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rfc:rfc3678

Network Working Group D. Thaler Request for Comments: 3678 Microsoft Category: Informational B. Fenner

                                                         AT&T Research
                                                              B. Quinn
                                                          Stardust.com
                                                          January 2004
      Socket Interface Extensions for Multicast Source Filters

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.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

 The Internet Group Management Protocol (IGMPv3) for IPv4 and the
 Multicast Listener Discovery (MLDv2) for IPv6 add the capability for
 applications to express source filters on multicast group
 memberships, which allows receiver applications to determine the set
 of senders (sources) from which to accept multicast traffic.  This
 capability also simplifies support of one-to-many type multicast
 applications.
 This document specifies new socket options and functions to manage
 source filters for IP Multicast group memberships.  It also defines
 the socket structures to provide input and output arguments to these
 new application program interfaces (APIs).  These extensions are
 designed to provide access to the source filtering features, while
 introducing a minimum of change into the system and providing
 complete compatibility for existing multicast applications.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Design Considerations. . . . . . . . . . . . . . . . . . . . .  3
     2.1 What Needs to be Added . . . . . . . . . . . . . . . . . .  4
     2.2 Data Types . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3 Headers. . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.4 Structures . . . . . . . . . . . . . . . . . . . . . . . .  4
 3. Overview of APIs. . . . . . . . . . . . . . . . . . . . . . . .  5

Thaler, et al. Informational [Page 1] RFC 3678 Multicast Source Filter API January 2004

 4. IPv4 Multicast Source Filter APIs . . . . . . . . . . . . . . .  6
    4.1 Basic (Delta-based) API for IPv4. . . . . . . . . . . . . .  6
         4.1.1 IPv4 Any-Source Multicast API. . . . . . . . . . . .  7
         4.1.2 IPv4 Source-Specific Multicast API . . . . . . . . .  7
         4.1.3 Error Codes. . . . . . . . . . . . . . . . . . . . .  8
    4.2 Advanced (Full-state) API for IPv4. . . . . . . . . . . . .  8
         4.2.1 Set Source Filter. . . . . . . . . . . . . . . . . .  8
         4.2.2 Get Source Filter. . . . . . . . . . . . . . . . . .  9
 5: Protocol-Independent Multicast Source Filter APIs . . . . . . . 10
    5.1 Basic (Delta-based) API . . . . . . . . . . . . . . . . . . 10
         5.1.1 Any-Source Multicast API . . . . . . . . . . . . . . 11
         5.1.2 Source-Specific Multicast API. . . . . . . . . . . . 11
    5.2 Advanced (Full-state) API . . . . . . . . . . . . . . . . . 11
         5.2.1 Set Source Filter. . . . . . . . . . . . . . . . . . 11
         5.2.2 Get Source Filter. . . . . . . . . . . . . . . . . . 12
 6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 13
 7.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 13
 8.  Appendix A: Use of ioctl() for full-state operations . . . . . 14
     8.1. IPv4 Options. . . . . . . . . . . . . . . . . . . . . . . 14
     8.2. Protocol-Independent Options. . . . . . . . . . . . . . . 15
 9.  Normative References . . . . . . . . . . . . . . . . . . . . . 16
 10. Informative References . . . . . . . . . . . . . . . . . . . . 16
 11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
 12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 18

1. Introduction

 The de facto standard application program interface (API) for TCP/IP
 applications is the "sockets" interface.  Although this API was
 developed for Unix in the early 1980s it has also been implemented on
 a wide variety of non-Unix systems.  TCP/IP applications written
 using the sockets API have in the past enjoyed a high degree of
 portability and we would like the same portability with applications
 that employ multicast source filters.  Changes are required to the
 sockets API to support such filtering and this memo describes these
 changes.
 This document specifies new socket options and functions to manage
 source filters for IP Multicast group memberships.  It also defines
 the socket structures to provide input and output arguments to these
 new APIs.  These extensions are designed to provide access to the
 source filtering features required by applications, while introducing
 a minimum of change into the system and providing complete
 compatibility for existing multicast applications.
 Furthermore, RFC 3493 [1] defines socket interface extensions for
 IPv6, including protocol-independent functions for most operations.

