PM67781: EZBITDSA(UK68305 11.151)+000084 S4C5/74FF2000 WHEN SENDING IPV6 PACKET AFTER IPV6 ICMP MESSAGE FOR PACKET TOO BIG

Obtain the fix for this APAR.

APAR status

• Closed as unreproducible in next release.

Error description

• TCPIP received an IPv6 ICMP message indicating "packet too big".
  The MTU received in the ICMP message was 1272.  EZBTCRDE called
  EZB6POUT to send the packet with the smaller MTU.  Since the
  packet was less than the minimum IPv6 MTU of 1280, the send got
  rejected with a EMSGSIZE error and an MTU of 1272.  This caused
  a loop, eventually running out of DSA storage.
  .
  VERIFICATION STEPS:
  1) 4C5 abend will occur
  2) Calling sequence of abend thread will be a loop:
   INDEX    DUAE     DUCB     DUSA     ASCB     ASID JOBNAME
  ABEND
  STATUS
   10000428 42D19568 7F5A9000 7F5A91A0 00F83C00 000F DMP50030
  004C5000 Ab
  Iu Su
   RSA  7F5A96E8  Prev 424802A0  Next 7F5A97D8  Mod EZBIFRC1
   RSA  7F5A97E0  Prev 7F5A96E8  Next 7F5AA790  Mod EZBIFINB
   RSA  7F5AA798  Prev 7F5A97E0  Next 7F5ABD18  Mod EZBTCRD:
   RSA  7F5ABD20  Prev 7F5AA798  Next 7F5AC550  Mod EZBTCRDE
   RSA  7F5AC558  Prev 7F5ABD20  Next 7F5AD720  Mod EZB6POUT
   RSA  7F5AD728  Prev 7F5AC558  Next 7C4A3038  Mod EZBTCRDE
   RSA at 7C4A3040 is outside address range of DUSA
   RSA  7C4A3040  Prev 7F5AD728  Next 7C4A4208  Mod EZB6POUT
   RSA  7C4A4210  Prev 7C4A3040  Next 7C4A4A40  Mod EZBTCRDE
   RSA  7C4A4A48  Prev 7C4A4210  Next 7C4A5C10  Mod EZB6POUT
   RSA  7C4A5C18  Prev 7C4A4A48  Next 7C4A6448  Mod EZBTCRDE
   RSA  7C4A6450  Prev 7C4A5C18  Next 7C4A7618  Mod EZB6POUT
   RSA  7C4A7620  Prev 7C4A6450  Next 7C4A7E50  Mod EZBTCRDE
   RSA  7C4A7E58  Prev 7C4A7620  Next 7C4A9020  Mod EZB6POUT
   RSA  7C4A9028  Prev 7C4A7E58  Next 7C4A9858  Mod EZBTCRDE
   RSA  7C4A9860  Prev 7C4A9028  Next 7C4AAA28  Mod EZB6POUT
   RSA  7C4AAA30  Prev 7C4A9860  Next 7C4AB260  Mod EZBTCRDE
  3) Ctrace with option INTERNET will show record
    GERMANY   INTERNET  300101BC  12:03:12.764131   Cant fragment
  IPv6 outbound packet
   HASID..0010      PASID..0010      SASID..0010
  USER...DMP50030
   TCB....00000000  MODID..EZB6POUT  REG14..4317F6AC
  DUCB...4B001422
   CID....0098A7B2  PORT...0         CPUA...00
   IPADDR. 2000:16:11:16::16:110
  ADDR...00000000  00000000_7C9F6FBC  LEN....000004  payload len
        +0000  000004F8                                | ...8
  ADDR...00000000  00000000_7C9F6F84  LEN....000004  RTE MTU
        +0000  000004F8                                | ...8
  ADDR...00000000  00000000_7C9F6F6C  LEN....000004  dest cache
        +0000  00000000                                | ....
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of the IBM Communications Server   *
  *                 for z/OS Version 1 Release 13 IP:            *
  *                 IPv6                                         *
  ****************************************************************
  * PROBLEM DESCRIPTION: ABENDS4C5 RSN74FF2000 in EZBITDSA when  *
  *                      sending an IPv6 packet of 1280 bytes    *
  *                      or larger without a fragment header     *
  *                      and the destination is known to have    *
  *                      a usable MTU of less than 1280.         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  ABENDS4C5 RSN74FF2000 in EZBITDSA when sending an IPv6 packet
  of 1280 bytes or larger without a fragment header and the
  destination is known to have a usable MTU of less than 1280.
  
  An IPv6 TCP connection sends a packet and an intermediate hop
  determines the packet must be encapsulated.  The encapsulation
  grows the packet size beyond the MTU of the forwarding link and
  an ICMPv6 Packet Too Big message is returned to the originating
  TCP/IP.  The ICMPv6 message also includes the usable MTU taking
  into account the encapsulation.  When the usable MTU is less
  than 1280 bytes TCP will retransmit the original packet and
  IPv6 fails to add a fragment header.  The sending logic in
  EZB6POUT determines the packet is to big and generates an
  internal ICMPv6 Packet Too Big message, driving the TCP layer
  to immediately retransmit the packet again.  The retransmitted
  packet size is not adjusted and the same process repeats itself
  until the abend occurs.
  
  When sending a packet of 1280 bytes to a destination that has
  a usable MTU of less than 1280 requires the originator of the
  IPv6 packet to include a fragment header.  The fragment header
  should be added in this scenario and the internal error should
  not be generated.
  +-------------------------------------------------------------+
  + Please check our Communications Server for OS/390 homepages +
  + for common networking tips and fixes.  The URL for these    +
  + homepages can be found in Informational APAR II11334.       +
  +-------------------------------------------------------------+
  

Problem conclusion

Temporary fix

Comments

• EZB6POUT has been amended to add a fragment header when sending
  a packet that is 1280 bytes and the known usable MTU to the
  destination is less than 1280.
  

APAR Information

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

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

  UK80653

Modules/Macros

• EZB6POUT
  

Fix information

• Fixed component name

  TCP/IP V3 MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R1D0 PSY UK80653

     UP12/09/08 P F209

Fix is available

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