1. ECE 4450:427/527 - Computer Networks Spring 2015 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 6.1: Internetworking Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 1

2. Internetworking: Discussions Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 2 Up to now, we have learned how to Connect one node to another Connect one node to an existing network, e.g. Ethernet “Last-mile” link: Connect to a modest number of node together: Ethernet 1024 nodes, point- to-point: 2 nodes Next, we will learn how to build networks of global scale: How to interconnect different types of links and networks: Internetworking

3. Internetworking: Discussions Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 3 For Internetworking, we shall look at few sub- problems: Interconnect links of the same type: Switches We consider an important of class switch: Bridges to interconnect Ethernet segments. We also look a way to interconnect disparate networks and links: Gateways, or now mostly known as routers. We shall focus on the IP Once we are able to interconnect a whole lot of links and networks with switches and routers, we will look at a way to find a suitable path, or route through a new working: Paths that are efficient, loop free, etc.: Routing

4. Internetworking: Our Goal Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 4 Understanding the functions of switches, bridges and routers Discussing Internet Protocol (IP) for interconnecting networks Understanding the concept of routing

5. Switch Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 5 – A mechanism that allows us to interconnect links to form a large network – A multi-input, multi-output device which transfers packets/frames from an input to one or more outputs – How about Hub/Repeater?

6. Switching Topology Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 6 A switch implements a star topology Switches are MIMO devices Ports are numbered

7. Properties Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 7 – Even though a switch has a fixed number of inputs and outputs, which limits the number of hosts that can be connected to a single switch, large networks can be built by interconnecting a number of switches – Adding a new host to the network by connecting it to a switch does not necessarily mean that the hosts already connected will get worse performance from the network

8. Properties Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 8 The last claim cannot be made for the shared media network (discussed in Chapter 2) – It is impossible for two hosts on the same Ethernet to transmit continuously at 10Mbps because they share the same transmission medium – Every host on a switched network has its own link to the switch So it may be entirely possible for many hosts to transmit at the full link speed (bandwidth) provided that the switch is designed with enough aggregate capacity

9. Primary Job Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 9 A switch’s primary job is to receive incoming packets on one of its links and to transmit them on some other link – This function is referred as switching and forwarding

10. Switches Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 10 Link-layer device: smarter than hubs, take active role – store, forward Ethernet frames – examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment transparent plug-and-play, self-learning

11. Properties Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 11 hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex – each link is its own collision domain switching: A-to-A’ and B-to-B’ simultaneously, without collisions – not possible with dumb hub A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6 Allows multiple simultaneous transmissions

12. Example: Institutional Network Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 12 to external network router IP subnet mail server web server

13. Switching and Forwarding Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 13 How does a switch decide which output port to place each packet on? – It looks at the header of the packet for an identifier that it uses to make the decision – Two common approaches Datagram or Connectionless approach Virtual circuit or Connection-oriented approach – A third approach source routing is less common

14. Datagram/Connectionless Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 14 Assumptions – Each host has a globally unique address, i.e. MAC – There is some way to identify the input and output ports of each switch We can use numbers We can use names

15. Datagram/Connectionless Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 15 Datagrams – Key Idea Every packet contains enough information to enable any switch to decide how to get it to destination – Every packet contains the complete destination address

16. Datagram/Connectionless Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 16 No dedicated connection between communicating hosts: Packets may follow independent paths to the destination Packets are sent to the switch at any time (no contention) Source is not aware of the state of the destination Less prone to switch failures if alternative paths exist

17. Example Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 17 An example network To decide how to forward a packet, a switch consults a forwarding table (sometimes called a routing table)

18. Forwarding Table Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 18 DestinationPort ------------------------------------- A3 B0 C3 D3 E2 F1 G0 H0 Forwarding Table for Switch 2 Entry: MAC address of host, port/interface to reach host: looks like a routing table! Q: how are entries created, maintained in switch table?

19. Switch: Self-Learning Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 19 switch learns which hosts can be reached through which interfaces – when frame received, switch “learns” location of sender: incoming LAN segment – records sender/location pair in switch table A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr interface Switch table (initially empty) A 1

20. Switch: Frame Filtering/Forwarding Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 20 When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

21. Self-Learning/Forwarding Example Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 21 A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr Port TTL Forwarding table (initially empty) A 1 A A’ Frame destination unknown: flood A’ A  Destination A location known: A’ 4 selective send

22. Interconnecting Switches Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 22 switches can be connected together A B  Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3 ?  A: self learning! (works exactly the same as in single-switch case!) S1S1 C D E F S2S2 S4S4 S3S3 H I G

23. Multi-Switch Forwarding Tables Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 23 Suppose C sends frame to I, I responds to C A B S1S1 C D E F S2S2 S4S4 S3S3 H I G 1 2  Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4

24. Datagram/Connectionless: Summary Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 24 Characteristics of Connectionless (Datagram) Network – A host can send a packet anywhere at any time, since any packet that turns up at the switch can be immediately forwarded (assuming a correctly populated forwarding table) – When a host sends a packet, it has no way of knowing if the network is capable of delivering it or if the destination host is even up and running – Each packet is forwarded independently of previous packets that might have been sent to the same destination. Thus two successive packets from host A to host B may follow completely different paths – A switch or link failure might not have any serious effect on communication if it is possible to find an alternate route around the failure and update the forwarding table accordingly

25. Virtual Circuit Switching Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 25 – Uses the concept of virtual circuit (VC) – Also called a connection-oriented model – First set up a virtual connection from the source host to the destination host and then send the data

