Abstract:
A method for improving the reliability of a low latency handoff is disclosed. The method comprises receiving a first message from a first device. Upon receipt of the first message, transmitting a registration request via the first device to a home agent for pre-registration of a co-located care-of address associated with a second device, wherein the registration request comprises the co-located care-of address associated with the second device. If the first message is received again from the first device prior to attaching to the second device, re-transmitting the registration request via the first device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/438,402, filed May 15, 2003, of the same title. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to a method for improving the reliability of low latency handoffs.  
       BACKGROUND OF THE INVENTION  
       [0003]     Today, developers are focused more on providing all Internet Protocol (“IP”) solutions to roaming nodes (e.g., radios, telephones, laptops, personal digital assistants, etc.). As such, it is necessary to have a seamless IP-based mobility solution. Seamless mobility for mobile nodes from one location to another is hence becoming increasingly important. Thus, fast and reliable handoffs for the mobile node are becoming more important.  
         [0004]     One of the ways to achieve low latency handoffs is to make use of layer  2  triggers to perform layer  3  handoffs. A pre-registration technique has been proposed in an Internet Draft to address low latency handoffs. The pre-registration technique exploits early triggers from the link layer that indicate a handoff is going to occur. The pre-registration technique uses this early trigger to start the mobile IP registration process from the mobile node&#39;s first foreign agent and/or first access node (i.e., current point of attachment), even though the registration itself is in reference to a new point of attachment, e.g. a second foreign agent and/or second access node. This early launch of the mobile IP registration process helps reduce overall handoff delay since more time is provided for the mobile IP registration to complete.  
         [0005]     As illustrated in  FIG. 1 , the first foreign agent  100  acts as a proxy for the second foreign agent  102  so that the mobile node  104  and the first foreign agent  100  can exchange messages on behalf of the second foreign agent  102 . It is important to note that where foreign agents are used that the first foreign agent does not generate the messages and simply relays the messages to and from the mobile node  104 .  
         [0006]     Summarily, where access nodes are used, if a mobile node tries to perform pre-registration for a new co-located care-of-address on a new link, e.g. with a second access node, the mobile node must wait for a timeout before it attempts to register again with a home agent. In such a case, the message sent on the old link may have been lost and so handoff is delayed.  
         [0007]     A disadvantage of the pre-registration technique as described in the Internet draft (and as known with access nodes) is that it relies on control signaling between the mobile node  104  and the first foreign agent  100  after the link layer has indicated that a handoff is imminent. In other words, the quality of the air interface link between the mobile node  104  and the first point of attachment, e.g. a first foreign agent  100  is likely deteriorating rapidly. The deterioration of the link between the mobile node  104  and the first point of attachment increases the likelihood of dropped messages. Under these conditions, it may not always be possible for the mobile node  104  to complete the pre-registration process prior to the “link down” with the first point of attachment (e.g., the mobile node  104  may be moving too fast). Under the Internet Draft, when pre-registration fails, the handoff time is longer than with standard mobile IP. This shortcoming makes the pre-registration technique as described in the Internet draft less than desirable, especially for mission critical applications.  
         [0008]     Further, in the case of a pre-registration failure, one of the following needs to occur before the mobile node  104  can perform a mobile IP registration: either the registration request timer expires and the mobile node  104  solicits another agent advertisement from the second point of attachment, e.g. second foreign agent  102 , or the second point of attachment transmits its regular periodic router and/or agent advertisement. A disadvantage to this technique is that both of these events could take in the order of a second to occur, which would significantly slow the handoff process.  
         [0009]     Alternatively, the mobile node  104  could always transmit an agent solicitation upon “link up” with the second point of attachment, and transmit a registration request to the home agent  106  upon receiving the advertisement. If the home agent  106  already has the care of address registered, the home agent  106  would simply refresh the state and send a reply. A disadvantage to this technique is that the number of registration requests could potentially double, which may lead to home agent  106  loading issues. Another disadvantage to this technique is wasted resources in case where the pre-registration succeeds.  
