Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension
How a DLEP modem can describe links to destinations that sit beyond a gateway, not just on the local Layer 2 network.
Taylor, R. and S. Ratliff, "Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension", RFC 8703, DOI 10.17487/RFC8703, February 2020, <https://www.rfc-editor.org/info/rfc8703>.
@misc{rfc8703,
series = {Request for Comments},
number = 8703,
howpublished = {RFC 8703},
publisher = {RFC Editor},
doi = {10.17487/RFC8703},
url = {https://www.rfc-editor.org/info/rfc8703},
author = {R. Taylor and S. Ratliff},
title = {{Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension}},
pagetotal = 9,
year = 2020,
month = feb,
} RFC text
Contents
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.
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, as they describe your rights and restrictions with respect
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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
AI ELI5
On this page
§1Why Link Identifiers Exist
The Dynamic Link Exchange Protocol (DLEP) lets modems tell attached routers about the state and quality of wireless links. The core specification, RFC 8175, assumes every modem in the radio network has an attached DLEP router, and it requires the MAC address of that router's DLEP interface to identify each destination.
RFC 8703 describes an extension for modems that do not meet that strict requirement. It lets them describe link availability and quality to one or more destinations reachable beyond a device on the Layer 2 domain. As with core DLEP, the router can use this knowledge to influence routing or flow-control decisions, understanding that the traffic flows via Layer 3.
The old rule said every friend must be named by a hardware address. That does not work for friends who live behind another house, so this gives the radio a way to name them anyway.
§1.1Terminology and Use Cases
The RFC defines three terms. The local Layer 2 domain links the router and modem of the current DLEP session. A Layer 3 DLEP Destination is a destination that cannot be addressed directly in that domain but is reachable through a node that can. The Gateway Node is the last device with a MAC address on the path from the router towards that destination. It is commonly the DLEP peer modem, but could be another DLEP Destination in the Layer 2 domain.
The extension was designed mainly for three cases: a radio system that provides Layer 3 connectivity instead of running in Layer 2 bridge mode, a point-to-multipoint tunnel system such as an SD-WAN where the tunnel provider acts as a modem, and a modem such as an LTE device or 802.11 station that links to a remote wide-area network where a Layer 2 reachable remote router does not apply. The RFC says this list is not exhaustive.
The gateway is the front door of a building, and the Layer 3 destination is a person inside. You can only find the door's address, so you also need a label for the person.
§2How Destinations Are Referred To
To refer to a Layer 3 DLEP Destination, a participant adds a Link Identifier Data Item to the relevant Destination Message and, as usual, includes a MAC Address Data Item. Paired with a Link Identifier, the MAC address MUST be that of the Gateway Node. When the router wants to send a packet to the destination through the modem network, it MUST use the Gateway Node's MAC address as the Layer 2 destination.
Only modems know about Layer 3 destinations at first, so a new link MUST first appear in a Destination Up Message from modem to router. A router MUST NOT send a Destination Announce Message with a Link Identifier for a link not mentioned in an earlier Destination Up Message. A modem that receives one MUST terminate the session with a Session Termination Message carrying status code 131 ('Invalid Destination'). If a router receives a Destination Up Message that reuses an existing Link Identifier, it MUST answer with a Destination Up Response Message carrying status code 130 ('Invalid Data') and move to the Session Termination state.
Because the MAC address is not the final destination's address, every Destination Up Message with a Link Identifier MUST carry Layer 3 information: IPv4 or IPv6 Address Data Items, or Attached Subnet Data Items such as 0.0.0.0/0 or ::/0 for upstream backbone networks. Addresses MUST be those used on the public side of any Network Address Translation. Implementations SHOULD also announce at least one valid IP address of the Gateway Node, so the router need not run an extra address resolution protocol before adding the route.
You write the front door's address on the envelope, and then add a name tag for the person inside. If someone mentions a name tag nobody has introduced, the radio hangs up the call.
§2.2Identifier Rules and Negotiation
A Link Identifier is, by default, 4 octets long. A modem that wants a different length announces it with the Link Identifier Length Data Item in its Session Initialization Response Message, and the length MUST stay constant for the whole session. How identifiers are generated is a modem implementation matter, and routers must not assume anything about their meaning.
Within one session, identifiers MUST be unique per MAC address, so a Layer 3 destination is identified by the pair {MAC Address, Link Identifier}. A pair MUST NOT be reused within the lifetime of a single DLEP peer-to-peer session.
As with all DLEP extensions, support is announced at session initialization. A router includes value 3 ('Link Identifiers') in the Extension Data Item of its Session Initialization Message, and a modem does the same in its Session Initialization Response Message. Only if both advertise it can Link Identifier Data Items be used. If a modem needs the extension but the router did not advertise it, the modem MUST NOT send Link Identifier Data Items, and SHOULD NOT immediately end the session. It SHOULD use Session Messages and Attached Subnet Data Items to give general information instead.
Both sides must say 'I know about name tags' before using them. Each tag must be used for one person only, and never handed out again during the same visit.
§3The Two New Data Items
The extension adds two DLEP Data Items. The Link Identifier Length Data Item (type 26) has a Length of 2 and carries one field, the length in octets of Link Identifiers for this session. It is used only in the Session Initialization Response Message, only if the router advertised support, and only if the length is not the default of 4. A router that did not advertise support means it MUST NOT be sent. An item that specifies 4 octets is valid even though it has no effect.
The Link Identifier Data Item (type 27) carries the unique identifier of the Layer 3 DLEP Destination. Its Length is 4 by default, or whatever length was announced at session initialization. The identifier has no implicit meaning and is only used to tell links apart. It MAY be used wherever core DLEP allows a MAC Address Data Item.
One note says how long the name tags will be. The other is the name tag itself, which is just a label so you can tell the links apart.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Data Item Type | 2 bytes | 26 | Identifies the data item. |
LENGTH | Length | 2 bytes | 2 | Length of the data item content. |
LINK-ID-LENGTH | Link Identifier Length | 2 bytes | Length in octets of Link Identifiers used by the modem for this session. |
§4Security Considerations
The RFC adds no security mechanisms of its own. Since it extends core DLEP, the security considerations of RFC 8175 apply, and the document states that none of the features it introduces require extra security considerations from an implementation.
This add-on does not change how safe the system is. The same safety rules as before still apply.
§5IANA Registrations
IANA assigned value 3 ('Link Identifiers') in the Extension Type Values registry, and types 26 ('Link Identifier Length') and 27 ('Link Identifier') in the Data Item Type Values registry. Both registries sit within the Dynamic Link Exchange Protocol (DLEP) Parameters registry, and the new values fall in the range with the 'Specification Required' policy.
The official number list now has three new entries, so nobody else picks the same numbers.
- 3Link Identifiers — Extension Type value announced during session initialization.
- 26Link Identifier Length — Data Item announcing the identifier length.
- 27Link Identifier — Data Item identifying a Layer 3 DLEP Destination.