Internetworking, WANs, and Dynamic Routing

Transcription

1 Internetworking, WANs, and Dynamic Routing Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya and lecture slides from Anan Phonphoem 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. The McGraw-Hill Companies, Inc.

2 Internetworks Internetwork = Network of networks Two or more networks connected become an internetwork, or internet KU Network TU Network CU Network 2

3 Internetworks Internetworking two LANs with a MAN or a WAN Bangkhen Obvious example The Internet Kampangsaen 3

4 The Internet (Conceptual View) ISP: Internet Service Provider NAP: Network access point (switching station) 4

5 Wide Area Network (WAN) Enterprise Network: WAN owned by a company 5

6 Traditional WAN Architecture 6

7 Traditional WAN Architecture LAN WAN 7

8 WAN Connection DCE generates clock for DTE generates clock WAN DTE DCE DCE DTE DCE Data Circuit-terminating Equipment DTE Data Terminal Equipment 8

9 WAN Devices CSU/DSU or Modem (DCE) Router (DTE) To WAN V.35 serial cable 9

10 Example of WAN Topology These packet switches form a packet switching network 10

11 Store and Forward Paradigm A packet switch stores packets in memory The forward operation occurs once a packet has arrived and is waiting in memory. The processor examines the packet determines its destination and sends the packet over the I / O interface that leads to the destination 11

12 Addressing in a WAN WANs addresses follow a key concept that is used in the Internet: hierarchical addressing Hierarchical addressing divides each address into two parts: (site, computer at the site) In practice, instead of a identifying a site, each packet switch is assigned a unique number first part of an address identifies a packet switch second part identifies a specific computer 12

13 Addressing in WAN 13

14 Next-Hop Forwarding Alaska New York Germany Bangkok Bangkok Germany Bangkok New Alaska York Alaska Next hop keep changing 14

15 Source Independence Next hop depends on destination of the packet Not the source! Source Independence Bangkok Germany New York Alaska Forwarding packet uses the destination address in the packet 15

16 Next-Hop Forwarding Forwarding Table in Switch 2 Source E [2,1] Destination C [3,2] 16

17 Routing Tables The next-hop table is called Routing Table Process of forwarding packet Routing Large network Routing table can be very large 17

18 Dynamic Routing in a WAN We use the term routing software to describe software that automatically reconfigures forwarding tables Route computation in a WAN is to think of a graph that models the network Each node corresponds to a packet switch (individual computers are not part of the graph) An edge (link) denotes a direct connection between a pair of packet switches 18

19 WAN Routing node Edge WAN A Graph representation 19

20 Routing Table node Edge = (u,v) 20

21 Default Routes One default Lowest priority > 1 destination with same next-hop 21

22 Routing Table Construction Static Routing Manual configure Simple and low overhead Inflexible Dynamic Routing Automatic changing Change according to network problems Mostly use 22

23 Distributed Route Computation In practice, networks need to perform distributed route computation All packet switches must participate in distributed route computation No central entity to do computation There are two general forms: Link-State Routing (LSR) Distance-Vector Routing (DVR) 23

24 Link-State Routing (LSR) Also known as Shortest Path First (SPF) routing Dijkstra algorithm used it to characterize the way it works To use LSR, packet switches periodically send messages across the network that carry the status of a link Every switch collects incoming status messages and uses them to build a graph of the network 24

25 Dijkstra's Algorithm Uses a greedy approach to select the next node into the shortest path tree Assumes nonnegative weight edges 25

26 Dijkstra s Algorithm Animation 26

27 Distance Vector Routing (DVR) Uses Distributed Bellman-Ford Algorithm Like LSR, DVR arranges for packet switches to exchange messages periodically In DVR, a switch sends a complete list of destinations and the current cost of reaching each When it sends a DVR message a switch is sending a series of individual statements, of the form: I can reach destination X, and its current distance from me is Y 27

28 DVR Concept 28

29 Hop Count 1 hop 2 hops 29

30 Routing table distribution 30

31 Updating routing table For router A 31

32 Final routing tables 32

33 Updating the routing table Example 33

34 Routing Problems In theory, either LSR or DVR will compute shortest paths Furthermore, each approach will eventually converge meaning that the forwarding tables in all packet switches agree However, problems do occur For example, if LSR messages are lost, two packet switches can disagree about the shortest path DVR problems can be more severe because a link failure can cause two or more packet switches to create a routing loop in which each packet switch thinks the next packet switch in the set is the shortest path to a particular destination As a result, a packet can circulate among the switches indefinitely 34

35 WAN Technologies ARPANET X.25 Frame Relay Switched Multi-Megabit Data Service (SMDS) Asynchronous Transfer Mode (ATM) Multi-Protocol Label Switching (MPLS) Integrated Services Digital Network (ISDN) 35