Routing in the Internet

Transcription

1 ÉOLE POLYTEHNIQUE FÉÉRLE E LUSNNE Routing in the Internet Jean-Yves Le oudec 1 ontents. Introduction. Unicast Routing: istance Vector. Unicast Routing: Link State. Multicast Routing E. The Spanning Tree lgorithm for ridges F. Interdomain Routing: GP G. Load dependent routing 2

2 oncepts You Should Know... after this lecture distance vector link state reverse path multicast interior, exterior protocol topology database count to infinity what is OSPF, IGRP, RIP, GP the spanning tree algorithm for bridges 3. Introduction onnectionless Network Layer assumes routing tables are maintained at hosts and routers used by Packet Forwarding Routing = control method maintain routing tables automatically in routers t host normally done by default rules plus IMP redirect in old times: was done also by a routing protocol (RIP) LNs connected by bridges operate at layer 2 like connectionless packet forwarders how do they maintain routing information? 4

3 Internal Routing Used inside an administrative domain other methods used to find routes between domains Problem solved by a routing protocol find reachable destinations find best paths towards destinations best in the sense of some metric in this chapter, best means along shortest path, for some additive metric (number of hops, delay) 5 Simple Routing Methods static configuration for toy networks only flooding each packet duplicated on each outgoing link; loops prevented by packet id or other mechanism ; duplicated packets destroyed at destination simple and robust no need for routing tables robust - tolerates link or router failures optimal in some sense the first packet has found the shortest path to the destination costly many duplicated packets little useful traffic used as an ingredient by mobile ad-hoc routing methods (OV, OLSR) source routing source writes route into packet header router reads next hop from packet header, moves pointer route discovered by flooding 6

4 Source Routing S RI data 2 IS IS IS IS 4 3 Loop free routes from to : 7 Route iscovery in Token Rings R2 6 R5 R6 R1 R4 5 4 ll Route roadcast: -R1-1-R2-2-R3 3-R5-6-R6 5-R4 4-R4-5-R5-3-R2-2-R3 6-R6 5 frames generated 2 copies reach route_1 = R1.1.R2.3.R5.6.R6 route_2 = R1.4.R4.5.R5.6.R6 explorer packet reaches and accumulates route sends response to along reverse path now knows a route to 8 R3

5 Other Methods istance vector (ellman-ford) routers only know their local state link metric and neighbor estimates internal routing protocols (RIP, IGRP) Link state knowledge of the global state topology database global optimization (Shortest Path First - ijkstra) internal routing protocols (OSPF, PNNI (TM)) Path vector no knowledge of the global state path: sequence of S with attributes global optimization and policy routing external routing protocols (GP) 9 Metrics istance vector and link state find paths that minimize a metric Static metric - does not depend on the network state; for example: number of hops link capacity and static delay cost ynamic metric- depend on the network state link load current delay see end of section

6 . istance Vector omputes best paths to all destinations uses distributed ellman-ford classical setting: concatenation = addition best = smallest in usual sense each router receives aggregated information from its neighbors individual link cost is setup by network management 11 The ellman-ford entralized lgorithm The original ellman-ford theorem computes the best path from i to j for any couple (i,j). Let (i,j) be the cost of going from i to j, defined for i j. We assume (i, j) > 0 and (i,j) = for i 1 when i and j are not connected. Take for example j=1 and define p k (i) as the cost of the best path from i to 1 in at most k hops. Let p 0 (1) = 0, p 0 (i) = for i 1. Theorem 1 F (ellman Ford) 1. For k =1,2,..: p k (i) = min j i [(i,j) + p k-1 (j)] for i 1 and p k (1)=0 2. If the network is fully connected, for k n-1 : p k (i) = p(i) where n is the number of nodes 3. The shortest path from i 1 to 1 is defined by pred(i) = rgmin j i [(i,j) + p(j)]. Proof: p k (i) is the distance from i to 1 in at most k hops. 12

7 Example pply the theorem: write p k (i), pred(i) and draw the shortest paths to node Impact of Initial onditions Example: does the algorithm converge to the shortest path with initial condition as shown? i\k i\k

8 Theorem 2 Impact of Initial ondition The algorithm converges in a finite number of steps to the correct values for all initial conditions such that p 0 (1)=0 and for every node i that is connected to 1 If there is no path from i to 1, the algorithm tends to infinity 15 Proof of Theorem 2 We do the proof assuming all nodes are connected. 1. Let p k be the vector p k [i], i=2,. Let be the mapping that transforms an array x[i] i=2 into the array x defined for i 1 by x[i]=min j i, j 1 [(i,j) + x(j)] Let b be the array defined for i 1 by b[i]= (i,1) The algorithm can be rewritten in vector form as (1) p k = p k-1 b where is the pointwise minimum 2. Eq (1) is a min-plus linear equation and the operator satisfies (x y)= x y. Thus, Eq(1) can be solved using min-plus algebra into (2) p k = k p 0 k-1 b b b 3. efine the array e for i 1 by e[i]=. Let p 0 =e. Eq (2) becomes (3) p k = k-1 b b b. Now we have the ellman Ford algorithm with classical initial conditions, thus, by Theorem 1: (4) for k n-1: k-1 b b b = q where q[i] is the distance from i to We can rewrite Eq(2) for k n-1 as (5) p k = k p 0 q 5. k p 0 [i] can be written as [i,i 1 ]+ [i 1,i 2 ]+ + [i k-1,i k ]+ p[i k ] thus (6) k p 0 [i] k a, where a is the minimum of all [i,j]. Thus k p 0 [i] tends to when k grows. Thus for k large enough, k p 0 is larger than and can be ignored in Eq(5). In other words, for k large enough : (6) p k = q 16

9 istributed ellman Ford F can be used in a centralized algorithm to compute p(i) i.e. find the spanning tree. However, this is not its main interest, because there is a better algorithm (ijkstra) that can be used in a centralized method ut: it can be distributed, as follows. istributed ellman-ford lgorithm every node, say i, maintains an estimate q(i) of p(i); initially, q(i) = p 0 (i); also pred(i)=i initially; whenever q(i) is modified (or initialized), i sends the new value q(i) to all its neighbours when node i receives a new value q(j) from a neighbour j it updates q(i) by q(i) := min (q(i), (i,j)+q(j)) i.e. node i sees whether q(j) can be used to provide a better estimate. if the received new value improves q(i), pred(i):=j Theorem: if the time to reliably send a message is bounded by T, the algo converges to the same result as the centralized version in at most nt time units (if the network is fully connected) 17 istributed ellman-ford istributed ellman-ford algorithm initially: (i,n) = 0 if i directly connected to n and (i,n) = + otherwise node i receives from neighbour k latest values of (k,n) for all n (distance vector) node i computes the best estimates (i,n) = min k (c(i,k) + (k,n)) converges if network is stable hello mechanism to reset computation after changes c(i,1) 1 (1,n) i c(i,k) k (k,n) n c(i,m) m (m,n) 18

10 Example 1 net dist nxt n1 0 n1, n4 0 n4, net dist nxt n1 0 n1, n2 0 n2, n1 net dist nxt n4 m3 n3 n2 m1 m2 net dist nxt n3 0 n3, n4 0 n4, m3 0 m3, n2 0 n2, n3 0 n3, m1 0 m1, m2 0 m2, 19 Example 1 net dist nxt n1 0 n1, n4 0 n4, from n1 0 n4 0 net dist nxt n1 0 n1, n2 0 n2, n4 1 n1, n1 net dist nxt n3 0 n3, n4 0 n4, m3 0 m3, n4 m3 n3 n2 m1 from n3 0 n4 0 m3 0 m2 net dist nxt n2 0 n2, n3 0 n3, m1 0 m1, m2 0 m2, n4 1 n3, m3 1 n3, 20

11 Example 1 net dist nxt net dist nxt n1 0 n1, n4 0 n4, n1 0 n1, n2 0 n2, n3 1 n2, n4 1 n1, m1 1 n2, m2 1 n2, m3 2 n2, net dist nxt n3 0 n3, n4 0 n4, m3 0 m3, n4 m3 n1 n3 n2 m1 from n2 0 n3 0 m1 0 m2 m2 0 n4 1 m3 1 net dist nxt n2 0 n2, n3 0 n3, m1 0 m1, m2 0 m2, n4 1 n3, m3 1 n3, 21 Example 1 - Final net dist nxt n1 0 n1, n2 1 n1, n3 1 n4, n4 0 n4, m1 2 n4, m2 2 n4, m3 1 n4, net dist nxt n1 0 n1, n2 0 n2, n3 1 n2, n4 1 n1, m1 1 n2, m2 1 n2, m3 2 n2, n1 net dist nxt n1 1 n4, n2 1 n3, n3 0 n3, n4 0 n4, m1 1 n3, m2 1 n3, m3 0 m3, m3 n4 n3 n2 m1 m2 net dist nxt n1 1 n2, n2 0 n2, n3 0 n3, m1 0 m1, m2 0 m2, n4 1 n3, m3 1 n3, 22

