PH37918: IGVH100E ECSA HAS EXCEEDED THE THRESHOLD PERCENTAGE OF 90%

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Selective ACKnowledgement (SACK) retransmission incorrectly
  retransmits the same packet in a loop.  The retransmitted
  packets consume some ECSA and due to the loop condition it can
  cause an ECSA shortage very rapidly.
  ECSA storage growth with TCPIP mapping outbound packets in
  SKMB/SKDB/IUDR and SKMB control blocks.
  IGVH100E ECSA HAS EXCEEDED THE THRESHOLD PERCENTAGE OF 90%
  

Local fix

• BYPASS/CIRCUMVENTION:
  Disable SACK dynamically using a VARY TCPIP,,OBEYFILE command
    TCPCONFIG NOSELECTIVEACK
  All new connections will honor the NOSELECTIVEACK setting.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 5 IP:  SELECTIVEACK (SACK)                         *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Rapid ECSA storage growth due to TCPIP loop retransmitting a *
  * packet when SACK (Selective ACKnowledgement) is enabled on   *
  * the connection.  The code changes in PH22344 introduced this *
  * problem and the PTFs have been marked PE.  The scenario      *
  * causing the probem is a TCP connection where the application *
  * issues a setsockopt( ) for TCP_NODELAY, issues multiple      *
  * small sends with a combined total less than a full size      *
  * packet, one or more packets are lost.  The duplicate ACKs    *
  * trigger SACK retransmission and if the retransmitted packet  *
  * contains all of the data that has been previously sent on    *
  * the connection it will loop building the same packet.        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Disable SACK dyanmically using a VARY TCPIP,,OBEYFILE        *
  * command with a file containing TCPCONFIG NOSELECTIVEACK      *
  * Alter the TCPCONFIG statement in the event of a TCPIP        *
  * restart with NOSELECTIVEACK                                  *
  * APPLY PTF when available and reenable SACK                   *
  ****************************************************************
  

Problem conclusion

• EZBTCRDF has been amended to exit the packet building loop when
  all previously sent bytes have been retransmitted.
  

Temporary fix

• *********
  * HIPER *
  *********
  

Comments

APAR Information

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

  PH36583

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

  UI75877

Modules/Macros

• EZBTCRDF
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R250 PSY UI75877

     UP21/07/21 P F107 ¢

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.