Abstract:
The present invention discloses a method of implementing traversal of multimedia protocols through Network Address Translation device, wherein the Network Address Translation (NAT) device establishes a mapping relationship between address of a terminal in a private network and address of a terminal in a public network, and enables the terminal in the private network to access the public network with a common address, so as to implement the interaction of media streams between the terminal in the private network and the terminal in the public network, in which the common address is a address of the private network for accessing the public network.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to multimedia communication technology, particularly to a method of implementing traversal of multimedia protocols through Network Address Translation device.  
       BACKGROUND OF THE INVENTION  
       [0002]     All multimedia communication protocols involve transmission of media streams (audio/video streams); and it is more suitable to use UDP protocol for transmission of media streams in IP networks. Therefore, presently, most of the multimedia communication protocols employ UDP to carry media streams. However, as UDP is a non-connection-oriented transport layer protocol, Network Address Translation (NAT) devices are unable to determine which private network the incoming UDP packets should be forwarded to; as a result, UDP packets transmitted from a public network to a private network will be rejected by the NAT devices, resulting in the final fail of multimedia communication.  
         [0003]     The modes for address translation of NAT device can be classified into: static NAT, dynamic NAT, and NAPT (Network Address Port Translation). At present, solutions for traversal of multimedia communication protocol through NAT in either of the first two NAT modes described above are available: static NAT solution and dynamic NAT solution.  
         [0004]     The static NAT solution is implemented as follows: The private network IP addresses of terminals requesting for multimedia communication are mapped to public network IP addresses at NAT, that is, NAT reserves public network IP addresses for the multimedia terminals in private networks in advance.  
         [0005]     Based on the static NAT solution, a dynamic NAT solution is introduced: when there is multimedia communication (detected at a known port, for example, the calling known port for H.323 protocol is 1720), NAT assigns a certain public network IP address to the multimedia terminal in the private network dynamically, till the multimedia communication is terminated. Though this solution has been improved compared with the static NAT solution, it is still impossible to save IP address resources in public networks. This is because once a certain public network IP address is assigned to a certain private network terminal, other terminals can not use this public network IP address before the multimedia communication of this terminal is terminated.  
         [0006]     However, for NAPT mode, there is still no solution for traversal of multimedia communication protocol through NAT. NAT devices are designed to save public network IP address resources. But it is impossible for a public network IP address to be shared by multiple multimedia communication terminals in a private network in the existing solutions. In many cases involving private network terminals (or other H.323 nodes), waste of public network IP addresses is inevitable, causing the major function of NAT devices saving public network IP addresses - to be meaningless in multimedia applications.  
       SUMMARY OF THE INVENTION  
       [0007]     The object of the present invention is to provide a method of implementing traversal of multimedia protocols through Network Address Translation device, so as to solve the problem that multimedia communication protocols are unable to traverse through NAT device in NAPT mode in the prior art.  
         [0008]     The present invention provide a technical solution to solve the above problem, the technical solution is as follows:  
         [0009]     A method of implementing traversal of multimedia protocols through Network Address Translation device, the Network Address Translation (NAT) device establishes a mapping relationship between address of a terminal in a private network and address of a terminal in a public network, and enables the terminal in the private network to access the public network with a common address, so as to implement the interaction of media streams between the terminal in the private network and the terminal in the public network, in which the common address is a address of the private network for accessing the public network, wherein the method comprises the following steps:  
         [0010]     the terminal in the private network acquires the address of the terminal in the public network for sending media streams;  
         [0011]     the terminal in the private network sends a redirect packet in which destination address of the redirect packet is the address of the terminal in the public network for sending media streams;  
         [0012]     the NAT device replaces source address in the redirect packet with the common address and sends the replaced redirect packet to the terminal of the public network, and establishes a mapping relationship between the addresses in the redirect packet before the replacement and the addresses in the redirect packet after the replacement;  
         [0013]     the terminal in the public network sends media stream, in which destination address of the media stream is the common address, the NAT device replacing the destination address in the media stream with the address of the terminal in the private network in accordance with the mapping relationship.  
         [0014]     In accordance with the solution described above:  
         [0015]     The source address in the redirect packet is the receiving port number and the private network IP address of the terminal of the private network.  
         [0016]     The terminal in the private network sends a request message to the terminal in the public network to acquire the address of the terminal in the public network for sending media streams.  
         [0017]     The terminal of the private network sends the request when the media stream sent from the terminal of the public network is not received within a preset time period after the ‘Open Media Channel’ signaling from the terminal of the public network has been received.  
         [0018]     The preset time period is shorter than the time for which the terminal of the private network will disconnect automatically when there is no code stream received.  
         [0019]     The terminal of the private network sends redirect packets periodically, in order to refresh the timeout time for the mapping relationship in the NAT device.  
         [0020]     The terminal of the private network performing interaction through an extension field in protocol message when the terminal of the private network sends a request to the terminal of the public network to obtain the source address for sending media streams.  
         [0021]     The present invention implements successful traversal of protocols without the need of modifying firewall (FW)/NAT device. With the present invention, the users can perform multimedia communication (e.g., H.323 communication) between public networks and private networks without the need of purchasing any new device; therefore, not only the problem of communication failure of multimedia communication protocols passing through NAT can be solved, but also one common public network IP address can be used, so as to save IP address resources of public networks.  
     
