Proposed Scheme
3.3 Proxy-based Fast Handover for Hierarchical Mobile IPv6
In this section, the proposed handover scheme over IEEE 802.11 network is shown for both predictive and reactive mode. It is assumed that the MN handover scenario is under the network architecture shown in the previous section. As shown in Figure 3.1, several scenarios can lead to a MN handover, and we discuss these handover scenarios in the following sections.
Furthermore, for simplicity, we consider only one MN attached to the AR. In fact, there is no difference between one MN and multiple MNs in the proposed scheme.
3.3.1 MN Enters a New AR Domain
In the first scenario, a MN enters a new proxy-based HMIPv6 (PHMIPv6) domain. Figure 3.4 shows the message exchanges when the MN enters a PHMIPv6 domain. It is similar to PMIPv6 procedure, the RtrSol message from the MN may arrive at any time after the MN’s attachment. The detailed description of message exchanges is as follows:
1. when an MN enters a new PHMIPv6 domain and attaches to the AR on an access link, the AR identifies the MN and obtains its identity. After identifying the MN’s identity, the system will determine whether the MN is authorized for the network-based mobility
service. If the MN is authorized for network-based mobility service, the network will guarantee that the MN using any of the address configuration mechanisms permitted to obtain the address configuration on the connected interface and move anywhere in that PHMIPv6 domain;
2. in order to register current location of the MN and store this mapping in LMA, we de-fine a Local Proxy Care-of-Address (LPCoA) for each AR. LPCoA is the global address configured on the outgoing interface of the AR and is the transport endpoint of the tun-nel between the MAP and the AR. After the MN attached to the AR, it sends a LPBU message including MN’s HoA and AR’s LPCoA to the MAP;
3. upon accepting this LPBU message, the MAP views the LPCoA as the care-of-address of the MN and registers it in the binding cache entry for the MN, and then the MAP sends a PBU message to the LMA;
4. upon accepting this PBU message, the LMA replies a PBA message including the MN’s home network prefix to the MAP. At this time the LMA also creates the BCE and sets up its endpoint of the bidirectional tunnel to the MN’s MAP;
5. on receiving the PBA message, the MAP sets up its endpoint of the bidirectional tunnel to the LMA and start to forward MN’s traffic, and then sends a Local Proxy Binding Acknowledgement (LPBA) message including the MN’s home network prefix to the AR;
6. also, on receiving the LPBA message, the AR sets up its endpoint of the bidirectional tunnel to the MAP and forwarding for the MN’s traffic. At this point, the AR has all the required information for emulating the MN’s home network. The AR replies the RtrAdv message to the MN and advertises the MN’s HNP as the hosted on-link prefix.
MN MAP
Figure 3.4 Signal Flow of Mobile Node Attachment.
3.3.2 MN Enters another AR under same MAP Domain
In the second scenario, the MN enters another PHMIPv6 domain under the same MAP. In this section, we discuss fast handover in both predictive mode and reactive mode.
3.3.3 Predictive Mode
Although packet buffering in FMIPv6 and FPMIPv6 avoids the on-the-fly packet loss, the MN still experiences the packet ordering problem. The reason is that the packets from the MAP after a new registration may arrive at MN before the buffered on-the-fly packets forwarded by the PAR. To solve the packet ordering problem, PFHMIPv6 uses an additional packet buffering at NAR to store the packets forwarded by MAP. In predictive fast handover of PFHMIPv6, the bidirectional tunnel will be established between the PAR and NAR before the MN attachment to the NAR.
Therefore, in order for the predictive fast handover to work efficiently, the MN requires ca-pable of reporting link-layer information, e.g. MAC address to the AN, and the AN is caca-pable of sending the HI message to the PAR at an appropriate timing. Figure 3.5 shows PFHMIPv6
handover procedure in predictive mode.
Figure 3.5 Predictive Fast Handover with proposed scheme.
