Chapter 4 Network Layer

Transcription

1 Chapte 4 Netwok Laye Reti di Elaboatoi Coso di Lauea in Infomatica Univesità degli Studi di Roma La Sapienza Canale A-L Pof.ssa Chiaa Petioli Pate di queste slide sono state pese dal mateiale associato al libo Compute Netwoking: A Top Down Appoach, 5th edition. All mateial copyight J.F Kuose and K.W. Ross, All Rights Reseved Thanks also to Antonio Capone, Politecnico di Milano, Giuseppe Bianchi and Fancesco LoPesti, Un. di Roma To Vegata Netwok Laye 4-1

2 Pevious lectue. Summay: Distibuted Belman Fod Based on Distibuted Bellman Fod Equation Cost associated to the (X,Z) link X D (Y,Z) = = distance fom X to Y, via Z as next hop Z c(x,z) + min {D (Y,w)} w D x (Y,Z) ecomputed: Upon eception of updates fom the neighbos Upon link cost change min z D X (Y,Z) communicated to the neighbos wheneve its value changes, o peiodically Netwok Laye 4-2

3 Distance Vecto Routing: oveview Iteative, asynchonous: each local iteation caused by: local link cost change message fom neighbo: its least cost path change fom neighbo Distibuted: each node notifies neighbos only when its least cost path to any destination changes m neighbos then notify thei neighbos if necessay Each node: wait fo (change in local link cost of msg fom neighbo) ecompute distance table if least cost path to any dest has changed, notify neighbos Netwok Laye 4-3

4 Distibuted Bellman Fod coectness Completely asynchonous Stating fom abitay estimates of the cost of the best oute fom node i to the destination, if: m m m links weights ae constant fo enough time fo the potocol to convege stale info expie afte a while once in a while updated info ae sent fom a node to its neighbos the Distibuted Bellman Fod algoithm conveges, i.e. each node coectly estimates the cost of the best oute to the destination Netwok Laye 4-4

5 Bellman-Fod Given a gaph G=(N,A) and a node s finds the shotest path fom s to evey node in N. A shotest walk fom s to i subject to the constaint that the walk contains at most h acs and goes though node s only once, is denoted shotest(<=h) walk and its length is D h i. Bellman-Fod ule: Initiatilization D h s =0, fo all h; w i,k = infinity if (i,k) NOT in A; w k,k =0; D 0 i =infinity fo all i!=s. Iteation: D h+1 i =min k [w i,k + Dh k ] Assumption: non negative cycles (this is the case in a netwok!!) The Bellman-Fod algoithm fist finds the one-ac shotest walk lengths, then the two-ac shotest walk length, then the thee-ac etc. distibuted vesion used fo outing Netwok Laye 4-5

6 Pevious lectue. Summay: Distibuted Belman Fod Based on Distibuted Bellman Fod Equation X D (Y,Z) Cost associated to the (X,Z) link = = distance fom X to Y, via Z as next hop Z c(x,z) + min {D (Y,w)} w D x (Y,Z) ecomputed: Upon eception of updates fom the neighbos Upon link cost change min z D X (Y,Z) communicated to the neighbos wheneve its value changes, o peiodically How long does it take fo the algoithm to convege? good news tavel fast, bad news may not count to infinity Netwok Laye 4-6

7 Distance Vecto: link cost changes Subtitle: Distibuted Bellman Fod conveges but how fast? Link cost changes: node detects local link cost change updates distance table (line 15) if cost change in least cost path, notify neighbos (lines 23,24) 1 X 4 Y 50 1 Z good news tavels fast algoithm teminates Ydetects change infoms neighbos Z eceives update ppdates table, new least Cost infoms neighbos Y eceives new least cost; no modification in the outing table, Netwok Laye 4-7 No updates sent

