Patent Publication Number: US-7908475-B2

Title: Method and apparatus for transferring a communicaton session

Description:
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records) but otherwise reserves all copyright rights whatsoever. 
     REFERENCE TO A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISC 
     The computer program listing appendix contained within file “code_listing.txt” on compact disc “1 of 1”, which has been filed with the United States Patent and Trademark Office in duplicate, is hereby incorporated herein by reference. This file was created on May 23, 2001, and is 56 KB in size. The appendix describes an alternate embodiment of the invention discussed below in the Detailed Description of the Preferred Embodiments. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to a method and apparatus for transferring a communication session from a first destination device to a second destination device in a computer network with minimal disruption to the communication session, and more particularly to a method and apparatus for transferring a communication session between a Transferring Node and a Correspondent Node in an Internet Protocol (IP) based network from the Transferring Node to a Target Node without disrupting the communication session with respect to the Correspondent Node. 
     Over the past decade, the number and size of computer networks, such as the Internet, Intranets, Wide Area Networks (WAN&#39;s), and Local Area Networks (LAN&#39;s), have increased dramatically. In addition, the number of users who access these networks from various locations over the network has grown. For example, many users are accessing their networks from other office buildings on and off corporate campuses, from their homes, from client offices, etc. The increase in the number and size of networks, as well as the increase in the number of mobile users has introduced several problems including the inability to provide uninterrupted access to the network for Mobile Nodes. One of the main problems confronted has been the inability to allow a user to disconnect from one IP sub-network in a large IP network and reattach to the large IP network from another sub-network without interrupting ongoing network communication sessions. For example, users may be required to manually change IP addresses and reinitialize sessions when they move about the network in order to be recognized as an authorized network user or to access their network files. A solution to this problem was presented in the form of an extension to the Internet Protocol called Mobile IP. 
     Mobile IP is a recently standardized method for enabling what this description terms Session Inter-Subnet (SIS) mobility in IP networks. SIS mobility refers to the ability of a device to disconnect from an IP network such as the Internet and reattach to the IP network on another sub-network without interrupting ongoing sessions. Mobile IP is documented in Internet Engineering Task Force (IETF) Request For Comment (RFC) 2002, 2005, and 2006. Textbooks such as “Mobile IP: The Internet Unplugged” ISBN 0-13-856246-6 by James D. Solomon also document Mobile IP. Mobile IP is one of the dominant IETF standards in mobility and operates as a highly secure and dynamic packet data device mobility service. It provides a network layer solution for moving active sessions with a device as the device moves between IP sub-networks. Therefore, the user does not have to manually change an IP address and reinitialize sessions when moving around within the IP network, nor do Correspondent Nodes need to take action to maintain communication with Mobile Nodes. 
     According to Mobile IP, a network connection can be broken down into a Correspondent Node and a Mobile Node connected via an IP network. The Correspondent Node is a node located elsewhere on the IP network that is currently communicating with another node called the Mobile Node. When the Mobile Node is at its home location, in other words directly attached to its home sub-network, it receives communications from the Correspondent Node via routers that direct the communications to a home router, which in turn directs this information to the Mobile Node. During this active communication session, the Mobile Node may wish to move to a different sub-network, called a foreign network, while continuing to maintain the active session throughout the move. In order to do so without disrupting the Correspondent Node and the current communication session, the Mobile Node must continue to be reachable via the IP address it has been using throughout its communication session with the Correspondent Node. This task is accomplished in Mobile IP by use of a Home Agent. 
     The Home Agent (HA) intercepts communications meant for the Mobile Node and forwards these communications to the Mobile Node wherever it currently is located. The HA accomplishes this by use of a low-level protocol within the Internet Protocol (IP) suite called Address Resolution Protocol (ARP). ARP associates or “binds” the physical address of a node on the local sub-network with its IP address. Multiple IP addresses can be associated in this fashion with a single physical address. By using ARP, the HA associates its physical address with the IP address(es) of the Mobile Node(s). Following this association, the HA will receive traffic (or data) sent from the Correspondent Nodes to the Mobile Node&#39;s home address and the Mobile Node will register a “care of” address with the HA identifying its current location. The Mobile Node keeps the HA aware of its current care-of address by sending registration messages to the HA. The HA then forwards communications to the “care-of” address on the foreign network via an IP tunnel. Various types of IP tunnels exist in the art (IETF RFC&#39;s 1701, 2003, 2004), but each is essentially used to encapsulate the original communication as an inner IP packet (inner packet) or payload section of another larger IP packet (outer packet) which is addressed from the tunnel starting point to the tunnel endpoint. The tunnel endpoint node removes the outer packet header information, restoring the original packet (or communication). 
     One possible endpoint for the tunnel carrying forwarded packets for the Mobile Node from the Home Agent is a Foreign Agent (FA). The FA is a router with at least one interface on the Mobile Node&#39;s current foreign sub-network. Upon receipt of the tunneled packet, the Foreign Agent can provide a care-of address for the Mobile Node so the packet will be properly delivered. The FA may also de-tunnel packets forwarded by the Home Agent for the Mobile Node by removing the outer packet header, thus restoring the packets back to the packets originally intercepted by the Home Agent. The FA then sends these de-tunneled packets directly to the Mobile Node on its current foreign sub-network via the Mobile Node&#39;s link-layer address. 
     Another possible endpoint for the tunnel from the Home Agent is the Mobile Node itself. In this case, the Mobile node may acquire a temporary IP address on the foreign sub-network. This temporary address, called a co-located care-of address, functions as the care-of address for the Mobile Node while connected to the foreign sub-network. The Mobile Node may acquire this address by any means for generating IP addresses known in the art such as a Dynamic Host Configuration Protocol (DHCP) which is documented in IETF RFC 2131. Using this method of acquiring an address, the Mobile Node contacts a DHCP server requesting an unused IP address on the Mobile Node&#39;s current foreign sub-network. DHCP is increasingly common on current IPv4 networks, and future networks employing IPv6 will allow for nodes to dynamically create IP addresses without needing to acquire the addresses from a server. Thus, co-located care-of address generation is an increasingly viable option for receiving tunneled packets from the Home Agent, freeing the Mobile Node from the constraint of only visiting foreign sub-networks that have FA&#39;s. 
     The operation of Mobile IP outlined above was designed to enable a specific type of mobility, but there remain other types of mobility that have not been addressed. Mobile IP partially fills the mobility gap in the IP protocol suite by accommodating device movement between IP network attachment points or sub-networks while maintaining all of the device&#39;s IP communication sessions. For example, Mobile IP enables a notebook PC to move from its office Ethernet connection to a wide-area wireless packet data network with all of its network communications sessions with multiple Correspondent Nodes in tact. This Session Inter-Subnet (SIS) mobility has become increasingly common, thereby increasing the need for Mobile IP. However, Mobile IP does not address the problem of moving active communication sessions between different devices. For example, a user may wish to transfer a video conference from a fixed desktop machine to a notebook PC or Personal Digital Assistant (PDA) as the user leaves the office in order to continue participating. This description terms moving active IP networking sessions between devices such as in the above example “Session Inter-Device” (SID) mobility. 
