PI90866: SENDTO() SOCKET API CALL FOR RAW TCP-PROTOCOL PACKET NOT GETTINGEMSGSIZE ERROR WHEN ITS PACKET LENGTH EXCEEDS ROUTE'S MTU SIZE

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• If an application builds a RAW TCP-protocol packet with its
  length larger than a outgoing routing interface's MTU and
  uses the sendto() socket API call for the write, it gets no
  EMSGSIZE error for the response. The IP/RAW layer in the TCPIP
  stack simply discards the packet silently. If the EMSGSIZE
  error had been returned, then the application will know
  that it needs to carve the large packet into smaller segments
  without requiring fragmentation.
  

Local fix

• Use packet length of 576 (smallest MTU) for all RAW TCP-protocol
  packets. That is, the application has to do the TCP segmentation
  (without fragmentation) that is not being done at the lower
  IP/RAW layer by design.
  
  KEYWORDS:
  RAW TCP SENDTO SOCKET API EZBRWWRI TORUWP EMSGSIZE MTU MSS
  PACKET
  

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * All users of the IBM Communications Server for z/OS Version  *
  * 2 Release 2 and 3 IP: Raw sockets                            *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * RAW socket application sends of TCP or UDP protocol packets  *
  * do not fail with EMSGSIZE if the packet must be fragmented   *
  * and has the don't fragment bit set in the IP header.         *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Apply PTF                                                    *
  ****************************************************************
  The raw sockets API does not support opening a raw socket with
  protocol 6 (IPPROTO_TCP) or protocol 17 (IPPROTO_UDP).  To send
  packets with the TCP or UDP protocol the raw socket must be
  opened with protocol 255 (IPPROTO_RAW) and the IP header must be
  supplied by the application.  The TCPIP stack will discard a
  packet when the don't fragment flag (IP_DF) is set in the
  supplied IP header and the length of the packet being sent is
  larger than the MTU defined for the route selected to reach the
  destination.  When this happens the EMSGSIZE error is not
  returned to the application send operation, thus allowing it to
  complete successfully.  The internally  generated error is not
  presented as an ICMP protocol message so raw sockets open on
  protocol 1 (IPPROTO_ICMP) and protocl 255 (IPPROTO_RAW) are
  unable to detect the error condition.  The raw application has
  no method of determining the send operation has failed to send
  the packet into the network.  If an ICMP message was generated
  in the network it could have been delivered to the raw socket
  application, allowing the application to make adjustments to the
  packets being sent.
  

Problem conclusion

• The raw sockets layer has been amended to fail a send operation
  when a TCP or UDP packet is discarded by the TCPIP stack.
  

Temporary fix

Comments

APAR Information

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

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

  UI57364 UI57365

Modules/Macros

• EZBRWRD
  

Fix information

• Fixed component name

  TCP/IP MVS

• Fixed component ID

  5655HAL00

Applicable component levels

• R230 PSY UI57365

     UP18/10/09 P F810

• R220 PSY UI57364

     UP18/10/09 P F810

Fix is available

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