Ed note: Sites like Slashdot are pitching this FCC filing as fresh news, but it is not. Comcast transitioned to the following mechanism by December 31, 2008. We are covering it today for the sake of discussion, and to offer facts–not FUD–to the public about what the US’ largest ISPs are doing to manage their networks.

A 2008 filing by Comcast (PDF) with the US Federal Communications Commission sheds light on the company’s throttling practices. The filing came as a result of the FCC’s 2008 Internet Policy Statement (PDF) which obligates ISPs to be transparent about their network management practices.

The entire throttling mechanism is centered around the Cable Modem Termination System, or CMTS. Comcast’s network contains 3300 CMTS units in the US, each serving approximately 4400 customers.

The throttling process begins when a CMTS approaches a congested state described as greater than 15 minutes of  >70% upstream utilization or >80% downstream utilization. Comcast calls this a Near Congested State, or NCS.

A CMTS in an NCS searches the last 15 minutes of IPDR data, which contains bandwidth usage records for connected subscribers. The IPDR logs are then used to identify users consuming a disproportionate amount of bandwidth relative to other users on the node. Any subscribers found to have exceeded 70% of their provisioned upstream bandwidth, or 80% of their provisioned downstream bandwidth for more than 15 minutes are switched to a “best effort” (BE) traffic state.

A subscriber in a BE traffic state will have his or her WAN traffic preempted by other users on the CMTS. If a subscriber’s usage dips below 50% of their provisioned bandwidth for more than 15 minutes, the user is returned to the default priority best effort (PBE) state.

It is important to understand that Comcast’s throttling practices do not actually scale back a customer’s provisioned bandwidth. Comcast’s technique simply alters the priority of a disproportionate user’s traffic so it can be preempted by users in an unmanaged state.

This sort of organic Quality of Service (QoS) allows Comcast to manage the congestion of its network without resorting to protocol inspection, a practice no longer observed by the ISP per FCC mandate.

The flowchart for a Comcast CMTS configured for congestion management.