     Networks other than IP networks provide transfer features and can enable movement of active communication sessions between devices. However, these networks do not address the variety of media and devices present in IP networks. For example, the International Telecommunications Union (ITU) H.323 protocol suite provides for call transfer based on the traditional telephony model and can accommodate transfer of multimedia sessions, and the IETF Session Initiation Protocol (SIP) family (IETF RFC 2543) with proposed extensions (IETF Draft “SIP Call Control Transfer” driaft-ietf-sip-cc-transfer-02) provides for call transfer accommodating multimedia sessions as well. However, both of these approaches operate at a level often requiring modification to applications, including applications running on Correspondent Nodes. Furthermore, traditional telephony call transfer methods involve a gap or “hold” in the communication session while the call is redirected. 
     Thus, there is a need for a system and method capable of providing selective transfer of active communication sessions between nodes on an IP network from one end device to another end device with minimal disruption of the selected and unselected communication sessions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description of the preferred embodiments and upon reference to the drawings, in which. 
         FIG. 1   a  is a block diagram of an IP based network system showing a communication session between a source device and a first destination device that can be transferred to a second destination device on a different sub-network where all communication sessions intended for the first destination device are transferred to the second destination device, 
         FIG. 1   b  is a block diagram of the network system of  FIG. 1   a  wherein both the first and second destination devices are on the same sub-network, 
         FIG. 1   c  is a block diagram of a Mobile IP based network system that allows a single destination device to maintain active sessions without interruption despite relocation within the network system, 
         FIG. 2   a  is a block diagram of an IP based network system showing a communication session between a source device and a first destination device that can be transferred to a second destination device on a different sub-network whereby the session(s) associated with an IP address of the first destination device are transferred to the second destination device; 
         FIG. 2   b  is a block diagram of the network system from  FIG. 2   a  wherein both the first and second devices are on the same sub-network; 
         FIG. 3   a  is a block diagram of an IP based network system showing a communication session between a source device and a first destination device that can be transferred to a second destination device on a different sub-network whereby selected sessions associated with an IP address of the first destination device are transferred to the second destination device; and 
         FIG. 3   b  is a block diagram of the network system from  FIG. 3   a  wherein both the first and second devices are on the same sub-network. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, network systems and methods are provided for transferring a communication session from a first destination device to a second destination device with minimal disruption. This transfer, which this description terms Session Inter-Device mobility or SID mobility, can be accomplished by transferring from the Transferring Node (or first destination device) the IP address(es) associated with active session(s) so that the network system will route the session communication(s) with the Correspondent Node(s) to the desired Target Node (or second destination device). The address can be transferred to the Target Node or to an intermediary agent, as will be discussed further hereinafter. 
     According to the SID mobility network system and method of  FIGS. 1   a  and  1   b , the Transferring Node participates in a communication session with a Correspondent Node using its IP address as the destination for communication with the Correspondent Node. When a decision is made to transfer the session from the Transferring Node to a Target Node, the Transferring Node transfers its IP address to the Target Node and stops receiving data packets addressed to its transferred IP address. The Target Node then requests and receives packets addressed to the transferred IP address of the Transferring Node. While the transfer is taking place and after it has been completed, the session with respect to the Correspondent Node continues without interruption. 
     In network systems where it is desired for the Transferring Node to retain the connectivity or capability of participating in additional sessions (e.g., ability to communicate with other Correspondent Nodes), an alternate form of a SID mobility network system is provided ( FIGS. 2   a - b ) in which a separate session specific IP address is used for each communication session. Once it has been determined that a Transferring Node and a Correspondent Node want to participate in a session together, the Transferring Node obtains a session specific IP address such as from an address generator. The Correspondent Node is then made aware of the session specific IP address of the Transferring Node and a session is initiated between the Correspondent Node and the Transferring Node using the session specific IP address. When a decision has been made to transfer the session from the Transferring Node to a Target Node, the session specific IP address is transferred to the Target Node and the Transferring Node stops receiving packets addressed to the session specific IP address while the Target Node begins receiving packets addressed to the session specific IP address. While the transfer is taking place and after it has been completed, the session with respect to the Correspondent Node continues without interruption. In addition, the Transferring Node can participate in sessions with additional Correspondent Nodes, each session using its own session specific IP address. 
     In networks requiring efficient use of network addresses (e.g., networks with limited numbers of addresses or networks where addresses are expensive), an alternate SID mobility network system is provided ( FIGS. 3   a - b ) wherein a single temporary IP address per Transferring Node is required to transfer a plurality of selected active sessions from a Transferring Node to a plurality of Target Nodes. When a decision has been made to transfer a session from the Transferring Node to a Target Node, the Transferring Node gains a temporary IP address while its original IP address is transferred to a Session Agent. The packets for the transferred session are no longer received by the Transferring Node but are instead tunneled by the Session Agent to the IP address of the Target Node while all of the other session packets are tunneled to the temporary IP address of the Transferring Node. The Session Agent accomplishes the forwarding of packets by receiving all packets for the original IP address of the Transferring Node and sorting them based on session specific information before tunneling them to different destination devices. This allows the Transferring Node using its temporary IP address to continue its participation in existing sessions, allows the Transferring Node to create new sessions, and allows the Target Node to receive packets for transferred sessions. While the transfer is taking place and after it has been completed, the sessions with respect to the Correspondent Nodes continue with minimal interruption. 
     Several instances exist in which the ability to transfer a session from a Transferring Node to a Target Node with minimal interruption would be advantageous. For example, a need exists for a SID mobility network system enabling a network system user the ability to transfer a video call from a desktop personal computer (PC) to another device, such as a laptop PC, a video equipped personal digital assistant (PDA), or a handheld videophone. This would enable the network system user the ability to participate in a video call while it is moved from a Transferring Node to a Target Node. One example of when such movement could become necessary is if the network system user was participating in the video call on a battery-powered device running low on battery power. The network system user could transfer to a line-powered device in order to conserve battery power and possibly improve the quality of service. 
     In another example a customer service representative on one networked device may want to forward the active communication session to a customer service technician on another device without losing the information the customer service representative has gathered thus far, (e.g., an order which was placed for the customer, the customer&#39;s name, product serial number, credit card number, etc.). Later the technician may transfer the session including the call and the database information to another technician for additional help, avoiding the need to ask the customer for this information again. 
     In yet another example, a video monitoring system may be installed at a hospital to monitor current patient conditions so that immediate diagnosis may be provided from a doctor if needed. For example, if an emergency situation occurred, the video session could be transferred to the nearest networked device to the doctor&#39;s current location so diagnosis and treatment could be made immediately. Other possible usage scenarios involve interactive and team based gaming, job sharing, and medical patient monitoring. The above examples require application state information transfer that will not be discussed wherein. In general, a SID mobility network system is most useful to a user requiring the need to transfer a session from a Transferring Node (or first destination device) to a Target Node (or second destination device) without establishing a new session and requiring minimal interruption of provided services. 
