Slide #1.

Network Layer Goals: Overview: • understand principles behind network layer services: • • • • • – – – – routing (path selection) dealing with scale how a router works advanced topics: IPv6, multicast • instantiation and implementation in the Internet network layer services routing principle: path selection hierarchical routing IP Internet routing protocols reliable transfer – intra-domain – inter-domain • what’s inside a router? • IPv6 • multicast routing
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Slide #2.

Network layer functions • transport packet from sending to receiving hosts • network layer protocols in every host, router three important functions: • 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 applicatio n 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 applicatio n transport network data link physical
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Slide #3.

Network service model service abstraction 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? The most important abstraction provided by network layer: ? ? ? virtual circuit or datagram?
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Slide #4.

Virtual circuits “source-to-dest path behaves much like telephone circuit” – performance-wise – network actions along source-to-dest path • 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
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Slide #5.

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 applicatio 5. Data flow begins n transport 4. Call connected network 1. Initiate call data link physical 6. Receive data applicatio n 3. Accept call 2. incoming call transport network data link physical
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Slide #6.

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 paths applicatio n transport network 1. Send data data link physical different applicatio n 2. Receive data transport network data link physical
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Slide #7.

Network layer service models: Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no • Internet model being extended: Intserv, Diffserv – Chapter 6
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Slide #8.

Datagram or VC network: why? Internet ATM • 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 • evolved from telephony • human conversation: – strict timing, reliability requirements – need for guaranteed service • “dumb” end systems – telephones – complexity inside network
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Slide #9.

Routing Routing protocol Goal: determine “good” path (sequence of routers) thru network from source to dest. Graph abstraction for routing algorithms: • graph nodes are routers • graph edges are physical links – link cost: delay, $ cost, or congestion level 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 • “good” path: – typically means minimum cost path – other definitions possible
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Slide #10.

Routing Algorithm classification 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 info with neighbors • “distance vector” algorithms Static or dynamic? Static: • routes change slowly over time (usually by humans) Dynamic: • routes change more quickly/automatically – periodic update – in response to link cost changes
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Slide #11.

A Link-State Routing Algorithm 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 destinations 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 along path from source to v, that is next v • N: set of nodes whose least cost path definitively known
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Slide #12.

Dijsktra’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 (of nodes adjacent to previous w) 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
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Slide #13.

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F) 2,A 1,A 5,A infinity infinity 2,A 4,D 2,D infinity 2,A 3,E 4,E 3,E 4,E 4,E 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2
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Slide #14.

Dijkstra’s algorithm, discussion Algorithm complexity: n nodes • each iteration: need to check all nodes, w, not in N • n*(n+1)/2 comparisons: O(n**2) • more efficient implementations possible: O(nlogn) Oscillations possible: • e.g., Suppose link cost = amount of carried traffic 1 D 0 1 A 0 0 C 1+e B e e initially 1 2+e A 0 D 1+e1 B 0 0 C … recompute routing (note: c(i,j) != c(j,i)) 0 D A 2+e 0 0 B 1 1+e C … recompute 2+e A 0 D 1+e1 B e 0 C … recompute
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