PI72242: SLOW TCP THROUGHPUT SHOWING A LARGE NUMBER OF DUPLICATE ACKS

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• .
  Problem can occur for high latency connections that experience a
  mix of lost and out-of-order packet transmissions.  TCPIP
  attempts to prevent FRR from enabling for connections that are
  experiencing only out-of-order packets, which is common for high
  latency connections.  The problem is this code doesn't handle
  the case when out-of-order packets occur along with true lost
  packets.  In this case we need FRR to engage for the lost packet
  case.
                                                                 .
  Symptoms of this from a packet trace show many DUP ACKs (even as
  many as over 100) received without FRR enabling.  Instead, we
  see the packet retransmitted only after the retransmission timer
  pops.  This leads to significant slowdown of the data transfer
  rate.
  

Local fix

• .
  If there is a high rate of packet loss on a connection,
  investigate the underlying cause in the network and correct.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 1 IP                                               *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * TCP throughput degradation after receipt of large number of  *
  * duplicate acknowledgements (DUP ACKs).                       *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply the PTF.                                               *
  ****************************************************************
  Sending side of a high latency TCP connection experiences both
  out-of-order and lost packets. As a result, a large number of
  duplicate acknowledgements are received. Code currently attempts
  to avoid entering FRR mode when it detects packets are
  out-of-order. This is preventing FRR from enabling in the cases
  when the DUP ACKs are a result of lost packets and degrades
  throughput.
  

Problem conclusion

• Code has been modified to not prevent FRR enablement if
  retransmission due to lost packets has occurred.
  

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:

  UI43295

Modules/Macros

• EZBTCRD
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R210 PSY UI43295

     UP17/02/21 P F702

Fix is available

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