Patent Publication Number: US-11659011-B2

Title: System and method for determining trust for SIP messages

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/175,430 filed Oct. 30, 2018 by Jan Hendrik Lucas Bakker, et al., entitled “System and Method for Determining Trust for SIP Messages”, which is a continuation of U.S. Pat. No. 10,135,885 issued on Nov. 10, 2018, which is a continuation of U.S. Pat. No. 9,401,935 issued on Jul. 26, 2016, which is a continuation of U.S. Pat. No. 8,407,354 issued on Mar. 26, 2013, which claims priority to and benefit of U.S. Provisional Application No. 61/168,798 filed Apr. 13, 2009 by Jan Hendrik Lucas Bakker, et al., entitled “System and Method for Determining Trust for SIP Messages”, all of which are incorporated herein as if reproduced in their entirety. 
    
    
     BACKGROUND 
     The IP (Internet Protocol) Multimedia Subsystem (IMS) is a standardized architecture for providing multimedia services and voice-over-IP calls to both mobile and fixed user agents (UAs). The Session Initiation Protocol (SIP) been standardized and governed primarily by the Internet Engineering Task Force (IETF) as a signaling protocol for creating, modifying, and terminating IMS-based calls or sessions. 
     As used herein, the terms “user agent” and “UA” might in some cases refer to mobile devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices that have telecommunications capabilities. Such a UA might be part of a UE (User Equipment). A UE may have multiple UAs. A UE may have removable memory module associated, such as but not limited to a Universal Integrated Circuit Card (UICC) that includes a Subscriber Identity Module (SIM) application, a Universal Subscriber Identity Module (USIM) application, a IP Multimedia Services Identity Module (ISIM) application, or a Removable User Identity Module (R-UIM) application, etc. Examples of such modules could include, but are not limited to, PC Card, CompactFlash I, CompactFlash II, SmartMedia, Memory Stick, Memory Stick Duo, Memory Stick PRO Duo, Memory Stick PRO-HG Duo, Memory Stick Micro M2, Multimedia Card, Reduced Size Multimedia Card, MMCmicro Card, Secure Digital card, SxS, Universal Flash Storage, miniSD card, microSD card, xD-Picture Card, Intelligent Stick, Serial Flash Module, p card, and NT Card. When the information is stored on a removable memory module, the contents of the module may be imaged into the UE. 
     Alternatively, such a UA might consist of the device itself without such a module. In other cases, the term “UA” might refer to devices that have similar capabilities but that are not transportable, such as fixed line telephones, desktop computers, set-top boxes, or network nodes. When one or more UAs are part of a network node, the network node could act on behalf of another function such as a UA or a fixed line device and simulate or emulate the UA or fixed line device. For example, for some UAs, the IMS SIP client that would typically reside on the device actually resides in the network and relays SIP message information to the device using optimized protocols. In other words, some functions that were traditionally carried out by a UA can be distributed in the form of a remote UA, where the remote UA represents the UA in the network. The term “UA” can also refer to any hardware or software component that can terminate a communication session that could include, but is not limited to, a SIP session. Also, the terms “user agent”, “UA”, “user equipment”, “UE”, and “node” might be used synonymously herein. Also, the terms “header” and “header field” might be used synonymously herein. Also, a SIP message is a SIP request or a SIP response. 
     A UA might connect to a SIP-based network that includes a plurality of other components such as a P-CSCF (Proxy Call Session Control Function), an S-CSCF (Serving CSCF), an IBCF (Interconnect Border Control Function), an Application Server (AS), and other components, any of which could be referred to as network nodes. A trust relationship might exist between nodes in a SIP network. That is, a group of nodes within a network might regard all messages received from other nodes in the group as legitimate. Such a group can be said to form a trust domain or one or more trusted networks. IETF RFC 3325 titled “Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks” discusses this subject further. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG.  1    is a diagram of a communications system including a plurality of network nodes according to an embodiment of the disclosure. 
         FIG.  2    is a call flow diagram illustrating a method for a UA to retrieve trust information according to an embodiment of the disclosure. 
         FIG.  3    illustrates a method for determining if a node outside a trust domain in an IMS network can be trusted according to an embodiment of the disclosure. 
         FIG.  4    illustrates a method for determining if a node outside a trust domain in an IMS network can be trusted according to an alternative embodiment of the disclosure. 
         FIG.  5    illustrates a processor and related components suitable for implementing the several embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although illustrative implementations of one or more embodiments of the present disclosure are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     A node within a trust domain in an IMS network might receive a message from a node outside the trust domain. In some cases, such a message might direct the node in the trust domain to perform one or more actions that may not be desirable for that node to perform. For example, a message may maliciously be sent to a plurality of UAs falsely informing the UAs that a server timeout has occurred. A UA receiving such a message might attempt to re-register with a SIP registrar even though re-registration is not actually necessary. If a large number of the UAs attempt to re-register, the registrar might become overloaded and fail. This could lead to major problems in the network since other UAs might then be unable to register. 
     In an embodiment, a message sent to a network node from outside the network node&#39;s trust domain can include a trust indicator that indicates the trustworthiness of the message. A trust indicator can also be a trust token, trust identifier or trust flag. Trust indicators can be one of two types. The presence of one type of trust indicator in a message indicates that the network node that sent the message can be trusted. The recipient of a message containing such a trust indicator need not perform any verification of the trust indicator. When the other type of trust indicator is present in a message, the recipient of the message compares the trust indicator with internally stored trust information/database. If the trust indicator matches the stored trust information, the trust indicator is verified, and the recipient/receiver knows that the network node that sent the message can be trusted. 
