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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: April 17, 2016
                                                                 S. Jury
                                                           Cisco Systems
                                                          D. Satterwhite
                                                                Broadcom
                                                               R. Taylor
                                                  Airbus Defence & Space
                                                        October 16, 2015


                 Dynamic Link Exchange Protocol (DLEP)
                        draft-ietf-manet-dlep-17

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 April 17, 2016.







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Copyright Notice

   Copyright (c) 2015 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.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   7
     1.2.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Mandatory Metrics . . . . . . . . . . . . . . . . . . . .   9
   4.  DLEP Signal and Message Processing  . . . . . . . . . . . . .  10
     4.1.  Transaction Model . . . . . . . . . . . . . . . . . . . .  11
   5.  DLEP Session Flow . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Peer Discovery State  . . . . . . . . . . . . . . . . . .  12
     5.2.  Session Initialization State  . . . . . . . . . . . . . .  13
     5.3.  In-Session State  . . . . . . . . . . . . . . . . . . . .  14
       5.3.1.  Heartbeats  . . . . . . . . . . . . . . . . . . . . .  15
     5.4.  Session Termination State . . . . . . . . . . . . . . . .  15
   6.  Extensions  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Experiments . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Scalability . . . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  DLEP Signal and Message Structure . . . . . . . . . . . . . .  17
     8.1.  DLEP Signal Header  . . . . . . . . . . . . . . . . . . .  18
     8.2.  DLEP Message Header . . . . . . . . . . . . . . . . . . .  19
     8.3.  DLEP Generic Data Item  . . . . . . . . . . . . . . . . .  19
   9.  DLEP Signals and Messages . . . . . . . . . . . . . . . . . .  20
     9.1.  Peer Discovery Signal . . . . . . . . . . . . . . . . . .  21
     9.2.  Peer Offer Signal . . . . . . . . . . . . . . . . . . . .  22
     9.3.  Session Initialization Message  . . . . . . . . . . . . .  22
     9.4.  Session Initialization Response Message . . . . . . . . .  23
     9.5.  Session Update Message  . . . . . . . . . . . . . . . . .  25
     9.6.  Session Update Response Message . . . . . . . . . . . . .  26
     9.7.  Session Termination Message . . . . . . . . . . . . . . .  26
     9.8.  Session Termination Response Message  . . . . . . . . . .  27
     9.9.  Destination Up Message  . . . . . . . . . . . . . . . . .  27



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     9.10. Destination Up Response Message . . . . . . . . . . . . .  28
     9.11. Destination Down Message  . . . . . . . . . . . . . . . .  29
     9.12. Destination Down Response Message . . . . . . . . . . . .  29
     9.13. Destination Update Message  . . . . . . . . . . . . . . .  30
     9.14. Heartbeat Message . . . . . . . . . . . . . . . . . . . .  31
     9.15. Link Characteristics Request Message  . . . . . . . . . .  31
     9.16. Link Characteristics Response Message . . . . . . . . . .  32
   10. DLEP Data Items . . . . . . . . . . . . . . . . . . . . . . .  33
     10.1.  Status . . . . . . . . . . . . . . . . . . . . . . . . .  34
     10.2.  IPv4 Connection Point  . . . . . . . . . . . . . . . . .  37
     10.3.  IPv6 Connection Point  . . . . . . . . . . . . . . . . .  38
     10.4.  Peer Type  . . . . . . . . . . . . . . . . . . . . . . .  39
     10.5.  Heartbeat Interval . . . . . . . . . . . . . . . . . . .  40
     10.6.  Extensions Supported . . . . . . . . . . . . . . . . . .  40
     10.7.  MAC Address  . . . . . . . . . . . . . . . . . . . . . .  41
     10.8.  IPv4 Address . . . . . . . . . . . . . . . . . . . . . .  42
     10.9.  IPv6 Address . . . . . . . . . . . . . . . . . . . . . .  43
     10.10. IPv4 Attached Subnet . . . . . . . . . . . . . . . . . .  44
     10.11. IPv6 Attached Subnet . . . . . . . . . . . . . . . . . .  45
     10.12. Maximum Data Rate (Receive)  . . . . . . . . . . . . . .  46
     10.13. Maximum Data Rate (Transmit) . . . . . . . . . . . . . .  46
     10.14. Current Data Rate (Receive)  . . . . . . . . . . . . . .  47
     10.15. Current Data Rate (Transmit) . . . . . . . . . . . . . .  48
     10.16. Latency  . . . . . . . . . . . . . . . . . . . . . . . .  49
     10.17. Resources (Receive)  . . . . . . . . . . . . . . . . . .  50
     10.18. Resources (Transmit) . . . . . . . . . . . . . . . . . .  50
     10.19. Relative Link Quality (Receive)  . . . . . . . . . . . .  51
     10.20. Relative Link Quality (Transmit) . . . . . . . . . . . .  52
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  52
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  53
     12.1.  Registrations  . . . . . . . . . . . . . . . . . . . . .  53
     12.2.  Expert Review: Evaluation Guidelines . . . . . . . . . .  54
     12.3.  Signal/Message Type Registration . . . . . . . . . . . .  54
     12.4.  DLEP Data Item Registrations . . . . . . . . . . . . . .  54
     12.5.  DLEP Status Code Registrations . . . . . . . . . . . . .  54
     12.6.  DLEP Extensions Registrations  . . . . . . . . . . . . .  54
     12.7.  DLEP Well-known Port . . . . . . . . . . . . . . . . . .  55
     12.8.  DLEP IPv6 Link-local Multicast Address . . . . . . . . .  55
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  55
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  55
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  55
     14.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  Discovery Signal Flows . . . . . . . . . . . . . . .  56
   Appendix B.  Peer Level Message Flows . . . . . . . . . . . . . .  56
     B.1.  Session Initialization  . . . . . . . . . . . . . . . . .  56
     B.2.  Session Initialization - Refused  . . . . . . . . . . . .  57
     B.3.  Router Changes IP Addresses . . . . . . . . . . . . . . .  57
     B.4.  Modem Changes Session-wide Metrics  . . . . . . . . . . .  57



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     B.5.  Router Terminates Session . . . . . . . . . . . . . . . .  58
     B.6.  Modem Terminates Session  . . . . . . . . . . . . . . . .  58
     B.7.  Session Heartbeats  . . . . . . . . . . . . . . . . . . .  59
     B.8.  Router Detects a Heartbeat timeout  . . . . . . . . . . .  60
     B.9.  Modem Detects a Heartbeat timeout . . . . . . . . . . . .  61
   Appendix C.  Destination Specific Signal Flows  . . . . . . . . .  61
     C.1.  Common Destination Signaling  . . . . . . . . . . . . . .  61
     C.2.  Multicast Destination Signaling . . . . . . . . . . . . .  62
     C.3.  Link Characteristics Request  . . . . . . . . . . . . . .  62
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

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.

