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How To Do High Speed Multicast Right!

How To Do High Speed Multicast Right!. Lothar.Zier@gmd.de Gundula.Doerries@gmd.de. High-Speed Multicast. High-Speed Multicast Distributing data to groups of receivers Single multicast streams with 100+ Mbit/s Outline Challenges and risks Practical technologies Hot spots Summary.

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How To Do High Speed Multicast Right!

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  1. How To Do High Speed Multicast Right! Lothar.Zier@gmd.de Gundula.Doerries@gmd.de

  2. High-Speed Multicast • High-Speed Multicast • Distributing data to groups of receivers • Single multicast streams with 100+ Mbit/s • Outline • Challenges and risks • Practical technologies • Hot spots • Summary

  3. High-Speed Multicast • Challenges • Software-based multicast was restricted to a few Mbit/s • New hardware allows multicast with hundreds of Mbit/s • Risks • “Multicast disaster” • Server sends much less than expected • Production network is down • Client system does not even seem to receive much of the data • Implementation goals for multicast • End systems send + receive multicast with high data rates • Restricted distribution of multicast inside the network

  4. Practical High-Speed Multicast Technologies • Three technologies available for 100+ MBit/s multicast distribution • ATM • Native ATM (Winsock2, XTI APIs) • LAN Emulation (LANEv1; separate VLAN) • Gigabit Ethernet (GE) • IGMP snooping / separate VLAN • IP multicast routing

  5. Protocol Packet size/byte Throughput / Mbit/s XTI 50 282.6 1472 536.7 8192 541.5 XTI without Padding 1432 539.5 8152 542.0 UDP/LANE 50 188.4 1472 519.9 9190 538.2 UDP/LANE without Padding 1436 523.6 9212 539.5 UDP/Gigabit Ethernet 50 480.8 1472 964.6 Multicast Throughput - Theoretical Limits ATM 622 Mbit/s + Gigabit Ethernet

  6. Practical Environment / Measurements • Gigabit Testbed West in Germany • Networking Technologies: ATM (622 MBit/s, 2.4 Gbit/s), Gigabit Ethernet, IP routing (GE + 622 Mbit/s ATM) • End systems: UNIX workstations: Sun, SGI • Network equipment: CISCO, Fore (Marconi) • Multi-Generator (MGEN) Toolset • Tools for sending + receiving multicast packets • ftp://manimac.itds.nrl.navy.mil/Pub/MGEN/ • Extension: • Native ATM – XTI • Bigger buffers

  7. Test Environment

  8. Sender Performance • Measurements • Sending rate depending on network technology + packet size • XTI/native ATM, LANE, Gigabit Ethernet • Packet sizes: 50 – 1472 (- 8192, - 9000) bytes • Maximum sending rate • Native ATM: ~540 Mbit/s, >5000 bytes packet • LANE: ~510 Mbit/s, >6000 bytes packet • Gigabit Ethernet: ~340 Mbit/s, 1472 bytes packets • Sending performance is limited by bandwidth + number of packets (number of interrupts) • Comparison of technologies • Gigabit Ethernet better for a fixed packet size • LANE / XTI better overall throughput (because of bigger packet sizes)

  9. Native ATM (XTI) Send Rate

  10. Comparing the Technologies

  11. LAN Emulation and Multicast • Multicast with LAN Emulation (Version 1) • Multicast data is send to the BUS and distributed to all clients • The sender also receives its own multicast data – which is discarded by the ATM driver • Comparing multicast and unicast sending rate • Maximum throughput for 1472 byte packets is reduced ~20 % • 190 Mbit/s with multicast traffic • 240 Mbit/s with unicast traffic

  12. Comparing LANE Unicast and Multicast

  13. Receiver Performance • High data losses in overloaded receivers • Two techniques for improving • Interrupt Coalescing – network adapter aggregates packets before triggering an interrupt • Increasing the (UDP) receive buffer • Improvement • Reduces receiver losses • Smoothing traffic peaks • No cure-all for slow receivers

  14. Losses without/with Interrupt Coalescing

  15. IP Multicast Routing Protocols • Methods for constructing IP multicast distribution trees • Flood & prune • Flood all (!) multicast packets in regular intervals in the whole multicast network; prune the distribution tree • DVMRP, PIM dense-mode • Explicit receiver registration • Send only data to explicitly registered receivers • PIM sparse-mode • Flood & prune protocols may cause severe traffic bursts in the whole multicast network • PIM sparse-mode should be used

  16. PIM dense-mode Traffic Burst

  17. Congestion Avoidance • No feedback mechanism for multicast sender in bottleneck situations • Other traffic flows (especially TCP) are severely damaged • Intelligent queuing mechanisms reduce congestion • Weighted Fair Queuing (WFQ) / Per VC Queuing • Allocating equal bandwidths between traffic flows • Implemented in many modern ATM switches • Improvement • Multicast and TCP traffic get equal bandwidth resources

  18. Parallel TCP and Multicast Traffic with WFQ

  19. Summary • Modern hardware allows high-speed multicast with hundreds of megabits • Several technologies to realize high-speed multicast in practice • ATM (native, LAN Emulation), Gigabit Ethernet, IP Router • No clear winner • Traffic management for high-speed multicast distribution is absolutely necessary • Protect your (unicast) production traffic • Native ATM, VLANs, IGMP-snooping, PIM sparse-mode • Applications • Use large packets • Feedback mechanisms useful to avoid receiver/network overloading • What data rates are realistic? • 200 – 500 Mbit/s for test programs • Your application may reduce this to 100 Mbit/s and below

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