrinfo() as well as setsockopt().  (For setsockopt, a different
   approach could have been chosen, but that would still require the
   same approach for getaddrinfo.)

   Note that this document does not directly modify the destination
   address selection rules described in [RFC3484].  An analysis has been
   done to see which destination address rules may be altered by the
   applications.  Rule number 4(prefer home address), 8(prefer smaller
   scope), 7(prefer native interfaces) of default address selection
   document [RFC3484] were taken into consideration for destination
   address alteration.  But as of this writing, there was not enough
   practical usage for applications to alter destination address
   selection rules directly by applying the setsockopt() with a
   preferred destination type of address flag.  However, this document
   does not rule out any possibility of adding flags for preferred
   destination address selection.  However, [RFC3484] destination
   address selection rules are dependent on source address selections,
   thus by altering the default source address selection by using the
   methods described in this document, one indirectly influences the
   choice of destination address selection.  Hence, this document

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   explains how getaddrinfo() can be used to select the destination
   address while taking the preferred source addresses into
   consideration (Section 11).

   This document specifies extensions only to the Basic IPv6 socket API
   specified in [RFC3493].  The intent is that this document serves as a
   model for expressing preferences for attributes of IP addresses that
   also need to be expressible in other networking API, such as those
   found in middleware systems and the Java environment.  A similar
   model is also applicable for other socket families.

2.  Definition Of Terms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Address preference flag:
      A flag expressing a preference for a particular type of address
      (e.g., temporary, public).

   Opposite flags:
      Each flag expressing an address preference has an "opposite flag"
      expressing the opposite preference:

      *  Home address preference flag is the opposite of the care-of
         address preference flag.

      *  Temporary address preference flag is the opposite of the public
         address preference flag.

      *  CGA address preference flag is the opposite of the non-CGA
         address preference flag.

   Contradictory flags:
      Any combination of flags including both a flag expressing a given
      address preference and a flag expressing the opposite preference
      constitutes contradictory flags.  Such flags are contradictory by
      definition of their usefulness with respect to source address
      selection.  For example, consider a set of flags, including both
      the home address preference flag and the care-of address
      preference flag.  When considering source address selection, the
      selected address can be a home address, or a care-of address, but
      it cannot be both at the same time.  Hence, to prefer an address
      that is both a home address and a care-of address is

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3.  Usage Scenario

   The examples discussed here are limited to applications supporting
   Mobile IPv6, IPv6 Privacy Extensions, and Cryptographically Generated
   Addresses.  Address selection document [RFC3484] recommends that home
   addresses should be preferred over care-of address when both are
   configured.  However, a mobile node may want to prefer a care-of
   address as the source address for a DNS query in the foreign network,
   as it normally means a shorter and local return path compared to the
   route via the mobile node's home-agent when the query contains a home
   address as the source address.  Another example is the IKE
   application, which requires a care-of address as its source address
   for the initial security association pair with a Home Agent [RFC3775]
   while the mobile node boots up at the foreign network and wants to do
   the key exchange before a successful home-registration.  Also, a
   Mobile IPv6 aware application may want to toggle between the home
   address and care-of address, depending on its location and state of
   the application.  It may also want to open different sockets and use
   the home address as the source address for one socket and a care-of
   address for the others.

   In a non-mobile environment, an application may similarly prefer to
   use a temporary address as the source address for certain cases.  By
   default, the source address selection rule selects "public" address
   when both are available.  For example, an application supporting Web
   browser and mail-server may want to use a "temporary" address for the
   former and a "public" address for the mail-server, as a mail-server
   may require a reverse path for DNS records for anti-spam rules.