         [0010]     Thus, there exists a need for a new method for improving the reliability of low latency handoffs. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]     An embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:  
         [0012]      FIG. 1  illustrates a bounce diagram of the signaling between a mobile node, a first foreign agent, a second foreign agent, and a home agent;  
         [0013]      FIG. 2  illustrates a flowchart of the signaling transmitted and/or received by the mobile node in accordance with one embodiment of the present invention;  
         [0014]      FIG. 3  illustrates a flowchart of the signaling transmitted and/or received by the first foreign agent in accordance with one embodiment of the present invention;  
         [0015]      FIG. 4  illustrates a flowchart of the signaling transmitted and/or received by the second foreign agent in accordance with one embodiment of the present invention;  
         [0016]      FIG. 5  illustrates a bounce diagram of the signaling between a mobile node, a first access node, a second access node, and a home agent;  
         [0017]      FIG. 6  illustrates a flowchart of the signaling transmitted and/or received by the mobile node in accordance with one embodiment of the present invention;  
         [0018]      FIG. 7  illustrates a flowchart of the signaling transmitted and/or received by the first access node in accordance with one embodiment of the present invention; and  
         [0019]      FIG. 8  illustrates a flowchart of the signaling transmitted and/or received by the second access node in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.  
         [0021]     An embodiment of the present invention modifies the signaling algorithms in the mobile node  104 , the current point of attachment  100 , and the new point of attachment  102  from the Internet Draft to address the shortcomings of the pre-registration technique as described above. It should be noted that in accordance with an embodiment of the present invention, the signaling changes in the current point of attachment  100  are required prior to “link down” with the mobile node  104 , and the signaling changes in the new point of attachment  102  are required after “link up” with the mobile node  104 . It is also important to note that the signaling outlined below with respect to  FIGS. 1-4  occurs in parallel. Let us now turn to the details and figures of the present invention.  
         [0022]     As in standard mobile IP applications, upon “link up”, the mobile node  104  solicits agent advertisements; in other words, the mobile node  104  determines if any mobile IP agents (home agent or foreign agent) are on the same network. If the mobile node  104  is on its home network, the mobile node  104  will receive the agent advertisement from its home agent  106 . Upon receiving the agent advertisement from its home agent  106 , the mobile node  104  transmits a registration request to its home agent  106 . The home agent  106  transmits a registration reply to the mobile node  104  indicating a successful registration with the home agent  106 . When on the home network, the mobile node  104  actually de-registers with the home agent  106  if it has previously registered since the home agent  106  does not need to tunnel any packets to the mobile node  104  via foreign agents.  
         [0023]     If the mobile node  104 , however, is on a network away from home, the mobile node  104  will receive the agent advertisement from a foreign agent  100  on the “visiting” network. Upon receipt of the agent advertisement from the foreign agent  100  on the “visiting” network, the mobile node  104  transmits the registration request to the home agent  106  via the foreign agent  100 . Upon receipt of the registration request, the home agent  106  transmits a registration reply to the mobile node  104  indicating a successful registration with the foreign agent  100 . At this point, for purposes of the following discussion, let us assume that the mobile node  104  has successfully registered with, and thus is currently attached to, the foreign agent  100 .  
         [0024]     Let us now assume that the mobile node  104  begins to move away from the first foreign agent (i.e., its current point of attachment)  100  towards a second foreign agent (i.e., its future point of attachment)  102 . It is desirable for the mobile node  104  to begin pre-registration with the second foreign agent  102  upon notice of a handoff to reduce latency. As illustrated in  FIG. 1 , if the mobile node  104  receives a trigger that a handoff is about to occur, the mobile node  104  transmits a proxy router solicitation message to the first foreign agent  100 ; the proxy router solicitation message solicits the first foreign agent  100  for the address of the second foreign agent  102 . In return, the mobile node  104  receives a proxy router advertisement from the first foreign agent  100  comprising the address of the second foreign agent  102 . Alternatively, if the first foreign agent  100  receives the trigger  108  that a handoff is about to occur, rather than the mobile node  104 , the first foreign agent  100  transmits the proxy router advertisement to the mobile node  104  without requiring the mobile node  104  to transmit the proxy router solicitation message. In one embodiment, the link layer triggers the network layer that the handoff is imminent, however, the trigger can alternatively be implemented at other layers as well.  
