The
The increasing popularity of mobile devices, such as PDAs, handhelds, and digital cellular phones, is beginning to change our perceptions of the Internet. A need has been generated to allow users to attach to any domain convenient to their current location. Confident access to the Internet anytime and anywhere will help free users from the ties that bind them to their desktops. A mobile networking environment has the potential not just to extend that flexibility but to fundamentally change the way the world communicates across administrative boundaries and geographical constraints. Users may now begin to enjoy the convenience of seamless and continuous connectivity to the Internet.
Mobility for IPv4
Mobile nodes were not considered when the Internet Protocol (IPv4) was designed. Then and now, a node's IP address, which indicates its point of attachment to the Internet, is assumed to remain unchanged for the duration of a session.
Mobile IP, a standard proposed by the Internet Engineering Task Force (IETF), was designed to solve this problem by allowing the mobile node to use two IP addresses: a fixed Home Address and a Care-of Address (COA) that changes at each new point of attachment to the Internet.
How Mobile IP Works
Mobility Support for IPv4 defines a protocol that allows transparent routing of IP datagrams to Mobile Nodes as they move about from one domain to another on the Internet. When a Mobile Node moves into a foreign network, its computing activities are not disrupted. Instead, all the needed reconnection occurs automatically and without user interaction.
Roles
The Role of the
A MN is responsible for detecting change in network connectivity and acquiring a care-of address. It initiates the process of informing its Home Agent of it current Care-of Address. Mobile Nodes using colocated a Care-of Address also needs to perform tunneling and encapsulation of packets.
The Role of the Foreign Agent
A Foreign Agent relays location updates and acknowledgments between the Home Agent and Mobile Node. If it is also the Care-of Address for the Mobile Node, the Foreign Agent forwards encapsulated packets destined to the Mobile Node. The Foreign Agent generally serves the Mobile Node as its default router.
The Role of the Home Agent
A Home Agent processes and coordinates mobility services for the Mobile Node. The Home Agent receives location updates from the Mobile Node, and acknowledges the updates with the result. The Home Agent recives packets that arrive on the network destined for a Mobile Node that it serves and tunnels them to the Mobile Node’s Care-of Address.
How a
When the Mobile Node is not attached to its home network, the Home Agent receives all packets destined for the Mobile Node's home address. The Home Agent encapsulates the original IP packet and directs it to the Mobile Node's Care-of Address. When the packet arrives at the Care-of Address, the original IP packet is extracted and delivered to the Mobile Node. This encapsulation is also called tunneling.
How a
Tunneling is generally not required when the Mobile Node sends a packet. The Mobile Node transmits an IP packet with its Home Address as the source IP address. The packet is routed through the Mobile Node's default router in the foreign network.
In networks that do source IP address checking, reverse tunneling may be desirable. With reverse tunneling, packets from the Mobile Node are encapsulated by the Care-of Address and sent to the Home Agent. The HA decapsulates these packets and routes them to the original destination.
Figure 1.1 shows the data path of an IP packet from a Correspondent Node to a Mobile Node.
Figure 1.1 Data path of an IP packet sent from a Correspondent Node to a Mobile Node
Home Agents and Foreign Agents, at regular intervals (every few seconds), broadcast on their subnet, messages known as Agent Advertisements. The Agent Advertisement is designed as an extension of the already existing ICMP router advertisement [RFC 1256] message. The agent advertisement conveys the following information:
o Whether the agent is a Home Agent or a Foreign Agent.
o A list of available Care-of Addresses (in case of the Foreign Agents).
Agent Solicitations
The Mobile Node may also broadcast or multicast an Agent Solicitation message. Any Home Agent or Foreign Agent that receives the agent solicitation message responds with an Agent Advertisement.
