Article ID: 314825 - View products that this article applies to.
This article was previously published under Q314825
For a Microsoft Windows 2000 version of this article, see 159211
This article defines the term "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.
Client-side ErrorThe client could not establish a connection to the remote computer. The most likely causes for this error are:
Locating a Black Hole RouterYou can use the Ping utility to locate a black hole router, by setting the -f and -l parameters when you type the ping command.
ping computer_name or IP_address -f -l 1472For all local IP addresses, the expected results are as follows:
(http://support.microsoft.com/kb/314496/EN-US/ )Default MTU Size for Different Network Topology
Fixing or Working Around a Black Hole RouterImportant This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/322756/ )How to back up and restore the registry in Windows
The following three methods are ways to either fix or work around a black hole router.
Method 1Enable PMTU Black Hole Detection on the Windows-based hosts that will be communicating over a WAN connection. Follow these steps:
Method 2Configure intermediate routers to send ICMP Type 3 Code 4 messages ("destination unreachable, don't fragment (DF) bit sent and fragmentation required"). This might require a router software or firmware upgrade, router reconfiguration, or router replacement.
Method 3Set the MTU of the host interface to be the largest size that the black hole router can handle, to guarantee that the largest possible packet size is sent over that connection. However, note that local traffic then uses smaller packets than necessary, as will traffic that uses the routed connections without problems.
This workaround assumes that you have identified the MTU and the state of all possible links that the host might use. After you identify the largest MTU size that is supported, manually set the MTU. Follow these steps:
For additional information manually setting the MTU, click the article number below to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/314053/EN-US/ )TCP/IP and NBT Configuration Parameters for Windows XP
For additional information, see Internet RFC 1191 and RFC 1435, which are available from the following Internic Web site:
Article ID: 314825 - Last Review: July 7, 2005 - Revision: 1.5