griffin BGP tutorial

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Published on October 7, 2007

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Interdomain Routing and BGP:  Interdomain Routing and BGP The slides here were created by Timothy G. Griffin of AT&T and presented at a SIGCOMM 2001 Tutorial Session. The slides have been edited by me (D. Szajda). Any errors introduced as a result of these minor changes are my responsibility. An Introduction to Interdomain Routing and BGP:  An Introduction to Interdomain Routing and BGP Timothy G. Griffin griffin@research.att.com http://www.research.att.com/~griffin/interdomain.html SIGCOMM 2001 Tutorial Session August 28, 2001 Caveat:  Caveat These slides have been edited by me. Any errors introduced as a result of these minor changes are my responsibility. Acknowledgements:  Acknowledgements Thanks to Jay Borkenhagen, Randy Bush, Anja Feldmann, Matt Grossglauser, Madan Musuvathi, Jennifer Rexford, Shubho Sen, and Jia Wang for many helpful comments My opinions should not be taken to represent AT&T policy Errors are my own BGP Generalities:  BGP Generalities BGP is a path-vector protocol Generally, vectoring refers to creating routes without having global knowledge of the tolopology Based on neighbors reported paths, you choose preferred path and report that to neighbors BGP implements policy-based routing talk of “minimum paths” is ill defined. configure routers with route preferences Policy Based Routing:  Policy Based Routing Ways to configure routers that effect which routes get computed Route preferences: e.g. don’t use any path going through AS X Which destinations should not be reported to which neighbors? How a path should be edited when passed to a particular neighbor E.G. maybe add several AS numbers into path to discourage, but not prevent, others from using path Policy Based Routing:  Policy Based Routing Is there a better way? R. Perlman supports link-state system plus source specified routes. Link-state gives complete information Sources can then determine routes that meet their specifications Instead of influencing neighbors with advertisements, simply drop traffic you don’t want to forward Common View of the Telco Network:  Common View of the Telco Network Brick Common View of the IP Network (Layer 3):  Common View of the IP Network (Layer 3) What This Tutorial Is About:  What This Tutorial Is About Goal:  Goal Understand how layer 3 connectivity is maintained in the global Internet This tutorial will not say much about the applications that exploit this connectivity. It will be restricted to IPv4 unicast routing. Part I : The basics of interdomain routing and BGP Part II : BGP in practice: Issues of Scale Outline Part I:  Outline Part I Forwarding vs. Routing IP addressing Autonomous Systems (basic units of interdomain routing) The Border Gateway Protocol (BGP) BGP fundamentals BGP route attributes Implementing policy with BGP A wee bit of theory Outline Part II:  Outline Part II Scaling internal BGP BGP table growth Address aggregation vs. Multihoming Growth in number of autonomous systems Dynamics of BGP Route flapping BGP convergence Rates of BGP updates Best Effort Connectivity :  Best Effort Connectivity This is the fundamental service provided by Internet Service Providers (ISPs) All other IP services depend on connectivity: DNS, email, VPNs, Web Hosting, … IP traffic 135.207.49.8 192.0.2.153 Routing vs. Forwarding:  Routing vs. Forwarding R R R A B C D R1 R2 R3 R4 R5 E Net Nxt Hop R4 R3 R3 R4 Direct R4 Net Nxt Hop A B C D E default R2 R2 Direct R5 R5 R2 Net Nxt Hop A B C D E default R1 Direct R3 R1 R3 R1 Default to upstream router A B C D E default Forwarding: determine next hop Routing: establish end-to-end paths Forwarding always works Routing can be badly broken How Are Forwarding Tables Populated to implement Routing?:  How Are Forwarding Tables Populated to implement Routing? Statically Dynamically Routers exchange network reachability information using ROUTING PROTOCOLS. Routers use this to compute best routes Administrator manually configures forwarding table entries In practice : a mix of these. Static routing mostly at the “edge” + More control + Not restricted to destination-based forwarding - Doesn’t scale - Slow to adapt to network failures + Can rapidly adapt to changes in network topology + Can be made to scale well - Complex distributed algorithms - Consume CPU, Bandwidth, Memory - Debugging can be difficult - Current protocols are destination-based Routers Talking to Routers:  Routers Talking to Routers Routing info Routing info Routing computation is distributed among routers within a routing domain Computation of best next hop based on routing information is the most CPU/memory intensive task on a router Routing messages are usually not routed, but exchanged via layer 2 between physically adjacent routers (internal BGP and multi-hop external BGP are exceptions) Before We Go Any Further …:  Before We Go