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tcpudpipip-fragmentation

IP fragments failure on network?

Exam question (with no additional info):

When an bunch of IP datagram fragments are being sent over the network and only one of them does not get to it's destination, what will happen then?

I'm not sure if ICMP is involved here or not. Does ICMP send an error report reporting to the source that it needs to resend that same fragment (only this one fragment)?

The problem is here that I don't know if the IP fragments use UDP or TCP therefore I don't know the answer to the question.

(I've posted on the networkengineering.stackexchange but my question was rejected)

Solution

  • Points to cover:

    • After a timer triggered by the receipt of the first fragment has expired, the reassembling host will discard all the of fragments.
    • The reassembling host may generate an ICMP Time Exceeded (Fragment reassembly time exceeded).
    • The first fragment will need to have been received for the ICMP to include the first 8 bytes of the triggering payload. IPv6 will not generate the ICMP Time Exceeded unless the first fragment was received.
    • With IPv6 if the reassembled datagram would be larger than 1500 bytes then it may be silently discarded.
    • If a higher level protocol with reliable delivery was used (e.g. TCP), then the originating host should retransmit datagrams for which no acknowledgement has been received.