Windows NT Router May Drop IP Packets While Awaiting ARP Response

ID: Q194881

The information in this article applies to:

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 


Keywords          : kbnetwork 
Version           : WinNT:4.0
Platform          : winnt 
Issue type        : kbinfo

Last Reviewed: February 22, 1999