Abstract:
Controllers in a Software Defined Network (SDN) automatically discover and authenticate each other as part of the control plane operations, without any need for manual configuration of other controllers. The new capability of such controllers is based on Controller-Advertisement messages transferred between controllers in the same or different SDNs. In doing so, multiple controllers can be used for load sharing, improved reliability under failure of a controller by enabling seamless take over and recovery. Moreover, controllers can cooperate to enable end-to-end services to work across multiple SDNs. Controller-Advertisement messages are also used to check the health statuses of discovered controllers in the same or different SDNs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The disclosed invention generally relates to the controller component of a Software Defined Network (SDN). In particular, it relates to such controllers automatically discovering and authenticating each other as part of the control plane operations, without any need for manual configuration of other controllers as it is being done in prior art. The system of this invention is a new capability of the controller that receives Controller-Advertisement messages from other controllers in the same or different SDN, performing authentication and subsequently performing control plane information exchange. The Controller-Advertisement messages used for discovery are also used to periodically check the health of the pool of discovered controllers. Multiple controllers can be used for load sharing, improved reliability under failure of a controller by enabling seamless take over and recovery. Also, controllers can cooperate to enable end-to-end services to work across multiple SDNs. 
     2. Discussion of Related Art 
     Software-Defined Networking (SDN) is a new paradigm where the control of computer and communication networks is accomplished via programmatic interfaces. There are a number of approaches to SDN. In the most popular approach, the data and control planes that typically reside in a switch are separated and the control plane is moved to a separate device, commonly referred to as the controller as described in the article “OpenFlow: Enabling Innovation in Campus Networks” by Nick McKeown et al., which appeared in ACM SIGCOMM Computer Communication Review, vol. 38, no. 2, April 2008 and also in the Pre-Grant Application by Casado et al. (2009/0138577). When triggered, the controller calculates the most appropriate route through the network between two nodes and programs all switches along this route accordingly. The controller can be a physical server, a virtual machine or an appliance. There can be multiple controllers in a computer network in this approach. It is possible to divide the network into regions and have different controllers control each region. It is also possible to have multiple controllers control a single switch for reliability and/or performance issues, where each controller may control different ports, or different flow types, or flows with different end nodes. In this approach, a well-defined protocol, OpenFlow, is used for communication between the network forwarding devices and the controller. The controller also has a northbound application programming interface so that different, custom control applications that use the OpenFlow data may be installed on the controller. This approach is advocated by the Open Networking Foundation (ONF) that aims to standardize the OpenFlow protocol. The latest version of the OpenFlow protocol may be accessed via the ONF web page. It is also possible to use a protocol other than OpenFlow to enable proper operation of this approach.  FIG. 1  illustrates the OpenFlow-based SDN concept with a single controller  100  and a plurality of switches  109 ,  110 ,  111 , and  112 . In  FIG. 1 , dotted lines  101 ,  102 ,  103  and  104  represent control plane connections between the controller and the switches, while solid lines  105 ,  106 ,  107 , and  108  represent the switch-to-switch data plane connections. All interactions in  FIG. 1  between the controller and the switches use a clearly defined protocol, such as OpenFlow. This protocol allows for the control plane operations, wherein the controller programs the switches to specify the route of data paths (or flows, slices). The controller programs the switch by simply uploading one or more forwarding table(s) (or table entries). 
     In a second approach to SDN, virtual switches, as described in the Pre-Grant Application by Casado et al. (2010/0257263), are deployed as part of an overlay network on top of the existing network infrastructure using protocols such as VXLAN or NVGRE, which are defined in IETF Internet Drafts, draft-mahalingam-dutt-dcops-vxlan-00.txt and draft-sridharan-virtualization-nvgre-01.txt, respectively. In this approach, the data and control planes are still kept separate in the overlay, and the controller (or a group of controllers) is used to control the virtual switches. 
     In a third SDN approach, the data and control planes are not physically separated. Instead, every forwarding device in the network uses programmatic interfaces for network control, on which custom control applications may be installed. This vendor specific approach may be visualized as having an SDN controller in each forwarding device and using a vendor-specific protocol for communication between the controller and the forwarder. 
     The Pre-Grant Publication to Kato (2012/025049) describes a Load Distribution System, Load Distribution Method, and Program for SDN. In Kato, the load distribution amongst controllers is made possible using a proxy server that is situated between the switches and the controllers. The proxy server enables transparency of the different controllers to each of the switches. It also enables seamless load-balancing amongst the controllers unbeknownst to the switches in the network. 
     The Pre-Grant Publication to Yamato et al. (2011/0317701) describes one possible method to formulate routing decisions in a SDN architecture where multiple controllers are deployed. The routing decision, as calculated by the controller that controls the switch from which the route originates, is communicated to the other, relevant controllers in the network by this controller using a message that includes the forwarding path information. 
     In Kato as well as Yamata et al., as with the remainder of the prior art, when multiple controllers are used, a manual configuration step is needed to program each controller with the information of the other controller(s). The present invention eliminates this manual step by providing a system and method that allow each controller to automatically discover and authenticate other controllers in the network. 
