Clarifying Misconceptions of the New Comcast Congestion Mgmt System

[David Farber <dave () farber net>]

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.

Regards,
Jason

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





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