IS 3'6 Network Services

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IS 3.6Network Services
Application
Presentation
Session
Transport
Network

• M.H. Rahman

Data Link
Physical
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Network Services

• There are a variety of service provided by the
  network. Some of the services are
• FTP and NFS
• Domain Name Service
• Electronic Mail.
• Directory Service.
• HTTP.

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FTP (File Transfer Protocol)

• A file transfer service moves a copy of a file
  from one computer to another.
• The most popular file transfer service is FTP. It
  uses TCP ports 20 data) 21(control).
• FTP requires a users to establish a control
  connection to a remote computer before files can
  be transferred.
• The control connection remains in place until the
  file transfer is complete.

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NFS (Network File Service)

• FTP can be used to transfer a file from one
  computer to another, but it does not allow one to
  modify the file in location.
• If a file needs to be modified, it must be copied
  to a local computer, modified and then
  transferred back again. This is not very
  convenient, especially for large files.
• NFS allows a computer to have direct access to a
  file in another computer. One can open a file,
  read, modify, rewrite without transferring the
  whole file to a local computer.

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Domain Name Service

• Although each computer in the Internet has an IP
  address (32 bit binary), it is not a convenient
  notation for use by humans. For convenience,
  computer are given names for use by humans.
• The scheme used in the Internet is called Domain
  Name System. The associated server is called the
  Domain Name Server

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Domain Name Service

• Domain names are organized in an hierarchical
  structure.
• Each computer, associated with an IP address, is
  given a name.
• The DNS specifies values for the most significant
  segment, called level DNS. These are registerd by
  a central authority, ICANN (Internet Corporation
  for Assignment of Names and Numbers).

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Top Level Domain Names

• To level categories

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DNS hierarchy (an example)
Footbar is the company with two branches. Candy
has three different factories. Each one has a
separate computer. Note that only footbar.com is
registered with ICANN
com
footbar
soap
candy
walnut
peanut
almond
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DNS Client -Server Model

• One of the main features of DNS is autonomy. The
  naming hierarchy helps achieving such autonomy by
  allowing an organization to control names with a
  particular suffix
• Footbar can create candy and soap without
  informing the central authority.
• Whenever an application needs to translate a name
  in to IP address, the application becomes a
  client and of the naming system. It places names
  to the DNS server, which in turn provides the IP
  address.

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DNS Server Hierarchy
Root server
com
Server for footbar.com
footbar
soap
candy
Server for candy.footbar.com
walnut
peanut
almond
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Link among servers

• All Domain name servers are linked together to
  form a unified system. Each server knows how to
  reach a root server and how to reach servers that
  are authorities for names further down the
  hierarchy

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Resolving a name

• The resolver software in a host (server)
  always requests recursive resolution in which a
  name is equivalent to address. When it becomes
  client of another server, a server can request
  iterative resolution to step through the server
  hierarchy one level at a time.

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Type of DNS entry

• Each entry in a DNS database consists of three
  items a domain name, a record type and a value.
• The record type specifies how the value is to be
  interpreted.
• e.g. Type A binds a domain name to an IP address.
• Type MX binds a domain name found in an email
  address to an IP address.
• A domain name server only returns an IP address
  if the type matches.

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Final remark

• There are many issues related to DNS that have
  not been covered. You will find some of them in
• Computer Networks and Internet by Douglas E.
  Comer, Prentice Hall, 1998.

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Electronic Mail

• A basic (single system) electronic mail system
  performs four functions
• Creation A user creates and edits a message
  using an editor or word processor.
• Sending The user designates the recipient of the
  message, and the facility stores the message
• Reception The intended recipient may invoke the
  email facility to access and read the message
• Storage Both sender and recipient may choose to
  save the message

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Electronic Mail

• To extend the system for network mail one also
  needs
• Some sort of network for transfer of message to
  another native email system
• Every users system then needs some communication
  I/O.
• The email software is an application layer
  software and the network support will be part of
  the layer 1 to 6 of the ISO model (1 to 4 of the
  TCP/IP model).

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Electronic Mail

• Conceptual structure of an email system

comm. I/O
terminal handler
Terminals
native mail
mail transfer
File I/o
mailboxes
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Electronic Mail Functional view of the X.400
model
Message Handling Environment

user
UA
MHS
MTS
MTA
UA
user
MTA
MTA
UA
user
UA
user
MTA
MTA
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Electronic Mail

• Definitions
• User People, process
• UA User Agent
• Application process which makes the service of
  the MTS accessible to the user
• It generates, sends, receives and archives
  messages
• MTS Message transfer system
• Provides the message transfer service from sender
  UA to recipient UA

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• Definitions (contd.)
• MTA Message transfer agent
• Application process which forwards incoming
  messages to the next MTA or the destination

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Electronic Mail

• Examples of physical realization of an email syst.

UA
MTA
UA
UA
MTA
UA
MTA
Single system
Single user PCs
UA
UA
MTA
MTA
UA
UA
Multiple users
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Electronic Mail

• X.400 message structure

envelope (MTL)
Rahman-mh_at_rmc.ca
envelope
Date 00-10-31 From Hafiz Rahman To Bernard
heading (UAL)
P2 report
control message
actual message
Body
IP message format
example
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Electronic Mail

• Envelop
• contains the information required in the
  switching and forwarding of the message via MTS
• Body
• contains the information generated by the user
• Heading IP message ID, From To etc.
• IPM Report Receipt notification, error,
  non-delivery etc.

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Electronic MailNames and Addresses

• Name Unique designation of an object (user)
• Address Unique designation of an object and its
  location. The address have the following
  information
• Country name
• Administrative domain name
• private domain name
• organization name
• organizational unit name
• personal name

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X.500 Directory Services

• Address
• Position of a real object, needed to generate
  communication relationships between real objects
• Directory Service
• The main task is the assignment of a set of
  values to the name of a real object address,
  additional info.

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X.500 Directory Services

• Contents of a DIT (an example)

Root
CCA
Country
Organization
ODND
Organizational Unit
OURMC
Nrahman-mh
Name
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X.500 Directory Services

• Access to Directory System

USER
DUA
USER
DUA
Directory System
USER
USER
DUA
DUA
USER
DUA
USER
DUA
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X.500 Directory Services

• The Directory system provides access to the user
  in the form of operations via an access point
• A user uses these operations with the assistance
  of an DUA (Directory User Agent)

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X.500 Directory Services

• Directory system are distributed and maintained
  by various institutions
• Directory system is made up of DSA (Directory
  System Agents)

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X.500 Directory Services

• Directory System

USER
DUA
USER
DUA
DSA
DSA
USER
USER
DUA
DUA
DSA
DSA
USER
DUA
USER
DUA
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X.500 Directory Services

• Top level DIT for DND

Root
CCA
Country Canada
OGC
Organization Government of Canada
OUDND.MDN
Organizational Unit Dept. of national
Defence Ministere de la Defence nationale
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HTTP

• Web documents use the HyperText Markup Language
  (HTML) to store documents. HTML contains general
  guidelines for display.
• When a browser interacts with a web server, the
  two programs follow a specific protocol, called
  HyperText Transport Protocol (HTTP). HTTP allows
  a browser to request a specific item which the
  server returns.
• Unlike FTP, HTTP connections are for a short
  duration. The connection remains long enough to
  transfer the specific item requested.
• If the browser need more items, another
  connection is set up.

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HTTP
Components of a web browser
D r I v e r
Output to display
HTML Interpreter
Controller
Input from keyboard
Optional interpreter
HTTP Client
Optional client
Network Interface
Communication with network server