SC250 Computer Networking I. Network Layer and IP. Prof. Matthias Grossglauser LCA/I&C.

Transcription

1 SC250 Computer Networking I Network Layer and IP Prof. Matthias Grossglauser LCA/I&C 1

2 Network layer Host Router Host Application Application Transport Transport Network Network Network Link/Physical L/P L/P Link/Physical 2

3 Objectives understand principles behind network layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, mobility instantiation and implementation in the Internet Overview: network layer services routing principles: path selection hierarchical routing IP protocol Internet routing protocols intra-domain inter-domain Network Layer 4-3

4 Network Layer and IP Introduction and Network Service Models 4

5 Network layer functions transport packet from sending to receiving hosts network layer protocols in every host, router ap p lication tran s p or t n etw ork data lin k p h ys ical net w ork dat a link physical net w ork dat a link physical net w ork dat a link physical three important functions: path determination: route taken by packets from source to dest. Routing algorithms forwarding: move packets from router s input to appropriate router output call setup: some network architectures require router call setup along path before data flows net w ork dat a link physical net w ork dat a link physical net w ork dat a link physical net w ork dat a link physical net w ork dat a link physical ap p lication tran s p ort n etw or k data lin k p h ys ical Network Layer 4-5

6 Network service model service ab stract ion Q: What service model for channel transporting packets from sender to receiver? guaranteed bandwidth? preservation of interpacket timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to sender? Th e most impor tan t abstr act ion pr ovided by n et wor k layer :??? vir tual cir cuit or dat agr am? Network Layer 4-6

7 Virtual circuits source-to-dest path behaves much like telephone circuit performance-wise network actions along source-to-dest path call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host ID) every router on source-dest path maintains state for each passing connection transport-layer connection only involved two end systems link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like perf. Network Layer 4-7

8 Virtual circuits: signaling protocols used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today s Internet 5. Dat a f low b egin s 6. Receive dat a 4. Call con n ect ed 3. Accep t call 1. In it iat e call 2. in com in g call Network Layer 4-8

9 Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections no network-level concept of connection packets forwarded using destination host address packets between same source-dest pair may take different paths 1. Sen d dat a 2. Receive dat a Network Layer 4-9

10 Network layer service models: Network rchitecture Service Model Guarantees? Bandwidth Loss Order Timing Congestion feedback Internet ATM ATM ATM ATM best effort CBR VBR ABR UBR none no constant yes rate guaranteed yes rate guaranteed no minimum none no no yes yes yes yes no yes yes no no no (inferred via loss) no congestion no congestion yes no Internet model being extended: Intserv, Diffserv Chapter 6 in book Netw ork Layer 4-10

11 Datagram or VC network: why? Internet data exchange among computers elastic service, no strict timing req. smart end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at edge many link types different characteristics uniform service difficult ATM evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service dumb end systems telephones complexity inside network Netw ork Layer 4-11

12 Network Layer and IP Routing Principles: Link State and Distance Vector Routing 12

13 Routing Graph abstraction for routing algorithms: Rout in g pr otocol Goal: determine good path (sequence of routers) thru network from source to dest. graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level A B 2 D C 1 E good path: 5 2 typically means minimum cost path other def s possible F Netw ork Layer 4-13

14 Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, link cost info link state algorithms Decentralized: router knows physicallyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes Netw ork Layer 4-14

15 A Link-State Routing Algorithm Dijkstra s algorithm net topology, link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths from one node ( source ) to all other nodes gives routing table for that node iterative: after k iterations, know least cost path to k dest. s Notation: c(i,j): link cost from node i to j. cost infinite if not direct neighbors D(v): current value of cost of path from source to dest. V p(v): predecessor node along path from source to v, that is next v N: set of nodes whose least cost path definitively known Netw ork Layer 4-15

16 Dijsktra s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(a,v) 6 else D(v) = infinity 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N Netw ork Layer 4-16

17 Dijkstra s algorithm: example S tep start N A A D A D E A D E B A D E B C A D E B C F D (B ),p(b ) D (C ),p(c ) D (D ),p(d ) D (E ),p(e ) D (F),p(F) A 2,A 2,A 2,A B 2 D 3 1 5,A 4,D 3,E 3,E 3 C 1 E 5 2 1,A F infin ity 2,D infinity infinity 4,E 4,E 4,E Netw ork Layer 4-17

18 Dijkstra s algorithm, discussion Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic 1 1 A 1+ e D 0 0 B 0 e C 1 e in itially A 2 + e 0 D 1+ e1 B 0 0 C r ecomput e r outin g A e D 0 0 B 1 1+ e C r ecomput e A 2 + e 0 D 1+ e1 B 0 e C r ecomput e Netw ork Layer 4-18