Any Further … IP ROUTING PROTOCOLS DO NOT DYNAMICALLY ROUTE AROUND NETWORK CONGESTION IP traffic can be very bursty Dynamic adjustments in routing typically operate more slowly than fluctuations in traffic load Dynamically adapting routing to account for traffic load can lead to wild, unstable oscillations of routing system Autonomous Routing Domains:  Autonomous Routing Domains A collection of physical networks glued together using IP, that have a unified administrative routing policy. Campus networks Corporate networks ISP Internal networks … Autonomous Systems (ASes):  Autonomous Systems (ASes) An autonomous system is an autonomous routing domain that has been assigned an Autonomous System Number (ASN). AS Numbers (ASNs):  AS Numbers (ASNs) ASNs are 16 bit values. 64512 through 65535 are “private” Genuity: 1 MIT: 3 Harvard: 11 UC San Diego: 7377 AT&T: 7018, 6341, 5074, … UUNET: 701, 702, 284, 12199, … Sprint: 1239, 1240, 6211, 6242, … … ASNs represent units of routing policy Currently over 11,000 in use. Architecture of Dynamic Routing:  Architecture of Dynamic Routing AS 1 AS 2 BGP EGP = Exterior Gateway Protocol IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco) Policy based: BGP The Routing Domain of BGP is the entire Internet OSPF EIGRP Slide23:  Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Each router knows little about network topology Only best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all next-hop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP Link State Vectoring Technology of Distributed Routing The Gang of Four:  The Gang of Four Many Routing Processes Can Run on a Single Router :  Many Routing Processes Can Run on a Single Router Forwarding Table OSPF Domain RIP Domain BGP OS kernel Forwarding Table Manager IPv4 Addresses are 32 Bit Values :  IPv4 Addresses are 32 Bit Values IPv6 addresses have 128 bits Classful Addresses :  Classful Addresses 0nnnnnnn 10nnnnnn nnnnnnnn nnnnnnnn nnnnnnnn 110nnnnn hhhhhhhh hhhhhhhh hhhhhhhh hhhhhhhh hhhhhhhh hhhhhhhh n = network address bit h = host identifier bit Class A Class C Class B Leads to a rigid, flat, inefficient use of address space … RFC 1519: Classless Inter-Domain Routing (CIDR):  RFC 1519: Classless Inter-Domain Routing (CIDR) IP Address : 12.4.0.0 IP Mask: 255.254.0.0 Use two 32 bit numbers to represent a network. Network number = IP address + Mask Usually written as 12.4.0.0/15 Which IP Addresses are Covered by a Prefix?:  Which IP Addresses are Covered by a Prefix? 12.4.0.0/15 12.5.9.16 12.7.9.16 12.5.9.16 is covered by prefix 12.4.0.0/15 12.7.9.16 is not covered by prefix 12.4.0.0/15 CIDR = Hierarchy in Addressing:  CIDR = Hierarchy in Addressing Classless Forwarding :  Classless Forwarding Destination =12.5.9.16 ------------------------------- payload even better OK better best! IP Address Allocation and Assignment: Internet Registries :  IP Address Allocation and Assignment: Internet Registries IANA www.iana.org RFC 2050 - Internet Registry IP Allocation Guidelines RFC 1918 - Address Allocation for Private Internets RFC 1518 - An Architecture for IP Address Allocation with CIDR ARIN www.arin.org APNIC www.apnic.org RIPE www.ripe.org Allocate to National and local registries and ISPs Addresses assigned to customers by ISPs Nontransit vs. Transit ASes:  Nontransit vs. Transit ASes ISP 1 ISP 2 Nontransit AS might be a corporate or campus network. Could be a “content provider” NET A Traffic NEVER flows from ISP 1 through NET A to ISP 2 (At least not intentionally!) Internet Service providers (often) have transit networks Selective Transit:  Selective Transit NET B NET C NET A provides transit between NET B and NET C and between NET D and NET C NET A NET D NET A DOES NOT provide transit Between NET D and NET B Most transit networks transit in a selective manner… Customers and Providers:  Customers and Providers Customer pays provider for access to the Internet provider customer Customers Don’t Always Need BGP:  Customers Don’t Always Need BGP provider customer Nail up default routes 0.0.0.0/0 pointing to provider. Nail up routes 192.0.2.0/24 pointing to customer 192.0.2.0/24 Static routing is the most common way of connecting an autonomous routing domain to the Internet. This helps explain why BGP is a mystery to many … Customer-Provider Hierarchy:  Customer-Provider Hierarchy IP traffic provider customer The Peering Relationship:  The Peering Relationship Peers provide transit between their respective customers Peers do not provide transit between peers Peers (often) do not exchange $$$ traffic allowed traffic NOT allowed Peering Provides Shortcuts:  Peering Provides Shortcuts Peering also allows connectivity between the customers of “Tier 1” providers. Peering Wars:  Peering Wars Reduces upstream transit costs Can increase end-to-end performance May be the only way to connect your customers to some part of the Internet (“Tier 1”) You would rather have customers Peers are usually your competition Peering