Users Guide

15–Data Center Bridging
DCB Capabilities
Doc No. BC0054508-00 Rev. R
January 21, 2021 Page 242 Copyright © 2021 Marvell
DCB Capabilities
DCB capabilities include ETS, PFC, and DCBX, as described in this section.
Enhanced Transmission Selection (ETS)
Enhanced transmission selection (ETS) provides a common management
framework for assignment of bandwidth to traffic classes. Each traffic class or
priority can be grouped in a priority group (PG), and it can be considered as a
virtual link or virtual interface queue. The transmission scheduler in the peer is
responsible for maintaining the allocated bandwidth for each PG. For example, a
user can configure FCoE traffic to be in PG 0 and iSCSI traffic in PG 1. The user
can then allocate each group a specific bandwidth. For example, 60 percent to
FCoE and 40 percent to iSCSI. The transmission scheduler in the peer will ensure
that in the event of congestion, the FCoE traffic will be able to use at least 60
percent of the link bandwidth and iSCSI to use 40 percent. See additional
references at:
http://www.ieee802.org/1/pages/802.1az.html
Priority Flow Control (PFC)
Priority flow control (PFC) provides a link-level flow control mechanism that can
be controlled independently for each traffic type. The goal of this mechanism is to
ensure zero loss due to congestion in DCB networks. Traditional IEEE 802.3
Ethernet does not guarantee that a packet transmitted on the network will reach
its intended destination. Upper-level protocols are responsible to maintain the
reliability by way of acknowledgment and retransmission. In a network with
multiple traffic classes, it becomes very difficult to maintain the reliability of traffic
in the absence of feedback. This is traditionally tackled with the help of link-level
flow control.
When PFC is used in a network with multiple traffic types, each traffic type can be
encoded with a different priority value and a pause frame can refer to this priority
value while instructing the transmitter to stop and restart the traffic. The value
range for the priority field is from 0 to 7, allowing eight distinct types of traffic that
can be individually stopped and started. See additional references at:
http://www.ieee802.org/1/pages/802.1bb.html