Mobile Internet basics: Transport layer mobility challenges - Embedded.com

Mobile Internet basics: Transport layer mobility challenges

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.


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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.


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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.


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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

Mobile IPv4
Mobile IPv4 was first standardized by the IEEE in RFC 2002, published in 1996. Subsequent RFCs have updated the original RFC 2002, and the current Mobile IPv4 RFC is 3344.

With the growth of cellular systems, it was recognized that there was a need to support some mechanism of mobility in which the mobile node could continue to communicate with either a static or another mobile node without forcing new session establishment. (Note : All RFCs are available at http://www.ietf.org/rfc/rfcxxxx.txt, where xxxx is the number of the RFC. )

Mobile IPv4 provides this mechanism while imposing no requirements on static nodes. That is, a static node has no awareness that the node with which it is communicating has changed point of attachment. This has allowed Mobile IPv4 to be deployed as an overlay on top of the existing IP/Internet model without any impact to the routing and addressing structure.

Mobile IPv4 Technology Overview
Mobile IPv4 uses numerous mobility-specific terms and definitions to address mobility, and also introduces a number of new elements into the IP network architecture. This new terminology can broadly be classified as: 1) Network-specific terms; 2) Network element-specific terms, and 3) Addressing-specific terms

Network-Specific Terms . For the purpose of understanding, Mobile IPv4 has employed numerous network reference points to better clarify where the mobile node is located:

Reference Point #1: The home network has a network prefix matching that of the mobile node’s home address.  Traffic will be routed normally to the mobile node’s home address when the node is attached to the home network.

Reference Point #2 : The virtual network typically resides on the home agent (see the following section for more detail), but might also reside as a nonphysical entity on any router in the home network. The router that hosts the virtual network advertises reachability to the virtual network to foreign networks.

Reference Point #3: A foreign network is any network other than the mobile node’s home network. The mobile node might be communicating with a node residing in a foreign network when it changes point of attachment. Mobile IPv4 allows the mobile node to move transparently without any node in the foreign network being aware.

A visited network is a foreign network to which the mobile node is connected. Figure 5-4  below illustrates the domains of a Mobile IPv4 network.


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Figure 5-4. Mobile IPv4 domains

Network Element–Specific Terms . Mobile IPv4 networks consist of four network entities:

Entity #1: The mobile node refers to the device that changes its point of attachment across multiple networks. This point of attachment change might or might not result in the change of IP address, depending on whether link layer connectivity can be maintained. The mobile node can be a host device or a router.

Entity #2: The foreign agent is a router in a mobile node’s visited network. The foreign agent provides Layer 3 routing functions to the mobile node during the life of the node’s association. For traffic originated from the mobile node, the foreign agent also acts as the default gateway. For traffic destined to the mobile node, the foreign agent establishes a tunnel to the home agent.

Entity #3: The home agent is a router in the mobile node’s home network. The home agent provides tunneled delivery of mobile-destined traffic through the established tunnel to the foreign agent when the mobile node is outside the home network. The home agent also maintains a database that contains the current location information for any mobile node that has previously registered.

Entity #4: The correspondent node is any node (static or mobile) with which the mobile node is communicating. The function of Mobile IPv4 is to allow the mobile node to change point of attachment without the correspondent node being aware. This allows TCP sessions to continue to functional normally.


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Figure 5-5. Mobile IPv4 network elements.

Addressing-Specific Terms. To facilitate the routing of packets to the mobile node, both within and outside the home network, Mobile IPv4 relies on multiple IP address assignments:

The home address (HoA) is assigned by the home agent to the mobile node. This address is maintained by the mobile node for the entire length of its session. The home address does not change, regardless of the mobile node’s point of attachment to the network.


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Figure 5-6. Mobile IPv4 addressing.

The care-of address (CoA) is the termination point of the tunnel from the home agent. The CoA can refer to an address on the foreign agent itself, known as the foreign agent CoA, or to an address assigned locally to an interface on the mobile node, known as the colocated care-of address (CCoA).

Mobile IPv4 Operation
As discussed earlier, a home agent relies on an internal database to track a mobile node location and determine the appropriate way to route traffic to the mobile node. The Mobile IPv4 protocol has a number of processes that take place to populate the database in the home agent. These processes can be characterized into three functions:

#1: Mobile IPv4 Agent Discovery
#2: Mobile IPv4 Registration and Authentication, Authorization, and Accounting (AAA)
#3: Mobile IPv4 Tunnels, Bindings, and Datagram Forwarding

The sections that follow cover these three functions in more detail.

