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Wireless LANs

Characteristics Infrastructure based

MAC (MACA)

Routing (Mobile IP)

Transport (TCP variants) Adhoc networks

Routing protocols

Transport issues

Implementations 802.11, HIPERLAN, Bluetooth

Other issues

Security

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Wireless LANs: Characteristics

Types

Infrastructure based

Adhoc

Advantages

Flexible deployment

Minimal wiring difficulties

More robust against disasters (earthquake etc)

Historic buildings, conferences, trade shows,

Disadvantages Low bandwidth compared to wired networks (1-10 Mbit/s)

Proprietary solutions

Need to follow wireless spectrum regulations

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Infrastructure vs. Adhoc Networks

infrastructure

network

ad-hoc network

APAP

AP

wired network

AP: Access Point

Source: Schiller

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Transmission: Infrared vs. Radio

Infrared

uses IR diodes, diffuse light,

multiple reflections (walls,furniture etc.)

Advantages

simple, cheap, available in many

mobile devices

no licenses needed

simple shielding possible

Disadvantages

interference by sunlight, heat

sources etc.

many things shield or absorb IR

light

low bandwidth

Example

Ir DA (Infrared Data Association)

interface available everywhere

Radio

typically using the license free

ISM band at 2.4 GHz Advantages

experience from wireless WAN

and mobile phones can be used

coverage of larger areas possible

(radio can penetrate walls,

furniture etc.)

Disadvantages

very limited license free frequency

bands

shielding more difficult,

interference with other electrical

devices

Example

Wave LAN, HIPERLAN,

Bluetooth

Source: Schiller

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Wireless MAC: Motivation

Can we apply media access methods from fixed networks?

Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection

send as soon as the medium is free, listen into the medium if acollision occurs (original method in IEEE 802.3)

Medium access problems in wireless networks

signal strength decreases proportional to the square of the distance

sender would apply CS and CD, but the collisions happen at thereceiver

sender may not hear the collision, i.e., CD does not work

CS might not work, e.g. if a terminal is hidden

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Difference Between Wired and Wireless

If both A and C sense the channel to be idle at the same time, they

send at the same time.

Collision can be detected at senderin Ethernet.

Half-duplex radios in wireless cannot detect collision at sender.

A B C

A

B

C

Ethernet LAN Wireless LAN

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Hidden terminals

A and C cannot hear each other.

A sends to B, C cannot receive A.

C wants to send to B, C senses a free medium (CS fails)

Collision occurs at B.

A cannot receive the collision (CD fails).

A is hidden for C.

Solution?

Hidden terminal is peculiar to wireless (not found in wired)

Need to sense carrier at receiver, not sender!

virtual carrier sensing: Sender asks receiver whether it canhear something. If so, behave as if channel busy.

Hidden Terminal Problem

BA C

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Exposed Terminal Problem

Exposed terminals

A starts sending to B.

C senses carrier, finds medium in use and has to wait for A->B toend.

D is outside the range of A, therefore waiting is not necessary.

A and C are exposed terminals.

A->B and C->D transmissions can be parallel; no collisions

A B

CD

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MACA: Multiple Access with Collision Avoidance

MACA uses signaling packets for collision avoidance RTS (request to send)

sender request the right to send from a receiver with a shortRTS packet before it sends a data packet

CTS (clear to send)

receiver grants the right to send as soon as it is ready to receive

Signaling (RTS/CTS) packets contain

sender address

receiver address packet size

Variants of this method are used in IEEE 802.11

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MACA avoids the problem of hidden terminals A and C want to send to B

A sends RTSto B

B sends CTSto A

C overhears CTSfrom B C waits for duration of As transmission

MACA avoids the problem of exposed terminals

B wants to send to A, C to D C hears RTS from B->A

C does not hear CTSfrom A

C sends RTSto D

MACA Solutions [Karn90]

A B C

RTS

CTSCTS

A B CRTS

CTS

RTS

D

D

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MAC: Reliability

Wireless links are prone to errors. High packet loss rate detrimental to

transport-layer performance. Solution: Use of acknowledgements

When B receives DATA from A, B sends an ACK.