     Embodiments of the present invention are an improvement over prior art systems and methods. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for a first controller in a first subnetwork to automatically discover a second controller of a second subnetwork, said first subnetwork further comprising at least a first switch and said second subnetwork further comprising at least a second switch, said method as implemented in said first controller comprising the steps of: (a) receiving a message from said first switch identifying activation of a first port, which is due to establishment of a connection between a first port in said first switch and a second port in said second switch; (b) generating a Controller-Advertisement message carrying information required to identify and access said first controller; (c) forwarding said generated Controller-Advertisement message from said first controller to said first switch, said first port in said first switch further forwarding said Controller-Advertisement message to said second port in said second switch, said second switch further forwarding said Controller-Advertisement message to said second controller along with an identification of said second port over which said Controller-Advertisement message was received; wherein said second controller in said second subnetwork identifies said first controller in said first subnetwork based on information carried in said Controller-Advertisement, and said second controller generating a Controller Information Base (CIB) entry containing information required to identify and access said first controller. 
     Reciprocally (and independently of the Controller-Advertisement message originating from the first controller), the present invention also provides the steps of (a) receiving a message from said second switch identifying activation of a second port, which is due to establishment of a connection between a second port in said second switch and a first port in said first switch; (b) generating a Controller-Advertisement message carrying information required to identify and access said second controller; (c) forwarding said generated Controller-Advertisement message from said second controller to said second switch, said second port in said second switch further forwarding said Controller-Advertisement message to said first port in said first switch, said first switch further forwarding said Controller-Advertisement message to said first controller along with an identification of said first port over which said Controller-Advertisement message was received; wherein said first controller in said first subnetwork identifies said second controller in said second subnetwork based on information carried in said Controller-Advertisement, and said first controller generating a CIB entry containing information required to identify and access said second controller. 
     The present invention also provides a method to check health of automatically discovered controllers comprising the steps of: (a) a first controller generating a first Controller-Advertisement message carrying the information required to identify and access said first controller when a new switch port becomes active; (b) said first controller sending said generated first Controller-Advertisement to a first switch having said newly activated port; (c) said first controller, reciprocally and independently of said first Controller-Advertisement message, receiving at least a second Controller-Advertisement message from at least a second controller; (d) said first controller periodically generating a third unsolicited Controller-Advertisement message with its access information and solicit-flag set to check health of said second controller; (e) said first controller receiving at least a fourth Controller-Advertisement message from said second controller indicating health information of said second controller, wherein first controller&#39;s CIB is updated with said health information. 
     The present invention also provides a method for a first controller in a first subnetwork to automatically discover a second controller of a second subnetwork, said first subnetwork further comprising at least a first switch and said second subnetwork further comprising at least a second switch, said method as implemented in said first controller comprising the steps of: (a) receiving a message from said first switch identifying activation of a first port, which is due to establishment of a connection between a first port in said first switch and a second port in said second switch; (b) generating a Controller-Advertisement message carrying information required to identify and access said first controller; (c) forwarding said generated Controller-Advertisement message from said first controller to said first switch, said first port in said first switch further forwarding said Controller-Advertisement message to said second port in said second switch, said second switch further forwarding said Controller-Advertisement message to said second controller along with an identification of said second port over which said Controller-Advertisement message was received; (d) if said first controller and said second controller are a single controller, then: (i) updating said single controller&#39;s Network Information Base (NIB) with a new entry showing new connection information between said first switch, first port and said second switch, second port, utilizing the information carried in the optional switch-id and port-no fields of said Controller-Advertisement message, and (ii) discarding any Controller-Advertisement messages that are looped to itself; (e) if said first controller and said second controller are different controllers, then said second controller: (i) identifying said first controller using information carried in said Controller-Advertisement message originated from said first controller; and (ii) generating a new CIB entry containing information for said first controller. 
