Digital Multimedia, 2nd edition Nigel Chapman & Jenny Chapman Chapter 17 This presentation © 2004, MacAvon Media Productions Multimedia and Networks

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. Protocols 591

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 591

Internet Protocol, 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 IP 592–593

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 593

Transmission Control Protocol 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 593–594

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 595

User Datagram Protocol 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 595–596

Real-Time Transport Protocol 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 596–597

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 Multicasting 597–599

Unicasting and Multicasting 598

HyperText Transport Protocol 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 600–601

String of 8-bit characters Request line (request) or status line (response) One or more headers Blank line Body (optional) HTTP Messages 601

Request line –Method – name of service being requested e.g. GET –Identifier – resource being requested e.g. path name of a file –Version e.g. HTTP/1.1 e.g. GET DMM2/links/index.html HTTP/1.1 HTTP Requests 601

Header name : arguments Host: Connection: keep-alive User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en-us) AppleWebKit/85 (KHTML, like Gecko) Safari/85 Accept: */* Accept-Language: en-us, ja;q=0.33, en;q=0.67 etc Headers 601–602

Status line –Protocol version –Numerical status code 200 = OK, 400 = bad request, 404 = not found, etc –Status description e.g. HTTP/ OK HTTP Responses 602–603

Headers – same format as for requests –e.g. Server: Apache/ (Darwin) Body contains data, e.g. HTML code of a requested page HTTP Responses 602

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 604–605

Real Time Streaming Protocol '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 606

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) 609–611

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 611–614