In the Value data box, type 0, and then click OK. Type EnablePMTUDiscovery, and then press ENTER. To disable PMTU Discovery, follow these steps: However, performance will be lower than if PMTU Discovery was enabled and the path supported an MTU size larger than 576. Most of the time, the performance will be at acceptable levels with a packet size of 576. This method affects packets sent to all destinations. This enables the routers to fragment the packet and send the packet across the networks. If you disable PMTU Discovery, TCP will only send packets that have an MTU size of 576 and that do not have the Don't Fragment set. Quit Registry Editor, and then restart the computer. In the Value data box, type 1, and then click OK. Type EnablePMTUBHDetect, and then press ENTER. On the Edit menu, point to New, and then click DWORD Value. HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters Locate the following key in the registry: To enable PMTU black hole detection, follow these steps:Ĭlick Start, click Run, type regedit, and then click OK. Black hole detection increases the maximum number of retransmissions for a specific segment. This method is preferred because the packet size is lowered for only the problematic segment. If a segment is acknowledged, the maximum segment size (MSS) will be reduced and the Don't Fragment bit will be set in future packets on the connection. TCP will try to send these segments if several retransmissions of a segment go unacknowledged. If you enable the Path Maximum Transfer Unit (PMTU) black hole detection feature, TCP will try to send segments that do not have the Don't Fragment bit set. Method 1: Enable Path Maximum Transfer Unit (PMTU) black hole detection The following recommendations may reduce the network throughput. Note You should not use the following recommendations if you are not experiencing one of these scenarios. Under these special circumstances, consider using one of the following recommendations. If the destinations to which the network connectivity is lost has any ICMP destination unreachable message with the next hop MTU value of less than 576, you are experiencing such scenarios. You can use tools such as Network Monitor to detect whether you are experiencing such scenarios by analyzing a network trace. In these WAN scenarios, network connectivity may be lost. In these WAN scenarios, the MTU size might be less than 576. However, an MTU size that is restricted to 576 bytes may affect certain WAN scenarios, such as satellite links. A very low MTU size could cause a severe decrease in performance. An ICMP-based attack could reduce the MTU size to very low value. The MTU size is restricted to prevent an ICMP-based attack.
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This security update restricts the lowest MTU size to 576 bytes. The MS05-019 security update modifies how the operating system validates Internet Control Message Protocol (ICMP) requests.
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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:ģ22756 How to back up and restore the registry in Windows Then, you can restore the registry if a problem occurs. For added protection, back up the registry before you modify it. Therefore, make sure that you follow these steps carefully. However, serious problems might occur if you modify the registry incorrectly. Important This section, method, or task contains steps that tell you how to modify the registry. This article describes the recommended TCP/IP settings for wide area network (WAN) links with a Maximum Transmission Unit (MTU) size of less than 576. This article describes three methods that you can use to help prevent the computer from losing network connectivity when the MS05-019 security update is installed. However, under special circumstances, this security update may cause the computer to lose network connectivity. This security update prevents an ICMP-based attack.
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