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What is Flow control?

The mechanism by which a sender adjusts how much or how fast it sends so that the receiver is not overrun. In TCP the receiver advertises its free buffer space (the receive window), and the sender does not send beyond it. RTS/CTS on a UART, Ethernet PAUSE frames and credit-based flow control in PCI Express are also forms of flow control. It is distinct from congestion control, which adapts to how busy the network as a whole is.

Japanese name: フロー制御

Flow control appears in 1 practice question across 1 topic, including Computer Networking Basics.

Field: Computer Networking Basics

Try it: questions about Flow control

In TCP flow control, what is the purpose of the receiver advertising its window size (receive window) to the sender?

  1. ATo detect network congestion and make routers along the path slow down forwarding
  2. BTo set the maximum length of data sent in one segment so that IP fragmentation is avoided
  3. CTo tell the sender how much data it may send without waiting for acknowledgments (ACKs), so the receiver's buffer does not overflow
Show answer and explanation

Answer: C. To tell the sender how much data it may send without waiting for acknowledgments (ACKs), so the receiver's buffer does not overflow

A TCP sender can send several segments in a row without waiting for acknowledgments (ACKs). But if the receiving application reads data slowly, the receive buffer can overflow. So with every ACK the receiver reports the free buffer space as the window size, and the sender keeps the unacknowledged data from exceeding that amount. When the free space reaches 0, the sender waits until it opens up. This is flow control, an adjustment between the two parties, sender and receiver. Adjusting the amount sent to network congestion is a separate mechanism, congestion control, and the maximum length of one segment is announced as the MSS at connection setup.

Flow control in the explanations

Questions that mention Flow control (1)

  1. In TCP flow control, what is the purpose of the receiver advertising its window size (receive window) to the sender?

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Related terms

TCP (Transmission Control Protocol) Congestion control ARP ARQ (automatic repeat request) CIDR CSMA/CA Default gateway DHCP DNS (Domain Name System) Ethernet

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