draft-perkins-dccp-rtp-00.txt   draft-perkins-dccp-rtp-01.txt 
Network Working Group C. Perkins Network Working Group C. Perkins
Internet-Draft University of Glasgow Internet-Draft University of Glasgow
Expires: April 20, 2006 October 17, 2005 Expires: September 7, 2006 March 6, 2006
RTP and the Datagram Congestion Control Protocol (DCCP) RTP and the Datagram Congestion Control Protocol (DCCP)
draft-perkins-dccp-rtp-00.txt draft-perkins-dccp-rtp-01.txt
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
This memo specifies a mapping of RTP onto DCCP, along with associated The Real-time Transport Protocl (RTP) is a widely used transport for
signalling. real-time media on IP networks. The Datagram Congestion Control
Protocol (DCCP) is a newly defined 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.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in this Memo . . . . . . . . . . . . . . . . 3 3. Conventions Used in this Memo . . . . . . . . . . . . . . . . 4
4. RTP over DCCP: Framing . . . . . . . . . . . . . . . . . . . . 3 4. RTP over DCCP: Framing . . . . . . . . . . . . . . . . . . . . 4
4.1. RTP Data Packets . . . . . . . . . . . . . . . . . . . . . 3 4.1. RTP Data Packets . . . . . . . . . . . . . . . . . . . . . 4
4.2. RTP Control Packets . . . . . . . . . . . . . . . . . . . 4 4.2. RTP Control Packets . . . . . . . . . . . . . . . . . . . 5
4.3. Multiplexing Data and Control . . . . . . . . . . . . . . 5 4.3. Multiplexing Data and Control . . . . . . . . . . . . . . 6
4.4. RTP Sessions and DCCP Connections . . . . . . . . . . . . 5 4.4. RTP Sessions and DCCP Connections . . . . . . . . . . . . 7
4.5. RTP Profiles . . . . . . . . . . . . . . . . . . . . . . . 6 4.5. RTP Profiles . . . . . . . . . . . . . . . . . . . . . . . 7
5. RTP over DCCP: Signalling using SDP . . . . . . . . . . . . . 6 5. RTP over DCCP: Signalling using SDP . . . . . . . . . . . . . 8
5.1. Protocol Identification . . . . . . . . . . . . . . . . . 6 5.1. Protocol Identification . . . . . . . . . . . . . . . . . 8
5.2. Service Codes . . . . . . . . . . . . . . . . . . . . . . 7 5.2. Service Codes . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Connection Management . . . . . . . . . . . . . . . . . . 8 5.3. Connection Management . . . . . . . . . . . . . . . . . . 10
5.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . . . 15
1. Introduction 1. Introduction
The Real-time Transport Protocol (RTP) [1] is widely used in video The Real-time Transport Protocol (RTP) [1] is widely used in video
streaming, telephony, and other real-time networked applications. streaming, telephony, and other real-time networked applications.
RTP can run over a range of lower-layer transport protocols, and the RTP can run over a range of lower-layer transport protocols, and the
performance of an application using RTP is heavily influenced by the performance of an application using RTP is heavily influenced by the
lower-layer transport protocol. The Datagram Congestion Control choice of lower-layer transport. The Datagram Congestion Control
Protocol (DCCP) [2] is a newly specified transport protocol which Protocol (DCCP) [2] is a newly specified transport protocol that
provides desirable properties for real-time applications running on provides desirable properties for real-time applications running on
unmanaged best-effort IP networks. This memo describes how RTP can unmanaged best-effort IP networks. This memo describes how RTP can
be framed for transport using DCCP. It also describes how the be framed for transport using DCCP, and discusses some of the
Session Description Protocol (SDP) [3] can be used to signal such implications of such a framing. It also describes how the Session
sessions. Description Protocol (SDP) [3] can be used to signal such sessions.
