Mobile Ad hoc Networks Working Group                          S. Ratliff
Internet-Draft                                                VT iDirect
Intended status: Standards Track                                B. Berry
Expires: November 14, 2015 January 7, 2016
                                                                 S. Jury
                                                           Cisco Systems
                                                          D. Satterwhite
                                                                Broadcom
                                                               R. Taylor
                                                  Airbus Defence & Space
                                                            May 13,
                                                            July 6, 2015

                 Dynamic Link Exchange Protocol (DLEP)
                        draft-ietf-manet-dlep-14
                        draft-ietf-manet-dlep-15

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

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   This Internet-Draft will expire on November 14, 2015. January 7, 2016.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   7
     1.2.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Core Features and Optional Extensions  . . . . . . . . . . . .  10
     3.1.  Negotiation of Optional Extensions  . . . . . . . . . . .  10
     3.2.  Protocol Extensions . . . . . . .
     3.1.  Experiments . . . . . . . . . . . .  11
     3.3.  Experimental Signals and Data Items . . . . . . . . . . .  11  10
   4.  Metrics . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Mandatory Metrics . . . . . . . . . . . . . . . . . . . .  12
   5.  DLEP Session Flow . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  DLEP Router session flow -  Peer Discovery case State  . . . . . . . .  13
     5.2.  DLEP Router session flow - Configured case . . . . . . .  13
     5.3.  DLEP Modem session flow . . .  12
     5.2.  Session Initialization State  . . . . . . . . . . . . . .  14
     5.4.  Common Session Flow  13
     5.3.  In-Session State  . . . . . . . . . . . . . . . . . . .  15
   6.  DLEP Signal Structure and Processing  . .  14
     5.4.  Session Termination State . . . . . . . . . .  16
     6.1.  DLEP Signal Header . . . . . .  16
   6.  DLEP Signal and Message Processing  . . . . . . . . . . . . .  16
     6.2.
   7.  DLEP Generic Data Item  . . . Signal and Message Structure . . . . . . . . . . . . . .  17
   7.
     7.1.  DLEP Signals  . . . . Signal Header  . . . . . . . . . . . . . . . . . . .  18
     7.2.  DLEP Message Header .  17
     7.1.  Peer Discovery Signal . . . . . . . . . . . . . . . . . .  18
     7.2.  Peer Offer Signal
     7.3.  DLEP Generic Data Item  . . . . . . . . . . . . . . . . .  19
   8.  DLEP Signals and Messages . . .  19
     7.3.  Peer Initialization Signal . . . . . . . . . . . . . . .  19
     7.4.
     8.1.  Peer Initialization ACK Discovery Signal . . . . . . . . . . . . .  20
     7.5.  Peer Update Signal . . . . .  20
     8.2.  Peer Offer Signal . . . . . . . . . . . . . .  22
     7.6.  Peer Update ACK Signal . . . . . .  21
     8.3.  Session Initialization Message  . . . . . . . . . . .  23
     7.7.  Peer Termination Signal . .  21
     8.4.  Session Initialization Response Message . . . . . . . . .  22
     8.5.  Session Update Message  . . . . . .  24
     7.8.  Peer Termination ACK Signal . . . . . . . . . . .  24
     8.6.  Session Update Response Message . . . .  25
     7.9.  Destination Up Signal . . . . . . . . .  25
     8.7.  Session Termination Message . . . . . . . . .  25
     7.10. Destination Up ACK Signal . . . . . .  25
     8.8.  Session Termination Response Message  . . . . . . . . . .  26
     7.11.
     8.9.  Destination Down Signal Up Message  . . . . . . . . . . . . . . . . .  27
     7.12.  26
     8.10. Destination Down ACK Signal . . Up Response Message . . . . . . . . . . . . .  27
     7.13.
     8.11. Destination Update Signal Down Message  . . . . . . . . . . . . . . . .  28
     7.14. Heartbeat Signal
     8.12. Destination Down Response Message . . . . . . . . . . . .  28
     8.13. Destination Update Message  . . . . . . . .  29
     7.15. Link Characteristics Request Signal . . . . . . .  29
     8.14. Heartbeat Message . . . .  29
     7.16. Link Characteristics ACK Signal . . . . . . . . . . . . .  30
   8.  DLEP Data Items . . .  30
     8.15. Link Characteristics Request Message  . . . . . . . . . .  30
     8.16. Link Characteristics Response Message . . . . . . . . . .  31
     8.1.
   9.  DLEP Version Data Items . . . . . . . . . . . . . . . . . . . . . . .  32
     8.2.
     9.1.  Status  . . . . . . . . . . . . . . . . . . . . . . . . .  33
     8.3.
     9.2.  IPv4 Connection Point . . . . . . . . . . . . . . . . . .  34
     8.4.  35
     9.3.  IPv6 Connection Point . . . . . . . . . . . . . . . . . .  35
     8.5.  36
     9.4.  Peer Type . . . . . . . . . . . . . . . . . . . . . . . .  36
     8.6.  37
     9.5.  Heartbeat Interval  . . . . . . . . . . . . . . . . . . .  36
     8.7.  38
     9.6.  Extensions Supported  . . . . . . . . . . . . . . . . . .  37
     8.8.  Experimental Definition . . . . . . . . . . . . . . . . .  38
     8.9.  39
     9.7.  MAC Address . . . . . . . . . . . . . . . . . . . . . . .  38
     8.10.  39
     9.8.  IPv4 Address  . . . . . . . . . . . . . . . . . . . . . .  39
     8.11.  40
     9.9.  IPv6 Address  . . . . . . . . . . . . . . . . . . . . . .  40
     8.12.  41
     9.10. IPv4 Attached Subnet  . . . . . . . . . . . . . . . . . .  40
     8.13.  42
     9.11. IPv6 Attached Subnet  . . . . . . . . . . . . . . . . . .  41
     8.14.  42
     9.12. Maximum Data Rate (Receive) . . . . . . . . . . . . . . .  42
     8.15.  43
     9.13. Maximum Data Rate (Transmit)  . . . . . . . . . . . . . .  43
     8.16.  44
     9.14. Current Data Rate (Receive) . . . . . . . . . . . . . . .  43
     8.17.  44
     9.15. Current Data Rate (Transmit)  . . . . . . . . . . . . . .  44
     8.18.  45
     9.16. Latency . . . . . . . . . . . . . . . . . . . . . . . . .  45
     8.19.  46
     9.17. Resources (Receive) . . . . . . . . . . . . . . . . . . .  46
     8.20.  47
     9.18. Resources (Transmit)  . . . . . . . . . . . . . . . . . .  46
     8.21.  47
     9.19. Relative Link Quality (Receive) . . . . . . . . . . . . .  47
     8.22.  48
     9.20. Relative Link Quality (Transmit)  . . . . . . . . . . . .  48
     8.23.  49
     9.21. Link Characteristics ACK Response Timer . . . . . . . . . . . . .  48
   9.  49
   10. Credit-Windowing  . . . . . . . . . . . . . . . . . . . . . .  49
     9.1.  50
     10.1.  Credit-Windowing Signals  . Messages  . . . . . . . . . . . . . . .  49
       9.1.1.  51
       10.1.1.  Destination Up Signal Message . . . . . . . . . . . . . . . .  49
       9.1.2.  51
       10.1.2.  Destination Up ACK Signal . . Response Message  . . . . . . . . . . . .  50
       9.1.3.  51
       10.1.3.  Destination Update Signal . Message . . . . . . . . . . . . .  50
     9.2.  51
     10.2.  Credit-Windowing Data Items  . . . . . . . . . . . . . . .  50
       9.2.1.  52
       10.2.1.  Credit Grant . . . . . . . . . . . . . . . . . . . .  51
       9.2.2.  52
       10.2.2.  Credit Window Status . . . . . . . . . . . . . . . .  52
       9.2.3.  53
       10.2.3.  Credit Request . . . . . . . . . . . . . . . . . . .  52
   10.  54
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  53
   11.  55
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  53
     11.1.  55
     12.1.  Registrations  . . . . . . . . . . . . . . . . . . . . .  53
     11.2.  55
     12.2.  Expert Review: Evaluation Guidelines . . . . . . . . . .  54
     11.3.  Signal  56
     12.3.  Signal/Message Type Registration . . . . . . . . . . . . . . . .  54
     11.4.  56
     12.4.  DLEP Data Item Registrations . . . . . . . . . . . . . .  55
     11.5.  56
     12.5.  DLEP Status Code Registrations . . . . . . . . . . . . .  56
     11.6.
     12.6.  DLEP Extensions Registrations  . . . . . . . . . . . . .  56
     11.7.
     12.7.  DLEP Well-known Port . . . . . . . . . . . . . . . . . .  57
     11.8.
     12.8.  DLEP Multicast Address . . . . . . . . . . . . . . . . .  57
   12.
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  57
   13.
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  57
     13.1.
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  57
     13.2.
     14.2.  Informative References . . . . . . . . . . . . . . . . .  57
   Appendix A.  Peer Level  Discovery Signal Flows . . . . . . . . . . . . . .  57
     A.1.  Discovery . . . . . . . . . .  58
   Appendix B.  Peer Level Message Flows . . . . . . . . . . . . . .  57
     A.2.  58
     B.1.  Session Initialization  . . . . . . . . . . . . . . . . .  58
     A.3.
     B.2.  Session Initialization - Refused  . . . . . . . . . . . .  59
     A.4.
     B.3.  Router Changes IP Addresses . . . . . . . . . . . . . . .  59
     A.5.
     B.4.  Modem Changes Session-wide Metrics  . . . . . . . . . . .  59
     A.6.
     B.5.  Router Terminates Session . . . . . . . . . . . . . . . .  60
     A.7.
     B.6.  Modem Terminates Session  . . . . . . . . . . . . . . . .  60
     A.8.
     B.7.  Session Heartbeats  . . . . . . . . . . . . . . . . . . .  61
     A.9.
     B.8.  Router Detects a Heartbeat timeout  . . . . . . . . . . .  62
     A.10.
     B.9.  Modem Detects a Heartbeat timeout . . . . . . . . . . . .  63
   Appendix B. C.  Destination Specific Signal Flows  . . . . . . . . .  63
     B.1.
     C.1.  Common Destination Signaling  . . . . . . . . . . . . . .  63
     B.2.
     C.2.  Multicast Destination Signaling . . . . . . . . . . . . .  64
     B.3.
     C.3.  Link Characteristics Request  . . . . . . . . . . . . . .  64
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  65

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 cable/DSL 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 2 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.
   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 signal message to its router via the DLEP
   protocol.  The signal 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 signal, message, the local router
   may take whatever action it deems appropriate, such as initiating
   discovery protocols, and/or issuing HELLO messages to converge the
   network.  On a continuing, as-needed basis, the modem devices use
   DLEP to report any characteristics of the link (datarate, latency,
   etc.) that have changed.  DLEP is independent of the link type and
   topology supported by the modem.  Note that the DLEP protocol is
   specified to run only on the local link between router and modem.
   Some over the air signaling may be necessary between the local and
   remote modem in order to provide some parameters in DLEP signals 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

1.1.  Protocol Overview

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

   The protocol is defined as a collection of type-length-value (TLV)
   based formats, specifying the signals messages that are exchanged between a
   router and a modem, and the data items associated with the signal. message.
   This document specifies transport of DLEP signals messages and data items via
   the TCP transport, with a UDP-based discovery mechanism.  Other
   transports for the protocol are possible, but are outside the scope
   of this document.

