Abstract:
A method is disclosed that enables the transmission of media and signaling packets in a Proxy Mobile Internet Protocol-based (PMIP-based) network. A mobile access gateway is enhanced to differentiate between different types of packet traffic and to act as a Session Initiation Protocol (SIP) proxy on behalf of a mobile node. For example, the gateway is able to handle quality-of-service-sensitive (QoS-sensitive) traffic such as voice or video media packets differently from QoS-insensitive traffic such as SIP signaling packets. In the case of traffic packets that are not QoS-sensitive, the gateway engages standard PMIPv6 procedure. In the case of traffic packets that are in fact QoS-sensitive, the gateway acts as a SIP proxy for each mobile node attached to it, thereby avoiding tunneling of the packets between the gateway and the mobile node&#39;s local mobility anchor, and thereby reducing the amount of delay that affects the packets.

Description:
FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to providing enhanced mobility management at a PMIPv6 Mobile Access Gateway (MAG) that is capable of using Session Initiation Protocol (SIP). 
     BACKGROUND OF THE INVENTION 
     The development of mobile communication networks and the growth of Wireless Local Area Network-based services are driving forces behind the increase in the number of users that communicate wirelessly by using mobile stations, which are also referred to as “mobile nodes.” Mobile Internet Protocol (IP) has been developed to facilitate mobility management, in particular by managing the locations of the mobile nodes and by providing communication for the mobile nodes during handover from one wireless network to another. Mobile IP advantageously allows a mobile node to transmit and receive data seamlessly while maintaining the same IP address. There are various versions of Mobile IP protocol, including Mobile IPv4 (MIPv4), MIPv6, and Proxy Mobile IPv6 (PMIPv6). 
       FIG. 1  depicts a schematic diagram of telecommunications system  100  in the prior art. System  100  comprises mobile node  101 ; correspondent node  102 ; foreign networks  103 - 1  through  103 -N, wherein N is a positive integer; local mobility anchor (LMA)  104 ; mobile access gateway (MAG)  105 ; home network  106 ; and Internet network  107 . The elements are interconnected as shown. 
     Mobile node  101  is a device that is capable of handling a telephone call, or another type of media transmission, on behalf of its user. Typically, node  101  is either a mobile device or portable device such as a cellular phone, a wireless handset, a laptop computer with or without a resident softphone, or another type of telecommunications appliance that is capable of exchanging media signals such as voice, video, and so forth. Node  101  is able to call, or be called by, another node within telecommunications system  100  such as correspondent node  102 . Node  102  is another telecommunications device that is capable of exchange media signals. For example, node  101  is able to dial a telephone number that routes to node  102  and to then exchange media signals with node  102 . Both mobile node  101  and corresponding node  102  are endpoints that must be IPv6-capable, but the endpoints themselves do not have to be capable of supporting mobility-related signaling protocols such as MIPv6. 
     Foreign network  103 - n , wherein n has a value between 1 and N, inclusive, provides the connectivity between visiting mobile nodes that are attached to one or more gateways within the network, such as mobile node  101 , or between an attached mobile node and a correspondent node or nodes in another network, such as correspondent node  102 . The communications signals transported within network  103 - n  convey bitstreams of encoded media, such as audio, video, and so forth. Network  103 - n  comprises an Internet Protocol-based (IP-based) network for the purpose of transporting the media signals. Network  103 - n  comprises one or more interconnected data-processing systems such as switches, servers, routers, and gateways. These data-processing systems, and therefore network  103 - n , operate in accordance with Proxy Mobile Internet Protocol, version 6 (i.e., PMIPv6). 
     Two of the types of data-processing systems that operate in accordance with PMIPv6 within system  100  are local mobility anchor  104  and mobile access gateway  105 , the mobility access gateway that serves network  103 - 1  being depicted. 
