Transport Layer Improvements in 802.11 Networks

Eryk Schiller

Bartosz Wieczorek

Outline

• Wireless technologies• TCP basics• Impact of transmission errors on TCP

performance• Approaches to improve TCP

performance in Wireless networksClassificationDiscussion of selected approaches

Protocol Stack

source: 802.11-1999

Wireless Technologies

• Wireless local area networksIEEE 802.11 WLAN Standardlast hop to a wireless hostwireless connectivity between hosts on the

LAN

Wireless Technologies (2)

• Multi-Hop Wireless May need to traverse multiple links to

reach a destinationMobility causes route changes (MANETs)

IP – Internet Protocol

• IP – Best effort deliveryPackets may be delivered out-of-orderPackets may be lostPackets may be duplicated

TCP – Transmission Control Protocol

• TCP – Reliability Reliable ordered deliveryImplements congestion avoidance and

controlReliability achieved by means of

retransmissions if necessary

TCP acknowledgments

• Acknowledgments sent to TCP sender confirm delivery of data received by TCP receiver

• ACK for data is sent only after data has reached receiver

• TCP assigns byte sequence numbers• ACK indicates the next expected byte that

receiver wants to receive

TCP acknowledgments (2)

• Single

S Rdata:n

ACK:n+1

data:n+1

ACK:n+2

S Rdata:n

data:n+2

ACK:n+3

data:n+1

• Cumulative

S Rdata:n

data:n+2

ACK:n

data:n+1

• Duplicate

ACK:n

Window Based Flow Control

• Sliding Window size determines how many segments can travel at the moment (can be unacknowledged)

• Prevents from overflowing receiver’s buffer• Window size minimum of

receiver’s advertised window - determined by available buffer space at the receiver

congestion window CWND - determined by the sender, based on feedback from the network, bounds the amount of data that can be sent per round-trip time RTT, actual throughput depending on CWND/RTT

Sliding Window - example

•Window size = 6

Sent and already acknowledged

Sent but not yet acknowledged

Already allowed to be sent

Not yet allowed to be sent

Slow Start and Congestion Avoidance

• Slow Startinitially, cwnd = 1 MSS (Max. segment size)increment window size by 1 MSS on each new

ACK (exponential growth)slow start phase ends when window size

reaches the slow-start threshold

• Congestion AvoidanceOn each new ACK, increase cwnd by 1/cwnd

packets (linear increase of 1 MSS per RTT)

Slow Start and Congestion Avoidance (2)

time [RTTs]

CW

ND

[seg

men

ts]

Slow start

Congestion avoidance

Slow start threshold

Congestion Control

• On detecting a packet loss, TCP sender assumes that network congestion has occurred and drastically reduces the congestion window

• Reducing congestion window reduces amount of data that can be sent per RTT - throughput may decrease

Congestion Control - Packet losses

• Causes of packet losses:• packet corruption (less probable)• buffer overflows on the network

• Detecting losses:• exceeding RTO (Retransmission Time-Out)

RTO is dynamically calculated, based on RTT (Round Trip Time) changes, RTO > RTT

• receiving duplicate acknowledgments (dupacks)

Congestion Control - Timeout

• SSThresh = ½ CWND (Slow Start Threshold)• CWND = 1 MSS• The slow start is initiated

0

5

10

15

20

25

0 5 10 15 21 26

Time [RTTs]

CW

ND

[se

gmen

ts]

SSThresh = 8SSThresh = 10

CWND = 20After timeout

Congestion Control - Fast Retransmit

• Fast retransmit [RFC 2001]• Initiated after receiving 3 dupacks• The lost packet is immediately

retransmitted • No need to wait for RTO to expire• Often followed by Fast Recovery

Congestion Control - Fast Recovery

• Fast recovery [RFC 2001]• SSThresh = ½ cwnd• cwnd = SSThresh + number of dupacks• cwnd = SSThresh (avoiding Slow-start Stage)

0

5

10

15

20

0 2 4 6 8 10 12 14

Time [RTTs]

CW

ND

[se

gmen

ts]

3 dupacks receivces dupacks

ACK new data

TCP Reno / TCP New Reno

• TCP Reno• Slow-start• Congestion avoidance regimen• Fast retransmit [RFC 2001]• Fast recovery [RFC 2001]

• Improvement of TCP Reno TCP New RenoIn TCP New Reno case Partial ACK indicates

another packets losses. Algorithm stay in Fast Recovery stage until cumulative ACK. Multiple losses/RTT may cause RTO. Motivation to create extension that will tell more about losses.