   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.  This router/modem
   session provides a carrier for information exchange concerning
   'destinations' that are available via the modem device.  A
   'destination' can be either physical (as in the case of a specific
   far-end router), or a logical destination (as in a Multicast group).
   As such, all of the destination-level exchanges in DLEP can be
   envisioned as building an information base concerning the remote
   nodes, and the link characteristics to those nodes.

   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.

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

   Routers and modems that exist as part of the same node (e.g., that
   are locally connected) can use a discovery technique to locate each
   other, thus avoiding a priori configuration.  The router is
   responsible for initializing the discovery process, using the Peer
   Discovery signal (Section 7.1). 8.1).

   DLEP uses a session-oriented paradigm.  A router and modem form a
   session by completing the discovery and initialization process.  This
   router-modem session persists unless or until it either (1) times
   out, based on the timeout values supplied, or (2) is explicitly torn
   down by one of the participants.  Note that while use of timers in
   DLEP is optional, it is strongly RECOMMENDED that implementations
   choose to run with timers enabled.

   DLEP assumes that the MAC address for delivering data traffic is the
   MAC specified in the Destination Up signal message (Section 7.9). 8.9).  No
   manipulation or substitution is performed; the MAC address supplied
   in Destination Up is used as the OSI Layer 2 Destination MAC address.
   DLEP also assumes that MAC addresses MUST be unique within the
   context of a router-modem session.  Additionally, 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).

   DLEP uses UDP multicast for single-hop discovery, discovery signalling, and TCP
   for transport of the control signals. 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.

   Destinations can be identified by either the router or the modem, and
   represent a specific destination (e.g., an address) that exists on
   the link(s) managed by the modem.  A destination MUST contain a MAC
   address, it MAY optionally include a Layer 3 address (or addresses).
   Note that since a destination is a MAC address, the MAC could
   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 on
   destinations accessible via the modem.

   The DLEP signals 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.  The
   information base would contain addressing information (i.e.  MAC
   address, and OPTIONALLY, Layer 3 addresses), link characteristics
   (metrics), and OPTIONALLY, flow control information (credits).

   DLEP assumes that any signal message not understood by a receiver MUST
   result in an error indication being sent to the originator, and also
   MUST result in termination of the session between the DLEP peers.
   Any DLEP data item not understood by a receiver MUST also result in
   termination of the 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]).

   This document specifies an implementation of the DLEP signals and
   data items messages
   running over the TCP transport.  It is assumed that DLEP running over
   other transport mechanisms would be documented separately.

3.  Core Features and Optional Extensions

   DLEP has a core set of signals signals, messages and data items that MUST be processed
   parsed without error by an implementation in order to guarantee
   interoperability and therefore make the implementation DLEP
   compliant.  This document defines the core this set of signals signals, messages and
   data items, listing them as 'mandatory'. 'core'.  It should be noted that some
   core signals signals, messages and data items might not be used during the
   lifetime of a single DLEP session, but a compliant implementation
   MUST support them.

   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.  To
   support future extension of DLEP, this document describes an
   extension negotiation capability to be used during session
   initialization via the Extensions Supported data item, documented in
   Section 8.7 of this document.

   All extensions are considered OPTIONAL.  Only the DLEP functionality
   listed as 'mandatory' is required by implementation in order to be
   DLEP compliant.

   This specification defines one extension, Credit windowing, exposed
   via the Extensions Supported mechanism that implementations MAY
   choose to implement, or to omit.

3.1.  Negotiation of Optional Extensions

   Optional extensions supported by an implementation MUST be declared
   to potential DLEP peers using the Extensions Supported data item
   (Section 8.7) during the session initialization sequence.  Once both
   peers have exchanged initialization signals, an implementation MUST
   NOT emit any signal or data item associated with an optional
   extension that was not specified in the received initialization
   signal from its peer.

3.2.  Protocol Extensions

   If/when

   If interoperable protocol extensions are required, they should MUST be
   standardized either as an update to this document, or as an
   additional stand-alone specification.  The requests for IANA-controlled IANA-
   controlled registries in this document contain sufficient reserved Reserved
   space, both in terms of DLEP
   signals and DLEP signals, messages, data items, items and status
   codes, to accommodate future extensions to the protocol and the data
   transferred.

3.3.  Experimental Signals and Data Items

   All extensions are considered OPTIONAL.  Extensions may be negotiated
   on a per-session basis during session initialization via the
   Extensions Supported mechanism.  Only the DLEP functionality listed
   as 'core' is required by an implementation in order to be DLEP
   compliant.

   This specification defines one extension, Credit Windowing, that
   devices MAY choose to implement.

3.1.  Experiments

   This document requests Private Use numbering space in both the DLEP signal and
   signal/message, data item and status code registries for experimental
   items.  The intent is to allow for experimentation with either (1) new signals, (2) new data items,
   or (3) both new signals and new
   messages, data items, and/or status codes, while still retaining the
   documented DLEP behavior.  If a given experiment proves successful,
   it SHOULD be documented as an update to this document, or as a stand-
   alone specification.

   Use of the experimental signals, messages, data items, status codes,
   or behaviors MUST be announced by inclusion of an Experimental Definition data item
   (Section 8.8) as Extensions, using extension
   identifiers from the Private Use space in the Extensions Supported
   registry (Table 4), during session initialization with a value agreed
   upon (a priori) between the participating peers.  The exact mechanism for a priori communication
   of the experimental definition formats is beyond the scope of this
   document.

   Multiple Experimental Definition data items experiments MAY appear be announced in the Peer
   Initialization/Peer Session Initialization ACK sequence.
   messages.  However, use of multiple experiments in a single peer session
   could lead to interoperability issues or unexpected results (e.g., redefinition
   clashes of experimental signals and/or signals, messages, data items), 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 peer session, or to establish a
   precedence of the conflicting definitions) if such conflicts arise.

4.  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 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 'modem-wide' per-session (those that apply to all destinations accessed via
   the modem).  Most metrics can be further subdivided into transmit and
   receive metrics.  Metrics supplied on DLEP Peer signals are, by
   definition, modem-wide;  In cases where metrics supplied on Destination signals are,
   by definition, used 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 specific logical destination only.
   originator.

   DLEP modem implementations MUST announce all supported metric items, items that will
   be reported during the session, and provide default values for those
   metrics, in the Peer Session Initialization ACK signal Response message
   (Section 7.4). 8.4).  In order to introduce use a new metric type, DLEP type that was not included
   in the Session Initialization Response message, modem implementations
   MUST terminate the session with the router (via the Peer Session Terminate signal
   message (Section 7.7)), 8.7)), and
   allow for session re-establishment. establish a new session.

   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).

   The approach of allowing for different contexts for metric data
   increases

   A DLEP participant MAY send metrics both in a session context (via
   the flexibility Session Update message) and the complexity of using metric
   data.  This document details the mechanism whereby the data is
   transmitted, however, the a specific algorithms (precedence, etc.) destination context (via
   Destination Update) at any time.  The heuristics for
   utilizing the dual-context applying
   received metrics are out of scope and not addressed
   by this document. is left to implementations.

4.1.  Mandatory Metrics

   As mentioned above, DLEP modem implementations MUST announce all
   supported metric items during session initialization. the Session Initialization state.
   However, an
   implementation a modem MUST include the following list of metrics: metrics in the
   Session Initialization Response message (Section 8.4):

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

5.  DLEP Session Flow

   For routers supporting DLEP, support of Discovery is optional.
   Discovery is initiated in the

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

   o  Peer Discovery Signal (Section 7.1)

   o  Session Initialization

   o  In-Session

   o  Session Termination

   The Peer Discovery state is OPTIONAL to a well-known multicast address.
   However, support implement for receipt and processing of the signal routers.  If it
   is used, this state is optional
   in the router (see Appendix A for flow diagrams of the discovery
   signal).  Due to the optional (on the router) support for discovery,
   normal session flow initial state.  If it is described for both not used, then
   one or more preconfigured address/port combinations SHOULD be
   provided to the 'Discovery case', router, and the 'Configured case'.  Again, for modem implementations of DLEP, device starts in the Session
   Initialization state.

   Modems MUST support of Discovery is mandatory; therefore, that is the only case
   to be described. Peer Discovery state.

5.1.  DLEP Router session flow -  Peer Discovery case

   If the DLEP router implementation is utilizing the optional discovery
   mechanism, then State

   In the implementation will initialize a UDP socket,
   binding it to an arbitrary port.  This UDP socket is used to Peer Discovery state, routers send the UDP packets containing a
   Peer Discovery signal (Section 7.1) 8.1) to the DLEP link-local well-known multicast
   address (Section 12.8) and port (TBD).  The implementation number (Section 12.7) then await a
   unicast UDP packet containing a Peer Offer signal (Section 8.2) from
   a modem.  While in the Peer Discovery state, Peer Discovery signals
   MUST be sent repeatedly by a router, at regular intervals; every
   three (3) seconds 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 (Section 7.2), which to
   the source address of the received UDP packet.  Peer Offer signals
   MAY contain the unicast address and port for TCP-based communication
   with a DLEP modem, via the IPv4 Connection Point data item (Section 8.3) 9.2)
   or the IPv6 Connection Point data item (Section 8.4). 9.3), on which it is
   prepared to accept an incoming TCP connection.  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.

   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 preceeding 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 MAY
   contain multiple IP Connection Point data items. or from a priori configuration to
   establish a new TCP connection to the modem.  If more than one IP
   Connection Point data items modem
   address/port combinations is in the Peer Offer, available, router implementations MAY
   use their own heuristics to determine the best order in which they are
   tried.  If a TCP connection cannot be achieved using any of the
   address/port combination. combinations and the Discovery mechanism is in use, then
   the router SHOULD resume issuing Peer Discovery signals.  If no IP
   Connection Point data items are included in the Peer Offer signal,
   the receiver router MUST use the origin address of the signal as the IP
   address, and the DLEP well-known port
   number (Section 11.7) to establish the number.