   Another complicating factor for mobile networks are the different
   methods of physically connecting the modem devices to the router.



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   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
   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,



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   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 A|                     | Modem A+---+
            |    | Device |  <===== // ======>  | Device |   |
            |    +--------+      P-2-P Link     +--------+   |
        +---+----+                                       +---+----+
        | Router |                                       | Router |
        |        |                                       |        |
        +---+----+                                       +---+----+
            |     +--------+                     +--------+  |
            +-----+ Modem B|                     | Modem B|  |
                  | Device |   o o o o o o o o   | Device +--+
                  +--------+    o  Shared   o    +--------+
                                 o Medium  o
                                  o       o
                                   o     o
                                    o   o
                                      o
                                 +--------+
                                 | Modem B|
                                 | Device |
                                 +---+----+
                                     |
                                     |
                                 +---+----+
                                 | Router |
                                 |        |
                                 +--------+

            Figure 2: DLEP Network with Multiple Modem Devices




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1.1.  Protocol Overview

   As mentioned earlier, DLEP defines a set of messages used by modems
   and their attached routers.  The messages are used 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.

   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 a heartbeat, or (2) is explicitly torn down by
   one of the 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 (e.g., an address) that
   can be reached via the link(s) managed by the modem.  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.  As for a
   logical destination, the best example is that of Multicast.

   Multicast traffic destined for the variable-quality network (the
   network accessed via the DLEP 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
   (albeit a logical one) 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 multicast (or logical) destinations
   are outside the scope of this specification, and is left to specific
   implementations to decide.





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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 (e.g.,
   multicast) destinations accessible via the modem device, as well as
   the link characteristics to those destinations.

1.2.  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.  Assumptions

   DLEP specifies UDP multicast for single-hop discovery signalling, and
   TCP for transport of the control 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.  Other reliable transports for the protocol are possible,
   but are outside the scope of this document.

   DLEP assumes that the MAC address for delivering data traffic is the
   MAC address used by DLEP to identify the destination.  No
   manipulation or substitution is performed; the MAC address supplied
   in a Destination Up message (Section 9.9) message is used as the OSI
   Layer 2 Destination MAC address.  DLEP also assumes that MAC
   addresses are unique within the context of a router-modem session.

   DLEP assumes that security on the session (e.g., authentication of
   session partners, encryption of traffic, or both) is dealt with by
   the underlying transport mechanism (e.g., by using a transport such
   as TLS [RFC5246]).

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 with 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



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   receive metrics.  In cases where metrics are provided at session
   level, the receiver MUST propagate the metrics to all entries in its
   information base for destinations that are accessed via the
   originator.

   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 modem-wide basis, if all
   connections via the modem are of this static nature).

   DLEP modem implementations MUST announce all metric items that will
   be reported during the session, and provide default values for those
   metrics, in the Session Initialization Response message
   (Section 9.4).  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 9.7)), and establish a new session.

   A DLEP participant 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 receiver gets metrics in a specific
   destination context (via Destination Update), then the specific
   destination is updated with the new metric. 2.  If the receiver gets
   metrics in a modem-wide context (via Peer Update), then the received
   metrics for all destinations accessed via the modem MUST be updated
   to the newly received value.

3.1.  Mandatory Metrics

   As mentioned above, DLEP modem implementations MUST announce all
   supported metric items during the Session Initialization state.
   However, a modem MUST include the following list of metrics in the
   Session Initialization Response message (Section 9.4):

   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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4.  DLEP Signal and Message Processing

   Most messages in DLEP are members of a request/response pair, e.g.
   Destination Up message (Section 9.9), and Destination Up Response
   message (Section 9.10).  As mentioned before, session message pairs
   control the flow of the session through the various states, e.g. an
   implementation MUST NOT leave the Session Initialization state until
   a Session Initialization message (Section 9.3) and Session
   Initialization Response message (Section 9.4) have been exchanged.

   Destination message pairs describe the arrival and departure of
   logical destinations, and control the flow of information about the
   destinations in the several ways.  A destination MUST contain a MAC
   address, it MAY optionally include a Layer 3 address (or addresses).
   The MAC address MAY reference a logical destination, as in a derived
   multicast MAC address, as well as a physical device.  As destinations
   are discovered, DLEP routers and modems build an information base of
   destinations accessible via the modem.

   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).

   Prior to the exchange of a pair of Destination Up and Destination Up
   Response messages, no messages concerning the logical destination
   identified by the MAC Address data item (Section 10.7) may be sent.
   An implementation receiving a message with such an unannounced
   destination MUST terminate the session by issuing a Session
   Termination message (Section 9.7) with a status code of 'Invalid
   Destination', see Table 3, and transition to the Session Termination
   state.

   The receiver of a Destination Up message MAY decline further messages
   concerning a given destination by sending a Destination Up Response
   with a status code of 'Not Interested', see Table 3.  Receivers of
   such responses MUST NOT send further messages concerning that
   destination to the peer.

   After exchanging a pair of Destination Down (Section 9.11) and
   Destination Down Response (Section 9.12) messages, no messages
   concerning the logical destination identified by the MAC Address data
   item may be a sent without a previously sending a new Destination Up
   message.  An implementation receiving a message about a destination
   previously announced as 'down' MUST terminate the session by issuing




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   a Session Termination message with a status code of 'Invalid
   Destination' and transition to the Session Termination state.

4.1.  Transaction Model

   DLEP defines a simple message transaction model: Only one (1) request
   per destination may be in progress at a time.  A message transaction
   is considered complete when a response matching a previously issued
   request is received.  If a peer receives a request for a destination
   for which there is already an outstanding request, the peer MUST
   terminate the session by issuing a Session Termination message
   (Section 9.7) with a status code of 'Unexpected Message', see
   Table 3, 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 peers 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 (1) session request, e.g. a Session
   Initialization message (Section 9.3) may be in progress at a time.
   As above, a session transaction is considered complete when a
   response matching a previously issued request is received.  If a peer
   receives a session request while there is already a session request
   in progress, the peer MUST terminate the session by issuing a Session
   Termination message with a status code of 'Unexpected Message', and
   transition to the Session Termination state.  Only the Session
   Termination message may be issued when a session transaction is in
   progress.  Heartbeat messages (Section 9.14) MUST NOT be considered
   part of a session transaction.

   DLEP transactions do not time out and are not cancellable.  An
   implementation can detect if a peer has failed in some way by use of
   the session heartbeat mechanism during the In-Session state, see
   Section 5.3.

