- Ip Fragmenting


COMMAND

Ip Fragmenting

SYSTEMS AFFECTED

AIX 3.2.55
4.1.x
4.2.x


PROBLEM

The  specification  for  the  Internet  Protocol  (IP) says that a
packet may be up to 65
535  (2^16 - 1) bytes in length
including
the  packet  header.   But  the  specifications  for  most network
technologies in  use today  do not  allow packets  that big.   For
example
the maximum Ethernet packet size is 1
500 bytes.

To allow large packets to be  sent
IP allows the sender to  break
a large  packet up  into several  smaller packets.   Each fragment
packet contains  an offset  value that  says where  in the  larger
packet this fragment  belongs -- the  first fragment will  have an
offset of zero
the second  fragment will have an offset  equal to
the length  of the  first fragment
and so  on.   Note that  this
makes  it  possible  to  combine  a  valid  offset with a suitable
fragment size such  that (offset +  size) is greater  than 65
535

the maximum size of a packet.

The problem arises in the way packet fragmentation is  implemented
by most  systems.   Typically
they  do not  attempt to  process a
packet until all the fragments  have been received and an  attempt
has been made to reassemble them into one big packet.  This  opens
these systems to the  possibility for overflow of  16-bit internal
variables
resulting in system crashes
protocol hangs
and  other
problems.

This problem was first discovered  in the context of sending  ICMP
ECHO REQUEST  packets
commonly  called "ping"  packets after  the
application program used  to send them.   Most implementations  of
ping
will  not  allow  improperly-sized  packets  to  be  sent

although there are  several exceptions to  this (and many  systems
can be  modified to  allow it
in any  case).   Because sending a
single
large (65
510  bytes) "ping" packet  to many systems  will
cause them to hang or even crash
this problem was quickly  dubbed
the "Ping o' Death."



SOLUTION

Get patch.  If you are running AIX 4.x
you can determine  whether
or not you  have these fixes  installed on your  system by issuing
the command

instfix -ik APAR_ID

where "APAR_ID" is the applicable "IXnnnnn" number for patch.
For more information on FixDist
and to obtain fixes via the
Internet
please reference

http://service.software.ibm.com/aixsupport/
or send electronic mail to "aixserv@austin.ibm.com" with the  word
FixDist
in the "Subject:" line.