Mobile IPv4 Agent Discovery
Mobile Agent Discovery is the method used by the mobile node to determine the network to which it is currently connected. This discovery takes place when a mobile node is first turned on or when a mobile node changes network point of attachment. There are two ways that the mobile node can discover its location—advertisement or solicitation.

Agent Advertisements. Agent Advertisement is the method used by the mobility agents to advertise which services it has available. When the mobile node first connects to a new network, it listens for Agent Advertisement messages. Agent Advertisements are unauthenticated multicast messages sent to the “all systems on this link” multicast address (224.0.0.1) or the “limited broadcast” address (255.255.255.255).

The Agent Advertisement messages are actually extensions of the Internet Control Message Protocol (ICMP) Router Advertisement message. These ICMP Router Advertisement messages are defined in the ICMP Internet Router Discovery Protocol (IRDP), defined in RFC 1256.

Figure 5-7 ICMP Router Advertisement
The extension includes information on registration lifetime, whether the advertising router is a home agent or foreign agent, and the tunnel encapsulation type. In addition, if the mobility agent is a foreign agent, the Agent Solicitation message also includes whether reverse tunneling is supported, the foreign agent CoA, and whether the foreign agent has available capacity to accept new registrations. Figure 5-8 below illustrates the format of the Agent Advertisement message.

Figure 5-8 MIPv4 Router Advertisement Extension ( To view larger image, click here)
Agent Advertisement messages are required when a mobile node cannot discover, through a Layer 2 protocol, both its own local IP address and a local mobility agent. 3rd Generation Partnership Project 2 (3GPP2) and other mobility standards rely on link layer mechanisms, such as Point to Point Protocol (PPP), to establish connectivity and Internet Protocol Control Protocol (IPCP) to configure IP over the PPP link.

The IP address contained in the IPCP negotiation can be used to determine the network to which the mobile node is connected—home or foreign. After the PPP session is established, Agent Advertisements can be sent over the PPP session. More information on Mobile IPv4 in practice can be found later in this series.

Agent Solicitations. Because multicast and broadcast Agent Advertisements consume network bandwidth, the rate at which these messages are sent is often limited so as not to consume a significant amount of bandwidth.

If the mobile node does not receive any Agent Advertisements for a period of time, it can optionally send an Agent Solicitation message to discover a CoA.

Solicitation is a method by which the mobile node solicits an Agent Advertisement message. This Agent Advertisement message can be sent unicast to the mobile node.

Figure 5-9. Mobile IPv4 Agent Solicitation
The Agent Solicitation message is identical to the ICMP Router Solicitation message defined in the IRDP standard. Figure 5-9 illustrate s the format of the Agent Solicitation message.

References:
1. RFC 3588, “The Diameter Base Protocol.”
2. Understanding IKEv2: Tutorial and Rationale for Decisions,” draft-ietf-ipsec-ikev2-tutorial-01.txt.
3. RFC 4621, “Design of the MOBIKE Protocol.”

To read Part 2 , go to “Mobile IPv4 registration and AAA .”
To read Part 3, go to “Mobile IPv4 tunners,bindings and diagrams .
To read Part 4 , go to “Mobile IPv6 Technology Overview
Toread Part 5, go to “ Mobile IPv6 in Practice
To read Part 6, go to“IKEv3 and MOBIKE

This series of articles is from the book “Building the Mobile Internet”, by Mark Grayson, Kevin Shatzkamer and Klass Wierenga.Copyright 2011, used by permission of Pearson Education, Inc.. Written permission from Pearson Education, Inc. is required for all other uses.

Mark Grayson is a distinguished consulting engineer at Cisco Systems with responsibility for leading Cisco’s mobile architecture strategy. With 20 years experience in the wireless industry, he holds first class honors in electronics and communications engineering from the University of Birmingham (England) as well as a PhD, in radio engineering.

Kevin Shatzkamer is a distinguished system architect at Cisco Systems with responsibility for long term strategy and architectural evolution of mobile wireless networks. He holds a Bachelor of engineering degree from the University of Floriga and a MBA from Indiana University.

Klaas Wierenga is a senior consulting engineer in the office of the CTO at Cisco. His 15 plus years of experience include planning, analysis and design of systems in the fields of mobility, security and identify. He holds a Master’s degree in consumer science from the University of Groningen, The Netherlands.

This article provided courtesy of Embedded.com and EmbeddedSystems Design Magazine. Sign up for subscriptionsand newsletters. Copyright © 2012 UBM–All rights reserved.

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