5.  DLEP Session Flow

   All DLEP peers transition through four (4) distinct states during the
   lifetime of a DLEP session:

   o  Peer Discovery

   o  Session Initialization



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   o  In-Session

   o  Session Termination

   The Peer Discovery state is OPTIONAL to implement for routers.  If it
   is used, this state is the initial state.  If it is not used, then
   one or more preconfigured address/port combinations SHOULD be
   provided to the router, and the device starts in the Session
   Initialization state.

   Modems MUST support the Peer Discovery state.

5.1.  Peer Discovery State

   In the Peer Discovery state, routers MUST send UDP packets containing
   a Peer Discovery signal (Section 9.1) to the DLEP well-known IPv6
   link-local multicast address (Section 12.8) and port number
   (Section 12.7), setting the packet source address to a valid local
   IPv6 address and the source port to an unused port in the range 49152
   to 65535.  If the router implementation supports IPv4, then they MAY
   also broadcast Peer Discovery signals in UDP packets to the IPv4
   broadcast address (255.255.255.255), setting the packet source
   address to a valid local IPv4 address and the source port to an
   unused port in the range 49152 to 65535.

   The implementation then waits for a unicast UDP packet containing a
   Peer Offer signal (Section 9.2) from a potential peer modem.  While
   in the Peer Discovery state, Peer Discovery signals MUST be sent
   repeatedly by a router, at regular intervals; every three (3) seconds
   with some jitter is RECOMMENDED.

   In the Peer Discovery state, the modem waits for incoming Peer
   Discovery signals on the DLEP well-known multicast address and port.
   On receipt of a valid signal, it MUST unicast a Peer Offer signal to
   the source address and port of the received UDP packet.  Peer Offer
   signals MAY contain the unicast address and port for TCP-based
   communication with a modem, via the IPv4 Connection Point data item
   (Section 10.2) or the IPv6 Connection Point data item (Section 10.3),
   on which it is prepared to accept an incoming TCP connection.  If the
   modem does not include an IPv4 Connection Point data item, nor a IPv6
   Connection Point data item, then the source address of the packet
   containing the Peer Offer signal MUST be set to the address on which
   the modem is willing to accept TCP connections.

   The modem then begins listening for incoming TCP connections, and,
   having accepted one, enters the Session Initialization state.
   Anything other than Peer Discovery signals received on the UDP socket
   MUST be silently dropped.



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   Modems SHOULD 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.  The modem
   MUST accept a TCP connection on only one (1) address/port combination
   per 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.  It is RECOMMENDED that an implementation attempt to connect
   to any announced IPv6 address/port combinations before attempting to
   use IPv4 combinations.  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 origin address of the UDP packet containing the signal as the IP
   address, and the DLEP well-known port number.

   Once a TCP connection has been established with the modem, the router
   begins a new session and enters 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.

   It should be noted that the peer discovery process operates using
   link-local multicast and is hence inapplicable if the potential peers
   are separated by more than one hop.

5.2.  Session Initialization State

   On entering the Session Initialization state, the router MUST send a
   Session Initialization message (Section 9.3) to the modem.  The
   router MUST then wait for receipt of a Session Initialization
   Response message (Section 9.4) from the modem.  Receipt of the
   Session Initialization Response message containing a Status data item
   (Section 10.1) with value 'Success', see Table 3, 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 and successful parsing of a Session Initialization message,
   the modem MUST send a Session Initialization Response message, and
   the session MUST transition to the In-Session state.




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   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 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.

   If the router receives any message other than a valid Session
   Initialization Response, it MUST send a Session Termination message
   (Section 9.7) with a relevant status code, e.g.  'Unexpected
   Message', see Table 3, and transition to the Session Termination
   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, then restart at the
   Peer Discovery state.

   As mentioned before, the Session Initialization Response message MUST
   contain metric data items for all metrics that will be used during
   the session.  If an additional metric is to be introduced after the
   session has started, the session between router and modem MUST be
   terminated and restarted, and the new metric described in the next
   Session Initialization Response message.

5.3.  In-Session State

   In the In-Session state, messages can flow in both directions between
   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.

   In addition to the session messages, the participants will transmit
   messages concerning destinations in the network.  These messages
   trigger creation/maintenance/deletion of destinations in the
   information base of the recipient.  For example, a modem will inform
   its attached router of the presence of a new destination via the
   Destination Up message (Section 9.9).  Receipt of a Destination Up
   causes the router to allocate 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 loss of a
   destination is communicated via the Destination Down message
   (Section 9.11), and changes in status to the destination (e.g.,
   varying link quality, or addressing changes) are communicated via the
   Destination Update message (Section 9.13).  The information on a
   given destination will persist in the router's information base until



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   (1) a Destination Down message is received, indicating that the modem
   has lost contact with the remote node, or (2) the router/modem
   transitions to the Session Termination state.

   As well as receiving 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 9.15), 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.

   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 9.7)), or

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

   The implementation MUST then transition to the Session Termination
   state.

5.3.1.  Heartbeats

   In order to maintain the In-Session state, periodic Heartbeat
   messages (Section 9.14) MAY be exchanged between router and modem.
   These messages are intended to keep the session alive, and to verify
   bidirectional connectivity between the two participants.  Each DLEP
   peer 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 traffic on the router/modem session before terminating the
   session by issuing a Session Termination message with a status code
   of 'Timed Out', and then transition to the Session Termination state.

5.4.  Session Termination State

   When a DLEP implementation enters the Session Termination state after
   sending a Session Termination message (Section 9.7) as the result of
   an invalid message or error, it MUST wait for a Session Termination
   Response message (Section 9.8) from its peer.  If Heartbeat messages
   (Section 9.14) are in use, senders SHOULD allow four (4) heartbeat
   intervals to expire before assuming that the peer is unresponsive,
   and continuing with session termination.  If Heartbeat messages are



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   not in use, then if is RECOMMENDED that an interval of eight (8)
   seconds be used.

   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.

   The sender and receiver of a Session Termination message MUST release
   all resources allocated for the session, and MUST eliminate all
   destinations in the information base accessible via the peer
   represented by the session.  Destination Down messages (Section 9.11)
   MUST NOT be sent.

   Any messages received after either sending or receiving a Session
   Termination message MUST be silently ignored.

   Once Session Termination messages have been exchanged, or timed out,
   the device MUST terminate the TCP connection to the peer, and return
   to the relevant initial state.

6.  Extensions

   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 behaviour,
   messages, data items and/or status codes that are not defined in this
   document.

   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.




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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 4), 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.,
   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.





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   Both signals and messages consist of a header followed by an
   unordered list of data items.  Headers consist of Type and Length
   information, while data items are encoded as TLV (Type-Length-Value)
   structures.  In this document, the data items following a signal or
   message header are described as being 'contained in' the signal or
   message.

