Patent Publication Number: US-9432239-B2

Title: End-point identifiers in SIP

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 13/221,620, filed Aug. 30, 2011, which is a continuation of application Ser. No. 12/169,584, filed Jul. 8, 2008, now U.S. Pat. No. 8,024,470, which is a continuation of application Ser. No. 10/387,238, filed Mar. 12, 2003, now U.S. Pat. No. 7,412,521, which applications are incorporated herein by reference in their entirety. 
    
    
     AREA OF THE INVENTION 
     The present invention relates generally to communication between devices over a computer network, and more particularly, to enabling identification and authentication of endpoint devices across the computer network. 
     BACKGROUND OF THE INVENTION 
     The Session Initiation Protocol (SIP) is a signaling protocol that provides a mechanism for a computing device to locate another device it wants to communicate with over a computer network and to establish a communication session therewith. In particular, SIP is an Internet Engineering Task Force (IETF) standard protocol for initiating interactive user-sessions in a number of scenarios. For example, SIP is used for Internet conferencing, telephony, gaming, virtual reality, event notification, and instant messaging. The SIP protocol enables call setup initiation, routing, authentication and other feature messages to endpoints within an IP domain. 
     Like HTTP or SMTP, SIP works in the Application Layer of the Open Systems Interconnection (OSI) communications model. As such, SIP can establish multimedia sessions or Internet telephony calls, and modify or terminate them. The SIP protocol can also invite participants to unicast or multicast sessions that do not necessarily involve the initiator. Because the SIP supports name mapping and redirection services, users initiate and receive communications and services from any location and networks are capable of identifying users within the network. 
     Although SIP has been widely implemented for various applications, the current SIP protocol has a deficiency whereby there is no unique identifier for a device carried in the SIP signal. The standard SIP solution is to use the device&#39;s IP address, however, this is not an adequate solution because in many situations the device itself remains the same yet the device&#39;s IP address changes, as in the case of a reboot. 
     As such, it is desirable to identify endpoint devices which are involved in a security association (SA) (i.e., a point to point association which is not shared across multiple devices for a user). In particular, tying the SA to the IP-level communication address means the SA is lost when the communication address changes dynamically. Having a unique identifier would make it possible to re-use an existing SA for situations where the device and user credentials have not changed. 
     It is also desirable to support independent send and receive communication addresses as in the case of using TCP as a transport wherein it is typical to use different send and receive ports. Because of the difference in the ports, the combination of IP address and port is not suitable for uniquely identifying the device and a single device would be perceived as having two different identities. Using the IP address alone precludes the ability to have two different SIP stacks on the same node. As such, having a unique device identifier would enable independent send and receive communication addresses. 
     It is further desirable to distinguish between multiple and single point of presence for a user as in the case where a registrar/presence agent server chooses to enforce policies that restrict a user to having only a single point of presence. In such a situation, a unique device identifier would be beneficial in allowing the registrar to distinguish when a user has registered multiple clients versus when a user has registered the same client multiple times with a different communication address. 
     It is still further desirable to allow routing of a request to one of several devices belonging to a particular user. In particular, it is desirable to allow the originator of the request to target a specific destination device and to allow a device to specify that future communications within a dialog should go directly to that device even in situations where the communication address for that device changes over time. 
     It is yet further desirable to support reliable communications over potentially transient HTTPS connections. Because a unique device identifier is useful for identifying peers in a TLS connection, the remote end of this connection can use the device identifier to determine the appropriate connection to use when forwarding messages, even when the connection is broken and re-established. Similarly, it is desirable to use the unique device identifier to manage a reboot/restart of a client such that changes in the communication address are differentiated from changes in the device. 
