in the following description of the example embodiments , reference is made to the accompanying drawings which form a part hereof , and in which is shown by way of illustration various manners in which the invention may be practiced . it is to be understood that other embodiments may be utilized , as structural and operational changes may be made without departing from the scope of the present invention . generally , the present invention provides a method and apparatus for utilizing a shared processing stack for multiple client applications that utilize a common procedure for session establishment . one protocol in particular that can be used for establishing sessions is the session initiation protocol ( sip ). each client establishes a connection used to send and receive sip messages to and from the sip processing stack . the clients also communicate their capabilities to the sip stack . the sip processing stack receives all incoming sip messages and forwards the message to the appropriate client based on the message type and the client capabilities . the sip processing stack can also send outgoing sip messages , as well as handling multiple request - response exchanges required by sip . using a sip processing stack frees the clients from having to deal with the states and timing of these exchanges . the primary purpose of session protocols such as sip is to establish sessions for end - to - end data communications between networked devices . sip can also be used for such applications as event notifications , instant messaging and presence . one or more of the endpoints may be mobile , e . g . moving from location to location and from network to network . mobile endpoints include all manner of digital communication devices . although the present invention is described in terms of sip , it is appreciated that concepts according to the present invention can be implemented using any form of session establishment procedure , and descriptions of the use of sip as defined by ietf is provided for purposes of illustration , not of limitation . in general , digital communication devices are electronic apparatuses that can exchange data with other devices . the data can be transmitted through various communication mediums such as wire , optical fiber , or through the air as electromagnetic or light waves . increasingly , communication devices include some sort of computing hardware such as a microprocessor . the growth of microprocessor controlled devices has been steadily growing in the field of mobile communication devices ( cellular phones , pdas , etc .). by and large , most mobile communications devices use microprocessors and can therefore be considered mobile data processing devices . fig1 illustrates a representative system environment 100 in which the principles of the present invention may be employed . in the representative system environment 100 , sip messages 102 may be communicated between devices in any number of known manners . these manners include via a landline network ( s ) 104 , which may include a global area network ( gan ) such as the internet , one or more wide area networks ( wan ), local area networks ( lan ), and the like . any computing device or other electronic device that supports sip messages 102 may be the target system that utilizes the present invention , such as servers 106 , desktop computers 108 or workstations , laptop or other portable computers 110 , or any other similar computing device capable of communicating via the network 104 , as represented by generic device 112 . the data 102 may be provided via one or more wireless networks 114 , such as global system for mobile communications ( gsm ), universal mobile telecommunications system ( umts ), personal communications service ( pcs ), time division multiple access ( tdma ), code division multiple access ( cdma ), or other mobile network transmission technology . again , any mobile electronic device that can be used to communicate using sip can interface with a target system that utilizes concepts according to the present invention , such as laptop or other portable computers 116 , mobile phones 118 a and other mobile communicators , personal digital assistants ( pda ) 120 , or any other similar computing device capable of communicating via the wireless network 114 , as represented by generic device 122 . the sip message 102 may be transferred between devices using short - range wireless technologies 124 , such as bluetooth , wireless local area network ( wlan ), infrared ( jr ), etc . the sip message 102 can also be distributed using direct wired connections , such as depicted by connection path 126 . the present invention is applicable regardless of the manner in which the sip message 102 is provided or distributed between the target devices . an example of a target device that utilizes concepts according to the present invention is illustrated as the mobile phone 118 b . the device 118 b includes , for example , a radio transceiver 134 and hardware ( including the processor ) coupled to an operating system ( os ) 130 . the present invention may include a sip processing stack 132 implemented as firmware , a module , or a program running on the os 130 . in reference now to fig2 , an example communications device 200 is shown which illustrates concepts of the present invention . the communication device 200 includes a network interface 202 for communication with a network 204 . the network 204 can be any type of digital communications network . the network 204 in this example provides some manner of sip services , typically by having one or more sip servers 220 and clients 222 , 224 , 226 on the network 204 . the current version of the sip specification is defined in internet engineering task force ( ietf ) rfc 3261 , dated june 2002 . rfc 3261 defines