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RFC 8175

Proposed STD
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Dynamic Link Exchange Protocol (DLEP)

Part 1 of 4, p. 1 to 18
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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.

[Page 2] 
Copyright Notice

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

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

Table of Contents

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

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

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

1.  Introduction

   There exist today a collection of modem devices that control links of
   variable data rate and quality.  Examples of these types of links
   include line-of-sight (LOS) terrestrial radios, satellite terminals,
   and broadband modems.  Fluctuations in speed and quality of these
   links can occur due to configuration, or on a moment-to-moment basis,
   due to physical phenomena like multipath interference, obstructions,
   rain fade, etc.  It is also quite possible that link quality and
   data rate vary with respect to individual destinations on a link and
   with the type of traffic being sent.  As an example, consider the
   case of an IEEE 802.11 access point serving two associated laptop
   computers.  In this environment, the answer to the question "What is
   the data rate on the 802.11 link?" is "It depends on which associated
   laptop we're talking about and on what kind of traffic is being
   sent."  While the first laptop, being physically close to the access

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   point, may have a data rate of 54 Mbps for unicast traffic, the other
   laptop, being relatively far away or obstructed by some object, can
   simultaneously have a data rate of only 32 Mbps for unicast.
   However, for multicast traffic sent from the access point, all
   traffic is sent at the base transmission rate (which is configurable
   but, depending on the model of the access point, is usually 24 Mbps
   or less).

   In addition to utilizing links that have variable data rates, mobile
   networks are challenged by the notion that link connectivity will
   come and go over time, without an effect on a router's interface
   state (Up or Down).  Effectively utilizing a relatively short-lived
   connection is problematic in IP routed networks, as IP routing
   protocols tend to rely on interface state and independent timers to
   maintain network convergence (e.g., HELLO messages and/or recognition
   of DEAD routing adjacencies).  These dynamic connections can be
   better utilized with an event-driven paradigm, where acquisition of a
   new neighbor (or loss of an existing one) is signaled, as opposed to
   a paradigm driven by timers and/or interface state.  DLEP not only
   implements such an event-driven paradigm but does so over a local
   (1-hop) TCP session, which guarantees delivery of the event messages.

   Another complicating factor for mobile networks are the different
   methods of physically connecting the modem devices to the router.
   Modems can be deployed as an interface card in a router's chassis, or
   as a standalone device connected to the router via Ethernet or serial
   link.  In the case of Ethernet attachment, with existing protocols
   and techniques, routing software cannot be aware of convergence
   events occurring on the radio link (e.g., acquisition or loss of a
   potential routing neighbor), nor can the router be aware of the
   actual capacity of the link.  This lack of awareness, along with the
   variability in data rate, leads to a situation where finding the
   (current) best route through the network to a given node is difficult
   to establish and properly maintain.  This is especially true of
   demand-based access schemes such as Demand Assigned Multiple Access
   (DAMA) implementations used on some satellite systems.  With a
   DAMA-based system, additional data rate may be available but will not
   be used unless the network devices emit traffic at a rate higher than
   the currently established rate.  Increasing the traffic rate does not
   guarantee that additional data rate will be allocated; rather, it may
   result in data loss and additional retransmissions on the link.

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   Addressing the challenges listed above, the Dynamic Link Exchange
   Protocol, or DLEP, has been developed.  DLEP runs between a router
   and its attached modem devices, allowing the modem devices to
   communicate (1) link characteristics as they change and
   (2) convergence events (acquisition and loss of potential routing
   next hops).  Figures 1 and 2 illustrate the scope of DLEP packets.

      |-------Local Node-------|          |-------Remote Node------|
      |                        |          |                        |
      +--------+       +-------+          +-------+       +--------+
      | Router |=======| Modem |{~~~~~~~~}| Modem |=======| Router |
      |        |       | Device|          | Device|       |        |
      +--------+       +-------+          +-------+       +--------+
               |       |       | Link     |       |       |
               |-DLEP--|       | Protocol |       |-DLEP--|
               |       |       | (e.g.,   |       |       |
               |       |       | 802.11)  |       |       |

                          Figure 1: DLEP Network

   In Figure 1, when the local modem detects the presence of a remote
   node, it (the local modem) sends a message to its router via DLEP.
   The message consists of an indication of what change has occurred on
   the link (e.g., the presence of a remote node detected), along with a
   collection of DLEP-defined Data Items that further describe the
   change.  Upon receipt of the message, the local router may take
   whatever action it deems appropriate, such as initiating discovery
   protocols and/or issuing HELLO messages to converge the network.  On
   a continuing, as-needed basis, the modem devices use DLEP to report
   any characteristics of the link (data rate, latency, etc.) that have
   changed.  DLEP is independent of the link type and topology supported
   by the modem.  Note that DLEP is specified to run only on the local
   link between router and modem.  Some over-the-air signaling may be
   necessary between the local and remote modem in order to provide some
   parameters in DLEP Messages between the local modem and local router,
   but DLEP does not specify how such over-the-air signaling is carried
   out.  Over-the-air signaling is purely a matter for the modem
   implementer.

   Figure 2 shows how DLEP can support a configuration where routers are
   connected with different link types.  In this example, Modem Device
   Type A implements a point-to-point link, and Modem Device Type B is
   connected via a shared medium.  In both cases, DLEP is used to report
   the characteristics of the link (data rate, latency, etc.) to
   routers.  The modem is also able to use the DLEP session to notify
   the router when the remote node is lost, shortening the time required
   to reconverge the network.

