Accelerating Service Discovery in Ad Hoc Zero Configuration Networking

Se Gi Hong, Suman Srinivasan, and Henning Schulzrinne

Columbia University

Contents

• Wireless mobile ad-hoc networks• Overview of Zeroconf (Zero Configuration Networking)• Zeroconf in Mobile Ad Hoc Networks• Analysis of the mean service loss probability• Overview of joining process in 802.11• Accelerating Zeroconf• Implementation and performance

wireless mobile ad-hoc networks

• No Infrastructure– Allocate IP addresses without a DHCP server– Translate between domain names and IP addresses

without a DNS Server– Find service without a directory service

• Service discovery– The usage of service discovery has been increasing– e.g., iTunes (Apple’s music file sharing), iChat

• Solution– Zeroconf (Zero Configuration Networking)

What is Zeroconf?• IETF Zero Configuration Networking (Zeroconf) WG• 4 requirements

– IP interface configuration• Auto-configuration of IPv4 link-local address without a central

server (DHCP server) within the 169.254/16 subnet– Translation between host name and IP address

• Auto-configuration of a host name without a central server (DNS server)

– IP multicast address allocation• Range of IP multicast: 224.0.0.0 to 239.255.255.255

– Service discovery• Implementation

– Bonjour• Apple’s implementation of zero-configuration networking • iTunes, iChat

– Avahi• Open source for Linux

What is Zeroconf?• Service discovery

– Users can discover available service instances of a service types.

– 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

Zeroconf in Mobile Ad Hoc Networks

• High mobile environment – frequently join/leave for a short time

• Zeroconf does not include algorithm to detect network topology changes

• To discovery services, services should be announced and looked up frequently -> overhead

Analysis of the mean service loss probability

• Assumption:– Node arrival rate to a local ad-hoc area follows an exponential

distribution with rate – Node residence time follows exponential distribution with rate – Service announcing and browsing interval follows exponential

distribution with rate – Current number of nodes in an area is n

Time

T

X

Y

Service browsingof node i

Service browsingof node k

Node A arrives

Node A leaves

Node A’s nextService announcing

Analysis of the mean service loss probability

• The service loss probability

• The mean service loss probability

Q(n)Pr(T X)Pr(Y X) Pr(T X)Pr(Y X)dxdy0

0

n

Q(n)Pr(N n)n0

n

n0

n

n!

exp

Analysis of the mean service loss probability

(a) Average residence time is 60 minutes (b) Average residence time is 10 minutes

Average number of users Average number of users

Mea

n se

rvic

e lo

ss p

roba

bilit

y

Mea

n se

rvic

e lo

ss p

roba

bilit

y

What do we need?• Current Service Discovery of Zeroconf is not enough for high

mobile environment since Zeroconf does not have the algorithm for detecting network topology changes

• We need a method to recognize new peers (services) in real-time while reducing overhead.

• Solution– We can use the existing Beacon frames of 802.11

What happens when a device joins in a local ad-hoc network?

• Ad-hoc network– BSSID (Basic service set identification) is required for

communication each other.– TSF (Time Synchronization Function) keeps the timer for all

node in same BSS for beacon period.– BSSID and TSF timer value are same for all nodes in the same

BSS (Basic Service Set)– Beacon frames contains BSSID and TSF timer value

• How to join in the group?– Initially, each device randomly chooses BSSID– Sending probe request and scanning (listening) Beacon frames

in all channels– Adopt the information, such as BSSID and the TSF timer, only if

the SSID is matched and the value of timestamp in the received beacon frames or probe requests is later than the node’s TSF timer

What happens when a device joins in a local ad-hoc network?

• Node A joins

802.11MAC/PHY

Node B Node CNode A

BSS1

BSS2

BSSID = 02:02:2D:0D:6B:0ETSF timer = 1000

BSSID = 00:35:0E:3S:5D:5ETSF timer = 2000

What happens when a device joins in a local ad-hoc network?

• Node A joins

802.11MAC/PHY

Node B Node CNode A

BSS2

BSSID = 00:35:0E:3S:5D:5ETSF timer = 2000

802.11MAC/PHY

Accelerating Zeroconf• Monitoring current BSSID

– Changes of BSSID means that it joins in a new local ad-hoc network.

– If (old_BSSID != current_BSSID), then re-trigger service discovery.

• Standard wireless extensions in Linux– ioctl(): SIOCGIWAP option – get BSSID

• Windows WLAN API– WlanGetNetworkBssList()

Accelerating Zeroconf• Accelerating service discovery architecture

Zeroconf Software

IP assignNaming

Service Discovery

Monitoring BSSID

Beacon Frames

ApplicationLayer

Data LinkLayer

Zeroconf Software

IP assignNaming

Service Discovery

Monitoring BSSID

Beacon Frames

User A User B

Performance• Scenarios

– Three laptops (A, B, and C)– Laptop A joins a local ad-hoc networks– Laptop A announces a service and B and C browse for the service

• Compare to theoretical analysis of delay in Zeroconf with periodic service announcing and browsing– Service announcing and browsing interval: exponential distribution

with rate – D is the delay– is the next announcing or browsing time of A, B and C

– Mean of the delay is

tA , tB , tC

Pr(D t)Pr(min(tA ,tB ,tc ) t)Pr(tA t)Pr(tB t)Pr(tC t)

e 3t .

1

3

Performance• Delay measurement of the service discovery protocol