relationships may require periodic renegotiation Peering struggles are by far the most contentious issues in the ISP world! Peering agreements are often confidential. Peer Don’t Peer BGP-4:  BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol Is the de facto EGP of today’s global Internet Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes. 1989 : BGP-1 [RFC 1105] Replacement for EGP (1984, RFC 904) 1990 : BGP-2 [RFC 1163] 1991 : BGP-3 [RFC 1267] 1995 : BGP-4 [RFC 1771] Support for Classless Interdomain Routing (CIDR) BGP Operations (Simplified) :  BGP Operations (Simplified) Establish session on TCP port 179 Exchange all active routes Exchange incremental updates AS1 AS2 While connection is ALIVE exchange route UPDATE messages BGP session Four Types of BGP Messages:  Four Types of BGP Messages Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. announcement = prefix + attributes values BGP Attributes:  BGP Attributes Value Code Reference ----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] ... 255 reserved for development From IANA: http://www.iana.org/assignments/bgp-parameters This tutorial will cover these attributes Not all attributes need to be present in every announcement Attributes are Used to Select Best Routes :  Attributes are Used to Select Best Routes 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! 192.0.2.0/24 pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route (Note: it could reject them all!) Two Types of BGP Neighbor Relationships:  Two Types of BGP Neighbor Relationships External Neighbor (eBGP) in a different Autonomous System Internal Neighbor (iBGP) in the same Autonomous System AS1 AS2 eBGP iBGP iBGP is routed (using IGP!) iBGP Peers Must be Fully Meshed:  iBGP Peers Must be Fully Meshed iBGP neighbors do not announce routes received via iBGP to other iBGP neighbors. iBGP is needed to avoid routing loops within an AS Injecting external routes into IGP does not scale and causes BGP policy information to be lost BGP does not provide “shortest path” routing Is iBGP an IGP? NO! BGP Next Hop Attribute:  BGP Next Hop Attribute Every time a route announcement crosses an AS boundary, the Next Hop attribute is changed to the IP address of the border router that announced the route. AS 6431 AT&T Research 135.207.0.0/16 Next Hop = 12.125.133.90 AS 7018 AT&T AS 12654 RIPE NCC RIS project 12.125.133.90 135.207.0.0/16 Next Hop = 12.127.0.121 12.127.0.121 Join EGP with IGP For Connectivity:  Forwarding Table Forwarding Table Join EGP with IGP For Connectivity AS 1 AS 2 192.0.2.1 135.207.0.0/16 10.10.10.10 10.10.10.10 192.0.2.0/30 destination next hop 135.207.0.0/16 Next Hop = 192.0.2.1 192.0.2.0/30 135.207.0.0/16 destination next hop 10.10.10.10 + 192.0.2.0/30 10.10.10.10 Router IP address Network address (for AS 2) Arrows show direction of route advertisement Next Hop Often Rewritten to Loopback:  Forwarding Table Forwarding Table Next Hop Often Rewritten to Loopback AS 1 AS 2 192.0.2.1 135.207.0.0/16 10.10.10.10 EGP 127.22.33.44 135.207.0.0/16 destination next hop 10.10.10.10 127.22.33.44 destination next hop 135.207.0.0/16 destination next hop 10.10.10.10 + 127.22.33.44 10.10.10.10 127.22.33.44 135.207.0.0/16 Next Hop = 192.0.2.1 135.207.0.0/16 Next Hop = 127.22.33.44 Loopback refers to directing traffic back to advertising router Implementing Customer/Provider and Peer/Peer relationships:  Implementing Customer/Provider and Peer/Peer relationships Enforce transit relationships Outbound route filtering Enforce order of route preference provider < peer < customer Two parts: Import Routes :  Import Routes From peer From peer From provider From provider From customer From customer Export Routes :  Export Routes To peer To peer To customer To customer To provider From provider provider route customer route peer route ISP route How Can Routes be Colored? BGP Communities!:  How Can Routes be Colored? BGP Communities! Very powerful BECAUSE it has no (predefined) meaning Community Attribute = a list of community values. (So one route can belong to multiple communities) RFC 1997 (August 1996) Used for signaling within and between ASes Communities Example:  Communities Example 1:100 Customer routes 1:200 Peer routes 1:300 Provider Routes To Customers 1:100, 1:200, 1:300 To Peers 1:100 To Providers 1:100 AS 1 Import Export Blackholes:  192.0.2.0/24 192.0.2.0/24 Accidental or malicious announcement of your prefix can blackhole your destinations in large part of the Internet Need Filter Here! legitimate not legitimate Blackholes Mars Attacks!:  Mars Attacks! 