PQ72867: TCPIP DOES NOT RESPOND TO ICMP ECHO REQUEST (PING) PACKET WITH DF (DON'T FRAGMENT) BIT SET

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Certain TCPIP implementations, in this case AIX, use ICMP echo
  (PING) packets with DF (don't fragment) set to establish Path
  MTU discovery. The problem is TCPIP is not responding to the
  PING with DF set and the outbound reply is getting discarded.
  

Local fix

• verification steps:
  Ctrace with option internet will have the following exception
  trace record:
  !Outbound_Error with error code x'6f' (eacces) set and the
  IP header of the PING reply packet with protocol set to x'01'
  at offset x'09' into the ip header.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for OS/390 Release 10 IP                     *
  ****************************************************************
  * PROBLEM DESCRIPTION: ICMP echo requests with the IP header   *
  *                      flag DF (don't fragment) set on may not *
  *                      receive a response.                     *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  ICMP echo requests that arrive with the IP header DF set
  (don't fragment) may not receive a response.  The stack
  generates the response IP header by copying the inbound header
  and switching the source and destination addresses.  This causes
  the DF flag to be on in the outbound IP header.  If the
  response packet is larger than the MTU of the outbound device
  it will be discarded because it cannot be fragmented.
  The ping will timeout and the stack will cut an exception ctrace
  record "!Outbound_Error with error code x'6f' (eacces)".
  The 'Datagrams Failing Fragmentation' count from the NETSTAT
  STATS will reflect the discarded packet(s).
  +-------------------------------------------------------------+
  + 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

• EZBIPICM has been amended to unconditionally set the DF bit off
  in echo response IP headers.
  
  * Cross Reference between External and Internal Names
  

Temporary fix

Comments

APAR Information

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

  PQ67784

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

  UQ76165

Modules/Macros

• EZBIPICM
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R50A PSY UQ76165

     UP03/05/15 P F305

Fix is available

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