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Packet Switching Packet Switching Switching and Forwarding Bridges and LAN Switches Cell Switching Switching Hardware Spring 2019 © CS 438 Staff, University of Illinois 1 Where are We? n Directly connected networks ¡ Point-to-point connectivity n Limitations ¡ Number of hosts ¡ Geographic area n Next Goal ¡ Communication between hosts that are not directly connected Spring 2019 © CS 438 Staff, University of Illinois 2 Next Topic: Switching n Motivation ¡ Why not just one direct link network? n Basic approach ¡ How can we extend the direct link abstraction? n Challenges ¡ What problems must we address? n Examples ¡ Where are these issues addressed in real networks? n Details of the switch ¡ What are the goals in design? How are these goals typically addressed? Spring 2019 © CS 438 Staff, University of Illinois 3 After Switching n Heterogeneity ¡ Switching allows multiple physical networks ¡ But assumes one switching strategy n Scale ¡ Direct link networks: O(100) hosts ¡ Packet-switched networks: O(100,000) hosts n Beyond the basics ¡ Quality of service ¡ Congestion and performance analysis ¡ Network trends and their importance Spring 2019 © CS 438 Staff, University of Illinois 4 How can many hosts communicate? n Best: use one direct link network n Naïve approach: full mesh n Problem: doesn't scale Spring 2019 © CS 438 Staff, University of Illinois 5 How can many hosts communicate? n Slightly better solution: shared media n But, how to deal with larger networks (more hosts, larger geographic area)? n How to deal with heterogeneous media? (different physical/MAC technologies? Spring 2019 © CS 438 Staff, University of Illinois 6 Real Solution: Switching n Most basic function: allow communication between arbitrary endpoints ¡ Provide illusion of one (reliable) physical network ¡ On demand rather than connecting all pairs in advance ¡ Share infrastructure across users (statistical multiplexing) Spring 2019 © CS 438 Staff, University of Illinois 7 Real Solution: Switching Telephone operator at Aberdeen proving grounds, ~1940 Telephone exchange in 1893 Spring 2019 © CS 438 Staff, University of Illinois 8 Real Solution: Switching n Computer networks: more critical benefits ¡ Translates between different link technologies, allowing heterogeneity ¡ Isolates different physical networks to improve scalability and modularity Spring 2019 © CS 438 Staff, University of Illinois 9 Switches n Build network from stars Spring 2019 © CS 438 Staff, University of Illinois 10 Challenges New York Which way should they go? Boston first challenge: forwarding Spring 2019 © CS 438 Staff, University of Illinois 11 Challenges bookstore host (books.com) coffee shop (chat server) host second challenge: How do we maintain routing forwarding information? Spring 2019 © CS 438 Staff, University of Illinois 12 Challenges New York Who goes first? Boston third challenge: contention Spring 2019 © CS 438 Staff, University of Illinois 13 Challenges n Efficient forwarding ¡ Switch with several output ports ¡ Decide which output port to use n Routing in dynamic network ¡ Need information for forwarding ¡ Construct and maintain the information n Handling contention ¡ Multiple packets destined for one output port ¡ Decide which packet goes first ¡ Decide what to do with others Spring 2019 © CS 438 Staff, University of Illinois 14 Network Layers and Switches Application Application Host Presentation Presentation Session Session Transport Switch Transport Network Network Network Data Link Data Link Data Link Physical Physical Physical Spring 2019 © CS 438 Staff, University of Illinois 15 Network Layers and Switches switch between different physical layers Switch Switch Network Network Data Link A Data Link B Physical A Physical B Spring 2019 © CS 438 Staff, University of Illinois 16 Switching task: Translate betw. lower-layer technologies Router Network Data Link A Data Link B Physical A Physical B Ethernet802.11 IP TCP interface #1 data interface #2 Header header header(wireless 802.11) (wired ethernet) Spring 2019 © CS 438 Staff, University of Illinois 17 Switching and Forwarding n Switch ¡ A switch forwards packets from input ports to output ports n Port selection is based on the destination address in the packet header ¡ Advantages n Can build networks that cover large geographic areas n Can build networks that support large numbers of hosts n Can add new hosts without affecting the performance of existing hosts T3 T3 Input T1 Switch T1 Output Ports STS-1 STS-1 Ports Spring 2019 © CS 438 Staff, University of Illinois 18 Router/switch Architecture Update 12.0.0.0/8 Protocol daemon RIB 12.0.0.0/8 à IF 1 FIB 12.0.0.0/8 à IF 1 IF 1 IF 3 12.10.2.3 Data packet IF 2 IF 4 Spring 2019 Interconnection© CS 438 Staff, University network of Illinois 19 Switching and Forwarding