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Mobile Internet basics: Transport layer mobility challenges

Mark Grayson, Kevin Shatzkamer and Klaas Wierenga, Cisco Systems

January 10, 2012

Mark Grayson, Kevin Shatzkamer and Klaas Wierenga, Cisco SystemsJanuary 10, 2012

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 1: Dealing with transport layer mobility.

The Internet Protocol (IP) is the most commonly known example of a Layer 3 protocol, and it provides the foundation upon which the Internet itself was built. IP is responsible for connectionless transfer of packets from an end node, through a network, to another end node.

For the network to identify a specific node, an addressing scheme (IP addressing) is used. The IP address uniquely identifies an endpoint connected to the network, and all packets are sent across the network indicating both the source IP address and destination IP address. In addition, TCP, which resides at the transport layer, creates connections, identified by a 4-tuple (source IP address, source port, destination IP address, destination port), used to identify the transmission session.

If neither endpoint is mobile, sending traffic from source to destination is trivial. Routers use the hierarchical structure of Internet addressing to locate a path from the source endpoint to the destination endpoint, and the packet is sent to the next hop in that path.

The IP address assigned to the endpoint, in essence, serves two purposes. For TCP, the IP address serves as an endpoint identifier upon which sessions can be established and maintained. At the network layer, however, the IP address is used in making routing decisions. Figure 5-1 below illustrates how IP addresses are assigned and used at the network layer.



Click on image to enlarge.

Figure 5-1. Network layer connectivity

For this reason, a unique mobility presents problem to the network. Layer 3 mobility refers to an end node that changes point of attachment in a way that is visible to Layer 3. Layer 3 creates a two-dimensional challenge:

Challenge #1: The mobile node keeps its IP address. If this were the case, the hierarchical structure of Internet addressing is no longer aligned with real Internet topology, and the network cannot properly route to the mobile node. Figure 5-2 below illustrates the problem presented by mobility at the network layer.


Click on image to enlarge.

Figure 5-2. Network layer mobility problem

Challenge #2: The mobile node changes its IP address. If this were the case, all TCP sessions built on the original IP address can no longer continue and are broken. Figure 5-3 illustrates the problem presented by network mobility at the transport layer.

Because this series covers the topic of network layer mobility, you will learn about seamless mobility—that is, persistence of the TCP session as an endpoint changes point of attachment. Because it is unreasonably complex to expect that the network routing decisions and topology can adapt to reflect the mobility of every endpoint, it is necessary to preserve the endpoint’s original IP address, regardless of point of attachment.

More specifically, it is necessary to preserve the endpoint’s original IP address, as perceived by the other node communicating over the established TCP session.



Click on image to enlarge.

Figure 5-3. Transport layer mobility problem

A number of mechanisms have been standardized to provide seamless mobility. Each of these methods have one thing in common—they utilize a separate IP address for endpoint identification than for routing.

This series of articles will discuss four mechanisms designed for network layer mobility as well as the associated architectures, deployment examples, and use cases for each: Mobile IPv4, Mobile IPv6, Dual Stack Mobile IP (DSMIP), IKEv2 Mobility and the Multihoming (MOBIKE) Protocol

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