The remainder of this memo is structured as follows: we begin with a The remainder of this memo is structured as follows: we begin with a
rationale for the work in Section 2, describing why a mapping of RTP rationale for the work in Section 2, describing why a mapping of RTP
onto DCCP is needed. Following a description of the conventions used onto DCCP is needed. Following a description of the conventions used
in this memo in Section 3, the specification begins in Section 4 with in this memo in Section 3, the specification begins in Section 4 with
the definition of how RTP packets are framed within DCCP; associated the definition of how RTP packets are framed within DCCP; associated
signalling is described in Section 5. We conclude with a discussion signalling is described in Section 5. We conclude with a discussion
of security considerations in Section 6, and IANA considerations in of security considerations in Section 6, and IANA considerations in
Section 7. Section 7.
2. Rationale 2. Rationale
TODO: rationale why RTP over DCCP is useful, referencing [13] and With the widespread adoption of RTP have come concerns that many real
comparing with [14]. time applications do not implement congestion control, leading to the
potential for congestion collapse of the network [14]. The designers
of RTP recognised this issue, stating that [4]:
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.
Two approaches have been used to provide congestion control for RTP:
1) develop new RTP profiles that incorporate congestion control; and
2) provide mechanisms for running RTP over congestion controlled
transport protocols. The RTP Profile for TCP Friendly Rate Control
[15] is an example of the first approach, extending the RTP packet
formats to incorporate feedback information such that TFRC congestion
control [16] can be implemented at the application level. This
approach has the advantage that congestion control can be added to
existing applications, without needing operating system or network
support, and offers flexibility to experiment with new congestion
control algorithms as they are developed. Unfortunately, there is
also the consequent disadvantage that the complexity of implementing
congestion control is passed onto the application author, a burden
which many would prefer to avoid.
The other 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 [5], but the reliable nature of TCP and the
dynamics of its congestion control algorithm make this inappropriate
for most interactive real time applications (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
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.
3. Conventions Used in this Memo 3. Conventions Used in this Memo
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4]. document are to be interpreted as described in RFC 2119 [6].
4. RTP over DCCP: Framing 4. RTP over DCCP: Framing
The following section defines how RTP and RTCP packets are framed for The following section defines how RTP and RTCP packets can be framed
transport using DCCP. It also describes the different between an RTP for transport using DCCP. It also describes the differences between
session and a DCCP connection, and the impact this has on application RTP sessions and DCCP connections, and the impact these have on the
design. design of applications.
4.1. RTP Data Packets 4.1. RTP Data Packets
Each RTP data packet MUST be conveyed in a single DCCP datagram. Each RTP data packet MUST be conveyed in a single DCCP datagram.
Fields in the RTP header MUST be interpreted according to the RTP Fields in the RTP header MUST be interpreted according to the RTP
specification, and any applicable RTP Profile and Payload Format. specification, and any applicable RTP Profile and Payload Format.
Header processing is not affected by DCCP framing (in particular, Header processing is not affected by DCCP framing (in particular,
note that the semantics of the RTP sequence number and the DCCP note that the semantics of the RTP sequence number and the DCCP
sequence number are not compatible, and the value of one cannot be sequence number are not compatible, and the value of one cannot be
inferred from the other). inferred from the other).
A DCCP connection is opened when an end system joins an RTP session A DCCP connection is opened when an end system joins an RTP session,
and SHOULD remain open for the duration of the session. To ensure and it remains open for the duration of the session. To ensure NAT
NAT bindings are kept open, an end system SHOULD send periodic low bindings are kept open, an end system SHOULD send periodic low rate
rate zero length DCCP-Data packets during periods when it has no RTP zero length DCCP-Data packets during periods when it has no RTP data
data to send. This removes the need for RTP no-op packets [15] when to send. This removes the need for RTP no-op packets [17] when using
using RTP over DCCP. RTP over DCCP.
RTP data packets MUST obey the dictates of DCCP congestion control. RTP data packets MUST obey the dictates of DCCP congestion control.
In some cases, the congestion control will require a sender to send In some cases, the congestion control will require a sender to send
at a rate below that which the payload format would otherwise use. at a rate below that which the payload format would otherwise use.
To support this, an application should use either a rate adaptive To support this, an application should use either a rate adaptive
payload format, or a range of payload formats (allowing it to switch payload format, or a range of payload formats (allowing it to switch
to a lower rate format if necessary). Details of the rate adaptation to a lower rate format if necessary). Details of the rate adaptation
policy for particular payload formats are outside the scope of this policy for particular payload formats are outside the scope of this
memo. memo.