the behavior and data formats that can be used by a communication device according to embodiments of the present invention . the sip specification includes protocols for creating , modifying , and terminating sessions with one or more participants . sip can also be used for such applications as event notifications , instant messaging and presence . sip is designed for use with networked data processing devices ; therefore a device 200 according to the present invention will deal with sip communications over the network interface 202 . the network interface 202 may be any physical or logical interface allowing communications with other networked devices . network interfaces commonly utilize the internet protocol ( ip ) at the network layer of the international standards organization ( iso ) networking protocol stack . typically coupled with ip are transmission control protocol ( tcp ) or user datagram protocol ( udp ) at the transport layer of the osi stack . it is appreciated , however , that the sip specification is independent of any type of networking protocols , and may be used with various other network technologies known in the art . the communications device 200 includes a sip processing stack 206 ( also referred to herein as a “ sip stack ”) that communicates with the network 204 through the network interface 202 . the sip stack 206 can be arranged to handle all sip communications for the communications device 200 . the sip stack 206 also communicates with clients 210 on the communications device 200 . clients 210 ( also known as “ client applications ”) are typically applications that are running on the communications device 200 . clients 210 deal with , among other things , formatting and presenting data to the users . a client 210 may include programs handling tasks as voice / video communications , text messaging , web services , etc . the sip stack 200 may be configured to communicate with an external client 212 that is not necessarily running on the communications device 200 . the external client 212 may be connected to the communications device 200 via a network interface or a data i / o bus , such as used by plug - in peripheral devices . such peripheral devices may include storage devices such smart cards , pc cards , ram / rom modules , disk drives . autonomous devices such as phones , pdas , computers , etc . may also be attached to the communication device 200 as peripherals or via a network to interface an external client 212 with the sip stack 200 . various techniques known in the art can be used for communications between clients 210 , 212 and the sip processing stack 206 . the sip processing stack 206 and clients 210 , 212 may be separately running processes . it is also possible for one or more clients to run in a single process with each other or with the sip stack 206 by using light weight processes ( lwp ) or threads . a device 200 according to the present invention may use any combination of separate processes and / or threads of execution within processes . communications between client processes or threads can be accomplished through inter - process communications ( ipc ). ipc mechanisms allow separate processes to interact . such interaction can be synchronous or asynchronous . computer architectures offer various methods and mechanisms to facilitate ipc , such as network sockets , shared memory , signals , semaphores , pipes , messaging , etc . various standard ipc protocols are known in the art , including java rmi , corba , com / dcom , remote procedure calls ( rpc ), etc . various operating systems and development environments provide specific ipc features , such as ipc within the symbian operating system for mobile devices . ipc protocols often allow programmers to transparently invoke methods on remote processes by making function calls . the processes can define application program interface ( api ) methods usable by another ipc aware processes . these api methods can be used to initiate events , transfer data , process queries , etc . in reference to fig2 , the clients 210 , 212 and sip stack 206 can exchange data related to sip using predefined api methods . one type of data exchanged between clients 210 , 212 and the sip stack 206 are client capabilities . these capabilities can be used by the sip stack 206 for determining a destination client for receiving incoming sip messages . the capabilities can be communicated to the sip stack 206 at runtime from the clients 210 , 212 . the capabilities may also be placed in persistent storage 215 for retrieval at any time by the sip stack 206 . the sip stack 206 can also be used to start and stop clients 210 , 212 using various mechanisms particular to the operating system incorporated by the device 200 . for example , many libraries provide implementations of the standard c language procedures of fork ( ) and exec ( ). other ways of starting clients 210 , 212 may include invoking system calls to the operating system to start an executable or batch file . clients can also be stopped by making system calls , or by use of a c function such as kill ( ). automatic starting and stopping of clients 210 , 212 advantageously allows the device to conserve memory and processing cycles by only running those clients that are needed for current communication sessions . it is appreciated that the clients 210 , 212 may contain their own network connections ( not shown ) in order to carry out data transfer used during communications . sip is used along with other protocols , such as the session description protocol ( sdp ), to provide session - oriented parameters such as data path definition , user availability , and allowable data types supported at both ends of a session . after sip and sdp ( or similar protocols ) have established the session , it is up to the clients 