Comment by viraptor
5 days ago
> The IPv4+ could pass through a router that doesn't know about it
It couldn't do that reliably. We don't have any flags left for that that. Options are not safe. We've got one reserved flag which is anyways set to 0, so that's not safe either.
> We don't have any flags left for that that.
There's the reserved bit (aka the evil bit[1]). Are you saying gear out there drops packets with reserved bit set to 1? Wouldn't surprise me, just curious.
Seems like IPv4+ would have been a good time to use that bit. Any IPv4+ packets could have more flags in the + portion of the header, if needed.
[1]: https://en.wikipedia.org/wiki/Evil_bit
That bit is currently defined as "Bit 0: reserved, must be zero", so there will be network gear out there, that either drops the packet otherwise or resets the bit to 0 when forwarding.
That makes it effectively impossible to ever use then, so a waste of a bit. Too bad they made that mistake when writing the spec. Would have been better if they specified it like most APIs, ie ignore if you get it, carry it if you forward, and set it to zero if you send it.
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