2. Protocol Overview
DLEP defines a set of Messages used by modems and their attached routers to communicate events that occur on the physical link(s) managed by the modem: for example, a remote node entering or leaving the network, or that the link has changed. Associated with these Messages are a set of Data Items -- information that describes the remote node (e.g., address information) and/or the characteristics of the link to the remote node. Throughout this document, we refer to modems/routers participating in a DLEP session as "DLEP Participants", unless a specific distinction (e.g., modem or router) is required.
DLEP uses a session-oriented paradigm between the modem device and its associated router. If multiple modem devices are attached to a router (as in Figure 2) or the modem supports multiple connections (via multiple logical or physical interfaces), then separate DLEP sessions exist for each modem or connection. A router and modem form a session by completing the discovery and initialization process. This router-modem session persists unless or until it either (1) times out, based on the absence of DLEP traffic (including heartbeats) or (2) is explicitly torn down by one of the DLEP participants.
While this document represents the best efforts of the working group to be functionally complete, it is recognized that extensions to DLEP will in all likelihood be necessary as more link types are used. Such extensions are defined as additional Messages, Data Items, and/or status codes, and associated rules of behavior, that are not defined in this document. DLEP contains a standard mechanism for router and modem implementations to negotiate the available extensions to use on a per-session basis.
2.1. Destinations
The router-modem session provides a carrier for information exchange concerning "destinations" that are available via the modem device. Destinations can be identified by either the router or the modem and represent a specific, addressable location that can be reached via the link(s) managed by the modem.
The DLEP Messages concerning destinations thus become the way for routers and modems to maintain, and notify each other about, an information base representing the physical and logical destinations accessible via the modem device, as well as the link characteristics to those destinations.
A destination can be either physical or logical. The example of a physical destination would be that of a remote, far-end router attached via the variable-quality network. It should be noted that for physical destinations the Media Access Control (MAC) address is the address of the far-end router, not the modem.
The example of a logical destination is Multicast. Multicast traffic destined for the variable-quality network (the network accessed via the modem) is handled in IP networks by deriving a Layer 2 MAC address based on the Layer 3 address. Leveraging on this scheme, multicast traffic is supported in DLEP simply by treating the derived MAC address as any other destination in the network.
To support these logical destinations, one of the DLEP participants (typically, the router) informs the other as to the existence of the logical destination. The modem, once it is aware of the existence of this logical destination, reports link characteristics just as it would for any other destination in the network. The specific algorithms a modem would use to derive metrics on logical destinations are outside the scope of this specification; these algorithms are left to specific implementations to decide.
In all cases, when this specification uses the term "destination", it refers to the addressable locations, either logical or physical, that are accessible by the radio link(s).
2.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.