   There is no restriction on the order of data items following a
   header, and the 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.

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+44, U+4C,
      U+45, U+50.

   Signal Type:  An 16-bit unsigned integer containing one of the DLEP
      Signal/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
      signal.  This length SHALL NOT include the length of the header
      itself.

   The DLEP signal header is immediately followed by one or more DLEP
   data items, encoded in TLVs, as defined in this document.

   If an unrecognized, or unexpected signal is received, or a received
   signal contains unrecognized, invalid, or disallowed duplicate data
   items, the receiving peer MUST ignore the signal.





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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:  An 16-bit unsigned integer containing one of the DLEP
      Signal/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 SHALL NOT include the length of the header
      itself.

   The DLEP message header is immediately followed by one or more DLEP
   data items, encoded in TLVs, as defined in this document.

   If an unrecognized, or unexpected message is received, or a received
   message contains unrecognized, invalid, or disallowed duplicate data
   items, the receiving peer MUST issue a Session Termination message
   (Section 9.7) with a Status data item (Section 10.1) containing the
   most relevant status code, and transition to the Session Termination
   state.

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:  An 16-bit unsigned integer field specifying the type
      of data item being sent.




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   Length:  The length in octets, expressed as an 16-bit unsigned
      integer, of the value field of the data item.  This length SHALL
      NOT include the length of the header itself.

   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 and messages are:































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   +-------------+-----------------------------------------------------+
   | Type Code   | Description                                         |
   +-------------+-----------------------------------------------------+
   | 0           | Reserved                                            |
   | 1           | Peer Discovery signal (Section 9.1)                 |
   | 2           | Peer Offer signal (Section 9.2)                     |
   | 3           | Session Initialization message (Section 9.3)        |
   | 4           | Session Initialization Response message (Section    |
   |             | 9.4)                                                |
   | 5           | Session Update message (Section 9.5)                |
   | 6           | Session Update Response message (Section 9.6)       |
   | 7           | Session Termination message (Section 9.7)           |
   | 8           | Session Termination Response message (Section 9.8)  |
   | 9           | Destination Up message (Section 9.9)                |
   | 10          | Destination Up Response message (Section 9.10)      |
   | 11          | Destination Down message (Section 9.11)             |
   | 12          | Destination Down Response message (Section 9.12)    |
   | 13          | Destination Update message (Section 9.13)           |
   | 14          | Heartbeat message (Section 9.14)                    |
   | 15          | Link Characteristics Request message (Section 9.15) |
   | 16          | Link Characteristics Response message (Section      |
   |             | 9.16)                                               |
   | 17-65519    | Reserved for future extensions                      |
   | 65520-65534 | Private Use. Available for experiments              |
   | 65535       | Reserved                                            |
   +-------------+-----------------------------------------------------+

                    Table 1: DLEP Signal/Message types

9.1.  Peer Discovery Signal

   A Peer Discovery signal SHOULD be sent by a router to discover DLEP
   modems in the network.  The Peer Offer signal (Section 9.2) is
   required to complete the discovery process.  Implementations MAY
   implement their own retransmit heuristics in cases where it is
   determined the Peer Discovery signal has timed out.

   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 the following data item:

   o  Peer Type (Section 10.4)








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9.2.  Peer Offer Signal

   A Peer Offer signal MUST be sent by a DLEP modem in response to a
   valid Peer Discovery signal (Section 9.1).

   The Peer Offer signal MUST be sent to the unicast address of the
   originator of the Peer Discovery signal.

   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 the following data item:

   o  Peer Type (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)

   o  IPv6 Connection Point (Section 10.3)

   The IP Connection Point data items indicate the unicast address the
   receiver of Peer Offer MUST use when connecting the DLEP TCP session.
   If multiple IP Connection Point data items are present in the Peer
   Offer signal, 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 receiver MUST use the origin
   address of the signal as the IP address, and the DLEP well-known port
   number (Section 12.7) to establish the TCP connection.

9.3.  Session Initialization Message

   A Session Initialization message MUST be sent by a 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.

   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 receiver of the message MUST conclude
   that there is no support for extensions in the sender.

   Implementations supporting the Heartbeat Interval (Section 10.5)
   should understand that heartbeats are not fully established until
   receipt of Session Initialization Response message (Section 9.4), and




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   should therefore implement their own timeout and retry heuristics for
   this message.

   To construct a Session Initialization message, the Message Type value
   in the message header is set to 3, from Table 1.

   The Session Initialization message MUST contain one of each of the
   following data items:

   o  Heartbeat Interval (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)

   A Session Initialization message MUST be acknowledged by the receiver
   issuing a Session Initialization Response message (Section 9.4).

   As an exception to the general rule that an implementation receiving
   an unrecognized data item in a message terminating the session with
   an error, see Section 8.2, 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.

9.4.  Session Initialization Response Message

   A Session Initialization Response message MUST be sent in response to
   a received Session Initialization message (Section 9.3).  The Session
   Initialization Response message completes the DLEP session
   establishment; the sender of the message should transition to the In-
   Session state when the message is sent, and the receiver should
   transition to the In-Session state upon receipt (and successful
   parsing) 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
   'modem-wide' basis.  This can be viewed as the modem 'declaring' all
   supported metrics at DLEP session initialization.  Receipt of any
   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.





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   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 receiver of the message MUST conclude that
   there is no support for extensions in the sender.

   After the Session Initialization/Session Initialization Response
   messages have been successfully exchanged, implementations MUST only
   use extensions that are supported by BOTH peers.

   To construct a Session Initialization Response message, the Message
   Type value in the message header is set to 4, from Table 1.

   The Session Initialization Response message MUST contain one of each
   of the following data items:

   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 (Receive) (Section 10.17)

   o  Resources (Transmit) (Section 10.18)

   o  Relative Link Quality (Receive) (Section 10.19)

   o  Relative Link Quality (Transmit) (Section 10.20)

   The Session Initialization Response message MAY contain one of each
   of the following data items:

   o  Status (Section 10.1)

   o  Peer Type (Section 10.4)

   o  Extensions Supported (Section 10.6)



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   A receiver of a Session Initialization Response message without a
   Status data item MUST behave as if a Status data item with code
   'Success' had been received.

9.5.  Session Update Message

   A Session Update message MAY be sent by a DLEP peer to indicate local
   Layer 3 address changes, or metric changes on a modem-wide basis.
   For example, addition of an IPv4 address to the router MAY prompt a
   Session Update message to its attached DLEP 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 9.13) to their local routers with the new (or
   deleted) addresses.  Modems that do not track Layer 3 addresses
   SHOULD silently parse and ignore Layer 3 data items.  The Session
   Update message MUST be acknowledged with a Session Update Response
   message (Section 9.6).

