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BGP

BGP. EE 122, Fall 2013 Sylvia Ratnasamy http://inst.eecs.berkeley.edu/~ee122 / Material thanks to Ion Stoica , Scott Shenker , Jennifer Rexford, and many other colleagues. BGP: The story so far. Destinations are IP prefixes (12.0.0.0/8) Nodes are Autonomous Systems ( ASes )

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BGP

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  1. BGP EE 122, Fall 2013 Sylvia Ratnasamy http://inst.eecs.berkeley.edu/~ee122/ Material thanks to Ion Stoica, Scott Shenker, Jennifer Rexford, and many other colleagues

  2. BGP: The story so far • Destinations are IP prefixes (12.0.0.0/8) • Nodes are Autonomous Systems (ASes) • Links represent both physical connections and business relationships • customer-provider or peer-to-peer • BGP • path-vector protocol • policy-driven route selection

  3. BGP: Today • BGP policy • typical policies, how they’re implemented • BGP protocol details • Issues with BGP

  4. Route export Policy imposed in how routes are selected and exported • Selection: Which path to use? • controls whether/how traffic leaves the network • Export: Which path to advertise? • controls whether/how traffic enters the network Route selection Customer 1 Can reach 128.3/16 blah blah 10 Competitor 5

  5. Typical Selection Policy • In decreasing order of priority • make/save money (send to customer > peer > provider) • maximize performance (smallest AS path length) • minimize use of my network bandwidth (“hot potato”) • … • …

  6. Typical Export Policy We’ll refer to these as the “Gao-Rexford” rules (capture common -- but not required! -- practice!)

  7. Gao-Rexford providers peers customers With Gao-Rexford, the customer-provider graph is a DAG (directed acyclic graph) and routes are “valley free”

  8. BGP: Today • BGP policy • typical policies, how they’re implemented • BGP protocol details • stay awake as long as you can… • BGP issues

  9. Border router Internal router Who speaks BGP? Border routers at an Autonomous System

  10. What does “speak BGP” mean? • Implement the standardized BGP protocol • read more here: http://tools.ietf.org/html/rfc4271 • Specifies what messages to exchange with other BGP “speakers” • message types: e.g., route advertisements • message syntax: e.g., first X bytes for dest prefix; next Y for AS path, etc. • And how to process these messages • e.g., “when you receive a message of type X, apply this selection rule, then…” • as per BGP state machine in the protocol spec + policy decisions, etc.

  11. BGP “sessions” “eBGP session” A border router speaks BGP with border routers in other ASes

  12. BGP “sessions” “iBGP session” A border router speaks BGP with other (interior and border) routers in its own AS

  13. eBGP, iBGP, IGP • eBGP: BGP sessions between border routers in different ASes • Learn routes to external destinations • iBGP: BGP sessions between border routers and otherrouters within the same AS • distribute externally learned routes internally • assume a full all-to-all mesh of iBGP sessions • IGP: “Interior Gateway Protocol” = Intradomain routing protocol • provide internal reachability • e.g., OSPF, RIP

  14. Some Border Routers Don’t Need BGP • Customer that connects to a single upstream ISP • The ISP can advertise prefixes into BGP on behalf of customer • … and the customer can simply default-route to the ISP Provider Install routes 130.132.0.0/16 pointing to Customer Install default routes 0.0.0.0/0 pointing to Provider Customer 130.132.0.0/16

  15. 3 2 2 4 9 6 3 1 Putting the pieces together • Provide internal reachability (IGP) • Learn routes to external destinations (eBGP) • Distribute externally learned routes internally (iBGP) • Travel shortest path to egress (IGP)

  16. Basic Messages in BGP • Open • Establishes BGP session • BGP uses TCP [will make sense in 1-2weeks] • Notification • Report unusual conditions • Update • Inform neighbor of new routes • Inform neighbor of old routes that become inactive • Keepalive • Inform neighbor that connection is still viable

  17. BGP Operations Opensession on TCP port 179 AS1 BGP session Exchange all active routes AS2 While connection is ALIVE exchange route UPDATE messages Exchange incremental Updates

  18. Route Updates • Format <IP prefix: route attributes> • attributes describe properties of the route • Two kinds of updates • announcements: new routes or changes to existing routes • withdrawal: remove routes that no longer exist

  19. Route Attributes • Routes are described using attributes • Used in route selection/export decisions • Some attributes are local • i.e., private within an AS, not included in announcements • e.g., LOCAL PREF, ORIGIN • Some attributes are propagated with eBGP route announcements • e.g., NEXT HOP, AS PATH, MED, etc. • There are many standardized attributes in BGP • We will discuss a few

  20. Attributes (1): ASPATH • Carried in route announcements • Vector that lists all the ASes a route announcement has traversed (in reverse order) • e.g., “7018 88” AS 7018 AT&T AS 88 AS 12654 Princeton, 128.112/16 IP prefix = 128.112.0.0/16 AS path = 88 128.112.0.0/16 AS path = 7018 88

