Introduction to Network Layer

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!!!

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

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?

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

Network layer functions transport packet from sending to receiving hosts network layer protocols in every host, router Important functions: addressing: each host/router interface must have a GLOBALLY unique network address path determination: route taken by packets from source to dest. Routing algorithms switching: move packets from router’s input to appropriate router output call setup: some network architectures require router call setup along path before data flows network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical

Figure 4.13 Services provided at the source computer

Figure 4.14 Processing at each router

Figure 4.15 Processing at the destination computer

Network service model Q: What service model for “channel” transporting packets from sender to receiver? guaranteed bandwidth? preservation of inter-packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to sender? ? ? ? virtual circuit or datagram? The most important abstraction provided by network layer: service abstraction

Virtual circuits call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host ID) every router on source-dest path maintains “state” for each passing connection –(in contrast, transport-layer connection only involved two end systems) link, router resources (bandwidth, buffers) may be allocated to VC –to get circuit-like performance “source-to-dest path behaves much like telephone circuit” –performance-wise –network actions along source-to-dest path

Virtual circuits: signaling protocols used to set up, maintain, and tear down VC used in ATM, frame-relay, X.25 not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

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 Incoming Interface Incoming VC # Outgoing interface Outgoing VC # VC table at R2:

Distinction between VC setup at the network layer and connection setup at the transport layer Connection setup at the transport layer only involves the two end systems in which they agree to communicate and together determine the parameters (e.g., initial sequence number, flow control window size) of their transport level connection before data actually begins to flow on the transport level connection Although the two end systems are aware of the transport- layer connection, the switches within the network are completely oblivious to it On the otherhand, with a virtual-circuit network layer, packet switches are involved in virtual-cicuit setup, and each packet switch is fully aware of all the VCs passing through it

Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections –no network-level concept of “connection” packets typically routed using destination host ID –packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

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 C C Forwarding table at R2: D D C C C C D D D Destination Address Outgoing interface Next Hop A1A C3R3 D3

Datagram or VC network: why? Internet 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” easier to connect many link types –different characteristics –uniform service difficult ATM evolved from telephony human conversation: –strict timing, reliability requirements –need for guaranteed service “dumb” end systems –telephones –complexity inside network