Solutions for Improving TCP Performance over Emerging

NetworksK.R.Renjish Kumar

kaleelaz@comp

Centre for Internet Research

Outline

Introduction Emerging Networks TCP TCP over Emerging Networks

– Issues – Solutions

TCP HACK MBM

Introduction

Internet has come a long way with:– Different kinds of networks– increasing number of users – innumerable applications which require better

quality of service.

Emerging Networks

Lossy Link networks– Wireless networks like wavelans– Satellite networks

Quality of Service(QoS) model based networks– Integrated Services (IntServ): Provides QoS in a per

connection basis– Differentiated Services (DiffServ): Provides QoS for an

aggregate flow. Much more scalable compared to IntServ.

Transmission Control Protocol (TCP)

De facto Transport layer Protocol– 95% of data byte

share in the Internet

– Common applications like Telnet,FTP and HTTP uses TCP

TCP contd…

TCP Header– Sequence Number :

the first byte of data in this segment.

– Options: For any additional features.

Rate Control– Slow start,fast

retransmit,recovery, congestion

avoidance.– Segments send equal

to the minimum of the sender’s window(cwnd) and receivers window.

Emerging Networks-Lossy Networks

Access Networks are increasingly becoming wireless eg: wavelans

Acts as bottlenecks. Bandwidth limited. Prone to higher rate of corruption

TCP over Lossy Links …

Issues – performs poorly in lossy links where the rate of corruption of

packets are higher Reason: TCP assumes that loss of packets are due to

congestion and not corruption, thus reducing the cwnd and thus reducing the throughput.

Solution

TCP HACK: TCP Header Checksum Option to Improve Performance over Lossy Links.IEEE INFOCOM 2001,Anchorage,Alaska

R.K.Balan,B.P.Lee,K.R.R.Kumar,L.Jacob,W.K.G.Seah,

A.L.Ananda, IEEE INFOCOM 2001,Anchorage,Alaska

TCP HACK

Key ObservationsFor most data transfers, the data portion of a

TCP packet is much larger than the header portion of the packet (may not be true for interactive traffic)

As such, corruptions are far more likely to occur in the data portion and not the header portion

Given this scenario, can we make use of the intact header to make TCP perform better?

TCP Hack Algorithm contd..

On receiving this specially marked ACK packet, the sender will know that packet corruption and not congestion has occurred

As such, the sender will retransmit the corrupted packet and will not half its congestion window

Header Checksum option

 

Kind=14 

 

Length=41’s complement checksum of TCP header and pseudo-IP header

 

Kind=15 

 

Length=632-bit sequence number of corrupted segment to resend

TCP Header Checksum option

TCP Header Checksum ACK option

TCP Sender

Header checksum option enabled?

Continue as per normal

1) Calculate header checksum of segment2) Continue as per normal

Yes

No

Data segment to be sent

Modifications to the TCP sender

TCP Receiver

TCP segment corrupted?

Continue as per normal

1) Recover sequence number of corrupted segment from header. 2) Generate ‘special’ ACK (option 15) containing the sequence number of the corrupted segment.

Yes

No

Data segment received

Header portion corrupted?

Discard PacketYes

No

ACK Processing

Option 15 present in the segment?

1) Extract sequence number of corrupted segment2) Selectively retransmit the segment 3) ACK is discarded without further processing

Yes

No

ACK segment received

Modification to the ACK processing

Note

For TCP HACK to be effective, link layer retransmissions and error recovery should be turned off

Corrupted packets should be sent up to the TCP layer where TCP HACK will do the retransmissions

Experimental Setup

Implemented into the Linux 2.2.10 kernel

Test bed was set up comprising of 3 machines

Client and server were running TCP HACK

2 different models were used– Single packet

corruption. – Burst corruption with

differing burst lengths

Client ServerError / Delay Box

Experimental test bed

Low Latency Links

0

10

20

30

40

50

60

70

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Percentage Packet Loss (%)

Th

rou

gh

pu

t (K

Byte

s/s

)

sack

hack+sack

hack

new reno

1

2

34

1

2

3

4

Throughput versus percentage packet loss for short latency (10 ms) link with random single packet errors

High Latency Links

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Percentage Packet Loss (%)

