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<pre>Internet Engineering Task Force (IETF) C. Perkins
Request for Comments: 5762 University of Glasgow
Category: Standards Track April 2010
ISSN: 2070-1721
<span class="h1">RTP and the Datagram Congestion Control Protocol (DCCP)</span>
Abstract
The Real-time Transport Protocol (RTP) is a widely used transport for
real-time multimedia on IP networks. The Datagram Congestion Control
Protocol (DCCP) is a transport protocol that provides desirable
services for real-time applications. This memo specifies a mapping
of RTP onto DCCP, along with associated signalling, such that real-
time applications can make use of the services provided by DCCP.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc5762">http://www.rfc-editor.org/info/rfc5762</a>.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
<span class="grey">Perkins Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
<a href="#section-2">2</a>. Rationale .......................................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Conventions Used in This Memo ...................................<a href="#page-4">4</a>
<a href="#section-4">4</a>. RTP over DCCP: Framing ..........................................<a href="#page-4">4</a>
<a href="#section-4.1">4.1</a>. RTP Data Packets ...........................................<a href="#page-4">4</a>
<a href="#section-4.2">4.2</a>. RTP Control Packets ........................................<a href="#page-5">5</a>
<a href="#section-4.3">4.3</a>. Multiplexing Data and Control ..............................<a href="#page-7">7</a>
<a href="#section-4.4">4.4</a>. RTP Sessions and DCCP Connections ..........................<a href="#page-7">7</a>
<a href="#section-4.5">4.5</a>. RTP Profiles ...............................................<a href="#page-8">8</a>
<a href="#section-5">5</a>. RTP over DCCP: Signalling using SDP .............................<a href="#page-8">8</a>
<a href="#section-5.1">5.1</a>. Protocol Identification ....................................<a href="#page-8">8</a>
<a href="#section-5.2">5.2</a>. Service Codes .............................................<a href="#page-10">10</a>
<a href="#section-5.3">5.3</a>. Connection Management .....................................<a href="#page-11">11</a>
<a href="#section-5.4">5.4</a>. Multiplexing Data and Control .............................<a href="#page-11">11</a>
<a href="#section-5.5">5.5</a>. Example ...................................................<a href="#page-11">11</a>
<a href="#section-6">6</a>. Security Considerations ........................................<a href="#page-12">12</a>
<a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-13">13</a>
<a href="#section-8">8</a>. Acknowledgements ...............................................<a href="#page-14">14</a>
<a href="#section-9">9</a>. References .....................................................<a href="#page-14">14</a>
<a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-14">14</a>
<a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-15">15</a>
<span class="grey">Perkins Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
The Real-time Transport Protocol (RTP) [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>] is widely used in video
streaming, telephony, and other real-time networked applications.
RTP can run over a range of lower-layer transport protocols, and the
performance of an application using RTP is heavily influenced by the
choice of lower-layer transport. The Datagram Congestion Control
Protocol (DCCP) [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>] is a transport protocol that provides desirable
properties for real-time applications running on unmanaged best-
effort IP networks. This memo describes how RTP can be framed for
transport using DCCP, and discusses some of the implications of such
a framing. It also describes how the Session Description Protocol
(SDP) [<a href="#ref-3" title=""SDP: Session Description Protocol"">3</a>] can be used to signal such sessions.
The remainder of this memo is structured as follows: it begins with a
rationale for the work in <a href="#section-2">Section 2</a>, describing why a mapping of RTP
onto DCCP is needed. Following a description of the conventions used
in this memo in <a href="#section-3">Section 3</a>, the specification begins in <a href="#section-4">Section 4</a> with
the definition of how RTP packets are framed within DCCP. Associated
signalling is described in <a href="#section-5">Section 5</a>. Security considerations are
discussed in <a href="#section-6">Section 6</a>, and IANA considerations in <a href="#section-7">Section 7</a>.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Rationale</span>
With the widespread adoption of RTP have come concerns that many
real-time applications do not implement congestion control, leading
to the potential for congestion collapse of the network [<a href="#ref-15" title=""IAB Concerns Regarding Congestion Control for Voice Traffic in the Internet"">15</a>]. The
designers of RTP recognised this issue, stating in <a href="./rfc3551">RFC 3551</a> that [<a href="#ref-4" title=""RTP Profile for Audio and Video Conferences with Minimal Control"">4</a>]:
If best-effort service is being used, RTP receivers SHOULD monitor
packet loss to ensure that the packet loss rate is within
acceptable parameters. Packet loss is considered acceptable if a
TCP flow across the same network path and experiencing the same
network conditions would achieve an average throughput, measured
on a reasonable timescale, that is not less than the RTP flow is
achieving. This condition can be satisfied by implementing
congestion control mechanisms to adapt the transmission rate (or
the number of layers subscribed for a layered multicast session),
or by arranging for a receiver to leave the session if the loss
rate is unacceptably high.