     Local mobility anchor (LMA)  104  serves as the home agent for mobile node  101  in the proxy mobile IPv6 domain. It is the topological anchor point for node  101 &#39;s home network prefix, and manages the node  101 &#39;s binding state. As such, LMA  104  acts as the entry point for all traffic packets destined for mobile node  101 . The presence of LMA  104  is necessary because when node  101  moves out of its home address network (i.e., network  106 ), mobile node  101 &#39;s home address cannot be used for direct communication by another node. This is because the home address is an invalid Internet Protocol address in the visited network (i.e., network  103 - 1  in the case of node  101 ). LMA  104  receives the traffic packets with the home address of node  101  as the destination address, and then forwards those packets to the mobile access gateway with which mobile node  101  is currently attached (i.e., MAG  105 ). 
     Mobile access gateway (MAG)  105  manages the mobility-related signaling for mobile nodes (e.g., node  101 ) attached to the gateway&#39;s access link. MAG  105  is responsible for tracking the mobile node&#39;s movement on the access link and for signaling the mobile node&#39;s local mobility anchor (e.g., LMA  104 ) on behalf of the mobile node. In addition, MAG  105  establishes a “tunnel” with the local mobility anchor, in order to enable the mobile node to use an address from its home network prefix, and then emulates the mobile node&#39;s home network. 
     For example, when mobile node  101  first enters PMIP network  103 - 1  and registers, and after determining that node  101  is eligible for service, MAG  105  transmits a proxy binding update (PBU) to LMA  104 , in order to inform the LMA of the new location of node  101 . LMA  104  transmits a binding acknowledgment (BA) message with node  101 &#39;s home-network prefix, creates a Binding Cache Entry (BCE), and sets up a bi-directional tunnel entry point. MAG  105  transmits a remote access (RA) message to node  101 , in order to simulate the node&#39;s home network. Whenever LMA  104  detects packets addressed to node  101 &#39;s home address, such as from correspondent node  102 , the local mobility anchor intercepts those packets, encapsulates those packets, and sends them to node  101 &#39;s proxy, which is at the “care-of” address assigned by MAG  105  to node  101 . MAG  105  de-encapsulates each packet received and routes the packet to node  101 . In handling packets originating from mobile node  101 , MAG  105  acts as a default router in that all of the traffic sent from node  101  to the destination node (correspondent node  102 ) has to be routed through it. MAG  105  encapsulates the packets received from node  101  and transmits them to LMA  104 . LMA  104  then de-encapsulates the tunneled packets and routes them to correspondent node  102 . 
     PMIPv6 provides an improvement over previously standardized mobility management protocols such as MIPv6 and MIPv4. However, since PMIPv6 focuses only on the Internet Protocol layer, it does not take into consideration the impact of handover on the higher protocol layer applications such as real-time communications based on Session Initiation Protocol (SIP). In PMIPv6, all of the traffic is tunneled through a bi-directional tunnel when a mobile node changes its point of attachment. Although this tunneling is adequate for some non-real-time traffic such as email, the tunneling can add overhead and delay to the packets, thereby affecting quality of service and making communication that involves real-time traffic problematic. 
     What is needed is a technique for transmitting packets in a Proxy Mobile Internet Protocol-based network, without some of the disadvantages in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention enables the transmission of different types of packets, such as media packets and signaling packets, in a Proxy Mobile Internet Protocol-based (PMIP-based) network, without some of the disadvantages in the prior art. In particular, a mobile access gateway is enhanced to differentiate between different types of packet traffic and to act as a Session Initiation Protocol (SIP) proxy on behalf of a mobile node. In accordance with the illustrative embodiment of the present invention, the gateway is able to handle quality-of-service-sensitive (QoS-sensitive) traffic such as voice or video media packets differently from QoS-insensitive traffic such as email or SIP signaling packets. 
