Whenever a packet is transmitted, the globally unique quality of service option is added to the packet. The sequencing preferred bit and the low transit delay bit are set in this option.
If a packet is forwarded containing the globally unique quality of service option, and the interface through which the packet will be transmitted has a queue length greater than
congest_threshold, then the congestion experienced bit is set in the quality of service option.
The threshold value stored in
congest_threshold may be tuned.
When a packet is received with the globally unique quality of service option present, and the congestion experienced bit is set, then the transport congestion control function is called.