19 Distance Vector Routing Algorithm iterative: continues until no nodes exchange info. self-terminating: no signal to stop asynchronous: nodes need not exchange info/iterate in lock step! distributed: each node communicates only with directly-attached neighbors Distance Table data structure each node has its own row for each possible destination column for each directlyattached neighbor to node example: in node X, for dest. Y via neighbor Z: X D (Y,Z) = = distance from X to Y, via Z as next hop Z c(x,z) + min {D (Y,w)} w Netw ork Layer 4-19

20 Distance Table: example A 1 7 B 8 E 1 2 C D 2 cost to destination via D E() A B D A B D E(C,D) D E(A,D) = c(e,d) + min {D D(C,w)} w = 2+2 = 4 c(e,d) + min {D D = (A,w)} w = 2+3 = 5 loop! destination C D D E(A,B) c(e,b) + min {D B = (A,w)} w = 8+6 = 14 loop! Network Layer 4-20

21 Distance table gives routing table cost to destination via D E() A B D A A Outgoing link to use, cost A,1 B B D,5 destination C D destination C D D,4 D,4 Distan ce table Rout in g t able Network Layer 4-21

22 Distance Vector Routing: overview Iterative, asynchronous: each local iteration caused by: local link cost change message from neighbor: its least cost path change from neighbor Distributed: each node notifies neighbors only when its least cost path to any destination changes neighbors then notify their neighbors if necessary Each n ode: wait for (change in local link cost of msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Network Layer 4-22

23 Distance Vector Algorithm: At all n odes, X: 1 Initialization: 2 for all adjacent nodes v: 3 D X (*,v) = infinity /* the * operator means "for all rows" */ 4 D X(v,v) = c(x,v) 5 for all destinations, y 6 send min D X (y,w) to each neighbor /* w over all X's neighbors */ w Network Layer 4-23

24 Distance Vector Algorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) if (c(x,v) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D X(y,V) = D X (y,v) + d else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min w DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D X (Y,V) = c(x,v) + newval if we have a new min D X w (Y,w)for any destination Y 24 send new value of min D X w (Y,w) to all neighbors forever Network Layer 4-24

25 Distance Vector Algorithm: example X 2 Y 7 1 Z Network Layer 4-25

26 Distance Vector Algorithm: example X 2 Y 7 1 Z D X(Y,Z) c(x,z) + min {D Z = w (Y,w)} = 7+1 = 8 D X(Z,Y) c(x,y) + min {D Y = (Z,w)} w = 2+1 = 3 Network Layer 4-26

27 Distance Vector: link cost changes Link cost changes: node detects local link cost change updates distance table (line 15) if cost change in least cost path, notify neighbors (lines 23,24) 1 X 4 Y 50 1 Z good n ews tr avels fast algorit h m t ermin at es Network Layer 4-27

28 Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - count to infinity problem! 60 X 4 Y 50 1 Z algorit h m con t in ues on! Network Layer 4-28

29 Distance Vector: poisoned reverse If Z routes through Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) will this completely solve count to infinity problem? 60 X 4 Y 50 1 Z algorit h m t ermin at es Network Layer 4-29

30 Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O (ne) msgs sent each DV: exchange between neighbors only convergence time varies Speed of Convergence LS: O(n 2 ) algorithm requires O(nE) msgs may have oscillations DV: convergence time varies may be routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS: DV: node can advertise incorrect link cost each node computes only its own table DV node can advertise incorrect path cost each node s table used by others error propagate thru network Network Layer 4-30

31 Network Layer and IP Hierarchical Routing 31

32 Hierarchical Routing Our routing study thus far - idealization all routers identical network flat not true in practice scale: with 200 million destinations: can t store all dest s in routing tables! routing table exchange would swamp links! administrative autonomy internet = network of networks each network admin may want to control routing in its own network Network Layer 4-32

33 Hierarchical Routing aggregate routers into regions, autonomous systems (AS) routers in same AS run same routing protocol intra-as routing protocol routers in different AS can run different intra- AS routing protocol gateway r outer s special routers in AS run intra-as routing protocol with all other routers in AS also responsible for routing to destinations outside AS run inter-as routing protocol with other gateway routers Network Layer 4-33

34 Intra-AS and Inter-AS routing a C C.b b d A A.a a b A.c c B.a a B c b Gateways: per f or m in ter - AS r out in g amon gst th emselves per f or m in tr a- AS r out er s with oth er r out er s in th eir AS in t er- AS, in t ra- AS rou t in g in gat ew ay A.c n et w ork layer lin k layer p h ysical layer Network Layer 4-34

35 Intra-AS and Inter-AS routing a Host h 1 C C.b b A.a In ter - AS r outin g bet ween A an d B A.c a d A b c In tr a- AS r outin g with in AS A B.a a B c b Host h 2 In t r a- AS r outin g with in AS B We ll examine specific inter-as and intra-as Internet routing protocols shortly Network Layer 4-35

36 Network Layer and IP The IP Protocol 36

37 The Internet Network layer Host, router network layer functions: Tran sp ort layer: TCP, UDP Net wor k layer Routing protocols path selection RIP, OSPF, BGP forwarding table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling Lin k layer p h ysical layer Network Layer 4-37

