Users Guide

A CNA is a computer input/output device that combines the functionality of a host bus adapter (HBA) with a network interface controller
(NIC). Multiple adapters on different devices for several traffic types are no longer required.
Data center bridging satisfies the needs of the following types of data center traffic in a unified fabric:
• LAN traffic consists of a large number of flows that are generally insensitive to latency requirements, while certain applications, such as
streaming video, are more 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 based on Fibre Channel media uses the 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.
• Servers use InterProcess Communication (IPC) traffic within high-performance computing clusters to share information. Server traffic
is extremely sensitive to latency requirements.
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
NOTE: In Dell Networking OS version 9.4.0.x, only the PFC, ETS, and DCBx features are supported 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 large amounts of queued LAN traffic do not cause storage traffic to be dropped, and that storage
traffic does not result in high latency for high-performance computing (HPC) traffic between servers.
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.
The following illustration shows how PFC handles traffic congestion by pausing the transmission of incoming traffic with dot1p priority 3.
Figure 1. Priority-Based Flow Control
In the system, PFC is implemented as follows:
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Data Center Bridging (DCB)