12 Example 1 - Failure net dist nxt We show only the router in the next hop field n1 0 n2 0 n3 1 n4 1 m1 1 m2 1 m3 2 n1 net dist nxt n1 1 n2 1 n3 0 n4 0 m1 1 m2 1 m3 0 n4 m3 n3 n2 m1 m2 net dist nxt n1 1 n2 0 n3 0 m1 0 m2 0 n4 1 m Example 1 - Failure net dist nxt n1 0 n2 0 n3 1 m1 1 m2 1 m3 2 timeout n1 timeout net dist nxt n2 1 n3 0 n4 0 m1 1 m2 1 m3 0 n4 m3 n3 n2 m1 m2 net dist nxt n1 1 n2 0 n3 0 m1 0 m2 0 n4 1 m3 1 24

13 Example 1 - Failure net dist nxt n1 0 n2 0 n3 1 m1 1 m2 1 m3 2 n1 net dist nxt n1 2 n2 1 n3 0 n4 0 m1 1 m2 1 m3 0 m3 n4 n3 From : n1 1 n2 0 n3 0 m1 0 m2 0 n4 1 m3 1 n2 m1 m2 net dist nxt n1 1 n2 0 n3 0 m1 0 m2 0 n4 1 m Example 1 - fter Failure net dist nxt n1 0 n2 0 n3 1 n4 2 m1 1 m2 1 m3 2 n1 net dist nxt n1 2 n2 1 n3 0 n4 0 m1 1 m2 1 m3 0 n4 m3 n3 n2 m1 m2 net dist nxt n1 1 n2 0 n3 0 m1 0 m2 0 n4 1 m3 1 26

14 Example 1 illustrates Example 1: conclusions how ellman Ford is mapped to the network concepts how topology changes are taken into account most recent announcement replaces previous ones non refreshed announcements become obsolete how distance vector carries reachability information 27 dest link cost local 0 l1 1 l3 1 l1 2 E l1 2 Example 2 To simplify, we identify destination with router ssume algorithm has converged cost =1 l3 dest link cost local 0 l3 1 l3 2 l3 3 E l6 1 cost =1 l1 l6 cost =1 l4 E dest link cost local 0 l1 1 l2 1 E l4 1 l1 2 cost =1 l2 l5 cost =5 E dest link cost E local 0 l4 2 l4 1 l6 1 l4 2 dest link cost local 0 l2 2 l2 1 l2 3 E l2 2 28

15 Example 2 we now show only table entries: to link 2 fails updates its table l1 2 l2 l1 l3 l6 l4 E l5 local 0 l3 3 l Example 2: Link failure Just before updates its table, broadcasts its table with cost 2 to accepts l1 2 l1 3 from : l1 2 l1 l3 l6 l4 E l5 local 0 l3 3 l4 2 30

16 Example 2: Link failure sends update to and E and E accept l1 4 l1 3 from : l1 3 l1 l3 l6 l4 E l5 local 0 from : l1 3 l3 3 l Example 2: Link failure sends update it is ignored by E because it it less good l1 4 l1 3 l1 l3 l6 l4 E l5 local 0 from : local 0 l3 3 l4 4 32

17 Example 2: Link failure broadcasts its table with cost 4 to accepts we have a loop between and cost is increase by 2 at every iteration l1 4 l1 5 from : l1 4 l1 l3 l6 l4 E l5 local 0 l3 3 l Example 2: Link failure E now accepts announcement from l1 6 l1 7 l1 l3 l6 l4 E l5 local 0 from : local 0 l3 7 l5 5 34

18 Example 2: Link failure E sends announcements to and and send announcements to the algorithm has converged stable state l1 7 l4 6 from : l4 6 l1 from E: l5 5 l3 l6 l4 E l5 local 0 from E: l5 5 l6 6 l onclusions from Example 2 the algorithm converges after modification of the topology, but the convergence may be very slow bounce effect Q: during convergence time, how are routing tables? 36

19 Example 3 ssume now all link costs are equal to 1 Links l1 and l6 fail detects failure and sets costs to infty dest link cost dest link cost local 0 l3 3 l3 1 l3 3 E l3 2 local 0 l4 3 l2 1 E l4 1 l4 2 dest link cost l3 l4 E E l2 l5 dest link cost dest link cost local 0 l5 3 l2 1 l5 2 E l5 1 local 0 l3 1 l3 l6 E l6 E local 0 l6 2 l4 1 l6 1 l Example 3 dest link cost local 0 l3 3 l3 1 l3 3 E l3 2 from : dest cost 0, 3 1 E 2 dest link cost local 0 l3 1 l3 4 l3 4 E l3 3 l3 dest link cost local 0 l3 5 l3 1 l3 5 E l3 4 from : dest cost 1, 4 0 E 3 dest link cost local 0 l3 1 l3 4 l3 4 E l3 3 l3 dest link cost local 0 l3 3 l3 1 l3 3 E l3 2 from : dest cost 0, 5 1 E 3 dest link cost local 0 l3 1 l3 6 l3 6 E l3 5 l3 38

20 onclusion from Example 3 The costs to,, E grow unbounded ount to Infinity the true costs are infinite onvergence to a stable state if we set = large number e.g. RIP: = 16 Split Horizon a heuristic to prevent this if routes packets to X via, it does not announce this route to 39 Example 3: with Split Horizon dest link cost dest link cost local 0 l3 3 l3 1 l3 3 E l3 2 local 0 l4 3 l2 1 E l4 1 l4 2 dest link cost l3 l4 E E l2 l5 dest link cost dest link cost local 0 l5 3 l2 1 l5 2 E l5 1 local 0 l3 1 l3 l6 E l6 E local 0 l6 2 l4 1 l6 1 l5 1 40

21 Example 3: with Split Horizon dest link cost local 0 l3 3 l3 1 l3 3 E l3 2 from : dest cost 0 l3 dest link cost local 0 l3 1 l3 l6 E l6 41 Split horizon dest link cost local 0 l3 l3 1 l3 E l3 Split horizon cuts the process of counting to infinity from : dest cost 0,,E l3 dest link cost local 0 l3 1 l3 l6 E l6 42

22 Split horizon may fail from E: dest cost 1 1 dest link cost local 0 l4 l2 1 E l4 1 l4 l4 E E l2 l5 dest link cost dest link cost local 0 l5 3 l2 1 l5 2 E l5 1 E local 0 l6 l4 1 l6 l Split horizon may fail dest link cost local 0 l2 4 l2 1 E l4 1 l2 3 from : dest cost 3 2 E 1 l4 E E l2 l5 dest link cost E local 0 l6 l4 1 l6 l5 1 dest link cost local 0 l5 3 l2 1 l5 2 E l5 1 from : dest cost 1 44

23 Split horizon may fail l4 E from : dest cost dest link cost local 0 l2 4 l2 1 E l4 1 l2 3 E l2 l5 dest link cost E local 0 l4 5 l4 1 l4 4 l5 1 dest link cost local 0 l5 3 l2 1 l5 2 E l onclusion: istance Vector convergence to stable state may be slow after changes count to infinity must be prevented by setting a maximum distance 46

24 istance Vector Protocols RIP v1 istance vector protocol Metric - hops Network span limited to 15 = 16 Split horizon estination network identified by IP address no prefix/subnet information - derived from address class Encapsulated as UP packets, port 520 Largely implemented (routed on Unix) roadcast every 30 seconds or when update detected Route not announced during 3 minutes cost becomes 47 Message format 31 0 command version zero address family IP address zero zero metric zero May be repeated 25 times ommand REQUEST - 1 (sent at boot to initialize) RESPONSE - 2 (broadcast each 30 sec) 48

25 Missing netmask ( ) packet to E F ( ) and E can forward to Packet to can go through F or if sent to, it goes through and If link - broken, no route to destination 49 RIP v2 Subnetworks take into account IR prefixes and netmasks uthentication 50

26 Message format 31 0 command version routing domain address family IP address netmask next router metric route tag ommand, version unchanged One address family - authentication Routing domain and next router distinguish different addressing domains (e.g. S) used at the border of S Route tag for external routes (used by GP) 51 nnouncing netmasks ( ) packet to E F ( ) E can forward to Packet to can go through F 52

27 Use of Next-Hop Field This This is is a a simple simple example example of of the the use use of of the the next next hop hop field field in in a a rip rip entry. entry IR1 IR1 IR2 IR2 IR3 IR3 XR1 XR1 XR2 XR2 XR3 XR < RIP < RIP ssume ssume that that IR1, IR1, IR2, IR2, and and IR3 IR3 are are all all "internal" "internal" routers routers which which are are under under one one administration administration (e.g. (e.g. a a campus) campus) which which has has elected elected to to use use RIP-2 RIP-2 as as its its IGP. IGP. XR1, XR1, XR2, XR2, and and XR3, XR3, on on the the other other hand, hand, are are under under separate separate administration administration (e.g. (e.g. a a regional regional network, network, of of which which the the campus campus is is a a member) member) and and are are using using some some other other routing routing protocol protocol (e.g. (e.g. OSPF). OSPF). XR1, XR1, XR2, XR2, and and XR3 XR3 exchange exchange routing routing information information among among themselves themselves such such that that they they know know that that the the best best routes routes to to networks networks N1 N1 and and N2 N2 are are via via XR1, XR1, to to N3, N3, N4, N4, and and N5 N5 are are via via XR2, XR2, and and to to N6 N6 and and N7 N7 are are via via XR3. XR3. y y setting setting the the Next Next Hop Hop field field correctly correctly (to (to XR2 XR2 for for N3/N4/N5, N3/N4/N5, to to XR3 XR3 for for N6/N7), N6/N7), only only XR1 XR1 need need exchange exchange RIP-2 RIP-2 routes routes with with IR1/IR2/IR3 IR1/IR2/IR3 for for routing routing to to occur occur without without additional additional hops hops through through XR1. XR1. Without Without the the Next Next Hop Hop (for (for example, example, if if RIP-1 RIP-1 were were used) used) it it would would be be necessary necessary for for XR2 XR2 and and XR3 XR3 to to also also participate participate in in the the RIP-2 RIP-2 protocol protocol to to eliminate eliminate extra extra hops. hops. RF Simple authentication 31 command version routing domain (zero) xffff authentication type = 2 0 password on 16 bytes onfiguration of gated (/etc/gated.conf) rip yes { interface all version 2 multicast authentication simple "qptszwmz" } 54

