Shigetomo Kimura

A proposal of DNS-based adaptive load balancing method for mirror server systems and its implementation

In order to protect a server to become overloading due to concentration of accesses, a mirror server system is widely used for distributing the load to multiservers. For such a system, it is very important for users to select an appropriate one among the servers. In the previous systems, the DNS (domain name system) Round-Robin mechanism is widely adopted for this purpose, otherwise users must choose a server by themselves. Even in the former method, however, since all servers are evenly assigned to clients without any cares of the performance and process status of the servers, the load of each server is deflected. To solve the above problem, this article proposes a new adaptive load balancing method based on the ordinary DNS Round-Robin mechanism. In this proposal, a DNS server for the mirror-server system periodically probes the status of each server. For a query from each client, the DNS server changes its response from the probed results and IP address of the client. Finally, the prototype system of the proposal is implemented and the file transfer speeds of both the proposed method and Round-Robin DNS method are estimated for the mirror server system of RingServer Project in Japan.

Adaptive two-level unequal error protection convolutional code scheme for wireless ATM networks

Because the wireless asynchronous transfer mode (ATM) networks are often constrained with the limited link bandwidth and error-prone characteristics, forward error correction (FEC) is needed to improve their bit error rate performance. In this paper, a two-level punctured convolutional code (PCC) scheme is constructed to apply to a wireless ATM cell header and various payloads with the flexible unequal error protection (UEP). Since their perforation matrices can be programmable, the code rates of PCC are applicable to the different sensitivity of source-encoded symbols, even if only the same encoding and decoding hardware is used. Finally, the performance on Gaussian and fading channels is analyzed, which shows significant reductions in cell loss rate (CLR) and good balances for CLR and the payload bit error rate (BER).

An approach to mobility management in Cellular IP networks utilising power-save mode of IEEE 802.11

Although FMIPv6, HMIPv6, and F-HMIPv6 have been proposed as enhancements to Mobile IPv6, they still have some technical barriers for fast and seamless handovers. We propose an intelligent mobility protocol benefiting from Cellular IP. To speed up the entire handover procedure, our proposed scheme enables L2 and L3 handovers to be performed in parallel by utilising power-save mode of IEEE 802.11. We provide analytical considerations on the handover latency and packet delivery cost of each protocol. Through our simulation experimental results, we have verified that our proposed scheme shows much better performance over the others under various parameter sets.

An introduction of compression algorithms into SSL/TLS and proposal of compression algorithms specialized for application

The specification of SSL/TLS defines that data from the upper layer can be compressed in the record layer before they are encrypted. Since only the no-compression is provided for the compression algorithm in the specification, nobody can compress transmission data at the SSL/TLS protocols unless communication peers have a special agreement to use a compression algorithm. However, the compression mechanism should be useful for the users of narrower bandwidth communication lines such as ordinary analog telephone lines and N-ISDN. In order to solve this problem, the paper first introduces general-purpose compression algorithms into SSL/TLS, and shows that the average transfer times for several Japanese text files via HTTP are much improved, especially for narrow bandwidth communication lines. We also compare it with the compression mechanism on HTTP, and the resulting difference between the results of the SSL/TLS compression and HTTP compression is rather small. For improving the compression ratio, we next propose compression algorithms specialized for application protocols. As examples, we focus to HTTP and HTML, and construct the tree structures of the field names and field values. From the tree, the static dictionaries for the compression algorithms are defined. Finally, we implement the compression algorithms and show the compression ratios when several HTML files are transferred.

Advanced semisoft handoff method of cellular IP access networks

Mobile IP is the current standard for supporting global mobility, however the performance degrades to support micro mobility. Cellular IP could be a good solution to accommodate efficiently hundreds of, thousands of mobile hosts that perform fast and frequent handoff in a small local area network. In semisoft handoff of cellular IP, a mobile host sends a semisoft packet to the new base station before performing actual handoff to guarantee that the routing cache mapping associated with the new base station is created before actual handoff takes place. But packet loss and handoff delay are still occurred by this model. We propose an advanced semisoft handoff method with no packet loss but with additional small packet delay just before or after handoff. In this method, the cross-over node calculates the time for the last message to be delivered to the mobile host via the old base station just before handoff based on four typical network models. On receiving that packet, the mobile host performs handoff to the new base station. Our simulation experiment results show that the proposed semisoft handoff method actually outperforms the traditional semisoft handoff method in both handoff packet loss and handoff delay. We study the performance of the advanced semisoft handoff method with the network models.

