Protocols in IPTV

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Protocols in IPTV

Manish Das

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Video encoding



 Video in its most basic form is a sequence of images, whichVideo in its most basic form is a sequence of images, which are displayed in a sequential order. The technical term for one are displayed in a sequential order. The technical term for one of these video images is “a frame.” The human eye in general of these video images is “a frame.” The human eye in general can comfortably watch TV at around 25 frames per second can comfortably watch TV at around 25 frames per second (fps).

(fps).



 Devices called video encoders are used to compress videoDevices called video encoders are used to compress video content contained in each of the frames, while maintaining a content contained in each of the frames, while maintaining a high level of picture quality.

high level of picture quality.



 Standard practice is to use Motion Picture Expert GroupStandard practice is to use Motion Picture Expert Group (MPEG) encoding.

(MPEG) encoding.

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Video encoding

MPEG uses three fundamental techniques to

MPEG uses three fundamental techniques to achieve compression:achieve compression:

S

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is less sensitive to eye S

Sppaattiiaal cl coommpprreessssiioonn RReemmoovvees res redduunnddaannt it innffoormrmaattiioonn within frames using the property within frames using the property that

that pixels pixels within within a a single single frameframe are related to their

are related to their neigboursneigbours Tem

Temporporal compal compressressionion ReRemovmoves redues redundndant infant informormatiationon between frames

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Video encoding

Compression achieved: Compression achieved:

C

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MPPEEGG--22 33..5 5 MMbbppss 1199..3 3 MMbbppss M

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Video packetising



_{In order for the elementary streams to be}

transmitted over the digital network, each

elementary stream is converted into an

interleaved stream of time stamped Packetized

Elementary Stream (PES) packets.



A PES packet may be a fixed (or variable) sized

block, with up to 65536 bytes per packet. This

includes an allocation of 6 bytes for the header

with the remainder of the packet used to carry

content.

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MPEG Transport Stream (TS) construction



The next layer deals with building a transport

stream, which consists of a continuous stream of

packets.



These packets, commonly called TS packets, are

formed by breaking up the PES packets into

fixed-sized TS packets of 188 bytes

sized TS packets of 188 bytes

..



Each TS packet comprises 184 bytes of payload

and a 4 byte header.

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MPEG Stream Encoder Flow

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MPEG Transport Stream (TS)

Header (4

bytes)

Video Payload (184 bytes)

188 bytes 188 bytes

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Formation of IP Packets

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 Once the TS has been structured and formatted it is passedOnce the TS has been structured and formatted it is passed down to either the transport layer (UDP) directly or a layer down to either the transport layer (UDP) directly or a layer that uses the Real-Time Transport Protocol (RTP).

that uses the Real-Time Transport Protocol (RTP).

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 RTP delivers end-to-end streams of audio and video byRTP delivers end-to-end streams of audio and video by encapsulating the content into a particular format called a encapsulating the content into a particular format called a packet. Each packet consists of a header and the payload packet. Each packet consists of a header and the payload IPTV data.

IPTV data.

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 To improve bandwidth efficiency, the payload typicallyTo improve bandwidth efficiency, the payload typically includes seven MPEG-TS packet.

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RTP encapsulation

RTP Header RTP Header (12 bytes) (12 bytes) MPEG TS MPEG TS (188 (188 bytes) bytes) MPEG TS MPEG TS (188 (188 bytes) bytes) MPEG TS MPEG TS (188 (188 bytes) bytes) MPEG TS MPEG TS (188 bytes) (188 bytes) MPEG TS MPEG TS (188 bytes) (188 bytes) MPEG TS MPEG TS (188 (188 bytes) bytes) MPEG MPEG TS TS (188 (188 bytes) bytes)

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Formation of IP Packets: Transport Layer

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 RTP packets form the input to the trRTP packets form the input to the transport layer UDPansport layer UDP or TCP.

or TCP.

