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Structure of the Internet

Structure of the Internet. Update for 1 st H/Wk We will start lab next week Paper presentation at the end of the session Next Class MPLS. Other Attempts at the problem. IDPR (and not IDRP) Link state! Prunes the ASes and keeps only the transit ones Assumes information is very static

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Structure of the Internet

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  1. Structure of the Internet • Update for 1st H/Wk • We will start lab next week • Paper presentation at the end of the session • Next Class MPLS

  2. Other Attempts at the problem • IDPR (and not IDRP) • Link state! • Prunes the ASes and keeps only the transit ones • Assumes information is very static • Topology databases need not be synchronized • Use source routing in order to avoid loops • Establish a “connection” through the ASes in the Path • Between Border routers • And repair it when it breaks • More complex than OSPF and BGP-4 together • Did not go anywhere

  3. How is the Internet in reality? • Provider relationships • PoPs and Internet structure? • Provider internal structure • Traffic Egnineering • BGP monitoring tools • Examples

  4. AS relationships • The internet is a BIG AS graph • How does it look? • Ideally, we would like to see a nice hierarchy: customer, local ISP, regional ISP, national ISP, transit ISP • Not really… • Locality is determined based on cost • Reliability requires multiple redundant paths • To whom I talk can have important business implications

  5. Types of relationships • Customer – Provider • Customer pays money for the service • Customer is usually smaller than the provider • Paid Transit • ISP A and B pay ISP C to connect them • Transit ISPs have big global networks (tier-1) • Peering • Two ISPs exchange routes that they originate into the internet • I.e. their own customer routes • No upstream routes • Nobody pays

  6. Things are complicated • Network connectivity does not imply reachability • Policies may prevent it, for example a multi-homed customer can not transit traffic between its providers • Need to know the relationships between ASes and this is not easy: • Policies are not widely advertised • Treated as sensitive business information

  7. Tiers • Large transit ISPs are Tier-1 • (MCI, AT&T) • They have no parent provider • Smaller national/regional ISPs are Tier-2 • GEANT • And small local ISPs are Tier-3 or edge • ForthNet • It is possible to find more structure in Tier-1 • See “characterizing the internet hierarchy from multiple vantage points” • Some tier-1 form the “dense core” of the Internet • Almost fully connected graph, tier-0 • Then tier-1 and tier-2, less connected large ISPs • Then small ISPs and customers • In 2001, 20 ISPs in the dense core

  8. ISPs need to talk to each other • Depends on the relationship • Customer provider over a single link • Transit and peering? • Do it in Internet Exchanges • Also known as Network Access Points (NAPs) and Points of Presence (PoPs) • No need for n^2 connections • Exchange provides a stable environment for peering • Backup power, administration etc • Providers need to “co-locate” in the exchange • Exchanges are not free • Although peering is • Can always have private peerings between two ISPs

  9. Exchange architectures • Centralized • A single or multiple routers • Router may have to enforce policies, not too good • Switched • Just connectivity, BGP enforces the policies • Need to co-locate • More expensive • Co-location costs and cost to send traffic to the exchange • Distributed • No need to co-locate • Not so stable as the centrally administered exchange

  10. Peering Costs • Peering • How to share the cost of an end-to-end path • Cost of sending a packet is almost 0 • Try to split the cost down the middle between the sender and the receiver • Zero cost peering • Slowly emerging paid peering

  11. Peering economics • When is it better to peer? • How much traffic I will be able to send through the peering • So I will not pay for it anymore? • Hard to measure how must traffic goes “behind” certain peers • How much will I have to pay for the exchange peering • Transit costs, exchange costs, operational costs

  12. How to charge? • Charging models in customer-provider • Say I have a OC-12 (622 Mbit/sec) connection • Pay flat rate for the whole thing • Expensive probably • Pay for a fraction of it (say 200 Mbit/sec only) • Can not send more • Burstable fractional • Pay for a fraction but I can send more • Extra traffic charged per Mbyte • 95% charging • Drop the 5% highest samples and use the next one to charge • For the whole month! • How often do I sample the traffic? Usually 5 min… • Volume based charging • And flat rate (DSL style)

  13. Structure of provider networks • Three levels (example) • Aggregation • Distribution • Core • Make sure IGP scales • Do not send it full BGP routes • Neither customer prefixes • Aggregation and distribution may not run iBGP • Core has to run iBGP • In transit ISPs core carries full BGP routes • In edge ISPs core may not have to run iBGP

  14. Routing policy best practices • Do not re-advertise to provider B routes you learn from provider A • Customers should not allow transit • Do not advertise internal networks • Do not advertise prefixes that are aggregated • If you have a single provider no need for full routes • Always check routes you get for bogons • Limit the maximum number of routes you receive from other so that their errors do not kill you

  15. Multi-homing • Two types • Provider assigned prefix • Secondary provider has to agree to advertise it • Provider independent prefix • Both providers will advertise it • But connectivity is only part of the problem • How to I use this multi-homing effectively? • How do I decide where I send traffic? • How can I control how I receive traffic?

  16. Traffic Engineering and BGP • BGP conveys only connectivity information • Can not tell me which is the best/cheapest/least load path to use • I have minimum influence on what paths are used to reach me from other providers • There are some hacks to do something about it • AS prepending: make some paths I export longer so they are not used too much • Selectively advertise my external networks • Breaks aggregation • Use help from my providers • They may advertise communities that allow me to have little bit of control on the incoming path • By telling provider where to advertise my paths • These are not real solutions • Spawned a market for route analytics • But these only address my outbound traffic

  17. Traffic Engineering inside the AS • Need to be able to control how transit and customer traffic flows in my network • It was believed that it is necessary to have circuit based transit to achieve this • ATM • Now MPLS • But IGP may be sufficient if I set the weights in a smart way • More to come…

  18. Hot Potato Routing • IGP cost to reach a BGP next-hop can make all the difference • May affect a lot of traffic and cause instabilities • And cause BGP forwarding loops • BGP routers compute their paths on a timer, in between route computations there may be inconsistencies • IGP cost is low in the BGP path selection process • Paths have to be otherwise the same • Common in tier-1 providers • Also rule for preferring eBGP over iBGP can result in asymmetric paths

  19. BGP Tools • WHOIS • Registration information for AS • Some examples, show the community stuff too • Looking glass • A provider opens ups its routing tables • I can see how my routes look from there • RADB • Route policy registry, some providers do not accept announcements that do not have correct entries there • BGP reports for scaling, CIDR etc • http://bgp.potaroo.net • http:/www.cidr-report.org

  20. General Tools • Ping • Trace route

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