draft-ietf-dccp-rtp-00.txt   draft-ietf-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: January 18, 2007 July 17, 2006 Intended status: Standards Track October 22, 2006
Expires: April 25, 2007
RTP and the Datagram Congestion Control Protocol (DCCP) RTP and the Datagram Congestion Control Protocol (DCCP)
draft-ietf-dccp-rtp-00.txt draft-ietf-dccp-rtp-01.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
The Real-time Transport Protocol (RTP) is a widely used transport for The Real-time Transport Protocol (RTP) is a widely used transport for
real-time media on IP networks. The Datagram Congestion Control real-time multimedia on IP networks. The Datagram Congestion Control
Protocol (DCCP) is a newly defined transport protocol that provides Protocol (DCCP) is a newly defined transport protocol that provides
desirable services for real-time applications. This memo specifies a desirable services for real-time applications. This memo specifies a
mapping of RTP onto DCCP, along with associated signalling, such that mapping of RTP onto DCCP, along with associated signalling, such that
real-time applications can make use of the services provided by DCCP. 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 . . . . . . . . . . . . . . . . 4 3. Conventions Used in this Memo . . . . . . . . . . . . . . . . 4
4. RTP over DCCP: Framing . . . . . . . . . . . . . . . . . . . . 4 4. RTP over DCCP: Framing . . . . . . . . . . . . . . . . . . . . 4
4.1. RTP Data Packets . . . . . . . . . . . . . . . . . . . . . 4 4.1. RTP Data Packets . . . . . . . . . . . . . . . . . . . . . 4
4.2. RTP Control Packets . . . . . . . . . . . . . . . . . . . 5 4.2. RTP Control Packets . . . . . . . . . . . . . . . . . . . 5
4.3. Multiplexing Data and Control . . . . . . . . . . . . . . 6 4.3. Multiplexing Data and Control . . . . . . . . . . . . . . 6
4.4. RTP Sessions and DCCP Connections . . . . . . . . . . . . 7 4.4. RTP Sessions and DCCP Connections . . . . . . . . . . . . 7
4.5. RTP Profiles . . . . . . . . . . . . . . . . . . . . . . . 8 4.5. RTP Profiles . . . . . . . . . . . . . . . . . . . . . . . 7
5. RTP over DCCP: Signalling using SDP . . . . . . . . . . . . . 8 5. RTP over DCCP: Signalling using SDP . . . . . . . . . . . . . 8
5.1. Protocol Identification . . . . . . . . . . . . . . . . . 8 5.1. Protocol Identification . . . . . . . . . . . . . . . . . 8
5.2. Service Codes . . . . . . . . . . . . . . . . . . . . . . 9 5.2. Service Codes . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Connection Management . . . . . . . . . . . . . . . . . . 10 5.3. Connection Management . . . . . . . . . . . . . . . . . . 10
5.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.4. Example . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13 9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16 Intellectual Property and Copyright Statements . . . . . . . . . . 16
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
choice of lower-layer transport. The Datagram Congestion Control choice of lower-layer transport. The Datagram Congestion Control
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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, and discusses some of the be framed for transport using DCCP, and discusses some of the
implications of such a framing. It also describes how the Session implications of such a framing. It also describes how the Session
Description Protocol (SDP) [3] can be used to signal such 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
With the widespread adoption of RTP have come concerns that many real With the widespread adoption of RTP have come concerns that many real
time applications do not implement congestion control, leading to the time applications do not implement congestion control, leading to the
potential for congestion collapse of the network [14]. The designers potential for congestion collapse of the network [16]. The designers
of RTP recognised this issue, stating that [4]: of RTP recognised this issue, stating that [4]:
If best-effort service is being used, RTP receivers SHOULD monitor If best-effort service is being used, RTP receivers SHOULD monitor
packet loss to ensure that the packet loss rate is within packet loss to ensure that the packet loss rate is within
acceptable parameters. Packet loss is considered acceptable if a acceptable parameters. Packet loss is considered acceptable if a
TCP flow across the same network path and experiencing the same TCP flow across the same network path and experiencing the same
network conditions would achieve an average throughput, measured network conditions would achieve an average throughput, measured
on a reasonable time-scale, that is not less than the RTP flow is on a reasonable time-scale, that is not less than the RTP flow is
achieving. This condition can be satisfied by implementing achieving. This condition can be satisfied by implementing
congestion control mechanisms to adapt the transmission rate (or congestion control mechanisms to adapt the transmission rate (or
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While the goals are clear, the development of TCP friendly congestion While the goals are clear, the development of TCP friendly congestion
control that can be used with RTP and real-time media applications is control that can be used with RTP and real-time media applications is
an open research question with many proposals for new algorithms, but an open research question with many proposals for new algorithms, but
little deployment experience. little deployment experience.
