Fragmentation

Question

Even If one fragment of packet is lost....will sender send the whole packet again?

Comments

No,after time out only that fragment will be sent again

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So how will you identify lost packet?

Answer 1

If packet size> MTU fragmentation at Network layer.

For reliability (i.e. Information about packet loss, destination unreachable, etc) ICMPs are there. But ICMPs are not generated for fragment of the packet. So even ICMP won't help. So for sending a particular packet (if so) how will you identify lost packet?

Comments

"No ICMP error wil be generated for a fragmented datagram that is not the first fragment".

So it sends an error message only for the first fragment. Else it will have to send the entire datagram again, correct?

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if packet is lost, then ICMP is generated, but if ICMP is lost, ICMP for the lost ICMP is not generated, the sender(Source) may or may not be notified about the loss of the packet => IP with ICMP is still unreliable,
but if the ICMP reaches the source saying packet size> MTU  then source will re-send the packet right ? or will it be ignored ? @Digvijay Pandey 

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if a fragment is lost complete packet has to retransmitted

Answer 2

If the MTU size is sufficient, then normally the intermediate routers don't care about fragmentation. They just pass it on what they receive to the appropriate next destination. In the case when one or more fragments are lost, the intermediate routers won't care about it. Only the receiver can realize the missing fragment at the time of reassembly of fragments. So, it will discard the whole IP datagram.

Whether sender will resend or not depends upon the transport layer protocol being used. On a TCP/IP stack, transport layer will request a retransmission from the sender when it discovers that the segment corresponding to the rejected datagram is missing.

References
https://www.netometer.com/qa/ipfrag.html#A11
https://networkengineering.stackexchange.com/questions/30424/how-does-a-router-behave-when-some-of-the-fragments-of-a-packet-is-lost