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IDMR Working Group                                     Dave Thaler
INTERNET-DRAFT                                           Microsoft
Expires January 2002                                   Bill Fenner
Type: Informational                                  AT&T Research
                                                         Bob Quinn
                                                      Stardust.com
                                                      20 July 2001





     Socket Interface Extensions for Multicast Source Filters
                 <draft-ietf-idmr-msf-api-02.txt>





Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time.  It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as "work
in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.



Copyright Notice






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Copyright (C) The Internet Society (2001).  All Rights Reserved.

1.  Abstract

IGMPv3 for IPv4 adds 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.  It
is expected that in the future, the same capability will be
available in IPv6 as well.

This document specifies new socket options and ioctl commands 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, while introducing
a minimum of change into the system and providing complete
compatibility for existing multicast applications.


2.  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 ioctl commands 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 2553 [1] defines socket interface extensions for
IPv6, including protocol-independent functions for most
operations.  However, while it defines join and leave functions





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for IPv6, it does not provide protocol-independent versions of
these operations.  Such functions will be described in this
document.


3.  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).


3.1.  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.





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The new API design must enable this functionality for both IPv4
and IPv6.


3.2.  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 Draft 6.6 (March 1997) of POSIX 1003.1g
are used: uintN_t means an unsigned integer of exactly N bits
(e.g., uint32_t).  We also assume the argument data types from
1003.1g when possible (e.g., the final argument to setsockopt() is
a size_t value).


3.3.  Headers

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


3.4.  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
precaution, nonstandard members could be verified by Feature Test
Macros as described in IEEE Std 1003.1.  (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.


4.  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.

IPv4 Multicast Source Filter APIs:

 o Basic (Delta-based): Use setsockopt() and reference a single
   source and group address pair to make incremental changes





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   + Any-Source:  Data accepted from any source by default, but
     source filter control is available

   + Controlled-Source:  A source filter is required

 o Advanced (Full-state): Use ioctl() and reference the entire set
   of sources with the group address to affect membership changes

Protocol-Independent Multicast Source Filter APIs:

 o Basic (Delta-based): Use setsockopt() and reference a single
   source and group address pair to make incremental changes

   + Any-Source:  Data accepted from any source by default, but
     source filter control is available

   + Controlled-Source:  A source filter is required

 o Advanced (Full-state): Use ioctl() and reference the entire set
   of sources

One might ask why the protocol-independent APIs cannot accomodate
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 accomodate the differences
     between IPv4 interface addresses (e.g., in the ip_mreq
     structure), and interface indices.


5.  IPv4 Multicast Source Filter APIs

Version 3 of the Internet Group Management Protocol (IGMPv3) [2]
provides 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, so when the source-filter API is available, applications





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can always benefit from using it.

There are two categories of the IPv4 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    The "Basic" (Delta-based) API is the simpler of the two and
     allows an application to reference a single source address in
     each operation.

o    The "Advanced" (Full-state) API allows an application to
     define a source-filter comprised of zero or more source
     addresses.


5.1.  Basic (Delta-based) API for IPv4

Some applications desire the simplicity of a delta-based API in
which each function call references a single source address along
with the multicast group address on which to listen.  Such
applications typically fall into either of two categories:

Any-source:
     By default, all sources are accepted.  Individual sources may
     be turned off and back on as needed over time.

Controlled-source:
     Only sources in a given list are allowed.  The list may
     change over time.


5.1.1.  Any-Source IPv4 Applications

The following socket options are defined in <netinet/in.h> for
applications in the any-source 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





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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).

The argument types of these options are defined as a result of
including the <netinet/in.h> header.

struct ip_mreq {
        struct  in_addr imr_multiaddr;  /* IP multicast address of group */
        struct  in_addr imr_interface;  /* local IP address of interface */
};

struct ip_mreq_source {
        struct  in_addr imr_multiaddr;  /* IP multicast address of group */
        struct  in_addr imr_sourceaddr; /* IP address of source */
        struct  in_addr imr_interface;  /* local IP address of interface */
};


5.1.2.  Controlled-Source IPv4 Applications

The following socket options are available for applications in the
Controlled-source 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

These options would be used, for example, by "single-source" style
applications such as audio/video broadcasting.  They can also be
used for logical multi-source sessions where each source
independently allocates its own source-specific group address.

