lecture5.pdf

CMPE 257 Winter'11 1

CMPE 257: Wireless and Mobile Networking

Katia Obraczka Computer Engineering

UCSC Baskin Engineering Lecture 5


Announcements •  Project proposals. •  Student presentations.

– 10 students so we need 5 lectures. •  2 students per lecture.

– Topics: •  Security. •  Mobility management. •  Hybrid networks. •  Energy management. •  DTNs


Today

•  Finish MAC. •  Unicast routing.

IEEE 802.11

•  Provides 2 types of medium access: – DCF: distributed coordination function. – PCF: point coordination function.

•  DCF is contention-based. •  PCF is polling-based.

– Collision free. –  Implemented atop DCF.


DCF PCF

IEEE 802.11 DCF

•  Physical carrier sensing: – Stations listen to channel before

transmitting (CS of CSMA/CA). •  Virtual carrier sensing:

– CA OF CSMA/CA. –  “Reserve” channel for transmission. – Use RTS/CTS handshake.



IEEE 802.11 MAC Protocol: CSMA (no CA)

802.11 sender 1 if sense channel idle for DIFS then

transmit entire frame (no CD) 2 if sense channel busy then

start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval,

repeat 2

802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to

hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

IEEE 802.11 MAC Protocol: CSMA/CA

•  Physical CS + virtual CS. – Sense channel for DIFS.

•  RTS/CTS handshake before sending data.

•  RTS is 20 bytes and CTS is 16 bytes. •  Maximum data frame is 2,346 bytes.


Note: This is only for unicast transmissions. Broadcast Transmissions do not use virtual carrier sensing.

CSMA-CA Examples


A

C D B

E

Scenario: A wants to transmit to C. . A sends RTS. . D defers. . C sends CTS. . B defers.

F


IEEE 802.11 Wireless LAN •  802.11b

–  2.4-5 GHz unlicensed spectrum –  up to 11 Mbps –  direct sequence spread

spectrum (DSSS) in physical layer

•  all hosts use same chipping code

•  802.11a –  5-6 GHz range –  up to 54 Mbps

•  802.11g –  2.4-5 GHz range –  up to 54 Mbps

•  802.11n: multiple antennae –  2.4-5 GHz range –  up to 200 Mbps

•  All use CSMA/CA for multiple access. •  All have base-station and ad-hoc network

modes.


CSMA Variants •  1-persistent (IEEE 802.3):

–  If medium idle, transmit. –  If medium busy, keep listening; when medium idle,

transmit with probability 1. •  p-persistent:

–  Same as above but with probability p. •  Non-presistent:

–  If medium idle, transmit. –  If medium busy, wait a random period before re-

trying.


MAC: A Bird’s Eye View


Solutions to Hidden/Exposed Nodes in CSMA

•  Use only virtual CS: – RTS/CTS (Request-To-Send/Clear-To-Send) – Used by MACA (Multiple Access Control

Avoidance) and MACAW (MACA for Wireless LANs).

•  Use both physical- and virtual CS: – CSMA/CA, IEEE 802.11.


Dynamic Reservation Approaches: Sender- vs. Receiver-initiated

•  Sender-initiated: –  A node wanting to send data takes the initiative of

setting up the reservation. –  Most existing schemes.

•  Receiver-initiated: –  A receiving node polls a potential transmitting node

for data. –  A node can send data after being polled. –  E.g., MACA-By Invitation.


Single vs. Multiple Channel Protocols

•  Single channel protocols: control and data use the same channel.

•  Multiple channel protocols: separate channels for control & data transmission; data transmission on separate channels.


Other criteria for classification

•  Power-aware. –  E.g., PAMAS.

•  Directional or omnidirectional antennas. •  QoS-aware

–  End-to-end (E2E) delay –  Packet loss rate (or the probability) –  Available bandwidth –  Challenges: lack of centralized control, limited bandwidth, node

mobility, power/computational constraints, error-prone nature of wireless media.


MACAW

•  [Bharghavan, 1994]. •  Proposed as improvement to MACA

[Karn, 1990]. •  Note that first IEEE 802.11 standard

(IEEE 802.11 “legacy”) released in 1997.


MACA

•  Proposed as alternative to CSMA. •  Introduced CA.

– RTS/CTS handshake (2-way).


MACA •  If node A wants to transmit to B, it first sends an RTS

packet to B, indicating the length of the data transmission to follow.

•  B returns a CTS packet to A with the expected length of the transmission.

•  A starts transmission when it successfully receives CTS. –  RTS and CTS packets are much shorter than data packets.

