Supporting IP Multicast Streaming Using Overlay Networks Marc Brogle, Dragan Milic, Torsten Braun University of Bern Institute of Computer Science and Applied Mathematics Neubrueckstrasse 10 CH-3012 Bern, Switzerland brogle|milic|[email protected] ABSTRACT decades ago. Unfortunately, even today IP Multicast has In this paper we present our solution for providing IP Mul- not been widely deployed in networks of commercial Inter- ticast on end systems in the Internet. The goal of the pro- net service providers (ISP). Some reasons for this are: posed solution is not to replace IP Multicast, but to provide • IP Multicast must be supported by all routers on the an IP Multicast interface to applications on end systems in path from source to destination. the current Internet environment, where IP Multicast is not available. Our solution, called Multicast Middleware, is a • Additional inter-ISP coordination is required (policy software, which is based on using Application Level Multi- issues of inter-domain routing). cast (ALM) for transporting IP Multicast traffic. The use of the Multicast Middleware is transparent for applications • IP Multicast routing can be very resource intensive. on end systems, since our Multicast Middleware uses a vir- As a transition between the Internet without IP Multicast tual network interface to intercept native IP Multicast com- and its full availability to the end-user, the MBONE [9] ap- munication. In this paper we also present a performance proach has been proposed. In MBONE, the Internet is con- evaluation of our Multicast Middleware. The results of this sidered as a set of isolated IP Multicast enabled islands. evaluation show that our Multicast Middleware is able to These \islands" are interconnected by an overlay network provide high bandwidth throughput to applications. This of tunnels. The overlay network is used to tunnel the IP makes our Multicast Middleware a viable solution for sup- Multicast traffic between the MBONE islands over parts of porting multimedia streaming services, etc. the Internet that support only unicast traffic. MBONE tun- nels are implemented using the loose source routing (LSRR Categories and Subject Descriptors IP option) or by encapsulating IP Multicast packets in uni- C.2.4 [Distributed Systems]: Distributed applications; cast packets. The drawback of this approach is that tunnels C.2.5 [Local and Wide-Area Networks]: Internet have to be set up manually. As a consequence the tun- nel end-points must be permanently available and require fixed IP addresses. This prohibits most Internet end-users General Terms from using the MBONE, since usually the end-users are not Performance permanently (e.g. modem users) and do not have fixed IP addresses assigned. Keywords Although IP Multicast is not widely available, there exist numerous applications using it. The MBONE video confer- transparent overlay multicast, performance, peer-to-peer, encing tools for example include among others vat (visual multimedia streaming audio tool), nv (Network video tool), vic (video conferenc- ing tool), etc. The Access Grid Project, for example, offers 1. INTRODUCTION the Multicast Application Sharing Tool (MAST). Microsoft The multicast communication paradigm represents a com- research developed the advanced collaboration and interac- munication where a small group of senders (usually one) is tive distance learning software called ConferenceXP. This sending information to a large group of receivers. In the In- software also uses IP Multicast to achieve an efficient com- ternet this has been realized in the form of IP Multicast [8]. munication between the collaborating parties. The video IP Multicast has been proposed and specified almost two lan client (VLC) is an IP Multicast enabled video broad- casting and video playback tool, which we also used to test and evaluate our Multicast Middleware. Since MBONE was not able to provide multicast commu- Permission to make digital or hard copies of all or part of this work for nication to the Internet end-users, numerous solutions were personal or classroom use is granted without fee provided that copies are proposed to address this problem. The rising popularity not made or distributed for profit or commercial advantage and that copies of Peer-to-Peer (P2P) networks lead to a revival of mul- bear this notice and the full citation on the first page. To copy otherwise, to ticast in the form of Application Level Multicast (ALM) republish, to post on servers or to redistribute to lists, requires prior specific [2,3,6,14,20,23{25]. ALM mechanisms use similar methods permission and/or a fee. Qshine’07, August 14-17, 2007, Vancouver , Canada as IP Multicast for data dissemination, but move the replica- Copyright 2007 ACM 978-1-59593-756-8 ...