     If the first type of trust indicator is present in a message or if the second type is present and is verified, the network node that receives the message performs the actions that are typically associated with receipt of the message or the message and its contents. If no trust indicator is present or if the trust indicator is not verified, the network node that receives the message might not perform one or more of the actions that are typically associated with receipt of the message or the message and its contents. 
     In an embodiment, the network node that receives the message is a UA that maintains trust information related to network nodes outside the UA&#39;s trust domain. Upon receiving a message from outside its trust domain, the UA can compare the trust indicator that might be included with the message with the trust information that the UA maintains. If the UA verifies that the trust indicator matches the trust information that it maintains, the UA performs the actions that are typically associated with receipt of the message or the message and its contents. If the UA cannot verify that the trust indicator matches the trust information that it maintains, the UA does not perform at least one action that is typically associated with receipt of the message or the message and its contents. 
     These embodiments are illustrated in  FIG.  1   , where a UA  110  is capable of communicating with a network node B  130 , which is capable of communicating with a network node A  120 . The UA  110 , network node A  120 , and network node B  130  might be components in an IMS-based network, and network node A  120  and network node B  130  might be outside the UA&#39;s trust domain. While only two other network nodes are shown, other numbers could be present. In this embodiment, network node A  120  generates a message A  140  and includes a trust indicator A  145  in the message A  140 . Network node A  120  then sends the message A  140  to network node B  130 . Receipt of message A  140  causes network node B  130  to generate a message B  150  containing a trust indicator B  155 , and message B  150  is then sent to the UA  110 . Message A  140  may or may not be the same as message B  150 , and trust indicator A  145  may or may not be the same as trust indicator B  155 . In other words, network node B  130  might simply pass on the trust indicator A  145  that is received from network node A  120 , or network node B  130  might generate a new trust indicator B  155  based on the trust indicator A  145  or other information received from the network node A  120  and/or other network nodes. 
     In other embodiments, network node A  120  does not include the trust indicator A  145  in the message A  140 . Instead, network node B  130  generates the trust indicator B  155  without regard to any information included in message A  140 , and network node B  130  then includes trust indicator B  155  in the message B  150  sent to the UA  110 . In other words, the trust indicator that the UA  110  receives might have been generated by the network node with which the UA  110  is in direct communication, might have been generated by another network node and then passed on without modification by the network node which the UA  110  is in direct communication, or might have been generated by another network node and then passed on with modification by the network node which the UA  110  is in direct communication. 
     In some embodiments, upon receiving a message that contains a trust indicator, the UA  110  performs the actions that are typically associated with receipt of the message. In other embodiments, upon receiving a message that contains a trust indicator, the UA  110  compares the trust indicator to trust information  115  that the UA  110  has previously received and stored. When a match is found between the trust indicator in the message and the stored trust information  115 , the UA  110  performs the actions that are typically associated with receipt of the message. When a match is not found between the trust indicator and the stored trust information  115 , the UA  110  does not perform at least one action that is typically associated with receipt of the message. 
     In an embodiment, the trust indicator and/or the trust information  115  might be a Uniform Resource Identifier (URI), or some other type of identifier, of a trusted network node. A network node might include its URI in a message sent to the UA  110 . The UA  110  might have previously received trust information  115  in the form of a list of trusted URIs. Upon receiving a message with a trust indicator in the form of a URI, the UA  110  might compare the URI in the message with the URIs in the list of URIs. If a match is found, the UA  110  might trust the network node that sent the message. 
     The UA  110  might not be capable of identifying whether a URI belongs to a P-CSCF, an S-SCSF, an IBCF, or some other type of network node. Some network nodes (such as an IBCF) may or may not include their URI information. Hence, the UA may not be sure which URI represents which network node. In order to determine this, some conventions might be followed or an additional indicator may be added. A SIP REGISTER request and its response (and the header field values included in the response or request) typically should not leave the trusted domain. A third party REGISTER request triggered by the original REGISTER request may leave the trusted domain. In an embodiment, measures are set up to prevent contamination of the information in the responses to the REGISTER in such a case. For example, the fact that a URI represents a known network node could be indicated by a URI parameter to a SIP message. For instance, for an S-CSCF, the URI parameter “scscf” could be added. Alternatively, a URI parameter such as “fe” could be set to a value or a list of values such as fe=“scscf” or fe=“pcscf,scscf”. Here, a network node is referred to as a functional element, or “fe”. When the SIP Service-Route header is used, the message might take a form such as the following:
         Service-Route: sip:orig@scscf1.home1.com;Ir;scscf
 
