BROCADE IP PRIMER

The Ethernet PAUSE Functionresponsible for monitoring data transmitted between the FTP client and server,and if any data is lost, corrupted, duplicated, or arrives out of order, TCP willdetect and correct it. It will request the retransmission of data if necessary,using the IP and Ethernet layers below it to move the data from node to node.It will continue to monitor, send, and request transmissions until all the necessarydata has been received reliably by the FTP application.The Ethernet PAUSE FunctionA decision was made by the IEEE committee members that lossless behaviorfor Ethernet would be implemented by using a variant of the PAUSE functioncurrently defined as part of the Ethernet standard. The PAUSE function allowsan Ethernet node to send a PAUSE frame to an adjacent node. The PAUSEsemantics require the receiving node not to send any additional traffic until acertain amount of time has passed. This time is specified by a field in thePAUSE frame.Using this approach, lossless behavior can be provided if a receiving nodeissues PAUSE requests when it doesn’t have any buffer space available toreceive frames. It assumes that by the time the PAUSE request expires, therewill be sufficient buffer space available. If not, it is the responsibility of thereceiving node to issue ongoing PAUSE requests until sufficient buffer spacebecomes available.The PAUSE command provides a mechanism for lossless behavior betweenEthernet nodes, but it’s only suited for links carrying one type of data flow.Remember that one of the goals of FCoE is to allow for I/O consolidation, withTCP/IP and Fibre Channel traffic converged onto the same media. If the PAUSEcommand is used to guarantee that Fiber Channel frames are not dropped asis required by that protocol, then as a side effect, TCP/IP frames will also bestopped once a PAUSE command is issued. The PAUSE command doesn't differentiatetraffic based on protocols. It pauses all traffic on the link betweentwo nodes, even control commands.So a conflict between what must be done to accommodate storage traffic inFCoE and TCP/IP traffic—both of which need to coexist on the same segmentof media. And problems could arise because one type of network traffic mayinterfere with the other.Suppose, for example, that storage traffic is delayed because of a slow storagedevice. In order to not lose any frames relating to the storage traffic, aPAUSE command is issued for a converged link carrying both FCoE and TCP/IPtraffic. Even though the TCP/IP streams may not need to be delayed, they willbe delayed as a side effect of having all traffic on the link stopped. This in turncould cause TCP time-outs and may even make the situation worse asretransmit requests for TCP streams add additional traffic to the already congestedI/O link.The solution to this problem is to enable Ethernet to differentiate between differenttypes of traffic and to allow different types of traffic to be pausedindividually if required.Brocade IP Primer 79