PI38540: 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 1 Release 13 IP             *
  ****************************************************************
  * PROBLEM DESCRIPTION: When using VIPAROUTE, packets can be    *
  *                      fragmented when they are forwarded      *
  *                      from the distributing node to the       *
  *                      target node.                            *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  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.
  
  +-------------------------------------------------------------+
  + Please check our Communications Server for OS/390 homepages +
  + for common networking tips and fixes.  The URL for these    +
  + homepages can be found in Informational APAR II11334.       +
  +-------------------------------------------------------------+
  

Problem conclusion

Temporary fix

Comments

• 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.
  

APAR Information

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

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

  PI39519 UI28929

Modules/Macros

• AFSNDINT EZACDDNE EZACDNE0 EZACDNE1 EZACDNE2
  EZACDNE6 EZACDNMS EZACDNM6 EZACDONE EZACDTNE EZACFMMN EZACFMSG
  EZACFPAR EZACFPSC EZACFPTK EZACFYAC EZBAFINB EZBAFUTL EZBAFWLM
  EZBDGPRF EZBIETI  EZBITINI EZBTCFCN EZBTCFWR EZBTCICT EZBTCPCN
  EZBTCPTM EZBTCRDE EZBTCSND EZBTCSTR EZBTIGBL EZBTLFWR EZB2BLOG
  EZB2MTCB EZB2PKTS EZB2RPRT EZB2TCB  EZB2TCBB EZB2TCBS EZB2UWCX
  IOCTLI   TCSNDINT TOSAPIDS TOTCPDS  YACCFYAC ZZTOTCP
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R1D0 PSY UI28929

     UP15/07/29 P F507

Fix is available

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