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In the RFC791 (Internet Protocol Specification) on page 12 said:

"For example, this size allows a data block of 512 octets plus 64 header octets to fit in a datagram. The maximal internet header is 60 octets, and a typical internet header is 20 octets, allowing a margin for headers of higher level protocols."

I understand why maximal IPv4 header is 60 octets, but I can't understand where is there 4 bytes more of header from this string:

"plus 64 header octets to fit in a datagram"

Maybe someone can explain me where this 4 bytes lost/or used?

IOWA
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    You did not highlight the important part: "_For example, this size allows a data block of 512 octets plus 64 header octets to fit in a datagram. The maximal internet header is 60 octets, and a typical internet header is 20 octets, **allowing a margin for headers of higher level protocols**._" – Ron Maupin Jul 31 '21 at 18:04
  • Thanks, but where can I read about this option more, for understanding how it works? Or could you please explain me? – IOWA Jul 31 '21 at 18:36
  • TCP uses up to 60 octets for its header options. Se RFC 793. – user207421 Aug 01 '21 at 06:37
  • @user207421, But TCP it's next level which lived in IP Playload data field, in header only one special filed "Protocol", no additional options. – IOWA Aug 01 '21 at 16:29

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This RFC was written way back in 1981. Back then, apparently, this was a typical system block/header size. Today, virtual all systems support much larger datagrams, and probably block sizes as well (some other experts can address that part, maybe).

--I re-read this. I see what you are getting at. I think the answer is in the RFC: "allowing a margin for headers of higher level protocols."

If the 60 in "internet header is 60 octets, and a typical internet header is 20 octets" refers to TCP (vs IP) header, then there should be an errata on the RFC. There is not.

Andrew
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  • The IPv4 header has not changed in this respect since 1981. Doesn't answer the question. – user207421 Aug 01 '21 at 06:38
  • Not entirely true. Options have since been added, e.g. router alert. Also, interpretation has changed, e.g. DSCP. Typical MTU sizes have also changed. I guess I don't see where this is headed--simply an academic Q? – Andrew Aug 01 '21 at 13:01
  • Yes @Andrew, there is no practical reason right now, just for a deep understanding.. – IOWA Aug 03 '21 at 07:42