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
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
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
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
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
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
Each AS has: One BGP speaker that advertises: local networks BGP 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 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 Example 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
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
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 W A B C X legend: provider network customer network: 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
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 W A B C X legend: provider network customer network: Y Figure 4.5-BGPnew: a simple BGP scenario 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! 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
Integrating Interdomain and Intradomain Routing All routers run ibgp and an intradomain routing protocol. Border routers (A, D, E) also run ebgp to other ASs Internal v.s. External BGP R1 R2 R3 BGP R4 BGP can be used by R3 and R4 to learn routes. How do R1 and R2 learn routes? Option 1: Inject routes in IGP only works for small routing tables Option 2: Use I-BGP Internal BGP (I-BGP) Same messages as E-BGP Different rules about re-advertising prefixes: prefix learned from E-BGP can be advertised to I-BGP neighbor and vice-versa, but prefix learned from one I-BGP neighbor cannot be advertised to another I-BGP neighbor reason: no AS PATH within the same AS and thus danger of looping 11
Internal BGP (I-BGP) R1 R2 R3 I-BGP E-BGP R4 R3 can tell R1 and R2 prefixes from R4 R3 can tell R4 prefixes from R1 and R2 R3 cannot tell R2 prefixes from R1 R2 can only find these prefixes through a direct connection to R1 Result: I-BGP routers must be fully connected (via TCP)! contrast with E-BGP sessions that map to physical links Integrating Interdomain and Intradomain Routing BGP routing table, IGP routing table, and combined table at router B 12