PI57886: SYN-ACK RESPONSE MAY CONTAIN TSOPT WHEN IT SHOULD NOT

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• z/VM TCP/IP is always using the TSopt and including a timestamp
  in the SYN-ACK packet that it sends back to any incoming SYN
  packet. According to RFC 7323, a TCP should only send a TSopt
  in the SYN-ACK if it received a TSopt in the initial SYN packet
  that it received for the connection.  There are some instances
  where this "invalid" SYN-ACK that gets sent back will get
  discarded by the TCP system that is attempting to connect.
  This results in the connection attempt timing out.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the z/VM TCP/IP stack           *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  ****************************************************************
  * RECOMMENDATION: APPLY PTF                                    *
  ****************************************************************
  z/VM TCP/IP is always using the TSopt and including a timestamp
  in the SYN-ACK packet that it sends back to any incoming SYN
  packet. According to RFC 7323, a TCP should only send a TSopt
  in the SYN-ACK if it received a TSopt in the initial SYN packet
  that it received for the connection.  There are some instances
  where this "invalid" SYN-ACK that gets sent back will get
  discarded by the TCP system that is attempting to connect.
  This results in the connection attempt timing out.
  

Problem conclusion

• When sending the SYN_ACK packet out, the updated code will check
  whether it has received a TSopt in the initial SYN packet that
  it received for the connection. If not, the SYN_ACK packet will
  not contain TSopt.
  

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:

  UI35843 UI35844 UI35845

Modules/Macros

• FPTCPDOW F6TCPDOW
  

Fix information

• Fixed component name

  TCP/IP V2 FOR V

• Fixed component ID

  5735FAL00

Applicable component levels

• R540 PSY UI35843

     UP16/03/03 I 1000

• R620 PSY UI35844

     UP16/03/03 I 1000

• R630 PSY UI35845

     UP16/12/02 P 1602

Fix is available

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