Hi, Bandwidth shaping on PPPoE links is difficult, because you don't know what the modem will do to you packets.
On my DSL PPPoE links the DSL modem sends the ethernet frames over an ATM circuit and adds additional, but not constant overhead to each packet in the process. The consequence is that the raw IP throughput (which is limited by the traffic shaper) which will saturate the link is smaller for small IP packets and larger for large IP packets. Now, one could determine the ATM bandwidth needed by a certain packet by calculating the number of ATM cells needed to encapsulate the packet. I made some measurements on my upstream link (224 kbaud). The extreme cases are: 261pps of very small (56 byte) TCP/IP packets. This makes 14.27 kbps of IP traffic. But 527 ATM cells are used per second. 17pps of the largest possible (1448 byte) TCP/IP packets. This makes 24.04 kbps of IP traffic. But 522 ATM cells are used per second. So the number of used ATM cells would be a better measure for traffic shaping on PPPoE(oA) links. I'm thinking of adding an option to altq that would estimate bandwidth usage by calculating the number of needed ATM cells. Is this the right approach and is there a chance to get this into the tree? Regards, Christopher Zimmermann