•  A neighboring node overhearing an RTS defers its own transmission until the corresponding CTS would have been finished.

•  A node hearing the CTS defers for the expected length of the data transmission.


MACA (Cont’d)

•  Nodes close to sender: –  If no CTS heard, OK to transmit. –  Avoid exposed terminal problem: nodes that hear

only RTS can transmit simultaneously with RTS sender.

•  Nodes close to receiver: –  Upon hearing CTS, defer till after data. –  Avoid hidden terminal.

•  Binary exponential backoff (BEB). –  Possible unfair channel allocation (starvation).


MACAW

•  Inspired 802.11. •  2 basic changes to MACA:

– Additional signaling. – Modified backoff algorithm.


MACAW Backoff •  Tries to avoid BEB’s unfairness. •  Proposed fix: sharing congestion information

among nodes. –  Backoff counter information propagated in packet

header. –  After successful transmission, neighbors have the

same backoff counter. •  Tries to prevent large variations of the back-

off value. –  Multiplicative increase (1.5), linear decrease

(decremented by 1).


Data Transmission in MACAW

•  Added ACK. – Reliability at layer 2. –  If ACK not received:

•  Retransmit frame. •  Increment backoff timer.


Data Transmission in MACAW •  Added small “Data Sending” (DS) control frame.

–  Addresses exposed terminal problem. •  In MACA, exposed node (received RTS but not CTS) is allowed to

transmit. –  Example: S1->R1 and S2->R2

•  CTS from R2 may collide with transmission S1->R1. •  S2 backs-off.

–  Fix: make sure S2 knows RTS-CTS exchange between S1 and R1 was successful.

•  S1 sends small control frame, DS with data exchange duration. •  When S2 receives DS, defers its transmission.

R1 S1 S2 R2


Data Transmission in MACAW •  Added “Request for Request-to-Send” (RRTS). •  R2 contends on behalf of S2 if it received RTS from S2 when it

could not have responded because deferring due to S1->R1 exchange.

•  When S2 receives RRTS from R2, proceeds with RTS, etc.

S2 R2 R1 S1 RRTS RRTS


FAMA Protocols •  Floor Acquisition Multiple Access.

– Floor acquisition = gain control of channel. •  MACA is an example of a FAMA

protocol. – Floor acquisition on packet-by-packet

basis. – No physical CS; only virtual CS. – For collision freedom, RTS needs to be at

least 2*channel-propagation-delay. •  .

FAMA Paper (Garcia-Luna et al.)

•  FAMA non-persistent packet sensing, or FAMA-NPS. – No carrier sensing, i.e., MACA. – Uses ALOHA to transmit RTS.



FAMA-NTR •  FAMA non-persistent transmit request. •  Sender can send packet bursts. •  Combines non-persistent CS + RTS/

CTS exchange. •  Enforces waiting periods at sender and

receiver. •  For both data and control frames. •  Waiting period proportional to maximum

propagation time.


FAMA-NTR (cont’d)

•  Before sending: –  Node senses

channel. –  If channel busy,

backs-off for random period and retries later => non-persistent.

–  If channel free, node sends RTS.

•  Node waits CTS for 1 RTT. –  If CTS not received,

node backs-off. –  Otherwise, transmits

data burst (up to a maximum size).


FAMA-NTR

•  To allow bursts, receiving station waits after processing each data packet. –  Waiting period (T) = maximum propagation time.

•  Transmitting node waits for 2T after any control frame. –  Allows enough time for RTS-CTS exchange.


Unicast Routing in MANETs


Why MANET routing is challenging?

•  No fixed infrastructure. – Nodes can have unlimited mobility. – So?

•  Multiple hops to destination. •  Unreliable communication medium. •  All nodes need to participate in routing/

forwarding. – Also, security issues.


Mobility

•  Mobility patterns may vary widely. –  Stationary nodes (e.g., sensor nodes). –  Highly mobile nodes (e.g., vehicles). –  Discrete versus continuous mobility. –  Structured versus unstructured mobility.

•  Mobility characteristics: –  Speed. –  Direction. –  Pause time.


MANET Routing Requirements •  Dissemination of routing information:

– Multi-hop paths. – Loop free all the time, or almost loop-free. – Limited signaling overhead.

•  Self configuring, and adaptive to dynamic topology.

•  Efficiency, e.g.,low consumption of communication bandwidth, energy. – Scalable with number of nodes. – Localized effect of topology or flow change.


