PH38136: SEGMENTATION OFFLOADED PACKETS ARE SENT OUTBOUND OUT OF ORDER BEFORE SMALLER NON OFFLOADED PACKETS

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• For streaming type connections that are NOT registered to IWQ
  as bulkmode/streaming, packets may be sent out of order
  outbound, causing many duplicate acks from the remote host. This
  causes unnecessary FRR (fast retransmit/recovery) to be done on
  the zOs sending side, which effect throughput. This issue can
  occur when segmentation offload is enabled, and the application
  has data flows that include large data sends (larger then mss)
  mixed with small sends (mss or smaller). If the small sends
  issued executed by the application prior to the segmentation
  offloaded sends, there are cases where the large sends are sent
  on the network before the small sends due to the TCPIP blocking
  factor.
  
  KNOWN IMPACT:
  streaming application throughput, duplicate acks
  
  VERIFICATION STEPS:
  packet trace shows that multiple packets using segentation
  offload are being sent after a small packet has been sent.
  However, once the packets actually sent to the OSA, they are
  sent out of order. The remote peer responds with duplicate ACKs
  indicating that it has not received the small packet that was
  sent prior to the large segmentation ofload packets
  

Local fix

• BYPASS/CIRCUMVENTION:
  Insure that the OSA interface(s) being used for the outbound
  traffic are enabled for IWQ in the event that the traffic meets
  the outbound bulk criteria. If the traffic is not getting
  registered to the OSA IWQ and the issue still occurs, this apar
  is applicable
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 5 IP: TCP                                          *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * For a TCP connection, segmentation off-loaded packets are    *
  * being sent out-of-order outbound, before smaller,            *
  * non-segmentation off-loaded packets.                         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply the PTF                                                *
  ****************************************************************
  Application messages where the segmentation (larger than 1 MSS
  messages) can be off-loaded to the outgoing adapter are sent
  immediately. Smaller messages, (less than 1 MSS size) can be
  optimized on the sending path by bundling them together before
  sending outbound. For a specific TCP connection that sends a mix
  of these application messages sizes this can lead to
  out-of-order packets. The segmentation off-loaded packets can be
  sent out before the smaller message even though the smaller
  message is sent by the application first. This can lead to
  sub-optimal performance for this connection.
  

Problem conclusion

• Code has been modified for connections with a mix of small and
  large message sizes to try and send the messages out in order as
  they arrive from the applicaiton when SEGMENTATIONOFFLOAD is
  enabled.
  

Temporary fix

Comments

APAR Information

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

  PH36176

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

  UI76000

Modules/Macros

• EZBT6SND EZBT6LWR EZBTCFWR EZBTLFWR EZBT6FWR EZBTCSND
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R250 PSY UI76000

     UP21/08/17 P F108

Fix is available

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