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 _{It is also possible to map MPEGTS packets directly into}_{It is also possible to map MPEGTS packets directly into}

the transport layer protocol

the transport layer protocol payload. This effectivelypayload. This effectively avoids the RTP layer completely. But a

avoids the RTP layer completely. But a number of number of service providers use RTP to

service providers use RTP to address the inherentaddress the inherent unreliabil

unreliability of ity of the UDP protocol.the UDP protocol.

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 _{In the context of IPTV, UDP is}_{In the context of IPTV, UDP is useful when a data}_{useful when a data}

center needs to send IP video content to multiple users center needs to send IP video content to multiple users and is the

and is the most popular transport level protocolmost popular transport level protocol employed by IPTV service providers.

employed by IPTV service providers.

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 An RTP header is identifiable with a value of 5004 in aAn RTP header is identifiable with a value of 5004 in a User Datagram Protocol (UDP) header.

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UDP encapsulation

UDP Header UDP Header (8 bytes) (8 bytes) RTP Header RTP Header (12 bytes) (12 bytes) Optional Optional 7 X MPEG TS

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Why UDP?

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 IPTV is a real-time application and does not IPTV is a real-time application and does not tolerate delays. TCP can often introducetolerate delays. TCP can often introduce latency into the delivery of

latency into the delivery of IP video content due to fact that IP video content due to fact that the protocol employs flowthe protocol employs flow control mechanisms.

control mechanisms.

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 UDP ensures that UDP ensures that delivery of IPTV content delivery of IPTV content is not delayed even if theris not delayed even if there is delayed or e is delayed or damaged packets contained in the network traffic, whereas in using TCP, TV viewers damaged packets contained in the network traffic, whereas in using TCP, TV viewers are faced with a pause as

are faced with a pause as they wait for a delayed packet they wait for a delayed packet or picture frame to arrive or or picture frame to arrive or wait for the damaged packet to be replaced.

wait for the damaged packet to be replaced.

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 Low overhead: The size of the header is only Low overhead: The size of the header is only 8 bytes when compared to the TCP8 bytes when compared to the TCP header, which occupies 20 bytes of

header, which occupies 20 bytes of data.data.

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 Speedy connection setup: The Speedy connection setup: The establishment and teardown of connections betweenestablishment and teardown of connections between IPTVCDs (set top box) and IPTV data center networking components takes place in IPTVCDs (set top box) and IPTV data center networking components takes place in a very short time period. Therefore, the delivery of video packets using the UDP a very short time period. Therefore, the delivery of video packets using the UDP protocol is generally quicker compared to using the TCP protocol.

protocol is generally quicker compared to using the TCP protocol.

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 Easy implementation: From a technical perspective UDP is pretty easy to iEasy implementation: From a technical perspective UDP is pretty easy to i mplementmplement because it is

because it is not required to keep track of video packets once not required to keep track of video packets once they are sent onto thethey are sent onto the IP network.

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The IP Layer

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Next task is IP

Next task is IP encapsulation.

encapsulation.

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20 byte header is added to the payload.

IPv4 IPv4 Header Header (20 bytes) (20 bytes) UDP UDP Header Header (8 bytes) (8 bytes) RTP Header RTP Header (12 bytes) (12 bytes) Optional Optional

7 X MPEG TS Packets (188 bytes)

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To Recapitulate

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Now the delivery mechanism

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In standard broadcast systems all of the normal

broadcast channels are delivered to the STB in

the home (via Cable, Satellite or Terrestrial).



There could be hundreds of channels, all of

which are delivered simultaneously. The STB

tunes to the desired channel in response to

requests from the viewer’s remote control.

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Now the delivery mechanism



In order to preserve bandwidth over the final link

to the house, IPTV systems are

to the house, IPTV systems are designed to

designed to

deliver only the requested channel to the STB.



In order to

In order to change channels, special commands

change channels, special commands

are sent into the Access network requesting a

change of channel.



