Added PathService and initial implementation; tests coming soon.

+package org.onlab.onos.net.path;
+
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.net.HostId;
+import org.onlab.onos.net.Path;
+import org.onlab.onos.net.topology.LinkWeight;
+
+import java.util.Set;
+
+/**
+ * Service for obtaining pre-computed paths or for requesting computation of
+ * paths using the current topology snapshot.
+ */
+public interface PathService {
+
+    /**
+     * Returns the set of all shortest paths, precomputed in terms of hop-count,
+     * between the specified source and destination devices.
+     *
+     * @param src source device
+     * @param dst destination device
+     * @return set of all shortest paths between the two devices
+     */
+    Set<Path> getPaths(DeviceId src, DeviceId dst);
+
+    /**
+     * Returns the set of all shortest paths, computed using the supplied
+     * edge-weight entity, between the specified source and destination devices.
+     *
+     * @param src source device
+     * @param dst destination device
+     * @return set of all shortest paths between the two devices
+     */
+    Set<Path> getPaths(DeviceId src, DeviceId dst,
+                       LinkWeight weight);
+
+
+    /**
+     * Returns the set of all shortest paths, precomputed in terms of hop-count,
+     * between the specified source and destination end-stations.
+     *
+     * @param src source device
+     * @param dst destination device
+     * @return set of all shortest paths between the two end-stations hosts
+     */
+    Set<Path> getPaths(HostId src, HostId dst);
+
+    /**
+     * Returns the set of all shortest paths, computed using the supplied
+     * edge-weight entity, between the specified source and end-stations.
+     *
+     * @param src source host
+     * @param dst destination host
+     * @return set of all shortest paths between the two end-station hosts
+     */
+    Set<Path> getPaths(HostId src, HostId dst, LinkWeight weight);
+
+}

+package org.onlab.onos.net.trivial.path;
+
+import com.google.common.collect.Lists;
+import com.google.common.collect.Sets;
+import org.apache.felix.scr.annotations.Activate;
+import org.apache.felix.scr.annotations.Component;
+import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Reference;
+import org.apache.felix.scr.annotations.ReferenceCardinality;
+import org.apache.felix.scr.annotations.Service;
+import org.onlab.onos.net.ConnectPoint;
+import org.onlab.onos.net.DefaultEdgeLink;
+import org.onlab.onos.net.DefaultPath;
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.net.EdgeLink;
+import org.onlab.onos.net.Host;
+import org.onlab.onos.net.HostId;
+import org.onlab.onos.net.Link;
+import org.onlab.onos.net.Path;
+import org.onlab.onos.net.PortNumber;
+import org.onlab.onos.net.host.HostService;
+import org.onlab.onos.net.path.PathService;
+import org.onlab.onos.net.provider.ProviderId;
+import org.onlab.onos.net.topology.LinkWeight;
+import org.onlab.onos.net.topology.Topology;
+import org.onlab.onos.net.topology.TopologyService;
+import org.slf4j.Logger;
+
+import java.util.List;
+import java.util.Set;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static org.slf4j.LoggerFactory.getLogger;
+
+/**
+ * Provides implementation of a path selection service atop the current
+ * topology and host services.