210 , 212 to perform the functions of transferring data and presenting data to users referring now to fig3 , an example is shown of how a session may be established between a sip stack 304 and clients 306 , 308 . a network interface 302 connects the sip stack 304 and clients 306 , 308 to a sip network ( not shown ). the client 308 ( client 2 ) first communicates 310 its capabilities to the sip stack 304 . this communication 310 is illustrated in fig3 as an example api call , setsipcapabilities ( ) invoked from client 2 308 . the sip stack 304 adds 312 this client &# 39 ; s capabilities to an internal state , using an example method called setclient ( ). similar processes of communicating capabilities 314 and adding the client 316 is repeated for client 1 306 . the capabilities communicated between clients 306 , 308 and the sip stack 304 may include standard components of the sip headers as defined in various ietf rfcs . well known capabilities supported by clients such as sip version , sip methods , language , encoding , content - disposition , media type , content type , etc ., are embedded in the sip message . other client capabilities can be embedded anywhere in the sip message , including the start line , headers , and message body . the api utilized by the sip stack 304 would typically have a flexible way of communicating these capabilities , thereby allowing support for future sip extensions without changing the api methods . the communication 310 , 314 of client capabilities can occur at any time during operation of a communications device . in one arrangement , the clients 306 , 308 may communicate their capabilities just once upon installation of the clients 306 , 308 . this type of communication may be accomplished by writing data to persistent storage such as a file or system registry . by accessing persistent storage , the sip stack 304 discovers and sets 312 , 316 the current client capabilities on startup , and therefore the clients 306 , 308 need not be running . it may also be desirable to have clients 306 , 308 dynamically added and removed at run - time . in this scenario , the clients 306 , 308 would communicate 310 , 314 capabilities upon each client startup and remove those capabilities at client shutdown . the sip stack 304 can be arranged to handle all sip related communications over the network interface 302 . for example , the register request 317 can be sent by the sip stack 304 to inform sip related hosts of the current system capabilities . handling all sip communications through the sip stack 304 has numerous advantages . for example , developers of client applications are freed from having to deal with including a sip stack with each client . updates and fixes can be applied to the sip stack alone for the benefit of all client applications . having a central processing point for sip messages also reduces potential contention for network resources . the sip stack 304 can be used for making outgoing connections using sip as well as listening for incoming connections . outgoing connections involve finding and connecting to a host on the network . listening to incoming connections involves waiting on a predetermined network port for connection attempts . although the concept of network ports is generally used in reference to tcp / ip and udp / ip , as used herein the concept of a port refers to any unique identifier used to isolate incoming connections at the transport layer . when used in tcp / ip and udp / ip , these ports are 16 - bit unsigned integers embedded in the protocol headers . tcp and upd ports may be “ well known ”, such as 5060 used for sip , or may be some other number prearranged for use by two or more hosts . when processing incoming connections , the sip stack 304 maintains an internal state of various client capabilities . this internal state can be used to respond to requests such as the sip options request . the options request can be initiated by another user on the sip network to query capabilities of the user device . the sip stack 304 can use the stack &# 39 ; s internal state to respond to the options request with all client capabilities of the current state . the internal state is also used to direct incoming messages to the appropriate client . an example of an incoming connection is shown by an invite message received 318 from the network interface 302 . the invite message is typically sent by a sip - aware host on the network in an attempt to initiate a session . the sip stack 304 analyzes the message and selects 320 the client that best suits the capabilities required to process the invite request . this selection 320 is indicated by an example method called getclient ( ). in this example , the invite message is forwarded 322 to client 1 306 through an api call , sipincomingmessage ( ). although the term “ forward ” is often meant to construe the message is sent unchanged , in this context “ forward ” is meant to indicate that the substantive data is communicated between clients and sip stack 304 . it is appreciated that when using an api , some parts of the data contained in a sip message may be altered to conform to the api . in general , the forwarding or communicating of sip messages involves communicating that sip data required by the api or other ipc arrangement , which may include sending the message unchanged . the sip stack 304 may handle various responses of the sip protocol such as the sending the “ 100 trying ” 324 and “ 180 ringing ” 326 in response to the invite message . when client 1 306 is ready to establish the session , it communicates 328 this to the sip stack 304 using an api call such as sipsession ( ). the sipsession ( ) call can be used to communicate establishment , changing , and termination of sip sessions to the sip stack 304 . after the sip stack 