Compuiter network 6 network

Computer Networks Network Layer Where are we? Will Layer Networking Suffice? Motivation Connect various link technologies to form a larger internetwork Universal addressing scheme required General purpose use Hides underlying technologies from end user Facilitate communicate between autonomous domains Able to move packets between any host on the internetwork Connecting Heterogeneous Networks Computer System used Special purpose Dedicated Works with LAN or WAN technologies Known as router gateway Illustration of a Router Cloud denotes an arbitrary network One interface per network Important Idea A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes or frame formats The Internet Concept Key Functions of the Network Layer Global Addressing Fragmentation Routing We’ll be primarily concerned with addressing and routing Example Network Layer: Internet Protocol (IP) Standardized by IETF as RFC 791 Most popular Layer protocol Core protocol used on the public Internet Connectionless protocol datagrams contain identity of the destination each datagram sent/handled independently Of utmost importance for this class! 10 Routing Protocol Requirements Efficient routing table size Efficient routing control messages Robustness and reliability prevent loops avoid black holes reconvergence time is short 41 Source of Route Table Information Manual Table created by hand Useful in small networks Useful if routes never change Automatic software creates/updates tables Needed in large networks Changes routes when failures occur 42 Compute Shortest/Best Path Possible metric geographic distance economic cost capacity 43 Algorithms for Computing Shortest Path Distance Vector Exchange routing tables with neighboring routers e.g., RIP, RIPv2 Link State Routers exchange link status information e.g., OSPF, IS-IS 44 Distance Vector Routers periodically advertise and learn about IP networks Cost of the route is based on hops to the network (number of routers to pass) Recalculation occurs when links fail 45 Count to Infinity Problem What happens when link 15 goes down? Does think it can get to through 2? 46 Solving the Count to Infinity Problem Hold down Wait for a period of time before switching paths Advertise route cost as infinity Based on timers Report the entire path Guarantees no loops, but expensive Split horizon Do not advertise routes to neighbors if the route was received from that neighbor Not foolproof 47 Other Distance Vector Improvements Triggered updates Advertise changes as soon as you learn of them May help convergence time May create routing instability for flapping routes Poison reverse Used with split horizon Report infinity rather than nothing at all Diffusing Update ALgorithm (DUAL) Somewhat like hold down, but routers are alerted of broken paths Complex Not popular 48 Example Distance Vector Protocol: RIP Standardized in RFC 1058 and 2453 An interior gateway protocol (IGP) Simple RIPv2 includes subnet mask in updates Hop count based (> 15 = unreachable) Widely used in small to medium sized organizations 49 Link State Routers distribute link cost and topology information to all other routers in its area All routers have complete information about the network Each router computes its own optimal path to destinations Ensures loop free environments 50 Link State Procedure Each router is responsible for meeting its neighbors and learning their names Each router advertises LSPs which contain costs to its neighbors Most current LSPs are stored by all routers Each router now computes routes 51 Dijkstra Algorithm: Databases Link State Database contains latest LSPs from all other routers PATH (permanent) Database Consists of ID, path cost and forwarding direction TENT (tentative/temporary) Database Consists of ID, path cost and forwarding direction Forwarding Database Contains ID and forwarding direction 52 Dijkstra Algorithm: Procedure Each router starts with itself as root of the tree by putting its ID and cost and forwarding direction in PATH For each node placed in PATH, examine its LSP and place those neighbors in TENT if not already in PATH or TENT If TENT is empty, terminate, otherwise find the ID in TENT with the lowest cost and move it to PATH 53 Example Link State Protocol: Open Shortest Path First (OSPF) Standardized in RFC 2328 An interior gateway protocol (IGP) Used in medium to large organizations Supports internal/external routes Supports TOS based routing, but never implemented in practice Two-tier hierarchy for scalability 54 Network Layer: Final Notes Basic IP requirements on an end host IP address, mask, gateway, DNS BOOTP ICMP BGP IPX AppleTalk CLNP 55 ... Examples: Network: 140.192.9.0 Subnet mask: 255.255.255.0 Network: 140.192.9.0 Subnet mask: 255.255.255.128 Network and mask: 140.192.9.0/24 26 More Subnet Examples 63 .85.18.5/22 - What is the network? ... CIDR blocks Decreases the routing table size Advertise 199.242 .64 .0/22 instead of 199.242 .64 .0,199.242 .65 .0, 199.242 .66 .0 and 199.242 .67 .0 The CIDR Report: http://www.employees.org/~tbates/cidrreport.html... (a) 36 Routing Table Size Since each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in the internetwork