Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams

In this series of six articles, the authors of “Building the Mobile Internet“ provide a tutorial on
extending Internet connectivity into mobile networking by using extensions of
protocols such as IPv4 and IPv6 as well as mobile specific protocols such as
DSMIP, IKEv2 and MoBIKE.
Part 3: Mobile IPv4 Tunnels, Bindings, & Datagram Forwarding.


Upon successful registration to the mobility agent as outlined in Part 1 and Part 2, the mobile node is then able to send traffic to a corresponding node.


From a mobility perspective, the challenge is no longer related to identifying the point of attachment and signaling layer process, but instead a bearer plane problem-delivering all packets from correspondent nodes to the CoA of the mobile node. This traffic is encapsulated between the CoA and the home agent in either IP in IP, generic routing encapsulation (GRE), or minimal encapsulation.


The mobile node routes traffic to its default router. When using a foreign agent CoA, the default router might be the foreign agent CoA or be selected from the list provided in the ICMP Router Advertisement portion of the Agent Advertisement message. Figure 5-16 below illustrates the bearer plane functions of Mobile IPv4 with foreign agent CoA.

Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams
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on image to enlarge.


Figure 5-16. Module IPv4 routing with foreign agent clock


When the mobile node is using a CCoA, the default router is selected from the list provided in the ICMP Router Advertisement portion of the Agent Advertisement message, as long as the network prefix of the router selected matches the network prefix of the mobile node CoA message.


Figure 5-17 below illustrates the bearer plane functions of Mobile IPv4 with CCoA. It is important that the mobile node does not issue any broadcast Address Resolution Protocol (ARP) messages while connected to a foreign network.

Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams
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on image to enlarge.


Figure 5-17 Mobile IPv4 Routing with CCoA


Tunneling and Reverse Tunneling

By default, the Mobile IPv4 tunnel established between the CoA and home agent is unidirectional. The mobile node sends traffic directly to a correspondent node, while the correspondent node sends traffic to the home agent. Figure 5-18 below illustrates triangular routing.

Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams
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on image to enlarge.


Figure 5-18. Mobile IPv4 triangular routing.


As you can see in Figure 5-18, the routing path forms a triangle with the following vertices:


1) Traffic from the mobile node to the correspondent node is sent directly through the foreign agent.
2) Traffic from the correspondent node to the mobile node is first sent to the home agent.
3) The home agent then encapsulates the traffic and forwards the traffic to the mobile node CoA.

Triangular routing is not just inefficient, but it also causes problems for many network elements that rely on bidirectional communication flows or topologically accurate source/destination address pairs.


For example, firewalls and other border routers at network ingress points can discard flows destined for the home agent because the mobile-initiated connection originally exited the network through a different border gateway.


To resolve the triangular routing problem, Mobile IPv4 reverse tunneling, standardized in RFC 3024, is used. Reverse tunneling forces traffic to be routed symmetrically, through the home agent, in both the forward and reverse paths.

Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams
Click
on image to enlarge.


Figure 5-19. Mobile IPv4 reverse tunneling


The mobile node, when configured for reverse tunneling, uses the foreign agent as its default gateway, and the foreign agent encapsulates all traffic and sends it to the home agent.


In this way, the home agent essentially acts as the border router for the Mobile IPv4 domain. Figure 5-19 above illustrates Mobile IPv4 reverse tunneling.

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