towards understanding tcp performance on...

Towards Understanding TCP Performance on LTE/EPC Mobile

Networks!

Binh Nguyen1, Arijit Banerjee1, Vijay Gopalakrishnan2, Sneha Kasera1, Seungjoon Lee2, Aman Shaikh2, and Jacobus (Kobus) Van der Merwe1

1School of Computing, University of Utah

2AT&T Labs - Research

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Motivation

2

Motivation

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“In 2013,…, they (4G-connections) already account

for 30 percent of mobile data traffic”

“4G traffic will be more than half of the total mobile

traffic by 2018.”

Motivation

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• Does TCP work well in LTE/EPC networks? • TCP only utilize 50% of the available bandwidth in cellular

networks (Huang et al. SIGCOMM’ 13) • SPDY is not faster in cellular networks because of the interaction

between TCP with cellular networks (Erman et al. CoNEXT’ 13)




traffic by 2018.”

Motivation

2




• Treat the cellular network as a black-box. • Speculate the root causes of performance problems.




traffic by 2018.”

Motivation

2




• Treat the cellular network as a black-box. • Speculate the root causes of performance problems.




traffic by 2018.”

We look “under the hood” to answer: how cellular specific events impact TCP’s performance?

We studied two cellular scenarios

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• On-going TCP connection when a user is handed-over between eNodeBs.

• Seamless handover.

• Lossless handover.

3


• On-going TCP connection when a user is handed-over between eNodeBs.

• Seamless handover.

• Lossless handover.

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• Sudden load increase on an eNodeB:

• When neighbor eNodeBs fail and users are handed to a survived eNodeB.

LTE/EPC network architecture

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Evolved Packet Core (EPC)Radio Network

(E-UTRAN) Internet

PDN !gateway

Serving !gateway

S1-U

X2

eNodeB

UE

LTE stack

PDCP: header compression, ciphering

RLC: segmentation/concatenation, ARQ

MAC: MAC multiplexing, HARQ

PHY: coding, modulation

IP packet

eNodeB

scheduler

5

LTE stack

PDCP: header compression, ciphering

RLC: segmentation/concatenation, ARQ

MAC: MAC multiplexing, HARQ


IP packet

UEeNodeB

radio bearers

logical channels

transport channels

scheduler

PDCP: decompression, deciphering

RLC: reassembly, ARQ

MAC: MAC demultiplexing, HARQ

PHY: decoding, demodulation

IP packet

5

LTE stack

PDCP: header compression,

ciphering

RLC: segmentation/concatenation,

MAC: MAC multiplexing,

HARQ


IP

UEeNodeB

radio

logical

transport scheduler

PDCP: decompression,

deciphering

RLC: reassembly, ARQ

MAC: MAC demultiplexing,

HARQ

PHY: decoding, demodulation

IP

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PDCP seamless handover

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Source eNodeB Target eNodeB


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X2 link

• Un-transmitted SDUs are forwarded to the target eNodeB via the X2 link.


7


X2 link

!

X

• Un-transmitted SDUs are forwarded to the target eNodeB via the X2 link.

• SDUs in the retransmission queue are discarded.

PDCP lossless handover

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8



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X2 link

• Both un-transmitted SDUs and SDUs in the retransmission queue are forwarded to the target eNodeB via the X2 link.

(1)

(2)

Methodology

• Use simulations (NS-3) for better controls and modifications.

• Real network stack: Linux v2.6.26 with TCP CUBIC.

• Workload: downlink bulk-transfer TCP application.

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Modifications to NS3

• Radio Link Control Acknowledged Mode (RLC AM).

• Lossless handover according to 3GPP TS 36.323 2009.

• Retrieving TCP parameters from the Network Simulation Cradle (NSC) integration.

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Scenario 1: PDCP handovers

• Source eNB - target eNB: 750m.

• X2 interface: 6ms delay.

• UE’s velocity: normal distribution with mean 40 km/h, variance 5 km/h.

• PDCP retransmission queue size: 64KB to 4096KB.

target eNB

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Serving gateway

S1-U

X2

source eNB

TCP in seamless handover

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Congestion window and throughput

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TCP in lossless handover

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Sent sequence numberAck sequence number


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Seamless HO suffers from a throughput reduction while lossless HO experiences a high delay.

TCP’s performance during handovers

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- In seamless handovers, the higher the PDCP queue size, the lower TCP good-put.

- In lossless handovers, the higher the PDCP queue size, the larger end-to-end delay.

Implications

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Implications• Different types of handover has different impacts on

TCP’s performance:

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TCP’s performance:• Seamless handover: low delay but hurts throughput.

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TCP’s performance:• Seamless handover: low delay but hurts throughput.• Lossless handover: high delay but guarantee

throughput and delivery.

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• What about applications?

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• What about applications?• Low delay is important: Skype, real-time interactive

games, etc.

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games, etc.• High throughput and guaranteed delivery are

favorable (with a certain bound of delay): File transfer, HTTP.

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Can applications specify the type of handover?







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Can applications specify the type of handover?No: same radio bearer type for all applications.







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Can applications specify the type of handover?No: same radio bearer type for all applications.

Need a closer interaction between applications and the network?

Scenario 2: Sudden load increase in an eNodeB

• Assume an area with 250m of radius from the eNodeB where UEs are uniformly distributed.

• Users mobility: Levy-walk pedestrian mobility.

• UEs are attached to the eNodeB at the beginning of the experiment and start downloading data at a specific time.

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Conclusions

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Conclusions• Cellular specific phenomena impacts TCP’s performance:

• Different types of handover has different impacts on TCP’s performance: delay and throughput trade-off.

• Possible future work:

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Conclusions• Cellular specific phenomena impacts TCP’s performance:

• Different types of handover has different impacts on TCP’s performance: delay and throughput trade-off.

• Possible future work:

• More investigations on the TCP’s performance problems.

• Cross-layer interactions: e.g., applications and the mobility stack.

• Base station congestion prediction and mitigation.

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Thank you!

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