Thaler, et al. Informational [Page 2] RFC 3678 Multicast Source Filter API January 2004

 However, while it defines join and leave functions for IPv6, it does
 not provide protocol-independent versions of these operations.  Such
 functions will be described in this document.
 The reader should note that this document is for informational
 purposes only, and that the official standard specification of the
 sockets API is [2].

2. Design Considerations

 There are a number of important considerations in designing changes
 to this well-worn API:
    o  The API changes should provide both source and binary
       compatibility for programs written to the original API.  That
       is, existing program binaries should continue to operate when
       run on a system supporting the new API.  In addition, existing
       applications that are re-compiled and run on a system
       supporting the new API should continue to operate.  Simply put,
       the API changes for multicast receivers that specify source
       filters should not break existing programs.
    o  The changes to the API should be as small as possible in order
       to simplify the task of converting existing multicast receiver
       applications to use source filters.
    o  Applications should be able to detect when the new source
       filter APIs are unavailable (e.g., calls fail with the ENOTSUPP
       error) and react gracefully (e.g., revert to old non-source-
       filter API or display a meaningful error message to the user).
    o  Lack of type-safety in an API is a bad thing which should be
       avoided when possible.
 Several implementations exist that use ioctl() for a portion of the
 functionality described herein, and for historical purposes, the
 ioctl API is documented in Appendix A.  The preferred API, however,
 includes new functions.  The reasons for adding new functions are:
    o  New functions provide type-safety, unlike ioctl, getsockopt,
       and setsockopt.
    o  A new function can be written as a wrapper over an ioctl,
       getsockopt, or setsockopt call, if necessary.  Hence, it
       provides more freedom as to how the functionality is
       implemented in an operating system.  For example, a new
       function might be implemented as an inline function in an

Thaler, et al. Informational [Page 3] RFC 3678 Multicast Source Filter API January 2004

       include file, or a function exported from a user-mode library
       which internally uses some mechanism to exchange information
       with the kernel, or be implemented directly in the kernel.
    o  At least one operation defined herein needs to be able to both
       pass information to the TCP/IP stack, as well as retrieve
       information from it.  In some implementations this is
       problematic without either changing getsockopt or using ioctl.
       Using new functions avoids the need to change such
       implementations.

2.1. What Needs to be Added

 The current IP Multicast APIs allow a receiver application to specify
 the group address (destination) and (optionally) the local interface.
 These existing APIs need not change (and cannot, to retain binary
 compatibility).  Hence, what is needed are new source filter APIs
 that provide the same functionality and also allow receiver multicast
 applications to:
    o  Specify zero or more unicast (source) address(es) in a source
       filter.
    o  Determine whether the source filter describes an inclusive or
       exclusive list of sources.
 The new API design must enable this functionality for both IPv4 and
 IPv6.

2.2. Data Types

 The data types of the structure elements given in this memo are
 intended to be examples, not absolute requirements.  Whenever
 possible, data types from POSIX 1003.1g [2] are used: uintN_t means
 an unsigned integer of exactly N bits (e.g., uint32_t).

2.3. Headers

 When function prototypes and structures are shown, we show the
 headers that must be #included to cause that item to be defined.

2.4. Structures

 When structures are described, the members shown are the ones that
 must appear in an implementation.  Additional, nonstandard members
 may also be defined by an implementation.  As an additional

Thaler, et al. Informational [Page 4] RFC 3678 Multicast Source Filter API January 2004

 precaution, nonstandard members could be verified by Feature Test
 Macros as described in [2].  (Such Feature Test Macros are not
 defined by this RFC.)
 The ordering shown for the members of a structure is the recommended
 ordering, given alignment considerations of multibyte members, but an
 implementation may order the members differently.