If A fails to receive an ACK, A retransmits the DATA.

Both C and Dremain quiet until ACK(to prevent collision of ACK).

Expected duration of transmission+ACK is included in RTS/CTSpackets.

This approach adopted in many protocols [802.11].

A B C

RTS

CTS CTS

DATA

D

RTS

ACK

Collision of RTS/CTSpackets can happen (hidden terminal). If no CTS, retransmit RTSafter backoff.

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MAC: Collision Avoidance

With half-duplex radios, collision detection is not possible Collision avoidance:Once channel becomes idle, the node waits for a

randomly chosen duration before attempting to transmit

IEEE 802.11 DCF

When transmitting a packet, choose a backoff interval in the range [0,cw];cwis contention window

Count down the backoff interval when medium is idle

Count-down is suspended if medium becomes busy

When backoff interval reaches 0, transmit RTS

Time spent counting down backoff intervals is part of MAC overhead

large cwleads to larger backoff intervals

small cwleads to larger number of collisions

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DCF Example

data

wait

B1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervals

at nodes 1 and 2cw = 31

B2 = 10

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MAC: Congestion Control

Number of nodes attempting to transmit simultaneously may change

with time; some mechanism to manage congestion is needed. IEEE 802.11 DCF: Congestion control achieved by dynamically

choosing the contention window cw

Binary Exponential Backoff in DCF:

When a node fails to receive CTSin response to its RTS, it increases thecontention window

cwis doubled (up to a bound CWmax)

Upon successful completion data transfer, restore cwto CWmin

Optimization: MACAW 802.11: cwreduces much faster than it increases

Backoff: multiply cwby 1.5 (instead of doubling)

Restore: Reduce cwby 1 (instead of CWmin)

cwreduces slower than it increases. Exponential increase linear decrease

Avoids wild oscillations of cwwhen congestion is high.

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MAC: Energy Conservation

Wireless nodes need to conserve power (resource poor).

Typical solution: Turning the radio off when not needed

Power Saving Mode in IEEE 802.11 (Infrastructure Mode)

An Access Point periodically transmits a beacon indicating which

nodes have packets waiting for them

Each power saving (PS) node wakes up periodically to receive the

beacon

If a node has a packet waiting, then it sends a PS-Poll After waiting for a backoff interval in [0,CWmin]

Access Point sends the data in response to PS-poll

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MAC Protocols: Summary

Wireless medium is prone to hidden and exposed terminalproblems

Protocols are typically based on CSMA/CA

RTS/CTS based signaling Acks for reliability

Contention window is used for congestion control

IEEE 802.11 wireless LAN standard Fairness issues are still unclear

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Routing and Mobility

Finding a path from a source to a destination

Issues

Frequent route changes

amount of data transferred between route changes may bemuch smaller than traditional networks

Route changes may be related to host movement

Low bandwidth links

Goal of routing protocols decrease routing-related overhead

find short routes

find stable routes (despite mobility)

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Mobile IP (RFC 2002): Motivation

Traditional routing

based on IP destination address

network prefix determines physical subnet

change of physical subnet implies

change of IP address (conform to new subnet), or

special routing table entries to forward packets to new subnet

Changing of IP address DNS updates take to long time

TCP connections break

security problems

Changing entries in routing tables

does not scale with the number of mobile hosts and frequent changes inthe location

security problems

Solution requirements

retain same IP address, use same layer 2 protocols

authentication of registration messages,

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Mobile IP: Basic Idea

Router

1

Router

3

Router

2

S MN

Home

agent

Source: Vaidya

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Mobile IP: Basic Idea

Router

1

Router

3

Router

2

S MN

Home agent

Foreign agent

move

Packets are tunneled

using IP in IP

Source: Vaidya

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Mobile IP: Terminology

Mobile Node (MN) node that moves across networks without changing its IP address

Home Agent (HA)

host in the home network of the MN, typically a router

registers the location of the MN, tunnels IP packetsto the COA

Foreign Agent (FA)

host in the current foreign network of the MN, typically a router

forwards tunneled packets to the MN, typically the default routerfor MN

Care-of Address (COA) address of the current tunnel end-pointfor the MN (at FA or MN)

actual location of the MN from an IP point of view

Correspondent Node (CN)

host with which MN is corresponding (TCP connection)

