Running MPROUTE, TCP/IP Stack Does Not Honor Different MTU Size

 
 APAR Identifier ...... PQ64237      Last Changed ........ 02/12/03
 RUNNING MPROUTE, TCPIP STACK DOES NOT HONOR DIFFERENT MTU SIZE
 
 Symptom ...... IN INCORROUT         Status ........... CLOSED  PER
 Severity ................... 2      Date Closed ......... 02/08/13
 Component .......... 5735FAL00      Duplicate of ........
 Reported Release ......... 430      Fixed Release ............ 999
 Component Name TCP/IP V2 FOR V      Special Notice
 Current Target Date ..              Flags
 SCP ...................
 Platform ............
 
 Status Detail: APARCLOSURE - APAR is being closed.
 
 PE PTF List:
 
 PTF List:
 Release 430   : UQ68996 available 02/12/03 (4302 )
 
 Parent APAR:
 Child APAR list:
 
 ERROR DESCRIPTION:
 Multiple devices attached to the same SUBnet are defined with
 different MTU sizes. VM TCPIP is not honoring the MTUs defined.
 The smallest MTU appears to be always selected for all
 interfaces.
 
 LOCAL FIX:
 MT03356
 
 PROBLEM SUMMARY:
 ****************************************************************
 * USERS AFFECTED: All users of MPROUTE for z/VM 430            *
 ****************************************************************
 * PROBLEM DESCRIPTION: The TCP/IP stack does not honor         *
 *                      interface MTUs when running with        *
 *                      two devices attached to the same        *
 *                      subnet and using MPROUTE.               *
 ****************************************************************
 * RECOMMENDATION: APPLY PTF                                    *
 ****************************************************************
 When running with MPROUTE and having more than one interface
 attached to the same subnet, all interfaces will use the
 lowest MTU among them for data transfers.  When the routes
 are equal cost this is acceptable, but TCP/IP needs to honor
 the specified interface MTUs when the routes are not equal
 cost.
 
 PROBLEM CONCLUSION:
 Procedures RemEquCostVisitNode() and DoMprIoctl() in TCIPDOW
 PASCAL have been modified to properly adjust the MTU size.
 In RemEquCostVisitNode(), the packet size is explicitly set to
 the MTU of the interface that will be used.  In most cases
 it should already have the correct value, but an odd
 scenario could have caused it to use an incorrect MTU.
 DoMprIoctl() was changed so that when a route is deleted from
 the multipath structure, TCP/IP cycles through the remaining
 routes in that structure and set the packet size to the lowest
 MTU remaining.
 
 TEMPORARY FIX:
 
 COMMENTS:
 
 MODULES/MACROS:   TCIPDOW  TCPIP
 
 SRLS:      NONE
 
 RTN CODES:
 
 CIRCUMVENTION:
 
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