dslreports logo

story category
AP Discovers Comcast Traffic Shaping
Talks to our forum user who first discovered practice...
by Karl Bode 11:47AM Friday Oct 19 2007 Tipped by jjoshua See Profile
A few months ago, an astute user in our forums started noticing that Comcast (in addition to their invisible download limits) was using Sandvine traffic-shaping hardware, installed at the CMTSs, to limit the effectiveness of BitTorrent seeding. The goal is to manage BitTorrent traffic without tipping off mainstream users that it's being done. Here's how it works, according to resident user Robb Topolski, who has been dissecting the practice for months:

"The Sandvine application reads packets that are traversing the network boundary. If the application senses that outbound P2P traffic is higher than a threshold determined by Comcast, Sandvine begins to interrupt P2P protocol sequences that would initiate a new transfer from within the Comcast network to a peer outside of the Comcast network. The interruption is accomplished by sending a perfectly forged TCP packet (correct peer, port, and sequence numbering) with the RST (reset) flag set. This packet is obeyed by the network stack or operating system which drops the connection."
When asked about the practice, Comcast consistently denies any application blocking, but chooses their words carefully. Ultimately, our users found they could get around the practice by enabling forced encryption on many BitTorrent clients. So far, the game of cat and mouse had been ignored by major outlets, given that the sanctity of TCP/IP doesn't make compelling mainstream news fodder.

The Associated Press changed that today by testing and confirming the practice using a copy of the Bible. The AP reporter gets the stock response from Comcast about the use of Sandvine gear, but also speaks to Topolski and BitTorrent companies (some of them obviously video competitors) impacted by the practice. It didn't take long for network neutrality supporters to lambast Comcast.

121 comments .. click to read

Recommended comments


Bryn Mawr, PA
·Verizon FiOS

2 edits

2 recommendations

reply to LeftOfSanity


By doing what they are doing, Comcast is FORGING data! They are pretending it comes from YOU, when it does not. Your comment of: "It's their network" does not hold a DROP of water! I'll bet if you said on the phone: "I love you" to your girlfriend or wife and Verizon forged the packets so in your voice it came out: "FU*K you, bitch!", you'd be screaming bloody murder! But why can't they do that? After all, it's their network!

R I G H T???

Digital Plumber
Minneapolis, MN
·Vitelity VOIP

1 edit

3 recommendations

P2P and Oversubscription are natural enemies

P2P traffic mitigation techniques are not going away anytime soon. Broadband networks are built to accommodate "normal" traffic patterns which have random bursts that tend to balance out the load. That's how with a single 38mbps downstream channel cable companies are able to provision multiple 16mbps connections with everyone appearing to be able to hit their maximum transfer speed.

P2P software operates under the assumption that there's all this "idle" bandwidth available to be tapped for transfers. The problem is that on an oversubscribed network your "idle" capacity tends to be your neighbors' "use" capacity. For being a "free" method of distributing content, P2P has expensive implications on capacity planning and network architecture.

There's only a few approaches to take with this, and any option that gets chosen is going to be unpopular.

1) Throttle traffic types that disrupt the experience for the overwhelming majority of your customer base
2) Convert to a usage-based billing system to fund infrastructure upgrades in areas where heavy use occurs
3) Increase the rates for everyone so that the oversubscription ratios can be lowered.

Tavistock NJ

1 edit

3 recommendations

Net Neutrality is NOT about being protocol neutral

Net neutrality was about being content provider neutral. It had nothing to do with being protocol neutral. Trying to tie protocol neutrality in to Net neutrality is just an attempt by the music and video thieves to protect the mechanisms of theft(that is their P2P systems).

Comcast is merely protecting their network from a peer to peer protocol that is extremely network management unfriendly.
Internet News
My Web Page