Peer-to-Peer Traffic in Operator Networks - Semantic Scholar

Eighth International Conference on Peer-to-Peer Computing (P2P'08)

Peer-to-Peer Traffic in Operator Networks Ayodele Damola, Victor Souza, Per Karlsson, Howard Green Ericsson Research Packet Systems Stockholm, Sweden {ayodele.damola, victor.souza, per.p.karlsson, howard.green}@ericsson.com

Abstract

the core network. The aggregation network is made up of L2 switches as this helps ensure low cost and simplicity. The end users are connected to the aggregation network by means of access nodes (e.g., DSLAMs) and the aggregation network is connected to the core by edge nodes which perform IP routing. A widely used policy in Ethernet access networks is to force all unicast and broadcast traffic via the aggregation network to the edge nodes where routing, policing and traffic shaping is performed. This is implemented with a technology like MAC-forced forwarding [5]. This common sort of setup brings about a so-called tromboning effect. The result of this effect is that end user traffic destined to another end user connected to the same access node would first be forced up to the edge node via the aggregation network after which it returns back via the aggregation network to the recipient end user. Moreover, the traffic between end users in the same subnet but connected to different access nodes must also go through the edge node. In this setup the P2P traffic between two users connected to the same access network would hence be sent twice via the aggregation network which is clearly suboptimal.

This paper examines the issues encountered by network operators when content providers utilize peer-to-peer (P2P) technologies to distribute media. After outlining the various causes of the conflict a set of potential solutions are presented.

1. Background P2P technologies are increasingly being used for legal distribution of media content. The generated traffic consists of bandwidth intensive file downloads as well as video streams, examples include Joost [2] and BBC’s iPlayer [1]. For fixed broadband network operators this traffic has brought about a set of problems. Managed operator networks are usually partitioned into at least two parts. One partition provides guaranteed quality of service used for running traffic of value added services such as IPTV and VoIP and a best effort partition with no service guarantees provides data transport for end user Internet traffic. The outstanding issue is the introduction of contention for bandwidth in the best effort class in the operators network between regular Internet traffic (e.g., web surfing, email) and P2P traffic. The fair usage of the best effort section of the network is hence disrupted as users of P2P applications tend to use up a disproportionately greater share of the available bandwidth. The fair usage issue arises primarily due to the way P2P applications are designed, namely their greedy behavior whereby tens of parallel TCP sessions are simultaneously opened to speed up content delivery.

2.2. Peer proximity and caching Solutions addressing P2P traffic have included peer proximity awareness. The peers that are in close proximity are encouraged to exchange content between each other rather than fetching it from more distant peers. This reduces the amount of links in the access network over which P2P traffic flows and produces a clustering effect. Network caching has been proposed as a solution to seeding the P2P network with popular content. The result of caching is the reduction of the amount of duplicate pieces of content sent over the links. The strategic placement of caches and content in the network makes it possible to redirect close-by peers to these caches. An obstacle in order to enable proximity awareness in P2P networks is the choice of the proximity metric and how to obtain that kind of information. A simple solution would

2. Problem analysis and solution space 2.1. Tromboning effect DSL Ethernet based broadband access networks consist of an aggregation network and an edge node connected to

978-0-7695-3318-6/08 $25.00 © 2008 IEEE DOI 10.1109/P2P.2008.51

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2.4. Transit costs

seem to be to use IP addresses and network prefixes assuming peers in the same IP subnet are deemed to be in closer proximity. In markets with competitive equal access provision different ISPs allocate IP addresses from different prefix spaces, and thus physical locality has nothing to do with address locality. Also due to the prior mentioned tromboning effect, in an access network proximity information based on IP addresses does not necessarily give the desired result as traffic still follows a suboptimal path due to traffic force-forwarding policies.

P2P traffic also brings about an extra cost to the operators; this is the cost of traffic generated at transit points. As most of the P2P applications have no proximity awareness, a content request might be served from a location outside of the operators network incurring transit costs even though the content might be available in the network. A model where a group of ISPs with friendly (zero-cost) peering agreements band together to mitigate the P2P issue based on cooperative caches has been proposed [3]. The solution entails the selection of a cache in a neighboring ISP with whom the peering agreements are favorable over a content source out in the Internet.

Another possible way for enabling peer proximity is by mapping the position of peers to the existing network topology. Since the operators are interested in a more optimal flow of P2P traffic in their networks a proposal has been to expose via an API the topology of the underlying physical network to the P2P application [6]. This requires modifications to be made to the P2P applications to use such information and also requires for the operators to share their topology information. Operators are known to be hesitant towards the latter requirement due to security, privacy and business concerns.

