16 bgp

16 bgp

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  1 Policy-Based Path-Vector Routing Reading:Sections 4.3.3 COS 461: Computer Networks Spring 2007 (MW 1:30-2:50 in Friend 004) Jennifer Rexford Teaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/
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  2 Goals of Today’sLecture • Challenges of interdomain routing – Scale, privacy, and policy – Limitations of link-state and distance-vector routing • Path-vector routing – Faster loop detection than distance-vector routing – More flexibility than shortest-path routing • Border Gateway Protocol (BGP) – Incremental, prefix-based, path-vector protocol – Programmable import and export policies – Multi-step decision process for selecting “best” route • Multiple routers within an AS • BGP convergence delay
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  3 Interdomain Routing • AS-leveltopology –Destinations are IP prefixes (e.g., 12.0.0.0/8) –Nodes are Autonomous Systems (ASes) –Edges are links and business relationships 1 2 3 4 5 67 Client Web server
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  4 Challenges for Interdomain Routing •Scale –Prefixes: 150,000-200,000, and growing –ASes: 20,000+ visible ones, and growing –AS paths and routers: at least in the millions… • Privacy –ASes don’t want to divulge internal topologies –… or their business relationships with neighbors • Policy –No Internet-wide notion of a link cost metric –Need control over where you send traffic –… and who can send traffic through you
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  5 Shortest-Path Routing is Restrictive •All traffic must travel on shortest paths • All nodes need common notion of link costs • Incompatible with commercial relationships Regional ISP1 Regional ISP2 Regional ISP3 Cust1Cust3 Cust2 National ISP1 National ISP2 YES NO
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  6 Link-State Routing is Problematic •Topology information is flooded –High bandwidth and storage overhead –Forces nodes to divulge sensitive information • Entire path computed locally per node –High processing overhead in a large network • Minimizes some notion of total distance –Works only if policy is shared and uniform • Typically used only inside an AS –E.g., OSPF and IS-IS
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  7 Distance Vector ison the Right Track • Advantages –Hides details of the network topology –Nodes determine only “next hop” toward the dest • Disadvantages –Minimizes some notion of total distance, which is difficult in an interdomain setting –Slow convergence due to the counting-to-infinity problem (“bad news travels slowly”) • Idea: extend the notion of a distance vector –To make it easier to detect loops
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  8 Path-Vector Routing • Extensionof distance-vector routing –Support flexible routing policies –Avoid count-to-infinity problem • Key idea: advertise the entire path –Distance vector: send distance metric per dest d –Path vector: send the entire path for each dest d 3 2 1 d “d: path (2,1)” “d: path (1)” data traffic data traffic
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  9 Faster Loop Detection •Node can easily detect a loop –Look for its own node identifier in the path –E.g., node 1 sees itself in the path “3, 2, 1” • Node can simply discard paths with loops –E.g., node 1 simply discards the advertisement 3 2 1 “d: path (2,1)” “d: path (1)” “d: path (3,2,1)”
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  10 Flexible Policies • Eachnode can apply local policies –Path selection: Which path to use? –Path export: Which paths to advertise? • Examples –Node 2 may prefer the path “2, 3, 1” over “2, 1” –Node 1 may not let node 3 hear the path “1, 2” 2 3 1 2 3 1
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  11 • Interdomain routingprotocol for the Internet –Prefix-based path-vector protocol –Policy-based routing based on AS Paths –Evolved during the past 17 years • 1989 : BGP-1 [RFC 1105], replacement for EGP • 1990 : BGP-2 [RFC 1163] • 1991 : BGP-3 [RFC 1267] • 1995 : BGP-4 [RFC 1771], support for CIDR • 2006 : BGP-4 [RFC 4271], update Border Gateway Protocol
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  12 BGP Operations Establish sessionon TCP port 179 Exchange all active routes Exchange incremental updates AS1 AS2 While connection is ALIVE exchange route UPDATE messages BGP session
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  13 Incremental Protocol • Anode learns multiple paths to destination –Stores all of the routes in a routing table –Applies policy to select a single active route –… and may advertise the route to its neighbors • Incremental updates –Announcement  Upon selecting a new active route, add node id to path  … and (optionally) advertise to each neighbor –Withdrawal  If the active route is no longer available  … send a withdrawal message to the neighbors
