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Questions and Answers
Congestion can result in lost packets due to buffer overflow at routers.
True
Sender retransmission of timed-out packets does not affect overall data flow.
False
A scenario with finite buffers will not experience queuing delays.
False
The selective repeat protocol allows a receiver to accept duplicate data as new.
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Having perfect knowledge of router buffer availability allows for better control of data transmission.
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Packets can be lost and dropped at a router due to full buffers.
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The sender resends packets regardless of whether they are known to be lost.
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The seq #’s indicate the order of packets in the selective repeat protocol.
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If the buffer space is free, packets will be dropped at the router.
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Timeouts are important for retransmitting packets in selective repeat.
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The sender can time out prematurely, leading to duplicate packet sending.
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Every packet in the selective repeat protocol is processed as it arrives without regard to its sequence number.
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In the given scenarios, full buffers lead to reduced throughput.
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The FIN bit in a TCP segment is used to signify the establishment of a connection.
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Flow control refers to the management of data transmission speed between network devices.
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Congestion control is the same as flow control in TCP networking.
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Study Notes
Congestion Manifestations
- Lost packets can be caused by buffer overflow at routers.
- Long delays can be caused by queueing in router buffers.
Causes of Congestion: Scenario 1
- Two senders and two receivers communicate with each other over a shared output link with infinite buffers.
- The output link capacity is R.
- Each sender can achieve a maximum throughput of R/2 due to shared resources.
- Large delays can occur as the arrival rate approaches the output link capacity (R).
Causes of Congestion: Scenario 2
- Two hosts communicate with a router that has finite buffers.
- Retransmissions occur when packets time out.
- The original data rate is 'lin', and the data rate including retransmissions is 'l'in.
- The output link capacity is R.
- The router can act optimally when perfectly aware of its available buffer space, limiting the sender's output to its available space.
- The sender can achieve a throughput of R/2 by sending only when buffer space is available.
- Packets can be lost due to full buffers, requiring retransmission.
- The sender can achieve a rate of R/2 asymptotically where some packets are lost and some are retransmitted because the goodput remains R/2.
- Packets can be duplicated due to premature timeouts, where a sender retransmits before confirmation of loss.
TCP Segment Structure
- TCP segments include a sequence number to track data flow.
- TCP segments are reliably delivered, ensuring data reaches the destination in the order they are sent.
- TCP segments include a checksum for data integrity.
TCP 3-way Handshake
- The TCP 3-way handshake is a process for establishing a connection between two hosts.
- The handshake involves three message exchanges:
- Client sends a SYN (synchronization) packet to establish a connection.
- The server sends a SYN-ACK packet, confirming receipt of the SYN packet and acknowledging the connection attempt.
- The client acknowledges the SYN-ACK packet, completing the connection establishment.
TCP: Closing a Connection
- Client and server independently close their respective ends of a connection.
- This is done by both sending a FIN (finish) segment with the FIN bit set to 1.
- Each party acknowledges the received FIN segment in a separate ACK segment.
- FIN and ACK can be combined in a single segment if they are received and sent simultaneously.
- Closing a connection progresses through a series of states to ensure a clean shutdown.
Principles of Congestion Control
- Congestion occurs when networks are overloaded with traffic.
- Congestion control mechanisms aim to prevent or minimize congestion by managing traffic flow.
Approaches to Congestion Control
- End-to-end congestion control relies on end systems to infer congestion based on loss or delay, without explicit feedback from the network.
- Network-assisted congestion control involves routers providing explicit feedback to end systems, including congestion indicators or rate control information.
ATM ABR Congestion Control
- ATM (Asynchronous Transfer Mode) ABR (Available Bit Rate) is an elastic service where senders adjust their rate based on network conditions.
- Resource Management (RM) cells are used in conjunction with data cells.
- RM cells contain bits set by switches to indicate congestion levels:
- NI (No Increase) bit: Mild congestion, sender should not increase rate.
- CI (Congestion Indication) bit: Severe congestion, sender should reduce rate.
- RM cells are returned to senders by receivers to provide congestion feedback.
TCP Congestion Control: Additive Increase, Multiplicative Decrease
- TCP uses an approach of additive increase and multiplicative decrease to automatically adjust its transmission rate (window size) in response to network conditions.
- Additive increase: The window size is increased by 1 MSS (Maximum Segment Size) every Round Trip Time (RTT) until loss is detected.
- Multiplicative decrease: The window size is cut in half after a loss event.
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Description
This quiz covers the manifestations and causes of network congestion, including scenarios where routers handle packet transmission. It explores the impact of buffer sizes, shared resources, and retransmission effects on data flow and delays in networking. Test your understanding of these critical concepts in network management.