     The forms of the SID mobility network systems and methods of operation enable SID mobility in IP-based networks, thereby enhancing the capabilities of any IP-based product. Within the IP protocol suite, a “flow” can be defined as a sequence of packets having a common addressing 5-tuple consisting of the following: source IP address, source port number, destination IP addresses, destination port number, and transport layer protocol (e.g., TCP or UDP). A “session” is a communication involving one or more flows between the same two devices and related to the same application. For example, a video call session may consist of separate flows in both directions (from sender to receiver and vice versa) for audio and video information. One possibility for transferring a session from a first destination device (Transferring Node) to a second destination device (Target Node) is to change the 5-tuple of each pertinent flow to reflect a port and address for the Target Node instead of the Transferring Node. The IP protocol suite, however, assume the 5-tuple remains constant throughout the duration of the session. Therefore, there is no mechanism for changing the 5-tuple in the IP protocol suite without interrupting the flow. It may be possible to modify the IP protocol suite to allow such a change, however, such modifications would be extensive and difficult to deploy in legacy systems. A brief overview of the previously mentioned Mobile IP standard is given in the next paragraph for the purpose of contrasting the described embodiments and is followed by an overview and then a more detailed description of the preferred embodiments. 
     In  FIG. 1   c , a block diagram of a Mobile IP network system is shown generally at reference numeral  10 . The Mobile IP network system  10  allows a single mobile device  12  to move from its home subnet  14  to a foreign subnet  16  while continuing to receive data sent to its home address. This is accomplished in-part by having a Home Agent (HA)  18  on the home subnet  14  intercept the IP packets (or communications) addressed to the mobile device  12  from a corresponding device (or Correspondent Node)  20 . The HA  18  tunnels the communications to the device  12  (now on the foreign subnet  16 ) using a “care-of” address, which the device has obtained and registered with the HA  18 . The “care-of” address may be a foreign agent care-of address, which is obtained from a Foreign Agent (FA)  22  on the foreign subnet  16 , or a co-located care-of address, which is obtained using some other means, (e.g., via a DHCP server). This scheme, however, creates the possibility of denial-of-service attacks where a foreign device on a foreign subnet registers with the HA  18 , posing as the mobile device  12  from the home subnet  14  which has moved, thereby stealing packets or communications intended for the mobile device  12 . Mobile IP includes security measures in an attempt to prevent this type of attack. In particular, the HA  18  only accepts registration requests from a mobile device  12  which the HA  18  can authenticate as originating from a legitimately relocated mobile device  12 . This authentication is accomplished using a security association between the HA  18  and the mobile device  12  based on secret key encryption. 
     The Mobile IP network operation of having a HA  18  intercept packets sent to a device  12  whose home location is on the HA&#39;s subnet  14  and tunneling them to that device  12  when it has relocated on a foreign subnet  16 , (discussed above regarding  FIG. 1   c ), can be reused to help enable SID mobility. However, in order to achieve SID mobility, the underlying Mobile IP concept of only transferring the communications to the actual mobile device  12  located on the foreign subnet  16  must be changed so that the receiving device on the foreign subnet (second destination device, or Target Node) can in fact be different than the device on the home subnet (first destination device, or Transferring Node) to which the packets&#39; were addressed. In doing so, any open (or selected) communication sessions with the Transferring Node will be transferred to the Target Node, thus enabling the network user to transfer a session from a first destination device to a second destination device with minimal interruption, (i.e. SID mobility). In one sense, this approach can be viewed as legitimizing the denial-of-service attack described above, but only by a Target Node which is implicitly trusted by the Transferring Node. If the Target Node were not trusted, then the HA would not forward the session. In order to accomplish this, a new method is required for establishing a security association between the Target Node and the HA so that the Target Node can register with the HA as though it were actually the relocated home device (i.e. Transferring Node). 
     Turning now to  FIG. 1   a , in which a SID mobility IP network system is shown generally designated with reference numeral  50 . The network  50  includes a Correspondent Node (or device)  52 , a home subnet  54  comprising a Transferring Node (or first destination device)  56   5  having a Transferring Node IP address  58  and a Agent  60 , a home IP router  62 , a foreign subnet  64  including a Target Node (or second destination device)  66  having a Target Node IP address  68 , and a foreign IP router  70 . As will be discussed in further detail below, a method is provided for enabling SID mobility in the network  50  which incorporates leveraging mobile IP functionally as discussed above and establishing a security association between the Agent  60  and the Target Node  66 . To ensure the security of the network  50 , the security association between the Agent  60  and the Target Node  66  is time limited to the duration of the transferred session so that the Target Node  66  cannot re-register with the Agent  60  at some later time to transfer the Transferring Node&#39;s future sessions without permission. Also, there is means of terminating the session transfer when the session has ended, or when the Transferring Node  56  wants to cancel the session transfer. 
     According to the network  50  of  FIG. 1   a , a session is initiated between the corresponding node  52  and the Transferring Node  56  using the normal IP suite of operations. When the Transferring Node  56  determines that it would like to transfer the session, it negotiates the session transfer with the Target Node  66 . Either the Transferring Node  56  or the Target Node  66  may initiate the negotiation. During the negotiation, a session key must be established that will form the basis of a temporary security association between the Target Node  66  and the Agent  60  for the duration of the session transfer. For example, a random number is selected by one of the nodes  56 / 66  to serve as a session key and is sent to the other node  66 / 56  in encrypted form using an existing Transferring Node  56 /Target Node  66  security association. The session key is transferred in encrypted form to prevent an unauthorized node from intercepting the session key. An existing Transferring Node  56 /Target Node  66  security association is required to encrypt the session key and may be accomplished by using a shared secret key, if one exists, or by using a public key encryption. If there is no shared secret key and neither device knows the other&#39;s public key, then one device will have to first learn the other&#39;s public key from a trusted third party such as a certificate authority. The learning of the public key would only have to be done once, (e.g., before the first time that the Transferring Node  56  transfers a session to the Target Node  66 ) in order for these devices to transmit sessions between one another. In practice, it will often be the case that the Transferring Node  56  and Target Node  66  are either owned by the same user or under control of the same administration authority, in which case a Transferring Node  56 /Target Node  66  security association may be readily available. 
     During the negotiation, the Transferring Node  56  will also inform the Target Node  66  of the Agent&#39;s IP address, and whether the Target Node  66  has permission to use the services of the Agent  60  to further transfer the session to another device. Next, the Transferring Node  56  will request the Agent  60  to transfer the session to the Target Node  66 . This request will contain the Transferring Node&#39;s IP and link layer addresses, the Target Node&#39;s IP address, the session key encrypted using a Transferring Node  56 /Agent  60  security association based on a shared secret key or public key encryption, (similar to that described above), and an indication of whether further transferring, (e.g. from the Target Node  66  to another device), is permitted. The Agent  60  authenticates that the Transferring Node  56  is the source of this request using the Transferring Node  56 /Agent  60  security association. 
