Posts Tagged ‘DHCP
Essentially, IP addresses need to be assigned to the client’s machines. Depending on the ISP one or more of the following alternatives are provided to the client, also depending on the subscription agreement.
1. One public static IP address: In this case, the ISP reserves an IP address from its public network address range for the specific home client. The client node is always assigned the same IP address, which is also routable from the public Internet. This option is good for hosting application servers on the client’s machine, since there is guarantee that it can always be accessed from any other machine on the Internet. However, a permanent address makes this client more vulnerable to repeated security attacks.
2. One public dynamic IP address: As in the first case, the client’s machine is assigned a public, Internet routable IP address, however this is not static. This means that it usually changes every time the machine is connected on the network, but it can also change at any time, depending on the ISP’s configuration. The request and assignment of an IP address to the client’s machine is done using the Dynamic Host Configuration Protocol (DHCP). There are two main reasons for using this dynamic IP assignment. Firstly, it is done for public IP address re-usage, since on IPv4 there are not many available addresses any more. The ISP assumes that not all customers are connected at the same time on the network and so it can have a small number of public IP addresses. The second reason is that most people meet their requirements with a dynamic IP address, except those advanced users who want to run servers on top of their connection. Servers would obviously cause extra traffic on the ISP’s network and thus the ISP wants to sell to these users the more expensive option of the public static IP address.
3. Multiple public IP addresses: The first two cases covered most home users until recently, since for most of them Internet connectivity meant access via a single home computer. However, modern households have more than one device that can access the Internet. In this case, more than one IP address is needed – one per device – if the users wants to simultaneously use those devices for accessing the Internet. Some ISPs provide the option of multiple public, either static or dynamic, IP addresses per connection. However, this is a rare case and usually costs extra money.
4. Multiple private dynamic IP addresses: This is probably the worst connectivity option a home user can get, since the ISP is assigning to the customer’s machines IP addresses that are not routable from the public Internet. In this case, the user can still use typical Internet services (web browsing, e-mail, etc), but cannot necessarily use more advanced services that require end-to-end connectivity, such as some audio/video conferencing applications. Also client cannot easily run any application servers on the home machines, since those cannot be accessed from the public Internet. Note, that there are some partial solutions for this home service accessibility, based on techniques above the network/IP layer.
One aspect that has been overlooked in mobile research is link layer access. Most mobility solutions assume that the link layer configuration will be automatic and base trigger mechanisms in the presence of network layer connectivity.We believe that there is the need for a framework for link layer access, to standardize the operating system interface, creating an unified API to report the presence of access point in the vicinity of the mobile, and to do AAA(Authentication, Authorization and Accounting). A multiplexing transport protocol has to be aware of new link layers that become available, and of link layers that can no longer be used, to add and remove these interfaces from protocol processing. To this end, a link-layer aware transport protocol needs the following support:
Link layer management: a management entity can usedirect information (by probing or listening to the link layer for the presence of access points) or indirect information(by using an existing connection to query the infrastructure for the existence of additional access points) to find new access points. This is called link layer discovery. Management also encompasses measuring signal strength and possibly location hints to rule that a link layer is nolonger usable. This is called link layer disconnection.
Network layer management: before using a link layer,the mobile has to acquire an IP address for that interface. The most common protocol for acquiring a network addressin broadcast media is DHCP (Dynamic Host Configuration Protocol). For point-to-point links, such as infrared, acquiring a network address also entails creatinga point-to-point link. In this case, the link will only be created on demand, as creating the link precludes other mobiles from using the same access point.
Transport layer notification: the transport layer has to benotified of new access points (in the form of a new IP address it can use) and of the loss of an active access point(an IP that can no longer be used). The transport protocols can also notify a management entity about the available bandwidth of each link. Because this bandwidth is closely tied with the available bandwidth of the last hop, by controlling the maximum bandwidth each protocol instance can use the management entity to enforce usage policies for cooperating protocols.