3. Design of the QUIC Transmission Machinery

All transmissions in QUIC are sent with a packet-level header, which indicates the encryption level and includes a packet sequence number (referred to below as a packet number). The encryption level indicates the packet number space, as described in Section 12.3 of [QUIC-TRANSPORT]. Packet numbers never repeat within a packet number space for the lifetime of a connection. Packet numbers are sent in monotonically increasing order within a space, preventing ambiguity. It is permitted for some packet numbers to never be used, leaving intentional gaps.

This design obviates the need for disambiguating between transmissions and retransmissions; this eliminates significant complexity from QUIC's interpretation of TCP loss detection mechanisms.

QUIC packets can contain multiple frames of different types. The recovery mechanisms ensure that data and frames that need reliable delivery are acknowledged or declared lost and sent in new packets as necessary. The types of frames contained in a packet affect recovery and congestion control logic:

• All packets are acknowledged, though packets that contain no ack-eliciting frames are only acknowledged along with ack-eliciting packets.

• Long header packets that contain CRYPTO frames are critical to the performance of the QUIC handshake and use shorter timers for acknowledgment.

• Packets containing frames besides ACK or CONNECTION_CLOSE frames count toward congestion control limits and are considered to be in flight.

• PADDING frames cause packets to contribute toward bytes in flight without directly causing an acknowledgment to be sent.

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