Transcript Network slides

• 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.
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
network
– 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
internet
– Set of four numbers,
uniquely identifying the
network and host
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
• 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
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
• 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
•
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
Unicasting & 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
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
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'
– 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)
• Quantifies the amount of
–
–
–
–
Delay
Jitter
Packet loss
an application can tolerate
• ATM (Asynchronous Transfer
Mode) networks can offer QoS
guarantees
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