The Basics of BGP (Border Gateway Protocol) Routing and its Performance in Today’s Internet

1. The Basics of BGP (Border Gateway Protocol) Routing and its Performance in Today’s Internet Presenter: Sophia Poku Slides taken from presentation by Nina Taft

2. Outline 1. Highlights 2. Addressing and CIDR 3. BGP Messages and Prefix Attributes 4. BGP Decision and Filtering Processes 5. I-BGP 6. Route Reflectors 7. Multihoming 8. Aggregation 9. Routing Instability 10. BGP Table Growth

3. IGP: Interior Gateway Protocol. Examples: IS-IS, OSPF I-BGP R5 R R1 R4 R2 R3 AS1 announce B AS3 border router internal router B Routing Protocols A E-BGP AS2 AS (Autonomous System) - a collection of routers under the same technical and administrative domain. EGP (External Gateway Protocol) - used between two AS’s to allow them to exchange routing information so that traffic can be forwarded across AS borders. Example: BGP

4. Routers used • Internal Router: directly connects networks belonging to the same area • It runs a single copy of the basic routing protocol • Border/Boundary Router: exchanges routing information with routers belonging to other AS

5. you can reachnet A via me R R2 R1 R3 traffic to A table at R1: destnext hop A R2 Purpose: to share connectivity information AS2 AS1 BGP A border router internal router

6. BGP Sessions • Primary function is to exchange network-reachability information (includes AS #s) • Uses TCP to establish connection • Initially … node advertises ALL routes it wants neighbor to know (could be >50K routes) • Ongoing … only inform neighbor of changes • One router can participate in many BGP sessions. AS1 AS3 AS2

7. Configuration and Policy • A BGP node has a notion of which routes to share with its neighbor. It may only advertise a portion of its routing table to a neighbor. • A BGP node does not have to accept every route that it learns from its neighbor. It can selectively accept and reject messages. • What to share with neighbors and what to accept from neighbors is determined by the routing policy, that is specified in a router’s configuration file.

8. 0 8 0 network host 0 16 0 network host 0 24 0 network host Addressing Schemes • Original addressing schemes (class-based): • 32 bits divided into 2 parts: • Class A • 0xxx or 1-126 in decimal; subnet mask:255.0.0.0 • Class B • 10xx or 128-192 in decimal • Subnet mask:255.255.0.0 • Class C • 110x or 192-223 in decimal, Subnet Mask:255.255.255.0 ~2 million nets 256 hosts

9. CIDR (Classless Inter-Domain Routing) CIDR introduced to solve 2 problems: • exhaustion of IP address space • size and growth rate of routing table

10. Problem #1: Lifetime of Address Space • Example: an organization needs 500 addresses. A single class C address not enough (256 hosts). Instead a class B address is allocated. (~64K hosts) That’s overkill -a huge waste. • CIDR allows networks to be assigned on arbitrary bit boundaries. • permits arbitrary sized masks: 178.24.14.0/23 is valid • requires explicit masks to be passed in routing protocols • CIDR solution for example above: organization is allocated a single /23 address (equivalent of 2 class C’s).

11. 232.71.0.0 232.71.1.0 232.71.2.0 ….. 232.71.255.0 232.71.0.0 232.71.1.0 232.71.2.0 ….. 232.71.255.0 service provider service provider Global internet Global internet Problem #2: Routing Table Size Without CIDR: With CIDR: 232.71.0.0 232.71.1.0 232.71.2.0 ….. 232.71.255.0 232.71.0.0/16

12. CIDR: Classless Inter-Domain Routing • Address format <IP address/prefix P>. The prefix denotes the upper P bits of the IP address. • E.g. in CIDR address 206.13.01.48/25, the “/25” indicates the first 25 bits are used to identify a unique network, the remaining bits are host’s • Idea - use aggregation - provide routing for a large number of customers by advertising one common prefix. • This is possible because nature of addressing is hierarchical • Summarizing routing information reduces the size of routing tables, but allows to maintain connectivity. • Aggregation is critical to the scalability and survivability of the Internet

13. 188.00011000.0.0 settable 13th bit Address Arithmetic: Address Blocks • The <address/prefix> pair defines an address block: • Examples: • 128.15.0.0/16 => [ 128.15.0.0 - 128.15.255.255 ] • 188.24.0.0/13 => [ 188.24.0.0 - 188.31.255.255 ]consider 2nd octet in binary: • Address block sizes • a /13 address block has 232-13 addresses(=524288) (/16 has 232-16 =65536) • a /13 address block is 8 times as big as a /16 address blockbecause 232-13 = 232-16 * 23

