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Versions: (draft-sratliff-manet-dlep) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 RFC 8175

Mobile Ad hoc Networks Working Group                          S. Ratliff
Internet-Draft                                                VT iDirect
Intended status: Standards Track                                B. Berry
Expires: September 9, 2016
                                                                 S. Jury
                                                           Cisco Systems
                                                          D. Satterwhite
                                                                Broadcom
                                                               R. Taylor
                                                  Airbus Defence & Space
                                                           March 8, 2016


                 Dynamic Link Exchange Protocol (DLEP)
                        draft-ietf-manet-dlep-20

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.  A bidirectional, event-
   driven communication channel between the router and the modem is
   necessary.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 9, 2016.

Copyright Notice





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   Copyright (c) 2016 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
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   7
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  DLEP Session Flow . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Peer Discovery State  . . . . . . . . . . . . . . . . . .  11
     4.2.  Session Initialization State  . . . . . . . . . . . . . .  12
     4.3.  In-Session State  . . . . . . . . . . . . . . . . . . . .  12
       4.3.1.  Heartbeats  . . . . . . . . . . . . . . . . . . . . .  13
     4.4.  Session Termination State . . . . . . . . . . . . . . . .  13
     4.5.  Session Reset state . . . . . . . . . . . . . . . . . . .  13
       4.5.1.  Unexpected TCP connection termination . . . . . . . .  14
   5.  Transaction Model . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Extensions  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Experiments . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  Scalability . . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  DLEP Signal and Message Structure . . . . . . . . . . . . . .  16
     8.1.  DLEP Signal Header  . . . . . . . . . . . . . . . . . . .  17
     8.2.  DLEP Message Header . . . . . . . . . . . . . . . . . . .  17
     8.3.  DLEP Generic Data Item  . . . . . . . . . . . . . . . . .  18
   9.  DLEP Signals and Messages . . . . . . . . . . . . . . . . . .  18
     9.1.  General Processing Rules  . . . . . . . . . . . . . . . .  20
     9.2.  Status code processing  . . . . . . . . . . . . . . . . .  20
     9.3.  Peer Discovery Signal . . . . . . . . . . . . . . . . . .  21
     9.4.  Peer Offer Signal . . . . . . . . . . . . . . . . . . . .  21
     9.5.  Session Initialization Message  . . . . . . . . . . . . .  22
     9.6.  Session Initialization Response Message . . . . . . . . .  23
     9.7.  Session Update Message  . . . . . . . . . . . . . . . . .  24
     9.8.  Session Update Response Message . . . . . . . . . . . . .  25
     9.9.  Session Termination Message . . . . . . . . . . . . . . .  26
     9.10. Session Termination Response Message  . . . . . . . . . .  26
     9.11. Destination Up Message  . . . . . . . . . . . . . . . . .  26



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     9.12. Destination Up Response Message . . . . . . . . . . . . .  27
     9.13. Destination Announce Message  . . . . . . . . . . . . . .  28
     9.14. Destination Announce Response Message . . . . . . . . . .  29
     9.15. Destination Down Message  . . . . . . . . . . . . . . . .  30
     9.16. Destination Down Response Message . . . . . . . . . . . .  30
     9.17. Destination Update Message  . . . . . . . . . . . . . . .  31
     9.18. Link Characteristics Request Message  . . . . . . . . . .  32
     9.19. Link Characteristics Response Message . . . . . . . . . .  33
     9.20. Heartbeat Message . . . . . . . . . . . . . . . . . . . .  34
   10. DLEP Data Items . . . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  Status . . . . . . . . . . . . . . . . . . . . . . . . .  35
     10.2.  IPv4 Connection Point  . . . . . . . . . . . . . . . . .  37
     10.3.  IPv6 Connection Point  . . . . . . . . . . . . . . . . .  38
     10.4.  Peer Type  . . . . . . . . . . . . . . . . . . . . . . .  39
     10.5.  Heartbeat Interval . . . . . . . . . . . . . . . . . . .  39
     10.6.  Extensions Supported . . . . . . . . . . . . . . . . . .  40
     10.7.  MAC Address  . . . . . . . . . . . . . . . . . . . . . .  41
     10.8.  IPv4 Address . . . . . . . . . . . . . . . . . . . . . .  41
     10.9.  IPv6 Address . . . . . . . . . . . . . . . . . . . . . .  42
     10.10. IPv4 Attached Subnet . . . . . . . . . . . . . . . . . .  43
     10.11. IPv6 Attached Subnet . . . . . . . . . . . . . . . . . .  44
     10.12. Maximum Data Rate (Receive)  . . . . . . . . . . . . . .  45
     10.13. Maximum Data Rate (Transmit) . . . . . . . . . . . . . .  46
     10.14. Current Data Rate (Receive)  . . . . . . . . . . . . . .  46
     10.15. Current Data Rate (Transmit) . . . . . . . . . . . . . .  47
     10.16. Latency  . . . . . . . . . . . . . . . . . . . . . . . .  48
     10.17. Resources  . . . . . . . . . . . . . . . . . . . . . . .  48
     10.18. Relative Link Quality (Receive)  . . . . . . . . . . . .  49
     10.19. Relative Link Quality (Transmit) . . . . . . . . . . . .  50
     10.20. Maximum Transmission Unit (MTU)  . . . . . . . . . . . .  50
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  51
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  52
     12.1.  Registrations  . . . . . . . . . . . . . . . . . . . . .  52
     12.2.  Signal Type Registration . . . . . . . . . . . . . . . .  53
     12.3.  Message Type Registration  . . . . . . . . . . . . . . .  53
     12.4.  DLEP Data Item Registrations . . . . . . . . . . . . . .  53
     12.5.  DLEP Status Code Registrations . . . . . . . . . . . . .  53
     12.6.  DLEP Extensions Registrations  . . . . . . . . . . . . .  53
     12.7.  DLEP Well-known Port . . . . . . . . . . . . . . . . . .  54
     12.8.  DLEP IPv4 Link-local Multicast Address . . . . . . . . .  54
     12.9.  DLEP IPv6 Link-local Multicast Address . . . . . . . . .  54
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  54
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  54
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  54
     14.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  Discovery Signal Flows . . . . . . . . . . . . . . .  55
   Appendix B.  Peer Level Message Flows . . . . . . . . . . . . . .  56
     B.1.  Session Initialization  . . . . . . . . . . . . . . . . .  56



