Protocols in IPTV_manishdas

8/9/2019 Protocols in IPTV_manishdas

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Protocols in IPTVProtocols in IPTV

Manish DasManish Das


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deconstructiondeconstruction

de-packetizingde-packetizing

decodingdecoding


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Video encodingVideo encoding Video in its most basic form is a sequence of images,Video in its most basic form is a sequence of images,

which are displayed in a sequential order. The technicalwhich are displayed in a sequential order. The technicalterm for one of these video images is a frame. Theterm for one of these video images is a frame. The

human eye in general can comfortably watch TV at aroundhuman eye in general can comfortably watch TV at around

25 frames per second (fps).25 frames per second (fps).

Devices called video encoders are used to compress videoDevices called video encoders are used to compress videocontent contained in each of the frames, while maintainingcontent contained in each of the frames, while maintaining

a high level of picture quality.a 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. An uncompressed video signal is compressed and an aAn uncompressed video signal is compressed and an a

continuous real-time digital signal is outputted from thecontinuous real-time digital signal is outputted from the

encoder. This is known asencoder. This is known as Elementary StreamElementary Stream..


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Video encodingVideo encodingMPEG uses three fundamental techniques to achieve compression:MPEG uses three fundamental techniques to achieve compression:

SubsamplingSubsampling Reduces colour information whichReduces colour information whichis less sensitive to eyeis less sensitive to eye

Spatial compressionSpatial compression Removes redundant informationRemoves redundant informationwithin frames using the propertywithin frames using the propertythatthat pixels within a single framepixels within a single frameare related to their neigboursare related to their neigbours

Temporal compressionTemporal compression Removes redundant informationRemoves redundant informationbetween framesbetween frames


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Video encodingVideo encodingCompression achieved:Compression achieved:

Coding versionCoding version SDTVSDTV HDTVHDTV

MPEG-2MPEG-2 3.5 Mbps3.5 Mbps 19.3 Mbps19.3 Mbps

MPEG-4MPEG-4 2 Mbps2 Mbps 8 Mbps8 Mbps


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Video packetisingVideo packetising In order for the elementary streams to beIn order for the elementary streams to be

transmitted over the digital network, eachtransmitted over the digital network, eachelementary stream is converted into anelementary stream is converted into aninterleaved stream of time stamped Packetizedinterleaved stream of time stamped PacketizedElementary Stream (PES) packets.Elementary Stream (PES) packets.

A PES packet may be a fixed (or variable) sizedA PES packet may be a fixed (or variable) sizedblock, with up to 65536 bytes per packet. Thisblock, with up to 65536 bytes per packet. Thisincludes an allocation of 6 bytes for the headerincludes an allocation of 6 bytes for the headerwith the remainder of the packet used to carrywith the remainder of the packet used to carrycontentcontent.


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MPEG Transport Stream (TS) constructionMPEG Transport Stream (TS) construction

The next layer deals with building a transportThe next layer deals with building a transport

stream, which consists of a continuous stream ofstream, which consists of a continuous stream of

packets.packets.

These packets, commonly called TS packets, areThese packets, commonly called TS packets, areformed by breaking up the PES packets into fixed-formed by breaking up the PES packets into fixed-

sized TS packets of 188 bytessized TS packets of 188 bytes..

Each TS packet comprises 184 bytes of payloadEach TS packet comprises 184 bytes of payload

and a 4 byte header.and a 4 byte header.


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MPEG Transport Stream (TS)MPEG Transport Stream (TS)

Header (4Header (4bytes)bytes)

Video Payload (184 bytes)Video Payload (184 bytes)

188 bytes


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Formation of IP PacketsFormation of IP Packets

Once the TS has been structured and formatted it isOnce the TS has been structured and formatted it ispassed down to either the transport layer (UDP) directly orpassed down to either the transport layer (UDP) directly ora layer that uses the Real-Time Transport Protocol (RTP).a layer that uses the Real-Time Transport Protocol (RTP).

