Add .NET 4.7.1 Networking known issue 534719

releases/net471/KnownIssues/534719-Networking ServicePoint.ConnectionLimit default behavior with loopback changed unexpectedly.md

@@ -0,0 +1,73 @@
+# ServicePoint.ConnectionLimit default behavior with loopback changed unexpectedly
+
+## Symptoms
+The limit for HTTP connections per endpoint is controlled by the `ServicePointManager.DefaultConnectionLimit` property.
+This value defaults to 2. In the .NET Framework 4.7 and earlier versions, the limit applied only to non-loopback 
+addresses such as http://www.microsoft.com.  For loopback addresses such as http://localhost, the limit for connections
+was always int.MaxValue (2,147,483,647) unless changed by calling the `ServicePoint.ConnectionLimit` API.
+
+In the .NET Framework 4.7.1 when using HttpClient APIs, the connection limit for loopback addresses now matches the limit
+for non-loopback addresses. Thus, the default limit is 2. This can cause applications to run slower or hang when
+doing multiple, parallel, requests to the http://localhost addresses.
+
+Here is code that shows the problem:
+
+```c#
+using System;
+using System.Net;
+using System.Net.Http;
+
+namespace ConsoleApp
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            Console.WriteLine($"ServicePointManager.DefaultConnectionLimit: {ServicePointManager.DefaultConnectionLimit}");
+
+            var uriLoopback = new Uri("http://localhost");
+            var uriExternal = new Uri("http://www.microsoft.com");
+
+            Console.WriteLine("Before using HttpClient APIs");
+            ServicePoint spLoopback = ServicePointManager.FindServicePoint(uriLoopback);
+            Console.WriteLine($"{uriLoopback.AbsoluteUri}, ConnectionLimit (should be {int.MaxValue}): {spLoopback.ConnectionLimit}");
+
+            ServicePoint spExternal = ServicePointManager.FindServicePoint(uriExternal);
+            Console.WriteLine($"{uriExternal.AbsoluteUri}, ConnectionLimit (should be {ServicePointManager.DefaultConnectionLimit}): {spExternal.ConnectionLimit}");
+
+            Console.WriteLine("Use HttpClient APIs");
+            var client = new HttpClient();
+            try
+            {
+                HttpResponseMessage response = client.GetAsync(uriLoopback).Result;
+            }
+            catch (Exception)
+            {
+                // Ignore any network error since there is probably not a loopback server present.
+            }
+
+            Console.WriteLine("After using HttpClient APIs");
+
+            // BUG - due to the bug in .NET Framework 4.7.1, the ConnectionLimit for this loopback ServicePoint is changed
+            // unexpectedly.
+            Console.WriteLine($"{uriLoopback.AbsoluteUri}, ConnectionLimit (should be {int.MaxValue}): {spLoopback.ConnectionLimit}");
+        }
+    }
+}
+```
+
+## Cause
+Changes in .NET Framework 4.7.1 to the `System.Net.Http.HttpClientHandler` class caused this problem.
+
+## Resolution
+To work around the problem using HttpClient APIs, you can use the following code to increase the connection limit.
+This code should be added before calling any HttpClient APIs.
+
+```c#
+ServicePointManager.DefaultConnectionLimit = 20; // Actual value should be based on your requirements.
+```
+
+Note: This will also change the limits for non-loopback addresses.
+
+## More information
+We will fix this problem in a future .NET Framework Quality Rollup release.

releases/net471/dotnet471-known-issues.md

@@ -12,3 +12,4 @@ This document lists the known issues that you may experience after you install t
 - [521954 - BCL - CultureAwareComparer with ignore casing serialized on previous versions of .NET Framework does not correctly deserialize on .NET Framework 4.7.1](https://github.com/Microsoft/dotnet/blob/master/releases/net471/KnownIssues/521954%20-%20BCL%20CultureAwareComparer%20with%20ignore%20casing%20on%20serialized%20on%20previous%20versions%20of%20.NET%20do%20not%20correctly%20deserialize%20on%20.NET%204.7.1.md)
 - [517815 - BCL - Applications making heavy use of System.Diagnostics.StackTrace or Exception.StackTrace might run more slowly on the .NET Framework 4.7.1](https://github.com/Microsoft/dotnet/blob/master/releases/net471/KnownIssues/517815-BCL%20Applications%20making%20heavy%20use%20of%20System.Diagnostics.StackTrace%20might%20run%20more%20slowly%20on%20.NET%204.7.1.md)
 - [523633 - Setup - Application crashes occur when running on the .NET Framework 4.7.1 after upgrading from Windows 10 Anniversary Update to Windows 10 Creators Update](https://github.com/Microsoft/dotnet/blob/master/releases/net471/KnownIssues/523633%20-%20Setup%20-%20OS%20upgrade%20to%20Windows%2010%20gets%20the%20product%20in%20bad%20state.md)
+- [534719 - Networking - ServicePoint.ConnectionLimit default behavior with loopback changed unexpectedly](https://github.com/Microsoft/dotnet/blob/master/releases/net471/KnownIssues/534719-Networking%20ServicePoint.ConnectionLimit%20default%20behavior%20with%20loopback%20changed%20unexpectedly.md)