PH30154: ORS (OUTBOUND RIGHT SIZING) IS NOT GETTING ENABLED DUE TO RTT CALCULATION ERROR

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• In order to enables TCP ORS for a TCP connection the RTT (round
  Trip Time) needs to be calculated to see if it meets the minimum
  requirement for ORS (2ms by default). The RTT value is being
  incorrectly calculated at the time where the ORS enablement
  logic is processing and it is being set to less then 2ms. This
  caused ORS to not be enabled for the respective TCP outbound
  streaming connection as seen in the netstat all output where
  TCPPRF2 s set to 2x. Instead of 7x which would be expected for
  the connection.
  
  ANALYSIS:
  tcprtt
  
  KNOWN IMPACT:
   not being enabled when it should be
  
  VERIFICATION STEPS:
  systcpip component trace with option TCP. The trace collected at
  the point where ORS logic is checking RTT shows x01 bit set at
  offset x5F3 into the TCB for the connection, at offset x635 x20
  indicating that the connection is ORS eligible and the RTT value
  being less then 2 at offset xA4 into the TCB. After this point
  the TCP send buffer size is not being dynamically increased for
  the connection, leading to throughput issues
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 3 and 4 IP                                         *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * TCP connection round trip time (RTT) calculation may occur   *
  * for an ACK packet that does not acknowledge the segment      *
  * being timed.  The first sequence number in the segment being *
  * timed is recorded in the connection.  When an ACK packet     *
  * arrives that acknowledges that sequence number the RTT can   *
  * be calculated.  The check to determine if the RTT            *
  * calculation should be performed is incorrectly comparing for *
  * the ACK being equal to or greater than the saved sequence    *
  * number of the segment being timed.  This can result in the   *
  * RTT being calculated when an ACK packet contains the         *
  * sequence number that is equal to the segment being timed.    *
  * When the RTT calculation is performed for the equals case it *
  * will be smaller than the real RTT.                           *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  

Problem conclusion

• EZBTCRD has been amended to use an ACK that acknowledges the
  segment being timed.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UI72121 UI72122

Modules/Macros

• EZBTCRD
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R230 PSY UI72121

     UP20/11/17 P F011

• R240 PSY UI72122

     UP20/11/17 P F011

Fix is available

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