Cisco Cisco IOS Software Release 12.2(4)XF Release Notes

The DOCSIS 1.1 QoS framework is based on the following objects:

Service class: A collection of settings maintained by the CMTS that provide a specific QoS service 
tier to a cable modem that has been assigned a service flow within a particular service class 

Service flow: a unidirectional sequence of packets receiving a service class on the DOCSIS link 

Packet classifier: A set of packet header fields used to classify packets onto a service flow to which 
the classifier belongs 

PHS rule: A set of packet header fields that are suppressed by the sending entity before transmitting 
on the link, and are restored by receiving entity after receiving a header-suppressed frame 
transmission. Payload Header Suppression increases the bandwidth efficiency by removing repeated 
packet headers before transmission 

In DOCSIS 1.1, the basic unit of QoS is the service flow, which is a unidirectional sequence of packets 
transported across the RF interface between the cable modem and CMTS. A service flow is characterized 
by a set of QoS parameters such as latency, jitter, and throughput assurances. 

Every cable modem establishes a primary service flow in both the upstream and downstream directions. 
The primary flows maintain connectivity between the cable modem and CMTS at all times. 

In addition, a DOCSIS 1.1 cable modem can establish multiple secondary service flows. The secondary 
service flows can either be permanently created (they persist until the cable modem is reset or powered 
off) or they can be created dynamically to meet the needs of the on demand traffic being transmitted. 

Each service flow has a set of QoS attributes associated with it. These QoS attributes define a particular 
class of service and determine characteristics such as the maximum bandwidth for the service flow and 
the priority of its traffic. The class of service attributes can be inherited from a preconfigured CMTS 
local service class (class-based flows), or they can be individually specified at the time of the creation 
of the service flow. 

Each service flow has multiple packet classifiers associated with it, which determine the type of 
application traffic allowed to be sent on that service flow. Each service flow can also have a Payload 
Header Suppression (PHS) rule associated with it to determine which portion of the packet header will 
be suppressed when packets are transmitted on the flow.

In Cisco IOS Release 12.2(4)XF1, the following new or enhanced software features are implemented for 
DOCSIS 1.1 functionality. (For more information, see the feature module DOCSIS 1.1 for 
Cisco Release 12.2(4)XF1 Series Universal Broadband Routers).

The cable arp and cable proxy-arp commands control whether the Cisco Release 12.2(4)XF1 series 
router allows ARP requests on the cable interfaces and whether the router serves as a proxy ARP server 
for cable modems, so that cable modems on the same subnet can communicate with each other, without 
having to send the traffic through the Cisco Release 12.2(4)XF1 series router. 

The cable flap list is a patented tool that is incorporated in the Cisco IOS software for the Cisco Release 
12.2(4)XF1 series universal broadband routers for troubleshooting cable modem connectivity problems. 
The flap list tracks cable modems that have intermittent connectivity problems (known as “flapping”) 
that could indicate a problem with the cable modem or with the upstream or downstream portion of the 
cable plant.