0.0.0.0/0: default 10.0.0.0/8: private 172.16.0.0/12: private 192.168.0.0/16: private 127.0.0.0/8: loopbacks 128.0.0.0/16: IANA reserved 192.0.2.0/24: test networks 224.0.0.0/3: classes D and E ….. Martian list often includes Martian list gives addresses that should be considered suspect in an advertisement (See RFC 1149) Import Routes (Revisited):  Import Routes (Revisited) From peer From peer From provider From provider From customer From customer provider route customer route peer route ISP route xxxxxx xxxxxx xxxxxx xxxxxx xxxxxx xxxxxx Customer address filters cccccc cccccc cccccc potential blackhole Martian So Many Choices:  So Many Choices Which route should Frank pick to 13.13.0.0./16? AS 1 AS 2 AS 4 AS 3 13.13.0.0/16 Frank’s Internet Barn BGP Route Processing:  BGP Route Processing Best Route 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 Tweak Tweak Tweak:  Tweak Tweak Tweak For inbound traffic Filter outbound routes Tweak attributes on outbound routes in the hope of influencing your neighbor’s best route selection For outbound traffic Filter inbound routes Tweak attributes on inbound routes to influence best route selection outbound routes inbound routes inbound traffic outbound traffic In general, an AS has more control over outbound traffic Route Selection Summary:  Route Selection Summary Highest Local Preference Shortest ASPATH Lowest MED (Multi-Exit Discriminator attribute) i-BGP < e-BGP Lowest IGP cost to BGP egress Lowest router ID traffic engineering Enforce relationships Throw up hands and break ties Back to Frank …:  Back to Frank … AS 1 AS 2 AS 4 AS 3 13.13.0.0/16 local pref = 80 local pref = 100 local pref = 90 Higher Local preference values are more preferred Local preference only used in iBGP Implementing Backup Links with Local Preference (Outbound Traffic) :  Implementing Backup Links with Local Preference (Outbound Traffic) Forces outbound traffic to take primary link, unless link is down. AS 1 primary link backup link Set Local Pref = 100 for all routes received from AS 1 AS 65000 Set Local Pref = 50 for all routes received from AS 1 We’ll talk about inbound traffic soon … Multihomed Backups (Outbound Traffic) :  Multihomed Backups (Outbound Traffic) Forces outbound traffic to take primary link, unless link is down. AS 1 primary link backup link Set Local Pref = 100 for all routes received from AS 1 AS 2 Set Local Pref = 50 for all routes received from AS 3 AS 3 provider provider ASPATH Attribute:  ASPATH Attribute AS7018 135.207.0.0/16 AS Path = 6341 AS 1755 Ebone AT&T AS 3549 Global Crossing 135.207.0.0/16 AS Path = 7018 6341 135.207.0.0/16 AS Path = 3549 7018 6341 AS 6341 135.207.0.0/16 AT&T Research Prefix Originated AS 12654 RIPE NCC RIS project AS 1129 Global Access 135.207.0.0/16 AS Path = 7018 6341 135.207.0.0/16 AS Path = 1239 7018 6341 135.207.0.0/16 AS Path = 1755 1239 7018 6341 135.207.0.0/16 AS Path = 1129 1755 1239 7018 6341 Interdomain Loop Prevention:  Interdomain Loop Prevention BGP at AS YYY will never accept a route with ASPATH containing YYY. AS 7018 12.22.0.0/16 ASPATH = 1 333 7018 877 Don’t Accept! AS 1 Traffic Often Follows ASPATH:  Traffic Often Follows ASPATH AS 4 AS 3 AS 2 AS 1 135.207.0.0/16 135.207.0.0/16 ASPATH = 3 2 1 IP Packet Dest = 135.207.44.66 … But It Might Not:  … But It Might Not AS 4 AS 3 AS 2 AS 1 135.207.0.0/16 135.207.0.0/16 ASPATH = 3 2 1 IP Packet Dest = 135.207.44.66 AS 5 135.207.44.0/25 ASPATH = 5 135.207.44.0/25 AS 2 filters all subnets with masks longer than /24 135.207.0.0/16 ASPATH = 1 From AS 4, it may look like this packet will take path 3 2 1, but it actually takes path 3 2 5 Shorter Doesn’t Always Mean Shorter:  In fairness: could you do this “right” and still scale? Exporting internal state would dramatically increase global instability and amount of routing state Shorter Doesn’t Always Mean Shorter AS 4 AS 3 AS 2 AS 1 Mr. BGP says that path 4 1 is better than path 3 2 1 Duh! Shedding Inbound Traffic with ASPATH Padding Hack:  Shedding Inbound Traffic with ASPATH Padding Hack Padding will (usually) force inbound traffic from AS 1 to take primary link AS 1 192.0.2.0/24 ASPATH = 2 2 2 customer AS 2 provider 192.0.2.0/24 backup primary 192.0.2.0/24 ASPATH = 2 Padding May Not Shut Off All Traffic :  Padding May Not Shut Off All Traffic AS 1 192.0.2.0/24 ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2 customer AS 2 provider 192.0.2.0/24 192.0.2.0/24 ASPATH = 2 AS 3 provider AS 3 will send traffic on “backup” link because it prefers customer routes and local preference is considered before ASPATH length! Padding in this way is often used as a form of load balancing backup primary COMMUNITY Attribute to the Rescue!:  COMMUNITY Attribute to the Rescue! AS 1 customer AS 2 provider 192.0.2.0/24 192.0.2.0/24 ASPATH = 2 AS 3 provider backup primary 192.0.2.0/24 ASPATH = 2 COMMUNITY = 3:70 Customer import policy at AS 3: If 3:90 in COMMUNITY then set local preference to 90 If 3:80 in COMMUNITY then set local preference to 80 If 3:70 in COMMUNITY then set local preference to 70 AS 3: normal customer local pref is 100, peer local pref is 90 Hot Potato Routing: Go for the Closest Egress Point :  Hot Potato Routing: Go for the Closest Egress Point 192.44.78.0/24 15 56 IGP distances egress 1 egress 2 This Router has two BGP routes to 192.44.78.0/24. Hot potato: get traffic off of your network as Soon as possible. Go for egress 1! Getting