IP Multicasting Concepts

• Used when sending same data to multiple receivers • Better bandwidth utilization • Less host/router processing • Used when receivers’ addresses unknown • Used when simultaneous deliver

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1

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

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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

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Copyright  2000, Cisco Systems, Inc IP Multicasting Concepts 1-3

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

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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

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Unicast vs Multicast Unicast vs Multicast

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

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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

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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

# 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)

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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)

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Multicast 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 i ssue

•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 applicati ons 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 Applic ations 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

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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)

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Summary

After completing this section, the student should be able to:

n Compare traditional unicast delivery to multicast 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 sufficiently in the past?

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IP Multicast Application types

Objectives

n Identify the types of applications suitable for implementation in an IP multicast environment

n Assess the requirements of those applications

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Types of Multicast Applications Types of Multicast Applications

two or more (n) receivers

sending to the same multicast group hosts are members of the group as well (sender = receiver)

receivers sending data back to a source (via unicast or multicast)

There are different types of multicast applications Two most common models are:

n The one-to-many model, where one sender sends data to many receivers or

n The many-to-many model, where a host can be a sender as well as a receiver simultaneously

Other models (e.g many-to-one, where many receivers are sending data back to one sender or few-to-many) are used as well, especially in financial applications / networks

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Corporate Broadcasts Corporate Broadcasts

Distance Learning

Training Training

Video Conferencing

Whiteboard/Collaboration

Multicast File Transfer

Data and File Replication

Real

Real -Time Data Delivery Time Data Delivery — Financial Financial

Video -On On - Demand Demand

Live TV and Radio Broadcast Live TV and Radio Broadcast

to the Desktop

IP Multicast Applications

Many new multipoint applications are emerging as demand for them grows:

n Real-time applications include live broadcasts, financial data delivery, whiteboard collaboration, and video conferencing

n Non-real-time applications include file transfer, data and file replication, and video-on-demand

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One-to-Many Multicast One-to-Many Multicast

• The simplest type of an application

•A single source is sending to a multicast group (“broadcasting”)

•Receiver population typically not known –

If a one-to-many application needs a feedback from receivers, it becomes a to-many application

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many-1-16 IP Multicast Technology Copyright  2000, Cisco Systems, Inc

Many-to-Many Multicast Many-to-Many Multicast

• Enables the most unique and powerful applications

•Any host can send to a multicast group as well as receive from it – receivers known

•Simultaneous reception from several sources – complex coordination

•Multimedia conferencing, collaboration, synchronization of resources, etc.

• Foundation for a set of new applications

Many-to-many multicast applications are those where two or more receivers also act as senders

Receiving data from several sources increases the complexity of applic ations and creates different management challenges

Using a many-to-many multicast concept as a foundation, a whole new range of applications can be built (e.g collaboration, concurrent processing, distributed interactive simulations, etc.)

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Other Multicast Types Other Multicast Types

• Many-to-one is a special type that has several challenges

•Typically two way request / response either end (many or one) generates request

-•Implosion problem / response storm – responses arrive simultaneously

•Resource discovery, Data collection, Auctions, Polling, etc.

• Can be combined with other types;

responses can be sent “out-of-band”

The many-to-one multicast model may be used for resource discovery, data collection, and similar applications, but building them imposes several challenges:

n Maintaining a Multicast State in networks with many senders can cause scaling problems in the network that are transparent to the application

n Many senders can cause message implosion problems at the receiver side There are some solutions to these problems (i.e bi-directional trees in multicast routing), but application developers must be aware of these problems

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Multicast Applications Requirements

• One-to-many applications typically more tolerant to delays

•Audio / video more sensitive to jitter

•Real-time monitoring less tolerant

• Many-to-many applications intolerant to delays since they are bi-directional

•Conference applications very strict – delays higher than 500 ms disturbing

Basically one-to-many applications (e.g file distribution) are tolerant to delays, but there are also some applications that are sensitive to jitter (e.g audio/video

conferencing) and some that are not tolerant to delays at all (e.g real-time monitoring, financial applications)

Many-to-many applications are intolerant to delays because they are bi-directional Delays in multimedia conferencing higher than 500 ms can be very disturbing to the audience

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Multicast Applications Requirements (cont.)

