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
November 2025
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) 2025 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
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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
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 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 composed of 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 (CLI) 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 control and flow 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,
November 2025, <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, November 2025,
<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,
November 2025, <https://www.rfc-editor.org/info/rfc9895>.
Acknowledgments
The Sub-Data Item format was inspired by Rick Taylor's "Data Item
Containers". He also proposed the separation of credit windows from
traffic classification at IETF 98. 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