Laila Daniel

Using Quick-Start to Improve TCP Performance with Vertical Hand-offs

Vertical hand-offs between different wireless access technologies have become more relevant after the introduction of multi-access mobile terminals with wireless LAN (WLAN) and wireless WAN (WWAN) technologies. While the IP mobility mechanisms are rather well known, the performance of TCP still has problems when moving between WLAN and WWAN accesses. First, with a high-latency WWAN link technology such as GPRS it takes several seconds before the TCP congestion window has reached the path capacity. Second, when the notification of the first packet loss arrives at the TCP sender, several packets have already been lost due to the slow-start overshoot and the TCP sender needs to retransmit a large number of the packets from the last transmission window. Third, after a vertical hand-off the path characteristics might have changed dramatically in which case the TCP congestion control state is not valid any more. In this paper we investigate Quick-Start, a mechanism for avoiding the initial slow-start delay, in the context of wireless multi-access terminals. We also propose an enhancement to Quick-Start to alleviate the effects of slow-start overshoot and apply Quick-Start after a vertical hand-off to quickly learn the available capacity on the new end-to-end path. An explicit cross-layer hand-off notification is employed to trigger Quick-Start when the hand-off completes. We conduct simulations with different hand-off models, and our simulations yield promising results with Quick-Start

Adapting TCP for Vertical Handoffs in Wireless Networks

With the growing use of multiradio mobile terminals a vertical handoff between different wireless access technologies is becoming increasingly common. The vertical hand-off may result in a significant change in the access link characteristics that can affect the performance of TCP dramatically as its behaviour depends on the end-to-end path properties that also change consequently. We propose a number of simple enhancements to the TCP sender algorithm which make use of explicit information about the change in the link characteristics due to handoff. We study the effectiveness of the enhancements in a simulated WLAN-GPRS environment with different handoff scenarios. The enhancements are shown to improve TCP performance significantly

Employing cross-layer assisted TCP algorithms to improve TCP performance with vertical handoffs

In this paper we study the performance of TCP with a vertical handoff between access networks with widely varying link characteristics. TCP being an end-to-end protocol has performance problems as its behaviour depends on the end-to-end path properties which are likely to be affected by a vertical handoff. We propose a set of enhancements to the TCP sender algorithm that leverage on explicit cross-layer information regarding the changes in the access link delay and bandwidth. We carry out a systematic study to identify the problems of regular TCP and compare the performance of the regular TCP with the performance of the enhanced TCP algorithms in various handoff scenarios between access networks having different bandwidth and delay characteristics. Our experiments show that with cross-layer notifications TCP performance can be improved significantly in most vertical handoff scenarios.

Experimental evaluation of alternative congestion control algorithms for Constrained Application Protocol (CoAP)

The wide range of IoT applications creates a demand for various types of communication. While both types of communication, unreliable and reliable, are important, it is crucial to extend scalable congestion control in the Internet to cover IoT communication also. Constrained Application Protocol (CoAP), a web transfer protocol for constrained devices and networks proposed by Internet Engineering Task Force (IETF), has optional reliability based on retransmission timeout (RTO) together with exponential RTO backoff to implement a simple basic congestion control mechanism. While the CoAP basic congestion control is simple, it is relatively conservative and potentially inefficient. Hence, there is a need for a more efficient congestion control alternative and CoCoA has been proposed as an option. In this paper we experimentally evaluate the efficiency and scalability of alternative congestion control algorithms for CoAP, including CoCoA and two TCP-based mechanisms. Our results show that while all the alternatives are scalable, they are more aggressive than the default congestion control mechanism of CoAP resulting in more efficient operation particularly at higher congestion level(s).

Using cross-layer information to improve TCP performance with vertical handoffs

this paper we study the performance of TCP with a vertical handoff between access networks with widely varying link characteristics. TCP being an end-to-end protocol has performance problems as its behaviour depends on the end-to-end path characteristics which are likely to be affected by a vertical handoff. We propose a set of enhancements to the TCP sender algorithm that leverage on explicit cross-layer information regarding the change in the access link delay and bandwidth. We apply our enhanced algorithms and carry out a systematic study comparing the performance of the regular TCP to the performance of the enhanced TCP algorithms in various handoff scenarios between access networks having widely different bandwidth and delay characteristics. Our experiments show that with cross-layer notifications TCP performance can be improved significantly in most vertical handoff scenarios.

