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 2: Mobile IPv4 registration and AAA.
As discussed in Part 1, after Agent Discovery has been completed, the mobile node has successfully determined whether it is connected to its home network or a foreign network.
While mobile nodes might use regular IP when in the home network, and Mobile IPv4 when in a foreign network, mobile standards organizations, such as 3GPP2, treat all networks as foreign networks and therefore require Mobile IPv4 to be used at all times. RFC 3344 defines three registration events for the mobile node: Registration, Deregistration, and Reregistration.
Mobile IPv4 Registration
A mobile node initiates a Registration Request (RRQ) when either the outcome of its Agent Discovery process concludes that it is connected to a foreign network or a link layer mechanism results in network layer establishment.
This RRQ allows the mobile node to request service from a foreign agent (optional), inform the home agent of its current CoA (registration), renew a registration that is about to expire (reregistration), or deregister.
RRQ messages originate from a mobile node and are destined for a mobility agent. The mobility agent might be a foreign agent or a home agent, depending on both the mobile node’s preference for CCoA and the foreign agent’s preference for registration.
Table 5-1 below includes the different scenarios that determine whether the mobile node registers through a foreign agent relay or directly with a home agent.
Table 5-1 Policy Enforcement Interaction Descriptions
When registering directly with the home agent, RRQs and RRPs are exchanged directly between the mobile node and home agent. When registering with the foreign agent, all RRQs originated by the mobile node are sent to the foreign agent, which acts as a relay to the home agent.
The home agent replies with a Registration Reply (RRP) message. The foreign agent also acts as a relay for the RRP toward the mobile node. Figure 5-10 below illustrates the foreign agent’s role in the Mobile IPv4 registration process.
Click on image to enlarge.
Figure 5-10 Mobile IPv4 Registration Process
The registration process also has several other optional capabilities:
Option #1: Allowing the mobile node to discover its home address
Option #2: Maintaining multiple registrations simultaneously for tunneling multiple copies of the same IP packet to each registered CoA
Option #3: Deregistering specific CoAs
Option #4: Discovering the home agent address dynamically