PQ51688: SLOW PERFORMANCE WHEN PMTUDISCOVERY IS ACTIVE

Obtain the fix for this APAR.

APAR status

• Closed as unreproducible in next release.

Error description

• Slow performance when PMTU Discovery is active.
  This is caused by the wrong route being chosen originally
  and the MTU being set incorrectly.
  VERFICATION STEPS:
  In a Packet trace you will see strange outbound patterns
  showing out of order packets.
  ex.
  A      I 2968952165 2968952694 368640     0   0.003041 10:52:5
  AP  F  O 2968965194 2968952165 368640     0   2.348429 10:53:0
  AP     O 2968952694 2968952165 368640  8140   6.819436 10:53:0
  -
  CTRACE TCP & INTERNET will show a message built and on the send
  attempt the below INTERNET CTRACE entry is cut:
  INTERNET  30010050  10:53:00.827013  !Outbound_Error
  These sends will not appear in the Packet trace.
  This occurs because we have created a packet that is too large
  for the PMTU route the send is going out and the don't fragment
  bit is on.
  -
  KEYWORDS: PMTU MTU MSS PERFORMANCE
  

Local fix

• WORK AROUND: Disable PMTUdiscovery
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the Communications Server for   *
  *                 OS/390 Release 10 IP and z/OS Version 1      *
  *                 Release 2 : PATHMTUDISCOVERY and MULTIPATH   *
  ****************************************************************
  * PROBLEM DESCRIPTION: Negotiated MSS is too large because     *
  *                      PathMTU Host route was not chosen when  *
  *                      session was initialized.                *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  PathMTU Host routes should be placed at the top of the list of
  applicable routes so the correct MSS values will be used during
  SYN SYN/ACK processing.  Failure to do so can result in
  session slowdowns because the MSS used to build the packet is
  larger then the MTU size of the actual PathMTU Route and the
  packet is discarded necessitating retransmits.
  +-------------------------------------------------------------+
  + 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

Temporary fix

Comments

• Route allocation logic will be amended to place PathMTU Host
  routes at the beginning of the applicable chain.
  

APAR Information

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

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

  UQ58451 UQ58452

Modules/Macros

• AFSNDINT EZBAFINB EZBAFUTL EZBAFWLM EZBIPRTE
  EZBPTREE EZBTCFWR EZBTCSND TCSNDINT
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R120 PSY UQ58451

     UP01/10/31 P F110

• R50A PSY UQ58452

     UP01/10/31 P F110

Fix is available

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