     The present invention also provides an article of manufacture having computer readable storage medium comprising computer readable program code executable by a computer to implement a method for a first controller in a first subnetwork to automatically discover a second controller of a second subnetwork, said first subnetwork further comprising at least a first switch and said second subnetwork further comprising at least a second switch, said medium comprising: (a) computer readable program code executed by said computer to receive a message from said first switch identifying activation of a first port, which is due to establishment of a connection between a first port in said first switch and a second port in said second switch; (b) computer readable program code executed by said computer to generate a Controller-Advertisement message carrying information required to identify and access said first controller; (c) computer readable program code executed by said computer to forward said generated Controller-Advertisement message from said first controller to said first switch, said first port in said first switch further forwarding said Controller-Advertisement message to said second port in said second switch, said second switch further forwarding said Controller-Advertisement message to said second controller along with an identification of said second port over which said Controller-Advertisement message was received; wherein said second controller in said second subnetwork identifies said first controller in said first subnetwork based on information carried in said Controller-Advertisement, and said second controller generating a CIB entry containing information required to identify and access said first controller. 
     Method and system for controllers to discover each other within the same Software Defined Network (SDN), where there are at least two controllers; or across a plurality of interconnected SDNs, where each SDN is controlled by one or more controllers. The method comprises the steps of: a switch controlled by a controller in an SDN establishing a new data connection to another switch controlled by another controller in the same or different SDN, each of these two interconnected switches sending a notification message to its respective controller about the newly established data connection; each controller responding to the received notification message by a Controller-Advertisement message carrying information required to identify and access the controller; the switch receiving the Controller-Advertisement message passing that message to the other interconnected switch which in turn propagates it to the other controller. Mutually doing so, each controller learns the other controller&#39;s connection information to establish a direct (peer controller) control plane connection, to authenticate and share control plane data such as routing table or service capabilities. While OpenFlow defines control plane messages to enable the controller to switch dialog, this invention describes additional control plane messages between controllers, which can be used with protocols other than OpenFlow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a prior art SDN. 
         FIG. 2A  illustrates a SDN with two subnetworks managed by different controllers. 
         FIG. 2B  illustrates the establishment of a data connection between a first port of a first switch and a second port of a second switch. 
         FIGS. 3A-B  illustrate a flowchart outlining the steps describing the controller discovery process. 
         FIG. 4  depicts the sequence of messages involved in one example of a controller discovery process. 
         FIG. 5  depicts the sequence of messages involved in another example of a controller discovery process. 
         FIG. 6  illustrates the messaging sequence when a new connection is established between switches that are managed by the same controller. 
         FIG. 7  illustrates a SDN with two subnetworks managed by different controllers, where the second subnetwork is managed by two controllers for redundancy and/or performance issues. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is illustrated and described in a preferred embodiment, the invention may be produced in many different configurations. There is depicted in the drawings, and will herein be described in detail, a preferred embodiment of the invention, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and the associated functional specifications for its construction and is not intended to limit the invention to the embodiment illustrated. Those skilled in the art will envision many other possible variations within the scope of the present invention. 
     In the prior art, when multiple controllers are used in a Software Defined Network (SDN) (or several SDNs are interconnected, each with different controllers), a manual configuration step is needed to program each controller with the information of the other controller(s). The present invention eliminates that manual step by providing a system and method that allows each controller to automatically discover and authenticate other controllers in the network. An Autonomous System (AS) may be represented as a single SDN. Alternatively, it may be comprised of many interconnected SDNs. Thus, two SDNs that are interconnected may be in the same AS or in different Autonomous Systems. If the interconnected SDNs are in the same AS, their controllers may exchange all the control information they contain. Doing so, the controllers will be replicas of each other. However, policies may be defined to limit the control plane information each such controller shares with the other controllers. If the SDNs are in different Autonomous Systems, their controllers will only exchange limited control information. For example, the internal network of the AS may be represented to the foreign controller in a more summarized or aggregated form. Also, there will be processes in place to ensure the other controller is trusted. The method of exchange of control information amongst controllers, once they discover each other, could be similar to exchange of routing information amongst routers in the classical Internet using various routing protocols, and therefore is kept outside the scope of this patent application. 
     The controller discovery process covered in this invention can be triggered by a change in the network (for example, when a new connection is established) and/or, periodically, at regular intervals. The former is more efficient in terms of messages injected into the network, but at the same time, it is more prone to message losses. Although some rules are defined in the following paragraphs to tolerate potential losses, it is advisable to perform controller discovery at regular intervals that are not short, besides being triggered by changes in the network. 
     The information gathered through the controller discovery process can be stored in the CIB, which is similar to the NIB used to store the information required to control the underlying switches. CIB can be considered as a table whose entries contain the information necessary to identify and access the discovered controllers, as shown in TABLE I below: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 An example CIB 
               