A Fast Authentication Method for Secure and Seamless Handoff

In mobile networks such as Mobile IP, a symmetric-key-based authentication method is a typical solution for identifying genuine users of the networks, but restricts scalability of the communications. From this reason, several asymmetric-key-based authentication methods are proposed to release the restriction. However, these methods still suffer from the heavily procedures, especially in the handoff, and public keys can not be distributed efficiently in advance. This paper introduces a key distribution and authentication mechanism for the handoff method with a certificate-based authentication. The computer simulations show that the proposed method provides low key distribution overhead of the authentication with no communications disruption.

A Cross-Layer Retransmission Control for Improving TCP Performance in Wireless LAN

In this paper we propose a cross-layer retransmission control for TCP communication over a wireless link. With our proposed control, a retransmission delay for lost packet is reduced, packet losses in the wireless link are eliminated and all packets are delivered in the correct order. No change is required to TCP itself or to the sender. Our proposed method is implemented in a queue between the media access control (MAC) layer and logical link layer in a base station, and is designed to assist local retransmission control in the MAC layer. Computer simulations show that our proposed method can maximally use the bandwidth of the wireless link under high bit error rates conditions with conventional TCP control. The fairness problem of TCP communication between connections with different bit error rates in a wireless link is also improved, and MAC level fairness is also controllable.

A compression method designed for SMTP over TLS

TLS (Transport Layer Security) is a well-known protocol used to provide authentication and private communication for application protocols like HTTP (Hypertext Transfer Protocol) and SMTP (Simple Mail Transfer Protocol) The specification of TLS lays down that in the TLS record protocol, data transferred from the upper layer can be compressed before it is encrypted However, since there are certain restrictions on the transfer of an E-mail, such as the need for text encoding of binary data and the recompression of already compressed data, using the compression mechanism on SMTP over TLS may result in a reduction in the compression performance In order to solve these problems, this paper introduces a compression method specifically designed for use with SMTP over TLS In network experiments, compression ratios and file transfer times for three kinds of E-mail were observed, with emulated bandwidth restrictions to represent three typical communication media, and the results show that the proposal method works efficiently especially for narrow bandwidth networks.

On-demand QoS hybrid ARQ for multimedia WATM networks

Since wireless channel is always band-limited with multi-path fading and highly burst errors, flexible error control is needed in wireless asynchronous transfer mode (WATM) networks. This paper proposes a hybrid automatic repeat request (ARQ) strategy using rate-compatible punctured convolutional (RCPC) codes, which has on-demand quality of services (QoS). In order to increase the throughput of WATM networks, the packet size for the retransmission of ARQ protocol varies according to the types of services, and the coding rate for the payload of the WATM cell is adjusted to match the varying channel. In case of cell loss rate (CLR) being constrained, such as 10/sup -6/, the throughput is analyzed and found to be better than the existing schemes.

A Minimum Contention Window Control Method for Lowest Priority Based on Collision History of Wireless LAN

Because of the widespread adoption of mobile devices, many applications have provided support for wireless LAN (WLAN). Under these circumstances, one of the important issues is to provide good quality of service (QoS) in WLAN. For this purpose, Dhurandher et al. improved the distributed coordination function (DCF). In this method, the contention window (CW) is divided into multiple ranges. Each range is independent of all other ranges and is assigned to a different priority. Although the highest-priority throughput increased using this method, throughput for the other priorities significantly decreased. To overcome this problem, this paper proposes a minimum contention window control method for two (high and low) priorities. In the method, all nodes are assumed to use real-time applications or data transmission. The former real-time frames are high priority and are sent by UDP. The latter data frames are low priority and are sent by TCP. The purpose of the proposed method is not only to provide good QoS for the highest priority but also to prevent decreasing the QoS for other priorities in WLAN. For this purpose, the proposed method keeps the CW for the high priority at a low value and controls the CW for the low priority based on the collision history. Finally, the network simulations demonstrated that the proposed method reduces the decrease in the average total throughput of the low priority frames as well as reducing the packet drop rate of both priorities, compared with those for DCF and Dhurandhers method.