• Bandwidth requirements depend on the size of a multicast stream

• When the bandwidth is scarce, the source can adjust the encoding techniques

• Feedback information is needed – audio / video distribution uses RTP / RTCP (Real-Time Protocol / Real-Time Control Protocol)

Different multicast applications have different bandwidth needs If there’s not enough bandwidth, applications can adjust the encoding techniques in order to use less bandwidth That’s why applications need the feedback information about how much of bandwidth is available

Audio/video applications use Real Time protocol (RTP) / Real Time Control Protocol (RTCP) to get the feedback about available bandwidth from the network

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Multicast Applications Conceptual Scheme

Multicast Applications Conceptual SchemeMulticast applications

audio video other White

board

G.7xx, PCM,

H.261, MPEG, .

RTP/RTCP Other

transports Reliable

Transport (SRM)

UDP IP

Just as its name “Whiteboard” implies, this is a form of electronic Whiteboard application that can be shared by members of the multicast group “Whiteboard” uses a form of Reliable Multicast

Reliable Multicast transport is necessary to ensure no loss of critical graphic information occurs “Whiteboard” uses the Scalable Reliable Multicast (SRM) protocol to ensure reliable delivery

Most multimedia multicast applications simply use UDP, “Best Effort” datagram delivery mechanisms because of the time critical nature of the media However,

“wb” application needs a reliable method to distribute the graphic images drawn on the electronic whiteboard

Audio and video distribution applications use different encoding techniques to minimize the bandwidth use In order to get the information about the available bandwidth and to reserve it for their use, they use RTP/RTCP protocols

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Example Multicast Applications Example Multicast Applications

• MBone Multicast Tools – first set of multicast applications:

• Sdr—session directory

– Lists advertised sessions and launches multicast application(s)

• Vat (Visual Audio Tool) and Rat (Robust Audio Tool) - audio conferencing

– Various types of audio compression (PCM, DVI, GSM, LPC4, )

• Vic (VIdeo Conferencing) - video conferencing

– H.261 video compression

• Wb (WhiteBoarding) - whiteboarding

– Shared drawing tool

– Uses Reliable Multicast (SRM – Scalable Reliable Multicast)

First multicast applications emerged in MBone (Multicast backbone of the Internet) including:

n SDR: Session Directory is a tool that allows participants to view advertised multicast sessions and launch appropriate multicast applications to join an existing session Originally, it was based on SD, but SD and SDR are not compatible, because SDR implements a later version of the 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 collaborate interactively in 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

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VAT- Audio Conferencing VAT- Audio Conferencing

The slide above shows the VAT audio conferencing tool The window on the left

is the main window for the session It contains a speaker gain slider widget and an output VU bar-graph meter along with a microphone gain slider widget and VU meter When one wishes to address the conference, one usually presses the right mouse button on the workstation

The window on the right is a menu that can be brought up by pressing the “Menu” button on the main window This menu allows various parameters about the session to be adjusted including encoding format

Notice that there are several members of this session listed in the main window even though only the second person is talking (Indicated by the blackened square next to the name.) This points out that all members of the session are multicast sources even though they may never speak and only listen to the session This is because VAT uses the RTP/RTCP model to transport Real-Time audio data In this model, all members of the session multicast member information and reception statistics to the entire group in an RTCP “back-channel”

Almost all multimedia multicast applications use the RTP/RTCP model including:

n VAT (and its cousin application RAT)

n VIC

n WB - (Whiteboard)

n IP/TV

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VIC - Video Conferencing VIC - Video Conferencing

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, each with its own

“thumbnail” image

The window on the left is a larger version of the thumbnail image from the main window

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WB - Whiteboard

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 text files and put their contents

on the whiteboard

The window on the right is a control window where participants are displayed as well as the last one who changed the contents of the whiteboard

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H.32x Videoconferencing

Framework

H.32x Videoconferencing

Framework

• ITU-T standard for videoconferencing

• H.32x – x=1 ISDN, x=3 LAN, x=4 Analog

• Audio (G.7xx) and video (H.26x) encoding, control protocols

• Support for whiteboarding (T.12x)

• Can employ IP multicast as a transport

• Elements implemented in Cisco routers

as Multimedia Conference Manager

ITU-T H.32x recommendations specify technical requirements for narrow-band visual telephone systems and terminal equipment, typically for videoconferencing and videophone services The transport media is defined by x (x = 1 ISDN, x = 3 LAN, x = 4 analog)

n The series of recommendations G.7xx specifies audio codecs and consist of standards including G.711, which uses PCM encoding at a bit-rate from 48 to