+ */
+@Component(immediate = true)
+@Service
+public class PathManager implements PathService {
+
+    private static final String DEVICE_ID_NULL = "Device ID cannot be null";
+    private static final String HOST_ID_NULL = "Host ID cannot be null";
+
+    private static final ProviderId PID = new ProviderId("org.onlab.onos.core");
+    private static final PortNumber P0 = PortNumber.portNumber(0);
+
+    private final Logger log = getLogger(getClass());
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected TopologyService topologyService;
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected HostService hostService;
+
+    @Activate
+    public void setUp() {
+        log.info("Started");
+    }
+
+    @Deactivate
+    public void tearDown() {
+        log.info("Stopped");
+    }
+
+    @Override
+    public Set<Path> getPaths(DeviceId src, DeviceId dst) {
+        checkNotNull(src, DEVICE_ID_NULL);
+        checkNotNull(dst, DEVICE_ID_NULL);
+        Topology topology = topologyService.currentTopology();
+        return topologyService.getPaths(topology, src, dst);
+    }
+
+    @Override
+    public Set<Path> getPaths(DeviceId src, DeviceId dst, LinkWeight weight) {
+        checkNotNull(src, DEVICE_ID_NULL);
+        checkNotNull(dst, DEVICE_ID_NULL);
+        Topology topology = topologyService.currentTopology();
+        return topologyService.getPaths(topology, src, dst, weight);
+    }
+
+    @Override
+    public Set<Path> getPaths(HostId src, HostId dst) {
+        return getPaths(src, dst, null);
+    }
+
+    @Override
+    public Set<Path> getPaths(HostId src, HostId dst, LinkWeight weight) {
+        checkNotNull(src, HOST_ID_NULL);
+        checkNotNull(dst, HOST_ID_NULL);
+
+        // Resolve the source host, bail if unable.
+        Host srcHost = hostService.getHost(src);
+        if (srcHost == null) {
+            return Sets.newHashSet();
+        }
+
+        // Resolve the destination host, bail if unable.
+        Host dstHost = hostService.getHost(dst);
+        if (dstHost == null) {
+            return Sets.newHashSet();
+        }
+
+        // Get the source and destination edge locations
+        EdgeLink srcEdge = new DefaultEdgeLink(PID, new ConnectPoint(src, P0),
+                                               srcHost.location(), true);
+        EdgeLink dstEdge = new DefaultEdgeLink(PID, new ConnectPoint(dst, P0),
+                                               dstHost.location(), false);
+
+        // If the source and destination are on the same edge device, there
+        // is just one path, so build it and return it.
+        if (srcEdge.dst().deviceId().equals(dstEdge.src().deviceId())) {
+            return edgeToEdgePaths(srcEdge, dstEdge);
+        }
+
+        // Otherwise get all paths between the source and destination edge
+        // devices.
+        Topology topology = topologyService.currentTopology();
+        Set<Path> paths = weight == null ?
+                topologyService.getPaths(topology, srcEdge.dst().deviceId(),
+                                         dstEdge.src().deviceId()) :
+                topologyService.getPaths(topology, srcEdge.dst().deviceId(),
+                                         dstEdge.src().deviceId(), weight);
+
+        return edgeToEdgePaths(srcEdge, dstEdge, paths);
+    }
+
+    // Produces a set of direct edge-to-edge paths.
+    private Set<Path> edgeToEdgePaths(EdgeLink srcLink, EdgeLink dstLink) {
+        Set<Path> endToEndPaths = Sets.newHashSetWithExpectedSize(1);
+        endToEndPaths.add(edgeToEdgePath(srcLink, dstLink));
+        return endToEndPaths;
+    }
+
+    // Produces a direct edge-to-edge path.
+    private Path edgeToEdgePath(EdgeLink srcLink, EdgeLink dstLink) {
+        List<Link> links = Lists.newArrayListWithCapacity(2);
+        links.add(srcLink);
+        links.add(dstLink);
+        return new DefaultPath(PID, links, 2);
+    }
+
+    // Produces a set of edge-to-edge paths using the set of infrastructure
+    // paths and the given edge links.
+    private Set<Path> edgeToEdgePaths(EdgeLink srcLink, EdgeLink dstLink, Set<Path> paths) {
+        Set<Path> endToEndPaths = Sets.newHashSetWithExpectedSize(paths.size());
+        for (Path path : paths) {
+            endToEndPaths.add(edgeToEdgePath(srcLink, dstLink, path));
+        }
+        return endToEndPaths;
+    }
+
+    // Produces an edge-to-edge path using the specified infrastructure path
+    // and edge links.
+    private Path edgeToEdgePath(EdgeLink srcLink, EdgeLink dstLink, Path path) {
+        List<Link> links = Lists.newArrayListWithCapacity(path.links().size() + 2);
+        links.add(srcLink);
+        links.addAll(path.links());
+        links.add(dstLink);
+        return new DefaultPath(path.providerId(), links, path.cost() + 2);
+    }
+
+}