Browse Prior Art Database

Method for handling FICON exchange overlap

IP.com Disclosure Number: IPCOM000013974D
Original Publication Date: 2000-Jun-01
Included in the Prior Art Database: 2003-Jun-19
Document File: 3 page(s) / 30K

Publishing Venue

IBM

Abstract

Disclosed is a method for handling the exchange overlap which may occur between consecutive operations on the same logical path across a FICON link. An exchange overlap occurs when an FC-4 Information Unit (IU) is received opening a new exchange before the FC-4 IU is received closing the currently open exchange. The method to handle the overlap is for the receiving entity, be it channel or control unit, to respond to the IU opening the new exchange with a Link Busy IU, and then discard all other IUs received on that exchange until the LACK IU in response to the Link Busy IU is received. The sending entity, be it control unit or channel, after sending the LACK IU in response to the Link Busy IU, redrives the IU opening the new exchange, and now the operation proceeds normally. In the examples below, I will use the term Outbound Exchange (OBE) to denote an exchange from the channel and Inbound Exchange (IBE) to denote an exchange from the control unit. An example of an overlap which a control unit would need to handle is: A Status Accept (SAC) IU has been sent closing the existing Outbound Exchange (OBE) and a new Command IU opening a new OBE is sent to the same device, but the Command IU is received first. As Sent by Channel: Channel Device

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Method for handling FICON exchange overlap

Disclosed is a method for handling the exchange overlap which may
occur between consecutive operations on the same logical path
across a FICON link. An exchange overlap occurs when an FC-4
Information Unit (IU) is received opening a new exchange before the
FC-4 IU is received closing the currently open exchange. The
method to handle the overlap is for the receiving entity, be it
channel or control unit, to respond to the IU opening the new
exchange with a Link Busy IU, and then discard all other IUs
received on that exchange until the LACK IU in response to the Link
Busy IU is received. The sending entity, be it control unit or
channel, after sending the LACK IU in response to the Link Busy
IU, redrives the IU opening the new exchange, and now the operation
proceeds normally.

In the examples below, I will use the term Outbound Exchange (OBE)
to denote an exchange from the channel and Inbound Exchange (IBE)
to denote an exchange from the control unit.

An example of an overlap which a control unit would need to handle
is: A Status Accept (SAC) IU has been sent closing the existing
Outbound Exchange (OBE) and a new Command IU opening a new OBE is
sent to the same device, but the Command IU is received first.

As Sent by Channel:
Channel Device
------------------------------------

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

Data IU on Existing OBE --->

                                     <--- Early End Status IU
Closing Existing IBE

SAC IU Closing Existing OBE --->
Command on New OBE --->
Data IU on New OBE --->

As Received by Device:
Channel Device
------------------------------------

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

Data IU on Existing OBE --->

                                     <--- Early End Status IU
Closing Existing IBE

Command on New OBE --->
SAC IU Closing Existing OBE --->
Data IU on New OBE --->

An example of an overlap which a channel would need to handle is: A
DACK IU to a Cancel IU has been sent by the device closing the

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existing Inbound Exchange (IBE), followed immediately by the
presentation of status by that device on a new IBE, but the Status
IU is received first.

As Sent by Device:
Channel Device
------------------------------------

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

                                     <--- Data IU on Existing
IBE

                                     <--- Data IU on Existing
IBE

Cancel IU Closing Existing OBE --->

                                     <--- Data IU on Existing
IBE

                                     <--- Data IU on Existing
IBE

                                     <--- DACK IU to Cancel
Closing Existing IBE

                                     <--- Status Opening New
IBE

As Received by Channe...