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Application Layer Protocols

Domain Name Server (DNS)

It is used to keep track of computer , services , resource both in LAN and WAN environment , to map an URL to an IP address and both TCP as well as UDP services on port 53.It use 4 server for application:
(i) Root server
(ii) Top level domain server
(iii) Authoritative server
(iv) Local DNS

It offer 4 services:
(i) Name Translation
(ii) Host Aliasing
(iii) Mail Aloasing
(iv) Load Balancing
It use distributed database to store information in terms of records.

File Transfer Protocol (FTP)

It takes services of TCP
  - Data(20)
  - Control(21)
It has set of status codes and error codes.
It has 3 modes of operation:
  (i) Active
  (ii) Passive,and
  (ii) Extended passive mode
There are 2 modes of access:

Hyper Text Transfer Protocol (HTTP)

It takes services of TCP on port 80.
It is "stateless protocol"since it does not have any mapping from one transaction onto other and treats a request and reply as a pair every-time.
It offers security through secure socket layer ( a protocol for transmitting private document via internet).It is used to access web-pages from www.
It has a set of status codes to keep track of operation and It has 8 methods for its operation.They are  
   (i) Head: Used to describe client
  (ii) Get: Used to get resources from authoritative server
  (iii) Put: Used to create object.
  (iv) Post: Used to modify the existing order or to enhance the object
  (v) Delete: Used to remove the object.
  (vi) Trace: Used by developers only to check the error + debug the network.
  (vii) Connect: Offers secured services 
  (viii) Options: Optimizes the website option i.e. to design website optimally.   
There are two type of connection in HTTP.
  (i )Non Persistent: For every process the connection is established newly.
  (ii) Persistent: A single TCP connection is set on which multiple request and response can be made.

Simple Mail Transfer Protocol

It uses port 25 on TCP.
Components of SMTP are:
   (i) User Agent (UA)
   (ii) Mail Transfer Agent (MTA)
   (iii) Multipurpose Internet Mail Extension (MIME)
   (iv) Mail Acess Protocol (MAP)
       (a) POP3
       (b) Internet MAP (IMAP4)

Internet Control Messaging Protocol (ICMP)

It is used for error reporting and query messages which help in network debugging and encapsulated into an IP data gram and then transmitted into the network,if the protocol field in IP datagram is 1 then the IP data-gram is said to be carrying ICMP message.It uses the services of TCP amd UDP on port 7 as ping command.

Type of Messages:
(i) Error reporting:
   (a) Destination unreachable
   (b) Source quench
   (c) Parameter problem
   (d) Time exceeded
   (e) Redirection
(ii) Query Message:
   (a) Router Solicitation and Router Advertisement
   (b) Address Mask Request and Reply
   (c) Time Stamp Echo Request and Reply
   (d) Echo Request and Reply

Post office Protocol Version 3 (POP 3)

It is a pull protocol which takes services of TCP on port 110.
At present IMAP4 is most used for MAP services.

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