_{Thus in IPTV systems the channel}

_{Thus in IPTV systems the channel change is}

_{change is}

made in the network and not on the local STB.

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Now the delivery mechanism

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 IP communication is normally one to one – unicast.IP communication is normally one to one – unicast.

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 This method of communications is not effective for This method of communications is not effective for

delivering traditional broadcast style channels because delivering traditional broadcast style channels because the duplication of point-to-point sessions would

the duplication of point-to-point sessions would overwhelm the network.

overwhelm the network.

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 So IPTV uses multicast to provide a single So IPTV uses multicast to provide a single broadcast TVbroadcast TV channel to multiple clients

channel to multiple clients simultaneously.simultaneously.

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 The primary purpose of multicast technologies is toThe primary purpose of multicast technologies is to ensure that end users are able

ensure that end users are able to instantaneously andto instantaneously and reliably switch channels during a TV

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Features of IP Multicast in IPTV network

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 _{Only a single copy of every video stream needs to be}_{Only a single copy of every video stream needs to be}

sent to a router,

sent to a router, which in turn makes a copy of which in turn makes a copy of thatthat stream for

stream for the requesting devices.the requesting devices.

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 _{Multicast not only reduce the}_{Multicast not only reduce the bandwidth requiremen}_{bandwidth requirements of}_{ts of}

the network but the processing power of the content the network but the processing power of the content server can also be kept relatively low because it only server can also be kept relatively low because it only transmits one copy of an IPTV stream

transmits one copy of an IPTV stream at a time.at a time.

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 _{But it increases the}_{But it increases the workload and processing}_{workload and processing}

requirements of routers as

requirements of routers as they handle additional tasksthey handle additional tasks such as replicating video streams and keeping track of such as replicating video streams and keeping track of multiple copies of video packets. Processing the various multiple copies of video packets. Processing the various tasks associated with IP

tasks associated with IP multicasting adds a significantmulticasting adds a significant burden to the workload of IP routers.

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A simple example

•• multiple unicasts

multiple unicasts

•• IP multicastIP multicast

S S SS R R R R R R R R

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Channel zapping in IPTV

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Every channel has a multicast group IP address.

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Internet Group Management Protocol (IGMP) is

used for zapping channels.



IGMP v2 is widely used in IPTV.



Whenever an end user changes a channel, t

Whenever an end user changes a channel, the

he

SETOPBOX responds by sending two

commands to the central equipment:

(1) To leave the existing video multicast.

(2) To join the desired new video multicast.

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IGMP message format

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Encapsulation of IGMP packet

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Typical IPTV System: Selection of first channel

Transport Transport H Heeaaddeenndd AAcccceessss HHoommee Transport Transport Network

Network xDSL CPE/xDSL CPE/ FTTH FTTH Computer Computer STB STB IGMP IGMP Router Router MPEG MPEG Encoders Encoders Apps Apps Servers Servers IGMP DSLAM IGMP DSLAM Video Video Servers Servers Other Other Content Content Sources Sources VoIP VoIP Subscriber Subscriber presses presses channel channel number number The IP set-top box

The IP set-top box accepts the channel accepts the channel request command request command

and sends this and sends this instruction in the form instruction in the form

of an IGMP of an IGMP join join request to the request to the DSLAM. DSLAM. IGMP JOIN IGMP JOIN

DSLAM examines the DSLAM examines the request to see if the request to see if the requested channel requested channel already exists at one of already exists at one of

its ports. If this is the its ports. If this is the case, then the DSLAM case, then the DSLAM

simply copies the simply copies the stream and sends to stream and sends to the requesting device. the requesting device.

