Why KCP Beats TCP and UDP for Real‑Time Apps: Low Latency, High Throughput

Overview

In modern internet applications such as online games, video conferencing, and live streaming, real‑time performance and smoothness are critical. Traditional TCP provides reliable transmission but its congestion control and retransmission mechanisms cause high latency, while UDP is fast but lacks reliability, leading to packet loss and reordering.

Why Introduce KCP

Before presenting KCP, we review TCP and UDP characteristics: TCP guarantees reliability without guaranteeing transmission efficiency; UDP maximizes transmission speed but cannot ensure reliability.

How can we achieve both reliability and high transmission efficiency?

KCP Introduction

The KCP protocol was created to fill the gaps of TCP and UDP, offering lower latency and higher throughput while maintaining reliability.

KCP Protocol Overview

KCP (KCP Protocol) is a UDP‑based reliable transmission protocol open‑sourced by skywind3000 in 2014. It borrows TCP’s reliable mechanisms such as ACK, timeout retransmission, and flow control, but streamlines and optimizes them for real‑time scenarios.

Key Features of KCP

Low latency: aggressive retransmission reduces waiting time.

High throughput: dynamically adjusts sending rate according to network conditions.

Reliability: ensures ordered delivery and supports selective retransmission.

Flexibility: rich configuration options allow tuning of delay, throughput, and reliability trade‑offs.

How KCP Works

KCP implements TCP‑like reliability on top of UDP, including:

ACK: receiver sends acknowledgment packets after receiving data.

Timeout retransmission: sender retransmits if ACK is not received within a set time.

Flow control: sliding‑window mechanism prevents buffer overflow.

Congestion control: simple algorithm adjusts sending rate based on network status.

Application Scenarios

Mobile games (e.g., Honor of Kings) – reduces latency and improves player experience.

Video conferencing – minimizes video stutter and delay.

Live streaming – lowers stream latency and enhances interactivity.

IoT – improves efficiency and reliability of device communication.

Pros and Cons

Advantages :

Low latency

High throughput

High reliability

Configurable flexibility

Disadvantages :

Implementation complexity higher than UDP

Additional bandwidth overhead

Sensitivity to network jitter

How to Use KCP

Repository: https://github.com/xtaci/kcptun

// Initialize KCP object; conv is a session identifier, user is a callback pointer
ikcpcb *kcp = ikcp_create(conv, user);

// Define the lower‑level output function that KCP calls to send data
int udp_output(const char *buf, int len, ikcpcb *kcp, void *user) {
    // ...
}
// Set the output callback
kcp->output = udp_output;

// Periodically call ikcp_update to advance KCP state (e.g., every 10 ms)
ikcp_update(kcp, millisec);

// When a UDP packet is received, feed it to KCP
ikcp_input(kcp, received_udp_packet, received_udp_size);

Summary

For mobile scenarios where server latency cannot stay under 2 ms and RTT cannot be consistently 3 ms, introducing KCP can replace TCP to achieve 1‑RTT message exchange, shortening app response time. Several commercial products already use KCP for optimization, including:

Genshin Impact – miHoYo uses KCP to reduce game message latency.

SpatialOS – large‑scale multiplayer engine uses KCP for data acceleration.

Westward Studios – employs KCP for game data acceleration.

NetEase CC – uses KCP to speed up video streaming.

NetEase BOBO – KCP accelerates live‑streamer broadcasting.

NetEase UU – UU accelerator uses KCP/KCPTUN for remote transmission acceleration.

Alibaba Cloud – GRTN video transmission acceleration service uses KCP for audio/video data optimization.

By sacrificing an additional 3‑8% bandwidth, KCP provides stable latency and reduces packet loss. Further performance comparisons between TCP and KCP under varying network conditions will be covered in the next article.