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I have trouble understanding why sometimes it occurs that during IP fragmentation certain fragments take different routes. I was following this the second example: https://www.gatevidyalay.com/ip-fragmentation-fragmentation-in-networking/

If the fragmentation is done by router-2 as I concluded it should, how does the second fragment take a detour over router-3 and network z with a different MTU? The provided explanation led me to believe that the fragments are returned to the source and repeatedly sent, but then how isn't the first fragment traveling directly and not taking a detour?

I am pretty new to networking and until now I was into backend programming while I graduated math so I am sorry in advance if I ask redundant stuff.

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    – Zac67 ♦
    Commented Aug 18, 2022 at 5:18

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On the routing level, each fragment is routed as an independent packet - remember that routing is a stateless process.

If an intermediate router uses equal-cost-multi-path routing (ECMP), different fragments can take different paths.

That page doesn't really provide sufficient information on why they think that happens in the example. Likely, they're just trying to show the possibility. And no, fragments (or packets) are never returned to the source.

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  • does that mean that in this example the second fragment could go from router-2 to network z and than returns to router-2 or event goes the extra mile to router-3 and again to network x when it comes again to router-3? how doesn't that mess up the order of fragments? is it important whether fragments arrive in a particular order?
    – mato_kan
    Commented Mar 20, 2022 at 19:15
  • Packets (or fragments) never ever go back to the source - unless there's a routing loop (then they'll circle until the TTL is exhausted). Yes, the order of fragments may change along a path. Packets should arrive in their original order but they don't need to. Usually, there's a performance penalty when they don't.
    – Zac67 ♦
    Commented Mar 20, 2022 at 20:16

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