In a packet-based network, it is quite common that theIn a packet-based network, it is quite common that theroute for transporting the packets is not always the same;route for transporting the packets is not always the same;packets may arrive at different times and out of order. Thepackets may arrive at different times and out of order. TheRTP protocol allows out-of-order arrival of the packets.RTP protocol allows out-of-order arrival of the packets.Since every RTP packet has a sequence number, as longSince every RTP packet has a sequence number, as longas the delay did not exceed the size of the receivingas the delay did not exceed the size of the receivingdecoder buffer, the packet can be processed and placed indecoder buffer, the packet can be processed and placed in

the right position for decoding.the right position for decoding.


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RTP encapsulationRTP encapsulation

RTP HeaderRTP Header

(12 bytes)(12 bytes)

MPEG TSMPEG TS

(188(188bytes)bytes)

MPEG TSMPEG TS


MPEG TSMPEG TS


MPEG TSMPEG TS


MPEG TSMPEG TS


MPEG TSMPEG TS


MPEGMPEGTSTS



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Formation of IP Packets: Transport LayerFormation of IP Packets: Transport Layer

RTP packets form the input to the transport layer UDPRTP packets form the input to the transport layer UDPor TCP.or TCP.

It is also possible to map MPEGTS packets directly intoIt is also possible to map MPEGTS packets directly intothe transport layer protocol payload. This effectivelythe transport layer protocol payload. This effectivelyavoids the RTP layer completely. But a number ofavoids the RTP layer completely. But a number of

service providers use RTP to address the inherentservice providers use RTP to address the inherentunreliability of the UDP protocol.unreliability of the UDP protocol.

In the context of IPTV, UDP is useful where theIn the context of IPTV, UDP is useful where thetransport network is career grade and is the mosttransport network is career grade and is the mostpopular transport level protocol employed by IPTVpopular transport level protocol employed by IPTV

service providers.service providers. 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.User Datagram Protocol (UDP) header.


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UDP encapsulationUDP encapsulation

UDP HeaderUDP Header

(8 bytes)(8 bytes)



OptionalOptional

7 X MPEG TS Packets (188 bytes)7 X MPEG TS Packets (188 bytes)


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Why UDP?Why UDP?

IPTV is a real-time application and does not tolerate delays. TCP can oftenIPTV is a real-time application and does not tolerate delays. TCP can oftenintroduce latency into the delivery of IP video content due to fact that theintroduce latency into the delivery of IP video content due to fact that theprotocol employs flow control mechanisms.protocol employs flow control mechanisms.

UDP ensures that delivery of IPTV content is not delayed even if there isUDP ensures that delivery of IPTV content is not delayed even if there isdelayed or damaged packets contained in the network traffic, whereas indelayed or damaged packets contained in the network traffic, whereas inusing TCP, TV viewers are faced with a pause as they wait for a delayedusing TCP, TV viewers are faced with a pause as they wait for a delayed

packet or picture frame to arrive or wait for the damaged packet to bepacket or picture frame to arrive or wait for the damaged packet to bereplaced.replaced.

Low overhead: The size of the header is only 8 bytes when compared to theLow overhead: The size of the header is only 8 bytes when compared to theTCP header, which occupies 20 bytes of data.TCP header, which occupies 20 bytes of data.

Speedy connection setup: The establishment and teardown of connectionsSpeedy connection setup: The establishment and teardown of connectionsbetween IPTVCDs (set top box) and IPTV data center networkingbetween IPTVCDs (set top box) and IPTV data center networkingcomponents takes place in a very short time period. Therefore, the deliverycomponents takes place in a very short time period. Therefore, the deliveryof video packets using the UDP protocol is generally quicker compared toof video packets using the UDP protocol is generally quicker compared tousing the TCP protocol.using the TCP protocol.


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The IP LayerThe IP Layer

Next task is IP encapsulation.Next task is IP encapsulation. 20 byte header is added to the payload.20 byte header is added to the payload.

IPv4IPv4

HeaderHeader


UDPUDP

HeaderHeader

(8 bytes)(8 bytes)



OptionalOptional

7 X MPEG TS Packets (188 bytes)7 X MPEG TS Packets (188 bytes)


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To RecapitulateTo Recapitulate


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Now the delivery mechanismNow the delivery mechanism

In standard broadcast systems all of the normalIn standard broadcast systems all of the normalbroadcast channels are delivered to the STB inbroadcast channels are delivered to the STB in

the home (via Cable, Satellite or Terrestrial).the home (via Cable, Satellite or Terrestrial).

