Computer Science

Q.  What are the main differences between OSI and TCP/ IP reference models? Explain briefly.

Ans: We will be focusing only on the key differences between the two references models. Three concepts are central to OSI model: services, interfaces and protocols. OSI model makes the clear distinction between these three concepts. The TCP/ IP model did not originally clearly distinguish between services, interface, and protocol. For example the only real services offered by the Internet layer are SEND IP packet and RECEIVE IP packet. The OSI reference model was devised before the protocols were invented. This ordering means that model was not biased towards one particular set of protocols, which made it quite general. With TCP/ IP reverse was true: the protocol came first, and the model was really just a description of the existing protocols. So problem was model did not fit for any other protocol stack. Another difference is in the area of connectionless versus connection oriented communication. The OSI model supports both connectionless and connection oriented communication in network layer, but only connection oriented in the transport layer. The TCP/ IP model has only connectionless mode in network layer but supports both the mode in transport layer.

Q.4 Define a socket? How read and write is performed using sockets?

Ans: An application program interface specifies the details of how an application program interacts with protocol software. Socket API is a defacto standard. Once a socket has been established the application can transfer information.
recv( ) and send( ) are used to read and write the data.
recv(socket , buffer, length, flags)
The socket is the descriptor of the socket, buffer specifies the address in memory where incoming message should be placed and length specifies the size of the buffer, flags al lows the caller to control details.
send(socket , data, length, f lags)
Here data is the address of data to be sent and other arguments are same. Sockets also al lows read( ) and write( ) to transfer data like send ( ) and recv( ). read( ) and write() have three arguments: a socket descriptor, the location of the buffer in the memory and the length of the memory buffer.

Q.  When web pages containing emails are sent out they are prefixed by MIME Header. Why?

Ans: Initially email consisted messages containing simple text written in English and expressed in ASCII. Now a days on world wide internet messages can be sent in languages with accents like French and German, languages without alphabet like Chinese and Japanese etc. the basic idea of MIME is to add structure to the message body and define encoding rule for non- ASCII messages.
MIME defines five additional message headers to the RFC 822 format .

Q.  What is trivial file transfer protocol. Explain briefly?

Ans: Trivial File Transfer Protocol (TFTP) is useful for bootstrapping a hardware device that does not have a disk on which to store system software. Al l the device needs is a network connect ion and a small amount of read only memory (ROM) into which TFTP, UDP and IP are hardwired. Al though TFTP is les powerful than FTP. TFTP does have two advantages. First , TFTP can be used in environments where UDP is available, but TCP is not . Second the code for TFTP requires less memory than the code for FTP.

Q.  What are the various address Translation schemes? Explain which scheme is used in Internet?

Ans: Translation from a computer’s protocol address to an equivalent hardware address is known as address resolution. Address resolution algorithms can be grouped into three basic categories:

Table lookup- Bindings or mapping are stored in a table in memory, which the software searches when it needs to resolve an address.

Closed Form Computation- The protocol address assigned to a computer is chosen carefully so the computer’s hardware address can be computed from the protocol address using basic Boolean and arithmetic operations.

Message Exchange- Computer exchange messages across a network to resolve an address. One computer sends a message that requests an address binding (i.e.,translation) , and another computer sends a reply that contains the requested information.TCP/ IP can use any of the three methods; the method chosen for a particular network depends on the addressing scheme used by underlying hardware. Generally third scheme is used over internet. The TCP/ IP suite contains a standard address resolution protocol (ARP). ARP defines the format of the messages that computers exchange to resolve an address as well as rules for handling ARP messages.

Q.  How many Octets does the smallest possible IPV6 datagram contain? Explain the significance IPV6 over IPV4.

Ans: The maximum size of an Ipv6 datagram is 65575 bytes, including the 0 bytes Ipv6 header. Ipv6 also define a minimum reassembly buffer size: the minimum datagram size that we are guaranteed any implementation must support . The minimum size for Ipv6 datagram is 1500 bytes.

Despite retaining the basic concepts from IPv4, IPv6 changes al l the details. IPv6 uses larger addresses and an entirely new datagram format . IPv6 uses a series of fixed-length headers to handle header information. Thus, unlike IPv4, which places key information in fixed fields of the header and only appends variable-length opt ions for less important information, the IPv6 header is always variable size.