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

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 595
Transport Addresses
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

8. 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

9. 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

10. 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

11. Unicasting and Multicasting
598
Unicasting and Multicasting

12. 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

13. 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)

14. 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

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

16. 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/ OK

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

18. 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

19. 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)

20. 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

21. 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