TODO: should we give more guidance here? TODO: provide more guidance on implementation of congestion control
within an RTP application.
DCCP allows an application to choose the checksum coverage, using a DCCP allows an application to choose the checksum coverage, using a
partial checksum to allow an application to receive packets with partial checksum to allow an application to receive packets with
corrupt payloads. Some RTP Payload Formats (e.g. [16]) can make use corrupt payloads. Some RTP Payload Formats (e.g. [18]) can make use
of this feature in conjunction with payload-specific mechanisms to of this feature in conjunction with payload-specific mechanisms to
improve performance when operating in environments with frequent non- improve performance when operating in environments with frequent non-
congestive packet corruption. If such a payload format is used, an congestive packet corruption. If such a payload format is used, an
RTP end system MAY enable partial checksums at the DCCP layer, in RTP end system MAY enable partial checksums at the DCCP layer, in
which case the checksum should cover at least the DCCP and RTP which case the checksum MUST cover at least the DCCP and RTP headers
headers. Partial checksums MUST NOT be used unless supported by to ensure packets are correctly delivered. Partial checksums MUST
mechanisms in the payload format. NOT be used unless supported by mechanisms in the RTP payload format.
4.2. RTP Control Packets 4.2. RTP Control Packets
The RTP Control Protocol (RTCP) SHOULD be used in the usual manner The RTP Control Protocol (RTCP) is be used in the usual manner with
with DCCP. RTCP packets MUST be grouped into compound packets, as DCCP. RTCP packets MUST be sent grouped into compound packets, as
described in Section 6.1 of [1]. Each compound RTCP packet MUST be described in Section 6.1 of [1]. Each compound RTCP packet MUST be
transported in a single DCCP datagram. transported in a single DCCP datagram.
The usual RTCP timing rules apply, subject to the constraint that The usual RTCP timing rules apply, subject to the constraint that
RTCP packets MUST be subject to DCCP congestion control. RTCP packets MUST be subject to DCCP congestion control.
TODO: is the RTP "session bandwidth" affected by congestion control? TODO: RTCP relies on a configured nominal "session bandwidth" in the
Dynamically adapting the RTCP transmission interval based on DCCP is calculation of the reporting interval. It is not clear that this is
difficult, since it will involve frequent reconsideration of the next appropriate for congestion controlled sessions, since the actual RTP
RTCP send time. Also, might be a problem if the RTP session spans a rate may vary significantly over time, and may differ for each half
range of links, with widely varying capacities. of a DCCP connection. There are two options: 1) use the nominal RTP
session bandwidth to drive RTCP, accepting that this may over- or
under-estimate the RTCP reporting interval; or 2) modify the RTCP
reporting interval based on the session bandwidth as determined by
the congestion control algorithm. The second choice introduces some
considerable complexity, since there is no longer a simple definition
for the reporting interval on which all participants will agree, but
it will better control the RTCP sending rate.
As noted in Section 17.1 of [2], there is the potential for overlap As noted in Section 17.1 of [2], there is the potential for overlap
between the information conveyed in RTCP report packets and that between the information conveyed in RTCP packets and that conveyed in
conveyed in DCCP acknowledgement options. In general this is not an DCCP acknowledgement options. In general this is not an issue: RTCP
issue: RTCP packets contain media-specific data that is not present packets contain media-specific data that is not present in DCCP
in DCCP acknowledgements, and DCCP options contain network-level data acknowledgement optionss, and DCCP options contain network-level data
that is not present in RTCP. There is no overlap between the RTCP that is not present in RTCP. Indeed, there is no overlap between the
packet types defined in [1] and the DCCP options defined in [2]. five RTCP packet types defined in the RTP specification [1] and the
standard DCCP options defined in [2]. There are, however, other
There are, however, other cases of overlap: most clearly between the cases where overlap does occur: most clearly between the optional
optional RTCP Extended Reports Loss RLE Blocks [17] and DCCP Ack RTCP Extended Reports Loss RLE Blocks [19] and the DCCP Ack Vector
Vector options. If there is overlap between RTCP reports and DCCP option. If there is overlap between RTCP report packets and DCCP
acknowledgements, an application SHOULD either use RTCP feedback or acknowledgements, an application should use either RTCP feedback or
request DCCP acknowledgements, but not both (use of both types of DCCP acknowledgements, but not both (use of both types of feedback
feedback will waste available network capacity, but is not otherwise will waste available network capacity, but is not otherwise harmful).
harmful).
