RAS Server Routing Client IP Packets Degrades Performance

• Microsoft Windows NT Workstation version 3.51
• Microsoft Windows NT Server version 3.51

SYMPTOMS

You may notice that the performance of your Remote Access Service (RAS) server degrades when you are using it to route IP packets from several RAS clients to one or more local area network (LAN) segments.

CAUSE

This problem occurs when a large number of packets are sent from the RAS server or a system on the LAN to a RAS client.

For example, when RAS clients use Internet browsers through a RAS server, the browsers typically open several TCP connections for each page. The browsers send several small request packets through the RAS IP router to the Web server. The Web server then responds by sending lots of frames with the content of the Web page.

RESOLUTION

To resolve this issue, perform the following steps to edit or add the appropriate registry entries:

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

1. Install the latest Windows NT 3.51 Service Pack.

   For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q128465

      TITLE     : How To Obtain Windows NT Version 3.51 U.S. Service Pack
   
   

2. Start Registry Editor (Regedt32.exe) and select the following subkey:

   HKEY_Local_Machine\System\CurrentControlSet\Services\NdisWanX

   where X is the adapter number.

   NOTE: There may be several NdisWanX subkeys. The one you need to change will have the subkeys Parameters\Tcpip\. This is often the last NdisWan entry.

3. Go to the Parameters subkey of NdisWanX (that includes the following):

   a. A value with the following information:

         Value name: ServerAdapter
         Data Type:  REG_DWORD
         Data:       1
   
      b. the TCPIP subkey.
   
   

4. Select the TCPIP subkey, click Edit, and then click Add Value.

5. Enter the following:

   Value Name: MaxForwardPending Data Type: REG_DWORD

      Data:       <Range is 1 to 0xFFFFFFFF>
   
      NOTE: The default value for this key before Service Pack 2 (SP2) is 20
      (decimal); however, for Windows NT before SP2, it was a hard-coded limit
      of 5.
   
      You can only change MaxForwardPending, but your queue will be limited to
      50 packets. Setting it to more than 50 will not provide any additional
      queuing because there are other buffers that become exhausted at 50. If
      you need more than 50, follow steps 6-8 below. Otherwise, you can skip
      to steps 9-10.
   
   

6. In the registry, locate the following key:

   HKEY_Local_Machine\System\CurrentControlSet\Services\Tcpip\Parameters

7. Click Edit, click Add value, and then type the following:

      Value Name:   NumForwardPackets
      Data Type:    REG_DWORD
      Data:         <Range is 1 to 0xFFFFFFFF>
      Default:      50
   
      Description: This parameter determines the number of IP packet headers
      allocated for the router packet queue. When all headers are in use, the
      router will begin to discard packets at random from the queue. This
      value should be at least as large as the ForwardBufferMemory value
      divided by the maximum IP data size of the networks connected to the
      router. It should be no larger than the ForwardBufferMemory value
      divided by 256, because at least 256 bytes of forward buffer memory are
      used for each packet. The optimal number of forward packets for a given
      ForwardBufferMemory size depends on the type of traffic carried on the
      network and will be somewhere between these two values. This parameter
      is ignored and no headers are allocated if the router is not enabled.
   
      NOTE: This value should be set equal to MaxForwardPending for
      adapter key.
   
   

8. Click Edit, click Add value, and then type the following:

      Value Name:   ForwardBufferMemory
      Data Type:    REG_DWORD
      Data:         <Range is network MTU>
      Default:      74240 (enough for fifty 1480-byte packets, rounded
                    to a multiple of 256)
   
      Description: This parameter determines how much memory IP allocates to
      store packet data in the router packet queue. When this buffer space is
      filled, the router begins discarding packets at random from its queue.
      Packet queue data buffers are 256 bytes in length, so the value of this
      parameter should be a multiple of 256. Multiple buffers are chained
      together for larger packets. The IP header for a packet is stored
      separately. This parameter is ignored and no buffers are allocated if
      the IP router is not enabled. Ideally you would multiply
      NumForwardPackets times 1480 bytes.
   
   

9. Click OK, and then quit Registry Editor.

10. Shut down and then restart Windows NT.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 3.51. This problem has been corrected in the latest U.S. Service Pack for Windows NT version 3.51. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K

MORE INFORMATION

When RAS client computers connect to Web servers on the RAS server's IP network, the backlog becomes extremely large. Web browsers on RAS client computers use the HTTP protocol, which can have many requests pending at the same time. In addition, Web browsers transmit small request packets that generates many large response packets. Consequently, using Web browsers can increase the number of packets being sent and queued on the RAS server for each RAS client computer.

The IP router of the Windows NT TCP/IP stack places IP packets in a queue and, by default, forwards 20 IP packets at once to an NDIS network driver. The number of packets that are forwarded depends on the MaxForwardPending registry value. This value was originally set to 5. After you upgrade to Windows NT 3.51 Service Pack 2 or greater, the value defaults to 20 and is user configurable in the registry.

The default value for MaxForwardPending is sufficient for local network speed and throughput. However, it is not sufficient for RAS NDIS drivers. For example, if 1,000 packets are sent to the RAS server for one RAS client, there is a long delay in completing the transmission of the packets and a huge backlog on the RAS server. Several third-party multiport serial interface vendors have developed drivers that can transmit more packets at once than a standard network card (which uses the Windows NT default value of 20 packets). These drivers place packets in a queue on the individual serial connector for each RAS client computer. This process reduces the load on the higher software layers and places the heavy load on the hardware.

For example, if you have a RAS server that uses a 64 multiport serial interface and one network connection, 64 RAS client computers using Internet Explorer that can make 5 possible simultaneous connections, and a Web site with a Web page that contains 5 graphic files (each is 10 KB in size), the MaxForwardPending data value is 1600 (decimal).

64 clients * 5 connections/client * 5 files/connection = 1600

To monitor your RAS IP router backlog queue, you need to use Performance Monitor. However, first you must install Network Monitor Agent on the RAS server. This is necessary because it adds additional network interface counter objects to Performance Monitor.

Start your test on your normal RAS traffic. Monitor the queue lengths on the chart. RAS should be the highest value. If the queue length hits your value for MaxForwardPending it will cause all RAS clients to drop packets. If this occurs, you should go back to the RESOLUTION section earlier in this article and increase all your registry parameters.