Abstract:
Improved techniques are disclosed for providing network address translation (NAT) in a session initiation protocol (SIP) based application. For example, a method for use in a router for performing NAT on a SIP message associated with a session being initiated between a first device and at least a second device, wherein the first device is part of a first network and the second device is part of a second network, includes the following steps. A SIP message is obtained from the first device, wherein a header field of the SIP message includes a local address and local port number usable by the first device on the first network. A port number to be associated with the session being initiated is randomly selected. The randomly selected port number is usable on the second network. An association (e.g., a binding) is generated between the randomly selected port number and a global address usable on the second network, the local address and the local port number. The local address and the local port number are substituted in the header field of the SIP message with the global address and the randomly selected port number thereby generating an address-translated SIP message. The address-translated SIP message is sent to the second device on the second network. The randomly selected port number may serve as a unique identifier for the session being initiated.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of multimedia communication systems, and more particularly to techniques for providing Network Address Translation (NAT) in a Session Initiation Protocol (SIP)-based application. 
       BACKGROUND OF THE INVENTION 
       [0002]    The Session Initiation Protocol (SIP) is an application-level control protocol for setting up and tearing down sessions with one or more participants. Such sessions may typically involve Internet telephone calls (i.e., so-called Voice over Internet Protocol or VoIP calls) or video calls. However, SIP may be used in other applications where session initiation is required. Also, SIP is merely a signaling (control) protocol which serves to create, modify and terminate sessions, while other session protocols (e.g., Real-time Transport Protocol or RTP) are used to handle the actual transfer of the media content (e.g., voice or video data). Further, SIP acts as a carrier for the Session Description Protocol (SDP), which describes the media content of the session. 
         [0003]    Network Address Translation (NAT) is a process that rewrites the source and/or destination addresses of Internet Protocol (IP) packets as they pass through a router or firewall to a global public IP address. The NAT process enables multiple hosts on a private network to access the Internet using the single public IP address. NAT also provides a degree of security by masking the internal private addresses from the public domain. 
         [0004]    A fundamental problem exists between SIP applications and traditional NAT. That is, a SIP message may include one or more header fields. SIP header fields, such as “Via” and “Contact,” are used to convey message attributes and modify message meaning. The header fields contain private IP addresses which are sent by a session offering device to an answering device when a session is being initiated. The SIP messages (and thus header fields) are embedded in the bodies of one or more IP packets. However, traditional NAT only translates addresses in the header of an IP packet, not in the body of an IP packet. Therefore, the private addresses in the SIP messages are not translated and are thus unroutable in the public domain. This disables the session initiation process. 
         [0005]    There are some existing solutions to this problem. Simple Traversal of User Datagram Protocol (UDP) Through NAT or STUN is a protocol that attempts to address this problem. However, STUN does not work with symmetric NAT. With symmetric NAT, all requests from the same internal IP address and port to a specific destination IP address and port are mapped to a unique external source IP address and port. If the same internal host sends a packet with the same source address and port to a different destination, a different mapping is used. Only an external host that receives a packet can send a packet back to the internal host. 
         [0006]    Another solution to the SIP/NAT problem that is intended to work with symmetric NAT is the Traversal Using Relay NAT or TURN protocol. However, TURN requires relaying data though a server that resides on the public Internet. This adds to the complexity and cost of initiating a session. 
         [0007]    Accordingly, there is a need for improved techniques for providing NAT in a SIP-based application. 
       SUMMARY OF THE INVENTION 
       [0008]    Principles of the present invention provide improved techniques for providing NAT in a SIP-based application. 
         [0009]    For example, in one aspect of the invention, a method for use in a router for performing network address translation on a session initiation protocol (SIP) message associated with a session being initiated between a first device and at least a second device, wherein the first device is part of a first network and the second device is part of a second network, includes the following steps. A SIP message is obtained from the first device, wherein a header field of the SIP message includes a local address and local port number usable by the first device on the first network. A port number to be associated with the session being initiated is randomly selected. The randomly selected port number is usable on the second network. An association (e.g., a binding) is generated between the randomly selected port number and a global address usable on the second network, the local address and the local port number. The local address and the local port number are substituted in the header field of the SIP message with the global address and the randomly selected port number thereby generating an address-translated SIP message. The address-translated SIP message is sent to the second device on the second network. Advantageously, the randomly selected port number may serve as a unique identifier for the session being initiated. 
