PI21269: FRR IS NOT CORRECTLY RE-SENDING LOST PACKETS, INSTEAD "REAL" TCP RE-TRANSMISSION IS OCCURRING

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• TCP is not immediately re-sending lost packets during FRR
  (receipt of 3 dup ACKS). Instead the lost packet(s) are being
  re-transmitted using the TCP retransmission timer, which is
  500ms by default.
  
  verification steps:
  
  Packet trace will show zOS as sending host. If packet loss is
  occurring the remote host will have sent at least 3 duplicate
  Acknowledgements, but the lost packets(s) will not be
  immediately sent as part of FRR processing.
  
  
  
  
  
  
  Additional Symptom(s) Search Keyword(s): FRR dup ack rexmit
  retransmission re-transmission dupacks dupack
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 1 IP                                               *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Slow TCP performance                                         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply PTF                                                    *
  ****************************************************************
  TCP streaming applications such as file transfers, can
  experience performance degradation if there is packet loss in
  the network. A check in EZBTCRD to determine if the connection
  was in Fast Retransmit and Recovery (FRR) was mistakenly
  deleted, causing the faster retransmits to be bypassed.
  

Problem conclusion

• EZBTCRD will be changed to restore the check to see if the
  connection is in FRR and retransmit the packets before the
  normal
  timeout retransmit.
  

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:

  UI19687

Modules/Macros

• EZBTCRD
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R210 PSY UI19687

     UP14/09/03 P F409

Fix is available

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