     Thus, there is a need to extend the SIP signaling protocol for the purpose of adding an identifier for use in uniquely identifying the end-point device to overcome these and other obstacles. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention provides a new scheme for uniquely identifying a SIP device by extending the SIP communications protocol. The extension involves a parameter typically carried in the header of the SIP signal. The extension, referred to as the “End Point Identifier” or “epid,” impacts the routing, registration, subscription, and authentication logic in the client, proxy, and server. The end point (device) of a given user epid can be uniquely identified by creating a key from an epid and a user&#39;s address-of-record (URI). This in turn enables improved connection management and security association management when the connections/IP addresses are transient, such as when HTTPS tunneling is used. Although HTTPS will be discussed in certain examples herein, it will appreciated that the transient nature of the connections or IP addresses in those embodiments is not unique to that protocol, and thus the examples are merely illustrative and not limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended claims set forth the features of the present invention with particularity. The invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a is a block diagram generally illustrating an exemplary computer system on which embodiments of the invention may be implemented; 
         FIG. 2  is a schematic diagram showing a Session Initiation Protocol (SIP) system including an SIP client (C 1 ), a proxy server (HP 1 ), load balancers (LB 1 , LB 2  and LB 3 ), load distributors (LD 1 , LD 2  and LD 3 ) and SIP registrars (HS 1 , HS 2  and HS 3 ) in which aspects of embodiments of the invention may be incorporated; 
         FIG. 3  is a schematic diagram showing exchange of signaling messages between an SIP client (C 1 ) and the SIP registrar (HS 1 ); 
         FIG. 4  is a schematic diagram showing a Session Initiation Protocol (SIP) system including SIP clients (C 1  and C 2 ), proxy servers (HP 1  and HP 2 ), load balancers (LB 1 , LB 2  and LB 3 ), load distributors (LD 1 , LD 2  and LD 3 ) and SIP registrars (HS 1 , HS 2  and HS 3 ) in which aspects of embodiments of the invention may be incorporated; 
         FIG. 5  is a schematic diagram showing exchange of signaling messages between an SIP client (C 1 ) and an SIP registrar (HS 2 ); and 
         FIG. 6  is a schematic diagram showing an exemplary RTC server architecture for using an EPID via SIP according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In some situations, it is beneficial to authenticate and identify incoming SIP messages and sign outgoing SIP messages. However, in order to this, there is a need to uniquely identify SIP devices such that connection management and security association management is improved. 
     Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     The following description begins with a description of a general-purpose computing device that may be used in an exemplary system for implementing the invention, and the invention will be described in greater detail with reference to  FIGS. 2-6 . Turning now to  FIG. 1 , a general purpose computing device is shown in the form of a conventional personal computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the personal computer  20 , such as during start-up, is stored in ROM  24 . The personal computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk  60 , a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. 
     The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the personal computer  20 . Although the exemplary environment described herein employs a hard disk  60 , a removable magnetic disk  29 , and a removable optical disk  31 , it will be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories, read only memories, storage area networks, and the like may also be used in the exemplary operating environment. 
     A number of program modules may be stored on the hard disk  60 , magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more applications programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and a pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB) or a network interface card. A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, personal computers typically include other peripheral output devices, not shown, such as speakers and printers. 
     The personal computer  20  preferably operates in a networked environment using logical connections to one or more remote computers, such as a remote computer  49 . The remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the personal computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 1 . Examples of such devices according to certain embodiments of the invention will be described below. The logical connections depicted in  FIG. 1  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
     When used in a LAN networking environment, the personal computer  20  is connected to the local network  51  through a network interface or adapter  53 . When used in a WAN networking environment, the personal computer  20  typically includes a modem  54  or other means for establishing communications over the WAN  52 . The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . Program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device if such is present. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used in embodiments of the invention. 
     In the description that follows, the invention will be described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that various of the acts and operations described hereinafter may also be implemented in hardware. 
     Referring now to  FIG. 2 , the present invention is directed to a mechanism for integrating a unique End Point Identifier into an incoming request message under the Session Initiation Protocol (SIP), e.g. to enable an SIP registrar (hereinafter referred to as a “home server”)  160   a - c  to respond to a request from an SIP client (C 1 )  112  using the same connections on the same route as that used by the SIP client (C 1 )  112  to initiate the connection. The SIP End Point Identifier is defined in an Internet Engineering Task Force (IETF) Request For Comments (RFC) 3261, which is hereby incorporated by reference in its entirety. IETF RFC 2543 is also instructive with respect to the Session Initiation Protocol, and is also incorporated herein by reference in its entirety. 
