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Campus Wireless Network Multicast Service Discovery Measurements

Analyzing traffic overhead & efficiency of multicast service discovery in a campus wireless network using measurements & channel utilization data.

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Campus Wireless Network Multicast Service Discovery Measurements

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  1. Measurements of Multicast Service Discovery in a Campus Wireless Network Se Gi Hong, Suman Srinivasan, and Henning Schulzrinne Columbia University

  2. Multicast service discovery • Problem • Applications using multicast service discovery are widely deployed • DNS-based service discovery (DNS-SD/mDNS) • iTunes, Printer setup by browsing • Multicast service discovery generates network traffic overhead • Princeton University filters DNS-SD/mDNS discovery traffic • DNS-SD/mDNS degrades campus network service • Wireless network • same channel interference, channel 1, 6, 11 • No formal measurement and analysis of the overhead of multicast service discovery traffic • Measurements • Columbia University’s wireless network (IEEE 802.11 b/g) • single subnet

  3. Multicast service discovery Host naming on a local network without a central DNS server Multicast DNS (mDNS) Windows, Linux, Mac OS X Link-local multicast name resolution (LLMNR) Windows Vista, Windows CE DNS-based service discovery (DNS-SD) Used with mDNS (DNS-SD/mDNS) iTunes, Printer setup by browsing Record types: PTR, SRV and TXT records PTR lookup: Discover service instances, <Service type>.<Domain> SRV records: Provide port number, IP address TXT records: Provide additional information Implementation of DNS-SD/mDNS Bonjour, Apple Inc.

  4. Average rate of multicast packets

  5. mDNS packet rate Average number of mDNS users seen in networks during measurement s when a total of 944 users are seen in the network ~70% of mDNS packets

  6. mDNS packet rate

  7. Channel utilization of mDNS packets on wireless networks • Channel utilization • The ratio of the sum of all the busy periods of mDNS packets to a unit time, U • Same channel interference • Channel 1, 6, 11. • A station sees multiple APs on the same channel • Each of the APs sends out the same multicast packet to the stations • Packets from all co-channel APs consume bandwidth at the station.

  8. mDNS packet rate with multiple same channel APs Bandwidth usage of mDNS packets: 13% of total bandwidth

  9. Comparison of service discovery models Model C: periodic announcements and browsing Model A: periodic announcements Model B: periodic browsing

  10. Comparison of service discovery models • Low service discovery delay: • B & C • Low traffic overhead: A • Average lifetime of nodes is high: • C gives low traffic overhead and • service discovery delay • Average lifetime of nodes is low: • C generates high traffic overhead

  11. Conclusion • Multicast service discovery • “Popular” • iTunes (~70% of mDNS) • “Chatty” • Need to evaluate network traffic overhead • 13% bandwidth usage • Define and analyze service discovery models • Provide insights for making design choice with different trade-offs • Traffic overhead, service discovery delay, network size, lifetime • Distributed system • Comparison between distributed system and client-server system • Network size, churn rate, lifetime, overhead

  12. backup

  13. mDNS Host naming on a local network Auto-configuration of a host name without a central DNS server Host names to be mapped into IP addresses and vice versa Resolution of naming conflict Works on single subnet Multicast DNS (mDNS) “single-dns-label.local.”: e.g., segihong.local. segihong.local. mDNS: Standard query response A 169.254.18.87 PTR segihong.local. mDNS: Standard query ANY segihong.local.

  14. DNS-Based service discovery Service discovery on a local network Users can discover services and choose the services without knowing the location of the service provider in advance to communicate with the provider DNS-Based service discovery (DNS-SD) Used with mDNS (DNS-SD/mDNS) Record types: PTR, SRV and TXT records PTR lookup: Discover service instances, <Service type>.<Domain> SRV records: Provide port number, IP address TXT records: Provide additional information

  15. DNS-Based service discovery Service discovery • SRV – 0 0 3689 segihong.local. • TXT – txtvers=1 • segihong.local. A 169.254.153.82 mDNS standard query response PTR: segi._daap._tcp.local. mDNS standard query: _daap._tcp.local. PTR? • segi._daap._tcp.local. SRV? • segi._daap._tcp.local. TXT? mDNS standard query response PTR: suman._daap._tcp.local.

  16. Channel utilization of mDNS packets on wireless networks

  17. What is Zeroconf? • IETF Zero Configuration Networking (Zeroconf) WG • 4 requirements • IP interface configuration • Translation between host name and IP address • IP multicast address allocation • Service discovery • Implementation • Bonjour • Apple’s implementation of zero-configuration networking • iTunes, iChat • Avahi • Open source for Linux

  18. What is Zeroconf • IP interface configuration • Auto-configuration of IPv4 link-local address and netmask without a central server (DHCP server) • A host randomly selects an IP address within the 169.254/16 subnet • ARP announcements/responses for address conflict resolution • Windows and Mac OS implement the auto-configuration of IPv4 link-local addressing 169.254.18.87 ARP: Who has 169.254.18.87? Gratuitous ARP ARP: Who has 169.254.18.87? Tell 0.0.0.0

  19. What is Zeroconf? • Translation between host name and IP address • Auto-configuration of a host name without a central server (DNS server) • Host names to be mapped into IP addresses and vice versa • Resolution of naming conflict • Bonjour • Multicast DNS (mDNS) • “single-dns-label.local.”: e.g., segihong.local. segihong.local. MDNS: Standard query response A 169.254.18.87 PTR segihong.local. MDNS: Standard query ANY segihong.local.

  20. What is Zeroconf? • IP multicast address allocation • Range of IP multicast: 224.0.0.0 to 239.255.255.255 • Bonjour • mDNS multicast address • 224.0.0.251

  21. What is Zeroconf? • Service discovery • Users can discover services and choose the services without knowing the location of the service provider in advance to communicate with the provider • Bonjour • Multicast DNS-Based service discovery (mDNS-SD) • PTR, SRV and TXT records • PTR lookup: <Service type>.<Domain> • SRV records: port number, host name • TXT records: additional information

  22. What is Zeroconf? • Service discovery • SRV – 0 0 3689 segihong.local. • TXT – txtvers=1 • segihong.local. A 169.254.153.82 • segi._daap._tcp.local. SRV? • segi._daap._tcp.local. TXT? MDNS standard query: _daap._tcp.local. PTR? MDNS standard query response PTR: segi._daap._tcp.local. MDNS standard query response PTR: suman._daap._tcp.local.

  23. Channel utilization of mDNS packets on wireless networks

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