Download Polygraph: Automatically Generating Signatures for

How (not) to Simulate
Wireless Networks with ns
Brad Karp
[email protected]
University College London
ns Workshop
MSR Cambridge
9th December, 2005
Outline
• Overview of ns wireless functionality
– History
– Simple example
ns overview material largely borrowed:
– MobileNode configuration
ns Manual (Chapter 16)
– Controlling mobility
Padmatraffic
Haldar’s
tutorial (11/2002)
– Defining
workloads
– MobileNode architecture
– Trace file format
• Role of simulation in wireless research
• Case study of simulation’s pitfalls: GPSR
2
History of ns Wireless Support
• David Johnson’s Monarch group at CMU:
–
–
–
–
–
Free-space, two-ray ground reflection channel model
802.11 MAC layer
Random waypoint mobility model
ARP
Ad hoc routing: Dynamic Source Routing (DSR),
TORA, …
– &c.
• Other major supported protocols and links
(outside scope of today’s talk): Mobile IP,
Directed Diffusion, satellite links, &c.
3
ns Wireless Architecture
• MobileNode at core of mobility support
• MobileNodes can move in a given topology,
receive/transmit signals to/from wireless
channels
• Wireless network stack consists of LL, ARP, MAC,
IFQ, &c.
• Allows simulations of multi-hop ad hoc networks,
wireless LANs, sensor networks etc
4
Wireless Example:
Ad hoc Routing
• Scenario
– 2 mobile nodes
– moving within 500m x 500m flat topology
– using DSDV ad hoc routing protocol
– Random Waypoint mobility model
– TCP traffic
• Examples:
– ns-2/ns-tutorial/examples/simple-wireless.tcl
– ns-2/tcl/ex/wireless-demo-csci694.tcl
5
An Example – Step 1
# Define Global Variables
# create simulator
set ns [new Simulator]
# create flat topology in 670m x 670m area
set topo [new Topography]
$topo load_flatgrid 500 500
6
An Example – Step 2
# Define wireless ns trace
# ns trace
set tracefd
[open simple.tr w]
$ns trace-all $tracefd
7
GOD (General Operations Director)
• Stores all-pairs Dijkstra shortest path
lengths
• Allows comparison of path length with
optimal
• Automatically generated, contained in
scenario file
• set god [create-god <no of mnodes>]
• $god set-dist <from> <to> <#hops>
8
Example –Step 3
• Create god
set god [create-god 2]
$ns at 900.00 “$god setdist 1 2 1”
• Create wireless channel
set thechan [new Channel/WirelessChannel]
9
An Example – Step 4
# Define how a mobile node is configured
$ns node-config \
-adhocRouting DSDV \
-llType LL \
-macType Mac/802_11 \
-ifqLen 50 \
-ifqType Queue/DropTail/PriQueue \
-antType Antenna/OmniAntenna \
-propType Propagation/TwoRayGround \
-phyType Phy/WirelessPhy \
-channel $thechan \
-topoInstance $topo
-agentTrace ON \
-routerTrace OFF \
-macTrace OFF \
-movementTrace OFF
10
An Example – Step 5
# Use “for” loop to create 3 nodes:
for {set i 0} {$i < 2} {incr i} {
set node($i) [$ns node]
# disable random motion
$node($i) random-motion 0
}
11
MobileNode Movement
• Node position defined in 3D
• Today, z axis not used
$node set X_ <x1>
$node set Y_ <y1>
$node set Z_ <z1>
$node at <time> setdest <newx>
<newy> <speed>
• Lots of these events over a simulation;
manual generation tedious…
12
Random Waypoint Model
• Place nodes uniformly at random
• Node moves to uniformly randomly chosen
destination, at velocity chosen uniformly
at random
• Between move events, node stays put for
“pause time”
• Meant to mimic user behavior: move,
work, move…
13
Scenario Generator: Movement
• setdest: MobileNode movement generator
setdest -n <num_of_nodes> -p
pausetime -M <maxspeed> -t
<simtime> -x <maxx> -y <maxy>
Source: ns-2/indep-utils/cmu-scengen/setdest/
14
Example setdest Output
$node_(2) set Z_ 0.000000000000
$node_(2) set Y_ 199.373306816804
$node_(2) set X_ 591.256560093833
$node_(1) set Z_ 0.000000000000
$node_(1) set Y_ 345.357731779204
$node_(1) set X_ 257.046298323157
$node_(0) set Z_ 0.000000000000
$node_(0) set Y_ 239.438009831261
$node_(0) set X_ 83.364418416244
$god_ set-dist 0 1 1
$ns_ at 50.000000000000 "$node_(2) setdest 369.463244915743
170.519203111152 3.371785899154"
$ns_ at 51.000000000000 "$node_(1) setdest 221.826585497093
80.855495003839 14.909259208114"
$ns_ at 33.000000000000 "$node_(0) setdest 89.663708107313
283.494644426442 19.153832288917"
15
Scenario Generator: Traffic
• Generating traffic pattern files
– CBR/TCP traffic
ns cbrgen.tcl [-type cbr|tcp] [-nn nodes] [-seed
seed] [-mc connections] [-rate rate (pkt/s)]
– CBR traffic
ns cbrgen.tcl –type cbr –nn 20 –seed 1 –mc 8
-rate 4
– TCP traffic
ns cbrgen.tcl –type tcp -nn 15 -seed 0 –mc 6
Default packet size: 512 bytes, hardwired in script!
