How do SCTP and QUIC handle congestion control and flow control differently?

1 What is SCTP?

SCTP stands for Stream Control Transmission Protocol, and it is a message-oriented, reliable, and secure protocol that supports multiple streams of data within a single connection. SCTP was designed to overcome some of the limitations of TCP, such as head-of-line blocking, poor support for multihoming, and vulnerability to SYN flooding attacks. SCTP also provides features such as unordered delivery, partial reliability, and network-level fault tolerance.

2 What is QUIC?

QUIC stands for Quick UDP Internet Connections, and it is a connection-oriented, reliable, and secure protocol that runs over UDP. QUIC was developed by Google to improve the performance of web applications, especially over high-latency and lossy networks. QUIC offers features such as multiplexing, encryption, congestion control, and flow control at the application layer, reducing the dependency on the underlying transport layer. QUIC also enables fast connection establishment, zero-RTT resumption, and seamless migration across IP addresses.

3 How do they control congestion?

Both SCTP and QUIC use a variant of the TCP congestion control algorithm, which adjusts the sending rate based on the feedback from the receiver and the network. However, they differ in how they implement and adapt the algorithm to their specific characteristics.

SCTP uses a separate congestion window for each destination address in a multihomed association, allowing it to balance the load and cope with failures. SCTP also uses a split fast retransmit algorithm, which can trigger a retransmission based on either the number of duplicate acknowledgments or the expiration of a timer. SCTP can also switch between different congestion control modes, such as slow start, congestion avoidance, fast recovery, or limited transmit, depending on the network conditions and the sender's state.

QUIC uses a single congestion window for the entire connection, regardless of the number of streams or IP addresses involved. QUIC also uses a hybrid loss detection algorithm, which combines packet number thresholds and time thresholds to determine when a packet is lost. QUIC can also adapt its congestion control algorithm to different scenarios, such as initial, handshake, or application data phases, or when migrating to a new path. QUIC also supports multiple congestion control algorithms, such as Reno, Cubic, or BBR, and allows the sender to choose the best one for its application.

4 How do they control flow?

Both SCTP and QUIC use a window-based flow control mechanism, which limits the amount of data that the sender can transmit without receiving an acknowledgment from the receiver. However, they differ in how they apply and manage the flow control window.

SCTP uses a per-stream flow control window, which is negotiated during the association establishment and can be updated dynamically by the receiver. SCTP also uses a per-association flow control window, which is derived from the sum of the per-stream windows and the receiver's buffer size. SCTP ensures that the sender does not exceed either of these windows, and that the receiver does not suffer from buffer overflow or underflow.

QUIC uses a per-stream flow control window, which is initialized by the receiver and can be increased or decreased by sending flow control frames. QUIC also uses a connection-level flow control window, which is applied to the aggregate data across all streams. QUIC ensures that the sender respects both of these windows, and that the receiver advertises them in a timely manner. QUIC also allows the sender to send data beyond the flow control window if it is needed for reliability or security reasons, such as retransmitting lost packets or completing the handshake.