How a 3270 EIS Application May Affect Outbound Pacing (181737)

SUMMARY

When using the SNA Server 3270 EIS interface, a 3270 application has the option of choosing the Application Pacing option in the Connection Information Control Block when sending its OPEN(PLU) response, as documented in section 3.3.1 of the 3270 EIS online guide. This option can be used by 3270 applications that have limited resources (for example, a printer) to allow the application to affect SNA Server pacing responses on outbound host data, preventing the host from sending more data than the 3270 application can handle.

However, the 3270 application must implement this option properly, or SNA Server does not respond to host pacing requests, causing the SNA session to hang (stop responding).

The 3270 application sets the Application Pacing option in the Open(PLU) response by setting dataru[21] in the first element to 0x01. After it sets this option, the emulator must send Status(Resource) messages to the SNA Server to indicate that it can accept further outbound messages. The emulator must supply enough credit to the SNA Server for the server to accept a full pacing window of messages from the host (or the SNA Server does not respond to the host pacing indicator request).

If this happens, the 3270 application hangs with an XCLOCK indication, because the host is unable to send any further messages on the session.

MORE INFORMATION

The SNA pacing responses that the SNA Server sends to the host are not like the credit (Status Resource) messages that the 3270 application sends to the SNA Server:

In Status Resource messages, the 3270 application specifies how much credit it is giving. It can specify as much or as little as it likes in pacing responses to the SNA Server, but it cannot specify the number of messages the host can now send.
An SNA Server pacing response (or IPR) tells the host that it can now send the SNA Server one more pacing window's worth of messages (where the size of the pacing window was established in the SNA session BIND).

The idea behind the pacing window is that the SNA Server will not grant the host another pacing window's worth of credit until it can guarantee to be able to immediately pass every message in that pacing window to the 3270 application. To be able to guarantee this, the 3270 application must have given the SNA Server enough credit for those messages, and any the SNA Server has already guaranteed to send, before the SNA Server sends the host the pacing response for them.

Here is a scenario that could cause a 3270 application to hang, by not providing sufficient credit responses to SNA Server:

Assume that the host can send seven messages to SNA Server before needing a pacing response to send more. The SNA Server can send eight messages to the 3270 application before it needs a credit (Status Resource) message to send more.

When the session is first started, SNA Server has "given" seven pacing credits to the host and "received" eight application credits from the 3270 application. At this point, SNA Server has one credit "in hand" for the host, but cannot send a pacing response until it has seven credits "in hand" from the 3270 application.

Next, assume the host sends seven messages to SNA Server, which are passed to the 3270 application, and the application responds with a Status Resource message providing a further three credits. After this, the host has no (0) pacing credits, and the SNA Server has (8 - 7 + 3 =) 4 credits from the 3270 application, which are insufficient to grant the host another pacing window of seven. The SNA Server will not send the host a pacing response (that is, Isolated Pacing Response, or IPR) until it gets (at least) another three credits from the 3270 application.

As a side note, it is possible to capture messages sent by the emulation product by enabling SNA Server 3270 message tracing on the SNA Server itself, or on the client. Here is an example of an Open PLU Response message, as traced in an SNA Server 3270 message trace:

   FMI   ----------------------------------------------- 14:38:27.0546
   FMI   41120F00->01023205 OPEN  PLU  RSP OK
   FMI                      FMI  LU#10 CredR:0 CredS:8
   FMI
   FMI   ---- Header  at address 0146DD74, 2 elements ----
   FMI   0202020A 00010000 000800AF 01003000     <..............0.>
   FMI
   FMI   ---- Element at address 01980C88, start 1, end 25 ----
   FMI   20202020 20202020 20200000 00000000     <          ......>
   FMI   00000020 01010000 02                    <... .....       >
   FMI
   FMI   ---- Element at address 0197E49C, start 13, end 59 ----
   FMI   31010303 B1903080 000787C7 80000280     <1.....0...gG....>
   FMI   00000000 18500000 7E000008 E3E2D6F1     <.....P..~...TSO1>
   FMI   F0F1F1F8 000008E3 C3F3F0F0 F0F0C1       <0118...TC30000A >

In the above trace, the Application Pacing option is set, because byte 21 of the first element is set to 01.

Here is an example of a Status Resource message, as appearing in an SNA Server 3270 message trace:

   FMI   ----------------------------------------------- 14:38:27.0703
   FMI   41120F00->01023205 FMIST RSRC
   FMI                      Credit:3
   FMI
   FMI   ---- Header  at address 0146E1B8, 0 elements ----
   FMI   04020003 00010000 00080100 01000000     <................>

In this message, the 3270 emulator is informing SNA Server that it can accept three more messages on this session.