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Introduction to BGP

Introduction to BGP. What Is BGP?. Border Gateway Protocol BGP-4 The de-facto interdomain routing protocol BGP enables policy in routing: Which information gets advertised and how BGP is a Distance Vector like protocol Within an AS, Internal Gateway Protocol (IGP or I-BGP).

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Introduction to BGP

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  1. Introduction to BGP

  2. What Is BGP? • Border Gateway Protocol BGP-4 • The de-facto interdomain routing protocol • BGP enables policy in routing: • Which information gets advertised and how • BGP is a Distance Vector like protocol • Within an AS, Internal Gateway Protocol (IGP or I-BGP)

  3. Internet Structure Original idea Backbone service provider Consumer ” ISP “ Consumer ISP ” Large corporation “ Small “ ” Consumer ISP “ ” Consumer ISP corporation Small Small Small corporation corporation corporation CS 640

  4. Large corporation “ ” Consumer ISP Peering point Backbone service provider Peering point Consumer ” ISP “ “ Consumer ISP ” Large corporation Small corporation Internet Structure Today CS 640

  5. Route Propagation in the Internet • Autonomous System (AS) • corresponds to an administrative domain • examples: University, company, backbone network • assign each AS a 16-bit number • Two-level route propagation hierarchy • interior gateway protocol (each AS selects its own) • exterior gateway protocol (Internet-wide standard) • Routes information is propagated at various levels • hosts know local router • local routers know site routers • site routers know core router • core routers know everything CS 640

  6. Popular Interior Gateway Protocols • RIP: Route Information Protocol • distributed with BSD Unix • distance-vector algorithm • based on hop-count (infinity set to 16) • OSPF: Open Shortest Path First • recent Internet standard • uses link-state algorithm • supports load balancing • supports authentication CS 640

  7. EGP: Exterior Gateway Protocol • Overview • Original standard for Internet routing protocol (c 1983) • designed for tree-structured Internet • Single backbone • concerned with reachability, not optimal routes • Protocol messages • neighbor acquisition: one router requests that another be its peer; peers exchange reachability information • neighbor reachability: one router periodically tests if the another is still reachable; exchange HELLO/ACK messages; • uses a k-out-of-n rule: ¼ to stay up, ¾ to establish • routing updates: peers periodically exchange their routing tables (including route weights) using a basic distance vector method • There can be multiple connections between ASs CS 640

  8. Limits of EGP • At first glance, EGP seems like a distance vector protocol since updates carry lists of destinations and distances – but distances are NOT reliable. • EGP was designed to support tree topologies, not meshes • False routes injected by accident can have really bad consequences (black holes) – there is no easy way for dealing with this problem • Loops can easily occur – all we are doing is forwarding routing tables • EGP was not designed to easily support fragmented IP packets – all data is assumed to fit in MTU. • Solutions to these and other EGP problems were all manual CS 640

  9. BGP-4: Border Gateway Protocol • BGP-1 developed in 1989 to address problems with EGP. • Assumes Internet is an arbitrarily interconnected set of ASs • AS traffic types • Local • starts or ends within an AS • Transit • passes through an AS • AS Types • stub AS: has a single connection to one other AS • carries local traffic only • multihomed AS: has connections to more than one AS • refuses to carry transit traffic • transit AS: has connections to more than one AS • carries both transit and local traffic CS 640

  10. BGP-4 contd. • Each AS has: • one or more border routers • Handles inter-AS traffic • one BGP speaker for an AS that participates in routing • BGP speaker establishes BGP sessions with peers and advertises: • local network names • other reachable networks (transit AS only) • gives path information including path weights (MEDs) • withdrawn routes • BGP goal: find loop free paths between ASs • Optimality is secondary goal • It’s neither a distance-vector nor a link-state protocol • Hard problem • Internet’s size (~12K active ASs) means large tables in BGP routers • Autonomous domains mean different path metrics • Need for flexibility CS 640

  11. 128.96 Customer P 192.4.153 (AS 4) Regional provider A (AS 2) Customer Q 192.4.32 (AS 5) 192.4.3 Backbone network (AS 1) Customer R 192.12.69 (AS 6) Regional provider B (AS 3) Customer S 192.4.54 (AS 7) 192.4.23 BGP Example • Speaker for AS2 advertises reachability to P and Q • network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2 • Speaker for backbone advertises • networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path (AS1, AS2). • Speaker can cancel previously advertised paths CS 640