    
     BRIEF DESCIPTION OF THE DRAWINGS  
       [0022]     FIG. 1  is a schematic diagram of networking to embody the present invention;  
         [0023]     FIG. 2  is a flow diagram of an embodiment of present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0024]     Hereinafter the present invention is described in an embodiment, with H.323 protocol which is a multimedia communication protocol that is most widely used as an example.  
         [0025]     Referring to  FIG.1 , terminal T 1  and T 2  are two H.323 terminals in a private network; terminal T 3  is a H.323 terminal in a public network; GK is a GateKeeper in H.323 protocol.  
         [0026]     Referring to  FIG.2 , the process that terminal T 1  of the private network calls terminal T 3  of the public network is as follows:  
         [0027]     Terminal T 1  of the private network sends a setup signaling to terminal T 3  of the public network;  
         [0028]     When receiving the setup signaling from terminal T 1  of the private network, terminal T 3  of the public network returns a connect signaling to terminal T 1  of the private network;  
         [0029]     T 1  establishes a H.245 connection with T 3  by utilizing the H.245 address of terminal T 3  of the public network carried in the connect signaling; then, T 1  and T 3  perform H.245 signaling interaction with each other.  
         [0030]     The above process fully complies with the specification of H.323 protocol. ‘ITU -T Recommendation H.323’ may be referred to for detailed information about the process.  
         [0031]     As the interaction is fully compliant with the standard protocol, it is impossible for media streams to reach terminals of the private network from the public network. The present invention achieves the object of free communication between terminals of the public and private network by performing interaction of some extension information (not specified in the protocol) on the basis of the standard protocol. The process of the extension information interaction is detailed as follows:  
         [0032]     1. When a terminal of the private network receives an ‘Open Media Channel Ack (OMC Ack)’ signaling from a opposite terminal (i.e. the terminal of the public network with which it connects), Timer  0  is started, in which the time interval of the timer should be shorter than the time for which the terminal disconnects automatically when there is no code stream received, preferably 5s.  
         [0033]     Timer  0  will be stopped if a correct media stream is received within the time that Timer  0  is time out. That the terminal has received the media stream means that there is no NAT device which doesn&#39;t support H.323 protocol, normal communication can be performed without utilizing the method provided in the present invention; therefore, the calling process after Timer  0  is stopped follows the standard H. 323 protocol, and the following steps are not necessary.  
         [0034]     If terminal T 1  of the private network has not received any media stream from terminal T 3  of the public network after Timer  0  is timeout, it means there is (are) NAT device(s) that doesn&#39;t (don&#39;t) support H.323 protocol; thus, the method provided in the present invention has to be used to enable media streams to traverse through NAT device(s) successfully, and the process has to be continued to step  2 .  
         [0035]     2. T 1  requests for the source IP address (i.e., the source IP of the UDP packet header that carries the media stream) and source port (i.e., source port of the UDP packet header that carries the media stream) of the media stream through an extension field in H.245 message after Timer  0  is time out.  
         [0036]     3. Similarly, terminal T 3  of the public network notifies the terminal of the private network of the source IP address and source port (ip_b, port_b 1 ) of the media stream through the extension field in H.245 message when it receives the request sent from terminal T 1  of the private network.  
         [0037]     4. The terminal of the private network sends periodically redirect packets which are defined in the following table to the source address(ip_b, port_b  1 ) of the media stream of the opposite terminal when it receives the H.245 message carrying the extension field sent from the opposite terminal:  
                                       UDP Header   Value   Description                   Source   ip_a   IP address of terminal T1 of the       IP address       private network (an address in               the private network)       Destination   ip_b   the redirect packet receiving       IP address       IP address of terminal T3               of the public network (extracted               from the extension field in H.245               message, i.e., the source IP address               when T3 sends media stream to T1)       Source port   Port_a   Rtp receiving port number of T1       Destination   Port_b1   The redirect packet receiving port       port       of terminal T3 of the public network               (extracted from the extension field               in H.245 message, i.e., the source               port number when T3 sends media               stream to T1)                  
 
         [0038]     NAT replaces the source address in the redirect packet with common public network address and establishes the mapping relationship between the source address and destination address before the replacement and the source address and destination address in the redirect packet after the replacement, when the redirect packet passes through the NAT device. If the firewall also employs NAT processing, the method is also applicable to the traversal through the firewall. The address mapping relationship formed is  
                                                                     Before firewall/NAT   After firewall/NAT            src ip   src port   dst ip   dst port   src ip   src port   dst ip   dst port               ip_a   port_a   ip_b   port_b1   ip_t   port_t   ip_b   port_b1                  
 
         [0039]     Wherein, ip_t and port_t are public network addresses.  
         [0040]     Since there is time-out time in the address mapping relationship in firewall/NAT device, terminal T 1  of the private network refreshes the timeout time by sending redirect packets periodically, and it is possible to ensure that the terminal of public network can receive the redirect packets even when some of the redirect packets are lost.  
         [0041]     5. Terminal T 3  of the public network sets the destination address to which a certain media stream is sent to the source IP (ip_t) and source port (port_t) which extract the UDP header of a redirect packet from the redirect packet when T 3  receives the redirect packet at the sending address (ip_b, port_b 1 ) of the media stream. Then, the media stream sent by Terminal T 3  of the public network after redirection will reach terminal T 1  of the private network through the reverse address mapping of the firewall/NAT device.  
         [0042]     Though the present invention is described in above embodiment with H.323 protocol as an example, the calling process in other multimedia communication protocols, such as SIP protocol, is similar, except that the specific signaling name and field name are different; therefore, the present invention is also applicable to other multimedia communication protocols.