01-10-2014, 11:12 AM
Mobile IP Data Encapsulation and Tunneling
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Introduction:
Once a mobile node on a foreign network has completed a successful registration with its home agent, the Mobile IP datagram forwarding process described in the general operation topic will be fully “activated”. The home agent will intercept datagram intended for the mobile node as they are routed to its home network, and forward them to the mobile node. This is done by encapsulating the datagram and then sending them to the node's care-of address
The Mobile Node sends packets using its home IP address, effectively maintaining the appearance that it is always on its home network. Even while the Mobile Node is roaming on foreign networks, its movements are transparent to correspondent nodes.
Data packets addressed to the Mobile Node are routed to its home network, where the Home Agent now intercepts and tunnels them to the care-of address toward the Mobile Node. Tunneling has two primary functions: encapsulation of the data packet to reach the tunnel endpoint, and decapsulation when the packet is delivered at that endpoint. The default tunnel mode is IP Encapsulation within IP Encapsulation. Optionally, GRE and minimal encapsulation within IP may be used.
Typically, the Mobile Node sends packets to the Foreign Agent, which routes them to their final destination, the Correspondent Node, as shown in Figure
Mobile IP Data Encapsulation Techniques
Encapsulation is required because each datagram we intercept and forward needs to be resent over the network to the device's care-of address. In theory, the designers might conceivably have done this by just having the home agent change the destination address and stick it back out on the network, but there are various complications that make this unwise. It makes more sense to take the entire datagram and wrap it in a new set of headers before retransmitting. In our mail analogy, this is comparable to taking a letter received for our traveling consultant and putting it into a fresh envelope for forwarding, as opposed to just crossing off the original address and putting a new one on.
The default encapsulation process used in Mobile IP is called IP Encapsulation within IP, defined in RFC 2003 and commonly abbreviated IP-in-IP. It is a relatively simple method that describes how to take an IP datagram and make it the payload of another IP datagram. In Mobile IP, the new headers specify how to send the encapsulated datagram to the mobile node's care-of address
IP-in-IP encapsulation
IP in IP is an IP tunneling protocol that encapsulates one IP packet in another IP packet. To encapsulate an IP packet in another IP packet, an outer header is added with SourceIP, the entry point of the tunnel and the Destination point, the exit point of the tunnel. While doing this, the inner packet is unmodified (except the TTL field, which is decremented). The Don't Fragment and the Type Of Service fields should be copied to the outer packet. If the packet size is greater than the Path MTU, the packet is fragmented in the encapsulator, as the outer header should be included. The decapsulator will reassemble the packet.
Minimal Encapsulation
As seen with IP-in-IP encapsulation , several fields are redundant. For example, TOS is just copied, fragmentation is often not needed etc. Therefore , minimal encapsulation as shown in figure is an optional method for mobile IP.The tunnel entry point and endpoint are specified. In this case, the field for the type of the following header contains the value 55 for the minimal encapsulation protocol. The inner header is different for minimal encapsulation. The type of the following protocol and the address of MN are needed. If the S bit is set, the original sender address of the CN is included as omitting the source is quite often not an option. No field for fragmentation offset is left in the inner header and minimal encapsulation does not work with already fragmented packets.
Generic Routing Encapsulation
Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol internetwork.
GRE encapsulates packets into IP packets and redirects them to an intermediate host, where they are de-encapsulated and routed to their final destination. Because the route to the intermediate host appears to the inner datagrams as one hop, Juniper Networks EX Series Ethernet switches can operate as if they have a virtual point-to-point connection with each other. GRE tunnels allow routing protocols like RIP and OSPF to forward data packets from one switch to another switch across the Internet. In addition, GRE tunnels can encapsulate multicast data streams for transmission over the Internet.
Optimization:
The route optimization extension adds a conceptual data structure, the binding cache, to the correspondent node and to the foreign agent. The binding cache contains bindings for mobile nodes' home addresses and their current care-of addresses. With the binding the correspondent node can tunnel datagram directly to the mobile node's care-of address.
Every time the home agent receives a datagram that is destined to a mobile node currently away from home, it sends a binding update to the correspondent node to update the information in the correspondent node's binding cache. After this the correspondent node can directly tunnel packets to the mobile node. Thus direct bi-directional communication is achieved with route optimization, as shown in Figure 5
Conclusion:
An IP tunnel is an Internet Protocol (IP) network communications channel between two networks. It is used to transport another network protocol by encapsulation of its packets.
IP tunnels are often used for connecting two disjoint IP networks that don't have a native routing path to each other, via an underlying routable protocol across an intermediate transport network. In conjunction with the IPsec protocol they may be used to create a virtual private network between two or more private networks across a public network such as the Internet. Another prominent use is to connect islands of IPv6 installations across the IPv4 Internet.
In IP tunneling, every IP packet, including addressing information of its source and destination IP networks, is encapsulated within another packet format native to the transit network.
At the borders between the source network and the transit network, as well as the transit network and the destination network, gateways are used that establish the end-points of the IP tunnel across the transit network. Thus, the IP tunnel endpoints become native IP routers that establish a standard IP route between the source and destination networks. Packets traversing these end-points from the transit network are stripped from their transit frame format headers and trailers used in the tunneling protocol and thus converted into native IP format and injected into the IP stack of the tunnel endpoints. In addition, any other protocol encapsulations used during transit, such as IPsec or Transport Layer Security, are removed