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

Protocols Rules governing the exchange of data over networks 591 Protocols 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.

591 Packets 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

IP Internet Protocol, defines Basic unit of transfer, datagram 592–593 IP 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

593 IP 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

TCP Transmission Control Protocol 593–594 TCP 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

595 Transport Addresses Packets must be sent to the right application (e.g. Web browser, not email 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

UDP User Datagram Protocol 595–596 UDP 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

RTP Real-Time Transport Protocol 596–597 RTP 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

597–599 Multicasting 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 and Multicasting 598 Unicasting and Multicasting

HTTP HyperText Transport Protocol 600–601 HTTP 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 Messages String of 8-bit characters 601 HTTP Messages String of 8-bit characters Request line (request) or status line (response) One or more headers Blank line Body (optional)

HTTP Requests Request line Method – name of service being requested 601 HTTP Requests 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

Headers Header name : arguments Host: www.digitalmultimedia.org 601–602 Headers Header name : arguments Host: www.digitalmultimedia.org 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

HTTP Responses Status line Protocol version Numerical status code 602–603 HTTP Responses Status line Protocol version Numerical status code 200 = OK, 400 = bad request, 404 = not found, etc Status description e.g. HTTP/1.1 200 OK

HTTP Responses Headers – same format as for requests 602 HTTP Responses Headers – same format as for requests e.g. Server: Apache/1.3.27 (Darwin) Body contains data, e.g. HTML code of a requested page

604–605 Caching 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

RTSP Real Time Streaming Protocol 'Internet VCR remote control' 606 RTSP 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)

Quality of Service (QoS) 609–611 Quality of Service (QoS) Quantifies the amount of Delay Jitter Packet loss an application can tolerate ATM (Asynchronous Transfer Mode) networks can offer QoS guarantees

Server-side Computation 611–614 Server-side Computation 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