Understanding Priority-Based Flow Control; Reliability Of Packet Delivery In Standard Ethernet Networks And In Layer - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

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Understanding Priority-Based Flow Control

Reliability of Packet Delivery in Standard Ethernet Networks and in Layer 2 Networks
Calculations for Buffer Requirements When Using PFC PAUSE
Copyright © 2010, Juniper Networks, Inc.
Native FC traffic from storage devices flows to the FCF FC ports, and the FCF encapsulates
that traffic in Ethernet as FCoE traffic. The FCoE traffic flows through the transit switch
to the appropriate server, and the server decapsulates the traffic.
Understanding FIP Snooping on page 3579
Priority-based flow control (PFC), IEEE 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, PFC
allows you to selectively pause traffic according to its class.
This topic describes:

Reliability of Packet Delivery in Standard Ethernet Networks and in Layer 2

Networks on page 3583
Calculations for Buffer Requirements When Using PFC PAUSE on page 3583
How PFC and Congestion Notification Profiles Work on page 3584
Standard Ethernet does not guarantee that a packet injected into the network will arrive
at its intended destination. Reliability is provided by upper-layer protocols. Generally, a
network path consists of multiple hops between the source and destination. A problem
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:
Chapter 127: Fibre Channel over Ethernet (FCoE)—Overview
3583