





Internet Engineering Task Force (IETF)                        S. Ratliff
Request for Comments: 8175                                    VT iDirect
Category: Standards Track                                        S. Jury
ISSN: 2070-1721                                            Cisco Systems
                                                          D. Satterwhite
                                                                Broadcom
                                                               R. Taylor
                                                  Airbus Defence & Space
                                                                B. Berry
                                                               June 2017


                 Dynamic Link Exchange Protocol (DLEP)

Abstract

   When routing devices rely on modems to effect communications over
   wireless links, they need timely and accurate knowledge of the
   characteristics of the link (speed, state, etc.) in order to make
   routing decisions.  In mobile or other environments where these
   characteristics change frequently, manual configurations or the
   inference of state through routing or transport protocols does not
   allow the router to make the best decisions.  This document
   introduces a new protocol called the Dynamic Link Exchange Protocol
   (DLEP), which provides a bidirectional, event-driven communication
   channel between the router and the modem to facilitate communication
   of changing link characteristics.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8175.










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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
   2. Protocol Overview ...............................................7
      2.1. Destinations ...............................................8
      2.2. Conventions and Terminology ................................9
   3. Requirements ....................................................9
   4. Implementation Scenarios .......................................10
   5. Assumptions ....................................................10
   6. Metrics ........................................................11
   7. DLEP Session Flow ..............................................12
      7.1. Peer Discovery State ......................................12
      7.2. Session Initialization State ..............................14
      7.3. In-Session State ..........................................14
           7.3.1. Heartbeats .........................................15
      7.4. Session Termination State .................................15
      7.5. Session Reset State .......................................16
           7.5.1. Unexpected TCP Connection Termination ..............16
   8. Transaction Model ..............................................16
   9. Extensions .....................................................17
      9.1. Experiments ...............................................18
   10. Scalability ...................................................18
   11. DLEP Signal and Message Structure .............................18
      11.1. DLEP Signal Header .......................................19
      11.2. DLEP Message Header ......................................20
      11.3. DLEP Generic Data Item ...................................20
   12. DLEP Signals and Messages .....................................21
      12.1. General Processing Rules .................................21
      12.2. Status Code Processing ...................................22
      12.3. Peer Discovery Signal ....................................22
      12.4. Peer Offer Signal ........................................23
      12.5. Session Initialization Message ...........................23




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      12.6. Session Initialization Response Message ..................24
      12.7. Session Update Message ...................................26
      12.8. Session Update Response Message ..........................27
      12.9. Session Termination Message ..............................28
      12.10. Session Termination Response Message ....................28
      12.11. Destination Up Message ..................................28
      12.12. Destination Up Response Message .........................30
      12.13. Destination Announce Message ............................30
      12.14. Destination Announce Response Message ...................31
      12.15. Destination Down Message ................................32
      12.16. Destination Down Response Message .......................33
      12.17. Destination Update Message ..............................33
      12.18. Link Characteristics Request Message ....................35
      12.19. Link Characteristics Response Message ...................35
      12.20. Heartbeat Message .......................................36
   13. DLEP Data Items ...............................................37
      13.1. Status ...................................................38
      13.2. IPv4 Connection Point ....................................41
      13.3. IPv6 Connection Point ....................................42
      13.4. Peer Type ................................................43
      13.5. Heartbeat Interval .......................................45
      13.6. Extensions Supported .....................................45
      13.7. MAC Address ..............................................46
      13.8. IPv4 Address .............................................47
           13.8.1. IPv4 Address Processing ...........................48
      13.9. IPv6 Address .............................................49
           13.9.1. IPv6 Address Processing ...........................50
      13.10. IPv4 Attached Subnet ....................................51
           13.10.1. IPv4 Attached Subnet Processing ..................52
      13.11. IPv6 Attached Subnet ....................................53
           13.11.1. IPv6 Attached Subnet Processing ..................54
      13.12. Maximum Data Rate (Receive) .............................55
      13.13. Maximum Data Rate (Transmit) ............................56
      13.14. Current Data Rate (Receive) .............................56
      13.15. Current Data Rate (Transmit) ............................57
      13.16. Latency .................................................58
      13.17. Resources ...............................................59
      13.18. Relative Link Quality (Receive) .........................60
      13.19. Relative Link Quality (Transmit) ........................60
      13.20. Maximum Transmission Unit (MTU) .........................61
   14. Security Considerations .......................................62
   15. IANA Considerations ...........................................63
      15.1. Registrations ............................................63
      15.2. Signal Type Registrations ................................63
      15.3. Message Type Registrations ...............................64
      15.4. DLEP Data Item Registrations .............................65
      15.5. DLEP Status Code Registrations ...........................66




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      15.6. DLEP Extension Registrations .............................67
      15.7. DLEP IPv4 Connection Point Flags .........................68
      15.8. DLEP IPv6 Connection Point Flags .........................68
      15.9. DLEP Peer Type Flags .....................................68
      15.10. DLEP IPv4 Address Flags .................................69
      15.11. DLEP IPv6 Address Flags .................................69
      15.12. DLEP IPv4 Attached Subnet Flags .........................69
      15.13. DLEP IPv6 Attached Subnet Flags .........................70
      15.14. DLEP Well-Known Port ....................................70
      15.15. DLEP IPv4 Link-Local Multicast Address ..................70
      15.16. DLEP IPv6 Link-Local Multicast Address ..................70
   16. References ....................................................71
      16.1. Normative References .....................................71
      16.2. Informative References ...................................71
   Appendix A. Discovery Signal Flows ................................73
   Appendix B. Peer-Level Message Flows ..............................73
     B.1. Session Initialization .....................................73
     B.2. Session Initialization - Refused ...........................74
     B.3. Router Changes IP Addresses ................................74
     B.4. Modem Changes Session-Wide Metrics .........................75
     B.5. Router Terminates Session ..................................75
     B.6. Modem Terminates Session ...................................76
     B.7. Session Heartbeats .........................................77
     B.8. Router Detects a Heartbeat Timeout .........................78
     B.9. Modem Detects a Heartbeat Timeout ..........................78
   Appendix C. Destination-Specific Message Flows ....................79
     C.1. Common Destination Notification ............................79
     C.2. Multicast Destination Notification .........................80
     C.3. Link Characteristics Request ...............................81
   Acknowledgments ...................................................82
   Authors' Addresses ................................................82

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



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   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.










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   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.




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                 +--------+                     +--------+
            +----+ 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

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



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   (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



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   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.

3.  Requirements

   DLEP MUST be implemented on a single Layer 2 domain.  The protocol
   identifies next-hop destinations by using the MAC address for
   delivering data traffic.  No manipulation or substitution is
   performed; the MAC address supplied in all DLEP Messages is used as
   the Destination MAC address for frames emitted by the participating
   router.  MAC addresses MUST be unique within the context of the
   router-modem session.

   To enforce the single Layer 2 domain, implementations MUST support
   the Generalized TTL Security Mechanism [RFC5082], and implementations
   MUST adhere to this specification for all DLEP Messages.

   DLEP specifies UDP multicast for single-hop discovery signaling and
   TCP for transport of the Messages.  Modems and routers participating
   in DLEP sessions MUST have topologically consistent IP addresses
   assigned.  It is RECOMMENDED that DLEP implementations utilize IPv6
   link-local addresses to reduce the administrative burden of address
   assignment.

   DLEP relies on the guaranteed delivery of its Messages between router
   and modem, once the 1-hop discovery process is complete -- hence, the
   specification of TCP to carry the Messages.  Other reliable
   transports for the protocol are possible but are outside the scope of
   this document.








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4.  Implementation Scenarios

   During development of this specification, two types of deployments
   were discussed.

   The first can be viewed as a "dedicated deployment".  In this mode,
   DLEP routers and modems are either directly connected (e.g., using
   crossover cables to connect interfaces) or connected to a dedicated
   switch.  An example of this type of deployment would be a router with
   a line-of-sight radio connected into one interface, with a satellite
   modem connected into another interface.  In mobile environments, the
   router and the connected modem (or modems) are placed into a mobile
   platform (e.g., a vehicle, boat, or airplane).  In this mode, when a
   switch is used, it is possible that a small number of ancillary
   devices (e.g., a laptop) are also plugged into the switch.  But in
   either event, the resulting network segment is constrained to a small
   number of devices and is not generally accessible from anywhere else
   in the network.

   The other type of deployment envisioned can be viewed as a "networked
   deployment".  In this type of scenario, the DLEP router and modem
   (or modems) are placed on a segment that is accessible from other
   points in the network.  In this scenario, not only are the DLEP
   router and modem(s) accessible from other points in the network; the
   router and a given modem could be multiple physical hops away from
   each other.  This scenario necessitates the use of Layer 2 tunneling
   technology to enforce the single-hop requirement of DLEP.

5.  Assumptions

   DLEP assumes that a signaling protocol exists between modems
   participating in a network.  This specification does not define the
   character or behavior of this over-the-air signaling but does expect
   some information to be carried (or derived) by the signaling,
   such as the arrival and departure of modems from this network,
   and the variation of the link characteristics between modems.
   This information is then assumed to be used by the modem to
   implement DLEP.

   This specification assumes that the link between router and modem is
   static with respect to data rate and latency and that this link is
   not likely to be the cause of a performance bottleneck.  In
   deployments where the router and modem are physically separated by
   multiple network hops, served by Layer 2 tunneling technology, DLEP
   statistics on the RF links could be insufficient for routing
   protocols to make appropriate routing decisions.  This would





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   especially become an issue in cases where the Layer 2 tunnel between
   router and modem is itself served in part (or in total) with a
   wireless backhaul link.

6.  Metrics

   DLEP includes the ability for the router and modem to communicate
   metrics that reflect the characteristics (e.g., data rate, latency)
   of the variable-quality link in use.  DLEP does not specify how a
   given metric value is to be calculated; rather, the protocol assumes
   that metrics have been calculated by a "best effort", incorporating
   all pertinent data that is available to the modem device.  Metrics
   based on large-enough sample sizes will preclude short traffic bursts
   from adversely skewing reported values.

   DLEP allows for metrics to be sent within two contexts -- metrics for
   a specific destination within the network (e.g., a specific router),
   and "per session" (those that apply to all destinations accessed via
   the modem).  Most metrics can be further subdivided into transmit and
   receive metrics.  In cases where metrics are provided at the session
   level, the router propagates the metrics to all entries in its
   information base for destinations that are accessed via the modem.

   DLEP modems announce all metric Data Items that will be reported
   during the session, and provide default values for those metrics, in
   the Session Initialization Response Message (Section 12.6).  In order
   to use a metric type that was not included in the Session
   Initialization Response Message, modem implementations terminate the
   session with the router (via the Session Termination Message
   (Section 12.9)) and establish a new session.

   A DLEP modem can send metrics in both (1) a session context, via the
   Session Update Message (Section 12.7) and (2) a specific destination
   context, via the Destination Update Message (Section 12.17), at any
   time.  The most recently received metric value takes precedence over
   any earlier value, regardless of context -- that is:

   1.  If the router receives metrics in a specific destination context
       (via the Destination Update Message), then the specific
       destination is updated with the new metric.

   2.  If the router receives metrics in a session-wide context (via the
       Session Update Message), then the metrics for all destinations
       accessed via the modem are updated with the new metric.







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   It is left to implementations to choose sensible default values based
   on their specific characteristics.  Modems having static
   (non-changing) link metric characteristics can report metrics only
   once for a given destination (or once on a session-wide basis, if all
   connections via the modem are of this static nature).

   In addition to communicating existing metrics about the link, DLEP
   provides a Message allowing a router to request a different data rate
   or latency from the modem.  This Message is the Link Characteristics
   Request Message (Section 12.18); it gives the router the ability to
   deal with requisite increases (or decreases) of allocated
   data rate/latency in demand-based schemes in a more deterministic
   manner.

7.  DLEP Session Flow

   All DLEP participants of a session transition through a number of
   distinct states during the lifetime of a DLEP session:

   o  Peer Discovery

   o  Session Initialization

   o  In-Session

   o  Session Termination

   o  Session Reset

   Modems, and routers supporting DLEP discovery, transition through all
   five of the above states.  Routers that rely on preconfigured TCP
   address/port information start in the Session Initialization state.

   Modems MUST support the Peer Discovery state.

7.1.  Peer Discovery State

   Modems MUST support DLEP Peer Discovery; routers MAY support the
   discovery signals or rely on a priori configuration to locate modems.
   If a router chooses to support DLEP discovery, all signals MUST be
   supported.