     By way of example, as shown by the topology in  FIG. 2 , in a session initiation operation, a client (C 1 )  112  (in customer  1 &#39;s network  110 ) that registers with an SIP home server  160   a - c  sends an incoming request message (e.g., SUBSCRIBE message) to the SIP home server  160   a - c . The incoming request message travels over a computer network comprising a number of components such as load balancers  140   a - c  and load distributors  150   a - c . Note that although the REGISTER and SUBSCRIBE methods are sometimes used as examples herein, the described embodiments apply as well to any other SIP method, including but not limited to NOTIFY, BYE, ACK, CANCEL, INVITE, REFER, MESSAGE, INFO, OPTIONS, PRACK, UPDATE, and PUBLISH. 
     The load balancers  140   a - c  are typically IP-level load balancers while the load distributors  150   a - c  are typically application-level entities. Load distributors as discussed herein preferably embody a “location service” as described in the SIP protocol specification (RFC 3261). A common deployment is to have the client (C 1 )  112  use the HTTP proxy server (HP 1 )  115  to make an outbound HTTPS connection to one of the IP based load balancers  140   a - c  after passing through a firewall  130 . As depicted in  FIG. 2 , a connection is made with Load Balancer (LB 2 )  140   b . The Load Balancer (LB 2 )  140   b  then makes an HTTPS connection to one of the SIP based Load Distributor servers  150   a - c . As depicted in  FIG. 2 , a connection is made between Load Balancer (LB 2 )  140   b  and Load Distributor (LD 3 )  150   c . The Load Distributor (LD 3 )  150   c  subsequently looks up the home server for the client (C 1 )  112 , such as via a directory service such as an ACTIVE DIRECTORY®, a product of Microsoft Corp. of Redmond, Wash. Alternatively, the Load Distributor (LD 3 )  150   c  may use LDAP, an external database, an in-memory database, or some other external data source to acquire this information. Thereafter, the Load Distributor (LD 3 )  150   c  makes a TCP connection to the SIP home server (HS 1 )  160   a  and forwards the incoming request message. 
     With the use of the End Point Identifier, also referred to as the “epid” or “End-point ID,” the outgoing response message from the SIP home server (HS 1 )  160   a  is then sent back to the client (C 1 )  110  using the same connections on the same route even in situations where the firewall  130  or proxy server  115  drops the connection. According to the present invention, the epid parameter is used in conjunction with the user URI (i.e., “Uniform Resource Identifier”) to uniquely identify an end point device in the system. A device can be any user agent or SIP client. In one embodiment of the invention, the epid parameter is generated by the end point device. 
     Alternatively, the epid parameter is supplied by a server. In this embodiment, the epid is typically supplied by the registrar (e.g. home server (HS 1 )  160   a ) for the user. In this embodiment, the epid value can for example be returned to the client (C 1 )  110  in the 200 OK response to a request such as a REGISTER request, to then be used by the client (C 1 )  110  in future communications. 
     To obtain the benefits of the epid solution across a client reboot, the epid parameter for a device is stored persistently. According to the present invention, generation of the epid parameter occurs the first time the client is initialized for a given user URI and should be saved for subsequent uses of the client by that user. As the epid is associated with a particular user URI, the epid parameter should be stored along with this user URI. The epid is not meaningful by itself; typically its uniqueness is only assured in combination with a user URI. 
     To be effective, the epid value is preferably unique for each end-point for a user. According to the present invention, the epid parameter can be any encoded string or substantially random value (i.e. either random or generated by a technique such as hashing that typically ensures a low probability of closely spaced repeat values if any) that follows the syntax of a “token.” Using a sufficiently random value reduces the likelihood of collisions where two end points chose the same end-point value. In one embodiment of the invention, the epid parameter is an 8 character hex encoded string of an unsigned 32-bit random value. In another embodiment, the epid parameter is a hex encoding of the 48-bit MAC address of the device. The persistence and substantial uniqueness of the MAC address ensures that the associated hex encoded string is persistently associated with a particular device and is substantially unique. 
     After generation, the epid is used as a parameter of the SIP header. In one embodiment of the invention, the epid is used as a parameter of the SIP From: or To: header. It is the responsibility of the client to select/generate an epid and insert the epid parameter in the From: header of each request it generates, such as the REGISTER request. The SIP specification ensures that the epid parameter is copied to the From: header of the response message for the request. Copying the epid parameter allows the originator&#39;s epid parameter to be available in both directions and end-to-end along the signaling path. 
     An example of this usage of the epid parameter is:
         From: “Bob”&lt;sip:bob@domain.com&gt;;tag=342994;epid=2a56e788       

