In a mobile network, when a user is using a network outside their home network, they are deemed to be ‘roaming’. Any media sent from the user's mobile node in a visited network is sent via a visited domain gateway to a home domain gateway. The home domain gateway is located in the user's home network. The media is then passed to a network node for onward transmission or as the final destination of the media.
The term “local breakout (LBO)” is used to refer to a situation when traffic from a mobile node (MN) in a visited network is not routed via the user's home network, but is instead routed in a different way. It is advantageous for the control and management of this routing to remain with the home network, in order to apply any relevant policies regarding local breakout. LBO can reduce the load on the home network if media is not routed via the home network.
LBO is not deployed in many mobile networks, although different mechanisms can be employed to provide local breakout. An example network in which it would be advantageous to employ LBO is a 3GPP Systems Architecture Evolution (SAE) architecture (see 3GPP TR 23.882 V1.9.0, System Architecture Evolution), which is a hierarchical mobile network. The SAE architecture may use Proxy-Mobile IP (P-MIP) as a mobility protocol.
The roaming architecture of 3GPP SAE provides two SAE gateways serving the MN. The first is a Home Packet Data Network (PDN) SAE Gateway, and the second is a Visited PDN SAE Gateway. Each of the PDN SAE Gateways has an interface (SGi) towards the PDN associated with that Gateway.
Consequently, two IP addresses can be assigned to the MN, because the SAE architecture is designed to apply IP mobility management in a hierarchical manner. An IP address assigned by the Home Public Land Mobile Network (HPLMN) can be used for global communication. An additional IP address assigned by the Visited PLMN (VPLMN) is used by the MN for LBO. Each IP address has different roles, characteristics and meanings; the former IP address provided by the HPLMN is a permanent IP address, whereas the IP address provided by the VPLMN is a transient IP address used only for the duration of the MN's connection with the VPLMN.
The IP address provided by the VPLMN is used for LBO. However it is cumbersome for the MN to use two IP addresses, because it is not always clear which IP address can be used, and for what purpose.
Mobile IP (MIP), which is described in IETF RFC 3344, allows users of mobile communications devices to move from one network to another whilst maintaining a permanent IP address, regardless of which network they are in. This allows the user to maintain connections whilst on the move. For example, if a user were participating in a Voice Over IP (VoIP) session and, during the session the user moved from one network to another, without MIP support the user's IP address may change. This would lead to problems with the VoIP session.
A Mobile Node (MN) is allocated two IP addresses: a permanent home address and a care-of address (CoA). The CoA is associated with a node in the network that the user is currently visiting. To communicate with the MN, packets are sent to the MN home address. These packets are intercepted by a Home Agent in the home network, which has knowledge of the current CoA. The Home Agent then tunnels the packets to the CoA of the MN with a new IP header, whilst preserving the original IP header. When the packets are received by the MN, it removes the new IP header and obtains the original IP header. The MN sends packets directly to another node via a foreign agent in the visited network. The foreign agent maintains information about visiting MNs, including the CoA of each visiting MN.
As described above, a P-MIP network (IETF draft-sgundave-mip6-proxymip6-01) can be used in a SAE network for IP mobility management. A P-MIP network is similar to a MIP network, but uses a Proxy Mobile Agent (PMA) function. This function emulates home link properties in order to make a MN behave as though it is on its home network and allows support for mobility on networks that would not otherwise support MIPv6.
A PMA is usually implemented at the access router. The PMA sends and receives mobility related signalling on behalf of a MN. When a MN connects to an access router having a PMA, the MN presents its identity in the form of a Network Access Identifier (NAI) as part of an access authentication procedure. Once the MN has been authenticated, the PMA obtains the user's profile from a policy store. The PMA, having knowledge of the user profile and the NAI, can now emulate the MN's home network. The MN subsequently obtains its home address from the PMA. The PMA also informs the MN's Home Agent of the current location of the MN using a Binding Update message. The Binding Update message uses the NAI of the MN. Upon receipt of the Binding Update message, the Home Agent sets up a tunnel to the PMA and sends a binding acknowledgement to the PMA. On receipt of the Binding Acknowledgement, the PMA sets up a tunnel to the Home Agent. All traffic from the MN is routed to the Home Agent via the tunnel.
The Home Agent receives any packet that is sent to the MN, and forwards the received packet to the PMA through the tunnel. On receipt of the packet, the PMA removes the tunnel header and sends the packet to the MN. The PMA acts as a default router on the access link. Any packets sent from the MN are sent via the PMA to the Home Agent, which then sends the packet on to its ultimate destination.
Where a P-MIP network is used with SAE, it would be advantageous to provide LBO without the possible problems caused by using two IP addresses for the MN. These problems include advertising multiple IP prefixes to the MN. The MN may be capable of using multiple IP addresses but it may not be knowledgeable about how each IP address should be used.
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