add support for parallel Connect

src/libraries/Common/tests/System/Net/Sockets/SocketTestExtensions.cs

@@ -102,6 +102,9 @@ internal class PortBlocker : IDisposable
         private const int MaxAttempts = 16;
         private Socket _shadowSocket;
         public Socket MainSocket { get; }
+        public Socket SecondarySocket => _shadowSocket;
+
+        public int Port;
 
         public PortBlocker(Func<Socket> socketFactory)
         {
@@ -126,7 +129,11 @@ public PortBlocker(Func<Socket> socketFactory)
                     _shadowSocket = new Socket(shadowAddress.AddressFamily, MainSocket.SocketType, MainSocket.ProtocolType);
                     success = TryBindWithoutReuseAddress(_shadowSocket, shadowEndPoint, out _);
 
-                    if (success) break;
+                    if (success)
+                    {
+                        Port = port;
+                        break;
+                    }
                 }
                 catch (SocketException)
                 {

src/libraries/System.Net.Sockets/ref/System.Net.Sockets.cs

@@ -6,6 +6,11 @@
 
 namespace System.Net.Sockets
 {
+    public enum ConnectAlgorithm
+    {
+        Default = 0,
+        Parallel = 1,
+    }
     public enum IOControlCode : long
     {
         [System.Runtime.Versioning.SupportedOSPlatformAttribute("windows")]
@@ -343,6 +348,7 @@ public void Connect(string host, int port) { }
         public System.Threading.Tasks.ValueTask ConnectAsync(System.Net.IPAddress[] addresses, int port, System.Threading.CancellationToken cancellationToken) { throw null; }
         public bool ConnectAsync(System.Net.Sockets.SocketAsyncEventArgs e) { throw null; }
         public static bool ConnectAsync(System.Net.Sockets.SocketType socketType, System.Net.Sockets.ProtocolType protocolType, System.Net.Sockets.SocketAsyncEventArgs e) { throw null; }
+        public static bool ConnectAsync(System.Net.Sockets.SocketType socketType, System.Net.Sockets.ProtocolType protocolType, System.Net.Sockets.SocketAsyncEventArgs e, System.Net.Sockets.ConnectAlgorithm connectAlgorithm) { throw null; }
         public System.Threading.Tasks.Task ConnectAsync(string host, int port) { throw null; }
         public System.Threading.Tasks.ValueTask ConnectAsync(string host, int port, System.Threading.CancellationToken cancellationToken) { throw null; }
         public void Disconnect(bool reuseSocket) { }

src/libraries/System.Net.Sockets/src/Resources/Strings.resx

@@ -315,4 +315,7 @@
   <data name="net_sockets_address_small" xml:space="preserve">
     <value>Provided SocketAddress is too small for given AddressFamily.</value>
   </data>
+  <data name="net_sockets_invalid_connect_algorithm" xml:space="preserve">
+    <value>Provided ConnectAlgorithm {0} is not valid.</value>
+  </data>
 </root>

src/libraries/System.Net.Sockets/src/System.Net.Sockets.csproj

@@ -19,6 +19,7 @@
 
   <ItemGroup Condition="'$(TargetPlatformIdentifier)' != ''">
     <!-- All configurations -->
+    <Compile Include="System\Net\Sockets\ConnectAlgorithm.cs" />
     <Compile Include="System\Net\Sockets\SocketReceiveFromResult.cs" />
     <Compile Include="System\Net\Sockets\SocketReceiveMessageFromResult.cs" />
     <Compile Include="System\Net\Sockets\SocketsTelemetry.cs" />

src/libraries/System.Net.Sockets/src/System/Net/Sockets/ConnectAlgorithm.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Net.Sockets
+{
+    /// <summary>
+    /// Specifies the algorithm used to establish a socket connection.
+    /// </summary>
+    public enum ConnectAlgorithm
+    {
+        /// <summary>
+        /// The default connection mechanism, typically sequential processing.
+        /// </summary>
+        Default = 0,
+
+        /// <summary>
+        /// Uses a Happy Eyeballs-like algorithm to connect, attempting connections in parallel to improve speed and reliability.
+        /// </summary>
+        Parallel = 1,
+    }
+}

src/libraries/System.Net.Sockets/src/System/Net/Sockets/Socket.cs

@@ -2977,9 +2977,16 @@ internal bool ConnectAsync(SocketAsyncEventArgs e, bool userSocket, bool saeaCan
             return pending;
         }
 
-        public static bool ConnectAsync(SocketType socketType, ProtocolType protocolType, SocketAsyncEventArgs e)
+        public static bool ConnectAsync(SocketType socketType, ProtocolType protocolType, SocketAsyncEventArgs e) =>
+                            ConnectAsync(socketType, protocolType,  e, ConnectAlgorithm.Default);
+        public static bool ConnectAsync(SocketType socketType, ProtocolType protocolType, SocketAsyncEventArgs e, ConnectAlgorithm connectAlgorithm)
         {
             ArgumentNullException.ThrowIfNull(e);
+            if (connectAlgorithm != ConnectAlgorithm.Default &&
+                connectAlgorithm != ConnectAlgorithm.Parallel)
+            {
+                throw new ArgumentException(SR.Format(SR.net_sockets_invalid_connect_algorithm, connectAlgorithm), nameof(connectAlgorithm));
+            }
 
             if (e.HasMultipleBuffers)
             {
@@ -3001,7 +3008,7 @@ public static bool ConnectAsync(SocketType socketType, ProtocolType protocolType
                 e.StartOperationConnect(saeaMultiConnectCancelable: true, userSocket: false);
                 try
                 {
-                    pending = e.DnsConnectAsync(dnsEP, socketType, protocolType);
+                    pending = e.DnsConnectAsync(dnsEP, socketType, protocolType, connectAlgorithm);
                 }
                 catch
                 {