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ITC242 – Introduction to Data Communications Internet Operation 1 Last Week • SMTP - transmits messages to appropriate hosts via TCP, attempts to provide error-free transmission. • MIME - Intended to resolve problems with SMTP, provides info about body of message, defines multiple content formats, and encodings • HTTP - Stateless protocol, flexible format handling, Proxy, Gateway, Tunnel, Cache • SIP - Manages real-time sessions over IP, enable Internet telephony/VoIP, HTTP-like request/response transaction model 2 Last Week • Client/server - user-friendly client applications, centralized databases, open and modular applications, the network is fundamental • Intranet - internet-based client/server technology within an organization, immensely successful • Extranets – Extend intranet concept to outside community, e.g customers and suppliers, enables sharing of information between companies, TCP/IP enabled form of EDI. 3 Topic 8 – Internet Operation Learning Objectives • Describe the characteristics of an Internet Address • Describe the different classes of IP addresses • Explain the purpose of subnet masks. 4 Network Layer • transport segment from sending to receiving host • on sending side encapsulates segments into datagrams • on rcving side, delivers segments to transport layer • network layer protocols in every host, router • router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network network data link data link physical physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical 5 Two Key Network-Layer Functions • forwarding: move packets from router’s input to appropriate router output( within a single router) • routing: determine route taken by packets from source to dest. – routing algorithms analogy: • routing: process of planning trip from source to dest • forwarding: process of getting through single interchange 6 Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 0111 1 3 2 7 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol •addressing conventions •datagram format •packet handling conventions Routing protocols •path selection •RIP, OSPF, BGP forwarding table ICMP protocol •error reporting •router “signaling” Link layer physical layer 8 IP protocol: IP Addresses • IP (Version 4) addresses are 32 bits long • IP addresses are hierarchical – They contain a network ID and a host ID • IP addresses are assigned statically or dynamically (e.g. DHCP) • IP (Version 6) addresses are 128 bits long 9 IP protocol: IP Addresses 223.1.1.1 • Interface: connection between host/router and 223.1.2.1 physical link 223.1.1.2 223.1.1.4 223.1.2.9 – router’s typically have multiple interfaces 223.1.2.2 223.1.3.27 223.1.1.3 – host typically has one interface – IP addresses associated with each interface 223.1.3.2 223.1.3.1 • Every interface has a unique IP address: – A computer might have two or more IP 223.1.1.1 = 11011111 00000001 00000001 00000001 addresses 223 1 1 1 – A router has many IP 10 addresses IP Address Classes Originally there were 5 classes: 1 CLASS “A” Host-ID 0 Net ID 00000000-01111111(127): 1126 CLASS “B” 100000000-10111111: 128-191 2^14=16,384 Class B addresses CLASS “C” 2 10 3 110 11000000-11011111: 192-223 2^21=2,097,152 CLASS “D” CLASS “E” 0 24 7 A 16 14 Host-ID Net ID 8 21 Host-ID Net ID 4 28 1110 Multicast Group ID 5 27 11110 Reserved B C D 232-1 11 IP Addresses Examples Class “A” address: www.mit.edu 18.181.0.31 (18<128 => Class A) Class “B” address: mekong.stanford.edu 171.64.74.155 (128<171<128+64 => Class B) 12 IP Address Some Problems: • • Address classes were too “rigid”. For most organizations, Class C were too small and Class B too big. Led to inefficient use of address space, and a shortage of addresses. Small organizations wanted Class B in case they grew to more than 255 hosts. But there were only about 16,000 Class B network IDs. 13 Solution ? Subnetting within an organization to subdivide the organization’s network ID. 14 Subnets CLASS “B” e.g. Company e.g. Site 2 10 2 10 Net ID 0000 Subnet ID (20) e.g. Dept 2 10 Subnet ID (22) 2 Host-ID 10 16 000000 2 Host-ID Subnet Host ID (10) 16 14 Net ID 1111 Subnet ID (20) Subnet Host ID (12) 14 Net ID Host-ID Net ID 16 14 16 14 10 Subnet Host ID (12) 16 14 Net ID Subnet ID (26) Host-ID 1111011011 Host-ID Subnet Host ID (6) 15 Subnets • IP address: – subnet part (high order bits) – host part (low order bits) 223.1.1.1 223.1.2.1 223.1.1.2 