Mobile IP is a standard communications protocol, developed by the Internet Engineering Task Force (IETF) and designed to allow mobile-device users to move from one network to another while maintaining their permanent IP address.
Mobile IP is an enhancement of the Internet Protocol (IP). It allows for Internet traffic to be forwarded to mobile devices, also called mobile nodes, when they are connecting through networks other than their home network. In wireless computing, Mobile IP is the technology that enables a user to receive information, such as e-mails and files directly to one’s laptop, without the sender’s knowledge of the serving network IP address.
It is predicted that wireless access will soon become the dominant means of connecting to the Internet. As it becomes more common, mobile users will expect similar levels of connectivity and service quality to wireline users, and Internet protocols will need to be further developed to meet the technological challenges ahead. Mobile IP, as it currently stands, presents a problem in terms of performance and scalability.
The problems with Mobile IP
Mobile IP has several weaknesses when it comes to supporting mobile device mobility, which can be categorized under the following headings:
- address space
- quality of service
- unnecessary overheads
Mobile IP requires changes to its forwarding function in order to adequately support node mobility. The required changes are based on the care-of address (COA). This is the IP address that marks the mobile node’s current location on the network. The care-of address, in Mobile IP, is used
- to advertise the mobile device’s new point of attachment during registration or it is stored for future use at the mobile node’s home agent
- by the home agent to tunnel data traffic from the home network to the new network indicated by the care-of address
The association between the mobile node and the care-of address it receives as it moves location is known as binding. When a mobile device changes network, it registers with its home agent each time. This can be a slow process, resulting in a latent period, and for mobile devices that continually change networks, this registration process can become totally inefficient. This need for frequent re-registration is one of the main criticisms leveled at the Mobile IP protocol.
Mobile IP registration
Mobile IP also requires a pool of valid addresses to serve as COAs inside each domain. As the Internet continues to grow, the Mobile IPv4 address space is now reaching its limit. Mobile IPv6 has been developed to try to resolve this problem by using 128-bit addresses, but it’s rollout is proving to be slow and IPv4 is expected to be around for some time yet.
Quality of service
For mobile device users, the quality of service (QOS) is affected by re-registration – the mobile node continually changes COA, leading to a heavy signaling load and latency issues that are incompatible with the provision of a quality service.
The Mobile IP protocol tunneling mechanism causes an increase in overhead costs due to delays, packet loss, and signaling problems. Packet loss and delays during handoff can be caused by the creation of new tunnels. Packet loss is likely if the mobile device is far away from its home network. Delays are due to the roundtrip of the registration request (it is sent to the home agent and the response is sent back to the foreign agent). Signaling problems are encountered when the constant registration-requests process places a burden on the core network.
In mobile networking, security is a major concern. For a network operation to be authorized, correct authentication is essential. Mobile IP has some security vulnerabilities, which means that mobile devices are vulnerable to security breaches. One of the most common security breaches is a denial-of-service attack.
Micromobility protocols are the proposed solution to the problems encountered with Mobile IP. By using a micromobility protocol, private addresses can be used – the micromobility protocol is transparent to the network outside a domain. This provides a cheap and effective solution to the address space problem.
By using a micromobility protocol, the mobile node does not need to re-register as it moves within a domain. Registration will occur only when the mobile device changes domain. This improves the quality of service.
Two of the most common micromobility protocols are TeleMIP and Cellular IP. TeleMIP provides lower handoff latency and signaling overhead compared to Mobile IP. TeleMIP is also designed to address the Mobile IP address space limitations. Cellular IP combines the efficiency and scalability of IP with features found in cellular networks, such as seamless handoff support, passive connectivity, and paging.
These protocols remove the latency issues associated with Mobile IP by using a two-layer hierarchical framework to manage mobility. This ensures that every change in connectivity doesn’t have to be communicated back to the home network. This, in turn, facilitates faster handoff.
Mobile IP introduces delays through the need for constant registration when the mobile node changes location on a network. Micromobility protocols circumvent this delay because they don’t interact with the Mobile IP enabled Internet. In the micromobility model, the mobile node receives a local COA when it connects to a domain. While the mobile node is in this domain, the COA remains valid. The mobile node needs to make only one registration when it first connects to the domain. This eliminates the need for registration during handoff, which has two effects:
- it significantly reduces delay and packet loss
- it reduces the signaling load experienced by the core network
This reduction in signaling load is important because as the numbers of wireless users increase, so does the signaling overhead associated with mobility management.
The benefits of micromobility protocols
Faster handoff and a reduction in registration are two of the main aims of micromobility protocols. These enhancements are essential so the Internet can support very large numbers of wireless users. The following issues need to be considered when developing micromobility protocols:
- fast handoff
- fast security/AAA
- quality of service
An effective mobility management solution should be able to support fast handoff by redirecting packets to the mobile node’s new point of attachment on the network with very little or no delay. As discussed, a time lag is inherent in Mobile IP due to the roundtrip of the registration request. But by using micromobility protocols, the need for registration is almost eliminated. It is necessary for the smooth functioning of real-time IP applications, such as Voice-over-IP, that latency is eliminated.
One of the key features of micromobility protocols is the support for fast handoff. Fast handoff reduces delay and packet loss. The handover performance can be affected by a number of design choices, such as
- handoff control
- buffering and forwarding techniques
- radio behavior
- movement detection and prediction
- coupling and synchronization between IP and radio layers
Usually, devices such as desktop PCs and laptops maintain “always on” connections to the Internet, even when they aren’t in use. This allows the users to be contactable around the clock and to have constant access to Internet resources. Mobile-device users will expect a similar service. For mobile devices, there are two issues surrounding “always on” connections – bandwidth use and battery power.
With fixed location desktops and laptops, the location information doesn’t change and the power source is unlimited. To be continuously contactable on a mobile device, this location information needs to be continually updated, which uses up the limited battery power. When the mobile devices are idle, then much needed battery power and bandwidth are unnecessarily consumed because of the need to broadcast each new location. The ideal solution is to have the mobile node transmit nothing unless it’s in an active state. But then the network would be unable to efficiently forward packets to the mobile node because the node location would be unknown for large periods of time.
A solution employed by GSM networks is to divide geographical areas into what are called “paging areas”. Idle devices are required to register only if they change paging area, and not when they move within the same paging area. Paging thereby reduces registration to a minimum, which serves to lower the signaling overhead and conserve battery power.
Security is important when it comes to designing mobile networking protocols and systems. Micromobility protocols aim to support fast handoff control for mobile devices. Location update messages need to be authenticated, although data encryption may not be necessary in each instance. Authentication with encryption can be used to guard the privacy of mobile users who do not want to make known their current location. Micromobility protocols deal with security issues that address network performance, device performance, quality of service, manageability, and the extent of support for AAA.
When fast handoff is sought, the security mechanisms must perform the authentication operation in the short space of time available. In the traditional authentication, authorization and accounting (AAA) model, the security-aware servers are located in distant locations, which is unlikely to facilitate fast handoff. Authenticating items, such as session keys, need to be immediately available to ensure fast handoff. The extent of support the micromobility protocols have for AAA has a big impact on the applicability of the protocol.
Quality of service
The use of micromobility protocols leads to increased quality of service. With Mobile IP, a new registration must be performed each time the mobile device changes its COA. This produces a heavy load of traffic on the network. With micromobility protocols, registrations do not have to be completed each time a mobile device moves within a domain, only when it changes domain. This considerably reduces the amount of network traffic and leads to better service quality for the mobile user.