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: MESSAGE TO SUBMITTER: