Browsing by Author "Tsao, SC"

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Assessing and improving TCP rate shaping over edge gateways
(2004-03-01)
Wei, HY
;
Tsao, SC
;
Lin, YD
;
資訊工程學系
;
Department of Computer Science
Computers installed with commercial/open-source software have been widely employed as organizational edge gateways to provide policy-based network management. Such gateways include firewalls for access control, and bandwidth managers for managing the narrow Internet access links. When managing the TCP traffic, pass-through TCP flows can introduce large buffer requirements, large latency, frequent buffer overflows, and unfairness among flows competing for the same queue. So, how to allocate the bandwidth for a TCP flow without the above drawbacks becomes an important issue. This study assesses and improves TCP rate shaping algorithms to solve the above problems through self-developed implementations in Linux, testbed emulations, live Internet measurements, computer simulations, modeling, and analysis. The widely deployed TCP Rate control (TCR) approach is found to be more vulnerable to Internet packet losses and less compatible to some TCP sending operating systems. The proposed PostACK approach can preserve TCR's advantages while avoiding TCR's drawbacks. PostACK emulates per-flow queuing, but relocates the queuing of data to the queuing of ACKs in the reverse direction, hence minimizing the buffer requirement up to 96 percent. PostACK also has 10 percent goodput improvement against TCR under lossy WAN environments. A further scalable design of PostACK can scale up to 750Mbps while seamlessly cooperating with the link-sharing architecture. 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 shaping TCP traffic at edge gateways
(2004)
Wei, HY
;
Tsao, SC
;
Lin, YD
;
資訊工程學系
;
Department of Computer Science
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 TCR's advantages while avoiding TCR's 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-the-fly TCP path selection algorithm in access link load balancing
(2004)
Lin, YD
;
Tsao, SC
;
Leong, UP
;
資訊工程學系
;
Department of Computer Science
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 on 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 to choose 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%.
Pre-order Deficit Round Robin: a new scheduling algorithm for packet-switched networks
(2001-02-01)
Tsao, SC
;
Lin, YD
;
資訊工程學系
;
Department of Computer Science
In recent years, many packet fair queueing algorithms have been proposed to approximate generalized processor sharing (GPS). Most of them provide a low end-to-end delay bound and ensure that all connections share the link in a fair manner. However, scalability and simplicity are two significant issues in practice. Deficit Round Robin (DRR) requires only O(1) work to process a packet and is simple enough to be implemented in hardware. However, its large latency and unfair behavior are not tolerated. In this work, a new scheme, Pre-order Deficit Round Robin, is described, which overcomes the problems of DRR. A limited number, Z, of priority queues are placed behind the DRR structure to reorder the transmission sequence to approximate packet by packet generalized processor sharing (PGPS). We provide an analysis on latency and fairness, which shows our scheme as a better alternative to DRR. In most cases PDRR has a per-packet time complexity of O(1), and O(log Z) in other specific cases. Simulation results are also provided to further illustrate its average behavior. (C) 2001 Elsevier Science B.V, All rights reserved.