SNA Server and 802.2 Connection Timers (t1, t2, ti)

• Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11 and 3.0

SUMMARY

By default, the SNA Server Admin program sets the t1, t2 and ti timers on all 802.2 connections to a value of "Default". This article describes the actual timer values used by SNA Server when this default setting is configured, as well the various DLC timers and default settings.

Tuning the SNA Server 802.2 connection and/or DLC timers may be necessary if 802.2 connections are dropping due to timeout problems. A timeout may be occurring if SNA Server logs Event 23, Qualifier = AE or AF within the Windows NT application event log. However, a network monitor or sniffer trace would be necessary to confirm if a timeout problem is occurring (as opposed to an underlying network or bridge/router problem).

If an SNA Server 802.2 connection timer is changed, the change will take effect when the SNA Server service is stopped and restarted.

MORE INFORMATION

DLC Timer Parameters

The following information describes various DLC timer parameters and how DLC timer values are calculated.

The DLC class II protocol uses three timers:

   t1 - Response timer
   t2 - Acknowledgment timer
   ti - Inactivity timer

The actual length of each timer (t1, t2, ti) is selected with parameters provided by the DLC application (SNA Server in this case) when the DLC_OPEN_SAP, DLC_OPEN_STATION, or DLC_MODIFY commands are issued. In these commands, the timer value is selected with a number between 1 and 10. If the number is between 1 and 5, the short timer tick (xxTickOne) is used. The timer value is equal to the selected number multiplied by the short timer tick value:

   timer value = selected number * xxTickOne * 40ms

   timer value = (selected number - 5) * xxTickTwo * 40ms

The link station starts t1 when it transmits either an Information (I-frame) or Supervisory (S-frame, such as a RR/RNR/REJ) with the poll bit set. The t1 will be reset when the station receives a response frame with the final bit set. If t1 expires, the link station sends an S-frame with the poll bit set in order to query the status of the remote link station. The link station then restarts t1. If there is no recovery after the specified number of retries, the link station assumes that the link is inoperative and disconnects the link.

The duration of t1 must take into account any delays introduced by the source routing bridges or routers separating SNA Server and the remote system.

The link station uses the acknowledgment timer (t2) to delay the sending of an acknowledgment for a received I-frame. The t2 timer is started when a link station receives an I-frame. The t2 timer is reset when a link station sends an acknowledgment in an I-frame or in an S-frame. If the t2 expires, the link station must send an acknowledgment as soon as possible.

The value of t2 must be less than the value of t1 of the remote station. This ensures that the remote link station will receive the acknowledgment before its response timer expires.

The inactivity timer (ti) is used to detect an inoperative condition in the remote link station or in the transmission medium. If a station does not receive any S- or I-frames before its ti expires, it must query the status of the remote link station with an S-frame with the poll bit set. If it does not get a response after specified number of retries, it disconnects the link.

SNA Server and 802.2 Connection Timers

When an SNA Server 802.2 connection is activated, SNA Server sends a TEST frame to the remote network address configured for the connection.

On Token Ring, the local ring is tried first. If there is no response to the TEST frame, SNA Server resends the TEST frame with the "all routes broadcast" setting enabled which is then forwarded by source routing bridges. If there is source routing data present in the TEST frame response from the remote system, then long tick timers are used on the connection. Otherwise, short tick timers are used.

On Ethernet, since there is no source routing data, the short tick timers are always used.

Here are the short tick and long tick timers used by SNA Server:

   Parameter   Short Tick Timer   Long Tick Timer
   ---------   ----------------   ---------------
   t1          2                  7
   t2          2                  6
   ti          5                  10

   t1TickOne = 5
   t1TickTwo = 25   (hex = 0x19)
   t2TickOne = 1
   tT2TickTwo = 10   (hex = 0xa)
   tiTickOne = 25   (hex = 0x19)
   tiTickTwo = 125  (hex = 0x7d)

   NOTE: These values are defined in the HKEY_LOCAL_MACHINE subtree of the
   registry in the  \SYSTEM
                      \CurrentControlSet
                        \Services
                          \DLC
                            \Parameters
                              \<network_adapter> directory.

   WARNING: Using the Registry Editor incorrectly can cause serious,
   system-wide problems that may require you to reinstall Windows NT to
   correct them. Microsoft cannot guarantee that any problems resulting
   from the use of the Registry Editor can be solved. Use this tool at your
   own risk.

If SNA Server is connecting to the remote system over Ethernet or Token Ring where no source routing bridges are involved, then short tick timers are used, giving the following default timer settings:

  t1 timer = 2 * 5  * 40ms = 400ms  (.4 seconds)
  t2 timer = 2 * 1  * 40ms = 80ms   (.08 seconds)
  ti timer = 5 * 25 * 40ms = 5000ms (5 seconds)

  t1 timer = (7-5)  *  25 * 40ms = 2000ms  (2 seconds)
  t2 timer = (6-5)  *  10 * 40ms = 400ms   (.4 seconds)
  ti timer = (10-5) * 125 * 40ms = 25000ms (25 seconds)

  t1 timer = (10-5) *  25 * 40ms = 5000ms  (5 seconds)
  t2 timer = (10-5) *  10 * 40ms = 2000ms  (2 seconds)
  ti timer = (10-5) * 125 * 40ms = 25000ms (25 seconds)