28 M5 authentication 31 command version routing domain (zero) xffff authentication type = 3 packet length key Id auth. length increasing sequence no. zero zero route info 0 xffff x01 seal 55 M5 authentication Seal M5 digest on the message using a shared secret sequence number avoids replay attacks onfiguration of gated (/etc/gated.conf) rip yes { interface all version 2 multicast authentication md5 "qptszwmz" } 56

29 IGRP (Interior Gateway Routing Protocol) Proprietary protocol by ISO Metric that estimates the global delay Maintains several routes of similar cost load sharing Takes into account netmasks No limit of 15 number of routers included in messages roadcast every 90 sec 57 Metric example Metric Trans = /andwidth (time to send Kb) delay = (sum of elay)/ m = [K 1 *Trans + (K 2 *Trans )/(256-load) + K 3 *delay] default: K1=1, K2=0, K3=1, K4=0, K5=0 if K5 0, m = m * [K 5 /(Reliability + K 4 )] andwidth in Kb/s, elay in µs t Venus: Route for /16: Metric = /784 + ( )/ = t Saturn: Route for 12./8: Metric = /224 + ( )/ =

30 onclusion Main distance vector protocols Largely deployed (Unix S routed) Simplicity Slow convergence Not suited for large and complex networks Link State protocols should be used instead 59 Review Questions Explain the following terms: distance vector bounce effect count to infinity split horizon ellman Ford RIP, IGMP source routing Explain why shortest path routing is not necessarily a globally optimum What is the raess paradox? 60

31 . Link State Routing Principle of link state routing each router keeps a topology database of whole network link state updates flooded, or multicast to all network routers compute their routing tables based on topology often uses ijkstra s shortest path algorithm Used in OSPF (Open Shortest Path First) and PNNI (TM routing protocol) 61 (a) Topology atabase Synchronization Neighbouring nodes synchronize before starting any relationship Hello protocol; keep alive initial synchronization of database description of all links (no information yet) Once synchronized, a node accepts link state advertisements contain a sequence number, stored with record in the database only messages with new sequence number are accepted accepted messages are flooded to all neighbours sequence number prevents anomalies (loops or blackholes) 62

32 Example network Each router knows directly connected networks n3 n6 E n2 n4 n5 n7 F n1 63 Initial routing tables net type n3 Ether n2 P-to-P n4 P-to-P n3 net type n6 Ether n5 P-to-P n6 E net type n6 Ether n7 Ether E net type n1 Ether n2 P-to-P n2 n1 n4 n5 net type F n7 F net type n1 Ether n7 Ether n1 n4 n5 Ether P-to-P P-to-P 64

33 fter Flooding The local metric information is flooded to all routers fter convergence, all routers have the same information rtr rtr net net cost cost n1 n1 n2 n2 0 0 n3 n3 n2 n2 0 0 n4 n4 0 0 n1 n1 n4 n4 0 0 n5 n5 0 0 n6 n6 n5 n5 0 0 E E n6 n6 E E n7 n7 F F n1 n1 F F n7 n7 n2 n3 n1 n4 n5 n6 F E n7 65 (b) Topology graph stub network point to point link rrows routers-to-nets with a given metric except P-to-P, stub, and external networks From nets to routers, metric = 0 n n n1 0 0 E 0 n7 F 54 external network external network broadcast network 66

34 (b) Path omputation Performed locally, based on topology database omputes one or several best paths to every destination from this node est Path = shortest for OSPF OSPF uses ijkstra s shortest path the best known algorithm for centralized operation Paths are computed independently at every node synchronization of databases guarantees absence of persistent loops every node computes a shortest path tree rooted at self 67 Simplified graph Only arrows with metrics between routers Every node executes the shortest path computation on the graph same graph, but different sources E F 68

35 ijkstra s Shortest Path lgorithm The nodes are 0...N and the algorithm computes best paths from node 0 c(i,j) is the cost of (i,j), pred(i) is the predecessor of node i on the tree M being built m(j) is the distance from node 0 to node j. m(0) m(0) = 0; 0; M = {0}; {0}; for for k=1 k=1 to to N { find find (i0, (i0, j0) j0) that that minimizes m(i) m(i) + c(i,j), with with i in in M, M, j not not in in M m(j0) m(j0) = m(i0) m(i0) + c(i0, c(i0, j0) j0) pred(j0) = i0 i0 like ellman-ford, M = M works {j0} {j0} for any min-plus algebra } 69 Example: ijkstra at 0 m()=0 0 0 m()= E F init: M = { } step 1: i0= j0= m()= M = {, } 70

36 Example: ijkstra at E i0= j0=f m(f)= M = {,,F} m()=0 m()= F m(f)= 71 Example: ijkstra at m(f)=20 E i0=f j0=e m(e)=20 M = {,,F,E} m()=0 m()= F m(f)= 72

37 Example: ijkstra at 0 m()= m(e)=20 E i0=e j0= m()=40 M = {,,F,E,} m()=0 m()= F m(f)= 73 Example: ijkstra at m()=0 0 m()= m(e)=20 E i0= j0= m()=0 M = {,,F,E,,} m()=0 m()= F m(f)= 74

38 Routing table of net next n1 direct n2 direct n3 n4 n5 n6 F n7 F n2 n3 n4 n5 n6 E n7 F n1 75 Test Your Understanding Q1: Run ijkstra at Q2: What are the routing tables at 76

39 Why divide large networks? ost of computing routing tables ivide large networks update when topology changes SPF algorithm n routers, k links complexity O(n*k) size of, update messages grows with the network size Limit the scope of updates and computational overhead divide the network into several areas independent route computing in each area inject aggregated information on routes into other areas 77 Hierarchical Routing large OSPF domain can be configured into areas one backbone area (area 0) non backbone areas (areas numbered other than 0) ll inter-area traffic goes through area 0 strict hierarchy Inside one area: link state routing as seen earlier one topology database per area X1 X1 1 X4 X3 1 area 1 area 2 2 X4 X3 2 area 0 78

40 Principles Routing method used in the higher level: distance vector no problem with loops - one backbone area Mapping of higher level nodes to lower level nodes area border routers (inter-area routers) belong to both areas Inter-level routing information summary link state advertisements (LS) from other areas are injected into the local topology databases 79 Example ssume networks n1 and n2 become visible at time 0. Show the topology databases at all routers 1 X1 6 X3 6 X2 6 X4 1 n1 2 area 1 6 X5 6 6 X6 area 2 2 n2 area 0 80

41 Hints ll routers in area 2 propagate the existence of n1 and n2, directly attached to 1 (resp. 2). raw the topology database in area 2. rea border routers X4 and X6 belong to area 2, thus they can compute their distances to n1 and n2 rea border routers X4 and X6 inject their distances to n1 and n2 into the area 0 topology database (item 3 of the principle). The corresponding summary link state record is propagated to all routers of area 0. raw now the topology database in area 0. ll routers in area 0 can now compute their distance to n1 and n2, using their distances to X4 and X6, and using the principle of distance vector (item 1 of the principle). o the computation for X3 and X5. rea border routers X3 and X5 inject their distances to n1 and n2 into the area 1 topology database (item 3 of the principle). raw now the topology database in area omments istance vector computation causes none of the RIP problems strict hierarchy: no loop between areas External and summary LS for all reachable networks are present in all topology databases of all areas most LSs are external can be avoided in configuring some areas as terminal: use default entry to the backbone rea partitions require specific support partition of non-backbone area is handled by having the area 0 topology database keep a map of all area connected components partition of backbone cannot be repaired; it must be avoided; can be handled by backup virtual area 0 links through non backbone area 82

42 Problems - link failure area 0 1 X1 6 X3 6 X2 6 X4 1 n1 2 area 1 6 X5 6 6 X6 area 2 2 n2 Link 1-2 fails, link X3-X6 is not in area 1. rea 1 is partitioned X3 has a route to 1, X5 to 2 cannot pass to X5 a packet to 1 and to X3 a packet to 2 Solution X3 and X5 will advertise only distances to connected networks as though there were two separated areas 83 Problems - partitioned backbone area 0 1 X1 X3 6 X2 6 X4 1 n1 2 area 1 6 X5 6 X6 area 2 2 n2 No connectivity between areas via backbone There is a route through rea 2 Virtual link X4 and X6 configure a virtual link through rea 2 virtual link entered into the database, metric = sum of links 84

