Fix and clarify default value for DontFragment property on UdpClient and Socket.

[StephenCleary]

Summary

The default value of DontFragment (for both UdpClient and Socket) is false. AFAICT, it's always been false.

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[learn-build-service-prod]

Learn Build status updates of commit d44ab53:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Sockets/Socket.xml	✅Succeeded	View
xml/System.Net.Sockets/UdpClient.xml	✅Succeeded	View

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[rzikm]

While we don't explicitly set DontFragment in the source code, the default value can be set by the OS.

using System.Net.Sockets;

var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

System.Console.WriteLine($"Sock.DontFragment: {sock.DontFragment}");

produces different values on different OSes, true on windows and false on linux. So unfortunately neither the original version nor your proposed change is correct.

@dotnet/ncl Should we make the defaults consistent across OSes? I know there are some platform-dependent behaviors on sockets but this seems like something we can reasonably unify.

[StephenCleary]

Good catch on checking TCP sockets. I only checked UDP, which was false on both Windows and Linux.

[wfurt]

@dotnet/ncl Should we make the defaults consistent across OSes? I know there are some platform-dependent behaviors on sockets but this seems like something we can reasonably unify.

I see value in keeping OS defaults. With that we would respect OS setting - Windows registry, /proc, sysctl or whatever.
People often compare the behavior to other tools - like curl & browsers and consistency may be beneficial. I don't know if there is benefit of barcoding everything in .NET to make it same. I do see benefit of documenting the value/behavior better. So people do not need to duplicate research and testing.