             
          
           
               
                 Remote 
                 Remote 
                 Local 
                 Local 
                   
               
               
                 Controller-ID 
                 Controller-IP 
                 Switch-ID 
                 Port-No 
                 Status 
               
               
                   
               
               
                 Controller1 
                 Controller1-IP 
                 Switch2 
                 p20 
                 Live 
               
               
                 Controller3 
                 Controller3-IP 
                 Switch3 
                 p37 
                 Live 
               
               
                 Controller4 
                 Controller4-IP 
                 Switch3 
                 p39 
                 Live 
               
               
                   
               
             
          
         
       
     
     Below are the possible fields of a CIB entry that stores the access information for a discovered controller:
         Controller-ID: ID of the discovered controller, which should be unique within the scope of the discovery process.   Controller-IP: IP address of the discovered controller.   Switch-ID: ID of the local switch from where the discovered controller can be accessed. This local switch is managed by the controller that maintains the CIB.   Port-no: Port number of the local switch from where the discovered controller can be accessed.   Status: Status of the discovered controller, which can take values such as ‘Live’, ‘Dead’, ‘Not Accessible’, or any other status values depending on the requirements.       

     The controllers listed in the CIB may change status under different conditions. For example, upon receiving a Controller-Advertisement message from a remote controller, the status of the remote controller will be set as ‘Live’, whereas not receiving a Controller-Advertisement message from a previously discovered remote controller in the health-check process will change the status to ‘Dead’. Alternatively, if a data link that was previously used to interconnect the switches which enabled the control plane connectivity between the controllers is broken (i.e., the port of the switch in the CIB table above associated with a remote controller) and access between the controllers thereby becomes unavailable, the status of the remote controller will be set as ‘Not Accessible’, upon receiving a message indicating the corresponding change in the network. While there are other values that can possibly be used for representing the status of a controller, it may also suffice to use only two values, which are ‘Live’ and ‘Dead’. 
     The discovery process involves a single message, namely the Controller-Advertisement message, which is used by controllers to advertise their presence together with connection information. Since there is no explicit reply message of Controller-Advertisement, message losses can be tolerated by adhering to the following rules:
         A controller that receives a Controller-Advertisement message with its solicit-flag set should respond with its own Controller-Advertisement message with its solicit-flag cleared, unless it does not want to participate in the controller discovery process.   A controller that has sent a Controller-Advertisement message over a connection should resend the advertisement message with its solicit-flag set, in case no remote Controller-Advertisement message is received from the same connection after a specific period.       

     The Controller-Advertisement message will have the following structure, in addition to the IP header, if encapsulated as an IP packet: 
     
       
         
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                   
                 CONTROLLER_ADVERTISEMENT &lt; 
               
               
                   
                    IP-Header: &lt; 
               
               
                   
                       src-addr: Controller-IP; 
               
               
                   
                       dst-addr: All-nodes multicast address; &gt; 
               
               
                   
                    controller-id: Controller-ID; 
               
             
          
           
               
                   
                    solicit-flag: 
                 ON/OFF; 
               
               
                   
                    algorithm-id: 
                 ID of the cryptographic algorithm 
               
             
          
           
               
                   
                 that generates the signature; 
                 # Optional 
               
             
          
           
               
                   
                    signature: 
                 cryptographically generated 
               
             
          
           
               
                   
                 authentication code; 
                 # Optional 
               
             
          
           
               
                   
                    switch-id: Switch-ID; 
                 # Optional 
               
               
                   
                    port-no: port-ID; 
                 # Optional 
               
               
                   
                    optional-fields; &gt; 
                 # Optional 
               
               
                   
               
             
          
         
       
         
         
           
             src-addr: Carries the access information for the controller that sends the advertisement message. 
             dst-addr: Multicast address to reach all nodes. 
             controller-id: ID of the controller, which should be unique within the multicast scope. 
             solicit-flag: This flag determines if the receiver of the advertisement message should respond with its own advertisement message or not. If set, the receiving controller should prepare its own Controller-Advertisement message and respond with it to the sender, unless it does not want to participate in the controller discovery process. If not set, no response should be sent. Typically, controller discovery is performed with the solicit-flag not set. On the other hand, Controller-Advertisement for a controller&#39;s health-check (towards an already discovered controller) should have its solicit-flag set to force a response from the discovered controller. Similarly, a Controller-Advertisement message, which is resent after a specific period because no remote Controller-Advertisement message is received from the newly activated switch-port, should have its solicit-flag set. 
             algorithm-id: ID of the cryptographic algorithm that generates the signature. This can be either a symmetric-key or an asymmetric-key (public-key) algorithm. This field is optional together with the following signature field. 
             signature: Cryptographically generated code that authenticates the advertisement message. This code can be constructed by the given cryptographic algorithm, using the src-addr, controller-id, switch-id and port-no fields. The controller that receives a Controller-Advertisement message with an existing signature value can validate the authenticity and the integrity of the received message. Since this field is optional, it is up to the controller to discard a received advertisement message with a null signature value. 
             switch-id: ID of the switch, from where the advertisement message is sent. The ID should be unique within the scope of the controller that originates the message. This field can be used for local topology discovery, that is, the discovery of connections between the switches managed by the same controller, together with port-no field. If a controller receives its own Controller-Advertisement message that is previously constructed, then it can utilize the switch-id and port-no fields to learn the existing connection between the sending and the receiving switches. This field is optional. 
             port-no: ID of the port, from where the advertisement message is sent. The ID should be unique within the scope of the sending switch. This field is optional. 
             optional-fields: Additional fields that can be utilized for controller-to-controller information exchange, which may be performed during the discovery process rather than being done after completing the discovery process. ID of the subnetwork that a controller manages and capability information of a controller are examples to such optional fields.
 