While the goals are clear, the development of TCP friendly congestion
control that can be used with RTP and real-time media applications is
an open research question with many proposals for new algorithms, but
little deployment experience.
<span class="grey">Perkins Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
Two approaches have been used to provide congestion control for RTP:
1) develop RTP extensions that incorporate congestion control; and 2)
provide mechanisms for running RTP over congestion-controlled
transport protocols. An example of the first approach can be found
in [<a href="#ref-16" title=""RTP with TCP Friendly Rate Control"">16</a>], extending RTP to incorporate feedback information such that
TCP Friendly Rate Control (TFRC) [<a href="#ref-17" title=""TCP Friendly Rate Control (TFRC): Protocol Specification"">17</a>] can be implemented at the
application level. This will allow congestion control to be added to
existing applications without operating system or network support,
and it offers the flexibility to experiment with new congestion
control algorithms as they are developed. Unfortunately, it also
passes the complexity of implementing congestion control onto
application authors, a burden which many would prefer to avoid.
The second approach is to run RTP on a lower-layer transport protocol
that provides congestion control. One possibility is to run RTP over
TCP, as defined in [<a href="#ref-5" title=""Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport"">5</a>], but the reliable nature of TCP and the
dynamics of its congestion control algorithm make this inappropriate
for most interactive real-time applications (the Stream Control
Transmission Protocol (SCTP) is inappropriate for similar reasons).
A better fit for such applications may be to run RTP over DCCP, since
DCCP offers unreliable packet delivery and a choice of congestion
control. This gives applications the ability to tailor the transport
to their needs, taking advantage of better congestion control
algorithms as they come available, while passing the complexity of
implementation to the operating system. If DCCP should come to be
widely available, it is believed these will be compelling advantages.
Accordingly, this memo defines a mapping of RTP onto DCCP.
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Conventions Used in This Memo</span>
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 <a href="./rfc2119">RFC 2119</a> [<a href="#ref-6" title=""Key words for use in RFCs to Indicate Requirement Levels"">6</a>].
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. RTP over DCCP: Framing</span>
The following section defines how RTP and RTP Control Protocol (RTCP)
packets can be framed for transport using DCCP. It also describes
the differences between RTP sessions and DCCP connections, and the
impact these have on the design of applications.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. RTP Data Packets</span>
Each RTP data packet MUST be conveyed in a single DCCP datagram.
Fields in the RTP header MUST be interpreted according to the RTP
specification, and any applicable RTP Profile and Payload Format.
Header processing is not affected by DCCP framing (in particular,
<span class="grey">Perkins Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
note that the semantics of the RTP sequence number and the DCCP
sequence number are not compatible, and the value of one cannot be
inferred from the other).
A DCCP connection is opened when an end system joins an RTP session,
and it remains open for the duration of the session. To ensure NAT
bindings are kept open, an end system SHOULD send a zero-length DCCP-
Data packet once every 15 seconds during periods when it has no other
data to send. This removes the need for RTP no-op packets [<a href="#ref-18" title=""A No-Op Payload Format for RTP"">18</a>], and
similar application-level keepalives, when using RTP over DCCP. This
application-level keepalive does not need to be sent if it is known
that the DCCP CCID in use provides a transport-level keepalive, or if
the application can determine that there are no NAT devices on the
path.
RTP data packets MUST obey the dictates of DCCP congestion control.
In some cases, the congestion control will require a sender to send
at a rate below that which the payload format would otherwise use.
To support this, an application could use either a rate-adaptive
payload format, or a range of payload formats (allowing it to switch
to a lower rate format if necessary). Details of the rate adaptation
policy for particular payload formats are outside the scope of this
memo (but see [<a href="#ref-19" title=""Strategies for Streaming Media Applications Using TCP-Friendly Rate Control"">19</a>] and [<a href="#ref-20" title=""Datagram Congestion Control Protocol (DCCP) User Guide"">20</a>] for guidance).
