1. 1 Internet History, Architecture, and Routing ECON 425/563 & CPSC 455/555 9/25/2008 ECON 425/563 & CPSC 455/555 9/25/2008

2. 2 Internet History Late 1960s and early 1970s: ARPANET –US Department of Defense –Connects small ARPA-sponsored data networks –Ground breaking testbed for network ideas and designs Early 1980s: Other wide-area data networks are established (e.g., BITNET and Usenet). Late 1980s and early 1990s: –“ARPANET” fades out. –US Gov’t sponsors NSFNET, which connects large regional networks. –Commercial data networks become popular (e.g., Prodigy, Compuserve, and AOL). Mid-1990s: Unified “Internet”

3. 3 Internet Protocols Design Philosophy Ordered set of goals: 1. multiplexed utilization of existing networks 2. survivability in the face of failure 3. support multiple types of communications service 4. accommodate a variety of network types 5. permit distributed management of resources 6. cost effective 7. low effort to attach a host 8. account for resources Not all goals have been met

4. 4 Packets! Basic decision: use packets not circuits (Kleinrock) Packet (a.k.a. datagram) –self contained –handled independently of preceding or following packets –contains destination and source internetwork address –may contain processing hints (e.g., QoS tag) –no delivery guarantees –net may drop, duplicate, or deliver out of order –reliability (where needed) done at higher levels Dest Addr Src Addr payload

5. 5 Telephone Network Connection-based Admission control Intelligence is “in the network” Traffic carried by relatively few, “well-known” communications companies Internet Packet-based Best effort Intelligence is “at the endpoints” Traffic carried by many routers, operated by a changing set of “unknown” parties

6. 6 Directly Connected Machines (a) Point-to-point: e.g., ATM (b) Multiple-access: e.g., Ethernet Can’t build a network by requiring all nodes to be directly connected to each other; need scalability with respect to the number of wires or the number of nodes that can attach to a shared medium (a) (b)

7. 7 Switched Network Circuit switching vs. packet routing Hosts vs. “the network,” which is made of routers Nice property: scalable aggregate throughput routers hosts

8. 8 Interconnection of Networks Recursively build larger networks gateway hosts

9. 9 Some Hard Questions How do hosts share links? How do you name and address hosts? Routing: Given a destination address, how do you get to it? gateway hosts

10. 10 IP Addresses and Host Names Each machine is addressed by an integer, its IP address, written down in a “dot notation” for “ease” of reading, such as 128.36.229.231 IP addresses are the universal IDs that are used to name everything. For convenience, each host also has a human-friendly host name. For example, 128.36.229.231 was concave.cs.yale.edu. Question: How do you translate names into IP addresses?

11. 11 Domain Hierarchy Initially, name-to-address mapping was a flat file mailed out to all the machines on the Internet. Now, we have a hierarchical name space, just like a UNIX file-system tree. Top-level names (historical influence): heavily US-centric, government- centric, and military-centric view of the world educomgovmilorgnetukfr Yale MIT Cisco... Yahoo Math CS Physics concave cyndra netra

12. 12 DNS Zones and Name Servers Divide up the name hierarchy into zones. Each zone corresponds to one or more name servers under the same administrative control. educomgovmilorgnetukfr Yale MIT Cisco... Yahoo Math CS Physics concave cyndra netra

13. 13 Hierarchy of Name Servers Clients send queries to name servers. Name servers reply with answers or forward requests to other name servers. Most name servers perform “lookup caching.” Root name server Yale name server CS name serverEE name server Cisco name server...

14. 14 Application-Level Abstraction What you have: hop-to-hop links, multiple routes, packets, can be potentially lost, can be potentially delivered out-of-order What you may want: application-to-application (end-to-end) channel, communication stream, reliable, in-order delivery host application host application

15. 15 Basic Architectural Principle: Layering Internet Protocol Transmission Control Protocol User Datagram Protocol Telnet HTTP (Web) SONETATM Ethernet Simple Network Management Domain Name Service

16. 16 Interdomain Routing Establish routes between autonomous systems (ASes). Currently done with the Border Gateway Protocol (BGP). AT&T Qwest Comcast Verizon

17. 17 Why is Interdomain Routing Hard? Route choices are based on local policies. Autonomy: Policies are uncoordinated. Expressiveness: Policies are complex. AT&T Qwest Comcast Verizon My link to UUNET is for backup purposes only. Load-balance my outgoing traffic. Always choose shortest paths. Avoid routes through AT&T if at all possible.

18. 18 BGP Route Processing (1) The computation of a single node repeats the following: Receive routes from neighbors Update Routing Table Choose “Best” Route Send updates to neighbors Paths go through neighbors’ choices, which enforces consistency. Decisions are made locally, which preserves autonomy. Uncoordinated policies can induce protocol oscillations. (Much recent work addresses BGP convergence.) Recently, private information, optimization, and incentive-compatibility have also been studied.

19. 19 Apply Policy = filter routes & tweak attributes BGP Route Processing (2) Routing Table Apply Import Policies Best Route Selection Apply Export Policies Install forwarding entries for best routes Receive BGP updates Storage of routes Transmit BGP updates Based on attribute values IP Forwarding Table Apply Policy = filter routes & tweak attributes Open-ended programming: constrained only by vendor configuration language

20. 20 Example: Convergence 1 2 d Prefer routes through 2 Prefer direct route to d

21. 21 Example: Oscillation 1 2 d BGP might oscillate forever between 1d, 2d and 12d, 21d Prefer routes through 2 Prefer routes through 1

22. 22 Example: Convergence 1 2 d Prefer routes through 2 Prefer routes through 1

23. 23 Dispute Wheels Nodes u i, hub routes R i, and spoke routes Q i. Each u i prefers R i Q i+1 to Q i. “No dispute wheel” —> robust convergence

24. 24 Gao-Rexford Framework (1) Neighboring pairs of ASes have one of: – a customer-provider relationship (One node is purchasing connectivity from the other node.) – a peering relationship (Nodes have offered to carry each other’s transit traffic, often to shortcut a longer route.) peerproviders customers peer

25. 25 Gao-Rexford Framework (2) Global constraint: no customer-provider cycles Local preference and scoping constraints, which are consistent with Internet economics: Gao-Rexford conditions => BGP always converges [GR01] Preference Constraints... i d R1R1 R2R2 k2k2 k1k1 If k 1 and k 2 are both customers, peers, or providers of i, then either ik 1 R 1 or ik 2 R 2 can be more valued at i. If one is a customer, prefer the route through it. If not, prefer the peer route. Scoping Constraints d k i j Export customer routes to all neighbors and export all routes to customers. Export peer and provider routes to all customers only. m.. peer customer provider

26. 26 Ongoing Research Challenge Fully characterize the conditions under which BGP converges (robustly). “No dispute wheel” is sufficient but not necessary. Is it enforceable? On those instances on which BGP is guaranteed to converge, how many rounds does it take to converge?