TBR is currently intended for ensuring that each competing node receives an equal share of channel occupancy time based on max-min fairness over the long run. As we later demonstrate in Section 5, TBR works well when competing flows last for hundreds of packets.
Although we believe that long-lived flows (e.g. file transfer applications) are usually the cause of congestion in enterprise and university networks, we acknowledge that congestion in hot-spot access networks may be caused by many short-lived flows with diverse data rates, each sending only dozens of packets.
Responsiveness of TBR relies on how it adjusts the token rate assigned to each competing node and how often (see ADJUSTRATEEVENT). Furthermore, the burst period (bucketi) in which node i can transmit successively also influences the responsiveness of TBR as well as short-term fairness. Special attention must be paid to a packet-level interaction between TBR and the underlying MAC so that TBR can respond to varying traffic conditions in the order of tens of packet transfer time. In the future, we plan to understand each of these issues in detail and make TBR responsive for very short-lived flows as well.
Large bucketi can exacerbate the short-term unfairness, i.e. some competing nodes do not achieve their desired fair shares within a very short interval, commonly found in 802.11 WLANs [17]. Short-term unfairness in its most severe form leads to TCP ack compression in which multiple TCP acks arrive at the sender, which then sends several TCP packets successively, leading to undesirable packet drops at the bottleneck queue. However, the TCP ack compression problem can be effectively solved by pacing TCP packets [5].
TBR can potentially be modified to provide each competing node with the desired share of channel occupancy time (not necessarily equal). Therefore, QoS mechanisms may use TBR to provide QoS at existing AP-based WLANs. We also note that although the current implementation of TBR allocates channel time to nodes, it can be extended to allocate channel time among various flows of each node.
We note that the 802.11e standard [12] currently being drafted defines quality of service support for the 802.11 MAC. Using 802.11e, competing nodes acquire Transmission Opportunities (TXOP), each of which is defined as an interval of time when a station has the right to initiate transmissions. TXOPs are allocated via contention or granted through the centralized coordinator like the AP. 802.11e differentiates the probability of channel access based on the traffic categories. TBR can be integrated with 802.11e by choosing appropriate traffic categories for each competing node according to their fair share of channel occupancy time.