NNSquad - Network Neutrality Squad
[ NNSquad ] Comcast traffic management
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From: David Farber <dave@farber.net>
To: "ip" <ip@v2.listbox.com>
Subject: [IP] Clarifying Misconceptions of the New Comcast Congestion Mgmt
System
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.
Regards,
Jason
Jason Livingood - jason_livingood@cable.comcast.com
Comcast
National Engineering & Technical Operations
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<html><body style=3D"word-wrap: break-word; -webkit-nbsp-mode: space; -webk=
it-line-break: after-white-space; "><br><div><br><div>Begin forwarded messa=
ge:</div><br class=3D"Apple-interchange-newline"><div><div style=3D"margin-=
top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font =
face=3D"Helvetica" size=3D"3" color=3D"#000000" style=3D"font: 12.0px Helve=
tica; color: #000000"><b>From: </b></font><font face=3D"Helvetica" size=3D"=
3" style=3D"font: 12.0px Helvetica">"Livingood, Jason" <<a href=3D"mailt=
o:Jason_Livingood@cable.comcast.com">Jason_Livingood@cable.comcast.com</a>>=
</font></div><div style=3D"margin-top: 0px; margin-right: 0px; margin-botto=
m: 0px; margin-left: 0px; "><font face=3D"Helvetica" size=3D"3" color=3D"#0=
00000" style=3D"font: 12.0px Helvetica; color: #000000"><b>Date: </b></font=
><font face=3D"Helvetica" size=3D"3" style=3D"font: 12.0px Helvetica">Septe=
mber 24, 2008 10:59:19 AM EDT</font></div><div style=3D"margin-top: 0px; ma=
rgin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font face=3D"Helv=
etica" size=3D"3" color=3D"#000000" style=3D"font: 12.0px Helvetica; color:=
#000000"><b>To: </b></font><font face=3D"Helvetica" size=3D"3" style=3D"fo=
nt: 12.0px Helvetica">"David Farber" <<a href=3D"mailto:dave@farber.net"=
>dave@farber.net</a>></font></div><div style=3D"margin-top: 0px; margin-rig=
ht: 0px; margin-bottom: 0px; margin-left: 0px; "><font face=3D"Helvetica" s=
ize=3D"3" color=3D"#000000" style=3D"font: 12.0px Helvetica; color: #000000=
"><b>Subject: </b></font><font face=3D"Helvetica" size=3D"3" style=3D"font:=
12.0px Helvetica"><b>Clarifying Misconceptions of the New Comcast Congesti=
on Mgmt System</b></font></div><div style=3D"margin-top: 0px; margin-right:=
0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div> =
</div> <div> <div><font size=3D"2" face=3D"Arial"><span class=3D"625374713-=
24092008">Hi Dave -</span></font></div> <div><span class=3D"625374713-24092=
008"></span><font face=3D"Arial"><font size=3D"2"></font></font> </div=
> <div><span class=3D"625374713-24092008"><font size=3D"2" face=3D"Arial">I=
wanted to try to clear up a misconception about how the new Comc=
ast congestion management system works. I believe we have both heard =
people complain that they fear that they will be unable to use their provis=
ioned 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 conf=
usion over how a user's traffic enters a lower priority QoS state, so I hop=
e to clarify that here.</font></span></div> <div><font size=3D"2" face=3D"A=
rial"></font> </div> <div><font size=3D"2" face=3D"Arial"><span class=
=3D"625374713-24092008">In order for any traffic to be placed in a lower pr=
iority state, there must first be relatively high utilization on a given CM=
TS 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 shar=
e the same downstream port, and about 100 cable modems share the same upstr=
eam port (see page 5 of Attachment B of our Future Practices filing with th=
e FCC, available at <a href=3D"http://downloads.comcast.net/docs/Attachment=
_B_Future_Practices.pdf">http://downloads.comcast.net/docs/Attachment_B_Fut=
ure_Practices.pdf</a>). We define a utilization threshold for downstr=
eam and upstream separately. For downstream traffic, a port must aver=