8 Distance Vecto: link cost changes Link cost changes: good news tavels fast Y bad news tavels slow - count 4 1 to infinity poblem! X Z algoithm continues on! Y detects link cost Incease but think can Reach X though Z at a total cost of 6 (wong!!) The path is Y-Z-Y-X Netwok Laye 4-8

9 Count-to-infinity an eveyday life example Which is the poblem hee? the info exchanged by the potocol!! the best oute to X I have has the following cost (no additional info on the oute) A Roman example -assumption: thee is only one oute going fom Colosseo to Altae della Patia: Via dei Foi Impeiali. Let us now conside a netwok, whose nodes ae Colosseo., Altae della Patia, Piazza del Popolo Colosseo 1 Km 1 Km Altae Patia Piazza del Popolo Netwok Laye 4-9

10 Count-to-infinity eveyday life example (2/2) 1Km 1Km Colosseo Al.Patia P.Popolo The Colosseo. and Alt. Patia nodes exchange the following info Colosseo says the shotest oute fom me to P. Popolo is 2 Km Alt. Patia says the shotest path fom me to P. Popolo is 1Km Based on this exchange fom Colosseo you go to Al. Patia, and fom thee to Piazza del Popolo OK Now due to the big dig they close Via del Coso (Al. Patia P.Popolo) Al. Patia thinks I have to find anothe oute fom me to P.Popolo. Look thee is a oute fom Colosseo to P.Popolo that takes 2Km, I can be at Colosseo in 1Km I have found a 3Km oute fom me to P.Popolo!! Communicates the new cost to Colosseo that updates OK I can go to P.Popolo via Al. Patia in 4Km VERY WRONG!! Why is it so? I didn t know that the oute fom Colosseo to P.Popolo was going though Via del Coso fom Al.Patia to P.Popolo (which is closed)!! Netwok Laye 4-10

11 Distance Vecto: poisoned evese If Z outes though Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t oute to X via Z) will this completely solve count to infinity poblem? 60 X 4 Y 50 1 Z algoithm teminates Infinity is advetized by Y (poisoned evese) Netwok Laye 4-11

12 Split hoizon poison evese failue A C D B Line CD goes down 1) because of split hoizon ule, A and B tell C that dist(d)=inf 2) C concludes that D is uneachable and epots this to A and B 3) but A knows fom B that dist(d)=2, and sets its dist=3 4) similaly, B knows fom A distance fom D C estimates new value 4; A and B again though C estimate a value of 5.then again 1) etc until distance = infinite Regadless the hack used, thee is always a netwok topology that makes the tick fail! Netwok Laye 4-12

13 Compaison of LS and DV algoithms Message complexity LS: global exchange of infomation DV: exchange between neighbos only m convegence time vaies Speed of Convegence LS: O(n 2 ) algoithm m may have oscillations DV: convegence time vaies m may be outing loops m count-to-infinity poblem Robustness: what happens if oute malfunctions? LS: DV: m node can advetise incoect link cost m each node computes only its own table m DV node can advetise incoect path cost m each node s table used by othes eo popagate thu netwok Netwok Laye 4-13

14 Chapte 4: Netwok Laye 4. 1 Intoduction 4.5 Routing algoithms 4.2 Vitual cicuit and datagam netwoks 4.3 What s inside a oute 4.4 IP: Intenet Potocol m Datagam fomat m IPv4 addessing m ICMP m IPv6 m Link state m Distance Vecto m Hieachical outing 4.6 Routing in the Intenet m RIP m OSPF m BGP 4.7 Boadcast and multicast outing Netwok Laye 4-14

15 Hieachical Routing Ou outing study thus fa - idealization all outes identical netwok flat not tue in pactice scale: with 200 million destinations: can t stoe all dest s in outing tables! outing table exchange would swamp links! administative autonomy intenet = netwok of netwoks each netwok admin may want to contol outing in its own netwok Netwok Laye 4-15

16 Hieachical Routing aggegate outes into egions, autonomous systems (AS) outes in same AS un same outing potocol m inta-as outing potocol m outes in diffeent AS can un diffeent inta- AS outing potocol Gateway oute Diect link to oute in anothe AS Netwok Laye 4-16