Start time uniform in [0, 180] s, hardwired in script!
• To evaluate routing, which traffic type would you use?
• Source: ns-2/indep-utils/cmu-scen-gen/
16
A Traffic Scenario
set udp_(0) [new Agent/UDP]
$ns_ attach-agent $node_(0) $udp_(0)
set null_(0) [new Agent/Null]
$ns_ attach-agent $node_(2) $null_(0)
set cbr_(0) [new Application/Traffic/CBR]
$cbr_(0) set packetSize_ 512
$cbr_(0) set interval_ 4.0
$cbr_(0) set random_ 1
$cbr_(0) set maxpkts_ 10000
$cbr_(0) attach-agent $udp_(0)
$ns_ connect $udp_(0) $null_(0)
$ns_ at 127.93667922166023 "$cbr_(0) start"
…….
17
An Example – Step 6
# Define node movement model
source <movement-scenario-files>
# Define traffic model
source <traffic-scenario-files>
18
An Example – Step 7
# Tell ns the simulation stop time
$ns at 200.0 “$ns halt”
# Start your simulation
$ns run
19
Energy Extension
• Makes MobileNode energy-aware
• Enable by adding options:
$ns_ node-config \
–energyModel EnergyModel
-initialEnergy
100.0
-txPower
0.6
-rxPower
0.2
20
Outline
• Overview of ns wireless functionality
–
–
–
–
–
–
–
History
Simple example
MobileNode configuration
Controlling mobility
Defining traffic workloads
MobileNode architecture
Trace file format
• Role of simulation in wireless research
• Case study of simulation’s pitfalls: GPSR
21
Wireless Internals
• MobileNode
– Basic entity with address and port de-muxes,
routing agent, &c.
– Stack of network components, including IFQ,
LL, MAC, NetIF, radio propagation model, &c.
• Wireless channel
22
Portrait of A Mobile Node
Node
port
classifier
protocol
agent
Classifier: Forwarding
255
addr
classifier defaulttarget_
LL
Agent: Protocol Entity
routing
agent
ARP
Node Entry
LL
IFQ
IFQ
IFQ: Interface queue
MAC
MAC: Mac object
PHY
PHY: Net interface
MAC
PHY
MobileNode
CHANNEL
Propagation
and antenna
models
LL: Link layer object
Prop/ant
Radio propagation/
antenna models 23
Mobile Node : Components
• Classifiers
• defaulttarget_ points to routing agent object
• 255 is the port id assigned for rtagent_
• Routing agent
• Ad hoc routing protocol, e.g., AODV, DSDV,
DSR; or directed diffusion
24
Mobile Node: Components
• Link Layer
– Same as LAN, but with a separate ARP module
– Looks up IP-to-MAC mappings using ARP
• ARP
– Resolves IP address to hardware (MAC) address
– Broadcasts ARP query
• Interface queue (IFQ)
– Gives priority to routing protocol packets
– Has packet filtering (search and remove) capacity
25
Mobile Node: Components
• MAC
– 802.11
• IEEE RTS/CTS/DATA/ACK for unicast
• Sends DATA directly for broadcast
• Network interface (PHY)
– Used by MobileNode to access channel
– Stamps outgoing packets with meta-data
– Interface with radio/antenna models
26
Mobile Node: Components
• Radio Propagation Model
– Friss-space model: attenuation at near distance
– Two-ray ground reflection model: attenuation at
far distance
– Shadowing model: probabilistic
• Antenna
– Omni-directional, unity-gain
27
Wireless Channel
• Duplicate packets to all mobile nodes
attached to the channel except the sender
• It is the receiver’s responsibility to decide
if it will accept the packet
– Collision is handled at individual receiver
– O(N2) computation!
28
Wireless Trace Support
• CMU trace format differs from standard ns trace format!
• Configurable tracing:
–
–
–
–
MAC (very voluminous!)
Router forwarding
Agent events
Movement events
• Format described in detail in ns manual
• When in doubt, no substitute for reading the code!
• Shortest path lengths logged per pkt rx (cost in-core?)
• “New” wireless trace format
– appears not to be in wide use yet
– same information, formatted differently
29
Outline
• Overview of ns wireless functionality
–
–
–
–
–
–
–
History
Simple example
MobileNode configuration
Controlling mobility
Defining traffic workloads
MobileNode architecture
Trace file format
• Role of simulation in wireless research
• Case study of simulation’s pitfalls: GPSR
30
Simulation as Way to Enable Area
• Ad hoc routing: huge system
deployment/debugging cost
• 1998 Broch et al. paper a watershed for
ad hoc routing: first even comparison of
wide field of protocols
• CMU ns wireless “extensions” the basis of
most ad hoc networking research for next
5+ years
31
Simulation as Way to Kill Area
• Simulators don’t evaluate motivation!
