seamless communication with overlay networks

Proceedings of the IEEE International Conference ICPWC 2005 New Delhi Jan 23-25, 2005.

PERFORMANCE ANALYSIS OF MOBILE WIRELESS OVERLAY NETWORKS BASED ON APPLICATION SERVICES By

A.K.Mukhopadhyay ,

D. Saha

College of Engg. & Management, Kolaghat [[email protected]]

Indian Institute of Management, Calcutta [[email protected]]

Abstract: Overlay network constructed from underlaying wireless base networks has become of great interest to the researchers for integration of heterogeneous networks under global perspective. Overlaying can be implemented either directly in the network layer or in the application layer. Application layer overlay is becoming more attractive due to its flexibility and expanded functionalities at the higher layer. However, the main cost paid for these is the Relative Delay Penalty (RDP). This paper prepares an overlay tree and makes a comprehensive analysis between the network services and application services. The various metrics for the application overlay networks are discussed based on some simulation results. Key Words: Application layer overlay, network services, overlay tree, relative delay penalty, link stress INTRODUCTION Intra-network and inter-network mobility on user terminal and integration of the heterogeneous networks are of great interest to the researches, service providers and even the users. Overlay network model may be an effective approach to implement these. A mobile wireless overlay network is constructed for a mix of heterogeneous underlay wireless networks, forming a logical hierarchy where low bandwidth networks with wider coverage areas are layered on top of high bandwidth, smaller coverage area networks [1]. This enhances the connectivity of mobile wireless devices by providing them with interfaces for each layer and then dynamically switching to upper layer where lower layers become unavailable. Single or multiple overlays for different purposes may be created over a common base network to provide network or application services to all, or a subset of the base network terminal users. Overlay networks can be configured using either at the IP layer (network overlay) or directly application layer. IP overlay network provides service of end-to-end delivery of IP packets by virtualization where all IP networks look like the same, despite heterogeneity at the data link layer. Therefore, network layer with mobile IP is the lowest layer required for global internetworking [2]. At the IP layer, the limited address space of IPv4 is resolved by the Network Address Translation (NAT) through the use of private addresses. But the main limitation of NAT is security which is again solved by IPv6 with its huge

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address space and in-built IPSec. The network overlay provides additional and advanced services of the Internet beyond the capability of its base network such as multicast routing, IPv6 addressing, secure data transmission etc. It uses IP-within-IP encapsulation between the overlay routers for data transmission. An overlay network is formed when a set of routers connected to a network of layer 2 switches are configured as a full mesh at the IP layer and the nodes send probes to determine the network characteristics between each other to route traffic through other nodes. The current developments in overlay networks are on application layer with some works on IP layer. Common applicationspecific overlay networks include multicasting, content distribution networks and peer-to peer file sharing services. The first generations of overlay networks (1990) are on IP layer with static routing tables and manual configuration. The second generation, (2000), signifies multicasting through application layer overlays. In this paper we propose an overlay tree according to their functionalities and make a comparative analysis on the network and application services in respect of node services, node location, addressing scheme, overlay goal, connection topology, coordination rules and layer connectivity. Finally, performance metrics are discussed based on various simulation results. APPLICATION OVERLAYS Application layer overlay provide multicast functionality above IP layer and data is transmitted between neighbors in the overlay without any need of multicast in the overlay network. The potential benefits are scalability, easier to deploy, and simplified support for higher layer functionality. Network overlay networks are composed of per-overlay routing daemon software installed at selected sites, connected by IP-encapsulation tunnels. Application overlay networks use existing networking infrastructure together with application-layer `routing', e.g. , to interconnect proxies of a cache distribution system. Interference between overlays is managed by system administrators. Data is disseminated from one source to multiple receivers through the multicast tree at the application layer instead of IP multicast which is not widely available in the IP backbone. The end systems are

Proceedings of the IEEE International Conference ICPWC 2005 New Delhi Jan 23-25, 2005.

organized into a logical overlay network and realize data transfer along the edges of the overlay network using unicast transport services. The overall flexibility in design and algorithm in the application layer improves its overall performance, provides variety of QoS features and makes it most bandwidth efficient. Thus, a large volume of data can be disseminated via application layer overlay networks. The most common way to maximize the throughput of an overlay multicast session is to split the bulk data and feed them into multiple trees. Thus, multi-tree solutions increase bandwidth utilization in application layer overlay network [3].

taking into account of .only the most common overlay networks as shown in figure 1. Parameter

Application Overlay

Network Overlay

Node services:

Application server with permanent storage, GUI, and complex services. Intelligent end Host TCP virtual circuits and UDP port pairs Application specific: URL, channels ID More user friendly; load balancing, scalable addressing, low latency, service redundancy Unidirectional hierarchies and meshes. Session redirection, content switching, replication, caching rules, agents

Packet transmission implemented by forwarding router. Intermediate nodes IP tunnels

