Black Hole Router
A black hole router is a router that discards IP packets that would have to be fragmented for forwarding without any response.
The Internet Protocol is designed so that it via various protocols of the network layer (Layer 3 in the OSI model can be transported). This can be, for example, Ethernet , WLAN or Frame Relay . Each of these protocols has a certain maximum packet size, namely the Maximum Transmission Unit (MTU). If data from a layer 3 protocol is to be transmitted via another layer 3 protocol whose maximum permitted packet size is smaller, the IP packets must be broken down into processable packet sizes.
Fragmentation has a detrimental effect on the performance of the receivers as it uses their processor and memory resources. For this reason, the DF flag (DF: Don't fragment) can be set to the value 1 for TCP data, which signals the router not to break up the packet.
A router that is to transmit an IP packet that would have to be dismantled but may not be dismantled due to the DF flag can reply to the sender according to RFC 792 or RFC 1191 with the ICMP message Destination Unreachable-Fragmentation Needed and DF Set . A router that does not send a reply but silently discards the packet is called a black hole router.
Black hole routers also prevent the automatic determination of the largest possible MTU along a network path with the aid of the Path MTU Discovery process.
