Overview/Questions What does Internet Protocol actually do? What is - PDF document

CS101 Lecture 6: Internetworking: Internet Protocol, IP Addresses, Routing, DNS John Magee 8 July 2013 Some images courtesy Wikimedia Commons 1 Overview/Questions – What does Internet Protocol actually do? – What is an IP address? – How do packets get where they need to go? – How is an Internet domain name related to an IP address? 2 1

What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Q: Other human protocols? Introduction 1-3 Network Protocols Protocol A set of rules that defines how data is formatted and processed on a network. From next week’s video: – Bob Kahn/BBN had been developing many different packet switching networks in 1970s – Each network had its own hardware, node- addressing scheme, and protocols ( http://en.wikipedia.org/wiki/Packet_switched_network) 2

Internet structure: network of networks  roughly hierarchical  at center: small # of well-connected large networks  “tier - 1” commercial ISPs (e.g., Verizon, Sprint, AT&T, Qwest, Level3), national & international coverage  large content distributors (Google, Akamai, Microsoft)  treat each other as equals (no charges) IXP IXP Tier-1 ISPs & Large Content Tier 1 ISP Large Content Content Distributor Distributor (e.g., Google ) (e.g., Akamai ) Distributors, interconnect (peer) privately Tier 1 ISP … or at Internet Tier 1 ISP Exchange Points IXPs Introduction 1-5 Analogy: Intermodal Transport What do these have in common? 6 3

Network Protocols Vint Cerf and Bob Khan created Internet Protocol, to connect different packet switching networks. – "A Protocol for Packet Network Interconnection”, published in 1974 Open System A system which is based on published standards for how protocols should work. – Interoperability of hardware and software from different vendors Internet Protocol Core ideas in Internet Protocol – Each node has a logical IP address – Separates application data from physical transport mechanism – IP packets can travel across multiple networks – Does not guarantee packet delivery  “best effort” 4

Internet Protocol Layer Model A four-layer model of network interaction to facilitate communication standards. Each layer deals with a particular aspect of network communication. Network protocols are usually specific to one layer of this model. IP is a layer 2 protocol. 9 Transport vs. network layer Household analogy:  network layer: logical communication between 12 kids sending letters to 12 hosts kids  transport layer: logical  processes = kids communication between  app messages = letters in processes envelopes – relies on, enhances,  hosts = houses network layer services  transport protocol = Ann and Bill who demux to in- house siblings  network-layer protocol = postal service Transport Layer 3-10 5

Transport Layer Protocols Break messages into packets, hands them off to the IP software for delivery, and then orders and reassembles the packets at their destination. Transmission Control Protocol (TCP) A reliable transport protocol: – TCP guarantees delivery of packets as well as data integrity. User Datagram Protocol (UDP) An unreliable transport protocol. – UDP does not guarantee delivery of packets. – Does provide data integrity of packets 11 Internet transport-layer protocols  reliable, in-order delivery application transport network (TCP) data link physical network – congestion control data link network physical data link – flow control physical – connection setup  unreliable, unordered network data link physical delivery: UDP network data link physical – no- frills extension of “best - network data link application effort” IP physical network transport data link network  services not available: physical data link physical – delay guarantees – bandwidth guarantees Transport Layer 3-12 6

TCP vs UDP Which to use? Examples… 13 Application Layer Protocols The application layer creates data and communicates to other applications on the same or another host. Layering of key network protocols (diagram showing internet (2), transport (3), and application (4) layers 14 7

Application-Level Protocols Port A numeric designation that corresponds to a particular high-level protocol Some high-level protocols which rely upon Internet Protocol, and the ports they use. 15 Network Addresses Recall that a network is a collection of nodes. Each node on a network must be uniquely identifiable. Hostname A name made up of words separated by dots which uniquely identifies a computer on the Internet. Hostnames for a given organization share some common part called a domain name e.g. bu.edu for BU. Examples: www.bu.edu, smtp.bu.edu 16 8

