INFO: How SNA Server Determines Max BTU Over 802.2 DLC (148972)


The information in this article applies to:

  • Microsoft SNA Server 2.0, when used with:
    • the operating system: Microsoft Windows NT
  • Microsoft SNA Server 2.1, when used with:
    • the operating system: Microsoft Windows NT
  • Microsoft SNA Server 2.11, when used with:
    • the operating system: Microsoft Windows NT
  • Microsoft SNA Server 3.0, when used with:
    • the operating system: Microsoft Windows NT
  • Microsoft SNA Server 4.0, when used with:
    • the operating system: Microsoft Windows NT
This article was previously published under Q148972

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).
Modification Type:MinorLast Reviewed:3/8/2005
Keywords:kbinfo KB148972
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