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  14 BGP Route • Destinationprefix (e.g,. 128.112.0.0/16) • Route attributes, including – AS path (e.g., “7018 88”) – Next-hop IP address (e.g., 12.127.0.121) AS 88 Princeton 128.112.0.0/16 AS path = 88 Next Hop = 192.0.2.1 AS 7018 AT&T AS 12654 RIPE NCC RIS project 192.0.2.1 128.112.0.0/16 AS path = 7018 88 Next Hop = 12.127.0.121 12.127.0.121
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  15 ASPATH Attribute AS7018 128.112.0.0/16 AS Path= 88 AS 1239 Sprint AS 1755 Ebone AT&T AS 3549 Global Crossing 128.112.0.0/16 AS Path = 7018 88 128.112.0.0/16 AS Path = 3549 7018 88 AS 88 128.112.0.0/16 Princeton Prefix Originated AS 12654 RIPE NCC RIS project AS 1129 Global Access 128.112.0.0/16 AS Path = 7018 88 128.112.0.0/16 AS Path = 1239 7018 88 128.112.0.0/16 AS Path = 1129 1755 1239 7018 88 128.112.0.0/16 AS Path = 1755 1239 7018 88
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  16 BGP Path Selection •Simplest case –Shortest AS path –Arbitrary tie break • Example –Three-hop AS path preferred over a five-hop AS path –AS 12654 prefers path through Global Crossing • But, BGP is not limited to shortest-path routing –Policy-based routing AS 3549 Global Crossing 128.112.0.0/16 AS Path = 3549 7018 88 AS 12654 RIPE NCC RIS project AS 1129 Global Access 128.112.0.0/16 AS Path = 1129 1755 1239 7018 88
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  17 BGP Policy: ApplyingPolicy to Routes • Import policy –Filter unwanted routes from neighbor  E.g. prefix that your customer doesn’t own –Manipulate attributes to influence path selection  E.g., assign local preference to favored routes • Export policy –Filter routes you don’t want to tell your neighbor  E.g., don’t tell a peer a route learned from other peer –Manipulate attributes to control what they see  E.g., make a path look artificially longer than it is
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  18 BGP Policy: Influencing Decisions BestRoute Selection Apply Import Policies Best Route Table Apply Export Policies Install forwarding Entries for best Routes. Receive BGP Updates Best Routes Transmit BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values IP Forwarding Table Apply Policy = filter routes & tweak attributes Open ended programming. Constrained only by vendor configuration language
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  19 Import Policy: LocalPreference • Favor one path over another – Override the influence of AS path length – Apply local policies to prefer a path • Example: prefer customer over peer AT&T Sprint Yale Tier-2 Tier-3 Local-pref = 100 Local-pref = 90
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  20 Import Policy: Filtering •Discard some route announcements – Detect configuration mistakes and attacks • Examples on session to a customer – Discard route if prefix not owned by the customer – Discard route that contains other large ISP in AS path AT&T Princeton USLEC 128.112.0.0/16
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  21 Export Policy: Filtering •Discard some route announcements – Limit propagation of routing information • Examples – Don’t announce routes from one peer to another – Don’t announce routes for network-management hosts AT&T Princeton Sprint 128.112.0.0/16 UUNET network operator
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  22 Export Policy: AttributeManipulation • Modify attributes of the active route – To influence the way other ASes behave • Example: AS prepending – Artificially inflate the AS path length seen by others – To convince some ASes to send traffic another way AT&T Princeton USLEC 128.112.0.0/16 Sprint 88 88 88
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  23 BGP Policy Configuration •Routing policy languages are vendor-specific – Not part of the BGP protocol specification – Different languages for Cisco, Juniper, etc. • Still, all languages have some key features – Policy as a list of clauses – Each clause matches on route attributes – … and either discards or modifies the matching routes • Configuration done by human operators – Implementing the policies of their AS – Business relationships, traffic engineering, security, … – http://www.cs.princeton.edu/~jrex/papers/policies.pdf
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  24 AS is Nota Single Node • AS path length can be misleading – An AS may have many router-level hops AS 4 AS 3 AS 2 AS 1 BGP says that path 4 1 is better than path 3 2 1
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  25 An AS isNot a Single Node • Multiple routers in an AS –Need to distribute BGP information within the AS –Internal BGP (iBGP) sessions between routers AS1 AS2 eBGP iBGP
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  26 Internal BGP andLocal Preference • Example – Both routers prefer the path through AS 100 on the left – … even though the right router learns an external path I-BGP AS 256 AS 300 Local Pref = 100 Local Pref = 90 AS 100 AS 200