Two approaches have been used to provide congestion control for RTP: Two approaches have been used to provide congestion control for RTP:
1) develop new RTP profiles that incorporate congestion control; and 1) develop new RTP profiles that incorporate congestion control; and
2) provide mechanisms for running RTP over congestion controlled 2) provide mechanisms for running RTP over congestion controlled
transport protocols. The RTP Profile for TCP Friendly Rate Control transport protocols. The RTP Profile for TCP Friendly Rate Control
[15] is an example of the first approach, extending the RTP packet [17] is an example of the first approach, extending the RTP packet
formats to incorporate feedback information such that TFRC congestion formats to incorporate feedback information such that TFRC congestion
control [16] can be implemented at the application level. This control [18] can be implemented at the application level. This will
approach has the advantage that congestion control can be added to allow congestion control to be added to existing applications without
existing applications, without needing operating system or network operating system or network support, and it offers the flexibility to
support, and offers flexibility to experiment with new congestion experiment with new congestion control algorithms as they are
control algorithms as they are developed. Unfortunately, there is developed. Unfortunately, it also passes the complexity of
also the consequent disadvantage that the complexity of implementing implementing congestion control onto application authors, a burden
congestion control is passed onto the application author, a burden
which many would prefer to avoid. which many would prefer to avoid.
The other approach is to run RTP on a lower-layer transport protocol The other approach is to run RTP on a lower-layer transport protocol
that provides congestion control. One possibility is to run RTP over that provides congestion control. One possibility is to run RTP over
TCP, as defined in [5], but the reliable nature of TCP and the TCP, as defined in [5], but the reliable nature of TCP and the
dynamics of its congestion control algorithm make this inappropriate dynamics of its congestion control algorithm make this inappropriate
for most interactive real time applications (SCTP is inappropriate for most interactive real time applications (the Stream Control
for similar reasons). A better fit for such applications may be to Transmission Protocol (SCTP) is inappropriate for similar reasons).
run RTP over DCCP, since DCCP offers unreliable packet delivery and a A better fit for such applications may be to run RTP over DCCP, since
choice of congestion control. This gives applications the ability to DCCP offers unreliable packet delivery and a choice of congestion
tailor the transport to their needs, taking advantage of better control. This gives applications the ability to tailor the transport
congestion control algorithms as they come available, while passing to their needs, taking advantage of better congestion control
complexity of implementation to the operating system. If DCCP should algorithms as they come available, while passing complexity of
come to be widely available, it is believed these will be compelling implementation to the operating system. If DCCP should come to be
advantages. Accordingly, this memo defines a mapping of RTP onto widely available, it is believed these will be compelling advantages.
DCCP. 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 [6]. 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 can be framed The following section defines how RTP and RTCP packets can be framed
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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 it remains open for the duration of the session. To ensure NAT 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- 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 RTP 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 [17], and data to send. This removes the need for RTP no-op packets [19], and
similar application level keep-alives, when using RTP over DCCP. similar application level keep-alives, when using 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 (but see [20] and [21] for guidance).
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. [18]) can make use corrupt payloads. Some RTP Payload Formats (e.g. [22]) 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 MUST cover at least the DCCP and RTP headers which case the checksum MUST cover at least the DCCP and RTP headers
to ensure packets are correctly delivered. Partial checksums MUST to ensure packets are correctly delivered. Partial checksums MUST
NOT be used unless supported by mechanisms in the RTP payload format. NOT be used unless supported by mechanisms in the RTP payload format.