         [0025]      FIG. 2  illustrates a flowchart of the signaling transmitted and/or received by the mobile node  104  in accordance with one embodiment of the present invention. Upon receipt of the proxy router advertisement from the first foreign agent  100  (at step  200 ), the mobile node  104  transmits a registration request to the home agent  106  for registration with the second foreign agent  102  via the first foreign agent  100  (at step  202 ) and waits for “link up” with the second foreign agent  102  (at step  204 ).  
         [0026]     If the mobile node  104  receives a subsequent proxy router advertisement  114  from the first foreign agent  100  prior to “link up” with the second foreign agent  102  (at step  206 ), it is assumed that the first foreign agent  100  did not receive the registration request from the mobile node  104 . As a result, the mobile node  104  determines the status of the communication link between the mobile node  104  and the first foreign agent  100  (at step  208 ). If the mobile node  104  determines that the communication link with the first foreign agent  100  has deteriorated (i.e., the communication link between the mobile node  104  and the first foreign agent  100  is no longer operational), the mobile node  104  no longer attempts to communicate with the first foreign agent  100 , and waits for a “link up” with the second foreign agent  102  (at step  210 ).  
         [0027]     If the mobile node  104 , however, determines that the communication link with the first foreign agent  100  is still operational (at step  208 ), the mobile node  104  retransmits the registration request  116  to the home agent  106  for registration with the second foreign agent  102  via the first foreign agent  100  (at step  202 ) and continues to wait for “link up” with the second foreign agent  102  (at step  204 ). It is important to note that, in accordance with the present invention, as long as the communication link between the mobile node  104  and the first foreign agent  100  is operational, the mobile node  104  will retransmit its registration request  116  for registration with the second foreign agent  102  via the first foreign agent  100  upon receipt of a proxy router advertisements  112 ,  114  from the first foreign agent  100 .  
         [0028]     Upon “link up” at the second foreign agent  102  (at step  210 ), the mobile node  104  waits idly until it receives a registration reply from the second foreign agent  102  indicating a successful registration with the second foreign agent  102 . If the mobile node  104  receives a unicast foreign agent advertisement message (addressed directly to the mobile node  104 ) from the second foreign agent  102  while waiting for the registration reply from the home agent via the second foreign agent  102  (at step  212 ), the mobile node  104  transmits its registration request to the home agent  106  for registration with the second foreign agent  102  via the second foreign agent  102  directly (at step  214 ), and continues to wait to receive the registration reply. In one embodiment of the present invention, the mobile node  104  should only receive the unicast foreign agent advertisement for registration with the second foreign agent  102  from the second foreign agent  102  if the second foreign agent  102  did not receive the registration request from the mobile node  104  (i.e., the first foreign agent  100  did not successfully receive and/or forward the previous registration request for registration with the second foreign agent  102  from the mobile node  104 ).  
         [0029]     Upon receipt of the registration reply (at step  216 ), the mobile node  104  is successfully registered with the second foreign agent  102 ; as a result, subsequent communication with the mobile node  104  is routed through the second foreign agent  102 .  
         [0030]     Thus, the present invention reduces latency in handoffs by allowing the mobile node  104  to recognize the reception of subsequent (extra) proxy router advertisements  114  from the first foreign agent  100  as an indication that the first foreign agent  100  has not yet received the registration request from the mobile node  104  (possibly due to the registration request being lost due to the communication link rapidly deteriorating, high error rate, or the like). Once attached to the second foreign agent  102 , the mobile node  104  can also recognize the reception of an unsolicited unicast foreign agent advertisement from the second foreign agent  102  as an indication that another registration request for registration with the second foreign agent  102  needs to be transmitted. Thus, the mobile node  104  is able to retransmit its registration request, when requested, for registration with the second foreign agent  102  as soon as possible.  
