Wireless & Mobile Networking CS 752/852 - Spring 2011

Wireless & Mobile Networking

CS 752/852 - Spring 2011

Tamer NadeemDept. of Computer Science

Wireless TCP

Page 2 Spring 2011 CS 752/852 - Wireless and Mobile Networking

The OSI Communication Model


Recall 1: PHY and MAC

MAC MAC

PHY PHY

• Spread Spectrum radios (DS and FH)• RTS/CTS and Carrier Sensing for Hidden Terminals• Directional antennas to reduce interference• Rate control to extract max capacity from available SINR• Power control for spatial reuse & energy savings – topology control• TDMA scheduling, multi-channel use, encryption security

… and many more


Recall 2: Network Layer

Routing

• The first view of the network

• Coping up with (uncontrolled) user mobility-Flooding the network reactively, or proactive updation

• Mobile IP, coping with handoffs, etc.

• Ad hoc routing – discovery, optimal metric, maintenance, caching• Secure routing – Routes bypassing malicious nodes

Routing

Routing

Routing

Routing


Role of Transport Layer

TCP

• Transport packets quickly and reliably over this network

• Network properties often unknown (or difficult to track)- Where is the congestion ? How much cross traffic ?- What is the bottleneck bandwidth ?- How much buffers at intermediate nodes ?

Motivation for end to end TCP

TCP

NETWORK

Data

Data

Data


Some transmission methods

• Stop & Wait

• Pipelined

• Go Back N

• Selective Repeat


Stop-and-wait operation

first packet bit transmitted, t = 0

sender receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

U sender

= .008

30.008 = 0.00027

microseconds

L / R

RTT + L / R =


Pipelined protocols

Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts

• range of sequence numbers must be increased

• buffering at sender and/or receiver

• Two generic forms of pipelined protocols: go-Back-N, selective repeat


Pipelining: increased utilization

first packet bit transmitted, t = 0

sender receiver

RTT

last bit transmitted, t = L / R

first packet bit arriveslast packet bit arrives, send ACK

ACK arrives, send next packet, t = RTT + L / R

last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK

U sender

= .024

30.008 = 0.0008

microseconds

3 * L / R

RTT + L / R =

Increase utilizationby a factor of 3!


Go-Back-N

Sender:

• k-bit seq # in pkt header

• “window” of up to N, consecutive unack’ed pkts allowed

ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK” may receive duplicate ACKs

timer for each in-flight pkt timeout(n): retransmit pkt n and all higher seq # pkts in window

GBN inaction


Selective Repeat

• receiver individually acknowledges all correctly received pkts

• buffers pkts, as needed, for eventual in-order delivery to upper layer

• sender only resends pkts for which ACK not received

• sender timer for each unACKed pkt

• sender window

• N consecutive seq #’s

• again limits seq #s of sent, unACKed pkts

Selective repeat: sender, receiver windows


Selective repeat

data from above :

• if next available seq # in window, send pkt

timeout(n):

• resend pkt n, restart timer

ACK(n) in [sendbase,sendbase+N]:

• mark pkt n as received

• if n smallest unACKed pkt, advance window base to next unACKed seq #

sender

pkt n in [rcvbase, rcvbase+N-1]

send ACK(n) out-of-order: buffer in-order: deliver (also deliver

buffered, in-order pkts), advance window to next not-yet-received pkt

pkt n in [rcvbase-N,rcvbase-1]

ACK(n)

otherwise: ignore

receiver

Selective repeat in action


TCP


TCP Congestion Control

• Problem Definition

• How much data should I pump into the network to ensure

• Intermediate router queues not filling up

• Fairness achieved among multiple TCP flows

• Why is this problem difficult?

• TCP cannot have information about the network

• Only TCP receiver can give some feedbacks


The Control Problem

• Two main components in TCP

• Flow Control and Congestion Control

• Flow Control

• If receiver’s bucket filling up, pour less water

• Congestion Control

• Don’t pour too much if there are leaks in intermediate pipes

• Regulate your flow based on how much is leaking out

• Aggressive pouring calls for retransmission of lost packets

• Conservative pouring lower e2e capacity

• Challenge: At what rate(t) should you pour ?


