KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub
Q148972: INFO: How SNA Server Determines Max BTU Over 802.2 DLC

Article: Q148972
Product(s): Microsoft SNA Server
Version(s): 2.0,2.1,2.11,3.0,4.0
Operating System(s): 
Keyword(s): 
Last Modified: 13-FEB-2002

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft SNA Server, versions 2.0, 2.1, 2.11, 3.0, 4.0, on platform(s):
   - the operating system: Microsoft Windows NT 
-------------------------------------------------------------------------------

SUMMARY
=======

When you communicate over 802.2 Token Ring or Ethernet, the following parameters
affect the Maximum Basic Transmission Unit (BTU) or "frame" size used by SNA
Server for the 802.2 connection:

- The "Max BTU" value configured within SNA Server Admin 802.2 Connection
  Advanced dialog.

- The maximum frame size supported by the Microsoft Windows NT NDIS driver and
  Token Ring adapter.

- The maximum frame size configured on the remote system, if the SNA Server
  receives a Format 3 XID message from the remote system during connection
  activation.

The maximum frame size supported by SNA Server will be the minimum value
encountered above.

NOTE: See Chapter 2 of the [ASCII 147]IBM SNA Formats Guide[ASCII 148]
(GA27-3136) for a description of Format 0 and Format 3 SNA XID messages.

MORE INFORMATION
================

The following is more detail on how SNA Server determines the Maximum BTU size
on an 802.2 connection:

- In SNA Server Admin, the administrator configures the Maximum BTU size in the
  Advanced dialog for the 802.2 connection.

  SNA Server supports a maximum of 16393 bytes for 16Mbps Token Ring, 4195 for
  4Mbps Token Ring, and 1493 for Ethernet. The Token Ring limits match the
  maximum values supported by the IBM Token Ring 16/4 adapter. The Ethernet
  limit is due to architectural limitations of the Ethernet media. The Max BTU
  size determines the maximum frame size that SNA Server will send.

- When the SNA Server 802.2 link service opens the adapter (using the DLC
  DIR_OPEN_ADAPTER command), it retrieves the max_i_field size from the DHB
  buffer for the adapter. The Windows NT DLC transport gets this max_i_field
  size from the NDIS driver (presumably, the NDIS driver determines the maximum
  frame size from the adapter configuration, or using an internal default).

  NOTE: Some NDIS drivers may be hard-coded to support a maximum frame size of 4
  kilobytes.


- SNA Server compares the Max BTU size configured on the connection with the
  value returned by the DIR_OPEN_ADAPTER command (for example, the value
  supported by the NDIS driver), and uses the value that is the minimum. The
  SNA Server 802.2 link service then issues a DLC_OPEN_SAP command and sets the
  maximum BTU size in the max_i_field in the DLC buffer.


- SNA Server then initiates a SNA XID exchange with the remote system as part
  of the connection activation process. If SNA XID Format 3 messages are used
  in the exchange (SNA Server defaults to use XID Format 3), then the XID
  indicates the maximum BTU size (in Byte 21-22 of the Format 3 XID) that the
  system can receive. If the remote system's Format 3 XID indicates a Max BTU
  size that is *less* than the Max BTU size currently set in SNA Server, the
  minimum value is chosen.

  NOTE: When you communicate with an IBM host system, the maximum frame size
  that can be received by the host is configured in the MAXDATA parameter in
  the VTAM PU definition. When you communicate with an IBM AS/400 system, the
  maximum frame size that can be received by the AS/400 is configured in the
  MAXFRAME parameter in the AS/400 line description (this is not the same as
  the MAXFRAME parameter within the APPC Controller description).


If the SNA Format 0 XIDs are used during connection activation, then the max BTU
size is not indicated in the XID message; as a result, the local and remote
system's configurations should match to prevent connection problems.

Definition of Max BTU Size and VTAM MAXDATA Size
------------------------------------------------

The Max BTU size and the MAXDATA refer to the size of LLC Information frame
(I-frame) that can be transmitted. This includes the SNA Transmission Header
(TH) , Request/Response Header (RH), and at least a portion of the
Request/Response Unit (RU). The I-frame does not include the Mac and LLC header.
For example:

 ---------------  ---------------  -----------------------------
 | TH (6 bytes)|  | RH (3 bytes)|  |  RU (as long as you like) |
 ---------------  ---------------  -----------------------------

If the total length exceeds the Max BTU size, then the data is segmented into two
or more I-frames similar to the following:

 ---------------  ---------------  --------------------
 | TH (6 bytes)|  | RH (3 bytes)|  |  RU (some of it) |
 ---------------  ---------------  --------------------

 ---------------  ----------------------
 | TH (6 bytes)|  | RU (rest of it)    |
 ---------------  ----------------------

The TH contains flags indicating whether the frame is part of a segmented RU or
not.

In each connection between two end points, there are potentially two MAXDATA
sizes; each one being the size which one end can receive. It is perfectly legal
for one end to be able to receive 265 bytes, and the other end 521 bytes. When
the Max BTU size is configured on SNA Server, that size refers to the size of
data you can send.

It is possible for SNA Server to have control over the size of data it receives
if the connection uses Format 3 XIDs---in these you specify the maximum BTU size
you can receive. If the connection uses Format 0 XIDs then it is up to the
configuration to make sure that one end does not send the other end a larger
frame than it can accept.

SNA Server Downstream Physical Units (DSPU) Considerations
----------------------------------------------------------

When you connect Downstream PU 2.0 devices through the SNA Server DSPU feature,
the handling of Max BTU size is more complicated because there are two SNA
Server connections involved---the SNA Server host connection and the SNA Server
Downstream PU connection. Most of the time the maximum BTU sizes on these two
connections do not effect each other, because the SNA Server in the middle will
do any segmentation as needed.

--------        -------------          -----------
| DSPU |--------|    SNA    |----------|   HOST  |
--------        |   SERVER  |          -----------

               -------------

Consider the case of the diagram above. If the DSPU (downstream PU2.0 device)
supports a frame size of 521 bytes, and the host connection supports a frame
size of 265 bytes, then when the DSPU sends SNA Server a frame of 521 bytes, SNA
Server will segment it into two frames before sending it to the host. In this
case, there is no relationship between the DSPUs MAXDATA configuration setting
and the host's MAXDATA value configured on the VTAM Physical Unit description.

There is one exception. Some SNA sessions do not support segmentation; these are
the SSCP-PU and the SSCP-LU sessions. If Format 0 XIDs are used during
connection activation (the only value supported by PU2.0 devices), SNA Server is
not notified of the maximum BTU size supported by the DSPU device. If the host
sends a USSMSG10 SSCP "banner" screen with an RU size of 512 bytes, but the DSPU
only supports a Max BTU size of 265 bytes, then the DSPU may reject the logon
screen with a frame reject (FRMR), or possibly hang if it does not handle this
properly. To work around this, configure the RU size on the SSCP-LU session no
larger than the DSPUs MAXDATA size (taking the TH and RH into account).

Additional query words: 2.00 2.10 2.11 prodsna snadlc

======================================================================
Keywords          :  
Technology        : kbAudDeveloper kbSNAServSearch
Version           : :2.0,2.1,2.11,3.0,4.0
Issue type        : kbinfo

=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.