Jesper Dangaard Brouer wrote:

The Linux traffic's control engine inaccurately calculates
transmission times for packets sent over ADSL links.  For
some packet sizes the error rises to over 50%.  This occurs
because ADSL uses ATM as its link layer transport, and ATM
transmits packets in fixed sized 53 byte cells.

I could have sworn that DSL uses its own framing protocol that is 
similar to the frame/superframe structure of HDSL ( T1 ) lines and over 
that you can run ATM or ethernet.  Or is it typically ethernet -> ATM -> 
HDSL?

In any case, why does the kernel care about the exact time that the IP 
packet has been received and reassembled on the headend?