     After receiving and authenticating the request, the Agent  60  will internally set up a binding (or transfer agreement), which associates the Transferring Node&#39;s IP Address  58  with the Target Node&#39;s IP Address  68 . This binding will be valid for a finite lifetime, after which it will expire. The initial value of this lifetime may be determined in one of several ways. For example, it may be negotiated between the Transferring Node  56  and the Agent  60  as part of the registration process, subject to a maximum value configured in the Agent  60 . Alternatively, it may set to an initial default value configured in the Agent  60 . 
     At this point, the Transferring Node  56  stops looking for its IP address  58  on its subnet, (e.g., stops using Address Resolution Protocol (ARP) to cause other nodes on its subnet including the home IP router  62  to associate its IP address  58  with its link layer address), and the Agent  60  begins sending out requests for the Transferring Node&#39;s IP address  58 , (e.g., using ARP to cause other nodes on its subnet to associate its link layer address with the Transferring Node&#39;s IP address  58 ). As a result of the Transferring Node  56  and Agent  60  ARP behavior, the Agent  60  then begins intercepting data packets sent to the Transferring Node&#39;s IP address  58 . Using encapsulation methods well known in the art, the Agent  60  then tunnels these packets to the Target Node&#39;s IP address  68 . For each tunneled packet, the starting point of the tunnel is the Agent  60  and the ending point of the tunnel is the Target Node  66 . All IP traffic (or data packets) addressed to the Transferring Node&#39;s IP address  58 , (including the Correspondent Node  52 /Transferring Node  56  session traffic), is thus transferred to the Target Node  66 . Therefore, the session transfer is accomplished without interrupting the session or otherwise affecting the Correspondent Node  52 . 
     The Agent  60  will continue to intercept data packets sent to the Transferring Node&#39;s IP address  58  and tunnel them to the Target Node&#39;s IP address  68  until the internal binding associating these two IP addresses as described above expires. At any point prior to the binding expiration, the Target Node  66  may send a registration request message to the Agent  60  to request that the binding lifetime be extended. The Agent  60  will then extend the lifetime and send a registration reply message to the Target Node  66  to inform it of the new lifetime value. This process may be repeated as needed to prevent the binding from expiring. The session key described above is used as the basis of a Agent  60 /Target Node  66  security association which is used to encrypt and authenticate the registration request and reply messages. This use of the security association is needed to prevent unauthorized nodes from spoofing and/or tampering with the registration request and reply messages to adversely affect the lifetime value and thus interfere with the session transfer. While the binding is valid, the session key may also be used for any other Agent  60 /Target Node  66  security needs. 
     When the Correspondent Node  52 /Transferring Node  56  session has ended, the Target Node  66  terminates the session transfer by sending a registration request to Agent  60  requesting that the binding lifetime be extended by 0, which the Agent will react to by immediately causing the binding to expire. Alternatively, the Target Node  66  may simply stop sending registration requests to the Agent  60  and let the binding expire at the end of its current lifetime. 
     When the binding expires, the Agent  60  sends a wake-up message to the Transferring Node  56  using the Transferring Node&#39;s IP and link layer addresses and stops intercepting packets addressed to the Transferring Node&#39;s IP address, (e.g. stops using ARP to associate its link layer address with the Transferring Node&#39;s IP address  58 ). After receiving the wake-up message, the Transferring Node  56  begins looking for communications addressed to its IP address, e.g. uses ARP to associate its link layer address with its IP address  58  and resumes its normal network connectivity. 
     Unfortunately, this termination scheme is subject to two potential problems. First, the Target Node  56  may maliciously continue to send registration requests to the Agent  60  extending the lifetime of the binding, thus allowing the Target Node  56  to keep receiving packets sent to the Transferring Node&#39;s IP address  58  longer than it should. Note however, it is unlikely that such an abuse will occur given that the Transferring Node  56  must have a high level of trust in the Target Node  66  prior to even allowing the transfer to take place. Therefore, it would seem that the chances of the Target Node  66  doing such a thing is unlikely if the Transferring Node  56  has transferred the session to the Target Node  66 . The second potential problem is that the Transferring Node&#39;s link layer address may have changed during the period of time that the session was being transferred. If such were the case, the Transferring Node  56  would not receive the wake-up message from Agent  60 . 
     To alleviate these problems, a new message is defined whereby the Transferring Node  56  may demand that the Agent  60  stop the session transfer. Upon receiving this message, the Agent  60  will cause the binding to expire, stop intercepting the communications addressed to the Transferring Node&#39;s IP address  58 , and stop tunneling packets to the Target Node  66 . The Transferring Node  56  may send this message at any time for a variety of reasons, including: (1) when the Transferring Node  56  suspects that the session has ended but that the Target Node  66  is not letting the binding expire; (2) when the Transferring Node  56  has changed link layer addresses and wishes to resume connectivity; (3) when the Transferring Node  56  wishes to override the session transfer and resume connectivity regardless of whether the session has ended or whether the Transferring Node&#39;s link layer address has changed; and (4) when the Transferring Node might have been off-line when the wake-up message was sent from Agent  60 . 
     Returning traffic from the transferred session can be sent directly from the Target Node  66  to the Correspondent Node  52  using the Transferring Node&#39;s IP address  58  as the source address in the packet header. This approach will not function properly if routers along the Target Node  66  to Correspondent Node  52  path, e.g. the foreign IP router  70 , use a process known in the art as ingress filtering to reject packets with a topologically inconsistent source IP address. Alternately, the returning traffic could be reverse tunneled to the Agent  60  for transmission from the Agent  60  to the Correspondent Node  52 . Again, this is done using the Transferring Node&#39;s IP address  58  as the source address in the packet header. These issues of handling the reverse path from Target Node  66  to Correspondent Node  52  are analogous to issues for Mobile IP (see  FIG. 1   c ) when the Mobile Node  12  visits a foreign network  16  and requires a path from the foreign network  16  to the Correspondent Node  20 . Thus, the reverse path issues and solutions are well known and discussed in the art, especially in Mobile IP references already given. 
     Once the session has been transferred to the Target Node  66 , it may be possible for the Target Node  66  to further transfer the session to another device, (e.g., a second Target Node) using the services of the Agent  60 . This type of transfer can occur if the Transferring Node  56  has granted permission for the Target Node  66  to do so. The granting of permission may be determined by looking to see if a flag has been set or enabled during the negotiation process between the Transferring Node  56  and the Target Node  66  (which takes place prior to the session transfer starting). If the Target Node has been given the authority to transfer the session, the transfer will be accomplished in a similar fashion to the initial transfer from the Transferring Node  56  to the Target Node  66 , (e.g., the Target Node  56  negotiates with the second Target Node, including the selection of a new session key for this transfer and negotiating whether further transfer permission is granted to the second Target Node; and the Target Node  66  communicates the necessary information to Agent  60 ; the Agent  60  sets up a new binding for this transfer, etc.). When the session is ended, the relevant binding expires and the Agent  60  sends a wake-up message to Transferring Node  56 . 
