Multicast Protocols and Applications

Transcription

1 Multicast Protocols and Applications Robert Elz Course Details Mid-Semester Exam 30% Final Exam 30% Participation 40% Exams open book Everything English Not an English course English ability not tested PLEASE ask questions until all understood! Participation: You do the teaching Later... Books Multicast Communication Protocols and Applications Ralph Wittmann & Martina Zitterbart Deploying IP Multicast In the Enterprise Thomas A Maufer IP Multicasting The Complete Guide to Interactive Corporate Networks Dave Kosiur Multicast Networking and Applications C Kenneth Miller

2 Group Communications Any communication with > 2 parties Conferences Games Distributed Databases Parties communicate with each other or one communicates with many Implementation Nothing special needed in network Normal connections work Need n *(n 1) connections n *(n 1) Really 2 reply to message from A to B the same as message from B to A Each packet transmitted (n 1) times. Implementation (2) Much extra work for sender transmitting packet many times Lots of load on network max bandwidth for communication network_bandwidth (n 1) Sender must know of all recipients Not useful for broadcast type communications TV Radio... Multicast One sender Many recipients intended to assist

3 Unix Multicast Applications vat Audio Conferencing vic Video Conferencing wb Shared Whiteboard imm File replication (images usually). VAT Visual Audio Tool Visual => GUI meters list of participants Several participants 10-50, sometimes more audio conference one talks, others listen anyone can talk Requirements Efficient path from each to all others low delay lower jitter low loss zero loss not required no recovery moderate bandwidth 32Kbps - 128Kbps VIC Video Conference Several participants Some with cameras 4-6 usually video only all cameras can transmit Requirements Efficient path from cameras to everyone delay/jitter not that important low loss mpeg suffers from lost frames no recovery fairly high bandwidth depends upon video compression and amount of movement

4 WB Shared Whiteboard Many participants 30-hundreds any can "scribble" any can create pages any can load files into pages Requirements Reasonable paths between all recipients Reliable transmission error detection and recovery lower loss -> less recovery delay/jitter unimportant for reasonable delay anyway sequencing Good scaling Low bandwidth usually IMM File replication Image replication First used to distribute satellite images Many participants Usually only one data source Requirements Good paths from source to recipients Reliable transmission Variable bandwidth depends upon images delay/jitter irrelevant General Requirements Possible privacy May be public event or may be private data still sent many places Minimum impact on net Ability to use tools together vic for image vat for audio wb for interaction requires co-operation similar delays

5 Other unix tools nte shared text editor requirements similar to wb rat another audio tool essentially identical to vat sdr session directory will investigate later Plus routing protocol implementations, etc System utilities, not tools Local Multicast Discovery Protocols Where is printer? Service Location svrloc Multicast Query Routing Protocols OSPF RIPv2 (etc) Multicast announcements To all routers on LAN replaces broadcast IPv6 advertisements discovery also broadcast replacements Multicast Similarities to Unicast Same packet formats Often same upper level protocols Specialist multicast transport exists Considered later Differences from Unicast Multicast address is special never used as source address address flipping impossible must test addresses Scaling is THE big issue always use care replying

6 Replies Unicast request/reply No special requirements Receive request send reply Multicast request/reply Same algorithm One request transmitted Overload at original sender Solutions Send no replies Seems absurd But actually frequently used Random Delay Avoid simultaneous replies Hard to achieve with many responders multicast nodes don t know Selective reply Nodes work out which should reply Sometimes uses multicast reply all nodes see first reply Multicast links Three types of link level Broadcast LAN ethernet (etc) Point to Point ppp, X.25,.. Multipoint ATM,... Multicast varies on each of these

7 Ethernet Transmission On an ethernet, all packets pass all nodes Even smart switches make it appear that way Ethernet Broadcast Broadcast is easy Nodes simply recognise magic address Capture & deliver packet All nodes receive broadcast packet Ethernet Multicast Multicast is almost as easy Nodes look for magic multicast address Capture & deliver packet if intended for this node Other nodes ignore packet Just like unicast packets

8 Ethernet Multicast Addr L G mmmmmm 0 1 mmmmmmmmmmmmmmmm g g g g g g g g g g g g g g g g g g g g g g g g G == 1 Indicates a group address Maybe more than one recipient Multicast Manufacturer code still used (if L == 0) Manufacturer assigns station bits Now called multicast group Ethernet Interfaces Ethernet interface usually has programmable MAC address The unicast address to match "Receive all Frames" switch Promiscuous mode Receive Broadcast switch A method of identifying multicast addresses Multicast address selection Table of local multicast addresses Fixed size Limited number of entries If node needs to receive more multicast packets Use promiscuous mode Hash Table Hash desired multicast address Take result modulo table size set bit to 1 to indicate multicast wanted Either way software needs to be able to filter addresses

9 Joining a Multicast Group Software instructs ethernet interface receive packets addressed to multicast address Hardware then captures packets with desired destination address Delivers packets to software Maybe some additional packets as well Easy When everything is on one ethernet Will return to other issues later. eg: Switches Point to Point Links Multicast trivial Just two nodes If other node wants traffic Send it there No need for multicast link level Multicast moves to network level Multipoint Link Levels ATM Frame Relay Hard Packets only go where sent One solution Treat as point to point Build circuits Use link level mesh as physical layer Ignore capacity to communicate with anyone

10 Multicast ATM Can emulate point to point Want to do more ATM (can have) multipoint transmission Send to multiple nodes at once Sounds perfect? No - ATM still circuit oriented Sender creates virtual circuit Then transmits Same for multipoint Sender must know all receivers scales badly Multicast ATM LAN Emulation (LANE) ATM mode to emulate Ethernet Allows ATM to appear to be a LAN can have multiple LANS in one ATM Like VLANS in modern switches Irrelevant here Implements broadcast Must to emulate LAN Uses server node Several server nodes Use multicast helper node Packets all sent there Redistributed to recipients Fairly complex high overheads must emulate everything Multicast ATM - LANE No multicast Use broadcast Unwanted frames ignored Just like "dumb" ethernet interface Several server nodes Configuration Server Lan Emulation Server Broadcast & Unknown Server Register with Config Server Told other server ATM addresses Register with those LAN Emulation server emulates ARP provides ATM address upon request Broadcast server receives and retransmits (to everyone)

11 Multicast ATM - Classic IP/ATM LANE not used But similar in operation Simpler - only IP supported ARP server maps IP address -> ATM address Nodes register when connected Multicast 2 approaches Multicast Router Multicast Address Resolution Server ATM Multicast Router Use router as multicast server For multicast Router treats links inside ATM cloud as p2p Uses network level multicast processing Forwards packets to correct nodes Once IP multicast is understood will make more sense Multicast Router Example

12 ATM MARS Multicast Address Resolution Server Performs ARP like function for a multicast address Maintains connections to each system bi-directional one connection each way systems announce when they join multicast groups leave multicast groups Maintains multipoint connection to everyone MARS broadcasts config changes ATM MARS To send multicast Request address list from MARS server Establish multipoint connection to addresses Transmit over new multipoint no redistribution server needed Many multipoint connections does not scale well Can use multicast server MARS returns only its address Multicast then only to it Multicast server redistributes No end node changes MARS Example

13 More recent changes Recent ATM Receiver initiated multicast join ATM signalling protocol adds multicast ATM addresses allows receivers to join multicast groups potential to simplify multicast over ATM but may be too late