Presentation on theme: "CS 4251: Computer Networking II Nick Feamster Spring 2008"— Presentation transcript:
1 CS 4251: Computer Networking II Nick Feamster Spring 2008 Interdomain RoutingCS 4251: Computer Networking II Nick Feamster Spring 2008
2 Today’s Lecture: Interdomain Routing Today’s interdomain routing protocol: BGPBGP route attributesUsageProblemsBusiness relationshipsSee (Chapter ) for good coverage of today’s topics.
3 Internet Routing The Internet AbileneGeorgiaTechComcastAT&TCogentLarge-scale: Thousands of autonomous networksSelf-interest: Independent economic and performance objectivesBut, must cooperate for global connectivity
4 Internet Routing Protocol: BGP Autonomous Systems (ASes)Route AdvertisementDestination Next-hop AS Path/16174… 2637SessionTrafficDiagram of routing table is very confusing because it’s not pointing to anythingGreen arrow shorter, and too thick… green is a msgMore intuition about how the system actually works.Don’t say “interdomain”DESTINATION-BASED RoutingTables look like a set of possible routes and a rankings over these routes(pop up a simplified table fragment)
5 Question: What’s the difference between IGP and iBGP? Two Flavors of BGPiBGPeBGPExternal BGP (eBGP): exchanging routes between ASesInternal BGP (iBGP): disseminating routes to external destinations among the routers within an ASQuestion: What’s the difference between IGP and iBGP?
6 Internal BGP (iBGP) Default: “Full mesh” iBGP. Doesn’t scale. Large ASes use “Route reflection”Route reflector: non-client routes over client sessions; client routes over all sessionsClient: don’t re-advertise iBGP routes.Put “RR” and “C” in the bubbles to make clear what’s a route reflectorWhat problem arises?If the client receives something from a route reflector, it doesn’t re-advertise it
7 Example BGP Routing Table The full routing table> show ip bgpNetwork Next Hop Metric LocPrf Weight Path*>i i*>i i*>i / i* i / i> show ip bgpBGP routing table entry for /16Paths: (1 available, best #1, table Default-IP-Routing-Table)Not advertised to any peerfrom ( )Origin IGP, metric 0, localpref 150, valid, internal, bestCommunity: 10578: :950Last update: Sat Jan 14 04:45:Specific entry. Can do longest prefix lookup:PrefixAS pathNext-hop
8 Routing Attributes and Route Selection BGP routes have the following attributes, on which the route selection process is based:Local preference: numerical value assigned by routing policy. Higher values are more preferred.AS path length: number of AS-level hops in the pathMultiple exit discriminator (“MED”): allows one AS to specify that one exit point is more preferred than another. Lower values are more preferred.eBGP over iBGPShortest IGP path cost to next hop: implements “hot potato” routingRouter ID tiebreak: arbitrary tiebreak, since only a single “best” route can be selected
9 Other BGP AttributesNext-hop:Next-hop:iBGPNext-hop: IP address to send packets en route to destination. (Question: How to ensure that the next-hop IP address is reachable?)Community value: Semantically meaningless. Used for passing around “signals” and labelling routes. More in a bit.
10 Local Preference Control over outbound traffic Higher local prefPrimaryDestinationBackupLower local prefControl over outbound trafficNot transitive across ASesCoarse hammer to implement route preferenceUseful for preferring routes from one AS over another (e.g., primary-backup semantics)
11 Communities and Local Preference PrimaryDestinationBackup“Backup” CommunityCustomer expresses provider that a link is a backupAffords some control over inbound trafficMore on multihoming, traffic engineering in Lecture 7
12 AS Path LengthTrafficDestinationAmong routes with highest local preference, select route with shortest AS path lengthShortest AS path != shortest path, for any interpretation of “shortest path”
13 AS Path Length Hack: Prepending TrafficAS 2AS 3AS Path: “1”AS Path: “1 1”AS 1DAttempt to control inbound trafficMake AS path length look artificially longerHow well does this work in practice vs. e.g., hacks on longest-prefix match?
14 Multiple Exit Discriminator (MED) Dest.TrafficSan FranciscoNew YorkMED: 20MED: 10ILos AngelesMechanism for AS to control how traffic enters, given multiple possible entry points.
15 Hot-Potato Routing Prefer route with shorter IGP path cost to next-hop Idea: traffic leaves AS as quickly as possibleDest.New YorkAtlantaTrafficCommon practice: Set IGP weights in accordance with propagation delay (e.g., miles, etc.)105IWashington, DC
16 Problems with Hot-Potato Routing Small changes in IGP weights can cause large traffic shiftsDest.New YorkAtlantaTrafficQuestion: Cost of sub-optimal exit vs. cost of large traffic shifts11105IWashington, DC
17 What policy looks like in Cisco IOS eBGP SessionInbound “Route Map” (import policy)
18 Internet Business Model (Simplified) ProviderPreferences implemented with local preference manipulationFree to usePay to usePeerGet paid to useCustomerDestinationCustomer/Provider: One AS pays another for reachability to some set of destinations“Settlement-free” Peering: Bartering. Two ASes exchange routes with one another.
19 Relationship #1: Customer-Provider FilteringRoutes from customer: to everyoneRoutes from provider: only to customersFrom other destinationsTo the customerFrom the customerTo other destinationsprovidersprovidersadvertisementstrafficcustomercustomer
20 Relationship #2: Peering FilteringRoutes from peer: only to customersNo routes from other peers or providersadvertisementspeerpeertrafficcustomercustomer
21 The Business Game and Depeering Cooperative competition (brinksmanship)Much more desirable to have your peer’s customersMuch nicer to get paid for transitPeering “tiffs” are relatively common31 Jul 2005: Level 3 Notifies Cogent of intent to disconnect.16 Aug 2005: Cogent begins massive sales effort andmentions a 15 Sept. expected depeering date.31 Aug 2005: Level 3 Notifies Cogent again of intent todisconnect (according to Level 3)5 Oct :50 UTC: Level 3 disconnects Cogent. Masshysteria ensues up to, and including policymakers inWashington, D.C.7 Oct 2005: Level 3 reconnects CogentDuring the “outage”, Level 3 and Cogent’s singly homed customers could not reach each other. (~ 4% of the Internet’s prefixes were isolated from each other)
22 Depeering Continued Resolution… …but not before an attempt to steal customers!As of 5:30 am EDT, October 5th, Level(3) terminated peering withCogent without cause (as permitted under its peering agreement withCogent) even though both Cogent and Level(3) remained in fullcompliance with the previously existing interconnection agreement.Cogent has left the peering circuits open in the hope that Level(3)will change its mind and allow traffic to be exchanged between ournetworks. We are extending a special offering to single homed Level 3 customers.Cogent will offer any Level 3 customer, who is single homed to theLevel 3 network on the date of this notice, one year of full Internettransit free of charge at the same bandwidth currently being suppliedby Level 3. Cogent will provide this connectivity in over 1,000locations throughout North America and Europe.