Mobile IP is the method devised to provide seamless Internet access to the mobile wireless network. The idea of Mobile IP first emerged in 1995. It is still under development.
The basic idea of Mobile IP is as follows: if a mobile node has an IP address of 120.54.32.1 and if it suddenly moves out of its subnet, then there are two solutions — either the mobile node may be assigned a new IP address, or through some mechanism it may keep on using its old IP address (e.g., 120.54.32.1).
If the moving mobile node is involved in a file transfer with a Web server, receiving a multimedia transmission and downloading e-mail, then giving it a new IP address according to the first solution will result in termination of all such connections based on TCP, which is simply not acceptable.
Research has proved that the second solution is much better. This second solution is known as Mobile IP. In Mobile IP, the home address of an MN remains unchanged, and all TCP/IP connections can be made to it.
When an MN moves to a network different from its home network, then a new IP address known as careof address (COA) is assigned to it. The MN notifies the HA at its home network of its new position.
The HA stores this information in a table and uses it to send traffic to the MN. Therefore, other hosts on the Internet do not need to know the COA of the Mobile IP. They can simply send their traffic to the home address of the mobile agent, where the HA will send it to the COA of the MN.
There is also a FA responsible for transmitting this traffic to the MN that is currently attached to that foreign network. The HA is mostly implemented on a router in the home network. The network architecture for a wireless LAN is depicted list below:
- Mobile IP/ Nomadic router.
- Network layer: VC support.
- Network layer: routing (DSDV, AODV, etc.)
- Link layer (Acks, Priority, etc.)
- MAC layer (MAC, MACA/PR, 802.3, etc.)
- Clustering (cluster TDMA)
- Connectivity.
- Radio channel.
In Mobile IP, three major functions are used:
- Dynamic COA allocation for the MN.
- Registration of the COA with the HA.
- Delivery of packets from the HA to the MN.
Mobile IPv4
As is obvious from its name, Mobile IPv4 uses the normally used 32-bit IP addresses. The HAs and the FAs broadcast advertisements at regular intervals to announce their availability.
As soon as an MN receives such an advertisement, it knows that its network has changed. These advertisement messages are an extended form of the Internet Control Message Protocol (ICMP). Router discovery messages are also known as agent advertisements.
The agent advertisements perform the following functions:
- Detect HAs and FAs.
- Provide COAs.
- Make available special features provided on foreign networks to the MN.
- Let the MN know its current location.
Registration
The registration messages are of two types — registration request and registration reply. They are sent to User Datagram Protocol (UDP) port 434. UDP provides better performance than TCP, in wireless environments. During registration, the following may be performed by an MN:
- Contacting the HA and telling about its current location.
- Temporarily routing services like Web browsing by using FA.
- Renewing registration.
- Deregistering from a foreign network after returning to the home network.
There are two cases in which deregistration may be required: first, when an MN leaves a foreign network and returns to its home network and second, when it leaves a foreign network and enters another foreign network.
In the second case, deregistration from the first foreign network and reregistration at the second network is essential. This function is performed by the MN if it receives its COA from a DHCP server and by the FA if it allocates the COA for the MN.
Tunneling
After registration is performed, the packets destined for the MN will arrive at its HA, which will send them to the COA of the HA through the process of tunneling. This is performed through encapsulation. The original IP packet is encapsulated inside another packet by IP-within-IP encapsulation, as explained in RFC 2003.
A special tunnel IP header is used with a value 4 in the outer protocol field. Except for the TTL field, the inner IP packet header is not modified. There are also other encapsulation techniques such as Minimal Encapsulation protocol and the Generic Routing Encapsulation (GRE) protocol.
The Minimal Encapsulation protocol combines the information from the tunnel header with the information from the inner Minimal Encapsulation header to reconstruct the original IP header.
The GRE protocol works in the following way. A source route entry (SRE) is provided in the tunnel header. By using the SRE, an IP source route, which includes the intermediate destinations, can be specified.
ARP by HA
HA uses a proxy Address Resolution Protocol (ARP) to enable nodes located in the home network to communicate with the mobile. This is accomplished by sending gratuitous ARP messages.
