



Internet Engineering Task Force (IETF)                         R. Taylor
Request for Comments: 8703                        Airbus Defence & Space
Category: Standards Track                                     S. Ratliff
ISSN: 2070-1721                                            February 2020


    Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension

Abstract

   The Dynamic Link Exchange Protocol (DLEP) is a protocol for modems to
   advertise the status of wireless links between reachable destinations
   to attached routers.  The core specification of the protocol (RFC
   8175) assumes that every modem in the radio network has an attached
   DLEP router and requires that the Media Access Control (MAC) address
   of the DLEP interface on the attached router be used to identify the
   destination in the network, for purposes of reporting the state and
   quality of the link to that destination.

   This document describes a DLEP extension that allows modems that do
   not meet the strict requirement above to use DLEP to describe link
   availability and quality to one or more destinations reachable beyond
   a device on the Layer 2 domain.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8703.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Applicability
     1.3.  Requirements Language
   2.  Operation
     2.1.  Identifier Restrictions
     2.2.  Negotiation
   3.  New Data Items
     3.1.  Link Identifier Length Data Item
     3.2.  Link Identifier Data Item
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Authors' Addresses

1.  Introduction

   The Dynamic Link Exchange Protocol (DLEP) is a protocol for modems to
   advertise the status of wireless links between reachable destinations
   to attached routers.  The core specification of the protocol
   [RFC8175] assumes that every modem in the radio network has an
   attached DLEP router and requires that the MAC address of the DLEP
   interface on the attached router be used to identify the destination
   in the network, for purposes of reporting the state and quality of
   the link to that destination.

   This document describes a DLEP extension that allows modems that do
   not meet the strict requirement above to use DLEP to describe link
   availability and quality to one or more destinations reachable beyond
   a device on the Layer 2 domain.

   As with core DLEP [RFC8175], a router can use this knowledge to
   influence any routing or flow-control decisions regarding traffic to
   this destination, understanding that such traffic flows via Layer 3.

1.1.  Terminology

   Local Layer 2 domain:  The Layer 2 domain that links the router and
      modem participants of the current DLEP session.

   Layer 3 DLEP Destination:  A DLEP Destination that is not directly
      addressable within the local Layer 2 domain but is reachable via a
      node addressable within the local Layer 2 domain.

   Gateway Node:  The last device with a MAC address reachable in the
      local Layer 2 domain on the path from the DLEP router participant
      towards the Layer 3 DLEP Destination.  This device is commonly the
      DLEP peer modem but could be another DLEP Destination in the Layer
      2 domain.

1.2.  Applicability

   This extension was designed primarily to address the following use
   cases:

   1.  A radio system that does not operate in Layer 2 bridge mode but
       instead provides Layer 3 connectivity between destinations, often
       using its own embedded Layer 3 routing function.

   2.  A point-to-multipoint tunnel system, such as a software-defined
       wide-area network (SD-WAN) deployment, where the tunnel provider
       acts as a modem that has knowledge of the characteristics of the
       underlay network and provides that information as availability
       and metrics between tunnel endpoints in the overlay network.

   3.  A modem that provides connectivity to a remote wide-area network
       via a wireless link, but the concept of a Layer 2 reachable
       remote router does not apply.  An example of such a modem would
       be an LTE device or 802.11 station that provides variable
       connectivity to the Internet.

   This list of use cases is not exhaustive, and this extension may well
   be applicable to future, currently unforeseen, use cases.

1.3.  Requirements Language

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

2.  Operation

   To refer to a Layer 3 DLEP Destination, the DLEP session participant
   adds a Link Identifier Data Item (Section 3.2) to the relevant
   Destination Message and (as usual) includes a MAC Address Data Item.
   When paired with a Link Identifier Data Item, the MAC Address Data
   Item MUST contain the MAC address of the Gateway Node.

   As only modems are initially aware of Layer 3 DLEP Destinations, Link
   Identifier Data Items referring to a new link MUST first appear in a
   DLEP Destination Up Message from the modem to the router.  Once a
   link has been identified in this way, Link Identifier Data Items may
   be used by either DLEP participant during the lifetime of a DLEP
   session.  Because of this, a router MUST NOT send a DLEP Destination
   Announce Message containing a Link Identifier Data Item referring to
   a link that has not been mentioned in a prior DLEP Destination Up
   Message.  If a modem receives such a message, it MUST terminate the
   session by issuing a Session Termination Message containing a Status
   Data Item with status code set to 131 ('Invalid Destination') and
   transition to the Session Termination state.  If a router receives a
   Destination Up Message specifying a Link Identifier that has already
   been used, the router MUST respond with a Destination Up Response
   Message containing a Status Data Item with status code set to 130
   ('Invalid Data') and transition to the Session Termination state.

   Because the MAC address associated with any DLEP Destination Message
   containing a Link Identifier Data Item is not the Layer 2 address of
   the final destination, all DLEP Destination Up Messages containing a
   Link Identifier Data Item MUST contain Layer 3 information.  In the
   case of modems that provide Layer 3 wide area network connectivity
   between devices, this means one or more IPv4 or IPv6 Address Data
   Items providing the Layer 3 address of the final destination.  When
   referring to some upstream backbone network infrastructures, this
   means one or more IPv4 or IPv6 Attached Subnet Data Items, for
   example: '0.0.0.0/0' or '::/0'.  This mechanism allows the DLEP peer
   router to understand the properties of the link to those routes.  The
   address or addresses in the IPv4 or IPv6 Address Data Items MUST be
   the addresses in use on the public side of any Network Address
   Translation.