         [0031]      FIG. 3  illustrates a flowchart of the signaling transmitted and/or received by the first foreign agent  100  in accordance with one embodiment of the present invention. As noted above, the signaling illustrated in  FIG. 3  is being performed in parallel with the signaling illustrated in  FIG. 2 . Starting with discovery that a handoff with the mobile node  104  is imminent, the first foreign agent  100  transmits a proxy router advertisement  112  to the mobile node  104  comprising at least the address of the second foreign agent  102  (at step  300 ). Upon transmission of the proxy router advertisement  112 , the first foreign agent  100  waits to receive a registration request from the mobile node  104  for registration with the second foreign agent  102 . If the registration request is received from the mobile node  104  (at step  302 ), the first foreign agent  100  forwards the request to the second foreign agent  102  (at step  304 ).  
         [0032]     If the registration request, however, is not received from the mobile node  104  within a predefined period of time after transmitting the proxy router advertisement  112  (at step  302 ), the first foreign agent  100  determines whether the communication link with the mobile node  104  is still operational (at step  306 ). If the communication link is no longer operational, the first foreign agent  100  no longer attempts to communicate with the mobile node  104 . If, however, the communication link is still operational with the mobile node  104 , the first foreign agent  100  retransmits the proxy router advertisement  114  (at step  300 ). It should be noted that, in accordance with the present invention, the first foreign agent  100  will continue to retransmit its proxy router advertisement  114  to the mobile node  104  until a registration request for registration with the second foreign agent  102  is received from the mobile node  104 , or until the communication link with the mobile node  104  is no longer operational, whichever is sooner.  
         [0033]     Thus, the present invention reduces latency in handoffs by setting a timer in the first foreign agent  100 . If the first foreign agent  100  does not receive the registration request for registration with the second foreign agent  102  from the mobile node  104  prior to expiration of the timer, the first foreign agent  100  transmits a subsequent proxy router advertisement  114  to the mobile node  104  to facilitate the pre-registration prior to handoff. Thus, the first foreign agent  100  informs the mobile node  104  that it has not received its registration request during the time frame in which it was expected.  
         [0034]      FIG. 4  illustrates a flowchart of the signaling transmitted and/or received by the second foreign agent  102 . Again, as stated above, the signaling illustrated in  FIG. 4  is performed in parallel with the signaling illustrated in  FIG. 2 . Starting with the mobile node  104  attaching to the second foreign agent  102 , the second foreign agent  102  receives a “link up” trigger identifying attachment of a new mobile node  104  (at step  400 ). Upon receipt of the “link up” trigger, the second foreign agent  102  determines whether a registration is pending for the mobile node  104  (at step  402 ). If registration is currently pending for the mobile node  104 , the second foreign agent  102  waits to receive a registration reply from the home agent  106  (at step  410 ). The second foreign agent  102  will eventually receive a registration reply from the home agent  106  indicating that the mobile node&#39;s registration with the second foreign agent  102  is successful, in which the second foreign agent  102  forwards the registration reply to the mobile node  104  (at step  412 ).  
         [0035]     If registration, however, is not currently pending for the mobile node  104  (i.e., the second foreign agent  102  never received the forwarded registration request for the mobile node  104 ), the second foreign agent  102  transmits an unsolicited unicast foreign agent advertisement to the mobile node  104  (at step  404 ). In response to transmitting the foreign agent advertisement, the second foreign agent  102  waits to receive the registration request for registration with the second foreign agent  102  from the mobile node  104 . Upon receipt of the registration request (at step  406 ), the second foreign agent  102  forwards the registration request to the home agent  106  (at step  408 ). The second foreign agent  102  will eventually receive a registration reply from the home agent  106  indicating that the mobile node&#39;s registration with the second foreign agent  102  is successful (at step  410 ), in which the second foreign agent  102  forwards the registration reply to the mobile node  104  (at step  412 ).  