If the requested channel If the requested channel is not available in the is not available in the

IGMP JOIN

Router examines the Router examines the request to see if the request to see if the requested channel requested channel already exists at one of already exists at one of

its ports. If this is the its ports. If this is the case, then the Router case, then the Router

simply copies the simply copies the stream and sends to stream and sends to the requesting device the requesting device If the requested If the requested channel is not channel is not available in the available in the Router, the request Router, the request is sent to the is sent to the upstream routers. upstream routers. IGMP JOIN IGMP JOIN Video Stream Video Stream

The request for the The request for the channel finally ends up channel finally ends up at the IPTV data center at the IPTV data center

where all broadcast where all broadcast channels are available. channels are available. The IP address of the The IP address of the subscriber’s IP set-top subscriber’s IP set-top box get added to the box get added to the multicast list. The multicast list. The channel is then copied channel is then copied and sent onward to the and sent onward to the

IP set-top box. IP set-top box.

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Channel selection: Possible steps that may occur

across a DSL based IPTV network

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 Once a subscriber wishes to select a channel they press a channel number Once a subscriber wishes to select a channel they press a channel number on their remote control or select from an EPG application. These commands on their remote control or select from an EPG application. These commands are received by the infrared receiver.

are received by the infrared receiver.

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 The IP set-top box accepts the channel changing command and sends thisThe IP set-top box accepts the channel changing command and sends this instruction in the form of an IGMP

instruction in the form of an IGMP join join request to the DSLAM. The DSLAMrequest to the DSLAM. The DSLAM sees the request and will either pass onward or else examine the request to sees the request and will either pass onward or else examine the request to see if the requested channel already exists at one of its ports. If this is the see if the requested channel already exists at one of its ports. If this is the case, then the DSLAM simply copies the stream and sends to the requesting case, then the DSLAM simply copies the stream and sends to the requesting device.

device.

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 If the requested channel is not available in the DSLAM, the request is sent toIf the requested channel is not available in the DSLAM, the request is sent to the upstream routers.

the upstream routers.

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 When the router located at the regional office receives a request that has notWhen the router located at the regional office receives a request that has not been serviced by the various downstream network components, it also has been serviced by the various downstream network components, it also has two options, namely, to copy the stream to the correct interface or pass the two options, namely, to copy the stream to the correct interface or pass the request upstream to the distribution router in the event that the channel is request upstream to the distribution router in the event that the channel is unavailable at its downstream interface.

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Channel selection: Possible steps that may occur

across a DSL based IPTV network

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 The request for the channel finally ends up at The request for the channel finally ends up at the IPTV data center wherethe IPTV data center where all broadcast channels are available. It is important to note t

all broadcast channels are available. It is important to note that thehat the channel is generally identified by an IP address. The IP

channel is generally identified by an IP address. The IP address of theaddress of the subscriber’s IP set-top box get added to the multicast list. The channel is subscriber’s IP set-top box get added to the multicast list. The channel is then copied and sent onward to the IP

then copied and sent onward to the IP set-top box.set-top box.

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 The set-top receives the new IP stream. It then buffers in memory, andThe set-top receives the new IP stream. It then buffers in memory, and waits for an I-frame to arrive before decoding starts. The I-frame contains waits for an I-frame to arrive before decoding starts. The I-frame contains all of the necessary information required to reconstr

all of the necessary information required to reconstruct the original pictureuct the original picture frame. Once the I-frame is received the IP set-top box, which can take frame. Once the I-frame is received the IP set-top box, which can take between a half and two full seconds, can s

between a half and two full seconds, can start the process of tart the process of displayingdisplaying the channel.

the channel.

