Buffer to Buffer credits
Define the maximum amount of data that can be sent prior to an acknowledgement
Buffer credits are physical ASIC port or card memory resources and are finite in number as a function of cost
Within a fabric, each port may have a different number of buffer credits
The number of available buffer credits is communicated at fabric logon (FLOGI)
One buffer credit allows a device to send one 2112 byte frame of data
Before any data frames are sent, the transmitter sets a counter equal to the BB-credit value.
For each data frame sent by the transmitter, the counter is decremented by one.
Upon receipt of a data frame, the receiver sends a status frame (R_RDY) to the transmitter indicating that the data frame was received AND the buffer can receive another data frame.
For each R_RDY received by the transmitter, the counter is incremented by one.
BB_Credits are the “admission control” mechanism in FC to ensure that FC switches don’t run out of buffers (FC Switches cannot drop frames)
For Devices operating at FC Class 3 (most devices), Buffer Credits are negotiated at login.
BB_Credits are the only flow-control mechanism for FC Class 3.
End to End Flow Control
BB flow control doe not provide a way for a destination port to control frame transmission by a source port
unless the ports are directly connected in a point-to-point or arbitrated loop configuration. In a fabric topology, BB flow control provides a mechanism to control the flow of frames between the node port and fabric port, but not between the two node ports.
End-to-end flow control is a source node port to destination node port flow control mechanism that is available for managing the rate of frame transmission between two node ports.
The purpose of end-to-end flow control is to manage data frame transmission between node ports. End-to-end credit is managed on a per-port basis and is granted during N-Port Login(PLOGI) with the associated port. This credit is referred to as the login EE_Credit and is used to initialize EE_Credit for that port.
Define the maximum amount of data that can be sent prior to an acknowledgement
Buffer credits are physical ASIC port or card memory resources and are finite in number as a function of cost
Within a fabric, each port may have a different number of buffer credits
The number of available buffer credits is communicated at fabric logon (FLOGI)
One buffer credit allows a device to send one 2112 byte frame of data
Before any data frames are sent, the transmitter sets a counter equal to the BB-credit value.
For each data frame sent by the transmitter, the counter is decremented by one.
Upon receipt of a data frame, the receiver sends a status frame (R_RDY) to the transmitter indicating that the data frame was received AND the buffer can receive another data frame.
For each R_RDY received by the transmitter, the counter is incremented by one.
BB_Credits are the “admission control” mechanism in FC to ensure that FC switches don’t run out of buffers (FC Switches cannot drop frames)
For Devices operating at FC Class 3 (most devices), Buffer Credits are negotiated at login.
BB_Credits are the only flow-control mechanism for FC Class 3.
End to End Flow Control
BB flow control doe not provide a way for a destination port to control frame transmission by a source port
unless the ports are directly connected in a point-to-point or arbitrated loop configuration. In a fabric topology, BB flow control provides a mechanism to control the flow of frames between the node port and fabric port, but not between the two node ports.
End-to-end flow control is a source node port to destination node port flow control mechanism that is available for managing the rate of frame transmission between two node ports.
The purpose of end-to-end flow control is to manage data frame transmission between node ports. End-to-end credit is managed on a per-port basis and is granted during N-Port Login(PLOGI) with the associated port. This credit is referred to as the login EE_Credit and is used to initialize EE_Credit for that port.
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