Th

rou

gh

pu

t (K

Byte

s/s

) sack

hack+sack

hack

new reno

1

2

3

4

1

2

3

4

Throughput versus percentage packet loss for long latency (300 ms) link with random single packet errors

Number of slow starts

Fig. 7: Average number of slow-starts for long latency link with random single packet errors

0

10

20

30

40

50

60

70

0 3 6 9 12

Percentage Packet Loss (%)

Av

era

ge N

um

ber o

f S

low

-S

tarts

newreno

sack

hack

hack+sack

Observations

Both SACK and TCP-HACK are better than NewReno

Hence NewReno was removed from the tests when burst errors were introduced

Burst errors

12.988

20.0838 19.7595

15.5958

3.5406

16.436

19.9486

0.5402

4.6428

0

5

10

15

20

25

2 5 10Length of Burst Error (packets)

Th

rou

gh

pu

t (K

B/s

)

hack+sackhacksack

Throughput for 2% burst error for various burst lengths

11.016211.6934

8.531

5.95827.6964

1.4022

0.21120.7454

8.5608

0

2

4

6

8

10

12

2 5 10Length of Burst Error (packets)

Th

rou

gh

pu

t (K

B/s

)

hack+sackhacksack

Throughput for 5% burst error for various burst lengths

Throughput vs Time

0

100

200

300

400

500

0 400 800 1200 1600 2000 2400 2800

Time (s)

Th

rou

gh

pu

t (K

b/s

) hack+sack

hack

sack

Throughput versus Time graph for various TCP implementations (5% packet error rate with burst length of 5)

Inference

How is TCP HACK doing retransmissions a good idea?– Could help stabilize TCP and prevent it’s RTT

calculation from fluctuating widely (especially useful when using a long latency link)

– Provides more information to TCP, thus preventing it from unnecessarily timing out

– Distinguishes between congestion and corruption – Improves overall TCP performance

Emerging QoS Network Architecture

Differentiated Services (DiffServ)– Provides QoS for aggregate flows using different

per hop forwarding behaviours at the core routers.

Basic building blocks of DiffServ– Classifiers– Meters– Markers– Policers

DiffServ

Classifier

Meter

Marker PolicerPacket

Logical View of a Packet Classifier and Traffic Conditioner

TCP over QoS based Networks

Issues– Bias against connections with long Round Trip Time (RTT)

Reason:Long RTT flows takes longer time to ramp up.

– Bias against connections with smaller window sizes Reason: Smaller windows mean smaller throughput

– Protection from unruly traffic like UDP traffic. Reason: UDP traffic has no rate control mechanism and

hence kills TCP traffic.

Solution

A Memory-Based Approach for a TCP-Friendly Traffic Conditioner in DiffServ Networks.to appear in Proc. of the 9th IEEE International Conference on Network Protocols (ICNP 2001), Riverside, California. 

K.R.R.Kumar, A.L.Ananda, Lillykutty Jacob

Topology

Memory-Based Marker (MBM)

Key observations– Markers, one of the building blocks of a traffic conditioner play a major role for resource allocation in a DiffServ network. – TCP performance is highly influenced by two

parameters, namely RTT, and window size. – Markers are mostly deployed at the edge routers,

which cannot easily decide the window size and RTT of the various TCP connections passing through.

What is needed?

Marking algorithm– Least sensitive to both the marker and TCP

parameters– Should be transparent to end hosts.– Maintain optimum marking– Tracks the TCP dynamics

Why memory-based marker?

During the period when TCP flows experience

congestion, either or both of the following occurs:

a) The cwnd reduces reducing the value of W

b) The RTT increases

causing a decrease in throughput or rate of flow.

Why memory-based marker?

The TCP window size W and the round trip time RTT are related to the throughput by the equation:

BW = ¾*(MSS*W)/(RTT) where W is expressed in

number of segments. Any variation in W or RTT is reflected as subsequent

changes in BW, i.e., in our case, the avg_rate. The parameter previous average rate (par) is compared with

the present average rate to track any change in the rate of flow and thus indirectly extract the variations in RTT or W.

MBM Algo. explained

In the expression for the marking probability mp,– (par – avgrate)/avgrate tracks the variations in

RTT and window size (W) and thus increases or decreases the marking probability according to the changes in the flow rate.

– (1- avgrate)/cir constantly compares the average rate observed with the target rate to keep the rate closer to the target.

