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RFC 4570

Proposed STD
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Session Description Protocol (SDP) Source Filters


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Network Working Group                                           B. Quinn
Request for Comments: 4570                       
Category: Standards Track                                   R. Finlayson
                                                     Live Networks, Inc.
                                                               July 2006

           Session Description Protocol (SDP) Source Filters

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes how to adapt the Session Description Protocol
   (SDP) to express one or more source addresses as a source filter for
   one or more destination "connection" addresses.  It defines the
   syntax and semantics for an SDP "source-filter" attribute that may
   reference either IPv4 or IPv6 address(es) as either an inclusive or
   exclusive source list for either multicast or unicast destinations.
   In particular, an inclusive source-filter can be used to specify a
   Source-Specific Multicast (SSM) session.

1.  Introduction

   The Session Description Protocol [SDP] provides a general purpose
   format for describing multimedia sessions in announcements or
   invitations.  SDP uses an entirely textual data format (the US-ASCII
   subset of [UTF-8]) to maximize portability among transports.  SDP
   does not define a protocol, but only the syntax to describe a
   multimedia session with sufficient information to discover and
   participate in that session.  Session descriptions may be sent using
   any number of existing application protocols for transport (e.g.,
   Session Announcement Protocol (SAP), SIP, Real Time Streaming
   Protocol (RTSP), email, and HTTP).

   Typically, session descriptions reference an IP multicast address for
   the "connection-address" (destination), though unicast addresses or
   fully qualified domain names (FQDNs) MAY also be used.  The "source-

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   filter" attribute defined in this document qualifies the session
   traffic by identifying the address (or FQDN) of legitimate sources
   (senders).  The intent is for receivers to use the source and
   destination address pair(s) to filter traffic, so that applications
   receive only legitimate session traffic.

   Receiver applications are expected to use the SDP source-filter
   information to identify traffic from legitimate senders, and discard
   traffic from illegitimate senders.  Applications and hosts may also
   share the source-filter information with network elements (e.g., with
   routers using [IGMPv3]) so they can potentially perform the traffic
   filtering operation further "upstream," closer to the source(s).

   The "source-filter" attribute can appear at the session level and/or
   the media level.

1.1.  Motivation

   The purpose of a source-filter is to help protect receivers from
   traffic sent from illegitimate source addresses.  Filtering traffic
   can help to preserve content integrity and protect against Denial of
   Service (DoS) attacks.

   For multicast destination addresses, receiver applications MAY apply
   source-filters using the Multicast Source Filter APIs [MSF-API].
   Hosts are likely to implement these APIs using protocol mechanisms to
   convey the source filters to local multicast routers.  Other
   "upstream" multicast routers MAY apply the filters and thereby
   provide more explicit multicast group management and efficient
   utilization of network resources.  The protocol mechanisms to enable
   these operations are beyond the scope of this document, but their
   potential provided motivation for SDP source-filters.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [REQMNT].

3.  The "source-filter" Attribute

   The SDP source-filter attribute does not change any existing SDP
   syntax or semantics, but defines a format for additional session
   description information.  Specifically, source-filter syntax can
   prescribe one or more unicast addresses as either legitimate or
   illegitimate sources for any (or all) SDP session description
   "connection-address" field values.

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   Note that the unicast source addresses specified by this attribute
   are those that are seen by a receiver.  Therefore, if source
   addresses undergo translation en route from the original sender to
   the receiver - e.g., due to Network Address Translation (NAT) or some
   tunneling mechanism - then the SDP "source-filter" attribute, as
   presented to the receiver, will not be accurate unless the source
   addresses therein are also translated accordingly.

   The source-filter attribute has the following syntax:

       a=source-filter: <filter-mode> <filter-spec>

   The <filter-mode> is either "incl" or "excl" (for inclusion or
   exclusion, respectively).  The <filter-spec> has four sub-components:

       <nettype> <address-types> <dest-address> <src-list>

   A <filter-mode> of "incl" means that an incoming packet is accepted
   only if its source address is in the set specified by <src-list>.  A
   <filter-mode> of "excl" means that an incoming packet is rejected if
   its source address is in the set specified by <src-list>.

   The first sub-field, <nettype>, indicates the network type, since SDP
   is protocol independent.  This document is most relevant to the value
   "IN", which designates the Internet Protocol.

   The second sub-field, <address-types>, identifies the address family,
   and for the purpose of this document may be either <addrtype> value
   "IP4" or "IP6".  Alternately, when <dest-address> is an FQDN, the
   value MAY be "*" to apply to both address types, since either address
   type can be returned from a DNS lookup.

   The third sub-field, <dest-address>, is the destination address,
   which MUST correspond to one or more of the session's "connection-
   address" field values.  It may be either a unicast or multicast
   address, an FQDN, or the "*" wildcard to match any/all of the
   session's "connection-address" values.