   In the Peer Discovery state, routers that support DLEP discovery MUST
   send Peer Discovery Signals (Section 12.3) to initiate modem
   discovery.






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   The router implementation then waits for a Peer Offer Signal
   (Section 12.4) response from a potential DLEP modem.  While in the
   Peer Discovery state, Peer Discovery Signals MUST be sent repeatedly
   by a DLEP router, at regular intervals.  It is RECOMMENDED that this
   interval be set to 60 seconds.  The interval MUST be a minimum of
   1 second; it SHOULD be a configurable parameter.  Note that this
   operation (sending Peer Discovery and waiting for Peer Offer) is
   outside the DLEP transaction model (Section 8), as the transaction
   model only describes Messages on a TCP session.

   Routers receiving a Peer Offer Signal MUST use one of the modem
   address/port combinations from the Peer Offer Signal to establish a
   TCP connection to the modem, even if a priori configuration exists.
   If multiple Connection Point Data Items exist in the received Peer
   Offer Signal, routers SHOULD prioritize IPv6 connection points over
   IPv4 connection points.  If multiple connection points exist with the
   same transport (e.g., IPv6 or IPv4), implementations MAY use their
   own heuristics to determine the order in which they are tried.  If a
   TCP connection cannot be achieved using any of the address/port
   combinations and the Discovery mechanism is in use, then the router
   SHOULD resume issuing Peer Discovery Signals.  If no Connection Point
   Data Items are included in the Peer Offer Signal, the router MUST use
   the source address of the UDP packet containing the Peer Offer Signal
   as the IP address, and the DLEP well-known port number.

   In the Peer Discovery state, the modem implementation MUST listen for
   incoming Peer Discovery Signals on the DLEP well-known IPv6 and/or
   IPv4 link-local multicast address and port.  On receipt of a valid
   Peer Discovery Signal, it MUST reply with a Peer Offer Signal.

   Modems MUST be prepared to accept a TCP connection from a router that
   is not using the Discovery mechanism, i.e., a connection attempt that
   occurs without a preceding Peer Discovery Signal.

   Implementations of DLEP SHOULD implement, and use, Transport Layer
   Security (TLS) [RFC5246] to protect the TCP session.  The "dedicated
   deployments" discussed in "Implementation Scenarios" (Section 4) MAY
   consider the use of DLEP without TLS.  For all "networked
   deployments" (again, discussed in "Implementation Scenarios"), the
   implementation and use of TLS are STRONGLY RECOMMENDED.  If TLS is to
   be used, then the TLS session MUST be established before any Messages
   are passed between peers.  Routers supporting TLS MUST prioritize
   connection points using TLS over those that do not.

   Upon establishment of a TCP connection, and the establishment of a
   TLS session if TLS is in use, both modem and router enter the Session
   Initialization state.  It is up to the router implementation if Peer
   Discovery Signals continue to be sent after the device has



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   transitioned to the Session Initialization state.  Modem
   implementations MUST silently ignore Peer Discovery Signals from a
   router with which a given implementation already has a TCP
   connection.

7.2.  Session Initialization State

   On entering the Session Initialization state, the router MUST send a
   Session Initialization Message (Section 12.5) to the modem.  The
   router MUST then wait for receipt of a Session Initialization
   Response Message (Section 12.6) from the modem.  Receipt of the
   Session Initialization Response Message containing a Status Data Item
   (Section 13.1) with status code set to 0 'Success' (see Table 2 in
   Section 13.1) indicates that the modem has received and processed the
   Session Initialization Message, and the router MUST transition to the
   In-Session state.

   On entering the Session Initialization state, the modem MUST wait for
   receipt of a Session Initialization Message from the router.  Upon
   receipt of a Session Initialization Message, the modem MUST send a
   Session Initialization Response Message, and the session MUST
   transition to the In-Session state.  If the modem receives any
   Message other than Session Initialization or it fails to parse the
   received Message, it MUST NOT send any Message, and it MUST terminate
   the TCP connection and transition to the Session Reset state.

   DLEP provides an extension negotiation capability to be used in the
   Session Initialization state; see Section 9.  Extensions supported by
   an implementation MUST be declared to potential DLEP participants
   using the Extensions Supported Data Item (Section 13.6).  Once both
   DLEP participants have exchanged initialization Messages, an
   implementation MUST NOT emit any Message, Signal, Data Item, or
   status code associated with an extension that was not specified in
   the received initialization Message from its peer.

7.3.  In-Session State

   In the In-Session state, Messages can flow in both directions between
   DLEP participants, indicating changes to the session state, the
   arrival or departure of reachable destinations, or changes of the
   state of the links to the destinations.










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   The In-Session state is maintained until one of the following
   conditions occurs:

   o  The implementation terminates the session by sending a Session
      Termination Message (Section 12.9), or

   o  Its peer terminates the session, indicated by receiving a Session
      Termination Message.

   The implementation MUST then transition to the Session Termination
   state.

7.3.1.  Heartbeats

   In order to maintain the In-Session state, periodic Heartbeat
   Messages (Section 12.20) MUST be exchanged between router and modem.
   These Messages are intended to keep the session alive and to verify
   bidirectional connectivity between the two DLEP participants.  It is
   RECOMMENDED that the interval timer between Heartbeat Messages be set
   to 60 seconds.  The interval MUST be a minimum of 1 second; it SHOULD
   be a configurable parameter.

   Each DLEP participant is responsible for the creation of Heartbeat
   Messages.

   Receipt of any valid DLEP Message MUST reset the heartbeat interval
   timer (i.e., valid DLEP Messages take the place of, and obviate the
   need for, additional Heartbeat Messages).

   An implementation MUST allow a minimum of 2 heartbeat intervals to
   expire with no Messages from its peer before terminating the session.
   When terminating the session, a Session Termination Message
   containing a Status Data Item (Section 13.1) with status code set to
   132 'Timed Out' (see Table 2) MUST be sent, and then the
   implementation MUST transition to the Session Termination state.

7.4.  Session Termination State

   When an implementation enters the Session Termination state after
   sending a Session Termination Message (Section 12.9) as the result of
   an invalid Message or error, it MUST wait for a Session Termination
   Response Message (Section 12.10) from its peer.  A sender SHOULD
   allow 4 heartbeat intervals to expire before assuming that its peer
   is unresponsive and before continuing with session termination.  Any
   other Message received while waiting MUST be silently ignored.






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   When the sender of the Session Termination Message receives a Session
   Termination Response Message from its peer or times out, it MUST
   transition to the Session Reset state.

   When an implementation receives a Session Termination Message from
   its peer, it enters the Session Termination state, and then it MUST
   immediately send a Session Termination Response and transition to the
   Session Reset state.

7.5.  Session Reset State

   In the Session Reset state, the implementation MUST perform the
   following actions:

   o  Release all resources allocated for the session.

   o  Eliminate all destinations in the information base represented by
      the session.  Destination Down Messages (Section 12.15) MUST NOT
      be sent.

   o  Terminate the TCP connection.

   Having completed these actions, the implementation SHOULD return to
   the relevant initial state:

   o  For modems: Peer Discovery.

   o  For routers: either Peer Discovery or Session Initialization,
      depending on configuration.

7.5.1.  Unexpected TCP Connection Termination

   If the TCP connection between DLEP participants is terminated when an
   implementation is not in the Session Reset state, the implementation
   MUST immediately transition to the Session Reset state.

8.  Transaction Model

   DLEP defines a simple Message transaction model: only one request per
   destination may be in progress at a time per session.  A Message
   transaction is considered complete when a response matching a
   previously issued request is received.  If a DLEP participant
   receives a request for a destination for which there is already an
   outstanding request, the implementation MUST terminate the session by
   issuing a Session Termination Message (Section 12.9) containing a
   Status Data Item (Section 13.1) with status code set to
   129 'Unexpected Message' (see Table 2) and transition to the Session




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   Termination state.  There is no restriction on the total number of
   Message transactions in progress at a time, as long as each
   transaction refers to a different destination.

   It should be noted that some requests may take a considerable amount
   of time for some DLEP participants to complete; for example, a modem
   handling a multicast Destination Up request may have to perform a
   complex network reconfiguration.  A sending implementation MUST be
   able to handle such long-running transactions gracefully.

   Additionally, only one session request, e.g., a Session
   Initialization Message (Section 12.5), may be in progress at a time
   per session.  As noted above for Message transactions, a session
   transaction is considered complete when a response matching a
   previously issued request is received.  If a DLEP participant
   receives a session request while there is already a session request
   in progress, it MUST terminate the session by issuing a Session
   Termination Message containing a Status Data Item with status code
   set to 129 'Unexpected Message' and transition to the Session
   Termination state.  Only the Session Termination Message may be
   issued when a session transaction is in progress.  Heartbeat Messages
   (Section 12.20) MUST NOT be considered part of a session transaction.

   DLEP transactions do not time out and are not cancellable, except for
   transactions in flight when the DLEP session is reset.  If the
   session is terminated, canceling transactions in progress MUST be
   performed as part of resetting the state machine.  An implementation
   can detect if its peer has failed in some way by use of the session
   heartbeat mechanism during the In-Session state; see Section 7.3.

9.  Extensions

   Extensions MUST be negotiated on a per-session basis during session
   initialization via the Extensions Supported mechanism.
   Implementations are not required to support any extensions in order
   to be considered DLEP compliant.

   If interoperable protocol extensions are required, they will need to
   be standardized as either (1) an update to this document or (2) an
   additional standalone specification.  The IANA registries defined in
   Section 15 of this document contain sufficient unassigned space for
   DLEP Signals, Messages, Data Items, and status codes to accommodate
   future extensions to the protocol.

   As multiple protocol extensions MAY be announced during session
   initialization, authors of protocol extensions need to consider the
   interaction of their extensions with other published extensions and
   specify any incompatibilities.



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9.1.  Experiments

   This document registers Private Use [RFC5226] numbering space in the
   DLEP Signal, Message, Data Item, and status code registries for
   experimental extensions.  The intent is to allow for experimentation
   with new Signals, Messages, Data Items, and/or status codes while
   still retaining the documented DLEP behavior.

   During session initialization, the use of the Private Use Signals,
   Messages, Data Items, status codes, or behaviors MUST be announced as
   DLEP extensions, using extension identifiers from the Private Use
   space in the "Extension Type Values" registry (Table 3), with a value
   agreed upon (a priori) between the participants.  DLEP extensions
   using the Private Use numbering space are commonly referred to as
   "experiments".

   Multiple experiments MAY be announced in the Session Initialization
   Messages.  However, the use of multiple experiments in a single
   session could lead to interoperability issues or unexpected results
   (e.g., clashes of experimental Signals, Messages, Data Items, and/or
   status code types) and is therefore discouraged.  It is left to
   implementations to determine the correct processing path (e.g., a
   decision on whether to terminate the session or establish a
   precedence of the conflicting definitions) if such conflicts arise.

10.  Scalability

   The protocol is intended to support thousands of destinations on a
   given modem/router pair.  On a large scale, an implementation should
   consider employing techniques to prevent flooding its peer with a
   large number of Messages in a short time.  For example, a dampening
   algorithm could be employed to prevent a flapping device from
   generating a large number of Destination Up / Destination Down
   Messages.

   Also, the use of techniques such as a hysteresis can lessen the
   impact of rapid, minor fluctuations in link quality.  The specific
   algorithms for handling flapping destinations and minor changes in
   link quality are outside the scope of this specification.

11.  DLEP Signal and Message Structure

   DLEP defines two protocol units used in two different ways: Signals
   and Messages.  Signals are only used in the Discovery mechanism and
   are carried in UDP datagrams.  Messages are used bidirectionally over
   a TCP connection between the participants, in the Session
   Initialization, In-Session, and Session Termination states.




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   Both Signals and Messages consist of a Header followed by an
   unordered list of Data Items.  Headers consist of Type and Length
   information, while Data Items are encoded as TLV (Type-Length-Value)
   structures.  In this document, the Data Items following a Signal or
   Message Header are described as being "contained in" the Signal or
   Message.

   There is no restriction on the order of Data Items following a
   Header, and the acceptability of duplicate Data Items is defined by
   the definition of the Signal or Message declared by the type in the
   Header.

   All integers in Header fields and values MUST be in network byte
   order.

11.1.  DLEP Signal Header

   The DLEP Signal Header contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      'D'      |      'L'      |      'E'      |      'P'      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Signal Type                   | Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: DLEP Signal Header

   "DLEP":  Every Signal MUST start with the following characters:
      U+0044, U+004C, U+0045, U+0050.

