Chapter 4 Network Layer Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)  If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Network Layer 4-1

Network Layer Introduction: Network layer  transport segment from sending to receiving host  on sending side encapsulates segments into datagrams  on receiving side, delivers segments to transport layer  network layer protocols in every host, router  router examines header fields in all IP datagrams passing through it application transport network data link physical application transport 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 network data link physical network data link physical network data link physical network data link physical

Network Layer Introduction: Two key network-layer functions  forwarding: move packets from router’s input to appropriate router output  routing: determine route taken by packets from source to destination  routing algorithms analogy:  routing: process of planning trip from source to destination  forwarding: process of getting through single interchange

Network Layer value in arriving packet’s header routing algorithm local forwarding table header value output link Introduction : Interplay between routing and forwarding routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router

Network Layer Introduction: The Internet network layer forwarding table host, router network layer functions: routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router “signaling” transport layer: TCP, UDP link layer physical layer network layer

4.1 Introduction: The Internet network layer  RIP (Routing Information Protocol) - for managing router information - uses hop count as a cost metric; each link has a cost of 1 - costs are actually from source router to a destination subnet  OSPF (Open Shortest Path First) - conceived as the successor to RIP - constructs a complete graph of the entire autonomous system and then determines a shortest-path tree to all subnets with a Dijkstra’s shortest-path algorithm  BGP (Border Gateway Protocol) - is the standard inter-AS routing protocol in today’s Internet - makes routing decisions based on paths, network policies or rule- sets configured by a network administrator Network Layer 4-6

4.1 Introduction: The Internet network layer  IP (Internet Protocol Protocol) - the principal communications protocol by which data is sent from one computer to another on the Internet  ICMP (Internet Control Message Protocol) - used by hosts and routers to communicate network-layer information to each other - most typical use for error reporting (ex) error message such as “Destination network unreachable” Network Layer 4-7

Network Layer 4-8 ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier header checksum time to live 32 bit source IP address head. len type of service flgs fragment offset upper layer 32 bit destination IP address options (if any) IP datagram format IP protocol version number header length (bytes) upper layer protocol to deliver payload to total datagram length (bytes) “type” of data for fragmentation/ reassembly max number remaining hops (decremented at each router) e.g. timestamp, record route taken, specify list of routers to visit. how much overhead?  20 bytes of TCP  20 bytes of IP  = 40 bytes + app layer overhead

Routers vs. Switches vs. Hubs  They look alike on the outside, but they’re very different on the inside.  Each is designed to allow computers to connect to it. Each features a number of physical ports on the front or back of the unit that provide the connection points for these computers  Hub - also known as a repeater, is a network device that can operate on layer-1 (I.e. the physical layer) to connect network devices for communication - commonly used to connect segments of a LAN - typically the least expensive, least intelligent, and least complicated - acts on individual bits rather than frames; when a bit arrives from one interface, the hub re-creates the bit and transmits the bit onto all the other interfaces Network Layer 4-9

Routers vs. Switches vs. Hubs  Switch - a.k.a. intelligent hub, is a network device that stores and forwards packets between LAN segments - operates at the link layer (layer 2) (i.e. forward packets using MAC addresses) - can identify the intended destination of the packet that they receive, so they send that packet to only the computers that are supposed to receive it  Router - network devices that operate at Layer-3 - as layer-3 protocols have access to logical address (IP addresses) so routers have the capability to forward data across networks - far more feature rich as compared to switches - maintain routing table for data forwarding Network Layer 4-10