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Data transfer to the mobile system

Internet

sender

FA

HA

MN

home network

foreign

network

receiver

1

2

3

1. Sender sends to the IP address of MN,HA intercepts packet (proxy ARP)

2. HA tunnels packet to COA, here FA,

by encapsulation

3. FA forwards the packet to the MN

Source: Schiller

CN

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Data transfer from the mobile system

Internet

receiver

FA

HA

MN

home network

foreign

network

sender

1

1. Sender sends to the IP address

of the receiver as usual,

FA works as default router

Source: Schiller

CN

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Reverse tunneling (RFC 2344)

Source: Schiller

Internet

receiver

FA

HA

MN

home network

foreign

network

sender

3

2

1

1. MN sends to FA

2. FA tunnels packets to HAby encapsulation

3. HA forwards the packet to the

receiver (standard case)

CN

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Mobile IP: Other Issues

Reverse Tunneling

firewalls permit only topological correct addresses a packet from the MN encapsulated by the FA is now topological correct

Agent Advertisement

HA/FA periodically send advertisement messages into their physical subnets

MN listens to these messages and detects, if it is in home/foreign network

MN reads a COA from the FA advertisement messages Registration

MN signals COA to the HA via the FA

HA acknowledges via FA to MN

limited lifetime, need to be secured by authentication

Optimizations Triangular Routing

HA informs sender the current location of MN

Change of FA

new FA informs old FA to avoid packet loss, old FA now forwardsremaining packets to new FA

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Multi-Hop Wireless Networks

May need to traverse multiple links to reach destination

Mobility causes route changes

Source: Vaidya

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Mobile Ad Hoc Networks (MANET)

Host movement frequent

Topology change frequent

No cellular infrastructure. Multi-hop wireless links.

Data must be routed via intermediate nodes.

A B A

B

Source: Vaidya

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Routing in MANET

Mobile IP needs infrastructure Home Agent/Foreign Agent in the fixed network

DNS, routing etc. are not designed for mobility

MANET

no default router available

every node also needs to be a router

Can we use traditional routing algorithms?

Distance Vector

periodic exchange of routing tables (destination, distance, next hop)

selection of the shortest path if several paths available

Link State

periodic notification about current state of physical links (flooding)

router get a complete picture of the network

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Traditional Routing

A routing protocolsets up a routing tablein routers

A node makes a local choice depending onglobal

topology

Source: Keshav

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Distance-vector & Link-state Routing

Both assume router knows address of each neighbor

cost of reaching each neighbor

Both allow a router to determine global routing

information by talking to its neighbors

Distance vector- router knows cost to each destination

Link state- router knows entire network topology and

computes shortest path

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Distance Vector Routing: Example

2

Source: Keshav

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Link State Routing: Example

Source: Keshav

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Extending Traditional Routing to MANET

Traditional routing protocols have been designed for fixed networkswith infrequent changes; typically assume symmetric links

MANET

dynamic topology:

frequent route changes no border routers

wireless medium:

variable connection quality

limited bandwidth (further reduced due to routing updates)

links may be asymmetric

resource poor mobile nodes:

routing table updates consume energy without contributing to data Tx

sleep modes difficult to realize

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MANET Routing Protocols

Reactive protocols

Determine route if and when needed

Source initiates route discovery

Example: DSR (dynamic source routing)

Proactive protocols

Extension of traditional routing protocols

Maintain routes between every host pair at all times

Example: DSDV (destination sequenced distance vector)

Hybrid protocols

Adaptive; Combination of proactive and reactive

Example : ZRP (zone routing protocol)

Multicast routing

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Dynamic Source Routing (DSR) [Johnson96]

When source S wants to send a packet to destination D, but does not knowa route to D, S initiates a route discovery

S floods Route Request (RREQ)

Each node appends its own identifierwhen forwarding RREQ

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

RREPsent on route obtained by reversingthe route appended in RREQ

RREPincludes the routefrom S to D, on which RREQwas received by D

S on receiving RREP, cachesthe route included in the RREP When S sends a data packet to D, entire route is included in the header