MANET Unicast Routing •  Many protocols have been proposed. •  Many have been invented specifically

for MANETs. •  Many are adapted from protocols for

wired networks. •  Can any one protocol work well in all

MANET environments?


DV or LS?


DV or LS? •  Distance-Vector Algorithm: Routers

exchange their distances to known destinations; a router uses the distance vectors received from its neighbors to compute its own distances. Distributed computation is problem.

•  Link-State Algorithm: Routers exchange information about the state of the links in the network; a router uses this information to compute its distances to destinations. Distributed database problem.


Proactive or Reactive?


MANET Unicast Routing Taxonomy

•  Proactive protocols: –  A.k.a, table-driven. –  Traditional routing protocols are proactive. –  Compute and maintain routes independent on

traffic demand/patterns. –  E.g., OLSR.

•  Reactive protocols: –  Compute and maintain routes “on-demand”. –  E.g., DSR, AODV.

•  Hybrid protocols. –  E.g., ZRP.


Tradeoffs?

•  Latency of route discovery. – Proactive protocols may have lower latency

since routes are maintained at all times. – Reactive protocols may have higher

latency because a route from X to Y will be found only when X attempts to send to Y.


Tradeoffs?

•  Overhead of route discovery/maintenance. –  Reactive protocols may have lower overhead

because routes are determined only if needed. –  Proactive protocols may result in higher overhead

due to continuous route updating (depends on rate of changes).

•  Which approach achieves better trade-offs depends on the traffic and mobility patterns.


What about Flooding?


Flooding for Data Delivery

•  Sender broadcasts data packet “P” to all its neighbors.

•  Each node receiving “P” forwards it to its neighbors.

•  Sequence numbers used to avoid forwarding P more than once. Why?

•  P reaches destination if reachable from source. Destination does not forward P.


Flooding


Flooding


Flooding

•  Advantages: –  Simplicity. –  Efficient when rate of

information transmission lower than topology changes.

–  Robustness.

•  Disadvantages: –  High overhead. –  May result in network

congetion.


Flooding for the Control Plane

•  Many protocols perform flooding of control packets. – E.g., route discovery and maintenance.

•  Overhead of control packet flooding may be amortized over data packets transmitted.


Dynamic Source Routing (DSR) [Johnson96]

•  Reactive protocol. •  When node S wants to send a packet to

D, and does not have a route to D, node S initiates a route discovery.

•  S floods Route Request (RREQ). •  Each node appends own identifier when

forwarding RREQ.


Route Discovery in DSR

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L


B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z

Y

Broadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ



B

A

S E F

H

J

D

C

G

I K

•  Node H receives packet RREQ from two neighbors: potential for collision

Z

Y

M

N

L

[S,E]

[S,C]



B

A

S E F

H

J

D

C

G

I K

•  Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

[S,C,G]

[S,E,F]



B

A

S E F

H

J

D

C

G

I K

Z

Y

M

•  Nodes J and K both broadcast RREQ to node D •  Since nodes J and K are hidden from each other, their transmissions may collide

N

L

[S,C,G,K]

[S,E,F,J]



B

A

S E F

H

J

D

C

G

I K

Z

Y

•  Node D does not forward RREQ, because node D is the intended target of the route discovery

M

N

L

[S,E,F,J,M]



•  Destination D on receiving the first RREQ, sends a Route Reply (RREP).

•  RREP is sent on a route obtained by reversing the route appended to the received RREQ.

•  RREP includes the route from S to D on which RREQ was received by node D.



Route Reply in DSR

B

A

S E F

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D

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RREP [S,E,F,J,D]

Represents RREP control message


Route Reply in DSR •  RREP can be sent by reversing the route

in RREQ only if links are guaranteed to be bi-directional

•  If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for S from D. – Unless D already knows a route to S. –  If a route discovery is initiated by D for a route

to S, then the RREP is piggybacked on D’s RREQ.


Processing RREP

•  Node S on receiving RREP, caches the route. •  When node S sends a data packet to D, the

entire route is included in the packet header –  Hence the name source routing.

•  Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded.


Data Delivery in DSR

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

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DATA [S,E,F,J,D]

Packet header size grows with route length


DSR Optimization: Route Caching

•  Each node caches a new route it learns by any means. –  When node S finds route [S,E,F,J,D] to node D, node S also

learns route [S,E,F] to node F. –  When node K receives Route Request [S,C,G], K learns

route [K,G,C,S] to S. –  When node F forwards Route Reply RREP [S,E,F,J,D], F

learns route [F,J,D] to D. –  When node E forwards Data [S,E,F,J,D] it learns route

[E,F,J,D] to node D –  Nodes may also learn route when it overhears data.


