4. Relevant Differences between QUIC and TCP
Readers familiar with TCP's loss detection and congestion control will find algorithms here that parallel well-known TCP ones. However, protocol differences between QUIC and TCP contribute to algorithmic differences. These protocol differences are briefly described below.
4.1. Separate Packet Number Spaces
QUIC uses separate packet number spaces for each encryption level, except 0-RTT and all generations of 1-RTT keys use the same packet number space. Separate packet number spaces ensures that the acknowledgment of packets sent with one level of encryption will not cause spurious retransmission of packets sent with a different encryption level. Congestion control and round-trip time (RTT) measurement are unified across packet number spaces.
4.2. Monotonically Increasing Packet Numbers
TCP conflates transmission order at the sender with delivery order at the receiver, resulting in the retransmission ambiguity problem [RETRANSMISSION]. QUIC separates transmission order from delivery order: packet numbers indicate transmission order, and delivery order is determined by the stream offsets in STREAM frames.
QUIC's packet number is strictly increasing within a packet number space and directly encodes transmission order. A higher packet number signifies that the packet was sent later, and a lower packet number signifies that the packet was sent earlier. When a packet containing ack-eliciting frames is detected lost, QUIC includes necessary frames in a new packet with a new packet number, removing ambiguity about which packet is acknowledged when an ACK is received. Consequently, more accurate RTT measurements can be made, spurious retransmissions are trivially detected, and mechanisms such as Fast Retransmit can be applied universally, based only on packet number.
This design point significantly simplifies loss detection mechanisms for QUIC. Most TCP mechanisms implicitly attempt to infer transmission ordering based on TCP sequence numbers -- a nontrivial task, especially when TCP timestamps are not available.
4.3. Clearer Loss Epoch
QUIC starts a loss epoch when a packet is lost. The loss epoch ends when any packet sent after the start of the epoch is acknowledged. TCP waits for the gap in the sequence number space to be filled, and so if a segment is lost multiple times in a row, the loss epoch may not end for several round trips. Because both should reduce their congestion windows only once per epoch, QUIC will do it once for every round trip that experiences loss, while TCP may only do it once across multiple round trips.
4.4. No Reneging
QUIC ACK frames contain information similar to that in TCP Selective Acknowledgments (SACKs) [RFC2018]. However, QUIC does not allow a packet acknowledgment to be reneged, greatly simplifying implementations on both sides and reducing memory pressure on the sender.
4.5. More ACK Ranges
QUIC supports many ACK ranges, as opposed to TCP's three SACK ranges. In high-loss environments, this speeds recovery, reduces spurious retransmits, and ensures forward progress without relying on timeouts.
4.6. Explicit Correction for Delayed Acknowledgments
QUIC endpoints measure the delay incurred between when a packet is received and when the corresponding acknowledgment is sent, allowing a peer to maintain a more accurate RTT estimate; see Section 13.2 of [QUIC-TRANSPORT].
4.7. Probe Timeout Replaces RTO and TLP
QUIC uses a probe timeout (PTO; see Section 6.2), with a timer based on TCP's retransmission timeout (RTO) computation; see [RFC6298]. QUIC's PTO includes the peer's maximum expected acknowledgment delay instead of using a fixed minimum timeout.
Similar to the RACK-TLP loss detection algorithm for TCP [RFC8985], QUIC does not collapse the congestion window when the PTO expires, since a single packet loss at the tail does not indicate persistent congestion. Instead, QUIC collapses the congestion window when persistent congestion is declared; see Section 7.6. In doing this, QUIC avoids unnecessary congestion window reductions, obviating the need for correcting mechanisms such as Forward RTO-Recovery (F-RTO) [RFC5682]. Since QUIC does not collapse the congestion window on a PTO expiration, a QUIC sender is not limited from sending more in-flight packets after a PTO expiration if it still has available congestion window. This occurs when a sender is application limited and the PTO timer expires. This is more aggressive than TCP's RTO mechanism when application limited, but identical when not application limited.
QUIC allows probe packets to temporarily exceed the congestion window whenever the timer expires.
4.8. The Minimum Congestion Window Is Two Packets
TCP uses a minimum congestion window of one packet. However, loss of that single packet means that the sender needs to wait for a PTO to recover (Section 6.2), which can be much longer than an RTT. Sending a single ack-eliciting packet also increases the chances of incurring additional latency when a receiver delays its acknowledgment.
QUIC therefore recommends that the minimum congestion window be two packets. While this increases network load, it is considered safe since the sender will still reduce its sending rate exponentially under persistent congestion (Section 6.2).
4.9. Handshake Packets Are Not Special
TCP treats the loss of SYN or SYN-ACK packet as persistent congestion and reduces the congestion window to one packet; see [RFC5681]. QUIC treats loss of a packet containing handshake data the same as other losses.