4.2. RTP Control Packets 4.2. RTP Control Packets
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packet was sent is then used as the basis for calculating the next packet was sent is then used as the basis for calculating the next
RTCP transmission time. RTCP transmission time.
RTCP packets comprise only a small fraction of the total traffic in RTCP packets comprise only a small fraction of the total traffic in
an RTP session. Accordingly, it is expected that delays in their an RTP session. Accordingly, it is expected that delays in their
transmission due to congestion control will not be common, provided transmission due to congestion control will not be common, provided
the configured nominal "session bandwidth" (see Section 6.2 of [1]) the configured nominal "session bandwidth" (see Section 6.2 of [1])
is in line with the bandwidth achievable on the DCCP connection. If, is in line with the bandwidth achievable on the DCCP connection. If,
however, the capacity of the DCCP connection is significantly below however, the capacity of the DCCP connection is significantly below
the nominal session bandwidth, RTCP packets may be delayed enough for the nominal session bandwidth, RTCP packets may be delayed enough for
participants to time out due to apparant inactivity. In such cases, participants to time out due to apparent inactivity. In such cases,
the session parameters SHOULD be re-negotiated to more closely match the session parameters SHOULD be re-negotiated to more closely match
the available capacity, for example by performing a SIP re-invite the available capacity, for example by performing a SIP re-invite
with an updated "b=" line. with an updated "b=" line.
Since the nominal session bandwidth is chosen based on media codec Since the nominal session bandwidth is chosen based on media codec
capabilities, a session where the nominal bandwidth is much larger capabilities, a session where the nominal bandwidth is much larger
than the available bandwidth will likely become unusable due to than the available bandwidth will likely become unusable due to
constraints on the media channel, and so require negotiation of a constraints on the media channel, and so require negotiation of a
lower bandwidth codec, before it becomes unusable due to lower bandwidth codec, before it becomes unusable due to
constraints on the RTCP channel. constraints on the RTCP channel.
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 information conveyed in RTCP packets and that conveyed in between information conveyed in RTCP packets and that conveyed in
DCCP acknowledgement options. In general this is not an issue since DCCP acknowledgement options. In general this is not an issue since
RTCP packets contain media-specific data that is not present in DCCP RTCP packets contain media-specific data that is not present in DCCP
acknowledgement options, and DCCP options contain network-level data acknowledgement options, and DCCP options contain network-level data
that is not present in RTCP. Indeed, there is no overlap between the that is not present in RTCP. Indeed, there is no overlap between the
five RTCP packet types defined in the RTP specification [1] and the five RTCP packet types defined in the RTP specification [1] and the
standard DCCP options [2]. There are, however, cases where overlap standard DCCP options [2]. There are, however, cases where overlap
does occur: most clearly between the optional RTCP Extended Reports does occur: most clearly between the optional RTCP Extended Reports
Loss RLE Blocks [19] and the DCCP Ack Vector option. If there is Loss RLE Blocks [23] and the DCCP Ack Vector option. If there is
overlap between RTCP report packets and DCCP acknowledgements, an overlap between RTCP report packets and DCCP acknowledgements, an
application should use either RTCP feedback or DCCP acknowledgements, application should use either RTCP feedback or DCCP acknowledgements,
but not both (use of both types of feedback will waste available but not both (use of both types of feedback will waste available
network capacity, but is not otherwise harmful). network capacity, but is not otherwise 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. For these reasons, use of multiple ports complicates NAT traversal. For these reasons,
it is RECOMMENDED that RTP and RTCP flows be multiplexed onto a it is RECOMMENDED that the RTP and RTCP traffic for a single RTP
single DCCP connection. session is multiplexed onto a single DCCP connection following the
guidelines in [7], where possible (it may not be possible in all
RTP and RTCP packets multiplexed onto a single connection can be circumstances, for example when translating from an RTP stream over a
distinguished provided care is taken in assigning RTP payload types. non-DCCP transport that uses conflicting RTP payload types and RTCP
The RTP payload type and marker bit(s) occupy the same space in the packet types).
