Round Trip Time

Round-trip time (RTT) is a crucial metric in web performance, measuring the time it takes for a web request to travel from a client to a server and back. In simpler terms, it's the duration between sending a request and receiving the server's response. In modern browsers, it's exposed via the navigator object and estimates the round-trip time of the current connection.

The rtt read-only property [...] returns the estimated effective round-trip time of the current connection, rounded to the nearest multiple of 25 milliseconds. This value is based on recently observed application-layer RTT measurements across recently active connections.

- mdn web docs

Why RTT Matters

• RTT directly impacts loading times and overall user experience
• It's a key component of Time to First Byte (TTFB), another critical performance metric
• Lower RTT values generally indicate better network performance and faster page loads

Factors affecting RTT

1. Physical distance between client and server
2. Network congestion
3. Server response times (not counting backend processing, but pure respone time of the front-most server or proxy)

The relation between RTT and TTFB

Understanding the relationship between Round-Trip Time (RTT) and Time to First Byte (TTFB) is crucial for a comprehensive grasp of web performance. Let's dive into how these metrics are connected and why they matter for your website's speed.

What is TTFB?

Time to First Byte (TTFB) measures the duration from when a user's browser requests a page to when it receives the first byte of the response. It's a key indicator of your server's responsiveness and the efficiency of your network. Read more about TTFB here

The RTT-TTFB Connection

RTT is a fundamental component of TTFB. Here's why:

1. Connection Establishment: TTFB includes the time needed to set up a connection with the server, which involves at least one full round trip for the TCP handshake.

2. Request Transmission: Sending the HTTP request to the server requires another portion of the RTT.

3. Server Processing: While not directly part of RTT, the time the server takes to process the request is included in TTFB.

4. Response Initiation: The start of the server's response transmission contributes to the final part of TTFB.

In essence, TTFB can be roughly approximated as:

TTFB ≈ 1 RTT (for TCP) + 1 RTT (for HTTP request) + Server Processing Time

Note that this is only valid when using the latest HTTP version.

HTTP protocols and their its impact on RTT

The relationship between Round-Trip Time (RTT) and Time to First Byte (TTFB) has evolved significantly with different HTTP versions. In HTTP/1.1, a secure connection required multiple round trips:

TTFB ≈ 1 RTT (TCP) + 2 RTT (TLS) + 1 RTT (HTTP request) + Server Processing Time

HTTP/2 introduced multiplexing and header compression, improving efficiency after the initial connection, but the setup process remained similar to HTTP/1.1.

HTTP/3, based on QUIC, brings substantial improvements:

TTFB ≈ 1 RTT (combined handshake) + 1 RTT (HTTP request) + Server Processing Time

For repeat connections, HTTP/3 can achieve 0-RTT resumption, further reducing latency.

These protocol advancements significantly impact web performance strategies. Adopting newer HTTP versions can drastically reduce the effect of RTT on TTFB. Connection reuse becomes more critical, and technologies like Edge Computing effectively decrease RTT.

PageVitals now offers insights into protocol usage, allowing you to:

• View protocol distribution among your visitors
• Compare RTT across different protocols
• Analyze how protocol changes affect your TTFB

By understanding these metrics, you can make informed decisions to optimize your site's performance across various network conditions.

How to optimize RTT

Reducing Round-Trip Time (RTT) is crucial for improving your website's overall performance. Here are key strategies to optimize RTT:

1. Leverage Content Delivery Networks (CDNs): CDNs distribute your content across multiple geographic locations, bringing it closer to your users and significantly reducing RTT.

2. Strategic Server Location: Choose server locations that are physically close to your primary audience. This minimizes the distance data needs to travel, directly impacting RTT.

3. Implement Keep-Alive Connections: Use HTTP keep-alive to maintain persistent connections, reducing the need for repeated TCP handshakes and lowering RTT for subsequent requests.

4. Limit Connection Count: Minimize the number of connections required to load your site. Consolidate resources and use domain sharding to balance between connection limits and parallel loading.

5. Adopt HTTP/3: Support HTTP/3 on your server to take advantage of its improved connection establishment and 0-RTT resumption capabilities, significantly reducing RTT for repeat visitors.

6. Optimize DNS Resolution: Use a reliable, fast DNS provider and consider DNS prefetching for critical domains.

7. Implement Resource Hints: Use preconnect and dns-prefetch hints to establish early connections and reduce effective RTT.

By implementing these optimizations, you can substantially reduce RTT, leading to faster page loads and improved user experience. PageVitals can help you monitor the impact of these changes on your site's performance metrics.