PDLM for NBAR
An external Packet Description Language Module (PDLM) can be loaded at any time to extend the NBAR list of recognized protocols. PDLMs can also be used to enhance an existing protocol recognition capability. PDLMs allow NBAR to recognize new protocols without requiring a new Cisco IOS image or a router reload, hence PDLMs allow the router to gain the functionality of recognizing applications at the application layer for the protocols which when the router was shipped, was either not available or have changed in its function so much that an update is required.
There are a number of examples, such as Citrix, Skype, Bittorrent, eDonkey, etc. This type of traffic would have been hard to classify using standard QoS techniques, either to minimize the impact of such programs on bandwidth, to drop them or to allocate the most amount of bandwidth to. PDLMs give the router this added ability to recognize the traffic specified by it as well as some other types of traffic pre-defined in the IOS.
NBAR in QoS