     When traffic packets arrive at the enhanced mobile access gateway, the gateway performs packet inspection followed by traffic separation. In the case of traffic packets that are not QoS-sensitive (e.g., delay-sensitive, etc.), the enhanced mobile access gateway of the illustrative embodiment engages standard PMIPv6 procedure, in order to transmit the packets through a tunnel when the transmitting mobile node is away from its home network. In the case of traffic packets that are in fact QoS-sensitive, the enhanced gateway acts as a SIP proxy for all of the mobile nodes that are attached to it, in order to avoid tunneling and the accompanying slowdown in processing that can affect the QoS-sensitive packets. 
     When a mobile node is out of its home network and is visiting a foreign network, the mobile node&#39;s home address (i.e., its own Internet Protocol address) cannot be used for direct communication with a correspondent node. This is because the home address is an invalid Internet Protocol address within the visited foreign network. Therefore, in accordance with the illustrative embodiment, when the mobile node initiates a Session Initiation Protocol session using its home address, the enhanced mobile access gateway changes the source address in each session packet to the care-of address that it assigned to the mobile node, and then sends the packet out. All of the SIP messages that are destined to any mobile node attached to the gateway are sent to the gateway itself, rather than to the home address of the mobile node. In turn, the enhanced gateway routes the packets to the correct mobile node. 
     When a mobile node moves from one enhanced mobile access gateway to another, the mobile node&#39;s home address stays the same, but the mobile node gets a new care-of address from the new gateway. If the move occurs during a call, in addition to performing a routine message exchange with the local mobility anchor (e.g., transmitting a binding update, etc.), the enhanced gateway sends either a SIP RE-INVITE message or a SIP UPDATE message to the correspondent node, in which the message contains the care-of address assigned to the mobile node by the new gateway. 
     In some embodiments, both the media packets and signaling packets are transmitted without the tunneling between the mobile access gateway and the local mobility anchor that is characteristic of standardized PMIPv6, while in some alternative embodiments the signaling packets are still transmitted via tunneling. 
     Advantageously, by basing the techniques of the illustrative embodiment at the mobile access gateway, there are no changes required at the local mobility anchor or at any other SIP component in the networks. By forcing at least some of the media packets not to be tunneled, as well as possibly some of the non-media packets, the mobile access gateway of the illustrative embodiment is able to mitigate the potential transmission and/or processing delays that are associated with some gateways in the prior art. 
     The illustrative embodiment of the present invention comprises: assigning, at a first mobile access gateway, a care-of address to a mobile node that is attaching to the first mobile access gateway, the mobile node also having a home address; receiving a packet having a source address and a destination address; when the source address comprises the home address of the mobile node, replacing the source address of the packet with the care-of address assigned to the mobile node, resulting in a modified packet; and transmitting the modified packet to the destination address. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of telecommunications system  100  in the prior art. 
         FIGS. 2A and 2B  respectively depict a first and second schematic diagram of telecommunications system  200  in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a first optimized call flow, in which local mobility anchor  204  is bypassed entirely, in accordance with the illustrative embodiment of the present invention. 
         FIG. 4  depicts a second optimized call flow, in which local mobility anchor  204  is bypassed by the media packets. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 2A and 2B  depict schematic diagrams of telecommunications system  200  in accordance with the illustrative embodiment of the present invention. System  200  comprises mobile node  201 ; correspondent node  202 ; foreign networks  203 - 1  through  203 -N, wherein N is a positive integer; local mobility anchor (LMA)  204 ; enhanced mobile access gateway (MAG)  205 ; home network  206 ; Internet network  207 ; and Session Initiation Protocol (SIP) Server  208 . The elements are interconnected as shown. 
     The distinction between  FIGS. 2A and 2B  is in the type of packet traffic that bypasses local mobility anchor  204 . In  FIG. 2A , local mobility anchor  204  is bypassed entirely by both the SIP signaling packets and the media packets that are exchanged between mobile node  201  and correspondent node  202 , as denoted by the dashed lines. In  FIG. 2B , local mobility anchor  204  is bypassed by at least some of the media packets that are exchanged between mobile node  201  and correspondent node  202 , but the SIP signaling traffic still passes through the local mobility anchor. An optimized call flow corresponding to  FIG. 2A  is described below and with respect to  FIG. 3 , while an optimized call flow corresponding to  FIG. 2B  is described below and with respect to  FIG. 4 . 
