TN3270/E Server Service Incorrectly Handles Courtesy Acks

• Microsoft SNA Server for Windows NT, version 2.11 Service Pack 1

SYMPTOMS

When you use a client that supports TN3270/E, the client connection may drop abnormally.

CAUSE

The TN3270/E Service may improperly handle acknowledgments between SNA Server and a TN3270E client, causing the SNA Server to report an invalid sequence number error to the TN3270/E Service. This error results in a dropped TN3270E client session.

RESOLUTION

Made a change to the TN3270E Server Service to not treat the RUI_WRITE as an implicit acknowledgment.

STATUS

Microsoft has confirmed this to be a problem in Microsoft SNA Server version 2.11 Service Pack 1. This problem was corrected in the latest Microsoft SNA Server 2.11 U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K

MORE INFORMATION

The Way TN3270 (Not TN3270E) Clients Get Acknowledged by the TN3270/E Server Service

RUI applications, such as the TN3270/E Server service, are required to send courtesy acknowledgments for RQE data, so that the node can clear out its internal correlation table.

When the TN3270 Server is connected to a TN3270 client, the SNA Server Service issues a courtesy acknowledgment for each RU of data it receives from the SNA host just as soon as it has passed the data on to the TN3270 client.

The Way TN3270/E Clients Get Acknowledged by the TN3270/E Server Service

When the TN3270 Server is connected to a TN3270/E client, acknowledgments are done differently. In TN3270E (unlike TN3270), data is passed to the TN3270E client with a flag specifying whether the data is RQD or RQE. The TN3270E terms for these are ALWAYS-RESPONSE and ERROR-RESPONSE. In order to match up responses from the TN3270E client, there is also a sequence number on each data message between the server and the client. This sequence number does not match the SNA sequence number in the SNF field (as defined by the IBM SNA Formats Guide).

Therefore, the effect of this is that the TN3270E Server cannot immediately send a courtesy acknowledgment for each RU of data it receives from the node, because the TN3270E client might, at some later time, send a negative response to the data, and the server is supposed to pass this negative response on to the SNA host. Therefore, the TN3270E Server has to keep its own correlation table---mapping TN3270E sequence numbers to SNA sequence numbers, just in case a TN3270E client decides to send a negative response to an ERROR-RESPONSE message. This correlation table has a fixed number of entries, and whenever this table begins to fill up the TN3270 Server clears out some of it (but only the oldest messages), and sends a courtesy acknowledgment at that point for the messages it is removing from the table.

The following diagram illustrates the message flow of the problem:

TN3270E client              TN3270E Service               SNA Server

                                                          <- RUI_READ Data
                                                          (RQE, SNF=001E)

                            <- RUI_READ OK (Data,
                            RQE, SNF=001E)

                            <- Data (RQE, SNF=001E)

                            Data (ACK, SNF=001E) ->

                            Implicit ACK (SNF=001E).   ->
                            See Note 2.
                                                          <- Error (Invalid
                                                          Sequence Number)

                            <- Error (Invalid Sequence Number)

• This is an RUI_WRITE and is treated as a courtesy ack for the RUI_READ that was received earlier. You can find out which data is being acknowledged by looking at the snf field, and matching it up with the snf field for the previous RUI_READ (001E).
• The TN3270E Server's algorithm does not take a crucial fact into account. This fact is that the node treats inbound data (that is, data from an TN3270E emulator) as an implicit courtesy acknowledgment for any outbound data it has previously passed to the emulator. In this case, there has been an exchange of outbound and inbound data (from the TN3270E emulator), and by the time that the TN3270E Server issues a courtesy acknowledgment to the SNA Server Service then the message for which it is sending the acknowledgment has already been implicitly acknowledged by some inbound data (RUI_WRITE). This causes the error which causes the session to end.