     In this example, the epid parameter is outside the angle brackets. The epid parameter is intended to be a unique identifier for an end point of the user in the From: or To: URI depending on which header the epid is present in. According to the present invention, the epid parameter is the same across all SIP sessions for this end point even if the tag parameter is different for each SIP session. In one embodiment of the invention, the epid parameter is treated as an opaque token by the receiving party. Alternatively, the epid parameter is case-insensitive. 
     The epid parameter provided at registration time in the From: will be inserted by the registrar in the To: header on behalf of the client when routing requests to that client. In general, the originator of a request will not know the epid for the end point of the destination of the request. The registrar inserts the parameter when it routes the request to the destination allowing downstream proxies to take advantage of this information. In one example, in user-to-user sessions established using the INVITE method, the registrar copies the intended recipient&#39;s epid into the TO header, aiding routers along the route in identifying security associations, and in executing other client-specific functions. 
     Turning to  FIG. 3 , a schematic diagram  200  showing an exemplary exchange of signaling messages between the SIP client (C 1 )  110  and the SIP home server (HS 1 )  160   a  (depicted in  FIG. 2 ) according to an embodiment of the invention is illustrated. In one embodiment of the invention, when the SIP client (C 1 )  110  initiates registration with a Web site  112  (e.g., the Acme Co. network), it does a Domain Name System (DNS) server lookup for sip.acmeco.net and obtains the IP address for the SIP home server (HS 1 )  160   a . Since Acme Co. publishes all of its front end servers in the DNS, it is possible that the SIP client (C 1 ) message may pass through many servers (e.g., proxy servers, load balancing servers, load distributor servers) before reaching the home server (HS 1 )  160   a.    
     As depicted in  FIGS. 2 and 3 , the incoming request message from SIP client (C 1 )  110  passes through Proxy server (HP 1 )  115  via path  205 . Alternatively, the incoming request message passes through a forwarding proxy in the customer network. As described above, the Proxy server (HP 1 ) routes the incoming request message to one of the load balancing servers  140   a - c , in particular, Load Balancer (LB 2 )  140   b  via path  210 . Load Balancer (LB 2 ) then routes the incoming request message to one of the load distributor servers  150   a - c , in particular, Load Distributor (LD 3 )  150   c  via path  215 . Load Distributor (LD 3 )  150   c  looks up the home server information for client (C 1 )  112  and proxies the request to client C 1 &#39;s home server (HS 1 )  160   a  via path  220 . 
     According to one aspect of the invention, the Load Distributor (LD 3 )  150   c  has Record-Route enabled so that it adds itself to the incoming request message. As such, the route for registration at the IP layer is C 1 -HP 1 -LB 2 -LD 3 -HS 1  whereas at the SIP layer it is C 1 -LD 3 -HS 1  because the proxy server (HP 1 ) and Load Balancer (LB 2 ) are not SIP servers. Using the route information ensures that the return path through the various network components is the same as the original path from the client (C 1 )  112  to the home server (HS 1 )  160   a.    
     To accomplish this goal, client (C 1 )  112  generates an epid and inserts the epid in the From field to uniquely identify client (C 1 )  112  from other clients. The epid allows the home server (HS 1 )  160   a  to uniquely identify each instance of the client (C 1 )  112  that registers with the home server (HS 1 )  160   a . In one embodiment of the present invention, the client (C 1 )  112  will also use the epid as a tag value for uniquely identifying the registration session. In another embodiment of the invention, the client (C 1 )  112  uses the same epid across instantiations for the same profile. Alternatively, if persistence is difficult across instantiations, the client (C 1 )  112  generates a new epid every time for every instantiation. 
     An example of usage of the epid parameter in a REGISTER request message sent from the client (C 1 )  112  to the Load Distributor (LD 3 )  150   c  is: 
                                            REGISTER sip:sip.tradewinds.net SIP/2.0           To: C1 &lt;sip:C1@tradewinds.net&gt;;epid=2af5c32b           From: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T1C1;epid=2af5c32b           Call-ID: 1           CSeq: 1 REGISTER           Contact: &lt;sip:10.1.1.1:2734;transport=TLS&gt;           Max-Forwards: 70           Expires: 300                        
As can be seen from this example, the epid parameter generated and added to the header is 2af5c32b. After the Load Distributor (LD 3 )  150   c  receives the incoming request message, with the Record-Route enabled, the Load Distributor (LD 3 )  150   c  adds a Record-Route header to the REGISTER request message. An example of usage of the epid parameter in a REGISTER request message sent from the Load Distributor (LD 3 )  150   c  to the home server (HS 1 )  160   a  is:
 