   When the DLEP peer router wishes to route packets to the Layer 3 DLEP
   Destination, the MAC address associated with the Gateway Node MUST be
   used as the Layer 2 destination of the packet if it wishes to use the
   modem network to forward the packet.

   As routers populate their Routing Information Base with the IP
   address of the next-hop router towards a destination, implementations
   supporting this extension SHOULD announce at least one valid IPv4 or
   IPv6 addresses of the Gateway Node; this removes the need for the
   router to use an additional IP address resolution protocol before
   adding the route to its Routing Information Base.

2.1.  Identifier Restrictions

   A Link Identifier is, by default, 4 octets in length.  If a modem
   wishes to use a Link Identifier of a different length, it MUST be
   announced using the Link Identifier Length Data Item (Section 3.1)
   contained in the DLEP Session Initialization Response Message sent by
   the modem to the router.

   During the lifetime of a DLEP session, the length of Link Identifiers
   MUST remain constant, i.e., the Length field of the Link Identifier
   Data Item MUST NOT differ between destinations.

   The method for generating Link Identifiers is a modem implementation
   matter and out of scope of this document.  Routers must not make any
   assumptions about the meaning of Link Identifiers or how Link
   Identifiers are generated.

   Within a single DLEP session, all Link Identifiers MUST be unique per
   MAC address.  This means that a Layer 3 DLEP Destination is uniquely
   identified by the pair: {MAC Address,Link Identifier}.

   Link Identifiers MUST NOT be reused, i.e., a {MAC Address,Link
   Identifier} pair that has been used to refer to one Layer 3 DLEP
   Destination MUST NOT be used again within the lifetime of a single
   DLEP peer-to-peer session.

2.2.  Negotiation

   To use this extension, as with all DLEP extensions, the extension
   MUST be announced during DLEP session initialization.  A router
   advertises support by including the value 3 ('Link Identifiers')
   (Section 5), in the Extension Data Item within the Session
   Initialization Message.  A modem advertises support by including the
   value 3 ('Link Identifiers') in the Extension Data Item within the
   Session Initialization Response Message.  If both DLEP peers
   advertise support for this extension, then Link Identifier Data Items
   can be included in DLEP Messages.

   If a modem requires support for this extension in order to describe
   destinations and the router does not advertise support, then the
   modem MUST NOT include a Link Identifier Data Item in any DLEP
   Message.  However, the modem SHOULD NOT immediately terminate the
   DLEP session; rather, it SHOULD use a combination of DLEP Session
   Messages and DLEP Attached Subnet Data Items to provide general
   information.

3.  New Data Items

   This extension introduces two new DLEP Data Items: 1) the Link
   Identifier Length Data Item (Section 3.1) used to announce the length
   of Link Identifiers at session initialization and 2) the Link
   Identifier Data Item (Section 3.2) used to identify a Layer 3 link at
   or beyond a destination.

3.1.  Link Identifier Length Data Item

   The Link Identifier Length Data Item is used by a DLEP modem
   implementation to specify the length of Link Identifier Data Items.
   If the router advertised support by including the value 3 ('Link
   Identifiers') in the Extension Data Item inside the Session
   Initialization Message, this Data Item MAY be used in the Session
   Initialization Response Message if the specified length is not the
   default value of 4 octets.  If the router did not specify support by
   including the value 3 ('Link Identifiers') in the Extension Data
   Item, this Data Item MUST NOT be sent.

    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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Identifier Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  26 (see Section 5)

   Length:  2

   Link Identifier Length:  The length, in octets, of Link Identifiers
      used by the DLEP modem for this session.

   A Link Identifier Length Data Item that specifies a Link Identifier
   Length of 4 octets (the default) is valid, even if it has no effect.

3.2.  Link Identifier Data Item

   The Link Identifier Data Item MAY be used wherever a MAC Address Data
   Item is defined as usable in core DLEP [RFC8175].

    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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Link Identifier...                          :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Item Type:  27 (see Section 5)

   Length:  The length of the Data Item, by default 4, but may be
      different if a Link Identifier Length Data Item (Section 3.1) has
      been announced during session initialization.

   Link Identifier:  The unique identifier of the Layer 3 DLEP
      Destination.  This Link Identifier has no implicit meaning and is
      only used to discriminate between multiple links.

4.  Security Considerations

   As an extension to core DLEP [RFC8175], the security considerations
   of that protocol apply to this extension.  This extension adds no
   additional security mechanisms or features.

   None of the features introduced by this extension require extra
   security considerations by an implementation.

5.  IANA Considerations

   IANA has assigned the following value to the "Extension Type Values"
   registry within the "Dynamic Link Exchange Protocol (DLEP)
   Parameters" registry.  This new value is in the range with the
   "Specification Required" [RFC8126] policy.

   +------+------------------+
   | Code | Description      |
   +======+==================+
   | 3    | Link Identifiers |
   +------+------------------+

       Table 1: Addition to
        the Extension Type
         Values Registry

   IANA has assigned two new values to the "Data Item Type Values"
   registry within the "Dynamic Link Exchange Protocol (DLEP)
   Parameters" registry.  These new values are in the range with the
   "Specification Required" [RFC8126] policy.

   +-----------+------------------------+
   | Type Code | Description            |
   +===========+========================+
   | 26        | Link Identifier Length |
   +-----------+------------------------+
   | 27        | Link Identifier        |
   +-----------+------------------------+

       Table 2: Additions to the Data
         Item Type Values Registry

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,
              <https://www.rfc-editor.org/info/rfc8175>.

6.2.  Informative References

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Authors' Addresses

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

   Email: rick.taylor@airbus.com


   Stan Ratliff