age over 80% utilization for 15 minutes or more. For upstream traffic=
, a port must average over 70% utilization for 15 minutes or more. </=
span></font></div> <div><font size=3D"2" face=3D"Arial"><span class=3D"6253=
74713-24092008"></span></font> </div> <div><font size=3D"2" face=3D"Ar=
ial"><span class=3D"625374713-24092008">When one of these threshold conditi=
ons has been met, we consider that individual port (not all ports on t=
he CMTS) to be in a so-called Near Congestion State. This simply mean=
s that the pattern of usage is predictive of that network port approac=
hing a point of high utilization, where congestion could soon occur. =
Then, and only then, do we search the most recent 15 minutes of user traffi=
c 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 p=
rovisioned speed, we mean the "up to" or "burst to" speed of their service =
tier. This is typically something like (1) 8Mbps downstream / 2Mbps u=
pstream or (2) 6Mbps downstream / 1Mbps upstream.</span></font></div> <div>=
<font size=3D"2" face=3D"Arial"><span class=3D"625374713-24092008"></span><=
/font> </div> <div><font size=3D"2" face=3D"Arial"><span class=3D"6253=
74713-24092008">So how does this work in action? Let's say that a dow=
nstream port has been at 85% utilization for more than 15 minutes.&nbs=
p; That specific downstream port is identified as being in a Near Congestio=
n State since it exceeded an average of 80% over that time. We then l=
ook 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 o=
r 4.2Mbps, respectively. So let's use the example of a user with 8Mbp=
s/2Mbps service on this port. In order for their traffic to be m=
arked with a lower priority on this downstream port, they must be consuming=
5.6Mbps in the downstream direction for 15 minutes or more, while sai=
d port is highly utilized. </span></font></div> <div><font size=3D"2"=
face=3D"Arial"><span class=3D"625374713-24092008"></span></font> </di=
v> <div><font size=3D"2" face=3D"Arial"><span class=3D"625374713-24092008">=
Once that condition has been met, that user's downstream traffic is now tag=
ged with the lower priority QoS level. This will have *no* effect wha=
tsoever on the traffic of that user, until such time as an actual cong=
estion moment subsequently occurs (IF it even occurs). Should congest=
ion subsequently occur, traffic with a higher priority is handled first, fo=
llowed by lower priority (and this is not a throttle to X speed).</span></f=
ont></div> <div><font size=3D"2" face=3D"Arial"><span class=3D"625374713-24=
092008"></span></font> </div> <div><font size=3D"2" face=3D"Arial"><sp=
an class=3D"625374713-24092008">I hope this helps. You can others can=
feel free to contact me directly if you have any questions.</span></font><=
/div> <div><font size=3D"2" face=3D"Arial"><span class=3D"625374713-2409200=
8"></span></font> </div> <div align=3D"left"> <div align=3D"left"><fon=
t size=3D"2" face=3D"Arial">Regards,</font></div> <div align=3D"left"><font=
size=3D"2" face=3D"Arial">Jason</font></div> <div align=3D"left"><font siz=
e=3D"2" face=3D"Arial"></font> </div> <div align=3D"left"><font face=
=3D"Arial"><font size=3D"2">Jason Livingood<span class=3D"625374713-2409200=
8"> - <a href=3D"mailto:jason_livingood@cable.comcast.com";>jason_livingood@=
cable.comcast.com</a></span></font></font></div> <div align=3D"left"> <div =
align=3D"left"><font size=3D"2" face=3D"Arial">Comcast </font></div></div> =
<div align=3D"left"><font size=3D"2" face=3D"Arial">National Engineering &a=
mp; Technical Operations</font></div> <div align=3D"left"><font size=3D"2" =
face=3D"Arial"></font> </div></div></div></div><br><div style=3D"paddi=
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