1 IP Multicasting Concepts Overview This lesson represents an entry point to IP multicast services, presents the functional model of IP multicasting, and gives an overview of technologies present in IP multicasting. The student will grasp the idea of IP multicasting, its benefits and associated caveats, and will determine various types of multicast applications. The student will gain an understanding of the IP multicast conceptual model and its implementation prerequisites. Objectives Upon completion of this chapter, the student will be able to: n Explain the benefits of multicasting and identify possible caveats n List various types of multicast applications and their requirements n Present the basic conceptual IP multicast model n Identify the addressing issues of IP multicasting n Explain the way multicast applications work and identify the prerequisites that have to be fulfilled for an implementation 1-2 IP Multicast Technology Copyright 2000, Cisco Systems, Inc. IP Multicast – Benefits and Caveats Objectives Upon completion of this section, the student will be able to: n Compare traditional unicast delivery to multicast data distribution n Explain the benefits of IP multicasting n Indicate possible problems associated with IP multicasting Copyright 2000, Cisco Systems, Inc. IP Multicasting Concepts 1-3 © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 5 Why Multicast? Why Multicast? •Used when sending same data to multiple receivers •Better bandwidth utilization •Less host/router processing •Used when receivers’ addresses unknown •Used when simultaneous delivery for a group of receivers is required (“simulcast”) Multicast may be used to send the same data packets to multiple receivers. By sending to multiple receivers the packets are not duplicated for every receiver, but are sent in a single stream where downstream routers take care of packet multiplication over receiving links. Routers process fewer packets because they receive only a single copy of the packet. This packet is then multiplied and sent on outgoing interfaces where there are receivers. Because downstream routers take care of packet multiplication and delivery to receivers, the sender or source of multicast traffic does not have to know the receivers’ unicast addresses. Simulcast – simultaneous delivery for a group of receivers - can be used for several purposes including audio / video streaming, news and similar data delivery, deploying software upgrades, etc. 1-4 IP Multicast Technology Copyright 2000, Cisco Systems, Inc. © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 6 Sending to Multiple Destinations Sending to Multiple Destinations •Pure unicast: Send the same copy of data multiple times •Web technologies: webcasting - “push” the same data to multiple destinations • Traditionally, receivers subscribed to data and “pulled” it down periodically • The underlying transport for those technologies has been unicast In order to send data to multiple destinations using unicast we have to send for each receiver its own data flow. The sender has to make copies of the same packet and send them once for each receiver. Some web technologies (e.g. webcasting) use a push method to deliver the same data to multiple users. Instead of users clicking on a link to get the data, the data is delivered automatically. Users first had to subscribe to a channel in order to receive the data, and, after that, the data is periodically “pushed” to the user. The problem with the webcast is that the transport is still done using unicast. Copyright 2000, Cisco Systems, Inc. IP Multicasting Concepts 1-5 © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 7 Unicast vs. MulticastUnicast vs. Multicast Host Router Unicast Host Router Multicast Unicast transmission sends multiple copies of data, one copy for each receiver: The top example shows a host transmitting three copies of data and a network forwards each packet to three separate receivers. The host can only send to one receiver at a time because it has to create a different packet destination address for each receiver. Multicast transmission sends a single copy of data to multiple receivers. The lower example shows a host transmitting one copy of data and a network replicates the packet at the last possible hop for each receiver. Each packet exists only in a single copy on any given network. The host can send to multiple receivers simultaneously because it’s sending only one packet. 