Windows NT Router May Drop IP Packets While Awaiting ARP Response (194881)
The information in this article applies to:
- Microsoft Windows NT Server 4.0
- Microsoft Routing and Remote Access Service Update for Windows NT Server 4.0
This article was previously published under Q194881 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:
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
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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
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Additionally, RFCs may be requested through electronic mail from the
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| Modification Type: | Major | Last Reviewed: | 8/10/2001 |
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| Keywords: | kbinfo kbnetwork KB194881 |
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