---
title: Improving network reliability using Multipath TCP
framework: foundation
role: article
role_heading: Article
path: foundation/improving-network-reliability-using-multipath-tcp
---

# Improving network reliability using Multipath TCP

Use the available radios in iOS devices to improve your app’s network reliability and performance.

## Overview

Overview When using your app on an iOS device, users are likely to move in and out of range of Wi-Fi, switching to cellular networks and back again. Multipath TCP improves the performance of your app when a user is in a location with limited Wi-Fi and while their device is transitioning to and from cellular data usage. In its default configuration, a URL session uses a single radio for a network call, preferring Wi-Fi over cellular. However, with Multipath TCP enabled, the URL session initiates the request on both radios and selects the more responsive of the two with a preference for Wi-Fi. Enable Multipath TCP in your app These steps are required: Enable the Multipath Entitlement in the Xcode Capabilities pane for your app target. Set the multipathServiceType property in URLSessionConfiguration to a mode other than none. For most cases, the handover mode is the best option. This mode provides your users with a seamless transition between Wi-Fi and cellular, allowing uninterrupted usage of your app. (For information about other modes, see URLSessionConfiguration.MultipathServiceType.) While the user’s device is connected to a reliable Wi-Fi network, your app uses only Wi-Fi, and no cellular data is consumed. As the user leaves the range of the Wi-Fi network and the signal starts to deteriorate, Multipath TCP begins transitioning data usage to the cellular network to provide a seamless transition. important: Multipath TCP requires the destination server to have Multipath TCP enabled. Many commercial load balancers already support Multipath TCP, and you can enable it in the configuration. For Linux-based servers, the Linux kernel with Multipath TCP is available at the Multipath TCP website. Limitations when using Multipath TCP with Wi-Fi Assist Because Wi-Fi Assist is a user selectable, device-wide option, there are a few limitations when using Multipath TCP with that option enabled: Wi-Fi Assist prevents flows from using cellular data when your app is in the background. Wi-Fi Assist limits the amount of data that your app can send over cellular networks. When the limit is reached, Multipath TCP is disabled. Wi-Fi Assist is integrated into the URLSession API and doesn’t require any changes to the session configuration. At the start of each URL session task, Wi-Fi Assist determines whether to use Wi-Fi or cellular data for the connection. After the URL session task begins, Wi-Fi Assist doesn’t change from Wi-Fi to cellular or from cellular to Wi-Fi. Multipath TCP extends the performance-improving capabilities of Wi-Fi Assist to provide seamless radio transition and improved throughput. note: When using a developer-enabled device for testing, you can disable the Wi-Fi Assist data limitation in the developer section of the Settings app.

## See Also

### Supporting Multipath TCP

- [multipathServiceType](foundation/urlsessionconfiguration/multipathservicetype-swift.property.md)
- [URLSessionConfiguration.MultipathServiceType](foundation/urlsessionconfiguration/multipathservicetype-swift.enum.md)
- [Multipath Entitlement](bundleresources/entitlements/com.apple.developer.networking.multipath.md)