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Understanding TCP/IP by Libor Dostálek, Alena Kabelová

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5.4 Optional Entries in the IP Header

Optional entries in the IP header are one of the more interesting facets of TCP/IP protocols. We will demonstrate how dangerous it can be to use optional entries, and why many Internet providers throw away IP datagrams that contain them. However, from the pure TCP/IP protocol point of view, this practice is inexcusable (even if performed in good faith), and it can be compared to requiring everyone to carry crutches in case they break their leg.

If a recipient receives an IP datagram with any of these options, it should also use the option in its reply. Optional entries widen the IP header. Due to the IP header’s maximum length of 60 B (out of which 20 B is mandatory), the optional entries are limited to 40 ...

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