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     B.2.  Session Initialization - Refused  . . . . . . . . . . . .  56
     B.3.  Router Changes IP Addresses . . . . . . . . . . . . . . .  57
     B.4.  Modem Changes Session-wide Metrics  . . . . . . . . . . .  57
     B.5.  Router Terminates Session . . . . . . . . . . . . . . . .  58
     B.6.  Modem Terminates Session  . . . . . . . . . . . . . . . .  58
     B.7.  Session Heartbeats  . . . . . . . . . . . . . . . . . . .  58
     B.8.  Router Detects a Heartbeat timeout  . . . . . . . . . . .  59
     B.9.  Modem Detects a Heartbeat timeout . . . . . . . . . . . .  59
   Appendix C.  Destination Specific Message Flows . . . . . . . . .  60
     C.1.  Common Destination Notification . . . . . . . . . . . . .  60
     C.2.  Multicast Destination Notification  . . . . . . . . . . .  61
     C.3.  Link Characteristics Request  . . . . . . . . . . . . . .  61
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  62

1.  Introduction

   There exist today a collection of modem devices that control links of
   variable datarate 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
   datarate 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 802.11 access point, serving two associated laptop
   computers.  In this environment, the answer to the question "What is
   the datarate on the 802.11 link?" is "It depends on which associated
   laptop we're talking about, and on what kind of traffic is being
   sent."  While the first laptop, being physically close to the access
   point, may have a datarate of 54Mbps for unicast traffic, the other
   laptop, being relatively far away, or obstructed by some object, can
   simultaneously have a datarate of only 32Mbps 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 24Mbps or less).

   In addition to utilizing variable datarate links, 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 routing protocols tend to rely
   on interface state and independent timers at OSI Layer 3 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



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   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 datarate, leads to a situation where finding the
   (current) best route through the network to a given destination 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 datarate 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 additional datarate will be allocated; rather, it may
   result in data loss and additional retransmissions on the link.

   Addressing the challenges listed above, the co-authors have developed
   the Dynamic Link Exchange Protocol, or DLEP.  The DLEP protocol runs
   between a router and its attached modem devices, allowing the modem
   to communicate link characteristics as they change, and convergence
   events (acquisition and loss of potential routing destinations).  The
   following diagrams are used to 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 the DLEP
   protocol.  The message consists of an indication of what change has
   occurred on the link (e.g., presence of a remote node detected),
   along with a collection of DLEP-defined data items that further



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   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 (datarate, latency,
   etc.) that have changed.  DLEP is independent of the link type and
   topology supported by the modem.  Note that the DLEP protocol 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 A
   implements a point-to-point link, and Modem B is connected via a
   shared medium.  In both cases, the DLEP protocol is used to report
   the characteristics of the link (datarate, 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 re-
   converge the network.

                 +--------+                     +--------+
            +----+ Modem  |                     | Modem  +---+
            |    | Device |                     | Device |
            |    | Type A |  <===== // ======>  | Type A |   |
            |    +--------+      P-2-P 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 |
                                 +---+----+
                                     |
                                     |



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                                 +---+----+
                                 | Router |
                                 |        |
                                 +--------+

            Figure 2: DLEP Network with Multiple Modem Devices

1.1.  Requirements

   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, RFC 2119 [RFC2119].

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 a modems/routers participating in a DLEP session as 'DLEP Peers',
   or 'DLEP Participants', unless a specific distinction (e.g. modem or
   router) is required.

   DLEP uses a session-oriented paradigm between the modem device and
   its associated router.  If multiple modem devices are attached to a
   router (as in Figure 2), or the modem supports multiple connections
   (via multiple logical or physical interfaces), then separate DLEP
   sessions exist for each modem or connection.  A router and modem form
   a session by completing the discovery and initialization process.
   This router-modem session persists unless or until it either (1)
   times out, based on the absence of DLEP traffic (including
   heartbeats), or (2) is explicitly torn down by one of the DLEP
   participants.

   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.








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   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.

   DLEP indentifies destinations by using the MAC address for delivering
   data traffic.  No manipulation or substitution is performed; the MAC
   address supplied in all destination Messages is used as the OSI Layer
   2 Destination MAC address.  DLEP therefore requires that MAC
   addresses are unique within the context of a router-modem session.

   The reliance on MAC addresses by DLEP forces the requirement that
   participating DLEP peers are on a single segment (either physical or
   logically, via tunneling protocols) at Layer 2.

   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 MAC address is the address of the far-
   end router, not the modem.

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

   To support these logical destinations, one of the DLEP participants
   (typically, the router) informs the other as to the existence of the
   logical destination.  The modem, once it is aware of the existence of
   this logical destination, reports link characteristics just as it
   would for any other destination in the network.  The specific
   algorithms a modem would use to derive metrics on logical
   destinations are outside the scope of this specification, and is left
   to specific implementations to decide.

   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 rules of behavior,
   Messages, Data Items and/or status codes 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.





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2.1.  Assumptions

   DLEP specifies UDP multicast for single-hop discovery signaling, and
   TCP for transport of the Messages.  Therefore, DLEP assumes that the
   modem and router 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.

   DLEP further assumes that security of the implementations (e.g.,
   authentication of stations, encryption of traffic, or both) is dealt
   with by utilizing Layer 2 security techniques.  This reliance on
   Layer 2 mechanisms secures all DLEP Messages - both the UDP discovery
   Signals and the TCP control Messages.

3.  Metrics

   DLEP includes the ability for the router and modem to communicate
   metrics that reflect the characteristics (e.g., datarate, 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.

   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 session
   level, the router MUST propagate the metrics to all entries in its
   information base for destinations that are accessed via the modem.

   DLEP modem implementations MUST 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 9.6).  In order to use a metric type that was not included
   in the Session Initialization Response Message, modem implementations
   MUST terminate the session with the router (via the Session Terminate
   Message (Section 9.9)), and establish a new session.








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   A DLEP modem MAY send metrics both in a session context (via the
   Session Update Message) and a specific destination context (via
   Destination Update) at any time.  The most recently received metric
   value MUST take 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 MUST be updated with the new metric.

   It is left to implementations to choose sensible default values based
   on their specific characteristics.  Modems having static (non-
   changing) link metric characteristics MAY 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 datarate
   or latency from the modem.  This Message is the Link Characteristics
   Request Message (Section 9.18), and gives the router the ability to
   deal with requisite increases (or decreases) of allocated datarate/
   latency in demand-based schemes in a more deterministic manner.