     Thus far, the discussion has focused on the case where a first destination device transfers a session to a second destination device located on a different subnet. It is also possible, however, that the Target Node (which the session is transferred to) may be located on the same subnet as the Transferring Node Referring now to  FIG. 1   b , a session would be initiated between the Correspondent Node  52 ′ and the Transferring Node  56 ′ using the normal IP suite of operations. The Transferring Node  56 ′ and the Target Node  66 ′ would negotiate the session transfer, (again, either may initiate the negotiation), and the Target Node  66 ′ would be informed of the Transferring Node&#39;s IP address  58 ′ and link layer address. At this point, the Transferring Node would stop looking for communications addressed to its IP address  58 ′ on the subnet, (e.g., the Transferring Node stops using ARP to associate its link layer address with its IP address  58 ′), and the Target Node  66 ′ would begin looking for and requesting the communications addressed to the Transferring Node&#39;s IP address  58 ′, (e.g. the Target Node starts using ARP to associate its link layer address with its IP address  58 ′). The Target Node  66 ′ then assumes the Transferring Node&#39;s IP address  58 ′ and begins receiving all IP traffic addressed to the Transferring Node  56 ′, including the Correspondent Node  52 ′/Transferring Node  56 ′ session traffic. It should be appreciated that the session transfer in such a network may be accomplished without an Agent and without interrupting the Correspondent Node  52 ′/Transferring Node  56 ′ session. 
     When the Correspondent Node  52 ′/Transferring Node  56 ′ session has ended, the Target Node  66 ′ sends a wake-up message to the Transferring Node  56 ′ using the Transferring Node&#39;s IP and link layer addresses and stops requesting the communications addressed to the Transferring Node&#39;s IP address  58 ′, (e.g., stops using ARP). After receiving the wake-up message, the Transferring Node  56 ′ begins requesting the communications addressed to its IP address  58 ′ and resumes its normal role in the network. Alternatively, the Transferring Node  56 ′ may send the Target Node  66 ′ a message at any time demanding to stop the session transfer, and then resuming its role in requesting communications addressed to its IP address  58 ′. 
     The transmission of the wake-up message requires the transmitting device (Agent  60  in the different subnet case, and the Target Node  66 ′ in the same subnet case) to create a packet with the Transferring Node&#39;s IP address as the destination and to send this packet over the subnet to the Transferring Node using the Transferring Node&#39;s link layer address. Normally, a transmitting device determines the link layer address to be used for a given destination IP address by consulting its ARP table. However, this may not work for the wake-up message because, at this point in time, the transmitting device&#39;s link layer address will be associated with the Transferring Node&#39;s IP address in its ARP table. Thus the transmitting device may bypass the normal ARP table lookup procedure when creating the wake-up message and explicitly plug in the Transferring Node&#39;s link layer address, which it has previously stored for this purpose. Requiring this bypass in either the Agent or the Target Node may be undesirable. To avoid this, an alternative wake-up procedure could be used wherein the Transferring Node obtains and requests (e.g., via DHCP) for an interim IP address prior to the session transfer. The Transferring Node would then send the interim address in lieu of its link layer address during the session transfer setup. The interim IP address could be used as the destination for the wake-up message. In this case, the normal ARP table lookup procedure would associate the Transferring Node&#39;s link layer address with the interim address, and the Transferring Node would then receive the wake-up message. After receiving the wake-up message, the Transferring Node could then return the interim address, (although it is also possible for the Transferring Node to use the interim address for other connectivity). The use of an interim address avoids the need to bypass the normal ARP table lookup procedure when creating a wake-up message, and it avoids the problem of the Transferring Node not receiving the wake-up message due to its link layer address having changed during the session transfer. However, this method obviously consumes an additional IP address for at least the duration of the session transfer and may therefore not be desirable. 
     Another possibility for enabling SID mobility is to allow the Target Node to use the same address and the same set of ports that the Transferring Node is using for a session, so that packets pertaining to the session are seen by both devices. In such a case, the user could participate in the session using whichever device he or she chooses, and is free to switch devices without interrupting the session. However, IP routing protocols use the destination IP address to determine routes for transmitting packets through the network to the destination device. With the exception of some special cases, (e.g., broadcasting links where packets sent to any device on the link are visible to all devices on the link, and IP multicasting), there is no support for routing packets to multiple devices sharing an address. In the first special case, devices are required to be on the same broadcast link. In the second special case, the need for SID mobility must be anticipated prior to the beginning of the session so that the session may be initiated as a multicast session. In both cases, reliable communication to multiple devices is not supported, as there is no mechanism for handling acknowledgments from multiple devices for the same session. 
     When the Transferring Node transfers a session to the Target node, all of the Transferring Node&#39;s IP traffic is transferred to the Target Node. This is convenient when a session has multiple flows, as all of the flows are transferred together. However, if the Transferring Node simultaneously has other active sessions, they will also be transferred. For simple devices that only have one active session at a time, such as a wireless videophone, this may not be an issue. However, for more complex services, a method and apparatus supporting a plurality of IP addresses at the first destination device whereby a unique IP address from the plurality of IP addresses is assigned to a unique transferable session may be desirable. Such a system is depicted in  FIG. 2   a  and  FIG. 2   b.    
     A SID mobility IP network system and method are shown in  FIG. 2   a  and  FIG. 2   b , wherein a different IP address is used for each transferable session. In  FIG. 2   a , a SID mobility network and method are shown generally designated with reference numeral  100 . In this network system  100 , a session specific IP address  114  is obtained for each session and the Transferring Node  106  retains its base IP address  108  so that it may continue to establish and participate in other active sessions. In other words, by using a different IP address for each session, additional active sessions that the Transferring Node does not transfer may remain with the Transferring Node rather than being transferred as would happen in the SID mobility network system as described by  FIG. 1   a  and  FIG. 1   b . Furthermore, the base IP address  108  provides an initial contact address for a Correspondent Node(s) to establish new sessions that will use session specific addresses. With such a SID mobility network system and method, a seamless transfer of a communication session at the network layer can occur in which the session transfer remains transparent to the corresponding node, which continues to send all packets (or communications) to the session specific IP address. 
