DLEP Link Identifier Extension
draft-ietf-manet-dlep-lid-extension-05

Abstract

The Dynamic Link Exchange Protocol (DLEP) describes a protocol for modems to advertise the status of wireless links between reachable destinations to attached routers. The core specification of the protocol 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 allowing 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on January 27, 2020.

Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Terminology
• 1.2. Applicability
• 1.3. Requirements
• 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. Normative References
• Authors' Addresses

1. Introduction

The Dynamic Link Exchange Protocol (DLEP) describes a protocol for modems to advertise the status of wireless links between reachable destinations to attached routers. The core specification of the protocol 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 allowing 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, 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 an SD-WAN deployment, where the tunnel provider acts as a modem, having knowledge of the characteristics of the underlay network, and providing 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

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 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.

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 infrastructure, this means one or more IPv4 or IPv6 Attached Subnet Data Items, for example: '0.0.0.0/0' or '::/0'. This allows the DLEP peer router to understand the properties of the link to those routes.

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 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 recycled to refer to a different destination within the lifetime of a single DLEP 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 'Link Identifiers', TBD1, in the Extension Data Item within the Session Initialization Message. A modem advertises support by including the value '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: the Link Identifier Data Item used to identify a Layer 3 link at or beyond a destination, and the Link Identifier Length Data Item used to announce the length of Link Identifiers at session initialization.

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. It MUST be used during Session Initialization, contained in a Session Initialization Response Message, if the specified length is not the default value of 4 octets.

 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:

TBD2

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.

 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:

TBD3

Length:

The length of the Data Item, by default 4, but may be different if a Link Identifier Length Data Item 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 the core DLEP protocol, 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 consideration by an implementation.

5. IANA Considerations

Upon approval of this document, IANA is requested to:

• Assign a new DLEP Extensions Registry value (TBD1) from the Specification Required section, named "Link Identifiers".
• Assign a new DLEP Data Item Type Values Registry value (TBD2) from the Specification Required section, named "Link Identifier Length".
• Assign a new DLEP Data Item Type Values Registry value (TBD3) from the Specification Required section, named "Link Identifier".

6. Normative References

Authors' Addresses