17 Inteconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Inta-AS Routing algoithm AS1 Fowading table Inte-AS Routing algoithm 2a 2c AS2 2b fowading table configued by both inta- and inte-as outing algoithm m inta-as sets enties fo intenal dests m inte-as & inta-as sets enties fo extenal dests Netwok Laye 4-17

18 Inte-AS tasks suppose oute in AS1 eceives datagam destined outside of AS1: m oute should fowad packet to gateway oute, but which one? AS1 must: 1. lean which dests ae eachable though AS2, which though AS3 2. popagate this eachability info to all outes in AS1 Job of inte-as outing! 3c 3a 3b AS3 1a 1c 1d 1b AS1 2a 2c AS2 2b Netwok Laye 4-18

19 Example: Setting fowading table in oute 1d suppose AS1 leans (via inte-as potocol) that subnet x is eachable via AS3 (gateway 1c) but not via AS2. inte-as potocol popagates eachability info to all intenal outes. oute 1d detemines fom inta-as outing info that its inteface I is on the least cost path to 1c. m installs fowading table enty (x,i) 3c 3a 3b AS3 1a 1c 1d x 1b AS1 2a 2c 2b AS2 Netwok Laye 4-19

20 Example: Choosing among multiple ASes now suppose AS1 leans fom inte-as potocol that subnet x is eachable fom AS3 and fom AS2. to configue fowading table, oute 1d must detemine towads which gateway it should fowad packets fo dest x. m this is also job of inte-as outing potocol! 3c 3a 3b AS3 1a 1c 1d x 1b AS1 2a 2c AS2 2b Netwok Laye 4-20

21 Example: Choosing among multiple ASes now suppose AS1 leans fom inte-as potocol that subnet x is eachable fom AS3 and fom AS2. to configue fowading table, oute 1d must detemine towads which gateway it should fowad packets fo dest x. m this is also job of inte-as outing potocol! hot potato outing: send packet towads closest of two outes. Lean fom inte-as potocol that subnet x is eachable via multiple gateways Use outing info fom inta-as potocol to detemine costs of least-cost paths to each of the gateways Hot potato outing: Choose the gateway that has the smallest least cost Detemine fom fowading table the inteface I that leads to least-cost gateway. Ente (x,i) in fowading table Netwok Laye 4-21

22 Chapte 4: Netwok Laye 4. 1 Intoduction 4.5 Routing algoithms 4.2 Vitual cicuit and datagam netwoks 4.3 What s inside a oute 4.4 IP: Intenet Potocol m Datagam fomat m IPv4 addessing m ICMP m IPv6 m Link state m Distance Vecto m Hieachical outing 4.6 Routing in the Intenet m RIP m OSPF m BGP 4.7 Boadcast and multicast outing Netwok Laye 4-22

23 Inta-AS Routing also known as Inteio Gateway Potocols (IGP) most common Inta-AS outing potocols: m RIP: Routing Infomation Potocol m OSPF: Open Shotest Path Fist m IGRP: Inteio Gateway Routing Potocol (Cisco popietay) Netwok Laye 4-23

24 Chapte 4: Netwok Laye 4. 1 Intoduction 4.5 Routing algoithms 4.2 Vitual cicuit and datagam netwoks 4.3 What s inside a oute 4.4 IP: Intenet Potocol m Datagam fomat m IPv4 addessing m ICMP m IPv6 m Link state m Distance Vecto m Hieachical outing 4.6 Routing in the Intenet m RIP m OSPF m BGP 4.7 Boadcast and multicast outing Netwok Laye 4-24

25 RIP ( Routing Infomation Potocol) distance vecto algoithm included in BSD-UNIX Distibution in 1982 distance metic: # of hops (max = 15 hops) Fom oute A to subnets: z u A C B D v y w x destination hops u 1 v 2 w 2 x 3 y 3 z 2 Netwok Laye 4-25

