CS 372 COMPUTER COMMUNICATION AND NETWORKS Muneera Bano Assistant Professor, DSE, FBAS, IIUI

Application Layer In TCP/IP, the Application Layer contains all protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network using the Transport Layer protocols to establish underlying host-to-host connections. E.g. Domain Name Service (DNS) World Wide Web (HTTP) Electronic Email (SMTP, POP) File Transfer (FTP) Remote Login (Telnet) December 23, 2010 CS 372 Computer Communication and Networks

Name Space A Name space that maps each address to unique name can be organized in two ways: Flat Name Space: Name is assigned to an address Name is a sequence of characters without structure. Hierarchical Name Space: Each name is made of several parts First part can define the nature of the organization, second part can define the name, the third part can define the department and so on. December 23, 2010 CS 372 Computer Communication and Networks

Domain Name Space To have hierarchical name space, domain name space is designed. Names are defined in an inverted-tree structure with the root at the top. Tree can have only 128 levels December 23, 2010 CS 372 Computer Communication and Networks

Domain name space December 23, 2010 CS 372 Computer Communication and Networks

Domain Names and Labels String with maximum of 63 characters. Root label is a null string. Domain Name: Sequence of labels seperated by dots(.) Names are always read from the node up to the root. December 23, 2010 CS 372 Computer Communication and Networks

FQDN(Fully Qualified Domain Name) If a label is terminated by a null string, it is called a FQDN. It is a domain name that contains the full name of a host. Contains all the labels, from the most specific to the most general Example: Challenger.atc.fhda.edu December 23, 2010 CS 372 Computer Communication and Networks

PQDN(Partially Qualified Domain Name) If a label is not terminated by a null string, it is called a PQDN. Starts from a node, but doesnot reach the root. Used when name to be resolved belongs to the same site as a client. Resolver can supply the missing part called as a suffix. December 23, 2010 CS 372 Computer Communication and Networks

Hierarchy of Name Servers Information is distributed among many computers called DNS servers. Whole space is divided into many domains based on first level. DNS allows domains to be divided further into smaller domains. December 23, 2010 CS 372 Computer Communication and Networks

Cont… December 23, 2010 CS 372 Computer Communication and Networks

DNS in the Internet DNS is a protocol that can be used in different platforms. In the internet the DNS(tree) is divided into three different sections Generic domain Country domain Inverse domain December 23, 2010 CS 372 Computer Communication and Networks

Cont…… Generic Domain: Define registered hosts according to their generic behaviour. December 23, 2010 CS 372 Computer Communication and Networks

Country Domain Follows the same format as generic but uses two-character country abbreviations in place of three-character organizational abbreviations . December 23, 2010 CS 372 Computer Communication and Networks

Inverse Domain Used to map an address to a name. December 23, 2010 CS 372 Computer Communication and Networks

Recursive Resolution Client (resolver) can ask for recursive answer from a name server. If the server is the authority for the domain name, it checks its database and responds. If server is not the authority, it sends the request to another server and waits for response. When query is finally resolved, response travels back until it finally reaches the requesting client. December 23, 2010 CS 372 Computer Communication and Networks

Cont… December 23, 2010 CS 372 Computer Communication and Networks

Iterative Resolution If server is an authority for a name, it sends the name. If it is not, it returns the IP Address of the server that it thinks can resolve the query. The client is responsible for repeating the query to this second server. December 23, 2010 CS 372 Computer Communication and Networks

Cont… December 23, 2010 CS 372 Computer Communication and Networks

Caching Each time a server receives a query for a name that is not in its domain, it needs to search its database for a server IP address. DNS handles this with Caching. DNS stores the information in its cache memory before sending it to client. Reduction of search time increases the efficiency. December 23, 2010 CS 372 Computer Communication and Networks

nslookup Utility nslookup is a network administration command-line tool available for many computer operating systems for querying the Domain Name System (DNS) to obtain domain name or IP address mapping or for any other specific DNS record. December 23, 2010 CS 372 Computer Communication and Networks

nslookup Utility December 23, 2010 CS 372 Computer Communication and Networks

Q & A

Reference Chapter 25,26 “Application Layer” Book: Data Communication and Networking (4th Edition) Auther: Behrouz A. Forouzan CS 372 Computer Communication and Networks