• By: Naeem Khademi (GS20561) • Supervisor: Prof. Dr. Mohamed Othman • Date/Time: 10 November 2009 9:00 AM • Duration : 30 min • Faculty Of Computer Sc. & Information Technology (FSKTM) • University Putra Malaysia (UPM)
IEEE 802.11 WLAN technology is the prevalent technology used by wireless users. TCP is the dominant transport protocol in Internet. Distributed Coordination Function (DCF) defined in IEEE 802.11 MAC provides all WLAN stations equal opportunity to access the transmission medium. Cross-layer interaction of TCP with DCF leads to flow level unfairness in WLAN.
Two main types of unfairness occur due to the cross-layer interaction of TCP and DCF in 802.11 MAC protocol [1]. 1. Size-based unfairness in benefit of long-lived flows against short- lived flows. 2. Direction-based unfairness in benefit of upstream flows against downstream flows.
Several solutions have been proposed to solve each type of unfairness phenomenon [2-4] each of which only solves one aspect of unfairness (either size-based or direction-based) while may deteriorating another aspect. The closest work to our research is done by Wu et al. [2]. It proposes a queue management policy at Access Point (AP) called Least Attained Service scheduling (LAS). LAS always gives service to the job that has received the least service so far without prior knowledge of job size .
LAS is used by [2] to provide direction-based fairness for upstream and download flows. Threshold-based LAS (TLAS) is used by [2] to provide size-based fairness. The main idea of TLAS is to give the newly arriving flow service priority up to a certain threshold (e.g. 50 packets). Once the threshold is reached, FIFO scheduling is employed on this flow. The reason behind TLAS proposal is to prevent starvation of long-lived TCP flows.
One of the key challenges that IEEE 802.11 WLAN technology faces is the lack of a unique solution to solve two typical types of unfairness which are sized based unfairness among flows in benefit of long-lived flows as well as direction based unfairness in benefit of upstream flows. Most proposed solutions solve one of these issues while deteriorated another one or at least unable to improve it to an acceptable level.
The main objectives of this research are three-fold: 1. We demonstrate and evaluate different types of unfairness phenomenon (short-lived versus long-lived flows as well as upstream versus downstream flows) in IEEE 802.11 WLAN. 2. We propose a novel queue management policy called “Threshold based Least Attained Service scheduling – Selective Acknowledgment Filtering (TLAS-SAF)” to alleviate mentioned types of unfairness to an acceptable level 3. We evaluate the efficiency and validation of our scheme using various simulation scenarios.
In the 802.11 WLAN, a bandwidth asymmetry exists between contending upload and download flows. the MAC layer contention parameters are all equal for the AP and the stations. If K stations and an AP are always contending for the access to the channel, each host ends up having approximately 1/(K + 1) share of the total transmit opportunities over a long time interval. Results in K/(K +1) of the transmissions being in uplink, while only 1/(K +1) of the transmissions belong to the downlink flows. Thus, overall upstream transmit opportunities will be 𝐿 times more than downstream.
802.11 WLAN is half-duplex. At AP downstream queue, data packet loss (downstream) leads to Triple-Duplicate ACK mechanism and halving Congestion Window. ACK loss impact on flow throughput can be seen negligible relying on cumulative nature of TCP ACK. When upstream ACKs fill the AP buffer, downstream data packets will be dropped. Several consecutive loss of data packets result in RTO which sets congestion window to one. This leads to aggressive upstream flows and conservative downstream flows.
The minimum buffer size to achieve direction-based fairness among all flows is determined by Pilosof et al. [4] as where 𝜖 is number of ACK packet per data packets in TCP and 𝑋 is the TCP advertised receiver window for all flows and 𝑂 and 𝑁 refers to the number of downstream and upstream flows respectively.
TCP congestion and flow control mechanism is designed in such a way that large data flows can enjoy from the best effort bandwidth allocation. Studies have revealed that Internet traffic shows a high variability property and most of the TCP flows are short (e.g. popular web transfer), while more than half of the bytes are carried by less than 5% of the largest flows [5,6]. The high variability of Internet traffic negatively affect the end user experience in general due to the nature of TCP transport protocol and FIFO scheduling mechanism used in most network routers.
small files data transfers often don’t reach to the steady state and congestion avoidance phase and their transfer terminate while they are in their slow start phase. They are also prone to packet loss because of their small congestion window (mostly results in RTO). This makes small file data transfers (short-lived flows) conservative in contrast to the aggressive long-lived flows. In WLAN the situation is even worse due to the burstiness of bit errors in wireless channel. short-lived flows suffer from higher variability of transfer time compare to the long-lived flows.
LAS is a preemptive scheduling policy that favors short jobs without prior knowledge of the job sizes. the mean response time of LAS highly depends on the job size distribution. Based on LAS definition in network flows, the next packet to be served is the one that belongs to the flow that has received the least amount of service so far. LAS significantly reduces the mean transfer time and loss rate of short TCP flows as compared to DropTail first in-first out (FIFO) scheduler.
Slow start phase for short TCP flows under both LAS and FIFO with no congestion
Slow start phase for short TCP flows under both LAS and FIFO with congestion
A small increase in mean transfer time of large flows happens but under moderate load values a large flow under LAS is not starved when competing with short TCP flows. However, the performance of the long-lived flow under LAS deteriorates severely when competing at high load with short flows. One drawback of LAS is that one newly arriving long-lived flow can block all other existing long-lived flows until the time it receive the same service they achieved. In this case, all other long-lived flows get starved which causes severe unfairness among long-lived flows.
The main idea of Threshold-based least attained service (TLAS) scheduling is to give the newly arriving flow service priority up to a certain threshold (e.g. 50 packets). Once the threshold is reached, FIFO scheduling is employed on this flow. Authors in [2] showed that TLAS can guarantee fairness for short-lived flows as well as long-lived flows. In our research we show that TLAS in unable to provide direction- based fairness. Because it gives the equal service to all flows until a certain threshold which normally is a small value and it behaves the same as FIFO for the rest of transmission period resulting in direction- based unfairness for most part of the transmission period (specifically for long-lived downstream flows).
Barrows its main idea from TLAS and Selective Packet Marking-ACK filtering (SPM) [7,8]. Sets a minimum threshold for service that should be guaranteed for every network flow and it behaves the same as LAS with all packets below that threshold. Once service threshold is reached for a flow it behaves with flow as SPM-AF (specified in [2]) does. TLAS-SAF inherits TLAS capability to provide size-based fairness while employing SPM-AF characteristics for larger flows to provide direction-based fairness as well.
In addition to giving priority to short-flows, TLAS-SAF also gives priority to the data packets which their congestion window of their flows is smaller than four . Data packet loss in these flows result in retransmission timeout (RTO) while loss of other data packets will trigger fast retransmission mechanism. It also frees the buffer space from repetitive ACKs of a flow relying on cumulative nature of ACKs.
TLAS-SAF procedure is as follow: 1. Gives service priority to newly arriving packets of a flow until a certain threshold (e.g. 50 packets). 2. Once threshold is reached, FIFO scheduling will be imposed on this flow . 3. If an arriving packet of a flow below threshold encounters to a full buffer, a packet of a below threshold flow which has received the maximum service so far (similar to TLAS) will be found and discarded. However, If received service for such a packet flow was less than received service of arriving packet flow, the arriving packet will be discarded instead. 4. TLAS-SAF gives priority to data packets which their congestion window is smaller than four for the flows have already achieved the minimum threshold service.
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