Networks - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

Calculations for Buffer Requirements When Using PFC PAUSE
How PFC and Congestion Notification Profiles Work
Copyright © 2010, Juniper Networks, Inc.
arises when transmitters send packets faster than receivers can accept them. When
receivers run out of available buffer space to hold incoming flows, they silently drop
additional incoming packets. This problem is generally resolved by upper-layer protocols
that detect the drops and request retransmission.
Applications that require reliability in Layer 2 must have flow control that includes
feedback from a receiver to a sender regarding buffer availability. Using IEEE 802.3x
Ethernet PAUSE control frames, a receiver can generate a MAC control frame and send
a PAUSE request to a sender when a specified threshold of receiver buffer has been filled
in order to prevent buffer overflow. Upon receiving a PAUSE frame, the sender stops
transmissions of any new packets until the receiver has sufficient buffer space to accept
them again. The disadvantage of using Ethernet PAUSE is that it operates on the entire
link, which might be carrying multiple traffic flows. Some traffic flows do not need flow
control in Layer 2, because they are carrying applications that rely on upper-layer protocols
for reliability. PFC enables you to configure Layer 2 flow control selectively for the traffic
that requires it, such as Fibre Channel over Ethernet (FCoE) traffic, without impacting
other traffic on the link. You can also enable PFC for other traffic types, such as iSCSI.
Receivers must ensure that a PFC PAUSE frame is sent while there is sufficient receive
buffer to absorb the data that might continue to be received while the system is
responding to the PFC PAUSE.
When you calculate buffer requirements, consider the following factors:
Processing and queuing delay of the PFC PAUSE—In general, the time to detect the
lack of sufficient buffer space and to transmit the PFC PAUSE is negligible. However,
delays can occur if the switch detects reduced buffer space occurs just as the
transmitter is beginning to transmit a maximum length frame.
Propagation delay across the media—The delay amount depends on the length and
speed of the physical link.
Response time to the PFC PAUSE frame
Propagation delay across the media on the return path
NOTE: We recommend that you configure at least 20 percent of the buffer
for the queue that is using PFC.
PFC is triggered when the incoming frame has a User Priority (UP) field that matches the
three-bit pattern specified for the PFC congestion notification profile, which you have
configured. Table 445 on page 3400 shows the one-to-one mapping between the UP field
of an IEEE 802.1Q tagged frame, the traffic class, and the egress queue. In addition to
setting a PFC congestion notification profile on an ingress port, you must set a forwarding
class to match the priority specified in the PFC congestion notification profile and to
forward the frame to the appropriate queue.
Chapter 113: Class of Service (CoS)—Overview
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