Observed once on Oracle Linux 7 x64 with jdk 11 b04 repeated runs.

Output as below:
  DatagramServer addr: localhost/127.0.0.1: 45381
  serverMulticastSocket:: fd: java.io.FileDescriptor@447be3e9, fd: 7, cleanup: java.net.SocketCleanable@bac6c4
socketImplClass: class java.net.PlainDatagramSocketImpl
socketImpl class name not matched: java.net.PlainDatagramSocketImpl != java.net.TwoStacksPlainDatagramSocketImpl
  clientMulticastSocket:: fd: java.io.FileDescriptor@153b9167, fd: 8, cleanup: java.net.SocketCleanable@999d25c
socketImplClass: class java.net.PlainDatagramSocketImpl
socketImpl class name not matched: java.net.PlainDatagramSocketImpl != java.net.TwoStacksPlainDatagramSocketImpl
echo received from: /127.0.0.1:45381
----------System.err:(13/908)----------
java.lang.AssertionError: incorrect data received: expected: 2, actual: 1
at UnreferencedMulticastSockets.main(UnreferencedMulticastSockets.java:137)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:115)
at java.base/java.lang.Thread.run(Thread.java:842)