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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10

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

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