         [0036]     It should be noted that if by chance the second foreign agent  102  receives the registration reply for the mobile node  104  from the home agent  106  prior to the mobile node  104  “linking up” with the second foreign agent  102 , the second foreign agent  102  buffers the registration reply until “link up” by the mobile node  104 . The second foreign agent  102  buffering the registration reply does not slow down the handoff because layer  2  handoff needs to be completed before any information can be transmitted to and/or received from the mobile node  104  via the second foreign agent  102 . Rather, buffering the registration reply until the mobile node  104  “links up” with the second foreign agent  102  increases the likelihood that the reply will not get lost due to a communication link deterioration, high error rates, or the like.  
         [0037]     In another embodiment,  FIG. 5  illustrates the case where the mobile node, e.g. mobile node  504 , uses co-located care-of addresses when communicating with access nodes, e.g.  500 ,  502 . As is known to one of ordinary skill in the art, the term access nodes encompasses the foreign agents mentioned with respect to  FIGS. 1-4 . However, as used herein, the term access nodes specifically refers to entities at the edge (as opposed to the core) of a network that the mobile node has a point of presence relationship with. One example of an access node is an edge router that provides IP layer connectivity for the mobile node. Another example, is a wireless local area network (WLAN) access point, where the WLAN access point provides wireless connectivity to the infrastructure for the mobile node. As is known to one of ordinary skill in the art, a co-located care-of address (CCoA) is a temporary IP address assigned to the mobile node when the mobile node is not in its home network, e.g. served by home agent  506 .  
         [0038]     Referring to  FIG. 5 , the mobile node  504  acquires a CCoA when the mobile node is attached to a first access node  500  where the CCoA has a prefix belonging to the first access node  500 , e.g. a router. As shown in  FIG. 5 , upon receiving a link layer trigger  508  (e.g. indicating that a handoff is pending), the mobile node  504  sends a proxy router solicitation to the first access node  500 . The first access node  500  sends a proxy router advertisement  512  in response to the proxy router solicitation, just as described with respect to  FIGS. 1-4 . Since the mobile node  504  is moving to a new point of attachment, the mobile node  504  needs to acquire a new CCoA belonging to the prefix of the new point of attachment, namely a second access node  502 . In one embodiment, the mobile node  504  may be able to autoconfigure a new CCoA based on information contained in the proxy router advertisement  512 . Alternatively, the mobile node  504  may use other methods such as Dynamic Host Control Protocol (DHCP) to acquire a new CCoA for use with the second access node  502  where in such a case, the proxy router advertisement  512  may comprise the new CCoA acquired on behalf of the mobile node  504 . In any case, there a number of ways that the mobile node  504  may acquire the new CCoA and such ways are known to one of ordinary skill in the art.  
         [0039]     Continuing with  FIG. 5 , the mobile node  504  then sends a registration request to pre-register (also termed a pre-registration) its new CCoA via the first access node  500 . If the registration request is dropped, the first access node  500  will retransmit the proxy router advertisement  514  to the mobile node  504 . The mobile node  504  will then retransmit the registration request  516  via the first access node  500  (additionally the second access node  502 , if necessary) to the home agent  506 , e.g. as a registration request or a regional registration request. The home agent  506  sends a registration reply in response to the registration request  516 . This registration reply is sent to the mobile node&#39;s new CCoA. As such, in one embodiment, the registration reply may be intercepted by the second access node  502  and forwarded to the mobile node  504 . However, the mobile node  504  may move prior to receiving the registration reply. In such a case, the second access node  502  may buffer the registration reply and upon attachment of the mobile node  504 , the second access node  502  will send the registration reply to the mobile node  504 .  
