1 RCP (Reception Control Protocol) A Receiver-Centric Transport Protocol for Mobile Hosts with heterogeneous Wireless Interfaces MOBICOM 2003 H. Hsieh,

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RCP(Reception Control Protocol)

A Receiver-Centric Transport Protocol for Mobile Hosts with heterogeneous Wireless InterfacesMOBICOM 2003H. Hsieh, K. Kim, Y. Zhu, and R. Sivakumar, Georgia Institute of Technology

Presented for ECE299.02 by Thilee Subramaniam

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OUTLINE of Presentation

Problem Statement Quick TCP Review Case for RCP Protocol Details Performance Discussion Q&A

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What is the Problem? Wired: low BER Wireless: high BER (channel error, Fading,

Handoffs) TCP

was originally proposed for wired networks assumes losses are due to congestion corrective measures address congestion

only! TCP over wireless: does not perform well TCP over wireless: can it be improved?

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TCP Recap

End-to-end argument Cater to the end-point

specific requirements

Flow Control Congestion Control

Various flavors of TCP Reliable in-order

delivery

data packetACK

receiversendercwnd

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5678

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Typical wireless setup

FH: Fixed Host BS: Base Station MH: Mobile Host

Assumption: Most data flow is from FH to MH

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TCP and wireless networks

Sender does Congestion control Sender reacts to feedback from receiver Assumption: any loss is due to congestion

BS <-> MH connection highly error prone MH to FH feedback takes time and resources Sender ends up sending less data! Reduces network performance. Not Good!!

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Attempts for Wireless friendly TCP Snoop

Link-layer of BS buffers un-ACKed data and DupACKs Retransmit Data, Suppress DupACK from MH to FH Prevent FH from decreasing CWND drastically Issues: IPSec incompatibility, mess with RTT estimate

on FH, requires change in BS which is difficult

WTCP Do local recovery like Snoop BS adds timestamp to ACKs from MH to FH, informing

FH to correctly estimate RTT Issues: require change in BS, assume all losses are

due to errors while congestion is possible!

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Attempts for Wireless friendly TCP.. ELN (Explicit Loss Notification)

BS sets an ELN bit in DupACK header FH does not decrease CWND for DupACK with ELN set Issue: Still need BS change, consumes resources for

feedback

Freeze-TCP Moves intelligence to MH Primary focus: handoff; FH still does Congestion Control

Syndrome BS simply adds the number of packets sent to MH MH can differentiate W loss and WL loss Handoff is still a problem

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Motivation for Receiver-Centric Protocol Why does Wireless TCP under-perform?

Sender over-reacts by reducing CWND when not necessary

Why cannot Receiver provide more information? Magnitude Cost Latency

The closer the problem solver is to the problem, the easier it is to solve

Client specific congestion control mechanisms Sender cannot provide custom mechanisms Generic mechanism cannot make everyone happy

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More Reasons

Multi-homed Devices (Notebook with Wifi, WIMAX, and 3G cards)

Technology specific congestion control schemes may be required. Receiver-centric more useful

Users can move from one technology coverage-area to another Receiver-centric transport protocol can better handle

handoffs

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Even More Reasons..

Power issue with mobile devices Limited Battery Wireless Transmission/Reception consume high

% energy

Wireless errors happen in bursts Need to cut down transmission Costly to inform the sender to cut down CWND Receiver dropping packets: affects throughput

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Therefore…

More appropriate to let receiver handle How Much data? Which data?

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TCP: Connection and Data transfer

SYNISN=X

SYNISN=X

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SYNISN=Y ACK=X+1

SYNISN=Y ACK=X+1

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ACK=Y+1ACK=Y+1 3tim

e

DATA X+3DATA X+3 4

ACK=X+4ACK=X+4 5

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RCP: Transposition of TCP

SYNISN=X

SYNISN=X

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SYNISN=Y ACK=X+1

SYNISN=Y ACK=X+1

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ACK=Y+1ACK=Y+1 3tim

e

REQ REQ 4

DATADATA 5

Either party can initiate connection

Receiver initiates with transfer Cumulative mode Pull mode

SenderReceiver

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RCP: Flow Control

In TCP, receiver advertises its buffer size Sender makes sure that sent data is limited by this

No need to advertise this in RCP Buffer internally handled by receiver.

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RCP: Congestion Control

TCP: CWND limits amount of unacknowledged data

RCP: CWND limits number of outstanding REQ packets from receiver

Slow start, congestion avoidance, fast retransmit, fast recovery etc. same as TCP

Receiver adjusts the CWND based on channel conditions Requested DATA arrives: increase CWND (AI) Wireless error detected: sleep or set CWND=0 or .. Wired link error detected: decrease CWND (MD)

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RCP: Reliability

TCP: Receiver performs re-sequencing Sender ensures reliability

RCP: Receiver has full control on data Any loss recovery mechanism can be used that

optimizes the wireless environment

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RCP: Is that all?

Extension: R2CP, the Radial RCP Enables functionality gains in multi-homed

systems Abstracts a single interface to upper layers

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More about R2CP

Receiver centric Manages multiple RCP instances Maintain state specific details of each RCP Exports connection & reliability to upper

layers Single connection Multiple interfaces

Utilizes underlying RCP congestion control Packet scheduling between different RCPs

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R2CP: What can it do?

New functionality gains Seamless handoffs between interfaces Server migration Bandwidth aggregation

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Sounds good. Will it work? What to look for?

TCP friendly? Loss recovery? Scalable Congestion control? Power Management?

Ns-2 simulator 20 data flows, reverse flows, varying RCP:TCP

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RCP: Performance (1)

TCP Friendliness: TCP Throughput same irrespective of RCP:TCP

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TCP Friendliness: Throughput constant with reverse traffic

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Error recovery: With ELN, MH can determine exact cause of loss, hence better throughput

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Power Management Transmit=1.65W,Recv=1.4W, Sleep=0.045W Vary channel condition using a statistical model

(good & bad states) Check-(Sleep/Wakeup)-(transmit/receive) cycle

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R2CP: Performance

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So, what do we think about RCP & R2CP?

Sound arguments for receiver-centric transport Hybrid RCP-TCP protocol (same binary) for

upstream traffic Error recovery still needs BS change Unanswered questions

How can sender guarantee fairness to multiple users? How to address REQ attacks? How bad it will be to the NW if the receiver sends REQ

and, before data is delivered, goes to sleep on seeing bad channel?

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1 RCP (Reception Control Protocol) A Receiver-Centric Transport Protocol for Mobile Hosts with heterogeneous Wireless Interfaces MOBICOM 2003 H. Hsieh,