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 include RTP mixers or translators bringing other participants into
the session. The use of a unicast DCCP connection does not imply the session. The use of a unicast DCCP connection does not imply
that the RTP session will have only two participants, and RTP end that the RTP session will have only two participants, and RTP end
systems must assume that multiple synchronisation sources may be systems must assume that multiple synchronisation sources may be
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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
non-trivial. non-trivial.
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 can 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 with the following exceptions:
RTCP reporting interval or the RTP packet transmission rules, since
this may conflict with DCCP congestion control. If an RTP profile o An RTP profile that changes the RTCP reporting interval or the RTP
conflicts with DCCP congestion control, that profile MUST NOT be used transmission rules may conflict with DCCP congestion control. For
with DCCP. example, the RTP Profile for TFRC [17] conflicts with DCCP by
changing the RTP packet transmission rules to provide alternate
congestion control semantics.
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 which 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 Of the profiles currently defined, the RTP Profile for Audio and
Video Conferences with Minimal Control [4], the Secure Real-time Video Conferences with Minimal Control [4], the Secure Real-time
Transport Protocol [7], the Extended RTP Profile for RTCP-based Transport Protocol [8], the Extended RTP Profile for RTCP-based
Feedback [8], and the Extended Secure RTP Profile for RTCP-based Feedback [9], and the Extended Secure RTP Profile for RTCP-based
Feedback [9] MAY be used with DCCP. The RTP Profile for TFRC [15] Feedback [10] MAY be used with DCCP (noting the potential conflict
MUST NOT be used with DCCP, since it conflicts with DCCP congestion between DCCP partial checksums and the integrity protection provided
control by providing alternative congestion control semantics. by the secure RTP variants -- see Section 6). The RTP Profile for
TFRC [17] MUST NOT be used with DCCP, since it conflicts with DCCP by
providing alternative congestion control semantics (DCCP CCID 3 [11]
provides similar congestion control within the DCCP framework).
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
[10] are widely used to negotiate RTP sessions (for example, using [12] are widely used to negotiate RTP sessions (for example, using
the Session Initiation Protocol [20]). This section describes how the Session Initiation Protocol [24]). This section describes how
SDP is 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 = %x6d "=" media SP port ["/" integer] SP proto media-field = %x6d "=" 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 [5] and [11] this memo defines the following five sent. Following [5] and [13] 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
skipping to change at page 9, line 24 skipping to change at page 9, line 19
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 [4] 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 [7] running over DCCP. Secure Real-time Transport Protocol [8] 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 [8] running over DCCP. Extended RTP Profile for RTCP-based Feedback [9] 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 [9] running over Extended Secure RTP Profile for RTCP-based Feedback [10] running over
DCCP. DCCP.
By default, a single DCCP connection on the specified port is used A single DCCP connection can be used to transport multiplexed RTP and
for both RTP and RTCP packets. The "a=rtcp:" attribute [12] MAY be RTCP packets. Such multiplexing MUST be signalled using an "a=rtcp:"
used to specify an alternate DCCP port for RTCP, in which case a attribute [14] according to the rules in [7]. If multiplexed RTP and
separate DCCP connection is opened to transport the RTCP data. RTCP is not to be used, then the "a=rtcp:" 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.
Port 5004 is registered for use by RTP and SHOULD be the default port Port 5004 is registered for use by RTP and SHOULD be the default port
chosen by applications. If more than one RTP session is active from chosen by applications. If more than one RTP session is active from
a host, ports in the dynamic range SHOULD be used for the other a host, ports in the dynamic range SHOULD be used for the other
sessions. If RTCP is to be sent on a separate DCCP connection to sessions. If RTCP is to be sent on a separate DCCP connection to
RTP, the RTCP connection SHOULD use port 5005 by default. RTP, the RTCP connection SHOULD use port 5005 by default.