4.  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 (5) 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.




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4.1.  Peer Discovery State

   In the Peer Discovery state, routers that support DLEP discovery MUST
   send UDP packets containing a Peer Discovery Signal (Section 9.3) 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 router implementation then waits for a unicast UDP packet
   containing a Peer Offer Signal (Section 9.4) from a potential DLEP
   peer modem.  While in the Peer Discovery state, Peer Discovery
   Signals MUST be sent repeatedly by a DLEP router, at regular
   intervals.  The interval MUST be a minimum of one 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, as the Transaction Model only describes Messages on a TCP
   session.

   Routers MUST use one or more of the modem address/port combinations
   from the Peer Offer Signal or from a priori configuration to
   establish a new TCP connection to the modem.  If more than one modem
   address/port combinations is available, router 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 IPv4
   Connection Point Data Items, nor IPv6 Connection Point Data Items are
   included in the Peer Offer Signal, the router MUST use the source
   address of the UDP packet containing the 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 link-local
   multicast address and port.  On receipt of a valid Peer Discovery
   Signal, it MUST unicast a Peer Offer Signal to the source address and
   port of the received UDP packet.

   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.

   Upon establishment of a TCP connection, 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 transitioned to the Session Initialization state.
   Modem implementations MUST silently ignore Peer Discovery Signals
   from a router with which it already has a TCP connection.




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4.2.  Session Initialization State

   On entering the Session Initialization state, the router MUST send a
   Session Initialization Message (Section 9.5) to the modem.  The
   router MUST then wait for receipt of a Session Initialization
   Response Message (Section 9.6) from the modem.  Receipt of the
   Session Initialization Response Message containing a Status Data Item
   (Section 10.1) with status code set to 0 'Success', see Table 4,
   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 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 6.  Extensions supported by
   an implementation MUST be declared to potential DLEP peers using the
   Extensions Supported Data Item (Section 10.6).  Once both DLEP peers
   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.

4.3.  In-Session State

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

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

   o  The implementation terminates the session by sending a Session
      Termination Message (Section 9.9)), or,

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

   The implementation MUST then transition to the Session Termination
   state.



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4.3.1.  Heartbeats

   In order to maintain the In-Session state, periodic Heartbeat
   Messages (Section 9.20) 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 peers.

   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).

   Implementations SHOULD allow two (2) heartbeat intervals to expire
   with no Messages from the peer before terminating the session by
   issuing a Session Termination Message containing a Status Data Item
   (Section 10.1) with status code set to 5 'Timed Out', see Table 4,
   and then transition to the Session Termination state.

4.4.  Session Termination State

   When an implementation enters the Session Termination state after
   sending a Session Termination Message (Section 9.9) as the result of
   an invalid Message or error, it MUST wait for a Session Termination
   Response Message (Section 9.10) from its peer.  Senders SHOULD allow
   four (4) heartbeat intervals to expire before assuming that the peer
   is unresponsive, and continuing with session termination.  Any other
   Message received while waiting MUST be silently ignored.

   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 enters the Session Termination state having
   received a Session Termination Message from its peer, it MUST
   immediately send a Session Termination Response and transition to the
   Session Reset state.

4.5.  Session Reset state

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

   o  Release all resources allocated for the session.






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   o  Eliminate all destinations in the information base represented by
      the session.  Destination Down Messages (Section 9.15) MUST NOT be
      sent.

   o  Terminate the TCP connection.

   Having completed these actions the implementation SHOULD return to
   the relevant initial state: Peer Discovery for modems; either Peer
   Discovery or Session Initialization for routers, depending on
   configuration.

4.5.1.  Unexpected TCP connection termination

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

5.  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 9.9) containing a
   Status Data Item (Section 10.1) with status code set to 2 'Unexpected
   Message', see Table 4, and transition to the Session Termination
   state.  There is no restriction to 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 9.5), may be in progress at a time per session.  As
   above, 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 2 '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



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   Messages (Section 9.20) MUST NOT be considered part of a session
   transaction.

   DLEP transactions do not time out and are not cancellable.  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 4.3.

6.  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 extension in order to
   be considered DLEP compliant.  An extension document, describing the
   operation of a credit windowing scheme for flow control, is described
   in [CREDIT].

   If interoperable protocol extensions are required, they MUST be
   standardized either as an update to this document, or as an
   additional stand-alone specification.  The requests for IANA-
   controlled registries in this document contain sufficient Reserved
   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 MUST consider the
   interaction of their extension with other published extensions, and
   specify any incompatibilities.

6.1.  Experiments

   This document requests Private Use 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.

   Use of the Private Use Signals, Messages, Data Items, status codes,
   or behaviors MUST be announced as DLEP Extensions, during session
   initialization, using extension identifiers from the Private Use
   space in the Extensions Supported registry (Table 5), with a value
   agreed upon (a priori) between the participating peers.  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, use of multiple experiments in a single session
   could lead to interoperability issues or unexpected results (e.g.,



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   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 to establish a
   precedence of the conflicting definitions) if such conflicts arise.

7.  Scalability

   The protocol is intended to support thousands of destinations on a
   given modem/router pair.  At large scale, implementations SHOULD
   consider employing techniques to prevent flooding a peer with a large
   number of Messages in a short time.  It is recommended that
   implementations consider a dampening algorithm to prevent a flapping
   device from generating a large number of Destination Up/Destination
   Down Messages, for example.  Implementations SHOULD also consider
   techniques such as a hysteresis to lessen the impact of rapid, minor
   fluctuations in link quality.  The specific algorithms to be used for
   handling flapping destinations and minor changes in link quality are
   outside the scope of this specification.

8.  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 bi-directionally
   over a TCP connection between two peers, in the Session
   Initialization, In-Session and Session Termination states.

   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 multiplicity 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.









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8.1.  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 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 associated with 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.

8.2.  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 associated with this
      Message.  This length MUST NOT include the length of the Message
      Header itself.




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

8.3.  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, which contains data specific to a
      particular Data Item.

9.  DLEP Signals and Messages

   As mentioned above, all DLEP Signals begin with the DLEP Signal
   Header, and all DLEP Messages begin with the DLEP Message Header.
   Therefore, in the following descriptions of specific Signals and
   Messages, this Header is assumed, and will not be replicated.