Controller Discovery Process
 
           
         
       
    
     When a new data connection is established between two switches in an SDN network, a physical connection is established between the ports of these switches. Upon such a change in the port status (i.e., the port changing status from idle to active) of the said switches, they notify their respective controllers of the corresponding change. Then, the controllers can act accordingly to discover the change in the network. 
       FIG. 2A  illustrates the scenario where there exist two subnetworks managed by different controllers, Controller 1    201  and Controller 2    202 . These subnetworks may belong to the same or different SDNs. In this scenario, when two subnetworks interconnect, their controllers (i.e., Controller 1    201  on the first subnetwork and Controller 2    202  on the second subnetwork) will need to discover one another, and authenticate/exchange routing information to properly route packets from one subnetwork to the other. 
     In  FIG. 2A , Switch 1    205  is controlled by Controller 1    201  on the first subnetwork, whereas Switch 2    206  and Switch 3    207  are controlled by Controller 2    202  on the second subnetwork. The OpenFlow protocol is run on control links between the controllers and the switches, shown in dashed lines numbered with  200 ,  203 , and  204 . Initially, as shown in  FIG. 2A , Switch 1    205  and Switch 2    206  are not connected, and Controller 1    201  and Controller 2    202  are not aware of each other. Thereafter, as depicted in  FIG. 2B , a data connection, numbered with  208 , is established between port p 10  of Switch 1    205  and port p 20  of Switch 2    206 . When this connection is established, the controller discovery process is automatically triggered. An exemplary flowchart, showing the discovery process initiated by Controller 1    201  upon connecting ports p 10  and p 20 , is depicted in  FIG. 3A  and  FIG. 3B . It should be noted that although  FIGS. 3A-B  depict a discovery process initiated by Controller 1    201 , a reciprocal discovery process (not shown) is also initiated by Controller 2    202 , and is within the scope of the present invention. 
     The steps below describe the controller discovery process initiated by Controller 1    201 , as illustrated in  FIGS. 3A-B . Reference is also made to  FIG. 4  as it depicts the sequence of messages.
         Step  300 : Begin method.   Step  302 : Switch 1    205 , port p 10  is connected to Switch 2    206 , port p 20 .   Step  304 : Upon port p 10  being connected to port p 20 , Switch 1    205  sends Port-Status message to Controller 1    201 . This OpenFlow message is intended to inform the controller that a previously idle port is now active.   For example, the corresponding OpenFlow (version 1.0.0) message from Switch 1    205  to Controller 1    201  contains:       

     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 OFPT_PORT_STATUS &lt; 
               
               
                   
                   reason: OFPPR_MODIFY; 
               
               
                   
                   port-no: p10; 
               
               
                   
                   state: OFPPS_STP_LISTEN &gt; 
               
               
                   
               
             
          
         
       
         
         
           
             Step  306 : In response to receiving the Port-Status message sent from Switch 1    205 , Controller 1    201  prepares a Controller-Advertisement message that carries its own ID and optionally the ID of Switch 1    205  and port p 10 , according to an aspect of this invention. The solicit-flag should be cleared (i.e., its value should be OFF), since this is the initial message. This message will carry the IP address of Controller 1    201  in its IP header. The content of the prepared Controller-Advertisement message is as follows: 
           
         
       
    
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 CONTROLLER_ADVERTISEMENT &lt; 
               
               
                   
                   IP-Header: &lt; 
               
               
                   
                       src-addr: Controller1-IP; 
               
               
                   
                       dst-addr: All-nodes; &gt; 
               
               
                   
                 controller-id: Controller1; 
               
               
                   
                 solicit-flag: OFF; 
               
               
                   
                 algorithm-id: Algorithm1; 
               
               
                   
                 signature: authentication-code; 
               
               
                   
                 switch-id: Switch1; 
               
               
                   
                 port-no: p10; &gt; 
               
               
                   
               
             
          
         
       
         
         
           
             Step  308 : Controller 1    201  hands over the Controller-Advertisement message to Switch 1    205 , to be sent out from port p 10 . Using OpenFlow protocol, this message can be sent inside the Packet-Out message, as shown below: 
           
         
       
    
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 OFPT_PACKET_OUT &lt; 
               
               
                   
                   type: OFPAT_OUTPUT; 
               
               
                   
                   port: p10; 
               
               
                   
                   data: CONTROLLER_ADVERTISEMENT &lt; . . . &gt; &gt; 
               
               
                   
               
             
          
         
       
         
         
           
