Abstract:
The present invention relates to the field of communication technologies and discloses a method, device, and system for connectivity check. The method includes the following: a Media Gateway (MGW) receives an Additional Connectivity Check message sent by a Media Gateway Controller (MGC); and the MGW performs an additional connectivity check according to the Additional Connectivity Check message. With the connectivity check method, device, and system of the present invention, the efficiency of the connectivity check may be improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/CN2009/074616, filed on Oct. 26, 2009, which claims priority to Chinese Patent Application No. 200910001787.2, filed on Jan. 12, 2009, both of which are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of communication technologies, and in particular, to a method, device, and system for connectivity check. 
       BACKGROUND 
       [0003]    With a softswitching device as a core, the Next Generation Network (NGN) emerges as driven by service development and under the trend that networks converge. 
         [0004]    As shown in  FIG. 1 , in the prior art, an NGN structure includes a Media Gateway (MGW) and a Media Gateway Controller (MGC). If MGW 1  and MGW 2  are in the same private or public bearer network, Internet Protocol (IP) packets exchanged between MGW 1  and MGW 2  can be sent to each other directly. However, if MGW 1  and MGW 2  are in different bearer networks, for example, if MGW 1  is in a public network and MGW 2  is in a private network, or if both MGW 1  and MGW 2  are in a private network but IP packets cannot be sent directly in both ways between the two networks, media streams between MGW 1  and MGW 2  may possibly be one-way connective or non-connective. 
         [0005]    A common method for the transmission of IP packets between a private network and a public network is to use Network Address Translation (NAT) that translates internal private network addresses into IP addresses of external public networks. With the development of NAT, NAT traversal techniques, such as Simple Traversal of UDP Through Network Address Translators (STUN), Traversal Using Relay NAT (TURN), and Interactive Connectivity Establishment (ICE), emerge, and NAT traversal can connect the private IP address of a private network terminal to a public network through an egress Network Access Translator (NAT) or a firewall (FW). 
         [0006]    ICE is not a new protocol. ICE uses some of the preceding protocols comprehensively so that the protocols can work in the most proper conditions, thus offsetting the inherent defects coming with the use of any one of the protocols. Being independent of a specific signaling protocol, the traversal of media streams through NAT happens between two clients: the session initiator and the session responder. When a media stream is delivered, the session participants exchange their local transmission address and source transmission address to generate an address pair of the local candidate and remote candidate. A connectivity check is performed on all address pairs according to their priorities, and the connectible address pair with the highest priority is chosen for the transmission of the media stream. 
         [0007]    During the process of connectivity check, an MGW sends an STUN request from addresses of local candidates to addresses of remote candidates in sequence; and the remote candidate carries a mapped address in a response message. If the mapped address differs from the addresses of all local candidates, the mapped address is a new local candidate and is referred to as a peer reflexive candidate. 
         [0008]    During the implementation of the present invention, the inventor discovers the following problem: When a connectivity check is being performed or has been completed, an MGC may instruct an MGW to add a new stream. The new stream or new peer reflexive candidate is an update to the candidate pair. When a new connectivity check is initiated, some candidate pairs that are already checked are checked again. This affects the efficiency of the connectivity check. 
       SUMMARY 
       [0009]    Embodiments of the present invention provide a method, device, and system for connectivity check to improve the efficiency of the connectivity check. 
         [0010]    Embodiments of the present invention adopt the following technical solution: 
         [0011]    A method for connectivity check includes: 
         [0012]    receiving, by an MGW, an Additional Connectivity Check message sent by an MGC; and 
         [0013]    performing, by the MGW, an additional connectivity check according to the Additional Connectivity Check message. 
         [0014]    A method for connectivity check includes: 
         [0015]    generating, by an MGC, an Additional Connectivity Check message; and 
         [0016]    sending, by the MGC, the Additional Connectivity Check message to an MGW, so as to enable the MGW to perform an additional connectivity check. 
         [0017]    An MGW includes: 
         [0018]    a message receiving unit, configured to receive an Additional Connectivity Check message sent by an MGC; and 
         [0019]    a connectivity check unit, configured to perform an additional connectivity check according to the Additional Connectivity Check message. 
         [0020]    An MGC includes: 
         [0021]    a message generating unit, configured to generate an Additional Connectivity Check message; and 
         [0022]    a message sending unit, configured to send the Additional Connectivity Check message to an MGW, so as to enable the MGW to perform an additional connectivity check. 
         [0023]    A system for connectivity check includes an MGW and an MGC, where: 
         [0024]    the MGW is configured to receive an Additional Connectivity Check message sent by the MGC and perform an additional connectivity check according to the Additional Connectivity Check message; and 
         [0025]    the MGC is configured to generate an Additional Connectivity Check message and send the Additional Connectivity Check message to the MGW, so as to enable the MGW to perform an additional connectivity check. 
         [0026]    In the method, device, and system for connectivity check provided in the embodiments of the present invention, an MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by an MGC. Therefore, with the technical solution provided in embodiments of the present invention, a connectivity check can be performed on a new candidate according to the Additional Connectivity Check message, thus improving the efficiency of the connectivity check. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    To better explain the technical solution provided in embodiments of the present invention, the accompanying drawings required in the description of embodiments are briefly described. It is apparent that the accompanying drawings illustrate only some exemplary embodiments of the present invention. Those skilled in the art may derive other drawings from these drawings without creative efforts. 
           [0028]      FIG. 1  is a schematic diagram of an NGN network in the prior art; 
           [0029]      FIG. 2  is a schematic diagram of a system for connectivity check according to a first embodiment of the present invention; 
