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Routing vs. Switching. S. Keshav Cornell University IEEE INFOCOM ‘97. Router look up destination port based on destination address send variable length packet to destination port RSVP signaling for establishing QoS state for scheduling schedule variable length packet. Switch

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Routing vs switching l.jpg

Routing vs. Switching

S. Keshav

Cornell University

IEEE INFOCOM ‘97


What s the difference l.jpg

Router

look up destination port based on destination address

send variable length packet to destination port

RSVP signaling for establishing QoS state for scheduling

schedule variable length packet

Switch

look up destination port based on VCI

send fixed length packet to destination port

UNI signaling to establish QoS state for scheduling

schedule fixed length packet

What’s the difference?


Four differences l.jpg
Four differences

  • Lookup

  • Data movement: fixed vs. variable length

  • Signaling: RSVP vs. UNI

  • Scheduling: fixed vs. variable length

  • Differences are rapidly disappearing


Lookup l.jpg
Lookup

  • VCI lookup was much faster and cheaper

  • Not any more!

  • Several fast lookup schemes are known

    • (all are probably being patented!)


Switching l.jpg
Switching

  • Variable size is harder to switch

  • But we can segment and reassemble within a router

  • Or shared memory allows fixed-size headers to be switched


Signaling l.jpg
Signaling

  • Both UNI and RSVP are complex

  • Timers make tuning and debugging hard

  • UNI 4.0 and RSVP are converging


Scheduling l.jpg
Scheduling

  • FIFO is easy for both

  • More complicated scheduling (such as FQ) is harder with variable size packets

    • but ASICs solve the problem

    • may need them anyway even with ATM

  • Large packets cause jitter in slow lines

    • not a problem with non-interactive apps or faster trunks


Bottom line l.jpg
Bottom line

  • Technical reasons to prefer ATM switching are fading fast

  • IP has a greater established base

  • Is it time to bury ATM?


Another grave problem l.jpg
Another grave problem

  • Do we really need QoS in the network?

  • Big and dumb may be the answer

  • A rising tide raises all ships


Research agenda l.jpg
Research agenda

  • Fast IP routers

  • Retrofit a smidgeon of QoS

  • Capacity planning

  • Pricing

  • (Lightweight signaling)


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