Configuring Qcn; Basic Concepts; Qcn Message Format; Data Flow Format - HP FlexFabric 5700 series Configuration Manual

Configuring QCN

Quantized Congestion Notification (QCN) is an end-to-end congestion notification mechanism that can
reduce packet loss and delay in Layer 2 networks by actively sending reverse notifications. As part of
data center standards, QCN is primarily used in data center networks.

Basic concepts

Reaction point (RP)—A source end host that supports QCN.
•
Congestion point (CP)—A congestion detection device that is enabled with QCN.
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Congestion notification message (CNM)—A message transmitted by a CP to an RP when a queue
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on the CP is congested.
• Congestion controlled flow (CCF)—A flow of frames with the same priority value. A CP assigns
frames of the same CCF to one queue before forwarding them.
Congestion notification tag (CN tag)—Identifies a CCF. Devices in a CND must be able to process
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packets with a CN tag.
Congestion notification priority (CNP)—An 802.1p priority that is enabled with QCN. The value of
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that 802.1p priority is called a Congestion Notification Priority Value (CNPV).
Congestion notification domain (CND)—A set of RPs and CPs with QCN enabled for a CNPV.
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Congestion point identifier (CPID)—An 8-byte unique identifier for a CP in the network.
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• Quantized feedback (QntzFb)—A 6-bit quantized feedback value indicating the extent of
congestion.

QCN message format

Data flow format

An RP can add CN tags to outgoing Ethernet frames to distinguish between CCFs. A CN tag defines a
CCF.
As shown in
EtherType—Indicates the Ethernet type of the data packet, 2 bytes in length and assigned a value
•
of 0x22E9.
• RPID—Locally assigned and 2 bytes in length. When receiving a CNM, the RP uses this field to
identify the CCF that causes congestion and then rate limits that CCF.
When only one CCF exists, the RP may not add a CN tag to packets. In this case, the triggered CNM
carries a CN tag with the RPID as 0.
A CN tag is confined within its CND. When a packet leaves a CND, the CN tag is stripped off.
Figure 29, the CN tag contains the following fields:
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