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Unformatted text preview: unique value for each IP datagram that the sender transmits. This
number is copied into each fragment of a particular datagram. (We now see the use for this
field.) The flags field uses one bit as the "more fragments" bit. This bit is turned on for each
fragment comprising a datagram except the final fragment. The fragment offset field contains
the offset of this fragment from the beginning of the original datagram. Also, when a datagram
is fragmented the total length field of each fragment is changed to be the size of that fragment.
Finally, one of the bits in the flags field is called the "don't fragment" bit. If this is turned on, IP
will not fragment the datagram. Instead the datagram is thrown away and an ICMP error
("fragmentation needed but don't fragment bit set," Figure 6.3) is sent to the originator. We'll
see an example of this error in the next section.
When an IP datagram is fragmented, each fragment becomes its own packet, with its own IP
header, and is routed independently of any other packets. This makes it possible for the
fragments of a datagram to arrive at the final destination out of order, but there is enough
information in the IP header to allow the receiver to reassemble the fragments correctly.
Although IP fragmentation looks transparent, there is one feature that makes it less than
desirable: if one fragment is lost the entire datagram must be retransmitted. To understand why
this happens, realize that IP itself has no timeout and retransmission-that is the responsibility of
the higher layers. (TCP performs timeout and retransmission, UDP doesn't. Some UDP
applications perform timeout and retransmission themselves.) When a fragment is lost that
came from a TCP segment, TCP will time out and retransmit the entire TCP segment, which
corresponds to an IP datagram. There is no way to resend only one fragment of a datagram.
Indeed, if the fragmentation was done by an intermediate router, and not the originating system,
there is no way for the originating system to kno...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12