           [0030]      FIG. 3  is a flowchart of a method for connectivity check according to a third embodiment of the present invention; 
           [0031]      FIG. 4  is a flowchart of a method for connectivity check according to a fifth embodiment of the present invention; 
           [0032]      FIG. 5  is a flowchart of a method for connectivity check according to a sixth embodiment of the present invention; 
           [0033]      FIG. 6  is an architecture diagram of an MGW according to a seventh embodiment of the present invention; 
           [0034]      FIG. 7  is a schematic diagram of an MGW according to the seventh embodiment of the present invention; 
           [0035]      FIG. 8  is an architecture diagram of an MGC according to an eighth embodiment of the present invention; and 
           [0036]      FIG. 9  is a schematic diagram of an MGC according to the eighth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    To better explain the technical solution provided in embodiments of the present invention, the embodiments of the present invention are described in detail with reference to the accompanying drawings. It is apparent that the embodiments herein are only some exemplary embodiments of the present invention. Based on the embodiments herein, those skilled in the art may derive other embodiments without creative efforts and such other embodiments all fall within the protection scope of the present invention. 
       Embodiment  1 
       [0038]    To improve the efficiency of a connectivity check, a first embodiment of the present invention provides a system for connectivity check. As shown in  FIG. 2 , the system includes an MGW  21  and an MGC  22 . 
         [0039]    The MGW  21  is configured to receive an Additional Connectivity Check message sent by the MGC  22  and perform an additional connectivity check according to the Additional Connectivity Check message; the MGC  22  is configured to generate an Additional Connectivity Check message and send the Additional Connectivity Check message to the MGW  21  to instruct the MGW  21  to perform an additional connectivity check. 
         [0040]    In the system provided in this embodiment of the present invention, the Additional Connectivity Check message may be generated by the MGC according to a new candidate discovered in a connectivity check process of the MGW. For example, when detecting a new peer reflexive candidate, the MGW instructs the MGC to generate the Additional Connectivity Check message. Or, when a new media stream is added and/or when an existing media stream is modified, the MGC generates the Additional Connectivity Check message according to practical requirements. The media stream may be referred to as a stream and the new media stream may be referred to as a new stream. Modifying an existing media stream may be referred to as modifying an existing stream. A specific operation of stream addition is as follows: The MGC adds a new stream in a media descriptor in a message sent to the MGW. For example, the existing stream is only a voice stream and now a video stream is added. A specific operation of stream modification is as follows: When a peer device discovers a peer reflexive candidate, the peer device instructs a local device to update information of the remote candidate in the stream; or the peer device adds a new media capability in the existing stream or modifies the existing media capability to cause change of the candidate. Stream modification includes modification to the local candidate and/or remote candidate in the existing stream. 
         [0041]    It can be seen from the preceding description that, in the system for connectivity check in this embodiment of the present invention, the MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by the MGC. Therefore, by using the technical solution in this embodiment of the present invention, a connectivity check can be performed on a new candidate according to the Additional Connectivity Check message, thus improving the efficiency of the connectivity check. 
       Embodiment  2 
       [0042]    A second embodiment of the present invention provides a method for connectivity check, including the following steps: 
         [0043]    Step  31 : An MGW receives an Additional Connectivity Check message sent by an MGC. 
         [0044]    As described in the first embodiment, the Additional Connectivity Check message is generated by the MGC according to a new candidate discovered in a connectivity check process of the MGW. Or, when a new media stream is added or when an existing media stream is required to be modified, the MGC spontaneously generates the Additional Connectivity Check message according to practical requirements. 
         [0045]    Step  32 : The MGW performs an additional connectivity check according to the Additional Connectivity Check message. 
         [0046]    The following describes how a connectivity check is performed in the two scenarios of step  31  with reference to the third and fourth embodiments of the present invention. 
       Embodiment  3 
       [0047]    As shown in  FIG. 3 , a method for connectivity check in the third embodiment of the present invention includes the following steps: 
         [0048]    Step  40 : The MGW sends an STUN message to the peer device in a connectivity check process. 
         [0049]    Step  41 : The MGW receives an STUN response message sent by the peer device, where the response message carries a mapped address. 
         [0050]    Step  42 : The MGW determines that a peer reflexive candidate is generated according to the mapped address carried in the response message. 
         [0051]    In this step, the MGW compares the mapped address in the response message with local candidate addresses one by one. If the mapped address is different from all local candidate addresses, the MGW determines that a peer reflexive candidate is generated; if the mapped address is identical with one of all local candidate addresses, the MGW determines that no peer reflexive candidate is generated. 
         [0052]    Step  43 : The MGW sends a peer reflexive candidate report message to the MGC, so as to enable the MGC to instruct the MGW to update its candidate address pair. The sequence of the MGC instructing the peer device to update its candidate address pair and the MGC instructing the MGW to update its candidate address pair may be inversed. When the candidate of one end of the connectivity check changes, the candidate addresses stored on both ends must be refreshed so that the candidate address pair on both the local and the peer devices can be changed. 
         [0053]    The peer reflexive candidate report message carries a new peer reflexive candidate event extended from the H.248 protocol by the embodiment of the present invention. For example, a New Peer Reflexive Candidate (nprc) event is defined by extending the H.248 protocol to notify the MGC that a peer reflexive candidate is generated in the connectivity check process. Furthermore, the reported nprc event carries one or more kinds of information listed in Table 1 through the parameters of the report event. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Data Type 
                   
