DLEP Diffserv credits: Dynamic Link Exchange Protocol (DLEP) Diffserv Aware Credit Window Extension
How a modem tells a router which queue to send to, matched by destination and Diffserv class.
Cheng, B., Wiggins, D., Berger, L., and D. Eastlake 3rd, Ed., "Dynamic Link Exchange Protocol (DLEP) Diffserv Aware Credit Window Extension", RFC 9894, DOI 10.17487/RFC9894, January 2026, <https://www.rfc-editor.org/info/rfc9894>.
@misc{rfc9894,
series = {Request for Comments},
number = 9894,
howpublished = {RFC 9894},
publisher = {RFC Editor},
doi = {10.17487/RFC9894},
url = {https://www.rfc-editor.org/info/rfc9894},
author = {B. Cheng and D. Wiggins and L. Berger and D. Eastlake 3rd},
title = {{Dynamic Link Exchange Protocol (DLEP) Diffserv Aware Credit Window Extension}},
pagetotal = 6,
year = 2026,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) B. Cheng
Request for Comments: 9894 MIT Lincoln Laboratory
Category: Standards Track D. Wiggins
ISSN: 2070-1721
L. Berger
LabN Consulting, L.L.C.
D. Eastlake 3rd, Ed.
Independent
January 2026
Dynamic Link Exchange Protocol (DLEP) Diffserv Aware Credit Window
Extension
Abstract
This document defines an extension to the Dynamic Link Exchange
Protocol (DLEP) that enables a Diffserv aware credit window scheme
for destination-specific and shared flow control.
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/rfc9894.
Copyright Notice
Copyright (c) 2026 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
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Key Words
2. Extension Usage and Identification
3. Management Considerations
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
The Dynamic Link Exchange Protocol (DLEP) is defined in [RFC8175].
The protocol provides the exchange of link-related control
information between DLEP peers. DLEP peers consist of a modem and a
router. DLEP defines a base set of mechanisms as well as support for
possible extensions. This document defines one such extension.
The DLEP specification does not include any flow control capability.
Various flow control techniques are theoretically possible with DLEP.
This document defines a DLEP extension that provides a Diffserv-based
flow control mechanism for traffic sent from a router to a modem.
Flow control is provided using one or more logical "credit windows",
each of which will typically be supported by an associated virtual or
physical queue. A router will use traffic flow classification
information provided by the modem to identify which traffic is
associated with each credit window. Credit windows may be shared or
dedicated on a per-flow basis. See [RFC9895] for an Ethernet-based
version of credit window flow control. As specified in Section 2.3.1
of [RFC9892], when both Diffserv and Ethernet traffic classification
are specified for a flow, the Ethernet information takes precedence.
This document uses the traffic classification and credit window flow
control mechanisms defined in [RFC9892] and [RFC9893] to provide
credit-window-based flow control based on DLEP destinations and
Differentiated Services Code Points (DSCPs) [RFC2475]. The defined
mechanism allows for credit windows to be shared across traffic sent
to multiple DLEP destinations and DSCPs, or used exclusively for
traffic sent to a particular destination and/or DSCP. The extension
also supports the "wildcard" matching of any DSCP.
The extension defined in this document is referred to as the
"Diffserv Aware Credit Window" or, more simply, the "DA Credit"
extension. The reader should be familiar with both the traffic
classification and credit window flow control mechanisms defined in
[RFC9892] and [RFC9893].
This document defines a new DLEP Extension Type value that is used to
indicate support for the extension. See Section 2.
1.1. Key Words
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. Extension Usage and Identification
The extension defined in this document is built on the mechanisms and
processing defined in [RFC9892] and [RFC9893]. To indicate that the
Diffserv Aware Credit Window Extension is to be used, an
implementation MUST include the Diffserv Aware Credit Window
Extension Type value in the Extensions Supported Data Item (see
Section 13.6 of [RFC8175]). The Extensions Supported Data Item is
sent and processed according to [RFC8175]. Any implementation that
indicates the use of the Diffserv Aware Credit Window Extension MUST
support all message types, Data Items, the Diffserv Traffic
Classification Sub-Data Item, and all related processing defined in
[RFC9892] and [RFC9893].
The Diffserv Aware Credit Window Extension Type value is 6. See
Section 5.
3. Management Considerations
This section provides several network management guidelines for
implementations supporting the Diffserv Aware Credit Window
Extension.
If this extension is supported, that support MUST be declared using
the Extensions Supported Data Item (see Section 13.6 of [RFC8175]).