     The network system  100  includes a Correspondent Node  102  and a home subnet  104  including a Transferring Node  106 . The Transferring Node  106  has a permanent Transferring Node IP address (or base Transferring Node IP address)  108  which the Correspondent Node designates as the destination address when it wishes to initiate a communication session with the Transferring Node  106 . The network system also includes a Agent  110 , an IP address generator  112 , (e g., a DHCP server, etc.), capable of generating a session specific IP address  114 , and a home IP router  116 . The home IP router  116  is used to route data about the network, including packets belonging to the Correspondent Node  102 /Transferring Node  106  sessions As will be discussed in further detail below, the Transferring Node  106  will obtain a session specific IP address from the address generator when a communication session is desired to take place between the Correspondent Node  102  and the Transferring Node  106 . With the advent of technologies such as DHCP servers that can allocate IP addresses dynamically to clients, obtaining a session specific IP address can be achieved and operated effectively by allowing the allocated IP addresses to be returned to the DHCP server when the lease time (or check out time) has expired, and reused by the server. In addition to DHCP, other methods for generating and reusing session specific addresses are possible, including the assignment of multiple IP addresses to each Transferring Node specifically for use as session specific addresses. Alternatively, in networks employing IPv6, each destination device can dynamically create IP addresses. 
     The SID mobility network system  100  further includes a foreign subnet  118  including a Target Node  120  having a Target Node IP address (or base Target Node IP address)  122 , and a foreign IP router  124 . If a transfer of the communication session from the Transferring Node  106  to the Target Node  120  is desired, the Transferring Node  106  and Target Node  120  will negotiate the transfer and the Transferring Node&#39;s end point of the session will be transferred from the Transferring Node  106  to the Target Node  120 . Either the Transferring Node  106  or the Target Node  120  may initiate the negotiation. 
     The Mobile IP network operation of having a Home Agent (HA) intercepting packets sent to a device whose home location is on the HA&#39;s subnet and tunneling them to that device when it has relocated on a foreign subnet can be modified to enable the SID mobility network system  100 . In order to achieve SID mobility, the underlying Mobile IP concept of re-routing sessions associated with a specific destination device as it moves through a network must be changed such that sessions can be re-routed to different destination devices than the original destination device. Rather than transferring the Transferring Node&#39;s base IP address  108 , a separate session specific IP address  114  is created with which the Transferring Node  106  will communicate with the Correspondent Node  102 . When a Correspondent Node initiates a session with the Transferring Node, the Transferring Node&#39;s base address  108  is initially contacted, and a new protocol is used to assign the session specific address  114  to the session instead of the base address  108 . When a Transferring Node initiates a session with a Correspondent Node, the Transferring Node will first obtain the session specific address  114  and use it to establish the session. This configuration will allow the Transferring Node  106  to retain its base IP address  108  to establish and participate in additional sessions with other Correspondent Nodes, yet retain the ability to transfer the session with the Correspondent Node  102  by merely transferring the session specific IP address  114  to the Target Node  120 . 
     According to this form, SID mobility is accomplished by moving the session specific IP address  114  through a combination of ARP and Mobile IP from the Transferring Node  106  to a Target Node  120 . In order to accomplish this, a protocol must be added to initiate a transferable session and, in order to initiate a communication session, the Correspondent Node  102  must first contact the Transferring Node  106  using its base IP address  108 . After receiving notification that the Correspondent Node  102  wishes to conduct a session, the Transferring Node  106  will obtain a session specific IP address  114  from an IP address generator, (e g., a DHCP server). As a result, the Transferring Node  106  will have two IP addresses, a base IP address  108  for general communication and a session IP address  114  specific to the active Correspondent Node/Transferring Node session. 
     The Transferring Node  106  must map the session specific IP address  114  to the same network interface as its permanent IP address  108 . This mechanism, commonly known as IP-aliasing, will enable all communications destined for the aliased address  114  to be passed up the protocol stack. Once IP aliasing has been accomplished, the Transferring Node  106  will notify the Correspondent Node  102  of the valid session IP address  114  so that the communication session can begin. The actual session is then begun using the session specific IP address  114  instead of the permanent address  108 . All communications from the Correspondent Node  102  for that session will use the session specific address  114  to reach the destination (initially the Transferring Node) instead of using the Transferring Node&#39;s permanent address  108 . 
     When a session transfer is desired, the Transferring Node  106  negotiates with the Target Node  120  to transfer the session specific IP address  114  to the Target Node  120 . Either the Transferring Node  106  or the Target Node  120  may initiate the negotiation. When the Transferring Node and the Target Node are on different subnets, the Target Node  120  will register the session address with the Transferring Node&#39;s Agent  110 . For security reasons, the Transferring Node  106  and the Target Node  120  will negotiate a session key and convey this information to the Agent  110 . This key is used by the Agent  110  to authenticate the registration messages it receives from the Target Node  120 . The Transferring Node  106  must also convey any additional information needed to maintain the session address to the Target Node  120  and/or the Agent  110 , (e.g., any information needed to keep the session specific IP address checked out). For example, if a DHCP server is being used, the server must receive any “keep-alive” messages required to allow the continued use of the session address  114  which either the Agent  110  or the Transferring Node  106  may send. 
     After a session IP address  114  has been transferred, the session specific traffic will be intercepted by the Agent  110  and tunneled to the Target Node  120 . The Target Node  120  must then properly decapsulate (or de-tunnel) the packets to remove the outer header used to tunnel packets to the Target Node  120  so that the original packet can be received. Once this step is complete, the packet is passed up the protocol stack of the Target Node. With such a configuration, the Transferring Node  106  is able to retain a base IP address thereby allowing it to participate in other sessions. The return traffic for the transferred session can be sent directly from the Target Node  106  to the Correspondent Node  102  using the session IP address  114  as the source address in the packet header. Alternately, the return traffic could be reverse tunneled to the Agent  110  for transmission from the Agent  110  to the Correspondent Node  102  using the session IP address  114  as the source address in the packet header. If the Transferring Node is involved in more than one session, (e g, an additional session with the same, or an additional, Correspondent Node), the return traffic from the Transferring Node  106  pertaining to the additional session can be sent directly to the additional Correspondent Node by using the session IP address which was obtained for the additional session as the source address in the packet header. 
     Once the session has ended, the Target Node  120  should de-register with the Agent  110 . Similarly, in order to transfer the session back to a node on the Transferring Node&#39;s subnet, the Target Node  120  must notify the new target and de-register with the Agent  110 . To transfer the session to another node not on the Transferring Node&#39;s subnet, (if such permission has been granted), the Target Node  120  must notify the new Target Node and the new Target Node must register the new session address location with the Agent  110 . Once the session terminates, the last Target Node should release the session IP address  114 . In the case of DHCP server, the session IP address  114  will be released back to the DHCP server for future use. If the last Target Node is not on the Transferring Node&#39;s subnet, then it must also de-register the session address  114  with the Agent  110 . 