or
   Service-Route: sip: orig@pcscf1.home1.com;Ir;fe=“pcscf,scscf”
 
in deployments where the P-CSCF and S-CSCF (and possibly other) functional elements are collocated on one physical equipment.
       

     As an alternative, after receiving trust information  115  in the form of a list of URIs, the UA  110  could query a database or other data repository to determine the network nodes and/or trust indicator and/or the trust information that correspond to the listed URIs. The database could be a network node in the network or a database on the device stored in memory that is either internal or in a removable memory module. 
     In another embodiment, the SIP Config Framework, the SIP Policy Framework, an Extensible Authentication Protocol (EAP) based policy retrieval mechanism, an Extensible Markup Language Configuration Access Protocol/HyperText Transfer Protocol (XCAP/HTTP) based server, or an Open Mobile Alliance (OMA) device management (DM) object could be used to convey trust indicators and/or the trust information and/or the network nodes that correspond to the listed URIs to the UA  110 . 
     The UA  110  might receive the trust information  115  in one or more of several different ways. In some embodiments, the trust information  115  might be provided to the UA  110  in response to a SIP REGISTER request submitted by the UA  110 . In some variations of these embodiments, the response might be a SIP 200 OK response, and the trust information  115  might be included directly in the 200 OK response. The trust information  115  could be included in the 200 OK response by a network node, such as an application server, that received the REGISTER request because the request was routed through it. Alternatively, the application server might have received a third party registration request as configured by initial Filter Criteria on an S-CSCF. 
     In other variations of these embodiments, the 200 OK response that the UA  110  receives in response to a REGISTER request might contain information that informs the UA  110  how the trust information  115  can be obtained. Such an embodiment is illustrated in  FIG.  2   . At event  210 , the UA  110  registers with an IMS network by sending a REGISTER request to network node B  130 , which might be an S-CSCF. As part of the registration procedure  220 , a home subscriber server (HSS)  200 , or a similar component, might download to network node B  130  the trust information  115  that is to be used by the UA  110 . At event  230 , the registration is complete, and network node B  130  sends the UA  110  a 200 OK response. In the embodiment of  FIG.  2   , the 200 OK response might contain a URI, or some other type of identifier, that identifies a location where the trust information  115  can be obtained. In other embodiments, as mentioned above, the 200 OK response might directly include the trust information  115 . 
     Alternatively, as shown at event  240 , as part of the SIP registration, the UA  110  might subscribe to the SIP Reg Event package, which can deliver information back to the UA  110 . In response to the Subscribe message at event  240 , network node B  130 , at event  250 , might return a message such as a Notify message. The Notify message might contain the location of the trust information  115  that was downloaded from the HSS  200  as described above. 
     When the UA  110  has received the location of the trust information  115 , either via the 200 OK message at event  230 , via the Notify message at event  250 , or via another SIP method, the UA  110  can retrieve the trust information  115  from the specified location. In this case, the specified location is network node A  120 , but in other cases, the trust information  115  could be retrieved from other network nodes. At event  260 , the UA  110  sends a message, such as an HTTP GET message, to retrieve the trust information  115  from network node A  120 . At event  270 , network node A  120  sends the trust information  115  to the UA  110 . The UA  110  can then store the trust information  115  in internal or removable memory, where the trust information  115  will be available for future use by the UA  110  in determining whether a network node is trusted. 
