CS 457 Networking and the Internet Fall 2016 The Global Internet - PDF document

10/4/16 CS 457 Networking and the Internet Fall 2016 The Global Internet (Then) The tree structure of the Internet in 1990 The Global Internet (And Now) A simple multi-provider Internet 1

10/4/16 The Global Internet • Some large corporations connect directly to one or more of the backbone, while others connect to smaller, non-backbone service providers. • Many service providers exist mainly to provide service to “consumers” (individuals with PCs in their homes), and these providers must connect to the backbone providers • Often many providers arrange to interconnect with each other at a single “peering point” Autonomous Systems • Internet is organized as autonomous systems (AS) each of which is under the control of a single administrative entity • Autonomous System (AS) • corresponds to an administrative domain • examples: University, company, backbone network • A corporation’s internal network might be a single AS, as may the network of a single Internet service provider Autonomous Systems A network with two autonomous system 2

10/4/16 Route Propagation • Idea: Provide an additional way to hierarchically aggregate routing information in a large internet. – Improves scalability • Divide the routing problem in two parts: – Routing within a single autonomous system – Routing between autonomous systems • Another name for autonomous systems in the Internet is routing domains – Two-level route propagation hierarchy • Inter-domain routing protocol (Internet-wide standard) • Intra-domain routing protocol (each AS selects its own) Routing by AS • Uses an interior gateway protocol (IGP) and common metrics to route packets within the AS (Intra-AS) • Uses an exterior gateway protocol (EGP) to route packets to other AS’s (Inter-AS) • AS may use multiple IGPs and metrics, but appears as single AS to other AS ’ s IGP and EGP Example 3

10/4/16 Why Different Intra- and Inter- AS routing ? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • 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 Inter-AS Routing - EGP and BGP • Exterior Gateway Protocol (EGP) – Forced a tree-like topology onto the Internet – Did not allow for the topology to become general • Tree like structure: there is a single backbone and autonomous systems are connected only as parents and children and not as peers • Border Gateway Protocol (BGP) – Assumes that the Internet is an arbitrarily interconnected set of ASs. – Today’s Internet consists of an interconnection of multiple backbone networks (they are usually called service provider networks, and they are operated by private companies rather than the government) • Sites are connected to each other in arbitrary ways BGP • The goal of Inter-domain routing is to find any path to the intended destination that is loop free – We are concerned with reachability than optimality – Finding path anywhere close to optimal is considered to be a great achievement • Why? 4

10/4/16 Path Vectors • Each routing update carries the entire path • Loops are detected as follows: – When AS gets route check if AS already in path • If yes, reject route • If no, add self and (possibly) advertise route further • Advantage: – metrics are local - AS chooses path, protocol ensures no loops BGP Philosophy • Scalability: An Internet backbone router must be able to forward any packet destined anywhere in the Internet – Having a routing table that will provide a match for any valid IP address • Autonomous nature of the domains – It is impossible to calculate meaningful path costs for a path that crosses multiple ASs – A cost of 1000 across one provider might imply a great path but it might mean an unacceptable bad one from another provider • Issues of trust – Provider A might be unwilling to believe certain advertisements from provider B BGP • BGP does not belong to either of the two main classes of routing protocols (distance vectors and link-state protocols) • BGP advertises complete paths as an enumerated lists of ASs to reach a particular network 5

10/4/16 BGP-4: Border Gateway Protocol • Assumes the Internet is an arbitrarily interconnected set of AS's. • Define local traffic as traffic that originates at or terminates on nodes within an AS, and transit traffic as traffic that passes through an AS. • We can classify AS's into three types: • Stub AS : an AS that has only a single connection to one other AS; such an AS will only carry local traffic • Multihomed AS : an AS that has connections to more than one other AS, but refuses to carry transit traffic • Transit AS : an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic BGP Multihomed AS Transit AS Stub AS BGP Example Example of a network running BGP 6

10/4/16 BGP Each AS has: • One BGP speaker that advertises: – local networks – other reachable networks (transit AS only) – gives path information • In addition to the BGP speakers, the AS has one or more border “gateways” which need not be the same as the speakers • The border gateways are the routers through which packets enter and leave the AS BGP Example • Speaker for AS 2 advertises reachability to P and Q • Network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS 2. • Speaker for backbone network then advertises • Networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path <AS 1, AS 2>. • Speaker can also cancel previously advertised paths BGP Issues • It should be apparent that the AS numbers carried in BGP need to be unique • For example, AS 2 can only recognize itself in the AS path in the example if no other AS identifies itself in the same way • AS numbers are 16-bit numbers assigned by a central authority 7

10/4/16 Policy With BGP • BGP provides capability for enforcing various policies • Policies are not part of BGP: they are provided to BGP as configuration information • BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS ’ s Examples of BGP Policies • A multi-homed AS refuses to act as transit – limit path advertisement • A multi-homed AS can become transit for some AS ’ s – only advertise paths to those AS ’ s • An AS can favor or disfavor certain AS ’ s for traffic transit from itself – Pick appropriate routes by examining path vectors BGP Is NOT Needed If: • Single homed network (stub) • AS does not provide downstream routing • AS uses a default route 8

10/4/16 Routing Information Bases (RIB) • Routes are stored in RIBs • Adj-RIBs-In: routing info that has been learned from other routers (unprocessed routing info) • Loc-RIB: local routing information selected from Adj-RIBs-In (routes selected locally) • Adj-RIBs-Out: info to be advertised to peers (routes to be advertised) BGP Messages • Open – Opens a BGP connection (establishes a TCP connection) • Update – Withdrawn routes – New routes that include path attributes e.g., origin, path • Notification – Used for error notification - TCP connection is closed immediately after notification • Keep alive – Sent periodically to peers to ensure connectivity – sent in place of an update message BGP: Controlling Who Routes To You legend: provider B network X W A customer network: C Y Figure 4.5-BGPnew : a simple BGP scenario ❒ 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 9

10/4/16 Internet Inter-AS routing: BGP Suppose: gateway X sends its path to peer gateway W • W may or may not select path offered by X – cost, policy (don’t route via competitors AS), loop prevention reasons . • If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) • Note: X can control incoming traffic by controlling it route advertisements to peers: – e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z BGP: Controlling Who Routes To You legend: provider B network X W A customer network: C Y ❒ A advertises to B the path AW Figure 4.5-BGPnew : a simple BGP scenario ❒ 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! BGP Operation Q: What does a BGP router do? • Receiving and filtering route advertisements from directly attached neighbor(s). • Route selection. – To route to destination X, which path (of several advertised) will be taken? • Sending route advertisements to neighbors. 10