   Once a TCP connection.  At this
   point, connection has been established with the modem, the router implementation MAY either destroy
   begins a new session and enters the UDP socket,
   or continue Session Initialization state.  It
   is up to issue the router implementation if Peer Discovery signals continue
   to be sent after the link-local
   address/port combination.  In either case, the TCP session
   initialization occurs as in the configured case.

5.2.  DLEP Router session flow - Configured case

   When a DLEP router implementation device has the address and port
   information for a TCP connection transitioned to a modem (obtained either via
   configuration or via the discovery process described above), the
   router will initialize and bind a TCP socket.  This socket is used to
   connect to Session
   Initialization state.

5.2.  Session Initialization State

   On entering the DLEP modem software.  After a successful TCP connect, Session Initialization state, the router implementation MUST issue send a Peer
   Session Initialization signal message (Section 7.3) 8.3) to the DLEP modem.  After sending the Peer
   Initialization, the  The
   router implementation MUST then wait for receipt of a
   Peer Session Initialization ACK signal
   Response message (Section 7.4) 8.4) from the modem.  Receipt of the Peer
   Session Initialization ACK signal Response message containing a Status data item
   (Section 8.2) 9.1) with value 'Success', see Table 3, indicates that the
   modem has received and processed the Peer Initialization, Session Initialization message,
   and the session router MUST transition to the 'in session' In-Session state.  At this point, signals
   regarding destinations in

   On entering the network, and/or Peer Update signals
   (Section 7.5), can flow on Session Initialization state, the DLEP session between modem MUST wait for
   receipt of a Session Initialization message from the router.  Upon
   receipt and router,
   and Heartbeat signals can begin to flow, if Heartbeats are used.  The
   'in session' state is maintained until one successful parsing of a Session Initialization message,
   the following
   conditions occur:

   o  The session is explicitly terminated (using Peer Termination), or

   o  The session times out, based on supplied timeout values.

5.3.  DLEP Modem session flow

   DLEP modem implementations MUST support the discovery mechanism.
   Therefore, the normal flow is as follows:

   The implementation will initialize send a UDP socket, binding that socket Session Initialization Response message, and
   the session MUST transition to the In-Session state.

   As mentioned before, DLEP link-local multicast address (TBD) and provides an extension negotiation
   capability to be used in the DLEP well-
   known port number (also TBD).  The Session Initialization state.
   Extensions supported by an implementation will then
   initialize a TCP socket, on a unicast address and port.  This socket
   is used MUST be declared to listen for incoming TCP connection requests.

   When
   potential DLEP peers using the modem Extensions Supported data item
   (Section 9.6).

   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 Peer Discovery signal
   (Section 7.1) on the UDP socket, valid Session
   Initialization Response, it responds by issuing MUST send a Peer Offer
   signal Session Termination message
   (Section 7.2) 8.7) with a relevant status code, e.g.  'Unexpected
   Message', see Table 3, and transition to the sender of the Peer Discovery signal.  The
   Peer Offer signal MAY contain the unicast address and port of Session Termination
   state.

   If the
   listening TCP socket, as described above.  A DLEP modem
   implementation MAY respond with ALL address/port combinations that
   have an active TCP listen posted.  Anything receives any message other than Peer Discovery
   signals received on the UDP socket MUST be silently dropped.

   When Session Initialization,
   or it fails to parse the DLEP modem implementation accepts a connection via TCP, received message, it MUST wait for receipt of a Peer Initialization signal (Section 7.3),
   sent by the router.  Upon receipt NOT send any
   message, and successful parsing of a Peer
   Initialization signal, the modem MUST respond with a Peer
   Initialization ACK signal (Section 7.4).  The terminate the TCP connection, then restart at the
   Peer Discovery state.

   As mentioned before, the Session Initialization ACK
   signal Response message MUST
   contain metric data items for ALL supported metrics. metrics that will be used during
   the session.  If an additional metric is to be introduced, introduced after the
   session has started, the DLEP session between router and modem MUST be
   terminated and restarted, and the new metric described in a Peer Initialization ACK signal.  Once the Peer
   Initialization signal (Section 7.3) and Peer next
   Session Initialization ACK
   signal (Section 7.4) have been exchanged, Response message.

5.3.  In-Session State

   In the session is transitioned In-Session state, messages can flow in both directions between
   peers, indicating changes to the 'in session' state.  As in session state, the router case, when arrival or
   departure of reachable destinations, or changes of the 'in
   session' state is reached, signals regarding destinations in the
   network, and/or Peer Update signals (Section 7.5), can flow on of the
   DLEP session between modem and router, and Heartbeat signals can
   begin
   links to flow, if Heartbeats are used.  The 'in session' state
   persists until the session is explicitly terminated (using Peer
   Termination), or it times out (based on timeout values).

5.4.  Common Session Flow destinations.

   In order to maintain the session between router and modem, In-Session state, periodic Heartbeat signals
   messages (Section 7.14) 8.14) MAY be exchanged. exchanged between router and modem.
   These signals messages are intended to keep the session alive, and to verify
   bidirectional connectivity between the two participants.  If heartbeat signals are
   exchanged, they do not begin until the DLEP peer session has entered
   the 'in session' state.  Each DLEP
   peer is responsible for the creation of heartbeat signals. messages.  Receipt
   of any valid DLEP signal SHOULD message MUST reset the heartbeat interval timer
   (i.e., valid DLEP signals messages take the place of, and obviate the need
   for, Heartbeat signals). messages).

   DLEP also provides a Peer Session Update signal message (Section 7.5), 8.5), intended to
   communicate some change in status (e.g., a change of layer 3 address
   parameters, or a modem-wide link change).

   In addition to the local (Peer level) signals above, session messages, the participants will transmit DLEP signals
   messages concerning destinations in the network.  These signals 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 signal message (Section 7.9). 8.9).  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 signal message
   (Section 7.11), 8.11), and changes in status to the destination (e.g.,
   varying link quality, or addressing changes) are communicated via the
   Destination Update signal message (Section 7.13). 8.13).  The information on a
   given destination will persist in the router's information base until
   (1) a Destination Down signal message is received, indicating that the modem
   has lost contact with the remote node, or (2) the router/modem session
   terminates, indicating that the router has lost contact with its own
   local modem.

   Metrics can be expressed within the context of a specific destination
   via the Destination Update signal, or on a modem-wide basis via the
   Peer Update signal.  In cases where metrics are provided at peer
   level, the receiver MUST propagate the metrics
   transitions to all entries in its
   information base for destinations that are accessed via the
   originator.  A DLEP participant MAY send metrics both in a router/
   modem session context (via the Peer Update signal) and a specific
   destination context (via Destination Update) at any time.  The
   heuristics for applying received metrics is left to implementations. Session Termination state.

   In addition to receiving metrics about the link, DLEP provides a
   signal
   message allowing a router to request a different datarate, or
   latency, from the modem.  This signal message is referred to as the Link
   Characteristics Request signal message (Section 7.15), 8.15), 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 8.7)), or

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

   The implementation MUST 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 8.7) as the result of
   an invalid message or error, it MUST wait for a Session Termination
   Response message (Section 8.8) from its peer.  If Heartbeat messages
   (Section 8.14) 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
   not in use, then if is RECOMMENDED that an interval of eight (8)
   seconds be used.

   When a DLEP 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.  No Destination Down messages
   (Section 8.11) are 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.  DLEP Signal Structure and Message Processing

   Communication between

   Most messages in DLEP peers consists are members of a bidirectional stream request/response pair, e.g.
   Destination Up message (Section 8.9), and Destination Up Response
   message (Section 8.10).  These pairs of messages define an implicit
   transaction model for both session messages and destination messages.

   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 8.3) and Session Initialization Response message
   (Section 8.4) 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.

   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 9.7) may be sent.
   An implementation receiving a message with such an unannounced
   destination MUST terminate the session by issuing a Session
   Termination message (Section 8.7) 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 8.11) and
   Destination Down Response (Section 8.12) 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 down
   destination MUST terminate the session by issuing a Session
   Termination message with a status code of 'Invalid Destination' and
   transition to the Session Termination state.

7.  DLEP Signal and Message Structure

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

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

   All integer values structures MUST be in network byte-order. signal or
   message.

   There is no restriction on the order of data items following a
   signal,
   header, and the multiplicity of duplicate data items is defined by
   the definition of the signal or message declared by the type in the signal
   header.

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

7.1.  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 terminate the session by issuing a
   Peer Termination signal (Section 7.7) with a Status data item
   (Section 8.2) containing the most relevant status code, and then
   close ignore the TCP connection.

6.1. signal.

7.2.  DLEP Signal Message Header

   The DLEP signal 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Signal Message Type                  | Length                        |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: 4: DLEP Signal Message Header

   Signal

   Message Type:  An 8-bit 16-bit unsigned integer containing one of the DLEP
      Signal
      Signal/Message Type values defined in this document.

   Length:  The length, length in octets, expressed as a 16-bit unsigned
      integer, of all of the DLEP data items associated with this signal.
      message.  This length
      does not SHALL NOT include the length of the header itself
      itself.

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

6.2.

   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 8.7) with a Status data item (Section 9.1) containing the
   most relevant status code, and transition to the Session Termination
   state.

7.3.  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| Type                | Length                        | Value...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Value...                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: 5: DLEP Generic Data Item

   Data Item Type:  An 8-bit 16-bit unsigned integer field specifying the type
      of data item being sent.

   Length:  The length, length in octets, expressed as an 8-bit 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 <Length> octets, which contains data specific to a
      particular data item.

7.

8.  DLEP Signals and Messages

   As mentioned above, all DLEP signals begin with the DLEP signal
   header structure.
   header, and all DLEP messages begin with the DLEP message header.
   Therefore, in the following descriptions of specific signals, signals and
   messages, this header structure is assumed, and will not be replicated.