Intermediate nodes use the source route in the packetheader to determine

to whom a packet should be forwarded

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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

Source: Vaidya

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Route Discovery in DSR

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

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Route Discovery in DSR

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]

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Route Discovery in DSR

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 RREQonce

Z

Y

M

N

L

[S,C,G]

[S,E,F]

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Route Discovery in DSR

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]

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

Node D does not forwardRREQ, because node D

is the intended targetof the route discovery

M

N

L

[S,E,F,J,M]

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Route Reply in DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

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

Represents RREP control message

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Data Delivery in DSR

B

A

S E

F

H

J

D

C

G

I

K

Z

Y

M

N

L

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

Packet header size grows with route length

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DSR Issues

Optimizations: cacheroutes learnt by any means

When S finds route [S,E,F,J,D]to D, S also learns route [S,E,F] to F

When K receives RREQ[S,C,G] for G, K learns route [K,G,C,S] to S

When F forwards RREP [S,E,F,J,D], F learns route [F,J,D] to D

When E forwards Data [S,E,F,J,D], E learns route [E,F,J,D] to D

Advantages

Routes maintained only between nodes who need to communicate

Reduces overhead of route maintenance

Caching (at intermediate nodes) can further reduce route discovery overhead

Disadvantages

Packet header size grows with route length due to source routing

Flood of route requests may potentially reach all nodes in the network

Route Reply Stormproblem: Many intermediate nodes reply from local cache

Stale caches will lead to increased overhead

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Destination-Sequenced Distance-Vector (DSDV)

[Perkins94Sigcomm]

Each node maintains a routing table which stores next hop, cost metric towards each destination

a sequence number that is created by the destination itself

Each node periodically forwards routing table to neighbors

Each node increments and appends its sequence numberwhen sending itslocal routing table

Each route is tagged with a sequence number; routes with greatersequence numbers are preferred

Each node advertises a monotonically increasing even sequencenumber for itself

When a node decides that a route isbroken, it increments the sequencenumber of the route and advertises it with infinite metric

Destination advertises new sequence number

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Destination-Sequenced Distance-Vector (DSDV)

When X receives information from Y about a route to Z Let destination sequence number for Z at X be S(X), S(Y) is sent

from Y

If S(X) > S(Y), then X ignores the routing information received

from Y

If S(X) = S(Y), and cost of going through Y is smaller than the

route known to X, then X sets Y as the next hop to Z If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is

updated to equal S(Y)

X Y Z

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Reactive v/s Proactive Trade-offs

Reactive protocols Lower overhead since routes are determined on demand

Significant delay in route determination

Employ flooding (global search)

Control traffic may be bursty

Proactive protocols

Always maintain routes

Little or no delay for route determination

Consume bandwidth to keep routes up-to-date

Maintain routes which may never be used

Which approach achieves a better trade-off depends on the traffic andmobility patterns

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Zone Routing Protocol (ZRP) [Haas98]

ZRP combines proactive and reactive approaches

All nodes within hop distance at most d from a node X are

said to be in the routing zoneof node X

All nodes at hop distance exactly dare said to beperipheralnodes of node Xs routing zone

Intra-zone routing: Proactively maintain routes to all nodes

within the source nodes own zone.

Inter-zone routing: Use an on-demand protocol (similar to

DSR or AODV) to determine routes to outside zone.

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ZRP: Example

Radius of routing zone = 2

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MANET Routing: Summary

Protocols

Typically divided into proactive, reactive and hybrid

Plenty of other routing protocols: location-aided, power-aware, .

