TCP Connections Require Extra Time to Close (94246)
This article was previously published under Q94246
SUMMARY
The Microsoft LAN Manager TCP/IP stack has configurable parameters to
select the number of NetBIOS sessions and TCP connections available. The
parameters are configurable because each connection requires MS-DOS
memory for buffering data and memory is often tight. When a TCP connection
is closed, there is a short delay before the same connection can be used
again.
This can create an interesting situation for LAN Manager MS-DOS clients
using logon scripts with many TCP connections. If you are already logged
on and enter
net logon user password /y
some of the old connections may still be pending when the script attempts
to log you on and establish new ones. In some instances, this can result
in errors, including "DOS error 53".
SYMPTOMS
A "DOS error 53" or "bad username or password" error is displayed when
you re-log on, or a long pause may occur when you executing batch files
that connect and disconnect to many resources.
CAUSE
This problem occurs because LAN Manager adheres to RFC 793 and RFC 1122,
which stipulate how TCP connections are to be closed in systems
configured with insufficient TCP connections. This method is best
explained by the following excerpt from RFC 1122, Section 4.2.2.13:
A TCP connection may terminate in two ways: (1) the normal TCP close
sequence using a FIN handshake, and (2) an "abort" in which one or
more RST segments are sent and the connection state is immediately
discarded. If a TCP connection is closed by the remote site, the
local application MUST be informed whether it closed normally or was
aborted.
The normal TCP close sequence delivers buffered data reliably in both
directions. Because the two directions of a TCP connection are closed
independently, it is possible for a connection to be "half closed,"
(that is, closed in only one direction), and a host is permitted to
continue sending data in the open direction on a half-closed connection.
A host MAY implement a "half-duplex" TCP close sequence, so an
application that has called CLOSE cannot continue to read data from the
connection. If such a host issues a CLOSE call while received data is
still pending in TCP, or if new data is received after CLOSE is called,
its TCP should send an RST to show that data was lost.
When a connection is closed actively, it MUST linger in TIME-WAIT state
for a time 2xMSL (maximum segment lifetime). However, it MAY accept a
new SYN from the remote TCP to reopen the connection directly from TIME-
WAIT state, if it:
- Assigns its initial sequence number for the new connection to be
larger than the largest sequence number it used on the previous
connection incarnation
and
- Returns to TIME-WAIT state if the SYN turns out to be an old
duplicate
RESOLUTION
MS-DOS systems using TCP/IP may need to be configured for extra
TCPCONNECTIONS in the PROTOCOL.INI file to prevent exhausting this
resource. Alternatively, to re-logon you may "net logoff" first, wait 5
seconds, then "net logon".
The parameter TCPCONNECTIONS defaults to (NBSESSIONS + 1 + NUMSOCKETS +
NSESSIONS).
* NBSESSIONS = The number of NetBIOS sessions, defaults to 6.
* NUMSOCKETS = The number of sockets, only present if Microsoft
TCP/IP Utilities for LAN Manager are installed,
defaults to 8 in 2.1 and 2.1a, 4 in 2.2. This
parameter is set in TCPUTILS.INI.
* NSESSIONS = The number of telnet connections, only present if
Microsoft TCP/IP Utilities for LAN Manager are
installed, defaults to 2. This parameter is set in
TCPUTILS.INI.
Thus, systems with the utilities default TCPCONNECTIONS to 17, which is
usually adequate. Systems without the utilities default to 7, which is
often too low for heavy network users.
TCPCONNECTIONS is best adjusted by altering one of the above parameters.
In LAN Manager 2.1, 2.1a, and 2.2, TCPCONNECTIONS itself can be set in
PROTOCOL.INI, but Microsoft may remove it as a configurable parameter in a
future version. Therefore you should alter one of the other parameters and
let TCPCONNECTIONS be determined by the formula.
For more information, query on the following words in the Microsoft
Knowledge Base:
tuning and TCP/IP and tcpwindowsize and tcpsegmentsize
| Modification Type: |
Major |
Last Reviewed: |
7/30/2001 |
| Keywords: |
kbnetwork KB94246 |
|