   Similarly, a node may be configured to use Cryptographically
   Generated Addresses [RFC3972] by default, as in Secure Neighbor
   Discovery [RFC3971], but an application may prefer not to use it; for
   instance, fping [FPING], a debugging tool that tests basic
   reachability of multiple destinations by sending packets in parallel.
   These packets may end up initiating neighbor discovery signaling that
   uses SEND if used with a CGA source address.  SEND performs some
   cryptographic operations to prove ownership of the said CGA address.
   If the application does not require this feature, it would like to
   use a non-CGA address to avoid potentially expensive computations
   performed by SEND.  On the other hand, when a node is not configured
   for CGA as default, an application may prefer using CGA by setting
   the corresponding preference.

4.  Design Alternatives

   Some suggested to have per-application flags instead of per-socket
   and per-packet flags.  However, this design stays with per-socket and
   per-packet flags for the following reasons:

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   o  While some systems have per-environment/application flags (such as
      environment variables in Unix systems) this might not be available
      in all systems that implement the socket API.

   o  When an application links with some standard library, that library
      might use the socket API while the application is unaware of that
      fact.  Mechanisms that would provide per-application flags may
      affect not only the application itself but also the libraries,
      hence, creating risks of unintended consequences.

   Instead of the pair of 'flag' and 'opposite flag' for each rule that
   can be modified, the socket option could have been defined to use a
   single 'flag' value for each rule.  This would still have allowed
   different implementations to have different default settings as long
   as the applications were coded to first retrieve the default setting
   (using getsockopt()), and then clear or set the 'flag' according to
   their preferences, and finally set the new value with setsockopt().

   But such an approach would not be possible for getaddrinfo() because
   all the preferences would need to be expressible in the parameters
   that are passed with a single getaddrinfo() call.  Hence, for
   consistency, the 'flag' and 'opposite flag' approach is used for both
   getaddrinfo() and setsockopt().

   Thus, in this API document, an application has three choices on
   source address selection:

      a) The application wants to use an address with flag X: Set flag
      X; unset opposite/contradictory flags of X if they are set before.

      b) The application wants to use an address with 'opposite' or
      contradictory flag of X: Set opposite or contradictory flag of X;
      unset flag X, if already set.

      c) The application does not care about the presence of flag X and
      would like to use default: No need to set any address preference
      flags through setsockopt() or getaddrinfo(); unset any address
      preference flags if they are set before by the same socket.

5.  Address Preference Flags

   The following flags are defined to alter or set the default rule of
   source address selection rules discussed in default address selection
   specification [RFC3484].

      IPV6_PREFER_SRC_HOME /* Prefer Home address as source */

      IPV6_PREFER_SRC_COA /* Prefer Care-of address as source */

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      IPV6_PREFER_SRC_TMP /* Prefer Temporary address as source */

      IPV6_PREFER_SRC_PUBLIC /* Prefer Public address as source */

      IPV6_PREFER_SRC_CGA /* Prefer CGA address as source */

      IPV6_PREFER_SRC_NONCGA /* Prefer a non-CGA address as source */

   These flags can be combined together in a flag-set to express more
   complex address preferences.  However, such combinations can result
   in a contradictory flag-set, for example:






   Examples of valid combinations of address selection flags are given





   If a flag-set includes a combination of 'X' and 'Y', and if 'Y' is
   not applicable or available in the system, then the selected address
   has attribute 'X' and system default for the attribute 'Y'.  For
   example, on a system that has only public addresses, the valid
   combination of flags:


   would result in the selected address being a public home address,
   since no temporary addresses are available.

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6.  Additions to the Socket Interface

   The IPv6 Basic Socket API [RFC3493] defines socket options for IPv6.
   To allow applications to influence address selection mechanisms, this
   document adds a new socket option at the IPPROTO_IPV6 level.  This
   socket option is called IPV6_ADDR_PREFERENCES.  It can be used with
   setsockopt() and getsockopt() calls to set and get the address
   selection preferences affecting all packets sent via a given 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 address selection specification.

