Chapter 4 Network Layer

Transcription

1 Chapter 4 Network Laer A note on the use of these ppt slides: The notes used in this course are substantiall based on powerpoint slides developed and coprighted b J.F. Kurose and K.W. Ross, Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesle, Jul Network Laer 4-1

2 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-2

3 Two Ke Network-Laer Functions Forwarding: move packets router s input to appropriate router output Routing: determine route taken b packets source to dest Routing algorithms Analog: Routing: process of planning trip source to dest Forwarding: process of getting through single interchange Network Laer 4-3

4 Interpla between Routing and Forwarding routing algorithm local forwarding table header value output link value in arriving packet s header Network Laer 4-4

5 Connection Setup The 3 rd important function in some network architectures: ATM, frame rela, X.25 Before datagrams flow, two end hosts and intervening routers establish virtual connection Routers get involved Network vs. transport laer connection service: Network: between two hosts (ma also involve intervening routers in case of VCs) Transport: between two processes Network Laer 4-5

6 Network Service Model Q: What service model for channel transporting packets sender to receiver? Guaranteed bandwidth? Preservation of inter packet timing (no jitter)? Loss free deliver? In order deliver? Congestion feedback to sender? service abstraction The most important abstraction provided b network laer:??? virtual circuit or datagram? Network Laer 4-6

7 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-7

8 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 address) Ever router on source dest path maintains state for each passing connection Link, router resources (bandwidth, buffers) ma be allocated to VC (dedicated resources = predictable service) Network Laer 4-8

9 Virtual Circuits: Signaling Protocols Used to setup, maintain teardown VC Used in ATM, frame rela, X.25 Not used in toda s Internet application transport network data link phsical 5. Data flow begins 6. Receive data 4. Call connected 3. Accept call 1. Initiate call 2. incoming call application transport network data link phsical Network Laer 4-9

10 Datagram Networks No call setup at network laer 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 ma take different paths application transport network data link phsical 1. Send data 2. Receive data application transport network data link phsical 4-10 Network Laer

11 Datagram or VC Network: Wh? Internet (datagram) Data echange among computers Elastic service, no strict timing req. Smart end sstems (computers) Can adapt, perform control, error recover Simple inside network, compleit at edge Man link tpes Different characteristics Uniform service difficult ATM (VC) Evolved telephon Human conversation: Strict timing, reliabilit requirements Need for guaranteed service Dumb end sstems Telephones Compleit inside network Network Laer 4-11

12 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-12

13 Router Architecture Overview Two ke router functions: Run routing algorithms/protocol (RIP, OSPF, BGP) Forwarding datagrams incoming to outgoing link Network Laer 4-13

14 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-14

15 The Internet Network Laer Host, router network laer functions: Transport laer: TCP, UDP Network laer Routing protocols Path selection RIP, OSPF, BGP forwarding table IP protocol Addressing conventions Datagram format Packet handling conventions ICMP protocol Error reporting Router signaling Link laer phsical laer Network Laer 4-15

16 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-16

17 IP Datagram Format IP protocol version number header length (btes) tpe of data (QoS) ma number remaining hops (decremented at each router) upper laer protocol to deliver paload to How much overhead with TCP? 20 btes of TCP 20 btes of IP = 40 btes + app laer overhead ver head. len 16-bit identifier time to live tpe of service upper laer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address Options (if an) data (variable length, tpicall a TCP or UDP segment) total datagram length (btes) for fragmentation/ reassembl E.g. timestamp, record route taken, specif list of routers to visit. Network Laer 4-17

18 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-18

19 IP Addressing: Introduction IP address: 32 bit identifier for host, router interface Interface: connection between host/router and phsical link Router s tpicall have multiple interfaces Host tpicall has one interface (ma have multiple toda) IP addresses associated with each interface, not with host or router = Network Laer 4-19

20 Subnets IP address: Subnet part (high order bits) Host part (low order bits) What s a subnet? Device interfaces with same subnet part of IP address Can phsicall reach each other without intervening router subnet network consisting of 3 subnets (for IP addresses starting with 223, first 24 bits are network address) Network Laer 4-20

