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Multimedia Communications over the Internet. IP Packet-Switching Networks Packet-switching protocols based on the Internet Protocol (IP) generally consist.

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Presentation on theme: "Multimedia Communications over the Internet. IP Packet-Switching Networks Packet-switching protocols based on the Internet Protocol (IP) generally consist."— Presentation transcript:

1 Multimedia Communications over the Internet

2 IP Packet-Switching Networks Packet-switching protocols based on the Internet Protocol (IP) generally consist of a variety of different subnetworks of various technologies – The datalink and physical layer are largely outside the scope of the Internet efforts IP is a connectionless (datagram) network layer – Different packets may traverse various routes between source and destination (they may arrive out of order or be duplicated) – Packets are transmitted on a best effort basis (no guarantee that a packet will arrive at destination

3 IP Networks (cont’d) IP provides for the transmission of blocks of data called datagrams from source to destination, where source and destination are hosts identified by fixed-length addresses Datagrams can be as large as 64 KB, but usually the are ~1500 bytes long. IP also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through small packet networks. IP basically runs over any Media Access Control (MAC) protocol

4 Internet Protocol Relationships (Non Real-Time) At the highest level, the users invoke application programs that access services available across the Internet. An application interacts with the transport-level protocol to send or receive data. Each application program chooses the size of transport needed, which can be either a sequence of individual messages or a continuous stream of bytes.

5 IP Relationships (cont’d) Above IP, there are two transport layer options: TCP and UDP. – TCP provides end-to-end communication. It takes care of reliable, error-free transfer of data, and in- sequence delivery. UDP has less overhead compared to TCP, but does not guarantee transfers. – Both protocols support multiplexing, i.e. they allow several distinct streams of data between two hosts Streams are labeled by source and destination port numbers

6 UDP The User Datagram Protocol (UDP) offers only minimal services beyond those provided by the network layer – Multiplexing via port number – Checksum for the payload in the packet IP provides only a checksum for the IP header

7 TCP The Transmission Control Protocol (TCP) offers a reliable, sequenced byte stream service between two Internet hosts Reliability is achieved by retransmission of lost or erroneous packets – Requires acknowledgment of received data Flow control is achieved by means of a sliding window mechanism – Also used for congestion control: the sender reduces the window size in the case of traffic congestion The sender probes for the currently available bandwidth by gradually increasing its window size until it senses packet loss - at which point it quickly reduces the window size

8 TCP (cont’d) TCP is less suited to multimedia data than UDP – It trades reliability for delay jitter – Since it enforces in-order delivery, a single lost packet can hold up packets arrived after it – To improve delay behavior, an implementation is free to accept out-of-order packets and to acknowledge packets that have not been received but are excessively delayed However, this may interfere with the TCP congestion control – The TCP congestion control mechanism imposes throughput limitations that may change on very short time scales

9 Connectivity Requirements Connectivity requirements examples – Accommodating a listening audience of 25,000 streams daily with an average listening time of 6 minutes per stream requires, on average, only 100 concurrent streams – Accommodating a daily listening audience of 250,000 streams, with an average listening time of 20 minutes per stream, requires an average of 6,000 concurrent streams

10 Connectivity Requirements (cont’d) Due to variable traffic patterns, load is not constant throughout the day, therefore the peak number of streams required may be twice or three times the average When implemented, broadcast protocols (to an entire subnetwork) or multicast protocols (to members of a multicast group in various scattered subnetworks) allow to reduce the load on the network by allowing many user to share a single stream from the server

11 IP Multicast IP Multicast allows a sender to transmit an IP packet to multiple receivers. Three possible ways for multicast: – Setting up multiple virtual circuits (not possible in Internet) – Including list of addresses in packet header (what happens if 1,000,000 addresses?) – Radio-like approach (used by IP Multicast) Sender chooses an IP address from the class-D set ( to Receivers subscribe to multicast finding out the multicast address. An IP multicast group can have any number of senders and receivers. Membership is dynamic. IP Multicast is independent of transport mechanism (but TCP cannot be used)

12 HTTP ‧ HTTP (Hyper-Text Transfer Protocol) is built on top of TCP/IP ‧ For transmission through firewalls, often multimedia must be encapsulated in a HTTP envelope, lacking most of the required real-time features ‧ Multimedia data is downloaded on the client computer. Fast-start techniques can be used to begin playback as soon as enough of the content has been downloaded to the client UDP and TCP protocols are otherwise used – UDP is sometimes blocked by firewalls

