Mobile Ad Hoc Networks (MANETs) for device-to-radio communications address the challenges faced when merging IP routing and mobile radio communications in ad hoc networking applications:

• Optimal route selection based on Layer 2 feedback from the radio network

• Faster convergence when nodes join and leave the network because devices are able to respond faster to network topology changes

• Efficient integration of point-to-point, directional radio topologies with multihop routing

• Flow-controlled communications between each radio and its partner device enables applications such as voice and video to work better because outages caused by moving links are reduced or eliminated. Sessions are more stable and remain active longer

Through the device-to-radio link, the radio can inform the device immediately when a node joins or leaves, and this enables the device to recognize topology changes more quickly than if it had to rely on timers. Without this link-status notification from the radio, the device would likely time out while waiting for traffic. The link-status notification from the radio enables the device to respond faster to network topology changes. Metric information regarding the quality of a link is passed between the device and radio, enabling the device to more intelligently decide on which link to use.

With the link-status signaling provided by the device-to-radio link, applications such as voice and video work better because outages caused by topology changes are reduced or eliminated. Sessions are more stable and remain active longer.

Cross-layer feedback for device-to-radio integration of Radio-Aware Routing (RAR) takes advantage of the functions defined in RFC 5578. The RFC 5578 is an Internet Engineering Task Force (IETF) standard that defines PPP over Ethernet (PPPoE) extensions for Ethernet-based communications between a device and a mobile radio, that operates in a variable-bandwidth environment and has limited buffering capabilities. These extensions provide a PPPoE session-based mechanism for sharing radio network status such as link-quality metrics and establishing flow control between a device and an RAR-compliant radio.

An RAR-compliant radio initiates a Layer 2 PPPoE session with its adjacent device on behalf of every device and radio neighbor discovered in the network. These Layer 2 sessions are the means by which radio network status for each neighbor link is reported to the device. The radio establishes the correspondence between each PPPoE session and each link to a neighbor.

Mobile Ad Hoc Networks (MANETs) enable users deployed in areas with no fixed communications infrastructure to access critical voice, video, and data services. For example, soldiers in the field can employ unified communications, multimedia applications, and real-time information dissemination to improve situational awareness and respond quickly to changing battlefield conditions. Disaster managers can use video conferences, database access, and collaborative tools to coordinate multiagency responses within an Incident Command System (ICS) framework. For event planners and trade show managers, MANETs represent a cost-effective way to accommodate mobile end users on a short-term basis.

In MANET environments, highly mobile nodes communicate with each other across bandwidth-constrained radio links. An individual node includes both a radio and a network device, with the two devices interconnected over an Ethernet. Because these nodes can rapidly join or leave the network, MANET routing topologies are highly dynamic. Fast convergence in a MANET becomes a challenge because the state of a node can change well before the event is detected by the normal timing mechanisms of the routing protocol.

Radio link quality in a MANET can vary dramatically because it can be affected by a variety of factors such as noise, fading, interference, and power fluctuation. As a result, avoiding congestion and determining optimal routing paths also pose significant challenges for the device network.

Directional radios that operate on a narrow beam tend to model the network as a series of physical point-to-point connections with neighbor nodes. This point-to-point model does not translate gracefully to multihop, multipoint device environments because it increases the size of each device’s topology database and reduces routing efficiency.

Effective networking in a MANET environment therefore requires mechanisms by which

• Devices and radios can interoperate efficiently, and without impacting operation of the radio network.

• Radio point-to-point and device point-to-multipoint paradigms can be rationalized.

• Radios can report status to devices for each link and each neighbor.

• Devices can use this information to optimize routing decisions.

The Mobile Ad Hoc Network (MANET) implementation uses PPP over Ethernet (PPPoE) sessions to enable intranodal communications between a device and its partner radio. Each radio initiates the PPPoE session as soon as the radio establishes a radio link to another radio. After the PPPoE sessions are active, a PPP session is established end-to-end (device-to-device). This is duplicated each time a radio establishes a new radio link. The virtual multipoint interface (VMI) on the device can aggregate multiple PPPoE sessions and multiplex them to look like a single interface to the routing processes. Underneath the VMI are virtual access interfaces that are associated with each of the PPP and PPPoE connections.

A PPPoE session is established between a device and a radio on behalf of every other device and radio neighbor located in the MANET. These Layer 2 sessions are the means by which radio network status gets reported to the Layer 3 processes in the device. The figure below shows the PPPoE session exchange between mobile devices and directional radios in a MANET network.

This capability requires that a Radio-Aware Routing (RAR)-compliant radio be connected to a device through Ethernet. The device always considers the Ethernet link to be up. If the radio side of the link goes down, the device waits until a routing update timeout occurs to declare the route down and then updates the routing table. The figure below shows a simple device-to-radio link topology.

The routing protocols optimized for VMI PPPoE are Enhanced Interior Gateway Routing Protocol (EIGRP) (IPv4, IPv6) and Open Shortest Path First version 3 (OSPFv3) for IPv4 and IPv6.

The virtual multipoint interface (VMI) provides services that map outgoing packets to the appropriate PPP over Ethernet (PPPoE) sessions based on the next-hop forwarding address for that packet. The VMI also provides a broadcast service that emulates a set of point-to-point connections as a point-to-multipoint interface with broadcast ability. When a packet with a multicast address is forwarded through the VMI in aggregate mode, VMI replicates the packet and sends it through the virtual access interfaces to each of its neighbors.