                                            REGISTER sip:sip.tradewinds.net SIP/2.0           To: C1 &lt;sip:C1@tradewinds.net&gt;;epid=2af5c32b           From: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T1C1;epid=2af5c32b           Call-ID: 1           CSeq: 1 REGISTER           Contact: &lt;sip:10.1.1.1:2734;transport=TLS&gt;           Record-Route:           &lt;sip:LD1.tradewinds.net:1234;transport=TCP&gt;           Max-Forwards: 69           Expires: 300                        
To send an outgoing response message from home server (HS 1 ) to the client (C 1 ), the header information is parsed to identify the route information.
 
     Turning to  FIG. 4 , in a session subscription operation, a client (C 1 )  112  in customer  1 &#39;s network  110  has on its buddy list a client (C 2 )  122  in customer  2 &#39;s network  120 . A SUBSCRIBE message is sent by client (C 1 ) using the same steps as that described for the registration scenario explained with reference to  FIGS. 2 and 3 . The SUBSCRIBE request follows the same route as that for registration, namely client (C 1 )  112  routes the SUBSCRIBE request to the proxy server (HP 1 )  115  which routes the request to the Load Balancer (LB 1 )  140   a  which, in turn, routes the request to the Load Distributor (LD 2 )  150   b . The Load Distributor (LD 2 )  150   b  then routes the request to the home server (HS 2 )  160   b  for client (C 2 )  122 . The Load Distributor (LD 2 )  150   b  preferably has Record-Route enabled so that it adds its route information to the SUBSCRIBE request. 
     To build the SUBSCRIBE message, the client (C 1 )  112  uses the epid generated previously in the registration function, namely 2af5c32b, and inserts the epid along with a tag into the From field of the header. Using the same epid allows the home sever (HS 1 )  160   a  of the client (C 1 )  112  to identify the instance of the client that the message (i.e., NOTIFY message) needs to be sent to. The PAS uses the existence of the epid parameter in the From: header as an indication that NOTIFY messages for this subscription should be routed via the home server (HS 1 )  160   a  for client (C 1 )  112 . 
     The home server (HS 1 )  160   a  also uses the SIP user URI of the user instead of the IP address in the contact field because routing of NOTIFY messages associated with this SUBSCRIBE is done based on the current registration state of the client and not based on the IP address of the client at the time the SUBSCRIBE was sent. 
     An example of usage of the epid parameter in a SUBSCRIBE request message sent from the client (C 1 )  112  to the Load Distributor (LD 1 )  150   a  is: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 SUBSCRIBE sip:C2@tradewinds.net SIP/2.0 
               