               
               
                   
                 of 
                   
               
               
                 Parameter Name 
                 Parameter 
                 Description of Parameter 
               
               
                   
               
             
             
               
                 streamID 
                 Integer 
                 Identifies the stream carrying the new 
               
               
                   
                   
                 peer reflexive candidate. 
               
               
                 groupID 
                 Integer 
                 Indicates the group ID of the new peer 
               
               
                   
                   
                 reflexive candidate in the Session 
               
               
                   
                   
                 Description Protocol (SDP) descriptor 
               
               
                   
                   
                 in the H.248 message. 
               
               
                 foundationID 
                 Integer 
                 Identifies the foundation of the 
               
               
                   
                   
                 connectivity check of the new peer 
               
               
                   
                   
                 reflexive candidate. 
               
               
                 componentID 
                 Integer 
                 Identifies the component number of the 
               
               
                   
                   
                 connectivity check of the new peer 
               
               
                   
                   
                 reflexive candidate. 
               
               
                 connection-ad 
                 Integer 
                 Includes an IP address and a port number. 
               
               
                 dress 
                   
                 The IP address and port number are 
               
               
                   
                   
                 separated by a colon, for example, 
               
               
                   
                   
                 “10.11.1.100:10000”. 
               
               
                   
               
             
          
         
       
     
