Functions of Network layer in computer networks

network access layer functions and network layer functions and protocols and also functions of network layer in osi model pdf
Dr.ShivJindal Profile Pic
Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
Your Website URL(Optional)
Comment
Computer Communication Networks (CCN) Network Layer (Routing) 1Network layer functions - 1 application transport network data link network physical data link network • transport packet network physical data link data link physical physical from sending to network data link network physical receiving hosts data link physical network • network layer network data link data link physical physical protocols in every network application data link transport physical host, router network data link physical Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 2Network layer functions - 2 three important functions: • path determination: route taken by packets from source to dest. Routing algorithms • Switching (forwarding): move packets from router‟s input to appropriate router output • call setup: (optional) some network architectures require router call setup along path before data flows Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 3Network service model Q: What service model for “channel” transporting The most important packets from sender to abstraction provided receiver? by network layer: • guaranteed bandwidth? virtual circuit • preservation of inter-packet ? or timing (no jitter)? ? datagram? • loss-free delivery? ? • in-order delivery? • congestion feedback to sender? Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 4Datagram networks: the Internet model - 1 • 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 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 5Datagram networks: the Internet model - 2 application application transport transport network network 1. Send data 2. Receive data data link data link physical physical Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 6Routing Routing protocol Goal: determine “good” path 5 (sequence of routers) thru 3 network from source to dest. B C 5 2 A 2 1 F 3 • Graph abstraction for 1 2 D E routing algorithms: 1 • graph nodes are routers “good” path: • graph edges are typically means physical links minimum cost path other def‟s possible • link cost: delay, cost, or congestion level Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 7Routing Algorithm classification - 1 Global or decentralized information? Global: • all routers have complete topology, link cost info • “link state” algorithms Decentralized: • router knows physically-connected neighbors, link costs to neighbors • iterative process of computation, exchange of partial info with neighbors • “distance vector” algorithms Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 8Routing Algorithm classification - 2 Static or dynamic? Static: • routes change slowly over time Dynamic: • routes change more quickly – periodic update – in response to link cost changes Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 9A Link-State Routing Algorithm - 1 Dijkstra‟s algorithm • net topology, link costs known to all nodes – accomplished via “link state broadcast” – all nodes have same info • computes least cost paths from one node („source”) to all other nodes – gives routing table for that node – iterative: after k iterations, know least cost path to k dest.‟s Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 10A Link-State Routing Algorithm - 2 Notation: • c(i,j): link cost from node i to j. cost infinite if not direct neighbors • D(v): current value of cost of path from source to dest. V • p(v): predecessor node (neighbor of v) along path from source to v • N: set of nodes whose least cost path definitively known Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 11Dijkstra‟s algorithm: example D(B),p(B) D(D),p(D) Step D(C),p(C) D(E),p(E) start N D(F),p(F) 2,A 1,A 0 5,A infinity A infinity 2,A 1 4,D 2,D AD infinity 2 2,A 3,E ADE 4,E 3 3,E ADEB 4,E 4 ADEBC 4,E 5 ADEBCF 5 3 B C 5 2 A 2 1 F 3 1 2 D E 1 Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 12Dijsktra‟s Algorithm 1 Initialization: 2 N = A 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 / new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v / 15 until all nodes in N Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 13Dijkstra‟s algorithm: discussion Algorithm complexity: n nodes • each iteration: need to check all nodes, w, not in N • n(n+1)/2 comparisons: O(n2) • more efficient implementations possible: O(nlogn) Oscillations possible: • e.g., link cost = amount of carried traffic Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 14Distance Vector Routing Algorithm - 1 iterative: • continues until no nodes exchange info. • self-terminating: no “signal” to stop asynchronous: • nodes need not exchange info/iterate in lock step distributed: • each node communicates only with directly- attached neighbors Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 15Distance Vector Routing Algorithm - 2 Distance Table data structure • each node has its own • row for each possible destination • column for each directly-attached neighbor to node • example: in node X, for dest. Y via neighbor Z: distance from X to = X Y, via Z as next hop D (Y,Z) Z c(X,Z) + min D (Y,w) = w Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 16Distance table: example cost to destination via 1 E B C D () A B 7 D A 2 8 1 A 1 14 5 E D 2 B 7 8 5 E D c(E,D) + min D (C,w) D (C,D) = w C 6 9 = 2+2 = 4 4 E D c(E,D) + min D (A,w) D (A,D) = w D 4 11 2 = 2+3 = 5 loop E B c(E,B) + min D (A,w) D (A,B) = w = 8+6 = 14 loop Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 17Distance table gives routing table cost to destination via Outgoing link E to use, cost D () A B D A 1 14 5 A A,1 B 7 8 5 B D,5 C 6 9 4 C D,4 D 4 11 2 D D,4 Routing table Distance table Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 18Distance Vector Routing: overview - 1 Iterative, asynchronous: each local iteration caused by: • local link cost change • message from neighbor: its least cost path change from neighbor Distributed: • each node notifies neighbors only when its least cost path to any destination changes – neighbors then notify their neighbors if necessary Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 19Distance Vector Routing: overview - 2 Each node: wait for (change in local link cost of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Rensselaer Polytechnic Institute © Shivkumar Kalvanaraman & © Biplab Sikdar 20