Introduction 1-1 Chapter 1 Part 2 Network Core These slides derived from Computer Networking: A Top Down Approach, 6 th edition. Jim Kurose, Keith Ross Addison-Wesley, March Comp 365 Computer Networks Fall 2014

Introduction 1-2 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge  end systems, access networks, links 1.3 Network core  circuit switching, packet switching, network structure 1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models 1.6 Networks under attack: security 1.7 History

Introduction 1-3 The Network Core  mesh of interconnected routers  the fundamental question: how is data transferred through net?  circuit switching: dedicated circuit per call: telephone net  packet-switching: data sent thru net in discrete “chunks” Resource s reserved Resources allocated on demand. Core Resources: buffers, link transmission rate

Introduction 1-4 The Network Core  The internet is packet switched  Telephone network is circuit switched  We need to understand circuit switching to know why it’s not used for the internet

Introduction 1-5 Network Core: Circuit Switching End-end resources reserved for “call”  link bandwidth, switch capacity  dedicated resources: no sharing  circuit-like (guaranteed) performance  call setup required  State maintained  Data transferred at a guaranteed rate

Introduction 1-6 Network Core: Circuit Switching End-end resources reserved for “call”  Links between circuit switches  Each link can support n circuits  There can be n simultaneous connections.  Each circuit thus gets 1/n of the link’s bandwidth.

Introduction 1-7 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to calls  resource piece idle if not used by owning call (no sharing)  When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts.  dividing link bandwidth into “pieces”  frequency division  time division

Introduction Alternative core: circuit switching end-end resources allocated to, reserved for “call” between source & dest:  In diagram, each link has four circuits.  call gets 2 nd circuit in top link and 1 st circuit in right link.  dedicated resources: no sharing  circuit-like (guaranteed) performance  circuit segment idle if not used by call (no sharing)  Commonly used in traditional telephone networks 1-8

Introduction 1-9 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example: FDM: frequency-division multiplexing TDM: time-division multiplexing Each connection gets one band of frequencies Each connection gets one time slot

Introduction 1-10 Circuit Switching: FDM and TDM FDM frequency time 4 users Example: telephone networks FDM: frequency-division multiplexing 4kHz 4kHz = 4,000 hertz = 4,000 cycles per second Radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz

Introduction 1-11 Circuit Switching: FDM and TDM TDM frequency time 4 users Example: TDM: time-division multiplexing One time slot One frame Circuit gets same time slot in every frame Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot. Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate? Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot. Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate? 64 kbps

Introduction 1-12 Numerical example  How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?  All links have transmission rate of Mbps  Each link uses TDM with 24 slots/sec, 24 slots/frame.  500 msec to establish end-to-end circuit Let’s work it out!

Introduction 1-13 Numerical example  How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?  All links have transmission rate of Mbps  Each link uses TDM with 24 slots/sec, 24 slots/frame.  500 msec to establish end-to-end circuit  Soln:  determine how many slots we need. To do this must figure out how many bits/slot: (1.536 x 106 bits/sec) / (24 slots/sec) = 64,000 bits / slot  Now can determine how many slots we need: 640,000 bits / (64,000 bits/slot) = 10 slots.  So need 10 seconds msec = 10.5 seconds.  This does not account for propagation delays.

Circuit switching: analysis  Disadvantages:  Network resources are wasted during “silent” times (when no one is talking but still connected)  Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software.  Advantage:  Guaranteed bandwith and transmission time  Necessary for some applications (streamed music/video) Introduction 1-14

Introduction 1-15 Network Core: Packet Switching transmission:  Each end-end data stream divided into packets  Each packet travels through communication links  Links connected by packet switches  Routers  Or link-level switches.  Switches use store-and-forward transmission  Switch receives entire packet before it transmits any of it again

Introduction 1-16 Network Core: Packet Switching transmission:

Introduction 1-17 Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth  resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation

Introduction 1-18 Network Core: Packet Switching Switching delays  Assume packet has L bits  Assume there are Q switches  Assume a switch transmits at rate of R  Takes each switch how long to transmit the packet?  L/R  Total delay is:  QL/R

Introduction 1-19 Packet-switching: store-and-forward This Figure:  takes L/R seconds to transmit (push out) packet of L bits on to link at R bps  store and forward: entire packet must arrive at router before it can be transmitted on next link  delay = 3L/R (assuming zero propagation delay) Example:  L = 7.5 Mbits  R = 1.5 Mbps  transmission delay = ??  15 sec R R R L more on delay shortly …

Introduction 1-20 Network Core: Packet Switching Switching delays:  Each switch has multiple links  Each link has buffer  If packet arrives and another packet is already being transmitted on that link, must wait in queue  Called queuing delay.  Varies depending on network congestion  Packet loss: queue is full when packet arrives.

Introduction 1-21 Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand  statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. A B C 100 Mb/s Ethernet 1.5 Mb/s D E statistical multiplexing queue of packets waiting for output link On-demand sharing of resources is called statistical multiplexing

Network Layer4-22 Two key network-core functions forwarding : move packets from router’s input to appropriate router output routing: determines source- destination route taken by packets  routing algorithms routing algorithm local forwarding table header value output link dest address in arriving packet’s header

Introduction 1-23 Packet switching versus circuit switching  1 Mb/s link  For each user assume:  100 kb/s when “active”  active 10% of time  circuit-switching:  10 users  packet switching:  with 35 users, probability > 10 active at same time is less than.0004 Packet switching allows more users to use network! N users 1 Mbps link Q: how did we get value ? A: see this pdf: 65/lectures/Prob10orMoreUsers.pdf

Introduction 1-24 Packet switching versus circuit switching  packet switching:  with 35 users, probability > 10 active at same time is less than.0004  With 10 or fewer simultaneously active users, aggregate arrival rate of data is less than or equal to 1 Mbps.  No loss of time  Allows essentially same performance as circuit switching  But allows > 3 times number of users Packet switching allows more users to use network!

Introduction 1-25 Packet switching versus circuit switching  great for bursty data  resource sharing  simpler, no call setup  excessive congestion: packet delay and loss  protocols needed for reliable data transfer, congestion control  Q: How to provide circuit-like behavior?  bandwidth guarantees needed for audio/video apps  still an unsolved problem (chapter 7) Is packet switching a “slam dunk winner?” Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Most networks use packet-switching; even telephone networks are going to this!

Routing  How do packets make their way through packet-switched Networks? Introduction 1-26

Internet structure: network of networks  End systems connect to Internet via access ISPs (Internet Service Providers)  Residential, company and university ISPs  Access ISPs in turn must be interconnected.  So that any two hosts can send packets to each other  Resulting network of networks is very complex  Evolution was driven by economics and national policies  Let’s take a stepwise approach to describe current Internet structure

Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … …

Internet structure: network of networks Option: connect each access ISP to every other access ISP? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … … … … … connecting each access ISP to each other directly doesn’t scale: O(N 2 ) connections.

Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement. global ISP

Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. ISP B ISP A ISP C

Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. which must be interconnected ISP B ISP A ISP C IXP peering link Internet exchange point

Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and regional networks may arise to connect access nets to ISPS ISP B ISP A ISP C IXP regional net

Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users ISP B ISP A ISP B IXP regional net Content provider network

Introduction Internet structure: network of networks  at center: small # of well-connected large networks  “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage  content provider network (e.g, Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs 1-35 access ISP access ISP access ISP access ISP access ISP access ISP access ISP access ISP Regional ISP IXP Tier 1 ISP Google IXP

Introduction Tier-1 ISP: e.g., Sprint … to/from customers peering to/from backbone … … … … POP: point-of-presence 1-36