1 Switches and Bridges!COS 461: Computer Networks Spring 2009 (MW 1:30-2:50 in COS 105) Guest Lecture Jennifer Rexford2 Goals of Todayʼs Lecture!• Devices that shuttle data at different layers – Repeaters and hubs – Bridges and switches – Routers • Switch protocols and mechanisms – Dedicated access and full-duplex transfers – Cut-through switching – Self learning of the switch table – Spanning trees • Virtual LANs (VLANs)3 Message, Segment, Packet, and Frame!HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP IP Ethernet interface Ethernet interface SONET interface SONET interface host host router router HTTP message TCP segment IP packet IP packet IP packet Ethernet frame Ethernet frame SONET frame4 Shuttling Data at Different Layers!• Different devices switch different things – Network layer: packets (routers) – Link layer: frames (bridges and switches) – Physical layer: electrical signals (repeaters and hubs) Application gateway!Transport gateway!Router! Bridge, switch!Repeater, hub!Frame#header!Packet#header!TCP#header!User!data!5 Physical Layer: Repeaters!• Distance limitation in local-area networks – Electrical signal becomes weaker as it travels – Imposes a limit on the length of a LAN • Repeaters join LANs together – Analog electronic device – Continuously monitors electrical signals on each LAN – Transmits an amplified copy6 Physical Layer: Hubs!• Joins multiple input lines electrically – Designed to hold multiple line cards – Do not necessarily amplify the signal • Very similar to repeaters – Also operates at the physical layer hub hub hub hub7 Limitations of Repeaters and Hubs!• One large shared link – Each bit is sent everywhere – So, aggregate throughput is limited – E.g., three departments each get 10 Mbps independently – … and then connect via a hub and must share 10 Mbps • Cannot support multiple LAN technologies – Does not buffer or interpret frames – So, can’t interconnect between different rates or formats – E.g., 10 Mbps Ethernet and 100 Mbps Ethernet • Limitations on maximum nodes and distances – Shared medium imposes length limits – E.g., cannot go beyond 2500 meters on Ethernet8 Link Layer: Bridges!• Connects two or more LANs at the link layer – Extracts destination address from the frame – Looks up the destination in a table – Forwards the frame to the appropriate LAN segment • Each segment can carry its own traffic host host host host host host host host host host host host Bridge9 Link Layer: Switches!• Typically connects individual computers – A switch is essentially the same as a bridge – … though typically used to connect hosts, not LANs • Like bridges, support concurrent communication – Host A can talk to C, while B talks to D switch A!B!C!D!10 Dedicated Access and Full Duplex!• Dedicated access – Host has direct connection to the switch – … rather than a shared LAN connection • Full duplex – Each connection can send in both directions – Host sending to switch, and host receiving from switch – E.g., in 10BaseT and 100Base T • Completely supports concurrent transmissions – Each connection is a bidirectional point-to-point link11 Bridges/Switches: Traffic Isolation!• Switch breaks subnet into LAN segments • Switch filters packets – Frame only forwarded to the necessary segments – Segments can support separate transmissions hub hub hub switch/bridge segment segment segment12 Advantages Over Hubs/Repeaters!• Only forwards frames as needed – Filters frames to avoid unnecessary load on segments – Sends frames only to segments that need to see them • Extends the geographic span of the network – Separate segments allow longer distances • Improves privacy by limiting scope of frames – Hosts can “snoop” the traffic traversing their segment – … but not all the rest of the traffic • Can join segments using different technologies13 Disadvantages Over Hubs/Repeaters!• Delay in forwarding frames – Bridge/switch must receive and parse the frame – … and perform a look-up to decide where to forward – Storing and forwarding the packet introduces delay – Solution: cut-through switching • Need to learn where to forward frames – Bridge/switch needs to construct a forwarding table – Ideally, without intervention from network administrators – Solution: self-learning • Higher cost – More complicated devices that cost more money14 Motivation For Cut-Through Switching!• Buffering a frame takes time – Suppose L is the length of the frame – And R is the transmission rate of the links – Then, receiving the frame takes L/R time units • Buffering delay can be a high fraction of total delay – Propagation delay is small over short distances – Making buffering delay a large fraction of total – Analogy: large group walking through NYC A!B!switches15 Cut-Through Switching!• Start transmitting as soon as possible – Inspect the frame header and do the look-up – If outgoing link is idle, start forwarding the frame • Overlapping transmissions – Transmit the head of the packet via the outgoing link – … while still receiving the tail via the incoming link – Analogy: different folks crossing different intersections A!B!switches16 Motivation For Self Learning!• Switches forward frames selectively – Forward frames only on segments that need them • Switch table – Maps destination MAC address to outgoing interface – Goal: construct the switch table automatically switch A!B!C!D!17 Self Learning: Building the Table!• When a frame arrives – Inspect the source MAC address – Associate the address with the incoming interface – Store the mapping in the switch table – Use a time-to-live field to eventually forget the mapping A!B!C!D!Switch learns how to reach A.!18 Self Learning: Handling Misses!• When frame arrives with unfamiliar destination – Forward the frame out all of the interfaces – … except for the one where the frame arrived – Hopefully, this case won’t happen very often A!B!C!D!When in doubt, shout!!19 Switch Filtering/Forwarding!When switch receives a frame: index switch table
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