Mobile IP

Mobile IP is a network protocol standard of the Internet Engineering Task Force ( IETF ), which was designed to allow users of mobile devices (eg laptops ) to switch from one computer network to another and at the same time a fixed IP address to keep.

Mobile IP provides an efficient mechanism for scaling and the mobility of computers on the Internet. With Mobile IP, it is possible that mobile hosts change their point of access to the Internet and still retain their (static ) IP address. This ensures that the compounds remain in the transport layer as a power -cut occurs.

  • 5.1 transmission from the transmitter to the home network
  • 5.2 Transfer from the home agent to the mobile host
  • 6.1 Problems of the standard procedure
  • 6.2 Reverse Tunneling
  • 7.1 ARP in the home network
  • 7.2 Co Located Care -of -Address
  • 7.3 Foreign Agent Care -of Address
  • 7.4 Finding the Foreign Agent

Initial situation

IP came at a time when mobile computer that would access any time and any place on the Internet, were barely conceivable. However, a mobile computer may no longer receive data without change to the original protocols, as soon as he leaves the network for which it was originally configured. In a network based on TCP / IP basis he has like all the other nodes have a unique IP address, which can recognize the affiliation to a specific subnet. If he leaves the existing network, it must obtain a new topologically correct IP address whose network portion is part of the address range of the new subnet. Manually change the configuration is not reasonable for many users, the protocol stack should react autonomously to the change.

DHCP ( Dynamic Host Configuration Protocol ) support mobility by allowing the automatic configuration of newly connected computer. The change of location results even though a change in the IP address. Provides a mobile computer itself provides services, he may then not be found from its communication partners in the network. Although enables DNS (Domain Name System ) is a mapping of a logical name to an IP address, but it takes a while to have spread in the network changes. Frequent, rapid changes of the mapping would be too time-consuming due to a number of necessary update messages.

In addition, higher layer protocols rely on IP addresses, eg TCP, whose connections through pairs of IP addresses and ports (sockets) are identified. A change of IP address during an open TCP connection can break this. Maintains a node but in spite of the temporary movement his address, the path changes through the net. The routing fails.

Considering these problems, IP should be expanded in terms of mobility support.

Motivation and requirements

With Mobile IP is to be achieved, that a mobile computer is always accessible via a permanent IP address, even if it moves from network to network. This allows the maintenance of compounds at higher layers, such as TCP. The following requirements must be met:

  • Compatibility: changes to the existing infrastructure (nodes, applications, protocols ) should not be necessary. It should have the same address formats and routing methods can be used.
  • Transparency: The mobility should not be visible layers above the network layer, so they can work unchanged. For TCP this as meaning that the computer to which you are connected, retains its IP address.
  • Scalability: The number of mobile terminals may change, without the process must be modified.
  • Efficiency: The amount of data transferred should be minimal.
  • Security: All packets that are used for inserting the mobile computer in the network must be authenticated. This includes the integrity of the data and the evidence of their origin.

Overview and basic concepts

The operation of Mobile IP can be compared to a redirection of mail. A receiver changes its location and gets a new address. He instructed the post office at his old residence with a redirection. The recipient address of the letters addressed to him there is pasted over with the new address, so that broadcasts can be forwarded to the new address. The post at the new location, the program may then forward it to the recipient without the sender knows the move of the recipient.

Similarly, it allows Mobile IP to address mobile computer even if you change the network under a fixed (the " original " ) address, without the communication partners gained knowledge of its mobility. The ongoing communication can be maintained.

The subnet for which the mobile computer was originally configured is called the home network ( Home Network). The subnet in which the mobile computer is staying current is called the foreign network (Foreign Network).

The home agent ( Home Agent ) is a node in the home network to which a mobile computer area, and if it is in a foreign network. It is designed to catch addressed to the mobile host packets and forward them to him. Moves the mobile computer into a foreign network, it must be there at a foreign agent ( Foreign Agent ) sign. This stores information about the mobile computer, eg its topologically correct IP address ( care-of address). The foreign agent can simultaneously serve as a default router for the mobile computer if it wants to send data from the foreign network beyond itself.

The IP address of the mobile computer in the home network is referred to as the home address ( Home Address). They will be maintained when you switch to another network. The care-of address ( CoA Care -of- Address) is the topologically correct IP address, the mobile computer can be reached in a foreign network. It usually corresponds to an IP address of the foreign agent, in which the mobile computer has been logged.

The developed over IP protocol extensions include three main mechanisms:

  • Agent Discovery: Allows a mobile computer, locate potential agents.
  • Registration: Allows the mobile host to inform its home agent of its care-of address.
  • Tunneling: Agreements forwarding rules of packets.

Preparatory measures

Before the data transport between a communication partner and the mobile computer which has left its home network, you can start making some preparations are necessary.