Directional radios are frequently used in applications that require greater bandwidth, increased power-to-transmission range, or reduced probability of detection. These radios operate in a point-to-point mode and generally have no broadcast capability. However, the routing processes in Mobile Ad Hoc Networks (MANETs) operate most efficiently because the network link is treated as point-to-multipoint, with broadcast capability. For the device, modeling the MANET as a collection of point-to-point nodes has a dramatic impact on the size of its internal database.

The VMI within the device can aggregate all of the per-neighbor PPPoE sessions from the radio Ethernet connection. The VMI maps the sessions to appear to Layer 3 routing protocols and applications as a single point-to-multipoint, multiaccess, broadcast-capable network. However, the VMI preserves the integrity of the PPPoE sessions on the radio side so that each point-to-point connection can have its own quality of service (QoS) queue.

The VMI also relays the link-quality metric and neighbor up/down signaling from the radio to the routing protocols. The VMI signals are used by the Enhanced Interior Gateway Routing Protocol (EIGRP) for IPv4 and IPv6 neighbors and the Open Shortest Path First version 3 (OSPFv3) for IPv6 neighbors.

You can configure virtual multipoint interfaces (VMIs) with IPv6 addresses only, IPv4 addresses only, or both IPv4 and IPv6 addresses.

IPv6 addresses are assigned to individual device interfaces and enable the forwarding of IPv6 traffic globally on the device. By default, IPv6 addresses are not configured and IPv6 routing is disabled.

Note

The ipv6-address argument in the ipv6 address command must be in the form documented in RFC 2373 where the address is specified in hexadecimal using 16-bit values between colons. The /prefix-length argument in the ipv6 address command is a decimal value that indicates how many of the high-order contiguous bits of the address comprise the prefix (the network portion of the address) A slash mark must precede the decimal value.

The Open Shortest Path First version 3 9OSPFv3) address family feature is implemented according to RFC 5838 and enables the concurrent routing of IPv4 and IPv6 prefixes.

When this feature is enabled with Mobile Ad Hoc Networks (MANETs), IPv6 packets are routed in mobile environments over OSPFv3 using IPv4 or IPv6 addresses.

For configuration details, see the IP Routing: OSPF Configuration Guide.

Mobile Ad Hoc Networks (MANETs) are highly dynamic environments. Nodes might move into, or out of, radio range at a fast pace. Each time a node joins or leaves, the network topology must be logically reconstructed by the devices. Routing protocols normally use timer-driven hello messages or neighbor timeouts to track topology changes, but MANETs reliance on these mechanisms can result in unacceptably slow convergence.

The neighbor up/down signaling capability provides faster network convergence by using link-status signals generated by the radio. The radio notifies the device each time a link to another neighbor is established or terminated by the creation and termination of PPP over Ethernet (PPPoE) sessions. In the device, the routing protocols (Open Shortest Path First version 3 [OSPFv3] or Enhanced Interior Gateway Routing Protocol [EIGRP]) respond immediately to these signals by expediting formation of a new adjacency (for a new neighbor) or tearing down an existing adjacency (if a neighbor is lost). For example, if a vehicle drives behind a building and loses its connection, the device immediately senses the loss and establishes a new route to the vehicle through neighbors that are not blocked. This high-speed network convergence is essential for minimizing dropped voice calls and disruptions to video sessions.

When virtual multipoint interfaces (VMIs) with PPPoE are used and a partner node has left or a new one has joined, the radio informs the device immediately of the topology change. Upon receiving the signal, the device immediately declares the change and updates the routing tables. The signaling capability provides these advantages:

• Reduces routing delays and prevents applications from timing out

• Enables network-based applications and information to be delivered reliably and quickly over directional radio links

• Provides faster convergence and optimal route selection so that delay-sensitive traffic such as voice and video are not disrupted

• Reduces impact on radio equipment by minimizing the need for internal queueing and buffering

• Provides consistent quality of service for networks with multiple radios

The messaging allows for flexible rerouting when necessary because of these factors:

• Noise on the radio links

• Fading of the radio links

• Congestion of the radio links

• Radio link power fade

• Utilization of the radio

The figure below shows the signaling sequence that occurs when radio links go up and down.

Each radio initiates a PPP over Ethernet (PPPoE) session with its local device as soon as the radio establishes a link to another radio. Once the PPPoE sessions are active for each node, a PPP session is then established end-to-end (device-to-device). This process is duplicated each time a radio establishes a new link.

The carrying capacity of each radio link might vary due to location changes or environmental conditions, and many radio transmission systems have limited buffering capabilities. To minimize the need for packet queueing in the radio, PPPoE protocol extensions enable the device to control traffic buffering in congestion situations. Implementing flow-control on these device-to-radio sessions allows use of quality of service (QoS) features such as fair queueing.

The flow-control solution utilizes a credit-granting mechanism documented in RFC 5578. When the PPPoE session is established, the radio can request a flow-controlled session. If the device acknowledges the request, all subsequent traffic must be flow controlled. If a flow-control session is requested and cannot be supported by the device, the session is terminated. Typically, both the radio and the device initially grant credits during session discovery. Once a device exhausts its credits, it must stop sending until additional credits are granted. Credits can be added incrementally over the course of a session.

Metrics scaling is used with high-performance radios that require high-speed links. The radio can express the maximum and current data rates with different scaler values. Credit scaling allows a radio to change the default credit grant (or scaling factor) of 64 bytes to its default value. You can display the maximum and current data rates and the scalar value set by the radio in the show vmi neighbor detail command output.

You can use the following commands to verify the virtual multipoint interface (VMI) configuration:

• show pppoe session all

• show interface vmi

• show vmi neighbors

• show vmi neighbors detail

• show ip eigrp interfaces

• show ip eigrp neighbors

• show ipv6 eigrp interfaces

• show ipv6 eigrp neighbors

• show ipv6 ospf interface