   The fourth sub-field, <src-list>, is the list of source
   hosts/interfaces in the source-filter, and consists of one or more
   unicast addresses or FQDNs, separated by space characters.

   The format and content of these semantic elements are derived from
   and compatible with those defined in [SDP].  For more detail, see
   Appendix A of this document.

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3.1.  Processing Rules

   There are a number of details to consider when parsing the SDP
   source-filter syntax.

   The <dest-address> value in a "source-filter" attribute MUST
   correspond to an existing <connection-field> value in the session
   description.  The only exception to this is when a "*" wildcard is
   used to indicate that the source-filter applies to all
   <connection-field> values.

   When the <dest-address> value is a multicast address, the field value
   MUST NOT include the sub-fields <ttl> and <number of addresses> from
   the <connection-address> value.  If the <connection-address>
   specifies more than one multicast address (in the <number of
   addresses> field), then a source filter, if any, for each such
   address must be stated explicitly, using a separate "a=source-filter"
   line for each address (unless a "*" wildcard is used for
   <dest-address>).  See section 3.2.4 for an example.

   When the <addrtype> value is the "*" wildcard, the <dest-address>
   MUST be either an FQDN or "*" (i.e., it MUST NOT be an IPv4 or IPv6
   address).  See section 3.2.6 for an example.

   As has always been the case, the default behavior when a source-
   filter attribute is not provided in a session description is that all
   traffic sent to the specified <connection-address> value should be
   accepted (i.e., from any source address).  The source-filter grammar
   does not include syntax to express either "exclude none" or "include

   Like the standard <connection-field> described in [SDP], the location
   of the "source-filter" attribute determines whether it applies to the
   entire session or only to a specific medium (i.e., "session-level" or
   "media-level").  A media-level source-filter will always completely
   override a session-level source-filter.

   A "source-filter" need not be located at the same hierarchy level as
   its corresponding <connection-field>.  So, a media-level
   <source-filter> can reference a session-level <connection-field>
   value, and a session-level "source-filter" can be applied to all
   matching media-level <connection-field> values.  See section 3.2.3
   for an example.

   An SDP description MUST NOT contain more than one session-level
   "source-filter" attribute that covers the same destination address,
   or more than one media-level "source-filter" attribute that covers
   the same destination address.

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   There is no specified limit to the number of entries allowed in the
   <src-list>; however, there are practical limits that should be
   considered.  For example, depending on the transport to be used for
   the session description, there may be a limit to the total size of
   the session description (e.g., as determined by the maximum payload
   in a single datagram).  Also, when the source-filter is applied to
   control protocols, there may be a limit to the number of source
   addresses that can be sent.  These limits are outside the scope of
   this document, but should be considered when defining source-filter
   values for SDP.

3.2.  Examples

   Here are a number of examples that illustrate how to use the source-
   filter attribute in some common scenarios.  We use the following
   session description components as the starting point for the examples
   to follow.  For each example, we show the source filter with
   additional relevant information and provide a brief explanation.

      <session-description> =
           o=The King <>
           s=Elvis Impersonation
           i=All Elvis, all the time
           t=0 0

      <media-description 1> =
           m=audio 54320 RTP/AVP 0

      <media-description 2> =
           m=video 54322 RTP/AVP 34

3.2.1.  Source-Specific Multicast Example

   Multicast addresses in the Source-Specific Multicast [SSM] range
   require a single unicast sender address for each multicast
   destination, so the source-filter specification provides a natural
   fit.  In this example, a session member should receive only traffic
   sent from to the multicast session address


      c=IN IP4
      a=source-filter: incl IN IP4

      <media-description 1>

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   This source-filter example uses an inclusion list with a single
   multicast "connection-address" as the destination and single unicast
   address as the source.  Note that the value of the connection-address
   matches the value specified in the connection-field.

   Also note that since the connection-field is located in the session-
   description section, the source-filter applies to all media.

   Furthermore, if the SDP description specifies an RTP session (e.g.,
   its "m=" line(s) specify "RTP/AVP" as the transport protocol), then
   the "incl" specification will apply not only to RTP packets, but also
   to any RTCP packets that are sent to the specified multicast address.
   This means that, as a side effect of the "incl" specification, the
   only possible multicast RTCP packets will be "Sender Report" (SR)
   packets sent from the specified source address.

