Communication Networks ( ) / Spring 2011 The Blavatnik School of Computer Science, Tel-Aviv University

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1 ommunication Networks ( ) / Sprin 0 The lavatnik School o omputer Science, Tel-viv University llon Waner urose & Ross, hapters (5 th ed.) Tanenbaum & Wetherall, hapters (5 th ed.) Many slides adapted rom:. urose &. Ross \ omputer Networkin: Top own pproach (5 th ed.) ddison-wesley, pril 009. opyriht ,. urose and.w. Ross, ll Rihts Reserved. ubs physical-layer ( dumb ) repeaters: bits comin in one link o out all other links at same rate all nodes connected to hub can collide with one another no rame buerin no SM/ at hub: host Ns detect collisions hub twisted pair Switch link-layer device: smarter than hubs, take active role store, orward thernet rames examine incomin rame s M address, selectively orward rame to one-or-more outoin links when rame is to be orwarded on sement, uses SM/ to access sement transparent hosts are unaware o presence o switches plu-and-play, sel-learnin switches do not need to be coniured ata Link Layer 5-3 ata Link Layer 5-4 Switch: allows multiple simultaneous transmissions hosts have dedicated, direct connection to switch switches buer packets thernet protocol used on each incomin link, but no collisions; ull duplex each link is its own collision domain switchin: -to- and - to- simultaneously, without collisions not possible with dumb hub switch with six interaces (,,3,4,5,6) Switch Table Q: how does switch know that reachable via interace 4, reachable via interace 5? : each switch has a switch table, each entry: (M address o host, interace to reach host, time stamp) looks like a routin table! Q: how are entries created, maintained in switch table? somethin like a routin protocol? switch with six interaces (,,3,4,5,6) ata Link Layer 5-5 ata Link Layer 5-6

2 Switch: sel-learnin switch learns which hosts can be reached throuh which interaces when rame received, switch learns location o sender: incomin LN sement records sender/location pair in switch table M addr interace TTL Source: est: Switch table (initially empty) Switch: rame ilterin/orwardin When rame received:. record link associated with sendin host. index switch table usin M dest address 3. i entry ound or destination then { i dest on sement rom which rame arrived then drop the rame else orward the rame on interace indicated } else lood orward on all but the interace on which the rame arrived ata Link Layer 5-7 ata Link Layer 5-8 Sel-learnin, orwardin: example rame destination unknown: lood destination location known: selective send M addr interace TTL Source: est: Switch table (initially empty) nterconnectin switches switches can be connected toether S S Q: sendin rom to - how does S know to orward rame destined to via S 4 and S 3? : sel learnin! (works exactly the same as in sinle-switch case!) S 4 S 3 ata Link Layer 5-9 ata Link Layer 5-0 Sel-learnin multi-switch example Suppose sends rame to, responds to nstitutional network S S S 4 S 3 to external network router mail server web server P subnet Q: show switch tables and packet orwardin in S, S, S 3, S 4 ata Link Layer 5- ata Link Layer 5-

3 Question ride = {,a};{,};{,b} c ride = {,};{,c};{,b} ind all errors in the table anxplain why? ride name rror in table xplain ll rrors ride = {,a};{,};{,b} c ride = {,};{,c};{,b} oes a messae reaches destination? rom to rom to rom to What will happen to the tables? rom to rom to ride = {,a};{,};{,b}{,c} c ride = {,};{,c};{,b} ride = {,a};{,};{,b} c ride = {,};{,c};{,b} 3

4 rom to ride = {,a};{,};{,b} c ride = {,};{,c};{,b} What will happen with loops? ncorrect learnin,, Lecture 3 #0 What will happen with loops? rame loopin What will happen with loops? rame loopin,??,??,, Lecture 3 # Lecture 3 # Loop-ree: tree Loop-ree: tree : messae rom will mark s location messae rom will mark s location Lecture 3 #3 Lecture 3 #4 4

5 Loop-ree: tree Loop-ree: tree : : : : : : : messae rom will mark s location messae rom will mark s location Lecture 3 #5 Lecture 3 #6 Loop-ree: tree Loop-ree: tree : : : : : : : : : : messae rom will mark s location So a messae to will o by marks messae rom will mark s location Lecture 3 #7 Lecture 3 #8 ntroduction Perlman, hapters ( th ed.) Peterson & avie, hapter 3.. (3 th ed.) eveloped by Radia Perlman Standardized in 80. We will learn the main concepts, and skip some o the technical drudery Reer to Perlman s book or to the standard or a complete description rides run a distributed spannin tree alorithm Select which ports (and brides) should actively orward rames ynamic, adapts itsel to topoloy chanes 5

