IP reassembly at destination host

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Oct 25th, 2009
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I happened to read that " ip reassembly happens at the destination host and not at the intermediate routers. " The book doesnt say why nor does it talk about the reason for the reassembly of ip. Could you please let me know why is reassembly of ip done and in why only at the destination host ?

Correct Answer by Lucien Avramov about 7 years 8 months ago

The goal of ip reassembly is to re assemble fragmented packets.


This should happen at the destination host.

If it happens on the intermediate routers, then fragmentation will have to re-occur again and this is cpu intensive and time consuming, will result in delays. Hence this is done at the destination

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Correct Answer
Lucien Avramov Mon, 10/26/2009 - 00:04
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The goal of ip reassembly is to re assemble fragmented packets.


This should happen at the destination host.

If it happens on the intermediate routers, then fragmentation will have to re-occur again and this is cpu intensive and time consuming, will result in delays. Hence this is done at the destination

Joseph W. Doherty Mon, 10/26/2009 - 04:29
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There's another reason why reassembly of ip fragments is done only at the destination host. Also keep in mind there might be multiple paths through the network so intermediate routers might not "see" all the fragmented packets.


As to why reassembly of ip is done, it's done to reconstruct the original packet before that packet is further processed. Until the packet is reassembled, it's not useful because one or more parts of it would be missing. (Also remember why fragementation is done to begin with, because the original packet couldn't not be transmitted, full size, across some network link. It's broken down into as many maximum size fragments that can be processed across a network link. Also, as Lucien noted for the extra processing load placed on the receiving host reassembling fragments, this is also true for the device the fragments them, which is why something like PMTU exists, i.e. to notify sender its packets are too large.)

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