lee hyo jin 2001 fall mobile networks 발표자료 nov/28/2001 prof. young-joo suh

Postech Postech DP&NM LabDP&NM Lab

Freeze-TCP: a true end-to-end TCP enhancement mechanism for mobile envi

ronments

Goff, T.; Moronski, J.; Phatak, D.S.; Gupta, V. INFOCOM 2000

Lee Hyo Jin2001 Fall Mobile Networks 발표자료

Nov/28/2001 Prof. Young-Joo Suh

Freeze-TCP (2) Postech Postech DP&NM LabDP&NM Lab

Reference• Tom Goff et el, "Freeze-TCP: A True End-to-End TCP Enh

ancement Mechanism for Mobile Environments," INFOCOM'00.

• K. Brown and S. Singh, “M-TCP: TCP for Mobile Cellular Networks,” ACM Computer Communications Review (CCR), vol. 27, no. 5, 1997.

• Ajay Bakre and B.R. Badrinath, “I-TCP: Indirect TCP for mobile hosts,” Tech. Rep., Rutgers University, May 1995,


Contents• Introduction.• Requirement• Key concepts.• TCP window management.• Introduce to existing solutions.• Details of Freeze-TCP.• Experimental result.• Conclusion and Discussion.


Introduction • Need to optimize TCP for mobility.• Not true end-to-end scheme.

– Intermediaries. ( like Base Stations )• To monitor the TCP traffic and participate in flow control to

enhance TCP performance.

– Not applicable when IP payload is encrypted.(IPSEC)• Security associations between sender and receiver.

• Require changes TCP/IP code at intermediate node.– It is difficult for mobile clients to inter-operate with the

existing infrastructure.


Requirements

• True end to end scheme.• Interoperate existing infrastructure.

– TCP code must change in mobile client (MH)• Need to performance enhancement.

We need a new scheme.


Key Concepts

• No help with base stations in hand-off.• To detect an impending handoff at client.( MH )• ZWA(MH): zero window advertisement.• ZWP (FH) : zero window probes.• TR-ACKs : Triplicate acks.• True end to end semantics.• Performance enhancement.


TCP window management -1• The window size

– minimum of receiver’s advertised buffer size – perceived network congestions.

• The receiver run out of its buffer-space and advertise a window size of zero. ( ZWA )

• The sender should freeze all retransmit-timers and enter a persist-mode on seeing ZWA.


TCP window management -2

Ack1 win4

12

43 4 5 6

8

DATA3 ~ 6 win4

Data1 win4

Ack6 win0Ack6 win4

9

10 11 12 13

DATA10 ~13 win4 ZW

A

sender

receiver


TCP window management -3• ZWP• Sending probes until the receiver’s window opens up.• Sender want to knows receiver’s window opened or not, in

advance. • Interval

– exponential back-off until it reaches 1 minute– remains constant after 1 minute.

• Receiver responds to a ZWP with a non-zero window size.• Sender will continue transmission using a window size

consistent with the advertised value.



Ack1 win4

12

43 4 5 6

8

DATA3 ~ 6 win4

Data1 win4

Ack6 win0Ack6 win4

9

10 11 12 13

DATA10 ~13 win4 ZW

A

ZWP



8Ack6 win0

9

10 11 12 13 DATA10 ~13

win4

ZWP

Probe response (win4)

Original ack


Existing Solutions• SNOOP • I-TCP ( Indirect TCP )• EBSN ( Explicit bad state notifications )• Delayed dupacks• M-TCP


I-TCP• Split the connection

– FH-BS : Standard TCP.– BS-MH : Standard TCP ,Optimi

zing protocol.(MTCP)

• Retain a little RTT – Fast recovery about cwnd size d

egradations.

• Need to exchange the status information – Long delay time.– MSR buffer size is small. (to re

duce handoff time)– MSR : Mobility Support Router

s.

