• 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• 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

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

• 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

• 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

Transport Addresses

• 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

• 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

• 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

• 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

• 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

• 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

• 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