Abstract:
The present invention advantageously provides several systems and methods for solving the trombone routing issues within an IMS/MMD network. These approaches avoid trombone routing, speed up handoff, and increase the efficiency of signaling and overall performance of an IMS/MMD network. These solutions can broadly be divided into the following categories. Piggy-backing SIP registration over MIP (Split at FA); Selective Reverse Tunneling and Tunneling between FA and P-CSCF; the SIP-based mobility protocol; use of CoA during SIP registration and call up in MIPv6; Piggy-backing SIP registration when HA and S-CSCF Co-exist; Using Dynamic Home Agents in MIPv4 FA-CoA; and the Interceptor-Caching Approach.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present invention claims the benefit of U.S. provisional patent application 60/841,784 filed Aug. 31, 2006, and of U.S. provisional patent application 60/844,955 filed Sep. 15, 2006, the entire contents and disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to IMS/MMD architecture, and more specifically to mitigation techniques for trombone routing encountered in IMS/MMD networks.  
       BACKGROUND OF THE INVENTION  
       [0003]     An IMS/MMD (IP Multimedia Subsystem/Multimedia Domain) network or architecture primarily comprises several signaling entities such as proxy-call session control function (P-CSCF), interrogating-CSCF (I-CSCF), serving-CSCF (S-CSCF), and home subscriber service (HSS) which is usually a database or other repository for user or subscriber information such as authorization data, including information related to services provided to a user. Roaming service and mobility are supported by a combination of Session Initiation Protocol (SIP) components such as the signaling entities, P-CSCF, S-CSCF, I-CSCF, and mobile IP components or nodes, such as home agent (HA) and foreign agent (FA). IMS/MMD architecture mandates that there should be security association (SA) between the mobile and P-CSCF. Secure Internet Protocol (IPSec) is one way of providing SA for signaling and media traffic.  
         [0004]     In IMS, authentication of a user, or user&#39;s equipment (UE), can be achieved using authentication and key agreement (AKA). Authentication is achieved between the UE, generally a mobile, and its home network even though the SIP messaging is transported over the Serving, or visiting, network. This allows home network-based control of access to IMS resources, while the visited network controls bearer resources over the packet data servicing nodes (PDSN). SIP Registration and Response messages are used to transport the IMS/AKA protocol payloads. These messages are sent from the UE to the S-CSCF and vice versa. The S-CSCF queries the HSS to obtain security related parameters for the UE. IMS AKA uses a challenge response mechanism to authenticate the UE to the Home Network. The UE uses a long term key to compute a response to a challenge sent by the S-CSCF via the P-CSCF. The P-CSCF plays no role in challenge generation aside from acting as a forwarding element.  
         [0005]     In an IMS/MMD network, the signaling and media travel to their destination, such as to S-CSCF, via the HA which usually resides on the home network. This long route or path from a Mobile Node or correspondent node (CN) to a destination through the home network is a phenomenon called trombone routing. Because trombone routing impacts both registration and call setup methods, trombone routing hinders efficiency during a mobile&#39;s boot-strapping (registration, re-registration, call setup) in the visited network and during its movement from one subnet to another. This causes both an initial call setup delay, and a handoff delay when the mobile moves from one visited subnet to another. In addition, during a mobile&#39;s movement between subnets, AKA is performed as part of registration; hence, a faster registration will help establish an expedited SA, thus reducing the handoff delay.  
         [0006]      FIG. 1  shows an example of the inefficiency associated with the trombone routing in the Mobile IPv4 (MIPv4) foreign agent-care-of address (FA-CoA) case. Here, even if the P-CSCF is situated in the same visited network as the mobile node (MN), the signaling related to registration has to traverse all the way to HA in the home network before getting routed to P-CSCF. This inefficiency is partly due to the reverse tunneling associated with the FA-CoA case. Similarly, any incoming call or INVITE signaling message from a CN traverses, via P-CSCF, to HA in the home network before being delivered to the Mobile Node in the visited network. This traversal increases the call setup delay. Since registration is delayed due to trombone routing, the handoff is also delayed as the mobile moves to a new network and sets up a new SA.  
         [0007]     Hence, as shown in  FIG. 1   a ), the path of a SIP registration message with trombone routing in MIPv4 FA-CoA is:  
         [0008]     MN→FA 1 →HA→P-CSCF→S-CSCF  
         [0009]     and the path of a SIP registration Reply message is:  
         [0010]     S-CSCF→P-CSCF→HA→FA 1 →MN.  