   Because of this, an SDP description for a Source-Specific Multicast
   (SSM) RTP session SHOULD also include an

      a=rtcp-unicast ...

   attribute, as described in [RTCP-SSM] (section 10.1).  This specifies
   that RTCP "Reception Report" (RR) packets are to be sent back via

3.2.2.  Unicast Exclusion Example

   Typically, an SDP session <connection-address> value is a multicast
   address, although it is also possible to use either a unicast address
   or FQDN.  This example illustrates a scenario whereby a session
   description indicates the unicast source address in an
   exclusion filter.  In effect, this sample source-filter says,
   "destination should accept traffic from any sender


      c=IN IP4
      a=source-filter: excl IN IP4

      <media-description 1>

3.2.3.  Multiple Session Address Example

   This source-filter example uses the wildcard "*" value for
   <dest-addr> to correspond to any/all <connection-address> values.
   Hence, the only legitimate source for traffic sent to either

Top       Page 7 or multicast addresses is  Traffic
   sent from any other unicast source address should be discarded by the


      a=source-filter: incl IN IP4 *

      <media-description 1>

      c=IN IP4

      <media-description 2>

      c=IN IP4

3.2.4.  Multiple Multicast Address Example

   In this example, the <connection-address> specifies three multicast
   addresses:,, and  The first and third
   of these addresses are given source filters.  However, in this
   example the second address - - is *not* given a source


      c=IN IP4
      a=source-filter: incl IN IP4
      a=source-filter: incl IN IP4

      <media-description 1>

3.2.5.  IPv6 Multicast Source-Filter Example

   This simple example defines a single session-level source-filter that
   references a single IPv6 multicast destination and source pair.  The
   IP multicast traffic sent to FFOE::11A is valid only from the unicast
   source address 2001:DB8:1:2:240:96FF:FE25:8EC9.


   c=IN IP6 FF0E::11A/127
   a=source-filter incl IN IP6 FF0E::11A 2001:DB8:1:2:240:96FF:FE25:8EC9

   <media-description 1>

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3.2.6.  IPv4 and IPv6 FQDN Example

   This example illustrates use of the <addrtype> "*" wildcard, along
   with multicast and source FQDNs that may resolve to either an IPv6 or
   IPv4 address, or both.  Although typically both the multicast and
   source addresses will be the same (either both IPv4 or both IPv6),
   using the wildcard for addrtype in the source filter allows asymmetry
   between the two addresses (so an IPv4 source address may be used with
   an IPv6 multicast address).


      c=IN IP4
      c=IN IP6
      a=source-filter: incl IN *

      <media-description 1>

3.3.  Offer-Answer Model Considerations

   The "source-filter" attribute is not intended to be used as an
   'offer' in an SDP offer-answer exchange [OFFER], because sets of
   source addresses do not represent 'capabilities' or 'limitations' of
   the offerer, and because the offerer does not, in general, have a
   priori knowledge of which IP source address(es) will be included in
   an answer.  While an answerer may include the "source-filter"
   attribute in his/her answer (e.g., to designate a SSM session), the
   answerer SHOULD ignore any "source-filter" attribute that was present
   in the original offer.

4.  Interoperability Issues

   Defining a list of legitimate sources for a multicast destination
   address represents a departure from the Any-Source Multicast (ASM)
   model, as originally described in [IGMPv1].  The ASM model supports
   anonymous senders and all types of multicast applications (e.g.,
   many-to-many).  Use of a source-filter excludes some (unknown or
   undesirable) senders, which lends itself more to one-to-many or few-
   to-few type multicast applications.

   Although these two models have contrasting operational
   characteristics and requirements, they can coexist on the same
   network using the same protocols.  Use of source-filters do not
   corrupt the ASM semantics but provide more control for receivers, at
   their discretion.

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5.  Security Considerations

   See [SDP] for security considerations specific to the Session
   Description Protocol in general.  The central issue relevant to using
   source address filters is the question of address authenticity.

   Using the source IP address for authentication is weak, since
   addresses are often dynamically assigned and it is possible for a
   sender to "spoof" its source address (i.e., use one other than its
   own) in datagrams that it sends.  Proper router configuration,
   however, can reduce the likelihood of "spoofed" source addresses
   being sent to or from a network.  Specifically, border routers are
   encouraged to filter traffic so that datagrams with invalid source
   addresses are not forwarded (e.g., routers drop datagrams if the
   source address is non-local) [FILTERING].  This, however, does not
   prevent IP source addresses from being spoofed on a Local Area
   Network (LAN).

   Also, as noted in section 3 above, tunneling or NAT mechanisms may
   require corresponding translation of the addresses specified in the
   SDP "source-filter" attribute, and furthermore, may cause a set of
   original source addresses to be translated to a smaller set of source
   addresses as seen by the receiver.

   Use of FQDNs for either <dest-address> or <src-list> values provides
   a layer of indirection that provides great flexibility.  However, it
   also exposes the source-filter to any security inadequacies that the
   DNS system may have.  If unsecured, it is conceivable that the DNS
   server could return illegitimate addresses.

