Converged networks with Fibre Channel over Ethernet and Data Center Bridging

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for all but the very highest priority traffic classes. This includes converged networks that handle block
storage traffic using a layer 2 encapsulation protocol, like FCoE.
One common misunderstanding about many modern Ethernet devices, particularly Ethernet switches, is that
they already have bandwidth control and traffic shaping capabilities that support layer 2 protocols like
FCoE. But these devices typically define traffic classes based on layer 3 (IP) or layer 4 information in
frames, not by the priority field of the IEEE 802.1Q tag field or the Ethertype (protocol) field in the Ethernet
frame header.
The Enhanced Transmission Selection (ETS) standard formally defines how the port transmit logic of an
Ethernet device selects the next frame to send from one or more priority/traffic class queues for layer 2, or
MAC based, protocols. This lets the device allocate bandwidth between layer 2 defined traffic classes and
support strict priority scheduling for traffic classes requiring it. ETS refines the existing TCs. ETS adds a
bandwidth-sharing algorithm that you can assign to each of the supported TCs. When you configure a TC
to use the ETS bandwidth-sharing algorithm, you must provide a bandwidth percentage.
Traffic class queues that are part of TCs assigned a strict priority-scheduling algorithm (typically the default
algorithm) are processed in strict priority order. They have three typical uses:
• Extremely high priority network control or management traffic
• Low-bandwidth/low-latency
• Jitter (variable latency) sensitive or intolerant
The ETS standard specifies that once all the strict priority TC queues are empty, the device sends frames
from the TCs assigned an ETS scheduling algorithm. A single ETS TC can have more than one priority
queue.
There is a common misconception about the ETS bandwidth-sharing algorithm. Some people think that the
bandwidth percentage assigned to an ETS traffic class is a percentage of link bandwidth for the port. That
is not true. ETS bandwidth percentages represent the percentage of available bandwidth after satisfying all
of the strict priority TCs. That is, if the strict priority TCs take up 4 Gb/s of the link bandwidth of a 10 Gb/s
link, the ETS queue assigned 50 percent bandwidth is asking for 50 percent of the remaining 6 Gb/s of the
link bandwidth, or 3 Gb/s.
The ETS standard does not specify the bandwidth allocation algorithm that DCB-enabled Ethernet devices
must use to select frames from the TCs. Device vendors get to decide the best algorithms for their products.
The standard does suggest that deficit weighted round robin (DWRR) and a handful of other algorithms
would suffice. The ETS standard also does not specify the algorithm for selecting frames for transmit from
multiple priority queues assigned to the same TC. The standard suggests that using a strict priority algorithm
between these queues is one possibility.
As Ethernet frames of varying priority queue up for transmission on a port, the device maps them into
priority queues and traffic classes. The device then places the frames into independent priority or traffic
class queues. Network administrators responsible for managing the port on the network device are
responsible for configuring these assignments. The ETS standard specifies that these administrators are also
responsible for assigning the scheduling algorithm for each traffic class.
In Figure 7, priority 5 frames are in TC4, and priority 1 frames are in TC1. Strict priority was the
scheduling algorithm for both TCs, so the device sends their frames before any frames of TCs assigned the
ETS scheduling algorithm. In this case, the device sends frames for TC4 before any frames from TC1. If
there are no frames in the queue for TC4, then the device sends frames in TC1 before any frames in any of
the other TCs. TCs assigned with ETS scheduling (TC0, TC2, and TC3) have been allocated 50, 40, and 10
percent of the available bandwidth, respectively. These allocations are the percentage of bandwidth
available after the transmit requirements of TC4 and TC1are satisfied.