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Introduction• Why TCP ?
– Most widely used reliable transport protocol– Lessons learnt apply to other transport protocol
• Implements congestion avoidance and control• Reliability achieved by means of retransmissions if
necessary• End-to-end semantics
– Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver
– Ack for data sent only after data has reached receiver
The Presentation
• Issues in the wireless world
•Wireless TCP approaches
• Wireless TCP protocols
• I-TCP (Indirect TCP)
• Snoop Protocol (handoffs)
Issues in the Wireless World:• High bit-error rate
– channel fading– noise – interference
• Intermittent Connectivity– Handoffs for mobile hosts
• Asymmetric links (Space Communications)– Bandwidth– Delay– Packet loss
TCP in the Wireless World• Packet loss due to:
– Motion– Bit errors
• This packet loss mistaken for congestion
• Congestion control mechanism initiated
• Result: -decrease in throughput - increase in delay- multiple packet loss leads to
bursts
FH FHRouter
Loss =/=Congestion
Loss = Congestion
MH
Wireless Topology
Sender
MH
BS-1 BS-2
Wireless TCP Issues• Maintain end to end semantics• No or minimum changes to the TCP
implementation of the fixed host• Earlier implementations concentrated on FH
to MH transfers only• Earlier, MH to FH transfers used NACK • Recently, work started on MH to FH data
transfers (ELN)– researchers at Daimler Benz are working on a
prototype of a Mercedes car that runs a Web server
Wireless TCP Schemes
End to End
•ARQ/FEC
Link Layer
Wired TCP
Wireless TCP
Split Connection
Split Connection • TCP connection
terminates at base station• Hide wireless link from
sender: sender NOT aware of non-congestion losses
• Separate reliable connection between base station and mobile host
I-TCP: Indirect TCP
• Breaks the TCP connection between FH and MH into two connections at MSR
• Connection between FH and MSR is regular TCP
• Connection between MSR and MH is any transport layer protocol tuned for wireless links.
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I-TCP indirect TCP (Cont.)
• BS cache packets from FH and send back ACK for MH.
• if MH switches to another cell the center point of the connection moves to the new MSR, no need of reconnection.
• FH is completely unaware of the indirection and is not affected even when the MH switches cells.
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Snoop Protocol
• Basic idea– Modify the IP layer in the BS, and let BS cache
the TCP packets sent from FH before route it to the MH.
– If packet lost on wireless link, IP layer on the BS will retransmit the packet .
– BS suppress DUPACKs sent from MH to FH.– BS use shorter local timer for local timeout.
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Three kinds of data packets from FH
• A new packet in the normal TCP sequence. (common case)
• An out-of-sequence packet that has been cached earlier. (sender retransmission)
• An out-of-sequence packet that has not been cached earlier. (congestion loss)
SNOOP• No modifications at the fixed host• Currently in use• Optimizations continue• FH to MH : Snoop• MH to FH : ELN• FH to BS : TCP
Transport
Network
Link
Physical
MH to FHLink-aware TCP
(ELN)FH to MHTCP-aware link
(Snoop agent at BS)
BS (Snoop Agent)
Mobile Host
Fixed Host
“Wire” TCP
BSSnoop_data()Snoop_Ack()
Modified TCP
SNOOP OVERVIEW
Snoop Agent
• Cache unacknowledged TCP data (from FH)
•Local re-transmissions
•Suppress DUPACK from FH
•Monitor TCP data from MH
•Set ELN and forward ACK to MH
Snoop_data() Snoop_Ack()
Conclusion• Various Link Layer and Transport Level proposals for improving
the performance of TCP over Wireless Links have been studied.• I-TCP• Snoop Scheme• The split-connection approach, with regular TCP used for
the wireless hop, shields the sender from wireless losses.
• to improve the performance of TCP in networks with wireless links and mobile hosts.
References
Karunaharan Ratnam and Ibrahim Matta. WTCP:An Efficient Transmission Control Protocol for Networks with Wireless Links. Technical Report NU-CCS-97-11, Northeastern University. July 1997.
Nitin Vaidya, Miten Mehta, Charles Perkins and Gabriel Montenegro. Delayed Duplicate Acknowledgements: A TCP-Unaware Approach to Improve Performance of TCP over Wireless. Technical Report 99-003, February 1999.
Ajay Bakre and B.R. Badrinath. I-TCP: Indirect TCP for Mobile Hosts.DCS-TR-314, Rutgers University, October 1994.
Elan Amir, Hari Balakrishnan, Srinivasan Seshan and Randy H. Katz. Efficient TCP over Wireless Links.
Bikram S. Bakshi, P. Krishna, N.H. Vaidya and D.K. Pradhan. Improving Performance of TCP over Wireless Networks. Texas A & M University.
Thank
You18