What is mean by Fragmented Packet?
Background: everything that is sent over the Internet is sent as
small messages, known as Internet Protocol (or IP) packets, that
are in a common format regardless of the kind of link over which
the packet is currently being transmitted.
In version 4 of the Internet Protocol (which is the version that
is most widely used today) a packet may be too big for the kind of
link over which it is about to be sent. For instance, traditional
Ethernet imposes a maximum size for IP packets of 1500 bytes. If a
packet that is about to be sent over an Ethernet link is bigger
than that, the router which is about to send the packet over that
link will fragment the packet. What this means is that the router
will split the packet up into smaller messages (known as fragments)
that are each small enough to be transmitted over the link. When
the fragments arrive at their destination (the computer to which
they are being sent), that computer can reassemble the fragments to
recover the original message - assuming none of the messages are
lost in transit.
IP version 4 also has a "don't fragment" option which tells
routers to not fragment packets that have that option set. If this
option is set, and the router wants to send the packet over a link
for which the packet is too large, the router will not send the
packet at all. Instead, the router will send a message back to the
sender of the packet that was too large. The sending computer can
then respond to this by sending out smaller packets. This is known
as "path MTU discovery" because it enables the sending computer to
determine the largest packet size that will work for the path
between sender and receiver. (MTU stands for maximum transmission
For best efficiency the sending computer would like to send out
packets that are as large as possible, but not so large that any of
the packets will be fragmented.
Fragmentation is of dubious value because it slows down routers,
and because none of the fragments will be useful to the receiver
computer unless all of the fragments arrive there intact. Path MTU
discovery is generally considered preferable to fragmentation.
However there are problems with Path MTU discovery. One is that
many sites (for no good reason) filter the messages that are sent
back to the senders when a packet is too large for a link. Another
is that path MTU discovery is inefficient for short conversations
consisting of just a packet or two in each direction. Yet another
problem is that the path between sender and receiver can change
during a conversation in a way that renders the old path MTU
Internet Protocol version 4 (IPv4) is in the process of being
supplanted by Internet Protocol version 6 (IPv6). In IPv6, routers
do not fragment packets, and path MTU discovery is mandatory.