         [0011]     Similarly, the path of a SIP INVITE, as shown in  FIG. 1   b ), is:  
         [0012]     CN→S-CSCF→P-CSCF→HA→FA 1 →MN  
         [0013]     and the path of a SIP OK is:  
         [0014]     MN→FA 1 →HA→P-CSCF→S-CSCF→CN  
         [0015]      FIG. 2  shows trombone routing in the Mobile IPv6 (MIPv6) case, and illustrates how trombone routing affects the efficiency when MIPv6 is used. Unlike the MIPv4 case, MIPv6 does not use FA. While using MIPv6, it is customary to use the Mobile Node&#39;s home address in the contact field during the Session Initiation Protocol (SIP) registration and re-registration process even if Mobile Node obtains a new CoA from the access router during each handoff. Thus, during the re-registration process, a new P-CSCF&#39;s address is provided to the HSS, while the contact address of the Mobile Node remains same. HA, of course, keeps a mapping of Mobile Node&#39;s home address and its most recent CoA by means of MIP registration.  
         [0016]     Since there is no FA in the visited network in MIPv6, the mobile obtains the new CoA using stateless auto-configuration. When a mobile registers with S-CSCF in the home network, the mobile provides its home address as its contact address. Since there is a reverse tunneling between the mobile and HA, both the call setup and registration (re-registration) process are subjected to trombone routing.  
         [0017]     As shown in  FIG. 2   a ), the path of a SIP registration message with trombone routing in MIPv6 is:  
         [0018]     MN→HA→P-CSCF→S-CSCF  
         [0019]     and the path of a SIP registration Reply message is:  
         [0020]     S-CSCF→P-CSCF→HA→MN.  
         [0021]     Similarly, the path of a SIP INVITE, as shown in  FIG. 2   b ), is:  
         [0022]     CN→S-CSCF→P-CSCF→HA→MN  
         [0023]     and the path of a SIP OK is:  
         [0024]     MN→HA→P-CSCF→S-CSCF→CN  
         [0025]     Thus, just like the case of MIPv4, the trombone routing will affect the performance. As is evident from both of these cases, trombone routing is undesirable.  
         [0026]     Similarly, there is an inherent trombone routing problem with data or media, because the reverse tunneling is used by default.  FIG. 3  shows trombone routing associated with media delivery for both the MIPv6 without route optimization, and the MIPv4 FA CoA-based approach. In MIPv4, MIP data is tunneled between Visited  1  and Home, and then the data travels, non-tunneled, from Home to CN. In MIPv6, the data travels from MN, in Visited  1 , to CN in Visited  2 , through HA, Home, so that the data passes through the home network when traveling from visited Network  1  to visited Network  2 . Although reverse tunneling can offer advantages, this trombone routing contributes to the handoff delay because it necessitates traversing a long path via the home network.  
         [0027]      FIG. 4  illustrates another affect of trombone routing in MIPv4. When an IMS mobile node, MN, moves from network A to network B as shown in  FIG. 4 , the MIP registration and SIP re-registration must be completed at network B as follows. First, Mobile Node detects its mobility through the FA advertisement from the FA at network B. Once Mobile Node detects the mobility, it invokes a MIP registration through the FA and dynamic host configuration protocol (DHCP)-client operation to get the internet protocol (IP) address of the new P-CSCF at network B. At this point, the routing table of the Mobile Node has been updated through the MIP operation, and the tunnel between the FA and HA has been established, so that the Mobile Node can be reachable from any node in the network. After getting the IP address of the P-CSCF from the DHCP server at network B, the Mobile Node invokes a SIP re-registration by sending a SIP registration message to the new P-CSCF. The P-CSCF forwards the SIP message to the S-CSCF that, in turn, replies back to the P-CSCF with a SIP response message. Accordingly, the Mobile Node receives the SIP response message and the SIP re-registration is completed.  
         [0028]     In this handoff process, there are two issues. The first is slow handoff. As shown schematically in  FIG. 5 , the sequential operations of FA advertisement detection, MIP registration, DHCP, and SIP registration increase the handoff delay.  
         [0029]     The second issue is inefficient routing. Because of the reverse mode of tunneling between the FA and HA, the SIP messages between the Mobile Node and P-CSCF take the trombone routing path. Hence, as shown in  FIG. 6 , the path of a SIP message from the Mobile Node to the P-CSCF is:  
         [0030]     MN→FA→Gateway in Network B→Gateway in Home Network→HA→Gateway in Home Network→Gateway in Network B→P-CSCF  
         [0031]     A SIP message from the P-CSCF to the Mobile Node takes the reverse path.  
         [0032]     Thus, trombone routing causes inefficiencies and delays in both registration and handoff.  
         [0033]     The following abbreviations are used throughout. 