14. CIDR: longest prefix match • Because prefixes of arbitrary length allowed, overlapping prefixes can exist. • Example: router hears 124.39.0.0/16 from one neighborand 124.39.11.0/24 from another neighbor • Router forwards packet according to most specific forwarding information, called longest prefix match • Packet with destination 124.39.11.32 will be forwarded using /24 entry. • Packet w/destination 124.39.22.45 will be forwarded using /16 entry

15. Will CIDR work ? • For CIDR to be successful need: • address registries must assign addresses using CIDR strategy • providers and subscribers should configure their networks, and allocate addresses to allow for a maximum amount of aggregation • BGP must be configured to do aggregation as much as possible • Factors that complicate achieving aggregation • multihoming, proxy aggregation, changing providers

16. Four Basic Messages • Open: Establishes BGP session (uses TCP port #179) • Notification: Report unusual conditions • Update:Inform neighbor of new routes that become activeInform neighbor of old routes that become inactive • Keepalive: Inform neighbor that connection is still viable

17. BGP Database 1.Neighbor table List of BGP neighbors 2. BGP forwarding table List of all networks learned from each neighbor 3. IP routing table List of best path to destination networks

18. OPEN Message • During session establishment, two BGP speakers exchange their • AS numbers • BGP identifiers (usually one of the router’s IP addresses) • Router ID • Holdtime • Open messages are confirmed using a keep-alive message sent by a peer and must be confirmed before updates • A BGP speaker has option to refuse a session • Select the value of the hold timer: • maximum time to wait to hear something from other end before assuming session is down. • authentication information (optional)

19. NOTIFICATION and KEEPALIVE Messages • NOTIFICATION • Indicates an error • terminates the TCP session • gives receiver an indication of why BGP session terminated • Examples: header errors, hold timer expiry, bad peer AS, bad BGP identifier, malformed attribute list, missing required attribute, AS routing loop, etc. • KEEPALIVE • protocol requires some data to be sent periodically. If no UPDATE to send within the specified time period, then send KEEPALIVE message to assure partner that connection still alive

20. UPDATE Message • Updates are sent using TCP to ensure delivery • used to either advertise and/or withdraw unfeasible prefixes from routing table • path attributes: list of attributes that pertain to ALL the prefixes in the Reachability Info field FORMAT: Withdrawn routes length (2 octets) Withdrawn routes (variable length) Total path attributes length (2 octets) Path Attributes (variable length) Reachability Information (variable length)

21. Advertising a prefix • When a router advertises a prefix to one of its BGP neighbors: • information is valid until first router explicitly advertises that the information is no longer valid • BGP does not require routing information to be refreshed • if node A advertises a path for a prefix to node B, then node B can be sure node A is using that pathitself to reach the destination.

22. BGP Attributes • Attributes: routes learned via BGP have associated properties that are used to determine the best route to a destination when multiple paths exist to a particular destination • Local Preference • Multi-Exit Discriminator (MED) • Origin • AS-path • Next-hop

23. Attribute: ORIGIN • ORIGIN: • Who originated the announcement? Where was a prefix injected into BGP? – indicates how BGP learned about a particular route • IGP: route is interior to the originating AS. This value is Value set using network router configuration command to inject router into BGP • EGP: route learned via the External Gateway Protocol • Incomplete (often used for static routes): origin of routes unknown or learned in some other way

24. Attributes: AS_PATH • a list of AS’s through which the announcement for a prefix has passed • each AS prepends its AS # to the AS-PATH attribute when forwarding an announcement • useful to detect and prevent loops

25. 209.15.1.0/24 A 1.1.1.1 3.3.3.3 D EBGP 2.2.2.2 C B 140.20.1.0/24 IBGP Attribute: NEXT HOP • IP address used to reach the advertising router • For EBGP session, NEXT HOP = IP address of neighbor that announced the route. • For IBGP sessions, if route originated inside AS, NEXT HOP = IP address of neighbor that announced the route • For routes originated outside AS, NEXT HOP of EBGP node that learned of route, is carried unaltered into IBGP. BGP Table at Router C: IP Routing Table at Router C:

26. Next-Hop Cont’d • Router C advertises 172.16.1.0 with next hop 10.1.1.1 • A propagates it within its AS

27. when AS’s interconnected via 2 or more links AS announcing prefix sets MED enables AS2 to indicate its preference AS receiving prefix uses MED to select link a way to specify how close a prefix is to the link it is announced on Attribute: Multi-Exit Discriminator (MED)

28. Used to prefer an exit point from the local AS Used to indicate preference among multiple paths for the same prefix anywhere in the internet. The higher the value the more preferred Exchanged between IBGP peers only. Local to the AS. Often used to select a specific exit point for a particular destination Attribute: Local Preference

29. Routes received from peers Routes sent to peers Input policy engine Routes used by router Decision process Routing Process Overview Choose best route accept, deny, set preferences forward, not forward set MEDs Output policy engine BGP table IP routing table

30. Input Policy Engine • Inbound filtering controls outbound traffic • filters route updates received from other peers • filtering based on IP prefixes, AS_PATH, community • denying a prefix means BGP does not want to reach that prefix via the peer that sent the announcement • accepting a prefix means traffic towards that prefix may be forwarded to the peer that sent the announcement • Attribute Manipulation • sets attributes on accepted routes • example: specify LOCAL_PREF to set priorities among multiple peers that can reach a given destination

31. BGP Decision Process 1. Choose route with highest LOCAL-PREF 2. If have more than 1 route, select route with shortest AS-PATH 3. If have more than 1 route, select according to lowest ORIGIN type where IGP < EGP < INCOMPLETE 4. If have more than 1 route, select route with lowest MED value 5. Select min cost path to NEXT HOP using IGP metrics 6.If have multiple internal paths, use BGP Router ID to break tie.

32. Output Policy Engine • Outbound Filtering controls inbound traffic • forwarding a route means others may choose to reach the prefix through you • not forwarding a route means others must use another router to reach the prefix • may depend upon whether E-BGP or I-BGP peer • example: if ORIGIN=EGP and you are a non-transit AS and BGP peer is non-customer, then don’t forward • Attribute Manipulation • sets attributes such as AS_PATH and MEDs

33. ISP1 ISP2 r1 r3 r2 r2 AS1 AS1 AS1 ISP1 ISP2 r1 r1 r2 r2 r3 r1 r3 r2 r1 r3 AS2 r3 r2,r3 r2,r1 r1 r3 r2 r2 Transit vs. Nontransit AS Transit traffic = traffic whose source and destination are outside the AS Nontransit AS: does not carry transit traffic Transit AS: does carry transit traffic • Advertise own routes only • Do not propagate routes learned from other AS’s • case 1: • Advertises its own routes PLUS routes • learned from other AS’s • case 2:

34. Internal BGP (I-BGP) • Used to distribute routes learned via EBGP to all the routers within an AS • I-BGP and E-BGP are same protocol in that • same message types used • same attributes used • same state machine • BUT use different rules for readvertising prefixes • Rule #1: prefixes learned from an E-BGP neighbor can be readvertised to an I-BGP neighbor, and vice versa • Rule #2: prefixes learned from an I-BGP neighbor cannot be readvertised to another I-BGP neighbor

35. I-BGP: Preventing Loops and Setting Attributes • Why rule #2? To prevent announcements from looping. • In E-BGP can detect via AS-PATH. • AS-PATH not changed in I-BGP • Implication of rule: a full mesh of I-BGP sessions between each pair of routers in an AS is required • Setting Attributes: The router that injects the route into the I-BGP mesh is responsible for • setting the LOCAL-PREF attribute • prepending AS # to AS-PATH

36. RR A B C Route Reflectors • Problem: requiring a full mesh of I-BGP sessions between all pairs of routers is hard to manage for large AS’s. • Solution: • group routers into clusters. • Assign a leader to each cluster, called a route reflector (RR). • Members of a cluster are called clients of the RR • I-BGP Peering • clients peer only with their RR • RR’s must be fully meshed RR RR clusters clients

37. Route Reflectors: Rule on Announcements • Provides mechanisms for minimizing the # of updates messages transmitted within an AS and reducing the amount of data propagated in each message. • If received from RR, reflect to clients • If received from a client, reflect to RRs and clients • If received from E-BGP, reflect to all - RRs and clients • RR’s reflect only the best route to a given prefix, not all announcements they receive. • helps size of routing table • sometimes clients don’t need to carry full table

38. Avoiding Loops with Route Reflectors • Loops cannot be detected by traditional approach using AS-PATH because AS-PATH not modified within an AS. • Announcements could leave a cluster and re-enter it. • Two new attributes introduced: • ORIGINATOR_ID: router id of route’s originator in ASrule: announcement discarded if returns to originator • CLUSTER_LIST: a sequence of cluster id’s. set by RRs.rule: if an RR receives an update and the cluster list contains its cluster id, then update is discarded.