     Mobile node  201  is a device that is capable of handling a telephone call, or another type of media transmission, on behalf of its user. In accordance with the illustrative embodiment, node  201  is a mobile device, or at least a portable device, such as a cellular phone, a wireless handset, a laptop computer with or without a resident softphone, or another type of telecommunications appliance that is capable of exchanging media signals such as voice, video, and so forth. Node  201  is able to call, or be called by, another node within telecommunications system  200  such as correspondent node  202 . Node  202  is another telecommunications device that is capable of exchanging media signals. For example, node  201  is able to dial a telephone number that routes to node  202  and to then exchange media signals with node  202 . Mobile node  201  is reachable via its home address, which is within the address space of the home network of node  201 —that is, home network  206 . Both mobile node  201  and corresponding node  202  are endpoints that must be IPv6-capable, but the endpoints themselves do not have to be capable of supporting mobility-related signaling protocols such as MIPv6 in order to operate within telecommunications system  200 . 
     In the illustrative embodiment, mobile node  201  is a mobile or portable device. Corresponding node  202 , however, can be either a mobile/portable device such as a cell phone or a non-portable device such as a landline deskset, as those who are skilled in the art will appreciate. In any event, it will be clear to those skilled in the art how to make and use mobile node  201  and corresponding node  202 . 
     For pedagogical purposes, telecommunications system  200  features a single mobile node (i.e., node  201 ) that is able to communicate with a correspondent node (i.e., node  202 ). However, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention to support concurrently more than one mobile node or more than one correspondent node, or both. 
     Also for pedagogical purposes, correspondent node  202 , which represents a node with which mobile node  201  is corresponding, is shown as being associated with the Internet network  207 . However, it will be clear to those skilled in the art how to make and use embodiments of the present invention in which mobile node  201  is able to correspond with one or more nodes associated with a network other than the Internet. Furthermore, as with mobile node  201 , correspondent node  202  might itself be in another network than its own home network; as a result, some or all of the call flows of the illustrative embodiment might also apply to the correspondent node as if it were a mobile node attached to a mobile access gateway. 
     Foreign network  203 - n , wherein n has a value between 1 and N, inclusive, provides the connectivity between visiting mobile nodes that are attached to one or more gateways within the network, such as mobile node  201 , or between an attached mobile node and a correspondent node or nodes in another network, such as correspondent node  202 . The communications signals transported within network  203 - n  convey bitstreams of encoded media, such as audio, video, and so forth. Network  203 - n  comprises the nodes and PMIPv6 functionality as described earlier and with respect to network  103 - n.    
     Three of the types of data-processing systems that operate in accordance with PMIPv6 within system  200  are local mobility anchor  204 , enhanced mobile access gateway  205 , and Session Initiation Protocol (SIP) server  208 , in accordance with the illustrative embodiment of the present invention. The mobility access gateway that serves network  203 - 1  is depicted. 
     Local mobility anchor  204 , in some embodiments, serves as the home agent for mobile node  201  in the proxy mobile IPv6 domain, and comprises the functionality described earlier and with respect to local mobility anchor  104 . It will be clear to those skilled in the art how to make and use local mobility anchor  204 . 
     Enhanced mobile access gateway (MAG)  205  detects a mobile node&#39;s movements and initiates mobility-related signaling on behalf of the mobile node. The enhanced gateway comprises the functionality described earlier and with respect to MAG  105 . In addition, the enhanced gateway performs the tasks of the illustrative embodiment described below and with respect to  FIGS. 3 and 4 . It will be clear to those skilled in the art, after reading this specification, how to make and use enhanced mobile access gateway  205 . 