Diffserv Aware Credit Window Extension Data Items MUST NOT be emitted
by a DLEP participant unless such support was specified in the
initialization message received from its peer. The use of the
extension defined in this document SHOULD be configurable on both
modems and routers. That configuration can be implemented using a
proprietary Command-Line Interface or by implementing a YANG module.
The definition of the YANG module is out of scope for this document.
Modems SHOULD support the configuration of mapping a DSCP to a credit
window (queue).
Modems MAY support the configuration of the number of credit windows
(queues) to advertise to a router.
Routers may have limits on the number of queues that they can support
and limits on supported credit window combinations. Per-destination
queues might not be supported at all. When credit window information
provided by a modem exceeds the capabilities of a router, the router
SHOULD use a subset of the provided credit windows. Alternatively, a
router MAY reset the session and indicate that the extension is not
supported. In either case, any mismatch in capabilities SHOULD be
reported to the user via normal network management mechanisms, such
as user interface messages or error logging.
In all cases, if credit windows are in use, traffic for which credits
are not available MUST NOT be sent to the modem by the router.
4. Security Considerations
This document defines a DLEP extension that uses DLEP mechanisms and
the credit window flow control mechanisms defined in [RFC9892] and
[RFC9893]. See also the Security Considerations sections of those
documents.
The defined extension is exposed to vulnerabilities similar to
existing DLEP messages and discussed in the Security Considerations
section of [RFC8175], such as an injected message resizing a credit
window to a value that results in a denial of service. The security
mechanisms documented in [RFC8175] can be applied equally to the
mechanism defined in this document.
Wildcards for matching Priority Code Point (PCP) and VLAN Identifier
(VID) fields (see [RFC9895]) are provided. Note that wildcards may
be convenient for matching a number of packet flows but could
inadvertently match unexpected flows or new flows that appear after
the wildcard matching has been set up. It is therefore RECOMMENDED
that wildcards not be used unless needed.
5. IANA Considerations
IANA has assigned the following code point in the "Extension Type
Values" registry in the "Dynamic Link Exchange Protocol (DLEP)
Parameters" registry group:
+======+==============================+
| Code | Description |
+======+==============================+
| 6 | Diffserv Aware Credit Window |
+------+------------------------------+
Table 1: Extension Type Value
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>.
[RFC9892] Cheng, B., Wiggins, D., Berger, L., and D. Fedyk, Ed.,
"Dynamic Link Exchange Protocol (DLEP) Traffic
Classification Data Item", RFC 9892, DOI 10.17487/RFC9892,
January 2026, <https://www.rfc-editor.org/info/rfc9892>.
[RFC9893] Cheng, B., Wiggins, D., Ratliff, S., Berger, L., and E.
Kinzie, Ed., "Dynamic Link Exchange Protocol (DLEP)
Credit-Based Flow Control Messages and Data Items",
RFC 9893, DOI 10.17487/RFC9893, January 2026,
<https://www.rfc-editor.org/info/rfc9893>.
6.2. Informative References
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC9895] Wiggins, D., Berger, L., and D. Eastlake 3rd, Ed.,
"Dynamic Link Exchange Protocol (DLEP) IEEE 802.1Q Aware
Credit Window Extension", RFC 9895, DOI 10.17487/RFC9895,
January 2026, <https://www.rfc-editor.org/info/rfc9895>.
Acknowledgments
Many useful comments were received from contributors to the MANET
Working Group, notably Ronald in 't Velt.
We had the honor of working too briefly with David Wiggins on this
and related DLEP work. His contribution to the IETF and publication
of the first and definitive open-source DLEP implementation have been
critical to the acceptance of DLEP. We mourn his passing on November
26, 2023. We wish to recognize his guidance, leadership, and
professional excellence. We were fortunate to benefit from his
leadership and friendship. He shall be missed.
Authors' Addresses
Bow-Nan Cheng
MIT Lincoln Laboratory
Massachusetts Institute of Technology
244 Wood Street
Lexington, MA 02421-6426
United States of America
Email: bcheng@ll.mit.edu
David Wiggins
Lou Berger
LabN Consulting, L.L.C.
Email: lberger@labn.net
Donald E. Eastlake 3rd (editor)
Independent
2386 Panoramic Circle
Apopka, FL 32703
United States of America
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
ISO Plain Language
On this page
§1Introduction
The Dynamic Link Exchange Protocol (DLEP), defined in RFC 8175, lets two peers exchange control information about a link. A DLEP session runs between a modem and a router. DLEP defines a base set of mechanisms and a way to add extensions, and this document defines one such extension.