223.1.1.4 223.1.1.3 223.1.3.27 223.1.2.2 subnet • What’s a subnet ? – device interfaces with same subnet part of IP address – can physically reach each other without intervening router 223.1.2.9 223.1.3.1 223.1.3.2 network consisting of 3 subnets 16 Subnets Recipe • To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnet mask: /24 17 Subnets & Subnet Masks • Allows for subdivision of internets within an organization • Each LAN can have a subnet number, allowing routing among networks • Host portion is partitioned into subnet and host numbers 18 Subnet Mask Calculations 19 Example of Subnetworking 20 Subnet masks 21 Source: http://zdnetasia.com/insight/network/0,39044847,39372217,00.htm IP addresses: how to get one? Q: How does network get subnet part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 Organization 1 Organization 2 ... 11001000 00010111 00010000 00000000 11001000 00010111 00010010 00000000 11001000 00010111 00010100 00000000 ….. …. 200.23.16.0/23 200.23.18.0/23 200.23.20.0/23 …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 a.b.c.d/x where x bits constitute the network portion of The IP address, and often referred to as the prefix of the address 22 IP addresses: how to get one? Q: How does host get IP address? • hard-coded by system admin in a file – Wintel: control-panel->network->configuration->tcp/ip>properties • DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server – “plug-and-play” – Goal: allow host to dynamically obtain its IP address from network server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on” Support for mobile users who want to join network 23 IP addressing: the last word... Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers – allocates addresses – manages DNS – assigns domain names, resolves disputes 24 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer IP protocol •addressing conventions •datagram format •packet handling conventions Routing protocols •path selection •RIP, OSPF, BGP forwarding table ICMP protocol •error reporting •router “signaling” Link layer physical layer 25 “A” The Problem “B” R2 R1 How does R1 choose a route to host B? R4 R3 26 Routing Metrics • Metrics – Delay to send an average size packet (Make high speed links attractive, but closeness counts) – Bandwidth – Link utilization – Stability: Is a link (or path) up or down? • Today: about 1/3 of Internet routes are asymmetric 27 Technique 1: Naïve Approach Flood! -- Routers forward packets to all ports except the ingress port. R1 Advantages: Simple. Every destination in the network is reachable. Disadvantages: Some routers receive a packet multiple times. Packets can go round in loops forever. Inefficient. 28 Technique 2: Bellman-Ford Algorithm Objective: Determine the route from (R1, …, R7) to R8 that minimizes the cost. Examples of link cost: Distance, data rate, price, congestion/delay, … 1 R1 1 R2 R4 2 2 R3 4 4 R6 3 R5 2 R7 3 2 R8 29 Example network In this simple case, solution is clear from inspection A 1 R1 1 R2 R4 2 2 R3 4 4 R6 3 R5 2 R7 3 2 R8 B 30 So what about this network...!? The public Internet in 1999 Learn more at http://www.lumeta.com 31 Technique 3: Dijkstra’s Shortest Path First Algorithm • • • The algorithm identifies the least costly paths between source and destination, given that costs are assigned to the edges. Routers send out update messages whenever the state of a link changes. Hence the name: “Link State” algorithm. Each router calculates lowest cost path to all others, starting from itself. 32 The problem • How to route in the Internet? 33 Internet Routing Protocols • Responsible for receiving and forwarding packets between interconnected networks • Must dynamically adapt to changing network conditions 34 Autonomous Systems (AS) • Key characteristics – Set of routers and networks managed by single organization – group of routers exchanging information via a common routing protocol – connected (in a graph-theoretic sense); that is, there is a path between any pair of nodes 35 Autonomous System Example 36 Directed Graph of Example 37 Routing in the Internet The Internet uses hierarchical routing • The Internet is split into Autonomous Systems (AS’s) • Within an AS, the administrator chooses an Interior Gateway Protocol (IGP) Examples of IGPs: RIP (rfc 1058), OSPF (rfc 1247). • Between AS’s, the Internet uses an Exterior Gateway Protocol AS’s today use the Border Gateway Protocol, BGP-4 (rfc 1771) 38 Routing