     In an alternative embodiment, the trust information  115  might be provided during the registration of the UA  110  in one or more fields in the SIP Path header or the SIP Service-Route header. For example, a SIP REGISTER request originated by the UA  110  might be routed through at least a P-CSCF and an S-CSCF, where the S-CSCF performs the role of REGISTRAR. The response (such as a 200 OK response) that the UA  110  receives to its REGISTER request might include an indicator (such as a new P-header, an existing header, or Extensible Markup Language (XML))) conveying information about the network nodes (such as a P-CSCF and an S-SCSF) on the path the REGISTER request was routed over. In addition, one or more fields in the SIP Service-Route header might contain at least the addresses of the P-CSCF or S-CSCF that actually perform any services. The addresses of the S-CSCF on the Service-Route header field and the S-CSCF on the Path header field are not necessarily the same. 
     In some cases, an S-CSCF acting as a REGISTRAR may not be the S-CSCF that replies to other requests from the UA  110 . More generally, not all network nodes that are capable of transmitting a trusted message may be included on the path over which the REGISTER request or its response is routed. However, if a network node transmits a trusted message, it may be advantageous to populate a header field (such as a SIP P-Asserted-Identity header field) or a URI parameter or a SIP body part with a value representative of the originator. Several means exist to enable the UA  110  to determine that some value representative of the originator could only be known or only be inserted by the originator. For example, a value in the P-Asserted-Identity header field could be compared against a value on the Service-Route header field. 
     When a trust indicator is not present in a message received by the UA  110  from a network node outside the UA&#39;s trust domain, or when a trust indicator is present but does not match the UA&#39;s stored trust information  115 , the UA  110  could react in several different ways. In some cases the UA  110  might deny, discard, or terminate the message. In other cases, the UA  110  might return an error message to the network node that sent the message. In still other cases, the UA  110  might remove portions of the message that might cause undesirable actions to be taken and might process the remainder of the message. In some cases, various combinations of these actions could be taken. 
       FIG.  3    illustrates an embodiment of a method  300  for determining if a node outside a trust domain in an IMS network can be trusted. At block  310 , a UA receives from the network node a message containing a trust indicator. At block  320 , the UA determines whether the trust indicator matches trust information stored in the UA. At block  330 , when the trust indicator matches trust information stored in the UA, the UA performs all actions typically associated with receipt of the message. At block  340 , when the trust indicator does not match trust information stored in the UA, the UA refrains from performing at least one action typically associated with receipt of the message. 
       FIG.  4    illustrates an alternative embodiment of a method  400  for determining if a node outside a trust domain in an IMS network can be trusted. At block  410 , a UA receives a message from the network node. At block  420 , the UA determines whether a trust indicator is present in a message. At block  430 , when the trust indicator is present in the message, the UA performs all actions typically associated with receipt of the message. At block  440 , when the trust indicator is not present in the message, the UA refrains from performing at least one action typically associated with receipt of the message. 
     Returning to the example mentioned above where a message is maliciously sent to a plurality of UAs falsely informing the UAs that a server timeout has occurred, the embodiments described herein might prevent the UAs from unnecessarily attempting to re-register with the network. When one of the UAs receives the malicious message, the UA might use a technique described herein to determine whether the sender of the message can be trusted. Since, in this case, the sender would not be trusted, the UA would not perform one or more actions typically associated with receipt of the message. In this case, the UA would not re-register. 
     One possible reflection for the UE could be as follows in 3GPP TS 24.229, subclause 5.1.2A.1.6, titled “Abnormal cases”: 
     In the event the UE receives a 504 (Server Time-out) response containing: 
     