43 Stub area Many networks are connected only via one router Stub area all external networks aggregated into default route this reduces routing table sizes e.g. route to n1, n2 or any other network in rea 0 and 2 goes through X3 1 X1 X3 6 6 X2 6 X4 1 n1 2 area X6 area 2 2 n2 area 0 85 OSPF (Open Shortest Path First) The OSPF Protocol IETF standard for internal routing used in large networks (ISPs) Link State protocol + Hierarchical The network is represented using the following principles separate hosts and routers consider different types of networks broadcast (Ethernet), NM (TM, X.25), point-to-point (PPP) divide large networks into several areas independent route computing in each area 86

44 Multiple paths are possible LS (comments) modification of ijkstra s algorithm to keep lists of paths instead of just a spanning tree even non-shortest paths are possible. For routing to be loop free, at node i, for a packet sent to n,we require that the next hop k satisfies d(k,n) < d(i,n) LS can use other algorithms than ijkstra s, but complexity of. is generally less ijkstra: O(N lnm), N=number of nodes, M=number of links F: O(MN) 87 LS: Summary ll nodes compute their own topology database represents the whole network strongly synchronized ll nodes compute their best path tree to all destinations Routing tables are built from the tree used for next hop routing only LS versus V LS avoids convergence problems of V supports flexible cost definitions; can be used for routing TM connections LS is much more complex 88

45 Multicast packet forwarding. Multicast Routing given source address and multicast destination address, send packet to all relevant interfaces relevant is indicated by the multicast routing method End-system group membership (as receiver) is known to routers via IGMP LS method: Multicast-OSPF (M-OSPF) topology database contains info about group membership OSPF computes a tree for one source to all destinations (standard LS routing) parts of tree leading to no destination are removed routing table built accordingly works only in one area using one LS routing algorithm Other methods exist for global multicast routing RPF (Revrese Path Forwarding) with pruning core based tree 89 Reverse Path Multicasting (RPM) RPM = Reverse Path with pruning compute one tree per source S using reverse of direct path tree to S then suppress (prune) branches that are not needed Router R receives packet src addr = S, dest addr = M on interface i if i is not the next hop towards S { discard packet; send PRUNE (S; M) to i; } else { if there is a relevant interface send packet to all relevant interfaces else send PRUNE (S; M) to i } relevant interface: member: leaf: at least one member exists host with IGMP routers that did not send PRUNE (S; M) Implemented with VMRP (Mbone) or PIM-dense (global Internet) VMRP uses its own V algo for path computation PIM-dense uses existing point-to-point routing 90

46 RPM Example 1 2 S S H2 H2 H1 E F H1 E F 3 4 S S PRUNE H2 PRUNE H2 H1 E F PRUNE H1 E F Showing the Shortest Path Tree for traffic to S, as given by local tables (links not on tree : ) H1 and H2 have subscribed to the multicast address 91 The figure shows the Shortest Path Tree for traffic to S, as given by local tables (links not on the shortest path tree to S are dashed. H1 and H2 have subscribed to the multicast address M (1) S sends IP packets to M. (2) The first packet is forwarded by to all its ports (3) E and F do not accept packets arriving via ports not on the spanning tree. The corresponding links are removed from the multicast distribution tree F does not send to if it can compute shortest paths one step ahead: F knows that will not accept the packet from F because, for, F is not on the path to S (4) suppresses its link to because it has no host listening and the only connected routers has sent a PRUNE message for (S, M). ll other packets follow the marked line. The effect of PRUNE messages disappears after a timeout ( for example 1 mn), which causes steps (2) and (3) to be repeated at regular intervals. 92

47 PIM sparse mode PIM Sparse Mode designate one router as rendez-vous point (RP) receivers send membership messages to RP, build tree along the path senders send to group by encapsulating packets to RP; RP sends on tree one tree per group but a destination can decide to build an additional tree towards a given source the location of RP is critical protocol exists for repairing RP failures 93 E: Transparent ridging (T) Interconnect systems beyond one LN segment, keeping main characteristics of LN without additional addresses M addresses used to identify end systems preserve sequence integrity End systems ignore that there are transparent bridges bridge is transparent M frames not changed by bridges frames not sent to bridge, but rather: bridge is promiscuous (listens to all frames) T operation connectionless forwarding, unstructured addresses bridges are plug and play: no address configuration (no IP address needed) 94

48 T: Learning ridge bridge builds routing table by reading all traffic table built by learning from S field in M frame learnt addresses timeout if not re-learnt broadcast forwarding if unknown port 1 ridge port 2 port 3 Forwarding Tables 95 Extension to Several Learning ridges an the learning bridge be extended to a network of bridges? How does 2 see the network? port 1 port 2 1 port 1 port 2 2 X Y Z 96

49 Loop-Free topology Learning bridge works well on Loop-Free topology only idirectional graph: node = bridge, edge = connection through LN Loop free - bidirectional graph = bidirectional tree examples: line, star On a tree, there is only one path from to Proposition: If bridge topology is loop-free, then there exists only one path from any end system to any bridge Loop-free topology is required and sufficient for 97 Transparent ridging ased on learning bridge: table driven forwarding, flooding if unknown or multicast, learning Forces topology to a tree Spanning Tree algorithm run by all bridges Some links blocked to prevent loops ports that are allowed to forward frames (in either way) are said to be in the forwarding state or called forwarding ports 98

50 T Forwarding Method opy all frames on all forwarding ports Individual PU forwarding Frame received on port i -> /* port i is forwarding */ If is unicast, is in forwarding table with port j and j is a forwarding port then copy to port j else flood all forwarding ports i Update forwarding table with (i, S) ontrol method Maintain spanning tree and port states Learn addresses on reading traffic 99 T Spanning Tree Specification Set of bridges with - bridge Id and prio - bridge ports on LNs - LN costs T Spanning Tree One bridge selected as root On every bridge - one root port - designated ports (other ports are blocked) ridges viewed as a bidirectional graph (nodes = bridges) Selection of the root bridge lowest priority with lowest identifier Spanning Tree = shortest path tree from root to all bridges edge costs set by management, high cost = less traffic based on distributed ellman Ford Root port on one bridge = port towards root, shortest path in case of equal costs, lowest id chosen esignated bridge ports: on one LN, shortest path to root Ports other than root or designated are blocked 0

51 Example cost = 3 X cost = cost = 3 T Z cost = 3 3 cost = 3 Y cost = 1 every bridge has one root port + some designated ports; other ports are blocked for individual frame forwarding all ports are active for control flows (PUs) 1 Solution cost = 3 X cost = cost = 3 T Z cost = 3 3 Y cost = cost = 1 Forwarding Tables: 41 1X 2YZ 3T 84 1XYZT 92 1XZT 2Y 81 1XYZT 90 2XZT 3Y 99 dxzt gy 2

52 Spanning Tree lgorithm istributed in all bridges ridges exchange messages with neighbours in order to both elect a root determine shortest path tree to root root port = port towards root on shortest path tree designated port = port for which bridge was designated 3 ridge PUs ontrol method uses control frames called ridge PUs (PUs) M = all bridges (multicast) SP = 0000 PUs are not forwarded by bridges unlike all other frames PUs are sent by one bridge to all bridges on the same LN segment reminder: a data frame is never sent to bridge by end system onfiguration PU contain root Id with priority cost to root (from sender of config PU) Id of sender with port number 4

53 Initialization of Spanning Tree (1) ridge initially assumes self is root ridge computes own new config PU based on received information determine best root so far distance to root with ellman Ford On every port, ridge transmits config PU until it receives a better config PU on that port better closer to root On every port, bridge maintains copy of best config PU sent or received 5 Initialization of Spanning Tree (2) Example: ridge 90 prepares config PU and sends on all ports; 90 configuration tables: < ,3 > < > Root Port : 1 esignated Ports : 2,3 locked Ports : Root Port : 2 esignated Ports : 3 locked Ports : 1 message received on port 1: 1 < message format: root.cost_to_root.sender 6

54 asic ST Procedure config PU received on any port or port enabled -> compute new root; compute new cost to root; /* ellman Ford */ build new_config_pu; for all ports i do if new_config_pu better than stored_config[i] then store and send on port i; end compute root port /* smaller distance to root */ designated ports = ports where config PU was sent blocked ports = other ports r.c.s better than r.c.s iff (r<r ) or (r=r and c<c ) or (r=r and c=c and s<s ) 7 New configurations Topology changes new PUs trigger basic procedure Failures, partitions: centralized control + distributed monitoring assume bridge 99 fail: is ST recomputed? 8

55 ST: Support for Reconfiguration configuration monitoring triggered by root root refreshes validity of stored configuration timeout causes recomputation of spanning tree Root sends config PUs every hello_timer; ridge receives config PU on root port i -> Reset timer on stored_config[i] for all designated ports j sends own config PU resets timer on stored_config[j] ridge.timeout on stored_config[j]-> delete stored_config[j]; performs basic ST procedure; 9 Example 99 powered off; stored_config at 90 : timeout [2], [3] ,3 > < Root Port : esignated Ports : 2 3 Spanning Tree 3 after failure locked? Ports : 3 locked Port configis do not timeout: port 1 becomes root on this bridge. If it would be wrong, then it would timeout 1

