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:
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