Multimedia Protocols

1. Multimedia Protocols Foreleser: Carsten Griwodz Email: griff@ifi.uio.no 1

2. Non-QoSMultimedia Networking RTP – Real-Time Transfer Protocol 2

3. Real-time Transport Protocol (RTP) • Real-time Transport Protocol (RTP) • RFC 1889 • Designed for requirements of real-time data transport • NOT real-time • NOT a transport protocol • Two Components • Real-Time Transfer Protocol (RTP) • RTP Control Protocol (RTCP) • Provides end-to-end transport functions • Scalable in multicast scenarios • Media independent • Mixer and translator support • RTCP for QoS feedback and session information

4. application media encapsulation RTP RTCP TCP UDP ATM IPv4/6 ST-2 AAL5 Ethernet ATM Real-time Transport Protocol (RTP) • No premise on underlying resources • layered above transport protocol • no reservation / guarantees • Integrated with applications • RTP follows principles of • Application Level Framing and • Integrated Layer Processing

5. RTP • RTP services are • Sequencing • Synchronization • Payload identification • QoS feedback and session information • RTP supports • Multicast in a scalable way • Generic real-time media and changing codecs on the fly • Mixers and translators to adapt to bandwidth limitations • Encryption • RTP is not designed for • Reliable delivery • QoS provision or reservation

6. RTP Functions • RTP with RTCP provides • Support for transmission of real-time data • Over multicast or unicast network services • Functional basis for this • Loss detection – sequence numbering • Determination of media encoding • Synchronization – timing • Framing - “guidelines” in payload format definitions • Encryption • Unicast and multicast support • Support for stream “translation” and “mixing” (SSRC; CSRC)

7. a byte RTP Packet Format Typical IETF RFC bit-exact representation a longword (32 bit)

8. RTP Packet Format Padding indicator bitif set, number of padding bytes is in last byte of payload Version number, Always 2 Header extension bit True if header extension is present 7 bit payload type Allows identification of the payload’s content type Marker bit Meaning depends on payload profile, e.g. frame boundary 4 bit CSRC count, indicates the number of contributing sources in the header

9. RTP Packet Format 16 bit sequence number 32 bit timestamp 32 bit SSRC Synchronization source identifier, a random number identifying the sender Several 32 bit CSRC Contribution source identifier, the number is indicated by CC A mixer copies the original sources’ SSRCs here Header extension multiples of 32 bit

10. Marker bitMeaning depends on payload profile, e.g. frame boundary 7 bit payload type Allows identification of the payload’s content type 32 bit timestamp 16 bit sequence number RTP Architecture Concepts • Integrated Layer Processing • Typical layered • Data units sequentially processed by each layer • Integrated layer processing • Adjacent layers tightly coupled • Therefore, RTP is not complete by itself: requires application-layer functionality/information in header

11. RTP Packet Format • Relatively long header (>40 bytes) • overhead carrying possibly small payload • header compression • other means to reduce bandwidth (e.g. silence suppression) • No length field • Exactly one RTP packet carried in UDP packet • Can use TCP or ATM AAL5 • do-it-yourself packaging • Header extensions for payload specific fields possible • Specific codecs • Error recovery mechanisms

12. RTP Profile (RFC 1890) • Set of standard encodings and payload types • Audio: e.g. PCM-u, GSM, G.721 • Video: e.g. JPEG, H.261 • Number of samples or frames in RTP packet • Sample-based audio: no limit on number of samples • Frame-based audio: several frames in RTP packet allowed • Clock rate for timestamp • Packetized audio: default packetization interval 20 ms • Video: normally 90 kHz, other rates possible

13. Additional Payload Profiles Formats Dynamic Payload Types AV Profile PT mapping outside RTP RTP / RTCP (e.g. SDP) RTP Profiles • Payload type identification • RTP provides services needed for generic A/V transport • Particular codecs with additional requirements • Payload formats defined for each codec: syntax and semantic of RTP payload • Payload types • Static: RTP AV profile document • Dynamic: agreement on per-session basis • Profiles and Payload Formats in RTP Framework

14. RTP Profiles • General • Associated with a media type. • Provides association between PT field and specific media format • Defines sampling rate of timestamp • May also define or recommend a definition for the “marker” bit • Video Profile • Marker bit recommended to mean last packet associated with a timestamp • Timestamp clock: 90000 Hz • Defines PT mapping for a number of different video encoding types