1-6 IP Multicast Technology Copyright 2000, Cisco Systems, Inc. © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 8 IP Multicast – How Does it Work? IP Multicast – How Does it Work? •The sender (source) sends one copy of a single packet addressed to a group of receivers - multicast group •Multicast routers replicate and forward the packet to all the branches where receivers (might) exist •Receivers express their interest in multicast traffic by sending control messages to routers In multicast, the sender sends only one copy of a single data packet addressed to a group of receivers – multicast group. Downstream multicast routers replicate and forward the data packet to all those branches where receivers (might) exist. Receivers express their interest in multicast traffic by registering at their first-hop router using the IGMP protocol. Copyright 2000, Cisco Systems, Inc. IP Multicasting Concepts 1-7 © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 9 Example: Audio Streaming All clients listening to the same 8 Kbps audio • Enhanced Efficiency: Controls network traffic and reduces server and CPU loads • Optimized Performance: Eliminates traffic redundancy • Distributed Applications: Makes multipoint applications possible 0 0.2 0.4 0.6 0.8 Traffic Mbps 1 20 40 60 80 100 # Clients Multicast Unicast Multicast Advantages Multicast Advantages Multicast transmission provides many advantages over unicast transmission in a one-to-many or many-to-many environment: n Enhanced Efficiency: available network bandwidth is utilized more efficiently because multiple streams of data are replaced with a single transmission n Optimized Performance: fewer copies of data require forwarding and processing n Distributed Applications: multipoint applications will not be possible as demand and usage grows because unicast transmission will not scale (traffic level and clients increase at a 1:1 rate with unicast transmission) 1-8 IP Multicast Technology Copyright 2000, Cisco Systems, Inc. © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 10 Multicast Advantages (cont.) Multicast Advantages (cont.) •Fewer resources required – bandwidth and host processing power (at sender) •Almost simultaneous delivery is assured (one packet is simultaneously forwarded across the networks) •Foundation for a whole range of new applications not possible in the past Other multicast advantages are: n For the equivalent amount of multicast traffic, the sender needs much less processing power and bandwidth n Multicast packets don’t impose as high bandwidth utilization as unicast packets, so there’s a greater possibility they will arrive almost simultaneously at the receivers n Multicast enables a whole range of new applications that were not possible on unicast (e.g. video on demand) Copyright 2000, Cisco Systems, Inc. IP Multicasting Concepts 1-9 © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 11 Multicast DisadvantagesMulticast Disadvantages • Multicast is UDP Based!!! • Best Effort Delivery: Drops are to be expected. Multicast applications should not expect reliable delivery of data and should be designed accordingly. Reliable Multicast will address this issue • No Congestion Avoidance: Lack of TCP windowing and “slow-start” mechanisms can result in network congestion. If possible, Multicast applications should attempt to detect and avoid congestion conditions • Duplicates: Some multicast protocol mechanisms result in the occasional generation of duplicate packets. Multicast applications should be designed to expect occasional duplicate packets • Out of sequence delivery: Network topology changes affect the order of delivery – the application must propery address the issue. There are also some disadvantages of multicast that need to be considered. n Most Multicast Applications are UDP based. This results in some undesirable side-effects when compared to similar unicast, TCP applications n Best Effort Delivery results in occasional packet drops. Many multicast applications that operate in real-time (e.g. Video, Audio) can be affected by these losses. Also, requesting retransmission of the lost data at the application layer in these not quite real-time applications is not feasible – Heavy drops on voice applications result in jerky, missed speech patterns that can make the content unintelligible when the drop rate gets high enough – Moderate to heavy drops in Video is sometimes better tolerated by the human eye and appear as unusual “artifacts” in the picture. However, some compression algorithms can be severely affected by even low drop rates; this causes the picture to become jerky or to freeze for several seconds while the decompression algorithm recovers n No Congestion Control can result in overall network degradation as the popularity of UDP based multicast applications grow n Duplicate packets can occasionally be generated as multicast network topologies change – Applications should expect occasional duplicate packets to arrive and should be designed accordingly n Out of Sequence delivery of packets to the application can also result during network topology changes or other network events that affect the flow of multicast traffic 1-10 IP Multicast Technology Copyright 2000, Cisco Systems, Inc. © 2000, Cisco Systems, Inc. www.cisco.com IP multicasting concepts Page 12 Multicast Disadvantages (cont.) Multicast Disadvantages (cont.) •Reliability is a special issue not addressed in original IP multicast research •Security is another area in IP multicast not sufficiently solved in the past •Proper solutions to the above issues will open space for several commercial applications (e.g. financial data delivery) Other multicast disadvantages are: n UDP has no reliability mechanisms, so reliability issues have to be addressed in multicast applications where reliable data transfer is necessary n Restricting multicast traffic to only a selected group of receivers. Eavesdropping issues are not sufficiently solved yet n Only when reliability and security issues are properly solved will some commercial applications be possible (e.g. financial data delivery) [...]