6 STP s oal reate a spannin-tree o the LN sements in the extended LN This is done by loically removin ports rom the network in order to reduce it to an acyclic raph ata traic is discarded upon receipt in ports not selected or inclusion in the spannin tree. Sometimes, an entire bride will be removed rom the network STP preliminaries rides reularly exchane rames known as oniuration ride Protocol ata Units (oniuration PUs). very bride has a unique. The bride with the smallest is the root bride. ach coni messae transmitted by bride contains: Root : o the bride R which currently considers to be the root ost: Least cost path rom to R (o all the paths is currently aware o) Transmittin ride : s. STP preliminaries (cont.) bride initially assumes itsel to be the root and transmits rames on all ports indicatin it to be the root with cost 0. ach bride saves or each port the best rame it had seen so ar on this port. est means: Smaller root root s are equal: lower cost to the root ost is the sum o the cost indicates in the messae had received, and the cost o the link on which it was received Oten cost is measured in hops, and thus is simply cost in ms +. root s and costs are equal: transmittin bride has lower still a tie: break ties by port identiier or simplicity s sake, we ll inore that in our presentation xample (adapted rom Perlman) Root 9 ost Transmitter Root ost Transmitter n all three case, coni messae is better than (assumin cost is hops). Smaller root. Smaller distance to the same root 3. Same root and distance, but throuh ride with smaller xample (adapted rom Perlman) ride 9 has 5 ports t saves the best coni messae it has seen on each port (root. ost to root. Transmitter ) ssume all link costs are. port : port : xample (cont.) ride 9 assumes the root is 4 and best distance to it is + = 3. 4 can be reached in cost 3 either via or via. is chosen (smaller than ). 9 can transmit the messae port : port :

7 Selection o Root Port very bride accepts one bride as the root the smallest root indicatin on any o the ports, or itsel i its is even smaller can now select its Root Port the port which indicated the least cost to R will be used to transer messaes towards the root Selection o esinated Ports or every LN sement is connected to decides whether it is the desinated bride or that LN: i its coni messae is the best it had seen on this LN n that case, the correspondin port becomes the desinated port or that LN. will be responsible to deliverin data rames rom the root towards this LN via the desinated port desinated port is never also a root port (Why?) ll the ports o the root bride are desinated (Why?) Selectin spannin tree ports orwards data messaes only on its root port and its desinated ports. Other ports discard data messaes upon receipt. t miht even be that entire brides are removed rom the network xample (cont.) Recall that 9 assumes 4 is the root, with cost 3 throuh. ts coni messae is Port 4 is the root port 9 decides it is the desinated bride on the LN sements connected to ports,. These are desinated ports. port : port : xample (cont.) Ports,, 4 set to orwardin state Ports 3, 5 set to blockin state xample (cont.) the bride s had been 5, it would have decided it was the root port, transmittin the messae ll its ports would have become desinated port : port : port : port :

8 xample: esinated port / Root Port is the root bride 3 and 5 are both connected to LN, but 5 is the desinated port since it's closer to root 5 and 7 are both connected to LN, but 5 is the desinated port due to smaller (equal distance). nd 3 nd this one 6 5 What are these 7 4 nd these STP Stabilization Recall that ride initially assumes itsel to be the root and transmits rames on all ports indicatin it to be the root with cost 0. When ride accepts another bride R as the root, it stops eneratin coni messaes on its own. transmits coni messaes on all its desinated ports when triered to (by an you see why only on the desinated ports? t adds the relevant link cost (oten simply +) to the cost indicated in them When the alorithm stabilizes, only the real root R enerates coni messaes, and the other brides orward them on their desinated ports STP Run ind Root sends PU sends PU sends PU 4, sends PU 8 sends PU 9 9 L 8 8 M STP Run lock Ports : 5, 0, :, 0, 3: LO 7: 7, 0, 5: 5, 0, 7: LO ata Laptop Messae to 9 9 L L M M Laptop 8

9 Topoloy chanes STP dynamically adapts to link / bride additions: new bride / ride with a new port will think it is the root / the desinated bride or the LN until receivin the appropriate coni messaes ctually, there are some technicalities here we will not o into Topoloy chanes (cont.) STP dynamically adapts to link / bride removal Recall that every bride saves or each port the best messae it had seen on this port The entry also holds an ae ield (which is reset by resh coni messaes). but not always to 0 aain, technicalities When a link / bride ails or is removed rom the network resh coni messaes miht stop arrivin on some o s ports: ae is increased radually until the entry expires the STP alorithm kicks in aain: reselects its assumed root, cost to root. 9