         
         
           
             a P-Asserted-Identity header field set to:
           a value equal to a value on the Service-Route or Path header field received during registration; or   
         
             a Content-Type header field set according to subclause 7.6 (i.e. “application/3gpp-ims+xml”), independent of the value or presence of the Content-Disposition header field, independent of the value or presence of Content-Disposition parameters, then the default content disposition, identified as “3gpp-alternative-service”, is applied as follows:
           if the 504 (Server Time-out) response includes an IM CN subsystem XML body as described in subclause 7.6 with the type element set to “restoration” and the action element set to “initial-registration”, then the UE:
               shall initiate restoration procedures by performing an initial registration as specified in subclause 5.1.1.2; and   may provide an indication to the user based on the text string contained in the reason element.   
               
         
           
         
       
    
     One possible reflection for the P-CSCF could be as follows in 3GPP TS 24.229, subclause 5.2.6.3.2A, titled “Abnormal cases”: 
     When the P-CSCF is unable to forward an initial request for a dialog or a request for a standalone transaction to the next hop in the Service-Route header, as determined by one of the following: 
     
         
         
           
             there is no response to the service request and its retransmissions by the P-CSCF; 
             a 3xx response or 480 (Temporarily Unavailable) response is received for the request; or 
             by unspecified means available to the P-CSCF;
 
and:
 
             the P-CSCF supports restoration procedures;
 
then the P-CSCF:
 
             1) shall reject the request by returning a 504 (Server Time-out) response to the UE; 
             2) shall assume that the UE supports version 1 of the XML Schema for the 3GPP IM CN subsystem XML body if support for the 3GPP IM CN subsystem XML body as described in subclause 7.6 in the Accept header field is not indicated; and 
             3) shall include in the 504 (Server Time-out) response:
           a) a Content-Type header field with the value set to associated MIME type of the 3GPP IM CN subsystem XML body as described in subclause 7.6.1;   b) a P-Asserted-Identity header field set to the value of the SIP URI of the P-CSCF included in the Path header field during the registration of the user whose UE sent the request causing this response; and   c) a 3GPP IM CN subsystem XML body containing:
               i) an &lt;alternative-service&gt; element, set to the parameters of the alternative service;   ii) a &lt;type&gt; child element, set to “restoration” to indicate that restoration procedures are supported;   iii) a &lt;reason&gt; child element, set to an operator configurable reason; and   iv) an &lt;action&gt; child element, set to “initial-registration”
 
NOTE: These procedures do not prevent the usage of unspecified reliability or recovery techniques above and beyond those specified in this subclause.
   
               
         
           
         
       
    
     One possible reflection for the S-CSCF could be as follows in 3GPP TS 24.229, subclause 5.4.3.2, titled “Requests initiated by the served user”: 
     When the S-CSCF receives a request initiated by the served user for which the S-CSCF does not have the user profile or does not trust the data that it has (e.g. due to restart), the S-CSCF shall attempt to retrieve the user profile from the HSS. If the S-CSCF fails to retrieve the user profile and the S-CSCF supports restoration procedures, then the S-CSCF shall:
         1) reject the request by returning a 504 (Server Time-out) response to the UE;   2) assume that the UE supports version 1 of the XML Schema for the 3GPP IM CN subsystem XML body if support for the 3GPP IM CN subsystem XML body as described in subclause 7.6 in the Accept header field is not indicated; and   3) a P-Asserted-Identity header field set to the value of the SIP URI of the S-CSCF included in the Service-Route header field during the registration of the user whose UE sent the request causing this response;   4) include in the 504 (Server Time-out) response:
           a) a Content-Type header field with the value set to associated MIME type of the 3GPP IM CN subsystem XML body as described in subclause 7.6.1; and   b) a 3GPP IM CN subsystem XML body:
               i) an &lt;alternative-service&gt; element, set to the parameters of the alternative service;   ii) a &lt;type&gt; child element, set to “restoration” to indicate that restoration procedures are supported;   iii) a &lt;reason&gt; child element, set to an operator configurable reason; and   iv) an &lt;action&gt; child element, set to “initial-registration”   
               
               

     In addition, the following modifications might be made to 3GPP TS 24.229, subclause 5.10.4.1, titled “General”:
         NOTE 1: THIG functionality is performed in I-CSCF in Release-5 and Release-6 and is compatible with the procedures specified in this subclause.
 
