1. Introduction

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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.