Dissecting the cable giant's traffic shaping efforts...
A few months ago, users in our forums started noticing that Comcast (in addition to their invisible caps
) was using Sandvine traffic-shaping hardware installed at the CMTSs to limit the effectiveness
of BitTorrent seeding. Here's how it works, according to resident user funchords
, who has been dissecting the practice for weeks:
"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."
Comcast's goal is to manage peak p2p traffic, not destroy it. While Comcast issued some very carefully worded denials
, using semantics to diffuse attention, insiders have privately been confirming that this information is accurate. Funchords continues to dissect Comcast's efforts, and has since come up with some solutions
•Encrypted tunnels to a point outside the Comcast network (VPN, SSH tunnel, etc.)•Forcing encryption
.•When downloading, make sure you've met your uploading goal by the time that the download completes. The easiest way to do this is to set a download rate slower than the uploading rate.•Tolerating the up to 40% rate RSTs when using BitTorrent to upload a file and not forcing encryption. He says he still hits his 16 KB/s (256 Kbps) preferred upload limit.
The question now becomes whether this evolves into a game of cat and mouse
now familiar to subscribers of Canadian cable operator Rogers, who took traffic shaping to an entirely new level by using deep packet inspection to strangle all VPN and encrypted traffic in order to thwart such workarounds.