Real Time Protocols
Tarik Cicic
University of Oslo
December 2001
Overview
• IETF-suite of real-time protocols
• data transport:
– Real-time Transport Protocol (RTP)
• connection establishment and control:
– Real Time Streaming Protocol (RTSP)
– Session Initiation Protocol (SIP)
• comparison with the ITU suite (H.323)
2
Introduction
• Two major standardization bodies for the
multimedia protocols “in packet switched
networks” / the Internet:
– International Telecommunication Union
– Internet Engineering Task Force
• partly competing, partly completing each
other
3
1
Components and protocols
file.rtsp
SP
RT
Streaming
Streaming
(RTSP)
(RTSP)
server
server
XP
RTP
Client
RTCP
Media
Media
source
source
4
Real time protocols
• Real-time Transport Protocol (RTP): carrier
of media information
• Real-Time Control Protocol (RTCP): carrier
of control information
• Real-Time Streaming Protocol (RTSP):
establishes and controls multimedia
sessions (“remote control”)
5
RTP in a nutshell
• Transport protocol for data with real-time
characteristics
• often transported in UDP/IP
• multicast friendly
• many encoding possibilities
• RTP mixers: many streams to one
• RTP translators: media format conversion
6
2
RTP functions
• Sequencing (loss & reorder detection)
• Intramedia synchronization (time stamps)
• payload identification (dynamic shift of
format)
• frame indication (e.g. synchronized data
delivery to higher layers)
• source identification (multicast support)
7
RTP header
• Payload type: 7 bits
Version, CSRC,
payload type, flags Sequence number
• timestamp: sampling
Timestamp
instance of the first octet,
resolution is payload
Synchronization source ID
dependent
Contributing source ID1
• first sequence number,
SSR and CSR chosen
Contributing source IDn
randomly
32 bits
8
(+ extensions)
RTCP functionality
• Provides feedback on the quality of data
distribution (packet loss, jitter)
• helps to localize network problems
• intermedia synchronization (“lip sync”)
• carries a persistent source ID
(“tarikc@129.240.68.135”)
• scaling problem (max 5% of total traffic!)
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3
RTCP packets
• five formats: sender report, receiver report,
source description, “bye” and “application
specific”
• compound packets, e.g. SR+SDES, even
from different sources
SR
Sender report SDES
CNAME, phone,
BYE
location
reason
3 packets compound, UDP encapsulated
10
RTSP (1)
• Controls synchronized streams of
continuous media
• data originates from multimedia servers, but
also multicast sessions
• typically used together with RTP, but is not
limited to it
11
RTSP (2)
• similar syntax to HTTP 1.1
• maintains state
• various commands (e.g. DESCRIBE, SETUP,
PLAY, PAUSE, TEARDOWN)
• example:
SETUP rtsp://godemine.ifi.uio.no/audio RTSP/1.0
Cseq: 1
Transport: RTP/AVP/UDP;unicast;client_port=5000-5001
12
4
RTSP (3)
•
•
•
•
•
•
extendibility
security (reuses Web security mechanisms)
transport independence
multi-server capability
recording possibility
transport negotiation
13
Protocol layers
RTSP
RTP
RTCP
RSVP
TCP
UDP
IP
14
Session Description Protocol
• Text protocol for multimedia session description
• used in many other protocols
• v=0
• o=mhandley 2890844526 2890842807 IN IP4
126.16.64.4
• s=SDP Seminar
• e=mjh@isi.edu (Mark Handley)
• c=IN IP4 224.2.17.124
• t=2873397496 2873404696
• a=recvonly
• m=audio 49170 RTP/AVP 0
• m=video 51372 RTP/AVP 31
• m=application 32416 udp wb
• a=orient:portrait
15
5
Session Initiation Protocol
• Signaling protocol
• establishes a connection between
communicating parts
• main functions:
– user location detection
– session establishment
– session negotiation
16
SIP operation
tarikc logged in
at uio.no
SIP redirect
server
IT E
INV
step
bird
c@
arik
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:
p
si
SIP
client
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SIP VIT
IN
proxy
. no
u io
kc@
tari
:
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i
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tarikc logged as
tarik.cicic@ifi.uio.no
SIP
proxy
Data
UIO
SIP
17client
SIP
proxy
H.323 protocol suite
• Serves the same purpose as the IETF suite
presented so far
• comes from a different planet
– tight architecture
– complete technical specifications
– includes parts of the existing ITU standards
18
6
H.323 interconnect
Gatekeeper
PSTN
H.323 / PSTN
gateway
Internet
Plain
phone
IP router
H.323
terminal
IP router
H.323 / H.320
gateway
H.323
terminal
H.320 network
(ISDN)
ISDN
videophone
19
H.323 terminal protocol stack
Audio
I/O
Video
I/O
Audio
Codecs
(e.g.
G.711)
Vidao
Codecs
(e.g.
H.261)
RTP
Data
I/O
System Control Unit
Q.931
Call
Signaling
RTCP
H.225
H.245
T.120
Control
Signaling
Data
Service
Transport protocol(s) and lower layers
20
Voice over IP (VoIP)
• Technology for telephony support in IP
networks, often using H.323
Internet
R
VoIP GW
VoIP GW
PSTN
PSTN
21
7
VoIP tasks
• Basic:
– Data path:
• sample and encode audio data
• pack it in RTP packets and send on the Internet
– Control path:
•
•
•
•
find the address of the remote peer
map between PSTN and IP addresses
alert the callee
start up and tear down the connection
• Advanced:
– QoS control (negotiation, RSVP)
– directory services +++ 22
IETF vs. ITU
• Example: in VoIP, SIP+RTP (IETF) and
H.323 (ITU) perform the same task:
–
–
–
–
Audio communications over IP
both suites can be used
SIP has a functionality similar to H.225 / H.245
however, H.323 dominates VoIP (CISCO)
23
IETF vs. ITU multimedia protocols
IETF
ITU
Architecture
Loosely coupled
components
Network distribution Easy
Extensibility,
modifications
Implementational
complexity
Simple
Large
Tightly coupled
components
Some components
cannot be distributed
Difficult
Large, but precise
specifications
24
8
Summary
• Multimedia communications in the Internet
need multiple protocols:
– multimedia data transport
– connection setup
– communication control
• resource reservation comes in addition
• IETF (“hackers”) and ITU (“ties”)
standardization bodies
25
Discussion: IETF vs. ITU multimedia
• IETF has in many ways shaped idea of multimedia
on the Internet
• There are many IETF protocols for transport and
control of multimedia streams
• IETF-protocol-based applications as WWW and email are ubiquitous today.
• Why the ITU-based Internet protocols enjoy
large popularity?
•
(you may not want to show this to early):
• Preciseness? Connection to existing technologies?
• More control and easier billing?
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