Chapter 4: Network Layer: Part II

Transcription

1 4: Network Laer Chapter 4: Network Laer: Part II (last revision 9/04/05. v3) 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

2 4: Network Laer Interpla between routing and forwarding routing algorithm local forwarding table header value output link value in arriving packet s header 0 3

3 4: Network Laer Graph abstraction 5 Graph: G = (N,E) u v 3 3 w 5 N = set of routers = { u, v, w,,, } E = set of links ={ (u,v), (u,), (v,), (v,w), (,w), (,), (w,), (w,), (,) } Remark: Graph abstraction is useful in other network contets Eample: PP, where N is set of peers and E is set of TCP connections

4 4: Network Laer Graph abstraction: costs u 5 v 3 3 w 5 c(, ) = cost of link (, ) -e.g., c(w,) = 5 cost could alwas be, or inversel related to bandwidth, or inversel related to congestion Cost of path (,, 3,, p ) = c(, ) + c(, 3 ) + + c( p-, p ) Question: What s the least-cost path between u and? Routing algorithm: algorithm that finds least-cost path

5 4: Network Laer Routing Algorithm classification Global or decentralied information? Global: all routers have complete topolog, link cost info link state algorithms Decentralied: router knows phsicallconnected neighbors, link costs to neighbors iterative process of computation, echange of info with neighbors distance vector algorithms Static or dnamic? Static: routes change slowl over time Dnamic: routes change more quickl periodic update in response to link cost changes

6 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

7 4: Network Laer A Link-State Routing Algorithm Dijkstra s algorithm net topolog, link costs known to all nodes accomplished via link state broadcast all nodes have same info computes least cost paths one node ( source ) to all other nodes gives forwarding table for that node iterative: after k iterations, know least cost path to k dest. s Notation: c(,): link cost node to ; = if not direct neighbors D(v): current value of cost of path source to dest. v p(v): predecessor node along path source to v N(v): set of neighbors of v N': set of nodes whose least cost path definitivel known

8 4: Network Laer Dijsktra s Algorithm Initialiation: N' = {u} 3 for all nodes v 4 if v N(u) 5 then D(v) = c(u,v) 6 else D(v) = 7 8 Loop 9 find w not in N' such that D(w) is a minimum 0 add w to N' update D(v) for all v N(w) and not in N' : D(v) = min( D(v), D(w) + c(w,v) ) 3 /* new v is either old v or known 4 shortest path w plus cost w to v */ 5 until all nodes in N'

9 4: Network Laer Dijkstra s algorithm: eample Step start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) D(C),p(C),A 5,A,A 4,D,A 3,E 3,E D(D),p(D),A D(E),p(E) D(F),p(F),D 4,E 4,E 4,E 5 A B D 3 3 C E 5 F

10 4: Network Laer Dijkstra s algorithm, discussion Algorithm compleit: n nodes, E links each iteration: need to check all nodes, w, not in N n(n+)/ comparisons: O(n ) more efficient implementations possible: O(nlogn+E) Oscillations possible: e.g., link cost = amount of carried traffic A +e D 0 0 B 0 e C e initiall +e A 0 D B 0 +e 0 C recompute routing 0 A +e D 0 0 B C +e recompute +e A 0 D B 0 +e e C recompute

11 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing

12 4: Network Laer Distance Vector Algorithm () Bellman-Ford Equation (dnamic programming) Define d () := cost of least-cost path to Then d () = min vєn() {c(,v) + d v () } where min is taken over all neighbors of

13 4: Network Laer Distance Vector Algorithm () u 5 v 3 3 w 5 Clearl, d v () = 5, d () = 3, d w () = 3 B-F equation sas: d u () = min { c(u,v) + d v (), c(u,) + d (), c(u,w) + d w () } Node that achieves minimum is net hop in shortest path forwarding table = min { + 5, + 3, 5 + 3} = 4

14 4: Network Laer Distance Vector Algorithm (3) D () = estimate of least cost to Distance vector: D = [D (): є N ] Node knows each neighbor v: c(,v) Node maintains D = [D (): є N ] Node also maintains its neighbors distance vectors For each neighbor v, maintains D v = [D v (): є N ]

15 4: Network Laer Comp 36, Spring 005 Bellman-Ford eample () Cost to 0 3 u v w u v w u w v Distance vectors stored at node