    AAA: authentication, authorization and accounting     AKA: authentication and key agreement     BSC: base station controller     BTS: base transceiver station     CDMA: code division multiple access     CN: correspondent node     CoA: care-of Address     DHA: dynamic home agent (aka mobility agent MA)     DHCP: dynamic host configuration protocol     DNS: domain name service     ESP: encapsulating security payload     FA: foreign agent     HA: home agent     HAA: Home-Agent-MIP-Answer     HAR: Home-Agent-MIP-Request     HHA: handover answer     HHR: handover request     HSS: home subscriber service     IMS: IP Multimedia Subsystem     IMS/MMD—combination of IMS and MMD     IPSec: suite of security protocols     MAC: message authentication code     MIPv4—Mobile IPv4     MIPv6—Mobile IPv6     MMD—Multimedia Domain     MN: mobile node     MPA: media independent pre-authentication     NAI: Network Access Identifier     PCF: packet control function     P-CSCF—Proxy Call Session Control Function     PDSN—Packet Data Serving Node     PPP: point to point protocol     RAN: radio access network     RTP: real-time transport protocol     SA: security association     S-CSCF—Serving Call Session Control Function     SIP: session initiation protocol     SRTP: secure real-time transport protocol     UE: user equipment     URI: Universal Resource Identifier    
 
       BRIEF SUMMARY OF THE INVENTION  
       [0074]     The present invention advantageously provides systems and methods for solving the trombone routing issues within an IMS/MMD network. These methods avoid trombone routing and increase the efficiency of signaling and overall performance of an IMS/MMD network.  
         [0075]     In one embodiment, a system and method for mitigating trombone routing in a MIPv4 FA-CoA network is presented in which the SIP registration message is attached to an MIP control message. The MIP control message, along with the attached SIP registration message, is transmitted to an application-specific relaying node that performs both MIP registration and SIP registration. In particular, the application-specific relaying node sends the SIP registration to the S-CSCF while simulaneously sending the MIP registration to the HA.  
         [0076]     In another embodiment, a system and method for mitigating trombone routing in a MIPv4 FA-CoA network using both tunneling and selective reverse tunneling is presented. In this approach, a bi-directional tunnel is created between FA and HA, an IP-IP tunnel is created from a mobile node to FA, and another tunnel is created from P-CSCF to FA. Packets in encapsulated delivery style are transmitted using the bi-directional tunnel from FA to HA and vice versa. Packets in direct delivery style are transmitted from mobile node to FA or from P-CSCF to FA.  
         [0077]     In another embodiment, a system and method for mitigating trombone routing using the SIP-base mobility protocol is presented. In this approach, when a mobile node bootstraps, it obtains its IP address either from a stateful DHCP server or from stateless auto-configuration, because there are no MIP entities such as HA or FA. Hence, the registration message and reply use the standard routing path, avoiding trombone routing.  
         [0078]     In another embodiment, a system and method for mitigating trombone routing in a MIPv6 network by using CoA in the mobile&#39;s registration message instead of the home address as the contact address is presented.  
         [0079]     In another embodiment, a system and method for mitigating trombone routing in a MIPv4 FA-CoA network having HA and S-CSCF on the same machine is presented. The SIP registration message is attached to an MIP control message. The MIP control message, along with the attached SIP registration message, is transmitted to the HA, and HA communicates with S-CSCF.  
         [0080]     In another embodiment, a system and method for mitigating trombone routing in a MIPv4 network having home agents, known as dynamic home agents, close the the mobile node&#39;s visiting networks is presented. Placing the home agents close to foreign agents minimizes the routing path.  
         [0081]     In another embodiment, a system and method for mitigating trombone routing in a MIPv4 and a MIPv6 network, a policy agent is added at the foreign agent. The policy agent decides whether to send the signaling via tunneling to the HA or directly to P-CSCF. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0082]     The invention is further described in the detailed description that follows, by reference to the noted drawings by way of non-limiting illustrative embodiments of the invention, in which like reference numerals represent similar parts throughout the drawings. As should be understood, however, the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:  
         [0083]      FIG. 1   a  illustrates a Trombone Routing in MIPv4 FA-COA during SIP Registration;  
         [0084]      FIG. 1   b  illustrates a Trombone Routing in MIPv4 FA-COA during SIP Invite;  
         [0085]      FIG. 2   a  illustrates a Trombone Routing in MIPv6 during SIP Registration;  
         [0086]      FIG. 2   b  illustrates a Trombone Routing in MIPv6 during SIP Registration and Call Setup;  
         [0087]      FIG. 3  illustrates a Trombone Routing for Media in MIPv4 and MIPv6;  
         [0088]      FIG. 4  illustrates a Mobile Host movement from one FA to another in MIPv4;  
         [0089]      FIG. 5  illustrates a typical Handoff Operational Sequence;  
         [0090]      FIG. 6  illustrates a Trombone Routing Path between MN and P-CSCF;  
         [0091]      FIG. 7  illustrates the operation of MIP and SIP Integration on Control Plane;  
         [0092]      FIG. 8  illustrates SIP and MIP Message Paths;  
         [0093]      FIG. 9  illustrates a typical deployment scenario of MIP and SIP Integration on Control Plane;  
         [0094]      FIG. 10  illustrates a schematic of Selective Reverse Tunneling;  
         [0095]      FIG. 11  illustrates a schematic of SIP Mobility that avoids trombone routing;  
         [0096]      FIG. 12   a  illustrates a schematic of the Selective Reverse Tunneling and CoA approach for MIPv6 during SIP Registration;  
         [0097]      FIG. 12   b  illustrates a schematic of the Selective Reverse Tunneling and CoA approach for MIPv6 during SIP Call Setup;  
         [0098]      FIG. 13  illustrates another Trombone Routing in MIPv4 FA-COA for SIR Registration;  
         [0099]      FIG. 14  illustrates another Trombone Routing in MIPv4 FA-COA for SIR INVITE;  
         [0100]      FIG. 15  illustrates Dynamic Home Agent Assignment;  
         [0101]      FIG. 16  illustrates Realization of Trombone Routing Mitigation for SIP INVITE; and  
         [0102]      FIG. 17  illustrates Realization of Trombone Routing Mitigation for SIP Registration Message. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0103]     In order to avoid the trombone routing and speed up the handoff, several solutions are presented that help mitigate the trombone routing effect. These can broadly be divided into the following:  
         [0104]     a) Piggy-backing SIP registration over MIP (Split at FA);  
         [0105]     b) Selective Reverse Tunneling and Tunneling between FA and P-CSCF;  
         [0106]     c) The SIP-based mobility protocol;  
         [0107]     d) Use of CoA during SIP registration and call up in MIPv6;  
         [0108]     e) Piggy-backing SIP registration when HA and S-CSCF Co-exist;  
         [0109]     f) Using Dynamic Home Agents in MIPv4 FA-CoA; and  
         [0110]     g) Interceptor-Caching Approach.  