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. A single new SDP DCCP connection has an associated service code. A single new SDP
attribute is defined to signal the service code: attribute is defined to signal the service code:
dccp-service-attr = %x61 "=dccp-service-code:" service-code dccp-service-attr = %x61 "=dccp-service-code:" service-code
service-code = hex-sc / decimal-sc / ascii-sc service-code = hex-sc / decimal-sc / ascii-sc
hex-sc = "SC=x" *HEXDIG hex-sc = %x53 %x43 "=" %x78 *HEXDIG
decimal-sc = "SC=" *DIGIT decimal-sc = %x53 %x43 "=" *DIGIT
ascii-sc = "SC:" *sc-char ascii-sc = %x53 %x43 ":" *sc-char
sc-char = %d42-43, %d45-47, %d63-90, %d95, %d97-122 sc-char = %d42-43, %d45-47, %d63-90, %d95, %d97-122
where DIGIT and HEXDIG are as defined in [13]. The service code where DIGIT and HEXDIG are as defined in [15]. The service code
should be interpreted as defined in Section 8.1.2 of [2]. The should be interpreted as defined in Section 8.1.2 of [2]. The
following DCCP service codes are registered for use with RTP: following DCCP service codes are registered for use with RTP:
o SC:RTPA an RTP session conveying audio data o SC:RTPA an RTP session conveying audio data and associated RTCP
o SC:RTPV an RTP session conveying video data o SC:RTPV an RTP session conveying video data and associated RTCP
o SC:RTPT an RTP session conveying textual data o SC:RTPT an RTP session conveying text media and associated RTCP
o SC:RTPO an RTP session conveying other types of media o SC:RTPO an RTP session conveying other media and associated RTCP
o SC:RTCP an RTCP connection, separate from the corresponding RTP
To ease the job of middleboxes, applications SHOULD use these service To ease the job of middleboxes, applications SHOULD use these service
codes to identify RTP sessions running within DCCP. 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.
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 [11], except that DCCP connections are being manner comparable with [13], 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 [11] to decide whether a new DCCP be used in a manner compatible with [13] 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 192.0.2.47 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 "RTPV": session, using RTP/AVP over DCCP with service code "RTPV". RTP and
RTCP packets are multiplexed onto a single DCCP connection:
v=0 v=0
o=alice 1129377363 1 IN IP4 192.0.2.47 o=alice 1129377363 1 IN IP4 192.0.2.47
s=- s=-
c=IN IP4 192.0.2.47 c=IN IP4 192.0.2.47
t=0 0 t=0 0
m=video 5004 DCCP/RTP/AVP 99 m=video 5004 DCCP/RTP/AVP 99
a=rtcp:5004
a=rtpmap:99 h261/90000 a=rtpmap:99 h261/90000
a=dccp-service-code:SC=x52545056 a=dccp-service-code:SC=x52545056
a=setup:passive a=setup:passive
a=connection:new a=connection:new
SDP offer showing use of RTP/SAVP over DCCP
An answerer at 192.0.2.128 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 192.0.2.128 o=bob 1129377364 1 IN IP4 192.0.2.128
s=- s=-
c=IN IP4 192.0.2.128 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=rtcp:5004
a=rtpmap:99 h261/90000 a=rtpmap:99 h261/90000
a=dccp-service-code:SC:RTPV a=dccp-service-code:SC:RTPV
a=setup:active a=setup:active
a=connection:new a=connection:new
SDP answer showing use of RTP/SAVP over DCCP
The end point at 192.0.2.128 then initiates a DCCP connection to port The end point at 192.0.2.128 then initiates a DCCP connection to port
5004 at 192.0.2.47. Note that DCCP port 5004 is used for both the 5004 at 192.0.2.47. Note that DCCP port 5004 is used for both the
RTP and RTCP data, and port 5005 is unused. RTP and RTCP data, and port 5005 is unused.
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. [4], [7], [8], or [9]) or payload format applicable RTP profile (e.g. [4], [8], [9], or [10]) or payload
apply when transporting RTP over DCCP. format 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], [10], [11] and [5]. considerations of [3], [12], [13], [5], [14], and [7].
It is not believed that any additional security considerations are The provision of effective congestion control for RTP through use of
introduced as a result of combining these protocols. Indeed, the DCCP is expected to help reduce the potential for denial-of-service
provision of effective congestion control for RTP will alleviate the present when RTP flows ignore the advice in [1] to monitor packet
potential for denial-of-service present when RTP flows ignore the loss and reduce their sending rate in the face of persistent
advice in [1] to monitor packet loss and reduce their sending rate in congestion.
the face of persistent congestion.
