Network Layer4-1 Chapter 4 Network Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 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 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) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form 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.  Modified by Merrie Bergmann 3/9/2003 Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved

Network Layer4-2 Network layer functions r transport packet from sending to receiving hosts r network layer protocols in every host, router three important functions: r path determination: route taken by packets from source to dest. Routing algorithms r forwarding: move packets from router’s input to appropriate router output r call setup: some network architectures (but not the Internet) require router call setup along path before data flows 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 application transport network data link physical application transport network data link physical

Network Layer4-3 Network service model Q: What service model for “channel” transporting packets from sender to receiver? r guaranteed bandwidth? r preservation of inter-packet timing (no jitter)? r loss-free delivery? r in-order delivery? r congestion feedback to sender? service abstraction

Network Layer4-4 ? ? ? virtual circuit or datagram? The most important abstraction provided by network layer:

Network Layer4-5 Virtual circuits r call setup, teardown for each call before data can flow r each packet carries VC identifier (not destination host ID) r every router on source-dest path maintains “state” for each passing connection r link, router resources (bandwidth, buffers) may be allocated to VC m to get circuit-like perf. “source-to-dest path behaves much like telephone circuit” m performance-wise m network actions along source-to-dest path

Network Layer4-6 Virtual circuits: signaling protocols r used to setup, maintain teardown VC r used in ATM, frame-relay, X.25 r not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

Network Layer4-7 Datagram networks: the Internet model r no call setup at network layer r routers: no state information about end-to-end connections m no network-level concept of “connection” r packets forwarded using destination host address m Routers maintain packet forwarding tables: show which link to send a packet out on for a given destination address m packets between same source-dest pair may take different paths (which means: the forwarding tables are dynamic) application transport network data link physical application transport network data link physical 1. Send data 2. Receive data

Network Layer4-8 Datagram or VC network: why? Internet (datagram) r many link types m different characteristics m uniform service difficult r data exchange among computers m “elastic” service, no strict timing req. r “smart” end systems (computers) m can adapt, perform control, error recovery m simple inside network, complexity at “edge” ATM (virtual circuit) r evolved from telephony r human conversation: m strict timing, reliability requirements m need for guaranteed service r “dumb” end systems m telephones m complexity inside network

Network Layer4-9 Routing principles Graph abstraction for routing algorithms: r graph nodes are routers r graph edges are physical links m link cost: delay, $ cost, or congestion level Goal: determine “good” path (sequence of routers) thru network from source to dest. Routing protocol A E D CB F r “good” path: m typically means minimum cost path

Network Layer4-10 Routing Algorithm classification Global or decentralized information? Global: r all routers have complete topology, link cost info r “link state” algorithms Decentralized: r router knows physically- connected neighbors, link costs to neighbors r iterative process of computation, exchange of info with neighbors r “distance vector” algorithms Static or dynamic? Static: r routes change slowly over time Dynamic: r routes change more quickly m periodic update m in response to link cost changes r Internet routing algorithms are typically dynamic

Network Layer4-11 A Link-State Routing Algorithm Dijkstra’s algorithm (single source shortest path) r net topology, link costs known to all nodes m accomplished via “link state broadcast” m all nodes have same info r computes least cost paths from one node (‘source”) to all other nodes m gives routing table for that node r iterative: after k iterations, know least cost path to k destinationss Notation:  cost(i,j): link cost from node i to j. cost infinite if not direct neighbors  Distance(v): current value of cost of path from source to destination V  pred(v): predecessor node of v along path from source to v  N: set of nodes whose least cost path definitively known

Network Layer4-12 Dijsktra’s Algorithm – where A is the source node 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then Distance (v) = cost(A,v) 6 else Distance (v) = infinity 7 8 Loop 9 find w not in N such that Distance(w) is a minimum 10 add w to N 11 update Distance(v) for all v adjacent to w and not in N: 12 Distance(v) = min( Distance(v), Distance(w) + cost(w,v) ) 13 /* new distance to v is either old distance to v or known 14 shortest path distance to w plus cost from w to v */ 15 until all nodes in N

Network Layer4-13 Dijkstra’s algorithm: example Step N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) 2,A D(C),p(C) 5,A 4,D 3,E D(D),p(D) 1,A D(E),p(E) infinity 2,D D(F),p(F) infinity 4,E A E D CB F

Network Layer4-14 H G F B D E C A Example: Use Dijstra’s algorithm to compute Shortest routes from node F to all other nodes

Network Layer4-15 Dijkstra’s algorithm, discussion Algorithm complexity: n nodes r each iteration: need to check all nodes, w, not in N r n*(n+1)/2 comparisons: O(n 2 ) (more efficient implementations possible: O(nlogn)) Pathology of this algorithm: oscillations possible: r e.g., link cost = amount of carried traffic r Consider the following scenario for routers B, C, D seeking shortest path to router A A D C B 1 1+e e 0 e A D C B 2+e e 1 A D C B 0 2+e 1+e A D C B 2+e 0 e 0 1+e 1 initially … results of recomputing routing … recompute, results … recompute, results

