1 Scalability is King

2 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

3 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

4 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)

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

6 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

7 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

8 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.

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

10 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

11 III. Dynamic vs. Static Routing 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) End2end (e2e) Delay Link delay Link utilization Available buffer space

12 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)

13 IV. QoS Aware Routing Quality of Service is tightly coupled with Reservations I need 10Mb/s with at most 100msec e2e delay Problems: Find a suitable path  Centrally easier,  Distributed: start reserving, then backtrack Reserving resources along the path Protecting resources from future flows

14 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