RTP extensions that provide application-level congestion control
(e.g., [<a href="#ref-16" title=""RTP with TCP Friendly Rate Control"">16</a>]) will conflict with DCCP congestion control, and MUST NOT
be used.
DCCP allows an application to choose the checksum coverage, using a
partial checksum to allow an application to receive packets with
corrupt payloads. Some RTP Payload Formats (e.g., [<a href="#ref-21" title=""RTP Payload Format and File Storage Format for the Adaptive Multi- Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs"">21</a>]) can make use
of this feature in conjunction with payload-specific mechanisms to
improve performance when operating in environments with frequent non-
congestive packet corruption. If such a payload format is used, an
RTP end system MAY enable partial checksums at the DCCP layer, in
which case the checksum MUST cover at least the DCCP and RTP headers
to ensure packets are correctly delivered. Partial checksums MUST
NOT be used unless supported by mechanisms in the RTP payload format.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. RTP Control Packets</span>
The RTP Control Protocol (RTCP) is used in the standard manner with
DCCP. RTCP packets are grouped into compound packets, as described
in Section 6.1 of [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>], and each compound RTCP packet is transported
in a single DCCP datagram.
<span class="grey">Perkins Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
The usual RTCP timing rules apply, with the additional constraint
that RTCP packets MUST obey the DCCP congestion control algorithm
negotiated for the connection. This can prevent a participant from
sending an RTCP packet at the expiration of the RTCP transmission
timer if there is insufficient network capacity available. In such
cases the RTCP packet is delayed and sent at the earliest possible
instant when capacity becomes available. The actual time the RTCP
packet was sent is then used as the basis for calculating the next
RTCP transmission time.
RTCP packets comprise only a small fraction of the total traffic in
an RTP session. Accordingly, it is expected that delays in their
transmission due to congestion control will not be common, provided
the configured nominal "session bandwidth" (see Section 6.2 of [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>])
is in line with the bandwidth achievable on the DCCP connection. If,
however, the capacity of the DCCP connection is significantly below
the nominal session bandwidth, RTCP packets may be delayed enough for
participants to time out due to apparent inactivity. In such cases,
the session parameters SHOULD be re-negotiated to more closely match
the available capacity, for example by performing a re-invite with an
updated "b=" line when using the Session Initiation Protocol [<a href="#ref-22" title=""SIP: Session Initiation Protocol"">22</a>] for
signalling.
Note: Since the nominal session bandwidth is chosen based on media
codec capabilities, a session where the nominal bandwidth is much
larger than the available bandwidth will likely become unusable
due to constraints on the media channel, and so require
negotiation of a lower bandwidth codec, before it becomes unusable
due to constraints on the RTCP channel.
As noted in Section 17.1 of [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>], there is the potential for overlap
between information conveyed in RTCP packets and that conveyed in
DCCP acknowledgement options. In general this is not an issue since
RTCP packets contain media-specific data that is not present in DCCP
acknowledgement options, and DCCP options contain network-level data
that is not present in RTCP. Indeed, there is no overlap between the
five RTCP packet types defined in the RTP specification [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>] and the
standard DCCP options [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>]. There are, however, cases where overlap
does occur: most clearly between the Loss RLE Report Blocks defined
as part of the RTCP Extended Reports [<a href="#ref-23" title=""RTP Control Protocol Extended Reports (RTCP XR)"">23</a>] and the DCCP Ack Vector
option. If there is overlap between RTCP report packets and DCCP
acknowledgements, an application SHOULD use either RTCP feedback or
DCCP acknowledgements, but not both (use of both types of feedback
will waste available network capacity, but is not otherwise harmful).
<span class="grey">Perkins Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Multiplexing Data and Control</span>
The obvious mapping of RTP onto DCCP creates two DCCP connections for
each RTP flow: one for RTP data packets and one for RTP control
packets. A frequent criticism of RTP relates to the number of ports
it uses, since large telephony gateways can support more than 32768
RTP flows between pairs of gateways, and so run out of UDP ports. In
addition, use of multiple ports complicates NAT traversal. For these
reasons, it is RECOMMENDED that the RTP and RTCP traffic for a single
RTP session is multiplexed onto a single DCCP connection following
the guidelines in [<a href="#ref-7" title=""Multiplexing RTP Data and Control Packets on a Single Port"">7</a>], where possible (it may not be possible in all
circumstances, for example when translating from an RTP stream over a
non-DCCP transport that uses conflicting RTP payload types and RTCP
packet types).