56 Transient States Transient periods cause loops or loss of connectivity during reconfiguration, topology is not yet (in general) loop free Even transient loops should be avoided T standard: forwarding state is not immediately operational pre-forwarding states: listening: wait for stabilization of ST (forwarding timer, 15 sec) learning: wait for addresses to be learnt (forwarding timer, 15 sec) ctions State Forward ST Learn locking Listening Learning Forwarding 112 Port FSM state event Port enabled by NM PU isabled locking Listening Learning Forwarding locking Port disabled isabled isabled isabled isabled (NM, failure) Port Selected as Root or designated Listening Port no longer locking locking locking root or designated Forwarding Learning Forwarding timer expires 113

57 Station ache Timer Station ache Timers as long as possible to avoid broadcasts along the spanning tree but wrong cache values cause station to be unreachable until timer expires example 1: station moved example 2: show that after failure of 99, U cannot reach Z until some entry in 90 is purged 114 Long and Short Timers Two timer values are used long timer (5mn): normal case short timer = forwarding timer (15 sec): after spanning tree updates Timer switching mechanism bridge detects change in ST -> maxlife = shorttimer how can bridges detect changes in ST? 115

58 Topology Update Mechanism Topology Update Mechanism : when one bridge port changes out of or into blocking state then bridge sends topology update PU towards root (upstream bridges repeat PU up to root) root forwards new config PU with topology change flag set for a time duration = forwarding timer + Maxge timer causes all bridges to use short timer value for caches until PU from root received with topology change flag cleared 116 F.1. Routing in the Internet The Internet is too large to be run by one routing protocol Hierarchical routing is used the Internet is split into omains, or utonomous Systems with OSPF: large domains are split into reas Routing protocols are said interior: (Internal Gateway Protocols, IGPs): inside Ss: RIP, OSPF (standard), IGRP (isco) exterior: between Ss: EGP (old) and GP-1 to GP-4 (today), IRP (tomorrow) 117

59 utonomous Routing omains utonomous Systems (Ss) R = routing domain under one single administration one or more border routers all subnetworks inside an R should be connected should learn about other subnetworks - the routing tables of internal routers should contain entries of all destination of the Internet S are R with a number ( S number ) 16 bits public: private: Rs with default route to the rest of the world do not need a number Examples S IG-GRENOLE, S1717, S Renater S559 - SWITH Teleinformatics Services S5511 OPENTRNSIT EPFL: one R, no number 118 the figure shows three domains, or Rs. Rs can be transit ( and ), stub () or multihomed (). Only non stub domains need an S number, as we can see on the GP slides later on. R 1 area area 1 area 2 2 GP OSPF 4 OSPF 2 GP-4 1 GP utonomous Routing omain 3 R GP-4 1 IGRP 4 3 R 2 119

60 Hierarchical Routing Hierarchical routing is different case by case, however, we can distinguish three elements 1. routing method used in the higher level 2. mapping higher level nodes to lower level nodes 3. inter-level routing information We know two examples hierarchical routing with OSPF (inside a large domain) entrally Organized inter-domain routing with GP-4 Self-Organized 120 Inter-omain Routing Inter domain routing hierarchies GP-4: one level of hierarchy (one R is a virtual node in GP) The R interconnection layer is self-organized IRP: several levels of hierarchy (Rs can be aggregated) The principles of GP-4 : 1. routing method used in the higher level: path vector with policy routing 2. mapping higher level nodes to lower level nodes border gateways (= GP speakers) 3. inter-level routing information summary link state records are injected into the interior routing protocol (OSPF, RIP, etc) 121

61 Path Vector Routing a message between neighbours is a set of: (path, dest) (called routes ) every node (here: one S) maintains a table of best paths known so far paths are announced to neighbours using the same principles as distance vector, ie. S announces the best paths it knows applies to inter-domain routing no global meaning for costs can be assumed (heterogeneous environment) Ss want control over which paths they use (see policy routing, later) Q. Explain how E chooses the paths to n1 and n2 Q. How can loops be avoided? n1, n2 :n1,n2 n5 :n1,n2 :n5 n4 E GP table in E dest dest S S path path n1 n1 n2 n2 n3 n3 n4 n4 n5 n5 :n1,n2 n3 :n1,n2 :n3 :n1,n2 : n3 : n4 122 order Gateways, E-GP and I-GP GP runs on routers called border gateways = GP speakers -- belong to one S only two border gateways per boundary Q: compare to OSPF In addition, GP speakers talks to each other inside the S using Internal- GP (I-GP) over TP connections full mesh called the GP mesh I-GP is the same as E-GP except for one rule: routes learned from a neighbour in the mesh are not repeated inside the mesh ( Q. why? ) Q: Is there a need for all GP speakers in one network to be adjacent? X:n1 X:n1 1 I-GP E-GP 3 E F 2 ->: 1,X:n1 ->E: 1,X: n1 E->G: 3,1,X:n1 G 4 E-GP H 5 123

62 F.2. Policy Routing Interconnection of Ss is self-organized point to point links between networks: ex: EPFL to Switch, Switch to Telianet interconnection points: NP (Network ccess Point), ME (Metropolitan rea Ethernet), IX (ommercial Internet exchange), GIX (Global Internet exchange), IXP, SFINX, LINX Mainly 3 types of relations, depending on money flows customer: EPFL is customer of Switch. EPFL pays Switch provider. Switch is provider for EPFL; Switch is paid by EPFL peer: EPFL and ERN are peers: costs of interconnection is shared 124 What is the Goal of Policy Routing? ISP 3 ISP ISP 2 2 n2 Example: ISP3-ISP2 is transatlantic link, cost shared between ISP2 and ISP 3 ISP 3- ISP 1 is a local, inexpensive link i is customer of ISPi, ISPs are peers It is advantageous for ISP3 to send traffic to n2 via ISP1 ISP1 does not agree to carry traffic from 3 to 2 ISP1 offers a transit service to 1 and a nontransit service to ISP 2 and ISP3 The goal of policy routing is to support this and other similar requirements peers provider customer 125

63 How does Policy Routing Work? ISP 3 ISP 2 Policy routing is implemented by rules imposed to GP speakers inside an S, who may refuse to import or announce some paths modify the attributes that control which path is preferred (see later) 3 n3 ISP 1 1 Example ISP 1 announces to ISP 3 all networks of 1 so that 1 2 can be reached by all sources in the world ISP 1 announces to 1 all routes it has learnt from ISP3 and ISP2 so that 1 can send traffic to all destinations in n2 the world ISP2 announces ISP2 n2 to ISP3 and ISP1 ; assume that ISP1 annouces ISP1 ISP2 n2 to ISP3. ISP 3 has two routes to n2: ISP2 n2 and ISP1 ISP2 n2 ; assume that ISP3 gives preference to the latter packets from n3 to n2 are routed via ISP1 undesired solution: ISP 1 announces to ISP3 only routes to ISP3 s customers 126 Typical Policy Routing Rules ISP 3 ISP ISP 2 2 Provider (ISP1) to customer (1) announce all routes learnt from other ISs import only routes that belong to domain 1 example: import from EPFL only one route /15 ustomer (1) to Provider (ISP1) announce all routes that belong to domain 1 import all routes Peers (ISP1 to ISP3) announce only routes to all customers of ISP1 import only routes to ISP3 s customer these routes are defined as part of peering agreement The rules are defined by every S (selforganization) and implemented in all GP speakers in one S 127

64 F.3. ggregation omains that do not have a default route (i.e. all transit ISPs) must know all routes in the world (> ) in IP routing tables unless default routes are used in GP announcements ggregation is a way to reduce the number of routes 128 ggregation Example 1 S1 S /24 S / / /23 S4 S1: /24 S_PTH: 1 S2: /24 S_PTH: 2 S3: /23 S_PTH: 3 {1 2} S4: /23 S_PTH: 4 3 {1 2} 129

65 ggregation Example 2 n2 S1 S / /22 S / /24 S4 n /22 S4 receives /22 S_PTH: 3 {1 2} /24 S_PTH: 2 oth routes are injected into S4 s routing tables Q: what happens to packets from n4 to n2? 130 ggregation Example 3 n2 S1 S / /24 S /24 S5 S / / /22 S4 n /24 S4 receives /22 S_PTH: 3 {1 2} /24 S_PTH: oth routes are received by S4; only shortest S paths routes are injected into routing tables Q: what happens to packets from n4 to n2? 131

66 Example Without ggregation S1 S / /24 S / / /24 S / / / /24 Q: If S3 does not aggregate, what are the routes announced by S 4? Is there any benefit? 132 F.4. GP (order Gateway Protocol) GP-4, RF 1771 S border router - GP speaker peer-to peer relation with another S border router connected communication on top of a TP connection, port 179 (vs. datagram (RIP, OSPF)) external connections (E-GP) with border routers of different S internal connections (I-GP) with border routers of the same S GP only transmits modifications 133

