MPEG RTP

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MPEG RTP
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Previously, on CS5248

• MPEG Compression
• Sequence, GOP, Picture, Slice, Macroblock, Block,
  DC/AC Coefficient
• I-Frame, P-Frame, B-Frame

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Previously, on CS5248

• RTP headers
• SSRC, Media Timestamp, Marker Bit, Payload Type
  ..
• Application-Level Framing

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You are Here
Encoder
Decoder
Middlebox
Sender
Receiver
Network
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Application-Level Framing
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How to send/recv?
Let the application decide, not protocol stacks.
Tennenhouse Clark
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Application Knows Best

• How to reorder packets
• Whether to ignore loss
• Which packet to retransmit

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Application Data Unit (ADU)

• Can be processed individually, even out-of-order
• 8-Bit PCM audio 1 ADU 1 Byte
• MPEG1 Video 1 ADU

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How to chop data into packets?

• Every received packet should be useful (even in
  very lossy environment)
• Ideally, 1 ADU in 1 packet

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RTP Payload Header
RTP Header
RTP Payload Header
RTP Payload

• MPEG-1? 2?
• Temporal Reference
• I? P? B?
• Begin of Slice? End of Slice?

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RTP Header

• Media Timestamp 32 bits
• the instant the first byte in this packet is
  captured
• 90 kHz timestamp (90000 1 second)

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RTP Header

• Marker Bit
• 1 if contains the last byte of a frame

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RTP Header

• Payload Type 7 bits
• 32 for MPEG-1

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RTP Payload Header

• MBZ (5 bits)
• Unused. Must be 0

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RTP Payload Header

• T (1 bit)
• 1 if there is a MPEG-2 Extension Header after
  this header.

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RTP Payload Header

• Temporal Reference (10 bits)
• The frame number of the current frame within
  the GOP

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RTP Payload Header

• AN bit and N bit
• Set to 0 for MPEG-1

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RTP Payload Header

• S (1 bit)
• Is there a sequence header in this packet?

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RTP Payload Header

• BS (1 bit) and ES (1bit)
• BS is 1 iff the first byte of this payload is a
  slice header
• ES is 1 iff the last byte of this payload is the
  end of a slice

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RTP Payload Header

• Picture Type (3 bits)
• I (1), P (2), B (3), D (4)

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RTP Payload Header

• Motion Vectors Information
• Get from most recent picture header

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Fragmentation Rules

• Sequence header at the start of payload
• GOP header at the start of a payload (or follows
  Sequence header)
• Picture header at the start of a payload (or
  follows Sequence/GOP header)

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Fragmentation Rules

• A slice must be either
• First data in the packet, or
• Follows integral number of slices
• A slice may be fragmented if exceeds the size of
  a packet

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Packet Size

• 1 MTU is 1500 bytes
• IP Header
• UDP Header
• RTP Header
• RTP Payload Header
• Payload Size

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Project 1

• Packetize MPEG-1 Video into RTP Packets

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Goal

• Read MPEG-1 video, output RTP packets with
  proper
• RTP headers
• RTP payload headers
• fragmentation

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Mnt (Media Networking Toy)

• C
• Tcl/OTcl
• UNIX

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Overview
OTcl
C
MntData
MntComponents
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MntComponent

• recv(MntData data)
• push(MntData data)

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MntPump

• Subclass of MntComponent
• No recv()
• pump_some( ) generate data push(data)
  pump_timer_.msched(time)

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Create and Link Components
set a new MntA set b new MntB a add_link_to
b a instproc on_stop_pumping puts
done! exit a start_pumping vwait forever
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Important Classes
MntComponent
MntRTPFileWriter
MntPump
MntRTPPayloadDumper
MntRTPFileReader
MntMPEGFileRTPizer
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Important Classes
MntData
MntRTPMPEGPacket
MntRTPHdr
MntRTPMPEGHdr
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MntRTPMPEGPacket
RTP Header
RTP Payload Header
RTP Payload
MntRTPHdr rtp_hdr_ MntRTPMPEGHdr pl_hdr_
unsigned char pl_data_ int pl_len_
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Example Tcl Scripts
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Dealing with Bits

• BitStream
• A buffer

buffer_
endDataPtr_
endBufPtr_
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Dealing with Bits

• BitParser
• Read bits from BitStream

BitParser
1011101
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Basic Usage
include mnt_bit_parser.h include
mnt_bit_stream.h BitStream bs new
BitStream(a.mpg) BitParser bp new
BitParser() bp-gtwrap(bs) int x Bp_GetInt(bp,
x)
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Dealing with MPEG Headers

• MntMPEG1SeqHdr
• MntMPEG1GopHdr
• MntMPEG1PicHdr
• MntMPEG1SliceHdr

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Basic Operations

• hdr-gtparse(bp)
• hdr-gtfind(bp)
• hdr-gtskip(bp)
• hdr-gtdump(bp1,bp2)

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ExampleMPEG Parsing
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Project Homepage

• Descriptions
• Skeleton Code
• Mnt Documentation
• IVLE Forums

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Advice

• Use the source wisely
• Start early