Q157462: RAS Server Routing Client IP Packets Degrades Performance
Article: Q157462
Product(s): Microsoft Windows NT
Version(s): WinNT:3.51
Operating System(s):
Keyword(s): kbnetwork
Last Modified: 10-AUG-2001
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The information in this article applies to:
- Microsoft Windows NT Workstation version 3.51
- Microsoft Windows NT Server version 3.51
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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.
Additional query words: prodnt
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Keywords : kbnetwork
Technology : kbWinNTsearch kbWinNTWsearch kbWinNT351search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS351 kbWinNTS351search
Version : WinNT:3.51
Issue type : kbbug
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