black hole connection. Some implementations of PMTUD attempt to prevent this problem by inferring that large payload packets have been dropped due to MTU rather than because of link congestion. However, in order for the Transmission Control Protocol (TCP) to operate most efficiently, ICMP Unreachable messages (type 3) should be permitted. A robust method for PMTUD that relies on TCP or another protocol to probe the path with progressively larger packets has been standardized in RFC 4821. A workaround used by some routers is to change the maximum segment size (MSS) of all TCP connections passing through links with MTU lower than the Ethernet default of 1500. This is known as MSS clamping. References Computer network analysis Internet protocols