Priority-Based Flow Control - Dell C9000 Series Networking Configuration Manual

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sensitive to latency. Ethernet functions as a best-effort network that may drop packets

in case of network congestion. IP networks rely on transport protocols (for example,

TCP) for reliable data transmission with the associated cost of greater processing

overhead and performance impact.

Storage traffic

Storage traffic based on Fibre Channel media uses the Small Computer System

Interface (SCSI) protocol for data transfer. This traffic typically consists of large data

packets with a payload of 2K bytes that cannot recover from frame loss. To successfully

transport storage traffic, data center Ethernet must provide no-drop service with

lossless links.

InterProcess

InterProcess Communication (IPC) traffic within high-performance computing clusters

Communication

to share information. Server traffic is extremely sensitive to latency requirements.

(IPC) traffic

To ensure lossless delivery and latency-sensitive scheduling of storage and service traffic and I/O

convergence of LAN, storage, and server traffic over a unified fabric, IEEE data center bridging adds the

following extensions to a classical Ethernet network:

802.1Qbb — Priority-based Flow Control (PFC)

802.1Qaz — Enhanced Transmission Selection (ETS)

802.1Qau — Congestion Notification

Data Center Bridging Exchange (DCBx) protocol

Dell Networking OS supports only the PFC, ETS, and DCBx features in data center bridging.

Priority-Based Flow Control

In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in

multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.

When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p

priority traffic to the transmitting device. In this way, PFC ensures that PFC-enabled priority traffic is not

dropped by the switch.

PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable flow control based on

802.1p priorities (classes of service). Instead of stopping all traffic on a link (as performed by the traditional

Ethernet pause mechanism), PFC pauses traffic on a link according to the 802.1p priority set on a traffic type.

You can create lossless flows for storage and server traffic while allowing for loss in case of LAN traffic

congestion on the same physical interface.

Data Center Bridging (DCB)