15. RTP Profiles • Audio Profile • Marker bit set on the first packet after a silence period where no packets sent • Timestamp equals sampling rate • Recommends 20ms minimum frame time • Recommends that samples from multiple channels be sent together • Defines PT for a number of different audio encoding types

16. RTP Profile for MPEG Video Payload GOP header Frame headers

17. RTP Profile for MPEG Video Payload • Fragmentation rules • Video sequence header • if present, starts at the beginning of an RTP packet • GOP sequence header • Either at beginning of RTP packet • Or following video sequence header • Picture header • Either at beginning of RTP packet • Following GOP header • No header can span packets • Marker Bit • Set to 1 if packet is end of picture

18. MPEG-1 Video specific payload header MBZ Must be zero TR Temporal reference The same number for all packets of one frame For ordering inside an MPEG GOP S 1 is sequence header is in this packet B 1 if payload starts with new slice E 1 if last byte of payload is end of slice P 3 bits that indicate picture type (I,P, or B) FBV, BFC, FFC, FFC Indicate how a P or B frame is related to other I and P frames 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MBZ | TR |MBZ|S|B|E| P | | BFC | | FFC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ FBV FFV MPEG Video Profile RTP Profile for MPEG Video Payload

19. Application Application Decoding Encoding Encoding Decoding RTP RTCP RTCP RTP UDP/IP UDP/IP RTP Quality Adaptation • Component interoperations for control of quality • Evaluation of sender and receiver reports • Modification of encoding schemes and parameters • Adaptation of transmission rates • Hook for possible retransmissions (outside RTP)

20. RTP Control Protocol (RTCP) • Companion protocol to RTP (tight integration with RTP) • Monitoring • of QoS • of application performance • Feedback to members of a group about delivery quality, loss, etc. • Sources may adjust data rate • Receivers can determine if QoS problems are local or network-wide • Loose session control • Convey information about participants • Convey information about session relationships • Automatic adjustment to overhead • report frequency based on participant count • Typically, “RTP does ...” means “RTP with RTCP does ...”

21. R SR / RR SDES BYE APP Compound (UDP) Packet RTCP Packets • Several RTCP packets carried in one compound packet • RTCP Packet Structure • SR Sender Report (statistics from active senders: bytes sent -> estimate rate) • RR Receiver Report (statistics from receivers) • SDES Source Descriptions (sources as “chunks” with several items like canonical names, email, location,...) • BYE explicit leave • APP extensions, application specific

22. Header Sender Information Reception Report ... Reception Report Profile Specific Extensions Header Reception Report ... Reception Report Profile Specific Extensions RTCP Sender / Receiver Reports • Sender report • Sender Information • Timestamps • Packet Count, Byte Count • List of statistics per source • Receiver report • For each source • Loss statistics • Inter-arrival jitter • Timestamp of last SR • Delay between reception of last SR and sending of RR • Analysis of reports • Cumulative counts for short and long time measurements • NTP timestamp for encoding- and profile independent monitoring

23. RTP Mixer • Mixer • Reconstructs constant spacing generated by sender • Translates audio encoding to a lower-bandwidth • Mixes reconstructed audio streams into a single stream • Resynchronizes incoming audio packets • New synchronization source value (SSRC) stored in packet • Incoming SSRCs are copied into the contributing synchronization source list (CSRC) • Forwards the mixed packet stream • Useful in conference bridges

24. MPEG Sink ATM MPEG UDP Source H.263 Sink Protocol Translator Profile Translator RTP Translator • Translation between protocols • e.g., between IP and ST-2 • Two types of translators are installed • Translation between encoding of data • e.g. for reduction of bandwidth without adapting sources • No resynchronization in translators • SSRC and CSRC remain unchanged

25. RTPIdentifiers SSRC chosen by sender S1 SSRC chosen by mixer M1 S1 S3 S3:19 S1:10 M1:33 (10,1) M1:33 (10,1) M1 R1 M2 T1 S4:13 M2:17 (19,13,33) S2:1 S4:13 S2 S4 Translators keep SSRCs and CSRCs CSRCs from mixed sources S1 and S2 CSRCs contain previous SSRCs, but not previous CSRCs