Burned by the Hot Potato:  Getting Burned by the Hot Potato 15 56 17 2865 High bandwidth Provider backbone Low bandwidth customer backbone Many customers want their provider to carry the bits! tiny http request huge http reply SFF NYC San Diego Cold Potato Routing with MEDs (Multi-Exit Discriminator Attribute):  Cold Potato Routing with MEDs (Multi-Exit Discriminator Attribute) 15 56 17 2865 192.44.78.0/24 192.44.78.0/24 MED = 15 192.44.78.0/24 MED = 56 This means that MEDs must be considered BEFORE IGP distance! Prefer lower MED values Note1 : some providers will not listen to MEDs Note2 : MEDs need not be tied to IGP distance Route Selection Summary:  Route Selection Summary Highest Local Preference Shortest ASPATH Lowest MED i-BGP < e-BGP Lowest IGP cost to BGP egress Lowest router ID traffic engineering Enforce relationships Throw up hands and break ties This is somewhat simplified. Hey, what happened to ORIGIN?? Policies Can Interact Strangely (“Route Pinning” Example) :  Policies Can Interact Strangely (“Route Pinning” Example) backup Disaster strikes primary link and the backup takes over Primary link is restored but some traffic remains pinned to backup 1 2 3 4 Install backup link using community customer News At 11:00 :  News At 11:00 BGP is not guaranteed to converge on a stable routing. Policy interactions could lead to “livelock” protocol oscillations. See “Persistent Route Oscillations in Inter-domain Routing” by K. Varadhan, R. Govindan, and D. Estrin. ISI report, 1996 Corollary: BGP is not guaranteed to recover from network failures. What Problem is BGP solving?:  What Problem is BGP solving? aid in the design of policy analysis algorithms and heuristics aid in the analysis and design of BGP and extensions help explain some BGP routing anomalies provide a fun way of thinking about the protocol X could A Wee Bit of Theory Separate dynamic and static semantics :  Separate dynamic and static semantics static semantics dynamic semantics See [Griffin, Shepherd, Wilfong] An instance of the Stable Paths Problem (SPP):  1 An instance of the Stable Paths Problem (SPP) 2 A graph of nodes and edges, Node 0, called the origin, For each non-zero node, a set of permitted paths to the origin. This set always contains the “null path”. A ranking of permitted paths at each node. Null path is always least preferred. (Not shown in diagram) When modeling BGP : nodes represent BGP speaking routers, and 0 represents a node originating some address block most preferred … least preferred (not null) Yes, the translation gets messy! A Solution to a Stable Paths Problem:  1 A Solution to a Stable Paths Problem 2 2 1 0 2 0 1 3 0 1 0 3 0 4 2 0 4 3 0 node u’s assigned path is either the null path or is a path uwP, where wP is assigned to node w and {u,w} is an edge in the graph, each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors. There may be many paths consistent with those assigned to neighbors. This condition limits which may be chosen A Solution need not represent a shortest path tree, or a spanning tree. A solution is an assignment of permitted paths to each node such that red path is the assigned path An SPP may have multiple solutions :  An SPP may have multiple solutions First solution 1 2 0 1 0 2 1 0 2 0 1 2 0 1 0 2 1 0 2 0 1 2 0 1 0 2 1 0 2 0 Second solution DISAGREE BAD GADGET : No Solution:  BAD GADGET : No Solution Why bad? Well… if 1 uses (1,3,0) then 2 must use path (2,0) (it can’t use (2,1,0)) and 3 must use (3,0), in which case 3 ends up assigned a consistent path that is NOT the highest ranked among consistent paths. if 1 uses (1,0), then 2 can use either (2,1,0) or (2,0). If 2 uses (2,1,0) then 3 must use (3,0), which means that the route (1,0) of 1 is consistent but not highest ranked. If 2 instead uses (2,0), then 3 will use (3,2,0), so the route (2,1,0) of 2 will be consistent with assignments to neighbors, but is not assigned to 2, creating a violation of the solution properties. SURPRISE : Beware of Backup Policies:  SURPRISE : Beware of Backup Policies 2 0 3 1 2 1 0 2 0 1 3 0 1 0 3 4 2 0 3 0 4 4 0 4 2 0 4 3 0 Becomes a BAD GADGET if link (4, 0) goes down. BGP is not robust : it is not guaranteed to recover from network failures. PRECARIOUS:  PRECARIOUS Has a solution, but can get “trapped” Part II :  Part II Issues of scale for BGP in the real world Big and Getting Bigger:  Big and Getting Bigger Scaling the iBGP mesh Confederations Route Reflectors BGP Table Growth Address aggregation (CIDR) Address allocation AS number allocation and use Dynamics of BGP Inherent vs. accidental oscillation Rate limiting and route flap dampening Lots and lots of noise Slow convergence time Scale Scale Scale Scale Scale Scale Scale Scale Scale Scale Scale Scale Scale iBGP Mesh Does Not Scale:  iBGP Mesh Does Not Scale eBGP update N border routers means N(N-1)/2 peering sessions Each router must have N-1 iBGP sessions configured The addition of a single iBGP speaker requires configuration changes to