Elsevier

Computer Communications

Volume 22, Issue 12, 25 July 1999, Pages 1111-1122
Computer Communications

Incorporating priority scheme in IPv6 over ATM networks

https://doi.org/10.1016/S0140-3664(99)00078-XGet rights and content

Abstract

An effective approach to support multiple QoS classes in IPv6 running over ATM networks is implemented by mapping the priority field of IPv6 into the ATM format. Such a scheme might not be necessary for simple congestion control in a network of equal capacity links, however, it might be important for multicasting to destinations reachable through a very diverse capacity network. When network congestion occurs, the cell loss requirements for different services can be satisfied by selectively discarding low priority cells. As the multi-priority scheme used in IPv6 is a “differentiated service” mechanism to enhance the QoS by best effort, its performance is certainly affected by the background traffic when these multi-priority traffic are multiplexed at the ATM level. This paper focuses on the performance with multi-priority scheme under different traffic conditions to determine the loss characteristics. The expected results are how QoS is affected and how priority can be given for more important traffic in the case of congested network.

Introduction

IPv6 is a new version of IP designed as an evolutionary step from IPv4. It is a natural increment to IPv4 and can be installed as a normal software upgrade in Internet devices and is inter-operable with the current IPv4. However, IPv6 is also designed to run well in high performance networks such as ATM and provides a platform for new Internet functionality to support real-time services.

A 4-bit priority field is designed in the IPv6 header [1], [2], [3] for originating nodes and/or forwarding routers to identify and distinguish between different classes of priority of IPv6 packets, so that it provides various forms of “differentiated service” for the IP packets, other than through the use of explicit flow set-up. Four priority bits representing 16 classes are divided into two categories: values 0 through 7 specify the priority of traffic for which the source provides congestion control, known as congestion controlled traffic, such as TCP traffic. The details of priority values recommended for particular applications in this category [1] are shown in Table 1.

On the contrary, values 8 through 15 specify the priority of traffic that does not back-off in response to congestion, and is known as non-congestion controlled traffic, such as “real-time” packets being sent at a constant rate. In this category, the lowest priority value 8 is used for those packets that the sender is most willing to discard under conditions of congestion such as high-fidelity video traffic. The highest priority value 15 is used for those packets that the sender is least willing to discard such as low-fidelity audio traffic [19].

By contrast, the standard ATM cell format only incorporates 1 bit as the cell loss priority (CLP) in the cell header. Hence, a two-level hierarchical coding scheme can easily be implemented in the mainstream of ATM specification. There are no direct provisions for a multilevel hierarchical coding or traffic priority scheme, because such a scheme might not be necessary for simple congestion control in a network of equal capacity links. However, it might be important for multicasting to destinations reachable through a very diverse capacity network [4]. When IPv6 is running over ATM network, the mapping of the priority field between IPv6 and ATM cell will have problem because the ATM cell has only 1 CLP bit, while the IPv6 priority levels require up to 16 CLP bits, using four priority bits. In order to adapt the multi-priority level defined in IPv6 in the ATM networks, we consider an approach which uses the ‘111’ reserved code in the PT field [5] of the ATM cell header to indicate the octet immediately following the ATM cell header containing the ATM priority level corresponding to the IPv6 priority field, as shown in Fig. 1.

Note that only the lower 4 bits of the first octet are defined for the priority levels, this is the same as the 4-bit priority field defined in IPv6. The upper 4 bits are reserved to facilitate the proposed change that the 4-bit priority field can be increased to an 8-bit traffic class field, as defined in the latest Internet Draft for RFC1883 [1]. Corresponding to the priority field defined in IPv6, the proposed ATM priority values are also sub-divided into two categories. The first priority category is scheduled to meet the different delay requirements in which the top priority class is given to the most delay sensitive traffic such as voice and video. By contrast, the second priority category is scheduled to meet the different cell loss rate requirements because in many data transfer applications such as file transfer, real time delivery is not of primary concern. Data traffic is more loss sensitive, and hence, must be received error free [6]. In these cases when network congestion occurs, different cell loss requirements can be satisfied by selectively discarding the ATM cells on priority basis.

