Switching via interconnection network s forwards multiple packets in arable go banyan networks, crossbar other interconnection nets initially developed to connect processors in multiprocessor ABc When packet from port A needs to forwarded to port y crossbar controller closes cross point at X Y Z intersection of two buses advanced design: fragmenting datagram into fixed Tength cells switch cells through the fabric Network Layer 4-27
Network Layer 4-27 Switching via interconnection network ❖ forwards multiple packets in parallel ❖ banyan networks, crossbar, other interconnection nets initially developed to connect processors in multiprocessor ❖ When packet from port A needs to forwarded to port Y, controller closes cross point at intersection of two buses ❖ advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. crossbar A B C X Y Z
Output ports datagram switch buffer link fabric layer protocol termination (send) queueing s buffering required when datagrams arrive from fabric faster than the transmission rate scheduling discipline chooses among queued datagrams for transmission Network Layer 4-28
Network Layer 4-28 Output ports ❖ buffering required when datagrams arrive from fabric faster than the transmission rate ❖ scheduling discipline chooses among queued datagrams for transmission line termination link layer protocol (send) switch fabric datagram buffer queueing
Output port queueing switch switch fabric fabric at t, packets more one packet time later from input to output s suppose Rswitch is N times faster than Rline ws still have output buffering when multiple inputs send to same output queueing(delay) and loss due to output port buffer overflow! Network Layer 4-29
Network Layer 4-29 Output port queueing ❖ suppose Rswitch is N times faster than Rline ❖ still have output buffering when multiple inputs send to same output ❖ queueing (delay) and loss due to output port buffer overflow! at t, packets more from input to output one packet time later switch fabric switch fabric
How much buffering? RFC 3439 rule of thumb: average buffering equal to"typical"rTT(say 250 msec) times link capacity C e.g., C=10 Gpbs link: 2.5 Gbit buffer s recent recommendation with N flows buffering equal to RTT·C Network Layer 4-30
Network Layer 4-30 How much buffering? ❖ RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C ▪ e.g., C = 10 Gpbs link: 2.5 Gbit buffer ❖ recent recommendation: with N flows, buffering equal to RTT C. N
工 nput port queuing fabric slower than input ports combined t queuing may occur at input queues queuing delay and loss due to input buffer overflow s Head-of-the-Line(HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward switch switch fabric fabric/ output port contention one packet time only one red datagram can be later: green packet transferred experiences HOL lower red packet is blocked blocking Network Layer 4-31
Network Layer 4-31 Input port queuing ❖ fabric slower than input ports combined queuing may occur at input queues ▪ queuing delay and loss due to input buffer overflow! ❖ Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward output port contention: only one red datagram can be transferred. lower red packet is blocked switch fabric one packet time later: green packet experiences HOL blocking switch fabric