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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 - PowerPoint PPT Presentation
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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.
5-3
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)
5-4
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
5-5
• 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
Congestiondetected
Fast retransmit/fast recovery
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Overview
Slow-start Congestionavoidance
Overview of TCP Concepts
5-6
TCP BasicsCongestiondetected
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Slow-start Congestionavoidance
5-7
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
5-8
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.