4.3. Multiplexing Data and Control 4.3. Multiplexing Data and Control
The obvious mapping of RTP onto DCCP creates two DCCP connections for The obvious mapping of RTP onto DCCP creates two DCCP connections for
each RTP flow: one for RTP data packets, one for RTP control packets. each RTP flow: one for RTP data packets, one for RTP control packets.
A frequent criticism of RTP relates to the number of ports it uses, A frequent criticism of RTP relates to the number of ports it uses,
since large telephony gateways can support more than 32768 RTP flows since large telephony gateways can support more than 32768 RTP flows
between pairs of gateways, and so run out of UDP ports. In addition, between pairs of gateways, and so run out of UDP ports. In addition,
use of multiple ports complicates NAT traversal. use of multiple ports complicates NAT traversal. For these reasons,
it is RECOMENDED that RTP and RTCP flows be multiplexed onto a single
DCCP connection.
If care is taken in the choice of payload type used, it is possible RTP and RTCP packets multiplexed onto a single connection can be
to multiplex RTP and RTCP onto the same port. This requires care in distinguished provided care is taken in assigning RTP payload types.
the application, but the added complexity there might be worthwhile. The RTP payload type and marker bit(s) occupy the same space in the
Is it appropriate to encourage this for RTP-over-DCCP? packet as does the RTCP packet type field. Provided the RTP payload
type is chosen such that the payload type, or the payload type plus
128 (when the marker bit is set), does not clash with any of the used
RTCP packet types, the two can be demultiplexed. With the RTCP
packet types registered at the time of this writing, this implies
that RTP payload types 64-65 and 72-79 must be avoided. None of the
registered static payload type assignments are in this range, and
typical practice is to make dynamic assignments in the range 96-127,
so this restriction is not typically problematic. This multiplexing
does not otherwise impact the operation of RTP or RTCP.
There may be circumstances where multiplexing RTP and RTCP is not
desired, for example when translating from an RTP stream over non-
DCCP transport that uses conflicting RTP payload types and RTCP
packet types. As specified in Section 5.1, the "a=rtcp:" SDP
attribute MAY be used to signal use of non-multiplexed RTCP.
4.4. RTP Sessions and DCCP Connections 4.4. RTP Sessions and DCCP Connections
An end system SHOULD NOT assume that it will observe only a single An end system should not assume that it will observe only a single
RTP synchronisation source (SSRC) because it is using DCCP framing. RTP synchronisation source (SSRC) because it is using DCCP framing.
An RTP session can span any number of transport connections, and can An RTP session can span any number of transport connections, and can
include RTP mixers or translators bringing other participants into a include RTP mixers or translators bringing other participants into
session. The use of a unicast DCCP connection does not imply that the session. The use of a unicast DCCP connection does not imply
the RTP session will have only two participants, and RTP end systems that the RTP session will have only two participants, and RTP end
SHOULD assume that multiple synchronisation sources may be observed systems must assume that multiple synchronisation sources may be
when using RTP over DCCP. observed when using RTP over DCCP.
A single RTP session may span multiple DCCP connections. The RTP An RTP translator bridging multiple DCCP connections to form a single
translator bridging those DCCP connections MUST be aware of the RTP session needs to be aware of the congestion state of each DCCP
congestion state of each connection, and MUST adapt the media to the connection, and must adapt the media to the available capacity of
available capacity of each. In general, transcoding is required to each. In general, transcoding is required to perform adaptation:
adapt the media: this is computationally expensive, induces delay, this is computationally expensive, induces delay, and generally gives
and generally gives poor quality results. Depending on the payload, poor quality results. Depending on the payload, it might be possible
it might be possible to use some form of scalable coding. Scalable to use some form of scalable coding. Scalable media coding formats
media coding formats are an active research area, and are not in are an active research area, and are not in widespread use at the
widespread use at the time of this writing. time of this writing.