   If metrics are supplied with the Session Update message (e.g.,
   Maximum Data Rate), these metrics are considered to be modem-wide,
   and therefore MUST be applied to all destinations in the information
   base associated with the router/modem session.

   To construct a Session Update message, the Message Type value in the
   message header is set to 5, from Table 1.

   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)

   o  Resources (Receive) (Section 10.17)

   o  Resources (Transmit) (Section 10.18)



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   o  Relative Link Quality (Receive) (Section 10.19)

   o  Relative Link Quality (Transmit) (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)

   A Session Update message MUST be acknowledged by the receiver issuing
   a Session Update Response message (Section 9.6).

9.6.  Session Update Response Message

   A Session Update Response message MUST be sent by implementations to
   indicate whether a Session Update message (Section 9.5) was
   successfully received.

   To construct a Session Update Response message, the Message Type
   value in the message header is set to 6, from Table 1.

   The Session Update Response message MAY contain one of each of the
   following data items:

   o  Status (Section 10.1)

   A receiver of a Session Update Response message without a Status data
   item MUST behave as if a Status data item with code 'Success' had
   been received.

9.7.  Session Termination Message

   A Session Termination message MUST be sent by a DLEP participant when
   the router/modem session needs to be terminated.

   To construct a Session Termination message, the Message Type value in
   the message header is set to 7, from Table 1.

   The Session Termination message MAY contain one of each of the
   following data items:

   o  Status (Section 10.1)

   A receiver of a Session Termination message without a Status data
   item MUST behave as if a Status of 'Unknown reason for Session
   Termination' has been received.



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   A Session Termination message MUST be acknowledged by the receiver
   issuing a Session Termination Response message (Section 9.8).

9.8.  Session Termination Response Message

   A Session Termination Response message MUST be sent by a DLEP peer in
   response to a received Session Termination message (Section 9.7).

   Receipt of a Session Termination Response message completes the
   teardown of the router/modem session.

   To construct a Session Termination Response message, the Message Type
   value in the message header is set to 8, from Table 1.

   The Session Termination Response message MAY contain one of each of
   the following data items:

   o  Status (Section 10.1)

   A receiver of a Session Termination Response message without a Status
   data item MUST behave as if a Status data item with status code
   'Success', implying graceful termination, had been received.

9.9.  Destination Up Message

   A Destination Up message can be sent either by the modem, to indicate
   that a new remote node has been detected, or by the router, to
   indicate the presence of a new logical destination (e.g., a Multicast
   group) in the network.

   A Destination Up message MUST be acknowledged by the receiver issuing
   a Destination Up Response message (Section 9.10).  When a Destination
   Up message is received and successfully processed, the receiver
   should add knowledge of the new destination to its information base,
   indicating that the destination is accessible via the modem/router
   pair.

   To construct a Destination Up message, the Message Type value in the
   message header is set to 9, from Table 1.

   The Destination Up message MUST contain one of each of the following
   data items:

   o  MAC Address (Section 10.7)

   The Destination Up message MAY contain one of each of the following
   data items:




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   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)

   o  Resources (Receive) (Section 10.17)

   o  Resources (Transmit) (Section 10.18)

   o  Relative Link Quality (Receive) (Section 10.19)

   o  Relative Link Quality (Transmit) (Section 10.20)

   The Destination Up 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)

   o  IPv4 Attached Subnet (Section 10.10)

   o  IPv6 Attached Subnet (Section 10.11)

   If the sender has IPv4 and/or IPv6 address information for a
   destination it SHOULD include the relevant data items in the
   Destination Up message, reducing the need for the receiver to probe
   for any address.

9.10.  Destination Up Response Message

   A DLEP participant MUST send a Destination Up Response message to
   indicate whether a Destination Up message (Section 9.9) was
   successfully processed.

   To construct a Destination Up Response message, the Message Type
   value in the message header is set to 10, from Table 1.

   The Destination Up Response message MUST contain one of each of the
   following data items:

   o  MAC Address (Section 10.7)




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   The Destination Up Response message MAY contain one of each of the
   following data items:

   o  Status (Section 10.1)

   A receiver of a Destination Up Response message without a Status data
   item MUST behave as if a Status data item with status code 'Success'
   had been received.

9.11.  Destination Down Message

   A DLEP peer MUST send a Destination Down message to report when a
   destination (a remote node or a multicast group) is no longer
   reachable.  A Destination Down Response message (Section 9.12) MUST
   be sent by the recipient of a Destination Down message to confirm
   that the relevant data has been removed from the information base.

   To construct a Destination Down message, the Message Type value in
   the message header is set to 11, from Table 1.

   The Destination Down message MUST contain one of each of the
   following data items:

   o  MAC Address (Section 10.7)

9.12.  Destination Down Response Message

   A DLEP participant MUST send a Destination Down Response message to
   indicate whether a received Destination Down message (Section 9.11)
   was successfully processed.  If successfully processed, the sender of
   the Response MUST have removed all entries in the information base
   that pertain to the referenced destination.

   To construct a Destination Down Response message, the Message Type
   value in the message header is set to 12, from Table 1.

   The Destination Down Response message MUST contain one of each of the
   following data items:

   o  MAC Address (Section 10.7)

   The Destination Down Response message MAY contain one of each of the
   following data items:

   o  Status (Section 10.1)






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   A receiver of a Destination Down Response message without a Status
   data item MUST behave as if a Status data item with status code
   'Success' had been received.

9.13.  Destination Update Message

   A DLEP participant 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 1.

   The Destination Update message MUST contain one of each of the
   following data items:

   o  MAC Address (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)

   o  Resources (Receive) (Section 10.17)

   o  Resources (Transmit) (Section 10.18)

   o  Relative Link Quality (Receive) (Section 10.19)

   o  Relative Link Quality (Transmit) (Section 10.20)

   The Destination Update message MAY contain one or more of the
   following data items, with different values:

   o  IPv4 Address (Section 10.8)



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   o  IPv6 Address (Section 10.9)

9.14.  Heartbeat Message

   While Heartbeat messages are not required by DLEP implementations, it
   is strongly RECOMMENDED that Heartbeat messages be used.

   A Heartbeat message SHOULD be sent by a DLEP participant every N
   seconds, where N is defined in the Heartbeat Interval data item of
   the Session Initialization message (Section 9.3) or Session
   Initialization Response message (Section 9.4).

   Note that implementations setting the Heartbeat Interval to 0
   effectively sets the interval to an infinite value, turning off
   Heartbeat messages.  Great care MUST be taken when exercising this
   option.

   The message is used by participants to detect when a DLEP session
   partner (either the modem or the router) is no longer communicating.
   Participants SHOULD allow two (2) heartbeat intervals to expire with
   no traffic on the router/modem session before initiating DLEP session
   termination procedures.