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 The first picture frame of the new channel is displayed on the TV The first picture frame of the new channel is displayed on the TV display.display. Note that the ITU-T FG IPTV group is recommending that the time taken Note that the ITU-T FG IPTV group is recommending that the time taken

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Typical IPTV System: Change of channel

Transport Transport H Heeaaddeenndd AAcccceessss HHoommee Transport Transport Network

Network xDSL CPE/xDSL CPE/ FTTH FTTH Computer Computer STB STB IGMP IGMP Router Router Digital and Digital and Analog Analog Receivers Receivers MPEG MPEG Encoders Encoders Apps Apps Servers Servers IGMP DSLAM IGMP DSLAM Video Video Servers Servers Other Other Content Content Sources Sources VoIP VoIP Subscriber Subscriber changes changes channel channel number number IGMP JOIN IGMP JOIN Video Stream Video Stream IGMP LEAVE IGMP LEAVE

The set-top box issues The set-top box issues an IGMP

an IGMP LeaveLeave message to terminate message to terminate the stream associated the stream associated with the old stream. with the old stream. The The termination of termination of the stream the stream takes place in takes place in the DSLAM. the DSLAM. A join A join message is message is then sent to then sent to start viewing start viewing the next the next channel. channel.

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Channel change: Possible steps that may occur

across a DSL based IPTV network

The process involved in changing channels in the middle of the TV The process involved in changing channels in the middle of the TV

viewing experience is quite similar to

viewing experience is quite similar to selecting a channel. There isselecting a channel. There is

however a couple of extra steps: however a couple of extra steps:



 When a viewer wants to When a viewer wants to change to another IPTV signal the remotechange to another IPTV signal the remote control button is pressed and the instruction is accepted by the IP control button is pressed and the instruction is accepted by the IP set-top box.

set-top box.



 The set-top box issues an IGMPThe set-top box issues an IGMP LeaveLeave message to terminate themessage to terminate the stream associated with the old stream.

stream associated with the old stream.



 The termination of the stream takes place The termination of the stream takes place in the DSLAM.in the DSLAM.



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Session

Announcement Protocol (SAP

Announcement Protocol (SAP)

)



 _{Session Announcement Protocol (SAP}_{Session Announcement Protocol (SAP) is used to}_{) is used to}

inform multicast enabled receivers periodically about inform multicast enabled receivers periodically about programs currently being multicast on a

programs currently being multicast on a network.network.



 _{The most important part of a SAP message is the}_{The most important part of a SAP message is the}

multicast address of the multicast stream. Once the user multicast address of the multicast stream. Once the user device has this address, it can send a

device has this address, it can send a request to therequest to the network to join that

network to join that multicast.multicast.



 By default, SAP communications always take place onBy default, SAP communications always take place on the multicast group address 224.2.127.254 on port

the multicast group address 224.2.127.254 on port

9875. Specialized software on the user device converts 9875. Specialized software on the user device converts the information received from SAP into a

the information received from SAP into a list of choiceslist of choices from which the user can select.

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MULTICASTING IPTV content across IPV6

networks



IPv6 deployment requires service providers to

use a multicasting signaling protocol called

Multicast Listener Discovery (MLD) when

delivering broadcast TV channels to IPTV end

users.



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D DSSLLAAM M # # 11 DDSSLLAAM M # # 22 DDSSLLAAM M # # 33 DDSSLLAAM M # # 44 T T22 TT22 T1 T1

Multicast TV

TV TV TV TV IGMP IGMP JOIN JOIN BNG BNG Video Server Video Server

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Time-shift TV

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Video on Demand (VoD)



 _{VoD is unicast delivery and uses}_{VoD is unicast delivery and uses RTSP.}_RTSP. 

 The Real Time Streaming ProtocolThe Real Time Streaming Protocol ((RTSPRTSP) is a) is a protocol for use in streaming media systems which protocol for use in streaming media systems which allows a client to r

allows a client to remotely control a streaming mediaemotely control a streaming media server, issuing VCR-like commands such as "play" and server, issuing VCR-like commands such as "play" and "pause", and allowing time-based access to files on a "pause", and allowing time-based access to files on a server

server



 RTSP requests are based on RTSP requests are based on HTTP requestsHTTP requests



 _{In RTSP we can distinguish follow}_{In RTSP we can}_{distinguish following requests:}_{ing requests:}

DESCRIBE, SETUP, PLAY, PAUSE, RECORD, DESCRIBE, SETUP, PLAY, PAUSE, RECORD, TEARDOWN.