The following procedures shall only be applied if network topology hiding is required by the network. The network requiring network topology hiding is called the hiding network.
   NOTE 2: Requests and responses are handled independently therefore no state information is needed for that purpose within an IBCF.
 
The IBCF shall apply network topology hiding to all header fields which reveal topology information, such as Via, Route, Record-Route, Service-Route, and Path.
 
Upon receiving an incoming REGISTER request for which network topology hiding has to be applied and which includes a Path header field, the IBCF shall add the routable SIP URI of the IBCF to the top of the Path header field. The IBCF may include in the inserted SIP URI an indicator that identifies the direction of subsequent requests received by the IBCF i.e., from the S-CSCF towards the P-CSCF, to identify the UE-terminating case. The IBCF may encode this indicator in different ways, such as, e.g., a unique parameter in the URI, a character string in the username part of the URI, or a dedicated port number in the URI.
   NOTE 3: Any subsequent request that includes the direction indicator (in the Route header field) or arrives at the dedicated port number, indicates that the request was sent by the S-CSCF towards the P-CSCF.
 
Upon receiving an incoming initial request for which network topology hiding has to be applied a SIP request or SIP response with a P-Asserted-Identity header field set the SIP URI of a functional element within its trust domain, the IBCF shall apply network topology hiding to the P-Asserted-Identity header field.
 
Upon receiving an incoming initial request for which network topology hiding has to be applied and which includes a Record-Route header field, the IBCF shall add its own routable SIP URI to the top of the Record-Route header field.
       

     The UE may receive a different value than the value stored by the network node as the IBCF may perform location hiding and replace the URIs in the SIP message (such as the Path or Service-Route header fields) with, for example, at least one of the values of the SIP URI of the IBCF. The IBCF would have to consistently perform this location hiding or replacing of URIs in order not to break the trust that is indicated. 
     When the SIP UA receives a SIP message, it will analyze a table within the function to see if any actions need to be performed for that SIP message, e.g., an INVITE request. The table or data structure identifies the indicators. These indicators could be, but are not limited to, SIP header fields, SIP specific values to look for, etc. For each field, there might also be an action or group of actions that could be performed but are not limited to: 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Remove 
                 Remove the item if it is not trusted 
               
               
                   
                 Ignore 
                 Ignore the item 
               
               
                   
                 Terminate 
                 Terminate the dialogue or refuse the  
               
               
                   
                   
                 dialogue to continue 
               
               
                   
                 Trusted 
                 Mark the field as trusted 
               
               
                   
                 Nontrust 
                 Mark the field as non trusted 
               
               
                   
                 Trust mess 
                 Mark Message as trusted 
               
               
                   
                 Trustnomess 
                 Mark Message as not trusted 
               
               
                   
                   
               
            
           
         
       
     
     For the latter two items, “Trust mess” and “Trustnomess”, all items in the SIP method have to be trusted. The method for identifying the message as trusted could be conveyed as:
         A) New Feature Tag Here a feature tag will be appended to the contact header with a value indicating the trustworthiness of the message.   B) New URI parameter   C) Body part (e.g., in XML)   D) New header field       

     An example embodiment of the data structure is below. 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                   
                   
                 SIP Method 
               
               
                   
                   
                 | 
               
               
                   
                   
                 |&gt; INVITE 
               
               
                   
                   
                 | | 
               
               
                   
                   
                 | |&gt; Field 1 : Value X 
               
               
                   
                   
                 | | |&gt; Action: Remove, ignore, terminate dialogue etc 
               
               
                   
                   
                 | |&gt; Field 2: Value k, c 
               
               
                   
                   
                 | 
               
               
                   
                   
                 |&gt;200 OK 
               
               
                   
                   
                 | | 
               
               
                   
                   
                 | |&gt;Field 3: Value..... 
               
