COMPUTER NETWORKS – UNIT -5 Y3/S5
UNIT – V
APPLICATION LAYER
The Application Layer- DNS - Name Space - Resource Records - Name Servers- E-Mail - Architecture And
Services – The User Agent –Message Format –Message Transfer –Final Delivery – WWW –Architecture –
Static Web Pages – Dynamic Web Pages And Web Applications –HTTP –Network Security –Introduction
To Cryptography –Substitution Ciphers-Transposition Ciphers-Public Key Algorithms – RSA –
Authentication Protocols –Authentication Using Kerberos.
DOMAIN NAME SYSTEM
This is primarily used for mapping host and e-mail destinations to IP addresses but can also be used
other purposes. DNS is defined in RFCs 1034 and 1035.
Working:-
To map a name onto an IP address, an application program calls a library procedure called Resolver,
passing it the name as a parameter.
The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP
address to the resolver, which then returns it to the caller.
Armed with the IP address, the program can then establish a TCP connection with the destination, or
send it UDP packets.
1. The DNS name space.
2. Resource Records.
3. Name Servers.
1. THE DNS NAME SPACE:
The Internet is divided into several hundred top level domains, where each domain covers many hosts.
Each domain is partitioned into sub domains, and these are further partitioned as so on. All these domains can
be represented by a tree, in which the leaves represent domains that have no sub domains. A leaf domain may
contain a single host, or it may represent a company and contains thousands of hosts. Each domain is named by
the path upward from it to the root. The components are separated by periods (pronounced “dot”)
Eg: Sun Microsystems Engg. Department = eng.sun.com.
The top domain comes in 2 flavours:-
Generic: com(commercial), edu(educational instructions), mil(the U.S armed forces, government), int
(certain international organizations), net( network providers), org (non profit organizations).
MASTER OF COMPUTER APPLICATIONS Page 1
, COMPUTER NETWORKS – UNIT -5 Y3/S5
Country: include 1 entry for every country. Domain names can be either absolute (ends with a period
e.g. eng.sum.com) or relative (doesn’t end with a period). Domain names are case sensitive and the
component names can be up to 63 characters long and full path names must not exceed 255 characters.
Figure 5-1. A portion of the Internet domain name space.
Insertions of a domain into the tree can be done in 2 days:-
• Under a generic domain ( Eg: cs.yale.edu)
• Under the domain of their country (E.g: cs.yale.ct.us)
2. RESOURCE RECORDS:
Every domain can have a sent of resource records associated with it. For a single host, the most common
resource record is just its IP address. When a resolver gives a domain name to DNS, it gets both the resource
records associated with that name i.e., the real function of DNS is to map domain names into resource records.
A resource record is a 5-tuple and its format is as follows:
Domain Name Time to live Type Class Value
Domain _name : Tells the domain to which this record applies.
Time- to- live : Gives an identification of how stable the record is (High Stable = 86400 i.e. no. of seconds
/day) ( High Volatile = 1 min)
Type: Tells what kind of record this is.
Class: It is IN for the internet information and codes for non internet information
Value: This field can be a number a domain name or an ASCII string
MASTER OF COMPUTER APPLICATIONS Page 2
, COMPUTER NETWORKS – UNIT -5 Y3/S5
3. NAME SERVERS:
It contains the entire database and responds to all queries about it. DNS name space is divided up into non-
overlapping zones, in which each zone contains some part of the tree and also contains name servers holding the
authoritative information about that zone.
Figure 5-2. Part of the DNS name space showing the division into zones.
When a resolver has a query about a domain name, it passes the query to one of the local name servers:
1. If the domain being sought falls under the jurisdiction of name server, it returns the authoritative resource
records (that comes from the authority that manages the record,and is always correct).
2. If the domain is remote and no information about the requested domain is available locally the name server
sends a query message to the top level name server for the domain requested.
E.g.: A resolver of flits.cs.vle.nl wants to know the IP address of the host Linda.cs.yale.edu
Figure 5-3. How a resolver looks up a remote name in eight steps.
