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

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 b/g) single subnet

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,. – SRV records: Provide port number, IP address – TXT records: Provide additional information Implementation of DNS-SD/mDNS – Bonjour, Apple Inc.

Average rate of multicast packets

mDNS packet rate Service typeNumber of users iTunes music sharing (_daap._tcp) 188 Apple file sharing (_afpovertvp._tcp) 83 iChat AV (_presence._tcp)25 Workgroup manager (_workstation._tcp) 49 Internet printing protocol (_ipp._tcp) 16 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

mDNS packet rate

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.

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

Comparison of service discovery models Model A: periodic announcementsModel B: periodic browsing Model C: periodic announcements and browsing Mod els Type of uplink packetsType of downlink packetsService discovery delay ABRABR A 0000 p/2 B00 immediate C

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

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

backup

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. mDNS: Standard query ANY segihong.local. mDNS: Standard query response A PTR segihong.local. segihong.local.

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,. SRV records: Provide port number, IP address TXT records: Provide additional information

DNS-Based service discovery Service discovery 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. segi._daap._tcp.local. SRV? segi._daap._tcp.local. TXT? SRV – segihong.local. TXT – txtvers=1 segihong.local. A

Channel utilization of mDNS packets on wireless networks Paramet ers Size (bits) Tx rate (Mb/s) Tx time (us) PLCP Preamble 1441 PLCP Header 481 SIFS--10 DIFS--50 ACK112Rc112/Rc

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

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 /16 subnet – ARP announcements/responses for address conflict resolution – Windows and Mac OS implement the auto-configuration of IPv4 link-local addressing ARP: Who has ? Gratuitous ARP ARP: Who has ? Tell

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. MDNS: Standard query ANY segihong.local. MDNS: Standard query response A PTR segihong.local. segihong.local.

What is Zeroconf? IP multicast address allocation – Range of IP multicast: to – Bonjour mDNS multicast address

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:. SRV records: port number, host name TXT records: additional information

What is Zeroconf? Service discovery 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. segi._daap._tcp.local. SRV? segi._daap._tcp.local. TXT? SRV – segihong.local. TXT – txtvers=1 segihong.local. A

Channel utilization of mDNS packets on wireless networks Paramet ers Size (bits) Tx rate (Mb/s) Tx time (us) PLCP Preamble 1441 PLCP Header 481 SIFS--10 DIFS--50 ACK112Rc112/Rc