26. Virtual Circuit (VC) Switching Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 26 Two-stage process – Connection setup – Data Transfer Connection setup – Establish “connection state” in each of the switches between the source and destination hosts – The connection state for single connection consists of an entry in the “VC table” in each switch through which the connection passes

27. VC Table Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 27 One entry in the VC table on a single switch contains – Incoming VC identifier (VCI) (identifies the connection per link) – Outgoing VC identifier (VCI) (possibly different that the incoming) – Incoming interface (different than the VC Identifier, similar to a port) – Outgoing interface (different than the VC identifier, similar to a port) The semantics for one such entry is – If a packet arrives on the designated incoming interface and that packet contains the designated VCI value in its header, then the packet should be sent out the specified outgoing interface with the specified outgoing VCI value first having been placed in its header Incoming Interface Incoming VCOutgoing Interface Outgoing VC 25111

28. VC Connection Setup Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 28 Permanent VC (PVC): Administrator sets up a permanent connection and configures VC table Switched VC (SVC): The host signals to the switches in order to establish a VC, dynamically.

29. PVC Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 29 Let’s assume that a network administrator wants to manually create a new virtual connection from host A to host B – First the administrator identifies a path through the network from A to B

30. PVC Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 30 The administrator then picks a VCI value that is currently unused on each link for the connection – For our example, Suppose the VCI value 5 is chosen for the link from host A to switch 1 11 is chosen for the link from switch 1 to switch 2 So the switch 1 will have an entry in the VC table Incoming Interface Incoming VCOutgoing Interface Outgoing VC 25111

31. PVC Example Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 31 Incoming InterfaceIncoming VCIOutgoing InterfaceOutgoing VCI 25111 Switch 1 Incoming InterfaceIncoming VCIOutgoing InterfaceOutgoing VCI 31127 Switch 2 Incoming InterfaceIncoming VCIOutgoing InterfaceOutgoing VCI 0714 Switch 3

32. Data Transfer Stage Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 32

33. SVC Example Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 33 A sends setup msg to switch 1 indicating the address of B Switch 1 setups incoming/outgoing interfaces and VCIs Connection setup msg is forwarded like a datagram to switch 2 Switch 2 repeats the setup process Once data stage is over, connection is torn down

34. SVC: Observations Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 34 Delay? Overhead (in terms of address)? A switch/link failure? How to get set up message to B?

35. Bridges Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 35 Bridges are special switches that interconnect Ethernet networks Act as simple nodes on each Ethernet What is Forwarding Table for this Bridge?

36. Problem with Flooding Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 36

37. Why Loop? Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 37 How does an extended LAN come to have a loop in it? – Network is managed by more than one administrator For example, it spans multiple departments in an organization It is possible that no single person knows the entire configuration of the network – A bridge that closes a loop might be added without anyone knowing – Loops are built into the network to provide redundancy in case of failures Solution – Distributed Spanning Tree Algorithm

38. Preliminaries on Graph Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 38 V1V1 V3V3 V2V2 V4V4 V5V5 V6V6 Path: {V 1, V 3, V 4 } Vertex Edge Graph G(V, E) : Collection of vertices and edges – V: Set of vertices – E: Set of edges, e(x, y) = 1, if x, y connected, 0 otherwise Path: sequence of vertices with an edge between subsequent vertices (can be defined as sequence of edges as well)

39. Connected Graph Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 39 A graph is said to be connected, if there is a path from any node to any other node V1V1 V3V3 V2V2 V4V4 V5V5 V6V6 V1V1 V3V3 V2V2 V4V4 V5V5 V6V6

40. (A)Cyclic Graph Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 40 V1V1 V3V3 V4V4 V5V5 V6V6 V1V1 V3V3 V4V4 V5V5 V6V6 Cyclic graph Tree Cycle: a path with the same first and last vertex Cyclic graph: A graph that contains at least one cycle Acyclic graph: A graph with no cycles Tree: An acyclic connected graph (spanning tree, spans all vertices) Forest: A disconnected acyclic graph (a graph with many trees)

41. Mapping Extended LAN to Graph Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 41 Think of the extended LAN as being represented by a graph that possibly has loops (cycles) A spanning tree is a sub-graph of this graph that covers all the vertices but contains no cycles – Spanning tree keeps all the vertices of the original graph but throws out some of the edges Example of (a) a cyclic graph; (b) a corresponding spanning tree.

42. Mapping Extended LAN to Graph Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 42 Bridges and LANs become vertices, ports become edges Goal: Make a tree that spans only LANs (red vertices): Spanning Tree Algorithm B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

43. Spanning Tree Algorithm Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 43 Each bridge has a unique identifier Pick bridge with smallest ID, make it the root of the tree F B1B1 H G E D

44. Spanning Tree Algorithm Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 44 Compute shortest path (in terms of hops) from each bridge to root B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

45. Spanning Tree Algorithm Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 45 Each LAN remains connected to the bridge with shortest path to the root. In case of a tie use smallest ID B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

46. Distributed Spanning Tree: Message Exchange Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527Computer Networks 46 Initially, all bridges consider themselves as roots Broadcast on all ports ID, root bridge, and distance to root If bridge hears a message from another bridge with smaller ID it adjusts root/distance, and forwards Ex., Let (Y, d, X) denote (root, distance, bridge ID) B3 receives (B2, 0, B2) B3 accepts B2 as root B3 adds one on its distance to B2, and fwds to B5 (B2, 1, B3) B2 accepts B1 as root and updates B3 with (B1, 1, B2) B5 accepts B1 as root and updates B3 with (B1, 1, B3) B3 accepts B1 as root, and note that both B2, B5 are closer to root in both ports. Hence, B3 blocks all ports