TCP OVER ADHOC NETWORK

TCP Basics

• TCP (Transmission Control Protocol) was designed to provide

reliable end-to-end delivery of data over unreliable networks.

• In Theory , TCP should be independent of the technology of the

underlying infrastructure

• In practice, it does matter because most TCP deployments have been

carefully designed based on assumptions that are specific to wired

networks.

• Ignoring the properties of wireless transmission can lead to TCP

implementations with poor performance.

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TCP Basics• Byte Stream Delivery

• Connection-Oriented: Two communicating TCP entities (the sender and the receiver) must first agree upon the willingness to communicate

• Full-Duplex: TCP almost always operates in full-duplex mode, – TCP exhibit asymmetric behavior only during connection start and

close sequences (i.e., data transfer in the forward direction but not in the reverse, or vice versa)

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Reliable TCP Guarantees• A number of mechanisms help provide the guarantees:

– Checksums: To detect errors with either the TCP header or data

– Duplicate data detection: Discard duplicate copies of data that has already been received

– Retransmissions: • For lost and damaged data • Due to lack of positive acknowledgements• Timeout period calls for a retransmission

– Sequencing: To deliver the byte stream data to an application in order

– Timers: Various static and dynamic timers used for deciding when to retransmit

– Window: For flow control in the form of a data transmission window size

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• Conventional TCP: Tahoe, Reno, New-Reno• Sending rate is controlled by

– Congestion window (cwnd): limits the # of packets in flight

– Slow-start threshold (ssthresh): when CA start

• Loss detection– 3 duplicate ACKs (faster, more efficient)– Retransmission timer expires (slower, less

efficient)• Overview of congestion control mechanisms

– Slow-start phase: cwnd start from 1 and increase exponentially

– Congestion avoidance (CA): increase linearly

– Fast retransmit and fast recovery: Trigger by 3 duplicate ACKs

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Overview

Slow-start Congestionavoidance

Overview of TCP Concepts

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TCP BasicsCongestiondetected

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Congestion Control• Slow Start (SS): A mechanism to control the transmission rate)

– When TCP connection starts (Initial Value): CWND =1, – congestion window increases by one segment for each

acknowledgement returned

• Congestion Avoidance(CA): Used to reduce the transmission rate– When Slow Start drops one or more packets due to congestion

• Fast Retransmit: Sender receiving triple duplicate ACKs– Immediate transmission of missing packet without waiting for the

Retransmission Timeout to expire

• Fast Recovery: In SS or CA when sender receiving triple duplicate ACKs Sender only enters Congestion Avoidance mode

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1. Mobility– Route stability and availability

2. High bit error rate– Packets can be lost due to “noise”

3. Unpredictability/Variability– Difficult to estimate time-out, RTT, bandwidth

4. Contention: packets compete for airtime– Intra-flow and inter-flow contentions

5. Long connections have poor performance– More than 4 hops thruput drops dramatically

Overview

What is Different in MANETs?

TCP and network cross layer proposals

• TCP Feedback is a feedback based approach to handle route failures in MANETs.

• This approach allows the TCP sender to distinguish between losses due to routes failure and those due to network congestion.

• When routing agent of a node detects the disruption of a route, it explicitly sends a Route Failure Notification (RFN) packet to the source.

• On receiving the RFN, the source goes into a snooze state.• TCP sender in snooze state will stop sending packets, and will

freeze all its variables, such as timers and congestion window size.

• The TCP sender remains in this snooze state until it is notified of the restoration of the route through Route Re-establishment Notification (RRN) packet

• On receiving the RRN, the TCP sender will leave the snooze

state and will resume transmission based on the previous sender window and timeout values.

• To avoid blocking scenario in the snooze state, the TCP sender, on receiving RFN, triggers a route failure timer.

• When this timer expires the congestion control algorithm is invoked normally.

TCP-BuS

• TCP Buffering capability and Sequence information uses the

network feedback in order to detect route failure events and to take

convenient reaction to this event.

• The novel scheme is the introduction of buffering capability in

mobile nodes.

• The following enhancements are proposed:

• Explicit notification

• Extending timeout values

• Selective retransmission request

• Avoiding unnecessary requests for fast retransmission:

• Reliable retransmission of control message

• Explicit notification:– Two control messages

1. ERDN

2. ERSN

• Extending timeout values:

• During the route reconstruction (RRC) phase, packets along the

path from the source to the PN are buffered.

• To avoid timeout events during the RRC phase, the retransmission

timer value for buffered packets is doubled.

• Selective retransmission request:

• As the retransmission timer value is doubled, the lost packet along

the path from the source to the PN are not retransmitted until the

adjusted retransmission timer expires.

• To overcome this, an indication is made to the source so that it can

retransmit these lost packet selectively.

• Avoiding unnecessary requests for fast retransmission:

• The destination node should continue to send ACK packets

containing expected sequence number until the expected

insequence packets arrive at the destination .

• Reliable retransmission of control message:

• The reliable transmission is done by overhearing the channel after

transmitting the control messages. If a node has sent a control

message but did not overhear this message relayed during a

timeout, it will conclude that the control message is lost and

it will retransmit this message.