Ad Hoc Networking

Tom Roeder

CS415 2005sp

Part IV questions?

What is an ad hoc network?

Nodes discover and maintain routes no use of infrastructure Every host is also a router (thus not all routers are trusted…)

Can be done over the infrastructure

Features of an ad hoc network

Change in reachability over time as nodes die and come back over space as nodes move

Often power constrained The screen is the constraint on your laptops, but

on many smaller machines, it is the network Oft cited: 1 packet for 3000 instructions

It adapts! Must not rely on static configurations

Applications

Sensor networks little, power-constrained “motes”

attached to animals in a park scattered on the ground from the air (military)

Rescue workers in a disaster area Educational apps (www.silicon-chalk.com) Operating systems

MagnetOS (Mobisys ’05) (www.cs.cornell.edu/people/egs/magnetos)

Overlays

leads in to P2P systems Why bother?

route around problems build multicast trees illegally share files gain anonymity build trust networks optimize RSS feeds

If this interests you, check out Copano

Types of ad hoc routing

Proactive DSDV, Link-state variants

Reactive DSR, AODV

Hybrid ZRP, HARP, SHARP

Overlay We’re not going to discuss this more

Costs and benefits: proactive

Pushes information low message latency high state overhead high message overhead

OK when network is small Full link state grows as n2

Can seriously impact throughput Not good for high mobility

Costs and benefits: reactive

Generates route at send time high initial latency

caching helps tremendously no wasted route information can lead to broadcast storms

brings the network down even faster at the end

Good for reasonably high mobility too fast and there’s nothing we can do

Widely used

802.11b MAC layer

To send a packet, must reserve the medium Uses a CSMA protocol

Additional optional protocol for 802.11b is RTS (Ready To Send) CTS (Clear To Send) Data ACK

Hidden terminal problem may get lower throughput than expected

Distance Vector protocols

Key Distance Vector idea: Instead of storing the full path, just keep direction “If I want to get to A, my next hop is B” Trade DV information with neighbors via flooding Based on distributed Bellman-Ford algorithm Can suffer from loops and counting-to-infinity

AODV finds distance vectors reactively Based on DSDV, which does it proactively Uses a sequence number to try to avoid problems

AODV information per node

A table (cache) of known distance vectors refresh rate will controll the message overhead

<seqnum, dest, hop, hopcount> seqnum: incremented on new information

used to avoid counting to infinity. Remember the last known seqnum for each cache elt

dest: the identifier of the destination note that identifiers are arbitrary

hop: the identifier of next hop to get to dest hopcount: how many hops on this route

AODV route requests

Node A wants to send a packet to B broadcasts a RREQ (with some TTL) heard by B, B sends a reply A sends directly to B

Node A wants to send a packet to C broadcasts a RREQ heard by B, but B just heard from C Sends reply <1, C, B, 1> A sends packet to B, who forwards it to C

AB

C

AODV route replies

A node receiving a RREQ sends a route reply from its cache if it has this route else it forwards the RREQ It also updates its path to the requestor with the

RREQ and TTL, if it is better If a node hears a better route reply

it doesn’t send its own it records the better route this helps avoid broadcast storms in flooding

AODV route caches

Think of the route cache as an optimization We could always choose to flood This would just have high latency and broadcast

storms, but would still be correct (don’t do it!) Timeout is critical

When a link goes down, the cache is wrong We don’t do explicit invalidation

real AODV does uses MAC link error info to guess at disconnection

AODV example

A

B

C

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQ

RREQRREQ

RREQ!RREP

RREP

RREP

AODV details to ignore

Counting to infinity is possible but hard see http://www.cse.ucsc.edu/research/

ccrg/publications/hari.icc.2005.pdf Don’t worry about security We are not managing the errors explicitly

This is clearly suboptimal, but easier See the AODV and DSR papers if you’re

interested Don’t worry if you get low throughput

AODV header spec

type (2 bits) RREP, RREQ, DATA

seqnum (4 bytes) incremented on new routes

from node (4 bytes) destination node (4 bytes) Time To Live (1 byte)

set to MAX_PATH_LENGTH

Layering and Abstraction

AODV is a layer below miniports it acts like IP for us it should encapsulate the miniports code

Other than AODV control packets, all packets should be miniports or minisockets we still are delivering to miniports on remote node

We have simply taken away the reliance on the IP routing infrastructure

Testing: over the infrastructure

We provide a “broadcast” layer for your code file format

saranac

heineken

dosequis

kingfisher

tecate

.xx.x

x.xx.

xx...

.x..x

x..x.

Testing: over the infrastructure

Use the network_broadcast_pkt To send to all reachable nodes When you don’t know the direction: RREQ For returning cached RREPs (optimization)

Use the network_send_pkt For returning RREPs For data packets

You implement miniroute_send_pkt Does AODV, then unicasts the packet

Testing: over the wireless

We do not have enough tablets to give you You would need a large network to test this We will do it in section We will schedule a few other times

Can also use any 802.11b Windows device laptop desktop with wireless card

Broadcast storm

Issues with flooding wireless networks May have already heard an answer, but unicast May have a better answer than one you hear n2 flooding is expensive to discover linear paths

What can we do? Damping Promiscuous unicast listening

Implementations and help

For real implementations, see AODV: moment.cs.ucsb.edu/AODV/aodv.html DSR: www.monarch.cs.cmu.edu/dsr-impl.html

To try out AODV without the hassle, see sns: www.cs.cornell.edu/People/egs/sns/

simulated implementation of AODV

Papers See the above sites for references or just google it