BROCADE IP PRIMER

Chapter 3: Data Center Bridging (DCB)For more about TRILL, see “Chapter 4” starting on page 89.Making Ethernet LosslessIn its original form, Ethernet was designed as best-effort delivery architecture.It does its best to be sure that a frame is correctly transmitted from one nodeto another until it has reached its final destination. As a result, Ethernet is alossy transport that is not suitable for transporting storage data, whichrequires a lossless medium to ensure data delivery and protect against loss ofvaluable customer data.As originally designed, many Ethernet nodes would listen to the same segmentof media and would copy all the frames they heard, but keep only thoseframes intended for that node. When a node wanted to transmit, it wouldbegin by listening to the media to be sure that no one else was transmitting. Ifso, it would wait until the media was silent and then begin to modulate theframe out onto the shared media. At the same time, the node would listen tothe media to be sure that it heard the same data that it was sending. If it heardthe same frame from beginning to end, then the node considered the framesent and would do no further work at the Ethernet layer to be sure that themessage arrived correctly.If the node happened to hear something other than what it was transmitting, itwould assume that another node began transmitting at about the same time.This node would then continue to transmit for a period to be sure that theother node was also aware of the collision but would then abort the frame.After a random time interval, the node would try to send the frame again. Byusing this approach, a node can be confident that a frame was sent correctlybut not whether the frame was received correctly.Ethernet implementations have moved from this shared-media approach toone in which each segment of media is shared by only two Ethernet nodes.Dual unidirectional data paths allow the two nodes to communicate with eachother simultaneously without fear of collisions. Although this approachaddresses how frames are delivered between Ethernet nodes, it doesn'tchange the behavior of how frames are treated once they're received.The rules of Ethernet allow a node to throw away frames for a variety of reasons.For example, if a frame arrives with errors, it's discarded. If a nonforwardingnode receives a frame not intended for it, it discards the frame. Butmost significantly, if a node receives an Ethernet frame and it has no data bufferin which to put it, according to the rules of Ethernet, it can discard theframe. It can do this because it's understood that nodes implementing theEthernet layer all have this behavior. If a higher-level protocol requires a losslesstransmission, another protocol must be layered on top of Ethernet toprovide it.Consider an implementation of the FTP protocol running across an Ethernetnetwork. FTP is part of the TCP/IP tool suite. This means that from the bottomlayer up, FTP is based on Ethernet, IP, TCP, and finally FTP itself. Ethernet doesnot guarantee that frames will not be lost and neither does IP. The TCP layer is78 Brocade IP Primer