PI88970: SACK (SLE/SRE) NOT HONORED FOR INBOUND TCP PACKETS DURING TRANSFER

APAR status

• Closed as fixed if next.

Error description

• After SACK has been permitted per the SYN and SYN-ACK packets
  in a 3-way handshake for a TCP connection, there can be times
  where unexpected retransmissions will occur repeatedly for the
  bulk data packets that have already been acknowledged according
  to the SACK's left edge (SLE) and right edge (SRE) for the range
  of acknowledged sequence numbers.
  
  Here's a sample TCPIP packet trace (SYSTCPDA CTRACE)
  illustrating the receipt of a packet already ACK'd in a
  previous SLE:
  ---------------------------------------------------------------
    ss.mmmmmm IPID Seq_num    Ack_num      Len Info
    45.823070 D4ED 3066213794 1915035215   118 1364->1410[ack]
    45.823124 73C7 1916033415 3066213794 56690 1410->1364[ack]
    45.823124 73EE 1916089887 3066213794  9201 1410->1364[psh,ack]
  | 45.823483 D4EE 3066213794 1915035215   142 1364->1410[ack]
  | [TCP DUP ACK 615532#01,SLE=1915050090,SRE=1915074706,SCNT=1]
    45.823503 D4EF 3066213794 1915035215   142 1364->1410[ack]
    [TCP DUP ACK 615532#02,SLE=1915050090,SRE=1915099322,SCNT=1]
    ...
    45.823792 73F5 1915035215 3066213794  1566
    ...
    45.826480 D4FC 3066213794 1915035215   142 1364->1410[ack]
    [TCP DUP ACK 615532#15,SLE=1915050090,SRE=1915312310,SCNT=1]
  | 45.826518 7406 1915051143 3066213794  1566 1410->1364[OOO,psh,
  |                ----------                             ack]
    45.826988 D500 3066213794 1915035215   142 1364->1410[ack]
    [TCP DUP ACK 615532#19,SLE=1915050090,SRE=1915377865,SCNT=1]
    ...
  | 45.840847 D532 3066213794 1916098970   142 1364->1410[ack]
  | [SLE=1915050090,SRE=1915051143,SCNT=1]
                        ----------
    ...
  ----------------------------------------------------------------
  Here, we see SEQ 191505051143 IPID x'7406' should not have been
  sent since receipt was already ACK'd in the previous SLEs with
  1915050090. That is, packet with IPID x'D4EE' shows SACK block
  range from SLE=1915050090 to SRE=1915074706.
  

Local fix

• None
  KEYWORDS:
  SACK Selective ACK TCP SLE SRE BULK RETRANSMIT RETRANSMISSION
  SYN SYNACK DUPACK OOO OUT-OF-ORDER TOTCPDS EZBTCRDF EZBTCSSK
  TCSNDINT
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Releases 1, 2 and 3 IP: SelectiveACK (SACK)                *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * Unnecessary retransmission of TCP packets, previously        *
  * acknowledged by the connection peer, may occur when SACK is  *
  * enabled for the connection.                                  *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * FIN                                                          *
  ****************************************************************
  

Problem conclusion

Temporary fix

Comments

• The solution for this APAR is included in CS for zOS Version 2
  Release 4.
  
  This APAR is being closed FIN (Fixed If Next) with concurrence
  from the submitting customer. This means that a fix to this APAR
  is expected to be delivered from IBM in a release (if any) to be
  available within the next 36 months.
  

APAR Information

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

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

Fix information

Applicable component levels