     Session Initiation Protocol (SIP) server  208  enables endpoint nodes, such as nodes  201  and  202 , to exchange SIP messages, to register user location, and, in the case of mobile node  201 , to move between networks. Server  208  enables the operators of telecommunications network  200  to install routing and security policies, authenticate users and manage user locations. Server  208  comprises one or more of proxy, redirect, and registrar server functionality, as are known in the art. In some alternative embodiments, server  208  comprises other functionality. It will be clear to those skilled in the art how to make and use SIP server  208 . 
     An example of standardized PMIPv6 operation involving a SIP flow between two nodes is provided here. At some point in time, mobile node  201  has registered its home address with SIP server  208 , and correspondent node  202  knows to contact mobile node  201  by using the home address of node  201 . When correspondent node  202  wants to send SIP signaling packets to mobile  201 , it contacts server  208 . Server  208  continues to send data to the home address of node  201  without having to know of node  201 &#39;s mobility. Whenever LMA  204  detects packets addressed to node  201 &#39;s home address, such as from correspondent node  202 , the LMA intercepts those packets, encapsulates those packets, and sends them to node  201 &#39;s proxy, which is the “care-of” address assigned by MAG  205  to node  201 . MAG  205  de-encapsulates each packet received and routes the packet to node  201 . In handling SIP packets originating from mobile node  201 , MAG  205  acts as a default router in that all of the traffic sent from node  201  to the destination node (correspondent node  202 ) has to be routed through it. MAG  205  encapsulates the packets received from node  201  and transmits them to LMA  204 . LMA  204  then de-encapsulates the tunneled packets and routes them to correspondent node  202 . 
     For pedagogical purposes, telecommunications system  200  features a single local mobility anchor (i.e., LMA  204 ), a single mobile access gateway (i.e., MAG  205 ) within foreign network  203 - 1 , and a single SIP server (i.e., server  208 ) interacting with one another. However, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention to support concurrently more than one local mobility anchor, more than one mobile access gateway (i.e., either within each foreign network  203 - n  or across multiple foreign networks), and/or more than one SIP server. 
     Furthermore, LMA  204 , MAG  205 , and SIP server  208  are shown as being associated with network  206 , network  203 - 1 , and network  206 , respectively. However, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention in which any or all of LMA  204 , MAG  205 , and SIP server  208  are associated with different networks than what is depicted. 
     In accordance with the illustrative embodiment, mobile node  201  and correspondent node  202  use the Session Initiation Protocol (SIP) to initiate sessions such as calls. However, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which a protocol other than SIP is used to initiate sessions between nodes. 
       FIGS. 3 and 4  depict call flows that involve mobile node  201  and correspondent node  202 , in which the call flows are optimized in accordance with the illustrative embodiment of the present invention. As those who are skilled in the art will appreciate, some of the tasks that appear in the figures can be performed in parallel or in a different order than that depicted. Moreover, those who are skilled in the art will further appreciate that in some alternative embodiments of the present invention, only a subset of the depicted tasks are performed. 
     The salient tasks depicted in  FIGS. 3 and 4  are described, for the most part, from the perspective of enhanced mobile access gateway  205  performing those tasks, in accordance with the illustrative embodiment of the present invention. As those who are skilled in the art will appreciate, in some alternative embodiments, some or all of those tasks can be performed at a different data-processing system. 
       FIG. 3  depicts a first optimized call flow, in which local mobility anchor  204  is bypassed entirely, with respect to both the SIP signaling packets and the media packets that are exchanged between mobile node  201  and correspondent node  202 , in accordance with the illustrative embodiment of the present invention. The net effect of bypassing LMA  204  is that neither type of packet (i.e., signaling or media) is tunneled between LMA  204  and mobile access gateway  205 . Correspondent node  202  is already registered with SIP server  208  in this first call flow. 
     At task  301 , MAG  205  receives a request from node  201  to attach to the foreign network represented by network  203 - 1 , in well-known fashion. 
     At task  302 , MAG  205  attaches node  201  to network  203 - 1  in well-known fashion. Part of the attaching process comprises assigning a care-of address to node  201 , in response to the receiving of the request at task  301 . 
