1、Network Layer,Goals: 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,Overview: network layer services routing principle: path selection hierarchical r
2、outing IP Internet routing protocols reliable transfer intra-domain inter-domain whats inside a router? IPv6 multicast routing,Network layer functions,transport packet from sending to receiving hosts network layer protocols in every host, router three important functions: path determination: route t
3、aken by packets from source to dest. Routing algorithms switching: move packets from routers input to appropriate router output call setup: some network architectures require router call setup along path before data flows,Network service model,Q: What service model for “channel” transporting packets
4、 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 importantabstraction provided by network layer:,service abstraction,Virtual circuits,call
5、 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, bu
6、ffers) 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
7、in todays Internet,1. Initiate call,2. incoming call,3. Accept call,4. Call connected,5. Data flow begins,6. Receive data,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 rou
8、ted using destination host ID packets between same source-dest pair may take different paths,1. Send data,2. Receive data,Network layer service models:,Network ArchitectureInternetATMATMATMATM,Service Modelbest effortCBRVBRABRUBR,Bandwidthnoneconstant rate guaranteed rate guaranteed minimum none,Los
9、snoyesyesnono,Ordernoyesyesyesyes,Timingnoyesyesnono,Congestion feedbackno (inferred via loss) no congestion no congestion yesno,Guarantees ?,Internet model being extended: Intserv, Diffserv Chapter 6,Datagram or VC network: why?,Internet data exchange among computers “elastic” service, no strict ti
10、ming 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 requirement
11、s need for guaranteed service “dumb” end systems telephones complexity inside network,Routing,Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level,Goal: determine “good” path (sequence of routers) thru network
12、from source to dest.,“good” path: typically means minimum cost path other definitions possible,Routing Algorithm classification,Global or decentralized information? Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physically-connected nei
13、ghbors, 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 chan
14、ges,A Link-State Routing Algorithm,Dijkstras 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, kn
15、ow 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 kn
16、own,Dijsktras 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(
17、 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,Dijkstras algorithm: example,Step 0 1 2 3 4 5,start N A AD ADE ADEB ADEBC ADEBCF,D(B),p(B) 2,A 2,A 2,A,D(C),p(C) 5,A 4,D 3,E 3,E,D(D),p(D) 1,A,D(E),
18、p(E) infinity 2,D,D(F),p(F) infinity infinity 4,E 4,E 4,E,Dijkstras 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 (note: c(i,j) != c(j,i),1,1+e,e,0,e,1,1,0,0,0,2+e,1+e,1,0,0,initially, recompute routing, recompute, recompute,
