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A fix is available


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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
    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


APAR Information

  • APAR number


  • Reported component name


  • Reported component ID


  • Reported release


  • Status


  • PE




  • Special Attention


  • Submitted date


  • Closed date


  • Last modified date


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

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

    UK90668 UK90669



Fix information

  • Fixed component name


  • Fixed component ID


Applicable component levels

  • R1C0 PSY UK90668

       UP13/02/02 P F302

  • R1D0 PSY UK90669

       UP13/02/02 P F302

Fix is available

  • Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.

Document information

More support for: z/OS family

Software version: 1D0

Operating system(s): z/OS

Reference #: PM76570

Modified date: 2013-03-04