  21. Attributes (2): NEXT HOP • Carried in a route update message • IP address of next hop router on path to destination • Updated as the announcement leaves AS 12.127.0.121 192.0.2.1 AS 7018 AT&T AS 12654 AS 88 Princeton, 128.112/16 IP prefix = 128.112.0.0/16 AS path = 88 Next Hop = 192.0.2.1 128.112.0.0/16 AS path = 7018 88 Next Hop = 12.127.0.121

  22. “Local Preference” Used to choose between different AS paths The higher the value the more preferred Local to an AS; carried only in iBGP messages Ensures consistent route selection across an AS Attributes (3): LOCAL PREF 140.20.1.0/24 AS1 AS3 AS2 AS4 BGP table at AS4:

  23. Example: iBGPand LOCAL PREF • Both routers prefer the path through AS 100 on the left AS1 AS 2 AS 3 Local Pref = 90 Local Pref = 100 I-BGP AS 4

  24. Records who originated the announcement Local to an AS Options: “e” : from eBGP “i” : from iBGP “?” : Incomplete; often used for static routes Typically: e > i > ? Attributes (4): ORIGIN

  25. “Multi-Exit Discriminator” Used when ASesare interconnected via 2 or more links to specify how close a prefix is to the link it is announced on Lower is better AS announcing prefix sets MED (AS2 in picture) AS receiving prefix (optionally!) uses MED to select link (AS1 in pic.) Attributes (5) : MED AS1 Link B Link A MED=50 MED=10 AS2 AS3 destination prefix

  26. dst 9 B B A A 4 D 3 8 10 3 4 G 8 E 5 F C Attributes (6): IGP cost • Used for hot-potato routing • Each router selects the closest egress point based on the path cost in intra-domain protocol hot potato

  27. IGP may conflict with MED NEXTHOP=BOS A MED=500 Dsf B NEXTHOP=SF MED=100

  28. Using Attributes • Rules for route selection in priority order

  29. BGP UPDATE Processing Open ended programming. Constrained only by vendor configuration language Filter routes & tweak attributes Receive BGP Updates Based on Attribute Values Transmit BGP Updates Apply Policy = filter routes & tweak attributes Best Routes Apply Import Policies Best Route Selection Best Route Table Apply Export Policies Install forwarding Entries for best Routes. IP Forwarding Table

  30. BGP: Today • BGP policy • typical policies, how they’re implemented • BGP protocol details • BGP issues

  31. Issues with BGP • Reachability • Security • Convergence • Performance

  32. Reachability • In normal routing, if graph is connected then reachability is assured • With policy routing, this does not always hold Provider Provider AS 1 AS 3 AS 2 Customer

  33. Security • An AS can claim to serve a prefix that they actually don’t have a route to (blackholing traffic) • Problem not specific to policy or path vector • Important because of AS autonomy • Fixable: make ASes“prove” they have a path • Note: AS can also have incentive to forward packets along a route different from what is advertised • Tell customers about fictitious short path… • Much harder to fix!

  34. Convergence • Result: If all AS policies follow “Gao-Rexford” rules, BGP is guaranteed to converge (safety) • For arbitrary policies, BGP may fail to converge!

  35. 1 2 3 Example of Policy Oscillation “1” prefers “1 3 0” over “1 0” to reach “0” 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  36. 1 2 3 Step-by-Step of Policy Oscillation Initially: nodes 1, 2, 3 know only shortest path to 0 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  37. 1 2 3 Step-by-Step of Policy Oscillation 1 advertises its path 1 0 to 2 1 3 0 1 0 advertise: 1 0 0 2 1 0 2 0 3 2 0 3 0

  38. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  39. 1 2 3 Step-by-Step of Policy Oscillation 3 advertises its path 3 0 to 1 1 3 0 1 0 advertise: 3 0 0 2 1 0 2 0 3 2 0 3 0

  40. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  41. 1 2 3 Step-by-Step of Policy Oscillation 1 withdraws its path 1 0 from 2 1 3 0 1 0 withdraw: 1 0 0 2 1 0 2 0 3 2 0 3 0

  42. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  43. 1 2 3 Step-by-Step of Policy Oscillation 2 advertises its path 2 0 to 3 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0 advertise: 2 0

  44. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  45. 1 2 3 Step-by-Step of Policy Oscillation 3 withdraws its path 3 0 from 1 1 3 0 1 0 withdraw: 3 0 0 2 1 0 2 0 3 2 0 3 0

  46. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  47. 1 2 3 Step-by-Step of Policy Oscillation 1 advertises its path 1 0 to 2 1 3 0 1 0 advertise: 1 0 0 2 1 0 2 0 3 2 0 3 0

  48. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0

  49. 1 2 3 Step-by-Step of Policy Oscillation 2 withdraws its path 2 0 from 3 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0 withdraw: 2 0

  50. 1 2 3 Step-by-Step of Policy Oscillation 1 3 0 1 0 0 2 1 0 2 0 3 2 0 3 0 We are back to where we started!

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