Piecewise Framework for End-to-End Network Awareness Service in Heterogeneous Data Networks

Liang Cheng and Ivan Marsic

Department of Electrical and Computer Engineering and the CAIP Center

Rutgers, The State University of New Jersey

94 Brett Rd., Piscataway, NJ 08854-8058

{chengl,marsic}@caip.rutgers.edu

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Abstract — Network awarenessis the capability of network applications to be aware of the characteristics of the networking environment. We present a piecewise framework for end-to-end network awareness service (NAS) that allows network applications on mobile devices to acquire end-to-end performance characteristics of heterogeneous data networks. Example piecewise techniques for awareness of available bandwidth, round-trip time, and packet loss rate are developed for the framework. Analytical results show that the framework can achieve substantial performance enhancement over traditional unitary frameworks in terms of wireless-bandwidth and battery-energy consumption. At the same time, the piecewise design does not reduce awareness agility. The framework is lightweight and scalable, and network applications can flexibly employ the NAS to adapt to dynamic environment in both proactive and reactive manners. Experiments with a prototype network-aware video-on-demand application exemplify the deployment of the NAS and demonstrate its near optimum performance.

Index Terms — Network awareness, heterogeneous data networks, adaptive applications, available bandwidth, round-trip time.

I.Introduction

Heterogeneous data networks consisting of both wired and wireless data networks are becoming ubiquitous with the proliferation of mobile devices. Generally, in a heterogeneous data network a mobile host with a wireless network interface card (NIC) or a modem may communicate with other hosts in the Internet via wireless link(s). A generic architecture of heterogeneous data networks can be abstracted as in Fig. 1. It has two main parts: the mobile wireless part connected to the fixed wired part via wireless interfaces. The wireless part may include mobile ad hoc networks (MANET) involving multiple-hop wireless communications [1].

A dedicated proxy or gateway as a service point is generally used for three reasons [2]: (i) the wireless connection quality is too poor to sustain a typical client-server application, (ii) the amount of data transferred to the mobile host must be filtered because of the orders of magnitude difference in bandwidth between the wired and wireless connections, and (iii) many portable computing devices have display and processing limitations that must be addressed by the proxy before sending the filtered response to the mobile host. Additionally, a proxy/gateway is necessary in case of protocol differences between the wired and wireless parts, and this will likely remain so in the next-generation wide-band wireless networks.

Wireless links in heterogeneous data networks generally exhibit the following characteristics:


  • Low bandwidth: The bandwidth of wireless connections ranges from several Kbps, e.g., in GSM and Palm.Net systems, to several hundred Kbps, e.g., in wireless LANs. More precisely, wireless Internet access via GSM at present has limited bandwidth from 9.6 Kbps to 115 Kbps using GPRS (General Packet Radio Service), and a CDPD channel of raw capacity 19.2 Kbps may be shared by up to 30 users [2].
  • Large transmission latency: Due to the low bandwidth and media-access-control (MAC) protocol overhead, the packet-transmission round-trip time (RTT) is large. For example, the round-trip latency on a GSM link is on the order of 410 ms [3].
  • Frequent and volatile bandwidth changing: Because of dynamic channel sharing and fading, the available bandwidth of a wireless link changes frequently and abruptly. Due to the movement of the mobile user, the connection may experience cell handoff and even blackout.

Additional fundamental constraints that need to be addressed are limitations of mobile devices, including low processing power, small memory, small display, and limited battery lifetime. If Moore’s law continues to hold, mobile devices in the near future will probably not be constrained by the low processing power and the memory size. However, the small displays and limited battery lifetime are long-term factors that should be considered.

A.Adaptive Applications

Considering the above limitations, network applications need to be adaptive to the network environment to make their performance acceptable or to be enhanced. A stock-trading scenario below illustrates these points.

While sitting in a taxi, a day trader connects his laptop to the Internet through a GSM network. He tracks his stock portfolio on an e-investment website. The intra-day charts of the stocks and market indices give him information for trading decisions. Assume that when he wants to get the chart of a stock quote, the taxi enters a handoff region. A problem occurs since the decreased bandwidth of the GSM link cannot support transmission of the chart image. After the connection switches to a new cell, he may have missed a trading opportunity even if the newly available bandwidth can again support the image data traffic. A more serious problem may occur if he places a stock trading order while the taxi crosses a tunnel and the connection experiences a blackout. The connection with the website is broken and the order gets lost! A solution to the above problems is to make the application adaptive to the heterogeneous data networks. If the link cannot afford the image transmission, then it can send the data in a table format or send just the key indices that reflect the dynamics of the stock curve. A solution for the broken connection due to a blackout may be communication protocol stack adaptation to the wireless network [2].