3. Overview of APIs

 There are a number of different APIs described in this document that
 are appropriate for a number of different application types and IP
 versions.  Before providing detailed descriptions, this section
 provides a "taxonomy" with a brief description of each.
 There are two categories of source-filter APIs, both of which are
 designed to allow multicast receiver applications to designate the
 unicast address(es) of sender(s) along with the multicast group
 (destination address) to receive.
    o  Basic (Delta-based): Some applications desire the simplicity of
       a delta-based API in which each function call specifies a
       single source address which should be added to or removed from
       the existing filter for a given multicast group address on
       which to listen.  Such applications typically fall into either
       of two categories:
       +  Any-Source Multicast: By default, all sources are accepted.
          Individual sources may be turned off and back on as needed
          over time.  This is also known as "exclude" mode, since the
          source filter contains a list of excluded sources.
       +  Source-Specific Multicast: Only sources in a given list are
          allowed.  The list may change over time.  This is also known
          as "include" mode, since the source filter contains a list
          of included sources.
          This API would be used, for example, by "single-source"
          applications such as audio/video broadcasting.  It would
          also be used for logical multi-source sessions where each
          source independently allocates its own Source-Specific
          Multicast group address.
    o  Advanced (Full-state): This API allows an application to define
       a complete source-filter comprised of zero or more source
       addresses, and replace the previous filter with a new one.

Thaler, et al. Informational [Page 5] RFC 3678 Multicast Source Filter API January 2004

       Applications which require the ability to switch between filter
       modes without leaving a group must use a full-state API (i.e.,
       to change the semantics of the source filter from inclusive to
       exclusive, or vice versa).
       Applications which use a large source list for a given group
       address should also use the full-state API, since filter
       changes can be done atomically in a single operation.
 The above types of APIs exist in IPv4-specific variants as well as
 with protocol-independent variants.  One might ask why the protocol-
 independent APIs cannot accommodate IPv4 applications as well as
 IPv6.  Since any IPv4 application requires modification to use
 multicast source filters anyway, it might seem like a good
 opportunity to create IPv6-compatible source code.
 The primary reasons for extending an IPv4-specific API are:
    o  To minimize changes needed in existing IPv4 multicast
       application source code to add source filter support.
    o  To avoid overloading APIs to accommodate the differences
       between IPv4 interface addresses (e.g., in the ip_mreq
       structure) and interface indices.

4. IPv4 Multicast Source Filter APIs

 Version 3 of the Internet Group Management Protocol (IGMPv3) [3] and
 version 2 of the Multicast Listener Discovery (MLDv2) protocol [4]
 provide the ability to communicate source filter information to the
 router and hence avoid pulling down data from unwanted sources onto
 the local link.  However, source filters may be implemented by the
 operating system regardless of whether the routers support IGMPv3 or
 MLDv2, so when the source-filter API is available, applications can
 always benefit from using it.

4.1. Basic (Delta-based) API for IPv4

 The reception of multicast packets is controlled by the setsockopt()
 options summarized below.  An error of EOPNOTSUPP is returned if
 these options are used with getsockopt().

Thaler, et al. Informational [Page 6] RFC 3678 Multicast Source Filter API January 2004

 The following structures are used by both the Any-Source Multicast
 and the Source-Specific Multicast API:
 #include <netinet/in.h>
 struct ip_mreq {
    struct in_addr imr_multiaddr;  /* IP address of group */
    struct in_addr imr_interface;  /* IP address of interface */
 };
 struct ip_mreq_source {
    struct in_addr imr_multiaddr;  /* IP address of group */
    struct in_addr imr_sourceaddr; /* IP address of source */
    struct in_addr imr_interface;  /* IP address of interface */
 };

4.1.1. IPv4 Any-Source Multicast API

 The following socket options are defined in <netinet/in.h> for
 applications in the Any-Source Multicast category:
 Socket option             Argument type
 IP_ADD_MEMBERSHIP         struct ip_mreq
 IP_BLOCK_SOURCE           struct ip_mreq_source
 IP_UNBLOCK_SOURCE         struct ip_mreq_source
 IP_DROP_MEMBERSHIP        struct ip_mreq
 IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP are already implemented on
 most operating systems, and are used to join and leave an any-source
 group.
 IP_BLOCK_SOURCE can be used to block data from a given source to a
 given group (e.g., if the user "mutes" that source), and
 IP_UNBLOCK_SOURCE can be used to undo this (e.g., if the user then
 "unmutes" the source).