26 RIP advetisements distance vectos: exchanged among neighbos evey 30 sec via Response Message (also called advetisement) each advetisement: list of up to 25 destination subnets within AS Netwok Laye 4-26

27 RIP: Example w x y A D B z Destination Netwok C Next Route Num. of hops to dest. w A 2 y B 2 z B 7 x Routing/Fowading table in D Netwok Laye 4-27

28 RIP: Example Dest Next hops w - 1 x - 1 z C Advetisement fom A to D w x y A D B z Destination Netwok C Next Route Num. of hops to dest. w A 2 y B 2 z B A 7 5 x Routing/Fowading table in D Netwok Laye 4-28

29 Diffeences wt Bellman Fod Since count to infinity not solved uppe bound on the netwok size Info on the cost of going though destination X though neighbo Z is maintained ONLY IF the path though Z is the cuent best (min cost) path m Diffeent way of updating costs Suppose cuent oute to dest has cost D and goes though G if an update aives fom X!=G then updates oute ONLY IF cost is <D if an update aives fom G always update oute cost PROBLEM: what if the oute we go though cashes? Route cost aging MUST be adopted Cost is maintained fo each subnetwok (athe than node) Peiodic exchange of messages Netwok Laye 4-29

30 The RIP algoithm (fom RFC) - Keep a table with an enty fo evey possible destination in the system. The enty contains the distance D to the destination, and the fist oute G on the oute to that netwok. Conceptually, thee should be an enty fo the entity itself, with metic 0, but this is not actually included. - Peiodically, send a outing update to evey neighbo. The update is a set of messages that contain all of the infomation fom the outing table. It contains an enty fo each destination, with the distance shown to that destination. - When a outing update aives fom a neighbo G', add the cost associated with the netwok that is shaed with G'. (This should be the netwok ove which the update aived.) Call the esulting distance D'. Compae the esulting distances with the cuent outing table enties. If the new distance D' fo N is smalle than the existing value D, adopt the new oute. That is, change the table enty fo N to have metic D' and oute G'. If G' is the oute fom which the existing oute came, i.e., G' = G, then use the new metic even if it is lage than the old one. Netwok Laye 4-30

31 RIP: Link Failue and Recovey If no advetisement head afte 180 sec --> neighbo/link declaed dead m outes via neighbo invalidated m new advetisements sent to neighbos m neighbos in tun send out new advetisements (if tables changed) m link failue info quickly (?) popagates to entie net m poison evese used to pevent ping-pong loops (infinite distance = 16 hops) Netwok Laye 4-31

32 Diffeences with Bellman Fod Split Hoizon m with Poison Revese m Simple split hoizon (omits cost of eaching destination when advetising though the oute it goes though) Clea implementation with point to point links. But conside the possibility that A and C ae connected by a boadcast netwok such as an Ethenet, and thee ae othe outes on that netwok. Is it a poblem? m If A has a oute though C, it should indicate that D is uneachable when talking to any othe oute on that netwok. The othe outes on the netwok can get to C themselves. They would neve need to get to C via A. m If A's best oute is eally though C, no othe oute on that netwok needs to know that A can each D. This is fotunate, because it means that the same update message that is used fo C can be used fo all othe outes on the same netwok. Thus, update messages can be sent by boadcast. Netwok Laye 4-32

33 An additional way to speed up convegence Tiggeed updates m Wheneve a oute changes the metic fo a oute it is equied to send update messages almost immediately must be implemented fo deleted outes Netwok Laye 4-33

34 RIP Table pocessing RIP outing tables managed by application-level pocess called oute-d (daemon) m pot numbe 520 advetisements sent in UDP packets, peiodically epeated outed outed Tanspt (UDP) netwok fowading (IP) table link physical fowading table Tanspt (UDP) netwok (IP) link physical Netwok Laye 4-34

35 Packet fomat Netwok Laye 4-35