CNM format
When a CP detects the congestion state by sampling frames, it sends CNMs to the RPs.
The CP constructs a CNM as follows:
Uses the source MAC address of the sampled frame as the destination MAC address.
Uses the destination MAC address of the sampled frame as the source MAC addresses.
Copies the VLAN tag and CN tag of the sampled frame.
Places the data as shown in Figure 32.
PDU EtherType—2 bytes in length. It indicates the Ethernet type of the PDU and has a value of 0x22E7.
CNM PDU—24 to 88 bytes of payload of the PDU.
Figure 32: CNM PDU format
As shown in Figure 33, a payload contains the following fields:
Field | Length | Description |
|---|---|---|
Version | 4 bits | Its value is fixed at 0. |
ReserverV | 6 bits | Its value is fixed at 0. |
Quantized Feedback | 6 bits | Quantized value indicating the extent of congestion. |
CPID | 8 bytes | Identifies the CP where congestion occurs. |
cnmQoffset | 2 bytes | Indicates the difference between instantaneous queue size at the sampling point and desired queue length. |
cnmQdelta | 2 bytes | Indicates the difference between instantaneous queue sizes at the current sampling point and at the previous sampling point. |
Encapsulated priority | 2 bytes | Priority of the sampled frame that triggered the CNM. |
Encapsulated destination MAC address | 6 bytes | Destination MAC address of the sampled frame that triggered the CNM. |
Encapsulated MSDU length | 2 bytes | Number of bytes in the Encapsulated MSDU field of the sampled frame that triggered the CNM. |
Encapsulated MSDU | 0 to 64 bytes | Initial bytes of the Encapsulated MSDU field of the sampled frame that triggered the CNM. |
Figure 33: CNM PDU format