21 IP Addresses Given notion of network, let s re eamine IP addresses: class-full addressing: class A 0network host B 10 network host C 110 network host D 1110 multicast address to to to to bits Network Laer 4-21

22 IP Addressing: CIDR Classful addressing: Inefficient use of address space, address space ehaustion E.g., class B net allocated enough addresses for 65K hosts, evenif onl 2K hosts in that network CIDR: Classless InterDomain Routing Subnet portion of address of arbitrar length Address format: a.b.c.d/, where is # bits in subnet portion of address subnet part host part /23 Network Laer 4-22

23 NAT: Network Address Translation rest of Internet local network (e.g., home network) / All datagrams leaving local network have same single source NAT IP address: , different source port numbers Datagrams with source or destination in this network have /24 address for source, destination (as usual) Network Laer 4-23

24 NAT: Network Address Translation Implementation: NAT router must: Outgoing datagrams: replace (source IP address, port #) of ever outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. Remember (in NAT translation table) ever (source IP address, port #) to (NAT IP address, new port #) translation pair Incoming datagrams: replace (NAT IP address, new port #) in dest fields of ever incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Laer 4-24

25 NAT: Network Address Translation 2: NAT router changes datagram source addr , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 3345 D: , : host sends datagram to , S: , 80 D: , : Repl arrives dest. address: , 5001 S: , 80 D: , : NAT router changes datagram dest addr , 5001 to , 3345 Network Laer 4-25

26 NAT: Network Address Translation Outside node cannot initiate the communication Reserved addresses: / / /16 Network Laer 4-26

27 NAT: Network Address Translation 16 bit port number field: 60,000 simultaneous connections with a single LAN side address! NAT is controversial: Routers should onl process up to laer 3 Violates end to end argument NAT possibilit must be taken into account b app designers, eg, P2P applications Address shortage should instead be solved b IPv6 Network Laer 4-27

28 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-28

29 ICMP: Internet Control Message Protocol Used b hosts & routers to communicate network level information Error reporting: unreachable host, network, port, protocol Echo request/repl (used b ping) Network laer above IP: ICMP msgs carried in IP datagrams ICMP message: tpe, code plus first 8 btes of IP datagram causing error Tpe Code Description 0 0 echo repl (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discover 11 0 TTL epired 12 0 bad IP header Network Laer 4-29

30 ICMP: Brief Summar ICMP is the control sibling of IP ICMP is used b IP and uses IP as network laer protocol ICMP is used for ping, traceroute, and path MTU discover Ping: Uses ICMP Echo request/repl messages Path MTU Discover Send a large IP datagram with No fragment bit set Reduce sie until success (No ICMP message received) Network Laer 4-30

31 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-31

32 IPv6 Initial motivation: 32 bit address space soon to be completel allocated Additional motivation: Header format helps speed processing/forwarding Header changes to facilitate QoS IPv6 datagram format: Fied length 40 bte header No fragmentation allowed Network Laer 4-32

33 Transition From IPv4 To IPv6 Not all routers can be upgraded simultaneous no flag das How will the network operate with mied IPv4 and IPv6 routers? Tunneling: IPv6 carried as paload in IPv4 datagram among IPv4 routers Network Laer 4-33

34 Tunneling Logical view: A B E F tunnel IPv6 IPv6 IPv6 IPv6 Phsical view: A B E F IPv6 IPv6 IPv6 IPv6 IPv4 IPv4 Network Laer 4-34

35 Tunneling Logical view: A B E F tunnel IPv6 IPv6 IPv6 IPv6 Phsical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F Src:B Dest: E Flow: X Src: A Dest: F Flow: X Src: A Dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 Network Laer B-to-C: IPv6 inside IPv4 E-to-F: IPv6 4-35

36 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-36

37 Graph Abstraction 5 Graph: G = (N,E) u 1 2 v w 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: P2P, where N is set of peers and E is set of TCP connections Network Laer 4-37