13 Requirements for Real-Time Multimedia Sequencing. Packets must be reordered in real time at the receiver. If a packet is lost, must be detected and compensated without retransmission Intramedia synchronization. Need some form of “time stamping” to know when to playback packets – Very important for VBR traffic Intermedia synchronization. E.g., audio must be synchronized with video (lip-sync). Payload identification. E.g., for media filtering Frame indication. For synchronized delivery, it is useful to indicate when a video frame (or audio segment) begins or end.

14 RTP ‧ RTP (Real-Time Protocol) is a transport protocol for audio and videoconferences and other multiparticipant real-time applications. ‧ Designed to run over multicast IP ‧ Light-weight protocol, without error correction, flow control, or guaranteed time delivery functionality. ‧ Offers services such as playout synchronization, demultiplexing, media identification, and active- party identification.

15 Functionalities of RTP Re-sequencing and loss detection Multicast-friendly Media-independent (voice, video,…) Explicit support for mixers and translator – Mixers: take media from several users and mix them into one media stream (e.g., conference bridge) – Translators: take a single media stream, convert it to another format, and send it out (e.g. media filtering)

16 Functionalities of RTP (cont’d) QoS feedback (via RTCP) – RTP sources can use this information to adjust their data rate (media scaling) Loose session control – Using RTCP, participants can periodically distribute identification information (name, e-mail address,…) – Provides awareness of who is participating in a session without maintaining a centralized conference participant registry Encryption

17 RTP (cont’d) RTP specifies a packet structure for packets carrying audio and video data – Payload type identification – Packet sequence numbering – Time-stamping RTP runs on top of UDP (can be viewed as a sub-layer of the transport layer) RTP encapsulation is only seen at the end systems (not at the intermediate routers)

18 RTP - Example Consider sending 64 Kb/s PCM-encoded voice over RTP Application collects the encoded data in chunks, e.g. every 20 ms = 160 bytes/chunk The audio chunk, along with the RTP header, forms the RTP packet, which is encapsulated into a UDP packet The RTP header indicates the type of audio encoding in each packet. Sender can change encoding during a conference RTP header also contains sequence number and timestamps

19 RTP Streams RTP allows each source (e.g., a camera or a microphone) to be assigned its own independent stream of packets – E.g., for a videoconference between two participants, 4 streams could be opened: two streams for audio (one in each direction) and two for video However, MPEG-1 and MPEG-2 bundle audio and video into a single stream during the encoding process - then only one RTP stream is generated in each direction.

20 Real-Time Control Protocol (RTCP) Works in conjunction with RTP Each participant in a RTP session periodically transmits RTCP control packets to all other participants. – Each RTCP packet contains sender and/or receiver reports with statistics useful to the application – E.g.: number of packets sent, number of packets lost, inter-arrival jitter, etc. This feedback information can be used to control performance and diagnostic purposes – E.g. the sender may modify its transmission based on feedback

21 RTCP (cont’d) RTCP can be used to synchronize different media stream within a RTP session – E.g.: videoconference where each sender generates one RTP stream for video and one for audio – Timestamps in RTP packets are tied to video/audio sampling clocks (not wall-clock time, ie real time) – Each RTCP sender-report packet contains, for the most recently generated packet in the associated stream, the timestamp of the RTP packet and the wall-clock time of when the packet was created – Receivers can use this association to synchronize the playout of audio and video

22 Protocols for Real-Time (cont’d) RTSP (Real-Time Streaming Protocol) provides methods to realize commands (play, fast-forward, fast-rewind, pause, stop) similar to the functionality provided by CD players or VCRs. – Can control either a single or several time- synchronized streams of continuous media. – Can act as a network remote control for multimedia servers and can run over TCP or UDP

23 Live vs. Video-On-Demand Live streaming is a live broadcast which allows users to join a session in which real time media is being sent over a network – Because these streams are live, users are not allowed to jump around to any point in time – Live streaming can exploit multicast or broadcast protocols Video-on-demand represents content stored on a streaming server which can be viewed at any time – User is allowed to jump to any point in time in the media – Only unicast protocols are allowed

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