         [0040]     As is known to one of ordinary skill in the art, in the case of  FIG. 5 , the registration request and registration reply messages are addressed to the destination (e.g. the home agent) and not to the intermediary access nodes, while in the case of foreign agents, the registration request sent by the mobile node and the registration reply sent by the home agent are addressed to the intermediary foreign agents. For example, the registration request  516  is addressed to the home agent  506 , whereas in the case of  FIG. 1 , the registration request  116  is addressed to the first foreign agent  100 . Further, for example, the registration reply  518  is addressed to the mobile node&#39;s new CCoA as identified in the registration request  516 . As such, the term “via” (as used herein) means that a message is passed through the intermediary access nodes with the intermediary access node&#39;s knowledge, but the message is not necessarily addressed to the intermediary access node. For example, the registration request  516  is transmitted via the first access node  500  and via means that the first access node  500  is aware of the registration request  516  but the registration request is addressed to the mobile node&#39;s new CCoA.  
         [0041]     In alternative embodiments, the mobile node, e.g. mobile node  104  or  504 , exchanges the proxy router solicitation and proxy router advertisement messages with the first access node  500  (and/or the first foreign agent  100 ) well in advance of an impending handoff. Exchanging these messages before an impending handoff removes this step from the critical handoff path. In such an embodiment, reception of a proxy router solicitation no longer implies an impending handoff. Hence, the first access node  500  (and/or the first foreign agent  100 ) cannot expect to receive a registration request from the mobile node shortly after sending the proxy router advertisement, e.g.  112  or  512 . In such an embodiment, the registration request may be sent in a confirmed delivery manner to the first access node  500  (and/or the first foreign agent  100 ). For example, where there is link layer reliability, the layer  2  acknowledgement may indicate successful transmission of the registration request on the link. The first access node  500  (and/or the first foreign agent  100 ) may not be directly attached to the mobile node on that link because there may be a base station or an access point to which the mobile node is attached. The first access node  500  (and/or the first foreign agent  100 ) may be on the wired link to which the base station or the access point is connected on the infrastructure side. Since the wireless link is the one most prone to errors and dropped packets, in one embodiment, the link layer acknowledgement may be sufficient to indicate delivery of the registration request.  
         [0042]     In alternate embodiments, where the link layer may not have reliable delivery mechanisms, where additional wireless backhaul links are present, where the link between the base station or access point and the first access node  500  (and/or the first foreign agent  100 ) cannot be considered reliable, then other means of confirmed delivery mechanisms may be required. In such cases, the registration request may indicate that the mobile node needs an acknowledgement from the first access node  500  (and/or the first foreign agent  100 ). When this indication is present, the first access node  500  (and/or the first foreign agent  100 ) will send an acknowledgement back to the mobile node upon receiving the registration request. As is known to one of ordinary skill in the art, although a few methods of confirmed delivery are discussed herein, a number of other means of confirmed delivery can be used for sending the registration request.  
         [0043]     In any case, when the mobile node  504  does not receive an acknowledgement of delivery of the registration request, the mobile node  504  will retransmit the registration request via the first access node  500 , if the mobile node  504  is still attached to the first access node  500 . If the mobile node  504  moves to the second access node  502  before the mobile node  504  can retransmit the registration request, the mobile node  504  will retransmit the registration request immediately upon attachment to the second access node  502 .  
         [0044]      FIG. 6  illustrates a flowchart of the signaling transmitted and/or received by the mobile node  504  in accordance with one embodiment of the present invention. Upon receipt of the proxy router advertisement from the first access node  500  (at step  600 ), the mobile node  504  transmits a registration request to the home agent  506  for pre-registration of the mobile node&#39;s future point of attachment, e.g. in this case, the second access node  502  via the first access node  500  (at step  602 ) and waits for “link up” with the second access node  502  (at step  604 ).  