IP_DROP_MEMBERSHIP can be supported, as a convenience, to drop all
sources which have been joined.  The operations are the same as if
the socket had been closed.









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5.1.3.  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 Controlled-Source 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.


5.2.  Advanced (Full-state) API for IPv4

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.

For this purpose 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).

     SIOCGIPMSFILTER could not be done with getsockopt(), since
     the group and interface must be passed down in order to
     retrieve the correct filter.  This can, however, be done with
     an ioctl(), and hence for symmetry, both gets and sets are
     done with an ioctl.





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5.2.1.  Set Source Filter

Ioctl option                   Argument type
SIOCSIPMSFILTER                struct ip_msfilter

The argument type of this option is defined as a result of
including the <netinet/in.h> header.

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>.

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

SIOCGIPMSFILTER cannot be done with getsockopt(), since the group
and interface must be passed down in order to retrieve the correct
filter.  This can, however, be done with an ioctl():

Ioctl option                  Argument type
SIOCGIPMSFILTER               struct ip_msfilter

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.  The result of this call 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.





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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.


6.  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.

While the Multicast Listener Discovery (MLD) protocol [3] for IPv6
does not currently support source-filters, the operating system
may provide filtering services with this API.  A future version of
MLD will support source-filters on routers, providing
functionality equivalent to IGMPv3 for IPv4.


6.1.  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().

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_SOURCE_GROUP     struct group_source_req
MCAST_LEAVE_SOURCE_GROUP    struct group_source_req

The argument types of these options are defined as a result of
including the <netinet/in.h> header.

struct group_req {
   uint32_t                gr_interface; /* interface index */
   struct sockaddr_storage gr_group;     /* group address */
};

struct group_source_req {





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   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 2553 [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.


6.2.  Advanced (Full-state) API

For the full-state API, the following ioctl() options are defined
in <sys/sockio.h>.  An ioctl() is required for obtaining the
filter on a group, since it requires both in and out parameter
fields, which cannot be done with getsockopt.  For symmetry, we
use ioctl() for both get and set operations.

Ioctl option                 Argument type
SIOCGMSFILTER                struct group_filter
SIOCSMSFILTER                struct group_filter

The argument types of these options are defined as a result of
including the <netinet/in.h> header.

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 */
};

#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 in section 5.2.2.









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7.  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 towards the "real" source, it will drop the packet.
However, this still does not provide any guarantee of protection.


8.  Acknowledgements

This draft was updated based on feedback from the IETF's Internet-
Draft Multicast Remnants (IDMR) Working Group.  Wilbert de Graaf
also provided many helpful comments.

9.  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









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10.  References

[1]  Gilligan, R., Thomson, S., Bound, J., and W. Stevens, "Basic
     Socket Interface Extensions for IPv6", RFC 2553, March 1999.

[2]  Cain, B., Deering, S., Fenner, B., Kouvelas, I., and A.
     Thyagarajan, "Internet Group Management Protocol, Version 3",
     Work in progress, draft-ietf-idmr-igmp-v3-07.txt, March 2001.

[3]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
     Discovery (MLD) for IPv6", RFC 2710, October 1999.


11.  Full Copyright Statement

Copyright (C) The Internet Society (2001).  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 implmentation 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 assigns.

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.










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Table of Contents


1: Abstract .................................................    2
2: Introduction .............................................    2
3: Design Considerations ....................................    3
3.1: What Needs to be Added .................................    3
3.2: Data Types .............................................    4
3.3: Headers ................................................    4
3.4: Structures .............................................    4
4: Overview of APIs .........................................    4
5: IPv4 Multicast Source Filter APIs ........................    5
5.1: Basic (Delta-based) API for IPv4 .......................    6
5.1.1: Any-Source IPv4 Applications .........................    6
5.1.2: Controlled-Source IPv4 Applications ..................    7
5.1.3: Error Codes ..........................................    8
5.2: Advanced (Full-state) API for IPv4 .....................    8
5.2.1: Set Source Filter ....................................    9
5.2.2: Get Source Filter ....................................    9
6: Protocol-Independent Multicast Source Filter APIs ........   10
6.1: Basic (Delta-based) API ................................   10
6.2: Advanced (Full-state) API ..............................   11
7: Security Considerations ..................................   12
8: Acknowledgements .........................................   12
9: Authors' Addresses .......................................   12
10: References ..............................................   13
11: Full Copyright Statement ................................   13






















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