rcallon@PARK-STREET.BBN.COM (Ross Callon) (09/09/87)
Yuko;
I think we need more experience in actually using the ISO IP before we say
which differences are the most important, but here are a few that come to mind.
I hope this is a useful start.
If you are doing a study of the differences between the DARPA IP and the ISO
IP, or looking at implementation issues with the ISO IP, I would be very
interested in seeing any result which you may come up with.
(1) As Paul mentioned, the ISO uses variable length addresses up to
20 octets, while the DOD uses fixed length 4 octet addresses. The
ISO addresses are much more flexible. For example, they may be used
to explicitly encode something like an "autonomous system" number,
and/or the IEEE 802 link address.
I think that the difference in addresses make the ISO IP more appropriate
for worldwide use with potentially hundreds of thousands of networks, but
make it more costly to run (bigger headers and more processing).
(2) The IPs use different checksums. The ISO checksum is computationally
more costly, but has a higher probability of detecting errors in which
only a small number of bits are changed (i.e., random bit errors as
opposed to burst errors). In addition, the ISO checksum can be "turned
off" by using all zeroes in the checksum field.
(3) The ISO IP doesn't have a source quench. This is because at the time
no one who went to the meetings where it was defined believed that
source quench was useful.
(4) The ISO IP has a small difference in the way that source routing is
implemented. The "destination address" field in the ISO IP always
contains the true final destination address, rather than the "next
hop" source routing address. This means that all of the gateways need
to implement source routing for it to work (rather than only those
gateways which are actually mentioned in the source route). Since
implementation of source routing is optional in the ISO IP, it is
unlikely that all gateway vendors will actually implement it.
(5) The ISO IP doesn't have an echo packet. The idea was that you could
get the same effect by source routing a packet to another gateway and
then back to yourself. Unfortunately, as mentioned in (4), this is not
likely to work since implementation of source routing is optional.
People in ANSI are aware of this problem and thus it is likely to be
fixed eventually (I hope).
(6) The encoding of many of the fields are different. For example, the
fields needed for reassembly (fragment offset, etc..) are only present
in the ISO IP if fragmentation is permitted. Similarly, the "security"
option had to take into account the fact that the ISO IP is standardized
for worldwide use, and therefore needs greater flexibility. Generally,
the ISO IP has opted for greater flexibility, and the cost of larger
packets and possibly greater cost in parsing the headers.
Ross