Many papers invoke the ACK compression phenomenon to explain the download performance drop in the presence of bidirectional traffic, whereas we argue that this symptom is merely a side effect of the core phenomenon that we analyze here. Interferences between upload and download is more and more frequent as it requires two commonplace ingredients: asymmetric links like most SOHO access links and long-lasting TCP uploads e.g. due to the presence of peer-to-peer traffic. We find that interaction between uploads and downloads is simple to analyze in the rare case where the (up)link buffer size is given in bytes. Otherwise, in the more common case where buffer size is a given number of packets, there is greater adverse interactions. We provide guidance for sizing the link buffers to mitigate detrimental interference. We provide experimental results that illustrate the case with buffer sizes in bytes using an asymmetric WiMAX link