Black hole (networking)

As a Black Hole Router A router is called, the IP packets need to be fragmented for immediate dispatch, discards without feedback.

The Internet Protocol is designed so that it can be transported on various technologies (Ethernet, Frame Relay, etc.) of the network layer ( Layer 3 in the OSI model ). Each of these technologies has a certain maximum packet size, namely the Maximum Transmission Unit (MTU). If data from a Layer 3 technology a different Layer 3 technology, the maximum permitted packet size is smaller than that of the first technology, to be transported, the decomposition ( fragmentation) of the IP packet is required.

Fragmentation, however, has an adverse effect on the performance of individual routers, as they claimed their processor and memory resources. For this reason, in TCP the DF flag (DF: Do not fragment - do not fragment ) set to the value 1, which prevents the router to the decomposition of the package. A router that is intended to provide an IP packet, which would have to be broken, due to the DF flag but must not be dismantled, the transmitter can respond according to RFC 792 or RFC 1191 contains the ICMP Destination Unreachable message - Fragmentation Needed and DF Set. A router that does not answer, but the package silently discards, Black hole router is called.

Such black- hole router also prevent the automatic determination of the maximum MTU along a network path using the Path MTU Discovery process.

• Network device