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

   The mandatory core DLEP signals and messages are:

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

   +-------------+-----------------------------------------------------+
   | Signal Type Code   | Description                                         | Mnemonic
   +-------------+-----------------------------------------------------+
   | Section 0           |
   +--------+--------------------+----------------------+--------------+ Reserved                                            | TBD
   | 1           | Peer Discovery     | DLEP_PEER_DISCOVERY  | Section 7.1 signal (Section 8.1)                 |
   | TBD 2           | Peer Offer         | DLEP_PEER_OFFER      | Section 7.2  |
   | TBD    | Peer               | DLEP_PEER_INIT       | Section 7.3 signal (Section 8.2)                     |
   | 3           | Session Initialization message (Section 8.3)        |
   | 4           |
   | TBD    | Peer               | DLEP_PEER_INIT_ACK   | Section 7.4  |
   |        | Session Initialization ACK Response message (Section    |
   |             | 8.4)                                                | TBD
   | Peer Update        | DLEP_PEER_UPDATE 5           | Section 7.5 Session Update message (Section 8.5)                |
   | TBD 6           | Peer Session Update ACK    | DLEP_PEER_UPDATE_ACK | Section 7.6 Response message (Section 8.6)       |
   | TBD 7           | Peer Session Termination   | DLEP_PEER_TERM       | Section 7.7 message (Section 8.7)           |
   | TBD 8           | Peer Session Termination   | DLEP_PEER_TERM_ACK   | Section 7.8  |
   |        | ACK                |                      | Response message (Section 8.8)  |
   | TBD 9           | Destination Up     | DLEP_DEST_UP         | Section 7.9 message (Section 8.9)                |
   | TBD 10          | Destination Up ACK | DLEP_DEST_UP_ACK     | Section 7.10 Response message (Section 8.10)      |
   | TBD 11          | Destination Down   | DLEP_DEST_DOWN       | Section 7.11 message (Section 8.11)             |
   | TBD 12          | Destination Down   | DLEP_DEST_DOWN_ACK   | Section 7.12 |
   |        | ACK                |                      | Response message (Section 8.12)    |
   | TBD 13          | Destination Update | DLEP_DEST_UPDATE     | Section 7.13 message (Section 8.13)           |
   | TBD 14          | Heartbeat          | DLEP_PEER_HEARTBEAT  | Section 7.14 message (Section 8.14)                    |
   | TBD 15          | Link               | DLEP_LINK_CHAR_REQ   | Section 7.15 |
   |        | Characteristics    |                      |              |
   |        | Request message (Section 8.15) |
   |              |
   | TBD 16          | Link Characteristics Response message (Section      | DLEP_LINK_CHAR_ACK
   | Section 7.16             | 8.16)                                               |
   | Characteristics 17-65519    | Reserved for future extensions                      |
   | 65520-65534 | Private Use. Available for experiments              | ACK
   | 65535       | Reserved                                            |
   +--------+--------------------+----------------------+--------------+
   +-------------+-----------------------------------------------------+

                    Table 1: DLEP Signal Values

7.1. Signal/Message types

8.1.  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 7.2) 8.2) is
   required to complete the discovery process.  Implementations MAY
   implement their own retry 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 DLEP_PEER_DISCOVERY in 1, from Table 1.

   The Peer Discovery signal MUST contain the following data item:

   o  DLEP Version (Section 8.1)

   The Peer Discovery signal MAY contain the following data item:

   o  Peer Type (Section 8.5)

7.2. 9.4)

8.2.  Peer Offer Signal

   A Peer Offer signal MUST be sent by a DLEP modem in response to a
   valid Peer Discovery signal (Section 7.1). 8.1).

   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 DLEP_PEER_OFFER in 2, from Table 1.

   The Peer Offer signal MUST contain the following data item:

   o  DLEP Version (Section 8.1)

   The Peer Offer signal MAY contain the following data item:

   o  Peer Type (Section 8.5) 9.4)

   The Peer Offer signal MAY contain one or more of any of the following
   data items, with different values:

   o  IPv4 Connection Point (Section 8.3) 9.2)

   o  IPv6 Connection Point (Section 8.4) 9.3)

   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 11.7) 12.7) to establish the TCP connection.

7.3.  Peer

8.3.  Session Initialization Signal Message

   A Peer Session Initialization signal message MUST be sent by a router as the
   first
   signal 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 Peer Session
   Initialization signal, message, the receiver of the signal message MUST conclude
   that there is NO no support for extensions in the sender.

   If any experimental signals or data items are used by the
   implementation, they MUST be enumerated in one or more Experimental
   Definition data items.  If there are no Experimental Definition data
   items in a Peer Initialization signal, the receiver of the signal
   MUST conclude that no experimental definitions are in use by the
   sender.

   Implementations supporting the Heartbeat Interval (Section 8.6) 9.5)
   should understand that heartbeats are not fully established until
   receipt of Peer Session Initialization ACK Signal Response message (Section 7.4), 8.4), and
   should therefore implement their own timeout and retry heuristics for
   this
   signal. message.

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

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

   o  DLEP Version (Section 8.1)

   o  Heartbeat Interval (Section 8.6) 9.5)

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

   o  Peer Type (Section 8.5) 9.4)

   o  Extensions Supported (Section 8.7)

   The Peer Initialization signal MAY contain one or more of any of the
   following data items, with different values:

   o  Experimental Definition (Section 8.8) 9.6)

   A Peer Session Initialization signal message MUST be acknowledged by the receiver
   issuing a Peer Session Initialization ACK signal Response message (Section 7.4).

7.4.  Peer 8.4).

8.4.  Session Initialization ACK Signal Response Message

   A Peer Session Initialization ACK signal Response message MUST be sent in response to
   a received Peer Session Initialization signal message (Section 7.3). 8.3).  The Peer Session
   Initialization ACK signal Response message completes the DLEP session
   establishment; the sender of the signal message should transition to an 'in-session' the In-
   Session state when the signal message is sent, and the receiver should
   transition to the
   'in-session' In-Session state upon receipt (and successful
   parsing) of an acceptable Peer Session Initialization ACK signal. Response message.

   All supported metric data items MUST be included in the Peer Session
   Initialization ACK signal, 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 signal message containing a metric data item NOT not included in the Peer
   Session Initialization ACK signal Response message MUST be treated as an error,
   resulting in the termination of the DLEP session between router and
   modem.

   If any optional extensions are supported by the modem, they MUST be
   enumerated in the Extensions Supported data item.  If an Extensions
   Supported data item does NOT not exist in a Peer Session Initialization ACK
   signal,
   Response message, the receiver of the signal message MUST conclude that
   there is NO no support for extensions in the sender.

   If any experimental signals or data items are used by the
   implementation, they MUST be enumerated in one or more Experimental
   Definition data items.  If there are no Experimental Definition data
   items in a Peer Initialization ACK signal, the receiver of the signal
   MUST conclude that NO experimental definitions are in use by the
   sender.

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

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

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

   o  DLEP Version (Section 8.1)

   o  Heartbeat Interval (Section 8.6) 9.5)

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

   The Peer Session Initialization ACK signal 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 8.19) 9.17)

   o  Resources (Transmit) (Section 8.20) 9.18)

   o  Relative Link Quality (Receive) (Section 8.21) 9.19)

   o  Relative Link Quality (Transmit) (Section 8.22) 9.20)

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

   o  Status (Section 8.2) 9.1)

   o  Peer Type (Section 8.5) 9.4)

   o  Extensions Supported (Section 8.7)

   The Peer Initialization ACK signal MAY contain one or more of any 9.6)

   A receiver of
   the following a Session Initialization Response message without a
   Status data items, item MUST behave as if a Status data item with different values:

   o  Experimental Definition (Section 8.8)

7.5.  Peer code
   'Success' had been received.

8.5.  Session Update Signal Message

   A Peer Session Update signal 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
   Peer
   Session Update signal 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 Peer Session Update signal 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
   signal
   message (Section 7.13) 8.13) 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 Peer Session
   Update
   Signal message MUST be acknowledged with a Peer Session Update ACK signal Response
   message (Section 7.6). 8.6).

   If metrics are supplied with the Peer Session Update signal 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.

   Supporting implementations are free to employ heuristics to
   retransmit Peer Session Update signals. messages.  The sending of Peer Session Update signals
   messages for Layer 3 address changes SHOULD cease when either
   participant (router or modem) determines that the other
   implementation does NOT not support Layer 3 address tracking.

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

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

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

   o  Resources (Receive) (Section 8.19) 9.17)
   o  Resources (Transmit) (Section 8.20) 9.18)

   o  Relative Link Quality (Receive) (Section 8.21) 9.19)

   o  Relative Link Quality (Transmit) (Section 8.22) 9.20)

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

   o  IPv4 Address (Section 8.10) 9.8)

   o  IPv6 Address (Section 8.11) 9.9)

   A Peer Session Update signal message MUST be acknowledged by the receiver issuing
   a
   Peer Session Update ACK signal Response message (Section 7.6).

7.6.  Peer 8.6).

8.6.  Session Update ACK Signal Response Message

   A Peer Session Update ACK signal Response message MUST be sent by implementations to
   indicate whether a Peer Session Update signal message (Section 7.5) 8.5) was
   successfully received.

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

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

   o  Status (Section 8.2) 9.1)

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

7.7.  Peer

8.7.  Session Termination Signal Message

   A Peer Session Termination signal message MUST be sent by a DLEP participant when
   the router/modem session needs to be terminated.  Implementations
   receiving a Peer Termination signal MUST send a Peer Termination ACK
   signal (Section 7.8) to confirm the termination process.

   The receiver of a Peer Termination signal MUST release all resources
   allocated for the router/modem session, and MUST eliminate all
   destinations in the information base accessible via the router/modem
   pair represented by the session.  Router and modem state machines are
   returned to the 'discovery' state.  No Destination Down signals
   (Section 7.11) are sent.

   The sender of a Peer Termination signal is free to define its
   heuristics in event of a timeout.  It may resend the Peer Termination
   or free resources and return to the 'discovery' state.

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

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

   o  Status (Section 8.2) 9.1)
   A receiver of a Peer Session Termination signal message without a Status data
   item MUST behave as if a Status of 'Unknown reason for Peer Session
   Termination' has been received.

   A Peer Session Termination signal message MUST be acknowledged by the receiver
   issuing a Peer Session Termination ACK signal Response message (Section 7.8).

7.8.  Peer 8.8).

8.8.  Session Termination ACK Signal Response Message

   A Peer Session Termination ACK signal Response message MUST be sent by a DLEP peer in
   response to a received Peer Session Termination signal message (Section 7.7). 8.7).

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

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

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

   o  Status (Section 8.2) 9.1)

   A receiver of a Peer Session Termination ACK signal 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.

7.9.

8.9.  Destination Up Signal Message

   A Destination Up signal 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 signal message MUST be acknowledged by the receiver issuing
   a Destination Up ACK signal Response message (Section 7.10). 8.10).  The sender of the
   Destination Up signal message is free to define its retry heuristics in
   event of a timeout.  When a Destination Up signal 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 signal, message, the Signal Message Type value in the
   signal
   message header is set to DLEP_DEST_UP in 9, from Table 1.

