Sometimes, network traffic is dropped without the source ever being informed that the traffic never reached its intended target. The error can only be detected by monitoring network traffic. Such a situation is referred to as a black hole.

One such scenario is when a host tries to connect to an IP address that was assigned to a host that is down or to an IP address that was never assigned to a host. Although TCP has mechanisms for communicating a failure to connect back to the original host, often the packets are just dropped. Moreover, if you are using a protocol such as UDP that is both connectionless and unreliable, then there are no means of communicating back to the original host that the IP address is dead.

Another common situation is with Maximum Transmission Unit (MTU) black holes. This happens when an MTU packet is larger than the maximum MTU size allowed on a network, and the Don't Fragment (DF) flag is set in the IP header. If this happens, any device whose MTU is smaller than the packet's size will drop the packet. The solution here is to make sure Path MTU Discovery (PMTUD) is running on all network devices. PMTUD solves this problem by sending back a Fragmentation Needed ICMP message back, thus causing the offending device to reduce its MTU size. Some network devices block ICMP messages for security reasons, however, and if this is the case on your network, you could end up with black hole connections: the TCP three-way handshake will be completed, but when data is transferred, the connection will hang because of the MTU size mismatch.

One possible solution is to use the RFC 4821 version of PMTUD. This version uses TCP or another protocol to probe the patch with progressively larger packets. Another solution is to change the maximum segment size (MSS) of all TCP connections lower than the Ethernet default of 1500.