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I wonder if the Don't Fragment flag is always set for TCP SYN packets.

By looking at some Wireshark captures this seems to be the case and according to Benefits of "Don't Fragment" on TCP Packets? the "DF flag is typically set on IP packets carrying TCP segments".

Is there a RFC about that?

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Nessaker
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  • There wouldn't be any point. The SYN packages carry no data so they aren't big enough to fragment in the first place. – user207421 Aug 25 '16 at 21:30
  • That's right, but I would like to know if it is somewhere stated. Also I would like to know if there are some OSes that don't set the DF flag. – Nessaker Aug 25 '16 at 21:52
  • PS: TCP SYN packets may carry data (see RFC 7413 (TCP Fast Open)) – Nessaker Aug 25 '16 at 21:53
  • You'd like to know if what is somewhere stated? RFC 793 states that SYN packets don't carry data, or at least implies it. RFC 7413 is experimental. – user207421 Aug 26 '16 at 01:19

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