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

   o  MAC Address (Section 8.9) 9.7)

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

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

   o  Resources (Receive) (Section 8.19) 9.17)

   o  Resources (Transmit) (Section 8.20) 9.18)

   o  Relative Link Quality (Receive) (Section 8.21) 9.19)

   o  Relative Link Quality (Transmit) (Section 8.22) 9.20)

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

   o  IPv4 Address (Section 8.10) 9.8)

   o  IPv6 Address (Section 8.11) 9.9)

   o  IPv4 Attached Subnet (Section 8.12) 9.10)

   o  IPv6 Attached Subnet (Section 8.13) 9.11)

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

7.10.

8.10.  Destination Up ACK Signal Response Message

   A DLEP participant MUST send a Destination Up ACK signal Response message to
   indicate whether a Destination Up signal message (Section 7.9) 8.9) was
   successfully processed.

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

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

   o  MAC Address (Section 8.9) 9.7)

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

   o  Status (Section 8.2) 9.1)

   A receiver of a Destination Up ACK signal Response message without a Status data
   item MUST behave as if a Status data item with status code 'Success'
   had been received.  Implementations are free to define retry heuristics
   when receiving a Destination Up ACK signal indicating an error.

7.11.

8.11.  Destination Down Signal Message

   A DLEP peer MUST send a Destination Down signal message to report when a
   destination (a remote node or a multicast group) is no longer
   reachable.  A Destination Down ACK signal Response message (Section 7.12) 8.12) MUST
   be sent by the recipient of a Destination Down signal message to confirm
   that the relevant data has been removed from the information base.
   The sender of the Destination Down signal message is free to define its
   retry heuristics in event of a timeout.

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

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

   o  MAC Address (Section 8.9)

7.12. 9.7)

8.12.  Destination Down ACK Signal Response Message

   A DLEP participant MUST send a Destination Down ACK signal Response message to
   indicate whether a received Destination Down signal message (Section 7.11) 8.11)
   was successfully processed.  If successfully processed, the sender of
   the ACK Response MUST have removed all entries in the information base
   that pertain to the referenced destination.

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

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

   o  MAC Address (Section 8.9) 9.7)
   The Destination Down ACK signal Response message MAY contain one of each of the
   following data items:

   o  Status (Section 8.2) 9.1)

   A receiver of a Destination Down ACK signal Response message without a Status
   data item MUST behave as if a Status data item with status code
   'Success' had been received.  Implementations are free to define retry
   heuristics when receiving a Destination Down ACK signal indicating an
   error.

7.13.

8.13.  Destination Update Signal Message

   A DLEP participant SHOULD send the Destination Update signal 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 signal message are:

   o  Change in link metrics (e.g., Data Rates)

   o  Layer 3 addressing change

   To construct a Destination Update signal, message, the Signal Message Type value in
   the signal message header is set to DLEP_DEST_UPDATE in 13, from Table 1.

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

   o  MAC Address (Section 8.9) 9.7)

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

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

   o  Resources (Receive) (Section 8.19) 9.17)

   o  Resources (Transmit) (Section 8.20) 9.18)

   o  Relative Link Quality (Receive) (Section 8.21) 9.19)

   o  Relative Link Quality (Transmit) (Section 8.22) 9.20)
   The Destination Update signal message MAY contain one or more of the
   following data items, with different values:

   o  IPv4 Address (Section 8.10) 9.8)

   o  IPv6 Address (Section 8.11)

   o  IPv4 Attached Subnet (Section 8.12)

   o  IPv6 Attached Subnet (Section 8.13)

7.14. 9.9)

8.14.  Heartbeat Signal Message

   A Heartbeat signal message SHOULD be sent by a DLEP participant every N
   seconds, where N is defined in the Heartbeat Interval data item of
   the Peer Session Initialization signal message (Section 7.3) 8.3) or Peer Session
   Initialization
   ACK signal Response message (Section 7.4). 8.4).

   Note that implementations setting the Heartbeat Interval to 0
   effectively set sets the interval to an infinite value, therefore, in those cases, therefore this signal
   message SHOULD NOT be sent.

   The signal 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 signal, message, the Signal Message Type value in the signal
   message header is set to DLEP_PEER_HEARTBEAT in 14, from Table 1.

   There are no valid data items for the Heartbeat signal.

7.15. message.

8.15.  Link Characteristics Request Signal Message

   The Link Characteristics Request signal 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 signal 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 ACK signal Response message (Section 7.16) 8.16) 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 ACK) Response) from its
   partner.

   The sender of a Link Characteristics Request signal message MAY attach a
   timer to the request using the Link Characteristics ACK Response Timer
   data item.  If a Link Characteristics ACK signal Response message is received
   after the timer expires, the sender MUST NOT assume that the request
   succeeded.  Implementations are free to define their retry heuristics
   in event of a timeout.

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

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

   o  MAC Address (Section 8.9) 9.7)

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

   o  Link Characteristics ACK Response Timer (Section 8.23) 9.21)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18)

7.16. 9.16)

8.16.  Link Characteristics ACK Signal Response Message

   A DLEP participant MUST send a Link Characteristics ACK signal Response message
   to indicate whether a received Link Characteristics Request signal message
   (Section 7.15) 8.15) was successfully processed.  The Link Characteristics
   ACK signal
   Response message SHOULD contain a complete set of metric data items,
   and MUST contain a full set (i.e. those declared in the Peer Session
   Initialization ACK signal Response message (Section 7.4)), 8.4)), 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 ACK signal 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' Denied', see Table 3, MUST be added to the signal. message.

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

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

   o  MAC Address (Section 8.9) 9.7)

   The Link Characteristics ACK signal Response message SHOULD contain one of each
   of the following data items:

   o  Maximum Data Rate (Receive) (Section 8.14) 9.12)

   o  Maximum Data Rate (Transmit) (Section 8.15) 9.13)

   o  Current Data Rate (Receive) (Section 8.16) 9.14)

   o  Current Data Rate (Transmit) (Section 8.17) 9.15)

   o  Latency (Section 8.18) 9.16)

   The Link Characteristics ACK signal Response message MAY contain one of each of
   the following data items:

   o  Resources (Receive) (Section 8.19) 9.17)

   o  Resources (Transmit) (Section 8.20) 9.18)

   o  Relative Link Quality (Receive) (Section 8.21) 9.19)

   o  Relative Link Quality (Transmit) (Section 8.22) 9.20)

   o  Status (Section 8.2) 9.1)

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

8.

9.  DLEP Data Items

   Following is the list of MANDATORY core data items that must 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.
   signal or message.

   The core DLEP data items are:

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

   +-------------+-----------------------------------------------------+
   | Data Item Type Code   | Description                                         | Section       |
   +------------+--------------------------------------+---------------+
   +-------------+-----------------------------------------------------+
   | TBD        | DLEP Version 0           | Section 8.1 Reserved                                            |
   | TBD 1           | Status                               | Section 8.2 (Section 9.1)                                |
   | TBD 2           | IPv4 Connection Point                | Section 8.3 (Section 9.2)                 |
   | TBD 3           | IPv6 Connection Point                | Section 8.4 (Section 9.3)                 |
   | TBD 4           | Peer Type                            | Section 8.5 (Section 9.4)                             |
   | TBD 5           | Heartbeat Interval                   | Section 8.6 (Section 9.5)                    |
   | TBD 6           | Extensions Supported                 | Section 8.7   |
   | TBD        | Experimental Definition              | Section 8.8 (Section 9.6)                  |
   | TBD 7           | MAC Address                          | Section 8.9 (Section 9.7)                           |
   | TBD 8           | IPv4 Address                         | Section 8.10 (Section 9.8)                          |
   | TBD 9           | IPv6 Address                         | Section 8.11 (Section 9.9)                          |
   | TBD 10          | IPv4 Attached Subnet                 | Section 8.12 (Section 9.10)                 |
   | TBD 11          | IPv6 Attached Subnet                 | Section 8.13 (Section 9.11)                 |
   | TBD 12          | Maximum Data Rate (Receive) MDRR)    | Section 8.14 (Section 9.12)    |
   | TBD 13          | Maximum Data Rate (Transmit) (MDRT)  | Section 8.15 (Section 9.13)  |
   | TBD 14          | Current Data Rate (Receive) (CDRR)   | Section 8.16 (Section 9.14)   |
   | TBD 15          | Current Data Rate (Transmit) (CDRT)  | Section 8.17 (Section 9.15)  |
   | TBD 16          | Latency                              | Section 8.18 (Section 9.16)                              |
   | TBD 17          | Resources (Receive) (RESR)           | Section 8.19 (Section 9.17)           |
   | TBD 18          | Resources (Transmit) (REST)          | Section 8.20 (Section 9.18)          |
   | TBD 19          | Relative Link Quality (Receive)      | Section 8.21  | (RLQR) (Section     |
   | (RLQR)             | 9.19)                                               |
   | TBD 20          | Relative Link Quality (Transmit)     | Section 8.22  | (RLQT) (Section    |
   | (RLQT)             | 9.20)                                               |
   | TBD 21          | Link Characteristics ACK Response Timer (Section 9.21)  | Section 8.23
   |
   +------------+--------------------------------------+---------------+

                      Table 2: DLEP Data Item Values

8.1.  DLEP Version

   The DLEP Version data item MUST appear in the Peer Discovery
   (Section 7.1), Peer Offer (Section 7.2), Peer Initialization
   (Section 7.3) and Peer Initialization ACK 22-24       | Credit Windowing (Section 7.4) signals.  The
   Version data item is used to indicate the version of the protocol
   running in the originator.  A DLEP implementation SHOULD use this
   information to decide if the potential session partner is running at
   a supported level.

   The DLEP Version 10) extension 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ items  | Data Item Type|    Length
   |         Major Version 25-65407    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Reserved for future extensions                      |       Minor Version
   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 65408-65534 | Private Use. Available for experiments              |
   | 65535       | Reserved                                            |
   +-------------+-----------------------------------------------------+

                       Table 2: DLEP Data Item Type:  TBD

   Length:  4

   Major Version:  The major version of the DLEP protocol, expressed as
      an 16-bit unsigned integer.

   Minor Version:  The minor version of the DLEP protocol, expressed as
      an 16-bit unsigned integer.

   Support of this draft is indicated by setting the Major Version to
   '1', and the Minor Version to '0' (i.e.  Version 1.0).