         [0010]    Further, the method may include the step of deleting the randomly selected port number association upon termination of the session initiated by the SIP message. Also, the method may include the step of listening for a response from the second device at the randomly selected port number associated with the session. The method may also include the step of substituting a media transport protocol port number in a session description portion of the SIP message with another randomly selected port number. The SIP message is typically embedded in the body of one or more packets sent by the first device. 
         [0011]    Still further, the session being initiated may include a voice call session or a video call session between the first device and the second device. The voice call session may be a Voice over Internet Protocol based session. Also, in one embodiment, the first network may be a private network and the second network may be a public network (e.g., Internet). 
         [0012]    Advantageously, the SIP message processing methodology of the invention can be employed in symmetric NAT and does not require relay servers, thus overcoming the drawbacks associated with the STUN and TURN protocols. 
         [0013]    These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIGS. 1A and 1B  illustrate an overload NAT methodology. 
           [0015]      FIGS. 2A and 2B  illustrate a SIP-enabled NAT methodology according to an embodiment of the invention. 
           [0016]      FIG. 3  illustrates SIP-based call signalling according to an embodiment of the invention. 
           [0017]      FIG. 4  illustrates a computing architecture of a device for use in implementing a SIP-enabled NAT methodology according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    It is to be appreciated that while the present invention will be described below in the context of IP telephony applications, the invention is not so limited. That is, the present invention is more generally applicable to any SIP-based application with which it would be desirable to implement NAT. 
         [0019]    Principles of the invention define the operation of a Network Address Translation (NAT) process (or Network Address Translator) targeted toward SIP-based applications. As mentioned above, SIP messages have embedded IP addresses of the sender in the “Via” and “Contact” header fields. When passing from the private domain to the public domain, local addresses need to be substituted with a global address (assigned by Internet Assigned Numbers Authority or IANA). To accomplish this, a NAT process needs to be capable of identifying outgoing SIP messages and substituting all relevant fields such that the SIP messages are routable over the Internet by one or more SIP servers. Such a process also should preferably be achieved while posing no special requirements on SIP phone specifications and in a manner that is simple and cost effective. As will be seen, principles of the invention achieve these and other advantages. First, we describe a non-SIP enabled NAT process in the context of  FIGS. 1A and 1B . 
         [0020]      FIGS. 1A and 1B  illustrate a so-called “overload” NAT process. In overload NAT, the NAT router/firewall uses only one global IP address, assigned to the site by IANA, and multiplexes different User Datagram Protocol (UDP) and Transport Control Protocol (TCP) sockets using their port numbers. Address substitution is limited to IP headers and does not involve the IP packet body. 
         [0021]    Thus,  FIG. 1A  shows a part of a private network  100  including a plurality of network devices such as laptop computers  102 -A and  102 -B, and desktop computers  102 -C and  102 -D. The network devices are coupled to NAT router  104 , which serves as a firewall between the private network and public network (Internet)  106 . While the invention is not limited to any particular private network configuration, it is to be appreciated that such configuration could be part of a private corporate network. 
         [0022]    Binding table  110  in  FIG. 1B  illustrates how router  104  performs NAT. Column  112  identifies the source computers A through D, respectively corresponding to network devices  102 -A through  102 -D. Column  114  identifies the unique private (local) IP address of each source computer. Column  116  identifies the port number of each source computer, which as shown is the same for each source computer. Column  118  identifies the NAT router&#39;s public IP address, i.e., the global IP address that is used as a substitute for the local IP addresses of the source computers. Column  120  identifies the NAT assigned port address statically set for each of the source computers. Thus, in the overload NAT approach illustrated in  FIG. 1B , each local private address (i.e., 192.168.42.1; 192.168.42.10, etc.) uses the same public IP address (i.e., 135.64.182.100), and the various data streams of the source computers are multiplexed using fixed port numbers (i.e., 1025, 2045, etc.). 
         [0023]    Unfortunately, as mentioned above, if the NAT approach shown in  FIG. 1B  were used for SIP-based applications, SIP messages would be sent using invalid “Via” and “Contact” headers, containing local addresses that would be unroutable in the public domain. Principles of the invention, as illustratively depicted in  FIGS. 2A and 2B , provide a solution to this problem. 
