Bootstrapping all the processes and establishing correct lines of communication is, by itself, the toughest aspect of this assignment. Before writing any code you should think carefully about how to start all the process and get them connected to each other. You will have accomplished a signi?cant ?rst step once you can reliably start all 20 processes and have them establish connections among themselves properly. Inter-process communication shoulbe via sockets whose type you choose.
I recommend that you make two executables, named host and router. host should accept a command line argument of one letter: either A, B, C, or D. router should accept a command line of R1 or R2. Because there are lots of parameters (e.g., IP addresses, link addresses), have each executable read its parameters from a common ?le once the executable has determined its identity. You may ?nd the combination of socketpair(2) and forking to be useful for starting and organizing the many processes. Here is a working example of the use of the socketpair system call to get three processes (app, IP, and link) connected to each other in a stack con?guration. The code below is available with the assignment in WebCT as ?le example.c. You are welcome to use all, part, or none of this code in your assignment.
#include
#include
#include
#include
#include
#include
int sock_fd[4];
#define APP_SOCK 0
#define IP_APP_SOCK 1
#define IP_LINK_SOCK 2
#define LINK_IP_SOCK 3
#define BUF_SIZE 1024
char buf[BUF_SIZE];
Once all connections are made, I suggest writing code that will format an option less IP datagram and forward from application to application, without ICMP error cases, fragmentation, or any other unusual cases such as TTL expiration, etc. Once you have the basic data ?ow working then you can start to add code one feature at a time for TTL, checksums, fragmentation, and routing errors. You will need to have lots of print statements to trace execution, but with 20 processes, writing to the console may prove problematic. If it does, consider writing debug statements into a log ?le. Be sure to use unbuffered output statements so that there is no possibility of output statements appearing in the log in a different order than they were executed in your programs.