     In some embodiments, MAG  205  obtains one or more quality-of-service (QoS) parameters that apply to a packet or packets to be exchanged between node  201  and node  202 . For the purposes of this specification and the appended claims, the quality of service parameters are defined as:
         i. bandwidth and its time derivatives, or   ii. latency and its time derivatives (e.g., “jitter”), or   iii. error rate and its time derivatives, or   iv. any combination of i, ii, and iii.
 
For example, MAG  205  might obtain, either by receiving parameters from another device or by determining on its own, latency-related information to the effect that the end-to-end delay between nodes  201  and  202  shall not exceed 40 milliseconds.
       

     In accordance with the illustrative embodiment, MAG  205  is enhanced with SIP processing and signaling capabilities, including separating SIP packets from non-SIP packets, modifying SIP packets, and supporting SIP signaling messages such as registration, binding update, and so forth. Accordingly at task  303 , MAG  205  notifies server  208  about the new location of node  201  by sending a binding update to server  208 . The binding update comprises the association of the care-of address, previously assigned at task  302 , with node  201 &#39;s home address. 
     Later, when correspondent node  202  needs to exchange SIP signaling with mobile node  201  (e.g., to make a call, etc.), MAG  205  at task  305  receives a SIP INVITE message, originated by node  202  at task  304  and forwarded by SIP server  208 . 
     At task  306 , MAG  205  forwards the received SIP INVITE message to node  201  in well-known fashion. 
     At task  307 , MAG  205  receives a 200 OK or ACK message from node  201 , with the node&#39;s home address in the session description. 
     At task  308 , MAG  205  replaces the home address in the session description of the received message, with the care-of address that it had assigned to node  201  at task  302 . MAG  205  then forwards the modified 200 OK or ACK message to server  208 . 
     In some embodiments, the replacing of the home address at task  308  can be dependent on one or more quality-of-service parameters (e.g., the types of parameters, their values, etc.) obtained at task  302 . By considering the QoS parameters, the enhanced gateway is essentially able to refine its handling of highly QoS-sensitive media traffic versus less QoS-sensitive media traffic. For example, if there is any latency parameter obtained, MAG  205  might replace the home address, but if there is no latency parameter obtained or if the latency parameter falls outside a predetermined threshold (e.g., 200 milliseconds or more, etc.), MAG  205  might not replace the home address. As those who are skilled in the art will appreciate, other combinations of quality-of-service parameter types and values can be used to determine whether to replace the home address with the care-of address. 
     Server  208  at task  309  then forwards the message to node  202 . 
     Mobile node  201  and correspondent node  202  are subsequently ready to exchange media packets. At task  310 , MAG  205  receives media packets transmitted from node  202 . At task  311 , MAG  205  forwards the received media packets to node  201 . 
     Likewise at task  312 , MAG  205  receives media packets transmitted from node  201 , in which node  201  has specified i) a source address equal to its home address and ii) a destination address equal of the correspondent node&#39;s home address. 
     At task  313 , MAG  205  replaces the home address in the source address field with the assigned care-of address in each media packet, and forwards the received media packets to node  202 , in accordance with the illustrative embodiment of the present invention. In some embodiments, the replacing of the home address can be dependent on a quality-of-service parameter, as discussed above and with respect to task  308 . 
     At some point when mobile node  201  moves out of the domain served by MAG  205 , the mobile access gateway informs server  208  to remove the association between node  201 &#39;s home address and the care-of address. 
     If node  201  moves into the domain served by a mobile access gateway (e.g., MAG  205 ) in the middle of the call, the new gateway sends a RE-INVITE message to correspondent node  202  on behalf of mobile node  201  with a new assigned care-of address as the new source address. The new gateway performs this in addition to conducting the routine message exchange for attaching the mobile node to the gateway. 