4.1.2. IPv4 Source-Specific Multicast API

 The following socket options are available for applications in the
 Source-Specific category:
 Socket option             Argument type
 IP_ADD_SOURCE_MEMBERSHIP  struct ip_mreq_source
 IP_DROP_SOURCE_MEMBERSHIP struct ip_mreq_source
 IP_DROP_MEMBERSHIP        struct ip_mreq
 IP_ADD_SOURCE_MEMBERSHIP and IP_DROP_SOURCE_MEMBERSHIP are used to
 join and leave a source-specific group.

Thaler, et al. Informational [Page 7] RFC 3678 Multicast Source Filter API January 2004

 IP_DROP_MEMBERSHIP is supported, as a convenience, to drop all
 sources which have been joined for a particular group and interface.
 The operations are the same as if the socket had been closed.

4.1.3. Error Codes

 When the option would be legal on the group, but an address is
 invalid (e.g., when trying to block a source that is already blocked
 by the socket, or when trying to drop an unjoined group) the error
 generated is EADDRNOTAVAIL.
 When the option itself is not legal on the group (i.e., when trying a
 Source-Specific option on a group after doing IP_ADD_MEMBERSHIP, or
 when trying an Any-Source option without doing IP_ADD_MEMBERSHIP) the
 error generated is EINVAL.
 When any of these options are used with getsockopt(), the error
 generated is EOPNOTSUPP.
 Finally, if the implementation imposes a limit on the maximum number
 of sources in a source filter, ENOBUFS is generated when an operation
 would exceed the maximum.

4.2. Advanced (Full-state) API for IPv4

 Several implementations exist that use ioctl() for this API, and for
 historical purposes, the ioctl() API is documented in Appendix A.
 The preferred API uses the new functions described below.

4.2.1. Set Source Filter

   #include <netinet/in.h>
   int setipv4sourcefilter(int s, struct in_addr interface,
                           struct in_addr group, uint32_t fmode,
                           uint32_t numsrc, struct in_addr *slist);
 On success the value 0 is returned, and on failure, the value -1 is
 returned and errno is set accordingly.
 The s argument identifies the socket.
 The interface argument holds the local IP address of the interface.
 The group argument holds the IP multicast address of the group.

Thaler, et al. Informational [Page 8] RFC 3678 Multicast Source Filter API January 2004

 The fmode argument identifies the filter mode.  The value of this
 field must be either MCAST_INCLUDE or MCAST_EXCLUDE, which are
 likewise defined in <netinet/in.h>.
 The numsrc argument holds the number of source addresses in the slist
 array.
 The slist argument points to an array of IP addresses of sources to
 include or exclude depending on the filter mode.
 If the implementation imposes a limit on the maximum number of
 sources in a source filter, ENOBUFS is generated when the operation
 would exceed the maximum.

4.2.2. Get Source Filter

   #include <netinet/in.h>
   int getipv4sourcefilter(int s, struct in_addr interface,
                           struct in_addr group, uint32_t *fmode,
                           uint32_t *numsrc, struct in_addr *slist);
 On success the value 0 is returned, and on failure, the value -1 is
 returned and errno is set accordingly.
 The s argument identifies the socket.
 The interface argument holds the local IP address of the interface.
 The group argument holds the IP multicast address of the group.
 The fmode argument points to an integer that will contain the filter
 mode on a successful return.  The value of this field will be either
 MCAST_INCLUDE or MCAST_EXCLUDE, which are likewise defined in
 <netinet/in.h>.
 On input, the numsrc argument holds the number of source addresses
 that will fit in the slist array.  On output, the numsrc argument
 will hold the total number of sources in the filter.
 The slist argument points to buffer into which an array of IP
 addresses of included or excluded (depending on the filter mode)
 sources will be written.  If numsrc was 0 on input, a NULL pointer
 may be supplied.

Thaler, et al. Informational [Page 9] RFC 3678 Multicast Source Filter API January 2004

 If the application does not know the size of the source list
 beforehand, it can make a reasonable guess (e.g., 0), and if upon
 completion, numsrc holds a larger value, the operation can be
 repeated with a large enough buffer.
 That is, on return, numsrc is always updated to be the total number
 of sources in the filter, while slist will hold as many source
 addresses as fit, up to the minimum of the array size passed in as
 the original numsrc value and the total number of sources in the
 filter.

