Scalability is King 1 Internet Scalability Rules Scalability

Scalability is King 1

Internet: Scalability Rules Scalability is : a critical factor in every decision Ease of deployment and interconnection The intelligence is in the outskirts of the network 2

Internet: Design Goals The goal: to interconnect multiple existing networks and technologies • Packet switching over circuit switching • Reasons: - the applications were suited for this: rlogin - Packet switching was well understood - More flexible, less committal “The design philosophy of the DARPA Internet protocol”, SIGCOMM 88 3

Internet Design: Secondary Level Goals Fault-tolerance (DARPA is the army) Support multiple communication services Accommodate various technologies & networks Allow distributed management Enable cost effective resource Ease of interconnection: adding a host Resources used in the Internet must be accountable (less attention) 4

Fault Tolerance: The Internet Premise: One Robust Connected Network Size of Largest Connected Component #Deleted nodes Robust to random, sensitive to focused failures The network tends to stay as one connected component 5

I. State: The Enemy of Scalability Routing state: the information that a router needs to keep in order to route Large routing state is bad • Consumes memory • Makes look-up slow State = information = intelligence The trade-off: intelligent vs scalable behavior 6

I. An Interesting Antithesis Telephone network: • End devices are stupid • Core is intelligent Internet: • End devices are intelligent • Core is stupid (forward packets, no guarantees) Telephone network: • Circuit switching • Ultra reliable, guarantee quality(real-time) • Centralized control • Difficult to add new users • Cumbersome to new apps Internet: • • • Packet switching Unreliable: no guarantees Decentralized Easy to add new users Easy to add new applications 7

I. Internet: Routing State What does a router know? Routing table: • IP prefix -> outgoing link What is an IP prefix? Scalability: • IP prefixes aggregate many IPs to one entry • Routers do not* keep per connection information * Some exceptions exist. 8

I. State Per Flow: pros and cons What would state per flow give us? Differentiate our behavior per flow • Provide Qo. S Be fair: share resources accordingly Monitor and measure: accountability - costing Ensure that packets follow the same path 9

II. Centralized vs Decentralized Routing Centralized • • All information exists in one place Decisions are taken there The decision involves the whole path OSPF: all routes have all the info: Decentralized • Each router makes a decision in isolation • Each router knows partial/aggregated information • RIP, BGP are some protocols 10

III. Dynamic vs. Static Routing is an optimization problem What are we trying to optimize? Static metrics (non time varying) • Number of hops (hopcount) • Link capacity • Buffer space Dynamic metrics (time varying) • • End 2 end (e 2 e) Delay Link delay Link utilization Available buffer space 11

III. Dynamic or Static Routing? Static routing • Stable • Arguably: utilizes resources well in the long run Dynamic routing • Flexible - adaptable • Better performance (short term at least) 12

IV. Qo. S Aware Routing Quality of Service is tightly coupled with Reservations • I need 10 Mb/s with at most 100 msec e 2 e delay Problems: • Find a suitable path - Centrally easier, - Distributed: start reserving, then backtrack • Reserving resources along the path • Protecting resources from future flows 13

For next time Refresh your memory: Kurose - Ross • Textbook: chapter 1: intro • Textbook: chapter 4: Network Layer and routing Read: Clark’s sigcomm ‘ 88 paper Case study: How stable is Internet routing: • Run traceroute (linux) and see what happens 14