         [0111]     Each of these solutions will be described below.  
         [0000]     A. Piggy-Backing SIP Registration Over MIP  
         [0112]     In this approach, MIP and SIP signaling on control plane are integrated, enabling SIP messages to be delivered as part of MIP control messages, bypassing the FA-HA tunnel. The transport of data is not affected. DHCP operation is dropped because the Mobile Node does not need the IP address of the P-CSCF. Instead, FA needs to know the IP address of the corresponding P-CSCF in advance, and, since FA and P-CSCF are stationary, the IP address of P-CSCF can be preconfigured in the FA. Accordingly, MIP and SIP registrations can be independent and their operations can perform simultaneously in parallel. Specifically, SIP registration does not need to wait until the routing paths for the SIP messages are completely established through the MIP.  
         [0113]      FIG. 7  illustrates the operation of MIP and SIP integration on a control plane.  FIG. 8  illustrates the route paths for both SIP and MIP registration messages.  FIG. 9  shows a typical deployment scenario of an integration of SIP and MIP.  
         [0000]     Operational Procedure of Piggy-Backing  
         [0114]     As shown in  FIG. 7 , the operation of the piggy-backing of SIP registration over MIP can be performed using the following steps. 
        1. MN  10  receives the FA advertisement  12  with a new P-CSCF address from the new FA  14 .     2. The MIP process (MIP-MN)  16  on the Mobile Node  10  informs the SIP User Agent (UA-SIP)  18  on the Mobile Node  10  of the detection of a new P-CSCF.     3. UA-SIP initiates a SIP registration by sending a signaling (e.g., sip_ua[p-cscf])  20  to the MIP-MN  16 .     4. MIP-MN  16  invokes MIP/SIP registration  22  with FA  14  through a MIP-specific messaging operation.     5. FA  14  invokes both MIP and SIP registration  24 ,  26  in parallel; the SIP registration message  24  is sent to P-CSCF  28 , and the MIP registration message  26  is sent to HA  30 . Note that here FA is not an IP layer forwarding node; instead, it plays a role as an application-specific relaying node.     6. The SIP registration message  24  is delivered to the S-CSCF  32 .     7. A SIP response message  34  is delivered to the P-CSCF  28 .     8. The P-CSCF  28  sends the SIP response message  34  to the FA  14  which is enabled to parse SIP messages; this SIP response message can be sent as an MIP message, UDP message, or SIP message.     9. FA  14  sends the SIP response message  34  to the Mobile Node  10  through MIP-MN  16 .     10. Finally, MIP-MN  16  informs the UA-SIP  18  of the notification of SIP response, and the SIP registration is completed.        
 
         [0125]     In this operating procedure, SIP messages from/to the Mobile Node to/from P-CSCF traverse according to the routes set by the regular IP routing not by MIP. Hence, no FA-HA tunnel is used for forwarding the messages.  
         [0126]     As shown in  FIG. 8 , the path of a SIP registration message  24  from the Mobile Node  10  to the P-CSCF  28  is:  
         [0127]     MN→Gateway in Network B→P-CSCF  
         [0000]     The path of a MIP registration message  26 , also shown in  FIG. 8 , from HA  30  to the Mobile Node  10  is:  
         [0128]     HA→Gateway in Home Network→Gateway in Network B→FA→MN  
         [0129]     This approach requires modifying both FA and the MIP client from their original configurations, for example, FA must be able to parse SIP messages. In addition, a filtering mechanism must be installed at the router. However, this approach offers a reduction in the number of signaling messages, and also enables two protocols, SIP and MIP, to be combined into one.  
