1rtp_rtcp
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RealReal--Time Transport andTime Transport and
Control ProtocolControl Protocol(RTP/RTCP)(RTP/RTCP)
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OverviewOverview
Introduction to RTP and RTCPIntroduction to RTP and RTCP RealReal--time Transport Protocol (RTP)time Transport Protocol (RTP)
RTP ImplementationsRTP Implementations
RTP in a Multimedia ContextRTP in a Multimedia Context
RTP Profiles and PayloadsRTP Profiles and Payloads
RTP Header FormatRTP Header Format
RTP Header Extension FormatRTP Header Extension Format
RealReal--time Transport Control Protocol (RTCP)time Transport Control Protocol (RTCP)
More on RTCPMore on RTCP
ReferencesReferences
ConclusionsConclusions
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Introduction to RTP/RTCPIntroduction to RTP/RTCP
The Real-time Transport Protocol (RTP) works
alongside with the Real-time Transport Control
Protocol (RTCP). In fact, RTP as a protocol is made up of 2 parts - a
data part (RTP) and a control part (RTCP).
Together, these protocols deliver real-time trafficwith timing information and feedback on the
reception quality.
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RealReal--time Transport Protocol (RTP)time Transport Protocol (RTP)
RTP is a transport protocol that meets the
requirements of voice and other real-time data.
RTP can transport data continuously rather than inbursts, and it can handle data delivery in multicast
environments.
RTP takes care of data that TCP cannot handle.
Such data can include voice, video, etc.
RTP was designed to run independently of the
underlying transport and network layers of the OSI
model.
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Functionalities of RTPFunctionalities of RTP
Timing recoveryTiming recovery
SynchronizationSynchronization
Content identificationContent identification
DemultiplexingDemultiplexing
Loss detection (errorLoss detection (error--recognition)recognition)
Security servicesSecurity services
Applicatio
/
UD
I
Li k layer
hysical layer
ra sport layer{Socket
i terface
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RTP PayloadRTP Payload
Stream TranslationStream Translation
TranslatorTranslator
MixerMixer
P eader RTP ayloadRTP eaderU P eader
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RTP ImplementationsRTP Implementations
Most implementations of RTP run in IP
environments on top of the ser Datagram Protocol
( DP).
But RTP can also be used with other protocols such
as the Asynchronous Transfer Mode (ATM), etc.
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RTP in a Multimedia contextRTP in a Multimedia context
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RTP in a Multimedia ContextRTP in a Multimedia Context
The RTCP packets report on the reception qualityfor a RTP session.
The routers on the network communicate through
RSVP (R
esource ReS
erV
ationP
rotocol) to set asidebandwidth for the transmission of the multimedia
data.
The RTP header provides timing information so
that data delivery can be properly synchronized.
The RTP header also specifies the payload type. In
this way multiple data and data compression types
are allowed.
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RTP Profile and PayloadRTP Profile and Payload
RTP is tailored to the application sending data by
means of supplementary profiles and payload
formats.
A payload format defines how a particular payload
has to be carried in RTP.
A profile assigns payload type numbers for the set
of payload formats contained in the application.Application-specific extensions or modifications to
RTP can also be defined by a profile.
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Required Bandwidth of Some EncodingRequired Bandwidth of Some Encoding
Formats (Throughput)Formats (Throughput)
PT Name of
Encoding Met od
T rougput
[kbit/s]
0 PCM 64
1 1016 4,8
2 G.726-32 32
3 GSM 13
6 DVI4 64
7 LPC 2,4
9 G.722 48-64
15 G.728 16
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RTP Header FormatRTP Header Format
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RTP Header FormatRTP Header Format
The first 12 octets are present in every RTP packet, while the
list of CSRC identifiers is present only when inserted by a mixer.
The fields have the following meaning:
The version (V) field identifies the version of RTP, and it consistsof 2 bits.
The padding (P) field consists of 1 bit. If this bit is set, the packet
contains one or more additional octets at the end which are not part
of the payload. The last octet of the padding contains a count of howmany padding octets should be ignored.
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RTP Header Format (Cont.)RTP Header Format (Cont.)
Padding may be needed by some encryption algorithms with fixedblock sizes or for carrying several RTP packets in a lower-layer
protocol data unit.
The extension (X) field consists of 1 bit and if this bit is set, the
fixed header is followed by exactly one header extension, with a
format defined later.
The CSRC count (CC) field consists of 4 bits and it contains the
number of CSRC identifiers that follow the fixed header.
The marker (M) field consists of 1 bit. The interpretation of the
marker is defined by a profile. It is intended to allow significantevents such as frame boundaries to be marked in the packet stream.
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RTP Header Format (Cont.)RTP Header Format (Cont.)
A profile may define additional marker bits or specify that there isno marker bit by changing the number of bits in the payload type
field.
The payload type (PT) field is of 7 bits and it identifies the RTP
payload format, and determines its interpretation by the application.