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  27 An AS isNot a Single Node • Multiple connections to neighboring ASes –Multiple border routers may learn good routes –… with the same local-pref and AS path length 1 2 3 4 5 67 Multiple links
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  28 Hot-Potato (Early-Exit) Routing •Hot-potato routing – Each router selects the closest egress point – … based on the path cost in intradomain protocol • BGP decision process – Highest local preference – Shortest AS path – Closest egress point – Arbitrary tie break hot potato A B C D G EF 4 5 3 9 3 4 108 8 A B dst
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  29 Joining BGP andIGP Information • Border Gateway Protocol (BGP) –Announces reachability to external destinations –Maps a destination prefix to an egress point  128.112.0.0/16 reached via 192.0.2.1 • Interior Gateway Protocol (IGP) –Used to compute paths within the AS –Maps an egress point to an outgoing link  192.0.2.1 reached via 10.1.1.1 192.0.2.1 10.1.1.1
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  30 IGP Joining BGP withIGP Information AS 7018 AS 88192.0.2.1 128.112.0.0/16 10.10.10.10 BGP 192.0.2.1128.112.0.0/16 destination next hop 10.10.10.10192.0.2.0/30 destination next hop 128.112.0.0/16 Next Hop = 192.0.2.1 + Forwarding Table 128.112.0.0/16 destination next hop 10.10.10.10 192.0.2.0/30 10.10.10.10
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  31 Some Routers Don’tNeed BGP • Customer that connects to a single upstream ISP – The ISP can introduce the prefixes into BGP – … and the customer can simply default-route to the ISP Qwest Yale University Nail up default routes 0.0.0.0/0 pointing to Qwest Nail up routes 130.132.0.0/16 pointing to Yale 130.132.0.0/16
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  32 Some Routers Don’tNeed BGP • Routers inside a “stub” network –Border router may speak BGP to upstream ISPs –But, internal routers can simply “default route” AT&T Princeton University 128.112.0.0/16 AS 88 USLEC BGP
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  33 Causes of BGPRouting Changes • Topology changes – Equipment going up or down – Deployment of new routers or sessions • BGP session failures – Due to equipment failures, maintenance, etc. – Or, due to congestion on the physical path • Changes in routing policy – Reconfiguration of preferences – Reconfiguration of route filters • Persistent protocol oscillation – Conflicts between policies in different ASes
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  34 BGP Session Failure •BGP runs over TCP – BGP only sends updates when changes occur – TCP doesn’t detect lost connectivity on its own • Detecting a failure – Keep-alive: 60 seconds – Hold timer: 180 seconds • Reacting to a failure – Discard all routes learned from the neighbor – Send new updates for any routes that change AS1 AS2
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  35 Routing Change: Beforeand After 0 1 2 3 0 1 2 3 (1,0) (2,0) (3,1,0) (2,0) (1,2,0) (3,2,0)
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  36 Routing Change: PathExploration • AS 1 – Delete the route (1,0) – Switch to next route (1,2,0) – Send route (1,2,0) to AS 3 • AS 3 – Sees (1,2,0) replace (1,0) – Compares to route (2,0) – Switches to using AS 2 0 1 2 3 (2,0) (1,2,0) (3,2,0)
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  37 Routing Change: PathExploration • Initial situation – Destination 0 is alive – All ASes use direct path • When destination dies – All ASes lose direct path – All switch to longer paths – Eventually withdrawn • E.g., AS 2 – (2,0)  (2,1,0) – (2,1,0)  (2,3,0) – (2,3,0)  (2,1,3,0) – (2,1,3,0)  null 1 2 3 0 (1,0) (1,2,0) (1,3,0) (2,0) (2,1,0) (2,3,0) (2,1,3,0) (3,0) (3,1,0) (3,2,0)
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  38 BGP Converges Slowly,if at All • Path vector avoids count-to-infinity – But, ASes still must explore many alternate paths – … to find the highest-ranked path that is still available • Fortunately, in practice – Most popular destinations have very stable BGP routes – And most instability lies in a few unpopular destinations • Still, lower BGP convergence delay is a goal – Can be tens of seconds to tens of minutes – High for important interactive applications – … or even conventional application, like Web browsing
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  39 Conclusions • BGP issolving a hard problem – Routing protocol operating at a global scale – With tens of thousands of independent networks – That each have their own policy goals – And all want fast convergence • Key features of BGP – Prefix-based path-vector protocol – Incremental updates (announcements and withdrawals) – Policies applied at import and export of routes – Internal BGP to distribute information within an AS – Interaction with the IGP to compute forwarding tables