1. 2001 Fall Mobile Networks 발표자료
Freeze-TCP: a true end-to-end TCP enhancement mechanism for mobile environments Goff, T.; Moronski, J.; Phatak, D.S.; Gupta, V. INFOCOM 2000
Lee Hyo Jin
2001 Fall Mobile Networks 발표자료
Nov/28/2001
Prof. Young-Joo Suh

2. Reference
Tom Goff et el, "Freeze-TCP: A True End-to-End TCP Enhancement 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,
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3. Contents Introduction. Requirement Key concepts.
TCP window management.
Introduce to existing solutions.
Details of Freeze-TCP.
Experimental result.
Conclusion and Discussion.
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4. 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.
End to end semantic이 무엇인가 설명함.
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5. 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.
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6. 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.
Disconnection에 임박했는지를 모바일 호스트가 직접 관찰하고
알아내서 한 다음에 경계로 갈수록 signal이 fading된다.
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7. 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.
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8. TCP window management -2
sender
receiver
Ack1 win4 1 2 4 8
DATA3 ~ 6 win4
Data1 win4
Ack6 win0
Ack6 win4 9
DATA10 ~13 win4
ZWA
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9. 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.
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10. TCP window management -6
Data1 win4 1
ZWP 2
Ack1 win4 4
DATA3 ~ 6 win4
Ack6 win0 8
Ack6 win4 9
DATA10 ~13 win4
ZWA
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11. TCP window management -78
Ack6 win0
ZWP
Probe response (win4) 9
Original ack
DATA10 ~13 win4
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12. Existing Solutions SNOOP I-TCP ( Indirect TCP )
EBSN ( Explicit bad state notifications )
Delayed dupacks
M-TCP
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13. I-TCP Split the connection Retain a little RTT
MH socket
(mhaddr, mhport, msr1addr, msr1port)
Split the connection
FH-BS : Standard TCP.
BS-MH : Standard TCP ,Optimizing protocol.(MTCP)
Retain a little RTT
Fast recovery about cwnd size degradations.
Need to exchange the status information
Long delay time.
MSR buffer size is small. (to reduce handoff time)
MSR : Mobility Support Routers. MH MH
Wireless TCP
MSR1or 2 mhsocket
(msr1addr, msr1port, mhaddr, mhport)
MSR 1
MSR 2
MSR1or 2 fhsocket
(mhaddr, mhport, fhaddr, fhport)
유선따로 무선따로.
Regular TCP FH
FH socket
(fhaddr, fhport, mhaddr, mhport)
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14. 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.
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15. 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
MSS
Cell MH
SH : Supervisor Host
MSS : Mobile Support Station
MH : Mbile Host
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16. 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 received an ack from MH. FH BS MH
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17. M-TCP (3) TCP Persist Mode Need help from BS.
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.
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18. TPC Performance enhancement degradation M-TCP Good
Re-packetization overhead at sender
SNOOP
I-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
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19. Picture of Freeze-TCP BS MH Fixed Host (Sender) ZWP ZWA Connection
Probe res
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20. ZWP Freeze-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 off ) until it reaches 1 minute.
ZWP reponse does not have receiver’s advertisement window size.
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21. Warning Period FreezeTCP (3)
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.
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22. TR-ACK -1 Freeze-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.
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23. TR-ACK ZWP ZWP Receiver window open Sending again sender receiver
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24. Estimate performance -1 Freeze-TCP (4)
Idle period avoided.
W • ts ≥ RTT , W ≥ RTT / ts
: ts ≈ packet-size / band width , W : sender window size
Receiver
Sender
RTT
W unACKed packets can be sent ts
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25. Estimate performance -2 Freeze-TCP (5)
Increased throughput.
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26. 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.
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27. 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 infrastructure is guaranteed.
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28. 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 )
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