             Step  310 : Controller 1    201  starts a timer t CA , so as to resend the Controller-Advertisement message prepared in step  306  unless a remote Controller-Advertisement message has been received from port p 10  of Switch 1    205  before t CA  times out. This timer is for tolerating potential message losses. 
             Step  312 : Controller 1    201  checks to see if a remote Controller-Advertisement message—in this case prepared by Controller 2    202 —has been received before t CA  times out. If no advertisement message is received in that period, then Controller 1    201  resends the Controller-Advertisement message prepared in step  306 , but this time with solicit-flag set (i.e., the solicit-flag is ON). 
             Step  314 : A remote Controller-Advertisement message, which is prepared by Controller 2    202 , is received at port p 10  of Switch 1    205 . When remote controller, namely Controller 2    202 , is discovered, there is no need to resend the Controller-Advertisement message prepared in Step  306 . 
             Step  316 : Switch 1    205  sends the Controller-Advertisement message prepared by Controller 1    201  to Switch 2    206  from port p 10 . 
             Step  318 : Switch 2    206  receives the Controller-Advertisement message sent by Switch 1    205  on port p 20 . 
             Step  320 : Switch 2    206  checks the Controller-Advertisement message to determine whether or not to forward the message to Controller 2    202 . The check involves looking up its forwarding table to see if there exists an entry that matches the message header. For the controller discovery to proceed, Switch 2    206  should forward the message to Controller 2    202 , because either there is no flow entry matching the message or the matching flow entry dictates the switch to forward the message to the controller. Otherwise, the discovery process ends in step  336 . This may occur because the Controller 2    202  has a policy that restricts the controller discovery process or there may also be other reasons not covered here. 
             Step  322 : In case Switch 2    206  decides to forward the Controller-Advertisement message to Controller 2    202 , it does so by utilizing the Packet-In message of the OpenFlow protocol. So, Controller 2    202  receives the advertisement message carried inside the OpenFlow (version 1.0.0) Packet-In message, as shown below: 
           
         
       
    
     
       
         
               
               
             
           
               
                   
               
             
             
               
                   
                 OFPT_PACKET_IN &lt; 
               
               
                   
                   in_port: p20; 
               
               
                   
                   data: CONTROLLER_ADVERTISEMENT &lt; . . . &gt; &gt; 
               
               
                   
               
             
          
         
       
         
         
           
             Step  324 : Controller 2    202  checks the authenticity of the received Controller-Advertisement message using the algorithm-id and signature fields of the message. If the message turns out to be valid, the process continues with Step  326 . Otherwise, the advertisement message is discarded, and the discovery process ends in step  336 . Also, if no authentication data is presented in the advertisement message, it is up to the controller—in this case, Controller 2    202 —to discard the message, or to continue with the discovery process. 
             Step  326 : Controller 2    202  checks the controller-id field of the Controller-Advertisement message, to determine if the message was originated by itself. 
             Step  328 : If Controller 2    202  determines that the received Controller-Advertisement message was its own message that is previously originated, as depicted in  FIG. 6 , then it can utilize the switch-id and port-no fields of the advertisement message and the in port field of the Packet-In message to discover the connection established between the sending and receiving switches. After that, Controller 2    202  can update its NIB accordingly, without making any changes in the CIB. 
             Step  330 : If Controller 2    202  determines that the received Controller-Advertisement message was originated by another controller—which is originated by Controller 1    201  in our case, then Controller 2    202  updates its CIB to reflect the newly discovered controller—Controller 1    201 . Assuming that the Controller 2    202  CIB was empty at the beginning, then the status of the CIB will be as in TABLE II after discovering Controller 1    201 : 
           
         
       
    
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Controller 2  202 CIB after discovering Controller 1  201 
               
             
          
           
               
                 Remote 
                 Remote 
                 Local 
                 Local 
                   
               
               
                 Controller-ID 
                 Controller-IP 
                 Switch-ID 
                 Port-No 
                 Status 
               
               
                   
               
               
                 Controller1 
                 Controller1-IP 
                 Switch2 
                 p20 
                 Live 
               
               
                   
               
             
          
         
       
         
         
           
             Since a reciprocal discovery process is also initiated by Controller 2    202 , the Controller 1    201  CIB will be as shown in TABLE III (shown below) after the discovery of Controller 2    202 , assuming that the CIB was empty at the beginning 
           
         
       
    
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Controller 1  201 CIB after discovering Controller 2  202 
               
             
          
           
               
                 Remote 
                 Remote 
                 Local 
                 Local 
                   
               
               
                 Controller-ID 
                 Controller-IP 
                 Switch-ID 
                 Port-No 
                 Status 
               
               
                   
               
               
                 Controller2 
                 Controller2-IP 
                 Switch1 
                 p10 
                 Live 
               
               
                   
               
             
          
         
       
         
         
           
             Step  332 : Controller 2    202  checks the solicit-flag of the received Controller-Advertisement message. If the value is OFF (i.e., if the solicit-flag is cleared), then the discovery process ends in step  336 . 
             Step  334 : If the solicit-flag of the received Controller-Advertisement message is ON (i.e., if the solicit-flag is set), then it means that Controller 1    201  has requested a Controller-Advertisement message from Controller 2    202 . In that case, Controller 2    202  prepares a Controller-Advertisement with its own information and solicit-flag cleared, and then sends the message to Controller 1    201  from port p 20  of Switch 2    206 . 
             Step  336 : The discovery process ends for the established connection. 
           