8.2. types

9.1.  Status

   The Status data item MAY appear in the Peer Session Initialization ACK
   Response (Section 7.4), Peer 8.4), Session Termination (Section 7.7), Peer 8.7), Session
   Termination ACK Response (Section 7.8), Peer 8.8), Session Update ACK Response
   (Section 7.6), 8.6), Destination Up ACK Response (Section 7.10), 8.10), Destination
   Down ACK Response (Section 7.12) 8.12) and Link Characteristics ACK Response
   (Section 7.16) signals. 8.16) messages.

   For the Peer Session Termination
   Signal message (Section 7.7), 8.7), the Status data
   item indicates a reason for the termination.  For all acknowledgement signals,
   messages, the Status data item is used to indicate the success or
   failure of the previously received
   signal. message.

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

   The Status data item contains the following fields:

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

   Data Item Type:  TBD

   Length:  1 + Length of text text, in octets

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

   Text:  UTF-8 encoded string, describing an problem, the cause, used for
      implementation defined purposes.  Since this field is used for a
      description of the problem,
      description, implementations SHOULD limit characters in this field
      to printable characters.  Implementations receiving this data item
      SHOULD check for printable characters in the field.

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

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

   +-------------+---------+-----------+-------------------------------+
   | Status Code | Value   | Failure   | Reason                        |
   +----------------+-------+------------------------------------------+
   |             |         | Mode      |                               |
   +-------------+---------+-----------+-------------------------------+
   | Success     | 0       | Success   | The signal message was processed     |
   |             |         |           | successfully.                 |
   | Unknown Signal     | TBD 1       | Terminate | The signal 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 8.3) in the  |
   |             |         |           | In-Session state.             |
   | Invalid Data     | TBD 3       | Terminate | One or more data items in the signal are |
   | Data        |         |           | message are invalid,          |
   |             |         |           | unexpected or incorrectly     |
   |             |         |           | duplicated.                   |
   | Unexpected Invalid     | TBD 4       | Terminate | The signal was destination provided in   |
   | Destination |         |           | the message does not expected while match a  |
   |             |         |           | previously announced          |
   |             |         |           | destination. For example, in  |
   |             |         |           | the Link Characteristic       |
   | Signal             |         | machine was           | Response message (Section     |
   |             |         |           | 8.16).                        |
   | <Reserved>  | 5-90    | Terminate | Reserved for future           |
   |             |         |           | extensions.                   |
   | <Private    | 91-99   | Terminate | Available for experiments.    |
   | Use>        |         |           |                               |
   | Not         | 100     | Continue  | The receiver is not           |
   | Interested  |         |           | interested in this state, e.g., message    |
   |             |         |           | subject, e.g. a Peer Destination   |
   |             |         | Initialization signal after session           | Up Response message (Section  |
   |             |         |           | 8.10) to indicate no further  |
   | establishment.             |         |           | messages about the            |
   |             |         |           | destination.                  |
   | Request Denied     | TBD 101     | Continue  | The receiver has not completed the refuses to       |
   | Denied      |         |           | complete the request.         |
   | Timed Out   | TBD 102     | Continue  | The request operation could not be completed in    |
   |             |         |           | completed in the time         |
   |             |         |           | allowed.                      |
   | Invalid <Reserved>  | TBD 103-243 | The destination provided in the signal Continue  | Reserved for future           | Destination
   |             | does not match a previously announced         |           | extensions.                   |
   | destination. For example, in the Link <Private    | 244-254 | Continue  | Available for experiments.    | Characteristic Request ACK signal
   | Use>        |         |           | (Section 7.16).                               |
   +----------------+-------+------------------------------------------+

8.3.
   | <Reserved>  | 255     | Terminate | Reserved.                     |
   +-------------+---------+-----------+-------------------------------+

                        Table 3: DLEP Status Codes

   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.

9.2.  IPv4 Connection Point

   The IPv4 Connection Point data item MAY appear in the Peer Offer
   signal (Section 7.2). 8.2).

   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 Type                |          IPv4 Address Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv4 Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   TCP Port Number (optional)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  4 (or 6 if TCP Port included)

   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 6, 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 4, the receiver MUST use the DLEP well-known port
   number (Section 11.7) 12.7) to establish the TCP connection.

8.4.

9.3.  IPv6 Connection Point

   The IPv6 Connection Point data item MAY appear in the Peer Offer
   signal (Section 7.2). 8.2).

   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 Type                |          IPv6 Address Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   TCP Port Number (optional)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  16 (or 18 if TCP Port included)

   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 18, 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 16, the receiver MUST use the DLEP well-known
   port number (Section 11.7) 12.7) to establish the TCP connection.

8.5.

9.4.  Peer Type

   The Peer Type data item MAY appear in the Peer Discovery
   (Section 7.1), 8.1) and Peer Offer (Section 7.2), Peer 8.2) signals, and the Session
   Initialization (Section 7.3) 8.3) and Peer Session Initialization ACK Response
   (Section 7.4) signals. 8.4) 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| Type                | Length                        | Peer Type
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Peer Type...                                                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  Length of peer type string. 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 implmentations SHOULD check
      the data for printable characters.

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

8.6.

9.5.  Heartbeat Interval

   The Heartbeat Interval data item MUST appear in both the Peer Session
   Initialization (Section 7.3) 8.3) and Peer Session Initialization ACK Response
   (Section 7.4) signals 8.4) 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
   signals
   messages (Section 7.14). 8.14).  By specifying an Interval value of 0,
   implementations MAY indicate the desire to disable Heartbeat signals messages
   entirely (i.e., the Interval is set to an infinite value).  However,
   it is strongly recommended RECOMMENDED that implementations use non-0 timer values.  Implementations MUST implement heuristics such that DLEP
   signals sent/received reset the timer interval.

   A DLEP session will be considered inactive, and MUST be torn down,
   via the Peer Termination procedure, by an implementation detecting
   that two (2) Heartbeat intervals have transpired without receipt of
   any DLEP signals.

   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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Interval            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  2
   Interval:  0 = Do NOT not use heartbeats on this DLEP session.  Non-zero
      = Interval, in seconds, for heartbeat signals.

8.7. messages.

9.6.  Extensions Supported

   The Extensions Supported data item MAY be used in both the Peer Session
   Initialization (Section 8.3) and Peer Session Initialization ACK signals. Response
   (Section 8.4) 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:

    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| Type                | Length                        | Extensions List
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions List...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

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

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

8.8.  Experimental Definition

   The Experimental Definition data item MAY be used in both the Peer
   Initialization and Peer Initialization ACK signals.  The Experimental
   Definition data item is used by the router and modem to indicate the
   formats to be used for experimental signals and data items for the
   given peer session.  The formats are identified by using a string
   that matches the 'name' given to the experiment.

   The Experimental Definition 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     | Experiment Name               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  Length of the name string for the Experiment.

   Experiment Name:  UTF-8 encoded string, containing the name of the
      experiment being implemented.

   An implementation receiving this data item MUST compare the received
   string to a list of experiments that it supports.

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

8.9.

9.7.  MAC Address

   The MAC address data item MUST appear in all destination-oriented
   signals
   messages (i.e., Destination Up (Section 7.9), 8.9), Destination Up ACK Response
   (Section 7.10), 8.10), Destination Down (Section 7.11), 8.11), Destination Down ACK
   Response (Section 7.12), 8.12), Destination Update (Section 7.13), 8.13), Link
   Characteristics Request (Section 7.15), 8.15), and Link Characteristics ACK
   Response (Section 7.16)). 8.16)).

   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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      MAC Address          |                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                      MAC Address                              |                              :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                MAC Address    :     (if EUI-64 used)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

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

   MAC Address:  MAC Address of the destination.

8.10.

9.8.  IPv4 Address

   The IPv4 Address data item MAY appear in the Peer Session Update
   (Section 7.5), 8.5), Destination Up (Section 7.9) 8.9) and Destination Update
   (Section 7.13) signals. 8.13) messages.

   When included in Destination signals, messages, this data item contains the
   IPv4 address of the destination.  When included in the Peer Session Update signal,
   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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Add/Drop      | IPv4 Address  |
   |               |                                  :
   | Indicator     |               |                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   : IPv4          |
   : Address       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  5
   Add/Drop:  Value indicating whether this is a new or existing address
      (1), or a withdrawal of an address (0).  Values other than 0 or 1
      MUST be considered as invalid.

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

8.11.

9.9.  IPv6 Address

   The IPv6 Address data item MAY appear in the Peer Session Update
   (Section 7.5), 8.5), Destination Up (Section 7.9) 8.9) and Destination Update
   (Section 7.13) signals. 8.13) messages.  When included in Destination signals, messages, this
   data item contains the IPv6 address of the destination.  When
   included in the Peer Session Update signal, 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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Add/Drop    | IPv6 Address  |
   |               |                                  :
   |   Indicator   |               |                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        IPv6 Address                           |                           :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   : IPv6 Address  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  17

   Add/Drop:  Value indicating whether this is a new or existing address
      (1), or a withdrawal of an address (0).  Values other than 0 or 1
      MUST be considered as invalid.

   IPv6 Address:  IPv6 Address of the destination or peer.

8.12.

9.10.  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, or that it has become
   aware of an IPv4 subnet being present at a remote destination.  The
   IPv4 Attached Subnet data item MAY appear in the Destination Up
   (Section 7.9) and Destination Update (Section 7.13) signals. 8.9) message.  Once an IPv4 Subnet has been declared on a
   device, the declaration can SHALL NOT be withdrawn without terminating withdrawing
   the destination (via the Destination Down signal message (Section 7.11)) 8.11)) and
   re-issuing the Destination Up signal. 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
   | Data Item Type                | Length                        | IPv4 Attached Subnet          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv4 Attached Subnet                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Prefix Len.   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  5

   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 speficied range MUST be considered as
      invalid.

8.13.

9.11.  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 7.9) and Destination Update (Section 7.13) signals. 8.9) message.  As in the case of the IPv4 attached Subnet
   data item above, once an IPv6 attached subnet has been declared, it can
   SHALL NOT be withdrawn without
   terminating withdrawing the destination (via the
   Destination Down signal message (Section 7.11)) 8.11)) and re-issuing the
   Destination Up signal. 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 Type                | IPv6 Attached Subnet Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv6 Attached Subnet                                     |                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                   IPv6 Attached Subnet                                     |                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                   IPv6 Attached Subnet                                     |                        :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                   IPv6 Attached Subnet                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Prefix Len.   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  17

   IPv4 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.

8.14.

