VM TCP/IP Discards Packets That Require Fragmentation But Have
The Don't Fragment Bit Set


 
 APAR Identifier ...... PQ37002      Last Changed ........ 01/02/21
 VM TCP/IP DISCARDS PACKETS THAT REQUIRE FRAGMENTATION BUT HAVE
 THE DON'T FRAGMENT BIT SET.
 
 Symptom ...... IN INCORROUT         Status ........... CLOSED  PER
 Severity ................... 3      Date Closed ......... 00/04/04
 Component .......... 5735FAL00      Duplicate of ........
 Reported Release ......... 320      Fixed Release ............ 999
 Component Name TCP/IP V2 FOR V      Special Notice
 Current Target Date ..              Flags
 SCP ...................
 Platform ............
 
 Status Detail: SHIPMENT - Packaged solution is available for
                           shipment.
 
 PE PTF List:
 
 PTF List:
 Release 240   : UQ42156 available 00/04/12 (1000 )
 Release 310   : UQ42157 available 00/04/12 (0002 )
 Release 320   : UQ42158 available 00/04/12 (0002 )
 
 Parent APAR:
 Child APAR list:
 
 ERROR DESCRIPTION:
 Hosts using Path MTU discovery fail if the route includes a
 VM TCP/IP system. Tracing the problem it was found that VM
 TCP/IP is not conforming to RFC1191 section 4 but instead is
 discarding packets if fragmentation is required but the Don't
 Fragment Bit is on.
 
 LOCAL FIX:
 
 PROBLEM SUMMARY:
 ****************************************************************
 * USERS AFFECTED: Users running VM TCP/IP in a network where   *
 *                 Path MTU discovery is used. In particular,   *
 *                 environments where Linux for S/390 is used   *
 *                 in conjunction with VM TCP/IP may experience *
 *                 this problem.                                *
 ****************************************************************
 * PROBLEM DESCRIPTION: VM TCP/IP does not conform with RFC     *
 *                      1191 section 4, "Router                 *
 *                      Specifications",which states:           *
 *                      "When a router is unable to forward a   *
 *                      datagram because it exceeds the MTU of  *
 *                      the next-hop network and its Don't      *
 *                      Fragment bit is set, the router is      *
 *                      required to return an ICMP Destination  *
 *                      Unreachable message to the source of    *
 *                      the datagram, with the Code indicating  *
 *                      'fragmentation needed and DF set'."     *
 *                      As a result, path MTU discovery cannot  *
 *                      always be performed in a network that   *
 *                      includes VM TCP/IP.                     *
 ****************************************************************
 * RECOMMENDATION: APPLY PTF                                    *
 ****************************************************************
 VM TCP/IP discards datagrams that require fragmentation but have
 their Don't Fragment bit set. As a result, a host using path MTU
 discovery cannot perform it if the route through the network
 includes a VM TCP/IP system.
 
 PROBLEM CONCLUSION:
 Code has been changed to generate the response required by RFC
 1191 when a datagram that requires fragmentation has its Don't
 Fragment bit set.
 
 TEMPORARY FIX:
 
 COMMENTS:
 
 MODULES/MACROS:   TCIPDOW  TCPIP
 
 SRLS:      NONE
 
 RTN CODES:
 
 CIRCUMVENTION:
 
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