1. Introduction
There exist today a collection of modem devices that control links of variable data rate and quality. Examples of these types of links include line-of-sight (LOS) terrestrial radios, satellite terminals, and broadband modems. Fluctuations in speed and quality of these links can occur due to configuration, or on a moment-to-moment basis, due to physical phenomena like multipath interference, obstructions, rain fade, etc. It is also quite possible that link quality and data rate vary with respect to individual destinations on a link and with the type of traffic being sent. As an example, consider the case of an IEEE 802.11 access point serving two associated laptop computers. In this environment, the answer to the question "What is the data rate on the 802.11 link?" is "It depends on which associated laptop we're talking about and on what kind of traffic is being sent." While the first laptop, being physically close to the access point, may have a data rate of 54 Mbps for unicast traffic, the other laptop, being relatively far away or obstructed by some object, can simultaneously have a data rate of only 32 Mbps for unicast. However, for multicast traffic sent from the access point, all traffic is sent at the base transmission rate (which is configurable but, depending on the model of the access point, is usually 24 Mbps or less).
In addition to utilizing links that have variable data rates, mobile networks are challenged by the notion that link connectivity will come and go over time, without an effect on a router's interface state (Up or Down). Effectively utilizing a relatively short-lived connection is problematic in IP routed networks, as IP routing protocols tend to rely on interface state and independent timers to maintain network convergence (e.g., HELLO messages and/or recognition of DEAD routing adjacencies). These dynamic connections can be better utilized with an event-driven paradigm, where acquisition of a new neighbor (or loss of an existing one) is signaled, as opposed to a paradigm driven by timers and/or interface state. DLEP not only implements such an event-driven paradigm but does so over a local (1-hop) TCP session, which guarantees delivery of the event messages.
Another complicating factor for mobile networks are the different methods of physically connecting the modem devices to the router. Modems can be deployed as an interface card in a router's chassis, or as a standalone device connected to the router via Ethernet or serial link. In the case of Ethernet attachment, with existing protocols and techniques, routing software cannot be aware of convergence events occurring on the radio link (e.g., acquisition or loss of a potential routing neighbor), nor can the router be aware of the actual capacity of the link. This lack of awareness, along with the variability in data rate, leads to a situation where finding the (current) best route through the network to a given node is difficult to establish and properly maintain. This is especially true of demand-based access schemes such as Demand Assigned Multiple Access (DAMA) implementations used on some satellite systems. With a DAMA-based system, additional data rate may be available but will not be used unless the network devices emit traffic at a rate higher than the currently established rate. Increasing the traffic rate does not guarantee that additional data rate will be allocated; rather, it may result in data loss and additional retransmissions on the link.
Addressing the challenges listed above, the Dynamic Link Exchange Protocol, or DLEP, has been developed. DLEP runs between a router and its attached modem devices, allowing the modem devices to communicate (1) link characteristics as they change and (2) convergence events (acquisition and loss of potential routing next hops). Figures 1 and 2 illustrate the scope of DLEP packets.
|-------Local Node-------| |-------Remote Node------|
| | | |
+--------+ +-------+ +-------+ +--------+
| Router |=======| Modem |{~~~~~~~~}| Modem |=======| Router |
| | | Device| | Device| | |
+--------+ +-------+ +-------+ +--------+
| | | Link | | |
|-DLEP--| | Protocol | |-DLEP--|
| | | (e.g., | | |
| | | 802.11) | | |
Figure 1: DLEP Network
In Figure 1, when the local modem detects the presence of a remote node, it (the local modem) sends a message to its router via DLEP. The message consists of an indication of what change has occurred on the link (e.g., the presence of a remote node detected), along with a collection of DLEP-defined Data Items that further describe the change. Upon receipt of the message, the local router may take whatever action it deems appropriate, such as initiating discovery protocols and/or issuing HELLO messages to converge the network. On a continuing, as-needed basis, the modem devices use DLEP to report any characteristics of the link (data rate, latency, etc.) that have changed. DLEP is independent of the link type and topology supported by the modem. Note that DLEP is specified to run only on the local link between router and modem. Some over-the-air signaling may be necessary between the local and remote modem in order to provide some parameters in DLEP Messages between the local modem and local router, but DLEP does not specify how such over-the-air signaling is carried out. Over-the-air signaling is purely a matter for the modem implementer.
Figure 2 shows how DLEP can support a configuration where routers are connected with different link types. In this example, Modem Device Type A implements a point-to-point link, and Modem Device Type B is connected via a shared medium. In both cases, DLEP is used to report the characteristics of the link (data rate, latency, etc.) to routers. The modem is also able to use the DLEP session to notify the router when the remote node is lost, shortening the time required to reconverge the network.
+--------+ +--------+
+----+ Modem | | Modem +---+
| | Device | | Device | |
| | Type A | <===== // ======> | Type A | |
| +--------+ Point-to-Point Link +--------+ |
+---+----+ +---+----+
| Router | | Router |
| | | |
+---+----+ +---+----+
| +--------+ +--------+ |
+-----+ Modem | | Modem | |
| Device | o o o o o o o o | Device +--+
| Type B | o Shared o | Type B |
+--------+ o Medium o +--------+
o o
o o
o o
o
+--------+
| Modem |
| Device |
| Type B |
+---+----+
|
|
+---+----+
| Router |
| |
+--------+
Figure 2: DLEP Network with Multiple Modem Devices