all other iBGP speakers Size of iBGP routing table can be order N larger than number of best routes (remember alternate routes!) Each router has to listen to update noise from each neighbor Currently four solutions: (0) Buy bigger routers! Break AS into smaller ASes BGP Route reflectors BGP confederations Route Reflectors:  Route reflectors can pass on iBGP updates to clients Each RR passes along ONLY best routes ORIGINATOR_ID and CLUSTER_LIST attributes are needed to avoid loops RR RR RR Route Reflectors BGP Confederations:  BGP Confederations AS 65501 AS 65502 AS 65503 AS 65504 AS 65500 AS 1 From the outside, this looks like AS 1 Confederation eBGP (between member ASes) preserves LOCAL_PREF, MED, and BGP NEXTHOP. BGP Table Growth:  BGP Table Growth Thanks to Geoff Huston. http://www.telstra.net/ops/bgptable.html on August 8, 2001 Large BGP Tables Considered Harmful :  Large BGP Tables Considered Harmful Routing tables must store best routes and alternate routes Burden can be large for routers with many alternate routes (route reflectors for example) Routers have been known to die Increases CPU load, especially during session reset Moore’s Law may save us in theory. But in practice it means spending money to upgrade equipment … Deaggregation Due to Multihoming May be a Leading Cause:  Deaggregation Due to Multihoming May be a Leading Cause AS 1 customer AS 2 provider 12.0.0.0/8 AS 3 provider 12.2.0.0/16 12.2.0.0/16 12.2.0.0/16 If AS 1 does not announce the more specific prefix, then most traffic to AS 2 will go through AS 3 because it is a longer match AS 2 is “punching a hole” in The CIDR block of AS 1 How Many ASNs are there?:  How Many ASNs are there? Thanks to Geoff Huston. http://www.telstra.net/ops on June 23, 2001 When will we run out of ASNs?:  64,511 2005? 2007? When will we run out of ASNs? What is to be done?:  What is to be done? Make ASNs larger than 16 bits How about 32 bits? See Internet Draft: “BGP support for four-octet AS number space” (draft-ietf-idr-as4bytes-03.txt) Requires protocol change and wide deployment Change the way ASNs are used Allow multihomed, non-transit networks to use private ASNs Use ASE (AS number Substitution on Egress ) See Internet Draft: “Autonomous System Number Substitution on Egress” (draft-jhaas-ase-00.txt) Works at edge, requires protocol change (for loop prevention) Makes some kinds of debugging harder! Multihomed and “Private”! (draft-jhaas-ase-00.txt):  Multihomed and “Private”! (draft-jhaas-ase-00.txt) In fairness: could you “do this right” and still scale? AS 2 AS 1 AS 65535 63.63.63.0/24 AS 3 Replace private ASN Choice of “private” ASN requires a bit of additional coordination between providers A non-transit network ASE-ORIGINATOR is a new attribute needed for “sender side loop detection” at AS 1 and 2 BGP Routing Tables :  BGP Routing Tables Use “whois” queries to associate an ASN with “owner” (for example, http://www.arin.net/whois/arinwhois.html) 7018 = AT&T Worldnet, 701 =Uunet, 3561 = Cable & Wireless, … Hey, we can use these paths to draw cool graphs! show ip bgp BGP table version is 111849680, local router ID is 203.62.248.4 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path . . . *>i192.35.25.0 134.159.0.1 50 0 16779 1 701 703 i *>i192.35.29.0 166.49.251.25 50 0 5727 7018 14541 i *>i192.35.35.0 134.159.0.1 50 0 16779 1 701 1744 i *>i192.35.37.0 134.159.0.1 50 0 16779 1 3561 i *>i192.35.39.0 134.159.0.3 50 0 16779 1 701 80 i *>i192.35.44.0 166.49.251.25 50 0 5727 7018 1785 i *>i192.35.48.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i *>i192.35.49.0 203.62.248.34 55 0 16779 209 7843 225 225 225 225 225 i *>i192.35.50.0 203.62.248.34 55 0 16779 3549 714 714 714 i *>i192.35.51.0/25 203.62.248.34 55 0 16779 3549 14744 14744 14744 14744 14744 14744 14744 14744 i . . . Thanks to Geoff Huston. http://www.telstra.net/ops on July 6, 2001 AS Graphs Can Be Fun:  AS Graphs Can Be Fun The subgraph showing all ASes that have more than 100 neighbors in full graph of 11,158 nodes. July 6, 2001. Point of view: AT&T route-server AS Graphs Depend on Point of View:  AS Graphs Depend on Point of View This explains why there is no UUNET (701) Sprint (1239) link on previous slide! peer peer customer provider 5 4 6 1 3 5 4 2 6 1 3 2 AS Graphs Do Not Show Topology!:  AS Graphs Do Not Show Topology! BGP was designed to throw away information! BGP Dynamics:  BGP Dynamics How many updates are flying around the Internet? How long Does it take Routes to Change? The goals of (1) fast convergence (2) minimal updates (3) path redundancy are at odds Daily Update Count :  Daily Update Count What is the Sound of One Route Flapping?:  What is the Sound of One Route Flapping? A Few Bad Apples …:  A Few Bad Apples … Thanks to Madanlal Musuvathi for this plot. Data source: RIPE NCC Typically, 80% of the updates are for less than 5% Of the prefixes. Most prefixes are stable most of the time. On this day, about 83% of the prefixes were not updated. Percent of BGP table