PN85260: TCP/IP IS NOT HANDLING MULTIPLE INCOMING CONNECTION REQUESTS (SYN)THAT ARE RECEIVED SIMULTANEOUSLY.

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• Under certain conditions, either received at a fast rate or
  simultaneuosly, some TCP connection requests are not processed
  and are dropped by MVS TCPIP. The remote host continues to
  retransmit SYN requests until it times out and resets the
  attempted connection to MVS TCP/IP. The problem is that TCPIP
  is not correctly redriving the Listen after an accept for a
  previously received SYN.
  Verification Steps:
  Customer configuration consists of an NSC Hyperchannel device
  which allows remote clients to connect to TCPIP. The remote
  clients are connecting to an IUCV socket application runing on
  MVS TCPIP. The remote clients sometimes receive TCPIP
  connection failure 61(ECONNREFUSED)after trying to connect to
  the TCPIP socket application. A TCPIP trace shows the client
  failing issuing a SYN to connect to TCPIP. The Syn is never
  ack'd by MVS TCPIP and the client retransmits this SYN packet
  until they eventually timeout and issue the econnrefused.During
  this time other client requests were seen to arrive to TCPIP
  on MVS, of which those were also dropped. Eventually TCPIP on
  MVS was able to ack the syn and the appliaction was able to
  accept new connection requests, but the previousily queued
  connections were lost and the clients effected were required to
  reissue their connect call.
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of TCP/IP for MVS                  *
  ****************************************************************
  * PROBLEM DESCRIPTION: Some Connection attempts are ignored.   *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  If two SYNs are received on the same Port, in quick succession,
  then often the second one was ignored because a new Socket had
  not yet been allocated and put into Listen state.
  

Problem conclusion

• TCPUP has been changed to allocate a new Socket and put it into
  Listen state immediately after moving a Socket from Listen state
  to SYNreceived state.
  
  The TCP/IP for MVS Version 3 Release 1 Messages and Codes manual
  (SC31-7132-01) and the TCP/IP for MVS Version 3 Release 2
  Messages and Codes manual (SC31-7132-03) should have the
  following messages added:
  
     EZB8486I AlmostQ = <1F31 SockAlmostAcceptQueue@.QueueSize>
              AcceptQ = <2F31 SockAcceptQueue.QueueSize>
              MaxBackLog = <3F31 SockMaxBacklog>
  
     Severity: Informational
     Explanation: If the sum of AlmostQ and AcceptQ equals or
     exceeds (MaxBacklog - 1), then a new socket will not be
     allocated and put into listen state immediately.
     System Action: TCPIP continues, but connection attempts from
     the network may be ignored for a while.
     User or Operator Response: None.
     System Programmer Response: You can reduce the frequency of
     this by increasing both the backlog parameter on the
     application's listen() and SOMAXCONN in PROFILE.TCPIP.
     Source Data Set: TCPUP
     Procedure Name: WorkOn
  
     EZB8487I Listen again: <1S100 SayBoolean(Ignored))> and
              AlmostAcceptQ
  
     Severity: Informational
     Explanation: A connection attempt from the network has been
     put on TCP's almost accept queue. If "listen again" is true,
     then a new socket has been allocated and put in LISTEN state.
     System Action: TCPIP continues, but connection attempts from
     the network may be ignored for a while if "listen again" is
     false.
     User or Operator Response: None.
     System Programmer Response: None.
     Source Data Set: TCPUP
     Procedure Name: WorkOn
  
  The equivalent softcopy file will also be updated.
  

Temporary fix

• EN85260
  

Comments

• Please contact the TCP/IP support center for a copy of the
  testfix.
  

APAR Information

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

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

  UN97120 UN97121

Modules/Macros

• EZAAA00K EZABB03G EZABB038
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R310 PSY UN97120

     UP96/11/20 P F611

• R320 PSY UN97121

     UP96/11/19 P F611

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.