Network Layer4-16 Distance Vector Routing Algorithm iterative: r continues until no nodes exchange new information r self-terminating: no “signal” to stop asynchronous: r nodes need not exchange information or iterate in lock step! distributed: r each node communicates only with directly-attached neighbors Distance Table data structure r each node has its own distance table r row for each possible destination r column for each directly- attached neighbor to node r example: in node X, for dest. Y via neighbor Z: D (Y,Z) X distance from X to Y, via Z as next hop c(X,Z) + min {D (Y,w)} Z w = =

Network Layer4-17 Distance Table: example A E D CB D () A B C D A1764A1764 B D5542D5542 E cost to destination via destination D (C,D) E c(E,D) + min {D (C,w)} D w = = 2+2 = 4 D (A,D) E c(E,D) + min {D (A,w)} D w = = 2+3 = 5 D (A,B) E c(E,B) + min {D (A,w)} B w = = 8+6 = 14 loop!

Network Layer4-18 Distance table gives routing table D () A B C D A1764A1764 B D5542D5542 E cost to destination via destination ABCD ABCD A,1 D,5 D,4 Outgoing link to use, cost destination Distance table Routing table

Network Layer4-19 Distance Vector Algorithm: 1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infinity /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */ X X X w At all nodes, X: What does this do? My table will list the distance to each of my neighbors via itself as the cost of the link to that neighbor, and all other distances are infinite. I will send to each neighbor the minimum distance that I have recorded to get to each destination – which will be the direct link if the destination is a neighbor, and infinity otherwise

Network Layer4-20 Distance Vector Algorithm: example X Z Y This column Shows the Init. results Init Info sent

Network Layer4-21 Distance Vector Algorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D (y,V) = D (y,V) + d else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min D V (Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors forever w X X X X X w w

Network Layer4-22 Distance Vector Algorithm: example X Z Y Adjust by newvalues

Network Layer4-23 Distance Vector Algorithm: example X Z Y D (Y,Z) X c(X,Z) + min {D (Y,w)} w = = 7+1 = 8 Z D (Z,Y) X c(X,Y) + min {D (Z,w)} w = = 2+1 = 3 Y

Network Layer4-24 Reminder: the distance vector algorithm is asynchronous! The previous slides make it look like there is synchronization, but it is not necessary! The algorithm will still work even if the updates are not synchronized!

Network Layer4-25 A B E C D EXAMPLE: COMPUTE DISTANCE VECTOR ALGORITHM

Network Layer4-26 Distance Vector Algorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D (y,V) = D (y,V) + d else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min D V (Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors forever w X X X X X w w

Network Layer4-27 Distance Vector: link cost changes Link cost changes: r node detects local link cost change r updates distance table (line 15) r if cost change in least cost path, notify neighbors (lines 23,24) X Z Y 1 algorithm terminates “good news travels fast”

Network Layer4-28 Distance Vector: link cost changes Link cost changes: r good news travels fast r bad news travels slow - “count to infinity” problem! X Z Y 60 algorithm continues on! -for how long?

Network Layer4-29 Distance Vector: poisoned reverse If Z routes through Y to get to X : r Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) r will this completely solve count to infinity problem? X Z Y 60 algorithm terminates

Network Layer4-30 Comparison of Link State and Distance Vector algorithms Message complexity r LS: with n nodes, E links, O(nE) msgs sent each (initial link state broadcast prior to Dijkstra’s algorithm) r DV: exchange between neighbors only m convergence time varies Speed of Convergence r LS: O(n 2 ) algorithm requires O(nE) msgs r DV: convergence time varies, can be very slow m may be routing loops m count-to-infinity problem Robustness: what happens if router malfunctions? LS: m node can advertise incorrect link cost m each node computes only its own table so incorrect calculations are local DV: m DV node can advertise incorrect path cost m each node’s table used by others Calculation error can propagate throughout network

Network Layer4-31 Hierarchical Routing scale: with 200 million destinations: r can’t store all destinations in routing tables! r routing table exchange would swamp links! administrative autonomy r internet = network of networks r each network admin may want to control routing in its own network Our routing study thus far - idealization r all routers identical r network “flat” … not true in practice

Network Layer4-32 Hierarchical Routing r aggregate routers into regions, “autonomous systems” (AS) r routers in same AS run same routing protocol m “intra-AS” routing protocol m routers in different AS can run different intra- AS routing protocol r special routers in AS r run intra-AS routing protocol with all other routers in AS r also responsible for routing to destinations outside AS m run inter-AS routing protocol with other gateway routers gateway routers

Network Layer4-33 Intra-AS and Inter-AS routing Gateways: perform inter-AS routing amongst themselves perform intra-AS routers with other routers in their AS a b b a a C A B d A.a A.c C.b B.a c b c

Network Layer4-34 Intra-AS and Inter-AS routing Host h2 a b b a a C A B d c A.a A.c C.b B.a c b Host h1 Intra-AS routing within AS A Inter-AS routing between A and B Intra-AS routing within AS B r We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly