I have a C++ network application running on linux using TCP sockets. I have been trying to fetch the DSCP marking from a received packet for TCP. We have IP_RECVTOS socket option for UDP. But I have tested in code that it is not working for TCP.

Why is this not implemented in kernel for TCP ?
What options do I have except raw sockets to address the issue ?
Is there a kernel patch available ?


Example code : The following is for UDP. I am looking for something similar for TCP.

//Set the socket option to receive IP_TOS:

unsigned char set = 0x03;
if(setsockopt(udpSocket, IPPROTO_IP, IP_RECVTOS, &set,sizeof(set))<0)
{
printf("cannot set recvtos\n");
}
else
{
printf("socket set to recvtos\n");
}

// and Retrieve the IP_TOS value from every packet header by:

struct PC_Pkt pkt;
int *ecnptr;
unsigned char received_ecn;
struct msghdr msg;
struct iovec iov[1];
memset(&msg, '\0', sizeof(msg));
msg.msg_iov = iov;
msg.msg_iovlen = 1;
iov[0].iov_base = (char *) &pkt;
iov[0].iov_len = sizeof(pkt);

int cmsg_size = sizeof(struct cmsghdr)+sizeof(received_ecn);
char buf[CMSG_SPACE(sizeof(received_ecn))];
msg.msg_control = buf;
msg.msg_controllen = sizeof(buf);

nRet = recvmsg(udpSocket, &msg, 0);

if (nRet > 0) {
struct cmsghdr *cmsg;
for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL;
cmsg = CMSG_NXTHDR(&msg,cmsg)) {
if ((cmsg->cmsg_level == IPPROTO_IP) &&
(cmsg->cmsg_type == IP_TOS) && (cmsg->cmsg_len) ){
ecnptr = (int *) CMSG_DATA(cmsg);
received_ecn = *ecnptr;
int isecn = ((received_ecn & INET_ECN_MASK) == INET_ECN_CE);

printf("received_ecn = %i and %d, is ECN CE marked = %d \n", ecnptr, received_ecn, isecn);

break;
}
}
}