   Signal Type:  A 16-bit unsigned integer containing one of the DLEP
      Signal Type values defined in this document.

   Length:  The length in octets, expressed as a 16-bit unsigned
      integer, of all of the DLEP Data Items contained in this Signal.
      This length MUST NOT include the length of the Signal Header
      itself.

   The DLEP Signal Header is immediately followed by zero or more DLEP
   Data Items, encoded in TLVs, as defined in this document.









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11.2.  DLEP Message Header

   The DLEP Message Header contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Message Type                  | Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: DLEP Message Header

   Message Type:  A 16-bit unsigned integer containing one of the DLEP
      Message Type values defined in this document.

   Length:  The length in octets, expressed as a 16-bit unsigned
      integer, of all of the DLEP Data Items contained in this Message.
      This length MUST NOT include the length of the Message Header
      itself.

   The DLEP Message Header is immediately followed by zero or more DLEP
   Data Items, encoded in TLVs, as defined in this document.

11.3.  DLEP Generic Data Item

   All DLEP Data Items contain the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Data Item Type                | Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Value...                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5: DLEP Generic Data Item

   Data Item Type:  A 16-bit unsigned integer field specifying the type
      of Data Item being sent.

   Length:  The length in octets, expressed as a 16-bit unsigned
      integer, of the Value field of the Data Item.  This length
      MUST NOT include the length of the Data Item Type and Length
      fields.

   Value:  A field of <Length> octets that contains data specific to a
      particular Data Item.




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12.  DLEP Signals and Messages

12.1.  General Processing Rules

   If an unrecognized or unexpected Signal is received or if a received
   Signal contains unrecognized, invalid, or disallowed duplicate Data
   Items, the receiving implementation MUST ignore the Signal.

   If a Signal is received with a TTL value that is NOT equal to 255,
   the receiving implementation MUST ignore the Signal.

   If an unrecognized Message is received, the receiving implementation
   MUST issue a Session Termination Message (Section 12.9) containing a
   Status Data Item (Section 13.1) with status code set to 128 'Unknown
   Message' (see Table 2) and transition to the Session Termination
   state.

   If an unexpected Message is received, the receiving implementation
   MUST issue a Session Termination Message containing a Status Data
   Item with status code set to 129 'Unexpected Message' and transition
   to the Session Termination state.

   If a received Message contains unrecognized, invalid, or disallowed
   duplicate Data Items, the receiving implementation MUST issue a
   Session Termination Message containing a Status Data Item with status
   code set to 130 'Invalid Data' and transition to the Session
   Termination state.

   If a packet in the TCP stream is received with a TTL value other than
   255, the receiving implementation MUST immediately transition to the
   Session Reset state.

   Prior to the exchange of Destination Up (Section 12.11) and
   Destination Up Response (Section 12.12) Messages, or Destination
   Announce (Section 12.13) and Destination Announce Response
   (Section 12.14) Messages, no Messages concerning a destination may be
   sent.  An implementation receiving any Message with such an
   unannounced destination MUST terminate the session by issuing a
   Session Termination Message containing a Status Data Item with status
   code set to 131 'Invalid Destination' and transition to the Session
   Termination state.

   After exchanging Destination Down (Section 12.15) and Destination
   Down Response (Section 12.16) Messages, no Messages concerning a
   destination may be sent until a new Destination Up or Destination
   Announce Message is sent.  An implementation receiving a Message
   about a destination previously announced as 'down' MUST terminate the




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   session by issuing a Session Termination Message containing a Status
   Data Item with status code set to 131 'Invalid Destination' and
   transition to the Session Termination state.

12.2.  Status Code Processing

   The behavior of a DLEP participant receiving a Message containing a
   Status Data Item (Section 13.1) is defined by the failure mode
   associated with the value of the status code field; see Table 2.  All
   status code values less than 100 have a failure mode of 'Continue';
   all other status codes have a failure mode of 'Terminate'.

   A DLEP participant receiving any Message apart from a Session
   Termination Message (Section 12.9) containing a Status Data Item with
   a status code value with failure mode 'Terminate' MUST immediately
   issue a Session Termination Message echoing the received Status Data
   Item and then transition to the Session Termination state.

   A DLEP participant receiving a Message containing a Status Data Item
   with a status code value with failure mode 'Continue' can continue
   normal operation of the session.

12.3.  Peer Discovery Signal

   A Peer Discovery Signal SHOULD be sent by a DLEP router to discover
   DLEP modems in the network; see Section 7.1.

   A Peer Discovery Signal MUST be encoded within a UDP packet.  The
   destination MUST be set to the DLEP well-known address and port
   number.  For routers supporting both IPv4 and IPv6 DLEP operation, it
   is RECOMMENDED that IPv6 be selected as the transport.  The source IP
   address MUST be set to the router IP address associated with the DLEP
   interface.  There is no DLEP-specific restriction on source port.

   To construct a Peer Discovery Signal, the Signal Type value in the
   Signal Header is set to 1 (see "Signal Type Registration"
   (Section 15.2)).

   The Peer Discovery Signal MAY contain a Peer Type Data Item
   (Section 13.4).











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12.4.  Peer Offer Signal

   A Peer Offer Signal MUST be sent by a DLEP modem in response to a
   properly formatted and addressed Peer Discovery Signal
   (Section 12.3).

   A Peer Offer Signal MUST be encoded within a UDP packet.  The IP
   source and destination fields in the packet MUST be set by swapping
   the values received in the Peer Discovery Signal.  The Peer Offer
   Signal completes the discovery process; see Section 7.1.

   To construct a Peer Offer Signal, the Signal Type value in the Signal
   Header is set to 2 (see "Signal Type Registration" (Section 15.2)).

   The Peer Offer Signal MAY contain a Peer Type Data Item
   (Section 13.4).

   The Peer Offer Signal MAY contain one or more of any of the following
   Data Items, with different values:

   o  IPv4 Connection Point (Section 13.2)

   o  IPv6 Connection Point (Section 13.3)

   The IPv4 and IPv6 Connection Point Data Items indicate the unicast
   address the router MUST use when connecting the DLEP TCP session.

12.5.  Session Initialization Message

   A Session Initialization Message MUST be sent by a DLEP router as the
   first Message of the DLEP TCP session.  It is sent by the router
   after a TCP connect to an address/port combination that was obtained
   either via receipt of a Peer Offer or from a priori configuration.

   To construct a Session Initialization Message, the Message Type value
   in the Message Header is set to 1 (see "Message Type Registration"
   (Section 15.3)).

   The Session Initialization Message MUST contain one of each of the
   following Data Items:

   o  Heartbeat Interval (Section 13.5)

   o  Peer Type (Section 13.4)

   If DLEP extensions are supported, the Session Initialization Message
   MUST contain an Extensions Supported Data Item (Section 13.6).




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   The Session Initialization Message MAY contain one or more of each of
   the following Data Items, with different values and with the Add/Drop
   (A) flag (Section 13) set to 1:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)

   If any optional extensions are supported by the implementation, they
   MUST be enumerated in the Extensions Supported Data Item.  If an
   Extensions Supported Data Item does not exist in a Session
   Initialization Message, the modem MUST conclude that there is no
   support for extensions in the router.

   DLEP Heartbeats are not started until receipt of the Session
   Initialization Response Message (Section 12.6), and therefore
   implementations MUST use their own timeout heuristics for this
   Message.

   As an exception to the general rule governing an implementation
   receiving an unrecognized Data Item in a Message (see Section 12.1),
   if a Session Initialization Message contains one or more Extensions
   Supported Data Items announcing support for extensions that the
   implementation does not recognize, then the implementation MAY ignore
   Data Items it does not recognize.

12.6.  Session Initialization Response Message

   A Session Initialization Response Message MUST be sent by a DLEP
   modem in response to a received Session Initialization Message
   (Section 12.5).

   To construct a Session Initialization Response Message, the Message
   Type value in the Message Header is set to 2 (see "Message Type
   Registration" (Section 15.3)).

   The Session Initialization Response Message MUST contain one of each
   of the following Data Items:

   o  Status (Section 13.1)

   o  Peer Type (Section 13.4)

   o  Heartbeat Interval (Section 13.5)



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   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Session Initialization Response Message MUST contain one of each
   of the following Data Items, if the Data Item will be used during the
   lifetime of the session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   If DLEP extensions are supported, the Session Initialization Response
   Message MUST contain an Extensions Supported Data Item
   (Section 13.6).

   The Session Initialization Response Message MAY contain one or more
   of each of the following Data Items, with different values and with
   the Add/Drop (A) flag (Section 13) set to 1:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)

   The Session Initialization Response Message completes the DLEP
   session establishment; the modem should transition to the In-Session
   state when the Message is sent, and the router should transition to
   the In-Session state upon receipt of an acceptable Session
   Initialization Response Message.

   All supported metric Data Items MUST be included in the Session
   Initialization Response Message, with default values to be used on a
   session-wide basis.  This can be viewed as the modem "declaring" all
   supported metrics at DLEP session initialization.  Receipt of any



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   further DLEP Message containing a metric Data Item not included in
   the Session Initialization Response Message MUST be treated as an
   error, resulting in the termination of the DLEP session between
   router and modem.

   If any optional extensions are supported by the modem, they MUST be
   enumerated in the Extensions Supported Data Item.  If an Extensions
   Supported Data Item does not exist in a Session Initialization
   Response Message, the router MUST conclude that there is no support
   for extensions in the modem.

   After the Session Initialization / Session Initialization Response
   Messages have been successfully exchanged, implementations MUST only
   use extensions that are supported by both DLEP participants; see
   Section 7.2.

12.7.  Session Update Message

   A Session Update Message MAY be sent by a DLEP participant, on a
   session-wide basis, to indicate local Layer 3 address changes and/or
   metric changes.

   To construct a Session Update Message, the Message Type value in the
   Message Header is set to 3 (see "Message Type Registration"
   (Section 15.3)).

   The Session Update Message MAY contain one or more of each of the
   following Data Items, with different values:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)

   When sent by a modem, the Session Update Message MAY contain one of
   each of the following Data Items:

   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)






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   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   When sent by a modem, the Session Update Message MAY contain one of
   each of the following Data Items, if the Data Item is in use by the
   session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   If metrics are supplied with the Session Update Message (e.g.,
   Maximum Data Rate), these metrics are considered to be session-wide
   and therefore MUST be applied to all destinations in the information
   base associated with the DLEP session.  This includes destinations
   for which metrics may have been stored based on received Destination
   Update messages.

   It should be noted that Session Update Messages can be sent by both
   routers and modems.  For example, the addition of an IPv4 address on
   the router MAY prompt a Session Update Message to its attached
   modems.  Also, for example, a modem that changes its Maximum Data
   Rate (Receive) for all destinations MAY reflect that change via a
   Session Update Message to its attached router(s).

   Concerning Layer 3 addresses and subnets: if the modem is capable of
   understanding and forwarding this information (via mechanisms not
   defined by DLEP), the update would prompt any remote DLEP-enabled
   modems to issue a Destination Update Message (Section 12.17) to their
   local routers with the new (or deleted) addresses and subnets.

12.8.  Session Update Response Message

   A Session Update Response Message MUST be sent by a DLEP participant
   when a Session Update Message (Section 12.7) is received.

   To construct a Session Update Response Message, the Message Type
   value in the Message Header is set to 4 (see "Message Type
   Registration" (Section 15.3)).

   The Session Update Response Message MUST contain a Status Data Item
   (Section 13.1).




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12.9.  Session Termination Message

   When a DLEP participant determines that the DLEP session needs to be
   terminated, the participant MUST send (or attempt to send) a Session
   Termination Message.

   To construct a Session Termination Message, the Message Type value in
   the Message Header is set to 5 (see "Message Type Registration"
   (Section 15.3)).

   The Session Termination Message MUST contain a Status Data Item
   (Section 13.1).

   It should be noted that Session Termination Messages can be sent by
   both routers and modems.

12.10.  Session Termination Response Message

   A Session Termination Response Message MUST be sent by a DLEP
   participant when a Session Termination Message (Section 12.9) is
   received.

   To construct a Session Termination Response Message, the Message Type
   value in the Message Header is set to 6 (see "Message Type
   Registration" (Section 15.3)).

   There are no valid Data Items for the Session Termination Response
   Message.

   Receipt of a Session Termination Response Message completes the
   teardown of the DLEP session; see Section 7.4.

12.11.  Destination Up Message

   Destination Up Messages MAY be sent by a modem to inform its attached
   router of the presence of a new reachable destination.