       FIG. 4  depicts a second optimized call flow, in which local mobility anchor  204  is bypassed by at least some of the media packets that are exchanged between mobile node  201  and correspondent node  202 , in accordance with the illustrative embodiment of the present invention. As in the first call flow discussed above, the net effect of bypassing LMA  204  is that media packets do not tunnel between LMA  204  and mobile access gateway  205 . In this second call flow, however, SIP signaling packets still tunnel between LMA  204  and MAG  205 . Correspondent node  202  is already registered with SIP server  208  in this second call flow. 
     In accordance with the second call flow of the illustrative embodiment, media packets are not tunneled but SIP signaling packets are still tunneled. It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments in which the distinction is made between i) QoS-sensitive packets (e.g., voice, video, etc.) which are not tunneled and ii) QoS-insensitive packets (e.g., SIP signaling, email messages, instant messages, Short-Message Service messages, etc.) which are still tunneled. It will also be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments in which such a distinction is made based on a different type of packet classification. 
     At task  401 , MAG  205  receives a request from node  201  to attach to the foreign network represented by network  203 - 1 , in well-known fashion. 
     At task  402 , MAG  205  attaches node  201  to network  203 - 1  in well-known fashion. Part of the attaching process comprises assigning a care-of address to node  201 , in response to the receiving of the request at task  401 . 
     In some embodiments, MAG  205  obtains one or more quality-of-service parameters that apply to a packet or packets to be exchanged between node  201  and node  202 , as described above and with respect to task  302 . 
     At task  403 , MAG  205  refrains from registering with server  208 , in contrast with the call flow depicted in  FIG. 3 . The refraining from registering has the effect of having any SIP signaling continue to be routed through local mobility anchor  204 . The refraining can be performed even when a SIP-related capability has been determined at task  402  to be present. 
     Later, when correspondent node  202  needs to exchange SIP signaling with mobile node  202  (e.g., to make a call, etc.), MAG  205  at task  406  receives a tunneled SIP INVITE message from LMA  204 , originated by node  202  at task  404  and intercepted by the local mobility anchor at task  405 . 
     At task  407 , MAG  205  forwards the received SIP INVITE message to node  201  in well-known fashion. 
     At task  408 , MAG  205  receives a 200 OK or ACK message from node  201 , with the node&#39;s home address in the session description. 
     At task  409 , MAG  205  replaces the home address in the session description of the received message, with the care-of address that it had assigned to node  201  at task  402 . In some embodiments, the replacing of the home address can be dependent on a quality-of-service parameter, as discussed above and with respect to task  308 . MAG  205  then forwards the modified 200 OK or ACK message to LMA  204  via tunneling. 
     LMA  204  at task  410  then forwards the tunneled message to server  208 , which then forwards the packet to node  202  at task  411 . When correspondent node  202  receives the message, it notes the care-of address that it sees in the session description section of the message, in order to use to for a direct exchange of media packets with MAG  205 . 
     Mobile node  201  and correspondent node  202  are subsequently ready to exchange media packets. At task  412 , MAG  205  receives media packets transmitted from node  202  with the assigned care-of address specified as the destination address in each packet. At task  413 , MAG  205  forwards the received media packets to node  201  based on the destination address in each packet being the care-of address of the node. 
     Likewise at task  414 , MAG  205  receives media packets transmitted from node  201 , in which node  201  has specified i) a source address equal to its home address and ii) a destination address equal to the correspondent node&#39;s home address. 
     At task  415 , MAG  205  replaces the home address in the source address field with the assigned care-of address in each media packet, and forwards the received media packets to node  202 , in accordance with the illustrative embodiment of the present invention. In some embodiments, the replacing of the home address can be dependent on a quality-of-service parameter, as discussed above and with respect to task  308 . 
     If node  201  moves into the domain served by a mobile access gateway (e.g., MAG  205 ) in the middle of the call, the new gateway sends a RE-INVITE message to correspondent node  202  on behalf of mobile node  201  with a new assigned care-of address as the new source address. The new gateway performs this in addition to conducting the routine message exchange for attaching the mobile node to the gateway. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.