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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.

Ratliff, et al. Standards Track [Page 1] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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

Ratliff, et al. Standards Track [Page 2] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

    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

Ratliff, et al. Standards Track [Page 3] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

    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

Ratliff, et al. Standards Track [Page 4] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 5] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 6] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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

Ratliff, et al. Standards Track [Page 7] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 (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

Ratliff, et al. Standards Track [Page 8] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 9] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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

Ratliff, et al. Standards Track [Page 10] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 11] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 12] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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

Ratliff, et al. Standards Track [Page 13] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 17] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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).

Ratliff, et al. Standards Track [Page 30] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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)

Ratliff, et al. Standards Track [Page 31] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 32] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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).

Ratliff, et al. Standards Track [Page 33] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 34] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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)

Ratliff, et al. Standards Track [Page 35] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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)                     |
 |             |                                                     |

Ratliff, et al. Standards Track [Page 37] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 | 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.

Ratliff, et al. Standards Track [Page 38] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.               |
 |          |             |                  |                       |

Ratliff, et al. Standards Track [Page 39] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 | 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.           |
 |          |             |                  |                       |

Ratliff, et al. Standards Track [Page 40] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 | 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.

Ratliff, et al. Standards Track [Page 41] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 42] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 45] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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)          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Ratliff, et al. Standards Track [Page 46] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 47] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 48] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 49] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 50] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 51] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 52] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 53] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 54] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 55] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 56] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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)                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Ratliff, et al. Standards Track [Page 57] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 58] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 59] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

Ratliff, et al. Standards Track [Page 60] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 61] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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.

Ratliff, et al. Standards Track [Page 62] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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".

Ratliff, et al. Standards Track [Page 63] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 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                 |
      |              |                                          |

Ratliff, et al. Standards Track [Page 64] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

      | 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                     |

Ratliff, et al. Standards Track [Page 65] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

   |                   |                                          |
   | 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                           |

Ratliff, et al. Standards Track [Page 66] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 |              |               |                                    |
 | 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

Ratliff, et al. Standards Track [Page 67] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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             |
         +------------+--------------------------------------+

Ratliff, et al. Standards Track [Page 68] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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                   |
         +------------+--------------------------------------+

Ratliff, et al. Standards Track [Page 69] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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".

Ratliff, et al. Standards Track [Page 70] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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>.

Ratliff, et al. Standards Track [Page 71] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

 [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>.

Ratliff, et al. Standards Track [Page 72] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

Appendix A. Discovery Signal Flows

Router Modem Signal Description

       ~ ~ ~ ~ ~ ~ ~                Router discovery timer expires
                                    without receiving Peer Offer.
——-Peer Discovery———→ Discovery Signal.
                               |
                               |    Modem receives Peer Discovery
                               |    Signal.
                               |
                               |    Modem sends Peer Offer with
←——-Peer Offer————-

: : 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

—-Session Initialization—–> Initialization Message.
                               |
                               |    Modem receives Session
                               |    Initialization Message.
                               |
                               |    Modem sends Session Initialization
←-Session Initialization Resp.-
«===========================⇒>

: : Heartbeats begin.

Ratliff, et al. Standards Track [Page 73] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

B.2. Session Initialization - Refused

Router Modem Message Description

—–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.–
———TCP close————

B.3. Router Changes IP Addresses

Router Modem Message Description

——-Session Update———→ Message to announce change of
                               |    IP address.
                               |
                               |    Modem receives Session Update
                               |    Message and updates internal
                               |    state.
                               |
←—Session Update Response—-

Ratliff, et al. Standards Track [Page 74] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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.

—-Session Update Response—→

B.5. Router Terminates Session

Router Modem Message Description

——Session Termination——>
——-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.—
———TCP close————

Ratliff, et al. Standards Track [Page 75] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

B.6. Modem Terminates Session

Router Modem Message Description

                               |    Modem sends Session Termination

|←—Session Termination——–| Message with Status Data Item.

—Session Termination Resp.—> Response with Status 'Success'.
                               |
                               |    Router stops sending Messages.
                               |
———TCP close————

Ratliff, et al. Standards Track [Page 76] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

B.7. Session Heartbeats

Router Modem Message Description

———-Heartbeat————>
———[Any Message]———>
←——–Heartbeat————-
       ~ ~ ~ ~ ~ ~ ~
←——-[Any Message]———-

Ratliff, et al. Standards Track [Page 77] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

B.8. Router Detects a Heartbeat Timeout

Router Modem Message Description

       X<----------------------|    Router misses a heartbeat.
X←———————
——Session Termination——>

: : Termination proceeds…

B.9. Modem Detects a Heartbeat Timeout

Router Modem Message Description

———————→X Modem misses too many
                               |    heartbeats.
                               |
                               |
←—-Session Termination——-

Ratliff, et al. Standards Track [Page 78] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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——
         ~ ~ ~ ~ ~ ~ ~
                               |    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.

——Destination Down Resp.—>

Ratliff, et al. Standards Track [Page 79] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

C.2. Multicast Destination Notification

Router Modem Message Description

—–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——
         ~ ~ ~ ~ ~ ~ ~
                               |    Modem detects change in multicast
                               |    destination metrics and sends

|←——Destination Update——| Destination Update Message.

          ~ ~ ~ ~ ~ ~ ~

| Router detects multicast

——–Destination Down——→ and sends Destination Down
                               |    Message.
                               |
                               |    Modem receives Destination Down,
                               |    updates internal state, and sends
←—-Destination Down Resp.—-

Ratliff, et al. Standards Track [Page 80] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

C.3. Link Characteristics Request

Router Modem Message Description

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

Ratliff, et al. Standards Track [Page 81] RFC 8175 Dynamic Link Exchange Protocol (DLEP) June 2017

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

Ratliff, et al. Standards Track [Page 82]

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