9.12.  Maximum Data Rate (Receive)

   The Maximum Data Rate (Receive) (MDRR) data item MUST appear in the
   Peer
   Session Initialization ACK signal Response message (Section 7.4), 8.4), and MAY appear
   in the
   Peer Session Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9),
   Destination Update (Section 7.13) 8.13) and Link Characteristics ACK Response
   (Section 7.16)
   signals 8.16) 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 Type                |          MDRR (bps) Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MDRR (bps)                             |                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        MDRR (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Data Item Type:  TBD

   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.

8.15.

9.13.  Maximum Data Rate (Transmit)

   The Maximum Data Rate (Transmit) (MDRT) data item MUST appear in the
   Peer
   Session Initialization ACK signal Response message (Section 7.4), 8.4), and MAY appear
   in the
   Peer Session Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9),
   Destination Update (Section 7.13) 8.13) and Link Characteristics ACK Response
   (Section 7.16)
   signals 8.16) messages to indicate the maximum theoretical data
   rate, in bits per second, that can be achieved while transmitting
   data on the link.

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

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

   Data Item Type:  TBD

   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.

8.16.

9.14.  Current Data Rate (Receive)

   The Current Data Rate (Receive) (CDRR) data item MUST appear in the
   Peer
   Session Initialization ACK signal Response message (Section 7.4), 8.4), and MAY appear
   in the
   Peer Session Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9),
   Destination Update (Section 7.13) 8.13) and Link Characteristics ACK Response
   (Section 7.16)
   signals 8.16) messages to indicate the rate at which the link is
   currently operating for receiving traffic.

   When used in the Link Characteristics Request signal message (Section 7.15), 8.15),
   CDRR represents the desired receive rate, in bits per second, on the
   link.

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

    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 Type                | CDRR (bps) Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        CDRR (bps)                             |                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        CDRR (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   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.

8.17.

9.15.  Current Data Rate (Transmit)

   The Current Data Rate Transmit (CDRT) data item MUST appear in the
   Peer
   Session Initialization ACK signal Response message (Section 7.4), 8.4), and MAY appear
   in the
   Peer Session Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9),
   Destination Update (Section 7.13), 8.13), and Link Characteristics ACK Response
   (Section 7.16)
   signals 8.16) messages to indicate the rate at which the link is
   currently operating for transmitting traffic.

   When used in the Link Characteristics Request signal message (Section 7.15), 8.15),
   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 Type                | CDRT (bps) Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        CDRT (bps)                             |                             :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                        CDRT (bps)                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   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.

8.18.

9.16.  Latency

   The Latency data item MUST appear in the Peer Session Initialization ACK
   signal
   Response message (Section 7.4), 8.4), and MAY appear in the Peer Session Update
   (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9), Destination Update
   (Section 7.13), 8.13), and Link Characteristics ACK Response (Section 7.16) signals 8.16)
   messages to indicate the amount of latency, in microseconds, on the
   link.

   When used in the Link Characteristics Request signal message (Section 7.15), 8.15),
   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 Type                | Latency Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Latency (cont.)                        |                            :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                            Latency (cont.)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Data Item Type:  TBD

   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.

8.19.

9.17.  Resources (Receive)

   The Resources (Receive) (RESR) data item MAY appear in the Peer Session
   Initialization ACK signal Response message (Section 7.4), Peer 8.4), Session Update
   (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9), Destination Update
   (Section 7.13) 8.13) and Link Characteristics ACK Response (Section 7.16) signals 8.16)
   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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RESR      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   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.

8.20.

9.18.  Resources (Transmit)

   The Resources (Transmit) (REST) data item MAY appear in the Peer Session
   Initialization ACK signal Response message (Section 7.4), Peer 8.4), Session Update
   (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9), Destination Update
   (Section 7.13) 8.13) and Link Characteristics ACK Response (Section 7.16) signals 8.16)
   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:

    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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     REST      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   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.

8.21.

9.19.  Relative Link Quality (Receive)

   The Relative Link Quality (Receive) (RLQR) data item MAY appear in
   the Peer Session Initialization ACK signal Response message (Section 7.4), Peer 8.4), Session
   Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9), Destination
   Update (Section 7.13) 8.13) and Link Characteristics ACK Response
   (Section 7.16) signals 8.16) 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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RLQR      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD
   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.

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

8.22.

9.20.  Relative Link Quality (Transmit)

   The Relative Link Quality (Transmit) (RLQT) data item MAY appear in
   the Peer Session Initialization ACK signal Response message (Section 7.4), Peer 8.4), Session
   Update (Section 7.5), 8.5), Destination Up (Section 7.9), 8.9), Destination
   Update (Section 7.13) 8.13) and Link Characteristics ACK Response
   (Section 7.16) signals 8.16) 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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     RLQT      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   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.

8.23.

9.21.  Link Characteristics ACK Response Timer

   The Link Characteristics ACK Response Timer data item MAY appear in the
   Link Characteristics Request signal message (Section 7.15) 8.15) to indicate the
   desired number of seconds the sender will wait for a response to the
   request.  If this data item is omitted, implementations supporting
   the Link Characteristics Request SHOULD choose a default value.

   The Link Characteristics ACK Response Timer 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| Type                | Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Interval      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  1

   Interval:  0 = Do NOT not use timeouts for this Link Characteristics
      request.  Non-zero = Interval, in seconds, to wait before
      considering this Link Characteristics Request lost.

9.

10.  Credit-Windowing

   DLEP includes an OPTIONAL optional Protocol Extension for a credit-windowing
   scheme analogous to the one documented in [RFC5578].  In this scheme,
   data plane traffic flowing between the router and modem is treated controlled
   by the availability of credits.  Credits are expressed as if two
   unidirectional
   windows. windows exist between the modem and router.  This
   document identifies these windows as the 'Modem Receive Window'
   (MRW), and the 'Router Receive Window' (RRW).

   If the OPTIONAL credit-windowing extension is used, credits MUST be granted by
   the receiver on a given window - that is, on the 'Modem Receive
   Window' (MRW), the modem is responsible for granting credits to the
   router, allowing it (the router) to send data plane traffic to the
   modem.  Likewise, the router is responsible for granting credits on
   the RRW, which allows the modem to send data plane traffic to the
   router.

   Credits are managed on a destination-specific basis; that is,
   separate credit counts are maintained for each destination requiring
   the service.  Credits do not apply to the DLEP session that exists
   between routers and modems. modems; they are applied only to the data plane
   traffic.

   Credits represent the number of octets, or an increment in the number
   of octets, that MAY be sent on the given window.  When sending data
   plane traffic to a credit-enabled peer, the sender MUST decriment the
   appropriate window by the size of the data being sent.  For example,
   when sending data plane traffic via the modem, the router MUST
   decriment the 'Modem Receive Window' (MRW) for the corresponding
   destination.  When the number of available credits to the destination
   reaches 0, a sender MUST stop sending data, data plane traffic to the
   destination, until additional credits are supplied.

   If a peer is able to support the OPTIONAL optional credit-windowing extension
   then it MUST include an Extensions Supported data item (Section 8.7) 9.6)
   including the value DLEP_EXT_CREDITS (value TBD) 1, from Table 4, in the appropriate
   Peer Session
   Initialization or Peer (Section 8.3) and Session Initialization ACK signal.

9.1. Response
   (Section 8.4) message.

10.1.  Credit-Windowing Signals Messages

   The credit-windowing extension introduces no additional DLEP signals. signals
   or messages.  However, if a peer has advertised during session
   initialization that it supports the credit-windowing extension then
   the following DLEP
   signals messages MAY contain additional credit-windowing
   data items:

9.1.1.

10.1.1.  Destination Up Signal Message

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

   o  Credit Grant (Section 9.2.1) 10.2.1)

   If the Destination Up signal message does not contain the Credit Grant data
   item, credits MUST NOT be used for that destination.

9.1.2.

10.1.2.  Destination Up ACK Signal Response Message

   If the corresponding Destination Up signal message contained the Credit
   Grant data item, the Destination Up ACK signal Response message MUST contain one
   of each of the following data items:

   o  Credit Window Status (Section 9.2.2)

9.1.3. 10.2.2)

10.1.3.  Destination Update Signal Message

   If the corresponding Destination Up signal message contained the Credit
   Grant data item, the Destination Update signal message MUST contain one of
   each of the following data items:

   o  Credit Window Status (Section 9.2.2) 10.2.2)
   If the corresponding Destination Up signal message contained the Credit
   Grant data item, the Destination Update signal message MAY contain one of
   each of the following data items:

   o  Credit Grant (Section 9.2.1) 10.2.1)

   o  Credit Request (Section 9.2.3)

9.2. 10.2.3)

10.2.  Credit-Windowing Data Items

   The credit-windowing extension introduces 3 additional data items.
   If a peer has advertised during session initialization that it
   supports the credit-windowing extension then it MUST correctly
   process the following data items.

          +------------+-----------------------+----------------+ items:

   +------------+------------------------------------------------------+
   | Data Item Type Code  | Description                                          | Section        |
          +------------+-----------------------+----------------+
   +------------+------------------------------------------------------+
   | TBD 23         | Credit Grant          | Section 9.2.1 (Section 10.2.1)                        |
   | TBD 24         | Credit Window Status  | Section 9.2.2 (Section 10.2.2)                |
   | TBD 25         | Credit Request (Section 10.2.3)                      | Section 9.2.3  |
          +------------+-----------------------+----------------+

9.2.1.
   +------------+------------------------------------------------------+

10.2.1.  Credit Grant

   The Credit Grant data item is sent from a DLEP participant to grant
   an increment to credits on a window.  The Credit Grant data item MAY
   appear in the Destination Up (Section 7.9) 8.9) and Destination Update
   (Section 7.13) signals. 8.13) messages.  The value in a Credit Grant data item
   represents an increment to be added to any existing credits available
   on the window.  Upon successful receipt and processing of a Credit
   Grant data item, the receiver MUST respond with a signal message containing
   a Credit Window Status data item to report the updated aggregate
   values for synchronization purposes, and if initializing a new credit
   window, granting initial credits.

   In

   When DLEP peers desire to employ the Destination Up signal, when credits are desired, credit-windowing extension, the
   originating
   peer originating the Destination Up message MUST set supply an initial,
   non-zero value as the initial credit value increment of the receive window it
   controls (i.e., the Modem Receive Recive Window, or Router Receive Window)
   to an initial, non-zero value.  If the receiver of a Destination Up
   signal with Window).
   When receiving a Credit Grant data item supports credits, on a Destination Up
   (#msg_dest_up) message, the receiver MUST either reject take one of the following
   actions:

   1.  Reject the use of credits for this destination, via a the
       Destination Up ACK response Response message containing a Status data item
       (Section 8.2) 9.1) with a status code of 'Request Denied', Denied'.  (See
       Table 3), or set

   2.  Initialize the
   initial appropriate window value from of zero, then apply the data contained
       increment specified in the Credit Window Status Grant data item.