3. Full solution Even though the interests of content providers and network providers are orthogonal in the P2P arena the general principles of a potential solution involves both business entities:

Indirect inference of the operator’s network topology is possible. This can be achieved by actively probing the network to discover the available bandwidth and determine more optimal paths for content delivery. Alternatively peers can prioritize sources that contribute with higher amounts of data. Peers can also be chosen based on lower latency times. These approaches are taken by most peer-to-peer applications with the goal of optimizing the end user experience (e.g., faster download times, shorter start-up delays).

• Popular media content should be moved closer to the end user. If content is closely available links up in the network are spared. This means that content should be cached or stored in the access network or in the end users terminal storage. This storage could be in desktop computers, residential gateways or set top boxes. Storage could be based on fast flash drives as the price for this type of storage decreases. • Peer-to-peer applications should be intelligent enough to obtain content from nearby sources, avoid congested links, and prefer cheaper network paths. This implies that peer-to-peer applications should be more network aware.

2.3. Connection asymmetry

DSL broadband connections and mobile connections are characterized by an asymmetry between the downlink and uplink speeds. Usually the downlink bandwidth is greater than the uplink. For a P2P system to function the overall available uplink capacity should be equal to the overall available downlink capacity. With asymmetric links this requirement is not satisfied.

• Access networks should be changed to accommodate the need for turning P2P traffic closer to end user. If content is destined to another host in the same subnet and access network there is no need to cross the aggregation network. Traffic control mechanisms deployed in the edge router can be moved down to the access node (such as policing and traffic shaping) thus avoiding congestion in the aggregation network.

The most obvious solution for this problem is to introduce support servers into the P2P system. These support servers are usually nodes with a large downlink capacity to compensate for insufficient uplink capacities of peers. They can also provide support to the application in terms of bootstrapping and sourcing of content when a peer leaves the network. Caches would be naturally collocated with such support servers. Examples of such servers are the media servers in Joost.

4. Research activities in the area Our research activities have been around the distribution of on demand IPTV using P2P technologies. A time shift TV prototype made possible by caching linear TV and redistributing it to STBs in a P2P manner has been demon-

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strated 1 . The prototype is based on caching (both in end user set top boxes and network nodes), proximity awareness and P2P content exchange. End users are capable of watching both linear and time shift TV services utilizing different content delivery mechanisms in the network. The approach shows that by utilizing the existing bandwidth at the edges of the network, bandwidth in the links in the high levels of aggregation is spared. The appropriate placement of caching servers can postpone network links upgrades and save costs. We leverage on the fact that storage prices have sharply dropped in the past years. The prototype makes clear the need for turning traffic closer to the end user. Also savings on central storage is made possible by utilizing the storage of the end terminals. Another prototype encompasses caches located in access network in an open Internet scenario. The content provider in this case is responsible for injecting content in the caching network which in its turn will distribute content in the caches considering content popularity in different areas of the network. Such kind of model could be used by content providers to off load central distribution servers and alleviate the demand for bandwidth between access and core networks. Yet another area of research is the utilization of a method for measuring end-to-end available bandwidth and bottleneck link capacity in real time. The Bandwidth Available in Real Time (BART) method [4] relies on actively probing the network path and by analyzing the observed effects of cross traffic on the probes BART updates the estimates of available bandwidth and bottleneck link capacity for each probe sample. The tool and method can be used to build and maintain optimal distribution trees for P2P streaming applications.

[4] S. Ekelin, M. Nilsson, E. Hartikainen, A. Johnsson, J. Maangs, B. Melander, and M. Bjorkman. Real-Time Measurement of End-to-End Available Bandwidth using Kalman Filtering. In Network Operations and Management Symposium. IEEE, April 2006. [5] M. T. and B. S. MAC-Forced Forwarding: A Method for Subscriber Separation on an Ethernet Access Network. RFC4562, June 2006. [6] H. Xie, A. Krishnamurthy, A. Silberschatz, and Y. R. Yang. P4P: Explicit Communications for Cooperative Control Between P2P and Network Providers. http://www.dcia.info/documents/.

4.1. Conclusions The conflict between network operators and peer-to-peer content providers is not settled by any means. While the technology needed to solve these issues is existent a good business model is still to be developed. Assuming storage is cheaper than bandwidth, network operators should start using caching of legal peer-to-peer content thereby reducing transit costs.

References [1] BBC iPlayer. http://www.bbc.co.uk/iplayer/. [2] Joost, Free online TV. http://www.joost.com/. [3] G. Dan. Cooperative Caching and Relaying Strategies for Peer-to-peer Content Delivery. In 7th International Workshop on Peer-to-Peer Systems, February 2008. 1 This prototype has been developed in partnership with the LARC, University of Sao Paulo

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