Transcript Wireless Routing (2)

Wireless Routing (2)
Philippe Jacquet
Reactive protocols
• Ad hoc On Demand distance Vector
(AODV)
– Routing tables contains only active routes
– Route discovery on demand based on
wireless flooding
– Route recovery procedure after route
failure
Distance vector revisited
• Count to infinity and routing loops
– Use of a route sequence number
• Incremented at each new demand or recovery
• The « good » route has the highest seq number
On demand routing
• A node S with a packet to a new destination
D
– If routing table lookup fails then
•
•
•
•
1. Starts a route discovery
2. Waits for new route notification
3. Insert new entry in routing table
4 forwards packet
– Meanwhile keeps packet to X in buffer.
– No best prefix routing.
– entries in routing table
D Next relay
• Expires if inactive
• Removed on route error notification
• Changed on updated seq(D)
dist
Seq(D)
Route discovery
• Two main control packets
– Route REQuest (RREQ) packet
– Route REPly (RREP) packet
• A third control packets
– Route ERRor (RERR) packet
• A node sequence number seq(X)
Route discovery
• RREQ from S for a route to D contains
(after IP headers)
– route originator ID (S),
– route destination ID (D)
– Hop count (set at zero on source)
– Seq(S) +1
– Eseq(D) (last known seq(D) +1)
Basic Route Discovery
• The RREQ packet is flooded in the network
– Each node retransmits once the RREQ
• Excepted the route destination
– Upon first reception of RREQ: on fly reverse route
update
• Route entry to S updated
• Hop count updated
S
Last relay
hop
S
A
B
C
D
(S:S,0)
(S:S,1)
(S:A,2)
(S:B,3)
(S:C,4)
Seq(S)
Basic Route Discovery
• Upon reception of RREQ
– Destination increments its own seq(S)
– Generates and send to D a RREP
– Use reverse route (using routing table)
• RREP contains
– A RREP hop count set to zero
– Current seq(S)
Basic Route Discovery
• RREP forwarding
– Update route to D
– Forward RREP
D
Last relay
hop
Seq(D)
• Routing table updated only on route nodes
S
A
B
C
D
(S:S,0)
(D:A,4)
(S:S,1)
(D:B,3)
(S:A,2)
(D:C,2)
(S:B,3)
(D:D,1)
(S:C,4)
(D:D,0)
On demand Distance vector
2
3
1
1
3
2
1
4
1
5
2
1
Route failure
• If a link breaks on an active route
– upward node sends a RERR packets
• To all nodes that are on the active route.
• Contains the list of unreachable destination.
– Active sources will generate new RREQ
– Possibility of local repair.
S
RERR(D)
A
RERR(D)
B
RERR(D)
C
D
Route discovery optimization
• Wireless flooding consumes too much
resource.
– 1. S Sends RREQ only to neighbor (TTL=1)
– 2. If neighbor X has route to D
• X sends RREP for D with hop count set to dist
• (optional) forwards RREP to D for reverse route to S.
S
(S:S,0)
(D:A,4)
A
B
C
D
(S:S,1)
(D:B,3)
(S:A,2)
(D:C,2)
(S:B,3)
(D:D,1)
(S:C,4)
(D:D,0)
Route discovery cost
Source
Destination
Route discovery optimization
• Expanding ring search
– First try with TTL=1
– With TTL=2
– With TTL=4…
Route optimization
• Hop count can be any
metric
– Basic route discovery
does not give shortest
path
– Improvement: upon
reception of a RREQ
copy with shorter metric
• Update route to D
• Sends a new RREP if
destination.
1
2
3
1
5
2
6
Route optimization
• Fails on
1
2
1
3
2
7
5
6
7
Super-flooding
• Retransmit RREQ when hop count is
shorter
– Node may retransmit RREQ several times
– Hop count:
• number of retransmissions may be quadratic
– General metric:
• Number of retransmissions may be exponential
Super-flooding
1
2
1
3
2
7
5
4
6
3
7
4
Further optimization
• MultiPoint Relay flooding
– RREQ flooded via MPRs
– Reduces flooding cost
Overhead in routing protocols
•Preliminary points
–Network: n mobile nodes in an area density
–Radio: average range R: average
neighborhood per node M  R 2

–Routing: average number of hops between
N
  1.63K

two random nodes
M

–Mobility: typical node speed s, average link
s

creation/termination
rate


R
•For uniform independent random walk

32


Classic BGP
• Topology dissemination
s
R
 NM
– Overall link rate creation and failure
– Message link state average size M
– Dissemination overhead per link
 change NM
– Total overhead  Rs N M
2
– With wireless
flooding

s
R
3
 N 2M2

OLSR
– Average multipoint relay subset size
M r  logM, or M
– Average link state TC message size:
– Dissemination overhead MrN
M
– Total overhead

s
R
 N 2 Mr2


Mr
1
3
Basic AODV

N
M
– Average route length
s
N
– Route failure rate
 
R
M
– Number of activeroutes per node rN N
– Dissemination overhead
N

s
R
 
N 3
N rN
M
– Ratio with OLSR


N NrN
M M r2