in the Internet The Internet uses hierarchical Gateway router routing • • Direct link to router in The Internet is split into Autonomous Systems (AS) another AS • aggregate routers into regions, AS routers in same AS run same routing protocol • – “intra-AS” routing protocol • routers in different AS can run different intra-AS routing protocol 39 Interconnected ASes 3c 3a 3b AS3 1a 2a 1c 1d 1b Intra-AS Routing algorithm 2c AS2 AS1 Inter-AS Routing algorithm Forwarding table 2b • forwarding table configured by both intra- and inter-AS routing algorithm – intra-AS sets entries for internal dests – inter-AS & Intra-As sets entries for 40 external dests Inter-AS tasks AS1 must: 1. learn which dests reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 Job of inter-AS routing! • suppose router in AS1 receives datagram dest outside of AS1 – router should forward packet to gateway router, but which one? 3c 3b 3a AS3 1a 2a 1c 1d 1b 2c AS2 2b AS1 41 Example: Setting forwarding table in router 1d • suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2. • inter-AS protocol propagates reachability info to all internal routers. • router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. – installs forwarding table entry (x,I) x 3c 3a 3b AS3 1a 2a 1c 1d 1b AS1 2c 2b AS2 42 Example: Choosing among multiple ASs • now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2. • to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. – this is also job of inter-AS routing protocol! x 3c 3a 3b AS3 1a 2a 1c 1d 1b 2c AS2 2b AS1 43 44 Internet inter-AS routing: BGP • BGP (Border Gateway Protocol): the de facto standard • maintain a table of IP networks or 'prefixes' which designate network reachability among AS. • BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all AS-internal routers. 3. Determine “good” routes to subnets based on reachability information and policy. • allows subnet to advertise its existence to rest of Internet: “I am here” 45 BGP basics • pairs of routers (BGP peers) exchange routing info over TCP connections: BGP sessions • when AS2 advertises prefix to AS1: – AS2 promises it will forward any addresses datagrams towards that prefix. – AS2 can aggregate prefixes in its advertisement eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b 46 Distributing reachability info • using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. – 1c can then use iBGP do distribute new prefix info to all routers in AS1 – 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session • when router learns of new prefix, creates entry for prefix in its forwarding table. eBGP session 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 1b 2c AS2 2b 47 Intra-AS Routing Protocols OSPF(Open Shortest Path First): A link-state protocal Link-state updates sent (using flooding) as and when required. A router broadcasts routing information to all other routers in the AS, not just to its neighboring routers. Every router locally runs Dijkstra’s algorithm to determine a shortest-path tree to all subnets. Authenticated updates: all OSPF messages authenticated (to prevent malicious intrusion) Autonomous system may be partitioned into “areas”. hierarchical OSPF in large domains 48 Hierarchical OSPF 49 Hierarchical OSPF • two-level hierarchy: local area, backbone. – Link-state advertisements only in area – each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. • area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. • backbone routers: run OSPF routing limited to backbone. 50 • boundary routers: connect to other AS’s. Topic 9 – LAN architecture and protocols Learning Objectives • Define the various types of Local Area Networks (LANs) • Discuss the different types of transmission media commonly used in LANs. 51 Backend & Storage Area Networks • • • • • • “Computer room networks” High data rate High-speed interface Distributed access Limited distance Limited number of devices 52 Storage Area Network (SAN) • A separate network to handle storage needs • Decouples storage tasks from specific servers • Creates a shared storage facility across a high-speed network 53 High-Speed Office Networks • Increased processing and transfer requirements in many graphics-intensive applications now require significantly higher transfer rates • Decreased cost of storage space leads to program and file bloat, increased need for transfer capacity • Typical office LAN runs at 10Mbps, high-speed alternatives run