16 4: Network Laer Comp 36, Spring 005 Bellman-Ford eample () Cost to 0 3 u v w u v w u w v u v w Routing table at node destination,, u, v,, hop, cost

17 4: Network Laer Distance vector algorithm (4) Basic idea: Each node periodicall sends its own distance vector estimate to neighbors When node receives new DV estimate neighbor, it updates its own DV using B-F equation: D () min v {c(,v) + D v ()} for each node N Under natural conditions, the estimate D () converges to the actual least cost d ()

18 4: Network Laer Distance Vector Algorithm (5) Iterative, asnchronous: each local iteration caused b: local link cost change DV update message neighbor Distributed: each node notifies neighbors onl when its DV changes neighbors then notif their neighbors if necessar Each node: wait for (change in local link cost or msg neighbor) recompute estimates if DV to an dest has changed, notif neighbors

19 4: Network Laer D () = min{c(,) + D (), c(,) + D ()} D () = min{c(,) + = min{+0, 7+} = D (), c(,) + D ()} node table = min{+, 7+0} = node table node table time

20 4: Network Laer Distance Vector: link cost changes Link cost changes: node detects local link cost change updates routing info, recalculates distance vector 4 50 if DV changes, notif neighbors good news travels fast At time t 0, detects the link-cost change, updates its DV, and informs its neighbors. At time t, receives the update and updates its table. It computes a new least and sends its neighbors its DV. At time t, receives s update and updates its distance table. s least costs do not change and hence does not send an message to.

21 good news travels fast w 7 4 Comp 36, Spring 005 node w table w node table w w w node table w node table w w Initial routing table (before change) 4: Network Laer

22 good news travels fast Algorithm converges in 3 steps. w node w table node table node table w w w w w w w w w w w w w w w node table w w w w w w Cost of link changes to Comp 36, Spring 005 4: Network Laer

23 good news travels fast Algorithm converges in 3 steps. w node w table node table node table w w w w w w w w w w node table w w w w Cost of link changes to Comp 36, Spring 005 4: Network Laer

24 4: Network Laer bad news travels slow count to infinit problem 4 50 node table node table node table Initial routing table

25 bad news travels slow Algorithm converges in 44 steps Cost of link changes to 60 Comp 36, Spring 005 node table node table node table node table node table node table node table node table node table node table node table node table : Network Laer

26 bad news travels slow Algorithm converges in 44 steps Cost of link changes to 60 Comp 36, Spring 005 node table node table node table node table node table node table node table node table node table node table node table node table : Network Laer

27 4: Network Laer Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies: see tet B suitabl increasing 50 b A and 60 b B, with A<B we can force algorithm to run as long as we want Real problem is that thinks its shortest path to is through, while thinks its shortest path to is through. The pingpong back and forth with this information. Not good!

28 4: Network Laer Distance Vector: poisoned reverse If Z routes through Y to get to X : 60 4 Y Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) X 50 Z will this completel solve count to infinit problem?

29 4: Network Laer 4 50 node table node table node table poisoned reverse Initial routing table

30 4: Network Laer Algorithm converges in 3 steps Cost of link changes to 60 node table node table node table node table node table node table node table node table node table node table node table node table

31 4: Network Laer Comparison of LS and DV algorithms Message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl convergence time varies Speed of Convergence LS: O(n ) algorithm requires O(nE) msgs ma have oscillations DV: convergence time varies ma be routing loops count-to-infinit problem Robustness: what happens if router malfunctions? LS: DV: node can advertise incorrect link cost each node computes onl its own table DV node can advertise incorrect path cost each node s table used b others error propagates thru network

32 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

33 4: Network Laer Hierarchical Routing Our routing stud thus far - idealiation all routers identical network flat not true in practice scale: with 00 million destinations: can t store all dest s in routing tables! routing table echange would swamp links! administrative autonom internet = network of networks each network admin ma want to control routing in its own network

34 4: Network Laer Hierarchical Routing aggregate routers into regions, autonomous sstems (AS) routers in same AS run same routing protocol intra-as routing protocol routers in different AS can run different intra- AS routing protocol gatewa routers 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 gatewa routers

35 4: Network Laer 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 Gatewas: perform inter-as routing amongst themselves b perform intra-as routing with other routers in their AS inter-as, intra-as routing in gatewa A.c network laer link laer phsical laer

