FIX: TOS Bits Cannot Be Set for Unconnected Sockets

MORE INFORMATION

The Type-of-Service field in an IP packet is an 8-bit field that is composed of three precedence bits, four TOS bits, and an unused bit that must be 0. The four TOS bits represent network services such as minimize delay, maximize throughput, maximize reliability, and minimize monetary cost. Only one of these four bits can be turned on at a time. If all four bits are 0, it implies normal service. RFC 1340 further describes how standard applications should set these bits.

You can also modify the following key in the Registry Editor to change the TOS value set in the header of outgoing IP packets:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\ 
   Value Name: DefaultTOS
   Value Type: REG_DWORD
   Valid Range: 0x00, 0x02, 0x04, 0x08, 0x10
				

The following code fragment demonstrates how to programmatically set TOS bits using the TCP/IP socket option IP_TOS:

   int tos, tos_len;
   tos_len = sizeof(tos);

   if (getsockopt(sock, IPPROTO_IP, IP_TOS, (char *)&tos,
                  &tos_len) == SOCKET_ERROR)
       printf("\nWarning: TCPIP stack does not support the "
              "IP_TOS option - %d\n", WSAGetLastError());

   if (setsockopt(sock, IPPROTO_IP, IP_TOS, (char *)&tos,
                  tos_len) == SOCKET_ERROR)
      printf("\nWarning: Fail to set TOS value: error - %d",
             WSAGetLastError());