There could be hundreds of channels, all ofThere could be hundreds of channels, all of

which are delivered simultaneously. The STBwhich are delivered simultaneously. The STB

tunes to the desired channel in response totunes to the desired channel in response to

requests from the viewers remote control.requests from the viewers remote control.

As a result of this local tuning the channelAs a result of this local tuning the channelchanges are almost instantaneous.changes are almost instantaneous.


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Now the delivery mechanism (contd.)Now the delivery mechanism (contd.)

In order to preserve bandwidth over the final linkIn order to preserve bandwidth over the final linkto the house, IPTV systems are designed toto the house, IPTV systems are designed to

deliver only the requested channel to the STB.deliver only the requested channel to the STB.

In order to change channels, special commandsIn order to change channels, special commands

are sent into the Access network requesting aare sent into the Access network requesting a

change of channel.change of channel.

Thus in IPTV systems the channel change isThus in IPTV systems the channel change is

made in the network and not on the local STBmade in the network and not on the local STB.


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Now the delivery mechanism (contd.)Now the delivery mechanism (contd.)

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

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

delivering traditional broadcast style channels becausedelivering traditional broadcast style channels because

the duplication of point-to-point sessions wouldthe duplication of point-to-point sessions would

overwhelm the network.overwhelm the network.

So IPTV uses multicast to provide a single broadcast TVSo IPTV uses multicast to provide a single broadcast TV

channel to multiple clients simultaneously.channel to multiple clients simultaneously.

The primary purpose of multicast technologies is toThe primary purpose of multicast technologies is to

ensure that end users are able to instantaneously andensure that end users are able to instantaneously and

reliably switch channels during a TV viewing sessionreliably switch channels during a TV viewing session.


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Features of IP Multicast in IPTV networkFeatures of IP Multicast in IPTV network

Only a single copy of every video stream needs to beOnly a single copy of every video stream needs to besent to a router, which in turn makes a copy of thatsent to a router, which in turn makes a copy of thatstream for the requesting devices.stream for the requesting devices.

Multicast not only reduce the bandwidth requirements ofMulticast not only reduce the bandwidth requirements ofthe network but the processing power of the contentthe network but the processing power of the content

server can also be kept relatively low because it onlyserver can also be kept relatively low because it onlytransmits one copy of an IPTV stream at a time.transmits one copy of an IPTV stream at a time.

But it increases the workload and processingBut it increases the workload and processingrequirements of routers as they handle additional tasksrequirements of routers as they handle additional taskssuch as replicating video streams and keeping track ofsuch as replicating video streams and keeping track ofmultiple copies of video packets. Processing the variousmultiple copies of video packets. Processing the varioustasks associated with IP multicasting adds a significanttasks associated with IP multicasting adds a significantburden to the workload of IP routers.burden to the workload of IP routers.


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A simple exampleA simple example

multiple unicasts IP multicast

S S

R

R

R

R

R

R


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Channel zapping in IPTV

In an IPTV network, each broadcast televisionIn an IPTV network, each broadcast televisionchannel is an IP multicast groupchannel is an IP multicast group.

Internet Group Management Protocol (IGMP) is usedInternet Group Management Protocol (IGMP) is used

for zapping channels.for zapping channels.

IGMP v2 is widely used in IPTV.IGMP v2 is widely used in IPTV. Whenever an end user changes a channel, the Set-Whenever an end user changes a channel, the Set-

top BOX responds by sending two commands to thetop BOX responds by sending two commands to the

central equipment:central equipment:

(1) To leave the existing video multicast.(1) To leave the existing video multicast.

(2) To join the desired new video multicast.(2) To join the desired new video multicast.


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IGMP DevicesIGMP Devices

Basic IGMP operation involves two devices:

IGMP host is a server or client, which issues messages

to join or leave a multicast group. The client also

responds to queries from the multicast router. A set-topbox is an example of an IGMP host.

IGMP router (or multicast router) responds to the join

and leave messages to determine if multicast groupsshould be forwarded out an interface.