MH

FH

MH

MH socket(mhaddr, mhport, msr1addr, msr1port)

MSR1or 2 mhsocket(msr1addr, msr1port, mhaddr, mhport)

MSR 1 MSR 2

MSR1or 2 fhsocket(mhaddr, mhport, fhaddr, fhport)

FH socket(fhaddr, fhport, mhaddr, mhport)

Regular TCP

Wireless TCP


EBSN• Explicit bad-state notifications.• BS sends an EBSN to sender when each failed

attempt to send a packet to a MH.• On receipt of each EBSN, the sender reset

retransmission timer to original value.• Prevent the sender from dropping congestion

window.


M-TCP (1)• Performance enhancement

during hand-off.• Low BER and Frequent

disconnections.• 3 level hierarchy structure.

– Reduce MSS functions– No need to exchange the

status info moving MSS in the same SH domain.

High-speed Network

SH SH

MH

MSS

Cell

SH : Supervisor Host

MSS : Mobile Support Station

MH : Mbile Host


M-TCP (2)• End to end TCP semantics.

– TCP connection is split at the BS– The SH does not send an ack FH unless BS has receive

d an ack from MH.

FH

MH

BS


M-TCP (3)• TCP Persist Mode

– When the positive window advertisement is received, sender exits persist mode.

– Retain RTO and congestion window size.• Need help from BS.

– BS detect disconnection or packet loss.– BS withholds ack for last one byte.– Re-packetization penalty at sender.– This ack uses to send to zero window advertisement at

hand off.


TPC Performanceenhancement degradation

M-TCP Good Re-packetization overhead at sender

SNOOPI-TCP( MTCP )

Handle Bit-error

Frequent hand-off or disconnections

Delayed dupacks

Single packet losses

Actual congestion losses

EBSN Significant duration or burst error

Random error


Picture of Freeze-TCP

Fixed Host

(Sender)

ZWA

ZWP

BS

BS

MH

MH MHMH

Connection

Probe res


ZWPFreeze-TCP (2)

• ZWP – ZWA force the sender into the ZWP (persist) mode.– To prevent it from dropping its congestion window.– To send ZWPs until the receiver’s opens up– The interval grows exponentially (exponential back of

f ) until it reaches 1 minute.– ZWP reponse does not have receiver’s advertisement w

indow size.


Warning PeriodFreezeTCP (3)

• Warning period.– How much in advance of the disconnection should the

receiver start advertising ZWA?– Ideally, long enough to ensure that exactly one ZWA

get across to the sender.– Longer : idle time prior to disconnections– Small : sender’s congestion window to drop.– RTT is reasonable. ( ref : Experimental result )– Only useful if a disconnection occurs while data is

being transferred.


TR-ACK -1Freeze-TCP (3)

• Triplicate Reconnection ACKs– ZWPs are exponentially backed off. – The possibility of idle time after reconnections.– To avoid this idle time, TR-ACKs implements.– Effect of standard TCP.


TR-ACK

ZWP

ZWP

sender

receiver

Receiver window open

Sending again


Estimate performance -1Freeze-TCP (4)

• Idle period avoided.– W • ts ≥ RTT , W ≥ RTT / ts

: ts ≈ packet-size / band width , W : sender window size

Receiver

SenderRTT

W unACKed packets can be sent

ts


Estimate performance -2Freeze-TCP (5)

• Increased throughput.


Experimental result

• Modifying the Linux 2.1.101 TCP source code.• Emulate the mobile host in a PC.• Freeze-TCP is not worsen performance by a

noticeable amount.


Conclusion and Discussion -1

• To enhance TCP performance in the present of disconnections and reconnections.

• True end-to-end signaling scheme.• Unnecessary intermediaries’s help.• Easy changing TCP code at receiver side • Easy to implement. • Almost no overheads and tradeoffs.• Complete inter-operability with existing infrastruc

ture is guaranteed.


Conclusion and Discussion -2

• Full rate with old window size on entering new unknown environment or not.

• Needs at receiver to predict impending disconnections. ( pro-active action/simulations )

lee hyo jin 2001 fall mobile networks 발표자료 nov/28/2001 prof. young-joo suh

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