Inter-Autonomous-System Routing: Border Gateway Protocol

Transcription

1 Inter-Autonomous-System Routing: Border Gateway Protocol Antonio Carzaniga Faculty of Informatics University of Lugano June 14, 2005

2 Outline Hierarchical routing BGP

3 Routing

4 Routing Goal: each router u must be able to compute, for each other router v, the next-hop neighbor x that is on the least-cost path from u to v

5 Routing Goal: each router u must be able to compute, for each other router v, the next-hop neighbor x that is on the least-cost path from u to v v x 3 x 4 u x 2 x 1

6 Network Model

7 Network Model So far we have studied routing over a flat network model

8 Network Model So far we have studied routing over a flat network model g h j d e f a b c Also, our objective has been to find the lowest-cost paths between sources and destinations

9 Network Model So far we have studied routing over a flat network model g 14 h j d e 9 f a 3 4 b c Also, our objective has been to find the lowest-cost paths between sources and destinations

10 More Realistic Topologies

11 More Realistic Topologies

12 Even More Realistic

13 Even More Realistic c 2001 Stephen Coast

14 An Internet Map c 1999 Lucent Technologies

15 Higher-Level Objectives

16 Higher-Level Objectives Scalability hundreds of millions of hosts in today s Internet

17 Scalability Higher-Level Objectives hundreds of millions of hosts in today s Internet transmitting routing information (e.g., LSAs) would be too expensive

18 Scalability Higher-Level Objectives hundreds of millions of hosts in today s Internet transmitting routing information (e.g., LSAs) would be too expensive forwarding would also be too expensive

19 Scalability Higher-Level Objectives hundreds of millions of hosts in today s Internet transmitting routing information (e.g., LSAs) would be too expensive forwarding would also be too expensive Administrative autonomy

20 Scalability Higher-Level Objectives hundreds of millions of hosts in today s Internet transmitting routing information (e.g., LSAs) would be too expensive forwarding would also be too expensive Administrative autonomy one organization might want to run a distance-vector routing protocol, while another might want to run a link-state protocol

21 Scalability Higher-Level Objectives hundreds of millions of hosts in today s Internet transmitting routing information (e.g., LSAs) would be too expensive forwarding would also be too expensive Administrative autonomy one organization might want to run a distance-vector routing protocol, while another might want to run a link-state protocol an organization might not want to expose its internal network structure

22 Hierarchical Structure Today s Internet is organized in autonomous systems (ASs) independent administrative domains

23 Hierarchical Structure Today s Internet is organized in autonomous systems (ASs) independent administrative domains Gateway routers connect an autonomous system with other autonomous systems

24 Hierarchical Structure Today s Internet is organized in autonomous systems (ASs) independent administrative domains Gateway routers connect an autonomous system with other autonomous systems An intra-autonomous system routing protocol runs within an autonomous system this protocol determines internal routes internal router internal router internal router gateway router gateway router gateway router

25 Inter-AS Routing An inter-autonomous system routing protocol determines routing at the autonomous-system level

26 Inter-AS Routing An inter-autonomous system routing protocol determines routing at the autonomous-system level Once computed by the gateway routers, inter-as routing information is propagated within an autonomous system all routers use both inter-as and intra-as routes to compile their forwarding tables

27 Hierarchical Routing Destinations within the same autonomous system are reached as usual

28 Hierarchical Routing Destinations within the same autonomous system are reached as usual What about a destination x outside the autonomous system?

29 Hierarchical Routing Destinations within the same autonomous system are reached as usual What about a destination x outside the autonomous system? routers use inter-as information to figure out that destination x is reachable through gateway G x

30 Hierarchical Routing Destinations within the same autonomous system are reached as usual What about a destination x outside the autonomous system? routers use inter-as information to figure out that destination x is reachable through gateway G x routers use intra-as information to figure how to send the packet to G x

31 Hierarchical Routing Destinations within the same autonomous system are reached as usual What about a destination x outside the autonomous system? routers use inter-as information to figure out that destination x is reachable through gateway G x routers use intra-as information to figure how to send the packet to G x in cases in which x is reachable through multiple gateway routers G x, G X,..., routers use the closest gateway hot-potato routing

32 Benefits of Hierarchical Routing Administrative autonomy

33 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use

34 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use an autonomous system needs to expose only minimal information about the internal structure of its network essentially only (sub)net addresses

35 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use an autonomous system needs to expose only minimal information about the internal structure of its network essentially only (sub)net addresses Scalability

36 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use an autonomous system needs to expose only minimal information about the internal structure of its network essentially only (sub)net addresses Scalability routers within an autonomous system need to know very little about the internal structure of other autonomous systems

37 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use an autonomous system needs to expose only minimal information about the internal structure of its network essentially only (sub)net addresses Scalability routers within an autonomous system need to know very little about the internal structure of other autonomous systems essentially only (sub)net addresses

