3310 Computer Network(4)

The main task for this week is a project in which we need to write a short program to implement the function of transmission between two entities, (ie. Two computers with different IP address named Robot and Student respectively) using two different transmission layer protocols – TCP and UDP. What’s more, for the bonus, we need to extend the program to implement the function that measures the different throughput for different receiver buffer size. The foal of the task is to help us get familiar with the usage of socket programming as well as some important properties of sending and receiving data such as buffer size and how port works as well.

To design the program, the most important part is to understand how the connection procedure and properties should be in either TCP or UDP protocols. In the past lecture, we should have known that the procedures for these two transmission modes should be like:

   

Figure 1 : functions flow of connection-oriented socket and connectionless Socket

     Implementing the functions shown above and we can accomplish our basic goal. Besides, there also have some detailed staff that we should take care of.

First of all, when considering the structure object sockaddr_in, it is describing and identifying the socket address family of either the server or the client such as IP address and port number for the socket.

For example, the definition of the family address in the program for the robot is like:

Figure 2 , address family of robot in the program

Secondly, for TCP socket, it is a stream socket which does not preserve message boundary, so in order to avoid the situation that the buffer load the information before all the data is correctly received, a loop is needed for receiving data in TCP protocol. It is the same for the sending procedure. Take the codes of receiving as an example, it is like :

                 Figure 3 The loop for TCP socket receiving

                

     Thirdly, in the bonus part, we need to let the robot continuing sending packets to the destination, lasting for 30 seconds. To solve this problem, a time function is needed, clock_t determines a variable corresponding to a certain time, in the mean time function clock() will return the current time from the computer. By using these two properties and a loop, the continuous sending procedure can be done. In the program, it is like:

   

         Figure 4 Codes fort he big packets transmission part

Through the programming project, though it is a hard and tiring work, my understanding of this course has been deeply developed. Although it is just a tiny little part of computer network, it stands for what is the work procedure like for the engineer to develop and implement those theories we are talking in the class. To discuss a method, or a tactic a easy, but to implement it is a not a easy work, which would always requires for deeply understanding and cautious habits.

TOPIC :Introduciton of Two Functions

In the project, one requirement is to find out the initial receiving buffer size and in addition to modify its value to see how the buffer size would effect the throughput. To accomplish this task, we need two functions which are included in the "winsock2.h" .They are getsockopt() and setsockopt() respectively.

For the function getsockopt(), it’s used to get the information in the socket bind by the user. It’s structure is as the figure shown below:

where:

sockfd : is the description the socket, it should be pointed to the socket

 you are willing to execute.

level :  is determine for the options of the socket, there exit

two levels that are SOL_SOCKET and IPPROTO_TCP.

   optname: the option that the user need to get from the socket

   optval : a pointer that points to the buffer location of the option value.

   optlen : a pointer that points to the length of the buffer area

For the function setsockopt(), it’s used to set the value in the socket bind by the user. It’s structure is as the figure shown below:

where:

sockfd : is the description the socket, it should be pointed to the socket

 you are willing to execute.

level :  is determine for the options of the socket, there exits four levels

which are SOL_SOCKET, IPPROTO_TCP, IPPROTO_IP and IPPROTO_IPV6

 optname: the option that the user need to get from the socket

 optval : a pointer that points to the buffer location of the option value.

 optlen : a pointer that points to the length of the buffer area

    By using these two functions, we can get the information that the default receive buffer size is 8192 bytes. Once the user increase or decrease the buffer size, it can be seen that the throughput of the packets would increase and decrease in pace with the buffer size.