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IGMP message formatIGMP message format


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Encapsulation of IGMP packetEncapsulation of IGMP packet


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Typical IPTV System: Selection of first channelTypical IPTV System: Selection of first channel

TransportHeadend Access Home

Transport

Network xDSL CPE/FTTH

Computer

STB

IGMP

Router

Digital and

Analog

Receivers

MPEGEncoders

AppsServers

IGMP DSLAM

VideoServers

OtherContentSources

VoIP

Subscriber

presses

channel

numberThe IP set-top box

accepts the channel

request command

and sends this

instruction in the form

of an IGMPjoinrequest to the

DSLAM.

IGMP JOIN

DSLAM examines the

request to 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 device.

If the requested channel

is not available in theDSLAM, the request is

sent to the upstream

routers.

IGMP JOIN

Router examines the

request to see if the

requested channel

already exists at one of

its ports. If this is the

case, then the Router

simply copies the

stream and sends to

the requesting device

If the requested

channel is not

available in the

Router, the request

is sent to the

upstream routers.

IGMP JOIN

Video Stream

The request for the

channel finally ends up

at the IPTV data center

where all broadcast

channels are available.

The IP address of the

subscribers IP set-top

box get added to the

multicast list. The

channel is then copied

and sent onward to theIP set-top box.


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Channel selection: Possible steps that may occurChannel selection: Possible steps that may occur

across a DSL based IPTV networkacross a DSL based IPTV network

Once a subscriber wishes to select a channel they press a channel numberOnce a subscriber wishes to select a channel they press a channel number

on their remote control or select from an EPG application. These commandson their remote control or select from an EPG application. These commands

are received by the infrared receiver.are received by the infrared receiver.

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 IGMPinstruction in the form of an IGMPjoinjoin request to the DSLAM. The DSLAMrequest to the DSLAM. The DSLAM

sees the request and will either pass onward or else examine the request tosees 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 thesee 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 requestingcase, then the DSLAM simply copies the stream and sends to the requesting

device.device.

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.

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 hasbeen serviced by the various downstream network components, it also hastwo options, namely, to copy the stream to the correct interface or pass thetwo 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 isrequest upstream to the distribution router in the event that the channel is

unavailable at its downstream interface.unavailable at its downstream interface.


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Channel selection: Possible steps that may occurChannel selection: Possible steps that may occur


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

all broadcast channels are available. It is important to note that theall broadcast channels are available. It is important to note that the

channel is generally identified by an IP address. The IP address of thechannel is generally identified by an IP address. The IP address of the

subscribers IP set-top box get added to the multicast list. The channel issubscribers IP set-top box get added to the multicast list. The channel is

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

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 containswaits for an I-frame to arrive before decoding starts. The I-frame containsall of the necessary information required to reconstruct the original pictureall of the necessary information required to reconstruct the original picture

frame. Once the I-frame is received the IP set-top box, which can takeframe. Once the I-frame is received the IP set-top box, which can take

between a half and two full seconds, can start the process of displayingbetween a half and two full seconds, can start the process of displaying

the channel.the channel.

The first picture frame of the new channel is displayed on the TV display.The first picture frame of the new channel is displayed on the TV display.

Note that the ITU-T FG IPTV group is recommending that the time takenNote that the ITU-T FG IPTV group is recommending that the time takento acquire a broadcast channel should not exceed 2.5 s.to acquire a broadcast channel should not exceed 2.5 s.


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Typical IPTV System: Change of channelTypical IPTV System: Change of channel

TransportHeadend Access Home

Transport

Network xDSL CPE/FTTH

Computer

STB

IGMP

Router

Digital and

Analog

Receivers

MPEGEncoders

AppsServers

IGMP DSLAM

VideoServers

OtherContentSources

VoIP

Subscriber

changes

channel

number

IGMP JOIN

Video Stream

IGMP LEAVE

The set-top box issues

an IGMP Leave

message to terminate

the stream associated

with the old stream.

The

termination of

the stream

takes place in

the DSLAM.

A join

message isthen sent to

start viewing

the next

channel.


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Channel change: Possible steps that may occurChannel change: Possible steps that may occur


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

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

however a couple of extra steps:

When a viewer wants to change to another IPTV signal the remote

control button is pressed and the instruction is accepted by the IPset-top box.

