FreezeTCP: a true endtoend TCP enhancement mechanism for mobile environments Goff, T' Moronski, J' P

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Freeze-TCP a true end-to-end TCP enhancement
mechanism for mobile environmentsGoff, T.
Moronski, J. Phatak, D.S. Gupta, V. INFOCOM
2000

• Lee Hyo Jin
• 2001 Fall Mobile Networks ????
• Nov/28/2001
• Prof. Young-Joo Suh

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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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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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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.

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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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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.

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TCP window management -1

• The window size
• minimum of receivers 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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TCP window management -2
sender
receiver
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TCP window management -3

• ZWP
• Sending probes until the receivers window opens
  up.
• Sender want to knows receivers 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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TCP window management -6
Data1 win4
1
2
Ack1 win4
4
3 4 5 6
DATA3 6 win4
8
Ack6 win0
Ack6 win4
9
DATA10 13 win4
10 11 12 13
ZWA
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TCP window management -7
8
Ack6 win0
ZWP
Probe response (win4)
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Original ack
10 11 12 13
DATA10 13 win4
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Existing Solutions

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

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I-TCP
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)
Regular TCP
FH
FH socket (fhaddr, fhport, mhaddr, mhport)
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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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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
SH Supervisor Host MSS Mobile Support
Station MH Mbile Host
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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.

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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.

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TPC Performance
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Picture of Freeze-TCP
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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 receivers opens up
• The interval grows exponentially (exponential
  back off ) until it reaches 1 minute.
• ZWP reponse does not have receivers
  advertisement window size.

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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 senders 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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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.

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TR-ACK
ZWP
ZWP
Receiver window open
Sending again
sender
receiver
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Estimate performance -1Freeze-TCP (4)

• Idle period avoided.
• W ts RTT , W RTT / ts
• ts packet-size / band width , W sender
  window size

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Estimate performance -2Freeze-TCP (5)

• Increased throughput.

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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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Conclusion and Discussion -1

• To enhance TCP performance in the present of
  disconnections and reconnections.
• True end-to-end signaling scheme.
• Unnecessary intermediariess 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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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 )