A single RTP session may also span a DCCP connection and some other 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 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 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 over UDP/IP multicast group on the other side of the translator. A
second example might be an RTP over DCCP connection that links PSTN second example might be an RTP over DCCP connection that links PSTN
gateways. The issues for such an RTP translator are similar to those gateways. The issues for such an RTP translator are similar to those
when linking two DCCP connections, except that the congestion control when linking two DCCP connections, except that the congestion control
algorithms on either side of the translator may not be compatible. algorithms on either side of the translator may not be compatible.
Implementation of effective translators for such an environment is Implementation of effective translators for such an environment is
nontrivial. nontrivial.
4.5. RTP Profiles 4.5. RTP Profiles
In general, there is no conflict between new RTP Profiles and DCCP In general, there is no conflict between new RTP Profiles and DCCP
framing, and most RTP profiles MAY be negotiated for use over DCCP. framing, and most RTP profiles can be negotiated for use over DCCP.
The only potential for conflict occurs if an RTP profile changes the The only potential for conflict occurs if an RTP profile changes the
RTCP reporting interval or the RTP packet transmission rules, since RTCP reporting interval or the RTP packet transmission rules, since
this may conflict with DCCP congestion control. If an RTP profile this may conflict with DCCP congestion control. If an RTP profile
conflicts with DCCP congestion control, that profile MUST NOT be used conflicts with DCCP congestion control, that profile MUST NOT be used
with DCCP. with DCCP.
The RTP Profile for TFRC [14] MUST NOT be used with DCCP, since it Of the profiles currently defined, the RTP Profile for Audio and
conflicts with DCCP congestion control by providing alternative Video Conferences with Minimal Control [4], the Secure Real-time
congestion control semantics. Transport Protocol [7] the Extended RTP Profile for RTCP-based
Feedback [8], and the Extended Secure RTP Profile for RTCP-based
Feedback [9] MAY be used with DCCP. The RTP Profile for TFRC [15]
MUST NOT be used with DCCP, since it conflicts with DCCP congestion
control by providing alternative congestion control semantics.
5. RTP over DCCP: Signalling using SDP 5. RTP over DCCP: Signalling using SDP
The Session Description Protocol (SDP) [3] and the offer/answer model The Session Description Protocol (SDP) [3] and the offer/answer model
[5] are widely used to negotiate RTP sessions (for example, using the [10] are widely used to negotiate RTP sessions (for example, using
Session Initiation Protocol [18]). This section describes how SDP is the Session Initiation Protocol [20]). This section describes how
used to signal RTP sessions running over DCCP. SDP is used to signal RTP sessions running over DCCP.
5.1. Protocol Identification 5.1. Protocol Identification
SDP uses a media ("m=") line to convey details of the media format 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 and transport protocol used. The ABNF syntax of a media line is as
follows (from [3]): follows (from [3]):
media-field = "m=" media SP port ["/" integer] SP proto media-field = "m=" media SP port ["/" integer] SP proto
1*(SP fmt) CRLF 1*(SP fmt) CRLF
The proto field denotes the transport protocol used for the media, The proto field denotes the transport protocol used for the media,
while the port indicates the transport port to which the media is while the port indicates the transport port to which the media is
sent. Following [6] and [7] this memo defines the following five sent. Following [5] and [11] this memo defines the following five
values of the proto field to indicate media transported using DCCP: values of the proto field to indicate media transported using DCCP:
DCCP DCCP
DCCP/RTP/AVP DCCP/RTP/AVP
DCCP/RTP/SAVP DCCP/RTP/SAVP
DCCP/RTP/AVPF DCCP/RTP/AVPF
DCCP/RTP/SAVPF DCCP/RTP/SAVPF
The "DCCP" protocol identifier is similar to the "UDP" and "TCP" The "DCCP" protocol identifier is similar to the "UDP" and "TCP"
protocol identifiers and describes the transport protocol, but not protocol identifiers and describes the transport protocol, but not
the upper-layer protocol. An SDP "m=" line that specifies the "DCCP" the upper-layer protocol. An SDP "m=" line that specifies the "DCCP"
protocol MUST further qualify the application layer protocol using a protocol MUST further qualify the application layer protocol using a
fmt identifier. A single DCCP port is used, as denoted by the port fmt identifier. A single DCCP port is used, as denoted by the port
field in the media line. The "DCCP" protocol identifier MUST NOT be field in the media line. The "DCCP" protocol identifier MUST NOT be
used to signal RTP sessions running over DCCP. used to signal RTP sessions running over DCCP.
