Hello @Ravi_916 ,
the source quench is a special subcode used in an ICMP reply sent to the sender of an ICMP request to signal to the original sender that the source quench sender is too busy to answer to ICMP requests of the original sender.
So source quench is an OSI L4 concept related to ICMP protocol (in IPv4).
In the ping output you will see the code "Q" for source quench and most TCP/IP stacks count them as lost packets.
Actually ICMP can send a train or burst of ICMP requests as a way to stress the path to the destination.
The pause control or IEEE pause control frames are an OSI layer 2 concept:
on a full duplex point to point ethernet link deviceA can send traffic as fast as line speed putting a pressure on device B input buffers and queues, so device B can send IEEE pause frames to device A in an attempt to buy the equivalent of 512 bytes of silence on wire.
I used the term attempt because deviceA is not obliged to react with silence to the reception of an IEEE pause frame: actually flow control on GE or above can be negotiated between the two parts i.e. deviceA and deviceB or it can be manually enabled or manually disabled.
When you examine the output of show interface gi1/0/5 on a cisco switch like a C3850 you will find two lines about flow control one for rx flow control and one for tx flow control.
As a result of this the IEEE pause frame does not guarantee to the device sending it that the other side will honor the 512 bytes equvalent silence.
It is best practice to have flow control negotiated both tx/rx and this also a reason for not using the manual speed command anymore in modern switched networks.
Hope to help
Giuseppe
