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Makefile
README
alphabet.txt
netcat_part.c
segments.eng

README

PROJECT 1: SOCKET PROGRAMMING: netcat_part

************************

Name: Sravya Gudipudi
uname: sgudipud

Name: Avinash Ravi
uname: avinravi

************************

What's in the code?
---------------------
This application enables to transfer data over the network from an end system to a remote server and 
has got two functions: a Client function and a Server function.

The CLIENT function when executed opens a socket and establishes connection with the server. It then 
reads the input from command line arguments( for the message mode) or from the file specified in the 
command line and stores it in a buffer. The buffer is now divided  into chunks of 512 bytes, HMAC is 
calculated for each chunk and appended  to the data in the front. Then we write this resulting chunk 
of data to the socket. The format of data we write over the socket is HMAC Digest Length + HMAC Digest 
+ 512 Bytes of Data. Here, a delay is introduced while writing second chunk of data into the socket to 
prevent the server from getting overloaded with the incoming data. Once the complete data is written 
to socket, the socket connection is closed.

The SERVER function opens a server connection and listens for incoming connections. When there is an 
incoming connection from the client, server accepts the connection if it is idle or places the request 
in a queue  if the server is busy. After a  connection is  established with  the client, it reads the 
data from the socket. The HMAC message digest, digest length and the actual message are extracted from 
the data received over the socket. HMAC digest is calculated for the incoming message and compared with 
digest received over the socket. If both the message digests are equal, the message is written to the 
output file. Once the complete message is received, the connection is closed.

Tasks Accomplished
---------------------
-> Establishing a TCP connention and sending/receiving data over ther network.
-> Reading data from the given file in Client and writing the received data to an output file at Server.
-> Sending data as chunks of 512 bytes from Client to Server.
-> At client, HMAC digest is calculated for each chunk of data and this digest along with the length of 
   the digest is appended infront of the actual message.
-> At Server, for each packet received, a new digest is calculated for the actual message and compared 
   with the digest sent along with the data by the client.

Compiling the Code
---------------------
The code is compiled as
  gcc -lssl netcat_part.c
The option -lssl specifies the compiler to link the 'ssl' library when compiling the code.

The code compilation can be done using the Makefile as 
  gmake all
When compiling using gmake, the executable is stored in netcat_part.

Execution and Interpretation of Output
---------------------------------------
SERVER can be started as
   ./netcat_part -l localhost <OutputFileName>
   Eg: ./netcat_part -l localhost result.txt

   Port number is an optional attribute and can be set using argument -p. The last argument is the output 
name, which can be specified by the user.

CLIENT can establish a connection to the server as - 
  For input to be read from command line argument
     ./netcat_part.c -m <Message> <ServerAddress>
	 Eg: ./netcat_part.c -m "Hello World" 127.0.0.1
  
  To transfer a file/ part of a file to server
     ./netcat_part.c <ServerAddress> <InputFileName>
	 Eg: ./netcat_part.c 127.0.0.1 alphabet.txt

A part of file can be sent by using -n, -o commands. -n specifies the number of bytes from the file to 
be sent to the server. -o sepcifies the offset from which the data is to be read to send to server.
  Eg: ./netcat_part.c -n 3 -o 3 127.0.0.1 alphabet.txt
The above example states that 3 bytes of data should be read from the input file starting from offset 3.

-v can be used in Client or Server to run them in the verbose mode.

The data sent to the server is written to the output file specified while starting the server.

Note: As per current implementation, server has to be started for every request from a new client.