# \[Pitch\] Ultra-Constrained Networking in URLSession on Darwin **URL:** \ **Category:** Related Projects\ **Tags:** foundation\ **Created:** [September 24, 2025, 9:52pm UTC](https://forums.swift.org/t/pitch-ultra-constrained-networking-in-urlsession-on-darwin/82345 "2025-09-24T21:52:39Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![travarin](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/travarin/32/42951_2.png) [@travarin](https://forums.swift.org/u/travarin)\ **Post date:** [September 24, 2025, 9:52pm UTC](https://forums.swift.org/t/pitch-ultra-constrained-networking-in-urlsession-on-darwin/82345/1 "2025-09-24T21:52:39Z")
Hi all, I’d like to share a proposal for introducing a new API on URLSession for using ultra-constrained interfaces. The API is not currently available on non-Darwin platforms. Please check out the details below. # URLSession Ultra-Constrained Network Access API - Proposal: SF-NNNN - Author: [Travis Langston](https://github.com/travarin) - Review Manager: TBD - Status: **Pitch** - Implementation: Pending ## Introduction Apps on Darwin are able to check if a network path uses an interface that is considered ultra-constrained by the system through the `isUltraConstrained` property on NWPath, and can then adjust whether a particular connection is allowed to use those interfaces through the `allowUltraConstrainedPaths` property on the `NWParameters`. In this proposal, we're introducing an API to allow apps that use URLSession on Darwin to also have this same capability of checking whether using ultra-constrained interfaces is currently allowed and specifying whether a particular session or request are allowed to use them. ## Proposed solution The proposed solution involves three changes: - Add a `allowsUltraConstrainedNetworkAccess` property on `URLSessionConfiguration`, to parallel existing properties like `allowsExpensiveNetworkAccess` and `allowsConstrainedNetworkAccess`. - Add a `allowsUltraConstrainedNetworkAccess` property on `URLRequest`, to parallel existing properties like `allowsExpensiveNetworkAccess` and `allowsConstrainedNetworkAccess`. - Add a new enum value `NSURLErrorNetworkUnavailableReasonUltraConstrained` for `NSURLErrorNetworkUnavailableReason`, to parallel existing values like `NSURLErrorNetworkUnavailableReasonExpensive` and `NSURLErrorNetworkUnavailableReasonConstrained`. These flags are not available on non-Darwin platforms at this moment. ## Example usage The following example allows an application to explicitly allow the task that follows to go over ultra-constrained interfaces. ```swift let configuration = URLSessionConfiguration.default configuration.allowsUltraConstrainedNetworkAccess = true let session = URLSession(configuration: configuration) let (data, response) = try await session.data(from: url) ``` Explicitly opting out a particular request, preventing it from being made over ultra-constrained interfaces. ```swift var request = URLRequest(url: URL(string: "https://www.example.com")!) request.allowsUltraConstrainedNetworkAccess = false let task = session.dataTask(with: request) { data, response, error in // Handle response } task.resume() ``` ## Detailed design We add a new property to `URLSessionConfiguration` that lets a caller control whether URLSession is allowed to use ultra-constrained network paths. ```swift open class URLSessionConfiguration : NSObject, NSCopying, @unchecked Sendable { ... @available(*, unavailable, message: "Not available on non-Darwin platforms") open var allowsUltraConstrainedNetworkAccess : Bool ... } ``` We also add a new property to `URLRequest` that lets a caller set if that particular request is allowed to use ultra-constrained network paths. Explicitly setting the property on the `URLRequest` used to create the `URLSessionTask` would override the `URLSessionConfiguration` setting. ```swift public struct URLRequest { ... @available(*, unavailable, message: "Not available on non-Darwin platforms") open var allowsUltraConstrainedNetworkAccess : Bool ... } ``` Finally we add a new enum value to NetworkUnavailableReason which can be returned in the userInfo dictionary with the key `NSURLErrorNetworkUnavailableReasonKey` to indicate connectivity was unavailable because the only routes available were ultra constrained and this did not meet the constraints set on the task. ```swift /// Reasons used by URLError to indicate that a URLSessionTask failed because of unsatisfiable network constraints. public enum NetworkUnavailableReason : Int, Sendable { @available(*, unavailable, message: "Not available on non-Darwin platforms") case ultraConstrained } ``` ## Source compatibility No impact. ## Implications on adoption This feature can be freely adopted and un-adopted in source code with no deployment constraints and without affecting source compatibility. ## Alternatives considered The biggest question here is the name of the property. "Ultra-constrained" is the name we have in the Network.framework API, and this proposal adds a corresponding API to URLSession. ---
**Author:** ![KeithBauerANZ](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/keithbaueranz/32/27897_2.png) [@KeithBauerANZ](https://forums.swift.org/u/KeithBauerANZ)\ **Post date:** [September 24, 2025, 10:29pm UTC](https://forums.swift.org/t/pitch-ultra-constrained-networking-in-urlsession-on-darwin/82345/2 "2025-09-24T22:29:32Z")
This feature seems to be entirely undocumented; this is all I can find, and says less than nothing: > **[isUltraConstrained | Apple Developer Documentation](https://developer.apple.com/documentation/network/nwpath/isultraconstrained)** > > Checks if the path uses an NWInterface that is considered to be constrained by user preference ---
**Author:** ![glhaynes](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/glhaynes/32/30039_2.png) [@glhaynes](https://forums.swift.org/u/glhaynes)\ **Post date:** [September 25, 2025, 1:09pm UTC](https://forums.swift.org/t/pitch-ultra-constrained-networking-in-urlsession-on-darwin/82345/3 "2025-09-25T13:09:59Z")
For those like me who weren’t previously familiar with the term “ultra constrained network”, I believe an example would be connection with a satellite. [Ultra-Constrained networks and URL… | Apple Developer Forums](https://developer.apple.com/forums/thread/795827) Might be a good idea to specify that in the pitch.