         [0054]    The MGW carries the information in Table 1 through the parameters of the reported nprc event to send to the MGC the information that the peer reflexive candidate is generated in the connectivity check. Other parameters may be extended in the nprc event to carry more information. Such parameters will not be listed here. 
         [0055]    A specific form of report parameter extension is as follows: 
         [0056]    A report event parameter Candidate is extended for the nprc event. The Candidate parameter is a character string in the following ABNF format: 
         [0057]    StreamID “|” GroupID “|” FoundationID “|” ComponentID “|” connection-address COLON PortNumber 
         [0058]    StreamID=UINT16 
         [0059]    GroupID=UINT16 
         [0060]    Foundation=UINT16 
         [0061]    Component-id=UINT16 
         [0062]    connection-address=IP 4-address 
         [0063]    PortNumber=UINT16 
         [0064]    In this way, a string carries all information described in the preceding table. 
         [0065]    In addition, the nprc event must be set on the MGW before being reported. The setting method includes: The MGC sets the event for the MGW, or the event is pre-configured on the MGW, for example, the event is set to a permanent event. 
         [0066]    Step  44 : The MGW receives an Update Connectivity Check Candidate message sent by the MGC and updates its candidate address pair. 
         [0067]    In practice, the Update Connectivity Check Candidate message may be a MODIFY message or other messages that can carry the Update Connectivity Check Candidate message, such as ADD message or MOVE message. The Update Connectivity Check Candidate message may carry the updated connectivity check candidate information through SDP in an H.248 message, including the update of the local candidate and/or remote candidate. The updated connectivity check candidate may cause refresh of the connectivity check candidate pair list. In this way, the connectivity check candidate pair shared by both ends of the connectivity check is modified. 
         [0068]    Step  45 : The MGW receives a Connectivity Check message Initiated by the MGC and performs a connectivity check on the candidate pair. 
         [0069]    In this step, the Connectivity Check message may be a connectivity check from beginning message, such as a Send Connectivity Check (scc) signal in an ostuncc package of H.248.50, or may be the Additional Connectivity Check message extended by the present invention. 
         [0070]    If the Connectivity Check message is a connectivity check from beginning message, the MGW performs a connectivity check on all candidate addresses from the beginning. 
         [0071]    If the Connectivity Check message is an Additional Connectivity Check message, the MGW performs an update connectivity check. That is, the MGW performs an additional connectivity check. The MGW checks the connectivity of new candidate pair and does not check candidate pairs whose connectivity is already checked. Under this circumstance, the MGW does not perform a connectivity check on candidate pairs that are already checked. On the basis that the connectivity check results of those already checked continue to be effective, the MGW performs a connectivity check on the connectivity check candidate pair list refreshed due to the change of candidates. Therefore, the connectivity check efficiency of the MGW is improved. 
         [0072]    In this embodiment of the present invention, the Additional Connectivity Check message carries a Send Additional Connectivity Check (sacc) signal extended in the present invention to indicate an additional connectivity check. Optionally, the sacc signal carries the same parameters as the scc signal, including two parameters, one is a parameter indicating the control role and the other is a parameter indicating the session participant. For a better description, in this embodiment of the present invention, the message that carries the sacc signal is described as an Additional Connectivity Check message. After receiving the sacc signal, the MGW performs an additional connectivity check. This process is the preceding update connectivity check. The connectivity check is not performed on candidate pairs that are already checked and is performed on only unchecked ones. Therefore, the connectivity check is named “additional connectivity check”. “Additional connectivity check” has the same meaning in other embodiments of the present invention. 
         [0073]    In this embodiment of the present invention, an MGW reports an nprc event to an MGC, to notify the MGC that a peer reflexive candidate is generated in the connectivity check process. After receiving the nprc event, the MGC sends an Update Connectivity Check Candidate message to the MGW. The MGW updates its candidate address pair according to the Update Connectivity Check Candidate message, thus changing the candidate address pair where the connectivity check is being performed. Under this circumstance, the connectivity check process requires intervention: When a new connectivity check from the beginning is required, the MGC may send an scc signal to the MGW, where the scc signal instructs the MGW to perform a connectivity check from the beginning; when an additional connectivity check is required, the MGC may send an sacc signal to the MGW, where the sacc signal instructs the MGW to perform an additional connectivity check, that is, the sacc signal instructs the MGW to perform a connectivity check on only the new candidate pair without checking the connectivity of those checked. 