         [0024]    Accordingly,  FIGS. 2A and 2B  illustrates a SIP-enabled NAT methodology according to an embodiment of the invention. The SIP-enabled NAT methodology enables the NAT router to parse SIP request messages and provides security against intrusion on listening ports. 
         [0025]    Thus,  FIG. 2A  shows a part of a private network  200  including a plurality of network devices, some of which are SIP-enabled devices, i.e., devices that are capable of running SIP-based applications such as a VoIP call participation. Thus, as shown, network  200  includes laptop computer  202 -A running a SIP softphone, laptop computer  202 -B, desktop computer  202 -C running a SIP softphone, desktop computer  202 -D and SIP phone  202 -E. The network devices are coupled to NAT router  204 , which serves as a firewall between the private network and public network (Internet)  206 . Again, while the invention is not limited to any particular private network configuration, it is to be appreciated that such configuration could be part of a private corporate network. 
         [0026]    Binding table  210  in  FIG. 2B  illustrates how router  204  performs NAT according to an embodiment of the invention. Column  212  identifies the source computers A through E, respectively corresponding to network devices  202 -A through  202 -E. Column  214  identifies the unique private (local) IP address of each source computer. Column  216  identifies the port number of each source computer, which as shown is the same for each source computer. Column  218  identifies the NAT router&#39;s public IP address, i.e., the global IP address that is used as a substitute for the local IP addresses of the source computers. Column  220  identifies a NAT assigned port address randomly chosen by router  204  for each of the source computers (e.g., randomly chosen between 1024 and 65535). Thus, advantageously, the NAT router recognizes a SIP request and creates a binding (such as the binding that each row of table  210  represents) between the private source address (column  214 ), the global IP address of the router (column  218 ) and the randomly selected port number (column  220 ) that is chosen to be the source port number to be sent over the Internet. 
         [0027]    It is to be understood that the term “binding” generally refers to a correspondence or association generated or established between two or more elements. For example, each row in tabular data structure  210  is considered a binding wherein the column elements (local address and port, and global address and port) in a row are relationally associated with one another. 
         [0028]    Advantageously, using the SIP-enabled NAT methodology of the invention, such randomly chosen port number will appear in the “Via” and “Contact” headers embedded in the bodies of IP packets and will substitute the original default port number (5060). The global IP address will substitute the address of the sender in the private domain. SIP-enabled NAT router  204  will then listen for responses to the request sent at the randomly chosen port number reserved to that particular dialog session. 
         [0029]    Such binding will remain active for the entire SIP session and, at the expiration of the session, the binding will be destroyed (removed or deleted). Destruction of the binding serves as a security mechanism to prevent active ports from being advertised outside the dialog in which they are involved. Accordingly, as a result of the randomly selected port number, each dialog has a separate randomly chosen identifier, such that termination of one dialog will not affect another dialog. 
         [0030]    For example, in one embodiment, the entries of binding table  210  are deleted in the following cases: 
         [0031]    (1) when a BYE Request (i.e., a request method of SIP that terminates a call) is received, after having sent a 200 OK (i.e., a response code of SIP); 
         [0032]    (2) when a BYE Request is sent, after having received a 481 or 408 Response (i.e., response codes of SIP), or after a client transaction timeout; 
         [0033]    (3) when a CANCEL Request (i.e., a request method of SIP that cancels searches and “ringing”) is sent, after 200 and 487 Responses (i.e., response codes of SIP) are received; or 
         [0034]    (4) when a CANCEL Request is received, if an ACK Request (a request method for SIP that confirms a final response for the INVITE Request) is received after having sent a 487 Response. 
         [0035]    Advantageously, the above-described NAT process uses randomly selected port numbers to uniquely identify different SIP ongoing dialogs. Such randomly selected port number and the global IP address are the values that substitute the private port number and the private IP address in each SIP request sent by NAT router  204  over the Internet. The binding of local and global IP addresses based on SIP transactions, such as the presence of a NAT entry during a SIP session, is guaranteed independent of the transport protocol (e.g., UDP, TCP). 
         [0036]      FIG. 3  illustrates an example of SIP-based call signalling using SIP-enabled NAT according to an embodiment of the invention. As shown, process  300  involves SIP client  302  which is part of the private network (e.g.,  202 -A in  FIG. 2A ), SIP-enabled router  304  (e.g., NAT router  204  in  FIG. 2A ) which includes an internal interface  306  and an external interface  308  where “internal” and “external” are from the perspective of the private network, and a SIP-remote server  310  which is located in the public (Internet) domain (e.g.,  206  in  FIG. 2A ). 
