Add an option to enable Multipath TCP on clients (#766)

Multipath TCP (MPTCP) is a TCP extension allowing to enhance the
reliability of the network by using multiple interfaces. This extension
provides a seamless handover between interfaces in case of deterioration
of the connection on the original one. In the context of iOS and Mac OS
X, it could be really interesting to leverage the capabilities of MPTCP
as they could benefit from their multiple interfaces (ethernet + Wi-fi
for Mac OS X, Wi-fi + cellular for iOS).

This contribution introduces patches to HTTPClient.Configuration and
establishment of the Bootstraps. A supplementary field "enableMultipath"
was added to the configuration, allowing to request the use of MPTCP.
This flag is then used when creating the channels to configure the
client.

Note that in the future, it might also be potentially interesting to
offer more precise configuration options for MPTCP on MacOS, as the
Network framework allows also to select a type of service, instead of
just offering the option to create MPTCP connections. Currently, when
enabling MPTCP, only the Handover mode is used.

---------

Co-authored-by: Cory Benfield <lukasa@apple.com>

Author: Aperence

Sources/AsyncHTTPClient/ConnectionPool/HTTPConnectionPool+Factory.swift

@@ -322,6 +322,7 @@ extension HTTPConnectionPool.ConnectionFactory {
         if #available(OSX 10.14, iOS 12.0, tvOS 12.0, watchOS 6.0, *), let tsBootstrap = NIOTSConnectionBootstrap(validatingGroup: eventLoop) {
             return tsBootstrap
                 .channelOption(NIOTSChannelOptions.waitForActivity, value: self.clientConfiguration.networkFrameworkWaitForConnectivity)
+                .channelOption(NIOTSChannelOptions.multipathServiceType, value: self.clientConfiguration.enableMultipath ? .handover : .disabled)
                 .connectTimeout(deadline - NIODeadline.now())
                 .channelInitializer { channel in
                     do {
@@ -338,6 +339,7 @@ extension HTTPConnectionPool.ConnectionFactory {
         if let nioBootstrap = ClientBootstrap(validatingGroup: eventLoop) {
             return nioBootstrap
                 .connectTimeout(deadline - NIODeadline.now())
+                .enableMPTCP(clientConfiguration.enableMultipath)
         }
 
         preconditionFailure("No matching bootstrap found")
@@ -415,6 +417,7 @@ extension HTTPConnectionPool.ConnectionFactory {
 
                 tsBootstrap
                     .channelOption(NIOTSChannelOptions.waitForActivity, value: self.clientConfiguration.networkFrameworkWaitForConnectivity)
+                    .channelOption(NIOTSChannelOptions.multipathServiceType, value: self.clientConfiguration.enableMultipath ? .handover : .disabled)
                     .connectTimeout(deadline - NIODeadline.now())
                     .tlsOptions(options)
                     .channelInitializer { channel in
@@ -443,6 +446,7 @@ extension HTTPConnectionPool.ConnectionFactory {
 
         let bootstrap = ClientBootstrap(group: eventLoop)
             .connectTimeout(deadline - NIODeadline.now())
+            .enableMPTCP(clientConfiguration.enableMultipath)
             .channelInitializer { channel in
                 sslContextFuture.flatMap { sslContext -> EventLoopFuture<Void> in
                     do {

Sources/AsyncHTTPClient/HTTPClient.swift

@@ -738,6 +738,10 @@ public class HTTPClient {
             }
         }
 
+        /// Whether ``HTTPClient`` will use Multipath TCP or not
+        /// By default, don't use it
+        public var enableMultipath: Bool
+
         public init(
             tlsConfiguration: TLSConfiguration? = nil,
             redirectConfiguration: RedirectConfiguration? = nil,
@@ -755,6 +759,7 @@ public class HTTPClient {
             self.decompression = decompression
             self.httpVersion = .automatic
             self.networkFrameworkWaitForConnectivity = true
+            self.enableMultipath = false
         }
 
         public init(tlsConfiguration: TLSConfiguration? = nil,

Tests/AsyncHTTPClientTests/HTTPClientTests.swift

@@ -3590,6 +3590,24 @@ final class HTTPClientTests: XCTestCaseHTTPClientTestsBaseClass {
         XCTAssertEqual(.ok, response.status)
     }
 
+    func testClientWithMultipath() throws {
+        do {
+            var conf = HTTPClient.Configuration()
+            conf.enableMultipath = true
+            let client = HTTPClient(configuration: conf)
+            defer {
+                XCTAssertNoThrow(try client.shutdown().wait())
+            }
+            let response = try client.get(url: self.defaultHTTPBinURLPrefix + "get").wait()
+            XCTAssertEqual(.ok, response.status)
+        } catch let error as IOError where error.errnoCode == EINVAL || error.errnoCode == EPROTONOSUPPORT || error.errnoCode == ENOPROTOOPT {
+            // some old Linux kernels don't support MPTCP, skip this test in this case
+            // see https://www.mptcp.dev/implementation.html for details about each type
+            // of error
+            throw XCTSkip()
+        }
+    }
+
     func testSingletonClientWorks() throws {
         let response = try HTTPClient.shared.get(url: self.defaultHTTPBinURLPrefix + "get").wait()
         XCTAssertEqual(.ok, response.status)