               
                   
               
            
           
         
       
     
     The UA  110  and other components described above might include a processing component that is capable of executing instructions related to the actions described above.  FIG.  5    illustrates an example of a system  1300  that includes a processing component  1310  suitable for implementing one or more embodiments disclosed herein. In addition to the processor  1310  (which may be referred to as a central processor unit or CPU), the system  1300  might include network connectivity devices  1320 , random access memory (RAM)  1330 , read only memory (ROM)  1340 , secondary storage  1350 , and input/output (I/O) devices  1360 . These components might communicate with one another via a bus  1370 . In some cases, some of these components may not be present or may be combined in various combinations with one another or with other components not shown. These components might be located in a single physical entity or in more than one physical entity. Any actions described herein as being taken by the processor  1310  might be taken by the processor  1310  alone or by the processor  1310  in conjunction with one or more components shown or not shown in the drawing, such as a digital signal processor (DSP)  1380 . Although the DSP  1380  is shown as a separate component, the DSP  1380  might be incorporated into the processor  1310 . 
     The processor  1310  executes instructions, codes, computer programs, or scripts that it might access from the network connectivity devices  1320 , RAM  1330 , ROM  1340 , or secondary storage  1350  (which might include various disk-based systems such as hard disk, floppy disk, or optical disk). While only one CPU  1310  is shown, multiple processors may be present. Thus, while instructions may be discussed as being executed by a processor, the instructions may be executed simultaneously, serially, or otherwise by one or multiple processors. The processor  1310  may be implemented as one or more CPU chips. 
     The network connectivity devices  1320  may take the form of modems, modem banks, Ethernet devices, universal serial bus (USB) interface devices, serial interfaces, token ring devices, fiber distributed data interface (FDDI) devices, wireless local area network (WLAN) devices, radio transceiver devices such as code division multiple access (CDMA) devices, global system for mobile communications (GSM) radio transceiver devices, worldwide interoperability for microwave access (WiMAX) devices, and/or other well-known devices for connecting to networks. These network connectivity devices  1320  may enable the processor  1310  to communicate with the Internet or one or more telecommunications networks or other networks from which the processor  1310  might receive information or to which the processor  1310  might output information. The network connectivity devices  1320  might also include one or more transceiver components  1325  capable of transmitting and/or receiving data wirelessly. 
     The RAM  1330  might be used to store volatile data and perhaps to store instructions that are executed by the processor  1310 . The ROM  1340  is a non-volatile memory device that typically has a smaller memory capacity than the memory capacity of the secondary storage  1350 . ROM  1340  might be used to store instructions and perhaps data that are read during execution of the instructions. Access to both RAM  1330  and ROM  1340  is typically faster than to secondary storage  1350 . The secondary storage  1350  is typically comprised of one or more disk drives or tape drives and might be used for non-volatile storage of data or as an over-flow data storage device if RAM  1330  is not large enough to hold all working data. Secondary storage  1350  may be used to store programs that are loaded into RAM  1330  when such programs are selected for execution. 
     The I/O devices  1360  may include liquid crystal displays (LCDs), touch screen displays, keyboards, keypads, switches, dials, mice, track balls, voice recognizers, card readers, paper tape readers, printers, video monitors, or other well-known input or output devices. Also, the transceiver  1325  might be considered to be a component of the I/O devices  1360  instead of or in addition to being a component of the network connectivity devices  1320 . 
     In an embodiment, a method is provided for performing initial registration. The method includes receiving a server timeout message, the server timeout message including at least a field set to a value equal to a value received during a first registration. The method further includes initiating restoration procedures by performing an initial registration. 
     In another embodiment, a UA is provided. The UA includes one or more processors configured such that in response to receiving a server timeout message when the server timeout message includes a field set to a value equal to a value received by the UA during registration, the UA initiates restoration procedures by performing an initial registration. 
     The following 3rd Generation Partnership Project (3GPP) Technical Specification (TS) is incorporated herein by reference: TS 24.229. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. 
     Also, techniques, systems, subsystems and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.