               
                   
                 To: C2 &lt;sip:C2@tradewinds.net&gt; 
               
               
                   
                 From: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T2C1;epid=2af5c32b 
               
               
                   
                 Call-ID: 2 
               
               
                   
                 CSeq: 1 SUBSCRIBE 
               
               
                   
                 Event: Presence 
               
               
                   
                 Contact: &lt;sip:C1@tradewinds.net&gt; 
               
               
                   
                 Max-Forwards: 70 
               
               
                   
                 Expires: 21600 
               
               
                   
                   
               
            
           
         
       
     
     In this case, no record-routing is required since the contact address of the client was set to point back to the home server. Since the route recorded for the Registration is used for routing messages, there is no need to Record-Route other requests. The Proxy can be configured such that it inserts Record-Route for REGISTER requests only, such as for use with the RTC registrar. An example of usage of the epid parameter in a SUBSCRIBE request message sent from the Load Distributor (LD 2 )  150   b  to the home server (HS 2 )  160   b  is: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 SUBSCRIBE sip:C2@tradewinds.net SIP/2.0 
               
               
                   
                 To: C2 &lt;sip:C2@tradewinds.net&gt; 
               
               
                   
                 From: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T2C1;epid=2af5c32b 
               
               
                   
                 Call-ID: 2 
               
               
                   
                 CSeq: 1 SUBSCRIBE 
               
               
                   
                 Event: Presence 
               
               
                   
                 Contact: &lt;sip:C1@tradewinds.net&gt; 
               
               
                   
                 Max-Forwards: 69 
               
               
                   
                 Expires: 21600 
               
               
                   
                   
               
            
           
         
       
     
     According to one aspect of the invention, when the presence state of client (C 2 )  122  changes, client (C 2 )  122  updates its presence state using a SERVICE request to its home server (HS 2 )  160   b . Home server (HS 2 )  160   b  sends NOTIFY messages to all watchers of client (C 2 )  122  which includes client (C 1 )  112 . The home server (HS 2 )  160   b  looks up the contact for client (C 1 )  112  in order to send the NOTIFY message. In one embodiment, the home server (HS 2 )  160   b  finds that the contact is associated with an epid parameter from the From header. This indicates that the NOTIFY message is sent to the home server (HS 1 )  160   a  for client (C 1 )  112  instead of using the Route specified in the SUBSCRIBE request. The home server (HS 2 )  160   b  looks up the home server for client (C 1 )  112  and finds that it is home server (HS 1 )  160   a . The home server (HS 2 ) uses the Contact field that was received in the SUBSCRIBE request for the Request-URI of the NOTIFY message. The home server (HS 2 ) then inserts the epid from the SUBSCRIBE request into the To: header of the NOTIFY and routes the NOTIFY message to home server (HS 1 )  160   a.    
     When the Load Distributor (LD 2 )  150   b  does a lookup for the home server of client (C 2 )  122 , it looks up the home server for client (C 1 )  112  as well and inserts it as a parameter in the SUBSCRIBE request before routing it to home server (HS 2 )  160   b . This is an optimization to avoid looking up the home server for C 1  when routing the NOTIFY request. An example of usage of the NOTIFY message sent by home server (HS 2 )  160   b  to home server (HS 1 )  160   a  is: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 NOTIFY sip:C1@tradewinds.net 
               
