diff --git a/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java b/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java
new file mode 100644
index 0000000..4e83263
--- /dev/null
+++ b/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java
@@ -0,0 +1,96 @@
+package net.onrc.onos.core.topology;
+
+import java.net.InetAddress;
+
+import net.floodlightcontroller.util.MACAddress;
+
+/**
+ * The northbound interface to the topology network graph. This interface
+ * is presented to the rest of ONOS. It is currently read-only, as we want
+ * only the discovery modules to be allowed to modify the topology.
+ */
+public interface NetworkGraph {
+    /**
+     * Get the switch for a given switch DPID.
+     *
+     * @param dpid the switch dpid.
+     * @return the switch if found, otherwise null.
+     */
+    public Switch getSwitch(Long dpid);
+
+    /**
+     * Get all switches in the network.
+     *
+     * @return all switches in the network.
+     */
+    public Iterable<Switch> getSwitches();
+
+    /**
+     * Get the port on a switch.
+     *
+     * @param dpid the switch DPID.
+     * @param number the switch port number.
+     * @return the switch port if found, otherwise null.
+     */
+    public Port getPort(Long dpid, Long number);
+
+    /**
+     * Get the outgoing link for a switch and a port.
+     *
+     * @param dpid the switch DPID.
+     * @param number the switch port number.
+     * @return the outgoing link if found, otherwise null.
+     */
+    public Link getLink(Long dpid, Long number);
+
+    /**
+     * Get the outgoing link from a switch and a port to another switch and
+     * a port.
+     *
+     * @param srcDpid the source switch DPID.
+     * @param srcNumber the source switch port number.
+     * @param dstDpid the destination switch DPID.
+     * @param dstNumber the destination switch port number.
+     * @return the outgoing link if found, otherwise null.
+     */
+    public Link getLink(Long srcDpid, Long srcNumber, Long dstDpid,
+			Long dstNumber);
+
+    /**
+     * Get all links in the network.
+     *
+     * TODO: Not clear if this method is needed. Remove if not used.
+     *
+     * @return all links in the network.
+     */
+    public Iterable<Link> getLinks();
+
+    /**
+     * Get the network devices for a given IP address.
+     *
+     * @param ipAddress the IP address to use.
+     * @return the network devices for the IP address.
+     */
+    public Iterable<Device> getDevicesByIp(InetAddress ipAddress);
+
+    /**
+     * Get the network device for a given MAC address.
+     *
+     * @param address the MAC address to use.
+     * @return the network device for the MAC address if found, otherwise null.
+     */
+    public Device getDeviceByMac(MACAddress address);
+	
+    /**
+     * Acquire a read lock on the entire topology. The topology will not 
+     * change while readers have the lock. Must be released using 
+     * {@link releaseReadLock()}. This method will block until a read lock is
+     * available.
+     */
+    public void acquireReadLock();
+	
+    /**
+     * Release the read lock on the topology.
+     */
+    public void releaseReadLock();
+}