38 Graph Abstraction: Costs u v w c(, ) = cost of link (, ) -e.g., c(w,) = 5 Cost could alwas be 1, or inversel related to bandwidth, or inversel related to congestion Cost of path ( 1, 2, 3,, p ) = c( 1, 2 ) + c( 2, 3 ) + + c( p-1, p ) Question: What s the least-cost path between u and? Routing algorithm: algorithm that finds least-cost path Network Laer 4-38

39 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-39

40 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': set of nodes whose least cost path definitivel known Network Laer 4-40

41 Dijkstra s Algorithm 1 Initialiation: 2 N' = {u} 3 for all nodes v 4 if v adjacent to 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 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 v is either old v or known 14 shortest path w plus cost w to v */ 15 until all nodes in N' Network Laer 4-41

42 Dijkstra s Algorithm: Eample Step start N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) D(C),p(C) 2,A 5,A 2,A 4,D 2,A 3,E 3,E D(D),p(D) 1,A D(E),p(E) D(F),p(F) 2,D 4,E 4,E 4,E 5 A 1 2 B D C E F Network Laer 4-42

43 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-43

44 Distance Vector Algorithm Bellman Ford Equation (dnamic programming) Define d () := cost of least cost path to Then d () = min {c (,v) + d v () } v where min is taken over all neighbors v of Network Laer 4-44

45 Bellman-Ford Eample u v w 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 () } = min {2 + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is net hop in shortest path forwarding table Network Laer 4-45

46 Distance Vector Algorithm D () = estimate of least cost to Node knows each neighbor v: c(,v) Node maintains distance vector D = [D (): є N ] Node also maintains its neighbors distance vectors For each neighbor v, maintains D v = [D v (): є N ] Network Laer 4-46

47 Distance Vector Algorithm (4) Basic idea: Each node periodicall sends its own distance vector estimate to neighbors When a 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 minor, natural conditions, the estimate D () converge to the actual least cost d () Network Laer 4-47

48 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 Network Laer 4-48

49 node table node table node table 71 0 time Network Laer 4-49

50 D () = min{c(,) + D (), c(,) + D ()} = min{2+0, 7+1} = 2 node table node table node table time D () = min{c(,) + D (), c(,) + D ()} = min{2+1, 7+0} = 3 Network Laer

51 D () = min{c(,) + D (), c(,) + D ()} = min{2+0, 7+1} = 2 node table node table node table time D () = min{c(,) + D (), c(,) + D ()} = min{2+1, 7+0} = 3 Network Laer

52 D () = min{c(,) + D (), c(,) + D ()} = min{2+0, 7+1} = 2 node table node table node table time D () = min{c(,) + D (), c(,) + D ()} = min{2+1, 7+0} = 3 Network Laer

53 Distance Vector: Link Cost Changes Link cost changes: Node detects local link cost change Updates routing info, recalculates distance vector 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 1, receives the update and updates its table. It computes a new least and sends its neighbors its DV. At time t 2, receives s update and updates its distance table. s least costs do not change and hence does not send an message to. Network Laer 4-53

54 bad news travels slow count to infinit problem node table node table node table Network Laer Initial routing table 4-54

55 bad news travels slow Algorithm converges in 44 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 Network Laer 4-55

56 bad news travels slow Algorithm converges in 44 steps node table node table node table node table node table node table node table node table node table node table node table node table Cost of link changes to 60 Network Laer 4-56

57 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 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 completel solve count to infinit problem? 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. Network Laer 4-57

58 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 2 ) 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 propagate thru network Network Laer 4-58

59 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-59

60 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 Network Laer 4-60

61 Interconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Intra-AS Routing algorithm AS1 Forwarding table Inter-AS Routing algorithm 2a 2c 2b AS2 Forwarding table is configured b both intraand inter AS routing algorithm Intra AS sets entries for internal dests Inter AS & Intra As sets entries for eternal dests Network Laer 4-61

62 Inter-AS Tasks Suppose router in AS1 receives datagram for which dest is outside of AS1 Router should forward packet towards one of the gatewa routers, but which one? AS1 needs: 1. To learn which dests are reachable through AS2 and which through AS3 2. To propagate this reachabilit info to all routers in AS1 Job of inter AS routing! 3c 3a 3b AS3 1a 1c 1d 1b AS1 2a 2c 2b AS2 Network Laer 4-62