         
       
    
       FIG. 4  depicts the Controller-Advertisement message traverses for the Controller 1    201  initiated discovery process as follows: [Controller 1    201 ]→[Switch 1    205 ]→[Switch 2    206 ]→[Controller 2    202 ]. For the reciprocal process, where Controller 2    202  initiates discovery, the Controller-Advertisement message traverses are as follows: [Controller 2    202 ]→[Switch 2    206 ]→[Switch 1    205 ]→[Controller 1    201 ]. This scenario is depicted in  FIG. 5 . In case Controller 1    201  and Controller 2    202  are a single controller (i.e., just Controller 1    201 ), the Controller-Advertisement message traverses, as per  FIG. 6 , as follows: [Controller 1    201 ]→[Switch 1    205 ]→[Switch 2    206 ]→[Controller 1    201 ]. 
     It is also possible for a switch to be managed by multiple controllers, for redundancy and/or performance issues.  FIG. 7  illustrates such a scenario, where a first subnetwork is managed by Controller 1    702  and a second subnetwork is managed by Controller 2    704  and Controller 3    706 . In this scenario, dotted lines  714 ,  716 ,  718 ,  720 , and  722  represent various control links. It should be noted that while Switch 1    708  has only one control link with Controller 1    702 , Switch 2    710  has two control links  716  and  720 , one for each of the controllers, Controller 2    704  and Controller 3    706  (i.e., Switch 2    710  is managed by Controller 2    704  and Controller 3    706 ). Similarly, Switch 3    712  has two control links  718  and  722 , one for each of the controllers, Controller 2    704  and Controller 3    706  (i.e., Switch 3    712  is also managed by Controller 2    704  and Controller 3    706 ). The controller discovery process covered in this invention covers such a scenario, such that the Controller-Advertisement message prepared by Controller 1    702  is forwarded to both Controller 2    704  and Controller 3    706  by Switch 2    710 . Moreover, Controller 2    704  and Controller 3    706  both prepare Controller-Advertisement messages to be forwarded to Switch 1    708  and eventually to Controller 1    702 , when notified by Switch 2    710  of the change in the port status (i.e., the port changing status from idle to active). 
     Additionally, the present invention provides for an article of manufacture comprising computer readable program code contained within implementing one or more modules to automatically discover multiple controllers in software defined networks (SDNs). Furthermore, the present invention includes a computer program code-based product, which is a storage medium having program code stored therein which can be used to instruct a computer to perform any of the methods associated with the present invention. The computer storage medium includes any of, but is not limited to, the following: CD-ROM, DVD, magnetic tape, optical disc, hard drive, floppy disk, ferroelectric memory, flash memory, ferromagnetic memory, optical storage, charge coupled devices, magnetic or optical cards, smart cards, EEPROM, EPROM, RAM, ROM, DRAM, SRAM, SDRAM, or any other appropriate static or dynamic memory or data storage devices. 
     Implemented in computer program code based products are software modules for: a) receiving a message from said first switch identifying activation of a first port, which is due to establishment of a connection between a first port in said first switch and a second port in said second switch; b) generating a Controller-Advertisement message carrying information required to identify and access said first controller; c) forwarding said generated Controller-Advertisement message from said first controller to said first switch, said first port in said first switch further forwarding said Controller-Advertisement message to said second port in said second switch, said second switch further forwarding said Controller-Advertisement message to said second controller along with an identification of said second port over which said Controller-Advertisement message was received; wherein said second controller in said second subnetwork identifies said first controller in said first subnetwork based on information carried in said Controller-Advertisement, and said second controller generating a Controller Information Base (CIB) entry containing information required to identify and access said first controller. 
     Implemented in computer program code based products are software modules for reciprocally (and independently of the Controller-Advertisement message originating from the first controller): (a) receiving a message from said second switch identifying activation of a second port, which is due to establishment of a connection between a second port in said second switch and a first port in said first switch; (b) generating a Controller-Advertisement message carrying information required to identify and access said second controller; (c) forwarding said generated Controller-Advertisement message from said second controller to said second switch, said second port in said second switch further forwarding said Controller-Advertisement message to said first port in said first switch, said first switch further forwarding said Controller-Advertisement message to said first controller along with an identification of said first port over which said Controller-Advertisement message was received; wherein said first controller in said first subnetwork identifies said second controller in said second subnetwork based on information carried in said Controller-Advertisement, and said first controller generating a Controller Information Base (CIB) entry containing information required to identify and access said second controller. 
     Also, implemented in computer program code based products are software modules for: a) receiving a message from said first switch identifying activation of a first port, which is due to establishment of a connection between a first port in said first switch and a second port in said second switch; b) generating a Controller-Advertisement message carrying information required to identify and access said first controller; c) forwarding said generated Controller-Advertisement message from said first controller to said first switch, said first port in said first switch further forwarding said Controller-Advertisement message to said second port in said second switch, said second switch further forwarding said Controller-Advertisement message to said second controller along with an identification of said second port over which said Controller-Advertisement message was received; d) if said first controller and said second controller are a single controller, then: (i) updating said single controller&#39;s Network Information Base (NIB) with a new entry showing new connection information between said first switch and said second switch, utilizing the information carried in the optional switch-id and port-no fields of said Controller-Advertisement message, and (ii) discarding any Controller-Advertisement messages that are looped to itself; e) if said first controller and said second controller are different controllers, then said second controller: (i) identifying said first controller using information carried in said Controller-Advertisement message originated from said first controller; and (ii) generating a new CIB entry containing information for said first controller. 
     CIB Maintenance 
     Once a controller has discovered a number of neighboring controllers, it should keep track of their statuses, for proper operation. This can be achieved by sending Controller-Advertisement messages, with solicit-flag set, to each of the previously discovered controllers, and waiting for them to send their Controller-Advertisement messages in return. 
     Returning to the discussion of  FIG. 2  through  FIG. 5  and considering the CIB given in TABLE IV (which is the same as the example in TABLE I), Controller 2    202  will prepare a Controller-Advertisement message with solicit-flag set, carrying its own access information, and then send the advertisement message to each of the controllers in the CIB. As neighboring controllers receive the Controller-Advertisement message with solicit-flag set, they will respond to Controller 2    202  with their Controller-Advertisement messages, with solicit-flag cleared. Upon receiving the advertisement messages from neighboring controllers, Controller 2    202  will maintain the statuses of these controllers. This process should be repeated at regular intervals for proper operation. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 Controller 2  202 CIB 
               
