Network Working Group B. Quinn
Request for Comments: 4570 BoxnArrow.com
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 (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.
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-
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.
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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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.
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.
3.1. Processing Rules
There are a number of details to consider when parsing the SDP
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
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.
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.
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.
o=The King <Elvis@example.com>
i=All Elvis, all the time
<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 192.0.2.10 to the multicast session address 18.104.22.168.
c=IN IP4 22.214.171.124/127
a=source-filter: incl IN IP4 126.96.36.199 192.0.2.10
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
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 192.0.2.10 in an
exclusion filter. In effect, this sample source-filter says,
"destination 192.0.2.11 should accept traffic from any sender
c=IN IP4 192.0.2.11
a=source-filter: excl IN IP4 192.0.2.11 192.0.2.10
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
188.8.131.52 or 184.108.40.206 multicast addresses is 192.0.2.10. Traffic
sent from any other unicast source address should be discarded by the
a=source-filter: incl IN IP4 * 192.0.2.10
c=IN IP4 220.127.116.11/127
c=IN IP4 18.104.22.168/63
3.2.4. Multiple Multicast Address Example
In this example, the <connection-address> specifies three multicast
addresses: 22.214.171.124, 126.96.36.199, and 188.8.131.52. The first and third
of these addresses are given source filters. However, in this
example the second address - 184.108.40.206 - is *not* given a source
c=IN IP4 220.127.116.11/127/3
a=source-filter: incl IN IP4 18.104.22.168 192.0.2.10
a=source-filter: incl IN IP4 22.214.171.124 192.0.2.42
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
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 channel-1.example.com/127
c=IN IP6 channel-1.example.com/127
a=source-filter: incl IN * channel-1.example.com src-1.example.com
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
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
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
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
Contact: Ross Finlayson
email: finlayson (at) live555.com
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
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.
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,
[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.
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].
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].
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