PI39519: FRAGMENTATION AVOIDANCE

Obtain the fix for this APAR.

APAR status

• Closed as new function.

Error description

• Fragmentation avoidance when VIPAROUTE is used
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 1 IP                                               *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * When using VIPAROUTE, packets can be fragmented when they    *
  * are forwarded from the distributing node to the target node. *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply PTF                                                    *
  ****************************************************************
  Packets that are forwarded from a distributing node to a target
  node using VIPAROUTE can be fragmented. The TCP maximum segment
  size (MSS) is negotiated during the TCP connection handshake.
  The MSS is based on the MTU size of the outbound interface for
  the SYN and SYN/ACK packets. If VIPAROUTE is used, the
  distributing node will add a 24 byte GRE header when it
  forwards the packet to the target stack. The addition of this
  GRE header can cause the total packet size to exceed the
  negotiated MSS. The packet will have to be fragmented in order
  to be forwarded to the target node. The fragmentation and
  reassembly of the packets causes peformance degradation.
  

Problem conclusion

• A new GLOBALCONFIG parameter, ADJUSTDVIPAMSS, will be added to
  allow the automatic adjustment of the TCP MSS value to avoid
  fragmentation. The ADJUSTDVIPAMSS parameter has three
  sub parameters:
  1. AUTO
  2. ALL
  3. NONE
  
  The sub parameters have the following effect:
  
  AUTO - Indicates that TCP/IP should decide if the advertised MSS
  should be adjusted to accommodate the length of the GRE header.
  For inbound connections on a target stack, the MSS
  will be adjusted if the destination address is a distributed
  DVIPA and VIPAROUTE is being used. For outbound connection from
  a target stack,  the MSS will be adjusted if the source
  IP address is a distributed DVIPA.
  
  ALL - Indicates that TCP/IP should adjust the MSS for any
  connections which use a DVIPA, distributed or not, as the
  local IP address.
  
  NONE - The MSS is not adjusted by TCP/IP. This is the default.
  

Temporary fix

Comments

APAR Information

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

  PI38540

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

  UI28533 UI28534

Modules/Macros

• EZBTCFWR EZBAFWLM EZBAFINB EZACFYAC EZBTCSND EZBTCPTM EZACFPSC
  EZACFMSG EZACFPAR EZBTCSTR EZBTCFCN EZBTCRDE EZBAFUTL EZBTLFWR
  EZBIETI  EZBTCICT EZACFMMN EZBTCPCN EZACDNMS EZACDDNE EZACDONE
  EZACDTNE EZACDNM6 EZACDNE0 EZACDNE1 EZACDNE2 EZBTIGBL EZACDNE6
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R210 PSY UI28533

     UP15/06/30 P F506

Fix is available

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