5. Protocol-Independent Multicast Source Filter APIs

 Protocol-independent functions are provided for join and leave
 operations so that an application may pass a sockaddr_storage
 structure obtained from calls such as getaddrinfo() [1] as the group
 to join.  For example, an application can resolve a DNS name (e.g.,
 NTP.MCAST.NET) to a multicast address which may be either IPv4 or
 IPv6, and may easily join and leave the group.

5.1. Basic (Delta-based) API

 The reception of multicast packets is controlled by the setsockopt()
 options summarized below.  An error of EOPNOTSUPP is returned if
 these options are used with getsockopt().
 The following structures are used by both the Any-Source Multicast
 and the Source-Specific Multicast API:  #include <netinet/in.h>
 struct group_req {
    uint32_t                gr_interface; /* interface index */
    struct sockaddr_storage gr_group;     /* group address */
 };
 struct group_source_req {
    uint32_t                gsr_interface; /* interface index */
    struct sockaddr_storage gsr_group;     /* group address */
    struct sockaddr_storage gsr_source;    /* source address */
 };
 The sockaddr_storage structure is defined in RFC 3493 [1] to be large
 enough to hold either IPv4 or IPv6 address information.
 The rules for generating errors are the same as those given in
 Section 5.1.3.

Thaler, et al. Informational [Page 10] RFC 3678 Multicast Source Filter API January 2004

5.1.1. Any-Source Multicast API

 Socket option               Argument type
 MCAST_JOIN_GROUP            struct group_req
 MCAST_BLOCK_SOURCE          struct group_source_req
 MCAST_UNBLOCK_SOURCE        struct group_source_req
 MCAST_LEAVE_GROUP           struct group_req
 MCAST_JOIN_GROUP and MCAST_LEAVE_GROUP are used to join and leave an
 any-source group.
 MCAST_BLOCK_SOURCE can be used to block data from a given source to a
 given group (e.g., if the user "mutes" that source), and
 MCAST_UNBLOCK_SOURCE can be used to undo this (e.g., if the user then
 "unmutes" the source).

5.1.2. Source-Specific Multicast API

 Socket option               Argument type
 MCAST_JOIN_SOURCE_GROUP     struct group_source_req
 MCAST_LEAVE_SOURCE_GROUP    struct group_source_req
 MCAST_LEAVE_GROUP           struct group_req
 MCAST_JOIN_SOURCE_GROUP and MCAST_LEAVE_SOURCE_GROUP are used to join
 and leave a source-specific group.
 MCAST_LEAVE_GROUP is supported, as a convenience, to drop all sources
 which have been joined for a particular group and interface.  The
 operations are the same as if the socket had been closed.

5.2. Advanced (Full-state) API

 Implementations may exist that use ioctl() for this API, and for
 historical purposes, the ioctl() API is documented in Appendix A.
 The preferred API uses the new functions described below.

5.2.1. Set Source Filter

   #include <netinet/in.h>
   int setsourcefilter(int s, uint32_t interface,
                       struct sockaddr *group, socklen_t grouplen,
                       uint32_t fmode, uint_t numsrc,
                       struct sockaddr_storage *slist);
 On success the value 0 is returned, and on failure, the value -1 is
 returned and errno is set accordingly.

Thaler, et al. Informational [Page 11] RFC 3678 Multicast Source Filter API January 2004

 The s argument identifies the socket.
 The interface argument holds the interface index of the interface.
 The group argument points to either a sockaddr_in structure (for
 IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP
 multicast address of the group.
 The grouplen argument gives the length of the sockaddr_in or
 sockaddr_in6 structure.
 The fmode argument identifies the filter mode.  The value of this
 field must be either MCAST_INCLUDE or MCAST_EXCLUDE, which are
 likewise defined in <netinet/in.h>.
 The numsrc argument holds the number of source addresses in the slist
 array.
 The slist argument points to an array of IP addresses of sources to
 include or exclude depending on the filter mode.
 If the implementation imposes a limit on the maximum number of
 sources in a source filter, ENOBUFS is generated when the operation
 would exceed the maximum.

