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BGP: Introduction and Issues. What Is BGP?. Border Gateway Protocol BGP-4 The de-facto interdomain routing protocol BGP includes specifications: Which information gets advertised and how BGP includes a routing protocol: Establishes and uses a routing table
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BGP: Introduction and Issues Advanced Networks
What Is BGP? • Border Gateway Protocol BGP-4 • The de-facto interdomain routing protocol • BGP includes specifications: • Which information gets advertised and how • BGP includes a routing protocol: • Establishes and uses a routing table • Internal Gateway Protocol (I-BGP in the book) Advanced Networks
Why Is There Such Fuss about BGP? • BGP dictates routing at the AS level • Absence of understanding: poor performance • BGP is complicated • Designed to be flexible • Involves multiple fields • Understanding BGP behavior is not intuitive • Implementation and business policies • The routing of the Internet relies on BGP Advanced Networks
Some Open Questions • How well does BGP work now? • How can I manage a BGP network? • How secure and robust is BGP? • Cyber-terrorism • How would we re-design BGP now? • How well will BGP scale for our future needs? Advanced Networks
Roadmap • Introduction to BGP • Highlights of BGP issues • Goal: instigate interest in BGP Advanced Networks
Some Basic Numbers • 17,000 Autonomous Systems approx. • Corporate Networks • ISP Internal Networks • National Service Providers • Identified by ASN a 16 bit value • Assigned by IANA • Superlinear growth (Huston, Siganos et al.) Advanced Networks
How A BGP graph Looks Like AS 2 AS 5 • Each AS has designated BGP routers • BGP routers of an AS communicate internally with another protocol (IGP) AS 4 AS 3 AS 1 Advanced Networks
IP Addresses and Prefixes • IP addresses have 32 bits: 4 octets of bits (IPv4) • A prefix is a group of IP addresses • 128.32.101.5 is an IP address (32 bits) • 128.32.0.0/16 is a prefix of the 16 first bits: • 128.32.0.0 – 128.32.255.255 (2^16 addresses) • 128.32.4.0/24 is a prefix of the 24 first bits - longer Advanced Networks
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 • 128.1.1.4 matches the first 8 bits – no match! • 128.32.0.0/16 match for 16 bits • 128.32.101.0/24 is a longer match Advanced Networks
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 Advanced Networks
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 Advanced Networks
What Does a Routing Table Look Like? • Origin AS “owns” the address • Routing tables can have peculiarities Advanced Networks
Route Advertising • Distance Vector style protocol • Hear advertisements: IP prefix, AS-path • Filter if desired (i.e. ignore) • Append yourself: IP prefix, myAS+AS-path • Forward to appropriate ASs Advanced Networks
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 Advanced Networks
Provider Customer Peer Peer The Internet as a Directed Graph • Every edge is bidirectional • Business relationships are represented Advanced Networks
The Initial Idea • Data flows between customers-providers • Top level providers are peers • They exchange information to ensure connectivity • What can possibly go wrong? Advanced Networks
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 Advanced Networks
Provider Customer Peer Peer Example: The Intended Use Advanced Networks
BGP Graph and Routing Policies • Up then down: 1, 10, 100, 200, 13, 4 • No valleys, no up-down-up, no more than 1 peer-peer 200 100 10 11 12 13 1 4 3 2 Advanced Networks
The Rules of BGP Routing • Transit traffic: traffic that does not go to my customers (or their customers) • A provider carries any traffic to, from customer • Peers exchange traffic only if between their customers Advanced Networks
Implementing BGP Rules • A customer advertises whatever she wants • A provider forwards everything from customer • So that world now where customer IPs are • A provider advertises whatever customer wants to customer • A peer hears but does not advertise further prefixes from a peer • A peer advertises only its customers to a peer Advanced Networks
Some Simple Policies: Transitivity ISP 1 ISP 2 • A customer should not be transit for its Providers • For this: ASX should not advertise ISP1 advertisements Not allowed AS X Advanced Networks
Basic AS Relationships Provider Customer 200 100 Peer Peer • Customer – Provider: customer pays and is always right • Peer to Peer: Exchange traffic only between their customers • Sibling-Sibling: Exchange traffic at will 10 11 12 13 1 4 3 2 Advanced Networks
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 How BGP Policy Restricts Routing Provider Customer Peer 100 Peer 200 10 11 13 12 1 3 4 2 Advanced Networks
How Policies Affect Routing Customer 1 • 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? ISP1 ISP2 Customer 2 Advanced Networks
BGP Path-Length Asymmetry • Consider number of AS traversed by a path • Asymmetry: 46% of pairs differ by at least one AS hop [Siganos 01] Advanced Networks
Determining The Logical Graph • The business relationships are critical • How can I find the relationships? • Infer relationships from routing tables • IRR database: manually maintained – error prone Advanced Networks
Two Inference Algorithms • Inference algorithm [Gao 00] • Using 1 routing table • Exploit the up-down path property in a routing path, assume highest degree node as peak • Inference using multiple tables [Subramanian02] • Use multiple points of observation to improve results Advanced Networks
Things Become Interesting:Traffic Engineering • How can I pick a route? • Local Preference: path attribute • AS2 wants to prefer fast thick link • Advertisement from right router of AS2 has higher Local Preference • Any BGP router in AS2 will prefer the AS 2 LP 100 LP 80 slow 208.1.1.0/24 AS 1 Advanced Networks
Load Balancing - Appetizer • I want to share traffic between my two providers • How can I do this? Advanced Networks
Load Balancing: Long Prefix Match Takes All! ISP 3 138.39/16 138.39.1/24 ISP 2 ISP 1 138.39.1/24 138.39/16 138.39.1/24 Customer 138.39.1/24 Advanced Networks
So How Can I Balance the Load? • Ask my provider to not aggregate my prefix • Will this work? • Split my prefix in two • 138.39.1.0/24 • A: 138.39.1.31/28 • B: 138.39.1.32/28 • Advertise only one part to ISP2 • ISP2 traffic destined for prefixes in A • ISP1 traffic destined for prefixes in B Advanced Networks
Summary Up to Now • BGP-4 is the de facto protocol for interdomain routing • BGP was developed to achieve: • Flexible policy implementation • Scalability via route aggregation given CIDR • There are many open issues • BGP is a hot research topic Advanced Networks
The Growth of BGP Table Advanced Networks
The BGP Growth: The Truth • Growth flattened out • Why? • Better management • Dot-com crash? Advanced Networks
Routing Table Variation • Larger ASes have significantly larger tables Advanced Networks
Update Activity Per Prefix • Measure rate of announcements + withdrawals + path updates • Compare relative update rate per prefix length to the relative number of prefixes of that length • >1 implies higher than average update rate (less stable) • <1 implies lower than average update rate(more stable) Advanced Networks
Measured Update Rate • Bursty! Advanced Networks
BGP robustness • Measuring the BGP updates Advanced Networks
Analyzing Messages By Content • Aggregated per 30 seconds Advanced Networks
Initial Observations • Updates show daily and weekly periodicity • There is no evidence BGP disturbance: • The Baltimore tunnel train 18 July that destroyed Internet lines • Sept 11 attack • There are some spikes at: • 19 July • 18-22 September Advanced Networks
BGP Updates Correlations Advanced Networks
BGP Under Attack Advanced Networks
Router CPU Activity Correlates… Advanced Networks
The Attack of The Worm Advanced Networks
Classification of Instabilities Advanced Networks
Isolating Instability: 1 unstable peer Advanced Networks
Detecting abnormal BGP activity Advanced Networks
The Worm Activity Advanced Networks
The Worm Correlates Again… Advanced Networks