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I'm trying to understand the PDU sizes as it traverse the TCP/IP model.

RFC 791 says the IP protocol is required to accept datagrams of 576 octets. ( 512 octets, plus up to 64 header octets). I know the IHL (Internet Header Length) field is 4 bits long and specifies the header length in 32 bit words.

So the IP Header has a max size of 60 bytes.... Can someone explain where this 64 octet header comes from and how it differs from the 60 byte header?

Thanks

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  • The 512+64 bytes were likely the original driver for defining a minimum MTU of 576, but the definite answer to your question is probably lost in time. Note that historical trivia is explicitly off topic here.
    – Zac67 ♦
    May 1 at 18:34
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    – Zac67 ♦
    May 31 at 19:12

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While there is no 64B header, RFC791's minimum datagram size allows for one. (otherwise there'd be odd power-of-two boundaries. eg. 64 = 2^6, 60 = 2^6 - 4 - not an even power)

(It's possible there was thinking that zero would equal 16, because IHL==0 would be nonsense. But that was never the case. In fact, IHL will never be less than 5.)

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  • Thanks for the reply... That makes the most sense out of any answer I've gotten about the subject!
    – Buster Poindexter
    May 2 at 13:43

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