     Referring now to  FIG. 2   b , in which the Transferring Node  106 ′ and the Target Node  120 ′ are on the same subnet, the Target Node  120 ′ can simply intercept the session traffic using a Proxy ARP message that binds the Target Node&#39;s link-layer address to the session IP address  114 ′. The Transferring Node  106 ′ will cease to ARP for the session specific address  114 ′ and the Target Node  120 ′ will begin using ARP to look for the session specific IP address  114 ′ instead. The session can also be passed on to other nodes on the Transferring Node&#39;s subnet in the same way or to nodes that are not on the Transferring Node&#39;s subnet using the method described above with respect to session specific transfers between nodes located on different subnets (if such permission has been granted). Again, once the session terminates, the last Target Node should release the session IP address  114 ′. If a DHCP server is being used, the Target Node  120 ′ will release the session specific IP address  114 ′ back to the DHCP server for future use. 
     Those skilled in the art will readily realize that additional transfers of a given session are possible. After the initial transfer, using the same techniques the session could be returned to the original Transferring Node or instead transferred to another Target Node. Once the session terminates, the last Target Node should release the session IP address  114 . In the case of DHCP server, the session IP address  114  will be released back to the DHCP server for future use. If the last Target Node is not on the original Transferring Node&#39;s subnet, then it must also de-register the session address  114  with the Agent  110 . 
     According to this form, the session transfer will remain transparent to the Correspondent Node because the session packets will be sent to the session IP address (regardless of whether that address is located at the Transferring Node or the Target Node). Thus, all “flows” in a session will transfer along with the session specific IP address, and all “ports” associated with the session specific IP address will move with that address. Therefore, many flows can be part of the mobile session. For example, a videoconference session may include numerous video, audio, and data flows associated with a session that would all be transferred together when the session moves from the Transferring Node to the Target Node. 
     Another method and apparatus for enabling SID mobility attempts to conserve IP addresses by only requiring a device to maintain at most two IP addresses. In order to achieve SID mobility by this method, two underlying Mobile IP Home Agent functions must be enhanced. First, referring to  FIG. 1   c , the concept of only transferring the communications to the actual mobile device  12  located on the foreign subnet  16  must be changed so that the receiving device on the foreign subnet can in fact be different than the device to which the packets were originally addressed. Second, the Mobile IP concept of non-selectively forwarding all sessions associated with a given device to its new location must be enhanced to sort and forward on a per-session basis using the IP flow identifying 5-tuples as means to identify sessions. 
     Referring to  FIG. 3   a , a SID mobility IP network system and method are shown generally designated with reference numeral  150 . In this embodiment, the network  150  is comprised of a first Correspondent Node  152  and a second Correspondent Node  154 , IP routers  156  and  158 , a home subnet  160  comprising a Transferring Node  162  having a Transferring Node IP address  164 , a Session Agent (SA)  166 , a temporary IP address generator  168  (e.g. a DHCP server, etc.) capable of generating a temporary IP address  170 , a home IP router  172 , a foreign subnet  174  comprising a Target Node  176  having a Target Node IP address  178 , and a foreign IP router  180 . 
     SID mobility IP network system  150  allows the diverting of IP traffic from a Transferring Node  162  to a Target Node  176 . This enables the network user to transfer all active sessions from one device to another device with minimal interruption, enabling part of SID mobility. In one sense, this functionality can be viewed as legitimizing the Mobile IP denial-of-service attack described above. However, only a Target Node  176  that is trusted by the Transferring Node  162  can receive traffic diverted by the SA  166 . As such, if the Target Node  176  were not trusted, then the SA  166  would not forward the session to it. In order to establish this trust, a new method is required for establishing a security association between the Target Node  176  and the SA  166 . 
     To ensure the security of the network  150 , the security association between the Transferring Node  162  and the Target Node  176  must be time limited to the duration of the forwarded session(s) so that the Target Node  176  cannot re-register with the SA  166  at some later time to transfer the Transferring Node&#39;s future sessions without permission (a form of replay attack). Also, there must be some means of terminating the session transfer when the session has ended, or when the Transferring Node  162  wants to cancel the session transfer. As will be discussed in further detail below, a method is provided for enabling SID mobility which incorporates leveraging Mobile IP functionally, establishing a per session security association between the SA  166  and the Target Node  176 , and enabling termination of the session transfer. 
     According to the network  150  of  FIG. 3   a , a session is initiated between a Correspondent Node  154  and the Transferring Node  162  using the normal IP suite of operations. When the Transferring Node  162  determines that it would like to transfer the session, it negotiates the session transfer with the Target Node  176 . In general, either the Transferring Node  162  or the Target Node  176  may initiate the negotiation. During the negotiation, a session key should be established that will form the basis of a temporary security association between the Target Node  176  and the SA  166 . For example, a random number is selected by one of the nodes ( 176  or  162 ) to serve as a session key and is sent to the other node ( 162  or  176 , respectively) in encrypted form using an existing Transferring Node  162 /Target Node  176  security association. The security association is required to encrypt the session key and may be accomplished by using a Transferring Node  162 /Target Node  176  shared secret key, if one exists, or by using a public key encryption. If there is no shared secret key and neither device knows the other&#39;s public key, then one device can first learn the other&#39;s public key from a trusted third party such as a certificate authority. The learning of the public key would only have to be done once, (e.g., before the first time that the Transferring Node  162  transfers a session to the Target Node  176 ) in order for these devices to exchange sessions between one another. In practice, it will often be the case that the Transferring Node  162  and Target Node  176  are either owned by the same user or under control of the same administration authority, in which case a Transferring Node  162 /Target Node  176  security association may be readily available. 
     The Transferring Node  162  will also inform the Target Node  176  of the SA&#39;s IP address, and whether the Target Node  176  has permission to use the services of the SA  166  to further transfer the session to another device. The Transferring Node  162  then obtains a temporary IP address  170  from the temporary IP address generator  168 . Next, the Transferring Node  164  will notify the SA  166  that the session is being transferred to the Target Node  176 . This notification will contain the Transferring Node&#39;s IP address  164  and Temporary IP address(es)  170 , the Target Node&#39;s IP address  178 , the session key encrypted using a Transferring Node  162 /SA  166  security association based on a shared secret key or public key encryption, (similar to that described above), a list of flows (5-tuples) defining the session(s) to be transferred, and a flag indicating whether further transferring, (e.g. from the Target Node  176  to another device), is permitted. The SA  166  authenticates that the Transferring Node  162  is the source of this notification using the Transferring Node  162 /SA  166  security association. 
     At this point, the Transferring Node  162  stops looking for packets sent to its IP address  164  on its subnet, (e.g., stops seeking sessions directed to its IP address via Address Resolution Protocol or ARP), and the SA  166  begins sending out requests for the Transferring Node&#39;s IP address  164 , using ARP for example, on its subnet. The SA  166  sets up a session binding and begins intercepting packets for flows and sessions sent to the Transferring Node&#39;s IP address  164 . The SA  166  sorts the packets by 5-tuple according to a list of transferred flows (each identified by a 5-tuple). Using encapsulation methods well known in the art, the SA tunnels flows that have been transferred to the Target Node  164  to the Target Node&#39;s IP address  178 . The SA also employs well-known techniques to tunnel flows that have not been transferred, for example sessions between another Correspondent Node  152  and the Transferring Node  162  or non-transferred sessions between the original Correspondent Node  154  and the Transferring Node  162 , to the Transferring Node  162  using the Transferring Node&#39;s temporary IP address  170 . Thus, only sessions selected for transfer are sent to the Target Node  178  while other sessions to the Transferring Node  162  proceed with minimal disruption. The session key is then used for all SA  166 /Target Node  176  security needs associated with the session transfer, (e.g., for authenticating successive registration requests, etc.). This session transfer is accomplished without interrupting the session or otherwise affecting any Correspondent Nodes  152  or  154 . 
