1 Network Layer: Host-to-Host Communication

2 Network Layer: Motivation Can we built a global network such as Internet by extending LAN segments using bridges? –No! Bridged networks do not scale 4 problems 1.We can only bridge certain link-layer technologies together Link layers to be bridged must have similar MAC address structure 2.Bridge table explosion Bridges use MAC addresses for forwarding and MAC addresses are flat, i.e., not hierarchical –the bridge table needs to have an entry per host in the network  bridge table explosion!!!

3 Network Layer: Motivation 3. Robustness Change of network topology requires a new spanning tree computation 4.Link-layer broadcast storms –Notice that a bridged network is still a single LAN! –A link-layer broadcast packet must still be delivered to ALL hosts in the network. –Can you imagine receiving a link-layer broadcast packet from a host 5000 km away at your host? –Bottom Line: Bridged/Switched LANs don’t scale! –What’s the solution? --- Next

4 A B C Bridge E F D Hub Switch L M Hub H I O N K G R1 R2 R3 R4 Network Core Separate LANs Each LAN is a separate LL Broadcast Domain Router A collision domain within a LAN Divide the network into separate LANs that are NOT part of the same “LL broadcast” domain Connect the LANs using “routers” –Notice that we CANNOT use bridges to connect separate LANs as bridged LANs form a single LL broadcast domain, which is what we are trying to avoid to achieve scalability How to achieve scalable growth?

5 A B C Bridge E F D Hub Switch L M Hub H I O N K G R1 R2 R3 R4 Network Core Separate LANs Each LAN is a separate LL Broadcast Domain Router A collision domain within a LAN How do two hosts on separate LANs, e.g., A and E, communicate? Recall that using the Link Layer (LL), only hosts that are neighbors, that is, hosts that are within the same LAN can communicate. Solution: Design a new layer, called the network layer, that would provide host-to-host packet delivery for hosts that are in separate LANs Communication Issue

6 Network Layer: Responsibility A B C Bridge E F D Hub Switch L M Hub H I O N K G R1 R2 R3 R4 Network Core Router link physical link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Responsibility: Deliver a packet from a sending host, e.g., A, to one (unicast) or more (multicast) receiving host(s)

7 Network Layer: Issues Addressing: Each host/router interface must have a GLOBALLY unique network address –Recall that within a LAN, this was achieved by globally unique MAC addresses at the LL –We need a similar GLOBALLY unique address at the network layer – e.g., IP address Packet Forwarding: How does a packet sent from host A to host E make it to host E? –Recall that within a LAN, this was achieved transparently by bridges/switches, with each bridge building a forwarding table with respect to MAC addresses on the fly –We need a similar “path determination” algorithm with respect to the destination network address. Called path determination or the routing problem.

8 Network Layer Service Model Every Network Layer must export a service model (interface) to the layers on top of it –guaranteed bandwidth? –preservation of inter-packet timing (no jitter)? –loss-free delivery: reliable vs. unreliable? –in-order delivery: ordered vs. unordered? –unicast vs. multicast delivery? –congestion feedback to sender?

9 ? ? ? virtual circuits or datagrams? The question that a network designer must answer to satisfy the chosen service model: Will the network be based on Network Layer Abstraction –That is, should we establish an end-to-end path through the network for the packets to flow? Yes: Virtual-Circuit Networks (X.25, Frame-Relay, ATM) No: Datagram Networks (the Internet)

10 Virtual Circuits Networks: Signaling Virtual Circuit Networks (e.g., X.25, Frame Relay, ATM) –Establish a path along which the packets will flow between the source and the destination. How? Use a signaling (virtual circuit establishment) protocol Ex: B tells its router (R1) that it wants to talk to C The call establishment message is forwarded by the routers in the network until it reaches C. Then a reply comes back from C to B. –Path established at call setup time remains fixed during packet exchange –Routers maintain state information for ongoing connections A B C D R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical

11 Virtual Circuits Networks: Forwarding A B C D Incoming Interface Incoming VC # Outgoing interface Outgoing VC # VC table at R1: R1 R2 R3 R4 R5 R6 R7 R8 R9 R –each packet carries tag (virtual circuit ID), which determines next hop –Path established at call setup time remains fixed during packet exchange –Routers maintain state information for ongoing connections Incoming Interface Incoming VC # Outgoing interface Outgoing VC # VC table at R2:

12 Datagram Networks: Idea A B C D R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 Datagram networks (e.g. the Internet): No call establishment before data exchange Simply put the destination address on top of the packet and submit it to the network for delivery Similar to postal service C C D D C C C C D D D CC C D D D D Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical Network link physical

13 Datagram Networks: Forwarding A B C D Destination Address Outgoing interface Next Hop B1B C2R3 D2 Forwarding table at R1: R1 R2 R3 R4 R5 R6 R7 R8 R9 R –Destination address is written on top of a packet and it is simply submitted to the network for delivery (like postal service) –Routers look at destination address in packet to determine the next hop –No connection-state information needed in the routers –Routes may change during session C C Forwarding table at R2: D D C C C C D D D Destination Address Outgoing interface Next Hop A1A C3R3 D3 CC

14 VC vs. Datagram network: why? Internet (Datagram) data exchange among computers –“elastic” service, no strict timing req. “smart” end systems (computers) –can adapt, perform control, error recovery –simple inside network, complexity at “edge” many link types –different characteristics –uniform service difficult X.25, Frame Relay, ATM (Virtual Circuit) evolved from telephony human conversation: –strict timing, reliability requirements –need for guaranteed service “dumb” end systems –telephones –complexity inside network

15 Network layer service models Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ? Internet model being extended: Intserv, Diffserv