– Why are 500 laptops in a gymnasium not
simply connecting to base stations?
• The founding of MobiHoc
– cf. the founding of SIGMETRICS…
• The founding of MobiSys
• Secret tip: building a real system
dramatically increases chances of
acceptance! (because it’s HARD)
32
Things to Investigate Using ns
• Coarse notion of capacity
• Coarse notion of effect of mobility
• Fine-grained behavior of your routing (or
other hop-by-hop) protocol
– Only place you can easily have “centralized’
view!
33
Things Not to Investigate Using ns
Broadcast Packet
Delivery Probability
• Realistic loss behavior (interference, multipath fading, …)
Roofnet
measurements
[SIGCOMM 2004]
Node Pair
34
Not in ns (cont’d)
• Realistic topology information
– Roofnet finding: RTS/CTS useless in outdoor
network!
– Hidden terminal problem still mainstay of
undergraduate networking courses
everywhere…
– GPSR free space ns simulation debacle (more
later)
• Fine-grained mobility behavior
– Random Waypoint corresponds little to reality
– GPSR density anomaly (more later)
35
When to Roll Your Own Simulator
• GPSR greedy-only simulator
– properties of node density, not of fine-grained
radio propagation
– ~250 lines of C, a morning’s work
– far faster than ns will ever be
• Simple visualization
– GPSR topology visualizer
• find connected components
• show planar subgraphs
• animate forwarding
36
Outline
• Overview of ns wireless functionality
–
–
–
–
–
–
–
History
Simple example
MobileNode configuration
Controlling mobility
Defining traffic workloads
MobileNode architecture
Trace file format
• Role of simulation in wireless research
• Case study of simulation’s pitfalls: GPSR
37
Greedy Perimeter Stateless Routing
(GPSR)
Central idea: Machines can know their geographic locations.
Route using geography. [MobiCom 2000]
• Packet destination field: location of destination
• Scalability:
Nodes all know tiny
own state
positions,
per e.g.,
router; low routing
– by GPS (outdoors)
protocol
overhead; approximates shortest
– by surveyed
(forsuccess
non-mobilerate
nodes)
paths;
high position
delivery
– by short-range localization (indoors, [AT&T Camb, 1997],
[Priyantha et al., 2000])
– &c.
• Two forwarding algorithms:
– Greedy mode simply chooses closest neighbor to destination
– Perimeter mode recovers when no closer next hop available, by
routing on planar subgraph of full network graph
38
Greedy Forwarding
• Nodes learn immediate neighbors’ positions from
beaconing/piggybacking on data packets
• Locally optimal, greedy next hop choice:
– Neighbor geographically nearest destination
D
x
y
39
Greedy Forwarding Failure
Greedy forwarding not always possible! Consider:
D
v circumnavigate voids?z
How can we
…based only on one-hop neighborhood?
void
y
w
x
40
Void Traversal: The Right-hand Rule
Well-known graph traversal: right-hand rule
Requires only neighbors’ positions
Doesn’t work when edges in graph cross
z
one another!y
x
41
Planarized Graphs: Example
200 nodes, placed uniformly at random on
2000-by-2000-meter region; 250-meter
radio range
Full Graph
GG Subgraph
RNG Subgraph
42
Packet Delivery Success Rate
(50, 200; Dense)
43
GPSR: Making it Real
• We implemented full GPSR for Berkeley mote
sensors [NSDI 2005]
– 3750 lines of nesC code
– also includes: simple link thresholding, ARQ
• Deployed on Mica 2 “dot” mote testbeds
– 23-node, 50-node subsets of 100-node network in
office building (Soda Hall; office walls; 433 MHz)
– 40-node network in office building (Intel Research
Berkeley; cubicles; 900 MHz)
• Delivery success workload: 50 packets
between all node pairs, serially
44
50-Node Testbed, Soda Hall
GAME OVER
Only 68.2% of node pairs connected!!
What’s going on here?!
45
Planar, but Partitioned
Absorption, reflection (multi-path),
interference, antenna orientation
differences, &c., lead to non-uniform
radio ranges.
RNG and GG partition graphs when radio
ranges not uniform!
Solution: entirely new protocol, CLDP, for
Output
of GPSR’s
removing crossing
edges
[NSDIDistributed
2005]. GG
Provably correct on
all connected
graphs.
(arrows
denote unidirectional
links)
46
Key Lessons and Conclusion
• Understand your algorithm’s correctness
assumptions
• Does the simulator model these properties
accurately, or at least pessimistically?
• Fading, interference, loss rate particular
pitfalls
• No substitute for building—even moreso
than for Internet congestion control, as
propagation too complex to model
accurately
47
UCL: A Tradition of Great Networking
Research in London
• Hiring junior and senior
networking faculty
• Application deadline:
5th January, 2006
• Details at
http://www.cs.ucl.ac.uk/
• Why UCL?
– Research: sdr, rat, Landmark
Routing, NAT, XORP, TFRC, XCP,
GPSR, CLDP, GHT, Autograph,
Polygraph, …
– Students (Paul Francis, Mark
Handley, Jon Crowcroft, …)
– LONDON
48