There is a potential conflict between the Secure RTP Profiles [8],
[10] 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 SRTP authentication. The confidentiality features
of the basic RTP specification cannot be used with DCCP partial
checksums, since bit 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 it requires authentication for security, and authentication is
incompatible with partial checksums.
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, The following DCCP service codes are registered: SC:RTPA, SC:RTPV,
SC:RTPT, and SC:RTPO (see Section 5.2 of this memo). SC:RTPT, SC:RTPO, and SC:RTCP (see Section 5.2 of this memo).
DCCP ports 5004 ("DCCP RTP") and 5005 ("DCCP RTCP") are registered DCCP ports 5004 ("DCCP RTP") and 5005 ("DCCP RTCP") are registered
for use as default RTP/RTCP ports (see Section 5.1 of this memo). for use as default RTP/RTCP ports (see Section 5.1 of this memo).
The four services codes listed above are to be associated with these The four services codes listed above are to be associated with these
ports. ports.
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. Thanks are due to to Philippe Gentric, Sciences Research Council. Thanks are due to to Philippe Gentric,
skipping to change at page 13, line 8 skipping to change at page 13, line 31
[4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video [4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003. Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[5] Lazzaro, J., "Framing RTP and RTCP Packets over Connection- [5] Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
Oriented Transport", RFC 4571, June 2006. Oriented Transport", RFC 4571, June 2006.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[7] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [7] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port",
draft-ietf-avt-rtp-and-rtcp-mux-00 (work in progress),
October 2006.
[8] 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.
[8] Ott, J., Wenger, S., Sato, N., and C. Burmeister, "Extended RTP [9] Ott, J., Wenger, S., Sato, N., and C. Burmeister, "Extended RTP
Profile for RTCP-based Feedback(RTP/AVPF)", RFC 4585, Profile for RTCP-based Feedback(RTP/AVPF)", RFC 4585,
June 2006. June 2006.
[9] Ott, J. and E. Carrara, "Extended Secure RTP Profile for RTCP- [10] 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-08
(work in progress), July 2005. (work in progress), October 2006.
[10] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [11] Floyd, S., Kohler, E., and J. Padhye, "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control ID 3:
TCP-Friendly Rate Control (TFRC)", RFC 4342, March 2006.
[12] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002. Session Description Protocol (SDP)", RFC 3264, June 2002.
[11] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the [13] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
Session Description Protocol (SDP)", RFC 4145, September 2005. Session Description Protocol (SDP)", RFC 4145, September 2005.
[12] Huitema, C., "Real Time Control Protocol (RTCP) attribute in [14] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003. Session Description Protocol (SDP)", RFC 3605, October 2003.
[13] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [15] 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
[14] Floyd, S. and J. Kempf, "IAB Concerns Regarding Congestion [16] 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.
[15] Gharai, L., "Use of the Extended RTP Profile for RTCP-based [17] Gharai, L., "RTP Profile for TCP Friendly Rate Control",
Feedback (RTP/AVPF) to Support TCP-Friendly Rate Control", draft-ietf-avt-tfrc-profile-06 (work in progress),
draft-ietf-avt-tfrc-profile-06 (work in progress), June 2006. September 2007.
[16] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP [18] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003. RFC 3448, January 2003.
[17] Andreasen, F., "A No-Op Payload Format for RTP", [19] 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.
[18] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "Real- [20] Phelan, T., "Strategies for Streaming Media Applications Using
TCP-Friendly Rate Control", draft-ietf-dccp-tfrc-media-01
(work in progress), October 2005.
[21] Phelan, T., "Datagram Congestion Control Protocol (DCCP) User
Guide", draft-ietf-dccp-user-guide-03 (work in progress),
April 2005.
[22] 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.
[19] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol [23] 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.
[20] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [24] 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
UK UK
Email: csp@csperkins.org Email: csp@csperkins.org
Intellectual Property Statement Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
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on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
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skipping to change at page 16, line 29 skipping to change at page 16, line 45
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This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
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 provided by the IETF
Internet Society. Administrative Support Activity (IASA).
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