How to use Fast Open Mechanism with MultipathTCP? - linux

Question

I would like to use Fast Open Mechanism with MultipathTCP.
Are there any options to do that?
I tried this:
int main(int argc, char *argv[])
{
...
struct msghdr msgh;
memset(&msgh, 0, sizeof(msgh));
struct cmsghdr *cmsg;
unsigned char buffer[1] = "X";
int size = 3;
struct sockaddr_in dst;
memset(&dst,0,sizeof(dst));
inet_pton(AF_INET, "127.0.0.1", &dst.sin_addr);
dst.sin_family = AF_INET;
dst.sin_port = htons(PORT);
/* Construct control information */
struct iovec msgiov = {};
struct unp_in_pktinfo {
struct in_addr ipi_addr; /* destination IPv4 address */
int ipi_ifindex; /* received interface index */
};
msgh.msg_name = &dst;
msgh.msg_namelen = sizeof(struct sockaddr_in);
msgiov.iov_base = buffer;
msgiov.iov_len = size;
msgh.msg_iov = &msgiov;
msgh.msg_iovlen = 1;
unsigned char control_buf[CMSG_LEN(sizeof(struct unp_in_pktinfo))] = {};
msgh.msg_control = &control_buf;
msgh.msg_controllen = CMSG_LEN(sizeof(struct unp_in_pktinfo));
cmsg = CMSG_FIRSTHDR(&msgh);
cmsg->cmsg_level = IPPROTO_IP;
cmsg->cmsg_type = IP_PKTINFO;
cmsg->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
memcpy((struct in_addr *)CMSG_DATA(cmsg), &(dst.sin_addr),
sizeof(dst.sin_addr));
cmsg = (struct cmsghdr *)((caddr_t) cmsg + CMSG_ALIGN(cmsg->cmsg_len));
sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_MPTCP);
ret = sendmsg(sock_fd, &msgh, MSG_FASTOPEN);
close(sock_fd);
return EXIT_SUCCESS;
}
It seems to be correct, but It doesn't work.
Could you please help me to find a solution?
Thank you!

Answer 1

There is existing approach to do that:
You need to apply the patch or use the kernel functionality (if exists). I applied patch from mptcp fastopen in linux kernel.
Instead of using msghdr, there is another solution posted here: mptcp fastopen in linux kernel. It is about using "sendto" fucntion with MSG_FASTOPEN:
ret = sendto(sock_fd, sendline, strlen(sendline), MSG_FASTOPEN,(struct sockaddr *) &daddr, sizeof(daddr));
Also the is another option (alternative) described in mptcp fastopen in linux kernel:
setsockopt(sock_fd, SOL_TCP, TCP_FASTOPEN_CONNECT, &enable, sizeof(enable));