         [0000]     B. Selective Reverse Tunneling and Tunneling Between FA and P-CSCF  
         [0130]     Another approach avoids trombone routing by using selective reverse tunneling and an encapsulation technique between the FA  14  and P-CSCF  28 .  FIG. 10  schematically illustrates selective reverse tunneling using one visiting network and one home network.  
         [0131]     As discussed above, the root cause of the trombone routing problem is the use of reverse tunneling at the FA  14 . The Internet Engineering Task Force (IEFT) protocol specifies Encapsulated Delivery style of packets between the Mobile Node  10  and FA  14 , wherein the Mobile Node  10  sets up a tunnel  36  to the FA  14 . The primary objective of this tunnel is to allow broadcast packets from the Mobile Node to be sent to their home network via reverse tunneling. In this approach, the IP header fields for packets received via the MN-FA tunnel  36  at the FA  14  are as follows: 
        Outer IP header: source=MN&#39;s home address, destination=FA&#39;s address     Inner IP header: source=MN&#39;s home address, destination=CN&#39;s address        
 
         [0134]     To minimize trombone routing problems, a means to leverage the Encapsulated Delivery style to perform Selective Reverse Tunneling is presented. This is intended to support packet delivery to local resources and can be used to optimize delivery to the P-CSCF in the visited network. In this case, the Mobile Node can request the FA to perform Selective Reverse tunneling as follows: 
        Packets meant to be reverse tunneled are sent using Encapsulated Delivery style via the MN-FA tunnel  36 . The FA  14  must reverse tunnel these to the HA  30 . The Mobile Node  10  can send all media packets using Encapsulated Delivery style of packets to ensure delivery to the CN  38  via the HA  30 .     Packets NOT meant to be reverse tunneled are sent using Direct Delivery style (not encapsulated). The FA will forward these and will not reverse tunnel them to the HA. Hence, the Mobile Node can send all packets meant for the P-CSCF using normal IP routing, because the FA will forward these as regular packets.        
 
         [0137]     Selective Reverse Tunneling with Encapsulated Delivery style of packets solves one part of the trombone routing problem by optimizing the route from the Mobile Node to the P-CSCF. However, packets from the P-CSCF to the Mobile Node will still be routed via the HA. The inefficiency and/or delay caused by this routing can be alleviated by establishing an IP-IP tunnel  40  between the P-CSCF  28  and the FA  14  for all packets destined for the Mobile Node  10  from the P-CSCF  28 . Using this approach, the IP header fields for packets received at the FA from the P-CSCF are as follows: 
        Outer IP header: source=P-CSCF&#39;s address, destination=FA&#39;s CoA     Inner IP header: source=P-CSCF&#39;s address, destination=MN&#39;s address        
 
         [0140]     The encapsulated packets received at the FA via the P-CSCF—FA tunnel  40  will be de-capsulated at the FA and forwarded to the MN. The de-capsulation is performed in a manner identical to that in which encapsulated packets received at the FA via the HA-FA tunnel are processed.  
         [0141]     The use of Selective Reverse Tunneling requires the following enhancements to the system architecture. 
        1) Establishment of MN-FA tunnel  36  for Encapsulated Delivery style of packets. This may be done after the Mobile Node  10  has registered with the FA  14 . In one embodiment, the tunnel establishment capability should be available in an RFC3024 compliant MN.     2) Use of Direct Delivery style for P-CSCF targeted packets at MN. This is generally an RFC3024 compliant capability. It can be set up after the Mobile Node registers with the FA and has received the P-CSCF address via DHCP. This requires the establishment of a P-CSCF specific route at the Mobile Node that bypasses the MN-FA tunnel.     3) Selective Reverse Tunneling at FA. This capability generally is RFC3024 compliant and should be activated after the Mobile Node registers with the FA.     4) Establishment of a bi-directional tunnel between P-CSCF and FA. This requires extending the P-CSCF&#39;s capability. The tunnel should be established after the SIP registration message (e.g., REGISTER) is received at the P-CSCF via the FA. In addition, a routing table entry should direct all packets to the Mobile Node via this tunnel. This will ensure delivery of SIP replies via the tunnel.        
 
         [0146]     This approach is somewhat complex because of the additional tunnels and the overhead they require. Like the piggy-backing approach, a filtering mechanism is needed at the router. This approach makes use of standard system features and does not necessitate any changes to the MIP protocol.  
         [0000]     C. The SIP-Based Mobility Protocol  
         [0147]     A third approach to avoid trombone routing is to use the SIP-based mobility protocol.  FIG. 11  illustrates a schematic of SIP registration and SIP Call Setup in SIP-based mobility, avoiding trombone routing.  
