Cisco Cisco Nexus 5010 Switch Libro bianco

White Paper

Page 1 of 8

Priority Flow Control: Build Reliable Layer 2 Infrastructure

What You Will Learn

This document describes priority flow control (PFC), one of the functions of IEEE 802.1 Data Center Bridging,

introduced to address reliability at Layer 2 on the Ethernet medium. PFC enables lossless semantics for a subset of

the Layer 2 flows carried on an Ethernet segment. The subset of flows subject to the lossless semantics is identified

by the IEEE 802.1p class of service (CoS) information found in the IEEE 802.1Q tags. PFC is currently being defined

in the IEEE 802.1Qbb workgroup. Although the Cisco

proposal for PFC has not achieved standards status yet, a

significant number of vendors have embraced the frame format proposed by Cisco, and therefore PFC

implementations can be deployed today without risk of future incompatibility in that respect. PFC is an important

requirement for enabling Fibre Channel over Ethernet (FCoE) for storage consolidation, and this document describes

how the Cisco Nexus

™

5000 Series Switches use PFC to build a lossless Layer 2 medium for FCoE.

What Is Priority Flow Control?

Traditional IEEE 802.3 Ethernet defines an unreliable communication medium; it does not offer guarantees that a

packet injected into the network will arrive at its intended destination. Reliability is expected by means of upper-layer

protocols and is outside the scope of the initial definition.

In a network path that normally consists of multiple hops between source and destination, lack of feedback between

transmitters and receivers at each hop is one of the main causes of unreliability. Transmitters can send packets faster

than receivers accept packets, and as the receivers run out of available buffer space to absorb incoming flows, they

are forced to silently drop all traffic that exceeds their capacity. These semantics work fine at Layer 2, so long as

upper-layer protocols handle drop-detection and retransmission logic.

For applications that cannot build reliability on upper layers, the addition of flow control functions at Layer 2 can offer

a solution. Flow control enables feedback from a receiver to its sender to communicate buffer availability. Its first

implementation in IEEE 802.3 Ethernet uses the IEEE 802.3x PAUSE control frames. IEEE 802.3x PAUSE is defined

in Annex 31B of the IEEE 802.3 specification. Simply put, a receiver can generate a MAC control frame and send a

PAUSE request to a sender when it predicts the potential for buffer overflow. Upon receiving a PAUSE frame, the

sender responds by stopping transmission of any new packets until the receiver is ready to accept them again.

IEEE 802.3x PAUSE works as designed, but it suffers a basic disadvantage that limits its field of applicability: after a

link is paused, a sender cannot generate any more packets. As obvious as that seems, the consequence is that the

application of IEEE 802.3x PAUSE makes an Ethernet segment unsuitable for carrying multiple traffic flows that might

require different quality of service (QoS). Thus, enabling IEEE 802.3x PAUSE for one application can affect the

performance of other network applications.

IEEE 802.1Qbb PFC extends the basic IEEE 802.3x PAUSE semantics to multiple CoSs, enabling applications that

require flow control to coexist on the same wire with applications that perform better without it. PFC uses the IEEE

802.1p CoS values in the IEEE 802.1Q VLAN tag to differentiate up to eight CoSs that can be subject to flow control

independently. The differences between IEEE 802.3x PAUSE and PFC frames are shown in Figure 1.