PK07355: TCPIP DOES NOT ACK A PACKET WITH THE SYN FLAG SET FOR AN ESTABLISHED CONNECTION

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• TCPIP drops a packet with the SYN flag set for an established
  connection, rather than sending back an ACK.  The SYN packet is
  dropped with dropit reason DROPIT_BAD_ACKNUM.  This is because
  a SYN packet has an ACK number of zero.  The code checks to
  ensure the ACK is in the window of data being received.  If it
  does not, the packet is dropped.  This apar will allow TCPIP to
  send back an ACK to this SYN packet.
  .
  KEYWORDS: acknowledgment ACK SYN existing
  .
  VERIFICATION STEPS:
  A packet trace will show the SYN is not ACKed and it gets
  retransmitted.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the Communications Server for   *
  *                 OS/390 Release 10 IP:  TCP connections       *
  ****************************************************************
  * PROBLEM DESCRIPTION: TCP sessions from systems that do not   *
  *                      terminate a connection and then         *
  *                      reopen a session using the same IP and  *
  *                      port combinations may fail to connect.  *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  Some systems that do not terminate a TCP connection and later
  attempt to reopen the same connection using the same IP
  addresses and port numbers may fail to connect. Code in the
  TCPIP stack views the inbound SYN packet as unacceptable because
  the ACK number of zero in the SYN packet that is trying to
  establish a new connection is not the next expected ACK number
  that would have been received during the previous connection on
  the same IP address and port four tuple. Since the ACK number is
  unacceptable, TCPIP discards the new SYN packet therefore
  preventing a new session from being established.
  +-------------------------------------------------------------+
  + 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

• EZBTCRD has been amended to allow initial SYN packets (a packet
  with the SYN bit set and the ACK bit unset) to bypass the ACK
  number check.
  
  * Cross Reference between External and Internal Names
  

Temporary fix

Comments

APAR Information

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

  PQ95609

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

  UK04713

Modules/Macros

• EZBTCRD
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R50A PSY UK04713

     UP05/07/22 P F507

Fix is available

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