         [0148]     In SIP-based mobility, the Mobile Node  10  does not use an MIP entity or mobile IP component for providing mobility binding. Thus, there is no HA or FA, nor any equivalent therefore. When the Mobile Node bootstraps, i.e. boots up, in a visited network  42 , Mobile Node must re-register and, if any session parameters have changed, Mobile Node must also re-INVITE. When the bootstrapping occurs, Mobile Node  10  obtains its IP address either from a stateful DHCP server, or by means of stateless auto-configuration. While obtaining its IP address, Mobile Node receives additional server configuration information, including the address of P-CSCF  28 , from the DHCP server (not shown), typically using DHCP INFORM.  
         [0149]     As Mobile Node  10  sends a registration message to S-CSCF  32 , it sends the new CoA as the new contact address and the address of P-CSCF in the network&#39;s subnet. Thus, at any point in time, HSS knows the new contact address of the mobile and its corresponding P-CSCF address. Since there is no HA or MIP, the registration message and reply follow the standard routing path, and neither are subjected to trombone routing.  
         [0150]     When a caller generates a new call or INVITE to the Mobile Node, the call is routed to S-CSCF using the Mobile Node&#39;s Universal Resource Identifier (URI). When the S-CSCF gets this new call, S-CSCF consults the registration database, and routes the call to the P-CSCF responsible for that Mobile Node. Since the contact address of the Mobile Node is still the new CoA obtained in the new network, P-CSCF looks up the contact address and forwards the call to the mobile using standard routing process. Thus trombone routing is avoided for both the call setup and the registration process. Using this procedure, delay during re-registration procedure is lessened, reducing the handoff delay during a Mobile Node&#39;s movement from one subnet to another.  
         [0151]     This approach supports only SIP-based applications such as VoIP, streaming, and chat, and, at present, is not yet standardized. However, no protocols, such as MIP or SIP, need to be changed to use this approach, and standard SIP signaling is used.  
         [0000]     D. Using CoA During SIP Registration and Call Up in MIPv6  
         [0152]     Next, a method for avoiding trombone routing by using CoA instead of the home address as the contact address in the Mobile Node&#39;s registration message is presented.  FIG. 12  shows a schematic diagram illustrating how trombone routing can be avoided using MIPv6 with CoA during SIP Call Setup.  
         [0153]     While it is mandatory that the media between CN  38  and Mobile Node  10  must travel via the HA  30 , having the signaling traverse through HA  30  may not be necessary. Hence, registration with CoA as the contact address in S-CSCF  32  could alleviate some of the trombone routing problems that are often observed in typical MIPv6 networks.  
         [0154]     The operation of this approach is as follows. While in a visited network, a Mobile Node sends a request to register, and receives a registration address that can be used as the Mobile Node&#39;s CoA, instead of HA as CoA. This CoA is stored in S-CSCF. Hence, when a call is placed to this Mobile Node, S-CSCF finds this mobile node using the stored CoA, and the request does not need to be transmitted to the HA to obtain a CoA for this Mobile Node. Hence, trombone routing is avoided in call setup.  
         [0155]      FIG. 12   a ) illustrates the path of a SIP registration message in MIPv6 to be:  
         [0156]     MN→P-CSCF→S-CSCF  
         [0157]     and the path of a SIP registration Reply message is:  
         [0158]     S-CSCF→P-CSCF→MN.  
         [0159]     Similarly, the path of a SIP INVITE, as shown in  FIG. 12   b ), is:  
         [0160]     CN→S-CSCF→P-CSCF→MN  
         [0161]     and the path of a SIP OK is:  
         [0162]     MN→P-CSCF→S-CSCF→CN  
         [0163]     This approach is limited to MIPv6 in which CoA is used, as opposed to MIPv4 which requires FA and CoA. Standard system features can be used and no protocol changes are necessary for implementation of this approach.  
         [0000]     E. Piggy-Backing SIP Registration when HA and S-CSCF Co-Exist  
         [0164]     This method borrows some of the concepts from the approach of Piggy-backing SIP registration over MIP, discussed above. In this situation, HA  30  and S-CSCF  32  co-exist on the same machine, and HA  30  has a binding cache. SIP registration URI and P-CSCF address is sent as part of the MIP update. As with the piggy-backing method discussed above, SIP registration information is attached to the MIP message, and all the SIP related registration information is sent as part of MIP binding update. However, the MIP/SIP message does not get split at FA  14  as with the prior piggy-backing approach. Instead, HA  30  passes the SIP related messages to the logical entity S-CSCF  32  that resides on the machine with HA  30 . Trombone routing is avoided by using Inter Process Communication instead of passing signals over the long distance.  
         [0165]     Operation of this approach is as follows. The SIP registration URI and P-CSCF address is attached to or bound with the MIP registration message. This MIP message is sent via FA to HA which passes the SIP registration information using Inter Process Communication.  
         [0166]     This approach requires that both HA and S-CSCF co-exist on the same machine, and the technique offers reduction of signaling and parallelization of processes.  