5.2.2. Get Source Filter

   #include <netinet/in.h>
   int getsourcefilter(int s, uint32_t interface,
                       struct sockaddr *group, socklen_t grouplen,
                       uint32_t fmode, uint_t *numsrc,
                       struct sockaddr_storage *slist);
 On success the value 0 is returned, and on failure, the value -1 is
 returned and errno is set accordingly.
 The s argument identifies the socket.
 The interface argument holds the local IP address of the interface.
 The group argument points to either a sockaddr_in structure (for
 IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP
 multicast address of the group.

Thaler, et al. Informational [Page 12] RFC 3678 Multicast Source Filter API January 2004

 The fmode argument points to an integer that will contain the filter
 mode on a successful return.  The value of this field will be either
 MCAST_INCLUDE or MCAST_EXCLUDE, which are likewise defined in
 <netinet/in.h>.
 On input, the numsrc argument holds the number of source addresses
 that will fit in the slist array.  On output, the numsrc argument
 will hold the total number of sources in the filter.
 The slist argument points to buffer into which an array of IP
 addresses of included or excluded (depending on the filter mode)
 sources will be written.  If numsrc was 0 on input, a NULL pointer
 may be supplied.
 If the application does not know the size of the source list
 beforehand, it can make a reasonable guess (e.g., 0), and if upon
 completion, numsrc holds a larger value, the operation can be
 repeated with a large enough buffer.
 That is, on return, numsrc is always updated to be the total number
 of sources in the filter, while slist will hold as many source
 addresses as fit, up to the minimum of the array size passed in as
 the original numsrc value and the total number of sources in the
 filter.

6. Security Considerations

 Although source filtering can help to combat denial-of-service
 attacks, source filtering alone is not a complete solution, since it
 does not provide protection against spoofing the source address to be
 an allowed source.  Multicast routing protocols which use reverse-
 path forwarding based on the source address, however, do provide some
 natural protection against spoofing the source address, since if a
 router receives a packet on an interface other than the one toward
 the "real" source, it will drop the packet.  However, this still does
 not provide any guarantee of protection.

7. Acknowledgments

 This document was updated based on feedback from the IETF's IDMR and
 MAGMA Working Groups, and the Austin Group.  Wilbert de Graaf also
 provided many helpful comments.

Thaler, et al. Informational [Page 13] RFC 3678 Multicast Source Filter API January 2004

8. Appendix A: Use of ioctl() for full-state operations

 The API defined here is historic, but is documented here for
 informational purposes since it is implemented by multiple platforms.
 The new functions defined earlier in this document should now be used
 instead.
 Retrieving the source filter for a given group cannot be done with
 getsockopt() on some existing platforms, since the group and
 interface must be passed down in order to retrieve the correct
 filter, and getsockopt only supports an output buffer.  This can,
 however, be done with an ioctl(), and hence for symmetry, both gets
 and sets are done with an ioctl.

8.1. IPv4 Options

 The following are defined in <sys/sockio.h>:
    o  ioctl() SIOCGIPMSFILTER: to retrieve the list of source
       addresses that comprise the source filter along with the
       current filter mode.
    o  ioctl() SIOCSIPMSFILTER: to set or modify the source filter
       content (e.g., unicast source address list) or mode (exclude or
       include).

Ioctl option Argument type SIOCGIPMSFILTER struct ip_msfilter SIOCSIPMSFILTER struct ip_msfilter

struct ip_msfilter {

 struct in_addr imsf_multiaddr;  /* IP multicast address of group */
 struct in_addr imsf_interface;  /* local IP address of interface */
 uint32_t       imsf_fmode;      /* filter mode */
 uint32_t       imsf_numsrc;     /* number of sources in src_list */
 struct in_addr imsf_slist[1];   /* start of source list */

};

#define IP_MSFILTER_SIZE(numsrc) \

 (sizeof(struct ip_msfilter) - sizeof(struct in_addr) \
 + (numsrc) * sizeof(struct in_addr))
 The imsf_fmode mode is a 32-bit integer that identifies the filter
 mode.  The value of this field must be either MCAST_INCLUDE or
 MCAST_EXCLUDE, which are likewise defined in <netinet/in.h>.

