Obile etworking 2015-10-151 M-TCP : TCP for Mobile Cellular Networks Kevin Brown and Suresh Singh...

obileobile etworking 23年 4月 19日 1

M-TCP : TCP for Mobile Cellular NetworksM-TCP : TCP for Mobile Cellular Networks

Kevin Brown and Suresh Singh

Department of Computer ScienceUniv. of South Carolina

ACM Computer Communications Review, 1997

2005.07.29Hyun-Jin Kim


Introduction – Introduction – TCP in Mobile EnvironmentTCP in Mobile Environment

Traditional TCPTCP is designed to work on wired networks

• Negligible medium loss

• Buffer overflow at routers leads network congestion

TCP under wireless mobile networks• High BER and frequent disconnections lead packet losses

• But, TCP simply interprets them as a indication of congestion

• Significant degradation of end-to-end performance

Approaches for improving TCP performanceSnoop TCP [1]

I-TCP[2] & MTCP[3]

M-TCP


Former Solutions – Former Solutions – SNOOP TCPSNOOP TCP

Snoop module resides at the BS (Base Station)Inspects TCP header of data packets and ACK packets

Keeps track of packets in both direction

Local retransmission between BS and MH (Mobile Host)

ProsPreserves end-to-end TCP semantics

Changes are restricted to BS and optionally to MH

ConsUnder long and frequent disconnections, sender times out

Frequent handoffs – snoop module needs to build its cache up


Former Solutions – Former Solutions – I-TCP & MTCPI-TCP & MTCP

Split connection protocolsFH to BS, BS to MHWireless connection can even use another transport protocol that suits wireless medium

ProsWhen a handoff occurs, the old BS can deliver the packets of its buffer to the new BSSender does not concern about wireless environment

ConsBreaks end-to-end TCP semanticsBS can be a bottleneck

BSFH MH

FH Socket MH Socket

Wireless TCPStandard TCP


M-TCPM-TCP - Overview - Overview

TCP connection is split in two at the SH

Maintains end-to-end TCP semantics

“Persist mode” – keeps timer of sender

SH (Supervisor Host)Maintains several MSS (Mobile Support Station) – reduces handoff overheads

Serves function of gateway

Bandwidth manager, local recovery

SHFH (Fixed Host) MH

SH-TCP M-TCP

M - TCP TCP

MSS


M-TCP M-TCP – The SH-TCP client– The SH-TCP client

Goal – keeps the FH’s congestion window size

FH uses unmodified TCP to send data to the SH

SH-TCP clientPasses packets from the FH on to M-TCP

Does not ACK those packets until the MH does• So, end-to-end TCP semantics are maintained

Sends ACK to the sender except ACK for one last byte

When the MH is disconnected, sends ACK for the last byte with a window size set to ‘0’ – “persist mode”

When a MH regains its connection, it sends a greeting packet• This packet allows the sender to leave “persist mode”


M-TCP M-TCP – M-TCP between the SH and MH– M-TCP between the SH and MH

Goal – fast local recovery from disconnection events

Has responds to notifications of wireless link connectivity

M-TCP at the MH is notified the connection is lostFreezes all M-TCP timer

Connection is regainedUnfreezes all M-TCP timers

MH sends a specially marked ACK to M-TCP at the SH which contains the highest received sequence number

How to determine the connection is lost?Assumes the SH assigns fixed bandwidth

No ACK from the MH within Timeout of M-TCP (See next slide)


M-TCPM-TCP –– How to estimate RTO ? How to estimate RTO ?

(1): FTCPRTO(2+4): STCPRTO(3): SM-TCPRTOTherefore (1) = (2+4)+(3)

FH SH MH

RTT(1)

(2)

(3)

(4)

Therefore, the SH can estimate RTO between itself and the MH

The SH can determine the connection is lost by using estimated RTO


M-TCPM-TCP – Other Issues – Other Issues

Compressed M-TCPPackets are compressed at the SH and decompressed at the MH

When a handoff occursFreezes connection state

Some of the state about the connection is passed to the new SH

Removes the contents of the socket buffers at the old SH

Unfreezes connection state

Connection setupTwo different operations for setting up

• FH SH and SH MH

Transparent SH• FH MH

• SH automatically creates sockets for the FA and the MH


M-TCPM-TCP - Normal Transmission (1/2) - Normal Transmission (1/2)

2 1 2 1

1 2

1 2

1

2

SH-TCP

M-TCP

This is for freezing sender’s window.

cwnd=2

SH

cwnd=3

(2,1) buffered


M-TCPM-TCP - Normal Transmission (2/2) - Normal Transmission (2/2)

4 3 4 3

3 4

3 4

2 3

4

SH-TCP M-TCP

This is for freezing sender’s window.

2

cwnd=3cwnd=5

SH (4,3) buffered


M-TCPM-TCP - Disconnection - Disconnection

8 7 6 5

4

SH-TCP M-TCP

4

Freezing

Notify disconnection

Freezing

Freezing

(8,7,6,5) buffered

cwnd=5

SH

cwnd=6


M-TCPM-TCP - Recovery - Recovery

5 6 7 8

8

SH-TCP M-TCP

5 6 7 8

5 6 7

16 15 14 13 12 11 10 9

Notify reconn.

5 6 7 84

cwnd=6cwnd=9

(8,7,6,5) buffered

SH


M-TCPM-TCP - Performance Evaluation - Performance Evaluation

SH FH1Distance Sender MH

FH2Close Sender

14 Hops

4 Hops

Emulated32kbps Link

Experimental set upAll nodes are Pentium PCs

Wireless link is emulated at the SH

32Kbps downlink, 8Kbps uplink

Disconnection length – 0.5 ~ 4.5 sec

Cell latency mean – 5 sec (12 disconnection events in 5 sec)



M-TCP vs TCP performance



Compressed M-TCP vs normal M-TCP



M-TCP processing time


ConclusionConclusion

M-TCPSolution to improve TCP in mobile networks

Splits TCP connection, but it makes an effort to maintain end-to-end TCP semantics

Persist mode

Still exists problemsWhen the MH sends cumulative ACK

When the FH finishes to send data

High processing at SH

Does M-TCP really maintain end-to-end TCP semantics?


ReferencesReferences

[1] H. Balakrishnan, S.Seshan, and Randy Katz, “Improving Reliable Transport and Handoff Performance in Cellular Wireless Networks”, Wireless Networks, Vol 1 No.4 December 1995

[2] A.Bakre and B.R.Badrinath, “I-TCP: Indirect TCP for Mobile Hosts”, IC on Distributed Computing Systems 1995

[3] R. Yavatkar and N. Bhagawat, “Improving End-to-End Performance of TCP over Mobile Internetworks”, IEEE Workshop on Mobile Computing Systems and Applications, 1994

Obile etworking 2015-10-151 M-TCP : TCP for Mobile Cellular Networks Kevin Brown and Suresh Singh...

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