   To construct a Heartbeat message, the Message Type value in the
   message header is set to 14, from Table 1.

   There are no valid data items for the Heartbeat message.

9.15.  Link Characteristics Request Message

   The Link Characteristics Request message MAY be sent by the 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.

   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.  A
   Link Characteristics Response message (Section 9.16) is required to
   complete the request.  Issuing a Link Characteristics Request with
   ONLY the MAC Address data item is a mechanism a peer MAY use to
   request metrics (via the Link Characteristics Response) from its
   partner.

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



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   To construct a Link Characteristics Request message, the Message Type
   value in the message header is set to 15, from Table 1.

   The Link Characteristics Request message MUST contain one of each of
   the following data items:

   o  MAC Address (Section 10.7)

   The Link Characteristics Request message MAY contain 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)

9.16.  Link Characteristics Response Message

   A DLEP participant MUST send a Link Characteristics Response message
   to indicate whether a received Link Characteristics Request message
   (Section 9.15) was successfully processed.  The Link Characteristics
   Response message SHOULD contain a complete set of metric data items,
   and MUST contain a full set (i.e. those declared in the Session
   Initialization Response message (Section 9.4)), if metrics were
   requested by only including a MAC address data item.  It MUST contain
   the same metric types as the request.  The values in the metric data
   items in the Link Characteristics Response message MUST reflect the
   link characteristics after the request has been processed.

   If an implementation is not able to alter the characteristics of the
   link in the manner requested, then a Status data item with status
   code 'Request Denied', see Table 3, MUST be added to the message.

   To construct a Link Characteristics Response message, the Message
   Type value in the message header is set to 16, from Table 1.

   The Link Characteristics Response message MUST contain one of each of
   the following data items:

   o  MAC Address (Section 10.7)

   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)



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   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:

   o  Resources (Receive) (Section 10.17)

   o  Resources (Transmit) (Section 10.18)

   o  Relative Link Quality (Receive) (Section 10.19)

   o  Relative Link Quality (Transmit) (Section 10.20)

   o  Status (Section 10.1)

   A receiver of a Link Characteristics Response message without a
   Status data item MUST behave as if a Status data item with status
   code 'Success' had been received.

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:



















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   +-------------+-----------------------------------------------------+
   | 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 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 (Receive) (RESR) (Section 10.17)          |
   | 18          | Resources (Transmit) (REST) (Section 10.18)         |
   | 19          | Relative Link Quality (Receive) (RLQR) (Section     |
   |             | 10.19)                                              |
   | 20          | Relative Link Quality (Transmit) (RLQT) (Section    |
   |             | 10.20)                                              |
   | 21-65407    | Reserved for future extensions                      |
   | 65408-65534 | Private Use. Available for experiments              |
   | 65535       | Reserved                                            |
   +-------------+-----------------------------------------------------+

                       Table 2: DLEP Data Item types

10.1.  Status

   The Status data item MAY appear in the Session Initialization
   Response (Section 9.4), Session Termination (Section 9.7), Session
   Termination Response (Section 9.8), Session Update Response
   (Section 9.6), Destination Up Response (Section 9.10), Destination
   Down Response (Section 9.12) and Link Characteristics Response
   (Section 9.16) messages.

   For the Session Termination message (Section 9.7), the Status data
   item indicates a reason for the termination.  For all acknowledgement
   messages, the Status data item is used to indicate the success or
   failure of the previously received message.





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   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

   Status Code:  One of the codes defined in Table 3 below.

   Text:  UTF-8 encoded string, 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.






















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   +-------------+---------+-----------+-------------------------------+
   | 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 expected  |
   | Message     |         |           | while the device was in the   |
   |             |         |           | current state, e.g., a        |
   |             |         |           | Session Initialization        |
   |             |         |           | message (Section 9.3) in the  |
   |             |         |           | In-Session state.             |
   | Invalid     | 3       | Terminate | One or more data items in the |
   | Data        |         |           | message are invalid,          |
   |             |         |           | unexpected or incorrectly     |
   |             |         |           | duplicated.                   |
   | Invalid     | 4       | Terminate | The destination provided in   |
   | Destination |         |           | the message does not match a  |
   |             |         |           | previously announced          |
   |             |         |           | destination. For example, in  |
   |             |         |           | the Link Characteristic       |
   |             |         |           | Response message (Section     |
   |             |         |           | 9.16).                        |
   | Timed Out   | 5       | Terminate | The session has timed out.    |
   | <Reserved>  | 6-90    | Terminate | Reserved for future           |
   |             |         |           | extensions.                   |
   | <Private    | 91-99   | Terminate | Available for experiments.    |
   | Use>        |         |           |                               |
   | Not         | 100     | Continue  | The receiver is not           |
   | Interested  |         |           | interested in this message    |
   |             |         |           | subject, e.g. a Destination   |
   |             |         |           | Up Response message (Section  |
   |             |         |           | 9.10) to indicate no further  |
   |             |         |           | messages about the            |
   |             |         |           | destination.                  |
   | Request     | 101     | Continue  | The receiver refuses to       |
   | Denied      |         |           | complete the request.         |
   | <Reserved>  | 102-243 | Continue  | Reserved for future           |
   |             |         |           | extensions.                   |
   | <Private    | 244-254 | Continue  | Available for experiments.    |
   | Use>        |         |           |                               |
   | <Reserved>  | 255     | Terminate | Reserved.                     |
   +-------------+---------+-----------+-------------------------------+

                        Table 3: DLEP Status Codes



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   A failure mode of 'Terminate' indicates that the session MUST be
   terminated after sending a response containing the status code.  A
   failure mode of 'Continue' indicates that the session SHOULD continue
   as normal.

10.2.  IPv4 Connection Point

   The IPv4 Connection Point data item MAY appear in the Peer Offer
   signal (Section 9.2).

   The IPv4 Connection Point data item indicates the IPv4 address and,
   optionally, the TCP port number on the DLEP modem available for
   connections.  If provided, the receiver MUST use this information to
   perform the TCP connect to the DLEP server.

   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 DLEP modem.

   TCP Port Number:  TCP Port number on the DLEP 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 receiver MUST use the DLEP well-known port
   number (Section 12.7) to establish the TCP connection.

   The Flags field is defined as:

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



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   T: Use TLS flag, indicating whether the TCP connection requires the
      use of TLS (1), or not (0).

   MBZ:  MUST be zero.  Reserved for future use.

10.3.  IPv6 Connection Point

   The IPv6 Connection Point data item MAY appear in the Peer Offer
   signal (Section 9.2).