   Following is the set of core Signals and Messages that MUST be
   recognized by a DLEP compliant implementation.  As mentioned before,
   not all Messages may be used during a session, but an implementation
   MUST correctly process these Messages when received.

   The core DLEP Signals are:

        +--------------+-----------------------------------------+
        | Type Code    | Description                             |
        +--------------+-----------------------------------------+
        | 0            | Reserved                                |
        | 1            | Peer Discovery Signal (Section 9.3)     |
        | 2            | Peer Offer Signal (Section 9.4)         |



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        | 3-65519      | Reserved for future extensions          |
        | 65520-65534  | Private Use. Available for experiments  |
        | 65535        | Reserved                                |
        +--------------+-----------------------------------------+

                        Table 1: DLEP Signal types

   The core DLEP Messages are:

   +-------------------+-----------------------------------------------+
   | Type Code         | Description                                   |
   +-------------------+-----------------------------------------------+
   | 0                 | Reserved                                      |
   | 1                 | Session Initialization Message (Section 9.5)  |
   | 2                 | Session Initialization Response Message       |
   |                   | (Section 9.6)                                 |
   | 3                 | Session Update Message (Section 9.7)          |
   | 4                 | Session Update Response Message (Section 9.8) |
   | 5                 | Session Termination Message (Section 9.9)     |
   | 6                 | Session Termination Response Message (Section |
   |                   | 9.10)                                         |
   | 7                 | Destination Up Message (Section 9.11)         |
   | 8                 | Destination Up Response Message (Section      |
   |                   | 9.12)                                         |
   | 9                 | Destination Announce Message (Section 9.13)   |
   | 10                | Destination Announce Response Message         |
   |                   | (Section 9.14)                                |
   | 11                | Destination Down Message (Section 9.15)       |
   | 12                | Destination Down Response Message (Section    |
   |                   | 9.16)                                         |
   | 13                | Destination Update Message (Section 9.17)     |
   | 14                | Link Characteristics Request Message (Section |
   |                   | 9.18)                                         |
   | 15                | Link Characteristics Response Message         |
   |                   | (Section 9.19)                                |
   | 16                | Heartbeat Message (Section 9.20)              |
   | 17-65519          | Reserved for future extensions                |
   | 65520-65534       | Private Use. Available for experiments        |
   | 65535             | Reserved                                      |
   +-------------------+-----------------------------------------------+

                        Table 2: DLEP Message types









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9.1.  General Processing Rules

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

   If an unrecognized Message is received, the receiving DLEP peer MUST
   issue a Session Termination Message (Section 9.9) containing a Status
   Data Item (Section 10.1) with status code set to 1 'Unknown Message',
   see Table 4, and transition to the Session Termination state.

   If an unexpected Message is received, the receiving DLEP peer MUST
   issue a Session Termination Message containing a Status Data Item
   with status code set to 2 'Unexpected Message', and transition to the
   Session Termination state.

   If a received Message contains unrecognized, invalid, or disallowed
   duplicate Data Items, the receiving DLEP peer MUST issue a Session
   Termination Message containing a Status Data Item with status code
   set to 3 'Invalid Data', and transition to the Session Termination
   state.

   Prior to the exchange of Destination Up (Section 9.11) and
   Destination Up Response (Section 9.12) Messages, or Destination
   Announce (Section 9.13) and Destination Announce Response
   (Section 9.14) Messages, no Messages concerning a destination may be
   sent.  A peer 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 4 'Invalid Destination', and transition to the Session
   Termination state.

   After exchanging Destination Down (Section 9.15) and Destination Down
   Response (Section 9.16) Messages, no Messages concerning a
   destination may be a sent until a new Destination Up or Destination
   Announce Message is sent.  A peer receiving a Message about a
   destination previously announced as 'down' MUST terminate the session
   by issuing a Session Termination Message with a Status Data Item with
   status code set to 4 'Invalid Destination', and transition to the
   Session Termination state.

9.2.  Status code processing









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   The behaviour of a DLEP participant receiving a Message containing a
   Status Data Item (Section 10.1) is defined by the failure mode
   associated with the value of the status code field, see Table 4.
   Except for the reserved value of 255, all status code values greater
   than or equal to 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 Session
   Termination Message (Section 9.9) containing a Status Data Item with
   a status code value with failure mode 'Terminate' MUST immediately
   issue a Session Termination Message containing an identical 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.

9.3.  Peer Discovery Signal

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

   To construct a Peer Discovery Signal, the Signal Type value in the
   Signal Header is set to 1, from Table 1.

   The Peer Discovery Signal MAY contain a Peer Type Data Item
   (Section 10.4).

   Implementations MUST implement their own retransmit heuristics in
   cases where it is determined the Peer Discovery Signal has timed out.

9.4.  Peer Offer Signal

   A Peer Offer Signal MUST be sent by a DLEP modem to the unicast
   address of the originator of a valid Peer Discovery Signal
   (Section 9.3).  The Peer Offer Signal is completes the discovery
   process.

   To construct a Peer Offer Signal, the Signal Type value in the Signal
   Header is set to 2, from Table 1.

   The Peer Offer Signal MAY contain a Peer Type Data Item
   (Section 10.4).

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

   o  IPv4 Connection Point (Section 10.2)



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   o  IPv6 Connection Point (Section 10.3)

   The IP Connection Point Data Items indicate the unicast address the
   router MUST use when connecting the DLEP TCP session.  If multiple IP
   Connection Point Data Items are present in the Peer Offer Signal,
   router implementations MAY use their own heuristics to select the
   address to connect to.  If no IP Connection Point Data Items are
   included in the Peer Offer Signal, the router MUST use the source
   address of the Signal as the IP address, and the DLEP well-known port
   number (Section 12.7) to establish the TCP connection.

9.5.  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, from Table 2.

   The Session Initialization Message MUST contain a Heartbeat Interval
   Data Item (Section 10.5).

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

   o  Peer Type (Section 10.4)

   o  Extensions Supported (Section 10.6)

   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 fully established until receipt of Session
   Initialization Response Message (Section 9.6), and therefore
   implementations must use their own timeout and retry heuristics for
   this Message.

   As an exception to the general rule governing an implementation
   receiving an unrecognized Data Item in a Message, see Section 9.1, if
   a Session Initialization Message contains one or more Extension
   Supported Data Items announcing support for extensions that the
   implementation does not recognize, then the implementation MAY ignore
   Data Items it does not recognize.



