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I have one example scenario, let consider i am sending 1500 bytes of frame over the internet. which is passing through 576 bytes MTU capable router.

My question is, Is that router will do the fragmentation and send to receiving host or it will send ICMP(code3 type4) to sending device by PMTU for fragmentation. How this thing is happening in real time in the world. Sometime in real world scenario router will block ICMP message by administrator , how they are overcoming this issue.

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 8 '17 at 3:14
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Router will respond with ICMP and drop the packet if it can't fragment the packet further, or it's prohibited to do so with the Don't Fragment bit set.

And yes, as you pointed out, PMTUD often breaks in real world, as people do filter ICMP messages - either in transit, or at their internet edges, shooting themselves in the foot.

Here's a great writeup on PMTUD and fragmentation for your reference: http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

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IPv4 fragments are allowed to be fragmented by routers unless the Don't Fragment bit is set. IPv6 always behaves as if the DF bit is set (the bit doesn't exist anymore) so routers never fragment.

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Sometime in real world scenario router will block ICMP message by administrator , how they are overcoming this issue.

Simple. Fire that dumb*** admin -- I've known many. If they have no understanding of why ICMP exists, they have no place saying firewall, much less managing one.

(But otherwise, yes, ICMP messages can end up lost just like any other packet. When that happens, other mechanisms have to kick in to generate any error(s) -- most often a connection timeout after a long delay.)

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