Node Location Link Addressing Overlay Objectives

NETWORK AND APPLICATION SERVICES Connectivity topology Coordination rules:

Single or several overlays for different purposes may be created over a common base network to provide application or network services to all or a subset of the base network clients with the properties such as server proximity, redundancy, load balancing, consistency, security, multicast communication, scalable addressing, etc. Overlays can be classified by the types of their services or functionality - network services and application services, and also the methods of application of this functionality - network layer and application layer overlays [4]. There are differences between the network service overlays and the application service overlays in functionalities such as per node service, address scheme, overlay goal, coordination rules and connectivity. Network service overlays provide a data packet transmission service with the nodes as packet-forwarding engines. Whereas application service overlays provide end-user applications with complex functionality with the overlays nodes as application server with access to persistent storage, graphical user interfaces, etc. Network service overlay nodes are identified by a network specific address scheme. Application service overlay nodes are identified by application specific address Schemes like universal resource identifiers, channels ids, etc. Network service overlays attempt to minimize end-to-end packet transmission latency and minimize overall bandwidth cost. Application service overlays aim to cater user expectations for reduced latency and service redundancy. Coordination rules for network service overlays imply applicability of routing rules to each packet, whereas application service overlays are coordinated by complex rules applied to application messages. Node services are connected in a specific way to achieve service goals using these coordination rules. Network service overlays topology is a bi-directional non-cyclic spanning tree, whereas application service overlays are unidirectional hierarchies or meshes. Overlay networks can be classified into many ways depending on applications, routing, quality of service, addressing and security. We have attempted to prepare a generalized overlay tree

Network Specific: IPv4, NAT, IPv6, Minimize end-to-end latency, minimize bandwidth usage Bidirectional noncyclic spanning trees. Packet routing.

Table-1 Service Oriented Overlays Classification Table EXAMPLES OF OVERLAY NETWORKS Network Overlays MBone is a set of all multicast routers connected by unicast path and is an overlay network at the IP layer providing multicast services. It creates virtual “tunnels” using the concept of IP tunneling (IP-in-IP encapsulation) over the Internet to connect networks that support native IP multicast, enabling a global multicast architecture. IP tunneling permits gradual deployment of multicast service. 6Bone is an IPv6 overlay that can be used to transmit IPv6 packets over an IPv4 network.. X-Bone is the system for automated deployment of overlay networks operating in the IP layer. It is a system for rapid, automated deployment and management of overlay networks. Application Overlays Overcast aims to provide wide area content distribution and bandwidth sensitive multicast services while utilizing the network bandwidth efficiently. Reliable Multicast for heterogeneous networks (RMX) provide scalable multicast services to real time heterogeneous receivers to reconcile the clients differences in capabilities and network connections. Resilient Overlay Network (RON) increases robustness in routing. Overlay nodes in RON are strategically placed in the Internet domain and form a complete graph and probe other nodes for lowest latency enabling faster detection and recovery from faults due to path outages and degraded performance. Other end-system multicast routing overlay Narada is aimed to small-sized groups. Yoid is a generic overlay architecture designed to support a variety of overlay applications that are diverse in nature – such as netnews, streaming broadcasts and

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Proceedings of the IEEE International Conference ICPWC 2005 New Delhi Jan 23-25, 2005.

bulk e-mail distribution. Planet-lab is a global testbed for developing and accessing new overlay network services. OPUS provides a large scale common overlay platform and necessary abstractions to service multiple distributed applications. It automatically configures overlay nodes to dynamically meet the performance and reliability requirements of competating applications. .Structured Peer-to-Peer (p2p) overlays such as Chord, CAN, Pastry , Tapestry , Bayeux, Scribe , PAST, [PeerSearch, Kademlia, Skipnet etc provide efficient routing over an abstract namespace. They assign a portion of the namespace to each node and provide a primitive to send messages to keys, which are points in the namespace. The overlay routes a message to the node responsible for the portion of the namespace that contains the destination key. Yallcast/Yoid , AMRoute build a shared tree. Chord, Pastry, CAN build an overlay using distributed hash tables (DHT) and embedded trees. SplitStream] is a high bandwidth content distribution overlay network. SQUIRREL] is an example of cooperative Web catching. There are main two main classes of p2p routing algorithms. Chord, Pastry, and Tapestry use a generalized form of hypercube routing with divide-and-conquer approach to route in a ring. CAN [5] uses a numerical distance metric to route through a Cartesian hyper-space by choosing a neighboring node closer to the destination at each hop. The different algorithms exploit network locality for efficiency with varying degrees of success but they all are scalable, fault resistant and self-organizing. Chord aims to locate efficiently the node that stores a particular data item. Data location can be easily implemented on top of Chord by associating a key with each data item, and storing the key/data item pair at the node to which the key maps. Scribe is a large scale and decentralized application level p2p overlay for group communication with event notification multicast infrastructure using tree building. Tapestry , a peer-to-peer overlay routing infrastructure offers efficient, scalable, locationindependent routing of messages directly to nearby copies of an object or service using only localized resources. It supports a generic decentralized object location and routing applications programming interface using a self-repairing, soft-state-based routing layer and exhibits stable behavior and performance as an overlay, despite the instability of the underlying network layers. OverQoS, an overlay network, may be utilized by a third party to provide QoS services to the customers using Controlled Loss Virtual Link (CLVL) technique. It ensures that the loss rate observed by aggregation is very small as long as the aggregate rate does not exceed a certain value. OverQoS can be employed to provide Internet QoS such as differentiated rate allocations, statistical bandwidth and loss assurance and can enable explicit-rate congestion control algorithms. A similar approach is Service Overlay Networks (SON) designed