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9.6.  Session Initialization Response Message

   A Session Initialization Response Message MUST be sent in response to
   a received Session Initialization Message (Section 9.5).

   To construct a Session Initialization Response Message, the Message
   Type value in the Message Header is set to 2, from Table 2.

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

   o  Status (Section 10.1)

   o  Heartbeat Interval (Section 10.5)

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)

   o  Maximum Transmission Unit (MTU) (Section 10.20)

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

   o  Peer Type (Section 10.4)

   o  Extensions Supported (Section 10.6)








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   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
   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 peers.

9.7.  Session Update Message

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

   To construct a Session Update Message, the Message Type value in the
   Message Header is set to 3, from Table 2.

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

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)





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   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)

   o  Maximum Transmission Unit (MTU) (Section 10.20)

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

   o  IPv4 Address (Section 10.8)

   o  IPv6 Address (Section 10.9)

   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.

   It should be noted that Session Update Messages can be sent by both
   routers and modems.  For example, addition of an IPv4 address to 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: If the modem is capable of
   understanding and forwarding this information (via proprietary
   mechanisms), the address update would prompt any remote DLEP modems
   (DLEP-enabled modems in a remote node) to issue a Destination Update
   Message (Section 9.17) to their local routers with the new (or
   deleted) addresses.  Modems that do not track Layer 3 addresses
   SHOULD silently ignore Address Data Items.

9.8.  Session Update Response Message

   A Session Update Response Message MUST be sent by by a DLEP
   participant when a Session Update Message (Section 9.7) is received.

   To construct a Session Update Response Message, the Message Type
   value in the Message Header is set to 4, from Table 2.

   The Session Update Response Message MUST contain a Status Data Item
   (Section 10.1).



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9.9.  Session Termination Message

   A Session Termination Message MUST be sent by a DLEP participant when
   the DLEP session needs to be terminated.

   To construct a Session Termination Message, the Message Type value in
   the Message Header is set to 5, from Table 2.

   The Session Termination Message MUST contain Status Data Item
   (Section 10.1).

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

9.10.  Session Termination Response Message

   A Session Termination Response Message MUST be sent by a DLEP
   participant when a Session Termination Message (Section 9.9) is
   recevied.

   To construct a Session Termination Response Message, the Message Type
   value in the Message Header is set to 6, from Table 2.

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

   Receipt of a Session Termination Response Message completes the tear-
   down of the DLEP session.

9.11.  Destination Up Message

   Destination Up Messages are 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, from Table 2.

   The Destination Up Message MUST contain a MAC Address Data Item
   (Section 10.7).

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

   o  IPv4 Address (Section 10.8)

   o  IPv6 Address (Section 10.9)





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

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)

   o  Maximum Transmission Unit (MTU) (Section 10.20)

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

   o  IPv4 Attached Subnet (Section 10.10)

   o  IPv6 Attached Subnet (Section 10.11)

   A router receiving a Destination Up Message allocates the necessary
   resources, creating an entry in the information base with the
   specifics (i.e. 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 9.15) is received, indicating that the modem has lost
   contact with the remote node, or the implementation transitions to
   the Session Termination state.

9.12.  Destination Up Response Message

   A router MUST send a Destination Up Response Message when a
   Destination Up Message (Section 9.11) is received.

   To construct a Destination Up Response Message, the Message Type
   value in the Message Header is set to 8, from Table 2.




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

   o  MAC Address (Section 10.7)

   o  Status (Section 10.1)

   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 4.

   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 100 'Not Interested'.

   A modem receiving a Destination Up Response Message containing a
   Status Data Item with status code of any value other than 0 'Success'
   MUST NOT send further Destination messages about the destination,
   e.g. Destination Down (Section 9.15) or Destination Update
   (Section 9.17) with the same MAC address.

9.13.  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 9.11) 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 used by a router to
   request information concerning a destination in which it has
   previously declined interest, via the 100 'Not Interested' status
   code in a Destination Up Response Message (Section 9.12), see Table
   4, or declared as 'down', via the Destination Down Message
   (Section 9.15).

   To construct a Destination Announce Message, the Message Type value
   in the Message Header is set to 9, from Table 2.

   The Destination Announce Message MUST contain a MAC Address Data Item
   (Section 10.7).

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




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   o  IPv4 Address (Section 10.8)

   o  IPv6 Address (Section 10.9)

   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.

9.14.  Destination Announce Response Message

   A modem MUST send a Destination Announce Response Message when a
   Destination Announce Message (Section 9.13) is received.

   To construct a Destination Announce Response Message, the Message
   Type value in the Message Header is set to 10, from Table 2.

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

   o  MAC Address (Section 10.7)

   o  Status (Section 10.1)

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

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)




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   o  Maximum Transmission Unit (MTU) (Section 10.20)

   If a modem is unable to report information immediately about the
   requested information, if the destination is not currently reachable,
   for example, the status code in the Status Data Item MUST be set to
   101 'Request Denied', see Table 4.

   After sending a Destination Announce Response Message containing a
   Status Data Item with 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.

9.15.  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.

   To construct a Destination Down Message, the Message Type value in
   the Message Header is set to 11, from Table 2.

   The Destination Down Message MUST contain a MAC Address Data Item
   (Section 10.7).

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

9.16.  Destination Down Response Message

   A Destination Down Response MUST be sent by the recipient of a
   Destination Down Message (Section 9.15) 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, from Table 2.

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




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   o  MAC Address (Section 10.7)

   o  Status (Section 10.1)

9.17.  Destination Update Message

   A modem SHOULD send the 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:

   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, from Table 2.

   The Destination Update Message MUST contain a MAC Address Data Item
   (Section 10.7).

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

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)

   o  Maximum Transmission Unit (MTU) (Section 10.20)

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




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   o  IPv4 Address (Section 10.8)

   o  IPv6 Address (Section 10.9)

   o  IPv4 Attached Subnet (Section 10.10)

   o  IPv6 Attached Subnet (Section 10.11)

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

9.18.  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, from Table 2.

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

   o  MAC Address (Section 10.7)

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

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   The Link Characteristics Request Message MAY contain either a Current
   Data Rate (CDRR or CDRT) Data Item to request a different datarate
   than what 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.









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9.19.  Link Characteristics Response Message

   A modem MUST send a Link Characteristics Response Message when a Link
   Characteristics Request Message (Section 9.18) is received.