<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. RTP Sessions and DCCP Connections</span>
An end system SHOULD NOT assume that it will observe only a single
RTP synchronisation source (SSRC) because it is using DCCP framing.
An RTP session can span any number of transport connections, and can
include RTP mixers or translators bringing other participants into
the session. The use of a unicast DCCP connection does not imply
that the RTP session will have only two participants, and RTP end
systems SHOULD assume that multiple synchronisation sources may be
observed when using RTP over DCCP, unless otherwise signalled.
An RTP translator bridging multiple DCCP connections to form a single
RTP session needs to be aware of the congestion state of each DCCP
connection, and must adapt the media to the available capacity of
each. The Codec Control Messages defined in [<a href="#ref-24" title=""Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)"">24</a>] may be used to
signal congestion state to the media senders, allowing them to adapt
their transmission. Alternatively, media transcoding may be used to
perform adaptation: this is computationally expensive, induces delay,
and generally gives poor-quality results. Depending on the payload,
it might also be possible to use some form of scalable coding.
A single RTP session may also span a DCCP connection and some other
type of transport connection. An example might be an RTP over DCCP
connection from an RTP end system to an RTP translator, with an RTP
over UDP/IP multicast group on the other side of the translator. A
second example might be an RTP over DCCP connection that links Public
Switched Telephone Network (PSTN) gateways. The issues for such an
RTP translator are similar to those when linking two DCCP
connections, except that the congestion control algorithms on either
side of the translator may not be compatible. Implementation of
effective translators for such an environment is non-trivial.
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<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>. RTP Profiles</span>
In general, there is no conflict between new RTP profiles and DCCP
framing, and most RTP profiles can be negotiated for use over DCCP
with the following exceptions:
o An RTP profile that is intolerant of packet corruption may
conflict with the DCCP partial checksum feature. An example of
this is the integrity protection provided by the RTP/SAVP profile,
which cannot be used in conjunction with DCCP partial checksums.
o An RTP profile that mandates a particular non-DCCP lower-layer
transport will conflict with DCCP.
RTP profiles that fall under these exceptions SHOULD NOT be used with
DCCP unless the conflicting features can be disabled.
Of the profiles currently defined, the RTP Profile for Audio and
Video Conferences with Minimal Control [<a href="#ref-4" title=""RTP Profile for Audio and Video Conferences with Minimal Control"">4</a>], the Secure Real-time
Transport Protocol [<a href="#ref-8" title=""The Secure Real-time Transport Protocol (SRTP)"">8</a>], the Extended RTP Profile for RTCP-based
Feedback [<a href="#ref-9" title=""Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)"">9</a>], and the Extended Secure RTP Profile for RTCP-based
Feedback [<a href="#ref-10" title=""Extended Secure RTP Profile for Real- time Transport Control Protocol (RTCP)-Based Feedback (RTP/ SAVPF)"">10</a>] MAY be used with DCCP (noting the potential conflict
between DCCP partial checksums and the integrity protection provided
by the secure RTP variants -- see <a href="#section-6">Section 6</a>).
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. RTP over DCCP: Signalling using SDP</span>
The Session Description Protocol (SDP) [<a href="#ref-3" title=""SDP: Session Description Protocol"">3</a>] and the offer/answer model
[<a href="#ref-11" title=""An Offer/Answer Model with Session Description Protocol (SDP)"">11</a>] are widely used to negotiate RTP sessions (for example, using
the Session Initiation Protocol [<a href="#ref-22" title=""SIP: Session Initiation Protocol"">22</a>]). This section describes how
SDP is used to signal RTP sessions running over DCCP.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Protocol Identification</span>
SDP uses a media ("m=") line to convey details of the media format
and transport protocol used. The ABNF syntax of a media line is as
follows (from [<a href="#ref-3" title=""SDP: Session Description Protocol"">3</a>]):
media-field = %x6d "=" media SP port ["/" integer] SP proto
1*(SP fmt) CRLF
The proto field denotes the transport protocol used for the media,
while the port indicates the transport port to which the media is
sent. Following [<a href="#ref-5" title=""Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport"">5</a>] and [<a href="#ref-12" title=""TCP-Based Media Transport in the Session Description Protocol (SDP)"">12</a>], this memo defines these five values of
the proto field to indicate media transported using DCCP:
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DCCP
DCCP/RTP/AVP
DCCP/RTP/SAVP
DCCP/RTP/AVPF
DCCP/RTP/SAVPF
The "DCCP" protocol identifier is similar to the "UDP" and "TCP"
protocol identifiers and denotes the DCCP transport protocol [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>], but
not its upper-layer protocol. An SDP "m=" line that specifies the
"DCCP" protocol MUST further qualify the application-layer protocol
using a "fmt" identifier (the "fmt" namespace is managed in the same
manner as for the "UDP" protocol identifier). A single DCCP port is
used, as denoted by the port field in the media line. The "DCCP"
protocol identifier MUST NOT be used to signal RTP sessions running
over DCCP; those sessions MUST use a protocol identifier of the form
"DCCP/RTP/..." as described below.