Several recent proposal in IETFs MANET Working Grouphttp://www.ietf.org/

Performance Studies

Typically studied by simulations using ns, discrete event simulator

Nodes (10-30) remains stationary for pause time seconds (0-900s) andthen move to a random destination (1500m X300m space) at a uniformspeed (0-20m/s). CBR traffic sources (4-30 packets/sec, 64-1024bytes/packet)

Attempt to estimate latency of route discovery, routing overhead

Actual trade-off depends a lot on traffic and mobility patterns Higher traffic diversity (more source-destination pairs) increases overhead

in on-demand protocols

Higher mobility will always increase overhead in all protocols

See Nitin Vaidyas MobiCom2000 tutorial

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TCP in Wireless Environments

TCP provides

reliable ordered delivery (by means of retransmissions, if necessary)

cumulative ACKs (an ACKacknowledges all contiguously receiveddata)

duplicate ACKs (whenever an out-of-ordersegment is received)

end-to-end semantics (receiver sends ACKafter data has reached)

implements congestion avoidance and control using congestion window

Factors affecting TCP performance in Wireless:

Wireless transmission errors

may cause fast retransmit, whichresults in reduction in congestionwindow size

reducing congestion window in response to errors is unnecessary Multi-hop routes on shared wireless medium

Longer connections are at a disadvantage compared to shorter ones,because they have to contend for wireless access at each hop

Route failures due to mobility

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40 39 3738

3533

TCP basics: Cumulative Acknowledgements

A new cumulative acknowledgement is generated only onreceipt of a newin-sequencepacket

41 40 3839

35 37

3634

3634

i data acki Source: Vaidya

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TCP: Delayed Acknowledgements

An ack is delayed until

another packet is received, or delayed ack timer expires (200 ms typical)

Reduces ack traffic

40 39 3738

3533

41 40 3839

35 37

New ack not produced

on receipt of packet 36,

but on receipt of 37

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TCP: Duplicate Acknowledgements

A dupack is generated whenever anout-of-ordersegment arrives at the receiver

40 39 3738

3634

42 41 3940

36 36

Dupack

(Above example assumes delayed acks)

On receipt of 38

TCP D li t A k l d t

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TCP: Duplicate Acknowledgements

Duplicate acks are not delayed

Duplicate acks may be generated when a packet is lost, or

a packet is delivered out-of-order (OOO)

40 39 3837

3634

41 40 3739

36 36

DupackOn receipt of 38

C i d d l C l

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TCP: Window Based Flow Control

Sliding window protocol

Window size minimum of

receivers advertised window- determined by available buffer space at the

receiver

congestion window- determined by the sender, based on feedback from

the network

Flow control is self-clocking new data sent only when old data is ACKd

congestion window sizebounds amount of data that can be sent per RTT

2 3 4 5 6 7 8 9 10 11 131 12

Senders window

Acks received Not transmitted

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TCP: Detection of packet loss

Retransmission timeout(RTO) sender sets retransmission timer for only one packet

if ACKnot received before timer expiry, the packet is assumed lost

RTO dynamically calculated, doubles on each timeout

Duplicate ACKs

may be generated due to packet loss or out-of-order delivery

sender assumes packet loss if it receives three consecutive dupacks

Fast Retransmit

RTO expiry may take too long

sender assumes packet loss if it receives three consecutive dupacks

On detecting a packet loss, TCP sender assumes that networkcongestion has occurred and drastically reduces the congestionwindow

TCP Sl St t d C ti A id

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TCP: Slow Start and Congestion Avoidance

Slow Start

initially, congestion window size cwnd= 1 MSS (maximum segt size)

increment window size by 1 MSS on each new ack

phase ends when window size reaches slow-start threshold (ssthresh)

cwndgrows exponentiallywith time during slow start

factor of 1.5 per RTT if every other packet ackd

factor of 2 per RTT if every packet ackd

Congestion Avoidance

On each new ack, increase cwndby 1/cwndpackets

cwndincreases linearlywith time during congestion avoidance

1/2 MSS per RTT if every other packet ackd

1 MSS per RTT if every packet ackd

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TCP: Congestion Control

On detecting a packet loss, TCP sender

assumes network congestion, drastically reduces the congestion window Reducing cwndreduces amount of data that can be sent per RTT