n Forwarding ¡ The task of specifying an appropriate output port for a packet n Datagram n Virtual Circuit Switching n Source Routing ¡ Each packet contains enough information for a switch to determine the correct output port ¡ Later n Building forwarding tables – routing. Packet Header Output Port Specification Spring 2019 © CS 438 Staff, University of Illinois 20 Forwarding n Packet switching ¡ Data traffic divided into packets n Each packet contains its own header (with address) n Packets sent separately through the network ¡ Destination reconstructs the message ¡ Example: sending a letter through the postal system n Circuit switching ¡ Source first establishes a connection to the destination n Each router on the path may reserve bandwidth ¡ Source sends data over the connection n No destination address, since routers know the path ¡ Source tears down connection when done ¡ Example: making a phone call on the telephone network Spring 2019 © CS 438 Staff, University of Illinois 21 Forwarding with Datagrams Destination is E, so I should forward out E DATA interface #2 Host Host A B 0 0 0 Host 3 a 1 3 b 1 3 g 1 Host G C 2 2 2 0 Host D Host 3 d 1 Host F E Destination is E, so I should 2 Destination is E, forward out so I should interface #1 forward out Spring 2019 © CS 438 Staff, University of Illinois interface #1 22 Forwarding with Datagrams n Connectionless ¡ Each packet travels independently n Switch ¡ Translates global address to output port ¡ Maintains table of translations n Used in traditional data networks ¡ i.e., Internet Spring 2019 © CS 438 Staff, University of Illinois 23 Forwarding Table Each switch maintains as Table bs Table gs Table ds Table a forwarding table that A 0 A 3 A 3 A 0 translates a host name B 1 B 0 B 3 B 0 to an output port C 1 C 1 C 1 C 0 A B D 1 D 1 D 2 D 0 0 0 0 3 1 3 1 3 1 G a b g C E 1 E 2 E 3 E 1 2 2 2 0 F 1 F 2 F 3 F 3 3 1 D F d E G 3 G 3 G 3 G 0 2 Spring 2019 © CS 438 Staff, University of Illinois 24 Forwarding TaBle Each switch maintains as Table bs Table gs Table ds Table a forwarding table that A 0 A 3 A 3 A 0 translates a host name B 1 B 0 B 3 B 0 to an output port C 1 C 1 C 1 C 0 E DATA A B D 1 D 1 D 2 D 0 0 0 0 3 1 3 1 3 1 G a b g C E 1 E 2 E 3 E 1 2 2 2 0 F 1 F 2 F 3 F 3 3 1 D F d E G 3 G 3 G 3 G 0 2 Spring 2019 © CS 438 Staff, University of Illinois 25 Forwarding with Datagrams A ® E A B A sends: E DATA 0 0 0 3 1 3 1 3 1 G a b g C C ® F 2 2 2 C sends: F DATA 0 3 1 D F d E B ® E 2 B sends: E DATA F ® G What happens to the F sends: G DATA last packet? A ® H A sends: H DATA Spring 2019 © CS 438 Staff, University of Illinois 26 Forwarding with Datagrams n Analogous to following signs n Advantages ¡ Can send packets immediately ¡ All necessary information in packet header ¡ Can route around failures dynamically n Disadvantages ¡ Header requires full unique address ¡ Might not be possible to deliver packet ¡ Successive packets may not follow the same route ¡ Global address to path translations requires significant storage Spring 2019 © CS 438 Staff, University of Illinois 27 Forwarding n Packet switching ¡ Data traffic divided into packets n Each packet contains its own header (with address) n Packets sent separately through the network ¡ Destination reconstructs the message ¡ Example: sending a letter through the postal system n Circuit switching ¡ Source first establishes a connection to the destination n Each router on the path may reserve bandwidth ¡ Source sends data over the connection n No destination address, since routers know the path ¡ Source tears down connection when done ¡ Example: making a phone call on the telephone network Spring 2019 © CS 438 Staff, University of Illinois 28 Forwarding with Circuit Switching Data from Circuit establishment interface #3 Only for A Þ E should be sent to interface #2 Host Host A B 0 0 0 Host 3 a 1 3 b 1 3 g 1 Host G C 2 2 2 0 Host D Host 3 d 1 Host F E Data from interface #0 2 Data from should be sent to interface #0 interface #1 should be sent to Spring 2019 © CS 438 Staff, University of Illinois interface #1 29 Forwarding with Virtual Circuits n Connection oriented ¡ Requires explicit setup and teardown ¡ Packets follow established route n Why support connections in a network? ¡ Useful for service notions ¡ Important for telephony Spring 2019 © CS 438 Staff, University of Illinois 30 Virtual Circuits: The Circuit n Logical circuit between source and destination ¡ Packets from different VCs multiplex on a link ¡ Used in ATM, MPLS, Intserv Link #14 Link #7 Link #8 Spring 2019 © CS 438 Staff, University of Illinois 31 Virtual Circuits: The Circuit n Identified by Virtual Circuit Identifier (VCI) ¡ Source setup: establish path for each VC ¡ Switch: mapping VCI to outgoing link ¡ Packet: fixed-length label in packet header 1:7 1:14 2:7
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