39. Multihoming

40. Single-homed vs. Multi-homed subscribers • A single-homed network has one connection to the Internet (i.e., to networks outside its domain) • A multi-homed network has multiple connections to the Internet. Two scenarios: • can be multi-homed to a single provider • can be multi-homed to multiple providers • Why multi-home? • Reliability • Performance

41. Subscriber called a “stub AS” Provider-Subscriber communication for route advertisement: static configuration. most common. Provider’s router is configured with subscriber’s prefix. good if customer’s routes can be represented by small set of aggregate routes bad if customer has many noncontiguous subnets can use BGP between provider and stub AS Single-homed AS Provider R1 R2 Subscriber

42. ISP 3 ISP 1 ISP 2 Customer Multihoming to Multiple Providers

43. Multihoming Issues • Load sharing • how distribute the traffic over the multiple links? • Reliability • if load sharing leads to preferencing certain links for certain subnets, is reliability reduced? • Address/Aggregation • which subnet addresses should the multihomed customer use? • how will this affect its provider’s ability to aggregate routes?

44. ISP R1 140.35/16 208.22/16 208.22/16 140.35/16 Load sharing from ISP to Customer using attributes • Goal: provider splits traffic across 2 links according to prefix • Implement this strategy using attributes • customer sets MEDs • provider sets LOCAL_PREF R2 R3 Customer

45. Load sharing from Customer to ISP using policy blue: announcements red: traffic • Goal: send traffic to ISP’s customers on one link; send traffic to the rest of the Internet on 2nd link • Implement using policy to control announcements ISP R1 R2 advertise customer routes only advertise default route 0/0 R3 traffic Customer

46. ISP 3 ISP 1 ISP 2 customer Address/Aggregation Issue • Where should the customer get its address block from? • 1. From ISP1 • 2. From ISP2 • 3. From both ISP1 and ISP2 • 4. Independently from address registry (cases 1 and 2 are equivalent)

47. ISP 3 200.50/16 140.20.6/24 140.20/16 ISP 1 ISP 2 140.20/16 200.50/16 140.20.6/24 140.20.6/24 customer 140.20.6/24 Case 1 & 2: Get address block from one ISP • example: customer gets address from ISP 1 • ISP 1’s aggregation is not broken • customer’s prefix not aggregatable at ISP 2 • longer prefix becomes a traffic magnet • How good is load sharing?If all ISP’s generate same amount of traffic for customer, then ISP2-customer link twice as loaded as ISP1-customer link

48. ISP 3 ISP 1 ISP 2 customer Case 3: Get address block from both ISPs • announcement policy: announce prefix only to its “parent” • advantage: both ISP’s can aggregate the prefix they receive • disadvantage: lose reliability • load balancing good? depends upon how much traffic sent to each prefix 140.20/16 200.50/16 140.20.1/24 200.50.1/24 140.20.1/24 200.50.1/24

49. ISP 3 100.20/16 200.50/16 150.55.10/24 ISP 1 100.20/16 ISP 2 200.50/16 150.55.10/24 customer 150.55.10/24 Case 4: obtain address block from registry • no aggregation possible • no traffic magnets created • good reliability • how to achieve load sharing? • customer breaks address block into 2 /25 blocks, and announce one per link (but may lose reliability) • OR use method of AS-PATH manipulation 150.55.10/24 150.55.10/24

50. ISP 3 150.55.10/24 - 2 33 150.55.10/24 - 1 33 33 AS 2 AS 1 150.55.10/24 - 33 33 customer 150.55.10/24 AS 33 AS-PATH manipulation • Idea: prepend your AS number in AS-PATH multiple times to discourage use of a link • makes AS-PATH seem longer than it is • recall BGP decision process uses shortest AS-PATH length as a criteria for selecting best path • Example: ISP 3 will choose path through AS 2 because its AS-PATH appears shorter 150.55.10/24 - 33