Transcript variable length

Routing vs. Switching
S. Keshav
Cornell University
IEEE INFOCOM ‘97
What’s the difference?
 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
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Four differences
 Lookup
 Data movement: fixed vs. variable length
 Signaling: RSVP vs. UNI
 Scheduling: fixed vs. variable length
 Differences are rapidly disappearing
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Lookup
 VCI lookup was much faster and cheaper
 Not any more!
 Several fast lookup schemes are known
(all are probably being patented!)
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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
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Signaling
 Both UNI and RSVP are complex
 Timers make tuning and debugging hard
 UNI 4.0 and RSVP are converging
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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
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Bottom line
 Technical reasons to prefer ATM switching are fading fast
 IP has a greater established base
 Is it time to bury ATM?
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Another grave problem
 Do we really need QoS in the network?
 Big and dumb may be the answer
 A rising tide raises all ships
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Research agenda
 Fast IP routers
 Retrofit a smidgeon of QoS
 Capacity planning
 Pricing
 (Lightweight signaling)
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