Congestion Avoidance

Congestion avoidance is the second method of flow control covered here. The computer receiving the data notices that its buffers are filling. This causes either a separate PDU, or field in a header, to be sent toward the sender, signaling the sender to stop transmitting. Figure 3-9 shows an example.

Figure 3-9 Congestion Avoidance Flow Control



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"Hurry up and wait" is a popular expression used to describe the process used in this congestion avoidance example. This process is used by Synchronous Data Link Control (SDLC) and Link Access Procedure, Balanced (LAPB) serial data link protocols.

A preferred method might be to get the sender to simply slow down instead of stopping altogether. This method would still be considered congestion avoidance, but instead of signaling the sender to stop, the signal would mean to slow down. One example is the TCP/IP Internet Control Message Protocol (ICMP) message "Source Quench." This message is sent by the receiver or some intermediate router to slow the sender. The sender can slow down gradually until "Source Quench" messages are no longer received.

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