B.Network Awareness

For an application on a mobile device to achieve behavior adaptation, it first needs to be aware of the characteristics of the heterogeneous data network. Awareness is the basis of adaptation. For example, a rate-adaptive application needs to be aware of the available bandwidth along the communication path. In addition to adaptation, resource reservation is another well-known technique for enhancing the end-to-end performance of network applications. Awareness is required even here, since the resource policy-making and admission-control are based on the awareness of the network characteristics. In this paper, network awareness is the capability of network applications on mobile devices to be aware of the end-to-end performance characteristics of heterogeneous data networks, such as available bandwidth and round-trip time. Awareness about dynamic context, such as the geographic location or proximity, may also be of interest, but it is not considered here.

An important consideration is that the cost of acquiring network awareness does not exceed its utility. If not obtained efficiently, awareness may impact the performance of heterogeneous data networks, especially the mobile devices, due to resource consumption overheads, including CPU, wireless bandwidth, and battery energy. An obvious example is the out-of-band method of measuring bottleneck bandwidth by saturating the communication path with a continuous stream of probe packets [4]. The cost/utility tradeoff is still an open research problem.

In this paper, we present a piecewise framework that partitions the end-to-end network awareness measurements into the wired and wireless components. This approach is a natural outcome of the observations about the difference between the characteristics and the dynamics of wired and wireless networks. The adaptive network applications on mobile devices use the network awareness service (NAS) to efficiently acquire end-to-end network conditions. NAS is designed as lightweight and scalable in terms of resource consumption and deployment overhead, so that it can be applied to small mobile devices.

Based on the separation principle[5], which states that media transfer, control, and management are functionally distinct architectural activities, this research focuses on the piecewise NAS framework rather than adaptation schemes for network applications. However, we show the feasibility and flexibility of employing the framework using adaptive applications in both proactive and reactive modes. Most of the existing adaptive applications adjust their behaviors according to the observed packet loss. They are called reactive because no action is taken unless the received quality of service (QoS) deteriorates. By taking advantage of the piecewise framework presented in this paper, the applications can also can effectively adapt to the dynamic environment in a proactive manner before the QoS deteriorates.

The paper is organized as follows. Section II describes the piecewise framework and relevant techniques for end-to-end network awareness service. Performance analysis is presented in Section III. A video-on-demand experiment in Section IV demonstrates the feasibility and flexibility of employing the NAS framework. Section V discusses related work. Finally, Section VI concludes the paper.

II.Network Awareness service (NAS)

A.Piecewise NAS Framework

Fig. 2 illustrates the piecewise framework for network awareness. The framework acquires, measures, integrates, and distributes the parameters that reflect the characteristics of heterogeneous data networks.


There are four main components: the NAS daemon, the NAS data repository, the NAS manager, and the NAS agent. The SNMP (Simple Network Management Protocol) agent and manager in Fig. 2 are optional in the piecewise framework.

  • The NAS daemons implement different end-to-end network awareness techniques, e.g., available bandwidth measurement. They are optional components at the mobile device, subject to the resource limitations.
  • The NAS data repository holds all data of interest that reflect network conditions. Data are acquired by NAS daemons and other service agents (e.g., SNMP agents) at the proxy or gateway, collected from the NAS agents at mobile devices, integrated via an inference engine, and distributed by the NAS manager. The repository has a hierarchical architecture conforming to the SNMP MIB standard for simplicity, extensibility, and compatibility.
  • The NAS manager manages the NAS data repository. It collects, integrates, and distributes the NAS data. The manager may communicate with other service agents, such as the SNMP agent or SNMP manager, to acquire additional information that the existing NAS daemons do not provide. Its inference engine integrates data collected from the NAS daemons and the SNMP manager or agent, if available. Once the manager is activated at the proxy or gateway, it advertises its existence through a well-known multicast channel defined in the NAS implementation. When the manger receives a query message from a NAS agent, it replies with the corresponding NAS data.
  • The NAS agent provides NAS data to applications. It sends registration messages to the NAS manager through the well-known multicast channel in activation. Applications may subscribe to the NAS agent, or send an explicit query message to it. If the information required by the query message cannot be obtained by the local NAS daemons, the NAS agent will send a query to the NAS manager to perform the awareness task and return the reply.

The NAS query/reply procedure can occur at the application’s startup phase and/or at the runtime phase. At startup, the applications deploy NAS in the proactive mode, while at runtime they may use NAS in both proactive and reactive modes.

