Download Speed

Technical Classification

Type: JavaScript Property (navigator.connection.downlink)

Data Source: Browser's Network Information API

When Available: Chromium-based browsers (Chrome, Edge, Opera, Brave), not supported in Firefox or Safari

Return Type: Number (megabits per second, Mbps)

Background & Purpose

The navigator.connection.downlink property returns an estimate of the effective bandwidth in megabits per second (Mbps). This is part of the Network Information API and represents the browser's best estimate of available download speed based on recent network activity.

Historical Context:

• Part of the W3C Network Information API specification
• Designed to enable adaptive content delivery and bandwidth-aware applications
• Based on actual observed network performance, not advertised connection speed
• Updates dynamically as network conditions change
• Primarily implemented in Chromium-based browsers
• Firefox and Safari chose not to implement due to privacy concerns

Key Characteristics:

• Represents an estimate, not a guarantee or measurement in real-time
• Based on recent network activity (typically last few seconds to minutes)
• Rounded to nearest 25 kbps increments (0.025 Mbps) to reduce fingerprinting
• Maximum reported value is typically capped at 10 Mbps
• May not reflect actual maximum bandwidth of connection

Downlink Speed Values

Typical Value Ranges:

Speed (Mbps)	Connection Type	Typical Usage Scenarios
0.05 - 0.1	Very slow 2G/EDGE	Text-only browsing, basic email
0.15 - 0.3	2G/EDGE	Basic web browsing, compressed images
0.4 - 0.7	Slow 3G	Web browsing, low-quality streaming
1.0 - 3.0	3G/HSPA	Standard definition video, moderate browsing
4.0 - 8.0	4G LTE/Fast Wi-Fi	HD video streaming, large file downloads
10.0+	Fast 4G/5G/Fiber/Cable	4K video, large downloads, gaming (often capped at 10.0)

Important Notes:

• Not Maximum Speed: Reports effective bandwidth, not connection's theoretical maximum
• Capped Value: Many browsers cap reported value at 10 Mbps even on gigabit connections
• Dynamic: Value changes throughout browsing session based on network activity
• Rounded: Values are rounded to reduce precision and fingerprinting potential
• Context-Dependent: Same connection may report different values based on congestion

Real-World Examples:

• Gigabit fiber at home: Often reports 10.0 (capped)
• Fast cable internet: 8.0-10.0
• Standard cable/DSL: 2.0-6.0
• 4G LTE with good signal: 5.0-10.0
• 3G or weak LTE: 1.0-3.0
• Public Wi-Fi (crowded): 0.5-2.0

Common Uses

Legitimate Uses:

• Adaptive Bitrate Streaming: Video services select appropriate quality based on bandwidth
• Progressive Image Loading: Serve high-res images only on fast connections
• Preloading Decisions: Decide whether to preload next page or resources
• Download Warnings: Warn users before large downloads on slow connections
• Feature Gating: Disable bandwidth-intensive features on slow connections
• Content Optimization: Serve compressed or simplified content on limited bandwidth
• Analytics: Understand user experience quality based on connection speed
• A/B Testing: Segment users by connection speed for performance testing

Example Implementations:

• Video Platforms: YouTube, Netflix use similar metrics for quality selection
• Image Services: Serve WebP, AVIF, or lower resolution based on downlink
• News Sites: Load images lazily or skip them entirely on <1 Mbps
• Cloud Storage: Adjust upload/download strategies based on available bandwidth

Example Code:

• if (navigator.connection.downlink < 1.5) { /* Load low-res images */ }
• if (navigator.connection.downlink > 5) { /* Preload next page */ }

Platform Differences

Browser Support:

Browser	Support	Notes
Chrome	Yes (v61+)	Full support, reliable implementation
Edge	Yes (v79+)	Full support on Chromium-based Edge
Opera	Yes (v48+)	Full support
Brave	Yes	Supported, may be limited in strict privacy modes
Firefox	No	Not implemented; navigator.connection undefined
Safari	No	Not implemented; privacy concerns cited
Mobile Chrome	Yes	Works on Android, more variable readings
Mobile Safari	No	Not supported on iOS

Why Limited Support?:

• Privacy Concerns: Can reveal information about user's network and location
• Fingerprinting: Adds another data point for tracking
• Accuracy Issues: Not always reliable or meaningful
• Philosophy Differences: Firefox/Safari prioritize privacy over adaptive features

Fallback Strategy:

• Always check for API availability: if (navigator.connection?.downlink)
• Provide sensible defaults for unsupported browsers
• Consider using effectiveType as alternative
• Implement feature detection and graceful degradation

Privacy Implications

Tracking Risk: LOW

Privacy Concerns:

• Network Profiling: Reveals information about internet service provider tier
• Location Context: Consistently low speeds may indicate rural area
• Economic Inference: Connection speed correlates loosely with socioeconomic factors
• Device Type Hints: Mobile connections typically slower than wired
• Fingerprinting Component: Adds entropy when combined with other data points
• Temporal Patterns: Speed variations throughout day may reveal behavior patterns

Why Low Risk:

• Values are rounded to reduce precision
• Most modern connections report high values (5-10 Mbps)
• Not supported in Firefox or Safari (limits cross-browser tracking)
• Changes frequently throughout browsing session
• Not unique enough for identification on its own
• Provides minimal information compared to other fingerprinting vectors

Entropy Analysis:

• Rounded to 0.025 Mbps increments: ~400 possible values from 0-10 Mbps
• In practice, most users cluster around 5-10 Mbps range
• Adds roughly 2-3 bits of entropy to fingerprint
• Most valuable when combined with effectiveType and RTT

User Control & Protection

How to Control Download Speed Reporting:

Browser Choice:

• Firefox: Does not support API - no data exposed
• Safari: Does not support API - no data exposed
• Chrome/Edge: No built-in option to disable
• Brave: Supported but may limit in aggressive privacy modes

Privacy Extensions:

• Browser fingerprinting protection extensions can block navigator.connection
• Canvas Defender, Trace, and similar tools may spoof or block this API
• uBlock Origin can block certain JavaScript APIs with advanced settings
• Check extension settings for "Network Information API" blocking

Testing Current Value:

• Open console: navigator.connection?.downlink
• Returns number in Mbps if supported, undefined if not
• Watch value change: setInterval(() => console.log(navigator.connection?.downlink), 5000)

Developer Tools:

• Chrome DevTools > Network > Throttling simulates speeds but doesn't affect reported downlink
• Some browser extensions can override JavaScript API responses
• Content scripts can intercept and modify API values

Limitations:

• No native Chrome/Edge setting to disable without extension
• Blocking may break adaptive loading features
• Spoofing can cause websites to serve inappropriate content quality
• Limited practical privacy benefit due to low uniqueness and frequent changes

Privacy Best Practices:

• Use browsers that don't expose this API (Firefox, Safari) if concerned
• Focus on higher-impact privacy measures (VPN, ad blockers, cookie management)
• Accept that on fast connections, many users report similar values
• Consider fingerprinting protection extensions for comprehensive protection

Additional Resources

• MDN: NetworkInformation.downlink

  Comprehensive documentation for the downlink property

• MDN: Network Information API

  Complete overview of the Network Information API

• W3C: Network Information API Specification

  Official W3C specification for the API

• Web.dev: Adaptive Loading with Service Workers

  Using network information for adaptive content delivery

• Can I Use: Network Information API

  Browser compatibility information

• Chrome Developers: Network Information API

  Google's guide to using the Network Information API