             
          
           
               
                 Remote 
                 Remote 
                 Local 
                 Local 
                   
               
               
                 Controller-ID 
                 Controller-IP 
                 Switch-ID 
                 Port-No 
                 Status 
               
               
                   
               
               
                 Controller1 
                 Controller1-IP 
                 Switch2 
                 p20 
                 Live 
               
               
                 Controller3 
                 Controller3-IP 
                 Switch3 
                 p37 
                 Live 
               
               
                 Controller4 
                 Controller4-IP 
                 Switch3 
                 p39 
                 Live 
               
               
                   
               
             
          
         
       
     
     In addition to sending advertisement messages per CIB entry periodically, network change events should be processed to maintain the statuses of discovered controllers. For instance, upon receiving a Port-Status message showing that a port has become inactive, the accessibility states of the neighboring controllers that are accessible through that port should be changed accordingly. 
     Secure Controller-to-Controller Connection 
     After discovering each other, two controllers should build a secure connection between themselves, like Transport Layer Security (TLS), over which controller-to-controller information such as routing, service and capability information will be exchanged. Establishing the secure connection is well-defined and beyond the scope of this invention. 
     Controller-to-Controller Information Exchange 
     After completing the discovery process and establishing a secure end-to-end connection, the two controllers may need to exchange information such as: 
     1—Routing information (NIB): The controllers may share a part of (aggregate) or all the routing information that they maintain, so as to enable packet routing between the networks that are in their control. 
     2—Network capability information: The controllers may exchange the capabilities of their networks, such as the maximum bandwidth or minimum latency that can be offered, so as to support end-to-end quality of service (QoS). 
     3—Controller capability information: Each controller may have different hardware and software-specific capabilities. Accordingly, each controller may be capable of handling only specific types of flows, and may have upper bounds on the traffic load that it can handle.
         4—Service offerings: The controller may provide certain services to the users of its subnetwork (such as location based services), which it may decide to make visible to the discovered controller so as to offer these services to the users of the other subnetwork, as well.       

     It is also possible to include some or all of this information in the Controller-Advertisement messages as part of the discovery process. This can be achieved by using the optional-fields defined in the Controller-Advertisement message. 
     CONCLUSION 
     A system and method has been shown in the above embodiments for the effective implementation of automatic discovery of multiple controllers in software defined networks (SDNs). While various preferred embodiments have been shown and described, it will be understood that there is no intent to limit the invention by such disclosure, but rather, it is intended to cover all modifications falling within the spirit and scope of the invention, as defined in the appended claims. For example, the present invention should not be limited by number of sub-networks, number of controllers, number of switches, number of switches per controller, number of controllers per switch, software/program, computing environment, or specific computing hardware.