PI16098: CONGESTION WINDOW SET TO 1 PACKET DURING SLOW START WHEN PACKET TRACE RUNNING ON A SHARED OSA WITH SEGMENTATION OFFLOAD ENABLED

APAR status

• Closed as fixed if next.

Error description

• Normally, the TCP congestion window (CWD) is initialized to 2
  packets during TCP slow start processing. However, when packet
  trace is enabled AND the peer located across a shared OSA
  (LPAR to LPAR), with segmentation offload enabled, the CWD is
  initialized to a single packet and this packet is delayed when
  sent to the peer host.
  
  
  Additional Symptom(s) Search Keyword(s): CWD congestion window
  packet trace segmentation offload slow start
  

Local fix

• turn off packet trace
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server                   *
  * for z/OS Version 2 Release 1 IP: Packet Trace                *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Outbound TCP/IP traffic is delayed when communicating        *
  * between two LPARs sharing a common OSA port when packet      *
  * trace is enabled.                                            *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Packet trace should not cause a delay in outbound traffic.   *
  ****************************************************************
  

Problem conclusion

Temporary fix

Comments

• The solution for this APAR is included in CS for zOS Version 2
  Release 2.
  

APAR Information

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

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

Fix information

Applicable component levels

• R1C0 PSN

     UP

• R1D0 PSN

     UP

• R210 PSN

     UP