PI65912: POTENTIAL FOR AN INCORRECT TCP WINDOW SIZE TO BE SENT

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• .
  When the TCP Receive Buffer size being used for a connection
  (either selected by the application via a setsockopt() call
  setting SO_RCVBUF or by the configured TCPRCVBUFRSIZE) is larger
  than half of the configured TCPMAXRCVBUFRSIZE value, the
  advertised TCP Window size in window updates can be
  significantly larger than they should be.  This can cause the
  remote system to send data beyond the correct window size.  In
  the worst case, this can cause disruptions when this system is
  receiving high rates of inbound data and the associated
  application is not reading the data as fast as it is arriving.
  
  Other symptoms:
  
   - In a packet trace, the unsolicited window updates sent by
     this system when the application reads a significant amount
     of data will have a large jump in the window size.  But the
     next ACK in response to data arrival will have a window size
     reduction that is larger than the amount of data that has
     arrived.
  
   - If the sender exceeds the actual window size (due to
     receiving the incorrectly large window), those packets will
     be discarded.  A NETSTAT STATS PROTOCOL TCP command will show
     increasing 'Segments Completely After Window' values (and
     possibly 'Segments Partially After Window' values as well).
     A packet trace with DISCARD option enabled will show these
     with a 8195 discard code (TCP_OUTSIDE_WINDOW).
  Addional symptoms:
  CSM storage growth in 32K HVCOMMON storage. All CSM 32K
  HVCOMM storage was related to only one connection of the
  application.
  

Local fix

• .
  Set the TCPMAXRCVBUFRSIZE to be at least twice what the affected
  application is using for the receive buffer size.  A NETSTAT ALL
  command filtered to list only this one socket will show this
  size as the ReceiveBufferSize if the x'40' bit of the TcpPrf
  value is OFF.  For FTP, the receive buffer size is 180K.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communication Server for z/OS Version 2 *
  * Releases 1 and 2 IP                                          *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Incorrect TCP window size is advertized causing inbound data *
  * to be discarded, impacting throughput on the connection.     *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply PTF                                                    *
  ****************************************************************
  The TCP window size calculated when an application reads data
  from the connection recieve queue may not take into account
  unread data remaining on the recieve queue.  The incorrectly
  calculated window is advertized in a window update ACK.  The
  remote peer sends data to fill the advertized window.  When the
  first data packet arrives a new window size is calculated
  correctly, taking into account unread data on the recieve queue.
  The newly calculated window will no longer accept the incoming
  data that was sent by the peer, cuasing packets to be dropped
  due to being outside the window.  The dropped packets require
  the peer to retransmit when the window allows, which negatively
  impacts the throughput on the connection.
  

Problem conclusion

• EZBTCFRD has been amended to take into account unread data on
  the receive queue when calculating the window size.
  

Temporary fix

Comments

APAR Information

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

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

  UI40995 UI40996

Modules/Macros

• EZBTCFRD
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R220 PSY UI40996

     UP16/10/25 P F610

• R210 PSY UI40995

     UP16/10/25 P F610

Fix is available

• Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.