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What is mean by Fragmented Packet?

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May 27, 2008 5:43PM

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

unit.)

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

obsolete.

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.


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