Introduction to Communication Networks Spring Unit 16 Global Internetworking
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1 Introduction to Communication Networks Spring 2007 Unit 16 Global Internetworking
2 Acknowledgements slides coming from: Based almost completely on slides from the earlier issues of the EECS 122 taught recently by Prof Jean Walrand, Prof David Tse, Prof Abhay Parekh, Prof Shyam Parekh, Prof Ion Stoica etc. Some slides borrowed from Timothy G. Griffins SIGCOM 2001 tutorial; August 2001, 2of 54
3 ISPs... [Steenkiste] 3of 54
4 ISPs, ISPs, But the hierarchy is not so strict there is a lot of choice... How many ISPs in my vicinity? in the 510 area code? 4of 54
5 How Are ISPs related? Peering & Transit Peering The business relationship whereby ISPs reciprocally provide to each other connectivity to each others local or inherited customers Transit The business relationship whereby one ISP provides (usually sells) access to all destinations in it s routing table William B. Norton, Internet Service Providers and Peering 5of 54
6 Peering & Transit :Peering Figure from William B. Norton, Internet Service Providers and Peering West and East Peer with USNet but they can t reach each other 6of 54
7 Peering & Transit :Transit Figure from William B. Norton, Internet Service Providers and Peering 7of 54
8 Peering & Transit: Benefits of Transit v/s Peering William B. Norton, Internet Service Providers and Peering 8of 54
9 Nontransit vs. Transit ASes [griffin] ISP 1 ISP 2 Internet Service providers (often) have transit networks Traffic NEVER flows from ISP 1 through NET A to ISP 2 (At least not intentionally!) NET A Nontransit AS might be a corporate or campus network. Could be a content provider IP traffic 9of 54
10 Hierarchical Routing The internet has many Administrative Domains B A 3 13 C of 54
11 Hierarchical Routing Border Routers B RIP BGP IGRP A C OSPF 11 of 54
12 Hierarchical Routing: Interdomain & Intradomain 2 3 BGP 4 InterDomain 4 InterDomain 6 B IntraDomain routing protocols: Local issue, different - InterDomain routing protocol: One common!!! 8 B 7 IntraDomain RIP 13 2 IntraDomain 1 3 IGRP A C OSPF IntraDomain 12 of 54
13 Why Hierarchical Routing? Is a natural way for routing to scale Size Network Administration Governance Allows multiple metrics at different levels of the hierarchy RIP Routing OSPF IGRP Inter Domain Exploits address aggregation and allocation 13 of 54
14 Internet Routing Internet organized as a two level hierarchy Some intra-domain routing approaches create their own hierarchy (see OSPF) Autonomous systems (AS s) AS region of network under a single administrative domain AS s run an intra-domain routing protocols Distance Vector, Communicate current distance estimates of node to every other node, uses Bellman_Ford,. e.g., RIP, IGRP,EIGRP Link State, Communicate the names and costs of neighbors. Each node maintains the entire topology, uses Dijkstra, e.g., OSPF, ISIS Between AS s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP) De facto standard today, BGP-4, uses Path Vector: Communicate current estimates of preferred paths from node to every other node, source routing! 14 of 54
15 Overview LINK STATE AA AA BB BB CC CC 1 3 DISTANCE VECTOR 1 3 PATH VECTOR AA 2 1 BB CC 1 1) Exchange Link States 2) Each node computes A: [B, 2], [C, 1] the shortest paths to B: [A, 2], [D, 1] the others DD C: [A, 1], [D, 3] D: [B, 1], [C, 3] 3 0 DD 0 D DD D AA AA BB 1 CC 3 3 B,D BB 1 CC C,D 3 DD 0 DD AA 2 1 Don t like B AA 2 1 BB CC BB CC DD DD 15 of 54
16 Link state vs. Distance Vector 16 of 54
17 Routing Sub-Functions Topology Update: Characterize and maintain connectivity Discover neighbors Measure distance (one or more metric) Disseminate Route Computation: Kind of path: Multicast, Unicast Centralized or Distributed Algorithm Policy Hierarchy Switching: Forward the packets at each node 17 of 54
18 OSPF: Topology update Flooding!!! [steenkiste] (Link ID and metric repeated for each link) Cost of the link might be split according to different Type of Service metrics: e.g. high for delay, low for bit error rate of 54
19 OSPF link-state advertisement [Peterson] LS Age Options Type=1 Link-state ID Advertising router LS sequence number LS checksum Length 0 Flags 0 Number of links Link ID Link data Link type Num_TOS Metric Optional TOS information More links Packet used to advertise links of 54
20 OSPF: Topology update processing [steenkiste] 20 of 54
21 Flooding- More issues [steenkiste] 21 of 54
22 Some Issues What happens if some routers are much faster at transmitting LSPs? What happens if sequence numbers wrap? What happens when a partitioned network is reconstituted? What about security? Etc., etc. Many lines of code 22 of 54
23 OSPF allows for some local hierarchy! This hierarchy simplifies routing (complexity of Dijkstra!!), but might introduce Longer routes of 54
24 Remember CIDR? 24 of 54
25 Hierarchical Adressing helps in Routing structuring 25 of 54
26 ISPs divide the available addresses of 54
27 ISPs... [Steenkiste] How can I learn who owns an IP address? Go to the interactive ARIN WHOIS data base: 27 of 54
28 Search Sample (line borders edited for readability!) Search results for: OrgName: Savvis OrgID: SAVVI-2 Address: 3300 Regency Parkway_ City: Cary_StateProv: NC_PostalCode: 27511_Country: US ReferralServer: rwhois://rwhois.savvis.net:4321/ NetRange: CIDR: /14 NetName: SAVVIS NetHandle: NET Parent: NET NetType: Direct Allocation NameServer: DNS01.SAVVIS.NET _NameServer: DNS04.SAVVIS.NET Comment: RegDate: _Updated: OrgAbuseHandle: ABUSE11-ARIN _OrgAbuseName: Abuse Org AbusePhone: _OrgAbuse OrgNOCHandle: NOC99-ARIN _OrgNOCName: SAVVIS Support Center OrgNOCPhone: _OrgNOC ipnoc@savvis.net OrgTechHandle: UIAA-ARIN OrgTechName: US IP Address Administration OrgTechPhone: _OrgTech ipadmin@savvis.net 28 of 54