B.Service/Data Distribution Model

The agility of the adaptive applications depends on the timely distribution of the NAS data. It is therefore important to distribute data effectively between different entities in the NAS framework. We use two distribution models: a push and a pull model.

In the push model, the data provider takes the initiative by “pushing” data to the consumer, while in the pull model the consumer “pulls” data from the supplier. In the NAS framework, applications subscribe to the NAS agent, thus implementing the push model, while the pull model is implemented through the applications’ querying the NAS agent (and the NAS manager). There are pros and cons for using either model. The push model requires runtime support for data delivery while the pull model needs only a well-defined pull interface. However, only a hookup with callback is needed to use the push model, while the time and frequency of polling must be specified in the pull model. In fact, these two models can be considered as the tradeoff between simplicity of implementation (pull model) and simplicity of use (push model). In the mobile wireless environment, it may be difficult to know when to push and when to pull. There are security/privacy concerns about the push model because it sometimes reveals the current status of the mobile user. However, the push model can provide timely updates for the information. We chose a combination data distribution model for the NAS framework; both models are used to distribute the NAS data.

Techniques for measuring network parameters are essential for the NAS. Three methods for end-to-end NAS are presented below: (i) available bandwidth estimation by processing the inter-packet time, (ii) round-trip time measurement of a data path, and (iii) awareness of packet loss type in heterogeneous data networks.

C.Piecewise Techniques for End-to-End NAS


Unlike a traditional, unitary awareness framework in which measurement tasks are solely performed at mobile hosts, techniques in the piecewise NAS framework acquire end-to-end network conditions in a piecewise way. Basically the network awareness is divided into two parts: awareness of the mobile wireless part and the fixed wired part, as illustrated in Fig. 1. The NAS manager or NAS agent integrates the intermediate results about these two parts and provides the result to the application. Here we present three example piecewise NAS techniques.

1)Available Bandwidth

The end-to-end available bandwidth is a frequently used parameter for adaptive applications. Generally, the end-to-end available bandwidth is the bottleneck bandwidth along the communication data path.

The essential idea of an awareness method for available bandwidth [4] uses inter-packet time to estimate the characteristics of the bottleneck link. If two packets (e.g., ICMP probe packets) travel together so that they are queued as a pair at the bottleneck link with no packet intervening between them, then their inter-packet spacing is proportional to the processing time required for the bottleneck link to transmit the second packet of the pair (Fig. 3).

We have implemented the inter-packet time method in the NAS framework. In our implementation, the probing packets can be payload packets, and explicit ICMP probe packets as well. When an application queries the NAS agent about the available bandwidth, the agent delegates the NAS manager to perform this awareness task by sending a query with the address of the destination host. The manager divides the task into three steps. First, it activates the NAS daemon at the proxy/gateway responsible for the awareness of available bandwidth to measure the available bandwidth between the mobile host and the proxy, say, b1. Next, it gets the available bandwidth between the proxy and the remote Internet host/server from the measurement by the NAS daemon, say, b2. Finally the manager computes the minimum of b1 and b2 and replies back to the NAS agent with the result. Note that the two steps can be done in parallel. Finally, the NAS agent replies with the result to the application.

2)Round-Trip Time and Packet Loss Rate

Similar to the awareness of the available bandwidth, the NAS agent delegates this task to the NAS manager. The RTTs are measured between the mobile host and the proxy/gateway (rtt1) and between the proxy/gateway and the remote Internet host/server (rtt2). The manager replies with the sum of rtt1 and rtt2 back to the NAS agent, which finally replies to the application.

Similarly, the packet loss rates are measured between the mobile host and the proxy/gateway (l1) and between the proxy/gateway and the remote Internet host/server (l2). The manager replies the result, 1(1l1)(1l2), back to the NAS agent, which forwards the reply to the application.

The NAS framework is piecewise not only because the NAS components are distributed between the mobile device and the proxy/gateway, but also since the awareness tasks are distributed from the NAS agent at the mobile host to the NAS manager at the proxy/ gateway. The next section shows that the wireless bandwidth and computing resources are preserved.

III.Performance Analysis

A.Performance Analysis Model


The performance analysis model for heterogeneous data networks is shown in Fig. 4. Although the wireless communication in MANET may be multi-hop, the data path between a mobile device and the proxy/gateway can still be abstracted as a one-hop wireless link in terms of the end-to-end packet-loss characteristics. Similar to Fig. 1, there are two parts: the mobile wireless part and the fixed wired part. The end-to-end path is between the mobile host and the remote host. Either of them could be the sender or the receiver. Assume that the path between the sender and the receiver is symmetric, meaning that the communication path characteristics are the same in both directions. In practice this is not true, though the difference is usually small.