               
                   
                 To: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T2C1;epid=2af5c32b 
               
               
                   
                 From: C2 &lt;sip:C2@tradewinds.net&gt; 
               
               
                   
                 Call-ID: 2 
               
               
                   
                 CSeq: 2 NOTIFY 
               
               
                   
                 Event: Presence 
               
               
                   
                 Max-Forwards: 70 
               
               
                   
                   
               
            
           
         
       
     
     In an embodiment of the invention, when the home server (HS 1 )  160   a  receives the NOTIFY message, it takes the Request-URI of the NOTIFY message and looks up the Registration database for all contacts registered for that SIP URI. When the home server (HS 1 )  160   a  finds a match, it compares the epid in the To of the NOTIFY message with each contact registered for that URI. When the epids match, the home server (HS 1 )  160   a  routes the NOTIFY using the Route stored in the Registration information for that contact. The home server (HS 1 )  160   a  then inserts the Route header with the contact information for client (C 1 )  112  received in the REGISTER request. In this case, the NOTIFY message is routed from home server (HS 1 )  160   a  to Load Distributor (LD 2 )  150   b  to Load Balancer (LB 1 )  140   a  to the proxy server (HP 1 )  115  and then to client (C 1 )  112 . 
     Thereafter, the home server (HS 2 )  160   b  preserves the From tag received in the SUBSCRIBE request and places it in the To field of the NOTIFY request. In this manner, the client can identify the SUBSCRIBE session to which the NOTIFY belongs. The home server (HS 2 )  160   b  also preserves the epid parameter from the From field in the SUBCRIBE request and places it in the To field of the NOTIFY request. If there are multiple clients for client (C 1 )  112  that have sent a SUBSCRIBE to the home server (HS 2 )  160   b , then the home server (HS 2 )  160   b  sends a NOTIFY for each instance of the client. Each instance of the client is uniquely identified by the epid parameter from the From field of the SUBSCRIBE request. 
     An example of usage of the NOTIFY message sent by the home server (HS 1 )  160   a  to the Load Distributor (LD 2 )  150   b  is: 
                                            NOTIFY LD1.tradewinds.net:1234;transport=TCP SIP/2.0           To: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T2C1;epid=2af5c32b           From: C2 &lt;sip:C2@tradewinds.net&gt;           Route: &lt;sip:10.1.1.1:2734;transport=TLS&gt;           Call-ID: 2           CSeq: 2 NOTIFY           Event: Presence           Max-Forwards: 69                        
When the Load Distributor (LD 2 )  150   b  receives the request, it rewrites the Request-URI with the Route header in the NOTIFY and routes the request over the same TLS connection that the client initiated when doing the registration. This connection is located using the epid parameter in the To: header as shown here:
 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 NOTIFY sip:10.1.1.1:2734;transport=TLS SIP/2.0 
               
               
                   
                 To: C1 &lt;sip:C1@tradewinds.net&gt;;tag=T2C1;epid=2af5c32b 
               
               
                   
                 From: C2 &lt;sip:C2@tradewinds.net&gt; 
               
               
                   
                 Call-ID: 2 
               
               
                   
                 CSeq: 2 NOTIFY 
               
               
                   
                 Event: Presence 
               
               
                   
                 Max-Forwards: 68 
               
               
                   
                   
               
            
           
         
       
