Why tunneling?
• Consider a situation when a Correspondent Node (CN) wants to send an IP packet to a Mobile Node (MN). All the CN knows about this MN is its IP address.
• The CN is totally unaware of the MN’s location. (Which in fact is a major requirement of Mobile IP) and so sends it as usual to MN’s IP address.
• The internet routes this packet to the Home router of the MN also called as Home Agent (HA).
• The HA now knowing that the MN is not in its home network send encapsulates and tunnels it to the COA.
• The Care-of-address (COA) defines the current location of the MN from an IP point of view (e.g. when a person Mr. XYZ stays as a guest in someone else’s home , the letters he receive will be marked as Mr. XYZ ,C/O i.e. care-of Mr. ABC)
• Since internet routes are created based on the header contents of an IP packet, to route it from HA to COA, we need a new to create header for the packet to be transmitted.
• The new header on top of the original header is made (refer diagram-1). Now this will enable us to set a new direct route (a tunnel) to the MN from the HA as it is roaming.
• Tunnelling: It is the process of creating a tunnel by the HA to the COA to route packets to the Mobile Node as it roams.
• It establishes a pipe (a data stream between two connected ends) wherein the data is inserted and moves in FIFO order.
• Encapsulation: Tunnelling 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. Let us study GRE technique.
(Note: the above section was expanded so that the reader understands well… in exam just write the definition…)
Generic Routing Encapsulation (GRE):
• The GRE encapsulation method supports other network layer protocols in addition to IP as compared to IP-in-IP and minimal encap. method in addition which supports only IP.
• GRE is a tunnelling mechanism which uses IP as the transport protocol and can be used for carrying many different passenger protocols.
• The packet of one protocol suite (which contains a header and data part) is taken and a new GRE header is prepared for it . Together they combine to form the data-part of the packet to be transmitted ( data=GRE header + Original Header +Original Data)
• After this the header of the second protocol suite (i.e IP protocol-check second point above) is put in front of this data.
• As seen here, the outer header (shaded white) is the standard IP header with HA as source and COA as destination.
• The protocol type field contains 47 which is value for GRE.
• The other fields like TTL, DOS can be copied from inner header.
• The GRE header contains several flags which indicate if certain fields are present or not like
- C -> checksum field contains a valid IP checksum
- R -> offset and routing fields are present
- K -> if K=1 key for authentication is used.
- S -> indicates if sequence number field is present
• rec.: The recursion control field is an important field that distinguishes GRE from other techniques. This field is a counter that contains the allowed number of recursive encapsulations.
o When a packet arrives for encapsulation it first checks this field if it is zero. If it is not zero then additional encapsulation is allowed. After encapsulation, counter decremented by 1.
o It prevents indefinite recursive encapsulation. It defaults value is 0 which means only one level of encapsulation.
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