Combating Packet Reordering in Vertical Handoff Using Cross-Layer Notifications to TCP

In this paper we propose an enhancement to the TCP sender algorithm to combat packet reordering that may occur due to a vertical handoff from a slow to a fast access link. The proposed algorithm employs cross-layer notifications regarding the changes in the access link characteristics. Our algorithm avoids unnecessary retransmissions by dynamically changing the dupthresh value according to the bandwidth and delay of the old and new access links involved in the handoff. In addition it uses DSACK information to infer that there are no congestion-related losses and selects proper values for cwnd and ssthresh after the handoff. Simulation results show that the unnecessary retransmissions caused by packet reordering in a vertical handoff can be effectively minimized over a wide range of bandwidth and delay ratios of the access links. In addition, our algorithm is effective in reducing the congestion-related packet losses due to a decrease in bandwidth-delay product (BDP) after a handoff.

The performance of multiple tcp flows with vertical handoff

The performance of an individual TCP flow with a vertical handoff has been studied in several papers. However, the effect of a vertical handoff on multiple TCP flows has been little studied. In this paper we study the behaviour of multiple competing TCP flows with a vertical handoff. As a part of this study we evaluate the cross-layer assisted TCP enhancements for a vertical handoff that we had earlier proposed and analyzed for a single TCP flow. We show that our algorithms can be adapted for multiple TCP flows with minor modifications and that they are effective in improving multiple flow-TCP performance in the presence of a vertical handoff.

Experimental Evaluation of the CoAP, HTTP and SPDY Transport Services for Internet of Things

Internet of Things (IoT) seeks to broaden the scope of the Internet by connecting a variety of devices that can communicate with the Internet. Transport services for IoT are crucial in enabling the applications to communicate reliably and in a timely manner while making efficient and fair use of the potentially scarce network resources. The communication with IoT devices is often implemented using HyperText Transfer Protocol (HTTP) or a specifically designed protocol such as Constrained Application Protocol (CoAP) that is a specialized web transfer protocol for constrained nodes and networks. In this paper we discuss various options for modifying or adapting HTTP to offer better transport service for IoT environments. We consider HTTP, SPDY that has been developed to speed up HTTP in general, IoT-HTTP and IoT-SPDY that are adaptations of HTTP and SPDY for IoT, and CoAP as transport services for IoT and experimentally evaluate their performance. The results of our experiments show that CoAP has the lowest object download times and the least number of bytes transferred compared to the other four transport services. IoT-HTTP and IoT-SPDY have around 50% shorter object download times and smaller number of bytes transferred compared to HTTP and SPDY.

TCP Behaviour with Changes in Access Link Bandwidth and Delay During Vertical Handoffs

In this paper we study TCP performance in the presence of vertical handoffs between access networks with widely varying link characteristics. TCP being an end-to-end protocol has performance problems as its behaviour depends on the end-to-end path characteristics which are likely to be affected by a vertical handoff. We carry out a systematic study of TCP behaviour and the consequent problems that arise in various handoff scenarios between networks with different bandwidth and delay characteristics. This study can serve as a basis to evaluate any modifications to the TCP algorithm to adapt to a vertical handoff.

Enhancing TCP with cross-layer notifications and capacity estimation in heterogeneous access networks

Finding the available network capacity for a TCP connection is an important research problem as it allows the connection to improve its throughput and fairness in addition to reducing packet losses. As flows arrive and depart randomly in the network, the state of a TCP connection at any instant is very dynamic and a good estimate of the available capacity can enable TCP to quickly adapt to the actual available capacity in the network. This is especially relevant to heterogeneous access network environments where the end-to-end path characteristics of a TCP connection may abruptly change due to the changes in the access link characteristics after a vertical handoff. In this paper, we present an approach that combines available network capacity estimation with cross-layer notifications to TCP about the access link bandwidth and delay to quickly determine a rough estimate of the available capacity for a TCP connection. Using simulation experiments we evaluate our algorithms in the different phases of a TCP connection where the available capacity is unknown such as in the beginning of a TCP connection and after a vertical handoff. Our results show that the proposed algorithms improve TCP throughput and reduce the transfer time after a vertical handoff in heterogeneous access networks.