Toru Osuga

A method for correcting minimum RTT of TCP congestion control in mobile networks

In mobile networks such as LTE and 3G, there is a problem in that the transmission control protocol (TCP) cannot sufficiently utilize network bandwidth and TCP throughput decreases. One cause of this degraded TCP throughput is the underestimation of minimum round trip time (RTT) in TCP congestion control algorithms. Handovers between mobile networks might increase propagation delay, so it is important to increase the minimum RTT appropriately. However, conventional methods cannot increase the minimum RTT because the minimum RTT is updated when a shorter RTT is observed. This causes many delay-based TCP variants to decrease the congestion window size, which in turn decreases TCP throughput. In this paper, we propose a method that revises the minimum RTT in mobile networks based on information available at the TCP sender node. Simulation results indicate that the proposed method can appropriately revise the minimum RTT.

A Cache Replacement Method for Crowded Streaming Cache Servers Responding to Rapidly Changing Access Patterns

The emergence of high quality video streaming services over the Internet has caused significant increases in the volume of traffic. Until now, cache technologies that have deployed frequently requested content in the proximity of users to mitigate traffic have been widely deployed on the Internet to improve user experiences and make them more effective. However, even when a cached object becomes a candidate for eviction due to lower access frequency, the object cannot be evicted since it is locked in working status until delivery is completed. This degrades the cache hit ratio and it has recently become much more common in high access traffic environments. This paper proposes a new method of replacing caches, called scheduled eviction of lingering caches (SELC). SELC selects an eviction candidate object based on replacement latency and shuts new access out by redirecting the access to an origin server so as to unlock the object by completing delivery for existing requests. Consequently, SELC can preserve cache space for emerging content. The results we obtained from simulations indicated that the proposed method could improve the cache hit ratio of crowded cache servers in an environment of rapidly changing access frequency.

Degree-Constrained ALM Tree Construction Algorithm for Heterogeneous Access Environment

Point-to-multipoint bulk data transfer using application-layer multicast (ALM) is discussed. In ALM, each host participating in an application session makes copies of received data by application-layer manipulation and forwards the copies to other hosts via unicast connections. Therefore, the performance, especially for access link speed, of hosts strongly affects overall delivery performance. In heterogeneous access environments, there is a strong possibility that some hosts with low-speed links decrease overall delivery performance. However, many previous solutions constructed ALM trees by introducing arbitrary degree allocations and by not well considering a diversity of access link speeds of hosts. This paper proposes an algorithm to construct an efficient ALM tree from hosts with heterogeneous access lines. This algorithm minimizes delivery completion time and lets many hosts complete reception of all data as fast as possible. The effectiveness of the algorithm is verified using simulations.