The set-top box issues an IGMP Leave message to terminate the

stream associated with the old stream.

The termination of the stream takes place in the DSLAM.

A join message is then sent to start viewing the next channel.


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SessionSessionAnnouncement Protocol (SAPAnnouncement Protocol (SAP))

Session Announcement Protocol (SAPSession Announcement Protocol (SAP) is used to) is used toinform multicast enabled receivers periodically aboutinform multicast enabled receivers periodically about

programs currently being multicast on a network.programs currently being multicast on a network.

The most important part of a SAP message is theThe most important part of a SAP message is the

multicast address of the multicast stream. Once the usermulticast address of the multicast stream. Once the user

device has this address, it can send a request to thedevice has this address, it can send a request to the

network to join that multicast.network to join that 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 portthe multicast group address 224.2.127.254 on port

9875. Specialized software on the user device converts9875. Specialized software on the user device converts

the information received from SAP into a list of choicesthe information received from SAP into a list of choices

from which the user can select.from which the user can select.


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MULTICASTING IPTV content across IPV6MULTICASTING IPTV content across IPV6

networksnetworks

IPv6 deployment requires service providers touse a multicasting signaling protocol called

Multicast Listener Discovery (MLD) when

delivering broadcast TV channels to IPTV end

users. MLD has been derived IGMP.


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Multicast TVMulticast TV

DSLAM # 1 DSLAM # 2 DSLAM # 3 DSLAM # 4

T2 T2

T1

PC PC

TVTV

IGMP

JOIN

PC

BNG

SETTOP

BOX

ADSL MODEM

Video Server


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Time-shift TVTime-shift TV


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Video on Demand (VoD)Video on Demand (VoD)

VoD is unicast delivery and uses RTSP. The Real Time Streaming Protocol (RTSP) is a

protocol for use in streaming media systems whichallows a client to remotely control a streaming mediaserver, issuing VCR-like commands such as "play" and

"pause", and allowing time-based access to files on aserver

RTSP requests are based on HTTP requests In RTSP we can distinguish following requests:

DESCRIBE, SETUP, PLAY, PAUSE, RECORD,

TEARDOWN. RTSP servers use RTP among other things the transport

protocol for the actual audio/video data and RTCP tomonitor quality of streaming.


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Example:RTSP with UDP-Based RTP DeliveryExample:RTSP with UDP-Based RTP DeliveryDESCRIBE

rtsp://192.168.1.25:554/mp

eg4/movies/3idiots.mpg

RTSP/1.0

CSec: 101

Accept: application/sdp

RTSP/1.0 200

OK

CSec: 101

Content-Base:

rtsp://192.168.1.25:554/mpeg4/m

ovies/3idiots.

mpg

Content-Type:

application/sdp

Content-Length:

320

OPTIONS

rtsp://192.168.1.25:554/mpeg4/movies/3idiots.mpg

CSec: 102

RTSP/1.0 200OK

CSec: 102

Public:

DESCRIBE,

OPTIONS,

PAUSE,

PLAY,

SETUP,

TEARDOWN,

ANNOUNCE

SETUP

rtsp://192.168.1.25:554/mp

eg4/movies/3idiots.mpg

CSeq: 103

Transport:

RTP/UDP;unicast;client_p

ort=4042-4043

RTSP/1.0

200 OK

CSeq: 103

Session:

1234567891;

timeout=10

Transport:

RTP/UDP;un

icast;mode=

play;

client_port=4

042-4043;

server_port=

5072-5073

PLAY

rtsp://192.168.1.25:554/m

peg4/movies/3idiots.mpg

RTSP/1.0

CSeq: 104Session: 1234567891

RTSP/1.0

200 OK

CSeq: 104

Session:

1234567891

PAUSE

rtsp://192.168.1.25:554/m


CSeq: 105

Session: 1234567891RTSP/1.0200

OK

CSeq:105

Session:

1234567891

TEARDOWN

rtsp://192.168.1.25:554/m


RTSP/1.0

CSeq: 106

Session: 1234567891RTSP/1.0 200OK

CSeq: 106

Session:

1234567891


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Thank You!Thank You!

Protocols in IPTV_manishdas

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