1. SYST5030/4030 Switching Circuit vs. Packet Types: –Datagram –Virtual circuit –Routers and routing –Organizing autonomous systems –Tracing routes

2. SYST5030/4030 Circuit versus Packet Switching Circuit Switching Source: Tannenbaum, Computer Networks, 3rd Ed., 1996 Packet Switching

3. SYST5030/4030 Boulder Atlanta Chicago

4. SYST5030/4030 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. SYST5030/4030 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. SYST5030/4030 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. SYST5030/4030 Switch (Catalyst 5500 switch from Cisco) Source: http://www.visi.com/~gje/atwork.html

8. SYST5030/4030 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. SYST5030/4030 Virtual Circuit Packet Switching [Source: Tannenbaum, Computer Networks, 3ed.]

10. SYST5030/4030 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. SYST5030/4030 Warriors of the Net

12. SYST5030/4030 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. SYST5030/4030 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. SYST5030/4030 The Workings of one Router Source: Tannenbaum, Computer Networks, 3rd Ed., 1996

15. SYST5030/4030 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. SYST5030/4030 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. SYST5030/4030 Congestion in packet switching networks Perfect Desirable Congested Packets Sent Packets Delivered How to handle congestion Monitor system for problem points Transmit this information regularly Adjust operation: i.e. reduce load, degrade service, reschedule.

18. SYST5030/4030 What is the impact of packet size on message transmission delay 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. Should we transmit the message in one large packet or several small ones?

19. SYST5030/4030 General Formula Total message delay = Transmission time for one packet over a link x (# of packets + # of links – 1)

20. SYST5030/4030 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. SYST5030/4030 Traceroute Program output > traceroute ufl.edu traceroute to ufl.edu (128.227.128.24), 30 hops max, 40 byte packets 1 cns-gw (128.138.129.1) 2 ms 2 ms 2 ms 2 cuatm-gw (128.138.138.10) 3 ms 2 ms 2 ms 3 ncaratm-cuatm.co.westnet.net (204.131.62.14) 3 ms 3 ms 3 ms 4 border2-hssi1-0.Denver.mci.net (204.70.29.5) 4 ms 5 ms 4 ms 5 core1-fddi-1.Denver.mci.net (204.70.3.113) 4 ms 5 ms 5 ms 6 pacbell-nap-atm.SanFrancisco.mci.net (204.70.1.202) 29 ms 31 ms 29 ms 7 pacbell-nap-atm.SanFrancisco.mci.net (204.70.1.202) 29 ms 30 ms 29 ms 8 ca-nap.grid.net (198.32.128.28) 33 ms 34 ms 36 ms 9 206.80.168.74 (206.80.168.74) 112 ms 117 ms 112 ms 10 172.17.80.38 (172.17.80.38) 122 ms 115 ms 121 ms 11 172.21.208.10 (172.21.208.10) 166 ms 124 ms 120 ms 12 ssrbso-cis-e0-b138.nerdc.ufl.edu (128.227.138.1) 136 ms 153 ms 127 ms 13 ssrb-cis-f30-b252.nerdc.ufl.edu (128.227.252.10) 135 ms 137 ms 134 ms 14 name.ufl.edu (128.227.128.24) 137 ms 147 ms 132 ms (Note: this command is called tracert on DOS)

22. SYST5030/4030 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.]