4.2.2.7 Fragmentation: RFC 791 Section 3.2
Fragmentation, as described in [INTERNET:1], MUST be supported by a
router.
When a router fragments an IP datagram, it SHOULD minimize the
number
of fragments. When a router fragments an IP datagram, it SHOULD
send
the fragments in order. A fragmentation method that may
generate one
IP fragment that is significantly smaller than the other MAY cause
the first IP fragment to be the smaller one.
DISCUSSION
There are several fragmentation techniques in common use in the
Internet. One involves splitting the IP datagram into IP
fragments with the first being MTU sized, and the others being
approximately the same size, smaller than the MTU. The
reason for
this is twofold. The first IP fragment in the sequence will be
the effective MTU of the current path between the hosts, and the
following IP fragments are sized to minimize the further
fragmentation of the IP datagram. Another technique is to split
the IP datagram into MTU sized IP fragments, with the last
fragment being the only one smaller, as described in
[INTERNET:1].
A common trick used by some implementations of TCP/IP is to
fragment an IP datagram into IP fragments that are no larger
than
576 bytes when the IP datagram is to travel through a router.
This is intended to allow the resulting IP fragments to pass the
rest of the path without further fragmentation. This would,
though, create more of a load on the destination host, since it
would have a larger number of IP fragments to reassemble into
one
IP datagram. It would also not be efficient on networks
where the
MTU only changes once and stays much larger than 576 bytes.
Examples include LAN networks such as an IEEE 802.5 network
with a
MTU of 2048 or an Ethernet network with an MTU of 1500).
One other fragmentation technique discussed was splitting the IP
datagram into approximately equal sized IP fragments, with the
size less than or equal to the next hop network's MTU. This is
intended to minimize the number of fragments that would result
from additional fragmentation further down the path, and assure
equal delay for each fragment.
Routers SHOULD generate the least possible number of IP
fragments.
Work with slow machines leads us to believe that if it is
necessary to fragment messages, sending the small IP fragment
first maximizes the chance of a host with a slow interface of
receiving all the fragments.