Routing(2) Inter-domain Routing

Transcription

1 Routing(2) Inter-domain Routing Information Network I Youki Kadobayashi 1

2 Outline Continued from previous lecture on: Distance vector routing Link state routing IGP and EGP Interior gateway protocol, Exterior gateway protocol Path vector routing BGP: Border Gateway Protocol Scaling tricks 2

3 Hierarchical Routing: a natural result of growth Routing domain Defines the boundary between domains Fault isolation, route aggregation Distinction between intra-domain routing protocol and inter-domain routing protocol IGP (Interior Gateway Protocol) EGP (Exterior Gateway Protocol) IGP IGP IGP EGP 3

4 Hierarchical Routing: in reality it is two-tier routing EGP It s like: train network plus airline network IGP IGP 4

5 IGP and EGP has different goal IGP Focus on propagating the state of each link/router as fast as possible RIP-2, RIPng: distance vector routing OSPF, IS-IS: link state routing EGP Focus on routing stability of the entire Internet BGP4, BGP4+: path vector routing 5

6 Path Vector Routing: a small trick for stable routing Derived from the Bellman-Ford algorithm Exchanged information: Distance Vector Path Vector (prefix, metric) (prefix, path, attributes) Attaches distance as well as path information to the route information Embodies routing without loops This protocol prioritizes route that has the shortest path vector. Representing route from S to D (D, { T 1, T 2 },...) S T 1 T 2 D (D, { T 3, T 4, T 5 },...) T 3 T 5 T 4 6

7 Path Vector Routing: Background Multiple alternative routes T 1 Dense connections between ISPs S Which route should we prioritize? T 2 constrained by cost, contract, load, etc. T 3 Routing policy Encodes the intention of the intermediate ISPs Route selection policies enable each domain to select a particular route among multiple routes Policy can t be expressed by scalar cost. Cost of loops Convergence time from transient state RTT D 7

8 Loop Avoidance in Path Vector Routing A B C D Path vectors from A to C (C, { B }) (C, { D, B }) When B-C link fails, B is deleted from path vector. Rejects path vector that include itself Loop avoidance 8

9 BGP: Border Gateway Protocol RFC 4271, 4760 Algorithm Path vector Transport TCP TCP provides retransmission and acknowledgement Adjacency: manually configured (i.e., no auto discovery) Routing information: (prefix, path, attributes) Topology: full mesh (internal), arbitrary (external) 9

10 Adjacency Relationship of BGP Adjacency relationship is defined by ISP operator Adjacency relationship must be explicitly configured Why? c.f. OSPF : if parameters match, adjacency relationship is enabled... R1 R2... R3 R4 10

11 State Transition of BGP: Establishment of Adjacency Relationship OPEN Idle Connect OpenSent OPEN Active OpenConfirm NOTE: KEEPALIVE and NOTIFICATION are omitted from this diagram for simplicity Established 11

12 BGP: Border Gateway Protocol Algorithm Transport Adjacency relationship and state transition Route information efficient path vector expression Topology 12

13 Representing Path Vector in BGP AS (Autonomous System) is represented as an AS number AS: routing domain that is operated by single policy BGP concisely encodes (prefix, AS-path, attributes) (AS-path, attributes, { prefix1, prefix2,... } ) Reduction of traffic 13

14 Example of Path Vector in BGP Try it yourself w/ looking glass 14

15 Hands on: see how BGP works Looking glass Traceroute a Analyze the routes and depict your findings in one page diagram 15

16 Q&A 16

17 BGP: Border Gateway Protocol Algorithm Transport Adjacency relationship and state transition Route information Topology 17

18 Topology of BGP: IBGP and EBGP IBGP EBGP IBGP AS2500 peer AS290 18

19 Topology Constraints in IBGP and EBGP IBGP must establish peering between all IBGP routers in the same AS. doesn t need physical adjacency. All IBGP routers have same routing information. EBGP establishes peering with other ASes requires physical adjacency in principle Routers don t share the same route information with adjacent ASes. 19

20 Q&A 20

21 Policies in BGP Routing policy Encode policies of transit providers Express route selection policy among alternative routes Policies transmitted to other providers MED Policies within IBGP Local preference, admin distance, route map, etc.

22 Limitation of policy in BGP: dispute wheel Rectangle denotes ordered list of path preferences Can we reach D? N. Feamster et al., Implications of Autonomy for the Expressiveness of Policy Routing, SIGCOMM 05.

23 Reducing routing information 23

24 Route Reduction: Default route /0 (IPv4), ::0/0 (IPv6) Longest prefix match matches in the end of route search Results in hiding of routes and reduction of the number of routes R2 Transit AS Stub AS R1 prefix ::0/0 next-hop R1 24

25 When and where routes can t be reduced? Default-free No default route Tier-1 ISPs, North American backbone (source: old UUNet network maps, 25

26 Aggregation /24 and /24 are aggregated into / / / / /18 26

27 Route Aggregation at Domain Edge Routes are aggregated at the edge of the routing domain. OSPF backbone OSPF area 1 R /18 via R1 (Summary LSA) 27

28 Hierarchical Aggregation of Routes OSPF areas R1 OSPF backbone OSPF backbone BR /18 via R1 (Summary LSA) /16 via BR (BGP advertisement) 28

29 Challenges in Route Aggregation Route aggregation depends on address assignment Planned address assignment is important for route aggregation Can we predict the extent of future growth of NAIST? Can we predict future growth of an ISP? Prefix renumbering Renumber to aggregatable addresses Provide operational means to unanticipated changes Technology under development Manual operation is necessary for route aggregation Internet full-route : 250,000 BGP table growth trends - Telstra 29

30 Q&A 30

31 Highlight of Assignment 4 achievements 31

32 Summary Hierarchical routing concepts IGP, EGP Path Vector Routing Loop-free, policy-aware BGP State transition, route information and topology Limitation of policy-based routing Scaling trick: Route aggregation Aggregation concepts, challenges 32

33 Assignment Pick two web sites, investigate inter-domain routes with two looking glasses, and then visualize AS paths. Mapping AS number to provider name: $ whois -h radb.ra.net. AS2500 aut-num: AS2500 as-name: WIDE descr: WIDE Project in Japan Optionally, pick a slow web site and investigate its reason, by using above tools and tcpdump or wireshark You may find Deadline: May 17, 17:00 S T 3 T 4 T 2