The base DLEP specification includes no flow control at all, and that is the gap this document fills. The extension provides flow control for the traffic a router sends to a modem, using one or more logical credit windows. Each window is typically supported by an associated virtual or physical queue. A credit is permission to send: the router uses the traffic classification information supplied by the modem to work out which window a packet belongs to, and sends that packet only while the window has credits left.
The mechanism reuses the traffic classification and credit window flow control mechanisms defined in RFC 9892 and RFC 9893, and adds the Diffserv dimension on top. The result is called the Diffserv Aware Credit Window extension, or more simply the DA Credit extension. Where both Diffserv and Ethernet traffic classification are specified for a flow, as in Section 2.3.1 of RFC 9892, the Ethernet information takes precedence.
DLEP is how a router and a modem exchange link information. It has no built-in way to slow the router down when the modem's queues fill. This document adds credit windows: the modem lists them, and the router checks that the window a packet belongs to still has credit before sending it.
| Credit window | What it covers |
|---|---|
| Shared | Traffic sent to multiple DLEP destinations and DSCPs. |
| Dedicated | Traffic sent to a particular destination and/or DSCP. |
| Wildcard DSCP | Any DSCP. |
| Diffserv and Ethernet both set | The Ethernet information takes precedence. |
§2Extension Usage and Identification
An implementation signals that it uses the extension by including the Diffserv Aware Credit Window Extension Type value in the Extensions Supported Data Item, which is described in Section 13.6 of RFC 8175. That data item is sent and processed according to RFC 8175.
Support is all or nothing in a specific sense. An implementation that indicates use of the Diffserv Aware Credit Window extension MUST support all message types, data items, the Diffserv Traffic Classification Sub-Data Item and all related processing defined in RFC 9892 and RFC 9893. The Extension Type value assigned to the extension is 6.
A router and a modem say they support the extension by listing it in the data item that carries supported extensions. Once listed, the implementation has to handle everything the two supporting documents define, not just the Diffserv matching.
§3Management Considerations
Support for the extension MUST be declared using the Extensions Supported Data Item, and Diffserv Aware Credit Window data items MUST NOT be emitted by a DLEP participant unless that support was specified in the initialization message received from its peer. The use of the extension SHOULD be configurable on both modems and routers. That configuration can be implemented with a proprietary command-line interface or by implementing a YANG module, whose definition is out of scope for this document.
On the modem side, the document says modems SHOULD support configuring the mapping of a DSCP to a credit window, that is, to a queue. Modems MAY support configuring the number of credit windows to advertise to a router.
Routers have limits of their own. A router may support only a few queues, may not support per-destination queues at all, and may not support every combination of credit windows. When the credit window information a modem provides exceeds the capabilities of a router, the router SHOULD use a subset of the provided credit windows. Alternatively, a router MAY reset the session and indicate that the extension is not supported. In either case, any mismatch in capabilities SHOULD be reported to the user through normal network management mechanisms, such as user interface messages or error logging.
One rule holds in all cases: while credit windows are in use, traffic for which credits are not available MUST NOT be sent to the modem by the router.
Both ends should let an operator turn the feature on or off, and a modem should let an operator map each Diffserv class to a queue. If a modem advertises more windows than a router can handle, the router uses the ones it can and tells the operator about the mismatch. The rule that never bends: a router does not send traffic that has no credit left.
§4Security Considerations
The extension uses DLEP mechanisms and the credit window flow control mechanisms defined in RFC 9892 and RFC 9893, so the Security Considerations sections of those documents apply, as do the ones in RFC 8175.
The document names one inherited weakness: an injected message could resize a credit window to a value that results in a denial of service. The security mechanisms documented in RFC 8175 can be applied equally to the mechanism defined here.
It also warns about wildcards. Wildcards for matching the Priority Code Point (PCP) and VLAN Identifier (VID) fields, which come with the Ethernet-based credit window extension in RFC 9895, are convenient, because one rule can match a number of packet flows. They can also inadvertently match unexpected flows, or flows that appear only after the wildcard matching has been set up. The document therefore RECOMMENDS that wildcards not be used unless they are needed.
An attacker who can inject DLEP messages could resize a credit window in a way that stops traffic, and the protections in the base DLEP document are the ones to use. The document also warns that a wildcard rule can match more traffic than you meant, including traffic that starts later, so it recommends using wildcards only when there is no alternative.
§5IANA Considerations
IANA has assigned one code point in the Extension Type Values registry, in the Dynamic Link Exchange Protocol (DLEP) Parameters registry group. Code point 6 is the Diffserv Aware Credit Window extension.
The extension is registered with IANA as number 6 in the registry of DLEP extension types, so peers can recognise it from the number alone.
- 6DLEP Extension Type — Diffserv Aware Credit Window.