SNA Server and 802.2 connection timers (t1, t2, ti)

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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 timer values are specified in timer ticks. Each timer requires a short timer tick (xxTickOne, where xx is t1, t2, or ti) and long timer tick (xxTickTwo, where xx is t1, t2 or ti). The length of these timer ticks are specified within the Windows NT DLC Transport driver (in the registry) in 40 millisecond increments.

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
				

If the selected number is between 6 and 10, the long timer tick (xxTickTwo) is used. In this case the timer value is equal to the selected number minus 5 multiplied by the long timer tick value.

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

The response timer (t1) is used to detect a failure to receive an acknowledgment or a response from the remote link station.

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. In some networks the T1 value will be increased to allow for propogation delay. DLC extends the T1 value using an algorithm based on the average poll response time of the connection:

Average Poll response time:     Response delay added to T1 and (Ti):
   40 ms                            0
   80 ms                           16 * 40 = 640 ms
  120 ms                           16 * 40 = 640 ms
  160 ms                           32 * 40 = 1280 ms
  200 ms                           32 * 40 = 1280 ms
  240 ms                           48 * 40 = 1920 ms
  ...
				

The poll response time is the time it takes for the remote end to respond to a frame sent with a Poll bit.

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
				

These multiplier values are used in conjunction with Windows NT Data Link Control transport settings to calculate the actual timer value used on the connection. The Windows NT DLC transport is configured with the following default timer values:

   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.

These default settings yield the following timeout periods:

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)
				

If SNA Server is connecting to the remote system over Token Ring across source routing bridges, then long tick timers are used, giving the following default timer settings:

  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)
				

The SNA Server ti, t2 and ti timers can be manually set for each connection. The maximum value of each timer is 10, which yields the following timeout periods:

  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)
				

To increase these timers further, the DLC transport timer settings must be increased within the registry. However, caution should be taken before changing these settings.