TCP Vegas

• Modifications of TCP Reno• Linearly adjust TCP’s cwnd upwards/downwards• Detect packet loss after first dupack and decrease

cwnd to ¾ instead of ½ • Halves SS to avoid packet losses

• TCP Vegas was created to avoid congestion so the impact on TCP performance in 802.11 Networks is not so significant

TCP Westwood

• Westwood improves TCP performance in wired and wireless networks

• TCP Westwood relies on end-to-end bandwidth estimation to discriminate cause of packet loss

• Bandwidth estimation is calculated by a measure of returning ACK flow

• Bandwidth estimation is then used to correctly set SSThresh and cwnd after timeout or dupacks.

SACK – Selective Acknowledgments

• SACK [RFC 2018]• Advertised in the option field during the

handshake• Must be approved by two sides • Every dupack should bear SACK option• SACK contains information about

unacknowledged, out of order contiguous block of data

• Only three blocks are announced in 1 SACK option (length of TCP header is limited)

• Recent block should be announced in SACK• Each block should be announced 3 times

SACK – Selective Acknowledgments (2)

S R

data

data

data

data

data

ACK

ACK

ACK

SEQ = 0

SEQ = n

SEQ = 2n

SEQ = 3n

SEQ = 4n

ACK = n

ACK = n, SACK = { [2n, 3n) }

ACK = n, SACK = { [2n, 3n), [4n, 5n) }

SACK – Selective Acknowledgments (3)

• Hardware• Celeron 2,4 GHz Linux 2.6.12 (Workstation)

with Ethernet Adapter• AMD 64-bit 1,8 GHz Linux 2.6.12 (Notebook)

with Wi-Fi 802.11g Network Adapter• Linksys WAP54G

• Conditions• Signal Level: -84dBm• Rate: 54Mbps• 64-bit WEP

SACK – Selective Acknowledgments (4)

SPLIT CONNECTION APPROACH

• End-to-end TCP connection is broken into two TCP connections - one on the wired part of route and the other over wireless part of the route

• If wireless link is not last on route, then more than two TCP connections may be needed

• Connection between wireless mobile host MH and fixed host FH goes through base station BS

FH MHBS

Base Station Mobile HostFixed Host

SPLIT CONNECTION APPROACH

• Split connection results in independent flow control for the two parts

• Flow/error control protocols, packet size, time-outs, may be different for each part

wireless

physical

link

network

transport

application

physical

link

network

transport

application

physical

link

network

transport

application rxmt

TCP connection TCP connection

SPLIT CONNECTION - Advantages

• BS-MH connection can be optimized independent of FH-BS connection

• Different flow / error control on the two connections

• Local recovery enables faster recovery due to relatively shorter RTT on wireless link

• Good performance achievable using selective acks when multiple packet losses occur per window (preventing from timeouts stalling BS-MH connection)

SPLIT CONNECTION - Disadvantages

• Ack may be delivered to sender, before data delivered to the receiver

• BS failure can result in loss of data (unreliability):

• BS fails to send a packet to MH

• BS acks successful receipt to FH

• FH deletes the ack’d packet from the buffer

• The packet will not be restored by MH

• Buffer space needed at BS for each TCP connection (overload for many slow connections)

• May not be useful if data and acks traverse different paths (both do not go through the base station)

SNOOP PROTOCOL• Retains local recovery of the Split Connection approach

• Buffers data packets at the base station BS to allow link layer retransmission

• When dupacks received by BS from MH, retransmit on wireless link, if packet present in buffer

0

400

800

1200

1600

2000

16

K

32

K

64

K

12

8K

25

6K

no

erro

r

1/error rate

kb

its

/se

c

base TCP

Snoop

2 Mbps Wireless link