$5.00. tion of multicast data from routers to the end systems. The IP Multicast Application Level Multicast Sender Sender Receiver Router Receiver Receiver Router Receiver Router Router Router Router Router Router Router Router Router Router Router Router Router Router Receiver Receiver Receiver Receiver Receiver Receiver Multicast Packet Flow Physical Link P2P Link Figure 1: IP Multicast with data replication in the routers vs. Application Level Multicast advantage of this approach is that ALM mechanisms do not Multicast Middleware. The Multicast Middleware enables require multicast support by the routers. On the other hand, the transparent use of ALM mechanisms for all IP Multi- the replication of data only on end systems is not as efficient cast enabled applications on end systems. as the replication in the routers. For example, the right side This is achieved by a virtual network interface intercept- in Fig. 1 shows a typical ALM scenario, which uses only ing and forwarding multicast packets to the Multicast Mid- unicast communication between the end systems to enable dleware. This mechanism can be used for high bandwidth multicast services. In comparison to IP multicast (presented and real time multimedia streaming as presented in [15], on the left side in Fig. 1), there is some redundancy of the where we mainly described how the Multicast Middleware data that is sent over the physical links. The reason for this can be used for video streaming, but without using an elabo- is that the replication of data is only done on end systems rate P2P / ALM infrastructure and not having implemented and not on routers. Although ALM mechanisms can never Quality of Service (QoS) mechanisms. In this paper we achieve the efficiency of IP Multicast regarding the usage of investigate performance enhancements regarding multiple network resources, it is still much more efficient than unicast hops in an overlay network, discuss the usage of a widely communication between the sender and all receivers. used P2P / ALM infrastructure called Scribe / Pastry and The advantages of ALM mechanisms are, that they are de- briefly show how the different QoS mechanisms can be ap- signed to be self-organizing and fault-tolerant, which make plied. them easier to deploy than the MBONE. This makes ALM The remainder of the paper is organized as follows: In the mechanisms better candidates for deploying multicast ser- next Section we describe communication obstacles in the vices to end-users. The drawback of using ALM mechanisms Internet. Section 3 contains a description of our proposed is that the used protocols and APIs are not standardized, Multicast Middleware. In Section 4 we present the perfor- which makes application development dependent on specific mance evaluation results. In the last section we summarize ALM protocols. Another drawback is that existing IP Mul- results and give an overview of future work. ticast enabled applications would have to be adapted to the specific API of ALM protocols. An overview and classifica- tion of peer-to-peer content distribution technologies can be 2. COMMUNICATION OBSTACLES IN THE found in [1]. INTERNET To use the best of both worlds our solution uses an ALM for Besides the lack of global IP Multicast support, there are transporting multicast traffic over the Internet. At the same further limiting factors for the communication in the In- time it offers a standard IP Multicast interface to applica- ternet. The most severe are the existence of Firewalls and tions on end systems. This solution is based on a so-called Network address translators (NATs). Firewalls are network devices that filter portions of network usual network device driver does not link to physical hard- traffic based on protocol header information and/or the data ware such as an Ethernet card, but forwards the traffic to payload of IP packets. They are used to protect network a user-space process. In our case this user space process is devices within a private network from intrusions from the the Multicast Middleware. For the applications this virtual Internet. Firewalls normally limit the ability of hosts in the network interface acts like a \real" network interface. Data Internet to connect to hosts behind them. In some networks, forwarding to the Multicast Middleware and processing is they are also used to prevent communication of hosts behind completely transparent to the operating system as well as to the firewall with hosts in the Internet. the involved applications. The Multicast Middleware uses Such installations are used as a preventive measure against the TAP mechanism to mimic an IP Multicast router at- Trojan horses, Internet worms, etc. This also limits the use tached to an Ethernet network, primarily by implementing of P2P applications on hosts behind firewalls, since these the Internet Group Management Protocol (IGMP) [5, 11]. applications assume universal connectivity between hosts. By setting appropriate routing table entries for IP multi- The address space of the IP protocol version 4 is limited to cast addresses, those packets are directed to the virtual net- 232 addresses. In addition there are IP address ranges that work interface instead of the real physical network interface.