   To construct a Destination Up Message, the Message Type value in the
   Message Header is set to 7 (see "Message Type Registration"
   (Section 15.3)).

   The Destination Up Message MUST contain a MAC Address Data Item
   (Section 13.7).








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   The Destination Up Message SHOULD contain one or more of each of the
   following Data Items, with different values:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   The Destination Up Message MAY contain one of each of the following
   Data Items:

   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Destination Up Message MAY contain one of each of the following
   Data Items, if the Data Item is in use by the session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   The Destination Up Message MAY contain one or more of each of the
   following Data Items, with different values:

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)

   A router receiving a Destination Up Message allocates the necessary
   resources, creating an entry in the information base with the
   specifics (MAC Address, Latency, Data Rate, etc.) of the destination.
   The information about this destination will persist in the router's
   information base until a Destination Down Message (Section 12.15) is
   received, indicating that the modem has lost contact with the remote
   node or that the implementation transitions to the Session
   Termination state.





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12.12.  Destination Up Response Message

   A router MUST send a Destination Up Response Message when a
   Destination Up Message (Section 12.11) is received.

   To construct a Destination Up Response Message, the Message Type
   value in the Message Header is set to 8 (see "Message Type
   Registration" (Section 15.3)).

   The Destination Up Response Message MUST contain one of each of the
   following Data Items:

   o  MAC Address (Section 13.7)

   o  Status (Section 13.1)

   A router that wishes to receive further information concerning the
   destination identified in the corresponding Destination Up Message
   MUST set the status code of the included Status Data Item to
   0 'Success'; see Table 2.

   If the router has no interest in the destination identified in the
   corresponding Destination Up Message, then it MAY set the status code
   of the included Status Data Item to 1 'Not Interested'.

   A modem receiving a Destination Up Response Message containing a
   Status Data Item with a status code of any value other than
   0 'Success' MUST NOT send further Destination Messages about the
   destination, e.g., Destination Down (Section 12.15) or Destination
   Update (Section 12.17) with the same MAC address.

12.13.  Destination Announce Message

   Usually, a modem will discover the presence of one or more remote
   router/modem pairs and announce each destination's arrival by sending
   a corresponding Destination Up Message (Section 12.11) to the router.
   However, there may be times when a router wishes to express an
   interest in a destination that has yet to be announced, typically a
   multicast destination.  Destination Announce Messages MAY be sent by
   a router to announce such an interest.

   A Destination Announce Message MAY also be sent by a router to
   request information concerning a destination (1) in which the router
   has previously declined interest, via the 1 'Not Interested' status
   code in a Destination Up Response Message (Section 12.12) (see
   Table 2) or (2) that was previously declared as 'down', via the
   Destination Down Message (Section 12.15).




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   To construct a Destination Announce Message, the Message Type value
   in the Message Header is set to 9 (see "Message Type Registration"
   (Section 15.3)).

   The Destination Announce Message MUST contain a MAC Address Data Item
   (Section 13.7).

   The Destination Announce Message MAY contain zero or more of the
   following Data Items, with different values:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   One of the advantages of implementing DLEP is to leverage the modem's
   knowledge of the links between remote destinations, allowing routers
   to avoid using probed neighbor discovery techniques; therefore, modem
   implementations SHOULD announce available destinations via the
   Destination Up Message, rather than relying on Destination Announce
   Messages.

12.14.  Destination Announce Response Message

   A modem MUST send a Destination Announce Response Message when a
   Destination Announce Message (Section 12.13) is received.

   To construct a Destination Announce Response Message, the Message
   Type value in the Message Header is set to 10 (see "Message Type
   Registration" (Section 15.3)).

   The Destination Announce Response Message MUST contain one of each of
   the following Data Items:

   o  MAC Address (Section 13.7)

   o  Status (Section 13.1)

   The Destination Announce Response Message MAY contain one or more of
   each of the following Data Items, with different values:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)




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   The Destination Announce Response Message MAY contain one of each of
   the following Data Items:

   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Destination Announce Response Message MAY contain one of each of
   the following Data Items, if the Data Item is in use by the session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   If a modem is unable to report information immediately about the
   requested information -- for example, if the destination is not
   currently reachable -- the status code in the Status Data Item MUST
   be set to 2 'Request Denied'; see Table 2.

   After sending a Destination Announce Response Message containing a
   Status Data Item with a status code of 0 'Success', a modem then
   announces changes to the link to the destination via Destination
   Update Messages.

   When a successful Destination Announce Response Message is received,
   the router should add knowledge of the available destination to its
   information base.

12.15.  Destination Down Message

   A modem MUST send a Destination Down Message to report when a
   destination (a remote node or a multicast group) is no longer
   reachable.

   A router MAY send a Destination Down Message to report when it
   no longer requires information concerning a destination.





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   To construct a Destination Down Message, the Message Type value in
   the Message Header is set to 11 (see "Message Type Registration"
   (Section 15.3)).

   The Destination Down Message MUST contain a MAC Address Data Item
   (Section 13.7).

   It should be noted that both modem and router may send a Destination
   Down Message to their peer, regardless of which participant initially
   indicated the destination to be 'up'.

12.16.  Destination Down Response Message

   A Destination Down Response Message MUST be sent by the recipient of
   a Destination Down Message (Section 12.15) to confirm that the
   relevant data concerning the destination has been removed from the
   information base.

   To construct a Destination Down Response Message, the Message Type
   value in the Message Header is set to 12 (see "Message Type
   Registration" (Section 15.3)).

   The Destination Down Response Message MUST contain one of each of the
   following Data Items:

   o  MAC Address (Section 13.7)

   o  Status (Section 13.1)

12.17.  Destination Update Message

   A modem SHOULD send a Destination Update Message when it detects some
   change in the information base for a given destination (remote node
   or multicast group).  Some examples of changes that would prompt a
   Destination Update Message are as follows:

   o  Change in link metrics (e.g., data rates)

   o  Layer 3 addressing change

   To construct a Destination Update Message, the Message Type value in
   the Message Header is set to 13 (see "Message Type Registration"
   (Section 15.3)).

   The Destination Update Message MUST contain a MAC Address Data Item
   (Section 13.7).





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   The Destination Update Message MAY contain one of each of the
   following Data Items:

   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Destination Update Message MAY contain one of each of the
   following Data Items, if the Data Item is in use by the session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   The Destination Update Message MAY contain one or more of each of the
   following Data Items, with different values:

   o  IPv4 Address (Section 13.8)

   o  IPv6 Address (Section 13.9)

   o  IPv4 Attached Subnet (Section 13.10)

   o  IPv6 Attached Subnet (Section 13.11)

   Metrics supplied in this Message overwrite metrics provided in a
   previously received Session Message, Destination Message, or Link
   Characteristics Message (e.g., Session Initialization,
   Destination Up, Link Characteristics Response).

   It should be noted that this Message has no corresponding response.










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12.18.  Link Characteristics Request Message

   The Link Characteristics Request Message MAY be sent by a router to
   request that the modem initiate changes for specific characteristics
   of the link.  The request can reference either a real destination
   (e.g., a remote node) or a logical destination (e.g., a multicast
   group) within the network.

   To construct a Link Characteristics Request Message, the Message Type
   value in the Message Header is set to 14 (see "Message Type
   Registration" (Section 15.3)).

   The Link Characteristics Request Message MUST contain a MAC Address
   Data Item (Section 13.7).

   The Link Characteristics Request Message MUST also contain at least
   one of each of the following Data Items:

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Link Characteristics Request Message MAY contain either a Current
   Data Rate (Receive) (CDRR) or Current Data Rate (Transmit) (CDRT)
   Data Item to request a different data rate than is currently
   allocated, a Latency Data Item to request that traffic delay on the
   link not exceed the specified value, or both.

   The router sending a Link Characteristics Request Message should be
   aware that a request may take an extended period of time to complete.

12.19.  Link Characteristics Response Message

   A modem MUST send a Link Characteristics Response Message when a Link
   Characteristics Request Message (Section 12.18) is received.

   To construct a Link Characteristics Response Message, the Message
   Type value in the Message Header is set to 15 (see "Message Type
   Registration" (Section 15.3)).

   The Link Characteristics Response Message MUST contain one of each of
   the following Data Items:

   o  MAC Address (Section 13.7)

   o  Status (Section 13.1)



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   The Link Characteristics Response Message SHOULD contain one of each
   of the following Data Items:

   o  Maximum Data Rate (Receive) (Section 13.12)

   o  Maximum Data Rate (Transmit) (Section 13.13)

   o  Current Data Rate (Receive) (Section 13.14)

   o  Current Data Rate (Transmit) (Section 13.15)

   o  Latency (Section 13.16)

   The Link Characteristics Response Message MAY contain one of each of
   the following Data Items, if the Data Item is in use by the session:

   o  Resources (Section 13.17)

   o  Relative Link Quality (Receive) (Section 13.18)

   o  Relative Link Quality (Transmit) (Section 13.19)

   o  Maximum Transmission Unit (MTU) (Section 13.20)

   The Link Characteristics Response Message MUST contain a complete set
   of metric Data Items, referencing all metrics declared in the Session
   Initialization Response Message (Section 12.6).  The values in the
   metric Data Items in the Link Characteristics Response Message MUST
   reflect the link characteristics after the request has been
   processed.

   If an implementation is not able to alter the characteristics of the
   link in the manner requested, then the status code of the Status Data
   Item MUST be set to 2 'Request Denied'; see Table 2.

12.20.  Heartbeat Message

   A Heartbeat Message MUST be sent by a DLEP participant every
   N milliseconds, where N is defined in the Heartbeat Interval Data
   Item (Section 13.5) of the Session Initialization Message
   (Section 12.5) or Session Initialization Response Message
   (Section 12.6).

   To construct a Heartbeat Message, the Message Type value in the
   Message Header is set to 16 (see "Message Type Registration"
   (Section 15.3)).

   There are no valid Data Items for the Heartbeat Message.



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   The Heartbeat Message is used by DLEP participants to detect when a
   DLEP session peer (either the modem or the router) is no longer
   communicating; see Section 7.3.1.

13.  DLEP Data Items

   The core DLEP Data Items are as follows:

   +-------------+-----------------------------------------------------+
   | Type Code   | Description                                         |
   +-------------+-----------------------------------------------------+
   | 0           | Reserved                                            |
   |             |                                                     |
   | 1           | Status (Section 13.1)                               |
   |             |                                                     |
   | 2           | IPv4 Connection Point (Section 13.2)                |
   |             |                                                     |
   | 3           | IPv6 Connection Point (Section 13.3)                |
   |             |                                                     |
   | 4           | Peer Type (Section 13.4)                            |
   |             |                                                     |
   | 5           | Heartbeat Interval (Section 13.5)                   |
   |             |                                                     |
   | 6           | Extensions Supported (Section 13.6)                 |
   |             |                                                     |
   | 7           | MAC Address (Section 13.7)                          |
   |             |                                                     |
   | 8           | IPv4 Address (Section 13.8)                         |
   |             |                                                     |
   | 9           | IPv6 Address (Section 13.9)                         |
   |             |                                                     |
   | 10          | IPv4 Attached Subnet (Section 13.10)                |
   |             |                                                     |
   | 11          | IPv6 Attached Subnet (Section 13.11)                |
   |             |                                                     |
   | 12          | Maximum Data Rate (Receive) (MDRR) (Section 13.12)  |
   |             |                                                     |
   | 13          | Maximum Data Rate (Transmit) (MDRT) (Section 13.13) |
   |             |                                                     |
   | 14          | Current Data Rate (Receive) (CDRR) (Section 13.14)  |
   |             |                                                     |
   | 15          | Current Data Rate (Transmit) (CDRT) (Section 13.15) |
   |             |                                                     |
   | 16          | Latency (Section 13.16)                             |
   |             |                                                     |
   | 17          | Resources (RES) (Section 13.17)                     |
   |             |                                                     |




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   | 18          | Relative Link Quality (Receive) (RLQR)              |
   |             | (Section 13.18)                                     |
   |             |                                                     |
   | 19          | Relative Link Quality (Transmit) (RLQT)             |
   |             | (Section 13.19)                                     |
   |             |                                                     |
   | 20          | Maximum Transmission Unit (MTU) (Section 13.20)     |
   |             |                                                     |
   | 21-65407    | Unassigned (available for future extensions)        |
   |             |                                                     |
   | 65408-65534 | Reserved for Private Use (available for             |
   |             | experiments)                                        |
   |             |                                                     |
   | 65535       | Reserved                                            |
   +-------------+-----------------------------------------------------+

                       Table 1: DLEP Data Item Types

13.1.  Status

   For the Session Termination Message (Section 12.9), the Status Data
   Item indicates a reason for the termination.  For all response
   messages, the Status Data Item is used to indicate the success or
   failure of the previously received Message.