The "DCCP/RTP/AVP" protocol identifier refers to RTP using the RTP The "DCCP/RTP/AVP" protocol identifier refers to RTP using the RTP
Profile for Audio and Video Conferences with Minimal Control [8] Profile for Audio and Video Conferences with Minimal Control [4]
running over DCCP. running over DCCP.
The "DCCP/RTP/SAVP" protocol identifier refers to RTP using the The "DCCP/RTP/SAVP" protocol identifier refers to RTP using the
Secure Real-time Transport Protocol [9] running over DCCP. Secure Real-time Transport Protocol [7] running over DCCP.
The "DCCP/RTP/AVPF" protocol identifier refers to RTP using the The "DCCP/RTP/AVPF" protocol identifier refers to RTP using the
Extended RTP Profile for RTCP-based Feedback [10] running over DCCP. Extended RTP Profile for RTCP-based Feedback [8] running over DCCP.
The "DCCP/RTP/SAVPF" protocol identifier refers to RTP using the The "DCCP/RTP/SAVPF" protocol identifier refers to RTP using the
Extended Secure RTP Profile for RTCP-based Feedback [11] running over Extended Secure RTP Profile for RTCP-based Feedback [9] running over
DCCP. DCCP.
By default, a single DCCP connection on the specified port is used
for both RTP and RTCP packets. The "a=rtcp:" attribute [12] MAY be
used to specify an alternate DCCP port for RTCP, in which case a
separate DCCP connection is opened to transport the RTCP data.
5.2. Service Codes 5.2. Service Codes
In addition to the port number, specified on the SDP "m=" line, a In addition to the port number, specified on the SDP "m=" line, a
DCCP connection has an associated service code. To signal the DCCP connection has an associated service code. A single new SDP
service code, one new SDP attribute is defined: attribute is defined to signal the service code:
dccp-service-attr = "a=dccp-service-code:" 1x8HEXDIG dccp-service-attr = "a=dccp-service-code:" service-code
where HEXDIG is as defined in [12]. service-code = hex-sc / decimal-sc / ascii-sc
The "a=dccp-service-code:" attribute conveys the numeric value of the hex-sc = "SC=x" *HEXDIG
DCCP service code, in network byte order, expressed as a sequence of
eight hexadecimal digits. This representation was chosen since DCCP decimal-sc = "SC=" *DIGIT
services codes are not necessarily comprised of printable characters.
ascii-sc = "SC:" *sc-char
sc-char = %d42-43, %d45-47, %d63-90, %d95, %d97-122
where DIGIT and HEXDIG are as defined in [13]. The service code
should be interpreted as defined in Section 8.1.2 of [2]. The
following DCCP service codes are registered for use with RTP:
o SC:RTPA an RTP session conveying audio data
o SC:RTPV an RTP session conveying video data
o SC:RTPT an RTP session conveying textual data
o SC:RTPO an RTP session conveying other types of media
To ease the job of middleboxes, applications SHOULD use these service
codes to identify RTP sessions running within DCCP.
The "a=dccp-service-code:" attribute is a media level attribute which The "a=dccp-service-code:" attribute is a media level attribute which
is not subject to the charset attribute. is not subject to the charset attribute.
TODO: Should this memo register service codes for each RTP profile?
Or should they be assigned to applications?
5.3. Connection Management 5.3. Connection Management
The "a=setup:" attribute indicates which of the end points should The "a=setup:" attribute indicates which of the end points should
initiate the DCCP connection establishment (i.e. send the initial initiate the DCCP connection establishment (i.e. send the initial
DCCP-Request packet). The "a=setup:" attribute MUST be used in a DCCP-Request packet). The "a=setup:" attribute MUST be used in a
manner comparable with [7], except that DCCP connections are being manner comparable with [11], except that DCCP connections are being
initiated rather than TCP connections. initiated rather than TCP connections.