The "DCCP/RTP/AVP" protocol identifier refers to RTP using the RTP
Profile for Audio and Video Conferences with Minimal Control [<a href="#ref-4" title=""RTP Profile for Audio and Video Conferences with Minimal Control"">4</a>]
running over DCCP.
The "DCCP/RTP/SAVP" protocol identifier refers to RTP using the
Secure Real-time Transport Protocol [<a href="#ref-8" title=""The Secure Real-time Transport Protocol (SRTP)"">8</a>] running over DCCP.
The "DCCP/RTP/AVPF" protocol identifier refers to RTP using the
Extended RTP Profile for RTCP-based Feedback [<a href="#ref-9" title=""Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)"">9</a>] running over DCCP.
The "DCCP/RTP/SAVPF" protocol identifier refers to RTP using the
Extended Secure RTP Profile for RTCP-based Feedback [<a href="#ref-10" title=""Extended Secure RTP Profile for Real- time Transport Control Protocol (RTCP)-Based Feedback (RTP/ SAVPF)"">10</a>] running over
DCCP.
RTP payload formats used with the "DCCP/RTP/AVP", "DCCP/RTP/SAVP",
"DCCP/RTP/AVPF", and "DCCP/RTP/SAVPF" protocol identifiers MUST use
the payload type number as their "fmt" value. If the payload type
number is dynamically assigned, an additional "rtpmap" attribute MUST
be included to specify the format name and parameters as defined by
the media type registration for the payload format.
DCCP port 5004 is registered for use by the RTP profiles listed
above, and SHOULD be the default port chosen by applications using
those profiles. If multiple RTP sessions are active from a host,
even-numbered ports in the dynamic range SHOULD be used for the other
sessions. If RTCP is to be sent on a separate DCCP connection to
RTP, the RTCP connection SHOULD use the next higher destination port
number, unless an alternative DCCP port is signalled using the
"a=rtcp:" attribute [<a href="#ref-13" title=""Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)"">13</a>]. For improved interoperability, "a=rtcp:"
SHOULD be used whenever an alternate DCCP port is used.
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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Service Codes</span>
In addition to the port number, specified on the SDP "m=" line, a
DCCP connection has an associated service code. A single new SDP
attribute ("dccp-service-code") is defined to signal the DCCP service
code according to the following ABNF [<a href="#ref-14" title=""Augmented BNF for Syntax Specifications: ABNF"">14</a>]:
dccp-service-attr = %x61 "=dccp-service-code:" service-code
service-code = hex-sc / decimal-sc / ascii-sc
hex-sc = %x53 %x43 "=" %x78 *HEXDIG
decimal-sc = %x53 %x43 "=" *DIGIT
ascii-sc = %x53 %x43 ":" *sc-char
sc-char = %d42-43 / %d45-47 / %d63-90 / %d95 / %d97-122
where DIGIT and HEXDIG are as defined in [<a href="#ref-14" title=""Augmented BNF for Syntax Specifications: ABNF"">14</a>]. The service code is
interpreted as defined in Section 8.1.2 of [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>] and may be specified
using either the hexadecimal, decimal, or ASCII formats. A parser
MUST interpret service codes according to their numeric value,
independent of the format used to represent them in SDP.