         [0037]    In step  312 , SIP client  302  generates an INVITE Request with a local port equal to 5060 (i.e., default port address shown in column  216  of  FIG. 2B ). This message is sent to NAT router  304 . 
         [0038]    In step  314 , NAT router  304  generates a random port binding (e.g., for source computer  202 -A in  FIG. 2A , the binding represented by the first row of table  210  in  FIG. 2B ). 
         [0039]    In step  316 , the one or more packets that constitute the INVITE Request are inspected by the NAT router, and the private IP address and port are respectively substituted with the global IP address and randomly selected port, as per the generated binding. 
         [0040]    In step  318 , the translated INVITE Request is then forwarded to the final destination, i.e., SIP-remote server  310 . 
         [0041]    Assuming a call was setup and occurred between SIP client  302  and SIP-remote server  310 , the SIP client in step  320  sends a BYE Request message to terminate the call. The packets in the BYE message are subjected to the same translation in step  322  as the packets in step  316 , i.e., they are translated using the same port number randomly selected for the binding generated in step  314 . The translated BYE message is then forwarded to SIP-remote server  310  in step  324 . 
         [0042]    The remote server, in step  326 , then sends a 200 OK response to NAT router  304 . In step  328 , the NAT router deletes the binding for that dialog and forwards the 200 OK response to the SIP client in step  330 , thus ending the call. 
         [0043]    It is to be appreciated that while the random port generation process of the invention is illustrated above for use in translating header fields (e.g., Via and Contact header fields) in a SIP message, the random port generation process can be applied to translation of other port numbers in the SIP message. For example, the process can be applied to the RTP port number in the SDP portion of a SIP message. It is to be understood that the SDP portion describes the session to be initiated. The IP address for the SDP portion of the SIP message will be the same as that for the signalling portion (i.e., header fields). 
         [0044]    Turning now to  FIG. 4 , a computing architecture  400  of a network device for use in implementing a SIP-enabled NAT methodology, according to an embodiment of the invention, is illustrated. That is,  FIG. 4  may be considered a computing architecture used to implement NAT router  204  as shown in  FIG. 2A . The computing architecture may also represent a SIP client and/or a SIP-remote server ( FIG. 3 ). However, it is to be understood that the invention is not limited to any particular computing system implementation. 
         [0045]    In this illustrative implementation, a processor  402  for implementing at least a portion of the methodologies of the invention is operatively coupled to a memory  404  and a network interface  406  via a bus  408 , or an alternative connection arrangement. 
         [0046]    It is to be appreciated that the term “processor” as used herein is intended to include any processing device, such as, for example, one that includes a central processing unit (CPU) and/or other processing circuitry (e.g., digital signal processor (DSP), microprocessor, etc.). Additionally, it is to be understood that the term “processor” may refer to more than one processing device, and that various elements associated with a processing device may be shared by other processing devices. 
         [0047]    The term “memory” as used herein is intended to include memory and other computer-readable media associated with a processor or CPU, such as, for example, random access memory (RAM), read only memory (ROM), fixed storage media (e.g., hard drive), removable storage media (e.g., diskette), flash memory, etc. 
         [0048]    Further, the phrase “network interface” as used herein is intended to include, for example, one or more devices capable of allowing the computing system  400  to communicate with other computing systems. Thus, the network interface may comprise a transceiver configured to communicate with a transceiver of another computer system via a suitable communication protocol. 
         [0049]    Accordingly, one or more computer programs, or software components thereof, including instructions or code for performing the methodologies of the invention, as described herein, may be stored in one or more of the associated storage media (e.g., ROM, fixed or removable storage) and, when ready to be utilized, loaded in whole or in part (e.g., into RAM) and executed by the processor  402 . 
         [0050]    In any case, it is to be appreciated that the techniques of the invention, described herein and shown in the appended figures, may be implemented in various forms of hardware, software, or combinations thereof, e.g., one or more operatively programmed general purpose digital computers with associated memory, implementation-specific integrated circuit(s), functional circuitry, etc. Given the techniques of the invention provided herein, one of ordinary skill in the art will be able to contemplate other implementations of the techniques of the invention. 
         [0051]    Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope or spirit of the invention.