67 Routes Route - unit of information; contains: destination (subnetwork prefix) path to the destination (S-PTH) attributes degree of preference (LOL-PREF) origin of announcement (ORIGIN) others, see late dvertised between a pair of GP speakers Stored locally in RIs (Routing Information ase) Every GP speaker can add or modify the path attributes, using its decision process 134 Routing Information ases One GP Speaker updates in updates out dj-ri-in dj-ri-in ecision Process: one best route per destination Loc-RI ggregation ompression dj-ri-out dj-ri-out IGP Static onfiguration Routing Table 135

68 Operation of GP Speaker GP speaker : stores received routes in dj-ri-in one per GP peer (internal or external) applies decision process and stores results in Loc-RI (global to GP speaker) decide which routes to accept decide how to rank them (set LOL-PREF) decide which routes to export and with which attributes dispatches results per outgoing interface into dj-ri-out (one per GP peer), after aggregation and information reduction maintains adjacency to peers (over TP connection): open, keep-alive sends updates when dj-ri-out changes Write forwarding entries in its routing table; redistributes routes learnt from E-GP from Loc-RI into IGP and vice-versa, unless other mechanisms are used (See Examples) 136 GP messages 4 types OPEN KEEPLIVE NOTIFITION UPTE Size: from 19 to 4096 bytes Security by M5 137

69 Route ttributes Well-known Mandatory ORIGIN (route learnt from IGP, GP or static) S-PTH NEXT-HOP (see later) Well-known iscretionary LOL-PREF (see later) TOMI-GGREGTE (= route cannot be dis-aggregated) Optional Transitive MULTI-EXIT-IS (ME)(see later) GGREGTOR (who aggregated this route) Optional Nontransitive WEIGHT (see later) 138 NEXT-HOP R5 E-GP R6 I-GP S x S z R1 I-GP R2 E-GP R3 R4.1/16.2/16 S y R3 advertises.2/16 to R1, NEXT-HOP = R4 IP address R6 advertises.2/16 to R5, NEXT-HOP = R6 IP address Q. where is such a scenario likely to happen? 139

70 MULTI-EXIT-IS (ME) packet to packet to S x R1 R2.1/16 ME=.2/16 ME=50 E-GP E-GP.1/16 ME=50.2/16 ME= R3.1/16.2/16 R4 S y One S connected to another over several links ex: multinational company connected to worldwide ISP S y advertises its prefixes with different MEs (low = preferred) If S x accepts to use MEs put by Sy: traffic goes on preferred link 140 ME Example Q1: by which mechanisms will R1 and R2 make sure that packets to Sy use the preferred links? Q2: router R3 crashes; can.1/16 still be reached? explain the sequence of actions. 141

71 ME Question Q1: ssume now Sx and Sy are peers (ex: both are ISPs). Explain why Sx is not interested in taking ME into account. Q2: y which mechanisms can Sx pick the nearest route to Sy? 142 LOL-PREF pref=0 R6 E-GP I-GP R1 E-GP S x I-GP E-GP R2 pref= Used inside an S to select a best S path ssigned by border router when receiving route over E-GP Propagated without change over I-GP Example R6 associates pref=0, R2 pref= R1 chooses the largest preference bgp default local-preference pref-value 143

72 LOL-PREF Example Q1: The link S2-S4 is expensive. How should S 4 set local-prefs on routes received from S 3 and S 2 in order to route traffic preferably through S 3? Q2:Explain the sequence of events for R1, R2 and R3. S1:.1/16.1/16 S 1 S1:.1/16 S 2 R1 S 4 R2 S 3 R3 S LOL-PREF Question Q: ompare ME to LOL-PREF 145

73 hoice of the best route one by decision process ; result is: route installed in Loc- RI t most one best route to exactly the same prefix is chosen Only one route to 2.2/16 can be chosen ut there can be different routes to 2.2.2/24 and 2.2/16 ecision Process uses the following priorities (for example) 1. Highest LOL-PREF 2. Shortest S-PTH 3. Lowest ME, if taken seriously by this network 4. E-GP > I-GP 5. Shortest path to NEXT-HOP, according to IGP 6. Lowest GP identifier 146 F.5. Interaction GP IGP Packet Forwarding There are three interactions between GP and internal routing that you have to know Redistribution: routes learnt by GP are passed to IGP (ex: OSPF) alled redistribution of GP into OSPF OSPF propagates the routes using type 4 LSs to all routers in OSPF cloud Injection: routes learnt by GP are written into the forwarding table of this router Routes do not propagate; this helps only this router Synchronization: see later 147

74 Redistribution Example R5 E-GP R6 S x IGP 18.1/16 I-GP (OSPF) R2 S z R1 IGP R5 advertises 18.1/16 to R6 via E-GP (OSPF) E-GP R6 transmits it to R2 via I-GP TP connection between R6 and R2 (redistribute GP into IGP) R6 injects 18.1/16 into IGP R4 (OSPF) OSPF propagates 18.1/16 (type 4 LS) and updates forwarding tables fter OSPF converges, R1, R2 now have a route to 18.1/6 S y R2 advertises route to R4 via E-GP (synchronize with IGP) R2 must wait for the OSPF entry to 18.1/6 before advertising via E-GP Packet to 18.1/16 from S y finds forwarding table entries in R2, R1 and R6 148 Example with Re-istribution by, F learns from G the route 2-1-n1 redistributes the external route 2:n2 into OSPF by, learns the route 2:n2; by learns the route Y2:n2; selects 2:n2 and does not redistribute it to OSPF by, learns the route 2:n2 by, learns the route X:2:n2 by, L learns the route to n2 via 1 = EPFL n1 1: n1 L X 1: n1 2 : n2 E Y 2: n2 Y G 2 = U F n2 2 : n2 physical link TP connection 149

75 Re-istribution onsidered Harmful In practice, operators avoid re-distribution of GP into IGP Large number of routing entries in IGP Reconvergence time after failures is large if IGP has many routing table entries classical solution is based on recursive table lookup When IP packet is submitted to router, the forwarding table may indicate a NEXT-HOP which is not on-link with router second table lookup needs to be done to resolve the next-hop into an on-link neighbour in practice, second lookup is done in advance not in real time by preprocessing the routing table 150 Example: Recursive Table Lookup t R1, data packet to.1.x.y is received The forwarding table at R1 is looked up Q: what are the next events? To NEXT-HOP layer-2 addr.1/ N/ x09:f1:6:33:76:21 R R2 R /16 151

76 void Redistribution: ombine Recursive Lookup and NEXT-HOP R5 E-GP R6 S x IGP 18.1/16 I-GP (OSPF) R2 S z R5 advertises 18.1/16, NEXT-HOP = to R6 via E-GP R6 transmits route to R2 via I-GP R6 injects 18.1/16, NEXT-HOP = into its local forwarding table, does not re-distribute into OSPF R2 injects 18.1/16, NEXT-HOP = into its local forwarding table ata packet to is received by R2 Recursive table lookup at R2 can be used Q: there is a problem at R1: how can we solve it? R1 IGP E-GP S y R4 152 void Redistribution: Practical Solution R5 S x E-GP R IGP 18.1/16 I-GP (OSPF) R2 S z Practical solution often deployed ll core routers also run I-GP (in addition to IGP) Edge routers change NEXT-HOP to self Routes learnt from GP are injected into local forwarding table Recursive table lookup is done at all routers Q: repeat the sequence of previous slide with this new assumption Potential problem: I-GP mesh -> use reflectors IGP handles only internal networks very few R1 IGP E-GP S y R4 153

77 F.6. Other Mechanisms in GP Route Flap ampening Route modification propagates everywhere Sometimes routes are flapping successive UPTE and WITHRW caused for example by GP speaker that often crashes and reboots Solution: decision process eliminates flapping routes How withdrawn routes are kept in dj-rin-in if comes up again soon (ie : flap), route receives a penalty penalty fades out exponentially used to suppress or restore routes 154 Route Flap ampening penalty suppress-limit reuse-limit time t 1 t 2 Route suppressed at t 1, restored at t 2 155

78 void I-GP Mesh: onfederations S P1 S P2 S P3 E-GP E-GP I-GP I-GP I-GP E-GP S z E-GP E-GP S decomposed into sub-s private S number similar to OSPF areas I-GP inside sub-s (full interconnection) E-GP between sub-s 156 void I-GP Mesh : Route reflectors I-GP RR RR RR I-GP I-GP I-GP I-GP I-GP cluster 1 cluster 2 cluster 3 E-GP S z E-GP E-GP luster of routers one I-GP session between one client and RR LUSTER_I Route reflector re-advertises a route learnt via I-GP to avoid loops ORIGINTOR_I attribute associated with the advertisement 157

79 I-GP configuration lo0 I-GP lo0 I-GP S z I-GP configured on loopback interface (lo0) interface always up IP address associated with the interface IGP routing guarantees packet forwarding to the interface 158 void E-GP mesh: Route server E-GP t interconnection point Instead of n(n-1)/2 peer to peer E-GP connections n connections to Route Server To avoid loops VERTISER attribute indicates which router in the S generated the route 159

