Mathias Fischer Resilient Networks 3.1 Resilient Network Design - Intro Prepared along: Michal Pioro and Deepankar Medhi - Routing, Flow, and Capacity Design in Communication and Computer Networks, The Morgan Kaufmann Series 1 in Networking, 800 pages, 2004
Outline Motivation Introduction to Network Design Problems – Traffic and Demand – Network Design and Routing – Multi-layer Networks – Network Management Notations and mathematical formulation – Link-Path Formulation – Link-Demand-Path-Identifier-based Notation Some basic design problems – Minimizing costs of network links – Capacitated design problems – Shortest path routing Summary 2
A Network Analogy Frankfurt Moscow Peking NY San Francisco Kuala Lumpur Sao Paolo Johannesburg Sydney • New links are expensive • Economies of scale • Store and forward concept • Hop-by-Hop routing 3
Network Basics Communication network carries traffic Network has different links of different capacity (bandwidth) – Economies of scale principle applies Traffic may be routed via different paths to destination – According to store-and-forward principle – Hop-by-hop We need to have enough bandwidth to carry all data We might want to reduce the average packet traversal delay 4
Network Design Questions Can we find better routes? Where should we add more bandwidth? Where and when should we add new nodes (and links) in the network? How does the inherent property of a network technology or protocol can affect our decision making? What level of abstraction is appropriate for a particular network for modeling purpose so that meaningful results can be obtained How to design cost-effective, resilient core/backbone networks taking into consideration properties of the network? - How to do network design? 5
Routing in the Internet – Autonomous Systems Internet consists out of connected Autonomous Systems (AS) – Stub AS : small organization (one connection to the Internet) – Multi-homed AS : large organization (several connections, no transit traffic) – Transit AS : Provider (several connections , transit traffic) The Internet today consists out of 44.000 AS – Of different size – AS Exchange data packets as • Peers • Provider AS and Customer AS Each AS has a unique ID (number) Each AS needs to know at least one route to any other AS Bildquelle: caida.org 6
Communication Networks and Network Providers (1) Packet sending in the Internet Involves series of different AS networks Each network with own switches or routers Packet routing depends completely on the specific network Network design problems are confined within each network or administrative domain AS 1 AS 2 AS 3 7
Communication Networks and Network Providers (2) Layered (or Hierarchy of) Networks Private networks of large companies and corporations Telecommunication network providers that operate transport or transmission networks Different ISPs may use the same transport network Multi-layer network environment AS 2 AS 4 AS 5 AS 3 AS 1 Transport Transport Provider 1 Provider 2 8
Autonomous Systems in Germany Bildquelle: Institut für Internet-Sicherheit, Fachhochschule Gelsenkirchen 9
Communication Networks and Network Providers (3) Network providers Design and manage their networks Need to know – traffic demands in their networks – Considering all network nodes, they need to know traffic volume between any two such nodes Traffic volume matrix or demand volume matrix as input to network design required 10
Traffic and Demand (1) Task of a network – Route packets from one end to another, – without considering the reliability of delivery Reasons for lost packets – Physical transmission errors – Traffic congestion and routers running out of buffer space Task of network designer (our task) – Keep delay low – Design networks to prevent or at least limit congestion at routers 11
Traffic and Demand (2) What do we need for this? Prediction of traffic demand, e.g., via statistical approaches Estimating traffic volume by capturing statistics about traffic arrival distribution We need this information in between different network points Necessary Measurements – Average arrival rate of packets – Average size of packets – Both influence delay 12
Traffic and Demand (3) Example: M/M/1 queuing system Packet arrival follows Poisson process Packet size is exponentially distributed D average delay in seconds λ p average packet arrival in seconds μ p average link service rate C link capacity per second K p average packet size Average delay increases drastically when average arrival rate is closer to average service rate of the link 13
Traffic and Demand (4) What is the acceptable delay users would like to tolerate? – If acceptable delay is 15 ms, then the acceptable average utilization can be no more than 64,5% on the link Good News – At least for purpose of network design maximum link utilization can be used as alternative criterion to the delay However, traffic arrival does not follow Poisson process – Realistic delay is worse than the calculated one – In reality average utilization has to be kept lower, e.g., at 50% to achieve the 15 ms 14
Traffic and Demand (5) We need to know whether observed utilization is higher than acceptable threshold for a particular link type In single-link networks – Measurement is easy – Bandwidth can be easily added In multi-link networks – Utilization is further impacted by routing of traffic flows – Adding bandwidth becomes complex network design problem Lesson learned / What we need to solve network design problems – Average arrival rate in between different nodes in the network – Traffic demand volume as input for all network design problems 15
Network Design and Routing (1) In reality we have many source-destination (egress-ingress) traffic demands between various points in the network Traffic-demand matrix required as input to network design Goal : Determine a network with enough capacity and connectivity to route traffic, so that acceptable service guarantees can be provided – In single-link networks it is sufficient to determine link utilization threshold for given traffic demand – However, this is not the case in multi-link networks anymore 16
Network Design and Routing (2) Role of network design Distinction between (usually continuous) Demand Volume Units (DVU) and (usually discrete) Link Capacity Units (LCU) Three node network with traffic demand 1 of 300Kbps between each node 300 Kbps 300 Kbps – QoS goal: utilization threshold below 60% 300 Kbps 2 3 – Three T1 links (LCU: each 1.54 Mbps) 300 Kbps/1.54Mbps ≈ 19,5% – Two T1 links (2-1, 1-3) (300 +300)Kbps / 1.54 Mbps ≈ 39% 1 300 Kbps 300 Kbps Network design also depends on 300 Kbps routing capabilities 2 3 17
Network Design and Routing (3) Some Definitions Candidate path list : All possible paths between two points Route : Particular path chosen as valid path by network design Flow : The amount of traffic associated with a route 18
Multi-layer Networks (1) End- End- system system SMTP/ POP3 / IMAP/ HTTP / … Layer 5 Layer 5 Application Layer Application Layer UDP / TCP Traffic networks Layer 4 Layer 4 • Aka service level Transport Layer Transport Layer • Traffic arrival IP stochastic in nature • Has switching / routing Layer 3 Layer 3 capabilities Network Layer Network Layer OpenFlow / MPLS / … Layer 2 Layer 2 Transport networks Ethernet / GMPLS Data Link Layer • service level traffic as Data Link Layer OC-1 / OC-3 / OC-48 / T- 1 / … demand input • High-data rate services that Optical Networks Layer 1 Layer 1 are required to be set up at Physical Layer Physical Layer permanent or semi- permanent basis • Traffic deterministic or precise in nature over time 19
Multi-layer Networks (2) The architecture of communication networks A network (or layer) on top another A network may look logically diverse, but may not be diverse in another layer Implications in protection and restoration design (network resilience) due to inter-relation between layers Multi-layer network design as important problem to consider 20
Multi-layer Networks (3) - Traffic and Demand Traffic, Demand, and layered Networks Output bandwidth requirement for each Internet, telephone, and private- line service from service networks, is input demand to layer beneath Capacity requirement of one network becomes traffic demand volume for network below 21
Network Management Cycle (1) Network management is the entire process from planning a network, to deploying it, and to operate it on a day-to-day basis. Requires network management systems and protocols Different management tasks run at different time granularity 22
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