Chapter 9: Flow Control and Quality of Service

Cisco Press

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Ethernet switches often employ "tail-drop" to manage flows. Tail-drop is not a mechanism per se, but rather a behavior. Tail-drop is the name given to the process of dropping packets that need to be queued in a queue that is already full. In other words, when a receive queue fills, additional frames received while the queue is full must be dropped from the "tail" of the queue. ULPs are expected to detect the dropped frames, reduce the rate of transmission, and retransmit the dropped frames. Tail-drop and the Pause Opcode often are used in concert. For example, when a receive queue fills, a Pause Opcode may be sent to stem the flow of new frames. If additional frames are received after the Pause Opcode is sent and while the receive queue is still full, those frames are dropped. For more information about Ethernet flow control, readers are encouraged to consult the IEEE 802.3-2002 specification.

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