The TCP Protocol (in a nutshell)

• T transmits few packets, waits for ACK

• Called slow start

• R acknowledges all packet till seq #i by ACK i (optimizations possible)

• ACK sent out only on receiving a packet

• Can be Duplicate ACK if expected packet not received

• ACK reaches T indicator of more capacity

• T transmits larger burst of packets (self clocking) … so on

• Burst size increased until packet drops (i.e., DupACK)

• When T gets DupACK or waits for longer than RTO

• Assumes congestion reduces burst size (congestion window)


TCP Timeline

Host A

one segment

RT

T

Host B

time

two segments

four segments

Think of a blind

person trying to

stand up in a low

ceiling room

Objective:

Don’t bang your

head, but stand

up quickly

Think of a blind

person trying to

stand up in a low

ceiling room

Objective:

Don’t bang your

head, but stand

up quickly


When waited for > RTO

0

5

10

15

20

25

Time (round trips)

Con

gest

ion

win

dow

(se

gmen

ts)

ssthresh = 8 ssthresh = 10

cwnd = 20

After RTO timeout


The TCP Protocol (in a nutshell)

• DupACK not necessarily indicator of congestion

• Can happen due to out of order (OOO) delivery of packets

• If 3 OOO pkts, then CW need not be cut drastically

• The DupACK packet retransmitted

• Continue with same pace of transmission as before

(fast recovery)

• R advertizes its receiver window in ACKs

• If filling up, T reduces congestion window


Fast Recovery on 3 OOO DupACKs

0

2

4

6

8

10

Time (round trips)

Win

dow

size

(seg

men

ts)

Receiver’s advertized window

After fast recovery


TCP Round Trip Time and Timeout

EstimatedRTT = (1- )*EstimatedRTT + *SampleRTTEstimatedRTT = (1- )*EstimatedRTT + *SampleRTT

Exponential weighted moving average influence of past sample decreases exponentially fast typical value: = 0.125


Example RTT estimation:

RTT: gaia.cs.umass.edu to fantasia.eurecom.fr

100

150

200

250

300

350

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106

time (seconnds)

RTT

(mill

isec

onds

)

SampleRTT Estimated RTT


TCP Round Trip Time and Timeout

Setting the timeout

• EstimtedRTT plus “safety margin”

• large variation in EstimatedRTT -> larger safety margin

• first estimate of how much SampleRTT deviates from EstimatedRTT:

TimeoutInterval = EstimatedRTT + 4*DevRTT

DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT|

(typically, = 0.25)

DevRTT = (1-)*DevRTT + *|SampleRTT-EstimatedRTT|

(typically, = 0.25)

Then set timeout interval:


Several flavors of TCP: combines options / optimizations

Reno, Vegas, Eifel, Westwood …

Overall TCP has worked well – proven on the internet

Then why study it again

for wireless networks ?


Renewed Challenge

• Key assumption in TCP

• A packet loss is indicative of network congestion

• Source needs to regulate flow by reducing CW

• Assumption closely true for wired networks

• BER ~ 10 -6

• With wireless, errors due to fading, fluctuations

• Need not reduce CW in response …

• But, TCP is e2e CANNOT see the network

• Thus, TCP cannot classify the cause of loss CHALLENGE


The Problem Model

wireless

physical

link

network

transport

application

physical

link

network

transport

application

physical

link

network

transport

application

rxmt

TCP connection

Wireline


Impact of Misclassification

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

0 10 20 30 40 50 60

Time (s)

Se

que

nce

nu

mb

er

(byt

es)

TCP Reno(280 Kbps)

Best possible TCP with no errors(1.30 Mbps)

2 MB wide-area TCP transfer over 2 Mbps WaveLAN


The Solution Space

• Much research on TCP over wireless

• Difficult to cover complete ground

• We peek into some of the key ideas

• Link layer mechanisms

• Split connection approach

• TCP-Aware link layer

• TCP-Unaware approximation of TCP-aware link layer

• Explicit notification

• Receiver-based discrimination

• Sender-based discrimination


Link Layer Mechanisms


Link Layer Mechanisms

• Forward error corrections

• Add redundancy in the packets to correct bit-errors

• TCP retransmissions can be alleviated

• Link layer retransmissions

• MAC layer ACKnowledgments

• Overhead only when errors occur (unlike FEC)

Such mechanisms require no change in TCP

Is that breaking e2e argument ??


Issues with Link Layer Mechanisms

• Link layer cannot guarantee reliability

• Have to drop packets after some finite limit

• What is the retransmission limit (??)

