PM76570: CALCULATED ROUND TRIP TIME INCORRECT

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• The Round Trip Time (RTT), Smoothed Round Trip Time (SRTT),
  and Round Trip Time Variance (RTTV) are all greatly
  exaggerated.
  .
  Additional Symtpoms:
  ABEND0C9 in EZBTMQTD + x'307E' on DR instruction attempting to
  calculate the RTT for a Telnet connection display.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for z/OS Version 1 Release(s) 12 and 13 IP   *
  ****************************************************************
  * PROBLEM DESCRIPTION: TCP connection round trip information   *
  *                      is incorrectly calculated.              *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  TCP connection round trip information is incorrectly calculated.
  
  The TCP round trip time (RTT), smooth round trip time (SRTT) and
  round trip time variance (RTTV) is incorrectly calculated for
  a connection that is Dynamic Right Sizing (DRS) eligible.  To
  be DRS eligible a TCP connection must have a receive buffer size
  of 64K or larger.
  
  The real round trip time is calculated when an ACKnowledgement
  is received for an outbound data packet.  The DRS function will
  calculate an estimated round trip time for a connection that
  is not sending data packets, or sends only 1 data packet in any
  single send operation.  When a single data packet is sent the
  remote peer TCP/IP stack may perform DelayACK processing which
  can delay the ACK and cause the calculated round trip time to
  be higher then the actual round trip time.  If DRS calculates
  the estimated round trip time and it is smaller than the
  previously calculated round trip time it will be used and the
  real round trip time calculation will be skipped.
  
  A timing condition exists where a single outbound data packet
  is sent causing the real round trip starting information to
  be saved.  An inbound data packet that does not contain the
  ACK for the sent data packet arrives resulting in an outbound
  ACK being sent and the DRS estimated round trip information
  is saved.  The ACK arrives inbound for the data packet and
  the round trip calculation routine (TCPRTT) detects DRS is
  estimating the round trip and skips the calculation of the
  real round trip time, but fails to clear the real round
  trip saved information.  At some later point in time another
  outbound data packet is sent and detects the saved round trip
  information so new information is not saved.  When the ACK
  arrives for this outbound data packet the round trip time
  is calculated using the arrival time of the ACK and the
  saved round trip information.  The resulting RTT, SRTT and
  RTTV will reflect the incorrectly calculated values.
  +-------------------------------------------------------------+
  + Please check our Communications Server for OS/390 homepages +
  + for common networking tips and fixes.  The URL for these    +
  + homepages can be found in Informational APAR II11334.       +
  +-------------------------------------------------------------+
  

Problem conclusion

• TCPRTT has been amended to calculate the real round trip time
  in addition to the DRS estimated round trip time and use the
  smallest value.
  
  * Cross Reference between External and Internal Names
  

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:

  UK90668 UK90669

Modules/Macros

• EZBCTFME EZBITTRC EZBTCRD  EZBTCSYN TCPRTT
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R1C0 PSY UK90668

     UP13/02/02 P F302

• R1D0 PSY UK90669

     UP13/02/02 P F302

Fix is available

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