Thaler, et al. Informational [Page 14] RFC 3678 Multicast Source Filter API January 2004

 The structure length pointed to must be at least IP_MSFILTER_SIZE(0)
 bytes long, and the imsf_numsrc parameter should be set so that
 IP_MSFILTER_SIZE(imsf_numsrc) indicates the buffer length.
 If the implementation imposes a limit on the maximum number of
 sources in a source filter, ENOBUFS is generated when a set operation
 would exceed the maximum.
 The result of a get operation (SIOCGIPMSFILTER) will be that the
 imsf_multiaddr and imsf_interface fields will be unchanged, while
 imsf_fmode, imsf_numsrc, and as many source addresses as fit will be
 filled into the application's buffer.
 If the application does not know the size of the source list
 beforehand, it can make a reasonable guess (e.g., 0), and if upon
 completion, the imsf_numsrc field holds a larger value, the operation
 can be repeated with a large enough buffer.
 That is, on return from SIOCGIPMSFILTER, imsf_numsrc is always
 updated to be the total number of sources in the filter, while
 imsf_slist will hold as many source addresses as fit, up to the
 minimum of the array size passed in as the original imsf_numsrc value
 and the total number of sources in the filter.

8.2. Protocol-Independent Options

 The following are defined in <sys/sockio.h>:
    o  ioctl() SIOCGMSFILTER: to retrieve the list of source addresses
       that comprise the source filter along with the current filter
       mode.
    o  ioctl() SIOCSMSFILTER: to set or modify the source filter
       content (e.g., unicast source address list) or mode (exclude or
       include).
 Ioctl option                 Argument type
 SIOCGMSFILTER                struct group_filter
 SIOCSMSFILTER                struct group_filter
 struct group_filter {
    uint32_t                gf_interface; /* interface index */
    struct sockaddr_storage gf_group;     /* multicast address */
    uint32_t                gf_fmode;     /* filter mode */
    uint32_t                gf_numsrc;    /* number of sources */
    struct sockaddr_storage gf_slist[1];  /* source address */
 };

Thaler, et al. Informational [Page 15] RFC 3678 Multicast Source Filter API January 2004

 #define GROUP_FILTER_SIZE(numsrc) \
    (sizeof(struct group_filter) - sizeof(struct sockaddr_storage) \
    + (numsrc) * sizeof(struct sockaddr_storage))
 The imf_numsrc field is used in the same way as described for
 imsf_numsrc above.

9. Normative References

 [1]  Gilligan, R., Thomson, S., Bound, J., McCann, J. and W.
      Stevens, "Basic Socket Interface Extensions for IPv6", RFC 3493,
      February 2003.
 [2]  IEEE Std. 1003.1-2001 Standard for Information Technology --
      Portable Operating System Interface (POSIX).  Open Group
      Technical Standard: Base Specifications, Issue 6, December 2001.
      ISO/IEC 9945:2002.  http://www.opengroup.org/austin

10. Informative References

 [3]  Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
      Thyagarajan, "Internet Group Management Protocol, Version 3",
      RFC 3376, October 2002.
 [4]  Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
      (MLDv2) for IPv6", Work in Progress, December 2003.

Thaler, et al. Informational [Page 16] RFC 3678 Multicast Source Filter API January 2004

11. Authors' Addresses

 Dave Thaler
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052-6399
 Phone: +1 425 703 8835
 EMail: dthaler@microsoft.com
 Bill Fenner
 75 Willow Road
 Menlo Park, CA  94025
 Phone: +1 650 867 6073
 EMail: fenner@research.att.com
 Bob Quinn
 IP Multicast Initiative (IPMI)
 Stardust.com
 1901 S. Bascom Ave. #333
 Campbell, CA 95008
 Phone: +1 408 879 8080
 EMail: rcq@ipmulticast.com

Thaler, et al. Informational [Page 17] RFC 3678 Multicast Source Filter API January 2004

12. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Thaler, et al. Informational [Page 18]

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