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Switching

Switching. Circuit vs. Packet Types: Datagram Virtual circuit Routers and routing Organizing autonomous systems Tracing routes. Packet Switching. Circuit Switching. Packet Switching. Circuit versus Packet Switching. Source: Tannenbaum, Computer Networks, 3rd Ed., 1996. Chicago.

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Switching

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  1. Switching • Circuit vs. Packet • Types: • Datagram • Virtual circuit • Routers and routing • Organizing autonomous systems • Tracing routes

  2. Packet Switching Circuit Switching Packet Switching Circuit versus Packet Switching Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

  3. Chicago Boulder Atlanta

  4. Packet Switching • The Internet is a packet switching network. • At sending end: • Following information is attached to each packet: • Message is split into packets (say, 128 bytes long) • - Address of destination • - Packet number (i.e., Packet 5 of 9) • - Error checking field (16 bits long) • At destination: • Packets are reassembled in the correct order • Additional information is stripped away • Original message is reconstructed. • Packet switching technologies: TCP/IP, X.25, Frame relay, ATM.

  5. Comparison between packet switching networks and circuit switching networks • Circuit Switching • Dedicated transmission path • Continuous transmission • No storage of data/messages • Same path for duration of • connection • Call setup required • Overload may block call • Busy signal if callee busy • Fixed bandwidth transmission • Charging by minute • Fast enough for Interactive • Packet Switching • No dedicated path • Transmission of packets • Packets may be stored • Each packet may take • different route • No call setup needed • overload increases packet delay • no busy signal • dynamic use of bandwidth • Charging by packet • Fast enough for Interactive (?)

  6. Advantages of packet switching networks over circuit switching networks • No end to end connection • Greater flexibility in routing • Greater reliability • Better circuit utilization • Less congestion • Better response time

  7. Switch (Catalyst 5500 switch from Cisco) Source: http://www.visi.com/~gje/atwork.html

  8. Types of packet switching • Datagram packet switching: • Each packet may follow its own route, no notion of a call setup. • This is called connectionless, no-acknowledgement form of communication. • Virtual Circuit packet switching: • A call is established (setup) yet there is no continuous connection. • The call setup determines the routing path of the packets and all packets follow the same path for the duration of the call. • A virtual circuit has a virtual circuit identifier (VCI) at each link to identify the circuit. • After transmission is complete, the call is cleared. • Virtual circuits can be switched (SVC) or permanent (PVC). • A PVC is like a leased line connection conceptually. • Useful for large file transmission, e.g. with ftp.

  9. Virtual Circuit Packet Switching [Source: Tannenbaum, Computer Networks, 3ed.]

  10. Routing in packet switching networks • Two routing strategies • Centralized routing: one node acts as routing manager and calculates “optimal” paths. • Problem: • -- Routing manager is bottleneck. • -- What if it fails? • -- Anomalies can arise because of delays in communicating • new routes to nodes. • Distributed routing: • Each node acts as a routing manager and calculates best route. Exchanges status information with other nodes periodically. • -- This works better in practice. Routing information is stored in routing tables.

  11. Warriors of the Net

  12. Routing Algorithms • Nonadaptive Algorithms (static routing) • Choice of route computed in advance • Choice downloaded to routers upon booting • Adaptive algorithms • Change routing decisions to reflect: • Changes in topology • Changes in traffic • Get information from adjacent routers or all routers

  13. Specific Algorithms • Shortest path routing • Number of hops/physical distance/fastest path • Flooding • Every incoming packet sent on every outgoing line, except the one it came in on • Hop counter used to avoid over-saturation • Distance vector • Each router maintains table of best known distance to each destination, and which output line to use • Flow-based, count-to-infinity, split horizon hack, link state, hierarchical, broadcast, multicast

  14. The Workings of one Router Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

  15. Organizing the Internet as Autonomous Systems AS - Autonomous System. BGP - Border Gateway Protocol Each AS handles its own routing within the AS. BGP protocol does routing across various ASes.

  16. More on Autonomous Systems • An autonomous system is a set of routers having a single routing policy, running under a single technical administration • The main goal of an interior gateway protocol is to route efficiently, while the exterior gateway protocols have to worry about “politics” • BGP4 is the de facto standard for exterior gateway protocol in the Internet.

  17. Congestion in packet switching networks Perfect Desirable Packets Delivered Congested Packets Sent • How to handle congestion • Monitor system for problem points • Transmit this information regularly • Adjust operation: i.e. reduce load, degrade service, reschedule.

  18. What is the impact of packet sizeon message transmission delay Should we transmit the message in one large packet or several small ones? • Large packet size implies fewer packets. • Each packet has a fixed overhead (number of bytes). • Fewer packets implies fewer total number of bits. • However, if there are fewer packets, there is less parallelism, i.e., there is less simultaneous transmission. • Need to evaluate the tradeoff based on actual parameters.

  19. General Formula Total message delay = Transmission time for one packet over a link x (# of packets + # of links – 1)

  20. Traceroute Program • Way to determine the path to a remote host. • Your host sends three packets (datagrams) to the remote host • Output of program displays: • All the routers along the way which received the packet • Round-trip time to reach those routers. • If no response received in 5 seconds • Asterisk is displayed, and • Another datagram is sent [Check out www.visualroute.com for a visual traceroute program.]

  21. Summary Packet switching is a very important idea because the entire Internet is based on it. Increasingly even real time applications like voice and video are possible on packet switching networks. Issues: 1. Switching 2. Routing 3. Delay 4. Congestion 5. Packet size Tools Traceroute, ping. [Note: We will discuss routing in more detail later in the semester and study specific protocols.]

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