     
     An exemplary RTC server architecture  600  for using an epid via SIP according to an embodiment of the invention is illustrated in greater detail in the schematic diagram of  FIG. 6 . The architecture is implemented in a number of portions. The primary portion of the architecture is the SIP Proxy  601 . At the SIP Proxy  601 , the epid parameter is used to identify a security association between the device endpoint and the proxy. This security association is used to sign outgoing messages and to verify the signature of incoming messages when appropriate. 
     Above the proxy layer  601  is an application, the registrar  603 , which also makes use of the epid parameter. Since the epid parameter uniquely identifies an endpoint for a given user, the registrar  603  examines this parameter to easily determine whether or not a given request comes from an endpoint it has previously seen. This allows the registrar  603  to make policy decisions regarding whether to allow the request or not. It also allows the registrar  603  to determine the proper signaling path (connection) that it should use for forwarding requests destined to a given user. This is particularly valuable in a situation where the connection may be changing rapidly due to unreliable network conditions or otherwise. Thus, the epid information is especially important for connection management in environments that make use of NATs, firewalls, and DHCP. 
     The proxy  601  and/or the registrar  603  maintain a set of tables to aid in connection management. In particular, in an embodiment of the invention, the tables include a connection table  605 , a security association table  607 , and an endpoint data table  609 . Each table is indexed by user URI and epid. These tables allow the proxy  601  and the registrar  603  to modify their operation to account for plurality, capability, and presence of the devices associated with each user. Finally, at the lowest level, the proxy  601  and registrar  603  interface with the SIP protocol stack  611  which is responsible for receipt of incoming messages  613  and transmission of outgoing messages  615 . 
     It should be noted that although the discussion has primarily focused on the use of epids by client devices, the same basic technique can provide numerous benefits when applied to servers as well. For example, each server may insert its own epid value in a Record-Route or Route header, such as, for example:
     Route:&lt;sip:proxy.domain.com&gt;;lr;epid=4353536. Use of an epid parameter in this manner confers benefits distinct from those conferred by the use of epids with respect to clients. For example, use of server epids allows a stateful server to indicate when it has rebooted and lost its prior state. Moreover, server epids can provide unique identification of a server, such as when that server is just one of a plurality of servers in a cluster having a single common “virtual” IP address.   

     In addition, the use of server epids facilitates classification of a connection by a server as belonging to a group of connections that should be treated identically from the perspective of a peer connecting to that server. Finally, use of server epids allows a peer of a server to reuse an existing connection to that server, even if the destination IP address of the server is different, such as when the server is multi-homed or has multiple NICs. 
     It will be appreciated that the unique mechanism described herein allows functionality beyond that permitted by using a device&#39;s IP address as its identifier. For example, the device&#39;s IP address can change, as in the case of a reboot, but the device identifier formed of the user identifier and the epid can remain the same in such situations. Moreover, while tying a security association to an IP-level communication address will cause the SA to be lost when the communication address changes dynamically, the epid allows re-use of an existing SA for situations where the device and user credentials have not changed. 
     Moreover, according to a further embodiment of the invention, use of the epid allows independent send and receive communication addresses, such as when using TCP as a transport over different send and receive ports. In this case, an endpoint maintains two separate Session Initiation Protocol stacks, having two endpoint identifiers, so that one stack is used for sending and the other is used for receiving. 
     In an embodiment of the invention, the epid also allows a distinction between multiple and single point of presence for a user, such as when a registrar/presence agent server chooses to enforce policies that restrict a user to having only a single point of presence. In such a situation, the epid allows the registrar to distinguish between a user with multiple registered clients and a user who has registered the same client multiple times with a different communication address. 
     The epid moreover allows the originator of a request to target a specific destination device in an embodiment of the invention and allows a device to specify that future communications within a dialog should go directly to that device even in situations where the actual communication address for that device changes over time. Finally, the epid also supports reliable communications over potentially transient HTTPS connections in an embodiment of the invention. In particular, the remote end of a TLS connection can use the epid to determine the appropriate connection to use when forwarding messages, even when the connection is broken and re-established. 
     Illustrative embodiments of the present invention and certain variations thereof have been provided in the Figures and accompanying written description. The present invention is not intended to be limited to the disclosed embodiments. Rather the present invention is intended to cover the disclosed embodiments as well as others falling within the scope and spirit of the invention to the fullest extent permitted in view of this disclosure and the inventions defined by the claims appended herein below.