29 The hierarchy again... This hierarchy simplifies routing (complexity of Dijkstra!!), but might introduce Longer routes of 54
30 What about the Forwarding Table? 30 of 54
31 Inter-AS tasks Suppose router in AS1 receives datagram for dest outside of AS1 Router should forward packet towards an ASborder router, but which one? AS1 needs: 1. to learn which dests are reachable through AS2 and which through AS3 2. to propagate this reachability info to all routers in AS1 Job of inter-as routing! 3c 3a 3b AS3 1a 1c 1d 1b AS1 2a 2c 2b AS2 31 of 54
32 Concept Advertising a route means readiness to carry traffic!!! you can reach net A via me AS2 AS1 traffic to A R1 BGP R2 A R3 table at R1: dest next hop A R2 R border router internal router Share connectivity information across ASes TOC IP Routing Types Interdomain EECS 122 BGP SPRING Concept of 54
33 Border Routers have to communicate 33 of 54
34 Join EGP with IGP For Connectivity [griffin] /16 Next Hop = / Forwarding Table destination next hop / destination + EGP next hop / Forwarding Table destination next hop AS 1 AS / / / of 54
35 Danger: Blackholes peer provider peer customer Need Filter Here! /24 legitimate Accidental or malicious announcement of your prefix can blackhole your destinations in large part of the Internet /24 not legitimate 35 of 54
36 Internet inter-as routing: BGP The de facto standard: Border Gateway Protocol (BGP) BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs 2. Propagate reachability information to all routers in the AS 3. Determine good routes to subnets based on reachability information and routing policy. Allows a subnet to advertise its existence to rest of the Internet: I am here Issues: Which routing algorithm? How are routes advertised? How to implement routing policies? 36 of 54
37 Some BGP principles. Pairs of routers (BGP peers) exchange routing info over semipermanent TCP connections: BGP sessions Note that BGP sessions do not correspond to physical links 37 of 54
38 BGP is a path vector protocol Distance vector algorithm with extra information Pure distance vector does not enable policies Pure link state does not scale and exposes policies When advertising a prefix, advert includes BGP attributes. prefix + attributes = route Two important attributes: AS-PATH: contains all ASs along the way: AS 67 AS 17 NEXT-HOP: Indicates the specific internal-as router to next-hop AS. When gateway router receives route advertisement, uses import policy to accept/decline Can make decision based on ASes on path Can easily avoid loops 38 of 54
39 BGP Routing Table 39 of 54
40 Peers exchange BGP messages using TCP OPEN: opens TCP conn. to peer authenticates sender UPDATE: advertises new path (or withdraws old) Path attributes: e.g. multiple exit discriminators KEEPALIVE: keeps connections alive in absence of UPDATES, ACKs OPEN request NOTIFICATION: reports errors in previous msg; closes a connection Process: Initialization: Open => Updates for all routes Ongoing: Updates for changed routes 40 of 54
41 The Philosophy: Reachability Interdomain routing is about implementing policies of reachability Routing efficiency and performance is important, but not essential ISPs could be competitors and do not want to share internal network statistics such as load and topology Router learn > 1 route to some prefix Router must select what they believe is the best route! Elimination rules: 1.Local preference value attribute: policy decision 2.Shortest AS-PATH (the minimal number of AS on the way ) 3.Best MED (multi-exit-discriminator) (announced preferred entry router) 4.Closest NEXT-HOP router: hot potato (out of my AS!) routing 5.Additional criteria 6.IP address of peer router (just solving the stalemate ) 41 of 54
42 BGP Route Processing Receive BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values Best and Alternate Routes Apply policies only to Best Routes! Transmit BGP Updates Apply Import Policies Best Route Selection BGP Route Table Apply Export Policies Install Best Routes IP Forwarding Table 42 of 54
43 Routing policy Reflects goals of network provider which routes to accept from other ASes how to manipulate the accepted routes how to propagate routes through network how to manipulate routes before they leave the AS which routes to send to another AS 43 of 54
44 BGP routing policy an example B legend: network provider W A X customer network: C Y B, C = Peers 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 from B via X to C.. so X will not advertise to B a route to C 44 of 54
45 BGP routing policy, example (cont) W A B X legend: provider network customer network: C B, C = Peers Y A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No way! 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 only to/from its customers! 45 of 54
46 Local Preference Attribute AS /16 AS 200 AS 300 D E / > / A AS 400 B Allows providers to prefer routes within his AS - Path with highest local preference wins C Local preference? Frequently Hot potato shortest way out of my network!!
47 Multi-exit discriminator of 54
48 Why different Intra- and Inter-AS routing? Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. 1.Local preference value attribute: policy decision 2. Shortest AS-PATH (the minimal number of AS on the way ) 3. Best MED (multi-exit-discriminator) (announced preferred entry router) 4. Closest NEXT-HOP router: hot potato (out of my AS!) routing 5. Additional criteria 6. IP address of peer router (just solving the stalemate ) Intra-AS: single admin, so no policy decisions needed Scale: hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance We need BOTH! 48 of 54
49 Midterm Scores: Avg=64, Sdev= 13.2 occurencies Series1 The Leader: Points*10 49 of 54
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