After that, every node will have updated information about the mobile node. Then the HA will use a proxy ARP to intercept node messages for the MN and will tunnel them to the COA of the MN.
Mobile IPv6
Work on IPv6 was started in 1994 to address various problems with IPv4. IPv6 permits 128-bit addresses as compared to 32-bit addresses of IPv4. This is an increase of the address space by a factor of 296 . This allows on the order of 6 × 1023 unique addresses per square meter of the surface of the Earth. This seems to solve the address allocation problem once and for all.
Major features of IPv6 are as follows:
- Route optimization.
- Transparent use of the COA in the packet.
- Use of COA as the source address.
- Neighbor discovery and address auto configuration, hence eliminating the use of FAs.
- Extensive use of IPSec for all security requirements.
- Use of IPv6 routing header instead of encapsulation by a node to directly send traffic to the MN.
- Use of anycast mechanism by mobile IPv6.
Extensions to Mobile IP
Several extensions are proposed for Mobile IP. Significant overhead is imposed by sending every packet destined for the MN through the mobile agent and using regular routing while sending the packet back from the MN. Therefore, certain extensions are proposed for Mobile IP so that a node may send traffic to the MN directly.
The HA notifies all the related nodes about the COA of the MN through the binding update message. This poses a problem. If a node sends some packets to an MN on its COA and simultaneously the MN leaves that network, then those packets would be lost.
Another extension to Mobile IP solves this problem also. By this extension, packets sent to a moved MN are directly forwarded to the new COA of the MN. Steps performed to achieve smooth data transfer between a network node and wireless nodes are as follows:
- A binding warning control message is sent by the corresponding node to the HA which tells the HA that the corresponding node is unaware of the new COA of the mobile node.
- A binding request message is sent by the corresponding node to the HA to request for the refreshment of its binding.
- An authenticated binding update message is sent by the HA to the corresponding nodes. This contains the current COA of the MN.
- The MN performs handoff with the corresponding node.
When an MN leaves a foreign network, it can either deregister itself or just perform handoff. When it reaches a new foreign network, it gets the address of the FA and sends a registration request to its HA by using the address of the FA as a COA. The HA, after processing, sends a registration reply.
Multicasting Using Mobile IP
Multicasting using Mobile IP is also possible. There are many protocols for this purpose, such as MoM (Mobile Multicast) protocol, MMA (Multicast by Multicast agent), MNG (Mobile Network Gateway), etc.
The current IETF Mobile IP multicast requires remote subscription in which the mobile host (MH) is required to resubscribe to the multicast group on each FA by using a co-located COA. This has the advantage of providing the most efficient delivery of multicast datagrams, but it has a high price.
The HA must also be a multicast router. Subscriptions must be done through the HA. For MoM, the HA need not forward a separate copy for each MH that it serves but only one copy for each foreign network at which its MH group members reside. The HA and FA maintain tables.
MoM Protocol Details
- MH arrives at foreign network.
- MH returns to its home network.
- MH times out at a foreign network.
- A unicast packet for MH arrives at MH’s FA.
- A multicast packet for group G arrives at HA.
- A tunneled packet arrives at FA from HA.
Problems and Issues of MoM
- The tunnel convergence problem.
- The duplication problem.
- Disruptions of Multicast Service.
- Packets that are sent and received by MHs must always traverse the home network, making routing nonoptimal.
- Multiple unicasts are used by the HA to tunnel multicast packets to FAs of MHs that are group members.
Deficiencies in Mobile IP
Mobile IP has been proved to be slow. Recent research has proposed that IP should take support from underlying wireless network architecture to achieve good performance. Data-link layer may be used to provide support to Mobile IP. This is also known as IP micro mobility and paging protocols.
Following are the issues of Mobile IPv4 related to macro mobility:
- Asymmetric routing.
- Inefficient direct routing.
- Inefficient HA notification.
- Inefficient binding deregistration.
Mobile IP does not provide capabilities for QoS provisioning, which is necessary to provide real-time support on mobile networks. Security is also an important issue for Mobile IP.
Mobile IPv4 does not provide reliable authentication. Ingress filtering and location privacy are also required. Some of these issues have been resolved in Mobile IPv6, such as reducing overhead and enhancing security.
