Multimedia and Networks

Protocols (rules) Rules governing the exchange of data over networks Conceptually organized into stacked layers – Application-oriented services (e.g. file transfer, Web browsing) – Transfer of raw data – Physical signals over wires, optical fibres, etc.

TCP/IP networks, including the Internet, are packet-switched networks – Messages split into small pieces called packets, sent separately – Messages are multiplexed – Enables network bandwidth to be shared efficiently between many messages Packets

IP I nternet P rotocol, defines – Basic unit of transfer, datagram – Mechanism for getting datagrams from source to destination host through a network of networks, via routers Hosts are identified by IP addresses – Set of four numbers, uniquely identifying the network and host network internet

Attempts to deliver each datagram individually from source to destination host Datagrams not delivered after specified time are discarded – Message may arrive with some packets missing Routes calculated dynamically – Packets may arrive in the wrong order IP

T ransmission C ontrol P rotocol Provides reliable delivery of sequenced packets – Requests retransmission of missing packets – Puts packets back into correct order Based on acknowledgements, using a sliding window of unacknowledged packets – May lead to some packets being sent more than once TCP

Packets must be sent to the right application (e.g. Web browser, not client) – IP address only identifies right host IP address extended with a port number, identifying an application running on the host IP address + port number = transport address Transport Addresses

U ser D atagram P rotocol Ensures packets are delivered to right application – Uses transport addresses Does not guarantee delivery – Suitable for networked multimedia where lost packets more acceptable than overhead of TCP UDP

R eal-Time T ransport P rotocol Runs on top of UDP, adds extra features for sequencing etc Header identifies the type of payload (video, audio, etc) – Format of payload optimized for the type of data Sequence numbers and timestamps used to reorder packets and synchronize separately transmitted streams RTP

Unicast – server sends a copy of e.g. video data stream to every client – Many copies of the data sent over network Multicast – server sends a single copy, which is only duplicated when necessary, when routes to different clients diverge – Hosts must be assigned to host groups, using a range of reserved IP addresses – Needs enhanced routers Unicasting & Multicasting

Unicasting and Multicasting

H yper T ext T ransport P rotocol Client opens TCP connection to the server – Server's name is usually extracted from a URL, mapped to an IP address via DNS Client sends a request and receives a response – Requests and responses are both messages HTTP

Copies of pages that have been retrieved are kept in a cache on user's machine or proxy – How to tell if version on server is newer than version in the cache? If-Modified-Since header in conditional request Status = 304, Not Modified – Browser displays page from cache Caching

R eal T ime S treaming P rotocol 'Internet VCR remote control' – Start, stop, pause media stream – Go to point identified by timecode – Schedule time to start display Messages syntactically similar to HTTP Data stream transmitted separately (using RTP) RTSP

Quantifies the amount of – Delay – Jitter – Packet loss – an application can tolerate ATM (Asynchronous Transfer Mode) networks can offer QoS guarantees Quality of Service (QoS)

Allows an HTTP server to communicate with other resources (e.g. databases) to generate Web pages dynamically CGI (Common Gateway Interface) – Receives data from HTTP request (e.g. form data) – Returns HTTP response PHP, ASP, JSP, ColdFusion, etc Server-side Computation