80 ommunities Other attributes can be associated with routes in order to simplify rules. They are called «communities» Pre-defined: Example: NO-EXPORT ( a well known, pre-defined attribute) see later for an example efined by one S (a label of the form SN:x where S= S number, x = a 2 byte number) 160 NO EXPORT Written on E-GP by one S, transmitted on I-GP by accepting S, not forwarded Example: S2 has different routes to S1 but S2 sends only one aggregate route to S3 simplifies the aggregation rules at S2 What is the route followed by a packet sent to received by R4? R / /17 NO-EXPORT 2.2/16 R3 R4 2.2/ /17 R5 R /17 NO-EXPORT 2.2/16 161

81 F.7. Examples ual Homing Hot potato routing 162 Ex1: Stub rea R2 Operator S R1 lient S GP not needed between lient and Operator No S number for client R2 learns all prefixes in lient by static configuration or RIP on link R1 R2 Example: EPFL and Switch Q: what if R1 fails? 163

82 Ex2: Stub rea, ual Homing to Single Provider S x R1 Provider R2.1/16 ME=.2/16 ME=50 E-GP E-GP.1/16 ME=50.2/16 ME= R3.1/16 lient.2/16 R4 S y With numbered lient S Use ME to share traffic from ISP to lient on two links Use lient IGP configuration to share traffic from lient to two links Q1: is it possible to avoid distributing GP routes into lient IGP? Q2: is it possible to avoid assigning an S number to lient? Q3: is it possible to avoid GP between lient and Provider? 164 Ex3: Stub rea, ual Homing to Several Providers S z S x R1 Providers S y R2 E-GP E-GP R3 2.0/17 lient 2.1/17 S 0 R4 lient has own address space and S number Q: how can routes be announced between S 0 and S x? S x and S z? Q: assume lient wants most traffic to favour S x. How can that be done? 165

83 Ex4: Hot Potato Routing ustomer 1 R11 ISP1 R12 ustomer 2 R21 ISP2 R22 Packets from ustomer 2 to ustomer 1 oth R21 and R22 have a route to ustomer 1 Shortest path routing favours R21 Q1: by which mechanism is that done? Q2: what is the path followed in the reverse direction? 166 F.8. Illustrations: Switch 167

84 n Interconnection Point

85 from all routes to /15 on RIPE Route ollectors S TP re im Type epefath ix er Next HOP ME Origin ommunity RR I / :05:49Z Not defined : : :3000 RIPE N / :16:00Z IGP : :2 3356: : : :2076 Netnod / :16:17Z IGP :1653 Netnod / :16:37Z IGP :1653 Netnod / :21:08Z IGP : : :155 Netnod / :21:17Z IGP :1653 Netnod / :24:06Z Not defined : Some statistics Number of routes : exponential increase : IR : linear increase (000/year) : return to exponential increase (42% per year) since 2001: return to linear increase, ~120,000 Number of Ss 51% per year for 4 last years S effectively used Number of IP addresses 162,128,493 (Jul 2002) 7% per year 171

86

87 Number of hosts 174 GP statistics GP routing table entries examined: Total Ses present in the Internet Routing Table: Origin-only Ses present in the Internet Routing Table: Transit Ses present in the Internet Routing Table: 1883 Transit-only Ses present in the Internet Routing Table: 63 verage S path length visible in the Internet Routing Table: 5.3 Max S path length visible: 23 Number of addresses announced to Internet: Equivalent to 70 /8s, 128 /16s and 147 /24s Percentage of available address space announced: 31.9 Percentage of allocated address space announced:

88 Prefix length distribution /1:0 /2:0 /3:0 /4:0 /5:0 /6:0 /7:0 /8:17 /9:5 /:8 /11:12 /12:46 /13:90 /14:239 /15:430 /16:7308 /17:1529 /18:2726 /19:7895 /20:7524 /21:5361 /22:8216 /23:9925 /24:64838 /25:185 /26:221 /27:126 /28:5 /29:85 /30:93 /31:0 /32:

89 Exercise What Ss does EPFL receive service from? What Ss does Switch receive service from? Find the names of the networks that have these S numbers 178 Exercise Lookup to find out the relationships between Switch and other providers How does the software on this site decide whether a relationship is client, provider or peer? 179

90 G. Load ependent Routing Instead of maximizing a path quality metric (nb hops, delay) assume we want to maximize the total network utility for example: total transported flows see congestion control chapter for other definitions how should routing be done? Q1: show an example where shortest path routing does not provide the optimal total flow (where path cost is static) One solution might be to take delay as the path cost high load on a link => high cost => link is less used however, this does not solve the problem: there is the raess paradox 180 raess Paradox (1) ssume all flows pick the route with shortest delay ssume parallel paths exist and flows can make use of them elay is function of load as given below; link 5 is (temporarily) closed Total offered load is b0 = 6 Gb/s For example, if we split traffic into : route 1-3: b = 1, route 2-4 b = 5 the delay along route 1-3 is 61, along route 2-4 is 5 thus the link costs will change and routing decisions will change also Eventually, there will be an equilibrium (called Wardrop Equilibrium ) delay is equal on all competing routes Q: compute the equilibrium traffic flow on every link 181

91 raess Paradox (2) Q: same question when we open link 5 with delay function: 182 raess Paradox and eyond With shortest delay routing, adding a new link may decrease overall throughput shortest delay routing is not a global optimum The global optimum problem: minimize total delay subject to flow constraints this is a well posed optimization problem the optimal solution depends on all flows but it can be implemented in a distributed algorithm similar to TP congestion control [ertsekasgallager ] In practice, it can be implemented in a network through a centralized network management procedure that updates the link costs (used by distance vector routing). given link costs c_i and traffic matrix compute total throughput or average delay ( a hard optimization problem, solved with heuristics) every few minutes, update the link costs in all routers let the routing algorithm compute new paths 183

92 Further Reading Slow convergence after route suppression GP path exploration, similar to (but worse than) distance vector slow convergence. Is in the nature of path vector routing with explicit suppression. raig Labovitz, bha huja, bhijit ose, Farnam Jahanian: elayed Internet routing convergence. IEEE/M Trans. Netw. 9(3): (2001) Route flap dampening slows down convergence Zhuoqing Morley Mao, Ramesh Govindan, George Varghese, Randy H. Katz: Route flap damping exacerbates internet routing convergence. SIGOMM 2002: Path vector + policy may suffer from incompatibilities (loops) The stable paths problem and interdomain routing Griffin, T.G.; Shepherd, F..; Wilfong, G. M/IEEE ToN pril 2002, Page(s): References Timothy Griffin s home page at Intel RF 1771 (GP-4). Huitema, Le Routage dans l Internet John W. Stewart III GP 4 : S paths aggregation statistics map of Internet rpsl.info.ucl.ac.be relations between Ss 185

93 Solutions 186 Example pply the theorem: write p k (i,1), pred(i) and draw the shortest paths to node 1. i\k i pred(i)

94 Impact of Initial onditions Example: does the algorithm converge to the shortest path with initial condition as shown? i\k i\k onclusions from Example 2 Q: during convergence time, how are routing tables? : they are incorrect there are loops packets are discarded (TTL expires) 189

95 Test Your Understanding Q1: Run ijkstra at : (final step) Q2: What are the routing tables at m()=0 m()=30 E m(f)= m()= m()=0 F m(f)= 190 Test Your Understanding Q2: What are the routing tables at : n3 n6 E n2 n4 n5 n7 F n1 net next n1 direct n2 n3 n4 direct n5 direct n6 F n7 F 191

96 Solution area 0 1 X1 6 X3 6 X2 6 X4 1 n1 2 area 1 6 X5 6 X6 6 area 2 2 n2 area 1 topology database n1, d=28 n2, d=22 n1, d=22 n2, d=16 n1, d= n2, d=16 n1, d=16 n2, d= n1 n2 area 2 topology database area 0 topology database 192 Path Vector Routing Q. Explain how E chooses the paths to n1 and n2. E receives the routes n1 and n1. E selects as best routes the ones with shorter S path. Q. How can loops be avoided? : GP routers recognize looping announcements by the repetition of the same S in the path. Such announcements are discarded n1, n2 :n1,n2 n5 :n1,n2 :n5 n4 E GP table in E dest dest S S path path n1 n1 n2 n2 n3 n3 n4 n4 n5 n5 :n1,n2 n3 :n1,n2 :n3 :n1,n2 : n3 : n4 193

97 order Gateways, e-gp and I-GP GP runs on routers called border gateways = GP speakers -- belong to one S only Q: compare to OSPF : there is one single inter-area router per area boundary: it belongs to both areas In addition, GP speakers talks to each other inside the S using Internal- GP (I-GP) over TP connections I-GP is the same as E-GP except for one rule: routes learned from a neighbour in the mesh are not repeated inside the mesh ( Q. why? ) : otherwise loops cannot be avoided (same S number!) Q: Is there a need for all GP speakers in one network to be adjacent? : no, they are generally not. The mesh is over TP connections. X:n1 X:n1 1 2 E F 3 ->: 1,X:n1 ->E: 1,X: n1 E->G: 3,1,X:n1 G 4 H ggregation Example 2 n2 S1 S / /22 S / /24 S4 n /22 S4 receives /22 S_PTH: 3 {1 2} /24 S_PTH: 2 oth routes are injected into S4 s routing tables Q: what happens to packets from n4 to n2? : it depends on the attributes set by the rules in S4; by default, the direct route to n2 is preferred (fewer ss in path). There are two routing entries in S4 routers: one for /22 and one for /24. Longest prefix match in the packet forwarding algorithm ensures that packets to n2 go on the direct route. 195