36 4: Network Laer Intra-AS and Inter-AS routing a Host h C C.b b A.a Inter-AS routing between A and B A.c a d A b c Intra-AS routing within AS A B.a a B c b Host h Intra-AS routing within AS B We ll eamine specific inter-as and intra-as Internet routing protocols shortl

37 4: Network Laer Inter-AS tasks Suppose router in AS receives datagram for which dest is outside of AS Router should forward packet towards one of the gatewa routers, but which one? AS needs:. to learn which dests are reachable through AS and which through AS3. to propagate this reachabilit info to all routers in AS Job of inter-as routing! 3c 3a 3b AS3 a c d b AS a c b AS

38 4: Network Laer Eample: Setting forwarding table in router d Suppose AS learns the inter-as protocol that subnet is reachable AS3 (gatewa c) but not AS. Inter-AS protocol propagates reachabilit info to all internal routers. Router d determines intra-as routing info that its interface I is on the least cost path to c. Adds entr (,I) to forwarding table

39 4: Network Laer Eample: Choosing among multiple ASes Now suppose AS learns the inter-as protocol that subnet is reachable AS3 and AS. To configure forwarding table, router d must determine towards which gatewa it should forward packets for dest. This is also the job of inter-as routing protocol! Hot potato routing: send packet towards closest of two routers. Learn inter-as protocol that subnet is reachable via multiple gatewas Use routing info intra-as protocol to determine costs of least-cost paths to each of the gatewas Hot potato routing: Choose the gatewa that has the smallest least cost Determine forwarding table the interface I that leads to least-cost gatewa. Enter (,I) in forwarding table

40 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

41 4: Network Laer Routing in the Internet The Global Internet consists of Autonomous Sstems (AS) interconnected with each other: Stub AS: small corporation: one connection to other AS s Multihomed AS: large corporation (no transit): multiple connections to other AS s Transit AS: provider, hooking man AS s together Two-level routing: Intra-AS: (within AS) administrator responsible for choice of routing algorithm within network Inter-AS: (between Ass) unique standard for inter-as routing: BGP

42 4: Network Laer Internet AS Hierarch Inter-AS border (eterior gatewa) routers Intra-AS (interior) routers

43 4: Network Laer Intra-AS Routing Also known as Interior Gatewa Protocols (IGP) Most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gatewa Routing Protocol (Cisco proprietar)

44 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

45 4: Network Laer RIP ( Routing Information Protocol) Distance vector algorithm Included in BSD-UNIX Distribution in 98 Distance metric: # of hops (ma = 5 hops) u A C B D v w destination hops u v w 3 3

46 4: Network Laer RIP advertisements Distance vectors: echanged among neighbors ever 30 sec via Response Message (also called advertisement) Each advertisement: list of up to 5 destination nets within AS

47 4: Network Laer RIP: Eample w A D B Destination Network C Net Router Num. of hops to dest. w A B B Routing table in D

48 RIP: Eample Dest Net hops w C Advertisement A to D w A D B Destination Network Comp 36, Spring 005 C Net Router Num. of hops to dest. w A B B A Routing table in D 4: Network Laer

49 4: Network Laer RIP: Link Failure and Recover If no advertisement heard after 80 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickl propagates to entire net poison reverse used to prevent ping-pong loops (infinite distance = 6 hops)

50 4: Network Laer RIP Table processing RIP routing tables managed b application-level process called route-d (daemon) advertisements sent in UDP packets, periodicall repeated routed routed Transprt (UDP) network forwarding (IP) table link phsical forwarding table Transprt (UDP) network (IP) link phsical

51 4: Network Laer RIP Table eample (continued) Router: giroflee.eurocom.fr Destination Gatewa Flags Ref Use Interface UH lo U 3 fa U le U 5 qaa U 3 0 le0 default UG Three attached class C networks (LANs) Router onl knows routes to attached LANs Default router used to go up Route multicast address: Loopback interface (for debugging)

52 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

53 4: Network Laer OSPF (Open Shortest Path First) open : publicl available Uses Link State algorithm LS packet dissemination Topolog map at each node Route computation using Dijkstra s algorithm OSPF advertisement carries one entr per neighbor router Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directl over IP (rather than TCP or UDP

54 4: Network Laer OSPF advanced features (not in RIP) Securit: all OSPF messages authenticated (to prevent malicious intrusion) Multiple same-cost paths allowed (onl one path in RIP) For each link, multiple cost metrics for different TOS (e.g., satellite link cost set low for best effort; high for real time) Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topolog data base as OSPF Hierarchical OSPF in large domains.

