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Export, PA
·Comcast Business..
reply to bradyr

Re: [Business] Seesaw download pattern

Yes, I've tried different computers, OSs, interfaces (wi-fi, ethernet) and the pattern is always the same. The length of the "tooth" may be different but the shape is the same.

Another observation: The upload is not affected.

Seems like this has something to do with Comcast traffic shaping and I'm not the only one:

»What's up with this "sawtooth" download pattern?

»[Speed] Wierd download speed pattern


It's possible that you have a congested node... IIRC way back they added in a feature that would throttle your connection to 50% if the node was overloaded and you were in the top percentage of users. Your download would be the perfect example of this because it's a long sustained connection eating up all of the bandwidth. By throttling it slightly everyone else's traffic can remain unaffected while just making your download take a bit longer.

here is the article:

»New Comcast Throttling System 100% Online

In line with their traffic management website Comcast has confirmed to us they've installed their new broadband throttling system across all markets. The system, which we first profiled back in September, throttles a user's connection if a particular CMTS port is congested, and if that user has been identified as a primary reason why. This two-condition throttling system replaces Comcast's old, FCC-criticized system of using forged TCP packets to throttle upstream P2P services for all users, regardless of consumption.

According to Comcast's filings (pdf) with the FCC, they've deployed new hardware and software close to the company's Regional Network Routers (RNRs). This hardware will flip a user from the standard "Priority Best-Effort" traffic (PBE) to lower quality of service (QoS) "Best-Effort" traffic (BE) for fifteen minutes if they're a major reason congestion exists.

While certainly a slightly more transparent system to those paying attention, the new system is probably going to confuse the American public, many of whom don't even know what a gigabyte is. Comcast used a bus metaphor to explain the difference between best effort and priority best effort traffic to the FCC:
If there is no congestion, packets from a user in a BE state should have little trouble getting on the bus when they arrive at the bus stop. If, on the other hand, there is congestion in a particular instance, the bus may become filled by packets in a PBE state before any BE packets can get on. In that situation, the BE packets would have to wait for the next bus that is not filled by PBE packets.
Comcast says that sustained use of 70% of your up or downstream throughput triggers the BE state, at which point you'll find your traffic priority lowered until your usage drops to 50% of your provisioned upstream or downstream bandwidth for "a period of approximately 15 minutes." A throttled Comcast user being placed in a BE state "may or may not result in the user's traffic being delayed or, in extreme cases, dropped before PBE traffic is dropped."

Note that upstream and downstream bandwidth are managed separately. Also note that the differentiation between PBE and BE traffic occurs in two millisecond increments. According to Comcast, even if the packets for a best effort throttled user missed 50 "busses," the delay would only be about one-tenth of a second.

In addition to the new throttling system, Comcast has also a 250GB monthly usage cap for all users. As we mentioned last Friday, Comcast has confirmed that a web portal-based bandwidth tracker is currently in beta among Comcast employees -- but has yet to give an official launch date. A Comcast insider had previously given us leaked screenshots of the monitor, and said it was originally scheduled to go live on January 5 (today).

Comcast has confirmed to us that they've completed the upgrade to the new system.


Export, PA
I don't think this should apply to me - I have a 12/2 business account. If it does apply, and my 12/2 connection is "primary source of congestion" then Comcast has a serious over subscription problem.

Bueller? Bueller?
El Dorado Hills, CA
I saw this a LONG time ago on a DSL line. It was due to a mismatched buffer size between the DSLAM and the ISP's equipment or something like that.

It definitely looks to me like some sort of misconfigured traffic shaping.

Would it be possible for you to get a pcap dump (tcpdump/wireshark) and post it here? It should be sufficient to capture for only 1 or 2 cycles of the sawtooth.

I did notice with some TCP stacks or congestion algorithms, powerboost can cause issues where the speed constantly ramps up and drops off. But you're observing this on multiple OSes, so that's likely not the issue.
"Women. Can't live with 'em, pass the beer nuts." -Norm


Export, PA
I'll try to capture something tonight.


Export, PA
·Comcast Business..
reply to lacibaci
Click for full size
Well, so much for trying to capture packets tonight. Ran a test download from same site as yesterday and got almost perfect flat line ;) Also ran shaperprobe:

lac@heron:~/Desktop/shaperprobe$ ./prober -v
DiffProbe release. January 2012.
Shaper Detection Module.

Connected to server

Estimating capacity:
Upstream: 7187 Kbps.
Downstream: 130849 Kbps.

The measurement will last for about 3.0 minutes. Please wait.

Checking for traffic shapers:

Upstream: Burst size: 9614-9797 KB; Shaping rate: 2090 Kbps.

Downstream: Burst size: 11582-12848 KB; Shaping rate: 35168 Kbps.

For more information, visit: »www.cc.gatech.edu/~partha/diffpr ··· iffprobe

I hope this was just a temporary hiccup. I'll keep an eye on this.

Thank you all for your suggestions.


Stuarts Draft, VA
That does look more normal.

Ukiah, CA
reply to ExoticFish
said by ExoticFish:

Let me rephrase that... Powerboost will reappear on business accounts after the initial "boost" whereas on residential accounts it's only available for the first part of a download.

That's what I remember reading somewhere anyway.

The only difference I've noticed with PowerBoost on business is that it's basically uncapped. For example, residential 25/4 Blast hits about 35/6 with PB. I have business 22/5 and just now it hit 93/20. Other than that, it's the same. It basically looks like the picture mrschultz02 posted above, just with a much higher initial spike.

Aptos, CA
reply to lacibaci
said by lacibaci:

I hope this was just a temporary hiccup. I'll keep an eye on this.

Thank you all for your suggestions.

Next time it happens try doing three or more downloads at the same time, to three different servers - if the combined download speed sawtooths, it's something specific to your end, or the path leading up to your end inside of comcast. If not, it's on the other end.
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