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 () 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 RTSP Streaming (RTSP) server XP Client RTP RTCP Media 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, timestamp: sampling payload type, flags Sequence number instance of the first octet, Timestamp resolution is payload dependent Synchronization source ID first sequence number, Contributing source ID 1 SSR and CSR chosen randomly Contributing source ID n 32 bits (+ extensions) 8 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!) 9 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 CNAME, phone, Sender report SDES BYE location 3 packets compound, UDP encapsulated reason 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 TCP RTP UDP RTCP RSVP 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 INVITE sip:tarikc@birdstep.com client operation proxy 17 redirect server INVITE sip:tarikc@uio.no Data client proxy tarikc logged in at uio.no tarikc logged as tarik.cicic@ifi.uio.no UIO 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 Internet IP router H.323 / PSTN gateway PSTN Plain phone H.323 terminal H.323 terminal IP router H.323 / H.320 gateway 19 H.320 network (ISDN) ISDN videophone H.323 terminal protocol stack Audio I/O Video I/O System Control Unit Data I/O Audio Codecs (e.g. G.711) RTP Vidao Codecs (e.g. H.261) Q.931 Call Signaling H.245 Control RTCP H.225 Signaling T.120 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 VoIP GW PSTN R VoIP GW 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, +RTP (IETF) and H.323 (ITU) perform the same task: Audio communications over IP both suites can be used has a functionality similar to H.225 / H.245 however, H.323 dominates VoIP (CISCO) 23 IETF vs. ITU multimedia protocols Architecture Network distribution Extensibility, modifications Implementational complexity IETF Loosely coupled components Easy Simple Large ITU 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 e- mail 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? 26 9