Dynamic Link Exchange Protocol (DLEP) Diffserv Aware Credit Window Extension
Extracted elements (17)
As specified in RFC 9892 Section 2.3.1, when both Diffserv and Ethernet traffic classification are specified for a flow, the Ethernet information takes precedence; this document builds on that precedence rule.
The extension uses logical 'credit windows', each typically backed by an associated virtual or physical queue, allowing flow control to be applied per-destination, per-DSCP, or shared across multiple destinations and DSCPs.
Per-destination queues might not be supported by all routers. When a router's capabilities are exceeded by the modem's advertised credit windows, the router should use a subset or reset the session, ensuring the extension degrades gracefully.
The DA Credit extension is built on RFC 9892 (Traffic Classification Data Item) and RFC 9893 (Credit-Based Flow Control); both must be fully supported when this extension is declared. An Ethernet-based analog is defined in RFC 9895.
Any implementation indicating 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.
Any mismatch in capabilities between modem and router SHOULD be reported to the user via normal network management mechanisms such as user interface messages or error logging.
If credit windows are in use, traffic for which credits are not available MUST NOT be sent to the modem by the router.
If the extension is supported, that support MUST be declared using the Extensions Supported Data Item. Diffserv Aware Credit Window Extension Data Items MUST NOT be emitted unless such support was specified in the initialization message received from the peer.
It is RECOMMENDED that wildcards for matching PCP and VID fields not be used unless needed, as they may inadvertently match unexpected or new flows.
Modems MAY support the configuration of the number of credit windows (queues) to advertise to a router.
Modems SHOULD support the configuration of mapping a DSCP to a credit window (queue).
The use of the Diffserv Aware Credit Window extension SHOULD be configurable on both modems and routers, implementable via a proprietary CLI or a YANG module.
To indicate use of the Diffserv Aware Credit Window Extension, an implementation MUST include the Diffserv Aware Credit Window Extension Type value in the Extensions Supported Data Item, sent and processed according to RFC 8175.
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, the router MAY reset the session and indicate the extension is not supported.
IANA has assigned Extension Type Value 6 ('Diffserv Aware Credit Window') in the 'Extension Type Values' registry within the 'Dynamic Link Exchange Protocol (DLEP) Parameters' registry group.
The extension is exposed to vulnerabilities similar to existing DLEP messages, including injected messages that resize a credit window to a value causing denial of service. The security mechanisms documented in RFC 8175 apply equally to this extension.
Wildcards for matching PCP and VID fields may be convenient but could inadvertently match unexpected flows or new flows appearing after wildcard matching is set up, potentially disrupting intended traffic classification.