After the initial offer/answer exchange, the end points may decide to After the initial offer/answer exchange, the end points may decide to
re-negotiate various parameters. The "a=connection:" attribute MUST re-negotiate various parameters. The "a=connection:" attribute MUST
be used in a manner compatible with [7] to decide whether a new DCCP be used in a manner compatible with [11] to decide whether a new DCCP
connection needs to be established as a result of subsequent offer/ connection needs to be established as a result of subsequent offer/
answer exchanges, or if the existing connection should still be used. answer exchanges, or if the existing connection should still be used.
5.4. Example 5.4. Example
An offerer at 10.0.0.47 signals its availability for an H.261 video An offerer at 192.0.2.47 signals its availability for an H.261 video
session, using RTP/AVP over DCCP with service code "RTP ": session, using RTP/AVP over DCCP with service code "RTPV":
v=0 v=0
o=alice 1129377363 1 IN IP4 10.0.0.47 o=alice 1129377363 1 IN IP4 192.0.2.47
s=- s=-
c=IN IP4 10.0.0.47 c=IN IP4 192.0.2.47
t=0 0 t=0 0
m=video 51372 DCCP/RTP/AVP 99 m=video 51370 DCCP/RTP/AVP 99
a=rtpmap:99 h261/90000 a=rtpmap:99 h261/90000
a=dccp-service-code:52545020 a=dccp-service-code:SC=x52545056
a=setup:passive a=setup:passive
a=connection:new a=connection:new
An answerer at 10.2.5.128 receives this offer and responds with the An answerer at 192.0.2.128 receives this offer and responds with the
following answer: following answer:
v=0 v=0
o=bob 1129377364 1 IN IP4 10.2.5.128 o=bob 1129377364 1 IN IP4 192.0.2.128
s=- s=-
c=IN IP4 10.2.5.128 c=IN IP4 192.0.2.128
t=0 0 t=0 0
m=video 9 DCCP/RTP/AVP 99 m=video 9 DCCP/RTP/AVP 99
a=rtpmap:99 h261/90000 a=rtpmap:99 h261/90000
a=dccp-service-code:52545020 a=dccp-service-code:SC:RTPV
a=setup:active a=setup:active
a=connection:new a=connection:new
The end point at 10.2.5.128 then initiates a DCCP connection to port The end point at 192.0.2.128 then initiates a DCCP connection to port
51372 at 10.0.0.47. 51370 at 192.0.2.47. Note that DCCP port 51370 is used for both the
RTP and RTCP data, and port 51371 is unused.
TODO: is DCCP port 9 registered as discard?
6. Security Considerations 6. Security Considerations
The security considerations in the RTP specification [1] and any The security considerations in the RTP specification [1] and any
applicable RTP profile (e.g. [8], [9], [10], or [11]) or payload applicable RTP profile (e.g. [4], [7], [8], or [9]) or payload format
format apply when transporting RTP over DCCP. apply when transporting RTP over DCCP.
The security considerations in the DCCP specification [2] apply. The security considerations in the DCCP specification [2] apply.
The SDP signalling described in Section 5 is subject to the security The SDP signalling described in Section 5 is subject to the security
considerations of [3], [5], [7] and [6]. considerations of [3], [10], [11] and [5].
It is not believed that any additional security considerations are It is not believed that any additional security considerations are
introduced as a result of combining these protocols. Indeed, the introduced as a result of combining these protocols. Indeed, the
provision of effective congestion control for RTP will alleviate the provision of effective congestion control for RTP will alleviate the
potential for denial-of-service present when RTP flows ignore the potential for denial-of-service present when RTP flows ignore the
advice in [1] to monitor packet loss and reduce their sending rate in advice in [1] to monitor packet loss and reduce their sending rate in
the face of persistant congestion. the face of persistant congestion.
7. IANA Considerations 7. IANA Considerations
The following SDP "proto" field identifiers are registered: "DCCP", The following SDP "proto" field identifiers are registered: "DCCP",
"DCCP/RTP/AVP", "DCCP/RTP/SAVP", "DCCP/RTP/AVPF" and "DCCP/RTP/SAVPF" "DCCP/RTP/AVP", "DCCP/RTP/SAVP", "DCCP/RTP/AVPF" and "DCCP/RTP/SAVPF"
(see Section 5.1 of this memo). (see Section 5.1 of this memo).
