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EE 122: Switching and Forwarding Kevin Lai September 23, 2002
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laik@cs.berkeley.edu2 Direct Link Network Review Data link layer presents a single media (e.g., single wire) network model Problem and solutions -Framing character stuffing, byte counting, bit stuffing, clocked framing -Error detection parity, checksum, CRC -Reliability stop and go, sliding window -solutions also apply to similar problems in higher layers problems can not be completely solved at data link layer only implemented in data link layer as optimization
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laik@cs.berkeley.edu3 Limitations of Direct Link Networks distance -distance increases propagation delay -large propagation delay causes large coordination delay -e.g., Ethernet collision detection requires 2*prop_delay number of hosts -More hosts increases the probability of collisions -collisions decrease efficiency of link bandwidth -bandwidth of link is shared among all connected nodes single media type -different media (e.g., fiber, Ethernet, wireless) have different tradeoffs for performance, cost, etc.
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laik@cs.berkeley.edu4 Direct Link Networks v.s. Switching Direct Link NetworkSwitched Network Single link n links SwitchEmulates clique
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laik@cs.berkeley.edu5 Definitions switch (aka bridge) -does switching -operates at data link layer -router also does switching, but at network layer switching consists of -forwarding read data from input links, decide which output link to forward on, and ¢ examine packet header or incoming circuit, and ¢ look up in forwarding table transmit it on one of the output links (unicast) -routing how the switch/router builds up its forwarding table
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laik@cs.berkeley.edu6 Properties spans larger physical area than direct link network (DLN) -can connect multiple switches together supports more hosts than DLN -hosts on separate links can transmit at same time higher aggregate bandwidth than DLN -approaches (n/2)*b instead of b, n = number of switched links, b = bandwidth of one link supports more than one media type -more expensive for bridge than router
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laik@cs.berkeley.edu7 Bridge/Router Comparison Router interconnects different link layer protocols more easily Ethernet 802.11b ATM SONET Ethernet 802.11b ATM SONET E-to-8 E-to-A E-to-S O(n 2 ) converters n = different link types Ethernet 802.11b ATM SONET Ethernet 802.11b ATM SONET Switch Router O(n) converters E-to-IPIP-to-E 8-to-IP S-to-IP A-to-IP IP-to-8 IP-to-S IP-to-A E-to-E
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laik@cs.berkeley.edu8 Forwarding Techniques packet switching -aka [packet|datagram|connectionless] [switching|forwarding] source routing virtual circuit switching -aka virtual circuit forwarding circuit switching despite names, all ways for switch to decide which output port to forward data
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laik@cs.berkeley.edu9 Packet Switching Data is separated into packets Each packet is forwarded independently of previous packets -packets between two hosts can follow different paths On link failure, adjoining switches select new route and continue forwarding packets Statistical multiplexing -any one host may use 100% of a link’s bandwidth
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laik@cs.berkeley.edu10 Statistical Multiplexing v.s. Resource Reservations Reserve explicit amount of resources (e.g., bandwidth) -get exactly that amount Statistical multiplexing: get whatever is available...... S H9 H0 10Mb/s / 10Mb/s... Statistical Multiplexing Resource Reservations AdvantageProblem 10Mb/s...... S H9 H0 congestion, packet loss... 10Mb/s...... S H9 H0... 1Mb/s 10Mb/s 1Mb/s...... S H9 H0... 1Mb/s 10Mb/s 0Mb/s 10Mb/s / 10Mb/s 1Mb/s / 10Mb/s low utilization
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laik@cs.berkeley.edu11 Packet Switching Operation Each switch maintains a forwarding table -forwarding entry: (address, output port) Upon packet arrival -input port forwards the packet to the output port whose address matches packet’s destination address exact match longest prefix match -forwarding entry: (address prefix, output port) -forward packet to the output port whose address matches packet’s destination address in the longest number of bits 2 12.xxx.xxx.xxx 1 12.82.xxx.xxx 1128.16.120.111 1 2 12.82.100.101
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laik@cs.berkeley.edu12 Packet Switching Properties Expensive forwarding -forwarding table size depends on number of different destinations -must lookup in forwarding table for every packet Robust -link and router failure may be transparent for end-hosts High bandwidth utilization -statistical multiplexing No service guarantees -Network allows hosts to send more packets than available bandwidth congestion dropped packets