   In addition, if source-filtering is implemented by sharing the
   source-filter information with network elements, then the security of
   the protocol(s) that are used for this (e.g., [IGMPv3]) becomes
   important, to ensure that legitimate traffic (and only legitimate
   traffic) is received.

   For these reasons, receivers SHOULD NOT treat the SDP "source-filter"
   attribute as being its sole mechanism for protecting the integrity of
   received content.

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6.  IANA Considerations

   As recommended by [SDP] (Appendix B), the new attribute name
   "source-filter" has been registered with IANA, as follows:

   The following contact information shall be used for all registrations
   included here:

        Contact:      Ross Finlayson
                      email: finlayson (at)
                      phone: +1-650-254-1184

      SDP Attribute ("att-field"):
        Attribute name:     source-filter
        Long form:          Source Filter
        Type of name:       att-field
        Type of attribute:  Session level or media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          This document
        Values:             See this document, and registrations below

7.  Acknowledgements

   The authors would like to thank Dave Thaler and Mark Handley, whose
   input provided much of the substance of this document.  Magnus
   Westerlund also provided valuable feedback during editing.

8.  Normative References

   [ABNF]      Crocker, D., Ed. and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", RFC 4234, October 2005.

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

   [SDP]       Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
               Description Protocol", RFC 4566, July 2006.

   [UTF-8]     Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, RFC 3629, November 2003.

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9.  Informative References

   [FILTERING] Ferguson, P. and D. Senie, "Network Ingress Filtering:
               Defeating Denial of Service Attacks which employ IP
               Source Address Spoofing", BCP 38, RFC 2827, May 2000.

   [IGMPv1]    Deering, S., "Host extensions for IP multicasting", STD
               5, RFC 1112, August 1989.

   [IGMPv3]    Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
               Thyagarajan, "Internet Group Management Protocol, Version
               3", RFC 3376, October 2002.

   [MSF-API]   Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
               Extensions for Multicast Source Filters", RFC 3678,
               January 2004.

   [OFFER]     Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
               with Session Description Protocol (SDP)", RFC 3264, June

   [RTCP-SSM]  Chesterfield, J., E. Schooler, J. Ott, "RTCP Extensions
               for Single-Source Multicast Sessions with Unicast
               Feedback", Work in Progress, October 2004.

   [SSM]       Bhattacharyya, S., "An Overview of Source-Specific
               Multicast (SSM)", RFC 3569, July 2003.

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Appendix A.  Source-Filter Attribute Syntax

   This appendix provides an Augmented BNF [ABNF] grammar for expressing
   an exclusion or inclusion list of one or more (IPv4 or IPv6) unicast
   source addresses.  It is intended as an extension to the grammar for
   the Session Description Protocol, as defined in [SDP].  Specifically,
   it describes the syntax for the new "source-filter" attribute field,
   which MAY be either a session-level or media-level attribute.

   The "dest-address" value in each source-filter field MUST match an
   existing connection-field value, unless the wildcard connection-
   address value "*" is specified.

   source-filter =  "source-filter" ":" SP filter-mode SP filter-spec
                    ; SP is the ASCII 'space' character
                    ;  (0x20, defined in [ABNF]).

   filter-mode =    "excl" / "incl"
                    ; either exclusion or inclusion mode.

   filter-spec =    nettype SP address-types SP dest-address SP src-list
                    ; nettype is as defined in [SDP].

   address-types =  "*" / addrtype
                    ; "*" for all address types (both IP4 and IP6),
                    ;  but only when <dest-address> and <src-list>
                    ;  reference FQDNs.
                    ; addrtype is as defined in [SDP].

   dest-address =   "*" / basic-multicast-address / unicast-address
                    ; "*" applies to all connection-address values.
                    ; unicast-address is as defined in [SDP].

   src-list =       *(unicast-address SP) unicast-address
                    ; one or more unicast source addresses (in
                    ;  standard IPv4 or IPv6 ASCII-notation form)
                    ;  or FQDNs.
                    ; unicast-address is as defined in [SDP].

   basic-multicast-address =   basic-IP4-multicast / basic-IP6-multicast
                               / FQDN / extn-addr
                               ; i.e., the same as multicast-address
                               ;  defined in [SDP], except that the
                               ;  /<ttl> and /<number of addresses>
                               ;  fields are not included.
                               ; FQDN and extn-addr are as defined
                               ;  in [SDP].

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   basic-IP4-multicast =       m1 3( "." decimal-uchar )
                               ; m1 and decimal-uchar are as defined
                               ;  in [SDP].

   basic-IP6-multicast =       hexpart
                               ; hexpart is as defined in [SDP].

Authors' Addresses

   Bob Quinn
   31 Caldwell Road
   Waltham, MA 02453

   Phone: +1-781-577-1539

   Ross Finlayson
   Live Networks, Inc.
   650 Castro St., suite 120-196
   Mountain View, CA 94041


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