55 4: Network Laer Hierarchical OSPF

56 4: Network Laer Hierarchical OSPF Two-level hierarch: local area, backbone. Link-state advertisements onl in area each nodes has detailed area topolog; onl know direction (shortest path) to nets in other areas. Area border routers: summarie distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone. Boundar routers: connect to other AS s.

57 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe)

58 4: Network Laer Inter-AS routing in the Internet: BGP R4 R5 R3 BGP AS3 AS (RIP intra-as routing) R BGP R AS (OSPF intra-as routing) (OSPF intra-as routing) Figure 4.5.-new: BGP use for inter-domain routing

59 4: Network Laer Internet inter-as routing: BGP BGP (Border Gatewa Protocol): the de facto standard BGP provides each AS a means to:. Obtain subnet reachabilit information neighboring ASs.. Propagate the reachabilit information to all routers internal to the AS. 3. Determine good routes to subnets based on reachabilit information and polic. Allows a subnet to advertise its eistence to rest of the Internet: I am here

60 4: Network Laer In BGP, destination are not individual hosts, the are networks! A network is represented b a CIDR prefi, e.g., /4 If a gatewa router broadcasts a BGP message stating that it is /4, it is advertising that it can deliver messages to an host in subnet /4. BGP messages between routers in same AS are called (interior) ibgp messages BGP messages between routers in diff AS are called (eterior) ebgp messages

61 4: Network Laer BGP basics Pairs of routers (BGP peers) echange routing info over semipermanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to phsical links. When AS advertises a prefi to AS, AS is promising it will forward an datagrams destined to that prefi towards the prefi. AS can aggregate prefies in its advertisement 3c 3a 3b AS3 a AS c d b a c b AS ebgp session ibgp session

62 4: Network Laer Distributing reachabilit info With ebgp session between 3a and c, AS3 sends prefi reachabilit info to AS. c can then use ibgp do distribute this new prefi reach info to all routers in AS b can then re-advertise the new reach info to AS over the b-to-a ebgp session When router learns about a new prefi, it creates an entr for the prefi in its forwarding table. 3c 3a 3b AS3 a AS c d b a c b AS ebgp session ibgp session

63 4: Network Laer Path attributes & BGP routes When advertising a prefi, advert includes BGP attributes. prefi + attributes = route Two important attributes: AS-PATH: contains the ASs through which the advert for the prefi passed: AS 67, AS 7, NEXT-HOP: Indicates the specific internal-as router to net-hop AS. (There ma be multiple links current AS to net-hop-as.) When gatewa router receives route advert, uses import polic to accept/decline.

64 4: Network Laer BGP route selection Router ma learn about more than route to some prefi. Router must select route. Elimination rules:. Local preference value attribute: polic decision. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato routing 4. Additional criteria

65 4: Network Laer BGP messages BGP messages echanged using TCP. BGP messages: OPEN: opens TCP connection to peer and authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also used to close connection

66 4: Network Laer BGP: Controlling who routes through ou B legend: provider network W A C X customer network: Y Figure 4.5-BGPnew: a simple BGP scenario A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks X does not want to route B via X to C.. so X will not advertise to B a route to C

67 4: Network Laer BGP: Controlling who routes through ou B legend: provider network W A C X customer network: Y Figure 4.5-BGPnew: a simple BGP scenario A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No wa! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route onl to/ its customers!

68 4: Network Laer Wh different Intra- and Inter-AS routing? Polic: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no polic decisions needed Scale: hierarchical routing saves table sie, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: polic ma dominate over performance

69 4: Network Laer Chapter 4: Network Laer 4. Introduction 4. Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing (mabe: See tetbook)

70 4: Network Laer Network Laer: summar What we ve covered: network laer services routing principles: link state and distance vector hierarchical routing IP Internet routing protocols RIP, OSPF, BGP what s inside a router? IPv6 Net stop: the Data link laer!