   The following flags are defined to alter or set the default rule of
   source address selection rules discussed in default address selection
   specification [RFC3484].  They are defined as a result of including
   the <netinet/in.h> header:

      IPV6_PREFER_SRC_HOME /* Prefer Home address as source */

      IPV6_PREFER_SRC_COA /* Prefer Care-of address as source */

      IPV6_PREFER_SRC_TMP /* Prefer Temporary address as source */

      IPV6_PREFER_SRC_PUBLIC /* Prefer Public address as source */

      IPV6_PREFER_SRC_CGA /* Prefer CGA address as source */

      IPV6_PREFER_SRC_NONCGA /* Prefer a non-CGA address as source */

   NOTE: No source preference flag for the longest matching prefix is
   defined here because it is believed to be handled by the policy table
   defined in the default address selection specification.

   When the IPV6_ADDR_PREFERENCES is successfully set with setsockopt(),
   the option value given is used to specify the address preference for
   any connection initiation through the socket and all subsequent
   packets sent via that socket.  If no option is set, the system
   selects a default value as per default address selection algorithm or
   by some other equivalent means.

   Setting contradictory flags at the same time results in the error

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7.  Additions to the Protocol-Independent Nodename Translation

   Section 8 of the Default Address Selection [RFC3484] document
   indicates possible implementation strategies for getaddrinfo()
   [RFC3493].  One of them suggests that getaddrinfo() collects
   available source/destination pairs from the network layer after being
   sorted at the network layer with full knowledge of source address
   selection.  Another strategy is to call down to the network layer to
   retrieve source address information and then sort the list in the
   context of getaddrinfo().

   This implies that getaddrinfo() should be aware of the address
   selection preferences of the application, since getaddrinfo() is
   independent of any socket the application might be using.

   Thus, if an application alters the default address selection rules by
   using setsockopt() with the IPV6_ADDR_PREFERENCES option, the
   application should also use the corresponding address selection
   preference flags with its getaddrinfo() call.

   For that purpose, the addrinfo data structure defined in Basic IPV6
   Socket API Extension [RFC3493] has been extended with an extended
   "ai_eflags" flag-set field to provide the designers freedom from
   adding more flags as necessary without crowding the valuable bit
   space in the "ai_flags" flag-set field.  The extended addrinfo data
   structure is defined as a result of including the <netdb.h> header:

    struct addrinfo {
        int ai_flags;             /* input flags */
        int ai_family;            /* protocol family for socket */
        int ai_socktype;          /* socket type */
        int ai_protocol;          /* protocol for socket */
        socklen_t ai_addrlen;     /* length of socket address */
        char *ai_canonname;       /* canonical name for hostname */
        struct sockaddr *ai_addr; /* socket address for socket */
        struct addrinfo *ai_next; /* pointer to next in list */
        int ai_eflags;            /* Extended flags for special usage */

   Note that the additional field for extended flags are added at the
   bottom of the addrinfo structure to preserve binary compatibility of
   the new functionality with the old applications that use the existing
   addrinfo data structure.

   A new flag (AI_EXTFLAGS) is defined for the "ai_flags" flag-set field
   of the addrinfo data structure to tell the system to look for the
   "ai_eflags" extended flag-set field in the addrinfo structure.  It is
   defined in the <netdb.h> header:

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      AI_EXTFLAGS /* extended flag-set present */

   If the AI_EXTFLAGS flag is set in "ai_flags" flag-set field of the
   addrinfo data structure, then the getaddrinfo() implementation MUST
   look for the "ai_eflags" values stored in the extended flag-set field
   "ai_eflags" of the addrinfo data structure.  The flags stored in the
   "ai_eflags" field are only meaningful if the AI_EXTFLAGS flag is set
   in the "ai_flags" flag-set field of the addrinfo data structure.  By
   default, AI_EXTFLAGS is not set in the "ai_flags" flag-set field.  If
   AI_EXTFLAGS is set in the "ai_flags" flag-set field, and the
   "ai_eflags" extended flag-set field is 0 (zero) or undefined, then
   AI_EXTFLAGS is ignored.