   The Status Data Item includes an optional Text field that can be used
   to provide a textual description of the status.  The use of the Text
   field is entirely up to the receiving implementation, e.g., it could
   be output to a log file or discarded.  If no Text field is supplied
   with the Status Data Item, the Length field MUST be set to 1.

   The Status Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Status Code   | Text...                                       :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  1

   Length:  1 + Length of Text, in octets.

   Status Code:  One of the status codes defined in Table 2 below.





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   Text:  UTF-8 encoded string of Unicode [RFC3629] characters,
      describing the cause, used for implementation-defined purposes.
      Since this field is used for description purposes, implementations
      SHOULD limit characters in this field to printable characters.

   An implementation MUST NOT assume that the Text field is a
   NUL-terminated string of printable characters.

   +----------+-------------+------------------+-----------------------+
   | Status   | Failure     | Description      | Reason                |
   | Code     | Mode        |                  |                       |
   +----------+-------------+------------------+-----------------------+
   | 0        | Continue    | Success          | The Message was       |
   |          |             |                  | processed             |
   |          |             |                  | successfully.         |
   |          |             |                  |                       |
   | 1        | Continue    | Not Interested   | The receiver is not   |
   |          |             |                  | interested in this    |
   |          |             |                  | Message subject,      |
   |          |             |                  | e.g., in a            |
   |          |             |                  | Destination Up        |
   |          |             |                  | Response Message      |
   |          |             |                  | (Section 12.12) to    |
   |          |             |                  | indicate no further   |
   |          |             |                  | Messages about the    |
   |          |             |                  | destination.          |
   |          |             |                  |                       |
   | 2        | Continue    | Request Denied   | The receiver refuses  |
   |          |             |                  | to complete the       |
   |          |             |                  | request.              |
   |          |             |                  |                       |
   | 3        | Continue    | Inconsistent     | One or more Data      |
   |          |             | Data             | Items in the Message  |
   |          |             |                  | describe a logically  |
   |          |             |                  | inconsistent state in |
   |          |             |                  | the network -- for    |
   |          |             |                  | example, in the       |
   |          |             |                  | Destination Up        |
   |          |             |                  | Message               |
   |          |             |                  | (Section 12.11) when  |
   |          |             |                  | an announced subnet   |
   |          |             |                  | clashes with an       |
   |          |             |                  | existing destination  |
   |          |             |                  | subnet.               |
   |          |             |                  |                       |






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   | 4-111    | Continue    | <Unassigned>     | Available for future  |
   |          |             |                  | extensions.           |
   |          |             |                  |                       |
   | 112-127  | Continue    | <Reserved for    | Available for         |
   |          |             | Private Use>     | experiments.          |
   |          |             |                  |                       |
   | 128      | Terminate   | Unknown Message  | The Message was not   |
   |          |             |                  | recognized by the     |
   |          |             |                  | implementation.       |
   |          |             |                  |                       |
   | 129      | Terminate   | Unexpected       | The Message was not   |
   |          |             | Message          | expected while the    |
   |          |             |                  | device was in the     |
   |          |             |                  | current state, e.g.,  |
   |          |             |                  | a Session             |
   |          |             |                  | Initialization        |
   |          |             |                  | Message               |
   |          |             |                  | (Section 12.5) in     |
   |          |             |                  | the In-Session state. |
   |          |             |                  |                       |
   | 130      | Terminate   | Invalid Data     | One or more Data      |
   |          |             |                  | Items in the Message  |
   |          |             |                  | are invalid,          |
   |          |             |                  | unexpected, or        |
   |          |             |                  | incorrectly           |
   |          |             |                  | duplicated.           |
   |          |             |                  |                       |
   | 131      | Terminate   | Invalid          | The destination       |
   |          |             | Destination      | included in the       |
   |          |             |                  | Message does not      |
   |          |             |                  | match a previously    |
   |          |             |                  | announced destination |
   |          |             |                  | -- for example, in    |
   |          |             |                  | the Link              |
   |          |             |                  | Characteristics       |
   |          |             |                  | Response Message      |
   |          |             |                  | (Section 12.19).      |
   |          |             |                  |                       |
   | 132      | Terminate   | Timed Out        | The session has       |
   |          |             |                  | timed out.            |
   |          |             |                  |                       |
   | 133-239  | Terminate   | <Unassigned>     | Available for future  |
   |          |             |                  | extensions.           |
   |          |             |                  |                       |







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   | 240-254  | Terminate   | <Reserved for    | Available for         |
   |          |             | Private Use>     | experiments.          |
   |          |             |                  |                       |
   | 255      | Terminate   | Shutting Down    | The peer is           |
   |          |             |                  | terminating the       |
   |          |             |                  | session, as it is     |
   |          |             |                  | shutting down.        |
   +----------+-------------+------------------+-----------------------+

                        Table 2: DLEP Status Codes

13.2.  IPv4 Connection Point

   The IPv4 Connection Point Data Item indicates the IPv4 address and,
   optionally, the TCP port number on the modem available for
   connections.  If provided, the router MUST use this information to
   initiate the TCP connection to the modem.

   The IPv4 Connection Point Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |               IPv4 Address...                 :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :  ...cont.     |   TCP Port Number (optional)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  2

   Length:  5 (or 7 if TCP Port Number included).

   Flags:  Flags field, defined below.

   IPv4 Address:  The IPv4 address listening on the modem.

   TCP Port Number:  TCP port number on the modem.

   If the Length field is 7, the port number specified MUST be used to
   establish the TCP session.  If the TCP Port Number is omitted, i.e.,
   the Length field is 5, the router MUST use the DLEP well-known port
   number (Section 15.14) to establish the TCP connection.







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   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |T|
   +-+-+-+-+-+-+-+-+

   T:  Use TLS flag, indicating whether the TCP connection to the given
       address and port requires the use of TLS [RFC5246] (1) or
       not (0).

   Reserved:  MUST be zero.  Left for future assignment.

13.3.  IPv6 Connection Point

   The IPv6 Connection Point Data Item indicates the IPv6 address and,
   optionally, the TCP port number on the modem available for
   connections.  If provided, the router MUST use this information to
   initiate the TCP connection to the modem.

   The IPv6 Connection Point Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |                IPv6 Address                   :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :  ...cont.     |   TCP Port Number (optional)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  3

   Length:  17 (or 19 if TCP Port Number included).

   Flags:  Flags field, defined below.

   IPv6 Address:  The IPv6 address listening on the modem.

   TCP Port Number:  TCP port number on the modem.




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   If the Length field is 19, the port number specified MUST be used to
   establish the TCP session.  If the TCP Port Number is omitted, i.e.,
   the Length field is 17, the router MUST use the DLEP well-known port
   number (Section 15.14) to establish the TCP connection.

   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |T|
   +-+-+-+-+-+-+-+-+

   T:  Use TLS flag, indicating whether the TCP connection to the given
       address and port requires the use of TLS [RFC5246] (1) or
       not (0).

   Reserved:  MUST be zero.  Left for future assignment.

13.4.  Peer Type

   The Peer Type Data Item is used by the router and modem to give
   additional information as to its type and the properties of the
   over-the-air control plane.

   With some devices, access to the shared RF medium is strongly
   controlled.  One example of this would be satellite modems -- where
   protocols, proprietary in nature, have been developed to ensure that
   a given modem has authorization to connect to the shared medium.
   Another example of this class of modems is governmental/military
   devices, where elaborate mechanisms have been developed to ensure
   that only authorized devices can connect to the shared medium.
   Contrasting with the above, there are modems where no such access
   control is used.  An example of this class of modem would be one that
   supports the 802.11 ad hoc mode of operation.  The Secured Medium (S)
   flag is used to indicate if access control is in place.

   The Peer Type Data Item includes a textual description of the peer;
   it is envisioned that the text will be used for informational
   purposes (e.g., as output in a display command).












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   The Peer Type Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags         | Description...                                :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  4

   Length:  1 + Length of Description, in octets.

   Flags:  Flags field, defined below.

   Description:  UTF-8 encoded string of Unicode [RFC3629] characters.
      For example, a satellite modem might set this variable to
      "Satellite terminal".  Since this Data Item is intended to provide
      additional information for display commands, sending
      implementations SHOULD limit the data to printable characters.

   An implementation MUST NOT assume that the Description field is a
   NUL-terminated string of printable characters.

   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |S|
   +-+-+-+-+-+-+-+-+

   S:  Secured Medium flag, used by a modem to indicate whether the
       shared RF medium implements access control (1) or not (0).  The
       Secured Medium flag only has meaning in Signals and Messages sent
       by a modem.

   Reserved:  MUST be zero.  Left for future assignment.













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13.5.  Heartbeat Interval

   The Heartbeat Interval Data Item is used to specify a period in
   milliseconds for Heartbeat Messages (Section 12.20).

   The Heartbeat Interval Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Heartbeat Interval                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  5

   Length:  4

   Heartbeat Interval:  The interval in milliseconds, expressed as a
      32-bit unsigned integer, for Heartbeat Messages.  This value
      MUST NOT be 0.

   As mentioned before, receipt of any valid DLEP Message MUST reset the
   heartbeat interval timer (i.e., valid DLEP Messages take the place
   of, and obviate the need for, additional Heartbeat Messages).

13.6.  Extensions Supported

   The Extensions Supported Data Item is used by the router and modem to
   negotiate additional optional functionality they are willing to
   support.  The Extensions List is a concatenation of the types of each
   supported extension, found in the IANA registry titled "Extension
   Type Values".  Each Extension Type definition includes which
   additional Signals and Data Items are supported.
















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   The Extensions Supported Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions List...                                            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  6

   Length:  Length of the Extensions List in octets.  This is twice (2x)
      the number of extensions.

   Extensions List:  A list of extensions supported, identified by their
      2-octet values as listed in the "Extension Type Values" registry.

13.7.  MAC Address

   The MAC Address Data Item contains the address of the destination on
   the remote node.

   DLEP can support MAC addresses in either EUI-48 or EUI-64 format
   ("EUI" stands for "Extended Unique Identifier"), with the restriction
   that all MAC addresses for a given DLEP session MUST be in the same
   format and MUST be consistent with the MAC address format of the
   connected modem (e.g., if the modem is connected to the router with
   an EUI-48 MAC, all destination addresses via that modem MUST be
   expressed in EUI-48 format).

   Examples of a virtual destination would be (1) a multicast MAC
   address or (2) the broadcast MAC address (FF:FF:FF:FF:FF:FF).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      MAC Address                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                MAC Address    :     (if EUI-64 used)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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   Data Item Type:  7

   Length:  6 for EUI-48 format or 8 for EUI-64 format.

   MAC Address:  MAC address of the destination.

13.8.  IPv4 Address

   When included in the Session Update Message, this Data Item contains
   the IPv4 address of the peer.  When included in Destination Messages,
   this Data Item contains the IPv4 address of the destination.  In
   either case, the Data Item also contains an indication of whether
   this is (1) a new or existing address or (2) a deletion of a
   previously known address.

   The IPv4 Address Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags         | IPv4 Address                                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :    ...cont.   |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  8

   Length:  5

   Flags:  Flags field, defined below.

   IPv4 Address:  The IPv4 address of the destination or peer.

   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |A|
   +-+-+-+-+-+-+-+-+

   A:  Add/Drop flag, indicating whether this is a new or existing
       address (1) or a withdrawal of an address (0).

   Reserved:  MUST be zero.  Reserved for future use.





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13.8.1.  IPv4 Address Processing

   Processing of the IPv4 Address Data Item MUST be done within the
   context of the DLEP peer session on which it is presented.

   The handling of erroneous or logically inconsistent conditions
   depends upon the type of the message that contains the Data Item,
   as follows:

   If the containing message is a Session Message, e.g., a Session
   Initialization Message (Section 12.5) or Session Update Message
   (Section 12.7), the receiver of inconsistent information MUST issue a
   Session Termination Message (Section 12.9) containing a Status Data
   Item (Section 13.1) with status code set to 130 'Invalid Data' and
   transition to the Session Termination state.  Examples of such
   conditions are:

   o  An address Drop operation referencing an address that is not
      associated with the peer in the current session.

   o  An address Add operation referencing an address that has already
      been added to the peer in the current session.

