PH21216: A RACE CONDITION TO CLOSE A TCP/IP SOCKET EXISTS WHICH COULD CAUSE TIME_WAIT STATUS ON THE SERVER

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• A race condition exists between IBM MQ clients and queue manager
  when closing down a TCP/IP network connection.
  The network resource at the end which issues the close first
  enters a TIME_WAIT state for a period of time in which delayed
  TCP packets will be discarded. If the queue manager side is the
  first to close the network resource across many connections
  within a short period of time, this state could eventually lead
  to a shortage of available ephemeral ports to accept new
  connections.
  This is equivalent APAR to distributed APAR IT27924.
  

Local fix

• Clients using SHARECNV(0) have a greater chance of the client
  closing the network connection first and so a TIME_WAIT on the
  server is usually avoided, but can't be guaranteed.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of IBM MQ for z/OS Version 8       *
  *                 Release 0 Modification 0.                    *
  ****************************************************************
  * PROBLEM DESCRIPTION: A large number of channel connections   *
  *                      to a queue manager are left in a        *
  *                      TIME_WAIT state when disconnecting,     *
  *                      resulting in a potential shortage of    *
  *                      ephemeral ports.                        *
  ****************************************************************
  A race condition exists when sending and receiving the final MQ
  protocol flows to indicate the end of a TCP/IP connection. The
  first side of a channel that closes the network connection will
  cause the ephemeral port that was used for the connection to
  enter a TIME_WAIT state for a short period of time to prevent
  its reuse. The duration of the TIME_WAIT state is relative to
  the maximum segment lifetime, this is to prevent delayed TCP
  packets from interfering with a new conversation reusing the
  same destination port.
  
  As a port in the TIME_WAIT state may be unavailable for reuse
  for a few minutes, it is desirable for MQ clients to be the side
  of the connection to close the network resources, this is so
  that the the range of ephemeral ports available for new
  connections is not diminished on the network interface being
  used by the queue manager.
  

Problem conclusion

• Responding message channel agents (MCAs) have been modified to
  wait briefly for the connection to be closed by the other end of
  the channel when it is deemed appropriate to do so. Under normal
  circumstances these changes cause TCP/IP TIME_WAIT state to
  occur on the initiating end of the channel.
  
  The changed behaviour will only be seen if the fix (or the
  distributed equivalent IT27924) is applied at both the sending
  and receiving end of a channel.
  

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:

  PH27932 UI71584

Modules/Macros

• CSQIRECP CSQMSTRT CSQXCCAX CSQXCCCX CSQXCCIT CSQXCCXD CSQXRMSS
  

Fix information

• Fixed component name

  IBM MQ Z/OS V8

• Fixed component ID

  5655W9700

Applicable component levels

• R000 PSY UI71584

     UP20/10/13 P F010

Fix is available

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