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Q194881: WinNT Router May Drop IP Packets While Awaiting ARP Response

Article: Q194881
Product(s): Microsoft Windows NT
Version(s): WinNT:4.0
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 Server version 4.0 
- Microsoft Routing and Remote Access Service Update for Windows NT Server version 4.0 
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SUMMARY
=======

When Windows NT Server version 4.0 is configured as a router, some routed IP
packets may be dropped while the server awaits an ARP response from the
destination or next hop router.

MORE INFORMATION
================

If a Windows NT router does not have an Ethernet (media access control) address
entry for the destination or next hop router in its ARP cache, it will have to
send an ARP Request. While Windows NT awaits an ARP Response, ARP will "queue"
the IP packet that needs to be sent. When Windows NT receives the ARP Response,
it will only transmit the "latest" or last packet that it received in its "ARP
Packet Queue" for any given destination host.

A common example of this phenomenon would be a ping (ICMP) message larger than
the MTU of a given network segment. This will force TCP/IP to fragment the
message into multiple packets. When Windows NT receives packet 1 of 2, it will
send an ARP Request for the destination or next hop router. Before it receives
the ARP Response, it will receive packet 2 of 2 of the ping message. When
Windows NT finally receives the ARP Response, it will only transmit the "latest"
or last packet it received -- the rest would be discarded.

REFERENCES
==========

For detailed information about the Address Resolution Protocol (ARP) in Windows
NT Server version 4.0, please download the "TCP/IP Implementation Details" white
paper from the Microsoft FTP Site:

  ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/TCPIMP2.EXE

For more information about the ARP Packet Queue, please see RFC 1122 -
"Requirements for Internet Hosts -- Communication Layers".

RFCs may be obtained via the Internet as follows:

Paper copies of all RFCs are available from the NIC, either individually or on a
subscription basis (for more information, contact NIC@NIC.DDN.MIL). Online
copies are available via FTP or Kermit from NIC.DDN.MIL as rfc/rfc####.txt or
rfc/rfc####.PS (#### is the RFC number without leading zeros).

Additionally, RFCs may be requested through electronic mail from the automated
NIC mail server by sending a message to SERVICE@NIC.DDN.MIL with a subject line
of "rfc ####" for text versions, or a subject line of "rfc ####.PS" for
PostScript versions. To obtain the RFC index, the subject line of your message
should read "rfc index".

Additional query words: ntrouter

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbAudDeveloper kbRRASNTSearch kbRRASNT400
Version           : WinNT:4.0
Hardware          : ALPHA x86
Issue type        : kbinfo

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