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 802.11 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, <Service type>.<Domain>– 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 type Number 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.

U

TTρ UD

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 announcements Model B: periodic browsing Model C: periodic announcements and browsing

Models

Type of uplink packets Type of downlink packets Service discovery

delayA B R A B R

A0 0 0 0

p/2

B 0 0immediate

Cimmediate

PNa

1

PNa

1

PNa

1

PNa

1

PN

1

PN

1

PN

1

PNNN

PNN aaaa

1)(

1)1(

N

NNN

N

NN a

aa

a

)1(

PNN

1)1(

)1( NP

N1

AP.each with associated users ofnumber Average : network., in the users ofnumber Average :

period. browse andnt announceme Service:.,ondistributiPoission rate,Join :

aNN

P

Comparison of service discovery models

minutes 30

10

sAssumption

P

Na

•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 169.254.18.87 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, <Service type>.<Domain>• 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 – 0 0 3689 segihong.local.TXT – txtvers=1segihong.local. A 169.254.153.82

Channel utilization of mDNS packets on wireless networks

header. and preamble PLCP theoflength Time :T

ACK. of rateon Transmissi :R

rate.nsmission uplink tra andDownlink :R ,R

ACK. frames, control ofLength :L

header.layer MAC including sizepacket mDNS Average :L

U.unit time,in packet uplink anddownlink mDNS ofnumber Average :N ,N

SIFS. and DIFS oflength Time :T ,T

TR

LT ACK, frames, control of on timeTransmissi

TR

LT packet,uplink mDNS one of on timeTransmissi

N)TTT(TT time,nsmission Uplink tra

TR

LT packet,downlink mDNS one of on timeTransmissi

N)T(TT n time,ransmissioDownlink tU

TTρ n,utilizatio Channel

PLCP

c

ud

c

m

ud

SIFSDIFS

PLCPc

cACK

PLCPu

mu

uACKuSIFSDIFSU

PLCPd

md

ddDIFSD

UD

Parameters

Size (bits)

Tx rate (Mb/s)

Tx time (us)

PLCP Preamble

144 1 144

PLCP Header

48 1 48

SIFS - - 10

DIFS - - 50

ACK 112 Rc 112/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 169.254/16 subnet– ARP announcements/responses for address conflict resolution– Windows and Mac OS implement the auto-configuration of IPv4 link-local

addressing

ARP:

Who has 169.254.18.87? Gratuitous ARP

169.254.18.87ARP:

Who has 169.254.18.87? Tell 0.0.0.0

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 169.254.18.87 PTR segihong.local.

segihong.local.

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

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

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 – 0 0 3689 segihong.local.TXT – txtvers=1segihong.local. A 169.254.153.82

Channel utilization of mDNS packets on wireless networks

header. and preamble PLCP theoflength Time :T

ACK. of rateon Transmissi :R

rate.nsmission uplink tra andDownlink :R ,R

ACK. frames, control ofLength :L

header.layer MAC including sizepacket mDNS Average :L

U.unit time,in packet uplink anddownlink mDNS ofnumber Average :N ,N

SIFS. and DIFS oflength Time :T ,T

TR

LT ACK, frames, control of on timeTransmissi

TR

LT packet,uplink mDNS one of on timeTransmissi

N)TTT(TT time,nsmission Uplink tra

TR

LT packet,downlink mDNS one of on timeTransmissi

N)T(TT n time,ransmissioDownlink tU

TTρ n,utilizatio Channel

PLCP

c

ud

c

m

ud

SIFSDIFS

PLCPc

cACK

PLCPu

mu

uACKuSIFSDIFSU

PLCPd

md

ddDIFSD

UD

Paramet

ersSize (bits)

Tx rate (Mb/s)

Tx time (us)

PLCP Preamble

144 1 144

PLCP Header

48 1 48

SIFS - - 10

DIFS - - 50

ACK 112 Rc 112/Rc