PH01208: APPLICATIONS USING RECV WITH MSG_WAITALL MAY HANG IF DATA ARRIVES TOO SLOWLY FROM A PEER

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• An application that is using recv with msg_wait may hang
  without receiving all of the data from a peer. The issue arises
  if data arrives slower than normal inbound. This will cause
  TCPIP to move the unread application data to pageable buffers.
  In this scenario, if the msg_waitall size is larger than the
  TCP receive buffer size (either coded in the TCPIP profile or
  programatically by the application) can cause the receive
  buffer size calculated by TCPIP to be set smaller than the
  msg_waitall size. This can cause a zero TCP window to be sent
  to the peer, resulting in the flow of data from the peer to
  stop.
  

Local fix

• Insure that the TCP receive buffer size is set to atleast the
  size of the msg_waitall size, either in the TCPIP profile or
  programatically by the application
  
  verification steps:
  
  a dump will show a receive event (x'20') with a msgwait_all size
  at offset x'10, which is larger than the TCB_RCVBUF_SIZE and
  TCB_SO_RCVBUF_SIZE. A TCB_RCV_WND of zero will also be set with
  data on the TCB_RECV_QLEN less then the msgwait_all size
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * IBM Communications Server for z/OS Version 2 Release 2 and 3 *
  * IP                                                           *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * TCP connection receive type operation with the MSG_WAITALL   *
  * flag does not complete                                       *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  Application issues a receive socket operation with the
  MSG_WAITALL flag and a buffer size that is greater than the
  connection ReceiveBufferSize.  The receive operation will not
  complete until the buffer size is satisfied or the connection
  terminates.  If the arrival of data on the connection is slow or
  consists of multiple small size packets the TCPIP stack will
  consolidate the pending receive data into pullup buffers for
  better memory management.  The connection receive window is
  limited to the connection ReceiveBufferSize when the receive
  data has been pulled up and will remain limited until an
  applicate receive operation completes.  The limited window
  prevents the peer from sending the data that is needed to
  satisfy the pending receive operation.
  The application should not issue a receive operation with a
  buffer size that is larger than the ReceiveBufferSize.  The
  ReceiveBufferSize can be manipulated using setsockopt() with the
  so_rcvbuf option and the value can be retrieved using
  getsockopt() with the so_rcvbuf option.  The value can only be
  manipulated prior to the connect() call on a client connect and
  prior to the listen on a server connection.  The maximum value
  that can be set using setsockopt() is limited by the TCPIP
  profile configuration value specified on TCPCONFIG
  TCPMAXRCVBUFRSIZE.
  

Problem conclusion

• EZBTCFRD has been amended to expand the connection receive
  buffer size to the buffer size specified on a receive operation
  with the MSG_WAITALL flag set, limited by the TCPMAXRCVBUFRSIZE.
  Applications issuing a receive operation with the MSG_WAITALL
  flag and a buffer size larger than TCPMAXRCVBUFRSIZE will not
  benefit from this change.
  

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:

  UI60907 UI60908

Modules/Macros

• EZBTCFRD
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R230 PSY UI60908

     UP19/04/23 P F904

• R220 PSY UI60907

     UP19/04/23 P F904

Fix is available

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