         [0045]     If the mobile node  504  receives a subsequent proxy router advertisement  514  from the first access node  500  prior to “link up” with the second access node  502  (at step  606 ), it is assumed that the first access node  500  did not receive the registration request from the mobile node  504 . As a result, the mobile node  504  determines the status of the communication link between the mobile node  504  and the first access node  500  (at step  608 ). If the mobile node  504  determines that the communication link with the first access node  500  has deteriorated (i.e., the communication link between the mobile node  404  and the first access node  500  is no longer operational), the mobile node  504  no longer attempts to communicate with the first access node  500 , and waits for a “link up” with the second access node  502  (at step  610 ).  
         [0046]     If the mobile node  504 , however, determines that the communication link with the first access node  500  is still operational (at step  608 ), the mobile node  504  retransmits the registration request  516  to the home agent  506  for registration with the second access node  502  via the first access node  500  (at step  602 ) and continues to wait for “link up” with the second access node  502  (at step  604 ). It is important to note that, in accordance with the present invention, as long as the communication link between the mobile node  504  and the first access node  500  is operational, the mobile node  504  will retransmit its registration request  516  for pre-registration of the mobile&#39;s node&#39;s future point of attachment, e.g. in this case, the second access node  502 , upon receipt of a proxy router advertisements  512 ,  514  from the first access node  500 .  
         [0047]     Upon “link up” at the second access node  502  (at step  610 ), the mobile node  504  waits idly until it receives a registration reply from the second access node  502  indicating a successful registration of the mobile node&#39;s new CCoA associated with the second access node  502 . As used herein, associated with means that the CCoA has the same prefix as the subnet that the second access node is attached to. If the mobile node  504  receives a unicast router advertisement message (addressed directly to the mobile node  504 ) from the second access node  502  while waiting for the registration reply from the home agent via the second access node  502  (at step  612 ), the mobile node  504  transmits its registration request to the home agent  506  for registration with the new CCoA associated with the second access node  502  (at step  614 ), and continues to wait to receive the registration reply  518 . In one embodiment of the present invention, the mobile node  504  should only receive the unicast router advertisement for registration with the new CCoA at the second access node  502 , if the second access node  502  did not receive the registration request from the mobile node  504  (i.e., the first access node  500  did not successfully receive and/or forward the previous registration request for registration with the second access node  502  from the mobile node  504 ).  
         [0048]     Upon receipt of the registration reply (at step  616 ), the mobile node  504  is successfully registered with the second access node  502 ; as a result, subsequent communication with the mobile node  504  is routed through the second access node  502 .  
         [0049]     Thus, the present invention reduces latency in handoffs by allowing the mobile node  504  to recognize the reception of subsequent (extra) proxy router advertisements  514  from the first access node  500  as an indication that the first access node  100  has not yet received the registration request from the mobile node  104  (possibly due to the registration request being lost due to the communication link rapidly deteriorating, high error rate, or the like). Once attached to the second access node  102 , the mobile node  504  can also recognize the reception of an unsolicited unicast router advertisements from the second access node  502  as an indication that another registration request for registration of the new CCoA associated with the second access node  502  needs to be transmitted. Thus, the mobile node  504  is able to retransmit its registration request, when requested, for registration as soon as possible.  
         [0050]      FIG. 7  illustrates a flowchart of the signaling transmitted and/or received by the first access node  500  in accordance with one embodiment of the present invention. As noted above, the signaling illustrated in  FIG. 7  is being performed in parallel with the signaling illustrated in  FIG. 6 . Starting with discovery that a handoff with the mobile node  504  is imminent, the first access node  500  transmits a proxy router advertisement  512  to the mobile node  504  comprising at least the address of the second access node  502  (at step  700 ). Upon transmission of the proxy router advertisement  512 , the first access node  500  waits to receive a registration request from the mobile node  504  for registration with the second access node  502 . If the registration request is received from the mobile node  504  (at step  702 ), the first access node  500  sends the registration request to the home agent (step  704 ) by either a) forwarding the request to the second access node  502  for delivery to the home agent  506  or b) optimally, forwarding the registration request to the home agent  506  directly and notifying the second access node that a registration request is pending (e.g. by message  520 ). However, forwarding the request to the second access node  502  for delivery to the home agent  506  (namely step a) may be required to avoid the registration request being dropped by a router in the network if the routers in the network apply ingress filtering.  