   If the initialization completes successfully, the receiver MUST
   respond to the Destination Up signal message with a Destination Up ACK signal Response
   message that contains a Credit Window Status data item, initializing
   its receive window.

   The Credit Grant 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 Type                |       Credit Increment Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Credit Increment                         |                         :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                      Credit Increment                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  8

   Reserved:  A 64-bit unsigned integer representing the additional
      credits to be assigned to the credit window.

   Since credits can only be granted by the receiver on a window, the
   applicable credit window (either the MRW or the RRW) is derived from
   the sender of the grant.  The Credit Increment MUST NOT cause the
   window to overflow; if this condition occurs, implementations MUST
   set the credit window to the maximum value contained in a 64-bit
   quantity.

9.2.2.

10.2.2.  Credit Window Status

   If the credit-window extension is supported by the DLEP participants
   (both the router and the modem), the Credit Window Status data item
   MUST be sent by the participant receiving a Credit Grant for a given
   destination.

   The Credit Window 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 Type                | Modem Receive Window Value Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Modem Receive Window Value                  |                  :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                   Modem Receive Window Value                  | Router Receive Window Value   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Router Receive Window Value                    |                 :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |
   :                   Router Receive Window Value                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  16

   Modem Receive Window Value:  A 64-bit unsigned integer, indicating
      the current number of credits available on the Modem Receive
      Window, for the destination referred to by the signal. message.

   Router Receive Window Value:  A 64-bit unsigned integer, indicating
      the current number of credits available on the Router Receive
      Window, for the destination referred to by the signal.

9.2.3. message.

10.2.3.  Credit Request

   The Credit Request data item MAY be sent from either DLEP
   participant, via the Destination Update signal message (Section 7.13), 8.13), to
   indicate the desire for the partner to grant additional credits in
   order for data transfer to proceed on the session.  If the
   corresponding Destination Up signal message (Section 7.9) 8.9) for this session
   did NOT not contain a Credit Window Status data item, indicating that
   credits are to be used on the session, then the Credit Request data
   item MUST be silently dropped by the receiver.

   The Credit Request 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     | Reserved, MUST|
   |               | Type                | be set to 0 Length                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  TBD

   Length:  1

   Reserved:  This field is currently unused and MUST be set to 0.

10.  0

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.

   The protocol itself does not contain any mechanisms for security
   (e.g., authentication or encryption).  The protocol encryption), as it assumes that any
   security would be implemented in the underlying transport (for
   example, by use an
   appropriate level of TLS or some other mechanism), authentication and non-repudiation is therefore
   outside the scope acheived
   by use of this document.

11. [TLS] when necessary.  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.

11.1.

12.1.  Registrations

   This specification defines:

   o  A new repository for DLEP signals, signals and messages, with sixteen (16)
      values currently assigned.

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

   o  A new repository for DLEP data items, with twenty-six 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 seven 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.

   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.

11.2.

12.2.  Expert Review: Evaluation Guidelines

   No additional guidelines for expert review are anticipated.

11.3.  Signal

12.3.  Signal/Message Type Registration

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

   All signal and message values are in the range [0..255].

   Valid signals are:

   o  Peer Discovery

   o  Peer Offer

   o  Peer Initialization

   o  Peer Initialization ACK

   o  Peer Update

   o  Peer Update ACK

   o  Peer Termination

   o  Peer Termination ACK

   o  Destination Up

   o  Destination Up ACK

   o  Destination Down

   o  Destination Down ACK

   o  Destination Update

   o  Heartbeat
   o  Link Characteristics Request

   o  Link Characteristics ACK

   It is also requested that the repository contain space for
   experimental signal types.

11.4. [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..255].

   Valid data items are:

   o  DLEP Version

   o  Status

   o  IPv4 Connection Point

   o  IPv6 Connection Point

   o  Peer Type

   o  Heartbeat Interval

   o  Extensions Supported

   o  Experimental Definition

   o  MAC Address

   o  IPv4 Address

   o  IPv6 Address

   o  IPv4 Attached Subnet

   o  IPv6 Attached Subnet

   o  Maximum Data Rate (Receive)

   o  Maximum Data Rate (Transmit)

   o  Current Data Rate (Receive)

   o  Current Data Rate (Transmit)
   o  Latency

   o  Resources (Receive)

   o  Resources (Transmit)

   o  Relative Link Quality (Receive)

   o  Relative Link Quality (Transmit)

   o  Link Characteristics ACK Timer

   o  Credit Window Status

   o  Credit Grant

   o  Credit Request

   It is also requested that the registry allocation contain space for
   experimental data items.

11.5. [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].

   Valid status codes are:

   o  Success (value 0)

   o  Unknown Signal

   o  Invalid Data

   o  Unexpected Signal

   o  Request Denied

   o  Timed Out

   o  Invalid Destination

11.6. [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..255].

   Valid extensions [0..65535].  Current
   allocations are:

   o  DLEP_EXT_CREDITS -

   +-------------+-----------------------------------------------------+
   | Code        | Description                                         |
   +-------------+-----------------------------------------------------+
   | 0           | Reserved                                            |
   | 1           | Credit windowing

11.7. Windowing (Section 10)                       |
   | 2-65519     | Reserved for future extensions                      |
   | 65520-65534 | Private Use. Available for experiments              |
   | 65535       | Reserved                                            |
   +-------------+-----------------------------------------------------+

                       Table 4: DLEP Extension types

12.7.  DLEP Well-known Port

   It is requested that IANA allocate a well-known port number for DLEP
   communication.

11.8.

12.8.  DLEP Multicast Address

   It is requested that IANA allocate a multicast address for DLEP
   discovery signals.

12.

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.

13.

14.  References

13.1.

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

13.2.

14.2.  Informative References

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5578]  Berry, B., Ratliff, S., Paradise, E., Kaiser, T., and M.
              Adams, "PPP over Ethernet (PPPoE) Extensions for Credit
              Flow and Link Metrics", RFC 5578, February 2010.

Appendix A.  Peer Level  Discovery Signal Flows

A.1.  Discovery

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.

A.2.

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 Peer Session Initialization
|-------Peer
|----Session Initialization----->|    signal.    message.
                                 |
                                 |    Modem receives Peer Session Initialization
                                 |    signal.    message.
                                 |
                                 |    Modem sends Peer Session Initialization
                                 |    ACK, with compatible extensions,
|<----Peer
|<--Session Initialization ACK----|    and Resp.-|    Response, with Success status data item.
|                                |
|<<============================>>|    Session established. Heartbeats
:                                :    begin.

A.3.

B.2.  Session Initialization - Refused

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

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

A.4.

B.3.  Router Changes IP Addresses

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

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

A.5. Response.

B.4.  Modem Changes Session-wide Metrics
Router                       Modem    Signal Description
========================================================================

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

A.6. Response.

B.5.  Router Terminates Session

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

|                                     Router sends Peer Session Termination
|-------Peer Termination-------->|    signal
|------Session Termination------>|    message with Status data item.
|                                |
|-------TCP shutdown (send)--->  |    Router stops sending signals. messages.
                                 |
                                 |    Modem receives Peer Session Termination,
                                 |    stops counting received heartbeats
                                 |    and stops sending heartbeats.
                                 |
                                 |    Modem sends Peer Session Termination ACK
|<-----Peer Response
|<---Session Termination ACK------| Resp.---|    with Status 'Success'.
|
|
|  <----TCP shutdown (send)------|                                     Modem stops sending signals. messages.
|
||------------------------------||
||---------TCP close------------||    Session terminated.

A.7.

B.6.  Modem Terminates Session
Router                       Modem    Signal Description
========================================================================

                                 |    Modem sends Peer Session Termination
|<------Peer Termination---------|    signal
|<----Session Termination--------|    message with Status data item.
|
|
|  <----TCP shutdown (send)------|                                     Modem stops sending signals. messages.
|
|                                     Router receives Peer Session Termination,
|                                     stops counting received heartbeats
|                                     and stops sending heartbeats.
|
|                                     Router sends Peer Session Termination ACK
|------Peer Response
|---Session Termination ACK----->| Resp.--->|    with Status 'Success'.
                                 |
                                 |
|-------TCP shutdown (send)--->  |    Router stops sending signals. messages.
                                 |
||------------------------------||
||---------TCP close------------||    Session terminated.

A.8.

B.7.  Session Heartbeats
Router                       Modem    Signal Description
========================================================================

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

         ~ ~ ~ ~ ~ ~ ~

|----------[Any signal]--------->|

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

         ~ ~ ~ ~ ~ ~ ~

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

         ~ ~ ~ ~ ~ ~ ~

|<---------[Any signal]----------|

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

A.9.

B.8.  Router Detects a Heartbeat timeout

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

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

|       ||<----------------------|    Router misses too many heartbeats
|
|
|-------Peer Termination-------->|
|------Session Termination------>|    Router sends Peer Session Termination
|                                     signal                                     message with 'Timeout' Status
|                                     data item.
:
:                                     Termination proceeds as above.

A.10.

B.9.  Modem Detects a Heartbeat timeout

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

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

|---------------------->||       |    Modem misses too many heartbeats
                                 |
                                 |
|<-------Peer Termination--------|
|<-----Session Termination-------|    Modem sends Peer Session Termination
                                 |    signal    message with 'Timeout' Status
                                 |    data item.
                                 :
                                 :    Termination proceeds as above.

Appendix B. C.  Destination Specific Signal Flows

B.1.

C.1.  Common Destination Signaling

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

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

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

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

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

B.2. Response message.

C.2.  Multicast Destination Signaling

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

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

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

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

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

B.3. Response message.

C.3.  Link Characteristics Request
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 signal. message.
                                 |
                                 |    Modem attempts to adjust link
                                 |    status to meet the received
                                 |    request, and sends a Link
                                 |    Characteristics Request ACK Response
|<---Link Char. Request ACK------|    signal Characteristics Resp.--|    message with the new values.

Authors' Addresses

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

   Email: sratliff@idirect.net

   Bo Berry

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

   Email: sjury@cisco.com

   Darryl Satterwhite
   Broadcom

   Email: dsatterw@broadcom.com
   Rick Taylor
   Airbus Defence & Space
   Quadrant House
   Celtic Springs
   Coedkernew
   Newport  NP10 8FZ
   UK

   Email: rick.taylor@airbus.com