Black hole router, describes a method of locating black hole routers, and suggests three ways to avoid the data loss that can occur because of a black hole router.
On a TCP/IP-based wide area network (WAN), communication over some routes may fail if an intermediate network segment has a maximum packet size that is smaller than the maximum packet size of the communicating hosts--and if the router does not send an appropriate Internet Control Message Protocol (ICMP) response to this condition or if a firewall on the path drops such a response. Such a router is sometimes known as a "black hole" router.
You can locate a black hole router by using the Ping utility, which is a standard utility that is installed with the Microsoft Windows TCP/IP protocol. You can then use one of three methods of fixing or working around black hole routers.
When a network router receives a packet that is larger than the size of the Maximum Transmission Unit (MTU) of the next segment of a communications network, and that packet's IP layer "don't fragment" bit is flagged, the router is expected to send an ICMP "destination unreachable" message back to the sending host.
If the router does not send a message, the packet might be dropped, causing a variety of errors that vary with the program that is communicating over the unsuccessful link. (These errors do not occur if a program connects to a computer on a local subnet.) The behavior may seem intermittent, but closer examination shows that the behavior can be reproduced, for example, by having a client read a large file that is sent from a remote host.