   The IPV6 source address preference values (IPV6_PREFER_SRC_*) defined
   for the IPV6_ADDR_PREFERENCES socket option are also defined as
   address selection preference flags for the "ai_eflags" extended flag-
   set field of the addrinfo data structure, so that getaddrinfo() can
   return matching destination addresses corresponding to the source
   address preferences expressed by the caller application.

   Thus, an application passes source address selection hints to
   getaddrinfo by setting AI_EXTFLAGS in the "ai_flags" field of the
   addrinfo structure, and the corresponding address selection
   preference flags (IPV6_PREFER_SRC_*) in the "ai_eflags" field.

   Currently, AI_EXTFLAGS is defined for the AF_INET6 socket protocol
   family only.  But its usage should be extendable to other socket
   protocol families -- such as AF_INET or as appropriate.

   If contradictory flags, such as IPV6_PREFER_SRC_HOME and
   IPV6_PREFER_SRC_COA, are set in ai_eflags, the getaddrinfo() fails
   and return the value EAI_BADEXTFLAGS, defined as a result of
   including the <netdb.h> header.  This error value MUST be interpreted
   into a descriptive text string when passed to the gai_strerror()
   function [RFC3493].

8.  Application Requirements

   An application should call getsockopt() prior to calling setsockopt()
   if the application needs to be able to restore the socket back to the
   system default preferences.  Note that this is suggested for
   portability.  An application that does not have this requirement can
   just use getaddrinfo() while specifying its preferences, followed by:

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      uint32_t flags = IPV6_PREFER_SRC_TMP;

      if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
                     (void *) &flags, sizeof (flags)) == -1) {
          perror("setsockopt IPV6_ADDR_REFERENCES");

   An application that needs to be able to restore the default settings
   on the socket would instead do this:

      uint32_t save_flags, flags;
      int optlen = sizeof (save_flags);

      /* Save the existing IPv6_ADDR_PREFERENCE flags now */

      if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
                     (void *) &save_flags, &optlen) == -1 {
          perror("getsockopt IPV6_ADDR_REFERENCES");

      /* Set the new flags */
      flags = IPV6_PREFER_SRC_TMP;
      if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
                  (void *) &flags, sizeof (flags)) == -1) {
          perror("setsockopt IPV6_ADDR_REFERENCES");

       *  Do some work with the socket here.

      /* Restore the flags */

      if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDR_PREFERENCES,
                  (void *) &save_flags, sizeof (save_flags)) == -1) {
          perror("setsockopt IPV6_ADDR_REFERENCES");

   Applications should not set contradictory flags at the same time.

   In order to allow different implementations to do different parts of
   address selection in getaddrinfo() and in the protocol stack, this
   specification requires that applications set the semantically
   equivalent flags when calling getaddrinfo() and setsockopt().  For
   example, if the application sets the IPV6_PREFER_SRC_COA flag, it
   MUST use the same for the "ai_eflag" field of the addrinfo data

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   structure when calling getaddrinfo().  If applications are not
   setting the semantically equivalent flags, the behavior of the
   implementation is undefined.

9.  Usage Example

   An example of usage of this API is given below:

    struct addrinfo hints, *ai, *ai0;
    uint32_t preferences;

    preferences = IPV6_PREFER_SRC_TMP;

    hints.ai_flags |= AI_EXTFLAGS;
    hints.ai_eflags = preferences;  /* Chosen address preference flag */
    /* Fill in other hints fields */

    getaddrinfo(....,&hints,. &ai0..);

    /* Loop over all returned addresses and do connect  */
    for (ai = ai0; ai; ai = ai->ai_next) {
        s = socket(ai->ai_family, ...);

        setsockopt(s, IPV6_ADDR_PREFERENCES, (void *) &preferences,
                   sizeof (preferences));

        if (connect(s, ai->ai_addr, ai->ai_addrlen) == -1){
            close (s);
            s = -1;



10.  Implementation Notes

   o  Within the same application, if a specific source address is set
      by either bind() or IPV6_PKTINFO socket option, while at the same
      time an address selection preference is expressed with the
      IPV6_ADDR_PREFERENCES socket option, then the source address
      setting carried by bind() or IPV6_PKTINFO takes precedence over
      the address selection setting.