The sequence number field consists of 16 bits and it increments
by one for each RTP data packet sent.
The timestamp field which consists of 32 bits reflects the
sampling instant of the first octet in the RTP data packet. The
sampling instant must be derived from a clock that incrementsmonotonically and linearly in time to allow synchronization and
jitter calculations.
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RTP Header Format (Cont.)RTP Header Format (Cont.)
The SSRC field identifies the synchronization source.This identifier is chosen randomly, with the intent that no
two synchronization sources within the same RTP session
will have the same SSRC identifier.
CSRC list: 0 to 15 items, 32 bits eachThe CSRC list identifies the contributing sources for the
payload contained in this packet. The number of
identifiers is given by the CC field. If there are more than
15 contributing sources, only 15 may be identified.
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tkzsi valsznsgChoosing Synchronization Source (SSRC)Choosing Synchronization Source (SSRC)
IdentifiersIdentifiersNumber of sources, L is the length of the identifier (32 bit)
P
Ns
s
L!
1
2
2
1exp
Ns
Its probability that two sources independently get the same SSRC
(in case of large number of sources):
In case of joining new sourc (when the others have already unique
identifier), the collision probability:
Pc $ v2 10 7Ps $
10 4
P Nc sL! 2
For Ns ! 1000 :
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RTP Header Extension FormatRTP Header Extension Format
If the X bit in the RTP header is one, a variable-length
header extension is appended to the RTP header,
following the CSRC list if present. The header extensioncontains a 16-bit length field that counts the number of
32-bit words in the extension, excluding the four-octet
extension header (therefore zero is a valid length).
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RTP Header Extension Format (Cont.)RTP Header Extension Format (Cont.)
Only a single extension may be appended to the RTP
data header. To allow multiple inter-operating
implementations to each experiment independently with
different header extensions, or to allow a particularimplementation to experiment with more than one type
of header extension, the first 16 bits of the header
extension are left open for distinguishing identifiers or
parameters. The format of these 16 bits is to be definedby the profile specification under which the
implementations are operating.
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Application of MPEGApplication of MPEG--H.261 TranslatorH.261 Translator
Lege
Translator
Host
64 k s WAN
Local ampus
Network
G-H.261
ra slator
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RealReal--time Transport Control Protocoltime Transport Control Protocol
As mentioned earlier, RTCP provides information
on the quality of reception of the data transmitted.
This information is very useful to the applications asthey can change the way they behave on basis of
RTCP reports
The RTCP feedback information can also be auseful tool for network managers
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Traffic of RTP and RTCP PackagesTraffic of RTP and RTCP Packages
Sender Receiver
Receiver
Internet
RTCPRTCP
RTCP
RTP
RTP
RTP
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More on RTCPMore on RTCP
RTCP is also used to keep track of the
participants in a RTP session. It does this by
carrying the RTP source identifier, called thecanonical name (CNAME), and the synchronization
source identifier (SSRC).
RTCP also controls the rate at which participants
in a RTP session transmit RTCP packets. The aim ofthis is to keep control traffic to a maximum of 5
percent of overall session traffic.
A RTCP BYE packet is transmitted for
termination.
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Controlling RTCP Transmission Intervals,Controlling RTCP Transmission Intervals,
Bandwidth FittingBandwidth Fitting The control traffic is limited to the maximum 5 % of the wholeThe control traffic is limited to the maximum 5 % of the wholetraffic by the modification of the RTCP package sending ratetraffic by the modification of the RTCP package sending rate
The available bandwidth of the RTCP is allocated to senders andThe available bandwidth of the RTCP is allocated to senders and
receivers, the part of the senders:receivers, the part of the senders:
all receivers together:all receivers together:
At sender: sending interval of RTCP packages:At sender: sending interval of RTCP packages:
number of senders:number of senders:
bandwidth of the whole session:bandwidth of the whole session: BB
average RTCP package size:average RTCP package size: At receiver the sending intervalAt receiver the sending interval
RTCP packages:RTCP packages:
number of receivers:number of receivers:
T pN
B rs
s
s
! 0 05,
pN
B (1r )r
r
s
! 0 05,
rs
! 25%
1 rs
Ns
p
Nr
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Estimation of Packet LossEstimation of Packet Loss
The known expression for the TCP traffic can be used for theestimating the lpacket loss ration of other traffic which uses
the same path
Let c=1,22, RTT=200 ms, the encoding method is DVI (ADPCMversion), 40 ms data packet, RTP/ DP/IP header 40 bytes,
codec state 4 bytes, media data 160 bytes, thus p=204 bytes,
sendign rate 25 packet/s,
thus the bandwith required by the stream:
From these the packet loss ratio: l= 6,0 %
lp c
B RTT!
1
2
Bpacket
s
byte
packet
byte
s1 25 204 5100! v !
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ConclusionsConclusions
RTP and RTCP work together
Together, these protocols deliver real-time
traffic with timing information and feedback on
the reception quality.
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