PI91114: Incorrect Smoothed Round-Trip-Time for low activity sessions

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• .
  For TCP connections that have large receive buffers, little to
  no outbound data, and infrequent inbound data in small packets,
  the Smoothed Round-Trip-Time (SRTT) can be significantly larger
  than it should be (much larger than a PING response, in some
  cases over an hour).  This will also affect the calculated
  Round-Trip-Variance as well.
  
  These values are reported in:
  
   - NETSTAT ALL output as the 'Smooth trip time' and
     SmoothTripVariance fields.
  
   - SMF 119 subtype 2 (TCP Connection Termination) records in the
     SMF119AP_TTRTT and SMF119AP_TTRVA fields.  These are also
     provided by the SYSTCPCN NMI.
  
  If any packet loss occurs for outbound data, the subsequent
  retransmission times will be affected.  In the most severe
  cases, it will always use the applicable MAXIMUMRETRANSMITTIME
  interval (default is two minutes).
  

Local fix

• <None>
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of thed IBM Communications Server for z/OS Version *
  * 2 Releases 2 and 3 IP                                        *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Incorrect Round-Trip-Time (RTT) calculation for low activity *
  * sessions can lead to elongated retransmit times.             *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply the PTF                                                *
  ****************************************************************
  This scenario is a connection with all inbound traffic (or at
  most a few singleton outbound packets).  If the receive buffer
  is large relative to the size of the packets, it can be a long
  time before the initial window's worth of data has been
  received. In this case, the DRS estimated RTT time will grow for
  that entire period, and eventually gets used to calculate the
  RTT and SRTT for the connection.  The incorrect RTT and SRTT can
  cause the retransmit timer to be set to a very large value, and
  the round trip times may also be reported in Netstat ALL and SMF
  output.
  

Problem conclusion

• The fix ensures real round trip time (tcb_rtt) and smooth round
  trip time (tcb_srtt) are not calculated from the dynamic right
  sizing (DRS) estimated RTT.
  

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:

  UI53051 UI53052

Modules/Macros

• EZBTCRD  EZBTCSYN
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R220 PSY UI53051

     UP18/03/17 P F803

• R230 PSY UI53052

     UP18/03/17 P F803

Fix is available

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