         [0051]     If the registration request, however, is not received from the mobile node  504  within a predefined period of time after transmitting the proxy router advertisement  512  (at step  702 ), the first access node  500  determines whether the communication link with the mobile node  504  is still operational (at step  706 ). If the communication link is no longer operational, the first access node  500  no longer attempts to communicate with the mobile node  504 . If, however, the communication link is still operational with the mobile node  504 , the first access node  500  retransmits the proxy router advertisement  514  (at step  700 ). It should be noted that, in accordance with the present invention, the first access node  500  will continue to retransmit its proxy router advertisement  514  to the mobile node  504  until a registration request for pre-registration of the new CCoA associated with the second access node  502  is received from the mobile node  504 , or until the communication link with the mobile node  504  is no longer operational, whichever is sooner.  
         [0052]     Thus, the present invention reduces latency in handoffs by setting a timer in the first access node  500 . If the first access node  500  does not receive the registration request for pre-registration of the CCoA associated with the second access node  502  from the mobile node  504  prior to expiration of the timer, the first access node  500  transmits a subsequent proxy router advertisement  514  to the mobile node  504  to facilitate the pre-registration prior to handoff. Thus, the first access node  500  informs the mobile node  504  that it has not received its registration request during the time frame in which it was expected.  
         [0053]      FIG. 8  illustrates a flowchart of the signaling transmitted and/or received by the second access node  502 . Again, as stated above, the signaling illustrated in  FIG. 8  is performed in parallel with the signaling illustrated in  FIG. 6 . Starting with the mobile node  504  attaching to the second access node  502 , the second access node  502  receives a “link up” trigger identifying attachment of a new mobile node  504  (at step  800 ). Upon receipt of the “link up” trigger, the second access node  502  determines whether a registration is pending for the mobile node  504  (at step  802 ) (e.g. by message  520 ). If registration is currently pending for the mobile node  504 , the pending registration indicates that the pre-registration was successful. If registration is not currently pending for the mobile  504 , then the second access node  502  transmits an unsolicited unicast router advertisement to the mobile node  504  (at step  804 ). In response to transmitting the unicast router advertisement, the second access node  502  waits to receive notification of a pending registration request as described with respect to  FIG. 6 , namely step  614  (step  806 )). If the second access node  502  receives a registration reply  518  from the home agent  506  indicating that the mobile node&#39;s registration with the new CCoA associated with the second access node  502  is successful (step  810 ), the second access node  502  forwards the registration reply to the mobile node  504  (at step  812 ).  
         [0054]     It should be noted that if by chance the second access node  502  receives the registration reply for the mobile node  504  from the home agent  506  prior to the mobile node  504  “linking up” with the second access node  502 , the second access node  502  buffers the registration reply until “link up” by the mobile node  504 . The second access node  502  buffering the registration reply does not slow down the handoff because layer  2  handoff needs to be completed before any information can be transmitted to and/or received from the mobile node  504  via the second access node  502 . Rather, buffering the registration reply until the mobile node  504  “links up” with the second access node  502  increases the likelihood that the reply will not get lost due to a communication link deterioration, high error rates, or the like.  
         [0055]     The present invention can be implemented in hardware, firmware and/or software. For example, a storage medium having stored thereon a set of instructions which, when loaded into a hardware device (e.g., a microprocessor), causes the hardware device/communication device to perform the functions of the present invention. Such a hardware device is located at the mobile node, the first foreign agent and/or the second foreign agent as one such means to implement the present invention.  
         [0056]     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.