計算機學校--英文講座-資料下載頁
2024-12-08 12:15本頁面
【正文】 , lookup output port using forwarding table in input port memory ? goal: plete input port processing at ?line speed? ? queuing: if datagrams arrive faster than forwarding rate into switch fabric Physical layer: bitlevel reception Data link layer: ., Ether see chapter 5 Introduct by Weifeng Sun Network layer 28/36 Three types of switching fabrics Introduct by Weifeng Sun Network layer 29/36 Switching Via Memory First generation routers: ? traditional puters with switching under direct control of CPU ?packet copied to system?s memory ? speed limited by memory bandwidth (2 bus crossings per datagram) Input Port Output Port Memory System Bus Introduct by Weifeng Sun Network layer 30/36 Switching Via a Bus ? datagram from input port memory to output port memory via a shared bus ? bus contention: switching speed limited by bus bandwidth ? 32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers Introduct by Weifeng Sun Network layer 31/36 Switching Via An Interconnection Network ? overe bus bandwidth limitations ? Banyan works, other interconnection s initially developed to connect processors in multiprocessor ? advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. ? Cisco 12022: switches 60 Gbps through the interconnection work Introduct by Weifeng Sun Network layer 32/36 Output Ports ? Buffering required when datagrams arrive from fabric faster than the transmission rate ? Scheduling discipline chooses among queued datagrams for transmission Introduct by Weifeng Sun Network layer 33/36 Output port queueing ? buffering when arrival rate via switch exceeds output line speed ? queueing (delay) and loss due to output port buffer overflow! Introduct by Weifeng Sun Network layer 34/36 How much buffering? ? RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C – ., C = 10 Gps link: Gbit buffer ? Recent remendation: with N flows, buffering equal to RTT C . N Introduct by Weifeng Sun Network layer 35/36 Input Port Queuing ? Fabric slower than input ports bined queueing may occur at input queues ? HeadoftheLine (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward ? queueing delay and loss due to input buffer overflow! Introduct by Weifeng Sun Network layer 36/36 Network Layer 436 The Inter Network layer forwarding table Host, router work layer functions: Routing protocols ?path selection ?RIP, OSPF, BGP IP protocol ?addressing conventions ?datagram format ?packet handling conventions ICMP protocol ?error reporting ?router “signaling” Transport layer: TCP, UDP Link layer physical layer Network layer
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