1 Internet Routing 11/11/2009

Admin. r Assignment 3 2

Recap r Distance vector protocol r Link state protocol 3

Outline  Routing in the Internet  overview

Routing in the Internet r The Global Internet consists of Autonomous Systems (AS) interconnected with each other m An AS is identified by an AS Number (ASN), e.g. Yale ASN is 29 m try % whois -h whois.arin.net “a Yale“

6 Internet ISP Connectivity

7 Challenge of the Internet: Characterizing Internet Topology

Routing with AS r Internet routing is divided into intra-AS routing and inter-AS routing r Intra-AS m A protocol running insides an AS is called an Interior Gateway Protocol (IGP) RIP: Routing Information Protocol E/IGRP: Interior Gateway Routing Protocol (Cisco) OSPF: Open Shortest Path First IS-IS: very similar to OSPF (or should we say OSPF is very similar to IS-IS?) r Inter-AS m a protocol runs among AS’s is also called an Exterior Gateway Protocol (EGP) m for global connectivity, a single interdomain routing protocol

Intra-AS and Inter-AS: Example a b b a a C A B d c A.a A.c C.b B.a c b border (exterior gateway) routers interior (gateway) routers

b a AS C (RIP intra routing) Routing in the Internet: Example c AS B (OSPF intra routing) AS A (OSPF intra routing) eBGP iBGP Gateway routers participate in intradomain to learn internal routes. Gateway routers of diff. AS’s exchange routes using eBGP gateway routers of same AS share learned external routes using iBGP. i e f g h d

Many Routing Processes on a Single Router Forwarding Table OSPF domain RIP domain BGP OS kernel RIP process RIP routing table Forwarding Table Manager OSPF process OSPF Routing table BGP process BGP routing table

inter-AS routing between A and B Intra-AS and Inter-AS Routing Host h2 a b b a a C A B d c A.a A.c C.b B.a c b Host h1 Intra-AS routing within AS A intra-AS routing within AS B border (exterior gateway) routers interior (gateway) routers

Why Partition into Intra- and Inter-AS Routing? r This partition allows ASes flexibility to choose their own intra-AS routing protocols m autonomy r By aggregating many destinations inside an AS into a single destination in interdomain routing, it improves scalability m the partition is a type of hierarchical routing m hierarchical routing improves scalability: only a small number of routers are involved with outside

Yale Internet Connectivity Yale Qwest default routes /0 pointing to provider / /16 AT&T Internet2

Outline  Routing in the Internet o overview  BGP

BGP Setup

Internet Interdomain Routing: BGP r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: m similar to Distance Vector protocol m a border gateway sends to a neighbor entire path (i.e., a sequence of ASes) to a destination, e.g., gateway X sends to neighbor N its path to dest. Z: path (X,Z) = X,Y1,Y2,Y3,…,Z m if N selects path(X, Z) advertised by X, then: path (N,Z) = N, path (X,Z) X N Z

BGP Operations (Simplified) Establish session on TCP port 179 Exchange all active routes Exchange incremental updates AS1 AS2 while (connection is ALIVE) exchange UPDATE message select best available route if route changes, export to neigh. BGP session

BGP Messages r Four types of messages m OPEN: opens TCP connection to peer and authenticates sender m UPDATE: advertises new path (or withdraws old) m KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request m NOTIFICATION: reports errors in previous msg; also used to close connection

Why Path Vector? r Path vector prevents counting-to-infinity problem r Path vector allows an AS to define local policies on the ASes of a given path

BGP Routing Decision Process routing cache select best path export path to neighbors route selection policy: rank paths export policy: which paths export to which neighbors

BGP Route Selection/Export Policies r Route selection m algorithm to select the best currently available route from routing cache (routing information base) m typical (Cisco) route selection policy highest local pref shortest AS path length prefer eBGP over iBGP … r Route export policy: m since the export of a path to a neighbor is an indication that the AS is willing to transport traffic for the neighbor, an AS may not export some routes to some neighbors (more later) Yale QwestAT&T Internet2

23 Routing: Example AS A (OSPF) AS B (OSPF intra routing) AS D AS C i b b->i: I can reach hosts in D; my path: BCD a1 a2 d d->a2: I can reach hosts in D; my path: D a1->i: I can reach hosts in D; my path: AD E F Export to E: i->e: I can reach hosts in D; path: IBCD AS I a2->a1: I can reach hosts in D; path: D choose BCD using i2 b->i2: I can reach hosts in D; my path: BCD i2 i2->i: I can reach hosts in D; path: BCD No Export to F

Hierarchical Routing May Pay a Price for Path Quality AS 4 AS 3 AS 2 AS 1

Outline r Admin. r BGP m Overview m BGP instability and stability condition 25

BGP Policy Interactions preferred less preferred The BAD GADGET example: - 0 is the destination - the route selection policy of each AS is to prefer its counter clock-wise neighbor Policy interaction causes routing instability !

Understanding Instability: P-Graph r Nodes in P-graph are feasible paths r A directed edge from path N 1 P 1 to P 1 m intuition: to let N 1 choose N 1 P 1, P 1 must be chosen and exported to N 1 r A directed edge from a lower ranked path to a higher ranked path m intuition: the higher ranked path should be considered first P-graph

Partial Order Graph and Convergence  If the P-graph has no loop, then BGP policy converges.  intuition: choose the node (i.e., a path) from the partial order graph with no out-going edge, choose the path, remove the path and all other lowered ranked paths of the same node; remove nodes if suffix removed; continue  Example: suppose we swap the order of 30 and

Partial Order Graph and BGP Convergence Preview: A reason we do not often see instability in the Internet is that: the current Internet ISP economy implies no loop in P-graph !

Internet Economy: Two Types of Business Relationship r Customer provider relationship m a provider is an AS that connects the customer to the rest of the Internet m customer pays the provider for the transit service m e.g., Yale is a customer of AT&T and QWEST r Peer-to-peer relationship m mutually agree to exchange traffic between their respective customers m there is no payment between peers provider customer peer provider provider to customer

Implication of Business Relationship on Policies r Route selection (ranking) policy: m the typical route selection policy is to prefer customers over peers/providers to reach a destination, i.e., Customer > pEer/Provider

customer provider customer peer Typical Export Policies provider customer peer provider customer peer routes learned from a customer are sent to all other neighbors routes learned from a provider are sent only to customers routes learned from a peer are sent only to customers case 1: routes learned from customer case 2: routes learned from provider case 3: routes learned from peer

Typical Export -> No-Valley Routing P1P1 A P2P2 IP traffic A advertises path to C, but not P 2 A advertises to C paths to P 1 and P 2 A advertises path to C, but not P 1 Suppose P 1 and P 2 are providers of A; A is a provider of C C A learns paths to C, P1, P2

Yale Internet Connectivity Yale Qwest default routes /0 pointing to provider / /16 AT&T Internet2

Typical Export Policies Imply Patterns of Routes r Assume a BGP path SABCD to destination AS D. Consider the business relationship between each pair: r Three types of business relationships: m PC (provider-customer) m CP (customer-provider) m PP (peer-peer) 35 S A B C D