   To construct a Link Characteristics Response Message, the Message
   Type value in the Message Header is set to 15, from Table 2.

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

   o  MAC Address (Section 10.7)

   o  Status (Section 10.1)

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

   o  Maximum Data Rate (Receive) (Section 10.12)

   o  Maximum Data Rate (Transmit) (Section 10.13)

   o  Current Data Rate (Receive) (Section 10.14)

   o  Current Data Rate (Transmit) (Section 10.15)

   o  Latency (Section 10.16)

   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 10.17)

   o  Relative Link Quality (Receive) (Section 10.18)

   o  Relative Link Quality (Transmit) (Section 10.19)

   o  Maximum Transmission Unit (MTU) (Section 10.20)

   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 9.6).  The values in the
   metric Data Items in the Link Characteristics Response Message MUST
   reflect the link characteristics after the request has been
   processed.






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   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 101 'Request Denied', see Table 4.

9.20.  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 10.5) of the Session Initialization Message (Section 9.5) or
   Session Initialization Response Message (Section 9.6).

   To construct a Heartbeat Message, the Message Type value in the
   Message Header is set to 16, from Table 2.

   There are no valid Data Items for the Heartbeat Message.

   The Message is used by DLEP peers to detect when a DLEP session peer
   (either the modem or the router) is no longer communicating.  DLEP
   participants SHOULD allow two (2) heartbeat intervals to expire with
   no Messages from the DLEP peer before initiating DLEP session
   termination procedures.

10.  DLEP Data Items

   Following is the list of core Data Items that MUST be recognized by a
   DLEP compliant implementation.  As mentioned before, not all Data
   Items need be used during a session, but an implementation MUST
   correctly process these Data Items when correctly associated with a
   Signal or Message.

   The core DLEP Data Items are:

   +--------------------+----------------------------------------------+
   | Type Code          | Description                                  |
   +--------------------+----------------------------------------------+
   | 0                  | Reserved                                     |
   | 1                  | Status (Section 10.1)                        |
   | 2                  | IPv4 Connection Point (Section 10.2)         |
   | 3                  | IPv6 Connection Point (Section 10.3)         |
   | 4                  | Peer Type (Section 10.4)                     |
   | 5                  | Heartbeat Interval (Section 10.5)            |
   | 6                  | Extensions Supported (Section 10.6)          |
   | 7                  | MAC Address (Section 10.7)                   |
   | 8                  | IPv4 Address (Section 10.8)                  |
   | 9                  | IPv6 Address (Section 10.9)                  |
   | 10                 | IPv4 Attached Subnet (Section 10.10)         |
   | 11                 | IPv6 Attached Subnet (Section 10.11)         |
   | 12                 | Maximum Data Rate (Receive) MDRR) (Section   |



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   |                    | 10.12)                                       |
   | 13                 | Maximum Data Rate (Transmit) (MDRT) (Section |
   |                    | 10.13)                                       |
   | 14                 | Current Data Rate (Receive) (CDRR) (Section  |
   |                    | 10.14)                                       |
   | 15                 | Current Data Rate (Transmit) (CDRT) (Section |
   |                    | 10.15)                                       |
   | 16                 | Latency (Section 10.16)                      |
   | 17                 | Resources (RES) (Section 10.17)              |
   | 18                 | Relative Link Quality (Receive) (RLQR)       |
   |                    | (Section 10.18)                              |
   | 19                 | Relative Link Quality (Transmit) (RLQT)      |
   |                    | (Section 10.19)                              |
   | 20                 | Maximum Transmission Unit (MTU) (Section     |
   |                    | 10.20)                                       |
   | 21-65407           | Reserved for future extensions               |
   | 65408-65534        | Private Use. Available for experiments       |
   | 65535              | Reserved                                     |
   +--------------------+----------------------------------------------+

                       Table 3: DLEP Data Item types

10.1.  Status

   For the Session Termination Message (Section 9.9), 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, i.e., 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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Code          | Text...                                       :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  1

   Length:  1 + Length of text, in octets



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   Status Code:  One of the codes defined in Table 4 below.

   Text:  UTF-8 encoded string of UNICODE [UNIV8] characters, describing
      the cause, used for implementation defined purposes.  Since this
      field is used for description, implementations SHOULD limit
      characters in this field to printable characters.  Implementations
      receiving this Data Item SHOULD check for printable characters in
      the field.

   An implementation MUST NOT assume the Text field is NUL-terminated.

   +---------------+---------+-----------+-----------------------------+
   | Status Code   | Value   | Failure   | Reason                      |
   |               |         | Mode      |                             |
   +---------------+---------+-----------+-----------------------------+
   | Success       | 0       | Success   | The Message was processed   |
   |               |         |           | successfully.               |
   | Unknown       | 1       | Terminate | The Message was not         |
   | Message       |         |           | recognized by the           |
   |               |         |           | implementation.             |
   | Unexpected    | 2       | Terminate | The Message was not         |
   | Message       |         |           | expected while the device   |
   |               |         |           | was in the current state,   |
   |               |         |           | e.g., a Session             |
   |               |         |           | Initialization Message      |
   |               |         |           | (Section 9.5) in the In-    |
   |               |         |           | Session state.              |
   | Invalid Data  | 3       | Terminate | One or more Data Items in   |
   |               |         |           | the Message are invalid,    |
   |               |         |           | unexpected or incorrectly   |
   |               |         |           | duplicated.                 |
   | Invalid       | 4       | Terminate | The destination included in |
   | Destination   |         |           | the Message does not match  |
   |               |         |           | a previously announced      |
   |               |         |           | destination. For example,   |
   |               |         |           | in the Link Characteristic  |
   |               |         |           | Response Message (Section   |
   |               |         |           | 9.19).                      |
   | Timed Out     | 5       | Terminate | The session has timed out.  |
   | <Reserved>    | 6-90    | Terminate | Reserved for future         |
   |               |         |           | extensions.                 |
   | <Private Use> | 91-99   | Terminate | Available for experiments.  |
   | Not           | 100     | Continue  | The receiver is not         |
   | Interested    |         |           | interested in this Message  |
   |               |         |           | subject, e.g. in a          |
   |               |         |           | Destination Up Response     |
   |               |         |           | Message (Section 9.12) to   |
   |               |         |           | indicate no further         |



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   |               |         |           | Messages about the          |
   |               |         |           | destination.                |
   | Request       | 101     | Continue  | The receiver refuses to     |
   | Denied        |         |           | complete the request.       |
   | <Reserved>    | 102-243 | Continue  | Reserved for future         |
   |               |         |           | extensions.                 |
   | <Private Use> | 244-254 | Continue  | Available for experiments.  |
   | <Reserved>    | 255     | Terminate | Reserved.                   |
   +---------------+---------+-----------+-----------------------------+

                        Table 4: DLEP Status Codes

10.2.  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 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 12.7) to establish the TCP connection.