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   o  setsockopt() and getaddrinfo() should silently ignore any address
      preference flags that are not supported in the system.  For
      example, a host that does not implement Mobile IPv6, should not
      fail setsockopt() or getaddrinfo() that specify preferences for
      home or care-of addresses.  The socket option calls should return
      error (-1) and set errno to EINVAL when contradictory flags values
      are passed to them.

   o  If an implementation supports both stream and datagram sockets, it
      should implement the address preference mechanism API described in
      this document on both types of sockets.

   o  An implementation supporting this API MUST implement both
      getaddrinfo() extension flags and socket option flags processing
      for portability of applications.

   o  The following flags are set as default values on a system (which
      is consistent with [RFC3484] defaults):




11.  Mapping to Default Address Selection Rules

   This API defines only those flags that are deemed to be useful by the
   applications to alter default address selection rules.  Thus, we
   discuss the mapping of each set of flags to the corresponding rule
   number in the address selection document [RFC3484].

      Source address selection rule #4 (prefer home address):

      IPV6_PREFER_SRC_HOME (default)


      Source address selection rule #7 (prefer public address):

      IPV6_PREFER_SRC_PUBLIC (default)


   At this time, this document does not define flags to alter source
   address selection rule #2 (prefer appropriate scope for destination)
   and destination address selection rule #8 (prefer smaller scope), as
   the implementers felt that there were no practical applications that

Top      ToC       Page 15 
   can take advantage of reverting the scoping rules of IPv6 default
   address selection.  Flags altering other destination address
   selection rules (#4, prefer home address and #7, prefer native
   transport) could have applications, but the problem is that the local
   system cannot systematically determine whether a destination address
   is a tunnel address for destination rule #7 (although it can when the
   destination address is one of its own, or can be syntactically
   recognized as a tunnel address, e.g., a 6-to-4 address.)  The flags
   defined for source address selection rule #4 (prefer home address)
   should also take care of destination address selection rule #4.
   Thus, at this point, it was decided not to define flags for these
   destination rules.

   Also, note that there is no corresponding destination address
   selection rule for source address selection rule #7 (prefer public
   addresses) of default address selection document [RFC3484].  However,
   this API provides a way for an application to make sure that the
   source address preference set in setsockopt() is taken into account
   by the getaddrinfo() function.  Let's consider an example to
   understand this scenario.  DA and DB are two global destination
   addresses and the node has two global source addresses SA and SB
   through interface A and B respectively.  SA is a temporary address
   while SB is a public address.  The application has set
   IPV6_PREFER_SRC_TMP in the setsockopt() flag.  The route to DA points
   to interface A and the route to DB points to interface B. Thus, when
   AI_EXTFLAGS in ai_flags and IPV6_PREFER_SRC_TMP in ai_eflags are set,
   getaddrinfo() returns DA before DB in the list of destination
   addresses and thus, SA will be used to communicate with the
   destination DA.  Similarly, getaddrinfo() returns DB before DA when
   AI_EXTFLAGS and ai_eflags are set to IPV6_PREFER_SRC_PUBLIC.  Thus,
   the source address preference is taking effect into destination
   address selection as well as source address selection by the
   getaddrinfo() function.

   The following numerical example clarifies the above further.

   Imagine a host with two addresses:

      1234::1:1 public

      9876::1:2 temporary

   The destination has the following two addresses:



Top      ToC       Page 16 
   By default, getaddrinfo() will return the destination addresses in
   the following order:



   because the public source is preferred and 1234 matches more bits
   with the public source address.  On the other hand, if ai_flags is
   set to AI_EXTFLAGS and ai_eflags to IPV6_PREFER_SRC_TMP, getaddrinfo
   will return the addresses in the reverse order since the temporary
   source address will be preferred.

