Transcript 17.Networks.Chapman

Multimedia and
Networks
Digital Multimedia, 2nd edition
Nigel Chapman & Jenny Chapman
Chapter 17
This presentation © 2004, MacAvon Media Productions
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591
Protocols
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Rules governing the exchange of data over
networks
Conceptually organized into stacked layers
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Application-oriented services (e.g. file
transfer, Web browsing)
Transfer of raw data
Physical signals over wires, optical fibres,
etc.
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Packets
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TCP/IP networks, including the Internet, are
packet-switched networks
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Messages split into small pieces called
packets, sent separately
Messages are multiplexed
Enables network bandwidth to be shared
efficiently between many messages
© 2004, MacAvon Media Productions
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592–593
IP
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Internet Protocol, defines
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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
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Set of four numbers, uniquely identifying
the network and host
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IP
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Attempts to deliver each datagram individually
from source to destination host
Datagrams not delivered after specified time
are discarded
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Message may arrive with some packets
missing
Routes calculated dynamically
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Packets may arrive in the wrong order
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593–594
TCP
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Transmission Control Protocol
Provides reliable delivery of sequenced packets
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Requests retransmission of missing packets
Puts packets back into correct order
Based on acknowledgements, using a sliding
window of unacknowledged packets
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May lead to some packets being sent more
than once
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Transport Addresses
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Packets must be sent to the right application
(e.g. Web browser, not email client)
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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
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UDP
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User Datagram Protocol
Ensures packets are delivered to right
application
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Uses transport addresses
Does not guarantee delivery
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Suitable for networked multimedia where
lost packets more acceptable than overhead
of TCP
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596–597
RTP
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Real-Time Transport Protocol
Runs on top of UDP, adds extra features for
sequencing etc
Header identifies the type of payload (video,
audio, etc)
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Format of payload optimized for the type of
data
Sequence numbers and timestamps used to
reorder packets and synchronize separately
transmitted streams
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597–599
Multicasting
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Unicast – server sends a copy of e.g. video
data stream to every client
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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
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Hosts must be assigned to host groups,
using a range of reserved IP addresses
Needs enhanced routers
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Unicasting and Multicasting
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600–601
HTTP
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HyperText Transport Protocol
Client opens TCP connection to the server
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Server's name is usually extracted from a
URL, mapped to an IP address via DNS
Client sends a request and receives a response
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Requests and responses are both messages
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HTTP Messages
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String of 8-bit characters
Request line (request) or status line (response)
One or more headers
Blank line
Body (optional)
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HTTP Requests
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Request line
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Method – name of service being requested
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e.g. GET
Identifier – resource being requested
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e.g. path name of a file
Version
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e.g. HTTP/1.1
e.g. GET DMM2/links/index.html HTTP/1.1
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Headers
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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
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Accept: */*
Accept-Language: en-us, ja;q=0.33, en;q=0.67
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602–603
HTTP Responses
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Status line
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Protocol version
Numerical status code
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200 = OK, 400 = bad request, 404 = not
found, etc
Status description
e.g. HTTP/1.1 200 OK
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HTTP Responses
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Headers – same format as for requests
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e.g. Server: Apache/1.3.27 (Darwin)
Body contains data, e.g. HTML code of a
requested page
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604–605
Caching
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Copies of pages that have been retrieved are
kept in a cache on user's machine or proxy
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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
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Browser displays page from cache
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RTSP
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Real Time Streaming Protocol
'Internet VCR remote control'
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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)
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609–611
Quality of Service (QoS)
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Quantifies the amount of
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Delay
Jitter
Packet loss
an application can tolerate
ATM (Asynchronous Transfer Mode) networks
can offer QoS guarantees
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611–614
Server-side Computation
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Allows an HTTP server to communicate with
other resources (e.g. databases) to generate
Web pages dynamically
CGI (Common Gateway Interface)
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Receives data from HTTP request (e.g. form
data)
Returns HTTP response
PHP, ASP, JSP, ColdFusion, etc
© 2004, MacAvon Media Productions