       Embodiment 4 
       [0074]    In the third embodiment of the present invention, the MGC instructs the MGW to initiate an additional connectivity check because the MGW detects a new peer reflexive candidate, so that the candidate pair is updated. In the fourth embodiment of the present invention, however, the candidate pair may also be updated in other circumstances. For example, a new media stream (or referred to as stream) is added, or an existing media stream (or referred to as stream) is modified. For example, the MGC adds a new stream through a MODIFY message when a connectivity check is being performed or has been completed on the existing stream. Or, when a connectivity check is being performed or has been completed on the existing stream, the MGC updates the existing stream through a MODIFY message. In the preceding circumstances, a connectivity check must be initiated to check the connectivity of the new candidate pair. The connectivity check is not required to be performed on candidate pairs that are already checked. 
         [0075]    In the preceding circumstances, the MGC instructs the MGW to perform an additional connectivity check. The instruction is carried in an sacc signal in this embodiment of the present invention. The MGW performs the additional connectivity check according to the instruction. 
         [0076]    Before the MGC instructs the MGW to perform the additional connectivity check, or in the same H.248 message, the MGC can optionally send a message about the addition of a new media stream or modification of an existing stream to the MGW, so as to enable the MGW and the peer device to update their candidate address pairs. 
         [0077]    From the preceding description, in the method for connectivity check in this embodiment of the present invention, the MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by the MGC. With the technical solution in this embodiment of the present invention, when a new candidate is added or an existing candidate is modified, the connectivity check candidate pair list is refreshed. Connectivity checks may have already been performed on some candidate pairs included in the refreshed candidate pair list and are not repeated on these candidate pairs, so that the connectivity check efficiency is improved. 
       Embodiment 5 
       [0078]    As shown in  FIG. 4 , a method for connectivity check in the fifth embodiment of the present invention includes the following steps: 
         [0079]    Step  51 : An MGC generates an Additional Connectivity Check message. 
         [0080]    As described in the preceding embodiment of the present invention, the Additional Connectivity Check message is generated by the MGC according to a new candidate discovered in a connectivity check process of the MGW. Or, when a new media stream is added or when an existing media stream is required to be modified, the MGC spontaneously generates the Additional Connectivity Check message according to practical requirements. 
         [0081]    Step  52 : The MGC sends the Additional Connectivity Check message to the MGW, so as to instruct the MGW to perform an additional connectivity check. 
         [0082]    If the Additional Connectivity Check message is generated by the MGC according to a new candidate discovered in the connectivity check process of the MGW, before step  51  where the MGC generates the Additional Connectivity Check message, the method further includes: 
         [0083]    Step  50   a : The MGC receives a peer reflexive candidate report message sent by the MGW. 
         [0084]    The peer reflexive candidate report message can be realized by extending the nprc event in the H.248 protocol. The method of extending the nprc event and the parameters carried in the event and related descriptions are the same as those in preceding descriptions and will not be further described here. 
         [0085]    Step  50   b : The MGC instructs the MGW and the peer device to update their candidate address pairs according to the peer reflexive candidate report message. 
         [0086]    The MGC may instruct the MGW and the peer device to update their candidate address pairs in at least two approaches: 
         [0087]    Approach 1: The MGC sends a Session Initiation Protocol (SIP) Update message to the peer device according to the peer reflexive candidate report message, so as to enable the peer device to update its candidate address pair; then, the MGC sends an Update Connectivity Check Candidate message to the MGW, so as to enable the MGW to update its candidate address pair. 