As the multi-priority scheme used in IPv6 is a “differentiated service” mechanism to enhance the QoS by best effort rather than QoS guaranteed by bandwidth allocation, the QoS will certainly be affected by the background traffic when these multi-priority traffic are multiplexed at the ATM level. Hence, the performance study, to determine the loss characteristics and comparison of the effects on different priority classes, with multi-priority scheme under different traffic conditions is one of the most important issues on the IPv6 traffic running over ATM networks. This paper focuses on how the QoS is affected by traffic characteristics when IPv6 streams are transferred over ATM network.

Section snippets

Adaptation of IPv6 multi-priority traffic in the ATM network

The overall framework of IPv6 over the ATM network is shown in Fig. 2a. Logical link control (LLC) encapsulation is needed when several protocols are carried over the same virtual channel [7]. In order to allow the receiver to process the incoming AAL5_CPCS_PDU properly, the payload field must contain information necessary to identify the protocol of the PDU [18].

In the LLC encapsulation, the protocol of the routed PDU is identified by prefixing the PDU using an IEEE 802.2 LLC header, which is

Modeling and performance analysis with multi-priority scheme

The performance of incorporating priority scheme in IPv6 over ATM networks is evaluated using push-out scheme [9], [10], [11], [12], [13], [14]. However, this scheme does not reduce the total cell loss rate. Its objective is to protect the high priority traffic from cell loss, while allowing the performance of the low priority traffic to degrade as little as possible [20].

As shown in Fig. 4, the push-out scheme operates only when the buffer is full, i.e. an arrival of low priority cell is

Conclusion

The introduction of a priority scheme in the IPv6 over ATM networks adds a quality of service flexibility. From the study, we show that incorporating priority scheme in IPv6 over ATM networks improve the performance of high priority traffic. These schemes do not reduce the total cell loss. Rather, they protect the high priority traffic from cell loss, while allowing the performance of the low priority traffic to degrade as little as possible. The behavior of priority scheme is studied

Dr. Liren Zhang is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng. degree from Shandong University of Technology in 1982, M.Eng degree from the University of South Australia in 1988, and Ph.D. from the University of Adelaide, Australia in 1990, all in electrical engineering. From 1990 to 1995 he was a Senior Lecturer in the Department of Electrical and Computer Systems Engineering, Monash

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Dr. Liren Zhang is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng. degree from Shandong University of Technology in 1982, M.Eng degree from the University of South Australia in 1988, and Ph.D. from the University of Adelaide, Australia in 1990, all in electrical engineering. From 1990 to 1995 he was a Senior Lecturer in the Department of Electrical and Computer Systems Engineering, Monash University, Australia. Dr. Zhang has vast experience as an engineer, academic and researcher in the field of multimedia communications, switching and signaling, teletraffic engineering, network modeling and performance analysis for ATM networks, high speed data networks, mobile networks and satellite networks. He has published more than 50 research papers in international journals and conferences.

Mr. Kee Way Ng received his B.Eng. (Hons.) and M.Eng. from Nanyang Technological University, Singapore in 1997 and 1999, respectively, all in electrical and electronic engineering. His research interests include computer communications and ATM networks.

Chee-Hock Ng is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng. (Hons.) degree from the National University of Singapore in 1981 and M.Sc. (Computer Engineering) degree from the University of Southern California in 1987. He has vast experience as an engineer, academic and researcher in the field of data communications, networking and network performance analysis. He has published many technical papers in international journals and conferences. He is also the author of a book “Queueing Modelling Fundamentals”, published by John Wiley in 1996.

Regu Subramanian is currently an Associate Professor at the School of EEE, Nanyang Technological University, Singapore. He received a B.Eng. (First Class Hons.) degree from University of Madras and M.Sc. from the University of Alberta, Canada. From 1969 to 1976, he was with Alberta Government Telephones, where he acquired extensive experience in telecommunication networks and systems. Before joining NTU, he was with Bell-Northern Research from 1976 to 1984, where he was Manager of the Access Network Evolution Planning Department, conducting system research in telecommunication network characterization, evolution, strategic planning and fiber optic applications. He is a Chartered Engineer and a Professional Engineer. The areas of his current interests are computer networks, fiber optic communication and management of technology.

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