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Clarifying Misconceptions of the New Comcast Congestion Mgmt System
From: David Farber <dave () farber net>
Date: Wed, 24 Sep 2008 12:36:38 -0400

Begin forwarded message:

From: "Livingood, Jason" <Jason_Livingood () cable comcast com>
Date: September 24, 2008 10:59:19 AM EDT
To: "David Farber" <dave () farber net>
Subject: Clarifying Misconceptions of the New Comcast Congestion Mgmt System

Hi Dave -

I wanted to try to clear up a misconception about how the new Comcast congestion management system works. I believe we have both heard people complain that they fear that they will be unable to use their provisioned speeds during off-peak hours, for example, or at all times of the day, or that users are somehow throttled to a set speed. Neither of these two things are correct. Part of the problem appears to be confusion over how a user's traffic enters a lower priority QoS state, so I hope to clarify that here.

In order for any traffic to be placed in a lower priority state, there must first be relatively high utilization on a given CMTS port. A CMTS port is an upstream or downstream link, or interface, on the CMTS in our network. The CMTS is basically an access network router, with HFC interfaces on the subscriber side, and GigE interfaces on the WAN/ Internet side. Today, on average, about 275 cable modems share the same downstream port, and about 100 cable modems share the same upstream port (see page 5 of Attachment B of our Future Practices filing with the FCC, available at http://downloads.comcast.net/docs/Attachment_B_Future_Practices.pdf) . We define a utilization threshold for downstream and upstream separately. For downstream traffic, a port must average over 80% utilization for 15 minutes or more. For upstream traffic, a port must average over 70% utilization for 15 minutes or more.

When one of these threshold conditions has been met, we consider that individual port (not all ports on the CMTS) to be in a so-called Near Congestion State. This simply means that the pattern of usage is predictive of that network port approaching a point of high utilization, where congestion could soon occur. Then, and only then, do we search the most recent 15 minutes of user traffic on that specific port, in order to determine if a user has consumed more that 70% of their provisioned speed for greater than 15 minutes. By provisioned speed, we mean the "up to" or "burst to" speed of their service tier. This is typically something like (1) 8Mbps downstream / 2Mbps upstream or (2) 6Mbps downstream / 1Mbps upstream.

So how does this work in action? Let's say that a downstream port has been at 85% utilization for more than 15 minutes. That specific downstream port is identified as being in a Near Congestion State since it exceeded an average of 80% over that time. We then look at the downstream usage of the ~275 cable modems using that downstream port. That port has a mix of users that have been provisioned either 8Mbps or 6Mbps, so 70% of their provisioned speed would be either 5.6Mbps or 4.2Mbps, respectively. So let's use the example of a user with 8Mbps/2Mbps service on this port. In order for their traffic to be marked with a lower priority on this downstream port, they must be consuming 5.6Mbps in the downstream direction for 15 minutes or more, while said port is highly utilized.

Once that condition has been met, that user's downstream traffic is now tagged with the lower priority QoS level. This will have *no* effect whatsoever on the traffic of that user, until such time as an actual congestion moment subsequently occurs (IF it even occurs). Should congestion subsequently occur, traffic with a higher priority is handled first, followed by lower priority (and this is not a throttle to X speed).

I hope this helps. You can others can feel free to contact me directly if you have any questions.


Jason Livingood - jason_livingood () cable comcast com
National Engineering & Technical Operations

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