38 Benefits of Hierarchical Routing Administrative autonomy each autonomous system decides what intra-as routing to use an autonomous system needs to expose only minimal information about the internal structure of its network essentially only (sub)net addresses Scalability routers within an autonomous system need to know very little about the internal structure of other autonomous systems essentially only (sub)net addresses External subnet addresses are likely to aggregate in groups that admit compact representations this process is called supernetting

39 Inter-AS Routing in the Internet

40 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet

41 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs

42 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs transmits reachability information to all internal routers within an AS

43 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs transmits reachability information to all internal routers within an AS determines good routes to all outside subnets

44 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs transmits reachability information to all internal routers within an AS determines good routes to all outside subnets based on reachability information

45 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs transmits reachability information to all internal routers within an AS determines good routes to all outside subnets based on reachability information based on policies

46 Inter-AS Routing in the Internet The Border Gateway Protocol (BGP) is the inter-as routing protocol in today s Internet provides reachability information from neighbor ASs transmits reachability information to all internal routers within an AS determines good routes to all outside subnets based on reachability information based on policies BGP is a distance-vector protocol (or better path-vector)

47 BGP Architecture and Terminology BGP session: a semi-permanent connection between two routers

48 BGP Architecture and Terminology BGP session: a semi-permanent connection between two routers BGP peers: two routers engaged in a BGP session BGP sessions are established over TCP

49 BGP Architecture and Terminology BGP session: a semi-permanent connection between two routers BGP peers: two routers engaged in a BGP session BGP sessions are established over TCP BGP external session (ebgp): a session across two autonomous systems

50 BGP Architecture and Terminology BGP session: a semi-permanent connection between two routers BGP peers: two routers engaged in a BGP session BGP sessions are established over TCP BGP external session (ebgp): a session across two autonomous systems BGP internal session (ibgp): a session within an autonomous system note that internal sessions carry inter-as information intra-as routing is a separate protocol (e.g., OSPF)

51 BGP Architecture and Terminology (2) BGP advertisement: a router advertises a routes to networks, much like an entry in a distance-vector destinations are denoted by address prefixes

52 BGP Architecture and Terminology (2) BGP advertisement: a router advertises a routes to networks, much like an entry in a distance-vector destinations are denoted by address prefixes an AS may or may not forward an advertisement for a foreign network. Doing so means being willing to carry traffic for that network

53 BGP Architecture and Terminology (2) BGP advertisement: a router advertises a routes to networks, much like an entry in a distance-vector destinations are denoted by address prefixes an AS may or may not forward an advertisement for a foreign network. Doing so means being willing to carry traffic for that network this is where a router may aggregate prefixes (a.k.a., supernetting ) E.g., } / / /24

54 BGP Architecture and Terminology (2) BGP advertisement: a router advertises a routes to networks, much like an entry in a distance-vector destinations are denoted by address prefixes an AS may or may not forward an advertisement for a foreign network. Doing so means being willing to carry traffic for that network this is where a router may aggregate prefixes (a.k.a., supernetting ) E.g., } / / / / / /22

55 BGP Architecture and Terminology (2) BGP advertisement: a router advertises a routes to networks, much like an entry in a distance-vector destinations are denoted by address prefixes an AS may or may not forward an advertisement for a foreign network. Doing so means being willing to carry traffic for that network this is where a router may aggregate prefixes (a.k.a., supernetting ) E.g., } / / / / / / /20

56 BGP Architecture and Terminology (2) Autonomous system number (ASN): a unique identifier for each AS (with more than one gateway)

57 BGP Architecture and Terminology (2) Autonomous system number (ASN): a unique identifier for each AS (with more than one gateway) BGP attributes: a route advertisement includes a number of attributes AS-PATH: sequence of ASNs through which the advertisement has been sent

58 BGP Architecture and Terminology (2) Autonomous system number (ASN): a unique identifier for each AS (with more than one gateway) BGP attributes: a route advertisement includes a number of attributes AS-PATH: sequence of ASNs through which the advertisement has been sent NEXT-HOP: specifies the interface to use to forward packets towards the advertised destination. This is used to resolve ambiguous cases where an AS can be reached through multiple gateways (interfaces)

59 Route Selection 1. Router preference: routes are ranked according to a preference value configured at the router or learned from another router within the same AS essentially a configuration parameter for the AS

60 Route Selection 1. Router preference: routes are ranked according to a preference value configured at the router or learned from another router within the same AS essentially a configuration parameter for the AS 2. Shortest AS-PATH

61 Route Selection 1. Router preference: routes are ranked according to a preference value configured at the router or learned from another router within the same AS essentially a configuration parameter for the AS 2. Shortest AS-PATH 3. Closest NEXT-HOP router

62 Route Selection 1. Router preference: routes are ranked according to a preference value configured at the router or learned from another router within the same AS essentially a configuration parameter for the AS 2. Shortest AS-PATH 3. Closest NEXT-HOP router 4....