RHEL: TCP_MAXSEG in setsockopt() does not appear to have any effect

Issue

• We are changing the TCP_MAXSEG size in a socket in our C program, like this:

n = setsockopt(sockfd, IPPROTO_TCP, TCP_MAXSEG, &requested_mss, sizeof(requested_mss));

• We then retrieve it here:

getsockopt(sockfd, IPPROTO_TCP, TCP_MAXSEG, &actual_mss, &actual_mss_len)

• And actual_mss is always 1448, no matter if we make requested_mss 500, or 5000, or whatever. It also doesn't matter if we run the program as root.
• Can the the MMS be changed after the connection is already established?
  • No the MSS is part of the TCP options which are negotiated via the initial 3 way handshake.