prefixes Two BGP Mechanisms for Squashing Updates:  Two BGP Mechanisms for Squashing Updates Rate limiting on sending updates Send batch of updates every MinRouteAdvertisementInterval seconds (+/- random fuzz) Default value is 30 seconds A router can change its mind about best routes many times within this interval without telling neighbors Route Flap Dampening Punish routes for misbehaving Effective in dampening oscillations inherent in the vectoring approach Must be turned on with configuration 30 Second Bursts:  30 Second Bursts How Long Does BGP Take to Adapt to Changes? :  How Long Does BGP Take to Adapt to Changes? Thanks to Abha Ahuja and Craig Labovitz for this plot. Two Main Factors in Delayed Convergence:  Two Main Factors in Delayed Convergence Rate limiting timer slows everything down BGP can explore many alternate paths before giving up or arriving at a new path No global knowledge in vectoring protocols Why is Rate Limiting Needed?:  Why is Rate Limiting Needed? Updates to convergence MinRouteAdvertisementInterval 0 Time to convergence MinRouteAdvertisementInterval 0 SSFNet (www.ssfnet.org) simulations, T. Griffin and B.J. Premore. To appear in ICNP 2001. Rate limiting dampens some of the oscillation inherent in a vectoring protocol. Current interval (30 seconds) was picked “out of the blue sky” (yet “impact on BGP convergence time is tremendous”) Route Flap Dampening (RFC 2439) :  Route Flap Dampening (RFC 2439) Routes are given a penalty for changing. If penalty exceeds suppress limit, the route is dampened. When the route is not changing, its penalty decays exponentially. If the penalty goes below reuse limit, then it is announced again. Can dramatically reduce the number of BGP updates Requires additional router resources Applied on eBGP inbound only Route Flap Dampening Example:  Route Flap Dampening Example penalty for each flap = 1000 Q: Why All the Updates?:  Q: Why All the Updates? Networks come, networks go There’s always a router rebooting somewhere Hardware failure, flaky interface cards, backhoes digging, floods in Houston, … This is “normal” --- exactly what dynamic routing is designed for… Q: Why All the Updates?:  Q: Why All the Updates? Misconfiguration Route flap dampening not widely used BGP exploring many alternate paths Software bugs in implementation of routing protocols BGP session resets due to congestion or lack of interoperability: BGP sessions are brittle. One malformed update is enough to reset session and flap 100K routes. (Consequence of incremental approach) IGP instability exported by use of MEDs or IGP tie breaker Sub-optimal vendor implementation choices Secret sauce routing algorithms attempting fancy-dancy tricks Weird policy interactions (MED oscillation, BAD GADGETS??) Gnomes, sprites, and fairies …. A: NO ONE REALLY KNOWS … IGP Tie Breaking Can Export Internal Instability to the Whole Wide World:  IGP Tie Breaking Can Export Internal Instability to the Whole Wide World 15 56 192.44.78.0/24 AS 4 AS 3 AS 2 AS 1 10 FLAP FLAP FLAP FLAP 192.44.78.0/24 ASPATH = 4 2 1 192.44.78.0/24 ASPATH = 4 3 1 MEDs Can Export Internal Instability:  MEDs Can Export Internal Instability 15 17 2865 192.44.78.0/24 192.44.78.0/24 MED = 15 192.44.78.0/24 MED = 56 OR 10 56 10 FLAP FLAP FLAP FLAP FLAP FLAP Implementation Does Matter!:  Implementation Does Matter! Thanks to Abha Ahuja and Craig Labovitz for this plot. stateless withdraws widely deployed stateful withdraws widely deployed How Long Will Interdomain Routing Continue to Scale?:  How Long Will Interdomain Routing Continue to Scale? ... the existing interdomain routing infrastructure is rapidly nearing the end of its useful lifetime. It appears unlikely that mere tweaks of BGP will stave off fundamental scaling issues, brought on by growth, multihoming and other causes. A quote from some recent email: Is this true or false? How can we tell? Research required… Summary:  Summary BGP is a fairly simple protocol … … but it is not easy to configure BGP is running on more than 100K routers (my estimate), making it one of world’s largest and most visible distributed systems Global dynamics and scaling principles are still not well understood Addressing and ASN RFCs:  Addressing and ASN RFCs RFC 1380 IESG Deliberations on Routing and Addressing (1992) RFC 1517Applicability Statement for the Implementation of Classless Inter- Domain Routing (CIDR) (1993) RFC 1518 An Architecture for IP Address Allocation with CIDR (1993) RFC 1519 Classless Inter-Domain Routing (CIDR) (1993) RFC 1467 Status of CIDR Deployment in the Intrenet (1983) RFC 1520 Exchanging Routing Information Across Provider Boundaries in the CIDR Environment (1993) RFC 1817 CIDR and Classful routing (1995) RFC 1918 Address Allocation for Private Internets (1996) RFC 2008 Implications of Various Address Allocation Policies for Internet Routing (1996) RFC 2050 Internet Registry IP Allocation Guidelines (1996) RFC 2260 Scalable Support for Multi-homed Multi-provider