         [0000]     F. Using Dynamic Home Agents in MIPv4 FA-CoA  
         [0167]     As discussed above, the root cause of the trombone routing problem is that, without MIPv4 route optimization, every packet needs to traverse through the HA  30 . Accordingly, another approach to mitigating the problem of such additional routing can be found by introducing Home Agents close to Mobile Node&#39;s visiting networks. These HAs are commonly known as Dynamic Home Agents (DHA)  44 , or Mobility Agents (MA)  44 . Dynamic home agent assignments are supported by the MIPv4 protocol. By placing Home Agents  30  close to Foreign Agents  14 , one can minimize the routing path to a great extent.  
         [0168]      FIGS. 13 and 14  depict the scenario whereby MAs  44  are deployed in the visited core network  46  of MIPv4 FA-CoA, thereby reducing the path traversal for both signaling and media. Hence, as shown in  FIG. 13 , the path of a SIP registration message with MA (or DHA) in MIPv4 FA-CoA is:  
         [0169]     MN→FA 1 →MA→P-CSCF→S-CSCF  
         [0170]     and the path of a SIP registration Reply message is:  
         [0171]     S-CSCF→P-CSCF→MA→FA 1 →MN.  
         [0172]     Similarly, the path of a SIP INVITE, as shown in  FIG. 14 , is:  
         [0173]     CN→FA 1 →MA→P-CSCF→S-CSCF→MN  
         [0174]     and the path of a SIP OK is:  
         [0175]     MN→S-CSCF→P-CSCF→MA→FA 1 →CN  
         [0176]     It is important to note that it is not necessary to deploy MA  44  in every subnet. Depending upon the topology and size of the visiting network  42 , multiple MAs can be deployed. However, one MA  44  can handle multiple FAs  14  since it will be usually placed one level higher than subnet level. Several mobility optimization protocols published in the literature, such as Regional Registration, IDM, use the same concept of Dynamic Home Agent. In the sections below, DHA assignment procedures, registration procedure with the old HA, functionalities of MA, packet handling procedures at MA, and dynamic DNS update are described.  
         [0177]     Dynamic Home Agent Assignment Procedures  
         [0178]     For DHA assignment, FA in the visiting network must include the ‘D’ flag in the agent advertisement message. The ‘D’ bit occupies the first reserved bit after the other flag bits specified in RFC 3220. FA must also include its Network Access Identifier (NAI) in the agent advertisement message. By comparing the advertising FA&#39;s NAI with its own NAI, Mobile Node can determine whether or not it is in its home domain.  
         [0179]     The signaling procedures and message exchanges for DHA assignment are shown in  FIG. 15 , including the detailed call flow. The operation of these procedures and message exchanges is as follows: 
        1. When the Mobile Node  10  arrives on the visiting network  42 , Mobile Node  10  sends a dynamic HA handover request (HHR) message to the FA  14 .     2. After receiving this message, FA  14  constructs an AA-Mobile-Node-Request (AMR) message in a pre-defined format, and sends AMR to the serving Authentication, Authorization, and Accounting (AAA)  48  server.     3. The AAA server  48  authenticates the Mobile Node  10  and assigns it an MA  44 . The AAA server  48  then sends a Home-Agent-MIP-Request (HAR) message to the assigned MA  44 .     4. The MA  44  will then assign a new home address to the Mobile Node  10  and return this new address in a Home-Agent-MIP-Answer (HAA) message to the AAA server  48 .     5. AAA server  48  will then send an AA-Mobile-Node-Answer (AMA) message to the FA  14 .     6. Subsequently, Mobile Node  10  will receive a dynamic HA handover answer (HHA) message from the FA  14 .        
 
         [0186]     Hence, the signaling message sequence is:  
         [0187]     HHR→AMR→HAR→HAA→AMA→HHA.  
         [0188]     In one embodiment, the HHR message can be defined as the Registration Request message in RFC 2002, but with the following changes:  
                                       Type   4 (Dynamic HA Handover Request)       Home Address   The current address for simultaneous HA bindings,           otherwise           null address (0.0.0.0)       HA Address   null address (0.0.0.0)       CoA   Care of address of FA       Extension   Mobile Node NAI Extension                  
 
         [0189]     The Mobile Node must include its NAI or fully qualified domain name (FQDN) in the extension for authentication purpose. The Mobile Node must set ‘S’ bit in its HHR message if it requires simultaneous HA bindings.  
         [0190]     On the other hand, if Mobile Node obtains a temporary address from either DHCP or point to point protocol (PPP) before the HA assignment and wants to use this temporary address as its new home address, Mobile Node must set the home address field in the HHR message to be this dynamically allocated temporary address. When FA sends the AMR message to the AAA server, the Mobile-Node-Home-Address-Requested flag in the MIP-Feature-Vector AVP must be set to zero to indicate that no further home address needs to be assigned to the Mobile Node.  
         [0191]     Registration Procedure with Old HA  
         [0192]     For seamless handover, Mobile Node must send a registration request message to the old HA. The registration is sent directly to the old HA with the fields as specified below:  
         [0193]     Home Address: the old Home Address  
         [0194]     HA Address: the old HA Address  
         [0195]     CoA: the new Home Address (e.g., MA Address)  
         [0196]     Since the CoA of the old home address is set to the new home address, all the packets destined to the old home address will be redirected to the new home address of the Mobile Node by the old address after successful registration. The MA will then intercept all packets destined to the Mobile Node and forward them to the current location of the Mobile Node.  