The following DCCP service codes are registered for use with RTP:
o SC:RTPA (equivalently SC=1381257281 or SC=x52545041): an RTP
session conveying audio data (and OPTIONAL multiplexed RTCP)
o SC:RTPV (equivalently SC=1381257302 or SC=x52545056): an RTP
session conveying video data (and OPTIONAL multiplexed RTCP)
o SC:RTPT (equivalently SC=1381257300 or SC=x52545054): an RTP
session conveying text media (and OPTIONAL multiplexed RTCP)
o SC:RTPO (equivalently SC=1381257295 or SC=x5254504f): an RTP
session conveying any other type of media (and OPTIONAL
multiplexed RTCP)
o SC:RTCP (equivalently SC=1381253968 or SC=x52544350): an RTCP
connection, separate from the corresponding RTP
To ease the job of middleboxes, applications SHOULD use these service
codes to identify RTP sessions running within DCCP. The service code
SHOULD match the top-level media type signalled for the session
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(i.e., the SDP "m=" line), with the exception connections using media
types other than audio, video, or text, which use SC:RTPO, and
connections that transport only RTCP packets, which use SC:RTCP.
The "a=dccp-service-code:" attribute is a media-level attribute that
is not subject to the charset attribute.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Connection Management</span>
The "a=setup:" attribute indicates which of the endpoints should
initiate the DCCP connection establishment (i.e., send the initial
DCCP-Request packet). The "a=setup:" attribute MUST be used in a
manner comparable with [<a href="#ref-12" title=""TCP-Based Media Transport in the Session Description Protocol (SDP)"">12</a>], except that DCCP connections are being
initiated rather than TCP connections.
After the initial offer/answer exchange, the endpoints may decide to
re-negotiate various parameters. The "a=connection:" attribute MUST
be used in a manner compatible with [<a href="#ref-12" title=""TCP-Based Media Transport in the Session Description Protocol (SDP)"">12</a>] to decide whether a new DCCP
connection needs to be established as a result of subsequent offer/
answer exchanges, or if the existing connection should still be used.
<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. Multiplexing Data and Control</span>
A single DCCP connection can be used to transport multiplexed RTP and
RTCP packets. Such multiplexing MUST be signalled using an "a=rtcp-
mux" attribute according to [<a href="#ref-7" title=""Multiplexing RTP Data and Control Packets on a Single Port"">7</a>]. If multiplexed RTP and RTCP are not
to be used, then the "a=rtcp-mux" attribute MUST NOT be present in
the SDP offer, and a separate DCCP connection MUST be opened to
transport the RTCP data on a different DCCP port.
<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>. Example</span>
An offerer at 192.0.2.47 signals its availability for an H.261 video
session, using RTP/AVP over DCCP with service code "RTPV" (using the
hexadecimal encoding of the service code in the SDP). RTP and RTCP
packets are multiplexed onto a single DCCP connection:
v=0
o=alice 1129377363 1 IN IP4 192.0.2.47
s=-
c=IN IP4 192.0.2.47
t=0 0
m=video 5004 DCCP/RTP/AVP 99
a=rtcp-mux
a=rtpmap:99 h261/90000
a=dccp-service-code:SC=x52545056
a=setup:passive
a=connection:new
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An answerer at 192.0.2.128 receives this offer and responds with the
following answer:
v=0
o=bob 1129377364 1 IN IP4 192.0.2.128
s=-
c=IN IP4 192.0.2.128
t=0 0
m=video 9 DCCP/RTP/AVP 99
a=rtcp-mux
a=rtpmap:99 h261/90000
a=dccp-service-code:SC:RTPV
a=setup:active
a=connection:new
The end point at 192.0.2.128 then initiates a DCCP connection to port
5004 at 192.0.2.47. DCCP port 5004 is used for both the RTP and RTCP
data, and port 5005 is unused. The textual encoding of the service
code is used in the answer, and represents the same service code as
in the offer.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
The security considerations in the RTP specification [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>] and any
applicable RTP profile (e.g., [<a href="#ref-4" title=""RTP Profile for Audio and Video Conferences with Minimal Control"">4</a>], [<a href="#ref-8" title=""The Secure Real-time Transport Protocol (SRTP)"">8</a>], [<a href="#ref-9" title=""Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)"">9</a>], or [<a href="#ref-10" title=""Extended Secure RTP Profile for Real- time Transport Control Protocol (RTCP)-Based Feedback (RTP/ SAVPF)"">10</a>]) or payload
format apply when transporting RTP over DCCP.
The security considerations in the DCCP specification [<a href="#ref-2" title=""Datagram Congestion Control Protocol (DCCP)"">2</a>] apply.