   If the containing message is a Destination Message, e.g., a
   Destination Up Message (Section 12.11) or Destination Update Message
   (Section 12.17), the receiver of inconsistent information MAY issue
   the appropriate response message containing a Status Data Item with
   status code set to 3 'Inconsistent Data' but MUST continue with
   session processing.  Examples of such conditions are:

   o  An address Add operation referencing an address that has already
      been added to the destination in the current session.

   o  An address Add operation referencing an address that is associated
      with a different destination or the peer in the current session.

   o  An address Add operation referencing an address that makes no
      sense -- for example, defined as not forwardable in [RFC6890].

   o  An address Drop operation referencing an address that is not
      associated with the destination in the current session.

   If no response message is appropriate -- for example, the Destination
   Update Message -- then the implementation MUST continue with session
   processing.

   Modems that do not track IPv4 addresses MUST silently ignore IPv4
   Address Data Items.



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13.9.  IPv6 Address

   When included in the Session Update Message, this Data Item contains
   the IPv6 address of the peer.  When included in Destination Messages,
   this Data Item contains the IPv6 address of the destination.  In
   either case, the Data Item also contains an indication of whether
   this is (1) a new or existing address or (2) a deletion of a
   previously known address.

   The IPv6 Address Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags         | IPv6 Address                                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   : IPv6 Address  |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  9

   Length:  17

   Flags:  Flags field, defined below.

   IPv6 Address:  The IPv6 address of the destination or peer.

   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |A|
   +-+-+-+-+-+-+-+-+

   A:  Add/Drop flag, indicating whether this is a new or existing
       address (1) or a withdrawal of an address (0).

   Reserved:  MUST be zero.  Reserved for future use.





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13.9.1.  IPv6 Address Processing

   Processing of the IPv6 Address Data Item MUST be done within the
   context of the DLEP peer session on which it is presented.

   The handling of erroneous or logically inconsistent conditions
   depends upon the type of the message that contains the Data Item,
   as follows:

   If the containing message is a Session Message, e.g., a Session
   Initialization Message (Section 12.5) or Session Update Message
   (Section 12.7), the receiver of inconsistent information MUST issue a
   Session Termination Message (Section 12.9) containing a Status Data
   Item (Section 13.1) with status code set to 130 'Invalid Data' and
   transition to the Session Termination state.  Examples of such
   conditions are:

   o  An address Drop operation referencing an address that is not
      associated with the peer in the current session.

   o  An address Add operation referencing an address that has already
      been added to the peer in the current session.

   If the containing message is a Destination Message, e.g., a
   Destination Up Message (Section 12.11) or Destination Update Message
   (Section 12.17), the receiver of inconsistent information MAY issue
   the appropriate response message containing a Status Data Item with
   status code set to 3 'Inconsistent Data' but MUST continue with
   session processing.  Examples of such conditions are:

   o  An address Add operation referencing an address that has already
      been added to the destination in the current session.

   o  An address Add operation referencing an address that is associated
      with a different destination or the peer in the current session.

   o  An address Add operation referencing an address that makes no
      sense -- for example, defined as not forwardable in [RFC6890].

   o  An address Drop operation referencing an address that is not
      associated with the destination in the current session.

   If no response message is appropriate -- for example, the Destination
   Update Message -- then the implementation MUST continue with session
   processing.

   Modems that do not track IPv6 addresses MUST silently ignore IPv6
   Address Data Items.



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13.10.  IPv4 Attached Subnet

   The DLEP IPv4 Attached Subnet Data Item allows a device to declare
   that it has an IPv4 subnet (e.g., a stub network) attached, that it
   has become aware of an IPv4 subnet being present at a remote
   destination, or that it has become aware of the loss of a subnet at
   the remote destination.

   The DLEP IPv4 Attached Subnet Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Flags        |          IPv4 Attached Subnet                 :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :    ...cont.   |Prefix Length  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  10

   Length:  6

   Flags:  Flags field, defined below.

   IPv4 Attached Subnet:  The IPv4 subnet reachable at the destination.

   Prefix Length:  Length of the prefix (0-32) for the IPv4 subnet.  A
      prefix length outside the specified range MUST be considered as
      invalid.

   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |A|
   +-+-+-+-+-+-+-+-+

   A:  Add/Drop flag, indicating whether this is a new or existing
       subnet address (1) or a withdrawal of a subnet address (0).

   Reserved:  MUST be zero.  Reserved for future use.







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13.10.1.  IPv4 Attached Subnet Processing

   Processing of the IPv4 Attached Subnet Data Item MUST be done within
   the context of the DLEP peer session on which it is presented.

   If the containing message is a Session Message, e.g., a Session
   Initialization Message (Section 12.5) or Session Update Message
   (Section 12.7), the receiver of inconsistent information MUST issue a
   Session Termination Message (Section 12.9) containing a Status Data
   Item (Section 13.1) with status code set to 130 'Invalid Data' and
   transition to the Session Termination state.  Examples of such
   conditions are:

   o  A subnet Drop operation referencing a subnet that is not
      associated with the peer in the current session.

   o  A subnet Add operation referencing a subnet that has already been
      added to the peer in the current session.

   If the containing message is a Destination Message, e.g., a
   Destination Up Message (Section 12.11) or Destination Update Message
   (Section 12.17), the receiver of inconsistent information MAY issue
   the appropriate response message containing a Status Data Item with
   status code set to 3 'Inconsistent Data' but MUST continue with
   session processing.  Examples of such conditions are:

   o  A subnet Add operation referencing a subnet that has already been
      added to the destination in the current session.

   o  A subnet Add operation referencing a subnet that is associated
      with a different destination in the current session.

   o  A subnet Add operation referencing a subnet that makes no sense --
      for example, defined as not forwardable in [RFC6890].

   o  A subnet Drop operation referencing a subnet that is not
      associated with the destination in the current session.

   If no response message is appropriate -- for example, the Destination
   Update Message -- then the implementation MUST continue with session
   processing.

   Modems that do not track IPv4 subnets MUST silently ignore IPv4
   Attached Subnet Data Items.







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13.11.  IPv6 Attached Subnet

   The DLEP IPv6 Attached Subnet Data Item allows a device to declare
   that it has an IPv6 subnet (e.g., a stub network) attached, that it
   has become aware of an IPv6 subnet being present at a remote
   destination, or that it has become aware of the loss of a subnet at
   the remote destination.

   The DLEP IPv6 Attached Subnet Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags       |         IPv6 Attached Subnet                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                   IPv6 Attached Subnet                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                   IPv6 Attached Subnet                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                   IPv6 Attached Subnet                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :    ...cont.   | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  11

   Length:  18

   Flags:  Flags field, defined below.

   IPv6 Attached Subnet:  The IPv6 subnet reachable at the destination.

   Prefix Length:  Length of the prefix (0-128) for the IPv6 subnet.  A
      prefix length outside the specified range MUST be considered as
      invalid.













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   The Flags field is defined as:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |  Reserved   |A|
   +-+-+-+-+-+-+-+-+

   A:  Add/Drop flag, indicating whether this is a new or existing
       subnet address (1) or a withdrawal of a subnet address (0).

   Reserved:  MUST be zero.  Reserved for future use.

13.11.1.  IPv6 Attached Subnet Processing

   Processing of the IPv6 Attached Subnet Data Item MUST be done within
   the context of the DLEP peer session on which it is presented.

   If the containing message is a Session Message, e.g., a Session
   Initialization Message (Section 12.5) or Session Update Message
   (Section 12.7), the receiver of inconsistent information MUST issue a
   Session Termination Message (Section 12.9) containing a Status Data
   Item (Section 13.1) with status code set to 130 'Invalid Data' and
   transition to the Session Termination state.  Examples of such
   conditions are:

   o  A subnet Drop operation referencing a subnet that is not
      associated with the peer in the current session.

   o  A subnet Add operation referencing a subnet that has already been
      added to the peer in the current session.

   If the containing message is a Destination Message, e.g., a
   Destination Up Message (Section 12.11) or Destination Update Message
   (Section 12.17), the receiver of inconsistent information MAY issue
   the appropriate response message containing a Status Data Item with
   status code set to 3 'Inconsistent Data' but MUST continue with
   session processing.  Examples of such conditions are:

   o  A subnet Add operation referencing a subnet that has already been
      added to the destination in the current session.

   o  A subnet Add operation referencing a subnet that is associated
      with a different destination in the current session.








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   o  A subnet Add operation referencing a subnet that makes no sense --
      for example, defined as not forwardable in [RFC6890].

   o  A subnet Drop operation referencing a subnet that is not
      associated with the destination in the current session.

   If no response message is appropriate -- for example, the Destination
   Update Message -- then the implementation MUST continue with session
   processing.

   Modems that do not track IPv6 subnets MUST silently ignore IPv6
   Attached Subnet Data Items.

13.12.  Maximum Data Rate (Receive)

   The Maximum Data Rate (Receive) (MDRR) Data Item is used to indicate
   the maximum theoretical data rate, in bits per second (bps), that can
   be achieved while receiving data on the link.

   The Maximum Data Rate (Receive) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MDRR (bps)                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        MDRR (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  12

   Length:  8

   Maximum Data Rate (Receive):  A 64-bit unsigned integer, representing
      the maximum theoretical data rate, in bits per second, that can be
      achieved while receiving on the link.












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13.13.  Maximum Data Rate (Transmit)

   The Maximum Data Rate (Transmit) (MDRT) Data Item is used to indicate
   the maximum theoretical data rate, in bits per second, that can be
   achieved while transmitting data on the link.

   The Maximum Data Rate (Transmit) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MDRT (bps)                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        MDRT (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  13

   Length:  8

   Maximum Data Rate (Transmit):  A 64-bit unsigned integer,
      representing the maximum theoretical data rate, in bits per
      second, that can be achieved while transmitting on the link.

13.14.  Current Data Rate (Receive)

   The Current Data Rate (Receive) (CDRR) Data Item is used to indicate
   the rate at which the link is currently operating for receiving
   traffic.

   When used in the Link Characteristics Request Message
   (Section 12.18), Current Data Rate (Receive) represents the desired
   receive rate, in bits per second, on the link.















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   The Current Data Rate (Receive) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        CDRR (bps)                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        CDRR (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  14

   Length:  8

   Current Data Rate (Receive):  A 64-bit unsigned integer, representing
      the current data rate, in bits per second, that can currently be
      achieved while receiving traffic on the link.

   If there is no distinction between Current Data Rate (Receive) and
   Maximum Data Rate (Receive) (Section 13.12), Current Data Rate
   (Receive) MUST be set equal to Maximum Data Rate (Receive).  Current
   Data Rate (Receive) MUST NOT exceed Maximum Data Rate (Receive).

13.15.  Current Data Rate (Transmit)

   The Current Data Rate (Transmit) (CDRT) Data Item is used to indicate
   the rate at which the link is currently operating for transmitting
   traffic.

   When used in the Link Characteristics Request Message
   (Section 12.18), Current Data Rate (Transmit) represents the desired
   transmit rate, in bits per second, on the link.

   The Current Data Rate (Transmit) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        CDRT (bps)                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        CDRT (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   Data Item Type:  15

   Length:  8

   Current Data Rate (Transmit):  A 64-bit unsigned integer,
      representing the current data rate, in bits per second, that can
      currently be achieved while transmitting traffic on the link.

   If there is no distinction between Current Data Rate (Transmit) and
   Maximum Data Rate (Transmit) (Section 13.13), Current Data Rate
   (Transmit) MUST be set equal to Maximum Data Rate (Transmit).
   Current Data Rate (Transmit) MUST NOT exceed Maximum Data Rate
   (Transmit).

13.16.  Latency

   The Latency Data Item is used to indicate the amount of latency, in
   microseconds, on the link.

   The Latency value is reported as transmission delay.  The calculation
   of latency is implementation dependent.  For example, the latency may
   be a running average calculated from the internal queuing.

   The Latency Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Latency                            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                            Latency                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  16

   Length:  8

   Latency:  A 64-bit unsigned integer, representing the transmission
      delay, in microseconds, that a packet encounters as it is
      transmitted over the link.









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13.17.  Resources

   The Resources (RES) Data Item is used to indicate the amount of
   finite resources available for data transmission and reception at the
   destination as a percentage, with 0 meaning 'no resources remaining'
   and 100 meaning 'a full supply', assuming that when Resources reaches
   0 data transmission and/or reception will cease.

   An example of such resources is battery life, but this could also
   include resources such as available memory for queuing, or CPU idle
   percentage.  The specific criteria to be used for this metric is out
   of scope for this specification and is implementation specific.