38 IP Addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host may have multiple interfaces IP addresses associated with each interface = Network Layer 4-38

39 IP Addressing IP address: network part (high order bits) host part (low order bits) What s a network? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router LAN n et w ork con sist in g of 3 IP n et w orks (f or IP addresses st artin g w it h 2 2 3, f irst 2 4 b it s are n et w ork address) Network Layer 4-39

40 IP Addressing How to find the networks? Detach each interface from router, host create islands of isolated networks In t ercon n ect ed syst em con sist in g of six n et w orks Netw ork Layer 4-40

41 IP Addresses given notion of network, let s re-examine IP addresses: class- full addr essin g: class A B C D 0 network host 10 network host 110 network host 1110 multicast address 32 bits t o t o t o t o Netw ork Layer 4-41

42 IP addressing: CIDR Classful addressing: inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network CIDR: Classless InterDomain Routing network portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in network portion of address n et w ork p art h ost p art / 23 Netw ork Layer 4-42

43 IP addresses: how to get one? Q: How does host get IP address? hard-coded by system admin in a file Wintel: control-panel->network- >configuration->tcp/ip->properties UNIX: /etc/rc.config DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play (more shortly) Netw ork Layer 4-43

44 IP addresses: how to get one? Q: How does network get network part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /20 Organization /23 Organization /23 Organization / Organization /23 Netw ork Layer 4-44

45 Hierarchical addressing: route aggregation Hierarch ical addressin g allow s ef f icien t advertisem en t of rou t in g in f orm at ion : Organization / 23 Organization / 23 Organization / 23 Organization Fly- By- Night- ISP Send m e anything with addresses beginning / 20 Internet / 23 ISPs- R- Us Send m e anything with addresses beginning / 16 Netw ork Layer 4-45

46 Hierarchical addressing: more specific routes ISPs- R- Us h as a m ore sp ecif ic rou t e t o Organ izat ion 1 Organization / 23 Organization / 23 Organization / Organization / 23 Fly- By- Night- ISP ISPs- R- Us Send m e anything with addresses beginning / 20 Send m e anything with addresses beginning / 16 or / 23 Internet Netw ork Layer 4-46

47 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes Netw ork Layer 4-47

48 Getting a datagram from source to dest. forwarding table in A Dest. Net. next router Nhops IP datagram: m isc f ields sou rce IP addr dest IP addr dat a datagram remains unchanged, as it travels source to destination addr fields of interest here A B E Netw ork Layer 4-48

49 Getting a datagram from source to dest. Starting at A, send IP datagram addressed to B: misc fields data look up net. address of B in forwarding table find B is on same net. as A link layer will send datagram directly to B inside link-layer frame B and A are directly connected A B forwarding table in A Dest. Net. next router Nhops E Netw ork Layer 4-49

50 Getting a datagram from source to dest. m isc dat a f ields Starting at A, dest. E: look up network address of E in forwarding table E on different network A, E not directly attached routing table: next hop router to E is link layer sends datagram to router inside linklayer frame datagram arrives at continued.. A B forwarding table in A Dest. Net. next router Nhops E Netw ork Layer 4-50

51 Getting a datagram from source to dest. m isc f ields dat a Dest. Net router Nhops interface Arriving at , destined for look up network address of E in router s forwarding table E on same network as router s interface router, E directly attached link layer sends datagram to inside link-layer frame via interface datagram arrives at !!! (hooray!) A B forwarding table in router E Netw ork Layer 4-51

52 IP datagram format IP p rot ocol version n u m b er h eader len gt h (b yt es) typ e of dat a m ax n u m b er rem ain in g h op s (decrem en t ed at each rou t er) u p p er layer p rot ocol t o deliver p ayload t o how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver 32 b it s h ead. t yp e of len service 16- b it iden t if ier f lgs t im e t o live u p p er layer len gt h f ragm en t of f set In t ern et ch ecksu m 32 b it sou rce IP address 32 b it dest in at ion IP address t ot al dat agram len gt h (b yt es) f or f ragm en t at ion / reassem b ly Op t ion s (if an y) E.g. t im est am p, record rou t e dat a t aken, sp ecif y (var iable len gth, list of rou t ers t ypically a TCP t o visit. or UDP segmen t) Netw ork Layer 4-52

53 IP Fragmentation & Reassembly network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly f ragmen t at ion : in : on e large dat agram out : 3 smaller dat agram Netw ork Layer 4-53

54 IP Fragmentation and Reassembly Example 4000 byte datagram MTU = 1500 bytes length =4000 ID =x length =1500 fragflag =0 ID =x offset =0 On e large dat agram b ecom es several sm aller dat agram s fragflag =1 offset =0 length =1500 ID =x fragflag =1 offset =1480 length =1040 ID =x fragflag =0 offset =2960 Netw ork Layer 4-54