at 100Mbps, 1 Gbps, 10Gbps 54 Backbone Local Networks • • • • Used instead of single-LAN strategy Better reliability Higher capacity Lower cost 55 Factory Networks • • • • • High capacity Ability to handle a variety of data traffic Large geographic extent High reliability Ability to specify and control transmission delays 56 Tiered LANs • Cost of attachment to a LAN tends to increase with data rate • Alternative to connecting all devices is to have multiple tiers • Multiple advantages – Higher reliability – Greater capacity (less saturation) – Better distribution of costs based on need 57 Tiered LAN Diagram 58 The Media • The Transmission Media is the physical path between transmitter and receiver • Can be classified as guided or unguided • For both transmission is with electromagnetic waves. • Guided Media – waves are guided along a solid medium, e.g. cables • Unguided Media – wireless transmission 59 Guided Media • Twisted Pair Wires • Coaxial Cable • Fibre Optic Cable 60 Twisted Pair Wires • Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs • Often used at customer facilities and also over distances to carry voice as well as data communications • Low frequency transmission medium 61 Types of Twisted Pair • STP (shielded twisted pair) – the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference 62 Types of Twisted Pair • UTP (unshielded twisted pair) – each wire is insulated with plastic wrap, but the pair is encased in an outer covering 63 Ratings of Twisted Pair • Category 3 UTP – data rates of up to 16mbps are achievable • Category 5 UTP – data rates of up to 100mbps are achievable – more tightly twisted than Category 3 cables – more expensive, but better performance • Category 5e UTP – 1Gbps • Category 6 UTP- Up to 10 Gbps • STP – More expensive, harder to work with 64 Twisted Pair Advantages • Inexpensive and readily available • Flexible and light weight • Easy to work with and install 65 Twisted Pair Disadvantages • Susceptibility to interference and noise • Attenuation problem – For analog, repeaters needed every 5-6km – For digital, repeaters needed every 2-3km 66 Coaxial Cable (or Coax) • Used for cable television, LANs, telephony • Has an inner conductor surrounded by a braided mesh • Both conductors share a common center axial, hence the term “coaxial” • Traditionally used for LANs, but growth of twisted pair for local nets and optical fiber for larger nets has reduced coax use 67 Fiber Optic Cable • Fiber optic cable is used for modular light transmission. Instead of transmitting electrical signals, it transmits pulses of light that represent bits. • Advantages – Greater capacity – Smaller size/lighter weight – Lower attenuation – Electromagnetic isolation • Operate in the range of about 1014 to 1015 Hz; (portions of the infrared and visible spectrums) 68 Fiber Optic Layers • consists of three concentric sections plastic jacket glass or plastic fiber core cladding 69 Fiber Optic Types • single-mode fiber – A single-mode cable uses lasers to generate light. It allows just one mode of light to pass through it at a time, but is capable of greater bandwidth and greater distances than multimode cable. It is more expensive than multimode cable, and has a maximum cable length of 60 kilometers • multimode fiber – Multimode cable allows multiple light modes to pass along its fibers. Favored in workgroup applications, multimode cable uses light emitting diodes (LEDs) to generate light. A multimode fiber optic cable cannot exceed 2 kilometers. 70 Fiber Optic Signals fiber optic multimode step-index fiber optic single mode 71 Comparison of Media • Twisted pair cable is a common cable type - it is available as shielded twisted pair (STP) or unshielded twisted pair (UTP). STP cable combines the techniques of twisting wires and shielding. UTP cable is a copper wirebased cable used in a variety of networks. Coaxial cable operates over relatively large distances, and transmits data at speeds of up to 100 Mbps. Installing coaxial cable is more expensive than installing twisted pair cable. Fiber optic cable transmits bits in the form of modulated light data. Light is refracted along the cable and can go around bends. Fiber optic cables are available as single-mode or multimode cable. Wireless signals are radio frequencies and infrared waves that can travel through air. They are a growth area in network communications and represent the future of communication media. 72