Agent Discovery

The mobile computer must at all times be able to determine his current location as well as potential agents. This information process is known as agent discovery and consists of two types of messages, the Agent Advertisement and Agent Solicitation. Both use an extension of ICMP ( Internet Control Message Protocol ), a protocol for the exchange of error messages and messages for controlling data transfer between network devices.

Under Agent Advertisement is meant as a remote broadcast at regular intervals messages from home or foreign agents who offer their service possible mobile computers in their subnet. By listening to these messages the mobile computer can determine if it is in its home network or a foreign subnet and which is its current foreign agent. It can also learn whether his whereabouts has changed since the last advertisement. Agent Advertisements may contain the following information:

  • Whether the agent is as a home or foreign agent available
  • Whether as the mobile host must register with the foreign agent, even when using a co -located care-of address,
  • Supported encapsulation,
  • The announcement of an available care-of address (in the case of a foreign agent ),
  • The validity of a registration.

Alternatively, the mobile computer can be an agent solicitation emit an explicit invitation to potential agents to send an Agent Advertisement. So he can enforce that give potential agents immediately to recognize and to ensure that waiting time is reduced.

Allocation of a care-of address

To the foreign network to be accessible, the mobile computer requires a care-of address, which belongs to the address space of the current subnet. It defines where ultimately addressed to the mobile host packets are sent. Has the mobile computer registered in the foreign network with a foreign agent, a foreign agent care-of address it can now be assigned. This is generally an IP address of the foreign agent. Several mobile computers can use the same foreign agent care-of address.

Ultimately owns a mobile computer so two IP addresses, the permanent home address and a temporary care-of address. Possible communication partners is the mobile computer known only by its home address.

Registration of the mobile computer in the home network

After the mobile host has received a care-of address, the active foreign agent can forward a registration request to the home agent to inform him about the new location of the mobile computer. This is necessary so that packets can be routed correctly to him. The registration request contains, inter alia, the home address of the mobile computer, the IP address of the home agent and the care-of address as the endpoint of the tunnel. In addition, the tunneling process is not specified, the data transport between home and foreign agent to run as well as parameters for authentication.

The home agent saves this information in a registration table ( Table Binding ) and can thus reach any mobile computer that has registered with it. Then, the home agent sends a response back to the foreign agent, which forwards it to the mobile computer. Hand, a mobile computer later returned to his home network, he volunteers for a Discovery Agent directly with the home agent and receives itself again intended for it packages. There is no support Mobile IP longer necessary.

Messaging

After completion of the preparatory measures of data transfer by means of Mobile IP can begin.

Transmission from the transmitter to the home network

The transmitter may reside either inside or outside of the home network of the mobile computer. Of course, he only knows the home address of the mobile computer, not its current care -of address. Want it send data to the mobile computer, and it carries its home address as the destination address, its own address as the source address. The packet passes through the ordinary IP routing process to the router, which is in charge of the home network and ultimately always first in the home network. This is possible because the home address logically belongs to the network that is connected to the home agent.

Transfer from the home agent to the mobile host

The home agent intercepts addressed to the mobile host or IP packets from representative. He enters through proxy ARP as the mobile computer. It is in the ARP message, although the IP address of the mobile computer to, however, its own MAC address. Instead of the intercepted package now forwarded to a node that is physically located in the home network, the packet is " redirected " toward the foreign agent ( tunneling). For forwarding encapsulation is necessary. By this is generally understood that a received packet to the payload of a new packet, will provide a new outer header precedes. The reverse case is referred to as de-encapsulation. Both mechanisms usually apply when data packets to under - or overlying layers of the layer model are passed. In Mobile IP, however this is done within the same protocol layer.

The received IP packet is preceded by an outer header. Header and payload of the received packet is to the payload of the new package. The routers see only the IP addresses in the outer header. The home agent checks its registration table an entry for the specified in the packet's destination address ( home address ) and reads the corresponding care-of address. This is now entered in the outer header destination address, the address of the home agent as the source address. The only change to the inner header is to reduce the lifetime of the packet ( TTL, Time to Live ) by 1 From the perspective of the original package so, the entire tunnel, regardless of the actual number of traversed routers, a length of 1 to. Thus, the mobile computer can behave as if it were directly connected to the home network, which also corresponds to the demand for transparency. A tunnel is a virtual point-to- point connection between two network nodes between which any number of subnets must lie. The enclosure is necessary for the packets to be transmitted to be understood at the start and end point of the tunnel. The package is finally conveyed through the tunnel from the home agent (starting point) to the foreign agent (end point). The foreign agent as the end point of the tunnel and the holder of the care-of - address receiving the packet removes the outer header ( decapsulation ), and finally passes further through the connection layer to the hardware address of the mobile computer. The foreign agent may not necessarily be input router of the foreign network, it can also be located at another location within the foreign network. The mobile computer is aware of its mobility so far not aware when he receives the packet with the same address information, as would have been the case in the home network.

