Real Time Streaming Protocol

1
Real Time Streaming Protocol

• Akshat Sikarwar
• Columbia University
• November 12, 2008

2
Overview

• Introduction
• Protocol properties
• RTSP messages
• Important messages

3
Introduction

• RTSP is like a network remote control for
  multimedia servers
• RTSP establishes and controls streams of
  continuous media
• Streaming data is carried out of band
  (interleaving is possible)

4
Protocol Properties

• RTSP is text based and HTTP like
• Transport independent (can use UDP or TCP)
• Unlike HTTP commands can be sent in either
  direction
• RTSP is a stateful protocol. Server maintains
  session state.

5
RTSP Connections

• RTSP messages can be sent over
• One persistent transport connection for several
  request-response transactions
• One connection per transaction
• Connectionless mode
• Transport connection ! Session
• Server can send message to client only in
  persistent connection

6
RTSP Reliability

• If using reliable transport message is sent just
  once.
• If using unreliable transport, RTSP will
  retransmit if it does not receive ACK
• Timeout is initially set to 500 ms
• Can re-compute timeout based on RTT like TCP
• For retransmission CSeq is not incremented
• Timestamp is used to overcome retransmission
  ambiguity problem

7
RTSP URI

• rtsp//media.example.com554/twister/audio
• rtsp identifies reliable protocol
• rtspu identifies unreliable protocol
• Default port is 554
• Presentation twister
• Stream audio

8
RTSP Message

• Text based messages
• Messages use Unicode (ISO 10646) character set
  encoded using UTF-8
• Lines are terminated by CRLF
• Messages can be request or response by either
  client or server
• Message structure is HTTP like

9
Request Message

• Method URI Version CRLF
• Header1Value CRLF
• Header2Value CRLF
• CRLF
• Body
• First line is Request line
• URI is always absolute (unlike HTTP)
• URI can be
• Example OPTIONS RTSP/1.0

10
Response Message

• Version Status-Code Reason-Phrase CRLF
• Status-Codes are HTTP like
• 1xx Informational
• 2xx Success
• 3xx Redirection
• 4xx Client error
• 5xx Server error
• Reason-Phrase is the textual description

11
RTSP at Work
HTTP GET
Client
Web server
Session description
Media server
PLAY
RTP
RTCP
PAUSE
CLOSE
From http//www.cs.columbia.edu/hgs/teaching/ais/
slides/2003/RTSP.pdf
12
RTSP Methods

• DESCRIBE Get (low level) description of media
  object
• OPTIONS Get available methods
• SETUP Establish transport
• PLAY Start playback, reposition
• PAUSE Halt delivery, but keep state
• RECORD Start recording
• ANNOUNCE Change description of media object
• REDIRECT Redirect client to new server
• GET_PARAMETER Retrieves presentation parameter
  value
• SET_PARAMETER Sets presentation parameter value
• TEARDOWN Remove state

From http//www.cs.columbia.edu/hgs/teaching/ais/
slides/2003/RTSP.pdf
13
Method Summary
From RFC 2326
14
Messages in Action

• Following messages play central role
• SETUP
• PLAY
• PAUSE
• TEARDOWN

15
SETUP

• Specifies the transport mechanism to be used for
  the streamed media
• Transport header specifies the transport
  parameters acceptable to the client
• Response contains transport parameters selected
  by the server
• Server generates session identifiers in response
  to SETUP requests
• Client can issue SETUP to change parameters for
  already streaming media

16
SETUP Message

• Client -gt Server
• SETUP rtsp//example.com/foo/bar/baz.rm RTSP/1.0
• CSeq 302
• Transport RTP/AVPunicastclient_port4588-4589
• Server -gt Client
• RTSP/1.0 200 OK
• Cseq 302
• Date 23 Jan 1997 153506 GMT
• Session 47112344
• Transport RTP/AVPunicast
• client_port4588-4589server_port6256-6257

From RFC 2326
17
PLAY

• Tells server to start sending data via mechanism
  specified in SETUP
• Plays from beginning to end of range specified
• Pauses at end as if PAUSE has been issued
• Requests may be pipelined
• Range header can have time parameter
• Legal to have PLAY without Range header
• Scale header can be used to change viewing rate
• Can be used for fast forward or rewind

18
PLAY Message

• Client -gt Server
• PLAY rtsp//audio.example.com/twister.en RTSP/1.0
• CSeq 833
• Session 12345678
• Range smpte01020-time19970123T153600Z
• Server -gt Client
• RTSP/1.0 200 OK
• CSeq 833
• Date 23 Jan 1997 153506 GMT
• Range smpte01022-time19970123T153600Z

From RFC 2326
19
PAUSE

• Causes stream delivery to halt
• Can pause entire presentation or selected stream
• Server resources continue to be reserved
• Server will terminate session after timeout
  period expires (specified in SETUP)
• Can contain Range header with one value to
  specify when to pause
• Must fall in one of the PLAY ranges

20
TEARDOWN

• Stops delivery of stream
• Frees up resources on the server
• Can stop single stream or entire presentation
• Example
• Client -gt Server
• TEARDOWN rtsp//example.com/fizzle/foo RTSP/1.0
• CSeq 892
• Session 12345678
• Server -gt Client
• RTSP/1.0 200 OK
• CSeq 892

21
References

• RFC 2326
• Internet Media on Demand The Real Time
  Streaming Protocol http//www.cs.columbia.edu/hg
  s/teaching/ais/slides/2003/RTSP.pdf
• RTSP FAQ http//www.cs.columbia.edu/hgs/rtsp/faq
  .html