Congestion Control: Timeout

timeout occurs when no more packets are getting across

ssthreshis set to half the window size before packet loss

cwndis reduced to the initial value of 1 MSS slow startis initiated

Congestion Control: Fast Retransmit

Fast retransmitoccurs when multiple (>= 3) dupacksare received

Fast recoveryfollows fast retransmit (different from timeout)

a packet is lost, but latter packets get through ack clock is still there; no need to slow start

ssthreshis set to half the window size before packet loss

missing segment is retransmitted (fast retransmit)

cwndis reduced to ssthresh(by half) when a new ACK is received

enter congestion avoidancephase

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Impact of Multi-hop Wireless Paths

TCP throughput degrades with increase in number of hops

Packet transmission can occur on at most one hop amongthree consecutive hops

Increasing the number of hops from 1 to 2, 3 results in increased

delay, and decreased throughput

Increasing number of hops beyond 3 allows simultaneoustransmissions on more than one link, however, degradationcontinues due to contention between TCP Data and ACKs

traveling in opposite directions

When number of hops is large enough (>6), throughputstabilizes [Holland99]

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mobility causeslink breakage,

resulting in route

failure

TCP data and acks

en route discarded

Impact of Node Mobility

TCP sender times out.

Starts sending packets again

Route is

repaired

No throughput

No throughput

despite route repair

TCP throughput degrades with increase in mobility but not always

Larger route repair

delays are especially

harmful

I d Th h t ith I d M bilit

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Improved Throughput with Increased Mobility

Low speed: (Route from A to D is broken for ~1.5 seconds)

When TCP sender times after 1 second, route still broken.

TCP times out after another 2 seconds, and only then resumes

High speed: (Route from A to D is broken for ~0.75 seconds)When TCP sender times out after 1 second, route is repaired

TCP timeout interval somewhat (not entirely) independent of speed

Network state at higher speed may sometimes be more favorable than lower speed

C

B

D

A

C

B

D

A

C

B

D

A

I t f R t C hi

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Impact of Route Caching

TCP performance typically degrades when caches are used for route repair

When a route is broken, route discovery returns a cached route from

local cache or from a nearby node

After a time-out, TCP sender transmits a packet on the new route.

However, typically the cached route has also broken after it wascached

Another route discovery, and TCP time-out interval

Process repeats until a good route is found

timeout due

to route failure

timeout, cached

route is brokentimeout, second cached

route also broken

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Caching and TCP performance

Caching can result in fasterroute repair Faster does not necessarily mean correct

If incorrect repairs occur often enough, caching performs poorly

If cache accuracy is not high enough, gains in routingoverhead may be offset by loss of TCP performance due to

multiple time-outs

Need mechanisms for determining when cached routes arestale

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Impact of Acknowledgements

TCP ACKs (and link layer acks) share the wireless bandwidth withTCP data packets

Data and ACKs travel in opposite directions

In addition to bandwidth usage, ACKs require additional receive-sendturnarounds, which also incur time penalty

Reduction of contention between data and ACKs, and frequency ofsend-receive turnaround

Mitigation [Balakrishnan97]

Piggybacking link layer acks with data

Sending fewer TCP acks- ack every d-th packet (dmay be chosendynamically)

Ack filtering- Gateway may drop an older ack in the queue, if a new ackarrives

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TCP Parameters after Route Repair

Window Sizeafter route repair Same as before route break: may be too optimistic

Same as startup: may be too conservative

Better be conservativethan overly optimistic

Reset window to small value; let TCP learn the window size

Retransmission Timeout (RTO) after route repair

Same as before route break: may be too small for long routes

Same as TCP start-up: may be too large and respond slowly to

packet loss new RTO could be made a function of old RTO and route lengths

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Improving TCP Performance in Wireless

Classification 1: based on nature of actions taken to improve performance

Hide error losses from the sender

if sender is unaware of the packet losses due to errors, it willnot reduce congestion window

Let sender know, or determine, cause of packet loss if sender knows that a packet loss is due to errors, it will not

reduce congestion window

Classification 2:

based on where modifications are needed

At the sender node only At the receiver node only

At intermediate node(s) only

Combinations of the above

Indirect TCP (I TCP)

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Indirect TCP (I-TCP)