The mobile computer as a transmitter

Want the mobile host itself to send data, so it does not send them to the home agent, but directly to its communication partner. He wears as the source address of an IP packet his home address as the destination address, the address of the recipient. The package will now be via the foreign agent - transmitted through standard IP routing to the recipient - if this acts as a default router for the mobile computer.

Problems of the standard procedure

If you compare the reception and transmission operations in accordance with this description graphically, resulting in a triangular structure, which is why the method is also called Triangle Routing. However, the efficient and at first glance very simple mechanism is not always applicable.

Routers or firewalls can filter packets for security with suspicious IP addresses. This can apply to both source and destination addresses on. The specified as the source address from the mobile computer when sending home address is in the foreign network, of course, not topologically correct, so could well be interpreted as a forgery. Further, an ingress filter can be switched in order to protect networks from unauthorized input traffic. As an example may be invalid Packets are considered whose source address is in its own network, but from outside. This is to prevent that other people's computers spend as internal computer. The specified as the source address in the packets of the mobile computer's home address may therefore well be interpreted as an attempt at deception. This can cause the mobile computer can not send packets in its own home network and thus interfere with the operation of Mobile IP. To remedy this, the filter needs to be configured so that it can pass packets whose source address belongs indeed to your own network, the sender but is also registered as a mobile computer in the home network.

Apart from this, the packet lifetime is a problem. Is the mobile computer in the home network, the packets need a certain life to reach their respective recipients. Moves the mobile computer is now in a foreign network, more hops could ( jumps) are as previously required in the home network to reach the receiver. The specified package in life would possibly be increased, however, the demand for transparency contradicts.

Reverse tunneling

To solve the above problems, reverse tunneling is designed as an additional process and is described in RFCs 3024 and 3344. These packets of the mobile computer on the way back again to be tunneled to the home agent. The foreign agent encapsulates the packets of the mobile computer and helps in the outer header source address as the care-of address as the destination address, the address of the home agent. At the end of the tunnel to the home agent receives the packet, de-encapsulates it and sends the original packet that contains the destination address, the address of the communication partner and as the source address, the home address, in the direction of the destination address via conventional routing method to the communication partner. Obviously, this method is less efficient than the triangle routing, but can be required for the reasons mentioned above.

Operation

In order to ensure the retention of the fixed IP address of each terminal is equipped with two mobile IP addresses: the primary address is the so-called home address, which is the secondary care-of - address (COA).

If the mobile computer ( Mobile Host ) leaves his home network and logs in a foreign network, he gets out of this one COA assigned, which he must notify its home agent ( a special computer in his home network ). When traffic this home agent now redirects incoming data packets to the COA and thus to the mobile host on ( IP -to- IP encapsulation ).

Will send the other hand, the mobile computer packages so he can normally without going through the home agent send, because the normal IP routing mechanisms ensure that the package reaches its destination.

Hand, the mobile computer returns to its home network, it registers with the foreign agent again, so that packets addressed to him again without going through the home agent to be delivered.

ARP in the home network

Due to the entrainment of the Home Address by the mobile computer, the result for computers that are located in the home network of the mobile computer, the problem that they do not achieve this, after he has left the home network. Through the use of gratuitous ARP and an ARP proxy, however, the home network remote mobile node can still be reached. To this end are from the Home Agent in ARP requests, ARP proxying, as the missing mobile computer.

For the management of traffic, there are two variants.

Co Located Care -of -Address

Here takes the Mobile Host the Forward Management. It uses the Home Address ( for the higher protocol layers ) and the COA ( for low ) at the same time. The COA is ( eg via DHCP ) assigned to the mobile host this case of a local IP address pool, as would happen even with a stationary host. Therefore, the local router can not distinguish whether the host is a mobile or stationary.

The advantage here is that it can be placed on the local infrastructure and no foreign agent is required.

Foreign Agent Care -of Address

Here the mobile host is assigned the COA of a special computer (Foreign Agent ) of the foreign network. For this purpose, the mobile host must first log in to the Foreign Agent ( He gets NO address from the foreign network ). The foreign agent accepts in this variant, the forwarding of the data from the HA to the mobile host (and reverse tunneling and vice versa from the Mobile Host to HA).

In order to register with the foreign agent, the mobile host first needs to know the address.

Find the Foreign Agent

Future

For IPv6, there are advanced versions such as Mobile IPv6, Hierarchical Mobile IPv6 and Fast Mobile IPv6. Thanks to the advanced routing capabilities of IPv6 is here, and others added the possibility to send packets without going through the home agent directly to the mobile computer. Above all, packet loss can be reduced or avoided in order to use time-critical applications without interruption to the AC line. With Proxy Mobile IPv6 (RFC 5213 ), the mobility management is entirely implemented on grid side, and the improved mobility can be provided so that any changes to the devices available to all users.

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