   Other source address rules (that are not mentioned here) were also
   deemed not applicable for changing its default on a per-application

12.  IPv4-Mapped IPv6 Addresses

   IPv4-mapped IPv6 addresses for AF_INET6 sockets are supported in this
   API.  In some cases, the application of IPv4-mapped addresses are
   limited because the API attributes are IPv6 specific.  For example,
   IPv6 temporary addresses and cryptographically generated addresses
   have no IPv4 counterparts.  Thus, the IPV6_PREFER_SRC_TMP or
   IPV6_PREFER_SRC_CGA are not directly applicable to an IPv4-mapped
   IPv6 address.  However, the IPv4-mapped address support may be useful
   for mobile-IPv4 applications shifting the source address between the
   home address and the care-of address.  Thus, the IPV6_PREFER_SRC_COA
   and IPV6_PREFER_SRC_HOME are applicable to an IPv4-mapped IPv6
   address.  At this point, it is not well understood whether this
   particular API has any value to IPv4 addresses or AF_INET family of
   sockets, but a similar model still applies to AF_INET socket family
   if corresponding address flags are defined.

13.  Validating Source Address Preferences

   Sometimes an application may have a requirement to only use addresses
   with some particular attribute, and if no such address is available,
   the application should fail to communicate instead of communicating
   using the 'wrong' address.  In that situation, address selection
   preferences do not guarantee that the application requirements are
   met.  Instead, the application has to use a new call that binds a
   socket to the source address that would be selected to communicate
   with a given destination address, according to its preferences, and
   then explicitly verify that the chosen address satisfies its
   requirements using a validation function.  Such an application would
   go through the following steps:

Top      ToC       Page 17 
   1.  The application specifies one or more IPV6_PREFER_SRC_* flags and
       AI_EXTFLAGS ai_flags with getaddrinfo().

   2.  The application specifies the same IPV6_PREFER_SRC_* flags with

   3.  The application calls the stack to select a source address to
       communicate with the specified destination address, according to
       the expressed address selection preferences.  This is achieved
       with a connect() call, or a bind2addrsel() call as specified
       below.  The connect() function must not be used when the
       application uses connection-oriented communication (e.g., TCP)
       and want to ensure that no single packet (e.g., TCP SYN) is sent
       before the application could verify that its requirements were
       fulfilled.  Instead, the application must use the newly
       introduced bind2addrsel() call, which binds a socket to the
       source address that would be selected to communicate with a given
       destination address, according to the application's preferences.
       For datagram-oriented communications (e.g., UDP), the connect()
       call can be used since it results in the stack selecting a source
       address without sending any packets.

   4.  Retrieve the selected source address using the getsockname() API

   5.  Verify with the validation function that the retrieved address is
       satisfactory as specified below.  If not, abort the
       communication, e.g., by closing the socket.

   The binding of the socket to the address that would be selected to
   communicate with a given destination address, according to the
   application preferences, is accomplished via a new binding function
   defined for this purpose:

      #include <netinet/in.h>

      int bind2addrsel(int s, const struct sockaddr *dstaddr,
                       socklen_t dstaddrlen);

   where s is the socket that source address selection preferences have
   been expressed by the application, the dstaddr is a non-NULL pointer
   to a sockaddr_in6 structure initialized as follows:

   o  sin6_addr is a 128-bit IPv6 destination address with which the
      local node wants to communicate;

   o  sin6_family MUST be set to AF_INET6;

Top      ToC       Page 18 
   o  sin6_scope_id MUST be set if the address is link-local;

   and dstaddrlen is the size of the sockaddr structure passed as

   The bind2addrsel() call is defined to return the same values as the
   bind() call, i.e., 0 if successful, -1 otherwise while the global
   variable errno is set to indicate the error.  The bind2addrsel() call
   fails for the same reasons that the bind() call.