Use of Route Caching •  When S learns that a route to D is broken, it

uses another route from its local cache, if such a route to D exists in its cache; otherwise, S initiates route discovery.

•  Node X on receiving a RREQ for some node D can send a RREP if X knows a route to D.

•  Use of route cache –  Can speed up route discovery. –  Can reduce propagation of route requests.


Use of Route Caching

A

E

D G

[P,Q,R]: Represents cached route at a node

M

N

L

[S,E,F,J,D] [E,F,J,D]

[C,S]

[G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

Z

K H

B

S

F C

I

J


Route Caching: Speed up Route Discovery,

Reduce RREQ Flooding

A

E

J

D K

M

N

L

[S,E,F,J,D] [E,F,J,D]

[C,S] [G,C,S]

[F,J,D],[F,E,S]

[J,F,E,S]

RREQ When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route

[K,G,C,S] RREP

Route caches at K and J limit the flooding of Z’s RREQ.

Z

H

B C

S

F

I

G


Route Error (RERR)

B

A

S E F

H

J

D

C

G

I K

Z

Y

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N

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RERR [J-D]

J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails. Nodes hearing RERR update their route cache to remove link J-D


Route Caching: Beware! •  Stale caches can adversely affect

performance. – With time and host mobility, cached routes

may become invalid. – A sender host may try several stale routes

(obtained from local cache, or replied from cache by other nodes), before finding a good route.


DSR: Advantages •  Routes maintained only between nodes who

need to communicate. –  Reduces overhead of route maintenance.

•  Route caching can further reduce route discovery overhead.

•  Single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches.


DSR: Disadvantages •  Packet header size grows with route

length. •  Flood of route requests may potentially

reach all nodes in the network. – Care must be taken to avoid collisions

between route requests propagated by neighboring nodes. •  Insertion of random delays before forwarding

RREQ.


DSR: Disadvantages •  Increased contention if too many route

replies come back due to nodes replying using their local cache. –  “RREP” storm problem. – Reply storm may be eased by preventing a

node from sending RREP if it hears another RREP with a shorter route.


DSR: Disadvantages •  An intermediate node may send RREP

using a stale cached route, thus polluting other caches. – This problem can be eased if some

mechanism to purge (potentially) invalid cached routes is incorporated. •  Static timeouts. •  Adaptive timeouts based on link stability.


AODV •  Route Requests (RREQ) are forwarded

similarly to DSR. –  When a node re-broadcasts a RREQ, it sets up a

reverse path pointing towards the source. –  AODV assumes symmetric (bi-directional) links.

•  When the intended destination receives a RREQ, it replies by sending a RREP.

•  RREPs travel along the reverse path set-up when RREQ is forwarded.


Route Requests in AODV

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L



B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z

Y

Broadcast transmission

M

N

L


B

A

S E F

H

J

D

C

G

I K

Represents links on Reverse Path

Z

Y

M

N

L



AODV Route Discovery: Observations

•  RREQ contains source and destination IP address, current destination seq. number (incremented as a result of loss of prior route), and broadcast id (incremented for every RREQ). –  Source IP + bcast id uniquely identifies RREQ: nodes do not

forward RREQs they have forwarded recently. –  RREQ processing: node creates reverse route table entry for

RREQ source with TTL. –  If node has “unexpired” route to destination in its table with

sequence number >= RREQ’s, it replies to RREQ with Route Reply (RREP) back to source.

–  Otherwise, broadcast RREQ onward.


Destination Sequence Number

•  When node D receives route request with destination sequence number N, D sets its sequence number to N, unless it is already larger than N.

•  Node’s own sequence number is monotonically increasing. –  Sequence number is incremented after

neighborhood topology change.


Reverse Path Setup in AODV

B

A

S E

D G

I K

•  Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

H

C F

J



B

A

S E F

H

J

D

C

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I K

Z

Y

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L



B

A

S E F

H

J

D

C

G

I K

Z

Y

•  Node D does not forward RREQ, because node D is the intended target of the RREQ

M

N

L


Route Reply in AODV •  An intermediate node has current route to destination,

responds to RREQ with RREP. •  RREP contains source and destination IP, current

sequence number, number of hops to destination. –  If destination, then destination seq. #. –  Else, node’s current record of destination’s seq. #.

•  Node receiving RREP sets up forward path to destination.

•  If multiple RREPs received, node forwards first one. Later RREPs discarded unless greater seq. # or smaller # of hops.


