Routing(2) Inter-domain Routing

Transcription

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

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

3 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 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. T 1 Representing route from S to D S (D, { T 1, T 2 },...) T (D, { T 3 3, T 4, T 5 },...) T 4 T 2 T 5 D 6

4 Path Vector Routing: Background! Multiple alternative routes! Dense connections between ISPs S! Which route should we prioritize? constrained by cost, contract, load, etc.! 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 T 1 T 2 T 3 D 7 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

5 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 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

6 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 11 Example of Path Vector in BGP Try it yourself w/ looking glass 12

7 Hands on: see how BGP works! (optional hands on with BGP) 13 Q&A 14

8 Reducing routing information 15 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 16

9 When and where routes can t be reduced?! Default-free! No default route! Tier-1 ISPs, North American backbone ( (source: old UUNet network maps, 17 Aggregation! /24 and /24 are aggregated into / / / / /18 18

10 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) 19 Hierarchical Aggregation of Routes OSPF areas R1 OSPF backbone OSPF backbone BR /18 via R1 (Summary LSA) /16 via BR (BGP advertisement) 20

11 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 21 Q&A 22

12 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 23