   This Data Item is designed to be used as an indication of some
   capability of the modem and/or router at the destination.

   The Resources Data Item contains the following fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RES       |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  17

   Length:  1

   Resources:  An 8-bit unsigned integer percentage, 0-100, representing
      the amount of resources available.  Any value greater than 100
      MUST be considered as invalid.

   If a device cannot calculate Resources, this Data Item MUST NOT
   be issued.















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13.18.  Relative Link Quality (Receive)

   The Relative Link Quality (Receive) (RLQR) Data Item is used to
   indicate the quality of the link to a destination for receiving
   traffic, with 0 meaning 'worst quality' and 100 meaning 'best
   quality'.

   Quality in this context is defined as an indication of the stability
   of a link for reception; a destination with high Relative Link
   Quality (Receive) is expected to have generally stable DLEP metrics,
   and the metrics of a destination with low Relative Link Quality
   (Receive) can be expected to rapidly fluctuate over a wide range.

   The Relative Link Quality (Receive) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RLQR      |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  18

   Length:  1

   Relative Link Quality (Receive):  A non-dimensional unsigned 8-bit
      integer, 0-100, representing relative quality of the link for
      receiving traffic.  Any value greater than 100 MUST be considered
      as invalid.

   If a device cannot calculate Relative Link Quality (Receive), this
   Data Item MUST NOT be issued.

13.19.  Relative Link Quality (Transmit)

   The Relative Link Quality (Transmit) (RLQT) Data Item is used to
   indicate the quality of the link to a destination for transmitting
   traffic, with 0 meaning 'worst quality' and 100 meaning 'best
   quality'.

   Quality in this context is defined as an indication of the stability
   of a link for transmission; a destination with high Relative Link
   Quality (Transmit) is expected to have generally stable DLEP metrics,
   and the metrics of a destination with low Relative Link Quality
   (Transmit) can be expected to rapidly fluctuate over a wide range.



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   The Relative Link Quality (Transmit) Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RLQT      |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  19

   Length:  1

   Relative Link Quality (Transmit):  A non-dimensional unsigned 8-bit
      integer, 0-100, representing relative quality of the link for
      transmitting traffic.  Any value greater than 100 MUST be
      considered as invalid.

   If a device cannot calculate Relative Link Quality (Transmit), this
   Data Item MUST NOT be issued.

13.20.  Maximum Transmission Unit (MTU)

   The Maximum Transmission Unit (MTU) Data Item is used to indicate the
   maximum size, in octets, of an IP packet that can be transmitted
   without fragmentation, including headers, but excluding any
   lower-layer headers.

   The Maximum Transmission Unit Data Item contains the following
   fields:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Data Item Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             MTU               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  20

   Length:  2

   Maximum Transmission Unit:  The maximum size, in octets, of an
      IP packet that can be transmitted without fragmentation, including
      headers, but excluding any lower-layer headers.



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   If a device cannot calculate Maximum Transmission Unit, this Data
   Item MUST NOT be issued.

14.  Security Considerations

   The potential security concerns when using DLEP are as follows:

   1.  An attacker might pretend to be a DLEP participant, either at
       DLEP session initialization or by injection of DLEP Messages once
       a session has been established.

   2.  DLEP Data Items could be altered by an attacker, causing the
       receiving implementation to inappropriately alter its information
       base concerning network status.

   3.  An attacker could join an unsecured radio network and inject
       over-the-air signals that maliciously influence the information
       reported by a DLEP modem, causing a router to forward traffic to
       an inappropriate destination.

   The implications of attacks on DLEP peers are directly proportional
   to the extent to which DLEP data is used within the control plane.
   While the use of DLEP data in other control-plane components is out
   of scope for this document, as an example, if DLEP statistics are
   incorporated into route cost calculations, adversaries masquerading
   as a DLEP peer and injecting malicious data via DLEP could cause
   suboptimal route selection, adversely impacting network performance.
   Similar issues can arise if DLEP data is used as an input to policing
   algorithms -- injection of malicious data via DLEP can cause those
   policing algorithms to make incorrect decisions, degrading network
   throughput.

   For these reasons, security of the DLEP transport must be considered
   at both the transport layer and Layer 2.

   At the transport layer, when TLS is in use, each peer SHOULD check
   the validity of credentials presented by the other peer during TLS
   session establishment.  Implementations following the "dedicated
   deployments" model attempting to use TLS MAY (1) need to consider the
   use of pre-shared keys for credentials, (2) provide specialized
   techniques for peer identity validation, and (3) refer to [RFC5487]
   for additional details.  Implementations following the "networked
   deployment" model described in "Implementation Scenarios" (Section 4)
   SHOULD refer to [RFC7525] for additional details.







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   At Layer 2, since DLEP is restricted to operation over a single
   (possibly logical) hop, implementations SHOULD also secure the
   Layer 2 link.  Examples of technologies that can be deployed to
   secure the Layer 2 link include [IEEE-802.1AE] and [IEEE-802.1X].

   By examining the Secured Medium flag in the Peer Type Data Item
   (Section 13.4), a router can decide if it is able to trust the
   information supplied via a DLEP modem.  If this is not the case, then
   the router SHOULD consider restricting the size of attached subnets,
   announced in IPv4 Attached Subnet Data Items (Section 13.10) and/or
   IPv6 Attached Subnet Data Items (Section 13.11), that are considered
   for route selection.

   To avoid potential denial-of-service attacks, it is RECOMMENDED that
   implementations using the Peer Discovery mechanism (1) maintain an
   information base of hosts that persistently fail Session
   Initialization, even though those hosts have provided an acceptable
   Peer Discovery Signal and (2) ignore any subsequent Peer Discovery
   Signals from such hosts.

   This specification does not address security of the data plane, as it
   (the data plane) is not affected, and standard security procedures
   can be employed.

15.  IANA Considerations

15.1.  Registrations

   IANA has created a new protocol registry for the Dynamic Link
   Exchange Protocol (DLEP).  The remainder of this section details the
   new DLEP-specific registries.

15.2.  Signal Type Registrations

   IANA has created a new DLEP registry, named "Signal Type Values".
















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   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

          +--------------+--------------------------------------+
          | Type Code    | Description/Policy                   |
          +--------------+--------------------------------------+
          | 0            | Reserved                             |
          | 1            | Peer Discovery Signal                |
          | 2            | Peer Offer Signal                    |
          | 3-65519      | Unassigned / Specification Required  |
          | 65520-65534  | Reserved for Private Use             |
          | 65535        | Reserved                             |
          +--------------+--------------------------------------+

15.3.  Message Type Registrations

   IANA has created a new DLEP registry, named "Message Type Values".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

        +--------------+------------------------------------------+
        | Type Code    | Description/Policy                       |
        +--------------+------------------------------------------+
        | 0            | Reserved                                 |
        |              |                                          |
        | 1            | Session Initialization Message           |
        |              |                                          |
        | 2            | Session Initialization Response Message  |
        |              |                                          |
        | 3            | Session Update Message                   |
        |              |                                          |
        | 4            | Session Update Response Message          |
        |              |                                          |
        | 5            | Session Termination Message              |
        |              |                                          |
        | 6            | Session Termination Response Message     |
        |              |                                          |
        | 7            | Destination Up Message                   |
        |              |                                          |
        | 8            | Destination Up Response Message          |
        |              |                                          |
        | 9            | Destination Announce Message             |
        |              |                                          |
        | 10           | Destination Announce Response Message    |
        |              |                                          |
        | 11           | Destination Down Message                 |
        |              |                                          |



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        | 12           | Destination Down Response Message        |
        |              |                                          |
        | 13           | Destination Update Message               |
        |              |                                          |
        | 14           | Link Characteristics Request Message     |
        |              |                                          |
        | 15           | Link Characteristics Response Message    |
        |              |                                          |
        | 16           | Heartbeat Message                        |
        |              |                                          |
        | 17-65519     | Unassigned / Specification Required      |
        |              |                                          |
        | 65520-65534  | Reserved for Private Use                 |
        |              |                                          |
        | 65535        | Reserved                                 |
        +--------------+------------------------------------------+

15.4.  DLEP Data Item Registrations

   IANA has created a new DLEP registry, named "Data Item Type Values".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

     +-------------------+------------------------------------------+
     | Type Code         | Description/Policy                       |
     +-------------------+------------------------------------------+
     | 0                 | Reserved                                 |
     |                   |                                          |
     | 1                 | Status                                   |
     |                   |                                          |
     | 2                 | IPv4 Connection Point                    |
     |                   |                                          |
     | 3                 | IPv6 Connection Point                    |
     |                   |                                          |
     | 4                 | Peer Type                                |
     |                   |                                          |
     | 5                 | Heartbeat Interval                       |
     |                   |                                          |
     | 6                 | Extensions Supported                     |
     |                   |                                          |
     | 7                 | MAC Address                              |
     |                   |                                          |
     | 8                 | IPv4 Address                             |
     |                   |                                          |
     | 9                 | IPv6 Address                             |
     |                   |                                          |
     | 10                | IPv4 Attached Subnet                     |



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     |                   |                                          |
     | 11                | IPv6 Attached Subnet                     |
     |                   |                                          |
     | 12                | Maximum Data Rate (Receive) (MDRR)       |
     |                   |                                          |
     | 13                | Maximum Data Rate (Transmit) (MDRT)      |
     |                   |                                          |
     | 14                | Current Data Rate (Receive) (CDRR)       |
     |                   |                                          |
     | 15                | Current Data Rate (Transmit) (CDRT)      |
     |                   |                                          |
     | 16                | Latency                                  |
     |                   |                                          |
     | 17                | Resources (RES)                          |
     |                   |                                          |
     | 18                | Relative Link Quality (Receive) (RLQR)   |
     |                   |                                          |
     | 19                | Relative Link Quality (Transmit) (RLQT)  |
     |                   |                                          |
     | 20                | Maximum Transmission Unit (MTU)          |
     |                   |                                          |
     | 21-65407          | Unassigned / Specification Required      |
     |                   |                                          |
     | 65408-65534       | Reserved for Private Use                 |
     |                   |                                          |
     | 65535             | Reserved                                 |
     +-------------------+------------------------------------------+

15.5.  DLEP Status Code Registrations

   IANA has created a new DLEP registry, named "Status Code Values".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

   +--------------+---------------+------------------------------------+
   | Status Code  | Failure Mode  | Description/Policy                 |
   +--------------+---------------+------------------------------------+
   | 0            | Continue      | Success                            |
   |              |               |                                    |
   | 1            | Continue      | Not Interested                     |
   |              |               |                                    |
   | 2            | Continue      | Request Denied                     |
   |              |               |                                    |
   | 3            | Continue      | Inconsistent Data                  |
   |              |               |                                    |
   | 4-111        | Continue      | Unassigned / Specification         |
   |              |               | Required                           |



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   |              |               |                                    |
   | 112-127      | Continue      | Private Use                        |
   |              |               |                                    |
   | 128          | Terminate     | Unknown Message                    |
   |              |               |                                    |
   | 129          | Terminate     | Unexpected Message                 |
   |              |               |                                    |
   | 130          | Terminate     | Invalid Data                       |
   |              |               |                                    |
   | 131          | Terminate     | Invalid Destination                |
   |              |               |                                    |
   | 132          | Terminate     | Timed Out                          |
   |              |               |                                    |
   | 133-239      | Terminate     | Unassigned / Specification         |
   |              |               | Required                           |
   |              |               |                                    |
   | 240-254      | Terminate     | Reserved for Private Use           |
   |              |               |                                    |
   | 255          | Terminate     | Shutting Down                      |
   +--------------+---------------+------------------------------------+

15.6.  DLEP Extension Registrations

   IANA has created a new DLEP registry, named "Extension Type Values".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

          +--------------+--------------------------------------+
          | Code         | Description/Policy                   |
          +--------------+--------------------------------------+
          | 0            | Reserved                             |
          | 1-65519      | Unassigned / Specification Required  |
          | 65520-65534  | Reserved for Private Use             |
          | 65535        | Reserved                             |
          +--------------+--------------------------------------+

                       Table 3: DLEP Extension Types













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15.7.  DLEP IPv4 Connection Point Flags

   IANA has created a new DLEP registry, named "IPv4 Connection Point
   Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Use TLS [RFC5246] indicator          |
           +------------+--------------------------------------+

15.8.  DLEP IPv6 Connection Point Flags

   IANA has created a new DLEP registry, named "IPv6 Connection Point
   Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Use TLS [RFC5246] indicator          |
           +------------+--------------------------------------+

15.9.  DLEP Peer Type Flags

   IANA has created a new DLEP registry, named "Peer Type Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Secured Medium indicator             |
           +------------+--------------------------------------+








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15.10.  DLEP IPv4 Address Flags

   IANA has created a new DLEP registry, named "IPv4 Address Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Add/Drop indicator                   |
           +------------+--------------------------------------+

15.11.  DLEP IPv6 Address Flags

   IANA has created a new DLEP registry, named "IPv6 Address Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Add/Drop indicator                   |
           +------------+--------------------------------------+

15.12.  DLEP IPv4 Attached Subnet Flags

   IANA has created a new DLEP registry, named "IPv4 Attached Subnet
   Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Add/Drop indicator                   |
           +------------+--------------------------------------+









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15.13.  DLEP IPv6 Attached Subnet Flags

   IANA has created a new DLEP registry, named "IPv6 Attached Subnet
   Flags".