MASTER OF COMPUTER APPLICATIONS Page 3
UNIT – V
APPLICATION LAYER
The Application Layer- DNS - Name Space - Resource Records - Name Servers- E-Mail - Architecture And
Services – The User Agent –Message Format –Message Transfer –Final Delivery – WWW –Architecture –
Static Web Pages – Dynamic Web Pages And Web Applications –HTTP –Network Security –Introduction
To Cryptography –Substitution Ciphers-Transposition Ciphers-Public Key Algorithms – RSA –
Authentication Protocols –Authentication Using Kerberos.
DOMAIN NAME SYSTEM
This is primarily used for mapping host and e-mail destinations to IP addresses but can also be used
other purposes. DNS is defined in RFCs 1034 and 1035.
Working:-
To map a name onto an IP address, an application program calls a library procedure called Resolver,
passing it the name as a parameter.
The resolver sends a UDP packet to a local DNS server, which then looks up the name and returns the IP
address to the resolver, which then returns it to the caller.
Armed with the IP address, the program can then establish a TCP connection with the destination, or
send it UDP packets.
1. The DNS name space.
2. Resource Records.
3. Name Servers.
1. THE DNS NAME SPACE:
The Internet is divided into several hundred top level domains, where each domain covers many hosts.
Each domain is partitioned into sub domains, and these are further partitioned as so on. All these domains can
be represented by a tree, in which the leaves represent domains that have no sub domains. A leaf domain may
contain a single host, or it may represent a company and contains thousands of hosts. Each domain is named by
the path upward from it to the root. The components are separated by periods (pronounced “dot”)
Eg: Sun Microsystems Engg. Department = eng.sun.com.
The top domain comes in 2 flavours:-
Generic: com(commercial), edu(educational instructions), mil(the U.S armed forces, government), int
(certain international organizations), net( network providers), org (non profit organizations).
MASTER OF COMPUTER APPLICATIONS Page 1
, COMPUTER NETWORKS – UNIT -5 Y3/S5
Country: include 1 entry for every country. Domain names can be either absolute (ends with a period
e.g. eng.sum.com) or relative (doesn’t end with a period). Domain names are case sensitive and the
component names can be up to 63 characters long and full path names must not exceed 255 characters.
Figure 5-1. A portion of the Internet domain name space.
Insertions of a domain into the tree can be done in 2 days:-
• Under a generic domain ( Eg: cs.yale.edu)
• Under the domain of their country (E.g: cs.yale.ct.us)
2. RESOURCE RECORDS:
Every domain can have a sent of resource records associated with it. For a single host, the most common
resource record is just its IP address. When a resolver gives a domain name to DNS, it gets both the resource
records associated with that name i.e., the real function of DNS is to map domain names into resource records.
A resource record is a 5-tuple and its format is as follows:
Domain Name Time to live Type Class Value
Domain _name : Tells the domain to which this record applies.
Time- to- live : Gives an identification of how stable the record is (High Stable = 86400 i.e. no. of seconds
/day) ( High Volatile = 1 min)
Type: Tells what kind of record this is.
Class: It is IN for the internet information and codes for non internet information
Value: This field can be a number a domain name or an ASCII string
MASTER OF COMPUTER APPLICATIONS Page 2
, COMPUTER NETWORKS – UNIT -5 Y3/S5
3. NAME SERVERS:
It contains the entire database and responds to all queries about it. DNS name space is divided up into non-
overlapping zones, in which each zone contains some part of the tree and also contains name servers holding the
authoritative information about that zone.
Figure 5-2. Part of the DNS name space showing the division into zones.
When a resolver has a query about a domain name, it passes the query to one of the local name servers:
1. If the domain being sought falls under the jurisdiction of name server, it returns the authoritative resource
records (that comes from the authority that manages the record,and is always correct).
2. If the domain is remote and no information about the requested domain is available locally the name server
sends a query message to the top level name server for the domain requested.
E.g.: A resolver of flits.cs.vle.nl wants to know the IP address of the host Linda.cs.yale.edu
Figure 5-3. How a resolver looks up a remote name in eight steps.
MASTER OF COMPUTER APPLICATIONS Page 3