I-TCP splitsthe TCP connection

no changes to the TCP protocol for wired hosts TCP connection is split at the wireless interface (foreign agent)

into 2 connections, (one from CN to FA and other from FA to MN)

hosts in wired network do not notice characteristics of wireless part

no real end-to-end connection any longer

mobile hostaccess point

(foreign agent) wired Internet

wireless TCP standard TCP

Source: Schiller

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I-TCP socket and state migration

mobile host

access point2

Internet

access point1

socket migrationand state transfer

Source: Schiller

I-TCP: Issues

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I-TCP: Issues

Advantages

no changes in the fixed network necessary no changes for the hosts (TCP protocol) necessary

all current optimizations to TCP still work

transmission errors on wireless link do not propagate into the fixednetwork

simple to control, mobile TCP is used only for one wireless hop a very fast retransmission of packets is possible (short delay on

the mobile hop is known)

Disadvantages

loss of end-to-end semantics, ACKto sender does not guarantee thatthe packet was received (FA may crash)

buffering and forwarding of packets from one FA to another, mayincrease latency

Snooping TCP

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Snooping TCP

Transparent extension of TCP within the foreign agent

buffer packets sent to the mobile host

lost packets on the wireless link (both directions) are retransmitted

immediately by the mobile host or foreign agent, respectively

foreign agent snoops the packet flow and recognizes ACKs in

both directions; it also filters ACKs

Source: Schiller

wired Internet

buffering of data

end-to-end TCP connection

local retransmission correspondent

hostforeign

agent

mobile

host

snooping of ACKs

Snooping TCP: Issues

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Snooping TCP: Issues

Data transfer to the mobile host

FA buffers data until it receives ACK of the MH

FA detects packet loss via dupacksor timeout

fast retransmission possible, transparent for the fixed network

Data transfer from the mobile host

FA detects packet loss on the wireless link via sequence numbers

FA answers directly with a NACK to the MH

MH can now retransmit data with only a very short delay

Integration of the MAC layer

MAC layer can detect and discard duplicated packets

Problems

snooping TCP does not isolate the wireless link as good as I-TCP

snooping might be useless depending on encryption schemes

Mobile TCP (M TCP)

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Mobile TCP (M-TCP)

Special handling of lengthy and/or frequent disconnections

M-TCP splits as I-TCP does unmodified TCP fixed network to supervisory host (SH)

optimized TCP SH to MH

Supervisory host

no caching, no retransmission

monitors all packets, if disconnection detected

set sender window size to 0

sender automatically goes into persistent mode

old or new SH reopens the window

Advantages maintains semantics, supports disconnection, no buffer forwarding

Disadvantages

loss on wireless link propagated into fixed network

adapted TCP on wireless link

Impact of Handoffs on Schemes to Improve TCP

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Impact of Handoffs on Schemes to Improve TCP

Split connection (I-TCP) hard stateat base station must be moved to new base station

Snoop protocol

soft stateneed not be moved

while the new foreign agent builds new state, packet losses maynot be recovered locally

Frequent handoffs a problem for schemes that rely on

significant amount of hard/soft state at base stations

hard state should not be lost

soft state needs to be recreated to benefit performance

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Improving TCP Throughput

Network feedback Network knows best(why packets are lost)

Need to modify transport & network layer to receive/send feedback

- Need mechanisms for information exchangebetween layers

Inform TCP of route failure by explicit message

Let TCP know when route is repaired

Probing

Explicit notification

Better route caching mechanisms

Reduces repeated TCP timeouts and backoff

Impact of Mobility on TCP: Summary

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Impact of Mobility on TCP: Summary

TCP assumes congestionif packets are dropped

Random/transmission errors may cause fast retransmit which results in retransmission of lost packet

reduction in congestion window

Reducing congestion window in response to errors is unnecessary

Reduction in congestion window reduces the throughput

Mobilityitself can cause packet loss

e.g. when a node moves from one access point to another while packetsare in transit, and forwarding is not possible

Performance of an unchanged TCP degrades severely

TCP cannot be changed fundamentally, due to widespread deployment

Several adaptation proposals exist

See Nitin Vaidyas MobiCom99 tutorial

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Wireless LAN Implementations

IEEE 802.11 (WaveLan)

HIPERLAN

Bluetooth