One new SDP attribute ("a=dccp-service-code:") is registered (see One new SDP attribute ("a=dccp-service-code:") is registered (see
Section 5.2 of this memo). Section 5.2 of this memo).
The following DCCP service codes are registered: SC:RTPA, SC:RTPV,
SC:RTPT, and SC:RTPO (see Section 5.2 of this memo).
8. Acknowledgements 8. Acknowledgements
This work was supported in part by the UK Engineering and Physical This work was supported in part by the UK Engineering and Physical
Sciences Research Council, under a GridNet2 award (administered by Sciences Research Council. Thanks are due to to Philippe Gentric,
the UK National e-Science Centre). Magnus Westerlund and the other members of the DCCP working group for
their comments.
9. References 9. References
9.1. Normative References 9.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64, "RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003. RFC 3550, July 2003.
[2] Kohler, E., "Datagram Congestion Control Protocol (DCCP)", [2] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-11 (work in progress), March 2005. draft-ietf-dccp-spec-11 (work in progress), March 2005.
[3] Handley, M., "SDP: Session Description Protocol", [3] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005. draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement [4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Levels", BCP 14, RFC 2119, March 1997. Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[6] Lazzaro, J., "Framing RTP and RTCP Packets over Connection- [5] Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06 Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06
(work in progress), September 2005. (work in progress), September 2005.
[7] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Session Description Protocol (SDP)", RFC 4145, September 2005. Levels", BCP 14, RFC 2119, March 1997.
[8] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[9] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [7] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
[10] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based [8] Ott, J. and S. Wenger, "Extended RTP Profile for RTCP-based
Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in Feedback(RTP/AVPF)", draft-ietf-avt-rtcp-feedback-11 (work in
progress), August 2004. progress), August 2004.
[11] Ott, J. and E. Carrara, "Extended Secure RTP Profile for RTCP- [9] Ott, J. and E. Carrara, "Extended Secure RTP Profile for RTCP-
based Feedback (RTP/SAVPF)", draft-ietf-avt-profile-savpf-02 based Feedback (RTP/SAVPF)", draft-ietf-avt-profile-savpf-02
(work in progress), July 2005. (work in progress), July 2005.
[12] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [10] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[11] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
Session Description Protocol (SDP)", RFC 4145, September 2005.
[12] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[13] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
9.2. Informative References 9.2. Informative References
[13] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion [14] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion
Control for Voice Traffic in the Internet", RFC 3714, Control for Voice Traffic in the Internet", RFC 3714,
March 2004. March 2004.
[14] Gharai, L., "RTP Profile for TCP Friendly Rate Control", [15] Gharai, L., "RTP Profile for TCP Friendly Rate Control",
draft-ietf-avt-tfrc-profile-04 (work in progress), July 2005. draft-ietf-avt-tfrc-profile-04 (work in progress), July 2005.
[15] Andreasen, F., "A No-Op Payload Format for RTP", [16] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
[17] Andreasen, F., "A No-Op Payload Format for RTP",
draft-wing-avt-rtp-noop-03 (work in progress), May 2005. draft-wing-avt-rtp-noop-03 (work in progress), May 2005.
[16] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real- [18] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real-
Time Transport Protocol (RTP) Payload Format and File Storage Time Transport Protocol (RTP) Payload Format and File Storage
Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi- Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-
Rate Wideband (AMR-WB) Audio Codecs", RFC 3267, June 2002. Rate Wideband (AMR-WB) Audio Codecs", RFC 3267, June 2002.
[17] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol [19] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
Extended Reports (RTCP XR)", RFC 3611, November 2003. Extended Reports (RTCP XR)", RFC 3611, November 2003.
[18] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [20] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002. Session Initiation Protocol", RFC 3261, June 2002.
Author's Address Author's Address
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
Department of Computing Science Department of Computing Science
17 Lilybank Gardens 17 Lilybank Gardens
Glasgow G12 8QQ Glasgow G12 8QQ
skipping to change at page 13, line 41 skipping to change at page 15, line 41
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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