64 kbps, and G.723.1, which uses MPE/ACELP encoding at 5.3 kbps and several others

n Recommendation H.261 specifies a video codec for audiovisual services at p x

64 kbit/s and H.263 specifies video coding for low bit rate communication

n Recommendations T.12x specify a multipoint communication service which includes whiteboarding applications and multipoint file transfers

H.32x is an application standard which does not define the underlying transport protocol, hence IP multicast can be used

The H.32x standard is supported by Cisco IOS as well and components of this standard are integrated into Cisco routers as Multimedia Conference Manager software

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Quality / Security in Multicast Quality / Security in Multicast

• Feedback information is needed by senders to:

•Determine the quality of reception and adjust the rate of sending or encoding

•Provide recovery in case of packet loss (reliable multicast)

•Learn / identify the population of receivers

• Security: when receivers are not known, encryption is the only way to secure data

Senders can use the feedback information from receivers to:

n Learn about the quality of reception and adjust the rate of sending or encoding

n Provide recovery in a case of packet loss

n Identify the population of receivers Multicast flows from the sender as well as from receivers can be encrypted for security reasons If the receivers are not known to the sender, encryption can be done only one way

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Real-Time Protocol - RTP Real-Time Protocol - RTP

• Real-Time Protocol (RFC-1889 / 1890) is used as a transport protocol for audio / video

• Provides sequence and timestamping without overhead required for reliable sessions – does not ensure reliability

• Enables synchronization of separate streams

• Strong industry support

Real Time Protocol (RTP) provides network independent end-to-end transport services for real-time applications Therefore it can be used as a transport protocol for audio or video data transfer

Services that RTP provides are:

n Data sequencing

n Timestamping

n Payload type identification

n Delivery monitoring It’s also important to know the services that RTP does not provide:

RTP enjoys strong industry support

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RealTime Control Protocol

• Sent to the same multicast group

• RTCP packets can use up to five (5) percent of available bandwidth

• Every receiver is a sender as well – traffic is asymmetrical

• Enables the source to adapt to quality of the network and to log the members

When a host wants to request a specific quality of service from the network, the Real Time Control Protocol (RTCP) can be used

All participants in the multicast session (senders and receivers) periodically send RTCP control packets Control packets are used to provide the feedback loop to the sender as well as to the receivers because it’s sent as a multicast

As each participant hears control packets from all other participants, it may use that information to limit the rate it sends them to the network RTCP packets can use only up to 5% of available bandwidth By limiting the number of control packets, the scalability of RTCP is assured

Because every receiver is also a sender, the traffic is asymmetric It flows towards the receivers as well as towards the sender

When the sender receives a control packet, it can adapt to changes in the network (available bandwidth, congestion, etc.) and keep track of the receivers

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Summary

After completing this section the student should be able to:

n Identify the types of applications suitable for implementation in an IP multicast environment

n Assess the requirements of those applications

Review Questions

n Which type of multicast application is the most common and emulates a traditional broadcast?

n Why is delay an important issue for many-to-many multicast applications?

n How do participants of an audio conference (using Mbone tools) identify each other?

n List some applications where feedback information is a requirement

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The Basic Model of IP Multicast

Objectives

n Describe the need for splitting multicast delivery path into various segments

n Identify functions needed on source, network, and receiver segments

n Explain the protocols used within the IP multicasting model

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IP Multicast Service Model

• RFC 1112 (Host Extensions for Multicast Support)

• Each multicast group identified by a class-D IP address

• Members join and leave the group and indicate this to the routers

• Routers listen to all multicast addresses and use multicast routing protocols to manage groups

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 to 239.255.255.255) Addresses are allocated dynamically and represent receiver groups, not the individual hosts

Receivers can dynamically join/leave a multicast group at any time using IGMP messages Messages are sent to the routers, which manage group membership Senders are not necessarily included in the multicast group they are sending to Many applications have the characteristics of receivers also becoming senders e.g RTCP streams from IP/TV clients and Tibco RV

Routers use multicast routing protocols (e.g DVMRP, MOSPF, CBT, etc.) to efficiently forward multicast data to multiple receivers They listen to all multicast addresses and create multicast distribution trees, which are used for multicast packet forwarding

Tiêu đề	IP Multicasting Concepts
Trường học	Cisco Systems, Inc.
Chuyên ngành	Computer Networks
Thể loại	lecture notes
Năm xuất bản	2000

Định dạng
Số trang	62
Dung lượng	3,2 MB