304 is informed the session will be established , the required ok response 330 and acknowledgement 332 responses can be sent by the sip stack 304 to complete the exchange . at this point , client 1 306 establishes 334 a media session via the network interface 302 . other media sessions may be established with client 2 308 , as shown by processing another invite request 336 , selecting the client 338 , and forwarding the message 340 . as previously described , the sip stack 304 will process incoming sip messages and forward those messages to the appropriate client . the sip stack 304 may look at various parts of the sip messages , including the start line and header . also , the sip message body may contain data ( such as sdp headers ) that can be examined by the sip stack 304 to determine appropriate clients for the sip message . in another example , the body may contain extensible markup language ( xml ) tags that provide a schema for classifying the session data . any combination of sip message headers , start lines , and message body content may be used by the sip stack 304 when determining the appropriate destination client for a sip message . fig4 and 5 illustrate an example of how part of this processing may occur . fig4 is a flowchart showing analysis of various sip message start line and header values to determine the appropriate client for receiving a message , such as shown in the getclient ( ) procedure 320 in fig3 . the procedure starts 402 with the sip message and a list of all clients and associated capabilities . the sip version is checked 404 and if any clients do not support this version , the clients are eliminated 406 from the list . this process is repeated over the remaining portion 408 of the procedure by checking for various client capabilities such as sip methods , sip extensions , language , etc . any clients that do not support the tested capability are removed from the list . finally , the content type is tested 410 and if not type sdp , then the message can be forwarded to the client . in order to forward the message , the list is first checked 412 to see if any clients remain . if so , the message is forwarded to the client 414 , otherwise an error message is sent back 416 to the network . if the content type is examined 410 and found to be of type sdp , further processing 412 is required . fig5 is a flowchart showing the continued processing 412 indicated in fig4 . as with the flowchart in fig4 , procedures 502 involve testing various fields in the sdp headers such as sdp version , network type , address type , bandwidth modifiers , encryption , media , transport , etc . any clients in the list that do not support those capabilities indicated in the sdp headers are eliminated . a “ while ” loop 504 proceeds similarly , testing various attribute lines of the sdp header , such as encoding name , whiteboard orientation , conference type , character set , sdp language , ftmp attribute , etc . again , any clients in the list that do not support these capabilities are eliminated . after processing in the “ while ” loop 504 , the list of clients is checked 506 to see if any clients remain . if so , the message is forwarded to the client 508 , otherwise an error message is sent back 510 to the network . it is appreciated that the example routines shown in fig3 , 4 and 5 are shown for purposes of illustration , and not of limitation . various changes may be made to the operations and program flow while still keeping within the spirit of the present invention . the sip stack may be amenable to an object - oriented approach rather than the procedural approach illustrated . for example , each client could be represented by a client object , and each client object can process the sip and sdp headers to determine which client should receive the message . using the description provided herein , the invention may be implemented as a machine , process , or article of manufacture by using standard programming and / or engineering techniques to produce programming software , firmware , hardware or any combination thereof . any resulting program ( s ), having computer - readable program code , may be embodied on one or more computer - usable media such as resident memory devices , smart cards or other removable memory devices , or transmitting devices , thereby making a computer program product or article of manufacture according to the invention . as such , “ computer readable mediums ” as used herein are intended to encompass a computer program that exists permanently or temporarily on any computer - usable medium or in any transmitting medium which transmits such a program . as indicated above , memory / storage devices include , but are not limited to , disks , optical disks , removable memory devices such as smart cards , sims , wims , semiconductor memories such as ram , rom , proms , etc . communication mediums include , but are not limited to , communications via wireless / radio wave communication networks , the internet , intranets , telephone / modem - based network communication , hard - wired / cabled communication network , satellite communication , and other stationary or mobile network systems / communication links . from the description provided herein , those skilled in the art are readily able to combine software created as described with appropriate general purpose or special purpose computer hardware to create a data processing device and / or computer subcomponents embodying the invention , and to create a data processing device and / or computer subcomponents for carrying out the method of the invention . the foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching . it is intended that the scope of the invention be limited not with this detailed description , but rather by the claims appended hereto .