Jetty/Feature/NPN

Introduction

The Jetty project provides an implementation of the Next Protocol Negotiation TLS Extension (NPN) for OpenJDK 7 or greater. NPN allows the application layer to negotiate which protocol to use over the secure connection.

NPN currently negotiates using SPDY as an application level protocol on port 443, and also negotiates the SPDY version. However, NPN is not SPDY specific in any way. Jetty's NPN implementation, although hosted under the umbrella of the Jetty project, is independent of Jetty (the servlet container); you can use it in any other Java network server.

Feature

Starting the JVM

To enable NPN support, you need to start the JVM with:

java -Xbootclasspath/p:<path_to_npn_boot_jar> ...

where path_to_npn_boot_jar is the path on the file system for the NPN Boot jar file, for example, one at the Maven coordinates org.mortbay.jetty.npn:npn-boot.

Understanding the NPN API

Applications need to interact with NPN TLS extension protocol negotiations. For example, server applications need to know whether the client supports NPN, and client applications needs to know the list of protocols the server supports, and so on.

To implement this interaction, Jetty's NPN implementation provides an API to applications, hosted at Maven coordinates org.eclipse.jetty.npn:npn-api. You need to declare this dependency as provided, because the npn-boot jar already includes it (see the previous section), and it is therefore available in the boot classpath.

The API consists of a single class, org.eclipse.jetty.npn.NextProtoNego, and applications need to register instances of SSLSocket or SSLEngine with a ClientProvider or ServerProvider (depending on whether the application is a client or server application). Refer to NextProtoNego javadocs and to the examples below for further details about client and server provider methods.

Client Example

SSLContext sslContext = ...;
SSLSocket sslSocket = (SSLSocket)context.getSocketFactory()
        .createSocket("localhost", server.getLocalPort());
 
NextProtoNego.put(sslSocket, new NextProtoNego.ClientProvider()
{
    @Override
    public boolean supports()
    {
        return true;
    }
 
    @Override
    public void unsupported()
    {
    }
 
    @Override
    public String selectProtocol(List<String> protocols)
    {
        return protocols.get(0);
    }
});

The NPN implementation calls NextProtoNego.ClientProvider methods supports(), unsupported() and selectProtocol(List<String>), so that the client application can decide:

• whether to support NPN.
• whether the server supports NPN.
• to select one of the protocols the server supports.

The example for SSLEngine is identical, and you just need to replace the SSLSocket instance with an SSLEngine instance.

Server Example

SSLSocket sslSocket = ...;
NextProtoNego.put(sslSocket, new NextProtoNego.ServerProvider()
{
    @Override
    public void unsupported()
    {
    }
 
    @Override
    public List<String> protocols()
    {
        return Arrays.asList("http/1.1");
    }
 
    @Override
    public void protocolSelected(String protocol)
    {
        System.out.println("Protocol Selected is: " + protocol);
    }
});

The NPN implementation calls NextProtoNego.ServerProvider methods unsupported(), protocols() and protocolSelected(String), so that the server application can

• know whether the client supports NPN.
• provide the list of protocols the server supports.
• know which protocol the client chooses.

Implementation Details

It is common that the NextProtoNego.ServerProvider (and the NextProtoNego.ClientProvider) are implemented as (anonymous) inner classes, and that their method's implementations require references to the the sslSocket (or sslEngine), either directly or indirectly.

Since the NextProtoNego class holds [sslSocket/sslEngine, provider] pairs in a WeakHashMap, if the value (i.e. the provider implementation) holds a strong (even indirect) reference to the key, then the WeakHashMap entries will never be removed, leading to a memory leak.

For example in Jetty the implementation of NextProtoNego.ServerProvider requires a reference to a org.eclipse.jetty.io.nio.SslConnection that in turn holds a reference to the sslEngine. Therefore the NextProtoNego.ServerProvider implementation does not use the SslConnection directly, but instead via an AtomicReference that is cleared upon connection close.

Note that declaring the SslConnection as a final local variable and referencing it from within the anonymous NextProtoNego.ServerProvider class generates a hidden field in the anonymous inner class, causing the memory leak, so it must be avoided.

Using Unit Tests

You can write and run unit tests that use the NPN implementation. The solution that we use with Maven is to specify an additional command line argument to the Surefire plugin:

<project ...>
 
  <properties>
    <npn-version>7.6.2.v20120308</npn-version>
  </properties>
 
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-surefire-plugin</artifactId>
        <configuration>
          <argLine>-Xbootclasspath/p:${settings.localRepository}/org/mortbay/jetty/npn/npn-boot/${npn-version}/npn-boot-${npn-version}.jar</argLine>
        </configuration>
      </plugin>
 
      ...
 
    </plugins>
  </build>
 
  ...
 
</project>

Debugging

You can enable debug logging for the NPN implementation in this way:

NextProtoNego.debug = true;

Since the NextProtoNego class is in the boot classpath, we chose not to use logging libraries because we do not want to override application logging library choices; therefore the logging is performed directly on System.err.

License Details

The NPN implementation relies on modification of a few OpenJDK classes and on a few new classes that need to live in the sun.security.ssl package. These classes are released under the same GPLv2+exception license of OpenJDK.

The NextProtoNego class is released under same license as the classes of the Jetty project.