BROCADE IP PRIMER

Enhanced Tranmission SelectionFrom another perspective, consider that PFC attempts to emulate VirtualChannel (VC) technology widely deployed in current Brocade Fibre ChannelSANs. While borrowing the lossless aspect of VCs, PFC retains the option ofbeing configured as lossy or lossless. PFC is an enhancement to the currentlink level of Ethernet flow control mechanism defined in IEEE 802.3x (PAUSE).Current Ethernet protocols support the capability to assign different prioritiesto different applications, but the existing standard PAUSE mechanism ignoresthe priority information in the Ethernet frame.Triggering the PAUSE command results in the link shutting down, whichimpacts all applications even when only a single application is causing congestion.The current PAUSE is not suitable for links in which storage FCoE andnetworking applications share the same link, because congestion caused byany one of applications shouldn't disrupt the rest of the application traffic.IEEE 802.1Qb is tasked with enhancing the existing PAUSE protocol to includepriority in the frames contributing to congestion. PFC establishes eight prioritiesusing the priority code point field in the IEEE 802.1Q tags, which enablethe control of individual data flows, called flow control, based on the frame'spriority. Using the priority information, the peer (server or switch) stops sendingtraffic for that specific application, or priority flow, while other applicationsdata flows continue without disruption on the shared link.The new PFC feature allows FC storage traffic encapsulated in FCoE frames toreceive lossless service from a link shared with traditional LAN traffic, which isloss tolerant. In other words, separate data flows can share a common losslessEthernet, while each is protected from flow control problems of the otherflows—and everyone is happy. Note that LAN traffic priorities can be configuredwith PFC off, allowing for lossy or lossless LAN transmissions.Enhanced Tranmission SelectionWith the use of Priority Flow Control, it is possible to combine eight differentlevels or classes of traffic onto the same converged link. Each of these classescan be paused individually if necessary without interfering with other classes.PFC does not, however, specify how the bandwidth is to be allocated to separateclasses of traffic.Suppose, for example, that a particular application happens to hit a hot spotthat causes it to send a large number of TCP/IP messages. There is a goodchance that the transmission of all these messages could interfere with theoperating system's attempt to either retrieve or store block information fromthe storage network. Under these or similar circumstances, bandwidth starvationcould cause either the application or the operating system to crash.This situation doesn’t occur if separate channels are used for storage and nonstoragetraffic. A Fibre Channel-attached server could access its block trafficindependent of the messages traveling across an Ethernet TCP/IP connection.Competition for bandwidth occurs only when these two ordinarily independentstreams share a common link.Brocade IP Primer 81