   The Flags field is defined as:




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



   Reserved:  MUST be zero.  Reserved for future use.

10.3.  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 included)

   Flags:  Flags field, defined below.

   IPv6 Address:  The IPv6 address listening on the modem.

   TCP Port Number:  TCP Port number on the modem.

   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 12.7) to establish the TCP connection.



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   The Flags field is defined as:

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



   Reserved:  MUST be zero.  Reserved for future use.

10.4.  Peer Type

   The Peer Type Data Item is used by the router and modem to give
   additional information as to its type.  The peer type is a string and
   is envisioned to be used for informational purposes (e.g., as output
   in a display command).

   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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Peer Type...                                                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  4

   Length:  Length of peer type string, in octets.

   Peer Type:  UTF-8 encoded string of UNICODE [UNIV8] 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, and receiving
      implementations SHOULD check the data for printable characters.

   An implementation MUST NOT assume the Peer Type field is NUL-
   terminated.

10.5.  Heartbeat Interval

   The Heartbeat Interval Data Item is used to specify a period in
   milliseconds for Heartbeat Messages (Section 9.20).

   The Heartbeat Interval Data Item contains the following fields:



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    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.

10.6.  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 DLEP Extensions repository.
   Each Extension Type definition includes which additional Signals and
   Data Items are supported.

   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.

   Extension List:  A list of extensions supported, identified by their
      2-octet value as listed in the extensions registry.









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10.7.  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,
   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 a multicast MAC address,
   or the broadcast MAC (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                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                MAC Address    :     (if EUI-64 used)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  7

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

   MAC Address:  MAC Address of the destination.

10.8.  IPv4 Address

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



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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.

10.9.  IPv6 Address

   When included in Destination Messages, this Data Item contains the
   IPv6 address of the destination.  When included in the Session Update
   Message, this Data Item contains the IPv6 address of the peer.  In
   either case, the Data Item also contains an indication of whether
   this is a new or existing address, or is 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                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   : IPv6 Address  |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  9

   Length:  17

   Flags:  Flags field, defined below.

   IPv6 Address:  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.

10.10.  IPv4 Attached Subnet

   The DLEP IPv4 Attached Subnet 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




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   Length:  6

   Flags:  Flags field, defined below.

   IPv4 Subnet:  The IPv4 subnet reachable at the destination.

   Prefix Length:  Length of the prefix (1-32) 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.

10.11.  IPv6 Attached Subnet

   The DLEP IPv6 Attached Subnet 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 Len.   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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 (1-128) for the IPv6 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.

10.12.  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, 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





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

10.13.  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
      (bps), that can be achieved while transmitting on the link.

10.14.  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 9.18),
   Current Data Rate (Receive) represents the desired receive rate, in
   bits per second, on the link.

   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                        |



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        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 and maximum receive data
   rates, current data rate receive MUST be set equal to the maximum
   data rate receive.

10.15.  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 9.18),
   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)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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.



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   If there is no distinction between current and maximum transmit data
   rates, current data rate transmit MUST be set equal to the maximum
   data rate transmit.

10.16.  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.

10.17.  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 might be battery life, but could equally
   be magic beans.  The list of resources that might be considered is
   beyond the scope of this document, and is left to implementations to
   decide.

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



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   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 SHOULD NOT be
   issued.

10.18.  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 as a percentage, 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



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   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 the Relative Link Quality (Receive),
   this Data Item SHOULD NOT be issued.

10.19.  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 as a percentage, 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.

   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 the Relative Link Quality (Transmit),
   this Data Item SHOULD NOT be issued.

10.20.  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



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   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.

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

11.  Security Considerations

   The potential security concerns when using DLEP are:

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

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

   Since DLEP is restricted to operation over a single (possibly
   logical) hop at layer 2, implementations requiring authentication and
   /or encryption of traffic MUST take steps to 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].

   To avoid potential denial of service attack, it is RECOMMENDED that
   implementations using the Peer Discovery mechanism maintain an
   information base of hosts that persistently fail Session
   Initialization having provided an acceptable Peer Discovery Signal,
   and ignore subsequent Peer Discovery Signals from such hosts.



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   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.

12.  IANA Considerations

   This section specifies requests to IANA.

12.1.  Registrations

   This specification defines:

   o  A new repository for DLEP Signals, with three (3) values currently
      assigned.

   o  Reservation of a Private Use numbering space within the above
      repository for experimental DLEP Signals.

   o  A new repository for DLEP Messages, with seventeen (17) values
      currently assigned.

   o  Reservation of a Private Use numbering space within the above
      repository for experimental DLEP Messages.

   o  A new repository for DLEP Data Items, with twenty one (21) values
      currently assigned.

   o  Reservation of a Private Use numbering space within the Data Items
      repository for experimental Data Items.

   o  A new repository for DLEP status codes, with eight (8) currently
      assigned.

   o  Reservation of a Private Use numbering space within the status
      codes repository for experimental status codes.

   o  A new repository for DLEP extensions, with one (1) value currently
      assigned.

   o  Reservation of a Private Use numbering space within the extension
      repository for experimental extensions.

   o  A request for allocation of a well-known port for DLEP TCP and UDP
      communication.

   o  A request for allocation of a link-local multicast IPv4 address
      for DLEP discovery.




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   o  A request for allocation of a link-local multicast IPv6 address
      for DLEP discovery.

12.2.  Signal Type Registration

   A new repository must be created with the values of the DLEP Signals,
   entitled "Signal Type Values for the Dynamic Link Event Protocol
   (DLEP)".  The repository is to be managed using the "Specification
   Required" policy documented in [RFC5226].

   All Signal values are in the range [0..65535], defined in Table 1.