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laik@cs.berkeley.edu13 Source Routing Each packet specifies the sequence of routers, or alternatively the sequence of output ports, from source to destination 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 4 source 34 434 434
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laik@cs.berkeley.edu14 Source Routing (cont’d) Gives the source control of the path Not scalable -Packet overhead proportional to the number of routers -Typically, require variable header length which is harder to implement Hard for source to have complete information Loose source routing sender specifies only a subset of routers along the path
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laik@cs.berkeley.edu15 Virtual Circuit (VC) Switching Packets not switched independently -establish virtual circuit before sending data Forwarding table entry -(input port, input VCI, output port, output VCI) -VCI – Virtual Circuit Identifier Each packet carries a VCI in its header Upon a packet arrival at interface i -Input port uses i and the packet’s VCI v to find the routing entry (i, v, i’, v’) -Replaces v with v’ in the packet header -Forwards packet to output port i’
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laik@cs.berkeley.edu16 VC Forwarding: Example 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 5 …… 114 …… … 3 … in out in-VCI 11 out-VCI … … 5 …… 73 …… … 2 … in out in-VCIout-VCI … … 11 7 …… 14 …… … 1 … in out in-VCIout-VCI … … 7 1 source destination
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laik@cs.berkeley.edu17 VC Forwarding (cont’d) A signaling protocol is required to set up the state for each VC in the routing table -A source needs to wait for one RTT (round trip time) before sending the first data packet Can provide per-VC QoS -When we set the VC, we can also reserve bandwidth and buffer resources along the path
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laik@cs.berkeley.edu18 Virtual Circuit Switching Properties Less expensive forwarding -forwarding table size depends on number of different circuits -must lookup in forwarding table for every packet Much higher delay for short flows -1 RTT delay for connection setup Less Robust -end host must spend 1 RTT to establish new connection after link and router failure Flexible service guarantees -either statistical multiplexing or resource reservations
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laik@cs.berkeley.edu19 Circuit Switching Packets not switched independently -establish circuit before sending data Circuit is a dedicated path from source to destination -e.g., old style telephone switchboard, where establishing circuit means connecting wires in all the switches along path -e.g., modern dense wave division multiplexing (DWDM) form of optical networking, where establishing circuit means reserving an optical wavelength in all switches along path No forwarding table
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laik@cs.berkeley.edu20 Circuit Switching Properties Cheap forwarding -no table lookup Much higher delay for short flows -1 RTT delay for connection setup Less robust -end host must spend 1 RTT to establish new connection after link and router failure Must use resource reservations
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laik@cs.berkeley.edu21 Forwarding Comparison pure packet switching virtual circuit switching circuit switching forwarding cost highlownone bandwidth utilization highflexiblelow resource reservations noneflexibleyes robustnesshighlow
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laik@cs.berkeley.edu22 Routing Update forwarding/routing tables Manual configuration -simple, error prone, work for administrator Learning bridges -all that is needed for single bridge Spanning Tree -necessary for multiple bridges Described in internetworking section -Distance Vector -Link State
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laik@cs.berkeley.edu23 Learning Bridges HostPort 1 2 3 1 2 3 AB AB AB HostPort A1 CA CA
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laik@cs.berkeley.edu24 Learning Bridge Problem B0 B1 B2 B3 H0 H1 HostPort HostPort HostPort HostPort 01 2 1 0 2 0 1 0 1 H1H0 H1H0 H1H0 HostPort H10 HostPort H11 HostPort H12 H0 H1H0 HostPort H10 HostPort H11 H0 H1H0
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laik@cs.berkeley.edu25 Spanning Tree B0 B1 B2 B3 H0 H1 HostPort H12 HostPort H11 HostPort H10 HostPort H11 01 2 1 0 2 0 1 0 1 RootDistPort B00- H1H0 RootDist B30 RootDist B20 “As if. I am” “Uh, no. B0/1 is.” RootDist B11 “I’m root” “B1/1 is root” RootDist B12 RootDist B02 “B1/2 is root” “Get out. I am.”“Cha. B0/3 is.” RootDistPort B10- RootDistPort B012
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laik@cs.berkeley.edu26 Summary Switching -overcome limitations of direct link networks Forwarding techniques -packet switching -source routing -virtual circuit switching -circuit switching Routing techniques
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