     For the case where the Transferring Node  162  already has at least one other session registered with SA  166 , the process of registering a new session to be transferred becomes simplified. For example, the SA  166  has already assumed the Transferring Node&#39;s IP address  164  and the Transferring Node  162  already has established a temporary address  170 . 
     Therefore, after negotiating with the session&#39;s accepting device (or new Target Node), the Transferring Node  162  simply registers the session with the SA  166  by providing the 5-tuples of all flows which comprise the session and the new Target Node&#39;s IP address. 
     When the Correspondent Node  154 /Transferring Node  162  session has ended, the Target Node  176  terminates the session transfer by sending a registration request to SA  166  with a lifetime field set to 0, thereby causing the transferred session binding (or transfer agreement) to expire. Alternatively, the Target Node  176  may simply stop sending registration requests to the SA  166  and let the binding expire since all SA registrations have a finite lifetime subject to a maximum configured in the SA  166 . 
     Once SA  166  has the Transferring Node  162 &#39;s IP address  164 , it retains the address until all session transfers are either terminated or transferred back to the original Transferring Node  162 . In other words, all session transfer bindings have expired. At this point, the SA  166  sends a wake-up message to the Transferring Node  162  using the Transferring Node&#39;s temporary IP address  170  and stops intercepting communications addressed to the Transferring Node&#39;s IP address  164 . (In other words, the SA  166  stops using ARP to associate its physical address with the Transferring Node&#39;s IP address  164 ). After receiving the wake-up message, the Transferring Node  162  begins looking for communications addressed to its IP address  164  (by using ARP to associate its physical address with the Transferring Node&#39;s IP address  164 ), and resumes its normal role in the network. At this point, the Transferring Node  162  can also return its temporary IP address  170  to the Temporary IP Address Generator  168 . Unfortunately, this termination scheme is subject to a potential problem. The Target Node  178  may maliciously continue to send registration requests to the SA  166  extending the lifetime of the binding thereby continuing to receive all traffic for the transferred session and perhaps also for new sessions with the same 5-tuple information. Note however, it is unlikely that such an abuse will occur given that the Transferring Node  162  must have a high level of trust in the Target Node  176  prior to even allowing the transfer to take place. Therefore, it would seem that the chances of the Target Node  176  abusing this trust unlikely if the Transferring Node  162  has transferred the session to the Target Node  176 . 
     To alleviate this problem, a new message is defined whereby the Transferring Node  162  may demand that the SA  166  stop the session transfer. Upon receiving this message, the SA  166  will stop intercepting the communications addressed to the Transferring Node&#39;s IP address  164  and will stop tunneling packets to the Target Node  176 . The Transferring Node  162  may send this message at any time for a variety of reasons, including: (1) when the Transferring Node  162  suspects that the session has ended but that the Target Node  176  is not letting the SA  166 /Target Node  176  binding expire; (2) when the Transferring Node  162  wishes to override the session transfer and resume connectivity regardless of whether the session has ended; and (3) when the Transferring Node  162  might have been off-line when the wake-up message was sent from SA  166 . 
     Returning traffic from the transferred session can be sent directly from the Target Node  176  to the Correspondent Node  154  using the Transferring Node&#39;s IP address  164  as the source address in the packet header. This approach will not function properly if routers along the Target Node  176  to Correspondent Node  154  path (router  180 , for example) use a process known in the art as ingress filtering to reject packets with a topologically inconsistent source IP address. Alternately, the returning traffic could be reverse tunneled to the SA  166  for transmission from the SA  166  to the Correspondent Node  154 . Again, this is done using the Transferring Node&#39;s IP address  164  as the source address in the packet header. These issues of handling the reverse path from Target Node  176  to Correspondent Node  154  are analogous to issues for Mobile IP (see  FIG. 1   c ) when the Mobile Node  12  visits a foreign network  16  and requires a path from the foreign network  16  to the Correspondent Node  20 . Thus, the reverse path issues and solutions are well known and discussed in the art, especially in Mobile IP references already given. 
     Once the session has been transferred to the Target Node  176 , it may be possible for the Target Node  176  to further transfer the session to another device, (e.g., a second Target Node not shown in  FIG. 3   a ) using the services of the SA  166 . This type of transfer can occur if the Transferring Node  162  has granted permission for the Target Node  176  to do so. The granting of permission may be determined by looking to see if a flag has been set or enabled during the negotiation process between the Transferring Node  162  and the Target Node  162  that takes place in preparation for the transfer. If the Target Node  176  has been given the authority to transfer the session via the SA  166 , the transfer will be accomplished in an similar fashion to the initial transfer from the Transferring Node  162  to the Target Node  176 , (e.g., the Target Node  176  negotiates with the second Target Node not shown, including the selection of a new session key for this transfer and negotiating whether further transfer permission is granted to the second Target Node, and the Target Node  176  communicates the necessary information to SA  166 , etc.). When this session is ended SA  166 /second Target Node binding expires, and the SA  166  sends a wake-up message to Transferring Node  162 . 
     Thus far, the discussion has focused on the case where one device transfers a session to another device located on a different subnet. The techniques used for this embodiment are fully general and would apply to the case where the Transferring Node and the Target Node are on the same subnet as discussed in the previous embodiments and shown in  FIG. 3   b . However, those skilled in the art will readily realize that more efficient though less general techniques for communication between the Target Node, Transferring Node and Session Agent would be available in the same subnet case. 
     In summary, this embodiment of the invention includes a network which uses a “Session Agent” to enable SID mobility. Using a SA  166 , Transferring Node  162  can transfer one or more sessions. The SA  166  then intercepts all packets for the Transferring Node  162  and transfers them for the transferred sessions to the appropriate Target Nodes while delivering all packets for non-transferred sessions to the Transferring Node  162 . This method enables the transfer of multiple sessions. In addition, the SID Mobility network system  150  uses IP addresses very efficiently so that an extra (or temporary) IP address is only used when a decision to transfer a session from the Transferring Node to a Target Node has been made. With such a configuration, this network system  150  may even be used on network systems where the IP addresses are scarce or expensive. 
     Thus, it is apparent that there has been provided, in accordance with the invention, methods and apparatuses for transferring a communication session that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims. For example, the Agent or SA functionality could be integrated with either a Transferring Node or a router on the home subnet, or the functionality of the address generation box (e.g., DHCP) could be integrated into one of the other boxes of the block diagram.