         [0088]    Approach 2: The MGC sends an Update Connectivity Check Candidate message to the MGW according to the peer reflexive candidate report message, so as to enable the MGW to update its candidate address pair; then, the MGC sends an SIP UPDATE message to the peer device, so as to enable the peer device to update its candidate address pair. 
         [0089]    In practice, the Update Connectivity Check Candidate message may be a MODIFY message where SDP carries information of the updated connectivity check candidates, or other messages that can carry the updated connectivity check candidate information. 
         [0090]    Or, when a media stream is added and/or an existing media stream is modified, after or during step  51  where the MGC generates the Additional Connectivity Check message, the method further includes the following: The MGC sends a message that carries information about the addition of a new media stream and/or the modification of an existing media stream to the MGW, so as to enable the MGW and the peer device to update their candidate address pairs; then the MGW performs an additional connectivity check according to the instruction of the MGC. 
         [0091]    From the preceding description, in the method for connectivity check in this embodiment of the present invention, the MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by the MGC. Therefore, with the technical solution in this embodiment of the present invention, when a peer candidate pair is generated or when a service changes, a connectivity check may be performed on the new candidate pair so that the connectivity check efficiency is improved. 
       Embodiment 6 
       [0092]    A procedure of the method for connectivity check provided in the sixth embodiment of the present invention will be described with reference to  FIG. 5 . 
         [0093]    As shown in  FIG. 5 , the method for connectivity check in the sixth embodiment of the present invention includes the following steps: 
         [0094]    Step  71 : An MGW sends an STUN request to a peer device, requesting to perform a connectivity check on a candidate pair. 
         [0095]    Step  72 : The peer device returns an STUN response that carries a mapped address. 
         [0096]    Step  73 : The MGW determines that a peer reflexive candidate is generated according to the mapped address carried in the response message. 
         [0097]    Step  74 : The MGW reports the new peer reflexive candidate to the MGC through a report event in the H.248 protocol. 
         [0098]    Step  75 : The MGC sends a response message to the MGW. 
         [0099]    Step  76 : After knowing that a peer reflexive candidate is generated, the MGW sends an SIP UPDATE message or other messages to instruct the peer device to update its local candidate address pair. 
         [0100]    Step  77 : The peer device sends a 200 OK message in response to the UPDATE message. 
         [0101]    Step  78 : The MGC sends a MODIFY message to the MGW to update the connectivity check candidate information in SDP. 
         [0102]    Step  79 : The MGW sends a 200 OK message to the MGC. 
         [0103]    Step  710 : The MGC sends a Start Connectivity Check message to the MGW, instructing the MGW to perform a connectivity check on the candidate address pair. In this embodiment of the present invention, the Start Connectivity Check message is an Additional Connectivity Check message that instructs the MGW to check only the updated candidate address pair. 
         [0104]    In the preceding process, the sequence of steps  76  and  77  and steps  78  and  79  may be inversed. That is, the MGC may first instruct the MGW to update its candidate address pair (the candidate pair stored on the MGW), and then instruct the peer device to update its candidate address pair (the candidate pair stored on the peer device). 
         [0105]    From the preceding description, in the method for connectivity check in this embodiment of the present invention, the MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by the MGC. Therefore, with the technical solution in this embodiment of the present invention, a connectivity check may be performed on a new candidate pair through the Additional Connectivity Check message, thus improving the efficiency of the connectivity check. In addition, in the connectivity check process, the peer reflexive candidate is discovered and used only in a connectivity check message and is not combined with another remote candidate to form a new candidate pair for a connectivity check. With the technical solution in this embodiment of the present invention, in a connectivity check process, the new peer reflexive candidate may be combined with another local candidate to form a candidate pair for a connectivity check, thus further improving the accuracy of the connectivity check. 
         [0106]    Those skilled in the art may understand that all or part of the steps of the methods provided in the embodiments of the present invention may be implemented by hardware under the instruction of a computer program. The program may be stored in a computer readable storage medium and when the program is carried out, the steps of the methods in the preceding embodiments of the present invention are carried out. The storage medium may be a magnetic disk, a Compact Disk-Read Only Memory (CD-ROM), a Read Only Memory (ROM), or a Random Access Memory (RAM). 
