RFC 8703

Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension

📅 published February 2020, from ietf

Okay bestie, this is RFC 8703 💖, published by the IETF (Internet Engineering Task Force). The authors are R. Taylor from Airbus Defence & Space and S. Ratliff ✨. It's a Standards Track doc, dated February 2020, with ISSN 2070-1721. Its header doesn't list anything it obsoletes or updates, so it doesn't replace or change any other RFC. 🌸

Abstract ✨

Okay so DLEP is the protocol where modems tell attached routers how their wireless links to reachable destinations are doing 📡. The core spec, RFC 8175, assumes every modem in the radio network has an attached DLEP router, and that the MAC address of the DLEP interface on that router gets used to identify the destination. That's how the state and quality of the link to that destination get reported 🦋.

This doc adds an extension for modems that don't fit that strict requirement. With it, they can still use DLEP to describe link availability and quality to one or more destinations reachable beyond a device on the Layer 2 domain. It's giving flexibility 💅

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.

Info about the current status of this document, any errata, and how to give feedback on it can be found at https://www.rfc-editor.org/info/rfc8703 🧚

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

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

Okay so DLEP is, again, the protocol for modems to advertise the status of wireless links between reachable destinations to attached routers 📶. The core spec [RFC8175] assumes that every modem in the radio network has an attached DLEP router. It also requires that the MAC address of the DLEP interface on the attached router gets used to identify the destination in the network, so the state and quality of the link to that destination can be reported.

This doc describes a DLEP extension for modems that don't meet that strict requirement above. It lets them 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 about traffic to this destination. Just know 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 that is 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 it could be another DLEP Destination in the Layer 2 domain 🧚

1.2. Applicability 🦋

This extension was designed mainly for these 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 knows the characteristics of the underlay network. It 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, where the concept of a Layer 2 reachable remote router doesn't apply. An example would be an LTE device or 802.11 station that provides variable connectivity to the Internet 📱

This list of use cases is not exhaustive. This extension may well be applicable to future use cases nobody has thought of yet 🔮

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. It also (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 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 hasn't 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. For 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 lets the DLEP peer router 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 wants 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 wants 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 address 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 wants 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. In other words, 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 is out of scope for this document. Routers must not make any assumptions about the meaning of Link Identifiers or how they are generated 🙈

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

Link Identifiers MUST NOT be reused. That is, 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. Instead, 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. The first is the Link Identifier Length Data Item (Section 3.1), used to announce the length of Link Identifiers at session initialization. The second is the Link Identifier Data Item (Section 3.2), used to identify a Layer 3 link at or beyond a destination 👑

3. New Data Items (continued) 💖

okay so the Link Identifier Length Data Item is how a DLEP modem tells everyone how long its Link Identifiers are for the session 🦋 if the router said "hey, I support Link Identifiers" (that's value 3, 'Link Identifiers', inside the Extension Data Item in the Session Initialization Message), then this Data Item MAY show up in the Session Initialization Response Message, but only if the length isn't the default of 4 octets 🧚 and if the router never said it supported value 3, this Data Item MUST NOT be sent. no exceptions, bestie 🙅‍♀️

    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 the Link Identifiers the DLEP modem is using for this session ✨

a Link Identifier Length Data Item that says 4 octets (the default) is valid 🌸 even though it doesn't actually change anything, lol. it's giving "I'm here, I'm valid, I'm just not doing anything" 🙈

the Link Identifier Data Item MAY be used anywhere the MAC Address Data Item is allowed in core DLEP [RFC8175] 💜 it's basically the MAC Address Data Item's cool cousin 😎

    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. it's 4 by default, but it can be different if a Link Identifier Length Data Item (Section 3.1) was 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 tell multiple links apart 🦋 so don't go reading tea leaves into it, periodt 🫖

4. Security Considerations 🔥

okay this section is short but it's SERIOUS 😭 as an extension to core DLEP [RFC8175], the security considerations of that protocol apply to this extension too 💀 this extension doesn't add any extra security mechanisms or features of its own 🙈

and the good news? none of the features introduced by this extension need any extra security considerations from an implementation 👑 we love a low-maintenance spec ✨

5. IANA Considerations 🌈

IANA assigned one new value to the "Extension Type Values" registry, which lives inside the "Dynamic Link Exchange Protocol (DLEP) Parameters" registry 🎀 this new value falls in the range that uses the "Specification Required" [RFC8126] policy 📋

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

    Table 1: Addition to
     the Extension Type
      Values Registry

IANA also assigned two new values to the "Data Item Type Values" registry, inside that same "Dynamic Link Exchange Protocol (DLEP) Parameters" registry 💖 these also sit 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 💜

6.2. Informative References ✨

Authors' Addresses 🫶

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

Email: rick.taylor@airbus.com

Stan Ratliff

📦 where this page comes from