The Evolution of Multicast From the MBone to InterDomain Multicast to Internet Deployment Kevin C Almeroth Department of Computer Science University of California Santa Barbara CA almerothcsucsbedu Octob er Abstract Without a doubt multicast communicationthe onetomanyormanytomany delivery of datahas b ecome a hot topic It is of interest in the research community among standards groups and to network service providers For all the attention multicast has received there are still issues that have not b een completely resolved One result is that proto cols are still evolving and some standards are not yet nished From a deployment p ersp ective the lackof standards has slowed progress but eorts to deploymulticast as an exp erimental service are in fact gaining momentum The question nowishow long it will b e b efore multicast b ecomes a true Internet service The goal of this pap er is to describ e the past present and future of multicast Starting with the Multicast Backb one MBone we describ e how the emphasis has b een on developing and rening intradomain multicast routing proto cols Starting in the middle to late s particular emphasis has b een placed on developing interdomain multicast routing proto cols We provide a functional overview of the currently deployed solution The future of multicast may hinge on several research eorts that are working to make the provision of multicast less complex by fundamentally changing the multicast mo del We briey survey these eorts Finally attempts are b eing made to deploy nativemulticast routing in b oth Internet networks and the commo dity Internet We examine howmulticast is b eing deployed in these networks Intro duction Without a doubt multicast communicationthe onetomanyormanytomanydelivery of data has b ecome a hot topic It is the fo cus of intense study in the researchcommunity It has b ecome a highly desired feature of manyvendors network pro ducts It is growing into a true deployment challenge for Internet engineers It is evolving into a highly touted service b eing oered by some Internet Service Providers ISPs And nally it is starting to b e used byanumb er of companies oering largescale Internet applications and services From almost all p ersp ectives multicast is developing into one of the most interesting Internet services For all the p otential multicast has and for all the advocacy multicast has received there are still some concerns First by Internet standards multicast is an old concept yet by most measures deployment has been very slow To put deployment in p ersp ective compare multicast to the World Wide Web WWW and the Hyp erText Transfer Proto col HTTP IP multicast was rst intro duced in Steve Deerings PhD dissertation in and tested on a wide scale during an audio cast at the Internet Engineering Task Force IETF meeting in San Diego The rst WWW browser was written in and in there were ab out sites on the WWW So while multicast and the WWW are roughly the same age multicast is considered to b e in the early stages of evolution while the WWWs success inuence and use seem totally p ervasive Second IP multicast is one of the rst services to be deployed which requires additional intelligence in the network Multicast requires a nontrivial amount of state and complexity in b oth core and edge routers These requirements are at o dds with the longstanding b elief that intelligence should b e pushed to the edges of the network While many in the Internet community realize that the new generation of network services will put demands on the network the dicultyisindeploying and managing these services in an infrastructure that has a lengthy history of only oering b esteort unicast service With these concerns in mind the image of multicast may seem somewhat tarnished Is multicast then more trouble than its eciency gains and economies of scale are worth This question is esp ecially relevantifmulticast is to b e used as a moneymaking enterprise for commercial companies The challenges are to dene elegant proto cols to supp ort an infrastructure on top of which new applications can be develop ed to continue to investigate new ways of increasing eciency and reducing complexity Doing multicast the rightway is a noble endeavor and an appropriate long term research topic but the demand for working multicast has created an environment in which even shortterm functional solutions are very attractive In this pap er we attempt to describ e the past present and future of multicast The history of multicast should help the reader understand how multicast has evolved into its current state Relevant topics include a description of the Multicast Backb one MBone and an overview of the common intradomain multicast routing proto cols More recently multicast evolution has been primarily fo cused in the area of interdomain proto col development Multicast in the present can be characterized as an eort to deploy multicast on a wide scale using a triumvirate of routing proto cols These deployments have b een carried out in the two Internet backb one networksthe very high sp eed Backb one Network Service vBNS and Abileneas well as in the commo dity Internet so designated in order to distinguish it from Internet networks The future of multicast is ro oted in the continued development evaluation and standardization of new proto cols However unlike current eorts which are fo cused primarily on routing future eorts are likely to include other issues such as address allo cation management and billing We are already starting to see some eorts in these areas The remainder of this pap er is organized as follows Section describ es the early evolution of multicast in particular the development of intradomain multicast The fo cus of Section is on interdomain multicast including the b est current practices and several of the eorts to dene the next generation of proto cols Section details interdomain deployment eorts in the commo dity Internet and in Internet networks Section is the conclusion of the pap er Evolution of IntraDomain Multicast From the rst Internetwide exp eriments in to the middle of standardization and de ploymentinmulticast fo cused on a single at top ology This top ology is in contrast to the Internet top ology which is based on a hierarchical routing structure The initial multicast proto col re search and standardization eorts were aimed at developing routing proto cols for this at top ology Beginning in when the multicast community realized the need for a hierarchical multicast infrastructure and interdomain routing the existing proto cols were categorized as intradomain proto cols and work began on standardizing an interdomain solution In this section we describ e the standard IP multicast mo del and the evolution and characterization of intradomain multicast proto cols The Standard IP Multicast Mo del Stephen Deering is resp onsible for describing the standard multicast mo del for IP networks This mo del describ es how end systems are to send and receive multicast packets The mo del includes both an explicit set of requirements and several implicit requirements An understanding of the mo del will help the reader understand part of the evolutionary path multicast has taken The mo del is as follows IPStyle Semantics A source can send multicast packets at any time with no need to register or to schedule transmission IP multicast is based on UDP not TCP so packets are delivered using a b esteort p olicy Op en Groups Sources only need to knowa multicast address They do not need to know group memb ership and they do not need to be a member of the multicast group to which they are sending A group can haveanynumb er of sources Dynamic Groups Multicast group memb ers can join or leave a multicast group at will There is no need to register synchronize or negotiate with a centralized group management entity The standard IP multicast mo del is an endsystem sp ecication and do es not discuss require ments for how the network should p erform routing The mo del also do es not sp ecify anymechanisms for providing quality of service security or address allo cation Birth of the Multicast Backb one Interest in building a multicastcapable Internet motivated by Deerings work b egan to achieve critical mass in the late s This work led to the creation of multicast in the Internet and the creation of the Multicast Backb one MBone In March the MBone carried its rst worldwide event when sites received audio from the meeting of the Internet Engineering Task Force IETF in San Diego While the conferencing software itself represented a considerable accomplishment the most signicantachievement here was the deploymentof a virtual multicast network The multicast routing function was provided byworkstations running a daemon pro cess called mrouted pronounced mrouted which received unicastencapsulated multicast packets on an incoming interface and then forwarded packets over the appropriate set of outgoing interfaces Connectivity among these machines was provided using pointtop oint IPencapsulated tunnels Each tunnel connected twoendpoints via one logical link but could cross several Internet routers Once a packet is received it can b e sent to other tunnel endp oints or broadcast to lo cal memb ers Routing decisions were made using the Distance Vector Multicast Routing Proto col DVMRP An example of connectivity provided via a virtual top ology is shown in Figure In this earliest phase of the MBone all tunnels were terminated on workstations and the MBone top ology was such that sometimes multiple