... data distribution n List the benefits of IP multicasting n Indicate possible problems associated with IP multicasting Review Questions n How many copies of the same data are sent by an IP multicast-enabled sender? n How many copies of the same packet are forwarded by IP multicast routers? n List the benefits and explain the disadvantages of IP multicast n Which IP multicast issues were not addressed... bandwidth from the network Copyright © 2000, Cisco Systems, Inc IP Multicasting Concepts 1-19 Multicast Applications Conceptual Scheme Multicast applications White board audio video other G.7xx, PCM, H.261, MPEG, Reliable Transport (SRM) RTP/RTCP Other transports UDP UDP IP IP © 2000, Cisco Systems, Inc www.cisco.com IP multicasting concepts Page 24 Just as its name “Whiteboard” implies, this is... Session Description Protocol (SDP) n VAT: Audio Conferencing allows multiple participants to share audio interactively VAT is based on the RTP n VIC: Video Conferencing allows multiple participants to share video and audio interactively VIC was designed with a flexible and extensible architecture to support heterogeneous environments and configurations n WB: Whiteboarding allows multiple participants to... a text and graphical environment Documents can be either in PostScript or plain ASCII text There are also some other MBone multicast applications, e.g RAT (Robust Audio Tool) and CU-SeeMe Copyright © 2000, Cisco Systems, Inc IP Multicasting Concepts 1-21 VAT- Audio Conferencing © 2000, Cisco Systems, Inc www.cisco.com IP multicasting concepts Page 26 The slide above shows the VAT audio conferencing... Systems, Inc IP Multicasting Concepts 1-23 WB - Whiteboard © 2000, Cisco Systems, Inc www.cisco.com IP multicasting concepts Page 28 The picture above shows an example of the “WB” whiteboard tool The window on the left is the main window for the whiteboarding session After deselecting the Receive Only checkbox in the right window we can draw or type in the main window We can also import PostScript or ASCII... VAT (and its cousin application RAT) IP/ TV IP Multicast Technology Copyright © 2000, Cisco Systems, Inc VIC - Video Conferencing © 2000, Cisco Systems, Inc www.cisco.com IP multicasting concepts Page 27 The slide above shows an example of the “VIC” video conferencing tool The window on the right is the main window for the video conferencing session Notice that multiple video streams are being received,... sufficiently in the past? Copyright © 2000, Cisco Systems, Inc IP Multicasting Concepts 1-11 IP Multicast Application types Objectives Upon completion of this section, the student will be able to: n n 1-12 Identify the types of applications suitable for implementation in an IP multicast environment Assess the requirements of those applications IP Multicast Technology Copyright © 2000, Cisco Systems, Inc... Inc www.cisco.com IP multicasting concepts Page 37 RFC 1112 specifies the host extensions to IP protocol in order to support multicast: n Allows hosts to join a group that receives multicast packets n Allows users to dynamically register (join/leave multicast groups) based on the applications they use n Uses IP datagrams to transmit data Each multicast group is defined by a class D IP address (224.0.0.0... Copyright © 2000, Cisco Systems, Inc IP Multicasting Concepts 1-31 IP Multicast Service Model (cont.) • Multicast network (routers) are distinct from source and receiver segments • Sources simply start sending data without any indication • First hop routers forward data • Receivers report their membership to last hop routers • Last hop (leaf) routers communicate group membership to the network © 2000, Cisco... © 2000, Cisco Systems, Inc www.cisco.com Receiver segement IP multicasting concepts Page 39 The example above shows a source connected to a first hop router that forwards multicast packets into the network Packets traverse the SPT on their way to the receivers towards the last hop router Copyright © 2000, Cisco Systems, Inc IP Multicasting Concepts 1-33 Functions of a Multicast Network • Learn about . benefits of IP multicasting n Indicate possible problems associated with IP multicasting Copyright 2000, Cisco Systems, Inc. IP Multicasting Concepts 1-3. 1 IP Multicasting Concepts Overview This lesson represents an entry point to IP multicast services, presents the functional model of IP multicasting,