Re: [quicwg/base-drafts] Server's first flight should be capped by INITCWND (#3639)

[Kazuho Oku <[e-mail]>]

@ekr 
>> Server cannot rely on CC until the client's address is validated, because an attacker might spoof Initial ACKs.
> 
> Just to clarify, only an on-path attacker can do this, right?
 
An off-path attacker can do that. An attacker can easily guess the packet number that the server uses, and spoof an ACK without actually receiving a packet from the server. Note also that there is no guarantee that there would be enough entropy in server-generated CID, even though a server can generate CIDs in such a manner. That's why we postpone the validation of the path until the server receives a Handshake packet.

@ianswett I think I agree with you that the limit can be defined as `min(3x, CC)`.

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