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● 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 28. Priority-Based Flow Control
In the system, PFC is implemented as follows:
● PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface. However, only two
lossless queues are supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged traffic and one for
Internet Small Computer System Interface (iSCSI) storage traffic. Configure the same lossless queues on all ports.
● A dynamic threshold handles intermittent traffic bursts and varies based on the number of PFC priorities contending for
buffers, while a static threshold places an upper limit on the transmit time of a queue after receiving a message to pause a
specified priority. PFC traffic is paused only after surpassing both static and dynamic thresholds for the priority specified for
the port.
● By default, PFC is enabled when you enabled DCB. When you enable DCB globally, you cannot simultaneously enable TX and
RX on the interface for flow control and link-level flow control is disabled.
● Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
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Data Center Bridging (DCB)