  12. Some BGP details • Path vectors are most important innovation in BGP • Enables loop prevention in complex topologies • If AS sees itself in the path, it will not use that path • Routes can be aggregated • Based on CIDR (classless) addressing • Routes can be filtered • Runs over TCP • Most of the same messages as EGP • Open, Update, Notify, Keepalive • BGP session have only recently been made secure CS 640

  13. BGP in practice • 10-20 “tier 1” ASs which are the Internet backbone • Clearly convergence is an issue – why? • Black holes are always a potential problem • There are lots of BGP updates every day! • BGP is really the heart of the Internet • BGP is a means by which network operators control congestion in the Internet. • BGP is really a big problem! CS 640

  14. How A BGP graph Looks Like AS 2 • Each AS has designated BGP routers • BGP routers of an AS communicate internally with another protocol (IGP) AS 5 AS 4 AS 3 AS 1

  15. What is different with BGP? • BGP goal: enable business relationships • Opts for: flexibility, scalability • Performance optimization is secondary

  16. Some Basic Numbers • ~20,000 Autonomous Systems approx. • Corporate Networks • ISP Internal Networks • National Service Providers • Identified by ASN a 16 bit value • Assigned by IANA • Superlinear growth

  17. Size of the Routing Table at the core of the Internet Source: http://www.telstra.net/ops/bgptable.html

  18. Routing is Based on Prefixes • A BGP Routing table has prefixes for entries • For a IP address of a packet, find longest match • Example: packet IP 128.32.101.1 • 128.32.0.0/16 match for 16 bits • 128.32.101.0/24 is a longer match • No matches: • 128.1.1.4 • 128.32.5.0/24

  19. Prefix Matching in More Detail • For a IP address of a packet, find longest match • Example: Compare • packet IP 128.32.101.1 • With 128.32.0.0/16 • IP : 01000000. 001000000. 01100101 .00000001 • Mask : 11111111. 111111111. 00000000 .00000000 • AND : 01000000. 001000000. 00000000 .00000000 • Prefix : 01000000. 001000000. 00000000. 00000000 • Equal? Yes

  20. Advertising Routing Information • Each AS advertises what it can reach from each BGP router • Policies I: filter what you advertise • Policies II: filter from what you hear advertised • Build up a BGP routing table • Remember which prefix you hear from which link

  21. What Does a Routing Table Look Like? • Origin AS “owns” the address

  22. Basic AS relationships • Customer – Provider • Customer pays Provider for service • The Customer is always right • Peer to Peer: mutual cooperation • Ex. MCI and AT&T • Sibling-Sibling • Ex. AT&T research and AT&T wireless

  23. Provider Customer Peer Peer The Internet as a Directed Graph • Every edge is bidirectional • Business relationships are represented

  24. The Initial Idea • Data flows between customers-providers • Top level providers are peers • They exchange information to ensure connectivity • What can possibly go wrong?

  25. And then came the rain… • Thousands of ASs • Complicated relationships • Multiple providers for one AS!! • Multihoming • Traffic engineering • I want to use multiple paths and load balance

  26. AS Relationships Provider Customer 200 100 • Customer – Provider: customer pays and is always right • Peer to Peer: Exchange traffic only between their customers • Sibling-Sibling: Exchange traffic at will Peer Peer 10 11 12 13 1 4 3 2

  27. The Rules of BGP Routing • Transit traffic: traffic that does not go to my customers (or their customers) • A provider carries any traffic to or from a customer • Peers exchange traffic only if between their customers

  28. How BGP Policy Restricts Routing • Routing rules: • Provider accept everything • Peer only if it is for its customers • Path Properties: • Up then down • No up-down-up, at most 1 peer-peer steps Provider Customer Peer 100 Peer 200 10 11 13 12 1 3 4 2

  29. Implementing BGP Rules • What do you do with an advertisement: • Through customer link • Advertise to all (customers, peers, providers) • Through provider link • Advertise to customer only (and possibly siblings) • Through peer link • Advertise to customer only (and possibly siblings) • Through sibling link • Advertise to all

  30. How Policies Affect Routing • A Provider will get rid of traffic as soon as possible, • But a Provider will carry the traffic for its customer • Did anyone say traffic is asymmetric? Customer 1 ISP1 ISP2 Customer 2

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