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In RFC 791 page 14 it says

The copied flag indicates that this option is copied into all fragments on fragmentation.

  0 = not copied
  1 = copied

If I understand correctly it is possible to specify a different route with the strict or loose source routing options. If that is done, is it required to set these options in every fragment?

Or is there any other example where an option must be in every fragment? Is it the same as options in RFC 791 which "Must be copied on fragmentation." ?

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  • IPv4 options are really obsolete. The Internet routers will ignore them or drop the packets. You may be able to use IPv4 options on your own network, as long as you have properly configured any equipment.
    – Ron Maupin
    Sep 9 '19 at 1:09
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Source routing is most often filtered or ignored for its security issues.

You've already linked RFC 791 answering your question:

Loose Source and Record Route

...

Must be copied on fragmentation. Appears at most once in a datagram.

Strict Source and Record Route

...

Must be copied on fragmentation. Appears at most once in a datagram.

Of course, you'd want every fragment to take the same route, so each needs to carry the source routing option.

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