On 08/02/2017 09:51 AM, David Laight wrote:
Sending pause frames just tells the adjacent switch not to send you packets (because you'll discard them). Since the idea is to avoid the discards, the switch will buffer the packets it would have sent. The buffers in the switch then fill up with packets it isn't sending you.
I was under the impression that the switch forwards the pause frames to other devices, so that the transmitting NIC can stop sending the data, but your explanation makes a lot more sense. If the EMAC never stops sending pause frames, then the switch's buffers will fill up, disabling all other devices. If the switch does not have per-port buffers, then it makes sense when the buffer is full, it blocks all ports.
The switch then runs out of buffers, it has 2 choices: 1) Throw the packets away. 2) Send 'pause' frames to the sources. If it sends 'pause' frames the entire network will very quickly lock up. If it discards the packets they might as well have been discarded by the receiving MAC. Doesn't this mean that pause frames are 99.999% useless??
Pause frames are intended for situations where the receiving CPU is temporarily overwhelmed and just needs a second or two to resume processing incoming packets. That makes sense on a dinky single-core 32-bit CPU.
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