63 Eample: Choosing Among Multiple ASes Now suppose AS1 learns the inter AS protocol that subnet is reachable AS3 and AS2 To configure forwarding table, router 1d must determine towards which gatewa it should forward packets for dest This is also the job on 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 Network Laer 4-63

64 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-64

65 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 ASes) unique standard for inter AS routing: BGP Network Laer 4-65

66 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) Network Laer 4-66

67 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-67

68 RIP (Routing Information Protocol) Distance vector algorithm Included in BSD UNIX Distribution in 1982 Distance metric: # of hops (ma = 15 hops) From router A to subsets: u A C B D v w destination hops u 1 v 2 w Network Laer 4-68

69 RIP Advertisements Distance vectors: echanged among neighbors ever 30 sec via Response Message (also called advertisement) Each advertisement: list of up to 25 destination nets within AS Network Laer 4-69

70 RIP: Link Failure and Recover If no advertisement heard after 180 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 = 16 hops) Network Laer 4-70

71 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-71

72 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 Network Laer 4-72

73 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 Network Laer 4-73

74 Hierarchical OSPF Network Laer 4-74

75 Chapter 4: Network Laer 4. 1 Introduction 4.5 Routing algorithms 4.2 Virtual circuit and datagram networks 4.3 What s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP IPv6 Network Laer 4-75

76 Inter-AS Routing in the Internet: BGP R4 R5 R3 BGP AS3 AS1 (RIP intra-as routing) R1 BGP R2 AS2 (OSPF intra-as routing) (OSPF intra-as routing) Figure new2: BGP use for inter-domain routing Network Laer 4-76

77 Internet Inter-AS Routing: BGP BGP (Border Gatewa Protocol): the de facto standard BGP provides each AS a means to: 1. Obtain subnet reachabilit information neighboring ASs. 2. Propagate reachabilit information to all ASinternal routers. 3. Determine good routes to subnets based on reachabilit information and polic. Allows subnet to advertise its eistence to rest of Internet: I am here Network Laer 4-77

78 BGP Basics Pairs of routers (BGP peers) echange routing info over semipermanent TCP connections: BGP sessions BGP sessions need not correspond to phsical links When AS2 advertises a prefi to AS1, AS2 is promising it will forward an datagrams destined to that prefi towards the prefi AS2 can aggregate prefies in its advertisement ebgp session ibgp session 3c 3a 3b AS3 1a AS1 1c 1d 1b 2a 2c AS2 2b Network Laer 4-78

79 Distributing Reachabilit Info With ebgp session between 3a and 1c, AS3 sends prefi reachabilit info to AS1 1c can then use ibgp do distribute this new prefi reach info to all routers in AS1 1b can then re advertise new reachabilit info to AS2 over 1b to 2a ebgp session When router learns of new prefi, creates entr for prefi in its forwarding table ebgp session ibgp session 3c 3a 3b AS3 1a AS1 1c 1d 1b 2a 2c AS2 2b Network Laer 4-79

80 Path Attributes & BGP Routes When advertising a prefi, advert includes BGP attributes. prefi + attributes = route Two important attributes: AS PATH: contains ASs through which prefi advertisement has passed: AS 67 AS 17 NEXT HOP: Indicates specific internal AS router to net hop AS. (There ma be multiple links current AS to net hop AS.) When gatewa router receives route advertisement, uses import polic to accept/decline Network Laer 4-80

81 BGP Route Selection Router ma learn about more than 1 route to some prefi. Router must select route Elimination rules: 1. Local preference value attribute: polic decision 2. Shortest AS PATH 3. Closest NEXT HOP router: hot potato routing 4. Additional criteria Network Laer 4-81

82 BGP Routing Polic 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 Network Laer 4-82

83 BGP Routing Polic (2) 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! Network Laer 4-83

84 Polic: Wh different Intra- and Inter-AS routing? 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 Network Laer 4-84

85 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 Network Laer 4-85