Connectivity (1998) RFC 2519 A Framework for Inter-Domain Route Aggregation (1999) RFC 1930 Guidelines for creation, selection, and registration of an Autonomous System (AS) RFC 2270 Using a Dedicated AS for Sites Homed to a Single Provider Selected BGP RFCs:  Selected BGP RFCs IDR : http://www.ietf.org/html.charters/idr-charter.html RFC 1771 A Border Gateway Protocol 4 (BGP-4) Latest draft rewrite: draft-ietf-idr-bgp4-12.txt RFC 1772 Application of the Border Gateway Protocol in the Internet RFC 1773 Experience with the BGP-4 protocol RFC 1774 BGP-4 Protocol Analysis RFC 2796 BGP Route Reflection An alternative to full mesh IBGP RFC 3065 Autonomous System Confederations for BGP RFC 1997 BGP Communities Attribute RFC 1998 An Application of the BGP Community Attribute in Multi-home Routing RFC 2439 Route Flap Dampening Internet Engineering Task Force (IETF) http://www.ietf.org Titles of Some Recent Internet Drafts:  Titles of Some Recent Internet Drafts Dynamic Capability for BGP-4 Application of Multiprotocol BGP-4 to IPv4 Multicast Routing Graceful Restart mechanism for BGP Cooperative Route Filtering Capability for BGP-4 Address Prefix Based Outbound Route Filter for BGP-4 Aspath Based Outbound Route Filter for BGP-4 Architectural Requirements for Inter-Domain Routing in the Internet BGP support for four-octet AS number space Autonomous System Number Substitution on Egress BGP Extended Communities Attribute Controlling the redistribution of BGP routes BGP Persistent Route Oscillation Condition Benchmarking Methodology for Basic BGP Convergence Terminology for Benchmarking External Routing Convergence Measurements BGP is a moving target … Selected Bibliography on Routing :  Selected Bibliography on Routing Internet Routing Architectures. Bassam Halabi. Second edition Cisco Press, 2000 BGP4: Inter-domain Routing in the Internet. John W. Stewart, III. Addison-Wesley, 1999 Routing in the Internet. Christian Huitema. 2000 ISP Survival Guide: Strategies for Running a Competitive ISP. Geoff Huston. Wiley, 1999. Interconnection, Peering and Settlements. Geoff Huston. The Internet Protocol Journal. March and June 1999. BGP Stability and Convergence:  BGP Stability and Convergence The Impact of Internet Policy and Topology on Delayed Routing Convergence. Craig Labovitz, Abha Ahuja, Roger Wattenhofer, Srinivasan Venkatachary. INFOCOM 2001 An Experimental Study of BGP Convergence. Craig Labovitz, Abha Ahuja, Abhijit Abose, Farnam Jahanian. SIGCOMM 2000 Origins of Internet Routing Instability. C. Labovitz, R. Malan, F. Jahanian. INFOCOM 1999 Internet Routing Instability. Craig Labovitz, G. Robert Malan and Farnam Jahanian. SIGCOMM 1997 Analysis of Interdomain Routing:  Analysis of Interdomain Routing Cooperative Association for Internet Data Analysis (CAIDA) http://www.caida.org/ Tools and analyses promoting the engineering and maintenance of a robust, scalable global Internet infrastructure Internet Performance Measurement and Analysis (IPMA) http://www.merit.edu/ipma/ Studies the performance of networks and networking protocols in local and wide-area networks National Laboratory for Applied Network Research (NLANR) http://www.nlanr.net/ Analysis, tools, visualization. IRTF Routing Research Group (IRTF-RR) http://puck.nether.net/irtf-rr/ Internet Route Registries:  Internet Route Registries Internet Route Registry http://www.irr.net/ Routing Policy Specification Language (RPSL) RFC 2622 Routing Policy Specification Language (RPSL) RFC 2650 Using RPSL in Practice Internet Route Registry Daemon (IRRd) http://www.irrd.net/ RAToolSet http://www.isi.edu/ra/RAToolSet/ Some BGP Theory:  Some BGP Theory Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000. (Also USC Tech Report, Feb. 1996) Shows that BGP is not guaranteed to converge An Architecture for Stable, Analyzable Internet Routing. Ramesh Govindan, Cengiz Alaettinoglu, George Eddy, David Kessens, Satish Kumar, and WeeSan Lee. IEEE Network Magazine, Jan-Feb 1999. Use RPSL to specify policies. Store them in registries. Use registry for conguration generation and analysis. An Analysis of BGP Convergence Properties. Timothy G. Griffin, Gordon Wilfong. SIGCOMM 1999 Model BGP, shows static analysis of divergence in policies is NP complete Policy Disputes in Path Vector Protocols. Timothy G. Griffin, F. Bruce Shepherd, Gordon Wilfong. ICNP 1999 Define Stable Paths Problem and develop sufficient condition for “sanity” A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001 Dynamic solution for SPVP based on histories Stable Internet Routing without Global Coordination. Lixin Gao, Jennifer Rexford. SIGMETRICS 2000 Show that if certain guidelines are followed, then all is well. Inherently safe backup routing with BGP. Lixin Gao, Timothy G. Griffin, Jennifer Rexford. INFOCOM 2001 Use SPP to study complex backup policies Thank You!:  Thank You! Companion links: http://www.research.att.com/~griffin/interdomain.html

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