98 ggregation Example 3 n2 S1 S / /24 S /24 S5 S / / /22 S4 n /24 S4 receives /22 S_PTH: 3 {1 2} /24 S_PTH: oth routes are received by S4; only shortest S paths routes are injected into routing tables Q: what happens to packets from n4 to n2? : now packets to n2 go via S3 196 Example Without ggregation S1 S / /24 S / / /24 S / / / /24 Q: If S3 does not aggregate, what are the routes announced by S 4? Is there any benefit? : /23 S_PTH: /24 S_PTH: /24 S_PTH: 4 2 : there is no benefit since all routes go via S 4 anyhow. S4 should aggregate. 197

99 NEXT-HOP R5 E-GP R6 I-GP S x S z R1 I-GP R2 E-GP R3 R4.1/16.2/16 S y R3 advertises.2/16 to R1, NEXT-HOP = R4 IP address R6 advertises.2/16 to R5, NEXT-HOP = R6 IP address Q. where is such a scenario likely to happen? : in interconnection points with many providers interconnected on one LN 198 ME Example Q1: by which mechanisms will R1 and R2 make sure that packets to Sy use the preferred links? : R1 and R2 exchange their routes to X y via I-GP R1 has 2 routes to.1/16, one of them learnt over E-GP; prefers route via R1; injects it into IGP R1 has 2 routes to.2/16, one of them learnt over E-GP; prefers route via R2; does not inject a route to.2/16 into IGP Q2: router R3 crashes; can.1/16 still be reached? explain the sequence of actions. : R1 clears routes to Sy learnt from R1 (keep-alive mechanism) R2 is informed of the route suppression by I-GP R2 has now only 1 route to.1/16 and 1 route to.2/16;. keeps both routes in its local RI and injects them into IGP since both were learnt via E-GP traffic to.1/16 now goes to R2 199

100 ME Question Q1: ssume now Sx and Sy are peers (ex: both are ISPs). Explain why Sx is not interested in taking ME into account. : Sx is interested in sending traffic to Sy to the nearest exit, avoiding transit inside Sx as much as possible. Thus Sx will choose the nearest route to Sy, and will ignore MEs Q2: y which mechanisms can Sx pick the nearest route to Sy? : it depends on the IGP. With OSPF: all routes to Sy are injected into OSPF by means type 5 LSs. These LSs say: send to router R3 or R4. Every OSPF router inside Sx knows the cost (determined by OSPF weights) of the path from self to R3 and R4. Packets to.1/16 and.2/16 are routed to the nearest among R3 and R4 (nearest = lowest OSPF cost). 200 LOL-PREF Example Q1: The link S2-S4 is expensive. How should S 4 set local-prefs on routes received from S 3 and S 2 in order to route traffic preferably through S 3? : for example: set LOL-PREF to 0 to all routes received from S 3 and to 50 to all routes received from S 2 S1:.1/16.1/16 S 1 S1:.1/16 Q2: Explain the sequence of events for R1, R2 and R3 S 2 R1 S 3 R2 S 4 R3 S3 S1:.1/16 S 5 : R1 receives the route S2 S1.1/16 over E-GP; sets LOL-PREF to 50 R2 receives the route S3 S1.1/16 over E-GP; sets LOL-PREF to 0 R3 receives S2 S1.1/16, LOL- PREF=50 from R1 over I-GP and S3 S1.1/16, LOL-PREF=0 from R1 over I-GP R3 selects S3 S1.1/16, LOL- PREF=0 and installs it into local-ri R3 announces only S3 S1.1/16 to S 5 201

101 LOL-PREF Question Q: ompare ME to LOL-PREF : ME is used between Ss (i.e. over E-GP); LOL-PREF is used inside one S (over I-GP) ME is used to tell one provider S which entry link to prefer; LOL- PREF is used to tell the rest of the world which S path we want to use, by not announcing the other ones. 202 Example with Re-istribution by I-GP, F learns from G the route to 2-1-n1 redistributes the external route 2:n2 into OSPF; by I-GP, learns the route 2:n2; by E-GP learns the route Y2:n2; selects 2:n2 and does not redistribute it to OSPF by I-GP, learns the route 2:n2 from by E-GP, learns the route X:2:n2 by OSPF, L learns the route to n2 via I-GP - internal GP E-GP - external GP 1 = EPFL n1 1: n1 L X 1: n1 2 : n2 E Y 2: n2 Y G 2 = U F n2 2 : n2 physical link TP connection 203

102 Example: Recursive Table Lookup t R1, data packet to.1.x.y is received The forwarding table at R1 is looked up Q: what are the next events? : first, the nex-hop is found; a second lookup for is done; the packet is sent to M address x09:f1:6:33:76:21 To NEXT-HOP layer-2 addr.1/ N/ x09:f1:6:33:76:21 R R2 R / void Redistribution: ombine Recursive Lookup and NEXT-HOP R5 E-GP R6 S x IGP 18.1/16 I-GP (OSPF) R2 S z ata packet to is received by R2 Recursive table lookup at R2 can be used Q: there is a problem at R1: how can we solve it? : the problem is. Packet is sent by R2 towards R1, which is the neighbour towards t R1, there is no forwarding entry to 18.1/16 blackhole! solution would be to use loose source routing: R2 adds as loose source routing info into packet. In practice however, source routing is not used with IPv4. See later in the section for another solution. R1 IGP E-GP S y R4 205

103 void Redistribution: Practical Solution R5 S x E-GP R IGP 18.1/16 I-GP (OSPF) R2 S z Q: repeat the sequence of previous slide with this new assumption R5 advertises 18.1/16, NEXT-HOP = to R6 via E- GP R6 transmits 18.1/16, NEXT-HOP = to R1 and R2 via I-GP R6 injects 18.1/16, NEXT-HOP = into its local forwarding table R2 injects 18.1/16, NEXT-HOP = into its local forwarding table Independently, IGP finds that, at R2, packets to should be sent to R1 ata packet to is received by R2 t R2, recursive table lookup determines that packet should be forwarded to R1 t R1, recursive table lookup determines that packet should be forwarded to R6 t R6, recursive table lookup determines that packet should be forwarded to R1 IGP E-GP S y R4 206 NO EXPORT Q: What is the route followed by a packet sent to received by R4? : the packet is sent via R3 and R1 R / /17 NO-EXPORT 2.2/16 R3 R4 2.2/ /17 R5 R /17 NO-EXPORT 2.2/16 207

104 Ex2: Stub rea, ual Homing to Single Provider S x Provider R1 R2.1/16 ME=.2/16 ME=50 E-GP E-GP.1/16 ME=50.2/16 ME= R3.1/16 lient.2/16 R4 S y Q1: is it possible to avoid distributing GP routes into lient IGP? : yes, for example: configure R3 and R4 as default routers in lient S; traffic from lient S is forwarded to nearest of R3 and R4. If R3 or R4 fails, to the remaining one Q2: is it possible to avoid assigning an S number to lient? : Yes, it is sufficient to assign to lient a private S number: Provider translates this number to its own. Q3: is it possible to avoid GP between lient and Provider? : Yes, by running a protocol like RIP between lient and Provider and redistributing lient routes into Provider IGP. Thus Provider pretends to the rest of the world that the prefixes of lient are its own. 208 Ex3: Stub rea, ual Homing to Several Providers S z S x R1 Providers S y R2 E-GP E-GP R3 2.0/17 lient 2.1/17 S 0 R4 lient has own address space and S number Q: how can routes be announced between S 0 and S x? S x and S z? : R3 announces 2.0/17 and 2.0/16; traffic from S x to 2.0/17 will flow via S x; if R3 fails, it will use the longer prefix and flow via sy. Sx announces 2.0/17 and 2.0/16 to S z Q: assume lient wants most traffic to favour S x. How can that be done? : R3 announces an artificially inflated path: : 2.0/17. S z will favour the path via sy which has a shorter S path length 209

105 Ex4: Hot Potato Routing ustomer 1 R11 ISP1 R12 ustomer 2 R21 ISP2 R22 Packets from ustomer 2 to ustomer 1 oth R21 and R22 have a route to ustomer 1 Shortest path routing favours R21 Q1: by which mechanism is that done? : «hoice of the best route» (criteria 5), assuming alll routers in ISP2 run GP Q2: what is the path followed in the reverse direction? : see picture. Note the asymmetric routing 2 Exercise What Ss does EPFL receive service from? from the previous routes, we find S 559 (Switch) What Ss does Switch receive service from? from the previous routes we see that there are at least: S 1299 S S 3549 Find the names of the networks that have these S numbers from whois on S 1299: Telianet S 20965: Geant S 3549: Global rossing 211

106 Exercise Lookup to find out the relationships between Switch and other providers How does the software on this site decide whether a relationship is client, provider or peer? S X is client of Switch if S X accepts NY path and announces only self (S X) S X is provider of Switch if S X announces NY path and accepts only S Switch S X is a peer if S X accepts and announces only a small set of routes