How a Mobile Node gets the Care-of Address
A Mobile Node, when attaching to a foreign network, must acquire a Care-of Address on that network. There are two ways of achieving this:
1. Foreign Agent Care-of Address: A Foreign Agent Care-of Address is a Care-of Address acquired by the Mobile Node from a Foreign Agent’s advertisement broadcast. The foreign Care-of Address is registered with the Home Agent and the Foreign Agent serves as the endpoint in the tunnel for encapsulated packets sent from the Home Agent to the Mobile Node.
터널 구간 : HA <-> FA (FA의 인터페이스 주소가 CoA주소)
같은 CoA주소 공유(주소 부족 문제 해결)
2. Co-located Care-of Address: A co-located Care-of Address is a Care-of Address acquired by the Mobile Node as a local IP address through some external means, such as DHCP [RFC2131], that the Mobile Node then associates with one of its own network interfaces. When using a co-located Care-of Address, the Mobile Node serves as the endpoint of the tunnel and performs decapsulation of datagrams tunneled to it.
DHCP처럼 스스로 주소 할당
터널 구간 : HA <-> MN
Location Updates - Registering the COA
When a Mobile Node attaches to a new foreign network, it sends a Registration Request to its Home Agent to register its Care-of Address. If the Mobile Node is using a Foreign Agent Care-of Address, the Registration Request is sent via the Foreign Agent. The Registration Request includes an extension with a cryptographic authentication value (HMAC-MD5 or keyed MD5). The Mobile Node calculates the authentication value based on fields in the Registration Request and a static key (shared secret key) that is specified on both the Home Agent and the Mobile Node.
The Home Agent authenticates the Mobile Node’s Registration Request by calculating its own authentication value and comparing it with the authentication value from the Mobile Node. After the Home Agent authenticates the Registration Request, it sends a Registration Reply message to the Mobile Node. The Registration Reply also includes a lifetime for the registration.
Deregistering the Care-of Address
A Mobile Node, upon returning to its home network or upon session termination, sends the Home Agent a Mobile IP Registration Request message for deregistration. The Home Agent removes its mobility binding for the Mobile Node. Deregistration with the Foreign Agent occurs automatically when the registration lifetime expires.
IP Encapsulation and Tunneling
When the mobile node is away from its home network, the Home Agent receives all packets destined for the Mobile Node's home address. The Home Agent encapsulates each packet in a new packet. The destination address of this encapsulated packet is set to the Mobile Node’s Care-of Address. This is called IP Encapsulation.
This encapsulated packet is then routed to the Mobile Node’s Care-of Address. When this encapsulated packet arrives at the Mobile Node’s Care-of Address, the original IP packet is extracted (or decapsulated) and the original packet is routed to the Mobile Node.
This use of encapsulation and decapsulation of a datagram is frequently referred to as tunneling the datagram, and the encapsulator and decapsulator are then considered to be the endpoints of the tunnel.
Reverse Tunneling [RFC 3024]
A Mobile Node, when communicating with a Correspondent Node, typically sends packets directly back to the Correspondent Node. With reverse tunneling, the IP packet is encapsulated in a new packet with the destination address set to the Home Agent. The Home Agent decapsulates the tunneled datagram and routes it to the original destination.
Reverse tunneling is useful for installations with ingress filtering (filters that check inbound packets for topologically correct source addresses). With reverse tunneling, the packet leaving the foreign network will have a topologically correct source address, because the source address in the outermost IP header will be an address from the foreign network (the Care-of Address) instead of the Mobile Node’s home address.
Route Optimization
HP-UX Mobile IPv4 facilitates the routing of datagrams directly from the Correspondent Node to the Mobile Node’s Care-of Address without going through the Mobile Node’s home network. This improves the data transmission rates between the Correspondent Node and Mobile Node and reduces congestion in the home network.
Normally, packets from the Correspondent Node to the Mobile Node are sent through the Home Agent (and through the Mobile Node’s home network). When Route Optimization is used, the Home Agent sends an authenticated message to the Correspondent Node with the Mobile Node’s current Care-of Address. This message informs the Correspondent Node to form a route optimization tunnel to the Mobile Node's Care-of Address.
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