Route Reply Example

B

A

S E F

H

J

D

C

G

I K

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Y

Represents links on path taken by RREP

M

N

L


Forward Path Setup in AODV

B

A

S E F

H

J

D

C

G

I K

Z

Y

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Forward links are setup when RREP travels along the reverse path

Represents a link on the forward path


Data Delivery in AODV

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

Routing table entries used to forward data packet. Route is not included in packet header.

DATA


Timeouts •  A routing table entry maintaining a reverse path is

purged after a timeout interval. –  Timeout should be long enough to allow RREP to come

back.

•  Routing table entry maintaining a forward path is purged if not used for active_route_timeout interval. –  If no data being sent using a particular routing table entry,

that entry will be deleted from the routing table (even if the route may actually still be valid).


Link Failure Reporting •  Link failures are propagated by means of Route Error

messages, which also update destination sequence numbers. –  RERR lists destinations now unreachable.

•  If upstream node has neighbors as precursors for the affected destinations, it broadcasts RERR.

•  Nodes receiving the RERR update cost to destination to infinity and forward RERR if needed.

•  Upon receiving RERR, source will initiate route discovery if still needs route.


Route Error •  When node X is unable to forward packet P (from

node S to node D) on link (X,Y), it generates a RERR message.

•  Node X increments the destination sequence number for D cached at node X.

•  The incremented sequence number N is included in the RERR.

•  When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N.


Link Failure Detection •  Hello messages: neighbor nodes

periodically exchange hello messages. •  Absence of hello message is used as

an indication of link failure. •  Alternatively, failure to receive several

MAC-level ACKs may be used as an indication of link failure.


AODV Packet Header

•  RREQ: –  RREQ id. –  Destination IP

address. –  Destination

sequence number. –  Originator IP

address. –  Originator sequence

number.

•  RREQ id + originator IP uniquely identifies the RREQ.

•  Originator sequence number.

•  Destination sequence number.


Destination Sequence Number

•  Avoid using stale routes. •  Node updates its destination seq. # when:

–  It generates a RREQ. •  Prevents conflicts previously established reverse routes.

–  It generates a RREP. •  New-seq-# = max(current seq #, RREQ dest. seq #).


Sequence Numbers in AODV

•  To prevent formation of loops

–  Assume that A does not know about failure of link C-D because RERR sent by C is lost

–  Now C performs a route discovery for D. A receives the RREQ (say, via path C-E-A)

–  A will reply since it knows a route to D via B. –  Results in a loop (for instance, C-E-A-B-C )

A B C D

E

A B C D

E


Optimization: Expanding Ring Search

•  RREQs are initially sent with small Time-to-Live (TTL) field, to limit their propagation. – DSR also includes a similar optimization.

•  If no RREP is received, then larger TTL tried.


Does The Sequence Numbering Work?

•  To some extent: – Sequence numbering scheme is not very

efficient. – Scheme requires that any given node A

either never forgets a destination sequence number it learns, or is able to wait “long enough” so that it cannot possibly attempt to reach a destination D through a path involving a node B that uses A to reach D.


Summary: AODV

•  Routes not included in packet header. •  Nodes maintain routing table entries for

“active” routes. •  At most one route (next hop) maintained

at each node. •  Unused routes expire even if topology

does not change.


Optimized Link State Routing (OLSR) [RFC 3626]

•  Overhead of flooding link state information reduced by having fewer nodes forward the information.

•  Broadcast from X only forwarded by its multipoint relays (MPRs).

•  Overhead is also reduced as the size of the LS updates is reduced: LS updates contain only info on MPRs.


OLSR •  OLSR floods information through MPRs. •  Flooded information contains links connecting

nodes to respective MPRs. –  I.e., node sends info on nodes that selected it as

their MPR. –  Periodic HELLO messages inform nodes which

other nodes selected it as their MPR. •  Routes used by OLSR only include multipoint

relays as intermediate nodes.


MPRs •  Multipoint relays of node X are its

neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X. – Each node transmits its neighbor list in

periodic beacons, so that all nodes know their 2-hop neighbors.

•  MPRs of X are 1-hop neighbors of X covering X’s 2-hop neighbors.


Optimized Link State Routing (OLSR)

•  C and E are multipoint relays of A.

A

B F

C

D

E H

G K

J

Node that has broadcast state information from A



•  Nodes C and E forward information received from A.

A

B F

C

D

E H

G K

J




•  E and K are multipoint relays for H. •  K forwards information received from H.

– E has already forwarded the same information once.

A

B F

C

D

E H

G K

J


L

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