PPTP Drops Packet Fragments That Arrive Out Of Order (292788)

SYMPTOMS

Microsoft Point-to-Point Tunneling Protocol (PPTP) discards all packets that are received out of sequence. This is likely to happen over networks, such as the Internet, where multiple packets may take different routes to a common destination.

This can cause the following symptoms:

The ping command may not work if the data buffer cannot fit into a single frame. For example, ping target_name -l 5000
Slow data transfer.
A high number of packet retransmissions.
Failed logon and security authorization using Kerberos. Users may receive the following error message:
Event Log Error 5719
Source NETLOGON
No Windows NT or Windows 2000 Domain Controller is available for domain domain name. The following error occurred:
There are currently no logon servers available to service the logon request.

CAUSE

PPTP relies on the Internet Generic Routing Encapsulation (GRE) protocol. Microsoft is strictly following RFC 2890 "Key and Sequence Number Extensions to GRE" which states in section 2.2:

When the decapsulator receives an out-of sequence packet it SHOULD be silently discarded.

RESOLUTION

To resolve this problem, obtain the latest service pack for Windows 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

260910 How to Obtain the Latest Windows 2000 Service Pack

The English version of this fix should have the following file attributes or later:

   Date         Time   Version        Size    File name
   ------------------------------------------------------
   07-Aug-2001  09:32  5.0.2195.4080  89,680  Ndiswan.sys
   07-Aug-2001  09:34  5.0.2195.4080  46,640  Raspptp.sys

NOTE: Install this hotfix on your Windows 2000 PPTP clients and your Windows 2000 PPTP servers.

WORKAROUND

For the Netlogon and Kerberos errors that are caused by this problem, you can use the following steps to reduce the impact. As described by the RFC, Kerberos uses the UDP protocol. If the network fragments large UDP packets, they could arrive out of order. If this happens over a VPN connection, PPTP will drop the frame, and the Kerberos operation will fail.

Performance will improve if you force Kerberos to use TCP instead of UDP. PPTP will still drop out of sequence frames. However, with TCP, Kerberos will retransmit the dropped packets, increasing the probability of subsequent in-order arrival.

MORE INFORMATION

The hotfix that is described in this article relaxes the requirements that are described in the "Cause" section of this article, and enables reassembly of fragmented frames that arrive out of order over PPTP and GRE.

For additional information about how to obtain a hotfix for Windows 2000 Datacenter Server, click the article number below to view the article in the Microsoft Knowledge Base:

265173 The Datacenter Program and Windows 2000 Datacenter Server Product

For additional information about how to install multiple hotfixes with only one reboot, click the article number below to view the article in the Microsoft Knowledge Base:

296861 Use QChain.exe to Install Multiple Hotfixes with One Reboot

For additional information about how to install Windows 2000 and Windows 2000 hotfixes at the same time, click the article number below to view the article in the Microsoft Knowledge Base:

249149 Installing Microsoft Windows 2000 and Windows 2000 Hotfixes