The SDP signalling described in <a href="#section-5">Section 5</a> is subject to the security
considerations of [<a href="#ref-3" title=""SDP: Session Description Protocol"">3</a>], [<a href="#ref-11" title=""An Offer/Answer Model with Session Description Protocol (SDP)"">11</a>], [<a href="#ref-12" title=""TCP-Based Media Transport in the Session Description Protocol (SDP)"">12</a>], [<a href="#ref-5" title=""Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport"">5</a>], and [<a href="#ref-7" title=""Multiplexing RTP Data and Control Packets on a Single Port"">7</a>].
The provision of effective congestion control for RTP through use of
DCCP is expected to help reduce the potential for denial of service
present when RTP flows ignore the advice in [<a href="#ref-1" title=""RTP: A Transport Protocol for Real-Time Applications"">1</a>] to monitor packet
loss and reduce their sending rate in the face of persistent
congestion.
There is a potential conflict between the Secure RTP profiles ([<a href="#ref-8" title=""The Secure Real-time Transport Protocol (SRTP)"">8</a>],
[<a href="#ref-10" title=""Extended Secure RTP Profile for Real- time Transport Control Protocol (RTCP)-Based Feedback (RTP/ SAVPF)"">10</a>]) and the DCCP partial checksum option, since these profiles
introduce, and recommend the use of, message authentication for RTP
and RTCP packets. Message authentication codes of the type used by
these profiles cannot be used with partial checksums, since any bit
error in the DCCP packet payload will cause the authentication check
to fail. Accordingly, DCCP partial checksums SHOULD NOT be used in
conjunction with Secure Real-time Transport Protocol (SRTP)
authentication. The confidentiality features of the basic RTP
specification cannot be used with DCCP partial checksums, since bit
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errors propagate. Also, despite the fact that bit errors do not
propagate when using AES in counter mode, the Secure RTP profiles
SHOULD NOT be used with DCCP partial checksums, since the profiles
require authentication for security, and authentication is
incompatible with partial checksums.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
The following SDP "proto" field identifiers have been registered (see
<a href="#section-5.1">Section 5.1</a>):
Type SDP Name Reference
---- -------- ---------
proto DCCP [<a href="./rfc5762">RFC5762</a>]
DCCP/RTP/AVP [<a href="./rfc5762">RFC5762</a>]
DCCP/RTP/SAVP [<a href="./rfc5762">RFC5762</a>]
DCCP/RTP/AVPF [<a href="./rfc5762">RFC5762</a>]
DCCP/RTP/SAVPF [<a href="./rfc5762">RFC5762</a>]
The following new SDP attribute ("att-field") has been registered:
Contact name: Colin Perkins <csp@csperkins.org>
Attribute name: dccp-service-code
Long-form attribute name in English: DCCP service code
Type of attribute: Media level.
Subject to the charset attribute? No.
Purpose of the attribute: see <a href="./rfc5762#section-5.2">RFC 5762, Section 5.2</a>
Allowed attribute values: see <a href="./rfc5762#section-5.2">RFC 5762, Section 5.2</a>
The following DCCP service code values have been registered (see
<a href="#section-5.2">Section 5.2</a>):
1381257281 RTPA RTP session conveying audio [<a href="./rfc5762">RFC5762</a>]
data (and associated RTCP)
1381257302 RTPV RTP session conveying video [<a href="./rfc5762">RFC5762</a>]
data (and associated RTCP)
1381257300 RTPT RTP session conveying text [<a href="./rfc5762">RFC5762</a>]
media (and associated RTCP)
1381257295 RTPO RTP session conveying other [<a href="./rfc5762">RFC5762</a>]
media (and associated RTCP)
1381253968 RTCP RTCP connection, separate from [<a href="./rfc5762">RFC5762</a>]
the corresponding RTP
<span class="grey">Perkins Standards Track [Page 13]</span></pre>
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The following DCCP ports have been registered (see <a href="#section-5.1">Section 5.1</a>):
avt-profile-1 5004/dccp RTP media data [RFC3551, <a href="./rfc5762">RFC5762</a>]
avt-profile-2 5005/dccp RTP control protocol [RFC3551, <a href="./rfc5762">RFC5762</a>]
Note: ports 5004/tcp, 5004/udp, 5005/tcp, and 5005/udp have existing
registrations, but incorrect descriptions and references. The IANA
has updated the existing registrations as follows:
avt-profile-1 5004/tcp RTP media data [RFC3551, <a href="./rfc4571">RFC4571</a>]
avt-profile-1 5004/udp RTP media data [<a href="./rfc3551">RFC3551</a>]
avt-profile-2 5005/tcp RTP control protocol [RFC3551, <a href="./rfc4571">RFC4571</a>]
avt-profile-2 5005/udp RTP control protocol [<a href="./rfc3551">RFC3551</a>]
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgements</span>
This work was supported in part by the UK Engineering and Physical
Sciences Research Council. Thanks are due to Philippe Gentric,
Magnus Westerlund, Sally Floyd, Dan Wing, Gorry Fairhurst, Stephane
Bortzmeyer, Arjuna Sathiaseelan, Tom Phelan, Lars Eggert, Eddie
Kohler, Miguel Garcia, and the other members of the DCCP working
group for their comments.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-1">1</a>] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
<a href="./rfc3550">RFC 3550</a>, July 2003.
