<PackageReference Include="NETStandard.Library" Version="2.0.0-preview2-25401-01" />

System.Security.Authentication.SslProtocols

public enum SslProtocols
namespace System.Security.Authentication { public enum SslProtocols { Default = 240, None = 0, Ssl2 = 12, Ssl3 = 48, Tls = 192, Tls11 = 768, Tls12 = 3072 } }