Connect() functionality of socket communication - c

Question

A question on the wallclock time of socket communication.
I am having a function, which finds the servers registered at a central server.
I am adding a layer of network check over this function by extracting the URL and port number of the servers and trying to connect to them by behaving like a simple TCP client.
If the return value is greater than 0, then it means that the network is working fine; if -1, then the network is broken.
printf("--Checking for network connectivity--\n");
for(size_t i = 0; i < serverOnNetworkSize; i++) {
UA_ServerOnNetwork *server = &serverOnNetwork[i];
A[i] = (char *)UA_malloc(server->discoveryUrl.length+1);
memcpy(A[i],server->discoveryUrl.data,server->discoveryUrl.length);
A[i][server->discoveryUrl.length] = 0;
int length = strlen(A[i]);
//discovery URLs are of the form : opc.tcp://hostname:port
//new addition to extract port
B[i] = A[i] + 10;
//printf("Hostname: %s\n", B[i]);
char *p = strrchr(B[i], ':');
int port = strtoul(p+1, NULL, 10);
//printf("%d\n",port);
B[i][length-5]='\0';
//printf("Hostname: %s\n", B[i]);
//removing the port
A[i][length-5]='\0';
//without initial tcp binding
C[i] = A[i] + 10;
//printf("Hostname: %s\n", C[i]);
// FIND IP OF THAT HOST
if(i!=0){
char ip_address[50];
find_ip_address(C[i],ip_address);
socketCommunication(ip_address,C[i],port);
}
}
printf("--Checks done!--\n");
Global Funcitons:
int find_ip_address(char *hostname, char *ip_address)
{
struct hostent *host_name;
struct in_addr **ipaddress;
int count;
if((host_name = gethostbyname(hostname)) == NULL)
{
herror("\nIP Address Not Found\n");
return 1;
}
else
{
ipaddress = (struct in_addr **) host_name->h_addr_list;
for(count = 0; ipaddress[count] != NULL; count++)
{
strcpy(ip_address, inet_ntoa(*ipaddress[count]));
return 0;
}
}
return 1;
}
void socketCommunication(char *ip_address,char *hostname, int port){
int clientSocket,ret;
struct sockaddr_in serverAddr;
char buffer[1024];
clientSocket = socket(AF_INET,SOCK_STREAM,0);
if(clientSocket<0){
printf("Error in connection \n");
exit(1);
}
//printf("Client socket is created\n");
memset(&serverAddr,'\0',sizeof(serverAddr));
serverAddr.sin_port = htons(port);
serverAddr.sin_family=AF_INET;
serverAddr.sin_addr.s_addr=inet_addr(ip_address);
ret = connect(clientSocket,(struct sockaddr*)&serverAddr,sizeof(serverAddr));
if(ret<0){
printf("\nLOOKS LIKE NETWORK CONNECTION HAS FAILED. HAVE A LOOK AT THE NETWORK CONNECTIVITY at host : %s\n",hostname);
printf("\n----Updated Status Information----:\n");
printf("Discovery URL : opc.tcp://%s:%d\n",hostname,port);
printf("Status:CONNECTON TIMED OUT\n");
printf("\n");
}
To test this, I switch off the network from one of the registered servers.
When I measure the time, it shows inconsistent values of 18seconds,24,38 seconds etc.
These values occur when I switch the network of the server and run my application. On a second run of the same application, the value reduces to 2seconds or 1 second sometimes.
Output:
LOOKS LIKE NETWORK CONNECTION HAS FAILED. HAVE A LOOK AT THE NETWORK CONNECTIVITY at host : o755-gksr
----Updated Status Information----:
Discovery URL : opc.tcp://o755-gksr:4841
Status:CONNECTON TIMED OUT
--Checks done!--
Time measured: 18 seconds.
Output on another try
--Checking for network connectivity--
LOOKS LIKE NETWORK CONNECTION HAS FAILED. HAVE A LOOK AT THE NETWORK CONNECTIVITY at host : o755-gksr
----Updated Status Information----:
Discovery URL : opc.tcp://o755-gksr:4841
Status:CONNECTON TIMED OUT
--Checks done!--
Time measured: 0 seconds.
My question is : Why does it show inconsistent values? If the connection is not possible, should it not return -1 and show the error quickly?
Is there any background process, which tries to establish the connection for a finite number of times before coming to a halt?
Please let me know.
Regards,
Rakshan

Answer 1

The connect() behavior and its timeouts highly depends on underlying network. There are more reasons why connect() fails when the target machine is down. Errors in most cases are:
ETIMEDOUT - it means the client sent SYNs but it does not receive any response at all. It is a TCP timeout and can be quite long (minutes).
EHOSTUNREACH - it means local ARP query failed or the client sent SYN and ICMP error Host Unreachable was received. ARP query failure is detected in a few seconds. ICMP error Host Unreachable is usually returned by a remote router when its ARP query fails.
So what happen in your case if the server is in the same network as your client :
The client has server's MAC address in its ARP cache.
You "switch off the network from one of the registered servers.". You probably disconnect a cable from the server or something like that.
The client calls connect. SYN is sent directly to the MAC address from the ARP cache and in worst case the connect returns with ETIMEDOUT after two minutes.
Client delete the entry in the ARP cache.
Subsequent connect needs ARP resolution. Either it fails after 3 ARP request (3 seconds) or it fails immediately if the negative entry in ARP cache is valid. It may be valid for a few seconds only.
If the server is in remote network then the situation is similar. The ARP cache of the remote router is guilty in this case. If the remote router cannot resolve IP address to MAC address then it send ICMP Host Unreachable almost immediately but if the remote router still has the destination IP in its ARP cache it takes some time than it realizes the cache entry is obsolete and MAC address is not available.