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                 +--------+                     +--------+
            +----+ Modem  |                     | Modem  +---+
            |    | Device |                     | Device |   |
            |    | Type A |  <===== // ======>  | Type A |   |
            |    +--------+ Point-to-Point Link +--------+   |
        +---+----+                                       +---+----+
        | Router |                                       | Router |
        |        |                                       |        |
        +---+----+                                       +---+----+
            |     +--------+                     +--------+  |
            +-----+ Modem  |                     | Modem  |  |
                  | Device |   o o o o o o o o   | Device +--+
                  | Type B |    o  Shared   o    | Type B |
                  +--------+     o Medium  o     +--------+
                                  o       o
                                   o     o
                                    o   o
                                      o
                                 +--------+
                                 | Modem  |
                                 | Device |
                                 | Type B |
                                 +---+----+
                                     |
                                     |
                                 +---+----+
                                 | Router |
                                 |        |
                                 +--------+

            Figure 2: DLEP Network with Multiple Modem Devices

2.  Protocol Overview

   DLEP defines a set of Messages used by modems and their attached
   routers to communicate events that occur on the physical link(s)
   managed by the modem: for example, a remote node entering or leaving
   the network, or that the link has changed.  Associated with these
   Messages are a set of Data Items -- information that describes the
   remote node (e.g., address information) and/or the characteristics of
   the link to the remote node.  Throughout this document, we refer to
   modems/routers participating in a DLEP session as "DLEP
   Participants", unless a specific distinction (e.g., modem or router)
   is required.

   DLEP uses a session-oriented paradigm between the modem device and
   its associated router.  If multiple modem devices are attached to a
   router (as in Figure 2) or the modem supports multiple connections

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   (via multiple logical or physical interfaces), then separate DLEP
   sessions exist for each modem or connection.  A router and modem form
   a session by completing the discovery and initialization process.
   This router-modem session persists unless or until it either
   (1) times out, based on the absence of DLEP traffic (including
   heartbeats) or (2) is explicitly torn down by one of the DLEP
   participants.

   While this document represents the best efforts of the working group
   to be functionally complete, it is recognized that extensions to DLEP
   will in all likelihood be necessary as more link types are used.
   Such extensions are defined as additional Messages, Data Items,
   and/or status codes, and associated rules of behavior, that are not
   defined in this document.  DLEP contains a standard mechanism for
   router and modem implementations to negotiate the available
   extensions to use on a per-session basis.

2.1.  Destinations

   The router-modem session provides a carrier for information exchange
   concerning "destinations" that are available via the modem device.
   Destinations can be identified by either the router or the modem and
   represent a specific, addressable location that can be reached via
   the link(s) managed by the modem.

   The DLEP Messages concerning destinations thus become the way for
   routers and modems to maintain, and notify each other about, an
   information base representing the physical and logical destinations
   accessible via the modem device, as well as the link characteristics
   to those destinations.

   A destination can be either physical or logical.  The example of a
   physical destination would be that of a remote, far-end router
   attached via the variable-quality network.  It should be noted that
   for physical destinations the Media Access Control (MAC) address is
   the address of the far-end router, not the modem.

   The example of a logical destination is Multicast.  Multicast traffic
   destined for the variable-quality network (the network accessed via
   the modem) is handled in IP networks by deriving a Layer 2 MAC
   address based on the Layer 3 address.  Leveraging on this scheme,
   multicast traffic is supported in DLEP simply by treating the derived
   MAC address as any other destination in the network.

   To support these logical destinations, one of the DLEP participants
   (typically, the router) informs the other as to the existence of the
   logical destination.  The modem, once it is aware of the existence of
   this logical destination, reports link characteristics just as it

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   would for any other destination in the network.  The specific
   algorithms a modem would use to derive metrics on logical
   destinations are outside the scope of this specification; these
   algorithms are left to specific implementations to decide.

   In all cases, when this specification uses the term "destination", it
   refers to the addressable locations, either logical or physical, that
   are accessible by the radio link(s).

2.2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Requirements

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

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

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

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

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4.  Implementation Scenarios

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

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

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

5.  Assumptions

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

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

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

6.  Metrics

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

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

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

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

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

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

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

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

7.  DLEP Session Flow

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

   o  Peer Discovery

   o  Session Initialization

   o  In-Session

   o  Session Termination

   o  Session Reset

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

   Modems MUST support the Peer Discovery state.

7.1.  Peer Discovery State

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

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

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

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

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

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

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

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

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

7.2.  Session Initialization State

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

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

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

7.3.  In-Session State

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

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

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

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

   The implementation MUST then transition to the Session Termination
   state.

7.3.1.  Heartbeats

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

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

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

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

7.4.  Session Termination State

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

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

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

7.5.  Session Reset State

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

   o  Release all resources allocated for the session.

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

   o  Terminate the TCP connection.

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

   o  For modems: Peer Discovery.

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

7.5.1.  Unexpected TCP Connection Termination

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

8.  Transaction Model

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

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

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

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

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

9.  Extensions

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

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

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

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9.1.  Experiments

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

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

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

10.  Scalability

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

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



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