26. Protocol Development • Changes and extensions to RTP • Scalability to very large multicast groups • Congestion Control • Algorithms to calculate RTCP packet rate • Several profile and payload formats • Efficient packetization of Audio / Video • RTCP-based retransmission • Loss / error recovery

27. Non-QoSMultimedia Networking Signalling Protocols: RTSP and SIP 27

28. Signaling Protocols • Applications differ • Media delivery controlled by sender or receiver • Sender and receiver “meet” before media delivery • Signaling should reflect different needs • Media-on-demand • Receiver controlled delivery of content • Explicit session setup • Internet telephony and conferences: • Bi-directional data flow, live sources • (mostly) explicit session setup, mostly persons at both ends • Internet broadcast • Sender announces multicast stream • No explicit session setup

29. Real-Time Streaming Protocol (RTSP) • Internet media-on-demand • Select and playback streaming media from server • Similar to VCR, but • Potentially new functionality • Integration with Web • Security • Varying quality • Need for control protocol • Start, stop, pause, … • RTSP is also usable for • Near video-on-demand (multicast) • Live broadcasts (multicast, restricted control functionality) • ...

30. RTSP Approach • In line with established Internet protocols • Similar to HTTP 1.1 in style • Uses URLs for addressing: rtsp://video.server.com:8765/videos/themovie.mpg • Range definitions • Proxy usage • Expiration dates for RTSP DESCRIBE responses • Other referenced protocols from Internet (RTP, SDP) • Functional differences from HTTP • Data transfer is separate from RTSP connection • typically via RTP • Server maintains state – setup and teardown messages • Server as well as clients can send requests

31. RTSP Features • Rough synchronization • Media description in DESCRIBE response • Timing description in SETUP response • Fine-grained through RTP sender reports • Aggregate and separate control of streams possible • Virtual presentations • Server controls timing for aggregate sessions • RTSP Server may control several data (RTP) servers • Load balancing through redirect at connect time • Use REDIRECT at connect time • Caching • Only RTSP caching so far • Data stream caching is under discussion

32. RTSP Methods

33. HTTP server RTSP server data source media server web browser RTSP Integration HTTP GET presentation description file RTSP SETUP RTSP OK RTSP PLAY RTSP plug-in RTSP OK RTSP TEARDOWN RTSP OK AV subsystem RTP VIDEO RTP AUDIO

34. Session Initiation Protocol (SIP) • Lightweight generic signaling protocol • Internet telephony and conferencing • Call: association between number of participants • Signaling association as signaling state at endpoints (no network resources) • Several “services” needed • Name translation • User location • Feature negotiation • Call control • Changing features

35. SIP Basics • Call user • Re-negotiate call parameters • Forwarding (manual and automatic) • Call center • Supports personal mobility (change of terminal) • Through proxies or redirection • Terminate / transfer calls • ASCII (readable) protocol – SIP vs. H.323 • Similarities (request/response, proxies ...) • Differences (server state, server may initiate actions ...) • Control, location and media description (via SDP) • Extensible towards • Usage for IP-IP, POTS-IP, inter-gateway interaction with firewalls, billing system, ... • Different modes • Proxy mode • Redirect mode

36. SIP Operation – Proxy Mode Location Server 2. Where? 5. “Ring” 4. Invite user@host 1. Invite u@domain 3. user@host Site A 6. Ok 7. Ok 8. ACK u@domain 9. ACK user@host User with “symbolic name” calls another Proxy Mode 10. Ok • Proxy forwards requests • Possibly in parallel to several hosts • Cannot accept or reject call • Useful to hide location of callee 11. Ok Site B

37. SIP Operation – Redirect Mode Location Server 2. Where? Site A 3. user@host 1. Invite u@domain Redirect Mode 4. Moved temporarily Location: user@domain2 User with “symbolic name” 5. ACK u@domain calls another 7. “Ring” 6. Invite user@domain2 7. Ok 8. ACK user@domain2 Site B

38. SIP – Methods • Basic Methods (RFC 2543) TABLE • Additional Methods (partially standardized) • INFO: carry information between User Agents • REFER: ask someone to send an INVITE to another participant • SUBSCRIBE: request to be notified of specific event • NOTIFY: notification of specific event