Shih-Chiang Tsao

Taxonomy and Evaluation of TCP-Friendly Congestion-Control Schemes on Fairness, Aggressiveness, and Responsiveness

Many TCP-friendly congestion control schemes have been proposed to pursue the TCP-equivalence criterion, which states that a TCP-equivalent flow should have the same throughput with TCP if it experiences identical network conditions as TCP. Additionally, the throughput should converge as fast as TCP when the packet-loss conditions change. This study classifies eight typical TCP-friendly schemes according to their underlying policies on fairness, aggressiveness, and responsiveness. The schemes are evaluated to verify whether they meet TCP-equivalence and TCP-equal share. TCP-equal share is a more realistic but more challenging criterion than TCP-equivalence and states that a flow should have the same throughput with TCP if competing with TCP for the same bottleneck. Simulation results indicate that one of the selected schemes, TCP-friendly rate control (TFRC), meets both criteria under more testing scenarios than the others. Additionally, the results under non-periodic losses, low-multiplexing, two-state losses, and bursty losses reveal the causes that bring fault cases to the schemes. Finally, appropriate policies are recommended for an ideal scheme.

Multiple-resource request scheduling for differentiated QoS at website gateway

Differentiated quality of service is a way for a website operator to provide different service levels to its clients. Traditional HTTP request scheduling schemes can achieve this, but they schedule requests to manage only one server resource, such as CPU or disk I/O. Actually, processing a request on the server will consume multiple resources. This paper presents a multiple-resource request scheduling algorithm, called mQoS, for differentiating the utilization of the server resource. The mQoS scheduler consists of several sub-schedulers and a main scheduler. Each sub-scheduler manages a server resource to differentiate its utilization among the classes. The main scheduler checks the availability of every server resource and triggers an appropriate sub-scheduler to balance the utilization of server resources. The implementation of the mQoS gateway is based on Squid and Linux. The evaluation compares the mQoS scheduling with no scheduling (nQoS) and single-resource request scheduling (sQoS). The mQoS scheduling reveals the accurate differentiation on every server resource. In addition, the total server throughput in the mQoS scheduling is improved by 21%, compared with the sQoS scheduling. The average user-perceived latency of the mQoS scheduling is also shorter than other scheduling.

On shaping TCP traffic at edge gateways

Many security and QoS functions have been deployed at edge gateways to provide policy-based network management. For QoS functions, the bandwidth management system can manage the narrow WAN access links. When managing the TCP traffic, pass-through TCP flows can introduce large buffer requirement, latency, buffer overflows, and unfairness among flows competing for the same queue. This study evaluates possible TCP-aware approaches through self-developed implementations in Linux, testbed emulation, and live WAN measurement. The widely deployed TCP rate control (TCR) approach is found to be more vulnerable to WAN packet losses and less compatible to several TCP sending operating systems. The proposed PostACK approach can preserve TCRs advantages while avoiding TCRs drawbacks. PostACK emulates per-flow queuing but relocates the queuing of data to the queuing of ACKs in the reverse direction, hence minimizes buffer requirement up to 96%. PostACK also has 10% goodput improvement against TCR under lossy WAN. Experimental results can be reproduced through our open sources: (1) tcp-masq: a modified Linux kernel; (2) wan-emu: a testbed for conducting switched LAN-to-WAN or WAN-to-LAN experiments with RTT/loss/jitter emulations.

On applying fair queuing discipline to schedule requests at access gateway for downlink differential QoS

Scheduling packets is a usual solution to allocate the bandwidth on a bottleneck link. However, this solution cannot be used to manage the downlink bandwidth at the user-side access gateway, since the traffic is queued at the ISP-side gateway but not the user-side gateway. An idea is scheduling the requests at the user-side gateway to control the amount of the responses queued in the ISP-side gateway. This work first investigates the possibility of applying the class-based fair queuing discipline, which was widely and maturely used in scheduling packets, to schedule requests. However, we found that simply applying this discipline to schedule requests would encounter the timing and ordering problems at releasing requests and may not satisfy high-class users. Thus, we propose a minimum-service first request scheduling (MSF-RS) scheme. MSF-RS always selects the next request from the class receiving the minimum service to provide user-based weighted fairness, which ensures more bandwidth for high-class users. Next, MSF-RS uses a window-based rate control on releasing requests to maintain full link utilization and reduce the user-perceived latency. The results of analysis, simulation and field trial demonstrate that MSF-RS provides fairness while reducing 23-30% of user-perceived latency on average. Besides, a MSF-RS gateway can save 25% of CPU loading.

Multiple-Resource Request Scheduling for Differentiated QoS at Website Gateway

Differentiated quality of service is a way for a website operator to provide different service levels to its clients. Traditional HTTP request scheduling schemes can achieve this, but they schedule requests to manage only one server resource, such as CPU or disk I/O. Actually, processing a request on the server will consume multiple resources. This paper presents a multiple-resource request scheduling algorithm, called mQoS, for differentiating the utilization of the server resource. The mQoS scheduler consists of several sub-schedulers and a main scheduler. Each sub-scheduler manages a server resource to differentiate its utilization among the classes. The main scheduler checks the availability of every server resource and triggers an appropriate sub-scheduler to balance the utilization of server resources. The implementation of the mQoS gateway is based on Squid and Linux. The evaluation compares the mQoS scheduling with no scheduling (nQoS) and single-re source request scheduling (sQoS). The mQoS scheduling reveals the accurate differentiation on every server resource. In addition, the total server throughput in the mQoS scheduling is improved by 21%, compared with the sQoS scheduling. The average user-perceived latency of the mQoS scheduling is also shorter than other scheduling.

On-the-fly TCP path selection algorithm in access link load balancing

Many enterprises install multiple access links for fault tolerance or bandwidth enlargement. Dispatching connections through good links is the ultimate goal in utilizing multiple access links. The traditional dispatching method is only based on the condition of the access links to ISPs. It may achieve fair utilization of the access links but poor performance on connection throughput. In this work, we propose a new approach to maximize the per-connection end-to-end throughput by the on-the-fly round trip time (RTT) probing mechanism. The RTTs through all possible links are probed by duplicating the SYN packet during the three way handshaking stage of a TCP connection. Combined with the statistical packet loss ratio and the passively collected link metrics, our algorithm can real-time select a link which provides the maximum throughput for the TCP connection. The experiment results show that the accuracy of choosing the best outgoing access link is over 71%. If the second best link is chosen, it is usually very close to the best, thus achieving over 89% of the maximum possible throughput. The average per-connection throughput for our algorithm and the traditional round-robin algorithm is 94% vs. 69%.

Erratum to Multiple-resource request scheduling for differentiated QoS at website gateway

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