Data Center Bridging Protocols

The following DCB Data Center Bridging. DCB is a collection of standards developed by IEEE for creating a converged data center network using Ethernet. protocols are leveraged to build lossless fabrics on Ethernet Ethernet is a network protocol for data transmission over LAN. fabrics:

Priority-based Flow Control

IEEE Institute of Electrical and Electronics Engineers. standard 802.1Qbb is a link-level flow control mechanism. The flow control mechanism is similar to that used by IEEE 802.3x Ethernet PAUSE, but it operates on individual priorities. Instead of pausing all traffic on a link, Priority Flow Control (PFC) allows you to selectively pause traffic according to its class.

Enhanced Transmission Selection

Enhanced Transmission Selection (ETS) provides a configurable bandwidth guarantee for all queues. For a PFC queue, this ensures a guaranteed percentage of the link bandwidth is available, so that the lossless flow is not starved for bandwidth by flows in other queues. Through this configuration, each traffic queue gets a minimum amount of bandwidth.

Data Center Bridging Exchange

Data Center Bridging Exchange (DCBx) protocol helps to ensure that the NIC Network Interface Card. NIC is a hardware component that allows a device to connect to the network. and the switch are configured correctly.Data Center Bridging Capability Exchange protocol (DCBX) is a discovery and exchange protocol for communicating configuration and capabilities among neighbors to ensure consistent configuration across the data center bridging network.The protocol allows auto exchange of Ethernet parameters and discovery functions between switches and endpoints. If the server NIC has enabled DCBx willing mode, DCBx will ensure the server NIC knows how to mark and treat traffic on that link after the switch is configured with the needed DCB and traffic marking parameters.

Quantized Congestion Notification

Quantized Congestion Notification (QCN) protocol provides a means for a switch to notify a source that there is congestion on the network. The source then reduces the flow of traffic. This helps to keep the critical traffic flowing while also reducing the need for pauses. This is only supported in pure layer-2 environments and seen very rarely now that RoCE v2 is the predominant RoCE solution.

Aruba CX switches with DCB-based solutions do not support QCN.

IP Explicit Congestion Notification

IP Explicit Congestion Notification (IP ECN) is not officially part of the DCB protocol suite, however, RoCE v2 supports ECN and sends Congestion Notification Packets (CNP) to an endpoint when congestion is signaled via the IP ECN bits on traffic originating from that endpoint. ECN must be enabled on both endpoints and on all the intermediate devices between endpoints for ECN to work properly. ECN notifies end nodes and connected devices about congestion with the goal of reducing packet loss and delay by signaling the sending device to decrease the transmission rate until congestion clears. When used in conjunction with PFC, ECN helps endpoints adjust their transmit rate before a PFC pause becomes necessary thereby improving performance.

The following table summarizes the DCB protocols for RoCE v2:

Table 1: DCB Protocols for RoCE v2

Parameter RoCE v2 Notes
PFC Yes Must use always. If not used, in times of congestion the RDMA advantages will not be achieved.
ETS Yes Must use in converged environments. If not used, lossless traffic classes can get starved for bandwidth.
DCBx Yes Not mandatory, but recommended.
QCN No

Congestion Notification (CN Common Name. CN is the primary name used to identify a certificate. ) helps to address pause unfairness and victim flow issues. When used with PFC, PFC acts as fast acting mechanism to address microbursts, while CN smooths out traffic flows helping to reduce pause storms under normal load.

NOTE: (Not Supported on Aruba CX)

IP ECN Yes

Highly recommended for L3 RoCE v2.

ECN helps to address pause unfairness and victim flow issues. When used with PFC, PFC acts as fast acting mechanism to address microbursts, while CN smooths out traffic flows helping to reduce pause storms under normal load.