       Embodiment 7 
       [0107]    As shown in  FIG. 6 , an MGW provided in a seventh embodiment of the present invention includes a message receiving unit  81  and a connectivity check unit  82 . The message receiving unit  81  is configured to receive an Additional Connectivity Check message sent by an MGC. The connectivity check unit  82  is configured to perform an additional connectivity check according to the Additional Connectivity Check message. 
         [0108]    If the Additional Connectivity Check message is generated by the MGC according to a new candidate discovered in a connectivity check process of the MGW, the message receiving unit  81  is further configured to receive an STUN response message that is sent by the peer device and carries a mapped address. Under this circumstance, as shown in  FIG. 7 , the MGW further includes: a peer reflexive candidate determining unit  83 , configured to determine that a new peer reflexive candidate is generated according to the mapped address carried in the response message; an information reporting unit  84 , configured to send a peer reflexive candidate report message to the MGC, so as to enable the MGC to instruct the MGW to update its candidate address pair; and a candidate address pair updating unit  85 , configured to receive an Update Connectivity Check Candidate message sent by the MGC and update its candidate address pair. 
         [0109]    In addition, when a new media stream is added or an existing media stream is modified, the message receiving unit  81  is further configured to receive a message that is sent by the MGC and carries information about the addition of the new media stream and/or modification of the existing media stream. 
         [0110]    The functions of the MGW in the second, third, and fourth embodiments of the present invention may be implemented by the MGW provided in this embodiment of the present invention. 
         [0111]    The MGW in this embodiment of the present invention performs an additional connectivity check according to the Additional Connectivity Check message sent by the MGC. Therefore, with the technical solution in this embodiment of the present invention, when a peer candidate pair is generated or when a service changes, a connectivity check may be performed on the new candidate pair so that the efficiency of the connectivity check is improved. 
       Embodiment 8 
       [0112]    As shown in  FIG. 8 , an MGC provided in an eighth embodiment of the present invention includes a message generating unit  91  and a message sending unit  92 . The message generating unit  91  is configured to generate an Additional Connectivity Check message. The message sending unit  92  is configured to send the Additional Connectivity Check message to the MGW, so as to enable the MGW to perform an additional connectivity check. 
         [0113]    The message generating unit  91  generates an Additional Connectivity Check message according to the previous connectivity check results of the MGW or according to service requirements. 
         [0114]    In addition, as shown in  FIG. 9 , the MGC further includes: 
         [0115]    a message receiving unit  93 , configured to receive a peer reflexive candidate report message sent by the MGW; 
         [0116]    an instructing unit  94 , configured to instruct the MGW and the peer device to update their candidate address pairs according to the peer reflexive candidate report message; and 
         [0117]    an information sending unit  95 , configured to send a message that carries information about the addition of a new media stream and/or the modification of an existing media stream to the MGW. 
         [0118]    The functions of the MGC in the fifth, sixth, and seventh embodiments of the present invention may be implemented by the MGC provided in this embodiment of the present invention. 
         [0119]    To sum up, in the method, device, and system for connectivity check in the embodiments of the present invention, the MGW performs an additional connectivity check according to an Additional Connectivity Check message sent by the MGC. Therefore, with the technical solution in the embodiments of the present invention, when a peer candidate pair is generated or when a service changes, a connectivity check may be performed on the new candidate pair so that the efficiency of the connectivity check is improved. 
         [0120]    In addition, the embodiments of the present invention are mainly applicable to NGN and IP Multimedia Subsystem (IMS) networks but are not limited to such networks. 
         [0121]    Although the present invention has been described in detail through some exemplary embodiments, the present invention is not limited to such embodiments. It is apparent that those skilled in the art can make various modifications and variations to the invention without departing from the spirit and scope of the present invention. The invention is intended to cover the modifications and variations provided that they fall within the protection scope defined by the claims or their equivalents.