TEARDOWN.



 _{RTSP servers use RTP among other things the transport}_{RTSP servers use RTP among other things the transport}

protocol for the actual

protocol for the actual audio/videaudio/video data and o data and RTCP toRTCP to monitor quality of

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Example:RTSP with UDP-Based RTP Delivery

_DESCRIBE_DESCRIBE

rtsp://192.168.1.25:554/mp rtsp://192.168.1.25:554/mp eg4/movies/3idiots.mpg eg4/movies/3idiots.mpg RTSP/1.0 RTSP/1.0 CSec: 101 CSec: 101 Accept: application/sdp Accept: application/sdp RTSP/1.0 200 RTSP/1.0 200 OK OK CSec: 101 CSec: 101 Content-Base: Content-Base: rtsp://192.168.1. rtsp://192.168.1. 25:554/mpeg4/m 25:554/mpeg4/m ovies/3idiots. ovies/3idiots. mpg mpg Content-Type: Content-Type: application/sdp application/sdp Content-Length: Content-Length: 320 320 <SDP <SDP Data...> Data...> OPTIONS OPTIONS rtsp://192.168.1.25:554/mp rtsp://192.168.1.25:554/mp eg4/movies/3idiots.mpg eg4/movies/3idiots.mpg CSec: 102 CSec: 102 RTSP/1.0 200 RTSP/1.0 200 OK OK CSec: 102 CSec: 102 Public: Public: DESCRIBE, DESCRIBE, OPTIONS, OPTIONS, PAUSE, PAUSE, PLAY, PLAY, SETUP, SETUP, TEARDOWN, TEARDOWN, ANNOUNCE ANNOUNCE SETUP SETUP rtsp://192.168.1.25:554/mp rtsp://192.168.1.25:554/mp eg4/movies/3idiots.mpg eg4/movies/3idiots.mpg CSeq: 103 CSeq: 103 Transport: Transport: RTP/UDP;unicast;client_p RTP/UDP;unicast;client_p ort=4042-4043 ort=4042-4043 RTSP/1.0 RTSP/1.0 200 OK 200 OK CSeq: 103 CSeq: 103 Session: Session: 1234567891; 1234567891; timeout=10 timeout=10 Transport: Transport: RTP/UDP;un RTP/UDP;un icast;mode= icast;mode= play; play; client_port=4 client_port=4 042-4043; 042-4043; server_port= server_port= 5072-5073 5072-5073 PLAY PLAY rtsp://192.168.1.25:554/m rtsp://192.168.1.25:554/m peg4/movies/3idiots.mpg peg4/movies/3idiots.mpg RTSP/1.0 RTSP/1.0 CSeq: 104 CSeq: 104 Session: 1234567891 Session: 1234567891 RTSP/1.0 RTSP/1.0 200 OK 200 OK CSeq: 104 CSeq: 104 Session: Session: 1234567891 1234567891 PAUSE PAUSE rtsp://192.168.1.25:554/m rtsp://192.168.1.25:554/m peg4/movies/3idiots.mpg peg4/movies/3idiots.mpg CSeq: 105 CSeq: 105 Session: 1234567891 Session: 1234567891 R R T TSSPP//11..00 220000 O OKK C CSSeeqq:: 110055 S Seessssiioonn:: 1 12233 4 45566 7 7889911 TEARDOWN TEARDOWN rtsp://192.168.1.25:554/m rtsp://192.168.1.25:554/m peg4/movies/3idiots.mpg peg4/movies/3idiots.mpg RTSP/1.0 RTSP/1.0 CSeq: 106 CSeq: 106 Session: 1234567891 Session: 1234567891 RTSP/1.0 200 RTSP/1.0 200 OK OK

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