Network overview

1
Network overview

• Teacher
• Jerry Xu
• New college
• Southampton University

2
Course resources

• Web page http//members.lycos.co.uk/daxu/
• Email
• jc1552002_at_YAHOO.COM
• daxu_at_soton.ac.uk

3
More about books

• More about books
• Data communications, computer networks and open
  systems 4th Edition Fred Halsall,
  Addison-Wesley (1996) ISBN 020142293X
• Data and Computer Communications 6th
  EditionWilliam Stallings Macmillan (1994) ISBN
  0024154415

4
Question

• Seven layers

5
This Session

• Network examples
• Application layer
• HTTP is the most important

6
Packet vs Circuit switching

• Circuit switching
• circuit switching an end-to-end circuit must be
  set up before the call can begin
• A fixed share of network resources is reserved
  for the call
• No other call can use those resources until the
  original connection is closed.

7
Packet vs Circuit switching

• Packet switching
• the data stream is broken up into packets of
  about 200 bytes (on average)
• Each packet contains a header'' with
  information necessary for routing.
• each packet in a data stream is independent
• permits statistical multiplexing'' on the
  communications lines (share a line )

8
Cost-Effective Resource Sharing

• Must share (multiplex) network resources (nodes
  and
• links) among multiple users.
• Common Multiplexing Strategies
• Time-Division Multiplexing (TDM)
• Frequency-Division Multiplexing (FDM)

9
Statistical Multiplexing

• Time-division, but on demand rather than fixed
• Reschedule link on a per-packet basis
• Packets from different sources interleaved on the
  link
• Buffer packets that are contending for the link
• Packet queue may be processed FIFO
• Buffer overflow is called congestion

10
The simple network

• Serial line communication
• Serial port
• Parallel line communication
• Parallel port

11
The simple network

• Use modem to connect serial line
• 
  communication

Serial line
12
Limitations of modem

• Speed
• Need telephone line
• Not good for company usage

13
An office network example
Hub or router
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16
Application layers
17
Application Layer

• network applications and protocols
• www http 1.1
• DNS domain name services
• Email SMTP, POP3
• FTP services
• Stream
• RTSP
• Teleconferences
• SIP
• H.323
• SNMP

18
HTTP Review
SERVER
CLIENT

• HTTP response
• HTTP/1.0 200 OK
• Date Thus, 08 Oct 2002 181752 GMT
• Server Apache/1
• content-type text/html
• content-length 1300
• Last-modified Mon, 03 Oct 2002 120000 GMT
• hello.html

• HTTP request
• Get hello.html HTTP/1.1
• User-Agent IE 4.0
• Host www.example.com
• Accept image/gif,, /

19
Naming Examples

• Hosts
• www.hotmail.com -gt 64.4.53.7
• 64.4.53.7 -gt8023ab335b9F
• Users
• Jerry -gt daxu_at_hotmail.com

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DNS

• Domain Name System
• DNS name space
• Resource Records
• Name Servers

21
DNS Name Space
22
How DNS works
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Resource Records

• Each NS implements the zone information as a
  collection of Resource Records
• A Resources Record is a name-to-value binding
• Name, Value,Type, Class, TTL
• Name cisco.com
• Value
• Type specify how the Value should be
  interpreted
• A IP Address
• NS next Name Server to connect
• CNAME aliases (convenient, level of indirection,
  easy to move server without affecting users)
• MX mail server
• Class IN (Internet)
• TTL How long this resource record is valid.

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Record examples

• ltcs.princeton.edu,gnat,cs.princeton.edu,NS,INgt
• ltgnat.cs.princeton.edu,192.13,19.44,A,INgt
• ltee.princeton.edu,helios.ee.princeton.edu,NS,INgt
• lthelios.ee.princeton.edu,192.14.44.32,A,INgt
• ltcic.cs.princeton,cicada.cs.princeton.edu,CNAME,IN
  gt
• ltcicada.cs.princeton.edu,192.12.96.60,A,INgt

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Name Servers

• DNS name space showing the division into zones

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DNS example
27
Electronic Mail (Email)

• Architecture and Services
• User Agent
• Message Format
• Message Transfer

28
Message Formats

• RFC822 defined the header fields
• For message transport
• MIME
• Multipurpose Internet Mail Extensions
• International languages
• Messages contain multimedia

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Message Transfer Example

• Transfer a message from a_at_abc.com to
• b._at_xyz.com
• S 220 xyz.com SMTP service ready
• HELO abc.com
• S 250 xyz.com says hello
• MAIL FROM lta_at_abc.comgt
• S 250 sender ok

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Message Transfer Example

• RCPT TO b_at_xyz.com
• S 250 recipient OK
• DATA
• S 354 Send mailend with . on a line by itself
• From a_at_abc.com
• To b_at_xyz.com
• MIME version1
• Message ID
• Content-type
• Subject hello
• Hello
• .
• S 250 message accepted
• QUIT
• S221 xyz.com closing connection

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POP3

• Fetch messages
• SOK POP3 server ready
• C USER a
• S OK
• PASS b
• SOK login successful
• CList

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POP3

• S 1 3456
• 2 14321
• C RETR 1
• ssends message 1
• C QUIT
• s OK POP3 server disconnected

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Email System
34
Telnet and FTP

• Telnet
• A little old
• FTP
• Transfer a copy of a file on one computer to
  another

35
H.323

• technology for the transmission of real-time
  audio, video, and data communications over
  networks.
• H.323 can be applied in a variety of mechanisms
  audio only (IP telephony) audio and video (video
  telephony) audio and data.

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H.323
37
H.323
38
Network tip

• How to use ping?
• Why useful?
• When your network is dead
• Ping 127.0.0.1
• Ping your IP
• Ping your gateway
• Ping your DNS
• Attack people

39
Summary

• Application layer
• http
• Email
• DNS
• Sockets