• Retransmission can take quite long

• Can be significant fraction of RTT

• TCP can timeout and retransmit the same packet again

• Increasing RTO can avoid this

• But that impacts TCP’s recovery from congestion

• Head of the line blocking

• Link layer has to keep retransmitting even if bad channel

• Blocks other streams


Findings

• Link layer retransmission good

• When channel errors infrequent

• When retransmit time << RTO

• When modifying TCP is not an acceptable solution


Split Connection Approach


1 TCP = ½ TCP + ½ (TCP or XXX)

wireless

physical

link

network

transport

application

physical

link

network

transport

application

physical

link

network

transport

application rxmt

Per-TCP connection state

TCP connection TCP connection

Base Station


Splitting Approaches

• Indirect TCP [Baker97]

• Fixed host (FH) to base station (BS) uses TCP

• BS to mobile host (MH) uses another TCP connection

• Selective Repeat [Yavatkar94]

• Over FH to BS: Use TCP

• Over BS to MH: Use selective repeat on top of UDP

• No congestion control over wireless [Haas97]

• Also use less headers over wireless

• Header compression


Issues with Splitting

• E2E totally broken

• 2 separate connections

• BS maintains hard state for each connection

• What if MH disconnected from BS ?

• Huge buffer requirements at BS

• What if BS fails ?

• Handoff between BS requires state transfer

• What if Data and ACK travel on different routes ?

• BS will not see the ACK at all – splitting not feasible


TCP-Aware Link Layer


Snoop

• Link layer at BS buffers un-acknowledged packets

• Now, BS peeks into every returning TCP ACK from MH

• If DupACK

• Retransmits the necessary packet

• Drops the DupACK

• DupACK does not reach sender

• Prevents fast retransmit


Snoop : Example

FH MHBS

40 39 3738

3634

Example assumes delayed ack - every other packet ack’d

36

37

38

35 TCP statemaintained at

link layer


Snoop : Example

41 40 3839

3634

36

37

38

35 39


Snoop : Example

42 41 3940

36

Duplicate acks are not delayed

36

dupack

37

38

39

40


Snoop : Example

40

363636

Duplicate acks

4143 42

37

38

39

40

41


Snoop : Example

FH MHBS

41

3636

3744 43

36

37

38

39

40

41

42

Discarddupack

Dupack triggers retransmissionof packet 37 from base station

BS needs to be TCP-aware to

be able to interpret TCP headers


Snoop : Example

37

36

36

4245 44

36

37

38

39

40

41

42

43

36


Snoop : Example

42

36

36

4346 45

36

37

38

39

40

41

42

43

41

36

44

TCP sender does notfast retransmit


Snoop : Example

43

3636

4447 46

36

37

38

39

40

41

42

43

41

36

44

TCP sender does notfast retransmit

45


Snoop : Example

FH MHBS

44

3636

4548 47

36

42

43

41

36

44

45

43

46


Snoop [Balakrishnan95acm]

0

400000

800000

1200000

1600000

2000000

16

K

32

K

64

K

12

8K

25

6K

no

erro

r

1/error rate (in bytes)

bit

s/s

ec

base TCP

Snoop

2 Mbps Wireless link


Issues with Snoop

• Link layer needs to be TCP aware

• Smelling cross layer

• Link layer needs to buffer and perform sliding window

• Not useful when TCP headers encrypted

• Not feasible when Data and ACK travel different routes

• RTT estimates can still go up due to link layer retransmission

• Affects performance of Snoop


Wireless TCP

• WTCP attempts to nullify RTT estimation problem

• When data packets are lost due to errors

• Link layer includes own time stamp in ACK packet

• ACK packets that have BS time stamps indicate a wireless loss

• RTT of these packets not considered for RTO calculation

• But then, what if wireless hop is also congested !!!!!!

• Time stamping cannot take care of that


Quick look at other schemes

TCP-unaware schemes

Explicit notification

Receiver-based


TCP-Unaware, ELN

• Delayed DupACKs

• Receiver waits for sometime before sending DupACK

• If link retransmission solves problem

• Then TCP sender does not send redundant packet

• Explicit Loss Notification (ELN)

• BS remembers only packet’s sequence numbers

• When DupACKs return through them, they check

• If packet was received by BS, then colors the DupACK

• Sender realizes that packet lost on wireless link

• Does not cut down CW, just retransmits that packet


Closing Thoughts

• Reliable and in-order packet delivery important

• TCP aims to support these features

• Implements congestion control and flow control

• TCP widely tuned for wireline networks

• Proven to be efficient on the internet

• When network periphery has wireless “last mile”

• TCP exhibits myriad problems

• Mainly because of

“misclassification between congestion and channel errors”

• Several solution approaches but many open problems


What’s Hot Now ??

• TCP over wireless multihop (mesh)

• Each hop has contention-based MAC

• Unpredictable delays and congestion

• Fairness between TCP e2e flows a very challenging problem

• Mobility can significantly affect TCP

(Very difficult set of open problems)

• More fundamental: Is TCP the way to go for wireless

• Strong ongoing debate in community

• Useful queuing solutions in ad hoc networks

• Neighborhood RED solution

… and many many more …


Questions ?

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Wireless & Mobile Networking CS 752/852 - Spring 2011

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