PK66210: TCPIP MAY SUBTRACT SPACE FOR GRE HEADERS TWICE WHEN CALCULATING THE MTU FOR DISTRIBUTED DVIPA DESTINATIONS

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Space for GRE headers may be subtracted twice when TCPIP
  calculates the MTU to advertise in a ICMP Fragmentation
  needed message when the original packet has the Do Not
  Fragment bit set.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for z/OS Version 1 Release 7, 8, 9, and 10   *
  *                 IP: Sysplex Distributor                      *
  ****************************************************************
  * PROBLEM DESCRIPTION: Space for GRE headers may be subtracted *
  *                      twice. As a result, the optimal MTU     *
  *                      will not be used and additional         *
  *                      fragmentation may occur.                *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  When using VIPAROUTEs, a distributor encapsulates packets
  for a DVIPA in a GRE header. As a result, the MTU is reduced
  by 28 bytes to account for a GRE header. If a packet arrives
  that needs to be fragmented, it is possible that TCPIP will
  subtract space for the GRE headers twice. As a result, a less
  than optimal MTU may be used, and additional fragments may
  be created.
  +-------------------------------------------------------------+
  + 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

• EZBIPINB has been amended to only subtract the space for GRE
  headers once.
  
  * 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:

  UK37924 UK37925 UK37926 UK37927

Modules/Macros

• EZBIPOUT
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R1A0 PSY UK37924

     UP08/08/19 P F809

• R170 PSY UK37925

     UP08/09/11 P F809

• R180 PSY UK37926

     UP08/09/11 P F809

• R190 PSY UK37927

     UP08/09/11 P F809

Fix is available

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