   The following table provides initial registry values and the
   policies, as defined by [RFC5226], that apply to the registry:

           +------------+--------------------------------------+
           | Bit        | Description/Policy                   |
           +------------+--------------------------------------+
           | 0-6        | Unassigned / Specification Required  |
           | 7          | Add/Drop indicator                   |
           +------------+--------------------------------------+

15.14.  DLEP Well-Known Port

   IANA has assigned the value 854 in the "Service Name and Transport
   Protocol Port Number Registry" found at
   <http://www.iana.org/assignments/service-names-port-numbers/> for use
   by "DLEP", as defined in this document.  This assignment is valid for
   TCP and UDP.

15.15.  DLEP IPv4 Link-Local Multicast Address

   IANA has assigned the IPv4 multicast address 224.0.0.117 in the
   registry found at
   <http://www.iana.org/assignments/multicast-addresses> for use as
   "DLEP Discovery".

15.16.  DLEP IPv6 Link-Local Multicast Address

   IANA has assigned the IPv6 multicast address FF02:0:0:0:0:0:1:7 in
   the registry found at
   <http://www.iana.org/assignments/ipv6-multicast-addresses> for use as
   "DLEP Discovery".















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16.  References

16.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
              ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
              November 2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <http://www.rfc-editor.org/info/rfc5082>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <http://www.rfc-editor.org/info/rfc8174>.

16.2.  Informative References

   [IEEE-802.1AE]
              "IEEE Standards for Local and Metropolitan Area Networks:
              Media Access Control (MAC) Security",
              DOI 10.1109/IEEESTD.2006.245590,
              <http://ieeexplore.ieee.org/document/1678345/>.

   [IEEE-802.1X]
              "IEEE Standards for Local and metropolitan area networks--
              Port-Based Network Access Control",
              DOI 10.1109/IEEESTD.2010.5409813,
              <http://ieeexplore.ieee.org/document/5409813/>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.





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   [RFC5487]  Badra, M., "Pre-Shared Key Cipher Suites for TLS with
              SHA-256/384 and AES Galois Counter Mode", RFC 5487,
              DOI 10.17487/RFC5487, March 2009,
              <http://www.rfc-editor.org/info/rfc5487>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <http://www.rfc-editor.org/info/rfc6890>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
              May 2015, <http://www.rfc-editor.org/info/rfc7525>.




































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Appendix A.  Discovery Signal Flows

Router                       Modem    Signal Description
========================================================================

|                                     Router initiates discovery,
|                                     starts a timer, sends Peer
|-------Peer Discovery---->X          Discovery Signal.

         ~ ~ ~ ~ ~ ~ ~                Router discovery timer expires
                                      without receiving Peer Offer.

|                                     Router sends another Peer
|-------Peer Discovery---------->|    Discovery Signal.
                                 |
                                 |    Modem receives Peer Discovery
                                 |    Signal.
                                 |
                                 |    Modem sends Peer Offer with
|<--------Peer Offer-------------|    Connection Point information.
:
:                                     Router MAY cancel discovery timer
:                                     and stop sending Peer Discovery
:                                     Signals.

Appendix B.  Peer-Level Message Flows

B.1.  Session Initialization

Router                       Modem    Message Description
========================================================================

|                                     Router connects to discovered or
|                                     preconfigured Modem Connection
|--TCP connection established--->     Point.
|
|                                     Router sends Session
|----Session Initialization----->|    Initialization Message.
                                 |
                                 |    Modem receives Session
                                 |    Initialization Message.
                                 |
                                 |    Modem sends Session Initialization
|<--Session Initialization Resp.-|    Response with 'Success' Status
|                                |    Data Item.
|                                |
|<<============================>>|    Session established.
:                                :    Heartbeats begin.



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B.2.  Session Initialization - Refused

Router                       Modem    Message Description
========================================================================

|                                     Router connects to discovered or
|                                     preconfigured Modem Connection
|--TCP connection established--->     Point.
|
|                                     Router sends Session
|-----Session Initialization---->|    Initialization Message.
                                 |
                                 |    Modem receives Session
                                 |    Initialization Message and
                                 |    will not support the advertised
                                 |    extensions.
                                 |
                                 |    Modem sends Session Initialization
                                 |    Response with 'Request Denied'
|<-Session Initialization Resp.--|    Status Data Item.
|
|
|                                     Router receives negative Session
|                                     Initialization Response, closes
||---------TCP close------------||    TCP connection.

B.3.  Router Changes IP Addresses

Router                       Modem    Message Description
========================================================================

|                                     Router sends Session Update
|-------Session Update---------->|    Message to announce change of
                                 |    IP address.
                                 |
                                 |    Modem receives Session Update
                                 |    Message and updates internal
                                 |    state.
                                 |
|<----Session Update Response----|    Modem sends Session Update
                                 |    Response.










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B.4.  Modem Changes Session-Wide Metrics

Router                       Modem    Message Description
========================================================================

                                 |    Modem sends Session Update Message
                                 |    to announce change of session-wide
|<--------Session Update---------|    metrics.
|
|                                     Router receives Session Update
|                                     Message and updates internal
|                                     state.
|
|----Session Update Response---->|    Router sends Session Update
                                 |    Response.

B.5.  Router Terminates Session

Router                       Modem    Message Description
========================================================================

|                                     Router sends Session Termination
|------Session Termination------>|    Message with Status Data Item.
|                                |
|-------TCP shutdown (send)--->  |    Router stops sending Messages.
                                 |
                                 |    Modem receives Session
                                 |    Termination, stops counting
                                 |    received heartbeats, and stops
                                 |    sending heartbeats.
                                 |
                                 |    Modem sends Session Termination
|<---Session Termination Resp.---|    Response with Status 'Success'.
|
|                                     Modem stops sending Messages.
|
||---------TCP close------------||    Session terminated.














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B.6.  Modem Terminates Session

Router                       Modem    Message Description
========================================================================

                                 |    Modem sends Session Termination
|<----Session Termination--------|    Message with Status Data Item.
|
|                                     Modem stops sending Messages.
|
|                                     Router receives Session
|                                     Termination, stops counting
|                                     received heartbeats, and stops
|                                     sending heartbeats.
|
|                                     Router sends Session Termination
|---Session Termination Resp.--->|    Response with Status 'Success'.
                                 |
                                 |    Router stops sending Messages.
                                 |
||---------TCP close------------||    Session terminated.






























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B.7.  Session Heartbeats

Router                       Modem    Message Description
========================================================================

|----------Heartbeat------------>|    Router sends Heartbeat Message.
                                 |
                                 |    Modem resets heartbeats missed
                                 |    counter.

         ~ ~ ~ ~ ~ ~ ~

|---------[Any Message]--------->|    When the Modem receives any
                                 |    Message from the Router.
                                 |
                                 |    Modem resets heartbeats missed
                                 |    counter.

         ~ ~ ~ ~ ~ ~ ~

|<---------Heartbeat-------------|    Modem sends Heartbeat Message.
|
|                                     Router resets heartbeats missed
|                                     counter.

         ~ ~ ~ ~ ~ ~ ~

|<--------[Any Message]----------|    When the Router receives any
|                                     Message from the Modem.
|
|                                     Modem resets heartbeats missed
|                                     counter.



















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B.8.  Router Detects a Heartbeat Timeout

Router                       Modem    Message Description
========================================================================

         X<----------------------|    Router misses a heartbeat.


|        X<----------------------|    Router misses too many
|                                     heartbeats.
|
|
|------Session Termination------>|    Router sends Session Termination
|                                     Message with 'Timeout' Status
|                                     Data Item.
:
:                                     Termination proceeds...

B.9.  Modem Detects a Heartbeat Timeout

Router                       Modem    Message Description
========================================================================

|---------------------->X             Modem misses a heartbeat.


|---------------------->X        |    Modem misses too many
                                 |    heartbeats.
                                 |
                                 |
|<-----Session Termination-------|    Modem sends Session Termination
                                 |    Message with 'Timeout' Status
                                 |    Data Item.
                                 :
                                 :    Termination proceeds...
















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Appendix C.  Destination-Specific Message Flows

C.1.  Common Destination Notification

Router                       Modem    Message Description
========================================================================

                                 |    Modem detects a new logical
                                 |    destination is reachable and
|<-------Destination Up----------|    sends Destination Up Message.
|
|------Destination Up Resp.----->|    Router sends Destination Up
                                 |    Response.

           ~ ~ ~ ~ ~ ~ ~
                                 |    Modem detects change in logical
                                 |    destination metrics and sends
|<-------Destination Update------|    Destination Update Message.

           ~ ~ ~ ~ ~ ~ ~
                                 |    Modem detects change in logical
                                 |    destination metrics and sends
|<-------Destination Update------|    Destination Update Message.

            ~ ~ ~ ~ ~ ~ ~
                                 |    Modem detects logical destination
                                 |    is no longer reachable and sends
|<-------Destination Down--------|    Destination Down Message.
|
|                                     Router receives Destination Down,
|                                     updates internal state, and sends
|------Destination Down Resp.--->|    Destination Down Response Message.



















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C.2.  Multicast Destination Notification

Router                       Modem    Message Description
========================================================================

|                                     Router detects a new multicast
|                                     destination is in use and sends
|-----Destination Announce------>|    Destination Announce Message.
                                 |
                                 |    Modem updates internal state to
                                 |    monitor multicast destination and
|<-----Dest. Announce Resp.------|    sends Destination Announce
                                      Response.

           ~ ~ ~ ~ ~ ~ ~
                                 |    Modem detects change in multicast
                                 |    destination metrics and sends
|<-------Destination Update------|    Destination Update Message.

           ~ ~ ~ ~ ~ ~ ~
                                 |    Modem detects change in multicast
                                 |    destination metrics and sends
|<-------Destination Update------|    Destination Update Message.

            ~ ~ ~ ~ ~ ~ ~
|                                     Router detects multicast
|                                     destination is no longer in use
|--------Destination Down------->|    and sends Destination Down
                                 |    Message.
                                 |
                                 |    Modem receives Destination Down,
                                 |    updates internal state, and sends
|<-----Destination Down Resp.----|    Destination Down Response Message.


















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C.3.  Link Characteristics Request

Router                       Modem    Message Description
========================================================================

                                      Destination has already been
           ~ ~ ~ ~ ~ ~ ~              announced by either peer.

|                                     Router requires different
|                                     characteristics for the
|                                     destination and sends Link
|--Link Characteristics Request->|    Characteristics Request Message.
                                 |
                                 |    Modem attempts to adjust link
                                 |    properties to meet the received
                                 |    request and sends a Link
                                 |    Characteristics Response
|<---Link Characteristics Resp.--|    Message with the new values.

































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Acknowledgments

   We would like to acknowledge and thank the members of the DLEP design
   team, who have provided invaluable insight.  The members of the
   design team are Teco Boot, Bow-Nan Cheng, John Dowdell, and Henning
   Rogge.

   We would also like to acknowledge the influence and contributions of
   Greg Harrison, Chris Olsen, Martin Duke, Subir Das, Jaewon Kang,
   Vikram Kaul, Nelson Powell, Lou Berger, and Victoria Pritchard.

Authors' Addresses

   Stan Ratliff
   VT iDirect
   13861 Sunrise Valley Drive, Suite 300
   Herndon, VA  20171
   United States of America
   Email: sratliff@idirect.net

   Shawn Jury
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   United States of America
   Email: sjury@cisco.com

   Darryl Satterwhite
   Broadcom
   Email: dsatterw@broadcom.com

   Rick Taylor
   Airbus Defence & Space
   Quadrant House
   Celtic Springs
   Coedkernew
   Newport  NP10 8FZ
   United Kingdom
   Email: rick.taylor@airbus.com

   Bo Berry










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