iOS: Add support for the throttling in Network tab

[SuperPat45]

I notice the throttling option in the Network tab does not have any effects.

Would it be possible to make it work?

[auchenberg]

Thanks for filing this. I doubt it, as Webkit don't have such APIs per my understanding, but let's try to figure it out.

[auchenberg]

Looks like this is the way is to do it in iOS, https://nshipster.com/network-link-conditioner.

I wonder if there's any APIs available.

[auchenberg]

Note, we might be able to use Network.setEmulatedConditions. https://github.com/WebKit/WebKit/blob/8fec86d40eff362d8e510c24faedadb4e82bddf2/Source/JavaScriptCore/inspector/protocol/Network.json#L348C22-L348C43

[auchenberg]

Network Throttling for iOS

Overview

Investigation into adding network throttling support in the Network panel for iOS devices.

GitHub Issue: #134

Current State

• Network throttling dropdown exists in DevTools UI (Network panel)
• Selecting throttling presets has no effect on iOS
• Network.canEmulateNetworkConditions returns false
• Network.emulateNetworkConditions is not implemented

Investigation Findings

1. DevTools Frontend Flow

The throttling UI uses SDK.NetworkManager.MultitargetNetworkManager:

// ThrottlingManager.js - When user selects a throttling preset
SDK.NetworkManager.MultitargetNetworkManager.instance().setNetworkConditions(conditions);

// NetworkManager.js:1119 - Sends CDP command
networkAgent.invoke_emulateNetworkConditions({
  offline: this.isOffline(),
  latency: conditions.latency,
  downloadThroughput: conditions.download < 0 ? 0 : conditions.download,
  uploadThroughput: conditions.upload < 0 ? 0 : conditions.upload,
  connectionType: NetworkManager._connectionType(conditions)
});

2. CDP Network.emulateNetworkConditions

Parameter	Type	Description
offline	boolean	Simulate internet disconnection
latency	number	Minimum latency in milliseconds
downloadThroughput	number	Max download speed (bytes/sec), -1 disables
uploadThroughput	number	Max upload speed (bytes/sec), -1 disables
connectionType	ConnectionType	Network connection type hint

3. WebKit Network.setEmulatedConditions - NOT AVAILABLE

WebKit had an experimental throttling API:

// WebKit Inspector Protocol (REMOVED)
Network.setEmulatedConditions({
  bytesPerSecondLimit: number  // optional, limits download if positive
})

Timeline:

• July 2022: ENABLE_INSPECTOR_NETWORK_THROTTLING added to WebKit (commit a8912b8)
• February 5, 2024: Feature REMOVED from WebKit (webkit-changes)

Reason for removal:

"The current implementation is based on _bytesPerSecondLimit SPI on NSURLSessionTask which is incorrect since this is a per-task limit, and it's not intended to be a realistic simulation. New unified HTTP stack no longer supports this SPI."

4. Preset Throttling Values (Reference)

Preset	Download (bytes/sec)	Upload (bytes/sec)	Latency (ms)
No throttling	-1 (disabled)	-1 (disabled)	0
Offline	0	0	0
Slow 3G	50,000 (~400 kbps)	50,000	2,000
Fast 3G	180,000 (~1.4 Mbps)	84,375	562

Conclusion: Not Implementable

Network throttling via Web Inspector is NOT available on iOS.

The WebKit team removed the Network.setEmulatedConditions API in February 2024 because:

1. The underlying _bytesPerSecondLimit SPI was per-task, not realistic
2. The new unified HTTP stack doesn't support this SPI
3. No replacement API has been provided

Platform Support Matrix

Platform	CDP Method	WebKit Method	Support Level
Android	Network.emulateNetworkConditions	N/A (direct CDP)	Full
iOS/WebKit	N/A	Network.setEmulatedConditions	None

Alternative: Network Link Conditioner

The only way to throttle network on iOS is via Apple's Network Link Conditioner, a system-level tool.

Setup on iOS Device

1. Connect iOS device to Mac with Xcode installed
2. On iOS device: Settings → Developer → Network Link Conditioner
3. Enable and select a profile (3G, Edge, LTE, etc.)

Limitations

• Affects all network traffic on the device (not just the inspected page)
• Requires Xcode to be installed to enable Developer settings
• Not controllable via Web Inspector

Documentation

• Network Link Conditioner Guide
• BrowserStack: Network Throttling in Safari

Recommendation

Option 1: Keep Current Behavior (Recommended)

Keep canEmulateNetworkConditions returning false. This:

• Accurately reflects iOS capabilities
• Prevents user confusion when throttling doesn't work
• Is the current behavior

Option 2: Hide Throttling UI for iOS

Consider hiding the throttling dropdown entirely when connected to iOS targets to avoid confusion.

Option 3: Show Informational Message

When user tries to use throttling on iOS, show a message explaining:

• Throttling is not supported via Web Inspector on iOS
• Suggest using Network Link Conditioner instead

Future Considerations

If WebKit adds a new throttling API in the future, this spec should be revisited. Monitor:

• WebKit Bug Tracker
• WebKit Network.json

References

• CDP Network.emulateNetworkConditions: https://chromedevtools.github.io/devtools-protocol/tot/Network/#method-emulateNetworkConditions
• WebKit Throttling Added (July 2022): WebKit/WebKit@a8912b8
• WebKit Throttling Removed (Feb 2024): https://www.mail-archive.com/webkit-changes@lists.webkit.org/msg209724.html
• GitHub Issue: iOS: Add support for the throttling in Network tab #134
• Network Link Conditioner: https://nshipster.com/network-link-conditioner