[<a id="ref-2">2</a>] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion
Control Protocol (DCCP)", <a href="./rfc4340">RFC 4340</a>, March 2006.
[<a id="ref-3">3</a>] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", <a href="./rfc4566">RFC 4566</a>, July 2006.
[<a id="ref-4">4</a>] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, <a href="./rfc3551">RFC 3551</a>, July 2003.
[<a id="ref-5">5</a>] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and
RTP Control Protocol (RTCP) Packets over Connection-Oriented
Transport", <a href="./rfc4571">RFC 4571</a>, July 2006.
[<a id="ref-6">6</a>] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
<span class="grey">Perkins Standards Track [Page 14]</span></pre>
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[<a id="ref-7">7</a>] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", <a href="./rfc5761">RFC 5761</a>, April 2010.
[<a id="ref-8">8</a>] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
<a href="./rfc3711">RFC 3711</a>, March 2004.
[<a id="ref-9">9</a>] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control Protocol
(RTCP)-Based Feedback (RTP/AVPF)", <a href="./rfc4585">RFC 4585</a>, July 2006.
[<a id="ref-10">10</a>] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-
time Transport Control Protocol (RTCP)-Based Feedback (RTP/
SAVPF)", <a href="./rfc5124">RFC 5124</a>, February 2008.
[<a id="ref-11">11</a>] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", <a href="./rfc3264">RFC 3264</a>, June 2002.
[<a id="ref-12">12</a>] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
Session Description Protocol (SDP)", <a href="./rfc4145">RFC 4145</a>, September 2005.
[<a id="ref-13">13</a>] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", <a href="./rfc3605">RFC 3605</a>, October 2003.
[<a id="ref-14">14</a>] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>, January 2008.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-15">15</a>] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", <a href="./rfc3714">RFC 3714</a>,
March 2004.
[<a id="ref-16">16</a>] Gharai, L., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&q=inurl:draft-+%22RTP+with+TCP+Friendly+Rate+Control%22'>"RTP with TCP Friendly Rate Control"</a>, Work
in Progress, July 2007.
[<a id="ref-17">17</a>] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
<a href="./rfc5348">RFC 5348</a>, September 2008.
[<a id="ref-18">18</a>] Andreasen, F., Oran, D., and D. Wing, "A No-Op Payload Format
for RTP", Work in Progress, May 2005.
[<a id="ref-19">19</a>] Phelan, T., "Strategies for Streaming Media Applications Using
TCP-Friendly Rate Control", Work in Progress, July 2007.
[<a id="ref-20">20</a>] Phelan, T., "Datagram Congestion Control Protocol (DCCP) User
Guide", Work in Progress, April 2005.
<span class="grey">Perkins Standards Track [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc5762">RFC 5762</a> RTP over DCCP April 2010</span>
[<a id="ref-21">21</a>] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "RTP
Payload Format and File Storage Format for the Adaptive Multi-
Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio
Codecs", <a href="./rfc4867">RFC 4867</a>, April 2007.
[<a id="ref-22">22</a>] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>, June 2002.
[<a id="ref-23">23</a>] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
Extended Reports (RTCP XR)", <a href="./rfc3611">RFC 3611</a>, November 2003.
[<a id="ref-24">24</a>] Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec
Control Messages in the RTP Audio-Visual Profile with Feedback
(AVPF)", Work in Progress, October 2007.
Author's Address
Colin Perkins
University of Glasgow
Department of Computing Science
Glasgow G12 8QQ
UK
EMail: csp@csperkins.org
Perkins Standards Track [Page 16]
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