Linux – Rate-limiting incoming traffic

bandwidthlinuxnetworkingtraffic-shaping

I've never quite understood whether or not it's possible to rate-limit incoming traffic. I realize that there is no direct method whereby to control the remote server's rate of sending packets (unless you're in control of both endpoints), but taking this limitation into account, how exactly do download managers allow me to successfully set download speed limits?

Is there any link between TCP slow-start and rate-limiting incoming traffic? Is it possible to use the methods described by slow-start to artificially limit the sender's rate of sending packets?

As an additional consideration, it should be noted that the server on which I'd like to implement traffic shaping establishes the PPPoE connection itself, and acts as a router for the rest of the network.

Update: The answers thus far have given a fair overview of the questions I've asked, but I still don't know how download managers are able to limit incoming traffic, and more specifically, whether it's possible to implement a similar strategy on a Linux gateway box.

Best Answer

The download managers most likely work as explained in the the trickle paper.

A process utilizing BSD sockets may perform its own rate limiting. For upstream limiting, the application can do this by simply limiting the rate of data that is written to a socket. Similarly, for downstream limiting, an application may limit the rate of data it reads from a socket. However, the reason why this works is not immediately obvious. When the application neglects to read some data from a socket, its socket receive buffers fill up. This in turn will cause the receiving TCP to advertise a smaller receiver window (rwnd), creating back pressure on the underlying TCP connection thus limiting its data flow. Eventually this “trickle-down” effect achieves end-to-end rate limiting. Depending on buffering in all layers of the network stack, this effect may take some time to propagate.

If you occasionally need to rate-limit a single program on a UNIX system, a simple solution is trickle. Real traffic shaping, like you would perform on a gateway, can be done with tc. This is documented in Chapter 9. Queueing Disciplines for Bandwidth Management of the Linux Advanced Routing & Traffic Control HOWTO.

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