Mobile Internet basics: Mobile IPv4 Tunnels, Bindings & Datagrams -

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.

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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.

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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.

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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.

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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.

Mobile IPv4 and Layer 2 Interactions
While Mobile IPv4 does resolve mobility at the Internet Layer of the TCP/IP stack, it actually creates additional challenges at the lower layers.

Mobile IPv4 networks present a unique challenge for ARP. When a packet is routed into a network, the Mobile IPv4 home agent intercepts all packets destined for a mobile node and tunnels this traffic to the CoA.

However, when a local corresponding node on the same network attempts to send a packet to the mobile node, the correspondent node does not need to route the packet.

Because both the mobile node and correspondent node are on the same network, the correspondent node issues an ARP request to determine the hardware, or link layer, address of the mobile node. If the mobile node responds to this ARP request, the home agent would never see the packet.

Figure 5-20 below illustrates a local correspondent node forwarding packets to a mobile node under standard ARP functionality.

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Figure 5-20 Local Correspondent Node Standard ARP Functionality

This presents a challenge because the correspondent node does not need to understand or be aware of the Mobile IPv4 session on the mobile node.

If the mobile node changes point of attachment, the local correspondent node would not be aware and will continue to use its local ARP table to determine how to forward the packet. Figure 5-21 below illustrates the mobility challenge that arises while communicating with a local correspondent node.

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Figure 5-21 Mobility Challenge with Local Correspondent Node

For this reason, Mobile IP4 adopted specific ARP rules that both mobile nodes and mobility agents must follow, including the following:

1) The mobile node must not issue broadcast ARP messages while away from its home network.

2) The foreign agent must not issue broadcast ARP messages to determine the MAC address of the mobile node. Instead, the foreign agent must obtain the MAC address from either an Agent Solicitation message or RRQ message.

3) The foreign agent’s ARP cache for a mobile node must be as long as the RRP lifetime.

4) When the mobile node is away from the home network, the home agent uses Proxy ARP messages to reply to ARP requests for a mobile node’s link layer address.

A Proxy ARP message is an ARP reply sent by one node (the home agent) on behalf of another node (the mobile node). Figure 5-22 below illustrates how Proxy ARP messages issued by the home agent solve the forwarding challenge that arises by communicating with local correspondent nodes.

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Figure 5-22 Home Agent Proxy ARP

When the mobile node changes IP point of attachment, the home agent issues a gratuitous ARP message. This gratuitous ARP message updates local nodes as to the link layer address of the mobile node.

This link layer address points to the home agent, and local nodes associate the mobile node’s link layer address with the IP address of the home agent.

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Figure 5-23. Home Agent Gratuitous ARP

Figure 5-23 above illustrates how gratuitous ARP messages issued by the home agent solve the mobility challenge that arises by communicating with local correspondent nodes.

Mobile IPv4 in Practice
Mobile IPv4 is one of the most-implemented mobility protocols and the most-implement-ed network layer mobility protocol to date. Mobile IPv4 is standardized in the IETF and leveraged across numerous organizations, including both 3GPP2, the standards organization for today’s Code Division Multiple Access (CDMA) Evolution Data Only (EVDO) networks and WiMAX Forum Network Working Group (NWG).

While Mobile IPv4 has numerous deployment examples in both service provider and enterprise networks, the following sections discuss specific implementations of Mobile IPv4 relative to mobile standards organizations.

These sections will look at a specific example of Mobile IPv4, as implemented in CDMA networks (Figure 5-24 below) standardized by 3GPP2.

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Figure 5-24 CDMA Network Architecture

3GPP2 Implementation of Mobile IPv4
3GPP2 X.S011 defines the usage of Mobile IPv4 to provide mobility services in a CDMA network environment. A CDMA data network includes four main elements:

Base Transceiver Station (BTS): The BTS is the radio frequency (RF) node in the CDMA architecture.

Packet Control Function (PCF): The PCF is both a packet-routing node between the BTS and Packet Data Serving Node (PDSN) as well as a radio node that provides intelligence and channel assignment to the mobile node.

PDSN: The PDSN provides the network access gateway function to the packet data network. The PDSN terminates PPP sessions from the mobile node and provides the foreign agent function for the Mobile IPv4 session.

Home Agent (HA): The HA provides Mobile IPv4 standards-compliant functions for the packet data network, ensuring that the mobile node can seamlessly hand off between CDMA BTSs.

Figure 5-25 below illustrates the call flow for establishing a Mobile IPv4 connection in a CDMA network.

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Figure 5-25. CDMA call flow

When a mobile node attempts to connect to the CDMA network, it attempts to establish a connection over the A10 interface, which is a PPP interface used to authenticate the mobile subscriber for network access.

The PPP session typically does not use Challenge Handshake Authentication Protocol (CHAP) authentication. This is done to reduce the call setup time. Instead, the PDSN will send Agent Advertisement messages that include the Agent Advertisement Challenge extension after the PPP session is successfully negotiated.

After the PPP has been completed, the mobile node initiates an RRQ to the PDSN. The foreign agent function in the PDSN communicates with a AAA server to authenticate the subscriber, retrieve a home agent IP address (if a dynamic home agent is required), the FA-HA SPI.

The PDSN then proxies the RRQ message to the home agent, including the MN-FA challenge extension, NAI, and MN-AAA authentication extension. The home agent interacts with the AAA server again to authenticate the mobile node for mobile services.

After it is successfully authenticated, the home agent sends an RRP message to the PDSN/FA. The mobile node home IP address can be assigned at many different steps within the call flow, including the following:

 Mobile IPv4 RRQ: The mobile node can request a specific home IP address, known as a static IP address, from the mobility agents.

 FA-CHAP response from the AAA server: If configured for CHAP authentication, the AAA server can assign a home IP address to the mobile node during authorization by the PDSN.

 Mobile IPv4 RRP: The home agent can assign a home IP address, based on NAI, in the Mobile IPv4 RRP message.

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Figure 5-26 CDMA End-to-End Protocol Stack

Figure 5-26 above is the protocol stack for sending traffic between a mobile node and correspondent node. As it illustrates, shortly after it is established, the mobile node can communicate with corresponding nodes using reverse tunneling. All packets both to and from the mobile node are sent through the home agent.

Next in Part 4: Mobile IPv6 technology overview

To read Part 1 , go to “ Transport layer mobility challenges.
To read Part 2, go to “ Mobile IPv4 registration and AAA.”

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

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

Kevin Shatzkamer is a distinguished system architect atCisco Systems with responsibility for long term strategy and architecturalevolution of mobile wireless networks. He holds a Bachelor of engineering degreefrom the University of Floriga and a MBA from Indiana University.

Klaas Wierenga is a senior consulting engineer in theoffice 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.

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