Q153596: TCP/IP Performance Degrades When Resuming Large Data Transfer
Article: Q153596
Product(s): Microsoft Windows NT
Version(s): winnt:3.51
Operating System(s):
Keyword(s):
Last Modified: 08-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
========
Windows NT clients using TCP/IP may notice a considerable delay resuming data
transfer to client applications that take a long time to clear the TCP/IP
receive window.
A Protocol Analyzer trace of the slow performance will show the following
behavior:
1. The server sends a large data transfer.
2. The client runs out of receive windows space and indicates a zero-byte
window.
3. The server enters exponential backoff until it hits the maximum limit of 240
seconds between window probe packets. Because the time between probes is
longer than the 120-second Address Resolution Packet (ARP) cache life, each
new window probe now requires that the server ARP for the client's Media
Access Control (MAC) address.
4. The client clears its receive window and advertises additional window space.
5. The server begins transmitting data with an incorrect sequence number.
6. The client acknowledges a previous data send.
7. The server takes up to four minutes to send the correct sequence number.
CAUSE
=====
The server falls out of sequence because its window probes are 240 seconds apart
so the server has to ARP for the client's MAC address when the client advertises
its new window space. The reason this causes the server to fall out of sequence
is that the server begins sending data immediately after discovering that the
client has additional window space. Because the ARP table entry for the client
is in the resolving state when the server starts indicating data, TCP/IP cannot
send the data to the client.
In accordance with RFC the ARP cache will only buffer one packet when the
destination IP address is in the resolving state. Windows NT buffers the last
packet only, so all sends, except for the last, are dropped until the ARP entry
has been resolved. When the ARP entry for the client is resolved TCP/IP sends
the last packet that was cached to the client; the packet is out of sequence
because the prior sends were dropped while the ARP entry was resolving. The
excessive delay that occurs is caused by the server's taking 240 seconds to send
the correct sequence number.
The reason the server takes 240 seconds to send the correct sequence number is
that a retransmit timer has started running. The retransmit timer takes 240
seconds to complete before the packet with the correct sequence number can be
sent.
Note: For additional information on the ARP cache please refer to
RFC 1122.
MORE INFORMATION
================
Example Protocol Analyzer Trace:
1 0.000 Client -> Server
TCP .A...., len: 0, seq:2192824888, ack: 578989364, win: 0
2 153.452 Client -> Server
TCP .AP..., len: 512, seq:2192824888, ack: 578989364, win: 0
3 0.008 Client -> Server
TCP .A...., len: 0, seq:2192825400, ack: 578989364, win:14336
4 0.002 Client -> Server ARP Reply
5 0.001 Server -> Client
TCP .A...., len: 1460, seq: 578992284, ack:2192825400, win:48640
6 0.005 Client -> Server
TCP .A...., len: 0, seq:2192825400, ack: 578989364, win:14336
7 0.195 Client -> Server
TCP .A...., len: 0, seq:2192825400, ack: 578989364, win:14336
8 4.785 Server -> Client
TCP .A...., len: 357, seq: 578993744, ack:2192825400, win:48640
9 0.003 Client -> Server
TCP .A...., len: 0, seq:2192825400, ack: 578989364, win:14336
10 0.193 Client -> Server
TCP .A...., len: 0, seq:2192825400, ack: 578989364, win:14336
11 235.180 Client -> Server
ARP Reply
12 0.000 Server -> Client
TCP .A...., len: 1460, seq: 578989364, ack:2192825400, win:48640
STATUS
======
Microsoft has confirmed this to be a problem in Windows NT version 3.51. This
problem was corrected in the latest Windows NT 3.51 U.S. Service Pack. 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
Additional query words: 3.50 3.51 prodnt
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Keywords :
Technology : kbWinNTsearch kbWinNTWsearch kbWinNT351search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS351 kbWinNTS351search
Version : winnt:3.51
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