   The IPv6 Connection Point data item indicates the IPv6 address and,
   optionally, the TCP port number on the DLEP modem available for
   connections.  If provided, the receiver MUST use this information to
   perform the TCP connect to the DLEP server.

   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 DLEP modem.

   TCP Port Number:  TCP Port number on the DLEP 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 receiver 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
   +-+-+-+-+-+-+-+-+
   |    MBZ      |T|
   +-+-+-+-+-+-+-+-+

   T: Use TLS flag, indicating whether the TCP connection requires the
      use of TLS (1), or not (0).

   MBZ:  MUST be zero.  Reserved for future use.

10.4.  Peer Type

   The Peer Type data item MAY appear in the Peer Discovery
   (Section 9.1) and Peer Offer (Section 9.2) signals, and the Session
   Initialization (Section 9.3) and Session Initialization Response
   (Section 9.4) messages.

   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.  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.




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10.5.  Heartbeat Interval

   The Heartbeat Interval data item MUST appear in both the Session
   Initialization (Section 9.3) and Session Initialization Response
   (Section 9.4) messages to indicate the Heartbeat timeout window to be
   used by the sender.

   The Interval is used to specify a period (in seconds) for Heartbeat
   messages (Section 9.14).  By specifying an Interval value of 0,
   implementations MAY indicate the desire to disable Heartbeat messages
   entirely (i.e., the Interval is set to an infinite value).  However,
   it is RECOMMENDED that implementations use non-0 timer values.

   The Heartbeat Interval data item contains the following fields:

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

   Data Item Type:  5

   Length:  2

   Interval:  0 = Do not use heartbeats on this DLEP session.  Non-zero
      = Interval, in seconds, for heartbeat messages.

10.6.  Extensions Supported

   The Extensions Supported data item MAY be used in both the Session
   Initialization (Section 9.3) and Session Initialization Response
   (Section 9.4) messages.

   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:







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

10.7.  MAC Address

   The MAC address data item MUST appear in all destination-oriented
   messages (i.e., Destination Up (Section 9.9), Destination Up Response
   (Section 9.10), Destination Down (Section 9.11), Destination Down
   Response (Section 9.12), Destination Update (Section 9.13), Link
   Characteristics Request (Section 9.15), and Link Characteristics
   Response (Section 9.16)).

   The MAC Address data item contains the address of the destination on
   the remote node.  The MAC address MAY be either a physical or a
   virtual destination, and MAY be expressed in EUI-48 or EUI-64 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.



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10.8.  IPv4 Address

   The IPv4 Address data item MAY appear in the Session Update
   (Section 9.5), Destination Up (Section 9.9) and Destination Update
   (Section 9.13) messages.

   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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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
   +-+-+-+-+-+-+-+-+
   |    MBZ      |A|
   +-+-+-+-+-+-+-+-+

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

   MBZ:  MUST be zero.  Reserved for future use.







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10.9.  IPv6 Address

   The IPv6 Address data item MAY appear in the Session Update
   (Section 9.5), Destination Up (Section 9.9) and Destination Update
   (Section 9.13) messages.  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                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :                        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
   +-+-+-+-+-+-+-+-+
   |    MBZ      |A|
   +-+-+-+-+-+-+-+-+

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

   MBZ:  MUST be zero.  Reserved for future use.



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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 IPv4 Attached Subnet data item MAY appear in the
   Destination Up (Section 9.9) message.

   The DLEP IPv4 Attached Subnet data item contains the following
   fields:

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

   Data Item Type:  10

   Length:  6

   Flags:  Flags field, defined below.

   IPv4 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
   +-+-+-+-+-+-+-+-+
   |    MBZ      |A|
   +-+-+-+-+-+-+-+-+

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

   MBZ:  MUST be zero.  Reserved for future use.






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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, or that it has become
   aware of an IPv6 subnet being present at a remote destination.  The
   IPv6 Attached Subnet data item MAY appear in the Destination Up
   (Section 9.9) message.  As in the case of the IPv4 attached Subnet
   data item above, once an IPv6 attached subnet has been declared, it
   SHALL NOT be withdrawn without withdrawing the destination (via the
   Destination Down message (Section 9.11)) and re-issuing the
   Destination Up message.

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

   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:







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

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

   MBZ:  MUST be zero.  Reserved for future use.

10.12.  Maximum Data Rate (Receive)

   The Maximum Data Rate (Receive) (MDRR) data item MUST appear in the
   Session Initialization Response message (Section 9.4), and MAY appear
   in the Session Update (Section 9.5), Destination Up (Section 9.9),
   Destination Update (Section 9.13) and Link Characteristics Response
   (Section 9.16) messages 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

   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 MUST appear in the
   Session Initialization Response message (Section 9.4), and MAY appear
   in the Session Update (Section 9.5), Destination Up (Section 9.9),
   Destination Update (Section 9.13) and Link Characteristics Response
   (Section 9.16) messages to indicate the maximum theoretical data



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   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 MUST appear in the
   Session Initialization Response message (Section 9.4), and MAY appear
   in the Session Update (Section 9.5), Destination Up (Section 9.9),
   Destination Update (Section 9.13) and Link Characteristics Response
   (Section 9.16) messages to indicate the rate at which the link is
   currently operating for receiving traffic.

   When used in the Link Characteristics Request message (Section 9.15),
   CDRR represents the desired receive rate, in bits per second, on the
   link.

   The Current Data Rate (Receive) 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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        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 MUST appear in the
   Session Initialization Response message (Section 9.4), and MAY appear
   in the Session Update (Section 9.5), Destination Up (Section 9.9),
   Destination Update (Section 9.13), and Link Characteristics Response
   (Section 9.16) messages to indicate the rate at which the link is
   currently operating for transmitting traffic.

   When used in the Link Characteristics Request message (Section 9.15),
   CDRT represents the desired transmit rate, in bits per second, on the
   link.

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

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




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   Data Item Type:  15

   Length:  8

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

   If there is no distinction between current 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 MUST appear in the Session Initialization
   Response message (Section 9.4), and MAY appear in the Session Update
   (Section 9.5), Destination Up (Section 9.9), Destination Update
   (Section 9.13), and Link Characteristics Response (Section 9.16)
   messages to indicate the amount of latency, in microseconds, on the
   link.

   When used in the Link Characteristics Request message (Section 9.15),
   Latency represents the maximum latency desired on the link.

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

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

   Data Item Type:  16

   Length:  8

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






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10.17.  Resources (Receive)

   The Resources (Receive) (RESR) data item MAY appear in the Session
   Initialization Response message (Section 9.4), Session Update
   (Section 9.5), Destination Up (Section 9.9), Destination Update
   (Section 9.13) and Link Characteristics Response (Section 9.16)
   messages to indicate the amount of resources for reception (with 0
   meaning 'no resources available', and 100 meaning 'all resources
   available') at the destination.  The list of resources that might be
   considered is beyond the scope of this document, and is left to
   implementations to decide.