   The verification of temporary vs. public, home vs. care-of, CGA vs.
   not, are performed by a new validation function defined for this

      #include <netinet/in.h>

      short inet6_is_srcaddr(struct sockaddr_in6 *srcaddr,
                             uint32_t flags);

   where the flags contain the specified IPV6_PREFER_SRC_* source
   preference flags, and the srcaddr is a non-NULL pointer to a
   sockaddr_in6 structure initialized as follows:

   o  sin6_addr is a 128-bit IPv6 address of the local node.

   o  sin6_family MUST be set to AF_INET6.

   o  sin6_scope_id MUST be set if the address is link-local.

   inet6_is_srcaddr() is defined to return three possible values (0, 1,
   -1): The function returns true (1) when the IPv6 address corresponds
   to a valid address in the node and satisfies the given preference
   flags.  If the IPv6 address input value does not correspond to any
   address in the node or if the flags are not one of the valid
   preference flags, it returns a failure (-1).  If the input address
   does not match an address that satisfies the preference flags
   indicated, the function returns false (0.)

   This function can handle multiple valid preference flag combinations
   as its second parameter, for example, IPV6_PREFER_SRC_COA |
   IPV6_PREFER_SRC_TMP, which means that all flags MUST be satisfied for
   the result to be true.  Contradictory flag values result in a false
   return value.

   The function will return true for IPV6_PREFER_SRC_HOME even if the
   host is not implementing mobile IPv6, as well as for a mobile node
   that is at home (i.e., does not have any care-of address).

Top      ToC       Page 19 
14.  Summary of New Definitions

   The following list summarizes the constants, structure, and extern
   definitions discussed in this memo, sorted by header.

   <netdb.h>        AI_EXTFLAGS
   <netdb.h>        IPV6_PREFER_SRC_HOME
   <netdb.h>        IPV6_PREFER_SRC_COA
   <netdb.h>        IPV6_PREFER_SRC_TMP
   <netdb.h>        IPV6_PREFER_SRC_PUBLIC
   <netdb.h>        IPV6_PREFER_SRC_CGA
   <netdb.h>        IPV6_PREFER_SRC_NONCGA
   <netdb.h>        EAI_BADEXTFLAGS
   <netdb.h>        struct addrinfo{};

   <netinet/in.h>   IPV6_PREFER_SRC_HOME
   <netinet/in.h>   IPV6_PREFER_SRC_COA
   <netinet/in.h>   IPV6_PREFER_SRC_TMP
   <netinet/in.h>   IPV6_PREFER_SRC_PUBLIC
   <netinet/in.h>   IPV6_PREFER_SRC_CGA
   <netinet/in.h>   IPV6_PREFER_SRC_NONCGA
   <netinet/in.h>   short inet6_is_srcaddr(struct sockaddr_in6 *,
   <netinet/in.h>   int bind2addrsel(int, const struct sockaddr *,

15.  Security Considerations

   This document conforms to the same security implications as specified
   in the Basic IPv6 socket API [RFC3493] and address selection rules
   [RFC3484].  Allowing applications to specify a preference for
   temporary addresses provides per-application (and per-socket) ability
   to use the privacy benefits of the temporary addresses.  The setting
   of certain address preferences (e.g., not using a CGA address, or not
   using a temporary address) may be restricted to privileged processes
   because of security implications.

16.  Acknowledgments

   The authors like to thank members of Mobile-IP and IPV6 working
   groups for useful discussion on this topic.  Richard Draves and Dave
   Thaler suggested that getaddrinfo also needs to be considered along
   with the new socket option.  Gabriel Montenegro suggested that CGAs
   may also be considered in this document.  Thanks to Alain Durand,
   Renee Danson, Alper Yegin, Francis Dupont, Keiichi Shima, Michael
   Hunter, Sebastien Roy, Robert Elz, Pekka Savola, Itojun, Jim Bound,
   Jeff Boote, Steve Cipolli, Vlad Yasevich, Mika Liljeberg, Ted Hardie,
   Vidya Narayanan, and Lars Eggert for useful discussions and

