7–RoCE Configuration
Configuring DCQCN
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DCQCN Terminology
The following terms describe DCQCN configuration:
ToS
(type of service) is a single-byte in the IPv4 header field. ToS
comprises two ECN least significant bits (LSB) and six Differentiated
Services Code Point (DSCP) most significant bits (MSB). For IPv6, traffic
class is the equivalent of the IPv4 ToS.
ECN
(explicit congestion notification) is a mechanism where a switch adds
to outgoing traffic an indication that congestion is imminent.
CNP
(congestion notification packet) is a packet used by the notification
point to indicate that the ECN arrived from the switch back to the reaction
point. CNP is defined in the Supplement to
InfiniBand Architecture
Specification Volume 1 Release 1.2.1
, located here:
https://cw.infinibandta.org/document/dl/7781
VLAN Priority
is a field in the L2 vLAN header. The field is the three MSBs
in the vLAN tag.
PFC
(priority-based flow control) is a flow control mechanism that applies to
traffic carrying a specific vLAN priority.
DSCP-PFC
is a feature that allows a receiver to interpret the priority of an
incoming packet for PFC purposes, rather than according to the vLAN
priority or the DSCP field in the IPv4 header. You may use an indirection
table to indicate a specified DSCP value to a vLAN priority value.
DSCP-PFC can work across L2 networks because it is an L3 (IPv4) feature.
Traffic classes
, also known as priority groups, are groups of vLAN priorities
(or DSCP values if DSCP-PFC is used) that can have properties such as
being lossy or lossless. Generally, 0 is used for the default common lossy
traffic group, 3 is used for the FCoE traffic group, and 4 is used for the
iSCSI-TLV traffic group. You may encounter DCB mismatch issues if you
attempt to reuse these numbers on networks that also support FCoE or
iSCSI-TLV traffic. Cavium recommends that you use numbers 1–2 or 5–7 for
RoCE-related traffic groups.
ETS
(enhanced transition services) is an allocation of maximum bandwidth
per traffic class.
DCQCN Overview
Some networking protocols (RoCE, for example) require droplessness. PFC is a
mechanism for achieving droplessness in an L2 network, and DSCP-PFC is a
mechanism for achieving it across distinct L2 networks. However, PFC is deficient
in the following regards:
When activated, PFC completely halts the traffic of the specified priority on
the port, as opposed to reducing transmission rate.