The basic operation of predictive fast handover for PFHMIPv6 is as follows:
1. when a MN detected that a handover event is imminent, the MN reports its identifier (MN-ID) and target Access Point Identifier (t-AP ID) to which it is most likely to move to. The MN-ID could be the network access identifier, link-layer address, or any other suitable identifier. In this case, the MN-ID we used is link-layer address;
2. then the previous access network (P-AN) is the access point to which the MN is currently attached. P-AN sends a handover indication message including the MN-ID and New AP ID to inform the PAR of MN’s handover request;
3. the PAR derives the NAR from the t-AP ID, and sends an HI message to the NAR. The HI message includes the MN-ID, the HNP and the address of the MAP that is currently serving the MN;
4. upon receiving HI message, the NAR sends a Local Proxy Binding Update (LPBU) mes-sage to MAP. The LPBU mesmes-sage has the O flag bit set and includes the NAR’s address (LPCoA). In the meantime, the NAR replies HAck message to the PAR with P flag set;
5. a bidirectional tunnel is established between the PAR and NAR after receiving HAck message. The packets heading for the MN are forwarded from the PAR to the NAR over this tunnel. After packet de-capsulation, those packets will be buffered at the NAR;
6. when the MAP received a LPBU message, the MAP replies a Local Proxy Binding Ac-knowledgement (LPBA) message. The LPBA message has the O flag bit set and includes the MN’s HNP. It also creates the BCE and sets up its endpoint of the bidirectional tunnel to the NAR;
7. on receiving the LPBA message from MAP, the NAR sets its endpoint of the bidirectional tunnel to the MAP and then forwards the packet heading for the MN. After these packets are de-capsulated, they are also buffered at the NAR. At this moment, the NAR has all the required information for emulating the MN’s home network. It sends RtrAdv messages to the MN and advertises the MN’s HNP as the hosted on-link prefix;
8. when the MN performs a handover to the new access network, it establishes a physical link connection to the N-AN. If this link connection has not yet been established, it in turn triggers the link-layer connection establishment between the N-AN and NAR;
9. the NAR starts to forward packets heading for the MN via the new access network. Note that the NAR first forwards packets which are buffered at the PAR, and then forwards packets which are buffered at the MAP.
3.3.4 Reactive Mode
In the case of the reactive handover for PFHMIPv6, the NAR sends the HI message to the PAR after the MN has moved to the new link. The bidirectional tunnel was established between the PAR and NAR after the MN attached to the NAR. The establishment of bidirectional tunnel requires the information of the PAR beforehand. The information should be provided by the MN sending the AP’s ID on the old link or by the link-layer procedure between P-AN and N-AN.
Figure 3.6 illustrates the reactive fast handover procedure for PFHMIPv6, where the bidi-rectional tunnel is established by the NAR. The detailed description of the message exchange is as follows:
1. the MN is detached from the previous access network, and then performs handover from the previous access network to the new access network;
2. the MN established a connection with the N-AN, which then establishes the connection with the NAR. The MN-ID is transferred to the NAR at this time for the following pro-cedures. The old AP’s ID is also transferred to the NAR to help identify the PAR on the new link;
3. to reduce the loss of on-the-fly packets which are sent from the LMA after MN’s detach-ment, the NAR sends an HI message which sets the P flag bit and includes the MN-ID to the PAR for the establishment of a bidirectional tunnel. At the same time the NAR also sends a LPBU message to MAP for the establishment of bidirectional tunnel between the NAR and the MAP. The LPBU message sets the O flag bit and includes the NAR’s address;
4. upon receiving the HI message, the PAR sends a HAck message back to the NAR with the P flag set. The HAck message includes the HNP that is corresponding to the MN-ID in the HI message and includes the MN’s link-layer address;
5. then a bidirectional tunnel is established between the PAR and NAR. The packets heading for the MN are forwarded from the PAR to the NAR over this tunnel, and then buffered
at the NAR after de-capsulation;
6. on the other hand, the MAP received the LPBU message of the NAR and replied a LPBA message. The LPBA message sets the O flag bit and includes the MN’s HNP. It also updates the BCE and sets its endpoint of the bidirectional tunnel to the NAR; and
7. after the NAR receives the LPBA message, the following procedure is same as that in predictive mode. The NAR first releases the buffered packets at the PAR, and then re-leases the buffered packets at the MAP. Since then, the packets heading for the MN are forwarded from the MAP to the NAR over this tunnel, and then delivered to the MN via the new access network.
MN P-AN N-AN MAP
HACK (Disconnect)
MN-NAR connection establishment (substitute for UNA and FBU) (Connect)
PAR NAR
HI
Forward packets
LPBU
LPBA
(Setup BCE) Forward packets
Buffer packets
Figure 3.6 Reactive Fast Handover with proposed scheme.