CS 640: Introduction to Computer NetworksIntra-domain routingInter-domain Routing: HierarchyInternet’s HierarchyAn exampleThe ProblemHistoryBGP PreliminariesChoices for RoutingAS Numbers (ASNs)Distance Vector with PathHop-by-hop ModelPolicy with BGPExamples of BGP PoliciesBGP MessagesBGP UPDATE MessagePath Selection CriteriaLOCAL PREFAS_PATHMulti-Exit Discriminator (MED)MEDSlide 22Decision Process (First cut)A Logical View of the InternetInter-ISP Relationships: Transit vs. PeeringIllustrating BGP PoliciesPolicy I: Prefer Customer routingPolicy II: Import RoutesPolicy II: Export RoutesPolicy II: Valley-Free RoutesBGP Route Selection SummaryInternal vs. External BGPLink FailuresFailure on an E-BGP LinkFailure on an I-BGP LinkNext ClassCS 640: Introduction to Computer NetworksAditya AkellaLecture 11 -Inter-Domain Routing -BGP (Border Gateway Protocol)2Intra-domain routing•The Story So Far… –Routing protocols generate the forwarding table–Two styles: distance vector, link state–Scalability issues: •Distance vector protocols suffer from count-to-infinity•Link state protocols must flood information through network•Today’s lecture–How to make routing protocols support large networks–How to make routing protocols support business policies3Inter-domain Routing: Hierarchy•“Flat” routing not suited for the Internet–Doesn’t scale with network size•Storage Each node cannot be expected to store routes to every destination (or destination network)•Convergence times increase•Communication Total message count increases–Administrative autonomy•Each internetwork may want to run its network independently–E.g hide topology information from competitors•Solution: Hierarchy via autonomous systems4Internet’s Hierarchy•What is an Autonomous System (AS)?–A set of routers under a single technical administration•Use an interior gateway protocol (IGP) and common metrics to route packets within the AS•Connect to other ASes using gateway routers•Use an exterior gateway protocol (EGP) to route packets to other AS’s–IGP: OSPF, RIP (last class)–Today’s EGP: BGP version 45An example Intra-AS routing algorithm + Inter-AS routing algorithm Forwarding table3b3a3c1c1a1b1d2a2c2bAS 2AS 3AS 16The Problem•Easy when only one link leading to outside AS•Much harder when two or more links to outside ASes–Which destinations reachable via a neighbor?–Propagate this information to other internal routers–Select a “good route” from multiple choices–Inter-AS routing protocol•Communication between distinct ASes•Must be the same protocol!7History•Mid-80s: EGP–Reachability protocol (no shortest path)–Did not accommodate cycles (tree topology)–Evolved when all networks connected to NSF backbone•Result: BGP introduced as routing protocol–Latest version = BGP 4–BGP-4 supports CIDR–Primary objective: connectivity not performance8BGP Preliminaries•Pairs of routers exchange routing info over TCP connections (port 179)–One TCP connection for every pair of neighboring gateway routers–Routers called “BGP peers”–BGP peers exchange routing info as messages–TCP connection + messages BGP session•Neighbor ASes exchange info on which CIDR prefixes are reachable via them9Choices for Routing•How to propagate routing information?•Link state or distance vector?–No universal metric – policy decisions–Problems with distance-vector:•Very slow convergence–Problems with link state:•Metric used by ISPs not the same loops•LS database too large – entire Internet•BGP: Path vector10AS Numbers (ASNs)ASNs are 16 bit values64512 through 65535 are “private”ASNs represent units of routing policyCurrently over 15,000 in use•Genuity: 1 •MIT: 3•CMU: 9•UC San Diego: 7377•AT&T: 7018, 6341, 5074, … •UUNET: 701, 702, 284, 12199, …•Sprint: 1239, 1240, 6211, 6242, …•…11Distance Vector with Path•Each routing update carries the entire AS-level path so far–“AS_Path attribute”•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–Advertisement depends on metrics/cost/preference etc.•Advantage:–Metrics are local - AS chooses path, protocol ensures no loops12Hop-by-hop Model•BGP advertises to neighbors only those routes that it uses–Consistent with the hop-by-hop Internet paradigm–Consequence: hear only one route from neighbor •(although neighbor may have chosen this from a large set of choices)•Could impact view into availability of paths13Policy with BGP•BGP provides capability for enforcing various policies•Policies are not part of BGP: they are provided to BGP as configuration information•Enforces policies by –Choosing appropriate paths from multiple alternatives–Controlling advertisement to other AS’s14Examples 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 some AS’s•An AS can favor or disfavor certain AS’s for traffic transit from itself15BGP Messages•Open–Announces AS ID–Determines hold timer – interval between keep_alive or update messages, zero interval implies no keep_alive•Keep_alive•Sent periodically (but before hold timer expires) to peers to ensure connectivity.•Sent in place of an UPDATE message•Notification•Used for error notification•TCP connection is closed immediately after notification16BGP UPDATE Message•List of withdrawn routes•Network layer reachability information–List of reachable prefixes•Path attributes–Origin–Path–Local_pref–MED–Metrics•All prefixes advertised in message have same path attributes17Path Selection Criteria•Attributes + external (policy) information•Examples:–Policy considerations•Preference for AS•Presence or absence of certain AS–Hop count–Path origin18LOCAL PREF•Local (within an AS) mechanism to provide relative priority among BGP exit points•Prefer routers announced by one AS over another or general preference over routesR1 R2R3 R4I-BGPAS 256AS 300Local Pref = 500 Local Pref =800AS 100R5AS 20019AS_PATH•List of traversed AS’sAS 500AS 300AS 200 AS 100180.10.0.0/16 300 200 100170.10.0.0/16 300 200170.10.0.0/16 180.10.0.0/1620Multi-Exit Discriminator (MED)•Hint to external neighbors about the preferred path into an AS
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