to use overlay technique to provide value-added Internet services. A SON can purchase bandwidth with certain QoS guarantees from ISP to build a logical end-to-end service delivery overlay. PERFORMANCE ANALYSIS Several metrics are used to analyze performance of different application level multicast implementations. These metrics evaluate the delay to deliver multicast messages, the load on the network, and the load imposed on end nodes. Use of application-level multicast increases delay to deliver messages compared to IP multicast. Relative Delay Penalty (RDP) is the excess delay incurred in a particular overlay relative to direct transmission in the underlying IP network and measured as the ratio between the additional packet delay introduced by the application overlay and the direct transmission delay through the underlying IP network. 90th percentile RDP is the RDP seen by 90% of the pair of nodes. Figure 2 shows typical variation of RDP with respect to node size for random construction DHT overlays such as CAN, Chord, Pastry etc. [30]. RMD or RDPmax is the ratio between the maximum delay using application-level multicast and the maximum delay using IP multicast, while RAD or RDPave is the ratio between the average delay using application-level multicast and the average delay using IP multicast. Typical values of RDP for CAN and Pastry have been plotted by analyzing the simulation results in [31 ]. Figure 3 shows RMD and RAD values for Pastry with flooding as well as tree based multicast with respect to four values of bit number of the destination key that Pastry attempts to match at each hop. Load Balance Ratio is a metric to measure the distribution of the forwarding load. It is the ratio of the maximum routing load to the median routing load. This metric is less relevant for multicast but important for multiple point-to-point communication. Figure 4 shows typical load balance ratio against overlay size for various overlay networks. Stress is a measure of excess bandwidth consumption induced by the overlay during a multicast transmission. Application-level multicast also increases the load on the network relative to IP multicast. The load on the network is evaluated using the link stress metric measured by counting the number of packets sent for each directed link over the link in the network topology. In application-level multicast, end nodes are responsible for maintaining routing information and for forwarding and duplicating packets whereas routers perform these tasks in IP multicast. Figure 5 shows typical link stress values relative to RDP for various DHT based overlays. Node Stress is the stress imposed by application-level multicast on each node and is measured as the number of nodes in each node‟s routing table and the number of messages received by each node when members join the groups.

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Proceedings of the IEEE International Conference ICPWC 2005 New Delhi Jan 23-25, 2005.

Some of the application-level multicast implementations generate duplicate messages that both waste network resources and increase load on end nodes Duplicate is another metric measured as the number of duplicates received by end nodes. Overlay topology has a great impact on the performance and routing overhead. Simulation results show [32] that physical topology aware based overlay provide resilient routing service and Link-state based on-demand routing is the best choice for topology since it is always scalable in contrast to non-scalability in full mesh.

In this work, we have addressed the common performance parameters to evaluate application overlays. However, there are additional overlay specific metrics on complexity, bandwidth, routing, traffic, security, optimization and QoS not discussed here. Higher layer functionalities and utmost design flexibility have highly mooted the researches on distributed look-up based P2P application layer overlays. But virtualization in application overlay may destroy locality and discard useful application specific information. Therefore appropriate measures are to be taken while considering the metrics at the design stage.

CONCLUSION

OVERLAY NETWORKS Network Overlay

Application Overlay

Service Network

Peer-tp-Peer

Content Distribution & Web Catching

Asymmetric

Symmetric DHTBased

MBone

VPN

Over Qos Non Scalable

QRON

Scalable

Flooding Based

File Sharing & Storage

Measurement Based

KAZZA Freenet 6Bone

SON

Narada

PAST

Napster

Gnutella XBone

Active

CDN

VNS

NICE

OceanStore Multi dimensional Routing

Napster

Overcast

JXTA

AKamai

Passive

Squirrel

RON

AMRoute VAN CAN

One dimensional Routing

ServerFarm Tree-like

Co-op Catches

ABone Chord

Skiplist Like

Pastry

Figure 1. Overlay Tree

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Scribe

Brocade

Tapestry

Proceedings of the IEEE International Conference ICPWC 2005 New Delhi Jan 23-25, 2005.

Figure 2 Variation of RDP with application overlays size

Figure 3 RMD and RAD values for Pastry

Figure 4 Variation of load balance ratio with overlay size.

Figure 5 Variation of link stress with RDP

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