         [0197]     Functionalities of Mobility Agent  
         [0198]     As mentioned earlier, Mobility Agent (MA) is a home agent that is dynamically assigned and has similar functionalities to an HA. For example, MA accepts the home registration request from the Mobile Node with the old HA. MA also has some behaviors like FA. For example, after receiving the home registration request, MA relays the request to the old HA. However, MA does not broadcast any FA advertisement and also does not provide any CoA address to the Mobile Nodes. Typically, MA will be placed one level higher than FA.  
         [0199]     Packet Handling Procedures at MA  
         [0200]     The MA should maintain two user lists. One user list is for normal HA function and the other is for seamless HA handover. For seamless handover, MA keeps a binding list that has &lt;MN_Old_Home_Addr, MN_New_Home_Addr&gt; information. For seamless session, MA will receive packets that are encapsulated by the old HA. For encapsulated packets, MA compares the inner destination address of the encapsulated packet with MA&#39;s binding list and, if there is a match, MA determines that it has received a packet whose inner destination address is the old home address. Then MA can de-capsulate the encapsulated packet or datagram, and tunnel it to Mobile Node&#39;s current location, that is, re-capsulating the datagram with the current CoA of the Mobile Node. For un-encapsulated packets destined to new home address of Mobile Node, MA should function like a normal HA of the Mobile Node.  
         [0201]     In case of security association, MA needs to maintain two SAs: i) one is with the Mobile Node (a.k.a. MA-MN) employing the MN-HA registration key, and ii) another one is old HA (a.k.a. MA-OHA) employing the FA-HA registration key. During subsequent moves, for example, when Mobile Node changes the FA but does not change the MA, Mobile Node should send the registration request to the MA with Mobile Node&#39;s new CoA. When MA receives such registration request, MA should verify the previous home address in its HA binding list and respond with the registration reply accordingly.  
         [0202]     Dynamic DNS Update  
         [0203]     Mobile Node should perform a secure dynamic DNS update with its Authoritative Domain name Server (ADS) to update Mobile Node&#39;s name bindings after it gets a new home address. An alternative approach could be that a DHCP server can update the DNS if Mobile Node uses the new home address allocated by the DHCP server. By setting the ‘S’ bit in the DHCP_REQUEST, Mobile Node can delegate the DNS update to the DHCP server. In many cases where security is a concern for Mobile Node updating the DNS, delegating the DNS update may be a better approach.  
         [0000]     G. Interceptor-Caching Approach  
         [0204]     Yet another approach to trombone routing mitigation involves minimal changes to the FA and the mobile. Assuming reverse tunneling is mandatory for this approach, a policy agent or interceptor  50  at respective FA  14  is added. This policy agent  50  will snoop the incoming traffic and, based on the port number, the policy agent  50  will decide whether to send the traffic to the encapsulation agent or route it directly to P-CSCF  28 . For example, if SIP signaling is usually carried over port  5060 , then the policy agent  50  will have the ability to capture the packets, inspect each one, and selectively send these packets either to the encapsulating agent for tunneling it to HA  30  or send these packets directly to P-CSCF  28 . Thus, any traffic other than SIP traffic will be sent back to HA  30  via reverse tunneling. This may include the media traffic as well. Hence, SIP related signaling such as REGISTER and INVITE messages will also traverse to P-CSCF  28  without being tunneled via HA  30 , avoiding trombone routing. However, media traffic is a separate issue and needs to be addressed accordingly.  
         [0205]     By virtue of reverse tunneling, any traffic destined to the Mobile Node  10  goes to HA  30  and gets tunneled to FA  14  before being delivered to P-CSCF  28 . In this case, response messages for SIP REGISTER and SIP INVITE that are destined to Mobile Node  10  via P-CSCF  28  will also need to traverse to HA  30  before being intercepted by FA  14 . As discussed above, this situation also adds an extra traversal between P-CSCF  28  and HA  30 . In order to alleviate this problem, a caching functionality  52  at P-CSCF is introduced to map Mobile Node&#39;s HA  30  with the FA  14 , and any message with a certain port number (e.g.,  5060 ) that is destined to Mobile Node&#39;s HA  30  will be routed to FA  14  instead. The caching functionality at FA (not shown) and the dynamic routing ability will help route the packets of certain types destined to Mobile Node  10  via FA  14  instead of sending it to HA  30 , as it does normally.  FIGS. 16 and 17  illustrate trombone routing mitigation including some of the additional modules, e.g. interceptor  50  and caching agent  52 , needed at the FA  14  and P-CSCF  28  as well as the protocol interaction between FA  14  and P-CSCF  28 .  
         [0206]     While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the below claims.