在IP頭中，有一Type-of-Service字段，該字段描述了IP包的
優先級和QoS選項，使用IP_TOS可以來設定該字段的值：
使用setsockopt設置IP_TOS代碼如下：
unsigned char service_type = 0xe0 | IPTOS_LOWDELAY | IPTOS_RELIABILITY;
if(setsockopt(sock, SOL_IP/*IPPROTO_IP*/, IP_TOS, (void *)&service_type, sizeof(service_type)) < 0)
perror("setsockopt(IP_TOS) failed:");
使用getsockopt讀取IP_TOS代碼如下：
int optval = 0;
int optlen = sizeof(optval);
if(getsockopt(sock, SOL_IP, IP_TOS, (void *)&optval, &optlen) < 0)
perror("getsockopt(IP_TOS) failed:");
else
printf("OPTVAL=%x.\n", optval)
在socket設置IP_TOS後，用wireshark抓取該socket上發送的數據，查看IP頭部可以看到設置的值。
附man手冊中IP_TOS、SO_PRIORITY說明：
IP_TOS: SOL_IP / IPPROTO_IP(BSD); IP_TOS Set or receive the Type-Of-Service (TOS) field that is sent with every IP packet originating from this socket. It is used to prioritize packets
on the network. TOS is a byte. There are some standard TOS flags defined:
IPTOS_LOWDELAY
to minimize delays for interactive traffic,
IPTOS_THROUGHPUT
to optimize throughput,
IPTOS_RELIABILITY
to optimize for reliability,
IPTOS_MINCOST
should be used for "filler data" where slow transmission doesn't matter.
At most one of these TOS values can be specified. Other bits are invalid and shall be cleared. Linux sends
IPTOS_LOWDELAY datagrams first by default, but the exact behaviour depends on the
configured queueing discipline. Some high priority levels may require superuser
privileges (the CAP_NET_ADMIN capability). The priority can also be set in a protocol
independent way by the (SOL_SOCKET, SO_PRIORITY) socket option (see socket(7)).
discipline. For ip(7), this also sets the IP type-of-service (TOS) field for outgoing packets.
SO_PRIORITY: SOL_SOCKET; SO_PRIORITY Set the protocol-defined priority for all packets to be
sent on this socket. Linux uses this value to order the networking queues: packets with a
higher priority may be processed first depending on the selected device queueing
Setting a priority outside the range 0 to 6 requires the CAP_NET_ADMIN capability.