A client MUST NOT reset the packet number it uses for 0-RTT packets. The keys used to protect 0-RTT packets will not change as a result of responding to a Retry packet unless the client also regenerates the cryptographic handshake message. Sending packets with the same packet number in that case is likely to compromise the packet protection for all 0-RTT packets because the same key and nonce could be used to protect different content.
Receiving a Retry packet, especially a Retry that changes the connection ID used for subsequent packets, indicates a strong possibility that 0-RTT packets could be lost. A client only receives acknowledgments for its 0-RTT packets once the handshake is complete. Consequently, a server might expect 0-RTT packets to start with a packet number of 0. Therefore, in determining the length of the packet number encoding for 0-RTT packets, a client MUST assume that all packets up to the current packet number are in flight, starting from a packet number of 0. Thus, 0-RTT packets could need to use a longer packet number encoding.
A client SHOULD instead generate a fresh cryptographic handshake message and start packet numbers from 0. This ensures that new 0-RTT packets will not use the same keys, avoiding any risk of key and nonce reuse; this also prevents 0-RTT packets from previous handshake attempts from being accepted as part of the connection.
-A client MUST NOT send 0-RTT packets once it starts processing 1-RTT packets from the server. This means that 0-RTT packets cannot contain any response to frames from 1-RTT packets. For instance, a client cannot send an ACK frame in a 0-RTT packet, because that can only acknowledge a 1-RTT packet. An acknowledgment for a 1-RTT packet MUST be carried in a 1-RTT packet.
+A client MUST NOT send 0-RTT packets once it starts processing 1-RTT packets from the server. This means that 0-RTT packets cannot contain any response to frames from 1-RTT packets. For instance, a client cannot send an ACK frame in a 0-RTT packet, because that can only acknowledge a 1-RTT packet. An acknowledgment for a 1-RTT packet MUST be carried in a 1-RTT packet. A server MAY treat a violation of remembered limits as a connection error of an appropriate type (for instance, a FLOW_CONTROL_ERROR for exceeding stream data limits).