MBM Algorithm

For each packet arrival If avg_rate cir then mp=mp+(1-avg_rate/cir)+

(par- avg_rate)/avg_rate; par = avg_rate; mark the packet using: cp 11 w.p. mp cp 00 w.p. (1-mp)

else if avg_rate > cir

then

mp= mp + (par – avg_rate)/avg_rate;

par=avg_rate;

mark the packet using:

cp 11 w.p. mp

cp 00 w.p. (1-mp)

MBM Algo. Contd…

where,

avg_rate= the rate estimate upon each packet arrival

mp = marking probability (1)

cir = committed information rate (i.e., the target rate)

par = previous average rate

cp denotes ‘codepoint’ and w.p. denotes ‘with

probability’.

Experiments

We used FTP bulk data transfer for the TCP traffic in all our experiments.

We conducted simulation studies in NS for:– Assured service for aggregates with different

target rates.– Effect of different RTTs– Effect of different window sizes– Protection from best effort UDP flows– Effect of UDP flows with target rates.

Results

Expt #Rt 1 Rt 2 BE TCP flow Link goodput

Total Marked Total Marked (Mbps) (Mbps)1 1 1 2.85 1.45 3.35 1.97 2.94 9.142 1 2 2.93 1.76 3.6 2.7 2.64 9.173 1 3 2.93 2.08 4.08 3.44 2.2 9.214 1 4 2.93 2.21 4.29 3.84 1.93 9.155 1 5 2.8 2.32 4.89 4.64 1.51 9.26 2 2 3.4 2.58 3.56 2.73 2.49 9.457 3 3 3.75 3.34 3.53 3.08 1.85 9.138 4 4 3.88 3.7 3.94 3.7 1.31 9.139 5 5 4.38 4.38 4.35 4.35 0.42 9.15

10 6 6 4.35 4.35 4.5 4.5 0.34 9.199.192 Average link utilization = 92% (approx.)

Ra1 Ra2 Target Rates (Mbps) Achieved Rates (Mbps)

Achieved Rates (Ra) for different Target Rates (Rt).

Results….

RTT (ms) Ra 1 Ra 2

60 1.82 3.81 5.6380 1.49 3.74 5.23

100 1.52 3.52 5.04120 1.38 3.58 4.96140 1.43 3.45 4.88

25.74

Achieved Rates ( Mbps)

Total link goodput

per source pair goodput(Mbps)

Achieved Rates (Ra) for different RTT values

window size(KB ) Without MBM With MBMAchieved Rates Achieved Rates Ra (Mbps) Ra (Mbps)

384 0.58 1.88768 3.1 3.06

1125 3.21 2.871536 2.76 3.071920 1.25 2.93

13.81Total Link utilization 10.90

Achieved Rates (Ra) for different window sizes

More results

Target Rate (Mbps) Rt Ra(udp_be) Ra(tcp_be)

Total Marked2 3.83 2.03 2.95 2.64 4.85 4.13 2.91 1.666 5.76 5.6 2.84 0.818 7.13 7.13 2.22 0.04

10 7.94 7.94 1.4 0

Achieved Rates (Mbps) Ra(tcp_prio)

Achieved Rates in presence of BE UDP and TCP

Target Rate (Mbps)Ra(udp_prio) Ra(tcp_be)

Total Marked2 3.73 1.83 2.98 2.634 4.73 4.04 2.98 1.646 5.66 5.58 2.98 0.738 6.08 6.08 2.98 0.32

Achieved Rates (Mbps) Ra(tcp_prio)

Achieved Rates in presence of AS UDP and BE TCP

Inference

MBM– Achieves transparency from the end hosts, simplicity, and

least sensitivity to parameters of both TCP as well as its own parameters.

– helps in achieving the target rate, with a better fairness in terms of sharing the excess bandwidth among flows.

– provides the TCP flows, a greater degree of insulation

from differences in RTT and window sizes.– The overall link utilization also seems to be much better.

Future work

Working on a 3 colour marker, Memory-Based Three color Marker (MBTCM) which is apt for DiffServ with AF per hop behaviour in its core.

Study of TCP dynamics and its performance improvements over broadband networks.

Related Links…..

http://www.comp.nus.edu.sg/~srijith/cir/papers.html

http://www.comp.nus.edu.sg/~kaleelaz