Top      ToC       Page 20 
   suggestions.  Thanks to Remi Denis-Courmont, Brian Haberman, Brian
   Haley, Bob Gilligan, Jack McCann, Jim Bound, Jinmei Tatuya, Suresh
   Krishnan, Hilarie Orman, Geoff Houston, Marcelo Bungulo, and Jari
   Arkko for the review of this document and suggestions for

17.  References

17.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

17.2.  Informative References

   [FPING]    "Fping - a program to ping hosts in parallel", Online web

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

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Appendix A.  Per-Packet Address Selection Preference

   This document discusses setting source address selection preferences
   on a per-socket basis with the new IPV6_ADDR_PREFERENCES socket
   option used in setsockopt().  The document does not encourage setting
   the source address selection preference on a per-packet basis through
   the use of ancillary data objects with sendmsg(), or setsockopt()
   with unconnected datagram sockets.

   Per-packet source address selection is expensive, as the system will
   have to determine the source address indicated by the application
   preference before sending each packet, while setsockopt() address
   preference on a connected socket makes the selection once and uses
   that source address for all packets transmitted through that socket
   endpoint, as long as the socket option is set.

   However, this document provides guidelines for those implementations
   that like to have an option on implementing transmit-side ancillary
   data object support for altering default source address selection.
   Therefore, if an application chooses to use the per-packet source
   address selection, then the implementation should process at the
   IPPROTO_IPV6 level (cmsg_level) ancillary data object of type
   (cmsg_type) IPV6_ADDR_PREFERENCES containing as data (cmsg_data[]) a
   32-bit unsigned integer encoding the source address selection
   preference flags (e.g., IPV6_PREFER_SRC_COA | IPV6_PREFER_SRC_PUBLIC)
   in a fashion similar to the advanced IPV6 Socket API [RFC3542].  This
   address selection preference ancillary data object may be present
   along with other ancillary data objects.

   The implementation processing the ancillary data object is
   responsible for the selection of the preferred source address as
   indicated in the ancillary data object.  Thus, an application can use
   sendmsg() to pass an address selection preference ancillary data
   object to the IPv6 layer.  The following example shows usage of the
   ancillary data API for setting address preferences:

Top      ToC       Page 22 
   void *extptr;
   socklen_t extlen;
   struct msghdr msg;
   struct cmsghdr *cmsgptr;
   int cmsglen;
   struct sockaddr_in6 dest;
   uint32_t flags;

   extlen = sizeof(flags);
   cmsglen = CMSG_SPACE(extlen);
   cmsgptr = malloc(cmsglen);
   cmsgptr->cmsg_len = CMSG_LEN(extlen);
   cmsgptr->cmsg_level = IPPROTO_IPV6;
   cmsgptr->cmsg_type = IPV6_ADDR_PREFERENCES;

   extptr = CMSG_DATA(cmsgptr);

   flags = IPV6_PREFER_SRC_COA;
   memcpy(extptr, &flags, extlen);

   msg.msg_control = cmsgptr;
   msg.msg_controllen = cmsglen;

   /* finish filling in msg{} */

   msg.msg_name = dest;

   sendmsg(s, &msg, 0);

   Thus, when an IPV6_ADDR_PREFERENCES ancillary data object is passed
   to sendmsg(), the value included in the object is used to specify
   address preference for the packet being sent by sendmsg().

Appendix B.  Intellectual Property Statement

   This document only defines a source preference flag to choose
   Cryptographically Generated Address (CGA) as the source address when
   applicable.  CGAs are obtained using public keys and hashes to prove
   address ownership.  Several IPR claims have been made about such

Top      ToC       Page 23 
Authors' Addresses

   Erik Nordmark
   Sun Microsystems, Inc.
   17 Network Circle
   Menlo Park, CA 94025


   Samita Chakrabarti
   Azaire Networks
   3121 Jay Street, Suite 210
   Santa Clara, CA 95054


   Julien Laganier
   DoCoMo Euro-Labs
   Landsbergerstrasse 312
   D-80687 Muenchen


Top      ToC       Page 24 
Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
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