[quicwg/base-drafts] Confused about Fixed Bit in Version Negotiation Packet (#2995)

Ainiroad <notifications@github.com> Wed, 28 August 2019 08:02 UTC

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Subject: [quicwg/base-drafts] Confused about Fixed Bit in Version Negotiation Packet (#2995)
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Fixed Bit Must set to 1:
> Fixed Bit:
The next bit (0x40) of byte 0 is set to 1. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded.

the first byte in Version Negotiation Packet
|1|  Unused (7) |
there is 7 bits unused, including Fixed Bit.

> The value in the Unused field is selected randomly by the server. Clients MUST ignore the value of this field. Servers SHOULD set the most significant bit of this field (0x40) to 1 so that Version Negotiation packets appear to have the Fixed Bit field.

It's said that `Clients MUST ignore the value of this field.` and `Servers SHOULD set the most significant bit of this field (0x40) to 1`. 
If Client must ignore the value(including Fixed Bit), why server should set Fixed Bit?

I'm confused about the describe. I think maybe it want to express it like this:
> Clients MUST ignore the value of this field *except Fixed Bit(0x40)*. Servers SHOULD set the most significant bit of this field (0x40) to 1 .......

Do I have any misunderstanding? thanks a lot

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