   The Resources (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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RESR      |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  17

   Length:  1

   Resources (Receive):  An 8-bit integer percentage, 0-100,
      representing the amount of resources allocated to receiving data.
      Any value greater than 100 MUST be considered as invalid.

   If a device cannot calculate RESR, this data item SHOULD NOT be
   issued.

10.18.  Resources (Transmit)

   The Resources (Transmit) (REST) data item MAY appear in the Session
   Initialization Response message (Section 9.4), Session Update
   (Section 9.5), Destination Up (Section 9.9), Destination Update
   (Section 9.13) and Link Characteristics Response (Section 9.16)
   messages to indicate the amount of resources for transmission (with 0
   meaning 'no resources available', and 100 meaning 'all resources
   available') at the destination.  The list of resources that might be
   considered is beyond the scope of this document, and is left to
   implementations to decide.

   The Resources (Transmit) 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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     REST      |
   +-+-+-+-+-+-+-+-+

   Data Item Type:  18

   Length:  1

   Resources (Transmit):  An 8-bit integer percentage, 0-100,
      representing the amount of resources allocated to transmitting
      data.  Any value greater than 100 MUST be considered as invalid.

   If a device cannot calculate REST, this data item SHOULD NOT be
   issued.

10.19.  Relative Link Quality (Receive)

   The Relative Link Quality (Receive) (RLQR) data item MAY appear in
   the Session Initialization Response message (Section 9.4), Session
   Update (Section 9.5), Destination Up (Section 9.9), Destination
   Update (Section 9.13) and Link Characteristics Response
   (Section 9.16) messages to indicate the quality of the link for
   receiving data.

   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:  19

   Length:  1

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




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   If a device cannot calculate the RLQR, this data item SHOULD NOT be
   issued.

10.20.  Relative Link Quality (Transmit)

   The Relative Link Quality (Transmit) (RLQT) data item MAY appear in
   the Session Initialization Response message (Section 9.4), Session
   Update (Section 9.5), Destination Up (Section 9.9), Destination
   Update (Section 9.13) and Link Characteristics Response
   (Section 9.16) messages to indicate the quality of the link for
   transmitting data.

   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:  20

   Length:  1

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

   If a device cannot calculate the RLQT, this data item SHOULD NOT be
   issued.

11.  Security Considerations

   The potential security concerns when using DLEP are:

   1.  DLEP peers may be 'spoofed' by an attacker, either at DLEP
       session initialization, or by injection of messages once a
       session has been established, and/or

   2.  DLEP data items could be altered by an attacker, causing the
       receiving peer to inappropriately alter its information base
       concerning network status.





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   If the modem and router are separated by more than a single hop,
   session messages could be altered in order to subvert the behaviour
   of either or both DLEP participants.  Under these circumstances, DLEP
   participants MUST implement TLS [RFC5246].

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

   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 and messages, with sixteen (16)
      values currently assigned.

   o  Reservation of a Private Use numbering space for experimental DLEP
      signals and messages.

   o  A new repository for DLEP data items, with twenty-four (24) values
      currently assigned.

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





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

   o  A request for allocation of a multicast IP address for DLEP
      discovery.

12.2.  Expert Review: Evaluation Guidelines

   No additional guidelines for expert review are anticipated.

12.3.  Signal/Message Type Registration

   A new repository must be created with the values of the DLEP signals
   and messages.

   All signal and message values are in the range [0..65535], defined in
   Table 1.

12.4.  DLEP Data Item Registrations

   A new repository for DLEP data items must be created.

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

12.5.  DLEP Status Code Registrations

   A new repository for DLEP status codes must be created.

   All status codes are in the range [0..255], defined in Table 3.

12.6.  DLEP Extensions Registrations

   A new repository for DLEP extensions must be created.

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

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

                       Table 4: DLEP Extension types



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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 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 and Victoria Mercieca.

14.  References

14.1.  Normative References

   [CREDIT]   Ratliff, S., "Credit Windowing extension for DLEP", draft-
              ietf-manet-credit-window-00 IETF draft, October 2015.

   [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>.

14.2.  Informative References

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

   [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>.






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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
                                 |    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    Signal Description
========================================================================

|                                     Router connects to discovered or
|                                     pre-configured Modem Connection
|---------TCP connect---------->      Point.
|
|                                     Router sends Session Initialization
|----Session Initialization----->|    message.
                                 |
                                 |    Modem receives Session Initialization
                                 |    message.
                                 |
                                 |    Modem sends Session Initialization
|<--Session Initialization Resp.-|    Response, with Success status data item.
|                                |
|<<============================>>|    Session established. Heartbeats
:                                :    begin.




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B.2.  Session Initialization - Refused

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

|                                     Router connects to discovered or
|                                     pre-configured Modem Connection
|---------TCP connect---------->      Point.
|
|                                     Router sends Session Initialization
|-----Session Initialization---->|    message.
                                 |
                                 |    Modem receives Session Initialization
                                 |    message, and will not support the
                                 |    advertised extensions.
                                 |
                                 |    Modem sends Session Initialization
                                 |    Response, with 'Request Denied' status
|<-Session Initialization Resp.--|    data item.
|
|
|                                     Router receives negative Session
|                                     Initialization Response, closes
||---------TCP close------------||    TCP connection.

B.3.  Router Changes IP Addresses

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

|                                     Router sends Session Update message to
|-------Session Update---------->|    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












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Router                       Modem    Signal 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.

B.5.  Router Terminates Session

Router                       Modem    Signal 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 Response
|<---Session Termination Resp.---|    with Status 'Success'.
|
|                                     Modem stops sending messages.
|
||---------TCP close------------||    Session terminated.

B.6.  Modem Terminates Session

















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Router                       Modem    Signal 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 Response
|---Session Termination Resp.--->|    with Status 'Success'.
                                 |
                                 |    Router stops sending messages.
                                 |
||---------TCP close------------||    Session terminated.

B.7.  Session Heartbeats































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

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

         ~ ~ ~ ~ ~ ~ ~

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

         ~ ~ ~ ~ ~ ~ ~

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

         ~ ~ ~ ~ ~ ~ ~

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

B.8.  Router Detects a Heartbeat timeout

Router                       Modem    Signal 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.




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B.9.  Modem Detects a Heartbeat timeout

Router                       Modem    Signal 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 Signal Flows

C.1.  Common Destination Signaling

Router                       Modem    Signal 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 Signaling

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

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


















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Router                       Modem    Signal 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
                                 |    status 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







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

   Email: rick.taylor@airbus.com










































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