Network Addresses A hostname is a mnemonic for an IP Address. IP Address A logical address made up of four one-byte integers, which uniquely identifies a computer on the Internet. For human consumption, IP addresses are specified in dotted-decimal form. Example: 128.197.26.3 Is there a relationship between the parts of a hostname and an IP address? 17 Network Addresses The IP Address is made up of 2 components: the Network Number and the Host Number. An IP address is stored in four bytes Class A : first byte network number, and three bytes for host number. Class B : first two bytes for network number, and two bytes for host number Class C : first three bytes for network number, and one byte for host number 18 9

Network Addresses The IP Address space is divided into address classes, which determines the size of the address space for administrative purposes. Class Leading Bits Number of Networks Addresses/Network Class A 0 128 16,777,214 Class B 10 16,384 65,534 Class C 110 2,097,152 254 Large telecom companies got Class A addresses. Universities got Class B addresses. Smaller individual companies got Class C addresses. 19 IP Address IPv4 uses 32 bit addresses – How many addresses available? – Will we run out? – Causes – Solutions 20 10

IP Packet Header Each protocol encapsulates some user data along with a packet header. The packet header contains information used for routing and sequencing. 21 IP Packet Header The IP Packet Header is 20 bytes (160 bits) of data: The header specifies 13 fields, including: – packet length – source and destination IP addresses – fragment sequence – time to live (number of hops remaining). 22 11

Routing Routing Routing directs the forwarding of IP packets from their source to their destination. Example by analogy: Suppose you’re flying from Boston, MA to Portland, OR. How do you get there? 23 Example: BOS to PDX 24 United Airlines Route map, source: http://www.united.com/page/article/0,6722,1020,00.html 12

BOS to PDX: Possible Routes  BOS to IAD to PDX (2 hops)  BOS to DEN to PDX (2 hops)  BOS to SFO to PDX (2 hops)  BOS to ORD to PDX (2 hops)  BOS to IAD to SFO to PDX (3 hops)  BOS to IAD to DFW to DEN to PDX (4 hops) 25 BOS to PDX: Possible Routes Suppose you take the first flight to IAD, and then the flight from IAD to PDX was cancelled. Now what?  IAD to PDX (cancelled)  IAD to ORD to PDX (2 hops)  IAD to DEN to PDX (2 hops)  IAD to LAX to PDX (2 hops)  IAD to DFW to DEN to PDX (3 hops) 26 13

Routing Algorithms Routing Information Protocol (RIP) – Uses distance-vector calculation to minimizes the number of hops. – Maximum 15 hops, updates every 30 seconds. Open-Shortest Path First (OSPF) – Detects link failures and adapts quickly – Broadcasts updates to entire network – Incorporates delay and bandwidth 27 Domain Name System Domain Name The part of a hostname that specifies a specific organization or group. Example: bu.edu Domain Name System (DNS) A distributed system for managing hostname resolution – the process of converting a domain name to an IP address. – Analogous to a “phone book” for Internet hosts. 28 14

Domain Name System cs-people.bu.edu hostname domain name Top-Level Domain (TLD) TLD The last section of a Domain Name that specifies the type of organization or its country of origin. http://en.wikipedia.org/wiki/Top-level_domain 29 Domain Name System Domain Name Server Attempts to translate a hostname into an IP address. – In practice, a hostname resolution might require queries to several DNS servers, each one with more detailed information than the previous one. Client programs such as web browsers send requests to a DNS Resolver (in the operating system), which communicates with the DNS servers. 30 15

DNS Query The DNS Resolver typically maintains a cache to prevent unnecessary queries to the DNS Servers. DNS cache entries have a time to live (TTL) metric which prevents stale information from being used. 31 Domain Name System DNS administrators makes changes manually – It takes several hours for changes to propagate throughout the network. When DNS servers fail, users get a “host not found” type of error – DNS update errors have caused large “outages.” – DNS servers are prime targets of denial of service attacks. 32 16

Networking Tools – Windows: ipconfig arp ping tracert (traceroute) – Others such as: nslookup whois 33 Take-Away Points – Internet Protocol – Hostnames and IP Addresses – TCP/UDP – Routing – Domain Name System Combine these ingredients to get the Internet (use only as directed; results may vary). 34 17