12.3.  Message Type Registration

   A new repository must be created with the values of the DLEP
   Messages, entitled "Message Type Values for the Dynamic Link Event
   Protocol (DLEP)".  The repository is to be managed using the
   "Specification Required" policy documented in [RFC5226].

   All Message values are in the range [0..65535], defined in Table 2.

12.4.  DLEP Data Item Registrations

   A new repository for DLEP Data Items must be created, entitled "Data
   Item Type Values for the Dynamic Link Event Protocol (DLEP)".  The
   repository is to be managed using the "Specification Required" policy
   documented in [RFC5226].

   All Data Item values are in the range [0..65535], defined in Table 3.

12.5.  DLEP Status Code Registrations

   A new repository for DLEP status codes must be created, entitled
   "Status Code Values for the Dynamic Link Event Protocol (DLEP)".  The
   repository is to be managed using the "Specification Required" policy
   documented in [RFC5226].

   All status codes are in the range [0..255] , defined in Table 4.

   With the exception of the reserved value 255, all status codes with
   values >= 100 are marked as 'Continue' codes, others 'Terminate'.

12.6.  DLEP Extensions Registrations

   A new repository for DLEP extensions must be created, entitled
   "Extension Type Values for the Dynamic Link Event Protocol (DLEP)".
   The repository is to be managed using the "Specification Required"
   policy documented in [RFC5226].



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   All extension values are in the range [0..65535].  Current
   allocations are:

      +--------------+----------------------------------------------+
      | Code         | Description                                  |
      +--------------+----------------------------------------------+
      | 0            | Reserved                                     |
      | 1            | Credit Windowing                             |
      | 2-65519      | Unassigned. Available for future extensions  |
      | 65520-65534  | Private Use. Available for experiments       |
      | 65535        | Reserved                                     |
      +--------------+----------------------------------------------+

                       Table 5: DLEP Extension types

12.7.  DLEP Well-known Port

   It is requested that IANA allocate a single well-known port number
   for both TCP and UDP, for DLEP communication.  SCTP port allocation
   is not required.

12.8.  DLEP IPv4 Link-local Multicast Address

   It is requested that IANA allocate an IPv4 link-local multicast
   address for DLEP discovery Signals.

12.9.  DLEP IPv6 Link-local Multicast Address

   It is requested that IANA allocate an IPv6 link-local multicast
   address for DLEP discovery Signals.

13.  Acknowledgements

   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 Mercieca.

14.  References

14.1.  Normative References

   [CREDIT]   Ratliff, S., "Credit Windowing extension for DLEP", IETF
              draft draft-ietf-manet-credit-window-02, March 2016.



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   [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>.

   [UNIV8]    , "The Unicode Consortium. The Unicode Standard, Version
              8.0.0, (Mountain View, CA: The Unicode Consortium, 2015.
              ISBN 978-1-936213-10-8)",
              http://www.unicode.org/versions/Unicode8.0.0/, June 2015.

14.2.  Informative References

   [IEEE-802.1AE]
              , "IEEE Standards for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Security", DOI
              10.1109/IEEESTD.2006.245590, August 2006.

   [IEEE-802.1X]
              , "IEEE Standards for Local and Metropolitan Area
              Networks: Port based Network Access Control", DOI 10.1109/
              IEEESTD.2010.5409813, February 2010.

   [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>.

   [RFC5578]  Berry, B., Ed., Ratliff, S., Paradise, E., Kaiser, T., and
              M. Adams, "PPP over Ethernet (PPPoE) Extensions for Credit
              Flow and Link Metrics", RFC 5578, DOI 10.17487/RFC5578,
              February 2010, <http://www.rfc-editor.org/info/rfc5578>.

Appendix A.  Discovery Signal Flows

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

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

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

   |                                     Router sends another Peer
   |-------Peer Discovery---------->|    Discovery Signal.
                                    |
                                    |    Modem receives Peer Discovery



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

Appendix B.  Peer Level Message Flows

B.1.  Session Initialization

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

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

B.2.  Session Initialization - Refused

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

   |                                     Router connects to discovered or
   |                                     pre-configured Modem Connection
   |---------TCP connect---------->      Point.
   |
   |                                     Router sends Session
   |-----Session Initialization---->|    Initialization Message.
                                    |
                                    |    Modem receives Session
                                    |    Initialization Message, and will
                                    |    not support the advertised



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                                    |    extensions.
                                    |
                                    |    Modem sends Session Initialization
                                    |    Response, with 'Request Denied'
   |<-Session Initialization Resp.--|    Status Data Item.
   |
   |
   |                                     Router receives negative Session
   |                                     Initialization Response, closes
   ||---------TCP close------------||    TCP connection.

B.3.  Router Changes IP Addresses

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

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

B.4.  Modem Changes Session-wide Metrics

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

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









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B.5.  Router Terminates Session

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

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

B.6.  Modem Terminates Session

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

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

B.7.  Session Heartbeats

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



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

            ~ ~ ~ ~ ~ ~ ~

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

            ~ ~ ~ ~ ~ ~ ~

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

            ~ ~ ~ ~ ~ ~ ~

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

B.8.  Router Detects a Heartbeat timeout

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

           ||<----------------------|    Router misses a heartbeat


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

B.9.  Modem Detects a Heartbeat timeout





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

   |---------------------->||            Modem misses a heartbeat


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

Appendix C.  Destination Specific Message Flows

C.1.  Common Destination Notification

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

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

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

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

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




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C.2.  Multicast Destination Notification

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

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

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

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

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

C.3.  Link Characteristics Request

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

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

   |                                     Router requires different
   |                                     Characteristics for the
   |                                     destination, and sends Link
   |--Link Characteristics Request->|    Characteristics Request Message.
                                    |
                                    |    Modem attempts to adjust link



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                                    |    properties to meet the received
                                    |    request, and sends a Link
                                    |    Characteristics Response
   |<---Link Characteristics Resp.--|    Message with the new values.

Authors' Addresses

   Stan Ratliff
   VT iDirect
   13861 Sunrise Valley Drive, Suite 300
   Herndon, VA  20171
   USA

   Email: sratliff@idirect.net


   Bo Berry


   Shawn Jury
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: sjury@cisco.com


   Darryl Satterwhite
   Broadcom

   Email: dsatterw@broadcom.com


   Rick Taylor
   Airbus Defence & Space
   Quadrant House
   Celtic Springs
   Coedkernew
   Newport  NP10 8FZ
   UK

   Email: rick.taylor@airbus.com








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